Jean Le Bras

The Chemistry of Rubber the Interaction of Ethylenic Compounds and Rubber

Abstract It is known that the hydrocarbon of rubber has an ethylenic structure; its chemical reactions, the basic principles of which were brought out in 1902 by Weber, and which were the object, in 1930, of important work by Fisher, show that it has the general properties of ethylene derivatives, including the addition of hydrogen and metalloids of the first group (chlorine and bromine in particular), the addition of hydracids, scission by ozone, autoxidation, and isomerization by means of catalysts which isomerize ethylene derivatives, such as sulfuric acid and chlorides of metalloids. However, aside from these general reactions, rubber hydrocarbon reacts in other ways which likewise depend on the unsaturation of the molecule and on the macromolecular structure, and which in this particular case are of prime importance because it is on these properties that the processing and applications of rubber depend. As a good example, a fundamental change results from the action of sulfur, viz., vulcanization, wh...

The mechanism of protection by the deactivating effect

The mechanism of protection by the deactivating effect has been studied on the basis of data from continuous and discontinuous relaxation experiments on vulcanizates accelerated with mercaptobenzothiazol and with diphenylguanidine. It has been shown that the mechanism involves the formation of intermolecular crosslinks, which restore the damages due to the thermo-oxidative ruptures in the vulcanizate network. These intermolecular crosslinks are formed by the free sulfur present in the vulcanizates under the influence of the deactivator, which acts as a slow accelerator, as has been proved by free sulfur determinations carried out during aging. The resistance of these crosslinks to the thermooxidative degradation is specific for the deactivator; this allows for interpretation of the more or less pronounced effect of the deactivator upon aging as a function of the nature of the accelerator used during vulcanization. This explanation of the deactivating mechanism gives not only a better understanding of the phenomena, but also presents new ideas for the general interpretation of aging of vulcanized rubber.

Two Modes of Action of Antioxidants in Rubber

Abstract It is a commonly accepted belief that antioxidants protect oxidizable products from the harmful effects of oxygen by retarding the rate at which these oxidizable products combine with oxygen. Consequently, if the oxidizability of a rubber mixture containing no antioxidant and the oxidizability of the same mixture to which such an agent has been added are measured under the same conditions, the absorption of oxygen would be expected to be distinctly less rapid in the second case. This has, in fact, been proved to be true in general, not only for rubber, but also for other substances which are protected by antioxidants. In a systematic study of a series of antioxidants of differing chemical compositions, various observations were made which gave indications that these products react in two different ways. In these experiments a rubber mixture of chosen composition was prepared as a blank or control, and was compared with other mixtures of the same base composition to which various antioxidants sele...

The Effect of Chemical Changes in the Rubber Hydrocarbon on Its Oxidizability

Abstract The hydrocarbon of rubber, which contains one ethylene linkage or double bond per isoprene unit, has such a structure that it is very sensitive to the action of oxygen, and the investigati...

The Chemical Structure Responsible for the Deactivating Effect of Compounds Which Protect Rubber from Deterioration by Oxygen

Abstract One of the present authors has already offered evidence which indicates the existence of a deactivating effect, whereby vulcanized rubber is protected against deterioration by oxygen. This effect is evident with such compounds as mercaptobenzimidazole (I), mercaptobenzoxazole, and ethylene-bis (N,N′-phenylthiourea) (II), and the phenomenon seems to be connected in some way with the presence in the molecule of a thiol group united to a nitrogen atom under such conditions that the possible tautomerism between the thion and thiol forms (III) is probably displaced toward the thiol form. We have completed these earlier experiments by a more systematic study, which has included an examination of the influence of cyclization, the nature of the ring, and hetero atoms.

