Improving Sustainability and Rubber Security with New Eggshell/Silica Filled Guayule Natural Rubber Composites

Abstract

Natural rubber (NR) is irreplaceable in current human society. Currently, all U.S. NR is produced from tropical rubber trees and imported. Guayule natural rubber (GNR) is under development as a domestic NR source and is allergy-free. GNR can supplement and eventually even replace tropical NR and some synthetic rubber. Most rubber products are reinforced with fillers, such as precipitated silica (PS), which is neither renewable nor environmental-friendly. Eggshell (ES) derived filler is produced from a food industry waste, which is mostly landfilled. In the present study, micro-sized ES filler and PS were used to reinforce GNR composites. Various filler loadings and different ES/PS fractions were compounded with GNR and cured before characterization. Lower energy was consumed during compounding with higher ES fraction and with decreasing filler loading. Tensile strength and elongation at break increased with increasing ES fraction due to strong ES-GNR interaction and more uniform filler dispersion, which was evinced by scanning electron micrographs and increased gel content. Although modulus and hardness reduced as ES fraction increased, original values could still be achieved at higher filler loadings. The tan delta of ES-filled GNR determined by dynamic mechanical analysis was high at very low temperatures but low at high temperatures, which indicated higher rubber chain motion at low temperature and less energy loss at high temperature. ES-filled GNR composites may open new markets for high performance, low cost, sustainable rubber products, and encourage the development of guayule farming and processing.