ABSTRACT
Studies of physico-mechanical and morphological properties of pure and -cellulose reinforced blends of polystyrene (PS) and poly (vinyl acetate) (PVAc) are reported in this work. The compression moulded articles of the blends of different compositions (10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20 and 90:10, PS/PVAc) were tested for mechanical performance, absorption behaviour, void activity and morphological arrangements. Comparison of mechanical properties such as tensile strength, modulus of elasticity and elongation at break revealed apparent compatibility domains for 20:80 and 10:90, PS/PVAc for both pure and reinforced blends. However, the -cellulose filled blends have enhanced tensile strength for all the compositions. From the values of the breaking load (in kilonewtons-KN) Vs composition of the blends, there was considerable strength improvement with -cellulose filler content. The equilibrium sorption in four different solvents, showed a declining order in accordance to acid > water > base > acetone. However, solvent absorption increased with filler content in all the solvents, because the filler increased the gelation level, hence the sorption rise except in acetone which showed a sharp reduction in % absorption in the presence of -cellulose filler. The acetone seems to dissolve the crystalline portion of the blends and cannot be used to detect the differences between the polymer blends
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and their molecular structure. The void activity in the blend were estimated by density measurements, and they showed a definite pattern except that the filler increases the densities of the blends 30/70, 20/80, and 10/90, PS/ PVAc having the highest values than other compositions. Also, photo micrographs of the certain blends showed a two phase system appearing bright (PVAc phase) and the other appearing opaque (PS phase) in virtually all the micrographs, even though the heterogeneity due to phase inversions (phase changes) was relevant for some compositions. For other compositions, a domain distribution showed considerable miscibility within the range of compositions.
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TABLE OF CONTENTS
Chapter – – – – – – – – Page Title – – – – – – – – – i Declaration – – – – – – – – ii Certification- – – – – – – – iii Dedication – – – – – – – – iv Acknowledgment – – – – – – – v Table of contents – – – – – – – vi List of Tables – – – – – – – x List of Figures – – – – – – – xi Abstract – – – – – – – – xv CHAPTER ONE INTRODUCTION 1.1 Background with commercial examples – – 1 1.2 Definition of polymer blends – – – – 2 1.3 Two-component systems: definitions – – – 4 1.3.1 Blends-two phase and single phase – – – 4 1.4 Trends in the science and development of polymer blends – – – – – – 11 1.5 Structure development in polymer blends – – 21 1.6 Preparation and manufacture of polymer blends – 21 1.6.1 Melt blending – – – – – – 22 1.6.2 Casting from common solvent – – – – 24 1.6.3 Latex or dispersion mixing – – – – 24 1.6.4 Compounding and mixing processes – – 24 1.6.5 Uses of polymer blends – – – – – 25 1.6.6 Basic thermodynamics – – – – – 25 1.7 Polystyrene – – – – – – – 28 1.7.1. Properties of polystyrene – – – – 29 1.8 Blend formulations and processing – – – 39 1.8.1 Mixing – – – – – – – 40 1.8.2. Processing of blends – – – – – 42 1.8.3. Test methods – – – – – – 43
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1.9 Fillers as additive in polymer blends – – – 51 1.9.1 (alpha) -Cellulose as a filler in polymer cellulose, structure and properties – – – – – – 52 1.10 Blend morphology – – – – – – 54 1.10.1Morphology generation and control – – – 55 1.10.2 Morphology characterization – – – – 57 1.11 Property relationships in polymer blends – – 59 1.12 Statement of problem- – – – – – 61 1.13 Aim and scope of research – – – – 61 1.14 Significance and limitation of this study – – 62 1.15 Scope of research – – – – – – 63 1.16 Justification – – – – – – 64 CHAPTER TWO LITERATURE REVIEW 2.1 REVIEW OF PAST RELATED WORKS – – 66 CHAPTER THREE MATERIALS AND METHODS 3.1 MATERIALS AND THEIR SOURCES – – 81 3.2 Preparation of blend samples and testing – – 81 3.2.1 Sample preparation / blend formulations – – 81 3.2.2 Dry blending of blend formulations – – – 82 3.2.3 Compression moulding of PS/PVAc dry blends – 82 3.2.4 Tensile strength measurements – – – 83 3.2.5 Sorption measurements – – – – 83 3.2.6 Density measurement of unfilled and filled PS/PVAc blends – – – – – – – 85 3.2.7 Establishment of blend morphologies of unfilled and filled PS/PVAc blends. – – – – – 85 3.2.8 FTIR, XRD and surface topography characterization of the blends – — – – – – – 86 3.2.9 Electrical conductivity of the filled and unfilled PS/PVAc blends – – – – – – – 88 3.2.9.1Film casting and measurement – – – 90 3.2.9.2Swelling properties measurement – – – 91 3.2.9.3Electrical conductivity measurements – – 91
