Biodeterioration of wood-based composites and its prevention

Wood is inherently more durable than other cellulosic materials, but eventually deteriorates under the right conditions. The degradation rate depends on many factors including temperature, moisture content, and wood species. Builders have long understood the need to exclude moisture and use naturally durable wood species; however, increasingly complex building systems, which trap water, have sharply increased decay risks. This problem is clearly demonstrated with composite panel products used for sheathing and siding. While the resin in some of these products increases their decay resistance, many composites are composed of low durability species that rapidly decay when wet, thus producing costly failures. In general, problems associated with these materials are no different than those experienced with solid wood of the same species; however, improving panel durability poses a far greater challenge. Conventional treatments can induce permanent changes in properties such as swelling and are unsuitable for most composites. Biocides can be added to the furnish but many of these compounds affect bonding properties or lack the thermal stability to survive pressing or extrusion. Borates have been employed successfully in some materials, but these chemicals are less effective against mold fungi. Mold fungi do not weaken composites, but their presence can produce. health related building issues. Clearly, there is a continuing need for inexpensive, broadly effective, long lasting, and non-toxic protection strategies for composites. This paper examines the relative durability of various composites and discusses approaches to biodeterioration prevention.