Sulfur-cured gum natural rubber vulcanizates were devulcanized using two different concentrations of diallyl disulfide. The devulcanization process was performed at 110°C for 10 min in an open two-roll cracker-cummixing mill. Natural rubber vulcanizates having various sulfur/accelerator ratios were used to study the cleavage of monosulfide, disulfide, and polysulfide bonds. The properties of devulcanized natural rubber increased upon increasing the disulfide concentration and the mechanical properties of the revulcanized natural rubber increased upon decreasing the sulfur content in the original rubber vulcanizates. The scorch time and the maximum state of cure both increased when the ground vulcanizates were treated with higher amounts of disulfide. TGA and DMA were conducted to study the effects of the devulcanization on the thermal stability and the T g behavior of the vulcanizates. SEM analysis was conducted to study how the failure mechanism was affected by the devulcanization process. It was possible to recover 70-80% of the original gum rubber properties by using this process. From IR spectroscopic analysis, we observed that the oxidation of the main chains did not occur during high-temperature milling.
ABSTRACT:In this study, the prime factor determining the size, shape, and distribution of liquid-crystalline polymer (LCP) was the viscosity ratio at the processing conditions. The fiber-forming capacity of the LCP depended on the viscosity of the ethylene-propylene-diene monomer rubber (EPDM). With increasing LCP content, the tensile and tear strengths did not increase, perhaps because of incompatibility between the EPDM and LCP. The hardness increased because of the hard mesogenic groups in the LCP. The percentage swelling decreased as the LCP content increased. With increasing LCP content, processability became easier because of a lower melt viscosity. The scorch time increased at higher LCP levels. A higher percentage crystallinity was observed with increasing LCP content. Scanning electron microscopy clearly showed the fiber phase formation, which was two-dimensionally isotropic in nature, confirming fiber formation even in a shear field. The addition of LCP improved the thermal stability. The onset degradation temperatures shifted to higher values with increasing LCP content. Dynamic mechanical thermal analysis revealed that with the addition of LCP, the mechanical damping increased at its lower level. High-temperature processing increased the effective amorphous zone.
Using a suitable disulfide-based devulcanizing agent, which cleaved the sulfur cross-links in vulcanized rubber at high temperature, devulcanization of gum natural rubber was carried out. High sulfur and medium sulfur, as well as low sulfur-containing rubber vulcanizates were used to study the cleavage of sulfidic bonds. The cure characteristics and mechanical properties of vulcanized natural rubber and revulcanized natural rubber were studied. Thermal properties of the rubber were analyzed by thermogravimetric analysis (TGA), which indicates that the onset degradation temperature further increased on revulcanization with higher amount of disulfide. The properties of the revulcanized natural rubber increased with increasing disulfide concentration, also the mechanical properties of the devulcanized natural rubber were increased by decreasing the sulfur content in the original rubber vulcanizate. From the rheometric study increases in optimum cure time were observed when ground rubber vulcanizates were treated with higher amounts of disulfide. The scanning electronic microscopy (SEM) study suggested the change in failure mechanism as influenced by the type of cross-linking present and the devulcanizing agent used. From infrared (IR) spectroscopy it was observed that the oxidation of the main polymeric chain did not occur at the time of high temperature milling.
Devulcanization of waste rubber poses a challenging environmental, economical, and disposal problem in the world because of their cross-linked three-dimensional network structure. Devulcanization of gum natural rubber vulcanizate containing three different sulfur/accelerator ratios was carried out in 90°C for 10 min with the help of open two-roll cracker cum mixing mill in presence and absence of thiol acid as a devulcanizing agent. The vulcanizate properties markedly depended on devulcanizing agent and also devulcanization techniques. Revulcanized rubber obtained by devulcanizing with thiol acid offered better mechanical properties. Decrease in scorch time and increase in rheometric torque were observed for revulcanized rubber containing devulcanizing agent. The onset degradation temperature largely depended on presence of thiol acid. IR spectroscopic results revealed that the main polymeric chain did not oxidize at the time of milling above the room temperature. Increase in storage modulus and decrease in loss modulus were observed for revulcanized rubber from DMA study. The SEM was considered to study the failure mechanism and homogeneity of the vulcanizate. By adopting this devulcanization technique, more than 85% mechanical property of vulcanized natural rubber was retained.
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