Determination of optimal latex pH level and evaluation of natural rubber coagulum quality when using lactic acid for coagulation of rubber latex

The objectives of this study were to determine the optimal latex pH level for coagulation of rubber latex using lactic acid and to evaluate the effects of lactic acid on natural rubber coagulum quality. In this study, 3% lactic acid was added to the latex until the latex pH values reached 4.6, 4.9, 5.2, 5.5 and 5.8; meanwhile, the control treatment was added with 3% acetic acid until the latex pH value was at 5.4. The rubber coagula of the optimum and the control treatment were evaluated for mechanical, physical and chemical criteria based on the international standards of quality for natural rubber grades. The results showed that latex pH of 5.5 resulted from the addition of 3% lactic acid was the optimum for latex coagulation, which also helped to reduce the necessary amount of acid used in latex coagulation and therefore reducing the risk of water pollution. The quality of natural rubber coagulum that was coagulated by lactic acid met all requirements of technical standard for SVR 5 grade based on the Standard Vietnamese Rubber 3769:2004. The rubber coagulum coagulated by 3% lactic acid had a higher initial plasticity (Po) and a lower plasticity retention index (PRI) than that coagulated by 3% acetic acid.