EP1198329A1

EP1198329A1 - Process for producing compressed wood pieces

Info

Publication number: EP1198329A1
Application number: EP00944077A
Authority: EP
Inventors: Pertti Viitaniemi; Saila JÄMSÄ; Paavo Kontinen; Pentti Ek
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 1999-06-29
Filing date: 2000-06-29
Publication date: 2002-04-24
Also published as: AU5831300A; FI991483A7; WO2001000377A1; FI991483A0; FI110241B

Abstract

The invention relates to a method of manufacturing compressed wooden pieces. According to the method, the wooden piece is compressed into a desired shape and is conducted to heat treatment at an increased temperature, in order to set the change in shape caused by compressing. According to the invention, the wooden piece is compressed at an increased temperature, the wooden piece being kept in compression until its moisture content has decreased to less than 15%, and after compressing the wooden piece is heated at normal atmospheric pressure in the presence of steam at a temperature of more than 180°C, until a weight loss of at least about 3% has taken place in the product. By means of the invention, the dimensional stability of compressed wood can be improved. Compressed wooden pieces manufactured according to the invention swell very little when soaked in water and, after drying, the wooden pieces return to their original thickness, whereas traditionally manufactured compressed wooden pieces suffer from permanent thickness swelling.

Description

PROCESS FOR PRODUCING COMPRESSED WOOD PIECES,

The present invention relates to a method, according to the preamble to Claim 1, of manufacturing compressed wooden pieces.

According to the method, the wooden piece is compressed into a desired shape and is conducted to a heat treatment at an increased temperature, to set the change in shape caused by compressing.

Compressing can be used to increase the surface hardness of a wooden piece. Compressed wooden products are therefore used, for example, in flooring. A drawback with compressed wooden products is that, in damp conditions, the compressing returns at least partially to its original state. Thus, a typical permanent thickness swelling of a compressed wood is more than 30 % when soaked in water.

The thickness swelling can be reduced by impregnating the wooden piece with chemicals, such as acid anhydrides, before compressing. However, this technique can only be applied to soft species of timber, such as that of coniferous woods.

Youichi Ito and his workgroup have shown that the change of shape in compressed wood can be made permanent by treating the compressed wood with steam in an autoclave (Y. Ito et al.: Compressive-Molding of Wood by High-Pressure Steam- Treatment: Part 1 and Part 2, Holzforschung, 52 (1998) 211 - 221). This known solution includes first softening the wooden pieces with saturated steam at 150°C, after which they are compressed. The temperature is then increased to 190 - 200°C and steaming is continued for a few minutes at this temperature. In this known method, it is essential to keep the wooden pieces under pressure for the entire duration of the treatment and to carry out the heat treatment at an increased pressure. For this reason, to implement the method, Ito and his workgroup have had to develop new high-pressure, steam-pressing/moulding equipment, capable of withstanding pressures of up to 2 MPa. It is obvious that the investment costs of such special equipment are greater than those of equipment operating at normal atmospheric pressure. Nor should the risk of possible damage, due to the use of such high operating pressures, be overlooked. In the state of the art, heat treatment has been successfully used to reduce moisture expansion in wooden pieces. FI publication 955391 discloses a heat-treatment method, according to which wood dried to a moisture content of less than 15 % is treated at normal atmospheric pressure, at a temperature of more than 150°C, in the presence of water vapour, until a weight loss of at least 5 % has taken place in the product.

According to the application, treatment of this kind improves the mould and rot resistance of products, as well as their dimensional stability. However, the application only deals with dried, and thus not compressed, 'cellulose-based products'. Thus, there is naturally no reference in the application to the possibility of using heat treatment to set a change in shape.

The purpose of the present invention is to eliminate the drawbacks of the state of the art disclosed above and to create an entirely new type of solution for manufacturing compressed wooden pieces.