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CHAPTER FOUR RESULTS 4.1 SORPTION TEST MEASUREMENT FOR PURE AND FILLED PS/PVAc BLENDS IN WATER, ACETONE, 0.05M HCl AND 0.05M NaOH. – – -92 4.2 Density of the pure and -cellulose filled PS/PVAc blends – – – – – -102 4.3 The XRD and FTIR analyses of the pure and filled PS/ PVAc blends – – – – – – – -105 4.4 The Mechanical properties of the pure and filled PS/PVAc blends — – – – – – – -108 4.5 The sorption characteristics and densities of pure and filed PS/PVAc blends- – — – – -117 4.6 The morphological and electrical properties of pure and filled PS/PVAc blends – – – – – – 121 CHAPTER FIVE DISCUSSION 5.1 TENSILE STRENGTH AND RELATED PROPERTIES – – – – – 160 5.2 Effects of modification of unfilled and filled PS/PVAc blends on equilibrium sorption – – 163 5.3 Effect of filler composition on the density of unfilled and filled PS/PVAc blends – – – – 166 5.4 Morphological studies on the unfilled and filled PS/PVAc
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blends – – – – – 167 5.5 The swelling and electrical conductivity properties of the filled and unfilled PS/PVAc blends – – 170 CHAPTER SIX CONCLUSION 6.1 SUMMARY OF RESULTS – – – 172 6.2 Contribution to knowledge – – – 172 6.3 Conclusion – – – – — – 173 6.4 Recommendations – – – – 174 REFERENCES – – – – – 175 APPENDIX – – – – – – 195 GLOSSARY – – – – – — 201
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CHAPTER ONE
INTRODUCTION 1.1 BACKGROUND WITH COMMERCIAL EXAMPLES Blending procedures had been employed since time immemorial. The principle of blending is geared towards achieving property averaging. A blend is therefore the physical mixture of two or more substances, without a chemical bond. Linseed oil, a triglyceride of unsaturated fatty acids, is heated with a brittle resin, usually an unsaturated C20 mono carboxylic acid (Mantell, Kopf, Curtis and Rogers, 1942). The resin itself could be used as a coating, but the linseed oil is incorporated to reduce the brittleness of the product. On the other hand, the linseed oil by itself forms an oily, slow-to-harden coating. Thus, the principle of property averaging (blending) was recognized a long time ago. The resin contains unsaturation and, presumably during the heating with the oil and the subsequent oxidative cross-linking, is incorporated covalently. Nevertheless, no segregation occurs and clear, tough, films of high molecular weight result. Many modern vanishes use phenol-formaldehyde resins rather than natural resins. Modification of the basic phenol structure is required in order to obtain solubility of the resin in the linseed oil. parasubstitution with tert-butyl, phenyl, and other hydrocarbon groups increases the aliphatic character (reduces solubility parameter) enough to achieve solubility.
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In the non-convertible coatings (lacquers) area, the importance of polymer-polymer systems was recognized as soon as it was found that cellulose nitrate required plasticization to obtain adequate toughness for coatings. Shellac, a natural resin obtained from insects, was an early example of a polymeric additive to cellulose nitrate. The mixture forms clear, tough films and “the two appear to act as mutual plasticizers” (OCCA, Manual, 1961). Later it was observed that poly (vinyl acetate), poly (vinyl methylether), and poly (vinyl ethyl ether) performed the same function. As cellulose acetate made inroads into cellulose nitrate markets, it was found that not all the plasticizers for the later were suitable for the former. Phenolic resins, poly (-methylstyrene) and poly (vinylacetate) gave clear coatings with cellulose acetate. The coatings chemists also discovered the concept of using a third component to improve the miscibility of the two others. Phenolic resin, shellac, and styrene-co--methylstyrene was an example of such a ternary mixture (Olabisi, , Robeson and Shaw, 1979). 1.2 Definition of polymer blends
These are mixture of chemically different polymers and or copolymers with no covalent blending between them. Polymer blends can be classified as homologous, miscible, immiscible, partially miscible, compatible, non-compatible, interpenetrating polymer networks (IPN) or polymer alloys. Homologous polymer blends are a subclass of chemically identical polymers which
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exhibit single-phase behaviour. Immiscible polymer blend encompasses those blends, which exhibit two or more phases at all composition and temperatures. Partially miscible polymer blends are a subclass of polymer blend including those blends that exhibit a „window‟ of miscibility i.e. they are miscible only at some concentration. Compatible Polymer blends are a utilitarian term indicating commercially useful materials, mixture of polymer with strong repulsive forces that is homogenous to the eye. Interpenetrating Polymer Network (IPN) are a subclass of polymer blend reserved for a mixture of two polymers where both components form continuous phases and at least one is synthesized or cross linked in presence of the other while polymer alloys are a class of polymer blends, heterogeneous in nature with modified controlled, interfacial properties and/or morphology (Utracki, 1987). The polymer blends and alloys however must be distinguished from polymeric composites, which are defined as follows (Mathews, 1912)
i. It (composite) consists of two or more physically distinct and mechanically separable materials.
ii. Mixing the materials in such a way that the dispersion of one material in the other can be done in a controlled way to achieve optimum properties.
