Bulk fillers from food waste for polymeric bio-composites: The influence of filler type, particle size and volume ratio on furan-matrix composites

Bio-based composites provide promising low embodied-carbon alternatives to technical materials, but they generally rely on virgin biomass which raises concerns about agricultural land use for non-food crops. Bio-composites made from organic waste address these concerns by providing high carbon-sequestration opportunities with fewer virgin resources. But the sourcing of these waste streams and their impact on the mechanical and functional properties of the bio-composite are poorly understood. This study investigates food industry waste as bulk fillers in bio-composites with a furan resin matrix. Six waste streams were selected based on local availability and current underutilisation. Firstly, bio-composite samples for each filler type were prepared and tested for strength, water absorption and freeze-thaw resistance. Secondly, the two most promising fillers, walnut shell and spent coffee ground, were investigated further, assessing the influence of filler particle sizes and filler content fraction on bending and impact strength. Finally, a carbon impact analysis of the primary production and fabrication was performed to evaluate the carbon footprint of the developed bio-composites, compared to conventional construction materials. With a mean bending strength up to 58 MPa the walnut shells and spent coffee fillers produced the highest performance bio-composites, while variants of cacao bean shells and cherry pits showed blisters and cracking, resulting in lower mechanical properties and higher water absorption. Walnut-based composites benefited from a blend of grain sizes by improving packing density, requiring less resin, while maintaining mechanical performance. The carbon impact analysis showed that a bio-composite with 55 % walnut shell filler is a low-carbon alternative to construction materials such as ceramics, aluminium and steel within the considered life-cycle phases and use case. The findings demonstrate the feasibility of utilising food-industry waste in bio-composites and present the further research needed in the development of these more sustainable materials. • Bio-based bulk fillers from food industry waste were tested in furan resin composites for semi-structural applications. • Processing ease and composite surface quality varied significantly between filler types. • Filler particle size and filler content strongly influenced mechanical performance. • Composites with 55 % walnut shells could compete with comparable materials regarding CO2-eq emissions in early LCA stages. • The study highlights the need for pretreatment of reactive fillers and potential for blended grain size composites.