WO2025056365A1

WO2025056365A1 - Method for increasing fire-retardation of wood

Info

Publication number: WO2025056365A1
Application number: PCT/EP2024/074559
Authority: WO
Inventors: Hans Bay Thomsen
Original assignee: Ecokemi Aps
Current assignee: Ecokemi Aps
Priority date: 2023-09-11
Filing date: 2024-09-03
Publication date: 2025-03-20
Anticipated expiration: 2026-03-11

Abstract

The present invention relates to a method of increasing fire-retardation of wood, use of a fire-retardant having a boiling point above the boiling point of water in a wood impregnation method and the wood products, such as a veneer layer or a strand of wood, obtainable from the method or wood products comprising the veneer layers or strands of wood.

Description

Method for increasing fire-retardation of wood

Technical field of the invention

The present invention relates to a method for introducing fire-retardants into wood products. In particular, the present invention relates to wood products produced from such methods.

Background of the invention

The demand for enhancing the fire-retardant properties of wood is continually growing, driven by the increasing utilization of wood in indoor settings, including its use in furniture, interior decoration, and as a fundamental building material. Commonly used wood products are for instance plywood, laminated veneer lumber (LVL) and Oriented Strand Boards (OSB).

Plywood is a composite wood product composed of multiple thin layers of wood veneers bonded together with adhesive. Each veneer layer is oriented perpendicular to adjacent layers, enhancing the material's strength and stability. Another type of wood product consisting of veneer layers are LVL, wherein the veneer layers are oriented in the same direction.

OSB similar to plywood, is an engineered wood product. OSB panels are manufactured by arranging and compressing strands of wood with adhesive, creating a layered structure. OSB's distinctive composition and manufacturing process give it mechanical properties that make it a suitable alternative to plywood for various construction and industrial applications. The strands are typically prepared from green wood, and directly cut into strands, where after they may be dried. The strands are then typically laid out with a suitable resin and pressed into OSB.

The veneer layers for the production of plywood and LVL is commonly produced by softening logs of wood, for instance in large water baths to render the logs suitable for further processing, this is for instance done for 12 hours at around 80 degrees Celsius, however many softening methods exist with varying temperatures and durations. The softening step is followed by feeding the logs through a peeling machine, which provides wet veneer layers or strands of wood with a significant amount of water content. The wet veneer layers or strands of wood, i.e. the wood product or intermediate wood product, is then further processed.

A common subsequent step after introducing fire-retardants into the wood involves drying the intermediate wood products. The dried items are then resoaked in aqueous fire-retardant compositions and dried once more, resulting in a product displaying a sub-optimal integration of fire-retardant and yielding a brittle wood product. This process often leads to the noticeable presence of residual fire- retardant on the veneer's surfaces and a significant proportion of the brittle product needing to be discarded. Consequently, when assembling such a veneer layer alongside additional veneer layers to create plywood, the remaining fire- retardant contributes to a heightened risk of disintegration due to an inadequate binding strength, compounded by the increased brittleness resulting from the multi-step drying procedure.

In such facilities, multiple ovens are occupied by drying wood products, resulting in a bottleneck and a high energy usage.

Since the soaking method often results in a large amount of the fire-retardant being at the surface or close to the surface, when a plywood is sanded down and finished, a part of this fire-retardant is removed, resulting in an outer layer of the plywood with a lower fire-retardancy, than the inner layers.

In some products, this reduced fire-retardancy is increased by applying a surface treatment capable of foaming, however this cannot be done when the surface of the plywood is to be finished with a coating or a decorative panel.

Hence, an improved method of introducing fire-retardation into wood would be advantageous, and in particular a more efficient and/or reliable product for use in downstream processes would be advantageous.

Summary of the invention

Thus, an object of the present invention relates to the provision of improved methods for increasing fire-retardation of wood products. In particular, it is an object of the present invention to provide wood products that solve the above-mentioned problems of the prior art with waste products, sub- optimal fire-retardation and high energy usage.

The present invention relates to a method of increasing fire-retardation of wood, use of a fire-retardant having a boiling point above the boiling point of water in a wood impregnation method and the wood products, such as a veneer layer or a strand of wood, obtainable from the method or wood products comprising the veneer layers.

We have successfully generated an improved method of incorporating fire- retardant wood products, such as plywood. The method can be used on different amounts of fire-retardant composition, producing products with amazing results in fire-retardation tests. The veneer layers furthermore show homogenous coloration, improved brittleness and little to non residual fire-retardant composition on the surface, overall resulting in a product, such as plywood, with improved stability, strength and performance.

Example 1 shows a measurement of the boiling point of various fire-retardant compositions. Interestingly, it was found that fire-retardant compositions had a higher boiling point than water and increases in the percentage of fire-retardant in the compositions resulted in increases in boiling points.

Example 2 shows, in a test setup, an improved method for introducing fireretardation. The test showed surprisingly, by cooking the wet veneer layers, we were able to rapidly remove water from inside the wood and exchange the water with the fire-retardant compositions.

