Features of SKI-3 Vulcanization
Abstract Generated abstract
This study examines the vulcanization behavior of domestic cis-polyisoprene synthetic rubber SKI-3 as a substitute for natural rubber in rubber-technical products. Experimental and production formulations were compared by varying sulfur content, accelerator type, and accelerator combinations, followed by physicomechanical testing and heat-aging evaluation of vulcanizates. The results indicate that SKI-3 vulcanizes more slowly than natural rubber and is more sensitive to the accelerator-to-sulfur ratio, with improved properties obtained at higher accelerator levels and relatively low sulfur content. Effective systems included combinations involving Altax with diphenylguanidine, thiuram, or sulfenamide accelerators, and adjusted SKI-3 formulations for carbon-black-filled products met specifications and in some strength properties exceeded natural-rubber analogues.
Full Text
Features of Vulcanization of SKI-3
V. D. Bogoslovskii, V. P. Bakhtina
(Presented by the Scientific-Methodological Seminar of the Organic Departments)
Domestic cis-polyisoprene synthetic rubber SKI-3—an analogue of natural rubber (NR) not only in composition but also in macromolecular structure. At the same time, SKI-3, unlike NR, contains no high-molecular-weight fractions, does not require plasticization before mixing, and contains no natural antioxidants. To stabilize its properties during storage and processing, SKI-3 is charged at the manufacturing plant with a mixture of 0.5 parts by weight of Neozone D (phenyl-β-naphthylamine) and 0.5 phr DPPD (diphenyl-p-phenylenediamine) per 100 phr polymer. Its plasticity is 0.55 by Karрer. SKI-3 has found fairly broad application as a substitute for NR, mainly in rubbers for tire production [1].
The aim of the present work was to study the features of vulcanization of SKI-3 in order to select an accelerator–vulcanizing group for rubber compounds of rubber-technical products based on it.
A standard rubber compound based on SKI-3 was taken as the basis. The experimental rubber compounds differed from the standard one only in the dosage of accelerators and sulfur and in the vulcanization temperature of \(143 \pm 1^\circ\), which is accepted for RTI rubbers. The standard compound based on SKI-3 was vulcanized at the temperature recommended for it, \(133 \pm 1^\circ\mathrm{C}\). For comparison of the properties of rubbers based on natural rubber and SKI-3, compounds based on NR—both standard and experimental—were prepared and vulcanized. Vulcanization was carried out in a press.
The composition of the accelerator–vulcanizing groups of standard and experimental rubbers based on NR and SKI-3 is given in Table 1. Some physicomechanical characteristics of the vulcanizates are given in Table 3.
Three of the best accelerator–vulcanizing groups of experimental rubbers, providing high physicomechanical characteristics of the vulcanizates, were used in formulations of production rubbers with replacement of natural rubber by SKI-3.
The production rubbers had to meet the following requirements:
| Requirement | 3311 (B-16) | 2959 (B-20) | 1847 (B-24) |
|---|---|---|---|
| Tensile strength, kg/cm² | 150 | 160 | 160 |
| Relative elongation, % | 700 | 500 | 600 |
| Permanent elongation, % | 25 | 32 | 32 |
| Shore hardness | 30–45 | 35–50 | 35–50 |
| Vulcanization time at 143°C, min | 10 | 20 | 20 |
The last two rubbers are carbon-black-filled.
The results of physicomechanical tests of vulcanizates of production RTI rubbers based on NR and SKI-3 (with modified accelerator and sulfur dosages) are given in Table 4.
The composition of the accelerator–vulcanizing groups of rubbers for RTI is given in Table 2.
Physicomechanical tests of vulcanizates of experimental rubbers based on SKI-3 show that the best results are given by compounds prepared with vulcanization accelerators: a combination of diphenylguanidine (DFG) and benzothiazole disulfide (Altax), sulfenamide BT (benzothiazolesulfene diethylamide), Vulcaphor HBS (benzothiazolesulfene cyclohexylamide), and a combination of sulfenamide BT and Altax. The vulcanizates are characterized by stable values of elongation and hardness, high elasticity, and resistance to heat aging. The presence of accelerators—the combination of DFG with Altax, thiuram (tetramethylthiuram disulfide) in the absence of free sulfur, the combination of thiuram with Altax, and the combination of Altax with sulfenamides—imparts to rubber compounds based on SKI-3 the ability to resist heat aging.
