Update on Syntactic Foams
Bibin John C.P. Reghunadhan Nair
Smithers Rapra Update
Update on Syntactic Foams
Bibin Jo...
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Update on Syntactic Foams
Bibin John C.P. Reghunadhan Nair
Smithers Rapra Update
Update on Syntactic Foams
Bibin John C.P. Reghunadhan Nair
iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net
First Published in 2010 by
iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK
©2010, Smithers Rapra
All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder.
A catalogue record for this book is available from the British Library.
Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.
ISBN: 978-1-84735-120-3 (Hardback) 978-1-84735-121-0 (ebook)
Typeset by Wordsworth Prepress, India Printed and bound by Lightning Source Inc.
C
ontents
Preface ........................................................................................vii 1
Introduction ......................................................................... 1 1.1
Syntactic Foams: Definition ........................................ 1
1.2
Matrices used in Syntactic Foams ............................... 2
1.3
Microspheres in Syntactic Foams ................................ 2
1.4
Structure of Syntactic Foams ...................................... 5
1.5
General Methods for Preparation of Syntactic Foams.......................................................... 7
1.6
Properties of Syntactic Foams ................................... 11 1.6.1
Comparison with Conventional Foams ........ 12
1.6.2
Property Tailoring ........................................ 12
1.6.3
Mechanical Properties .................................. 13
1.6.4
Moisture Absorption.................................... 14
1.6.5
Isotropic Nature........................................... 15
1.6.6
Dielectric Properties ..................................... 15
1.6.7 Thermal Properties ....................................16 1.7 Factors Affecting the Performance of Syntactic Foams........................................................ 16 1.7.1 Volume Fraction of Filler ............................. 16 1.7.2 Nature of Filler ............................................ 17 1.7.3 Nature of Polymer Matrix ........................... 17 1.7.4 Quality of the Filler-matrix Interface ........... 18 1.7.5 Method of Processing................................... 18 References .......................................................................... 18 iii
Update on Syntactic Foams 2
Types of Syntactic Foams ................................................... 23 2.1
Epoxy Syntactic Foams............................................. 23
2.2
Phenolic Syntactic Foams ......................................... 31
2.3
Cyanate Ester Syntactic Foams ................................. 39
2.4
Polyimide Syntactic Foams ....................................... 43
2.5
Polyurethane Syntactic Foams .................................. 51
2.6
Polyester Syntactic Foams ......................................... 53
2.7
Silicone Syntactic Foams ........................................... 54
2.8
Elastomeric Syntactic Foams .................................... 54
2.9
Nylon Syntactic Foams ............................................ 55
2.10
Polybutadiene Syntactic Foams ................................. 55
2.11
Polypropylene Syntactic Foams ................................ 56
2.12
Carbon–carbon Syntactic Foams ............................. 56
References ............................................................................ 58 3
Recent Developments in the Field of Syntactic Foams ........ 63 3.1
Fibre-reinforced Syntactic Foams ............................. 63
3.2
Nanoclay-incorporated Syntactic Foams ................. 76
3.3
Rubberised Syntactic Foams ..................................... 82
3.4
Functionally Graded Syntactic Foams (FGSF) ........... 83
3.5
Syntactic Foam Core Sandwich Composites ............. 86
3.6
Cement-based Syntactic Foams ................................. 89
References .......................................................................... 91 4
iv
Applications of Syntactic Foams ......................................... 95 4.1
Syntactic Foams in Buoyancy Applications ............... 95
4.2
Syntactic Foam as Thermal Insulation Material ........ 99
4.3
Syntactic Foams in the Aerospace Industry ............ 100
4.4
Syntactic Foams in Radomes ................................. 102
4.5
Syntactic Foams in the Sports Industry ................... 103
4.6
Syntactic Foams for Furniture Applications ............ 104
Contents 4.7
Syntactic Foams as Synthetic Marble ...................... 105
4.8
Syntactic Foam for Air-equivalent Solid Backing ... 105
4.9
Shape Memory Syntactic Foams ............................. 106
4.10
Syntactic Foam Plug Assist Materials ..................... 107
4.11
Expandable Graphic Art Printing Media using Syntactic Foam ...................................................... 108
4.12
Syntactic Foams in Underwater Sound Transducers ........................................................... 108
4.13
Syntactic Foams in the Airbus ................................ 109
4.14
Miscellaneous Applications .................................... 109
References .......................................................................... 111 Abbreviations ........................................................................... 117 Index ........................................................................................ 119
v
P
reface
Syntactic foams, which are composite materials obtained by dispersing hollow spheres in a matrix, have emerged as an attractive material for diverse applications. This field has evinced a lot of interest in high tech areas. Though several research papers have emerged in this field, there is practically no book or report that compiles the developments in this area. In this context, we thought it would be appropriate to write a book compiling the important works carried out in this field. The book is divided into four chapters. The first chapter gives an introduction to syntactic foams. The basics of syntactic foams, matrix systems used in syntactic foams, different types of microballoons, structure of syntactic foams, general methods of preparation and properties of syntactic foams are covered in this chapter. Chapter 2 deals with syntactic foams based on different types of resin systems such as cyanate, epoxies, polyimides and so on. Chapter 3 focusses on recent advances in the field of syntactic foams like fibrereinforced syntactic foams, nanoclay-reinforced syntactic foams, rubberised syntactic foams, functionally graded syntactic foams, syntactic foam core sandwich composites and cement-based syntactic foams. The applications of syntactic foams in different fields are detailed in Chapter 4. The applications of some commercial grades of syntactic foams are presented in this chapter to give the readers a better understanding of the importance of these materials. We hope that this book will be a useful guide and reference material to those working in this field. We thank iSmithers, UK, for publishing this book. We thank the Director of the Vikram Sarabhai Space Centre, Thiruvananthapuram for giving permission to publish this book. We also thank our family members for their constant support during the preparation of this book. vii
1
Introduction
1.1 Syntactic Foams: Definition Lightweight materials with high specific strength are essential in aerospace, marine and other structural applications. The commonest method to produce such a material is by introducing a lightweight material in a matrix. This concept forms the basis of syntactic foams: hollow microspheres are incorporated into a matrix [1–2]. As per the definition from the American Society for Testing and Materials (ASTM), syntactic foam is a ‘material consisting of hollow sphere fillers in a resin matrix’. The term ‘syntactic foam’ was introduced in the 1960s [3]. ‘Syntactic’ is derived from the Greek word ‘syntaktikos’, meaning ‘to arrange together’ [4]. The term ‘foam’ is used because of the cellular nature of the material. Syntactic foams are also known as ‘foam composites’ because the hollow microspheres can be viewed as reinforcements in a matrix [5]. This type of material was previously used in the marine industry, providing buoyancy for subsea apparatus such as submersible vehicles and oceanographic equipment. Nowadays, syntactic foams find many applications ranging from tea cups to the limits of space travel. In syntactic foams, the matrix is reinforced with hollow spherical particles which have a controlled systematic arrangement in the matrix. Incorporating hollow particles having a lower density compared with binder material allows for the manufacture of lightweight materials with the increase of filler content. Thus, syntactic foam with a filler density that is lower compared with the binder can be considered to be a special type of particulate-filled polymer composite [6–8]. Syntactic foams are categorised as physical foams because the matrix is not foamed chemically but instead the gas-containing
1
Update on Syntactic Foams particles are filled mechanically into the matrix. They have the real advantage of being fabricated over a large density range and possess useful properties that can be tailored for specific applications [9].
1.2 Matrices used in Syntactic Foams The matrices used in syntactic foam include polymers, metals or ceramics. In this review, we are confining the discussion to polymeric matrix-based syntactic foams. Thermoplastic and thermosetting polymers have been employed to process syntactic foams. The important thermosetting resins used are epoxies, phenolics, cyanate esters, bismaleimides, unsaturated polyesters, and polyurethanes. Examples of the thermoplastic resin matrices used include polyethylene, polypropylene, polystyrene, and Nylons. Syntactic foams based on various resin systems are detailed in Chapter 2. Syntactic foams are mainly prepared by using thermosetting matrices because of the favourable processing conditions, i.e., avoiding breakage by gently blending the hollow microspheres with thermoset precursors of very low viscosity. From the processing viewpoint, thermosetting syntactic foams have many advantages compared with thermoplastic ones. For example, thermosetting syntactic foams can be processed at much lower temperatures compared with thermoplastic syntactic foams, thereby reducing the material and energy costs for processing. Also, thermosetting resins have less solvent sensitivity and are not negatively affected by cleaning solutions [10]. Nevertheless, some attempts have been made to process syntactic foams with a thermoplastic matrix by using a solvent, or even by using a twin-screw extruder [11]. The important advantage associated with a thermoplastic matrix is the higher toughness compared with thermosets.
1.3 Microspheres in Syntactic Foams Hollow microspheres give the syntactic foam its low density, high specific strength, and low moisture absorption. Microspheres may comprise glass, polymer, carbon, ceramic, or even metal [12]. Other terminologies are used in the literature to describe microspheres
2
Introduction (e.g., microballoons, cenospheres). All these terms are intermittently used throughout this review to indicate hollow microspheres. The microspheres have a burst pressure sufficient to withstand the forces imposed upon them during the formulation, mixing and dispensing processes. The main processing advantage of microspheres is that the viscosity of the systems with spherical fillers is always less than that of systems with fillers of any other shape [13]. Properties such as high temperature resistance, good strength-to-weight ratios, clean surface chemistry, low thermal conductivity, low dielectric constant, and low dissipation factor make microballoons an important reinforcing material in these composites. Microspheres are characterised by their particle size, wall thickness and density. In general, microspheres used in syntactic foams have a diameter of 1–50 μm, wall thickness of 1–4 μm, bulk density of 70–500 kg/m3, and apparent density of 50–500 kg/m3. Hollow macrospheres (diameter, 1–100 mm) are also used as fillers in syntactic foams [13]. The wall thickness of a microballoon is related to a parameter termed the ‘radius ratio’, η which is defined as the ratio of the inner radius, ri, to the outer radius, ro, of the microballoons: η=
ri ro
where ri is the inner and ro is the outer radius of the microballoon, respectively. Increase in the radius ratio corresponds to a decrease in wall thickness, which leads to a decrease in true particle density of the microballoon. Therefore, microballoons having a higher radius ratio give rise to lower-density syntactic foams and vice versa [14]. The parameter η is related to the microballoon density (ρmb) and the material that the microballoon is made of, and is given by: ρmb η = 1– ρg
1 3
where ρg is the density of the microballoon material [15]. 3
Update on Syntactic Foams The most commonly used microspheres in syntactic foam are glass microspheres due to their high specific strength, regularity of the surface, good wetting characteristics, and low viscosity of the resin– microballoon mixture, as well as energy absorption properties, low cost and ease of fabrication [16–18]. Also, hollow glass particles have very low densities as compared with corresponding hollow metallic and ceramic particles [8]. In general, syntactic foams based on glass microspheres exhibit much better mechanical properties than those made with polymeric microspheres due to the substantial difference between the elasticities of glass and polymer. Glass microspheres can be made by heating tiny droplets of dissolved water glass in a process known as ‘ultrasonic spray pyrolysis’, and properties can be improved by using an acid treatment to remove some of the sodium [19]. Polymeric microballoons are commonly made up of epoxy resin, unsaturated polyester resin, silicone resin, phenolics, melamineformaldehyde, polyvinyl chloride, polypropylene, or polystyrene [12, 20]. As a general rule, these microspheres are produced by spraying low-viscosity solutions and melts [13]. Among the various polymeric microballoons, phenolic microballoons have been widely used for processing syntactic foams. The main advantage of phenolic microspheres over those of glass is their lower density. Another type of polymeric microballoons is based on polyvinylidene chloride (PVDC). These are known as ‘saran microspheres’, are mechanically tougher and have high resistance to organics. Ultra low-density resilient PVDC hollow microspheres can be premixed in polyester and stored for long periods. They also find application in urethane and epoxy foams [21]. Organic microspheres are also used in syntactic foams. They suffer from many limitations and their use is limited. Some organic microspheres can be converted into carbon microspheres. Usually, these spheres are derived from phenolic microspheres or carbon pitch spheres. Conversion of the organic spheres to carbon microspheres is usually accomplished by heating in an inert atmosphere to 800–1000 °C. This is sufficient to convert most of the organic material to carbon but, to achieve graphitisation, further heating to temperatures >2000 °C is
4
Introduction required. Carbon microspheres obtained from phenolic resins have densities of about 0.15 g/cm3, whereas those obtained from pitch sources have densities of 0.05–0.25 g/cm3 [20]. Materials made from carbon microspheres of low apparent density have a substantially lower thermal conductivity (0.049–0.064 W/(m.K) for densities of 200–300 kg/m3) than other syntactic foams [13]. Another type of microballoon known as shirasu microballoon is the yield of secondary popping expansion of volcanic ash sand particularly available in Kagoshima, Japan [22]. Fly ash in the form of hollow particles is also used in syntactic foams [20]. The Philadelphia Quartz Company (USA) produces ‘Q-Cel’ quartz microspheres which are mechanically very strong and very cheap (half the price of glass microspheres and one-third the price of phenolic microspheres). Metal, metal oxide and metal salt microspheres have also been used in syntactic foams [13].
1.4 Structure of Syntactic Foams Syntactic foams are usually tertiary systems because the matrix and gas-filled microspheres are usually composed of different materials. They are classified as two-phase systems and three-phase systems [16]. The close-packed arrangement of hollow microspheres in the matrix gives rise to two-phase syntactic foams. The two-phase structure is schematically shown in Figure 1.1(a) but this is only a hypothetical situation. During the processing of syntactic foams, air entrapment is possible, which leads to voids in the foam structure. In some other cases, a thin film of resin may surround the cluster of microballoons so that the resin cannot penetrate into the cluster, leaving an empty space in between the microspheres. If resin concentration is very low, the space in-between the microspheres is not wet, giving rise to voids. Sometimes, depending upon the application, voids are intentionally incorporated to obtain lower density. The existence of voids makes syntactic foams a three-phase system (unlike conventional polymer foams which are binary) [9, 23]. The three-phase structure is shown schematically in Figure 1.1(b). Thus, two-phase syntactic
5
Update on Syntactic Foams
Microballoons
(a)
Matrix resin
Void
Microballoons
(b)
Figure 1.1 Schematic representation of (a) two-phase and (b) three-phase structures of syntactic foam. Reproduced with permission from N. Gupta, C.S. Karthikeyan, S. Sankaran and Kishore, Materials Characterization, 1999, 43, 4, 273 ©1999, Elsevier [9] foams consist of hollow microspheres dispersed in a resin, whereas three-phase syntactic foams comprise hollow microspheres dispersed in a resin containing finely dispersed air bubbles [24, 25]. Two-phase syntactic foams offer good mechanical properties, but their electrical properties are limited by the lowest possible density for a given combination of matrix and filler. Moreover, three-phase syntactic foams
6
Introduction are limited as far with respect to mechanical properties. However, they possess low dielectric and loss coefficients compared with two-phase foams [23]. Two-phase syntactic foams are less moisture-absorbing due to their structure, whereas three-phase types absorb more moisture because of the presence of open-type voids [5]. The microstructure of syntactic foams can be observed using a scanning electron microscope (SEM) (Figure 1.2). The figure shows the SEM image of a glass-microballoon syntactic foam. Glass microballoons can be clearly seen as round particles embedded in the polymer matrix. Figure 1.2 also depicts matrix and voids, as marked.
1.5 General Methods for Preparation of Syntactic Foams The general methods for the preparation of conventional polymer foams require the use of at least two chemical constituents: one to decompose into a gas to form the bubbles, and one to form the
Microballoon
Void
Matrix
Figure 1.2 SEM image of a syntactic foam
7
Update on Syntactic Foams cell walls. Syntactic foams use ‘prefabricated’ microballoons or microspheres that are mechanically combined with a resin to form a composite material [4], i.e., syntactic foams can be processed by incorporating microballoons in a matrix. The commonest methods used to prepare syntactic foams differ from those used for preparation of other polymer composites. Hollow-glass, organic or carbon microspheres do not withstand high pressures. It is therefore practically impossible to extrude or injection-mould mixtures containing hollow microspheres. Various manufacturing techniques are available for processing syntactic foams. These vary from simple blending of the components to novel coating methods of the resin on the microsphere surface [20]. The microballoon concentration and matrix composition was found to be crucial for the ease of manufacturing syntactic foams [26]. The choice of process parameters (temperature, mixing time, addition sequence) is also a main challenge in the processing of syntactic foams [13]. The binder materials used in syntactic foams must have low viscosity, readily controlled gelation time, small exothermal effect during curing, low curing shrinkage, good adhesion and wettability to the filler as well as compatibility with modifiers and fillers (including diluents, plasticisers, dyes, flame retardants) [13]. A common method for the preparation of syntactic foams is by impregnation of microspheres in a resin solution. This method ensures uniform coating of each sphere by the resin. Homogeneous dispersion of the resin among the microspheres is therefore achieved. The main difficulty of this method lies in the need to entirely remove the solvent before the final curing. Several impregnation techniques are in common use. One method is to fill a mould with an appropriate amount of microspheres and then a pre-measured amount of resin solution is poured over it. The solution penetrates into the intersphere voids due to gravity, capillary forces and sometimes with the aid of vacuum or positive pressure. Final curing is after solvent removal. In another method, a measured amount of microspheres is introduced into a dilute resin solution. After removal of most of the solvent, the system has the consistency of wet sea-sand. This
8
Introduction ‘wet sand’ is then filled manually into a mould and cured. The ratio of polymer to microspheres in the dry powder is determined by solution concentration, time of contact, and temperature [20]. However, these methods have many drawbacks. These include potential environmental safety and health hazards of volatile solvents; formation of defects and structural non-uniformities when the solvent is being driven off by heat from the interior of the syntactic foam; difficulty in obtaining inter-batch reproducibility; and additional energy or cost associated with transporting the solvent-laden pasty mass to the moulding or curing equipment [18]. When the resin is available in a powdery form, a solid mixture of the hollow microspheres and the resin powder is prepared first. Then, a weighed quantity of the solid mixture is transferred to a mould of predetermined volume, pressed and cured [20]. In this method, the volume of microballoons used for each sample was constant, with the purpose of completely filling the mould volume with the closest packing possible [24]. An alternative approach to this method, when using thermoplastics, is to add microspheres to a melt of the thermoplastic in an extruder or a kneader. In this method, the microspheres are extremely lightweight and very fine in size, so many problems are encountered. The fine airborne dust characteristic of microspheres poses environmental problems. There is the possibility of breakage of the relatively fragile hollow microspheres during the dry mechanical/ frictional mixing and rough handling. Settling of the higher-density thermoplastic particles can cause non-uniformity in composition. Also, lower-density syntactic foams are difficult to prepare by this method. This is due to the inability to mix-in higher amounts, e.g., any amounts higher than approximately 15–20 wt% of microspheres in the resin matrix because of the very high volume-to-weight ratio of the microballoons compared with the resin [18]. A coating method has also been used for processing syntactic foams which consists of resin coating, vacuum filtration and polymer precipitation. In the coating step, a thin film of the polymeric solution is adsorbed onto the sphere surface. Polymer precipitation at this stage would lead to undesired agglomeration. The slurry is
9
Update on Syntactic Foams then vacuum-filtered and rinsed with liquids while on the filter. The purpose of this liquid rinsing step is to precipitate the polymer on the sphere surface and to simultaneously remove the solvent by leaching. Vacuum-drying of the coated spheres gives a moulding powder of discrete particles. By this method, uniform resin coating is achieved on the spheres, which subsequently leads to uniform resin distribution in moulded articles [27]. US Patent 4,595,623 [28] describes processing of syntactic foams from liquid thermosetting resins without using solvent. The use of liquid resin in polymeric foam materials has disadvantages. For example, liquid resin materials when used to form syntactic foams do not produce highly uniform distributions of the solid materials in the foam. The mixture is heated to allow the thermosetting resin to flow and wet the microballoons in the mixture. The mixture is then cured to set and crosslink the thermosetting resin to form the syntactic foam [10]. This method has been used for liquid polyesters and silicones. The development of extrudable epoxy syntactic foam systems has also been reported. Such systems offer a faster and more reliable way of making the models. Instead of laying on plaster by hand, resin is simply extruded onto a frame, allowed to harden, and machined to the correct shape by a numerically controlled machine tool. These types of syntactic foams find possible application in aircraft and other industries [29]. Easily removable, environmentally safe and low-density syntactic foams are prepared by mixing insoluble microballoons with a solution of water and/or alcohol-soluble polymer to produce a pourable slurry. Vacuum-filtering the slurry in varying degrees removes unwanted solvent and solute polymer. Finally, drying removes the residual solvent. The method is used when the solute polymers are non-toxic and soluble in environmentally safe solvents such as water or low-molecular-weight alcohols. The syntactic foams produced by this method are particularly useful in those applications where ease of removability is beneficial, and could find application
10
Introduction in: packaging of recoverable electronic components; drilling and mining applications; art works; the entertainment industry for special effects; manufacturing as temporary fixtures [30]. Spraying methods on a surface of a liquid resin stream and a microballoon stream meeting at some point while still in the air are also possible. This will produce discrete resin-coated particles which behave as a free-flowing material serving as a regular moulding powder [31]. Cornerstone Research Group, Incorporated have developed a unique process to produce strong (yet lightweight) syntactic foams. This process involves a low-stress resin removal system in which the excess resin is extracted from the syntactic material before the resin is cured. Along with the excess resin, microspheres that may have been broken during the mixing process are also removed. The resulting foam has low density and low void content, essential for high-integrity composites [32].
