Wood surfaces exposed to natural physicochemical dynamics (light, rain, wind, air pollution, etc.) are susceptible to environmental degradation during a complicated phenomenon known as weathering. Although biotic factors such as fungal colonization also contribute, electromagnetic energy (specifically ultraviolet and visible light), moisture, and oxygen are the most significant causative agents in the weathering of exterior wood exposed above ground. Wood strongly absorbs ultraviolet light and can dissipate this energy through chemical processes involving free radical formation and reaction with molecular oxygen (O2). Formation of free radicals within the lignocellulosic fibers induces depolymerization of lignin and cellulose, the major structural components of wood. Photodegradation of wood also occurs beneath clear wood finishes, greatly reducing the durability of such coatings when used outdoors.
As part of a joint Forintek / CAWP project to develop a surface modification treatment that will improve the weather resistance of wood products (without masking the natural colour and texture of wood), this document provides an overview of the environmental factors and effects, and the mechanisms involved in wood weathering. A review of wood weathering protection strategies published in recent years is given, and future directions are outlined.