This invention relates to coated wood material, a manufacturing method of the material in question as well as use of the material or a coating contained by it. The invention particularly relates to providing wood material including pitch with a coating impervious to pitch such that the sticky, poorly-hardening pitch will not disturb the use of the material, especially painting or varnishing the material surface.

With paintable wooden surfaces, conifer pitch penetrating the coat of paint is particularly problematic at the points of pitch-rich knots. Despite their efforts, the paint industry has not yet found a solution for this problem. Hence, conifer wood cannot be used e.g. in high-quality furniture and decorative materials. If knotty conifer material is employed in manufacturing door or window frames, one has recently resorted to removing the knotty bits and using expensive finger joints. This decreases the advantage in costs otherwise obtainable by using knotty wood.

According to the invention, a solution for the pitch problem is to provide knotty conifer material with a coating impervious to pitch fastened in it.

Previously is known to provide various fibre-based boards used as building material with a protective film impregnated with resin, fastened by hot curing. Such protectable boards are, inter alia, plywood, chipboard, fibreboard, hardboard and OSB board. The protective film is typically of kraft paper or decor paper which is impregnated with phenol or amino resin and fastened onto a base board at increased pressure and temperature. The purpose of the protective film is to make the material endure mechanical stress, resist moisture penetration and/or remain unchanged in outdoor use. Furthermore, films impregnated with phenolic resin have been fastened onto building boards as a painting base to level the irregularities of the board surface and to provide a good adherence base for paint. For paintablity, the resin content of the painting bases is still lower than that of impregnated protective films, and their outdoor use is limited to the coating of good-quality birch plywood.

An example of said known board coating technique is specification FI20050172 which describes a coating intended for plywood or chipboard which is based on kraft paper impregnated with a resin compound. Particularly to solve a discolouration problem related to phenolic resin, the specification depicts the pigmentation of the resin compound and the colour printing of the kraft paper with a light, well-covering printing ink. The use of an impregnated film as an insulating film preventing pitch penetration is not described in the specification nor is its description known by the applicant anywhere else in prior art. Besides, pitch is not a problem in boards manufactured of chips and other such particle-formed material by gluing and pressing, such as chip or fibreboards, nor in hardwood- based materials in which the chemical composition of pitch (extractives) is different from the one of conifers. Conifer materials and especially conifer wood including knots, in which pitch is a problem, are not mentioned at all in said specification FI20050172.

According to the invention, a specific solution for the problem of pitch penetrating out of knots in conifer wood is that the wood material being knotty conifer wood is provided with a fibre-based insulating film impervious to pitch, impregnated with amino resin, which is fastened onto the surface of the material by hot curing. Those conifers which the pitch problem concerns are particularly the wood species of the genera picea, pinus, larix and pseudotsuga. Scandinavian examples of these are spruce {picea abies) and pine {pinus sylvestris).

The coating according to the invention prevents the exudation of pitch through a coat of paint or varnish besides at the points of knots but also at pitch pockets, which exist in conifer wood outside the knots, and the actual heartwood.

The invention is based on observations according to which insulation can be surprisingly provided with a fibre-based film particularly treated with amino resin, which insulation conifer pitch is not able to penetrate onto the material surface and thus damage the painted surfaces of door or window frames and other such demanding targets of usage. According to the observations, an equivalent property is lacking in phenol resins commonly employed in impregnated films. The use of amino resin in the impregnated coatings of building boards is known as such but, as already mentioned, it is not known having earlier used this type of coating as an insulator to prevent pitch penetration. In addition to eliminating the pitch problem, other advantageous properties of the coated wood material according to the invention include good resistance to weather and good mechanical durability. The coating protects the wooden surface from dirtying and makes the surface easily cleanable. According to the invention, the film insulating pitch is preferably impregnated with softened amino resin. Suitable softeners to increase the elasticity of the impregnated film and to prevent the cracking of the coating include caprolactam, ortho-/para-toluene sulphonamide, acetoguanamine, monoethylene glycol, diethylene glycol, polyethylene glycol, polyvinyl alcohol, polyacrylate, acrylic resin or phenol-formaldehyde resin.

Suitable aminoresins include melamine-formaldehyde resin and urea- formaldehyde resin and their mixtures. Melamine-formaldehyde resin is advantageous in outdoor use and urea-formaldehyde resin in indoor use. The fraction of amino resin in the impregnation mixture can be e.g. about 60-95% by weight and the fraction of softener about 5-40% by weight.

The insulating film impregnated with amino resin of the material according to the invention can as such form a material surface which can be painted with water- dilutable paint or be varnished. A colourless transparent impregnated film is particularly suitable for a surface to be varnished, and wood pitch cannot penetrate it to prevent the drying of varnish. If desired, the impregnated film or varnish can be included with pigment to tint the material surface.