The invention is based on the surprising observation that heat treatment will set a change of shape induced in a wooden piece by means of compressing. According to the method of the present invention, the wooden piece is first compressed into the desired shape. The compressing stage is carried out at an increased temperature, the wooden piece being simultaneously dried to a moisture content of less than 15 %. The change in shape created by compressing is set by means of heat treatment, which is carried out at normal atmospheric pressure in the presence of steam. The temperature of the heat treatment is more than 180°C. The heat treatment is continued, until a weight loss of at least about 3 % has taken place in the product.

More specifically, the method according to the invention is principally characterized by what is stated in the characterizing part of Claim 1.

Considerable advantages are gained with the aid of the invention. Compared to wood that has not been heat treated, there is a substantial improvement in dimensional stability. When soaked in water, compressed wooden pieces manufactured according to the invention swell considerably less than compressed wooden products that have not been heat treated. In addition, wooden pieces manufactured using the method according to the invention return to their original thickness after drying, whereas traditionally manufactured wooden pieces suffer from permanent thickness swelling.

The surface hardness of wooden pieces manufactured according to the invention is at least as good as wooden pieces than have been compressed, but not heat treated. In the solution according to the invention, there is no need to carry out heat treatment at an increased pressure. A process that operates at normal atmospheric pressure is safer and cheaper than the high-pressure method of the state of the art.

Wooden pieces manufactured using the method according to the invention can be used, for example, as floor planks, in parquet, in tables, and generally in applications requiring hardness and strength.

In the following, the invention is examined with the aid of a detailed description and examples, with reference to the accompanying drawings.

Figure 1 shows temperature as a function of time, during the heat treatment of compressed wood according to the invention.

Figure 2 shows the effect of heat treatment according to the invention on the dimensional stability of compressed birch.

Figure 3 shows the effect of heat treatment according to the invention of the dimensional stability of compressed pine.

Definitions

In connection with the present invention, the term 'wooden piece' is used to describe a piece of any species at all of timber, which may vary even very greatly in size and shape. Typically, it is solid timber, for example, sawn goods, such as a plank, board, log, or column, or it may be a very small or thin or irregularly shaped piece. The method according to the invention can be used to treat any species of timber at all, but the method is particularly suitable if the wooden piece is of birch, spruce, pine, aspen, cherry, oak, beech, maple, linden, rowan, ash, or alder.

'Compressed dimension' refers to the dimension by which the thickness of the wooden piece diminishes when compressed. 'Degree of compression' refers in turn to the compressed dimension divided by the original thickness of the wooden piece. The number in percent thus obtained is then multiplied by one hundred.

In connection with the present invention, the term 'total compressing time' refers to the period of time during which the wooden piece is subject to an external compressive force. The total compressing time includes not only the time during which the wooden piece is actively compressed, but also the time for which the counter-plate is already in contact with the stop and for which the press plates continue to be held closed.

'Thickness swelling' refers to the percentage increase in the thickness of the wooden piece when soaked in water. 'Permanent thickness swelling' refers in turn to thickness swelling that scarcely returns to its original state after the piece has been dried.

The complete process

In the compressing stage, it is preferable to use damp timber, which is compressed into the desired shape against gas-permeable surfaces, while the timber is being heated. Heat is used, so that the damp wooden piece will dry more rapidly. In terms of the implementation of the invention, there is no difference whether the wooden piece being pressed has been partly or entirely dried in a separate stage, but typically there is an attempt to save both time and energy, both of which are consumed in large amounts in separate drying. On the other hand, damp timber is also easier to compress. What is essential, is that the moisture content of the wooden piece before heat treatment is less than 15 %, preferably less than 12 %, and especially preferably 10 % at most.

Compressing can also be carried out without simultaneous heating, in which case the compressed wooden piece is heated only after pressing. However, the simultaneous heating of the piece to be compressed represents a quicker, and thus an economically more interesting solution.

