Polypropylene 267 _______~~ ~ ~ ~ Market Table 11.8 Market breakdown for USA polypropylene Production (based on data in Modern Plastics International) I987 1997 '000 tonnes % '000 tonnes % Injection moulding Fibre and filaments Film Blow moulding Other extrusions 650 111 245 55 223 34 40 12 3 11 1653 1 508 523 85 232 41 38 13 2 6 I I I I techniques are developed for dyeing, polypropylene may be expected to extend its range of fibre applications. Polypropylene monofilaments combine low density with a high tenacity and good abrasion resistance and are finding some application in ropes and netting. The polymer has found some small-scale outlets in other directions such as sheet, pipe and wire coating. Consumption of the polymer in these directions is, however, dependent on finding applications for which polypropylene is the most suitable material. Although similar to polyethylene both in its structure and its properties, polypropylene has developed different patterns of usage. Estimates for the market breakdown in the United States, which are similar to those in Western Europe, are given in Table 11.8. 11.1.7 Atactic and Syndiotactic Polypropylene Atactic polypropylene may be obtained either as a by-product of the manufacture of isotactic polypropylene or by specific processes designed for its direct production. Whilst completely atactic material would be amorphous, commercial materials have a small measure of crystallinity. This is often assessed in terms of insolubility in n-heptane which is usually of the order of 5~10%. Viscosity average molecular weights are in the range 20 000-80 000 and specific gravities are about 0.86 g/cm3. In appearance and on handling the material is somewhat intermediate between a wax and a rubber. It is also semi-tacky. Like isotactic polypropylene it is attacked by oxygen but unlike the isotactic material it swells extensively in aliphatic and aromatic hydrocarbons at room temperature. It is also compatible with mineral fillers, bitumens and many resins. For many years atactic polypropylene was an unwanted by-product but today it finds use in a number of markets and is specially made for these purposes rather than being a by-product. In Europe the main use has been in conjuction with bitumen as coating compounds for roofing materials, for sealing strips where it confers improved aging properties and in road construction where it improves the stability of asphalt surfaces. Less important in Europe but more important in USA is its use for paper laminating for which low-viscosity polymers are used, often in conjunction with other resins. Limestone/atactic 268 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers polypropylene blends in ratio 70130 are used as back coatings for self-laying carpet tiles. Here the requirements are non-slip characteristics, good dimensional stability and resistance to lateral compressive loads as well as low cost. Other uses are as sealing compounds, for adhesives and, in combination with felt or open-pore expanded plastics, for automobile vibration damping. High molecular weight atactic polyropylene is now available (Rexene- Huntsman). This is miscible with isotactic polypropylene in any proportion to give transparent blends of interest in packaging applications. In the early 1990s syndiotactic polypropylene became available from a number of sources (Fina, Mitsui Toastu, Sumitomo) and were joined in the late 1990s by Dow using metallocene catalyst systems. Interest in these materials is a consequence of their possessing greater toughness, clarity and heat resistance (softening point) than corresponding isotactic polypropylene. See Table 11.6 11.1.8 Chlorinated Polypropylene The chlorination of polypropylene has been the subject of several fundamental studies and a variety of products is obtainable according to the tacticity of the original polymer and to the extent of chlorination. The polymers have been offered by Sanyo Pulp of Tokyo as film-forming resins of good chemical resistance, and heat and light stability. Suggested uses include paint vehicles, printing ink binders, overprint varnishes, adhesives, additives to sealing compounds and waterproofing agents. 