The invention relates to a method for preparing a wood veneer plate. In the method, wood veneer surfaces are coated with resin. Then, wood veneers are piled on top of each other and, finally, they are pressed together for curing the resin.

Wood veneer plates are usually prepared by cutting or rotary-cutting wood into veneer. Before the cutting or rotary-cutting, sawn and barked wood is steamed or soaked to increase the temperature of wood in order for it to plasticise. The normal veneer thickness is about 0.5-5 mm. The veneers are dried, sorted and possibly plugged. Then, the veneers are coated with resin and assembled into plates. The plates are heated and pressed, whereby the resin cures and binds the veneers together. Finally, the plates are trimmed, polished and sorted.

In prior art, veneers are coated with resin and assembled such that the resin is applied on both sides of every second veneer and, except for one outer surface, on one side of each veneer, the veneers are placed on top of each other and the resin is cured by pressing and heating. Most common resins are phenol- formaldehyde resin and urea-formaldehyde resin. Melamine-formaldehyde resin is also used, particularly when strength and clearness are required of the glued joint. It is also known to impregnate a completed veneer plate to improve its stability, fire resistance, appearance, dimension stability and processibility. Then, micromolecule phenol-formaldehyde resin is often used, which has the particular property of penetrating the cell walls of wood and reacting to decrease the hygroscopicity of wood (Kirk-Othmer, 3 ^rd Ed., Vol. 14, pp. 10-15).

It is a requirement of the wood veneer plate that the strength of the joint between the veneers is sufficient. Only if breaking occurs in the wood and not in the glued joint when breaking off veneers from each other, the joint is strong enough. Strong resin glue as such does not guarantee that the breaking occurs in the wood. If there are weakening points, such as rotary-cut splits, in the wood veneer, the glue does not adhere or is excessively soaked in the wood and the breaking occurs in the glued joint.

Traditionally, wood veneer plates improved by impregnating have been prepared in the following way. First, dried wood veneers are enforced by impregnating them with impregnation resin which is absorbed within the veneer. Then, the impregnated wood veneers are dried, which is followed by coating them with resin glue. The resin glue is traditionally applied on both sides of every second veneer and, when piling the veneers, the coated veneer is placed between two uncoated veneers. Lastly, the piled veneer stack is prepressed and finally pressed in increased temperature for curing the resin glue.

Wood veneer plates prepared by traditional methods are not as strong as required. The breaking still occurs in the glued joint, not in the wood. Furthermore, separate impregnation and drying take too much time. Replacing them by increasing the dosage of resin glue did not succeed either. It was found in the tests that the wood break of the wood veneer plate (share of an off-broken surface) or its strength was not improved even though the dosage of resin glue was more than doubled.

The object of the invention is to provide a preparing method of a wood veneer plate which produces strong veneer but avoids awkward work stages. As mentioned at the beginning, in the invention, resin is applied on wood veneer surfaces, the wood veneers are piled on top of each other and hot pressed together for curing the resin.

Said object is now mainly achieved by applying resin on wood veneer surfaces which face each other when piled i.e. on both surfaces forming the same joint. Resin is applied on at least 50%, advantageously at least 75%, most advantageously 100% of the surfaces facing each other when piled and forming a glued joint when pressed.

Applying the resin on facing joint surfaces thus intensifies its absorption in the veneers and results in a stronger joint than the above preimpregnation of veneer and/or increasing the dosage of resin only on one joint surface.

Even though the gluing of facing surfaces according to the invention can be performed for preimpregnated veneers, preimpregnation is not necessary. The gluing of facing surfaces plugs and strengthens the veneers better than the combination of preimpregnation and one-sided gluing according to prior art. Thus, it is advantageous that the facing surfaces on which resin is applied are non- impregnated i.e. of wood which is raw or pre-processed in some other way than by impregnating.

On each surface facing each other when piled is applied advantageously 100-300 g/m ² , most advantageously 155-175 g/m ² of resin. For a joint is thus used 200- 600 g/m ² , advantageously 310-350 g/m ² of resin. These amounts have been experimentally found the best. The thickness of wood veneers employed in the invention is advantageously 0.5-5.0 mm, most advantageously 1.0-3.5 mm. The number of wood veneers is advantageously 3-70, most advantageously 12-24, and they are advantageously chosen of birch, aspen, pine and spruce or other wood species suitable for the method. The most advantageous wood species is birch.

In the invention, resin is applied on facing surfaces. In this context, resin refers to unmixed resin or a mixture substantially containing it. Resin is typically applied on wood veneer surfaces by spraying, casting, foaming, using liquid extrusion and/or by using rolls. The most advantageous coating method is application by means of rolls. In the invention, on surfaces facing each other is typically applied resin which is chosen from the group of heat-setting resins. Heat-setting resins include phenolic resins (reaction product of phenol, o-, m- and p-cresol, p-t-butyl phenol, p-octyl phenol, p-nonyl phenol, p-phenyl phenol, bisphenol-A and/or resorcinol with formaldehyde, acetaldehyde, paraldehyde, glyoxal and/or fural) and amino resins (reaction product of melamine, urea, benzoguanamine, dihydroxy ethylene urea, aniline and/or acrylamide with formaldehyde).

Advantageous resins are melamine resin i.e. the reaction product of melamine and formaldehyde and urea melamine resin i.e. the reaction product of urea, melamine and formaldehyde. Totally superior results are obtained with melamine resin, see examples. When using melamine resin, it is advantageously in the form of aqueous solution. The solids content of the aqueous solution is advantageously

45-70% by weight, most advantageously 55-65% by weight. The mole ratio of formaldehyde and melamine in melamine resin is preferably between 1.4-3.0 and, according to an embodiment, the resin has been modified with acetoguanamine, caprolactam, urea and/or glycol.

