Manufacture and Processing of PVC

MANUFACTURE AND PROCESSING OF PVC

MANUFACTURE AND PROCESSING OF PVC Edited by R.H.BURGESS Senior Research Chemist, PVC Research, Imperial Chemical Industries Ltd, Plastics Division, Bessemer Road, Welwyn Garden City, Herts., UK

ELSEVIER APPLIED SCIENCE PUBLISHERS LONDON and NEW YORK

ELSEVIER APPLIED SCIENCE PUBLISHERS LTD Crown House, Linton Road, Barking, Essex IG11 8JU, England This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” British Library Cataloguing in Publication Data Manufacture and processing of pvc. 1. Polyvinyl chloride I. Burgess, R.H. 668.4'237 TP118.V48 ISBN 0-203-49056-8 Master e-book ISBN

ISBN 0-203-79880-5 (Adobe eReader Format) ISBN 0-85334-972-X (Print Edition) WITH 28 TABLES AND 100 ILLUSTRATIONS © APPLIED SCIENCE PUBLISHERS LTD 1982 Reprinted 1986 Special regulations for readers in the USA This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside the USA, should be referred to the publisher. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher.

FOREWORD

In my long period of intimate involvement with the PVC industry I have seen the business expand from its small beginnings to the world-wide business of today. Much important research work was carried out in the early days laying the foundations for the sophisticated control of particle type both of the suspension and mass types of PVC and in emulsion polymerisation which is such a feature of the business today. Perhaps in no other large scale plastic are the physical forms in which the polymer powder can be produced (as opposed to the detailed chemical structure of the molecule) of such importance; and it is this feature which makes the interaction between what happens in the polymerisation vessel and the subsequent use of the polymer so crucial. The industry faced its most serious challenge in the early 1970s with the discovery of a link between exposure to vinyl chloride and a rare form of liver cancer. Its vigorous response to that challenge is a model for the way in which these problems should be tackled, and its successful outcome has ensured the continued growth of the business. The pace of change of the manufacturing process, in the understanding of the processes involved in converting raw PVC to the final article and in the processing equipment itself, remains high with new markets still being developed. All these factors are discussed in this book which has been written by five ICI colleagues who have worked with me for many years. I believe it represents a very fair and up-to-date view of the state of the art. A.W.BARNES Formerly ICI Plastics PVC Director and Chairman of CIA and European CEFIC Committees on VCM Toxicity

PREFACE

Polyvinyl chloride (PVC) has been produced commercially for 50 years but most of the expansion has taken place since the end of the 1939–45 War. From its early beginning in Germany the market has grown to the present 12 million tonnes per year worth £5 billion a year in turnover. It is manufactured by over 70 major companies in more than 30 countries. In turn each of these 70 manufacturers may have up to 100 customers who fabricate articles from PVC many of whom are themselves part of large organisations. The growth of this large market and the involvement of many major companies has stimulated much development work over the years on the manufacturing process, on the product produced and on its subsequent fabrication. For example, a typical manufacturer may have up to 100 or more technically qualified people working on various ways of improving the production and subsequent processing of PVC. Such a large effort has led to important advances in the technologies employed in the processes used to produce PVC and in the ways in which it is subsequently converted to the final article. ICI have been manufacturing PVC in the UK for approximately 40 years and throughout most of this period have developed their technology in close collaboration with Solvay, the leading continental European producer. Outside Europe, through subsidiary/associate companies and licensing arrangements, the ICI/ Solvay technology is also being operated in the USA, Argentina, Brazil, Australia, South Africa and Turkey. Consequently within the ICI organisation there is knowledge of the PVC industry world-wide. Five experts from ICI with experience ranging from 12 to 30 years in the business have contributed 10 chapters to this book covering the essential features of the major polymerisation processes for the production of PVC, including random and graft copolymers and blends of PVC with special resin and rubber additives. Particular attention is given to the so-called stripping step in the process, i.e. the effective removal of unreacted vinyl chloride—today a mandatory requirement. The important features of the subsequent processing of PVC resin to the final articles of commerce with which we are all familiar are also covered and the importance of the morphology of the PVC resin particle in this subsequent processing discussed. We are particularly grateful to Rhone-Poulenc, who contributed to the chapter on the bulk process for making PVC as this is not within ICI’s direct experience. Thus, this book contains in one volume the essential features of the technology of the PVC industry, including the most up-to-date thinking on the different established manufacturing processes and the subsequent fabrication of PVC into finished articles. The authors are grateful to ICI for permission to publish the information in the book and to J.H.Wilson who read the manuscript and made many useful suggestions which have been incorporated into the text. R.H.BURGESS

LIST OF CONTRIBUTORS

M.W.ALLSOPP, A.P.I., M.Sc. Senior Research Chemist, PVC Research, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. R.H.BURGESS, B.Sc., Ph.D. Senior Research Chemist, PVC Research, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. D.E.M.EVANS, B.Sc., Ph.D. Senior Research Chemist, PVC Research, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. V.G.LOVELOCK Consultant, 31 Westly Wood, Welwyn Garden City, Herts, UK. Formerly Senior Research Chemist, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. D.A.TESTER, B.Sc., Ph.D. PVC Development Group Leader, PVC Technical Service, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK.

CONTENTS

Foreword

iv

Preface

v

List of Contributors

vi

Introduction

xi

SUSPENSION POLYMERISATION OF VINYL CHLORIDE R.H.Burgess

1

1.1

Market for PVC and Principal Manufacturing Processes

1

1.2

Kinetics of Vinyl Chloride Polymerisation

2

1.3

Outline of Suspension Polymerisation Process

6

1.4

The Polymerisation Process

7

1.5

Cost of Manufacturing Suspension PVC

17

1.6

Use of Large Autoclaves

19

Acknowledgements

27

References

27

BULK PROCESSES FOR THE MANUFACTURE OF PVC M.W.Allsopp

28

2.1

Introduction

28

2.2

Development of Bulk Polymerisation of Vinyl Chloride

29

2.3

Evolution of St Gobain to Rhone-Poulenc

30

2.4

History of the Rhone-Poulenc Bulk Process for PVC

31

2.5

Rhone-Poulenc Two-stage Bulk Polymerisation Process

32

2.6

Control of Properties

36

2.7

Process Control

37

2.8

Degassing and Powder Handling

37

2.9

Morphology

38

Chapter 1

Chapter 2

viii

2.10

Copolymers

39

2.11

Cost Comparison of Suspension and Bulk Processes

40

2.12

Latest Development in the Two-stage Process: Vertical Autoclaves

40

2.13

Polymer Quality Status—Bulk Versus Suspension

42

2.14

Gas Phase Polymerisation

43

Acknowledgements

43

References

44

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS D.E.M.Evans

45

3.1

Introduction

45

3.2

Types of Polymer Produced Using the Emulsion Process

46

3.3

Applications of Paste/Emulsion Polymers

46

3.4

Production of Paste/Emulsion Polymers

47

References

58

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS R.H.Burgess

59

4.1

Introduction

59

4.2

Theory of Copolymerisation

60

4.3

Vinyl Acetate Copolymers

61

4.4

Olefin Copolymers

64

4.5

Other Copolymers

66

4.6

PVC Blends—General

67

4.7

Rubber Blends

67

4.8

Polymer Blends

70

4.9

Filler Blends

71

Acknowledgements

71

References

71

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC R.H.Burgess

72

5.1

Vinyl Chloride Toxicity

72

5.2

Protection of PVC Plant Operators

73

Chapter 3

Chapter 4

Chapter 5

ix

5.3

Vinyl Chloride Analysis

75

5.4

Removal of Residual VCM

77

Acknowledgements

85

References

85

ISOLATION PROCESSES FOR PVC V.G.Lovelock

87

6.1

Introduction

87

6.2

Suspension Polymerisation

87

6.3

Emulsion/Microsuspension Polymerisation

97

6.4

Bulk Polymerisation

107

6.5

PVC Dust—Possible Hazards

107

References

108

MORPHOLOGY OF PVC M.W.Allsopp

109

7.1

Introduction

109

7.2

Nomenclature

110

7.3

Classification of PVC Morphology

111

7.4

Mechanism of Suspension Polymerisation

116

7.5

Overall Morphology of Suspension PVC and Polymer Properties

125

7.6

Morphology of Rhone-Poulenc Bulk Polymer

128

7.7

Morphology of Gas Phase Polymer

128

7.8

Summary

130

Acknowledgements

132

References

132

MECHANISM OF GELATION OF RIGID PVC M.W.Allsopp

135

8.1

Introduction

135

8.2

Pre-mixing with Additives

135

8.3

Effect of Processing Parameters

137

8.4

Extruder Sampling Techniques

141

8.5

Twin-Screw Extruder Sampling

143

Chapter 6

Chapter 7

Chapter 8

x

8.6

Single-Screw Extruder Sampling

147

8.7

Other Extruders

150

8.8

Banbury High Shear Internal Mixer

150

8.9

Brabender Plasticorder

157

8.10

Two-roll mill

158

8.11

Summary

158

Acknowledgements

160

References

160

THE PROCESSING OF RIGID PVC D.A.Tester

161

9.1

Introduction

161

9.2

Feedstock Preparation

162

9.3

Extrusion

164

9.4

The Influence of Processing on Properties

165

9.5

The Influence of Formulation Ingredients on Processing Behaviour

170

9.6

Orientation

181

References

181

THE PROCESSING OF PLASTICISED PVC D.A.Tester

183

10.1

Introduction

183

10.2

Feedstock Preparation

183

10.3

Polymer/Plasticiser Interaction

185

10.4

Particle Morphology and Melt Flow

185

10.5

The Effect of Formulation Ingredients

188

10.6

The Processing of PVC Pastes

193

References

200

Index

201

Chapter 9

Chapter 10

INTRODUCTION

Polyvinyl chloride (PVC) has been produced commercially for 50 years. It was first produced in Germany in the early 1930s but its extensive use did not start until the 1939–45 War when mixtures with certain organic liquids (plasticisers) producing a flexible material found wide application as a rubber substitute, particularly in those countries denied access to natural rubber supplies. This early commercial success stimulated the development of cheap processes for the production of the monomer, vinyl chloride (VCM), initially based on the reaction of hydrochloric acid with acetylene, both materials cheap to produce and readily available. More recently cheaper processes involving the reaction of chlorine with ethylene (oxychlorination) have been developed. Many companies, interested in the electrolysis of brine to give chlorine and caustic soda, saw this new polymer as an important user of chlorine so maintaining the allimportant balance between sales of chlorine and caustic soda. It was soon realised that PVC/plasticiser mixtures could be used for a wide range of applications such as cable covering, raincoats, fabric coating, etc., where flexibility, toughness and, in some cases, transparency were required, and that these articles could be made on equipment already available for processing rubber. In many cases the product, used, for example, in cable covering, showed real advantages over that based on the materials used hitherto. The-combination of comparative cheapness, the link with the already important chlor-alkali business and the excellent properties of flexible products based on PVC persuaded a large number of major chemical companies to become interested in PVC. Their interest and that of their customers who fabricated the final articles, coupled with the excellent properties of the material, have seen the world market for PVC itself increase to its present 12 million tonnes per year (worth £5 billion a year in turnover). The market for PVC articles in total is worth several times this figure when machinery sales, stabilisers, lubricants, other additives, plasticisers, etc., are taken into account. This large growth has seen many changes to the polymerisation process, the fabrication techniques used, the types of articles produced and their properties. The first production process for PVC was based on emulsion polymerisation of VCM to produce a latex from which the PVC was separated by spray drying. This process was expensive since the product contained significant quantities of relatively high cost emulsifier and all water had to be removed by evaporating it with heat. Most manufacturers turned their attention to the suspension polymerisation process in which droplets of vinyl chloride were polymerised to produce coarser particles rather like grains of sand. This process is potentially cheaper because it uses much lower concentrations of additives and because much of the associated water can be removed by filtration. Commercially attractive suspension polymerisation processes were soon developed but, at least initially, the product was much more difficult to process into the final article than that based on the emulsion process, both in the flexible and rigid (i.e. unplasticised) applications, such as packaging film and foil, developing at that time. It was some time

xii

before the necessary control of particle structure was achieved to enable suspension PVC to be used for flexible and rigid applications but once this was achieved the cheaper suspension PVC largely superseded emulsion PVC in a wide range of PVC applications. The present suspension polymerisation process, including the addition polymerisation kinetics used in virtually all commercially important types of polymerisation, is described in Chapter 1. The rapid growth of the rigid PVC market in the last 30 years for such articles as film, sheet, pipe, rainwater goods, conduit and other profiles has been achieved in spite of the thermal instability of PVC at about the processing temperature needed to fabricate the PVC powder into the final rigid article. This problem has prompted the development of complex stabiliser systems, the production of intrinsically more stable PVC and detailed changes to the PVC grain structure to ensure rapid and uniform gelation of the PVC powder to give a tough final article. The morphology of PVC grains (the key property of PVC as sold) and the gelation mechanism are described in Chapters 7 and 8, respectively. PVC, when fabricated without plasticiser, produces a very tough, rigid article which is transparent if desired. However, the difficulty in processing the material normally means that only a narrow range of polymer molecular weights can be used, which limits both the toughness of the final product and the complexity of the article which can be produced. Toughness can be improved by blending rubbers with the PVC and the product melt flow increased by copolymerising vinyl chloride with other monomers, notably vinyl acetate. PVC blends and VCM copolymers are now used extensively for applications such as bottles, window profiles, calendered and extruded rigid film and foil, gramophone records, etc. These special polymers are normally based on the suspension polymerisation process and are discussed in Chapter 4. While these factors favour the growth of PVC manufacture using the suspension polymerisation process, such that 80 % of the total PVC made is produced by this process, the growth of markets such as PVC coated wallpaper, continuous vinyl flooring, etc., coupled with traditional applications such as gloves, bottle caps, fabric coating, etc., which are produced from PVC pastes (i.e. fine PVC particles suspended in plasticiser) has stimulated renewed interest in emulsion polymerisation processes. In order to obtain the required properties in the paste very sophisticated control of the polymerisation process and drying is required. About 10% of the present PVC made is produced by emulsion processes. The polymerisation processes used to make both emulsion and paste PVC are described in Chapter 3. The drying and milling processes which are such an important part of emulsion/paste polymer technology are described in Chapter 6 along with the centrifuging and drying steps used for suspension PVC. In the early 1960s a number of companies investigated the production of PVC in the absence of water. A major problem was ensuring that the correct PVC grain structure was achieved but one of these companies, Rhone-Poulenc, succeeded in developing a process in which a PVC seed particle was used to produce the desired final PVC grain morphology. This process is potentially cheaper than suspension PVC because of the absence of the drying step. However, the extra cost associated with the seed step and the comparative inflexibility of the process in terms of product properties have limited its penetration to less than 10% of the total PVC made. This process is described in Chapter 2. The discovery that the PVC monomer, VCM, represented a serious health hazard to man has posed many problems for the PVC industry. VCM is a gas at ambient temperatures and is usually handled as a liquid under pressure so that there is a high probability of some escape of the monomer into the environment of a VCM or PVC plant unless especial efforts are made to control that escape. The kinetics of VCM addition polymerisation are such that it is uneconomic to convert all the VCM to PVC so that PVC as made is always contaminated with VCM. In recent years processes have been developed to remove this VCM from the PVC very efficiently. The toxicity of VCM and the measures taken to prevent its escape from the PVC plant are described in Chapter 5.

xiii

Historically, PVC was first used mixed with plasticisers to make flexible articles. The market for these products has widened considerably since those early days to include flexible film, pipe, sheet, footwear and flooring applications, such that plasticised PVC articles still consume just over half the PVC produced. Although the basic steps of mixing PVC powder with plasticiser and other additives and fabricating the final article by the application of heat have been established for many years there has been constant development in the understanding of these steps and in fabrication techniques. Chapter 10 describes the processing of plasticised PVC from theoretical aspects such as polymer/plasticiser interaction and melt rheology. It also describes the processes used to make the final article with special emphasis on their production from PVC pastes. Over more recent years the growth of the rigid PVC market has been most rapid, presenting many problems to the processor because of the latent instability of PVC to heat. In addition to the important developments in blending, copolymerisation, PVC grain morphology and product heat stability already mentioned there has been much development in the machinery used to process rigid PVC and in the formulations of stabilisers and lubricants used to assist processing. Chapter 8 describes recent work on the mechanism ‘of gelation of rigid PVC while Chapter 9 deals more generally with the topic of making rigid PVC articles.

Chapter 1 SUSPENSION POLYMERISATION OF VINYL CHLORIDE R.H.BURGESS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

1.1 MARKET FOR PVC AND PRINCIPAL MANUFACTURING PROCESSES PVC has been produced commercially for over 40 years and world-wide sales have expanded greatly over that period with continued growth in the market forecast. The applications for which PVC is currently used span such a wide field that healthy growth is virtually assured. World sales of PVC are shown in Table 1.1, indicating the four-fold increase in sales over the past 15 years. TABLE 1.1 World Consumption of PVC (106 t per annum) Year Total world sales

1965 3.0

1970 6.0

1975 8.1

1980 12.0

1985 15 (estimate)

There are three major methods of manufacturing PVC; namely suspension, emulsion and bulk (mass) polymerisation. Limited quantities of PVC are made by solution polymerisation for speciality applications. Table 1.2 shows the proportion of PVC made by the three principal processes. These figures show the pre-eminence of the suspension polymerisation process in terms of the tonnage produced. The emulsion process, which leads to products with unique properties, also has a healthy growth record. The mass process, the most recent of these processes to become commercial, has grown rapidly over the last 20 years, but less rapidly in the TABLE 1.2 World Nameplate Capacity for the Manufacture of PVC by the Suspension, Emulsion and Mass Processes (103 t per annum) Year 1960 1965 1970 1975 1980

Suspensiona

Emulsiona

Mass

1430 360 12 2900 660 140 6200 1 160 340 10000 1450 1000 13200 1640 1200 a Approximate figures only since these capacities are, to some extent, interchangeable.

Total 1800 3700 7700 12500 16000

2

R.H.BURGESS

past five years than formerly. The products made by the mass process are similar to those made by suspension polymerisation and are used for the same market. Each of these major processes is described in detail in the following chapters. 1.2 KINETICS OF VINYL CHLORIDE POLYMERISATION Before proceeding to describe the suspension polymerisation process it is appropriate to discuss the kinetics of the vinyl chloride (VCM) polymerisation which are common to all the commercial processes. All are based on a free radical induced addition polymerisation described by the following six equations:1–6 (1.1) (1.2) (1.3) (1.4) (1.5) (1.6) where I are free radicals formed by decomposition of the initiator, R is a growing polymer free radical, M is a VCM monomer molecule and P is the final polymer molecule. The initiation step (eqn. 1.1) is commonly a first order decomposition of a labile molecule (peroxide or azo compound) but activation of this decomposition is commonly practised in emulsion polymerisation and is technically feasible in suspension polymerisation. Equations 1.2 and 1.3 represent the chain propagation step normally with similar rate constants, i.e. k2=k3. In some cases, for example azo initiators with a very short half-life for decomposition, the radical produced is so stable that initiation of the polymerisation is very inefficient (k2
SUSPENSION POLYMERISATION OF VINYL CHLORIDE

3

monomer and is precipitated immediately it 75/25 PVC/VCM ‘gel’. It is probable that this ‘gel’ contains a normal is formed. Moreover, this precipitated PVC is swollen by VCM to give a ca amount of initiator and hence free radicals formed by its decomposition. There are, therefore, two loci of polymerisation, at least up to 75% conversion of VCM to PVC, a VCM phase with a high termination rate constant for radical reactions k5 and a PVC/VCM phase with a low termination rate constant k6. Kuchanov and Bort5 give values of 1.3×1012 exp(−4200/RT) and 0.7×1012 exp(−8200/RT) litremole−1 s−1 for k5 and k6 respectively. Applying stationary state kinetics to eqns. 1.1–1.6 the rate of polymerisation is given by (1.7) and (1.8) Using eqns. 1.7 and 1.8 it is possible to predict the way in which the polymerisation rate changes with conversion assuming that: (a) the initiating radical concentration is constant and the same in both phases, (b) the termination rates in the two phases do not change with conversion even when the conversion of VCM to PVC exceeds 75%, and (c) that the effective monomer concentration [M] in the gel phase up to 75% conversion is 25% of that in the VCM phase and falls as the conversion exceeds 75%. Figure 1.1 shows the way in which the rate changes with conversion for various values of k5 and k6 ranging from k5=k6 to k5=1600k6. This Figure shows clearly the increase in rate with conversion up to 75%, which occurs whenever k5/k6 exceeds 16 and Kuchanov and Bort5 predict that k5/k6=950 at 50°C, 790 at 60°C and 660 at 70°C. This increase in rate is known as the Trommsdorff effect. Above 75% conversion the polymerisation rate falls as the VCM concentration falls. Both Ugelstad4 and Kuchanov5 believe that the radical concentration is not the same in the two phases and invoke radical transfer between the two phases to explain some of the observed kinetics. However, there is no general agreement between these workers and the model presented here represents a reasonable approximation of the kinetics actually observed. The rapid increase in rate with conversion shown in Fig. 1.1 (peak rate is seven times the rate in VCM for k5/k6=790 at 60°C) can be expressed in terms of the rate against time (Fig. 1.2.) when the rapid increase of rate up to 75% conversion characteristic of a VCM polymerisation is clearly seen, followed by a marked fall in rate beyond 75% conversion. The Kuchanov data predict a slightly higher, more ‘peaky’ reaction at lower temperature, but the essential shape of the reaction profile is largely unchanged as the polymerisation temperature changes. Normally the reaction is initiated by the decomposition of a peroxide or azo compound to give free radicals. Such decompositions exhibit first order kinetics with a rate falling with time as the undecomposed initiator concentration falls. The effect of this on the reaction rate shown in Figs. 1.1 and 1.2 is to reduce the overall rate with increasing time or conversion, with the reduction in rate being greatest for the fastest decomposing initiators. Figure 1.3 shows the effect on the overall reaction rate of using initiators which decompose to varying extents during the polymerisation, remembering that the instantaneous rate of polymerisation (eqns. 1.7 and 1.8) is proportional to the square root of the initiator concentration. The Figure shows diagrammatically the effect of using initiator which decomposes 10%, 40%, and 90% in arbitrary time. The initial rate of polymerisation is higher for the faster decomposing initiator but the peak

4

R.H.BURGESS

FIG. 1.1. The effect of conversion on reaction rate for various termination rate constant ratios.

FIG. 1.2. The effect of time on the reaction rate.

reaction rate falls since the amount of initiator remaining is less because of its faster decomposition. The rate beyond the peak is generally less once more because the initiator concentration is so low. Indeed, in an extreme case there may be insufficient initiator to complete the reaction and the reaction stops almost completely. Polymer molecular weight (Degree of Polymerisation, DP) is given by DP={k3[M](k1[I]/k5)1/2+2k1[I]}/k4[M](k1[1]/k5)1/2 in VCM (1.9) or DP={k3[M](k1[I]/k6)1/2+2k1[I]}/k4[M](k1[I]/k6)1/2 in VCM/PVC (1.10)

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

5

FIG. 1.3. The effect of initiator decomposition on reaction rate.

If polymer molecule production by chain transfer to monomer is assumed to be much more frequent than production by the termination reaction this simplifies to DP=k3/k4 (1.11) Kuchanov and Bort5 have obtained a value of 9.2×10−3 exp 7400/RT for k3/k4 implying a DP of 950 at 50° C, 670 at 60°C and 482 at 70°C, i.e. a molecular weight of 59000 at 50°C, 42000 at 60°C and 30000 at 70° C. Since k5 is known to be at least 500k6 the approximation leading to eqn. 1.11 is more likely to be valid at high conversion than at low conversion. Indeed, experiments show that product K-value changes by no more than 3K-units over the range 10–90% conversion. Since most commercial polymerisations are carried out to comparatively high conversion ( >70%), molecules produced by termination in the VCM phase at low conversion have little effect on the final product K-value. Similarly, K-value can be shown to vary by no more than 5K-units over the initiator concentration range 0.01–1% on VCM and to be virtually immeasurable over the commercially used range 0.01–0.1%. Consequently eqn. 1.11 is a good measure of the molecular weight achieved. This is further confirmed by the good agreement between the Kuchanov and Bort prediction and the experimental data of Freeman and Manning6 shown together with the ISO K-value temperature relationship in Fig. 1.4. It is possible to reduce the molecular weight at a given temperature by the use of chain transfer agents. Typical transfer agents which can be used are trichloroethylene, thiolesters and isobutyraldehyde but high concentrations are necessary (say 0.5−1% on VCM) to have any significant effect on polymer molecular weight. This is not surprising in view of the chain transfer properties of VCM itself and the fact that VCM is present, inevitably, in such high concentration. It is possible to increase the molecular weight by the addition of minor amounts of cross-linking agents7 (di- and tri-functional monomers such as divinylbenzene, glycol dimethacrylate, diallyl maleate and diallyl phthalate). Again, the technique is not widely used because of consequent changes in the product molecular weight distribution.

6

R.H.BURGESS

FIG. 1.4. The effect of temperature on molecular weight and K-value.

1.3 OUTLINE OF SUSPENSION POLYMERISATION PROCESS VCM is a gas at ambient temperature (BP—13°C)8 but is handled as a liquid under pressure. It forms explosive mixtures with air (explosive limits 3·6−26%v/v) and is toxic (see Chapter 5); consequently extreme care has to be taken in the design of the storage and transport equipment. VCM as produced is relatively stable and shows little tendency to polymerise. However, contamination with oxygen can give rise to vinyl chloride polyperoxide which decomposes and initiates polymerisation of the VCM. Consequently VCM manufacturers take steps to avoid contamination with oxygen, or if this is unavoidable, small quantities of stabiliser, usually phenol or phenol derivatives, can be added which prevent both peroxidation of the monomer and inhibit polymerisation. A very small quantity (2–10ppm) is normally sufficient to stabilise contaminated VCM against pre-polymerisation and is sometimes used to prevent polymerisation under long-term storage conditions. This stabiliser is not normally removed before

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

7

FIG. 1.5. Suspension polymerisation process. (Reproduced from Burgess, Developments in PVC Production and Processing—1, Whelan and Craft, Eds., Applied Science Publishers Ltd, 1977.)

the VCM is used in the polymerisation process. VCM is transported by pipeline from bulk storage (in spheres of up to 2000t capacity) or by road or rail tanker into stock tanks on the PVC plant. Since it is not economic to convert all the VCM charged to the polymerisation vessel to a quantity of VCM is always recovered from these vessels. This recovered VCM can be returned to the VCM plant for redistillation and blending with virgin VCM, or can be used in subsequent polymerisations as produced. The latter practice is normally followed, the recovered VCM being used blended with virgin VCM roughly in the proportions in which the recovered VCM is produced. In the suspension polymerisation process VCM is dispersed as droplets in water in a suitably designed pressure vessel and polymerised using free radical initiators until 80–90% of the VCM is converted to PVC. The residual VCM is then removed from the suspension of PVC in water by a process termed ‘stripping’, the stripped slurry is dewatered using a centrifuge, dried and then stored as required. This outline of the process is shown diagrammatically in Fig. 1.5. The stripping and polymer isolation processes are described in more detail in later chapters of this book and this chapter concentrates on the polymerisation process only. 1.4 THE POLYMERISATION PROCESS In a typical suspension polymerisation a known quantity of water, normally demineralised water or water of a known high quality, is charged to a pressure vessel (autoclave) and other polymerisation ingredients such as initiator, buffers and protective colloid are added. The autoclave is then sealed and evacuated. VCM is added either by metering, using a turbine meter, or by weight from a suitable weigh vessel. A typical recipe is shown in Table 1.3. TABLE 1.3 Typical Suspension Polymerisation Recipe for a 10m3 Autoclave Weight (kg) Water

5000

8

R.H.BURGESS

Weight (kg) Protective colloid Buffer Initiator Vinyl chloride

3.5 0.7 1.5 3500

Polymerise at 60°C for up to 6 h

The autoclave is heated up to the polymerisation temperature using a mixture of steam and water in the autoclave jacket. Once the autoclave reaches the set temperature, polymerisation starts and heat is evolved (heat of polymerisation of VCM (liquid) to PVC is—1540kJ/kg).9 This heat is removed by cooling water in the autoclave jacket. The autoclave pressure will be steady at the autogenous pressure of VCM for the polymerisation temperature ranging from 6·3 bar at 40°C to 15 bar at 80°C8 until ca 75% conversion of VCM to PVC, when there is no free VCM phase remaining. The pressure then begins to fall as the equilibrium vapour pressure of the PVC/VCM phase falls as its PVC content rises. At a given pressure below the autogenous pressure the polymerisation is terminated, usually by venting off the excess VCM. Since the polymerisation rate falls at high conversion (see Figs. 1.1 to 1.3) it is normally not economic to take the polymerisation beyond 90% and lower conversions are desirable for polymer quality reasons. Venting off the excess VCM reduces the polymerisation rate markedly and effectively stops the polymerisation. The remaining VCM is then stripped from the slurry either in the autoclave or in a separate stripping process (see Chapter 5). After discharge of the batch the autoclave is cleaned to remove any polymer remaining in the autoclave either as loose particles or adhering to the walls of the autoclave. The autoclave is then ready for the next polymerisation cycle. A typical autoclave cycle as operated in the 1950s and early 1960s is shown in Table 1.4. TABLE 1.4 Typical Suspension Polymerisation Cycle for a 10m3 Autoclave (1950–1960) Time (min) Charge aqueous phase Evacuate autoclave Charge VCM Heat up to 60°C Hold at 60°C Remove unreacted VCM Discharge autoclave Clean autoclave Total cycle:

30 15 15 60 360 120 30 60 — 690

Consequently such an autoclave is capable of producing about 3t of PVC every 690 min, corresponding to over 2000t per year assuming continuous operation and 90% equipment availability. A plant capable of producing, say, 20000t per year would thus consist of 10 such autoclaves or 20 for 40000t per year although it should be pointed out that normal practice today would be to use a smaller number of larger autoclaves with a shorter cycle (see Section 1.5).

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

9

1.4.1 Role of Water in the Process Water is present to act as a continuous phase so enabling the VCM to be broken up into discrete droplets, to act as a heat transfer medium and as a carrier for the protective colloid. The quantity of water used is determined to some extent by each of these activities but principally by the nature of the VCM and PVC particles, normally called granules or grains, produced during the process. Initially the VCM phase is broken up into 30–150 µ m droplets by the comminuting action of the agitation and prevented from coalescing by the action of the protective colloid. A minimum quantity of water is required to fill all the spaces between the spherical VCM particles and to provide some ‘free’ water to ensure a low viscosity mixture. Water:monomer ratios of 1:1 are perfectly adequate to achieve this aim. However, at the end of the polymerisation it is equally necessary for the PVC grains to be held apart by the water. The bulk density of PVC made varies widely depending on the market for which it is intended, but will normally be in the range 0·4−0·8 kg/litre. This in turn requires water: monomer ratios up to 1·75:1 to ensure a low viscosity mixture is present. Typically a water:monomer ratio of about 1·5:1 is used as shown in Table 1.3 (actually 1·43 in this example). The second major purpose of the water is to act both as a heat sink and as a heat transfer medium. Since the specific heat of VCM and PVC are approximately 0–25 cal/g and that of water is 1 cal/g, the heat capacity of the water at the typical 1·5:1 water: monomer ratio is six times that of the organic phase. This has a considerable moderating effect on any changes in temperature caused by either heat evolved by, or heat lost from, the system. However, the major purpose of the water is to act as a heat transfer medium. The total heat transfer (HT) from a polymerising system to the autoclave jacket is given by the relationship (1.12) where HI is the film coefficient on the inside of the autoclave, HM is the thermal conductivity of the autoclave wall itself and HJ is the jacket side film coefficient. Since HT has to be as high as possible for maximum autoclave output and HM and HJ are fixed and relatively high, it is vital that HI is as large as possible. HI for a well agitated slurry is at least 10 times that for a well agitated powder. The final role of the water is to carry the protective colloid. Most protective colloids, as described in the next section, are water soluble polymers and clearly can only act in an aqueous system. 1.4.2 Role of the Protective Colloid During the suspension polymerisation of VCM the VCM droplets are converted gradually from a non-sticky liquid (VCM), through a mixed droplet system consisting of a PVC/VCM phase with some free VCM, to PVC grains containing some VCM. This process has been described by a number of authors10–12 and can be shown schematically by Fig. 1.6, due to Sanderson. In the intermediate stages of this polymerisation the particles are viscous and sticky and tend to agglomerate. With most agitation systems this agglomeration process would occur to an uncontrolled extent such that large lumps of PVC would result. In the limit these lumps could not be agitated and heat transfer would be so poor that temperature control of the polymerisation would be lost. The presence of the protective colloid avoids this problem. Water soluble polymers are known which are capable of fully protecting the original droplets leading to PVC granules of low porosity and high packing density.

10

R.H.BURGESS

FIG. 1.6. VCM suspension polymerisation—precipitation of PVC within droplets. (Reproduced from Sanderson, British Polmer Journal, 12, 186 (1980).)

PVC is used for a wide variety of applications but there are two essential properties which for almost all applications it must possess. One is that the PVC grains must be capable of absorbing stabilisers, lubricants and in many cases large quantities of plasticiser during the blending steps which take place before the PVC is fabricated into the final article. The second is that the PVC must be capable of being transformed into the final article as rapidly as possible without degradation. The final point is especially critical since PVC is essentially a heat labile material processed, particularly for rigid applications, close to its decomposition temperature so that a very short heat history is necessary. Both of these requirements call for PVC grains which are more-or-less porous. The protective colloid thus has the dual responsibility of both stabilising the polymerising droplets against too much agglomeration and the production of porous grains. Figure 1.7 shows typical PVC granules of both dense and porous structure made under the same conditions except for a change of protective colloid.12 As has already been indicated, most protective colloids are water soluble polymers, the characteristic of which is an affinity for both the water and the organic phase. They tend to gravitate to the interface between the two phases stabilising the droplets by lowering the interfacial tension between the two phases. It is possible to demonstrate this simply by mixing a VCM simulant, say ethylene dichloride, with water in the presence of 0.1% of a water soluble polymer such as partially hydrolysed poly(vinyl acetate) and agitating the mixture. If agitation is stopped the ethylene dichloride droplets are stable for many hours. In the absence of the protective colloid they will coalesce almost instantaneously. In addition to special grades of poly(vinyl acetate), cellulose derivatives such as methyl cellulose, sodium carboxyethyl cellulose, hydroxypropyl methyl cellulose, etc., are also widely used as protective colloids. Undoubtedly other water soluble colloids such as polyvinylpyrollidone, ethylene/maleic anhydride copolymers, polyacrylates and the naturally occurring materials such as gelatin and gum arabic can be used. In practice gelatin was used for much early suspension PVC manufacture, but, as the demands of the market

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

11

FIG. 1.7. PVC suspension polymer—appearance under transmitted light. (Reproduced from Sanderson, British Polymer Journal, 12, 186 (1980).)

place for more reproducible and sophisticated grain structures have developed, there has been a complete change to the use of man-made colloids especially tailored to the demands of the process. It seems that in some cases the protective colloid acts by stabilising the VCM droplets against coalescence in the early stages of the polymerisation and controlling the aggregation step shown in Fig. 1.6, so enabling a fine grain (50µ m) of controlled internal structure to be achieved. As polymerisation proceeds a certain amount of coalescence occurs, so producing the desired final grain size of about 100–150 µ m and the desired grain porosity. In other cases the protective colloid stabilises the original VCM droplet throughout polymerisation but produces the desired grain porosity by controlling the contraction of the grain (there is a density change VCM PVC of 0.85 1.4 kg/litre) and allowing the precipitating PVC primary particles to form in a specially suitable way. In recent years increasing attention has been paid to the use of secondary granulating agents such as short chain surface active agents mixed with the primary substituted cellulosic or polyvinyl alcohol (PVA) protective colloid. Their role is almost certainly to control the aggregation step shown schematically in Fig. 1.6.13 1.4.3 Role of the Agitator In the earlier sections it has been mentioned that the agitator has the functions of producing the necessary droplet size, maintaining a suspension of these droplets and ultimately of PVC grains, and ensuring good heat transfer from the polymerising mass to the autoclave walls. The ability of the agitator to produce a particular droplet size is relatively easily predicted from classical fluid mechanics.14 Most breakdown of the VCM into droplets occurs in the area of turbulence near the tip edges and trailing edges of the impellor. As would be expected, the droplet size decreases as the agitator speed increases. Consequently much higher speeds are required to produce 50 µ m droplets, as required for some processes, than are required for the coarser 150 µ m grains.

12

R.H.BURGESS

Most of the protective colloids used for stabilising VCM droplets are very efficient so that droplet coalescence very rarely occurs, certainly at the beginning of the polymerisation. However, as the polymerisation proceeds the nature of the droplets changes as PVC is precipitated inside them and as they contract because of the density change VCM PVC. Basically the droplets become less stable, particularly at the intermediate stages of polymerisation. Also the system becomes much more viscous as the final PVC grains with their porous structure are formed. The requirements for the agitator system to prevent or at least control coalescence and to ensure good and uniform circulation are now much more onerous. This has called for the development of quite sophisticated agitation systems especially for the more modern, large autoclave plants. In the early days of PVC manufacture it was normal to agitate the vessel with a single simple 45° or 90° paddle on the end of a shaft entering from the top of the small (up to 10m3) vessel. Under these conditions the VCM passes through the area of high turbulence close to the impellor sufficiently frequently to ensure that the correct droplet size and distribution occur and to ensure adequate circulation of the whole batch. In these circumstances various protuberances inside the autoclave such as thermocouple pockets and the stirrer shaft itself undoubtedly helped the circulation by acting to some extent as baffles. As the autoclave size increased, the length of a shaft from the top of the vessel to the impellor became very long and the power required (perhaps l00hp for a 50m3 vessel) so large that the shaft and bearing became very expensive. For some years it has been common practice for new large autoclaves to be agitated by means of an impellor close to the bottom of the vessel driven by a short bottom entry shaft. At the same time the problem of ensuring adequate circulation within the vessel by a single impellor becomes greater as the autoclave volume increases. This problem can be overcome by using a second or third impellor above the first or by placing baffles in the autoclave. Most manufacturers use baffles of some sort and these greatly improve circulation by increasing the areas of moderate turbulence within the system. Most of the baffles take the form of a shape (cylinder, plate, etc.) on the inside of the vessel wall, usually in the top part of the autoclave. 1.4.4 Role of the Buffer It is impossible to polymerise VCM without the formation of some HC1. Additionally some initiators produce acidic products as they decompose. These processes are not under perfect control and the properties of most water soluble polymers can vary quite considerably with changing pH with consequent loss of control of the type of PVC granules produced. Thus it is quite normal to add inorganic salts to VCM suspension polymerisation in order to alter or control the pH of the system. Most additives are designed to raise the autoclave pH somewhat. Strictly speaking, not all of the additives used are buffers but merely bases. Sodium and magnesium hydroxides, carbonates and bicarbonates, phosphates and acetates have all been mentioned in the patent literature. A further virtue of the resulting higher pH of the slurry is that it is less likely to attack the plant equipment used to isolate the PVC from the slurry. In practice almost all the plant equipment from the autoclaves to the driers is constructed of stainless steel or is glass lined or epoxy coated. If coating is to be used, glass is the normal coating material where the environment is at its most active, such as in the autoclaves, while the cheaper epoxy coatings are used in less aggressive situations such as in the silos used to store the PVC. The coated substrate will normally be mild steel. Mild steel is, of course, attacked by aqueous systems but even stainless steel is attacked by PVC slurries in certain circumstances. This attack, called stress corrosion cracking, can lead to failure of the steel and is faster the higher the temperature and chloride ion

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

13

FIG. 1.8. A typical suspension polymerisation reaction.

concentration and the lower the pH. Since little can be done to prevent chloride ion formation, and high temperatures are also inevitable, some measure of pH control is desirable. 1.4.5 Role of the Initiator The choice of initiator for use in VCM suspension polymerisations is vitally important since it significantly affects the cost of the process. As described in Section 1.2 on the kinetics of VCM polymerisation (see Figs. 1.2 and 1.3), the reaction accelerates rapidly at a constant rate of initiation, because the locus of polymerisation moves from VCM to a PVC/VCM phase in which the kinetic termination step is markedly lower, as the VCM conversion to PVC proceeds. The reaction VCM PVC is exothermic giving out 1540 kJ/kg VCM polymerised, and it is necessary to remove this reaction heat at its rate of generation if the polymerisation temperature is to be maintained constant as is normally desired to produce the PVC K-value required. Removal of this heat is a problem in the commercial scale production of PVC because of the desire to minimise the reaction time and so make the maximum use of the autoclave. Since in most suspension polymerisation processes the heat of polymerisation is removed by cooling water circulating round the jacket of the autoclave, there is clearly a maximum rate of heat removal controlled by the heat transfer coefficient between the autoclave contents and the jacket and the temperature difference between the batch and the jacket. As the heat transfer coefficient between the contents and the jacket stays sensibly constant throughout a polymerisation, the rate of polymerisation is given by the temperature difference between the contents and the jacket. Typical polymerisation profiles are shown in Fig. 1.8. This shows the way in which the jacket temperature is used to increase the batch from ambient temperature to the

14

R.H.BURGESS

polymerisation temperature (in this case 60°C) and the role of the jacket water in removing the heat of polymerisation, the rate of that polymerisation being given roughly by the temperature difference between the contents and the jacket. This temperature difference increases with time and mirrors the reaction rate increase shown in the 40% curve of Fig. 1.3. The polymerisation kinetics (initiator used, etc.) are arranged so that the heat evolution at the maximum polymerisation rate can be controlled just using the minimum cooling water temperature (in this case 15°C). Subsequently the polymerisation rate falls as the free VCM concentration falls below the 75/25 PVC/ VCM equilibrium concentration. Of course, the batch pressure which up to that point is the vapour pressure of VCM at 60°C, falls also. This typical reaction profile as discussed earlier is achieved by using a free radical initiator which decomposes to give free radicals. Typical initiators which can be used are azo compounds such as azodiisobutyronitrile and azobis(2,4-dimethylvaleronitrile), peresters such as t-butyl perpivalate and t-butyl perneodecanoate, acid peresters such as lauroyl and benzoyl peroxide, peroxydicarbonates such as dibutyl, di(2-ethylhexyl), dicetyl, and di(t-butylcyclohexyl) peroxydicarbonates and special initiators such as acetylcyclohexylsulphonyl peroxide. Whereas the azo compounds decompose by the elimination of nitrogen the peroxides are decomposed by splitting the O—O bond in the peroxide. All the initiators mentioned above are characterised by decomposing according to first order kinetics, see eqn. (1.1), with a rate of reaction expressed by the equation (1.13) That is, the rate of initiation is proportional to the initiator concentration and this, of course, falls with time as the initiator decomposes. The rate of decomposition is given by the rate constant k1 which is a characteristic of each particular initiator. This is conveniently expressed as the initiator half-life, i.e. the time taken at a particular temperature for the initiator concentration to fall to half its original value. Table 1.5 shows the half-lives of the initiators mentioned over a range of temperatures used in VCM suspension polymerisations. As can be seen from the data in the Table, most of these initiators are characterised by having half-lives of about 2 h at one temperature in the range 50–70°C which is that commonly used for suspension PVC manufacture. Thus in a reaction time of 6h given in Table 1.4 as being typical for a 10m3 autoclave, the initiator may be decomposing at a rate such that its final concentration is only 10–15% of that initially added with the VCM charge. This change of initiation rate with conversion has a powerful effect on the rate of polymerisation, greatly moderating the increase in that rate which would otherwise result. This has the result of reducing the instantaneous polymerisation rate at any one time, as shown in Fig. 1.3, so facilitating heat removal. The overall effect is to make possible a TABLE 1.5 Half-lives of Initiators used in VCM Suspension Polymerisation15 Initiator Azodi-isobutyronitrile Azobis(2,4-dimethylvaleronitrile) t-Butyl perpivalate Dibutyl peroxydicarbonate Di(2-ethylhexyl) peroxydicarbonate Di(t-butylcyclohexyl) peroxydicarbonate Lauroyl peroxide

Half-life (min) 50°C

60°C

70°C

4200 420 1300 300 250 250 3000

1080 120 360 70 60 60 800

300 35 100 18 15 15 200

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

Initiator Benzoyl peroxide Acetyl cyclohexylsulphonyl peroxide

15

Half-life (min) 50°C

60°C

70°C

— 80

3000 18

800 4

higher average polymerisation rate so making better use of the cooling capacity of the autoclave and increasing autoclave productivity. An examination of the data in Table 1.5 also shows that if a half-life of, say, 2–3 h is desirable to maximise reactor productivity, a particular initiator may only be especially suitable for use over a relatively minor range of polymerisation temperatures. Thus for example azodiisobutyronitrile and lauroyl peroxide might be used at 70°C, t-butyl perpivalate and azobis(2,4-dimethylvaleronitrile) at 60°C and the peroxydicarbonates or acetylcyclohexylsulphonyl peroxide at 50°C. The use of mixtures of any of these initiators may be especially advantageous. For example a mixture of an initiator which decomposes rapidly with one which decomposes slowly at the polymerisation temperature should increase the polymerisation rate at the beginning where the rate is normally slow (see Fig. 1.2) while maintaining an adequate rate of initiation at the end of the polymerisation. Without the latter feature it is quite possible for the VCM polymerisation to become very slow at high conversions as shown in the bottom curve of Fig. 1.3. Half-life is not the only basis for the choice of initiator. Some initiators are less efficient at starting a VCM polymerisation than others. For example, while the peroxides in general have a high radical efficiency the azo compounds are less effective. This is basically because the free radicals produced when a peroxide decomposes are very energetic and are sufficiently active to overcome the energy barrier needed to open the VCM double bond. By comparison azo compounds decompose to give much less energetic radicals and their efficiency in starting a VCM polymerisation is correspondingly less. This problem is of no real consequence for the longer half-life azo initiators such as azodi-isobutyronitrile but azo compounds such as 2,2 -azobis(4-methoxy-2,4-dimethylvaleronitrile), which has a half-life at 50°C of 40min,16 produces such stable radicals that it is a very ineffective initiator for VCM polymerisation. Such an initiator can, of course, be employed but very high concentrations which would probably be uneconomic have to be used. Another major factor in the choice of initiator is its stability and storage properties. Not surprisingly, the more active the initiator, i.e. the faster it will decompose when exposed to heat, the more unstable it tends to be. For example, dibutyl peroxydicarbonate has to be stored at lower than ambient temperature while lauroyl peroxide can be stored at temperatures up to 30°C. However, there are exceptions to this general rule which are related, once more, to the energetics of the radicals produced and to the physical form of the initiator. For example, as is well known, benzoyl peroxide has to be stored wet but lauroyl peroxide is stable dry at room temperature. The liquid peroxydicarbonates have auto-ignition temperatures (i.e. temperatures above which they self-ignite) below ambient temperature while the solid peroxydicarbonates have high autoignition temperatures. Dissolving the solid peroxydicarbonates in a solvent greatly reduces their autoignition temperatures15 so that they are then comparable with the liquid materials. Clearly if one initiator, for example lauroyl peroxide, can be stored at ambient temperature and presents no explosion hazard, there is a powerful incentive to choose it rather than another which requires special cooled storage conditions with safety devices designed to guard against any breakdown of the cooling system. Also initiators which self-ignite at ambient temperature have to be handled especially carefully to avoid any spillage. Not only are these disadvantages for the PVC manufacturer but the initiator manufacturer will also have increased costs because of the safety problems which have to be overcome.

16

R.H.BURGESS

The initiator used in a PVC process tends in many cases to have a small effect on other features of the process. For example, the more water soluble initiators such as acetylcyclohexylsulphonyl peroxide tend to induce the formation of more fouling, i.e. deposition of PVC on the autoclave surface. Others may have an effect on the colour and heat stability of the PVC made and on the porosity of the PVC granules produced. A PVC manufacturer thus has to weigh many facts before choosing the initiator he will use. At the present time the most popular initiators are undoubtedly the peroxydicarbonates which are gradually replacing lauroyl peroxide, at least for the lower temperature polymerisations. t-Butyl perpivalate and azobis-2,4 (dimethylvaleronitrile) are used to some extent but this use is declining as is that of acetylcyclohexylsulphonyl peroxide. As is apparent from the earlier discussion a control over the initiation rate is very desirable to optimise autoclave productivity. There are a number of patents covering the activation of peroxides in a VCM polymerisation. For example, Rhone-Poulenc17 describe a process in which decomposition of lauroyl peroxide is induced with a mixture of ascorbic acid and a transition-metal salt. It is possible to activate the decomposition of benzoyl peroxide with aromatic amines. These and similar techniques are of considerable interest but they suffer from the disadvantage that the activation reaction only produces one free radical from each peroxide molecule instead of the normal two, and this significantly increases initiator cost. 1.4.6 Effect of the Evacuation Step It is customary to evacuate the autoclave before charging VCM in order to remove as much as possible of the oxygen otherwise present. This is a powerful inhibitor of the VCM polymerisation since it copolymerises very readily with VCM but only slowly propagates the chain reaction. mCH2=CHCl+nO2 → —(CH2—CHCl)m(O2)n— (1.14) This vinyl chloride polyperoxide is itself labile and breaks down either before or during the VCM homopolymerisation to give HC1 and various unsaturated or carbonyl containing PVC molecules, together with small quantities of carbon monoxide and formaldehyde. The low pH produced by this reaction is a major cause of corrosion attack on the PVC plant equipment since it can be responsible for stress corrosion cracking of stainless steel, direct attack on mild steel and, when concentrated by evaporation during the drying operation, direct attack on ducting, etc. The by-product PVC is also undesirable since unsaturated PVC or PVC containing carbonyl groups is known to be of poorer colour and lower thermal stability than the homopolymer itself. Carbon monoxide will copolymerise with VCM to give an unstable (to light and heat) copolymer. Since the large evacuation pumps used to remove air from PVC reactors are usually incapable of producing a perfect vacuum, manufacturers often reduce the oxygen further either by multiple evacuation steps with the back addition of nitrogen between evacuations or evacuate the autoclave when it contains warm water, so using the steam generated to sparge the air out of the autoclave. 1.4.7 Vinyl Chloride Quality Until around 1960 most VCM was produced from acetylene and hydrogen chloride in a single step addition process HC≡CH+HCl→CH2=CHCl (1.15)

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

17

but there was also some production of VCM by direct chlorination of ethylene to ethylene dichloride (EDC) followed by dehydrochlorination. CH2Cl—CH2Cl→CH2=CHCl+HCl (1.16) In some plants both processes were used in tandem, the HC1 from the cracking of EDC being combined with acetylene. Since 1965 virtually all major new VCM plants have been based on a combination of direct chlorination and oxychlorination of ethylene, the so-called balanced process. In the first step EDC is produced by the direct chlorination of ethylene. CH2=CH2+Cl2→CH2Cl—CH2Cl (1.17) In the second step the purified EDC is pyrolysed to give VCM and HC1. CH2Cl—CH2Cl→CH2=CHCl+HCl (1.18) In the third step this HC1, mixed with air or oxygen, is reacted with ethylene to form further amounts of EDC. CH2=CH2+2HCl+0.5 O2→CH2Cl—CH2Cl+H2O (1.19) The chief advantage of this process over those operated earlier is that it enables ethylene and chlorine plus air (or oxygen) to be used as the basic raw materials, and ethylene is cheaper than acetylene. There are also no major side products. However, the full economies of this process are only realised when it is operated on a very large scale (> 200ktpa) and at high occupacity. The process involving hydrochlorination of acetylene alone or coupled with EDC cracking involves lower capital investment and can be operated on a smaller scale. It tends, therefore, still to be used in some countries at an early stage in the development of their PVC industry. The acetylene route has also been favoured in South Africa, where the economy is based on coal. VCM as made by any of these processes is extremely pure and will normally be sold at greater than 99. 8% purity. The nature of the remaining impurities depends on the process used to make the VCM, the quality of the raw materials used in that process and the purification regime. They will consist principally of other chlorinated hydrocarbons and water. Neither is a major concern in suspension polymerisation although chlorinated hydrocarbons may act as chain transfer agents and contaminate either the PVC, if they remain in the polymer after VCM removal, or increase losses in the VCM recovery plant if they are removed during stripping. The major problem with water is that it promotes corrosion of the plant equipment which is itself serious but the resultant iron in the VCM may have more serious effects on the polymerisation especially for redox catalysed emulsion polymerisations. Other impurities are normally present at the ppm concentration level only. Nonetheless these small concentrations are thought to have a significant effect on the morphology of PVC grains produced by suspension polymerisation. The significance of each impurity will depend on the details of the suspension process and especially on the protective colloid used. 1.5 COST OF MANUFACTURING SUSPENSION PVC At the time of writing a ratio of monomer cost to PVC selling price of 60–65% is fairly normal. Assuming this ratio, the cost elements of producing PVC can be summarised very approximately as in Table 1.6. It is apparent from this table that the VCM cost is by far the biggest cost element, but that after that the next most important is associated with the

18

R.H.BURGESS

TABLE 1.6 Approximate Cost of Manufacturing Suspension PVC Percentage of selling price Monomer cost assuming 98 % efficiency Variable services+ainitiator+other chemicals Total variable cost Amortisation of capital Research and development Selling costs and all other overheads Total cost Profit element to reward capital investment

a

64 6 —— 70 7.5 2.5 10 —— 90 10 —— 100

Steam, electricity, water, etc.

amortisation of, and reward for, the capital employed. There is a powerful incentive, therefore, to reduce the capital cost of building PVC plants. It is possible to divide a suspension PVC plant into sections, namely VCM storage and recovery, polymerisation, VCM removal, slurry storage and drying, isolation and storage, and laboratories and offices. Ignoring isolation and storage, the cost of which can vary very considerably depending on whether bulk storage is required or not, the polymerisation sections can account for up to half the total cost of the plant. Consequently there is a strong case to reduce costs in this area. In principle there are three ways in which this can be accomplished. (1) By reducing the cost of the individual items used. However, this cost-cutting approach may not be compatible with the ever-improving PVC quality standards and the pressure to increase the safety standards under which the PVC process operates. For example the discovery that VCM is a human carcinogen has greatly increased the cost of the equipment needed on a PVC plant to reduce both deliberate and adventitious VCM leaks to a minimum, to ventilate plants more effectively and to monitor VCM concentrations. (2) By increasing the amount of PVC made in a particular autoclave. An examination of Table 1.4 shows that a 10m3 autoclave operating under those conditions will produce 2000t per year but that there are a number of ways in which that productivity can be increased. In the cycle described (Table 1.4) over half the time is spent polymerising the VCM. The correct choice of initiator, as described in Section 1.4.5, coupled with adequate polymerisation heat removal, can reduce that time significantly with 300 min or less being achievable. The second largest item in the cycle is the VCM removal step. The provision of a separate vessel for VCM removal will eliminate this item and the cycle time saving may more than offset the extra cost. Over the past ten years there has been much activity in the PVC industry aimed at preventing PVC deposits in the autoclave (build-up) which caused numerous problems and required interbatch autoclave cleaning. This will be discussed in more detail in Section 1.6.5 but most companies now have chemical systems for controlling build-up formation such that very greatly reduced cleaning is required. When VCM is converted to PVC there is a marked volume change (density of VCM is 0.85 g/ml—density of PVC is l.4g/ml) so that there is a large volume contraction in the autoclave contents. Advantage can be taken of this fact by increasing the initial VCM charge at the expense of the initial water

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

19

and injecting water during the polymerisation as the volume contracts. In this way increased quantities of PVC can be produced per batch. Taking all these ideas into account it is theoretically possible to increase the VCM charge from the 3500 kg in Table 1.3 to, say, 4000 kg and to reduce the overall cycle to 480 min instead of the 690 min in Table 1.4. This increases the autoclave output to 305t of PVC every 8 h corresponding to over 3400t per year at 90% autoclave availability. Even with this increase in productivity the costs remain very high for this size of autoclave. A l00000tpa plant would still require 30×10m3 autoclaves, the expensive associated equipment having to be replicated on each autoclave. Consequently most PVC manufacturers have today moved to the use of a smaller number of larger autoclaves. (3) Increasing autoclave size. Making the assumption that the VCM charge possible and, hence, the amount of PVC produced, increases pro rata with the size of the autoclave, the use of a large autoclave, say 100m3, is very attractive. Clearly the capital cost of such an autoclave must be significantly greater than that for a 10m3 autoclave but it is not 10 times as much. The autoclave section building, the agitator and its drive, the cooling water circulating pump, the monomer and water charge pumps, etc., will be more expensive but again not by a ×10 factor. Other items such as instruments may be no more expensive than those used on the 10m3 scale but there is a need to increase instrumentation with increasing autoclave size to ensure satisfactory safety standards when handling such large quantities of VCM. As a rough approximation it is reasonable to assume that capital costs will be roughly in proportion to the square root of the autoclave size, e.g. a 100m3 autoclave polymerisation section to give the same total autoclave volume would cost 10/ 10, i.e. 0.3 times as much to build as one based on 10m3 autoclaves. However, there are a number of problems associated with the use of large autoclaves which either must be solved before saleable product is produced or have to be taken into account when designing a plant based on large autoclaves. These have been discussed by Terwiesch.18 1.6 USE OF LARGE AUTOCLAVES 1.6.1 Size of Plant When designing a new PVC plant the first concern is to decide how much PVC production capacity is required. Since transport costs may be a relatively high proportion of the selling price of PVC it is normal for a plant to be built to service primarily only its local market area (say one country of Western Europe or a part of the USA). Since a number of companies may be competing for this same total market, the amount available to a particular firm may be limited and this acts as a constraint on the size of any new plant to be built. Even in the highly developed markets of Europe, USA and Japan, the capacity of a new plant is at present unlikely to be much greater than 100000tpa although the capacity may be considerably increased by subsequent extensions (for example to as much as 300000 tpa). In many of the developing countries a plant to produce as much as 100000 tpa would be too large, so plants with a capacity of 35000 tpa or less have been built in recent years in the developing countries. Accepting the data in section 1.5 which suggest that one 10m3 autoclave can produce 3400tpa of PVC, a plant containing 10–11 autoclaves would be required for 35000tpa and 30 for 100000tpa. Alternatively, assuming for the moment that a 100 m3 autoclave will produce 10 times as much PVC as one of 10m3 (and this is not true—see below), a 35000tpa plant would need one 100m3 autoclave and a plant of 100000tpa three 100m3 autoclaves.

