Structure properties of purified natural rubber

Abstract

Purifiednatural rubber (PNR) is natural rubber (NR) from which most of the nonrubber constituents are removed by repeated centrifugation. The study of PNR is of interest for two reasons. First, it has been reported that deproteinized natural rubber exhibited improved dynamic mechanical properties including heat buildup and flex-cracking resistance. However, the explanations for the observed improvement have not been given. Second, NR uncontaminated by nonrubber substances (mainly proteins and lipids) might be more suitable for medical applications than normal NR, which contains potential allergy-causing compounds, e.g., proteins. The present work was carried out with dual objectives: to understand the effect of nonrubber constituents on the network structures and properties of NR vulcanizates and to make the first assessment of the mechanical properties of the prepared PNR vulcanizates. The vulcanization system used was N-cyclohexylbenzothaizole-2-sulphenamide (CBS)-accelerated sulfur vulcanization system. Both the efficient vulcanization (EV) and conventional vulcanization (CV) systems were studied. It was found that vulcanization of PNR was strongly inhibited compared with normal NR, indicating significant influences of nonrubber compounds. For unfilled PNR, their tensile and tear properties were generally smaller than those of NR containing nonrubber constituents (WNR). PNR vulcanizates were also softer than WNR vulcanizates. Vulcanized PNR, however, exhibited distinct superiority in flex-cracking resistance than its WNR counterpart. Analysis of the network structures of the vulcanizates studied showed that for the EV system, the type of crosslinks [polysulfidic (Sx), disulfidic (S2), and monosulfidic (S)] in PNR vulcanizates were more evenly distributed than in WNR samples. The % Sx, S2, and S crosslinks were respectively 36.4, 25.0, and 38.7 in PNR samples compared with 6.6, 29.7, and 64.1 in WNR samples. For the CV vulcanization system, the differences in sulfur crosslink type were not as great but the tendency toward the formation of shorter sulfur crosslinks persisted in PNR vulcanizates. The more uniformly distribution of sulfur crosslink type was thought to be responsible for the observed superiority in flex-cracking resistance of PNR vulcanizates. For carbon black-filled PNR vulcanizates, similar trends existed with respect to their properties. Properties of PNR vulcanizates were generally lower than those of WNR vulcanizates, particularly when the CV vulcanization system was employed. The EV vulcanization system gave PNR properties comparable to those of WNR samples except for heat buildup where PNR showed better properties. Flex-cracking resistances of CB-filled PNR vulcanizates, however, still maintained their superiority over those of WNR counterparts of similar crosslink density. It was concluded, therefore, that the improved dynamic properties of filled PNR vulcanizates over those of normal NR are also likely to be due to more balanced formation of sulfidic crosslinks of different lengths, thus better cyclic load-bearing properties. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1495–1504, 2000