Carbon-black-filled rubbers prepared according to production formulations, but based on SKI-3 with adjustment of the accelerator–vulcanizing part of the formulation, meet the technical specifications and, in strength characteristics, surpass rubbers based on NR. Compounds based on SKI-3 prepared according to the carbon-black-free rubber 3311 formulation require further refinement of the accelerator–vulcanizing group, taking into account the presence in the rubber of the second antioxidant, flexon. For rubber 3311 based on SKI-3, at other vulcanization times the indices are worse in comparison with the compound based on NR.
The results of vulcanization under production conditions of RTI parts made from rubber 1847 (experimental formulation V-26) make it possible to recommend a broader replacement of natural rubber by SKI-3. In this case, it is necessary to adjust the accelerator–vulcanizing group of the formulation, taking into account the effect on vulcanization of other components of the rubber compound, such as antioxidants and fillers.
From a comparison of the physicomechanical properties of rubbers based on NR and on SKI-3, it follows that SKI-3 vulcanizes more slowly than natural rubber. The absence of natural vulcanization accelerators—the rubber resins—has an effect.
During vulcanization, SKI-3 rubber is more sensitive to the ratio between accelerator and sulfur. The best physicomechanical properties of vulcanizates of rubbers based on SKI-3 are observed at an increased content of vulcanization accelerator and a relatively low (in comparison with rubbers based on NR) sulfur content. Significant deviations in sulfur content from the optimum lead to a decrease in the physicomechanical properties of the rubbers.
In contrast to NR, in rubber compounds based on SKI-3 the use of thiuram as a vulcanizing agent is less effective. As a vulcanizing agent, thiuram increases the heat resistance of vulcanizates; the other physicomechanical properties, however, are inferior to those of compounds with the optimum sulfur dosage and a combination of accelerators.
Compounds based on SKI-3 are better vulcanized by combinations of accelerators or by accelerators of combined action of the sulfenamide type. Of those tested, active combinations are Altax with DFG, Altax with thiuram, and Altax with sulfenamide.
Table 1
Composition of accelerant groups for SKI-3 and NK
In parts by weight per 100 parts by weight of rubber
| Ingredients / Blend code | B-1* | B-2 | B-3 | B-4** | B-5 | B-6 | B-7 | B-8 | B-9 | B-10 | B-11 | B-12 | B-13 | B-14 | B-15 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| NK | — | — | 100 | 100 | — | — | — | — | — | — | — | — | — | — | — |
| SKI-3 | 100 | 100 | — | — | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| Sulfur | 1.0 | 1.0 | 1.0 | 3.0 | 3.0 | 0.6 | 1.0 | 1.0 | 1.0 | — | 0.6 | 1.0 | 1.0 | 1.0 | 1.0 |
| DOTG | 3.0 | — | 3.0 | — | — | 2.0 | — | — | 2.5 | — | — | — | — | — | — |
| Altax | 0.6 | 3.0 | 0.6 | — | — | 0.4 | — | 2.0 | 0.5 | — | — | — | 2.5 | — | 2.0 |
| Thiuram | — | — | — | — | — | — | 1.0 | 0.4 | — | 4.0 | 1.0 | — | 0.3 | — | — |
| Captax | — | — | — | 0.7 | 0.7 | — | — | — | — | — | — | — | — | — | — |
| Sulfenamide BT | — | — | — | — | — | — | — | — | — | — | — | 3.0 | — | — | 2.0 |
| Vulkanox HBS | — | — | — | — | — | — | — | — | — | — | — | — | — | 2.5 | — |
* — standard blend for SKI-3.
