RU2811112C1 - Method of manufacturing product from dense wood - Google Patents

Abstract

FIELD: wood products.

SUBSTANCE: group of inventions is related to a method for manufacturing a product from compacted wood, to a product from compacted wood that can be obtained using the method, to the use of an aqueous composition of phenol-formaldehyde resin for impregnating wood in the process of manufacturing products from compacted wood, and to the use of products from compacted wood. Vacuum impregnation of a wooden product with a resin composition is performed at a pressure below 0.01 bar and for a period of time from 10 to 240 minutes. The resin content in the wooden product after vacuum impregnation is preferably from 25 to 45, more preferably from 30 to 40% of the weight of the unimpregnated wooden product. The resulting impregnated wooden product is dried, including the drying stage in an oven at a temperature from 50 to 80°C for a period of 48 to 60 hours. A compressive force is applied to the dried impregnated wood product to reduce the thickness to a final thickness ranging from 25% to 80% of the initial thickness. The temperature of the dried impregnated wooden product is increased to cure the impregnated phenol-formaldehyde resin from 150 to 200°C. The resulting compacted wood product is cooled.

EFFECT: density, hardness, and abrasion resistance increase, the strength properties of wood improve, and recovery after shrinkage decreases.

15 cl, 3 dwg, 2 tbl

Description

FIELD OF APPLICATION OF THE INVENTION

1. Field to which the invention relates

[0001] The invention relates to a method for manufacturing a product from densified wood, including impregnation of porous low-density wood with an aqueous composition of phenol-formaldehyde resin and pressing. The invention also relates to a densified wood product produced by this method. The invention also relates to the use of a special aqueous phenol-formaldehyde resin composition in a method for manufacturing a densified wood product and to the use of a densified wood product or part of a densified wood product in high value-added applications.

2. Description of the background to the state of the art

[0002] Already in 1916, methods for impregnating wood with chemicals were discovered. US1198040 is a method of impregnating wood with substituted polynitrophenols under pressure or vacuum.

[0003] CN108582377A describes a method for densifying wood using a two-step pressure treatment without impregnating the wood with a chemical. The temperature is high and the pressure time is long, so that the hemicellulose and lignin of the wood are decomposed, and the wood becomes stable.

[0004] Hagreen, J.J. and Daniel, D.X. Wood and Fiber 1: 38-53, 1969 title: Simultaneous drying and compaction of sapwood reveals the process of compacting thin wooden boards. Wood is plasticized by heat, moisture and/or a thermosetting resin that swells in the wood. The treatment can be accomplished by soaking untreated sapwood in undiluted water-soluble phenol-formaldehyde resin. The material is then stored for more than 5 days. The specimens were end-coated and soaked in undiluted phenol-formaldehyde thermosetting resin for 16 and 72 hours. After soaking, samples were wrapped and stored at room temperature for 120 hours before compaction. Compaction was accomplished by pressing the wood in a press to the desired thickness and maintaining the position until the wood dried. After pressing, the product is conditioned under moderate humidity conditions to the desired final moisture content.

[0005] JPH11151703 discloses a method for introducing chemicals into compressed wood. The wood is heated by compression in a molding machine and immobilized, for example by cooling and subsequent injection of chemicals such as wood modifiers, fire retardants, insecticides, etc., for example by vacuum impregnation.

[0006] US4606388 discloses a method for densifying low density wood. An untreated solid wood piece with a high moisture content is impregnated with anhydrous ammonia to plasticize the wood, then placed in a cyclic press and subjected to multiple low-pressure compression cycles at temperatures below 100°C.

[0007] WO2004/056542 discloses a method for making a compressed wood product. The method includes two compression stages: a first compression stage; chemical diffusion impregnation/vacuum bath/tank stage; a second compression stage involving chemical infiltration; annealing, cooling and curing. The chemical used for impregnation is a fatty acid.

[0008] EP3470189 discloses a method for producing a decorative product from laminated veneer. The method involves impregnating layers of wood veneer with an aqueous phenol-formaldehyde resin containing a dye, applying adhesive to each layer, laying various layers according to color requirements into a veneer stack, and then compacting the veneer by hot pressing the entire stack at high pressure and temperature.

[0009] Manoj Kumar Dubey, in his PhD thesis in forestry at the University of Canterbury, May 2010, describes a wood densification process that involves impregnating the wood with vegetable oil. Thermal treatment of wood with oil resulted in increased water repellency and decreased hygroscopicity, which consequently reduced shrinkage recovery.

[0010] Thermohydromechanical (THM) processes that combine lining and impregnation with thermoset resins are currently on the market. Typically, the material is impregnated with low molecular weight resins such as phenol formaldehyde and then partially cured before pressing in a heated press to a density of approximately 1300 kg/m ³ . Applications include electric transmission mounts, liquefied natural gas (LNG) storage containers, machine and vehicle wear plates, machine molds, bulletproof barriers, musical instruments and building components.

[0011] A.F. Ang et al. in the article “Possibility of improving the properties of mahang wood (Macaranga sp.) using the phenolic composite method” (Sains Malaysiana, February 1, 2014 (02/01/2014), pages 219-225) describes the wood densification process in which mahang wood has very low density (Macaranga sp.) was impregnated with low solids phenol formaldehyde (15% w/v) with a molecular weight of 600 g/mol by pressure impregnation for 30 minutes to achieve maximum resin saturation. The impregnated wood was pre-cured in an oven at 65°C and dried to a moisture content of 10%, and then hot pressed at 150°C for 30 min. to a thickness of 10 mm in a hot press and subsequent conditioning at a relative humidity of 65% and 25 ° C to constant weight. The dimensional stability and mechanical properties of wood are increased.

