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Patent Searching and Data


Title:
METHOD FOR PRETREATING TIMBER
Document Type and Number:
WIPO Patent Application WO/2003/041926
Kind Code:
A1
Abstract:
The object of the invention is a method for pretreating timber, in which method wood is immersed into liquid and wood is bathed in suitable temperature before drying. Characteristic to the method in accordance with the invention is the fact, that wood is bathed in liquid at temperature of 40-100°C and under pressure of 0,5-10 bar for 6-72 hours, after which chemical reactions causing discoloration of wood have, at least partly, taken place and that after bathing wood is mechanically dried.

Inventors:
MOETTOENEN VEIKKO (FI)
Application Number:
PCT/FI2002/000890
Publication Date:
May 22, 2003
Filing Date:
November 11, 2002
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
MOETTOENEN VEIKKO (FI)
KAERKI TIMO (FI)
International Classes:
B27K5/00; B27K5/04; (IPC1-7): B27K5/00
Foreign References:
GB1283683A1972-08-02
DE3229826A11984-02-16
DE3826798A11989-02-16
DE349812C1922-03-09
FR2544222A11984-10-19
US2726169A1955-12-06
DE338809C1921-07-01
NL8403097A1986-05-01
DE341375C
Other References:
DATABASE WPI Week 200113, Derwent World Patents Index; Class A32, AN 1993-252135, XP002962532
DATABASE WPI Week 199702, Derwent World Patents Index; Class F09, AN 1987-011627, XP002962533
DATABASE WPI Week 199817, Derwent World Patents Index; Class C07, AN 1998-187923, XP002962534
Attorney, Agent or Firm:
PATENTTITOIMISTO PITKÄNEN OY (Kuopio, FI)
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Claims:
CLAIMS
1. A method for pretreating timber, in which method wood is immersed into liquid and wood is bathed in liquid with suitable temperature before drying, characterized in that wood is bathed in liquid at temperature of 40 100°C and under pressure of 0,510 bar for 672 hours, after which chemical reactions causing discoloration of wood have, at least partly, taken place and that after bathing wood is mechanically dried.
2. A method in accordance with claim 1, characterized in that wood from broadleaved tree species is preliminary treated with the method.
3. A method in accordance with claim 1 or 2, characterized in that wood is preliminary treated as logs with the method.
4. A method in accordance with any of claims 13, characterized in that as substance water is used.
5. A method in accordance with any of claims 13, characterized in that as substance for controlling acidity is used.
6. A method in accordance with any of claims 15, characterized in that wood from birch is bathed in liquid at temperature of 6585°C under pressure of 0,510 bar for 2448 hours.
7. A method in accordance with claims 4 or 6, characterized in that wood is piled into piling rack, in which pieces of wood to be bathed are placed in a distance from each other, the piling rack is moved to a basin to be closed with a cover, and in which there is water of temperature 6585°C, the cover is closed and the wood is left to bath under 0,510 bar pressure for 2448 hours, and that after bathing the piling rack with wood is lifted from the basin and transported after short settling time to mechanical drying machine.
Description:
METHOD FOR PRETREATING TIMBER The present invention relates to a method for pretreating timber, in which method sawn wood is immersed to liquid and the wood is bathed at suitable temperature and under suitable pressure in the liquid before drying.

It has been a tradition to dry wood by stocking it in an airy place. Recently various kinds of mechanical drying methods have been introduced. One of these is so called quick-drying method. One of the greatest disadvantage of mechanical drying is discolorations in wood. Usually discolorations are the greater the higher temperature in mechanical drying is used. Discolorations usually appear in the inner part of wood as darkenings or places turned into red. Usually they may not be noticed on the surface or on the outer layer of dried wood as the surface stays lighter as it dries quicker than the inner part under (from the discoloration point of view) the critical humidity before the temperature of the drying section is raised to separate the firm bonded water from wood. Discolorations created in the inner part of wood prevent the processing, for example to glue laminated board or other decorative products while they appear as spots and stripes on the surface of products.

Chemical reactions, in which certain extracts of wood polymerize in the presence of oxygen into coloured compounds are the reason for discoloration during the drying process. Favourable circumstances needed for polymerization are created during the drying process while the temperature raises when wood is still wet and as the amount of oxygen increases. The most important chemical reaction concerning the discoloration of wood, i. e. the polymerization of phenolic extracts into only slightly soluble and coloured compounds takes place in acid environment in presence of oxygen. The raise of temperature increases the acidity of wet wood and while the wood starts to dry the amount and availability of oxygen increase in wood.

Reactions, causing discolorations in wood take place in different ways depending on what the circumstances are at certain time in regard with temperature, humidity, acidity and oxygen content in wood. Compounds in polymerization may react also in the absence of oxygen if the temperature of wood is high enough and acidity low enough. If there is no oxygen available while polymerization takes place, the reaction products are colourless or only slightly coloured as less chromofors, side- chains of polymer causing colour, are created.

Discolorations during drying processes are tried to be avoided by slowing down the drying in the early state of the drying process to the saturation point of grain (PSK).

