Plastics Materials 7 Episode 9 potx

ABS Plastics 447 16.8.2 Processing of ABS Materials The processing behaviour of ABS plastics is largely predictable from their chemical nature, in particular their amorphous nature and the somewhat unpleasant degradation products. The main points to bear in mind are: (1) ABS is more hygroscopic than polystyrene. (It will absorb up to 0.3% moisture in 24 hours.) It must therefore be dried carefully before moulding or extrusion. (2) The heat resistance in the melt is not so good as that of polystyrene and unpleasant fumes may occur if the melt is overheated. This can occur at the higher end of the processing range (250-260°C) and when high screw speeds and high back pressures are used when injection moulding. Volatile decomposition products can also lead to bubbles, mica marks (splay marks), and other moulding defects. The problem is often worse with flame-retarding grades. It is usual to purge the material at the end of a run. (3) The flow properties vary considerably between grades but some grades are not free flowing. Flow path ratios in the range 80 to 150: 1 are usually quoted, generally being lower with the heat-resistant grades. (4) Being amorphous, the materials have a low moulding shrinkage (0.044-0.008 cm/cm). One particular feature of the material is the facility with which it may be electroplated. In order to obtain a good bond the ABS polymer is first treated by an acid etching process which dissolves out some of the rubber particles at or near the polymer surface. After sensitisation and activation electroless metal deposition processes are carried out. Much of the strength between the ABS and the plating depends on a mechanical press-stud type of effect. It is commonly observed that low peel strength usually arises not through failure at the interface but in the moulding just below the surface. It would seem that the greater the molecular orientation in such regions the lower the interlayer forces and hence the lower the peel strength. 16.8.3 Properties and Applications of ABS Plastics Because of the range of ABS polymers that may be produced, a wide range of properties is exhibited by these materials. Properties of particular importance are toughness and impact resistance, dimensional stability, good heat distortion resistance (relative to the major tonnage thermoplastics), good low-temperature properties and their capability of being electroplated without great difficulty. Several classes of ABS which show the above general characteristics but with specific attributes are recognised. One supplier for example classifies ABS materials into the following categories: general purpose grades fire retardant grades improved heat resistance grades enhanced chemically resistant grades static dissipation grades extrusion grades fire retardant extrusion grades 448 Plastics Based on Styrene transparent grades electroplating grades blow moulding grades. Over the years there has been some difference in the balance of use between UPVC and ABS in the United States compared with Western Europe. This was due largely to the earlier development in Western Europe of UPVC and in the United States of ABS. Thus, for example, whilst ABS consolidated its use for pipes and fittings in the United States, UPVC was finding similar uses in Europe. Whilst some of these traditional differences remain, ABS is now well established in both Europe and the United States. As well as unplasticised PVC, ABS also finds competition from polypropyl- ene. In recent years polypropylene has been the cheaper material on a tonnage basis and even more economic on the more relevant volume basis. On the other hand the properties listed above, in particular the extreme toughness and superior heat distortion resistance, lead to ABS being preferred in many instances. Because ABS, typically, has a higher flexural modulus than polypropylene, mouldings of the latter will have to a wall thickness some 15-25% greater in order to show an equal stiffness. It is also interesting to note that because of its higher specific heat as well as possessing a latent heat of fusion, polypropylene requires longer cooling times when processing (see Section 8.2.3). Applications of ABS are considered in more detail in Section 16.16. 