1.6 Properties of Syntactic Foams Syntactic foams possess many advantageous properties compared with conventional polymer foams and composites. The application of syntactic foams has centred around their ability to: reduce weight; increase stiffness; exhibit buoyancy; provide good nailability and screwability; reduce cost [33–34]. Syntactic foams are mechanically superior due to the rigidity of the microballoons. Hence, syntactic foams stand in-between conventional reinforced polymeric materials and foams with very high specific strengths [35–36]. Several other foamed materials offer better thermal insulation properties but none offer simultaneous high-strength properties [37]. They have excellent compressive and hydrostatic strength, good impact behaviour and damage tolerance compared with other closed-cell structured materials, which makes them very attractive for structural applications [33, 38–39]. They are found to be highly strain rate-sensitive, in contrast with metallic foams [14].
11
Update on Syntactic Foams
1.6.1 Comparison with Conventional Foams Blown and self-expanding foams develop a fairly random distribution of gas pockets of widely varying sizes and shapes. The porosity of syntactic foams can be much more closely controlled by careful selection and mixing of the microballoons with the matrix. The voids in syntactic foams are due to the cellular structure of the microsphere, so it is easier to predetermine and control the size and size distribution of the voids within syntactic foam [20, 31]. Ordinary foams are visibly porous, but syntactic foams have cells so small that the material looks like a homogeneous solid [4]. In conventional foams, cells are partially or fully connected whereas, in syntactic foams, the voids are enclosed within the walls of the microspheres and are therefore isolated from each other. The porosity of syntactic foams is typically at the microscopic level [40].
1.6.2 Property Tailoring The properties of syntactic foams can be tailored by changing a wide variety of parameters: matrix and microballoon materials, microballoon size, size distribution, wall thickness and volume fraction [25–26, 41–42]. The composition and density of syntactic foams can be predicted because these are fabricated by filling mechanically the gas-containing particles into the polymer matrix. Unlike most other foams, syntactic foam is a material whose density before curing is the same as that after curing [6]. The density of syntactic foam is proportional to the concentration of filler added. The limiting lower densities are determined by sphere density, the handling requirements of the uncured mix, and the property requirements of the final product. Sphere density also determines its efficiency as a bulking agent; the less dense the spheres, the lower the weight concentration required to achieve a specified composite density [34]. Syntactic foams can be designed and fabricated according to the physical and mechanical property requirements of the application. Depending upon the service conditions, the matrix resin can be
12
Introduction chosen from a wide range of thermosetting and thermoplastic resins. Similarly, microspheres of polymer, ceramic or metal can be chosen. Appropriate choice of fillers can produce composites with superior strength, damage tolerance, wear resistance and chemical resistance. The ability to cast a syntactic foam to any desired shape and the ease of machineability of the cured foam make syntactic foams very attractive structural materials [37].
1.6.3 Mechanical Properties Syntactic foams have excellent mechanical properties. They can absorb a considerable amount of energy under compressive loading [43–44]. The high specific compressive strength of syntactic foams derives from the resistance of microspheres to compressive loads [45–47]. The compressive properties of syntactic foams primarily depend on the properties of the microballoons, whereas the tensile properties depend on the matrix material used [26, 42]. Microspheres have an extremely strong structure and hence can withstand stresses very well. In the case of two-phase syntactic foams, properties are a function of the properties of the resin and the microballoon and their proportions. In the case of three-phase syntactic foams, the shape and content of voids play an important part. At lower resin concentration, failure is mainly that of structure collapse, whereas at higher resin concentration polymer properties become more important and more spheres break before the structure collapses [20]. A general stress–strain curve for the compression testing of syntactic foams is illustrated in Figure 1.3. A typical characteristic is a plateau region (marked as ‘densification region’ in the figure). This plateau region corresponds to energy absorption by the material in the process of compression. During the process of crushing, the hollow glass particles tend to fracture and expose the volume enclosed by them. This volume accommodates the material that is being compressed. Equilibrium between the new volume exposed and volume reduction due to compression is visible in the form of a plateau region in the stress-strain curve [37].
13
Update on Syntactic Foams
Densification region
40
Stress (MPa)
35 30 25 20 15
Elastic Deformation
10 5 0 0
0.05
0.1
0.15
Strain (mm/mm)
Figure 1.3 General stress-strain curve for the compression testing of a syntactic foam. Reproduced with permission from N. Gupta and E. Woldesenbet, Composite Structures, 2003, 61, 4, 311 ©2003, Elsevier [45]
1.6.4 Moisture Absorption The most significant property of syntactic foams is their low moisture absorption. Syntactic foams have completely closed cells and therefore absorb water to a lesser degree compared with foamed plastics in which the cells are open [12, 38, 45–46, 48]. The overall moisture absorption in syntactic foams is affected by many parameters. The most important ones are the nature of the resin–microsphere interface, and the concentration, size and strength of the microsphere. The chemical and physical properties of filler and binder are also decisive factors [49]. It is reported that for microballoon concentration <67 vol%, the water absorption in syntactic foam is independent of density, but above such concentration absorptivities increase rapidly due to loss in binder integrity and the appearance of cavities [5, 49]. Syntactic foams are found to absorb ~1% water by weight even after immersion in water at room temperature for long periods. Different methods
14
Introduction are used to lower the moisture absorption in syntactic foams such as addition of hydrophobic compounds to binder (e.g., alkyl alkoxy silane derivatives, amino or epoxy alkoxysilanes) and microsphere dressing (e.g., using agents such as aminoethoxysilanes) [13]. Syntactic foams absorb considerable amounts of water only at hydrostatic pressures above 100 MPa; they absorb very little between 20 and 100 MPa. They are stable in cyclic hydrostatic tests also. They can withstand up to 1000 cycles of alternating between 60 MPa and atmospheric pressure. Even under hydrostatic pressures up to 75% of the collapsing pressure, syntactic foams do not absorb large amounts of water [13].
1.6.5 Isotropic Nature Syntactic foams are isotropic materials due to the randomness of the microstructure, i.e., they tend to behave the same way on every load-bearing axis [50]. In syntactic foams, the filler is randomly dispersed in the matrix in a way to obtain homogeneous and isotropic macroscopic behaviour [16]. The properties of syntactic foams do not appear to be associated with any preferred direction in the material but rather with the orientation of the applied stresses [31].
1.6.6 Dielectric Properties Syntactic foams are also characterised by low dielectric constants. This enables their use in radomes and other products where lightweight materials of desired electrical properties are required. By using appropriate inorganic hollow spheres, very low loss tangents can be reached, thus avoiding major losses of radiation energy. Some silica microspheres are characterised by very low dissipation factors, making them suitable for the development of low-loss syntactic foams. Such foams can be used for radome applications where minimum absorption of electromagnetic energy is required [20, 24].
15
Update on Syntactic Foams
1.6.7 Thermal Properties In general, the thermal properties of syntactic foams are dominated by the matrix characteristics. Filler type also has some effect on the thermal properties of syntactic foams. For example, the replacement of glass microspheres by phenolic microspheres improves the thermal oxidation stability of epoxy foams, especially at 100–150 °C. Syntactic foams are less combustible than their chemically foamed counterparts for the same reason [13].
1.7 Factors Affecting the Performance of Syntactic Foams The properties of syntactic foams are influenced by several factors. The most important ones are given below.
1.7.1 Volume Fraction of Filler The volume fraction of filler affects the properties of syntactic foams. The density of syntactic foams is inversely proportional to the volume percentage of microballoons. The variation in microballoon concentration affects the mechanical, dynamic mechanical, thermal and water-absorption properties of syntactic foams. Variation in the properties with filler concentration is different for different types of polymers, and is described in detail in Chapter 2. The concentration of filler also plays an important part in processing. When the microsphere concentration exceeds a certain threshold value called the ‘space factor’, the mixture loses its fluidity and changes from a casting composition to a press moulding composition. Each type of microsphere has its own binder:filler ratio at which this transition occurs, marking the boundary between castable and mouldable compositions. The space factor is given by the volume that the microspheres occupy when packed the closest, as given by: Ks =
16
Φsph Φo
Introduction where Φsph is the true volume fraction of the spheres, and Φo is the closest packed volume fraction of the spheres [13].
1.7.2 Nature of Filler The properties of syntactic foams are also found to depend on the nature of the microballoon, i.e., microballoon material, shell thickness, size of the microballoon, and particle-size distribution. Different types of microballoons, namely polymeric, glass, carbon, ceramic, and metallic can be incorporated in syntactic foams to impart specific properties. In each class of microballoon, different properties can be achieved by suitably selecting chemical constitution of the material of the microballoon. The properties of syntactic foams are strongly dependent on the size of the spheres. Larger spheres cause some deterioration of the mechanical properties much more than the small ones [20, 51]. The shell thickness of a microballoon is also an important factor affecting the mechanical properties of syntactic foams. For the same volume percentage of microballoon, those with high thickness give syntactic foams of high density. The compressive strength and modulus increased with increase in shell thickness of microballoons [41, 52–55]. The type of filler used has some effect on the fire resistance of syntactic foams. The use of carbon microspheres instead of organic filler makes the foam less combustible [13].
1.7.3 Nature of Polymer Matrix For the same volume fraction of polymer, the properties of syntactic foams are different for different polymers. In the case of thermosetting polymers such as epoxy, the curing agent and cure cycle are important parameters [56]. The physical and mechanical properties of syntactic foams therefore depend upon their component type and proportions, and their structure. The properties of syntactic foams based on different polymer systems will be discussed in Chapter 2. Depending on the property requirements, an appropriate binder can be selected. The concentration/nature of the curative/hardener can also be decisive 17
Update on Syntactic Foams in determining properties. In some cases, blends of resins may also be used to impart additional properties [57].
1.7.4 Quality of the Filler-matrix Interface The properties of syntactic foams (particularly the mechanical ones) are directly dependent on the filler-binder interface. Microsphere strength had a strong effect on overall compressive properties, with interfacial strength playing a secondary (yet significant) part. The role of the interface is much more critical in the case of tensile and flexural properties. The binder-filler interface also has an important role in the moisture-absorbing properties.
1.7.5 Method of Processing The method of processing also has a profound influence on the properties of syntactic foams. The properties depend on the method used, e.g., casting or moulding. Syntactic foams made by moulding possess better mechanical properties compared with that prepared by casting. Nikhil and co-workers reported the effect of processing on the entrapment of voids in fibre-free and fibre-bearing syntactic foam systems [9].
References 1.
A.R. Luxmoore, M.F. Yeo and D.R.J. Owen, Composites, 1976, 7, 2, 110.
2.
R.F. Long and E.C. Mitchell, Syntactic Foams from Cenospheres, Admiralty Materials Laboratory Report No.1/73, 1973. [http://www.nationalarchives.gov.uk/catalogue/ displaycataloguedetails.asp?CATID=8095773&CATLN= 6&Highlight=&FullDetails=True]
3.
N. Gupta, Kishore, E. Woldesenbet and S. Sankaran, Journal of Materials Science, 2001, 36, 18, 4485.
18
Introduction 4.
R. Erikson, Mechnaical Engineering Magazine, 1999, January. [http://www.memagazine.org/backissues/ membersonly/january99/features/foams/foams.html]
5.
C.S. Karthikeyan and S. Sankaran, Journal of Reinforced Plastics and Composites, 2001, 20, 11, 982.
6.
E.M. Wouterson, F.Y.C. Boey, X. Hu and S-C. Wong, Composites Science and Technology, 2005, 65, 11–12, 1840.
7.
C.S. Karthikeyan, S. Sankaran, M.N.J. Kumar and Kishore, Journal of Applied Polymer Science, 2001, 81, 2, 405.
8.
R. Maharsia, Development of High Performance Hybrid Syntactic Foams: Structure and Material Property Characterization, Louisiana State University and Agricultural and Mechanical College, 2005. [PhD Thesis]
9.
N. Gupta, C.S. Karthikeyan, S. Sankaran and Kishore, Materials Characterization, 1999, 43, 4, 271.
10. E.S. Harrison, D.J. Bridges and J.L. Melquist, inventors; McDonnell Douglas Corporation, assignee; US 6,074,475, 2000. 11. T. Fine, H. Sautereau and V. Suavant-Moynot, Journal of Materials Science, 2003, 38, 12, 2709. 12. Kishore, R. Shankar and S. Sankaran, Materials Science and Engineering A, 2005, 412, 1–2, 153. 13. F.A. Shutov, Handbook of Polymeric Foams and Foam Technology, Ed., D. Klempner and K.C. Frisch, Hanser Publishers, New York, NY, USA, 1991, p.355. 14. E. Woldesenbet, N. Gupta and A. Jadhav, Journal of Materials Science, 2005, 40, 15, 4009. 15. N. Gupta and R. Nagorny, Journal of Applied Polymer Science, 2006, 102, 2, 1254.
19
Update on Syntactic Foams 16. E. Rizzi, E. Papa and A. Corigliano, International Journal of Solids and Structures, 2000, 37, 40, 5773. 17. Handbook of Reinforcement for Plastics, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987. 18. A.M. Matthews, inventor; E.I. Du Pont de Nemours and Company, assignee; US 5,356,958 (A), 1994. 19. H.K. Phlegm, The Role of the Chemist in Automotive Design, CRC Press, Taylor and Francis Group, Boca Raton, FL, USA, 2009. 20. M. Puterman, M. Narkis and S. Kenig, Journal of Cellular Plastics, 1980, 16, 4, 223 21. E.G. Melber, M.K. Gibbons and F.T. Anderson, Environmental Science and Technology, 1984, 7, 2, 19. 22. Y. Takeuchi in Proceedings of the 2003 IEEE Ultrasonics Symposium, Honolulu, HI, USA, 2003, p.1056. 23. A. Calahorra, O. Gara and S. Kenig, Journal of Cellular Plastics, 1987, 23, 4, 383. 24. M. Narkis, M. Puterman and S. Kenig, Journal of Cellular Plastics, 1980, 16, 6, 326. 25. N. Gupta, Characterization of Syntactic Foams and their Sandwich Composites: Modeling and Experimental Approaches, Louisiana State University and Agricultural and Mechanical College 2003. [PhD Thesis] 26. P. Bunn and J.T. Mottram, Composites, 1993, 24, 7, 565. 27. T.E. Cravens, Journal of Cellular Plastics, 1973, 9, 6, 260. 28. P.S. Du Pont, J.E. Freeman, R.E. Ritter and A. Wittmann, inventors; Hughes Aircraft Company, assignee; US 4,595,623. 20
Introduction 29. Modern Plastics, 1972, 2, 5, 52. 30. C. Arnold, Jr., D.K. Derzon, J.S. Nelson and P.B. Rand, inventors; US Army, assignee; US 5,432,205, 1995. 31. G.S. Mukherjee and M.N. Saraf, Popular Plastics and Packaging, 1994, 59. 32. CRG Technology, Dayton, OH, USA. [http://www.crgrp.com/ syntactics.shtml] 33. N. Gupta, R. Maharsia and H.D. Jerro, Materials Science and Engineering A, 2005, 395, 1–2, 233. 34. T.F. Anderson, H.A. Walters and C.W. Glesner, Journal of Cellular Plastics, 1970, 6, 4, 171. 35. C.S. Karthikeyan, S. Sankaran and Kishore, Polymers for Advanced Technologies, 2007, 18, 3, 254. 36. C.S. Karthikeyan, S. Sankaran and Kishore, Materials Letters, 2004, 58, 6, 995. 37. O.L. Ferguson and R.G. Shaver, Journal of Cellular Plastics, 1970, 6, 3, 125. 38. N. Gupta, E. Woldesenbet, Kishore and S. Sankaran, Journal of Sandwich Structures and Materials, 2002, 4, 3, 249. 39. P.R. Marur, Materials Letters, 2005, 59, 14–15, 1954. 40. M.A. El-Hadek and H.V. Tippur, International Journal of Solids and Structures, 2003, 40, 8, 1885. 41. N. Gupta, E. Woldesenbet and P. Mensah, Composites Part A: Applied Science and Manufacturing, 2004, 35, 1, 103. 42. N. Gupta and W. Ricci, Material Science and Engineering A, 2006, 427, 1, 331. 43. N. Gupta in Proceedings of the 2006 TMS Annual Meeting, San Antonio, TX, USA, 2006, p.3.
21
Update on Syntactic Foams 44. R. Maharsia and H.D. Jerro, Materials Science and Engineering A, 2007, 454/455, 416. 45. N. Gupta and E. Woldesenbet, Composite Structures, 2003, 61, 4, 311. 46. N. Gupta and E. Woldesenbet, Journal of Composite Materials, 2005, 39, 24, 2197 47. Y-J. Huang, L. Vaikhanski and S.R. Nutt, Composites Part A: Applied Science and Manufacturing, 2006, 37, 3, 488. 48. N. Gupta and E. Woldesenbet, Journal of Cellular Plastics, 2004, 40, 6, 461. 49. E.C. Hobaica and S.D. Cook, Journal of Cellular Plastics, 1968, 4, 4, 143. 50. S. Sankaran, K.R. Sekhar, G. Raju and M.N.J. Kumar, Journal of Materials Science, 2006, 41, 13, 4041. 51. Y. Watanabe and S. Agawa, Japan Plastic Age, 1972, p.12. 52. M. Koopman, K.K. Chawla, K.B. Carlisle and G.M. Gladysz, Journal of Materials Science, 2006, 41, 13, 4009. 53. N. Gupta and E. Woldesenbet in Proceedings of ASC 17th Annual Conference, Purdue University, West Lafayette, IN, USA, 2002. 54. R.S.V. Nascimento, J.R.M. d’Almeida and E.S.V. Abreu in Proceedings of the 3rd Ibero-American Polymer Symposium, Vigo, Spain, 1992, p.531. 55. J.R.M. d’Almeida, Composites Science and Technology, 1999, 59, 14, 2087. 56. D. Benderly, Y. Rezek, J. Zafran and D. Gorni, Polymer Composites, 2005, 25, 2, 229. 57. K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. 22
2
Types of Syntactic Foams
The properties of syntactic foams are very much dependent on the nature of polymer matrix employed. Studies have therefore focused on exploring different polymer matrices for syntactic foams. Apart from the nature of the matrix, concentration of the matrix, type of curative, and type of microballoon are critical parameters in determining properties. This chapter focuses on syntactic foams based on various polymer systems.
2.1 Epoxy Syntactic Foams Epoxy syntactic foams have been the most extensively studied because they can be formulated in various ways to give the desired end product. They are preferred to other matrix systems owing to their high strength and stiffness, thermal and environmental stability, creep resistance and lower shrinkage and water resistance [1]. The high viscosity of epoxy is a disadvantage, but the problem can be solved by adopting suitable processing techniques and by material selection [2–3]. Bisphenolic, novolac and other structural epoxy resins in combination with different microballoons have been used to process syntactic foams. Epoxy syntactic foams with polystyrene, carbon, phenolics (BJO-0930), glass and mineral microballoons have been reported [2, 4–5]. Epoxy syntactic foams have been widely used in sandwich composites. They have been used as a thermal protection system in rocket science, particularly to protect the substructures from the heat flux of the exhaust plumes. The performance of epoxy syntactic foams is dependent upon volume percentage, shell thickness and the material of microballoons.