11.2 POLYBUT-1-ENE Polybut-1-ene became available in the early 1960s as Vestolen BT produced by Chemische Werke Huls in Germany. Today it is manufactured by Shell in the United States. It is produced by a Ziegler-Natta system and the commercial materials have very high molecular weights of 770 000 to 3 000 000, that is about ten times that of the normal low-density polyethylenes. This polymer is typical of the aliphatic polyolefins in its good electrical insulation and chemical resistance. It has a melting point and stiffness intermediate between high-density and low-density polyethylene and a thermal stability intermediate between polyethylene and polypropylene. It is less resistant to aliphatic hydrocarbons than polyethylene and polypropyl- ene and in fact pipes may be solvent welded. At the same time the resistance to environmental stress cracking is excellent. Polybut-1-ene is unusual in that it exhibits three crystalline forms. One form is produced on crystallisation from the melt but this is unstable and on standing for 3-10 days this is replaced by a second crystal form. A third modification may be obtained by crystallising from solution. When first cooled from the melt the polymer has a density of 0.89g/cm3 and a melting point of 124°C but on reversion to the second form the density rises to 0.95 g/cm3 and the melting point to 135°C. Although ultimate tensile strength is unaffected by the change, stiffness, yield strength and hardness all increase. Freshly extruded and moulded material must be handled with care. From the technical point of view the outstanding property of polybut-1-ene is its creep behaviour. Possibly because of its very high molecular weight the polymer has a very high resistance to creep for an aliphatic polyolefin. One Polyisobutylene 269 advantage of this is that the wall thicknesses of polybut-1-ene pipes may be much less than for corresponding polyethylene and polypropylene pipes; they are thus sometimes flexible enough to be coiled. The processing behaviour of polybut- 1-ene is somewhat intermediate between the behaviour of high-density polyethylene and polypropylene. Processing temperatures are in the range 160-240°C. Both die swell and cooling shrinkage are greater than for polyethylene. The crystalline material formed initially on cooling from the melt is rather weak and must be handled with care on the haul off equipment. As mentioned above the polymer must be aged for about a week in order to allow the more stable crystalline form to develop. The main interest in polybut-1-ene is in its use as a piping material, where the ability to use a lower wall thickness for a given pressure requirement than necessary with other polyolefins, together with the low density, can lead in some cases to economic use. The principal application is for small-bore cold and hot water piping (up to 95°C) for domestic plumbing. Current world-wide sales are of the order of 16-20X lo3 tonnes per annum. 11.2.1. Atactic Polybut-1-ene Since only a small amount of atactic material is available as a by-product from the manufacture of isotactic polybut- 1 -ene, atactic polybut- 1 -ene is normally produced directly. Compared with atactic polypropylene it has a lower softening point (less than 100°C compared with 154°C when assessed by ball and ring methods), has better resistance to subzero temperatures and is completely soluble in aliphatic hydrocarbons. The molecular mass of atactic polybut-1-ene is about twice that of an atactic polypropylene of similar melt viscosity. It offers technical advantages over atactic polypropylene for roof coverings, sealing strips and sealing compounds. On the other hand the longer time required for it to reach a stable hardness after processing mitigates against extensive use in carpet backings. 11.3 POLYISOBUTYLENE In chronological terms polyisobutylene (PIB) was the first of the polyolefins. Low polymers were prepared as early as 1873 by Butlerov and Gorianov and higher molecular weight waxes in 1930 by Staudinger and Brunner. High molecular weight polymers were produced by IG Farben in the early 1930s using cationic polymerisation methods and polymers based on these methods are currently available from BASF (Oppanol) and Esso (Vistanex). The pair of opposing methyl groups leads to a low T, of about -73°C (c.f. -20°C for polybut-1-ene) and the lack of preference for any particular steric configuration inhibits crystallisation in the normal way although this can be induced on stretching. The methyl groups do, however, hinder rotation about the main chain bonds so the resulting material is, at sufficiently high molecular weights, a rather sluggish rubber. It has little use as a rubber in itself because of its high cold flow but copolymers containing about 2% of isoprene to introduce unsaturation for cross-linking are widely used (butyl rubber-see Section 11.9). 270 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers The homopolymer finds a variety of uses, as an adhesive component, as a base for chewing gum, in caulking compounds, as a tackifier for greases, in tank linings, as a motor oil additive to provide suitable viscosity characteristics and to improve the environmental stress-cracking resistance of polyethylene. It has been incorporated in quantities of up to 30% in high-density polyethylene to improve the impact strength of heavy duty sacks. 