Dual-waste valorisation of pine needle biomass and post-consumer PET into sustainable composites: Mechanical, dynamic mechanical and environmental performance

The use of lignocellulosic biomass with post-consumer polyethylene terephthalate (PET) is an attractive option for waste diversion and resource recovery. In this study, pine needle (PN) biomass was used to reinforce a commercial unsaturated polyester resin (UPR) made from recycled PET (rPET). The untreated (UPN) and NaOH-treated (TPN) fibres were added at 30 wt% using the vacuum-bagging method and subsequently characterised by tensile and flexural testing, dynamic mechanical analysis, Fourier-transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), and 45-day water immersion and soil burial ageing. Alkali treatment raised tensile strength from 7.32 ± 0.46 to 11.05 ± 0.53 MPa (+50.9%), flexural strength from 44.00 ± 3.16 to 64.20 ± 6.10 MPa (+45.9%), and the moisture-diffusion coefficient fell from 9.81 × 10 −13 to 3.76 × 10 −13 m 2 s −1 . TPN-rPET was superior to UPN-rPET under environmental ageing conditions. Alkaline modification, therefore, improves PN reinforcement, resulting in a waste-based composite material suitable for non-load-bearing and lightly loaded applications.