Practical Guide to Polypropylene

Rapra Practical Guide Series

Practical Guide to Polypropylene

Devesh Tripathi

Practical Guide to Polypropylene By

Devesh Tripathi

Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK Tel: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First published 2002 by Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

© 2002, Rapra Technology Limited ISBN: 1-85957-282-0 All rights reserved. Except as permitted under current legislation 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 prior permission from the copyright holder. Typeset, printed and bound by Rapra Technology Limited.

Contents 1 Introduction................................................................................................................. 1 1.1 Background ............................................................................................................ 1 1.2 Major Advantages .................................................................................................. 2 1.3 Major Disadvantages.............................................................................................. 3 1.4 Competitive Materials............................................................................................ 3 1.5 Applications ........................................................................................................... 5 1.6 Market Share and Consumption Trend .................................................................. 6 1.7 Major Suppliers...................................................................................................... 7 1.8 Material Price......................................................................................................... 8 2 Basic Types of PP ........................................................................................................ 9 2.1 Homopolymer ........................................................................................................ 9 2.2 Copolymer.............................................................................................................. 9 2.2.1 Random Copolymer ...................................................................................... 10 2.2.2 Block Copolymer .......................................................................................... 10 2.3 Elastomer-Modified Polypropylene..................................................................... 11 2.4 Controlled Rheology ............................................................................................ 11 2.5 Metallocene Polymers.......................................................................................... 12 2.6 Syndiotactic and Atactic PP................................................................................. 13 2.7 Filled Grades of PP .............................................................................................. 13 2.7.1 Talc Filled PP................................................................................................ 14 2.7.2 Calcium Carbonate Filled PP ........................................................................ 14 2.7.3 Glass Fibre Reinforced PP ............................................................................ 14 2.7.4 Mica Reinforced PP ...................................................................................... 15 2.8 Additives for PP................................................................................................... 15 2.9 Identification of PP Type ..................................................................................... 16 3 Structure .................................................................................................................... 19 3.1 Molecular Weight ................................................................................................ 19 3.2 Molecular Weight Distribution ............................................................................ 20 3.3 Crystallinity.......................................................................................................... 20 3.4 Orientation ........................................................................................................... 22 3.5 Isotacticity............................................................................................................ 22 4 Properties................................................................................................................... 24 4.1 Density ................................................................................................................. 24 4.2 Thermal Properties............................................................................................... 24 4.2.1 Glass Transition Temperature and Melting Point ......................................... 24 4.2.2 Maximum Continuous Use Temperature ...................................................... 27 4.2.3 Heat Deflection Temperatures and Softening Points .................................... 28 4.2.4 Brittle Temperature ....................................................................................... 29 4.2.5 Specific Heat ................................................................................................. 30 4.2.6 Thermal Conductivity ................................................................................... 31 4.2.7 Thermal Expansion ....................................................................................... 31

4.3 Mechanical Properties ..........................................................................................32 4.3.1 Short-term Mechanical Properties.................................................................32 4.3.1.1 The Effect of Test Speed .........................................................................33 4.3.1.2 The Effect of Temperature......................................................................33 4.3.1.3 Time-temperature Superposition............................................................34 4.3.2 Impact Strength .............................................................................................34 4.3.2.1 Falling Dart Impact Test........................................................................35 4.3.2.2 Notched Impact Strength........................................................................35 4.3.2.3 Tensile-impact Strength..........................................................................36 4.3.3 Creep .............................................................................................................36 4.3.4 Fatigue...........................................................................................................39 4.3.5 Dynamic Fatigue ...........................................................................................39 4.3.6 Mechanical Properties of Filled Grades ........................................................40 4.3.7 Biaxial Orientation ........................................................................................43 4.4 Electrical Properties .............................................................................................44 4.5 Optical Properties.................................................................................................46 4.5.1 Transparency .................................................................................................46 4.5.2 Gloss..............................................................................................................47 4.5.3 Haze...............................................................................................................47 4.6 Surface Properties ................................................................................................47 4.6.1 Hardness and Scratch Resistance ..................................................................47 4.6.2 Abrasion Resistance ......................................................................................48 4.6.3 Friction ..........................................................................................................49 4.7 Acoustic Properties ..............................................................................................49 4.8 Biological Behaviour............................................................................................50 4.8.1 Assessment Under Food and Water Legislation ...........................................50 4.8.2 Resistance to Microorganisms.......................................................................50 4.8.3 Physiological Compatibility..........................................................................51 4.9 Additives ..............................................................................................................51 4.9.1 Antistatic Agents ...........................................................................................51 4.9.2 Electromagnetic Interference/Radio Frequency Interference Shielding .......52 4.9.3 Slip and Antiblocking Agents .......................................................................53 4.9.4 Metal Deactivators and Acid Scavengers......................................................53 4.9.5 Blowing Agents.............................................................................................53 4.9.6 Nucleating Agents .........................................................................................54 4.9.7 Antifogging Agents .......................................................................................54 4.9.8 Biocides.........................................................................................................54 4.9.9 Flame Retardants...........................................................................................55 4.10 Performance in Service ......................................................................................56 4.10.1 Thermal or Heat Stability............................................................................56 4.10.2 Stability to Light and Ultraviolet Rays........................................................57 4.10.3 Chemical Resistance ...................................................................................59 4.10.4 Permeability ................................................................................................60

4.10.4.1 Permeability of Water and Liquids ...................................................... 60 4.10.4.2 Permeability of Gases .......................................................................... 61 4.10.5 Sterilisation ................................................................................................. 61 4.10.5.1 Autoclave and Ethylene Oxide Sterilisation......................................... 61 4.10.5.2 Radiation Sterilisation ......................................................................... 62 5 Design ......................................................................................................................... 65 5.1 Product Design..................................................................................................... 65 5.1.1 Design for Rigidity and Toughness............................................................... 65 5.1.2 Weld Lines .................................................................................................... 66 5.1.3 Shrinkage and Dimensional Stability............................................................ 66 5.1.4 Sinks and Voids ............................................................................................ 67 5.1.5 Design for Assembly..................................................................................... 68 5.1.6 Integral Hinges.............................................................................................. 68 5.1.7 Design to Avoid Failure and Durability........................................................ 69 5.1.8 Design Safety Factors ................................................................................... 69 5.2 Mould Design....................................................................................................... 70 5.2.1 Flow Length .................................................................................................. 70 5.2.2 Feed Systems................................................................................................. 71 5.2.3 Venting.......................................................................................................... 72 5.2.4 Mould Cooling .............................................................................................. 72 5.2.5 Taper and Ejection ........................................................................................ 73 5.2.6 Surface Finish ............................................................................................... 73 5.2.7 Filled Grades ................................................................................................. 74 6 Processing of PP ........................................................................................................ 75 6.1 Rheology .............................................................................................................. 76 6.1.1 Melt Flow Rate.............................................................................................. 76 6.1.2 Viscosity Versus Shear Rate ......................................................................... 76 6.2 Injection Moulding............................................................................................... 80 6.3 Extrusion .............................................................................................................. 82 6.3.1 Fibre and Filament ........................................................................................ 82 6.3.2 Film Extrusion............................................................................................... 82 6.3.2.1 Cast Film................................................................................................ 82 6.3.2.2 Blown Film............................................................................................. 83 6.3.2.3 Biaxially Oriented Film.......................................................................... 83 6.3.3 Coextrusion ................................................................................................... 83 6.3.4 Stretched Tapes ............................................................................................. 83 6.3.5 Sheet Extrusion ............................................................................................. 83 6.3.6 Pipes and Tubes ............................................................................................ 84 6.4 Blow and Stretch Blow Moulding ....................................................................... 85 6.5 Thermoforming and Vacuum Forming ................................................................ 86 6.6 Calendering .......................................................................................................... 87 6.7 Rotational Moulding ............................................................................................ 87 7 Post Processing and Assembly ................................................................................. 89

7.1 Joining..................................................................................................................89 7.1.1 Welding .........................................................................................................89 7.1.1.1 Heated Tool Welding..............................................................................90 7.1.1.2 Hot Gas Welding ....................................................................................90 7.1.1.3 Friction and Vibration Welding .............................................................90 7.1.1.4 Ultrasonic Welding ................................................................................91 7.1.1.5 Radio Frequency Welding......................................................................91 7.1.1.6 Other Welding Techniques .....................................................................91 7.1.2 Solvent Bonding............................................................................................91 7.1.3 Adhesive Gluing............................................................................................92 7.1.4 Sealability......................................................................................................92 7.2 Assembly and Fabrication....................................................................................92 7.2.1 Machining......................................................................................................92 7.2.2 Snap-fit Joints................................................................................................92 7.2.3 Mechanical Fastening....................................................................................93 7.3 Decorating ............................................................................................................93 7.3.1 Printability and Paintability...........................................................................93 7.3.2 Metallising and Electroplating ......................................................................94 7.3.3 Appliques ......................................................................................................94 8 Causes of Failure .......................................................................................................95 9 Product Development Issues ....................................................................................97 9.1 Material Selection ................................................................................................97 9.2 Design ..................................................................................................................97 9.3 Processing and Post Assembly.............................................................................98 9.4 Performance in Service ........................................................................................98 References ...................................................................................................................101 Abbreviations and Acronyms....................................................................................103

Practical Guide to Polypropylene

1 Introduction 1.1 Background Polypropylene (PP) was first produced by G. Natta, following the work of K. Ziegler, by the polymerisation of propylene monomer in 1954 (Figure 1). The macromolecule of PP contains 10,000 to 20,000 monomer units. The steric arrangement of the methyl groups attached to every second carbon atom in the chain may vary (see Figure 2). If all the methyl groups are on the same side of the winding spiral chain molecule, the product is referred to as isotactic PP. A PP structure where pendant methylene groups are attached to the polymer backbone chain in an alternating manner is known as syndiotactic PP. The structure where pendant groups are located in a random manner on the polymer backbone is the atactic form.

CH2 = CH CH3 Figure 1 Propylene monomer

CH3 CH2

CH

CH3

CH3 CH2

CH

CH2

CH

CH3 CH2

CH

n

isotactic polypropylene CH3 CH2

CH

CH3 CH2

CH

CH2

CH

CH2

CH3

CH n CH3

syndiotactic polypropylene CH3 CH2

CH

CH3 CH2

CH

CH3 CH2

CH

CH2

CH

n

CH3 atactic polypropylene Figure 2 PP polymer molecule in isotactic, syndiotactic and atactic forms 1

Practical Guide to Polypropylene

Only isotactic PP has the requisite properties required for a useful plastic material. Stereospecific or Ziegler-Natta catalysts are used to polymerise PP in this form. All the applications of PP described in this book are for isotactic PP, although brief mention is made of the main applications and properties of syndiotactic and atactic PP. The pendant methylene group in PP is replaced by a chlorine atom in polyvinyl chloride (PVC), by a benzene ring in polystyrene (PS) and by a hydrogen atom in polyethylene (PE). The pendant group significantly affects the properties of the polymer, and consequently the properties of PP are very different from other commodity plastics such as PE, PVC and PS (Section 4). In 1957, PP was commercially produced by Montecatini as Moplen. Recently, metallocenes have attracted widespread attention as the new generation of olefin polymerisation catalysts. Metallocene catalysts provide enhanced control over the molecular make up of PP, and grades with extremely high isotacticity and narrow molecular weight distribution (MWD) are possible. Properties of metallocenepolymerised PP are further discussed in Section 2.5. 1.2 Major Advantages PP is very popular as a high-volume commodity plastic. However, it is referred to as a low-cost engineering plastic. Higher stiffness at lower density and resistance to higher temperatures when not subjected to mechanical stress (particularly in comparison to high and low density PE (HDPE and LDPE)) are the key properties. In addition to this, PP offers good fatigue resistance, good chemical resistance, good environmental stress cracking resistance, good detergent resistance, good hardness (5 on the comparative ranking utilised in Table 4) and contact transparency and ease of machining, together with good processibility by injection moulding and extrusion. These advantages of PP are further elaborated in later sections. Table 1 Comparison of unmodified PP with other materials: Advantages [1] Property PP LDPE HDPE HIPS PVC ABS Flexural modulus (GPa) 1.5 0.3 1.3 2.1 3.0 2.7 Tensile strength (MPa) 33 10 32 42 51 47 Specific density 0.905 0.92 0.96 1.08 1.4 1.05 Specific modulus (GPa) 1.66 0.33 1.35 1.94 2.14 2.57 HDT at 0.45 MPa. (°C) 105 50 75 85 70 98 Maximum continuous use 100 50 55 50 50 70 temperature (°C) Surface hardness RR90 SD48 SD68 RM30 RR110 RR100 Cost (£/tonne) 660 730 660 875 905 1550 Modulus per unit cost 2.27 0.41 1.97 2.4 3.31 1.74 (MPa/£) ABS = acrylonitrile butadiene styrene RM = Rockwell M HIPS = high impact polystyrene SD = Shore Durometer RR = Rockwell R

2

Practical Guide to Polypropylene

The properties of unmodified PP are compared with other competitive thermoplastics in Table 1. It can be seen from the table that PP offers advantages over most of its competitive materials on the basis of specific modulus (modulus to density ratio), heat deflection temperature (HDT), maximum continuous use temperature or modulus to cost ratio. Environmental and food legislation may further tip the balance in favour of PP. 1.3 Major Disadvantages The major disadvantages of unmodified PP compared with other competitive thermoplastics are evident from Table 2. It can be seen that PP has significantly higher mould shrinkage, higher thermal expansion and lower impact strength, particularly at sub-ambient temperatures, than HIPS, PVC and ABS. However, PP has lower mould shrinkage and thermal expansion coefficient than HDPE and LDPE. Poor UV resistance and poor oxidative resistance in the presence of certain metals such as copper are other disadvantages of PP. As any semi-crystalline material, PP also suffers from high creep under sustained load in comparison to an amorphous plastic such as ABS or PVC. Other disadvantages of PP are difficult solvent and adhesive bonding, poor flammability, warpage, limited transparency, poor wear properties, unsuitability for frictional applications and poor resistance to gamma radiation. (Further discussion of the properties of PP may be found in Section 4). However, most of these disadvantages could be overcome, either completely or to a certain degree, by proper selection of material, sensible design and good processing. The processing of PP by thermoforming and blow moulding is difficult. Vacuum forming of PP is also difficult. Table 2 Comparison of unmodified PP with other materials: Disadvantages [1] Property PP LDPE HDPE HIPS PVC ABS Mould shrinkage (%) 1.9 3.0 3.0 0.5 0.4 0.6 -5 Thermal expansion (x10 ) 10 20 12 7 6 8 Notched Izod impact 0.07 >1.06 0.15 0.1 0.08 0.2 strength (kJ/m) at 23 °C

PP is not hazardous to health, however, it can release volatile organic compounds (VOCs) into the surrounding air during high-temperature processing. Workers at the processing plant can be subjected to these VOCs through inhalation or skin contact. Good ventilation using exhaust fans can minimise the exposure. Residual monomer and catalysts present in the resin can increase the toxicity. 1.4 Competitive Materials PP is most frequently compared with PE but other competitive materials are polystyrene and its derivatives, cellulose acetate (CA), cellulose acetate butyrate (CAB) and PVC. PP is used to replace engineering plastics, such as polyethylene terephthalate (PET), polyamide (PA), polycarbonate (PC) and ABS, etc., in kitchen appliances and domestic appliances. In non-plastics, PP faces competition from glass and metal. 3

Practical Guide to Polypropylene

Major competitive materials for PP and their crude advantages/disadvantages over PP are given in Table 3. This table is for broad comparison only. In many cases, polymers are filled or modified to improve properties or to reduce cost which makes the distinction between the properties of two polymers for a particular application quite blurred. Consequently, choice of a particular material for a given application will require a careful study of the product requirements, material properties and other commercial, environmental and legislative issues. Table 3 Comparative advantages/disadvantages of other thermoplastics to PP Polymer Advantages Disadvantages LDPE Higher impact resistance Lower strength and stiffness Lower brittle temperature Lower surface hardness Lower heat distortion temperature HDPE Lower cost Lower strength and stiffness Higher impact resistance Lower surface hardness Lower brittle temperature Lower heat distortion temperature HIPS Lower shrinkage and warpage Lower chemical resistance Better gloss Higher cost Better rigidity Environmental stress cracking PVC Better clarity Worse environmental acceptance Better processing window Lower solvent stress crack Better weather resistance resistance Lower heat deflection temperature PET Higher clarity Worse water barrier properties Better oxygen barrier Unsuitable for hot fill and Better impact properties sterilisation Higher price ABS Better stiffness Higher cost and weight Better gloss Lower solvent resistance Better processibility Lower heat resistance PA 6, 66 Higher toughness Higher water absorption Better feel Higher cost and density Better hydrocarbon resistance PC Better transparency Higher cost and density Higher toughness and modulus Notch sensitive Higher continuous use Lower fatigue resistance temperature CA Better transparency Lower solvent resistance Better impact strength at lower Greater moisture absorption temperatures Higher cost Higher modulus CAB Better transparency Lower solvent resistance Better gloss Greater moisture absorption Higher cost

A typical material selection involves many properties which are not easily quantifiable in numerical terms (such as weathering, warpage, surface finish, ease of machining, etc.) or which may have very obscure units (such as transparency, fatigue, wear, 4

Practical Guide to Polypropylene

bonding, detergent resistance, etc.). These properties for PP are compared with other competitive materials on a judgemental value basis on a scale of zero to nine in Table 4. Table 4 Comparative ranking of different plastics on a scale of 0 to 9 where 0 represents unfavourable property while 9 represents favourable property [2] PP homo- PP coProperty LDPE HDPE ABS PVC HIPS polymer polymer Bonding 5 5 5 5 8 9 9 Brittle temperature 1 2 7 7 5 7 3 Detergent resistance 8 7 4 4 5 9 7 Dimensional stability 4 4 5 5 9 5 7 Fatigue index 9 9 7 8 2 6 3 Flammability 1 1 1 1 2 8 2 Friction 5 5 1 6 1 2 1 Gamma radiation 2 2 5 4 6 7 8 Hydrolytic stability 8 8 9 9 8 7 8 Shrinkage 3 2 1 1 5 7 6 Surface finish 8 8 7 8 8 4 8 Toughness at room 4 6 9 6 7 5 6 temperature Toughness at –40 °C 3 4 8 7 7 3 6 Transparency 5 5 5 5 0 7 5 Weathering 3 3 2 3 3 7 3 Warpage 5 4 5 5 8 8 8 Water absorption 9 8 9 9 4 7 6 Wear 5 5 4 5 2 3 1 Extrusion 8 8 9 9 8 7 9 Injection moulding 8 8 9 8 8 3 8 Machining 8 8 5 8 9 5 4 Vacuum forming 3 2 3 4 8 9 9

1.5 Applications The main applications of PP in different market sectors are given in Table 5. Some of the critical requirements for these applications are explained in Table 6. Sector Household goods Automotive industry Fibres

Table 5 Typical applications of PP Typical applications Buckets, bowls, bottle crates, toys, bottle caps, bottles, food processor housing, video cassettes, luggage Radiator expansion tanks, brake fluid reservoirs fittings, steering wheel covers, wheel arch liner, bumpers, bumper covers, side strips, spoilers, mudguards, battery cases, tool boxes Artificial sport surfaces, monofilaments for rope and cordage, stretched tapes, woven carpet backing, packaging sacks and tarpaulins, staple fibres, coarse fibres, filament yarns, fine fibres

5

Practical Guide to Polypropylene Table 5 (cont.) Typical applications of PP Dishwasher parts such as top frame, basement, tubs, extruded gaskets, water duct, water softener compartment, etc. Domestic appliances

Washing machine parts such as detergent dispenser, door frames, inlet and outlet pipes, bellows, feet and wheel, housings and ducts, etc. Refrigerator parts such as boxes, containers, drawers, ducts, inlet and outlet pipes etc.

Packaging

Pipes and fittings Furniture

Microwave oven cabinet, irons and coffee maker body parts Margarine and ice-cream tubs, films, compartmentalised meal trays, thin-walled packaging for, e.g., disposable food trays, dessert cups and confectionery boxes, strapping tapes, blister packaging Solid rods, punching plates, hot wire reservoirs, tower packings for distillation columns, domestic wastewater pipes, pressure pipes, heat exchangers, corrugated pipes, small diameter tubing, e.g., biro cartridges, drinking straws Stackable chairs

Table 6 Critical requirements for applications where PP is one of the best choice of material Application Critical requirements Good rigidity, good toughness, colourability, mouldability in Chairs complex shapes High impact strength at low temperatures, excellent weathering, Car bumper high rigidity Hair dryers, irons and Rigidity, brilliant surface gloss, good heat ageing resistance, kitchen appliances antistatic properties, high HDT, mar resistance Disposable food Rigidity, transparency (if required), heat sterilisable, no taste, packaging good flow and fast cycling, low cost Syringes, tubes, Transparency, sterilisable and unbreakability (toughness), good cartridges flow length Video cassette boxes Fatigue strength, high flexibility, warpage Low frictional loss, good chemical resistance, high continuous Pipes and fittings use temperature, low noise Luggage Impact strength, warpage

1.6 Market Share and Consumption Trend Over the last four decades, PP has established itself as one of the major commodity plastics. PP is now the third largest consumed plastic material after PE and polyvinyl chloride. The consumption of PP in comparison to other plastics is shown in Figure 3 [3]. Demand for PP has grown consistently, managing an impressive growth even during recessions. Western European PP consumption in 1995 was estimated at about 5 million tonnes against a production capacity of about 6 million tonnes. Approximately 55% of PP is used in extrusion and the rest in injection moulding [3]. Sixty percent of 6

Practical Guide to Polypropylene

the PP consumed is homopolymer, 20% block copolymer, with the rest either compounded or random copolymer grades. It is estimated that the growth of PP in the coming decade will be around 6%, the strongest growth pattern for the bulk polymers.