By visually inspecting the veneer layers in aerated conditions, we were able to see how the fire-retardant composition was sucked up by the veneer layers. The resulting veneer layers were found to be far less brittle, and with little to no residual fire-retardant on the surfaces of the veneer layers.

Examples 3 and 5 shows the new method in a larger setup, with different concentrations of fire-retardant. The new method is capable of introducing a large amount of fire-retardant. The dry veneer layers show a homogenous coloration, a sign of a surprisingly even introduction of fire-retardant into the veneer. The resulting veneer layers are less brittle than veneer layers prepared by previous techniques, and with little to no residual fire-retardant on the surfaces of the veneer layers.

Example 4 and 6 shows measurements of the fire-retardation in plywood produced by the veneer layers of example 3 and 5. The plywood was found to have a very good performance in the EN 13823 test.

Examples 7 and 8 shows comparisons between products obtained from the method of the present disclosure and soaking method of the prior art. From the comparisons it is clear that plywood obtained from veneers produced from the method as disclosed herein has an improved performance.

Thus, one aspect of the invention relates to a method of increasing fireretardation of wood, the method comprising the steps: a) Providing at least one wood product intended for increasing the fire- retardancy, wherein the wood product is produced from either green wood or re-wetted wood; b) Cooking the at least one wood product in a fire-retardant composition at atmospheric pressure at a temperature above the boiling point of water and at or below the boiling point of the fire-retardant composition; c) Optionally, allowing the at least one wood product to cool in the fire- retardant composition, thereby providing an wood product with increased fire-retardation; and d) Drying the at least one wood product with increased fire-retardation to a moisture content of between 2% and 15%.

Another aspect of the present invention relates to the use of a fire-retardant having a boiling point above the boiling point of water in a wood impregnation method.

Yet another aspect of the present invention is to provide an intermediate wood product, such as a veneer layer or a strand of wood, obtainable from the method as described herein. Still another aspect of the present invention is to provide wood product comprising the intermediate wood product, such as plywood or oriented strand board.

Brief description of the figures

Figure 1 shows two pieces from a plywood with fire-retardant properties, produced from a soaking method. The plywood has disintegrated due to a bad gluing of veneer layers, due to residual fire-retardant.

Figure 2 shows residual fire-retardant on the surface of a veneer layer produced from a soaking method.

The present invention will now be described in more detail in the following.

Definitions

Prior to discussing the present invention in further details, the following terms and conventions will first be defined:

"Green wood" is understood as wood that has recently been cut from a living tree and still contains a significant amount of moisture or sap. It has not undergone the drying process to reduce its moisture content.

"Re-wetted wood" refers to wood that has already been dried or seasoned but is then exposed to moisture again, causing it to absorb water and regain some of its original moisture content.

If not indicated otherwise, percentages are to be understood as "by weight" or "(w/w)" or "(weight/weight)".

The logs or wood products described herein may be wood or wood-like materials derived from any relevant kind of tree or plant, such as birch, fir, spruce, beech tree, bamboo, hemp, hemphay, seaweed, cotton fibre or even common types of hay as a leftover after threshing the grain. Preferably the wood or wood-like materials comprises fibres. In the present context, the terms "flame-retardant", "fire retardant" and "fire- retardant" are used interchangeably. In the present context, the terms "flame retardant" and "fire retardant" relate to compounds or compositions, which are added to manufactured materials, such as wood or wood-based materials to prevent ignition of the materials. Flame/fire retardants may be activated by the presence of an ignition source and are intended to prevent or slow the further development of ignition by a variety of different physical and chemical methods.

Detailed description of the invention

A first aspect of the invention relates to a method of increasing fire-retardation of wood, the method comprising the steps: a) Providing at least one wood product intended for increasing the fire- retardancy, wherein the wood product is produced from either green wood or re-wetted wood; b) Cooking the at least one wood product in a fire-retardant composition at a temperature above the boiling point of water and at or below the boiling point of the fire-retardant composition; c) Optionally, allowing the at least one wood product to cool in the fire- retardant composition, thereby providing a wood product with increased fire-retardation; and d) Drying the at least one wood product with increased fire-retardation to a moisture content of between 2% and 15%.

Without being bound by theory, the water inside the wood product will evaporate and be replaced by fire-retardant. The method can be used on different amounts of fire-retardant composition, producing products with amazing results in fireretardation tests. A veneer layer produced from the method show homogenous coloration after drying, improved brittleness and little to non residual fire- retardant composition on the surface, overall resulting in a product with improved stability, strength and performance. A fire-retardant composition will typically comprise at least a solvent such as water, a buffer for regulating the pH and the compound responsible for providing the fire-retardation capability.

The skilled person will know that the specific type of fire-retardant will importanjt for the working of the present invention. The invention is thus not limited to the specific type of fire-retardant. Many different types of f ire -retard ants exist, for instance.

Phosphorous based, such as phosphoric acid H3PO⁴ and phosphonates OP(OR)2R). Boron based, such as boric acid H3BO3, metaboric acid HBO3, boric oxide B2O3, borax Na20-2B203- 10H20, anhydrous borax Na2B4O?, sodium tetraborate X- hydrate Na2 B4O?-XH2O, borax + boric acid (BBA) Na2O- 2B2O3- 10H2O + H3BO3 and sodium borate + boric acid Na2B4O?+ H3BO3.

