Unsaturated elastomer

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

Aromatic Amines. Antioxidants derived from -phenylenediarnine and diphenylamine are highly effective peroxy radical scavengers. They are more effective than phenoHc antioxidants for the stabilization of easily oxidized organic materials, such as unsaturated elastomers. Because of their intense staining effect, derivatives of -phenylenediamine are used primarily for elastomers containing carbon black (qv). 

Ai,Af-Disubstituted- -phenylenediamines, such as A/-phenyl-Af -(l,3-dimethylbutyl)-/)-phenylenediamine [793-24-8] (10), are used in greater quantities than other classes of antioxidants. These products protect unsaturated elastomers against oxidation as well as ozone degradation (see Antiozonants). 

Butyl Rubber. Butyl mbber was the first low unsaturation elastomer, and was developed ia the United States before World War II by the Standard Oil Co. (now Exxon Chemical). It is a copolymer of isobutylene and isoprene, with just enough of the latter to provide cross-linking sites for sulfur vulcanization. Its molecular stmcture is depicted ia Table 1. 

Chlorobutyl rubber is prepared by chlorination of butyl rubber (chlorine content is about 1 wt%). This is a substitution reaction produced at the allylic position, so little carbon-carbon double unsaturation is lost. Therefore, chlorobutyl rubber has enhanced reactivity of the carbon-carbon double bonds and supplies additional reactive sites for cross-linking. Furthermore, enhanced adhesion is obtained to polar substrates and it can be blended with other, more unsaturated elastomers. 

The blends are frozen in liquid nitrogen and then microtomed and stained with osmium tetraoxide, which stains only unsaturated elastomers. 

Amine anti-oxidants are more effective than hindered phenols. Thus they are used with unsaturated elastomers and rubbers, which readily form radicals. 

Properties Liquid Unsaturated elastomer flexible glass-filled compound Crystal... 

Like ldpe, polybutadienes are resistant to most nonoxidizing acids, alkalis, and salts. However, because they are unsaturated, the polyalkadienes are attacked by hydrochloric, hydrobromic, and hydrofluoric acids, as well as by hydrogen and chlorine. The reaction products, which are thermoplastic, have been used as commercial nonelastomcric plastics. NR and other diene elastomers are also attacked by peroxides and ozone. In the absence of an tioxidants and carbon black filler, these unsaturated elastomers are degraded in the sunlight. 

Studies of several investigators have shown that polyfunctional thiol compounds are useful as cross-linking agents for the radiation curing of unsaturated elastomers and polymers used in graphic arts, electronics, and coating industries. Already small amounts of these compounds enhance the... 

Cyclisation of unsaturated elastomers has been the subject of interesting research. Cyclisation yields hard resinous products which have commercial importance and are designated as cyclised rubber [49, 50]. Cyclisation can be carried out with cationic, radiation induced, photo-induced or by thermal methods. Among all these methods cationic cyclisation has been extensively reported probably because there are less side reactions. The generally accepted mechanism of cyclisation of 1,3 diene elastomers is shown in Scheme 4.3a. 

The technology of sulfur vulcanisation of unsaturated elastomers has evolved since Goodyear s invention in 1839. Scientific studies into the chemistry of sulfur vulcanisation started to appear in the late 1950s (for reviews see References [14-18]). Two experimental approaches can be distinguished the analysis of rubber vulcanisates themselves and the so-called low-molecular-weight model studies. 

Over the last decade the development of advanced analytical techniques, such as Fourier transform (FT) Raman and solid-state NMR spectroscopy, have been impressive, resulting in a great deal of progress in the field of the sulfur vulcanisation of unsaturated elastomers [22-25]. 

A specific attention has been paid to the mechanochemical generation of X in unsaturated elastomers, EPDM, ABS, PVC and polyolefins [243, 252]. This synthetical approach exploits processing operations producing free radicals. In situ chemical reactions can be performed during some important industrial processes like mastication of rubber and extrusion, mixing or reaction injection molding of plastics. 

Ditetrazoles of type (254), obtained from the reaction of potassium 5-phenyltetrazolate with 1,2-dibromoethane, have proved useful for curing unsaturated elastomers in a cross-linking process that involved thermal elimination of N2 from the ditetrazole to form dinitrilimine 1,3-dipoles (72MI41301). 

Ethanox 376 is a stabilizer that provides heat stability by preventing thermo-oxidative degradation during processing and service life. It provides compatibility with resins and extraction resistance. It can be applied in polyolefins, such as polyethylene, polypropylene, polybutene-1 and other polymers such as engineering plastics, styrenes, polyurethanes, saturated and unsaturated elastomers, styrenics, rubber modified styrenics, segmented block copolymers, and PVC. 

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