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NIR studies on short term weathering of wood

Anna Sandak 1 ; Jakub Sandak 2 ; Ingunn Burud 3 ; Lone Ross Gobakken 4
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Introduction
Weathering is the general term used to define the slow degradation of materials exposed to the weather. Wood exposed to weathering is degraded by various environmental agents such as solar radiation, cyclic wetting, atmospheric temperature and relative humidity changes, environmental pollutants, mechanical abrasion and certain micro-organisms. The rate of weathering varies within and between timber species, function of product, technical/design solution, finishing technology applied but most of all on the specific local conditions. Weathering process leads to a slow breaking down of surface structures, roughening of the surface and reduction of the glossiness. Rapid changes of color of untreated wood when exposed to weathering are mostly caused by photodegradation of lignin and wood extractives in the middle lamella. The goal of this work was an attempt for understanding and modeling the degradation kinetic of thin wood samples exposed to four geographical directions by means of FT-NIR.
Experimental
Experimental samples were prepared from one piece of Norway spruce wood (Picea abies) on the slicing planner (Marunaka). The thickness of samples was ~100μm and the efficient surface exposed to weathering was 30 x 35mm (width x length respectively). Four sets of samples were exposed in San Michele (Italy) for four geographical directions (NESW), and were collected before exposition and after 1, 2, 4, 7, 9, 11, 14, 17, 21, 24 and 28 days of weathering. Samples were conditioned after collection in the climatic chamber (20°C, 60%RH) to the equilibrium moisture content of ~12%.
Experimental samples were measured by using a Fourier transform near infrared spectrometer VECTOR 22-N produced by Bruker Optics GmbH. The spectral range measured was between 4000 and 12000 cm-1. The spectral resolution of the spectrophotometer was 8 cm-1, each spectrum was computed as an average of 32 successive measurements.
Results and discussion
NIR spectra were used for identification of the chemical changes in wood due to weathering. A novel approach using chemometric models was developed in parallel to the common spectra interpretation and evaluation. The PLS model was developed by regressing two points defining the border values for the weathering state:
• the initial stage (weathering index Wind= 0), modeled on the base of the un-weathered samples
• final stage modeled with spectra collected from the most degraded samples after 28 days of exposure (weathering index Wind= 1000).
The PLS model developed was used in the second phase for prediction of the Wind values for all the intermediate samples (weathered between 1 and 24 days). Quant 2 Analysis tool pack, part of the OPUS software was used for both, PLS model development and for prediction of the intermediate indexes.
Western and northern exposure sites are slightly less affected by weathering process. The most intense changes were observed in samples exposed to South direction at the early stage of the weathering. Understanding the mechanisms of weathering and the role of the altering factors is fundamental to assess the actual conditions of timber structures. It is essential to predict the future performance and to ensure a long-term preservation and maintenance.