Figure 3 Consumption of PP in comparison to other major plastics in the UK [3]

1.7 Major Suppliers The major manufacturers of PP and their trade names are given in Table 7. Manufacturer Atofina Basell Borealis BP Dow DSM Exxon Repsol Solvay

Table 7 Major PP manufacturers Trade name Appryl Novolen Borstar PP Acclear, Accpro, Acctuf Polypropylene Homopolymer, Impact Copolymer Stamylan P, Vestolen P Exxon Mobil PP Fortilene

7

Practical Guide to Polypropylene

1.8 Material Price The price of PP is compared with that of other competitive thermoplastics in Table 8. It can be seen from the table that commonly used engineering plastics, e.g., acetals, PC, PET and PA are more costly than PP. The different PEs are similar in price to PP, whilst styrenics and PVC are generally more costly. Since the prices of different materials depend on the grade, the quantity purchased, the supplier, etc., these prices should be taken for guidance only. The prices of different grades of PP are compared in Table 9. Table 8 Comparison of indicative prices of different raw materials Polymer Price (US cents/lb) PP homopolymer 30 PP copolymer 35 LDPE 50 HDPE 35 ABS 70 PS 45 HIPS 50 PVC 30 PA 6 125 Acetal 100 PC 140 PET 100 Source of data: Plastics News, March 11, 2002, 21

Table 9 Indicative prices of different types of PP PP type Price (US cents/lb) Homopolymer, injection 34 Extrusion grades Fibre 33 Film 35 Profile 39 Sheet 36 Random copolymer Blow moulding 39 Film 38 Injection 37 Source of data: Plastics News, www.plasticsnews.com, site visited April 2002

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Practical Guide to Polypropylene

2 Basic Types of PP PP, a semi-crystalline thermoplastic, is made in its homopolymer form by polymerising propylene monomer using stereospecific Ziegler-Natta catalysts. The catalyst system is termed stereospecific because it controls the position of the side (methyl) group in each propylene unit in the polymeric chain. A typical catalyst system may be prepared by combining titanium trichloride with tributyl aluminium or its variants. Most commercial PP is isotactic. The physical properties and processing characteristics of PP are mainly determined by the molecular weight (average number of propylene units in a chain), the molecular weight distribution (variation in average length of chains) and the type and amount of copolymerising monomer. The selection of the right grade of PP for a specific application involves •

choosing between homopolymer and copolymer,

•

choosing a reactor or controlled rheology grade,

•

defining the melt flow rate required and the appropriate additive system.

However, with changes in manufacturing technology, operating conditions and catalyst systems, the traditional differences between the properties of homopolymers and copolymers have blurred [4-6]. Hence, an open mind is necessary to select a proper grade for a particular application. 2.1 Homopolymer Homopolymer PP is made by polymerising propylene in the presence of a stereospecific catalyst. Homopolymers are more rigid and have better resistance to high temperatures than copolymers but their impact strength at temperatures below zero is limited (Section 4.3.2). Typical applications for homopolymer polypropylene include windshield washer tanks, shrouds for fans and steering columns, housings for domestic appliances such as hair dryers, sterilisers, irons, coffee makers, toasters, etc., extrusion of fibres and filaments for carpet backing, upholstery fabrics, clothing, geotextiles, disposable diapers, medical fabric and automotive interior fabrics. 2.2 Copolymer The properties of PP depend on the type and amount of comonomer. There are two basic types: random copolymer and heterophasic or block copolymer. The random polymers contain 1.5% to 6% by weight of ethylene or higher alkenes (such as butene1) in random distribution and in a single chemical phase. The essential difference between a random and a block copolymer is that the block copolymer contains 9

Practical Guide to Polypropylene

comonomer in the form of a dispersed rubber phase [7]. The structure of random and block copolymerised PP is shown schematically in Figure 4. —P—P—P—P—E—P—P—P—E—P—P—P—P—P—E—P—P—P—P—P—P—E—P—P— random copolymer —P—P—E—E—P—P—P—P—E—E—E—P—P—P—P—P—E—E—E—E—E—P—P—P— block copolymer

Figure 4 Structure of random and block copolymerised PP molecules. P and E represent propylene and ethylene monomer units, respectively

Copolymerised PP gives a softer feel to film and fibre products compared to homopolymers. However, PP copolymers are more expensive than the homopolymers (see Table 8). Typical applications of copolymerised PP are battery cases, bumper filler supports, interior trim, glove boxes, package trays and window mouldings, video cassette boxes, office chairs, disposable containers, boxes and appliance housings. 2.2.1 Random Copolymer The random copolymer of PP contains chains with a small number (~1.5–6%) of ethylene or higher olefin units (such as butene or hexane), dispersed randomly among the propylene units. The presence of ethylene in the polymer chain reduces the tendency to crystallise and results in improved impact strength, a softer feel, a wider range of heat sealability, resistance to creasing and improved clarity. Some of the inherent rigidity of the homopolymer is sacrificed by copolymerisation. Due to the lower crystallinity, random copolymers have a lower melting point and specific gravity than the homopolymer. This combination makes copolymers attractive for injectionmoulded houseware, thermoforming, stretch blow mouldings and films [4]. Random copolymer grade can be used to replace PVC, PS and PET in food packaging and stationary applications. 2.2.2 Block Copolymer PP homopolymer is copolymerised with ethylene. In block copolymers, the ethylene content is much higher than the random copolymers. The copolymerised part of the material is rubbery and forms a separate dispersed phase within the PP matrix. As a result, block copolymerised PP is much tougher than homopolymerised PP and can withstand higher impact even at low temperatures but at the expense of transparency and softening point. The main applications of the block copolymerised PP are similar to those of elastomer-modified PP but where the impact property requirement is not that critical.

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Practical Guide to Polypropylene

2.3 Elastomer-Modified Polypropylene Extremely high toughness at low temperatures can be achieved by modifying PP with elastomers, mainly ethylene propylene rubber (EPR), ethylene propylene diene rubber (EPDM) or plastomers. Plastomers are very low density (<0.88 g/cm3) copolymers of ethylene and an olefin produced using metallocene technology. Plastomers can have narrow molecular weight distribution and more long chain branching than EPR and EPDM. Modification of PP with elastomers causes loss of hardness and stiffness. If elastomermodified PP is considered for food-related applications, relevant national and international regulations should be checked for compliance. Shrinkage of elastomermodified grades is lower than the copolymer grades due to reduced crystallinity and increased free volume. Further, elastomer-modified grades allow good paint adhesion since the rubber phase provides sites for etching or surface treatment. For outdoor applications, black-coloured or light-stabilised materials are required. Low melt flow rate grades are used for extrusion and blow moulding where there are higher impact requirements than can be met with PP homopolymers. However, higher melt flow rate grades are more suitable for injection moulding, once again where better impact strength is justified in terms of application suitability. Elastomer blends are commonly used in the automotive industry for bumpers, bumper covers, protective side strips, spoilers, steering wheel covers, mudguards for tractors and lorries, and other parts which are likely to encounter high impact stresses. 2.4 Controlled Rheology The polymerisation techniques for PP lead to a wide range of molecular weight. The molecular weight distribution can be controlled by splitting the PP chains using hydrogen peroxide into smaller units in the post-reaction stage. This reduces molecular weight and narrows its distribution and, consequently, increases melt flow rate. Most of such controlled rheology (CR) grades have melt flow rate (MFR) values higher than 20 g/10 min at 230 °C at 2.16 kg load. It can be as high as 120–150 g/10 min or more. Moulding cycles for the CR grades can be up to 15% faster, and warpage and shrinkage is reduced because of reduced orientation of polymer chains in the flow direction and the reduction in injection pressure due to easy flow of the material. Reactor grades of PP have a broad molecular weight distribution (Mw to Mn ratio of 5–12), but CR grades offer a substantially lower ratio (~3–5). However, the breakdown of polymeric chains might lead to formation of low molecular weight polymers or oligomers that can cause odour problems (organoleptic problems) in PP (Section 4.8.1). The other problem with the CR grades is the reduction in impact strength due to the reduction in molecular weight. The impact properties of the CR grades should be carefully monitored, particularly at low temperatures. CR grades are available both as homopolymers and copolymers. Copolymer-based CR grades for injection moulding flow well and are highly resistant to warpage and internal stresses. These grades find application in thin-walled packaging for food and 11

Practical Guide to Polypropylene

pharmaceuticals, video cassettes, automotive parts, machine housing parts, suitcases, crates and freezer containers and other warpage prone parts. However, the arrival of metallocene-catalysed PP (which offers advantages such as better organoleptic properties and narrow molecular distribution) is set to challenge the use of CR grades in traditional applications. 2.5 Metallocene Polymers Metallocenes are a new generation of olefin polymerisation catalyst. They have attracted widespread attention because of their high activity and versatile performance with different monomers. The principal obstacles to their use in PP production have been that their melting point and molecular weight are too low. These problems are now solved with newly designed stereo-specific zircocenes making isotactic and syndiotactic PP of high molecular weight and varying stereoactivity. Metallocene catalysts provide enhanced control over the molecular make up of PP [8]. Reactor grades with extremely high isotacticity (~1% atacticity in comparison to a minimum 3–4% atacticity of conventionally polymerised PP) and narrow molecular weight distribution are possible. The narrow molecular weight distribution results in lower shear sensitivity of the PP resin and provides low melt elasticity and elongation viscosity in extrusion (Section 6.1.2). Metallocene-polymerised copolymers offer the same mechanical properties as the conventional Ziegler-Natta catalysed polymers, similar deflection temperature under load but with lower melting point (147–158 °C). Metallocene-catalysed PP significantly improves the property window of conventionally polymerised PP. Significant improvements in modulus and hot tack strength are observed while water vapour transmission rate, haze and heat seal initiation temperature is reduced [9]. High melt flow properties without the use of organic peroxides (as required by CR grades) means that the metallocene polymers offer superior organoleptic properties. The properties of metallocene-polymerised PP are compared with Ziegler-Natta homopolymer and copolymer PP in Table 10 [10]. It can be seen that the mechanical properties (tensile modulus and tensile yield strength) of metallocene-catalysed PP are similar to that of homopolymer PP while the optical properties (gloss and haze) are similar to random polymer. This unique combination of mechanical and optical properties is associated with ease of flow resulting from a higher MFR value and narrower molecular distribution. Because of the lower molecular weight distribution, the metallocene-based PP offers low warpage and is particularly suitable for thin-walled packaging products for dairy products such as yoghurts and cheese. Other targeted markets for metallocene polymers are medical products such as petri dishes and syringe bodies. Europe’s first commercial metallocene-catalysed PP was launched by Targor, the joint venture between BASF and Hoechst (now Basell), with the trade name of Metocene. Exxon Mobil (Achieve) and BP have also produced grades of metallocene-catalysed PP.

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Practical Guide to Polypropylene Table 10 Comparison of the properties of metallocene-polymerised PP with PP homopolymer and copolymer manufactured using Ziegler-Natta catalyst [10] Ziegler-Natta Ziegler-Natta Metallocene homopolymer random Property PP PP copolymer PP MFR (230 °C/2.16 kg) 60 48 48 Tensile yield strength (MPa) 35 35 29 Tensile modulus (GPa) 1.7 1.55 1.15 Charpy impact strength at 23 90 103 180 2 °C (kJ/m ) Haze (%) 7 60 7 Specular gloss at 20° (%) 77 57 65

2.6 Syndiotactic and Atactic PP Syndiotactic PP is available from, e.g., Fina Oils and Chemicals, and Mitsui Toatsu Chemicals, polymerised using metallocene catalysts. It is claimed that the syndiotactic structure provides better impact strength, greater flexibility, lower haze, lower heat deflection temperature and lower residual monomer content. However, the full properties of these polymers are still to be evaluated and it remains to be seen whether syndiotactic PP can offer properties which are unique enough to market it as superior to isotactic PP and which can provide justification for the higher cost of material [8, 11, 12]. Atactic PP is an amorphous material and has little strength. The main application of atactic PP is in coatings in conjunction with bitumen or asphalt. 2.7 Filled Grades of PP While most of the PP produced is used without mineral filler, the use of such materials is more common in PP than with PE. PE has very low modulus and stiffness. Consequently, the improvement in mechanical properties achieved by addition of fillers is not significant. By choosing the appropriate filler, PP type and compounding technology, it is possible to design products with properties approaching those of some engineering polymers. For these reasons, fillers are used not only to reduce the polymer content and cost but also to enhance its performance. As a result, a significant number of filled and reinforced PP grades has been developed and are successfully used in different applications. The improved stiffness and heat deformation resistance has led to the use of such compounds for the manufacture of heater housings, car mounting components and several domestic appliances. The main fillers and reinforcements for PP are discussed in this section. Their impact on its mechanical properties is discussed in Section 4.3.6.

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It is reported that products made from PP have no effect on the biosphere after landfill disposal. However, mineral fillers may remain on the disposal site for a very long time or build up in incinerators. 2.7.1 Talc Filled PP Control of the average particle size, the particle size distribution, the purity and the aspect ratio of the filler is necessary to achieve consistent product quality in talc filled PP. In some grades of talc filled PP, water absorption may be an important factor. This will affect the surface appearance of the moulded product and the adhesion of the resin to the filler. Grades filled with 10% to 40% talc by weight have been produced. Both homopolymer and copolymer grades of PP are used. Talc filled grades offer higher stiffness, better surface aesthetics, lower coefficient of thermal expansion, lower shrinkage, and improved scratch and mar resistance than non-filled grades. Heat deflection temperature and mould shrinkage are also improved by the addition of talc. Flexural modulus increases dramatically with added talc at the expense of tensile strength. In some cases, impact modifiers are added to maintain the impact strength, but at the expense of stiffness. Filled copolymer grades offer higher yield elongation at the expense of tensile yield strength. The main applications for talc filled PP grades are in car heater casing, motor housing, dryer drums, textile bobbins, industrial and agriculture plant components. Talc filled PP sheet is used as an alternative to carton board. 2.7.2 Calcium Carbonate Filled PP Calcium carbonate is also commonly used as a filler for PP. In comparison to the talc filled grades, the calcium carbonate filled grades are claimed to have higher impact strength, brighter colour, higher thermal stability and improved fatigue strength, but lower stiffness and tensile strength. Calcium carbonate is added to PP at the same loading as talc, from 10–40% by weight. However, in a highly filled system, nonuniformity of mechanical properties can result from poor dispersion during the compounding process. The main applications of calcium carbonate filled PP are in instrument panels, motor vehicle grills, heater boxes and garden furniture. 2.7.3 Glass Fibre Reinforced PP Glass fibres are used to confer enhanced strength and rigidity. These fibres are usually coated with silanes, lubricants, film formers and, sometimes, antioxidants and antistatic agents. These coatings provide better fibre-matrix adhesion, consequently enhancing the mechanical properties of the product. These coatings are also intended to reduce 14

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breakage of the glass fibre during manufacturing and processing. Substantial improvements in tensile strength and modulus are only realised after a coupling reaction takes place between organofunctional silanes on the glass fibre and reactive groups introduced into the PP molecule. There are many commercial glass fibre grades that impart enhanced performance in PP. The higher aspect ratio of glass fibre imparts higher reinforcing efficiency than talc, calcium carbonate or mica. Glass fibre reinforced PP has been successfully used to replace engineering thermoplastics in various applications. It has replaced PC, ABS, polyesters and PA in hand-held tools, automotive grill opening reinforcing panels and pump housings. Glass fibre reinforced grades are used for car and truck fan shrouds, car rear light housing, radiation expansion tank, grills, headlamp housing, furniture frames and washing machine components. 2.7.4 Mica Reinforced PP Mica is a generic term encompassing a family of minerals, mainly hydrated potassium aluminosilicates. Due to its high aspect ratio (about 50–100) mica gives higher flexural modulus than talc or calcium carbonate at the same loading. More significant improvement in tensile strength is obtained upon the use of appropriate coupling agents. Due to overall mechanical property profile and high temperature resistance, mica reinforced PP is used in several automotive applications, e.g., crash pad retainers, battery and fan shrouds. As mica is dark in colour, it is not suitable for light-coloured articles. 2.8 Additives for PP Many other additives can be added to PP to provide or improve different functionality. Commonly used functional additives are given in Table 11 and further discussed in Section 4. However, it should be noted that the improvement in a certain property (or properties) on addition of additives is generally at the expense of some other useful properties. Hence, any change in material should be considered thoroughly to understand its full impact on the product quality, specification and suitability for the intended application. The presence of additives in PP can significantly increase the toxicity of the resin. These substances can migrate into food or water through plastic packaging or to the body through medical devices. The handling of the additives might require special handling instructions and they can produce toxic degradation products during processing. Detailed information about the toxicity and hazard of special additive or material may be obtained from the Material Safety Data Sheets from the manufacturer. Some ingredients known to cause health and safety problems used in PP are blowing agents, peroxides, fillers (such as glass fibre), pigments (particularly lead- and cadmium-based pigments) and flame retardants.

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As a rule of thumb, if any additive is added to the formulation of the PP, it should be tested for its likely impact on food and medical applications. Resin should conform to the regulations for health and safety. Table 11 Commonly used functional additives for PP Functionality To reduce accumulation of dust and associated possible fire Antistatic agents hazard To decrease the friction between the film and the machinery Slipping agents during processing Antiblocking agents To avoid films sticking together Metal deactivators To reduce degradation due to the presence of metals Blowing agents To reduce density Nucleating agents To improve transparency and clarity Antifogging agents To prevent condensation forming Biocides To control the growth of micro-organisms and bacteria Flame retardants To reduce flammability of the material or to suppress smoke To prevent thermal oxidative degradation during processing and Antioxidants service Lubricants To lower melt viscosity and prevent sticking to metal surface UV stabilisers To protect against harmful UV radiation Light stabiliser To provide stability against visible light Additive

2.9 Identification of PP Type Identification of a plastic component may be required for various reasons, e.g., the identification of the material of a competitive product or defective products returned from the field. The simplest technique to identify PP is by burning a small specimen. PP burns with a blue flame with a yellow tip and smells of burning candle when the flame is extinguished. PP floats on water and can be easily cut providing smooth surfaces. PP is soluble in hot toluene. Most of the above observations for identification of PP are similar to those of PE. Hence, further tests are invariably required for confirmation of polymer type. The results from flame testing are further complicated by the presence of comonomers, fillers and additives such as flame retardants, blowing agents, lubricants and stabilisers. Hence, chemical and thermal analysis is required for positive identification of the polymer. Infrared (IR) spectroscopy is the most widely used technique for the positive identification of PP. Typical IR spectra (transmittance (T) plotted against wavenumber) for different types of PP are shown in Figure 5. IR spectroscopy can provide limited information about the fillers as well. Differential scanning calorimeter (DSC) thermograms may be required to confirm the presence of ethylene comonomer in the case of copolymerised PP or to measure the degree of crystallinity in the PP artefacts (Section 3.3). Further information about the fillers can be obtained from thermogravimetric analysis (TGA) and X-ray fluoroscence spectroscopy (XRF). In TGA, the weight loss and

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derivative weight loss of the polymer are measured as a function of temperature while XRF provides the elemental analysis of the polymer compound.

Figure 5a Typical IR spectrum for homopolymer PP [13]

Figure 5b Typical IR spectrum for copolymer PP [13]

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3 Structure Similar to PE, PP is a linear hydrocarbon polymer containing little or no unsaturation. It is, therefore, not surprising that PP and PE have many similarities in their properties, particularly in their swelling and solution behaviour and in their electrical properties. In spite of many similarities, the presence of a methyl group attached to alternate carbon atoms in the chain backbone does alter the properties of the polymer in a number of ways. For example, it causes slight stiffening of the polymer chain and interferes with the molecular symmetry. The first effect leads to an increase in the crystalline melting point whereas the interference with molecular symmetry would tend to depress it. However, the increase in the melting point due to the presence of pendent group is much higher than the corresponding reduction due to decrease in molecular symmetry. The melting point of PP is approximately 50 °C higher than that of PE. The melting point of HDPE ranges from 120–130 °C. The crystalline melting point of PP ranges from 160–170 °C. Further, due to the presence of pendant methyl groups, PP generally has higher tensile, flexural and compressive strength and higher modulii than PE. The methyl side groups can also influence some aspects of chemical behaviour. For example, the tertiary carbon atom provides a site for oxidation so that PP is less stable than PE to the influence of oxygen. Thermal oxidation (Section 4.10.1) and high-energy radiation (Section 4.10.5) lead to chain scission rather than crosslinking. The detailed discussion of the structure-property relationship is a very complex issue and is not within the scope of this book. Further details can be found in many textbooks [e.g., 14]. However, many aspects of structure such as molecular weight, molecular weight distribution, crystallinity, etc., significantly influence the properties of PP and, hence, are briefly discussed here. 3.1 Molecular Weight The molecular weight of PP is normally estimated from the simple measurement of viscosity. Intrinsic viscosity and limiting viscosity numbers can be established by solution techniques. Melt flow rate is more commonly used to measure the viscosity and is defined as the weight of the polymer which can be extruded through a defined orifice in a given time at a defined temperature and pressure. Melt flow rate is inversely related to molecular weight. Easy flowing grades are generally less tough than those of higher molecular weight and stiffer flow. More sophisticated techniques such as gel permeation chromatography are used for measuring the molecular weight (Section 3.2). The influence of molecular weight on the bulk properties of PP is often opposite to that experienced with most other well-known polymers. Although an increase in molecular weight leads to an increase in melt viscosity and impact strength, in accord with most other polymers, it also leads to a lower yield strength, lower hardness, lower stiffness and softening point. This effect is generally believed to be due to the fact that a high

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molecular weight polymer does not crystallise as easily as lower molecular weight material and it is the differences in the degree of crystallinity which affects the bulk properties. It may also be mentioned that an increase in molecular weight leads to a reduction in brittle point. 3.2 Molecular Weight Distribution The distribution of molecular weight in a polymer is a measure of the degree of variation in length of molecular chains since not all the chains grow to the same length during polymerisation. Molecular weight distribution is expressed in a number of ways. Polydispersity is the ratio of weight-average molecular weight (Mw) and numberaverage molecular weight (Mn) and can be determined by fractionation techniques, such as gel permeation chromatography, or by interpreting rheological data. A typical gel permeation chromatography curve for PP homopolymer is shown in Figure 6. Published data on PP indicate that molecular weight is in the range Mn = 38,000–60,000 and MW = 220,000–700,000, with values of MW/Mn from about 5.6–11.9 [14]. The controlled rheology grades have significantly lower MW/Mn ratio (3–5). The molecular weight distribution influences the processibility of the resin (Section 6).

