The present invention relates to a metlκ>d accc dmg to the preamble of c^^ binding modification chemicals and for preventing internal cracks in wood pieces which are treated at increased temperatures. The treated wood pieces are large enough for practical applications such as for use in pieces of furniture.

It is known in the art that wood products can be plasticized, e.g. for shaping, by using modification chemicals. Traditionally- water and ammonia have been used as plastificaάon chemicals, but also other chemicals suitable for the above purpose are disclosed in the patent literature. Thus, FI Patent Specification No. 43008 discloses a method for plastification of wood by the use of hydrazine, whereas FI Patent Specification No. 91503 anticipates a method of using aqueous solutions of maleic acid.

Immediately after impregnation in liquid state, the modification chemicals are usually fixed or bound to the wood by heatin or by using a combination of heat and pressure.

There are some problems associated with the prior art. Thus, depending on the concentration (in %) of the substance, considerable amounts of solvent are also absorbed into the wood. If the binding of the modification chemicals takes place at an increased temperature, the modification chemical is easily released from wood together with the solvent. In such a case, it is not possible to achieve decreased swelling and improved surface hardness, as desired. A large temperamre difference between the inner and the outer surfaces will cause cracking of wood pieces designated for practical applications. With small test pieces hardly any such cracking can be noted and the problem has not been given any consideration in the art.

It is an object of the present invention to eliminate the problems associated with the prior art and to provide an entirely novel method for treating wood, which method makes it possible to prevent release of modification chemicals from wood during reactions taking

place at high temperatures and simultaneously to prevent the forming of internal cracks in the wood.

The invention is based on the concept of drying a wood product which has been treated with modification chemicals until it is so dry that no cracks are formed during hardening of the product and no seeping of modification chemicals takes place. In particular, the wood product is dried at a temperature of less than 100 °C to a moisture content of less than 20 %.

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

The method according to the present invention is suitable for all solvent-based, in particular water-soluble, modification chemicals which are to be bound to the wood at increased temperatures. Typically, as a result of the heat treatment, ester bonds are formed between the modification chemicals and the OH groups of the wood (e.g. of cellulose and/or hemicellulose). Examples of suitable modification chemicals are maleic acid and maleic anhydride and maleic acid employed in aqueous solutions possibly together with glycerol. By using maleic acid and maleic anhydride it is possible to plasticize wood for (cf. the aforementioned FI Patent Specification No. 91503). Other modification chemicals are, for example, succinic anhydride and the corresponding acid which are suitable for thermoplasticizing of wood, as well as phthalic anhydride and the corresponding acid.

According to one particularly preferred embodiment, the wood product is impregnated essentially deeper than superficially with maleic anhydride, with a mixture of maleic anhydride and glycerol, with succinic anhydride or with phthalic anhydride.

It is contemplated within the scope of the present invention that term "the wood product which is to be treated" means a product made of wood, the shape or properties of which are to be modified. It may, for example, be desired to bring the wood product into a foπnable or plastic state in order to shape it, or the wood is treated in order to improve

its dήnensional stability or, in particular, its surface hardness. According to the invention, the product to be treated is typically at least moderately thick, usually its thickness is larger than about 5 mm (« massive wood piece). It can comprise a uniform piece of wood which is made of solid wood, but it can also comprise a laminar product (such as a chip board or wood fibre board) made with wood veneer or wood chips. The wood product can also comprise a structure whose individual wood layers are bound together to form a laminate. In the last mentioned cases the wood product means the layered or laminated product to be prepared which can be impregnated either before the product is formed or subsequent thereto.

In the following, the invention will be examined in more detail using maleic acid and maleic anhydride as an example. It should be pointed out that, subject to suitable modifications, other modification chemicals can also be used in the method.

The invention comprises three stages, viz. treatment of the wood pieces with modification chemicals, drying of the wood pieces and heat treatment of the wood pieces (binding of the modification chemicals to the wood by using heat).

