The Effect of Fresh Latex Coagulation Concentration and Coagulum Maturation Time on the Properties of Natural Rubber

To investigate the effects of latex coagulation concentration and coagulum maturation time on the properties of microbially coagulated natural rubber (NR). This study used fresh Reyan 73397 latex as the raw material and applied a microbial coagulation process. Four coagulation concentrations (35%, 25%, 20%, and 17%) and three coagulum maturation times (1 day, 2 days, and 3 days) were chosen. The evolution patterns of physicochemical indexes, non-rubber components contents, vulcanization characteristics, crosslink density and physical and mechanical properties of NR under the combined conditions of these two processing parameters were systematically analyzed. The results show that as the concentration used for fresh latex coagulation increases, the initial plasticity value (P 0 ), Mooney viscosity (M L(1+4)100°C ), nitrogen content, acetone content, gel content and fatty acid content of NR gradually increase, whereas the plasticity retention index (PRI), weight-average molecular weight (Mw), minimum torque (M L ), scorch time (t 10 ) and optimum curing time (t 90 ) gradually decrease. The number-average molecular weight (Mn), maximum torque (M H ), torque difference (ΔM), crosslink density, tensile strength, tear strength, modulus and Shore A hardness all first increase and then decrease, while the molecular weight distribution (PDI) initially decreases before reaching a plateau.. Vulcanizates exhibit optimal mechanical performance at a coagulation concentration of 20%. As coagulum maturation time increases, NR shows gradual increases in P 0 , M L(1+4)100°C , gel content, Mw, PDI, MH, ΔM, crosslink density, tensile strength and tear strength. Meanwhile, PRI, nitrogen content, acetone extract content, fatty acid content, Mn, M L , t 10 , t 90 , and elongation at break gradually decrease. The optimal comprehensive mechanical performance of vulcanized NR is achieved under the combined conditions of 20% coagulation concentration and 3 d of coagulum maturation. Under these conditions, tensile strength, tear strength, 100% modulus, 300% modulus and Shore A hardness reach 24.74 MPa, 30.20 kN/m, 0.88 MPa, 1.90 MPa and 39 HA, respectively. This indicates that the optimal combined processing conditions can effectively enhance the crosslinking of rubber molecular chains, create a dense and stable three-dimensional crosslinked network, and considerably improve the overall properties of NR. This finding provides a technical support for the process optimization and quality control of NRused in high-end rubber products.