Method for heat treatment of timber

Abstract: Procedure for heat treatment of timber, in which procedure the timber is dried to a humidity level below 15 %, whereupon the temperature of the timber is raised above a heat treatment threshold, the timber is kept at the required temperature for as long as is necessary for the treatment and the temperature of the timber is lowered below the heat treatment threshold. In the procedure, above the heat treatment threshold, both when the temperature is being raised and when it is being lowered, the difference between the internal and external temperatures of the timber is kept within certain limits.

Description: The present invention relates to a procedure for heat treatment of timber, as defined in the preamble of claim 1. Such a procedure is known, for example, from WO-A-95/31680. International patent application WO-A-94/27102 presents a procedure for improving the resistance to biological decay and the dimensional stability of timber via heat treatment. The specification presents a procedure for the drying of timber at a raised temperature. The procedure aims at ensuring that no cracks are formed in the product. According to the specification, this is achieved by continuously determining the temperatures in the interior parts of the timber and on its exterior surface and continuously maintaining a reasonably small difference between the temperatures. This principle is applied both when the temperature is being raised and when it is being lowered during the drying process. After this, according to the specification, the temperature of the timber is raised rapidly to a level of over 150 °C, and the treatment is continued at this temperature to achieve the desired treatment result. By the method described, certain properties of timber can be improved, but it has been established that the final result is not exactly homogeneous, in other words, material portions at different depths in the timber undergo somewhat different treatment. The object of the present invention is to eliminate the drawbacks described above. A specific object of the present invention is to present a new type of procedure for heat treatment of timber, which makes it possible to produce timber that has a very homogeneous and good quality throughout its thickness. As for the features characteristic of the invention, reference is made to claim 1. In analysing heat treatment and the changes occurring in the structure of wood and in the substances contained in it, it has unexpectedly been discovered that the way in which the temperature of the wood is raised to the temperature required for the heat treatment and the way in which it is lowered are equally essential in actual heat treatment of timber as the regulation of temperature is in drying. It has been found that timber behaves differently at different temperatures, its structures undergc different changes at different temperatures and its constituents undergo different changes and chemical reactions at different temperatures, and that the duration of action of the temperature is just as decisive for many changes as the level of the temperature. Therefore, in the procedure of the invention, the timber is first dried to a suitable humidity level below 15 %, whereupon the temperature of the timber is raised above a heat treatment threshold and maintained at the required level for as long as is necessary for the treatment. Finally, the temperature of the timber is lowered below the heat treatment threshold. Heat treatment threshold means the temperature above which the desired changes produced by the heat treatment begin to appear. It is generally about 140 - 150 °C. Furthermore, above the heat treatment threshold, both when the temperature is being raised and when it is being lowered, the difference between the internal and external temperatures of the timber is kept within certain limits. The limit is preferably 10 - 30 °C, e.g. about 20 °C. According to the invention, the heating and cooling phases above the heat treatment threshold are of equal duration and, in addition, the absolute value of the difference between the internal and external temperatures is maintained at a substantially equal magnitude at temperatures corresponding to each other during heating and cooling. Therefore, practically every part of the timber is at the same temperature, i.e. subject to the same kind of chemical and structural changes in the timber, for a substantially equal length of time. Thus, the heating and cooling processes of the timber are rendered about identical, i.e. mirror image processes above the heat treatment threshold. In this context, external temperature of timber means the temperature of the air surrounding the timber, which is substantially the same as the surface temperature of the timber, and internal temperature means the temperature in the central part of the timber. In a preferred embodiment of the invention, both the internal and external temperatures of the timber are raised and lowered in a linear fashion. Likewise, preferably a constant temperature difference is maintained for a substantial part of the time during which the temperature is being raised or lowered. In a preferred embodiment of the invention, the temperature difference is changed as a function of temperature. In this case, when the temperature is high, the temperature difference is maintained at a low value, e.g. between 5-20°C, and when the temperature is lower, the difference can be increased, e.g. to a value within the range 20-40°C. Before the heat treatment, the timber should in any case be dried to a humidity below 15 %, but preferably the timber is dried to a humidity below 10, even below 5 %. The heat treatment threshold, above which the timber begins to undergo changes due to heat, is about 140-150°C, but the high limit of heat treatment may vary considerably depending on the kind and size of the timber and the changes intended to be produced in it. If the aim is to increase the strength, surface hardness, flexural resistance and elastic flexural strength of the timber, the maximum temperature may be 150-200°C, depending on the case. On the other hand, if the main objective of the heat treatment is to improve the rot-proof properties of the timber, the temperature of the timber should be raised to a value between 200 - 300 °C. Further objectives in both of the above-mentioned temperature ranges may be to reduce the propensity of the timber to expand and contract due to humidity, to retard the absorption of water and to change the colour of the wood. The heat treatment is preferably carried out in a substantially non-pressurised room, in other words, the treatment room may preferably have an overpressure of max. 0.1 bar. Whereas in prior art it has been common to use a high pressure, e.g. about 10 bar, to prevent cracking of the wood, the procedure of the invention uses a controlled and steady treatment, so that no overpressure is needed but the timber endures the treatment at normal pressure without cracking. Therefore, the structures of the treatment chamber can be built e.g. only from thin and light boards, whereas prior-art structures need to have pressure-resistant frameworks and wall surfaces. As compared with prior art, the procedure of the invention has significant advantages. In the invention, the real behaviour of wood during heat treatment has been clarified, and from timber subjected to heat treatment by the method of the invention it is possible to produce timber material of very homogeneous quality, having the desired properties uniformly throughout its thickness. This has not been possible in any method known so far. In the following, the invention will be described in detail by referring to the attached drawing, which is graphical illustration of the process stages of the procedure of the invention. The drawing shows the external temperature T o and the internal temperature T i of the timber as functions of time. At instant t 1 , when the heat treatment begins, the timber has been dried in a suitable temperature e.g. to a humidity of 5% and it has a roughly uniform temperature, about 140 °C. At this point, the surrounding air is subjected to intensive heating, producing an initial impulse S that gives the process a fast start. The temperature increase is stopped at 180 °C at instant t 2 and the internal temperature T i is allowed to rise. It begins to rise steadily and at instant t 3 the difference ΔT between the internal and external temperatures is at the desired value, about 20 °C, and at this point the raising of the external temperature is continued. From this point onwards, the external temperature T o and the internal temperature T i rise at the same rate, with the difference ΔT between them remaining unchanged. At instant t 4 , the external temperature reaches the desired maximum temperature of 250 °C. At this point, the raising of the temperature is stopped and this level of the external temperature is maintained. The internal temperature continues co rise, reaching the external temperature at instant t 5 . The heat treatment of the timber is continued at this maximum temperature of 250 °C until instant t 6 , whereupon the lowering of the external temperature is started. The internal temperature follows the external temperature with a certain time delay, and the external temperature is lowered in a steady and linear fashion so that the temperature difference ΔT between the internal and external temperatures is the same as when the temperature was being raised. The temperatures are lowered steadily down to the heat treatment threshold of 140 °C, at which point the heat treatment of the timber has been completed. The essential point in this embodiment is that the graphs representing the rise and fall in the temperature are mainly mirror images of each other, in other words, the absolute value of the difference between the internal and external temperatures is of equal magnitude in the same temperature range during heating and cooling. Moreover, in the embodiment described, this difference is constant for most of the time during the process. As the heating and cooling phases are equal in duration, every part of the timber is at the same temperature, i.e. subject to the same kind of temperature changes, for a substantially equal length of time. In the embodiment described above, before the heat treatment, the timber has been dried at a high temperature of over 100 °C, so the initial impulse S that gives the process a fast start occurs above the heat treatment threshold of 140 °C. But if the initial temperature of the timber is relatively low, i.e. clearly below the heat treatment threshold, then the initial impulse and the levelling of the temperature difference can be effected below the heat treatment threshold. In this case, the graphs representing the rise and fall in the cemperature are even more exact mirror images of each other. In the foregoing, the invention has been described by way of example by the aid of the attached drawing while different embodiments of the invention are possible within the framework of the inventive idea defined by the claims.

