Effect of proteins on the structure of crosslinking junctions and mechanical properties of vulcanized natural rubber

Primary structure of vulcanized natural rubber was quantitatively analyzed by rubber state NMR spectroscopy to investigate a relationship between structure, strain-induced crystallization and mechanical properties of the rubber. The vulcanized natural rubber was prepared from deproteinized natural rubber (DPNR) and natural rubber, which were compounded with sulfur, ZnO, stearic acid, and N - tert -butyl-2-benzothiazole sulfenamide (TBBS), at temperatures of 110, 130, 150, and 170 °C under pressure of 15 MPa for optimal vulcanization time, t 90 . The vulcanized rubbers were characterized by rubber-state NMR spectroscopy, swelling method, and wide angle X-ray diffraction measurement. Carbon-sulfur bonds of the vulcanized rubbers were quantitatively compared with crosslink density, and abnormal groups such as trans -1,4 isoprene units were analyzed as a side reaction product. Contents of carbon-sulfur bonds and trans -1,4 isoprene units of the vulcanized rubbers increased as the vulcanization temperature rose; that is, a sequence length of cis -1,4 isoprene units was shortened with the temperature. Accordingly, the strain-induced crystallization was suppressed and the mechanical properties were reduced as the vulcanization temperature rose. The strain-induced crystallinity and tensile strength of the vulcanized DPNR were higher than those of the corresponding vulcanized natural rubber while the crosslink density and cis -1,4 isoprene unit content of the vulcanized DPNR were lower. • Shortening a sequence length of cis -1,4 isoprene units as the vulcanization temperature rose. • Suppressing the strain-induced crystallization as the vulcanization temperature rose. • Promoting the strain-induced crystallization of vulcanized natural rubber.