Figure 6 A typical gel permeation chromatography curve for PP showing molecular weight distribution

3.3 Crystallinity The molecular chains in PP are linear so they are able to pack together in an ordered crystal structure. Since chains may be entangled or otherwise imperfect (e.g., branching), the structure is not completely regular. Hence, PP is best described as a semi-crystalline polymer. The degree of crystallinity and crystal structure in a polymer depends on its thermal history. A rapid quenching gives a tough clear product since it suppresses the formation 20

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of crystals, while annealing or slow cooling of the product leads to a rather brittle and hazy product. Increased crystallinity increases hardness, modulus, strength, abrasion and wear resistance, creep resistance, barrier properties, shrinkage and density. Low crystallinity offers the advantages of good processibility, better transparency, economical melt processing and good thermoforming capability. Depending on the processing conditions, 60%–70% crystallinity in the finished product could be achieved. (a)

(b)

Figure 7 Typical differential scanning calorimeter thermograms for PP showing the effect of cooling rate on the formation of crystalline structure (a) Annealed specimen (b) Quenched specimen [15] 21

Practical Guide to Polypropylene

Crystallinity in the final moulded artefact could be measured using differential scanning calorimetry (Figure 7). In differential scanning calorimetry, the energy absorbed or produced is measured by monitoring the difference in energy input into the substance and into a reference material as a function of temperature. It can further provide information about melting, crystallisation and glass transition temperature. It can be seen from Figure 7 that the heat taken by the product to melt the crystals depends on the cooling rate of the sample. Quenching suppresses the formation of cystallites, reflected by the lower heat required for melting of crystals (19.87 cal/g compared to 21.87 cal/g). The morphological structure in an injection-moulded article can be quite complex, with graduated layers of different crystallinity. The details of the crystal structure depend on the shape of the article and the conditions under which it is moulded. Thicker sections in a moulding or extrusion may vary in crystallinity, with the rapidly cooled surface having a tough skin while the slower cooling interior has larger spherulites and is relatively brittle. Consequently, moulding shrinkage, internal stresses, dimensional stability and warpage depend on the crystalline structure (Section 5.1.3). PP is often referred to as warpolene because of the warpage problems associated with the processing of the material. The size of spherulites in PP may vary from 1 to 50 microns and can be seen using an optical microscope under a cross polariser. The use of nucleating agents can further modify the crystallinity and crystal structure of PP by providing numerous sites for growth of small spherulites during cooling from the melt. This results in less scattering of light. This technique is used in injection moulding to improve clarity and rigidity, and to reduce set-up time. Further details are given in Section 4.9.6. 3.4 Orientation PP may be oriented either in the melt phase or by stretching when it is solid. In both processes, the polymer chains are aligned in the perfect direction usually along the line of flow or stretch. Deliberately introduced orientation in fibres or oriented films can lead to dramatic changes in molecular and crystalline arrangements. As a result, major variation in the properties of the article can be expected. Orientation produced by stretching increases tensile strength and reduces elongation in the direction of stretch. Biaxial orientation of PP film improves clarity. Further effects of biaxial orientation on the mechanical properties of PP are explained in Section 4.3.7. 3.5 Isotacticity Isotacticity is the measure of the percentage of side methyl groups aligned on one particular side of the polymer chain. The isotacticity of commercially produced grades is measured in terms of isotactic index, the percentage of the polymer insoluble in nheptane. The isotacticity index for most commercially available grades of PP varies from 85% to 95%. It is understood that within the range of commercial polymers, the greater the amount of isotactic material, the greater the crystallinity, and hence the greater the softening point, stiffness, tensile strength, modulus and hardness [14]. 22

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4 Properties The mechanical and thermal properties of PP are dependent on the isotacticity, the molecular weight and its distribution, crystallinity, and the type and the amount of comonomer. Additionally, PP is, like other thermoplastics, a viscoelastic material. Consequently its mechanical properties are strongly dependent on time, temperature and stress. The properties of 7 commercial materials (all made by the same manufacturer and subjected to the same test methods) are compared in Table 12. These grades are of approximately the same isotactic content but differ in molecular weight (indicated by the change in melt flow rate) and in being either homopolymers, random, block copolymer or controlled rheology grades. 4.1 Density The typical density of PP is 0.9 g/cm3 and it is the lightest among the widely used thermoplastics (see Table 1). Therefore, it offers the advantage of being able to manufacture more items for a given weight of the polymer. Polymethylpentene (TPX), a commercially available semi-crystalline transparent thermoplastic, has a lower density (0.83 g/cm3) than PP. Unlike PE, where changes in the degree of crystallinity result in quite large variations in density, the density of PP changes little over the whole range of homopolymer and copolymers. The density of the random polymers is marginally lower than the homopolymer grades (Table 12). On the other hand, elastomer-modified, filled or reinforced grades might have significantly higher density depending on their formulation. For example, a 40% talc-filled grade has a density of 1.2 g/cm3. 4.2 Thermal Properties Unlike metals, plastics are extremely sensitive to changes in temperature. The mechanical, electrical or chemical properties of plastics cannot be considered without knowing the temperature at which the values are derived. The thermal properties of a polymer typically determine its low- and high-temperature applications, impact properties and processing characteristics. Typical low-temperature applications for PP are in refrigerator parts and food packaging for refrigerated shelves. The applications where high-temperature properties of PP are of particular interest include sterilisation, particularly steam sterilisation, microwave oven proof containers and parts for dishwashers and washing machine which are subjected to hot water in the presence of detergents. 4.2.1 Glass Transition Temperature and Melting Point The mechanical properties of PP at a particular temperature are dependent on the glass transition temperature. At very low temperature, the macromolecules are largely immobile. As the polymer is heated, the restricted macromolecular zones become progressively more mobile. At the transition temperatures, the material changes from a glassy hard state to a soft tough state because certain molecular segments become more 24

Practical Guide to Polypropylene

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Practical Guide to Polypropylene

mobile. A polymer above its glass transition temperature acts a tough ductile material while below it, the material is hard and glassy. On cooling, the glass transition temperature is sometimes known as the freeze temperature. Glass transition temperature is measured using a dynamic mechanical thermal analyser (DMTA) or differential scanning calorimeter (DSC). PP has the following transition temperatures: •

Second-order glass transition temperature at –10 °C (predicted). The actual value may be observed between 0 to 20°C depending on the frequency/heating rate.

•

Crystalline melting point between 160–170 °C depending on the grade and the frequency/heating rate.

•

Recrystallisation temperature on slow cooling of the melt between 115 °C and 135 °C.

A typical temperature curve for shear modulus and mechanical loss factor, measured using torsion pendulum, for different grades of PP is shown in Figure 8. It can be seen from the figure that a copolymerised grades of PP has two peaks in the mechanical loss factor curve while PP homopolymer has only one peak. The first peak, above 0 °C, denotes the glass transition temperature, similar to that of homopolymer PP. The secondary transition peak at –45 °C is due to the presence of comonomer which provides some mobility to polymer chains above –45 °C, thereby, giving enhanced impact properties.

Figure 8 A typical DMTA trace of PP showing different transition temperatures

PP copolymers, due to lower crystallinity, and metallocene-catalysed PP have lower melting points in comparison to homopolymerised PP. Recrystallisation temperature is quite important for injection moulding. Since the recrystallisation temperature of PP is between 115–135 °C, most of the crystallisation occurs during the cooling of the artefact in the mould. Since the recommended mould 26

Practical Guide to Polypropylene

temperature is in the region of 20–60 °C, this allows the possibility to restrict warpage and improve dimensional stability during processing (Section 5.1.3). In addition, PP continues to crystallise after processing at a rate varying with moulding conditions and storage or treatment temperature. Brittle temperature is very closely related to the glass transition temperature and determines the minimum temperature at which a semi-crystalline polymer could be used without significant loss of its impact properties. 4.2.2 Maximum Continuous Use Temperature Maximum continuous use temperatures are based upon the Underwriters’ Laboratories (UL) rating for long-term (100,000 hours) continuous use, and specifically on the elevated temperature which causes the ambient temperature tensile strength of the material to fall to half its unexposed initial value following exposure to that elevated temperature for 100,000 hours. The tests provide a continuous use temperature for a plastic in the absence of stresses. The maximum use temperature of PP is compared with other thermoplastics in Table 13. It can be seen that other commodity plastics and some other engineering plastics have a significantly lower maximum continuous use temperature than PP. However, polycarbonate has a higher maximum continuous use temperature in comparison to PP. Table 13 Maximum continuous use temperature of different plastics [1] Polymer °C PP 100 HDPE 55 LDPE 50 PVC 50 ABS 70 HIPS 50 PA 6 80 PA 66 80 PC 115

Occasionally it is required that the service life of the component is predicted at a temperature above its maximum continuous use temperature or vice-versa. As a rule of thumb, a 10 °C increase in temperature is equivalent to a decade increase in time. Since the maximum continuous use temperature of PP is 100,000 hours at 100 °C, this would be equivalent to 10,000 hours at 110 °C or 1,000,000 hours at 90 °C. Hence, certain grades of PP may be theoretically suitable for a very short-term use at 140 °C. However, the maximum use temperature of a polymer depends on the specific grade and its heat stabilisation system and should be carefully noted from the relevant trade literature. However, the functionality of the polymer for high temperature application might be quite limited in the presence of stresses.

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4.2.3 Heat Deflection Temperatures and Softening Points Heat deflection temperature is defined as the temperature at which a standard test bar deflects by a standard amount under a standard load. Generally loads of 0.45 and 1.80 MPa are used. The values of heat deflection temperature of various plastics are compared at different loads in Table 14. It can be seen from the table that the heat deflection temperature of PP is higher than the PE but, it is outranked by more expensive engineering thermoplastics. The Vicat softening temperature is the temperature at which a flat-ended needle of 1 mm2 circular cross-section area will penetrate a thermoplastic specimen to a depth of 1 mm under a specified load using a selected uniform rate of temperature rise. The Vicat softening point of PP lies between 90–95 °C and is considerably higher than the PEs. Above the Vicat softening point, the material becomes progressively softer and the crystalline melting point of PP homopolymer is about 165 °C, depending on the grade. The practical application of the Vicat softening point data is limited to quality control and material characterisation. However, it is taken as a rough estimate of the maximum temperature for ejection of the artefact from the injection moulding machine. Table 14 Thermal behaviour of other thermoplastics in comparison to PP [1] Heat deflection Heat deflection Polymer temperature at 0.45 MPa temperature at 1.8 MPa (°C) (°C) PP 88–95 50–60 HDPE 75 46 LDPE 50 35 PVC 70 67 ABS 98 85 HIPS 85 75 PA 6 200 80 PA 66 200 100 PC 143 137

Heat deflection temperature is a single point measurement and does not indicate longterm heat resistance of plastic material. However, it may be used to distinguish between those materials that are able to sustain light loads at high temperatures. The heat deflection temperature of a specimen is affected by the presence of residual stresses. Warpage of the specimen due to stress relaxation may lead to erroneous results. In addition, injection-moulded specimens tend to give a lower heat deflection temperature than compression-moulded specimens. This is because compression-moulded specimens are relatively stress free. The data obtained by these tests cannot be used to predict the behaviour of plastic materials at elevated temperature nor can it be used in designing a part or selecting and specifying material. If an article is subjected to high temperature in the absence of stresses, maximum continuous use temperature (Section 4.2.2) can provide a suitable 28

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criterion for the selection of material. In addition, if load bearing properties are required from the component at high temperatures, the modulus of the plastic as a function of temperature (Section 4.3.1.2) could provide data for the design calculations. The flexural modulus of different plastics as a function of temperature have been plotted in Figure 9. The remarkable difference between different polymers can be explained on the basis of amorphous and semi-crystalline polymers. It can be seen from the figure that the amorphous polymers, such as PVC and ABS, maintain their strength quite well up to their maximum use temperature. However, their strength falls sharply when they reach their glass transition temperature. On the other hand, semi-crystalline polymers, such as PE and PP, slowly lose their strength above the glass transition temperature. However, the residual strength of a semi-crystalline material may be higher than the amorphous material at a higher temperature, and an amorphous polymer may be stronger at the lower temperature.

Figure 9 Flexural modulus of different plastics as a function of temperature [2]

4.2.4 Brittle Temperature At low temperatures, all plastics tend to become rigid and brittle. This happens mainly because the mobility of polymer chains is greatly reduced. Brittle temperature is closely related to the second-order glass transition temperature (Section 4.2.1). Brittle temperature is defined as the temperature at which 50% of the specimens tested exhibit brittle failure under specified impact conditions. The brittle temperature of different grades of PP are given in Table 15. Table 15 Brittle temperature of different types of PP PP grade Brittle Temperature Homopolymer 5 to 15 °C Random copolymer –10 to 15 °C Block copolymer –40 to 10 °C 29

Practical Guide to Polypropylene

Due to the comparatively higher brittle temperature of PP, its use in low-temperature environments should be carefully considered and compared with other available thermoplastics. Low-temperature brittleness of some common thermoplastics is compared in Table 4. It can be seen that LDPE, HDPE, ABS and PVC offer lower brittle temperature. Impact strength at lower temperatures, e.g., –40 °C, should also be considered as a useful criterion for material selection for use under arctic conditions. 4.2.5 Specific Heat Many features of the processing behaviour of PP may be predicted by consideration of thermal properties. The specific heat of PP is lower than that of PE but higher than that of PS. Therefore, the plasticising capacity of an injection moulding machine using PP is lower than when PS is used but generally higher than with HDPE. The plasticising capacity is defined as the amount of the material which can be melted and plasticised in the barrel in a given time in a given injection moulding machine. Specific heat is a function of temperature below melting temperature. However, a significant rise in specific heat is observed near the melting point due to the partial crystalline nature of the polymer. However, the specific heat of the polymer melt is virtually independent of temperature. The specific heat, or more precisely the enthalpy, of the material controls the cooling of the artefact in the mould and predominantly the design of the cooling channels in the mould. The heat requirement for cooling of a PP artefact can be calculated from a graph such as that illustrated in Figure 10. To achieve faster cycles, mould cooling requirements should be considered from the beginning. The cooling system should balance the heat flow from the part to ensure uniform part cooling and minimise residual stresses, differential shrinkage and warpage. Other thermal properties of PP are given in Table 16. Table 16 Thermal properties of PP Property Specific heat (J/g °C) at 23 °C Specific heat (J/g °C) at 100 °C Thermal conductivity at 20 °C (W/m K) Linear coefficient of thermal expansion (/°C) 20–60 °C 60–100 °C 100–140 °C

30

Value 1.68 2.10 0.22 -5 10 x 10 -5 15 x 10 -5 21 x 10

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Figure 10 Specific heat of PP as a function of temperature

4.2.6 Thermal Conductivity The lower thermal conductivity of PP and other plastics compared to metals, gives protection against external temperature changes and so PP could be used for insulation applications. However, the use of PP, unless foamed, as a primary insulating material is rather limited (owing to cost factors). PP has been used for food packaging of refrigerated foodstuffs due to its suitability for food applications rather than its suitability as an insulating material. Lower thermal conductivity limits the production cycles and can result in cooling strains in thick sections, which may lead to warpage of the article. Similar to other plastic materials, the conductivity of the PP is a function of density and foamed PP has lower conductivity than the unfoamed PP. 4.2.7 Thermal Expansion The coefficient of thermal expansion is defined as the fractional change in length or volume of a material for a unit change in temperature. The coefficient of thermal expansion of plastics is considerably higher than metals, up to 6 to 10 times as high. This difference in the coefficient of thermal expansion can lead to internal stresses and stress concentrations. Consequently, premature failure may occur. Thermal expansion in PP gives significant volume changes on melting. It thus shrinks by 1–2% in moulding, this must be allowed for when designing the tool. Mould shrinkage and thermal expansion values for PP are compared with other thermoplastics in Table 2. The use of filler lowers the coefficient of thermal expansion considerably and brings the value closer to that of metals and ceramics (Section 4.3.6). The effect of thermal expansion on shrinkage, warpage and dimensional tolerances is discussed in Section 5.1.3. 31

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4.3 Mechanical Properties The mechanical properties of PP depend on several factors and are strongly influenced by the molecular weight. General observations suggest that an increase in molecular weight, keeping all other structural parameters similar, leads to a reduction in tensile strength, stiffness, hardness, brittle point but an increase in impact strength. This effect of molecular weight on the properties of PP is contrary to most other well-known plastics. The properties of some PP grades with different melt flow indices and structure are compared in Table 12. It can be observed that an increase in mechanical properties is not necessarily reflected in a trend predicted only on the basis of molecular weight, and other structural parameters, particularly crystallinity, play a very important role. Hence, the prediction of the mechanical properties on the basis of molecular weight or melt flow rate should be treated with caution. Appropriate data for the properties of the material should always be consulted. 4.3.1 Short-term Mechanical Properties A tensile test reveals that tensile force increases with increasing elongation, up to the yield point (Figure 11). After this, force initially decreases, i.e., the material can be further stretched with a smaller force. This is accompanied by a marked necking of the cross section of the test specimen. When this necking down has progressed along the entire length of the specimen, force increases again until elongation at break is reached. The second increase in deformation resistance is due to partial orientation of the macromolecules which strengthens the material. This typical behaviour of PP is similar to other ductile plastics. It can be seen from Table 12 that the mechanical properties of random and block copolymer grades are lower than the homopolymers for the same value of melt flow rate or molecular weight. The difference in their tensile stress/strain curves is highlighted in Figure 12.

Figure 11 A schematic tensile stress/strain curve for PP 32

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Figure 12 Tensile stress/strain curves for different types of PP

It can be seen from Table 1 that the flexural modulus and tensile strength of PP is lower than most of the plastic materials except LDPE and HDPE. However, PP offers an advantageously high flexural modulus to cost ratio which makes it an ideal candidate for replacement material to many engineering plastics on the cost reduction basis. The short-term stress/strain data of different grades of PP (and for other plastics) is of limited use and should only be used for pre-selection of material. In reality, plastic components are seldom designed and subjected to such high levels of strain as applied in short-term tensile tests. In addition, most of the cases of product failure are brittle in nature. Consequently, the long-term creep and fatigue properties of PP, discussed in Sections 4.3.3 to 4.3.5, are more important for structural applications. 4.3.1.1 The Effect of Test Speed Like other viscoelastic thermoplastics, the mechanical properties of PP depend on the speed of the test. For instance, raising the speed of the test decreases the observed flexibility and increases the observed brittleness. 4.3.1.2 The Effect of Temperature The stiffness of PP is a function of temperature. The variation of flexural modulus of different grades of PP as a function of temperature is shown in Figure 13. PP homopolymers are slightly stiffer than copolymers at room temperature. However, the difference between the two types is diminished as the temperature rises. The flexural modulus of elastomer-modified PP is significantly lower than the homopolymer or copolymer PP, and its service temperature is around 90 °C, much lower than that of homopolymer PP. PP becomes more ductile as the usage temperature increases, shown by an increase in elongation at break and decrease in ultimate tensile strength and yield stress.

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Figure 13 Flexural modulus of different grades of PP as a function of temperature [2]

4.3.1.3 Time-temperature Superposition PP is a viscoelastic material, and, consequently, its mechanical properties are strongly dependent on time, temperature, the level and type of applied stress and the testing speed. The apparent stiffness or elastic modulus of all plastics reduces with time under load due to the processes of stress relaxation and creep. Similarly, the modulus reduces with increasing temperature. In other words, the effect of time and temperature on the mechanical properties is interchangeable. The theory behind this behaviour of polymeric materials was given by Williams, Landel and Ferry. The detailed description of this theory can be found in standard textbooks [e.g., 14]. However, at this point, it will suffice to say that the effect of time during service could be simulated in the shortterm using high temperature. This superposition of time and temperature could be used in practice to predict the durability of the products. 4.3.2 Impact Strength The second-order transition temperature of PP homopolymer is –10 °C. This explains the drop in its impact strength at temperatures around 0 °C. The impact strengths of different grades of PP at different temperatures are given in Table 12. Several methods are used for measuring the impact strength of PP. However, none of the methods satisfactorily predict performance under conditions of end use. In the Izod or Charpy test, a notch is incorporated in the sample to concentrate stress; this normally leads to brittle failure. Impact strength is reduced as the notch gets sharper. Consequently, sharp corners in load-bearing sections must be avoided in the design of the article, as a general rule for all the plastics. The impact strength of an article depends on the inherent molecular structure of the grade used and the morphology arising from the processing conditions. Changes in the

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geometry of an item can have a major effect on its toughness rating. Impact strength increases with the molecular weight but more markedly with comonomer content. The most important way of improving the impact strength of PP is by incorporating a rubbery phase, as in heterophasic copolymers. Toughness increases rapidly with higher rubber content, and its transition from ductile to brittle failure occurs at lower temperatures. One of the major reasons for the failure of PP artefacts is the brittle failure. This is mainly caused by the incorrect selection the PP grade, particularly the use of PP homopolymer in place of copolymer or use of wrong material at the moulding floor. Infrared microscopy and gel permeation chromatography can quickly identify the source of the problem. 4.3.2.1 Falling Dart Impact Test The falling weight or dart drop test method simulates actual day-to-day abuse and can be carried out either on standard laboratory specimens or on the articles themselves. Failure may occur in various ways ranging from brittle to ductile failure (Figure 14). Particular care must be taken to avoid the brittle failure by proper selection of grade. At temperatures below –20 °C, elastomer-modified PP is more impact resistant than PP copolymer and homopolymer. Ductile

Bructile

Brittle

Figure 14 Example of ductile, bructile or brittle material failure Reproduced with permission from Shell, © Shell, website: www.basell.com

4.3.2.2 Notched Impact Strength PP copolymer has higher impact strength than homopolymers even at low temperature (Figure 15). Higher molar mass provides better impact and notched impact strength

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above 0 °C. Elastomer-modified PP shows high notched impact strength even at temperatures below 0 °C.

Figure 15 Typical notched impact strength of PP as a function of temperature

4.3.2.3 Tensile-impact Strength Impact strength tests permit no differentiation between specimens undergoing the test without failure. In this respect, the tensile-impact strength test is superior. Other test variables such as notch sensitivity, loss factor and specimen thickness are eliminated in the tensile-impact strength test. In addition, tensile-impact strength tests can be used for very thin specimens. The tensile-impact strength test consists of a specimen-in-head type of set up. In this case, the specimen is mounted in the pendulum and attains full kinetic energy at the point of impact. One end of the specimen is mounted in the pendulum and the other end be gripped by a crashing member, which travels with the pendulum until the instant of impact. The energy to break by impact in tension is determined by kinetic energy extracted from the pendulum in the process of breaking the specimen. The superior impact properties of elastomer-modified PP are, once again, observed. 4.3.3 Creep PP is a viscoelastic material and, like all other thermoplastics, it exhibits creep (or cold flow). Creep is the deformation or total strain which occurs after a stress has been applied. Its extent depends on the magnitude and nature of stress, the temperature and time for which the stress is applied. Over a period of time, PP undergoes deformation, even at room temperature and under relatively low stress. After removal of stress, a moulding more or less regains its original shape, depending on the time under stress and 36

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magnitude of stress. Recoverable deformation is known as elastic deformation and permanent deformation as plastic deformation. Typical creep curves plot deformation or creep against time on logarithmic scale for a range of loads or stresses. This basic creep data could be used to plot isochronous stress/strain curves, isometric stress curves or creep modulus as a function of time. In an isochronous graph, stress is plotted against strain at a constant series of time intervals (Figure 16). In isometric curves, stress or strain is plotted as a function of time for a series of constant strains or stresses. Creep modulus curves are the time-dependent value of modulus (Figure 17). As the properties of polymers are a function of temperature, these curves can be produced at different temperatures. This type of data is available from the raw material suppliers in most of the cases. However, sometime the creep data for the conditions which the component might observe in service are not available, hence the data is extrapolated to the required conditions. Care should be exercised in extrapolating the data to higher temperatures or longer durations outside the experimental creep data range. Copolymer type and melt flow rate also influence the creep behaviour. Copolymer grades of PP have substantially lower creep modulus than the homopolymer grades. PP has a similar modulus to high density PE, but its resistance to creep is much better and, at a equivalent time under similar load, the creep modulus of PP is more than that of high density PE. However, the creep resistance of amorphous plastics is much better than the semi-crystalline plastics such as PP and PE. Creep resistance of PP could be further improved by addition of fillers or reinforcements. The creep behaviour of moulded artefacts is affected by the residual stress or orientation effect in the moulded article.

