Surface checking of wood is increased by photodegradation caused by ultraviolet and visible light
The aim of this research was to test the hypothesis that exposure to solar radiation increases the checking of wood exposed to the weather, and to examine the causes and spectral dependency of such an effect. Lodgepole pine decking samples were exposed outdoors under filters, which blocked selected regions of the solar spectrum while allowing other weathering factors to act on samples. Surface checking in samples was quantified after 12, 24 and 36 weeks of exposure, and the chemical and micro-structural changes occurring at weathered wood surfaces were examined. Check numbers and dimensions were greater in samples exposed under a filter to the full solar spectrum than in samples exposed under filters that blocked the transmission of UV, visible or infrared radiation. Samples that were shielded from more energetic wavelengths developed less checking and also showed less delignification at the exposed wood surfaces. Checks developed at the margins of rays and propagated at the interface between adjacent tracheids, close to the middle lamella. We conclude that exposure to UV and visible light increases the tendency of wood to check during exterior exposure. Our findings point to a link between changes in cell micro-structure as a result of photodegradation of lignin and the development of visible checks in wood exposed outdoors.
Investigations into the photodegradation of wood using microtensile testing
Significant increases in the rates of strength loss of thin strips were observed as a result of exposure under conditions of high humidity and water saturation. However, under some circumstances, exposure of the strips at high levels of relative humidity could result in an initial increase in tensile strength. This strength increase appeared to be associated with cellulose changes and cross-linkings. Differences in photoresistance between the different wood species, and between heartwood and sapwood, were most clearly shown under the drier exposure conditions. These differences were progressively diminished with the pronounced effect of water.ZusammenfassungNach künstlicher Bewitterung von dünnen Brettchen unter feuchten und wassergesättigten Bedingungen ergab sich eine beschleunigte Erniedrigung der Festigkeitswerte. Unter gewissen Bedingungen zeigte sich allerdings sogar ein anfänglicher Anstieg der Zugfestigkeit. Diese Festigkeitserhöhung scheint mit Änderungen und Quervernetzungen der Cellulose verbunden. Unterschiede der Beständigkeit gegen Strahlungsschäden der verschiedenen Holzarten, sowie bei Kern- und Splintholz zeigten sich deutlich nur unter relativ trockenen Bedingungen. Mit zunehmendem Wassereinfluss verringerten sich diese Unterschiede.
Investigations into the photodegradation of wood using microtensile testing