** — standard blend for NK.
Table 2
Composition of accelerant groups for RTI based on NK and SKI-3
In parts by weight per 100 parts by weight of rubber
| Ingredients / Blend code | B-16 | B-17 | B-18 | B-19 | B-20 | B-21 | B-22 | B-23 | B-24 | B-25 | B-26 | B-27 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| NK | 100 | — | — | — | 100 | — | — | — | 100 | — | — | — |
| SKI-3 | — | 100 | 100 | 100 | — | 100 | 100 | 100 | — | 100 | 100 | 100 |
| Sulfur | 3.0 | 1.0 | 1.0 | — | 3.0 | 1.0 | 1.0 | 1.0 | 3.0 | 1.0 | 1.0 | 1.0 |
| DPG | — | — | 2.5 | — | — | — | 2.5 | — | — | — | 2.5 | — |
| Altax | — | — | 0.5 | — | — | — | 0.5 | — | — | — | 0.5 | — |
| Thiuram | 2.3 | — | — | 4.0 | — | — | — | — | 1.0 | — | — | 1.0 |
| Captax | — | — | — | — | 0.8 | — | — | 0.6 | — | — | — | — |
| Sulfenamide BT | — | 3.0 | — | — | — | 3.0 | — | — | — | 3.0 | — | — |
Table 3
Physico-mechanical characteristics of experimental compounds
Vulcanization time: 30 min
| Indicator / compound code | B-1 | B-2 | B-3 | B-4 | B-5 | B-6 | B-7 | B-8 | B-9 | B-10 | B-11 | B-12 | B-13 | B-14 | B-15 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Modulus at 600% elongation, kg/cm² | 58 | 53 | 96 | 56 | 16 | 50 | 46 | 123 | 46 | 60 | 28 | 127 | 87 | 118 | 136 |
| Tensile strength, kg/cm² | 260 | 248 | 262 | 241 | 100 | 230 | 171 | 202 | 203 | 179 | 170 | 264 | 210 | 267 | 267 |
| Relative elongation, % | 830 | 780 | 800 | 780 | 900 | 780 | 780 | 640 | 780 | 740 | 850 | 720 | 700 | 700 | 740 |
| Permanent elongation, % | 6 | 8 | 12 | 14 | 12 | 8 | 4 | 2 | 6 | 4 | 4 | 4 | 4 | 4 | 6 |
| Shore hardness | 35 | 32 | 36 | 32 | 25 | 32 | 34 | 35 | 34 | 34 | 30 | 38 | 34 | 34 | 36 |
| Schob elasticity | 62 | 60 | 62 | 60 | 52 | 62 | 60 | 62 | 60 | 58 | 54 | 62 | 60 | 62 | 60 |
| Aging coefficient, 96 h at 70°C, by tensile strength | 1.05 | 0.85 | 1.16 | 1.12 | 1.30 | 0.98 | 1.03 | 1.12 | 1.05 | 1.18 | 1.09 | 0.94 | 0.84 | 0.95 | 1.24 |
| Aging coefficient, 96 h at 70°C, by relative elongation | 0.97 | 0.92 | 1.04 | 0.98 | 0.88 | 0.95 | 0.96 | 0.95 | 0.94 | 1.02 | 0.98 | 0.96 | 1.05 | 0.88 | 1.15 |
Table 4
Physico-mechanical properties of vulcanizates of rubber-goods and tire compounds based on SKI-3.
Vulcanization time: 30 min
| Property / compound code | B-16 | B-17 | B-18 | B-19 | B-20 | B-21 | B-22 | B-23 | B-24 | B-25 | B-26 | B-27 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Modulus at 600% elongation, kg/cm² | 112 | 63 | 35 | 31 | 125* | 128* | 86* | 105* | 128* | 117* | 64* | 67* |
| Tensile strength, kg/cm² | 224 | 219 | 146 | 169 | 176 | 210 | 215 | 176 | 192 | 264 | 224 | 169 |
| Relative elongation, % | 740 | 720 | 800 | 840 | 530 | 580 | 650 | 550 | 760 | 680 | 770 | 680 |
| Permanent elongation, % | 12 | 10 | 4 | 4 | 16 | 16 | 20 | 12 | 16 | 6 | 16 | 6 |
| Shore hardness | 36 | 36 | 32 | 32 | 38 | 38 | 36 | 40 | 42 | 42 | 38 | 42 |
| Elasticity by Schob | 58 | 56 | 54 | 54 | 56 | 58 | 54 | 56 | 44 | 54 | 52 | 56 |
| Aging coefficient, 96 h at 70°C — by strength | 0.94 | 0.95 | 0.88 | 0.94 | 0.96 | 0.98 | 0.96 | 0.92 | 0.90 | 0.92 | 0.94 | 0.98 |
| Aging coefficient, 96 h at 70°C — by relative elongation | 0.88 | 0.80 | 0.90 | 0.90 | 0.94 | 0.93 | 0.94 | 0.90 | 0.86 | 0.87 | 0.90 | 0.94 |
* Modulus at 500% elongation, kg/cm².
The activity of a vulcanization accelerator depends on the structure of its molecule [2]. The presence in the accelerator molecule of a benzothiazole sulfide radical makes it active with respect to SKI-3. The presence of amine, imine, and dithiocarbamine groups in the ingredients that make up the rubber compound activates the main accelerator.
REFERENCES
-
N. D. Zakharov. New Types of Rubbers and Areas of Their Practical Use. Yaroslavl, TsBTI, 1962.
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G. A. Blokh. Organic Accelerators in Rubber Vulcanization. Moscow–Leningrad, “Khimiya,” 1964.