Colour changes and acid formation in wood during heating

Abstract: Heating wood has since ancient times been a method to dry and modify its properties. Nowadays heat is used in industrial processes for the same reasons. Treatment at temperatures above 150ºC can change the colour, improve resistance to biodegradation and enhance dimensional stability. However, losses in the mechanical strength of wood may also occur, and this drawback is a limitation for the use of heat-treated wood in a broad range of products. This thesis suggests that cellulose degradation can contribute to the loss of mechanical strength in wood under high-temperature treatment. The formation of formic and acetic acid during heat treatment of birch wood has been studied. Substantial amounts of acetic acid (at most 7.2% by weight) and formic acid (at most 1.1% by weight) were found in autoclave experiments at temperatures between 160ºC and 200ºC. It was also found that the average molecular size of both commercially heat-treated birch wood and birch wood treated in laboratory experiments under acidic pH conditions was considerably reduced (42%–53%) in comparison to untreated birch wood. It is reasonable to think that the formation of acid and the accompanying decrease in average cellulose molecular size have a crucial influence on the observed decrease of mechanical strength in heat-treated wood. The thesis suggests that wood can be heat-treated while maintaining mechanical strength through a process design that keeps the wood in neutral to alkaline conditions. This thesis also describes studies of colour development in birch, Norway spruce and Scots pine wood during hydrothermal treatment, with special reference to treatment at temperatures between 65ºC and 95ºC with high moisture content. The colour responses of wood that had been heat treated or kiln dried have been investigated, and the colour coordinates Lightness (L'), Chroma (Cab'), hue (h) and colour difference DEab' are presented. It is shown that colour changes associated with heat treatment at high temperatures can be obtained by treatment for long periods at temperatures around 100ºC. Such treatments will lead to changes in colour, but presumably no change in dimensional stability or resistance to biodegradation. The origin of colour formation in wood as a result of heating is briefly investigated and discussed. Colour stability during accelerated UV/Visible light exposure of heat-treated samples has been tested and the results are presented in this thesis. The colour responses of birch, Norway spruce and Scots pine wood were measured after drying in laboratory kiln experiments in the interval of 40º– 111ºC, and it was concluded that the average wood colour of a batch can be controlled by regulating time and temperature. There are some results that show an increase (around 20%) in the mechanical strength of birch wood for heat treatment around 180º–200ºC for approximately 1 hour and that colour measurements may be used as a way to monitor and control the phenomenon. However, further experiments will have to be made to confirm these indications.