Preparation of an Emulsion Copolymer of Chloroprene with Isoprene and Study of the Features of Its Vulcanization
Abstract Generated abstract
This study examines the emulsion copolymerization of chloroprene with isoprene using organic sulfides as plasticity regulators and evaluates the vulcanization behavior of the resulting rubbers with various accelerators and additives. Copolymers were prepared at a 20:1 chloroprene to isoprene ratio, isolated by granular coagulation, compounded with metal oxides and selected sulfur-containing additives, and tested for plasticity, tensile properties, elongation, hardness, swelling, elasticity, aging resistance, and optimum vulcanization time. A combination of lower molecular weight mercaptans gave the strongest plasticizing effect, although excessive regulation was associated with lower strength and undervulcanization under standard conditions. Altax was identified as an effective vulcanization accelerator, while several other additives either retarded curing or reduced strength, and the copolymer rubber was found to vulcanize somewhat more slowly than commercial nairite while showing broadly comparable or improved physicomechanical properties.
Full Text
PREPARATION OF AN EMULSION COPOLYMER OF CHLOROPRENE WITH ISOPRENE AND INVESTIGATION OF THE FEATURES OF ITS VULCANIZATION
V. D. Bogoslovskii, N. F. Antof’eva
(Presented by the Scientific-Methodological Seminar of the Organic Departments)
Chloroprene rubber imparts to vulcanizates a number of valuable service properties that make it indispensable for the manufacture of many rubber-technical products. New grades of chloroprene rubber are produced on the basis of copolymers of chloroprene with acrylonitrile, isoprene, and styrene [1].
The objectives of the work were: to obtain a product of the joint polymerization of chloroprene with isoprene in an emulsion in the presence of various organic sulfides as regulators of rubber plasticity; to vulcanize experimental rubber samples in the presence of various vulcanization accelerators; to determine the physicomechanical properties of the vulcanizates; to draw a conclusion about the relative reactivity of the organic sulfides studied as regulators of rubber plasticity in the emulsion copolymerization of chloroprene with isoprene and of vulcanization accelerators for copolymeric chloroprene rubber; and to identify and note the differences in the vulcanization and properties of vulcanizates of the experimental chloroprene rubber and the commercial nairit-chloroprene rubber regulated with sulfur.
Polymerization experiments were carried out at a monomer ratio of 20:1 by weight in an emulsion charge according to the nairit recipe. Sulfur was replaced by the corresponding organic sulfide or by their combination. The course of polymerization was monitored by the increase in the specific gravity of the latex. Polymerization was interrupted with an aqueous solution of sodium diethyldithiocarbamate. To stabilize the polymer after completion of polymerization, an aqueous emulsion of solutions of Neozone D (phenyl-β-naphthylamine) and Thiuram D (tetramethylthiuram disulfide) in benzene, or an emulsion of a benzene solution of P-23 (ditertiary butylphenol), was introduced into the latex. The polymer was isolated by the method of granular coagulation. After drying and milling for 5 minutes, the plasticity of the rubber was determined by the standard method [4].
Three experiments were carried out to obtain a copolymer of chloroprene with isoprene in the presence of one rubber-plasticity regulator—tertiary dodecyl mercaptan—and combinations of regulators: n-heptyl mercaptan with di-n-heptyl xanthogen disulfide and n-nonyl mercaptan with n-hexyl mercaptan.
The most active of the plasticity regulators tested proved to be the combination of mercaptans. The plasticity of the rubber obtained in the presence of this combination was the highest—0.69.
| Copolymer no. | Regulator and its dosage, % of monomer weight | Rubber plasticity by Karрer |
|---|---|---|
| 1. | combination: n-heptyl mercaptan — 0.26 and di-n-heptylxanthogen disulfide — 0.38 |
0.27 |
| 2. | tertiary dodecyl mercaptan — 0.80 | 0.57 |
| 3. | combination: n-nonyl mercaptan — 0.32 and n-hexyl mercaptan — 0.23 |
0.69 |
The dosages of the regulator combinations were equimolecular to a 0.8% dosage (calculated on the monomers) of tertiary dodecyl mercaptan.
