Abstract: This study demonstrated a green functionalization process of a cellulose substrate by combining the chitosan treatment and gamma radiation. To impart the UV protection characteristics in the cellulosic structure, natural chromophores derived from Banana floral stem (BFS) was grafted in the functionalized cellulose surface. Phytochemical screening was performed to confirm the types of UV protective natural chromophores (UVPNCs) presented in BFS. The result exhibited the presence of condensed tannin, flavonoids, anthocyanin, betacyanin, and anthraquinone as the major UVPNC components in BFS. The optimum conditions for maximum sorption of UVPNCs into the functionalized cellulose matrix were recorded at 80 °C for 60 min. The cationic biopolymer chitosan (2 g/L), different gamma absorbs doses (2, 4, and 6 kGy), and combined chitosan and gamma treatment into cellulose resulted around 15–23%, 44%, and 41–64% improved absorption of UVPNCs, respectively, as demonstrated by the change in color strength (K/S) compared to unmodified cellulose matrix. The concurrent treatments greatly improved the total crystallinity index (TCI), hydrogen bond intensity (HBI), hydrogen bonding energy (EH), hydrogen bonding distance (R) and asymmetric factor (AF) from 1.358 to 1.363, 0.971 to 0.988, 27.15 kJ–27.40 kJ (at 3274 cm−1), 2.766 A to 2.764 A (at 3274 cm−1) and 0.47 to 0.10, respectively. For gamma treatment, 6 kGy irradiation dose provided the best result for improved molecular orientation of cellulose and in combination of chitosan the substrate attained a maximum K/S of 1.98 after UVPNCs grafting. It was found that the UV protection factor (UPF) rating has a linear relationship with the UVPNCs absorption (K/S) and a maximum four-fold increase in UPF (165–506) was evident. The excellent bonding durability of the UVPNCs grafted samples was further ensured in terms of several colorfastness properties. This sustainable functionalization of cellulose with high UPF offers a great promise for applications in health care and photodegradation protection.
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