Figure 16a Isochronous stress/strain curves of PP at 23 °C

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Figure 16b Isochronous stress/strain curves of PP at 80 °C

Figure 17 Tensile creep modulus of PP as a function of time under stress (T1 = 23 °C, T2 = 65 °C and T3 = 110 °C)

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PP shows different responses to different stresses or combination of stresses. For PP, it is reported that, up to strains of 0.8%, stress is proportional to strain measured during the creep tests. Up to this level of deformation, stress/strain behaviour under both compressive and flexural stress can be approximately calculated from the tensile creep tests. 4.3.4 Fatigue An alternative case to creep, where the deformation of the material is measured as a function of time at a constant stress, is fatigue (or stress relaxation). In this case, the material is subjected to constant strain, the relaxation in the stress in the component is measured as a function of time. This scenario occurs in press fits, springs, interference fits, screws and washers, etc., which during service undergo stress relaxation. A typical stress relaxation curve for PP is shown in Figure 18. A significant relaxation in the tensile modulus occurs over the 10 year period depicted in this graph (logarithmic scale), the value dropping from around 1,000 N/mm2 to around 350 N/mm2.

Figure 18 Tensile stress relaxation modulus for PP homopolymer at 23 °C

4.3.5 Dynamic Fatigue Materials subjected to cyclic loads or stresses fail at a point far below the ultimate strength measured in short-term mechanical tests. The cyclic loads may be caused by periodic or intermittent loading in on-off situations. It is well known that the amorphous plastics are more susceptible to fatigue than semi-crystalline plastics such as PP. But it should be noted that the semi-crystalline materials also suffer from dynamic fatigue and the stress level decreases significantly as the number of cycles increases, though semicrystalline materials do not undergo the ductile to brittle transition of amorphous materials. The stress levels for cycles to failure for different plastics are compared in Figure 19. This figure clearly demonstrates that the amorphous plastics (PC and ABS) 39

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are seriously prone to fatigue related problems. However, most semi-crystalline plastics show a similar slope in the fatigue strength curve. A notable exception is acetal resin which shows a transition. However, it should be noted that PP is not a very stiff plastic, hence the safe stress level for PP under fatigue conditions is very low.

Figure 19 Stress levels for cycles to failure for different plastics

Fatigue data is usually published in the form of Wohler curves where stress or strain amplitude is plotted against the number of cycles to failure on a logarithmic scale. Dynamic fatigue is a complex issue. However, the following common observations can be made: •

Fatigue strength decreases with increasing temperature.

•

Fatigue strength is sensitive to stress concentration such as notches or sharp corners.

•

Fatigue strength depends on the stress frequency. The effect of fatigue at low frequencies is much more severe. In other words, at low frequencies the failure could occur earlier than that predicted using high frequency tests.

4.3.6 Mechanical Properties of Filled Grades The properties of filled or reinforced grades of PP are heavily influenced by the type and amount of the filler. For example, the density of a heavily filled grade can be up to 50% higher than the unfilled material. In the next paragraphs, the effect of fillers or reinforcing agents on the properties of PP is explained. The typical properties of filled grades of PP are given in Table 17.

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It can be seen from the table that the mechanical properties of filled grades of PP are substantially modified by the presence of filler. Depending on the type and content of filler or reinforcement, PP may even show a brittle failure at low applied strains at low testing rates. The improvement/reduction in the tensile strength of the filled grades is marginal (with the exception of 30% glass fibre reinforced PP with coupling agent) due to stress concentration effects. However, the modulus is significantly improved on addition of fillers and reinforcements, particularly for glass fibre reinforced grades with a suitable coupling agent. The impact properties of the glass fibre reinforced grades are reduced. However, reinforced copolymer grades provide good low-temperature impact properties but at the expense of rigidity. The notched impact strength of the glass fibre reinforced grades is better due to blunting of the crack propagation mechanism. Reinforced grades of PP have a distinctly higher surface hardness than the nonreinforced grades; hardness varies according to the type of reinforcing material and its proportion by weight. The reinforced materials can be used for sliding elements but the wear of other materials in contact may be very high. The crystalline melting temperature and glass transition temperature of reinforced PP is not substantially different to those of the unreinforced grades. However, substantial changes in HDT values are observed. Reinforced PP grades have reduced specific heat values since the reinforcing materials have considerably lower values than the base polymer. The coefficient of linear expansion, to a large degree, is dependent on the orientation and distribution of the reinforcing material. In general, it is lower for a reinforced material than for an unreinforced material. Shrinkage of the filled or reinforced grades of PP is dependent on the aspect ratio of the filler. Differential shrinkage, the difference between longitudinal and transverse shrinkage, is relatively low for talc or calcium carbonate filled grades in comparison to glass fibre or mica reinforced grades due to the higher aspect ratio of glass fibre and mica reinforcements. The effect of shrinkage due to molecular orientation in the partially crystalline matrix is encountered when reinforcing material is incorporated. As a result, with an increase in the proportion of spherical filler (e.g., talc or calcium carbonate), increasingly isotropic shrinkage occurs. Consequently, injection-moulded articles made from reinforced PP containing such a filler have less tendency to distort than the parts moulded from non-reinforced PP and can be manufactured with closer tolerances. On the other hand, during injection moulding, glass fibres are generally oriented in the flow direction. The glass fibres oriented in the melt bring about a very large reduction in shrinkage in the direction of orientation while a smaller reduction takes place at right angles to the direction of orientation. This difference between the shrinkage in two directions can give rise to distortion problems. Filled and reinforced grades of PP have a substantially higher creep modulus than unfilled grades. This is due to the higher initial modulus of the filled or reinforced 42

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grades and the reduced amount of the polymer in the material which is able to creep. The glass fibre reinforced grades offer higher creep modulus, particularly when used with proper coupling agents. The general discussion about the fatigue and creep of unmodified PP discussed in Sections 4.3.2–4.3.5 is also valid here. Choice of compounding method is very important to limit the effect of fibre length distribution on the mechanical properties of glass fibre reinforced PP. Since glass fibres are damaged under high shear compounding conditions and during injection moulding, knowledge of screw design and moulding conditions is essential to control the fibre attrition and reproducibility of product performance. The weld lines in the glass fibre reinforced components are particularly weak in comparison to the rest of the moulded articles since the reinforcing fibres are oriented perpendicular to the direction of flow. Thus the weld strength in reinforced artefact is similar to the unreinforced material. This is one of the major causes of weakness in reinforced articles and proper care is required in designing the gate. 4.3.7 Biaxial Orientation The second increase in deformation resistance of PP is exploited by uniaxially stretching of monofilament and polymer tapes, and uniaxial or biaxial stretching of film. The various microstructural changes occurring during stretching of PP are shown in Figure 20. The difference between the original length (or width) of a monofilament, tape or film and its length after stretching is known as the stretch ratio. After stretching, the material has considerably higher tensile strength and lower elongation at break in the stretch direction. By using suitable stretching rates and temperatures below the crystalline melting temperature, optimum stretch ratios and hence very high degrees of orientation can be obtained. Depending on the stretch ratio, tensile strength values several times higher than that of the unstretched material can be attained. Elongation at break is greatly reduced. Typical figures illustrating these effects are given in Table 18. Table 18 Comparison of cast, monoaxially oriented and biaxially oriented PP films [16] Monoaxiall Biaxially Property Cast film y oriented oriented Tensile strength—machine direction (MPa) 39 55 180 Tensile strength—transverse direction (MPa) 22 280 152 Elongation at break—machine direction (%) 425 300 80 Elongation at break—transverse direction (%) 300 40 65 Gloss (ASTM D523) 75–80 >80 >80

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Figure 20 Schematic representation of structural changes during stretching of PP

4.4 Electrical Properties PP is an excellent electrical insulator, as can be expected from a non-polar hydrocarbon. The electrical properties of PP are very similar to those of PE and are compared in Table 19. Table 19 Electrical properties of PP and PE [1] Property PP LDPE 17 16 Volume resistivity ( cm) 10 10 Dielectric strength (MV/m) 28 27 Dielectric constant at 1 kHz 2.28 2.3 Dissipation factor at 1 kHz 0.0001 0.0003

HDPE 17 10 22 2.3 0.0005

The following terms are commonly used to describe the electrical properties of a material: •

Dielectric strength is a measure of dielectric breakdown resistance of a material under an applied voltage. The dielectric voltage just before breakdown is divided by the specimen thickness to give the value in kV/mm or MV/m. However, thickness should be specified since the results depend on the thickness.

•

Volume resistivity is the electrical resistance when an electrical potential is applied between the opposite faces of a unit cube of material. It is usually measured in cm. 44

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•

Surface resistivity is a measure of the ability to resist the surface current. It is the resistance when a direct voltage is applied between two surfaces mounted electrodes of unit width and unit spacing. The value is expressed in ohms.

•

Arc resistance is the measure of the time (in seconds) required to make an insulating surface conductive under a high voltage, low current arc. When an electric current is allowed to travel across the surface of an insulator the surface will become damaged over time and become more conductive.

•

Dielectric constant, or relative permittivity, is defined as the ratio of the electric flux density produced in a material to the value in free space produced by the same electric field. It is a ratio, and thus dimensionless.

•

Power and dissipation factors are the measures of the fraction of energy absorbed per cycle by the dielectric from the field. These terms arise by considering the delay between the changes in the field and the change in polarisation which in turn leads to a current in a dielectric material leading the voltage across it. The angle of the lead is known as the phase angle (θ). The power factor is defined as Cos θ and dissipation factor is Tan(90-θ). The loss factor is the product of dielectric constant and dissipation factor.

Typical electrical properties of a few grades of PP are given in Table 12 and are a function of temperature and frequency. In general, PP shows outstandingly high resistivity, low dielectric constant and negligible power factor, all substantially unaffected by temperature, frequency and humidity over the usual range of service conditions. The electrical properties of PP are independent of melt flow rate. However, certain additives and fillers can have an adverse effect on the electrical properties. The electrical properties of the PP are unaffected by prolonged immersion in water. Further, low values of dielectric constant can also be achieved using PP in the form of structural foam. The power factor is critically dependent on the amount of catalyst residues in the polymer. Typical electrical application of PP is in insulating power cable, particularly for telephone wires. The other functional requirements for this application are high impact strengths at low temperature and heat stabilisation for use in contact with copper. However, with the increasing use of optical fibres, the use of PP in this application is limited. The dissipation factor of PP is low and is hardly affected by temperature and frequency. The low dissipation factor rules out the use of high frequency heating and welding of PP. Hence, special techniques are required for welding of PP, discussed in Section 7.1.

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4.5 Optical Properties PP granules are naturally white and translucent. However, the final appearance of the material can be very different and ranges from hard, fairly rigid, brightly coloured, glossy, flexible or transparent film to high tenacity fibre. Mouldings made from the natural coloured homopolymer are semi-transparent, depending on the thickness and other processing and material parameters. The optical properties of a material are defined in terms of refractive index, clarity or transparency, haze and gloss. The refractive index of PP is 1.49. The remaining catalyst residue in the resin may affect the opacity of the PP resin and produce yellowness. Different catalyst systems may have different effects on the transparency and yellowness of the resin. Hence, the optical properties of equivalent grades of PP may be different. 4.5.1 Transparency Transparency may be defined as the state permitting perception of objects through or beyond the specimen. It is often assessed as that fraction of the normally incident light transmitted with a deviation of less than 0.1 degree from the primary beam direction. A material with good transparency will have high transmittance and low haze. Transmittance is the ratio of transmitted light to incident light and is complementary to reflectance. Uncoloured PP is translucent in thick sections. In thin sections or films, it can be transparent or opaque depending on the grade and the processing conditions. Homopolymers can be converted into transparent film with good optical properties. The light scattering due to formation of crystal structure should be minimised. Because of their phase structure, copolymers which have high impact strength do not in general yield transparent films. However, single phase random copolymers which suppress the formation of the crystal due to their irregular structure usually have better clarity than the homopolymers. Furthermore, it is important that the refractive index is constant throughout the sample in the line of direction between the object in view and the eye. The presence of interfaces between regions of different refractive index will cause scattering of the light rays. A high melt flow rate material should be used. Biorientation of PP film improves transparency since layering of the crystalline structures reduces the variations in refractive index across the thickness of the film, and this in turn reduces the amount of light scattering. Transparency of PP articles can be improved by using moulds or dies that provide a very good surface finish. Further improvements can be made by choosing the processing conditions which restrict the formation of spherulites, e.g., rapid cooling, low melt and mould temperature. However, low mould temperature will reduce the surface gloss of the moulding. Nucleating agents and clarifying agents which suppress the formation of spherulites can further improve the transparency of PP artefacts.

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Transparency is improved by contact with liquids to the point where the liquid level inside a container can be seen from outside (contact transparency). 4.5.2 Gloss Gloss is defined as the relative luminous reflectance factor of a specimen at the specular angle. This method has been developed to correlate the visual observations of surface shininess made at different angles. A highly polished black glass is assigned a specular gloss value of 100. Three basic angles of incidence are used for measuring gloss: 20°, 60° and 85°. The gloss increases as the angle of incidence increases. The gloss is a function of the reflectance and the surface finish of the material which, in turn, depends on the finish of the mould. Gloss is an extremely important factor where replacement of ABS with PP articles is considered on cost grounds. 4.5.3 Haze Sometimes a polymer may have a cloudy or milky appearance, generally known as haze. It is often measured quantitatively as the amount of light deviating by more than 2.5 degrees from the transmitted beam direction. Haze is often the result of surface imperfections. Recent developments in sheet manufacturing machinery with two cooling lines, which polish both sides of PP sheet, have resulted in low haze and high gloss sheets. 4.6 Surface Properties 4.6.1 Hardness and Scratch Resistance Although PP can be scratched with a metal point, its hardness is sufficient to resist the rule of thumb which often distinguishes between the polyolefins. LDPE is easily marred by a thumbnail, HDPE is scratched in this way with difficulty but PP is marked little, if at all. Hardness is defined as the resistance of a material to deformation, particularly permanent deformation, indentation or scratching. Hardness is purely a relative term and should not be confused with wear and abrasion resistance of plastics. For example, polystyrene has a high hardness but a poor abrasion resistance. Many tests have been devised to measure hardness. However, Rockwell and Durometer hardness tests are commonly used. The Rockwell hardness test measures the net increase in depth impression as the load on an indentor is increased from a fixed minor load to a major load and then returned to a minor load. The hardness numbers derived are just numbers without units. Rockwell hardness numbers in increasing order of hardness are R, L, M, E and K scales. The higher the number in each scale, the harder is the material. The Durometer hardness test is based on the penetration of a specified indentor forced into 47

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the material under specified conditions. Two types of durometers are commonly used: Type A for softer materials and Type D for relatively harder materials. The hardness values of different PP grades are compared in Table 12. Furthermore, the indentation hardness of PP decreases with the temperature (Figure 21). The hardness of PP depends on its crystallinity. With decreasing molar mass, crystallinity decreases and so does the hardness.

Figure 21 Ball indentation hardness of PP as a function of temperature (°C)

4.6.2 Abrasion Resistance Resistance to abrasion is defined as the ability of a material to withstand mechanical action that tends to progressively remove material from its surface. Abrasion resistance of polymeric materials is a complex subject. The resistance to abrasion is closely related to other factors such as hardness, resiliency and the type and amount of added fillers and additives. Resistance to abrasion depends on factors such as test conditions, type of abradant and development and dissipation of heat during the test cycle. This all makes abrasion a difficult mechanical property to define as well as to measure adequately. A material’s ability to resist abrasion is most often measured by its loss in weight when abraded with an abrader. The Taber abrader is widely used to measure abrasion resistance. In the Taber abrasion test, the test specimen is placed on a revolving turntable with a suitable abrading wheel under a set certain dead weight. The weight loss after a large number of revolutions (at least 5000 revolutions) is measured. For softer materials, less abrasive wheels with a smaller load on the wheels may be used.

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Wear may be high when reinforced parts are in contact with unreinforced parts. This situation could lead to excessive wear of unreinforced parts. Use of lubricated reinforced grades may reduce wear when in contact with the unreinforced component. China clay additives may be used to improve scratch and mar resistance of PP. Scratch resistance is quite important for car body panels. Recently, development of highcrystallinity copolymer grades has occurred in response to the automotive sector requirement for low-cost, low-weight materials for interior trim that do not show evidence of scratching [17]. These grades are around 15% lighter than ABS, with higher stress crack resistance and the same toughness. Highly crystalline PP copolymers have similar scratch resistance to talc filled PP but, because they do not contain white filler, they do not show the evidence of scratch. However, disadvantages of the high-crystallinity copolymers are 30% less stiffness in comparison to talc filled counterpart and higher gloss, which is not preferred by the automotive manufacturers, greater shrinkage and less acoustic damping. 4.6.3 Friction PP has a friction coefficient between 0.25 to 0.45, higher than the friction coefficient for typical bearing materials (from 0.10 to 0.25). However, the friction performance of PP can be improved by addition of silicone oil and/or polytetrafluoroethylene (PTFE) additives. PTFE-modified grades of PP with very low dynamic and static friction coefficients are available from resin compounders. The poor mechanical properties of PP, particularly low shear yield strength, also contribute to its near zero use in sliding applications. One reported sliding application of PP is in a conveyor belt guide bar for the beverage industry. Friction characteristics are very important for fibre to fibre interaction in spin technology. Internal lubricants can be used to reduce the friction which can be quite important in slide applications such as medical syringes. 4.7 Acoustic Properties PP offers excellent acoustic damping properties with considerable rigidity at normal service temperatures. Articles made from PS, ABS or HDPE have their own resonance and tend to rattle. On the other hand, components made from PP are more or less acoustically dead because acoustic vibrations are heavily damped in this plastic. The DMTA or torsional pendulum curves (Figure 8) provide primary data about the acoustic properties of the material. A good loss factor with sufficient rigidity is required to achieve damping of vibrations. It is primarily the component’s own vibrations which are heavily damped, e.g., eigen tones or natural vibration frequencies of the housing. However, PP does not have good air-borne sound absorption because of its rigidity. To achieve effective sound insulation, additional measures are required such as suitable cladding or spring mounting of the noise source.

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4.8 Biological Behaviour 4.8.1 Assessment Under Food and Water Legislation PP is generally accepted as a non-toxic and non-carcinogenic material. PP homopolymers and copolymers are used in many food contact applications ranging from simple beverage closures to retortable pouch applications. The main requirements for contact with food are that the article must not impart odour or taste to the food and should be suitable for the intended application. The main reason for assessment of PP for contact with food or potable water comes from the use of additives in material formulation. Additives, monomers, catalyst residues, polymer degradation products, etc., can migrate to any food in contact if the concentration of these substances is lower in the food than in the plastic. The migration of these species is a function of time and temperature. The rate of migration of chemicals or additives is inversely proportional to the molecular weight of the PP. The migration of these species could produce toxicity or the formation of undesirable flavours or odours, commonly known as organoleptic problems. The application of PP in contact with food and water is covered by the relevant standards/regulations by different authorities in different countries. Health assessment of plastics under food legislation varies from one country to another. In USA, clearance from the Food and Drug Authority (FDA) is required while relevant European directives form the basis of suitability of resin for food contact applications. Normally, the migration/extraction of resins and additives is measured for contact with different food simulants, e.g., distilled water, vegetable oil or acetic acid. Although similar principles apply, it is advisable to check in each case with the raw material manufacturers. Many grades are available which have the material composition meeting the requirements of the various regulatory authorities. Provided the approved grades are used and compliance with relevant regulations is checked, there should not be any problem in using PP for food and water contact applications. However, the finished part must also meet the requirements of the relevant regulations. The degradation of material during processing, use of mould release agents, etc., can make the final product non compliant. 4.8.2 Resistance to Microorganisms PP is not a nutrient medium for microorganisms and is therefore not attacked by them. It cannot be penetrated by microorganisms provided the wall or film thickness is at least 0.1 µm. In thinner walls, small pores may be introduced during manufacture. Low molecular weight additives, such as plasticisers, lubricants, stablisers and antioxidants, may migrate to the surface of plastic components and encourage the growth of microorganisms. The detrimental effects can be readily seen through the loss of properties, change in aesthetic quality, loss of optical transmission and increase in brittleness. Preservatives, also known as fungicides or biocides, are added to plastic materials to prevent the growth of microorganisms. 50

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4.8.3 Physiological Compatibility Toxicology tests in acute and chronic animal feed trials have shown no damage attributable to the PP that could be detected in the organs of animals, even after a two year trial. Skin and mucous membrane compatibility tests have shown no signs of irritation. New medical devices must undergo thorough biocompatibility testing by the manufacturer in order to meet regulatory requirements. Biocompatibility tests are designed to determine the risk of adverse health affects from the use/abuse of the component. Toxicity of a material device depends on the toxicity of the added ingredients and their migration into the body. Potential toxic substances include plastic additives (e.g., antioxidants, stabilisers, plasticisers), substances added during manufacturing (e.g., lubricants, mould release agents, cleaning agents) and products from polymer degradation during storage or implantation. ISO 10993—Biological Evaluation of Medical Devices outlines the methods for evaluating the safety of medical devices, directly or indirectly in contact with the body or with bodily fluids. The extent of the testing depends on the duration of contact, the nature of contact with the body (externally or internally) and the body part in contact (e.g., skin, mucous membrane, breached or compromised surfaces, blood, tissue or bone, etc.). Most of the grades of PP approved for food contact applications are normally suitable to produce articles for pharmaceuticals and medicines. However, relevant manufacturer’s standards should always be checked. Parts correctly manufactured from PP can meet the requirements specified by the relevant controlling bodies for pharmaceutical packing and medical articles. Once again, the suitability of the article for the intended application should be checked rather than the starting raw material. 4.9 Additives 4.9.1 Antistatic Agents Owing to its high electrical resistance, PP tends to accumulate electrostatic charge, in common with other polymers. This is a disadvantage because dust can form on articles in a rather unattractive pattern. Furthermore, the possibility of sparks presents a hazard in application where explosive fumes may be present. Antistatic agents are added, either directly or in masterbatch form, to PP to overcome this problem. During processing, traces of antistatic agent migrate to the surface of the moulding and form a moisture absorbent, weakly conductive layer. This reduces the surface resistance, thus allowing the faster dissipation of accumulated electrostatic charges. For useful antistatic properties, the surface resistivity need to be in the range of 109 to 1014 /m2. The optimum antistatic effect is generally obtained after a storage period of about two or three weeks. In many cases, the optimum effect is attained only after a few days, but under arid conditions it is reached very much later and is diminished. The antistatic 51

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effect persists for a considerable time and articles made from this plastic can be washed in water without any marked decrease in antistatic activity. Grades with low crystallinity (e.g., random copolymers) allow faster migration of antistatic agents to surface, thereby, achieving the optimum performance quickly. The incorporation of an antistatic agent has little or no effect on the mechanical, chemical or thermal properties of PP and no effect at all on the processing conditions. However, the presence of antistatic agents may affect the transparency and printability and may make PP non compliant to FDA regulations, although FDA approved antistatic agents are available. With the inherent hygroscopic nature of antistatic agents, it is advisable to pre-dry the granules before processing. Antistatic agents can be ionic or non ionic in nature. Glyceryl monostearate, a non ionic antistatic agent, is commonly used in injection moulding of PP up to a loading of 1% or more. The exact amount of the antistatic agent added depends on other property requirements such as transparency, FDA approval, printability and its compatibility to the polymer. Artefacts can be coated with suitable antistatic agents also. To a lesser degree, lubricants can also act as an antistatic agent and vice versa by reducing the friction. Typical applications where antistatic properties are required are household items, housings for electrical and electronic appliances, parts which undergo friction or sliding, etc. 4.9.2 Electromagnetic Interference/Radio Frequency Interference Shielding There might be few occasions when a reduction in the dielectric properties of PP might be desirable, e.g., antistatic properties. However, for certain applications, the reduction in surface resistivity achieved by addition of antistatic agents may not be sufficient. The two cases where greater reduction might be required are •

to achieve electrostatic dissipation (ESD) where a PP component is in contact with semi-conductor devices or is used in a hazardous environment, or

•

to shield the component against electromagnetic interference (EMI) or radio frequency interference (RFI). Many electronic and electrical devices such as computers, mobile phones, etc., emit signals which may interfere with communications. Regulations now require that the plastic enclosures used to house these devices should eliminate or attenuate these signals.

