Flexible, Transparent, UV-Protecting, Water-Resistant Nanocomposite Films Based on Polyvinyl Alcohol and Kraft Lignin-Grafted Cellulose Nanofibers

Reinforcing and UV-absorbing kraft lignin-grafted TEMPO cellulose nanofibers (TKL) were synthesized using amidation of paper mill sludge-derived TEMPO-oxidized cellulose nanofibers (TCNFs) and aminated kraft lignin (AKL). Furthermore, modification of TKL was carried out to reduce colored chromophores of lignin using hydrogen peroxide treatment to obtain peroxide-bleached TKL (PBTKL100 and PBTKL400). These modified CNFs (TKL, PBTKL100, and PBTKL400) were introduced into the polyvinyl alcohol (PVA) matrix at various concentrations using aqueous solution processing and casting to obtain PVA nanocomposites. Compared to PVA films, the tensile strength and Young's modulus of the nanocomposite films were improved up to 5% modified CNF concentration. Maximum improvement in these properties (22 and 28%, respectively) was obtained for PBTKL400. The PBTKL400 addition also imparted UV-blocking properties to the PVA matrix by maintaining its high transparency. It was observed that for a nanocomposite containing 5% addition of PBTKL400, more than 92% of pure PVA transparency was retained with complete UV B screening and 88% of UV A screening. Also, the thermal properties such as glass transition temperature (Tg) and maximum weight loss temperature (Td) of PVA were increased by 4 and 9 °C, respectively. The modified CNF could prevent lignin leaching when thermally treated PVA nanocomposite films were immersed in water, improving their applicability in aqueous environment applications. This study presented a new approach to produce fully biobased, multifunctional CNFs suitable for applications requiring strong, flexible, transparent, UV-blocking packaging materials.