Effect of acid coagulating agents on the molecular structure and properties of bulk raw rubber and rubber compounds

Concerns about the quality of raw rubber produced by acid coagulation and the impact of acids on the mechanical properties of rubber compounds remain insufficiently addressed. This study investigated the effects of four acids commonly used to coagulate natural rubber latex: formic acid (HCOOH), acetic acid (CH3COOH), hydrochloric acid (HCl), and sulfuric acid (H2SO4). Field-coagulated latex served as a natural control. Crepe rubber was produced from the various types of coagulated latex. Vibrational spectroscopy combined with principal component analysis (PCA) revealed that strong mineral acids promoted oxidative modifications and chain scission. These findings were consistent with visual observations and plasticity retention index (PRI) values. H2SO4, in particular, was linked to sulfate radical formation, driving extensive chain scission and oxidation in crepe rubber. The crepe rubber samples were then compounded with additives and tested. Mechanical properties were compared with those of the standard benchmark, STR 20. Samples prepared with the weaker organic acids preserved higher Mooney viscosity, more stable curing behavior, higher cure efficiency, and superior tensile properties, whereas samples prepared with the stronger acids showed compromised molecular integrity, resulting in low crosslink density and inferior mechanical performance. Overall, this work provides systematic evidence connecting acid coagulation with molecular-level transformations and rubber compound properties, underscoring the importance of appropriate coagulant selection.