1 1.4 POLY-(4-METHYLPENT- 1 -ENE) Of all the branched aliphatic polyolefins higher than the polybutenes that have been prepared in the laboratory only one has so far achieved commercial status. This predominantly isotactic polymer of 4-methylpent-1-ene was introduced as TPX by IC1 in 1965, but since 1973 has been marketed by Mitsui. These materials are characterised by low density, high transparency, high melting point and excellent electrical insulation but are rather brittle, have poor aging characteristics, show a high gas permeability and are rather expensive, being at the time of writing about 3-4 times the price of low-density polyethylene. The monomers can be prepared by isomerisation of 4-methylpent-2-ene or reaction of tri-isobutylaluminium with ethylene but commercial interest appears to centre on the dimerisation of propylene (Figure 11.12). CH3 CH, \ \ CH + CH,- CH =CHI -+ CH - CH,- CH =CH, / CH, // CH2 Figure 11.12 Factors affecting laboratory polymerisation of the monomer have been discussed” and these indicate that a Ziegler-Natta catalyst system of violet TiCI3 and diethyl aluminium chloride should be used to react the monomer in a hydrocarbon diluent at atmospheric pressure and at 30-60°C. One of the aims is to get a relatively coarse slurry from which may be washed foreign material such as catalyst residues, using for example methyl alcohol. For commercial materials these washed polymers are then dried and compounded with an antioxidant and if required other additives such as pigments. 11.4.1 Structure and Properties The commercial poly-(4-methypent- 1-ene) (P4MP1) is an essentially isotactic material which shows 65% crystallinity when annealed but under more normal conditions about 40%. For reasons given later the material is believed to be a copolymer. In the crystalline state P4MP1 molecules take up a helical disposition and in order to accommodate the side chains require seven monomer units per two turns of the helix (c.f. three monomers per turn with polypropylene and polybut-1-ene). Because of the space required for this arrangement the density of the crystalline zone is slightly less than that of the amorphous zone at room temperature. Poly-(4-methylpent-l -ene) 27 1 From considerations of structure it will be recognised that as it is a paraffinic hydrocarbon the electrical insulation properties will be excellent, not unlike those of polyethylene, and that its chemical properties will also be typically paraffinic. However, like polypropylene, P4MP 1 possesses tertiary carbon atoms and the material is particularly sensitive to oxygen. Inferior in this respect even to polypropylene, this property is aggravated by the high processing temperatures required for processing and by the fact that many potential end uses involve elevated temperature conditions. The use of efficient antioxidant systems therefore becomes of paramount importance. It is claimed that current commercial materials will last about one day at 200°C and one year at 125°C. Aliphatic polyolefins in general have low densities and in the case of P4MP1 the open packing of the crystalline zones leads to the very low density of 0.83 g/cm3. Perhaps the most astounding property of this material is the high degree of transparency. This arises first because both molecules and crystals show little optical anisotropy and secondly because crystalline and amorphous zones have similar densities. They also have similar refractive indices and there is little scatter of light at the interfaces between amorphous and crystalline zones. It has, however, been observed that mouldings made from the homopolymers often show a lack of clarity. Such mouldings appeared to contain shells of voids which formed round the edges of the spherulites. It has been suggested that these arise from the different coefficients of thermal expansion of amorphous and crystalline zones. At the melting point the crystal zone has a density about 7% greater than the amorphous zone, at 60°C the densities are equal and at room temperature the amorphous zone is slightly denser. The strains set up at the boundaries will therefore cause the amorphous polymer to tear, thus setting up voids. Experiments were carried out" to investigate the transparency of various materials produced by copolymerising 4MP1 with other olefins such as but- 1 -ene, hex- 1 -ene and oct- 1 -ene. It was found that to varying degrees the other olefin units could co-crystallise with the 4MP1 units in the main chain, being most perfect in the case of hex-I-ene, and that in many cases much better clarity was obtained. This improvement in clarity through reduction in voidage has been ascribed to the retardation of spherulite growth on cooling. The rather 'knobbly' side groups have a stiffening effect on the chain and result in high values for T, (245°C) and TJ50-60"C). Copolymerisation with hex-1-ene, oct-1 -ene, dec-1 -ene and octadec-1 -ene which may be practised to reduce voidage causes some reduction in melting point and crystallinity as indicated in Table 11.9. Polymers below the glass transition temperature are usually rather brittle unless modified by fibre reinforcement or by addition of rubbery additives. In some polymers where there is a small degree of crystallisation it appears that the crystallines act as knots and toughen up the mass of material, as in the case of the polycarbonates. Where, however, there are large spherulite structures this effect is more or less offset by high strains set up at the spherulite boundaries and as in the case of P4MP1 the product is rather brittle. Compared with most other crystalline polymers the permeability of P4MPI is rather high. This is no doubt due to the ability of gas molecules to pass through the open crystal structure with the large molecular spacing. 