Refractometer Studies on Rubber-Pigment Mixtures

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Abstract
Several years ago, McPherson and Cummings published data on the refractive index of rubber and of rubber-pigment mixtures. They found no difference in the refractive index of rubber from various sources. The degree of mastication did not appreciably change the refractive index. Sulfur in combination with rubber altered the refractive index relatively more than free sulfur in rubber. With the exception of zinc oxide, the pigments and fillers they examined did not affect the refractive index. In the case of zinc oxide, there was a significant increase in refractive index and the fine particle size oxide (Kadox) developed a higher value than the coarser oxide (XX Red). They suggested that zinc oxide probably reacts with the nonrubber components to form salts which dissolve and change the refractive index. This laboratory found that, for a series of various particle size zinc oxides in rubber, the increase in refractive index was inversely proportional to the particle size of the oxides; that is, a fine particle size zinc oxide increased the refractive index somewhat more than a coarse oxide. It was also observed that the refractive index of zinc oxide-rubber mixtures increased proportionally with each addition of zinc oxide until the opacity of the zinc oxide-rubber mixture became too great (over 6 per cent) to allow measurement by transmitted light.

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