16.9 MISCELLANEOUS RUBBER-MODIFIED STYRENE-ACRYLONITRILE AND RELATED COPOLYMERS The commercial success of ABS polymers has led to the investigation of many other polyblend materials. In some cases properties are exhibited which are superior to those of ABS and some of the materials are commercially available. For example, the opacity of ABS has led to the development of blends in which the glassy phase is modified to give transparent polymers whilst the limited light aging has been countered by the use of rubbers other than polybutadiene. Notable among the alternative materials are the MBS polymers, in which methyl methacrylate and styrene are grafted on to the polybutadiene backbone. This has resulted in two clear-cut advantages over ABS. The polymers could be made with high clarity and they had better resistance to discolouration in the presence of ultraviolet light. Disadvantages of MBS systems are that they have lower tensile strength and heat deflection temperature under load. The MBS polymers are two-phase materials, with the components being only partially compatible. It is, however, possible to match the refractive indices providing the copolymerisation is homogeneous, i.e. copolymers produced at the beginning of the reaction have the same composition as copolymers produced at the end. Such homogeneity of polymerisation appears to be achieved without great difficulty. The poor aging of ABS appears to be due largely to oxidative attack at the double bonds in the polybutadiene backbone. Methyl methacrylate appears to inhibit or at least retard this process whereas acrylonitrile does not. Miscellaneous Rubber-modified Styrene-Acrylonitrile 449 Besides the MBS materials, related terpolymers have been prepared. These include materials prepared by terpolymerising methyl methacrylate, acrylonitrile and styrene in the presence of polybutadiene (Toyolac, Hamano 500); methyl methacrylate, acrylonitrile and styrene in the presence of a butadiene-methyl methacrylate copolymer (XT Resin), and methylacrylate, styrene and acrylo- nitrile on to a butadiene-styrene copolymer. Because the polybutadiene component is liable to oxidation, ABS materials are embrittled on prolonged exposure to sunlight. By replacing polybutadiene rubber with other elastomers that contain no main chain double bonds it has been possible to produce blends generally similar to ABS but with improved weathering resistance. Three particular types that have achieved commercial status are: (1) ASA polymers which utilise an acrylic ester rubber (see Chapter 15). (2) AES polymers which use an ethylene-propylene termonomer rubber (see (3) ACS polymers based on elastomeric chlorinated polyethylene. The ASA materials were introduced by BASF about 1970 as Luran S. Similar to ABS, they show improved light resistance and heat resistance (both during processing and in service). Because of their generally very good weatherability these materials have become best known for automotive grilles and mirror housings and have also been successfully used in garden equipment including pumps, marine equipment and satellite dishes. Other applications reported include chain covers and guards for agricultural machines, moped guards, housings for street lighting, road signs and mileage indicators. Where greater toughness is required alloys of ASA and polycarbonate resins (see also Section 20.8) are available from BASF (Luran SC). The extension of ABS-type materials into such exterior applications means that these products have to be considered alongside other plastics that show good weathering behaviour such as poly(methy1 methacrylate), cellulose acetate-butyrate and several fluorine- containing polymers. Whilst the ASA materials are of European origin, the AES polymers have been developed in Japan and the US. The rubber used is an ethylene-propylene terpolymer rubber of the EPDM type (see Chapter 11) which has a small amount of a diene monomer in the polymerisation recipe. The residual double bonds that exist in the polymer are important in enabling grafting with styrene and acrylonitrile. The blends are claimed to exhibit very good weathering resistance but to be otherwise similar to ABS. ACS polymers, developed primarily in Japan, are grafts of acrylonitrile and styrene onto elastomeric chlorinated polyethylene. Although the polymer has good weathering properties it is somewhat susceptible to thermal degradation during processing and to date these polymers have been of limited interest. Blends have also been produced containing neither acrylonitrile and styrene in the glassy phase nor polybutadiene in the rubbery phase. One such system involved grafting 70 parts of methyl methacrylate on to 30 parts of an 81-19 2-ethylhexyl acrylate-styrene copolymer. Such a grafted material was claimed to have very good weathering properties as well as exhibiting high optical transmission. Perhaps the greatest resistance to development with these materials is the strong competition offered by the clear impact-modified grades of unplasticised PVC which are generally less expensive. Chapter 11). 