Patent Description:
Plywood is a well-known wood based material. Being wood based, plywood as such is typically not very resistant to fire. It is known to apply fire retardant to plywood in order to improve fire resistance. The fire retardant may be applied to a plywood panel under high pressure and/or in vacuum followed by heat-assisted drying.

Wood material can be treated with fire retardant chemicals by e.g. vacuum pressure impregnation that is carried out in a pressure vessel. However, for plywood, this conventional method of applying preservative treatment is not appropriate due to the low penetration of the chemical through the glue-lines, the unsuitable geometry of the pressure vessels for plywood, and the batch type impregnation process. In addition, pressure impregnation and additional drying step needed may destroy the wood cell membranes, induce surface and internal cracks, which may adversely affect the strength of plywood.

The prior art further recognizes to treat plywood through the glue-lines, wherein the protecting chemical, such as fire retardant, is mixed with the binder composition used to glue together the veneers to form plywood. The glue-line impregnation method can be integrated into lay-up operations, but it requires the fire retardant chemical to be stable under extreme process conditions such as heat, pressure and high pH that occur during the lay-up and hot pressing steps of the plywood. These process conditions can result in degradation of the fire retardant and thus loss of its activity during the service life of the end product. In addition, the used fire retardant has to be able to penetrate from the binder composition into the wood material in order to provide the desired protection level.

In the field of plywood, the document <CIT> discloses plywood that has a number of veneers and a coating on at least one surface of the plywood. Therein the surface of at least the other of the outermost veneers of the plywood is sanded or sanded and a coating is provided on the sanded surface. The coating comprises a hydrophobic material and a film comprising a coating material.

In the field of fire-retardant plywood, the invention of the document <CIT> is a flame-retardant plywood, which is composed of a single board, a fire-proof adhesive and a flame-retardant. That invention uses a special fireproof adhesive, whereby the veneer does not need to be flame-retardant before hot pressing. The document <CIT> relates to wood-base materials made flame-retardant with halogen-free organic phosphorus compounds, and to compositions and processes for their production and their use. The invention of the document <CIT> relates to the flame retardant for wooden materials used for the incombustibility of wooden materials, such as a board or plate material, a plywood, the veneer for plywoods, a glulam, a particle board, a middle density fibre board (MDF), paper, and a pulp. In that invention the flame retardant is for wooden materials and is consists of aqueous solution containing phosphonic acid and ammonia. The flame retardant includes phosphonic acid of the range of <NUM>-<NUM> mass %, and ammonia of the range of <NUM>-<NUM> mass % with respect to whole quantity of an aqueous solution.

It has been noticed, that application of a high pressure decreases the strength of the plywood panel. Therefore, improving the fire resistance of plywood panels will at the same time reduce the strength. In the alternative, if pressure is not used, the level of impregnation of the fire retardant may remain so low that the fire resistance properties of the panel are not achieved.

The inventors have recognized the need for a method to protect plywood against the fire while remaining the other properties of the plywood required for its end-use applications.

A method for improving fire resistance of a plywood panel is disclosed. Sufficient level of impregnation of fire retardant is achieved without using pressure assisted impregnation. Sufficient level of impregnation of fire retardant is achieved without using heat assisted drying. Thus, the mechanical strength of the panel remains high in the process. Moreover, because high pressure is not used, a significant amount of the fire retardant remains in the surface veneer layers, which are most vulnerable to fire. In this way, since most of the fire retardant is used in the most vulnerable layers, a small amount of fire retardant suffices, which makes the process cost-efficient. Moreover, the density (kg/m<NUM>) of plywood remains low in the treatment, which helps moving the panels at a construction site. High amount of impregnation of the fire retardant into the surface veneer layers of the panel is achieved by a proper sanding of the surfaces of the plywood panel before applying the fire retardant. Furthermore, high amount of impregnation of the fire retardant into the surface veneer layers of the panel is achieved by applying the fire retardant at a certain temperature. Preferable amounts of fire retardant in connection with preferable surface veneer thickness are also disclosed.

The invention regarding the method is disclosed in independent claim <NUM>. The invention regarding a corresponding fire resistant panel is disclosed in independent claim <NUM>. Preferred embodiments are disclosed in the dependent claims and the description. Further embodiments are disclosed in the description.

The invention relates to a method for improving fire resistance of a plywood panel <NUM>. The invention relates to a fire resistant plywood panel <NUM>. In the method, fire retardant <NUM> is applied onto at lease a first surface <NUM> of a plywood panel <NUM> to improve fire resistance. Preferably, fire retardant <NUM> is applied onto two opposite surfaces <NUM>, <NUM> of a plywood panel <NUM> to improve fire resistance. The fire retardant <NUM> may be a liquid fire retardant solution or liquefied during application thereof in order to impregnate the panel <NUM> with the fire retardant <NUM>. In an embodiment, the fire retardant <NUM> is a liquid fire retardant solution <NUM>. In an embodiment, the liquid fire retardant solution <NUM> comprises phosphorous. The phosphorous is bound to a chemical compound. Thus, the term "bound phosphorous" is used in this description. Thus, the expression "comprises bound phosphorous" means "comprises a chemical compound comprising phosphorous". An amount of bound phosphorous refers, however, to the amount of only the bound phosphorous, not the total amount of the chemical compound, to which the phosphorous is bound. Both the un-treated plywood panel that is to be treated, and the treated plywood panel, i.e. a fire resistant plywood panel, are referred to as a plywood panel. In the product claims, the term plywood panel refers to a treated panel, i.e. a fire resistant plywood panel. When considered applicable, the terms treated panel and untreated panel are also used to clarify the issue.

<FIG> shows, as a side view, a plywood panel <NUM>. The method comprises receiving such a panel <NUM>. A plywood panel <NUM> comprises a first veneer layer <NUM>, a second veneer layer <NUM>, and adhesive <NUM> in between the first veneer layer <NUM> and the second veneer layer <NUM>. As is well known, a plywood panel <NUM> typically comprises at least three veneer layers, such as from <NUM> to <NUM> veneer layers. The embodiments, wherein the plywood panel <NUM> comprises from <NUM> to <NUM>, such as <NUM>, <NUM>, or <NUM> veneer layers, are particularly suitable for constructional purposes. For these purposes, the thickness of the plywood panel <NUM> is preferably from <NUM> to <NUM>.

The numbering of the veneer layers herein is such that the first veneer layer <NUM> forms a first surface <NUM> of the uncoated plywood panel <NUM> and the second veneer layer <NUM> forms a second surface <NUM> of an uncoated plywood panel <NUM>, if the panel is not coated. With the method, an uncoated plywood panel, or a plywood panel that is coated only from the second side, is treated to form a fire resistant plywood panel. However, such a treated plywood panel may be later coated e.g. by painting. A user of the panel should take into account that a coating may deteriorate the fire resistance of the panel. Therefore, the invention relates, in addition to the method, to plywood panels, of which at most one side is coated. The coating refers to a water-resistant coating, such as a polymer coating, in particular a coating comprising a polymerized resin, such as polymerized phenol formaldehyde and/or polymerized lignin-phenol formaldehyde resin. The treatment of a surface of an uncoated plywood panel by the fire retardant is not considered to form a coating onto the plywood panel, even if, at least on some locations of the treated surface, some fire retardant <NUM> may be present. For example, the second veneer layer <NUM> may be coated with polymerized phenol formaldehyde resin, while the wooden first surface <NUM> may be treated with the fire retardant <NUM>.

If coated, the surface <NUM>, <NUM> of the panel is coated by a coating <NUM>, <NUM> whereby the surface <NUM>, <NUM> is transformed to an interface <NUM>, <NUM> between the coating <NUM>, <NUM> and a surface veneer <NUM>, <NUM> (i.e. the first veneer <NUM> or the second veneer <NUM>). Moreover, if the panel is coated from the side of the first veneer <NUM> by a first coating <NUM>, the first veneer <NUM> extends, in the direction of the thickness tp of the panel <NUM> to the interface <NUM> between the first coating <NUM> and the first veneer <NUM>. Moreover, if the panel is coated by a second coating <NUM> from the side of the second veneer <NUM>, the second veneer <NUM> extends, in the direction of the thickness tp of the panel <NUM> to the interface <NUM> between the second coating <NUM> and the second veneer <NUM>. The surfaces of an uncoated panel are indicated in <FIG>, while the interfaces of a coated panel are indicated in <FIG>.