20

R.H.BURGESS

There are, however, a number of other factors which have an important bearing on the choice of autoclave; one is the peak service usage, a second is its effect on downstream equipment and a third is autoclave downtime. Since the PVC suspension polymerisation process is a batch process involving services to charge the autoclave, to heat the autoclave, to cool the autoclave and to discharge it, these services must be adequate to perform these tasks. For example, in the limit, the steam service required to heat a single 100m3 autoclave might be 10 times that required for ten 10m3 autoclaves all sequenced so that they were heated up in turn. A similar argument can be applied to other services. Although on a chemical complex with many other plants sharing facilities such as steam raising, cooling water, etc., this effect can be reduced, it is most unlikely that a single autoclave plant can be justified economically. The effect on downstream activities can also be very large. For example, a 100m3 autoclave will require slurry storage facilities of at least 100 m3 capacity if the autoclave is not to be under-used while awaiting slurry storage room. A 10 m3 autoclave based plant could use a storage tank of, say, 30m3 and this would permit batch blending which might be desirable on product consistency grounds. Finally a plant containing very few autoclaves would be much more seriously affected by a breakdown of any one of those autoclaves than a plant based on a multiplicity of small autoclaves. Taking all these effects into account it is rarely economic to build a PVC plant containing fewer than four autoclaves except as an addition to an existing plant. 1.6.2 Effect of Size on Heat Removal As the size of the autoclave increases, its surface area to volume ratio falls unless the size increase is achieved by simple lengthening of the vessel. This latter is not possible, except to a limited extent, on agitation grounds and most autoclave size increases are achieved by increasing volume at a constant length: diameter ratio. Assuming a constant length: diameter ratio of 2 it is possible to calculate the way in which the surface area changes with volume (see Table 1.7), assuming that the vessel is cylindrical in shape and that only the cylindrical section, and not the ends, is useful surface for cooling. TABLE 1.7 Effect of Autoclave Size on Heat Transfer Surface Area Autoclave volume (m3)

Length (m)

Diameter (m)

Surface area (m2)

Surface area: volume (m-1)

1 3 10 30 100 300

1·72 2·48 3·71 5·34 7·98 11·52

0·86 1·24 1·85 2·67 3·99 5·76

4·65 9·66 21·56 44·79 100 208

4·65 3·22 2·16 1·49 1·00 0·695

The Table shows that the available surface area for cooling as a function of the possible VCM charge (autoclave volume) falls by about a factor of 10 for the autoclave size range 1 to 300 m3 or more realistically by over a factor of 2 for the range 10 to 100m3. However, there is an additional factor in that the actual thickness of the autoclave wall must increase as the diameter of the vessel increases if the fabricated vessels are to have the same pressure rating. As already

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

21

discussed the maximum temperature used for VCM suspension polymerisation is about 80°C, corresponding to an autogenous VCM pressure of 15 bar. After allowing suitable safety margins so that the autoclave pressure can be relieved without rupturing the vessel itself, a typical suspension polymerisation autoclave might be designed to 25 bar working pressure. Such a vessel would require a vessel wall thickness of from 10 to 40mm if built of steel. Typically a cylindrical vessel of 2 m diameter would require a wall thickness of 16mm while one of 4m diameter would require 32mm. Given that the available heat removal capacity is a function of the heat transfer through the walls of the vessel, this can have a significant effect (see eqn. 1.12). A doubling of the wall thickness could reduce the overall heat transfer coefficient by up to 20% (assuming HI, HJ and HM are of similar magnitude at the original wall thickness). Consequently, increasing the autoclave size from 10 to 100m3 may reduce the overall ability of the jacket to remove heat, as a proportion of the heat generated (i.e. the VCM charge) by almost a factor of 3. If no steps are taken to improve this aspect of the process and if the smaller autoclave were already operating on a heat transfer limitation, then the polymerisation time in the 100m3 autoclave would have to increase markedly by up to a factor of 3. The effect on the idealised autoclave process described in Section 1.5 would be to increase the reaction time from 300 to, say, 800 min and the overall cycle to, say, 1000 min giving a theoretical autoclave output, assuming a 40t charge and 90% utilisation, of 21 000 tpa. This is markedly lower at 210 t/m3/year than the 340 t/m3/year theoretically achievable with 10m3 autoclaves. In practice, of course, this theoretical output is very rarely achievable so that a real comparison for a 100 000 tpa plant might be that it would require 6×100m3 autoclaves compared with say 30×10m3 autoclaves. Accepting that 3×100m3 autoclaves would cost only 10/10=0.3 times as much as 30×10m3 autoclaves, then 6×100m3 autoclaves at 0–6 times the cost of 30×10m3 autoclaves would represent a significant saving which would be greater if the heat transfer limitation were reduced. 1.6.3 Reduction of Heat Transfer Problem The heat transfer problem can be reduced either by reducing the peak heat removal rate or by increasing the ability of the autoclave to remove that heat. 1.6.3.1 Reduction of Peak Heat Removal Rate As has already been discussed earlier it is important in a VCM suspension polymerisation to maintain an adequate control of the polymerisation temperature and this means that it is necessary to be able to remove polymerisation heat at the peak rate at which it is generated. In earlier sections the tendency of the polymerisation rate to accelerate with conversion has been described and the correct choice of initiator to balance this effect has been stressed. Unfortunately there are no really satisfactory initiators available to exploit the high cooling capacity of the small autoclaves when polymerising at the low temperatures (50–58° C) needed to make PVC for plasticised applications so that rather longer polymerisation times than necessary, on heat transfer grounds, are common. With 0the poorer heat transfer of the larger autoclaves, especially for low temperature polymerisations, this problem is less severe because at the longer reaction times used initiators already available produce a more square reaction profile. Consequently the effect on reaction time of the poorer heat transfer availability is less severe than would otherwise occur.

22

R.H.BURGESS

1.6.3.2 Increase of Heat Transfer Coefficient The overall heat transfer coefficient HT is given by eqn. 1.12. HI, the autoclave side film coefficient, can be increased somewhat by careful control of agitation to increase turbulence and reduce stagnant boundary layers at the surface and by reducing fouling on the autoclave wall with PVC (see Section 1.6.5). Similarly HJ, the jacket side film coefficient, can be increased slightly by the use of special nozzles or baffles designed to keep the jacket contents in contact with the wall moving as much as possible. However, it is HM, the heat transfer coefficient across the jacket, which has received the most attention. In early days many PVC autoclaves were glass lined and although glass lining techniques have improved so that the effect of the low thermal conductivity of glass has been reduced, almost all large PVC autoclaves have stainless steel, which has a much higher thermal conductivity, on the autoclave side of the jacket. However, the thermal conductivity of stainless steel is itself significantly less than that of mild steel so that most large autoclaves are now constructed of mild steel with a thin cladding of stainless steel in contact with the actual polymerising batch. 1.6.3.3 Use of Low Temperature Coolants Most suspension PVC plants use site cooling water to cool the autoclave contents. Clearly the temperature of that water, which is normally and most cheaply obtained using a cooling tower, will depend on ambient conditions with much higher temperatures resulting on hot, humid days than on cool, dry days. Commonly 15°C may be achievable much of the time in temperate climates but in hotter climates the minimum cooling water temperature achievable may be as high as 30°C. Since the autoclave cooling is determined directly by the temperature difference between the autoclave contents and the jacket these changes in cooling water temperature can have a profound effect on the maximum rate of heat removal, especially for the lower temperature polymerisations. A simple and widely practised means of overcoming this problem is the use of specially chilled water. In this process some or all of the autoclave coolant is reduced in temperature by refrigeration. If the coolant used is water then temperatures as low as 4°C are possible although somewhat higher temperatures (say 5– 10°C) may be more economic. The use of 10°C chilled water on a plant where 30°C cooling water can only be guaranteed by the use of a cooling tower can double the heat removal rate for a 50°C polymerisation temperature and hence increase autoclave productivity by a very considerable amount (50–100%). Lower temperatures than 4°C are possible by changing the coolant from water to water/methanol or to other materials such as ethylene glycol. As far as is known to the author, systems employing cooling below 4°C are not used for PVC manufacture, presumably because suitable initiators are not available to exploit this large extra cooling capacity which is, inevitably, very expensive to operate. Indeed, the use of chilled water itself is expensive since refrigeration equipment is expensive to install, maintain and run unless extremely cheap electricity happens to be available. 1.6.3.4 Use of Condensers These objections to the high cost of chilled water have prevented many manufacturers from exercising this option and increasing attention has been paid in recent years to the use of condensers to remove extra heat. Since VCM is a liquefied gas under polymerisation conditions, its vaporisation and subsequent

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

23

reliquefaction in a condenser can be used to remove polymerisation heat. The latent heat of evaporation of VCM varies with temperature ranging from 336, through 310 to 279 kJ/kg at 20, 50 and 80°C respectively. This compares with the heat of polymerisation of 1540kJ/kg at 75°C, showing that it is necessary to condense all the VCM about five times during the course of the polymerisation to remove all the polymerisation heat by this route. It can be shown that a condenser with a surface area of about 1000m2 is capable of condensing 40t per hour of VCM using a temperature difference across the cooling surface of 30° C, i.e. sufficient to remove the polymerisation heat from a 100m3 autoclave containing 40t of VCM assuming a uniform polymerisation rate and a 5 h reaction. Such a condenser if of the conventional shell and tube construction is quite modest in size and potentially cheap. However, the use of condensers poses certain other problems such as the maintenance of a uniform product remembering that condensed VCM will contain non-typical concentrations of initiator and protective colloid, the avoidance of fouling of the condenser (which is difficult to clean)19 and the diminishing amount of VCM available for condensation especially after 75 % conversion when there is no free VCM phase. In spite of these problems a number of PVC manufacturers are known to use condensers. 1.6.3.5 External Cooling Loop The removal of heat by pumping polymerising slurry out of the reactor, through an external cooler and back into the reactor has been proposed.20 Such a process is clearly capable of significant heat removal but formidable problems such as fouling of the pipe and heat exchanger and, once more, an adequate control of PVC grain properties have to be solved. Despite the reported use of external cooling loops in PVC production it is not thought that this process is much used commercially at this time. 1.6.4 Agitation The VCM suspension polymerisation process is widely practised because its use of protective colloid dissolved in the aqueous phase permits the production of the types of final granule which are required for the wide range of applications for which PVC is used. While it is relatively easy to produce the desired product type from a particular size of autoclave, much greater expertise is required to match the granule structure produced in one autoclave size with that made in another. The basic reason for this is that the production of the final grain is controlled as much by the agitation as it is by the protective colloid and the agitation inevitably changes on changing autoclave size geometry. This problem has been discussed in Section 1.4.3. 1.6.5 Autoclave Fouling PVC is insoluble in VCM so that any PVC deposited on the autoclave surfaces during a polymerisation is unlikely to be removed from the autoclave subsequently. Moreover, since PVC is always swollen with VCM when free VCM is present any PVC deposits (termed build-up) on the autoclave surface are likely to grow rapidly in subsequent polymerisations. The formation of this build-up is a major and long-standing problem with PVC production since it will reduce the heat transfer capability of the autoclave jacket and will lead to quality problems with the PVC produced if any small quantity finds its way into the final product. When

24

R.H.BURGESS

large quantities have been formed the build-up may alter the agitation, prevent good temperature control and block the autoclave discharge valve. These problems are so severe that for many years it was the practice of suspension PVC manufacturers to clean the autoclave of build-up between every batch. This was formerly a manual operation involving a man entering the autoclave and scraping the build-up off with paint scrapers or the like. About 15 years ago alternative methods of removing the build-up either with jets of high pressure water (100–300 bars)18 or with solvents21 were developed and started to come into increasing service. The discovering of a link between VCM exposure and acroosteolysis, a degeneration of finger bone tissue, in the mid 1960s and, more particularly, angiosarcoma, a rare form of liver cancer, in 1974, rendered it unacceptable to expose operators to the relatively high VCM concentrations in air produced by releasing VCM dissolved in the autoclave build-up unless they were wearing suitable protective clothing and breathing apparatus. However, most companies from that time onwards adopted high pressure water or solvent cleaning and entry of the autoclaves became a rare occurrence, and then only under strictly controlled conditions. In practice high pressure water cleaning is the more popular because of the high cost of solvent systems, especially the cost of separation of solvent from the dissolved PVC. 1.6.5.1 High Pressure Water Cleaning The use of high pressure water (e.g. 300 bars) involves passing water through a constriction so that a jet of water with a strong cutting action is produced. While hand-held devices are available their usefulness is strictly limited since the need is to avoid operator exposure to the inside of the autoclave. Consequently, rotating heads have been developed which are lowered into the autoclave and automatically cover a large area of the surface usually by rotating in both the vertical and horizontal planes. These cleaning heads are a complex piece of equipment operating under very onerous conditions, and in the early days were subject to quite frequent breakdown. However, much development work has been carried out by PVC and equipment manufacturers both on the high pressure water pumps and the rotary heads to improve reliability in service. These rotating heads were formerly spaced centrally in the autoclave and their efficiency as autoclave cleaners depended on the very high pressures of water used at the nozzles. The input energy of the water falls very rapidly with increasing distance from the nozzle and there is a tendency for the jet to break up on passing through the air. This is potentially very serious for large autoclaves since a 100m3 autoclave has double the diameter of a 10m3 vessel (Table 1.7). Increasing the water pressure to compensate for increased autoclave diameter would present routine operation problems with the pump and serious erosion at the nozzles. Since the jet impact energy falls with distance from the nozzle principally because of the resistance of the gas through which it is passing and the droplet break-up which occurs, high pressure water cleaning in a vacuum or partial vacuum has been proposed.22 However, most attention has been given to equipment development designed to place the nozzles nearer the autoclave walls. 1.6.5.2 Chemical Build-up Suppression The major event of the last 10 years in the field of build-up control has been the development of chemical systems designed to prevent build-up formation. Hundreds of patents have been filled on these systems and it is very difficult to generalise on the methods chosen but a consideration of the possible mechanisms of build-up formation gives some clue to the reasons why these systems are effective. The VCM suspension

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

25

polymerisation process is intended to produce grains of PVC using an initiator which is dissolved in the VCM or PVC/VCM phase. Provided the droplets of VCM or the PVC/VCM particles formed are sufficiently stable and the initiator is only in the organic phase then no build-up on the autoclave should be formed. In Section 1.4.2 the possible mechanism of grain formation was discussed and the probability of controlled agglomeration of the original droplets was mentioned. This inevitably implies that the droplets are not very stable and since this step is needed for the production of PVC with the desired grain structure little can be done to change this situation. In fact the use of large quantities of protective colloid can be shown largely to eliminate build-up formation at the expense of producing very fine glassy grains of little commercial value. Another and more effective method of reducing build-up formation is by eliminating areas of high shear in the autoclave, so that the somewhat unstable polymerising particles are not shattered by the high shear. Indeed it is well known that build-up forms on all discontinuities inside the autoclave, e.g. brackets, impellor, etc., and the elimination of these, or at least streamlining, is known to reduce buildup formation.23 The initiators used in VCM suspension polymerisations are monomer soluble and all tend to be mainly dissolved in the VCM. However, there is normally an excess of water present in the system and many of the initiators are soluble at least to a small extent in the water. Since VCM is also soluble in water to a limited extent there is a finite possibility of PVC formation in the aqueous phase. Such PVC is not stabilised by the protective colloid and may well be attracted to the autoclave wall where it forms build-up. It is true as a generalisation that the more water soluble the initiator the more build-up is formed. Initiators such as acetylcyclohexylsulphonyl peroxide and t-butyl perpivalate are known to be particularly poor in this respect while highly organic molecules such as lauroyl peroxide and dicetyl peroxydicarbonate are good. Other initiators such as the azo compounds produce little build-up possibly because they are incapable of activated decomposition on the surface induced by iron (Fe3+). Certainly it is known that build-up is rich in iron and mild steel or other corrodable steels are especially bad for build-up. It will be clear from this that although the choice of initiator, the protective colloid system and details of the autoclave design and agitation are very important in reducing build-up, further action is usually necessary to eliminate build-up completely. The largest class of chemical build-up suppressant systems described in the literature is based on chemicals which are inhibitors for VCM polymerisation which are themselves located on the autoclave wall. Clearly if these materials are complete inhibitors for the polymerisation and are located only on the surfaces to which build-up normally adheres a complete build-up suppressant system has been developed. In practice neither of these criteria is easy to achieve but many systems have been developed aimed at this ideal. A wide range of phenolic and nitrogen containing inhibitors has been patented25–27 which either have an affinity for metal surfaces, for example Nigrosine dyestuffs,25 or can be physically located on the autoclave wall, for example phenol formaldehyde resins.26 Taking these two examples as typical it is possible to identify some of the problems encountered in the development of build-up suppressant systems. Nigrosine dyestuffs are highly coloured and very powerful inhibitors for VCM polymerisation. Many of them are indeed very good build-up suppressants but there is a tendency for some of the Nigrosine to end in the bulk phase producing discoloration of the PVC and some retardation of the polymerisation. Much effort has been expended trying to ‘fix’ the Nigrosine on the autoclave wall using, for example, resin carriers.25 The use of phenol/formaldehyde resins located on the autoclave wall may involve a cross-linking baking step in their application since there is a tendency for all organic materials to be swollen with VCM during the polymerisation which can lead to breakdown of the layer on the surface. Other patents are concerned with suppressing reactions/interactions between the autoclave wall and the PVC. Oxidising agents,28 reducing agents,29 and iron chelators such as EDTA,30 have all been patented.

26

R.H.BURGESS

It is clear that most major PVC manufacturers have now developed build-up suppressant systems. It seems that almost all are different in detail, possibly because of differences in the equipment used to make PVC but more probably because the suppressant system must be compatible with the rest of the PVC process, i.e. it must have at most only a minimum effect on the quality of the PVC made. The development of these build-up suppressant systems has enabled PVC suspension polymerisation without interbatch opening to be a possibility. At the end of a typical suspension polymerisation after discharge of the unstripped PVC slurry, the autoclave is full of VCM. This must be removed by evacuation and purging before the autoclave is opened for high pressure water cleaning. These operations take some considerable time and inevitably some loss of VCM to the environment results. Furthermore after clearing the autoclave it is necessary to remove residual air before the next batch can be charged. Both these steps can be avoided if there is no necessity for interbatch cleaning although as a consequence of this it is necessary to be able to charge the recipe ingredients automatically. While it is generally simple to dissolve many of the polymerisation ingredients, protective colloid, buffer, etc., in water, some ingredients, notably the initiator, are not soluble in water and it is undesirable to dissolve them in VCM for safety reasons. Many of the more active initiators are already supplied as solutions in phthalate plasticisers or mineral oils but many have to be stored and handled at lower than ambient temperatures if dangerous decompositions are to be avoided. Some solid initiators could be dissolved in solvent but the presence of solvent in the final PVC or the liquid effluent from the plant is undesirable and dissolving some of the ‘stable’ active initiators such as the solid peroxydicarbonates actually reduces their stability so that they behave like their liquid lower molecular weight analogues. Consequently dispersions of these solid initiators in water have been developed31 for use on automated plants operating with so-called closed lid technology.31 1.6.6 Instrumentation and Safety Consideration Over the past 10 years the availability of sophisticated computerised equipment has revolutionised the possibilities for controlling a PVC suspension polymerisation plant. Prior to that time the instrumentation on a PVC plant consisted of accurate but limited batch controllers which, for example, would control one function such as the autoclave temperature with the remainder of the operations being carried out manually by the plant operators. While a batch process such as the VCM suspension polymerisation is much more complicated to control than a continuous one, the use of fewer and larger autoclaves has made the overall task significantly less. The very large quantities of VCM in process and the toxicity of VCM have increased the need to ensure the safety of the process. Finally the high cost of process labour has provided an economic case for eliminating the more mundane tasks. These factors have combined so that the more recent PVC plants18 are almost always computer controlled and many old plants have been converted to at least semi-automatic operation with the minimum of operator involvement. These installations range in complexity from a replacement of the batch controllers by a computer through to the most complex systems involving both master and slave computer. Perhaps the most common is a dual computer system where the first computer is used to control the charging of the autoclave, the polymerisation and its discharge together with the cleaning operation necessary before the next batch can be charged. Usually such a process will involve extensive safety checking to ensure, for example, that water and VCM are charged to one autoclave at a time through a metering system which is checked to ensure that it is operating correctly. The autoclave temperature and pressure might be monitored carefully and automatic systems to prevent abnormal polymerisations automatically triggered. Other operations such as stripping and drying might also be controlled by the computer. The second computer would back up the

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

27

first in the event of a breakdown but might also be monitoring upstream and downstream activities so that a decision on discharging or charging an autoclave could be taken automatically. ACKNOWLEDGEMENTS The author is especially grateful to G.C. Maitland of Imperial College and to J.H.Wilson, P.D.Roberts and A.K.Sanderson of ICI Plastics Division for their help in preparing this chapter. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

TALAMINI, G., J. Polym. Sci., Part A2, 4, 535 (1966). CROSATO-ARNALDI, A., GASPARINI, P. and TALAMINI, G., Makromolek. Chemie, 117, 140(1968). ABDEL-ALIM, A.H. and HAMIELEC, A.E., J. Appl. Polym. Sci., 16, 783 (1972). UGELSTAD, J., LEVRICK, H., GARDINOVACKI, B. and SUND, E., Pure Appl. Chem, 26, 121 (1971). KUCHANOV, S.J. and BORT, D.N., Polym. Sci. USSR, 15, 2712 (1974). FREEMAN, M. and MANNING, P.P., J. Polym. Sci., A2, 2017 (1964). MONSANTO, British Patent 954983. DANA, L.T., BURDICK, J.N. and JENKINS, A.C., J. Amer. Chem. Soc., 49,2801 (1927). JOSHI, R.M., Ind. J. Chem., 2, 125 (1964). UEDA, T., TAKCUCHI, K. and KATO, M., J. Polym. Sci., Polymer Chem. Edn., 10, 2841 (1972). DAVIDSON, J.A. and WITENHAFER, D.E., J. Polym. Sci., Polymer Phys. Edn., 18, 51 (1980). SANDERSON, A.K., Brit. Polym. J., 12, 186 (1980). ZICHY, E.L., J. Macromol. Sci.—Chem., A11(7), 1205 (1977). UHL, V.W. and GRAY, J.B. (Eds.), Mixing Theory and Practice, Volume 2, Academic Press, London (1967). Data provided by Akzo Chemie. Data of Wako Pure Chemicals Industries Ltd. RHONE-PROGIL, British Patent 1435425. TERWIESCH, B., Hydrocarbon Processing, November 1976, 117. CONOCO, British Patent 1410128. ANIC, British Patent 2001659. IAMMARTINO, N.R., Chemical Engineering, 24 November 1975, 25. ICI, British Patent 1484866. GEORGIA PACIFIC, British Patent 1508818. HULS, German Patent Publication 2000397. SHIN ETSU, British Patents 1536160 and 1291145. ICI, British Patent 1439334; MITSUI TOATSU, Japanese Patent Publication 54 107991; GOODRICH, British Patent 1523041 GOODRICH, British Patent 1491115; UNIVERSAL PVC RESINS, US Patent 3778423. SHIN ETSU, British Patent 1373286; HULS, British Patent 1502335. WACKER, French Patent 2185678. Dow, Dutch Publication 68 10726. VERHELST, W.F., OOSTERWIJK, H.M.J. and VAN der BEND, D. Th., Kunstoffe, 70, 224(1980).

Chapter 2 BULK PROCESSES FOR THE MANUFACTURE OF PVC M.W.ALLSOPP Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

2.1 INTRODUCTION Vinyl chloride monomer (VCM) can be polymerised by any of the following techniques: suspension, bulk (mass or gas phase), emulsion and solution/diluent. The largest tonnage of PVC and its copolymers is made by the suspension polymerisation route which produces a coarse powder of 100–150 µ m mean size (see Chapter 1). A considerable quantity of PVC is made by the emulsion process, whose product is a latex containing particles below 1–2 µ m in size which can be used directly or, as is more usual, spray dried to produce a friable powder suitable for general purpose extrusion or for mixing with plasticisers to form pastes used in spreading and coating applications (see Chapter 3). A number of attempts have been made to produce PVC by the well-known bulk polymerisation route, which is used extensively for other monomers, e.g. styrene and methyl methacrylate, but the only process to reach commercial status is that developed by Pechiney St Gobain (PSG)—now Rhone-Poulenc Industries Ltd, this process being the subject of this chapter. A large tonnage of PVC is now made by this process, the majority by a number of PVC manufacturers who are licensees of the PSG process: see Table 2.1. Small quantities of vinyl acetate copolymers are produced by solution copolymerisation for coating applications but this is a rather specialised field involving only one major company, Union Carbide, and is not within the scope of this book. A few years ago much interest was created by the development of a bulk ‘gas phase’ polymerisation process based on a fluidised bed reactor but the process has not yet reached commercial status. This process is considered in more detail later in this chapter. All the above processes are operated in a batchwise form only, except for TABLE 2.1 Rhone-Poulenc Licensees of ‘Mass’ PVC Process Licensee

Country

Capacity (ktpa)

Approx. date of original licence

Wacker Wacker Wacker Toa Gosei Rio Rodanoa Dzerghinsk

Germany Germany Germany Japan Spain USSR

Small single-stage process plant Small two-stage process plant 60 ktpa two-stage process plant Small single-stage process plant 80 90

1956 1966 1971/72 1957 1966 1966

BULK PROCESSES FOR THE MANUFACTURE OF PVC

Licensee

Country

Calico Mills India BP Chemicals UK Hoechst Germany Huls Germany SCR Italy Polymeros Mexico Goodyear USA Goodrich USA Hooker USA Vinylplastika Yugoslavia Ohis Yugoslavia Certain Teed USA Diamond Shamrock Canada Rhone-Poulenc France Total mass capacity in World

Capacity (ktpa)

20 90 30 30 30 50 75 20 80 20 40 90–100 90–100 250 ca 106 a Rio Rodano is a Rhone-Poulenc affiliate.

29

Approx. date of original licence 1967 1967/68 1968 1968 1968 1968 1968 1968 1967/68 1969 1971 1972/73 1976 —

emulsion polymerisation, where a continuous process has also been in operation in Germany for many years (Chapter 3), and the solution polymerisation process. The different approaches to suspension and emulsion polymerisation are dealt with in Chapters 1 and 3 respectively whilst in this chapter bulk polymerisation techniques are described. 2.2 DEVELOPMENT OF BULK POLYMERISATION OF VINYL CHLORIDE Bulk polymerisation is defined as a single-phase process involving just monomer and the initiating species. Two modes of bulk polymerisation are possible, viz. homogeneous or heterogeneous. In the former case where the polymer is soluble in the monomer, e.g. polystyrene and poly(methyl methacrylate), bulk processes in which the monomer is converted to a very high viscosity polymer/monomer system result in a solid structureless mass of interspersed polymer chains. Both batch and continuous processes are well known1 and are characterised, in the case of polystyrene, by a fall in reaction rate as the conversion and viscosity of the system increases. Hence, the maximum rate of heat removal is required when the viscosity of the system is at its lowest. In contrast, in the heterogeneous mode where the polymer precipitates from the monomer at low conversion, e.g. poly(vinyl chloride) and polyacrylonitrile, the operation of a bulk process can present quite different problems. With PVC the polymer precipitates immediately as 0.1 µ m primary particles and the number of them is fixed at an early stage. They grow with conversion and then aggregate, so the viscosity of the system increases rapidly. Eventually a phase change results as the unreacted monomer is absorbed by the powdery polymer. For many years the problems of control of grain size and removing the heat of polymerisation at high viscosity retarded the development of a commercial bulk process for PVC. Many companies evaluated their own bulk polymerisation processes since the concept, compared to suspension, is simple especially as a continuous process for PVC. A number of attempts were made to overcome the problems but these met

30

M.W.ALLSOPP

with limited success and eventually most PVC manufacturers dropped the bulk process in favour of increased development of their existing suspension or emulsion systems. It was left to St Gobain to pursue the bulk polymerisation route but the early days were fraught with problems of particle size control, buildup and overheating. Three success criteria were apparent to those working in this field. Initially, control the primary particle agglomeration step to give a uniform product of the correct size at low conversion and prevent further agglomeration as conversion rises by uniform agitation. Secondly, remove the heat of polymerisation uniformly from the powdery phase so as to control reaction rate, molecular weight and polymer properties. Thirdly, prevent encrustation of the reactor walls. The breakthrough came after many years’ development in 1960 when the above criteria were largely satisfied by the introduction of the two-stage process by the company, then operating as Pechiney-St Gobain. 2.3 EVOLUTION OF ST GOBAIN TO RHONE-POULENC As some readers may be confused over the proliferation of company names which are associated with apparently the same process, a short company history of bulk process now follows. Also, the term ‘mass’ polymerisation is used to describe the bulk process particularly in France. St Gobain has a very long history in the chemical industry, one of its principal antecedents going back to the time of Louis XIV in the 17th century when it was principally involved in glass manufacture. A famous name in the history of the company is Gay-Lussac. By the early part of the 20th century the company had become known by the name Manufactures des Glaces et Produits Chimiques de St Gobain, Chauney et Cirey SA. In the post-Second World War period expansion of the company was rapid and links were established with other French companies as well as with companies of different national identities. The early development of the PVC bulk process was achieved by P.C. de St Gobain. In 1960 St Gobain and another large French chemical company, Pechiney, also with PVC interests, decided to pool their activities and a-new 50/50 company called Produits Chimiques Pechiney-St Gobain was formed. Pechiney and St Gobain both had a whole network of relationships with other French and nonFrench companies, one of these being Rhone-Poulenc. As the development of the two-stage PVC bulk process was achieved and exploited at about this time it came to be identified with Pechiney St Gobain, or PSG. In the late 1960s a series of mergers began to take place within the network of companies related to Pechiney and to St Gobain and in 1969 it was announced that Rhone-Poulenc was taking a controlling (51%) interest in PSG. This led to some problems for other companies in the network of relationships and as a result further realignments began to follow. In 1971 there was a merging of Rhone-Poulenc’s two largest subsidiaries, viz. PSG and Progil, and the new company became known as Rhone-Progil. From then on the PVC bulk process became attributed to Rhone-Progil rather than to PSG, probably because Pechiney became increasingly closely related to another grouping, Produits Chimiques Ugine Kuhlmann (PCUK), which was also producing PVC but by another process and in competition with Rhone-Progil. A formal regrouping of these companies took place in 1974 with PCUK splitting off completely. In January 1975 there was a complete merger of the subsidiary Rhone-Progil with the parent Usines Chimiques Rhone-Poulenc, and a new company called Rhone-Poulenc Industries was formed. As from January 1975 the bulk PVC process was the property of Rhone-Poulenc.

BULK PROCESSES FOR THE MANUFACTURE OF PVC

31

2.4 HISTORY OF THE RHONE-POULENC BULK PROCESS FOR PVC 2.4.1 Early Developments—the Single-Stage Process Work began in France in 1938 in the laboratories of Produits Chimiques St Gobain and by 1939 St Gobain had developed their first commercial process for the bulk polymerisation of VCM. At this point in time it consisted of a 0–8m3 vertical autoclave equipped with two agitators, a turbine type to mix the liquid phase and an anchor type to mix the precipitating solid phase. The product made in this way was reported to be of good quality but wall encrustation as well as problems of scale-up and of heat removal caused this system later to be abandoned, when the rapid expansion of the market demand brought the need for increased capacity. In 1956 a modified process was introduced by St Gobain. This involved the use of a 12m3 horizontal autoclave revolving about its own axis. Agitation was further assisted by the inclusion in the autoclave of a number of stainless steel balls weighted with lead. This process became known as the L51 process and it permitted a very marked increase in production capacity: without this development the mass process could not have remained competitive. A principal defect of the process, however, was that the speed of rotation of the autoclave had to be kept within strict limits and the conditions of agitation could not be varied, so the variations in grain structure were strictly limited. 2.4.2 Development of the Two-Stage Process From 1958 the chief research objective of St Gobain was to develop a process that permitted better control of particle morphology. This was achieved when it was shown to be possible to carry out the polymerisation in two steps: by the production of a seed polymer in stage 1, when conversion would be taken to 7–12%, and then transference of the seed in its VCM medium to a second autoclave to which more VCM was added and where conversion would be taken to 70–85%. For the first stage seed polymerisation a vertical stainless steel (SS) autoclave with a turbine type stirrer with flat blades giving strong agitation was found best for the right kind of nucleation. For the second stage polymerisation a horizontal SS autoclave of 12m3 capacity rotating as before on its axis and with steel balls to promote agitation was used initially. Later this was changed to a stationary autoclave with a slowly rotating (9 rpm) ribbon blender type of agitator. The new process was at first known simply as ‘modified L51’ but was renamed M60 when the shape of the agitator blades in the horizontal autoclave was changed and the shaft hollowed out to permit improved cooling. Such a stationary horizontal autoclave has been in commercial operation at St Fons since 1962. In 1966 the process was renamed M66 after being further modified by the addition of reflux condensers to both stage 1 and stage 2 autoclaves. The improved heat removal permitted shorter reaction times and therefore improved productivity. The next step in the development of the process was to use one pre-polymeriser, typically an 8m3 autoclave, to feed three horizontal autoclaves of 16m3 capacity and this combination formed a stream of ca 15ktpa capacity which could be duplicated to give a plant of ca 30ktpa capacity. Next there was a further increase in the number and size of the horizontal autoclaves that could be fed by the pre-polymeriser, which was itself increased in size. The preferred new combination comprised 1×17m3 pre-polymeriser and 5×30m3 horizontal autoclaves.

32

M.W.ALLSOPP

FIG. 2.1. General layout of the Rhone-Poulenc bulk polymerisation plant (top) and a detailed representation of the prepolymeriser and the second stage horizontal autoclave (bottom). CY=cyclone. (Reproduced in part from Marks, Developments in PVC Technology, Henson and Whelan, Eds., Applied Science Publishers Ltd, 1973.)

Further changes were also made in the agitator and cooling arrangements. Such a plant was receded N7 and its capacity was ca 60ktpa per stream. In the mid 1970s a further increase in the size of the stage 1 and stage 2 autoclaves was made, the preferred combination being 1×25m3 pre-polymeriser and 5×50m3 horizontal autoclaves. This new combination was renamed N8 and the capacity per stream was raised to 100ktpa. All further discussion in this chapter relates to the N8 process involving one pre-polymeriser feeding five horizontal reactors unless otherwise indicated. 2.5 RHONE-POULENC TWO-STAGE BULK POLYMERISATION PROCESS There are two distinct phases in the process; firstly the formation of the grains in a liquid phase and secondly, the growth of the grains in an essentially solid powdery phase. Since the agitation requirements in the two phases are fundamentally different, each step must be carried out in two distinct reactors. Figure 2.1 shows the overall layout and the two reactors in detail.2,3 The importance of agitation in the first stage with respect to polymer properties is best understood by first considering the mechanism of particle formation in a VCM polymerisation.

BULK PROCESSES FOR THE MANUFACTURE OF PVC

33

2.5.1 Mechanism of Particle Formation Vinyl chloride is polymerised in bulk by a free radical mechanism using an acyl peroxide or peroxydicarbonate as the initiating species. Since vinyl chloride boils at-13.4°C the polymerisation is carried out under pressure in stainless steel reactors at temperatures of 40–70°C and pressures of 5–12 bar (75–175psi). The vinyl chloride containing the initiator is rapidly brought to reaction temperature and polymerisation begins. After initiation the growing polymer chain rapidly becomes insoluble in the monomer and precipitates.4 Several polymer chains aggregate together to form the first identifiable species —the micro-domain or basic particle about 200 A in diameter. (See Chapter 7 for a full description of nomenclature.) These micro-domains contain a maximum of about 50 polymer chains and therefore cannot be representative of a single polymerisation site. They are negatively charged and are stable for a short period of time.5 After a brief growth stage the micro-domains coagulate to form primary particles, which contain approximately 103 micro-domains and are 0.1 to 0.2 µ m in diameter. This stage is attained before 2% conversion is reached; after completion of the aggregation step free microdomains are no longer observed and no new primary particles are formed. Therefore, all further growth takes place in, or on, the surface of the existing primary particles and two mechanisms are possible. If the monomer contains steric stabilisers further growth may continue to give monodisperse primary particles up to 1–2 µ m in diameter. In the absence of colloid stabilisers, where the system is electrostatically stabilised, the zeta potential gradually decreases with increase in particle size until the primaries reach a critical size at which van der Waals attractive forces exceed the electrostatic repulsive forces between the particles and coagulation into 1–2 µ m agglomerates results. Although the structure of each agglomerate is essentially close packed for minimum free energy, the inter-agglomerate structure depends very much on the level of agitation at the time of coalescence. Up to this point the mechanism of polymerisation of both bulk and suspension processes is the same but from this point onwards the objectives are different. In the suspension process (Chapter 1) agitation is used to control not only the coagulation of the primary particles but also the size, rate and degree of coalescence of the monomer droplets and hence the size and shape of the final grains. In the bulk process the rate and uniformity of the primary coagulation step alone defines the total morphology of the grains at this stage. In order to achieve a uniform grain this step is completed as rapidly and uniformly as possible by the use of very vigorous agitation in the first stage reactor—the pre-polymeriser (pre-po). This results in formation of the agglomerates at an earlier stage in the bulk process than in suspension. However, once formed the growth mechanisms converge and the agglomerates grow in size with conversion as a result of the growth of the component primary particles. The growth of the primaries from 0.2 µ m at low conversion to approximately 0.6–0.8 µ m results in the agglomerate dimensions increasing from 1–2 µ m when first formed up to 2–10 µ m (average 5µ m) at high conversion. The actual mechanism of growth with conversion is considered in more detail in Chapter 7. 2.5.2 The Influence of the First Stage Reactor (Pre-po) A flat blade turbine agitator is used in conjunction with baffles to prevent vortex formation. The diameter of the final grain depends on the tip speed used (Fig. 2.2).2 As well as grain size and size distribution it is very important to control the level of primary particle and aggregate cohesion since this parameter not only affects the final processing properties of the polymer but also determines the ability of the material to survive the severity of transfer to, and a long period of agitation

34

M.W.ALLSOPP

FIG. 2.2. Variation of grain size of bulk polymer as a function of pre-polymeriser agitation speed. (Reproduced from Marks, Developments in PVC Technology, Henson and Whelan, Eds., Applied Science Publishers Ltd, 1973.)

(and possibly attrition) in, the second stage reactor. This is achieved by control of the polymerisation temperature in the pre-po. As higher reaction temperatures are used the degree of cohesion between primary aggregates increases. Therefore, to ensure aggregate cohesion, the first stage is always polymerised above 62°C. This has little effect on the molecular weight of the final polymer, since, on average, only 5% of the total polymer is formed in the pre-po. The temperature of pre-polymerisation can be selected at will independently of the molecular weight of the final PVC produced. However, the use of temperatures above 62°C in the pre-po will limit the ultimate porosity that can be achieved since the openness of the aggregate network depends to a considerable extent on pre-po temperature (Fig. 2.3).3 The morphology created in the initial 2% of conversion in suspension or bulk has a major influence on the overall morphology of the grain. As the spacial arrangement of the aggregates in the grain is fixed early on in the pre-po the morphology created in this step is important even though it only contributes less than 5% of the total conversion. In fact, the degree of conversion in the pre-po plays only a minor role in the final porosity, which mainly depends on the final conversion because growth continues in the second stage reactor. Therefore, if porosity needs to be increased the pre-po temperature can be reduced, but not below 62°C. In the pre-po sufficient initiator just to complete the reaction to 8% conversion is used so that the seed can be stored for some time if necessary. Reaction time is less than half an hour. It is likely that for seed produced at the lower end of the 62–75°C range a highly active initiator, e.g. acetylcyclohexylsulphonyl peroxide, is used whereas di-isopropyl peroxydicarbonate is used at the higher end of the temperature range. The latter initiator is probably used in the second stage reactor.

BULK PROCESSES FOR THE MANUFACTURE OF PVC

35

FIG. 2.3. Cold plasticiser absorption versus pre-polymerisation temperature, K-value 65; conversion 80%. (Reproduced from Chatelain, British Polymer Journal, 5, 457, 1973.)

Cleaning of the pre-po is generally unnecessary since the conversion is very low and even if some material is held up from one batch to the next the slight increase in density and cohesion is unlikely to produce dispersion faults (‘fish eyes’) in the final product. The low viscosity (0.2cP at—14°C, 0.193cP at 25°C) system is easy to agitate initially but the viscosity increases rapidly with conversion so that above 15% the power requirement would prevent normal turbine operation. Conversion is normally 8% at which point grain cohesion is sufficient to allow transfer to the second stage reactor with a practical limit of 12%. The heat of polymerisation is removed by a combination of jacket cooling and reflux condenser. Experiments have shown that there is no need to submit all the monomer to the pre-polymerisation step. Generally, only half the monomer required for the polymerisation is charged to the pre-po. 2.5.3 The influence of the Second Stage Reactor The seed is transferred by gravity along with additional monomer, and the necessary initiator for the second step is injected in solution in plasticiser. Oxygen is removed from the reactor by vaporising a part of the monomer charge. To ensure that the pre-po remains free of polymer some of the additional monomer required can be used to flush the pre-po contents to the second stage reactor. In the second stage the grain becomes stronger due to fusion of the primary particle agglomerates which grow in size with subsequent infilling of the pores between them. Experiments have shown that the porosity is dependent on the temperature of polymerisation and on the degree of conversion in the second step (Fig. 2.4).3 If a high porosity level is required final conversion must be decreased, and/or a lower polymerisation temperature used. The physical nature of the material undergoes several changes with conversion. Initially a suspension of grains in VCM results, but this rapidly changes in appearance to that of a damp powder with no free monomer on the outside of the grain at about 20% conversion. As the reaction continues the liquid monomer is absorbed by the PVC grains leaving a ‘dry’ powdery material after a conversion of about 40%. Correct agitation of the powder is important at this stage to ensure that the monomer returned from the condenser is

36

M.W.ALLSOPP

FIG. 2.4. Cold plasticiser absorption versus conversion at two different second stage temperatures. Pre-polymerisation temperature 70°C. (Reproduced from Chatelain, British Polymer Journal, 5, 457, 1973.)

well distributed. This is performed by a complex three-bladed helical agitator with incurved blades to prevent jerks when penetrating the polymer powder (Fig. 2.1). These rotate with minimum wall clearance at 6–7 rpm for a 50m3 autoclave. The conversion continues to rise until the predetermined end point is reached, when the reaction is terminated by removal of unreacted VCM. Progress of the polymerisation is followed by measuring the heat generated during polymerisation and is used to indicate the moment for transfer from the pre-polymeriser to the second stage reactor and for stopping the polymerisation. The normal level of conversion reached is 80% and reaction time in the second stage is 3–5 h depending on temperature. Use of a high level of conversion reduces porosity and gives rise to a highly integrated grain structure. 2.6 CONTROL OF PROPERTIES For a certain grade of polymer, molecular weight (and K-value) and hence second stage polymerisation temperature are fixed, since K-value depends solely on temperature due to the dominance of chain transfer, as in suspension polymerisation (see Chapter 1). Grain size is controlled by the speed of agitation during the first stage (Fig. 2.2). Temperature during the first stage affects porosity (Fig. 2.3), and the cohesion of the grains. The latter limits the temperature to 62°C and above, which has a slight influence on the level of porosity that can be attained. Conversion in the first stage defines the quantity of seed available and ensures sufficient cohesion for transfer to the horizontal reactor. In the second stage an increase of conversion lowers porosity and increases bulk density. The temperature of polymerisation in the second stage controls the molecular weight or K-value (see Chapter 1) of the polymer as well as its porosity (see Fig. 2.4). In practice, the porosity expected at a pre-po temperature of 70°C and a conversion of 80% is known (Fig. 2.5).3 If the porosity is too low, the pre-po temperature has to be lowered but a reduction below 62°C would give rise to problems of grain attrition during transfer and second stage reaction. If the theoretical temperature needs to be below 62°C to attain a higher porosity, a temperature of 62°C is used but conversion in the second stage is reduced. The second stage reactor temperature cannot be changed because it defines the K-value of the final polymer.

BULK PROCESSES FOR THE MANUFACTURE OF PVC

37

FIG. 2.5. Cold plasticiser versus K-value. Pre-polymerisation temperature 70°C, conversion 80%. (Reproduced from Chatelain, British Polymer Journal, 5, 457, 1973.)

2.7 PROCESS CONTROL Rather than attempting to control the process temperature itself, pressure is monitored as it can be measured more accurately, particularly in the powdery stage, and its relationship with temperature is linear as long as conversion is not too high. Towards the end of the polymerisation when most of the vinyl chloride is used up the control system increases the temperature of the reactor in an attempt to maintain a constant pressure. The polymerisation of VCM is strongly exothermic. As the monomer is constantly in equilibrium with its vapour, it is easy to remove the heat of reaction by vaporisation and recondensation on the cool walls of the reactor or in a condenser so the reaction is carried out under reflux, which is possible as long as the degree of conversion remains below 88%. Therefore, if a grain becomes too hot the pressure in the immediate zone surrounding it becomes lower than that at saturation and some of the adsorbed monomer evaporates until equilibrium is again established. The condensation of the evaporated monomer can take place on any chilled surface and the normal cooling plan as the reaction accelerates is to bring progressively into operation firstly a cooled agitator, secondly the jacket, and finally the condenser(s). One condenser is used on reactors up to 30m3 capacity while two are used on the 50m3 scale. As in other processes, build-up can form on all surfaces if the correct precautions are not taken. In the bulk process build-up levels depend on monomer purity, initiator type and wall temperature; as long as the latter is kept low the build-up level is small. The pre-po does not require regular cleaning but the second stage reactor does. High pressure water cleaning is regularly used and operator entry is no longer required on a routine basis. 2.8 DEGASSING AND POWDER HANDLING Degassing takes place in the reactor and may be completed elsewhere. This step represents a significant proportion of the cycle which is a reflection of the problem of heating the polymer sufficiently rapidly at the end of the polymerisation in the absence of a good heat transfer medium. As the condenser removes the

38

M.W.ALLSOPP

FIG. 2.6. Optical morphology of typical suspension and bulk polymer: (a) suspension; (b) bulk.

majority of the exothermic heat of reaction, conversely, the other forms of heat exchange can only provide a limited degree of heating. Prior to the industry’s awareness of the VCM cancer problem, the bulk process resins were usually fairly high in residual VCM ex-factory, although because of the porous nature of the grains the VCM level fell fairly rapidly when the resins were packed and stored in paper sacks. Bulk handling of resin did present increased problems and longer degassing times were required for resins handled in this way. More recently the problem was largely overcome by injecting a small quantity of water (steam or liquid) or nitrogen at the degassing stage to act as carrier6 and mass polymers ex-factory are probably now little different from well stripped suspension resins in terms of residual VCM. After completion of the polymerisation in the horizontal autoclave the unreacted VCM is recovered and dry PVC conveyed to a resin receiver. From there the resin is screened to remove small amounts of oversize material. The main bulk of the resin (95%) goes to finished product storage. The particle size of this grade A material is slightly narrower than for an equivalent suspension resin. The oversize material is ground and rescreened enabling some of it to be recovered as grade A product. 2.9 MORPHOLOGY Although the subject is dealt with in depth in Chapter 7, it is worth acquainting the reader at this stage with aspects of bulk polymer morphology compared to suspension which make the former instantly recognisable. If a sample of polymer is immersed in plasticiser, allowed to come to equilibrium and then viewed by transmitted light, the optical morphology is seen (Fig. 2.6). Whereas suspension polymers appear rounded and slightly different from different manufacturers, mass polymer is always typified by ‘squaresided’ grains and a background of fines, whatever the source. Scanning electron microscopy also shows large differences in the appearance of the grains (Fig. 2.7).

BULK PROCESSES FOR THE MANUFACTURE OF PVC

39

FIG. 2.7. Scanning electron micrographs of typical suspension and bulk PVC grains: (a) suspension; (b) bulk.

2.10 COPOLYMERS Mass polymerisation is adaptable for the preparation of copolymers of vinyl chloride which are insoluble in their monomers but the reaction temperature must be lower than the softening point of the copolymer formed.

40

M.W.ALLSOPP

The copolymers assume the characteristics of homopolymers made by bulk polymerisation in that the structure of the seed is maintained. Hence, it is possible to obtain copolymers with vinyl acetate having a suitable porosity. As for all copolymerisation, it is necessary to take into account the characteristics of the monomers used, in particular their rate of polymerisation. The continuous introduction of the co-monomer which polymerises faster is a simple operation, since the bead during polymerisation is not protected by any barrier. The conditions of the reaction must obviously be adapted to the monomers present. In particular the speed of agitation in the pre-polymeriser must be adjusted since the viscosity of the medium is often different from that of vinyl chloride. It is necessary in copolymerisation to maintain a constant temperature rather than a constant vapour pressure, since vapour pressure varies during the reaction with the change in monomer concentration. 2.11 COST COMPARISON OF SUSPENSION AND BULK PROCESSES The chief advantage of the bulk process over the suspension process is the absence of water and therefore the saving of energy costs in drying PVC wet cake. Comparing two modern plants of ca l00ktpa capacity, the saving in energy costs is 1–2% average selling price of general purpose PVC. In terms of capital costs there is little to choose between the two processes. Superficially the saving of capital on the installation of centrifuges and driers for handling PVC slurry would appear to be significant, but the fairly elaborate system of PVC powder handling/conveying, classification and grinding required for bulk polymers tends to offset this saving. A detailed appraisal of the costs of suspension and bulk processes is given in reports issued by the Stanford Research Institute which are available to subscribers. 2.12 LATEST DEVELOPMENT IN THE TWO-STAGE PROCESS: VERTICAL AUTOCLAVES Since July 1978, vertical 50m3 autoclaves have been in operation at St Fons.7 Before reaching such a size, smaller autoclaves had been operated for 15 years and the agitation system has been progressively improved. Today, a single screw agitator enters from the top while a scraper agitator is introduced from the bottom. This system allows a fast discharge and an easy cleaning of the autoclave (see Fig. 2.8). Much research work has been carried out on this development. This allowed construction of a successful 8m3 autoclave pilot plant which was then scaled up to a 30m3 unit. The results proved satisfactory. Meanwhile improvements of the agitation have been performed which avoid the non-stirred areas along the shaft areas where crusts occur. The advantages of this screw design can be summed up: suppression of dead areas and deposits on the screw, improved mixing of the material, improved heat exchange via the screw. A new production line has been erected at St Fons: it includes one prepolymeriser (30m3) and five vertical 50m3 autoclaves and is shown in Fig. 2.9.7 The vertical position of these autoclaves, as well as the free space left between the screw and the walls, bring several advantages.

BULK PROCESSES FOR THE MANUFACTURE OF PVC

41

FIG. 2.8. General layout of Rhone-Poulenc 50m3 vertical reactor: (1) reactor shell and jacket; (2) upper screw agitator; (3) lower anchor agitator; (4) reflux condenser; (5) degassing filter; (6) maximum PVC resin level; (7) discharge valve; (8) manhole; (9) packing seal.

(1) The fact that the screw does not scrape the walls allows space for a thermo-probe giving the actual temperature of the powder, thus enabling control of the degassing without excessive heating and so maintaining satisfactory product colour. Moreover, the temperature at the bottom of the autoclave is also measured, giving interesting information on the progress of the polymerisation. (2) The vertical position of the autoclaves allows a very short discharge time. The flow-speed of the powder may reach at least 1 t/min. (3) The combination of these features (vertical position and free space between screw and walls) allows efficient cleaning of the reactor and agitator. For this purpose, two high pressure water cleaning heads are let down by two holes diametrically facing each other. These heads, fitted with two rotating nozzles, go down along the autoclave and thus remove the powder left at the end of the discharge. After each operation the autoclave is cleaned and the whole powder is thus removed. At the same time, the condenser is flushed by a separate line. The use of such a reactor, besides giving a reduction of investment costs due to simpler engineering and better productivity, allows for an appreciable improvement in the quality of resins. A higher efficiency in the cleaning of reactors eliminates totally the remaining PVC particles, which by overpolymerisation would lead to glassy beads difficult to degas and to gel. The improved control of temperature during the whole reaction, including the final degassing phase, permits the production of resins with a very low amount of residual monomer and good colour which are essential for most rigid and flexible applications.

42

M.W.ALLSOPP

FIG. 2.9. General layout of Rhone-Poulenc 50m3 vertical reactor plant. (Reproduced from Elastomers, March 1980, p. 26.)

2.13 POLYMER QUALITY STATUS—BULK VERSUS SUSPENSION Bulk density of resins is a function of the degree of aggregate cohesion of primary particles and the particle size distribution. Generally the combination of properties achieved in a polymer from the mass process can be closely matched in the suspension process by recipe modification and vice versa. The best polymers from each process are similar in performance. Mass polymers hold about 10% of the total market and are used in all the principal applications as are suspension polymers. For example, both mass and suspension polymers are used for rigid pipe production. However there are detailed differences between the two which mean that the one or the other type tends to be preferred for certain segments of the market. For example, mass polymers are preferred for rigid injection mouldings whereas suspension polymers are preferred for large sections of the extrusion blow moulding business. A point to note is that the two types of polymer are not compatible, mixtures leading to powder flow problems. Hence fabricators at any one time will use only mass or only suspension polymers and avoid mixing the two types in storage silos. The bulk process is terminated at a lower conversion than most suspension polymerisations, which will obviously give advantages in laboratory tests involving rapid ad/absorption of large quantities of plasticiser. It is often claimed that the pericellular skin surrounding suspension polymer grains (Chapter 7) retards plasticiser absorption relative to bulk but the skin contains many holes, particularly at the bottom of reentrant pores and many grains are covered by an incomplete membrane. Although the skin is porous to plasticiser it may retard the wetting out process slightly but this seems to have little effect in practice. If the conversion of suspension polymer is reduced to that of the mass polymer, plasticiser ad/absorption behaviour is very similar.