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The properties are superior and possibly unique in some specific respects to the properties of the individual components. 1.3 Two-component systems: definitions 1.3.1 Blends-two phase and single phase
An important case where the polymer-polymer mixture exhibits miscibility or solubility has provoked recent interest as miscible polymer blends are increasingly reported in the technical polymer literature. Until now, polymer-polymer miscibility has been treated as a special case in the field of polymer blends or alloys with the recent commercialization of new polymer-polymer miscible blends combined with very definite advances in the thermodynamics of polymer-polymer phase behaviour, the necessity for a treatise specifically related to polymer-polymer miscibility has therefore evolved. Several reviews of miscible polymer mixtures have been published in the past decade with listings of up to 50 mixtures exhibiting some degree of miscibility (Buckley, 1967, Bohn, 1968, Krause, 1972 and Krause, 1978). The most comprehensive reviews have been authored by Krause (1972 and 1978) and provide an excellent listing of both miscible and immiscible blends. The literature search for this treatise has revealed over 100 different mixtures with the necessary criteria to be considered miscible polymer mixtures. Polymer miscibility is not only important in the case of simple polymer mixtures, but also determines the physical
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nature of block and graft copolymers, interpenetrating networks, and thermosetting networks of polymer mixtures. It is evident that classification of the use of the term “miscibility” to describe single-phase, polymer-polymer blends is necessary. Prior studies and reviews have generally used the term “compatible” to describe single-phase behaviour. However, “compatibility” has been used by many other investigators involving various studies of polymer-polymer blends behaviour to describe good adhesion between the constituents, average of mechanical properties, behaviour of two-phase block and graft copolymers, and ease of blending. The term “solubility” which is more descriptive and exact than “compatibility”, could be another choice for describing molecular mixing in polymer-polymer blends. For single-phase, solvent-solvent and polymer-solvent mixtures, solubility is the accepted term. With polymer-polymer blends, ideal or random molecular mixing may not adequately describe the true nature of the blend even though the physical parameters of the blend would suggest true solubility. After much deliberation and discussions with many investigators involved with polymer-polymer blend research, the term miscibility has been chosen to describe polymer-polymer blend with behaviour similar to that expected of a single-phase system. The term miscibility used does not imply ideal molecular mixing but suggests that the level of molecular
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mixing is adequate to yield macroscopic properties expected of a single-phase material.
The level of molecular mixing existing in polymer blends that exhibit macroscopic properties indicative of single-phase behaviour is commanding considerable attention without widespread agreement. For specific blends, recent studies have been able to provide experimental evidence of the level of molecular mixing. Until recently, only microscopic (i.e. electron microscopy), techniques were utilized to provide further insight into the level of mixing. In some blends, heterogeneous structure was observed at high levels of magnification even though macroscopic properties implied single-phase behaviour. Heterogeneous structures (domains), however, have been observed in amorphous homopolymers (i.e. atactic polystyrene), thus confusing the interpretation of polymer blend miscibility (Geil, 1975). These conflicting conclusions from macroscopic and microscopic experiments have resulted in research directed toward answering a specific question (Couchman and Karasz, 1977 and Kaplan, 1976): How large is the size of a domain required to be in order to yield macroscopic properties (i.e. glass transition temperature) distinctly different from other domains of different molecular structures? In early investigations of the thermodynamics of polymer mixtures, serious deficiencies were observed with the application of methods used successfully for
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predicting solvent-solvent or solvent-polymer solubility. The single value solubility parameter approach was found to be quite unsuccessful in predicting polymer-polymer miscibility and new techniques that offered more promise were investigated. Equation of state thermodynamics has been applied recently to polymer phase behaviour and the result qualitatively agrees with the experimentally observed phase behaviour of polymer mixtures. This approach reveals definite differences from those one would expect via extrapolation of techniques commonly applied to solvent-solvent and solvent-polymer mixtures. Indeed, the thermodynamics of polymer-polymer mixtures requires much more rigorous treatment than the previously accepted methods for solvent-solvent and solvent-polymer mixtures.
Although the miscible blends of poly (vinyl chloride) (PVC) and butadiaene acrylonitrile copolymers have been commercial since the 1940s (Emmet, 1944 and Reed, 1949), recent interest has been provoked by the commercialization of a blend of poly (2,6-dimethyl-1, 4-phenylene oxide) (PPO) and polystyrene (PS) (Cizek, 1968) under the trade name Noryl, as well as the need for more permanent (i.e. polymeric) plasticizers for PVC (Graham, 1975, and Hammer, 1977). The best commercial advantages of a miscible polymer blend can best be summarized by the word “versatility”. With a specific polymer, the number of
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possible variations in usable properties is limited without resorting to composition changes. Random, block and graft copolymerization, polymer blends, and composites offer significant property diversification (Olabisi et al, 1979). With polymer blends exhibiting two-phase behaviour, definite advantages can be derived (e.g. impact polystyrene and acrylonitrile-butadiene-styrene (ABS) if mechanical compatibility is assured and a property compromise between the constituents is therefore achieved. With miscible polymer mixtures, mechanical compatibility is assured and a property compromise between the constituents is therefore achieved. Thus, with a miscible polymer-polymer blend, a range of price/performance characteristics between the component polymers can lead to a large number of potentially useful and different products. The versatility places miscible polymer blends into a unique situation with potential commercial importance. Single-phase polymer blend, the two-phase system must be defined and contrasted with miscible systems to delineate the two subjects and to establish the criteria for excluding what is certainly the vast majority of polymer-polymer mixtures. In most instances, the critical property will be the glass transition; blend with a single glass transition will be classified as miscible, blends of components having similar glass transition temperatures will provide ambiguous cases and other techniques must be employed. But experience has shown that immiscibility in
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polymer blends is rarely well concealed, revealing itself as opacity, delamination, double glass transition, or combination of these properties. From a thermodynamic point of view, every polymer has some solubility in every other polymer, but the magnitude in most cases is exceedingly low. For example, if polystyrene is fluxed on a mill with poly (methylmethacrylate), a two-phase mixture results, no matter how long or intensive the mixing. One could, in principle (but with difficulty in practice), separate the two phases, analyze the composition of each, and arrive at values for the mutual solubilities. In this case, as with hexane in water, the solubilities would be less than 1%(Okada, 1977). On the other hand, if one fluxes polystyrene on a mill with poly (2,6-dimethyl-1, 4-phenylene oxide) as the second component, one phase results. It is thermodynamically stable because no matter how slowly the mixer runs or how long one waits there is still only one phase (Ogawa, Kanaya and Matsuba, 2008).
