Artificial Ageing of Wood Monitored by DRIFT Spectroscopy and CIE L*a*b* Color Measurements. 1. Effect of UV Light

Abstract

The effect of UV irradiation up to 200hrs was monitored on five wood species (Picea abies, Pinus sylvestris, Larix decidua, Populus euramericana, Robinia pseudoacacia) by spectro colorimetry and diffuse reflectance infrared Fourier transform (DRIFT) spectrometry. The CIE L * a * b * color calculations exemplified the rapid change of total color difference (ΔE * ) in the first 25 to 50hrs of irradiation. All samples exhibited yellowing, i.e. a pronounced steep shift to positive Δa * values in the Δa * , Δb * coordinate system. Robinia p. showed an additional shift to redness, i.e. towards positive Δb * values. The colored compounds developed could be removed by water extraction only to a moderate extent. Thus it was concluded that the yellow color was mainly due to oligomeric chromophores, arisen probably from leuco chromophores of the lignin moiety of wood. In agreement with the literature, DRIFT measurements revealed the preferred degradation of aromatic structures and the formation of nonconjugated carbonyl and carboxyl groups. The DRIFT spectra indicate dehydration, oxidation to carboxyl groups and the formation of lactones or other esters. It was demonstrated by spectral subtraction that the increasing carbonyl bands consisted in the case of softwoods of two sub-bands with equal intensity at 1763 and 1710 cm -1 . The formation carbonyl bands of the hardwoods obeyed different rules. The formation of conjugated carbonyl groups below 1700 cm -1 was observed only to a limited extent. The carboxyl group containing photodegradation products could easily be removed by water extraction. This was interpreted that the oxidized compounds did not necessarily contribute to the formation of colored compounds. The plots of DRIFT band intensities, derived from baseline corrected and normalized spectra, vs. the irradiation time revealed pronounced intensity changes in the first 50 hrs of irradiation. The coherent intensity profiles were beveling above 100hrs of irradiation.