If required, it is possible to include a catalyst in the resin which facilitates the curing of the resin when hot pressing. The resin can also contain a filler. Typical fillers are inorganic fillers, such as chalk and kaoline, and organic fillers, such as wood flour, cellulose, starch and protein. The most advantageous filler is soy flour belonging to proteins. Mixtures of these fillers can also be employed. The amount of the filler is adjusted such that the viscosity of the mixture is obtained suitable, advantageously between 10-60 s, most advantageously between 20-40 s (Ford cup 6/20 ⁰ C). Furthermore, the resin can contain other ingredients, such as anti- microbe, anti-insect, anti-moisture and fire-resistant materials. The method described above can include other stages in addition to said stages. After having applied the resin, the glued veneers are let to stand without pressure. After piling and before hot pressing, the veneers are advantageously prepressed, advantageously for 5-40 minutes in the temperature of 10-40 ⁰ C and the pressure of 0.1-1.0 MPa. The hot pressing of wood veneers piled on top of each other for curing the resin according to the invention is performed advantageously for 5-40 minutes in the temperature of 100-140°C and the pressure of 1-3 MPa. Finally, the plates can be trimmed, polished and sorted.

Next, an exemplifying embodiment will be described with the sole purpose of illustrating the invention.

Example

The raw material was birch veneer with the thickness of 1.5 mm, aqueous solution of melamine resin with the pH of 9.5, with the viscosity of 77 mPas/20°C and the dry content of (1 g, 3 h, 105 ⁰ C) 62% by weight, and a filler.

The impregnation gluing mixture was prepared by mixing together 100 portions of the aqueous solution of melamine resin and 6 portions of the filler. Then, the resin content was 59% by weight, the dry content 64% by weight and the viscosity 30 s (Ford cup 6/20°C).

165 g/m ² of the impregnation gluing mixture was applied on all surfaces of all veneers except for the outer surface of the plate. There were 13 pieces of veneers per plate.

After the gluing, the veneers were piled into preform panels. The duration of the gluing and the following letting to stand was about 20 minutes. The preform panels were prepressed for 8-10 minutes in the pressure of 0.8 MPa. The web time i.e. the time between prepressing and hot pressing was about 30-100 minutes. The duration of hot pressing was 25 minutes and took place in the temperature of 125 ⁰ C and the maximum pressure of 1.8 MPa. The completed plate was recovered for testing.

As a comparison, two plates of the same size were used, which were prepared by applying a gluing mixture based on phenol-formaldehyde resin on both sides of every second veneer, by assembling the veneers such that the glued veneer remains between two unglued veneers and by pressing the veneers together. The tables enclosed in this specification show the preparing and properties of veneer plates according to the invention and according to prior art.

As it is evident from the test description and the tables, better veneer properties are obtained when all veneers are coated with resin, particularly when the resin is melamine resin.

Tables: Used resin compositions, pressing conditions and strength results in the method according to prior art and according to the invention

Veneer impregnation, test 6 M surface veneers not glued

MF = melamine formaldehyde, PF = phenol formaldehyde, PRF = phenol resorcinol formaldehyde

Resin composition

Mixture 1

MF resin 1 ,000 g

Soy flour 6O g

1 ,06O g

Resin % 57.5

Curing time, 100 ⁰ C, 2 h 6 min 30 s

Viscosity Fc 6 / 25°C 25-35 s

Viscosity when gluing

34.6 s (2 h)

1. gluing

Assembly 13 * 1.4 mm birch veneer

Veneer about 3% moisture (%) 4

Gluing All veneers glued Every second glued

Roll 200 g/nr 170 g/m ² 150 g/m ² 250 g/m ² application

Piling + open time from 20 / 40 20 20 40 rolling to prepressing, min

Prepressing 0.8 MPa / 10 min Hot pressing 125 ⁰ C 1.8 MPa 1.1 MPa 0.4 MPa

25 min (total 8 min 13 min 4 min time) (time in (time in (time in pressure) pressure) pressure)

κ

OO

V

V

VO

2. gluing

Gluing 2 pieces with mixture 1

Assembly 13 * 1.4 mm birch veneer

Moisture about 3%

Gluing All veneers glued

1. gluing g/m ²

Prepressing 0.8 MPa / 8 min

Web time 0.5-1 h

Hot pressing 125 ⁰ C 1.8 MPa 1.1 MPa 0.4 MPa

Total time 25 min 8 min 13 min 4 min

18 min 7 min 8 min 3 min

13 min 6 min 5 min 2 min

V

V

Veneer impregnation

Test 6: Gluing conditions

MIXTURES 1-5 MIXTURE 0

Assembly 13 * 1.4 mm birch veneer

Moisture about 3%

Gluing All veneers glued Surface veneers not glued Every second veneer glued

(invention)

200 g/m ² 155 g/m ^a Ki

Piling + open 20 min 20 min

Prepressing 0.8 MPa / 8 min 0 .8 MPa / 8 min

Web time 0.5-1 h 0.5-1 h

Hot pressing 125°C 1.8 MPa 1.1 MPa 0.4 MPa 128 ⁰ C 1 .8 MPa 1.1 MPa 0.4 MPa

25 min 8 min 13 min 4 min 15 min 6 min 7 min 2 min

V

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