RU2776928C1 - Fire-resistant plywood panel and method for improvement of fire resistance of plywood panel - Google Patents

Abstract

FIELD: building materials.

SUBSTANCE: group of inventions relates to the manufacture of fire-resistant plywood panels. The first surface of a plywood panel is ground in the first direction of grinding, which forms an angle of at least 30° with the first direction of wood fibers. After grinding of the first surface, a flame retardant is applied to the first surface having a temperature of at least 15°C. The plywood panel contains the first veneer layer containing the first surface of the plywood panel, so that all veneer layers of the plywood panel remain on one side of the first surface. The plywood panel contains the second veneer layer, so that all veneer layers of the plywood panel remain on one side of the second interface surface between the second veneer and coating or the second panel surface, wherein the second veneer layer contains the second surface. The plywood panel contains adhesive substance between the first veneer layer and the second veneer layer. The first surface contains traces indicating the grinding of the first surface of the plywood panel in the first direction of grinding. The first veneer layer contains at least 8 g/m² of bound phosphorus.

EFFECT: fire resistance of plywood panels is improved.

24 cl, 6 dwg, 2 tbl

Description

Technical field

The invention relates to plywood panels. The invention relates to plywood panels suitable for use as structural elements such as walls or floor coverings. The invention relates to fire-resistant plywood panels. The invention relates to methods for improving the fire resistance of plywood panels.

State of the art

Plywood is a well known wood based material. Being wood based, plywood as such is usually not very fire resistant. It is known to apply a flame retardant to plywood to improve fire resistance. The flame retardant can be applied to the plywood panel under high pressure and/or vacuum, then dried with heat.

The wood material can be treated with flame retardant chemicals, for example by vacuum pressure impregnation, which is carried out in a pressure vessel. However, for plywood, this conventional protective treatment method is not suitable due to the low penetration of the chemical through the adhesive layers, the geometry of pressure vessels unsuitable for plywood, and the batch type of impregnation method. In addition, pressure impregnation and the additional necessary drying step can destroy the wood cell membranes, leading to surface and internal cracks, which can adversely affect the strength of the plywood.

It is known in the art to treat plywood by applying adhesive layers, where a protective chemical, such as a flame retardant, is mixed with a binder composition used to glue veneer sheets to form plywood. The method of impregnating the adhesive layer can be included in the laying of sheets in a glue bag, however, it requires that the flame retardant chemical be stable under extreme process conditions, such as heat, pressure and high pH, which exist during the stages of laying sheets and hot pressing of plywood. . These process conditions can lead to the decomposition of the flame retardant and thus the loss of its activity during the life of the final product. In addition, the flame retardant used must be able to penetrate from the binder composition into the wood material in order to provide the required level of protection.

It has been observed that the application of high pressure reduces the strength of the plywood panel. Therefore, improving the fire resistance of plywood panels at the same time reduces the strength. Alternatively, if no pressure is used, the degree of impregnation with the flame retardant may remain so low that the fire retardant properties of the panel are not achieved.

The inventors have recognized the need for a method for protecting plywood from fire while maintaining other plywood properties required for its end use.

Brief description of the invention

A method for improving the fire resistance of a plywood panel is proposed. A sufficient degree of flame retardant impregnation is achieved without the use of pressure impregnation. Sufficient impregnation with the flame retardant is achieved without the use of heat drying. Thus, in this method, the mechanical strength of the panel remains high. Moreover, due to the fact that high pressure is not used, a significant amount of flame retardant remains in the surface layers of the veneer, which are most susceptible to fire. Thus, since most of the flame retardant is used in the most exposed layers, a small amount of flame retardant is sufficient, making the process economical. Moreover, the density (kg/m ³ ) of the plywood remains low during processing, which facilitates the movement of the panels to the construction site. A high degree of fire retardant impregnation of the surface layers of the panel veneer is achieved by proper sanding of the plywood panel surfaces prior to applying the fire retardant. In addition, a high degree of fire retardant impregnation of the surface layers of the panel veneer is achieved by applying the fire retardant at a certain temperature. Also disclosed is the preferred amount of flame retardant in connection with the preferred thickness of the veneer.

The method according to the invention is described in independent claim 1 of the claims. A corresponding fire-resistant panel according to the invention is described in independent claim 14 of the claims. Preferred embodiments are described in the dependent claims and the description. Additional embodiments are disclosed in the description.

Brief description of the drawings

On FIG. 1a shows a side view of a plywood panel,

in FIG. 1b shows a side view of a plywood panel,

in FIG. 2a shows a perspective view of the first surface of the plywood panel and the first sanded surface,

in FIG. 2b shows a perspective view of the second surface of the plywood panel and the second sanded surface,

in FIG. 3a shows a side view of the application of the liquid flame retardant solution to the first surface by spraying,

in FIG. 3b shows a side view of stacked plywood panels for pressing liquid flame retardant solution into the plywood panel,

in FIG. 4a shows a side view of the layers of plywood veneer and the direction of the wood fibers in it,

in FIG. 4b shows an axonometric view of the plywood veneer layers and the direction of the wood fibers in it,

in FIG. 4c shows a perspective view of a layer of plywood veneer,

in FIG. 4d shows an axonometric view of a veneer,

in FIG. 5a shows a plywood panel coated on both sides, not part of the invention,

in FIG. 5b shows a plywood panel coated on one side only and

in FIG. 6a-6c show three kinds of fire test conditions for a plywood panel.

Detailed description of the invention

The invention relates to a method for improving the fire resistance of a plywood panel 100. The invention relates to a fire retardant plywood panel 100. In this method, a fire retardant 300 is applied to at least the first surface 112 of the plywood panel 100 to improve fire resistance. Preferably, fire retardant 300 is applied to two opposite surfaces 112, 122 of plywood panel 100 to improve fire resistance. The flame retardant 300 may be a liquid flame retardant solution or it may be liquefied during application to impregnate the panel 100 with the flame retardant 300. In one embodiment, the flame retardant 300 is a liquid flame retardant solution 300. In one embodiment, the liquid flame retardant solution 300 contains phosphorus. Phosphorus is bound in a chemical compound. Thus the term "associated phosphorus" is used in this specification. Thus, the expression "comprises bound phosphorus" means "comprises a chemical compound containing phosphorus". However, the amount of bound phosphorus refers to the amount of bound phosphorus only, not to the total amount of the chemical compound in which the phosphorus is bound. Both the raw plywood panel to be treated and the treated plywood panel, that is, the fire-resistant plywood panel, are called plywood panels. In the product claims, the term "plywood panel" refers to the treated panel, i.e. the fire-resistant plywood panel. When considering the application, the terms "treated panel" and "raw panel" are also used to clarify the issue.

On FIG. 1a shows a side view of a plywood panel 100. The method includes providing such a panel 100. The plywood panel 100 includes a first veneer layer 110, a second veneer layer 120, and an adhesive 190 between the first veneer layer 110 and the second veneer layer 120. As is well known, plywood panel 100 typically contains at least three layers of veneer, such as 3 to 21 layers of veneer. Embodiments in which the plywood panel 100 comprises 5 to 11, for example 5, 7 or 9 layers of veneer are particularly suitable for construction purposes. For this purpose, the thickness of the plywood panel 100 is preferably 15 mm to 18 mm.

The numbering of the veneers herein is such that the first veneer layer 110 forms the first surface 112 of the uncoated plywood panel 100, and the second veneer layer 120 forms the second surface 122 of the uncoated plywood panel 100 if the panel is uncoated. With this method, an uncoated plywood panel or a plywood panel that is only coated on the second side is treated to form a fire-resistant plywood panel. However, such a treated plywood panel can be treated later, for example by painting. The user of the panel should be aware that the coating may degrade the fire resistance of the panel. The invention therefore relates, in addition to the method, to plywood panels in which no more than one side is covered. The coating refers to a waterproof coating such as a polymer coating, in particular a coating containing a polymerized resin such as a polymerized phenol-formaldehyde and/or a polymerized lignin-phenol-formaldehyde resin. Surface treatment of an uncoated plywood panel with a fire retardant is not considered to form a coating on the plywood panel, even though some fire retardant 300 may be present at least in some areas of the treated surface. surface 112 may be treated with flame retardant 300.

In the case of coating, the surface 112, 122 of the panel is covered with a coating 195, 196, while the surface 112, 122 becomes the interface 112, 122 between the coating 195, 196 and the surface of the veneer 110, 120 (i.e., the first veneer 110 or the second veneer 120 ). Moreover, if the panel is coated on the side of the first veneer 110 with the first coating 195, the first veneer 110 extends in the thickness direction tp of the panel 100 to the interface 112 between the first coating 195 and the first veneer 110. Moreover, if the panel is coated with the second coating 196 on the side of the second veneer 120, the second veneer 120 extends in the thickness direction tp of the panel 100 to the interface 122 between the second coating 196 and the second veneer 120. The surfaces of the bare panel are indicated in FIG. 1a and 1b, with the interfaces of the coated panel indicated in FIGS. 5a and 5b.

The thickness tp of the plywood panel 100 is defined between the first surface 112 and the second surface 122 (or the first surface 112 and the second interface 122 if the second surface 122 of the panel is coated with a coating 196). Other veneer layers 130, 140, 150, if present, are located between the first veneer layer 110 and the second veneer layer 120 in the thickness direction Sz of the thickness tp of the plywood panel 100, as shown in FIG. 1b. The panel thickness tp is smaller than the panel length lp and the panel width wp 100 (see Fig. 4b). Accordingly, all veneer layers of a plywood panel are located between its surfaces (112, 122) or, if the panel is coated on the second side, between surface 112 and interface 122 (see Fig. 1a and 5b).

Within the veneer layer of the plywood panel 100, the wood fibers of the wood material are oriented. In particular, the first veneer layer 110 contains wood fibers oriented in the first wood fiber direction D1, and the second veneer layer 120 contains wood fibers oriented in the second wood fiber direction D2. Such directions are shown, for example, in Fig. 2a, 2b, 4a and 4b.

The veneer layer 110, 120, 130 may contain wood fibers oriented in different directions. The term "grain direction" refers to the direction in which the wood from which the layers of veneer were obtained grew. This direction is obvious, for example, from the annual rings visible on the veneer. Alternatively, the direction of the wood fibers of the veneer layer refers to the average direction of the wood fibers, the average being calculated as a weight average. Alternatively, the direction of the wood fibers of the veneer layer refers to the median direction of the wood fibers, the median being calculated from the weight, so that 50 wt. % wood fibers of the veneer layer form a positive angle relative to the plane defined by the normal to the veneer layer and the direction of the wood fibers of the veneer layer, and 50 wt. % wood fibers of the veneer layer form a negative angle relative to the plane defined by the normal to the veneer layer and the direction of the wood fibers of the veneer layer.

