The invention relates to a method for coating a product, which method is presented in the preamble of the appended claim 1.

This idea relates particularly to coating of wood or wood-based prod¬ ucts (not paper), such as a continuous board or single boards, with a thermoplastic polymer, such as a thermoplastic, in molten state.

In the near future, wood-based materials with thermoplastic coating will play an important role in various applications of material technology. At present, the coatings used are primarily thermosetting plastics, which can be used for coating wooden boards in their normal manufacturing process.

Thermosetting plastics involve a number of problems in composite technology, and for this reason their use is becoming less and less common. Thermoplastic polymers and composites based on them are replacing thermosetting plastics, mainly for the following reasons: increased number of potential polymers, more possibilities with respect to properties, no storage problems, and reduced problems of disposal / recycling.

Finnish Patent No. 80637 discloses a method for coating a wooden board by using an extrusion die to produce a web of molten polymer which is then pressed for attachment against the board to be coated. Even with a short open time, a thermoplastic plastic is usually cooled down in such a degree that the viscosity will increase, making it more difficult to produce a smooth and even coating onto an uneven surface. When porous and/or moisture-containing products are being coated by a method, in which a die orifice supplying a molten coating opens to the surface of a piece, the problem arises that by the action of heat from the molten thermoplastic material, air and/or moisture in the pores forms bubbles in the coating layer which are allowed by the molten or sufficiently soft material to penetrate the layer all the way to the surface before hardening. This will produce holes and craters on the surface.

Methods of this kind, with one edge of the extrusion die consisting of a moving material to be coated, is presented e.g. in US Patents No. 4,143,187 and No. 3,544,669. In US-4,143,187, an attempt is made to eliminate the problem caused by pores by drying the wood before coating, and in US-3,544,669, a separate layer of vapour barrier is ap¬ plied before the coating layer.

The purpose of the present invention is to disclose a method for coating a stiff product, such as wooden board or wood-based panel, with melt processing techniques of thermoplastic polymers, i.e. thermoplastics and thermoelastics without prior drying in an oven or without a layer of vapour barrier, the method eliminating the above problems which in the worst case may cause poor adhesion and problems of appearance.

According to the invention, this aim is achieved by the characteristics presented in the characterizing part of the appended claim 1. The in¬ vention relates to a method, known as such, where from a die situated against the product applying a molten thermoplastic material is applied onto the surface of the piece, but wherein the pressing surface after the die acts also as a cooling surface which, simultaneously when prevent¬ ing the free expansion of the layer away from the surface to be coated and the formation of bubbles, causes congealing of the layer. There is thus no need for cooling devices, such as cooling rolls, presented e.g. in US Patent No. 4,143,187, but coating and cooling are in practice one and the same operation.

Other advantageous embodiments of the invention are presented in the appended dependent claims 2 — 7 and in the following description.

In the following, the invention will be explained with reference to the appended drawing which illustrates implementation of the method in a sectional view taken in the longitudinal direction of the coating appara¬ tus.

According to the method, a device 6 is placed against the product 1 to be coated, the device comprising a die 3 opening towards the piece and feeding molten thermoplastic polymer, such as a thermoplastic, onto the surface of the piece. In the moving direction A of the product 1

to be coated, in relation to the device 6, one edge of the orifice of the die 3 extends as a cooling and pressing surface 4 of a determined length, which while being at a certain distance from the surface of the product 1 simultaneously determines the thickness of the layer 2 to be formed between the surface of the product 1 and the cooling and pressing surface 4 facing the same. The cooling and pressing surface prevents the free upward expansion of the layer which would allow the formation of bubbles, and simultaneously cools down the layer rela¬ tively fast. However, adhesion is good when the molten material comes into contact with the product 1 at the orifice of the die 3. For improving the adhesion, the product 1 can be subjected to a pretreatment, e.g. an operation improving the adhesion of the surface, such as heating, oxidi¬ zation, corona treatment, flame treatment, or the like, and this optional pretreatment is indicated with an arrow 8.