Nitrogen based, such as melamine CsHeNe, urea CH4N2O, dicyandiamide C2H4N4 and guanidine CH5N3.

Combined nitrogen-phosphorous systems, such as monoammonium phosphate (MAP) NH4 H2PO4, diammoniumphosphate (DAP) (NH4)2HPO4, ammonium polyphosphates (APP) [NH4POs]n, poly ammoniumpolyphosphate (PAP) NH4O(NH4PO3)n NH4, melamine phosphate CsHeNe (HsO4P)n, guanylurea phosphate (GUP) C2H6N4O H3PO4 and guanidine phosphate CH5N3 H3PO4. In preferred embodiments of the disclosure, the fire-retardant composition is a Combined nitrogen-phosphorous system. In an even preferred embodiment, the fire-retardant is DAP.

Combined sulphur-nitrogen phosphate systems, such as ammonium sulphate (NH4)2SC>4 and ammonium sulphamate NH4SONH2, Silica based, such as sodium silicates Na2SiOs and potassium silicates SiCH, and Metal hydrates, such as aluminium hydroxide AI(OH)s and magnesium hydroxide Mg(OH)2. In addition to these there are other inorganic and organic salts that can be used as a fire retardant, such as aluminium sulphate AI2 (SO4)3, formates like potassium formate KCOOH and citrates [CeHsO?]^3-.

The method as described herein provides a finished wood product, or an intermediate product that can be used in a variety of downstream products, such as plywood, LVL, or OSB. The finished product is thus dried to a final moisture content. Typically, this may be done in a drying oven at temperatures above 130 degrees Celcius, such as above 140 degrees Celcius such as even up to 270 degrees Celcius. The moisture content may be between 2% and 15%, such as 2%-8%, such as 3%-6%, preferably a moisture content between 3%-4%. In other embodiments, the moisture content is 7-12%.

The moisture content may depend on the type of wood used, or when it is measured. For instance, the moisture content may increase after leaving the mill.

In one embodiment of the present disclosure, the wood product is an intermediate wood product. In one embodiment of the present disclosure, the wood product is an intermediate wood product in the process for developing plywood, laminated veneer lumber (LVL) or OSB plates, such as a veneer layer or a strand of wood. In one embodiment of the present disclosure, the wood product is an intermediate wood product in the process for developing plywood, such as a veneer layer. In one embodiment of the present disclosure, the wood product is an intermediate wood product in the process for developing plywood, laminated veneer lumber (LVL), OSB, Particleboard or Chipboard, such as standard Particleboard, Medium- Density Fiberboard (MDF), High-Density Fiberboard (HDF, Low-Density Fiberboards (LDF), Glued Laminated Timber (Glulam), Engineered Wood Products (EWP), Fiberboards such as Waferboard, Strandboard, Blockboard Honeycomb Core Panels, or Bamboo Plywood.

Depending on the type of final product, the wood products may be wood or woodlike materials derived from any relevant kind of tree or plant, such as birch, fir, spruce, beech tree, bamboo, hemp, seaweed or cotton fibre. Further types of wood can for instance also be teak, poplar, redwood, mahogany, walnut, maple, cherry, ash, oak, or cedar. In a preferred embodiment, the logs or wood products are wood derived from any kind of tree. In a preferred embodiment, the wood product is made from birch. In another embodiment, the wood product is made from fir.

Even further uses may for instance be to produce various insulation materials. Fibre comprising products, such as hemp, hemphay, seaweed, cotton fibre or even common types of hay as a leftover after threshing the grain, can for instance be impregnated, before being processed into insulation.

Without being bound by theory, wood of varying sizes may be impregnated by the method as described herein. The skilled person will be able to select different thicknesses of the wood product, such as veneer, depending on the final product. Typically, a plywood will use individual veneer layers of 1.5 -3.5 mm, preferably 1.5 mm. On the contrary, roof veneer may comprise 3.5 mm layers and LVL 4 mm layers. Thus, in one embodiment, the wood product has a thickness of up to 4 mm, such as 3.5 mm, such as 3 mm, such as 2 mm, such as 1.5 mm. In a preferred embodiment, the thickness of the wood product is up to about 2 mm, such as 1.5 mm.

When the skilled person is, for instance, making wood products for use in OSB, or chipboards, such as MDF, even smaller wood products are used. The strands of wood, chips and flakes employed in such embodiments may thus be of varying sizes and can be less than 1 mm in thickness, depending on the type of manufacture.

Insulation produced from fibrous products may for instance be produced from even smaller fragments, such as for instance when a refurbished clothing material is impreganted with fire-retardant as described herein.

In one embodiment of the present disclosure, the at least one wood product is cooked at a temperature between 101 degrees Celcius to 105 degrees Celcius. In another embodiment, the at least one wood product is cooked at a temperature between 100 degrees Celcius to 105 degrees Celcius. In another embodiment, the at least one wood product is cooked at a temperature between 100.5 degrees Celcius to 103 degrees Celcius. In another embodiment, the at least one wood product is cooked at a temperature between 100.8 degrees Celcius to 103 degrees Celcius.

