Protection of Natural Rubber against Atmospheric Agents. I. The Effects of Nickel Dibutyldithiocarbamate Alone and in Combination with Protective Agents
Authors: Pinazzi C. P. , Billuart M. 
Published: February 2011

Abstract
Latex-rubber mixtures vulcanized with zinc diethyldithiocarbamate as accelerator show excellent resistance to sunlight if they contain 1.5–5 per cent of NBC. Combinations of protective agents which were tested contributed nothing to the protective effect of NBC alone as far as resistance to cracking is concerned; in fact, under some conditions they had an unfavorable influence. Only with respect to the maintenance of mechanical properties do these combinations have any noticeable protective effect, as do combinations of NBC with DNPD and with methyl di (tert.-butyl)phenol. However, such combinations should not be disregarded a priori, for it is possible that some of them would impart an appreciable increase of resistance to heat. In the present investigation, natural aging in darkness at ordinary temperature for twenty-four months was excellent, and this is at variance with the accelerated aging tests, except for the oxidizability measurements. The behavior of dry-rubber vulcanizates accelerated by mercaptobenzothiazole differs greatly from that of corresponding latex-rubber vulcanizates. Only NBC showed any protective effect against cracking; on the contrary, it promoted a rapid loss of the mechanical properties of vulcanizates when they were exposed to light or were stored in darkness. The action of heat or oxygen under pressure is still more striking from this viewpoint. By the concurrent action of structurizing agents, among which DPPD has been found to be the most effective, it is possible to attain a final protective effect which is manifest by a notable retardation of the appearance of cracks on exposure to light and by outdoor weathering, and at the same time, by a satisfactory retention of good mechanical properties during aging. Aging at ordinary temperature in darkness is likewise good, since the loss of tensile strength in two years was only 2–3 per cent. By way of comparison, a control vulcanizate, protected by PBN and stored under the same conditions, lost approximately 1 per cent of its tensile strength. Recent experiments indicate that some carbon blacks have a very favorable effect. This should make it possible to utilize simultaneously effective protection against cracking, maintenance of good mechanical properties during exposure to light and during aging in darkness, and increased resistance to heat. This remarkable resistance to exposure cracking and to cracking by light of rubber-carbon black vulcanizates indicates the possibilities in the practical application of combinations of NBC and DPPD. In the case of reinforced mixtures, the combined effect of a complex nickel compound, a structurizing agent, and a deactivating agent seems to be the most effective system. The choice of the structurizing agent is very important, as are also the proportions of all the agents. A later study will be concerned with establishing the best proportions from the viewpoint of obtaining satisfactory protection with the minimum quantities of these agents. Finally, it must always be kept in mind that there may be other protective agents which would make superior combinations with NBC or with other organic nickel complexes.