This has required the drying temperature to be under 55°C until the average humidity of wood has decreased to be under 20%. Disadvantage in this kind of method is, naturally, the lengthening of drying time. Due to careful drying the drying process of birch wood for example, takes 2-3 weeks by using the method of warm air-drying. Another way to decrease discoloration is drying outside before mechanical drying until humidity of wood has decreased to be under 30%. This takes usually several months. Furthermore, results are unreliable as the drying depends on the weather. While considering of effective production the earlier mentioned hindering methods of discoloration cause for instance great costs as much drying capacity is needed and stocks are needed to be large.

The object of the invention is to provide a method for preliminary treatment of wood, by using of which the earlier mentioned disadvantages are eliminated.

Especially, the object of the invention is to provide a method for preliminary treatment of wood, by using of which discolorations created while using mechanical drying of wood may considerable be eliminated. Furthermore, the object of the invention is to provide a method for preliminary treatment of wood, which is clearly quicker and more effective than the earlier known methods for preventing of discolorations created while drying wood mechanically, and by using of which deformations and crackings due to those taking place in wood dried mechanically, are reduced as well as homogenous drying of various parts of wood is achieved.

The object of the invention is accomplished by a method, the characteristics of which are presented in the claims.

Characteristic to the method in accordance with the invention is the fact, that wood is bathed at temperature 40-100°C and under pressure 0,5-10 bar for 6-72 hour, after which chemical reactions causing discolorations of wood have taken place, at least partially, and that after bathing wood is dried mechanically. Chemical reactions do not cause disadvantageous discolorations if they take place in wood in oxygen-free substance. While using preliminary treatment at this temperature, under this pressure and for this duration of bathing, it is possible to have so many chemical reactions causing discoloration before mechanical drying, that clearly less disadvantageous discolorations takes place in mechanical drying than earlier, such avoiding problems caused by those later. This is why chemical reactions causing discoloration occur less when drying wood treated with method than when using wood without such treatment. Furthermore, this kind of method decreases disadvantageous deformations occurring during drying process, as bathing evens the differences in humidity in various parts of fresh wood.

In an advantageous application of the method in accordance with the invention wood from broad-leaved tree species is preliminary treated by the method. Tests indicate that it is possible to minimize discoloration more with broad-leaved tree species while using this kind of preliminary treatment than with coniferous tree species. Furthermore, broad-leaved tree species are broadly used in furniture industry, where discoloration of wood causes great costs, so that eliminating discoloration problems with broad-leaved tree species is very profitable.

In the second advantageous application of the method in accordance with the invention water serves as substance. Water is an economical substance due to acquisition and operation costs. While bathing wood in water the rising temperature, organic acids in wood and pressure rising lead to rising acidity. Due to rising acidity reactions having influence on discoloration increase they take place in absence of oxygen. Furthermore, preliminary treatment in water decreases disadvantageous deformations during drying process, because bathing in water equalizes differences in humidity in various parts of fresh wood. Furthermore, while using water as substance, the construction of equipment for preliminary treatment is able to be constructed simple as well as economical by manufacturing and operating costs.

In the third advantageous application of the method in accordance with the invention as substance liquid is used, by using which it is possible to regulate the acidity of treated wood suitable in order the chemical reactions connected with coloration to happen. This kind of liquid can be easily available, advantageous and non-toxic acidity controlling substance, for example, water solution of weak acid or salt solution. Furthermore, this kind of substance possesses the same earlier mentioned properties in equalizing differences in humidity of wood as water does.

In the fourth advantageous application of the method in accordance with the invention wood from birch is bathed susbstance with temperature 65-85°C under pressure 0,5-10 bar for 24-48 hours. Birch is a broad-leaved tree with light natural colour, widely used in furniture and building product industry, for example, as raw material for tables, cupboard doors, as well as raw material for parquet. In mechanical drying process wood gets uneven colour. This causes stripes and other colour flaws leading to rejection of raw material and products as well as to client reclamations. These kind of colour flaws may be eliminated by using preliminary treatment as above mentioned clearly better and more reliably than by using earlier known preliminary treatments.

In the fifth advantageous application of the method in accordance with the invention wood is piled in a piling rack in which pieces of wood to be bathed are placed in a distance from each other, the piling rack is moved to a reservoir with a cover on it, in which there is either water or acidity controlling substance at temperature 65- 85°C, the cover is closed and the wood is left in water bath for 24-48 hours. After bath the piling rack with wood is taken up from the reservoir and placed after a short settling time to a mechanical drying machine. While using this kind of piling rack substance will be in contact with all surfaces of all pieces of wood. Therefore this kind of method is suitable for preliminary treatment of large quantities of wood at one treatment time in rather short time period. Furthermore due to this even a large quantity of wood is possible to have homogenous in quality easily and reliably. Therefore, while using this kind of method preliminary treatment eliminating discoloration is possible to realize effectively, with high quality and exceptionally suitably for industrial manufacturing.

Next, the invention will be explained in more detail with reference to the accompanying drawings, in which results from one application of the method in accordance with the invention are illustrated in scheme.

