A. P. Kuriakose

Studies on a New Binary Accelerator System for the Sulfur Vulcanization of Styrene—Butadiene Rubber

Abstract The chemical reactions involved when a binary accelerator system is used in rubber vulcanization varies with the systems used. The exact mechanism of these reactions is not fully understood even now. Accelerators in which sulfur is combined as S—S, C—S—C, or S—N are generally inactive at low vulcanization temperatures because of the high thermal stability of their sulfur bonds. But sulfur-containing nucleophiles such as thiourea or its derivatives enable accelerators like TMTD or CBS to operate at lower vulcanization temperature, indicating a nucleophilic reaction mechanism in these vulcanization reactions. But, no conclusive evidence is given for such a postulate. In the present study, 1-phenyl-2,4-dithiobiuret and 1,5-diphenyl-2,4-dithiobiuret, which are more nucleophilic than thiourea but which vary in their nucleophilic reactivity, were tried as secondary accelerators in binary systems containing TMTD or CBS in sulfur vulcanization of SBR. Both the above dithiobiurets were found to reduce the...

Amidinothiourea as a secondary accelerator in a binary system for the sulphur vulcanization of natural rubber

A study of a binary accelerator system using amidinothiourea as a secondary accelerator with mercaptobenzothiazyl disulfíde (MBTS), in sulphur vulcanization of natural rubber has been undertaken. This system was found to boost MBTS accelerator activity considerably. From the results, a nucleophilic reaction mechanism was established for this system. Different compositions of the amidinothiourea were tried in different mixes using standard recipes, and reference mixes were also vulcanized. Different cure characteristics of the mixes and important physical properties of the vulcanizates were evaluated. Optimum concentrations of amidinothiourea required were derived and the experimental mixes were found to have better cure characteristics, including an advantageous reduction in the optimum cure time. Many of the physical properties of the vulcanizates studied were also found to be better compared to those of the reference mixes. Evaluation of these properties both before and after ageing showed good retention of these properties as well. An attempt was also made to correlate the variation in physical properties to chemical crosslink formation in the various vulcanizates.

Effect of fillers in the binary systems containing TMTD–amidinothiourea and MBTS–amidinothiourea in NR vulcanization

In earlier studies,1,2 we have shown that in MBTS–amidinothiourea and TMTD–amidinothiourea binary system of rubber vulcanization, amidinothiourea functions effectively as a secondary accelerator and improves the accelerator activity of these systems. These secondary accelerators enhance the physical properties of the vulcanizates and show better cure characteristics of the mixes compared to the reference systems studied. In this paper, we present the results of our study on the effect of various fillers in the sulfur vulcanization of natural rubber using amidinothiourea as the secondary accelerator. We have taken MBTS–amidinothiourea and TMTD–amidinothiourea binary accelerator systems and vulcanizates were prepared with carbon black, precipitated silica, and china clay as fillers. Different compositions of amidinothiourea were used in various mixes using standard recipes. Reference mixes were also studied. Curing characteristics of the mixes and various physical properties of the vulcanizates were evaluated. Optimum dosage of amidinothiourea required has also been derived. Among the different secondary accelerators tried including amidinothiourea, it may be stated generally that the more nucleophilic the secondary accelerator, the less the optimum cure times. This fact points to a nucleophilic reaction mechanism in the systems under review. In the evaluation of tensile and other physical properties of the vulcanizates, some of the systems containing amidinothiourea show better properties while others give comparable values with the reference mixes. Estimation of crosslink density also supports the above conclusion.

Use of rice-bran oil in the compounding of styrene butadiene rubber

The compounding of rubber involves mixing of activator, co-activator, accelerator, antioxidant, filler, processing aid, vulcanizing agent, etc., to produce the rubber. The replacement of three of the above ingredients by rice-bran oil in the compounding of styrene butadiene rubber has been investigated. The higher fatty acids and phenols present in rice-bran oil act as co-activator and antioxidant, respectively, in the vulcanization process. Addition of this oil together with fillers, also meets the requirement of processing aid. The results obtained were compared with those obtained with reference mixes containing stearic acid, styrenated phenol and aromatic/napthenic oil in carbon-black and silica-filled systems. Evaluation of the cure characteristics of the mixes indicates that substitution of the above ingredients with rice-bran oil did not affect the cure properties. Vulcanizates containing the rice-bran oil also showed physical properties comparable to the reference mixes. Processability studies using a plasticorder also showed similar trends. Considering the fact that rice-bran oil is cheaper than other conventional oils used as processing aids in rubber compounding, this non-toxic natural oil can be advantageously used in rubber product manufacture.

Studies on blends of melt-processable liquid crystalline polymers and thermoplastics. I. Blend of polyesteramide with polyethylene

The processability characteristics, physicomechanical properties, and thermal decomposition characteristics of blends of low-density polyethylene (LDPE) and polyesteramide (PEA), a thermotropic liquid crystalline polymer, were studied using various analytical techniques. Studies on a Brabender Plasticorder at temperatures ranging from 170 to 230°C showed good melt processability for the blends. The melt rheology of the blends containing 0–15% of PEA at 170°C was studied using a capillary rheometer (Goettfert) fitted with a circular die (L/D = 30/1) at apparent shear rates ranging from 12 to 2300 s−1. The samples containing PEA showed a comparatively lower die swell at high shear rates. X-ray diffraction measurements showed a reduction in crystallinity of LDPE in the presence of 2–4% of PEA. Scanning electron microscopic evaluation of the morphology of the fractured surface of the blend showed some degree of orientation, but not to the level typical of LCPs. However, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) indicated significant improvement in the resistance to thermooxidative decomposition of LDPE modified with PEA.

Effect of Substituted Dithiobiurets—Tetramethyl Thiuram Disulfide (TMTD) Binary Accelerator Systems in the Vulcanization of NR-SBR Blends

Abstract 1-Phenyl 2,4-dithiobiuret and 1,5-diphenyl 2,4-dithiobiuret were studied as secondary accelerators along with tetramethyl thiuram disulfide (TMTD) in the vulcanization of a blend of natural rubber (NR) and styrene-butadiene rubber (SBR). These binary systems were found to be very effective and reduced the optimum vulcanization time considerably. 1-Phenyl 2,4-dithiobiuret, which is more nucleophilic than 1,5-diphenyl 2,4-dithiobiuret, reduced the vulcanization time more, indicating a nucleophilic reaction mechanism in the vulcanization reactions under review. In both cases the optimum dosage of the secondary accelerator was derived. Physical properties such as modulus, tensile strength, elongation at break, hardness, compression set, heat buildup, resilience, etc., of the vulcanizates were studied before and after heat aging and compared with a reference mix. There is substantial increase in many of these properties compared with TMTD alone or with the reference mix. Chemical characterization of t...