Moisture Diffusion in Silica/Clay/Natural Rubber Hybrid Composites

This study investigated moisture diffusion in natural rubber (NR) hybrid composites filled with silica and bentonite clay. Natural bentonite (BNT) was treated with tetradecyldimethylammonium chloride and coco diethanolamide to produce modified bentonite (M-BNT). Varied proportions of silica, M-BNT, and BNT fillers were added to raw NR according to a third-degree simplex lattice mixture design of experiment. The addition of fillers affects the vulcanization characteristics, mechanical properties, and hardness of NR hybrid composites. Moisture diffusion behavior was studied by monitoring the water uptake of NR composites during immersion in deionized water at 80°C. Data from sorption experiments were fitted on the classical Fickian and Langmuir-type diffusion models. The Fickian model overestimates and underestimates the water uptake of NR composites in the early and later stages of moisture diffusion, respectively. On the other hand, the Langmuir-type model adequately captures the anomalous diffusion behavior of moisture in NR composites. Parameters of the Langmuir model (equilibrium water uptake and diffusion coefficient) vary with the composition of hybrid fillers. Optimum proportions of silica, M-BNT, and BNT in rubber composites were obtained by considering the effect of fillers on mechanical properties and moisture diffusion characteristics of NR.