Guayule resin as a bio-based modifier for soy protein adhesive systems

Guayule (Parthenium argentatum) is a promising source of natural rubber. It is a semi-arid perennial shrub native to the Southwest part of the United States and Mexico. Unfortunately, one of the reasons guayule rubber has not been commercialized is because there currently are no markets for the potential coproducts. Finding markets for the guayule resin and bagasse would greatly strengthen the economics of guayule rubber production. Hence, the aim of this preliminary study was to develop a new application for guayule resin as an additive for soy protein (SP) adhesives to increase the wet tensile strength. The resin was modified with NaOH in hexane, acetone, and water solutions, and subsequently mixed with soy protein adhesive at different concentrations. An addition of 7.5 % resin to soy protein adhesive showed the highest elastic modulus representing the highest intermolecular interactions. The highest water resistance was also observed for soy protein blended with 7.5 % resin. The wet adhesion strength significantly increased by 70 % compared to pure soy protein adhesive. Differential scanning calorimetry suggested that guayule resin strengthened soy protein network by interacting with 7S and 11S subunits of SP. The changes in soy protein properties were due to the interactions between guayule resin and protein's backbone and side chains. This study demonstrates the potential to utilize guayule resin as a bio-adhesive for wood industries.