23
Update on Syntactic Foams The properties of epoxy syntactic foams can be further tuned by appropriate choice of the resin system, blending with other polymer resins, and by suitably selecting the curative. The properties are also found to depend upon the process technique followed, i.e., casting and moulding. The effect of microballoon volume fraction on the properties of epoxy syntactic foams with microballoons of density 220, 320, 380, and 460 kg/m3 (designated SF22, SF32, SF38 and SF46, respectively), has been reported by Gupta and co-workers. These microballoons have identical size but different shell thickness. The tensile strength showed a decreasing trend when the microballoon volume percentage was increased from 30% to 60%. All types of syntactic foams showed a decrease in tensile strength of 60–80% compared with the neat resin. The specific strengths of SF22 and SF32 foams do not show variation with microballoon concentration in the range of 30–60 vol%. The values showed a decreasing trend for SF38 and SF46 foams, but these values are also closely spaced and the trend can be considered to be nearly constant. The modulus was found to increase with microballoon density [6]. Another study was conducted by Wouterson and co-workers for epoxy syntactic foams with three types of microballoons: 3M Scotchlite, K-15 and K-46 glass microballoons, and Phenoset BJO093 hollow phenolic microballoons [7]. They showed that tensile and flexural strength decreased with increasing filler content, and was not affected by the component microspheres. The specific tensile strength showed a maximum value at 10 vol% microsphere, whereas specific flexural strength decreased with microballoon concentration for all types of microspheres used (Figure 2.1a and 2.1b). Tensile and flexural moduli showed different trends for each type of microsphere with increasing filler content [7]. The compressive properties of epoxy syntactic foams were reported by Bunn and co-workers. They studied the compressive properties of syntactic foams made from phenolic microballoons with different volume fractions. The bulk density of the foams varied from
24
Specific Tensile Strength (MPa.cm3.g–1)
Types of Syntactic Foams
50
40
30
20
10
0 0
10
20
30
40
50
40
50
vol% microspheres Specific Flexure Strength (MPa.cm3.g–1)
(a) 80 70 60 50 40 30 20 10 0 0
10
20
30
vol% microspheres (b)
Figure 2.1 Specific (a) tensile and (b) flexural properties of epoxy syntactic foam containing () K15, () K46 and () phenolic microspheres. Reproduced with permission from E.M. Wouterson, F.Y.C. Boey, X. Hu and S-C. Wong, Composites Science and Technology, 2005, 65, 11–12, 1840. ©2005, Elsevier [7]
25
Update on Syntactic Foams 1.5 g/cm3 to 0.78 g/ cm3 when microballoon volume percentage was increased from 0% to 53%. A decrease in compressive strength was observed on increasing the microballoon volume fraction. The mean compressive yield strength ranged from 28 MPa to 71 MPa, and the mean initial tangent compressive modulus of elasticity ranged from 0.81 GPa to 2.7 GPa. Thus, the compressive yield strength and initial tangent modulus of elasticity were linearly dependent on the bulk density and the volume fraction of microballoons (Figure 2.2) [4]. Palumo and co-workers also observed a reduction in compressive strength from 70 MPa to 50 MPa as the weight percentage of the microballoons increased from 15% to 35% [8]. Zihlif and co-workers found that the density, elastic modulus, compressive yield stress and strain decreased with increase in the volume fraction of microballoons [9]. Studies by Wouterson and co-workers showed that the specific compressive yield strength decreased with increasing filler content for epoxy syntactic foams with phenolic and K15 microspheres. However, an upward trend is observed for syntactic foams with K46 microspheres. This is attributed to a relatively minor decrease in the compressive yield strength compared with the decrease in density. The difference in compressive strength of syntactic foams with K15 and K46 microspheres shows the influence of compressive strength on the wall thickness of microspheres. The specific compressive modulus increased with microballoon concentration for syntactic foams with K15 and K46, whereas in the case of phenolic microballoon syntactic foams, the modulus remained almost constant with microballoon concentration [7]. The compressive strength and modulus of epoxy syntactic foams was found to be higher for those processed using microballoons of higher shell thickness [7, 10–12]. The approach of changing the microballoon thickness while keeping the microballoon volume fraction constant is more effective than changing the microballoon volume fraction to change the syntactic foam density because it considerably increases the specific strength [11]. The moisture absorption of epoxy syntactic foams with S22 (microballoon density = 220 kg/m3) and K46 (microballoon density = 460 kg/m3) microballoons was found to be <1% at room temperature.
26
Compressive yield strength, σcy (MPa)
Types of Syntactic Foams
80 70 60 50 40 30 20 10 0
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.4
1.5
Bulk density, ρ (g cm ) –3
(a)
Compressive modulus of elasticity, E (GPa)
3.0
2.5
2.0
1.5
1.0
0.5
0.8
0.9
1.0
1.1
1.2
1.3
Bulk density, ρ (g cm–3) (b)
Figure 2.2 Variation of (a) compressive strength and (b) modulus with bulk density of epoxy syntactic foams. Reproduced with permission from P. Bunn and J.T. Mottram, Composites, 1993, 24, 7, 565. ©1993, Elsevier [4]
27
Update on Syntactic Foams At 70 °C, S22-type syntactic foam absorbed 6.7% and 2.5% moisture in deionised and salt water, respectively, whereas K46 syntactic foam absorbed 3.9% and 1.9%, respectively, (Figure 2.3). A considerable decrease in modulus is observed in wet samples compared with dry samples of the same type. For S22-type syntactic foam, the decrease is 49%, 51%, 65% and 68%, respectively, for low-temperature deionised water, low-temperature salt water, high-temperature deionised water and high-temperature salt water. The corresponding decrease in the case of K46 syntactic foams was measured to be 48%, 64%, 57% and 60%, respectively. No significant difference was observed in the peak compressive strength of low-temperature specimens compared with the dry specimens. However, hightemperature specimens showed a decrease in compressive strength
Moisture Content (Wt. %)
7 6 5 4 3 2 1
0
300
600
900
1200
1500
Time (Hrs) R.T., DI Water, S22
High T., DI Water, S22
R.T., Salt Water, S22
High T., Salt Water, S22
R.T., DI Water, K46
High T., DI Water, K46
R.T., Salt Water, S46
High T., Salt Water, K46
Figure 2.3 Moisture absorption of epoxy syntactic foams at room temperature and 70 °C. Reproduced with permission from N. Gupta and E. Woldesenbet, Composite Structures, 2003, 61, 4, 311. ©2003, Elsevier [13]
28
Types of Syntactic Foams of 36%, 33%, 34% and 31% for S22 deionised water-, S22 salt water-, K46 deionised water- and K46 salt water-immersed samples compared with the corresponding dry samples [13]. The fracture toughness of epoxy syntactic foams with glass microballoons of 220, 320, 380, and 460 kg/m3 density decreased from 2 MPa.m0.5 to 0.6 MPa.m0.5 as the volume percentage of microballoon increased from 30% to 65%. The decrease in fracture toughness was more (30%) as the volume percentage of the microballoons increased from 60% to 65% [14]. The fracture toughness of epoxy syntactic foams of lower microballoon concentrations, i.e., 0–20%, showed an increasing trend with increasing volume fraction of microballoons. This is attributed to crack front bowing mechanisms which assumes that microspheres can resist crack propagation and cause the crack front to bow out between the microspheres. Highdensity microballoons contribute to higher strength and stiffness in addition to crack bowing mechanisms due to the higher strength of the filler [15]. These two studies showed that there may be an optimum volume fraction of microballoons at which the fracture toughness reaches a maximum. Fracture toughness and fracture energy did not vary significantly with change in temperature in the range 50–125 °C [9]. The fracture toughness of epoxy (Epon 815) syntactic foams can be improved by changing the resin and curing agent without sacrificing mechanical and thermal properties. In syntactic foams in which epoxy was cured by anhydride (see Figure 2.4a and 2.4b), the addition of elastomer improved the fracture toughness whereas flexural strength was unaffected. For the cycloaliphatic amine-cured system (see Figure 2.4c), the addition of elastomer did not affect fracture toughness. The use of a cycloamine curing agent rather than a anhydride curing agent yielded an increase in fracture toughness of 30% due to better curing properties [16]. Dynamic mechanical analysis of epoxy syntactic foams shows a higher glass transition temperature (Tg) for the syntactic foams compared with the neat resin due to the reinforcing effect of the microballoons
29
Update on Syntactic Foams
O O O
O
O
O
(a)
(b)
H2 N
NH2
(c)
Figure 2.4 Structures of curing agents (a) nadic methyl anhydride, (b) hexahydrophthalic anhydride and (c) bis (p-aminocyclohexyl methane) that affects the mobility of the polymeric chains in the interphase region between the matrix and the microballoons. The reduction in modulus values with increasing temperature is much lower in syntactic foams compared with neat resins. The room-temperature tan values of the syntactic foams showed a relatively large decrease compared with the neat resin due to the presence of rigid glass microballoons that was also at high concentration levels [17]. The coefficient of thermal expansion (CTE) of epoxy syntactic foams is lower than that of neat epoxy. The CTE of epoxy syntactic foam with carbon microspheres decreases as the filler concentration increases. The CTE values for foams with 0 (pure binder), 15, 20, and 25 vol% filler are 55 × 10–6, 45 × 10–6, 37 × 10–6, and 13 × 10–6 °C–1, respectively. These values do not change for temperatures up to 370 °C, which is important for materials used as thermal insulation where the heat loads can vary abruptly [18]. The dielectric constant was decreased with increase in frequency and filler content. Surface energy was higher compared with neat epoxy [19]. 30
Types of Syntactic Foams
2.2 Phenolic Syntactic Foams Phenolic resins have an important role as matrix in composite materials for thermo-structural and ablative applications. These excellent properties are due to the chemical structure, which primarily comprises C-C bonds in aromatic rings, which is a characteristic of phenolic groups. Under thermal stress, the phenolic structure slowly breaks up, releasing carbon. Syntactic foams based on phenolic resins are very advantageous because they can act as low-density ablatives. They have been successfully used for the thermal protection of atmospheric reentry space vehicles and to prevent structures from the extreme heat flux of rocket exhaust plumes. They have the low thermal diffusivity and high char-forming properties desirable for good ablatives [20]. Phenolic syntactic foams and composites have become well-established in the offshore oil and gas industry over the past few years as highly fire-resistant, thermally insulative coatings and structures. Phenolic syntactic foams have good resistance to burning, and have improved thermal stability at 300 °C. The undesirable friable character of phenolic foams can be markedly improved by the introduction of microballoons [21]. Syntactic foams based on a phenolic binder (novolac) has a lower CTE (7 × 10–6 °C–1 for an apparent density of 200–300 kg/m3) compared with an epoxy-based system [18]. Phenolic syntactic foams based on silas microballoons, glass microballoons, IG-101 and K-37, amino microballoons, and fly ash have been reported [20–23]. Conventional condensate-cure phenolic resins are thermooxidatively fragile, so they were chemically modified to confer addition cure characteristics. One such polymer is propargyl ether novolac (PN) whose synthesis and structure are shown in Figure 2.5. The resin cures by an addition reaction of the propargyl group by a complex mechanism. The cured resin is stable up to 400 °C, unlike the condensate-cured one which starts losing weight at about 300 °C. We studied the properties of phenolic syntactic foams and PN resin syntactic foams with K-37 microballoons. The composition of the syntactic foams is given in Table 2.1. The mechanical properties of phenolic syntactic foams with different microballoon concentrations 31
32
H 2C
H 2C
H2 C
H 2C
O
CH 2
OH
CH 2
O
CH 2
CH2
OH
O
CH2
CH 2
CH 2
CH 2
O
Br
CH 2
O
K2CO 3
H2 C
H2C
CH 2
O
H2 C
O
CH 2
CH 2
Figure 2.5 Synthesis and curing of PN resin
CH2
O
CH 2
OH CH 2
O CH 2
PN Resin
O
CH 2
O
CH2
O
H
C
Update on Syntactic Foams
Types of Syntactic Foams
Table 2.1 Composition and density of the phenolic syntactic foams Volume percentage Resin
Microballoon
Void
Density (kg/m3)
P1
26.5
68.5
5.0
570
P2
22.3
72.1
5.6
540
P3
19.0
73.9
7.1
504
P4
16.8
76.0
7.2
482
P5
15.3
79.3
5.4
467
PNM
26.8
70.2
3.0
581
Code for syntactic foams
Reproduced with permission from B. John, C.P.R. Nair and K.N. Ninan, Cellular Polymers, 2007, 26, 4, 229. © 2007, Smithers Rapra Technology [20]
(P1 to P5) and that of PN resin syntactic foam (PNM) is given in Table 2.2 [20]. Though inferior to the properties of neat resin, tensile and flexural strength increased with volume fraction of microballoon and optimised at 72–74% by volume of microballoon. Both properties decreased with further addition of microballoon. The variation in tensile and flexural strength with microballoon volume fraction is due to the difference in interfacial bonding between microballoon and resin upon microballoon loading. The interfacial strength has been found to vary with microballoon volume fraction. When the resin content is high, smearing of the resin over microballoon occurs, making the interfacial strength excellent. Therefore, we expect tensile and flexural strength to decrease with increase in microballoon volume fraction. However, here the tensile and flexural strength were found to increase initially with increase in microballoon content. This has been ascribed to the inherent brittle nature of phenolic resin, which is reduced by adding microballoons at low volume percentages. Therefore, the microballoons act as a kind of reinforcing filler at
33
34
17 ± 2
24 ± 1
21 ± 1
18 ± 2
13 ± 1
9±1
8±2
13 ± 3
14 ± 2
13 ± 1
9±1
6±1
Neat resin
P1
P2
P3
P4
P5
PNM
12 ± 2
14 ± 1
17 ± 3
19 ± 2
25 ± 2
35 ± 3
70–210
Compressive strength (MPa)
10 ± 2
19 ± 2
27 ± 2
28 ± 4
24 ± 6
14 ± 4
18–58
Specific tensile strength (MPa/(kg/m3)) × 10–3
16 ± 2
28 ± 2
37 ± 4
42 ± 2
44 ± 2
30 ± 4
40–88
Specific flexural strength (MPa/(kg/m3)) × 10–3
21 ± 3
30 ± 2
35 ± 6
38 ± 4
46 ± 4
61 ± 5
58–175
Specific compressive strength (MPa/(kg/ m3)) × 10–3
581
467
482
504
540
570
1200
Density (kg/m3)
Reproduced with permission from B. John, C.P.R. Nair and K.N. Ninan, Cellular Polymers, 2007, 26, 4, 229. © 2007, Smithers Rapra Technology [20]
50–105
20–70
Sample reference
Flexural strength (MPa)
Tensile strength (MPa)
Table 2.2 Mechanical properties of neat phenolic resin, phenolic syntactic foam, and PN resin syntactic foam
Update on Syntactic Foams
Types of Syntactic Foams low loading levels. The decrease in tensile and flexural strength after attaining a maximum value has been ascribed to poor wetting of microballoons, which weakens the resin-microballoon interface. The syntactic foams failed easily at high volume fraction of microballoon due to easy debonding between microballoon and matrix, thereby reducing tensile and flexural strength [20]. Compressive strength showed a gradual decrease with increase in microballoon concentration. The compressive strength of threephase syntactic foams primarily depends upon the properties of the microballoons, and the degree to which they can be packed into a volume, i.e., density and strength of the bond holding the microballoons. Neat phenolic resin has a compressive strength in the range 58–175 MPa. However, when microballoons are incorporated, the compressive strength decreases due to the easy crushing of microballoons. Thus, inclusion of microballoons interrupts the transfer of load in the matrix, thereby acting as weak points for the failure of the material [20]. From Table 2.2, it is obvious that the mechanical properties of PN resin syntactic foams are inferior to those of phenolic syntactic foam of comparable density. The void content is lower in PN resin syntactic foam than in phenolic syntactic foam of almost identical density. The higher void content in phenolic syntactic foam can be attributed to the inherent property of resole phenolic resin to form voids during curing due to the evolution of volatiles. Therefore, for syntactic foams containing the same material composition, PN resin syntactic foams exhibited marginally higher density than phenolic syntactic foams [20]. The specific mechanical properties of the syntactic foams, which are very important in comparing syntactic foams with different densities, are shown in Table 2.2. The specific mechanical properties followed the same trend as the corresponding mechanical properties. Although the inclusion of microballoons considerably decreased the mechanical properties, the specific properties of some of the syntactic foams were comparable with those of the neat resin. This highlights the applicability of phenolic syntactic foams for use in lightweight structural materials [20].
35
Update on Syntactic Foams The dynamic mechanical properties of phenolic and PN resin syntactic foams were studied to assess their temperature capabilities. The typical dynamic mechanical analysis graphs of P1 and PNM in the temperature range 35–300 °C are shown in Figure 2.6(a) and 2.6(b), respectively. The storage modulus gradually decreased with increase in temperature for both syntactic foams. For PNM, the decrease in storage modulus with temperature is linear up to 300 °C whereas, for P1, there is a steep reduction in E′ in the temperature range 600 900 850
400
P1
300
800 750
PNM
700
200
E' (MPa) of PNM
E' (MPa) of P1
500
650
100
600 0 0
50
100
150
200
250
300
Temperature (°C)
(a) 0.25
0.020 P1
Tan delta of P1
0.015 PNM
0.15
0.010
0.10 0.005 0.05
Tan delta of PNM
0.20
0.000 0.00 –0.005 0
50
100
150
200
250
300
Temperature (°C)
(b)
Figure 2.6 Variation of (a) storage modulus and (b) tanδ with temperature for P1 and PNM
36
Types of Syntactic Foams 160–210 °C. The stiffness values of the syntactic foams are dominated by the rigid glass microballoons, which do not undergo transition in this temperature range. Thus, the steep decrease in storage modulus for P1 in the temperature range 160–210 °C is a feature of phenolic resins (which are known to undergo softening in this temperature range). The maximum use temperature (Tmax) of syntactic foams is taken as the temperature at which the storage modulus starts decreasing steeply in the thermograms. Tmax assessed this way was found to be 160 °C for P1. In the case of PNM, the modulus tends to continuously decrease (slowly) with temperature, unlike conventional phenolic resin. This implies weak bonding between the microballoon and resin, which relaxes the applied stress. The tanδ profile shows a maximum at 200 °C for PI, in tune with the Tg of the cured matrix. For the PN resin syntactic foam, the maximum occurs at 270 °C, close to the resin Tg. Analysis shows that tanmax values of syntactic foams are lower than those of neat resin materials due to diminution in overall flexibility as a result of rigid fillers [20]. In the case of phenolic syntactic foams with IG-101 and silas microballoons, flexural strength, modulus and compressive strength decreased with increase in concentration of microballoon. Except for syntactic foams with silas microballoons, the properties are superior to those of phenolic foams [21]. For syntactic foams prepared from blends of six phenolic resins and carbon microbubbles, the compressive strength of the phenolic resin foam is equivalent to the strength of foams made from a polyimide resin [24]. The combined effect of good specific mechanical properties and the likely ablative characteristics make phenolic syntactic foams important lightweight material for aerospace applications. Another class of addition cure phenolic resin that has also been explored for syntactic foams is polybenzoxazine. Benzoxazines have high thermal stability, easy processability, good electrical properties and flame retardance, improved toughness, low water absorption, near-zero shrinkage upon curing, and good mechanical integrity. These properties of the polybenzoxazine matrix render it a good candidate for syntactic foam composites. In general,
37
Update on Syntactic Foams benzoxazines are synthesised from phenol, aniline and formaldehyde. The polymerisation of bisphenol-A based benzoxazine is shown in Figure 2.7. Here, the polymerisation occurs by the ring-opening polymerisation of the benzoxazine ring [25]. Our research group has studied the mechanical properties of polybenzoxazine syntactic foams with varying concentrations of K-37 microballoons. The variation of specific mechanical properties with microballoon concentration is depicted in Figure 2.8. Tensile and compressive properties were optimised at about 68% by volume of microballoon while flexural strength decreased marginally on increasing the microballoon content. Although the specific tensile and compressive strength showed a maximum followed by a decrease, the specific flexural strength systematically increased
N
OH
O
OH N
O
N
OH
OH
Figure 2.7 Bisphenol A-based benzoxazine monomer and its polymer. Reproduced with permission from K.S.S. Kumar, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 108, 2, 1021. ©2008, Wiley [25] 38
Sp.Mechanical Strength (MPa/kg/m3)
Types of Syntactic Foams
Sp.Tens.str. Sp.Flex.str. Sp.Comp.str.
0.018
0.015
0.012
0.009
0.006
0.003
60
66
72
78
Microballoon volume (%)
Figure 2.8 Variation of specific mechanical strength of polybenzoxazine syntactic foams with microballoon content. Reproduced with permission from K.S.S. Kumar, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 108, 2, 1021. ©2008, Wiley [25] with microballoon content. Polybenzoxazine is basically a phenolic resin, so these composites could be potential candidates for lightweight thermostructural applications, as in ablative thermal insulation [25].
2.3 Cyanate Ester Syntactic Foams Cyanate esters constitute a high-temperature resin family traditionally associated with space applications because of their good thermal stability, low dielectric constant and extremely low moisture uptake compared with other resins of their class. Cyanate ester is known for its built-in toughness, micro-crack resistance and ease of processing [26]. Therefore, syntactic foams derived from these resins will be a
39
Update on Syntactic Foams better choice as the ‘core’ in sandwich composites. On curing, cyanate ester forms a polycyanurate network (Figure 2.9). We studied the mechanical properties of syntactic foams of cyanate ester of bisphenol A with varying volume fractions of glass microballoons. The effect of shell thickness on mechanical properties was also studied by processing syntactic foams using microballoons of two shell thicknesses: K-25 and K-37. Mechanical properties showed a gradual decrease with increase in volume fraction of microballoon. The specific strength values also manifested a similar order. The properties were found to be higher for those processed using microballoons of higher shell thickness for the same volume percentage of microballoon. The variation of tensile, flexural and compressive strength with microballoon volume percentage for K-25 and K-37 syntactic foams is shown in Figure 2.10 [26]. We subsequently evaluated cyanate ester syntactic foams as the core in sandwich composites for possible structural applications [27].