In a preferred embodiment of the invention, the compress apparatus used comprises press plates, the surfaces of which can be heated. Though heat can be alternatively applied by blowing, it is particularly preferable to use plates that can be heated. There are typically two press plates, which are typically located above and below the piece to be compressed, though in principle the wooden piece can also be compressed using more press plates and from any direction at all. The press surfaces are preferably covered with screens that are permeable to gas, especially water vapour. The screens can be, for instance, perforated plastic or metal, or wire mesh. The pieces to be compressed are placed between the screens. The extent of pressing is limited by means of stops located in the ends of the press plates. The thickness of the stop determines the degree of compression, when the stop catches on the opposing press plate, the wooden piece is no longer subject to such a compressive force that would reduce the thickness of the wood. The degree of compression of the wooden piece is preferably at least 20 %, especially preferably about 30 - 40 %, or even greater.

The press apparatus disclosed in FI patent 91947 is preferably used.

During the compressing stage, the temperature of the wood is about 80 - 170°C. The temperature of the wood is regulated during pressing, so that in the initial stage the temperature is typically at the upper limits of the above range, typically about 150 -

170°C, and in the final stage of compressing the temperature is lowered to the lower limits of the said range, typically about 80 - 125°C.

During compressing, it is preferable to ensure that splits do not form in the product. This can be achieved by continuously determining the temperatures of the inner part and correspondingly the outer surface of the wood and continuously keeping the difference between the temperatures reasonably small. The said difference is most suitably about 30°C, preferably about 10 - 30°C, and especially preferably about 15 - 25°C. This is done both when raising and lowering the temperature.

The speed at which the press plates close determines the compressing pressure. Typically, the closing speed of the plates is set to 5 - 10 mm/h, preferably 7 - 8 mm/h.

The pressure becomes higher the closer the press plates are pressed to each other.

Typically, it is attempted to maintain an even pressing speed even, but it is certainly also possible to vary the pressing speed during pressing. Thus, in the first stage, the damp wooden piece is compressed rapidly to the desired shape and in the second stage the pressing pressure is reduced to a level that will cause at least as great a change in shape as the shrinkage caused by drying. The first stage typically lasts for about 10 - 30 minutes, after which the pressing pressure is reduced to about one-quarter of the original pressure.

The duration of the pressing depends not only on the intended compressed dimension, but also on the desired moisture content of the compressed wood. In connection with this invention, a moisture content of less than 15 % is aimed at before heat treatment. This is typically achieved with a total pressing time of 3 - 10 h, preferably about 5 - 7 h.

After the compressing stage, the wooden piece is removed and taken to a separate heat- treatment stage. In the heat-treatment state, the wooden piece is no longer pressed.

Heat treatment is used to set the thickness swelling of the compressed wooden piece to a value of at most 10 %, preferably at most 8 %, and especially preferably at most 6 %, all of them after a soak period of 1 week. Usually, the swelling is in the order of 3 - 6 %. Heat treatment can be used to achieve a permanent thickness swelling of about 1 % or less, preferably at most about 0.5 %.

Heat treatment takes place in a kiln, which in principle can be of any known type whatever, or even a quite simple enclosed space. The kiln must have, however, inlet and outlet connections for water vapour, and devices for heating the wooden piece to a sufficiently high temperature. Typically, the kiln is heated by blowing hot air into it. Often, steam and heating air are fed through the same duct, in which case the heating air will be moist. A preferred kiln solution is disclosed in FI patent application 955391.

In the heat treatment, the temperature of the wood is raised rapidly to above 180°C (typically to about 180 - 250°C), at which temperature the treatment continues until a weight loss of at least 3 %, preferably at least 5 % (weight by dry substance) is achieved in the product being treated. The preferred operating temperature is more than 200°C, especially preferably about 220 - 250°C. Heat treatment takes place at essentially normal atmospheric pressure, in a saturated steam atmosphere. As feeding water vapour may cause a slight excess pressure in the kiln, the expression 'essentially normal atmospheric pressure' must be understood so that the pressure in the reaction space can vary to about 20 kPa less, or typically more than normal atmospheric pressure. In the heat treatment, the heating can be controlled in a similar manner to that used in pressing, i.e. by continuously determining the temperatures in the interior and correspondingly on the outer surface of the wood and continuously maintaining a reasonably small difference between them. In this stage too, the most suitable said difference is at most about 30°C, preferably about 10 - 30°C, and especially preferably about 15 - 25°C. This is done both when raising and lowering the temperature.

