Fire Retardant Treatment of Wood – State of the Art and Future Perspectives

Abstract

Outdoor and indoor exposed wooden structures are prone to the hazard of fire. Thisr is often inevitable and hardly avoidable by factors such as the design. However, wood is widely used as a structural element in buildings, it is present all-over public places and the main source for indoor furniture. Thus, and due to recent incidents, the demand for an effective and leaching-resistant fire protection is rising. In addition, fire protection technologies are desired, which survive mechanical processing. Considering the latter, protective surface coatings show a high fire protection, while on opposite they are very sensitive to mechanical damages. Therefore, various approaches based on a full impregnation of timber with fire retardants have been studied. In the past aluminum, boron, halogens (e.g. bromine) and more recently phosphorus and nitrogen, were shown to be effective fire retardants in wood. Nowadays, most conventional fire retardant systems are halogen-free, while boron is still used. However, boron shows a low resistance to leaching and is classified as a SVHC candidate, which brings up health and environmental issues. The same is true for formaldehyde. Concerning environmental issues, nitrogen and phosphorus were found to be promising alternatives and highly effective fire retardants. Leaching in service was slightly reduced compared to boron but a decrease in strength properties was detected after treatment of wood with those compounds. In general, an increased hygroscopicity of wood was found after any of the listed treatments, together with a leaching of the flame-retardant chemical which was still too high to guarantee a long-term fire protection in wood exposed outside. The overall aim of this study is to (1) give an overview about the past developments and most established fire retardant chemicals and (2) review recent findings and developments in terms of permanent fire retardant treatments of wood.