The Activating Influence of Metallic Oxides on Vulcanization in the Presence of Organic Accelerators

Abstract

Abstract 1. The activating influence of the oxides of six metals, zinc, cadmium, mercury, magnesium, calcium, and barium in Group 2 of the periodic table, has been investigated with raw and acetone-extracted rubber mixes containing accelerators representative of seven chemical classes. 2. Oxides of barium, calcium, and magnesium exert insignificant activating influence on vulcanization in the presence of all accelerators tested. 3. Cadmium oxide is superior to zinc oxide in activating effect when employed in normal rubber mixes accelerated with zinc isopropylxanthate, tetraethylthiuram disulfide, mercaptobenzothiazole, and ethylideneaniline. 4. Cadmium oxide proved superior to zinc oxide in activating all accelerators tested (other than diphenylguanidine) in a mix made from acetone-extracted rubber. 5. In a mercaptobenzothiazole stock, mercuric oxide gave greater activation than either zinc oxide or cadmium oxide. This superiority was more marked when using acetone-extracted rubber. 6. The activating influence of molar or semimolar proportions of twenty-four metallic oxides on vulcanization in the presence of diphenylguanidine and mercaptobenzothiazole was investigated and reported in the form of graphs showing the rigidity developed after varying times of vulcanization, with mixes containing either raw rubber or acetone-extracted rubber or rubber with additional stearic acid. 7. With mixes containing diphenylguanidine, the activating influence of oxides on vulcanization was more marked the lower the fat acid content. Most oxides gave the highest modulus in a mix containing acetone-extracted rubber and diphenylguanidine. 8. In mixes containing mercaptobenzothiazole, most oxides exhibited an increasing activating influence as the fat acid content of the rubber increased, although in several instances stearic acid in excess of that naturally present in the raw rubber merely acted as a softener. 9. Although development of modulus was retarded by increase in the fat acid content of a diphenylguanidine mix, it was frequently observed that the tensile strength and elongation at break were higher in the presence of stearic acid, suggesting that this improved dispersion of the oxide. 10. Of the twenty-four oxides tested, zinc oxide, cadmium oxide, red lead, and arsenious oxide proved most effective in the presence of diphenylguanidine, arsenious oxide giving greater rigidity than zinc oxide. 11. Bismuth trioxide, cadmium oxide, mercuric oxide, litharge, red lead, and zinc oxide were found satisfactory as activators in mixes incorporating mercaptobenzothiazole; and all the former gave stiffer vulcanizates than obtained with zinc oxide. 12. Cadmium oxide and red lead proved effective with both diphenylguanidine and mercaptobenzothiazole. 13. Cupric oxide showed distinct activating properties, but an equimolecular proportion of cuprous oxide caused such rapid deterioration during and subsequent to vulcanization that it was not practicable to complete physical tests.