NC O
Ar
Ar
OC N
N
O N
O N
O
n
Polycyanurate network
CH3
-Ar-
= CH3
Figure 2.9 Polymerisation reaction of cyanate ester
40
Types of Syntactic Foams K-37 K-25
6 5 4 3 2 1 60
K-37 K-25
35
Flexural Strength (MPa)
Tensile Strength (MPa)
7
30 25 20 15 10 5
65
70
75
80
85
90
60
Volume percentage of microballoon
65
70
(a)
80
85
90
(b)
50
Compressive Strength (MPa)
75
Volume percentage of microballoon
K-37 K-25
45
40 35 30 25 20 15 10 5 60
65
70
75
80
85
90
Volume percentage of microballoon (c)
Figure 2.10 Effect of microballoon concentration and shell thickness on (a) tensile, (b) flexural and (c) compressive strength of cyanate ester-based syntactic foams. Reproduced with permission from B. John, C.P.R. Nair, K.A. Devi and K.N. Ninan, Journal of Materials Science, 2007, 42, 14, 5398. ©2007, Springer [26]
The difference in strength of syntactic foams based on K-25 and K-37 microballoons is due to the higher shell thickness of K-37. This is evident from the SEM image of syntactic foams with K-25 (Figure 2.11(a)) and K-37 (Figure 2.11(b)) microballoons which contain almost the same volume percentage of microballoon. Fewer 41
Update on Syntactic Foams
(a)
(b)
Figure 2.11 (a) SEM image of a flexurally failed syntactic foam with K-25 showing broken microballoons. (b) SEM image of flexurally failed syntactic foam with K-37 showing fewer broken microballoons. Reproduced with permission from B. John, C.P.R. Nair, K.A. Devi and K.N. Ninan, Journal of Materials Science, 2007, 42, 14, 5398. ©2007, Springer [26]
42
Types of Syntactic Foams microballoons are broken under the applied load in the case of K-37 syntactic foams (as illustrated in the micrograph of the syntactic foam). K-25 microballoons break more easily, thereby reducing the strength of the material. Thus, the inherent property of the microballoon influences the properties of the resultant composites [26]. Cuming Microwave Corporation (USA) manufacture toughened cyanate ester syntactic foams suitable for curing operations at 121 °C. The dielectric and loss properties of the material can be adjusted to meet unique project applications. The material is ideal for encapsulating components due to its low thermal shrinkage. The material is cured at a much lower temperature than the conventional materials cured at 177 °C, so many problems associated with CTE of dissimilar materials can be eliminated. This material can be provided as a low loss base foam for electrically transparent applications, or can be provided with various lossy fillers for antenna pattern shaping, specular attenuation, or insertion loss applications [28]. Another grade of cyanate ester syntactic foam, BryteCor EX-1541, is extremely unique in the industry due to its extremely low density and good structural properties. It represents a no-compromise solution to stringent foam core applications that to date have been unachievable. The material does not require pressure during cure to achieve its mechanical properties and is readily machinable. BryteCor EX-1541 has low dielectric/low loss performance which makes it ideal for radomes, antennae and radar transparent structures. Its chemistry is based on its cyanate ester backbone which assures low moisture absorption and low outgassing (Figure 2.9). These latter two features coupled with the low, isotropic CTE make EX-1541 ideal for spacecraft and other dimensionally stable structures and tooling [29].
2.4 Polyimide Syntactic Foams Polyimide syntactic foams made with silica microballoons have good electrical and mechanical properties. They have been used
43
Update on Syntactic Foams for encapsulation of electrical devices [30]. They are remarkably mechanically stable at elevated temperatures. For example, HTF-60 (apparent density = 370 kg/m3), a type of polyimide syntactic foam, loses <20% of its initial strength as the temperature rises from 200 °C to 370 °C. These syntactic foams can remain intact under compressive strains (ε) up to 40% at 370 °C whereas, under tension at the same temperature, fracture at ε=1.8% with a tensile strength of 3.2 MPa. The material loses 10% of its weight when heated to 528 °C in air, or to 557 °C in an inert atmosphere. The limitations of the material are that it shrinks on curing by up to 20%, and curing takes a long time, with the evolution of toxic and flammable gases (acetic acid, acetic anhydride, N-methyl pyrrolidone). Polybenzimidazole foam with phenolic or glass microspheres also has good thermal properties. It is not combustible (starts to lose weight at 600 °C) and can retain good mechanical properties up to 350 °C [18]. Due to their porous structure, moisture may degrade the electrical properties of polyimide syntactic foams. To reduce moisture absorption, syntactic foams were made using silica microballoons having a Parylene C (di-(chloro-para-xylylene)) coating. Moisture absorption by the coated syntactic foam at 95% relative humidity at 50 °C was found to be 5 wt% after 60-day exposure. The parylene coating provides an efficient water vapour barrier to the three-phase syntactic foam [30]. The polyimide syntactic foam HTF-60 with the formulation 263.2 g RC-5081 (40 g polyimide solids), 60 g B35A glass bubbles, 52 g acetic anhydride, and 5.2 g 4-benzoylpyridine has been proven to be a robust system capable of being compressed to >40% strain without catastrophic failure. The combination of the polyimide and glass microballoons has shown excellent retention of strength at elevated temperatures. The improvement in strength retention of HTF-60 over the sintered polyimide is due to the high percentage of microballoons in HTF-60 [31]. A co-reacted blend of thermosetting resins comprising diallyl bisphenol A (DABA), epoxy phenol novolac (EPN) and 2,2- bis
44
Types of Syntactic Foams 4-(4-maleimidophenoxy) phenyl propane (BMIP) (structures shown in Figure 2.12) abbreviated as epoxy-allyl phenol-bismaleimide (EPB) was used as a matrix in processing syntactic foams [32]. Curing of the blend system occurs through a combination of phenol-epoxy reaction and Alder-ene reaction of DABA and BMIP (Figure 2.13) [33]. The strength and modulus of the EPB syntactic foam system under tensile, flexural, and compressive loading environments were found
CH3 C
HO
OH
CH3
(a)
H2C
O
O
O HC
CH2O
O
CH
CH2
H2C
OCH2
CH2
CH
CH2
CH2
(b)
O
O
CH3 N
O
C
O
N
CH3
O
O
(c)
Figure 2.12 Structures of (a) DABA (b) EPN and (c) BMIP. Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. ©2007, Wiley [32]
45
46
DABA
O
N
R
OH
O
O R
EPN
O
O
N
O O
Heat
O
N
O
O
O
R
OH
O
O
BMIP
O O
N
O
Figure 2.13 Reaction scheme of the EPB system. Reproduced with permission from K.A. Devi, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 106, 2, 1192. ©2007, Wiley [33]
HO
O
Update on Syntactic Foams
Types of Syntactic Foams
Mechanical Strength (MPa)
to decrease with increase in filler concentration. The effect of microballoon weight percentage on the tensile, compressive, and flexural properties of the foam composite is shown in Figures 2.14 and 2.15. The failure mode was found to be brittle in the case of tensile and flexural tests. Thus, at lower microballoon concentration, the failure was mainly due to interface failure, i.e., debonding between the matrix and microballoon. At higher microballoon concentration, the weak microballoons are not adequately protected by the binder matrix, thereby reducing the strength as a result of poor interfacial interaction between the matrix and the microballoon. The stress– strain curves obtained for the compression test (Figure 2.16) reveal that the failure is not of a brittle nature. After attaining the maximum strength, a slow rate of decrease in strength with increase in strain
Tensile Strength Flexural Strength Compressive Strength
60 50 40 30 20 10 40
45
50
55
60
65
70
Weight percentage of microballoon
Figure 2.14 Effect of microballoon (K-37) concentration on the flexural, compressive and tensile strength of the syntactic foam. Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. ©2007, Wiley [32]
47
Update on Syntactic Foams
(Tensile Modulus) (Flexural Modulus) (Compressive Modulus)
4000
Modulus (MPa)
3500 3000 2500 2000 1500 1000 500 0 35
40
45
50
55
60
65
70
75
Weight percentage of microballoon
Figure 2.15 Effect of microballoon (K-37) concentration on the modulus of EPB syntactic foams. Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. ©2007, Wiley [32]
was observed in all cases. A special feature observed in this case is that at strain values of the order of double the strain at maximum strength, the material can withstand almost 50% of its maximum load. The initial high slope of the compression stress–strain curve corresponds to the elastic deformation of the foam and the low slope/plateau regime is readily related to the crushing/densification of microspheres. Unlike the trend observed in fibre-reinforced composites (where kink banding, shear failure, matrix-fibre, interface failure are common failure modes in compression tests), the failure in these syntactic foam composites was crushing failure at the upper and lower surfaces of the specimen in contact with the platens of the compression test fixture, whereas no visual cracks were observed in the other areas [32].
48
Types of Syntactic Foams
Compressive stress (MPa)
60
50
40%
40
50%
60% 30
20 70% 10
0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
strain (mm/mm)
Figure 2.16 Effect of K-37 microballoon concentration on the compressive behaviour of EPB syntactic foam composites. Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. ©2007, Wiley [32] The specific heat values of the syntactic foam composites showed a marginal increasing trend with increase in microballoon concentration up to about 60% (Table 2.3). At the highest microballoon concentration, the specific heat value showed a reduction. The temperature dependence of the specific heat was evaluated up to 100 °C. Specific heat was found to be more or less insensitive to the test temperature [32]. The effect of microballoon composition and temperature on the coefficient of linear expansion of the foam composite is given in
49
Update on Syntactic Foams Table 2.4. The expansion was found to show a systematic decrease with increase in microballoon concentration. At a higher temperature regime, the relative expansion was more for a given composition. The expansion was less pronounced for higher filler loading. This implies that the role of the matrix is dominant in thermal expansion of the foam [32]. For EPB syntactic foams containing K-37 and K-25, tensile strength, flexural strength, and compressive strength values decreased with increase in the concentration of K-25 microballoons (Figure 2.17). For the same concentration of microballoon, foam composites with K-37 microballoons gave a higher density owing to their higher shell thickness and true density. The strength and the corresponding specific strength of K-37 composites were significantly higher than those of K-25 composites. The values of breaking strength are proportional to the shell thickness as well as the crushing strength values of the two types of microballoons, so microballoon breakage is confirmed as the cause for the foam failure at this filler loading (50%) [32].
Table 2.3 Effect of microballoon (K-37) weight percentage on the specific heat of EPB syntactic foam composites Microballoon content (wt%)
Specific heat (cal/g/°C) at different test temperatures 70 °C
80 °C
90 °C
100 °C
40%
0.37
0.37
0.37
0.38
50%
0.38
0.38
0.35
0.32
60%
0.41
0.39
0.36
0.33
70%
0.25
0.25
0.23
0.22
Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. ©2007, Wiley [32]
50
Types of Syntactic Foams
Table 2.4 Effect of microballoon (K-37) concentration and temperature on the coefficient of thermal expansion of EPB syntactic foams Microballoon content (wt%)
Linear expansion coefficient (°C–1) at different temperature ranges (10–5) 30–50 °C
50–100 °C
150–200 °C
40%
2.5
4.4
5.8
50%
2.2
3.6
3.7
60%
1.7
2.4
2.9
70%
1.1
1.5
2.5
Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. © 2007, Wiley [32]
2.5 Polyurethane Syntactic Foams Polyurethanes are also used in syntactic foams. They are mainly used for packaging applications [34–35]. US Patent Number 4,916,173 [36] discloses polyurethane syntactic foam compositions which have high Tg and low CTE. A typical polyurethane composition consists of a blend of (a) 30–55% of a polyisocyanate; (b) 9–35% of an amine-based polyol; (c) 5–40% of a polyether triol; (d) 3–10% of a molecular sieve and (e) 10–40% of a hollow microsphere filler (all percentages being by weight of the total composition). The molecular sieve functions as a ‘moisture scavenger’ which reduces foaming in the moulding process and yields a product of uniform density, whereas polyols provide high Tg and appropriate reactivity. The materials show improved mechanical strength to permit their use under autoclave pressures and temperatures. US Patent 6,706,776 [37] details polyurethane syntactic foams which are hydrolytically stable and have a high compression modulus (>2.068 MPa). These syntactic foams can be exposed to water
51
Update on Syntactic Foams
Mechanical strength (MPa)
55 Tensile Strength Flexural Strength Compressive Strength
50 45 40 35 30 25 20 15 10 0
20
40
60
80
100
Weight percentage of K-25 in syntactic foams
Figure 2.17 Effect of microballoon (K-25) concentration on the strength of EPB syntactic foams containing K-25 and K-37. Reproduced with permission from K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. ©2007, Wiley [32]
at temperatures from 0 °C to 40 °C for up to 10 years without degradation. They exhibit a long pot-life and short demoulding times (<10 min). In particular, at about the same hardness and density, for liquid diphenylmethane diisocyanate vs. polymethylene poly(phenylisocyanate) in combination with a low unsaturation difunctional polyether polyol, the resultant syntactic foams have higher tensile strength and elongation (about 50% more), higher tear strength (i.e., about threefold higher), lower Taber abrasion (30–50% less) and considerably lower water absorption (25–50% less in fresh water, and 20–30% less in salt water). A polyurethane syntactic foam with a combination of low density, strength, fluidity and flame retardancy was obtained by mixing an organic polyol, a polyisocyanate, a catalyst for the reaction of the 52
Types of Syntactic Foams polyol and polyisocyanate, microballoons and a flame retardant (e.g., tris(2-chloroethyl) phosphate). A combination of the polyol and polyisocyanate is liquid at 25 °C, and the amount of microballoons present is sufficient to provide a non-castable mixture in the absence of the flame retardant; the amount of flame retardant is sufficient to provide a castable mixture which will flow in a mould cavity. These syntactic foams are claimed to be suitable for structural purposes such as replacement for wood, and have the added advantage of being flame-retardant [38].
2.6 Polyester Syntactic Foams The polyesters are room temperature-cured with shrinkage, cracking, and exotherm being the major problems. Polyester syntactic foams have been used for floating devices. The low cost of the polyester systems has dictated their use in syntactic foams in large applications such as submarine structural void fillers [39]. It has been revealed that the hollow-sphere fraction of the fly ash from the Banhida Power Station in Hungary was suitable for preparation of polyester syntactic foams without further purification or refinement [40]. For the preparation of polyester syntactic foams, benzoyl peroxide was first mixed with polyester resin, followed by the required amount of microballoons. The mixture was poured into a mould cavity and cured at 100 °C for one hour followed by a further post cure at 100 °C for two hours [21]. A silane coupling agent has been shown to improve the performance and versatility of floating devices based on polyester syntactic foams. It improves the bond between the glass microballoons and the polyester resin, thereby making the spheres stronger without increasing their wall thickness. The use of a coupling agent also improves the flow of the material during moulding, resulting in fewer trapped air bubbles. Otherwise, these bubbles could produce stress in the wall of the sphere. The silane material, representing only 1% of total filler weight, forms bonds at the interface between the organic resin and the inorganic microballoons. This produces a 53
Update on Syntactic Foams strong composite and improves strength retention (particularly in wet environments) [41].
2.7 Silicone Syntactic Foams Silicone resins with their very attractive dielectric properties are very promising thermosetting resins for electromagnetic applications. They have been used as a matrix material in composites based on glass or quartz fibres for high-performance radomes. However, the use of silicone resins for syntactic foams has not been widely studied. For processing a silicone syntactic foam based on phenyl methyl polysiloxane, powdered silicone resin was first processed in a ball-mill and then dry-blended with the silica microballoon. The catalyst and curing agents were added to the resin before milling. Curing was carried out in a constant-volume mould at a maximum temperature of 200 °C and pressure of 0.206 MPa for 15–20 minutes. The compressive properties were studied using different catalyst combinations of dibutyl tin dilaurate, tin octoate (SnOc) and triethanolamine (TEA). The combination of SnOc and TEA results in better compressive properties (particularly at lower densities) [42]. Glass microballoon-filled polydimethyl siloxane, which is crosslinked at room temperature using methyl tris-(methyl ethyl ketoxime) silane in the presence of dibutyl tin dilaurate catalyst, showed tunable mechanical properties depending on the resin-to-filler ratio. Though the mechanical properties diminished on adding microballoons, the specific mechanical properties improved on embedding microballoons. The specific heat showed a decreasing trend with increase in microballoon concentration. The thermal conductivity also gets reduced by the incorporation of silica microballoons. This low-density system can withstand temperatures up to 427 °C, and can find extensive space applications [43].
2.8 Elastomeric Syntactic Foams Elastomeric syntactic foams were developed for use as stress-relief coatings on encapsulated electronic components. The high moduli 54
Types of Syntactic Foams and the ability to relieve high stresses make syntactic foams ideal for this application. These foams have been made with urethane, silicone, and polysulfide elastomers blended with glass and phenolic microballoons. The polysulfide/phenolic microballoon foam has good pot life and can be diluted with toluene to ease processing. It is useful for high-voltage applications where a good bond is required. The variety of compressive properties that are made available by varying the elastomer, microbubble, and the formulation has provided a useful engineering material that can be easily formulated for a particular application. The urethane elastomer/glass microbubble foam is a relatively high-strength material which is difficult to process because of its high viscosity and short pot life. The silicone/glass microbubble foam is made with a low-viscosity resin which has a long pot life [44].
2.9 Nylon Syntactic Foams Nylon syntactic foams made with phenolic microballoons are proposed for use in the nosecap of Scout re-entry vehicles produced by Langley Research Centre at NASA (USA). The formulation of the ablative material is 40% by weight of powdered Nylon 66–80 mesh average, 35% by weight of phenolic microspheres, and 25% by weight of powdered phenolic novolac resin-240 mesh. The syntactic foam exhibited several deficiencies as a plastic material such as low mechanical properties, high porosity, difficult processing, heterogeneous microstructure, and variability of raw materials [45].
2.10 Polybutadiene Syntactic Foams Syntactic foams based on a polybutadiene matrix whose microstructure contains <80% of 1, 2-units provide better control of the exothermicity during the curing of the resin. They are of particular importance when manufacturing articles of great size such as floating members. The microspheres may be of borosilicate glass, silica, carbon, or thermoplastic or thermoset resins, and whose diameter is 10–500 μm, and the ‘macrospheres’ of glass, or thermoplastic 55
Update on Syntactic Foams or thermoset resins, whose diameter is 1–100 mm. In general, the microballoon concentration is 5–50 phr [46].
2.11 Polypropylene Syntactic Foams Syntactic foams based on polypropylene have also been studied. The use of glass microballoons is suitable for obtaining low-density polypropylene syntactic foam without great loss in mechanical properties. The hollow space in polypropylene syntactic foams may also improve thermal insulating properties in addition to isotropy and better surface finishing [47]. Shell Chemicals UK Limited developed polypropylene syntactic foam (carizite) that can be extruded or injection-moulded. It contains tiny glass spheres (<50 μm in diameter) that can withstand extreme pressures. Carizite has been used for insulating oil transmission pipes in the North Sea. Its processability, extremely high compressive strength, and very low shrinkage make it a candidate for structural composites in automotive applications. Carizite has a density of 0.68 g/cm3, compressive strength of 33 MPa, ultimate tensile strength of 10.5 MPa, and dimensional stability of 0.2% at 70 °C [48].
2.12 Carbon–carbon Syntactic Foams Carbon-carbon syntactic foam consists of hollow carbon microspheres bounded by carbonaceous matter. It has high specific strength coupled with useful thermal insulating properties, and is particularly useful for high-temperature applications [49]. Carbon microspheres of various types, particle size, and wall thickness, along with different carbonaceous binder materials, solvents, and moulding pressures, are used for C-C syntactic foams [50]. Bruneton and co-workers detailed the processing of C-C syntactic foams. The precursor of the microspheres is phenolic resin. When the resin is heated, it transforms into a liquid or viscous product. Bubbles are produced by the evolution of volatile products. By controlling the
56
Types of Syntactic Foams heating rate, the starting material is converted into hollow individual spheres. The diameter of the spheres varies between micrometres to several tens of micrometres, and shell thickness is 1–5 μm. The matrix is also a phenolic resin. Phenolic resin and the microspheres are soaked in solvent, which dissolves the resin and disperses the spheres. The mixture is moulded, and excess resin as well as the solvent is removed. The resulting material is polymerised and then carbonised at a temperature close to 1000 °C. The density of the resulting foam was 0.3 g/cm3 [51]. It has been shown that the use of the resin-starch binder significantly increases the plasticity of the foam during the carbonisation step to minimise deleterious internal stress [52]. Compressive tests were carried out from room temperature to 3100 °C at a heating rate of 180 °C/s to obtain results close to atmospheric
2200°C
6
2300°C
2000°C
Stress (MPa)
5
20°C1800°C
4 3 2
3100°C
1 0 0
0.05
0.1
0.15
0.2
Strain
Figure 2.18 Influence of test temperatures on the compressive behaviour of carbon–carbon syntactic foams. Reproduced with permission from E. Bruneton, C. Tallaron, N.G. Naulin and A. Cosculluela, Carbon, 2002, 40, 11, 1919. ©2002, Elsevier [51] 57
Update on Syntactic Foams re-entry. Figure 2.18 shows the variation of compressive properties as a function of temperature. The figure shows that compressive behaviour changes as a function of the temperature from 20 °C to 3100 °C with respect to mechanical characteristics and curve shape [51].
References 1.
G. Li and M. John, Materials Science and Engineering A, 2008, 474, 1–2, 390.
2.
C.S. Karthikeyan, S. Sankaran, M.N. Jagdish Kumar and Kishore, Journal of Applied Polymer Science, 2001, 81, 2, 405.