BULK PROCESSES FOR THE MANUFACTURE OF PVC

43

2.14 GAS PHASE POLYMERISATION In the late 1960s and early 1970s several companies simultaneously studied a variation of the bulk process which came to be known as gas phase or partial pressure polymerisation. A low conversion, highly porous ‘seed’ polymer is produced using suspension or conventional bulk polymerisation. This seed is added to the gas phase reactor, a fluid bed unit in a loop system; a solution of initiator in VCM is injected into the centre of the bed and the whole fluidised with hot VCM vapour at a pressure about 1 bar below saturation. Some of the fluidising vapour is absorbed by the PVC seed so the pressure in the reactor starts to fall and more monomer is injected, whereupon it immediately vaporises, re-establishing the operating pressure. Once polymerisation has started, reaction temperature is maintained by injecting cold liquid monomer into the fluid bed and the heat of polymerisation is removed as latent heat of vaporisation. In order to maintain the correct operating partial pressure, monomer is continually condensed in another part of the loop reactor system. The kinetics of a suspension or bulk polymerisation (Chapter 1) are typified by a gradually accelerating rate with the maximum occurring just after pressure drop, after which the rate falls due to monomer starvation. By operating at about 1 bar below saturation the gas phase process works at the optimum kinetic point and very high rates of polymerisation are possible. The progress of the reaction can be followed very closely by measuring actual monomer consumption, which is not possible in other processes, and the batch terminated at an exact point when the seed has grown in weight by a factor of 8–15—the growth factor. Reaction rate is linear throughout the process and the same temperature/K-value relationship that applies to suspension or bulk is followed. Since the process is dry it is an ideal vehicle for the use of organometallic catalysts with the possibility of producing more stereoregular PVC. The fact that the process does not appear to have reached commercial operation is probably related to two factors. In the first place, plant capital costs are not significantly lower than those of an equivalent tonnage suspension process, and secondly the properties of the gas phase polymer are such that it cannot be handled in the kind of processing equipment used widely by the PVC fabricating industry. The morphology of PVC undergoes considerable changes with conversion (Chapter 7). Since the gas phase process operates under partial pressure conditions the growth mechanism is typified by that operating after pressure drop in the suspension process. In the latter we see a densification of the aggregate structure even though only 10–20% of the polymer is produced under these conditions. It is not surprising then that gas phase polymers consist of rather dense grains of low porosity since the whole process is carried out in this post-pressure drop region; see Chapter 7. Also, the material tends to be somewhat coarser than that currently available from the bulk and suspension processes since the seed doubles in size for a growth factor of 8. The gelation behaviour (see Chapter 8) of gas phase polymer tends to be inferior to suspension and bulk but the polymer could be processed to give, say, good quality pipe on a twin-screw extruder. However, the screw length required for adequate gelation is longer than that found on conventional machines and until longer barrelled machines are available the gas phase polymer will represent a processing problem. ACKNOWLEDGEMENTS The author is indebted to a colleague, Dr J.H.Wilson, who provided commercial data on the Rhone-Poulenc process, and to J.C.Thomas of Rhone-Poulenc Industries Limited who kindly agreed to check the text for technical accuracy, since ICI do not operate a bulk polymerisation process themselves.

44

M.W.ALLSOPP

REFERENCES 1. 2. 3. 4. 5. 6. 7.

RINGSDORF, H. Encyclopaedia of Polymer Science and Technology, Volume 2, Interscience, New York, pp. 642–66. MARKS, G.C. Developments in PVC Technology, J.H.L.Henson and A. Whelan (Eds.). Applied Science, London, 1973, pp. 17–40. CHATELAIN, J. Br. Polym. J., 5, 457–65 (1973). COTMAN, J.D., GONZALES, M.F. and CLAVER, G.C. J. Polym. Sci., 5, 1137–64 (1967). WILSON, J.C. and ZICHY, E.L. Polymer, 20, 264–5 (1979). RHONE-POULENC. British Patent 1506981. ANON., Elastomerics, March 1980, p. 26.

Chapter 3 THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS D.E.M.EVANS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

3.1 INTRODUCTION The first PVC produced commercially was manufactured using an emulsion polymerisation process. Work on PVC was pioneered in Germany in the early 1930s and rapid progress was made in this period, spurred on by the German requirement for natural rubber substitutes. Thus the successful development of an emulsion polymerisation process to produce ‘Buna’ rubber had a significant influence on the way in which the development of PVC proceeded and following pilot plant production in the early thirties, full scale production was achieved in 1937 by I.G. Farbenindustrie.1 Work on suspension polymerisation also began early in Germany outside I.G. Farbenindustrie, Wacker Chemie commencing semi-technical production in 1935. Commercial scale production by Wacker of suspension homo- and co-polymer had begun at Burghausen before the outbreak of the Second World War and by war end output had reached about 7 ktpa. There was a hiatus period then for a year or so but during 1946 Wacker brought their Burghausen plant back into production and production continues there to this day. In contrast to Germany, in the UK there was little incentive in the 1930s to search for synthetic substitutes. Nevertheless in 1938, ICI became interested in PVC as part of a general programme of development work on plastics. Initially to avoid patent difficulties, attention was concentrated on a suspension technique for producing PVC similar to that already in use for ‘Diakon’* (poly(methyl methacrylate)) but an emulsion process was also developed. The outbreak of the Second World War rendered the patent situation irrelevant and a pilot scale plant was commissioned at Runcorn in late 1940 using the emulsion process. A small commercial scale plant (ca 500tpa) was started up at Runcorn in 1942. Also in 1942, Malaya was lost to the Japanese and the UK supply of natural rubber was thus cut off. The need for producing high quality material for cable insulation therefore became more urgent and although resins made by the emulsion process could be used, the impurities in the resin, an inherent feature of the process (see later), impaired the electrical properties. Hence product from a suspension process became more attractive for electrical applications. Based on earlier work, ICI were able to develop a suspension process quickly and by 1943 commercial production had been introduced on the Runcorn plant. Nevertheless the emulsion route was still favoured by ICI at that time and a new larger capacity (5ktpa)

* ICI Trade Name.

46

D.E.M.EVANS

plant designed to make emulsion resins was built and came on stream in 1944—(this was ‘Corvic’ 1 plant, now shut down), allowing the smaller Runcorn plant to specialise in suspension resin production. A similar development of the suspension process took place in the USA during the war and for a period imports of suspension PVC for use in electrical applications came into Britain under the Lend Lease Agreement. After the war the suspension process was developed rapidly in the UK and the USA whereas in Germany in the early post-war period more attention continued to be paid to the emulsion process. However, in the 1950s in Germany, as elsewhere in the Western World, the emphasis switched to the suspension process. Today the emulsion process accounts for only 15–20% of the total PVC production (30– 35% in Germany still but declining) and represents the speciality sector of the market. An excellent account of the historical development of PVC manufactured by the emulsion polymerisation process is contained in books by Kaufman.2,3 3.2 TYPES OF POLYMER PRODUCED USING THE EMULSION PROCESS Two main types of polymer are produced using the emulsion polymerisation process: paste and so-called ‘emulsion’ polymers. Paste polymers and may then be spread on to substrates, deposited on formers produce stable suspensions (‘plastisols’) when mixed with plasticisers by dipping, rotationally cast, etc., prior to fusion in an oven. Stringent control of the particle size of the latex at the polymerisation stage is necessary in order to control the rheological properties essential for subsequent conversion into finished articles (see later). Emulsion polymers, on the other hand, do not form pastes and are used for calendering and the production of thin complex sections by extrusion or in the manufacture of car battery separators using a sintering process. Although both types of polymer can originate from a common polymerisation process, apart from latex particle size control at this stage, subsequent processing (drying, milling, etc.—see Chapter 6) determines the final properties and hence the applications. 3.3 APPLICATIONS OF PASTE/EMULSION POLYMERS Paste polymers, when stirred with plasticisers, form stable suspensions (pastes, plastisols) and it is in this form that they are used to produce finished articles. Fillers, diluents, stabilisers, etc., can all be mixed into the paste at this stage. The major applications are in spread coatings, i.e. the paste is spread at room temperature in a thin layer on to a suitable substrate and is subsequently cured in an oven for a short period of time (1–5 min) to give a coherent film. A reverse roll coater is normally used to spread the film. Typical examples of finished products are vinyl coated wallcoverings, artificial leathercloth, foamed flooring, etc. In the latter case, a chemical blowing agent is mixed into the paste which produces gas at the oven curing temperature and thus produces a foam layer. Various layers may be built up prior to the final curing by heating the spread paste to an intermediate temperature (e.g. using a heated roller) which allows the plasticiser to be absorbed into the paste polymer particles (grains) to produce a coating which is dry but with little strength. All the layers built up may then be cured together in the final oven which causes fusion of the grains to a homogeneous state. Other applications include rotational casting, e.g. in the production of toys. Here the plastisol is poured into a mould which is then rotated in an oven. The finished article may be removed by opening the mould. Dipping processes are also employed, e.g. for the production of gloves, where a former

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS

47

is dipped into a plastisol bath. After heating, the glove is then peeled off. A more detailed account of such applications is given in Chapter 10. Emulsion polymers are used mainly in the production of thin complex profiles by extrusion. They have an advantage over suspension polymers in this application due to their rapid gelation rates. Battery separators (for car batteries) are manufactured by spreading the polymer powder (grains) on to a band which then passes through an oven to produce sintered sheet. Using the correct grade of polymer and optimum process conditions, sheets of the correct porosity can be produced for this application. 3.4 PRODUCTION OF PASTE/EMULSION POLYMERS 3.4.1 Properties Required of Dried Product Since plastisols are used in a variety of applications by many different processes, different properties are required both at room temperature and in the fused state. The most important of these at room temperature are the plastisol viscosity, and viscosity/shear rate relationships. By considering the various applications it can be seen that vastly different viscosities may be required and also the shear rates encountered are quite different, e.g. for dipping compared with high speed spread coating. Whilst the viscosity properties can be controlled to a certain extent by using different plasticisers and different additives, a significant contribution arises from the polymer itself and different grades of product are manufactured and recommended for different applications. One of the most convenient ways of controlling the viscosity is to alter the size distribution of the particles in the final plastisol. Since we are dealing with a dried product from a latex (from the emulsion polymerisation), which after drying consists of agglomerates of the particles formed at the polymerisation stage, the degree to which these agglomerates break down in the paste is significant in the final particle size distribution. Thus the final distribution is a mixture of primary particles (distributions containing particles < 1.5 µ m formed during polymerisation) and secondary particles (grains—mean particle size 30–60 µ m formed during drying and subsequently reduced to 5–20 µ m (mean) by milling). The ease and degree to which the secondary particles break down in the paste is determined by the drying conditions used. Nevertheless, the primary particle size distribution plays a significant role in determining the paste viscosity. Thus, for unimodal size distribution, the larger the size, the lower the viscosity, but such a paste would become very dilatant as the shear rate was increased. In order to reduce the viscosity and approach Newtonian behaviour, it is necessary to broaden the distribution, but the number of distributions is limited in practice by the degree of control available at the polymerisation stage. Figure 3.1 shows the effect of latex particle size distribution on the viscosity behaviour of the plastisol. The requirements for emulsion polymers differ from those for paste polymers. For extrusion applications, the emphasis is on easy gelation, good powder flow properties and high packing density. In general this normally means producing a latex of small particle size but a coarse powder after drying. For battery separator applications the flow properties of the powder have to be tailored to the process being used to produce the separator. In addition, good sintering properties are required to give good mechanical strength to the separator together with a low pore size and optimum total porosity. Clearly the particle size and distribution of both primary and secondary particles is important.

48

D.E.M.EVANS

FIG. 3.1. Effect of particle size distribution on plastisol viscosity. Recipe: 50phr DOP. (Reproduced from Rangnes and Palmgren, Journal of Polymer Science, Part C, 33, 181, 1971.)

3.4.2 General Description of the Process A schematic diagram of the process is shown in Fig. 3.2. The manufacture of paste/emulsion PVC polymers may be divided into six main parts: (a) emulsion polymerisation or (b) microsuspension polymerisation; (c) VCM removal; (d) latex storage; (e) drying; (f) finishing. 3.4.2.1 Batch Process Emulsion Polymerisation

The principal ingredients in an emulsion polymerisation recipe are VCM, water, initiator and emulsifier. The polymerisation takes place in an autoclave capable of withstanding the vapour pressure of VCM at the temperature of polymerisation. This is usually 40–60°C, corresponding to pressures of 6.4–10.0 bar. The polymerisation temperature is chosen depending on the molecular weight required for a particular grade. A major step in the polymerisation of PVC is chain transfer of radicals with VCM. As the rate of this reaction increases more rapidly with temperature than the chain propagation reaction, this means that the polymerisation temperature controls the molecular weight and other variables, e.g. initiator concentration, have little effect. Further control can be obtained by deliberately adding chain transfer agents. For paste polymers, low molecular weights are necessary when low fusion temperatures are required whereas high molecular weights give increased wear resistance in fused layers. The contents of the autoclave are agitated and heat is supplied or removed via a jacket through which a mixture of water and steam are pumped. The polymerisation reaction is strongly exothermic, and the normal requirement is for cooling. As the size of the autoclave is increased, the cooling requirement presents a problem as a less favourable surface area:volume ratio results. These problems have been overcome by designing vessels with thinner walls, using chilled water, and installing an external condenser

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS

49

FIG.3.2. Diagram of emulsion mircosuspension polymersation process.(Reproduced from Burgess, Developments in PVC Proucation and Processing—1, Whelan and Craft, Eds., Applied Science Publisher Ltd, 1977.)

to condense the VCM vapour.4 The size of the autoclave is usually governed by economics but modern plants currently use vessels in the 30–80 m3 range, although a 200m3 vessel is used by Huls.4 Larger autoclaves

50

D.E.M.EVANS

FIG. 3.3. Effect of emulsifier concentration and type on latex particle size. (Reproduced from Rangnes and Palmgren, Journal of Polymer Science, Part C, 33, 191, 1971.)

also present mechanical problems with respect to agitation. The conventional top entry stirrer shaft would become long and very thick. This is usually overcome by employing a bottom entry stirrer fixed in the base of the autoclave and driven by a relatively thin shaft. These effects are discussed in more detail for suspension polymerisation in Chapter 1 (section 1.6). For emulsion polymerisation, the agitation conditions and system can be critical and a balance has to be achieved between good mixing/heat removal and the mechanical stability of the latex, otherwise excessive coagulation will occur. The initiator used in emulsion polymerisation is soluble in water and is normally potassium persulphate or ammonium persulphate. Initiation takes place in the aqueous’phase by sulphate ion free radicals. The rate may be increased significantly by the addition of a reducing agent. Such systems were developed by ICI and independently in other industrial laboratories in 1940 and the work was subsequently published after the war. Bacon5 called such a system a ‘reduction activation’ system but the term ‘redox catalysis’ is now commonly used.6 Three-component systems, involving the use of a metal salt also, give further benefits7 and typical systems are ammonium persulphate/sodium bisulphite/copper sulphate or hydrogen peroxide/ ascorbic acid/ferrous sulphate. Using such systems, polymerisation may be carried out at lower temperature or better control of reaction rates may be achieved at the higher temperatures (say 50°C), particularly if some of the compounds are injected continuously or as required.8 Typical quantities (based on monomer) for persulphate redox systems are 0.01–0.3% persulphate, 0.001–1% reducing agent and 0.05–10ppm metal ion. One of the most important components of the emulsion polymerisation recipe is the emulsifier. The quantity used has a major effect on the final latex particle size achieved, by determining the number of particles initiated. The nature of the emulsifier can also affect the number of particles initiated, particularly at low concentrations.9,10 This is illustrated in Fig. 3.3. However, the number of particles formed is strongly influenced by the concentration of free emulsifier in the aqueous phase and accurate metering during reaction provides an effective method of controlling particle size and allows the minimum amount of emulsifier to be used.11 Since the latex formed is spray dried, the emulsifier remains on the dried particles and can influence, therefore, the properties of the dried polymer. Typical properties influenced are heat stability, colour,

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS

51

FIG. 3.4. Electron micrograph of PVC latex primary particles.

transparency, quality of chemical foam, etc.12,13 Typical emulsifiers used are sodium or ammonium salts of alcohol sulphates, alkyl sulphonates, alkyl aryl sulphonates, sulphosuccinates, and fatty acids. Although the emulsifier has a strong influence on the number of particles initiated, and therefore on the final latex particle size distribution, the use of preformed seed latex in the polymerisation recipe is a useful technique for controlling particle size distribution, particularly if large particles are required. In this way, by suitable choice of size and quantity of seed latex, growth to a predicted size distribution can be achieved with minimum or controlled initiation of new particles. It has been shown that the absence of free emulsifier in the aqueous phase is not the only requirement for the suppression of fresh initiation of particles and the surface area of the seed latex employed plays a major role.14,15 The normal requirement for emulsion polymer is to produce primary particles at the polymerisation stage of <0.3 µ m depending on the application. For paste polymer, however, the presence of large particles is essential for the production of polymers which are used to prepare plastisols of low viscosity. In this case therefore the particle size distribution is wider and some particles >1.0 µ m are desirable. A typical particle size distribution of a PVC latex is shown in Fig. 3.4. The quantity and type of emulsifier are governed mainly by a balance of cost and the properties required for the finished product. Another consideration is the mechanical stability of the latex both during polymerisation and for the subsequent handling operations. As mentioned above, the type of agitation used will play a role, together with the amount of polymer adhering to the walls of the vessel. These considerations clearly set the lower limit for emulsifier concentration, whereas, although high concentrations will reduce build-up, the upper limit is governed by the product properties, both by its effect on primary particle size and by the residual amount on the dried product after spray drying. The normal range is approximately 1–3 % on PVC. The control of pH during polymerisation is also important. For persulphate-initiated polymerisations, faster reactions are achieved at alkaline pH whereas if hydrogen peroxide is used, acid conditions are often more favourable.16 Conventional buffer systems can be used and in some cases, e.g. sodium carbonate or

52

D.E.M.EVANS

sodium bicarbonate, these can also act as heat prestabilisers. It has been claimed also that pH during spray drying can affect finished product properties.17 The quantity of VCM converted to PVC per batch is normally in the range 85–95 %. It is not generally economic to carry out the polymerisation to higher conversion as the rate of reaction becomes slow at the end of the reaction cycle. At approximately 70% conversion, the pressure in the autoclave starts to fall as liquid monomer is used up. The remaining gaseous monomer is usually vented to a gas-holder when the pressure falls to 3–4 bar. The solids content is governed by the ratio of monomer:water used and also the monomer converted. It is normally in the range 40–45 % solids and is restricted by the particle size distribution, which affects the latex viscosity and the control of unwanted secondary initiation. Micro suspension Polymerisation

In general, the ingredients and polymerisation vessel used in microsuspension polymerisations are similar to those used for the emulsion reactors. The principal differences are that a monomer-soluble initiator is used and some or all of the monomer is emulsified to small droplets using a mechanical homogeniser prior to the start of the polymerisation. A typical procedure, therefore, is to homogenise a mixture of VCM, water, emulsifier, and monomersoluble initiator and pump this mixture into an autoclave. The mixture is then heated, with agitation, to the desired temperature which allows polymerisation to take place to yield a stable emulsion with a particle size distribution of 0.1–1.5 µ m. Homogenisation of the mixture before polymerisation is carried out by the application of high shearing forces. Suitable equipment is a high speed pump, colloid mill or high speed stirrer. For a number of years, the preferred initiator was of the long chain diacyl peroxide type, normally lauroyl peroxide. However, this gave slow reactions with unfavourable kinetics, e.g. fast rate at the end of the polymerisation and slow at the start. The use of the more active peroxydicarbonate initiators gave undesirable quantities of particles <0.1 µ m and large amounts of polymer adhering to the autoclave walls. A significant advance was made by the use of mixtures of long chain hydrocarbons with peroxydicarbonates (chain length >5) to overcome the problem of small particle formation and autoclave build-up.18 The long chain compounds were of the peroxide, ether, hydrocarbon, or carbonyl compound type. In this way significantly improved reaction kinetics were obtained. A further advance was the discovery that it was not necessary to homogenise all the VCM or water,19 thus reducing capital and energy costs associated with the homogenisation step. The scope for modifying the particle size distribution is more limited than for the emulsion process. A continuous distribution is obtained and the main opportunity for changing this is at the homogenisation stage. Nevertheless, the distributions obtained are suitable for making paste polymers giving medium-low viscosities and the products are useful for general applications. Recently, a process has been developed to alter the particle size distribution and to obtain a higher proportion of large particles which is analogous to the seeded emulsion process described in the previous section.ntf20 In this process a ‘seed’ latex is made by the microsuspension process described above but using a large excess of initiator, typically lauroyl peroxide. In the second stage, a portion of this seed is used together with VCM, water, and emulsifier. The polymerisation is activated using a redox system consisting of copper sulphate/ascorbic acid. As the copper sulphate/ascorbic acid forms a complex which is soluble in the monomer phase, the excess lauroyl peroxide contained in the seed particle is activated. In this way the seed latex can be grown to large particles without the problem of secondary initiation. A further development is to introduce an emulsion latex with the microsuspension seed latex to modify the particle size distribution further. In this case, very little

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS

53

polymerisation takes place on the emulsion seed and by suitable choice of size and quantity it is claimed that distributions which allow high solids contents (up to 55%) and low paste viscosities can be obtained.21 3.4.2.2 Continuous Emulsion Polymerisation Process The continuous emulsion process for the production of PVC was first developed by I.G.Farbenindustrie in Germany in the 1930s and has already been referred to in the Introduction (Section 3.1). Such a process is still operated in Germany by several companies which formerly constituted I.G.Farbenindustrie, disbanded after the Second World War. These companies have continued to develop the continuous emulsion process and some new plants have been built in recent years. However, outside Germany there is a preference for the batch emulsion polymerisation process and most of the new capacity has been of this type. Although good quality paste polymers can be produced using the continuous process, it does not possess the versatility of the batch process. Build-up formation can present problems and often higher emulsifier contents are necessary compared with the batch process (2–4% normally). Also residence time control can present problems. Continuous stirred tank reactors are generally preferred, usually with a height:diameter ratio of at least 3 and having a capacity of at least 5m3.22 VCM and other ingredients (similar to those used for the batch process) may be fed in separately or mixed/pre-emulsified separately shortly before introduction to the reactor. The portion of the reactor near the inlet is well agitated whereas the rest of the reactor is usually mildly agitated to maintain a homogeneous mixture together with adequate heat transfer. The reactants may be fed in at the top, although a recent patent22 claims that there is less build-up and a reduction in unwanted larger particles (‘pebble’) if the reactants are fed in at the bottom and the product removed from the side of the reactor at a level 30–90 % of the total internal height of the reactor above the feed inlet. If the outlet is too low then the product will contain excess unreacted material, whereas if it is too high, pressure regulation problems are encountered. The heat from the reactor is removed by a cooling jacket and, additionally, cold ‘fingers’ or condensers are used. High monomer conversion is desirable and this is measured either by computing the heat produced from the inlet/outlet temperatures and jacket flow rates23 or by taking samples at the outlet of the reactor and measuring the foam density, which is related to the free monomer remaining.24 The conversion is controlled by varying the monomer feed rate/reaction rate. In order to produce a consistent product, it is necessary to maintain a constant rate of reaction and conversion. Fluctuations in reaction rate are unavoidable in practice and it is desirable to have a highly reactive initiator system. Redox systems are normally suitable and some have been specially developed for the continuous process, e.g. hydrogen peroxide/ascorbic acid at a particular pH and in the absence of heavy metal salts.23 The process beyond the reactor is similar to the batch process although a continuous monomer stripping process is of course necessary. In the process as it is today operated conversions are usually > 90 % and the solids content approximately 45%, although in the early days much lower solids were normal. 3.4.3 VCM Removal In theory, because of the small size of the PVC latex particles produced by emulsion polymerisation, the removal of VCM from such polymers should proceed at a high rate. This in fact is achieved during the spray drying step and the VCM concentration in the finished product is often very low. However, this is not a

54

D.E.M.EVANS

suitable process for VCM removal since it is difficult to recover the monomer removed and thus there is an environmental problem. In practice therefore the VCM is removed at the end of the polymerisation stage, either in the autoclave or after transfer to a separate stripper vessel. The most efficient method of removing VCM at the autoclave stage is to boil the latex by applying sufficient heat and vacuum and to remove the VCM by first condensing the steam. The major problems encountered in practice are excessive foam formation due to the presence of anionic emulsifiers, and coagulation which in its mildest form produces excessive build-up formation in the vessel. Careful control of venting rates, temperature, agitation, etc., is therefore required and low levels of VCM in latex can be achieved but the cycle time can become unattractive with certain types of latex. Stripping external to the autoclave can be achieved by continuously spraying latex into an evacuated chamber against a countercurrent of steam.25,26 Foaming and build-up problems are reduced considerably using the latter technique. Removal of VCM from PVC latex is discussed in more detail in Chapter 5 (Section 5.4.3). 3.4.4 Drying of Latex In order to isolate the product from the latex it is necessary at some stage to remove the water. Although coagulation techniques have been investigated these are not in general use and can usually be applied only to certain types of latex. Also the structure of the particles (grains) formed by removing the water is important in determining the final properties of the plastisol. For this reason a spray drier is normally employed to isolate the solid product. As this process is expensive on energy costs, relatively small changes in the amount of water to be evaporated can lead to significant reduction in drying costs. Thus, it is desirable to produce latices with high solids contents at the polymerisation stage but the limitations of this technique have already been mentioned. Concentration of the latex after the autoclave stage is sometimes employed, e.g. thin film evaporation or ultrafiltration,27 but the choice of technique has to be carefully assessed if there are to be worthwhile savings. In general approximately 45–60% of the latex is water and this is finally removed by the drier. Using a spray drier a product containing <0–4% residual moisture is usually produced and any non-volatile ingredients added during the polymerisation remain in the solid product. Thus, although the drier is used as a means of isolating a solid product in the production of paste and emulsion polymers, it forms a part of the process which can significantly affect the final product properties and can in fact be used to control the properties required for various applications. For example, depending on the drying conditions used, it is possible to produce products suitable either for paste polymer applications or emulsion polymer applications. The secondary particles formed in a spray drier consist of aggregates of primary particles (formed in the latex by polymerisation—Fig. 3.5) which have been heat-treated in the drier. The external and internal structure of the secondary particles is shown in Figs. 3.5 and 3.6. Figure 3.7 shows the aggregate of primary particles in the grain. Both the particle size and the way in which the primary particles aggregate have a significant effect on the nature of the secondary particles and their subsequent behaviour when mixed with plasticiser. The secondary particle size is affected mainly by the degree of atomisation imparted to the latex, while the way in which these particles behave in plasticiser (i.e. degree of breakdown, etc.) is controlled by the primary particle size and heat treatment in the drier. The formation of secondary particles is complex. The particle structure is usually determined in the early stages of drying when enough water is present to allow the primary particles to pack together, and is controlled by the primary particle size distribution and the time interval available for packing. The latter depends on the temperature conditions.28 Thus the heat treatment given to the polymer during drying by fixing inlet and outlet temperatures determines to a large

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS

55

FIG. 3.5. External structure of secondary PVC particles (grains).

extent the behaviour of the resin particles when made into a plastisol. Gentle heat treatment gives a high degree of particle breakdown when mixed with plasticiser whereas severe heat treatment gives sintering and little breakdown.29,30 In both these extremes, the paste viscosity would probably be too high for commercial use. In practice, therefore, drying conditions have to be carefully optimised to suit the nature of the feed latex (solids content, particle size distribution, surface tension, etc.) in order to give the desired final properties in the plastisol. Two main types of atomisation are employed for the manufacture of PVC paste/emulsion polymers, one using a single disc spinning at high speeds31 (Chapter 6, Section 6.3), whereas the other uses a number of fine nozzles, which may be divided into two types, viz. pressure nozzles and two-fluid nozzles. For fine atomisation, two fluid nozzles using compressed air up to 4 bar as the atomising fluid can be employed.32 It is claimed that variations in nozzle design and operating conditions can produce very fine powders requiring no further classification33,34 but the economics of operating under special conditions using high ratios of compressed air:latex feed have to be carefully considered. Fine products can also be produced using pressure nozzles, but very high pressures (160–500 bar) are required in this case.35,36 For the production of certain types of emulsion polymer where a fine product is not required, single fluid pressure nozzles are used and it has been claimed that additives such as stabilisers and lubricants can be mixed more efficiently by introducing them during atomisation using double inlets.37 The type of atomiser used is governed by the type and range of products required from the drier, and by the economics. In general, nozzle systems tend to be more expensive to run due to the compressed air requirement. For maximum throughput it is desirable to have as high a difference between the inlet drying air temperature and its outlet temperature as possible but, as mentioned previously, these temperatures are usually determined by the quality of product required. Also, for maximum thermal efficiency, it is desirable to have as high an inlet temperature as possible. This is normally set by product quality and the importance

56

D.E.M.EVANS

FIG. 3.6. Internal structure of secondary PVC Particles (grains).

of heat treatment during drying has already been mentioned. It has been shown that the air flow pattern is particularly important; by modifying this it has been claimed that good quality polymer at an inlet temperature in excess of 180°C can be obtained.38 A more detailed account of the spray drying of PVC is given in Chapter 6 (Section 6.3). For general aspects of spray drying the book by Masters is recommended.31 3.4.5 Milling of Polymer Although the mean particle size of the secondary particles emerging from the spray drier is usually in the range 30–60 µ m, the number of larger particles in the plastisol will depend on the degree of breakdown, which in turn will depend on the drying conditions used. Also, due to excessive heat treatment, some particles will be very hard. If a plastisol containing large hard particles is spread as a very thin layer, as in paper coating, then streaks will appear in the finished product. Even if this is not a problem, the initial stages of mixing in plastisol formation involve breaking down the large agglomerates and the presence of large numbers of these will lead to excessive mixing times. Although it is possible to obtain an optimum balance of properties, e.g. paste viscosity, plastisol preparation mixing time, hard particle content, etc., for certain applications by suitable adjustment at the polymerisation/drying stages, for more critical applications this is often not possible and some treatment of the polymer emerging from the drier is necessary. Generally, the very coarse material, e.g.>300 µ m, is removed by sieving and, in some instances, the particle size distribution can be modified by using an air classifier system. In this way the coarse particles can be removed either as reject or for further treatment with a mill. Alternatively, all the powder

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS

57

FIG. 3.7. Internal structure of secondary PVC particles (grains) showing aggregates of primary particles.

after sieving may be subjected to a milling (grinding) process to remove the large hard particles. This has the advantage of breaking the relatively soft agglomerates also and thus improves the ease with which the polymer will form a plastisol. However, other properties, in particular plastisol viscosity, are significantly affected depending on the conditions and type of equipment used. In general plastisol viscosities are increased using polymer which has been milled, particularly at high shear rates. The equipment generally used is of the attrition mill type, e.g. hammer mills. It is essential to be able to control the degree of milling and, to produce polymer for critical applications, some form of classification is required. This is often built into the equipment although a separate unit can be used if necessary. 3.4.6 Powder Handling The final product emerging from the drier or mills may be stored either in bags or in silos. There are no serious handling problems with emulsion polymers and both bag and silo storage are used. For paste polymers, however, problems do arise since the powder has poor flow properties and compacts readily, making bulk handling difficult. Special equipment is therefore necessary to handle these polymers in bulk both at the manufacturing site and the off-loading point. A recent patent39 claims to have overcome these problems by co-spray drying latex under special conditions with a fine suspension polymer, normally used as an extender resin in plastisol formulations to reduce viscosity (see Chapter 10). However, such polymers would have limited applications as not all formulations can accept extender resins, e.g. thin layers for paper coating.

58

D.E.M.EVANS

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.

I.G.FARBENINDUSTRIE, German Patent 900019. KAUFMAN, M., The History of Polyvinylchloride, Maclaren & Sons, London, (1969), p. 62. KAUFMAN, M., The First Century of Plastics, The Plastics Institute, London (1963), p. 75. TERWIESCH, B., Hydrocarbon Processing, November 1976, 117. BACON, R.G. R., Trans. Faraday Soc., 42, 169 (1946). BACON, R.G. R., Quart. Rev., 9, 287 (1955). MORGAN, L.B., Trans. Faraday Soc., 42, 169 (1946). WACKER-CHEMIE, German Patent 2006966. RANGNES, P. and PALMGREN, O., J. Polym. Sci., Part C, 33, 181–92 (1971). DIAMOND SHAMROCK, US Patent 3981837. CHEMISCHE WERKE HULS, German Patent 1964029. CHEMISCHE WERKE HULS, Belgian Patent 857235. CHEMISCHE WERKE HULS, Belgian Patent 830223. GATTA, G., VIANOLLO, G., BENETTA, G., La Chemica e l’Industrie, 51(11), 1234–9 (1969). MONTECATINI EDISON, British Patent 1120410. DYNAMIT NOBEL AG, German Patent 1720524. CHEMISCHE WERKE HULS, German Patent 2531780. ICI, British Patent 978875. ICI, British Patent 1458367. RHONE-PROGIL, British Patent 1435425. RHONE-POULENC, British Patent 1503247. HOECHST, German Patent 2625149. CHEMISCHE WERKE HULS, British Patent 1411875. HOECHST, German Patent 2521862. ICI, British Patent 1553829. HOECHST, US Patent 4158092. HOECHST, German Patent 2440634. SHTARKMAN, B.P., MUKHIMA, I.A. and SHARIKOVA, L.L., Koll. Zhur., 31(4), 611(1969). SHTARKMAN, B.P., MUKHIMA, I.A., VISHNEVSKAYA, I.N., Akad. Nauk. SSSR, Sb Statei, 124–7(1966). WIESEBACH, H. et al.. Plaste u. Kaut., 21(8), 576–8 (1974). MASTERS, K., Spray Drying Handbook, George Godwin Ltd, London, 1979. HERTE, P., Chemischen Technik, 4(7), 327–30 (1952). CHEMISCHE WERKE HULS, US Patent 3805869. CHEMISCHE WERKE HULS, US Patent 3883494. CHEMISCHE WERKE HULS, Belgian Patent 820496. A/S NIRO ATOMISERS, Belgian Patent 867667. CHEMISCHE WERKE HULS, Belgian Patent 869496. A/S NIRO ATOMISER, Belgian Patent 854317. STAUFFER, British Patent 1410202.

Chapter 4 VINYL CHLORIDE COPOLYMERS AND PVC BLENDS R.H.BURGESS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

4.1 INTRODUCTION Historically, PVC gained wide commercial acceptance because it could be mixed with organic liquids (plasticisers) to give a tough flexible material not unlike rubber in its properties and the way in which it could be processed. The large market thus developed soon produced reductions in the cost of the VCM monomer and the manufacturing process so that it was natural for PVC manufacturers and users to consider its use for unplasticised applications. PVC in its unplasticised state when correctly fabricated is capable of producing tough rigid articles of excellent surface appearance and transparency if desired. However, it is thermally labile and will decompose quite rapidly at or near the temperatures needed to melt the PVC sufficiently to produce the finished articles. Hence, before the full commercial potential of PVC could be realised, much progress had to be made improving the basic heat stability of the PVC, developing improved heat stabilisers and process formulations, and developing equipment capable of fabricating the required finished products. As a result of this successful development work, it is now possible to produce rigid, i.e. unplasticised, articles of very considerable commercial importance (pipes, profiles, etc.) which are in widespread use. Even so, their performance is limited necessarily by the basic properties of PVC. Some of these basic properties are listed in Table 4.1 for the narrow range of molecular weight (expressed as Kvalue) used for rigid applications. Attempts to widen this property range by molecular weight change lead either to an unprocessable material (at K-values above 70) or a brittle material without the most desired property of PVC, its toughness (at K-values below 50). These basic properties of rigid PVC can be extended quite considerably TABLE 4.1 Properties of Rigid PVC K-value 5kg Vicat softening point (°C) PVC minimum torquea in Brabender plasticorder (g/m)

Failure stressb (MN/m2)

56 60 64 68 72

12–5 13–3 14–0 14–8 15–6

79 80–5 82 83 84

720 970 1200 1420 1640

60

R.H.BURGESS

K-value 5kg Vicat softening point (°C) PVC minimum torquea in Brabender plasticorder (g/m)

Failure stressb (MN/m2)

a

At equal gelation time. Fatigue resistance under cyclic on/off load 0–5 Hz. Fatigue stress at 106 cycles. b

either by copolymerisation or by blending with other polymers. Both these areas have been extensively studied and many products of commercial importance developed. 4.2 THEORY OF COPOLYMERISATION The copolymerisation of any two monomers is a complex process but the basic theory has been understood for some considerable time.1,2 Two concepts can be used, both of which recognise that copolymerisation in addition polymerisation is an event which must take account of the properties both of the monomer which is about to add to the chain and the nature of the group on the end of the chain. If one considers two monomers M1 and M2 then there will be two polymer radicals P1 and P2 containing M1 or M2 active ends respectively and four reactions possible with rate constants k11, etc. P1+M1→P2 k11 P1+M2→P2 k12 P2+M1→P1 k21 P2+M2→P2 k22 The first theory1 then defines reactivity ratios r1=k11/k12 and If this nomenclature is used it is possible to show that the copolymer produced (composition ratio M1/M2) will be related to the monomer mixture present ([M1] and [M2] being the respective concentrations of the two monomers) by the relationship M1/M2=[M1](r1[M1]+[M2])/[M2](r2[M2]+[M1]) (4.1) It is clear from this that only when r1[M1]+[M2]=r2[M2]+[M1] will the polymer composition be the same as the monomer mixture composition and only in the very special case of r1=r2=1 will this apply for all concentrations of M1 and M2. In virtually all practical situations these conditions are not met and the copolymer composition is not the same as that of the monomer mixture. Moreover a consequence of this is that the monomer mixture concentration changes continuously as one monomer is incorporated into the copolymer more rapidly than the other, which in its turn, changes the composition of the next copolymer produced. Thus copolymers of mixed comonomers content are produced unless special steps are taken to maintain a constant monomers composition. The usefulness of the reactivity ratio approach has been extended2 by postulating that the specific reactivity of a monomer is given by a resonance effect Q and the polar character of the radical adduct by e. Then for example : k12=P1Q2exp{−e1e2} (4.2) The reactivity ratios r1 and r2 would be given by the expressions r1=Q1exp{−e1(e1−e2)}/Q2 (4.3) r2=Q2 exp {−e2(e2−e1)}/Q1 (4.4) In principle the determination of the values of Q and e for a particular monomer in copolymerisation with any monomer can be used to predict its copolymerisation behaviour with any other monomer.

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS

61

Typical values for r1 and r2 and for Q and e for the more important VCM comonomers are given in Table 4.2. It can be seen that r1, r2, Q and e vary widely from monomer to monomer. For the easiest copolymerisation r1 and r2 should be as similar to one another as possible or the Q and e values of the two monomers to be polymerised (in this case VCM and one other) should be as similar as possible. Inspection of Table 4.2 shows this is rarely true. TABLE 4.2 Copolymerisation Parameters for Commonly Used Vinyl Chloride Comonomers assuming VCM is M1 Reactivity ratio Vinyl chloride Vinyl acetate Methyl methacrylate Vinyl isobutyl ether Vinylidene chloride Methyl acrylate Maleic anhydride Ethylene Propylene Styrene Butadiene

r1

r2

Q

e

__ 2 0·1 2 0·3 0·08 0·3 2·85 5·66 0·077 0·037

__ 0·25 10 0·02 3 9·0 0·008 0·055 0·0066 357 97

0·044 0·02 0·74 0·023 0·22 0·42 0·23 0·015 0·002 1 2·40

0·20 −0.59 0·40 −1.77 0·36 0·60 2·25 −0.20 −0.78 −0.8 −1.05

References 3 and 4 except where indicated.

4.3 VINYL ACETATE COPOLYMERS By far the most important copolymers commercially are those based on copolymerisation with vinyl acetate (VAM). These have been used in relatively large quantities for about 25 years, the VAM copolymerised markedly reducing the melt viscosity of the resultant polymer at the expense of softening point, and to a lesser extent heat stability and product toughness. Two main market areas have emerged, one requiring a product with the maximum melt flow for applications such as gramophone records and vinyl flooring, and the other a material of good melt flow, high melt elasticity and adequate mechanical properties mainly for packaging applications. The former market is met by a product of very low molecular weight (K-value 45– 50) with about 15% of copolymerised vinyl acetate, and the latter by a product of higher molecular weight (K-value 60) and a lower vinyl acetate content ( 10%). Typical properties of these products are listed in Table 4.3. The data in Table 4.2 show that when VCM and VAM are polymerised together there is a tendency for VCM to polymerise more rapidly than VAM. The reaction rate of either a VCM or VAM terminated polymerisation radical is more rapid with VCM than it is with VAM (r1 > 1,

62

R.H.BURGESS

TABLE 4.3 Properties of Commercially Available Vinyl Acetate Copolymers K-value

VAM content (%)

TG (ºC)

Flow pressure at 140° Ductility factora at 23° C (psi) C (mm)

57 62 47 60

0 0 14·5 9·5

79 80 70 73

3220 3345 1070 2210

a

2·4 4·6 1·5 3·1

Ductility factor obtained by dividing the stress field intensity factor (KIc) by the yield stress y and squaring the result. Description of ductility factor to be published soon. Note that at low molecular weight (K-value) copolymers tend to have a higher ductility factor than homopolymers.

r2 < 1). The copolymer produced will consequently contain rather less VAM than is present in the monomer mixture. The two rates r1 and r2 are much more similar to one another, however, than are the rates for many other monomer mixtures (see Table 4.2) so that simple copolymerisation, in which both monomers are added at the beginning of the polymerisation, is possible and is commonly performed. Under these conditions a copolymer mixture with the range of compositions shown in Fig. 4.1 (which is discussed later) is produced. Both emulsion and suspension polymerisations are possible but for most applications the cheaper suspension process produces the type of product required and this method is consequently most used. The actual process is very similar to that described in Chapter 1 for suspension homopolymerisation with the same initiators, protective colloids and water:monomer ratios in common use. VAM is not a chain transfer agent and the preponderance of VCM used in the commercial processes means that the product molecular weight is still controlled primarily by chain transfer to VCM; consequently reaction temperatures are used as described in Chapter 1. Although the measured K-value for vinyl acetate copolymers is lower than for homopolymers of the same molecular weight, the K-value of the 15% vinyl acetate content copolymers required is still very low by homopolymer standards and rather higher polymerisation temperatures of up to 80°C are used. This may present a problem with autoclave temperature control so that additional chain transfer agents such as isobutyraldehyde or trichloroethylene have been used to reduce that temperature. The general reservations about impurities in the subsequent polymer (much trichloroethylene remains in the product after drying) have reduced the use of chain transfer agents in recent years. Fortunately the vapour pressure of a mixture of VAM and VCM is less than that of VCM alone, presumably because VCM is soluble in VAM to some extent, so that the maximum pressure of the polymerising mixture is little higher than the maximum generated in homopolymerisation in spite of the higher temperature used. Because of the high polymerisation temperatures, initiators such as lauroyl peroxide, benzoyl peroxide or azodi-isobutyronitrile are the most suitable for the 15% vinyl acetate copolymers (see Table 1.5 in Chapter 1). The more modest temperatures used for the K 60 ca 10% vinyl acetate copolymers require the normal initiators for the medium K-value range of homopolymers. The grain structure of PVC homopolymers is extremely important because it is a major factor in their subsequent processing. As discussed in Chapters 8, 9 and 10 this is related to the way in which the grains accept plasticiser, stabilisers, lubricants, etc., and the way in which the grains break down and fuse during processing. Vinyl acetate copolymers are rarely used for plasticised applications and because of their lower melt temperature and melt viscosity are much easier to process than homopolymers. Consequently there is not the need for the porous, easily broken down structure which is so important for PVC homopolymers. In practice the market is most interested in extruder output, where reasonably high packing density polymers are desirable and in the case of record manufacture very high packing density grades with dense spherical

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS

63

FIG. 4.1. Changes of composition during vinyl chloride/vinyl acetate copolymerisation.

grains are normally sold. This type of grain is very non-porous and processes such as those required to remove residual VCM are much more difficult to carry out satisfactorily than for homopolymers (see Chapter 5). This problem is even more acute if one considers the removal of the much less volatile residual VAM. Figure 4.1 shows the change of composition of polymer being produced throughout a VCM/VAM copolymerisation using the reactivity ratios given in Table 4.1 and a computer simulation technique (similar techniques using Q and e values give similar results8). The Figure shows that using an original VCM/VAM mixture containing 15% VAM by weight a copolymer with an average VAM content of 12% is produced after 90% of the original monomers charged have been converted to polymer. Polymer molecules are produced with a VAM content ranging from almost 8% VAM at the beginning to almost 24% at 90% conversion. At 90% conversion an unreacted monomer mixture containing 40% of VAM remains. This monomer mix is more soluble in the copolymer than VCM is in the homopolymer, so it exerts a lower vapour pressure. Removal of both VCM and VAM is difficult, especially so for the VAM which is both more soluble in the copolymer and being a liquid (BP 73°C) much more difficult to vaporise. It was common for up to 1–5% residual VAM to remain in the polymer after drying but the improved stripping techniques described in Chapter 5 and designed to remove VCM almost completely have greatly reduced this figure; however, 0–1% VAM is still quite common in the final polymer. In principle the homogeneity of the copolymer molecules produced can be improved by proper control of the monomer composition. For example, Fig. 4.1 shows that a 15% vinyl acetate copolymer requires a 73/27 VCM/VAM monomer mix. By maintaining such a composition all the copolymer made would have the desired copolymerised monomer mix. This can be achieved by constantly adding VCM to the VCM/VAM

64

R.H.BURGESS

polymerising mixture at a rate just sufficient to balance the extra VCM being copolymerised. This can be done either by measuring the extent of polymerisation, perhaps by sampling a few runs and then assuming a similar polymerisation rate in future batches, by measurement of the heat of polymerisation evolved or by an indirect method such as measuring the pressure generated when operating below the saturated vapour pressure of VCM. All these methods have been used, especially for applications where homogeneous compositions, for example for solution coating applications, are required. In practice the range of composition produced in a simple copolymerisation presents no problem for applications other than solution coating and is often used perhaps with one or more late additions of VCM to improve copolymer homogeneity somewhat. 4.4 OLEFIN COPOLYMERS Olefin copolymers have been extensively studied and their reactivity with VCM falls markedly with increasing olefin molecular weight. In fact some higher olefins are retarders for VCM polymerisations. Both ethylene and propylene can be polymerised relatively easily and have the property of reducing the copolymer melt viscosity in much the same manner as vinyl acetate with the advantage that the product mechanical properties and heat stability are not impaired as in the case of vinyl acetate. 4.4.1 Ethylene Table 4.2 shows that ethylene copolymerises with VCM much less readily than VAM. A comparison of the data for VAM in Fig. 4.1 with those shown for ethylene in Fig. 4.2 shows the magnitude of these differences. For example a 13–5% ethylene content monomer mix is required to produce a 5% ethylene copolymer, compared with 27% VAM monomer mix for a 15% VAM copolymer. Moreover, in a single copolymerisation in which both monomers are added at the beginning, a very large change of composition of the copolymer produced throughout the conversion occurs with the ethylene content steadily increasing. Figure 4.2 shows that a 7% ethylene content copolymer is produced at 90% conversion from a 10% ethylene monomer mix with copolymer varying from 3–5 to 22% ethylene content and with residual gas containing 60% ethylene. This increase in ethylene content produces a large change in the pressure of the monomer mixture. For example,9 in a polymerisation carried out at 50°C producing a 4% ethylene copolymer, the pressure before ethylene addition was 8 bar, it was 22 bar after ethylene was added and this peaked to 31 bar during the polymerisation. These high pressures in ethylene copolymerisations are inevitable but the increase in pressure with conversion for a simple copolymerisation and the heterogeneity of the copolymer composition produced are additional reasons compared with VAM copolymerisation for feeding VCM continuously. The very high pressures necessarily require thick wall autoclaves which reduce heat transfer to the cooling jacket so that very long polymerisations and uneconomic recipes have to be used. At the end of the polymerisation a mixture of VCM and ethylene has to be removed from the copolymer; the resultant recovered monomer gas mixture is very difficult to separate into the constituent monomers by conventional compression and reliquefaction but can be used for further copolymerisations as a gas mixture.

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS

65

FIG. 4.2. Changes of composition during vinyl chloride/ethylene copolymerisation.

4.4.2 Propylene Mixtures of VCM and propylene produce a much lower pressure than ethylene mixtures at polymerisation temperatures so that thinner wall autoclaves can be used. For example,9 in a polymerisation carried out at 50°C producing a 2–5% propylene copolymer the pressure of the polymerisation never rose above 10 bar. However propylene is less reactive with VCM than ethylene (see Table 4.2) so that higher propylene content mixes have to be used to produce the required propylene content copolymer. This is exemplified in Fig. 4.3 which shows that a 7.5% propylene copolymer requires a monomer mixture containing 34% propylene, i.e. almost 5 times as much, compared with less than 2 times for VAM and 2–3 times for ethylene. In the example shown a 10% propylene content mixture would produce a 4–8% average propylene content copolymer at 90% conversion with polymer ranging in composition from less than 2% to 21% with a residual gas mixture containing over 70% propylene. Because of its low reactivity, propylene slows down the rate of polymerisation, but even more serious is the fact that propylene is a significant chain transfer agent so that lower polymerisation temperatures than usual have to be used to produce the product of required molecular weight. As a consequence only very slow polymerisation rates can be achieved even with high initiator concentrations. As for ethylene copolymers, a mixture of unreacted monomers very rich in the comonomer is obtained which is difficult to separate and the mixture is normally used in the production of further VCM/propylene copolymer. Comparative testing of the melt flow characteristics of vinyl acetate, ethylene and propylene copolymers has shown that a similar reduction in melt viscosity is produced for the same molar concentration of the three comonomers. For example, the 15% VAM, 7.5% propylene or 5% ethylene copolymers described

66

R.H.BURGESS

FIG. 4.3. Changes of composition during vinyl chloride/propylene copolymerisation.

above would have similar melt viscosity characteristics if they have the same K-value but the difficulties in making the olefin copolymers with such high olefin content mean that comparisons have been made only at lower comonomer content (7–2% VAM, 3–5% propylene and 2–5 % ethylene). Of these three polymers both the ethylene and propylene copolymers show improved polymer heat stability, the ethylene polymer improved notched impact strength and the VAM copolymer the best deep draw characteristics.9 Both ethylene and propylene copolymers were marketed for some years by Union Carbide and Air Reduction Co. Ltd respectively, but had very little impact on the market, presumably because of the marginal improvement in product properties and their high cost compared with vinyl acetate copolymers. Nearly all have now been withdrawn from sale. 4.5 OTHER COPOLYMERS Copolymers of VCM with various acrylate and methacrylate esters can be made although all these common monomers tend to react more rapidly than VCM (see Table 4.2) so that lower acrylate content monomer mixes are required to make a given copolymer and the acrylate or methacrylate have to be added to the polymerising mixture if homogeneous copolymers are required. These copolymers are only of slight commercial interest although some are offered in latex form for coating applications and a copolymer resin of VCM and 2-ethylhexyl acrylate is sold because its superior processing eases the development of the optimum mechanical properties in the final product. Copolymers of VCM and methyl methacrylate are thermally unstable, yielding methyl chloride on heating.