Consider a third example: polystyrene plus poly (vinyl methyl ether) (PVME), if PS is fluxed with an equal amount of PVME on the mill at 800C, a clear, one-phase mixture results. However, if the temperature is raised to 1400C, two phases appear. A return to 800C restores one phase. This behaviour has been
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summarized for a range of compositions by the experimental cloud-point curves (Olabisi, Robeson and Shaw, 1979). These three examples provide an excellent basis on which to build a definition of polymer miscibility and hence polymer blend. The first represents an example of an immiscible blend, the second is a miscible blend; the third illustrates that miscibility and immiscibility can be exhibited by the same mixture depending on the ambient condition. Furthermore, the third example demonstrates that the driving forces for the transition from the one-phase (miscible) to the two-phase (immiscible) state are thermodynamic in origin and do not depend, for example, on the extent or intensity of mixing.
In spite of the seemingly unquestionable behaviour described in these examples, an exact definition of miscibility in polymer mixtures is a subject of considerable debate because it represents different characteristics to different investigators. For the investigator interested in macroscopic properties useful on practical industrial problems, a miscible polymer mixture is that which exhibits a single glass transition temperature (Tg) and it is irrelevant whether or not changes of state occur during the preparation of the sample or during measurement. Here, miscibility implies homogeneity of the mixture up to a scale whose dimension is similar to the segmental size responsible for the major glass transition.
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However, for the investigator interested in statistical thermodynamics, miscibility implies homogeneity on a scale equivalent to the range of intermolecular forces. Miscibility in this sense is not necessarily satisfied by the Tg criterion. Miscibility is not even absolutely satisfied by the usual qualitative criterion given in several texts that the Gibb‟s free energy of mixing, GM, must be negative for a system to be thermodynamically stable. Such criterion is only a degenerate form of Gibb‟s criteria, because it neither distinguishes unstable from stable states nor gives any understanding of the concept of metastability, which is so important in phase separation phenomena. Furthermore, it is known (Konigsveld and Kleintjens, 1976) that mixtures are often unstable at negative GM (not relative to the pure constituent but to some intermediate composition and that the GM becomes more negative as the mixtures phase-separate further. 1.4 Trends in the science and development of polymer blends
The oldest reference to blends of solid polymers appropriately is to impact resistant polystyrene which is today the poly blend in largest commercial production. In 1912, in a British patent, Mathews stated that the addition of rubber to polystyrene “gives hardness and toughness thereto in accordance with the
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proportion added.” Ostromislensky (1927) was issued a patent which describes the preparation of impact polystyrene by a method which today is termed a “graft copolymerization”. However, the first commercial blend was produced in 1846 by Sir Thomas Hancock, the discoverer of mastication by mixing natural rubber with gutta percha. It was used to apply on clothes for water proofing and was patented in the said year. This was the fist rubber/plastics blend. The next in the series came only in 1942 almost about a century later when it was discovered that acrylonitrile butadiene rubber could plasticize polyvinyl chloride (PVC) permanently. This spurred the rapid development of commercial thermoplastics-rubber blends. In the subsequent years, plasticised PVC was added to vulcanized nitrile butadiene rubber (NBR) to improve ultraviolet (UV) and oxidative stability. They are still utilized in sponge, wire, cable, footwear, belt and hose applications/appliances. This yielded compositions offering resistance to placticizer migration in adverse environments such as oil and water containment and food contact applications. This blend is found to be miscible. Other high molecular weight Polymers have since been used as permanent plasticizers for PVC such as ethylene/vinylacetate/acrylonitrile (EVA) copolymer containing 65 – 70 wt % vinylacetate and polyester based thermoplastic polyurethane (TPU). These blends are also found to be miscible. In 1942, Dow Chemical Corporation, introduced an alloy
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“Stralloy-22” an interpenetrating network (IPN) of polystyrene and polybutadiene. In 1944, R.F. Boyer polymerised styrene in the presence of styrene – butadiene rubber (SBR) to obtain high impact polystyrene (HIPS). In 1946, a mechanical mixture of NBR and poly (styrene-co-acrylonitrile) was produced to obtain acrylonitrile – butadiene – styrene (ABS) type Polymers (Nando, 2002). In 1947, a series of miscible blends were developed. Nitrocellulose with polyvinyl found a miscible pair. Similarly, nitrocellulose with polylmethyl methacrylate (PMMA) gave rise to a miscible blend. Polystyrene with benzylcellulose also formed a miscible pair.