3.
C.S. Karthikeyan, Kishore and S. Sankaran, Journal of Reinforced Plastics and Composites, 2000, 19, 9, 732.
4.
P. Bunn and J.T. Mottram, Composites, 1993, 24, 7, 565.
5.
N. Gupta, C.S. Karthikeyan, S. Sankaran and Kishore, Materials Characterization, 1999, 43, 4, 271.
6.
N. Gupta and R. Nagorny, Journal of Applied Polymer Science, 2006, 102, 2, 1254.
7.
E.M. Wouterson, F.Y.C. Boey, X. Hu and S-C. Wong, Composites Science and Technology, 2005, 65, 11/12, 1840.
8.
M. Palumbo, G. Donzella, E. Tempesti and P. Ferruti, Journal of Applied Polymer Science, 1996, 60, 1, 47.
9.
A.M. Zihlif and G. Ragosta, Polymer & Polymer Composites, 2001, 9, 5, 345.
10. N. Gupta and E. Woldesenbet in Proceedings of ME Graduate Student Conference, Louisiana State University, Baton Rouge, LA, USA, 2002. 11. N. Gupta and E. Woldesenbet, Journal of Cellular Plastics, 2004, 40, 6, 461. 58
Types of Syntactic Foams 12. J.R.M. d’Almeida, Composites Science and Technology, 1999, 59, 14, 2087. 13. N. Gupta and E. Woldesenbet, Composite Structures, 2003, 61, 4, 311. 14. V. Gorugantu, Fracture Toughness Characterisation of Syntactic Foams, Louisiana State University, LA, USA, 2005. [MSc Thesis] 15. E.M. Wouterson, F.Y.C. Boey, X. Hu and S-C.Wong, Journal of Cellular Plastics, 2004, 40, 2, 145. 16. D. Benderly, Y. Rezek, J. Zafran and D. Gorni, Polymer Composites, 2005, 25, 2, 229. 17. S. Sankaran, K. Ravisekhar, G. Raju and M.N.J. Kumar, Journal of Materials Science, 2006, 41, 13, 4041. 18. F.A. Shutov, Advances in Polymer Science, 1986, 73, 63. 19. S-J. Park, F-L. Jin and C. Lee, Material Science and Engineering A, 2005, 402, 1–2, 335. 20. B. John, C.P.R. Nair and K.N. Ninan, Cellular Polymers, 2007, 26, 4, 229. 21. K. Okuno and R.T. Woodhams, Journal of Cellular Plastics, 1974, 10, 6, 273. 22. Y-J. Huang, L. Vaikhanski and S.R. Nutt, Composites Part A: Applied Science & Manufacturing, 2006, 37, 3, 488. 23. S.D. Argade, K.N. Shivakumar, R.L. Sadler, M.M. Sharpe, L. Dunn, G. Swaminathan and U. Sorathia in Proceedings of International SAMPE Technical Conference, SAMPE 2004, Long Beach, CA, USA, 2004, p.1313. 24. H.M. McIlroy, Phenolic Resin Syntactic Foams, Technical report of Bendix Corporation, Kansas City, MO, USA, 1980. 59
Update on Syntactic Foams 25. K.S.S. Kumar, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 108, 2, 1021. 26. B. John, C.P.R. Nair, K.A. Devi and K.N. Ninan, Journal of Materials Science, 2007, 42, 14, 5398. 27. B. John, D. Mathew, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 110, 3, 1366. 28. Technical Bulletin 370-2 of Cuming Microwave Corp., 2007. http://www.cumingmw.com/pdf/370-Syntactic-Foams/ 370-2-cyanate-ester-syntactic-foams.pdf 29. Cyanate Ester Syntactic Foam, BryteCor EX-1541 Technical Data Sheet, Bryte Technologies, Inc., 2003. 30. A. Calahorra, O. Gara and S. Kenig, Journal of Cellular Plastics, 1987, 23, 4, 383. 31. P.B. Rand, Journal of Cellular Plastics, 1973, 9, 3, 130. 32. K.A. Devi, B. John, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 105, 6, 3715. 33. K.A. Devi, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2007, 106, 2, 1192. 34. D. Rittel, Materials Letters, 2005, 59, 14–15, 1845. 35. J. Adrien, E. Maire, N. Gimenez and V.S. Moynot, Acta Materialia, 2007, 55, 5, 1667. 36. E.L. Otloski and G.H. Sollner, inventors; Ciba-Geigy Corporation, assignee; US 4,916,173, 1990. 37. P.H. Markusch, R. Guether and T.L. Sekelik, inventors; Bayer Corporation, assignee; US 6,706,776, 2004. 38. J.R. Harper, inventor; Dow Corning Corporation, assignee; US 4,082,702, 1978.
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Types of Syntactic Foams 39. E.C. Hobaica and S.D. Cook, Journal of Cellular Plastics, 1968, 4, 4, 143. 40. P. Hirschberg, G.Y. Novotny, A. Kalló and G.Y. Vamos, Journal of Cellular Plastics, 1987, 23, 4, 399. 41. Elastomerics, 1980, 112, 12, 63. 42. S. Kenig, I. Raiter and M. Narkis, Journal of Cellular Plastics, 1984, 20, 6, 423. 43. S.C. Sukumaran, A. Mathew, R. Pravin and G. Prabhakaran in Proceedings of National Seminar on Aerospace-Expanding Frontiers, Technologies and Challenges, Trivandrum, India, 2009, p.115. 44. P.B. Rand, Journal of Cellular Plastics, 1978, 14, 5, 277. 45. L.B. Keller, Journal of Cellular Plastics, 1968, 4, 11, 418. 46. F. Dawans, inventor; Institut Francais Du Petrole, assignee; US 4,107,134, 1978. 47. C.R. Ana, N. Barboza and M.A. De Paoli, Polímeros, 2002, 12, 2, 130. 48. Modern Plastics International, 1994, 24, 1, 65. 49. J.J. Gebhardt and P.W. Juneau Jr., inventors; General Electric Co., assignee; US 4,442,165, 1984. 50. S.T. Benton and C.R. Schmitt, Carbon, 1972, 10, 2, 185. 51. E. Bruneton, C. Tallaron, N.G. Naulin and A. Cosculluela, Carbon, 2002, 40, 2, 1919. 52. W.B. Malthouse and D.R. Masters, inventors; Atomic Energy Commission, USA, assignee; US 3,832,426, 1974.
61
3
Recent Developments in the Field of Syntactic Foams
Syntactic foams have been used in a wide range of applications. Due to their wide range of possible applications, it is desirable to modify the properties of syntactic foams according to requirements. Recent studies have focussed on improving the properties of syntactic foams. Various types of filler materials have been used to modify the microstructure/properties of foams to attain certain desired properties. This chapter focuses mainly on recent developments in this field.
3.1 Fibre-reinforced Syntactic Foams Several works have been devoted to improving the performance of syntactic foams by fibre reinforcement [1–5]. Syntactic foams reinforced with fibre are better core for application in sandwich composites. The method of processing and fibre composition has a profound influence on the properties of syntactic foams. Karthikeyan and co-workers studied the mechanical properties of fibre-reinforced syntactic foams containing 0.9, 1.76, 2.54, 3.54 and 4.5 vol% fibre. Syntactic foam with 3.54% fibre was processed in two ways. In the first method, microballoons were added to the resin before the fibre. In the second method, fibres were added first, followed by microballoons. The results showed that the flexural modulus increased with fibre content, with the exception of 1.76% and 3.54% (processed by the first method) of fibres. This deviation was due to a higher void content for the 1.76% fibre system and a non-uniform distribution of fibres for the 3.5% fibre system. The difference in
63
Update on Syntactic Foams fibre distribution in the case of the 3.54% fibre system was because the fibres could not distribute themselves in a medium that contains microballoons dispersed in a resin. Here, the microballoons act as an obstacle for tangible distribution of fibres. Due to this clustering, fibres were not completely wet by the resin matrix; the fibres were therefore less effective in bearing the load transferred from the matrix while testing. Thus, the latter method was found to be more effective, as evident from the lower void content and good mechanical properties [2–3]. We investigated the effect of glass fibre on the mechanical properties of cyanate ester syntactic foams. Tensile and flexural strength increased with fibre concentration, reached a maximum at a fibre concentration of 16.6 wt%, and decreased on further addition (Figure 3.1(a)). The specific properties also manifested a similar trend (Figure 3.1(b)). The general pattern of increase in strength values is ascribed to the load-bearing capacity of the fibrous reinforcements, which are very effective in sustaining the load that is transferred onto them from the matrix. The diminution in strength beyond a fibre concentration of 16.6 wt% was ascribed to the poor wetting of fibre by resin, which leads to easy pull-out of fibres (Figure 3.2). Here, the resin concentration is too low to wet all the fibres and, as a result, the poor resin-fibre interface weakens the resin-fibre bond strength [6]. Wouterson and co-workers also observed an increase in the ultimate tensile strength and Young’s modulus with increasing fibre loading for epoxy (Epicote 1006) syntactic foams with hollow phenolic microspheres (Phenoset BJO-093). The ultimate tensile strength increased by 40% and Young’s modulus by 115% with the addition of 3 wt% short carbon fibre (SCF). For the Young’s modulus, a linear additive trend with the increase in SCF weight fraction was observed [7]. In the case of bare syntactic foams, a process involving resin fracture and resin–microballoon debonding rather than crushing of the microballoons dominated the tensile failure. Apart from these failure
64
Recent Developments in the Field of Syntactic Foams
Tensile/Flexural strength (MPa)
Tensile strength Flexural strength
18 16 14 12 10 8 6 4 4
6
8
10
12
14
16
18
20
22
Weight percentage of fibre
(a)
Specific tensile/flexural strength (MPa/(kg/m3)) x 10–3
Specific Tensile strength Specific Flexural strength
40 36 32 28 24 20 16 12 8 4
6
8
10
12
14
16
18
20
22
Weight percentage of fibre
(b)
Figure 3.1 Variation in (a) tensile and flexural strength and the corresponding (b) specific strength with fibre concentration. Reproduced with permission from B. John, C.P.R. Nair and K.N. Ninan, Polymers and Polymer Composites, 2008, 16, 431. ©2008, Smithers Rapra Technology Ltd [6]
65
Update on Syntactic Foams
Figure 3.2 Pull out of fibres as observed in a high fibre loading. Reproduced with permission from B. John, C.P.R. Nair and K.N. Ninan, Polymers and Polymer Composites, 2008, 16, 431. ©2008, Rapra Technology Ltd [6]
modes, pull-out of fibres must also be considered in the case of fibrereinforced syntactic foams. The failure mode that dominates in a particular syntactic foam is determined by the relative concentration of resin, microballoon, and fibre, as well as their distribution. For plain syntactic foams, the cracks propagate in a straight manner, whereas the cracks propagate in a deviated path for fibre-reinforced syntactic foams. For a fibre-reinforced system, instead of a clear split of the sample, a bent-like appearance is observed. These confirm that reinforced syntactic foams are sufficiently stronger to take more load than fibre-free syntactic foams. For similar density levels, reinforced syntactic foams exhibit fewer brittle microscopic features than unreinforced syntactic foams [6]. Karthikeyan and co-workers found that compressive modulus and specific compressive modulus increased with increase in fibre concentration despite the presence of voids (Table 3.1). The increase in compressive properties is due to the increased load-bearing ability 66
42
41
42
41
42
E1SF
E2SF
E3SF
E4SF
E5SF
48
47
48
47
48
53
Microballoon
4.5
3.5
2.7
1.7
0.9
—
Fibres
Composition of syntactic foam (vol%)
6
9.4
7.6
11
9.6
0.45
Voids
720
676
671
633
621
670
Density (kg/m3)
1.787
1.090
1.425
1.308
1.023
1.076
Compressive modulus (GPa)
25
16
21
21
16
16
Specific modulus (GPa/(kg/m3)) × 10–4
Reproduced with permission from C.S. Karthikeyan, S. Sankaran and Kishore, Materials Letters, 2004, 58, 6, 995. ©2004, Elsevier [4]
47
Matrix
SF
Sample code
Table 3.1 Composition and compressive properties of fibre-reinforced syntactic foams
Recent Developments in the Field of Syntactic Foams
67
Update on Syntactic Foams of the fibrous reinforcements. The poor compressive properties of E4SF are due to the difference in processing route adopted for this particular fibre-reinforced foam. Here the fibres were added to resin before microballoon addition during processing [4]. In another study, Karthikeyan and co-workers observed that fibre-free syntactic foams had a higher compressive strength than fibre-bearing syntactic foams, whereas moduli values differed only slightly. The difference in strength is correlated with the amount of voids present in the two foams. Another reason for low strength could be the clustering of fibres which occur due to non-uniform dispersion of fibres in the foam sample. Due to this clustering, the transfer of load is less effectively achieved and therefore a reduction in compressive strength was observed [8]. The compressive strength of fibre-reinforced syntactic foams after immersion in aqueous media consisting of saline or seawater shows that compressive strength decreased in samples exposed to water vapour, but the saline- or seawater-immersed samples showed higher compressive strength when compared with the dry sample. The decrease in strength in the water vapour-exposed case is ascribed to higher absorption of water and to debonding and damaging of interfaces. The enhanced strength values for the samples immersed in saltish media is due to the larger size of the chloride ion and resultant changes in the stress state around the fibre-bearing regions [9]. The length of fibre is also an important factor affecting the properties of fibre-reinforced syntactic foams. For cyanate ester syntactic foams, the effect of fibre length was studied by processing foam composites with different fibre length (5–25 mm) and maintaining the cyanate ester:microballoon:glass fibre ratio at a constant at 1:1:0.5 (by weight) [6]. The dependence of the tensile and flexural strength and the corresponding specific strength on fibre length is shown in Figures 3.3(a) and (b), respectively. The tensile and flexural strength increased with fibre length and reached a maximum at a fibre length of 20 mm. Further increase in fibre length leads to a diminution in tensile and flexural properties. The increase in tensile and flexural properties with increase in fibre length is attributed to the effective
68
Recent Developments in the Field of Syntactic Foams
Tensile/Flexural strength (MPa)
Tensile strength Flexural strength
20 18 16 14 12 10 8 6 4 5
10
15
20
25
Fibre length (mm)
Specific tensile/flexural strength (MPa(kg/m3)) x 10–3
(a) Specific Tensile strength Specific Flexural strength
35 30 25 20 15 10 5 5
10
15
20
25
Fibre length (mm)
(b)
Figure 3.3 Effect of fibre length on (a) tensile and flexural properties and the corresponding (b) specific properties of glass fibre-reinforced cyanate ester syntactic foams. Reproduced with permission from B. John, C.P.R. Nair and K.N. Ninan, Polymers and Polymer Composites, 2008, 16, 431. ©2008, Smithers Rapra Technology Ltd [6]
69
Update on Syntactic Foams transfer of the applied load along the length of fibre. In the case of fibre-reinforced composites, it is reported that the introduction of long fibre rather than short fibre provides a substantial improvement in properties such as creep resistance, impact strength, and shrinkage. When the fibre length is shorter, only a smaller portion of each fibre effectively resists the load in the composite. Therefore, shorter fibres will not work to full effect, and the strength gained from the addition of short fibres will be low. At low fibre length, the probability of fibre debonding occurring when a flexural load is applied is greater than in the case of long-fibre composites. If the fibre length increases, the area of a single fibre in contact with the resin increases. As a result, a good load will be effectively carried throughout the length of the fibre [6]. The decrease in the mechanical properties beyond a fibre length of 20 mm can be explained. As the fibre length increases, there is a chance that the fibre may curl. Also, the presence of microballoons acts as an obstacle to linear arrangement of fibres, which adds to the bending of fibres. Moreover, the fibres tend to bend during mixing of the constituents (resin, microballoon, and fibre). Figure 3.4a depicts the curling of fibres in the case of syntactic foam with a fibre length of 25 mm. The curling of fibres prevents appropriate alignment of fibres in syntactic foams. This leads to a reduction in the effective fibre length in the direction of the applied stress, which results in decreased tensile and flexural properties. Also, the probability of clustering of fibre with consequent resin starvation is greater in the case of long fibre-reinforced syntactic foams (Figure 3.4b) [6]. Yi-Jen Huang and co-workers studied phenolic syntactic foams with amino microspheres reinforced with aramid (Kevlar-49, length = approximately 12 mm) and carbon (C30, length = approximately 24 mm) fibres. The composition of the syntactic foams was kept constant (4 wt% fibre, 19 wt% phenolic resin, and 77 wt% amino microspheres). The density of all foams was 300 kg/m3. It was observed that fibre length had only a minor effect on compressive strength of the syntactic foams. Fibre length and orientations had major effects on shear properties in some foams. Higher moduli values were reported for long fibre-based foams (24 mm) compared 70
Recent Developments in the Field of Syntactic Foams
(a)
(b)
Figure 3.4 The flexurally failed surface of the syntactic foam with a fibre length of 25 mm, depicting (a) bending of fibres and (b) clustering of fibres. Reproduced with permission from B. John, C.P.R. Nair and K.N. Ninan, Polymers and Polymer Composites, 2008, 16, 431. ©2008, Smithers Rapra Technology Ltd [6] 71
Update on Syntactic Foams with shorter fibre-based foams (12 mm). If loaded perpendicular to the dominant fibre orientation, properties were enhanced 30–40% but, if load was applied parallel to the dominant fibre orientation, the compressive strength of the composite foam increased by up to 2; the tensile strength and modulus increased by up to 7 and 8, respectively, and the shear strength and modulus increased by up to 3.3 and 2.3, respectively. This shows that fibres were more effective in enhancing the strength of syntactic foams if loads were applied parallel to the dominant fibre orientation [1]. The fracture toughness, KIC, and energy release rate, GIC, increased by 95% and 90%, respectively, upon introduction of 3 wt% SCF in epoxy (Epicote 1600) syntactic foam, indicating the potent toughening potential for SCF in syntactic foam systems. The specific energy required to create new surfaces was enhanced by the presence of fibres and was the main contributor to the toughness of short fibre-reinforced syntactic foams (SFRSF) [7]. A study on thermal stability of SFRSF showed that thermal stability is independent of the fibre length and fibre weight fractions. The thermal stability of SFRSF is primarily governed by the matrix of the SFRSF. Storage modulus increased with the short fibre content (Figure 3.5a). Despite a slight increase in the glass transition temperature (Tg), compared with neat syntactic foam, the variations in fibre content and fibre length do not affect the Tg of SFRSF (Figure 3.5b) [7]. The effect of microballoon addition on the properties of silica fibre-reinforced polybenzoxazines has been reported. The variation in specific mechanical properties with increase in microballoon concentration is shown in Figure 3.6. Specific tensile strength enhanced upon inclusion of microspheres up to about 47% by volume of microballoon, beyond which a decreasing trend was observed. Specific compressive strength also followed a similar trend. Specific flexural strength decreases marginally and practically stagnates beyond 57% volume of microspheres. It can be concluded that a composition with about 55 vol% of microballoon is sufficient for moderate load-bearing thermostructural applications (low-density, moderate-strength materials) without great penalty on mass [10]. 72
Recent Developments in the Field of Syntactic Foams
Storage Modulus (GPa)
4.0
0 wt% SCF 1 wt% 3 mm 2 wt% 3 mm 3 wt% 3 mm 1 wt% 4.5 mm 2 wt% 4.5 mm 3 wt% 4.5 mm 1 wt% 10 mm 2 wt% 10 mm 3 wt% 10 mm
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 20
40
60
80
100
120
Temperature (°C) (a)
0 wt% SCF 1 wt% 3 mm 2 wt% 3 mm 3 wt% 3 mm 1 wt% 4.5 mm 2 wt% 4.5 mm 3 wt% 4.5 mm 1 wt% 10 mm 2 wt% 10 mm 3 wt% 10 mm
0.4
Tan δ
0.3
0.2
0.1
0.0 20
40
60
80
100
120
Temperature (°C) (b)
Figure 3.5 The variation of (a) storage modulus and (b) tan δ with temperature for short carbon fibre-reinforced syntactic foam with different fibre weight fractions and fibre lengths. Reproduced with permission from E.M. Wouterson, F.Y.C. Boey, X. Hu and S-C. Wong, Polymer, 2007, 48, 11, 3183. ©2007, Elsevier Science [7] 73
Sp.Mechanical Strength (MPa/kg/m3)
Update on Syntactic Foams 0.08 Sp.T Sp.F 0.06
Sp.C
0.04
0.02
0.00 35
40
45
50
55
60
65
70
Volume of microballoon (%)
Figure 3.6 Specific mechanical strength of microballoon-filled polybenzoxazine-silica fibre composites. Reproduced with permission from K.S.S. Kumar, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 107, 2, 1091. ©2008, Wiley [10]
The coefficient of thermal expansion (CTE) of a microballoon-filled benzoxazine-silica fibre system is lower than that of a benzoxazinesilica fibre system. The spherical microballoons in fibre-reinforced syntactic foams restrict the movement of the matrix and, as a whole, the dimensional change decreases and consequently the CTE value diminishes. Thus, fibre-embedded syntactic foams have better dimensional stability under thermal conditions than the corresponding fibre-incorporated polybenzoxazine [10]. Figure 3.7(a) and (b) show variation of the storage modulus and tan δ of silica fibre- and microballoon-filled polybenzoxazines with increase in temperature. The storage modulus of the polybenzoxazinesilica fibre system is considerably higher than that of neat resin. The storage modulus decreases with the introduction of microballoon. 74
Recent Developments in the Field of Syntactic Foams
Storage modulus (GPa)
10 BS 1 BSM 0.1
0.01 Bz-A 1E-3 0
100
200
300
400
Temperature (°C) (a) 0.6 Bz-A
tan δ
0.4 BS 0.2 BSM 0.0 0
100
200
300
Temperature (°C) (b)
Figure 3.7 Dynamic mechanical properties of polybenzoxazine (Bz-A), polybenzoxazine-silica fibre (BS2), and polybenzoxazinesilica fibre–microballoons (BSM) systems (a) Storage modulus and (b) tan δ. Reproduced with permission from K.S.S. Kumar, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 107, 2, 1091. ©2008, Wiley [10]
75
Update on Syntactic Foams The Tg of the neat polybenzoxazine was found to be 215 °C (Figure 3.7b). This was decreased marginally on the introduction of silica fibres and reverted back to the original value on microballoon incorporation. The Tg is higher for the microballoon-added systems and increases with microballoon content. This is due to the effect of the microballoons that affect the mobility of the polymeric chains in the interphases between the matrix and the microballoons. The lowering of the Tg for the polybenzoxazine-silica fibre system may due to the weak interphases between fibres and polybenzoxazine [10]. The void content plays a crucial part in determining the properties of fibre-reinforced syntactic foams. Besides voids, fibre distribution also has a great influence on the properties of fibre-reinforced syntactic foams. It is therefore essential to have control over voids and fibre distribution to achieve better mechanical properties. Thus, by carefully controlling the volume percentage of fibre, fibre length and processing parameters, fibre-reinforced syntactic foams with better properties compared with the bare syntactic foam could be achieved. In general, fibre incorporation improved the properties of the syntactic foam system without much variation in density, thereby making the reinforced syntactic foams act as improved core materials for sandwich and other structural applications.