A thickness tp of the plywood panel <NUM> is left in between the first surface <NUM> and the second surface <NUM> (or first surface <NUM> and the second interface <NUM>, if the second surface <NUM> of the panel is coated by the coating <NUM>). Other veneer layers <NUM>, <NUM>, <NUM>, if present, are arranged in between the first veneer layer <NUM> and the second veneer layer <NUM> in a direction Sz of thickness tp of the plywood panel <NUM>, as indicated in <FIG>. The thickness tp of the panel is smaller than a length lp of the panel and a width wp of the panel <NUM> (see <FIG>). Correspondingly, all veneer layers of the plywood panel are left in between the surfaces (<NUM>, <NUM>) thereof, or, if the panel is coated from the second side, in between the surface <NUM> and the interface <NUM> (see <FIG> and <FIG>).

Within a veneer layer of the plywood panel <NUM>, the wood material has a grain orientation. In particular, the first veneer layer <NUM> comprises grains oriented in a first grain direction D1 and the second veneer layer <NUM> comprises grains oriented in a second grain direction D2. Such directions are shown e.g. in <FIG>, <FIG>.

A veneer layer <NUM>, <NUM>, <NUM> may comprise grains oriented in different directions. The term "grain direction" refers to the direction into which the tree, from which the veneer has been peeled, has grown. The direction is evidenced e.g. by the annual rings visible in the veneers. In the alternative, the grain direction of a veneer layer refers to the average direction of grains, the average being calculated as a mass average. In the alternative, the grain direction of a veneer layer refers to the median direction of grains, the median being calculated on a mass basis such that <NUM> w% of the grains of the veneer layer form a positive angle relative to a plane defined by a normal of the veneer layer and the grain direction of the veneer layer; and <NUM> w% of grains of the veneer layer form a negative angle relative to a plane defined by a normal of the veneer layer and the grain direction of the veneer layer.

A problem with processed wood products, such as plywood, is that the surfaces <NUM>, <NUM> thereof, as a result of the manufacturing processes, e.g. hot-pressing, they have been subjected to, are often poor in e.g. wettability compared to those of freshly cut, polar wood. The surfaces <NUM>, <NUM> of plywood <NUM> may have a glazed appearance indicating that they have been inactivated by pressing at high temperatures. Especially during hot-pressing resinous extractives migrate to the surface, adhesives cure, and caul release agents remain on product surfaces and thus inactivate or block the surfaces from being wetted. During hot-pressing plywood <NUM> will be subjected to heat treatment, whereby the plywood surfaces <NUM>, <NUM> become more hydrophobic as a result of plasticisation of lignin and/or loss of residual water leading to reorganisation of the lignocellulosic components of the wood. As a result of increased hydrophobicity of plywood the wettability thereof decreases. The poor wettability may then adversely affect the following treatments such as the surface treatment process with fire retardant <NUM>.

It was found out that sanding of the surface <NUM> of the plywood, on which the fire retardant <NUM> is to be applied, results in roughness of the surface suitable for good coating of the surface. The surface roughness, as a result of the sanding, of the surface of the plywood has the advantage of enabling even and adequate spreading of the fire retardant chemical over the surface by rollers or spraying.

Referring to <FIG>, the method comprises sanding the first surface <NUM> of the plywood panel <NUM> in a first sanding direction R1 that forms an angle α1 of at least <NUM> degrees, at least <NUM> degrees, or at least <NUM> degrees with the first grain direction D1. The first sanding direction R1 is also perpendicular to a normal of the first surface <NUM>. In particular, the first surface <NUM> is sanded using a first sanding surface <NUM> such that the first sanding surface <NUM> and the first surface <NUM> of the plywood panel <NUM> move relative to each other in a direction that forms an angle of at least <NUM> degrees, at least <NUM> degrees, or at least <NUM> degrees with the first grain direction D1 and belongs to the plane of the first surface <NUM> of the plywood panel <NUM>. It has been found that sanding in such a direction R1 opens up the grains of the first surface <NUM>. It has been found that the liquid or liquefied fire retardant will impregnate well to a wooden surface sanded in this direction. This happens most likely because the sanding surface breaks the fibres. Correspondingly, if the sanding surface would move only parallel to the grains, some of the grains might be left intact. As indicated in <FIG>, preferably the angle α1 between D1 and R1 is about <NUM> degrees, and R1 is perpendicular to a normal of the first surface <NUM>. The plywood panel <NUM> may be sanded and treated with fire retardant <NUM> only from the first side.

Referring to <FIG>, an embodiment, wherein also the second surface <NUM> is treated with fire retardant, comprises sanding the second surface <NUM> of the plywood panel <NUM> in a second sanding direction R2 that forms an angle α2 of at least <NUM> degrees, at least <NUM> degrees, or at least <NUM> degrees with the second grain direction D2. The second sanding direction R2 is also perpendicular to a normal of the second surface <NUM>. In particular, the second surface <NUM> is sanded using a second sanding surface <NUM> such that the second sanding surface <NUM> and the second surface <NUM> of the plywood panel <NUM> move relative to each other in a direction that forms an angle of at least <NUM> degrees, at least <NUM> degrees, or at least <NUM> degrees with the second grain direction D2 and belongs to the plane of the second surface <NUM> of the plywood panel <NUM>. As indicated in <FIG>, preferably the angle α2 between D2 and R2 is about <NUM> degrees, and R2 is perpendicular to a normal of the second surface <NUM>.

Sanding may be done e.g. by moving the sanding surface (<NUM>, <NUM>) and/or the surface (<NUM>, <NUM>) to be sanded back and forth relative to each other. In the alternative, the sanding surface (<NUM>, <NUM>) may form a loop (i.e. a band) and may be moved only in one direction.

It was also surprisingly found out that sanding of the surface of the plywood on which the fire retardant chemical is to be applied, has the beneficial effect of enabling penetration of the fire retardant chemical into the voids and cells of the wood material.

In one embodiment of the present invention the sanded surface <NUM> (optionally also the surface <NUM>) is provided by using at the sanding step before providing the fire retardant on the sanded surface, an abrasive paper having a grit size of P20 - P220, preferably a grit size of P40 - P120, and more preferably a grit size of P40 - P80 in accordance with standard ISO <NUM>. The use of an abrasive paper with this kind of grit size results in a surface <NUM> into which the fire retardant <NUM> easily absorbs. If using a finer grit size, the surface could remain so-called closed for the absorption of the fire retardant chemical and the use of a coarser grit size could result in the fire retardant chemical being non-uniformly distributed with main part of the fire retardant chemical remaining in the valleys of the surface while the so-called tops or peaks would remain un-covered.

In one embodiment of the present invention, at least the first surface <NUM> of the plywood panel <NUM> is sanded in an essentially transverse direction in relation to the grain direction of the surface veneer. The term "essentially transverse" has been elaborated above. This has the beneficial effect of most grains on the surface of the plywood and many distinct points of one grain being sanded and so-called opened such that the fire retardant chemical easily absorbs into the grains.

Sanding in said essentially transverse direction offers better wetting properties of wood surface. Wetting is facilitated in the direction of abrasive scratches. The fibres are torn-out when sanded in the essentially transverse direction to the wood grains, while they are mostly stripped when sanded in the parallel direction to the wood grains. Presence of torn-out micro-fibrils, promote a better mechanical anchorage and also offer a greater actual surface available to fire retardant and wood interactions.

In addition to sanding direction, the roughness of the sanding surface(s) has been found to affect how well the liquid or liquefied fire retardant solution <NUM> becomes impregnated into the surface veneers. In an embodiment, the sanding (i.e. grinding) of the first surface <NUM> of the plywood panel <NUM> is performed with a first sanding surface <NUM> (e.g. a surface of sandpaper) having a grit size within the limits disclosed above (e.g. from P20 to P220 or from P40 to P120 in accordance with standard ISO <NUM>). The sanding of the second surface <NUM> of the plywood panel <NUM> may be performed with a sanding surface, such as the first sanding surface <NUM> or a second sanding surface <NUM>, having a grit size within the aforementioned limits. This ensures that the sanded surface of the plywood panel is no too smooth before applying the fire retardant, and, at the same time, the fire retardant is impregnated into the surface veneers substantially evenly.

Preferably, the method comprises sanding the first surface <NUM> with a primary first sanding surface <NUM> (e.g. a surface of a primary sandpaper) having a grit size of from P40 to P60 and thereafter sanding the first surface <NUM> with a secondary first sanding surface (e.g. a surface of a secondary sandpaper) having a grit size of from P60 to P120. In a similar manner, the method may comprise sanding the second surface <NUM> with a primary sanding surface having a grit size of from P40 to P60 and thereafter sanding the second surface <NUM> with a secondary sanding surface having a grit size of from P60 to P120.