Claims: Procedure for heat treatment of timber, in which procedure the timber is dried to a humidity level below 15 %, whereupon the temperature of the timber is raised above a heat treatment threshold above which the desired changes produced by the heat treatment begin to appear, the timber is kept at the required temperature for as long as is necessary for the treatment, the temperature of the timber is lowered below the heat treatment threshold, and above the heat treatment threshold, both when the temperature is being raised and when it is being lowered, the difference between the internal and external temperatures of the timber is kept within certain limits, characterised in that, above the heat treatment threshold, the heating and cooling phases are of equal duration and the absolute value of the difference between the internal and external temperatures is maintained at a substantially equal magnitude at temperatures corresponding to each other during heating and cooling so that every part of the timber is at the same temperature for a substantially equal length of time. Procedure as defined in claim 1 characterised in that the temperatures are raised and lowered in a linear manner. Procedure as defined in claim 1 or 2 characterised in that the temperature difference is kept constant for a substantial part of the time during which the temperature is being raised or lowered. Procedure as defined in claim 1 or 2 characterised in that the temperature difference is changed as a function of temperature. Procedure as defined in any one of claims 1 - 4, characterised in that the timber is dried to a humidity below 10, preferably below 5 %. Procedure as defined in any one of claims 1 - 5, characterised in that the heat treatment threshold is about 140 °C. Procedure as defined in any one of claims 1 - 6, characterised in that, during the heat treatment, the temperature of the timber is raised to a level in the range 150 - 200 °C to increase the strength, surface hardness, flexural resistance and elastic flexural strength of the timber. Procedure as defined in any one of claims 1 - 6, characterised in that, during the heat treatment, the temperature of the timber is raised to a level in the range 200 - 300 °C to improve the rot resistance properties of the timber. Procedure as defined in any one of claims 1 - 8, characterised in that the heat treatment is carried out in a substantially non-pressurised room.

Inventors: LALLUKKA TERO, LALLUKKA, TERO, Lallukka, Tero, c/o Stellac Oy
Applicants: STELLAC OY
Application year: 1998
Main classification: F26B21
IPC codes: B27K5, F26B21