3.2 Nanoclay-incorporated Syntactic Foams Nanoclay has been used to improve the properties of syntactic foams. The improvement in performance is attributed to the unique phase morphology and better interfacial properties of the resulting composites. In conventional composites, phase mixing occurs on a macroscopic scale whereas, in nanocomposites, phase mixing occurs on a nanometre-length scale. Many interfaces are created in a nanocomposite upon dispersion of nanoparticles, which leads to an increase in strength of the composite. In general, incorporation of nanoclay improves the strength, fracture strain and damage tolerance properties of syntactic foams without significantly affecting the density of syntactic foams. The highly damage-tolerant hybrid
76
Recent Developments in the Field of Syntactic Foams foams are useful as the core in sandwich composites for various applications. Maharsia and co-workers observed an increase in tensile strength with nanoclay addition (Figure 3.8a). This is attributed to the presence of exfoliated/intercalated nanoclay particles which restrict the mobility of polymer chains during tensile loading. The platey structure of nanoclay provides very high surface area, which reduces stress concentration in the matrix. A remarkable increase in the area of the interface with the surrounding matrix, along with better stress distribution in the matrix, results in increased tensile strength. Nanoclay particles have a significant role in delaying crack initiation and growth, resulting in enhanced tensile strength and toughness. The low tensile strength of S32-type foam with 2% nanoclay may be attributed to the presence of a significantly higher amount of void content in the samples. It is also observed that the modulus has decreased in all syntactic foams upon the addition of nanoclay particles (Figure 3.8b). These imply a significant increase in fracture strain of all nanoclay hybrid syntactic foams [11]. Studies done by Wouterson and co-workers showed a 13% reduction in tensile strength and 19.5% improvement in Young’s modulus by the introduction of 2 wt% of nanoclay to syntactic foam. The reduction in tensile strength was attributed to the presence of voids and agglomeration of the nanoclay particles. The tensile properties of these nanoclay-reinforced syntactic foam were not as good as similar properties observed for short fibre-reinforced syntactic foams [12]. Comparison of the flexural properties of syntactic foams with and without nanoclay is depicted in Figure 3.9. Addition of 2% nanoclay particles resulted in an overall reduction in flexural strength of the syntactic foams by about 11%, whereas addition of 5% nanoclay increased the strength of low-density syntactic foams by about 22%, with no considerable difference in the strength of nanoclay syntactic foams containing microballoons of higher shell thickness. Here, only a small fraction of nanoclay was exfoliated and most of the nanoclay was present as clusters. At higher volume fraction (5%), the intercalated and exfoliated content of nanoclay increase and thus 77
78 (b)
Figure 3.8 Comparison of (a) tensile strength and (b) modulus between nanoclay hybrid syntactic foams and pure syntactic foams containing different types of microballoons. Reproduced with permission from R. Maharsia and H.D. Jerro, Materials Science and Engineering A, 2007, 454–455, 1–2, 416. ©2007, Elsevier Science [11]
(a)
Update on Syntactic Foams
M38
M46
unmodified foam
(a)
C2 Nanoclay foam
C5 Nanoclay foam
0
M32
0
M22
500
1000
1500
2000
2500
3000
3500
5
10
15
20
25
Modulus (MPa)
unmodified foam
M22
M38
(b)
C2 Nanoclay foam
M32
C5 Nanoclay foam
M46
Figure 3.9 Comparison of flexural properties for various nanoclay-reinforced syntactic foam samples; (a) flexural strength, (b) flexural modulus. Reproduced with permission from R. Maharsia, N. Gupta and H.D. Jerro, Materials Science and Engineering A, 2006, 417, 1–2, 249. ©2006, Elsevier Science [13]
Flexural Strength (MPa)
30
Recent Developments in the Field of Syntactic Foams
79
Update on Syntactic Foams provide a strengthening effect. The fracture in the case of syntactic foams with microballoons of density 380 kg/m3 and 460 kg/m3 is governed by matrix failure so their strength should also be comparable. The flexural modulus in syntactic foams has increased by as much as 10%, upon the addition of nanoclay particles. The fracture strain is reduced by 15% due to the addition of 2% nanoclay particles. However, only a small change (<3%) is observed upon the addition of 5% nanoclay particles. Stiffness has increased by about 10% upon the addition of nanoclay particles [13]. Flexural toughness, which is measured as the area under the stress-strain curve, increased upon the addition of 5% nanoclay particles. However, the toughness showed a decrease upon the addition of 2% nanoclay particles [14]. Another study conducted by Maharsia and co-workers showed that foams containing 2% nanoclay particles have 10–20% lower compressive strength compared with plain syntactic foams. The strength of 5% nanoclay-containing foams is higher than the corresponding bare syntactic foams except in the case of syntactic foams with microballoons of density 380 kg/m3 and 460 kg/m3. Transmission electron microscopy showed partial intercalation of nanoclay in the specimens and also the clustering of the nanoclay particles. Such microstructure leads to nearly the same strength with considerable enhancement in fracture strain. Increase in fracture strain with little change in strength caused a considerable increase in toughness of the material. Hence, the toughness of the material, measured as the area under stress-strain curve, is found to increase by 80–200% [15]. A comparison of the KIC and GIC of nanoclay-reinforced syntactic foams and SFRSF is shown in Figure 3.10. Initially, a decrease in KIC is observed for syntactic foams containing 0–0.75 wt% of nanoclay. For foams containing 1–2 wt% nanoclay particles, an increase in KIC is observed compared with the pristine syntactic foam. KIC increases from 1.15 MPa.m0.5 for 0 wt% nanoclay to 1.63 MPa.m0.5 for 1 wt% nanoclay. KIC drops slightly for specimens containing >1 wt% nanoclay due to increased agglomeration of the nanoclay particles. Therefore, the presence of nanoclay or SCF in the syntactic foam
80
0.0
0.5
1.0
1.5
2.0
0.0
0.5
1.0
0.0 0.0
0.5
1.0
1.5
(b)
3.0
(a)
2.5
wt%
2.0
0.5
1.0
1.5
2.0
wt%
1.5
NCRSF SFRSF
GIC (kJ/m2)
2.0
2.5
NCRSF SFRSF
3.0
Figure 3.10 Comparison of the fracture properties of nanoclay-reinforced syntactic foam (NCRSF) and short fibre-reinforced syntactic foam (SFRSF) as a function of reinforcement concentration; (a) KIC and (b) GIC. Reproduced with permission from E.M. Wouterson, F.Y.C. Boey, S-C. Wong, L. Chen and X. Hu, Composites Science and Technology, 2007, 67, 14, 2924. ©2007, Elsevier Science [12]
KIC (MPa.m0.5)
2.5
Recent Developments in the Field of Syntactic Foams
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Update on Syntactic Foams resulted in substantial toughening effect. The GIC for syntactic foam containing various amounts of nanoclay is shown in Figure 3.10b. The trend in GIC versus wt% of nanoclay is similar to the trend observed for KIC. Introduction of up to 0.75 wt% nanoclay causes GIC to decrease from 0.66 kJ/m2 to 0.41 kJ/m2 (41% reduction). GIC shows a significant increase of 104% for specimens containing 1 wt% nanoclay compared with bare syntactic foams. The increase in KIC and GIC for syntactic foam containing 1 wt% nanoclay indicates the excellent toughening potential of nanoclay in syntactic foams [12]. The addition of nanoclay to cyanate ester syntactic foams has considerably improved the tensile, flexural and compressive properties. The specific properties also showed substantial improvement by nanoclay addition. The storage modulus is also improved by nanoclay addition. However, the Tg of the composites decreased as a result of plasticisation of the matrix by the modifier of the nanoclay [16].
3.3 Rubberised Syntactic Foams Epoxy systems (which are extensively used in syntactic foams) tend to form crosslinks when curing, which results in brittle behaviour. Therefore, the impact tolerance of epoxy-based foam is weak and the residual strength is low. Addition of rubber particles modifies the matrix of syntactic foams for achieving better impact- and fracturetoughening properties. The rubber particles absorb more impact energy through elastic deformation of the particles, which leads to a higher toughness of the matrix. Also, the lower-stiffness rubber particles serve as stress concentrators. When the stress exceeds the strength of the material, microcracks will initiate, consume a considerable amount of impact energy, and thus results in higher energy absorption capacity. The propagation of microcracks will be blunted, stopped, and arrested by the rubber particles through mechanisms such as rubber pinning and rubber bridging-over. Therefore, the addition of rubber particles is an effective method for absorbing impact energy [17]. Damage-tolerant hybrid foams based on rubberised syntactic foams will be useful for aerospace and marine structures.
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Recent Developments in the Field of Syntactic Foams Rubber hybrid foams processed using crumb rubber particles (rubber obtained from waste tyres), R40 (mean particle size = 40 μm) and R75 (mean particle size = 75 μm) have been reported. The rubber particle volume fraction in the composites is 2% and the ratio of rubber-to-polymer matrix volume fraction is nearly 5.7%. The variation in compressive properties with rubber particle concentration is shown in Figure 3.11. The compressive modulus is found to decrease by about 50%, whereas a decrease of about 10% is observed in compressive strength due to the incorporation of rubber particles. The decrease in modulus is due to the low modulus of rubber particles. Rubberised foams containing smaller rubber particles have higher strength and modulus due to their higher surface area compared with larger-size particles [14, 18]. Incorporation of rubber particles has a significant effect on the flexural properties of syntactic foams. Addition of R75- and R40-type rubber particles increased the flexural strength of syntactic foams by 15% and 18%, respectively. Foams with R40 rubber particles showed higher strength because smaller rubber particles have a higher probability of being engulfed in the process zone around a crack tip, which leads to crack bridging. Flexural modulus and stiffness have been found to increase by 10% by the addition of rubber particles. Flexural toughness also increases upon the addition of rubber particles. Smaller-sized rubber particles produce higher toughening of the foam matrix. The irregular structure of the rubber particle helps in developing mechanical interlocking between rubber particles and the cured epoxy resin apart from the interfacial bonding. R40 particles showed better results when used in 2 vol% [13–14].
3.4 Functionally Graded Syntactic Foams (FGSF) Functionally graded materials are gaining importance due to their increasing demands in modern engineering applications. They can be obtained by changing the particle volume fraction or the particle size along the thickness of the material. Functionally graded syntactic foams (FGSF) can be obtained by creating a gradient of
83
84
M22 M22
M32
M38
M46
(b)
0
(a)
M46
Hybrid Composite Type
M38
500
1000
1500
2000
2500
3000
Hybrid Composite Type
M32
R40
0 R75
Modulus (MPa)
Strength (MPa)
R40
0 R75
Figure 3.11 Comparison of (a) compressive strength and (b) modulus of hybrid foams containing R75 and R40 rubber particles with corresponding syntactic foams without rubber particles. Reproduced with permission from N. Gupta, R. Maharsia and H.D. Jerro, Materials Science and Engineering A, 2005, 395, 1–2, 233. ©2005, Elsevier Science [18]
0
10
20
30
40
50
60
70
80
Update on Syntactic Foams
Recent Developments in the Field of Syntactic Foams microballoon volume fraction (VF-type) or thickness [(radius ratio) RR-type] along the length or thickness of the syntactic foam to achieve variation in density and mechanical properties. The schematic representation of VF-type and RR-type are shown in Figure 3.12. VF-type systems have disadvantages such as the possibility of warping or localised swelling if exposed to varying temperature and moisture conditions due to the difference in resin concentration along the length or thickness. The approach also poses severe limitations on the minimum density achievable in such materials, when the filler particles have lower density than the matrix resin. It can also lead to premature fracture due to the non-uniform distribution of stress along the microballoon gradient and existence of resin/microballoon-rich sides in the material. For the RR-type, volume fraction is available as an additional parameter for controlling the properties of the FGSF [19–20].
Matrix
Microballoons
(a)
Increasing microballoon wall thickness (decreasing η)
Increasing microballoon volume fraction
Gupta and co-workers reported the compressive properties of VF-type and RR-type syntactic foams. VF-type FGSF showed a sharp
Matrix
Microballoons
(b)
Figure 3.12 Schematic representation of (a) VF-type and (b) RR-type functionally gradient syntactic foams. Reproduced with permission from N. Gupta and W. Ricci, Materials Science and Engineering A, 2006, 427, 1, 331. ©2006, Elsevier Science [19]
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Update on Syntactic Foams drop in stress (~40–60%) after the peak compressive strength value. However, such a feature is not observed in RR-type FGSF, leading to better control over strength and energy absorption. This type of syntactic foam can withstand compression for 60–75% strain without a considerable loss in strength. This material has the same volume fraction of microballoons throughout the structure, which eliminates the undesirable effects of functionally graded composites containing a gradient of particle volume fraction. The total energy absorption was found to be 3–5-times higher in RR-type FGSF compared with VF-type FGSF and plain syntactic foams. Thus, the use of FGSF in structural applications can significantly enhance the structural safety under compressive loading [11, 18]. The flexural properties of RR-type FGSF can be controlled more effectively. RR-type FGSF show better possibilities of tailoring their properties compared with conventional VF-type FGSF [21].
3.5 Syntactic Foam Core Sandwich Composites Syntactic foams have been widely used as core in sandwich composites due to their higher compressive strength, damage tolerance, low moisture absorption, high structural efficiency and good thermal insulation. Unlike honeycomb core sandwich composites (in which interfacial strength between the skins and core is critical due to the failure originating mainly at such places), the strength of syntactic foam core sandwich composites assumes importance because the material can fail at any point due to its structure [22]. Syntactic foams, due to their structure and formation, behave differently under compression compared with other traditionally used core materials. The damage tolerance of syntactic foams is found to be high, indicating their usefulness in aerospace applications [23]. The density and properties of the syntactic foam core (and hence those of the derived sandwich composites) can be tailored by varying the volume fraction of microspheres or by using microspheres of different shell thickness. The compressive modulus of syntactic foam core sandwich composites increases with decrease in the radius ratio
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Recent Developments in the Field of Syntactic Foams of microballoons due to an increase in their wall thickness [23]. We reported the variation in the mechanical properties of cyanate ester syntactic foam core sandwich composites as a function of volume percentage of microballoon. Sandwich composites with different core compositions (see Table 3.2 for composition) and property profiles are prepared by varying the volume percentage of microballoon in the core. The composition of the foam core influences the flatwise tensile strength, flatwise compressive strength, and edgewise compressive strength of sandwich composites (Table 3.3). Flatwise tensile strength increases with the increase in resin content of the core. However, the presence of voids at high resin concentration reverses this trend. The failure mode for flatwise tensile loading changes from core failure in the case of low resin content system (SA-1) to skin-to-core debonding in the case of high resin content systems (SA-2 to SA-5). The flatwise compressive strength and edgewise compressive strength and the corresponding modulus values increase with resin content of the core and are only slightly affected by the presence of voids at high resin loading. The failure mode under edgewise compression occurs by skin delamination followed by core crushing. Although the flatwise compressive modulus and edgewise compressive modulus
Table 3.2 Density and composition of sandwich composites Core composition (vol%) Cyanate ester
Microballoon
Void
Overall density (kg/m3)
SA1
10.9
82.1
7.0
560
SA2
16.3
79.4
4.3
610
SA3
21.1
68.3
10.6
630
SA4
26.5
57.4
16.1
640
SA5
33.0
46.0
21.0
680
Sample code
Reproduced with permission from B. John, C.P.R. Nair, D. Mathew and K.N. Ninan, Journal of Applied Polymer Science, 2008, 110, 3, 1366. ©2008, Wiley [24]
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Update on Syntactic Foams
Table 3.3 Mechanical properties of cyanate ester syntactic foam core sandwich composites Sample code
Ultimate Ultimate Flatwise Flatwise Edgewise flatwise edgewise tensile compressive compressive compressive compressive strength modulus modulus strength strength (MPa) (MPa) (MPa) (MPa) (MPa)
SA1
1.4 ± 0.3
16 ± 2
155 ± 3
12.5 ± 0.5
620 ± 15
SA2
3.0 ± 0.3
19 ± 1
181 ± 2
14.8 ± 0.5
640 ± 20
SA3
3.8 ± 0.2
21 ± 1
182 ± 1
17.8 ± 0.5
720 ± 20
SA4
4.2 ± 0.2
27 ± 1
381 ± 1
19.0 ± 0.4
750 ± 40
SA5
2.2 ± 0.2
42 ± 5
374 ± 2
19.4 ± 0.4
540 ± 10
Reproduced with permission from B. John, C.P.R. Nair, D. Mathew and K.N. Ninan, Journal of Applied Polymer Science, 2008, 110, 3, 1366. ©2008, Wiley [24]
of the sandwich composites increase with resin content, higher resin content is detrimental to these properties. Depending on the application and strength requirement, the suitable composition of core can be selected [24]. The porosity in syntactic foams is in the closed form, so they provide a relatively smoother surface compared with open-cell structured foams. The closed cell structure provides continuous contact between the skin and the core materials, improving interfacial strength compared with other cores (e.g., open-cell-structured foams). For open-cell structured foams, due to the presence of open porosity, depressions on the surface lead to point contact between the skin and the core while forming the sandwich structures. Therefore, nearplanar contact between skin and core can be achieved if syntactic foam is used as core material. This provides uniform stress distribution on the surface in the sandwich structure and superior bonding between skin and core compared with other polymeric foams. Lamination of the skins can be done directly over the core material to ensure integral
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Recent Developments in the Field of Syntactic Foams bonding between skin and core. For these reasons, use of syntactic foams as core materials for aeronautical structural applications is increasing. The ability of syntactic foams to keep the damage highly localised (especially in sandwich constructions) has led to the development of damage-tolerant designs [25]. Low moisture absorption is another advantage of using syntactic foams as core material. Even if the skin material is damaged and the core is exposed to the external environment, moisture absorption by the structure remains limited. In the case of the open-cell-structured foams and honeycomb-structured cores, damage to the skins may lead to considerable moisture absorption and can cause hygrothermal degradation and structural instability [23]. Syntactic foam core sandwich composites can be processed by onestep compression moulding or by a two-step process which involves bonding of the core to skin using an adhesive. If the one-step process is used, cross-linking of polymer between core and skin would provide adhesion strength level equal to the strength of the polymer. This provides the possibility of making the skin an integral part of the structure, eliminating the requirement of the adhesive. If an adhesive is used to bond the skin and core together, selection of adhesives becomes very important because they should be compatible with the skin and core materials. The adhesion must have the desired strength and should remain unaffected by the working environment [26].
3.6 Cement-based Syntactic Foams Cement-based syntactic foams have recently been introduced by Li and co-workers [27]. These types of syntactic foams are a multi-phase composite material with microballoons dispersed in a rubber latextoughened cement paste matrix. Other phases of these composites contain trace amounts of nanofibre and nanoclay. Cement-based syntactic foam has a higher capacity for dissipating impact energy without affecting the strength as compared with the control cement paste core.