In one method in accordance with the invention there is preliminary treatment equipment, which includes a basin-like treatment space where wood may be placed by means of a separate piling rack. The basin-like space includes a basin, which may be filled with liquid substance. Substance may be water or some other liquid solution depending on wood to be treated. The basin includes also a cover to be closed with a hatch, with which the open upper part of the basin may be closed after the piling rack has been immersed to the basin. Furthermore, the basin includes a heater, which is in this case, a spiral-like electric resistor placed, for example, on the bottom or on a wall of the basin and includes also temperature sensor for measuring the temperature of the substance. The purpose of the heater is to heat the substance and to keep it at the wanted temperature during bathing. For controlling the equipment there is a control unit connected to the equipment, which consists controlling and regulating appliances for controling the preliminary treatment process according to earlier given programme. Controlling unit may also be build up, for example, of a computer and controlling electronics connected to it, which automatically control the equipment according to given information from the computer and from the measuring data from the temperature sensor in the basin. To make the treatment easier the equipment has a suitable chamber piling rack separate from the chambers, on which the wood is piled in such a way, that there will be

empty space horizontally and vertically between the pieces of wood for substance penetrate round the wood pieces as required. This as even and efficient as possible absorption of substance to wood. Furthermore, the piling rack is constructed such a way, that the whole space of the basin may be utilized as exactly as possible.

Furthermore, there are suitable lifting lugs or some other corresponding lifting elements by means of which the piling rack with wood may be lifted to the basin and back from there. The piling rack used in preliminary treatment may advantageously be used in drying machine of wood. After preliminary treatment the piling rack is moved straight from the preliminary treatment space to drying chamber of a drying machine. This is how transporting of separate pieces of wood from one place to another is avoided.

While using the equipment explained above a suitable amount of wood, for example, saw material from birch is piled to a piling rack. After this the piling rack is moved to the basin with water in which the temperature is 65-85°C when saw material from birch is in question. Next the cover of the basin is closed and wood is left in water to bath for 24-48 hours depending on the cross-area of the pieces of wood and the water temperature. For example, for birch wood with cross-area 38 mm x 100 mm advantageous water temperature would be 76°C and bathing time 24 hours. After bathing the cover of the basin is opened and the piling rack is taken from the basin. After short settling time wood is moved to mechanical drying machine, where it is dried in usual way. After drying wood is moved from drying chamber, taken off from the piling rack and transported to a desired place, for example to an intermediate landing from it may be transported further to an end user for use for desired purpose.

In figure 1 there are schemes in which discolorations of birch saw material treated with the earlier explained method and discolorations of birch saw material without corresponding treatment used as comparison material are compared. The schemes present colour parametres of lightness (L*) and redness (a*) measured with a spectrophotometer from the inner part of 38 mm x 100 mm birch saw material, quick-dried at temperature of 120°C in preliminary treatment periods of various lengths. Initial situation (comparison) is the colour of saw material in mechanical dried wood without preliminary treatment. Lightness increases and redness decreases while preliminary treatment time gets longer.

The method in accordance with the invention may be realized in many various ways differing from above explained application example. The temperature of the liquid substance used in the treatment depends on the tree species of the treated wood,

because the temperature required for chemical reactions for discoloration to take place varies on various tree species. Furthermore, cross-section parametres of wood, humidity of wood as well as other qualitative facts have influence on temperature. Best results may be achieved when the temperature of the liquid used as substance is 40-100°C. Also treatment time varies. Treatment time depends, mostly, on cross-section parametres, but also on tree species and other facts, such as humidity of wood before starting the treatment. The most favourable treatment time would be 6-72 hours. In application examples explained above the treatment space was an open bath basin under normal pressure, but the treatment space may also be a closed space. Researches and tests indicate that bathing may also be realized by treating an amount of wood in a treatment chamber with 0,5-10 bar pressure. As the pressure increases the absorption of substance gets more effective and quicker, which shortens the treatment time and improves results of the treatment. The use of pressure increases the acidity of the used substance. This kind of pressure is easy to achieve while using the treatment equipment of wood in accordance with the invention. Furthermore, in some application of the method bathing may be carried out even before sawing when wood is in the form of logs, when wood may be dried straight after sawing.

The equipment used in the method in accordance with the invention may also be realized in many different ways differing from application examples explained above. The treatment space being a part of the equipment may be an open basin, basin to be closed with a cover or, for example, some suitable chamber-type space, which may be filled with liquid substance. Construction and size of the treatment space depends on the amount of wood to be treated at one time, but also on the way wood is transported to the treatment space for treatment. If the treatment is carried out under pressure the space has been built tight and to hold the used pressure. The heater for heating substance may also be other that an electrical heater. It may be, for instance a heating tube spiral or some other radiator-type heater using separate substance, with which the substance in the chamber is able to be heated and kept in desired temperature in functional way. In a chamber-type equipment the piling rack may be organized, for example, to be moved on rails or on wheels in and out from the chamber.

The invention is not limited to the presented advantageous application but it can vary within the frames of the idea of the invention formed in the claims.