Plant oils have significantly higher thermal conductivity than air, thus conducting heat more effectively to the surface and into wood. Although known for long time, drying in oil under vacuum is underestimated and the information on this type of drying is scarce. The objective of this study was to investigate the suitability of oil drying under vacuum for three wood species using cheap and available plant oil.
Boards of Scots pine, Norway spruce and oak wood with thickness of 26 mm and high initial moisture content (100-130%) were dried in rapeseed oil at temperature of 60-90ºC and under permanently or alternately applied vacuum. Final moisture content, distribution of moisture along the thickness of samples, internal stress, modulus of elasticity (MOE) and oil uptake were measured for the studied wood species. Scots pine and spruce boards were dried to 12% moisture content with negligible degradation for 12 h. The drying dynamic differs significantly from that of conventional kiln drying, e.g. the drying curves show only constant drying rate. The drying times were compared to that calculated by a drying model and it was concluded that drying in oil under vacuum is 4-5 times faster. MOE of the oil dried samples was measured by a static method and compared to air dried control samples. Oil drying had no negative effect on the MOE. The uptake of oil was in the range of 40-90 kg/m3. The tests showed that drying in oil is a promising process for softwood timber providing short drying time and insignificant degradation.
Use of oils as water repellent impregnation agents has industrial potential and is an environmentally friendly alternative to the traditionally employed chemical wood protection. The method facilitates further impregnation of wood with modified oils.
Keywords: drying quality, Norway spruce, oak, rapeseed oil, Scots pine
Authors
Kaden R.
University of Applied Science of Eberswalde, Eberswalde, Germany
Terziev N.
Swedish University of Agricultural Sciences, Department of Forest Products/Wood Science, Uppsala, Sweden
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