In the construction shown in the figure, the die 3 comprises a prelimi¬ nary chamber 3a for receiving the molten material, a choke 3b at the terminal end of the preliminary chamber, and an expanded melt cham¬ ber 3c located underneath the choke and opening against the surface of the product over the width of the area to be coated, the orifice of the same forming the actual die orifice. Between the rear edge of the melt chamber 3c and the upper surface of the product 1 to be coated, an air gap 9 is left which is so small that the molten thermoplastic will not flow backwards, thanks to the high flow resistance. The figure shows also cooling pipes 7 above the cooling surface 4, whereby the layer 2 to be formed can be quickly cooled by means of a cooling agent, e.g. water, circulating in the cooling pipes. It is also possible to use other ar¬ rangements for active heat transfer away from the pressing and cooling surface 4 in contact with the upper surface of the layer 2 in a way that after entering the free space after the surface 4, the layer 2 has been congealed so that vapour and/or gas cannot penetrate it from below.

In thermoplastic polymers, this change from the molten material must take place in connection with the cooling and pressing surface 4 in such a degree that the layer 2 is dimensionally stable against pressurized gas and/or vapour bubbles tending to penetrate it, e.g. at least up to an overpressure of 50 kPa. For crystalline or partly crystalline polymers, such as polypropylene and polyethylene, this will usually involve a state

below the melting temperature, and for amorphic thermoplastic poly¬ mers a state where the material is sufficiently solid to resist the pres¬ sure of the gas and/or vapour.

Furthermore, the possibility to apply onto the surface of the melt a gaseous substance increasing reactivity, such as air, oxygen, ozone or a corresponding oxidizing agent, is illustrated with a gap 11 opening to the melt chamber 3c, indicated by broken Iines.

A separate preliminary chamber 3a and melt chamber 3c are not nec¬ essarily required, but the die can be constructed as a straight die con¬ nected with the extruder, opening against the product 1 to be coated.

Moreover, broken Iines indicate the possibility to combine a means for producing pressure with the cooling and pressing surface 4, such as a roll 10, which can be located after the die 3 and before the cooling pipes 7 and which is loaded against the layer 2 with a certain force F. The roll can be arranged in the construction of the device 6 in a way that its circumferential surface forms at one point part of the cooling and pressing surface 4. Instead of the roll, also a loading structure extend¬ ing over a longer distance is feasible, such as an endless track struc¬ ture, or the whole section incorporating the pressing surface 4 can be loaded.

The thermoplastic can be melted (plastified) by some method into a homogeneous mass having a desired viscosity. For plastif ication, any extruder type can be used. If various additives must be added into the thermoplastic during the plastification phase, the mixing apparatus is selected accordingly. The plastic can be blended for modifying the coating properties. For example, crosslinking agents can be added into the plastic for crosslinking of the coating. Chemical or physical foaming agents can be added into the plastic for foaming of the coating.

Molten thermoplastic is continuously fed into the die 3. The product 1 to be coated passes the melt chamber 3c above the same, receiving simultaneously a thermoplastic layer with a controlled thickness on its surface.

The product to be coated can be moved in a channel located between the supporting base of the piece and the cooling and pressing surface 4 and having a desired total thickness, as long as the thermoplastic coating is congealed in such a degree that it is subject to no other de- formations caused by cooling than shrinkage. The surface 4 may also comprise a roll for providing a suitable pattern or text on the coating.

The figure illustrates by a broken line 12 a heat insulator between the part comprising the die 3 and the part comprising the cooling and pressing surface 4, to avoid harmful transfer of heat from the molten mass in the die 3 along the structure of the device 6 in the direction of the cooling surface. Altematively, also an air gap is conceivable as the heat insulator 12, but in a way that the continuity of the surface 4 is se¬ cured through an insulating section at this point.

Particularly when the cooling and pressing surface 4 and the chamber 3 are heat-insulated from each other by a thermal break or a heat insula¬ tor 12, it is conceivable that a long retention time in connection with the surface 4 would be sufficient as such for cooling of the layer. However, this will lead to long apparatus lengths as far as the surface 4 is con¬ cerned or low coating speeds, and consequently active cooling at the surface 4 is the most advantageous alternative.

The supporting base of the product 1 can be an ordinary conveyor 5. The distance between the device 6 and the supporting base is adjust¬ able according to the thickness of the product and the desired coating layer.

