Gamma-ray induced deterioration of natural rubber vulcanizates

Abstract

The influence of 7-ray irradiation on polymers has been studied extensively [1 -4 ] , and natural rubber (NR) is not an exception. However, the sofar-reported results of NR were obtained on the NR vulcanizates of filled stock, i.e. containing carbon black or some other flUers [1-3, 5 -7 ] , though in some examples the formulation recipes were not described in detail. The effect of radiation can be influenced by the presence of filler. We would like here to report the effect of radiation on purified NR vulcanizates which do not contain any fillers. RSS No. 1 was used as natural rubber for vulcanizations. It was mixed on an open two-roll mill with reagents shown in Table I. In Table I $1 means the formulation to obtain the mainly monosulphide-crosslinked vutcanizate, S= means that of mainly polysulphide crosslink, and B the carbon black stock. The compounds were press-cured at 140 ° C. The resultant vulcanizates were subject to solvent (acetone/methanol/chloroform = 35/28/ 29 in volume ratio [8]) extraction for a week to purify them. Extracted vulcanizates were vacuumdried to a constant weight. The weight decrease by extraction was below 5%. The 7-ray irradiation upon the purified vulcanizates was carried out in air at room temperature using 6°Co (12 000 Curie) as a source. The dose rate was 1.88Gyh -1. From the swelling in benzene at 25 ° C, the networkchain density (v) was determined [9]. The tensile test was conducted on an Instron-type tensile tester at the strain rate of 50mm min -1. Attenuated total-reflection infrared (ATR-IR) spectra were measured on an IR-A302 of Japan Spectroscopic Co. Ltd. with the ATR-6 attachment. Fig. 1 shows the stress-strain curves of NR vulcanizates before and after 100 kGy irradiation. The stress was not much affected by the irradiation