Epichlorohydrin rubbers properties

The development and optimisation is described of a new curing system to replace lead-based compounds used in epichlorohydrin elastomers currently used in automotive applications. The system is based on 2,4,6-trimercapto-1,3,5-triazine and dialkyldithiophosphate, which is shown to produce a scorch-safe curing system and which confers excellent physical and ageing properties on epichlorohydrin rubbers. Trials are conducted in formulations for multilayer fuel hoses. 9 refs. 

Uses of Epichlorohydrin Rubbers. Because of their properties and moderate price, epichlorohydrin rubbers are used in automotive apphcations such as seals, gaskets, hoses, and tubing. They are also used in coated fabrics and roll covers. 

Many times when good heat, fuel, and oil resistance are needed, epichlorohydrin rubber is selected, especially for seals. This elastomer is used to make gaskets and rollers. Because of the good dynamic properties that epichlorohydrin imparts to a compound, it is used to make belts as well. 

Blends of poly(3-hydroxyalkanoic acid)s (PHAs) with various natural and synthetic polymers have been reported as reviewed in Refs. [21,22]. By blending with synthetic polymers it is expected to control the biodegradability, to improve several properties, and to reduce the production cost of bacterially synthesized PHAs. The polymers investigated as the blending partners of PHAs include poly(ethylene oxide) [92, 93], poly(vinyl acetate) [94], poly(vinylidene fluoride) [95], ethylene propylene rubber [94, 96], po-ly(epichlorohydrin) [97, 98], poly(e-caprolactone) [99], aliphatic copolyesters of adipic acid/ethylene glycole/lactic acid [100] and of e-caprolactone/lactide... 

None of these rubbers has carbon-carbon double bonds. Consequently, they have relatively good aging properties, but, on the other hand, they cannot be vulcanized by the classical sulfur process. For this reason, some of these rubbers are cross-linked with the aid of peroxides, and, in this case, by polymerization of vinyl groups in the case of some silicone rubbers or by free radical transfer reactions in the case of ethylene/vinyl acetate or acrylic rubbers. Other speciality elastomers are cross-linked by reaction with diamines, for example, in the cases of acrylic, epichlorohydrin and fluorine rubbers. 

Epichlorohydrin. Commercial polyester elastomers inclnde both the homopolymer and the copolymer of epichlorohydrin with ethylene oxide. The very polar chloromethyl gronp creates basic resistance to oil for these polymers, and they have been extensively used in fuel lines however, the desire for lower fuel permeation is cansing a search to be made for other polymers (20). Epichlorohydrin (ECO) has excellent resistance to fuel and oil swell. The ECOs show a volume swell of 35% at room temperature compared to 70% for a medium ACN-nitrile rubber in ASTM Reference Fuel C. The copolymer has a low temperature brittle point of -40°C and the homopolymer, -15°C. An interesting property of these elastomers is a stable dynamic performance over a wide temperature range however, the electrical properties are only average. 

Synthetic rubbers are produced as commodities. Polybutadiene, polybutylene, polychloroprene and polyepichlorohydrin are examples of elastomeric homopolymers. Copolymeric rubbers comprise poly-(butadiene-co-styrene), poly(butadiene-co-acryloni-trile), poly(ethylene-co-propylene-co-diene), and poly-(epichlorohydrin-co-ethylene oxide). The unsaturated group in the comonomer provides reactive sites for the crosslinking reactions. Copolymers combine resilience with resistance to chemical attack, or resilience in a larger temperature range, and thermoplastic-like properties. There are several studies in the literature describing the preparation of blends and composites of elastomers and conductive polymers. A description of some significant examples is given in this section. 

When Hercules entered the specialty rubber business in 1968 with its two HERCIX)R epichlorohydrin elastomers (Table I), very little was known about their structure. As indicated previously, it should be possible to change the microstructure by altering the catalyst system and in this way optimize the balance of properties. 

Some of the discussed additives may affect electrical properties of the materials. There is not much information published on this subject. It is known from literature that fluoropolymer additives made a dramatic improvement in the processing rates of several polymers (ethylene oxide epichlorohydrin copolymer, silicone, polyacrylate, nitrile butadiene rubber, and ethylene propylene diene terpolymer) without affecting the dielectric constant and dissipation factors. It can be assumed that a similar effect can be obtained with some silicone additives, but in the remaining cases, these properties have to be analyzed if they are of importance. 

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