Acrylonitrile-butadiene rubber ozone resistance

The natural rubber does not generally exhibit all the desired properties for use in the rubber industry. Thus, it is possible to obtain better mechanical and physical properties at a lower cost by blending natural rubber with synthetic rubbers. Normally, natural rubber is deteriorated by ozone and thermal attacks due to its highly unsaturated backbone, and it also shows low oil and chemical resistances due to its non-polarity. However, these properties can be achieved by blending it with low unsaturated ethylene propylene diene monomer rubber, styrene butadiene rubber, carboxylate styrene butadiene rubber, nitrile butadiene rubber, chloroprene rubber, chlorosulfonated polyethylene rubber, and acrylonitrile butadiene rubber. 

Acrylonitrile butadiene rubber is susceptible to ozone attack, and therefore suitable anti-ozonants and/or waxes need to be added. Acrylonitrile butadiene rubber /EPDM blends (at 70/30 or 80/20 ratio) bave a better ozone resistance than NBR alone. 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

Blends of butadiene-acrylonitrile copolymer rubber (nitrile rubber or NBR) and PVC are among the oldest known examples of commercial elastomer/ thermoplastic blends. The shortage of natural rubber during World War II stimulated research in the USA on the compounding and modification of synthetic polymers to produce rubber-like materials. An outcome of this research was the commercial introduction of NBR/PVC blends by B.F. Goodrich in 1947 under the trade name of Geon Polyblends [Pittenger and Cohan, 1947]. The blend showed improved ozone resistance and melt processability compared to the nitrile rubber (Table 15.12). 

Nitrile rubber (acrylonitrile-butadiene copolymer) is a unique elastomer. The acrylonitrile content of the commercial elastomers ranges from 25% to 50% with 34% being a typical value. This nonhydrocarbon monomer imparts to the copolymer very good hydrocarbon oil and gasoline resistance. The oil resistance increases with increasing amounts of acrylonitrile in the copolymer. Nitrile rubber is also noted for its high strength and excellent resistance to abrasion, water, alcohols, and heat. Its drawbacks are poor dielectric properties and poor resistance to ozone. 

It is proposed that this is due to attack of carbonyl oxides, in their biradical form, on the rubber double bonds. Typical diene rubbers (polyisoprene and polybutadiene) have rate constants several orders of magnitude greater than polymers having a saturated backbone (polyolefins). Other unsaturated elastomers having high reaction rates with ozone include styrene-butadiene (SBR) and acrylonitrile-butadiene (NBR) rubbers. As an example, Polychloroprene (CR) is less reactive than other diene rubbers, and it is therefore inherently more resistant to attack by ozone. 

The most prominent application end-uses for PVC blends require permanent plasticisation. Butadiene/acrylonitrile copolymers have been compounded as permanent plasticisers for PVC wire and cable insulation, applications requiring food contact, and in pond liners used for oil containment [21]. Compounding with nitrile rubbers in plasticised PVC provides improved ozone, thermal ageing and chemical resistance for applications such as fuel hose covers, gaskets,... 

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