Optimisation of alkaline treatment conditions for lignocellulose enrichment from spent coffee grounds and its application in polymer composites

Abstract

Spent coffee grounds (SCG) are promising agro-industrial residues for composite reinforcement, but non-cellulosic components limit their direct polymer application. This study evaluated the relationship between alkaline treatment parameters, the structural modifications of SCG, and its reinforcing behaviour in PVC composites. The effects of NaOH concentration, treatment time, and temperature were evaluated via single-factor experiments and optimised using response surface methodology with a Box-Behnken design. The operationally optimal conditions were identified as 4.0% NaOH, 91 min, and 62°C, yielding an experimentally validated NDF-derived lignocellulose fraction of 88.389±0.0217%. Characterisation through Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) confirmed the removal of non-fibrous impurities, accompanied by structural, morphological, crystalline, and thermal degradation changes. When incorporated at 40 wt.% into PVC, the treated SCG composite exhibited marked mechanical improvements, achieving compressive and flexural strengths of 79.66 and 64.03 MPa, respectively, compared with 67.80 and 38.33 MPa for the untreated SCG counterpart. Overall, optimised alkaline pretreatment provides a practical strategy to modify SCG composition and structure for value-added applications in rigid thermoplastic composites.