A problem with treated wood products such as plywood is that their surfaces 112, 122, as a result of the manufacturing methods, such as hot pressing, to which they have been subjected, often have, for example, poor wetting properties compared to freshly sawn wood products. polar wood. Surfaces 112, 122 of plywood 100 may have a glossy appearance indicating that they have been pressure deactivated at high temperatures. Especially during hot pressing, the resinous extractives migrate to the surface, the adhesives cure and the pan lubricant remains on the surfaces of the product and thus deactivates or blocks the surfaces preventing wetting. During hot pressing, the plywood 100 is heat treated, whereby the plywood surfaces 112, 122 become more hydrophobic as a result of plasticization of the lignin and/or loss of residual water, resulting in a rearrangement of the lignocellulosic components of the wood. As a result of the increased hydrophobicity of plywood, its wettability decreases. Poor wettability can then adversely affect subsequent treatments such as surface treatment with flame retardant 300.

It has been found that sanding the surface 112 of the plywood to which the flame retardant 300 is to be applied results in a surface roughness suitable for good surface coverage. The surface roughness resulting from sanding the surface of the plywood has the advantage of ensuring that the fire retardant chemical is evenly and properly distributed over the surface by rollers or by spraying.

As for FIG. 2a, the embodiment includes sanding the first surface 112 of the plywood panel 100 in a first sanding direction R1 that forms an angle α1 of at least 30 degrees, at least 45 degrees, or at least 60 degrees with the first wood grain direction D1. The first grinding direction R1 is also perpendicular to the normal of the first surface 112. In particular, the first surface 112 is ground using the first grinding surface 332, so that the first grinding surface 322 and the first surface 112 of the plywood panel 100 move relative to each other in a direction that forms an angle along at least 30 degrees, at least 45 degrees, or at least 60 degrees with the first wood grain direction D1 and is in the plane of the first surface 112 of the plywood panel 100. Sanding in this direction R1 has been found to expose the wood fibers of the first surface 112. It was it has been found that the liquid or liquefied flame retardant well impregnates the wood surface sanded in this direction. This is most likely due to the fact that the grinding surface destroys the fibers. Accordingly, if the sanding surface moves only parallel to the wood fibers, some of the wood fibers may remain intact. As shown in FIG. 2a, preferably the angle α1 between D1 and R1 is approximately 90 degrees and R1 is perpendicular to the normal of the first surface 112. The plywood panel 100 can only be sanded and treated with the flame retardant 300 on the first side.

As for FIG. 2b, an embodiment in which the second surface 122 is also treated with a flame retardant comprises sanding the second surface 122 of the plywood panel 100 in a second sanding direction R2 that forms an angle α2 of at least 30 degrees, at least 45 degrees, or at least 60 degrees with the second direction of D2 wood fibers. The second grinding direction R2 is also perpendicular to the normal of the second surface 122. In particular, the second surface 122 is ground using the second grinding surface 334, so that the second grinding surface 324 and the second surface 122 of the plywood panel 100 move relative to each other in a direction that forms an angle along at least 30 degrees, at least 45 degrees, or at least 60 degrees with the second wood grain direction D2 and is in the plane of the second surface 122 of the plywood panel 100. As shown in FIG. 2b, preferably the angle α2 between D2 and R2 is approximately 90 degrees and R2 is perpendicular to the normal of the second surface 122.

Grinding can be carried out, for example, by moving the grinding surface (322, 324) and/or the grinding surface (112, 122) back and forth relative to each other. Alternatively, the grinding surface (322, 324) may form a loop (ie, a belt) and may only move in one direction.

It has also surprisingly been found that sanding the surface of the plywood to which the fire retardant chemical is to be applied has the advantage of allowing the fire retardant chemical to penetrate into the cavities and cells of the wood material.

In one embodiment of the present invention, a sanded surface 112 (in some cases also surface 122) is provided using, in the sanding step, prior to placing the flame retardant on the sanded surface, an abrasive paper having a P20-P20 grit, preferably a P40-P120 grit, and more preferably a P40 grit. - P80 in accordance with ISO 6344. Using abrasive paper with this grit grade results in a surface of 112, into which the 300 flame retardant is easily absorbed. Using a finer grit, the surface can remain so to speak closed to absorb the flame retardant chemical, and using a coarser grit leads to a non-uniform distribution of the flame retardant chemical, where the main part of the flame retardant chemical remains in the depressions of the surface, while the so-called tops or peaks remain uncovered.

In one embodiment of the present invention, at least the first surface 112 of the plywood panel 100 is sanded in a substantially transverse direction with respect to the grain direction of the veneer surface. The term "substantially transverse" has been specified above. This has a positive effect on most of the wood fibers on the surface of the plywood and on many individual sanding spots on the wood fiber and so-called open spots, so that the fire retardant chemical is readily absorbed into the wood fibers.

Grinding in said substantially transverse direction gives improved wetting properties of the wood surface. Wetting is facilitated in the direction of abrasive scratches. The fibers are pulled out during grinding in a direction substantially transverse to the wood fibers, while they are mainly stripped off during grinding in a direction parallel to the wood fibers. The presence of torn out microfibers promotes better mechanical bonding and also provides a larger real surface area available for flame retardant and wood interactions.

In addition to the direction of sanding, the roughness of the surface(s) being sanded has been found to affect how well the liquid or liquefied fire retardant solution 300 impregnates the surface of the veneer. In one embodiment, sanding (i.e., abrading) the first surface 112 of the plywood panel 100 is performed with a first sanding surface 322 (e.g., sandpaper surface) having a grit in the ranges described above (e.g., P20 to P220 or P40 to P120, according to with ISO 6344). Grinding of the second surface 122 of the plywood panel 100 can be performed with a sanding surface, such as the first sanding surface 322 or the second sanding surface 324, having a grit within the aforementioned ranges. This ensures that the sanded surface of the plywood panel is not too smooth before applying the flame retardant and at the same time the flame retardant substantially uniformly impregnates the surface of the veneer.

Preferably, the method includes sanding the first surface 112 with a primary first sanding surface 322 (e.g., a primary sandpaper surface) having a P40 to P60 grit, and thereafter sanding the first surface 112 with a secondary first sanding surface (e.g., a secondary sandpaper surface) having a grit from P60 to P120. In the same way, the method may include grinding the second surface 122 with a primary grinding surface having a grit of P40 to P60, and then grinding the second surface 122 with a secondary grinding surface having a grit of P60 to P120.

It has also been found that the liquid flame retardant solution impregnates the surface 112 (and, in some cases, 122) well when the surface temperature is high enough. One embodiment includes setting the temperature of the first surface 112 to at least 15° C. and, after said grinding the first surface and setting the first surface temperature, applying the liquid fire retardant solution 300 to the first surface 112. The liquid fire retardant solution 300 is applied to the first surface 112 when the temperature of the first surface 112 is at least 15°C. Preferably, the first surface temperature is between 30°C and 100°C when the fire retardant liquid solution 300 is applied to the first surface 112. More preferably, the first surface temperature is between 50°C and 95°C when the fire retardant liquid solution 300 is applied to the first surface. surface 112. Also preferably, the temperature of the liquid flame retardant solution 300 when applied to the first surface 112 is at least 15°C, such as at least 20°C or at least 25°C. Preferably, the temperature of the liquid fire retardant solution 300 when applied to the first surface 112 is at most 55°C or at most 50°C.

The increased temperature of the wood, and in some cases also of the flame retardant solution, can soften the resinous compounds present in the wood structure and facilitate fluid flow and therefore contribute to improved penetration. However, at higher temperatures there may be a risk of resinous compounds being released onto the surface of the wood. This can have the opposite effect on the penetration of the flame retardant.

One embodiment includes setting the temperature of the second surface 122 to at least 15° C. and, after said grinding the second surface and setting the temperature of the second surface, applying the liquid fire retardant solution 300 to the second surface 122. The liquid fire retardant solution 300 is applied to the second surface 122 when the temperature of the second surface 122 is at least 15°C. Preferably, the second surface temperature is between 30°C and 100°C when the fire retardant liquid solution 300 is applied to the second surface 122. More preferably, the second surface temperature is between 50°C and 95°C when the fire retardant liquid solution 300 is applied to the second surface. surface 122. Also preferably, the temperature of the liquid flame retardant solution 300 when applied to the second surface 122 is at least 15°C, such as at least 20°C or at least 25°C. Preferably, the temperature of the liquid flame retardant solution 300 when applied to the second surface 122 is at most 55°C or at most 50°C.

As for FIG. 3a, the fire retardant solution 300 may, for example, be sprayed onto the panel, i.e., its first surface 112. The fire retardant solution 300 may, for example, be sprayed onto the second surface 122. The sprinkler may include a conveyor 410. The sprinkler may include a nozzle 420 for spraying the solution. 300 flame retardant per panel. Alternatively or additionally, the flame retardant 300 may be applied using a coating roller. Alternatively, the panel 100 may be immersed in the flame retardant solution 300.

As for FIG. 3b, after the fire retardant solution 300 has been applied, the panels 100 may be stacked in the plywood panel stack 200 to press the liquid fire retardant solution 300 into (ie, impregnate) the plywood panel 100. As an example, in FIG. 3b shows a stack 200 of five panels 100 ₁ , 100 ₂ , 100 ₃ , 100 ₄ and 100 ₅ . The pressure of such a stack 200 forces the flame retardant 300 into the panels. However, usually the stack is so low that the pressure of the stack does not degrade the mechanical properties of the panels. In one embodiment, the panel is held in the stack 200 for at least 24 hours or at least 48 hours to allow the surface veneers 110, 120 to be impregnated with a flame retardant.

In the case where only the first surface 112 of the plywood panel 100 is treated with the fire retardant 300, it is possible to improve the impregnation of the fire retardant 300 of the treated panels 100 before stacking them. Impregnation with flame retardant 300 can be improved by using elevated temperature. It has been noted that temperature affects the properties of the fire retardant solution as well as the structure of the wood and therefore affects penetration. Therefore, the treated panel may be thermally treated at a temperature of at least 60° C. or at least 80° C. for at least 15 seconds or at least 30 seconds to improve the fire retardant impregnation 300 of the treated panels 100. The treatment temperature may be, for example , not more than 120°C or not more than 100°C, so as not to create high pressure in the treated wet wood due to boiling. However, even temperatures above 100° C. can be used in connection with a water-based fire retardant without degrading the internal structure of the panel, since with this method only little (or none) of the fire retardant penetrates into the middle layers of the veneer (for example, 130). Thus, the pressure generated by heat treatment does not appear to affect the mechanical properties of the panel.