The high dielectric strength, low dielectric constant and dissipation factor of PP make it a bad choice of material for EMI/RFI shielding. In these cases, carbon black can be added to provide increased electrical conductivity. This might be sufficient in many cases. However, secondary shielding methods, such as metal deposition using metallising or electroplating, or adding metal fillers or nickel coated graphite fibres might be required to achieve sufficient protection from EMI/RFI.

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4.9.3 Slip and Antiblocking Agents During the use of PP films and laminates, the line speeds and reliability of the packaging machine are very important. To decrease the friction of the film on the processing and conversion equipment, a lubricating agent known as a slip agent is added. Slip agents migrate to the surface and reduce the coefficient of friction. Once on the surface, slip agents are able to lubricate the machine allowing for the faster line speeds. Primary amides, secondary amides and ethylenebisamides are commonly used as the slip agents. Reduction in the coefficient of friction and thermal stability of the slip agents are important issues. Slip agents should not interfere with corona treatment or printing and it should not have a deleterious effect on the clarity of clear films. Slip agents should meet FDA or equivalent approval for the recommended application. Another important property in the manufacture of films is blocking. Blocking is a term describing the polymer film sticking to itself as a result of storage in roll form. Antiblocking agents are added to overcome this problem. Typical antiblocking agents are silica, talc or diatomaceous earth. These materials are inorganic, are not miscible with the polymer and migrate to the surface. They create microscopic roughness on an otherwise smooth film surface. Addition of just the right amount and good dispersion are the critical factors. 4.9.4 Metal Deactivators and Acid Scavengers As with most polyolefins and polydiene, the presence of metals has a strong adverse effect on PP and most antioxidants are relatively ineffective. Copper and cuprous alloys produce the greatest effects, but other metals such as iron and nickel also accelerate degradation. Metal deactivators are widely used to deactivate metal residues present in the formulation due to catalyst residues, impurities in additives, direct contact with copper in wire and cable applications and metal inserts. Metal deactivators work by chelation of the metal. Metal deactivators are organic molecules containing heteroatoms or functional groups such as hydroxyl or carboxyl groups. Good results may be achieved by the use of 1% of a 50:50 blend of phenol alkane and dilauryl thiodiproprionate instead of the 0.1–0.2% of antioxidants commonly used in PP. Acid scavengers (or antacids) are used in PP to neutralise acidic catalyst residues. Calcium or zinc stearate are commonly used, and also function as internal lubricant. Inserts should, therefore, be made of light metal or be nickel or chromium plated. No adverse effect with brass screw fittings has been detected at temperatures below 100 °C. 4.9.5 Blowing Agents Through the addition of suitable chemical blowing agents to PP, fine-celled foams with apparent density down to 0.6 g/cm3 can be produced by conventional extrusion or injection moulding. Injection-moulded foamed articles produce a very fine-celled

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structure creating a smooth surface finish. The blowing agent concentrates are generally added in amounts of 1% to 3%. High melt flow rate grades of PP are preferred. Extruded, foamed PP film and tape can be stretched and thermoformed for trays for meat applications, cups for drinks, tape yarns for carpet backing. Within the specified loading limits for blowing agent, the approval of PP for food applications is not compromised. 4.9.6 Nucleating Agents The use of nucleating agents can further modify the crystallinity and crystal structure of PP by providing numerous sites for growth of small spherulites during cooling of the melt. This results in less scattering of light. This technique is used in injection moulding to improve clarity and rigidity [18]. As seen in polarised light photomicrographs, PP containing nucleating agents has crystallites of much finer and more uniform size relative to unclarified PP. As the crystallites in the clarifier containing grades are generally smaller than the wavelength of visible light, scattering is reduced and clarity is greatly improved. As nucleating agents are generally organic in nature, they melt during normal processing and thus become completely and evenly distributed in the resin. Furthermore, many organic compounds and metal salts can act as nucleating agents, including pigments and residual monomer. The use of dimethyl benzylidene sorbitol, dibenzylidene sorbitol and sodium di 4-t-butylphenol phosphate has been reported. These additives are also used in resins to provide reduced manufacturing cost through increased productivity and reduced set up cost. 4.9.7 Antifogging Agents Fogging occurs when water droplets formed from exposure of the moisture in foods to low storage temperatures condense on the inside surfaces of packaging films. Use of polyglycerol ester or a sorbitan ester of a fatty acid has been reported for antifogging applications in refrigerator packaging. Slip agents can also, to a certain degree, act as antifogging agents. 4.9.8 Biocides Biocides in PP are not as common as in plasticised PVC. In traditional PP applications, the need for a biocide to control the growth of microorganisms is virtually non-existent. Some trash cans and other waste disposal containers make use of these components to inhibit bacterial growth. The improvement is both aesthetic and sanitary. The product is able to resist the formation of unpleasant and unsightly organisms.

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4.9.9 Flame Retardants PP is basically flammable and ignites at a temperature of about 600 °C, although its burning rate is slow. PP ignites in contact with flame and burns with a faintly luminous flame. It continues to burn when the ignition source is removed and melts with burning drips. The spontaneous burning temperature of PP is 360 °C and the temperature at which ignition is induced from an external source is 345 °C. Combustion of unfilled PP produces no environmentally relevant pollutants. The burning produces very little soot, unlike PS or styrene-acrylonitrile copolymer (SAN), and no char, unlike oxygencontaining polymers such as polyphenylene ether (PPE) or PC. Polymers with superior fire resistance are thermosetting resins, fluorinated plastics and other plastics containing sulphur such as polyether sulphone and polyphenylene sulphide. The flammability of the polymers is commonly measured using Underwriters’ Laboratories, Inc., (UL) specifications. The UL 94 standard covers classification of a material’s tendency to ignite in the presence of a flame and to continue to burn after the ignition source is removed. There are three distinct flame tests in UL 94 which are most often applied to a PP product: horizontal burn (94HB), vertical burn (94VB) and 125 mm vertical burn (94-5VA, 94-5VB). Recognition under 94HB does not imply any selfextinguishing character of the resin. However, the 94VB specification measures the self-extinguishing character of burning and V-0 is the most stringent specification. Flame retardants are added to reduce the flammability of PP. PP is one of the most difficult plastics to make flame retardant. A high level of flame retardant is required to achieve necessary protection against fire required by the application, a level which will impair the mechanical performance of the material. The unmodified PP grades generally have a 94HB rating up to 0.8 mm thickness. Suitably flame retarded grades with V-0 rating at 3.2 mm thickness are available from compounders. Flame retardants can reduce the processibility and interfere with the function of certain hindered amines as light stabilisers. Flame retardant grades of PP are generally not suitable for use in food contact applications. The integral hinge properties of flame retarded PP are greatly reduced. Flame retardants include halogens (bromine and chlorine), aluminium trihydrate, magnesium hydroxide, phosphates and antimony oxide. Each of these flame retardants has its own strengths and weaknesses when flame retarding PP products as well as on its effect on the stability of the resin. Brominated compounds commonly used for flame retardation of PP are decabromodiphenyl oxide and octabromodiphenyl oxide. Antimony oxide is generally added to halogenated flame retardants for a synergistic effect. The halogenated flame retardants act as free radical scavengers. Halogenated flame retardants have many problems. They are known to interfere with hindered amine light stabilisers due to the production of halogenated acids which could react with hindered amines, to cause the corrosion of the processing machinery, to produce toxic decomposition products such as brominated dioxins and furans. Magnesium and aluminium hydroxides dissociate in the presence of heat to form water and metal oxides. Water dilutes the combustion gases and takes away the heat from burning 55

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plastics. Aluminium oxide or magnesium oxide form an insulating char layer on the burning plastic. Use of magnesium hydroxide requires careful processing conditions since it dissociates at the processing temperature of PP. The limiting oxygen index (LOI) test is used to measure the minimum concentration of oxygen necessary for candle-like burning for 3 minutes or more. A higher LOI indicates that more oxygen is needed to support combustion. The presence of flame retardants increases the LOI. Unmodified PP has a LOI of 17. The key fire properties of some plastics are compared with PP in Table 20. Suitable flame retarded grades of PP can have a LOI of 28. Polymer PE PP PS UPVC CPVC ABS PC PPO/PS PA

Table 20 Fire performance of different plastics Flammability Limiting oxygen index HB 17 HB 17 HB 18 V-0 45 V-0 50 HB 19 V2 25 HB 20 HB 22

4.10 Performance in Service 4.10.1 Thermal or Heat Stability Owing to the high susceptibility to oxidation due to the pendant methyl group, unstabilised PP can begin to decompose at high temperatures. Unstabilised PP oxidises in the presence of air and the rate of oxidation increases with increased temperature. Oxidation leads to embrittlement, surface cracking, discolouration and loss of mechanical properties and clarity. High temperatures are encountered during melt processing or service. This degradation process is accelerated by contact with certain metals. All commercial grades of PP are incorporated with stabilisers which give protection against oxidation during processing. Stabilisers also provide protection during normal service conditions. It is, therefore, essential to determine the likely enduse conditions before the choice of the grade and stabilisation system is made. The mechanism of thermal oxidation in PP is through the formation of free radicals which react with environmental oxygen to produce peroxides. It could also occur due to radiation, light or the presence of metal residues. Primary antioxidants inhibit the oxidation reaction by combining with free radicals. Hindered phenolics (e.g., butylated hydroxytoluene (BHT)) are commonly used as primary antioxidants. BHT is FDA approved for many applications but suffers from high-temperature volatility. Furthermore, phenolics suffer from oxidation in the presence of metal residues left from catalysts and produce a yellow colour. Different residues or different amounts of 56

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residue can produce different extents of yellowness. High molecular weight phenolics are used for high-temperature processing or high-temperature service conditions. Secondary antioxidants, also called peroxide decomposers, inhibit oxidation of PP by decomposing hydroperoxides. Phosphites and thioesters are commonly used as secondary antioxidants. Secondary antioxidants are usually combined with primary antioxidants to produce a synergistic effect on oxidation. With proper selection of two antioxidants, it is possible to achieve protection against oxidation which is greater than the sum of protection given by the two antioxidants when working separately. Stabilisation with antioxidants may render PP unsuitable for food contact application as they may directly migrate into the food products, hydrolysing and imparting odour or taste to the food. Careful selection of a suitable stabiliser system is necessary for food contact applications. In addition, heat stabilisers could adversely affect the working of light stabilisers. In applications involving no undue mechanical stresses, PP articles will withstand 100 °C for a long period of time, depending on the stabiliser systems. Consequently, the heat and thermal stability of PP is closely related to its maximum continuous use temperature (Section 4.2.2). Short-term exposure to 140 °C is also possible. It has been observed that a properly heat stabilised and properly processed material can undergo up to five processing cycles without noticeable reduction in molecular weight or the level of antioxidant content. 4.10.2 Stability to Light and Ultraviolet Rays Most plastics are affected by ultraviolet (UV) light in the presence of air. PP is no exception and, when unstabilised, it very rapidly becomes brittle when exposed to sunlight. Degradation is accompanied by marked deterioration in mechanical properties. Mouldings generally lose gloss after short exposure; the surface and the material immediately beneath suffers the most. The appearance of chalky powder at the surface of the heavily degraded PP article has also been reported. The amount of the degradation depends on the duration of the exposure and whether the article is used behind glass. However, the screening effect of the glass may not be sufficient to prevent degradation if the stabilisation system is inadequate for the application. Like most plastics, PP exhibits little or no change under short-term exposure to radiation in the visible light range. Prolonged exposure to direct sunlight can, however, cause a deterioration in properties due mainly to UV radiation. UV absorbers are added to provide protection against harmful UV radiation. The UV absorbers absorb UV light and release the excess energy as heat. Commonly used UV absorbers for PP are derivatives of benzophenone, benzotriazoles and esters. The use of hindered amines has also been reported. The efficiency of the UV absorbers to protect the part depends on the part thickness. Stabilisation in thick parts is more effective than the thin parts, films or sheets. 57

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Carbon black in a suitable concentration can make the article sufficiently protected for 10 years continuous outdoor exposure in temperate climates. The use of carbon black is clearly restricted in fibres and films. In addition, the heat stability of the carbon black modified grades may be poor in comparison to unmodified grades. Hindered amine light stabilisers function as free radical scavengers and can double as thermal antioxidants. They are, though costly, effective at very low concentrations. Hindered amines can interact with other additives (e.g., phenolic antioxidants, titanium dioxide) in the PP to produce yellowishness. Halogenated flame retardants can react with hindered amine light stabilisers and render them ineffective. The protection of PP against UV radiation and heat is a very complex issue. The idea of this section is not to go in depth into formulation issues which are best left for resin manufacturers/formulator or stabiliser suppliers. However, from the above discussion, it can be appreciated that: •

The heat stabiliser and light stabiliser can interact with each other as well as other additives present in the PP formulation. The mechanical properties of the modified PP will depend on the amount of stabilisers.

•

The addition of stabilisers may make material unsuitable for food applications by leaching and migration.

The reduction in the molecular weight of the PP on exposure to harmful UV radiation can be easily seen using gel permeation chromatography (Figure 22). It can be seen from the figure that the reduction in molecular weight is more severe at the part surface. Molecular weight distribution

Figure 22 Gel permeation chromatogram showing the degradation of polymer caused by UV exposure

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Light stabilised grades have very good light, UV radiation and weathering resistance in both natural and colour formulations. These grades are supplied with different degrees of stabilisation to match the various application requirements. It should be noted that many additives and pigments can seriously impair the effectiveness of the stabiliser system. Hence a balance of stabilisation system, pigments and homogeneous distribution of additives is necessary to achieve optimum resistance to light, UV radiation and weathering. Light stabilised grades are generally unsuitable for food contact applications. However, formulations can be developed based on requirements. Compatibility of the light stabilisers is very important since they are added in significant quantities. Blooming and the migration of low molecular weight light stabilisers are important issues. The most accurate method to test the suitability of the material is the use of material in its intended environment for a long period of time. Due to the long-term nature of outdoor weathering, accelerated testing using weatherometers is common. Different light sources are used, such as xenon arc lamp, carbon arc lamp and fluorescent sun lamp. Filtered xenon most accurately reproduces the spectral energy distribution of sunlight. However, the results from accelerated testing may be different from the longterm outdoor testing. HDPE offers inherently better oxidation and UV resistance in comparison to PP. Whilst these properties may be greatly improved in PP by the use of additives, these may increase the cost of PP compounds to beyond that which is considered economically attractive. It is for this reason that HDPE has retained a substantial part of the crate market. Nevertheless, PP blow mouldings have been commercially used in horticultural sprayers and motor car parts. 4.10.3 Chemical Resistance As a non polar, high molecular weight paraffinic hydrocarbon, PP has outstanding chemical resistance, the best of all thermoplastics to organic chemicals. Indeed, there is no solvent for PP at room temperature, although it may swell in some cases. Since PP contains only hydrogen and carbon and does not contain polar atoms, non polar molecules (such as hydrocarbons and chlorinated solvents) are more easily absorbed by PP than polar molecules (such as soaps, wetting agents and alcohols), causing swelling, softening or surface crazing. PP is extremely resistant to inorganic environments. It is not affected by aqueous solutions of inorganic salts, nor by most mineral acids and bases, even when concentrated. It is, however, susceptible to attack by oxidising agents such as chlorosulphuric acid, pure fuming nitric and sulphuric acids, and the halogens. It must be remembered that stabilisers and other additives can be attacked by aggressive chemicals to which PP is resistant. This might affect the stability and properties of the material.

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Generally speaking, higher temperatures can considerably impair resistance depending on the chemical environment. Up to 60 °C, PP is resistant to many solvents but aromatic and halogenated hydrocarbons, certain fats, oils and waxes cause swelling. At temperatures up to 30 °C, the effect is only slight. The higher the degree of crystallinity in a PP material, the greater its chemical resistance. Consequently, homopolymers of PP have more chemical resistance than the random copolymer. Environmental stress cracking (ESC) is the surface initiated brittle fracture of a polymer under stress in contact with a medium in the absence of which fracture does not occur under the same conditions of stress. Combinations of external and/or internal stresses may be involved, and the sensitising medium may be gaseous, liquid or solid. A stress raiser or notch, an external and/or internal residual stress and a stress cracking medium must be present for ESC to occur. For example, PE products prematurely fail in the presence of detergents and other active environments. PP is widely used for packaging, fluid containment and transportation. However, PP is virtually free from environmental stress cracking observed in other polymers and attempts in the laboratory to identify a pure ESC agent for PP have failed. Many plastics are inclined to environmental stress cracking or embrittlement on prolonged contact with boiling detergent solutions. The PP components specially made for washing machines do not exhibit these disadvantages. A reflux test involving 1000 hours in boiling detergent solution is used to measure water absorption, embrittling and change of the dimensions. It has been reported that suitable grades show 0.5% higher water absorption than the normal grades when soaked in detergent solution. Furthermore, no embrittlement is observed and the yield stress, ultimate tensile strength, dimensions, surface hardness, rigidity and toughness of PP are not changed. 4.10.4 Permeability 4.10.4.1 Permeability of Water and Liquids PP is virtually impermeable to water and water-based products. It does not, therefore, swell when immersed in water. The water vapour permeability of PP film is compared with other plastics in Table 21. Changes in relative humidity have no effect on the properties of the material. The very slight water uptake can be determined when there is a change of temperature in a hot damp atmosphere. It is due entirely to surface adsorption. Fillers, reinforcements and additives may slightly increase the uptake of water. It is advisable to dry the material immediately before processing or to degas it during plasticisation. This particularly applies to carbon black pigmented PP. However, there is appreciable adsorption and permeation with certain organic solvents (especially if non-polar). The degree of permeation increases with temperature.

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Practical Guide to Polypropylene Table 21 Water vapour permeability of various plastics relative to the permeability of oriented PP 2 Polymer Water vapour (g/m 24 h) Oriented PP film 1.0 Cast PP film 2.0 HDPE 1.0 PVDC 0.10 LDPE 5.0 Rigid PVC 10.0 Plasticised PVC 30–70 EVA 12.0 PA 11 20 PA 6 100 Oriented PS 40 Cellulose acetate 400 Cellophane P 1000

4.10.4.2 Permeability of Gases The permeation rate of PP with different gases is compared to other polymers in Table 22. It can be seen that the resistance to permeability of gases improves with orientation. In addition, PP offers good resistant to permeability of gases which is comparable to HDPE but significantly better than LDPE. In PP packaging for solvent containing substances or strong smelling products, migration of solvent must be expected and, as a result, loss of weight in the contents during prolonged storage. Most of the flexible packaging materials require stiffness, good printability, high gloss and good barrier properties. Oriented PP is coextruded with polyvinylidene fluoride (PVDF) to achieve suitable product requirements where excellent barrier properties are required. 4.10.5 Sterilisation 4.10.5.1 Autoclave and Ethylene Oxide Sterilisation Devices may be sterilised by one of three main techniques: autoclave, ethylene oxide or radiation treatment. Steam autoclaving is generally carried out at temperatures of 120– 135 °C. Being resistant to high temperatures and water, PP is one of the obvious choices. It is entirely unaffected by the autoclaving as long as the treatment temperature is kept below its softening temperature. Random polymers have lower softening temperature than the homopolymer and block copolymers and, hence, are not preferred for steam autoclaving. Use of ethylene oxide sterilisation is now on the decline due to the mutagenic nature of the material. PP is generally unaffected by ethylene oxide treatment.

