Drying and gas or vapor contact with solids – Process – With nondrying treating of material
Patent
1995-11-09
1997-10-21
Sollecito, John M.
Drying and gas or vapor contact with solids
Process
With nondrying treating of material
34497, F26B 700
Patent
active
056783248
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method according to the preamble of claim 1 for improving the resistance of cellulosic products against mould and decay as well as to enhance the dimensional stability of the products.
According to a method of the present kind the cellulosic product is subjected to a heat treatment carried out at an elevated temperature.
It is well-known in the art that the dimensional stability of wood can be improved by heat treatments. As far as the prior art is concerned, reference is made to, for instance, the Finnish Patent Specification No. 68,122, which discloses a method for treating wood products at temperatures of 160.degree. to 240.degree. C. and at pressures of 3 to 15 bar. As a result of the treatment, the capability of wood to absorb water and thus to expand is considerably reduced. The effect of heat treatments on decay resistance of wood has also been studied. Mailun, N. P. and Arenas, C. V describe in their article "Effect of heat on natural decay resistance of Philippinean woods" (Philippinen Lumberman, Vol 20, No. 10, 1974, p. 18-19, 22-24) the treatment of Asian wood species in dry state at temperatures of 90.degree., 110.degree., 130.degree., 150.degree. and 175.degree. C. for 240 hours. As a result of the treatment the colour of the wood changes to chocolate brown. An extended treatment at 130.degree., 150.degree. and 175.degree. C. increased the resistance of the wood samples against two brown rot fungi. However, at the same time it made the wood weaker.
Because all kinds of wood are not suited for the conventional pressure impregnation methods, using substances which prevent the growth and spreading of fungi, heat treatment is an interesting alternative for protecting wood against decay.
The prior heat treatment processes, which call for the use of pressure and extended treatment times, have been too complicated for industrial applicability. It has also been ascertained that under the influence of high pressures and increased temperatures wood becomes brittle and it weakens. Furthermore, wood is easily ignited at high temperatures.
The present invention aims at eliminating the problems related to the prior art and to provide a completely novel solution for improving the dimensional stability of and the resistance against decay and mould (i.e. the biodegradation resistance) of cellulosic products.
The invention is based on the concept of carrying out the heat treatment of the cellulosic product in two stages: first the product is dried to desired moisture content, typically to below 15%. Then the temperature is rapidly raised above 150.degree. C. (typically to about 180.degree. to 250.degree. C.) and the treatment is continued at that temperature until the weight loss of the treated product amounts to at least 3%.
In particular the method according to the invention is principally characterized by what is stated in the characterizing part of claim 1.
As mentioned above, unseasoned timber or similar cellulosic products are used as starting materials for the method according to the present invention. The product can be dried at any suitable conditions (even outdoors at ambient temperature) to the desired moisture content of less than 15%. According to a preferred embodiment of the invention the product is, however, dried at elevated temperatures. The colour of the wood product will become darker during such drying. In connection with the drying due care is taken to avoid cracking of the product. This goal is advantageously attained by constantly determining the temperatures of the interior and the surface, respectively, of the wood and by maintaining the temperature difference at a reasonably small value. Preferably said difference amounts to about 10.degree. to 30.degree. C. This procedure is followed both when the temperature is raised and when it is lowered. Surprisingly, it has been found that said solution will even completely prevent the formation of cracks in (the interior parts of) the wood material. When larger amounts of wood are to be dried several samples sh
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Ek Pentti
Jamsa Saila
Viitanen Hannu
Viitaniemi Pertti
Gravini Steve
Sollecito John M.
Valtion teknillinen tutkimuskeskus
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