The thermal impregnation discussed above can also be used if the opposing surfaces 112, 122 of the panel 100 are treated sequentially. For example, it is possible to first apply a flame retardant to the first surface 112, then heat treat the panel 100 to improve impregnation, and then apply the flame retardant to the second surface 122.

The impregnation need not be complete. It is enough that the first surface feels dry. Alternatively or additionally, it may be sufficient that the treated surface (112, 122) does not stain objects that come into contact with the treated surface.

It has also been found that fire retardant 300, such as liquid fire retardant solution 300, is better at impregnating dry wood than wet wood. Moreover, it has been found that a flame retardant 300, such as a liquid flame retardant solution 300, is better at impregnating wood that is not too dry. Therefore, in the preferred embodiment, the first veneer layer 110 is substantially dry when the flame retardant 300 is applied. This also applies to the second veneer layer 120 if both surfaces are treated with a flame retardant. In one embodiment, prior to application of the flame retardant 300 to the first surface 112, the moisture content of the first veneer layer 110 is between 3% and 14%, preferably between 8% and 12%. In one embodiment, prior to application of the flame retardant 300 to the second surface 122, the moisture content of the second veneer layer 120 is 3% to 14%, preferably 8% to 12%. Typically, the term "moisture content" refers to the mass of water divided by the dry mass of the wood. The degree of dryness affects the impregnation, in particular when the liquid flame retardant solution 300 is used.

It has been found that in this way the first veneer layer 110 and the second veneer layer 120 are well impregnated with the fire retardant 300, thus forming a fire-resistant plywood panel 100. Such a panel is fire-resistant according to the classification standard EN 13501-1:2007+A1:2009, as indicated by the : combustibility B, smoke production s1 and flaming droplets d0. Also note that this standard references other standards, as detailed below. Moreover, preferably the aforementioned fire resistance classes (B-s1, d0) are achieved in the conventional arrangements described in EN 13823. Such arrangements are illustrated in FIG. 6a - 6s.

Preferably the aforementioned fire resistance classes (B-s1, d0) are achieved in all arrangements described in FIG. 6a - 6s. On FIG. 6a-6c, the plywood panel is mechanically fastened to wood or metal frames. The plywood panel is fixed using metal wood screws. On FIG. 6a shows a fire test of a plywood panel 100. The panel is mounted on a base 510 with a frame 520, or via a base 510 on a frame 520. The frame 520 may be a wood frame or a metal frame. Insulation 530 is placed in contact with panel 100. Insulation 530 has an A1 or A2-s1, d0 fire rating and a density of at least 30 kg/m ³ . A gap 550 is left between the two panels 100. Both horizontal and vertical gap arrangements are tested. In practice, a large plywood panel is sawn into smaller pieces, leaving gaps of 550 between them. Thus, it is important that the fire resistance of the treated plywood panel is good even after it has been sawn into pieces. In particular, fire resistance classes B-s1, d0 are confirmed in arrangements with both horizontal and vertical gaps.

The frame 520 is left in gap position 550 as shown in FIG. 6a - 6s. On FIG. 6b shows a fire test of a plywood panel 100. The panel is mounted directly on the base 510. The base 510 has an A1 or A2-s1, d0 fire rating and a density of at least 30 kg/m ³ . The base may be, for example, an insulating gypsum board. The base is connected to the frame 520. In FIG. 6c shows a fire test on a 100 plywood panel. The panel is mounted on a 510 base using a 520 frame. As specified in EN 13823, the 510 base must meet the requirements of EN 13238. The base can be, for example, gypsum board or concrete. The frame 520 may be a wooden frame or a metal frame. Thus, an air gap 540 is left between the panel 100 and the base 510. The width of the air gap 540 may be at least 40 mm, for example, or as described in the aforementioned standard.

In practice, the above classes (B-s1, d0) refer to the best class applicable to combustible material such as wood. Moreover, in one embodiment, the aforementioned fire ratings (B-s1, d0) are achieved at least in an arrangement in which an air gap is left between the panel 100 and the base 510 (FIG. 6c), which is usually the most difficult test condition to obtain these fire resistance classes.

Moreover, preferably the aforementioned fire resistance classes (B-s1, d0) are achieved without restrictions regarding the structures surrounding the panel or their absence, with the exception of the coating of the panel itself.

Thus, a fire-resistant plywood panel 100 is obtained. Such a fire-resistant plywood panel 100 comprises a first veneer layer 110 forming a first surface 112 of the plywood panel 100, a second veneer layer 120 forming a second surface 122 (or interface 122) of the plywood panel 100, and an adhesive 190 between the first veneer layer 110 and the second veneer layer 120. In one embodiment, the first veneer layer 110 contains at least 8 g/m ^{2 of} bound phosphorus. In an embodiment in which the second veneer layer 120 is also treated with a flame retardant, the second veneer layer 120 contains at least 8 g/m ^{2 of} bound phosphorus. Phosphorus is bound into a chemical compound, as explained in more detail below. Due to the way plywood panel 100 was made and/or due to the phosphorus, plywood panel 100 is fire resistant according to EN 13501-1:2007+A1:2009 as indicated by the classes:

flammability B,

smoke production s1 and

flaming drops d0.

According to EN 13501-1:2007+A1:2009 (see section 11.6), a panel is classified as belonging to fire class B when the parameter FIGRA ≤ 120 W/s and the parameter THR _600s ≤ 7.5 MJ as defined in the standard EN 13823:2002 and when the tongue of flame does not reach 150 mm above the point of impact of the flame in 60 seconds, as explained in detail in the standard in view of EN ISO 11925-2:2002. The FIGRA parameter mentioned above refers to the flame growth rate and the THR parameter to the total heat release. The parameter THR _600s refers to the total heat dissipation in 600 seconds (section 9.1 of EN 13823:2002).

According to EN 13501-1:2007+A1:2009 (see section 11.9.2), a panel is classified as having smoke production s1 when SMOGRA ≤30 ^m2 /s2 and TSP _600s ^{≤50 m2} as defined in EN 13823:2002 standard. The above parameter SMOGRA refers to the smoke growth rate and the parameter TSP refers to the total smoke production. The parameter TSP _600s refers to the total smoke production in 600 seconds (section 9.2 of EN 13823:2002).

According to EN 13501-1:2007+A1:2009 (see section 11.10.1), a panel is classified as having flaming droplets d0 when no flaming droplets or particles as described in EN 13823:2002 (section 9.3) are formed. in 600 seconds.

The above classification is valid for building material with the exception of flooring, that is, for plywood panels not intended to be used as a flooring element. For flooring elements, slightly different criteria apply. However, it has been observed that the panel 100 tested as a flooring element is fire resistant according to EN 13501-1:2007+A1:2009 as indicated by the classes

flammability B _fl ,

smoke production s1.

According to EN 13501-1:2007+A1:2009 (section 12.6) to achieve class B _fl critical heat flux ≥8.0 kW/ ^m2 according to EN ISO 9239-1. Moreover, the tongue of flame does not reach 150mm above the point of impact in 20 seconds, as defined in EN ISO 11925-2:2002.

According to EN 13501-1:2007+A1:2009 (section 12.9.2) to achieve class s1 the Smoke parameter is ≤750% × minimum according to EN ISO 9239-1.

To keep the cost of the panel, in particular the additional cost due to the use of flame retardant 300, low, in one embodiment, the first veneer layer 110 contains no more than 30 g/m ² , for example, no more than 25 g/m ² or no more than 20 g/m 2 m ^{2 of} associated phosphorus. In one embodiment, the second veneer layer 120 contains no more than 30 g/m ² , no more than 25 g/m ² , or no more than 20 g/m ^{2 of} bound phosphorus. In addition, the thickness of the surface veneer 110 can be chosen such that the first veneer 110 contains 2 kg/m ³ to 15 kg/m ³ , such as 3 kg/m ³ to 12 kg/m ³ , of bound phosphorus. Moreover, the thickness of the second veneer 120 can be chosen such that it contains from 2 kg/m ³ to 15 kg/m ³ , for example from 3 kg/m ³ to 12 kg/m ³ , of bound phosphorus.

The amount of phosphorus can be measured, for example, by taking a sample of the veneer layer, burning it to ash, and analyzing the bottom ash. This method is commonly referred to as SEM-EDS, short for scanning electron microscopy (SEM) - energy dispersive spectroscopy (EDS). After performing an SEM-EMF analysis, it is possible to determine the proportion of elements that have the same mass as sodium or are heavier than sodium. For example, in one embodiment, in such an analysis, the phosphorus content was at least 40 wt. % (one significant figure) of those elements in the ash that had an atomic mass at least equal to that of sodium (Na). This content was measured for ash using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

In one embodiment, plywood panel 100 has marks on first surface 112 indicating sanding of first surface 112 of plywood panel 100 in a first sanding direction R1 that forms an angle of at least 30 degrees, at least 45 degrees, or at least 60 degrees with the direction D1 wood fibers of the first layer 110 veneer. In one embodiment, the plywood panel 100 has marks on the second surface 122 indicating sanding of the second surface 122 of the plywood panel 100 in a second sanding direction R2 that forms an angle of at least 30 degrees, at least 45 degrees, or at least 60 degrees with the direction D2 wood fibers of the second layer 120 veneer. Traces may also indicate that the panel surfaces have been sanded with a sanding surface having the proper grit, as detailed above. However, if the surface of the panel is coated with coating 195 or coating 195, 196 (see FIG. 195), the coating(s) may cover the sanding marks, with the sanding marks and direction not necessarily visible on the panel.

Because of the adhesive 190, the liquid flame retardant solution 300 does not significantly spread from the surface layer (110, 120) of the veneer to another layer (130, 140, 150) of the veneer. As noted above, this helps to concentrate the flame retardant 300 where it is needed. In one embodiment, the plywood panel 100 includes a third veneer layer 130 located between the first veneer layer 110 and the second veneer layer 120. Moreover, the third veneer layer 130 does not contain added phosphorus (bound or free) or contains less phosphorus than the surface layer 110 or 120 of the veneer. In one embodiment, the content of c130 phosphorus in the third layer 130 of the veneer, measured in wt. %, less than the phosphorus content c110 of the first veneer layer 110, measured in wt. %. In one embodiment, the c130/c110 ratio of these values is at most 0.5, at most 0.25, or at most 0.1. The c130 and c110 content can be determined from the ash, as discussed above, and in proportion to only those ash elements that have an atomic mass at least equal to that of sodium (Na).