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Practical Guide to Polypropylene Table 22 Permeability to gases of various plastics Polymer N2 O2 H2O CO2 Oriented PP film 7.0 35 140 175 Cast PP film 18 90 360 450 HDPE 15 75 300 375 Polyvinylidene 0.06 0.3 1.2 1.5 chloride (PVDC) LDPE 50 250 1000 1250 Rigid PVC 1 5 20 25 Plasticised PVC 5–200 25–1000 100–4000 125–5000 Ethylene vinyl 80 400 1600 2000 acetate copolymer PA 11 3.0 15 60 75 PA 6 0.5 2.5 10 12.5 Oriented PS 20 100 400 500 Cellulose acetate 25 125 500 625 Cellophane P 10 50 200 250

SO2 280 720 600 2.4 2000 40 200–8000 3200 120 20 800 1000 400

Medical sterilisation at temperatures above 100 °C demands two opposing requirements from the polymer. The polymer should have high crystallinity to provide good tensile strength retention and heat resistance while low crystallinity is required to improve the transparency. PE plastomers based on metallocene technology have been blended with PP to achieve the optimum properties for syringe applications. The random copolymers have lower resistance to autoclave sterilisation due to their lower heat distortion temperature. Parts which are significantly stressed can deform during autoclave treatment. The stresses in a syringe may arise from pressure, vacuum, weight of the liquid or due to pressure exerted on syringe. 4.10.5.2 Radiation Sterilisation Radiation sterilisation is most damaging to PP. When PP is subjected to high energy ionising radiation, deterioration in its physical properties and changes in its molecular structure occur detectable by infrared (IR) spectroscopy. PP that will be radiation sterilised requires unique stabilisation packages. Since radiation sterilisation causes increased oxidation of the polymer, additive levels are also increased. Sterilisation of disposable articles is generally carried out with a radiation dose of about 25 kJ/kg or 2.5 Mrad. Although toughness suffers to some extent and shade changes are possible, articles properly made from PP still retain their serviceability. Suitable trials should be conducted to properly assess the effect of radiation. Thick-walled parts are less affected than thin-walled components. PP copolymers have markedly better resistance to high energy radiation than homopolymers, due to lower crystallinity and greater free volume. Flex to failure test is used for evaluating the radiation tolerance of PP. Main areas of application include syringes, needle shields, surgical trays and blow moulded containers. El Paso Products Co., Nested Chemicals International and Exxon Chemicals have reported development of suitable grades of gamma radiation sterilisable PP to

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satisfy the requirements for blow moulded containers and injection moulded articles [19, 20]. PE has considerable better radiation resistance than PP due to the absence of the tertiary carbon atom which initiates degradation of the material and loss of the mechanical properties. Irradiated PP has been used as a plant wrapping which gradually degrades in the soil. These wrappings are suitable for both manual and machine planting of forests and fields. PP can encounter radiation from various sources when used in the nuclear industry. Gamma radiation, used for medical sterilisation, is far more penetrating than beta, electron radiation or alpha radiation. The radiation dosage level harmful for PP is quite low. Hence, use of PP in the nuclear industry where high radiation occurs is not possible. Suitable resins for use in nuclear industry include phenolics, polystyrene, polyester, particular epoxies, silicone, etc.

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5 Design 5.1 Product Design 5.1.1 Design for Rigidity and Toughness Rigidity and toughness are quite opposing material properties. It can be seen from Table 12 and other manufacturers’ data sheets that the grades possessing higher impact strength have lower stiffness or rigidity. Occasionally high impact strength and good rigidity both might be required from the product. When both rigidity and toughness are required, relatively thick sections are usually necessary. The other plastics which have inherently high rigidity and toughness are PC, ABS, PPE and PA. The rigidity of PP products may be improved by selecting the appropriate grade of the material with high rigidity or opting for filled or reinforced grades which offer high rigidity at the expense of impact strength. The other methods for improving rigidity are: •

By increasing the wall thickness. This is effective in increasing the rigidity as well as impact strength. The disadvantages are an increase in the weight of the component and slower cooling of the product in the mould.

•

By incorporating ribs. Ribs are quite effective in increasing the rigidity with minimum increase in the weight of the component. However, the disadvantages are higher tool cost, risk of lower impact strength due to stress concentration and sink marks.

•

By incorporating curvature in design. Improvements in both rigidity and impact strength are possible. The improvement in rigidity is marginal and the impact strength goes up due to the absence of stress concentration.

•

By incorporating stepped sections. This is effective in increasing the rigidity of the structure while avoiding sink marks and giving lower tooling costs. However, design of cooling channels in the mould might be difficult.

•

By localised stiffening using metal or other reinforcing material. This is effective in providing high rigidity in localised regions.

The methods of improving impact strength based on design are: •

By providing thick sections where the structure is rigid by their shape or construction.

•

By providing a mechanism of deflection in less rigid structures. The deflection mechanism will depend on the direction of the impact.

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5.1.2 Weld Lines Weld or knit lines occur where flow fronts converge. They are inevitable in multi-gated parts and in parts with moulded-in holes, metal inserts and bosses. They are generally visible on close inspection and the more obvious they appear to be, the weaker they will be. The strength of the material at a weld line can be less than 50% of the reported strength of the material, and fibre reinforced plastics are more affected by the weld line problem because the fibres will not bridge the weld. Although the weld strength can be controlled by increasing the temperature and pressure at the flow fronts, the design of the mould and the product plays a very crucial role. A pure butt weld where two flow fronts meet at 180 degrees forms the weakest weld. The weld strength can be improved if the weld forms by the merging of two flow fronts. Some techniques used to overcome the problems of weld lines are knock-out holes and flow leaders. 5.1.3 Shrinkage and Dimensional Stability PP has relatively high mould shrinkage (see Table 2) and it can range from 0.8% to 2.5%. The moulding shrinkage is influenced by the processing conditions. Consequently, it is very difficult to achieve ultra-close dimensional tolerances in PP. However, PP is a tough, resilient polymer with good resistance to stress cracking, diminishing the need for very close tolerances. Dimensional stability cannot be seen separately from warpage. If a component distorts, the average dimensions may still be within the tolerances. However, individual dimensions may exceed tolerance. If a moulding deforms under light or no-load conditions, the cause is usually post-moulding differential shrinkage. This may result in warpage or internal stresses. Large area, shallow mouldings are more prone to warping, e.g., video cassette covers. This problem can be visible as soon as the article is ejected or may take a long time to appear depending on the service conditions such as temperature and load. It is possible to avoid these problems during the moulding stage, however, a proper design might be the better solution to start with. Curvatures can accommodate some degree of differential shrinkage unnoticeably whereas straight edges or flat surfaces can exhibit visible distortion under similar conditions [21]. It is usually good practice to maintain a constant wall thickness. This is necessary for two reasons: to avoid the possibility of air entrapment while moulding and to avoid warpage due to differential cooling and shrinkage. Thick sections cool more slowly than thin sections and, consequently, shrinkage of thick sections is much higher than that of thin sections. This can cause mouldings to bow, particularly if the difference in wall thickness extends over a considerable distance. However, there are exceptions to this general rule of maintaining constant wall thickness. Sometimes, the stresses created by differential shrinkage due to different wall thickness may be used to offset deformation induced by other causes. For example, the tendency of straight side walls on a box-shaped moulding is to bow inwards. External ribs or tapered wall thickness can stiffen the moulding, thus assisting in achieving undistorted wall thickness.

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To create the right dimensions in the moulded part, the dimensions of the mould cavity must be increased by an amount known as the shrinkage allowance. The material shrinks more in the direction of the flow than in the transverse direction. This is known as differential shrinkage and is a consequence of the long chain molecular structure which leads to partial orientation of chains and stretching of the chains during melt flow. Differential shrinkage results in a distorted or warped moulding, and is more marked in materials with a wide molecular weight distribution and is less in grades with narrower molecular weight distribution. The differential shrinkage in controlled rheology grades is remarkably less than the regular grades. The differential shrinkage is not easy to design for since the differences are small and prediction of the flow direction is required. Where close tolerances are required, the differential shrinkage should be accounted for in design. The injection pressure in the mould cavity decreases considerably with distance from the gate. The location, shape and number of gates influence melt orientation. This explains the very large variation in the shrinkage which sometimes occurs between different parts of a moulding, particularly complex mouldings. The phenomenon of post shrinkage may amount to about 1% and no measurements of critical dimensions should be made sooner than 24 hours after moulding. Changes may continue after 24 hours but the rate of change will be very slow at normal room temperature. Post shrinkage is time and temperature dependent. With increased storage temperature, post shrinkage accelerates and reaches its final value in a shorter time. Post shrinkage of filled and reinforced grades is limited to about 0.5% or less. 5.1.4 Sinks and Voids All injection-moulded thermoplastics are prone to sink marks and voids in areas where sudden changes in section thickness occur, or over ribs and bosses. Semi-crystalline polymers such as PP are more prone to sinks and voids. Voids occur when the external skin of the moulding is rapidly cooled and becomes sufficiently rigid to support the contraction of the underlying melt. Sinking or surface depression occurs in localised thick sections where internal mass contains sufficient heat to keep the polymer in molten stage and crystallises slowly producing sink marks. Once again, the moulder can control these defects to a certain extent. However, a product designer can play an important role in keeping the changes in wall thickness to a minimum. Ribs and bosses which introduce locally increased section thicknesses are the main items for consideration. The other methods for reducing the impact of sinking in stepped sections are camouflaging the sink marks in design by placing them in a section not visible directly, changing the wall thickness or putting beads or decorations on the possible locations of sink marks.

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5.1.5 Design for Assembly PP is a tough and resilient plastic and many types of mechanical fastening techniques, (Section 7.2), can be easily used. However, PP is prone to creep and screws used under high stress for assembly may slack due to creep. In these cases, metal inserts should be preferred since they increase the load-bearing area in PP. Metal inserts may be moulded in or pressed in after injection moulding. However, the stress concentration effect on the impact strength due to sharp-edged inserts should be carefully considered. Moulded-in internal or external threads can be used with PP. The design of threads is important to avoid the possibility of thread jumping. Thread jumping may occur if high torque is applied to the screw. If this happens, the moulded thread is still usually unharmed and can still be used. Acme or buttress type threads can be used to decrease the possibility of thread jumping. Design of undercut sections for snap-fit assembly parts is another crucial point in the design of PP artefacts. External undercuts can be easily demoulded using retracting segments in the mould, provided part geometry allows for this. Internal undercuts can be ‘jumped’ out of the mould and are limited in size and shape. In this case, the wall of the moulding should be free to flex outwards during ejection. 5.1.6 Integral Hinges PP is the best material for producing internal hinges. Thin sections of PP have shown excellent resistance to continued flexing. This has led to the introduction of a number of one-piece mouldings for boxes and cases in which the hinges are an integral part of the moulding. In this special, very cost-effective production process, two parts plus hinge are injection moulded in one moulding operation. After moulding, the integral hinge must be initially flexed and hence stretched, if possible while still hot. Only in this way are the polymer chains oriented along the hinge to give excellent flexural fatigue resistance. The stretched hinges can be flexed several million times without failure even when bent through large angles right back on themselves. This, of course, assumes correct design, suitable injection moulding conditions and exposure to flexural stress only. Where product design makes direct moulding impractical, the hinge may be subsequently formed or machined. To a limited extent, hinges can be formed from filled grades but the quality is unpredictable and they are not recommended. To ensure straightforward mould filling and easy mobility of the hinge, the film thickness should be between 0.25 mm and 0.8 mm. If transverse stresses and small bending angles are expected, the film thickness might be up to 1.8 mm. There should be fast, even flow through the hinge section when the melt is injected into the mould. With multipoint gating, e.g., on lids and boxes, merging of melt fronts should never occur in the hinge section. Moulded parts with hinges frequently require a higher melt flow rate material to ensure a rapid mould fill and good hinge quality. However, material with too high a flow reduces the strength of the hinge.

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5.1.7 Design to Avoid Failure and Durability In designing components of PP, the general guidelines in designing of plastics apply. It is essential that the plastic components are designed as structures. Even when applied loads and deformations are apparently negligible in comparison to metal or other construction materials, this procedure is recommended since the strength and stiffness of plastics are orders of magnitude less than for conventional materials. On the other hand, over design is costly. Furthermore, the properties of the plastics are time dependent. Consequently, it is not difficult to find examples where failure of the product occurs after months or years after design, manufacture and testing of a plastic product. Hence, a proper means of assessing durability should be used. Viscoelastic behaviour must be taken into account in the design of mouldings. In precision engineering, it is accepted that the deformation of the component over a given period of time should not exceed 0.25%. This condition is related to a given stress. If greater deformation is permissible within the same period of time, the stress may also be higher. It should be remembered that the mechanical properties of a plastic are dependent on time, temperature and stress. Long-term creep and fatigue data should be used to ascertain long-term durability of PP artefacts. Since PP offers a cost advantage as a replacement material for engineering plastics, it is anticipated that, in the future, PP will be used to replace many engineering components. Due to the lower modulus of PP in comparison to other engineering plastics, ascertaining durability of the artefacts will be a crucial aspect of the design. However, extrapolation of creep and fatigue data to longer durations of time or higher service temperatures is fraught with problems owing to the unknown transitions in the mechanical behaviour of material at longer durations/higher temperature. Tertiary creep can eventually cause the ultimate failure of the product. Hence, when extrapolating the long-term testing results, durability of the product should be checked using elevated temperature testing to avoid catastrophic failure due to the tertiary creep. The actual performance of the product may deviate from that predicted using short-term tests. Consequently, the properties from the short-term standard tests should be taken as guide only. Ideally, the properties of the finished product should be measured under service conditions. Fatigue and creep data for individual trade-named grades are normally available from the manufacturers. 5.1.8 Design Safety Factors In design calculations for moulded components, the property values determined from the long-term tests must be divided by a safety factor. The safety factor recommended in the designing of components for maximum permissible deformation is 1.2 while for calculations to provide safety against fracture, factors of 1.3 to 5 are advised, depending on the stress and the degree of risk to property and persons. It should be noted that these safety factors are based on the long-term strength of the material, not on the short-term

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yield strength. The long-term recommended design stress might be 6–10 times lower than the short-term yield strength. The safety factor incorporated in the design of reinforced PP parts is high in comparison to unreinforced PP. A safety factor of 2–3 for fracture and 1.2 for change in dimensions is recommended. 5.2 Mould Design 5.2.1 Flow Length The flow ratio is the flow length in the mould divided by the wall thickness of the moulding. A number of factors can affect the flow ratio such as melt flow rate, the processing conditions and the features of the mould. Thick sections enable higher flow ratios to be achieved while very thin sections have the opposite effect. The flow length of the material provides the mould designer with a guide to the minimum number or position of feed points which will be advisable. The effect of variations in thickness and melt temperature on the flow length of a typical PP sample is shown in Figure 23. The relationship between the flow length and MFR of CR grades is not simple. Generally, the flow length of CR grades is similar to those of normal grades of about two-thirds of the MFR quoted for the CR type. This is due to the narrow molecular weight distribution of CR grades.

Figure 23 Effect of thickness variation and temperature on flow length of a typical PP material (T1 is 275 °C, T2 is 200 °C, t1 is 3.0 mm, t2 is 2.0 mm and t3 is 1.0 mm) 70

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Another method for the practical assessment of the melt flow properties of PP is the spiral flow test. The plasticised melt is injected into a spiral mould cavity. Before reaching the end of this spiral channel, the melt freezes. The distance which the melt flows along the channel before freezing (spiral flow length) depends on the flow properties of the material and on the section thickness of the spiral. The flow properties, obviously, vary with the particular grade of PP. The spiral flow data for different materials is easily available from the raw material manufacturers. It is advisable to use only 0.7 to 0.8 times the flow path length determined experimentally in the spiral flow mould to leave adequate margin of safety to achieve complete filling of the mould. 5.2.2 Feed Systems The feed system to moulds has three principal components: sprue, runner and gate. The sprue is a tapered bore in line with the axis of the injection unit that conducts the melt to the parting line of the mould. The runner is a channel cut in the parting line of the mould to conduct material flow from the sprue to a point very close to the melt cavity. The gate is a relatively small and short channel which connects the runner to the mould cavity. Runners may be cold or hot, indicating the temperature at which the runner is maintained in comparison to the melt temperature. Runner layouts should be designed for all plastic materials to deliver the plastics melt at the same time and at the same temperature, pressure and velocity to each cavity of a multi-cavity mould. A typical runner diameter for unfilled PP is 4.8–9.6 mm. A balanced runner (all cavities fill at the same time at the same pressure) will usually consume more material than an unbalanced type but this disadvantage is outweighed by the improvement in the uniformity and quality of the mouldings. Unbalanced moulds usually cause overpacking, warpage, differential shrinkage, brittleness, dimensional inconsistency and parts that stick to the mould. If a naturally balanced runner is not possible, the artificial balancing of runner via size and length changes should be carried out to achieve good quality parts. Balancing by varying the gate size is not recommended. The position and type of gate(s) has a major bearing on the product quality. All possible gate types are possible with PP and the usual recommendations for other plastics are also valid for gates for PP artefacts. Some general recommendations for the design of gates are: • • • • • • •

place the gate near the thick sections so that they will fill completely, place the gate in a cosmetically acceptable position, the gate should be easily removable, maintain the symmetry in flow, avoid gas traps and weld lines, avoid gating in parts which are subjected to high stresses, and minimise the differential shrinkage. 71

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5.2.3 Venting Venting of the mould might be necessary to avoid a short shot or burning of the material in certain cases. In other cases, venting will allow easy and quick mould filling, possibly at lower injection pressure. When an injection mould fills, the incoming high velocity melt stream is resisted by and must displace the air in the feed systems and cavities. Often air gaps between the parting faces of the mould and between the assembled part of the core and cavity provide a mechanism for escape of air. In some cases, the location of vents will be quite obvious. In other complex cases, mould flow simulation packages can predict the location of vents before machining the mould. Vents are susceptible to corrosion and proper choice of the material, e.g., alloys or high chromium steel containing a chromium fraction greater than 10%, are preferred. However, if alloys containing copper are in contact with melt PP for an extended time they can cause degradation of the material. The location of vents is quite important for thin-walled mouldings where high injection speeds are necessary. 5.2.4 Mould Cooling Good design of cooling channels is very important for moulds for PP due to the higher heat capacity of the PP in comparison to most of the plastics, with the exception of HDPE. It is very difficult to overdesign cooling channels in a PP mould. Mould cooling is a very crucial requirement in the moulding of PP artefacts to achieve good quality moulding in minimum time cycle. Cooling time depends on the melt temperature, mould temperature, resin stiffness and rate of crystallisation. If the wall thickness of the component is doubled, it is expected that the cooling time is increased by 3 to 4 times. PP has a relatively high softening point and therefore a wide range of mould temperatures between 20–90 °C can be used. High mould temperatures are sometimes desirable to avoid distortion and to encourage flow over long distances. Surface gloss is improved by the use of high mould temperatures and voids may be eliminated by this means also; any sinking may be accentuated. Within very small limits, the size of the mouldings is controlled by the mould temperature; the amount of shrinkage increases as the rate of cooling decreases. The aim of the design of the cooling circuits is not to cool a moulding as quickly as possible. Very fast cooling may have undesirable effects on the physical properties or surface appearance of the product. The design of the cooling circuit should attempt to cool the whole moulding to the ejection temperature at the same time. The temperature of the plastic melt decreases with outward flow from the feed point, and the temperature of the water increases as it flows through the mould. This effect could be utilised to counter the temperature drop in the material so that all the portions of the moulding are cooled at the same rate, i.e., using a co-current system of cooling where the cold water enters to a point close to the feed systems and travels outward in the direction of melt flow. This would ensure, for all practical purposes, the

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simultaneous cooling of the whole moulding. Local cooling is required for thick sections to avoid shrinkage variations and to reduce the possible increase in cycle time. Narrow cores may prevent the incorporation of adequate cooling channels. In such instances, the cores tend to overheat and, apart from increasing the cooling time, this can cause the walls on each side of the core to shrink towards each other after the moulding is ejected. Advantage may be taken of the superior heat transmission characteristics of some alloys, such as beryllium-copper, to improve this situation. The core is then provided with generous cooling at the base. Turbulent flow in the cooling channels is more efficient than laminar flow. This can be achieved by decreasing the size of the channels or by increasing the flow rate. This runs counter to popular belief that large cooling channels are always better. It is difficult to achieve turbulent flow with oils or antifreeze solutions due to their higher viscosities. 5.2.5 Taper and Ejection Normally, a generous draft taper should be provided for easy ejection of the artefact, consistent with the design of the product. The ejection of PP artefacts is not very difficult and sometimes a taper of 0.5° is sufficient. If enough taper is not possible, mouldings can be produced with very low or even no taper, provided that ample loadbearing area is provided for the ejection and that the thickness of the moulding is sufficient to avoid buckling or permanent deformation. PP is a resilient plastic which stretches rather than cracks if ejector pins of inadequate area are employed. It is not a very hard or rigid material so the rule is to use ejectors that are as large as possible. Ejectors inevitably leave witness marks on the mouldings. Hence, it is preferred for ejectors to operate on side walls, ribs or bosses. When ejector pins are to be positioned in flexible areas of moulding, they should be of ample diameter. Cylindrical ejector pins are most commonly used. However, rectangular pins or blade ejectors could be used in constricted areas. A stripper plate acts on the entire wall of the part and so distributes the ejection force. Air assisted ejection of thin walled articles could be used. 5.2.6 Surface Finish In many moulded articles, textured surfaces are required. Electrodeposition, casting, photo-etching and spark-erosion are used to provide textured mould surfaces. Two good requirements for the faithful reproduction of the textured surface are low viscosity of the resin and good elasticity to permit the withdrawal of textured surface. PP is quite suitable for both these requirements. The taper of the mould should be increased to assist ejection of the items with textured surfaces.

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The design of the gating system for the component to be produced in the textured surface is very important since the merging flow fronts can cause surface defects on the moulded artefact. To avoid this fault, gates should be positioned to prevent converging streams of material. If this is not possible any converging lines should be directed into unseen areas of the moulding. 5.2.7 Filled Grades When designing mouldings in reinforced and filled PP, the general guidelines for design of the product and the mould are the same as those for unreinforced PP. However, filled and reinforced grades require some extra consideration: •

Avoid abrupt changes in cross-section to avoid degradation of reinforcing fibres.

•

Avoid melt accumulation to avoid the degradation of material. Filled grades are more temperature sensitive than the unreinforced grades and degradation temperature is lower.

•

Avoid notches such as transition, corner and edges. Use a rounding off radius of greater than 0.5 mm.

•

Provide more draft for ejection, e.g., 0.5–1°.

•

Carefully consider the orientation of fibres to improve weld line strength.

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6 Processing of PP A broad idea about the processibility of a plastic could be made from its crystalline nature. In general, the following comments could be made about the processibility of semi-crystalline and amorphous plastics. •

Semi-crystalline plastics have higher specific heat capacity than the amorphous plastics. Hence, the energy required for melting of the resin will be more leading to costly melt processing.

