Waste-Hair-Derived Natural Melanin/TiO₂ Hybrids as Highly Efficient and Stable UV-Shielding Fillers for Polyurethane Films

Melanin is a ubiquitous natural pigment that protects living organisms from intense sunlight. Although synthetic melanin with different sizes and morphologies has been developed for UV-shielding polymers, studies can rarely be found regarding their natural counterparts. In this study, the natural melanin (NM) granules were isolated from conventional textile scraps such as yak and alpaca hair wastes and well characterized. This was followed by integrating them with TiO₂ nanoparticles through hydrogen bonding using a simple physical mixing method, and the resultant NM/TiO₂ hybrids were embedded in polyurethane (PU) films. Our results suggested that the PU films containing 2.5 wt % NM/TiO₂ hybrids possessed highly efficient UV-shielding ability, improved photostability, and well-maintained transparency. The ultraviolet protection factor (UPF) values of the 2.5 wt %-NM/TiO₂/PU composite films were approximately four times higher than those of the 2.5 wt %-TiO₂/PU composite films. Moreover, although the 2.5 wt %-NM/TiO₂/PU and 1.25 wt %-NM/PU composite films shared a similar UPF value, the transmittance at 550 nm of 2.5 wt %-NM/TiO₂/PU was ∼2.5 times higher. Most importantly, the photoinduced chemiluminescence tests proved the excellent photostability of the NM/TiO₂/PU composite films. Owing to the abundant natural source of NM, the commercial availability of TiO₂, as well as the simple preparation protocol of the NM/TiO₂ hybrids, this work highlights the potential of hair-derived NM-based UV-shielding fillers for various polymeric materials.