EFFECT OF MERCAPTANS ON THE PLASTICITY OF SKMS-30RP RUBBER

V. D. BOGOSLOVSKII, A. S. MANUSHKINA

(Presented by the scientific-methodological seminar of the organic departments of the KhTF)

Aliphatic mercaptans and dialkyl xanthogen disulfides are the most widely used plasticity regulators for copolymer butadiene rubbers obtained by emulsion polymerization. The regulating activity of mercaptans and xanthogen disulfides was determined in the emulsion copolymerization of butadiene with styrene [1, 2, 3].

In domestic butadiene-α-methylstyrene rubbers of the SKMS grade, plasticity is regulated both by xanthogen disulfides and by mercaptans. In the copolymerization of butadiene with styrene or α-methylstyrene, the degree of conversion of monomers into rubber is limited by the quality of the copolymer obtained and usually does not exceed 55–65%. The necessity of isolating and returning unreacted monomers to production considerably complicates and increases the cost of the technology for obtaining rubber and its manufacture.

The aim of this work was, using the example of the copolymerization of divinyl with α-methylstyrene according to the SKMS-30 RP formulation, to investigate the comparative reactivity of certain mercaptans and their combinations as rubber plasticity regulators, depending on the size of the hydrocarbon radical; to test the possibility of increasing monomer conversion without deterioration of the physicomechanical properties of the rubber by rational selection of polymer plasticity regulators or their combinations.

The polymerization of divinyl with α-methylstyrene in emulsion was carried out in bottles in a water thermostat at 50°C according to the formulation adopted for obtaining rubber of the SKMS-30 RP grade. The monomer ratio was 68 : 32 by weight. The emulsifier was sodium soap of synthetic fatty acids and leukanol for stabilization of the latex. The initiator was potassium persulfate. The regulator dosage was 0.2–0.3 parts by weight per 100 parts by weight of monomers. The ratio of the aqueous phase to the hydrocarbon phase was 1 : 1. The excess alkalinity of the emulsion was no more than 0.03 parts by weight of caustic soda per 100 parts by weight of monomers. The polymerization charge was prepared from technical components used in production. The regulator was dissolved in α-methylstyrene and introduced into the polymerization charge all at once at zero monomer conversion (before the start of polymerization).

Polymerization at 50°C was carried out for 15–18 hours. The depth of conversion of monomers into polymer was monitored by the dry residue.

The latex was stabilized with neozone D (phenyl-β-naphthylamine) and degassed (the unreacted divinyl was removed). α-Methylstyrene, which had not entered into polymerization, was separated by decantation.

The distilled latex was coagulated. The coagulum was washed. The resulting rubber was dried. The dried rubber was tested for physicomechanical characteristics by the standard methods adopted at SK plants.

In the experimental polymerizations, the regulator dosages were taken in equimolecular amounts relative to the amount of regulator in the standard formulation. The formulation for producing SKMS-30 RP rubber with the regulator diproxide (diisopropyl xanthogen disulfide) at a dosage of 0.3 parts by weight per 100 parts by weight of monomers was taken as the standard.

The activity of mercaptans or their combinations as regulators of plasticity, and consequently of the average molecular weight and properties of the rubber, was determined by comparing the physicomechanical characteristics of the experimental rubbers and their vulcanizates with the physicomechanical characteristics of the copolymer obtained in the presence of 0.3 parts by weight of tertiary dodecyl mercaptan per 100 parts by weight of the monomer mixture.

The imported tertiary dodecyl mercaptan tested in the experimental polymerizations had a specific gravity of 0.8604, \(n = 1.4650\), and a sulfur content of 15.2% (theoretical—15.85%) [1]. The boiling points of the other mercaptans tested were as follows:
n-hexyl mercaptan, 174–176°C;
n-octyl mercaptan, 198–200°C, 98–100°C (22 mm);
n-nonyl mercaptan, 220–222°C, 98–100°C (15 mm);
n-decyl mercaptan, 88–90°C (2 mm), 96–97°C (5 mm).

SKMS-30 RP rubber must meet the following standards for physicomechanical properties:

Defo hardness for rubber of group I: 225–375;
” group II: 376–526;
” group III: 526–700.

