Modification Of Lignin And Its Application On Production Of Functional Film Materials

Due to the increasing consumption of fossil resources and environmental pollution,the development of biodegradable bio-based materials has become the key to maintaining the sustainable development of society.Lignin is the most abundant natural polymer on earth except for cellulose.Being a renewable natural resource,it has the advantages of low price,wide availability,and biodegradability.Polylactic acid(PLA)and polyvinyl alcohol(PVA)are two nontoxic,biodegradable polymers commonly used for the preparation of packaging materials.Adding lignin to polymers such as PLA and PVA to prepare composite materials can not only reduce the cost but also introduce other functions such as UV absorption to the composites.However,the poor dispersion of lignin in PLA and PVA limits its efficient application.With this regarding,organosolv lignin was modified to improve its compatibility with PLA,and lignin/PLA composite films were prepared;Similarly,alkali lignin was modified by hydroxymethylation and lignin/PVA composite films were prepared using cross-linking agents.The mechanical properties and UV absorption properties of the two composite films were investigated.The contents of this thesis and some important results are as follows:1.Composite films with UV absorption properties were prepared by modifying the organosolv lignin with oxyalkylation and blending with PLA.To improve the reactivity of organosolv lignin and L-lactide,the organosolv lignin was firstly modified by oxyalkylation using propylene carbonate,and then the oxyalkylated organosolv lignin(OOL)was polymerized with L-lactide by ring-opening to prepare oxyalkylated organosolv lignin-g-PLA copolymer(OOLPC).Finally,the copolymer was blended with PLA and the lignin/PLA composite film(OOLPCB)was prepared using the solvent casting method.31 P NMR characterization of the oxyalkylated organosolv lignin showed that the number of aliphatic hydroxyl groups in the lignin increased from 3.56 mmol·g-1to 3.92 mmol·g-1 and the number of phenolic hydroxyl groups decreased f from 4.73 mmol·g-1 to0.16 mmol·g-1 after modification.The results of GPC characterization of the resulting graft copolymers showed that the molecular weight of the copolymers increased with the increase of lignin mass fraction.The SEM was used to characterize the surface morphology of the composite film,and the results showed that the copolymer had good compatibility with PLA.When the loading of OOL in OOLPC was 5.0 %,the film had an increase in elongation at a break of 7.43%and a decrease in tensile strength of 12.03%.The effect of lignin addition on the UV absorption performance of the composite film was compared,and the results showed that the addition of lignin improved the UV absorption performance of the lignin/PLA composite film.2.Flexible lignin/PVA composite films were prepared with hydroxymethylated lignin.The alkali lignin was firstly modified by hydroxymethylation,and the effects of reaction temperature and mass ratio of alkali lignin/formaldehyde on the modification effect were investigated.Optimum reaction condition was at a reaction temperature of 80 °C and a mass ratio of 1:1 of alkali lignin/formaldehyde.The resulting hydroxymethylated alkali lignin had the highest amount of hydroxyl groups of 1.39 mmol·g-1.The hydroxymethylated alkali lignin/PVA composite films were prepared by solvent casting method.The effects of three cross-linking agents(formaldehyde,glutaraldehyde,and epichlorohydrin)on the mechanical properties of the composite films were investigated,and the results showed that epichlorohydrin as a cross-linking agent had a large improvement in the mechanical properties of the composite films.Compared with those of the uncross-linked composite films without cross-linking,the tensile strength and elongation at break of the resulting composite films were increased to 38.25 MPa and 686.12 %,respectively.SEM was used to characterize the surface morphology of the composite film.Smooth surface of the composite film with a cross-linker was observed compared to those without cross-linker.The three different crosslinkers were compared for their effects on the water absorption and degradation properties of the composite films,and the results showed that the composite films cross-linked with formaldehyde and glutaraldehyde were easily decomposed by water,while the composite films cross-linked with epichlorohydrin had better water resistance and more stable structure.In addition,the UV absorption rate of the lignin/PVA composite film could reach 100 % by adding 5wt% modified lignin.