Screening of mediterranean agro-food by-product powders as potential reinforcing fillers of biocomposites for footwear

Aim: Plastic materials contribute 4.5% to global greenhouse gas (GHG) emissions. The footwear industry contributes significantly to these emissions due to the high plastic use (38-71%) in shoe components. With the aim of reducing the footwear industry's environmental footprint, this study investigated the potential to produce and assess the potential of agro-food by-products (olive pomace (OP), grape stem (GP), vine shoots (VS)) as biocomposite fillers for footwear components. After milling these abundant Mediterranean agro-waste raw materials, physical and chemical composition analyses were performed to evaluate their potential as biocomposite fillers. Method: Particle size (sieves: 250,150, 10, and 50 μm), proximate composition (moisture, ash, protein, fat, and carbohydrates), and phenolic content were evaluated. Followed by extractives, cellulose, hemicellulose, and lignin content determination, according to Sluiter et al. 2008 and Sluiter et al. 2021 respectively. Four fractions were also produced and evaluated regarding their extractives, cellulose, hemicellulose, and lignin composition. FTIR analysis was also attained for the fractions with and without extractives.
Results: After milling, a higher fraction of particles above 250 μm (56%) was attained to OP, and VS had fewer particles above 250μm (37%). VS displayed the highest carbohydrate content (87.2%), followed by GS (77.3%) and OP (67.5%). VS also exhibited the highest cellulose content (39.7%) and the lowest extractive amount (16.1%). Instead, OP P showed a relatively high composition of extractives (38.2%) and lignin (20.5%). GS has a similar cellulose content (26.4%), lignin (30.6%), and extractives (30.3%) but reveals the highest phenolic content, followed by OP and VS. OP appeared as the most promising reinforcement filler due to its higher yield of smaller particle size fractions, which facilitated better interaction with polymers. However, VS had a more favourable lignocellulosic composition, with high cellulose, low extractives, and ling content. Conclusion: Overall, the investigated agro-food by-products hold promise as valuable sources of lignocellulosic fillers for biocomposite production in the footwear industry. Nevertheless, optimisation of filler preparation processes is essential. While VS and GP require enhanced particle size reduction, OP and GP would benefit from removing or reducing extractives, lignin, and hemicellulose to ensure optimal performance.