(411b) A Novel and Green Approach in Engineering Transparent and Homogenous Cellulose Nanocrystal-Lignin UV Protection Films

In the context of valorization of lignin produced from the pulp and paper industry, a simple approach to prepare the lignin and cellulose nanocrystals (CNC) based transparent and homogenous UV protection films is developed. The results demonstrated for the first time that CNC aqueous suspensions with and without containing lignin could be tuned through the addition of sodium hydroxide (NaOH) to produce transparent and homogenous films. Initially, CNC films were optimized for improving their transparency using various NaOH additions. It was observed that the addition of NaOH in the range of 3 to 4 wt.% resulted in the CNC films with maximum transparency. Moreover, NaOH addition enhanced the homogeneity of the films by uniformly dispersing lignin in the films. CNC/lignin films were prepared by using optimized CNC suspensions with lignin concentrations in the range 1 to 10 wt.%. CNC/AL (alkaline lignin) and CNC/SKL (softwood kraft lignin) with 10 wt.% lignin concentration provided complete UV blocking. The UV protection behavior of these films was stable under UV irradiation. Cross polarized optical microscopy and scanning electron microscopic images of films showed some degree of global alignment of CNC rods upon addition of NaOH, which remained unaffected by lignin addition. Acetylation of lignin reduced the lignin color with only slight reduction in their extinction coefficient. Incorporation of 10 wt.% Ac-SKL into CNC films increased visible light transmittance at 550 nm by 67% without significantly affecting the UV blocking property compared to 10 wt.% CNC/SKL films. Presence of lignin also provided the thermal and contact angle stability. 10 wt.% SKL addition increased the maximum weight loss temperature (T₁)of CNC by 5 °C, while 10 wt.% Ac-SKL, being more stable, increased T₁ by 20 °C. Due to the relatively more hydrophobic nature of lignin compared to CNC, contact angle of CNC/lignin films remained stable over time compared to CNC films; however, initial contact angle was unaffected by the presence of lignin. With further optimization in terms of lignin color reduction and increasing the CNC alignment, these CNC/lignin nanocomposites could be a potential biodegradable, low cost alternative as a coating/film material with UV blocking and optical polarization functionality for sunglasses, automobile windshields, home windows, contact lenses and UV sensitive polymers.