Effect Of Lignin Structure/Aggregation State Modulation On Efficient UV Protection And Its Application

At present,the UV shielding active substances used in commercial sunscreens are mainly inorganic UV sunscreens（such as titanium dioxide,zinc oxide）and organic UV sunscreens,but the above UV shielding active substances have problems such as poor photostability,harmful to the environment,and can produce harmful substances.Lignin is the second most abundant natural polymer in the world.It contains phenolic hydroxyl groups,ketone carbonyl groups and carboxyl groups,which make lignin a shielding absorber for the whole wavelength of UV.However,there are some problems with lignin as a UV shielding active substance,the Sun protection factors（SPF）of untreated lignin are too low to meet the needs of most consumers,and the structure of industrial lignin is destroyed after the pulping process,in which a large number of chromogenic groups and condensation structures are produced,making the lignin color darkening.In this study,we used industrial alkali lignin（AL）,which is non-biotoxic,chemically stable,environmentally friendly and degradable,as raw material,and used different organic solvents/water systems as solvents to purify alkali lignin,aiming to remove lignin impurities（such as carbohydrates and high molecular weight parts with severe molecular agglomeration）,reduce the molecular weight of lignin,and at the same time increase the content of lignin phenol hydroxyl group content.Then,the purified lignin from the organic solvent/water system was chemically modified by demethylation and acetylation treatment to further increase the phenolic hydroxyl functional group content in lignin,improve the UV shielding properties of lignin,and improve the appearance color of lignin without substantially affecting the UV shielding properties of lignin.Finally,lignin was homogeneously mixed with commercial creams to prepare lignin-based sunscreen samples,and the UV shielding properties,light stability properties and antibacterial properties of the samples were investigated and analyzed to explore the application prospects of lignin in the cosmetic field.The main findings of this thesis are as follows:1.Three organic solvents,methanol,ethanol and acetone,were used to separate and extract alkaline lignin with aqueous solution.The results showed that the color and UV shielding performance of AL soluble fraction was significantly better than the original and insoluble fractions,and the lignin color brightness of soluble fraction was increased by more than 20%,and the color difference with human skin was reduced to less than 5.After treatment with organic solvent/water system,the microstructures of soluble lignin obtained from three 75%concentration organic solvent systems separated showed nano-microsphere structures.The organic solvent classification could remove insoluble substances in alkaline lignin,such as small amounts of carbohydrates and high molecular weight lignin polymers,and the relative molecular masses of soluble lignin were all smaller than that of raw lignin.When the concentration of organic solvent was 50%and 75%,the phenolic hydroxyl content of the isolated soluble lignin was increased,among which,the UV absorption performance of soluble lignin was best for 75%acetone/water mixture.2.The UV shielding performance of lignin was improved by oxidizing the methoxy on the benzene ring of lignin to phenolic hydroxyl group using the Fenton reaction of hydrogen peroxide and ferrous ions with lignin.The optimal reaction conditions were obtained by controlling the reaction time and oxide concentration.The results showed that after the demethylation reaction,the phenolic hydroxyl content of lignin samples showed a trend of increasing and then decreasing,and the phenolic hydroxyl content reached the highest after 4h of the reaction.The corresponding phenolic hydroxyl content of lignin dissolved in methanol/water system and acetone/water system after demethylation was 4.35(DAL_{Me OH})and4.34(DAL_Acetone)mmol/g,respectively.while after acetylation,the content of guaiacyl phenolic hydroxyl in lignin samples was significantly reduced,and the degree of acetylation was about70%;compared with the lignin raw material,the demethylation The color brightness values of lignin after the reaction were all significantly decreased by 5-15%,and the brightness values of the acetylated lignin samples were significantly increased by 40-60%compared with those before the reaction;SEM images showed that even after the demethylation reaction and acetylation reaction,lignin still showed a nano-microsphere structure;after the demethylation reaction,the UV absorption properties of lignin were improved,and then after After the acetylation treatment,the UV absorption ability of lignin decreased,but it was still higher than that of the original lignin sample,in which the UV absorption performance of lignin treated with acetone/water system was the highest.3.The SPF values of the corresponding composite sunscreen showed a trend of increasing and then decreasing after the graded treatment with organic solvents,and the SPF value of soluble lignin reached the highest when the concentration of organic solvents was 75%.Among the three organic solvents soluble lignin,AL_Acetone exhibited the highest SPF value,followed by AL_{Me OH},and lignin grade dissolved in ethanol/water system(AL_{Et OH})had the lowest SPF,but both were higher than the pristine alkaline lignin,where the SPF values of AL_{Me OH} and AL_Acetone were 5.63 and 6.63.Compared with the pristine lignin,the addition of 5 wt%acetone/water system-extracted lignin gave the highest SPF values for the composite sunscreens,which were about 4.5-fold higher,along with acceptable cosmetic color.The UV absorption properties of the lignin samples were improved after the demethylation reaction,whose optimal reaction time was 4 h.The SPF values were all significantly increased,with DAL_Acetone having an SPF value of 11.23,which was 69.4%higher relative to the pre-reaction period.After the acetylation treatment,the UV absorption capacity of the lignin samples decreased,and the SPF value decreased by less than 10%,but it was still higher than that of the original lignin samples.Meanwhile,the lignin samples also had good UV aging resistance.After 3 h UV lamp irradiation,the SPF values of the demethylated lignin samples decreased by about 13%,while the SPF values of the acetylated demethylated lignin samples did not decrease.In addition,the lignin samples had bacterial inhibitory properties,and the inhibitory strength was:DAL_Acetone>AL_Acetone>ADAL_Acetone>AL.The phenolic hydroxyl content of the lignin samples after acetylation decreased,leading to a decrease in the bacterial inhibitory properties,and the inhibition The inhibition rate was only 34.38%.In summary,the graded treatment of lignin with organic solvents effectively increased the phenolic hydroxyl content of lignin,which led to the improvement of UV shielding performance and the obvious lightening of lignin appearance;the oxidation of methoxy on the benzene ring of lignin to hydroxyl group improved the UV shielding performance of lignin;in addition,the acetylation reaction could substantially increase the brightness value of lignin samples,and the lignin UV shielding material had good In addition,the acetylation reaction can substantially improve the brightness value of lignin samples,and the lignin UV shielding materials have good light stability and antibacterial properties.The research in this thesis can provide a theoretical basis and methodological reference for the development of green high performance UV shielding materials,indicating the potential value of industrial lignin in value-added applications such as UV protection and cosmetics.