Kinetic Studies of the Vulcanization of Natural and Synthetic Rubbers

Abstract

Abstract The author thanks the Editorial Committee of Rubber Reviews for the honor of reviewing all the work that has been carried out at the Technische Hochschule, Hannover in the field of vulcanization. He welcomes the opportunity to limit the discussion chiefly to our own investigations. Since research on the vulcanization of rubbers is still in full swing and since our own investigations can on no account be considered as finished, it would certainly be premature to make the results of kinetic studies the subject of an exhaustive treatise. Then too, it happens that in the meantime several excellent critical papers have appeared, which give the state of our knowledge at the present time, and in which the questions come forth which are yet awaiting a solution in the realm of vulcanization. For such reviews we have to thank Farmer, who in 1946 published a monograph dealing particularly with the organic chemistry of vulcanization, and Craig, who, in his paper which appeared in 1957, dealt chiefly with the results of physicochemieal and technological research on vulcanization. It therefore seems perhaps to be more appropriate to report thoroughly for once, a portion of the field of vulcanization research together with repetition of a part of the experimental data, that is, to recount the kinetics of the chemical reactions which take place during vulcanization, and with which the Rubber Institute of the Technische Hochschule, Hannover, has been primarily concerned since its founding in 1951. It is known that the results of kinetic studies arc not always adequate to completely and uniquely clarify the mechanism of a chemical reaction in all details; for they are frequently ambiguous, and especially so when complex processes are involved. This holds—as will appear—for vulcanization reactions, and recommends caution in the interpretation and the theoretical treatment of experimental results. For here in particular next to nothing is known about the overall reactions; and the pertinent knowledge supplied by organic chemistry, instructive though it may be, is only of a qualitative nature. Although studies of vulcanization kinetics are of great scientific and practical value, still the efforts of the physical and the organic chemists must supplement each other, so that some day it will be possible to present a complete picture of the vulcanization reactions. Kinetic studies of vulcanization with their quantitative results have always had technical significance. Indeed the logical understanding of the change in several characteristic aspects of the vulcanization process—in dependence on vulcanization time, temperature and concentration of the reactants which induce vulcanization—permits a general view of processes, which at this time are for the most part characterized through conventional technological tests rather than by a clear-cut measurement of the chemical transformations and exact physical quantities. Such a general view allows the interdependence of the processes to be recognized and their peculiarities to be disclosed. In other words, it is only in this way that the various phenomena which are observed in vulcanization reactions can be set in order and coordinated with the basic principals of our knowledge of the course of chemical reactions. Such a goal can naturally be reached only if broad, systematic investigations are made, starting with relatively simple vulcanization reactions. Proceeding in this way is also necessary for reasons peculiar to the kinetic method.