•

The amount of heat to be removed from the mould during cooling is more in the case of semi-crystalline material. Hence, the moulded artefacts will take longer to cool. Cooling method and design of cooling channels are quite important.

•

The shrinkage between the melt and solid state is more in semi-crystalline plastics. The dimensions of the mouldings are thus more difficult to predict. In addition, warping, sinking and voiding may occur. Higher die swell makes accurate extrusion profiling difficult.

•

The lower melt elasticity of semi-crystalline material means that forming by blow moulding and thermoforming is quite difficult.

•

Semi-crystalline materials have a narrow processing range.

PP is a semi-crystalline plastic, and hence, it would offer many disadvantages as shown by other semi-cystalline materials. The economics of processing depend on the thermal characteristics of the material. The thermal properties of PP are compared with some other plastics in Table 23. The heat transfer requirement for cooling from the melt temperature to mould temperature in the case of PP is much higher than those in the case of amorphous polymers such as ABS and PS. Hence, the processing of PP is costly. In addition, thermal conductivity determines the cooling time of the material in the mould. It can be seen that the thermal conductivity of PP is less than HDPE. It would require more cooling time, and hence, a slower production rate.

Polymer HDPE PP PVC PA 66 ABS PS

Table 23 Thermal properties of different plastics Heat transfer requirement Thermal conductivity at for cooling from the melt 20 °C (W/m K) temperature (kJ/kg) 750 0.43 640 0.22 280 0.16 800 0.23 370 0.18 310 0.17

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The purpose of this text is not to go into the detail of the injection moulding process and associated control but to highlight the complexities involved in injection moulding of PP and how it is different from other plastics. For details on injection moulding, several textbooks are available [e.g., 22]. 6.1 Rheology 6.1.1 Melt Flow Rate Melt flow rate is the traditional method of measuring the viscosity of PP. The melt flow rate of PP is determined by measuring the flow of molten PP through a standard orifice at 230 °C and 2.16 kg load. The processing technique basically decides the selection of melt flow rate. The melt flow indices of PP may range from as low as 0.5 g/10 min to as high as 1000 g/10 min. CR grades in general have higher MFRs. The current trend in grade specification design is to increase MFR without loss of mechanical and thermal properties. A grade with a low index is preferred for techniques requiring a self-supporting melt, like film blowing. High index grades give the easy flow needed for thin wall injection moulding, film casting and fibre spinning. As a rough guide to selection of grades, a MFR between 0.5–2 is chosen for pipes, sheets and blow moulding, 2–8 for films and fibre applications, and greater than 8 for extrusion coating, injection moulding and fibre spinning. Higher MFR grades are mainly used for thin-walled applications which require rapid mould filling. Unfortunately, a higher MFR means low toughness. It must be kept as low as is consistent with adequate processibility. Further, in making fibres from PP film, MFR influences the degree of fibrillation and this has to be balanced against tenacity. Molecular weight and its distribution is closely related to MFR. Controlled rheology grades of PP offer narrower molecular weight distribution than reactor grades. 6.1.2 Viscosity Versus Shear Rate The MFR test as discussed above has certain limitations. Since it is a one-point test only, it is quite possible for two materials with the same MFR to behave completely differently at shear stresses that are different from the ones used during the MFR measurements. Further, the low shear rates employed during the measurement cannot give a reliable measure of mouldability, owing to the higher shear rates encountered in injection moulding. The capillary rheometer measures apparent viscosity over an entire range of shear stresses and shear rates encountered in compression moulding, calendering, extrusion, injection moulding and other polymer melt processing operations. The shear rates generated during compression moulding are quite low while injection moulding operation generates very high shear rates. Shear rates during extrusion fall between compression moulding and injection moulding. At least three

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different loads and speed settings are used to generate a shear stress versus shear strain curve for a polymer in a capillary rheometer. PP is sensitive to shear. When the shear stress or shear strain on the material is increased, the viscosity of the melt decreases. The extent to which this occurs varies with molecular weight distribution, particularly when molecular weight is high. The viscosity versus shear rate curves for three homopolymers of PP with different MFRs are compared in Figure 24. It can be seen from the figure that the viscosity of the resin decreases non linearly as the shear rate increases and the extent of shear thinning depends on the molecular weight and molecular weight distribution of the polymer.

Figure 24 Viscosity versus shear rate curve for three homopolymer grades of PP with different melt flow rates

The viscosity versus shear strain curves for homopolymer and controlled rheology grades of PP are shown in Figure 25. It can be seen from the figure that the viscosity of the controlled rheology grades is lower than the conventional grades at low shear rates. The extent of shear thinning at high shear rates is less than the conventional grades due to the lower molecular weight distribution of controlled rheology grades. In other words, controlled rheology grades are less shear sensitive. PP grades with a wide molecular weight distribution respond the most to an increase in shear rate. Hence, for PP it would be more appropriate to deal with mould filling problems by increasing the injection speed rather than increasing the melt temperature. Material with broader MWD is processed more easily during injection moulding since the low molecular weight chains acts as plasticiser. However, narrow molecular weight distribution grades are preferred for extrusion applications.

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Figure 25 Viscosity versus shear rate curve for homopolymer and controlled rheology grades of PP

The rheological requirements from the resin for different processing routes are different. In injection moulding, the material is sheared between two surfaces, hence, shear viscosity is more important. Extensional flow and melt strength are useful requirements during thermoforming, extrusion coating, foaming, blow moulding and film blowing. As mentioned above, PP has low melt strength, hence, thermoforming, blow moulding and film blowing of PP are difficult. To improve the acceptance of PP in these applications, high melt strength grades have been introduced by the resin manufacturers. High melt strength grades have broader molecular weight distribution and show greater shear thinning (Figure 26). The long chain branching in high melt strength grades is similar to that in LDPE. However, it is thought that the branch chains are significantly longer and can be as long as the main chain itself. The extensional viscosity of conventional and high melt strength grades of PP is compared in Figure 27 and it shows that conventional grades have low extensional viscosity and melt strength. High melt strength grades break in elastic manner at high extension rates while conventional grades break in normal necking mode without significant recoil. High melt strength grades offer significant advantages over conventional grades for many processing routes: improved sag resistance and part uniformity for thermoforming, improved fine cell structure for low density foams, sag resistance and part uniformity for blow moulding and increased bubble stability for film blowing [23, 24].

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Figure 26 A comparison of viscosity profiles of conventional and high melt strength PP grades

Figure 27 Extensional viscosity of conventional and high melt strength grades of PP as a function of time 79

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The main use of the viscosity versus shear rate data is during the mould design to predict the behaviour of the mould during an actual filling cycle. Its use at the moulding shop floor is, however, limited. Consideration of the rheology of the material at the moulding floor by comparison of rheological behaviour of similar grades from different suppliers might reap sufficient economical dividend. For example, the final choice of the grade for injection moulding from different equivalent grades available from the resin manufacturers may be made from the injection pressure versus injection time curve for different grades. The material which offers lowest injection time at minimum injection pressure could be the final choice of material. Similarly, for blown film applications, screw speed, power required and head pressure estimation based on rheological characterisation may be useful. 6.2 Injection Moulding Injection moulding is a very commonly used technique for producing PP articles and almost a third of PP is processed by injection moulding. Although the process of injection moulding is very simple, injection moulding machines and moulds are very costly due to the high pressure needed for injection of the thermoplastic melt and the associated complex controls. However, the ability to produce articles at high speed tips the balance in favour of injection moulding. In injection moulding, the plastics material is heated until it becomes a viscous melt and it is then forced into a closed mould that defines the shape of the article to be produced. PP can be easily processed on all commercial injection moulding machines. The injection moulding machines for PP should be fitted with a standard three zone reciprocating screw to achieve good plasticisation of the material. In selection of an injection moulding machine for PP artefact, an estimation of the clamping force needed to keep the mould shut during injection of polymeric melt is required. The clamp force needed to keep the mould shut is a complex function of the projected area of the mould and the injection pressure. The projected area of the mould is the area of the mould cavities and feed systems at the mould parting line projected on the plane perpendicular to the clamp opening direction. The injection pressure is a function of part thickness, melt temperature and mould temperature. Further, the injection pressure can vary within the mould. The rule of thumb clamping force required for PP is 2 to 4 tonnes per square inch of projected area. However, this estimate is based on the projected area and should be taken as a very rough estimate only. A large safety factor of 25–50% is advisable. The use of computer-based flow simulation will provide a comparatively accurate estimation of the injection pressure and mould opening force. In this case, a lower safety margin for the clamping force might be adequate. PP, being a semi-crystalline material, undergoes a greater volume increase than amorphous material during melting, hence the compression ratio required for the injection moulding of PP is lower. The various machine and moulding parameters suggested for injection moulding of PP are given in Table 24. The hold up pressure and time for PP are relatively high (compared to other plastics) to avoid sinks and to compensate for the high volume reduction when a semi-crystalline material passes from 80

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the melt state to the solid state. Higher compression ratio screws (3:1 to 4:1) with length/diameter (L/D) ratio greater than 20 are required for filled grades and grades using masterbatches, to achieve good plasticisation and homogeneous melt. The shot volume for the injection machine is a function of screw diameter and its maximum retraction during plasticising. Quoted injection weights are usually referenced for PS. The melt densities of polystyrene and PP are 0.89 and 0.85 g/cm3, respectively. This means the shot weight for PP on a particular injection moulding machine will be less than for PS on the same machine. The shot weight for most plastics on a particular injection moulding machine, with the exception of LDPE and HDPE, is higher than PP. Shot volume is limited by the residence time. PP has a tendency to oxidise and is routinely protected by the inclusion of antioxidants in the polymer production process. Hence, PP cannot be exposed to very high residence times. Allowable residence time depends on the material temperature and the nature of heat stabilisation. Longer residence time is possible at lower temperature. The shot volume may be up to 85% of the machine maximum shot volume. The minimum shot volume depends on the residence time and can be as low as 15% of the maximum shot volume of the machine. Table 24 Recommended machine and moulding parameters for PP Parameter Typical value Compression ratio for screw 2.3–2.8 L/D ratio for screw 20:1 to 25:1 Injection pressure 120–180 MPa Hold up pressure 40–80% of injection pressure Back pressure 10–30 MPa

PP normally requires no predrying before injection moulding. Exceptions can arise after storage under unfavourable conditions or when processing filled grades. In the case of filled plastics or dark coloured plastics, additional enhancement of the surface quality could be achieved by predrying the compounds at 120 °C for 3 hours in a dried air circulating oven. Moulding conditions are quite unique to the article being moulded and depend on the part configuration, mould design, material properties, choice of the material and properties required from the finished part. PP can be injection moulded at a melt temperature of 200–300 °C and a mould temperature of 20–90 °C. However, a melt temperature of 220–260 °C and mould temperature of 20–40 °C are quite normal. Lower mould temperature may be used for fast cycling of parts with a high injection rate. However, high mould temperatures may be required for thick-walled parts where premature freezing of the walls may lead to the formation of internal voids. The melt temperature for flame retardant grades of PP should not exceed 220–230 °C. Higher temperatures can result in discolouration and thermal degradation. In the long run, higher temperatures can also lead to corrosion of the mould and the machine. It is

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always advisable not to let products stay in the heated machine for lengthy periods and to flush the barrel with unfilled grades of PP. The melt temperature of the easy flowing grades should be kept low, as recommended by the material manufacturer. 6.3 Extrusion Extrusion is the single most popular process for forming PP. Fibre and filament account for most of the PP extrusion. Sheet and profile extrusion account for only a minor share of the market. 6.3.1 Fibre and Filament A large portion of extruded PP goes into the manufacture of fibre and filament. The various versions of extruded PP fibre and filament are multifilament, continuous filament, bulk continuous filament, staple fibre and monofilament. The diameter of the monofilaments (75–5000 denier) is considerable higher than those of the staple fibres (1–10 denier). The MFR requirements from the resin are quite different and monofilaments are produced from resins with very low MFR. The main applications for fibres and filaments are carpet yarns, upholstery fabrics, disposable fabrics, woven sacks, rope, cord and strapping. 6.3.2 Film Extrusion The suitable screws for extrusion of PP should have a high L/D ratio of 24 to 30 and a high compression ratio of 3 to 4. Attention to the barrel can also improve the material conveying performance at the inlet, stepping up the output rate. A series of axial grooves in the barrel wall, extending for at least three screw diameters, is effective for PP. Normally PP is not likely to contain moisture, volatiles or entrained air so vented screws are not normally used in its processing. The main methods for manufacturing PP film are cast and blown film techniques. 6.3.2.1 Cast Film Cast film of PP may be produced by using chill roll or water quench techniques. Both of these processes are characterised by relatively high melt temperatures and rapid rates of film cooling. This results in films with low haze, good clarity and high gloss. Slit dies are used to produce the cast film. The film thickness is controlled by the gap between the die lips but also by the rotational speed of the chill roll. Relatively high melt temperatures between 240–270 °C are used to optimise the optical properties. When carefully controlled, the thickness uniformity of chill roll cast film is substantially superior to blown film. The rapid quenching of the film by direct water cooling reduces the crystallinity and produces a tough film.

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6.3.2.2 Blown Film In the blown film process, internal air pressure is used to produce a relatively thin film. Normally air is used as the coolant for PE films but it presents greater difficulty when used with PP due to the slower cooling speed and the greater thermal degradation of material. The water quench process is generally the preferred method of producing blown PP film. 6.3.2.3 Biaxially Oriented Film Biaxially oriented PP (BOPP) film is a highly transparent, stiff film. The film can be produced using a blown film or tenter process. Stretching in both the machine direction and transverse direction is carried out at a temperature below the melting point of the polymer and results in the partial orientation of molecules in the direction of stretch. The film produced by the blown process is nearly isotropic in both the directions while the film produced by the tenter process tends to be more highly oriented in the direction of machine. The cost of the tenter process equipment is much higher in comparison to blown film equipment. Biaxially oriented films are not readily heat sealable. This is a prime requirement for packaging films so BOPP is normally given a surface coating of a heat sealable polymer such as a coextruded PP random copolymer. 6.3.3 Coextrusion Coextruded film may be a very complex structure composed of many layers to provide different functionalities and of different tie layers which improve the bonding between adjacent layers. Separate extruders are required for extrusion of the different layers. Apart from the functionality provided by the resin, the main issues involved with the selection of different layers for coextrusion are the viscosity of the resin and the adherence between layers. 6.3.4 Stretched Tapes Strapping tapes, film tapes, monofilaments, fibres and nonwovens are usually stretched immediately after extrusion to achieve considerable increase in ultimate tensile strength in the stretching direction as a result of molecular orientation. The elongation at break correspondingly decreases. The film for stretched tapes can be produced by a cast film technique using either the water quench or chill roll process. 6.3.5 Sheet Extrusion The distinction between sheet and film arises from the thickness; sheets are usually thicker than 0.25 mm. PP sheets can be produced with slit dies in a range of widths and thicknesses. The width of the sheets may be as high as 2.5 m. Sheets may be used for 83

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further fabrication such as thermoforming, machining, welding and laminating. The sheets are generally produced from low MFR material (2–6 g/10 minutes). The key advantages of PP sheets are good rigidity to thickness ratio, toughness, moisture resistance, chemical resistance, non toxicity and good moisture barrier properties. 6.3.6 Pipes and Tubes PP pipes can normally be produced on plants which are designed to process HDPE. However, some special considerations are needed since PP degrades faster and is more sensitive to oxidation. PP has a similar heat capacity to HDPE with a lower thermal conductivity. Consequently, the output in the case of PP is 15% lower than that for HDPE. Applications for PP pipes include potable water supply, domestic waste systems, under-floor heating circuits, chemical effluent disposal systems and thermal spring conduits. The principal advantages of PP for use in pipe extrusion are good heat ageing, high heat distortion temperature, low frictional losses, light weight and good resistance to attack by chemicals. Flame retardant grades could be used for construction applications. PP pipes are very sensitive to degradation by UV radiation. Hence, the material should be stabilised against UV degradation for above ground piping and where the pipes are stored in the open for considerable time before installation. To maintain good weld strength in pipes, a long die length to increase die pressure and minimise flow variations is required for spider-type dies. During the selection of suitable pipe size and wall thickness, suitable care should be taken to provide an adequate safety factor. A safety factor of two based on the long-term strength of the pipe for the design life of the pipe is recommended. The variation in temperature should also be considered where the PP pipes are being used for under-floor heating or effluent waste water pipes; a fatigue situation might occur due to temperature variation. PP random copolymer has been used for piping systems for domestic hot and cold water supply [25]. The important issues for consideration are high drinking water quality, corrosion resistance, ease of installation and cost reduction in installation. The random copolymer is preferred over the block copolymer since it has better long-term internal pressure resistance at elevated temperature. The insensitivity of the random copolymer to the pH of the water, low noise from flowing water, low pressure loss and good thermal insulation are also contributory factors. Domestic piping systems are required to fulfil a service life of at least 50 years at a temperature of 60–70 °C at 1 MPa. Welding of the pipes is very crucial and is further discussed in Section 7.1.1.1. Pipes and fittings should have high impact strength, a homogeneously fine structure throughout the thickness of the pipe and low frozen-in stresses. Thick-walled pipes made of conventional PP tend to have coarse morphology in the middle of the wall. Frozen-in stresses are undesirable because pipes are subjected to internal pressure stresses in operation. Early failure may result where these are combined with external stresses. 84

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6.4 Blow and Stretch Blow Moulding Only a very small amount of PP is processed by the blow moulding technique. As a blow moulding material, PP has never enjoyed the success of HDPE. This is in large measure because mouldings of PP require more attention to equipment design and operating conditions. The initial development of the machinery for blow moulding was optimised for HDPE due to its earlier invention. The difficulty with PP is that its melt viscosity is far more sensitive to temperature and shear rate than is the case for PE. The basic difficulty is to reconcile the need for a homogeneous melt which requires high melt temperatures and shear mixing with the tendency of these conditions to cause polymer degradation. In order to work at the lowest practical temperatures, very good temperature control is necessary for all stages of the process and in addition the machine should be of robust construction. This is because at low temperatures, high pressures will be developed in the barrel. In particular, the thrust bearings must be well designed. To minimise wear the extruder barrels should have a continuous hardened steel liner. The continuous extrusion blow moulding process is commonly used for PP and involves a slow extrusion rate, running the risk of thinning as a parison stretches under its own weight. Hence this technique is not suitable for containers with a capacity of more than 10 litres. Intermittent extrusion might be the solution for long or heavy parisons or for thermoplastic materials with low melt viscosity or strength. The extrusion temperatures for homopolymer and block copolymers are in the range of 210 °C while the melt temperature for random PP is about 190 °C. A melt temperature greater than 230 °C are not normally used for blow moulding. The screw required for extrusion of the parison is similar to the screw requirements for the extrusion process in general. The critical requirements for the screw design are to achieve high production rates with good plasticisation of the material without degradation. Conventional extruders with screw lengths greater than or equal to 20 and smooth barrels, or with barrels with a grooved feed zone can be used for plasticisation. The latter offer advantage with regard to the output rate and uniformity. Conventional blow moulding imparts a degree of circumferential orientation, caused by the expansion of the parison in the mould cavity, but there is no preferential axial expansion or orientation. Stretch blow moulding processes are designed to produce biaxial orientation in the blown articles and are commonly used for PET bottles for carbonated drinks. The stretch blow moulding technique is now being developed for PP [26]. The main advantages achieved by biaxial orientation are discussed in Section 4.3.7. Stretch blow moulded PP containers have some important advantages. They can be sterilised at higher temperatures than those of competitive materials and are not prone to environmental stress cracking. OPP containers can be hot filled at 90 °C compared to 60 °C for the PET equivalent. A possible opportunity includes the replacement of PVC in non-carbonated water bottles.

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6.5 Thermoforming and Vacuum Forming The amount of PP presently used for thermoforming is very small and the reasons are similar to those discussed in Section 6.4. The low elasticity of semi-crystalline polymers at forming temperatures makes vacuum forming of the PP very difficult. More heat is required to make it formable and the cooling time is longer. Its low melt strength means that it is prone to excessive sagging when heated above its melting point. It is difficult to heat PP sheet evenly because it tends to buckle when warmed. The problem lies in the way the tensile strength of PP varies with temperature, which is different to the behaviour of amorphous materials. With PP, there is an extremely rapid change in tensile strength just below the crystalline melting temperature. The temperature window within which thermoforming is possible lies within this range. The material must on the one hand be soft enough to be deformed by the air pressure of about 0.6 MPa but on the other hand, it must retain sufficient strength for thermoforming. This temperature window is only about 5 °C in the case of PP. In contrast, the change in the mechanical properties of polystyrene is more gradual and it remains processible within a window of 50 °C. Increased die swell makes accurate extrusion profiling of PP difficult. Despite the inherent difficulties in thermoforming PP, the advances in the material and the machinery are making this a rapid growth area for PP [5, 27]. The good oxygen barrier properties of stretched PP, high softening point, good chemical resistance, low cost, excellent toughness, good stress crack resistance and light weight of the material are the advantageous properties which could be used to replace thermoformed polystyrene. PP containers can be hot filled and microwaved [28]. PP can be thermoformed in two distinct phases: in the solid phase below its crystalline melting point and in the melt phase above its crystalline melting point. Both techniques have advantages and disadvantages. Solid phase forming of PP is carried out in the temperature range of 155–165 °C. Under these conditions, the sheet is relatively strong and resistant to sagging. The disadvantage is that the vacuum may not be sufficient for forming so plug assistance will be required. The pressure required for forming is highly dependent on the forming temperature and a few degrees difference in temperature may make quite a large difference in pressure required. Solid phase thermoforming of PP requires specialised machinery. This uses high air pressure to form sheets which have been heated to a temperature below their melting point. Furthermore, the cut-in-place design is essential for PP. The part needs excessive cooling and can deform excessively. The cut-in-place technique separates excess material as soon as possible, thus reducing warpage. Melt phase forming of PP is carried out in the range of 175–185 °C and requires grades of PP with high melt strength. Conventional grades have insufficient strength to be formed successfully by this method. Sag resistance is related to the extensional viscosity of the material and to the molecular weight of the material. Long chain branching is preferred to achieve high melt strength. Grades with long chain branching 86

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(which offers improved sag resistance and drawability) are suitable for thermoforming of large parts. The residual stress level in melt phase forming of PP is quite low (less tendency to warp and distort on heating than with solid phase forming) and there is reduced wall thickness variation. Further, the variation in forming pressure with temperature is quite low during melt phase forming of PP. Crystallinity, the orientation of the chains and the clarity depend on the forming temperature. The shrinkage of PP thermoformed articles is complex and is a function of sheet crystallinity, orientation, thermoforming conditions and orientation arising from stretching of the sheet. Hence, a reliable figure for shrinkage is not possible. The specific heat of PP is very much higher than for the amorphous plastics. This is because with PP the glass transition temperature of the amorphous region has already been exceeded at room temperature and at the thermoforming temperature the crystalline region begins to melt. PP requires 2 to 2.5 times more heat than PS to raise its temperature from the ambient to the thermoforming temperature. Thermoforming moulds for PP should be made from material with high thermal conductivity such as aluminium. Furthermore, the moulds should be efficiently cooled. PP for thermoforming typically has a MFR in the range of 1.8 to 8.0. Suitable homopolymers, random and copolymer grades could be used offering their relative advantages and disadvantages [4]. Nucleated homopolymers are preferred. Higher stiffness of the homopolymer grades allows reduced wall thickness, thereby, achieving cost savings. 6.6 Calendering Controlled rheology grades of PP have been used in the production of calendered PP films [29]. Compared with conventional reactor grade material, the reduced molecular weight distribution of PP provides better flow and a slow rate of crystallisation, giving better processibility and films free from surface defects. However, the optical quality of the calendered film does not reach that of extruded film. The poor thermal oxidation resistance of PP in the presence of air is the major impediment in use of PP as a calendering material. 6.7 Rotational Moulding PP can be rotational moulded as an alternative to PA, polyester and crosslinked PE for such rotational mouldings as automotive parts, toys, food handling systems and chemical tanks [30]. It also provides higher heat/chemical resistance and stiffness than normally achieved by rotational mouldable LLDPE.