In one embodiment of the present disclosure, the boiling temperature of tap water is measured and set to 100 degrees Celcius, and the at least one wood product is cooked at a temperature of at least 100.5 degrees Celcius, such as 100.6 degrees Celcius, such as 100.7 degrees Celcius, such as even 100.8 degrees Celcius. In another embodiment of the present disclosure, the boiling temperature of tap water is measured at atmospheric pressure (1 atm = 101.325 pascals), and set to 100 degrees Celcius and the at least one wood product is cooked at a temperature of at least 100.5 degrees Celcius, such as 100.6 degrees Celcius, such as 100.7 degrees Celcius, such as even 100.8 degrees Celcius.

In one embodiment of the present disclosure, the cooking temperature of the fire- retardant composition is measured at the surface of the fire-retardant composition.

In one embodiment of the present disclosure, the wood product is only allowed to cool in the fire-retardant until the temperature is below the boiling point of water, such as 1-3 degrees Celsius below the boiling point of water, such as even 0.5 degrees Celcius below the boiling point of water.

In one embodiment of the present disclosure, the wood product is allowed to partially dry in aerated conditions, such as at least until fire-retardant composition residue on the surface of the wood product is allowed to be absorbed into the wood.

Some prior art methods are conceived by soaking multiple times in the same of different solution, at different temperatures. An advantage of the present invention is that additional soaking is not necessary, and thus additional soaking steps may be avoided. In one embodiment of the present disclosure, the method does not include a step of transferring the at least one wood product into a second fire-retardant composition and cooling the at least one wood product in said second fire-retardant composition. In one embodiment of the present disclosure, the cooking step and the optional cooling step are the only steps done to provide sufficient introduction of fire-retardant into the wood product.

As introduced in the background, the wood product may be prepared by different methods. In a particular embodiment, the method further comprises one or more of the following steps prior to cooking the wood product: i. Providing logs and a fire-retardant composition having a boiling point above the boiling point of water; ii. Softening the logs in water for a time sufficient and at a temperature sufficient to render the logs suitable for further processing, such as for 12 hours at around 80 degrees Celsius; and/or iii. Feeding the logs through a peeling machine, thereby providing at least one wood product, such as a wet veneer layer or a strand of wood.

Thus, in one embodiment of the present disclosure the method includes providing logs and a fire-retardant composition having a boiling point above the boiling point of water;

In one embodiment of the present disclosure the method includes softening the logs in water for a time sufficient and at a temperature sufficient to render the logs suitable for further processing, such as for 12 hours at around 80 degrees Celsius. The logs may also be softened in an autoclave, or at different temperatures, such as for instance 60 degrees Celcius.

In one embodiment of the present disclosure the method includes feeding the logs through a peeling machine, thereby providing at least one wood product, such as a wet veneer layer or a strand of wood.

In a particular embodiment, the method further comprises all the steps prior to cooking the wood product: i. Providing logs and a fire-retardant composition having a boiling point above the boiling point of water; ii. Softening the logs in water for a time sufficient and at a temperature sufficient to render the logs suitable for further processing, such as for 12 hours at around 80 degrees Celsius; and/or iii. Feeding the logs through a peeling machine, thereby providing at least one wood product, such as a wet veneer layer or a strand of wood.

An advantage of the present invention, is the use of a wood product with a high water content, since multiple drying steps often produces brittle wood products, and results in a high energy usage. In one embodiment of the present disclosure, there is no drying step between the peeling step and the cooking step.

In one embodiment of the present disclosure, the fire-retardant composition has a boiling point above the boiling point of water.

In one embodiment of the present disclosure, the wood product is cooked for a time sufficient to allow water inside the wood product to evaporate. In one embodiment of the present disclosure, the wood product is cooled for a time sufficient to allow fire retardant to penetrate into the wood product.

As shown in the examples, a batch of venner layers may be prepared by the method as described herein. When multiple layers are stacked, it may be important to cook for at sustained period of time, until enough water has evaporated from the veneer layers. In one embodiment of the present disclosure, the wood product is cooked for at least 10 minutes, such as for at least 20 minutes, such as at least 30 minutes, such as at least 40 minutes, such at least 50 minutes, preferably the wood product is cooked for 20-30 minutes.

The methods as shown in the examples are performed on batch preparations of veneer layers, however in some setups, a single veneer may be rolled into the cooking fire-retardant composition allowing for a much shorter time needed for the veneer to be inside the boiling fire-retardant compositions. In one embodiment of the present disclosure, the wood product is cooked for at least 30 second, such as for at least 1 minutes such as for at least 2 minutes.

In one embodiment of the present disclosure, the at least one wood product is cooled in the same composition as they are cooked.

As seen from examples 3 and 5, different concentrations of fire-retardant can be used.

