New Approach to the Dispersion of Silica in the Natural Rubber Matrix by Grafting with Biobased Compounds: The Compatibilizing Effect of l‑Cysteine.

Precipitated silica was subjected to a sustainable strategy of surface modification with l-cysteine or functional silanes: 3-aminopropyltriethoxysilane or 3-mercaptopropyltrimethoxysilane (as a reference way), and the effects were compared from the point of view of the degree of coverage and the number of functional groups present on the filler surface. Because the silane coupling agent requires relatively high doses, being additionally hazardous, flammable, susceptible to failure during storage, and has to be incorporated into the organic dispersants and prehydrolysis treatment, the application of more convenient and green modifiers is sought. Infrared spectra and thermogravimetric analysis confirm the effective grafting of silica surface, with the greatest extent observed in the case of l-cysteine. The presence of both amino and thiol groups in the amino acid molecule is likely to be responsible for its much greater degree of adsorption on the filler surface. A greater degree of modification of the silica surface with l-cysteine comes together with better compatibility of the amino acid with natural rubber (NR). Unlike silanes, l-cysteine does not seem to interfere with the sulfur vulcanization process. The proposed modification of silica resulted in an increase in the cross-link density of vulcanizates and a change in the cross-link structure. The replacement of silica-coupling agents with l-cysteine resulted in an improvement in the macrodispersion of filler, represented by the dispersion index and indicated by the Payne effect. The changes in the cross-link density and structure, as well as improved filler dispersion and unchanged bound rubber content, result in an increase in the tensile strength and elongation at break, but contrary to silanization, decreasing stiffness of the vulcanizates. The presented results confirm the possibility of replacing conventional silane compatibilizers with l-cysteine in rubber technology, opening a new direction of research on the application of cheap protein derivatives obtained from recycled natural products.