Structural, thermal, mechanical and physical performance of cellulose fibers based low-density polyethylene/recycled polyethylene terephthalate blend composites

Abstract Despite the increasing interest in recycled polymer composites and natural fiber reinforcement, relatively few studies have systematically examined low-density polyethylene (LDPE)/recycled polyethylene terephthalate (r-PET) blend systems reinforced with cellulose fibers recovered from used disposable paper cups, particularly through a direct comparison of untreated and methacrylic acid-grafted fibers. In this context, the present study developed sustainable composites based on LDPE and r-PET reinforced with cellulose fibers extracted from post-consumer paper cups and chemically modified with methacrylic acid. FTIR analysis indicated successful surface modification, as evidenced by the disappearance or reduction of lignin- and hemicellulose-related bands after bleaching and the appearance of a carbonyl band at approximately 1712 cm − 1 after grafting. XRD results showed a marked increase in crystallinity after bleaching, from 21.13% for washed fibers to 54.70% for bleached fibers. Grafted fibers retained an intermediate crystallinity of 46.75%, reflecting the partial disruption of the cellulose structure by grafting. TGA showed that methacrylic acid grafting improved the thermal stability of the fibers, increasing the main decomposition temperature from 327 °C for washed fibers to 354.48 °C for grafted fibers. DSC analysis was consistent with the enhanced thermal response and crystallization behavior of the composites, whereas r-PET incorporation increased the rigidity and thermal resistance relative to neat LDPE. Mechanically, blending LDPE with 25 wt% r-PET increased the ultimate tensile strength from 1.69 MPa to 7.40 MPa and Young’s modulus from 101.55 MPa to 272.17 MPa. The addition of grafted fibers further improved the tensile strength to 6.78 MPa and modulus to 218.58 MPa compared with untreated fibers. Water absorption tests showed that untreated fibers significantly increased moisture uptake, whereas methacrylic acid-grafted fibers markedly reduced water absorption and improved dimensional stability. Overall, the results suggest that integrating recycled PET and chemically modified cellulose fibers into LDPE matrices can yield composites with improved mechanical performance, enhanced thermal stability, and reduced water uptake relative to neat LDPE, thereby contributing to waste valorization and circular-economy-oriented applications within the limitations of the present system.