Typically, the entire heat treatment lasts about 1 - 40 hours, preferably about 10 - 30 hours. The wood's internal temperature is kept at the desired temperature for typically at least 0.5 h, preferably 1 - 10 h, and especially preferably about 2 - 10 h. Heat treatment time and temperature are dependent from each other. According to a preferred embodiment of the invention, the wood's internal temperature is kept at a value of 220 - 250°C for about 2 - 5 h, preferably for about 3 - 4 h.

The following examples illustrate the invention.

Example 1

Pressing of birch samples

In the experiment, two fresh birch plants were treated. Table 1 shows the dimensions and initial weights of the planks.

Table 1

No. Dimensions Initial weight Kl 51 mm * 102 mm * 630 mm 2380 g

K2 53 mm * 105 mm * 580 mm 2427 g

The conditions used in compression were:

Temperature: 170°C

Time: 3 Vi h Stops: 32.5 mm

Screens: Plastic screens The press was closed slowly at a speed of 7 mm/h. Closing was kept at the desired rate by increasing pressure when required. A compressed dimension of 10 mm (degree of compression 20 %), required a pressure of 1.5 MPa. A compressed dimension of 15 mm (degree of compression 30 %), required a pressure of 3.1 MPa.

The press heating was cut after 3 h, when the temperature measured inside plank Kl was 150°C, rising from this to a maximum of 165°C. When the press was opened, the temperature was 151°C. The samples were 34-mm thick, so the degree of compression was 35 %.

Example 2

Heat treatment of pine samples

The samples were fresh. Table 3 shows the dimensions and weights of the samples.

Table 2.

No. Dimensions Weight

Ml 63 mm * 152 mm * 550 mm 3267 g M2 " 2743 g

M3 " 2771 g

M4 " 3300 g

M5 63 mm * 152 mm * 500 mm 2549 g

Compressing took place under the following conditions

Temperature: 170°C

Time: 6 54 h

Stops: 40 mm

Screens: Plastic screens

An attempt was made to maintain a press closing speed of 8 mm/h. A compressed dimension of 13 mm (degree of compression 20 %) required a pressure of 0.8 MPa. In the final stage of compressing, the pressure was 1.4 MPa. Heating was cut after 2 h 40 min, when the temperature measured inside plank Ml was 150°C. The press was opened once the temperature had dropped to less than 100°C. The total pressing time was 6 54 h.

After compressing, the moisture content of the test planks was about 12 %. Test planks Ml, M4, and M5 had an even thickness of about 40 mm. Test planks M2 and M3 were about 40-mm thick at the ends, but were slightly thicker in the centre.

The final degree of compression was 37 %. Table 3 shows the weights of the samples after compression.

Table 3.

No. Weight

Ml 2515 g

M2 2420 g

M3 2391 g

M4 2449 g

M5 2123 g

Example 3

Heat treatment of the compressed samples

After the compressing described in Examples 1 and 2, the sides of the test planks were planed and they were cut transversely into two parts, the second part of each test plank being subject to heat treatment.

The heat treatment was carried out 'normally', so that the temperature measured inside the planks was 230°C for 3 h. Figure 1 shows the curves depicting the change in temperature.

Table 4 shows the weights of the samples before and after the heat treatment.

Table 4.

No. Weight before heat treatment Weight after heat treatment Mi x 1147 g 972 g

M2 x 1111 g 895 g

M3 x 1075 g 876 g

M4 x 1109 g 943 g

M5 x 1000 g 850 g

Kl x 916 g 767 g

K2 x 966 g 811 g

Example 4

Swelling of the compressed and heat treated pieces

After heat treatment, the samples were planed and 100-mm long pieces were sawn from their ends, from which the thickness swelling in water soaking was determined. After soaking in water for a week, the samples were dried absolutely dry and their thickness was measured, to determine if the samples had permanent thickness swelling.