The effect of organic sulfides on the vulcanization process of a chloroprene copolymer with isoprene was studied by comparing the physicomechanical properties of vulcanizates of experimental rubber compounds containing an additive of a substance affecting vulcanization with the same properties of a standard compound containing only magnesium oxide and zinc oxide, vulcanized at the same temperature of 143°C and for the same time. A similar procedure has also been used by other investigators.
To determine more clearly the specific features of the action of the additives of the organic sulfides studied on the vulcanization of the chloroprene copolymer with isoprene, the substances under study were introduced in dosages from 2.0 to 5.0% based on the rubber. In the work of other authors, substances affecting the vulcanization of chloroprene rubber were tested in dosages of 0.10—2.0% based on the rubber [2, 3].
The following recipe for a standard compound based on chloroprene rubber was taken as the basis:
| Component | Amount |
|---|---|
| Rubber | —100 parts by weight |
| Zinc whites | — 5 parts by weight |
| Magnesium oxide | — 7 parts by weight |
| Total | —112 parts by weight |
Organic sulfides, their salts, and other additives distinguishing the experimental rubber compounds from the standard one were introduced into the rubber simultaneously with the magnesium oxide.
The mixing schedule for the standard and experimental compounds on laboratory rolls at a roll temperature of 25—30°C was as follows:
| Operation | Time |
|---|---|
| Milling of the copolymer | — 7 min |
| Introduction of magnesium oxide and (additives) | — 4 min |
| Introduction of zinc oxide | — 4 min |
| Total | —15 min |
The dosages of the additives to the standard copolymer compound are given in Table 1.
The results of physicomechanical tests of the experimental rubber compounds and their vulcanizates are given in Table 2.
The physicomechanical properties of the vulcanizate of the standard compound with commercial sodium hyposulfite (sulfur regulation) were as follows: tensile strength 228 kg/cm²; relative elongation 840%; Shore hardness 46; rebound elasticity by Schob 28—30.
Analysis of the results presented in the tables shows that the plasticity of the copolymeric chloroprene rubber is increased best of all by the combination of n-nonyl mercaptan with n-hexyl mercaptan and by tertiary dodecyl mercaptan. The use of the combination of n-heptyl mercaptan with di-n-heptylxanthogen disulfide is less effective.
Table 1
Organic sulfides, their salts, and other additives in a standard rubber compound based on chloroprene-rubber copolymer
| Copolymer No. | Compound code | Name of additive | Dosage, % based on rubber |
|---|---|---|---|
| 2 | H-1 | Altax (benzothiazolyl disulfide) | 2.0 |
| » | H-2 | [[unclear: sulfur additive]] | |
| » | H-3 | Product of oxidation of captax (mercaptobenzothiazole) and xanthogenate of higher alcohols | 5.0 |
| » | H-4 | Product of oxidation of captax (mercaptobenzothiazole) and xanthogenate of higher alcohols | 2.0 |
| 1 | H-5 | Dipiperidylthiuram disulfide | 2.0 |
| » | H-6 | Cimat (zinc salt of diethyldithiocarbamate) | 2.2 |
| » | H-7 | Vulcanite R (piperidine pentamethylenedithiocarbamate) | 3.0 |
| » | H-8 | ||
| 3 | H-9 | ||
| » | H-10 | Vulcanite Mor (benzothiazolylsulfenmorpholide) | 3.0 |
| » | H-11 | Salt of captax with morpholine | 3.0 |
| » | H-12 | Product of oxidation of captax and morpholinedithiocarbamate | 2.2 |
| » | H-13 | Morpholine tetrasulfide | 2.0 |
Table 2
Physico-mechanical properties of rubber compounds and vulcanizates based on the copolymer of chloroprene with isoprene. Vulcanization at 143°C
| Property / compound code | H-1 | H-2 | H-3 | H-4 | H-5 | H-6 | H-7 | H-8 | H-9 | H-10 | H-11 | H-12 | H-13 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plasticity according to Carrer | 0,41 | 0,26 | 0,29 | 0,32 | 0,26 | 0,10 | 0,13 | 0,13 | 0,59 | 0,58 | 0,60 | 0,57 | 0,59 |