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Update on Syntactic Foams Compared with a polymer-based foam core having similar compositions, cement-based foam has comparable energy dissipation capacity. The rubber latex toughens the cement matrix and stores more elastic strain energy. The microballoons reduce the weight, retain water tightness and provide micro-length-scale mechanisms for energy absorption. The nanoclay improves the crystal structure of the cement hydrates. Microballoon and microfibre increase the number of sites for energy absorption through microballoon crushing, microballoon-matrix interfacial debonding, matrix microcracking, and fibre pull-out. Figure 3.13 shows the fractured surface of a cement-based syntactic foam where microballoon crushing, interfacial debonding, and fibre pull-out can be seen. These serve to absorb impact energy, leading to higher energy dissipation capacity. Matrix microcracks are also seen in the figure. The creation of microcracks absorbs impact energy. However, the microcracks did not propagate into macrocracks or catastrophic structural failure because of the toughness of the rubber-modified cement paste matrix. Cement paste cracking is the primary mechanism for absorbing impact
Figure 3.13 Micro length-scale mechanisms for energy absorption in the case of cement-based syntactic foams. Reproduced with permission from G. Li and V.D. Muthyala, Materials Science and Engineering A, 2008, 478, 1–2, 77. ©2008, Elsevier Science [27]
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Recent Developments in the Field of Syntactic Foams energy. Once the microcrack is initiated, there are no mechanisms to prevent it from propagating into a macrocrack, leading to fracture of the specimen with an insignificant amount of energy absorption. The higher the cement content, the lower the initiation energy and the higher the propagation energy for cement-based syntactic foams. The higher the rubber concentration (rubber-to-cement ratio) in the core, the higher is the ability of the sandwich to retain its bending strength. Cement-based syntactic foams could be a potential material as the core in sandwich composites. The higher energy dissipation capacity, together with the higher capacity to retain strength, make cement-based syntactic foam a viable alternative core for lower-cost sandwich structures [27].
References 1.
Y-J. Huang, L. Vaikhanski and S.R. Nutt, Composites Part A: Applied Science and Manufacturing, 2006, 37, 3, 488.
2.
C.S. Karthikeyan, S. Sankaran and Kishore, Polymer International, 2000, 49, 2, 158.
3.
C.S Karthikeyan, S. Sankaran and Kishore, Macromolecular Materials and Engineering, 2005, 290, 1, 60.
4.
C.S. Karthikeyan, S. Sankaran and Kishore, Materials Letters, 2004, 58, 6, 995.
5.
C.S. Karthikeyan, S. Sankaran, M.N.J. Kumar and Kishore, Journal of Applied Polymer Science, 2001, 81, 2, 405.
6.
B. John, C.P.R. Nair and K.N. Ninan, Polymers and Polymer Composites, 2008, 16, 431.
7.
E.M. Wouterson, F.Y.C. Boey, X. Hu and S-C. Wong, Polymer, 2007, 48, 11, 3183.
8.
C.S. Karthikeyan and S. Sankaran, Journal of Reinforced Plastics and Composites, 2000, 19, 9, 732. 91
Update on Syntactic Foams 9.
C.S. Karthikeyan and S. Sankaran, Journal of Reinforced Plastics and Composites, 2001, 20, 11, 982.
10. K.S.S. Kumar, C.P.R. Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 107, 2, 1091. 11. R. Maharsia and H.D. Jerro, Materials Science and Engineering A, 2007, 454–455, 416. 12. E.M. Wouterson, F.Y.C. Boey, S-C. Wong, L. Chen and X. Hu, Composites Science and Technology, 2007, 67, 14, 2924. 13. R. Maharsia, N. Gupta and H.D. Jerro, Materials Science and Engineering A, 2006, 417, 1–2, 249. 14. R. Maharsia, Development of High Performance Hybrid Syntactic Foams: Structure and Material Property Characterization, Louisiana State University and Agricultural and Mechanical College, Baton, Rouge, LA, USA, 2005. [PhD Thesis] 15. R. Maharsia and H.D. Jerro in the Proceedings of the ME Graduate Student Conference, Louisiana State University, Baton Rouge, LA, USA, 2005, p.1. 16. B. John, C.P.R. Nair and K.N. Ninan in Proceedings of Recent Advances in Polymeric Materials, MACRO 2009, IIT, Chennai, India, 2009. 17. G. Li and M. John, Materials Science and Engineering: A, 2008, 474, 1–2, 390. 18. N. Gupta, R. Maharsia and H.D. Jerro, Materials Science and Engineering A, 2005, 395, 1–2, 233. 19. N. Gupta and W. Ricci, Material Science and Engineering A, 2006, 427, 1, 331. 20. N. Gupta, Materials Letters, 2007, 61, 4–5, 979.
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Recent Developments in the Field of Syntactic Foams 21. N. Gupta, S.K. Gupta and B.J. Mueller, Materials Science and Engineering: A, 2008, 485, 1–2, 439. 22. N. Gupta and S. Sankaran, Journal of Reinforced Plastics and Composites, 1999, 18, 14, 1347. 23. E. Woldesenbet, N. Gupta and H.D. Jerro, Journal of Sandwich Structures and Materials, 2005, 7, 2, 95. 24. B. John, C.P.R. Nair, D. Mathew and K.N. Ninan, Journal of Applied Polymer Science, 2008, 110, 3, 1366. 25. N. Gupta, E. Woldesenbet, Kishore and S. Sankaran, Journal of Sandwich Structures and Materials, 2002, 4, 3, 249. 26. N. Gupta, Characterization of Syntactic Foams and their Sandwich Composites: Modeling and Experimental Approaches, Louisiana State University and Agricultural and Mechanical College, Baton, Rouge, LA, USA, 2003. [PhD Thesis] 27. G. Li and V.D. Muthyala, Materials Science and Engineering A, 2008, 478, 1–2, 77.
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4
Applications of Syntactic Foams
Syntactic foams offer an outstanding combination of properties when compared with conventional cellular foams and bulk materials. They can be readily moulded and machined into various desired shapes according to application requirements. The possibilities of making a wide range of densities are the vital to these materials gaining increasing attention. In general, syntactic foams find increasingly diverse applications, ranging from packaging materials for expensive components to core material in sandwich structures used in automobile, aerospace, transportation and deep-sea submersibles. This chapter gives an account of the different fields in which syntactic foams find application.
4.1 Syntactic Foams in Buoyancy Applications The most important application of syntactic foams is in deepsubmergence buoyancy where compressive and low moistureabsorption properties are of prime importance [1]. They have been used as the principal source of supplementary buoyancy in recently developed deep submergence vehicles. Glass microballoon-based syntactic foams are particularly useful for this application due to their high compressive strength [2]. The advantage of using syntactic foam is that the buoyancy can be increased by increasing the volume fraction of microballoons. Most of the several thousand tons of syntactic foams produced each year are used for flotation in offshore drilling rigs, buoys, small boats, and submarines. The combined effect of increased hydrostatic pressure, temperature and
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Update on Syntactic Foams water ingress found in ultra-deep water places a greater demand on syntactic foams. Trelleborg Emerson & Cuming, Inc., have introduced different grades of syntactic foams for undersea buoyancy applications. These syntactic foam blocks are used to fabricate large buoyancy modules that can be readily shaped to conform to hull contours and outfitted for installation in the forward and aft free-flood areas of submarines. Syntactic foams are also used to manufacture manned and unmanned submersibles due to their range of densities and ability to resist exposure to diesel fuels and hydraulic fluids. Syntactic foam counterbalances the heavy construction of the vehicle. Alvin (the vehicle used in the discovery of the Titanic) was constructed using Trelleborg’s man-rated syntactic foam buoyancy materials (Figure 4.1) [3]. DS Grade syntactic foams manufactured by Trelleborg Emerson & Cuming, Inc., use glass microballoons and multifunctional epoxy
Figure 4.1 A 2,218 pound piece of syntactic foam used in Alvin. Alvin Overhaul, Oceanus, Woods Hole, MA, USA, 2005. Reproduced with permission from Trelleborg Offshore [4]
96
Applications of Syntactic Foams resin to produce an ultra-high strength-to-weight ratio material for high-performance, deep-sea applications including unmanned submersibles rated to perform at ocean depths up to 20,000 feet as well as man-rated submersibles. TG grade syntactic foams are used to fabricate eyebrows for 688 class US nuclear submarines because of their buoyancy, acoustic profile and ability to significantly improve sonar functions. VF Grade epoxy syntactic foams combine microballoons and macrospheres for ultra-lightweight applications such as filling of control surfaces on class 688 US nuclear submarines [3]. Figure 4.2 shows the syntactic foam block being used in US nuclear submarines [5]. Syntactic foams have been a highly buoyant structural material for open-frame, remotely operated vehicles (ROV). The structural
Figure 4.2 Trelleborg’s syntactic foam being used in US nuclear submarines. It can withstand blasts from 60 lb of high explosives at 20 ft. Trelleborg – Syntactic Foam and Surface Buoys, Trelleborg, Lancashire, UK. Reproduced with permission from Offshore [5]
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Update on Syntactic Foams component is fabricated from advanced composite materials and syntactic foam. The buoyancy element employs a syntactic foam core, which is then over-wrapped with an advanced composite braid. The triaxial carbon braid is then impregnated with an epoxy resin and cured at room or elevated temperature. The advanced composite hybrid offers enhanced axial stiffness and strength properties comparable with metals as well as positive buoyancy [6]. In the area of naval and marine engineering, syntactic foams have been used for structural elements such as: hulls, ribs and decks; for components at deep depth such as submarines; submerged buoys; deep-sea platforms and pipe joints; and for shielding and repairing submerged apparatuses [7]. Oceanographers depend on syntactic foams to suspend instrumentation in deep ocean studies. Buoyancy modules made of syntactic foams have a dual role for the Placid Oil Company. They operate the world’s largest floating production system located in Green Canyon block 29, Gulf of Mexico. For the first time, syntactic foam buoyancy modules have been converted to the task of housing flow and service lines in addition to providing vital buoyancy for the riser string. Specially designed Eccofloat syntactic foam modules suited to the main riser were made by Grace Syntactics, Canton, USA [8]. Syntactic foams have been suggested as a foamed jacketing compound for a buoyant underwater communication cable for the US Navy. Here, an expandable thermoplastic microballoon consisting of a thermosetting acrylic shell that encapsulates isopentane was chosen as the syntactic foaming agent. A metallocene-catalysed polyolefin was used for the base material of the foamed jacketing material. During the extrusion process, the acrylic microballoons soften. The blowing agent (isopentane) within the sphere is heated, but is contained by the hydrostatic pressure exerted by the surrounding polymer phase as well as the modulus of the acrylic shell. Once the melt exits the die of the extruder, the isopentane expands until the modulus of the acrylic shell increases due to cooling of the extrudate and prevents further expansion [9].
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Applications of Syntactic Foams
4.2 Syntactic Foam as Thermal Insulation Material Syntactic foams have been used as thermal insulating material for deepwater subsea equipments and pipelines. The hollow spherical particles in syntactic foams encapsulate the insulating gas protecting the insulation from collapse due to the high pressures encountered in the subsea environment [10]. Components that are typically insulated include wet trees and valves, jumpers, sleds and pipelineend manifolds, risers and flowlines [11]. Polypropylene syntactic foam thermal insulation tapes have been launched by Trelleborg Emerson & Cuming, Incorporated as deepwater insulation. These glass microballoon-based tapes provide efficient insulation and are sufficiently robust to be incorporated within flexible pipe solutions. The thermal insulation materials manufactured by Trelleborg Emerson & Cuming, Inc., offer high insulation and buoyancy. For high temperature deepwater thermal insulation applications, a blend of Eccospheres glass microspheres and thermoplastic resins are specially formulated to meet the exact requirements [3]. A new underwater thermal insulation material has been designed that is a hybrid composite of two constituents: a syntactic foam and an insulating aerogel blanket. The hybrid insulation has thermal resistance that is significantly higher than neoprene foam and underwater pipeline insulation at atmospheric and elevated hydrostatic pressures (1.2 MPa). The total thermal resistance of the hybrid insulation decreased 32% at 1.2 MPa and returned to its initial value upon decompression. With modifications, the hybrid insulation could be used for wetsuit construction, shallow underwater pipeline insulation, or any underwater application where high thermal resistance, flexibility, and resistance to compression are required [12]. The offshore exploration and conveying of oil and gas resources in deep water require the use of insulated pipelines. If the pipeline temperature drops too low, heavy components in crude oil can
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Update on Syntactic Foams solidify into waxy material that can clog the line, and natural gas can form hydrates that can also cause pipeline blockage. These deep-water wells put stringent requirements on insulation products to withstand compressive loads and to offer heat loss resistance. Syntactic foams have been identified as suitable insulating materials for this application [13–14]. In the startup phase, the materials remain intact, with no significant volume change. As pressure and temperature in the well rise during operations, the materials begin to compress and relieve pressure in the narrow, confined space of the annulus. Finally, when conditions reach preset limits, the syntactic foam undergoes a sudden and dramatic collapse, preventing excessive overpressure, and protecting the steel casing. An important advantage of this material is that it is passive, requiring no controls or active intervention. It responds automatically to protect the well casing from overpressures and temperature spikes. The properties of the material can be adjusted to suit a wide range of conditions inside a given well, or from one well to another [15].
4.3 Syntactic Foams in the Aerospace Industry There is an increasing demand for syntactic foams in the aerospace industry. Depending on the resin system used, syntactic foam has the potential for an improved transition temperature and a low dielectric constant, making it ideal for use in aerospace applications. Syntactic foams are good potting compounds, and have been used in the aerospace industry as fillers for finishing holes and edges in honeycomb structures. For thin panels, they may be specified for entire sandwich cores [16]. Westland Aerospace (Isle of Wight, UK) have used syntactic foam plugs for several years. Here, the syntactic foam consists of small, hollow, phenolic-type spheres in a thermosetting resin binder (mineral-filled paste epoxy, a plasticised liquid epoxy and an appropriate hardener). This syntactic foam has very high compressive yield strength for several applications. The composition of the syntactic foam specified by Westland Aerospace is Araldite AV 121, 75 parts by weight;
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Applications of Syntactic Foams Araldite AY 103, 25 parts by weight; hardener HY 951, 5–6 parts by weight; and phenolic microballoons BJO-0930, 10 parts by weight of binder [17–18]. Epoxy- and phenolic-based syntactic foams have been successfully used for thermal protection of atmospheric re-entry space vehicles and to prevent structures from the extreme heat flux of rocket exhaust plumes. Phenolic syntactic foams have low thermal diffusivity and high char-forming properties which are desirable for a good ablative. Moreover, syntactic foams are sufficiently strong to encounter the aerodynamic shear and have adequate mass-saving advantage. The thermal protection systems having low density as syntactic foams are advantageous in reducing the weight of the total payload, or the lift-off weight of the launch vehicle. The Pathfinder used a phenolic honeycomb filled with a cork- and silica bead-filled epoxy, SLA-561, for the Mars entry heat shield [19]. Syntactic foam-based lightweight polymer composites have been successfully used in thermal protection of atmospheric re-entry spacecrafts such as ARD (ESA), Apollo (NASA) and Shenzhou, China. ARD and Apollo used phenolic resin as binder, but Shenzhou is reported to have used silicone for this purpose. An epoxy-based composite nose cap has been developed for the space shuttle solid rocket boosters (SRB). These consist of a core of epoxy-based syntactic foam (3M SC350G) sandwiched between composite laminates (Hexcel’s AGP370-8H/3501-6). The composite laminates provide structural strength and the syntactic foam serves as a thermal barrier. Studies show that a nose cap made of this sandwich and top coated with Hypalon paint should be an adequate replacement for the aluminium nose cap that is currently being used on the SRB [20]. Special formulations of syntactic foam are used by NASA to insulate the fuel tanks and solid rocket boosters of the Space Shuttle [21]. A ‘spray-on’ syntactic foam insulation that does not crack from repeated thermal cycling has been suggested for cryogenic applications. This material will be ideally suited for applications in which vacuumjacketed insulation proves uneconomical, and if current foams and
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Update on Syntactic Foams foam-glass fail prematurely due to thermal cracking of the vapour barrier, leading to moisture attack. The proposed insulation system can be engineered to each set of requirements, and economically installed and maintained in the stated environment. Successful development of an insulating syntactic foam capable of being applied by a spraying process without the use of solvents would provide a novel approach for the insulation of pipes and other cryogenic systems. This will mitigate the liquid oxygen, fire, and corrosion hazards associated with currently used insulations [22]. A design concept for composite wind tunnel compressor blades at NASA Ames Research Center was based on a sandwich construction composed of carbon fibre-reinforced polymer skins bonded to a syntactic foam core [23].
4.4 Syntactic Foams in Radomes Honeycomb structures which are used in sandwich panels of radomes are highly orthotropic (i.e., the properties vary with direction), which leads to unpredictable signal propagation in radomes. Also, the strength and stiffness of honeycomb sandwich panels vary with the direction of loading. Other properties such as the dielectric constant and thermal conductivity are also affected. All these problems can be solved by the use of syntactic foam core sandwich composites [24]. Radomes can be made entirely from syntactic foam with an outer protective rain-erosion covering. The density (and hence the dielectric constant) of syntactic foams can be adjusted and specified to suit the particular radome application. The British Aerospace Dynamics Group, UK, has developed a syntactic foam as a construction material in the manufacture of precisionmoulded radomes for use in broadband microwave applications such as electronic warfare systems. A low-pressure compression moulding technique enables syntactic foam radomes accurately to shape in one production operation. Radomes made by this method are strong and light, and possess consistent (and even microwave transmission)
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Applications of Syntactic Foams properties. Quartz microspheres and epoxy or polyimide resins are used mainly for high-performance radomes [2].
4.5 Syntactic Foams in the Sports Industry New applications are emerging in the sports industry based on syntactic foams (e.g., snow skis, Adidas soccer balls are some of the examples). Bayer and Adidas produced high-tech soccer balls for the EUFA EURO 2004 Championship. The new official ball, called the ‘Fevernova’, is highly resistant to abrasion and has very low water-absorption properties. Figure 4.3 shows the schematic of Fevernova. The surface material of the ball is a high-solid polyurethane coating. Beneath the surface is an elastic layer of Impranil (syntactic polyurethane foam with low-temperature flexibility). The foam consists of equal-sized, highly elastic, exceptionally resistant gas-filled microcells. This composite ensures particularly good ‘feel’ and good damping properties of the ball. According to Adidas, the ball returns energy equally at all points on the ball, which makes the flight of the ball more accurate and more predictable than any previous ball. Moreover, the foam is highly elastic, so more of the force is transferred to the ball when it is kicked, causing it to travel faster [25–26].
Figure 4.3 Different layers present in ‘Fevernova’. Players Bawl over Ball, BBC Sport, London, UK, 2002. Reproduced with permission from BBC Sport Online [27]
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Update on Syntactic Foams
4.6 Syntactic Foams for Furniture Applications The materials used in the construction of modern articles are undergoing a radical change. Therefore, attention has been focused by material scientists to develop polymer-based materials to substitute wood for various applications [28]. The use of cast thermoset resins to make furniture parts is growing rapidly despite the problems of high density and brittleness. Syntactic foams are examined as ways to solve these problems without loss of the inherent advantages of cast thermosets, i.e., short initiation time from master to production parts, low equipment costs, low mould cost, high-quality detail reproduction, easy finishing, low part cost, and fast cycle times [29]. Among the organic polymeric microspheres, Saran microballoons give the best results for the fabrication of synthetic wood. In general, saran microspheres are expandable thermoplastic spheres composed of polyvinylidene chloride (PVDC) co-reacted with other monomers to form a micron-sized shell encapsulating a low-boiling-point hydrocarbon. Ultra-low density PVDC microspheres with high resiliency have come into the market for the production of syntactic foam-based synthetic wood having improved toughness and nailability. The addition of even a small quantity (~3–4%) of PVDC microspheres to a typical epoxy or polyester casting formulation can provide the characteristics of wood in terms of natural feel and sound, acceptance of machine screws, nails and staples and desired level of density (approximately 0.64 g/cm3) [28]. In polyester synthetic wood castings, saran microspheres offer many advantages over other commonly used lightweight fillers. The cost per pound of microspheres is fairly high, but their cost per cubic foot is less compared with the cost per cubic foot of polyester resin, making them an excellent material for reducing the cost and density of the final product [30]. PVDC-based synthetic wood has been used in boat and other marine applications owing to its exceptional water-resistance, durability and mechanical strength; these characteristics are better than natural wood in many respects. They allow sandwich core hulls and decks with the same weight as a solid laminate but with stiffness 3–8 times
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Applications of Syntactic Foams greater and impact resistance up to 7–8 times greater. Even if PVDC microsphere-filled synthetic wood is damaged, there is no significant absorption of water by foam or water vapour transmission through the foam [28].
4.7 Syntactic Foams as Synthetic Marble The major drawbacks of natural and synthetic marble are their weight and low impact strength. A 0.5 m × 0.5 m tub back panel weighs >57 kg, whereas bathtubs start at >90 kg each. The use of microspheres in synthetic marble formulations can reduce the weight of parts produced to one-half or less while eliminating the need for titanium dioxide pigment. The products containing microspheres have a translucency resembling natural marble. They have about double the resistance to impact breakage and slightly lower flexural and compressive strength. If a gel-coat is used, stain resistance is comparable with that of the heavier synthetic marble [31]. The use of saran microspheres in the reinforcing backup for acrylic bathroom fixtures can provide significant improvements in part quality at a lower raw material and labour cost. The microspheres in the resin matrix produce a laminate with increased stiffness with a reduction in weight when compared with resin/glass backup systems. The improvements in quality obtained when using microspheres are no delamination during cure or edge trimming, lower exotherm, no warpage, lightweight and opacity [30].