It has also been found that the liquid fire retardant solution will impregnate well to a surface <NUM> (and optionally <NUM>) when the temperature of the surface is sufficiently high. The method comprises arranging a temperature of the first surface <NUM> to be at least <NUM> and after said sanding the first surface and arranging the temperature of the first surface, applying liquid fire retardant solution <NUM> onto the first surface <NUM>. The liquid fire retardant solution <NUM> is applied onto the first surface <NUM> when the temperature of the first surface <NUM> is at least <NUM>. Preferably, the temperature of the first surface is from <NUM> to <NUM> when the liquid fire retardant solution <NUM> is applied onto the first surface <NUM>. More preferably, the temperature of the first surface is from <NUM> to <NUM> when the liquid fire retardant solution <NUM> is applied onto the first surface <NUM>. Preferably also the temperature of the liquid fire retardant solution <NUM>, when applied onto the first surface <NUM> is at least <NUM>, such as at least <NUM> or at least <NUM>. Preferably, the temperature of the liquid fire retardant solution <NUM>, when applied onto the first surface <NUM> is at most <NUM> or at most <NUM>.

Increased temperature of wood and optionally also the fire retardant solution might soften the resinous compounds present within the wood structure and reduce the obstruction of liquid flow, and therefore contribute to improved penetration. At higher temperatures, however, risk of exudation of resinous compounds onto the wood surface may occur. This might have opposite effect on the fire retardant penetration.

An embodiment comprises arranging a temperature of the second surface <NUM> to be at least <NUM> and after said sanding the second surface and arranging the temperature of the second surface, applying liquid fire retardant solution <NUM> onto the second surface <NUM>. The liquid fire retardant solution <NUM> is applied onto the second surface <NUM> when the temperature of the second surface <NUM> is at least <NUM>. Preferably, the temperature of the second surface is from <NUM> to <NUM> when the liquid fire retardant solution <NUM> is applied onto the second surface <NUM>. More preferably, the temperature of the second surface is from <NUM> to <NUM> when the liquid fire retardant solution <NUM> is applied onto the second surface <NUM>. Preferably also the temperature of the liquid fire retardant solution <NUM>, when applied onto the second surface <NUM> is at least <NUM>, such as at least <NUM> or at least <NUM>. Preferably, the temperature of the liquid fire retardant solution <NUM>, when applied onto the second surface <NUM> is at most <NUM> or at most <NUM>.

Referring to <FIG>, the fire retardant solution <NUM> may be e.g. sprayed onto the panel, i.e. onto the first surface <NUM> thereof. The fire retardant solution <NUM> may be e.g. sprayed onto the second surface <NUM>. An applicator may comprise a conveyor <NUM>. An applicator may comprise a nozzle <NUM> for spraying the fire retardant solution <NUM> onto the panel. In the alternative or in addition, the fire retardant <NUM> may be applied with a roll coater. In the alternative, the panel <NUM> may be immersed into the fire retardant solution <NUM>.

Referring to <FIG>, after the fire retardant solution <NUM> has been applied, the panels <NUM> may be arranged into a stack <NUM> of plywood panels in order to press (i.e. impregnate) the liquid fire retardant solution <NUM> into the plywood panel <NUM>. As an example, <FIG> shows a stack <NUM> of five panels <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, and <NUM><NUM>. The pressure of such a stack <NUM> presses the fire retardant <NUM> into the panels. However, typically the stack is so low that the pressure of the stack will not deteriorate the mechanical properties of the panels. In an embodiment, the panel is held in the stack <NUM> for at least <NUM> hours or at least <NUM> hours to allow the fire retardant to impregnate into the surface veneers <NUM>, <NUM>.

In case only the first surface <NUM> of the plywood panel <NUM> is treated with the fire retardant <NUM>, is it possible to enhance impregnation of the fire retardant <NUM> into the treated panels <NUM> before stacking them to a stack. Impregnation of the fire retardant <NUM> may be enhanced in an elevated temperature. It has been noticed that the temperature influences the properties of fire retardant solution as well as structure of the wood and, therefore, affects the penetration. Therefore, the treated panel may be thermally treated at a temperature of at least <NUM> or at least <NUM> for at least <NUM> seconds or at least <NUM> seconds to enhance impregnation of the fire retardant <NUM> into the treated panels <NUM>.

The treatment temperature may be e.g. at most <NUM> or at most <NUM> in order not to generate high pressures into the treated, moist, wood by boiling. However, even a temperature more than <NUM> can used in connection with a fire retardant comprising water without deteriorating the inner structure of the panel, since with the method, only a little (if any) fire retardant is impregnated into the middle veneer layers (e.g. <NUM>). Thus, the pressure generated by the thermal treatment seems not to affect the mechanical properties of the panel.

Thermal impregnation, as discussed above, can also be used, if the opposite surfaces <NUM>, <NUM> of the panel <NUM> are treated subsequently. it is possible to apply fire retardant first to the first surface <NUM>, thereafter apply the heat treatment to the panel <NUM> to enhance impregnation, and thereafter apply fire retardant to the second surface <NUM>.

Impregnation needs not be complete. It suffices that the first surface <NUM> feels dry. In the alternative or in addition if may suffice that the treated surface (<NUM>, <NUM>) does not stain objects that contact the treated surface.

It has also been found that a fire retardant <NUM>, such as the liquid fire retardant solution <NUM>, is better impregnated into dry wood than into moist wood. Moreover, it has been found that a fire retardant <NUM>, such as the liquid fire retardant solution <NUM>, is better impregnated into wood that is not too dry. Therefore, in a preferable embodiment, the first <NUM> veneer layer is substantially dry when the fire retardant <NUM> is applied. This concerns also the second veneer layer <NUM>, if both surfaces are treated with fire retardant. In an embodiment, when starting to apply the fire retardant <NUM> onto the first surface <NUM>, the moisture content of the first veneer layer <NUM> is from <NUM> % to <NUM> %, preferably from <NUM> % to <NUM> %. In an embodiment, when starting to apply the fire retardant <NUM> onto the second surface <NUM>, the moisture content of the second veneer layer <NUM> is from <NUM> % to <NUM> %, preferably from <NUM> % to <NUM> %. As conventional, the term "moisture content" refers to the mass of water as divided by the dry mass of wood. The dryness affects impregnation in particular, when a liquid fire retardant solution <NUM> is used.

It has been found that in such a way, the first veneer layer <NUM> and the second veneer layer <NUM> are impregnated with the fire retardant <NUM> well, whereby a fire resistant plywood panel <NUM> is formed. Such a panel is fire resistant according the classification standard EN <NUM>-<NUM>:<NUM>+A1:<NUM> as indicated by the classes: fire behaviour B, smoke production s1, and flaming droplets d0. It is further noted that this standard refers to other standards, as detailed below. Moreover, preferably the aforementioned fire resistivity classes (B-s1, d0) are achieved in typical mountings described in the standard EN <NUM>. Such mountings are illustrated in <FIG>.

Preferably the aforementioned fire resistivity classes (B-s1, d0) are achieved in all mountings described in <FIG>. In <FIG>, the plywood panel is fixed mechanically to wooden or metallic frames. The plywood panel is fixed using metallic wood screws. <FIG> shows a fire resistance test for the plywood panel <NUM>. The panel is mounted to a substrate <NUM> with a frame <NUM> or via a substrate <NUM> to the frame <NUM>. The frame <NUM> may be a wooden frame or a metal frame. An insulation <NUM> is arranged to contact the panel <NUM>. The insulation <NUM> has a fire resistance of class A1 or A2-s1, d0, and a density of at least <NUM>/m<NUM>. A gap <NUM> is left in between two panels <NUM>. Arrangement with both horizontal and vertical gaps are tested. In practice, a large plywood panel is sawn to smaller pieces, in between which the gaps <NUM> remain. Thus, it is important that the fire resistance of the treated plywood panel is good also after having been sawn to pieces. In particular, the fire resistivity classes B-s1, d0 are applicable in arrangements with both horizontal and vertical gaps.

The frame <NUM> is left at the location of the gap <NUM>, as indicated in <FIG> shows a fire resistance test for the plywood panel <NUM>. The panel is mounted directly to a substrate <NUM>. The substrate <NUM> has a fire resistance of class A1 or A2-S1, d0, and a density of at least <NUM>/m<NUM>. The substrate may be e.g. an insulation, a gypsum board. The substrate is connected to a frame <NUM>. <FIG> shows a fire resistance test for the plywood panel <NUM>. The panel is mounted to a substrate <NUM> with a frame <NUM>. As indicated in the standard EN <NUM>, the substrate <NUM> shall meet the requirements of the standard EN <NUM>. The substrate may be e.g. a gypsum board or concrete. The frame <NUM> may be a wooden frame or a metal frame. In this way, an air gap <NUM> is left in between the panel <NUM> and the substrate <NUM>. A width of the air gap <NUM> may be e.g. at least <NUM>, or as described in the aforementioned standard.

In practice, the aforementioned classes (B-s1, d0) is the best class applicable to a burnable material, such as wood. Moreover, in an embodiment, the aforementioned fire resistivity classes (B-s1, d0) are achieved in at least a mounting, wherein an air gap is left between the panel <NUM> and the substrate <NUM> (<FIG>), which is typically the hardest test condition for obtaining these fire resistivity classes.

