Structure–property relationships in biowaste-filled natural rubber: effects of particle size and ageing on mechanical and thermal performance

Abstract

This study investigates the potential of spent coffee grounds (SCG) and spent tea grounds (STG) as biowaste-derived fillers for reinforcing natural rubber (NR) composites. The effects of filler contents (0–10 phr), particle sizes, and thermal ageing were examined using mechanical, morphological, and thermal analyses. At an optimal loading of 5 phr and particle size < 44 µm, STG-filled composites exhibited the best overall performance. Compared to unfilled NR, tensile strength increased from 15.41 MPa to 20.10 MPa, elongation at break improved from 531 to 608%, and cross-link density rose from 147.3 to 239.8 mol/m³. The bound rubber content reached 19.6%, indicating a stronger filler–rubber interfacial interaction. SEM images confirmed more uniform filler dispersion and reduced surface defects in fine-particle composites. FTIR analysis revealed decreased carbonyl absorption and improved retention of C = C double bonds after thermal ageing, particularly in STG-filled NR, indicating reduced oxidative degradation. These stabilising effects are attributed to the antioxidant activity of polyphenols and lignin derivatives inherent in the biowaste fillers. Although the improvements were moderate, the consistency of several properties supported the assumption that particle size plays a role in optimising the performance of the composite. These findings highlight the potential of SCG and STG as viable, eco-friendly, and reinforcing fillers that improve the durability and sustainability of soft rubber materials.

Graphical abstract