During the first stage of the method, the wood pieces are typically impregnated with an aqueous solution of maleic acid or maleic anhydride. Preferably, the modification agent also contains glycerol. This mixture is preferably made with maleic anhydride and glycerol. The substances are mixed in water, maleic anhydride being hydrolysed in aqueous solution to maleic acid. It is preferred to impregnate the wood raw material with an aqueous solution of maleic acid and glycerol having a concentration of 1 to 99 % , wherein the weight ratio of maleic acid to glycerol is 1 : 10 to 10: 1. In particular, a rather concentrated aqueous solution is used, having a concentration of about 20 to 60 wt-% and a weight ratio of about 1:5 to 5: 1 between the maleic acid and glycerol. The amount of solution impregnated into the wood product depends on the intended use of the wood product and on the wood raw material (wood species, wood density and wood moisture). Usually, the wood material is impregnated with so much solution mat it penetrates essentially deeper than superficially into the wood product. If the wood product is to be subjected to a mechanical shaping treatment, it is important that also the inner parts of

wood product are as soft as possible. Thus, the wood is preferably impregnated with at least about 20 kg, preferably about 25 to 300 kg and in particular about 30 to 140 kg of a maleic anhydride and glycerol per cubic metre of the wood. The humidity or moisture content of the wood subjected to impregnation is preferably 5 to 30 %, in particular about 6 to 7 %.

The impregnation can be carried out by first removing air in a pressure vessel from the wood material under reduced pressure; then feeding the impregnation solution into the vessel and allowing the solution to penetrate at normal pressure or, possibly, at excess pressure (a method known as the vacuum-pressure method). Alternatively, the wood material can be immersed into the impregnation solution for such a long period of time that the solution has time to penetrate deep into the wood (the immersion method). If the solution of maleic acid/glycerol is to be applied solely onto the surface of the wood, the impregnation can also be carried out by surface application.

After the treatment with the modification chemicals, the wood product is slowly dried at low temperature, whereby the solvent is removed leaving the active chemical in the wood. The drying is continued until the humidity of the wood is less than 20 %, preferably even below 15 to 17 % . The drying can be carried out at room temperature, but it is preferred to dry the wood product at an increased temperature of less than 100 °C. Preferably the drying is carried out at a temperature in the range of 20 to 99 °C, in particular at 30 to 95 °C. Any known heating device for drying the wood piece in the presence or absence of water vapour can be used for the heat treatment, such a drying oven, an incubator or an equivalent device.

According to one preferred embodiment, the drying of the wood product is carried out by continuously deteπnining the temperatures of the inner parts of the wood product and the surface of the wood, respectively, and by keeping their difference moderately small. In particular said temperature difference is maintained at 30 ^D C, at the most. The expression "the inner temperature of the wood" means the temperature of the wood at some distance from the surface of the wood. Depending on the shape of the wood piece, the "inner temperature" is determined typically at a depth of at least about 2 to 5 cm.

The "outer temperature" signifies the temperature on the wood surface or slightly below it. Thus, the "outer temperature" often corresponds to the ambient temperature. The temperature is gradually increased so that die temperature dϋ-fference between the surface of the wood and the interior thereof does not exceed 30 °C. Preferably said temperature difference is 10 to 30 ^C C. This embodiment can be carried out by ^g * ^ta -~ ^h ing at least two sensors to a test piece, one of which measures the inner temperanire of the wood and the other the surface temperature. For drying larger amounts of wood products, several test pieces should be equipped with sensors. On an industrial scale, a heating program can be determined for each group of wood products which takes into account the influence of the starting moisture on the process.

After drying, the impregnated and predried wood products are subjected to a heat treatment carried out at a temperature above 100 °C, preferably at about 100 to 200 °C.

When the substance is used for plasticizing wood, the heat of the wood generated during drying can be utilized. If the wood has cooled after the drying it has to be heated again to a temperature above 50 ^β C but preferably below 150 "C, e.g. to about 50 to 100 °C or even 120 °C. The heating time typically ranges from a few minutes to a couple of hours. The treatment time depends on the thickness of the piece and on the amounts of impregnation solution; normally a 30 to 90 minute treatment is enough, but also shorter or longer periods of time are possible. According to one preferred embodiment the heat treatment is carried out by steaming at normal pressure. The wood containing modification chemicals is then plasticized by heating, whereby it can be bem into desired shape and/or compacted. The bending of the wood after impregnation and heating is considerable.