In one embodiment of the present disclosure, the fire-retardant composition is an aqueous composition comprising between 20-45 % fire-retardant, such as 20- 30% fire-retardant. In a specific embodiment of the present disclosure, the fire- retardant composition is an aqueous composition comprising 23 % fire-retardant. In another specific embodiment of the present disclosure, the fire-retardant composition is an aqueous composition comprising 20 % fire-retardant. In another specific embodiment of the present disclosure, the fire-retardant composition is an aqueous composition comprising 30 % fire-retardant.

The skilled person is able to select the needed concentration, depending on the wood product to be produced. In some instances, it may for instance be preferred to use a fire-retardant composition with a high percentage, such as 42%, to prepare a veneer layer with increased fire-retardant properties. As shown in the examples, a veneer layer prepared as such may produce a higher amount of smoke, however such veneer layers can preferably be combined with non-treated veneer layers in a final plywood product.

The wood product used in the method is produced from either green wood or rewetted wood, which ensures a sufficient amount of water will be able to evaporate and create a vacuum when the wood product is cooled. In one embodiment of the present disclosure, the wood product is a wet product, such as having a wood moisture content of at least 50 %, such as at least 60 %, such as at least 70 %.

In one embodiment of the present disclosure, the at least one wood product is a wet veneer layer.

To allow for a better dispersion of fire-retardant composition in the cooking step, a stack of veneer layers may be tilted. Thus, in one embodiment of the present disclosure, the wood product, such as wet veneer layers, are positioned in the cooking step so that the at least one veneer layer is positioned perpendicular to the ground.

In one embodiment of the present disclosure, the at least one wood product is a strand of wood. Such strands of wood may be used to produce OSB plates, where the OSB plates will have an increased fire-retardancy.

Use of fire-retardant, and product by process

Another aspect of the present disclosure relates to a use of a fire-retardant composition having a boiling point above the boiling point of water in a wood impregnation method. The method may be a method as described herein.

Another aspect of the present disclosure relates an intermediate wood product, such as a veneer layer or a strand of wood, obtainable from the method as described herein.

An intermediate wood product produced according to the method as disclosed herein may typically show one or more of the following sign, since it has little to non residual fire-retardant composition on the surface, a uniform coloration and a low tendency to break due to a low brittleness.

Another aspect of the present disclosure relates a wood product comprising the intermediate wood product, such as plywood, oriented strand board or LVL. Any type of wood product can be made comprising such an intermediate product, for instance, plywood, laminated veneer lumber (LVL), OSB, Particleboard or Chipboard, such as standard Particleboard, Medium-Density Fiberboard (MDF), High-Density Fiberboard (HDF, Low-Density Fiberboards (LDF), Glued Laminated Timber (Glulam), Engineered Wood Products (EWP), Fiberboards such as Waferboard, Strandboard, Blockboard Honeycomb Core Panels, or Bamboo Plywood.

Even further uses may for instance be to produce various insulation materials. Fibre comprising products, such as hemp, hemphay, seaweed, cotton fibre or even common types of hay as a leftover after threshing the grain, can for instance be impregnated, before being process into insulation.

Such a wood product may comprise a combination of intermediate wood products as prepared by the method as described herein and intermediate wood product which are not impregnated as such.

Furthermore, such wood products may for instance comprise layers of veneer layers, prepared as described herein, and a cover for instance made of a finer appearing wood to provide a decorative plywood. Several other coatings may be applied to the surface.

Plywood may for instance also be prepared by combining layers with fireretardation and layers without fire-retardation.

It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention.

All patent and non-patent references cited in the present application, are hereby incorporated by reference in their entirety. The invention will now be described in further details in the following non-limiting examples.

Examples

Example 1 - boiling point of various fire-retardant compositions

Materials and methods

A test setup was constructed to measure the boiling points of different fire- retardant compositions. The test setup included a pot including the fire-retardant compositions, a lid mounted with a thermometer and a heating device. The boiling point of water was set to 100 degrees Celcius, and the measurements correlated accordingly thereto.

Results

Table 1 - boiling point of different fire-retardant compositions

SDA 440-11 being a diamr noniumphosphate (DAP) based fire-retardant, pH adjusted a suitable acid. FIRESTOP 22-75 and PW640 being fire-retardants from other providers.

Interestingly, it was found that fire-retardant compositions had a higher boiling point than water, and increases in the percentage of fire-retardant in the compositions resulted in increases in boiling points. Example 2 - improved method for introducing fire-retardation

In common dipping techniques where dry veneer layers are dipped into compositions for instance comprising 23% fire-retardant, it is normally expected that an uptake of fire-retardant is about 60 kg/M³ at 20 degrees Celcius or up to 80-85 kg/M³ at 50 degrees Celcius after 5 hours in the dipping solution. Where the uptake is measured after the second drying step. We thus set out to develop a technique capable of increasing the uptake of fire-retardant composition, or providing a dry wood product with an increased performance.

Materials and methods

A test setup was constructed to measure the introduction of fire-retardants by methods as disclosed herein.

Veneer layers were prepared by softening, and peeling to obtain wet birch veneer layers.

Wet birch veneer layers (1.5 mm) were cooked in fire-retardant compositions in temperatures above the boiling point of water, and either cooled in the same compositions or immediately removed and cooled in aerated conditions, i.e. not in a liquid solution.