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS

67

A very wide range of other comonomers have been studied, nearly always with the aim of reducing the product melt viscosity. Vinyl ethers and vinylidene chloride have been used and produce polymers with significantly lower melt viscosity. However, vinyl ether copolymers are expensive because of the high cost of the comonomer, and vinylidene chloride copolymers are more unstable to heat than vinyl acetate copolymers. Some comonomers have been studied in the hope of producing especial effects. VCM and carbon monoxide can be copolymerised together and the resultant copolymer is very unstable, especially to light. Copolymers with substituted N-phenylmaleimides produce copolymers with an increased softening point but these are thermally unstable. Methacrylic acid copolymers have been produced in the hope of matching the ‘ionomer’ properties of olefin copolymers.10 Homogeneous VAM copolymers are produced by solution polymerisation and the addition of a third monomer is of some commercial interest for solution coating applications (e.g. Vinylite resins from Union Carbide). These terpolymers containing, perhaps, 1% of a third monomer are used to improve, for example, metal adhesion. Copolymers of VCM, VAM and ethylene are of increasing interest in latex form for paint applications. Many potential comonomers, i.e. butadiene, styrene, -methylstyrene, higher olefins, etc., react relatively rapidly with polymerising VCM but the radical produced does not propagate the polymerisation chain so that such monomers are retarders for the polymerisation. In practice these comonomers are of some commercial importance, either because their presence as impurities in VCM has to be carefully controlled to avoid reaction time variability (e.g. butadiene) or because they are used as retarders to control potential runaway reactions (e.g. -methylstyrene). 4.6 PVC BLENDS—GENERAL PVC when processed is almost always used mixed with other materials to improve its heat stability or its processibility or to modify its properties. The formulation of PVC compositions to enable it to be processed is examined in detail by Matthews11 and discussed in Chapters 8–10 of this book. This section discusses the production of blends, with additives other than plasticisers, made at the polymerisation stage which are designed to significantly change the properties of the final composition. Basically there are three types of blends: (1) Those based on rubbers designed to increase product impact strength. (2) Those based on fillers designed to increase product rigidity. (3) Those based on compatible polymers where product properties intermediate between those of the blended polymers result. 4.7 RUBBER BLENDS 4.7.1 Butadiene Based Rubbers A large number of butadiene based rubbers are available for blending with PVC. These are either based on a butadiene/styrene or polybutadiene rubber grafted with poly(methyl methacrylate) or styrene/acrylonitrile

68

R.H.BURGESS

and these products are then blended, usually at the 5–15% concentration with suspension PVC and the usual other additives. Fabricated articles made from these blends, when examined optically, are seen to consist of a continuous PVC phase in which are suspended discrete rubbery particles. The product consequently retains most of the properties (rigidity, softening point) of rigid PVC with enhanced impact strength. By correct choice of the composition and particle size of the rubber phase it is possible to produce transparent blends with these materials. They are extensively used for applications such as bottles where high impact strength and transparency are required. While these blends are now produced exclusively by blending specially produced grafted rubbers as powders with PVC, much work was carried out some years ago on the production of intimate blends at the autoclave stage. The most successful of these techniques was to coagulate a suitable rubber latex in the presence of suspension PVC and then dry the resultant mixed material using a conventional suspension PVC drier.12 This technique, when applied to butadiene based rubber latices, offers no great technical or economic advantage over simple powder blending. Moreover the expertise in producing the optimum product properties resides almost entirely in the production of the grafted rubber so that a few companies only now produce the speciality rubbers required. 4.7.2 Chlorinated Polythene Unsaturated rubber blends such as those based on butadiene rubbers suffer from the disadvantage that they degrade quite rapidly when exposed to sunlight. Consequently, they are quite unsuitable for the type of outdoor applications where rigid PVC is widely used. A number of saturated rubbery materials have been examined for these applications. Of these, chlorinated polythene (CPE) initially gained wide acceptance for such applications as PVC window-frames, house siding, etc. These blends are based on physical blends of a suspension PVC with 8–10% CPE. The degree of chlorination is important in order to ensure the correct balance between compatibility (to avoid cheesy composition), and incompatibility (to maintain PVC type rigidity and a higher impact strength). While optimum product properties are produced from physical blends when CPE containing 35% combined chlorine is used, rather different results are obtained if the CPE is incorporated into the VCM before polymerisation. In general the mechanical properties of physical blends deteriorate the lower the chlorine content of the CPE is below 30%, whereas ‘graft’ blends with excellent mechanical properties can be produced from CPE containing as little as 24% chlorine.13 Again, no great technical or economic advantage has been demonstrated for such ‘graft’ blends. The market is dominated by the product sold under the trade name ‘Hostalit Z’, which is believed to be produced by physical blending of PVC and CPE. 4.7.3 Ethylene/Vinyl Acetate Rubbers These ideas have been developed further with ethylene/vinyl acetate (EVA) rubbers where once more a wide range of ethylene: vinyl acetate ratio rubbers are available. The optimum product properties have been obtained by polymerising VCM in the presence of a roughly 50:50 ethylene:vinyl acetate rubber.14 Such a rubber is quite incompatible with PVC and a physical blend has poor mechanical properties. Two manufacturing processes have been proposed. In one14 the EVA is dissolved in VCM before polymerisation, which is carried out using the usual suspension polymerisation procedures, to give a 5–15% EVA content blend which is then used to fabricate the final article or is blended with more suspension PVC

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS

69

to produce a lower EVA content blend, usually about 6% EVA. In the other,15 finely broken down EVA rubber is suspension polymerised in the presence of VCM below its saturated vapour pressure, i.e. so that there is no free VCM phase. In this procedure the EVA particles grow by swelling with VCM and subsequent polymerisation. Both these methods can be used to make much higher EVA content grafts (50%) which can then be used for dilution with 5–10 times as much PVC homopolymer to produce the final composition. Blends of EVA and PVC give compositions with similar mechanical properties to those based on CPE and being saturated rubbers are suitable for use outdoors. They are somewhat easier to process than the CPE blends and in recent years have captured the major share of the European market for such applications as window frames, etc. 4.7.4 Ethylene/Propylene Rubbers It is possible to make graft blends of ethylene/propylene rubber (EPR) with PVC by polymerising VCM in the presence of EPR by the partial pressure method. These blends show no advantage over the established EVA blends. 4.7.5 Acrylate Rubbers Increasing attention is being paid to the use of blends of PVC with acrylate rubbers. Blends of PVC with poly(butyl acrylate) (PBA) or PBA grafted with methyl methacrylate can be produced either by coagulating the PBA latex in the presence of suspension PVC and codrying, by polymerising VCM in the presence of PBA rubber as described in Sections 4.7.2, 4.7.3 and 4.7.4 for other saturated rubbers, or by blending PVC with commercially available graft PBA powders (e.g. 8–12% Paraloid KM323B). The Vinidur SZ range of polymers marketed by BASF are in the form of ‘graft’ blends of PBA and PVC and can be used at lower concentrations of PBA than Paraloid KM323B to give the same product mechanical properties. Fabricated articles based on blends of PVC with these PBA materials have mechanical properties similar to those obtained with EVA and CPE blends and the customer chooses which blend to use on minor differences such as surface gloss and die relaxation in the particular application. All the PVC/saturated rubber blends described here, when containing the 5–10% rubber normally required for enhanced impact properties, are opaque or strongly translucent. This is of no concern for woodsubstitute application such as window-frames but makes the blends of no interest for clear applications. However, blends containing a lower concentration of PBA are sufficiently transparent for some clear applications. Clear rigid foil for packaging applications can be produced either from emulsion PVC or suspension PVC. However, the latter is difficult to process by calendering to give the highest transparency because of sticking to the calender bowl if the high bowl temperatures necessary for high transparency are used. This problem can be overcome either by incorporating some methyl methacrylate grafted PBA powder into the formulations, e.g. Paraloid K175, by coagulating PBA latex in the presence of suspension PVC, or by polymerising VCM in the presence of some PBA.16

70

R.H.BURGESS

4.8 POLYMER BLENDS 4.8.1 Processing Aids Quite a wide range of polymers which are compatible with PVC act as processing aids when mixed at 1– 10% concentration in rigid PVC formulations. They apparently act by increasing the melt elasticity of the formulations and the molecular weight of the added polymer is very important (see Section 9.5.4). The most widely used additive polymer is poly(methyl methacrylate) (PMMA) or copolymers of methyl methacrylate and ethyl acrylate containing at least 90% MMA. These polymers are usually added as fine powders to suspension PVC at the mixing stage but they can be added to the polymerisation process either before polymerisation of the VCM or by coagulating a PMMA latex in the presence of suspension PVC. Styrene/ acrylonitrile polymers are also used as processing aids. 4.8.2 Poly(methyl methacrylate) Poly(methyl methacrylate) (PMMA) is compatible with PVC in all proportions. Consequently it is possible to produce any blend which is desired and the properties of these blends are intermediate between those of the constituent homopolymers. These blends can be produced by simple mixing of the respective powders followed by normal processing.17 They can also be produced by polymerising the second monomer in the presence of the first polymer. For example successive suspension polymerisations are possible in which VCM is polymerised to give the normal porous PVC granules, the VCM remaining is removed and MMA is added together, if desired, with extra initiator and chain transfer agent. The mixture is heated and an intimate mixture of PMMA and PVC is produced.18 Most compositions of commercial interest concentrated on 60–88 % PVC in the blend and have the expected properties of a cheaper, more flame resistant acrylic. Unfortunately the compositions share some of the less desirable features of their base polymers, i.e. the material is difficult to process and more expensive than PVC, and their commercial sale has now substantially ceased. These blends are potentially transparent although not all the products marketed were. Blends of PVC with PMMA have a slightly higher softening point than PVC itself. If the methyl methacrylate polymer used has a higher softening point than PMMA itself, for example by using a suitable MMA copolymer, a higher softening point blend results. 4.8.3 Chlorinated PVC The softening point of PVC can be increased by chlorination of the product either in a solvent or as a suspension. Normally one grade of chlorinated PVC (CPVC) containing 63% chlorine is produced with a softening point of 120°C compared with 80°C for PVC itself. This grade is difficult to process and expensive so that if a lower softening point is acceptable blends of this product and PVC will be used in the proportions necessary to give the desired softening point. Products of this type have not been very successful commercially but are used for speciality markets such as hot water systems in mobile homes in the USA and for fibre manufacture in France.

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS

71

4.9 FILLER BLENDS In principle the rigidity of a PVC composition can be increased by the addition of fillers such as talc, chalk, mica, glass, etc., to the mixture. These blends are of little commercial importance largely because of poor processing behaviour and the poor bonding between the filler and the PVC which gives poor long term properties. Various forms of finely divided calcium carbonate, surface treated with stearate to improve dispersion, are available and are extensively used at low concentration to improve toughness. It is possible to polymerise VCM in the presence of various fillers but no commercially important product has arisen even though this process should improve the PVC filler bond. ACKNOWLEDGEMENTS Thanks are due especially to R.W.Gould for his help in the preparation of the sections on blends. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

MAYO, F.R. and LEWIS, F.M., J. Amer. Chem. Soc., 66,1594 (1944); ALFREY. T. and GOLDFINGER, G., J. Chem. Phys., 12, 205 (1944). ALFREY, T. and PRICE, C.C., J. Polym. Sci., 2, 101 (1947). BRANDRUP, J. and IMMERGUT, E.H., Polymer Handbook, Interscience, New York (1966). FLEISCHER, G. and KELLER, F., Plaste u. Kaut., 20, 10 (1973). FLEISCHER, G. and KELLER, F., Plaste u. Kaut., 20, 7 (1973). SIELAFF, G. and HUMMEL, D.O., Makromol Chemie, 175, 1561 (1974). NASS, L.I., Encyclopedia of PVC, Marcel Dekker, New York (1976). HIBBERT, P.M., personal communication. DICKSON, J.T., unpublished work. EISENBERG, A. and King, M., Ion-Containing Polymers—Physical Properties and Structure, Academic Press, London (1977). MATTHEWS, G., Vinyl and Allied Polymers, Volume 2, Iliffe, London (1972). ICI, British Patent 1219352. MONSANTO, British Patent 1029634. BAYER, British Patents 1021324 and 1027710. CHEMISCHE WERKE BUNA, East German Patents 134232 and 135083. DYNAMIT NOBEL, German Patent 1595515; SOLVAY, British Patent 1530854. MONTECATINI, British Patent 1 043058. ICI, British Patent 1015334.

Chapter 5 THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC R.H.BURGESS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

5.1 VINYL CHLORIDE TOXICITY Vinyl chloride (VCM) is a gas at ambient temperature with a boiling point of−13°C. It is usually handled in commercial PVC manufacture as a liquid at pressures ranging from 2 to 15 bar. Since the PVC manufacturing process, as has been seen in Chapters 1–3 of this book, is a batchwise one, large quantities of VCM are handled in very complex pipework and pressure vessels. There are necessarily in these processes many opportunities for leaks of VCM from the pressure envelope to the surrounding atmosphere. Moreover, as has already been described (Chapter 1, section 1.2) the kinetics of VCM polymerisation are such that it is uneconomic to polymerise the VCM completely to PVC and substantial quantities of VCM (10–20% of original charge) have to be removed subsequent to the polymerisation step. Prior to the early 1960s VCM was known as a gas which presented a severe explosion risk (explosive limits in air 3.6−26% v/v), with anaesthetic properties but with a not-unpleasant ethereal smell apparent at concentrations of > 2000 ppm v/v in air. Plants were designed to avoid the build-up of concentrations of VCM in air so that explosive concentrations and concentrations capable of rendering an exposed person unconscious were avoided. Basically this involved attention to the pressure envelope to minimise major leaks and concentrations in the atmosphere of plants of 100–1000 ppm v/v VCM in air were common. The assumed low toxicity of VCM at that time can be evidenced by proposals to use it as an anaesthetic for medical use, and as an aerosol propellant. While its use as an anaesthetic did not develop, it was used as an aerosol propellant up to the early 1970s in some countries, especially in the USA. In the mid 1960s the occurrence of a bone deficiency called acroosteolysis (AOL) in a very small number of PVC plant operators was noticed. This disease occurred especially in plant operators engaged in cleaning PVC reactors and usually took the form of a degeneration of bone in the tips of the fingers of the operators. After removal of the operator from autoclave cleaning, bone recovery usually took place and no other symptoms were observed. This discovery stimulated the development of methods of cleaning autoclaves alternative to the manual scraping then generally used, and further research into the toxicity of VCM. The various steps taken to develop non-manual autoclave cleaning have been described fully in Chapter 1, Section 1.6.5, and took the form of cleaning the autoclaves either with high pressure water or with solvents. In more recent years the development of chemical systems of preventing autoclave fouling has further reduced the need for manual cleaning of autoclaves. These changes have been sufficient to overcome the problem of AOL and indeed where these changes have been implemented no case of AOL has been reported.

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

73

In the late 1960s Professor Viola of Solvay1 tried to reproduce AOL in rats by exposing them to 30000ppm of VCM; he failed to produce AOL but found that the rats developed tumours. In the early 1970s work by Maltoni,2 sponsored by ICI, Solvay, Montedison and Rhone-Poulenc, showed that VCM over a wide range of doses induced tumours in rats, especially a particularly rare form of tumour, angiosarcoma of the liver (ASL). Quite clearly, VCM was a proven animal carcinogen. The VCM/PVC industry was alerted to do epidemiology studies to discover whether there were any comparable effects in humans. In early 1974 the discovery of cases of ASL among workers on the B.F.Goodrich PVC plant at Louisville, Kentucky,3 alerted the world to the fact that VCM appeared to be a human carcinogen. Epidemiological studies of workers associated with the manufacture and polymerisation of VCM revealed further cases of ASL world-wide, in the USA and elsewhere.4 The majority of the cases occurred among workers in polymerisation rather than VCM monomer plants, and again especially among autoclave cleaners. These findings confirmed the link between ASL and heavy exposure to VCM. Since then further animal experimentation has been carried out.5 As is the case with most forms of cancer, ASL has a long latency period (20y average), so that cases are still occurring, usually among operators exposed to VCM in some PVC plant in the 1940s, 1950s and 1960s. A total of 84 cases of ASL, world-wide, have so far been confirmed up to mid-1980. Although further cases can be expected there is reason to hope that the steps taken in the late 1960s to prevent AOL among autoclave cleaners will reduce the total number of ASL cases. Following the discovery of the link between ASL and VCM exposure very considerable efforts were made to reduce the exposure to VCM of: (a) VCM and PVC plant operators, (b) PVC fabrication personnel, (c) the public and other plant operators around the VCM and PVC plants, and (d) the public at large. This has involved: (a) large changes in operating procedures, especially for PVC plants, (b) marked developments in the techniques used for the measurement of VCM concentration, and (c) the development of new processes for the removal of unreacted VCM from PVC immediately after polymerisation. 5.2 PROTECTION OF PVC PLANT OPERATORS In the section 5.1 the strong correlation between the cases of ASL and workers on PVC manufacturing plants has been noted. Accordingly in 1974 effort was directed at reducing the exposure to VCM of such plant operators. A gradual tightening of standards from a threshold limit value (TLV) of 500 ppm to 200 ppm in the 10 years prior to 1974 had reduced the number of leaks on a PVC plant, such that the previous industry-wide average of, perhaps, 100–1000 ppm worker exposure to VCM had fallen to the 20–200 ppm region. Further reductions were rapidly achieved by changing operating procedures to avoid ingress into the plant atmosphere and to remove such VCM as did escape. The elimination of normal autoclave cleaning either by using automatic cleaning or by the avoidance of the necessity to clean autoclaves has been referred to earlier, but these changes were rapidly completed after the ASL link was uncovered. Interbatch opening

74

R.H.BURGESS

of the autoclave is a major source of VCM emission into the plant. Changes of procedure designed either to degas the autoclave contents before opening or to ensure that air from the plant is always drawn into the autoclave when it is open have markedly reduced this source of VCM emission into the plant atmosphere. Some manufacturers have developed processes without interbatch opening which, of course, eliminate this source altogether. Other routine sources of VCM emission have been tackled by attempting to eliminate or reduce markedly the amount of the emission, but where this has not proved feasible controlled venting of the emission through tall stack pipes has greatly reduced operator exposure. VCM discharged in this way is quickly diluted very considerably and rapidly destroyed by the action of air and sunlight. A further source of VCM in the plant air is leaks from the many flanges and valves in the pipework and autoclave used to contain the VCM under pressure. Much attention has been paid to eliminating such leaks by regular inspection for VCM emissions, by improved maintenance and in many cases by replacement with more suitable equipment. Further reductions have been achieved by improved ventilation of the plant either by using forced ventilation (up to 10 air changes per hour) or by opening up the plant to the outside by removing parts of the plant walls. The latter step, of course, depends both on the detailed design of the plant and on the ambient conditions. Finally, residual VCM left in the polymer and, in the suspension polymerisation process, in the waste water, is a further source of VCM. The solution to this problem lies in the more efficient VCM removal by stripping after polymerisation. This is dealt with in detail later in the chapter (Section 5.4), but, where a significant source of residual VCM remains, emissions into the plant are reduced by local ventilation coupled with venting of contaminated air at high level. These changes enabled the PVC manufacturing industry to reduce operator exposure to VCM very considerably and meet the requirements of regulatory bodies in the various countries. For example, the British ‘Code of Practice’, arrived at after agreements between industry, workers’ representatives and Government, called for a maximum time-weighted average (TWA) of l0ppm v/v in the workplace, no excursions beyond 30ppm, and continuing efforts to produce as low concentrations as possible. This was superseded in December 1979 by the EEC Directive 78/610 which requires an annual average of <3 ppm in the workplace and alarm thresholds at 15 ppm over 1 h, 20 ppm measured over 20 min or 30 ppm measured over 2 min. The Occupational Safety and Health Administration (OSHA) in the USA specifies <1 ppm v/v as the maximum average personal exposure with no excursions beyond 5 ppm, while the German authorities call for <5 ppm annual average exposure in old plants and <2ppm annual average exposure on new installations but there, too, their regulation has been superseded by EEC 78/610. A comparison between these standards is not easy to make since the measurements are made on totally different bases and calculated in different ways, e.g. workplace concentration is not the same as personal exposure.6 Suffice it to say that all require basically similar standards for VCM exposure and, indeed, actual operator exposure is very similar in plants of similar age, design and product range, at least in the Western World. Not surprisingly, the more modern the plant the lower the VCM concentrations tend to be, because modern plants have been designed in the knowledge that VCM is a carcinogen and exposure to VCM must be minimised. The plant modifications required to produce these reductions in VCM exposure were expensive. In the UK £15–20 million in capital has been spent to make the necessary changes; other costs such as loss of capacity, additional running costs and research costs are not available but they are believed to be substantial. In a few cases manufacturers in some countries have chosen to close a plant rather than modify it because of the uneconomic cost of the necessary improvements. Where the closure option was chosen, the plants have tended to be small, old units containing many small autoclaves and other small pieces of equipment particularly difficult to modify economically.

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

75

These steps taken to reduce operator exposure on the PVC manufacturing plants generally have the effect of greatly reducing emissions to the atmosphere around the plants so protecting operators on adjoining plants and the general public who live near a PVC plant. 5.3 VINYL CHLORIDE ANALYSIS A major factor in the success of this work has been the development of very sensitive methods of measuring VCM in the working atmosphere and in the solids and liquids from a PVC plant.7 Reference 4, pp. 19–54, gives a particularly useful review of the present position. Infra-red spectroscopy, using the =C—H out-ofplane deformation band at 10.9 µ m, oxidation of VCM to chlorine and/or hydrochloric acid, thermal conductivity, chemiluminescence, photo-ionisation, mass spectrometry and flame ionisation have all been used. Many of these methods of detection are not specific for VCM so that separation techniques are often required before measurements are taken. Gas chromatographic separation is the technique most frequently used. 5.3.1 Atmospheric Testing Atmospheric testing can be subdivided into instantaneous testing, area monitoring and personal monitoring. Instantaneous testing takes a number of forms, depending largely on the information required. Grab samples are taken in which samples of the gas are collected in evacuated flasks, by gas-tight syringe or by pumping into plastic bags, but care has to be taken to ensure that the equipment is clean and made of suitable material to avoid loss of VCM by absorption. The sample is then analysed by any of the analytical procedures mentioned earlier. This type of testing might be used for areas of the plants or other locations to confirm that VCM is not present, for example in the atmosphere of PVC processing plants where significant concentrations of VCM are not normally present or in vessels such as polymerisation autoclaves where regular measurements are inappropriate. The sensitivity of the test will depend on the detection method but measurements down to 0–1 ppm v/v are possible. Colorimetric methods based on chemical reactions, in which the chlorine or hydrochloric acid formed by oxidation of VCM stains the familiar coloured crystals (Drager or Gastec) or an indicating paper (UEL), are commonly used to obtain on-the-spot analysis of the VCM concentration, although the sensitivity of these tests is no better than 1 ppm v/v. Wide use is made of portable VCM monitors such as the Century Systems Corporation OVA 108 organic vapour analyser to locate the source of VCM emissions from such places as the flanges and valves on lines containing VCM. These methods are not specific for VCM unless gas chromatographic separation is included and they are not very sensitive to low concentrations of organic vapour. Area monitoring of the atmosphere of PVC plants is required in many countries and is usually carried out by the measurement of instantaneous concentration in a number of locations on the plant in which the samples are analysed serially or as a mixture. The samples are pumped through plastic tubes to a central analyser where the VCM concentration is measured by infra-red spectroscopy, mass spectrometry or flame ionisation with or without gas chromatographic separation of the VCM from other possible organic vapours,8 usually to a concentration of 0.1 ppm v/v. These systems are commonly equipped with dataprocessing instrumentation which calculates time-weighted means and other values required by regulatory authorities. For example in the UK the ‘Code of Practice’ requires records of the area monitoring test results to be kept for periods up to 30 years. In recent years these automatic sequence analysers have also been used

76

R.H.BURGESS

to analyse regularly a particular area of the plant, for example where a leak of VCM is suspected or the atmosphere of an autoclave both before and during operator entry for some non-routine task. Personal monitoring is also frequently carried out, usually as confirmation that the area monitoring analyses reflect the actual exposure of plant operators to VCM. Here the measuring equipment is often a tube of active carbon through which a continuous flow of air is pumped. The equipment has to be light yet sturdy since it is worn by the person to be monitored for a working shift of up to eight hours. The average VCM concentration over the test period is then obtained by eluting absorbed materials from the carbon and analysing for VCM in the normal way. The technique is potentially capable of measuring very low VCM average concentrations (0–1 ppm v/v). In spite of great ingenuity on the part of the many manufacturers of this type of equipment these devices are, of necessity, somewhat cumbersome (weight 200 g) because of the need to incorporate a battery and pump. Diffusion badges have been developed in which VCM diffuses through a permeable membrane at a rate proportional to its concentration in the surrounding air and is trapped by a layer of absorbent material, the analysis of which is then carried out in the laboratory. Personal monitors which record the average concentrations of VCM using a colour detector are available (Drager), as is a portable monitor based on a sensitised moving paper tape; these techniques have the advantage of giving an instantaneous record of VCM exposure. A special case for VCM-in-atmosphere measurement is in the determination of VCM concentrations round PVC and VCM manufacturing plants in order to protect workers on surrounding plants and the general public against exposure to significant quantities of VCM emitted from the plants. All the methods described earlier can be used but especially accurate analysis is required because the concentration present is normally very low. Carbon absorption techniques are usually used coupled with traps designed to remove water vapour prior to the sample reaching the carbon in order to reduce the chance of VCM ‘breakthrough’. Sensitivities as low as 5 ppb v/v have been claimed, and the method can be incorporated into an eight-day system by changing the absorption tube every 24 h. 5.3.3 Vinyl Chloride in Solids and Liquids The VCM content of PVC may be determined by dissolving the PVC in a suitable solvent and directly injecting the solution into a gas chromatograph. This method is suitable for use down to 5 ppm w/w. However, the requirement for PVC containing the minimum quantity of VCM, to eliminate VCM exposure to PVC processors and users of PVC fabricated articles, calls for greater accuracy than this. This can be achieved by using head-space gas chromatographic analysis in which a sample of gas in equilibrium with PVC, a PVC solution, or other material (water, process effluent, etc.) suspected of containing VCM is analysed by the methods already described for atmospheric testing. Automatic methods with a limit of detection of 0–1 ppm w/w VCM in PVC are available, for example using a Perkin-Elmer F42 head-space analyser. These methods are in routine use by most PVC manufacturers who are now required to supply PVC containing no more than 10 ppm VCM for most applications and < 1 ppm for use in the manufacture of articles to be contacted with foodstuffs. The achievement of these low concentrations has involved considerable development work over the past few years on removing residual VCM from PVC; the methods are described in Section 5.4. While the rate of loss of VCM from PVC powder is fairly high, its loss rate from the final fabricated article to the atmosphere is usually extremely slow, so the VCM content of the fabricated article is of interest although lower than that of the PVC used in the fabrication. Similar head-space methods can be used to determine the VCM content of these articles.

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

77

The VCM content of foodstuffs which have been in contact with PVC is of considerable interest and much work has been carried out to develop test methods (ref. 7, p. 113). Both direct injection into a gas chromatograph and the head-space method have been used but the sensitivity obtained depends on the method used and the foodstuff involved. With some authorities insisting on a non-detectable concentration of VCM in the foodstuff much emphasis has been placed on very sensitive analysis with the current limit of detection by head-space chromatography being 5ppb for oils and fatty foods and 1 ppb for aqueous based materials. 5.4 REMOVAL OF RESIDUAL VCM 5.4.1 General The kinetics of polymerisation of VCM are such that it is impossible to polymerise all the VCM to PVC because the rate of polymerisation falls rapidly at high conversion (see Chapter 1). Consequently all VCM polymerisation processes have in common the feature that substantial quantities of VCM remain unreacted after the polymerisation process. In the case of both the suspension and mass processes there are marked changes in the polymer particle structure which occur progressively the further below saturation vapour pressure the polymerisation process proceeds. It was shown in Chapter 1 that subsaturation vapour pressure occurs whenever the PVC:VCM ratio exceeds 75:25. These changes in particle structure are damaging to the subsequent processing of the polymer (see Chapter 7) and there are other effects on the polymer properties which are undesirable. Both these economic and quality considerations require incomplete conversion of VCM to PVC and, in fact, 10–20% of the original VCM charged remains unreacted at the end of most commercial processes. Whilst in the very early days of PVC manufacture it was practice to vent this VCM to atmosphere both deliberately at an earlier stage and during the PVC drying step, PVC manufacturers have for many years recovered as much as possible of this unreacted VCM by gasification and subsequent reliquefaction. However, the VCM is held tenaciously by the PVC so that it was common practice prior to 1974 to leave at least 1% residual VCM in the PVC after this VCM recovery process. Most of this VCM was lost during subsequent transfer operations and especially during the drying operation necessary with the suspension and emulsion processes. Residual VCM concentrations in the freshly dried PVC were commonly 10–1000 ppm w/w in suspension PVC, lower in emulsion PVC but with higher concentrations in bulk PVC which, of course, has no drying step. The residual VCM concentration was shown to vary from grade to grade of PVC because of the grain structure of the grades, with the more porous grains, generally those intended for plasticised application, having the lowest values and the more dense grains, generally used for rigid applications, the highest values. Dried emulsion PVC contained low concentrations of residual VCM (<100 ppm) because the very small size of the PVC particles increased the ease of VCM loss. Concentrations as low as 1 ppm were found for grades where after-drying operations such as milling were used. All dried products continue to lose VCM on storage if stored in porous (e.g. paper) bags but there is no significant loss in bulk storage. This high residual VCM content of PVC slurry and latices\had the following consequences: (1) high VCM loss during slurry/latex storage or transfer, (2) high VCM emissions from the drier, and

78

R.H.BURGESS

FIG. 5.1. Process for removing VCM from PVC slurry or latex. (Reproduced from Burgess, Developments in PVC Production and Processing—1, Whelan and Craft, Eds., Applied Science Publishers Ltd, 1977.)

(3) the potential exposure of PVC fabricators and the general public to VCM contained in the PVC to be fabricated, and even the final fabricated article, which stimulated PVC manufacturers to improve the stripping processes employed. Figure 5.1 is a schematic diagram of the process of removing VCM from a PVC particle, in which the PVC particle is suspended in water. The studies of Berens and other workers9 have shown that in an equilibrium situation containing roughly equal quantities of PVC and water, at least 90% of the VCM present will be in the PVC. Hence the first process of VCM removal is the transfer of VCM from the PVC to the water (A in Fig. 5.1). The rate of this process depends on the diffusion path length, the temperature and the force driving the VCM from the PVC to the water. It is already established that emulsion PVC is easier to strip than suspension PVC because the particles (emulsion particles normally <0.1−1 µ m in size), and consequently the diffusion path lengths, are small. Suspension PVC, although normally consisting of grains with an average size of 100–150 µ m, is porous so that the diffusion path length is significantly less than the appropriate radius based on this figure. The known variation in final VCM concentration of the dry PVC is due to the different porosity of the various suspension PVC grades. The effect can be predicted by measuring the porosity of the grains by, for example, nitrogen adsorption surface area and relating this to the effective sphere diameter of the structures making up the PVC grains. Berens in his work10 has shown how VCM release can be affected by such changes and Table 5.1 shows how the effective PVC sphere diameter can change with grain type and its likely effect on the rate of loss of VCM.11 Clearly these changes in VCM loss rate are potentially very important and are significant for mass as well as for suspension PVC. Moreover

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

79

TABLE 5.1 Effect of Grain Type on VCM Loss Rate Sample Grain type description Cold plasticiser acceptance Effective sphere diameter (%) (µ m)

VCM loss rate (arb. units)

A B C D

3 9 44 >100

Completely dense Fairly dense Porous Emulsion

5 15 30 —

17 7 1 0.3

increasing the porosity of the PVC grains is an obvious way of easing stripping problems and reducing residual VCM in the final PVC. Much work has been carried out by PVC manufacturers modifying the structure of PVC grains, particularly in suspension polymerisation. The replacement of simple protective colloid systems based on the use of the cellulose derivatives or partially hydrolysed poly(vinyl acetate) with mixed protective colloid systems such as cellulosic materials with different substituted groups (methyl, hydroxyethyl, etc.),12 mixtures of cellulose and poly(vinyl acetate) types,13 mixtures of poly(vinyl acetates) of different degrees of hydrolysis14 and short chain surfactants such as the Span and Tween ethoxylated or esterified sorbitols mixed with the main protective colloid15 have all been suggested. Increasing the temperature of the slurry has the effect of increasing the vapour pressure of VCM over PVC as well as increasing the rate of diffusion of VCM through the PVC. The former is of most importance in stripping processes since any increase in VCM vapour pressure increases the potential rate of removal from the system by the normal sparging, venting and evacuation procedures used. Figures 5.2 shows the way in which VCM vapour pressure changes with changes in temperature and VCM concentration at low VCM-in-PVC concentrations according to both Berens9 and work in our own laboratories.11 Most VCM stripping processes now employ higher temperatures than formerly, usually in the range 70–120°C, although care has to be taken at the higher temperatures to ensure that the residence time of the PVC at that temperature is as short as possible to avoid the known bad effect of too much heat history on the thermal stability of the PVC. Finally the rate of transfer of VCM from the PVC particle to the water is determined by the force driving it into the water, i.e. the difference in the relative saturation of the PVC and the water phases. Figure 5.3 shows the vapour pressure of VCM over water as a function once more of the temperature and the VCM concentration in the water using the data of Berens,9 Zichy16 and Hayduk and Laudie.17 A comparison of Figs. 5.2 and 5.3 will show the relative solubility of VCM in PVC and water at any particular VCM vapour pressure. In practice, of course, during stripping the VCM concentration in the water is kept to the minimum by increasing its rate of removal (B in Fig. 5.1) as much as possible. This step is normally the rate controlling step of the traditional suspension and emulsion PVC stripping process and the new processes which have been developed are aimed, at least in part, at overcoming this problem. The application of these ideas to suspension, emulsion and bulk polymer stripping will now be discussed. 5.4.2 Stripping of Suspension PVC The traditional process for VCM removal, after suspension polymerisation to the normal 80–90% conversion VCM to PVC, involves venting of VCM gas from either the autoclave or a separate stripping vessel, either to a gasholder, i.e. essentially at atmospheric pressure, or directly to the inlet of a gas compressor when the effective pressure is somewhat higher. Both of these steps, which are common also to

80

R.H.BURGESS

FIG. 5.2. Equilibrium VCM vapour pressure as a function of the VCM content of PVC.

the emulsion and bulk VCM removal processes and to the newer processes, produce PVC containing, perhaps, 3% of unreacted VCM. This concentration can be reduced further by increasing the slurry/latex/ bulk PVC temperature as indicated by Fig. 5.2. In the traditional process a vacuum was then applied and further quantities of VCM were removed until 1–2% of residual VCM remained, the precise amount depending on the slurry temperature, the time of evacuation, the vessel agitation and the porosity of the PVC grains. Until 1974 this slurry was then dewatered and the resultant PVC wet cake dried, during which process most of the residual VCM was removed, but a significant quantity, perhaps 10–1000 ppm depending on polymer grade porosity and the drying conditions, remained in the PVC. Most of this residual VCM was lost to the atmosphere during storage, if stored in a porous package, from the fabricators’ blending or melt processing equipment and only a small quantity remained in the final PVC fabricated article. In order to reduce VCM emissions in and around PVC manufacturing plants and to eliminate problems of VCM concentration in the fabricators’ works and any risk of exposure to VCM of the public using articles made of PVC, considerable efforts have been made by PVC manufacturers around the world to improve the stripping process. Most of these improvements concentrated on increasing the rate of removal of VCM from the continuous aqueous phase. A number of patents18 advocate the addition of an organic liquid, the purpose of which is presumably to solubilise the VCM into the aqueous phase and to vaporise it rapidly under the action of vacuum and heat,

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

81

FIG. 5.3. Equilibrium VCM vapour pressure as a function of the VCM content of water.

so carrying the VCM out of the water into the vapour phase. It is thought that these processes are not used commercially because of the further problem of separating VCM from the organic liquid. A similar objection can be raised against other patents which propose either air or nitrogen sparging of the VCM-rich slurry19 where the air or nitrogen acts as a carrier gas to remove the VCM. A further objection to air sparging of very VCM-rich slurries is that potentially explosive VCM/air mixtures can be produced. Figure 5.420 shows the effect on VCM content of sparging PVC slurry with air at a range of temperatures and indicates the very large effect of that temperature. Concentrations of 10–100 ppm w/w VCM on PVC were obtained after a few minutes at temperatures of 90°C and above. Theoretically the log/ linear plots should be linear and the slowing down of the VCM loss rate with time is evidence either for an inefficient removal process of the VCM from the water or for a changing rate of diffusion from the PVC grains due to heterogeneity in their structure as proposed by Berens. The objections raised against the use of both solvents and these gases as aids to VCM removal can be overcome by using steam as the inert gas, in which case separation can be easily effected using a condenser to remove the steam. The use of steam overcomes the third rate controlling step (C in Fig. 5.1) by removing VCM from the gas space of the vessel. Steam sparging is now the basis for all the commercial stripping processes for suspension PVC. These processes take two forms, batch and continuous.

82

R.H.BURGESS

FIG. 5.4. Effect of sparging and temperature on VCM loss from PVC slurry. Cs is the concentration of VCM at time t and Cs° the concentration of VCM at time zero.

In the batch process21 the slurry is vented in the normal way and is heated up with steam usually to temperatures between 80 and 110°C. Large quantities of steam are then allowed to pass through the slurry or are removed from the slurry using an evacuation pump, the resulting steam/VCM mixture being separated using a condenser. In this way slurry with a residual VCM concentration as low as l0ppm w/w of PVC can be produced. The process is expensive in terms of steam and requires considerable equipment (pressure vessels, instrumentation, etc.). In recent years continuous processes have been developed22,23 in which PVC slurry is passed down a column, up which is passing steam (Fig. 5.5). Unstripped slurry containing, perhaps, 3% of unreacted VCM is fed to the top of a column containing a number of trays designed in such a way that they control both the average and the spread of residence time of the PVC in the column within close limits. Steam is fed into the bottom of the column and passes up it, both heating the slurry and removing VCM. A mixture of VCM and steam is released from the top of the column, steam is removed by condensation and the VCM gas is transferred to the recovery plant where it is reliquefied and re-used. Most of the steam is used to heat the slurry to the column operating temperature (normally 80–120°C) and only a small proportion is used to carry the VCM from the top of the column. Because most of the steam used to heat the slurry condenses in the top part of the column, the steam at the base of the column is all used to remove the last traces of VCM from the slurry. Consequently very low residual concentrations of VCM can be achieved (< 10 ppm w/w of PVC) at comparatively low steam usages. Short residence times are used (5–10 min) but careful control of the average and spread of polymer residence time in the process is necessary to prevent the formation of discoloured polymer. A number of forms of this basic process are now in use which differ essentially in the design of the trays within the column. Many patents22 specify the use of conventional sieve trays with weirs and downcomers, the residence time of the polymer in the device being controlled by slurry feed rate, the number of trays, their size and the height of the weir. Other patents use trays that contain no liquid inlet and outlet areas,23 both

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

83

FIG. 5.5. Column stripper for VCM removal.

the slurry flowing down the column and the steam up the column passing through the same holes. The residence time of the polymer in such a column is controlled by slurry feed rate, the size and number of holes in the tray and the rate of steam passing up the column. Both types of column are believed to be in commercial use. The use of processes of this type has enabled suspension PVC with a residual VCM content <5ppm to be generally available, with lower concentrations ( 1 ppm) in polymer intended for use in contact with foodstuffs. Most PVC copolymers are made by the suspension polymerisation route and commonly contain 8–16% vinyl acetate (see Chapter 4) to improve melt processing. These polymers tend to have very dense grain structures and this, coupled with their low softening point and consequential low degassing and drying temperatures, means that they are more difficult to strip than homopolymers and tend to retain a somewhat higher concentration of residual VCM. Nevertheless copolymers intended for applications involving contact with foodstuffs are now supplied with a residual VCM content 1 ppm. 5.4.3 Stripping of Emulsion PVC The principles described earlier (Sections 5.4.1 and 5.4.2) apply equally to the removal of residual VCM from emulsion PVC. However, the small size of the individual PVC particles (0.1–1 µ m compared to the effective 1–17 µ m for suspension PVC as shown in Table 5.1) means that the loss of VCM from the particles by diffusion is rapid and is rarely the rate controlling step. The low surface tension of latices presents a problem leading to foaming and skinning difficulties while in some cases the latex stability is low and excessive shear has to be avoided, otherwise local latex coagulation occurs. These latter difficulties far outweigh the

84

R.H.BURGESS

advantage of the fine PVC particle size and, in practice, VCM removal from emulsion PVC is much more difficult than from suspension PVC. These difficulties have been recognised by regulatory authorities, for example, the US Environmental Protection Agency suggested < 2000 ppm residual VCM after stripping latices compared with < 400 ppm for suspension and mass PVC. As in the case of suspension PVC two types of basic process, batch and continuous, have been proposed. In the batch process (indeed, as in the first stage of the suspension stripping process), residual VCM above atmospheric pressure is vented from the VCM/PVC/water mixture. Because of the tendency of the latex to foam, greater care is necessary to control the rate of off-take of gas than is the case with the suspension process; this normally means restricting the venting rate. As in the suspension process, heat and vacuum are then applied to the mixture, using rather lower temperatures (60–90°C) than are common for suspension since the diffusion rate and, indeed, the final achieved VCM concentration require less stringent conditions. During this stage of the process extreme care is necessary to prevent excessive foam formation and gas offtake rates are restricted to very low levels. Alternatively, foam controlling agents are added.24 In spite of these precautions such a process is very difficult to control and techniques designed to overcome the foaming problem have been extensively explored. The use of vibrating surfaces25 and other ideas26 have been patented. The use of high gas off-take rates and mechanical foam control have been proposed but here again, the equipment is expensive, coagulum formed by the high shear forces employed is difficult to avoid, and the process is difficult to operate. These processes are, in principle, capable of producing latex with a residual VCM content < 150ppm w/w, but commonly higher concentrations are achieved. The rate of loss of VCM during the subsequent drying and milling operation is so high that emulsion and paste PVC with a final VCM content < 10 ppm as supplied to the PVC fabricator is easily achieved. Continuous processes for VCM removal from emulsion PVC have been widely examined and take three basic forms. In the first form the latex is sprayed into a chamber cocurrently with steam27 and steam and VCM are removed. Processes of this type have been widely used for some years to concentrate latices and to remove residual impurities from other polymeric latices. As in the case of mechanical foam breakers this type of equipment inevitably subjects the latex to high shear forces and some coagulation, etc. is difficult to avoid. It is thought that devices of this type are not widely used for VCM removal. Continuous column processes, in principle similar to that described for suspension PVC in the previous section (Section 5.4.2 and Fig. 5.5), can be used for stripping VCM from emulsion PVC. Care has to be taken to avoid excessive foaming and coagulum formation but the rapid loss of VCM from the small latex particles means that relatively short residence times, low temperatures, and low steam usages, are possible. A number of patents have been taken out on the stripping of either fine droplets or thin films of PVC latex. These processes are shown diagrammatically in Fig. 5.6. Unstripped latex is pumped into a large chamber not unlike the chamber of a spray drier (see Chapter 6). Here it is either atomised using nozzles or a rotating disc,28 or it is sprayed in the form of a thin cone, or by other means,29 on to the wall of the vessel or metal cones inside the vessels. Normally a vacuum is applied to the vessel and a mixture of steam and VCM is removed. Heat is supplied for evaporating the water either directly by steam injection or by a combination of feeding hot latex and jacket heating. These processes are reported to be very efficient and capable of producing stripped latex containing very low VCM concentrations (<100ppm) without foam problems or significant coagulum formation.

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

85

FIG. 5.6. Diagram of spray or thin film stripper.

5.4.4 Stripping of Bulk (Mass) PVC In principle, the elimination of the aqueous phase removes one of the rate controlling steps as indicated by Fig. 5.1, namely, the transfer of VCM from the PVC to the water and from there to the gas space. This concept is the subject of a number of patents30 in which VCM is removed from suspension polymerisation wet cake rather than from slurry. The mass process apparently has the same advantage but the absence of any water and, hence, steam to act as a carrier gas, means that VCM removal is controlled by the quality of the vacuum achieved. On a large scale it is not easy to obtain a very good vacuum economically and higher residual VCM concentrations result. The addition of some water31 or organic non-solvents18 has been proposed. In spite of these additional problems VCM removal from mass PVC is now carried out efficiently and low VCM concentrations in the polymer sold to PVC fabricators are achieved. ACKNOWLEDGEMENTS Thanks are due to D.C.M.Squirrell, L.Ruddle and A.R.Jeffs for their help on analytical procedures, and to J.Stafford for information on VCM toxicity. REFERENCES 1. 2. 3. 4.

VIOLA, P.L., Med. Law., 61, 174 (1970); VIOLA, P.L., BIGOTTI, A. and CAPUTO, A., Cancer Res., 31, 516 (1971). MALTONI, C. and LEFEMINE, S., Environ. Res., 7, 387 (1974). CREECH, J.L. and JONSON, M.N. J., Occup. Med., 16, 150 (1974). GRICIUTE, L., Environmental Carcinogens, Selected Methods of Analysis, Volume 2. Vinyl Chloride, IARC Scientific Publication No. 22, Lyon (1978), pp. 3–11.

86

5.

6. 7.

8.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

R.H.BURGESS

CAPUTO, A., VIOLA, P.L. and BIGOTTI, A., IRCS, 2, 1582 (1974); HOLMBERG, B., KRONEVI, T. and WINELL, M., Acta. Vet. Scand., 17, 328 (1976); MALTONI, C., Environmental Pollution and Carcinogenic Risks, IARC Scientific Publications No.13, Lyon (1976), pp. 127–34. STAFFORD, J., personal communication; BARNES, A.W., in Vinyl Chloride and Safety at Work, BPF/PRI Joint Conference, British Plastics Federation, 28 May l975. SQUIRRELL, D.C.M. and THAIN, W., Environmental Carcinogens, Selected Methods of Analysis, Volume 2. Vinyl Chloride, IARC Scientific Publication No. 22, Lyon (1978), pp. 19–142; The Determination of Vinyl Chloride, A Plant Manual, 3rd Edition, Chemical Industries Association, London (1977). CRIDER, L.B., An Instrumental System for the Monitoring of Vinyl Chloride Monomer in Plant and Laboratory Environments, B.F.Goodrich Chemical Company, Avon Lake, Ohio, USA (1974); BAKER, G.L. and REITER,R. E.,Am. Ind. Hyd. Assoc. J., 38,24(1977); FIELD, M.A. and MOORE,R. C., Conf. Chem. Soc., London, December 1977. BERENS, A.R., Polymer Preprints, 15,203 (1975); PATEL, C.B., GRANDIN, R.E., GUPTAR, R., PHILLIPS, E.M., REYNOLDS, R.E. and CHAN, R.K. S., Polymer J., 11, 43 (1979). BERENS, A.R., Polymer Preprints, 15, 197 (1975). BURGESS, R.H., unpublished work. BASF, German Patent 2541372; SHIN ETSU, Belgian Patent 779861. HULS, German Patent 2528950. SIGMA, British Patent 1524492. FIRESTONE, US Patent 4000355. ZICHY, E.L., unpublished work. HAYDUK, W. and LAUDIE, H., J. Chem. Eng. Data, 19, 253 (1974). ELECTROCHEM. IND., Japanese Patent 1017950; SUN ARROW CHEM. K.V., Japanese Patent 1059986; UNILEVER, German Patent 2612096. MITSUI, Japanese Patent 1028890; MITSUI, Japanese Patent 1053588. PATEL, C.B., HONEK, R., GUPTAR, R. and CHAN, R.K. S., J. Polym. Sci., Polymer Chem. Edn., 17, 3775 (1979). SOLVAY, Belgian Patent 793505; HOECHST, German Patent 2429777; HULS, Belgian Patent 832866; TEKKOSHA, Japanese Patent 1055390. GOODRICH, Belgian Patent 843624; HOECHST, Belgian Patent 866469. HOECHST, German Patent 2521780; ICI Australia, Belgian Patent 857124. BORDEN, US Patent 4168373. STAUFFER, German Patent 2731186. WACKER, German Patent 2744462. B.P.CHEMICALS, Belgian Patent 853871. ICI, British Patent 1553829; TENNECO, US Patent 4031056; FIRESTONE, Belgian Patent 853357; DUNLOP, British Patent 1090976. HOECHST, US Patent 4158092; GOODYEAR, German Patent 2842868; KUREHA, US Patent 4032497; NIPPON OIL, German Patent 2162860. SHIN ETSU, Belgian Patent 831864; HULS, Belgian Patent 830689; NARA, Japanese Patent 1089587. RHONE-POULENC, German Patent 2612748.

Chapter 6 ISOLATION PROCESSES FOR PVC V.G.LOVELOCK Consultant, Welwyn Garden City, UK

6.1 INTRODUCTION Of the five established processes for the polymerisation of vinyl chloride monomer (VCM) two, the emulsion and microsuspension processes, can be grouped together for the purposes of discussing isolation of the polymer. Both require the removal, normally by hot air drying, of a large quantity of water, to produce a saleable product. The water to be removed may be as much as 2·5 times the weight of dry PVC produced. A third process, suspension polymerisation, produces a slurry of high water content from the reactor but the particle size and type is such that 60–80% of the water can be removed mechanically before hot air drying. A fourth process, solution polymerisation, as the name suggests, requires that an organic solvent be removed in order to isolate the PVC. This process is operated industrially only on a limited scale and is not dealt with here. In the fifth process, the mass or bulk process, the polymerisation occurs in the presence of VCM only and no water is involved. The polymer precipitates from the VCM and after removal of residual VCM the polymer is passed directly to the finished product handling. Consequently its isolation is perhaps much simpler than that of the other two main process types. 6.2 SUSPENSION POLYMERISATION 6.2.1 Treatment Prior to Drying The suspension polymerisation process produces a slurry in which the coarse grains (granules) of about 140 µ m mean size settle reasonably rapidly. In common with the emulsion process, the slurry can contain a small proportion of aggregates which have to be removed before further processing. In recent years problems associated with these coarse agglomerates have been reduced by the introduction of autoclave build-up suppressants so that the strainers used for the removal of build-up have been reduced in size. Such strainers normally consist of fairly coarse meshes of 1–3 mm aperture size and can be the conventional ‘candle’ type or flat plates such as are used in filter presses. Since the carcinogenicity problem of VCM has been recognised precautions are taken to minimise manual handling of the small quantities of coarse agglomerates by using automatic cleaning systems and by the provision of adequate ventilation.

88

V.G.LOVELOCK

FIG. 6.1. Effect of grain type on drying of suspension PVC.

Commercial filters and strainers are available for the process but many manufacturers prefer to design their own devices to deal with their specific products. All strainers and pipework configurations are designed so that slurry speeds are high enough to avoid product sedimentation and so that there are no dead spaces. Adequate water flushing facilities must be provided to ensure that no slurry remains in the devices or pipework should the apparatus be shut down. Since the granular PVC is separated from the aqueous phase before drying, dilution with extra water is unimportant provided the quantity is not excessive. After straining, the slurry is normally stored in large holding tanks which can contain a number of individual autoclave batches. This storage method enables a degree of blending to take place so that the effect of any small batch-to-batch variation is reduced to a minimum. 6.2.2 Morphology The morphology and size of suspension polymer grains is determined during the polymerisation process (see Chapters 1 and 7), it is unaffected by drying unless very high temperatures are used, but the morphology of the grains does affect the ease with which a given polymer can be dewatered and dried. Suspension polymer grains vary quite considerably in porosity. Any dewatering processes prior to drying are unable to remove the water trapped in the pores: thus, the higher the porosity of the grains, the more water must be removed by drying. On the other hand, the moisture level at which the drying curve changes from constant rate to falling rate is reduced. This is shown in Figs. 6.1 and 6.2, which apply only when the grains are dispersed in the drying gas. Figure 6.1 shows the large change in the rate of loss of water once surface water is all removed and only water trapped within the non-porous granules is left. This is shown more clearly by Fig. 6.2 where the change in the water loss rate, here expressed as the matter transfer coefficient K multiplied by the surface area of the PVC grains A so that a constant weight of PVC can be used, is plotted against the moisture content of the PVC. This shows that both porous and non-porous PVC lose water at the same rapid rate at high moisture content. However, these rapid rates fall dramatically at below 0.6% for non-porous PVC and at below 0.25% for porous PVC. This has a TABLE 6.1

ISOLATION PROCESSES FOR PVC

89

FIG. 6.2. Change of matter transfer when drying suspension PVC. Effect of Polymer Properties on Dewatering and Drying Water in wet-cake Wet basis Dry basis 30 42·8 25 33·3 20 25·0 15 17·6 10 11·1

Porosity of PVC (%) 37 28 20 12 6

Approx. drying parameters Temp. (°C) Time (s) 50 2–5 50 2–5 60 20–60 75 60–600 75 >2000

large effect on the drying conditions required to produce the 0.1–0.2% residual moisture needed for successful processing of PVC (Table 6.1). The Table indicates that drying can be accomplished in a few seconds for the porous grades. However, since the moisture level at which constant rate drying changes to falling rate increases as the polymer porosity falls, the drying time for the less porous grades must be considerably extended to achieve the desired levels of residual moisture in the product. Obviously, the drying rates and hence the times required could be reduced by using elevated temperatures but PVC is thermally sensitive. PVC temperatures in excess of 80°C are undesirable for any extended period of time, say about l0min, otherwise the material discolours. In addition, copolymers become soft and sticky at high temperatures so that the powdery material is likely to fuse and become an unmanageable lump. All the constraints described above limit the number of drier types which can be successfully used in industry. The preferred types are described in Section 6.2.4. 6.2.3 Solids Separation and Dewatering When a wet-cake is produced by mechanical separation from a PVC slurry its residual water content is located in three places (see Fig. 6.3). Water is contained in the grains due to their porosity (A), water is held by surface tension forces in irregularities on the surface of the grains (B) and small menisci of water are held by surface tension forces between the grains where they touch (C). The early PVC process industry

90

V.G.LOVELOCK

FIG. 6.3. Location of water in PVC wet-cake.

used vacuum filtration to separate the PVC solids from the aqueous phase of the slurry. This method of filtration did not apply sufficient forces to remove effectively the water from locations B and C, so the wetcake water content was not reduced to its minimum level, thus making subsequent drying costs unnecessarily high. Now, centrifugal methods of separation which rely on the density difference between PVC and water are used almost universally. Figure 6.4 serves to illustrate the basic construction of a typical continuous centrifugal separator. In all centrifuges of this type, the slurry containing the suspended solids enters the machine through a feed pipe, into a section which flings it to the inside periphery by centrifugal force. The position of the discharge end of the slurry feed pipe can be altered so that the fluid impinges on the inside of the bowl at a point where the pond meets the ‘beach’. This is usually the position where optimum separation takes place and where the bulk of the water phase is removed. A screw conveyor moves the solids towards the wetcake discharge end where they are thrown out into a wet-cake collecting hood. The liquid phase flows in the opposite direction to the solids at a level determined by weirs which are adjustable so that optimum conditions can be obtained. This part of the process determines the clarity of the filtrate as, obviously, the longer the fluids are in this portion of the machine, the greater the proportion of suspended solids which will be separated by centrifugal force and conveyed to the wet-cake ‘beach’ by the screw. Clearly, the longer the cylindrical section of the machine, the more effectively the filtrate is clarified. On the other hand, the longer the conical section or the longer the time the wet-cake ‘beach’ is draining, the drier the wet-cake will be. All machines have a conical section in order to produce a reasonably dry wet-cake but not all contain a cylindrical section. In fact commercial machines give a compromise between the highest filtrate clarity and the driest wet-cake. In principle, if a screen section is added to the conical section, a drier wet-cake can be produced. Such centrifuges are used in the PVC industry but the filtrate from the screen section contains some PVC fines and may have to be returned to the main slurry. All centrifugal methods of separation of solids from slurries can remove the maximum amount of grain surface moisture (B) and all the grain interstitial moisture (C) (Fig. 6.3). Therefore, they are capable of producing wet-cake of the minimum water content. Reference to Table 6.1 shows that for efficiently dewatered PVC slurry there are large differences between the wet-cake residual water concentration for different types of PVC and that this is related to granule porosity. This is due to water in the pores shown as A and B in Fig. 6.3. Clearly, this implies that granule porosity can be determined by examination of the residual moisture content of a wet-cake when separated in ideal conditions. When such a state exists the residual moisture is contained within the grains and the wet-cake will behave like a free flowing powder.

ISOLATION PROCESSES FOR PVC

91

FIG.6.4. A typical cetrifuge for dewatering PVC slurry.

Normally, there is no intermediate storage of wet-cake between the dewatering devices and the drier and the feed must be matched to the thermal loading capability of the drier.

92

V.G.LOVELOCK

The power consumption of centrifuges of the solid bowl type comprises three elements, power used to rotate the machinery, an element which is determined by acceleration of the feed to the speed of the machine and the torque needed to screw convey the solids from the bowl. The last is generally the largest component and can be used to determine the solids throughput and hence control the output of the machine into the drier. Small deviations in thermal load to the drier units can then be controlled by drier variables, notably the temperature gradient. 6.2.4 Drier Types Used Any type of drier suitable for isolating granular materials of mean size from 30 µ m to 1 mm can be used. Typically PVC suspension polymer has a mean size in the range 100–200 µ m but finer grades down to 30 µ m are produced. Table 6.2 gives a list of some typical types of drier with their advantages and limits. Continuous tray driers, band or through circulation units and batch or continuous ovens are not used industrially and are not discussed in the Table. All the units use air as the drying medium since PVC has not been considered to present a significant dust explosion hazard. Recent work by Bartknecht of Ciba Geigy has shown that PVC dust explosions can occur given sufficiently high ignition energies, i.e. 1500–10000 J for powders of TABLE 6.2 Advantages and Limits of Principal Drier Types Drier type 1. Rotary Driers (a) Indirect heat (Fig. 6.5a)

(b) Direct heat (Fig. 6.5b)

2. Flash Driers (a) Pneumatic conveying (Fig. 6.6)

(b) Spray (Fig. 6.9)

3. Fluidised Bed Driers (a) Direct heat (Fig. 6.7)

(b) Indirect heat (Fig. 6.7)

Advantages

Limits

Economical to operate. Can dry all grades of PVC.

Large rotating assembly. Difficult to clean. Prone to corrosion of the heating tubes. Difficult to control. Large rotating assembly. Tendency to polymer build-up round inlet air ports and through end seals. Less economical than (a).

Easy to operate and to clean. Can dry all grades of PVC.

Easy to operate and to clean. Very simple construction. Very high evaporative capacity. Useful when not required for emulsion polymer drying.

Occupy a small space. Easy to operate and to clean.

Occupy a small space. Can dry all grades of PVC. Economical to operate and maintain.

Can only dry porous PVC grades. High air temperatures cause danger of degrading some PVC. Can only dry porous PVC grades. Very large installations with complex atomisation of wet-cake. High air temperatures cause danger of degrading some PVC. Can dry all grades of PVC but has to be fed with a partially dry product. Degradation on fluidising plate is possible. Corrosion of heating tubes requires special attention. Time consuming

ISOLATION PROCESSES FOR PVC

Drier type

4. Combined Flash/Long Residence Time Driers (a) 2(a) or (b) plus 1(b) or 3(a)

Advantages

Limits to clean. Tendency to overdry product.

Very flexible in operation. Can dry all grades of PVC economically.

Has disadvantages of each component.

