PHOTOYELLOWING OF MILLED WOOD LIGNIN AND PEROXIDE-BLEACHED MILLED WOOD LIGNIN IN SOLID 2-HYDROXYPROPYLCELLULOSE FILMS AFTER SODIUM-BOROHYDRIDE REDUCTION AND CATALYTIC-HYDROGENATION IN SOLUTION - AN UV/VIS ABSORPTION SPECTROSCOPIC STUDY

UV absorption spectroscopy was used to determine the chromophore composition of tetrahydrofuran/water-soluble milled wood lignin (MWL) and peroxide-bleached lignin (LHO) using ionization difference UV spectroscopy. MWL and LHO were treated in solution with sodium borohydride to reduce the carbonyl chromophores which were shown to be prone to induce photoyellowing. Furthermore, the reduced solutions were hydrogenated in solution over Pd/C catalyst to eliminate the aromatic-conjugated double bonds that also induce photodiscoloration. The lignins were incorporated into 2-hydroxypropylcellulose (HPC) at a concentration near 0.5% and then irradiated with filtered light (lambda > 300 nm) in the form of solid, transparent films. The photochemistry of the different samples was followed by UV/Vis absorption spectroscopy and the kinetics of discoloration and aromatic ring destruction were established. The peroxide-bleached, reduced, and hydrogenated lignin was found to be almost completely stable in the solid carbohydrate matrix under irradiation. Furthermore, the kinetics show the importance of phenolic biphenyl chromophores and probably also of phenolic phenylcoumaran structures in the photoyellowing process.