Keratin-modified waterborne polyurethane: an alternative circular economy technology for adding value to cattle hair waste from leather tanneries

Waterborne polyurethane (WPU) has excellent performance because of its special soft and hard segments in polymer chains, but the relatively high cost, poor water vapor permeability, and inferior biocompatibility limit its application. The introduction of biomass materials can improve the performance of WPU. Keratin belongs to natural macro-molecule compounds which contain numerous peptide bonds and hydrophilic groups. It can make up for the shortcomings of WPU materials. In this study, keratin was extracted from the recycled cattle hair waste, and then it was chemically bonded with isocyanates, polyols, and other raw materials to prepare a keratin-modified WPU film which has excellent performance. First, polytetramethylene ether glycol (PTMEG) was reacted with 4.4-dicyclohexylmethane diisocyanate (HMDI) to form a prepolymer, then it was reacted with dimethylol propionic acid (DMPA), neopentyl glycol (NPG) and trimethylolpropane (TMP). The addition of keratin was in the emulsification process, and the structure of keratin-modified WPU was investigated. TG analysis results showed that the addition of keratin can improve the thermal stability of the WPU film with a higher residual carbon content at 600°C. The DMA analysis showed that the mechanical properties of the modified WPU film with a certain amount of keratin added (≤ 0.1%) were significantly improved. The yellowing resistance test showed that the addition of an appropriate amount (≤ 0.1%) of keratin can increase its stability to light, but the addition of an excessive amount of protein (≥ 0.2%) will result in a decrease in the yellowing resistance of the WPU film.