EFFECT OF CURE EFFICIENCY ON PROPERTIES OF GUM AND BLACK-FILLED NATURAL RUBBER VULCANIZATES

Effects of the sulfur cure efficiency on the reversion behavior and the normal and edge-cut tensile strength of gum and black filled natural rubber (NR) vulcanizates were studied. N, N-dicyclohexyl-2-benzothiazole sulfenamide (DCBS) was used as an accelerator. A series of five vulcanizates with high to low cure efficiencies was prepared by increasing the sulfur (S) to DCBS ratios within the range of 0.26-6.66. All vulcanizates were formulated to have the same crosslink density. The degree of reversion (%) calculated from cure curves of gum and black filled NR at 20 min above the cure time (t_c100) passed through maximum with decreasing cure efficiencies. For both gum and black filled NR, the highest degree of reversion (%) was observed at the S/DCBS ratio of 1.17.   For all compounds cured with different cure efficiencies, the incorporating 50 phr of carbon black (N330) into compounds resulted in an increase in the degree of reversion (%). The normal tensile strength of gum and black filled NR were directly proportional to the cure efficiency. The abrupt drop of normal tensile strength of gum NR vulcanizates with the increase in the testing temperature was observed. This temperature indicated that there was no strain induced crystallization of NR possible. The temperature at which the normal tensile strength significantly decreased shifted to the higher temperature with increasing cure efficiencies. For gum NR vulcanizates, the edge-cut tensile strength was markedly influenced by cure efficiency. Similar to the normal tensile strength, the gum NR vulcanizates cured with the lowest cure efficiency showed the lowest edge-cut tensile strength. Effect of the cure efficiency on the edge-cut tensile strength was less in the case of black filled NR vulcanizates. However, the black filled NR vulcanizates cured with the lowest cure efficiency also showed the lowest edge-cut tensile strength.  The cut tip characteristics of the fracture specimens were investigated using SEM. The gum specimens showed only the simple lateral cracking pattern, while all black filled specimens showed the longitudinal cracking pattern. Four different cracking patterns of the black filled specimens were identified. The distribution of cracking patterns depended strongly on the size of precut and the cure efficiency.