93

20–100 µ m particle size.1,2 However, these very high ignition energies suggest that there is very little chance of an explosion when drying normal particle size PVC made by the suspension process. This is borne out by the fact that no dust explosions involving PVC powder alone have occurred in nearly 50 years of PVC manufacture. At the same time it is important to recognise that the presence of other materials such as known inflammable dusts, plasticisers or significant quantities of inflammable gases, e.g. VCM or fuel gas, can reduce the ignition energy to a marked extent. 6.2.4.1 Indirect Heat Rotary Driers In indirect heat rotary driers (Fig. 6.5a) the thermal energy necessary to dry the PVC is supplied by surfaces heated by steam or hot water and the PVC conveyed from the feed to the discharge end by the slope of the rotating barrel. The driers are normally countercurrent, cool air being introduced at the dry product discharge end and being exhausted at the wet-cake feed end. They are very effective for drying materials of very low porosity as the polymer dwell time in them can be prolonged to up to about 90 min by using deep weirs at the discharge end. In this type of application low heat transfer surface temperatures of the order of 80°C are used to avoid overheating the PVC. Low temperatures are also necessary to avoid corrosion of the heating surfaces. Indirect heat, countercurrent units are very economical with regard to service usages but can be overloaded very easily. The countercurrent drying air can become saturated with water vapour at a higher temperature than the feedstock in the barrel at a point remote from the wet-cake feed and air exhaust point. Water vapour condenses from this air on to the cooler feedstock so that the solids become wetter and this super-wet material traverses to the polymer output point where it is discharged at high moisture level. In this way transient pulses of wet material are discharged. Very precise calculations must be carried out from experimental data obtained from the actual machine and product if it is desired to operate an indirect heat rotary drier in optimum economic conditions. These must be adhered to quite rigidly but this becomes very difficult industrially so this type of drier has not been favoured in recent times. 6.2.4.2 Direct Heat Driers In direct heat driers (Fig. 6.5b) all the thermal energy required is supplied by the sensible heat of the incoming, heated, drying air as the nomenclature implies. They consist of large rotating barrels up to 2–5 m in diameter and 12–20 m long, provided with ‘lifters’ and, sometimes, baffles on the inside to facilitate tumbling and blending of the materials being dried. The position and number of the lifters and/or baffles is normally determined from experimental data so that the solids’ dwell time, determined by the amount of nearly dry material at the discharge end of the barrel, is sufficient for drying and the removal of the last traces of residual monomer. Air dwell times of 12–60 s and polymer dwell times of up to 30 min are

94

V.G.LOVELOCK

FIG. 6.5. Typical rotary driers for suspension PVC.

normal. This type of rotary drier is cocurrent when used for PVC, the direction of flow of the drying gas being the same as for the drying solids. Two types of unit exist, those where all the product and the exit air are discharged from one central port or those where some of the product is discharged from the barrel over a weir into a conveying system, the remaining fine fractions being conveyed from the drier with the exit air. In both cases PVC is separated from the drying air by means of cyclones. Bag filter units are rarely used because the grains are coarse and dense enough for satisfactory separation by the cheaper cyclones. Both types of rotary drier operate at air inlet temperatures of 140–180°C. Higher temperatures can result in problems with build-up and burning around the hot air entry ports of the drier. Air exit temperatures in the range 60–90°C are used and these result in product exit temperatures of 50–85°C, these temperatures being chosen to ensure adequate drying and residual monomer removal. Air speeds in the barrel are adjusted so that the bulk of the solids are air conveyed down the barrel to a designed extent. For suspension PVC these air speeds range from 0.3–1.2m/ s. Barrel rotational speeds in the range 2–10rpm are normally used. 6.2.4.3 Flash Driers These types of unit depend on producing a finely dispersed suspension of the product grains in the hot drying air. They are all of the direct heat type and are cocurrent of necessity. Inlet and exhaust air temperatures similar to direct heat rotary driers are used but both the air and, of course, the polymer dwell times are very

ISOLATION PROCESSES FOR PVC

95

FIG. 6.6. Typical flash drier.

short, of the order of 4–30 s and, because of this and the dynamics of drying, polymer temperatures of the order of 45–60°C are produced. These driers induce very little thermal treatment of the product but suffer from the disadvantage that they will only remove water in the constant rate drying period and, therefore, are only satisfactory for polymers of high porosity. Flash driers can be divided into three types but the actual mechanism of operation is very similar. The first group, so-called pneumatic conveyor driers, consists of just a tube through which the drying air passes, carrying the polymer with it (see Fig. 6.6). The second type consists of spray driers (see Fig. 6.9) with specially designed atomisers. This application is very useful where spray drying capacity in excess of emulsion drying needs is available but suspension polymer isolation capacity is required. The third type consists of a series of what can be best described as cyclones in which the polymer solids spin on the periphery so that a proportion is always held longer than the average dwell time. Because the action is cyclonic and the granules spin due to centrifugal force, the heavier, that is the wetter, particles tend to be held up longer than the others. Thus more uniform drying is achieved. Since an air conveying drier of the type shown in Fig. 6.6 is fitted with cyclones necessary for product separation from the drying air the extra dwell time in the cyclones may make possible a foreshortening of the main drying tube. It is essential that the feedstock to all flash driers is of a friable nature and completely free from even loosely agglomerated lumps. This is because all flash driers depend on dispersing the solids uniformly and finely in the drying gas. Many drying problems associated with poor quality feedstock properties can arise and are not always truly identified. Very careful attention must be given to the design of the dewatering devices so that the moisture level is the minimum possible attainable and so that wet-cake friability is obtained.

96

V.G.LOVELOCK

FIG. 6.7. Typical fluid bed drier.

6.2.4.4 Fluid Bed Driers The simplest description of this type of drier (Fig. 6.7) is that of a tank with a perforated bottom through which air passes from a plenum chamber. The finely divided solids which occupy about one-third of the volume available above the fluidising plate are held in a condition which resembles a boiling fluid. In this way there is intimate contact between the drying air and the solids such that heat and matter transfer are particularly efficient. The design of the perforated fluidising plate is particularly important but is normally a compromise between the cost of making the plate and its required efficiency of operation. Thus it is found that the plate design can vary quite considerably between various drier manufacturers, some preferring a simple steel plate with small circular perforations, some using especially manufactured perforated plates such as ‘Conidur’, and yet others having a series of specially designed nozzles, mounted on the plate, to perform the necessary air introduction. Each design is specific to its operational requirements and must be chosen for its efficient function in the circumstances needed. For instance, the nozzle type plate will satisfactorily fluidise powders which are incapable of fluidisation by a simple perforated plate. However it is expensive to make, having about 100–250 individually made nozzles to the square metre. The two drying mechanisms used in fluidised bed driers are analogous to those used in rotary driers. There are the direct heat types using the sensible heat of the incoming air to supply all the energy for drying. Alternatively heated tubes or panels in the bed of fluidised solid can be used to supply up to 90% of the heat needed for drying, the balance being supplied by the sensible heat of the incoming air. Only the latter, indirect heat or ‘contact’ fluidised bed units, can dry materials delivered directly from the dewatering devices. Direct heat units are used in conjunction with flash driers which remove the bulk of the moisture from the feedstock so that the fluidised bed drier can be used to remove the last traces of water in the falling rate drying period. Both types of drier are widely used in the PVC industry.

ISOLATION PROCESSES FOR PVC

97

6.2.5 Product Collection The size range, shape and density of suspension PVC granules is such that high efficiency cyclones are capable of separating the airborne material from the drier exhaust gas. The requirements of the authorities concerned with elimination of environmental contamination can be met and a maximum concentration as low as 30mg/m3 can be achieved easily with good design. Most drier manufacturers supply driers as package units which include dust collection. The design of cyclones can vary a little between suppliers and the subject of design is dealt with very adequately in the literature. 6.2.6 Sieving, Product Conveying and Packing Suspension polymers are very easily sieved and conveyed and their properties are very close to those of fine sand in the dry state. Their moisture content remains low even on exposure to atmospheric air so that large static electrical charges can be generated on the granules and on any equipment handling them. All ducting is normally well earthed. Because the granules tend to develop mixed charges, problems due to sieve blinding are very apparent if suitable precautions are not taken. Antistatic agents are very effective in removing the electrical charges but they adversely affect the desired properties of the PVC. Corona discharge devices can be used over sieves but air humidification is the most widely used method. Bulk storage and transport are used to a considerable extent for suspension polymers and normally only the smaller tonnages are packed in paper sacks for delivery to customers. Silos of up to 1000t capacity are used for storage on the PVC plant and at the fabricators’ works. Road transport is by tankers of up to 25t capacity while in rail transport 60t tankers are normally employed. 6.3 EMULSION/MICROSUSPENSION POLYMERISATION 6.3.1 Latex Filtration Microsuspension and emulsion polymerisation processes produce suspensions of very small PVC particles, up to 1.0 µ m, suspended in an aqueous phase. This is normally referred to as a latex. These latices are normally of low viscosity and contain no oversize material. However, flocculation can be caused by excessive mechanical shear or by the addition of cationic electrolytes to the anionic latex. Flocculation will create lumps of coagulum and at worst a mud-like, unhandlable material when there is virtually complete coagulation. Subsequent handling of flocculated latices is difficult and certain properties of the final product are impaired by the presence of >150 µ m hard aggregates. Because of this the oversize material is invariably removed by a straining process before the latex is passed to further unit operations for isolation. The primary strainers for PVC latices take the form of sloping sieves supported on frames, special selfcleaning sieves or cloths made into the shape of filter candles using radial flow and with a water back-flush cycle. Standard steel cloths of aperture diameter 150 µ m–l mm are used.

98

V.G.LOVELOCK

6.3.2 Latex Concentration Since the emulsion and microsuspension processes typically produce latices of 30–46% solids of a fine particle size which cannot be concentrated by filtration or centrifuging, their drying involves the use of considerable thermal energy. Efforts to pre-concentrate such latices prior to drying have been in the direction of electro-decantation, flocculation and coagulation to expel water, dialysis using semipermeable membranes and simple vacuum concentration. To date, the first three of these processes have met with little success, due, it is thought, to the very strong forces of surface tension in the interstices of the flocculated particles. For instance it has been shown that the moisture content of a coagulum of PVC particles can be greater than that of the original latex. The additional water, of course, originates from the electrolyte solution which has to be added to cause the material to flocculate. Electro-decantation and dialysis can suffer from the fact that the membranes used become coated with a clay-like layer of aggregated latex particles which is not easily removed and consequently the membrane then becomes less and less effective. It is well known that PVC latices flocculate to varying degrees on freezing; hence this process is a possible method of concentration by transforming the suspension of fine particles of the latex into a suspension of coarser granules. These may be subsequently filtered and dried as a granular wet-cake consisting of angular particles. The friability of the granules depends on the PVC particle size of the original latex, the finer particle size feedstocks, 0.01–0.2 µ m, producing the stronger granules. The process would give an energy saving in that apart from sensible heat, it requires only the latent heat of freezing, which is much less than the latent heat of evaporation of water. However, different latex feedstocks produce granules which do not have all the product properties needed and thus the process has not been used extensively in industry. Concentration by vacuum evaporation can be more successful so that, using film type evaporators, the latex concentration can be increased to about 55% solids (82% moisture, dry basis). The obvious question which arises is why the polymerisation process has not been developed to produce latices of a higher solids content, say 50–55%. Clearly, it should be possible to make the particle size distribution suitable to produce a low viscosity latex, say 5–10 cP, at these higher solids levels. Indeed, latices of 55–57% solids have been produced commercially but, as will be described later, latex particle size distribution and the feedstock solids concentration affect some of the properties of the dried product and, therefore, this approach is not widely used. 6.3.3 The Properties of the Final Product The fact that, for emulsion polymers, the drying process coupled with milling creates PVC of the desired readily manageable granule (grain) size, say 5–100 µ m, is not always sufficiently appreciated. In addition, it is the way in which these grains behave in subsequent processing which determines their later use. Three widely differing requirements for emulsion polymers serve to illustrate how different the requirements of the dried/milled polymers can be (Table 6.3).

ISOLATION PROCESSES FOR PVC

99

TABLE 6.3 Different PVC Types Required for Various Applications Property

Product requirements for: Plasticised film and rigid extrusion

Latex particle size (µ m) Dried product grain shape Product grain strength Grain max. size (µ m) Grain mean size (µ m) Grain min. size (µ m)

0.02–0.2 Spherical and angular Not friable ca 100 50–60 Preferably 30 (not very dusty) Possible drying processes Spray drying Drum drying Freeze drying Preferred drying processes Spray drying Drum drying Milling process No

Battery separators PVC pastes 0.02–0.2 Spherical Hard ca 100 ca 35 3–4

0.2–1.0 Spherical and angular Varying degrees of softness 60 15–25 Size equivalent to latex particles

Spray drying

Spray drying Drum drying

Spray drying

Spray drying

No

Yes

6.3.4 Emulsion Polymer Drying Processes 6.3.4.1 Freeze Drying Table 6.3 notes three possible drying processes for emulsion polymers. Freeze drying is very expensive to operate and is not used industrially. Any additional advantage of the process so far as product properties are concerned, i.e. greater grain friability, are outweighed by the disadvantages such as excessive dustiness and poor powder manageability. 6.3.4.2 Drum Drying The drum drying equipment consists of a heated rotating drum which dips into a tray of the latex feedstock. The temperature, usually about 100–140°C for PVC drying, causes a film of polymer to adhere to the drum which is adjusted so that the film is dried when a rotation of about 260° has taken place. A scraper then detaches the dried film in the form of flakes and fine powder. The rate of drying, the thermal treatment of the product and the thickness of the film depend on the rate of rotation of the drum as well as its temperature and, of course, the solids content of the latex feedstock in the tray. With this process it is relatively difficult to achieve the correct balance between an economic drying rate and the needed thermal treatment. The product physical form is suitable for any subsequent compounding which does not specifically need spherical particles. It is usually very dusty, and in the case of paste polymers where the feedstock primary

100

V.G.LOVELOCK

particles are large (0.2–1.0 µ m), very friable. This latter property renders the material eminently suitable for blending with products dried by other means to form a mixture which will give the correct amount of granule breakdown when mixed with plasticiser to make PVC paste. Although the process is economical in terms of capital investment and service costs, it is not used to any great extent in industry due to the difficulty in producing product of the correct quality to be used by itself. 6.3.4.3 Spray Drying As the name suggests the operation of spray driers depends on introducing a fine spray of liquid feedstock into a hot drying gas which is usually air, but can be any gas into which evaporation will take place. The process has two major advantages over others which deal with the same type of feedstock. Firstly, because the sprayed droplets, which can be up to about 0–5 mm are dried as separate entities in the gas the resultant dried product is in the form of a readily manageable powder. More importantly in the process, because the droplets are small, the act of spraying creates a very large surface area per unit mass of feedstock. A simple calculation shows that the surface area in m2/litre of feed liquor is (6.1) where d is the droplet diameter in cm and p is the density of the feedstock. Thus 1 litre of water sprayed into 200 µ m droplets would produce a surface area of 30m2, which represents a very large surface for heat and matter transfer. The large surface determines that the constant rate evaporation occurs at a temperature only a few degrees removed from the adiabatic wetbulb temperature, which is of the order of 55°C at an air/gas drybulb temperature of 250°C. In this way the PVC is subjected to very little thermal treatment. The parameters involved in heat and matter transfer, thermal treatment and the drying time of droplets are adequately treated in the literature,3,4 reference 3 contains many further references to other workers in the field. The droplets dry to surface dryness in a very short time, of the order of 70–140ms. The dried product varies in form from hollow spheres (cenospheres) from PVC latices of up to 0.2µ m particle size to solid spheres (plerospheres) from latices of 0.5–1.0 µ m. They are more or less friable, depending on the size of the primary particles in the feedstock, the largest primaries making the more friable products, and the spray drier air exit temperature, the higher the temperature the harder the product. Spray driers

Line diagrams of typical drying installations are given in Fig. 6.8. There are two basic gas circulation systems, one balanced and the other not. In the former system inlet air as well as exit air fans are used for drying and either inlet air and exit air or exit air fans only (6.8(b)) can be used for conveying. Only exit air fans are needed for the unbalanced system (6.8(c)) and this makes the installation much cheaper but it suffers from considerable disadvantages. The two-fan system enables the balancing of any chosen part of the system to atmospheric pressure so that the design costs of the drying chamber can be reduced. This is because, if it is balanced to atmosphere, it does not require the heavier gauge metal which would be needed to withstand the small vacuum which is created by the unbalanced system. Obviously the balanced system is also far less critical of the physical state of the inlet air heater assemblies and it is easy to compensate for filter choking during operation by increasing the air input to the inlet fan to maintain a constant draught in the drying chamber. Therefore, it is easier to maintain consistent product quality using a balanced air throughput system.

ISOLATION PROCESSES FOR PVC

101

FIG.6.8. Gas flow system for spray dries.(a) Cocurrent balanced system—low powder load in gas from chamber. (b)but all exit gas from chamber: as(a) but all exit gas from chamber to bag filter unit i.e. no intermediate cyclone. (c) Cocurrent unbalanced system: as (b) but with no inlet air fans to drier or to conveying system.

Operation under an inert gas is possible by burning a combustible gas in air but this is not generally used

102

V.G.LOVELOCK

for PVC. Normally, air is used and the throughput rate is manually set and kept constant, thus ensuring a fixed gas dwell time. The drying rate is controlled by the temperature gradient in the drying chamber, the precise conditions being determined by the morphology of the PVC granules required. Atomisation to produce the necessary spray can be produced by a variety of devices which are described later. The dried product must be separated from the drying gas and this is achieved by a choice of the normal dust separation techniques such as cyclones, bag filters, wet scrubbers or, on rare occasions, electrostatic precipitators. Although cyclones have a major economic advantage in low installation and maintenance costs, they become very inefficient (recovery of the order of 70–80 %) in collecting low density (0.95–1.1) particles such as PVC of less than about 10 µ m. In some instances they are used followed by a bag filter or wet scrubber system. This reduces the dust loading and hence the size of the bag filter or wet scrubber. Product from the bag filter is normally returned to the main dry product stream but can be kept separate if desired either for special sale or to avoid contamination of the main product. Effluent from the wet scrubber can be returned to the latex feed when it has reached a suitable concentration. Of these two options bag filters are normally preferred. Bag filter units are capable of separating all particles down to 1 µ m at 99–9 % efficiency but are expensive to install and to maintain because they occupy a large space and contain many filter bags, which have to be changed periodically, say about once a year. For example, a bag filter designed for a 10000 tpa drier might have a bag area of 500 m2 and consist of up to 500 felt bags. The mechanisms used for cleaning the bags can be divided into two groups. The ‘Hersey’ type uses a ring with a series of slots through which compressed air is forced through the bag. The ring traverses up and down the outside of the bag and is of slightly smaller inside diameter so that the bag is slightly deformed. The dust-laden gas enters on the inside of the bag from the top while the cleaned gas is discharged into the cabinet holding each group of bags. This type of unit has the advantage that the gas being filtered does not have to be stopped while the bags are being cleaned. Other types of units employ various methods of shaking the bags during a discrete cleaning cycle during which the gas being cleaned is usually stopped from flowing through each cell. The dust-laden

ISOLATION PROCESSES FOR PVC

103

FIG. 6.9. Typical spray drier chamber assemblies, (a) Cocurrent: spinning disc atomiser, (b) Counter-current: nozzle atomiser, (c) Cocurrent: nozzle atomiser.

air travels from the outside to the inside of the bags so that all the bags have to be supported on wire cages to stop collapse. Earlier types of unit used a simple mechanical shaking mechanism which required fairly frequent maintenance. Modern units employ a system of ‘pulsed jet’, shock wave techniques. This consists of sending a sudden pulse of compressed air through a jet at the top and on the clean air side of the bags at about 2–5 min intervals. The jet of air creates a shock wave which traverses the inside of the bags and simulates shaking. Atomisation

Location in the drying chamber: The performance of the atomiser is of importance because it creates the spray of feedstock droplets which determine the grain size range of the product. The location of the atomiser or atomisers in the drying chamber can, together with the droplet size distribution, affect the thermal history of the particles which, as will be seen later, affects the morphology of final grains (Fig. 6.9). Spray driers normally operate in cocurrent conditions but pseudo-countercurrent conditions can be employed to make coarse material with a relatively high degree of thermal treatment. In the commonly used cocurrent types both drying air and latex enter through the top of the chamber. Atomiser design: The design of atomisers takes three basic forms illustrated in Fig. 6.10: (a) spinning discs, in the form of wheels consisting of a series of radial vanes or dishes and cups, (b) pressure nozzles in which the pressure applied to the feedstock provides the energy for creation of the surface area, and (c) twofluid nozzles where a gas, usually air, is used to break up a film of liquid feedstock formed at the nozzle tip. The operation and performance of all types of atomisers used in spray drying have been well studied.3 Spinning disc atomisers require less power than other types, are versatile in use, can be simply gravity fed but with rate control, and are less critical of variations in feedstock concentration or viscosity. The mean droplet size is proportional to about the cube root of the feed rate; hence, for a given evaporation rate, variation in feedstock solids content and thus feed rate have a minimum effect on droplet size. The droplet size is inversely proportional to a power of between 1–3 and 2 of the rotation speed. For comparison of the performance of various disc atomisers on an industrial scale an inverse square law is a close approximation. The discs can only produce a fine mean particle size if they spin at fairly high speeds, of the order of 15000

104

V.G.LOVELOCK

FIG. 6.10. Diagram of various types of atomisers, (a) Spinning disc, (b) Pressure nozzle, (c) Two-fluid nozzle.

to 20000rpm. This subjects them to very high centrifugal forces so that they must be very carefully designed and sometimes use special low-density, high-strength metals such as titanium. It has been shown3 that the droplet size is also proportional to the so-called wetted periphery which, in the case of vaned discs, is a function of the number of vanes, and their height. Obviously the diameter of the wheel also has an influence on performance so that, for a given feedstock, the following relationship can be used to compare the geometry of the two differing wheels if the product made by one is known.5 (6.2) where dm is the droplet mean diameter, Q is the feed rate, D the diameter of the wheel, N and H the number and height of the vanes and S the speed. Pressure nozzle atomisers have been used to some extent in the past but are now becoming less used for PVC manufacture. They are capable of producing a fine droplet mean diameter of about 50 µ m but this requires rather high pressures of up to 210 bar. Normally they are used only where a coarse final product is required. Most nozzles of this type have small ports inside them and, indeed the emergent hole from the swirl chamber, which discharges the spray, is also quite small. Thus, very efficient feedstock filtration is required and this can be difficult for PVC latices, particularly those of low stability. In many instances it is necessary to increase the emulsifier content of the PVC latex feedstock in order to overcome the possibility of coagulation due to the very high shear forces which occur in the swirl chamber. Pressure nozzles are not capable of operating satisfactorily at any more than small turn-down ratios of the order of 0.9. However, since a drier can have up to 100 nozzles it is relatively easy to turn down the output if desired by isolating a number of the nozzles.

ISOLATION PROCESSES FOR PVC

105

Two-fluid nozzles can produce droplet diameters finer than pressure nozzles with considerably fewer problems. Atomisation is produced by violent mixing of the secondary gaseous fluid with the feedstock liquor immediately exterior to the nozzles. No very small feed ports are required and no particularly fine filtration techniques are needed for the liquid feedstock, which can be gravity fed to the units although with accurate rate control. Droplet size is governed to a large extent by the air/latex feedstock weight ratio, usually of the order of 1.0–1.25. This type of atomiser is expensive to operate compared with spinning discs because sophisticated, heavy duty, air compressors with special pressure/flow characteristics and control have to be used. Atomisers using low air pressures and somewhat lower air/feedstock rates are available but they produce large droplets, and, hence, a coarse product which is suitable for limited applications only.6 The solids content of the latex has an effect on the final grain size and this and other factors have to be taken into account for the detailed design of the nozzle. 6.3.5 Product Morphology The drying process is responsible for the physical form of the grains of emulsion and microsuspension polymers. The structure of dried material consists of more or less coherent agglomerates of the primary latex particles. The glass transition temperature (TG) of pure PVC is of the order of 82°C, that is close to the drying temperature, so that thermal treatment can be used to make harder finished grains to some extent. Residual monomer7 and other PVC soluble contaminants may affect the TG and so the type of particle produced. The quantity and nature of water soluble additives, including emulsifiers, can also affect hardness of the grains; for instance, if the additive is of an oily or sticky nature in the dry state it can operate effectively as a type of glue. On the other hand, additives (viscosity depressants) can be used to reduce the adherence of the primary particles in order to assist breakdown of the grains when making PVC pastes, which as described in Chapter 3 consist of a dispersion of PVC particles in plasticiser. Very small menisci of fluid exist between the primary particles in the grains during the last period in the drying process. The smaller these menisci become the smaller is the radius of curvature and, hence, surface tension forces become of importance in pulling the particles together. These forces can be significant in creating the strength of the agglomerates of the primaries which form the grains. It is thought that this is a major contributory factor in determining the fragility of the grains and this is borne out by the fact that latices of high surface tension, 55–65 dynes/cm, consisting of primaries of up to 0.2 µ m in size, form relatively hard cenospheres with glassy walls from the spray drying process. They form hard, brittle films from a drum drying process and small angular granules resembling crystals from a freeze drying process. On the other hand, latices consisting of primary particles larger than 0.5 µ m or mixtures of particles with a predominating quantity of the larger sizes form plerospheres from spray driers, very friable angular agglomerates which give rise to considerable dustiness from drum driers and an incoherent mass of dust and small, fragile agglomerates from the freeze drying technique. There is considerable direct evidence5 that suggests that dried PVC from the drum process and the interior of the spherical grains from spray drying consists of primary particles randomly packed and of a loose structure provided the primary latex particles are greater than 0.5 µ m and so can be seen by microscopic examination. Statistical evidence5 has shown that, when latices of primary particle size less than 0.2 µ m maximum size are spray dried the relationship between dried grain diameter (Dp) and droplet diameter (Dd) is: (6.3) where

106

V.G.LOVELOCK

α=1.8–l.2/(l+0.01M) M being the moisture content of the latex, dry basis, between the limits 1900 % and 82% (latices of between 5 and 55% solids). This relationship suggests that a rigid crust forms on the droplets when the volume solids content of the droplet is about 58.8%, which is far below that for close rhombic packing of uniform spheres, and thus it also suggests a degree of random packing. Microscopical examination of spray dried material shows that it consists of a wide range of grain types ranging from cenospheres with completely unbroken shells, through cenospheres with incomplete shells to agglomerates called plerospheres. In general terms the cenosphere walls become thinner as the feedstock latex solids is reduced and less hard as the latex particle size increases. Generally, plasticiser can migrate freely through these walls, but, in the case of cenospheres with unbroken walls, small bubbles of air are entrapped within them and this limits the total quantity of plasticiser that can be absorbed per unit mass of the powder. Thus, when latex of small particle size and solids contents higher than about 40% is dried, there is an upper limit to which plasticised premixes can be made without them becoming sludgy and asphalt-like in consistency. At the other extreme when latices with a latex particle size >0.5 µ m are spray dried the granules break down during mixing with plasticiser or are already partially broken down by grinding. The size distribution is such that few cenospheres or plerospheres remain and the whole becomes a free flowing mass as described in Chapters 3 and 10. Obviously, intermediate behaviour is possible leading to higher paste viscosity with a high shear rate dependence. 6.3.6 Conveying and Screening The handling characteristics of dried emulsion polymers vary greatly, some being free flowing while others are not. The former can be conveyed by all the normal processes including standard pneumatic conveying and dense phase conveying as well as vibro and mechanical methods. Dense phase conveying is not normally used for the more unmanageable types and mechanical conveyors tend to become choked unless special types are used. Even simple discharge from small storage vessels may be very difficult. Since the drying process can produce small amounts of agglomerated material a sieving or screening process is usually incorporated in the process after drying. Principles similar to those governing conveying problems apply to these screening processes. Reciprocating, vibratory oscillating and rotating screens can be used. All can be made to operate continuously and are usually chosen from the results of experimental work using the specific types of polymer being considered. 6.3.7 Grinding or Milling and Blending Emulsion polymers are used for plasticised PVC film and certain rigid extrusions where very easy processing is needed, more so on the Continent of Europe than in the United Kingdom. They are also used for the manufacture of plate separators in batteries for the motor industry and to some extent in batteries for electrically powered vehicles. All these types of polymer are made by the spray drying process from latices of less than about 0.2 µ m particle size and are of the harder, glassy walled cenospherical grain type. They are not normally subjected to any further processing after drying. Paste polymers, predominantly made from latices of primary particle size up to 1.0 µ m, are ground before sale in most cases, although a few types are used in the paste making process without grinding. Both drum drying and spray drying processes can be used and products of varying grain structure can be blended

ISOLATION PROCESSES FOR PVC

107

before milling, although spray drying under a single set of conditions is normally used. The grinding process is carried out in order to produce a grain size and size distribution which will produce a manageable paste with the shortest possible mixing time with plasticiser and other additives. Hammer, fluid energy and pin type mills can be used, and in some cases the preferred system involves some method of air classification so that the unground or partly ground granules are returned to the grinding process. The product from the milling process is very fine and particularly dusty in nature since there is a significant proportion of particles of about 1–2 µ m. No system of dust separation other than bag filters is used for milled polymer. 6.4 BULK POLYMERISATION The bulk (mass) polymerisation process for making PVC differs from the suspension, emulsion or microsuspension processes in that there is no continuous water phase present. No isolation from a water phase by separation and/or drying processes is involved and the polymer goes directly to the finished product handling stage (see Chapter 2 for further details). 6.5 PVC DUST—POSSIBLE HAZARDS During the handling of PVC polymer powder, e.g. drying, weighing, bagging, etc., operators are exposed to finely divided PVC particles, more generally referred to as ‘PVC dust’. The emulsion process can give rise to particles in the 3–100 µ m range, suspension and mass processes 50–300 µ m and milled emulsion grade PVC 1–50 µ m. Most authorities have regarded PVC dust as a ‘nuisance paniculate’ as defined by the American Conference of Governmental Industrial Hygienists (ACGIH) and the nuisance dust hygiene standard of 10mg/m3 total dust has applied. Over the years, in animal and in vitro studies of PVC dust have been carried out and numerous observations made without any clear guidance for human exposure. Isolated cases have been reported of human respiratory disease alleged to be due to PVC dust exposure. Chest X-rays have been done in numerous PVC factories without any indication of problems until recently. In France, one case of a PVCexposed man whose chest radiograph showed micronodular shadows has been reported.8 In Italy, workers at Padua University9,10 studying Italian PVC workers on behalf of their trade union reported abnormalities in chest X-rays which were attributed to PVC dust exposure but the findings and causation are disputed. Perhaps the best disciplined study of the respiratory health of workers exposed to PVC dust has been done by the Institute of Occupational Medicine (IOM), Edinburgh, on 818 ICI employees at their Hillhouse factory. This is available as the Institute Report TM/79/2 and will shortly be published in the literature. The IOM found evidence that inhalation of PVC dust is associated with respiratory dysfunction and radiographic changes. There was no evidence of a serious progressive form of pulmonary fibrosis but there were radiographic abnormalities in a proportion of the exposed employees. Changes in lung function tests on average of the same order as that caused by cigarette smoking and about one-seventh that of ageing were discovered. The study is being continued to discover whether there is anything of clinical significance in the employees who were noted to have radiographic and lung function changes. While these results are not unduly alarming, it would be prudent to reduce exposures to PVC dust.

108

V.G.LOVELOCK

REFERENCES 1. 2. 3.

4. 5. 6. 7. 8. 9. 10.

BARTKNECHT, W., Explosionen, Ablauf und Schutz-massnahmen, Springer, Berlin, 1978. BARTKNECHT, W., paper presented at a conference organised by OYEZ on ‘Prevention and Protection’ entitled ‘The Hazards of Industrial Explosions from Dusts’, February, 1979, London. MARSHALL, W.R., Atomisation and Spray Drying, Amer. Inst. Chem. Eng. Monograph, 50(2), (1954); HERRING, W.M. and MARSHALL, W.R., J. Amer. Inst. Chem. Eng. (1955) 200; RUIS MIRO, A. et al., Anales Real. Soc. Espan. Quin. 5381, 73 and 87 (1957). KIRSHBAUM, E., Chem. Ing-Technik, 24, 3 (1952). LOVELOCK, V.G., unpublished work. MASTERS, K., Spray Drying Handbook, George Godwin Ltd, London, 1979, pp. 179–82. IBRAGIMOV, I.Ya. and BORT, D.N., Vysokomol. Soyed., B16, 376 (1974). ARNAUD, A. et al., Thorax, 33, 19 (1978). MAPP, C. et al., Med. Lavoro, 69, 151 (1978). MASTROANGELO, G. et al., Journal of Occupational Medicine, 21, 540 (1979).

Chapter 7 MORPHOLOGY OF PVC M.W.ALLSOPP Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

7.1 INTRODUCTION Each of the large-tonnage polymers produced today has particular properties which must be carefully controlled in order to ensure the technical acceptability of the material with customers. Whereas other polymers may be concerned with molecular weight distribution, rate or level of crystallisation, density, melt flow behaviour, etc., the success or otherwise of a PVC polymer depends largely on its grain structure or morphology. With the advent of a number of new techniques it is possible to characterise the morphology of PVC in a much more comprehensive way than was previously possible and this has led to a much greater technical awareness of the importance of morphology, particularly porosity and uniformity, with respect to processing behaviour (see Chapter 8). Vinyl chloride is polymerised by a free radical mechanism but the final powder form depends exclusively on the actual process used. From a morphological viewpoint there are five distinct methods for the production of PVC powder, viz. (1) suspension; (2) bulk or mass; (3) emulsion; (4) gas phase or partial pressure; (5) solution or diluent. Of these suspension polymerisation predominates commercially and an examination of polymers produced by this technique will form the main body of this chapter. Bulk and gas phase polymers contain features of particular morphological interest and will be included in order to demonstrate how process changes can affect the morphology of the final product. Emulsion polymer morphology is discussed in Chapter 3 and group (5) processes are not included since they are of little interest morphologically. TABLE 7.1 PVC Nomenclature Term

Approximate size

Origin or description

Previous terminology (with references)

Visible constituents of free flowing powders, made up of more than 1 monomer droplet. Polymerised monomer droplet. Formed during early stage of polymerisation by coalescence of

Granule2 Cellular grain3

Range (µ m) Average (µ m) Grain

50–250

130

Sub-grain Agglomerate

10–150 2–10

40 5

Sub-granule2 Unicell3 Aggregate2 Cluster3 Macroglobule4

110

M.W.ALLSOPP

Term

Approximate size

Origin or description

Previous terminology (with references)

Range (µ m) Average (µ m)

Primary particle 0.6–0.8

0·7

Domain

0.1–0.2

0·2 (200 nm) (2000 Å)

Micro-domain

0.01–0.02

0.02 (20nm) (200Å)

primary particles (1–2µ m). Grows with conversion to size shown. Grows from domain. Formed at low conversion (less than 2%) by coalescence of micro-domain: grows with conversion to size shown. Primary particle nucleus. Contains about 103 microdomains. Only observed at low conversion (less than 2%) or after mechanical working. Term only used to describe 0.1 µ m species; becomes primary particle as soon as growth starts. Smallest species so far identified. Aggregate of polymer chains— probably about 50 in number.

Microgranule Primary particle2 Granule3 Micro-globule4

Primary nucleus2 Granule3

Basic particle2 Particle3

Notes 1. The domain is not a feature of PVC morphology in high conversion polymer samples since a growth of this species with conversion obliterates all memory of it. It may only be ‘regenerated’ and observed after subsequent processing. 2. As soon as formation of the domain is complete and growth is registered it is preferable to call it a primary particle. Therefore, the term domain is often ignored in favour of primary particle even at the point of morphogenesis of the 0.2 µ m primaries at low conversion. 3. The reason for a separate identity for the domain is that it may be shown in future to contain an atypical morphological or molecular feature, e.g. higher level of crystallinity.

7.2 NOMENCLATURE In the technical literature the terminology used to describe the morphology of PVC is varied and very confusing. Since it is important to have a clear and unambiguous system of nomenclature this aspect of the subject will be covered in some detail here. Following the second international symposium on PVC at Lyon-Villeurbanne in 1976 a discussion of the terminology used to describe PVC morphology was summarised and published by Geil1 in an attempt to obtain consistency. Geil’s terminology forms the basis of the system used here (Table 7.1). In order that the reader is clear on this subject a series of micrographs is shown in Figs. 7.1–7.3 so that each species may be identified in turn. All these structures can be brought together in the idealised model of a PVC grain shown in Fig. 7.4. The problem with models of this type is that they are idealised and not drawn to scale. The problem of portraying a realistic model is seen if we just consider the primary, domain and micro-domain morphology drawn to scale (Fig. 7.5). Only then can it be appreciated how much growth of the domain or primary

MORPHOLOGY OF PVC

111

FIG. 7.1. PVC (a) grains and (b) sub-grains.

nucleus must take place with conversion in order to reach the 0.5–1.5 µ m (0.7µ m at 50°C) primary particles seen in high conversion commercial samples of PVC. Also it suggests that although the domain may be important in terms of the molecular ordering/crystallinity of PVC at low conversion and hence it may exert a considerable influence on say, rheological properties, it is not a fundamentally important feature of morphological texture of high conversion polymer. For the remainder of this chapter it will be considered simply as a primary particle nucleus and the latter term will be used as well as domain to describe this particular species. 7.3 CLASSIFICATION OF PVC MORPHOLOGY It is clear from the previous section that PVC morphology covers a wide variety of size scales and for the purposes of discussion in this chapter three levels are identified. Macro-scopic will refer to all size entities above 10 µ m, micro-scopic covers the range 10–0.1 µ m and sub-microscopic below 0.1 µ m. Polymers made by the four main routes of interest, as described in the introduction, tend to be significantly different in their macro-morphology, are much more similar in micro-structure and as far as evidence is available at present, virtually identical in the sub-micron range. Because of its commercial importance and the fact that the macro-morphology of suspension PVC is more varied than that from the other processes, the morphology of this material will be considered in detail initially.

112

M.W.ALLSOPP

FIG. 7.2. PVC (a) primary particles and (b) agglomerates.

The role of temperature in the control of PVC grain morphology is often ignored or not defined in papers on this subject. Initially, this chapter considers the morphology resulting from polymerisations conducted at 50°C where high K-value polymer is produced. Later, the effect of higher polymerisation temperatures in producing a more coherent structure of lower porosity is reviewed. 7.3.1 Macro-Morphology of Suspension Polymerised PVC In the suspension polymerisation of VCM (see Chapter 1) the bulk monomer phase is dispersed in water by vigorous agitation and the droplets produced are stabilised against coalescence by the presence of a granulating agent or protective colloid. An immense variety of agitation regimes and different protective colloid combinations are possible but in essence three main mechanisms are possible (Fig. 7.6): Route 1

The monomer droplets can be extremely well protected and once formed are completely stable against coalescence and survive throughout the whole polymerisation process as individual droplets to give an essentially spherical, fairly fine sub-grain of low porosity.

MORPHOLOGY OF PVC

113

FIG. 7.3. PVC micro-domains and primary particles from an ultra thin section.

FIG. 7.4. Model of PVC grain morphology. Route 2

An intermediate level of protection is applied so that the droplets undergo controlled coalescence during the polymerisation to give rise to a more irregularly shaped grain of intermediate size and higher porosity. A very wide variety of end products can be produced by subtle variations in the level of protection/time and extent of coalescence. Route 3

Inadequate protection is given in which case the monomer droplets coalesce freely at low conversion to produce the PVC manufacturer’s nightmare— one grain the same size as the reactor. Most commercial polymers are produced by route 2 and speciality resins, e.g. paste fillers, gramophone records, etc., by route 1.

114

M.W.ALLSOPP

FIG. 7.5. Scale model of PVC sub-microscopic structure.

The degree of protection afforded to the monomer droplets depends on the type and concentration of the protective colloid used. The level of agitation depends on stirrer speed, agitator type, size and shape and arrangement of baffles. In essence, the higher the level of agitation the smaller the VCM droplets and the greater the surface area to be protected. Different types of protective colloid have a considerable influence both on droplet formation and on the droplet protection stage. Highly surface active material, e.g. methyl cellulose or 80 mole % hydrolysis poly(vinyl alcohol) (PVA) considerably lowers interfacial tension of the two-phase system which gives it high dispersing powers and hence smaller monomer droplets are formed (route 2 type). When 88 mole % PVA is used, dispersion is less efficient but the level of droplet protection is higher (route 1 type). Neither parameter is specific in the control of polymer properties; instead the right balance of droplet protection is found by research. However, apart from grain size the shape and internal morphology of the PVC must be carefully controlled. On the macro-scale the shape of PVC grains is quite varied (Fig. 7.7). A useful classification of the range of possible structures was presented by Tregan and Bonnemayne (Fig. 7.8).3 If a comparison of Figs. 7.6 and 7.8 is made it is clear that the suspension polymers on the left hand side of Fig. 7.8 are made by a route 1 process, whereas the centre and right hand side polymers arise by intermediate and route 2 mechanisms respectively. A simple technique for studying the macro-structure of suspension PVC clarifies the classification of the grains on the left hand side of Fig. 7.8. A sample of the polymer is immersed in dialkyl(C7–C9) phthalate (DAP) plasticiser on a microscope slide and after equilibrium is viewed at low magnification (100 ×) by transmitted light. The choice of a plasticiser with a refractive index close to that of PVC(DAP=1.519, PVC=1.542) allows clear observation of the internal morphology of each grain (Fig. 7.9). This simple test allows PVC grains to be separated into three basic structures depending on their optical appearance: (i) ‘Blacks’—grains containing closed pores which are not connected to the surface and are not fully wetted by plasticiser leaving air pockets within the grain, giving two regions of very marked refractive index difference. Pronounced diffraction of the light rays at the PVC/air interface gives rise to ‘black’ zones in the grain. ii) ‘Translucent’—grains of high porosity where pores are evenly distributed throughout the grain. Since the refractive index of the PVC and the DAP are similar only slight diffraction occurs giving the grains a ‘translucent’ appearance. iii) ‘Clears’—solid grains containing little or no porosity. Light rays are diffracted at the surface only since the refractive index of the grain interior is almost constant giving a ‘clear’ appearance.

MORPHOLOGY OF PVC

115

FIG. 7.6. Schematic diagram of the effect of VCM droplet stability on the macromechanism of polymerisation.

FIG. 7.7. Typical variations in PVC grain macro-morphology.

These three grain types can be produced in any of the classifications shown in Fig. 7.8, not just route 1 grains, but as the mechanism of polymerisation tends towards the route 2 type the translucent type is more typical. It is possible in practice to get some grains with more than one of the types of structure (Fig. 7.9). This is particularly true of higher temperature polymerisations (see Section 7.5.1). In order to understand how these structures are formed it is necessary to investigate the mechanism of polymerisation in a typical suspension system.

116

M.W.ALLSOPP

FIG. 7.8. Classification of PVC grains. (Reproduced from Tregan and Bonnemayne, Plast. Mod. et Elast., 23(7) 220–47, 1971.)

7.4 MECHANISM OF SUSPENSION POLYMERISATION 7.4.1 On the Macro-scale In a suspension polymerisation the bulk monomer phase is converted into droplets by vigorous agitation in the presence of an aqueous solution of a protective colloid such as cellulosic derivatives or partially hydrolysed poly(vinyl acetate). The mean size of the droplets is about 40 µ m within a spread of 5–150 µ m. The suspension is continually agitated and maintained at a constant temperature (normally between 50 and 75° C). Polymerisation is initiated in the monomer phase by the presence of an oil soluble initiator and the reaction proceeds in the same way as that already described for bulk polymerisation in Chapter 2. Suspension polymerisation mechanism is similar to bulk since the former is in essence a mini-bulk system with the bulk monomer phase simply broken down into a large number of separate units.

MORPHOLOGY OF PVC

117

FIG. 7.9. Optical classification of PVC grains in DAP. (a) Blacks with clears; (b) blacks with translucents; (c) translucents; (d) translucents; (e) clears with blacks; (f) clears; (g) clears.

7.4.1.1 Droplet Stabilisation The droplets formed by agitation can be stabilised against coalescence in one of two ways. In emulsion polymerisation (Chapter 3) the mutual repulsion of electric charges generated by low molecular weight ionic surface active agents is used. In suspension polymerisation a protective barrier of a non-ionic water soluble polymer (protective colloid) is used. This method is usually described as steric stabilisation and is shown schematically in Fig. 7.10. Based on the Flory treatment of the thermodynamics of polymer solutions, the technique relies on the fact that when two monomer droplets, which have polymer molecules attached to their respective surfaces, approach one another in a liquid medium in which the attached polymer molecules are soluble, then an increase in free energy will take place as the soluble polymer molecules interpenetrate or are compressed. The resulting increase in the concentration of polymer segments in the zone of interpenetration will generate an osmotic pressure. To counteract this effect water diffuses into the regions of higher polymer concentration, forcing the particles apart until the steric barriers are no longer in contact. An effective stabiliser must be able to maintain a complete coverage of the droplet surface. Each solvated polymeric component must be firmly attached to the surface of the droplet so that it is neither desorbed from the surface nor displaced laterally when two droplets collide. The most successful types of protective colloids are those based on block copolymers of an amphipathic character. Partially hydrolysed poly(vinyl acetates)

118

M.W.ALLSOPP

FIG. 7.10. Schematic model of steric stabilisation.

FIG. 7.11. Adsorption of granulating agent from the aqueous phase.

have the two necessary components—one insoluble (poly(vinyl acetate)) and one soluble poly(vinyl alcohol) in the continuous phase. By virtue of the insolubility of one component, these colloids are adsorbed in an irreversible manner on to the surface of the VCM droplet so that a permanent lyophilic layer of polymer completely protects the droplet from coalescence. Experiments in our laboratories have shown that VCM droplets protected in this way are stable for many days, in the absence of any polymerisation (see later), and that the adsorbed layer of protective colloid (about 100–150 A in thickness) cannot be desorbed. Further experiments in which the rate of loss of poly

MORPHOLOGY OF PVC

119

FIG. 7.12. Membrane formation at low conversion.

(vinyl acetate) from the aqueous phase is monitored indicate that some 20% of the added stabiliser is adsorbed initially on to the monomer droplets surface. However, at the onset of polymerisation there is a rapid and considerable loss of poly(vinyl acetate) from the aqueous phase of up to 90% (Fig. 7.11). 7.4.1.2 Effect of Conversion To help elucidate the mechanism of polymerisation further, polymerising systems were observed and sampled during the reaction. A well defined membrane around the droplets is seen at low conversion and this has been shown to be a poly(vinyl acetate)/poly(vinyl chloride) graft copolymer. As conversion rises this becomes more coherent (Fig. 7.12). Depending on the recipe used, varying levels of coalescence can now take place (see Fig. 7.6) over the range of conversion 5–15%, which suggests that the nature of the steric barrier is modified by polymerisation to a point where the polymerising droplets can become unstable temporarily, after which coalescence ceases. At about 30% conversion the membrane is quite strong and stable. During this time density changes are occurring within the droplet as monomer of sg 0.85 is converted to PVC, sg 1.4. In theory this 39% contraction could be absorbed in two ways. A uniform contraction of the polymerising droplet and its contents could take place progressively throughout the reaction, but this would yield an almost solid grain structure of almost zero porosity. This does occur with one or two speciality polymers (see Fig. 7.9). Alternatively, if porosity is to be created in the final product the grain must either increase its dimensions or, as is more feasible, the contraction of the polymerising volume must be arrested at some

120

M.W.ALLSOPP

FIG. 7.13. Changes occurring in VCM droplets during the early stages of polymerisation.

stage. This is achieved by forming an open floc structure at the time the primary particles coagulate to form the continuum of agglomerates. As this grows stronger with conversion it reaches a point where it is coherent enough to resist the forces of contraction and no further movement/distortion of the floc is possible and the ‘polymerising volume’ is fixed. From this point onwards all further contraction creates potential porosity. Around 65–70% conversion, most of the unreacted monomer is dissolved in the PVC and as polymerisation proceeds, the pressure within the grain falls and the grain collapses with folding and rupturing of the surface, accompanied by intrusion of water. This accounts for the wrinkled surface appearance of the PVC grains shown in Fig. 7.1 and some of the following micrographs. If samples of grains produced before 70% conversion are examined before degassing of unreacted monomer a smooth surface is observed. As conversion rises further the extent of folding and rupturing of the grain increases slightly until the polymerisation is terminated by degassing unreacted monomer. For commercial polymers this is usually in the range 80–95%. As the conversion increases considerable changes in bulk polymer properties such as mean grain size and distribution, apparent density, packing density, powder flow, etc., can be related to the macro-structure changes (see later), but some properties, e.g. surface area and porosity, vary appreciably yet the changes seen on the macro-scale are really quite small. It is left to other levels of structure to provide an explanation. 7.4.2 Micro-scopic Scale While the macro-scale changes involving droplet formation, coalescence, growth and collapse are taking place a series of similar changes on the micro-scopic scale are also being enacted in the monomer phase

MORPHOLOGY OF PVC

121

FIG. 7.14. Agglomerate packing in PVC grains, (a) Porous; (b) dense.

(Fig. 7.13). Since the suspension process can be considered as a mini-bulk system in which the bulk VCM phase is simply reduced to a number of individual reaction zones there are marked similarities between the two processes, particularly on the micro- and sub-microscopic levels. Hence, the mechanism of polymerisation in the VCM phase in suspension is remarkably similar to that already described in Chapter 2, section 2.5.1. Within the droplet the first formed PVC coagulates to form 0.2 µ m primary particles (domains) at an early stage, less than 2% conversion. The whole droplet turns cloudy as this phase separation step occurs with the interfacial region a zone of particularly high activity. Graft copolymerisation of PVC on to the poly(vinyl acetate) protective colloid not only starts the process of forming the skin but begins to modify the nature of the interfacially adsorbed polymer, which lowers the covering power of the material and more is absorbed from the aqueous phase. Although subjected to intense Brownian motion, the primary particles remain discrete and are stable and continue to grow. In the range of 3–10 % conversion they become unstable and flocculate to form close packed agglomerates of 1–2 µ m diameter. Due to the conditions prevailing at this time, different spatial packing arrangements of these agglomerates is possible and this step is perhaps the most important in defining final grain morphology. If the agglomerates pack loosely the final grain is porous whilst close packing results in a dense grain of low porosity (Fig. 7.14). Further polymerisation after this stage by surface deposition and internal growth of existing aggregates and their constituent primary particles produces a gradual increase in size to 2–10 µ m in the final product, together with an increase in coherency of the whole network structure. Both temperature and the level of agitation

122

M.W.ALLSOPP

FIG. 7.15. Variation of porosity (CPA) with autoclave stirrer speed.

have a marked influence on grain structure. Our own results show that as the level of agitation progresses from low to high the grain structure changes from a dense spherical to a porous irregular type with a

MORPHOLOGY OF PVC

123

FIG. 7.16. Variation of porosity (CPA) with polymerisation temperature.

corresponding increase in porosity (Fig. 7.15). Likewise polymerisation temperature is an important variable used in practice to control molecular weight but this too has a marked effect on grain porosity (Figure 7.16). Changes in the micro-scopic level of structure have a marked influence on virtually every type of polymer property. Surface area, porosity, plasticiser adsorption, gelation, are all controlled by the subtle changes in the micro-structure of the PVC. Yet again, some properties, e.g. hot plasticiser absorption, solvent dissolution, and visco-elastic melt properties cannot be explained in terms of micro-structure and an examination of sub-microscopic texture is required for an explanation. 7.4.3 Sub-microscopic Scale With the advent of a number of new techniques in recent years the macro-and micro-scopic morphology of PVC is now fairly well characterised, although much still needs to be done to understand and relate these morphological features to processing behaviour and in-service properties. Recently, more emphasis has been placed on characterising the sub-microscopic morphology of PVC. This is proving to be a very difficult field of study as one of the main problems, especially in unplasticised PVC, is simply resolving those structures present. The main feature of interest at this scale of scrutiny is the micro-domain, some 100–200 A in diameter, and the domain, an order of magnitude larger. The former was first identified in our laboratories in 1956 by Cobbold et al.5 when examining the replicated surface of an experimental polymer by transmission electron microscopy (TEM) (Fig. 7.17). Since that time various literature references, for example 6, 7, 8 and 9, have reported that entities of this size can be found in certain polymer samples. During the course of further work in our laboratories in which bulk and suspension polymerisations were sampled at very low conversion, clear evidence for the existence of 0.2 µ m domains was obtained (Fig. 7.18). Further proof that the larger domain is produced by the agglomeration of smaller sub-species is given in Fig. 7.18. Existence of particles (domains) in the size range 0.1–0.2 µ m has also been reported by other workers using different techniques and appears to confirm that the domain (0.2 µ m) is a real structural entity.6–9,11 It must be remembered when considering photomicrographs of PVC morphology that the appearance of the nascent structure may be modified to varying degrees by the sampling technique used. This is no problem with the larger species, since techniques have been developed which allow in situ observation, or with samples of high conversion PVC because of its more coherent nature. Low conversion sampling is more

124

M.W.ALLSOPP

FIG. 7.17. Surface replica of micro-domains.

difficult, particularly when investigating micro-and sub-microscopic texture, since degassing will tend to produce coagulation of the small particulate species present. In Fig. 7.18, for example, the domains on the top left of the photomicrograph are probably largely unchanged by the sampling technique, but the true nascent form of the micro-domains is debatable. Perhaps the best evidence for domains in plasticised PVC has been provided by Geil and co-workers at Case Western Reserve University.6,8 By means of a freeze fracture technique micro-domains are clearly identified and are shown to increase in size with varying plasticiser content. Following the mechanism of polymerisation proposed earlier it should be rewarding to examine material produced during the early stages of polymerisation in relation to sub-microscopic structure. If, as has been suggested, there is a crystalline nucleus or core associated with the basic micro-domain structure, or even a lamellar texture, then these features should be easier to identify at the point of their morphogenesis at low conversion before additional growth and agglomeration take place and obliterate the structure. Using laser Raman spectroscopy on material isolated at about 2% conversion it is possible to show that this material produced at very low conversion is indeed more crystalline.12 Small angle X-ray scattering (SAXS) measurements on PVC can be attributed to a two-phase supermolecular structure of crystallites in an amorphous matrix. The arrangement of the crystallites can exhibit various degrees of order. When PVC is annealed under conditions that enhance crystallinity, the order is sufficient to produce a discrete, albeit broad, diffraction peak corresponding to spacings of about 100 Å.13It is not possible to tell from SAXS alone whether this peak is the result of a stacked crystalline lamellar morphology or whether it is from semi-ordered arrays of spherical crystalline nodules. Electron microscope evidence tends to favour the nodular structure in which case the 100 A spacing is typical of the internodule distance. Without these annealing conditions, the crystallite arrangement remains disordered and probably consists of irregularly packed nodules with a distribution of sizes. The mean size of the nodules is of the order of 30 Å. In addition to low conversion polymer, high conversion material has been examined both before and after processing. In high conversion polymer samples 1 µ m primary particles in 5 µ m agglomerates are easily

MORPHOLOGY OF PVC

125

FIG. 7.18. Domains and micro-domains in low conversion polymer.

identified but not the other size species. After processing it is particularly interesting that in addition to these species the 0.1 µ m domains are also seen. This suggests that the primary particle nucleus retains its original identity, even though it undergoes extensive growth with conversion (Fig. 7.5). It may suggest that the domain is the location for the more highly ordered material mentioned in reference 12 since the material examined was of this size range, but this is not fully proven at the present time. 7.5 OVERALL MORPHOLOGY OF SUSPENSION PVC AND POLYMER PROPERTIES The mode of formation of the types of grain shown in Fig. 7.8 can now be better understood in the light of the above mechanism of polymerisation. When droplet protection is complete a route 1 non-coalescing system results in grains of regular shape. Depending on the exact conditions of the polymerisation varying degrees of flocculation can take place within the droplet to give clear, black or translucent grains. When droplet protection is reduced to a point where coalescence takes place a variety of structures are formed. If coalescence occurs when a strong pericellular structure and aggregate floc structure have formed, an intermediate B type grain would result (Fig. 7.8). If the conversion at the time of coalescence is lower, the agitation higher, or the aggregate network and skin weaker, an intermediate A type grain is formed. At lower levels of droplet protection the route 2 fully coalescing system results in the 2A type of grain morphology. Type 2B is difficult to produce consistently because of variable levels of coalescence. Most commercial polymers tend to be of the Inter A or 2A type.