Cooked veneer layers were then dried at 35 degrees Celcius for 24 hours followed by 1 hour at 120 degrees Celcius.

The dry veneer layers where weighed and the uptake of fire-retardant calculated, assuming a dry weight of 540 kg/M³ of untreated wood.

Results

Table 2 - uptake of fire-retardant into wood Surprisingly, by cooking the wet veneer layers, we were able to rapidly remove water from inside the wood, and exchange the water with the fire-retardant compositions. By visually inspecting the veneer layers in aerated conditions, we were able to see how the fire-retardant composition was sucked up by the veneer layers. The resulting veneer layers were found to be far less brittle, and with little to no residual fire-retardant on the surfaces of the veneer layers.

A quick test with a gasburner indicated an improved fire-retardancy.

Example 3 - scaling up the improved method

Having seen the effect on from the test setup as described in example 2, the introduction of fire-retardant was tested in a larger setup, and plywood was generated for testing.

Materials and methods

A large steelvessel measuring 60X60X25 was mounted with 2X9 kW gasburners. A 42 % fire retardant composition, as described in example 1 was loaded into the vessel.

Results

30 wet birch veneer layers (1.5 mm) were cooked in a 42 % fire-retardant composition for 30 minutes. The temperature was measured at the middle of the boiling fire-retardant composition, the temperature reached 102 degrees Celcius. After 30 minutes, the energysupply was turned off and the composition allowed slightly to settle before removing the veneer layers from the fire-retardant composition. By visual inspection, it was then possible to see how the warm veneer layers absorbed the composition on the surface of the veneer layers.

The mass weight of the outer ten veneer layers were compared against the middle ten veneer layers and showed an equal amount of composition introduced into the veneer layers.

When the veneer layers were dried, they showed a homogenous coloration across the surface of individual veneer layers as well as when comparing the coloration between different veneer layers. The homogenous coloration is a sign of a surprisingly even introduction of fire-retardant into the veneer.

The resulting veneer layers were found to be far less brittle than veneer layers prepared by previous techniques, and with little to no residual fire-retardant on the surfaces of the veneer layers. The dry veneer layers were glued and pressed into plywood and sent off to test the fire-retardation properties.

Example 4 - measurement of the fire-retardation in plywood

Having produced a plywood product product where the veneer show homogenous coloration, improved brittleness and little to non residual fire-retardant composition on the surface, we wanted to the fire-retardation properties.

Materials and methods

The plywood obtained from example 3 was evaluated according to standard test methods (PN/EN 13823:2020-11) by Sychta Laboratorium on 12mm plywood.

Results

The test showed that the coated plywood received the results B-s2,d0.

It is noted that plywood and wood without any flame retardants would normally be classified as D or E.

The classification parameter FIGRA, Fire Growth RAte, is as a way to classify the fire properties of building products and is part if the test. It predicts burning behaviour of a large variety of building products in reference scenarios. These reference scenarios are in their turn related to real life fire scenarios. It is a part of the European harmonized directions under the construction products directive, CPD. Therefore, FIGRA is relevant for products in a very large market.

FIGRA is defined as the growth rate of the burning intensity, HRR, during a test (e.g. SBI). FIGRA is calculated as the maximum value of the function (heat release rate)/ (elapsed test time) and the unit is W/s.

Interestingly, the plywood received a score in the FIGRA test (0.2) of 69 W/s. The threshold in the FIGRA test for being classified as A2 or B is below 120 W/s.

However, a significant amount of smoke was generated due to the high amount of fire-retardant introduced into the wood. In sum, the coated wood material passes the fire test.

Table 3 - PN-EN 13823 test results

Example 5 - further tests on improved method

Now knowing that the method could work in larger scale, we set out to test whether lower concentrations could also facilitate a high introduction of fire- retardant.

Materials and methods

A large steelvessel measuring 80X60X25 was mounted with 2X9 kW gasburners. A 24% fire retardant composition, as described in example 1 was loaded into the vessel.

Results

90 wet birch veneer layers (1.5 mm) were cooked in a 24 % fire-retardant composition for 20 minutes. The stack of veneer layers were tilted 90 degrees, so that their longitudinal orientation was perpendicular to the ground, ensuring an improved dispersement of solution between individual layers. When the temperature was measured at the surface of the boiling fire-retardant composition, the temperature reached 100.8 degrees Celcius.

After 20 minutes, the energysupply was turned off. After 6 minutes, the temperature was below the boiling point of water (measured at 99.6 degrees Celcius), and the stack of veneer layers was lifted from the fire-retardant composition. By visual inspection, it was then possible to see how the warm veneer layers absorbed the composition on the surface of the veneer layers. The veneer layers were then dried at 145 degrees Celcius, the uptake was calculated to an average of 88 kg/M³.

As for example 3, the veneer layers showed an even coloration, less brittleness, and with little to no residual fire-retardant on the surfaces of the veneer layers.

The dry veneer layers were glued and pressed into plywood and sent off to test the fire-retardation properties.

Example 6 - measurement of the fire-retardation in plywood

Having produced another plywood product where the veneer shows homogenous coloration, improved brittleness and little to non residual fire-retardant composition on the surface, we wanted to the fire-retardation properties.

