A Study of Protein and Amino Acids in Guayule Natural Rubber

Natural rubber (NR) from Hevea brasiliensis (the Hevea rubber tree) is a critical agricultural material vital to United States industry, medicine, and defense, yet the country is dependent on NR imports to meet domestic needs. Parthenium argentatum(guayule), a woody desert shrub indigenous to the US, is now under development as a new agricultural crop in the semi-arid southwestern US. Guayule natural rubber (GNR) 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. In natural rubbers those properties can be strongly impacted by their non-rubber constituents, typically proteins and lipids, depending on the plant species and post-harvest processing. 

NR from Hevea typically contains high levels of proteins that create insoluble gel structure and contribute to cure acceleration and thermo-oxidative stability.  The presence of rubber gel has been attributed to outstanding physical and chemical properties which render Hevea NR uniquely suited for the most demanding rubber applications (eg aircraft tires). Low protein guayule NR was blended with a series of proteins and amino acids varying in chemical structure to study the impact of specific interactions (e.g. hydrogen bonds, disulfide bonds, and hydrophobic interactions) that may be formed in the blends. The bulk viscosity, thermal-oxidative stability and cure rate were impacted by addition of amino acids; in some cases green strength and gel were impacted as well. This study provides insights as to the mechanisms of non-rubber constituents’ interaction with polymers in natural rubber compositions.