FI104286B - Ways to prevent internal cracks in wood - Google Patents

Description

104286

A method for preventing internal wood splitting

The present invention relates to a process for the prevention of internal wood splitting at high temperatures according to the preamble of claim 1.

Heat is needed in wood drying, heat modification and generally in various manufacturing processes. When wood is heated, it expands. In general, the thermal expansion of wood has been kept low and no attention has been paid to heat treatment. In fact, however, 10 only the thermal expansion along the length of the tree is small; perpendicular to the reasons it is even larger than aluminum. The large thermal expansion in the transverse direction of the wood easily causes internal cracks unless it is taken into account in the development of various types of processes.

The object of the present invention is to eliminate the problems of the prior art and to provide a solution for controlling the phenomena related to the thermal expansion of wood, in particular to prevent internal cracks in wood in cases where the wood is heated above 90 ° C, typically above 100 ° C.

The invention is based on the idea that in processes where the wood is kept at elevated temperature, the temperatures of the inner wood and the outer surface of the wood, respectively, are continuously determined and their difference is kept consistently low. This is the case both in the heating step, in which the temperature of the wood is increased to dry the wood, for example, and in the cooling step, in which the wood is cooled back to the base temperature. Most preferably, said temperature difference is at most about 30 ° C. It has surprisingly been found that the solution presented can even completely prevent the formation of cracks in the wood.

More specifically, the method of the invention is essentially characterized by what is set forth in the characterizing part of claim 1.

In the present invention, the "indoor temperature" of wood is the temperature that exists in the wood some distance from the outside of the wood. Depending on the shape of the piece of wood, the "internal temperature" of 1042866 is typically measured at a depth of at least about 2 to 5 cm.

"Outdoor temperature" means the temperature on or just below the surface of the wood. Thus, in many cases, the "outdoor temperature" corresponds substantially to the ambient temperature.

5

When wood according to the prior art is heated, for example, to a degree of dryness of wood joinery at about 200 ° C, it quickly warms up to the same temperature, i.e. about 200 ° C. However, the heating of the wood interior stops at about 100 ° C until the humidity of the wood interior has dropped below 5%. It is only after this that the temperature inside the core begins to rise rapidly. As a result of the warming-up, the temperature differences, when combined with the drought of the wood, produce internal cracks which are not visible on the surface but which nevertheless render the wood unusable. In the invention, the temperature is gradually increased so that the temperature difference between the wood surface and the interior cannot be increased by 30 ° C. Preferably, the temperature difference is maintained at 10-30 ° C.

15

The solution according to the invention can be implemented by applying at least two sensors to the test piece, one measuring the internal temperature of the wood and the other measuring the surface temperature. When drying larger quantities of wood, more specimens should be equipped with sensors. On an industrial scale, it is advantageous to determine, on the basis of the measurement results, a suitable heating program for each timber, taking into account the effect of the initial humidity on the process.

According to an alternative embodiment, the actual heat treatment according to the invention is started only when the humidity of the wood interior has dropped below 10%, in some cases even below the above 5% limit. Thus, in the first step of this alternative, a piece of wood at room or factory temperature is placed in a drying oven at a temperature of about 110 to 140 ° C to remove most of the water. Thereafter, when the internal temperature of the wood has increased to at least that value, the temperature of the ambient, i.e., drying oven, is gradually increased, keeping the difference between the internal and external temperatures of the wood 30 to about 30 ° C.

The solution according to the invention is particularly applicable to heat drying of wood 3 104286 and heat treatment. In hot drying, the wood is dried, for example, to less than 15% moisture, in some cases preferably to less than 10% moisture.

The present invention achieves significant advantages. Thus, the invention is capable of accelerating normal drying, controlling special drying at temperatures above 100 ° C, and developing compression drying applications.

The invention will now be further explored by means of the accompanying drawings and a few embodiments.

10

Fig. 1 shows the temperatures measured at different points of the wood as a function of time when heating fresh spruce at 220 ° C for 5 hours according to the prior art, Fig. 2 shows corresponding temperature measurements of fresh spruce heated to the same temperature for 24 hours, The difference of -15 during heating has been kept below 30 ° C in accordance with the invention, Fig. 3 shows the measurement result (conventionally obtained) for the wound and Fig. 4 shows the temperature measurement results obtained with the method according to the invention.

20

Example 1: Treatment of spruce at elevated temperature

When wet spruce (50 x 100 x 1500 mm) with a humidity of about 40% was heated for 25 hours as shown in Figure 1 without any limitation on the temperature difference between the inner and outer surfaces, small internal cracks were observed in the specimen splitting. As a result of the treatment, the piece was dried to a final moisture content of less than about 5%.

When a wet specimen of the same size from a wet moon to a spot was heated for 24 hours in accordance with the invention, the solution of the invention was heated with a temperature difference of 30 ° C, with no difference in the specimen (Fig. 2). The dry specimen had a final moisture content of less than about 5%.

, 104286 4

Example 2

Wound Treatment at Elevated Temperature 5 Dry aspen (38 x 100 x 1500 mm) was heated for 7 hours using the solution of the invention, resulting in a quick and controlled end result with no cracks in the lumber. When the corresponding heat treatment was carried out for 3 hours without any limitation of the temperature difference between the inner and outer surfaces, small internal cracks were found in the interior of the pieces, which render the wood practically unusable.

10

In both cases, the final moisture content of the wood was less than 5%.

Claims (4)

5, 104286 A method for reducing the formation of internal cracks in wood during heat treatment above 90 ° C, characterized in that the internal wood part 5 is brought to less than 10% humidity, whereupon the ambient temperature is gradually increased so that the temperature measured inside and outside the wood the difference is not more than 30 ° C. A method according to claim 1, characterized in that said temperature difference is maintained at 10-30 ° C. Method according to claim 1, characterized in that the wood is subjected to drying in a drying oven at a temperature of about 110 ° C to 140 ° C before heat treatment to reduce the moisture content of the wood. 15 Method according to Claim 1, characterized in that the temperature difference is maintained at 10 to 30 ° C when the wood is cooled. • * 6 104286