Properties of Ebonite. XXXIV. Influence of Organic Accelerators of Vulcanization

Abstract

Abstract 1. An investigation was made of the influence of several organic accelerators, of the types commonly used in soft rubber, on the vulcanization of ebonite and the properties of the product. This includes the study of zinc and magnesium oxides as activators, and of variation in the proportion of accelerator used. 2. Most of the accelerators are effective in the vulcanization of ebonite, and it is definitely established that they accelerate the final “hard rubber” phase of the rubber-sulfur reaction. The degree of acceleration, however, is much less than in soft rubber, and the different accelerators do not show the same relative efficiencies. 3. Neither zinc oxide nor magnesium oxide functions as an accelerator activator in the hard-rubber reaction. 4. The effects of accelerators and metallic oxides on the maximum safe rate of vulcanization, i.e., consistent with avoiding excessive exothermic temperature rise, have been examined. All the accelerators improved the maximum safe rate of vulcanization; the most effective were butyraldehyde-aniline and diphenylguanidine, the former being considerably the better. Zinc oxide reduced the maximum safe rate when added to an accelerated mixing ; magnesium oxide caused a further improvement. Butyraldehyde-aniline plus magnesium oxide was thus the most effective combination for enabling the vulcanizing period to be reduced without risk of overheating due to the exothermic hard-rubber reaction. The fact that certain accelerators or accelerator metallic oxide combinations make possible quicker vulcanization without increased exothermic temperature rise suggests the possibility of developing special ebonite accelerators possessing this useful characteristic in an enhanced degree. 5. For speedy set-curing, a mixing containing either diphenylguanidine or butyraldehyde-aniline with magnesium oxide appears best, particularly the latter, since it can be used at a higher vulcanizing temperature without excessive overheating. The activating effect of zinc oxide in the early (soft-rubber) stage of vulcanization is of no use in speeding up set-curing ; indeed, zinc oxide usually delays the formation of a satisfactory set-cured product. 6. Accelerators do not alter the relationship between combined sulfur content and physical properties, as they do in soft rubber ; their function is simply to increase the rate of the rubber-sulfur reaction, and, perhaps as a consequence of this, to enable more of the added sulfur to become combined. 7. The technically important effects of accelerators on the properties of ebonite are : increase in resistance to plastic yield, decrease in impact strength, increase in resistance to swelling liquids, and increase (usually) in dielectric loss at normal temperatures, though the loss at elevated temperatures may be reduced. The first three of these effects result from the more complete combination of sulfur noted above.