450 Plastics Bused on Styrene 16.10 STYRENE-MALEIC ANHYDRIDE COPOLYMERS There has been some interest in random copolymers of styrene with small amounts of maleic anhydride. Manufacturers included Monsanto (Cadon), Dow (Resin XP.5272) and Dainippon (Ryurex X-15). However, the only current manufacturer of high molecular weight materials appears to be Arco, which markets its products under the trade name Dylarc. The abbreviation SMA is commonly used for these materials. The unmodified copolymers are transparent and have a Tg and deflection temperature under load in excess of 125°C. Toughened grades may be obtained by incorporating a graftable rubber during the polymerisation stage. Glass-fibre reinforcement of the copolymer is also common. Long glass-fibre grades have recently become available in addition to the more common grades obtained by melt blending of polymers with glass fibre. The processing of SMA materials is largely predictable from a consideration of the structure. The polymer is easy flowing but setting temperatures are somewhat higher than for polystyrene and thus facilitate short cycle times. The low shrinkage, typical of an amorphous polymer, does, however, require that excessive pressures and pressure holding times during injection moulding should not occur since this could hinder mould release. Styrene-maleic anhydride copolymers have achieved a good market penetra- tion in the USA for auto instrument panels, where factors such as good heat resistance, rigidity, predictable impact properties and dimensional stability are important. Commercial blends of SMA with polycarbonate resin have been marketed. Such blends have deflection temperatures about 15°C above those for straight SMA copolymers and are also attractive for their ductility, toughness and ease of mouldability. A composite material consisting of an SMA foamed core sandwiched between an elastomer-modified SMA compound has been of interest as a car roof lining. This interest arose from the ability to expose components to the elevated temperatures that occur in hot paint drying equipment and in metallising baths. Other applications include car heating and ventilating systems and transparent microwave packaging material. In addition to the above SMA materials, low molecular weight (1660-2500) copolymers with 25-50% maleic anhydride content have been made available (SMA Resins-Elf Atochem). These find use in such diverse applications as levelling agents in floor polishes, embrittling/anti-resoil agents in rug shampoos, and pigment dispersants in inks, paints and plastics. They are also used in paper sizing and metal coating. The suppliers of these materials lay emphasis on the reactivity of such materials. For example, the maleic anhydride groups may be esterified with alcohols, enabling a wide spectrum of chemical structures to be grafted onto the chain, neutralised with ammonia, or imidised by reaction with an amine. As with all styrene polymers, the benzene ring may also be subject to a number of chemical reactions such as sulphonation. Production of SMA materials is of the order of 25 000 t.p.a. and recent reports refer to an annual growth rate of the order of 10-15%. 16.11 BUTADIENE-STYRENE BLOCK COPOLYMERS Random copolymers of butadiene and styrene have been known for over half a century and such polymers containing about 25% of styrene units are well known Butadiene-Styrene Block Copolymers 45 1 as SBR (see Section 11.7.4). Styrene-butadiene-styrene triblock copolymers have also been known since 1965 as commercial thermoplastic elastomers (Section 11.8). Closely related to these but thermoplastic rather than rubber-like in character are the K-resins developed by Phillips. These resins comprise star-shaped butadiene-styrene block copolymers containing about 75% styrene and, like SBS thermoplastic elastomers, are produced by sequential anionic polymer- isation (see Chapter 2). An interesting feature of these polymers is that they have a tetramodal molecular mass distribution which has been deliberately built in and which is claimed to improve processability. This is achieved by the following procedure: (1) Initiating polymerisation of styrene with sec-butyl-lithium. (2) When the styrene has been consumed, to give living polymers of narrow molecular