Moreover, even though some of the flame retardant 300 may spread into the middle layers, such as when applied by dipping, due to the low pressure, the flame retardant is only applied near the edges of the panel. In one embodiment, the central region of the third veneer layer 130 contains no added phosphorus (bound or free) or contains less phosphorus than the surface of the veneer layer 110 or 120. Also untreated wood, such as spruce, contains a small amount of phosphorus. For example, the above c130/c110 ratio can be applied when a sample of the third veneer ply is taken from the central region of the third ply 130 veneer. The central region of the third veneer layer 130 refers to those parts of the third veneer layer 130 that are located at least 5 cm or at least 25 mm from those edges of the plywood panel that are perpendicular to the surfaces 112, 122 or interfaces 112, 122.

The fact that a high level of fire resistance is achieved without high pressure application of a flame retardant has the additional effect that the edges of the panel are not responsible for fire resistance. As stated above, in one embodiment, only no more than a reasonably necessary small boundary region of the middle veneer is impregnated with a flame retardant. Therefore, the panel can be sawn in the longitudinal and/or transverse direction(s) without loss of fire resistance properties. For example, the edges of a finished panel can be sawn off and the panel is still considered fire resistant at the same level. This has many advantages in building applications, as large panels that have been treated to be fire resistant can then be sawn to the proper size.

In one embodiment, the first veneer layer 110 comprises soft wood such as spruce or pine. In one embodiment, the second veneer layer 120 comprises soft wood such as spruce or pine. This has the effect that less flame retardant 300 is required to achieve the required level of fire resistance. Thus, preferably, at least the first veneer layer 110 comprises softwood. In one embodiment, the second veneer layer 120 also contains softwood. Preferably, both of these layers (110, 120) contain soft wood such as spruce or pine. In one embodiment, all veneer layers of plywood panel 100 are made from the same types of wood. What has been said about the plywood panel applies both to the panel that is processed in the process and to the panel that has been processed as described above. The invention has been found to work particularly well when the first veneer layer 110 contains wood from the species Norway Spruce (Picea Abies) or Scotch Pine (Pinus Sylvestris). In addition, the second layer 120 of the veneer may contain wood species of Scotch spruce or Scotch pine.

In one embodiment, at least one of the thickness tsv1 (see FIG. 1b) of the first veneer layer 110 and the thickness tsv2 of the second veneer layer 120 is between 1.5 mm and 3.2 mm. On the one hand, this thickness was found to be large enough to receive a sufficient amount of flame retardant 300. On the other hand, this thickness was found to be small enough to have the flame retardant concentration at the proper level without using an excess amount of flame retardant. In one embodiment, the thickness tsv1 of the first veneer layer 110 is equal to the thickness of the second veneer layer 120. In one embodiment, both the thickness tsv1 of the first veneer layer 110 and the thickness tsv2 of the second veneer layer 120 are between 1.5 mm and 3.2 mm.

As for FIG. 1b, the thickness tbv of the base veneer layer (ie, the veneer layer between the first and second layers 110, 120) may be different from the aforementioned thicknesses tsv1 and tsv2. For example, the thickness tbv of the veneer body layer may be greater than the thickness tsv1, tsv2 of the surface layer of the veneer (110, 120). In one embodiment, the panel comprises a third veneer layer 130 which is located, in the direction of the panel, between the first veneer layer 110 and the second veneer layer, where the thickness of the third veneer layer is between 1.5 mm and 4.0 mm. In one embodiment, the panel comprises at least a third veneer layer 130 and the thickness of each such veneer layer, which is located between the first veneer layer 110 and the second veneer layer, is from 1.5 mm to 4.0 mm.

However, as shown in FIG. 1a, all veneer layers of plywood panel 100 may be of equal thickness, or substantially equal thickness, at least prior to sanding.

Grinding the surface layers 110, 120 of the veneer typically reduces their thickness. This is advantageous in terms of the fire resistance of the panel. Since the surface layers of the veneer can be thin, for example due to sanding, the concentration of flame retardant in the surface layers increases. In one embodiment of the method, a fire retardant is applied to such a panel 100 which comprises a third veneer layer 130 between the first veneer layer 110 and the second veneer layer 120, the third veneer layer 130 having a thickness tbv and the first veneer layer 110 having a thickness tsv1 where the thickness tbv of the third the veneer layer 130 is greater than the thickness tsv1 of the first veneer layer 110. In one embodiment of the method, a fire retardant is applied to such a panel 100 that comprises a third veneer layer 130 between the first veneer layer 110 and the second veneer layer 120, the third veneer layer 130 having a thickness tbv and the second veneer layer 120 having a thickness tsv2, where the thickness tbv of the third the veneer layer 130 is greater than the thickness tsv2 of the second veneer layer 120. This is possibly the case, for example, when all veneer layers are of equal thickness before sanding and the thickness of the surface layers 110, 120 of the veneer is reduced by sanding. However, the flame retardant treatment may make the surface layer 110, 120 of the veneer slightly thicker.

In one embodiment, the adhesive 190 of the plywood panel 100 comprises at least one of a polymerized phenol formaldehyde resin and a polymerized lignophenol formaldehyde resin. This gives two results. First, the polymerized phenol-formaldehyde and lignin-phenol-formaldehyde resins are at least somewhat impervious to liquid (and waterproof), so the adhesive 190 prevents the flame retardant 300 from diffusing from the surface layer (110, 120) into the main part of the veneer layers (130, 140, 150, 160). This helps to maintain a high content of flame retardant in the surface layers 110, 120 of the veneer. Secondly, polymerized phenol-formaldehyde and lignin-phenol-formaldehyde resins are quite heat resistant and do not conduct heat well. Therefore, the adhesive 190 also protects the main part of the veneer layers (130, 140, 150, 160) in case of fire.

As for FIG. 4d and 4c, the veneer layer 130 may comprise a plurality of veneer sheets 131, 132, 133. The veneer sheets 131, 132, 133 are preferably arranged so that their wood grain directions are parallel. On FIG. 4d shows an example of a veneer sheet 131. Preferably, the first veneer layer 110, which is the surface layer of the veneer, contains only one sheet of veneer. This improves the appearance of the first side of the plywood panel 100. Preferably, the second veneer layer 120, which is the other surface layer of the veneer, contains only one sheet of veneer. This improves the appearance of the second side of the plywood panel 100. Moreover, the presence of the surface layer 110, 120 of the veneer formed from only one sheet of veneer improves impregnation with the flame retardant 300, since in this case both surface layers 110, 120 of the veneer do not contain adhesive. . The adhesive can make it difficult to impregnate with flame retardant 300.

As for FIG. 4b, the plywood panel may be cross-layered. This means that the veneer layer has a wood grain direction that is substantially perpendicular to the wood grain direction(s) of the adjacent layer(s). However, the internal structure of the plywood panel 100 may be different. For example, the directions of the wood fibers of two or more adjacent layers may be parallel. Typically, a plywood panel with an odd number of veneers (eg, 3, 5, 7, 9, 11, 13, or 15) is entirely cross-ply. Typically, a plywood panel with an even number of veneers (eg, 4, 6, 8, 10, 12, or 14) has cross plies, except for the two middle plies, which have the grain directions parallel to each other. In such structures, the wood fiber direction DG1 of the first veneer layer 110 is parallel to the wood fiber direction DG2 of the second veneer layer 120. A panel in which the wood grain direction DG1 of the first veneer layer 110 is parallel to the wood grain direction DG2 of the second veneer layer 120 is preferred 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 grinding process.

The internal structure affects the mechanical strength of the plywood panel. However, unlike some other methods, when the liquid flame retardant solution 300 is applied as above, the internal structure of the plywood panel is not destroyed during the fire retardant treatment, thus the mechanical properties of the panel remain largely unchanged.

If the panel is treated with a pressurized flame retardant, such as pressure impregnation, the panel is affected at least as follows:

- the flame retardant is also absorbed from the sides of the panel, thus also those layers that are located between the surface layers of the panel veneer are impregnated with the flame retardant, at least to some extent and at some distance from the border of the panel,

- the internal structure of the veneer layers changes, as evidenced, for example, by large cracks in the veneer layers,

- the structure of the fibers of the wood material of the veneer layers. If the flame retardant has been applied under pressure, some of the fibers of the wood material are permanently pressed in. This should in particular reduce the flexural rigidity of the wood material.

Thus, the panel embodiment only includes wood that has not been pressure impregnated with a flame retardant. The preferred embodiment of the panel includes only wood that has not been pressure impregnated with a flame retardant, where the impregnation pressure is at least 200 kPa abs. (2 bar abs.) or at least 1 MPa abs. (10 bar abs).

In addition, if the middle layers were impregnated with a flame retardant and the panel was then dried in an oven, this affects the panel at least as follows:

- the internal structure of the veneer layers changes, as evidenced, for example, by large cracks in the veneer layers,

- the structure of the fibers of the wood material of the veneer layers. If the flame retardant has been applied under pressure, some of the fibers of the wood material must be permanently pressed in. This should in particular reduce the flexural rigidity of the wood material.

Typically, the temperature used in such drying is from 110°C to 180°C. This can cause the flame retardant that has been impregnated with the middle layers of veneer to boil and thus damage the panel.

Drying by heating may be required only in cases where the panel has not been treated with a pressure of not more than 50 kPa abs. (0.5 bar abs), i.e. low or high vacuum. The preferred embodiment of the panel includes only wood that has not been pressure treated below 50 kPa abs. (0.5 bar abs).

The plywood panel may contain, for example, from 3 to 25 layers of veneer. The thickness tp of the plywood panel may be 4 mm to 75 mm, preferably 9 mm to 21 mm, more preferably 15 mm to 21 mm. In a preferred embodiment, the plywood panel contains at least five layers of veneer and has a thickness tp of at least 15 mm. Such a panel is usually strong enough to be used as a building material.

The specific gravity (ie, mass divided by area) of such a plywood panel can be between 6.5 kg/m ² and 10 kg/m ² (two significant figures). The specific gravity depends, for example, 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, for example, from 400 kg/m ³ to 550 kg/m ³ . In building applications, denser panels are more difficult to handle. This is another reason to use softwood and only a small amount of flame retardant.

In one embodiment, flame retardant 300 contains bound phosphorus. The term bound phosphorus refers to phosphorus bound in a chemical compound. In one embodiment, flame retardant 300 contains from 2 wt. % up to 30 wt. %, for example, from 5 wt. % up to 15 wt. % bound phosphorus measured according to SFS-EN ISO 11885 (referring to the latest version available from May 2017). Preferably, the flame retardant 300 is a liquid flame retardant solution 300.

In one embodiment, flame retardant 300 contains no more than 1 ppm or none of any heavy metal, boron, and halogenated compound. Accordingly, in one embodiment of the treated plywood panel, the first layer of veneer contains no more than 1 ppm or no added heavy metal, boron, and halogenated compound.