In 1951, the impact properties of crystalline, polypropylene (PP) was improved by blending with polyethylene or by copolymerizing with ethylene. The year 1960 saw rapid growth in the field of engineering polymer alloys and blends. Polystyrene was found to be miscible with poly (2, 6, dimethyl 1:4 phenylene ether) and this helped to process the PPE easily. The density as well as mechanical properties of the blends is found to be higher than that obtained by the additivity rule, exhibiting a significant synergistic behaviour. This densification has been assumed to be due to specific interaction between the blend constituents, which also increased the blend viscosity
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above that of log-normal additivity rule. A series of commercial blends were produced in 1965 by Mayer & Salwer General Electric Company in the name of Noryl. The upper use temperature of ABS polymer is 900C. This temperature can be improved by adding -methylstyrene acrylonitrile copolymer to ABS which also improves its load bearing capability. An azeotropic methyl styrene acrylonitrile has a Tg of 250C higher than the SAN matrix of ABS. This is known as „High heat ABS‟ developed in 1962. Similarly, -methyl styrene acrylonitrile is miscible with PVC. It has great demand in high heat resistance applications. ABS/PVC blends have been commercialized since 1969. In the same year, PP and ethylene / propylene/ diene monomer (EPDM) blends have been discovered by Coran and Patel and later commercialized by Monsanto, Chemical Co., in the name of „Santoprene‟. It improves the impact resistance of PP. They were widely used as bumpers, in automotive parts, suitcases, sports, thermoplastic rubbers and as construction materials. Polystyrene (PS) has a heat distortion temperature at 900C. It‟s Tg can be increased significantly by adding poly (2, 6 dimethyl 1:4 phenylene oxide) (PPO) with a Tg of 2100C. These blends are used in appliances, business machines, housing, electronic equipment, pump components and automotive applications.
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Blends of ABS/PVC was marketed in 1969 by Borg Warner. They are thermoformable, possessing good moldability and flame retardancy. Also they bear good weatherability. They are used in electronics housing, business machines and electronic parts. Super touch nylon was developed by Dupont in 1975 by blending Nylon66 with an elastomer. It becomes tougher and fracture behaviour changes on incorporating rubber. It is basically used in automotive applications. It was followed by toughening of polyethylene terephthalate (PET) by adding a small quantity of the polyolefinic elastomer. When reinforced with glass fibres (31%), it is used to make automotive body parts. In 1976, blends of PET with polybutylene terephthalate (PBT) were developed by General Electric Co., USA (Valox 500 or 700), and, when reinforced with glass fibres, provided good dimensional stability. Glass fibres were used to the extent of 65%.
Borg Warner first introduced blends of polycarbonate (PC) and ABS in 1977 (Banerjee, 2002), which was commercialized by Mobay Chemical Co., U.S.A. These blends were used in automotive applications, electronics and medical parts. When
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reinforced with glass fibres and aluminium, it attained excellent impact strength, and higher dimensional stability. These blends possess excellent moldability and are most suitable for injection moulding. They possess good low temperature impact strength and also sometimes used in electrical and electronic parts. ABS/PC blends confer high heat and impact resistant properties. In 1979, General Electric Co. introduced impact modified blends of PC with PBT and/or PET, under the trade name Xenoy. A 50:50 blend of PC: PBT improved the low temperature properties and when reinforced imparted good tensile strength and flexural modulus. Therefore, they are used in car bumpers, automobile and electronic components. When PC is blended with PET, and reinforced with glass fibres, it is used in car bumpers and in sports. The percent of glass fibre is limited to 30%. Celanese Co. in 1980, introduced toughened blends of elastomer with PBT (Celanex 500) and poly oxymethylene (POM) – Celcon-400. PBT/elastomer blends are used in car bumpers, automotive and electronic components and possess high impact resistance properties. They are quite often reinforced with glass fibres and other short fibres and mica. Blends of PBT/PET with glass fibre reinforcements are used in computer and other appliances.
POM-elastomer blends are used for low temperature impact strength property and POM with PBT/TPU are used for
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automotive and electronic industries. Quite often they are reinforced with particulate as well as fibrous fillers. In 1980, Union carbide, USA introduced blends of ABS with Polysulfone (PSO) which improved the processability and the blends imparted good hot water resistant property. In 1981, blends of (polypropylene ether) (PPE) copolymer and PS were introduced by Borg Warner for low temperature impact strength. It is also reinforced with glass fibres. They are used for equipment housing, office equipment and in electronics. They possess good processability, excellent impact strength and can be used continuously at a temperature of 1000C. PPE/PS blends were found to be miscible due to specific interaction. Densification and increased blend viscosity above the log-normal additivity rule evidences this. It improves flame resistance and solvent resistant properties too.