272 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers Table 11.9 Copolymerisation of 4MP1 and hex-1-ene" (a) Effect on % crystallisation and melting point (T,) Property Hex-1-ene (molar) I Crystullinity (%) I T, ("C) Value I I 0 5 10 20 65 60 57 53 245 238 235 228 (b) Effect of adding 5% comonomer to 4MPI I I I I Blank Hex-1-ene Oct- 1 -ene Dec- 1 -ene Octadec- 1-ene 65 60 50 46 25 245 238 234 229 225 11.4.2 General Properties" Some general properties of the commercial 4-methylpent- 1 -ene polymer (TPX) are given in Table I1 .IO. Many properties are temperature dependent. For example up to 100°C the yield stress drops with temperature at a faster rate than does the yield stress of polypropylene; however, it retains some strength up to 160°C. Table 11.10 Typical properties of commercial methylpentene polymer (tested according to ASTM procedures). I Specific gravity Transparency Tensile strength Elongation at break Tensile modulus Water absorption, 24h Crystalline melting point Vicat softening point Specific heat Mould shrinkage Thermal conductivity (by BS 874 test) Permittivity 20°C, 102-106Hz Volume resistivity Stress cracking 0.83 90 4000 (27.5) 15 0.07 240 179 2.18 0.015-0.030 16.7X104 2.12 1OlS Yes-similar to low density polyethylene 2.1 x 105(1500) Units % Ibf/in*(MPa) % Ibf/in'(MPa) % OC OC Jg-'OC-' cm cm-' Jcm s-' cm-2 "C-' Other Aliphatic Olefin Homopolymers 273 11.4.3 Processing Poly-(4-methylpent-l -ene) is a highly pseudoplastic material and in the usual processing range is of low melt viscosity. There is a narrow melting range and the viscosity is highly dependent on temperature. In injection moulding this results in the use of cylinder temperatures of the order of 27O-30O0C, mould temperatures of about 70°C and the use of restricted nozzles to prevent ‘drooling’. In extrusion, high-compression screws with a sharp transition from feed to metering zone are recommended. Melt temperatures of about 270°C are required for many operations. 11.4.4 Applications There are a number of occasions where a transparent plastics material which can be used at temperatures of up to 150°C is required and in spite of its relatively high cost, low impact strength and poor aging properties poly-(4-methylpent- 1 -ene) is often the answer. Like poly(viny1 chloride) and polypropylene, P4MP1 is useless without stabilisation and as with the other two materials it may be expected that continuous improvement in stabilising antioxidant systems can be expected. At the present time major uses are in transparent chemical plant, in electrical equipment which can withstand soldering and encapsulation processes, in transparent sterilisable medical equipment and for lamp covers. One widely publicised use has been for the cover of a car interior light. Requiring only intermittent heating the cover can be placed much nearer the light source than can competitive plastics materials because of the greater temperature resistance. This can cause a saving in the volume of material required for the moulding and also give increased design flexibility. Poly-(4-methylpent- 1 -ene) is not a major thermoplastic such as polyethylene but fulfils a more specialist role. 11.5 OTHER ALIPHATIC OLEFIN HOMOPOLYMERS A number of polymers have been produced from higher olefins using catalysts of the Ziegler-Natta type. Figure I1 .I3 shows the effect of increasing the length of the side chain on the melting point and glass transition temperature of a number of poly-a-olefins. As discussed previously the melting point of isotactic polypropylene is higher than that of polyethylene because the chain stiffness of the polymer has a more dominating influence than the reduction in symmetry. With an increase in side- chain length (polybut- 1 -ene and polypent- 1 -ene) molecular packing becomes more difficult and with the increased flexibility of the side chain there is a reduction in the melting point. A lower limit is reached with polyoct-1-ene and polynon-1 -ene, and with polymers from higher a-olefins the melting point increases with increase in the length of the side chain. This effect has been attributed to side-chain crystallisation. It is interesting to note that a polyolefin with n carbon atoms in the side chain frequently has a similar melting point to