Role of pH on hetero-aggregation of modified natural rubber and silica particles in latex medium and its physical and mechanical properties

Abstract

To achieve effective reinforcement of silica in natural rubber (NR) composites, it is essential to enhance the interaction between NR and silica particles to improve mechanical performance. In this study, NR latex was chemically modified to a low-degree epoxidation (5 mol%), designed as ENR-5. The average particle size (d₅₀) of ENR-5 latex increased to 6.1 µm—approximately ten times larger than that of unmodified NR—due to reduced surface charge and subsequent aggregation of rubber particles following epoxidation. The pH of ENR-5 latex, equal to 3, was further adjusted to 7, 10, and 12 using NaOH, resulting in progressive decrease of particle size to 3.4, 2.0, and 1.3 µm, respectively. A latex-based mixing of NR with 40 phr silica, named NR-40Si composite, exhibited homo-aggregation of NR-NR and silica-silica particles due to poor compatibility between the non-polar NR and polar silica surfaces. While ENR-5-pH3–40Si demonstrated pronounced hetero-aggregation between rubber and silica, driven by favorable interactions between epoxide and silanol groups. Increasing the pH of ENR-5 latex resulted in decreasing the hetero-aggregation sizes. Morphological analysis of the dried ENR-5-silica composites indicated that the dispersion of silica in the rubber matrix was closely related to the hetero-aggregation behavior in the latex phase. ENR-5-pH3 particles promoted the successful formation of hetero-aggregates of the rubber with silica, strengthening the rubber–silica interactions and enhancing thermal and mechanical properties better than other composites. This latex-stage mixing approach represents a simple, effective, and environmentally friendly (green) strategy for significantly improving the performance of silica-reinforced rubber composites.