Biorefining of rice husks into sustainable nanoparticles for reinforced polyvinyl alcohol composites

Rice husk is an attractive renewable feedstock for producing both fermentable sugars and functional materials. In this study, lignin nanoparticles (LNPs), silica nanoparticles (SiNPs), and lignin-silica hybrid nanoparticles (LSiNPs) were prepared from rice husks using acid-catalyzed glycerol pretreatment, in addition to sugar production. The resulting nanoparticles were evaluated as sustainable nanofillers for polyvinyl alcohol (PVA) composites. The results showed that adding SiNPs into PVA at a loading of 1.0 wt% enhanced tensile strength by 17.7%, elasticity by 43%, and temperature resistance to degradation by 10 °C. In contrast, adding LNPs and LSiNPs resulted in only marginal improvements in these composite properties. However, by employing a biofiller modification strategy using 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTMAC) as an intermolecular crosslinker in LNPs and LSiNPs, the tensile strength and elasticity of the PVA composites were significantly improved. For example, adding CHPTMAC-modified LNPs and LSiNPs into PVA films significantly improved tensile strengths by 24.2% and 26.4%, and substantially enhanced elasticities by 69.0% and 52.8%, respectively, compared to the pristine PVA film at the same non-modified filler loading. Further, the addition of 1% CHPTMAC-modified LNPs blocked more than 95% of UV light while the film remained transparent. These metrics were selected to reflect key performance requirements for packaging and protective films, namely mechanical integrity, UV shielding with visible transparency, thermal resistance margin, and reduced surface wettability under humid exposure. Overall, this study demonstrated that multiple sustainable nanofillers can be produced from renewable, low-cost rice husks to manufacture PVA composite films with various enhanced properties, suitable for a broad range of applications.