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.
Mit 5Figuren im TextSeit sich HEDVALL und TAMMANN den Reaktionen im festen Zustande eingehend und umfassend gewidmet haben, hat es nicht an Versuchen gefehlt, den Verlauf dieser Realrtionen naher zu erforschen. Schon vor etwa 6 Jahren hat der eine von uns Vorstellungen entwickelt, wie diese Art von Umsetzungen vor sich gehen.2) Diese Theorien haben sich bisher auch recht gut bewahrt. In der letzten Zeit ist aber weiteres Tatsachenmaterial im hiesigen und anderen Laboratorien zusammengekommen , das es notwendig erscheinen lafit, die Vorstellungen praziser und genauer zu entwickeln und zu erweitern, um sich ein moglichst einheitliches Bild uber dieses Gebiet machen zu konnen. Das durfte auch deshalb gut sein, da neuerdings das Interesse an den Reaktionen im festen Zustande stark gewachsen ist und auI3er den schon langer Arbeitenden, HEDVALL, TAMMANN, FISCHBECK und JANDER, eine groI3ere Anzahl von Fachgenossen Untersuchungen hieruber angestellt oder sie zur Darstellung von Verbindungen benutzt haben. Dabei sind das eine oder andere Ma1 Theorien aufgestellt worden, die mit dem gesammelten Tatsachenmaterial in keiner Weise vereinbar ~i n d .~) Im nachfolgenden wollen wir zunachst die neuen theoretischen Vorstellungen entwickeln, urn dann diejenigen Tatsachen zu bringen, durch die die neue Theorie gestutzt wird. Dann sollen diejenigen Erscheinungen kurz besprochen werden, die durch sie erklart werden kdnnen.
The present paper deals with the results of an orientating, quantitative investigation of sulfur vulcanization accelerated by thiuram disulfide, with tetramethylthiuram disulfide as the representative example. It was found: In the sulfur cure of natural rubber with tetramethylthiuram disulfide at different TMTD:S ratios, the rates of TMTD decrease and dithiocarbamate formation increase with increasing sulfur concentration, the TMTD content being kept constant. The rates practically do not change any further when the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The peak value of dithiocarbamate formation increases with the increase of sulfur concentration and reaches a constant end value of about 90 mole per cent based on the amount of original thiuram disulfide, when the stocks contain 4 gram atoms of sulfur per mole thiuram disulfide. This end value is identical to the end value of dithiocarbamate formation in the reaction of thiuram disulfide with zinc oxide (in the absence of rubber). The crosslinking, as measured by the change of reciprocal equilibrium swelling per time unit is also a reaction whose rate increases with the sulfur concentration to the point where the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The optimum degrees of crosslinking are roughly proportional to the sulfur concentration; at high sulfur levels the vulcanizates tend to revert. As in the pure TMTD vulcanization, the TMTD decrease as well as the dithiocarbamate formation are always first order reactions. The reversion at higher sulfur levels as well as the complicated course of the increase of combined sulfur during vulcanization render all but impossible an accurate determination of the reaction order for the crosslinking at higher sulfur levels. Nevertheless, in vulcanizations with 1 mole TMTD per 1 or 2 gram atoms of sulfur the crosslinking is a first order reaction.
Vulcanization of natural rubber with sulfur was studied in presence of six sulfenamides, to determine the effect of the chemical constitution of the sulfenamide on sulfur decrease and on crosslinking. The results can be condensed as follows: (1) The kinetics of sulfur disappearance is in every respect qualitatively independent of the chemical constitution of the sulfenamide. (2) For the sulfenamides investigated, the smallest and largest rate constants for sulfur decrease differed only by a factor of two. (3) Greater differences are encountered in the induction times for sulfur decrease and for crosslinking. The latter are notably longer than those for sulfur disappearance. (4) The same activation energy, 23 kcal/mole, is derived from the temperature dependence of the induction times for all the sulfenamides. (5) The dissociation of sulfenamides in solution and their reaction with mercaptobenzothiazole were investigated further. The results provide the basis for a proposed reaction mechanism, which is presented in detail and can account for a number of the features typical of sulfenamide-accelerated vulcanization. (6) The drop in sulfur concentration goes at practically the same rate, if one introduces, instead of N, N-dicyclohexyl-2-benzothiazolesulfenamide, the corresponding ammonium mercaptide in equimolar concentration.
This paper disproves an assertion by H. W. Zijp that a fundamental result of our investigations was wrong. The result in question is that the limiting value of dithiocarbamate formation (66 mole-percent of the thiuram disulfide added) is independent of the temperature and, in a certain range, is also independent of the concentration of thiuram disulfide, added. It is shown experimentally that a reaction of thiuram disulfide with rubber which may take place fortuitously during mixing cannot be of any influence on the limiting value of the dithiocarbamate formation.
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