Technical field

The disclosure generally relates to the field of forming of panels, e.g. building panels. More particular, the disclosure relates to a method of forming floor panels and floor panels produced by the method .

Known technology

Traditional laminated panels, intended to be used for e.g. flooring or furniture components, are produced by the following steps:

• applying a melamine formaldehyde resin impregnated kraft paper as balancing layer on one side of a core of wood fibre based material for example HDF;

• applying a by melamine formaldehyde resin impregnated printed decor paper on the other side of the core;

• applying a melamine formaldehyde resin impregnated transparent overlay paper with wear resistant particles, for example aluminium oxide, on the decor paper; and

• curing the resin by applying heat and pressure in a continuous or discontinuous press to obtain a laminated product.

Typical press parameters are 40 bar pressure and a temperature of 160-200 C° with a pressing time of 12-30 seconds. The surface layer has generally a thickness of 0, 1 -0,2 mm, the thickness of the HDF core varies between 6 - 12 mm and the balancing layer is about 0, 1 -0,2 mm thick.

This production method and product produced by such methods are generally referred to as the DPL process and DPL products (Direct Pressure Laminate) Recently new panels with a wood powder based surface have been developed. Powder comprising wood fibres, binders preferably melamine particles, aluminium oxide particles and colour pigments are scattered on a HDF board and pressed under heat and pressure in a continuous or discontinuous press to obtain a product with a paper free and solid surface layer. The press parameters are similar to DPL. A higher pressure of 40-80 bar and a longer pressing time of 20 - 40 seconds may be used when the surface is formed with deep embossing and a thickness of for example 0,4 - 0,6 mm. The pressing temperature is generally 160-200 C°. Such wood fibre based floors, generally referred to as WFF floors, have considerably better properties than traditional laminate floors since a thicker and more impact- resistant and wear-resistant surface may be produced in a cost efficient way.

Figures 1 a-1 d show production of a laminate or WFF floor panel according to known technology. A surface layer 2 of an impregnated overlay and a decorative paper or a powder layer is applied on the upper part of a HDF core 3. A balancing paper or powder-backing layer 4 is applied on the backside of the HDF core as shown in figure 1 a. The core 3 with the upper 2 and lower 4 layers is moved into a press and pressed under heat and pressure such that the layers are cured and attached to the board as shown in figure 1 b.

The layers on the front face and the rear face are exposed to a first shrinking when the thermosetting resin in the upper and lower layer cures during pressing. The backing layer at the rear face balances the tension that is created by the surface layer of the front face and the panel is substantially flat with a small convex backward bending when it leaves the press. Such first shrinking and balancing of the panel is below referred to as "pressing balancing". The second temperature shrinking, when the panels is cooled from about 160-200 C° to room temperature, is also balanced by the backing layer and the panel 1 is essentially flat as shown in figure 1 b. The second balancing is below referred to as "cooling balancing". A small convex backward bending is preferred (not shown) since this counteracts upward bending of the edges in dry conditions when the relative humidity may go down to 20% or lower during wintertime.

The problem is that this essentially flat panel comprises tension forces caused by the shrinking of the surface and balancing layers. The panel is generally cut into several elements and formed into floor panels with locking systems on long and short edges as shown in figure 1 c. The locking system may comprise, for example, generally a tongue 10 and a tongue groove 9 for vertical locking and a strip 6 with a locking element 8 that cooperates with a locking groove 14 for horizontal locking.

The surface and the core will swell in summertime when the indoor humidity is high and shrink in wintertime when the indoor humidity is low. The panels will shrink and expand and a cupping of the edges may take place. The balancing layer is used to counteract such cupping. This balancing is referred to as "climate balancing".

Figure 1d show that the internal tension may cause problems when the locking system is formed and especially when the panel is installed in a dry climate, which causes the layers to shrink. A so called topping of the edges may occur when the internal tension opens up the groove 9 and bends the tongue part 10 upwards due to the fact that these parts are no longer balanced when the locking system is formed. The internal tension may also cause a backward bending of the strip 6, which decreases the locking strength and the quality of the locking system. The tension may be higher than the internal bonding strength of the core, which may cause cracks C, primarily in horizontally extending grooves, such as the tongue groove 9.

Such internal tension may require a higher board quality than normal and this may increase the costs.

It would be a major advantage if the tension caused by the curing and temperature shrinking could be reduced or completely eliminated. Such a tension reduction would be especially favourable in WFF floors that generally comprise rather thick solid surface layers with considerable amount of thermosetting binders or high quality DPL floors for commercial application where the thickness and the resin content of the surface layer is higher than in DPL floors for domestic use. Summary of the Invention and Objects thereof

An overall objective of embodiments of the present invention is to provide a method to reduce or eliminate tension in panels with a surface comprising thermosetting resins that is cured and connected to a core under heat and pressure. Embodiments of the present invention may combine various process flexibility and product features as listed below.

