Contributions to the Analysis of Rubber. III. Determination of Copper in Raw Rubber, in Mixtures, and in Fillers

It is already well known that copper compounds have an extremely harmful effect on rubber articles. Since, however, very small quantities of copper are always present in fillers, in fact, in compounding ingredients in general and even in ordinary plantation rubber itself, it is not possible to manufacture a rubber article which is free of copper. In view of this, one must be content in each case with keeping the copper content within a certain allowable limit. However, it has been found that not only is the quantity of copper of decisive importance, but that still other factors play a part in the harmful effects brought about by the copper. Attention has been called repeatedly in the literature to the fact that copper compounds which are injurious are those which dissolve in the rubber mixture. Nevertheless, it is certain that the harmful effects of copper depend not only on solubility but also on other factors. But since so little is known about this problem, nothing can be done for the time being except to regard any copper which may be found as injurious. In the Netherlands Government Rubber Institute the permissible quantity of copper in rubber articles has been set at 0.001 per cent. Although with this percentage harmlessness is not guaranteed, it may still be expected that, with such small percentages of copper compounds, aging is not affected unfavorably. The present work is concerned chiefly with methods for determining copper.