DRYING OF WOOD IN OIL UNDER VACUUM

Nils GROTHE
Eng. - University of Applied Science
Address: Friedrich-Ebertstraße 28, Eberswalde, Germany
E-mail: nilsgrothe@web.de

Nasko TERZIEV
Dr.eng. - Swedish University of Agricultural Sciences, Department of Forest Products/Wood Science
Address: Box 7008, 750 07 Uppsala, Sweden
E-mail: nasko.terziev@sprod.slu.se

Ulrika RABERG
Dr.eng. - Swedish University of Agricultural Sciences, Department of Forest Products/Wood Science
Address: Box 7008, 750 07 Uppsala, Sweden
E-mail: ulrika.raberg@sprod.slu.se

Abstract: Use of oils as water repellent impregnation agents has industrial potential and is an environmentally friendly alternative to the traditionally employed chemical wood protection. This study deals with drying of wood in oil under vacuum, i.e. the initial and inevitable technological step prior to impregnation.
        Plant oils have significantly higher thermal conductivity than air, thus conducting the heat more effectively to the surface and into wood. The objective of this study was to investigate the suitability of oil drying under vacuum for various wood species using cheap and available plant oil.
        Boards of Scots pine, Norway spruce and oak wood with thickness within 25-30 mm and high initial moisture content were dried in rapeseed oil at temperature of 80ºC and permanently applied vacuum (60%). Final moisture content, internal stress, modulus of elasticity (MOE), oil uptake and penetration depth were measured for the studied wood species. Scots pine and spruce boards were dried to less than 20% moisture content with negligible degradation (checks and stress) for 6 h. For identical duration of drying, the oak samples were more degraded and dried to only 30-40% moisture content. MOE of the oil dried samples was measured by a dynamic and static method and compared to air dried control samples. Oil drying had no negative effect on the MOE. The uptake of oil was 36, 25 and 26 kg/m3 and penetration depth was measured to be 2.2, 0.7 and 1.7 mm for Scots pine, spruce and oak respectively. The initial tests showed that drying in oil is a promising process for softwood timber providing short drying time and insignificant degradation. Drying of oak is possible but the temperature and duration of drying must be adjusted to ensure better results. The method facilitates further impregnation of wood with modified oils.

Key words: drying in oil, rapseed oil, drying time, internal stresses, MOE.

REFERENCES
FIES, R. D. and JOHNSTON W. W. (1965). Process Of Drying Wood By Oil Immersion And Vacuum Treatment To Selected Moisture Content With Oil Recovery, US Patent No. 3,205,589, USA.
HAGER, B. O. (1971). Wood Treatment Method United States Patent 3,560,251, Sweden, Stockholm.
LOHMANN, U. (1998). Holz Handbuch, 5. Auflage, DRW Verlag, ISBN 3-87181-348-6, Germany, Rosenheim.
McDONALD, D. (1958). Method of Drying and Impregnating Wood, United States Patent 2,860,070, USA.
McMILLEN, J. M. (1947). Special Methods of Seasoning Wood; Boiling in Oil, Forest Products Laboratory Report (FPL) no. 1665, USA, Wisconsin.
*** DIN 52182 (1976). Prufung von Holz, Bestimmung der Rohdichte (Testing of Wood; Determination of Density, ISO 3131-1975).
*** DIN 52183 (1977). Prufung von Holz, Bestimmung des Feuchtigkeitsgehaltes (Testing of Wood; Determination of Moisture Content, ISO 3130-1975).
** DIN 52186 (1978). Prufung von Holz, Biegeversuch (Testing of Wood; Bending Test, ISO 3133-1975).