A method for coating a wooden plate and a wooden plate

This invention relates to a method for coating wood boards, in which method the surface of the wood board is coated with curable resin, and the resin is cured and simultaneously fixed to the wood board using heat and pressure. The invention also applies to wood board coated with cured resin.

Wood, used in the form of different wood products, is a beneficial and versatile material for many purposes, but it also has unfavourable properties. OH-hydroxy groups, methoxy groups, acid ester groups and other compounds contained in cellulose fibres are hydrophilic, binding abundant water, so they cause many kinds of damage to wood products. Because wood needs water as it grows, it contains vascular tissue along which water travels easily, going on to make hydrogen bonds with the polar groups in cell walls. This type of water bonding has certain ill effects in terms of the use of wood for human purposes. Changes in volume in the wood cells are disadvantageous in many uses. Absorbed water causes the wood to swell not only dimensionally but throughout the material, causing it to take an uneven shape. This phenomenon is greatly detrimental to coated boards if the coating material is not completely waterproof. Wood materials are generally glued and coated with thermoset plastic. For example the production of plywood consists mainly of the cross-wise adhesion of veneers using a liquid resin adhesive. Weather-proof exterior sizing can be carried out with phenol-formaldehyde resol adhesive. This resin adhesive contains alkaline catalysts such as sodium hydroxide (NaOH), which are powerfully hygroscopic, i.e. absorbent to water. They increase the wood's absorbent effect. Coatings are used to stop water from being absorbed and to increase the material's durability. For instance in the case of concrete casting, a coating helps to separate the wood board from the concrete surface.

The most common coating material is paper impregnated with phenol- formaldehyde resin, hardened into a glossy film by hot moulding.

Although this coating has many advantageous properties, it is not completely waterproof. When water is absorbed through the film, it causes uneven swelling of the underlying wood material, which is visible as an adverse rippling effect on the surface of the product. Uneven swelling also causes tensions and cracking in the coating, which in turn increases the absorbency of the wood tissue. When this kind of material is used in a concrete casting mould, the uneven swelling of the surface is reflected in the concrete cast's surface, causing aesthetic problems and additional work.

Natural wood material consists of grain fibres and clusters that contain intermittent crystal-like areas in which well-ordered fibres have bonded together with strong hydrogen bonds. These ordered areas alternate with amorphous areas in which the fibre chains are non-ordered. These areas have sufficiently large gaps to allow water molecules to enter easily and be absorbed due to the wood's hygroscopic groups. The water forms hydrogen bonds with the hydroxy groups in the cellulose, pushing weakly ordered fibre chains away. This causes the gaps to widen and allows more water to enter. This causes the swelling of the wood.

This can happen for instance with plywood boards (and other wood materials) if they are poorly coated with permeable films, paint or varnish layers. The abovementioned rippling effect is very disadvantageous in phenolic-film-coated wood composites when used for instance in concrete casting. The uneven ripples on the boards' surfaces negatively imprint on the finished concrete product.

Therefore, the surface of wood materials must be made impermeable with a coating such as a film pressed onto the wood's surface (e.g. phenolic films and other resin-impregnated papers). The coating is permeable enough not to stop the abovementioned rippling effect from taking place on the wood surface, being detrimental in many uses.

The aim of this invention is to present a method for treating the surface of wood materials and/or the inner components of wood materials such that the rippling effect is reduced or removed, and the swelling caused in wood by water is reduced.

The invention is characterised in that the surface of the wood board is treated before it is coated with curable resin, with a substance that contains a polyalkene glycol or modified acrylic resin. The wood board according to the invention is characterised in that the surface of the wood that touches the cured resin consists of a polyalkene glycol or modified acrylic resin.

In the first procedure according to the invention, the surface of the wood board is treated with a substance containing a polyalkene glycol. Alkenes include ethene, propene, butene, pentene, hexene, heptene, octene, nonene and decene. Of the substances presented in this application, the most favourable polyalkene glycol is polyethylene glycol (PEG). Polypropylene glycol (PPG) also has suitable properties. Mixtures of polyalkene glycols can also be used.

If the selected polyalkene glycol is polyethylene glycol, its molecular weight should be at least 400 (PEG 400), favourably at least 1 ,000 (PEG 1000) and most favourably at least 1 ,500 (PEG 1500). The greatest evenness is achieved for the wood board when the molecular weight of the polyethylene glycol is 1 ,500-2,500, which only allows the polyethylene glycol to penetrate the largest wood cells. As the molecular size increases, the melting point rises, which means that polyethylene glycols with a molecular weight equal to or greater than 1 ,000 are solid at room temperature. Therefore the treatment agent must be melted by heating or it must be made into an aqueous solution. The aqueous solution should be made such that the polyalkene glycol accounts for 35-75% by weight usually, or favourably 40-70% by weight. If using polyethylene glycol with a molecular weight of 1 ,000 or above, the water and polyethylene glycol must be heated to 50-60 ⁰ C to form the solution.

When the aqueous solution has been prepared, it is applied to the wood board's surface using a roller, or by spraying or pouring. A suitable amount for application is 60-100 g/m ² , favourably 70-90 g/m ² . Simple melted polyalkene glycol can be applied directly onto the wood board's surface. The wood board can be for instance a plywood board consisting of several interconnected layers of wood veneer. A particularly suitable material for use is birch plywood, as the treatment substance is absorbed less efficiently into softwood plywood.