mass distribution, more styrene and more catalyst is added. The styrene adds to the existing chains and also forms new polymer molecules initiated by the additional sec-butyl-lithium. (3) When the replenishing styrene had also been consumed butadiene is added to give a living diblock and when the monomer has been consumed the diblocks will have two modal molecular weights. (4) The linear diblocks are then coupled by a polyfunctional coupling agent such as epoxidised linseed oil to give a star-shaped polymer. As already mentioned, commercial materials of this type have a tetramodal distribution. Polymers of this sort possess an interesting combination of properties. They are clear and tough (although notch sensitive) and exhibit a level of flexibility somewhat higher than that of polypropylene. Typical properties are given in Table 16.6. The block copolymers are easy to process but in order to obtain maximum clarity and toughness attention has to be paid to melt and mould temperatures during injection moulding. Polymers of this type find application in toys and housewares and are of interest for medical applications and a wide variety of miscellaneous industrial uses. Table 16.6 Some typical properties of styrene-butadiene block copolymer thermoplastics (Phillips K-Resins) Value I ~~ ~ Property Specific gravity Tensile strength Tensile modulus Hardness (Rockwell R) Heat deflection temperature (at 1.81 MPa stress) Vicat softening point Water absorption (24 hours) Transparency i 1.04 27-30 MPa 1400 MPa 72 71°C 93°C 0.09% Transparent 452 Plastics Based on Styrene 16.12 MISCELLANEOUS POLYMERS AND COPOLYMERS In addition to the polymers, copolymers and alloys already discussed, styrene and its derivatives have been used for the polymerisation of a wide range of polymers and copolymers. Two of the more important applications of styrene, in SBR and in polyester laminating resins, are dealt with in Chapters 11 and 25 respectively. The influence of nuclear substituents on the properties of a homopolymer depends on the nature, size and shape of the substituent, the number of the substituents and the position of entry into the benzene ring. Table 16.7 shows how some of these factors influence the softening point of the polymers of the lower p-alkylstyrenes. Table 16.7 I Polymer I BS1524 softening point I Poly-(p-methylstyrene) Poly-@-n-propylstyrene) Pol y-(p-isopropylstyrene) Poly-(p-n-butylstyrene) Poly-(p-sec-butylstyrene) Poly-(p-tert-butylstyrene) 88°C R.T. 87°C rubber 86°C 130°C It will be seen that increasing the length of the n-alkyl side group will cause a reduction in the interchain forces and a consequent reduction in the transition temperature, and hence the softening point. Branched alkyl groups impede free rotation and may more than offset the chain separation effect to give higher softening points. Analogous effects have already been noted with the polyolefins and polyacrylates. Polar substituents such as chlorine increase the interchain forces and hinder free rotation of the polymer chain. Hence polydichlorostyrenes have softening points above 100°C. One polydichlorostyrene has been marketed commer- cially as Styramic HT. Such polymers are essentially self-extinguishing, have heat distortion temperatures of about 120°C and a specific gravity of about 1.40. A poly(tribromostyrene) with the bromine atoms attached to the benzene ring is marketed by the Ferro corporation as Pyro-Chek 68 PB as a heat-resisting fire retardant used in conjunction with antimony oxide. The polymer has an exceptionally high specific gravity, reputedly of 2.8, and a softening point of 220°C. The nuclear substituted methyl styrenes have been the subject of much study and of these poly(viny1 toluene) (Le. polymers of m- and p-methylstyrenes) has found use in surface coatings. The Vicat softening point of some nuclear substituted methyl styrenes in given in Table 16.8. In 1981 Mobil marketed p-methylstyrene monomer as a result of pressure on the chemical industry to replace benzene with toluene, which was less expensive. Miscellaneous Polymers and Copolymers 453 Polymer VSP ("C) Poly-(m-methylstyrene) Poly-(0-methylstyrene) Poly-(p-methylstyrene) Poly-(2,4-dimethylstyrene) Poly-(2,5 -dimethylstyrene) Poly-( 3,4-dimethylstyrene) Poly-(2,4,5-trimethyIstyrene) Poly-(2,4,6-trimethyIstyrene) Poly-(2,3,5,6,-tetramethylstyrene) 92 128 105 135 139 99 147 164 150 Whilst the homopolymer is similar to polystyrene, it does exhibit certain distinct differences, including: (1) Lower specific gravity (1.01 compared to 1.05). (2) Higher softening point (Vicat temperatures of 110-1 17°C compared to (3) Increased hardness. (4) Easier flow. Copolymers based on p-methylstyrene analogous to SAN (PMSAN) and to ABS (ABPMS) have also been developed by Mobil. The differences in properties reported are very similar to the differences between the homopolymers. Catalytic dehydrogenation of cumene, obtained by alkylation of benzene with propylene, will give a-methylstyrene (Figure 26.25). Both the alkylation and dehydrogenation may be camed out using equipment designed for the production of styrene. 