Chloroprene is a chlorine substituent of 1,3-butadiene. Butadiene rubber (BR) and chloroprene rubber (CR) composites were thermally aged at 60, 70, 80, and 90 o C for 2 -185 days in a convection oven and changes of the crosslink densities by the accelerated thermal aging were investigated. The crosslink densities increased as the aging time elapsed and as the aging temperature became higher. Degrees of the crosslink density changes of the BR composite were on the whole larger than those of the CR one except the short-term thermal aging at 60 and 70 o C. The crosslink densities abnormally increased after themal aging at high temperatures for a long time. Activation energies for the crosslink density changes of the rubber composites tended to increase with increase of the aging time and the variation showed a local minimum. The activation energies of the CR composite were lower than those of the BR one. The experimental results were explained with a role of ligand of chlorine atom of CR in a zinc complex, steric hindrance by chlorine atom of CR, and oxidation of rubber chain.

ABSTRACT：EPDM, IIR, and BIIR composites were thermally aged and the crosslink density changes were investigated. Crosslink densities of the EPDM composite increased with increasing the aging time and temperature, whereas those of IIR and BIIR composites for long-term aging at high temperatures tended to decrease. Activation energies for the crosslink density changes of the EPDM composite were higher than those of the BIIR one. The experimental results were explained with the number of allylic hydrogens, activation of the zinc complex, the steric hindrance effect, and oxidation of rubber chain. Ⅰ. IntroductionR group rubbers such as natural rubber (NR), butadiene rubber (BR), styrene-butadiene rubber (SBR), and acrylonitrilebutadiene rubber (NBR) have unsaturated hydrocarbon (C=C double bond) backbones. Due to the C=C double bond backbones, they have lots of allylic hydrogens which participate in sulfur crosslinking reaction. 1 Hence, degrees of crosslink density changes of the R group rubber composites by themal aging are relatively high, and their physical and chemical properties are also changed. 2-10 Of R group rubbers, isobutene-isoprene rubber (butyl rubber, IIR) has relatively good thermal resistance properties since it has only isoprene content with less than 3 wt%. 11,12 Crosslink type and degree of crosslink density of a rubber vulcanizate determine the physical properties such as modulus, hardness, resilience, elongation at break, heat build-up, and so forth. 13 Sulfide linkages, especially polysulfides, are dissociated by heating 14,15 and this brings about decrease of the crosslink density. Curatives, especially sulfur, in rubber vulcanizates make new crosslinks 16 and this results in increase of the crosslink density. Crosslink density of a rubber vulcanizate is changed by thermal aging. 4,[17][18][19][20] In general, when sulfur-cured rubber vulcanizates are thermally aged, the crosslink densities increase and the degree also increases as the aging temperature and time increase. [21][22][23] Crosslink density change by thermal aging is also one of reasons about permanent deformation of a rubber vulcanizate. [21][22][23] Brown and coworkers reported physical property changes of rubber composites after natural aging for 40 years and compared with the accelerated heat aging results. 24,25 Relative deviations for the physical property changes of naturally aged rubber vulcanizates were very big but the results were valuable. The big deviation of the physical property may be due to the difference in the initial states of the samples. A number of samples are needed to measure physical properties and to perform thermal aging for a long time. Sample size should be small to minimize the experimental errors, but relatively large sample size for measurement of the physical properties should