Have Mechanical Stresses Any Effect on the Oxidizability of Rubber

Abstract Subjection of rubber to mechanical stresses, whether static or dynamic, does not change its inherent oxidizability, at least within the limits of stress which were applied in the experiments described. Perhaps changes would have been observed if the range of stresses had been reached where crystallization phenomena became pronounced, for sufficient distortion or change in molecular state might have a certain influence on the oxidizability. For example, Williams and Dale, in a study of infrared absorption by rubber, pointed out that linear extensions greater than 400 per cent are necessary to bring about any appreciable increase in the vibration frequency of the C — C groups. However, the elongations employed in the present work cover, in general, the range of deformations to which vulcanized rubber products are normally subjected in service; hence there seemed to be no advantage in attempting to carry out the tests under more severe conditions. From another point of view, it would appear of inter...

II. Combination of the Two Modes of Action

Abstract The various experiments described in the present paper show that the combined use of an antioxygenic agent and a deactivating agent makes it possible to improve the aging of rubber mixtures to a considerable extent. This is, then, an experimental confirmation of the hypothesis which had previously been advanced as a result of observations on the relation between oxidizability and aging. It should be pointed out here that the present work is concerned only with static aging, with oxygen as the only factor playing any part in the deterioration occurring in ordinary artificial aging tests. In dynamic aging, other factors undoubtedly play a part, but it is true, none the less, that the action of oxygen is still of prime importance, as has been very clearly shown by Neal and Northam. The improved aging which was observed in the present work should, then, play a part in dynamic aging, and this has been confirmed by preliminary tests, the progress of which cannot be reported at this time. From another p...

Two Methods of Protection against Oxygen and the Improvement Which Results from Their Combined Use. I. Observations on the Mechanism of Protection

Abstract The observations which are recorded in the present paper represent an extension of the single case of litharge which has already been described. They show that, when small percentages of certain substances are added to rubber with a view to protecting the rubber from deterioration by oxygen, these substances are capable of directing the combination of oxygen with the rubber in different ways. This is shown by the fact that, as a result, a given percentage of combined oxygen does not lead to the same deterioration in physical properties. This difference in behavior can be explained logically on the basis of the antioxygenic theory by assuming that some agents act, not by retarding the rate of oxidation, but by deactivating the peroxides as soon as they are formed. By what term are these agents to be designated? First of all it should be recalled how an antioxygenic substance is defined. Every substance is an antioxygenic agent when it has the power, in small percentages, of retarding the rate of a...

The Autoxidation of Rubber and Mechanisms of Protection against Oxygen

Abstract The autoxidation of ethylenic hydrocarbons has been the object of various investigations, and various authors, among whom Engler and Stephens in particular might be mentioned, have studied the part played by peroxides. Up to 1936 it was generally believed that structurally these peroxides represent the fixation of one molecule of oxygen on the double bond. About this time, however, Criegee on the one hand, and Hock and Sehrader on the other, published papers almost simultaneously on the preparation and constitution of cyclohexene peroxide, in which they demonstrated that the latter can exist in the form of a hydroperoxide.Later these investigators offered further proof in favor of this view by extending the reaction to other hydrocarbons. These studies led to the important conclusion that ethylenic hydrocarbons have two sensitive points: the double bond itself, and the carbon in α-position to this double bond; hence there are two forms of the peroxide, the first of which retains its unsaturated c...

The Protection of Rubber against Oxygen by a Deactivating Effect

Abstract It has already been shown by one of the authors that certain substances protect vulcanized rubber against deterioration by oxidation and yet have practically no retarding effect on its rate of oxidation. The mechanism of the action of these agents is, therefore, different from that of antioxygenic agents, and it has been called a deactivating effect. The first substance which was found to manifest this hitherto unrecognized effect was mercaptobenzimidazole. The attempt has been made to gain a better understanding of the conditions under which mercaptobenzimidazole is active, both alone and in the presence of an antioxygenic substance. It was first found that mercaptobenzimidazole, in contrast to most antioxygenic agents, has no protective action against aging in the case of simple rubber sulfur vulcanizates, i.e., vulcanizates which do not contain any other ingredients which are ordinarily added, notably accelerators, zinc oxide and plasticizers. Systematic experiments with these various types of...