126

M.W.ALLSOPP

The overall size and shape of the final grain together with related properties such as powder flow depend largely on the macro-morphology produced in the droplet coalescence step. Clearly, these parameters depend on the extent to which the polymerisation follows routes 1 or 2 defined earlier (Figs. 7.6 and 7.8). Depending on the micro-morphology created at about the same time, a variety of internal structures can form. The measurement of porosity by mercury intrusion is a very powerful tool for characterisation of micro-structure. It can give a measure of mean pore diameter and pore size spread as well as the overall porosity of the grain. Since the porosity is a measure of the spaces between primary particle aggregates some ideas of how the aggregates packed when first formed can be gained. If the mean pore diameter is small, for example, it suggests that the degree of stability in the monomer phase was low at the time of flocculation, that a fairly dense floc of aggregates was formed and that a long period of growth, since flocculation occurred at low conversion, has produced considerable densification. Surface area measurements provide useful additional data since the technique measures the area of the aggregates which is accessible, e.g. to plasticiser. This test can also be used to show that the primary particles are solid PVC containing no micro-pores. Irregular translucent grains of the Inter A or 2A type have a high porosity and surface area ideal for plasticised applications (Chapter 10). In the rigid extrusion from dry blend (Chapters 8 and 9) a higher bulk or apparent density is required and grains of lower porosity of the IT or Inter B type are preferred. Some of these polymers can still be used in flexible applications if the method of processing the polymer involves a high shear compounding step (see Chapter 8). These general purpose resins are obviously a compromise between the ideal properties required for either system. In the area of high K-value polymers (ISO K > 65) made at the lower end of the polymerisation temperature range the polymer chemist has a wide choice of grain types available to him. However, high Kvalue polymers cannot be used for all applications because of melt viscosity limitations, but they are ideal for use in plasticised formulations where the melt viscosity is not a limiting factor. The level of absorption of formulation additives, e.g. heat stabilisers, lubricants, plasticisers, etc., has a tremendous effect on the compounding stages of processing (see Chapters 8, 9 and 10). In rigid formulations the presence of large quantities of external lubricant on the surface of the grain clearly prolongs the process of gelation (see Chapter 8) which, if delayed to a point too far along the extruder profile, will have a damaging effect on mechanical properties. In flexible formulations the effect is less subtle. A slow rate or low extent of plasticiser absorption will result in long dry-up times and high mixer temperatures if a free flowing mix or plasticised dry blend is to be achieved. The effect of cycle time and operating costs on a customer’s operation clearly demands that in virtually all flexible applications the maximum porosity possible at a given bulk or apparent density is highly desirable. In rigid applications the effect is more subtle as the porosity/apparent density balance is tuned to the exact processing conditions (Chapter 8) since the gelation behaviour does not depend on porosity alone, although this parameter does exert the major influence. This is due to the fact that gelation rate also depends on other polymer parameters such as powder flow, apparent or packing density and ease of grain densification and heat transfer (see Chapter 8). The control of morphology on the finer scale is less well established. Agitation, temperature and the presence in the monomer phase of monomeric or polymeric additives and secondary granulating agent have a considerable effect but control of this level of morphology is not as well advanced as that on the macroscale. In suspension polymerisation the change of morphology on one scale is often reflected by a change on the smaller or larger scale. The interrelationship between the scales of morphology is such that they must always be considered in total; no one parameter can be changed in total isolation from the others; even a change in molecular weight results in morphology changes.

MORPHOLOGY OF PVC

127

FIG. 7.19. The effect of polymerisation temperature on the macro-morphology of PVC grains.

7.5.1 The Effect of Higher Polymerisation Temperatures on Morphology When selecting a sample of PVC for a certain application the polymer chemist is faced with a number of possible choices. As a general rule the highest possible K-value material, compatible with the processing equipment to be used, is chosen in order to maximise mechanical properties. The increasing melt viscosity with increasing Kvalue will obviously impose limitations on some processes so that currently, for example, blow moulding machines for bottles cannot process ISO-K71, K67 or even K62 material, and therefore bottle polymers have no choice but to be made at about K57. Because of the marked effect of polymerisation temperature on morphology the porosity of K57 polymer is limited to the middle/bottom end of the range. However, by the correct choice of the polymerisation conditions and granulating agents (Chapter 1), low K-value polymers can be produced with a reasonable level of porosity. The advantage is two-fold since in addition to the improvements in processing and thermal stability which are gained by increasing the porosity, it also increases the ease with which unreacted VCM can be removed from the polymer during stripping (Chapter 5). The reason for the change in morphology with temperature is associated with the type of packing of primary particle agglomerates and the degree of fusion between them. We have already seen in Chapter 2, Section 2.5.2, how the polymerisation temperature in bulk polymerisation exerts a major control of agglomerate fusion. In fact, the success of this process depends to a high degree on the correct level of fusion being produced in the prepolymeriser. The same mechanism applies to suspension polymerisation. In addition, growth of the primaries at higher polymerisation temperatures produces a very dense morphology (Fig. 7.19). Thus there are two components producing a dense morphology at high temperature: a well fused mass of primary particles within each agglomerate and a closely packed arrangement of all primary particle agglomerates. However, the use of secondary granulating agents can offset this densification to considerable effect.

128

M.W.ALLSOPP

FIG. 7.20. External morphology of bulk polymer—‘Lucovyl’ GB 1150.

7.6 MORPHOLOGY OF RHONE-POULENC BULK POLYMER The overall morphology of bulk polymer is similar to that of suspension PVC of the ‘translucent’ grain type (Fig. 7.8). The bulk polymer does not have a pericellular membrane or ‘skin’ enclosing each grain and the external surface of bulk polymer appears to be very similar to its internal morphology (Figs. 7.20 and 7.21). However, using a technique which was developed to examine the degree of coherency of intra- and interagglomerate fusion it was discovered that some grades of bulk polymer contain a sub-surface ‘skin’, or more specifically, a layer of more highly fused material (Fig. 7.22). In this technique whole grains or 5 µ m thick sections are treated with solvent (cyclohexanone) at 25°C on a microscope stage. The changes occurring with time are recorded photomicrographically. Apart from showing that both suspension and bulk polymer contain surface regions which are slower to dissolve than the bulk of the grain (Fig. 7.22) other useful information can be derived from this technique. If polymers of different levels of conversion from either process are examined the gradual fusion of the primary particles within each agglomerate and the inter-agglomerate fusion can be appreciated (Figs.7.23 and 7.24). 7.7 MORPHOLOGY OF GAS PHASE POLYMER The morphology of gas phase polymer (Chapter 2) is particularly interesting to study since it gives a clear insight into the changes taking place after ‘pressure drop’ (70–75% conversion) in the conventional suspension process. Even though only 10–20% of the overall growth is contributed in this region the type of material produced has a considerable influence on polymer properties. Using a porous bulk polymerised seed polymer the changes taking place with increasing growth factor (Chapter 2) can be seen in Figs. 7.25 and 7.26. As well as the possibility of growth within the VCM/PVC gel phase there appears to be considerable growth on the surface of the existing primary particle

MORPHOLOGY OF PVC

129

FIG. 7.21. Internal morphology of bulk polymer—‘Lucovyl’ GB 1150.

FIG. 7.22. The effect of solvent on a 5 µ m section of bulk polymer (‘Lucovyl’GB1210).

agglomerates. It is difficult to establish the exact mechanism of growth for the following reason. When the agglomerate is first formed the point contact of fusion between each primary particle defines the geometry of the agglomerates and fixes the distance between each of the primary particle centres. As these grow they are unable to move apart and even it growth only occurred within the VCM/PVC gel the primaries would appear to fuse progressively together (Figs. 7.25 and 7.26). Perhaps this dilemma can be resolved if one compares the polymer produced if all the growth is under saturated conditions with that produced below the saturated vapour pressure (gas phase). In work2 in which

130

M.W.ALLSOPP

FIG. 7.23. The effect of solvent on a 5 µm section of suspension polymer at 50% conversion.

monomer was continuously fed in under these two regimes for long periods so as to obtain a wide range of conversion in the two systems, an analysis of the polymers made produces the following conclusions. Under saturated conditions the interface between the growing primary particle agglomerates and the surrounding medium consists of liquid monomer containing some undecomposed initiator. The precipitating PVC nuclei are accreted by the existing primary particles which are themselves growing as a result of gel phase polymerisation. Each agglomerate surface advances geometrically towards another, gradually filling the available spaces in between, i.e. the pores, with a slight preference for infilling of the smaller pores. This is responsible for the decrease in the degree of porosity with little or no change in the size of the grains; as a result the apparent density increases rapidly. In sub-saturation or starved conditions the new growth produces an increase in grain size, no marked change in apparent density and a slightly slower decline in the level of porosity. This suggests that a systematic infilling as under saturated conditions is not taking place; instead it indicates that the majority of the growth after pressure drop takes place within the confines of the gel phase and some of it in small pores, which produces an expansion of the gel phase and hence increases the grain size in these monomer feeding experiments. In the conventional suspension process when the gel phase contains virtually all the unconverted monomer after pressure drop and no further monomer is added, a gradual contraction and integration of the gel with conversion will result. 7.8 SUMMARY From the evidence gained so far it is clear that the growth of PVC takes place through a series of interconnected aggregation steps which can be represented by the scheme in Fig. 7.27. In the first instance the growing macro-radicals coil as they grow to give the first phase-separated species. The aggregation of some 50 of these to produce an unstable micro-domain (200 A) gives us the first species which can be

MORPHOLOGY OF PVC

131

FIG. 7.24. The effect of solvent on a 5 µ m section of suspension polymer at conversion.

identified as a separate morphological unit. This in turn flocculates in much greater numbers ( 1000) to produce the domain or primary particle nucleus. The next, and perhaps the most interesting step of all, is the growth of the primary particle, its flocculation into an agglomerate and the subsequent growth of the primary particles within an agglomerate. Some growth must inevitably occur in the VCM/PVC ‘gel phase’ since this contains a ready supply of monomer and some initiator ‘trapped’ within its loose confines. One can envisage the growth of single chains building up the PVC network, but what of the mechanism in the VCM phase? In the early stages this is overwhelmingly predominant in volume and is the main source of initiator. Although it has been clearly established that no new primary particles form with conversion is it possible that the growth element is the micro-domain? If so, some 50 chains would have to flocculate together to form this species before impinging on an existing primary particle surface and suffering capture. Certainly, if this were the mechanism one would expect to be able to identify micro-domains in high conversion polymer and this has yet to be achieved. Perhaps a smaller entity not yet identified is the growth species or are we left with the PVC macro-molecule itself as the growth unit? The subject of PVC micro-scopic and sub-microscopic morphology has seen a tremendous advance in the last decade in terms of the ability to identify the species concerned. Perhaps the next few years will see the relationship between morphology and rheology and mechanical properties being clearly established. Progress is already being made. In a recent IUPAC working party14 participants studied the effect of draw ratio on micro-morphology. The main conclusion of this work indicated that neither the 200 A microdomain nor the 0.2 µ m domain structure is deformable of itself but that the connective tissue between the 0. 2µ m domain is highly deformable. This evidence is consistent with the flow unit of unplasticised PVC being an approximately spherical domain of about 0.2µ m diameter which is not itself deformable during processing. The internal structure of these units is composed of an agglomerate of structure on the 200 Å scale. It has been suggested that the 200 Å micro-domain is held together internally by ‘crystallinity’ acting

132

M.W.ALLSOPP

FIG. 7.25. Ultra-microtome cross sections of gas phase polymer at various stages of growth.

as cross links10 but the connective tissue between the 0.2 µ m domains is far less ordered and easily deformable. ACKNOWLEDGEMENTS The author is indebted to a number of colleagues who supplied data for this chapter, in particular Arthur Cobbold and Tony Curson whose expertise in electron and optical microscopy contributed significantly to our understanding of PVC suspension morphology. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

GEIL, P.H., J. Macromol. Sci.—Phys., B14(1), 171 (1977). ALLSOPP, M.W., J. Macromol. Sci.—Chem., A11(7), 1223–34 (1977). TREGAN, R. and BONNEMAYNE, A., Plast. Mod. et Elast., 23(7), 220–47 (1971). BORT, D.N., MARININ, V.G., KALININ, A.I. and KARGIN, V.A., Vysokomol. Soyed., A10(11) 2574–83 (1968). COBBOLD, A.J., private communication. SINGLETON, C., ISNER, J., GEZOVICH, D.M., Tsou, P. K. C., GEIL, P. H. and COLLINS, E.A., Polym. Eng. Sci., 14(5), 371 (1974). BERENS, H., Plaste u. Kaut., 22(1), 2–7 (1975). GEZOVICH, D.M. and GEIL, P.H., Int. J. Polym. Mater., 1, 3–16 (1971). MUKHINA, I.A., SHTARKMAN, B.P. and VIDYAIKINCA, L.I., Vysokomol. Soyed., B11(5), 343–5 (1969).

MORPHOLOGY OF PVC

133

FIG. 7.26. Micro-particle morphology of bulk polymer seed increased in mass three times in gas phase reactor, (a) Bulk polymer ‘seed’ 25% conversion; (b) growth factor 3. 10. MUNSTEDT, H., J. Macromol. Sci.—Phys., B14(2), 195–213 (1977). 11. MENGES, G. and BERNDTSEN, N., Pure Appl. Chem., 49, 597–613 (1977). 12. ROBINSON, M.E. R., BOWER, D.I., ALLSOPP, M.W., WILLIS, H.A. and ZICHY, V. , Polymer, 19, 1225–9 (1978). 13. BLUNDELL, D.J., Polymer, 20, 934–8 (1979). 14. COGSWELL, F.N., Sub Primary Particles in PVC—Identification and Elucidation of their Role during Flow, Pure Appl. Chem., 52(8), p. 2033–50 (1980).

134

M.W.ALLSOPP

FIG. 7.27. Schematic representation of the mechanism of VCM polymerisation.

Chapter 8 MECHANISM OF GELATION OF RIGID PVC M.W.ALLSOPP Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

8.1 INTRODUCTION The importance of understanding the mechanism of gelation can be gauged by the fact that in virtually every fabrication process unplasticised PVC (UPVC) undergoes gelation prior to end use. During processing UPVC formulations are converted from the 130 µ m polymerised grains (Chapter 7) through a number of stages to the finished product on a wide variety of machines. The various processing routes involve three interconnected stages of pre-mixing with additives, compounding and shaping. Each step is not mutually exclusive of the others but there is considerable merging of stages particularly in modern machines where the latter two stages are often completed in a single step. In this chapter we initially consider pipe extrusion on a twin-screw extruder as a typical processing technique since this involves the largest single application use of PVC and is likely to retain this position over the next decade. The formulation used in this study is typically European in that it is based on a lead stabilised system but preliminary work on a tin stabilised formulation indicates that the overall mechanism is just the same. Other compounding and processing techniques such as single-screw extrusion, Banbury mixing, Brabender plasticorder and two-roll milling are examined in turn and their gelation mechanism compared to that of the twin-screw extruder. 8.2 PRE-MIXING WITH ADDITIVES In all UPVC processing techniques the first stage involves pre-mixing with additives. This step can involve a simple low speed mixer or ribbon blender TABLE 8.1 Formulation used in the Mixing Experiments (phr) Polymer Tribasic lead sulphate (TBLS) Lead stearate (PdS t) Calcium stearate (Ca S t)

100 1·5 0·7 0·5

136

M.W.ALLSOPP

FIG. 8.1. Microtome sections of ‘Corvic’ S67/111 grains after dry blending to 140°C. (a) Differential Interference Contrast illumination (D.I.C.); (b) common light. (N.B. (1) Fields of view are identical; (2) In (b), additives appear as black specks on grain surface; (3) Densification of grains, particularly in sub-surface regions.) TABLE 8.2 Powder Properties of ‘Corvic’ S67/111 Dry Blends Control polymer Premix Dry blends Mixing temperature (°C) Sieve analysis percentage retained on BS mesh no.

25

40

60

80

100

120

140

150

52

—

—

—

—

—

—

—

—

—

0·2

72 100 150

0·2 26 62

— — —

0·3 23 63

0·3 27 61

0·3 25 61

0·2 26 62

0·2 20 65

0·3 19 66

0·2 0·2 67

1 13 71

12 134

— —

14 129

12 130

15 130

12 134

15 127

15 126

14 127

14 122

500

—

541

541

563

563

571

580

606

702

Percentage passing BS mesh no. 150 Mean granule size (µ m) Bulk density (g/ litre)

M.W.ALLSOPP

137

Control polymer Premix Dry blends Mixing temperature (°C) Packing density (g/litre) Specific surface area (m2/g) Volatile matter (%) Net internal porosity (cm3/g)

25

40

60

80

100

120

140

150

606

—

645

625

635

645

656

667

702

833

1·14

1·18

1·14

1·10

1·19

1·16

1·14

1·06

0·79

0·08

0·16

—

0·16

0·15

0·12

0·10

0·07

0·09

0·09

—

0·252

—

0.251

0·260

0·244

0·279

0.234

0·229

0·186

V.low

through to high speed mixing at elevated temperatures. Twin-screw extruders are normally fed with dry blend which is produced in a high speed mixer of the Henschel or Papenmeier type. Depending on the dry blend properties required, the mix is taken to different temperatures before being discharged to the cooler. In our experiments an ISO-K 67 polymer of high porosity with an irregular translucent grain type (Chapter 7) was mixed with the lead formulation additives according to the formulation shown in Table 8.1. Several different blends were produced over a range of mixing temperatures and the powder properties of the resulting dry-blends measured and collated in Table 8.2. At first a gradual change of properties was registered but as the temperature increased a more rapid drop in porosity and a corresponding increase in packing density was recorded (Table 8.2). For all further work a dry blending temperature of 120°C, which is the temperature often used commercially, was selected. At the end of the dry blending step a good distribution of the additives between grains is assured but the optimum level is achieved only if the mixing temperature exceeds the melting point of one of the additives. An optical and analytical examination of the final dry blend clearly shows that the additives are found on the surface of the grains whatever conditions of temperature, time and mixing speed are used (Fig. 8.1). Apart from a general distribution of lubricant and stabiliser on the free surface an increased level is located in surface folds and particularly in deep re-entrant folds (Fig. 8.1). 8.3 EFFECT OF PROCESSING PARAMETERS The effects produced by each of the three fundamental processing parameters, heat, compression and shear were then assessed. If polymer powder is simply heated, relatively little change takes place until a temperature of 150°C is exceeded. Then a spontaneous loss of porosity occurs (Table 8.3), with slightly more obvious loss from the surface regions (Fig. 8.2). When heat and pressure are applied together the relationship between powder density and applied pressure over a range of temperatures shown in Fig. 8.3 is recorded. Obviously the glass transition temperature of PVC (~80°C) must be exceeded before an appreciable increase is seen but the more quickly the material exceeds 100 °C the lower is the applied pressure needed to produce a dense compacted powder. TABLE 8.3

138

M.W.ALLSOPP

FIG. 8.2. Densification of porous PVC grains by heat alone. (a) Untreated; (b) 160°C for 15min. Porosity of Oven Heated Polymer Samples Oven temperature (°C) 15min

Cold plasticiser Polymer code S67/111a

absorption (%) Polymer code BST 4875b

Unheated 100 120 140 150 160 180

26·13 26·06 25·75 25·41 24·06 21·11 7·06

9·06 8·83 8·62 8·77 8·79 7·68 5·82

a b

Grain type—porous. Grain type—dense.

Grain morphology is very important in controlling the compacted powder density. This is illustrated in Table 8.4: polymers of different grain density (porosity) are moulded under constant conditions to give strong coherent ‘biscuits’ of the densities shown. It is particularly significant that substitution of the dry blend for the base polymer results in a biscuit of the same relative density but very low coherency which breaks up spontaneously on removal from the mould. TABLE 8.4 Powder Densities of Compressed ‘Biscuits’ of Different Polymers

M.W.ALLSOPP

FIG. 8.3. Densities of ‘Corvic’ S67/111 mouldings produced at different temperatures and pressures. Moulding Temperature 120 °C Polymer code

Porosity cold plasticiser absorption (%)

Biscuit

Applied

pressure

(psi)

200

500

900

SB 65/40E (40 % conversion)

67

1·03

Density

(g/cm3)

—

1·24

139

140

M.W.ALLSOPP

FIG. 8.4. Weight of gelled material versus final temperature at two pressures in a shear cell. Polymer code

Porosity cold plasticiser absorption (%)

Biscuit

Applied

pressure

(psi)

200

500

900

S67/111 XD60/74 BST 4875 PB33/44

26 18 9 8

1.16 1.15 1·16 1.11

Density

(g/cm3)

1·26 — 1·29 1·20

1·29 1·31 1·34 1·28

If the shear component is now applied significant changes result. Experiments were carried out in a shear cell, in which an annular bed of powder is rotated against a heated shoe to which a load is applied.1 The shoe is restrained from rotating and the restraint (or torque) is measured continuously with a displacement transducer. A well fused layer forms against the shoe, gradually spreading through the powder bed. Apart from the quantity of fused material produced, the coefficient of friction of the material in contact with the shoe is continuously measured. The influence of pressure and hence shear on the gelation process is clearly seen in Fig. 8.4. At low temperatures the low applied pressure cannot compact the material sufficiently and shear is not transmitted. Instead the grains simply move relative to one another dissipating the applied shear although some frictional heating is generated. At higher pressure more compaction results and hence the shear force is applied more effectively resulting in higher temperatures and a larger quantity of fused material.

M.W.ALLSOPP

141

Therefore, in extrusion the quicker the dry blend reaches a temperature in excess of 100–120 °C, the lower is the pressure required before the material is sufficiently compressed into a compacted powder which can transmit shear. Once the shear component is added the rate of fusion increases rapidly. The effect of different combinations of pressure, shear and temperature can now be assessed using the range of processing techniques as listed in the introduction. 8.4 EXTRUDER SAMPLING TECHNIQUES As extruder sampling forms the main bulk of the work discussed in this chapter it is worth considering initially the theoretical differences between single- and twin-screw extruders since they convey and fuse material in distinct ways which will affect the nature of the strip-down samples produced. 8.4.1 Theoretical Aspects of Single- and Twin-screw Extrusion Single- and twin-screw extruders differ fundamentally in the way in which material is conveyed from one end of the machine to the other. The single-screw extruder conveys by a shear mechanism and can be considered as two parallel plates, one moving and one static, with the forward motion caused by the drag flow due to the moving plate. One plate is the barrel wall and the other is the screw root. Because a singlescrew extruder is essentially two parallel plates with open ends, its performance is greatly affected by the restriction produced by any head/die assembly with which it is fitted, the greater the head restriction the greater the melt flow: flow which operates in the reverse direction to the drag flow. Increases in head restriction also increase leakage flow back over the screw flights, but such flow is of much less importance than the drag and pressure back flow mentioned already. In contrast, a twin-screw extruder is theoretically a positive pump. In such a machine with contra-rotating screws the feedstock is conveyed in discrete banana-shaped segments. Because the two screws intermesh, material cannot be easily transferred from one segment to another and thus it is carried forward towards the die. The performance of a totally intermeshing machine would not be affected by the head/die restriction used; in fact no machine is totally intermeshing with leakage deliberately introduced to bring about mixing, so the head restriction does have an effect but it is not as marked as that with a single-screw machine. The continuous state of the material in a single-screw extruder lends itself to study and theoretical analysis. The discontinuous state in a twin-screw machine does not. For this reason the transfer mechanism in a single-screw extruder has been the subject of considerable study, whilst little work has been published on twin-screw extruders. In PVC2 these studies on single-screw machines have shown a melt pool to exist on the trailing edge of the flight with the rest of the channel filled with a bed of unchanged feedstock plus sintered material, except in certain specific circumstances brought about by processing additives which cause the melt pool to form on the leading edge. Single-screw extruders tend only to be used to process rigid PVC in dry blend form for products with thin walls which are made at relatively low weight/hour outputs, for example sub-soil drainpipe. In other areas dry-blends of PVC are usually processed using twin-screw extruders. This is because twin-screw extruders for a number of reasons are much more suited to such feedstock. PVC, as is well known, is unstable at the temperatures at which it is processed and additives are used to increase its stability. The greater the temperature and the shear to which it is subjected, the greater its instability. PVC also has a low coefficient of thermal conductivity which makes it more difficult to heat

142

M.W.ALLSOPP

uniformly than many other polymers without intensive mixing and high shear being introduced as well, the shear also being increased by the polymer’s very high melt viscosity. Twin-screw machines have a much higher surface area of contact between the barrel, the screw and the polymer than a single-screw machine and this gives much better thermal transfer, minimising the amount of energy which has to be supplied mechanically via the screw(s). As a twin-screw extruder operates by what is in theory a shear-free process, and is run at low screw speeds, the amount of shear to which the PVC is subjected can be kept at a minimum. In addition positive pumping action is ideal for the conveyance of powder and allows maintenance of much more stable outputs than would be possible with a single-screw machine. The 1.25 in Iddon and the Schloemann BT80 extruders used in this work are not identical to machines used widely in industry today for the processing of dry blends of rigid PVC, but they are similar to such machines in many ways. 8.4.2 The 1.25 inch Iddon Extruder The 1.25 in Iddon is a single-screw extruder. Single-screw extruders used to extrude dry-blends of rigid PVC normally are fitted with long barrels and with screws with fairly high compression ratios. For this reason the Iddon was fitted with a barrel extension and a 3:1 compression, 20:1 L:D screw. The screw configuration is typical of screws used to process dry blends of PVC. The Iddon was fitted with a 0.625 in tube head/die assembly. The great difference between the Iddon and machines used in production is size, the Iddon being much smaller. The surface area: volume ratio in the Iddon will therefore be larger, resulting in a greater fraction of the energy input originating from the heaters rather than being mechanical energy from the screw/motor. This will be true especially when large machines with deep screw channels intended for high outputs are considered. 8.4.3 The Schloemann BT80 Extruder The Schloemann BT80 is a twin-screw extruder manufactured in 1966. Although some machines identical to this are still used to process rigid compound into profiles, the machines now used to process dry-blend are significantly different. Firstly modern machines are much longer; the Schloemann has a barrel 6–5 D long, modern Schloemann BTSOs are 16D long. Secondly modern screws are two-stage screws with a vent, this BT80 is unvented. A vent allows entrapped air to be removed and thus higher outputs are achieved without problems with air entrapment. Thirdly, modern machines have screws whose temperatures are controlled; the screws of the Schloemann are not. The long barrel has two important consequences: more energy can be put in from the barrel heaters and the machine is feed controlled. This latter statement needs some explanation. In theory all twin-screw extruders are feed controlled, but in practice leakage is introduced into the screws, allowing the possibility of melt control in shorter machines. The Schloemann BT80 has a great deal of leakage between the screws, and is normally run with a head of sufficient restriction to cause some melt control. Despite these differences, past experience with long barrelled Schloemanns and the Schloemann BT80 suggest that the processing behaviour of the two types of machine is very similar. The screw design of all Schloemanns is essentially the same, being the Pasquetti arrangement, in which compression is produced by decreasing the pitch and number of starts progressively along the screw. It has a compression ratio of 2:1.

M.W.ALLSOPP

143

For the experiment the Schloemann was fitted with a 2 in Class C pipe head. This head produces a back pressure of 4000 psi at the output involved, and has a compression ratio of 15.5:1. The breaker plate produces an additional 875 psi. The temperature profile which was used is shown in Table 8–5, along with that used in the Iddon. TABLE 8.5 Extrusion Conditions (a) Schloemann BT80 2 in Class C pipe head and standard breaker plate. Temperature (°C)

1

Barrel

Head

2

3

2 170 164

3 170 179

4 170 174

Motor load (A)

6

16·5

Die 4

5

Set 165 175 165 175 190 Actual 168 170 166 181 188 (b) 1.25 in Iddon Tube die with the long barrel, 3:1 compression ratio screw and standard breaker plate. Temperature (ºC)

Barrel 1 Set Actual

Screw speed (rpm)

5 165 168

Head 6 165 169

Die 7 175 171

Screw speed (rpm)

180 185

50

8.5 TWIN-SCREW EXTRUDER SAMPLING The lead stabilised dry blend shown in Table 8.6 was fed to a Schloemann BT80 twin-screw extruder. The machine was run for 1 h to ensure that equilibrium conditions had been reached and that good quality pipe was being produced. A sample of pipe was taken and the machine stopped and stripped down as rapidly as possible to obtain samples of the whole extruder profile. The individual components of the head and adaptor were removed in succession and each part of the ‘melt’ retained for further examination. The barrel was then carefully ‘racked-off’ leaving the screws filled with powder in different stages of densification and fusion. Material in each turn was numbered, from the screw tips back to the powder end, and collected so that TABLE 8.6 Schloemann Dry Blend Formulation Used in Both Extruder Sampling Experiments (pts) Polymer (‘Corvic’ S67/1 1 1) TBLS powder

100

144

M.W.ALLSOPP

(pts) (tribasic lead sulphate) n-PbS t powder Hoechst wax E (ethylene glycol montanate) CaS t Hoechst wax PA 190 (polyethylene, M n=9 000) Mixed to 120°C, cooled to 50 °C

1·6 0·5 0·4 0·5 0·15

a complete profile of material from the feed to the die was retained for optical examination (Figs. 8.5 and 8.6). Little apparent change in the material took place in the early screw turns and even the transition from the 4-start to the 3-start screw channel region, which subjects the material to its first major compression, produced little change. This region, turns 21/20, is hidden within the feed section of the barrel which could not be removed and thus cannot be seen in Fig. 8.5. The second major compression zone, 3 to 2 starts in turns 15/14, produced a noticeable densification and small colour change. The material at this point appeared very heterogeneous and lumps of well fused polymer embedded in powdery material were clearly seen (turn 9 in Fig. 8.5). As the material reached the screw tips the colour improved slightly but bands of different colours were still evident (Fig. 8.5). During its passage through the head region the colour improved appreciably especially after passing through the breaker plate. The memory of each of the break plate filaments was retained for some considerable distance in the melt especially in the split shown in Fig. 8.6 where the material divides over the spider. Similarly the memory of the spider lines was retained in the melt until about 2–3 cm from the die. It was only in this final region of the head that the melt appeared homogeneous and well fused with an absence of memory of earlier flow discontinuities. 8.5.1 Core Samples Taken from Schloemann BT80 Twin-screw Extruder 8.5.1.1 Changes to Grain Structure Material from the whole length of the extruder from feed section to extruded die was represented by 27 samples. Eight of these were selected to represent regions where significant changes in structure of the material might be expected to occur. At an early stage in turn 18 (Fig. 8.7b) the polymer grains show signs of densification at their surfaces (Fig. 8.7b). This develops rapidly and, at the same time, the PVC blend forms a solid but very friable mass in the screw channels. The upper left micrograph in Fig. 8.8 is a section through the core at turn 15 and shows its structure to be made up primarily of whole grains. The top micrograph in Fig. 8.9, at higher magnification, illustrates the degree of densification which the grains have undergone at turn 15. At the beginning of the 2-start screw profile (turn 13) the material at the bottom of the channel is unaltered (Fig. 8.8) and its granular composition is clearly shown in the upper right micrograph. Evidence

M.W.ALLSOPP

145

FIG. 8.5. General views of the material at different stages of fusion in the screws of the Schloemann BT80.

of shear and fusion can be seen in the upper half of the channel and this is gradually developed along the screw to take in more of the material (turn 7—Fig. 8.8). Notice that in these micrographs the elongation of the grains indicates a rolling action of the poorly developed melt in the screw channel. Also, the higher magnification micrographs clearly show that although fusion is well advanced the original grain boundaries have not been lost but are highlighted by the concentration of additives (turns 13 and 7—Fig. 8.10). At turn 4 are found the first indications of a homogeneous melt but this represents only a small fraction of the material in the channel. The range of structure represented is from a loose granular agglomerate to a well gelled melt, the predominant structure being that of the loosely fused grains which have undergone some amount of shear. By the time the PVC has reached the ends of the screws the degree of fusion is quite high but memory of both grain boundaries and shear is still very much in evidence (turn 1—Fig. 8.10). As the material advances through the adaptor and head region towards the die the degree of mixing improves, elongation gradually encompasses all grains and the inhomogeneity of structure and additive distribution gradually disappears (compare Figs. 8.10 and 8.11 with Fig. 8.12). The final material leaving the die is composed of well fused homogeneous material with little residual grain memory under normal operating conditions. If any residual grain memory is present it is found in the centre of the pipe crosssection with the inner and outer surfaces characterised by the featureless structure of a well gelled melt.

146

M.W.ALLSOPP

FIG. 8.6. General views of the material from the head region of the Schloemann BT80.

8.5.1.2 The Development and Subsequent Fading of Fluorescence When PVC degrades it will fluoresce so that an examination of the samples from the BT80 by ultraviolet fluorescence will enable an assessment of the level of degradation to be made. Examination of the samples taken from the BT80 shows that fluorescence of the unmixed polymer builds up to a maximum at the screw ends. Subsequently, as the melt passes through the head and the additives become more uniformly dispersed, the fluorescence dies away considerably until it is more-or-less absent from the final extruded material. Figure 8.11 shows the level of fluorescence in the loosely bound granular structure occurring in turn 13. This is the first of the samples (working from the feed end of the screw) in which fluorescence is detectable. Where individual sections have been folded over the increase in intensity of the fluorescence arising from the double thickness is quite noticeable. The bright line surrounding the grains is a diffraction phenomenon. The upper micrograph in Fig. 8.10 illustrates the dispersion of the additives and shows that in general they are located at the surfaces of the grains. Further along the screw, at turn 7, fusion is more pronounced and there has been an increase in the degree of mixing of the additives into the melt. However, the level of fluorescence, at the centres of the identifiable grains into which the additives have not penetrated, has increased (compare turn 7—Figs. 8.10 and 8.11). Elsewhere the fluorescence has been suppressed according to the depth of penetration of the zone of effectiveness of the additives. Again the upper micrograph shows the level of dispersion of the additives. As the polymer/additive melt becomes more homogeneous in increasingly larger regions towards the ends of the screws the level of fluorescence of the well mixed ‘gel’ is reduced to a minimum whereas regions still devoid of additives have become markedly more intense (compare turn 1—Figs. 8.10 and 8.11). Passage through the breaker plate increases homogenisation and produces a reduction not only in the size of the regions of additive-free polymer but also in the intensity of their fluorescence (Fig. 8.12). The final extrudate exhibits little if any fluorescence.

M.W.ALLSOPP

147

FIG. 8.7. (a) Internal morphology of ‘Corvic’ S67/111 polymer used in sampling experiments; (b) Schloemann BT80— material from screw turn 18.

8.6 SINGLE-SCREW EXTRUDER SAMPLING The 1.25 in Iddon was set up on dry blend as mentioned previously and tuned to give good pipe. The extruder was then stopped and with the heaters still operating the machine was stripped down in a similar way to the BT80 except that the screw was pushed out of the barrel with a hydraulic jack. A partial profile of material from compacted powder (turn 14) to the melt in the breaker plate was collected. The first powder sample retained was from turn 14 and severe degradation was apparent on the external surface of the material in the screw channel where it contacts the barrel wall (Fig. 8.13). The material in contact with the screw was less discoloured, the main degradation band being located in the outer 10–20% of the channel. By the time turn 4 had been reached the brown discoloration of the surface layer was severe. From turn 14 onward a thin zone of pale yellow material was formed at the leading edge of the turn (Fig. 8.13) and this region gradually increased in size along the screw until it occupied approximately half of the channel volume at the screw end (turn 1). The colour of the remainder of the material changed to purple on the outside and dark brown on the inside of the channel. As the material passed through the breaker plate the intense coloration disappeared and the melt became creamy white. An examination of the melt just after passing through the breaker plate showed that the internal regions were still discoloured. In particular the centres of the filaments towards the mid point of the melt showed greatest discoloration.

148

M.W.ALLSOPP

FIG. 8.8. Schloemann BT80—cross sections through screw turns 15, 13, 7 and 1 (common light).

8.6.1 Core Samples from a 1.25 inch Iddon Single-screw Extruder Sections taken through the scroll from this extruder show it to develop two distinct regions of very different structure. The first indication of this was obtained from the appearance of the scroll as it was withdrawn from the extruder barrel. It was found to be divided along its entire length into two differently coloured regions, the boundary between them being fairly sharply defined (Fig. 8.13). The leading edge of the scroll had the typical colour of well gelled PVC whilst the trailing edge exhibited a brownish purple discoloration. 8.6.1.1 Initial Structure The material in the first channel examined (turn 14) was found to consist of lightly fused complete grains which had undergone a high degree of densification with most of the additives located at the grain boundaries (Fig. 8.14c). 8.6.1.2 The Development of the Dual Structure At the front of the screw channel

As the material progresses along the screw a region of reasonably well homogenised but highly fused PVC develops at the leading edge. This region increases in size as the mix approaches the breaker plate but it never fills the whole of the screw channel and at best represents about 50 % of the material within the channel (Fig. 8.14b). The material in this melt region is highly oriented across the channels and indicates a rolling action along the length of the scroll.

M.W.ALLSOPP

149

FIG. 8.9. Schloemann BT80—centre of screw turns 15 (top), 13 (middle) and 7 (bottom) (D.I.C.). (N.B. gradual loss of intra- and then inter-grain porosity).

Micrographs taken within this region and also at the boundary between the two regions clearly illustrate the difference in structure. Figure 8.15 shows the elongation of the grain structure referred to above which thus has been developed from the less homogeneous texture which is represented on the right of Fig. 8.15, and which is described fully in the next section. The boundary between the two regions is quite sharp and represents, perhaps, the interface between a slowly advancing bank of highly sheared melt and a static compressed mass of grains. At the rear of the screw channel

Within this region the structure of the PVC varies from that in screw turn 14 to that of screw turn 1 illustrated in Fig. 8.14, an intermediate level being located in channel 6. In the first of these, the grains still retain some of their internal porosity but densification and intergrain fusion has developed quite significantly. Further along the screw, at turn 6, densification of the grain has become complete with near total fusion of the grain although there still remain a few voids in the matrix, particularly at the original boundaries. The common light micrographs show that at all stages along the screw, irrespective of the degree of fusion, the additives in the rear portions of the channels remain as concentrations around the grain boundaries. This is true right up to the breaker plate. Finally, at the upper surface of the scroll, where the PVC is in contact with the inner surface of the extruder barrel, the degree of homogenisation of the polymer and additives is very advanced even in the powder bed region (Fig. 8.14d). This suggests that the material in this part of the channel has experienced a high shear.

150

M.W.ALLSOPP

FIG. 8.10. Schloemann BT80—centre of screw turns 13 (top), 7 (middle) and 1 (bottom) (common light). (N.B. (1) Additives appear as black specks; (2) Fields of view are identical to Fig. 8.11; (3) Highly elongated grains in turn 1.)

As the material leaves the screw and enters the head region the melt pool migrates to the outside and the powder bed to the inside of the melt cross section. Passage through the head improves the dispersion of additives as a result of grain elongation particularly in the breaker plate and final die taper regions. As a result the dispersion in the final pipe is much improved compared to the state of the material at the end of the screw but not as good as that achieved in a twin-screw machine. 8.7 OTHER EXTRUDERS Sampling experiments were repeated on several other twin screw extruders, e.g. Kestermann K86, Cincinatti-Milacron CT 110/8, Krauss-Maffei KMD 90 and KMD 25, and single-screw extruders, e.g. Reifenhauser S60 and 2 inch Bone. In each case the overall mechanism of gelation was the same but extruders differed in that the stages of fusion and elongation occurred at different points along the screw/ head profile. 8.8 BANBURY HIGH SHEAR INTERNAL MIXER A formulation consisting of ‘Corvic’ S67/111 100pts, TBLS Powder 2phr, PbSt 1 phr, was mixed in the Banbury mixer to different drop temperatures between 100 and 210°C. Each ‘dolly’ was then immediately

M.W.ALLSOPP

151

FIG. 8.11. Schloemann BT80—centre of screw turns 13 (top), 7 (middle) and 1 (bottom) (fluorescent light). (N.B. (1) Fields of view are identical to Fig. 8.10; (2) Increasing level of fluorescence in additive-free regions.)

‘masson’ cut whilst still hot and the resulting chip examined in a ram extruder at 155°C at various shear rates using a zero length die. The flow pressures were measured and a graph of flow pressure as a function of drop temperature was produced. Samples of the chip from each drop temperature were prepared by embedding it in an epoxy resin and microtoming thin (3 µ m) sections. These were then examined using differential interference contrast, dark ground and fluorescence microscopy. 8.7.1 Microscopical Examination of Samples Compounded to Different Temperatures between 100 and 200°C The sample dropped at 110°C is a free flowing powder consisting of essentially unmodified grains. Some signs of early comminution can be detected and the additives are located at the surfaces of the grains. The sample dropped at 120°C is also a free flowing powder but there is an increase in the amount of agglomeration that has taken place. Examination of the powder component shows this to consist of unmodified PVC grains with the additives located at their surfaces, and seen as light specks, (Fig. 8.16a). The agglomerates are loosely bound fragments of comminuted grains into which the additives have penetrated deeper than in the whole grains. In the sample dropped at 130°C the degree of agglomeration has increased until it represents about 50% of the total volume. The powder component consists of whole unmodified grains and remnants of grains

152

M.W.ALLSOPP

FIG. 8.12. Schloemann BT80—breaker plate, (a) Common light; (b) fluorescent light. (N.B. (1) Fields of view are identical; (2) Lower levels of grain memory and fluorescence compared to Figs. 8.10 and 8.11.)

FIG . 8.13. General view of material from screw turn 12 to the breaker plate of the 1–25 in Iddon extruder.

which have been broken up. The agglomerates are composed of loosely bound large and small fragments of comminuted grains in which the primary particle structure is still quite apparent. The additives are becoming more uniformly dispersed except where the grain fragments are large and retain the original grain skin in which case they remain on the original grain surfaces. In the sample dropped at 140°C, the agglomerates are beginning to account for the majority of the bulk of the material. The powder that is present contains a large proportion of fragments of grains, the remainder being whole grains (Fig. 8.16b). The primary particle structure is generally maintained but there are signs of densification (loss of inter-primary voiding). The additives remain on the surfaces of the complete grains but are well dispersed among the comminuted grains. The agglomerates in this sample are composed of a mass of small fragments of grains although the occasional near-complete grain can be detected. The primary particle structure is still well in evidence but there are signs of densification. This is the lowest drop

M.W.ALLSOPP

153

FIG. 8.14. 1.25 in Iddon—cross sections of material from screw turns 14((a) and (c)) and l((b) and (d)). (a)Common light (N.B. no melt pool); (b) common light (N.B. melt pool occupies 50% of section); (c) centre of screw turn 14— common light; (d) nature of material in contact with barrel wall in powder bed zone (fluorescent light).

FIG. 8.15. 1.25 in Iddon—screw turn 1. (a) Melt pool region (N.B. highly elongated grains); (b) melt pool/powder bed interface (N.B. distinct interface); (c) powder bed region (N.B. densified coherent grains identified by ribbons of additives) Identical fields of view: (1) common light; (2) fluorescent light.

temperature in which inter-primary voiding is noticeably decreasing. The additives are fairly well dispersed in these agglomerates but there are additive-free ‘islands’ corresponding to large fragments of grains and large voids. The sample dropped at 150°C contains only a little free powder which is made up of a few whole grains and fragments of grains in both of which can be seen obvious signs of densification. The additives remain on the surfaces of the whole grains but are well dispersed in the fragments. The agglomerates, which form the major part of the sample, still retain the primary particle structure together with the associated voiding. They are composed almost entirely of grain remnants and densification is apparent in isolated patches. The dispersion of the additives within the agglomerates is good with a few ‘islands’ created by near-complete grains and large voids. Very little free powder is present in the sample dropped at 160°C and what there is is made up of unmodified complete grains and small agglomerates of fragments in which densification is well advanced.

154

M.W.ALLSOPP

FIG. 8.16. Banbury mixer material at different temperatures: (a) 120°C; (b) 140°C; (c) and (d) 160°C; (e) and (f) 200° C.

The bulk of this sample is composed of grain fragments in which densification is again well advanced (Fig. 8.16d), although there still remains a large proportion (about 40%) of voids. Primary particle structure is still recognisable and the additives are well dispersed (Fig. 8.16c and d). No free powder exists in the sample dropped at 170°C, which is composed of a well densified PVC compound matrix. A number of large voids (>10 µ m) do exist, and at the surfaces of these, and at the surfaces of the agglomerate itself, the original primary particle structure is still evident. The additives are uniformly distributed throughout the matrix. The sample dropped at 180°C is a highly densified UPVC matrix containing a number of large voids at the boundaries of which may be seen memory of the primary structure. The additives are well dispersed. The sample dropped at 200 °C consists of an apparently homogenised matrix containing a number of large (ca 100 µ m) spherical voids (Fig. 8.16f). All memory of both grain and primary particle structure has been lost, and the additives are uniformly distributed (Fig. 16e and f).

M.W.ALLSOPP

155

8.8.2 Surface Area and Porosity of Banbury Compounds Surface area and porosity were measured on each of the samples from the Banbury, up to and including the material dropped at 170°C. The results, which are presented in Fig. 8.17, show a similar trend to the variation of flow pressure with drop temperature presented in Fig. 8.18. Both surface area and porosity levels begin to decrease around 140°C, as does the flow pressure, and around 170°C are virtually zero, whilst the flow pressure begins to increase around 160°C as chain entanglement increases. 8.8.3 The Gelation Process in the Banbury Mixer In the early stages of the process the additives are located on the surfaces of the grains which remain discrete and the action of the Banbury mixer results in attrition of the PVC grains, although there are no signs of actual abrasion. This results in the comminution of the more friable grains and the development, from the remnants of these, of small agglomerates. As the dwell time, and hence the temperature, of the mix within the Banbury increases the number of grains broken down becomes greater with a consequent increase in the uniformity of distribution of the additives within the polymer matrix. Although loosely bound agglomerates are formed as the grains break down, the structure of the compound retains the identity of the primary particles and their aggregates together with the characteristic inter-primary voiding. Consequently, the surface area and porosity remain almost constant. Within the temperature range 140–160°C, the bulk of the compound, which has become a mass of primary particle structure with little if any grain memory, gradually loses the inter-primary voiding and densifies thereby losing the primary structure identity, and accompanying surface area. However, the primary structure is still evident at the boundaries of the agglomerates and at the surfaces of the large internal voids which are developing. As the Banbury temperature increases further, even this memory is gradually lost until, ultimately, the boundaries become smooth and typical of a true melt. It is not until material compounded at temperatures between 140 and 150°C is run on the rheometer that an acceptable colour of extrudate is produced. The production of this white extrudate corresponds with the period during which a good dispersion of the additives is first achieved. It is known from the examination of the gelation process in extruders that early signs of degradation in UPVC can be suppressed by subsequent rapid dispersal of the stabiliser throughout the polymer. It is just such a phenomenon which is being reproduced in this instance. The deterioration in colour that follows in samples compounded above 150°C is the result of degradation due to excessive thermal treatment arising from the accumulative effects of higher Banbury temperatures and rheometer extrusion. Although fluorescence was not detected in the Banbury samples, as it was during extrusion, this does not mean that the initial stages of degradation were not present. It suggests that conjugated sequences of sufficient length are not formed due to the superior additive dispersion in the Banbury as a result of grain comminution. The densification mechanism operating in extruders allows the temporarily unstabilised regions in the middle of grains to degrade to the point of fluorescence before grain elongation improves additive dispersion and reduction of conjugated sequence length.

156

M.W.ALLSOPP

Fig 8.17. Variation with Babury compounding temperature of (a) S67/111 surface area; (b) S67/111 internal porosity

8.8.4 Relationship of Morphology with the Flow Pressure Curve Comparing the process of gelation with the zero length die flow curve (Fig. 8.18), it is found that the period during which attrition and comminution of the grains are the dominant factors corresponds to the initial plateau of the flow pressure graph. Porosity and surface area remain virtually constant over this region.

M.W.ALLSOPP

157

FIG. 8.18. The zero length die flow curve of ‘Corvic’ S67/111 (subdivided into regions on phenomenological and morphological features).

Following this plateau the flow pressure begins to fall in the last stages of this period but in this region there is to be found a growing dominance of the second period of gelation, namely that of densification of the primary structure, with loss of porosity and surface area. As this begins to dominate the process the flow pressure drops and forms the central depression of the flow pressure curve. With the densification of the primary structure complete, the flow pressure rapidly increases corresponding to the third gelation period: that of loss of primary structure memory. The onset and increasing severity of melt fracture corresponds to the period of progressive loss of memory of primary structure. It is in this region that the degree of surface chain entanglement and cohesion between the primaries is increasing, thereby reducing the mobility of these entities in the polymer. It is perhaps this very loss of mobility that gives rise to the melt fracture of the extrudate in that the material is increasingly unable to conform to the restriction of the die except by elastic deformation. If this is inhomogeneous in nature then when the pressure is released as the material emerges from the die, the nonuniform relaxation would result in a poor surface quality of the extrudate. 8.9 BRABENDER PLASTICORDER A similar series of experiments to that conducted on the Banbury mixer was completed on the Brabender Plasticorder. Using the same formulation the Brabender was charged with 37 g of dry blend at a temperature of 125°C. The temperature was increased at 4 °C per min and a pressure of 3500 g/cm2 applied to the ram. Both rotors were run at a speed of 40 rpm and a series of samples taken at discharge temperatures between 130 and 210°C at 10°C intervals. Each sample was examined optically and flow pressure measurements taken from a ram extruder.

158

M.W.ALLSOPP

FIG. 8.19. Two-roll milling (dark ground illumination).

The results obtained were remarkably similar to those recorded for the Banbury drop temperature series and for the sake of brevity will not be reiterated here. 8.10. TWO-ROLL MILL A two-roll mill set at 160°C was fed with a sample of the dry blend used for the twin-screw extruder sampling experiment and a sample of the ‘crumb’ taken after each passage through the mill rolls. A very rapid densification of the grains was observed followed immediately by extensive elongation (Fig. 8.19). No sign of grain comminution could be detected even though the rate and extent of elongation was greater than that recorded for extruders. 8.11 SUMMARY Previously, the mechanism of gelation of UPVC in extruders was considered to involve breakdown of the PVC grains into primary particles (1 µ m) and aggregates (2–10 µ m) and subsequent fusion of these entities into a ‘gelled’ mass. We have shown that this is not the normal mechanism of gelation of UPVC, instead a totally different process occurs involving the compaction, densification, fusion and elongation of grains, the

M.W.ALLSOPP

159

FIG. 8.20. Mechanism of gelation during processing.

FIG. 8.21. A classification of processing machinery in terms of the comminution densification mechanism of gelation.

CDFE mechanism, with little if any comminution of the PVC grains taking place (Fig. 8.20). This confusion has arisen since published work generally uses high shear/aggressive mixing equipment, e.g. Brabender, as the basis for mechanistic studies. In our work we have confirmed that a considerable degree of comminution takes place in both the Brabender Plasticorder and the laboratory scale Banbury mixer but the mechanism operating on this specialised equipment cannot be transposed to extrusion. The behaviour on different machines is summarised in Fig. 8.21 and is related to the temperature, pressure and shear profile of each machine. If conditions are such that pressure/shear is applied before densification is complete, i.e. Brabender, grain comminution results but in the less aggressive conditions in an extruder, grain densification is complete before shear forces can be applied. The importance of pressure

160

M.W.ALLSOPP

on gelation mechanism is seen in Fig. 8.21, where the large scale Banbury mixer and the ICI modified Brabender behave in an intermediate manner. In each case the machine is operated under a lower pressure regime than normal and a partial changeover in mechanism from comminution to densification is recorded, markedly so in the case of the ICI modified Brabender. Use of the latter ensures that more relevant studies can be completed on the small scale since the mechanism is more typical of that in extruders. ACKNOWLEDGEMENTS The enthusiastic co-operation of Tony Curson, Mike Hitch and Alan Skelton in this study is gratefully acknowledged. REFERENCES 1. 2.

HANSON, D., Polym. Eng. and Sci., 9(8), 405 (1969). GALE, G.M., Plastics and Polymers, 38, 183 (1970).