Moreover, preferably the aforementioned fire resistivity classes (B-s1, d0) are achieved without restrictions regarding the structures surrounding the panel or lack of them, except for a coating of the panel itself.

In such a way, a fire resistant plywood panel <NUM> is obtained. Such a fire resistant plywood panel <NUM> comprises the first veneer layer <NUM> forming the first surface <NUM> of the plywood panel <NUM>, the second veneer layer <NUM> forming the second surface <NUM> (or interface <NUM>) of the plywood panel <NUM>, and adhesive <NUM> in between the first veneer layer <NUM> and the second veneer layer <NUM>. The first veneer layer <NUM> comprises at least <NUM>/m<NUM> of bound phosphorous. In an embodiment, wherein also the second veneer layer <NUM> has been treated with a fire retardant, the second veneer layer <NUM> comprises at least <NUM>/m<NUM> of bound phosphorous. The phosphorous is bound to a chemical compound, as will be detailed below. Because of the way the plywood panel <NUM> has been manufactured and/or the phosphorous, the plywood panel <NUM> is fire resistant according the standard EN <NUM>-<NUM>:<NUM>+A1:<NUM> as indicated by the classes.

According to the standard EN <NUM>-<NUM>:<NUM>+A1:<NUM> (see section <NUM>), the panel is classified as belonging to the fire behaviour class B, when the parameter FIGRA ≤ 120W/s and the parameter THR<NUM> ≤ <NUM> MJ as defined in the standard EN13823:<NUM> and when a flame tip does not reach <NUM> above a flame application point within <NUM> as detailed in the standard in view of the standard EN ISO <NUM>-<NUM>:<NUM>. The aforementioned parameter FIGRA refers to fire growth rate, the parameter THR to total heat release. The parameter THR<NUM> refers to total heat release within <NUM> (section <NUM> of the standard EN13823:<NUM>).

According to the standard EN <NUM>-<NUM>:<NUM>+A1:<NUM> (see section <NUM>. <NUM>), the panel is classified as having a smoke production s1, when the parameter SMOGRA ≤ <NUM><NUM>/s<NUM> and TSP<NUM> ≤ <NUM><NUM> as defined in the standard EN13823:<NUM>. The aforementioned parameter SMOGRA refers to smoke growth rate and the parameter TSP refers to total smoke production. The parameter TSP<NUM> refers to total smoke production within <NUM> (section <NUM> of the standard EN13823:<NUM>).

According to the standard EN <NUM>-<NUM>:<NUM>+A1:<NUM> (see section <NUM>. <NUM>), the panels is classified as having flaming droplets d0, when no flaming droplets or particles as described in the standard EN13823:<NUM> (section <NUM>) are produced within <NUM>.

The aforementioned classification is valid for a constructional product except flooring, i.e. a plywood panel not intended for use as a flooring element. For flooring elements somewhat different criteria applies. However, it has been observed that the panel <NUM> is, if tested as a flooring element, fire resistant according the standard EN <NUM>-<NUM>:<NUM>+A1:<NUM> as indicated by the classes.

According to EN <NUM>-<NUM>:<NUM>+A1:<NUM> (section <NUM>), to achieve the class Bfl, the critical heat flux ≥ <NUM>,<NUM> kW/m<NUM> according to standard EN ISO <NUM>-<NUM>. Moreover, a flame tip does not reach <NUM> above a flame application point within <NUM> as defined in the standard EN ISO <NUM>-<NUM>:<NUM>.

According to EN <NUM>-<NUM>:<NUM>+A1:<NUM> (section <NUM>. <NUM>), to achieve the class s1, a parameter Smoke is ≤ <NUM> %×min according to standard EN ISO <NUM>-<NUM>.

To keep the costs of panel, in particular the additional cost for using the fire retardant <NUM>, low, in an embodiment, the first veneer layer <NUM> comprises at most <NUM>/m<NUM>, such as at most <NUM>/m<NUM> or at most <NUM>/m<NUM> of bound phosphorous. In an embodiment, the second veneer layer <NUM> comprises at most <NUM>/m<NUM>, at most <NUM>/m<NUM>, or at most <NUM>/m<NUM>, of bound phosphorous. In addition, the thickness of the surface veneers <NUM> may be selected in such a way that the first veneer <NUM> comprises from <NUM>/m<NUM> to <NUM>/m<NUM>, such as from <NUM>/m<NUM> to <NUM>/m<NUM>, bound phosphorous. Moreover, the thickness of the second veneer <NUM> may be selected such that it comprises from <NUM>/m<NUM> to <NUM>/m<NUM>, such as from <NUM>/m<NUM> to <NUM>/m<NUM>, bound phosphorous.

The amount of phosphorous can be measured e.g. by taking a sample of the veneer layer, burning it to ash, and analyzing the content of the ash. Such a method is commonly referred to as a SEM-EDS, an acronym for scanning electron microscopy (SEM) - energy dispersive spectroscopy (EDS). After the SEM-EDS analysis is carried out, one can determine the ratio of elements that are as heavy or heavier than sodium. For example, in an embodiment, when such analyzed, the content of phosphorous was at least <NUM> wt% (one significant digit) of such elements of the ash that had an atomic mass at least equal to the atomic mass of sodium (Na). Such content was measured from the ash using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

The plywood panel <NUM> comprises, on the first surface <NUM>, traces indicative of sanding the first surface <NUM> of the plywood panel <NUM> in a first sanding direction R1 that forms an angle of at least <NUM> degrees, at least <NUM> degrees, or at least <NUM> degrees with the grain direction D1 of the first veneer layer <NUM>. In an embodiment, the plywood panel <NUM> comprises, on the second surface <NUM>, traces indicative of sanding the second surface <NUM> of the plywood panel <NUM> in a second sanding direction R2 that forms an angle of at least <NUM> degrees, at least <NUM> degrees, or at least <NUM> degrees with the grain direction D2 of the second veneer layer <NUM>. The traces may also be indicative of a sanding the surfaces of the panel with a sanding surface having a proper grit, as detailed above. However, if the surface of the panel is covered by a coating <NUM> or coating <NUM>, <NUM> (see Fig. <NUM>) the coating(s) may cover the traces of sanding, whereby the traces and the sanding direction is not necessarily visible from the panel.

Because of the adhesive <NUM>, the liquid fire retardant solution <NUM> does not significantly propagate from a surface veneer layer (<NUM>, <NUM>) to another veneer layer (<NUM>, <NUM>, <NUM>). As indicated above, this helps to concentrate the fire retardant <NUM> to such locations where it is needed. In an embodiment, the plywood panel <NUM> comprises a third veneer layer <NUM> arranged in between the first veneer layer <NUM> and the second veneer layer <NUM>. Moreover, the third veneer layer <NUM> is free from added phosphorous (bound or free) or comprises less phosphorous than the surface veneer layer <NUM> or <NUM>. In an embodiment, the content c130 of phosphorous of the third veneer layer <NUM> as measured in wt%, is less than the content c110 of phosphorous of the first veneer layer <NUM> as measured in wt%. In an embodiment, the ratio c130/c110 of these contents is at most <NUM>, at most <NUM>, or at most <NUM>. The contents c130 and c110 may be measured from the ash, as discussed above, and in proportion of only of such elements of the ash that have an atomic mass at least equal to the atomic mass of sodium (Na).

Moreover, even if some fire retardant <NUM> may propagate into the middle layers e.g. when applied by immersion, because of low pressure, the fire retardant becomes applied only near the boundaries of the panel. In an embodiment, a central area of the third veneer layer <NUM> is free from added phosphorous (bound or free) or comprises less phosphorous than the surface veneer layer <NUM> or <NUM>. Also untreated wood, e.g. spruce, comprises a small amount of phosphorous. For example the aforementioned ratio c130/c110 may apply, when the sample of the third veneer layer is taken from a central area of the third veneer layer <NUM>. The central area of the third veneer layer <NUM> refers to such parts of the third veneer layer <NUM> that are located at least <NUM> or at least <NUM> away from such edges of the plywood panel that a perpendicular to the surfaces <NUM>, <NUM> or interfaces <NUM>, <NUM>.

The fact that the high level of fire resistance is achieved without high-pressure application of the fire retardant has the further effect, that the edges of the panel are not responsible for the fire resistance. As indicated above, only at most a reasonable small boundary area of a middle veneer is, in an embodiment, impregnated with the fire retardant. Therefore, the panel can be sawn in a longitudinal and/or a transversal direction/directions without losing the fire resistivity properties. For example, the edges of the treated panel may be sawn, and the panel would still be considered fire resistant to the same level. This has many benefits in constructional applications, since large panels that have been treated to be fire resistant can be sawn to proper size later on.

