To assess and predict the functional life of a natural rubber engine mount compound, the mechanical property changes were determined under accelerated aging conditions. The rubber was aged at temperatures ranging from 70 to 1108C for times ranging from 1 h to 5 weeks. Tensile and fatigue measurements were used to characterize the aging trends and mechanisms of the engine mount compound. With the time-temperature superposition approach, the activation energy was found to be about 98 kJ/mol for the elongation at break, 93 kJ/mol for the tensile strength, and 60 kJ/mol for the fatigue life. The tensile strength after aging for 13 weeks at 508C was predicted to be 18.73 MPa, which was very close to the experimental value of 19.04 6 2.25 MPa. With a 50% reduction in the tensile strength used as the failure criterion, it was predicted that the tensile strength of the engine mount compound would take 80 days to decrease by 50% at 708C. At 238C, it would last approximately 140 times (31 years) its lifetime at 708C.
Drying of natural rubber (NR) crumb (grade TSR10) requires high temperatures (100-120 • C). In order to determine the changes in bulk viscosity during the drying process, ageing kinetics at 120 • C were studied on model NR samples. During the process, changes in Mooney viscosity and weight-average molar mass (M W ) were monitored. Rubber from clones GT 1 and PR 107, with a bimodal inherent molar mass distribution (MMD 0 ), was degraded in a two-phase process. During the first phase, Mooney viscosity and M W increased, undoubtedly owing to a predominance of storage hardening over chain scissions [0 < t (min) < 120]. During the second phase, chain scissions predominated (t > 120 min) and Mooney viscosity and M W decreased. For rubber samples from clone PB 217, with a unimodal MMD 0 , no or reduced storage hardening was observed throughout the ageing process. These results showed that the key parameter involved in storage hardening seems to be the quantity of short polyisoprene chains and probably the nature of the chain ends.
ABSTRACT:The effects of the maturation and storage of naturally coagulated latex, generally termed cuplumps, on some bulk rheological properties and on parameters characterizing the macromolecular chain length were investigated for natural rubbers of different clonal typologies. The sensitivity of the clonal material to the degradative effects of cuplump maturation increased with the level of tree energetic metabolism, with the most metabolically active plant materials (e.g., PB 235) being most sensitive, whereas the less active clone PB 217 was less sensitive. A definite relationship evolves between Hevea tree metabolism and the stability of the structure of the biosynthesized rubber. The results presented here highlight at a macromolecular level the effects of maturation previously demonstrated on bulk processing parameters.
reEpoxidised liquid natural rubber (ELNR) constitutes a new family of polymers chemically derived from natural rubber. It is a highly viscous rubber of low molar mass generally used as plasticiser or processing aid, especially during processing of natural rubber and styrene/butadiene rubber stocks.The effect of incorporating this polymer rather than a standard plasticiser in a nitrile/butadiene rubber based mix has been investigated. The retention in mechanical properties following air and oil aging of the vulcanisates was also studied. Satisfactory processability of compounded stocks, mechanical properties, and aging resistance was maintained following incorporation of ELNR into nitrile/butadiene rubber formulations even at the lowest plasticiser concentrations. The improvement in aging behaviour was more pronounced when 25 mol-% ELNR was used at concentrations above 15 pphr. PRC/1641
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