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7 Post Processing and Assembly Moulded PP artefacts may be subjected to different operations, such as joining using welding or gluing, machining or decorating. The issues of post processing and assembly are very crucial and, in many cases, make a difference between the success of the product or failure. Like other plastics, PP has its own advantages and disadvantages in terms of post processing and assembly and is more suitable to certain methods. The suitability of common post processing methods for PP has been compared with those for other plastics in Table 25. From the table, it may be seen that PP is a difficult plastic to bond, paint and plate. However, it is easy to machine. Suitable welding techniques are friction, vibration, hot gas and hot plate welding. Ultrasonic welding is difficult while radio frequency welding is not possible. 7.1 Joining 7.1.1 Welding Like all other plastics, PP can be welded by application of heat and pressure. Ultrasonic, heated tool, hot gas, vibration or spin welding techniques can be used with PP. The choice of the welding technique depends on the area of joint, and the number and the shape of the parts to be joined. The strength of the welding depends on the geometry of the component, welding method and welding parameters. The selection of the welding method and the design of the component should be carefully considered at the design stage. Table 25 Comparison of post processing properties of PP with other competitive materials on a scale of 0 to 9 where 0 represents an unfavourable property while 9 represents a favourable property [2] Property PP LDPE HDPE HIPS PVC ABS PA 6 PET Bonding 5 5 5 9 9 8 5 6 Machining 8 5 8 4 5 9 8 4 Painting 4 1 4 8 6 8 5 7 Plating 0 0 0 1 0 6 0 0 Friction welding 9 8 8 8 8 8 9 8 Vibration 9 8 8 8 8 8 9 8 welding Hot gas welding 9 7 7 7 9 6 7 2 Hot plate 9 8 8 8 9 8 3 7 welding Ultrasonic 4 2 5 7 5 9 9 4 welding Radio frequency 0 0 0 0 8 0 0 0 welding

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7.1.1.1 Heated Tool Welding In heated tool welding, a heated platen is used to melt the joining surfaces of two thermoplastic parts. The two interfaces are then brought into contact and allowed to cool to provide a hermetic seal. The main variations of heated tool welding are hot plate, socket, butt fusion and saddle. The technique may be used to weld most thermoplastics. If the correct procedures are used, the weld strength may be equal to the tensile strength of the parent material. It is a relatively slow welding process; weld times can range from 10 seconds for small components to 60 minutes for components with large weld area. The main welding parameters are the temperature of the hot plate, the heating time, the welding pressure and the welding time. For welding PP pipes, socket welding is used where a heated pipe is inserted into a heated fitting. It is reported to be the preferred method for joining PP pipes for domestic installations. Electrofusion fittings are used in exceptional circumstances where socket welding is not possible. Heated element butt fusion is the most common method of joining PP pipes for industrial applications. The cut surfaces of the pipes are planed, heated in contact with a heated element, and then joined and cooled under pressure. Valves and other armatures can be incorporated by flanging, welding or threading. 7.1.1.2 Hot Gas Welding In hot gas welding, a stream of hot gas, typically air, is directed towards the joint between the two thermoplastics parts to be joined, where it softens or melts the polymer. A thermoplastic filler rod may also be used to form a weld. Hot gas welding is a manual process and the quality of the bond depends on the skills of the operator. The adjustable parameters are gas type, flow rate and temperature, and the angle of the filler rod to the parts being welded. The variations of the hot gas welding technique are hand welding, speed welding and extrusion welding. 7.1.1.3 Friction and Vibration Welding In friction welding, the heat needed to melt the thermoplastic material is generated by pressing one of the parts to be joined against the other and rapidly vibrating it. If the correct procedures are used, weld strength could be equal to the tensile strength of the parent material. The heat generated by the friction melts the material at the interface between 2–3 seconds. Further time is required to align the parts and to hold them together under pressure until a solid bond is formed. The main welding parameters are rotational speed, friction pressure, forge pressure, displacement and duration. The main variations of frictional welding are linear vibration welding, orbital friction welding (circular movement), spin welding and angular friction welding.

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7.1.1.4 Ultrasonic Welding In ultrasonic welding, the high frequency sound energy is used to soften or melt the thermoplastic at the joint. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20 to 40 kHz. It is a very fast welding process and can be easily automated. The main advantages are fast, strong, clean and reliable welds. Due to the semi-crystalline nature of PP with a sharp melting point, PP is not as easily weldable as PS. However, good results may be obtained by near field ultrasonic welding. PP does not transmit ultrasonic waves well enough to weld it using far field ultrasonic welding. 7.1.1.5 Radio Frequency Welding Radio frequency welding is also known as dielectric welding or high frequency welding. Resulting bonds can be as strong as the original material. Radio frequency welding requires high dielectric properties of the material to achieve the heating by high frequency alternating current. PP has a low power factor and is not suitable for radio frequency welding. 7.1.1.6 Other Welding Techniques In the laser welding technique, a high intensity laser beam is used to increase the temperature at the joint interface of thermoplastic material to or above the melting point. In IR welding, the joining surfaces are heated to the melting temperature using IR radiation. When melting begins, the parts are brought together under pressure, forming the weld on cooling. IR joining systems have been tried on PP, and joints with a welding factor greater than 0.9 (1 represents the ideal) at –40 °C testing temperature have been reported. Microwave welding uses high frequency electromagnetic radiation to heat material located at the joint surface. The generated heat melts thermoplastic materials at the joint surface, producing weld on cooling. The use of these welding techniques is not yet well established for PP. 7.1.2 Solvent Bonding The high solvent resistance of PP prevents the solvation of the moulding surface, and, therefore, solvent bonding is not possible.

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7.1.3 Adhesive Gluing For adhesive gluing, the surfaces to be joined must be adequately treated to improve adhesion. Flaming, corona discharge and primer are used for surface treatment. The methods for surface treatment are similar to those used for decorating and printing (Section 7.3). Adhesives used for bonding of PP are contact adhesives based on polyurethanes or synthetic rubber, two-pack adhesives based on epoxy resins, polyurethanes, etc., vinyl acetate copolymer hot melt adhesives, pressure-sensitive adhesives and polyurethane contact cements. The strength of a bonded joint is highly dependent on its geometry and the adhesive used. The factors affecting the choice of adhesive are resistance to various chemicals, moisture, high temperature and vibrations. Testing of the bonded joint is necessary to ensure optimum bonding is achieved. 7.1.4 Sealability Sealability is quite an important factor in film packaging applications. Copolymers of PP with ethylene offer improved sealability over homopolymers. However, copolymers of PP with alpha-olefins such as butene-1 are much more effective with a melting point of 115 °C and an approximate sealing temperature of 95 °C with a hot tack range of 90– 140 °C. 7.2 Assembly and Fabrication 7.2.1 Machining PP is a relatively soft material and is easy to machine. It can be easily machined with carbide-tipped or high-speed steel tools on conventional metalworking or woodworking machines. High cutting speed or low feed rates are required for optimal finishes. The heat generated can be dissipated by cooling with fluids such as water and cutting emulsions. Using sharp tools, PP can be machined using sawing, turning, milling, planing, chiseling and drilling. Reaming, grinding, filling and polishing are less suitable processes. 7.2.2 Snap-fit Joints In snap-fit fastening, two parts are joined through an interlocking configuration that is moulded into the parts. A protrusion on one part is briefly deflected during joining to catch in a depression or undercut moulded into the other part. The force required for joining varies depending on the part design. Depending on the locking angle, these connections are either easily released or permanent. The high level of elasticity of PP makes the material suitable for snap-fit joints. Snap-fit joints require more attention to the engineering design and can fail during assembly or use if not designed properly. They are commonly used in assembly of tools, housings, electronic components, 92

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medical devices, etc. The snap-fit joints may be designed to be stress free during use. If not stress free during use, it will be subjected to creep and stress relaxation. 7.2.3 Mechanical Fastening Two plastic parts may be joined using mechanical fasteners such as screws and bolts, self-tapping screws or inserts. The fasteners may be reusable, if required, and can be moulded in metal or plastic. Plastic fasteners are light in weight, corrosion resistant and impact resistant. Use of fasteners requires that the plastic component can withstand the stresses generated during insertion and the stresses left in the moulding near the fasteners. The high stress produced near the fasteners can make the parts more susceptible to chemical and thermal attack. The design stress, residual stress and assembly stresses in the component should not exceed the maximum allowable stress for the plastic. To reduce the stress concentration, inserts with smooth rounded surfaces produce less stresses than knurled inserts. In addition, knit lines should not be located in areas of the part that are being inserted. 7.3 Decorating PP can be decorated by a number of available techniques. The decoration may be in the form of a coating (e.g., printing, painting, metallising and electroplating), by appliques (e.g., surface coverings or adhesive films), or by impression (e.g., hot stamping). PP parts can be painted to provide colour for colour matching, finish, or to cover surface imperfections. Paints and coatings can provide enhanced properties such as improved chemical, abrasion or weathering resistance and electrical conductivity. Surface pretreatment by flaming using a very oxygen rich burner flame or application of electrical processes is usually required to improve the adhesion of printing inks and coating to PP [31]. However, use of pretreatment, in some cases, may not be required when primers are being used. A wide variety of equipment and devices are available for pretreatment of PP. Corona discharge is preferred in the case of film. Pretreatment of the film may be required before printing if the film has been stored for a long time. Chemical pretreatment by dipping in a chromosulphuric acid bath is also possible. UV irradiation treatment of a UV primer is also possible. It is reported as a reliable, efficient and economical method for obtaining an improved level of adhesion of paint on PP, even when the parts have complicated shapes [32]. The coating generally leads to a reduction in the impact strength. 7.3.1 Printability and Paintability Printing processes suitable for PP include pad transfer, screen printing, laser printing, dyeing, and fill and wipe. Due to the non polar nature of PP, the choice of primer or surface treatment is very crucial for printability and paintability.

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Elastomer-modified grades of PP are more suitable for printing, painting and other decoration techniques since etching of the rubber particles provides the location for bonding of the primer to the surface of PP artefact. PP bumpers made with an EniChem grade of EPR are said [33] to have paint cohesion 20 times higher than unmodified PP or a reactor-modified PP, and over three times better than PP/EPR grades available now. Baxenden Chemical Ltd., has a two-component PU adhesive system for print lamination with PP. This range consists of clear, fast-drying adhesives for the lamination of biaxially oriented and treated PP film to printed paper and board to give durable and glossy sheet or real laminates. The use of transparent lacquers for printing has also been reported. 7.3.2 Metallising and Electroplating Plastic surfaces can be given a metallic appearance using metallising and electroplating. In vacuum metallising, the metal is heated in a vacuum chamber to its vaporisation point, which is lower than the melt temperature of plastics. The metal vapour than deposits on the cooler plastic surface. In electroplating, an electric current is used to deposit metals from a metal salt solution onto a plastic rendered conductive by electroless plating. Other techniques for depositing metal layers on plastic substrates are flame/arc spraying and sputtering. Metallising requires a clean surface, free from mould release agents. Primers or pre-cleaners can be used to improve the adhesion of the metal to the polymer surface. The parts should have uniform walls and mouldings should be stress free. Normal commercial metallising equipment is suitable for metallising of plastic parts. 7.3.3 Appliques Appliques or surface coverings can be applied using heat or pressure by different methods, e.g., hot stamping, decals, hot transfer, water transfer, in-mould decorating. Decals are decorations or labels printed on carriers such as paper or plastic with a pressure-sensitive adhesives. The standard requirements for applying surface coverings are a clean surface free of mould release agents with minimal sink marks and projections. The adhesive backing must be compatible with the PP and should not cause cracking or crazing.

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8 Causes of Failure Like other plastics, PP is not free from product failure during service. In a recent study [34], the main reasons for failure in plastics, in general, are stated in decreasing order of failures as environmental stress cracking, dynamic fatigue, static notch failure, creep related failure, chemical attack, UV attack, heat degradation and wear/abrasion. Environmental stress cracking, which is the largest reason of failure in other plastics, is not very significant for PP. PP is virtually free from the problems of environmental stress cracking. Another major reason for the failure of plastic products is dynamic fatigue. However, being a semi-crystalline material, PP offers better fatigue resistance than amorphous thermoplastics. It should be noted that dynamic fatigue strength of PP is lower than that of engineering plastics. Impact-related failure in the absence or presence of notches is one of the biggest reasons, if not the biggest, of failure in PP artefacts. Impact-related failures arise from the choice of the incorrect grade of material for the service temperature, accidental mixing of different grades of material during processing, and poor product design causing stress concentration effects due to presence of sharp corners and notches. Like other semi-crystalline materials, PP undergoes significant creep. Elastomermodified grades are more prone to creep. PP offers good chemical resistance in general. However, it is strongly recommended that the effect of chemicals at service temperature should be checked. PP is not greatly used for wear and abrasive applications. Consequently, the examples of PP with failure related to wear are extremely limited. UV and heat degradation is another reason for short product life or catastrophic failure in PP artefacts. Correct choice of the material and proper understanding of the service environment is necessary. The UV resistance of PP depends on the thickness of the artefact. Flame retardant grades have poor UV resistance. Another cause of failure in PP artefacts is the poor quality of the weld lines. Weld lines are inherent weak positions due to the poor fusion of the material at molecular level. However, the strength of welds can be maximised by favourable moulding conditions such as sufficiently high melt temperature, high tool temperature, heated metal inserts and fast injection times. Similarly to other plastics, more attention should be paid to filled or reinforced grades of PP. PP artefacts are prone to severe shrinkage and warpage. Sensible product design and processing parameters are the solution to the problem. If batch-to-batch variation in the

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shrinkage and warpage is observed at similar moulding conditions, the material should be characterised for rheological behaviour. Product failure analysis is a complex subject. It is though comparatively easy to establish the main reason of product failure. However, when changes in the product are required to avoid the problem, several possibilities exist: choice of material, type of grade, product design, mould design, processing parameters and service conditions. Any change in one aspect or property of the material would, on many occasions, lead to compromise on another aspect of the product performance. Hence, careful consideration of different possibilities is required.

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9 Product Development Issues The important factors in the use of PP have been discussed throughout the text. However, the purpose of this section is to compile them on the basis of their relevance at different stages of the life of the PP artefacts. Further information about these sensitive issues can be obtained from the relevant sections of this book. The issues relating to a PP article during its lifetime can be divided into the following categories: material selection, design, processing and post assembly, performance in service and environmental and legislative issues. 9.1 Material Selection Transparency: Only contact transparency. Consider using PS, PMMA, TPX or PC if transparency is critical. PP random copolymers have better transparency. Nucleating agents further improve clarity. Material from different suppliers may have different optical properties. Transparency depends on the processing conditions and crystallisation as well. Heat deflection temperature: Good maximum operating temperature without stresses. However, it is severely limited in the presence of stresses. Consider using engineering plastics, e.g., acetal, PPE, polysulphone or polycarbonate. Impact properties: Significant cause of in-service failure. High brittle temperature. For impact demanding applications or sub-ambient applications, consider block copolymerised or elastomer-modified grades but improvement in impact properties at the expense of stiffness. Avoid accidental mixing of homopolymer and copolymer grades. Wear and friction: Not very suitable for wear and friction applications. Consider PA or acetals. 9.2 Design Warpage: Serious problem. May cause high rejection rate during moulding. Consider warpage at the design stage. Warpage reduced in controlled rheology or metallocenecatalysed grades. Consider computer-based flow simulation at the mould design level. Weld lines: Points of inherent weakness and lead to many failure problems. Particularly important in reinforced grades, particularly with glass fibre. Use process conditions or mould modification to improve weld strength. Creep: Significant creep in comparison to amorphous plastics, in general. Creep particularly important at high temperatures. Filled grades have lower creep. Consider HDPE or other engineering plastics.

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Dynamic fatigue: Low fatigue strength in comparison to engineering plastics. Temperature, stress concentration and stress frequency quite important. Always compare fatigue strength with short-term strength to see the full affect of fatigue. 9.3 Processing and Post Assembly Injection moulding: Processing cost high in comparison to amorphous materials. Mould cooling critical. Warpage and dimensional tolerances difficult. Extrusion: Requires extra cooling. Lower output in comparison to polystyrene. Thermoforming: Difficult. Narrow processing range, high forming pressure and extra cooling required. Specialised machinery might be required. Blow moulding: Difficult control. Melt strength critical. Calendering: Difficult. Thermal degradation of PP. Dimensional stability: Critical due to warpage and shrinkage. Consider processing parameters and mould design. Consider controlled rheology or metallocene-catalysed grades. Welding: Suitable welding techniques are friction, vibration, hot gas and hot plate welding. Ultrasonic welding difficult while radio frequency welding not possible. Adhesive gluing: Difficult to bond plastic, surface preparation very crucial. Proper selection of glue necessary. Consider improved design to avoid the need for gluing or use welding. Decoration: Difficult to paint and metallise/electroplate. Surface preparation very important. 9.4 Performance in Service Thermal oxidation and heat stability: Poor thermal oxidation and heat stability. Consider using thermally stabilised grades. Weather resistance: Poor weather or UV resistance. For short-term use, use UV stabilised grades. For long-term outdoor usage, choose black colour material. Significant cause of in-service failure. Fire resistance: Poor fire resistance. Fire retarded grades can offer up to V-0 rating. Heat resistance very critical. Consider materials which are inherently fire retarded, e.g., PVC.

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Medical sterilisation: Sterilisation possible using steam or gamma radiation. Careful selection of material grade necessary. Transparency requirements may contradict those for high-temperature resistance. Microwave applications: Use in disposable microwave containers possible, complete knowledge of in-service temperatures required. Food applications: Check raw material compliance with various national and international legislation. Pay special attention to elastomer-modified grades. Fire retarded grades are not suitable. Consider organoleptic properties which might be a problem with controlled rheology grades. Consider using metallocene-catalysed grades. Chemical resistance: Like dissolves like and vice versa. Environmental stress cracking: Very good environmental stress cracking resistance. No known environmental stress cracking agent. Be careful with hot detergents.

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References 1.

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2.

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21. J. Gosden, Proceedings of Polypropylene in Automotive Applications, Rapra Technology Limited, Shawbury, UK, 1992, Paper 15. 22. D.V. Rosato, D.V. Rosato and M.G. Rosato, Injection Molding Handbook, Third Edition, Kluwer Academic Publishers, 2001. 23. H.J. Yoo and D. Done, Proceedings of Antec ’92, Detroit, Michigan, USA, 1992, 569. 24. R.J. Koopmans, Proceedings of Antec ’97, Toronto, Ontario, Canada, 1997, 1006. 25. British Plastics and Rubber, July/August 1997, 16. 26. Plastics Flash, 1993, 28, 259, 44. 27. European Plastics News, 1996, 23, 6, 22. 28. T.M. Pontiff, Proceedings of Antec ’93, New Orleans, Louisiana, USA, 1993, 3047. 29. A. Wolfsberger, Macplas International, November 1993, 94. 30. Modern Plastics International, 1993, 23, 2, 46. 31. K.F. Lindsay, Modern Plastics International, 1992, 22, 4, 22. 32. F. Polato, Macplas International, February 1992, 70. 33. Modern Plastics International, 1997, 27, 1, 110. 34. D.C. Wright, Environmental Stress Cracking of Plastics, Rapra Technology Limited, Shawbury, UK, 1996. 35. D.C. Wright, Failure of Plastics and Rubber Products, Rapra Technology Limited, Shawbury, UK, 2001.

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Abbreviations and Acronyms ABS BHT BOPP CA CAB CPVC CR DMTA DSC EMI EPDM EPR ESC ESD EVA FDA HDPE HDT HIPS IR ISO L/D LDPE LLDPE LOI MFR MFI Mn MOT Mw MWD PA PC PE PET PMMA PP PPE PS PTFE PU PVC PVDF RFI

Acrylonitrile butadiene styrene Butylated hydroxytoluene Biaxially oriented polypropylene Cellulose acetate Cellulose acetate butyrate Chlorinated PVC Controlled rheology Dynamic mechanical thermal analysis Differential scanning calorimetry Electromagnetic interference Ethylene propylene diene terpolymer Ethylene propylene copolymer Environmental stress cracking Electrostatic dissipation Ethylene vinyl acetate copolymer Food and Drug Administration (USA) High density polyethylene Heat deflection temperature High impact polystyrene Infrared International Standards Organization Length/diameter Low density PE Linear low density polyethylene Limiting oxygen index Melt flow rate Melt flow index Number-average molecular weight Maximum continuous operating temperature Weight-average molecular weight Molecular weight distribution Polyamide Polycarbonate Polyethylene Polyethylene terephthalate Polymethylmethacrylate Polypropylene Polyphenylene ether Polystyrene Polytetrafluoroethylene Polyurethane Polyvinyl chloride Polyvinylidene fluoride Radio frequency interference 103

Practical Guide to Polypropylene

RM RR SAN SD TGA TPX UL UPVC UV VOC XRF

Rockwell M Rockwell R Styrene-acrylonitrile copolymer Shore Durometer Thermogravimetric analysis Polymethylpentene Underwriters’ Laboratories Unplasticised PVC Ultraviolet Volatile organic compound X-ray fluorescence spectroscopy

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