Polydiene rubbers crosslinking

These products find use in specialty applications requiring better thermal stability than available in the sulfur vulcanized elastomers. Other processes are also used to crosslink polydiene rubbers (Secs. 9-2c and 9-2d). 

Polydiene rubbers can also be crosslinked by heating with p-dinitrosobenzene, phenolic resins, or maleimides [Coran, 1978 Gan and Chew, 1979 Gan et al., 1977, 1978 Sullivan, 1966]. The crosslinking mechanism is similar to that for accelerated sulfur vulcanization, for example, for vulcanization by p-dinitrosobenzene... 

Crosslinking of EPDM and Polydiene Rubbers Studied by Optical Spectroscopy... 

The aim of this chapter is to review optical spectroscopy studies on sulfur and peroxide crosslinking of polydiene rubbers, such as NR and BR (Sections 6.2.1 and 6.3.1, respectively), and to discuss in detail recent FT-Raman and FT-IR spectroscopy studies into the sulfur and peroxide crosslinking of EPDM (Sections 6.2.2 and 6.3.2, respectively). The results of optical spectroscopy studies will also be discussed in the light of results obtained with other techniques. Finally, the elucidation of the chemical structures of the crosslinks formed will allow enhanced understanding of the mechanisms of crosslinking and some preliminary insight into the structure/property relationships of crosslinked rubber. 

The mechanism of the accelerated sulfur vulcanisation of EPDM is probably similar to that of the highly unsaturated polydiene rubbers. The vulcanisation of EPDM has been studied with emphasis on the cure behaviour and mechanical and elastic properties of the crosslinked EPDM. Hardly any spectroscopic studies on the crosslinking chemistry of EPDM have been published, not only because of the problems discussed in Section 6.1.3 but also because of the low amount of unsaturation of EPDM relative to the sensitivity of the analytical techniques. For instance, high-temperature magic-angle spinning solid-state 13C NMR spectroscopy of crosslinked EPDM just allows the identification of the rubber type, but spectroscopic evidence for the presence of crosslinks is not found [72]. 

Treatment of polydienes with osmium tetroxide (OSO4) was introduced by Andrews (1964) and Andrews and Stubbs (1964). A later development by Kato (1965 1966 1967) showed that OSO4 was able to stain the rubber component in polymer blends. Osmium tetroxide adds to the double bonds and the density of rubber phase is increased. Since it crosslinks the rubber polymer by reacting with two double bonds located in two different polymer chains (Fig. 11.10), the rubber is fixed and sectioning can be carried out without smearing. Osmium tetroxide... 

Of technical importance are radically crosslinking reactions on finished articles of polyolefins by means of electron beams in order to increase, for example, the thermostability. The technical importance of networks consisting of polydienes and other rubbers, polyurethanes, formaldehyde resins, alkyd resins, and silicones has already been explained in Sects. 1.3.3.3,4.1 and 4.2. 

Hydroxyl-terminated polydiene resins gelled by the reaction with orthosilicate esters have increased thermal stability. These polymeric gels, like silicone rubbers, exhibit outstanding electrical properties. The polymeric gels crosslinked at ambient temperature are castable as self-curing liquids. For example, they are used as binders for rocket solid fuels, in coatings for pipes, tanks, etc. They can be mixed with rubbers. 

In the same way halogenation and hydrohalogenation of polydienes are accompanied by side reactions thus the reaction of chlorine with natural rubber results in additions and cyclizations but in the case of polybutadiene crosslinking is prevalent. 

These studies are useful indicators of the reactivity of polydienes to free radicals. However, in the process for making HIPS or ABS the situation is more complex due to the fact that styrene acts as both a polymerizable monomer and an initiator. Stein et al. showed that the final 2% conversion of styrene is important for rubber crosslinking. There is competition between the monomer and the double bonds of the polydiene for the free radicals hence, only as the monomer approaches zero can addition of the free radicals to the elastomer double bonds occur, leading to crosslinking. 

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