Monsanto Chemical Co. in 1981 introduced blends of poly (styrene-co-maleicanhydride) (SMA) with ABS and Arco Chemical Co. introduced blends of SMA with polycarbonate (PC). The former blend improved moldability and paintability, whereas the later blend is transparent and is used in food contact applications. In 1982, Dupont introduced Selar, a modified and compatibilised amorphous polyamide to be used as an additive to polyolefins. It reduced the permeability of
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polyolefins. PA-6, 12/Elastomer and PA-66/Elastomer blends found applications in automotive and recreational apparatus (sports) when reinforced with 15 – 50% of glass fibres, its stiffness and impact resistant property increased. PA-6, 6/inomer blends are used in low temperature, impact strength, tubing and in cables. They improve moldability PA-6, 12/inomer blend is used for low water absorption also. In 1983, blends of PPE and PA by General Electric Co. started a new family of high performance blends called Noryl GTX or Noryl plus. PA:PPE in the ratio of 70:30, with filler reinforcement give rise to composites for auto panels, wheels and fenders. When reinforced with glass fibre, it found applications in automotive parts. PPE/PA blends are also used for high temperature impact resistance and electrical as well as in mechanical components. Quite often they are reinforced with carbon fibres and glass fibres. Blends of PBT/EVA/polymer ether block-amide (PEBA) and PA/PEBA are used for powder coating and in sports goods.
In 1984, Mobay Chemical Corporation developed blends of polyurethane elastomer (TPU) with polycarbonates (PC). It was found to be basically useful in automobile industry and as a thermoplastic rubber. In the same year, Borg Wagner developed blend of poly acrylates (PA) and acrylonitrile-styrene-butadiene (ABS) rubber for automotive body panel as they possess high heat, impact and chemical resistance properties. In 1985,
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terblends of PC with ASA i.e., poly (acrylate-co-styrene-co-acrylonitrile) were introduced by General Electric Co. and BASF. These blends are used in automotive and household applications. The coating industry has utilized miscible polymer systems for years to enlarge their market base. Nitrocellulose and cellulose acetate, butyrates were modified with natural products such as shellac and other synthetic polymers e.g. phenolics, poly (vinyl acetate) etc to improve their performance. Specific adhesive formulations based on miscible polymer blend systems have been developed e.g. nitrile rubber formulations with vinylchloride, co – and terpolymers and phenolic resins. For commercial applications weatherable films and poly (methyl methacrylate) – poly (vinylidene fluoride) have been proposed. Poly electrolyte complexes such as poly (vinyl benzyltrimethyl ammonium chloride) and sodium poly (styrene sulfonate) have been utilized for diverse applications such as titrations of macromolecules, battery separators, homodialyzers (artificial kidneys) and as a matrix for slow release of implantable drugs (Nando, 2002).
The commercial importance/development of Polymer blends and alloys is driven by more favourable economics than in the more conventional chemical routes to new products. Blend systems, comprising existing materials, can be developed about twice as rapidly as new polymers, allowing manufacturers to respond
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more rapidly at reduced cost, to new market requirement. Since the properties of an existing blend system are functions of composition, an existing blend can be easily and quickly modified to meet performance and cost objectives required by new or changing markets. Discounting research and development cost, new blend systems are particularly attractive when one of the compounds is much less expensive than the others because this allows the blend to be procured at a lower cost than that of the higher costing ingredient. Blends can also be commercially rewarding, even though the ingredients are comparably valued, if the blend offers improvements in processibility, performance and find some special application. Polymer blends are required to share the common goal of reducing the weight and dimensions of the moulded articles, of achieving multi-functionality and greater design freedom, better performance and improving cost efficiency.
New trend of polymer technology has appeared to make products having high resistance to heat, cold and chemicals. For purposes of miniaturization, manufacturers of electronic equipment particularly in the field of data processing, office machine and information technology are particularly interested in good resistance to heat, dimensional stability and excellent electrical properties. Medical technology requires light, reliable and sterilisable equipment. The aeronautics industry requires
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low weight, excellent mechanical properties, flame resistance, low smoke density in the event of fires and low toxicity smoke. The increased usage of instrumentation in all branches of industry has led in particular to the increased use of dimensionally stable polymer blends (Banerjee, 2002). The last few years have seen a remarkable increase in both the usage of polymer blend (Hess, 1993) as well as the development of many new types of blends which include elastomeric alloys (Coran, 1980, 1990, 1992) and IPN (Mc Donel, 1978). 1.5 Structure development in polymer blends It is convenient to classify Polymer blends are either miscible or immiscible. From rheological point of view, the miscible blend can be modeled as either solutions or homologous polymer blends. On the other hand, suspensions, emulsion and block copolymers can be used as models for immiscible blends (Ultracki, 1972). 1.6 Preparation and manufacture of polymer blends Polymer blends may be manufactured by variety of technique such as (Shaw, 1985 and Eanthos, 1991)
i. Melt blending
ii. Solution blending
iii. Latex or dispersion mixing
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Melt blending in intensive mixing extrusion equipment is the predominant commercial method of blend preparation. 1.6.1 Melt blending In melt mixing, two or more polymers plus any derived fillers, reinforcement and additives are referred by weight into a sheer intensive extruder. The constituents are mixed at elevated temperatures (i.e. above the melting points of the polymer constituents) by extruder screw, which exerts mechanical shearing forces and ensures even distribution and thorough blending of blend elements. The entire process of compound has four stages involving:
a. Preparation of ingredients (drying, sizing, heating, etc)
b. Premixing (dry blending, homogenization, breakage of agglomerates, fluxing etc)
c. Melt mixing in dispense and distributive fields (usually with degassing) and
d. Chopping e.g. granulating, peletizing or dicing.
