Structural changes in vulcanizates of carboxyl and methylmethacrylate rubbers on heating

TWO-STAGE vulcanization is fairly widely known in the rubber industry practice; the second stage in this process is accomplished in a thermostat at higher temperatures than the first. Two-stage vulcanization is used for example to obtain rubbers from polysiloxane, polyacrylie, and phosphorus-containing rubbers. The second stage of the vulcanization substantially improves the properties of the rubbers. The improvement in the properties in the second stage of vulcanization is generally connected [1-3] with an increase in the degree of threedimensional structure formation during preheating in air without pressure. Thus, if prolonged vulcanization of a polysiloxane rubber is carried out in the press, then this leads to an impairment of the equilibrium modulus; during prevulcanization in air, the modulus rises, which is evidence about three-dimensional structure formation taking place [3]. During heating in air, the elimination of moisture and other volatile substances takes place, for example, the decomposition products of peroxides and oxidized groups which give rise to processes involving the destruction of bonds or of the polymer chain [1, 2, 4]. I t had been shown previously [5-7] that in the case of certain types of vulcanizates of carboxyl-containing rubbers, chlorosulphopolyethylene and butadiene methylmethacrylate rubbers, the properties are substantially improved during their subsequent thermostatic t reatment after compression vulcanization. In all cases, a substantial improvement in properties takes place when the structural network of vulcanizates is formed principally by relatively weak bonds of the ionic type. A more detailed s tudy was made of the change in structure and properties of vulcanizates of the carboxyl-containing rubber SKS-30-1 as a result of its subsequent heating at various temperatures [5]. I t was established that in the case of rubbers having only (or principally) ionic bonds (vulcanized with magnesium oxide, a combination of magnesium oxide and the epoxide resin E-41, or a combination of magnesium oxide, hyperize and triethanolamine) a marked increase in their rupture resistance takes place with keeping in the thermostat; the relative elongation somewhat increases initially and thereafter