Exploring the effect of lignin as a filler on the mechanical properties and anisotropic nature of Glass/Polypropylene LFTs manufactured via direct compounded compression moulding (LFT-D)

Glass Long Fibre Thermoplastics (LFTs) are becoming increasingly popular as lightweight, high-performance recyclable materials. This trend has also led to greater interest in the use of biomaterials, such as bio-fillers like lignin, to help reduce the carbon footprint of these petroleum-based polymer composites. This study investigated the influence of lignin bio-filler on the mechanical properties of glass-reinforced polypropylene (glass/PP) LFTs. Three lignin weight percentages (0 %, 14 %, and 21 %) were evaluated while maintaining 30 % glass fibre content. Tensile and shear tests were conducted on samples from both charge and flow regions of compression-moulded plaques, considering 0°, +45°, −45° and 90° material directions. Fracture surface analysis was conducted by utilising a Scanning Electron Microscope (SEM) to understand the failure mechanisms and structural behaviour under mechanical stress. The results indicated a decrease of up to 36 % in tensile strength with increasing lignin content in the 0° direction, which was particularly significant at a lignin content of 21 %. The 0° direction consistently exhibited higher tensile strength in the range of 70 MPa–100 MPa than the 90° material directions followed by +45°, −45°. Shear strength remained largely unaffected for 14 % lignin content, with an approximately 10 % drop for 21 % lignin content samples. SEM analysis revealed distinct failure mechanisms across the different material directions (0°, +45°, −45°, and 90°). This study provides essential material characterisation, enabling more accurate numerical and analytical modelling of these materials with a lightweight, low-cost filler.