89-107°C). CH Y /CH3 c,H CH, I CH,= C Figure 16.15 It has not been found possible to prepare high polymers from a-methylstyrene by free-radical methods and ionic catalysts are used. The reaction may be carried out at about 40°C in solution. Polymers of a-methylstyrene have been marketed for various purposes but have not become of importance for mouldings and extrusions. On the other hand copolymers containing a-methylstyrene are currently marketed. Styrene-a -methylstyrene polymers are transparent, water-white materials with BS softening points of 104-106°C (c.f. 100°C for normal polystyrenes). These materials have melt viscosities slightly higher than that of heat-resistant polystyrene homopolymer. 454 Plastics Based on Styrene Many other copolymers are mentioned in the literature and some of these have reached commercial status in the plastics or some related industry. The reason for the activity usually lies in the hope of finding a polymer which is of low cost, water white and rigid but which has a greater heat resistance and toughness than polystyrene. This hope has yet to be fulfilled. 16.13 STEREOREGULAR POLYSTYRENE Polystyrene produced by free-radical polymerisation techniques is part syndio- tactic and part atactic in structure and therefore amorphous. In 1955 NattaI6 and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler17 at about the same time. The use of n-butyl-lithium as a catalyst has been described. ' * Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. In the crystalline region isotactic polystyrene molecules take a helical form with three monomer residues per turn and an identity period of 6.65 A. One hundred percent crystalline polymer has a density of 1.12 compared with 1.05 for amorphous polymer and is also translucent. The melting point of the polymer is as high as 230°C. Below the glass transition temperature of 97°C the polymer is rather brittle. Because of the high melting point and high molecular weight it is difficult to process isotactic polystyrenes. Various techniques have been suggested for injection moulding in the literature but whatever method is employed it is necessary that the moulding be heated to about 18O"C, either within or outside of the mould, to allow the material to develop a stable degree of crystallinity. The brittleness of isotactic polystyrenes has hindered their commercial development. Quoted Izod impact strengths are only 20% that of conventional amorphous polymer. Impact strength double that of the amorphous material has, however, been claimed when isotactic polymer is blended with a synthetic rubber or a polyolefin. 16.13.1 Syndiotactic Polystyrene The first production of syndiotactic polystyrene has been credited to research workers at Idemitsu Kosan in 1985 who used cyclopentadienyl titanium compounds with methyl aluminoxane as catalyst. Whereas the isotactic polymer has not been commercialised Dow were scheduled to bring on stream plant with a nameplate capacity of 37 000 t.p.a. in 1999 to produce a syndiotactic polystyrene under the trade name Questra. The particular features of this material are: Tg of about 100°C (similar to that of amorphous polystyrene) and T, of 270°C. Low density with crystalline and amorphous zones both having densities of about 1.0Sg/cm3. This is similar to that occurring with poly-4-methyl pentene-1, discussed in Chapter 11 and with both polymers a consequence of the spatial requirements in the crystal structure of the substantial side groups. Processing of Polystyrene 455 An advantage of the matching densities of the two zones or phases is that there is little warping and generally good dimensional stability. (c) While the unfilled polymer is somewhat brittle, impact strength is substantially increased by the use of glass fibres and/or impact modifiers. (d) While the heat deflection temperatures of unfilled materials are similar to Tgr that of glass-filled grades approaches T,. This is in line with observation made with other