Elastomer Considerations

While formulations with rubber compounded into the GPPS are effective, grafting the elastomer into the continuous phase is preferred. Commercial polymerization processes produce a polymer system that not only has an elastomer incorporated, but also a grafted species where short polystyrene side chains have been attached to the rubber domains. This grafting anchors 

When external forces act on HIPS, the rubber particles have a stress-relaxing action. To do this job, they must be bound sufficiently to the matrix and have a certain elasticity. This means that the rubber used must be capable of grafting and crosslinking. Both properties can be controlled within certain limits via the microstructure. These requirements mean that nowadays use is predominantly made of medium-m and sometimes also high-cw rubbers, which can be produced using organolithium initiators or coordinate Ziegler catalysts. 

High -cis polybutadiene has relatively high heat resistance, which is advantageous in the processing of HIPS. On the other hand, this type of polybutadiene crystallizes at about 0 °C, owing to its stereoregular structure, with the consequence that the low-temperature toughness of polystyrene, produced in this way, is reduced. 

Apart from pure polybutadiene rubbers, styrene-butadiene block copolymers are also used, enabling products having particle sizes of less than 1 xm to be produced. They have high gloss and high rigidity, but somewhat lower toughness for a comparable polybutadiene content. 

Other rubber systems have been commercially successful. Styrene block copolymers yield a HIPS product with a small particle size and provide high gloss. A mixed rubber system consisting of styrene-butadiene block rubber and/or ethylene-propylene diene modified (EPDM) rubber can be blended with the polybutadiene to form bimodal rubber particle size distribution for a 

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There are certain facts which should not be overlooked in any discussion of filled elastomers, consideration of which eliminates much unnecessary conjecture about the nature of the reinforcement process. These may be summarized as follows ... 

With vinyl chloride, the content of diene units in the elastomer considerably affects the yield of graft copolymer. With styrene, the values of the transfer constants of elastomers with different ENB contents suggest that this effect is much less. 

Impact Strength - Impact strength or impact resistance is the measure of a hard rubber s resistance to fracture imder sudden impact or force. The impact force is one of compression on one side of the rubber and tension on the other, so that fracture is a result of failure similar to that occuring when a tensile sample is elongated to break. The same filler-elastomer consideration as in tensile and flexural testing therefore apply. 

Until now we have restricted ourselves to consideration of simple tensile deformation of the elastomer sample. This deformation is easy to visualize and leads to a manageable mathematical description. This is by no means the only deformation of interest, however. We shall consider only one additional mode of deformation, namely, shear deformation. Figure 3.6 represents an elastomer sample subject to shearing forces. Deformation in the shear mode is the basis... 

Considerable work has also been conducted to try to find thermoplastic elastomers that can be used to simplify processing by enabling dry blending and melt casting instead of the conventional mixing and curing process (see Elastomers, synthetic). 

Moreover, commercially available triblock copolymers designed to be thermoplastic elastomers, not compatihilizers, are often used in Heu of the more appealing diblock materials. Since the mid-1980s, the generation of block or graft copolymers in situ during blend preparation (158,168—176), called reactive compatibilization, has emerged as an alternative approach and has received considerable commercial attention. 

Considerable amounts of EPM and EPDM are also used in blends with thermoplastics, eg, as impact modifier in quantities up to ca 25% wt/wt for polyamides, polystyrenes, and particularly polypropylene. The latter products are used in many exterior automotive appHcations such as bumpers and body panels. In blends with polypropylene, wherein the EPDM component may be increased to become the larger portion, a thermoplastic elastomer is obtained, provided the EPDM phase is vulcanked during the mixing with polypropylene (dynamic vulcani2ation) to suppress the flow of the EPDM phase and give the end product sufficient set. 

Ring S. In O-ring appHcations, the primary consideration is resistance to compression set. A fluorocarbon elastomer gum is chosen for O-ring apphcations based on its gum viscosity, cross-link density, cure system, and chemical resistance so that the best combination of processibiUty and use performance is obtained. Sample formulations for such uses are given in Table 4. 

Economic Considerations. Gum production by Ethyl Corp. is under 20 metric tons per year. Products are sold primarily for use in U.S. and N.A.T.O. naval appHcations. The materials are sold as finished goods, and thus elastomer pricing is not appropriate. Ethyl Corp. has aimounced that it will either divest or exit this business. Atochem has developed their technology and has constmcted a pilot plant to produce aryloxyphosphazenes. 

AGE-Gontaining Elastomers. The manufacturing process for ECH—AGE, ECH—EO—AGE, ECH—PO—AGE, and PO—AGE is similar to that described for the ECH and ECH—EO elastomers. Solution polymerization is carried out in aromatic solvents. Slurry systems have been reported for PO—AGE (39,40). When monomer reactivity ratios are compared, AGE (and PO) are approximately 1.5 times more reactive than ECH. Since ECH is slightly less reactive than PO and AGE and considerably less reactive than EO, background monomer concentration must be controlled in ECH—AGE, ECH—EO—AGE, and ECH—PO—AGE synthesis in order to obtain a uniform product of the desired monomer composition. This is not necessary for the PO—AGE elastomer, as a copolymer of the same composition as the monomer charge is produced. AGE content of all these polymers is fairly low, less than 10%. Methods of molecular weight control, antioxidant addition, and product work-up are similar to those used for the ECH polymers described. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Thermoplastic elastomers have now been available for over 30 years and the writer recalls organising a conference on these materials in 1969. In spite of considerable publicity since that time these materials still only comprise about 5-10% of the rubber market (equivalent to about 1-2% of total plastics consumption). It is important to appreciate that simply being a thermoplastic material (and hence being processed and reprocessed like a thermoplastic plastics material) is not enough to ensure widespread application. Crucially the material must have acceptable properties for a potential end-use and at a finished product price advantageous over other materials. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

Vulcanised rubbers possess a range of very desirable properties such as resilience, resistance to oils, greases and ozone, flexibility at low temperatures and resistance to many acids and bases. However, they require careful (slow) processing and they consume considerable amounts of energy to facilitate moulding and vulcanisation. These disadvantages led to the development of thermoplastic rubbers (elastomers). These are materials which exhibit the desirable physical characteristics of rubber but with the ease of processing of thermoplastics. 

Replacement of a vinylidene fluonde unit by an e ylene or propylene unit in a locally perfluonnated cham environment greatly reduces the acidity ot the methylene hydrogens Copolymers of TFE and propylene are therefore considerably more resistant to bases and polar solvents than VDF-based elastomers TFE and propylene form a highly altematmg structure... 

In multiphase polymeric systems, the properties of the end products do not solely depend on the properties of the pure components, but other various parameters also have a great impact (Fig. 1). In order to emphasize these factors, the following systems are taken into consideration (I) elastomer toughened styrene system, (2) elastomer toughened polycarbonate blends, and (3) direct reactive blend processing. 

The initial sealing force, item 4, should be roughly known from initial design studies, but it may be subject to considerable uncertainty and must be based on an assumption of the worst combination of seal-housing tolerance variables. The residual sealing force may be estimated if there is knowledge of the stress-relaxation rate of the elastomer, using... 

Internal bubble growth occurs because flaws at an elastomer-substrate interface act as nucleation sites where gas which is coming out of solution in the mbber can collect, following a reduction in pressure. If there are flaw sites, based on energy considerations, gas will take the easiest option— which is to collect at an already preexisting bubble and cause it to grow. 

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