Stearic Acid in Litharge-Cured Rubber Compounds

Abstract

provement. This has been found to be true. In compounds containing substantial amounts of alkali process reclaim, a low sulfur and a high accelerator ratio, there is sometimes a rapid reversion causing blowing after the proper cure has been reached. Compounds containing stearic acid show less tendency to reversion under these conditions than compounds containing other softeners. Conclusions1-Stearic acid does not compare with other softeners in plasticizing efficiency when used in contact with vulcanized rubber scrap during devulcanization.2-Stearic acid when added as a softener to devulcanized scrap on the mill prior to refining imparts properties which are very desirable. (a) Makes the reclaim batch more plastic, (b) improves tubing and calendering properties, (c) reduces nerve without production of excess tack, (d) improves curing properties (higher tensile strength, higher modulus, and improved molding properties) of reclaimed rubber, ( e ) when pigments are added it gives better dispersion with improved physical properties.3-Stearic acid when used in compounds containing reclaimed rubber improves the curing properties of these compounds.T HAS been known at least since 1912 that the resins present in crude rubber are essential to vulcanization with litharge. I n that year Weber ( 4 ) reported that deresinated rubber mill not cure and concluded that "resins play an active part in the vulcanization;" but he formulated no theory of the way in which they function. At that time the composition of the rubber resins was not so well known as now. I n 1916 Stevens (3) verified Weber's conclusions, but believed his findings supported the Esch and Auerbach ( 2 ) theory that litharge accelerated vulcanization through the rise in temperature occasioned by an exothermal reaction.It is now well understood that resins promote vulcanization through their content of organic acid. It was proposed by Bedford and Winkelmann (1) in 1924 that litharge vulcanization proreeds by the following steps: (a) reaction of lead oxide with an organic acid (naturally present in the resin or added during the mixing) to form a rubber-soluble soap; (b) reaction of the lead soap with hydrogen sulfide to form a hydrosulfide salt or a hydrosulfide; (c) reaction of the latter with sulfur to form a disulfide and then a polysulfide; (d) decomposition of the (unstable) polysulfide to yield a very active form of sulfur-the ultimate vulcanizer. The polysulfide theory has been generally accepted as accounting adequately for the known facts.It is clear, then, that the efficiency of litharge as an accelerator is absolutely dependent upon the existence of an organic acid in the rubber as mixed. The mere statement of this general principle, however, by no means gives the compounder all the information he desires in the matter of organic acids as related to litharge curing. Various questions of practical importance arise. I n the present paper an attempt will be made not comprehensively or systematically to survey all these numerous questions, but only to illus...