Effect of Protein Addition on Properties of Guayule Natural Rubber

Effect of Protein Addition on Properties of Guayule Natural Rubber Parthenium argentatum, commonly known as guayule, is a U.S. native desert shrub cultivated as a domestic source of natural rubber (NR) in the semi-arid southwestern United States. Guayule may be used to replace petroleum-based rubber or in place of Hevea NR, but substitution must take into consideration differences in physical and chemical properties. Currently, Hevea NR is required in tire applications, especially aircraft and truck tires, because of its high oxidative resistance, rapid cure acceleration, and exceptional stress-strain response. These outstanding features are due to the presence of non-rubber constituents, mainly proteins and lipids, which form insoluble gel in organic solvents, and lead to strain-induced crystallization. In contrast, guayule natural rubber is low in proteins, thus deprived of those attributes of Hevea. Addition of amino acids and proteins to guayule could potentially improve performance, and thereby widen the range of applications for use. In a previous study, amino acids blended with GNR latex improved thermo-oxidative stability, served as plasticizer and cure accelerator, and enhanced green strength slightly, but did not translate to tensile strength. Here, a series of bio-based commercial proteins (gelatin, soy, casein, zein, albumin, gluten and gliadin) were added to guayule as a latex blend. In general, protein addition reduced bulk viscosity and improved thermo-oxidative stability. Gel and green strength of the polymer-protein blends were increased, with the exception of gliadin, but not to levels observed for Hevea. Effects on vulcanization and mechanical properties in compounds were surprisingly influenced by the choice of antioxidants used. Our results suggest proteins and amino acids have potential as a new class of biobased, low environmental impact, rubber compounding chemicals.

For oral presentation at the American Chemical Society Rubber Division Meeting, October 10-13, 2016