1. The vulcanization of rubber in the presence of N,N-diethyl-2-benzothiazolylsulfenamide is characterized by an S-shaped curve for the addition of sulfur with an initial induction period in the reaction. The modulus and number of crosslinks are changed in an analogous manner to the structure of the vulcanizate. 2. The energy of activation of the addition of sulfur in the initial period is equal to 30 kcal per mole as against 14 kcal per mole in the main period. 3. The induction period is increased if the sodium-butadiene rubber is purified from alkali. 4. Molecular oxygen present in the compound being vulcanized decreases the induction period and increases the rate of the addition of the sulfur in the main period. An induction period is not observed when vulcanization is carried out in an atmosphere of pure oxygen. 5. The interaction of N,N-diethyl-2-benzothiazolylsulfenamide with rubber (in the absence of sulfur) at vulcanization temperatures is accompanied by the formation of MBT, diethylamine, and the addition of the elements of the accelerator to the rubber. The kinetics of this process were studied. 6. The interaction of N,N-diethyl-2-benzothiazolyl sulfenamide with rubber leads to the formation of chemical crosslinks between the molecules of rubber (the effect of vulcanization). 7. The change of N,N-diethyl-2-benzothiazolyl sulfenamide under the conditions of normal sulfur vulcanization has the same character as in the interaction of it with rubber. The kinetics of the formation of MBT have a maximum which coincides with the maximum rate of the addition of sulfur to the rubber. 8. A mechanism is presented for the vulcanization and acceleration actions of N,N-diethyl-2-benzothiazolyl sulfenamide which provides for the extraction of hydrogen by the accelerator radicals from the molecular chains of the rubber with the formation of MBT, diethylamine and polymer radicals which are able to interact with the sulfur.

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.

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.

1. We have synthesized and investigated as vulcanization accelerators the derivatives of 2-mercaptobenzothiazole (MBT) in which the thiol hydrogen is replaced by the nonpolar methyl radical, as well as compounds in which methyl hydrogen of the methyl derivative is replaced by various functional groups. 2. It has been shown that methyl-2-thiolbenzothiazole is not an accelerator. The replacement in this compound of one of the methyl hydrogens by a polar hydroxyl group substantially enhances the activity (see, however, editors note in the text). The substitution of hydrogen by a carboxyl group does not increase vulcanizing activity. 3. We have determined that replacement of a methyl hydrogen by an amino radical increases sharply the accelerating activity. The structure obtained as a result of this reaction, benzothiazolyl-2-thiolmethyldiethylamine (BTMA), is of great practical interest as an accelerator. 4. The accelerator BTMA in stocks of natural and SKS rubber gives vulcanizates which are substantially superior in their properties to rubbers cured with MBT, and are practically equal to vulcanizates obtained with sulfenamide accelerators—sulfenamide BT and sulfenamide Z (Santocure). 5. The accelerator BTMA is much cheaper than sulfenamide BT since its production requires much less diethylamine. 6. It has been determined that, just as in the case of 2-mercaptobenzothiazole derivatives, for the derivatives of dimethyldithiocarbamic acid containing analogous functional groups the same results are obtained for the change in activity depending upon the chemical structure of the accelerator.

The wide use in industry of new types of synthetic rubbers calls for the creation of a variety of vulcanization systems of specific action. The quest for such vulcanization systems and, in particular, for vulcanization accelerators with given properties is of course made more difficult by the fact that up to the present time no one has cleared up the question of the influence of the chemical structure of the accelerators upon their vulcanizing activity. As a result of this the provision of experimental data revealing the connection between the chemical structure and the action of vulcanization accelerators is of immediate theoretical and practical value.