It is also possible to implement the method in a way that the device 6 is moved while the product 1 remains stable. However, due to the ar¬ rangements for feeding molten material and the adjustment of a con¬ stant distance between the device 6 and the supporting base, a station¬ ary device 6 is more practical.

The method has the following advantages which will be manifested particularly when coating a wooden piece:

The molten thermoplastic is applied on the surface of the wooden piece without a so-called air gap (open time), dur¬ ing which the thermoplastic is usually cooled to such an ex¬ tent that the viscosity and adhesion of the molten plastic are substantially reduced. This situation is fully known from pa¬ perboard coating, but it cannot be eliminated in that proc¬ ess, because the process involves very strong thinning of the coating film after leaving the die. The method of the in¬ vention relates particularly to coatings with a thickness of 0.5 to 3 mm, whereby no thinning is required after the die.

In this method, the molten plastic is at no stage before the coating operation itself in any contact with air (oxygen) in an uncontrolled manner, whereby the properties of the mass cannot be impaired by the action of oxygen.

In this method, a plastified thermoplastic is applied on the surface of a wooden piece, whereby the fluidity is still very good at the time of coating. This makes it possible to achieve a fully even coating on an uneven surface. This is not possible in normal extrusion coating, because at the time when the extrusion-coating layer enters the surface, its viscosity is already so high that it is difficult to achieve an even coating on an uneven surface. In this context, the un- even surface refers to natural defects of a wooden surface.

In this method, the coating is forced to congeal in a channel according to a desired final thickness. Presently used meth¬ ods for coating with a thermoplastic are characterized by the following phenomenon: The temperature of the coating at the stage when it is applied on the surface to be coated, is usually much higher than 100°C. It is known from both ex¬ trusion coating and spray coating that in a porous material which may also contain moisture (wood usually contains a few percent of moisture), expansion of gases and vaporiza¬ tion of water takes place by the action of heat from the coating. This may have the effect that the thermoplastic coating in a plastified state starts to bubble. Now when the

expansion of the coating is prevented also during the whole cooling operation over a certain distance in the running di¬ rection of the material, it is not possible for the material to bubble.

In this method, the material used can be any polymer work¬ able in a molten state, such as a thermoplastic plastic or a thermoplastic elastomer or a composite based on these.

- This method can be combined with any possible way of modifying the surface of the wood before coating, such as flame, corona, electron beam, heating, drying, vacuuming etc.

- In comparison with normal extrusion coating, this method allows the use of substantially lower melt temperatures, be¬ cause the actual adhesion to the piece, such as a wooden board, takes place in the molten state and not after a certain air gap. In other words, this coating technique allows the use of materials with a substantially higher molecular weight

(lower melt index), whereby the coating can have substan¬ tially better properties.

In this method, the molten polymer to be applied on the sur- face can also be very hot, whereby its viscosity is as low as possible and penetration into the uneven surface of the board to be coated is thus excellent. Further, a sufficient quantity of coating is supplied from the die. Consequently, this method can be used to cover the defects on the surface of the board to be coated very well.

In normal extrusion coating, only one surface of the product can be coated. This method makes it possible to "encapsu¬ late" the product to be coated, and different continuous pro- files can be coated very easily.

By this method, it is possible also to make a multilayer coating with a minor modification of the coating device,

whereby a separate plastification unit and die is provided for each coating material.

This method can be used in the manufacture of coated wooden boards having both the good properties of wooden boards (strength, light weight, low price, availability) and the properties of thermoplastics (almost any combination of properties). However, the invention is not limited to the coating of wood only, but it can be used in coating other materials as well. The invention is particularly well suited for the coating of all stiff plate-like pieces, particularly porous pieces.

The invention will be illustrated in the following example which is non- restricting.

Example

A birch plywood board was coated in different tests with a polypropyl¬ ene film of 1 to 2 mm in thickness by a die shown in the figure. The temperature of the molten material varied in the range between 280 and 320°C. The peel strength of the film was measured by pulling it in a perpendicular direction away from the surface, and the strength values were 1 to 2 N/mm for polypropylene and 2 to 2.5 N/mm for polypropyl¬ ene mixed with a maleic acid anhydride modified polypropylene. When pulling off the film, splinters from the coated substrate were left on the lower film surface of each coating, which shows the strength of the joint.