1. It was established that in the joint use of vulcanization accelerators there takes place mutual activation of the accelerators, activation of one accelerator or additive action depending on the chemical structure of the accelerators. 2. Of the studied combinations of vulcanization accelerators the systems showing mutual activation are disulfides or mercaptans with nitrogen containing organic bases, and disulfides with sulfenamides. Sulfenamides with nitrogen containing organic bases result in activation of one of the accelerators. Examples of systems of accelerators with additive action are combinations of sulfenamides or disulfides with TMTM. 3. It was established that in the case of systems with mutual activation, such as MBTS with DPG or with CHBTS, the reaction between the accelerators produces MBT. For such systems, greater yields of MBT were observed under vulcanization conditions than were obtained by action of the accelerators alone. This indicates that the principal result of the chemical reaction of the accelerators is the formation of free radicals which are capable of splitting hydrogen from the rubber molecule with the formation of rubber high polymer radicals. 4. There is presented a tentative scheme for a radical mechanism of the joint action of vulcanization accelerators for the case where mutual activation of accelerators exists.

The efficiency and mechanism of the combined action of vulcanization accelerators are considered from the standpoint of their chemical structure. An investigation of the vulcanization of various types of butadiene‐styrene and natural rubbers showed that double accelerator systems may be divided into three groups; namely: systems with mutual activation of the accelerators, systems with activation of one of the accelerators, and systems with additive action of the accelerators. The vulcanization kinetics of the first two (nonadditive) groups of accelerators differ from those for the separate application of the accelerators by a retardation in the initial period and a sharp increase in rate of the main period of the vulcanization process. During interaction of accelerators of which one contains the benzothiazolyl group and the other is a hydrogen donor the formation of 2‐mercaptobenzothiazole is observed. Formation of this compound may also be noted on interaction of the accelerators in the rubber medium, the yield in this case being considerably higher than on separate interaction of the accelerators with the rubber. In the presence of sulfur (disulfide‐sulfenamide or disulfide‐organic nitrogenous base systems) a considerable rise in the yield of 2‐mercaptobenzothiazole is observed, accompanied by the formation of hydrogen sulfide and its subsequent interaction with the disulfide. The activity of double accelerator systems is the higher, the more intensive the formation of 2‐mercaptothiazole. The mutual activation of the accelerators is explained by the formation of intermediate complexes, decomposing to free radicals that initiate the interaction between sulfur and rubber and polymerization processes of the latter. A scheme of such reactions is presented.

1. Elementary sulfur, liberated in the nascent state at room temperature in the reactions of MBTS with H2S, of benzoyl peroxide with H2S and SO2 with H2S, does not bring about vulcanization of butadiene rubber. In the case of the system MBTS/H2S we observe combination of sulfur in amounts 1.2 to 1.6% to a small portion of the rubber, which does not lead to structurization. The main part of the rubber (about 90% by weight) does not, according to spectroscopic analysis, alter. The combination of sulfur with rubber observed in this case takes place, apparently, according to an ionic mechanism. 2. Low-temperature vulcanization (structurization) of rubber by the system MBTS/H2S becomes apparent with prior irradiation of solutions of rubber containing disulfide with diffuse or ultraviolet light. The rate of structurization depends upon the duration of irradiation and is governed by the interaction with the H2S of the polymeric rubber radicals which are formed as a result of the dehydrogenation of the rubber by the benzothiazolyl radicals which are formed in the photodissociation of the disulfide. 3. Structurization of rubber by the system benzoyl peroxide/hydrogen sulfide is observed in the presence of an amine, in particular PBNA, necessary for the formation of free benzoate radicals as a result of the reaction of the peroxide with the amine. The peroxide in the present case acts similarly to the benzothiazolyl radicals in the case of the system MBTS/H2S. 4. Peachey type low-temperature vulcanization (SO2/H2S) proceeds in the presence of the peroxides of the rubber itself. Prior heating of the solutions of rubber upsets structurization. 5. In the vulcanization of rubber by the systems MBTS/H2S and benzoyl peroxide/hydrogen sulfide we observe combination of sulfur with the rubber in amounts of 0.6 to 0.7% and a considerable loss of double bonds, reaching 60% for 1:4 type bonds and 75% for 1:2 type bonds. 6. Radical chain interaction schemes are put forward for the processes of low-temperature structurization (vulcanization) of rubber under the action of the systems MBTS/H2S, benzoyl peroxide/hydrogen sulfide and SO2/H2S. 7. The reaction of benzoyl peroxide with PBNA is studied. A new compound, O-benzoyl-N-phenyl-N-2-naphthylhydroxylamine, is obtained, which is a powerful inhibitor of rubber oxidation.

Reactivities of various rubbers (SKB, SKS-30 and NR) were studied at vulcanization temperatures, employing MBTS, CBS and sulfur as vulcanizing agents. The systems, rubber plus accelerator, and rubber plus accelerator plus sulfur were subjected to such temperatures. Reactivities of the rubbers were determined by measuring the degree of dehydrogenation indicated by formation of MBT. Reactivities were also determined by the addition of RS⋅ radicals to polymer chains, the variation in maxima of gelling and the kinetics of addition of elementary sulfur. Dehydrogenation resulting from addition of accelerator radicals to polymer chains in the rubbers assumed the following order: SKB > SKS-30 > NR. Reactivity of elementary sulfur to rubbers, in the presence of the accelerators employed, assumed the reverse order; NR > SKS-30 > SKB. The differences in the reactivities of the various rubbers is associated with the peculiarities of their structures.