Vulcanization saturated hydrocarbon

In addition to the vulcanization of diene hydrocarbon polymers using sulfur, other methods of crosslinking hydrocarbon polymers, which do not require a double bond and which do not use sulfur have been developed. Thus, saturated hydrocarbon polymers and, in particular, PE, are crosslinked by reactions resulting from the addition of a peroxide to the polymer at elevated temperatures (6). 

Sulfur and selenium react with many organic molecules. For example, saturated hydrocarbons are dehydrogenated. The reaction of sulfur with alkenes and other unsaturated hydrocarbons is of enormous technical importance hot sulfurization results in the vulcanization (formation of S bridges between carbon chains) of natural and synthetic rubbers. 

Rubbers are plasticized with petroleum oils, before vulcanization, to improve processability and adhesion of rubber layers to each other and to reduce the cost and increase the softness of the final product. Large quantities of these oil-extended rubbers are used in tire compounds and related products. The oil content is frequently about 50 wt% of the styrene-butadiene rubber. The chemical composition of the extender oil is important. Saturated hydrocarbons have limited compatibility with most rubbers and may sweat-out. Aromatic oils are more compatible and unsaturated straight chain and cyclic compounds are intermediate in solvent power. 

Saturated hydrocarbons are dehydrogenated when heated with sulfur, and further reaction with alkenes occurs. An application of this reaction is in the vulcanization of rubber, in which soft rubber is toughened by cross-linking of the poly-isoprene chains, making it suitable for use in, for example, t)Tes. The reactions of sulfur with CO or [CN] )deld OCS (16.13) or the thiocyanate ion (16.14), while treatment with sulfites gives thiosulfates (equation 16.21). 

Like the very important diene rubbers (polyisoprene, polybutadiene and SBR) the ethylene-propylene rubbers are hydrocarbons. They are therefore resistant to polar solvents but dissolve (when unvulcanized) or swell (when vulcanized) in hydrocarbons. Being saturated they are somewhat inert chemically and therefore have good resistance to oxygen, ozone, acids and alkalies. Besides being attacked by peroxide radicals they may also be halogenated. 

The properties of these elastomers are widely different. All require vulcanization. In general, sulfur is used only for unsaturated polymers, peroxides, quinones, metallic oxides, or cliisocyauates effect vulcanization with saturated types. Many are special-puipose rubbers, some can be used in tires when loaded with carbon black, others have high resistance to attack by heat and hydrocarbon oils and thus are superior to natural rubber for steam hose, gasoline and oil-loading hose. Most are available in latex form. See also Elastomers. 

Ethylene-propylene rubber is a synthetic hydrocarbon-based rubber made either from ethylene-propylene diene monomer or ethylene-propylene terpolymer. These monomers are combined in such a manner as to produce an elastomer with a completely saturated backbone and pendant unsaturation for sulfur vulcanization. As a result of this configuration, vulcanizates of EPDM elastomers are extremely resistant to attack by ozone, oxygen, and weather. 

Uncured ethylene-propylene copolymers are soluble in hydrocarbons and have rather poor physical properties useful technological properties are developed only on vulcanization. As mentioned above, the saturated copolymers are vulcanized by heating with peroxides whilst the terpolymers are vulcanized by conventional sulphur systems. The peroxide-cured rubbers have somewhat better heat aging characteristics and resistance to compression set but sulphur-cured rubbers are more convenient to process and allow greater compounding freedom. 

The vulcanization of polybutadiene rubbers deviates substantially from this reaction pattern, evidently because the bulk of the accelerator becomes irreversibly bound to the rubber at an early stage. This denudes the vulcanizing system of zinc accelerator-thiolate complexes and, therefore, prevents desulfuration from occurring. The crosslinks thus remain di- and poly-sulfidic and are apparently less prone to decomposition than in the case of the polyisoprene rubbers, since cyclic sulfides and conjugated hydrocarbon groupings seem not to be prominent products. The removal of accelerator leaves the system unresponsive to zinc and gives it the characteristics of unaccelerated vulcanization with the result that vicinal crosslinking and crosslinks with saturated chain junctions become important products. 

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