Photodegradation of thermally modified wood

Abstract

Natural wood, being biological material, undergoes rapid degradation by ultraviolet (UV) radiations and other environmental factors under outdoor exposure. In order to protect wood from such degradation, the chemical structure of wood is altered by chemical modification or heat treatment. In the present study, heat treated specimens of rubberwood (Hevea brasiliensis) were exposed to xenon light source in a weather-o-meter for different periods up to 300 h. Photostability of modified and unmodified wood was evaluated in terms of colour and chemical changes. Light coloured untreated wood became dark upon UV irradiation whereas, dark colour of heat treated wood lightened on UV exposure. CIE lightness parameter (L^*) decreased for untreated wood whereas its value increased for heat treated wood upon irradiation. Other colour coordinates a^* and b^* increased with exposure duration for both untreated and heat treated wood. The overall colour change (ΔE^*) increased for both untreated and heat treated wood. The Fourier Transform Infrared (FTIR) spectroscopic studies revealed severe lignin degradation of heat treated wood due to UV light exposure. Colour changes and FTIR measurements indicate that thermal modification of wood was ineffective in restricting light induced colour changes and photodegradation of wood polymers.

Introduction

Wood-based materials are used in vide variety of applications. Wood is a good engineering and structural material because of good mechanical strength, low thermal expansion and aesthetic appeal. However, being biological material, it is susceptible to environmental degradation. It is necessary to treat wood to provide specific surface properties depending on the intended application. One of the usual problem associated with wood during its outdoor application is degradation of its polymeric constituents resulting from UV light present in solar radiation, termed as photodegradation. Photodegradation is primary cause of weathering of wood. Though it is surface phenomenon, the photodegradation of wood polymers, mainly lignin is quite rapid. The rapid surface delignification occurs in few hours of exposure to accelerated weathering [1], [2], [3] or within few days of natural weathering [4], [5]. The process is characterised initially by colour changes; usually colour darkening, then the surface fibres loosen and erode. Wood surface further develop checks and raised grain or colonisation by mildew [6]. The primary cause of photo induced degradation is the presence of light absorbing chromophoric groups (α-carbonyl, biphenyl and ring conjugated double bond structures) in lignin. Light absorption by chromophores leads to the formation of free radicals which react with oxygen and are responsible for colour changes in wood [7], [8]. The influence of weathering on microscopic structure, colour changes and chemical changes has been investigated extensively [6], [7], [8], [9], [10], [11], [12], [13], [14].

In thermally modified wood, physical and chemical properties are altered by subjecting wood to high temperatures (180–250 °C) under inert or restricted air environment. Heat treatment increases dimensional stability and durability though strength properties decrease [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Thermal modification leads to decrease in amorphous polysaccharide content and also condensation, demethoxylation of lignin [18], [22], [24]. Heat treated wood can be used for outdoor applications because of its improved dimensional stability and durability as compared to unmodified wood [26]. In addition heat treated wood has uniform dark colour. However, behaviour of thermally modified wood against UV light exposure is still not clearly understood. It is important to understand the behaviour of modified wood under UV exposure for its outdoor applications.

Effect of natural and/or accelerated weathering on colour stability and chemical changes of heat treated wood has been studied by few workers [27], [28], [29], [30], [31], [32], [33]. The heat-treated wood has been reported to be more resistant to weathering than untreated wood [27], [28], [29], [32]. The modified chromophoric lignin structure due to heat treatment may interfere with light absorption process, thereby inducing photo-stability. However, some of studies have found thermally treated wood as vulnerable to photodegradation as untreated wood [31], [33].

The present work was carried out to examine the colour stability and UV resistance of thermally modified wood surfaces. Heat treated rubber wood specimens were exposed to xenon light source in a weather-o-meter. Colour changes on exposed wood surfaces were analysed using a spectro-colorimeter and chemical changes were monitored using FTIR spectroscopy.