In one embodiment, the application of flame retardant 300 does not affect the emission class of the plywood panel. The emission class describes the amount of emissions of the plywood panel used. Emission classes usually vary from one country to another. In this document, reference is made to the document "M1 Protocol for Chemical and Sensory Testing of Building Materials" published by Rakennustietosaatio RTS, i.e. The Building Information Foundation RTS, (version dated 1/22/2015) to define the emission class. The embodiment includes using such an amount of flame retardant 300 that the treated plywood panel has the same emission class as defined in the "M1 Protocol for Chemical and Sensory Testing of Building Materials" document for a similar untreated plywood panel. For a more specific meaning of the term "similar", see below.

In one embodiment, flame retardant 300 is a liquid flame retardant solution. In one embodiment, flame retardant 300 is an aqueous solution of a liquid flame retardant. In one embodiment, the liquid flame retardant solution 300 contains an acid or an acid salt compound containing phosphorus. Preferably, the liquid flame retardant solution 300 contains at least one of

1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and/or its salts,

ethylenediaminetetramethylenephosphonic acid (EDTMP) and/or its salts, and

diethylenetriaminepentamethylenephosphonic acid (DTPMF) and/or its salts.

More preferably, the liquid flame retardant solution 300 contains 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP). HEDP has the chemical formula CH ₃ C(OH)[PO(OH) ₂ ] ₂ and the following structure

It also holds the Chemical Abstracts Service (CAS) number 2809-21-4. HEDP is preferred because it is soluble in water, and surface veneers 110, 120 can be easily impregnated properly with this aqueous solution. In one embodiment, liquid flame retardant solution 300 contains from 10 wt. % up to 60 wt. % HEDP, for example, from 20 wt. % up to 40 wt. % HEDP. Moreover, as calculated from the chemical formula, pure HEDP contains 30 wt. % phosphorus (i.e. bound phosphorus). In one embodiment, the flame retardant 300 is an aqueous liquid solution containing from 10 wt. % up to 60 wt. % HEDP, for example, from 20 wt. % up to 40 wt. % HEDP.

Preferably, the liquid flame retardant solution 300 further comprises at least one of calcium, iron, potassium, sodium, sulfur, copper, and zinc. Materials that contain at least one of these elements can be used to improve the ability to absorb flame retardant into veneer layers. In addition, these materials help retain the flame retardant in the veneer, even in wet conditions. In one embodiment, the liquid flame retardant solution 300 contains at least 4 ppm of at least one of calcium, iron, potassium, sodium, sulphur, copper, and zinc as defined in SFS-EN ISO 11885. In one embodiment, the solution 300 liquid flame retardant contains at least 4 ppm of each element of calcium, iron, potassium, sodium, sulfur, and zinc as defined in SFS-EN ISO 11885. In one embodiment, liquid flame retardant solution 300 contains at least 10 mass. % or at least 20 wt. % HEDP and at least 4 ppm of at least one of calcium, iron, potassium, sodium, sulphur, copper, and zinc as defined according to SFS-EN ISO 11885. In one embodiment, the liquid flame retardant solution 300 contains at least measure 10 wt. % or at least 20 wt. % HEDP and at least 4 ppm of at least each of calcium, iron, potassium, sodium, sulfur and zinc, as determined according to SFS-EN ISO 11885.

In one embodiment, the first surface 112 is coated with an amount of flame retardant such that at least 8 g/m ² or at least 9 g/m ^{2 of} associated phosphorus is applied to the first surface 112. In one embodiment, an amount of fire retardant is applied to second surface 122 such that at least 8 g/m ² or at least 9 g/m ^{2 of} associated phosphorus is applied to second surface 122.

Such an amount is preferably obtained using a liquid flame retardant solution 300, which contains from 20 wt. % up to 40 wt. % 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP). In this case, at least 150 g/m ² or at least 180 g/m ² of the liquid flame retardant solution 300 is applied to the first surface 112. In one embodiment, at least 150 g/m ² or at least 180 g/m ² of liquid flame retardant solution 300 is also applied to second surface 122. However, it has been observed that sufficient fire resistance is obtained at this amount. In order to keep costs reasonable, preferably no more than 250 g/m ^{2 of} such liquid flame retardant solution 300 is applied to the surfaces. Even more preferably, at least 180 g/m ² and less than 200 g/m ^{2 of} such a solution is applied to at least the first (112) of the surfaces (112, 122).

Depending on the flame retardant 300, the excellent fire performance of the original fire retardant plywood panel may deteriorate over time, especially under outdoor conditions. This is due to the fact that the chemicals used to form the flame retardant properties may be hygroscopic and water soluble compounds. Thus, when exposed to high humidity, fire retardant plywood can acquire a high moisture content. High moisture content can cause fire retardant chemicals to migrate into the plywood and salts to crystallize on the surface of the product. Flame retardant chemicals can eventually leach out of the plywood. Even with moderate outdoor and indoor humidity, fire performance can be degraded due to the fact that fire retardant chemicals migrate from the surface towards areas of lower concentration deeper within the material, thus increasing the flammability of the product.

The use of a liquid solution containing HEDP as flame retardant 300 has the additional advantage that the flame retardant is not very hygroscopic. As mentioned above, a hygroscopic flame retardant may, over time, due to the migration of flame retardant chemicals, form salt on the surface of the product and thus reduce the fire resistance of the panel. In contrast, when the flame retardant 300 is a liquid containing HEDP, such problems are avoided, at least to a large extent.

The hygroscopicity of the flame retardant 300 can be tested by placing the treated plywood panel in a controlled environment for a predetermined time and observing the increase in the weight of the panel. Such a test is described in detail in the document "Nodrtest Method, build 504, Approved 2003-12". The test conditions of 90 ± 3% at 27 ± 2°C are used after adjustment to a relative humidity of 50 ± 3% at 23 ± 2°C (see Section 6.2). At the same time, it is necessary to observe the formation of salts on the surface of the panel or the seepage of water from the surfaces of the panel (i.e. its loss).

Using the results of this test, the panel can be classified as suitable for indoor use as specified in "NT Method: DURABILITY OF REACTION TO FIRE - PERFORMANCECLASSES OF FIRE-RETARDANT TREATED WOOD-BASEDPRODUCTS IN INTERIOR AND EXTERIOR END USEAPPLICATIONS; NT FIRE 054, Approved 2006" -06". A panel can be classified as suitable for indoor use when the weight gain in the above test is less than 30%. The increase in mass is due to the absorption of water by the panel.

Fire resistant panels that have been treated as described above have been observed to have "DFR Class INT" panel quality according to the NT Method: DURABILITY OF REACTION TO FIRE - PERFORMANCECLASSES OF FIRE-RETARDANT TREATED WOOD-BASEDPRODUCTS IN INTERIOR AND EXTERIOR END USEAPPLICATIONS; NT FIRE 054 Approved 2006-06. As a result, the mass increase in the above test "Nodrtest Method, build 504, Approved 2003-12" is less than 30%. In addition, after or during the test, there are no visible salts on the surface and no liquid is released. Moreover, the panel is inherently fire resistant according to EN 13501-1:2007+A1:2009

what do the classes point to

flammability B,

smoke production s1 and

flaming drops d0 or

what do the classes point to

flammability B _fl ,

smoke production s1.

As justified above, too much pressure can destroy the internal structure of the panel 100 so that its strength is reduced. This is undesirable for construction panels. Therefore, in one embodiment, the liquid fire retardant solution 300 is applied to the surfaces (112, 122) such that the pressure by which the fire retardant solution 300 is applied does not exceed 200 kPa abs. (2 bar abs.) or 1 MPa abs. (10 bar abs). As stated above, in one embodiment, the liquid fire retardant solution 300 is applied to the surfaces (112, 122) such that the pressure by which the fire retardant solution 300 is applied does not fall below 50 kPa abs. (0.5 bar abs). However, in stack 200, slight pressure improves impregnation without destroying the internal structure of the panel.

It has been found that, apart from the liquid flame retardant solution, no other means are required for sufficient fire resistance. Therefore, preferably the panel 100 does not contain aluminum foil. For example, the panel may contain no aluminum or contain less than 100 ppm aluminum. Preferably, the panel 100 does not include a synthetic fiber layer, such as a glass fiber layer. Preferably, the panel 100 comprises only layers of veneer, of which at least the first veneer 110 (and preferably also the second veneer 120) is impregnated with a flame retardant as discussed above, and an adhesive between the layers of the veneer. Naturally, the user of the panel can cover such a panel, for example, with a pattern. As shown in FIG. 5a, the coated plywood panel 100 comprises, in addition to the coating 195 and/or the coating 196, a first veneer layer 110 such that all veneer layers of the plywood panel 100 remain on one side of the first interface 112 between the first veneer 110 and the first coating 195, and the second layer 120 of the veneer so that all of the veneers of the plywood panel 100 remain on one side of the second interface 122 between the second veneer 120 and the second coating 196. It is naturally possible that the panel is only coated on one side, such as the first side, as shown in FIG. 5b. In such a case, all veneer layers of the plywood panel 100 remain on one side of the first interface 112 between the first veneer 110 and the first coating 195 and the second surface 122 of the panel, with the second surface 122 being represented by the second veneer 120.

When the surface (112, 122) of the panel is coated (eg, it has been patterned), the surface of the surface layer (110, 120) of the veneer does not form the surface of the panel. However, in such a case, the panel comprises a first veneer layer 110 so that all of the veneer layers of the plywood panel 100 remain on the same side of the first interface 112 between the first veneer layer 110 and the cover 195. Moreover, the panel includes a second veneer layer 120 so that all the veneer layers of the plywood panel 100 remain on one side of the second interface 122 between the second veneer layer 120 and the coating 196. Moreover, sanding marks may not be visible on the coated panel.

As an example, a 15 mm thick plywood panel containing five cross veneer layers each made of spruce was treated to be fire resistant as discussed above and strength was measured according to EN 310 (February 1993). This standard describes how to measure the modulus of elasticity E (section 7.1.1) and the flexural strength f _m (section 7.2.1) in the longitudinal and transverse directions (section 6.6). In the longitudinal dimension, the plywood panel is placed between two supports (see FIG. 1 of EN 310) so that the direction of the wood fibers of the surface layers 110, 120 of the veneer is from one support to the other. In the transverse dimension, the plywood panel is placed between two supports (see FIG. 1 of EN 310) so that the direction of the wood fibers of the surface layers 110, 120 of the veneer is parallel to the supports.

The panel had a first bending strength f _m|| from 37 N/mm ² to 52 N/mm ² measured in the EN 310 longitudinal test. The panel had a second bending strength f _m⊥ from 19 N/mm ² to 28 N/mm ² measured in the EN 310 transverse test The panel had a first modulus of elasticity E _|| from 4600 N/mm ² to 6000 N/mm ² measured in the EN 310 longitudinal test. The panel had a second modulus of elasticity E _⊥ from 1800 N/mm ² to 2700 N/mm ² measured in the EN 310 longitudinal test.

