Influence of Lubricating Oil Additives on Swell-Resistant Elastomers

Abstract

Abstract In the last few decades the demands made on lubricating oils have become increasingly severe; corresponding improvements in the quality of oils have been achieved primarily by adding additives of widely different composition and function. These substances, however, may severely attack elastomers which come into contact with lubricating oils, possibly making the elastomers useless within a short time. The influences of chemically defined lubricating oil additives on nitrile-butadiene, chloroprene, silicone, and acrylic rubbers are investigated at 140° C, ASTM Oil No. 1 being used as the medium. The various substances are found to have strong influences on the condition of the elastomers, depending on the structure of the polymer. See Figure 16. Sulfonated and chlorosulfonated fatty oils and hydrocarbons cause surface hardening in nitrile rubber, which results in crack formation under flexure or elongation. However, after pronounced initial deterioration of the mechanical properties, further penetration of the additive is prevented by the protective layer thus formed. Chloroprene rubber also takes part in a crosslinking reaction with these additives, though without undergoing surface hardening. Silicone and acrylic rubber, which have no double bonds in the polymer chain, are not attacked by these additives. Owing to a crosslinking reaction compounds which release hydrogen chloride, e.g., chlorinated paraffin, cause severe surface hardening in nitrile and chloroprene rubber vulcanizates, which is not restricted to the surfaces. In the case of nitrile rubber, hydrogen chloride combines with the vulcanizate. In the presence of chlorinated paraffin, acrylic rubber first softens but this is afterwards obscured by hardening. Chlorinated paraffin does not attack silicone rubber. Lead naphthenate damages acrylic and silicone rubber but stabilizes nitrile and chloroprene rubber. With reference to nitrile rubber, as an example, it is shown that the resistance of the elastomers greatly depends on the temperature and concentration of the additive and that both synergistic and antagonistic effects are possible when additives are used in conjunction with one another. The purposes of this study have been, firstly, to show that the differences in the behavior of elastomers in technical lubricating oils are usually due to the influence exerted by additives, and, secondly, to encourage the lubricating oils industry to consider and make allowances for the problems of the rubber industry.