However, to level the material surface and to improve its paintability, it is possible to fasten on top of the impregnated insulating film a painting base also impregnated with amino resin, whereby the material coating is double-layered. The resin concentrations of the layers can then be different; the resin content of the inner layer operating as the insulator can be higher to prevent pitch penetration while the resin content of the outer layer operating as the painting base is lower to improve the paintability of the surface. The coating layers are fastenable in the wood material at one single hot pressing step in which the impregnation resin of the inner layer operates as the glue fastening the outer layer. Hence, no separate adhesion layer is required between the coating layers impregnated with resin.

According to a further embodiment of the invention, the content of impregnated amino resin in the coating film fastenable in the wood material is low, whereby the film operates as such as a good painting base. The insulation of pitch is provided by spreading more amino resin onto the film surface coming against the wood material before fastening the film. The spread resin simultaneously operates in hot curing as the glue fastening the film onto the wood material. Thus, the result is a coating comprising one fibre-based insulating film the inner boundary surface of which having higher resin content and the outer surface to be painted having a lower one.

The wood material coated according to the invention can be solid wood or glue wood, providing which with a coating impregnated with resin is not previously known as far as is known.

The wood material coated according to the invention can also be knotty, pitch- containing conifer plywood. In the invention, solid wood, glue wood as well as plywood are most typically of spruce or pine. Spruce pitch is resols and pine pitch phenolic acid, which both dissolve in phenolic resins but not in amino resins. According to the invention, the film being the pitch insulator is most advantageously paper impregnated with amino resin, such as bleached or unbleached paper manufactured of chemical, chemi-mechanical or mechanical pulp or a pulp mixture. An advantageous example is kraft paper being of sulphate pulp. The film fastened on top of the insulating film as the painting base, preferably impregnated with the same amino resin can consist of thicker kraft paper.

The weight of paper used as the insulating film is most preferably between 40-100 g/m ² and the quantity of amino resin impregnated is between 70-170 g/m ² . The weight of paper forming the painting base is most preferably between 150-300 g/m ² and the quantity of amino resin impregnated is between 60-120 g/m ² . In the insulating film, the fraction of resin of the total weight can be above 45% by weight, preferably above 55% by weight and, in the painting base, below 45% by weight, preferably below 35% by weight, respectively.

It is characteristic of the method according to the invention for manufacturing the above coated wood material that pitch-containing knotty conifer material is provided with a fibre-based insulating film impervious to pitch, impregnated with amino resin, which is fastened onto the wood material surface by hot curing.

The various above embodiments related to the coated wood material according to the invention equally apply to the manufacturing method of material according to the invention. The invention also comprises the painting or varnishing or the paintability or varnishability of the surface of wood material coated with a simple impregnated insulating film or double-coated with an insulating film and an impregnated painting base. The intended uses of the invention comprise the use of the above coated conifer material as such, its use as painted or paintable building material or furniture material as well as the use of material in which pitch penetration out of wood onto the outer surface of the material is prevented. Also, the invention includes the use of a fibre-based film impregnated with amino resin and fastened by hot curing as the coating of conifer material to prevent pitch penetration out of wood onto the outer surface of the material.

The invention will be illustrated by means of the following embodiments and related tests. In each case, impregnation resin is mixed with quite a small quantity of softener which is included in said gram quantities of resin.

Example 1 (comparison)

Conifer material coated was knotty spruce plywood of the thickness of 9 mm. Plywood was provided with a double-layer coating the inner layer of which consisted of paper, weight 30 g/m ² , which was impregnated with 47 g/m ² of phenolic resin, and the outer layer (painting base) of thicker paper, weight 150 g/m ² , which was impregnated with 64 g/m ² of phenolic resin. The coating layers were fastened by hot pressing, in which compression was 15 bar, press temperature 135°C and press time 7 min.

The coating did not bubble or split between the layers nor did the plywood split. Pitch still penetrated at the points of knots.

Example 2 (comparison)

Conifer material coated was knotty spruce plywood of the thickness of 9 mm according to example 1. Plywood was provided with a double-layer coating the inner layer of which consisted of paper, weight 30 g/m ² , which was impregnated with 47 g/m ² of phenolic resin, and the outer layer (painting base) of thicker paper, weight 245 g/m ² , which was impregnated with 119 g/m ² of melamine resin. The coating layers were fastened by hot pressing, in which compression was 15 bar, press temperature 135°C and press time 8.5 min.

The coating did not bubble or split between the layers nor did the plywood split. Pitch still penetrated at the points of knots. Example 3

Conifer material coated was knotty spruce plywood of the thickness of 9 mm according to example 1. Plywood was provided with a double-layer coating the inner layer of which consisted of paper, weight 42 g/m ² , which was impregnated with 75 g/m ² of amino resin, and the outer layer (painting base) of thicker paper, weight 245 g/m ² , which was impregnated with 178 g/m ² of melamine resin. The coating layers were fastened by hot pressing, in which compression was 18 bar, press temperature 135°C and press time 8.5 min.

The coating did not bubble or split between the layers nor did the plywood split. Pitch did not penetrate at the points of knots. The coating is non-transparent, and the material is well suited for use as painted.

