Chemical and appearance changes of wood due to artificial weathering – Dose–response model

The aim of this study was to assess a model for the surface degradation kinetics of natural wood exposed to artificial
weathering. The photochemical and physical processes of weathering result in simultaneous changes of both the wood
matrix composition (i.e. lignin content, cellulose crystallinity index, cellulose polymerization degree) and wood’s appearance
(i.e. colour, gloss, roughness). European larch, a popular cladding material, was used for experimental samples. Weathering
was conducted in a QUV artificial weathering machine for 672 h according to the EN927-6 standard. The response of wood
was assessed using multi-sensor measurements. Results revealed that changes in colour are evident already after 28 h of
artificial weathering, with an increase in a* and b*, and a decrease in L*. That initial trend was reversed afterward, with the
colour changing to grey. Due to the particular shape of the curves representing colour coordinates, it is difficult to use CIE
L*a*b* parameters to model the weathering dose. However, the degradation curve established for selected near infrared
bands was useful to build a dose–response model of wood surface degradation. In all cases, a clear trend of change in
absorbance values that progressed along with the weathering dose was observed. These curves, unlike the CIE L*a*b*
values, are monotonic, meaning that dose value corresponded with only one value of absorbance. Principal component analysis of the near infrared spectra allowed clustering of the spectra by different dose classes, with the main differentiation in the reference class and in the final class (maximum dose). It was found that the discrimination between spectra at diverse cumulative weathering doses was more evident for earlywood than for latewood. This confirms the hypothesis that degradation kinetics differ depending on the anatomical configuration of wood. This research is a first step for establishing dose–response model for natural weathering, where precise quantification of weathering parameters is more challenging.