A first aspect of the invention is a method for producing a panel, for example a building panel, preferably a floor panel, with a wood based core and a surface layer comprising a thermosetting resin wherein the method

comprising the steps of:

• curing and connecting the surface layer to the core under heat and pressure in a first main pressing step;

• applying a bending force on the panel after the first main pressing step such that it bends backwards away from the surface layer in hot condition; and

• releasing the bending force such that the panel springs back in an

essentially flat shape.

In an embodiment, the surface layer may be directly connected to the core. In an embodiment, the surface layer may be connected to an intermediate layer, the intermediate layer being directly connected to the core.

The method may further comprise the step of decreasing the surface temperature during bending.

The temperature decrease may be about 20 C°.

The temperature decrease may be more than 20 C°. The resin may be a melamine formaldehyde resin.

The method may comprise a bending that is at least about 3 cm/m.

The surface layer may comprise wood fibres and wear resistant particles.

The surface layer may comprise wood fibres, wear resistant particles and colour pigments. A second aspect of the invention is a floor panel comprising a wood based core and a surface layer comprising a thermosetting resin produced according to the first aspect and that the panel has no balancing layer.

The floor panel according to the second aspect may have a back face, which is the wood based core.

The floor panel may be a DPL panel comprising a surface layer of

impregnated paper bonded to a HDF core.

The floor panel may be a WFF panel comprising a powder based surface layer. Brief description of the drawings

The disclosure will in the following be described in connection to exemplary embodiments and in greater detail with reference to the appended exemplary drawings, wherein:

Figures 1 a - 1 d show known production of a floor panel according to known technology.

Figures 2a - 2d show production of a panel according to a first embodiment of the invention.

Figures 3a - 3d show production of a panel according to a second

embodiment of the invention. Figures 4a - 4c show a method to measure internal tension in a panel.

Figures 5a - 5b show the panel temperature during pressing and cooling.

Figures 6a - 6d show a method to obtain plastic deformation in a panel.

Description of Embodiments of the Invention

Figure 2a shows a panel 1 that has been pressed with only a surface layer 2 applied on a core 3. The panel may be a DPL panel or a WFF panel. The shrinking of the surface layer 2 during pressing, when the thermosetting resin has cured, causes the panel edgesl to bend upwards. The bending may increase further when the panel 1 is cooled down to room temperature. The bending is such that the panel 1 is not possible to use for flooring production. The reason is that there is no backing layer that balances the shrinking of the surface layer 2 during pressing and cooling.

The panel has essentially the same surface temperature as the press table when it leaves the press. A normal pressing temperature is about 170 C°. The core has generally a much lower temperature of about 80 -100 C°. This means that the surface layer will cool down very quickly after the pressing. After 10 seconds, the surface temperature may be 120 C° and after 15-20 seconds only about 80 C°. The cooling of the surface and the core is effected by the pressing time. A longer pressing time will increase the core

temperature more than a short pressing time.

Figure 2b shows that the panel 1 is pressed in a second pressing step between curved pressing tables 5, 7 when the surface layer 2 is still hot after the first pressing step. If such post pressing is made at a temperature of about 70-90 C°, it will deform the hot HDF core and create a counter pressure that is sufficient to produce a flat panel without a backing layer. A plastic nonreversible deformation will take place.

A post pressing at higher surface temperatures will also stretch and deform the surface layer 2 and eliminate the tension caused by the shrinking in the first main pressing step. Such surface stretching is preferably made at temperatures around 140 - 160 C°

The second pressing and surface stretching step is preferably a cold pressing and it is an advantage if the panel is moved to the second pressing and bending station as soon as possible in order to maintain a high initial surface temperature when the bending starts. For example, the peak surface temperature may be in the range of 160-200°C, for example at about 170°C during the first pressing, where the surface layer is connected to the core. When the bending step starts the surface temperature may be at least at 140°C, or at least 120°C. For example, when the bending may step starts the surface temperature may be within 30°C, or within 40°C, or within 50°C, of the peak surface temperature during the first pressing, where the surface layer is connected to the core. A decrease of the temperature in bended and stretched position will generally be obtained automatically when the core cools down the surface. This will stabilize the surface layer in a stretched position and a permanent deformation may be obtained. Internal tension in the panel may be reduced considerably or completely eliminated and the panel may be completely flat when it is cooled to room temperature as shown in figure 2c.

The production balancing and the cooling balancing that is made with a backing layer may be replaced partly or completely with plastic deformation.

The bending may be such that the panel is convex and bends backwards when it leaves the second pressing station. The panel may be slightly bended in a third strep upwards towards the surface such that a predetermined shape is obtained. This type of overstretching and double bending may reduce the internal tension completely. The temperatures, bending time and bending magnitude should be adapted to the quality and specification of the surface and the core.