Example 4

Conifer material coated was knotty spruce plywood of the thickness of 9 mm according to example 1. Plywood was provided with one coating layer which consisted of paper, weight 42 g/m ² , which was impregnated with 73 g/m ² of amino resin. The coating was fastened by hot pressing, in which compression was 15 bar, press temperature 135°C and press time 7.5 min.

The coating did not bubble or split between the layers nor did the plywood split. Pitch did not penetrate at the points of knots. The coating is transparent, whereby the wood structure remains visible, and the material is well suited for used as varnished or as such.

Example 5 (comparison)

Conifer material coated was a knotty log panel planed of pine. The panel was provided with a double-layer coating the inner layer of which consisted of paper, weight 30 g/m ² , which was impregnated with 47 g/m ² of phenolic resin, and the outer layer (painting base) of thicker paper, weight 150 g/m ² , which was impregnated with 64 g/m ² of phenolic resin. A series of tests was performed in which the coating layers were fastened by hot pressing, in which compression was each time 14 bar, press temperature varied between 110-180°C and press time varied between 20 s-28 min. The conditions of tests 1-8 are in the following table: Table 1

The coating did not bubble or split between the layers. In all tests of the series, pitch still penetrated at the points of knots. A further series of tests was performed in which the pressing conditions were: compression 14 bar, press temperature 130 ^° C and press time 7 min, the outer coating layer (painting base) was in accordance with the above but the inner coating layer was varied such that the quantities of paper/resin were 30/47 g/m ² , 40/80 g/m ² , 60/107 g/m ² and 80/140 g/m ² . Also in this series, pitch penetrated at the points of knots in each test. As the thickness of the inner film increased, the penetration of pitch decreased but, in the last test, phenolic resin started to penetrate the painting base.

Example 6

A series of four tests was performed in which conifer material coated were knotty glue wood of pine (test 1 ) and a knotty log panel planed of pine according to example 5 (tests 2-4). In test 1 , a single-layer coating consisted of paper, weight 150 g/m ² , which was impregnated with 160 g/m ² of amino resin. In tests 2-4, the inner layer of a double-layer coating consisted of impregnated coating paper according to test 1 and the outer layer of paper, the weight of which was 60 or 100 g/m ² and which was impregnated with 114 or 170 g/m ² of amino resin. The coating layers were fastened by hot pressing, in which compression was each time 14 bar, press temperature was each time 130°C, press time in test 1 was 7.5 s and press time in tests 2 and 3 was 9.5 min. The test conditions are compiled in the following table: Table 2

The coating did not bubble or split between the layers. In test 1 , little pitch exuded at the points of knots. In tests 2-4, no pitch penetration occurred. The materials of tests 2-4 are particularly well suited for use as such, even though varnishing or painting them is also possible.

Example 7

Conifer material coated was glue wood of pine used for window frames. Glue wood was provided with a double-layer coating the inner layer of which consisted of paper, weight 42 g/m ² , which was impregnated with 75 g/m ² of amino resin, and the outer layer (painting base) of thicker paper, weight 245 g/m ² , which was impregnated with 178 g/m ² of melamine resin. The coating layers were fastened by hot pressing, in which compression was 15 bar, press temperature 135°C and press time 8.5 min. The coating did not bubble or split between the layers. Pitch did not penetrate at the points of knots.

Cracking test

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Coated glue wood according to example 7 was kept in a temperature cabinet in the temperature of 80°C for a week. Then, the sample was immersed in water at room temperature for 24 hours. Finally, the sample was again kept in the temperature cabinet in 80°C for a week. No cracks were formed in the sample during the test. Painting tests

Coated glue wood according to example 7 was painted once from both sides with water-dilutable paint (Aquatop 2600, marketed by Teknos Oy). The adhesion of paint in the wood was discovered to be good. A cold check was preformed for the painted wood with the following cycle: (a) 6 h of water immersion in the temperature of 23°C, (b) 16 h in a freezer in the temperature of -18°C, and (c) 24 h in an oven in the temperature of 60°C. The cycle (steps a-c) was repeated for the test sample for 10 times. In the test, the paint adhered well in the wood, and the test did not otherwise damage the wood detectably.

A 1 ,000-h climate chamber test was performed for the painted wood with repeating the cycle: (a) 102 min of heating with a lamp imitating the spectrum of light with the rating of 0.40 W/m ² , due to exposure, surface temperature of pieces 60°C, air temperature 40°C, relative humidity 65%; (b) 18 min of irrigation with cold tap water, (c) air blowing in the temperature of 50°C, (d) returning to step (a). The 1 ,000-h climate chamber test is evaluated to correspond the actual exposure to weather for about 5 years, except for frost periods. In the test, the paint adhered well in the wood, and the test did not otherwise damage the wood detectably.

According to the tests, the coated glue wood according to the invention wears well stresses due to weather and changes due to varying moisture when painted.