Tensile strength, kg/cm², for rubber of group I: not less than 210;
” group II: not less than 220;
Relative elongation, %: not less than 500;
Permanent elongation, %: not more than 28;
Shore elasticity, %: not less than 32.

In addition, in SKMS-30 RP rubber the following are standardized: content of divalent iron salts, content of the antioxidant neozone D, ash content, moisture content, and also the content of free and bound fatty acids. With respect to these chemical indices, the rubbers from the experimental polymerizations did not differ from the standard commercial rubber.

The physicomechanical characteristics of the rubbers and vulcanizates based on them, obtained as a result of the experimental polymerizations, were compared with the standards for the same indices for commercial rubber, and the effectiveness of the regulators studied was judged from the results.

It was established that the mercaptans tested exert a strong regulating effect on the plasticity and properties of divinyl-α-methylstyrene rubber obtained by emulsion polymerization.

The polymerization process in the charge with n-decyl mercaptan and tertiary dodecyl mercaptan proceeds faster than when a mercaptan with a smaller number of carbon atoms is used as the regulator (Table 1).

To achieve the required degree of conversion of monomers into polymer in the system regulated by mercaptans, less polymerization time is required in comparison with the system regulated by diproxide. At the same time, the properties of the resulting rubbers are approximately the same. In pri-

in the presence of tert-dodecyl mercaptan as a conversion regulator, the conversion of monomers reaches 70% after 15 hours of polymerization. The Defo hardness of the rubber is 680/38. When diproxide (diisopropylxanthogen disulfide) is used as the polymerization regulator, however, the conversion of monomers after 18 hours of polymerization reaches only 59%, while the Defo hardness of the rubber increases to 1100/39.

Table 1

Rate of experimental polymerizations in the presence of various regulators

The regulating activity of mercaptans increases with an increase in the number of carbon atoms in the hydrocarbon part of the molecule (Table 2).

With an increase in the mercaptan dosage in the polymerization charge, an increase in the polymerization rate (conversion of monomers) and an increase in the plasticity of the rubber are observed, i.e., a decrease in Defo hardness (Table 3).

Table 3

Dependence of monomer conversion
and hardness of the copolymer of divinyl
with α-methylstyrene on the dosage of the regulator—tert-dodecyl
mercaptan in the polymerization charge.
Polymerization time: 18 hours

Standard formulation—diproxide regulator.
Polymerization: 18 hours

In combination with one another, mercaptans are more effective regulators of the plasticity and properties of divinyl-α-methylstyrene rubber

Table 2

Physicomechanical characteristics of rubbers obtained from experimental polydivinyl with α-methylstyrene in the presence of various regulators, and of vulcanizates based on them. Copolymerization time 18 h. Vulcanization 100 min at 143°C

(Table 2). The dosages of various mercaptans in combinations require further refinement.

By using effective combinations of regulators—mercaptans—in formulations for the emulsion copolymerization of divinyl with α-methylstyrene, it is possible to obtain rubber with high physicomechanical properties at an increased conversion of monomers into polymer.

The experimental polymerizations carried out and the determinations of the physicomechanical characteristics of the polymers show the possibility of using mercaptans as regulators in producing SKMS-30RP rubber. In this case, the aqueous phase of the polymerization charge must not contain more than 0.03 parts by weight of free alkali per 100 parts by weight of monomers, in order to avoid conversion of the active regulator into the less active mercaptide.

As the number of carbon atoms in the hydrocarbon part of the regulator—mercaptan molecule—increases, the plasticity of the polymer increases. The Defo hardness decreases. Among the regulators studied, tertiary dodecyl mercaptan is the most active. A further increase in the effectiveness of regulation is possible through the use of combinations of mercaptans.

REFERENCES

1. M. Morton, R. V. V. Nicholls. Candian Journal of Research. 25, 2, 158—182, 1947.
2. S. M. Kolthoff, W. K. Harris. J. of Polymer Science. 2, 1, 49—71, 1947.
3. V. Vaclavek. Chemicky Prumysl. 10, 6, 327—331, 1960.