Oxidation solubilized unsaturates

The principal mbbers, eg, natural, SBR, or polybutadiene, being unsaturated hydrocarbons, are subjected to sulfur vulcanization, and this process requires certain ingredients in the mbber compound, besides the sulfur, eg, accelerator, zinc oxide, and stearic acid. Accelerators are catalysts that accelerate the cross-linking reaction so that reaction time drops from many hours to perhaps 20—30 min at about 130°C. There are a large number of such accelerators, mainly organic compounds, but the most popular are of the thiol or disulfide type. Zinc oxide is required to activate the accelerator by forming zinc salts. Stearic acid, or another fatty acid, helps to solubilize the zinc compounds. 

In order to confirm the reactivity and selectivity of lead ruthenates for the oxidation of isolated double bonds, two soluble, unsaturated carboxylic acids were chosen that contain a double bond far removed from the solubilizing carboxylate group. The two olefinic compounds, 1-undecylenic acid and 2-cyclopentene-l-acetic acid were both cleaved at the double bond as shown in Figure 13. 

The choice of ethoxylated sorbitan monoisostearate (ESMIS) and sorbitan monolaurate (SML) can be justified by the fact that they do not contain unsaturated bonds susceptible to oxidation. Besides, they are liquid at room temperature. Such a molecular structure facilitates their utilization, as they can be supplied to customers in the form of easily solubilized liquids. 

Typically, the activator stearic acid reacts with the activator zinc oxide during the curing process to solubilize the divalent zinc ion. This in turn reacts with the organic rubber accelerator to enable the eight-membered ring of the sulfur molecule to break up and rapidly establish sulfur crosslinks between the unsaturated rubber... 

Rubber-grade "stearic acid is usually a mixture of stearic acid (a Cl8 saturated fatty acid) and palmitic acid (a Cl6 saturated fatty acid) usually with a very small amount of oleic acid (a Cl8 fatty acid with one unsaturated site per molecule). Just as zinc oxide is ubiquitous in rubber recipes, so is rubber-grade stearic acid. Stearic acid and zinc oxide are almost always used together in rubber compounding. After these two ingredients are mixed in the rubber stock, they react with each other to solubilize the zinc (ion) into the rubber so that it will initiate the vulcanization process. 

The rate at which sulfur will react with the unsaturated polymer chains can be increased by the addition of activators a metal oxide plus fatty acid. The most common combination is zinc oxide and stearic acid, with the primary fimction of the fatty acid being to solubilize the zinc in the elastomer. In the presence of the metal, it is believed that the sulfur reacts as a cation at the double bond which results in charged and uncharged polysulfides, the latter of which could in turn form free radicals. Metal activated sulfur vulcanization will proceed more rapidly than crosslinking by sulfur alone, but still too slow for most production purposes. The metal oxide/fatty acid is, in practice, used not to activate the sulfur itself, but to activate the organic compounds used as vulcanization accelerators. 

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