As another example, an 18 mm thick plywood panel containing seven cross layers of spruce veneer was treated to be fire resistant as discussed above and the strength was measured according to EN 310. Such a panel had a first flexural strength f _{m ||} from 37 N/mm ² to 53 N/mm ² and a second bending strength f _m⊥ from 27 N/mm ² to 44 N/mm ² measured in longitudinal and transverse tests, respectively, and defined in EN 310. Except In addition, the panel had a first modulus of elasticity E _|| from 4800 N/mm ² to 6800 N/mm ² and a second modulus of elasticity E _⊥ from 2800 N/mm ² to 4300 N/mm ² measured in longitudinal and transverse tests, respectively, and defined in the EN 310 standard.

The plywood panel of one embodiment comprises five to nine layers of veneer, each made of spruce, and the thickness of the panel is 15 mm to 21 mm, for example 15 to 19 mm. Moreover, the panel has a first bending strength f _m|| at least 37 N/mm ² , the second bending strength f _m⊥ at least 19 N/mm ² , the first modulus of elasticity E _|| of at least 4600 N/mm ² and a second modulus of elasticity E _⊥ of at least 1800 N/mm ² measured and defined in the EN 310 standard. In this document "first" refers to the longitudinal test and "second" to the transverse test of the EN standard 310.

Moreover, as mentioned above, the application of a flame retardant does not significantly affect the mechanical properties of the panel. Therefore, in an embodiment of the method, the panel has a primary first modulus of elasticity E _||1 and a primary second modulus of elasticity E _⊥1 before applying the flame retardant. The term "primary" refers to the properties prior to application of the flame retardant. Moreover, the panel has a secondary first modulus of elasticity E _||2 and a secondary second modulus of elasticity _E⊥2 after application of the flame retardant. The term "secondary" refers to the properties after application of the flame retardant. As for the terms first and second, they refer to the orientation of the panel in the test, as discussed above. Moreover, the ratio of the primary first modulus of elasticity E _||1 to the secondary first modulus of elasticity E _||2 , i.e. E _||1 /E _||2 , is from 0.75 to 1.25, preferably from 0.9 to 1.2 or 0.9 to 1.1. In addition or alternatively, the ratio of the primary second elastic modulus E _{⊥ 1} to the secondary second elastic modulus E _{⊥ 2} , i.e. E _{⊥ 1} /E _{⊥ 2} , is from 0.75 to 1.25, preferably from 0.9 to 1, 2 or from 0.9 to 1.1.

The modulus of elasticity of the panel can be determined without destroying the panel. However, if flexural strength is also determined, the test passes with failure of the prototype. In order to determine the properties of one panel, the panel can be divided into a first part and a second part before flame retardant treatment, and only the second part can be treated with a flame retardant. The mechanical properties of the raw first part can be measured according to EN 310 to give the values of primary first modulus E _||1 , primary second modulus E _⊥1, primary first flexural strength f _m||1 and primary second flexural strength f _{m ⊥1.} The mechanical properties of the machined second part can be measured according to EN 310 to give secondary first modulus E _||2 , secondary second modulus E _⊥2, secondary first flexural strength f _m||2 and secondary secondary flexural strength f _{m ⊥2.}

Preferably, the bending strength values also remain unchanged. Thus, in one embodiment

the ratio f _m||1 /f _{m||2 of} the primary first bending strength (f _m||1 ) to the secondary first bending strength (f _m||2 ) is between 0.75 and 1.25 and/or

the ratio f _m⊥1 /f _{m⊥2 of} the primary second bending strength (f _m⊥1 ) to the secondary second bending strength (f _m⊥2 ) is from 0.75 to 1.25.

Alternatively, a raw similar plywood panel can be obtained and the primary first modulus E _||1 , the primary second modulus E _⊥1 , the primary first flexural strength f _m||1 and the primary second flexural strength f _m⊥1 according to EN 310 can be measured on an untreated similar panel. The mechanical properties of the treated panel can be measured according to EN 310 to give secondary first modulus E _||2 , secondary second modulus E _⊥2 , secondary first flexural strength f _m||2 and secondary second flexural strength f _{m⊥ 2} . For a more specific meaning of the term "similar", see below. The four relationships discussed above are obtained in embodiment. More specific:

E _||1 /E _||2 is from 0.75 to 1.25; preferably 0.9 to 1.2 or 0.9 to 1.1,

E _⊥1 /E _⊥2 is from 0.75 to 1.25; preferably 0.9 to 1.2 or 0.9 to 1.1,

f _m||1 /f _m|| is from 0.75 to 1.25; preferably 0.9 to 1.2 or 0.9 to 1.1,

f _m⊥1 /f _m⊥2 is from 0.75 to 1.25; preferably 0.9 to 1.2 or 0.9 to 1.1.

As stated above, the application of a flame retardant does not significantly affect the mechanical properties of the panel. This is true in particular for the corresponding average values obtained from a group of treated panels and a group of similar untreated panels. Therefore, in the test panels, the first group of sixty panel samples had an average primary first modulus of elasticity <E _||1 > before application of the flame retardant and the second group of sixty panel samples had an average primary second modulus of elasticity <E _⊥1 > before application of the flame retardant. . In this document, the average is calculated by measuring the modules for each of the sixty panel samples.

In this document, the term "panel sample" refers to samples obtained by sawing each panel of the first group of ten raw identical plywood panels 100 into twelve panel samples. All of the ten panels are similar to each other. Each of the ten panels of the first group is sawn so that six panel samples are suitable for the longitudinal test and the other six panel samples are suitable for the transverse test according to EN 310. Thus the first group of sixty panel samples (which are unfinished) are suitable for the longitudinal test and the second a group of sixty panel samples (which are unfinished) are eligible for the transverse test according to EN 310. The test can be destructive, with these panel samples being destroyed during the test. However, in the same test, in addition to the moduli of elasticity, it is possible to measure the flexural strength.

The term "similar" in connection with plywood panels means panels having the same number of layers of veneer, the same thickness of the panel, the same adhesive between the layers of veneer, and the layers of veneer are made from the same type of wood. Moreover, the thicknesses of the individual layers of veneer are the same and the waterproof coating 196 (if present) is made of the same material with the same thickness. This applies in particular when the panels are produced on the same production line as consecutive plywood panels. In particular, the plywood panel processed by the above method is the same as the plywood panel obtained and processed by this method before processing.

The second group of ten identical panels is subjected to fire retardant treatment. These panels are similar to each other and to the panels of the first group of ten similar raw plywood panels. Each of the ten panels of the second group of ten identical panels is sawn so that six panel samples are suitable for the longitudinal test and the other six panel samples are suitable for the transverse test. Thus, the third group of sixty panel samples (which are machined) are suitable for the longitudinal test and the fourth group of sixty panel samples (which are machined) are suitable for the transverse test according to EN 310.

Panel samples of the third group of sixty panel samples have, after applying the fire retardant, an average secondary first elastic modulus <E _||2 > and panel samples of the fourth group of sixty panel samples have, after applying a fire retardant, an average secondary second modulus of elasticity <E _{⊥ 2} >. Such an embodiment involves making a total of two hundred and forty identical plywood panel samples and processing one hundred and twenty of them as described above. In other words, twenty identical panels are made, ten of them are processed, and each of the twenty panels is sawn into twelve panel samples as above.

In one embodiment, for the third and first groups of sixty panel samples having a thickness of 15 mm, the average modulus of elasticity was <E _||2 > and <E _||1 >, respectively, and the ratio <E _||2 >/<E _{| |1} > was 0.95. For the fourth and second groups of sixty panel samples having a thickness of 15 mm, the average elastic modulus was < _E⊥2 > and < _E⊥1 >, respectively, and the ratio < _E⊥2 >/< _E⊥1 > was 0.88 .

In one embodiment, for the third and first groups of sixty panel samples having a thickness of 18 mm, the average modulus of elasticity was <E _||2 > and <E _||1 >, respectively, and the ratio <E _||2 >/<E _{| |1} > was 0.98. Moreover, for both the fourth and second groups of sixty panel samples having a thickness of 18 mm, the average elastic modulus was <E _⊥2 > and <E _⊥1 >, respectively, and the ratio of <E _⊥2 >/<E _⊥1 > was 1.04.

A similar test can be performed in connection with the first and second bending strengths f _m|| and f _m⊥ , respectively. As stated above, these values can be measured in the same test as the corresponding modulus E _|| and E _⊥ , respectively. In this way:

- the average primary flexural strength <f _m||1 >, i.e. the average value of the flexural strengths measured in the direction of the wood fibers of the surface layer of the veneer for the raw panel samples, is measured for the first group of sixty panel samples, as discussed above,

- the average secondary first bending strength <f _m||2 >, i.e. the average value of the bending strengths measured in the direction of the wood fibers of the surface layer of the veneer for the treated panels, is measured for the third group of sixty panel samples, as discussed above,

- the average primary second flexural strength <f _m⊥1 >, i.e. the average value of the flexural strengths, measured perpendicular to the direction of the wood fibers of the surface layer of the veneer for the raw panels, is measured for the second group of sixty panel samples, as discussed above, and

- average secondary second bending strength <f _m⊥2 >, i.e. the average value of bending strengths measured perpendicular to the direction of the wood fibers of the surface layer of the veneer for the treated panels, is measured for the fourth group of sixty panel samples, as discussed above.

In one embodiment, for sixty panel samples having a thickness of 15 mm, the ratio <f _m||2 >/<f _m||1 > was 0.97 and the ratio <f _m⊥2 >/<f _m⊥1 > was 0.88. In one embodiment, for sixty panel samples having a thickness of 18 mm, the ratio <f _m||2 >/<f _m||1 > was 0.98 and the ratio <f _m⊥2 >/<f _m⊥1 > was 1.07.

As evidenced by these values, in one embodiment, the above four ratios are within:

the ratio <E _||2 >/<E _||1 > is from 0.85 to 1.1 or from 0.9 to 1.1,

the ratio <E _⊥2 >/<E _⊥1 > is from 0.85 to 1.1 or from 0.9 to 1.1,

the ratio <f _m||2 >/<f _m||1 > is 0.85 to 1.1 or 0.9 to 1.1, and

the ratio <f _m⊥2 >/<f _m⊥1 > is 0.85 to 1.1 or 0.9 to 1.1.

Examples

Two types of plywood panels were made. The first type of plywood panels had five layers of spruce veneer and was 15 mm thick. As is obvious, the panels of the first type were similar to each other in the sense described above. Type 2 plywood panels had seven layers of spruce veneer and were 18 mm thick. As is obvious, the panels of the second type were similar to each other in the sense described above. Forty panels of both types were taken for processing.