Materials and methods

The plywood obtained from example 5 was evaluated according to standard test methods (PN/EN 13823:2020-11) by Sychta Laboratorium on 12mm plywood.

Results

The test showed that the coated plywood received the results B-sl,d0.

Interestingly, the plywood received a score in the FIGRA test (0.2) of 68 W/s. The threshold in the FIGRA test for being classified as A2 or B is below 120 W/s.

Of further importance, the lower amount of fire-retardant introduced into the wood resulted in less production of smoke.

In sum, the coated wood material passes the fire test.

Table 4 - PN-EN 13823 test results

Example 7 - comparison between method of the present disclosure with the prior art

As previously described, a common technique of increasing fire-retardancy into plywood includes soaking of dry veneer layers, followed by a second drying step before pressing into plywood. We wanted to compare plywood obtained from the soaking/dipping technique, with our improved method.

Materials and methods

Veneer layers were prepared by softening birch, peeling and drying.

Veneer with fire-retardancy was prepared by soaking at 50 degrees Celcius, in 23 % SDA 440-11 in 5 hours. Uptake of fire-retardant was app. 80-85 kg/M³.

Plywood was prepared by either pressing 9 layers of fire-retardant veneer and sanding down (sample A), or by pressing 7 layers of fire-retardant plywood between 2 layers of non-fire-retardant veneer and sanding down (sample B). After sanding down, sample B was coated with SF Ultra 50.

Samples A and B was evaluated according to standard test methods (PN/EN 13823:2020-11) by Sychta Laboratorium.

Results

Comparing the results it is clear that the plywood produced from the improved method as disclosed herein has a much better performance in the test, outperforming the soaking technique in the important parameters. The FIGRA0.2 was above the threshold for sample A, probably due to the outer surface being sanded down, and resulting in a loss of fire-retardant by the soaking technique. This may be salvaged by the application of the foam SF Ultra 50 on the surface, as shown by sample B. Unfortunately, sample B cannot be covered further, since SF Ultra 50 needs to be able to produce a foam in order to have an effect.

Table 5 - PN-EN 13823 test results

Example 8 - comparison of commercially available products and products produced from the method as disclosed herein

To further evaluate our improved products, we compared the performance with products produced by a competitor.

Materials and methods

The competitor's product was produced by soaking birch/alder plywood into a 60 % composition comprising the fire retardant FOSAN, with a density of 1.34 g/cm³, for 5 hours.

Results

Comparing the results it is clear that the plywood produced from the improved method as disclosed herein has a much better performance in the test, outperforming the competitor product in several parameters, most importantly, the FIGRA0.2.

Table 6 - PN-EN 13823 test results

Example 9 - conclusion We have succesfully generated an improved method of incorporating fire- retardant wood prodcuts, such as plywood. The method can be used on different amounts of fire-retardant composition, producing products with amazing results in fire-retardation tests. The veneer layers furthermore show homogenous coloration, improved brittleness and little to non residual fire-retardant composition on the surface, overall resulting in a product, such as plywood, with improved stability, strength and performance.

Items

1. A method of increasing fire-retardation of wood, the method comprising the steps: a) Providing at least one wood product intended for increasing the fire- retardancy, wherein the wood product is produced from either green wood or re-wetted wood; b) Cooking the at least one wood product in a fire-retardant composition at a temperature above the boiling point of water and at or below the boiling point of the fire-retardant composition; c) Optionally, allowing the at least one wood product to cool in the fire- retardant composition, thereby providing an wood product with increased fire-retardation; and d) Drying the at least one wood product with increased fire-retardation to a moisture content of between 2% and 15%.

2. The method according to item 1, wherein the at least one wood product is cooked at a temperature between 101 degrees Celcius to 105 degrees Celcius.

3. The method according to any of the proceeding items, wherein the wood product is an intermediate wood product in the process for developing plywood, laminated veneer lumber (LVL) or OSB plates, such as a veneer layer or a strand of wood.

4. The method according to any of the proceeding items, wherein the wood product is only allowed to cool in the fire-retardant until the temperature is below the boiling point of water, such as 1-3 degrees Celsius below the boiling point of water.

5. The method according to any of the proceeding items, wherein the wood product is allowed to partially dry in aerated conditions, such as at least until fire- retardant composition residue on the surface of the wood product is allowed to be absorbed into the wood.

6. The method according to any of the proceeding items, wherein the method does not include a step of transferring the at least one wood product into a second fire-retardant composition, and cooling the at least one wood product in said second fire-retardant composition.

7. The method according to any of the proceeding items, wherein the cooking step and the optional cooling step are the only steps done to provide sufficient introduction of fire-retardant into the wood product. 8. The method according to item 1, wherein the method further comprises one or more of the following steps prior to cooking the wood product: i. Providing logs and a fire-retardant composition having a boiling point above the boiling point of water; ii. Softening the logs in water for a time sufficient and at a temperature sufficient to render the logs suitable for further processing, such as for 12 hours at around 80 degrees Celsius; and/or iii. Feeding the logs through a peeling machine, thereby providing at least one wood product, such as a wet veneer layer or a strand of wood.

