Abstract
Year by year the complete definition of vulcanized rubber in terms of organic chemistry has become more desirable. The severity of the demands on rubber products makes it imperative to extend basic knowledge of vulcanization if we are to overcome the traditional defects or use limitations in this field. Chemical investigations of the vulcanization problem can be arranged into three lines of attack. In the first method the vulcanizate network is severed at various points by chemical or thermal means to liberate small portions of the material which can be studied as are small molecules. Ideally there should be no alteration of the linkages except where cutting of the chains takes place. Unfortunately no such “scissors” are known. Midgley, Henne, and Sheppard applied the thermal decomposition method to ebonite. Their study of the fragments, based on 1 per cent of the total material involved, was inclusive. Secondly, a small molecule reagent which swells the rubber may be used to penetrate the three-dimensional network and react with the various linkages in it. This method was used by Meyer and Hohenemser, who diffused methyl iodide into vulcanized rubber. This complicated reaction and its background with pure sulfur compounds were reported in the first two articles in the present series. The conclusions of this study were that the part of the combined sulfur which could be removed as trimethylsulfonium iodide was sulfide sulfur linked to a carbon atom alpha to a double bond. In most cases the less of this type of sulfur present, the higher the tensile strength of the vulcanizate. Methyl iodide was successful to a hitherto unattained degree because both its rate of reaction and products vary with the type of sulfur bond. This work left unanswered the question of the sulfur linkages which were not attacked by methyl iodide—in some cases the greater part of the combined sulfur. The third chemical line of attack is the study of model systems. A small molecule, such as an olefin, is reacted with sulfur and rubber-compounding ingredients; then, from identification of the products and study of the reaction, conclusions concerning vulcanizates are reached by analogy. The use of this method is old in chemical problems.
