Lignin nanoparticles (LNP) was isolated from wheat straw using metal chloride-catalyzed maleic acid pretreatment in this study. With the addition of metal chlorides, LNP with small particle size, high carboxyl content (4.83 mmol/g), good dispersion and containing different metal ions were prepared. Subsequently, LNP was added as reinforcing agents to CNF, and the composite films were successfully prepared by vacuum filtration method. The surface morphology, optical property, surface chromaticity value and mechanical property of pure CNF and composite films are compared and analyzed. The results show that the composite films could achieve more than 95% UVA shielding ratio and 99% UVB shielding ratio when LNP content is only 3wt%. Meanwhile, the addition of LNP significantly increase the tensile strength of composite film (up to 188.5 MPa). The interaction forces between CNF and different LNPs (276~ 406 nN) are all higher than those between CNFs (202 nN) as measured using AFM, which is consistent with the improvement in tensile strength of the composite films. In summary, LNP containing metal ions are introduced into CNF films to construct super-strong network with metal ion cross-linking and hydrogen bonding interactions, which provides a new idea for the development of lignin-reinforced cellulose-based film materials.
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