Study of the Reactivity of Certain Sulfides as Agents for the Cleavage of Sulfur Bonds in Polychloroprene

V. D. Bogoslovskii

(Presented by Prof. L. P. Kulev, Doctor of Chemical Sciences)

One of the stages in the production of chloroprene rubber—nairite—is the process of alkaline maturation of the latex, which consists in the latex, compounded with thiuram E (tetraethylthiuram disulfide), being held for a certain time at a temperature of about 30°C and pH 10, after which it is sent to coagulation.

Earlier studies [1] showed that the process of alkaline maturation of the latex ensures the production of a plastic rubber.

The mechanism of this process was studied by Mochel and Peterson [2] using labeled sulfur.

According to their ideas, in a neoprene molecule obtained with sulfur used as the polymerization regulator, the segments of the polychloroprene chains are joined by polysulfide bonds [3].

When thiuram E is introduced into the latex of a chloroprene–sulfur copolymer, according to the views of the same authors, the thiuram interacts with the polymer, causing the latter to break down at the sites of the polysulfide bonds.

In the production of nairite, thiuram E serves not only as a regulator of the plasticity of the rubber, but also plays the role of a stopper of the polymerization process and of a stabilizer during its storage and processing [4].

As a result, in the production of nairite it is necessary to add considerably more thiuram E than the amount required to obtain a plastic polymer [4].

It was of interest to examine the possibility of a complete or partial replacement of thiuram E in the production of nairite by other, cheaper and perhaps more active substances that exert a specific action on polychloroprene.

At the Department of Technology of Basic Organic Synthesis of the Tomsk Polytechnic Institute, the comparative reactivity of certain organic compounds containing sulfide sulfur is being studied as regulators of the properties of synthetic rubbers obtained by emulsion polymerization and as accelerators of vulcanization of rubber mixtures based on these rubbers.

The present study is a continuation of the indicated work and has the aim of establishing the relationship between the nature and

...the structure of organic sulfides and their activity as destructive agents for polysulfide bonds in nairite.

Experimental Part

For the tests, organic sulfides were taken which could presumably possess, to one degree or another, activity with respect to the destruction of polysulfide bonds.

As such compounds, substances widely used in the rubber industry as vulcanization accelerators were tested: thiurams, thiazoles, sulfenamides, salts of dithiocarbamic and xanthogenic acids, as well as substances used as polymerization regulators in the emulsion method of producing synthetic rubbers—bisethylxanthogen disulfide and diisopropylxanthogen disulfide.

These compounds were obtained from various rubber-industry plants, or were synthesized at NIISHP and in the laboratory of the Tomsk Polytechnic Institute; moreover, some of these substances, before testing, were preliminarily purified by recrystallization from various solvents.

In carrying out the work, latex $\alpha$—nairite L was used, which is an intermediate product in the production of nairite. The latex was stabilized only with neonoz D.

The substances under study were introduced into the latex in equimolecular amounts of 3% (calculated on the weight of dry substance in the latex) of thiuram E.

The weighed portions of organic sulfides were introduced into the latex in the form of a solution in 15 ml of water (in the case of water-soluble substances) or in the form of an emulsion of a benzene solution (in the case of substances soluble in hydrocarbons).

In those cases where the substances dissolved neither in water nor in hydrocarbons, they were introduced into the latex in the form of an aqueous dispersion stabilized with rosin soap.

The substances under study were uniformly distributed throughout the latex mass by intensive stirring of the latter with a mechanical stirrer. To carry out the process of alkaline maturation, the latex samples were placed in a water thermostat at a temperature of $30 \pm 2^\circ$C for 20 hours.

After alkaline maturation, the latex was coagulated in the form of granules with aqueous solutions of NaCl and CaCl$_2$, and the resulting pulp was acidified with a 2% HCl solution. The washed rubber samples, wrung out on rolls, were dried in an air dryer at a temperature of 120–130°C for 1–1.5 hours. Final drying of the samples was carried out on hot laboratory rolls, which ruled out plasticization of the nairite.

The dried samples were plasticized on standard laboratory rolls at a temperature of 20–30°C for 10 minutes and with a gap between the rolls of 0.5 mm.

As a criterion for evaluating the effectiveness of the destructive action on the polysulfide bonds of polychloroprene of the additives being tested, the plasticity index was adopted, determined according to Karff on a Williams plastometer.

Moreover, the more active the compound tested was, the higher the nairite plasticity index caused by its addition to the latex.

When doubtful results were obtained, the experiments were repeated.

With the substances that proved to be most effective, additional experiments were carried out with varied dosages. It was established that potassium xanthogenates are very active agents for de-

Table

Effect of additions of various organic sulfides on the plasticity of polychloroprene obtained using sulfur as the polymerization regulator

Continuation of the table

Continuation of the table

structures of the polysulfide bonds during the alkaline aging of nairite. The activity of xanthogenates increases with an increase in the number of carbon atoms in the hydrocarbon radical.

The methyl derivatives of xanthogenates and dithiocarbamates differ from the ethyl derivatives by their sharply reduced activity as additives that impart plasticity to nairite.

Among the dithiocarbamates, the most active are diethyldithiocarbamate, piperidinedithiocarbamate, and carbazoledithiocarbamate.

The presence in the radicals attached to the nitrogen atom of oxygen, as well as asymmetry and an increase in the molecular weight of the radical, reduce the activity of dithiocarbamates.

All the thiurams tested had activity of the same order. An increase in the number of sulfur atoms in the thiuram molecule has no noticeable effect on its activity.

The sulfenamides investigated, as well as the thiazoles, proved to be effective plasticizing agents during the alkaline aging of α-nairite L.

The experimental data are given in the table.

Conclusions

1. Work has been carried out to study the activity of certain classes of organic sulfides as agents for the destruction of polysulfide bonds during the alkaline aging of polychloroprene latex obtained using sulfur as a polymerization regulator.

2. It has been shown that xanthogenates with more than two carbon atoms in the alcohol group; diethyldithiocarbamate, piperidinedithiocarbamate, and carbazoledithiocarbamate; thiurams; altax; captax; sulfenamides; and product No. 284 are active agents for the destruction of polysulfide bonds in α-nairite L, causing strong plasticization of the polymer during alkaline aging of the latex.

References

1. S. I. Zurabyan, N. G. Karapetyan, A. N. Lyubimova. Izv. AN Arm. SSR, XI, 12, 3, 159, 1959.

2. W. E. Mochel, J. H. Peterson, J. Am. Chem. Soc., 71, 1426, 1949.

3. S. I. Zurabyan, N. G. Karapetyan, A. N. Lyubimova. Izv. AN Arm. SSR, XI, 12, 3, 15, 1959.

4. S. I. Zurabyan, N. G. Karapetyan, A. N. Lyubimova. Ibid., 12, 4, 241, 1958.