Zinc Oxide Testing of Latex

Abstract

Viscosity changes in concentrated natural rubber latex under different experimental conditions were investigated with the object of determining the mechanism of zinc oxide thickening and to find reasons for inconsistencies in correlating results of the zinc oxide thickening or ZOT test with other latex tests. It has been found that small variations of the amount of naturally occurring soap are of great importance in the thickening of latex containing zinc oxide, especially at the low ammonia content used for the ZOT test. The effect of added soap on the ZOT test was found to vary from latex to latex and to be different for different soaps. Less than about 0.3 per cent of various soaps added to latex prior to the zinc oxide reduced the degree and rate of thickening. With more than this amount of soap, the thickening depended on the soap used. A similar investigation in which the mechanical stability of latexes containing varying amounts of added soap and a standard amount of zinc oxide was measured, showed that the latexes varied anomalously with the amount and the type of soap added. The results of the work presented in this paper show that new tests for the zinc sensitivity of latexes must take into account a number of hitherto unsuspected factors, particularly when such tests are required to have a bearing on processing behavior.