In an embodiment, the first veneer layer <NUM> comprises softwood, such as spruce or pine. In an embodiment, the second veneer layer <NUM> comprises softwood, such as spruce or pine. This has the effect that less fire retardant <NUM> is needed to achieve a needed fire resistance level. Thus, preferably, at least the first veneer layer <NUM> comprises softwood. In an embodiment, also the second veneer layer <NUM> comprises softwood. Preferably both these layers (<NUM>, <NUM>) comprise softwood, such as spruce or pine. In an embodiment, all the veneer layers of the plywood panel <NUM> are made from the same wood species. What has been said about the plywood panel relates both to the panel that is treated in the method and to the panel that has been treated as indicated above. The invention has been found to work particularly well, when the first veneer layer <NUM> comprises wood from the species Picea Abies or Pinus Sylvestris. In addition, the second veneer layer <NUM> may comprise wood from the species Picea Abies or Pinus Sylvestris.

In an embodiment, at least one of the thickness tsv1 (see <FIG>) of the first veneer layer <NUM> and the thickness tsv2 of the second veneer layer <NUM> is from <NUM> to <NUM>. It has been found that this thickness is, on one hand sufficiently high for receiving a sufficient amount of fire retardant <NUM>. On the other hand, it has been found that this thickness is sufficiently low in order to have the concentration of fire retardant in a proper level without using excessive amounts of fire retardant. In an embodiment, the thickness tsv1 of the first veneer layer <NUM> is equal to the thickness of the second veneer layer <NUM>. In an embodiment, both the thickness tsv1 of the first veneer layer <NUM> and the thickness tsv2 of the second veneer layer <NUM> are from <NUM> to <NUM>.

Referring to <FIG>, the thickness tbv of a body veneer layer (i.e. veneer layer in between the first and second layers <NUM>, <NUM>) may be different from the aforementioned thicknesses tsv1 and tsv2. For example, the thickness tbv of a body veneer layer may be greater than the thickness tsv1, tsv2 of a surface veneer layer (<NUM>, <NUM>). In an embodiment, the panel comprises a third veneer layer <NUM> that is arranged, in the direction of the panel in between the first veneer layer <NUM> and the second veneer layer, wherein a thickness of the third veneer layer is from <NUM> to <NUM>. In an embodiment, the panel comprises at least a third veneer layer <NUM>; and a thickness of each such veneer layer that is arranged in between the first veneer layer <NUM> and the second veneer layer is from <NUM> to <NUM>.

However, as indicated in <FIG>, all the veneer layers of a plywood panel <NUM> may be equally thick or substantially equally thick, at least before sanding.

Sanding the surface veneer layers <NUM>, <NUM> typically reduces their thickness. This is beneficial from the point of view of fire resistance of the panel. Since the surface veneer layers may be thin, e.g. due to the sanding, the concentration of fire retardant in the surface layers will be increased. In an embodiment of the method, the fire retardant is applied to such a panel <NUM> that comprises a third veneer layer <NUM> in between first veneer layer <NUM> and the second veneer layer <NUM>; the third veneer layer <NUM> having a thickness tbv and the first veneer layer <NUM> having a thickness tsv1, wherein the thickness tbv of the third veneer layer <NUM> is greater than the thickness tsv1 of the first veneer layer <NUM>. In an embodiment of the method, the fire retardant is applied to such a panel <NUM> that comprises a third veneer layer <NUM> in between first veneer layer <NUM> and the second veneer layer <NUM>; the third veneer layer <NUM> having a thickness tbv and the second veneer layer <NUM> having a thickness tsv2, wherein the thickness tbv of the third veneer layer <NUM> is greater than the thickness tsv2 of the second veneer layer <NUM>. This may be the case, for example when all the veneer layers are, before sanding, equally thick, and the thickness of the surface veneer layers <NUM>, <NUM> is reduced by sanding. However, the treatment by fire retardant may make the surface veneer layer <NUM>, <NUM> somewhat thicker.

In an embodiment, the adhesive <NUM> of the plywood panel <NUM> comprises at least one of polymerized phenol-formaldehyde resin and polymerized lignin-phenol-formaldehyde resin. This has two effects. First, polymerized phenol-formaldehyde and lignin-phenol-formaldehyde resins are at least to some extent liquid-proof (and waterproof), whereby the adhesive <NUM> prevents the fire retardant <NUM> from diffusing from a surface layer (<NUM>, <NUM>) into a body veneer layer (<NUM>, <NUM>, <NUM>, <NUM>). This helps to keep the content of fire retardant in the surface veneer layers <NUM>, <NUM> high. Second, polymerized phenol-formaldehyde and lignin-phenol-formaldehyde resins are is reasonably heat resistant and conduct heat poorly. Therefore, the adhesive <NUM> also protects the body veneer layers (<NUM>, <NUM>, <NUM>, <NUM>) in case of fire.

Referring to <FIG>, a veneer layer <NUM> may comprise multiple veneers <NUM>, <NUM>, <NUM>. The veneers <NUM>, <NUM>, <NUM> of the layer are preferably arranged so that their grain directions are parallel. <FIG> shows an example of a veneer <NUM>. Preferably, the first veneer layer <NUM>, which is a surface veneer layer, comprises only one veneer. This improves the visual appearance of a first side of the plywood panel <NUM>. Preferably, the second veneer layer <NUM>, which is another surface veneer layer, comprises only one veneer. This improves the visual appearance of a second side of the plywood panel <NUM>. Moreover, having a surface veneer layer <NUM>, <NUM> formed only from one veneer improves the impregnation of the fire retardant <NUM>, since, in that case, both the surface veneer layers <NUM>, <NUM> are free from adhesive. Adhesive could hinder the impregnation of the fire retardant <NUM>.

Referring to <FIG>, a plywood panel may be cross laminated. This means that a veneer layer has a grain direction that is substantially perpendicular to the grain direction(s) of neighbouring layer(s). However, the internal structure of a plywood panel <NUM> may be different. For example, the grain directions of two or more neighbouring layers may be parallel. Typically, a plywood panel with an odd number (e.g. <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>) veneer layers are cross laminated throughout. Typically, a plywood panel with an even number (e.g. <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>) veneer layers is cross laminated except for the two middle layers, which have grain directions parallel to each other. Within such structures, the grain direction DG1 of the first veneer layer <NUM> is parallel to the grain direction DG2 of the second veneer layer <NUM>. A panel, in which the grain direction DG1 of the first veneer layer <NUM> is parallel to the grain direction DG2 of the second veneer layer <NUM> is beneficial for the method described above, since then also the first sanding direction R1 can be parallel to the second sanding direction R2. This simplifies the sanding process.

The internal structure affects the mechanical strength of the plywood panel. However, unlike in some other methods, when the liquid fire retardant solution <NUM> is applied as indicated above, the internal structure of the plywood panel does not break during the fire resistive treatment, whereby the mechanical properties of the panel remain substantially unaltered.

If a panel would be treated with fire retardant under pressure, e.g. by pressure impregnation, this would affect the panel in at least the following:.

Thus, an embodiment the panel comprises only such wood that has not been pressure impregnated with a fire retardant. The panel comprises only such wood that has not been pressure impregnated with a fire retardant, wherein the pressure of the pressure impregnation would have been at least <NUM> bar(a).

In addition, if the middle layers were impregnated with fire retardant, and the panel was dried in an oven later on, this would affect the panel in at least the following:.

These would reduce in particular the bending stiffness of the wood material. Typically, the temperature used in such drying is in the range from <NUM> to <NUM>. This may cause the fire retardant that has been impregnated into the middle veneer layers to boil and in this way deteriorate the panel.

Heat-assisted drying may be needed also in cases, where the panel has not been treated by a pressure of at most <NUM> bar(a), i.e. a poor or good vacuum. The panel comprises only such wood that has not been treated by a pressure that goes below <NUM> bar(a).

The plywood panel may comprise e.g. from <NUM> to <NUM> veneer layers. The thickness tp of the plywood panel may be from <NUM> to <NUM>, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>. In a preferred embodiment, the plywood panel comprises at least five veneer layers and has a thickness tp of at least <NUM>. Such a panel is typically sufficiently strong for use as a building material.

The specific mass (i.e. mass divided by area) of such a plywood panel may be from <NUM>/m<NUM> to <NUM>/m<NUM> (two significant digits). The specific mass depends e.g. on the thickness of the panel and on the thickness of the veneer, which affects the amount of adhesive used. The density of the panel may be e.g. from <NUM>/m<NUM> to <NUM>/m<NUM>. In constructional applications, more dense panels are hard to handle. This is another reason for using softwood and only a small amount of fire retardant.