Both surfaces of each panel were sanded. Grinding was performed in two stages. In the first step, a P60 grit surface was used in the sanding direction, which was perpendicular to the orientation of the wood fibers of the surface layer of the veneer that was being sanded. In the second step, a P80 grit surface was used in the sanding direction, which was perpendicular to the orientation of the wood fibers of the surface layer of the veneer that was being sanded.

From 180 g/m ² to 200 g/m ² liquid flame retardant 300 containing from 20 wt. % up to 40 wt. % HEDP was sprayed onto the first and second surfaces 112, 122 of each panel. The temperature of the surfaces 112, 122 of the panels and the temperature of the flame retardant 300 before treatment were above 15°C.

The panels were stacked 200 and held in the stack for 48 hours to allow the 300 surface layers of the veneer to be impregnated with the flame retardant. Ten panels of both types were randomly selected for fire testing according to EN 13823:2010+Al:2014, EN ISO 11925-2:2010 and EN ISO 9239-1:2010.

Fire test according to EN 13823:2010+A1:2014

The prototypes were prepared from ten panels according to EN 13823:2010+A1:2014 (see section 5). In the fire test, plywood specimens were mounted with standard vertical and horizontal joints on a 40 mm x 40 mm raw wood frame with metal screws. An air gap was used between the product and the standard drywall backing. Samples were brought to constant weight according to EN 13238:2010 (temperature 23±1°C, relative humidity 50±5%). The results of the fire test are shown in Table 1.

As shown in the table, the following criteria are met: FIGRA≤120 W/s, THR _600s ≤7.5 MJ, SMOGRA≤30 m ² /s ² and TSP _600s ≤50 m ² .

Fire test according to EN ISO 11925-2:2010

The prototypes were prepared from ten panels according to EN ISO 11925-2:2010 (see section 5). Samples were brought to constant weight according to EN 13238:2010 (temperature 23±1°C, relative humidity 50±5%). The flame source was applied to the surface or bottom edge of the samples. Three samples were tested for each wood fiber orientation and each flame application with both panel thicknesses. The flame application time was 30 seconds and the total test duration was 60 seconds. Flame damage (flame spread) ranged from 50 to 70 mm. The filter paper did not ignite.

Flame spread was less than 150 mm in all tested specimens. In connection with the data in Table 1, it can be seen that the samples met the set of criteria for the fire hazard class B-s1, d0 of the EN 13501-1:2007+A1:2009 standard.

Fire test according to EN ISO 9239-1:2010

The prototypes were prepared from ten panels according to EN ISO 9239-1:2010 (see section 6). Basics were not used. The samples were conditioned according to EN 13238:2010. The test results are shown in Table 2.

As shown in Table 2 and taking into account the test results according to EN ISO 11925-2:2010, the samples meet the criteria set for fire class B _fl-s1 of EN 13501-1:2007+A1:2009.

Test according to EN 310

Ten panels were also tested according to EN 310 to determine the longitudinal and transverse modulus of elasticity E _|| and E _⊥ , respectively, and longitudinal and transverse bending strength f _m|| and f _m⊥ , respectively. The results are shown above. Briefly, 15 mm panels had the first flexural strength f _m|| from 37 N/mm ² to 52 N/mm ² , the second bending strength f _m⊥ from 19 N/mm ² to 28 N/mm ² , the first modulus of elasticity E _|| from 4600 N/mm ² to 6000 N/mm ² and the second modulus of elasticity Ex from 1800 N/mm ² to 2700 N/mm ² . 18 mm panels had the first bending strength f _m|| from 37 N/mm to 53 N/mm, the second bending strength f _m⊥ from 27 N/mm ² to 44 N/mm ² , the first modulus of elasticity E _|| from 4800 N/mm ² to 6800 N/mm ² and the second modulus of elasticity E _⊥ from 2800 N/mm ² to 4300 N/mm ² .

Hygroscopicity test according to the NT method

Ten panels were subjected to the hygroscopicity test as described in "Nodrtest Method, build 504, Approved 2003-12". The weight gain in the test was less than 30%.

As evidenced by this value, the samples meet the criteria set for "DFR Class INT" of the document "NT Method: DURABILITY OF REACTION TO FIRE - PERFORMANCECLASSES OF FIRE-RETARDANT TREATED WOOD-BASEDPRODUCTS IN INTERIOR AND EXTERIOR END USEAPPLICATIONS; NT FIRE 054, Approved 2006- 06".

Test according to EN 310 standard for similar panels

The moduli of elasticity and flexural strength of the raw panels were also determined according to EN 310. The only difference between these panels and the processed panels was that the processed panels were processed. Thus, the panels were similar in the sense discussed above. As detailed in the description, the modulus of elasticity and flexural strength do not change as a result of the application of a flame retardant.

Claims (24)

