Fabrication Of Lignin-based Micelles And Investigation Of Their Excellent UV-blocking Applications

Lignin from plants contains a large number of benzene rings and conjugated structures such as carbonyls and double bonds and these fuctional stuctures can absorb the ultraviolet lights from sunshine effectively.The widely existed phenol hydroxyl groups in lignin can also scavenge free radicals.Both properties endow lignin excellent UV-blocking and antioxidant performance.The structure of lignin that separated from industrial process changes significantly and the molecular weight decreases rapidly,but industrial lignin still retains the functional structure and the amphiphilities turn better.However,the random structure and disordered aggregation cause the uneven surface polarity of industrial lignin,which makes lignin has poor compatibility with most of the materials and limits its large scale applications.In this study,both hydrophilic lignin colloidial spheres and hydrophobic lignin reverse micelles were successfully prepared by self-assembling.These two kinds of lignin micelles were applied in polar and non-polar material systems,respectively.The compatibilities of lignin and materials system increased significantly and the UV-blocking properties of lignin were also enhanced.Enzymatic hydrolysis lignin(EHL)and Organosolv lignin(OL)were self-assembled into nano-,submicron-and micron-scale hydrophilic lignin colloidial spheres in acetone-water system by changing the initial concentrations of lignin solutions and the ionic strengths of poor solvents.The results indicate that hydrophilic lignin colloidial spheres dispersed better in polar cream than original lignin.The UV-blocking performance of hydrophilic lignin colloidial spheres was also better and it increased with the decrement of their particle size.OL colloidial spheres had better UV-blocking and antioxidant property than EHL colloidial spheres because OL had more phenolic hydroxyl,methoxy groups and conjugated structures than EHL.Adding 10wt% OL nano-scale colloidial spheres(OL-S)into polar cream could prepare a sunscreen with sun protection factor(SPF)of 15,which meets the daily sun protection requirement.Small amount of chemical sunscreen can significantly enhanced the UV-absorbing performance of OL submicron-scale colloidial spheres(OL-M).The SPF value of sun cream containing 10wt% OL-M,2 wt% UVA and 2wt% UVB chemical sunscreen actives reached 48.Alkali lignin(AL)was self-assembled into spherical hydrophobic lignin reverse micelles(LRM)in dioxane-cyclohexane mixture by promotion of hydrophobic interaction and hydrogen bonding.Light scattering result indicated that the critical cyclohexane content of forming LRM was 7%.Results of contact angle,transmission electron microscopy(TEM)and Fourier transform infrared(FTIR)spectra indicated that LRM was spherical structure with a water droplet contact angle of ~90° and the chemical structure of lignin did not change after self-assembling into LRM.The UV-blocking ability of LRM was better than AL.This kind of hydrophobic LRM showed an excellent compatibility with non-polar materials such as high density polyethylene(HDPE).They can be dispersed well in HDPE and enhanced the UV blocking property of the composite material.Furthermore,the added LRM had little negative influence,but significantly improved the HDPE mechanical properties.With 5wt % LRM loading,Young's modulus increased from 1066 to 2104 MPa and the elongation at break increased from 671% to 1030%,respectively.The rheological properties of HDPE turned better in the meanwhile.The thermal stability of HDPE can be retained.