Chapter 9 THE PROCESSING OF RIGID PVC D.A.TESTER PVC Development Group Leader, 1CI Ltd, Welwyn Garden City, UK

9.1 INTRODUCTION No other thermoplastic and few naturally occurring materials can match the range of variety of PVC applications. But for familiarity, it would surely seem remarkable that water pipe, cable insulation, squash bottles and surgical gloves can all be manufactured from the same synthetic polymer, particularly one that seemed intractable and unattractive when first discovered. This vast exploitation of PVC has been made possible by the impressive formulation technology that has grown up, together with the increasing skills of the processors and the machine manufacturers that serve them. The most radical change that can be effected by additives is, of course, plasticisation, generally by liquid esters, to produce flexible products. Since plasticised PVC can be so different from the unplasticised polymer, both in ease of processing and in product properties, it will be considered in a separate chapter. Here, it is intended to deal only with rigid PVC, where, in spite of the importance of additives, poly(vinyl chloride) constitutes by far the major constituent of the plastic composition. The particular problems of processing rigid PVC arise from those fundamental properties of the polymer which are also the source of its particular advantages. Poly(vinyl chloride), as commercially manufactured, is a predominantly amorphous polymer, although there is a low but significant degree of crystallinity with regions of syndiotactic structure.1 In contrast with highly crystalline polymers it does not exhibit a sharp melting point, but gradually softens above TG, ultimately forming a viscous melt, which becomes more fluid with increasing temperature. In the absence of heat stabilisers, degradation through dehydrochlorination would generally occur before temperatures for satisfactory processing could be reached. Even with the addition of stabilisers, the temperature at which degradation and subsequent coloration occurs quite rapidly is not far above that reached in some melt processing operations. Thus, there is a requirement for careful temperature control, streamlined flow paths, and absence of stagnation in the processing of rigid PVC compositions. As explained in previous chapters, the grains or granules of around 100 µ m dimensions, isolated in manufacture of suspension and mass polymers, persist to a remarkable degree, and even a superficially homogeneous melt may contain significant granule memory. Further, there is evidence that the much smaller primary particles, from the early stages of polymerisation (see Chapter 7), can persist beyond the destruction of the grains. It is indeed likely that some degree of order, and memory of the smallest ‘particles’, persist through any practical melt processes. Crystalline melting points have been obtained by various techniques of extrapolation2,3 but are evidently in the region of 210–240°C for typical polymers, and

162

D.A.TESTER

therefore at temperatures where polymer decomposition becomes quite rapid. In these circumstances it is not surprising that the visco-elastic properties of PVC melts are complex and variable. The nature of this complex behaviour and its practical significance in processing and fabrication constitute a principal theme of the presentation that follows. The relatively broad transition between the solid and melt states, and the visco-elastic properties of the PVC melt, provide advantages as well as problems to the processor. Thus, the high melt stiffness of a rigid PVC extrudate facilitates handling and sizing. Whilst high melt viscosity and limited heat stability limit flow paths during injection moulding, the moulded article exhibits much less shrinkage or distortion than is encountered with many other plastics. PVC foil may be readily thermoformed, because of the controlled and partial softening that can be achieved on reheating. With continuing improvements in machine design, in formulation technology, and in the range of polymer grades available, rigid PVC is processed today by all the major techniques available to the plastics industry. These include the extrusion of pipe, complex profiles, sheet and foil, calendering, thermoforming, injection moulding and blow moulding. The application of these various techniques is fully described in standard texts4–6 whilst modern advances in particular areas are frequently discussed at PVC symposia. It is not proposed, therefore, to consider particular techniques in detail here, except where they illustrate the general principles which it is intended to explore. In this context, the most frequent reference will be to extrusion, because it is the most commercially important of the techniques used to convert unplasticised PVC into useful products, and because as a consequence it has been subject to the most detailed and fundamental study. 9.2 FEEDSTOCK PREPARATION It will be apparent that PVC requires to be intimately mixed with other minor, but essential, ingredients for the fabrication of rigid articles. The significance of these additives will be considered later, but it is necessary, at this juncture, to define the nature of the mixed feedstocks which are fed to processing/ fabricating equipment. Broadly, there are two basic techniques employed. The polymer and its additives may be mixed in the form of a free flowing powder, which is subsequently fed to machines capable of accepting this physical form for completion of the other processing steps. Alternatively, as a preliminary operation, the composition may be mixed and gelled, then chopped into pellets of about 3mm size, and this fully gelled ‘compound’ employed as feedstock for fabricating machines, in which it is remelted and shaped. 9.2.1 Compound Manufacture Historically, the earliest exploitation of rigid PVC was on machines designed to accept and process pelletised compound rather than powder. The compound was therefore manufactured as a separate operation, either by the fabricator, or more usually by the polymer supplier or an outside company. This technique is still widely employed, particularly where the fabricator requires to use a range of compounds to produce a variety of exacting products, e.g. in profile extrusion. The compound operation is required to achieve adequate and uniform gelation of the polymer and thorough dispersion of additives. It will be appreciated from the introductory remarks that the rheological properties of the melt will change with increasing processing, with alteration to the subsequent behaviour of the compound, so that close control of the heating and shearing treatment is necessary. The original process, following the experience of the rubber industry, entailed mastication of the mixed ingredients in a Banbury or similar internal mixer, followed by

THE PROCESSING OF RIGID PVC

163

sheeting on a two-roll mill with subsequent cooling and cutting of the compound. In some respects this twostage process can be highly effective, in that it facilitates partial separation, and therefore sensitive control, of the heating and shearing process, but there are obvious disadvantages. It is essentially a batch process, relatively labour intensive, and needing a separate cutting step to produce the small chip or pellet form required for feeding fabricating equipment. Today, this procedure has been largely displaced, in rigid PVC compounding, by the use of continuous compounding machines. These are exclusively screw machines, but may be broadly divided into single-stage machines where compounding and extrusion of the melt through the die take place in one unit, and two-stage machines. These separate the high shear plasticising homogenising section from a lower shear extrusion section which simply transports melt to the die and die face cutter, to produce the required pellets. Twin-screw and planetary screw machines, reciprocating screws, and a combination of mixing rotors and separate single-screw extruder in sequence are all in commercial use. Whatever equipment is chosen, there is, of course, a common requirement for intensive shearing and homogenising without localised overheating in the first stages, exact temperature control, and uniform treatment of all the material. 9.2.2 Powder Blend Manufacture In volume terms, the processing of rigid PVC is dominated today by the use of powder feedstocks, particularly in the area of twin-screw extrusion of pipe (see Section 9.3), which represents a major tonnage PVC application. The dry powder blend needs to be more than a superficial mix of the ingredients, and invariably requires the use of a ‘high speed’ mixer. The essential feature of such a mixer is that movement of the components is achieved by a rotor at the base of the chamber, rotating at speeds up to several thousand rpm. The powder is thrown out by the centrifugal action of the rotor and rises up the walls of the mixing chamber, to return down the central zone, creating a fluid like movement with a vortex. Frictional heat is developed, and can be augmented by heat from the jacket, to minimise cycle time in achieving ‘hot’ mixing (though not to such a temperature as to promote incipient gelation of the mass of polymer!). It is customary to discharge the mixed material, at the end of the mixing cycle, into an associated low speed cooling chamber, where it cools to a temperature at which it will not suffer agglomeration and compaction on standing. This process not only provides intimate and thorough mixing, but causes additives, particularly liquid or low melting point components, to adhere to, and in some cases to be absorbed by, the surface of the polymer grains. The performance of subsequent gelation/fabrication processes depends critically on the nature of the dry blend fed into them, which in turn is a function, not only of the efficiency of the dry blender, but of the properties of the polymer grade employed, and the judicious choice of additives. This aspect will be referred to later. Given the provision of suitable processing equipment, the use of dry blend feedstock has the obvious advantage over pelletised compound of lower capital and energy costs for feedstock preparation. It has the minor disadvantage of not being readily applicable to solid additives that are not provided in a powdered or easily friable form, and can create dusty working conditions. In large scale operations, the latter disadvantage is increasingly being obviated by the use of automatic and largely enclosed systems of powder handling.

164

D.A.TESTER

9.3 EXTRUSION Since studies of PVC processing relate so frequently to extrusion, it is pertinent to consider this process specifically. The use of screw extruders for rubber and plastics processing has been common since the earliest days of these industries, and the concept of extrusion will no doubt be familiar in some context to most readers. The earliest machines employed in the processing of rigid PVC were single-screw extruders, fed by compound. Because of the limitations of heat stability and high melt viscosity it was soon realised that particular care has to be exercised to ensure a continuous flow of material at all points of the flow path, with no stagnation through sharp corners or chips and scratches in metal surfaces. Obstructions in the flow path, such as the mandrel, torpedo or breaker plate, must be as streamlined as possible, whilst the screw tip should not be square but hemispherical or conical with a curved end. To avoid excessive frictional heating, compression ratios must be relatively low compared with plasticised PVC, whilst screws need to be long (e.g. employing an L/D ratio of over 20:1). Single-screw machines continue to be used most successfully for compound feedstocks, but for rigid powder blends twin-screw extruders are much more widely employed today. With the increasing use of dry blend feedstocks for the very large tonnage manufacture of pipe and conduit, twin-screw machines have attained a dominant role in rigid PVC extrusion during the last ten years. To appreciate the advantages of twin-screw machines in the extrusion of rigid PVC, particularly from powder blend, it is necessary to consider the very different mode of operation of single- and twin-screw extruders. A single-screw extruder has a screw rotating in a closely fitting barrel. If the molten polymer sticks to the screw and slips at the barrel surface the material will simply revolve with the screw and not be transported forward at all. To achieve maximum output, the polymer has to slip as freely as possible on the screw surface but have a high coefficient of friction at the wall. The rotational speed of the extrudate will then be less than that of the screw, so that the screw flights force the material along the barrel and through the die. The operation thus depends critically on friction between the polymer and barrel wall, and frictional heating supplies much of the heat input to the system. An increasing output requires increasing screw speed and therefore higher frictional heat generation, with increasing possibility of local overheating in a heat sensitive material. In an intermeshing contra-rotating twin-screw extruder, by contrast, material flow does not depend on friction against the barrel. As the material rotates with the screw, it reaches the point of intermeshing and is passed over to the other screw. The material is thus kept moving forward in discrete C-shaped segments, and the machine behaves very much as a positive displacement pump. The transport efficiency of the twinscrew machine is thus greater, whilst less frictional heat is generated. Heat input for melt formation is therefore supplied predominantly by conduction from the heated barrel, and temperature control is enhanced. For a given output, the twin-screw machine operates at a lower screw speed, and can employ deeper flighted screws. Whilst completely intermeshing screws would provide the most effective material transport, they would also minimise mixing outside each discrete chamber. A significant degree of leakage flow between the twin screws is therefore essential, but must to some extent compromise the desirable features described. Modern screw designs have sought to optimise performance by, for instance, providing close intermeshing in the initial solid transport and melting zones, with increased leakage in the final metering zone leading up to the head and die. In extrusion from powder blend it is necessary to eliminate polymer grains persisting in the gelling mass, and to thoroughly disperse powder additives within the melt. This process is facilitated by the mode of operation of the contra-rotating twin-screw extruder. It is at the points where the screws intermesh that the material encounters the highest shear rates. When the screws rotate in opposite directions, with some

THE PROCESSING OF RIGID PVC

165

FIG. 9.1. Schematic illustration of intermeshing twin screws.

leakage, the material is thus forced between them, rather in the manner of a twin-roll mill, and there is an effective dispersing action. Schematic diagrams of counter-rotating twin-screw operation are shown in Fig. 9.1. The foregoing treatment of twin-screw machines is of necessity simplistic and entirely in the context of PVC extrusion. Readers seeking a more fundamental analysis will find a detailed and comprehensive study of the subject in Twin Screw Extrusion by Leon Janssen.7 To summarise what has been said here, contrarotating twin-screw extruders provide high and consistent output rates with less shear heating than in singlescrew machines. With the optimisation of screw design and degree of leakage, they can also provide good fusion and mixing characteristics, without excessive sacrifice of positive pumping action. This makes them particularly suitable for the extrusion of large diameter pipe, where economics demand powder feeding and the minimum inclusion of relatively expensive heat stabilisers and lubricants, with a product of consistent wall thickness and quality. In typical mode of operation the output of the twin-screw extruder is governed by the achievable rate of mass transport of powder into the feed section, and thus by the bulk density of the powder blend. The gelation of the polymer and the development of the required properties in the extrudate also depend critically on the properties, proportions, and interaction of the ingredients of the powder blend. Before considering the role of the individual components of a PVC composition, it is appropriate to examine the changes occurring during gelation and the influence of gelation level on the properties of the melt and the final product. 9.4 THE INFLUENCE OF PROCESSING ON PROPERTIES As already stated in Chapter 8, gelation in screw machines proceeds predominantly through densification and fusion of the grains of suspension or mass polymer, which are generally of around 100 µ m dimensions (although in internal mixers, such as the Banbury, comminution of granules may also be

166

D.A.TESTER

FIG. 9.2. Different processing levels of UPVC examined using optical microscopy with dark ground illumination. (Reproduced from Gotham and Hitch, British Polymer Journal, 10, 47, 1978.)

involved). Although modern machines might be expected substantially to destroy granule memory in the melt, superficially ‘well gelled’ products may still contain discernible particles of polymer or inadequately dispersed ingredients. These may not affect bulk properties or appearance very much, but they can act as points of incipient weakness, influencing fracture behaviour. The effect of residual granules on the fatigue lifetime of rigid PVC was demonstrated by Gotham and Hitch.8 They obtained well processed and poorly processed pipe samples, by deliberate changes in extrusion conditions, detecting the differences by optical microscopy. Under transmitted dark field illumination, PVC shows as an area of darkness, and processing additives appear as bright specks. Figure 9.2 shows photographs obtained on thin sections taken from samples representing two markedly different processing

THE PROCESSING OF RIGID PVC

167

FIG. 9.3. The effect of processing level on the fatigue lifetime of UPVC, 23 °C, 50% RH. (Reproduced from Gotham and Hitch, British Polymer Journal, 10, 47, 1978.)

levels. In the poorly processed sample (level 2) it will be seen that there are discrete dark areas of around 100 µ m dimensions. These are undispersed polymer grains. The influence of these differences in processing level on dynamic fatigue behaviour was measured by subjecting plain specimens to a range of stresses, cycled between tension and zero, and plotting the number of cycles to fracture. The results, reproduced in Fig. 9.3, illustrate the superiority of the well processed sample. When sharply notched specimens were fatigue tested, however, there was very little difference in behaviour between the specimens. It was inferred from this observation that changes in processing, at this level, affect the crack initiation component of fracture, rather than crack propagation. Even assuming effective destruction of granule memory, there is evidence that much smaller primary particles of micrometre and sub-micrometre dimensions persist to appreciable extents throughout melt processing.9–11 It might be asked what significance ‘particles’ of micrometre dimensions can have in a continuous melt of a single polymer. It must be assumed that an appreciable degree of chain entanglement, or more important, chain alignment and pseudocrystallinity, exists within each primary particle, with initially little such interaction between particles. As gelation proceeds, however, there appears to be increasing interaction between particles, with their gradual diminution or destruction, and the build-up of a new ‘melt structure’. It is believed that this progressive breakdown of particulate boundaries, with the formation of network structures through the melt, produces the rheological changes that take place as the melt is subjected to high temperature processing. For experimental purposes a PVC composition may be subjected to different thermal histories by mixing to different drop temperatures in a Banbury mixer or Brabender Plasticorder, or by milling at a range of elevated temperatures. Workers using these techniques have invariably found that increasing thermal treatment has resulted in an increase in the pressure necessary, under standard conditions, to extrude the resultant compound through the die of a ram extruder such as a capillary rheometer or extrusion plastometer. Plots of extrusion pressure and stock temperature, although varying in detail with changes in composition and processing equipment, generally follow a

168

D.A.TESTER

FIG. 9.4. Rheometer pressure versus processing temperature.

similar pattern, as illustrated by the largely supportive evidence of two separate papers recently presented.12,13 An idealised form of the extrusion pressure/stock temperature curve is shown in Fig. 9.4. The minimum in the pressure/processing temperature plot may be considered a threshold for good gelation. Pressures rise quite steeply through the temperatures typical of unplasticised PVC processing, to give a maximum in the region of 190–210°C. The material state corresponding to this maximum is variously described as ‘fully gelled’, ‘highly processed’ or ‘100 % gelation level’. This is not to imply that this is an optimum condition to be achieved in all melt processing. Indeed, in Gray’s work13 it was noted that the extrudate quality in the rheometer tests passed from smooth to rough, and ultimately lumpy, with increasing process temperature. Such measurements are generally made with dies of very low length: diameter ratio, effectively with a zero length die, so that the measured pressures are dominated by the elastic deformation of the melt at the die entry. It is not surprising, therefore, to find that the pattern of elastic recovery and surface appearance of the extrudate leaving the die are also influenced by processing history. The appearance of true lumpiness is likely to be associated with heterogeneous shear heating, and therefore thermal history, within the material, leading to a heterogeneous build-up of network structures in the melt. Where compounds are manufactured for subsequent reprocessing in fabricating equipment, there is no universal level of gelation to be prescribed, beyond a necessary minimum. As previously stressed however, it is clearly essential to control heat/shear history to give consistent performance during further operations. But what of the effects of thermal history and gelation level (beyond substantial destruction of the original grains) on the mechanical properties of the final product? Needless to say, this has been of particular concern to the pipe manufacturers, especially for pressure pipe required to withstand rupture under internal pressure for many years of service. To predict long term performance, times to burst under high internal water pressure are commonly measured, and regression lines are extrapolated to indicate the times at which, after progressive fatigue, the pipe would fail at service pressures. Such tests cannot give very rapid results and are unsuitable for routine quality control. In practice, manufacturers must rely largely on tests of impact strength, which are cumbersome and not unequivocally related to long term performance, or on

THE PROCESSING OF RIGID PVC

169

observations of the resistance of the pipe to attack by powerful solvents, such as methylene chloride. The possibility that a quantitative measure of gelation level might provide a single control test for pipe quality during the extrusion process is obviously attractive, and has prompted detailed studies by Benjamin14 on the influence of processing on the properties of PVC pipe. He obtained samples of PVC compositions at differing gelation levels by preparation on a twin-roll mill over a range of temperatures, and extruded these samples on a capillary viscometer, to obtain pressure/processing temperature curves such as have already been discussed. For a particular PVC formulation he was thus able to define a minimum and maximum gelation level, as defined by this approach. Samples from the wall of pipe manufactured from the same formulation were then tested on the capillary viscometer in the same way to yield measurements of pressure. By relating these pressure readings to the maximum gelation level already determined from the relevant curve, a percentage gelation level could be derived for each pipe sample. The pipes were manufactured using varied extrusion conditions to produce four significantly different gelation levels, and a range of physical properties was measured. Properties concerned with brittle/ductile behaviour showed an optimum value and then decreased. Properties concerned with stiffness showed an increase over the whole gelation range but tended to a maximum value. It appeared that an optimum balance of properties required a gelation level of around 60%. This is clearly a very valuable approach, but in the opinion of the present author, the level of gelation, so defined, does not necessarily provide a complete definition of pipe quality. This is because the incidence of failure under stress will depend not only on the bulk properties of the material, but on the presence of particular faults or discontinuity in properties in the pipe wall. These may arise from the extruder itself, as when the melt flow, divided by obstructions such as the spider legs or breaker plate, does not completely ‘heal’, but retains memory of the flow defect. Granted that this is less likely where a coherent melt has already been established, there are also possibilities of other defects within the melt. The most obvious source is the presence of discrete particles, acting as points of crack initiation if they occur near the surface of the pipe wall. These might be caused by gross contamination, undispersed additives or abnormal polymer granules (further discussed in the following section) which resist gelation. There might also be heterogeneity of melt structure within a seemingly ‘well gelled’ material, arising from inhomogeneous heating and shearing in the screws, and inadequate melt mixing. In the extreme case, it would be possible, through a combination of inferior feedstock and faulty extrusion, to obtain a melt of good average gelation level that contained a mixture of highly gelled and poorly gelled material, and with a low but significant incidence of ungelled granules. To define pipe quality unequivocally it would therefore seem desirable to combine measurements of gelation level with other tests, including appraisal of extrudate appearance and optical examination of sections through the pipe wall.

170

D.A.TESTER

9.5 THE INFLUENCE OF FORMULATION INGREDIENTS ON PROCESSING BEHAVIOUR 9.5.1 The Polymer 9.5.1.1 Grain Size and Morphology The salient processes of PVC manufacture, and the characteristic polymer types produced, have been described in previous chapters. In the context of rigid PVC processing, the interest lies almost wholly with suspension and mass polymerised grades, with suspension polymer providing by far the greater tonnage. The fine particle polymers produced by emulsion polymerisation are also employed by some European fabricators, but only in a very minor proportion with suspension polymer, to assist the processing of the latter, as, for example, in the extrusion of exacting profiles. Primarily because of the higher cost of emulsion polymer, its use in the United Kingdom is almost entirely limited to plastisols or other specialised applications, and it does not feature in rigid formulations. In general, the average grain diameter of suspension or mass polymerised polymer is in the range of 100–160 µ m, but there is, of course, a distribution of grain size about the mean. Very fine grains should be avoided as far as possible, since they introduce dust problems in handling, and they also tend to impede free flow of the polymer and its powder blends. On the other hand, the powder should be substantially free of grossly oversize grains, which can provide difficulty in dispersion and gelation. Polymers in commercial production may vary, according to the particular grade, in typical shape of grain, from irregular to fairly uniform and spherical. Whatever the precise distribution of grain size and shape, it must be concordant with related aspects of particle behaviour, discussed below. The rate determining step in fabrication processes employing dry blend feedstocks is frequently found to be the achievable rate of mass transfer of powder into the feed section of the machine. In particular, output rate for twin-screw extrusion of pipe at a given screw speed, and under conditions of flood feeding, is proportional to the bulk density of the powder blend. This, in turn, is closely related to the bulk density of the initial PVC powder. The bulk density of the initial powder will not be of immediate concern to the fabricator employing a fully compounded feedstock, but even here the rate and efficiency of production of that compound may depend on the bulk density of the powder precursor. For a given particle shape and size distribution, the bulk density of the powder must depend upon the density of the individual granules. The grains of both mass and suspension polymers are to some extent porous, from their process of formation, and in the case of suspension polymers the degree of porosity can be varied between very wide limits. It might be argued that the ideal polymer for powder feedstocks would be completely devoid of porosity, to give maximum density. In practice, such grains would absorb very little heat stabiliser or lubricant during powder blending, would densify and fuse with difficulty, and for higher K-value at least, give unsatisfactory gelation. At the other extreme, very highly porous grains would give low bulk densities and excessive absorption of lubricant during blending and would be unsuitable for rigid applications employing dry blend. The bulk densities of polymer grades actually employed, particularly in pipe manufacture, are thus a compromise between the requirements of ease of gelation and output rate. There is, of course, no ideal combination of particle properties for all processing machines and applications. The paramount need in any particular grade is for consistency of properties, so that an optimum relationship of formulation, output and

THE PROCESSING OF RIGID PVC

171

quality can be established and maintained by the fabricator. In addition to the need for general consistency of particle morphology, satisfactory processing depends upon the avoidance of coarse glassy grains, even at trace levels. By persisting as ungelled inclusions in the gelling mass of normal grains, these hard particles can give rise to faults, initiating brittle failure in rigid pipes or sections, or appearing as visible ‘nibs’ in glass clear products. 9.5.1.2 K-value As might be expected, the molecular weight of the polymer has a marked influence on the processing behaviour of an unplasticised PVC composition. As manufactured commercially, the degree of chain branching and the molecular weight distribution of polymers made at a given temperature show little variation, so that a measure of the average molecular weight defines the polymer, in this respect. In practice, an indication of molecular weight is obtained from the relative viscosities of dilute polymer solutions, compared with pure solvent, the results being expressed as ISO Viscosity Numbers, or more commonly as derived ‘K-values’. The higher the K-value, the higher the molecular weight, the DIN K-value of most commercially exploited polymers lying between 50 and 75. A lower K-value gives lower melt viscosity at a given temperature, and in general, easier processing, whilst a higher K-value is generally associated with better mechanical properties in the fabricated product. The choice of K-value for any particular rigid PVC composition is thus a compromise, based on the competing demands of processability and product properties. Where ease of processing and absence of discoloration in the products are of greater importance than mechanical strength, or where the shearing and melt flow requirements of the process are particularly severe, a relatively low K-value is commonly chosen. Thus, for rigid injection moulding, blow moulding of bottles, and production of extruded or calendered foil, K-values in the 55–62 range are generally employed. For the extrusion of clear sheet, profiles and sections, K-values of 60–68, and particularly in the 62–66 range, are chosen. In the extrusion of pipe, particularly large diameter pipe, the mechanical properties of the product are of paramount importance, and polymers of K-values around 66–68 are generally offered. The most exacting product in this respect is large diameter pressure pipe, and for this application a K70 polymer is commonly employed in Western Germany. In principle, strength should increase progressively with K-value, but in practice, with decreasing ease of processing, it is debatable whether significant improvements are achieved much beyond K68. Apart from the particular German example mentioned, there seems to be little exploitation of polymers above K68 in rigid applications. As the K-value of suspension homopolymers is decreased, and therefore polymerisation temperatures are increased, the polymers tend inevitably to be of denser grain type. Whilst this tends also to give high packing densities, it militates against easy gelation. This trend would thus seem to be in opposition to the easier processing achievable through the reduced melt viscosity of lower molecular weight polymers, already discussed. It is indeed possible in some processes to show the deleterious effects on gelation of the denser grains of lower K-value polymers. In practice, however, low K-value polymers are not normally extruded from powder feedstocks into pipe or profiles, but are employed in blow moulding, injection moulding and the extrusion and calendering of foil. These are high shear processes that adequately break down the granular structure of low K-value polymers even where these granules are relatively dense. In such processes it is heat degradation of polymer and flow defects that are more important, and the low melt viscosity of the lower K-value polymers can help to minimise these. In applications such as glass-clear foil or bottles, where appearance is of paramount importance, it is particularly important to avoid dispersion faults. These may arise not only from very hard glassy particles or poorly dispersed additives, but from

172

D.A.TESTER

contamination by any polymer of appreciably higher K-value. The grains of the higher K-value polymer persist as separate entities in the melt, giving rise to characteristic ‘fish-eyes’ in the final foil or bottle wall. In the previous section, it was shown that the effects of thermal history on the level of gelation in a PVC compound could be assessed by plotting ram extrusion pressure through a zero length die against processing temperature. For a given process and rigid composition a threshold temperature could be identified, representing the beginning of ‘good’ gelation. This threshold is related to the polymer K-value as illustrated by the results of Moore,12 who noted the progression shown in Table 9.1, in measurements of flow in a capillary rheometer. He also noted a progressive increase in the maximum flow pressures, corresponding to ‘fully processed’ compounds, with increasing K-value. The change in melt modulus as a function of thermal history, with increasing K-value, was particularly pronounced. The K58 and K61 polymers were relatively insensitive to thermal history as far as melt modulus was concerned, but the compositions based on K68 and 71 polymers exhibited melt moduli that increased dramatically with processing temperature above 160°C. In practical terms, TABLE 9.1 Relation of Polymer K-values with Threshold Temperature Threshold temperature (°C)

Polymer K-value (ISO scale)

155 157 159 165

58 61 68 71

this suggests a particular sensitivity of the higher K-value polymers to variation of thermal history within the melt, which leads to marked heterogeneity of elastic properties, and consequent problems in fabrication, such as rippled or lumpy extrudates. 9.5.1.3 Copolymerisation It may seem strange, at first sight, to find the influence of comonomers relegated to the end of a discussion of polymer properties and processing. In practice, however, the vast bulk of rigid PVC applications are based upon homopolymers, and copolymers, although they perform a unique role, are restricted to a minority of specific applications. With the exception of certain specialised coating materials, such as the vinyl chloride-vinylidene chloride copolymers, commercial exploitation has been limited to two systems, copolymers with vinyl acetate and with olefins. Copolymers of vinyl chloride with 2–10 % of ethylene or propylene give products of lower melt viscosity, and therefore easier processing, which are rigid at ambient temperature, though with somewhat reduced softening points. Standard manufacturing processes for PVC are not easily adapted to the incorporation of ethylene or propylene, and production of these copolymers has never grown to high tonnages. There is currently little exploitation in rigid applications, and the ‘internal’ plasticisation of rigid PVC is achieved almost exclusively by incorporation of vinyl acetate. Vinyl acetate copolymers have a long and honourable history, being first introduced in 1928.6 It was realised at that early period that the presence of even a very minor proportion of vinyl acetate acted as an internal plasticiser, allowing the polymer to be processed at lower temperatures but still giving a rigid

THE PROCESSING OF RIGID PVC

173

FIG. 9.5. Effect of vinyl acetate on melt viscosity at constant K-value.

product over a useful temperature range. The role of vinyl acetate in assisting processing may be appreciated from Fig. 9.5, which illustrates the effect of vinyl acetate concentration on melt viscosity at 180° C at constant K-value. It will be seen that the effect is marked at low concentrations, but the rate of viscosity reduction falls beyond 6–7% vinyl acetate. In a detailed study of the melt rheology of vinyl acetate copolymers,15 Hollister found that they required less energy to process than homopolymers of the same Kvalue, and sustained higher shear rates before surface fracture effects became noticeable. Unfortunately, the advantages in melt behaviour are obtained at the expense of some deterioration in other aspects of performance. Firstly, the thermal stability of vinyl acetate copolymers is inherently inferior to that of homopolymers. Secondly, at a given K-value, copolymers have lower impact resistance than the corresponding homopolymer. Thirdly, there is inevitably a lowering of the softening temperature, the Vicat softening point dropping 6–7°C for incorporation of 10% vinyl acetate. There is also a price premium for copolymers, reflecting an increased monomer cost. It will be seen, therefore, that the use of copolymers is limited to those applications where ease of processing is more important than thermal and mechanical properties of the product, and where the advantage justifies an increased material cost. The application where vinyl acetate copolymers play a uniquely important role is the manufacture of gramophone records. In the moulding of vinyl records it is necessary to reproduce faithfully the complex contours of the record stamper and to meet stringent service demands in the use of the product. The width of groove in an LP record is about 50 µ m, whilst there may be as many as 14 grooves per millimetre. For a stereo record the pattern is even more complex and demanding, since the two walls of the groove undulate

174

D.A.TESTER

differently, one carrying left and the other right channel information, and the stylus moves up and down, as well as from side to side. Not only must the melt fill the most difficult intricacies of the mould cavity, but there must be minimal shrinkage or distortion of the moulding, whilst cycles must be as short as possible, for economic production. It is, in fact, possible to produce satisfactory records from homopolymers, but not at economic rates in commercial production. In practice, ‘high’ acetate copolymers, containing about 15% vinyl acetate, are employed, and to further facilitate ease of processing, very low K-values, typically K46– 48, are employed. A minor proportion of a copolymer of lower acetate level, or homopolymer, may be incorporated. There is a very stringent requirement for good dispersion, and avoidance of contamination by grit or by ‘foreign’ polymer. Imperfections of 30 µ m dimensions, or less, are sufficient to produce a continuous background noise. Whilst smaller records may be injection moulded, the bulk of 12 in records are still manufactured by compression moulding. Traditionally, the mould was fed with a preheated preform of fully gelled compound prepared as a separate operation, but more recently, extrusion compounding machines have been introduced, specifically for the in-line charging of the record presses. High acetate copolymers are also employed in a very different type of application, the manufacture of vinyl floor tiles. Here it is required to incorporate and bind together very high levels of filler. To achieve the necessary wetting of the filler particles, both a high acetate copolymer and a proportion of plasticiser are employed, to produce a rigid or semi-rigid product. ‘Medium’ acetate copolymers are employed in the production of calendered flooring, but find their most exacting application in the production of extruded foil, particularly clear foil for packaging applications. Here it is required to extrude the melt through a horizontal slit die, to be drawn on to a casting roller or into a simple calendering operation. The products are rigid foils, in the 50–500 µ m thickness range. To obtain good processing performance, together with adequate mechanical properties, vinyl acetate contents of 8– 12% are employed, at K-values around 58–62. Homopolymers of low K-value can also be used for the extrusion/semi-calendering of rigid foil, but copolymers show particular advantage in the subsequent thermoforming of the foil to produce nestings, tubs and blister-packs. Where it is required to obtain deep drawn formings the ability of the copolymer foil to be easily softened by reheating, and deformed into the required shape, allows the production of good mouldings at high output rates. 9.5.2 Lubricants The impressive and continuing increase in the volume of rigid PVC applications during the last 30 years would not have been possible without the effective use of lubricants in unplasticised PVC compositions. A considerable number of lubricants are now offered commercially and lubricant systems are generally based on combinations of several of these additives, so a casual observer could be forgiven for thinking formulation technology to be a black art rather than an exact science. Granted that working formulations are often arrived at by highly empirical methods, there are, however, indications today of a more systematic approach, and an increasing understanding of the function of lubricants. They may be divided broadly into internal and external types. The former are recognised as being highly compatible with PVC and influence flow through reduction in the bulk viscosity of the melt. In contrast, external lubricants are less compatible and have a selective activity at the interface between the melt and the internal surfaces of processing equipment. They are included to prevent sticking and to improve the surface finish of the product. Some substances cannot be categorised precisely as they show some measure of both internal and external activity, particularly where their compatibility changes rapidly with temperature. In general, however, predominantly ‘external’ or ‘internal’ lubricants can be differentiated, and show the expected differences in

THE PROCESSING OF RIGID PVC

175

chemical nature. Thus, internal lubricants are typically fatty esters or alcohols, and most compatible of all, secondary plasticisers at very low concentrations may be considered in this role. Metal soaps, amide wax and hydrocarbon waxes are typical types of external lubricant. It may be necessary to incorporate more than one lubricant of this type in a formulation, to give optimum performance through the whole fabricating process, because the efficiency of each one at a particular temperature is dependent on its melting point. The difference in role between external and internal lubricants has been most elegantly demonstrated by Chauffoureaux.16 He showed slip of a rigid PVC composition at the wall by optical observations of the displacement of tracers (glass microspheres) with the flow of the melt in a slit die. A special Couette device was also used to measure the sticking-slipping transition at the wall of the chamber. Both sets of measurements indicated the velocity at the wall to be a function of the shear stress and the kind and quantity of additives in the formulation. Providing the compatibility between the additive and the PVC did not vary with temperature, the velocity at the wall was found to be almost independent of temperature. An external lubricant (Wax OP) was found to change the velocity profile markedly, increasing the velocity close to the die wall. However, addition of an internal lubricant that also decreased the apparent viscosity of the melt did not change the flow velocity distribution at all. An external lubricant of very limited compatibility with PVC, but not completely insoluble, may be assumed to be present at increasing concentration close to the surface with the processing equipment, ultimately displacing the PVC melt entirely at this surface. At excessive concentrations, surface quality will deteriorate, finally giving ‘plate out’, whereby lubricant, sometimes carrying with it other ingredients of low solubility, builds up on the metal surface. 9.5.2.1 The Influence of Lubricants on Gelation In addition to their influence on melt behaviour, lubricants can have a considerable effect on the gelation of polymer in a dry blend feedstock. Whilst in some situations this may provide problems, it can also provide a means of controlling gelation behaviour, in a way to complement the action of the extruder screw. The influence of lubricants on gelation can be demonstrated on a laboratory scale by following the pattern of gelation in a Brabender Plasticorder. The majority of lubricants can be shown to produce some impairment of TABLE 9.2 Lubricant/Gelation Tests on Brabender Plasticorder with Jacket Temperature 165°C Average chain length of ester wax lubricant (number of C atoms)

Fluxing time (min) Kneading resistance (mkps-1)a Stock temp. (°C) 15 min after max. max.

30 31 32 35 39 43

1·6 4·33 2·0 3·93 2·0 3·54 8·7 2·63 17·8 2·20 19·5 — a mkps-1 — metre kiloponds per second, a rate of working.

15 min after max. 2·40 2·45 2·29 2·26 1·98 1·99

182·5 182·2 182·5 182·2 176·8 176·2

176

D.A.TESTER

gelation, but the effects vary considerably with chemical type. As might be expected, the smallest effects are with the strongly internal lubricants, whilst the most marked effects are generally found with strongly external lubricants, of low compatibility with PVC. Observed behaviour does not fit entirely with this simple classification, however, and other factors, such as the melting point of the lubricant, undoubtedly have an effect. The influence of decreasing compatibility with the PVC can be most clearly demonstrated by comparing the effects on gelation of lubricants of a common chemical type, but of increasing chain length. This is well illustrated in the technical information issued by Henkel on the ‘Loxiol’ range of ester wax lubricants.17 Influence on fluxing time in a Brabender kneading test is recorded by Henkel for a range of esters of increasing chain length, incorporated into simple PVC formulations. Table 9.2 shows the results obtained with 2phr of each ‘Loxiol’ ester wax, in a composition containing, in addition, 2phr of tribasic lead sulphate as stabiliser, and 0·3 phr of calcium stearate. A rotor speed of 40 rpm and a 32 g charge were employed. It will be seen that under the conditions of this test, the fluxing time was very considerably longer with the longer chained esters, of external lubricant type, than with the short chained, highly compatible members of the series. In discussing the effects of polymer granule type, it was stated that the more porous granules absorbed additives, particularly lubricants, more readily during the powder blending operation. It follows that at a given lubricant addition level, there will be less lubricant on the granule surfaces of a more porous polymer, to slow down gelation. Conversely, the external lubricant will less readily play its role at the surface of the melt, once formed. For a given grade of polymer, in particular processing equipment, there is thus an optimum balance and level of lubricants, which can vary among polymers even if intended for the same applications. In practice, fabricators operating well established large tonnage processes, such as pipe extrusion, are increasingly concerned to use fairly simple standard lubricant systems, at the lowest level concordant with satisfactory processing. They are also concerned not to become involved in drastic reformulation when using the products of different polymer suppliers. These factors tend towards simplification and rationalisation of lubricant systems, and towards a degree of uniformity in the performance of competing polymers for major rigid fabrication processes. 9.5.3 Heat Stabilisers As previously stated, the presence of heat stabilisers is essential to protect PVC melts from degradation at the temperatures encountered during processing. The process of heat degradation liberates hydrogen chloride, which has an autocatalytic effect on that degradation and many stabilisers act primarily by reaction with hydrogen chloride. Stabilisers may also react with double bonds or other chromophoric structures, or interfere with the propagation reaction, whilst some beneficial additives appear to operate as antioxidants or as chelators for metals or other impurities. There is a considerable range of heat stabilisers employed in practical formulations, including inorganic metal salts, metal soaps and complexes, organotin compounds, epoxy compounds and organic phosphites. The role of stabilisers in combating heat degradation is complex and cannot appropriately be discussed here. However, it must be pointed out that some, at least, of these additives can have a direct effect on the Theological properties of the melt and/or on the process of gelation. Metal soaps, of limited compatibility with the PVC melt, will act, to varying extents as external lubricants. Organotin compounds, generally liquid and more compatible with PVC, behave as internal lubricants. However, some of these additives also tend to

THE PROCESSING OF RIGID PVC

177

cause sticking of PVC compositions to hot metal surfaces, so in an external role they may actually act as antilubricants. Some epoxy additives, particularly epoxidised soya bean oil, are effective secondary plasticisers and may therefore be expected to behave as internal lubricants, at low concentrations. At the same time, additives chosen primarily as lubricants may have a significant stabilising action. As an example, long chained partial esters of polyols are effective lubricants for rigid PVC processing, and have a beneficial effect on heat stability. Certain well-known additives, such as dibasic lead stearate, can be equally well classed as lubricants or heat stabilisers. Since there is an interaction and an overlap of function between stabilisers and lubricants, it might be expected that specific combinations of lubricant systems and stabilisers would be particularly effective. Indeed, synergistic support between various lubricants and stabiliser systems has been claimed, and Worscheck20 tables evidence for such synergy in both tin and lead based systems. There is a growing exploitation of ‘one pack’ lubricant/stabiliser systems by fabricators, to simplify formulation technology and material handling, and this gives particular point to the development of simple but optimum combinations. The choice of heat stabiliser systems may be dictated by extraneous factors, which can thus influence the whole pattern of formulation technology. An obvious example arises from the different views in the UK and the USA as to the ingredients texicologically acceptable in potable water pipe. Whereas the UK, in common with the majority of European countries, accepts lead based stabilisers, which are virtually water insoluble, for pressure pipe, lead compounds are not permitted in this application in the USA, and tin based stabilisers are generally employed. From what has already been discussed, it will be appreciated that these two types of additive behave very differently in terms of absorption by the polymer grains and subsequent effects in the melt. As a result, there are differences in the balance of lubricants typically employed for pipe extrusion in the USA and in Western Europe, and even some difference in the optimum requirements of the polymers employed. 9.5.4 Processing Aids As the name indicates, these additives assist the melt processing of PVC compositions, but in a way that is quite distinct from the role of lubricants. Processing aids do not reduce melt viscosity, and may even produce some increase in viscosity, but they improve the elastic behaviour of the melt. Rigid PVC melts are prone to rupture under high accelerating stresses, and many of the practical processing problems encountered are caused by melt rupture. For example, the ‘sharkskin’ surface sometimes met in extrusion is the result of a cyclic rupture/recovery mechanism in the surface layer of the melt when it cannot stretch sufficiently to accommodate the velocity changes at the die exit.18 The break up of a rolling bank on a calender is likewise caused by melt rupture, but on a much larger scale, as the melt is drawn into the nip. The addition of a small proportion of a processing aid to a rigid PVC composition considerably reduces the incidence of processing defects associated with melt rupture. Thus, in the extrusion of profiles and sheet, processing aids can give freedom from ‘sharkskin’, better gloss, and easier post extrusion forming and shaping. In calendering of foil, flow marks and edge shredding are much reduced, and overall surface quantity improved. In injection moulding, rigid PVC melts are subjected to higher shear rates and steeper shear gradients than in any other fabrication processes, and here the quality improvements effected by the inclusion of processing aids are particularly marked. The benefits conferred in extrusion are made use of very frequently in the extrusion blow moulding of clear bottles, where the improved gloss and surface quality are especially valuable in obtaining good optical properties, whilst the greater melt extensibility increases the maximum blow ratio that can be used.

178

D.A.TESTER

FIG. 9.6. Post-extrusion swell using converging knife-edge D.E., (a) without processing aid, (b) with 3 phr processing aid: 1, linear Hookean region; RC, critical strain; RM, maximum strain. (Reproduced from Gould and Player, Kunstoffe, 69(7), 393, 1979.)

Two distinct groups of materials are commonly used as processing aids; copolymers based on styrene, and those based on methyl methacrylate.Within the latter group, copolymers of methyl methacrylate with minor proportions of ethyl acrylate are particularly widely used, as they give much improved processing without detracting from the properties of the end product. All these additives are characterised by a high level of compatibility with PVC, and much higher molecular weights than are encountered in the PVC resins themselves. It is reasonable, therefore, to consider these long chain molecules as helping to bind together the PVC melt structure, smoothing out stress concentrations and increasing the extensibility of the melt. Although the practical role of processing aids is so well established, very few quantitative studies of their effects on PVC melts have been published. The most comprehensive is that of Gould and Player.19 They determined the post-extrusion swell of a lace extrudate produced on a ram extruder, using a zero length die. The experimentally observed parameters, swelling ratio and flow pressure, were related to recoverable strain ( R) and the average extensional stress ( E) by established Theological equations. Plots of R against E showed two parts. At low stress, there was a linear Hookean region, up to a critical strain ( RC), followed by a non-linear region where the recoverable strain reached a maximum ( RM) and then tended to fall as the stress increased further. The addition of processing aid did not alter the slope (modulus) in the Hookean region, but RC and RM both increased in value, as did the corresponding stresses, (see Fig. 9.6) The critical strain ( RC) was considered to correspond to the onset of melt rupture in the die, and hence this parameter was taken as a measure of extensibility. In a less sophisticated technique these workers also measured the maximum draw down that could be achieved in screw extrusions carried out at a fixed rate, by means of a calibrated variable speed haul-off. The results obtained by these two approaches were in good agreement, increases in maximum draw ratio corresponding to increases in the critical and maximum recoverable strain. Typical results are shown in Table 9.3, which illustrates the improvements obtained by adding 3 phr of a methyl methacrylate/ethyl TABLE 9.319

THE PROCESSING OF RIGID PVC

179

Influence of a Processing Aid on the Elastic Behaviour of a PVC Melt Measurement

Control with no processing aid With 3phr acrylic processing aid

Maximum extrudate draw ratio at 170°C Critical recoverable strain RCat 170°C (%) Maximum recoverable strain RM at l70°C (%) Average extensional stress at RM at 170°C (MPa)

1·32 35 42 2·4

2·55 45 56 5·9

acrylate copolymer to a simple lead stabilised rigid composition based on a K67 polymer. In addition to improving melt behaviour, processing aids can produce a faster rate of plastication of a polymer premix in an extruder or internal mixer. The mechanism involved appears to be one of increasing particle to particle friction, combined with enhanced heat transfer. Gould and Player studied this gelation effect, using the Brabender plasticorder.19 The charge weight required to obtain a specific pattern of gelation, under fixed conditions of temperature and rotor speed, was determined, with and without processing aid. The addition of processing aid reduced the charge weight required, the difference being proportional to the increase in the rate of plastication. In these studies of methyl methacrylate/ethyl acrylate processing aids, the effects of molecular weight and comonomer content were examined. The maximum recoverable strain of a PVC melt containing a fixed level of processing aid was not very sensitive to ethyl acrylate content, but increased steadily with the molecular weight of the additive, up to a maximum at a MW of about 5×106, thereafter falling off. Thus the optimum MW for the processing aid would appear to be about 50 times greater than that of the base PVC. In contrast, the enhancement of gelation increased with decreasing MW and increasing ethyl acrylate content, suggesting that softening temperature of the processing aid was the controlling factor here. Processing aids which soften readily might be expected to provide a more efficient ‘heat flux’. It will be seen that for these acrylate additives, and presumably for other systems also, the chosen composition must represent, to some degree, a compromise between the requirements of easier gelation and improved melt elasticity. 9.5.5 Impact Modifiers The impact resistance of unplasticised PVC can be increased by the inclusion of impact modifiers, generally at a concentration of 5–15%. A variety of materials are used to improve the toughness of PVC, including acrylonitrile/butadiene/styrene terpolymers (ABS), methyl methacrylate/ butadiene/styrene (MBS), acrylate rubbers, ethylene/vinyl acetate copolymers (EVA), and chlorinated polythenes. Most of these additives have a rubbery nature, or are rubber/resin grafts, and their effect is believed to depend on their being present in a separate phase, as globules or strands of submicron dimensions, in the PVC matrix. This dispersed phase must be bound to, or at any rate effectively ‘wetted’ by, the surrounding PVC. The effect on the processing of PVC compositions depends upon the physical form of the impact modifier employed. MBS and ABS systems, for instance, are available as free flowing powders and are commonly incorporated in powder blend feedstocks. Chlorinated polythene, a more rubbery material, requires a mastication process for satisfactory in-corporation into PVC compositions, and is more applicable to compounded feedstocks. The requirements of adequate compatibility, but the existence of a separate phase, are sometimes met by polymerisation of vinyl chloride in the presence of the preformed modifier, so that a degree of grafting between the PVC and the incorporated modifier is achieved. Providing that the impact

180

D.A.TESTER

modifier is a very minor proportion of the total polymer composition, the modified PVC may be dried and handled like a normal PVC grade. This technique is commonly employed for high impact PVC compositions incorporating EVA. Although impact modifiers must be adequately dispersed, excessive compounding can actually detract from the impact properties, underlining the need to maintain the additive as a separate phase. The choice of impact modifier is generally dominated by considerations other than processing. Thus, MBS modifiers are very commonly employed in the manufacture of blow moulded bottles, since a careful matching of refractive index with that of the PVC allows production of glass-clear bottles. However, where retention of impact properties during prolonged external weathering is demanded, as in window frames, systems incorporating butadiene copolymers are normally avoided, and saturated polymer systems are employed. Although impact modifiers are chosen primarily for their effects on finished properties, they are frequently claimed to have a beneficial influence on processing, particularly in the case of ABS and MBS systems. Some increase in melt extensibility can certainly be observed when MBS modifiers are employed, and the effects may be compared, in this respect, with those of processing aids.21 As already mentioned, impact modifiers, to be effective, must be present predominantly as a separate phase, whilst processing aids are considered to be dissolved within the PVC melt. One would therefore expect the influence of impact modifiers on bulk melt properties to be very much reduced, at a given concentration, compared with processing aids. This is indeed the case, but it must be remembered that impact modifiers are generally used at much higher concentrations than processing aids, up to 10–15 phr for high impact compositions, so that a weak effect per unit part can be quite significant in practice. 9.5.6 Fillers In contrast to plasticised PVC and many other thermoplastics, rigid PVC compositions very rarely incorporate high levels of fillers. Relatively cheap mineral fillers, that could offer a significant volume cost saving, generally produce very poor impact resistance, even at low filler loadings. Fillers can certainly improve the stiffness of rigid PVC, but the enhancement of modulus is highly strain and time dependent, so that in any application which involves continuous loading the modulus advantage disappears. This behaviour is concordant with inadequate wetting of the filler by the PVC melt, and poor adhesion at the filler/PVC interface. Probably because fillers play a relatively small part in rigid formulations, there are few published studies of the effects of fillers on the processing of unplasticised PVC compositions. Nevertheless, some general observations can be made. Small additions of fine particle fillers can aid the flow of powder blend feedstocks, but higher levels of filler would be expected to compete with the polymer surface for lubricant, during the powder mixing. This in turn might be expected to facilitate faster gelation during processing, though in practice the evidence suggests that appreciable levels of filler, particularly coarse particle fillers, impede gelation. One may suppose the filler particles to be separating and preventing adhesion between the softened polymer grains. The need to achieve good dispersion of high filler loadings, given that product property considerations allow their use, must put particular demands on the compounding and fabricating processes, and militate against the use of powder blend feedstocks. Melt viscosity will generally tend to increase with increasing filler loading. There is an important exception to what has been said about the embrittling effect of fillers on rigid PVC. The incorporation of up to 30 phr of a very fine particle size calcium carbonate can actually increase the toughness of rigid PVC. Fine powders of this type tend to agglomerate during processing, but this can be prevented by coating the particles with a small percentage of stearic acid, or similar lubricant. This treatment

THE PROCESSING OF RIGID PVC

181

also improves the wetting by polymer during processing, and apart from a slight increase in melt viscosity, there are no deleterious effects on melt behaviour from incorporation of coated precipitated calcium carbonate at modest levels. Given thorough dispersion and optimum design of formulation, 10 phr of this filler have been shown to give a marked increase in toughness, as measured by some impact tests.22 9.6 ORIENTATION In the foregoing discussion, poly(vinyl chloride) has been treated as a predominantly amorphous material, although some degree of syndiotactic ordering of chains has been postulated as contributing to melt structure. However, when rigid PVC is stretched at temperatures above TG but below normal processing temperatures, say between 80° and 120°C, and then cooled rapidly, strain orientation is frozen into the polymer. This influences physical properties markedly, giving increased stiffness and yield strength. The effects of uniaxial stress are, of course, strongly anisotropic, but by introducing biaxial orientation, useful improvements in properties may be obtained. Brady23 has carried out a detailed study of the influence of degree, rate and temperature of biaxial stretching on the properties of rigid PVC, in the form of calendered sheet. He found that a stretch ratio of 2 x 2 gave optimum improvements. Whilst property/processing relationships exhibited a complicated interaction, low stretching temperatures (within the indicated range) and high stretching rates generally gave greatest enhancement of mechanical strength. Biaxial orientation of PVC sheet has been exploited commercially, and in recent years there has been a growing interest in processes to achieve orientation during the blow moulding of bottles. Various machines have been developed for this purpose, falling broadly into two types. The first type utilises injection moulding techniques. Preforms, in which the necks of the bottles can be injection moulded to their final dimensions, are reheated to the critical temperature for orientation before being blow moulded. Inflation is effected by high pressure air, whilst a telescopic mandrel may also be employed to stretch the material longitudinally. The second type comprises extrusion blow moulding machines specifically designed for the production of oriented bottles. Here, in a continuous operation, the extruded parison is first inflated in a preform mould, then, after the appropriate temperature conditioning, transferred to the blowing station where it is further blown to final dimensions, with longitudinal stretching assisted by a mandrel incorporated in the blowing. Advantages claimed over conventional extrusion blow moulding are not only increased strength, stiffness and impact resistance (or a lighter weight bottle for a given performance) but also better clarity and gas barrier properties. An application of orientation which is of considerable potential importance has recently been announced by Yorkshire Imperial Plastics Ltd. This company has developed a high strength PVC pipe which exploits a post-extrusion orientation process to achieve a significantly improved range of physical properties. The process is particularly applicable to the manufacture of pressure pipes, for which improved rupture strengths are claimed. For a given pressure rating, the pipe can thus be about 40 % lighter than conventional UPVC pipe, giving considerable savings in raw material costs, and possibly in installation costs also. The high strength pipe has also been shown to give higher impact strengths, and improved fatigue resistance under conditions of cyclic pressures. REFERENCES 1.

FULLER, C.S., Chem. Revs., 26, 162 (1940).

182

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

D.A.TESTER

WALTER, A.J., J. Polym. Sci., 13, 207 (1954). NAKAJIMA,A.,HAMADA, H.and HAYASKI, S., Makromol.Chem.,95, 40 (1966). PENN, W.S., PVC Technology, Applied Science Publishers, London (1971). NASS, L.I., Encyclopaedia of PVC, Marcel Dekker Inc, New York (1976). MATHEWS, G., Vinyl and Applied Polymers, Volume 2, Iliffe, London (1972). JANSSEN, L.P. B.M., Twin Screw Extrusion, Elsevier Scientific Publishing Co. (1978). GOTHAM, K.V. and HITCH, M.J., Brit. Polym. J., 10, 47 (1978). BERENS, A. and FOLT, V.L., Pol. Eng. Sci., 9(1), 27 (1969). MENGES, G. and BERNDTSEN, N., Kunstoffe, 66(11), 735(1976). MENGES, G. and BERNDTSEN, N., Pure Appl. Chem, 49, 597 (1977). MOORE, D.R., International Conference on PVC Processing, 1978, The Plastics and Rubber Institute, London, 1978. GRAY, A., International Conference on PVC Processing, 1978, The Plastics and Rubber Institute, London, 1978. BENJAMIN, P., International Conference on PVC Processing, 1978, The Plastics and Rubber Institute, London, 1978. HOLLISTER, E.H., 37th ANTEC, SPE, 365 (1979). CHAUFFOUREAUX,J.C., DEHENNAU,C. and RIJCKEVORSEL, J. VAN, J. Rheology, 23, 1 (1979). HENKEL TECHNICAL DATA BOOKLET, Plastics Report No. 1, p. 7, Dusseldorf, 1979. COGSWELL, F.N., J. Non-Newtonian Fluid Mechanics, 2, 37 (1977). GOULD, R.W. and PLAYER, J.M., Kunstoffe, 69(7), 393 (1979). WORSCHECK, K.F., International Conference on PVC Processing, The Plastics and Rubber Institute, London, 1978. PETRICH, R.P., International Conference on PVC Processing, The Plastics and Rubber Institute, London, 1978. HARPER, W.W., Kunstoffe, 56(10), 704 (1966). BRADY, T.E., 33rd ANTEC, SPE, 546 (1975).

Chapter 10 THE PROCESSING OF PLASTICISED PVC D.A.TESTER PVC Development Group Leader, ICI Ltd, Welwyn Garden Ctiy, UK

10.1 INTRODUCTION Whilst the use of plasticisers is not confined solely to PVC technology, the interaction of poly(vinyl chloride) with plasticisers plays a unique role in the exploitation of thermoplastics. Not only do plasticisers facilitate fusion and melt processing at lower temperatures, to yield softer and more flexible products, but the properties change progressively with plasticiser level, to give a wide range of useful compositions. Even at high plasticiser levels, PVC compositions retain a useful degree of strength and elasticity, with surprisingly good creep recovery. The plasticisers commonly employed are relatively non-volatile liquids which solvate and soften the polymer, but are not good solvents at ambient temperatures. The role of plasticiser type and concentration in PVC processing will be considered later, but it will be apparent from the multitude of flexible PVC applications that plasticisers have been identified to meet a vast range of product requirements, thermal, mechanical, electrical, toxicological and visual. As might be expected, all of the techniques employed for the processing of rigid PVC are applicable to plasticised compositions, generally with less likelihood of thermal degradation under conditions of intensive shear. There is one area of processing technology which is relevant only to plasticised PVC compositions; that is the formation and application of PVC pastes. Although paste processing and hot melt fabrication may sometimes serve the same application areas, the dominant aspects of processing are so different that PVC pastes will be considered in a separate section. As with the treatment of rigid PVC processing, it is not appropriate here to discuss in any detail individual processing and fabricating techniques, all of which are described in the standard texts to which reference has already been made. The intention, in the following pages, is to relate considerations of practical processing to polymer/plasticiser interaction and to the concepts of particle morphology and melt formation which have been discussed in previous chapters. 10.2 FEEDSTOCK PREPARATION Just as with rigid PVC, there are two types of feedstock employed in fabrication from plasticised PVC compositions. A fully gelled ‘compound’ of all the formulation ingredients may be manufactured in a separate process, and fed in pellet form to the fabrication equipment, to be remelted and shaped.

184

D.A.TESTER

Alternatively, a powder blend of the ingredients may be fed directly to fabrication equipment designed to effect initial fusion and melt homogenisation. 10.2.1 The Preparation of Powder Blends It might be thought that the presence of plasticiser would place severe restrictions on the exploitation of dry blends for flexible applications, but this is not necessarily so. Given suitable choice of polymer type and plasticiser, dry free-flowing powders can be produced in a high speed mixer from formulations containing up to at least 30 % of liquid plasticiser. The technique normally employed is to mix the polymer and minor solid ingredients at the beginning of the cycle, and then to run in the plasticiser, with any other liquid ingredients, at a prescribed temperature, generally between 60 and 90°C. Mixing is then continued to a peak temperature in the region of 90–110°C, after which the blend is transferred to the cooling chamber. This procedure facilitates the even dispersion and absorption of plasticiser into the polymer granules and thus minimises the likelihood of unplasticised or partially plasticised granules in the final product. The rate and effectiveness of the dry blending process depend critically on the nature of the polymer granules and the plasticisers employed, as will be exemplified in Section 10.5. 10.2.2 Compounding In the previous chapter it was stated that the use of dry blend feedstocks had grown to become the predominant practice for rigid compositions, largely through the volume of rigid dry blend processed in twin-screw extruders. For plasticised compositions, twin-screw extrusion is not normally employed, and extrusion development has been through the design of more sophisticated single screw machines, generally fed by fully gelled compounds. Whilst some aspects of processing are, of course, greatly eased by the presence of plasticiser, it is essential to achieve good dispersion and homogeneous plasticisation of the polymer particles to obtain satisfactory products. Where a fully compounded feedstock is employed, subsequent processing is less critical in this respect. Although plasticised dry blends have been utilised in most fabrication processes, compound feedstocks are more commonly employed, particularly in the large tonnage area of cable insulation and sheathing. It must be remembered, however, that high speed mixing, rather than the less effective ribbon blending or tumble blending, is increasingly employed to produce the feed for modern compounding operations. So even where the fabrication equipment is fed by pelletised compound, the ease and speed of achieving a satisfactory powder blend may still be of significance to the overall manufacturing process. As with rigid compositions, Banbury/mill compounding was used originally, and gave very satisfactory products. Today, extrusion compounding units are increasingly employed. Whatever the particular compounding route, it is important, as with rigid compositions, to maintain consistent control of the heat/ shear history of the resultant compounds, since this will very significantly affect the melt behaviour in subsequent processes.