a paraffin with 2n carbon atoms. Published datai3 on glass transition temperatures show similar but less dramatic changes. None of the polymers from unbranched olefins, other than ethylene, propylene or but-1-ene, has yet become important as a plastics material although some of them are of interest both as adhesives and release agents. One limitation of a 274 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers ? NUMBER OF CARBON ATOMS IN SIDE CHAIN Figure If .13. Effect of side-chain branching on the melting point and glass transition temperature of polyolefins (-CHR-CH2-)”- (R straight chain) (Ref 13) number of these materials is their tendency to undergo complex morphological changes on standing, with the result that fissures and planes of weakness may develop. Polyolefins with branched side chains other than P4MPl have been prepared (Figure 11.14). Because of their increased cohesive energy, ability for the molecules to pack and the effect of increasing chain stiffness some of these polymers have very high melting points. For example, poly-(3-methylbut-l -ene) melts at about 240°C and poly-(4,4-dimethylpent-l -ene) is reported to have a melting point of between 300°C and 350°C. Certain cyclic side chains can also -CH,-CH* -CH,-CH- I I CH CH2 I CH,- -CH, ’i CH, Poly-(3-methylbut- 1 -me) Poly-(4,4-dimethylpent- 1 -me) - CH,- CH- J\N CH,- CH- I I Poly-(vinylcyclohexane) Poly-(4-methylpent- 1 -ex) Figure 11.14 Copolymers Containing Ethylene 275 lead to high melting polymers; for example, poly(vinylcyc1ohexane) melts at 342"C13. Subsequent reviews even quoted a T, of 385°C together with a Tg of 80°C and a crystalline specific gravity of 0.95 for poly(viny1 cyclohexane). The polymer was also reported to have good dielectric loss properties over the range -180 to +160"C but to be subject to oxidative degradation. Some 40 years or more after its original discovery Dow announced in 1998 that they were undertaking developmental work on poly(viny1cyclohexane) but using the alternative name polycyclohexylethylene and the abbreviation PCHE. The Dow material is said to be amorphous and is being explored for use in optical discs where its hydrocarbon nature leads to a low specific gravity (0.947 cf. 1.21 for polycarbonate), negligible water absorption (one-tenth that of polycarbonate), 91.85% light transmission (cf. 89.81% for PC) and a flexural modulus of 3400 MPa (cf. 2500 MPa for PC). Emphasis is also being put on the stress optical coefficient which determines birefringence levels across a moulded disc. Compared to an optimum value of zero PCHE is quoted at -200 brewsters and polycarbonate at 5200 brewsters. Heat distortion temperatures are said to be similar to those of polycarbonate. In terms of processing there is no need for pre-drying PCHE granules, a standard extruder screw as used for polycarbonate may be used and discs are said to release well from the mould. Question marks remain on the oxidative stability of the polymer and on the quality of adhesion of the reflective layer but Dow claim that metallising is possible. 11.6 COPOLYMERS CONTAINING ETHYLENE Many monomers have been copolymerised with ethylene using a variety of polymerisation systems, in some cases leading to commercial products. Copolymerisation of ethylene with other olefins leads to hydrocarbon polymers with reduced regularity and hence lower density, inferior mechanical properties, lower softening point and lower brittle point. Two random copolymers of this type are of importance, ethylene-propylene copolymers and ethylene-but-1 -ene copolymers. The use and properties of polypropylene containing a small quantity of ethylene in stereoblocks within the molecule has already been discussed. Although referred to commercially as ethylene-propylene copolymers these materials are essentially slightly modified polypropylene. The random ethylene-propylene polymers are rubbery and are discussed further in Section 11.9. The Phillips process for the manufacture of high-density polyethylene may be adapted to produce copolymers of ethylene with small amounts of propylene or but-1-ene and copolymers of this type have been available since 1958. These soon found application in blown containers and for injection moulding. Properties of two grades of such copolymers are compared with two grades of Phillips-type homopolymer in Table 11 .ll. From this table it will be noted that in terms of the mechanical and thermal properties quoted the copolymers are marginally inferior to the homopolymers. They do, however, show a marked improvement in resistance to environmental stress cracking. It has also been shown that the resistance to thermal stress cracking and to creep are better than with the hom~polymer.'~ This has led to widespread use in detergent bottles, pipes, monofilaments and cables. 