Oil-resistant elastomers

PMA is a tough leathery resin with a low Tg and a solubility parameter of 10.5 H. In polymers of alkyl acrylates the solubility parameter decreases as the size of the alkyl group increases. The flexibility also increases with the size of the pendant groups but because of side chain crystallization this tendency is reversed when the alkyl group has more than ten carbon atoms. Polyalkyi acrylates are readily hydrolyzed by alkalis to produce salts of polyacrylic acid. The copolymer of ethyl acrylate (95%) and chloroethyl vinyl ether (5%) is a commercial oil-resistant elastomer. 

These ozone and oil-resistant elastomers may be cross-linked by amines. 

Polyolefin poiysulfides (Thiokol) are thio analogues of polyethers which are flexible, amorphous, oil-resistant elastomers. The number of sulfur links in the repeating unit, which is called rank, is always greater than two. The solvent resistance, resistance to gaseous permeation, and flexibility of these polyolefin sulfides increase with rank. The actual strength of polymers and copolymers of olefin sulfides varies. Many of these products contain the following repeating unit ... 

Some of the most useful polyphosphazenes are fluoroalkoxy derivatives and amorphous copolymers (11.27) that are practicable as flame-retardant, hydrocarbon solvent- and oil-resistant elastomers, which have found aerospace and automotive applications. Polymers such as the amorphous comb polymer poly[bis(methoxyethoxyethoxy)phosphazene] (11.28) weakly coordinate Li " ions and are of substantial interest as components of polymeric electrolytes in battery technology. Polyphosphazenes are also of interest as biomedical materials and bioinert, bioactive, membrane-forming and bioerodable materials and hydrogels have been prepared. 

Poly(alkylene sulfide)s are exceptionally oil-resistant elastomers. They also exhibit good resistance to solvents and to weathering. On the other hand, these elastomers lack the strength of synthetic rubbers and possess an unpleasant odor. 

Copolymerization of butadiene with acrylonitrile is, like that of styrene, also carried out in emulsion. This may be a discontinuous copolymerization by the cascade process or it may be continuous with withdrawal of the latex from the bottom of the reaction pot. Acrylonitrile and butadiene are used in the azeotropic ratio of 37 63. The copolymers are commercially available under the name of nitrile rubber, or the initials NBR, previously also Buna N or GR-N. They are oil-resistant elastomers. Latices produced with cation-active emulsifiers are used to coat or impregnate textiles and paper. 

Poly(l,l-dihydroperfluoroalkyl acrylates) are obtained in the following manner Aliphatic carboxylic acids are fluorinated electrochemically. The resulting perfluorocarboxylic acids, CF3(Cp2)jcCOOH, are hydrogenated in the form of their acyl chlorides or esters, and the resulting 1,1-dihydroper-fluoroalcohols are esterified with acrylyl chloride. The monomers are polymerized with K2S2O8 in aqueous emulsion. Oil-resistant elastomers result from vulcanization of these polymers with sulfur/triethylene tetramine. Poly(l,l-dihydroperfluorobutyl acrylate) and poly(3-perfluoromethoxy-l,l-dihydroperfluoropropyl acrylate) are commercially available. 

The major commercial use of propylene oxide is as a comonomer for copolymerization. The block copolymerization with ethylene oxide produces water-soluble detergents. The copolymerization of propylene oxide with non-conjugated dienes produces sulfur-vulcanizable, oil-resistant elastomers that remain rubber-like at low temperatures. The elastomers obtained by the copolymerization of propylene oxide with allyl glycidyl ether have only poor oil resistance, but have good ozone resistance and remain rubberlike at low temperatures. 

The quality of rubber stoppers is important in relation to the release of substances or particles into the liquid, the ease with which the rubber can be punctured and the flexibility around the puncture opening. The most often used material is bromine butyl rubber. For oily injections, closures made from an oil resistant elastomer (sDicmie rubber) must be used. The Ph. Eur. describes requirements for rubber closures [33]. Rubber closures for containers for aqueous parenteral preparations are classified in two classes type-I-closures and type-II-closures. Type-II-closures have additional mechanical characteristics, for example being suitable for multiple piercing. 

Williams, J. A. Oil-Resistant Elastomers with Low Permeability to Nitrogen. US Army Weapons Command, Technical Report SWERR-TR-72-58, August 1972. 

Nitrile and neoprene are high volume oil-resistant elastomers. Nitrile is superior to neoprene in resistance to oil, gasoline and aromatic solvents. However, it does not perform as well as neoprene in applications requiring exposure to weather, ozone and sunlight. Furthermore, it does not have inherent flame resistance. 

Fluoronitrosopolymers have potential primarily as low temperature, oil-resistant elastomers but although they have attracted a good deal of interest, these materials do not appear to have progressed beyond the pilot plant stage. The most thoroughly investigated nitrosopolymer is the copolymer of trifluoro-nitrosomethane and tetrafluoroethylene [4]. 

There are other specialty, oil-resistant elastomers that can substitute for epichloro-hydrin elastomers for the right price. Of course, these alternate elastomers would require another technical evaluation before use. 

DOA is mainly used to impart superior low-temperature properties to certain compounds that are based on polar, oil-resistant elastomers. 

The tire rubbers (natural rubber, SBR), when placed in oil, absorb the fluid slowly until either the oil is all gone or the rubber has disintegrated. They never reach equilibrium. The so-called oil-resistant elastomers absorb some oil, especially at elevated temperature, but only a limited amoimt. With some it may be negligible. Most end uses requiring oil-resistant elastomers can tolerate appreciable swelling or volume increase. Hence, volume increase is not a very significant way to measure the ability of a rubber article to perform its intended function in oil. 

Copolymerization is an important way to produce properties that are not possible with homopolymers. For example, the homopolymer of vinylidene chloride is highly crystalline, and though it has excellent moisture and oxygen barrier properties, it does not produces very strong film or fiber. Copolymerization with 15 percent vinyl chloride disrupts the regular structure of the homopolymer to produce a stronger, clearer, more flexible material. The copolymer retains much of the barrier properties of the homopolymer and finds wide use for food packaging and filament. Other commercial copolymers include styrene-acrylonitrile, discussed above vinylidene fluoride-hexafluoropropylene, a heat- and oil-resistant elastomer styrene-butadiene rubber ethylene-vinyl acetate hot melt adhesive and 2-ethylhexyl acrylate-vinyl acetate-acrylic acid pressure-sensitive adhesives. 

FIGURE 3.7 Comparison of oil-resistant elastomers in automotive transmission fluid. 

Finally, it is important to compare relative measures of cost of the various oil-resistant elastomers. Typically, the pound-volume cost method is used which is simply the cost of the compoimd multiplied by the specific gravity. This gives the true cost of the amount of compoimd needed to fiU a given volume such as a mold cavity. This is shown in Table 3.1 along with the upper high-temperature limits of the various materials. While Acrylic elastomers, ACMs and AEMs, are cost effective for use up to 150°C, the HNBRs are much tougher, more abrasion-resistant materials. The Huorocarbon, FKM, and Fluorosilicone, FVMQ, materials have excellent upper temperature limits, but the pound-volume costs are heavily influenced by the relatively... 

Pound-Volume Cost Comparisons of Oil-Resistant Elastomers... 

Most oil-field elastomers must also be oil resistant, and oil-resistant elastomers typically have poor low-temperature properties. When a polymer type is selected for its oil resistance and does not have acceptable low-temperature properties, plasticizers can be added to provide better low-temperature properties for the compound. Addition of plasticizers reduces modulus and changes other properties. Changing other compound ingredients, however, wiU help to compensate for these changes. 

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