Effects of Acetylation and Hydrothermal Treatment on Lignin as Revealed by Cellular UV-Spectroscopy in Norway Spruce (Picea abies [L.] Karst.)

Summary

The effect of two different wood modification treatments on the lignin of Norway spruce wood was studied by means of cellular UV-spectroscopy. The modification treatments chosen were acetylation using acetic anhydride and a two step hydrothermal treatment. Small cross sectional thin slides from the wood surface were taken and were subsequently analysed using a UV-micro-spectro-photometer. The modified samples were compared with a non-treated control sample. The results revealed changes in the absorption spectra of lignin after both modification treatments. Spectra of the compound middle lamella (CML) of acetylated wood revealed a decrease in the lignin-specific absorption maximum at 280nm, which is considered to be caused by superficial leaching of lignin due to acetylation. The retarding effect of acetylation on UV-degradation of wood is discussed. In hydrothermal-treated wood the 280nm maximum was increased within the S2 layer. This maximum remained almost unchanged in the CML while a lower maximum was observed at about 330nm. Changes in the chromophoric behaviour of lignin are probably caused by de-masking reactions within the S2 layer resulting from hydrolysis of carbohydrates and side chain reactions. Both treatments lead to a severe decrease in absorption at the lower end of the spectrum towards 250nm which is possibly attributed to changes or even splitting of biphenyls.