The most frequently used applications are single-screw extruders (usually with preblending stage), two screw extruders, intensive blenders and other machines. The advantages of single screw extruder are its price, wide commercial choice of the type and size; the disadvantage are poor adjustability of the pressure field and residence time and low degree of deformation. The twin
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screw extruders do not have this disadvantage. Excellent adjustment of the temperature and flow profiles, short residence time and flexibility can be achieved with them but their price is relatively high. Both extruders with one-screw and twin screws are continuous blenders, which can easily be built into a method with several phases in the case of high volume production. General purpose extruders have been produced recently having high performance mixing heads for single-screw extruders for Polymer blends. The other types of blending appliances for Polymer blends are batch sigma or internal blenders. Their advantage is uniform shear and deformation. Their disadvantage are price, low productivity, long blending cycle and low reproductivity of certain phases of production. Melt blending has several advantages over solution blending and latex mixing. A liquid blending or dispersion agent is eliminated thus reducing cost associated with solvent removal, recovery and losses. Additionally by combining only those elements desired in the final mixture, melt blending reduces the likelihood of contamination in the blend. The use of elevated temperatures in melt blending however, poses the possibility of considerable cross linking or grafting of the polymers. Further polymer degradation caused by chain scission can result in color shifts and poor mechanical properties if processing temperatures are not carefully controlled.
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1.6.2 Casting from common solvent
Casting of blend from common solvent is the simplest mixing method available and is practiced widely. Very small quantities of Polymers can be handled easily. If pure solvent and clear glass wares are used, contamination can be precluded. As temperatures never exceed ambient so degradation is not a problem. Contaminations of the blend with residual solvent and potential for phase separation or selective precipitation are hazards for this preparation method. 1.6.3 Latex or dispersion mixing This uses coagulation to give an intimate blend mixture. Films can also be cast. Mixing requires no extensive equipment, no high temperatures are required. Dispersion mixing is followed with melt mixing (or compounding) to produce polymer stands for pelletizing. 1.6.4 Compounding and mixing processes
Mixing and compounding are terms used interchangeably by different authors. Compounding is a major processing operation in fabricating plastics and their products. It is essentially simple mixing in which particles of two or more components are rearranged into a more random distribution without reducing the ultimate particle size (Mascia, 1974). The purpose of mixing in polymer processing is to attain an acceptable degree of
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homogeneity or uniformity of composition, (Holmes-Walker, 1975). According to reports, optimum results in mixing can be obtained when the mixing parameters, for example , time of mixing, temperature and speed are taken into account during speed mixing, (Titow, 1984). The expected degree of mixing is in most cases identified by visual appearance of the mix and the homogeneity of the product after subsequent processing. Several types of machines such as roll mills, high-speed mixers, extruders and others are used in mixing. Each method of mixing imposes its own pattern and degree of severity on the matrix materials and additives. The conventional warring blender will be used in this study for compounding the blends. 1.6.5 Uses of polymer blends Polymers blend is used to increase ductility and toughness of brittle polymers or to increase the stiffness of rubbery polymers and can be quickly and easily modified to meet performance and cost, objective required by new and changing markets. 1.6.6 Basic thermodynamics Equilibrium – phase behavior of mixtures is governed by the Gibbs free energy of mixing,
ΔGmix = ΔHmix – TΔSmix – – – (1)
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ΔHmix= Enthalpy of mixing, ΔSmix= Entropy of mixing and T is
the temperature.
Depending on how S and H are affected by composition and
temperature (T) for miscibility, ΔGmix must be negative and
satisfy the additional requirement:
δ2ΔGmix > 0 – – – – -(2)
δ 2
i T,P
In order to ensure stability against phase segregation (P =
pressure), the volume fraction of component i is employed.
1.6.6(i) Flory – Huggins’ Theory
The thermodynamic treatment of phase behaviour of mixtures
becomes more useful when specific models for the enthalpic and
entropic terms are used. The example of such model, which
introduces the most important elements needed for polymer
blends is that developed by Flory and Huggins originally for the
treatment of polymer solutions (Flory, 1953). It assumes that
only contribution to the entropy of mixing is combinatorial in
origin and approximated by:
ΔSmix = -R(VA –VB) A ln A + B ln B – – (3)
VA VB
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i refers to the volume fraction (i) in the mixture and V i is the
molar volume of i, which is related to molecular weight and
density. For simplicity each component is assumed to be
monodispersed, as more complex expressions result when
polydispersity is considered. The Flory – Huggins treatment
assumes that the heat of mixing follows a Van-Laar type
relation.
ΔHmix = (VA + VB)B A B – – – – – (4)
B is the interaction energy for mixing segments of the two
components and it is expressed as – parameters (Schrer and
Chinai 1955), where R is the gas constant.