After shaping of the wood, the modification chemicals are fixed or bound to the wood produα, for example in an incubator, at a temperanire of at least 150 ^B C, normally about 150 to 200 °C, preferably at about 170 to 180 °C. The duration of the heat treatment is typically about 10 min to 3 hours; depending on the thickness of the wood piece and on the amount* of impregnation solution even longer or shorter times can be used. During the heat treatment, volatile components are evaporated which enhances cross-linking

reactions within the wood.

The binding can also be carried out directly to a dried wood product, e.g. in a theπno- compactor or heat press at temperatures in excess of 100 °C. The pieces can also be compacted in the compressor.

Based on our tests, considerable advantages are obtained by the present invention. Thus, the test pieces are coloured through and their surface hardness is considerably increased. The treatment also rf™™th- ^» « swelling of the wood and improves its stability against decay. There are no inner cracks in the wood.

In the following, the invention will be examined in more detail with the aid of a few working examples.

Examples 1 to 3 concern a situation wherein the test pieces have been subjected to heat treatment at 150 °C immediately after the impregnation without any intermediate drying. Then the substance seeps out and inner cracks are formed in the pieces. The ASE values for decreased swelling are poor. Examples 4 and 5 show a situation wherein the test pieces are dried to a moisture content of less than 20 % before binding at an increased temperature; no seeping of the substance takes place.

Example 1 (comparison)

When test pieces of pine (size: 20 mm X 20 mm x 360 mm) were impregnated with a 40 % MG solution, the amount of the absorbed effective substance was 260 kg/m ³ . ASE value of decreased swelling in tangential direction was, after a two-day water immersion, 40 % and in radial direction 42 % in comparison to a control specimen saturated with water.

Example 2 (comparison)

The absorption of test pieces vacuum impregnated with a 40 % MG solution amounted to

130 kg/m ³ . Swelling in radial direction decreased by 18 % in comparison to an nntn^t^A sample.

Example 3 (comparison)

Wood pieces of alder and birch with a thickness of 20 to 23 mm, width of about 50 mm and a length of about 1 m, were impregnated with a 30 % maleic anhydride/glycerol mixture. The test pieces were hardened in a heat press at 160 °C and compacted by about 20 %. After the compression the thickness was about 18.5 mm. Cracks were formed in about 70 % of the compressed wood pieces during the initial phase of steaming and the substance seeped out.

Example 4

Test pieces (25x100x500 mm) of pine and spruce were impregnated with MG solutions having various concentrations of 20, 30 and 40 % in amounts of 100 to 300 kg effective substance per cubic metre. After impregnation, the test pieces were dried at a temperature of 70 °C for 2 to 7 days before the maleic acid was bound to the wood at 160 ^Q C (duration of treatment: 2 hours). After a one-day immersion into water the tangential swelling (ASE) in the birch pieces decreased by 81 to 96 % and radially by 80 to 94 %. The amount of absorbed water decreased by 70 to 95 % in comparison to an untreated sample. The ASE swelling of the pine pieces was 70 to 87 % in tangential direction. The amount of absorbed water decreased by 37 to 88 % in comparison to untreated wood.

The wood pieces treated in the above way are suitable for use in moist conditions outdoors and indoors, for instance in bathrooms.

Example 5

The intermediately dried pieces of Example 4 were compressed with about 20 % in a beat press at 160 °C. No substance seeped out of the test pieces nor could any inner

cracks be seen. The test pieces were immersed into water for 1 day. The thickness swelling of the birch pieces was 1.2 to 1.6 % and the breadth swelling was 0.5 to 1.1 %.

The thickness swelling of the pine pieces was 1.4 to 3.8 % and the breadth swelling 1.2 to 3.6 % . The untreated, compressed reference pieces almost completely recovered their former dimensions after one day of water immersion. The Brinell hardness of the birch and pine pieces was 5 to 10.

Due to intermediate drying the effective substance did not seep out during compression. As a result, enough of the substance is retained and the compression is not reversed even in humid conditions.