The Action of Certain Organic Accelerators in the Vulcanization of Rubber—III
Journal of Industrial & Engineering Chemistry
function in the same manner as the organic polysulfide accelerators may be classed with them as "primary accelerators." A third class consists of those compounds that are both primary and secondary accelerators. I. Secondary Accelerators-litharge, zinc oxide, etc., seem t o act no further than t o form the corresponding sulfides, in connection with hydrogen sulfide polysulfides. 11. Primary Accelerators-To this class belong the sulfides and hydrosulfides of the alkali and alkaline-earth metals.
... 111, Accelerators That Are Both Primary and Secondary-Inorganic oxides and hydroxides function first as secondary accelerators forming sulfides or hydrosulfides which then take up sulfur and act as primary accelerators. Such accelerators are sodium and calcium hydroxides, magnesium oxide and basic carbonate, etc. Secondary accelerators are believed t o function as aids to organic polysulfides by breaking them up into colloidal sulfur and the original nitrogen base. This may be illustrated by the decolorization of polysulfide solutions by litharge or zinc oxide. Ferric oxide does not act as a secondary accelerator, and neither does it readily decompose the polysulfide solutions. The solubility of organic accelerators in sulfur and rubber gives them much more intimate contact with hydrogen sulfide a t the time of its formation than is the case with the comparatively large particles of litharge or zinc oxide. Hydrogen sulfide is therefore available for the formation of organic polysulfides before being taken up by the secondary accelerators. The decomposition of a polysulfide by a secondary accelerator regenerates the free base, which with more hydrogen sulfide and sulfur re-forms the polysulfide. Secondary accelerators do not act as true catalysts; once formed into sulfides they do not react again with hydrogen sulfide. S U M M A R Y 1-All organic accelerators are believed t o function through the formation of some type of polysulfide. %-Or gani c bases and compounds that form bases during vulcanization are believed to form polysulfides through the aid of hydrogen sulfide. These are termed "hydrogenflsulfide polysulfide accelerators." 3--Thioureas, dithiocarbamates, thiurams, and mercaptan compounds are believed t o form polysulfides directly, or by first forming disulfides, and are termed "carbo-sulfhydryl polysulfide accelerators." 4-It is proposed that the function of such compounds as litharge and zinc oxide may lie in the decomposition of polysulfides into colloidal sulfur and amines. 5-Such inorganic compounds as sodium hydroxide, calcium hydroxide and magnesium oxide are believed t o function as "primary accelerators" through the formation of inorganic polysulfides. THE VULCANIZATION OF RUBBER-111' FALLS RUBBER Co., CUYAHOGA FALLS, OHIO It has for some time been generally recognized t h a t although aniline is effective as an accelerator in the 1 Presented before the Rubber Division at the 60th Meeting of the American Chemical Society, Chicago, Ill., September 6 to io, 1920. absence of zinc oxide, diphenylthiourea functions but mildly in the absence of, and strongly in the presence of this substance. Reference to this effect has already been made indirectly in the literature several times, and recently Twissl has given curves for physical test results which demonstrate quite clearly the effectiveness of diphenylthiourea as an accelerator in the presence of zinc oxide. His statement t h a t diphenylthiourea is practically inert in the absence of zinc oxide is, however, not in accord with our findings. In a previous paper of this series2 we have shown that in the acceleration of the vulcanization of a rubbersulfur mixture, the activity of one molecular part of diphenylthiourea is less than that of an equimolecular quantity of aniline, but equal t o t h a t of one molecular part of aniline and one molecular part of phenyl mustard oil. Our former experiments, however, were confined t o the determination of sulfur coefficients at one cure only. I n the present instance, we desired to compare the relative effects of aniline and diphenylthiourea over a series of cures, and to effect this comparison both by means of the sulfur coefficients and the physical prQperties of the various mixtures and cures. Further, it was desired to compare mixtures which contained zinc oxide, as well as the rubber-sulfur mixtures previously employed. I n the experimental part of this paper we have given results obtained with six different mixtures, as follows a rubber-sulfur control, a control which contained zinc oxide, and similar mixtures which contained either one molecular part of aniline or diphenylthiourea. All of the mixtures were vulcanized for various intervals over a wide range of time. After vulcanization, comparisons of sulfur coefficients and physical properties were made. Summarizing these results briefly, we found that, in a rubber-sulfur mixture, the accelerating effect of aniline is considerably greater than that of diphenylthiourea, when judged either by sulfur coefficients or on the basis of the physical properties of the vulcanized mixtures. I n mixtures which contained zinc oxide, however. the reverse was found to be true, and diphenylthiourea was more active than aniline when judged by either of the above criteria. It was also evident that in the case of the mixtures which contained zinc oxide, although the tensile strength of the mixture which was accelerated by diphenylthiourea increased more rapidly than in the case of the mixture accelerated by aniline, the same maximum tensile strength was attained by each. The sulfur coefficients a t their respective maxima were practically identical. While the maximum tensile strength of the rubbersulfur mixture which was accelerated by aniline was the same as t h a t obtained when zinc oxide was present in the mixture, it was attained only a t a much higher sulfur coefficient. Lastly, it was also found that the tensile strengths of the mixtures that contained zinc oxide and which were accelerated by either aniline or diphenylthiourea, particularly the latter, were in-1