In an embodiment, the fire retardant <NUM> comprises bound phosphorus. The term bound phosphorous refers to phosphorous as bound in a chemical compound. In an embodiment, the fire retardant <NUM> comprises from <NUM> wt% to <NUM> wt%, such as from <NUM> wt% to <NUM> wt%, bound phosphorus as measured according to the standard SFS-EN ISO <NUM> (referring to the latest version available in May <NUM>). Preferably, the fire retardant (<NUM>) is a liquid fire retardant solution (<NUM>).

In an embodiment, the fire retardant <NUM> comprises at most <NUM> ppm, or is free from, each one of a heavy metal, boron, and a halogenated compound. Correspondingly, in an embodiment of a treated plywood panel, the first veneer layer comprises at most <NUM> ppm, or is free from, added heavy metal, boron, and a halogenated compound.

In an embodiment, the application of the fire retardant <NUM> does not affect an emission class of the plywood panel. The emission class describes the amount of emissions of the plywood panel in use. Emission classes typically vary from one country to another. Herein, for the determination of the emission class, the document "M1 Protocol for Chemical and Sensory Testing of Building Materials", as published by Rakennustietosäätiö RTS, i.e. The Building Information Foundation RTS, (version dated <NUM>. <NUM>) is referred to. An embodiment comprises using such an amount of such a fire retardant <NUM> that the treated plywood panel has the same emission class as defined in the document "M1 Protocol for Chemical and Sensory Testing of Building Materials" as a similar untreated plywood panel. As for more specific meaning of the term "similar", see below.

In an embodiment, the fire retardant <NUM> is a liquid fire retardant solution. In an embodiment, the fire retardant <NUM> is an aqueous liquid solution of the fire retardant. In an embodiment, the liquid fire retardant solution <NUM> comprises an acid or acid salt compound comprising phosphorus.

The liquid fire retardant solution <NUM> comprises <NUM>-hydroxyethylidene-<NUM>,<NUM>-diphosphonic acid (HEDP). HEDP has the chemical formula CH<NUM>C(OH)[PO(OH)<NUM>]<NUM> and the following structure
<CHM>.

It is also has a Chemical Abstracts Service (<NPL>. HEDP is preferable, because it is water soluble, whereby the aqueous solution can be reasonably easily be impregnated into the surface veneers <NUM>, <NUM>. In an embodiment, the liquid fire retardant solution <NUM> comprises from <NUM> wt-% to <NUM> wt-% HEDP, such as from <NUM> wt-% to <NUM> wt-% HEDP. Moreover, as calculated from the chemical formula, pure HEDP comprises <NUM> wt% phosphorous (i.e. bound phosphorous). In an embodiment, the fire retardant <NUM> is an aqueous liquid solution comprising from <NUM> wt-% to <NUM> wt-% HEDP, such as from <NUM> wt-% to <NUM> wt-% HEDP.

Preferably, the liquid fire retardant solution <NUM> further comprises at least one of calcium, iron, potassium, sodium, sulphur, copper, and zinc. Materials that comprise at least one of these elements may be used to improve the capability of the fire retardant to impregnate into veneers. In addition, these materials help to keep the fire retardant within the veneer even in humid conditions. In an embodiment, the liquid fire retardant solution <NUM> comprises at least <NUM> ppm of at least one of calcium, iron, potassium, sodium, sulphur, copper, and zinc, as measured according to the standard SFS-EN ISO <NUM>. In an embodiment, the liquid fire retardant solution <NUM> comprises at least <NUM> ppm of each one of calcium, iron, potassium, sodium, sulphur, and zinc, as measured according to the standard SFS-EN ISO <NUM>. In an embodiment, the liquid fire retardant solution <NUM> comprises at least <NUM> wt-% or at least <NUM> wt-% of HEDP, and at least <NUM> ppm of one of calcium, iron, potassium, sodium, sulphur, copper, and zinc as measured according to SFS-EN ISO <NUM>. In an embodiment, the liquid fire retardant solution <NUM> comprises at least <NUM> wt-% or at least <NUM> wt-% of HEDP, and at least <NUM> ppm of each one of calcium, iron, potassium, sodium, sulphur, and zinc as measured according to SFS-EN ISO <NUM>.

In an embodiment, such an amount of the fire retardant <NUM> is applied onto the first surface <NUM> that at least <NUM>/m<NUM> or at least <NUM>/m<NUM> of bound phosphorous is applied onto the first surface <NUM>. In an embodiment, such an amount of the fire retardant <NUM> is applied onto the second surface <NUM> that at least <NUM>/m<NUM> or at least <NUM>/m<NUM> of bound phosphorous is applied onto the second surface <NUM>.

Such an amount is preferably obtained using a liquid fire retardant solution <NUM> that comprises from <NUM> wt-% to <NUM> wt-% <NUM>-hydroxyethylidene-<NUM>,<NUM>-diphosphonic acid (HEDP). In such a case, at least <NUM>/m<NUM> or at least <NUM>/m<NUM> of the a liquid fire retardant solution <NUM> is applied onto the first surface <NUM>. In an embodiment at least <NUM>/m<NUM> or at least <NUM>/m<NUM> of the a liquid fire retardant solution <NUM> is applied also onto the second surface <NUM>. However, it has been observed that a sufficient fire resistance is obtained by this amount. In order to keep the costs reasonable, preferably at most <NUM>/m<NUM> of such liquid fire retardant solution <NUM> is applied onto the surfaces. Even more preferably, at least <NUM>/m<NUM> and less than <NUM>/m<NUM> of such solution is applied onto at least the first one (<NUM>) of the surfaces (<NUM>, <NUM>).

Depending on the fire retardant <NUM>, the excellent fire performance of the virgin fire resistant plywood panel may degrade with time, especially in outdoor conditions. This is due to the fact that the chemicals used to generate the fire resistant properties may be water-soluble compounds and hygroscopic. Thus, when exposed to high humidity, fire resistant plywood may get a high moisture content. High moisture content may lead to migration of the fire retardant chemicals within the plywood and salt crystallisation on the product surface. The fire retardant chemicals may ultimately be leached out of the plywood. Even at moderate outdoor humidity and indoors, the fire performance may deteriorate because the fire retardant chemicals migrate away from the surface towards lower concentration regions deeper inside the material thus increasing flammability of the product.

Using a liquid solution comprising HEDP as the fire retardant <NUM> has the further beneficial effect that the fire retardant is not very hygroscopic. As indicated above, hygroscopic fire retardant may, in the long run, because of the migration of fire retardant chemicals, form salt on the product surface and in this way weaken the fire resistance of the panel. In contrast, when the fire retardant <NUM> is a liquid comprising HEDP, such problems are avoided, at least to great extent.

Hygroscopicity of the fire retardant <NUM> can be tested by applying the treated plywood panel to a controlled environment for a specified time and observing the weight increase of the panel. Such test is detailed in a document "Nodrtest Method, build <NUM>, Approved <NUM>-<NUM>". The test conditions <NUM> ± <NUM>% at <NUM> ± <NUM> are used after conditioning in <NUM> ± <NUM>% RH at <NUM> ± <NUM> (see section <NUM>). At the same time, formation of salts onto a surface of the panel, or dripping of water from the surfaces of the panel (i.e. lack thereof), must be observed.

Using the results of this test, the panel can be classified as being suitably for indoor use, as specified in the document "<NPL>". The panel can be classified as being suitably for indoor use, when a weight increase in the aforementioned test is less than <NUM> %. Weight increase is due to water absorption to the panel.

It has been observed that the fire resistant panels, having been treated as described above, have the property of being "DFR Class INT" panels according to<NPL>. As a consequence, the weight increase in the aforementioned test "<NPL>" is less than <NUM> %. In addition, after or during the test, there is no visible salt at surface and no exudation of liquid. Furthermore, the panel is initially fire resistant according the standard EN <NUM>-<NUM>:<NUM>+A1:<NUM>.

As motivated above, too high a pressure can break the internal structure of the panel <NUM> such that its strength becomes reduced. This is not desired for structural panels. Therefore, in an embodiment, the liquid fire retardant solution <NUM> is applied onto the surfaces (<NUM>, <NUM>) such that a pressure by which the fire retardant solution <NUM> is applied does not exceed <NUM> bar(a) or <NUM> bar(a). As indicated above, in an embodiment, the liquid fire retardant solution <NUM> is applied onto the surfaces (<NUM>, <NUM>) such that a pressure by which the fire retardant solution <NUM> is applied does not go below <NUM> bar(a). However, in a stack <NUM> a small pressure improves the impregnation without breaking the internal structure of the panel.

