The Theory of Vulcanization and the Action of Accelerators

1. Vulcanization of rubber by benzothiazolyl disulfide (without sulfur) is a radical process. The benzothiazolyl radicals formed during heat dissociation either are absorbed by a double bond or accept the mobile hydrogen of the α-methylene groups of the molecular chains of rubber. The polymer radicals formed thereby react with the other molecular chains, leading to combination of the molecules through the —C—C— bonds into spatial formations characteristic of the vulcanizate. 2. Kinetic curves were obtained which describe the conversion of benzothiazolyl disulfide into mercaptobenzothiazole and combination with rubber molecules. 3. Changes of viscosity and molecular weight during the vulcanization of rubber solutions were studied by light-scattering. It was established that the kinetic curve of viscosity has a minimum, while the molecular weight increases to three times its original value toward the end of the process. 4. The number of —C—C— cross-links in the vulcanizate was calculated from the swelling maximum and equilibrium modulus of elasticity. The data obtained indicate that, on the average, two and not more than five elementary acts of union of the molecular chains of rubber are necessary for each benzothiazolyl radical. 5. Experiments on stress relaxation at 130° established that the vulcanizate contains —C—C— cross-links between the molecular chains of rubber. 6. The isotopic exchange of a radioactive vulcanizate with the diffused benzothiazolyl disulfide demonstrates the existence of benzothiazolyl groups in the structure of the rubber. 7. A scheme of the elementary radical reactions between rubber and benzothiazolyl disulfide which lead to vulcanization is given. 8. The kinetics of vulcanization of rubber with sulfur in the presence of benzothiazolyl disulfide was studied. The combination of sulfur follows a monomolecular law and the kinetic constant depends linearly on the concentration of accelerator. 9. In the earliest stage of sulfur vulcanization, benzothiazolyl disulfide is converted into mercaptobenzothiazole, which is consumed as vulcanization proceeds. At the same time the rubber reacts with the benzothiazol radicals, which initiate polymerization processes with the formation of —C—C— links between the molecular chains of rubber. 10. The ratio between —C—C— bonds and sulfide bonds in a vulcanizate depends on the ratio between accelerator and sulfur. 11. As in the case of vulcanization with benzothiazolyl disulfide, in sulfur vulcanization in the presence of an accelerator, the reactions involving the α-methylene groups of the molecular chains are of considerable importance in structure formation. Thus vulcanization can not be regarded as a process which proceeds only at the double bonds of the rubber molecules. Activation of vulcanization by disulfides and sulfenamide accelerators is due to a large degree to reaction between these accelerators and the rubber. 12. A theory advanced in the present article together with experimental data reveal the radical mechanism of vulcanization and the action of accelerators, as well as the existence of polymerization phenomena during this process.