Liquid water flow and discolouration of wood during kiln drying

[摘要] The discolouration of South African softwood during kiln drying can reduce the value of furniture gradelumber. Thermal discolouration of wood, as found due to heat treatment, produces a homogeneouslybrowner colour in wood than is normally expected. This type of discolouration is attributed to reactions ofthe macromolecules present in wood and is found in both hard- and softwoods. Yellow stain and kiln brownstain can severely alter the colour of the outer few millimeters of a wooden board and is attributed to thereaction of water-soluble sugars and nitrogenous compounds, present in the wood sap, after deposition atthe wood surface due to liquid or capillary water flow during drying.A discussion of the mechanism of discolouration due to yellow stain and kiln brown stain would beincomplete without a good understanding of the liquid flow of water during drying above fibre saturationpoint. This thesis brings the two concepts of liquid water flow and discolouration in context and is presentedin four chapters:• an introduction motivating the aims of the investigation (Chapter 1);• a literature review of factors which may influence discolouration and liquid water flow duringdrying (Chapter 2);• original manuscripts describing the discolouration of South African softwood and liquid water flowin hard- and softwood (Chapter 3); and• a final conclusion that links up the results from the investigations (Chapter 4).The investigations into the occurrence of yellow stain and kiln brown stain showed that the intensity of thesetypes of discolouration was influenced by geographical origin (and/or climate), tree species, planing depthof dried lumber, and kiln schedule parameters like dry- and wet bulb temperature and time. Thecharacteristic discolouration pattern of yellow stain and kiln brown stain indicated that this stain type wasrelated to the wetline phenomenon that is found during the liquid water flow phase of drying wet wood.Thermal discolouration, on the other hand, occurred homogeneously throughout the volume of lumber andis, therefore, not related to free water flow, but to chemical changes of the macromolecules in wood.The results of the liquid water flow investigations support the invasion percolation theory of drying thatstates that the largest meniscus will retract into a drying liquid-filled capillary network until it is not thelargest meniscus anymore. Fluctuations in the rate of moisture loss from the cores of wood pieces abovefibre saturation point were also found. The pattern of fluctuation differed appreciably between Betulaverrucosa and Pinus radiata. In both cases, the start of the last phase in rate of moisture loss from the corecoincided with a reduction in the cross-sectional area of the drying wood piece. This behaviour is explainedby the hypothesis that distinct capillary size classes are emptied of free water, in order, from large to small.As smaller capillaries are emptied, the capillary forces become greater, to the point where the forces aregreat enough to cause permanent or temporary deformation of the remaining water-filled capillaries. Classification and regression tree analysis was a useful statistical technique to analyse a large multivariatedataset. The importance of kiln schedule temperatures and planing depth to control yellow stain and kilnbrown stain was clearly pointed out by the technique, which can help to simplify the control of colour qualityduring the industrial processing of wood.

[发布机构] Stellenbosch University