Thermally-induced cellulose nanofibril films with near-complete ultraviolet-blocking and improved water resistance

•Transparent UV shielding TOCN films were prepared via simple thermal treatment.•The yellow discoloration led to an excellent UV blocking property.•The UV shielding was persistent when exposed to UV irradiation.•The hornification are advantageous for improving water-resistance. In recent years, ultraviolet (UV) protection films made from cellulose nanofibril (CNF) have drawn significant attention, owing to its high transparency, high mechanical strength and relatively high thermostability. Generally, CNF films had poor UV-shielding performance, and required UV absorbent to enhance their UV-shielding ability. Herein, a simple thermal treatment is proposed to directly improve the UV blocking properties of CNF films without incorporating UV absorbent. After thermal treatment at 160 °C, the CNF films exhibited a near-complete UV blocking ability. In particular, they exhibited full absorption of ultraviolet A (UVA) and ultraviolet B (UVB), and also showed a high visible light transmittance of 72%. In addition, the UV-shielding films performed stable UV-blocking when exposed to UV irradiation. Simultaneously, the hornification induced by thermal treatment endowed an improved hydrophobicity for CNF films. However, the tensile strength of the CNF films decreased from 133 MPa to 81 MPa after thermal treatment at 160 °C.