9. The method according to item 8, wherein there is no drying step between the peeling step and the cooking step.

10. The method according to any of the proceeding items, wherein the cooking temperature of the fire-retardant composition is measured at the surface of the fire-retardant composition.

11. The method according to any of the proceeding items, wherein the fire- retardant composition has a boiling point above the boiling point of water.

12. The method according to any of the proceeding items, wherein the wood product is cooked for a time sufficient to allow water inside the wood product to evaporate.

13. The method according to any of the proceeding items, wherein the wood product is cooled for a time sufficient to allow fire retardant to penetrate into the wood product.

14. The method according to any of the proceeding items, wherein the wood product is cooked for at least 10 minutes, such as for at least 20 minutes, such as at least 30 minutes, such as at least 40 minutes, such at least 50 minutes, preferably 20-30 minutes.

15. The method according to any of the proceeding items, wherein the at least one wood product is cooled in the same composition as they are cooked.

16. The method according to any of the proceeding items, wherein the fire- retardant composition is an aqueous composition comprising between 20-45 % fire-retardant, such as 20-30% fire-retardant.

17. The method according to any of the proceeding items, wherein the fire- retardant is a phosphorous fire-retardant, such as DAP. 18. The method according to any of the proceeding items, wherein the cooking temperature is between 100-105 degrees Celsius, such as between 101-105 degrees Celcius.

19. The method according to any of the proceeding items, wherein the wood product is a wet product, such as having a wood moisture content of at least 50 %, such as at least 60 %, such as at least 70 %.

20. The method according to any of the proceeding items, wherein the at least one wood product is a wet veneer layer.

21. The method according to item 20, wherein the wet veneer layers are positioned in the cooking step so that the at least one veneer layer is positioned perpendicular to the ground.

22. The method according to any of items 1-19, wherein the at least one wood product is a strand of wood.

23. Use of a fire-retardant having a boiling point above the boiling point of water in a wood impregnation method.

24. An intermediate wood product, such as a veneer layer or a strand of wood, obtainable from the method according to any of the preceding items.

25. A wood product comprising the intermediate wood product according to item 24, such as plywood or oriented strand board.

Claims

1. A method of increasing fire-retardation of wood, the method comprising the steps: a) Providing at least one wood product intended for increasing the fire- retardancy, wherein the wood product is produced from either green wood or re-wetted wood; b) Cooking the at least one wood product in a fire-retardant composition at atmospheric pressure at a temperature above the boiling point of water and at or below the boiling point of the fire-retardant composition; c) Optionally, allowing the at least one wood product to cool in the fire- retardant composition, thereby providing a wood product with increased fire-retardation; and d) Drying the at least one wood product with increased fire-retardation to a moisture content of between 2% and 15%.

2. The method according to claim 1, wherein the boiling point of the fire-retardant composition is such that the at least one wood product is cooked at a temperature between 100.5 degrees Celcius to 105 degrees Celcius.

3. The method according to any of the proceeding claims, wherein the wood product is an intermediate wood product in the process for developing plywood, laminated veneer lumber (LVL) or OSB plates, such as a veneer layer or a strand of wood.

4. The method according to any of the proceeding claims, wherein the wood product is allowed to cool in the fire-retardant until the temperature is below the boiling point of water, such as 1-3 degrees Celsius below the boiling point of water.

5. The method according to any of the proceeding claims, wherein the wood product is allowed to partially dry in aerated conditions, such as at least until fire- retardant composition residue on the surface of the wood product is allowed to be absorbed into the wood.

6. The method according to any of the proceeding claims, wherein the method does not include a step of transferring the at least one wood product into a second fire-retardant composition, and cooling the at least one wood product in said second fire-retardant composition.

7. The method according to any of the proceeding claims, wherein the cooking temperature is measured at the surface of the fire-retardant composition.

8. The method according to any of the proceeding claims, wherein the wood product is cooked for at least 10 minutes, such as for at least 20 minutes, such as at least 30 minutes, such as at least 40 minutes, such at least 50 minutes, preferably 20-30 minutes.

9. The method according to any of the proceeding claims, wherein the at least one wood product is allowed to cool in the fire-retardant composition and cooled in the same composition as they are cooked.

10. The method according to any of the proceeding claims, wherein the fire- retardant composition is an aqueous composition comprising between 20-45 % fire-retardant, such as 20-30% fire-retardant.

11. The method according to any of the proceeding claims, wherein the fire- retardant is a phosphorous fire-retardant, such as DAP.

12. The method according to any of the proceeding claims, wherein the wood product is a wet product, such as having a wood moisture content of at least 50 %, such as at least 60 %, such as at least 70 %.

13. Use of a phosphorous fire-retardant composition having a boiling point above the boiling point of water in a wood impregnation method.

14. An intermediate wood product, such as a veneer layer or a strand of wood, obtainable from the method according to any of the preceding claims, wherein the wood product a uniform coloration.

15. A wood product comprising the intermediate wood product according to claim

14, such as plywood, LVL or oriented strand board.