Carbon Nitride Grafting Modification of Poly(lactic acid) to Maximize UV Protection and Mechanical Properties for Packaging Applications

Due to their biobased nature and biodegradability, poly(lactic acid) (PLA) rich blends are promising for processing in the packaging industry. However, pure PLA is brittle and UV transparent, which limits its application, so the exploration of nanocomposites with improved interfacial interactions and UV absorbing properties is worthwhile. We therefore developed and optimized synthesis routes for well-designed nanocomposites based on a PLA matrix and graphitic carbon nitride (g-C₃N₄; CN) nanofillers. To enhance the interfacial interaction with the PLA matrix, a silane-coupling agent (γ-methacryloxypropyl trimethoxysilane, KH570) is chemically grafted onto the CN surface after controlled oxidation with nitric acid and hydrogen peroxide. Interestingly, only 1 wt % of CNO-KH570, as synthesized under mild conditions, is needed to significantly improve the UV absorption, blocking even a large part of both UV-C, UV-B, and UV-A outperforming the UV absorption performance of PLA and, for instance, polyethylene terephthalate (PET). The low nanofiller loading of 1 wt % also results in a higher ductility with an increase in elongation at break (+73%), maintaining the tensile modulus. The results on a joint optimization of UV protection and mechanical properties are supported by a broad range of experimental characterizations, including FTIR, XRD, DSC, DSEM, FETEM, XPS, FTIR, TGA, and BET N₂ adsorption-desorption analysis.