Submitted to: Rubber Chemistry and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2015
Publication Date: 3/13/2015
Citation: Mcmahan, C.M., Lhamo, D. 2015. A study of amino acid modifiers in guayule natural rubber. Rubber Chemistry and Technology. 88:310-323.
Interpretive Summary: Parthenium argentatum (guayule) is a natural rubber-producing crop native to the southwestern US and northern Mexico that is growing in economic importance. Natural rubber derived from the guayule plant can provide supply security for a critical agricultural material as well as rural development in semi-arid southwest farmlands. Guayule rubber can replace petroleum-based rubber or imported natural rubber from Hevea trees, but substitution must take into consideration differences in physical and chemical properties, especially due to species-specific non-rubber components like proteins and amino acids. In this study, modification of guayule latex with amino acids impacted rubber and compound performance, in some cases providing valuable benefits. Amino acids can be considered biobased modifiers in rubber compounds for use as plasticizers, antioxidants, and as vulcanization aids. The results from our study introduce a new perspective for amino acids as biobased rubber compound additives.
Technical Abstract: Natural rubber from the Hevea tree is a critical agricultural material vital to United States industry, medicine, and defense, yet the country is dependent on imports to meet domestic needs. Guayule, a desert shrub indigenous to the US, is under development as an alternative source of natural rubber. Guayule rubber can replace petroleum-based rubber or Hevea rubber, 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. Rubber from Hevea typically contains high levels of non-rubber constituents that create insoluble gel structures and which contribute to thermal-oxidative stability, cure acceleration, and especially strain-induced crystallization. The latter has been attributed to compound properties which render Hevea natural rubber uniquely suited for the most demanding rubber applications (e.g. aircraft tires). Here, low-protein guayule latex was blended with a series of amino acids varying in chemical structure. The bulk viscosity, thermal-oxidative stability and cure rate were influenced by the addition of amino acids. Generally, gel formation, green strength, and tensile strength were not impacted. The results introduce a new perspective for amino acids as biobased rubber compound additives, and provide insights into naturally-occurring non-rubber constituents’ interaction with natural rubber polymers.