FIELD OF THE INVENTION AND PRIOR ART

The invention relates to a method to improve the quality of a wooden element, such as providing the wooden element with improved resistibility against decay, according to the preamble of claim 1 and a wooden element according to the preamble of claim 13.

Wood and constructions made of wood have in all times been subject to decay due to numerous reasons, including putrefaction, mold, attacks from insects etc. and many different steps have, during the history of man, been taken to slow down this decay. One of the first approaches to prolong the life-time of wooden elements was to place the wooden element in a fire, hence carbonize the surface of the element to enhance the durability of the wooden element against decay. Nowadays, the dominant method for wood preservation is impregnation with poisonous chemicals using methods such as CCA treatment (CCA stands for copper chromium arsenate). Huge amounts of chemicals are used for wood preservation today and it is important, from an environmental point of view, to find substitutes to impregnation.

Recent years some of the attention of the wood working industry has turned towards heat treatment of wood. US 6217939 B1 discloses a wood heat treatment method in which the wooden element is heat treated for several hours in oil having a temperature in the range of 180-260 ⁰ C. The result is a homogenous heat treatment of the wooden element, wherein the properties of the bulk as well as the surface of the wooden element is converted so as to improve the resistance against decay. EP 0576608 B1 discloses a method wherein wooden elements are heat treated in linseed oil or similar curable vegetable oil. The treatment results in a diffusion of the water naturally occurring in the wood out from the wooden element, whereby the oil is replacing the water in the cavities in the wood. The oil is thereafter made to cure and thereby to stiffen the wood. Insecticides and/or fungicides are also added to the oil for additional resistance against possible insect or fungus attack on the wood.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a new and favourable method to improve the quality of a wooden element, such as providing the wooden element with improved resistibility against decay. A further object of the invention is to provide a new and favourable wooden element.

This object is according to the invention achieved by means of a method having the features defined in claim 1 and a wooden element having the features defined in claim 13.

It can be assumed that said point will be located in the part of the wooden element where the distance from the centre to the surface of the wooden element is the shortest. The heat treatment will then be interrupted when the temperature at a point in the wooden element, located at a distance from the centre of the wooden element being 50%, preferably 70 %, of the distance from said centre to the part of the surface of the wooden element being closest to said centre, reaches 170 ⁰ C.

A well known problem associated with heat treatment of a wooden element, wherein the wooden element is brought to temperatures over 180 ⁰ C, is that the mechanical properties, such as the resistance to wear, the impact strength and the bending strength, of the wooden element deteriorate. The deterioration of the mechanical properties becomes more pronounced as the heat treatment temperature increases towards and over 200 ⁰ C. At the same time the desirable resistance to decay, by for instance fungal attack, of the wooden element also becomes more pronounced as the heat treatment temperature increases. Therefore, it has been a trade-off in the choice of heat treatment temperature when heat treating wooden elements. In order to achieve a resistance to decay in the heat treated wooden element comparable with the resistance to decay in an impregnated wooden element, the temperature for the heat treatment would need to be high, and using such high heat treatment temperature would result in deteriorations in the mechanical properties which are unacceptable for certain applications, such as using the wooden element as a load carrying beam or as posts for a fence. By using the method of the invention, only the surface portions of the wooden element is subjected to heat treatment, i.e. a heat induced change in the lignin-carbohydrate, which leads to a better resistance to decay, is only applied to the surface portions of the wooden element. The present invention has realized that since it is the surface portions of the wooden element which are exposed to moisture and agents in the environment which promote decay of the wooden element, i.e. fungal spores etc., it is sufficient if only said surface portions, and not the entire bulk of the wooden element, are heat treated to achieve a good resistance to decay in the wooden element. The mechanical properties of the wooden element are preserved in most of the wooden element material, since major parts of the element stay unexposed to the high temperature which induce changes in the lignin- carbohydrate and deteriorate the mechanical properties of the wooden element.

According to one embodiment of the invention the exposure of the wooden element to the heating medium is interrupted before the temperature at any said point in the wooden element reaches 160 ⁰ C, preferably 150 ⁰ C. According to another embodiment of the invention the wooden element is exposed to a heating medium in the form of an oil. An oil can be heated to a temperature in the range of 200-300 ⁰ C without being subject to decomposition. Furthermore, using oil as heating medium provide for a homogenous heating of the surface portions of the wooden element during heat treatment of said wooden element.

