Research On Green Preparation Of Lignin Nanoparticles And Their Application In Active Packaging

The use of natural bio-based polymers for the preparation of composite materials has attracted increasing interests in the field of food packaging due to the growing demand for sustainable development and health.Lignin exhibits excellent properties of good biocompatibility,anti-UV,anti-oxidation and thermal stability,making it a promising functional additive for food packaging composite films.However,the heterogeneity of the lignin structure,including its complex chemical structure and wide molecular weight distribution,lead to heterogeneity in the dispersion of lignin in polymer matrices and lower performance,which greatly limited its value-added applications.In this study,we developed a“lignin-first” strategy and obtained ethanolic lignin(OEL)from wheat straw using an oxygenenhanced ethanol-water system under mild conditions and characterized the molecular weight and chemical structure of OEL.Subsequently,tailed lignin nanoparticles(T-LNP)with novel morphology and long-term stability were prepared from OEL by a combination of fractionation treatment and solvent-antisolvent self-assembly.Finally,composite films were successfully prepared by compounding T-LNP with polyvinyl alcohol(PVA)and chitosan(CS),which exhibited excellent UV shielding and free radical scavenging abilities and had promising applications in food packaging materials.(1)Based on the “Lignin-first” strategy,the oxygen-enhanced ethanol delignification processes under mild conditions were comprehensively investigated.Results showed that lignin yield could achieve about 52.3 % under the optimum process conditions of ethanol concentration 80 %,temperature 90 ℃,liquid to wheat straw ratio 25:1 for powdery-scale wheat straw,which was 65.0 % higher than that for rod-scale wheat straw under the same conditions.The lignin structural and carbohydrate component results demonstrated the employment of oxygen induced great quantities of lignin dissolving out on the premise of little carbohydrate component(<1 %)and lignin structural(mainly β-O-4 units)changes.Moreover,based on the molecular weight and polydispersity comparison results,the dissolved oxygen could transfer homogeneously in mild organosolv system and resulted in lignin degradation uniformly.Besides,the employment of oxygen assisted in not only extending the massive lignin removal stage to 30 min and 50 min for P-OEEL and R-OEEL respectively,but also boost the delignification rate with comparison to P-EL and R-EL.Lastly,the economic calculations showed that the cost for lignin production were about 1.58 USD/g lignin from powdery-scale wheat straw,providing a competitive route for high-value utilize waste biomass.(2)Some studies had shown that lignin nanoparticles had potential applications in catalysis,energy storage,biomedicine and life health,etc.However,the toxic chemicals used in part of the preparation process limited its large-scale application.In this paper,P-OEEL was used as raw material,ethanol and water were used as solvent and antisolvent,and lignin nanoparticles were prepared by self-assembly.On this basis,T-LNP was prepared by introducing a new functional additive,syringaldehyde(SA).The results of scanning electron microscopy(SEM)showed that the morphology of T-LNP could be regulated by controlling the amount of SA.The results of dynamic light scattering(DSC)showed that the ζ potential increased with increasing SA concentration,which provided more negative charge for T-LNP.The red shift of Fourier infrared spectra(FTIR)showed that the main driving force of the selfassembly process was π-π interaction.The particle size of T-LNP could be controlled by adjusting the drop acceleration of the antisolvent,the stirring speed of the magnetic agitator and the final water content.Finally,T-LNP demonstrated stability in storage at different p H ranges(4～12)and temperatures(20～60 ℃),as well as long-term stability at room temperature.(3)Binary and ternary composite films were prepared by solvent casting method using TLNP,PVA and CS as raw materials,and their optical properties,mechanical properties,thermal properties,oxidation resistance and antibacterial activity were investigated.The optical properties showed that the addition of 3wt % T-LNP could achieve complete shielding against UVA and UVB and maintain good visible light transmission.When 3wt % T-LNP was added into the PVA system,the tensile strength of the composite film increased from 36.9 MPa to59.5 MPa,and the elongation at break reached 128.3 %,which was 64.9 % higher than that of pure PVA(77.8 %).The introduction of T-LNP into the CS system did not improve the tensile strength,but also toughened the CS composite film to some extent.In addition,the addition of3 wt % T-LNP significantly improved the thermal stability and hydrophobic properties of binary and ternary nanocomposite systems.The overall migration activity test showed that the maximum migration level of the composite film was 22.9 mg/kg simulator.DPPH scavenging test showed that adding 3wt % T-LNP could reduce DPPH radical to yellow compound diphenyl hydrazine,and the scavenging effect was significant(89.0 %).The antibacterial performance of the composite film increased with the extension of time,and reached about 99 %after 12 h,which could inhibit the growth of Escherichia coli.Finally,T-LNP and CS showed synergistic effects in the antioxidant reaction of thin films and had the potential to be used in biomedical applications(drug delivery,tissue engineering,wound healing).