It has been found that in addition to the liquid fire retardant solution, no other means is necessary for sufficient fire resistance. Therefore, preferably, the panel <NUM> does not comprise an aluminium foil. For example, the panel may be free from aluminium or comprise less than <NUM> ppm aluminium. Preferably, the panel <NUM> does not comprise a synthetic fibrous layer, such as a glass fibre layer. Preferably, the panel <NUM> only comprises the veneer layers, of which at least the first veneer <NUM> (and preferably also the second veneer <NUM>), has been impregnated with the fire retardant, as discussed above, and adhesive in between the veneer layers. Naturally, a user of the panel may coat such a panel, e.g. by painting. With reference to <FIG>, a coated plywood panel <NUM> comprises, in addition to the coating <NUM> and/or the coating <NUM>, a first veneer layer <NUM>, such that all veneer layers of the plywood panel <NUM> are left on the same side of a first interface <NUM> between the first veneer <NUM> and a first coating <NUM>, and a second veneer layer <NUM>, such that all veneer layers of the plywood panel <NUM> are left on the same side of a second interface <NUM> between the second veneer <NUM> and a second coating <NUM>. Naturally, it is possible that the panel is coated only from one side, e.g. the first side, as indicated in <FIG>. In such a case all veneer layers of the plywood panel <NUM> are left on the same side of a first interface <NUM> between the first veneer <NUM> and a first coating <NUM>, and a second surface <NUM> of the panel, the second surface <NUM> being comprised by the second veneer <NUM>.

When a surface (<NUM>, <NUM>) of the panel has been coated (e.g. painted), the surface of the surface veneer layer (<NUM>, <NUM>) does not form a surface of the panel. However, in such a case the panel comprises a first veneer layer <NUM>, such that all veneer layers of the plywood panel <NUM> are left on the same side of a first interface <NUM> between the first veneer layer <NUM> and the coating <NUM>. Moreover, the panel comprises a second veneer layer <NUM>, such that all veneer layers of the plywood panel <NUM> are left on the same side of a second interface <NUM> between the second veneer layer <NUM> and a coating <NUM>. Moreover, the traces of sanding may not be visible from a coated panel.

As an example, a <NUM> thick plywood panel having five cross laminated veneer layers, each made of spruce, was treated to be fire resistant as discussed above, and the strength was measured according to the standard EN <NUM> (February <NUM>). The standard describes how to measure a modulus of elasticity E (section <NUM>. <NUM>) and a bending strength fm (section <NUM>. <NUM>) in a longitudinal and transverse directions (section <NUM>). In the longitudinal direction measurements, the plywood panel is placed in between two supports (see <FIG> of the standard EN <NUM>) such that the grain direction of surface veneer layers <NUM>, <NUM> is directed from one support to the other. In the transverse direction measurements, the plywood panel is placed in between two supports (see <FIG> of the standard EN <NUM>) such that the grain direction of surface veneer layers <NUM>, <NUM> is parallel to the supports.

The panel had a first bending strength fm∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM> as measured in a longitudinal test of the standard EN310. The panel had a second bending strength fm⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, as measured in a transverse test of the standard EN310. The panel had a first modulus of elasticity E∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM> as measured in a longitudinal test of the standard EN310. The panel had a second modulus of elasticity E⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, as measured in a transverse test of the standard EN310.

As another example, a <NUM> thick plywood panel having seven cross laminated veneer layers made of spruce was treated to be fire resistant as discussed above, and the strength was measured according to the standard EN <NUM>. Such a panel had a first bending strength fm∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM> and a second bending strength fm⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, as measured and defined in the standard EN <NUM>, longitudinal and transversal tests, respectively. In addition the panel had a first modulus of elasticity E∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM> and a second modulus of elasticity E⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, as measured and defined in the standard EN <NUM>, longitudinal and transversal tests, respectively.

An embodiment of a plywood panel comprises from five to nine veneer layers, each made from spruce, and the thickness of the panel is from <NUM> to <NUM>, such as from <NUM> to <NUM>. Moreover, the panel has a first bending strength fm∥ at least <NUM> N/mm<NUM>, a second bending strength fm⊥ at least <NUM> N/mm<NUM>, a first modulus of elasticity E∥ at least <NUM> N/mm<NUM>, and a second modulus of elasticity E⊥ at least <NUM> N/mm<NUM>, as measured and defined in the standard EN <NUM>. Herein "first" refers to a longitudinal test and "second" to a transverse test of the standard EN <NUM>.

Moreover, as indicated above, the application of fire retardant does not significantly affect the mechanical properties of the panel. Therefore, in an embodiment of the method, the panel has, before the application of the fire retardant, a primary first modulus of elasticity E∥<NUM> and a primary second modulus of elasticity E⊥<NUM>. The term "primary" refers to the properties before the application of the fire retardant. Moreover, the panel has, after the application of the fire retardant, a secondary first modulus of elasticity E∥<NUM> and a secondary second modulus of elasticity E⊥<NUM>. The term "secondary" refers to the properties after the application of the fire retardant. As for the terms first and second, these refer to the orientation of the panel in the test, as discussed above. Furthermore, the ratio of the primary first modulus of elasticity E∥<NUM> to the secondary first modulus of elasticity E∥<NUM>, i.e. E∥<NUM>/E∥<NUM>, is from <NUM> to <NUM>; preferably from <NUM> to <NUM> or from <NUM> to <NUM>. In addition or alternatively, the ratio of the primary second modulus of elasticity E⊥<NUM> to the secondary second modulus of elasticity E⊥<NUM>, i.e. E⊥<NUM>/E⊥<NUM>, is from <NUM> to <NUM>; preferably from <NUM> to <NUM> or from <NUM> to <NUM>.

The elastic moduli of a panel can be tested without breaking the panel. However, if the bending strength is also tested, the test is destructive. To test the properties of a single panel before the treatment with the fire retardant, it is possible to divide a panel to a first part and a second part, and treat only the second part with the fire retardant. The mechanical properties of the untreated first part can be measured according to the standard EN <NUM>, resulting in values for a primary first modulus of elasticity E∥<NUM>, a primary second modulus of elasticity E⊥<NUM>, a primary first bending strength fm∥<NUM>, and a primary second bending strength fm⊥<NUM>. The mechanical properties of the treated second part can be measured according to the standard EN <NUM>, resulting in values for a secondary first modulus of elasticity E∥<NUM>, a secondary second modulus of elasticity E⊥<NUM>, a secondary first bending strength fm∥<NUM>, and a secondary second bending strength fm⊥<NUM>. Preferably also the values for the bending strength remain unchanged. Thus, in an embedment,.

In the alternative, it is possible to receive an untreated similar plywood panel, and measure the first modulus of elasticity E∥<NUM>, a primary second modulus of elasticity E⊥<NUM>, a primary first bending strength fm∥<NUM>, and a primary second bending strength fm⊥<NUM> according to the standard EN <NUM> from the untreated similar panel. The mechanical properties of the treated panel can be measured according to the standard EN <NUM>, resulting in values for the secondary first modulus of elasticity E∥<NUM>, the secondary second modulus of elasticity E⊥<NUM>, the secondary first bending strength fm∥<NUM>, and the secondary second bending strength fm⊥<NUM>. As for more specific meaning of the term "similar", see below. The four ratios as discussed above apply in an embodiment. More specifically:.

As indicated above, the application of fire retardant does not significantly affect the mechanical properties of the panel. This applies in particular for the corresponding average values as obtained from a set of treated panels and a set of similar untreated panels. Therefore, in a test regarding panels, a first set of sixty panel samples has, before the application of the fire retardant, an average primary first modulus of elasticity <E∥<NUM>> and a second set of sixty panel samples has, before the application of the fire retardant average primary second modulus of elasticity <E⊥<NUM>>. Herein the average is calculated by measuring the moduli for each of the sixty panel samples.

Herein the term "panel sample" refers to samples obtained by sawing a first set of ten un-treated similar plywood panels <NUM> to twelve panel samples each. Each one of the ten panels are similar to each other. Each of the ten panels of the first set is sawn such that six panel samples are suitable for a longitudinal test and other six panel samples are suitable for a transversal test according to EN <NUM>. In this way, first set of sixty panel samples (which are untreated) are suitable for a longitudinal test and second set of sixty panel samples (which are untreated) are suitable for a transversal test according to the standard EN <NUM>. The test may be destructive, whereby these panel samples are destroyed in the test. However, in addition to the moduli of elasticity, the bending strength can be measured in the same test.

The term "similar" in connection with plywood panels means panels having the same number of veneer layers, same thickness of panel, same adhesive in between the veneer layers, and the veneer layers are made from same wood species. Moreover, the thicknesses of individual veneer layers are the same, and the water resistant coating <NUM> (if any) is made of same material with the same thickness. This applies in particular, when the panels are obtained from the same production line as subsequent plywood panels. In particular, a plywood panel having been treated with the aforementioned method is similar to the plywood panel received and treated by the method before the treatment.

