Structural characterization of vulcanizates. Part III. The cis‐1,4‐polyisoprene tetramethylthiuram disulfide–zinc oxide system

Abstract

AbstractThe vulcanization of a purified synthetic cis‐1,4‐polyisoprene (100 parts) with tetramethylthiuram disulfide (TMTD; 4 parts) and zinc oxide (4 parts) at 140°C. yields a network which at short cure time (15 min.) is structurally complex, containing 13.5 atoms of sulfur and 4.3 atoms of nitrogen per chemical crosslink present. The network becomes progressively simpler in structure as crosslinking proceeds, and at full cure (10 hr.) contains 3.2 atoms of sulfur and 0.7 atoms of nitrogen per chemical crosslink present. The initial network complexity is due mainly to the presence of rubber‐bound intermediate compounds, R·SxC(:S)N(CH3)2 (R = polyisoprenyl; x ≥ 2), which yield sulfidic crosslinks and zinc dimethyldithiocarbamate (ZDMC) on heating with zinc oxide or zinc sulfide as revealed by the efficient crosslinking on further heating at 140°C. of extracted undercured vulcanizates. Evidence is presented that the intermediate compounds result from interaction of the polyisoprene with a sulfurating species formed by a prior reaction of TMTD with zinc oxide, and not, as previously supposed by others, by direct reaction of the polyisoprene with TMTD. Our knowledge of the detailed structure of the network obtained at full cure is presented, together with estimates of the principal extra‐network compounds (ZDMC, tetramethylthiourea, and zinc sulfide) formed at full cure; these compounds together with the sulfur and nitrogen combined in the network account for ca. 90% of sulfur and nitrogen originally present as TMTD.