B/RT = A = B = AB – – (5)
VA VB
Where A is the interaction parameter of polymer A, B is the
interaction parameter of polymer B and AB is the interaction
parameter of polymer blend AB.
Application of the thermodynamic requirement for phase
stability to this model reveals that miscibility of two polymers
occurs only when B is less than a critical value given by
equation (6)
Bc =
2
1 / 2
2
B
B
A
A
M M
RT
– – – – – (6)
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A and B are densities of A and B, and MA and MB molecular weights of A and B and R is the gas constant, T, the temperature. Some important conclusion can be learned from this simple model. First, it shows that Hmix does not depend on polymer molecular weight, whereas Smix does. Endothermic mixing, where B > 0, does not favour miscibility. Thus, forming a homogeneous mixture requires that molecular weights must be low enough that the favourable entropic contribution offsets the unfavourable enthalpic effect. For exothermic mixing, where B <0, this theory predicts that the condition for miscibility will be satisfied no matter how large the moelcular weights are. Thus, miscibility of high molecular weight polymers is only assured when mixing is exothermic. When the interaction energy density is positive, the polymer blend phase diagram is not predicted by this simple theory, unless B is temperature-dependent, in which case basic thermodynamic relations reveal that this quantity is not strictly an enthalpic parameter as defined in equation (6). 1.7 Polystyrene
Polystyrene is brittle, rigid, transparent, easy to process (shrinkage is low), free from odour and taste. It is thermally stable, with excellent electrical properties. All these properties are responsible for the commercial success of polystyrene.
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Polystyrene is sometimes referred to as crystal Polystyrene, these refers to the clarity of the finished product and does not imply that the molecular structure responsible for many of the good properties of polystyrene such as clarity of products, the low energy input required for processing and ease of processing with low shrinkage. 1.7.1. Properties of polystyrene i. General Polystyrene is an amorphous thermoplastic with a density of 1.05 g/cm3 with an extremely low moisture absorption (0.05%). Styrene polymers have some unique properties which makes them useful in a wide range of products. The single most important characteristic of general purpose polystyrene is that it is a glass like solid below 1000C. Above this temperature, commonly called the glass – transition temperature, the polymer chain (on a molecular level) has rotational freedom which allows large – chain – segment mobility. The polymer is thus fluid enough to be easily shaped into useful forms. Below the glass transition temperature, polystyrene possesses considerable mechanical strength, allowing it to be used in load bearing tasks in thousands of applications.
Rubber – modified polystyrene is a two phase system consisting of a dispersed rubber phase and a continuous polystyrene phase. This system uses a unique feature of polystyrene –
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elongation by the formation of energy absorbing crazes. The dispersed rubber particles initiated large numbers of crazes without crack formation, thus contributing to the development of very tough products. In addition to toughening, the rubber particles also increase the environmental stress – crack resistance because the microscopic rubber particles are placed in tension as they are cooled after fabrication, compressing the rigid phase. The particles try to shrink more than the rigid phase because rubber has a greater coefficient of expansion than polystyrene. Modern micrographic and analytical tools have been developed to measure and predict the complex interactions of these two – phase systems. Styrene readily copolymerizes with a variety of other monomers. The first well known copolymer was styrene – butadiene synthetic rubber. Other significant copolymers include tough, solvent – resistant copolymers with acrylonitrile; heat, resistant polymers with maleic anhydride; and rubber – modified, transparent systems with methyl methacrylase. Although there have been many studies concerning, multiple (more than two) comonomers, few significant commercial products exist.
Since styrene polymers are non polar, chemically inert, resistant to water, and easy to fabricate, they are the products of choice for electronic, medical, food packaging, appliance and
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automotive applications. Recent manufacturing trends provide improved processability and further decrease trace impurities. High – speed, efficient fabrication equipment is both reducing the cost of manufacturing and increasing the strength of the fabricated parts.
Polystyrene molecules can be oriented during fabrication. Modern processing equipment uses controlled orientation to produce tougher fabricated parts. Tensile strengths may double and elongation may increase by up to two orders of magnitude, resulting in tremendous increases in toughness. Toughening by orientation contributes to the success of polystyrene foam, now widely used in both insulation and as foam sheet in food packaging, and to the success of clear, thermoformed, biaxially oriented polystyrene. Pure polystyrene does not absorb ultra violet light in the terrestrial sunlight spectrum and would apparently have better ultraviolet stability if it were not for the presence of ultraviolet absorbing trace impurities. The presence of rubber tends to decrease the out door stability; this is countered by incorporating special rubbers and stabilizers. Anionic polymerization produces a more thermally stable polymer which can be made even more stable by proper selection of the end group, because most degradation begins at chain ends. Because of the commercial interest in polystyrene, its polymerization ease, and its relatively simple linear
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structure, polystyrene is one of the most thoroughly investigated polymer systems in the world. (Mark, Bikales, Overberger and Menges, 1989). ii. Mechanical Properties Polystyrene is hard, stiff and dimensionally stable but relatively inextensible material with a high tensile strength and low elongation at break. The mechanical strength is affected to a large degree by the processing conditions. The highest value can be obtained with free flowing materials at a low processing temperature.
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