Enhancing the Performance of Biodegradable Lignin Nanoparticle/PVA Composite Films via Phenolation Pretreatment of Lignin Using a Novel Ternary Deep Eutectic Solvent

As an environment-friendly biodegradable material, poly (vinyl alcohol) (PVA) has been focused on improving performance and expanding its applications. In this study, improved performance lignin nanoparticle/PVA composite film was prepared by phenolation of bagasse lignin (BL) using a novel ternary deep eutectic solvent (DES). The effects of introduction of DES-phenolated lignin (DL) nanoparticles with different additions (1, 3, 5, 10 wt%) on the properties of DL/PVA composite film were comprehensively studied by mechanical performance test, UV-shielding performance test, contact angle measurement, thermogravimetric analyses and DPPH free radical scavenging activity. The experimental results indicated that lignin nanoparticles (LNPs) were homogeneously distributed in a biodegradable PVA matrix due to hydrogen bonds between the PVA matrix and lignin nanoparticles. With the introduction of DES pretreatment on native bagasse lignin, the various performances of DL/PVA composite films, such as tensile strength, surface hydrophobicity, UV-shielding and thermal stability, were enhanced in comparison with both pure PVA film and BL/PVA composite film incorporated with DES-untreated BL. The tensile strength of DL/PVA composite film with 3 wt% addition increased to 97.79 MPa from 69.41 MPa for pure PVA film, and the water contact angle increased from 43.7° to 84.2°. DL/PVA composite film with 10 wt% addition shielded 95.8% of the UV spectrum (400–200 nm). Moreover, after incorporating the DL nanoparticles into the PVA matrix, the as-obtained DL/PVA composite films displayed good antioxidant activity by eliminating most of the DPPH free radicals. With 10 wt% addition of DL nanoparticles, the DPPH radical scavenging activity of DL/PVA composite film increased by about 76% compared with pure PVA film. These enhanced properties were attributed to the more phenolic hydroxyl groups of DL nanoparticles than of BL and the hydrogen-bonding interactions. In conclusion, the DES-phenolation pretreatment of lignin clearly improved the properties of PVA composite films. Furthermore, as both lignin and PVA are biodegradable, the lignin nanoparticle/PVA composite film may be a promising candidate for fully biodegradable robust coating materials with vital potential applications, such as UV-shielding and food packaging, etc.