At the same time, the thickness swelling of reference pieces was measured. Pieces of birch and pine, which had been compressed but not heat treated, were used as the reference pieces. Table 5 shows the results of the measurements.

Table 5

Time Swelling % Swelling %

(Compressed + heat treated piece) (Reference piece) BIRCH

1 day 1 25.5 1 week 4.5 47.2 Permanent 0.1 36.8 PINE 1 day 1.3 8.2

1 week 6.2 25.2 Permanent 0.3 18 The results shown in Table 5 are shown in graphic form in Figures 2 and 3. Figure 2 shows the effect of heat treatment on the dimensional stability of birch and Figure 3 shows the effect of heat treatment on the dimensional stability of pine.

Table 6 and the figures show that heat treatment has quite substantially improved the dimensional stability of the compressed wood. The thickness swellings of the compressed and heat-treated pine and birch are only a fraction of those of the compressed pine and birch. Following the drying of the samples after soaking, the thickness of the heat-treated wood has returned to very close to its initial thickness, whereas a large permanent thickness swelling has remained in the compressed (but not heat treated) wood.

Brinell hardness were determined from the second parts of the samples. Brinell hardness were also determined from a corresponding point in the compressed, but not heat- treated reference pieces, so that the hardness results show as clearly as possible if heat treatment has affected the hardness of the compressed wood.

Brinell hardness was measured with a 10-mm diameter ball, which was pressed against the surface of the wooden piece with a force of 50 kp for 30 seconds. The surface area (mm²) of the depression was measured, and Brinell hardness calculated by dividing the force used by the depression's surface area. Table 6 shows the Brinell hardness.

Table 6

Sample Brinell hardness N NVuJmber of tests

Average Standard deviation

PINE

Compressed + heat treated 1.52 0.63 30

Reference 1.42 0.54 30

BIRCH

Compressed + heat treated 2.55 0.55 12

Reference 2.56 0.69 12 The Brinell hardness of the compressed and heat-treated pieces have been of quite the same order as those of the reference pieces that were only compressed. Though in the case of pine the heat-treated pieces were even slightly harder than the reference pieces, but the difference cannot be regarded as significant (the t-test value of 0.65 is nowhere near the 'almost significant' limit of 2.00). In any event, the results in the table show clearly that heat treatment has not reduced the hardness of the pressed pine and birch.

Claims

Claims:

1. A method of manufacturing compressed wooden pieces, according to which method

- the wooden piece is compressed into the desired shape and

- it is conducted to heat treatment at an increased temperature, to set the change of shape caused by compressing,

characterized in that

- the wooden piece is compressed into shape at an increased temperature,

- the wooden piece is compressed, until its moisture content has reduced to less than 15 %, and

- after compressing, the wooden piece is heated at normal atmospheric pressure, in the presence of steam, at a temperature of more than 180°C, until a weight loss of at least about 3 % has taken place in the product.

2. A method according to Claim 1, characterized in that, in the first stage during compressing, the damp wooden piece is pressed rapidly into the desired shape and in the second stage the compression is reduced to a level that causes at least as great a change in shape as the shrinkage due to drying, and that as the compressing proceeds the temperature of the wood is lowered.

3. A method according to Claim l or2, ch ar ac t eri z e d in that during compressing the temperature of the wood is 80 - 170°C.

4. A method according to any of Claims 1 -3, characterized in that the wooden piece is compressed to achieve a degree of compression of at least 20 %.

5. A method according to any of the previous Claims, characterized in that the heat treatment of the compressed wooden piece takes place at more than 200°C.

6. A method according to any of the previous Claims, characterized in that the heat treatment of the wooden piece is continued, until a weight loss of at least about 5 % has taken place in the product.

7. A method according to any of the previous Claims, characterized in that during the heat treatment the temperature inside the wood is maintained within a range of 220 - 250°C for about 2 - 5 h, preferably for about 3 - 4 h.

8. A method according to any of the previous Claims, characterized in that the heat treatment is carried out in such a way that the permanent thickness swelling in the wooden piece is about 1 % or less.