Preparation of purified spent coffee ground and its reinforcement in natural rubber composite

Spent coffee ground (SCG) contains a variety of organic compounds such as fatty acids, amino acids, polyphenols, polysaccharides, etc. In this study, purification of SCG was carried out by alkalization and bleaching treatment and the purified SCG (PSCG) was characterized by various techniques, i.e., FTIR, XRD, BET, SEM and TGA. PSCG was later treated with Bis-triethoxysilylpropyl tetrasulfide (TESPT). Both PSCG and TESPT-treated PSCG were then incorporated into natural rubber (NR) to investigate their reinforcement magnitude in the bio-composite. Results revealed the eradication of lignin and other non-polysaccharide components after the purification leading to the significant increases in specific surface area and cellulose content of PSCG. Although the addition of PSCG into NR showed cure time reduction in association with the increased modulus and hardness, its reinforcement was not very high due to the large particle size and the abundance of hydroxyl groups in PSCG. The TESPT treatment significantly improved the reinforcement of PSCG due to the increases in rubber-filler interaction and crosslink density. However, the reinforcement of both PSCG and TESPT-treated PSCG is still relatively low compared to the commercial nanofillers and, thus, they can be considered as a cheap and eco-friendly filler in NR.