4.8 Syntactic Foam for Air-equivalent Solid Backing Syntactic foams are known to be useful materials as low-density, solid, spurious-echo-free backing materials for high-efficiency, air-equivalent backed transducers, typically, for imaging and CW Doppler systems, including 2D focal plane array (FPA) configurations. It was found that the shirasu microballoon-based epoxy syntactic
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Update on Syntactic Foams foam can easily make air-equivalent backing material whose acoustic impedance is <0.2 Mrail yet sufficiently ‘solid’ to support fragile piezoelectric substrate or array. Another designation can yield matched-to-water material good for lens and/or echo-free water vessel walls. Its cost advantage also benefits if quite a large-size component is required. However, careful filtering of larger balloons is required if t7–10 MHz frequency is required [32].
4.9 Shape Memory Syntactic Foams Shape memory syntactic foam systems have been introduced by Cornerstone Research Group (CRG), Inc., USA, under the trade name Verilyte. Verilyte can function as deployable or conformal composite foam core in its softened form, or as a structural foam core in its rigid state. Figure 4.4 illustrates the compressed and expanded form of shape memory syntactic foam. Current developmental formulations
Figure 4.4 Compressed and expanded forms of shape memory syntactic foam made by CRG. Reproduced with permission from Verilyte, CRG, Dayton, OH, USA. Source: www.crgrp.com/verilyte.shtml, ©Cornerstone Research Group, Inc. [33] 106
Applications of Syntactic Foams have densities ranging from 0.4 g/cm3 to 0.1 g/cm3, with expansion ranges up to 400%. These qualities provide the option to fabricate adaptive composite structures with a foam core, soften the structural foam element, alter its shape, and then return it to a rigid state in its new configuration. Verilyte can be reshaped several times. Possible applications include dynamic structural support, flexible foam core, and expandable foam fill [33].
4.10 Syntactic Foam Plug Assist Materials Trelleborg Emerson & Cuming, Inc., make syntactic foam plug assist materials known as Syntac using high-purity, hollow-glass microballoons in a high-performance epoxy matrix. Figure 4.5 shows a photograph of syntactic foam plug assist materials. These materials have outstanding consistency, low void content, and easy, low-dust, high-speed milling. The advantages of syntactic foam plug assists include even distribution of sheets, elimination of webbing, dimensional stability at high temperature, lower thermal conductivity, easy handling, and better durability than wood. Syntac maintains its hardness right up to its specified maximum running temperature and exhibits excellent abrasion resistance. It is a lightweight, durable and cost-effective alternative to wood, felt, aluminium and derlin.
Figure 4.5 Syntactic foam plug assist materials. Syntac® Syntactic Foam Plug Assist Materials, Trelleborg Offshore Boston, Inc., Mansfield, MA, USA. Reproduced with permission from Trelleborg Offshore [3] 107
Update on Syntactic Foams Syntac plug material is engineered to address problems inherent in traditional thermoforming tooling such as sticking, deformation and failure. Syntac plugs exhibit extremely low thermal conductivity, so they do not draw heat away from the sheet, virtually eliminating the primary cause of plug sticking or fouling. Syntac plugs are quick and easy to replace and are repairable if subject to minor damage [3]. Aircraft manufacturers (e.g., Airbus and Boeing) reinforce hollow areas within the aircraft with syntactic foam [21].
4.11 Expandable Graphic Art Printing Media using Syntactic Foam Syntactic foams are suitable for printing two- and three-dimensional graphic designs on various substrates, including paper and textile fabrics. Syntactic foam graphic arts print media are particularly suitable for application to a substrate using high-speed mechanical printing processes. The application involves adding to a basic print medium a minor amount of pre-expanded, non-friable, thermoplastic polyvinylidene chloride-based microspheres that are substantially heatstable between about –20 °C and 175 °C and having a particle size of about 10 μm to 100 μm in diameter to produce syntactic foams having an unlimited shelf-life. The print media thereby produced may be applied to any known substrate in any desired graphic pattern [34].
4.12 Syntactic Foams in Underwater Sound Transducers Underwater sound transducers are often employed under high hydropressure. To realise a transducer with the desired directivity in the deep sea, a new transducer construction has been devised in which a syntactic foam is attached to a sensitive cylindrical element. Syntech Materials, Inc., USA, combine syntactic foam technology with advanced acoustics theory to develop a novel class of underwater sound attenuators. These materials, known as the syntactic acoustic damping material (SADM) and syntactic acoustic transducer baffle (SATB) families of 108
Applications of Syntactic Foams syntactic foams, have repeatedly shown high levels of echo reduction and insertion loss over the 10–100 kHz range at ambient pressure and at operating depth. Two types of materials are available: the –1 series for broadband attenuation and –0.5 series for improved performance at higher frequencies (>30 kHz). These materials have been found to be extremely effective in several underwater acoustic applications. SADM has been used extensively as an anechoic acoustic test tank lining. SATB is designed for use as a transducer isolation mounting material [35].
4.13 Syntactic Foams in the Airbus The leading edge honeycomb panels of Airbus A330/340 have been built by The Aerostructures Corporation, France, since 1989. These honeycomb panels are relatively flat in contour and are located behind the curved metal portion of the leading edge. The upper leading edge panels are fabricated of woven fibreglass skins and Nomex honeycomb, whereas the lower edge panels are fabricated of woven carbon skins and Nomex honeycomb. In addition, around the panel periphery, the lower carbon skins use Syncore syntactic foam between carbon plies [36]. Some aircraft designs now use syntactic foams in place of honeycombs, especially in parts of the structure that are highly contoured or involved with communication, navigation, or radar antennae. Honeycombs are suited for simple flat panels but, for aircraft applications, with their many antenna installations and highly contoured mouldlines, there is no substitute for syntactic foams [16].
4.14 Miscellaneous Applications In the production of disposable drinking cups and plastic packaging, syntactic foam is used because it is a highly durable material capable of withstanding the millions of cycles associated with repetitive thermoforming [21]. TG Grade syntactic foams manufactured by Trelleborg Emerson & Cuming, Inc., have been used in mine neutralisation systems 109
Update on Syntactic Foams because of their zero magnetic and seawater-comparable acoustic signatures. If a military vehicle drives over a device such as a landmine or improvised explosive device, the outer metal structure often protects the occupants from injuries caused by shrapnel penetration. However, shock waves from the explosion can often result in serious injuries. Syntactic foams act as a shock absorber to reduce this risk of serious injury. The photograph of a syntactic foam blast protection system is shown in Figure 4.6. This system provides a field-installable protection against explosive devices for floorboard undercarriage shields, door guards and firewalls. Damage is minimised as are the effects of fire and secondary explosions [37]. Improved syntactic foam replaces graphite composite in the jet engine Nacelle. The syntactic foam loaded with high compressive strength microspheres is sandwiched between thin layers of composite. The foam core combines a 177 °C cure epoxy with the glass microspheres and has a thin non-woven Kevlar carrier. The material was developed by American Cyanamid (USA) [38]. Elastomeric syntactic foams have been developed for use as stress relief coating on electronic components in encapsulated assemblies.
Figure 4.6 Syntactic foam blast protection system. Syntactic Foam Blast Protection, AEM Product Group, Trelleborg, Offshore UK, Skelmersdale, Lancashire, UK. Reproduced with permission from Trelleborg Offshore [37]
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Applications of Syntactic Foams These foams have been made with various castable thermosetting elastomers and microbubbles [39–40]. Syntactic foams are also employed in electronics and telecommunications due to their superior thermal and dielectric properties [41]. They are also employed in electronics and telecommunication fields for shielding electronic components or cables from vibrations, electromagnetic fields, radiations and high temperatures [7]. Due to high specific energy absorption, and impact resistance, syntactic foams have been considered for army vehicles and for personnel protection [41]. Microwave transparency of syntactic foam gives an additional advantage in military aviation [42]. In the motor car industry, syntactic foams have been used for spoilers, dashboards and roofs [7]. Carbon–carbon syntactic foams find application as structural members for aerospace applications, furnace electrodes, insulant for hightemperature furnaces, and as filter material for corrosive chemicals and gases [27]. Syntactic foams based on Saran microspheres are used as fibreglass-reinforced backup for vacuum-formed acrylic structures, and in sandwich panel constructions [29]. Carbon microspheres combined with carbonaceous matrix, polyimide or epoxy resins are used as high-temperature insulation materials and, in some cases, due to their appropriate electronic conductivity, may be used for attenuating electromagnetic energy [2].
References 1.
O.L. Ferguson and R.G. Shaver, Journal of Cellular Plastics, 1970, 6, 3, 125.
2.
A. Calahorra, O. Gara and S. Kenig, Journal of Cellular Plastics, 1987, 23, 4, 383.
3.
Syntac® Syntactic Foam Plug Assist Materials, Trelleborg Offshore Boston, Inc., Mansfield, MA, USA.
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Update on Syntactic Foams [http://www.trelleborg.com/en/Emerson/ Products--Solutions/Syntac/] 4.
2005 Alvin Overhaul, Oceanus, Woods Hole, MA, USA. [http://www.whoi.edu/oceanus/viewSlideshow.do?clid=5712 &aid=8247&mainid=16953&p=16947&n=16959]
5.
Trelleborg – Syntactic Foam and Surface Buoys, Trelleborg, Lancashire, UK. [http://www.naval-technology.com/contractors/ advanced_materials/trelleborg/trelleborg3.html]
6.
J.B. Hinves and C.D. Douglas, IEEE, 1993, 3, 3, 468.
7.
E. Rizzi, E. Papa and A. Corigliano, International Journal of Solids and Structures, 2000, 37, 40, 5773.
8.
Oil and Gas Journal, 1988, 86, 32, 67.
9.
L.J. Trainer, D. Beauregard, S. Orroth and N. Schott in Proceedings of the SPE Annual Conference – Antec 2000, Orlando, FL, USA, 2000, Paper No.49.
10. J. Plummer, M. Toupin and J. Yeh, inventors; Emerson & Cuming Composite Materials Inc., assignee; US 6,284,809, 2001. 11. L. Watkins and E. Hershey, Oil and Gas Journal, 2001, 99, 23, 49. 12. E. Bardy, J. Mollendorf and D. Pendergast, Journal of Physics D: Applied Physics, 2006, 39, 9, 1908. 13. T. Fine, H. Sautereau and V.S. Moynot, Journal of Materials Science, 2003, 38, 12, 2709. 14. L. Watkins and E.E. Hershey in Proceedings of the 16th Annual Energy – Sources Technology Conference and Exhibition, Houston, TX, USA, 1993.
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Applications of Syntactic Foams 15. W-T. Wang and L. Watkins in Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering, 2003, 3, p.123. 16. R. Erikson, Mechanical Engineering Magazine, 1999, January. [http://www.memagazine.org/backissues/ membersonly/january99/features/foams/foams.html] 17. P. Bunn and J.T. Mottram, Composites, 1993, 24, 7, 565. 18. The Manufacture of Pre-moulded Lightweight Inserts for Honeycomb Sandwich Panels, Westland Group Process Specifications, Westland Aerospace, Isle of Wight, UK, 1987, 2, 369. 19. J.D. Guthrie, B. Battat and B.K. Severin, Advanced Materials and Process Technology, Defense Technical Information Centre, IIT Research Institute, New York, NY, USA, 2000. 20. C.I. Stuckey, T.R. Reinarts and D. Davis, AIP Conference Proceedings, 2001, 552, p. 298. 21. Syntactic Foam adds Strength and Buoyancy, Trelleborg AB, Trelleborg, Sweden. [http://www.trelleborg.com/en/Media/The-World-of-Trelleborg/ Syntactic-foam-adds-strength-and-buoyancy] 22. A Unique, Sprayable Syntactic-Foam Insulation for Cryogenic Application, Spacepda.net, [http://www.spacepda.net/abstracts/94/sbir_html/ 9400160-940117.html] 23. N.N. Jize, C. Hiel and O. Ishai, ASTM Special Technical Publication, 1996, 1274, 125. 24. J.B. Cattanach, inventor; Imperial Chemical Industries PLC, assignee; US 4,876,055, 1989.
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Update on Syntactic Foams 25. Bayer and Adidas Produce High Tech Soccer Ball for EUFA EURO 2004 Championship, A to Z of Materials, Sydney, Australia. [http://www.azom.com/Details.asp?ArticleID=2408] 26. Fuming over the World Cup’s Foam Ball, Conde Nast. [http://www.wired.com/culture/lifestyle/news/2002/05/52828] 27. Players Bawl over Ball, BBC Sport, London, UK, 2002. [http://news.bbc.co.uk/sport3/worldcup2002/hi/other_news/ newsid_2001000/2001924.stm] 28. G.S. Mukherjee and M.N. Saraf, Popular Plastics and Packaging, 1994, 39, 10, 59. 29. C.W. Glesner and T.F. Anderson in Proceedings of the 26th Annual Technical Conference – Society of the Plastics Industry, Reinforced Plastics/ Composites Division, Washington, DC, USA, 1971, p.8 30. T.E. Cravens, Journal of Cellular Plastics, 1973, 9, 6, 260. 31. T.F. Anderson, H.A. Walters and C.W. Glesner, Journal of Cellular Plastics, 1970, 6, 4, 171 32. Y. Takeuchi in Proceedings of the IEEE Ultrasonics Symposium, Atlanta, GA, USA, 2001, p.1039. 33. Verilyte, CRG, Dayton, OH, USA. [http://www.crgrp.com/ verilyte.shtml] 34. G.E. Melber, inventor; Pierce & Stevens Corp., assignee; US 4902722, 1990. 35. http://www.syntechmaterials.com/underwater.nxg.htm 36. P.K. Nelson in Proceedings of the 29th International SAMPE Technical Conference, Orlando, FL, USA, 1997, p.243.
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Applications of Syntactic Foams 37. Syntactic Foam Blast Protection, AEM Product Group, Trelleborg, Offshore UK, Skelmersdale, Lancashire, UK. [http://www.trelleborg.com/en/Advanced-EngineeringMaterials/Markets--Applications/Defence/ Syntactic-Foam-Blast-Protection/] 38. Advanced Materials Newsletters, 1988, 10, 13. 39. S. Kenig, I. Raiter and M. Narkis, Journal of Cellular Plastics, 1984, 20, 6, 423. 40. B.P. Rand, Journal of Cellular Plastics, 1978, 14, 5, 277. 41. M.A. El-Hadek and H.V. Tippur, Optics and Lasers in Engineering, 2003, 40, 4, 353. 42. N. Gupta, C.S. Karthikeyan, S. Sankaran and Kishore, Materials Characterization, 1999, 43, 4, 271.
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A
bbreviations
ASTM
American Society for Testing and Materials
BMI
Bismaleimide
BMIP
2,2-Bis 4-(4 maleimidophenoxy) phenyl propane
BS2
Polybenzoxazine-silica fibre
BSM
Polybenzoxazine-silica fibre-microballoon
BZ-A
Polybenzoxazine
CRG
Cornerstone Research Group
CTE
Coefficient of thermal expansion
DABA
Diallyl bisphenol A
EPN
Epoxy phenol novolac
FGSF
Functionally graded syntactic foams
FPA
Focal plane array
NASA
National Aeronautics and Space Administration
NCRSF
Nanoclay-reinforced syntactic foam
phr
Parts per hundred resin
PN
Propargyl ether novolac
PNM
Propargyl ether novolac resin syntactic foam
PVDC
Polyvinylidene chloride
ROV
Remotely operated vehicle(s)
RR
Radius ratio
SADM
Syntactic acoustic damping material
SATB
Syntactic acoustic transducer baffle
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Update on Syntactic Foams SCF
Short carbon fibre
SEM
Scanning electron microscope
SFRSF
Short fibre-reinforced syntactic foams
SnOc
Tin octoate
SRB
Solid rocket booster(s)
TEA
Triethanolamine
Tg
Glass transition temperature
Tmax
Maximum temperature
VF
Volume fraction
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I
ndex
A Aerogel blanket 99 Antenna pattern shaping 43
B Binder-filler interface 18 Binder materials 8, 56, Blowing agent, 98 Buoyancy modules, 96, 98
C Carbon braid, triaxial 98 Catastrophic failure 44 Cement matrix 90 Coefficient of thermal expansion 30, 51, 74 Compressive modulus edgewise 87, 88 flatwise 87, 88 Composites, fibre-reinforced 48, 70 Composites, foam 1, 50, 68 Coupling agent, silane 53 Crack propagation 29 Curing agent 17, 29, 30, 54 Cyanate esters 2, 39
E Elastomer, 29, 55, 111 Epoxy resins, bisphenolic 23 Extruder, twin-screw 2 119
Update on Syntactic Foams
F Fevernova soccer ball, 103 Fibre-bearing regions, 68 Fibre-reinforced system, 66 Fibre pull-out, 90 Filler concentration, 16, 30, 47 Flame retardant, 8, 53 Flexural modulus, 63, 79, 80, 83 Floating members, 55 Foams open-cell structured 89 phenolic 31, 37 silicone/glass microbubble 55
H Honeycomb structures, 100, 102 Hydrostatic tests, 15 Hygrothermal degradation, 89 Hypalon paint, 101
I Interfacial strength, 18, 33, 86, 88
M Microballoon-matrix interfacial debonding 90 Microballoons acrylic 98 crushing 90 phenolic 4, 24, 55, 101 polymeric 4 Saran 104 Shirasu 5, 105 Silas 31, 37 Microcracks, 82, 90 Microcracking, matrix 16, 90 Microlength scale mechanisms, 90 120
Index Microspheres carbon 4, 5, 8, 17, 30, 56, 111 dressing 15 glass 4, 5, 16, 44, 99, 110 phenolic 4, 5, 16, 55, 64, Saran 4, 104, 105, 111 silica 15 Moisture scavenger, 51
N Nomex honeycomb, 109 Novolac, 23, 31, 44, 55
O Organic filler, 17
P Parylene coating, 44 Pipelines, insulated 99 Pyrolysis, ultrasonic spray 4
R Radome application, 15, 102 Resin - fibre interface, 64 Resin - microballoon interface, 35 Resin - microsphere interface, 14 Resin-to-filler ratio, 54 Resins liquid thermosetting 10 neat 24, 29, 30, 33, 35, 37, 74 phenolic 5, 31, 37,
S Space factor, 16 Storage modulus, 36, 37, 72–75, 82 121
Update on Syntactic Foams Stress-strain curve, 13, 14, 47, 48, 80 densification region 13, 14 Subsea apparatus, 1 Syntactic foams bare 64, 80, 82 buoyancy materials, 96 carbon–carbon 56, 57, 111 cement-based 89–91 composites, 37, 48, 49 cyanate ester 39, 40, 43, 64, 68, 69, 82 core sandwich composites 86–89, 102, elastomeric 54, 110 epoxy-based system 31 fibre-free 66, 68 fibre-reinforced 63, 66–8, 70, 72, 74, 76, 77 functionally graded 83 nanoclay-incorporated 76 Nylon 55 polybutadiene 55 polyester 53 polyimide 43, 44 polypropylene 56 polyurethane 51 propargyl ether novolac resin 31, 33–37 rubberised 82 silicone 54 Syncore 109 three-phase 6 toughening potential, 72 two-phase 5–7, 13
T Thermal insulation material, 99 Thermal shrinkage, 43 Thermoplastic, 2, 9, 13, 55, 98, 99, 104 matrix, 2
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Index Thermosets, 2, 104 Thermoset resins, 55, 56, 104 Thermosetting, 2, 10, 13, 17, 44, 54, 98, 100, 111
V Vacuum-filtering, 10
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Published by iSmithers, 2010
Syntactic foams are polymer composites obtained by dispersing hollow spheres in a matrix. They have excellent properties such as low density, high specific strength, low moisture absorption, lower thermal coefficient of expansion, and in some case radar or sonar transparency for use in Stealth technology. One of their biggest advantages is the fact that they can be tailored to have the properties necessary for a specific product. Syntactic foams uses include: marine applications, aerospace, ground vehicles, and sports applications (snow skis and soccer balls). In fact, they can be used for anything that needs a high strength material. Update on Syntactic Foams will be of interest to all those who produce polymer products that need very high strength due to the adverse conditions that they are used in, as well as manufacturers of raw materials used in these products. This Update gives the reader a good insight into the properties, manufacture and uses of syntactic foams, including: * The basics of syntactic foams including the types of microballoons used, foam structure, methods of synthesis and the properties. * Syntactic foams based on different types of resin systems such as epoxy, phenolics, cyanate ester and so on. * Recent developments in the field such as fibre reinforcement, nanoclay reinforcement, functionally grading of foams, rubberisation, syntactic foam core sandwich composites and cement-based syntactic foams. * Applications of syntactic foams.
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