FIELD OF TECHNOLOGY

The invention concerns a board-making method, wherein a wooden board is coated with a thermoplastic layer.

BACKGROUND OF THE INVENTION

It is a known practice to coat wooden boards with a ther¬ moplastic layer so that the melted plastic layer is allowed when cooling to attach directly to the board surface.

However, the poor adhesion of plastic poses a problem.

DESCRIPTION OF THE INVENTION General description A board-making method as defined in claim l has now been invented. Some advantageous applications of the invention are presented in the other claims.

It is essential according to the method that a particle layer is attached to the surface of the wooden board with glue and a thermoplastic layer is melted on top of the par¬ ticle layer and it will adhere to the particles when cooling. In this way the plastic will adhere very strongly to the surface.

The board is suitable for use in structures where a good moisture-resistance is required. For example, various struc¬ tures in production premises of the food industry and concrete moulds are such structures.

Closer description A wooden board can be plywood or veneer, a chipboard, fibreboard or massive wood. Any kind of wood can be used, such as spruce, birch or pine.

The particle to be glued is preferably a wood-based par¬ ticle, such as woodfibre, dust, chip or paper chaff. The wood-based particle may be mixed with, for example, synthetic plastic, especially the same plastic as the one melted onto the surface. A suitable particle quantity is 10 - 500 g/sq.m. , preferably about 100 g/sq.m. The particle may also be, for example, a nonwoven fabric or a chopped strand mat.

The glue may be, for example, isocyanate, polyurethane or phenol or some other resin glue. MDI or FF glues are especially suitable for use. A suitable glue quantity is 10 -

300 g/sq.m., preferably about 50 g/sq.m. The thermoplastic is preferably a polyolefine, such as polyethylene. Naturally, it may also be a mixture. A suitable quantity is 100 - 5000 g/sq.m., preferably 1000 - 2000 g/sq.m. The plastic is preferably a granulate or a similar material. Recycled plastic may also be used. The method can be implemented in such a way that the glue is first spread out on the surface of the wooden board and particles are scattered on top of it and then a thermoplas¬ tic, whereupon the workpiece is put into a hot-press at such a high temperature that the thermoplastic will melt. The glue hereby binds the particles onto the surface of the wooden board and the melting thermoplastic surrounds the particles. Finally, the board is allowed to cool, whereby the hardening plastic adheres strongly to the particles. Cooling may take place, for example, in a cold-press or in a pile together with releasing sheets. Alternatively, the plastic may be melted before pressing, for example, using an IR heater. This has the advantage that moisture is allowed to evaporate freely during heating. It is more difficult to control this process in a hot-press. The wooden board can also be coated on both sides at the same time. When a fixed or removable frame is also used in the board press the board edges are also coated.

Adhesion of the coating can be further improved by treating the surface of the wooden board mechanically before glueing. The surface can be treated i.a. by roughening (for example, by wire brushing a wet wooden surface) , perforating, grooving or serrating. The surface is preferably treated by planing across the grain.

Example 1

MDI glue was spread out in a quantity of 50 g/sq.m. onto the surface of softwood plywood (thickness 12 mm) . Pinewood fibre in a quantity of 200 g/sq.m. and made with a Bauer grinder was spread out on top of the glue. Recycled

polyethylene granulate in a quantity of 2000 g/sq.m. was scattered on top of the fibre layer. The coated plywood was compressed under a pressure of 1.2 N/sq.mm. at a temperature of 170°C for 8 minutes and it was cooled immediately in a cold-press under the same pressure for 2 minutes.

In a cross-drawing test (BFU 20) the boards were broken away from the basic board.

The coating adhered well after a boiling test (BFU100) .

Example 2

A mixture of MDI glue and phenol resin (50 %) was spread out in a quantity of 100 g/sq.m. onto the surface of softwood plywood (thickness 12 mm) . The mixture contained 25 % of MDI or 75 % of the resin solution. Fine chips in a quantity of 100 g/sq.m. and made with a Pallman grinder were scattered onto the glued surface. Recycled polyethylene granulate was scattered onto the chip layer in a quantity of 1000 g/sq.m. The coated plywood was compressed under a pressure of 1.2 N/sq.mm. and at a temperature of 170°C for 6 minutes and it was cooled immediately in a cold-press under the same pres¬ sure for 2 minutes.

The boards broke away from the basic board in a cross- drawing test (BFU 20) .

The coating adhered well after a boiling test (BFU100) .

Example 3

A mixture of MDI glue and phenol resin (50 %) was spread out in a quantity of 50 g/sq.m. onto the surface of softwood plywood (thickness 12 mm) . The same mixture was also spread out onto the board edges. The mixture contained 25 % of MDI or 75 % of the resin solution. Fine chips in a quantity of 100 g/sq.m. and made with a Pallman grinder were scattered on top of the glued surface. The same chips were also scattered onto the edges of the board. Recycled polyethylene granulate in a quantity of 2000 g/sq.m. was scattered on top of the chip layer. The coated plywood was compressed under a pres¬ sure of 1.2 N/sq.mm. and at a temperature of 170°C for 6 minutes and it was cooled immediately in a cold-press under the same pressure for 2 minutes. When being compressed, the

board was in a metal frame which was wider by 2 mm at each edge. In this way the melted plastic entered between the board and the frame and the edges of the board were also coate . The boards broke away from the basic board in a cross- drawing test (BFU 20) .

The coating adhered well after a boiling test (BFU100) .