Influence of Amino Acids on Anti-Oxidative Properties of Green Natural Rubber and Natural Rubber Compound

Abstract

This research was to study the efficiency of amino acids such as alanine, asparagine and cysteine, which were applied as antioxidant both in green and vulcanized natural rubbers (NR) for STR5L and RSS3, comparing with the use of synthetic antioxidant, i.e., N-(1, 3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD). In the case of green NR, the rubber was mixed with antioxidants on a two-roll mill machine. Then, the specimens were shaped by hydraulic hot press. In the case of cured NR, conventional system of the sulfur vulcanization was carried out at 150°C for t95. Both green and vulcanized NRs specimens were subjected to heat aging test at 100°C for 24, 48, 72 and 96 hours. Another mixed green and cured NR were also tested the plasticity by Wallace plastometer and ozone resistance following ISO 1431/1, respectively. From the experiment, it was found that although plasticity retention index (PRI) of the green NR mixed with each type of amino acids were lower than those mixed with 6-PPD, alanine exhibits the best antioxidative property among all of amino acids. Considering at the reduction in tensile properties, stress at 200% strain of both raw STR5L and RSS3 mixed with 1 phr of 6-PPD after heat aging test were totally lower than those applied with alanine, asparagine, and cysteine. In addition, for the cured NR specimens, the tensile results are agreeable with the PRIs value of the raw NR. However, NR vulcanizates preserved with each amino acid inhibited the crack length under ozone test better than that mixed with 6-PPD. Anti-oxidative mechanism of amino acids as well as 6-PPD is available as chain breaking donors (CB-D) mechanism. This might be concluded that alanine can be applied as a natural antioxidant in the green and vulcanized NR better than other amino acids, but not for 6-PPD. Moreover, three types of amino acid can be used as a good antiozonant for the sulphur vulcanized NR.