According to another embodiment of the invention the oil is a mineral oil. Mineral oil provide good protection from moisture in the atmosphere, it also has good thermal properties which are required in the heat treatment of the wooden element. Furthermore, mineral oils are inexpensive.

According to another embodiment of the invention the oil is a non-curable vegetable oil, such as rape oil. Vegetable oils provide for an environmental friendly method for heat treatment of the wooden element. Using non-curable oils instead of curable oils results in less remaining oil in the wooden element after heat treatment since the non-curable vegetable oils do not cure in the cavities and on the surface of the wooden element. With remains of the oil in the wooden element after heat treatment, as would be the case when using curable vegetable oils as heating medium, the wooden element would become heavier; furthermore, the consumption of oil would be higher which results in a more costly production of heat treated wooden elements.

According to another embodiment of the invention the wooden element is exposed to a heating medium in the form of an inert gas.

In this description and the subsequent claims, the expression "inert gas" is defined as a gas which not promotes oxidative reactions in materials exposed to said gas, i.e. a non-oxidative gas. This means that in other cases reactive gases, such as carbon monoxide, gases of combustion, gaseous water and/or water steam, are also included in the definition.

Presence of oxygen during heat treatment increases the risk for combustion of the wooden element, wholly or partly, and the presence of oxygen also weakens the resistance to decay in the wooden element being subjected for heat treatment in the heating medium; therefore an oxygen free atmosphere is desirable.

According to another embodiment of the invention the inert gas is nitrogen or water steam. Nitrogen and water steam are easy to handle and non-expensive, which makes them useful as heating media in the method according to the invention.

According to another embodiment of the invention the heating medium having a temperature in the range of 220-280 ⁰ C, preferably 230-270 ⁰ C, most preferred 240-260 ⁰ C, is brought in contact with the wooden element. In order to achieve a good resistance to decay in the wooden element the temperature of the heating medium should be relatively high, and since the heat treatment is interrupted before the temperature in the bulk of the wooden element reaches a high temperature, this is possible to achieve without significant deterioration of the mechanical properties.

According to another embodiment of the invention the heating medium is heated to the temperature desired for the heat treatment prior to exposing the wooden element to the heating medium. By having the heating medium preheated to the desired temperature for heat treatment, the heat treatment of the wooden element can be localized to the surface portions of the wooden element. Usually in the prior art the heating medium is heated while the wooden element is exposed to said medium so as to heat treat the wooden element slowly and achieve a homogenous heat treatment of said element. Since the present invention relates to a method in which the heat treatment should not be performed homogenously on the wooden element, a rapid heating is desired, hence the heating medium is heated prior to exposing the wooden element to said medium.

According to another embodiment of the invention the exposure of the wooden element to the heating medium is continued at least until the temperature reaches 200 ⁰ C at a point in the wooden element, located 5 mm, preferably 7 mm, most preferred 10 mm from the surface of the wooden element. In order du achieve the desired resistance to decay in the wooden element it is necessary that the temperature of the surface portions of said wooden element reaches over 200 ⁰ C. Only then can the heat treated wooden element achieve a resistance to decay comparable with the resistance to decay observed for impregnated wooden elements.

According to another embodiment of the invention the exposure of the wooden element to the heating medium is continued at least until the temperature reaches 230 ⁰ C at a point in the wooden element, located 5 mm, preferably 7 mm, most preferred 10 mm from the surface of the wooden element.

According to another embodiment of the invention the wooden element is exposed to the heating medium for 1 -200, 3-150 or 5- 100 minutes. Heat treatments have previously been performed for several hours to achieve a homogenous heat induced change in the lignin-carbohydrate in the wooden element. By the method according to the invention a homogenous heat treatment is not desired, therefore heat treatment is performed under shorter periods of time, which may be dependent on parameters such as the thickness of the wooden element heat treated. The invention also relates to a wooden element with improved quality, the wooden element having been heat treated using the method according to the invention.

According to another embodiment of the invention the wooden element is elongated, such as a wooden beam or pole.

According to another embodiment of the invention the wooden element has an essentially square or circular cross-section, perpendicular to the length direction.

Other advantages and advantageous features of the invention will appear from the dependent claims and the subsequent description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples. It is to be understood that the drawings are schematically depicted, and should therefore not be considered to be represent the present invention in scale.

