Upcycling Leather Waste Through Zero-Waste Hydrolysis for Versatile 3D Printable Composites

The leather industry produces a substantial amount of solid waste, which is frequently disposed of via incineration or landfilling. While hydrolysis offers a valuable and sustainable method to chemically recycle leather waste, both acidic and alkaline processes present challenges due to the salts produced during neutralization. This study aims to upcycle leather scraps through hydrolysis, producing a powdered filler for versatile composites suitable for both LCD vat photopolymerization and Direct Ink Writing 3D printing technologies. A zero-waste hydrolysis process was adopted using sulfuric acid neutralized with calcium hydroxide, achieving a yield of 91.3%. The composites featured a matrix composed of polyethylene-glycol-diacrylate and glycerol dimethacrylate, with embedded leather hydrolysate powder at concentrations up to 20% w/w_matrix. Tensile tests conducted on neat resin and composites demonstrated the strengthening effect of leather hydrolysate filler. Additionally, rheological tests displayed a viscoelastic behavior suitable for the adopted 3D printing technologies. The composites were successfully 3D-printed using both Direct Ink Writing and vat photopolymerization techniques, showing promising printing accuracy. This work demonstrates the potential of valorizing leather waste, upcycled via a hydrolysis method, to produce composites suitable for additive manufacturing to advance the sustainability and the circularity of the fashion sector.