| Optimum vulcanization, min | 30—90 | 45—90 | 60—90 | 60—90 | 30—45 | 30—90 | 30—90 | 60 | 90 | 150 | 90 | 120—150 | 15—45 |
| Swelling in benzene for 24 h at 20°C, % | 490 | 520 | 390 | 470 | 400 | 360 | 380 | 400 | 600 | 320 | 720 | 700 | 690 |
| Tensile strength, kg/cm² | 172—196 | 191—200 | 160—183 | 172—200 | 131—144 | 73—77 | 84 | 174 | 145 | 69 | 72 | 148—152 | 137—154 |
| Relative elongation, % | 1150—940 | 1130—1010 | 1010—870 | 930—710 | 780—730 | 420—340 | 420—310 | 930 | 1250 | 1290 | 1210 | 1130—1100 | 1220—960 |
| Residual elongation, % | 22—16 | 20—16 | 24—20 | 20—16 | 16 | 12—8 | 12—8 | 16 | 24 | 28 | 28 | 20 | 20 |
| Shore hardness | 37—44 | 40—41 | 40—48 | 42—44 | 43—46 | 40—42 | 38—40 | 42 | 36 | 28 | 22 | 36 | 30—35 |
| Elasticity according to Shob | 28—30 | 26—28 | 28—30 | 25—28 | 28 | 24—26 | 30 | 36 | 30 | 26 | 30 | 26 | 24 |
| Coefficient of thermal aging resistance \(144\) h at \(70^\circ\mathrm{C}\), by strength | 1,20 | 1,29 | 0,82 | 0,83 | 0,49 | 1,26 | 1,08 | 0,91 | 0,68 | 1,15 | 0,68 | 0,71 | 1,06 |
The low plasticity of compounds based on rubber regulated with a combination of n-dodecyl mercaptan and di-n-heptyl xanthogen disulfide indicates insufficient stability of the rubber stabilized with P-23, and also a tendency of the compounds toward premature vulcanization and, consequently, toward a decrease in the physico-mechanical indices of the vulcanizates. The high plasticity of compounds based on the copolymer rubber regulated with a combination of mercaptans, together with high relative elongations, reduced strength, increased residual elongations, and lower hardness of the vulcanizates, characterize the rubber as overregulated (containing an increased proportion of low-molecular fractions) and undervulcanized under ordinary vulcanization conditions.
A combination of comparatively low-molecular mercaptans is a stronger plasticity regulator for chloroprene rubber of emulsion polymerization than a combination of mercaptan with xanthogen disulfide or tertiary dodecyl mercaptan.
Among the additives that affect the vulcanization of the copolymer of chloroprene with isoprene, Altax is the most effective; it accelerates rubber vulcanization. The oxidation product of Captax and xanthogenate of higher alcohols retards vulcanization. Sulfur pentasulfide reduces swelling and increases the elasticity of the vulcanizates. Vulcacit Mor and the salt of Captax with morpholine, unlike morpholine tetrasulfide and the oxidation product of Captax and morpholine dithiocarbamate, do not increase the strength of the vulcanizates and therefore are not effective in a blend with chloroprene rubber.
Vulcacit P imparts increased strength to the vulcanizate in comparison with the standard compound, but gives a very narrow vulcanization plateau. Dipiperidinium disulfide and cimate reduce the strength indices of chloroprene-rubber vulcanizates while favorably affecting their resistance to swelling. These accelerators provide a good degree of vulcanization and low residual elongation of the vulcanizates.
The copolymer of chloroprene with isoprene, while only slightly inferior to commercial nairite in strength, surpasses it in all other physico-mechanical properties. The swelling of vulcanizates of both types of chloroprene rubber is approximately the same.
The copolymer chloroprene rubber vulcanizes somewhat more slowly than nairite.
Compounds containing sulfur and decomposing into radicals at elevated temperature accelerate the vulcanization of the rubber—a copolymer of chloroprene with isoprene.
REFERENCES
- N. L. Ketton. Neopreny [Neoprenes], Leningrad, Goskhimizdat, 1958.
- A. M. Neal. Rubber Age N. I. 67, 5, 569—572, 1950.
- N. D. Zakharov, A. V. Makarov. “Kauchuk i rezina,” 1960, No. 6, pp. 23—26.
- Rezina. Metody ispytanii [Rubber. Testing Methods], Moscow, Standartgiz, 1960.