In the drawings:

Fig 1 shows a very schematic cross-sectional view of a wooden element in a heating medium during heat treatment according to the invention,

Fig 2 shows a very schematic cross-sectional view of a wooden element in a heating medium during heat treatment according to the invention, and

Fig 3 shows a schematic perspective view of heating medium in a partly cut tank wherein a wooden element is placed during heat treatment according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Explained herein are preferred embodiments of the invention, describing the method and the wooden element of the invention. The invention may, however, be embodied in many different forms and should not be construed as being limited to the exemplary embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

Fig 1 shows in a cross-sectional view a wooden element 1 in a heating medium 2 during heat treatment using the method according to the invention. The wooden element may of course have any geometry, but here it is depicted with an essentially square cross-sectional shape. The method comprises heat treatment of the wooden element 1 by exposure thereof to the heating medium 2 having a temperature in the range of 200-300 ⁰ C. The exposure of the wooden element 1 to the heating medium 2 is interrupted before the temperature at any point in the wooden element 1 , located at a distance from the centre 3 of the wooden element 1 being 50%, preferably 70%, of the distance from said centre 3 to the surface 4 of the wooden element 1 , reaches 170 ⁰ C. Said distance is for clarifying purposes depicted in Fig 1 as a boundary 5, the boundary 5 comprising all said points at said distance from the centre 3. The exposure of the wooden element 1 to the heating medium 2 is continued at least until the temperature reaches 200 ⁰ C at a point in the wooden element 1 , located 5 mm, preferably 7 mm, most preferred 10 mm from the surface 4 of the wooden element 1 and/or for 1 -200, 3-150 or 5-100 minutes. The wooden element 1 resulting from this heat treatment will have surface parts 6 that have been heat treated to such an extent that said surface 6 parts will have a better resistance to decay, equally good as impregnated wooden elements, than the same would have without the heat treatment. Core parts 7 of the wooden element have not been exposed to a temperature higher than 170 ⁰ C, therefore said core parts 7 essentially have the mechanical properties of wood which not has been heat treated, i.e. native wood.

Fig 2 shows in a cross-sectional view a wooden element 1 1 in a heating medium 12 during heat treatment using the method according to the invention. The geometry of the wooden element can of course be any geometry, but here it is depicted with a rectangular cross-sectional shape, two of the sides in the rectangle being longer than the two other sides. The method comprises heat treatment of the wooden element 1 1 by exposure thereof to the heating medium 12 having a temperature in the range of 200-300 ⁰ C. The exposure of the wooden element 1 1 to the heating medium 12 is interrupted before the temperature at any point in the wooden element 1 1 , located at a distance from the centre 13 of the wooden element 1 1 being 70% of the distance from said centre 13 to the surface 14 of the wooden element 1 1 , reaches 170 ⁰ C. Said distance is for clarifying purposes depicted in Fig 1 as a boundary 15, the boundary 15 comprising all said points at said distance from the centre 13. The exposure of the wooden element 1 1 to the heating medium 12 is continued at least until the temperature reaches 200 ⁰ C at a point in the wooden element 1 1 , located 5 mm, preferably 7 mm, most preferred 10 mm from the surface 14 of the wooden element 1 1 and/or for 1 -200, 3-150 or 5-100 minutes. The wooden element 1 1 resulting from this heat treatment will have surface parts 16 that have been heat treated to such an extent that said surface 16 parts will have a better resistance to decay, equally good as impregnated wooden elements, than the same would have without the heat treatment. Core parts 17 of the wooden element have not been exposed to a temperature higher than 170 ⁰ C, therefore said core parts 17 essentially have the mechanical properties of wood which not has been heat treated, i.e. native wood.

In Fig 3 a schematic perspective view is shown of a wooden element 21 exposed to a heating medium 22 in a partly cut tank 28. The tank 28 comprises heating means for controlling the temperature of the heating medium 22 to 200-300 ⁰ C before the wooden element 21 is placed in the tank 28 and exposed to said heating medium 22. The heating medium 22 depicted in Fig 3 is an oil, such as a mineral oil or a non-curable vegetable oil but can also consist of an inert gas, such as nitrogen or water steam.

Of course the method according to the invention is not limited to heat treatment of entire wooden elements but can also be applied to parts of a wooden element, e.g. posts for a fence can be heat treated on the parts which are subjected to deteriorating factors, such as the parts to be placed in the ground, while other parts of the same posts are not heat treated using the method of the invention. Also the thickness of the heat treated outer layer of the wooden element can be optimized for certain applications.

The invention is of course not in any way limited to the embodiments described above. On the contrary, several possibilities to modifications thereof should be apparent to a person skilled in the art without departing from the basic idea of the invention as defined in the appended claims.