THE PROCESSING OF PLASTICISED PVC

185

10.3 POLYMER/PLASTICISER INTERACTION Since the interaction of polymer and plasticiser is central to the formation of plasticised compositions, it is pertinent at this point to consider the mechanism of this interaction. The highly electronegative nature of the chlorine atoms leads to strong dipoles along the polymer chain, resulting in high concentrations of secondary valency forces, and therefore in a reduction of chain flexibility. At the same time, since chlorine atoms are relatively bulky, they separate the chains sufficiently to make van der Waals forces a comparatively insignificant source of cohesion. The simplest model of plasticisation envisages the plasticiser molecules, which have polar or polarisable groups, as bonding with the polymer dipoles, whilst the non-polar parts of the molecules act as shields between polymer dipoles. The result is thus a reduction of dipole bonding between polymer chains, less overall cohesion and a consequent increase in the freedom of molecular movement. There are, however, aspects of the behaviour of plasticised PVC which suggest a more complex model. Flexible products can result from the incorporation of quite low levels of plasticiser, which might be expected to interact with only a minority of the polymer dipoles. On the other hand, even lower levels of plasticiser (up to 10–15% by weight for di(2-ethylhexyl) phthalate) actually increase stiffness and produce ‘anti-plasticisation’. There is clear evidence that addition of low levels of plasticiser leads to an increase in the amount of crystallinity in the polymer, resulting in the effects observed.1 The presence of a small degree of crystallinity in plasticised PVC, and the possibility of orientation of these crystallites, was reported 30 years ago,2 and confirmed by subsequent studies. It would thus appear that there is some degree of microcrystalline structure in plasticised PVC, which has been envisaged3 as a polymeric mass with regions that are solvated and made flexible by plasticiser, and rigid, non-solvated crystalline regions. The crystallites result in a network structure which may be assumed to impart toughness and strength to plasticised compositions, and to be responsible for the broad range of properties available for poly(vinyl chloride). In the context of melt processing, we must consider whether this crystalline network persists in the plasticised melt at elevated temperatures, and how this concept relates to the observations of particle morphology and melt rheology discussed in the previous chapter. 10.4 PARTICLE MORPHOLOGY AND MELT FLOW Whereas it has been shown that a significant ‘memory’ of the original polymer granules can persist in the processing of rigid PVC compositions, this is much less true of plasticised PVC. Whilst excessively large or glassy particles may remain as partially plasticised ‘fish eyes’ or ‘gels’, a uniform melt, at this level of definition, will generally be obtained unless the polymer granules employed are very heterogeneous in type. There is, however, clear evidence for the persistence of the primary particles of micrometre dimensions. Summers et al.4.5 developed a technique for the identification of primary particles by swelling samples of extrudate in acetone, shearing between glass slides and examining optically. They prepared compounds containing 0, 30, 60 and 90 phr of dioctyl phthalate plasticiser, using the least rigorous compounding concordant with adequate dispersion. These compounds were then extruded on a small single-screw extruder, over a range of temperatures. Acetone swelling and microscopic examination of the extrudates showed unagglomerated particles in the samples corresponding to low extrusion temperatures, partially agglomerated particles in the intermediate samples, and a continuous melt for the highest-temperature extrudates. These particles were of the same size, allowing for the acetone swelling, as the primary particles identified in the original polymer granules. The same transitions took place in all the compounds, regardless

186

D.A.TESTER

of plasticiser level, except, of course, that with increasing plasticiser the temperature for each morphological condition was lowered. 10.4.1 Melt Rheology and Melt Structure The sets of extrusion conditions employed by Summers corresponded to visual differences in the extrudates. The low-temperature extrusions produced materials with a smooth dull finish. Increasing melt temperature produced increasing roughness, but ultimately an extrusion temperature was reached at which the extrudate became smooth and glossy. The transition to a rippled or lumpy extrudate with increasing thermal history is reminiscent of rigid compositions, where the observed behaviour was seen to be concordant with the build-up of a melt structure under conditions of heat and shear. It is now possible therefore to provide a model for the formation of plasticised PVC melts in accord with the facts described. The primary particles of the polymer persist during processing, even in the presence of high levels of plasticiser, over a wide range of temperature. These particles absorb and are softened by plasticiser, but the crystalline regions within them remain unsolvated up to very high temperatures. With increasing thermal history there is increasing interaction and loss of boundaries between the primary particles and build-up of a structure within the melt, which may be associated with persisting crystalline fragments or formation of fresh crystalline networks. If melt structures are built up in plasticised PVC compositions then, as with rigid compositions, we should expect to find marked changes in rheological properties with increasing thermal history. This is indeed the case, as illustrated by the results obtained by Moore6 in capillary rheometer studies. The previous chapter illustrated the typical results obtained when rigid compounds made at progressively higher temperatures are extruded through a ‘zero’ length die under standard conditions. The extrusion pressure/ processing temperature curve shows a minimum, corresponding to the so-called ‘threshold’ temperature for good processing, followed by a rise in pressure to a maximum corresponding to a very elevated temperature. Exactly the same pattern was observed for a range of TABLE 10.1 Threshold Temperature as a Function of Plasticiser6 Concentration BS softness

Plasticiser concentration (DIOP) (phr)

Threshold temperature (°C)

4 24 40 67

25 40 55 75

135 130 128 118

compositions plasticised by varying proportions of di-octyl phthalate (Table 10.1). As the level of plasticiser was increased, however, the curves became flatter, and the threshold temperature was progressively reduced. On a sample at each plasticiser level, with a heat history corresponding to the ‘highly processed’ state, apparent viscosity and tensile melt modulus measurements were carried out. As might be expected, melt viscosity at a given temperature and shear rate dropped progressively with increasing plasticiser content, as did melt modulus. However, for a given composition, at BS Softness 40, a comparison of the effects of high and low processing temperatures showed that the higher temperature gave a higher viscosity in subsequent

THE PROCESSING OF PLASTICISED PVC

187

FIG. 10.1. The effect of thermal history on the flow curves for a BSS 40 compound. Extrusion at 120°C, r=0.8mm, l=30.4mm. (Reproduced from Moore, PRI conference on PVC Processing, 1978.)

extrusion at 120°C. Also there was marked departure with the high-temperature sample from a simple power law relationship between shear stress and shear rate (Fig. 10.1). Tensile melt modulus at 120°C followed a similar trend, with the higher processing temperature giving the higher modulus, and a less ductile melt, since modulus at a specific stress may be inversely related to recoverable strain. 10.4.2 Product Properties Summers5 carried out measurements of mechanical properties on the extrudates he obtained at different extrudate temperatures. Maximum tensile strength and tear strength were found to increase with increasing melt temperature, especially for the more highly plasticised compounds. This increase in toughness and ductility was attributed to the molecular chain diffusion across primary boundaries, occurring increasingly with increasing melt temperature. In the previous chapter, the importance of developing optimum mechanical properties, particularly impact strength, in the processing of rigid compositions was stressed. In flexible compositions, mechanical properties, beyond a necessary minimum level, are generally less critical. Appearance of the product, and the rate at which acceptable product can be produced, are more likely to be dominant considerations. Given good dispersion and melt coherence, conditions giving a low level of melt structure will tend, in practice, to give higher output and better surface quality in the processing of plasticised compositions. High levels of melt structure, on the other hand, will generally give higher melt strength, and will, for instance, facilitate physical handling of extrudates. There is one area in which the use of the highest melt temperatures is definitely beneficial, and that is the production of transparent products of high clarity. It appears that the boundaries of the primary particles can give rise to light scattering, so that the destruction of the primary particles is necessary to remove haze and achieve really good clarity. At the same time, of course, this treatment must tend to build up a high level of melt structure. Where compounds are manufactured for further processing and fabrication, the optimum level of melt structure for the compound must depend on the heating and shearing that will be involved in the subsequent

188

D.A.TESTER

operations. There is therefore no unique level of melt structure to be sought in all compound manufacture. However, since melt rheology is so sensitive to thermal history, it is essential to maintain close control of compounding conditions, as it is with rigid formulations, to achieve consistent performance in subsequent fabricating processes.

10.5 THE EFFECT OF FORMULATION INGREDIENTS 10.5.1 The Polymer 10.5.1.1 K-value The requirements for acceptable processing behaviour are less restrictive on K-value than in the case of rigid compositions, and so relatively high K-values are generally employed in flexible compositions, to optimise product properties. K-values are typically in the range of 65–72, with the higher end of the range being employed where mechanical properties, particularly performance at elevated temperatures, are of paramount importance, and lower K-values where the emphasis is on easy processing. In practice, K-values across this range can be very successfully employed in many of the same applications, by minor adjustments to plasticiser level or other formulation variables. There is a limited use of very high K-values, of K80 and above, particularly in Japan, to obtain very tough rubbery products. These are, however, very much speciality products, and required mechanical properties can generally be achieved within the normal K-value range, by judicious formulation, and in certain areas, by incorporating other polymers or by crosslinking. In the previous chapter on rigid processing it was shown that the threshold for the formation of melt structure occurs at lower temperatures with low K-value polymers, but that the ultimate effects of melt structure on melt rheology are more pronounced with higher K-values. By analogy, one might expect a similar response to K-value in plasticised compositions, and this is, indeed, found to be the case. Whilst the use of a lower K-value will tend to facilitate the introduction of melt structure during processing, the most pronounced changes in melt properties, after severe thermal history, occur with the highest K-values employed. 10.5.1.2 Grain Morphology An essential quality in any polymer for flexible applications is that it should absorb plasticiser readily, but homogeneously. The plasticiser absorption characteristics are reflected not only in the performance during dry blend manufacture, but also in the ease with which a homogenous melt, free of dispersion faults, may be produced. A key requirement is that the polymer grains should possess a relatively high level of porosity. An increase in porosity produces an increase in the equilibrium uptake of plasticiser, at ambient temperature, and in practice the ‘cold plasticiser absorption’ (CPA) may be taken as a measure of grain porosity. Polymers in commercial production are often characterised by CPA, for which an ISO test

THE PROCESSING OF PLASTICISED PVC

189

procedure has been established. CPA levels of 30–35% are commonly encountered in polymers specifically designed for flexible applications, in comparison with typical levels of 15–25% for rigid polymers. Other things being equal, an increasing grain porosity must result in a lower bulk density, and densities of 450– 500 g/litre are typical of flexible polymers, as opposed to 550–600 g/litre for rigid grades. In the production of plasticised dry blends, temperatures in excess of TG are generally reached, as indicated in section 10.2.1, and plasticiser not only fills the available grain pores, but diffuses into, and swells, the polymer matrix. The most meaningful measure of polymer behaviour, in this context, is the so called ‘hot plasticiser absorption’. Quoted values for this property usually refer, not to the ultimate equilibrium absorption, but to the initial rate of absorption, at 75°C. At the beginning of this section, it was stated that polymers for flexible applications should absorb plasticiser readily, but also homogeneously. As important as the average grain porosity is the need for a high degree of conformity from grain to grain, since without this, particles which accept plasticiser less readily can remain inadequately dispersed in the rest of the melt, even after vigorous heating and shearing. This creates imperfections in the final product, and in the case of transparent materials it is a source of ‘fish eyes’, ‘nibs’ and ‘polyblobs’. As with polymers for rigid applications, extremely fine or grossly oversized grains should be avoided. Within the range normally encountered for suspension or mass polymers, however, grain size is a much less critical parameter than grain morphology. In general, finer polymers, with a larger surface area:volume ratio, would be expected to show some advantage in plasticised formulations, providing, of course, that extreme ‘fines’ and dustiness are avoided. 10.5.2 Plasticiser Type A wide variety of substances are employed as plasticisers for PVC. Whilst phthalate esters comprise the largest family, esters of aliphatic acids and organic phosphates are also widely employed. There are also various secondary plasticisers and extenders, whose limited compatibility with poly(vinyl chloride) dictates their being used only in conjunction with primary plasticisers. The choice of plasticiser is generally dictated by end product requirements. These may include not only mechanical properties at ambient temperature, but high-temperature or low-temperature performance, permanence, electrical properties, weathering and degree of flammability. For food applications, considerations of toxicity and taint may be dominant, and always, of course, cost may be the deciding factor. Within these numerous constraints, however, there is generally some scope to choose plasticisers and plasticiser combinations to optimise processing performance. In Section 10.3, the mechanism of polymer/plasticiser interaction was discussed in general terms. The degree of interaction will naturally depend upon the properties of the particular plasticiser, and there have been TABLE 10.2 Values and Equilibrium Swelling of Polymer at 74°C8 Plasticiser Dioctyl sebacate Chlorinated Paraffin Trioctyl trimellitate

Equilibrium swelling (% increase in area) 0·8 1·0 1·2

15 21·7 22·4

190

D.A.TESTER

Plasticiser Dioctyl adipate Dioctyl phthalate ‘Mesamoll’ Butyl benzyl phthalate Octyl diphenyl phosphate Dibutyl phthalate

Equilibrium swelling (% increase in area) 1·4 2·4 2·5 2·6 3·3 3·4

28·6 42 44·5 72 373 384

numerous reported studies of the factors determining plasticiser activity in this respect. Empirical methods of characterisation have generally been based on dilute solution viscosities, or on the temperature at which a mixture of polymer and diluent undergoes some apparent phase change. Theoretical approaches based on solution thermodynamics have led to the Hildebrand solubility parameter and the Flory-Huggins interaction parameter. Bigg 7–8 derived a quantitative activity parameter a from the Flory-Huggins parameter, which correlated well with empirical assessments, and particularly with equilibrium swelling of polymers in plasticiser at elevated temperature. Bigg developed a simple technique for measuring the absorption of plasticiser by individual grains of polymer,8 which provides a convenient empirical assessment of polymer/ plasticiser interaction and is directly relevant to the preparation of plasticised powder blends. The results obtained by Bigg for equilibrium swelling of a standard polymer at 74°C in a range of common plasticisers are shown in Table 10.2 together with the activity parameters he derived for these materials. It will be seen that equilibrium swelling correlated very well with the activity parameter. However, the rate of absorption, during the period when it was essentially linear, was found simply to vary inversely with the molar volume of the plasticiser, consistent with a diffusion controlled process. In practical terms, we may expect the absorption of plasticiser by polymer, at elevated temperatures, to be a function of both molar volume and solvent activity of the plasticiser, which are, of course, interrelated for any homologous series. TABLE 10.3 Flux Temperatures of PVC Plasticiser, as a Function of Activity Parameter Plasticiser Trioctyl trimellitate Dioctyl adipate Dioctyl phthalate Butyl benzyl phthalate Dibutyl phthalate Octyl diphenyl phosphate

Minimum flux temperature (for 100 phr) (°C) 1·2 1·4 2·4 2·6 3·4 3·3

132 120 106 71 68 67

The ease of initial gelation of a polymer/plasticiser system may also be shown to be a function of plasticiser type, as well as concentration. This can be demonstrated most easily by measuring the minimum temperature for fusion of a plastisol system. PVC pastes will be considered further in Section 10.6, but the important factor in the present context is that a plastisol may be gelled by application of heat alone, without the complications of compaction and shearing involved in melt processing. Fusion behaviour may therefore be studied conveniently by establishing a temperature gradient along a metal bar or plate, coated by plastisol, and noting the point at which fusion occurs. In Table 10.3, minimum fluxing temperatures, quoted

THE PROCESSING OF PLASTICISED PVC

191

in the literature,9 are compared with plasticiser activity parameters derived by Bigg.7 It will be seen that there is quite a good inverse relationship between the activity parameters and fluxing temperature. As previously mentioned, Moore6 demonstrated, in capillary rheometer studies, the effect of plasticiser concentration on the threshold temperature for good processing, and on the viscoelastic properties of the melt. But what of the effects of plasticiser type on the rheology of the melt? Moore compared a variety of plasticisers in compositions adjusted to give the same softness of BSS40. All the compositions were ‘well processed’ according to TABLE 10.4 Viscosity and Melt Modulus for Different Types of Plasticised Formulation that have been Well Processed6 Plasticiser type (used in BSS Plasticiser concentration 40 composition) (phr)

Newtonian viscosity (k Ns/ m2)

Tensile melt modulus (10MN/m2, 120°c)

Dioctyl phthalate Di(2-ethylhexyl) phthalate Dialkyl phthalate Di-tridecyl phthalate Di-isodecyl phthalate Di-iso-octyl adipate Phthalate of straight chain alcohol Dibutyl phthalate Epoxidised soya bean oil Alkyl sulphonic ester of phenol DIOP/chlorinated paraffin Mixed glycol adipate (alcohol chain stopped)

57 52 52 77 60 50 51

5·2 5·7 5·1 3·4 3·9 3·8 4·9

7·1 9·3 7·4 5·5 6·1 6·6 6·4

40 60 55

7·5 7·0 6·0

10·0 8·6 10·8

40:24 57

4·5 8·7

6·9 15·6

the criteria already discussed, and were then extruded through a capillary rheometer at 120°C. Viscosity and melt modulus results obtained are shown in Table 10.4. These results demonstrated that choice of plasticiser could have a pronounced effect on the rheological properties of the melt. In fact, the choice of plasticiser appeared to be more critical in this respect than the effect of thermal history. Very broadly, there would appear to be some correlation between the level of melt viscosity and modulus and the solvent activity of the plasticisers, as judged on general grounds or by reference to activity parameters. This is concordant with the very reasonable expectation that more active plasticisers will more easily facilitate the build-up of melt structure. However, there are anomalies that suggest this to be too simplistic a picture. In particular, the relatively high viscosity and melt modulus results recorded for epoxidised soya bean oil are unexpected, since this material is generally regarded as a secondary plasticiser or heat stabiliser, and is normally used in combination with more active plasticisers. In assessing these results, it must be noted that the extrusion temperature of 120°C is rather lower than would be employed in most practical processing, and that the compositions are adjusted to a constant product softness. Whilst the latter procedure is fully justified, since plasticised formulations must be judged against a required finished product performance, it would have been interesting, in addition, to compare the

192

D.A.TESTER

effects of the ester plasticisers at a constant molar concentration. This is clearly an area deserving further study, on both theoretical and practical grounds. No attempt has been made here to summarise or review the very considerable literature that exists on the properties and performance of particular commercial plasticisers, or combinations of plasticisers. It will be apparent, however, from the general principles discussed, that the practical choice of plasticisers will generally involve an element of compromise. Whilst the most active plasticisers may be expected to give higher absorption by polymers at elevated temperatures, and easier gelation, they may also tend to give rise to undesirably high levels of melt structure in some processes. Also, the lowest molecular weight members of a homologous series, which may be expected to give the fastest rate of absorption into polymer granules at elevated temperature, may also be expected to be most volatile and most prone to migration. 10.5.3 Fillers Mineral fillers are commonly employed in plasticised PVC formulations, and range in type from fine precipitated calcium carbonate to ground chalk and limestone, clays and talc. As a general rule, their incorporation leads to a deterioration in most mechanical properties, and their main role is to cheapen the composition. There are, however, important exceptions to this generalisation. Thus, some clays can increase the level of volume resistivity of plasticised PVC compositions very markedly, and are therefore valuable in electrical applications. Low levels of fine particle fillers can improve the flex resistance of soft compounds. Coarse particle fillers produce mattness in the surface of fabricated products, which may be considered a defect or an advantage, according to the visual effect intended. Many of the data presented on fillers have shown the effects of increasing filler loading on a base composition in which the concentration of plasticisers and other ingredients is held constant. Since the fabricator will generally require to keep the flexibility and softness fairly constant for a given end use, this is not a very realistic approach. In practice, other ingredients, particularly plasticiser level, will be adjusted to counteract the hardening and stiffening effect of the filler. With appropriate formulation, up to 40–50 phr of fillers can be incorporated, without severe problems of processing or excessive deterioration of properties, in a variety of end products. In certain products, such as flexible flooring, much higher levels are commonly employed. In the preparation of a powder premix, the filler particles will compete with polymer granules, to an extent dependent on particle size and nature, in absorbing plasticiser. It may therefore be necessary to mix thoroughly the plasticiser and polymer, before introduction of filler. The effect of fillers on the melt rheology of plasticised compositions depends upon the particle size and shape of the filler as well as concentration, and on the processing regime found necessary for adequate dispersion. Where compositions are formulated to a constant softness, in some cases the presence of filler can actually improve the balance of output rate and surface quality achievable in extrusion. 10.5.4 Other Ingredients The contribution of minor ingredients to processing performance is generally smaller than is the case with rigid compositions. The addition of internal lubricants is, of course, inapplicable, since the plasticiser provides the desired decrease in bulk viscosity of the melt to a very marked degree. On the other hand, external lubricants, to reduce and control adhesion at the metal/polymer interface, are commonly required in

THE PROCESSING OF PLASTICISED PVC

193

the processing of plasticised compositions. The external lubricants described in the previous chapter are mostly applicable, but effective concentrations tend to be somewhat higher than with rigid PVC. Processing aids can give significant improvements in melt elasticity, as with rigid compositions, but their effect in plasticised compositions is much less dramatic, and they are not commonly employed. 10.6 THE PROCESSING OF PVC PASTES PVC plastisols, generally referred to as ‘pastes’, consist essentially of stable dispersions of polymer particles in plasticiser, sometimes with the addition of fillers, diluents, and various minor ingredients. To produce plastisols, emulsion or microsuspension polymers are required. As previously described, suspension or mass polymers, with relatively large but porous granules, will absorb quite high levels of plasticiser to form free flowing powders, or at worst, a sticky agglomeration. The primary particles produced in emulsion, being dense but only of the order of 1 µ m in dimensions, will, in contrast, disperse in plasticiser to form stable and relatively fluid systems. The advantage offered by pastes over other PVC compositions is that processing can be carried out at or near room temperature, elevated temperatures being required only during the relatively short heating cycle for fusing the plastisol into a homogeneous material. PVC pastes are used in a variety of applications, but are of particular relevance where thin flexible coatings are to be produced by dipping, spraying or spreading techniques. Although achieving a melt state is essential to the development of final properties, it will be appreciated that processing considerations are dominated by the properties of the paste at ambient temperatures. The rheology of PVC pastes is complex. Whilst very dilute dispersions, containing more than 50% by weight of plasticiser, may be substantially Newtonian in behaviour, this is rarely true of the paste formulations employed in practice, which contain higher polymer loadings. At low shear rates they are generally pseudoplastic, viscosity reducing as shear rate increases. At intermediate shear rates they typically show dilatant (i.e. shear thickening) behaviour, but may become pseudoplastic again at even higher shear rates. In very general terms, it is reasonable to suppose some degree of interaction or structure in the static plastisol, which is broken down progressively as shear is applied. At higher shear rates however, the particle displacements required in paste flow may be expected to take place with increasing difficulty, particularly where the particle size distribution does not favour close packing, leading to dilatancy. Beyond such general observations, however, there appears to be no coherent theoretical treatment that satisfactorily explains all aspects of paste rheology. Readers wishing to explore the detailed models put forward to explain the viscosity behaviour of PVC pastes are recommended to read the standard texts and comprehensive reviews on this topic.10–13 Whilst there may be some uncertainty as to the exact mechanisms of PVC plastisol behaviour, there is an abundance of empirical observation. In particular, there is a considerable knowledge of the effects of polymer particle size, and size distribution, and plasticiser type and concentration, on the rheology of pastes. In practice, the optimum viscosity/shear rate behaviour for a plastisol will depend critically on the application for which it is intended. Not only are there obvious differences in shear rates, between say, glove dipping and high speed machine coating operations, but different manufacturing processes within the same broad application area will make specific demands on the rheological properties of a paste. Typical viscosity/shear rate curves for PVC plastisols, over a wide range of shear rates, have been illustrated by Cunningham in a review of PVC coating compounds.14 He recognised three typical families of curves, reproduced in Fig. 10.2. It will be seen that curve (a) corresponds to markedly pseudoplastic behaviour initially, showing a minimum viscosity at quite low shear rates, but with such severe dilatancy thereafter as

194

D.A.TESTER

FIG. 10.2. Typical viscosity: shear rate curves for PVC plastisols. (Reproduced from Cunningham, SATRA Materials conference, 26, 1978.)

to render the paste unusable at very high shear rates. Curve (b) shows a very typical viscosity profile, with initial pseudoplasticity followed by dilatancy, giving a viscosity peak at approximately 1000 s−1, beyond which there is a return to pseudoplastic behaviour. Curve (c) shows mildly pseudoplastic behaviour at low shear rates, but essentially Newtonian behaviour at high shear rates, with a virtual disappearance of the dilatancy peak. The various crossing points on these curves point to the necessity for comparing plastisols at a wide range of shear rates when attempting to match or characterise the performance of a plastisol for a given application. 10.6.1 The Effect of Formulation Ingredients on Plastisol Rheology 10.6.1.1 The Polymer Whilst the K-value of the polymer has little effect on the viscosity characteristics of the plastisol, the particle shape, size, and most particularly size distribution, have a dominant influence. It will be appreciated from Chapter 6 that the stable PVC latex obtained from emulsion or microsuspension polymerisations is generally converted to dry powder by driving off the water in a spray drier. Each particle obtained from the drier is thus an aggregate of the primary particles in an original latex droplet. These aggregate particles are, however, relatively fragile, and can be broken down to give primaries, by the shear introduced in the mixers employed to disperse the polymer in plasticiser. The ease with which this breakdown occurs depends critically on the fragility of the particles ex-drier, and therefore on the drying conditions used. Additionally,

THE PROCESSING OF PLASTICISED PVC

195

the dried polymers may be ground to reduce the size of these aggregate or ‘secondary’ particles. In considering the effective particle size distribution in a plastisol it is therefore necessary to take note of two regions of particle size, the primary particles of around 1 µ m or less in diameter and the proportion of remaining secondary particles, broadly of 2–50 µ m dimensions. Cunningham14 attributed his three typical viscosity: shear rate curves to three different categories of paste polymer, in commercial production. These correspond broadly to the ‘high viscosity’, ‘medium viscosity’ and ‘low viscosity’ polymers subjected to detailed size analysis by Underdal et al.15 These workers analysed the particle size distribution, after dispersion, of a range of commercial paste polymers, and related this analysis to viscosity measurements on plastisols. The high viscosity resins were found to have a narrow monodisperse primary particle size distribution, with quite small (<0.5 µ m) mean diameters. The much bigger secondary particles that remained were considered to have little significance for paste viscosity, with this type of polymer. The medium viscosity polymers all had a broad polydisperse distribution of primary particle size, with the median in the size range 0.8–1.5 µ m. The nature of the secondary particles was shown here to have some effect on viscosity at higher shear rates, with dilatancy most pronounced where they were relatively small. The low viscosity polymers had a broad primary particle size distribution, which tended to be bimodal. The proportion of remaining secondary particles was generally greater, and these were somewhat coarser than with the high and medium viscosity polymers. It was concluded that the primary particle size distribution was of principal importance in deciding the viscosity characteristics of paste polymers, but that in each viscosity category, significant differences could be obtained by varying the size and cohesive strength of the secondary particles. These results are in accord with the generally accepted concept of the viscosity of the dispersed system depending on the packing efficiency of the polymer particles, and hence the amount of plasticiser remaining to dilute the dispersion, once the interparticle voids have been filled. In this context, mention should be made of‘blending polymers’, which are sometimes employed to achieve a plastisol of high polymer content, but adequate fluidity. These are special grades made by suspension polymerisation, which are relatively fine and dense, though much coarser than the primary particles of the paste polymer. Minor proportions of blending polymers can thus behave almost as inert fillers, with minor effects on paste viscosity, but fully contributing to the properties of the final fused composition. Whilst emulsifiers are used in emulsion polymerisations primarily to facilitate the formation and stabilisation of the latex particles, it must be remembered that all of this emulsifier remains in a spray dried polymer. The effect of the emulsifier on the viscosity characteristics of paste polymers is significant, but complex and imperfectly understood. Paste viscosities at low shear rates may be observed to increase, or to decrease, with increasing emulsifier content in the polymer, according to the choice of emulsifier. The effects will depend, of course, on the surface activity of the emulsifier in an environment of plasticiser, and therefore on the solubility characteristics of the emulsifier in the plasticiser. This was clearly demonstrated by Underdal el al.,15 who compared the effects of sodium dodecyl sulphate with a sodium dodecyl ether sulphate containing six ethoxy groups. At low shear rates, the latter emulsifier, in a plastisol containing 70phr of DOP, gave a tenfold lower viscosity. 10.6.1.2 The Plasticiser For a given plasticiser, as the volume fraction of plasticiser in the plastisol increases, the viscosity will be lowered across the range of shear rates, and behaviour will tend to become more Newtonian. The choice of

196

D.A.TESTER

FIG. 10.3. Effect of plasticiser solvent power on the ageing of PVC pastes. (Reproduced from Bigg, Journal of Applied Polymer Science, 20, 565, 1976.)

plasticiser, at a given level, can influence the plastisol rheology in at least two ways. Firstly, other things being equal, a lower viscosity plasticiser yields a lower viscosity plastisol. Secondly, the viscosity of the plastisol will be influenced by the interaction between plasticiser and polymer. For a given polymer, we might expect this effect to be closely related to the solvating activity of the plasticiser, as previously discussed. In very general terms, this tends to be true, and, for instance, the phthalate plasticisers generally give higher viscosity pastes than the less active sebacates and adipates. However, there are so many anomalies in this approach as to suggest that in reality the situation is much more complex. Bigg16 compared the viscosities of freshly made pastes over a range of shear rates with the corresponding plasticiser activity parameters, already discussed in Section 10.5.2. He found no apparent relationship between plasticiser activity and the viscosities of the pastes, at any shear rate. He did, however, find a very clear relationship between plasticiser activity and the relative change of paste viscosity with time. As a general rule, the viscosity of a PVC paste increases on standing, but tends towards a limiting value after long periods at ambient temperature. This phenomenon of paste ‘ageing’ is of considerable importance in the practical storage and handling of pastes. Bigg demonstrated a dramatic increase in paste ageing at 23°C, with increasing plasticiser solvent power, as shown in Fig. 10.3. With increasing storage temperature (though still far below the temperature for rapid gelation) the rate of viscosity ageing increases markedly, introducing yet another variable into the complex relationships governing paste viscosity.

THE PROCESSING OF PLASTICISED PVC

197

10.6.1.3 Fillers Mineral fillers are frequently incorporated into paste formulations, sometimes to impart certain desirable qualities, but more generally for cost reduction. Whiting, precipitated calcium carbonate, clays, silicates and ground silica gel are all commonly used. Whilst some applications, e.g. carpet tile backing, can accommodate relatively coarse fillers, applications involving the spreading of thin coatings require fine fillers, thoroughly dispersed. The disadvantage of high loadings of filler, from the point of view of processing, lies in the increased viscosity which results. With fillers of small particle size and high absorption capacity, the reduction in the amount of free plasticiser in the plastisol, and therefore the viscosity increase, is most marked. 10.6.1.4 Other Ingredients A variety of viscosity depressants are available to the paste formulator. Small additions of these surface active materials can lower viscosity quite significantly, and sometimes also improve viscosity stability and air release properties. Polyethylene glycol derivatives are known to be effective, whilst some monohydric aliphatic alcohols suppress viscosity ageing to a useful extent. Among the commonly used viscosity depressants are numerous proprietary materials of undisclosed chemical composition. Although the reduction of viscosity is frequently sought in paste processing, there are certain applications where a composition is required with a high viscosity at low shear rates, but with a low viscosity at high shear rates. For example, in spray coating, the paste must flow easily through the nozzles at high shear rates, but must not sag once the surface film is deposited. To get the extreme pseudoplasticity required, special thickeners or fillers can be added in small proportions. Aluminium stearate and colloidal silica are particularly effective, and are believed to produce weakly bonded structures which readily break down under mechanical shear. As with other types of PVC compositions, a variety of minor additives may be used for particular effects, e.g. pigments, heat stabilisers, blowing agents and foam stabilisers. These may also have significant influence on the rheology of the paste, and this must be taken into account in the design of formulations. Finally, brief mention should be made of the use of diluents or thinners, to reduce paste viscosity. These are generally hydrocarbons with very little solvent power for the polymer, and sufficiently volatile to vaporise during the fusion stage of the fabrication process. Strictly speaking, systems containing diluents should be called ‘organosols’, as distinct from ‘plastisols’, but the general term ‘paste’ has, in any case, been used here, as in general parlance, to cover all compositions within this area of technology. 10.6.2 Fusion As a plastisol is heated, there is an initial reduction in viscosity due to the viscosity decrease of the plasticiser. As heating continues, the viscosity begins to rise and the polymer particles swell, until the mass reaches gelation point. ‘Gelation’, in the usage of paste technology, generally denotes the first disappearance of the liquid phase, although the solid mass at this point is quite weak and crumbly. As heating continues, there is further interaction of polymer and plasticiser, until complete fusion is reached, when clarity, gloss, and mechanical strength of the final composition have reached a maximum. The ease of fusion of plastisols has been very clearly related to the polarity and solvating power of the plasticiser

198

D.A.TESTER

used.17,18 As already shown in Section 10.5.2, in the discussion of plasticiser effects, minimum flux temperatures for a range of plasticisers in a standard paste composition may be inversely related to the activity parameters derived by Bigg. Lower K-value polymers and smaller particle size tend to give lower temperatures for initial gelation, with a given plasticiser, and lower K-value polymers also give shorter fusion times to develop optimum properties. Minimum temperatures for adequate fusion are, however, less sensitive to polymer K-value. Where it is important to achieve rapid processing at relatively low temperatures, e.g. in coating heat sensitive substrates, it may be necessary to employ a vinyl acetate copolymer. The incorporation of as little as 5% vinyl acetate in a copolymer can lower fusion temperatures by 20–30°C. Although paste processing is so largely dominated by the rheology of the ungelled paste, it is, of course, necessary, through the appropriate combination of formulation and thermal treatment, to achieve complete fusion. In certain processes, precise control of fusion rate and the resultant melt viscosity are critical in obtaining satisfactory products. This is particularly true of the manufacture of cellular materials by the incorporation of chemical blowing agents. If the blowing agent is completely decomposed before gelation of the paste, an open cell foam will be formed. The cell structure will be fine if the paste has developed a high viscosity at this decomposition temperature. Where decomposition of the blowing agent takes place at, or above, the temperature at which fusion takes place, a predominantly closed cell foam will be produced. The melt viscosity of the fused composition, together with the rate of blowing agent decomposition, then largely determine the cell size. 10.6.3 The Requirements of Major Paste Applications The intention in this presentation has been to indicate the dependence of paste rheology and fusion characteristics on formulation variables, and particularly on the nature of the polymer and plasticiser. The practical significance of these observations is best illustrated by reference to some of the more important applications for PVC pastes. The following comments provide a very brief guide to the general requirements of each application area; detailed formulations are generally the result of considerable experience of the particular process employed. 10.6.3.1 Spread Coating Processes These are generally concerned with paste coating of flexible substrates, by knife or roll, with fusion achieved by passage through a heated oven. Fabric coating

This is probably the earliest exploitation of paste spreading, particularly in the production of leathercloth for upholstered furniture, car seats and travel goods. Where direct spreading is employed it is important to avoid excessive penetration of the fabric. Paste viscosities must therefore not be too low at low shear rates, particularly where the fabric is of open weave construction. Medium/low viscosities at higher shear rates are desirable, although fabric coating processes do not employ the highest coating speeds encountered in paste spreading. A ‘medium viscosity’ polymer is therefore required, with marked pseudoplasticity at lower shear rates. Where a very soft, limp product is required, a high plasticiser level will be employed. To achieve the

THE PROCESSING OF PLASTICISED PVC

199

appropriate paste viscosity characteristics in this case, it may be necessary to use a ‘high viscosity’ polymer, or a blend of polymers. Wallpaper coating

The growing popularity of ‘vinyl’ wall covering has made this an important application for paste spreading. Thin coatings are applied, at high line speeds, so that paste behaviour at high shear rates is of dominant interest. Fillers and pigments are incorporated to give opacity and colour, and must be of fine particle size. The need is therefore for a ‘low viscosity’ polymer, with some pseudoplasticity at low shear rates, but with as near Newtonian behaviour as possible, at high shear rates. Since relatively coarse particles will cause grittiness in the surface of the product, blending resins cannot be employed. Foamed flooring

Whilst the use of solid PVC flooring has tended to decline in recent years, there has been considerable growth in the sales of foamed flooring. This consists essentially of a foamed interlayer spread on a suitable base, printed with the required colours and pattern, and protected by a top wear layer which must be tough and transparent. The foam layer, which is chemically blown, provides a cushion effect which is an attractive feature of this flooring. At least two different polymers are required for optimum performance in the manufacturing of foamed flooring. Coating speeds are generally lower than with paper coating, but the formulations are of fairly high solids content, so that ‘medium/low viscosity’ polymers are required. The essential requirement for the foam layer is the achievement of a foam of appropriate density and uniform closed cell structure. As previously mentioned, foam structure is controlled largely by the interaction of the rate of gas evolution at fusion temperatures and the fusion/melt viscosity characteristics of the composition. The nature and content of emulsifier in the polymer have an important influence on foaming behaviour, whilst use of a relatively low K-value polymer (<70) is generally found to be advantageous. In the final passage of the composite structure through the oven to achieve complete fusion, blowing of the foam layer takes place. It is important that the wear layer is sufficiently viscous at this point to prevent break through of bubbles from the foam beneath. For this reason a relatively high molecular weight polymer (Kvalue ~80) is advantageous for the wear layer. A high K-value is also of benefit in achieving the wear properties required in the finished product. 10.6.3.2 Dipping Essentially, this process consists of dipping a former, or an article to be coated, into a suitable paste, withdrawing it with an adhering layer of paste, allowing excess paste to drain and then gelling. In contrast to spreading processes, high shear rate behaviour is generally irrelevant, but low-shear rate viscosities are of critical importance. The optimum viscosity for the paste will depend upon the thickness of coating required. In general, fairly low viscosities are employed, but too low a viscosity would give excessive drainage and ‘tear drops’ on the ungelled surface. The formers or articles to be coated may be at ambient temperature, or may be heated to 100–160 °C, and the gelation properties of the paste must be adjusted to take account of this. Where a soft unsupported product is required, as in the manufacture of surgical gloves, a high plasticiser level will be employed, and a ‘high/medium viscosity’ polymer is needed to give an adequate paste viscosity. Where fabrics are being coated, as in the manufacture of fabric lined gloves, it is generally necessary to minimise penetration of the fabric liner by paste. A high viscosity paste is required, and a ‘high viscosity’ polymer is appropriate.

200

D.A.TESTER

10.6.3.3 Slush Moulding and Rotational Casting In slush moulding, one or more layers of paste are gelled on the inside of a hollow mould which is open at one end. The mould is commonly preheated before being filled to achieve rapid partial gelation. Ungelled paste may be poured off, and the residue completely fused by heating in an oven. To achieve thicker mouldings, the heating and filling operation may be repeated several times before completing gelation and fusion at a higher temperature. Rotational casting, which developed from slush moulding, involves the use of a closed hollow mould, usually openable into two halves, into which paste is poured. The closed mould is rotated about two axes at right angles to each other, whilst mounted in an oven to gel the paste. A layer of paste is picked up by the whole surface of the mould. Heating must be such as to give fairly rapid gelation, but not so fast as to prevent a uniform thickness of gelled coating being formed. As with dipping, only low-shear rates are involved in these processes, and pastes with low viscosities at these shear rates are required. At the same time, fairly rapid gelation is desirable. ‘Medium viscosity’ polymers are satisfactory, and where harder mouldings, and therefore pastes of higher polymer content, are required, the use of ‘blending polymers’ is advantageous to maintain adequately low paste viscosities. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

LEBEDER, V.P., DERLYUKOVA, L.Y., RAXINSKAYA, I.N., AKLADNOV, N.A. and SHTARKMAN, B.P., Vysokomol. Soyed, 7(2), 333 (1965). ALFREY, T., WIEDERHERN, N., STEIN, R. and TOBOLSKY, A., Ind. Eng. Chem., 41, 701 (1949). KOLESKE, J.V. and WORTMAN, L.H., Poly(vinyl chloride), Macdonald, London (1969), p. 67. SUMMERS, J.W., ISNER, J.D. and RABINOVITCH, E.B., 36thANTEC, SPE, 757 (1978). SUMMERS, J.W., ISNER, J.D. and CHAPMAN, T.E., 37th ANTEC, SPE, 370 (1979). MOORE, D.R., International Conference on PVC Processing, The Plastics and Rubber Institute, London, 1978. BIGG, D.C. H., J. Appl. Polym. Sci., 19, 3119 (1975). BIGG, D.C. H., J. Appl. Polym. Sci., 19, 1487 (1975). NASS, L.I., Encyclopaedia of PVC. Volume 1, Marcel Dekker Inc., New York (1976), Chapter 11. SARVETNICK, H.A., Plastisols and Organosols, Van Nostrand Reinhold, London (1972). MATHEWS, G., Vinyl and Applied Polymers, Volume 2, Iliffe, London (1972). GILLESPIE, T., J. Colloid Interface Sci., 22, 554 (1966). COLLINS, E.A. and HOFFMAN, D.J., Rubber Chem. Technol., 52(3), 676 (1979). CUNNINGHAM, J.A., SATRA Materials Conference, Brighton (England), 26 (1975). UNDERDAL, L., LARGE, S., PALMGREN, O. and THORSHAUG, N.P., International Conference on PVC Processing, The Plastics and Rubber Institute, London, 1978. BIGG, D.C. H., J. Appl. Polym. Sci., 20, 565 (1976). McKENNA, L.A., Modern Plastics, 35(10), 142 (1958). SEVERS, E.T. and SMITMANS, G., Paint and Varnish Production, 47(12), 54 (1957).

INDEX

Acroosteolysis, 102 Acrylate esters, 93 rubbers, 97, 240 Acrylonitrile/butadiene/styrene terpolymers (ABS), 240 Additives, 79, 172, 183–6 Aerosol propellant, 101 Ageing, 263 Agitation, 15–16, 56, 69, 72, 167 Air classifier, 80 Angiosarcoma of the liver, 102–3 Anti-plasticisation, 248 Antistatic agents, 136 Area monitoring, 106 Atmospheric testing, 105–7 Atomisation, 78–9, 143 Atomisers design, 143–6 pressure nozzle, 145 spinning disc, 144 Autoclaves capacity, 25 cleaning, 33, 102–3 cooling, 30 costs, 26 discharge valve, 32 discharging and charging, 37 fouling, 32–6 heat transfer problem, 29–32 heat transfer surface area, 28 number of, 27 output, 26 pressure, 37, 67 Autoclaves—contd. size, 26–37, 69 effect on heat removal, 28–9 stirrer speed, 167 temperature, 37

vertical, 56–8 volume, 28 Bag filter units, 142 Banbury high shear internal mixer, 204–10 Banbury mixer, 224 Batch process, 67 Battery plates, 148 separators, 65, 66 ‘Blacks’, 159 Blends and blending, 94–9, 148, 186, 218, 246, 253, 262 Brabender Plasticorder, 210–11, 224, 234, 240 Brownian motion, 167 Buffer, 16–17, 72 Bulk density, 51, 58 (mass) PVC, stripping processes, 119–20 polymerisation, 39, 41–2, 148–9 processes, 39–61 costs, 55–6 quality status, 58 properties, 165 Butadiene based rubbers, 95 Cable insulation, 64 Carcinogenicity problem, 102, 124 Centrifuges, 129 Chain transfer agents, 69, 87–8 Chemical build-up suppression, 34–5 Chlorinated polythene, 95–6, 240 Chlorinated PVC, 99 Cleaning systems, 124 ‘Clears’, 160 Coating compounds, 259–60 Coating processes, 265 Code of Practice, 104, 106 201

202

INDEX

Cold plasticiser absorption, 252 Colorimetric methods, 106 Computer system, 37 Condensers, 31 Conversion effects, 163–5, 177 Conveying, 136, 147–8 Coolants and cooling, 30–1, 69 Cooling loop, 32 Copolymerisation, 55, 91, 230–3 parameters, 86 theory, 84–5 Copolymers, 55, 83–97 Corona discharge devices, 136 Cost comparison, 55–6 Cyclones, 136, 142 Degassing, 52–3 Density control, 189 Dewatering, 126–9 Dipping processes, 65, 267 Driers, 129–35 advantages and limits of principal, 130 direct heat, 132 flash, 133 fluid bed, 134 indirect heat rotary, 131 Droplet stabilisation, 161 Drying chamber location, 143 conditions, 66, 126 drum, 139 freeze, 139 processes, 139–48 Drying—contd. rates, 126 spray, 70, 72, 75, 79, 139, 147 treatment prior to, 123–4 Dust explosions, 129 hazards, 149 e value, 85 EDTA, 35 EEC Directive, 104 Elastic behaviour, 239 deformation, 225 Electrical applications, 64 Emulsifiers, 70, 71, 262 Emulsion

polymerisation, 40, 63, 64, 68, 87, 123, 136 continuous process, 73–5 polymer types produced by, 64–5 principal ingredients, 67 polymers, 65, 71 applications, 65, 138 handling, 80 manufacture, 67 production, 66 PVC, stripping processes, 117–19 Ethyl acrylate, 98 Ethylene, 90–1 dichloride, 23 Ethylene/propylene rubbers, 97 Ethylene/vinyl acetate copolymers, 240 Ethylene/vinyl acetate rubbers, 96 2-Ethylhexyl acrylate, 93 Explosion hazard, 129 Explosion risk, 101 Extenders, 253 Extrudate, mechanical properties, 251 Extruder(s) 1·25 inch Iddon, 191–2, 201–4 sampling, 190–204 single-screw, 201–4 twin-screw, 193–200 Schloemann BT 80, 192, 194 Extrusion, 65, 66 pressure/processing temperature curve, 249 rigid PVC, 219–21 single-screw, 190–2 temperature, 256 theory, 190–200 twin-screw, 190, 204, 220, 227 Fabric coating, 266 Fatigue lifetime, 222, 223 Fillers, 99, 241–2, 257–8, 264 Film coefficient, 12 Filters, 124 First stage reactor (pre-po), 47–9 Fish eyes, 253 Flame ionisation, 106 Flocculation, 136, 171 Flooring, 267 Flory-Huggins interaction parameter, 254 Flow pressure, 210 properties, 66

INDEX

Fluorescence, 198–200, 210 Foil, 216, 233 Food applications, 253 Foodstuffs, 108 Freeze drying, 139 Friction coefficient, 189 Gas chromatographic analysis, 107 Gas chromatographic separation, 106 Gas-phase polymer, 177–80 Gas-phase polymerisation, 39, 59–60 Gay-Lussac, 42 Gel phase, 178, 180 Gelation, 60, 183–213, 221, 224–6, 234, 265 Glass transition temperature, 186 Gloves, 65 Grain appearance, 55 attrition, 51 size, 47, 50, 138, 180, 226–8, 253 structure. See Morphology Grinding, 148 Handling characteristics, 147 Handling problems, 52–3, 80, 149 Heat removal rate, 29 stabilisers, 236–7 transfer, 28–32, 52 coefficient, 30 treatment, 77–9 Hildebrand solubility parameter, 254 Ignition energy, 129–31 Impact modifiers, 240–1 Infra-red spectroscopy, 106 Initiators, 17–22, 25, 34, 36, 69, 73 Instrumentation, 36 ISO-K 67, 186 Isolation processes, 123–50 K-value, 50, 51, 60, 83, 86, 87, 172, 173, 228–33, 252, 260, 265, 267 Kinetics, 2–8, 59, 101, 108 Laser Raman spectroscopy, 170 Latex concentration, 137 drying, 75–9

203

filtration, 136 Lubricants, 79, 233–6 M66 process, 44 Macro-morphology, 156 Manufactures des Glaces et Produits Chimiques de St Gobain, Chauney et Cirey SA, 42 Manufacturing costs, 24–6 Manufacturing plant, 25 Manufacturing processes, 1 Market, 1 Mass spectrometry, 106 Mechanical properties, 251 Melt modulus, 256 Methacrylate esters, 93 Methyl methacrylate, 93–4, 97, 98 Methyl methacrylate/butadiene/ styrene, 240 Microscopical examination, 205 Microsuspension polymerisation, 68, 72–3, 123, 136 Milling, 79–80, 148 Modified L51 process, 44 Moisture content, 124, 147 Molecular weight, 7, 50, 51, 86, 98 Morphology, 48, 53–5, 60, 88, 124–6, 146, 152–82, 189, 226–8, 248–53 classification, 155–61 control, 173 external, 174 flow pressure, and, 210 gas-phase polymer, 177–80 internal, 174, 196 macro-scale, 161–5 micro-scopic, 165–8, 171–3, 181 nomenclature, 153–4 overall, 171–4 Rhone-Poulenc bulk polymer, 174–7 sub-microscopic, 168, 181 temperature effects, 156, 173–4 N7 process, 44 N8 process, 44 Nibs, 253 Nozzle design, 78–9, 145 Occupational Safety and Health Administration, 104 Olefin copolymers, 90–3 Organosols, 265 Orientation effects, 242–4

204

INDEX

Packing, 136 Paraloid K175, 97 Paraloid KM323B, 97 Partial pressure polymerisation, 59 Particle formation, 45–7 size, 64, 67, 72, 79 distribution, 66, 67, 70–3, 77, 80 Paste polymers, 64, 71, 148, 258–68 applications, 65 handling, 80 manufacture, 67 production, 66 see also Plastisols Pechiney St Gobain (PSG), 42 Personal monitoring, 106–7 pH control, 72 value, 74 N-Phenylmaleimides, 94 Plant operator protection, 103 Plasticisation, 247 Plasticised PVC, 245–69 compounding, 246 effect of formulation ingredients. 252–8 feedstock preparation, 246–7 melt flow, 248–51 rheology, 249–51 structure, 249–51 other ingredients, 258 plasticiser, choice of, 253–7 polymer characteristics, 252 polymer/plasticiser interaction, 247–8, 254 powder blends, 246 premix, 258 product properties, 251 Plasticisers, 65, 83, 159, 172, 245, 250, 253, 258, 262–3 Plastisols, 65, 66, 71, 78, 227, 255, 258 applications, 266 filler, choice of, 264 fusion, 265–6 other ingredients, 264 plasticiser, choice of, 262–3 polymer characteristics, 260 rheology, 259–65 see also Paste polymers Polar character, 85

Polyblobs, 253 Poly(butyl acrylate), 97 Polymerisation degree, 7 free radical induced addition, 2 kinetics, 2–8, 59, 101, 108 processes, 39, 151 Polymerising volume, 164 Poly(methyl methacrylate), 98 Poly(vinyl acetate), 161, 163, 166 Poly(vinyl alcohol), 157, 163 Porosity, 51, 167, 186, 187, 207, 227, 252 Porosity/apparent density, 173 Porosity measurement, 172 Powder handling. See Handling problems Pre-mixing, 183–6 Pressure drop, 177 effects, 91, 190 monitoring, 51–2 pipes, 243 Process control, 51–2 Processing aids, 98, 237–40, 258 Processing machinery, 213 Processing parameters, 186–90 Production capacity, 26 Produits Chimiques Pechiney-St Gobain, 42 Produits Chimiques Ugine Kuhlmann (PCUK), 42–3 Progil, 42 Propylene, 92–3 Protective colloid, 13–15, 156, 157, 161, 163, 166 Pseudoplastic behaviour, 260 PVC blends, 94 Q-value, 85 Quality status comparison, 58–9 Reactivity ratios, 84–5, 8 Redox catalysis, 69 Reduction activation, 69 Refractive index, 159 Resonance effect, 85 Respiratory disease, 149 Rheological properties, 64, 259–65 Rhone-Poulenc, 42 bulk polymer, 174–7 two-stage process, 44–50 Rhone-Progil, 43 Rigid PVC, 215–44

INDEX

compound manufacture, 217–18 copolymerisation, 230–3 extrusion, 219–21 feedstock preparation, 217–19 formulation ingredients on processing behaviour, influence of, 226–42 melt stiffness, 216 orientation effects, 242–4 pipe quality, 225–6 polymer characteristics, 226–37 power blend manufacture, 218 processing on properties, influence of, 221–6 processing problems, 215 Rotational casting, 65, 268 Rubber blends, 95–7 Safety consideration, 36–7 Saturation vapour pressure, 108 Screening, 147–8 Second stage reactor, 49–50 Shear effects, 189 Sieving process, 136, 148 Single-stage process, 43 Sintering properties, 67 Size distribution, 47 Slush moulding, 268 Small angle X-ray scattering, 170 Solids separation, 126–9 Solution copolymerisation, 39 Spray drying, 70, 72, 75, 79, 139, 147 Spread coatings, 65 ‘Square-sided’ grains, 53 St Gobain, 42 Stabilisers, 79 Steric stabilisation, 161 Strainers, 124, 137 Stretching effects, 243 Stripping processes, 89, 109–20 bulk (mass) PVC, 119–20 emulsion PVC, 117–19 suspension PVC, 113–17 Styrene/acrylonitrile polymers, 98 Subsaturation, 180 Subsaturation vapour pressure, 108 Surface area, 207 Suspension polymerisation, 8, 39, 40, 64, 87, 123–36, 151, 156

agitation, 32 agitator, role of, 15–16 buffer, role of, 16 costs, 55–6 evacuation step, 22 initiator, choice of, 20–2, 25 initiator, role of, 17–22 initiators, 34 mechanism of, 161–71 outline of process, 8–10 protective colloid, role of, 13–15 quality status, 58 typical cycle, 11 typical process, 10–24 typical recipe, 10 vinyl chloride, 1–38 water, role of, 12 Suspension polymers, macro-morphology, 156–61 Suspension PVC, 110 stripping processes, 113–17 Temperature control, 32, 47, 87, 216 effects, 50–1, 67, 69, 77, 88, 111, 156, 167, 173–4, 189, 207, 210–11, 243, 249–50, 263 Thermal treatment, 223–6 Thermoplastics, 245 Threshold limit value, 103 Threshold temperature, 249 Time-weighted average, 104 Toxicity, vinyl chloride, 101–3 Translucent grains, 160, 172, 174 Tumours, 102 Two-stage process, 43–50, 56–8 Unplasticised PVC, 183 Vacuum evaporation, 137 Vacuum filtration, 126 Vapour pressure, 67, 112 Vinidur SZ, 97 Vinyl acetate, 86, 231, 232 Vinyl acetate copolymers, 86–90 Vinyl chloride analysis, 105–8 quality aspects, 23–4 recovery, 109 removal, 75, 108–20 solids and liquids, in, 107–8

205

206

INDEX

suspension polymerisation, 1–38 toxicity, 101–3 Visco-elastic properties, 216 Viscosity, 242, 256, 260, 261, 264, 265 ageing, 263 behaviour, 66 control, 66 properties, 80 Viscosity/shear rate relationship, 66, 261 Wallpaper coating, 266 Water loss rate, 124–5 suspension polymerisation, in, 12 Wet-cake, 127–9