The fire resistant treatment is applied to a second set of ten similar panels. These panels are similar to each other and to the panels of the first set of ten un-treated similar plywood panels. Each of the ten panels of the second set of ten similar panels is sawn such that six panel samples are suitable for a longitudinal test and other six panel samples are suitable for a transversal test. In this way, a third set of sixty panel samples (which are treated) are suitable for a longitudinal test and a fourth set of sixty panel samples (which are treated) are suitable for a transversal test according to the standard EN <NUM>.

The panel samples of the third set of sixty panel samples have, after the application of the fire retardant, an average secondary first modulus of elasticity <E∥<NUM>>; and the panel samples of the fourth set of sixty panel samples have, after the application of the fire retardant, an average secondary second modulus of elasticity <E⊥<NUM>>. Such an embodiment comprises receiving, in total, two-hundred and forty similar plywood panel samples, and treating hundred and twenty of them as disclosed above. In other words, twenty similar panels are received, ten of them are treated, and each one of the twenty panels are sawn to twelve panel samples as indicated above.

In an embodiment, for the third and the first sets of sixty panel samples having the thickness of <NUM>, the average elastic moduli were <E∥<NUM>> and <E∥<NUM>>, respectively; and the ratio <E∥<NUM>>/<E∥<NUM>> was <NUM>. For the fourth and the second sets of sixty panel samples having the thickness of <NUM>, the average elastic moduli were <E⊥<NUM>> and <E⊥<NUM>>, respectively; and the ratio <E⊥<NUM>>/<E⊥<NUM>> was <NUM>.

In an embodiment, for the third and the first sets of sixty panel samples having the thickness of <NUM>, the average elastic moduli were <E∥<NUM>> and <E∥<NUM>>, respectively; and the ratio <E∥<NUM>>/<E∥<NUM>> was <NUM>. Moreover, and for the fourth and the second sets of sixty panel samples having the thickness of <NUM>, average elastic moduli were <E⊥<NUM>> and <E⊥<NUM>>, respectively; and the ratio <E⊥<NUM>>/<E⊥<NUM>> was <NUM>.

Similar test can be made concerning the first and second bending strengths fm∥ and fm⊥, respectively. As indicated above, these values can be measured in the same test as the corresponding elastic modulus E∥ and E⊥, respectively.

In an embodiment, for sixty panel samples having the thickness of <NUM>, the ratio <fm∥<NUM>>/<fm∥<NUM>> was <NUM> and the ratio <fm⊥<NUM>>/<fm⊥<NUM>> was <NUM>. In an embodiment, for sixty panel samples having the thickness of <NUM>, the ratio <fm∥<NUM>>/<fm∥<NUM>> was <NUM> and the ratio <fm⊥<NUM>>/<fm⊥<NUM>> was <NUM>.

As indicated by the values, in an embodiment, the aforementioned four ratios are in the limits:.

Two types of plywood panels were manufactured. Plywood panels of the first type had five veneer layers of spruce and a thickness of <NUM>. As evident, the panels of the first type were similar to each other in the meaning described above. Plywood panels of the second type had seven veneer layers of spruce and a thickness of <NUM>. As evident, the panels of the second type were similar to each other in the meaning described above. Forty panels of both types were received for treatment.

Both surfaces of each of the panels were sanded. Sanding was performed in two stages. In a first stage, a surface with a grit of P60 was used in a sanding direction that was perpendicular to the grain orientation of the surface veneer layer that was sanded. In a second stage, a surface with a grit of P80 was used in a sanding direction that was perpendicular to the grain orientation of the surface veneer layer that was sanded.

An amount of from <NUM>/m<NUM> to <NUM>/m<NUM> of fire liquid retardant <NUM> comprising from <NUM> wt-% to <NUM> wt-% of HEDP was sprayed onto the first and second surfaces <NUM>, <NUM> of each panel. The temperature of the surfaces <NUM>, <NUM> of the panels and the temperature of the fire retardant temperature <NUM> before the treatment were over <NUM>.

The panels were arranged in a stack <NUM>, and kept in the stack for <NUM> hours in order to allow the fire retardant <NUM> to impregnate to the surface veneer layers.

Ten panels of both types were randomly selected for fire test according to the standards EN <NUM>:<NUM> + A1:<NUM>, EN ISO <NUM>-<NUM>:<NUM>, and EN ISO <NUM>-<NUM>:<NUM>.

Test specimens were prepared from ten of the panels according to the standard EN <NUM>:<NUM> + A1:<NUM> (see section <NUM>). In the fire test, plywood specimens were mounted with standard vertical and horizontal joints on <NUM> × <NUM> untreated wood frame with metal screws. Air gap between product and standard gypsum plasterboard substrate was used. Specimens were conditioned according to standard EN <NUM>:<NUM> to constant mass (temperature <NUM> ± <NUM>, RH <NUM> ± <NUM>%). The results of the fire test are given in Table <NUM>.

As indicated in the table, the following criteria are fulfilled: FIGRA ≤ <NUM> W/s, THR<NUM> ≤ <NUM> MJ, SMOGRA ≤ <NUM><NUM>/s<NUM>, and TSP<NUM> ≤ <NUM><NUM>.

Test specimens were prepared from ten of the panels according to the standard EN ISO <NUM>-<NUM>:<NUM> (see section <NUM>). Specimens were conditioned to constant mass according to standard EN <NUM>:<NUM> (temperature <NUM> ± <NUM>, RH <NUM> ± <NUM>%). Flame source was applied to surface or bottom edge of the specimens. Three specimens per grain orientation and flame application were tested with both panel thicknesses. Flame application time was <NUM> and total test duration was <NUM>. Damage by flame (flame spread) varied between <NUM> to <NUM>. Ignition of filter paper did not occur.

Flame spread was less than <NUM> in all tested samples. In connection with the data of Table <NUM>, it is seen that the specimens fulfilled the criteria set for fire class B-s1, d0 of the standard EN <NUM>-<NUM>:<NUM>+ A1:<NUM>.

Test specimens were prepared from ten of the panels according to the standard EN ISO <NUM>-<NUM>:<NUM> (see section <NUM>). Substrates were not used. Specimens were conditioned according to standard EN <NUM>:<NUM>. Results of the test are indicated in Table <NUM>.

As indicated in Table <NUM> and taking into account the results from the tests according to standard EN ISO <NUM>-<NUM>:<NUM>, the specimens fulfilled the criteria set for fire class Bfl-s1 of the standard EN <NUM>-<NUM>:<NUM>+ A1:<NUM>.

Ten of the panels were also tested according to the standard EN <NUM> to determine the longitudinal and transverse elastic moduli E∥ and E⊥, respectively, and the longitudinal and transverse shear strength fm∥ and fm⊥, respectively. The results are given above. In short, the <NUM> panels had a first bending strength fm∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, a second bending strength fm⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, a first modulus of elasticity E∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, and a second modulus of elasticity E⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>. The <NUM> panels had a first bending strength fm∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, a second bending strength fm⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>, a first modulus of elasticity E∥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM> , a second modulus of elasticity E⊥ from <NUM> N/mm<NUM> to <NUM> N/mm<NUM>.

Ten of the panels were subjected to a hygroscopicity test as specified in the document "Nodrtest Method, build <NUM>, Approved <NUM>-<NUM>". The weight increase in the test was less than <NUM> %.

As indicated by this value, the specimens fulfilled the criteria set for the class "DFR Class INT" of the document "NT Method: DURABILITY OF REACTION TO FIRE - PERFORMANCE CLASSES OF FIRE-RETARDANT TREATED WOOD-BASED PRODUCTS IN INTERIOR AND EXTERIOR END USE APPLICATIONS; NT FIRE <NUM>, Approved <NUM>-<NUM>".

Claim 1:
A method for improving fire resistance of a plywood panel (<NUM>), the method comprising
- receiving a plywood panel (<NUM>), the plywood panel (<NUM>) comprising
• a first veneer layer (<NUM>),
• a second veneer layer (<NUM>), and
• adhesive (<NUM>) in between the first veneer layer and the second veneer layer,
• the first veneer layer (<NUM>) forming a first surface (<NUM>) of the plywood panel (<NUM>) and having grains oriented in a first grain direction (D1),
characterized by
- sanding the first surface (<NUM>) of the plywood panel (<NUM>) in a first sanding direction (R1) that forms an angle of at least <NUM> degrees with the first grain direction (D1), and
- after said sanding the first surface, applying fire retardant (<NUM>) onto the first surface (<NUM>) having a temperature of at least <NUM>, wherein
- the fire retardant (<NUM>) is a liquid fire retardant solution (<NUM>) that comprises <NUM>-hydroxyethylidene-<NUM>,<NUM>-diphosphonic acid (HEDP),
- the fire retardant (<NUM>) is applied onto the first surface (<NUM>) such that a pressure by which the fire retardant (<NUM>) is applied does not go below <NUM> bar(a), and
- the fire retardant (<NUM>) is not applied by pressure impregnation with a pressure of at least <NUM> bar(a).