The treatment agent is left to dry and/or dried before the next stage.

At the next stage the surface of the wood board is coated with curable resin. The curable resin can be in film form, for instance impregnated into paper, or in liquid form. In the latter case it is spread onto the wood board's surface and dried. The curable resin can be a urea, melamine or phenolic resin. Most favourably, the curable resin should be in film form.

The curable resin is pressed onto the wood board's surface at a temperature of 100-200 ⁰ C, applying a pressure of 1.0-2.5 N/mm ² . As the curable resin is attached to the wood board's surface, the treatment agent containing polyalkene glycol penetrates deeper into the wood material, which forms a polyalkene-glycol-treated layer at the wood material's surface.

In the second procedure according to the invention, the surface of the wood panel is treated with a substance containing a modified acrylic resin. The modified acrylic resin can be for instance carboxylated acrylic resin, hydroxylated acrylic resin, aminated acrylic resin, fatty- acid-modified acrylic resin or wax-modified acrylic resin. Suitable commercial brands include Hydrolast (AST Products, Inc., USA). The modified acrylic resin is applied to the wood board's surface using a roller, or by spraying or curtain coating. A suitable amount for

application is 30-400 g/m2, favourably 40-150 g/m2. 35 - 45 g/m ² is sufficient to slow down the absorption of water, but 95-105 g/m ² is needed for complete protection of the wood board. The wood board can be for instance a plywood board consisting of several interconnected layers of wood veneer. A particularly suitable material for use is birch plywood.

When the treatment agent has been spread onto the wood board, it is left to dry and/or dried before the next stage.

At the next stage the surface of the wood board is coated with curable resin. The curable resin can be in film form, for instance impregnated into paper, or in liquid form. In the latter case it is spread onto the wood board's surface and dried. The curable resin can be a urea, melamine or phenolic resin. Most favourably, the curable resin should be in film form. Suitable films include phenolic film with a basis weight of 120-220 g/m ² . All resin coatings with a weight of 60-400 g/m ² are suitable, as are thin plastic coatings. By thin plastic coatings we mean thermoset plastic and duroplastic with a thickness of less than 1 mm.

The curable resin is pressed onto the wood board's surface at a temperature of 100-200 ⁰ C applying a pressure of 1.0-2.5 N/mm ² .

Example 1.

PEG 200 and PEG 400 were compared. Coated wood boards treated with polyethylene glycol were examined with regard to surface evenness. Each board was rated. Ratings were given on a scale of 1 = worst and 5 = best.

Table 1.

Table 2.

The results show that PEG 400 gave better results than PEG 200 in every test, so the molecular weight of the used polyethylene glycol has an effect on the end result of the treatment.

Example 2.

Two different PEG treatments were given to samples. In each treatment, 80 g/m ² of PEG 400 was absorbed into the board's surface. The first sample piece was treated with 80 g/m ² of 100% PEG 400 and the second sample was treated with 130 g/m ² of 60% PEG 400. During spreading, no significant differences were observed in the absorption of the agent. After spreading, the samples were dried for 18 h. After drying, no significant differences were observed between the samples.

When the samples had dried for 18 h they were coated with paper impregnated with phenolic resin (Imprex® 40/120 FA5X). After the coating procedure, no significant differences were observed between the samples.

For ripple testing, the samples' coatings were scratched and subjected to water for half an hour. A similar coated wood board, not treated with polyethylene glycol, was used as a control sample.

After being subjected to water for half an hour, the control sample rippled substantially, while the rippling of the samples treated with polyethylene glycol was much more reduced. The results for 100% PEG were slightly better than for 60% PEG.

Example 3.

Four samples were treated with PEG 2000 and coated. The treatments are indicated in Table 3. The coating consisted of paper impregnated with phenolic resin (Imprex® 40/120 FA5X). The wood board consisted of birch plywood. After coating, the samples were scratched and subjected to water for half an hour. The test results are indicated in Table 3.

Table 3.

The results show that when the PEG concentration in the aqueous solution is high enough, the treatment has a significant reducing effect on the rippling of wood board due to moisture.

Example 4.

Three samples were treated with wax-modified acrylic resin such that 59 g/m ² , 86 g/m ² and 153 g/m ² of modified acrylic resin were spread with a roller. The modified acrylic resin was used undiluted. An untreated birch plywood board was used as a control sample.

The treated samples were left to dry; the suitable drying time ranges from 1 to 20 minutes depending on the amount applied. After this, a phenolic film with a basis weight of 120 g/m ² was pressed onto the surfaces by hot moulding. A similar phenolic film was pressed onto the control sample's surface.

The prepared boards were tested by making scratches with a depth of 150 and 225 μm. The scratches went through the coating. The scratches were covered with water-soaked paper and left in these moist conditions for one hour. After this the wet papers were removed and the results were evaluated.

It was observed that the control sample presented severe rippling. The application amounts of 59 and 86 g/m ² reduced rippling significantly. The application amount of 153 g/m ² stopped rippling altogether.