276 Aliphatic PolyoEefins other than Polyethylene, and Diene Rubbers Table 11.11 Comparison of major properties of ethylene-based copolymers with p~lyethylene'~ I Copolymer 1 Homopolymer I I I Specific gravity Melt flow index Tensile strength (MPa) Elongation (70) Vicat softening point ("C) Environmental stress cracking (F&) Izod impact (ft lbf/in-' notch) 0.95 0.95 0.3 4.0 24.8 24.8 70 30 255 255 400 20 4 0.8 0.96 0.96 0.2 3.5 30.3 30.3 30 15 260 260 60 2 5 1.5 The linear low-density polyethylenes discussed in the previous chapter might be considered as variations of this type of polymer. Ethylene has also been copolymerised with a number of non-olefinic monomers and of the copolymers produced those with vinyl acetate have so far proved the most significant commercially16. The presence of vinyl acetate residues in the chain reduces the polymer regularity and hence by the vinyl acetate content the amount of crystallinity may be controlled. Copolymers based on 45% vinyl acetate are rubbery and may be vulcanised with peroxides. They are commercially available (Levapren). Copolymers with about 30% vinyl acetate residues (Elvax-Du Pont) are flexible resins soluble in toluene and benezene at room temperature and with a tensile strength of about 10001bf/in2 (6.9MPa) and a density of about 0.95 g/cm3. Their main uses are as wax additives and as adhesive ingredients. Ethylene-vinyl acetate (EVA) polymers with a vinyl acetate content of 10-15 mole % are similar in flexibility to plasticised PVC and are compatible with inert fillers. Both filled and unfilled copolymers have good low-temperature flexibility and toughness and the absence of leachable plasticiser provides a clear advantage over plasticised PVC in some applications. Although slightly stiffer than normal rubber compounds they have the advantage of simpler processing, particularly as vulcanisation is unnecessary. The EVA polymers with about 11 mole % of vinyl acetate may also be used as wax additives for hot melt coatings and adhesives. A further class of ethylene-vinyl acetate copolymer exists where the vinyl acetate content is of the order of 3 mole %. These materials are best considered as a modification of low-density polyethylene, where the low-cost comonomer introduces additional irregularity into the structure, reducing crystallinity and increasing flexibility, softness and, in the case of film, surface gloss. They have extensive clearance as non-toxic materials. A substantial part of the market for the ethylene-vinyl acetate copolymer is for hot melt adhesives. In injection moulding the material has largely been used in place of plasticised PVC or vulcanised rubber. Amongst applications are turntable mats, base pads for small items of office equipment and power tools, buttons, car door protector strips and for other parts where a soft product of good appearance is required. Cellular cross-linked EVA is used in shoe parts. EVA polymers have been important for film manufacture. They are not competitive with normal film because of the high surface tack and friction which make them difficult to handle on conventional processing machinery. However, because of their somewhat rubbery nature, gloss, permeability, and good impact [...]... rubber (CR) Nitrile rubber (NBR) Olefin rubbers: Ethylene-propylene terpolymer (EPDM) Butyl rubber (IIR) Other rubbers All plastics materials (approx.) I9 87 I992 30 57 2503 903 120 292 189 4494 2 67 7 10 87 113 292 23 1 5229 2 67 0 1251 115 260 255 373 372 92 60 000 534 6 17 552 75 000 60 6 90 000 Note: (1) Separate data for butyl rubber not available after 1983 hut it is believed to be in decline (2) Data for... Notched Izod @ -40°C Deflection temp @0.45MPa I I Units MPa % % GPa Jlm JIm "C *Data is for BP Developmental grade Ketonex 2202 I Aliphatic I Nylon 66 conditioned Acetal polyketone* I I I 1.22-1.24 60 25 300 1 .7 140 27 180 1.425 70 15 45 2.8 80 60 172 60 15 >lo0 1 110 30 205 280 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers the effect of reducing the melting point by up to 50°C The... solvents being recycled Thermoplastics grades have a norbomene content in the range 60 -80% with Tg values from 60 -180°C, in this range the glass transition being almost linearly related to the norbornene content The modulus of elasticity increases with norbomene content and for commercial materials is in the range 260 0-3200 MPa but density (1.02 g/cm), tensile strength 66 MPa and water absorption ( . temperature: I9 87 CH, CH, II 30 57 2503 903 120 292 189 373 372 92 60 000 Buta-l,3-diene Isoprene 2,3-Dimethylbuta- 1,3-diene c1 4494 2 67 7 10 87 113 292 23 1 534 552 75 000 I. Butyl rubber (IIR) Olefin rubbers: Other rubbers All plastics materials (approx.) I992 5229 2 67 0 1251 115 260 255 6 17 60 6 90 000 Note: (1) Separate data for butyl rubber not. Thermal conductivity (by BS 874 test) Permittivity 20°C, 102-106Hz Volume resistivity Stress cracking 0.83 90 4000 ( 27. 5) 15 0. 07 240 179 2.18 0.015-0.030 16. 7X104 2.12 1OlS Yes-similar