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The effect of solar radiation on the surface checking of lodgepole pine Urban, Kathrin

Abstract

The weathering of wood is caused by a complex combination of chemical and mechanical effects. One of the first signs of weathering is the development of checks at wood surfaces. This study examined the effect of UV-B, UV-A, visible light and infrared light on the development of checks at the surface of lodgepole pine {Pinus contorta Dougl. ex Loud.) deck boards for three periods of twelve weeks, each starting in June 2004. Colour change and change in lignin content were analysed during exposure. A separate experiment examined the loss of weight and tensile strength of microveneers clamped together during exposure. SEM images provided information about structural changes. UV-B had the most significant effect on the quantity, total width, total length and total area of checks that developed at the surface of boards during exterior exposure. UV-B degraded lignin and there was a darkening and reddening of the exposed surface. Defibration of the cell structure at the microscopic level was also observed. UV-A and visible light degraded the surface to a lesser extent. Photodegradation was a surface phenomenon with UV-B only affecting the two upper layers of the veneer blocks (150 um). It can be concluded that selectively protecting wood from UV-B, UV-A and visible light using chemical means might result in less severe checking of lodgepole pine during weathering. Even the blockage of UV-B alone might lead to significantly improved behaviour of surfaces, when the natural look of decks is preferred.

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