Utilizing spent coffee grounds as sustainable fillers in biopolymer composites: influence of particle size and content†

This study presents the first systematic investigation of spent coffee grounds (SCG) particle size effects in polybutylene succinate (PBS) and polybutylene adipate terephthalate (PBAT) biocomposites, evaluating their potential as sustainable fillers in biodegradable polymers. Composites containing 30–60 wt% SCG were produced using unfractionated (SCG_m), coarse (SCG_L), and fine (SCG_S) particle size fractions. Thermogravimetric analysis (TGA) confirmed that both PBS and PBAT composites retained thermal stability up to processing temperatures of 220 °C, with onset degradation temperatures ranging from 266 °C to 294 °C for PBS and from 267 °C to 294 °C for PBAT. DSC analysis for PBS revealed an increase in glass transition temperature from −29.83 °C (neat) to −13.48 °C (60% SCG_m), while crystallinity remained stable (26.38–28.64%). Mechanical testing showed that SCG_m increased stiffness in both matrices. Young's modulus rose from 675 MPa (PBS) and 52 MPa (PBAT) up to 1016 MPa and 210 MPa, respectively. However, tensile strength declined from 34.5 MPa to 9.0 MPa (PBS) and from 18.8 MPa to 4.3 MPa (PBAT), and elongation at break dropped sharply, particularly in PBS (148% to 2.7%) and to a lesser extent in PBAT (446% to 12.4%). Finer SCG particles (SCG_S) enhanced ductility and water uptake (up to 3.40% for PBS and 4.42% for PBAT), while coarser particles (SCG_L) provided higher stiffness. Water contact angle and colour changes were minor across all samples. These results demonstrate that SCG can partially replace virgin biopolymer content in PBS and PBAT, enabling property tuning through particle size and promoting material circularity.