Property Of Degradable Polymer Films Reinforced With Modified Lignin Nanoparticles

Poly(vinyl alcohol)(PVA)and poly(lactic acid)(PLA)are two kinds of full degradable eco-friendly polymers,however,both of them have some disadvantages that restrict the applications.PVA is very sensitive to humidity and has poor resistance against water.PLA shows high brittleness and weak tenacity.Small and spherical lignin nanoparticles contribute to higher ultraviolet absorbance and antioxidant properties than micro-lignin.Thus,lignin nanoparticles prepared by acid hydrolysis were blended with polymer matrix to improve disadvantages of the neat polymer.Subsquently,surface modification of lignin nanoparticles was carried out by three different procedures to improve the compatibility with matrix.Acetylated lignin nanoparticles and citric acid modified lignin nanoparticles were simultaneously blended with PLA to prepare the ternary system,structure and properties of the ternary films were characterized.In addition,hydrophobic silica nanosphere were applied for modifying lignin nanoparticles.The methodology for preparing packaging films by manufacturing modified lignin nanoparticles as reinforcing nanofillers has been preliminarily established.The main research contents and main results are as follows:The immiscibility and incompatibility between micro lignin and PVA or PLA matrix resulted poor mechanical properties which limits the high value-added utilization of lignin.Thus,spherical lignin nanoparticles were synthesised by acid hydrolysis at various pH value(HC1 with pH 4.6 and 2.5,H2SO4 with pH 4.7 and 2.9,and H3PO4 with pH 3.3 and 2.6).The size of lignin nanoparticles prepared by different pH are as follows:32.8 nm(HC1 pH 2.5),58.9 nm(H2SO4 pH 2.9),54.1 nm(H3PO4 pH 2.6).Lignin nanoparticles obtained at HC1(pH 2.5)(64.5%)show higher free radical scavenging ability than pristine alkali lignin(47.5%).The suspension of lignin nanoparticles is extremely stable in water and can be stored for more than one year in the fridge.The obtained lignin nanoparticles have lower molecular weight and higher polydispersity than the pristine alkali lignin.The as-prepared lignin nanoparticles effectively inhibit the growth of tested Gram-negative bacterium.The antimicrobial ability may also correlate to the antioxidant property of phenolic hydroxyl groups.The lignin polyphenols can cause damage to the cell wall by lysis effect,resulting in the leakage of internal fluid.Meanwhile,some monophenolic compounds originated from lignin can penetrate and induce oxidative stress,stimulate the adenosine triphosphate of(ATP)thereby depleting ATP,resulating in the death of bacteria.The compatibility between lignin nanoparticles and PVA matrix would be weakened by adding more than 3%of lignin nanoparticles,which in turn affects the mechanical properties and water resistance of the composites.Therefore,citric acid was used for modifying lignin nanoparticles and changing the polarity,aiming at improving its compatibility with the polymer and prepare high performance packaging films.Citric acid is crosslinked with lignin nanoparticles mainly by etherification but also limited extent of esterification.The size of lignin nanoparticles before and after modification is 60 nm and 80 nm,respectively.Solubility of the modified lignin nanoparticles in methanol increases,but the visual appearance looks more turbid.Therefore,the polarity transformation of lignin nanoparticles was evaluated by further blending with two different polymer matrix.Results show that the polarity of the lignin nanoparticles was reduced,it becomes more hydrophobic,and the modified lignin nanoparticles/PVA composite films exhibit higher hydrophobicity and better antioxidant ability than unmodified films.Antimicrobial property of the films exhibit positive inhibition within 12 hours of bacteria static testing;citric acid modified lignin nanoparticles/PVA composite films meets the requirements for food direct contact plastics,and the prepared films have potential to be used in packaging application;Compared to neat PVA film,citric acid modified lignin nanoparticles/PVA nanocomposite films show higher young’s modulus,the tensile strength and elongation at break of modified nanocomposite films decreases with increasing the addition of modified lignin nanoparticles.Despite of the fact that citric acid modified nanolignin improves antioxidant property of the PVA-blended films,while compromises the toughness of the films,acetylated lignin nanoparticles was prepared and simultaneously blended with citric acid modified lignin nanoparticles and PLA to prepare binary and ternary films,aiming at overcoming brittleness of citric acid modified lignin nanoparticles that leading in the blended films.The results suggest that films blended with PLA and acetylated lignin nanoparticles are provided with poor antioxidant property and UV-blocking properties,the ternary nanocomposite films show satisfactory effect on UV protection and antioxidant properties compared to acetylated lignin nanoparticles blended films.Meanwhile,ternary nanocomposite films with an maximum reduction,56%and 63%respectively in oxygen transmission rate and water vapor permeability compared to neat PLA film.Nanofillers can slightly inhibit disintegration rate within 10 days of composting test.Nevertheless,all the films exhibit more than 90%of weight loss after 15 days of disintegration,indicating the biodegradable property of all the nanocomposite films.The ternary composite films overcome the brittleness of the citric acid modified lignin nanoparticles/PLA films and compensate for the low antioxidant property of the acetylated modified lignin nanoparticles/PLA films.The ternary composite films was used to exert the advantages of both nanofillers and synergistically promoted the toughness of the films,while no significant change occurred on the tensile strength and the elastic modulus of nanocomposite films.Therefore,new methods for improving the mechanical properties of PLA films was developed in the next experimental section.Modified silica nanosphere was grafted onto the surface and give more hydrophobic groups to lignin nanoparticles.Thus,better interfacial compatibility between modified nanofillers and PLA matrix and mechanical properties was desired.Results suggest that covalent bonding between silica nanosphere and lignin nanoparticles was formed.Silica nanosphere,modified lignin nanoparticles resulted in more transparency of nanocomposite films than unmodified lignin nanoparticles.While modified lignin nanoparticels blended nanocomposite films show better UV blocking property than neat PLA film,inferior to unmodified lignin nanocomposite films.Compared to neat PLA films,the addition of hydrophobic modified lignin nanoparticles resulted in improved tensile strength and elastic modulus of films,suggesting high rigidity of the modified nanocomposite films.Nevertheless,modified nanocomposite films lost the majority of antioxidant property due to entrappment of silica on the surface of lignin nanoparticles and blocking of phenolic hydyoxyl groups,new complementary substances is desired when photosensitivity packaging is necessary.In conclusion,three modification procedures upon lignin nanoparticles exhibit individually different advantages.Citric acid modified lignin nanoparticles imparts higher antioxidant properties to the films,acetylated lignin nanoparticles imparts better toughness to the films,and films from silica nanosphere grafted lignin nanoparticles show higher tensile strength and tensile modulus.Antioxidant property,UV protection,water resistance and mechanical properties are all very important sectors for the films used for packaging application.This study gives a variety of ideas and basis for application of lignin nanoparticles to the packaging field.