The Action of Copper Salts on Raw Rubber

Abstract

Abstract 1. An examination of the literature makes open to doubt the view of many investigators that copper salts exert a destructive action on rubber by functioning as catalysts which start the autoöxidation of rubber and then promote this autooxidation. The literature available indicates with just as good reason that there is another explanation of the effect of copper salts, viz., that the destructive action of these salts is characterized in its initial stage by a disaggregation of the. rubber. As a result of this disaggregation there is an increased surface of rubber micelles with an accumulation of double bonds. The rubber is then more readily susceptible to oxidation by atmospheric oxygen. 2. The detailed study in the present work of the action of copper salts on rubber solutions has shown that the complicated process as a whole can be divided into separate partial processes, i. e., into a desolvation, a micellar disaggregation, and a molecular disaggregation. 3. Observations of changes in weight and of the properties of rubber films proved conclusively that copper salts first bring about a desolvation of the rubber micelles and then a disaggregation. The latter proceeds without any accompanying oxidation of the rubber. Following this there is an oxidation of the irreversibly degenerated rubber. 4. The influence of oxygen on the appearance of stickiness is very slight, in fact tackiness appears both in the presence and the absence of oxygen. 5. The stickiness of rubber samples to which a copper salt has been added is always to be found in the interior of the samples, while a brittleness appears on the surfaces. 6. These facts lead to the conclusion that the stickiness of raw rubber appears as a result of disaggregation, whereas brittleness is the result of oxidation. 7. The destructive effect of copper salts depends to a large extent upon the state of dispersion of the copper salt in the rubber; the more finely dispersed is the salt in the rubber, the greater is its effect. 8. Copper salts accelerate vulcanization, without however increasing the proportion of combined sulfur. Upon standing a long time, rubber containing a copper salt loses completely its power of vulcanization. This latter phenomenon can be explained by the fact that copper compounds bring about in a relatively short time the disaggregation which is necessary to vulcanization, whereas after a longer time they destroy the rubber completely.