crystalline thermoplastics as discussed in Chapter 9. (e) Electrical, chemical and thermal properties and dimensional stability are similar to those of general purpose ('atactic') polystyrene and thus has some advantages over more polar crystalline, so-called, engineering plastics such as the polyamides and linear polyesters. Units Some typical properties are given in Table 16.9. Unfilled 30% glass 30% glass filled filled and impact modified Table 16.9 Some properties of syndiotactic polystyrene MPa MPa % "C in/in Property 42 121 105 3500 10000 7580 IO 96 117 100 249 232 1 1.5 3.4 1 .os 1.25 1.21 2.6 3.1 3.1 0.0002 0.001 0.001 0.0027 0.003 4.0037 0.004 ASTM method Tensile strength Tensile modulus Elongation @ break Notched Izod 23°C D256 Deflection temp. under load @ 1.82MPa Specific gravity Dielectric constant Dissipation factor Moulding shrinkage D638 D638 D638 Jlm D648 D792 D150 D150 D955 Potential applications for glass-filled grades include electronic/electrical connectors, coil bobbins, relays; automotive lighting and cooling system components and pump housings and impellers. Unfilled grades are of interest as capacitor film with a heat resistance that can withstand infra-red reflow soldering combined with excellent electrical insulation properties little affected by temperature and frequency. Non-woven fabrics with good heat, moisture and chemical resistance are of interest for filter media. There has also been some interest in melt blending with polyamides to increase the toughness but at some sacrifice to dimensional stability and moisture resistance. 16.14 PROCESSING OF POLYSTYRENE Polystyrene and closely related thermoplastics such as the ABS polymers may be processed by such techniques as injection moulding, extrusion and blow moulding. Of less importance is the processing in latex and solution form and the 456 Plastics Based on Styrene process of polymerisation casting. The main factors to be borne in mind when considering polystyrene processing are: (1) The negligible water absorption avoids the need for predrying granules. (2) The low specific heat (compared with polyethylene) enables the polymer to be rapidly heated in injection cylinders, which therefore have a higher plasticising capacity with polystyrene than with polyethylene. The setting-up rates in the injection moulds are also faster than with the polyolefins so that faster cycles are also possible. (3) The strong dependence of apparent viscosity on shear rate. This necessitates particular care in the design of complex extrusion dies. (4) The absence of crystallisation gives polymers with low mould shrinkage. (5) Molecular orientation. Although it is not difficult to make injection mouldings from polystyrene which appear to be satisfactory on visual examination it is another matter to produce mouldings free from internal stresses. This problem is common to injection mouldings of all polymers but is particularly serious with such rigid amorphous thermoplastics as polystyrene. Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil (‘relax’) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. To produce mouldings from polystyrene with minimum strain it is desirable to inject a melt, homogeneous in its melt viscosity, at a high rate into a hot mould at an injection pressure such that the cavity pressure drops to zero as the melt solidifies. Limitations in the machines available or economic factors may, however, lead to less ideal conditions being employed. A further source of stress may arise from incorrect mould design. For example, if the ejector pins are designed in such a way to cause distortion of the mouldings, internal stresses may develop. This will happen if the mould is distorted while the centre is still molten, but cooling, since some molecules will freeze in the distorted position. On recovery by the moulding of its natural shape these molecules will be under stress. A measure of the degree of frozen-in stresses may be obtained comparing the properties of mouldings with known, preferably unstressed, samples, by immersion in white spirit and noting the degree of crazing, by alternately plunging samples in hot and cold water and noting the number of cycles to failure or by examination under polarised light. Annealing at temperatures just below the heat distortion temperature followed by slow cooling will in many cases give a useful reduction in the frozen-in stresses. The main reason for extruding polystyrene is to prepare high-impact polystyrene sheet. 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