1. A method for improving the fire resistance of a plywood panel (100), including: providing a plywood panel (100), where the plywood panel (100) contains a first layer (110) of veneer, a second layer (120) of veneer, and an adhesive (190) between the first layer of veneer and a second layer of veneer, wherein the first layer (110) forms the first surface (112) of the plywood panel (100) and contains wood fibers oriented in the first direction (D1) of the wood fibers, and the method includes sanding the first surface (112) of the plywood panel ( 100) in a first grinding direction (R1) that forms an angle of at least 30 degrees with the first direction (D1) of the wood fibers, and after said grinding of the first surface, applying a flame retardant (300) to the first surface (112) having a temperature of at least measure 15°C. 2. The method according to claim 1, wherein the second layer (120) of the veneer forms the second surface (122) of the plywood panel (100) and contains wood fibers oriented in the second direction (D2) of the wood fibers, and the method includes sanding the second surface (122 ) a plywood panel (100) in a second grinding direction (R2) that forms an angle of at least 30 degrees with the second direction (D2) of wood fibers, and after said grinding of the second surface, applying a flame retardant (300) to the second surface (122), having a temperature of at least 15°C. 3. The method according to claim 1 or 2, including after said application of a fire retardant (300) placing a plywood panel (100), on the surface (112) or surface (112, 122) of which the fire retardant (300) was applied, in a stack ( 200) of plywood panels to press the flame retardant (300) into the plywood panel (100), preferably the panel is held in the stack (200) for at least 24 hours. 4. The method according to any one of paragraphs. 1-3, in which before applying the fire retardant (300) to the first surface (112), the moisture content of the first layer (110) of the veneer is from 3% to 14%, while the method includes impregnating the first layer (110) of the veneer with a fire retardant (300 ) to form a plywood panel (100) that is fire resistant according to EN 13501-1:2007+A1:2009, as indicated by fire classes B, smoke production s1 and flaming droplets d0, or as indicated by fire classes B _fl and smoke production s1. 5. The method according to any one of paragraphs. 1-4, in which the flame retardant (300) contains bound phosphorus, preferably the flame retardant (300) is a solution (300) of a liquid flame retardant that contains from 2 wt.% to 30 wt.%, for example from 5 wt.% up to 15 wt.% bound phosphorus, measured according to SFS-EN ISO 11885; in one embodiment, the flame retardant (300) is a liquid flame retardant solution (300) that further comprises at least 4 ppm of at least one of calcium, iron, potassium, sodium, sulfur, copper, and zinc, defined according to SFS-EN ISO 11885, in one embodiment, the flame retardant (300) is a liquid flame retardant solution (300) that further contains at least 4 ppm of each element of calcium, iron, potassium, sodium, sulfur, and zinc, as defined according to SFS-EN ISO 11885. 6. The method according to any one of claims 1 to 5, wherein the flame retardant (300) is a liquid flame retardant solution (300) that contains a phosphorus-containing acid or a phosphorus-containing acid salt compound, preferably the liquid flame retardant solution (300) comprises at least one compound of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) or a salt thereof, ethylenediaminetetramethylenephosphonic acid (EDTMP) or a salt thereof and diethylenetriaminepentamethylenephosphonic acid (DTPMF) or a salt thereof, preferably the flame retardant (300) contains at most 1 ppm or free of any heavy metal, boron and halogenated compound. 7. The method according to any one of paragraphs. 1-6, in which the flame retardant (300) is a solution (300) of a liquid flame retardant that contains 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), preferably a solution (300) of a liquid flame retardant contains from 10 wt. % to 60 wt.% 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP). 8. The method according to any one of paragraphs. 1-7, in which such an amount of fire retardant (300) is applied to the first surface (112) that at least 8 g/m ^{2 of} bound phosphorus is applied to the first surface (112), it is also preferably applied to the second surface (122) such an amount of flame retardant (300) such that at least 8 g/m ^{2 of} bound phosphorus is applied to the second surface (122). 9. The method according to any one of paragraphs. 1-8, in which the flame retardant (300) is a solution (300) of a liquid flame retardant that contains from 20 wt.% to 40 wt.% 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), and at least least 150 g/m ² of the liquid fire retardant solution (300) is applied to the first surface (112), preferably not more than 250 g/m ² of the liquid fire retardant solution (300) is applied to the first surface (112), more preferably from 150 g/ m ² up to 250 g/m ² solution (300) liquid flame retardant is also applied to the second surface (122). 10. The method according to any one of paragraphs. 1-9, in which the fire retardant (300) is applied to the first surface (112) so that the pressure at which the fire retardant (300) is applied does not exceed 200 kPa abs. (2 bar abs.) and/or does not fall below 50 kPa abs. (0.5 bar absolute). 11. The method according to any one of paragraphs. 1-10, in which the grinding of the first surface (112) of the plywood panel (100) is performed using the first grinding surface (322) having a grain size of not more than P220, for example from P20 to P220, according to ISO 6344, preferably grinding the second surface (122 ) of the plywood panel (100) is made with a sanding surface (322, 324) having a grit of not more than P220, for example from P20 to P220, according to ISO 6344. 12. The method according to any one of paragraphs. 1-11, in which before said application of the fire retardant (300) to the first surface (112), the plywood panel (100) has a primary first modulus of elasticity (E _║1 ) and a primary second modulus of elasticity (E _┴1 ), and the method includes applying fire retardant (300) on the first surface (112) in such a way that after said application of the fire retardant (300) on the first surface (112), the plywood panel (100) has a secondary first modulus of elasticity (E _║2 ) and a secondary second modulus of elasticity ( E _┴2 ), where the ratio of the primary first modulus of elasticity (E _║1 ) to the secondary first modulus of elasticity (E _║2 ) is from 0.75 to 1.25 and/or the ratio of the primary second modulus of elasticity (E _┴1 ) to the secondary the second modulus of elasticity (E _┴2 ) is from 0.75 to 1.25, preferably the method includes after said grinding of the first surface applying a flame retardant (300) to the first surface (112) in such a way that in a test including providing a similar n unfinished plywood panel, measurement of primary first modulus of elasticity (E _║1 ), primary second modulus of elasticity (E _┴1 ), primary first flexural strength (f _m║1 ) and primary second flexural strength (f _m┴1 ) according to the standard EN 310 for raw panel and measurement of secondary first modulus of elasticity (E _║2 ), secondary second modulus of elasticity (E _┴2 ), secondary first flexural strength (f _m║2 ) and secondary second flexural strength (f _m┴2 ) according to the EN 310 standard, for a panel on which a fire retardant (300) has been applied, the following is obtained: the ratio of the primary first modulus of elasticity (E _║1 ) to the secondary first modulus of elasticity (E _║2 ) is from 0.75 to 1.25, the ratio of the primary second modulus of elasticity (E _┴1 ) to the secondary second modulus of elasticity (E _┴2 ) is from 0.75 to 1.25, the ratio of the primary first flexural strength (f _m║1 ) to the secondary first flexural strength (f _m║2 ) is from 0.75 to 1.25 and the ratio of primary the second bending strength (f _m┴1 ) to the secondary second bending strength (f _m┴2 ) is from 0.75 to 1.25. 13. A method for improving the fire resistance of ten plywood panels, including processing at least ten similar plywood panels according to the method according to any one of paragraphs. 1-12 so that when ten treated plywood panels and ten unfinished panels similar to treated plywood panels are tested in a test involving cutting ten plywood panels that have not been treated to produce a first group of sixty panel samples suitable for longitudinal testing according to EN 310, and the second group of sixty panel samples suitable for transverse testing according to EN 310, cut ten plywood panels that were processed according to the method according to any one of paragraphs. 1-12 to obtain a third group of sixty panel samples suitable for the longitudinal test according to EN 310 and a fourth group of sixty panel samples suitable for the transverse test according to EN 310, for each panel sample of the first group of sixty panel samples measurement primary first modulus of elasticity (E _║1 ) and primary first flexural strength (f _m║1 ) according to EN 310 to determine the average primary first modulus of elasticity (<E _║1 >) and the average primary flexural strength (<f _{m ║1} >), for each panel sample of the second group of sixty panel samples, the measurement of the primary second modulus of elasticity (E _┴1 ) and the primary second flexural strength (f _m┴1 ) according to EN 310 to determine the average primary second modulus of elasticity (< E _┴1 >) and average primary second bending strength (<f _m┴1 >), for each panel sample of the third group of sixty panel samples, the second measurement of the first modulus of elasticity (E _║2 ) and the secondary first flexural strength (f _m║2 ) according to EN 310 to determine the average secondary first modulus of elasticity (<E _║2 >) and the average secondary first flexural strength (<f _{m ║1} >) and for each panel sample of the fourth group of sixty panel samples, measurement of the secondary second modulus of elasticity (E _┴2 ) and secondary secondary flexural strength (f _m┴2 ) according to EN 310 to determine the average secondary second modulus of elasticity (< E _┴2 >) and the average secondary second bending strength (<f _m┴2 >), the following is obtained: the ratio (<E _║2 >/<E _║1 >) of the average secondary first modulus of elasticity (<E _║2 >) to the average primary first modulus of elasticity (<E _║1 >) is from 0.85 to 1.1, the ratio (<E _┴2 >/<E _┴1 >) of the average secondary second modulus of elasticity (<E _┴2 >) to the average primary second modulus of elasticity (<E _┴1 >) is from 0.85 to 1.1, the ratio (<f _m║2 >/<f _m║1 >) of the average secondary first strength ty in bending (<f _m║2 >) to the average primary first bending strength (<f _m║1 >) is from 0.85 to 1.1 and the ratio (<f _m┴2 >/<f _m┴1 >) average secondary secondary bending strength (<f _m┴2 >) to average primary second bending strength (<f _m┴1 >) is from 0.85 to 1.1. 14. The method according to any one of paragraphs. 1-13, wherein the first veneer layer (110) comprises softwood such as spruce or pine, preferably also the second veneer layer (120) comprises softwood such as spruce or pine, preferably all veneer layers of the plywood panel are made from the same the same type of wood. 15. The method according to any one of paragraphs. 1-14, where the thickness of the plywood panel (100) is from 4 mm to 75 mm, for example from 9 mm to 21 mm, and the plywood panel (100) contains from three to twenty five layers (110, 120, 130) of veneer, each of which is made of soft wood, preferably the thickness of the plywood panel (100) is at least 15 mm, the thickness (tsv1) of the first layer (110) of veneer is from 1.5 mm to 3.5 mm and the thickness (tsv2) of the second layer ( 120) of veneer is between 1.5 mm and 3.5 mm, the plywood panel (100) contains at least five layers (110, 120, 130) of veneer, each of which is made of soft wood, and the panel has a first bending strength (f _m║ ) at least 37 N/mm ² , the second bending strength (f _m┴ ) at least 19 N/mm ² , the first modulus of elasticity (E _║ ) at least 4600 N/mm ² and the second modulus elasticity (E _┴ ) of at least 1800 N/mm ² measured and defined in EN 310. 16. The method according to any one of paragraphs. 1-15, where the adhesive (190) contains a polymerized phenol-formaldehyde resin and/or a polymerized lignin-phenol-formaldehyde resin. 17. Plywood panel (100) containing the first layer (110) of veneer containing the first surface (112) of the plywood panel (100), so that all layers (110, 120, 130) of the veneer of the plywood panel (100) remain on one side of the first surface (112), the second layer (120) of the veneer, so that all layers of the veneer of the plywood panel remain on one side of the second interface (122) between the second veneer (120) and the coating (196) or the second surface (122) of the panel (100) , and the second layer (120) of the veneer contains the second surface (122), the adhesive (190) between the first layer (110) of the veneer and the second layer (120) of the veneer, where the first surface (112) contains traces indicating the grinding of the first surface ( 112) of the plywood panel (100) in the first sanding direction (R1) that forms an angle of at least 30 degrees with the direction (D1) of the wood fibers of the first veneer layer (110), and the first veneer layer (110) contains at least 8 g /m ^{2 of} bound phosphorus, whereby the plywood panel (100) is fire resistant according to EN 13501-1:2007+A1:2009 as indicated by flammability classes B, smoke production s1 and flaming droplets d0 or as indicated by flammability classes B _fl and smoke production s1. 18. Plywood panel according to claim 17, in which the second layer (120) of veneer forms the second surface (122) of the plywood panel, the second surface (122) contains traces indicating grinding of the second surface (122) of the plywood panel (100) in the second direction (R2) sanding, which forms an angle of at least 30 degrees with the direction (D2) of the wood fibers of the second layer (120) veneer, and the second layer (120) veneer contains at least 8 g/m ² bound phosphorus, preferably the direction (D1 ) the wood fibers of the first veneer layer (110) parallel to the direction (D2) of the wood fibers of the second veneer layer (120). 19. Plywood panel (100) according to claim 17 or 18, containing a third veneer layer (130) located between the first veneer layer (110) and the second veneer layer (120), where [A] is the content (c130) of phosphorus in the third layer (130) of the veneer is not more than half or not more than 25% of the content (c110) of phosphorus in the first layer (110) of the veneer, and/or [B] the third layer (130) of the veneer contains not more than 5 g/m ² or not more 2 g/m ² bound or free phosphorus, and/or [C] the panel contains only such wood, on which the flame retardant is applied without impregnation under pressure of at least 200 kPa abs. (2 bar absolute). 20. Plywood panel (100) according to any one of paragraphs. 17-19, in which the first layer (110) of the veneer is treated with a solution containing 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), for example, the first layer (110) of the veneer contains HEDP, preferably the panel has a weight gain of less than 30% in the test, detailed in “Nodrtest Method, build 504, Approved 2003-12”. 21. Plywood panel (100) according to any one of paragraphs. 17-20, where [A] the plywood panel (100) contains five layers of spruce veneer, the thickness (t _p ) of the plywood panel (100) is 15 mm, and the plywood panel (100) has the following mechanical properties: first bending strength ( f _m║ ) from 37 N/mm ² to 52 N/mm ² measured in the longitudinal test of the EN 310 standard, the second bending strength (f _m┴ ) from 19 N/mm ² to 28 N/mm ² measured in the transverse test according to EN 310, the first modulus of elasticity (E _║ ) from 4600 N/mm ² to 6000 N/mm ² measured in the longitudinal test according to EN 310, and the second modulus of elasticity (E _┴ ) from 1800 N/mm ² to 2700 N /mm ² measured in the transverse test of EN 310, or [B] the plywood panel (100) contains seven layers of spruce veneer, the thickness (t _p ) of the plywood panel (100) is 18 mm, and the plywood panel (100) has the following mechanical properties: first bending strength (f _m║ ) from 37 N/mm ² to 53 N/mm ² measured in the longitudinal test of EN 310 standard, second bending strength (f _m┴ ) from 27 N/mm ² up to 44 N/mm ² measured in the transverse test of EN 310, the first modulus of elasticity (E _║ ) from 4800 N/mm ² to 6800 N/mm ² measured in the longitudinal test of EN 310 and the second modulus of elasticity (E _┴ ) from 2800 N/ ^mm2 to 4300 N/ ^mm2 measured in the EN 310 transverse test. 22. Plywood panel according to any one of paragraphs. 17-21, where the first veneer layer (110) contains soft wood such as spruce or pine, preferably also the second veneer layer (120) contains soft wood such as spruce or pine, preferably all veneer layers of the plywood panel are made from the same the same type of wood. 23. Plywood panel according to any one of paragraphs. 17-22, where the thickness of the plywood panel (100) is from 4 mm to 75 mm, for example from 9 mm to 21 mm, and the plywood panel (100) contains from three to twenty five layers (110, 120, 130) of veneer, each of which is made of soft wood, preferably the thickness of the plywood panel (100) is at least 15 mm, the thickness (tsv1) of the first layer (110) of veneer is from 1.5 mm to 3.5 mm and the thickness (tsv2) of the second layer ( 120) of veneer is between 1.5 mm and 3.5 mm, the plywood panel (100) contains at least five layers (110, 120, 130) of veneer, each of which is made of soft wood, and the panel has a first bending strength (f _m║ ) at least 37 N/mm ² , the second bending strength (f _m┴ ) at least 19 N/mm ² , the first modulus of elasticity (E _║ ) at least 4600 N/mm ² and the second modulus elasticity (E _┴ ) of at least 1800 N/mm ² measured and defined in EN 310. 24. Plywood panel according to any one of paragraphs. 17-23, where the adhesive (190) contains a polymerized phenol-formaldehyde resin and/or a polymerized lignin-phenol-formaldehyde resin.

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