Heat may be added in the second surface-stretching step in order to improve and control the stretching and to adapt the production method to various surface thickness and resins. The surface may also be cooled in order to stabilize the panel in a pre-determined condition. The second post pressing operation and the bending of the panel may be made in several ways. It is preferred that the bending and stretching is three- dimensional such that the bending is made along and across the panel. The press tables are preferably curved in two perpendicular directions like a top of a ball. The backward bending may also be made with pressing cylinders, frames, pressing against a rubber or silicone matrix, with rollers and in many other ways where the panel is bended in hot condition after the initial main pressing operation where the thermosetting resin is cured.

The thermosetting resin may be adapted such that it is easy to stretch. Resins similar to the types used in so-called post-forming laminate qualities may facilitate surface stretching by bending. The surface layer may also be stretched by lateral pulling forces applied at the panel edges.

It is also possible to heat up the panel again after cooling in a separate production step with for example a hot press table, infra-red lamps and similar in order to soften and stretch the surface layer in order to obtain a flat panel. A first cylinder may bend the panel backwards along the length and a second cylinder may after preferably an intermediate heating bend the panel along the width. The panels may also be held in a bended position in the cooling part after the pressing. Figure 2d shows that a moisture barrier 4 may be used of for example a plastic foil, treated paper, foam etc. in order to prevent moisture to penetrate into the panel from the sub floor. Such a backing layer may be used to obtain the climate balancing. In some application the bending of the HDF core may be sufficient even for the climate balancing. The Surface Stretching Process (SSP) according to embodiments of the invention offers one or more or all of the following advantages:

• a more cost efficient board with lower internal bonding strength may be used.

• topping that causes high wear at the edges and decreases locking strength due to internal tension may be eliminated.

• splitting of the board in the locking system may be avoided.

• cost savings may be obtained by eliminating the backing or replacing the paper/melamine backing with a thinner or more cost efficient material. Figure 3a shows an embodiment with a thin backing layer 4 that is not able to balance the panel 1 after initial pressing. Figure 3b shows bending and preferably also stretching of the surface layer 2. The result is a flat panel as shown in figure 3c with a backing layer 4 and reduced internal tension. The backing layer prevents moisture to penetrate into the backside of the panel 1 and will stabilize the panel when humidity changes. Figure 3d shows three- dimensional bending of a panel 1 . The magnitude of the bending B is preferably about 3 - 6 cm/m or more.

Figures 4a - 4c show a method to measure internal tension in a panel. A groove 1 1 is formed in the panel edge. A high internal tension will separate the edges as shown in figure 4b. A large difference between the inner panel thickness T1 and the outer panel thickness T2 means that the panel 1 has a large internal tension that may cause topping and cracks in the locking system, especially in dry conditions. Figure 4c shows a panel 1 with a low internal tension. For example, a panel with a reduced internal tension will have a T2 that is lower than a T2 for the same panel that did not have method, such as the bending step, performed on the panel.

This measuring method may be used to adjust the parameters of the backwards bending in the second pressing step by modifying initial pressing temperature, final pressing temperature, pressing time and the magnitude of the bending.

Figure 5a shows the temperature curve during pressing of a WFF floor panel with a 0,5 mm surface layer. The temperature A is measured in C° and the time B in minutes. The upper curve C is the temperature of the surface layer and the lower curve D is the temperature of the core measured 3 mm below the surface. The surface temperature is increasing during pressing to 170 C° and the core temperature to about 1 10 C°. The cooling of the surface layer takes place rather quickly as may be seen from figure 5b that shows the same temperature curves C and D during the first 60 seconds. For example, the bending step may start within 60 seconds or less, or 90 second or less, from when the first pressing starts. For example, the bending step may start within 30 seconds or less, from when the first pressing step ends. The plastic deformation is preferably made when the surface temperature is at least about 140 C° and when the core temperature, measured a few mm below the surface, is at least about 100 C°. It is possible to modify the resins in the core and in the surface layer such that plastic deformation may be made at lower temperatures for example at 120 - 140 C° for the surface layer and 80 - 100 C° for the core. Figures 6a - 6d show that vacuum cups 15 with adjustable cylinders 16 may be used to bend the pressed panel 1 after pressing. The vacuum cups may lift the panel immediately after pressing and form it to a pre-determined shape as shown in figure 6b and 6c. Such forming may be made within for example 10 - 20 seconds after the pressing when the panel surface still has a

temperature around and above 140 C°. The panel is kept in a bended position until it cools down to a suitable temperature, preferably below 100 C°, and a permanent non reversible plastic deformation is obtained such that the panel is essentially flat at room temperature as shown in figure 6d.

Example: Second pressing and stretching step.

Panel size 1 .200 ^* 2.400 ^* 9 mm Surface 500 gr. powder mix/m2

Initial pressing temperature 140 - 160 C° Final pressing temperature 80 - 140 C° Pressing time 5-15 sec Bending - width 5 cm Bending - length 14 cm