Reinforcement mechanisms of lignin nanoparticles in biodegradable cellulosic films for plastic replacements

Lignocellulose and its nanomaterials have received extensive interests for fabrication of biocomposite films due to their renewability, sustainability, and biodegradability. This study aimed to elucidate reinforcement mechanisms of lignin nanoparticle (LNPs) for cellulosic films. LNPs with varying particle sizes and chemistry were prepared in aprotic solvent with ultrasonication. Behaviors of LNPs incorporating into the matrix of cellulose nanofibers (CNFs), including surfaces and voids, were studied. Upon hot press, the LNPs melted and fully filled the voids of CNF matrix to form a dense, cross-linked structure where CNFs imparted tensile strength, and LNP that coated CNF imparted rigidity and hydrophobicity. By tuning LNP characteristics for optimal self-bonding and cross-linking, the resulting biocomposite film exhibited an outstanding tensile strength of 202 MPa, Young's modulus of 9.55 GPa, maximum degradation temperature of 375.1 °C, steady contact angle of ∼70°, and 100% UV blocking due to the effects of hydrogen bonding between CNFs and LNPs and self-bonding of LNPs. These findings indicated the great potential of the biocomposite film as an alternative to non-biodegradable plastics. LNPs can be used as a reinforcement agent to tailor the CNF network mechanically and chemically, leading to multifunctional biocomposite films as plastic replacements.