[0012] Several other wood densification processes have been developed that use heat and/or steam in combination with mechanical compression without resin or other chemicals. Examples of such processes are the CaLignum process, molding or 3D molding, wood densification and subsequent processing in a closed THM system, viscoelastic thermal compression (VTC) for surface densification.

[0013] Thus, it is known that the density of wood can be increased by compressing the porous structure under suitable humidity and temperature conditions to increase surface hardness, for example, for flooring. However, once compressed, warped wood material is sensitive to moisture and most of the warp is recovered in a humid environment, particularly when soaked in water. This recovery phenomenon, called established recovery, has been addressed in various ways in the prior art by modifying the wood after densification.

[0014] However, the problem underlying the invention is that the moisture-induced swelling of densified wood to its original shape (established recovery) is still too great. This is a particular problem in monolithic wood slabs used for high value-added products such as structural beams, furniture and flooring. High shrinkage recovery implies dimensional instability, resulting in deformation that is unacceptable where the timber must conform to the structure. These high-end applications also have stringent and stringent requirements for low chemical emissions. Therefore, tropical hardwoods continue to be widely used in such applications, which is becoming increasingly unacceptable due to the problem of deforestation. Shrinkage recovery of prior art densified wood products is a major obstacle preventing the widespread commercialization of densified wood products.

[0015] Therefore, there is still a need for an improved method of making a densified wood product and resulting products that do not have one or more of the problems mentioned above.

BRIEF DESCRIPTION OF THE INVENTION

[0016] According to the invention, one or more of the mentioned problems have been solved by providing a method for producing a densified wood product containing

i. By providing a wood product, the specified wood product:

a) has at least two substantially parallel surface planes having an initial thickness TH1, defined as the distance between the substantially parallel surface planes,

b) said initial thickness TH1 is preferably at least 5 mm,

c) is a low density porous wood having a density of less than 700 kg/ ^m3 , preferably from 100 to 600, more preferably from 200 to 500 kg/ ^m3 ,

ii. optionally conditioning the provided wood product to ensure equilibrium moisture content, preferably at a temperature of 15 to 25°C and a relative humidity (RH) of 55 to 80%, most preferably about 20°C and 65% RH. ow.,

iii. Providing an aqueous composition of phenol-formaldehyde (FF) resin, the specified aqueous composition of phenol-formaldehyde resin is:

a) has a solids content of 20 - 50 wt. %, preferably 30 - 40 wt. %, where wt. % is defined as the ratio of the mass of dry resin solids to the total mass of water and resin in the composition,

b) contains oligomers of phenol-formaldehyde resin having a low weight average molecular weight (Mm), measured by size exclusion chromatography, MEC, and amounting to 150 - 700, preferably 200 - 500 g mol ^-1 ,

c) has a period of unlimited dilution with water exceeding 4, preferably 6 weeks of storage at 25°C or 8 weeks at 20°C

d) with a low free monomer content, preferably with a free phenol and free formaldehyde content of less than 0.7 wt. % each, preferably less than 0.5 wt. % and more preferably with a total content of free phenol and free formaldehyde of less than 0.7 wt. % relative to the total weight of the FF resin composition,

e) contains an alkaline catalyst,

f) preferably has a pH value from 7.5 to 11,

iv. vacuum impregnation of a wooden product with a resin composition,

at a pressure Pi below 0.01 bar and for a time ti of 10 to 240 minutes, preferably P is about 0.001 bar and ti is about 60 minutes, preferably at room temperature,

wherein the resin content in the wooden product after vacuum impregnation is preferably from 25 to 45, more preferably from 30 to 40% by weight of the unimpregnated wooden product,

v. drying the resulting impregnated wooden product,

drying, including an oven drying step and, preferably, a preceding air drying step,

wherein the oven drying step is carried out at a temperature Td of 50 to 80°C, preferably about 60°C, for a time of 48 to 60 hours, and

wherein the optional preceding air drying step is carried out for at least 24 hours, preferably in ambient air,

vi. performing a thermo-hydro-mechanical treatment of the dried impregnated wood product to obtain a densified wooden product, said treatment comprising pressing the dried impregnated wood product in a press at an elevated temperature,

in this case, a compressive force is applied to the dried impregnated wooden product to reduce the thickness to the final thickness TH2 in the range from 25% to 80% of the initial thickness TH1,

wherein the temperature of the dried impregnated wood product _Twa is increased to cure the impregnated phenol-formaldehyde resin to a _Twa from 150 to 200°C, more preferably to about 170°C,

cooling the resulting compacted wood product,

vii. optional post-conditioning of the densified wood product under ambient conditions, preferably 20°C and 65% relative humidity.

[0017] The processing method of the invention results in low to medium density wood with increased density, hardness, abrasion resistance, improved strength properties and low shrinkage recovery. The method of the invention provides significant application for low density wood species that are otherwise underutilized.

[0018] The invention also relates to a densified wood product produced by this method. This densified wood product can replace tropical hardwoods such as ebony in many applications and can be used for flooring, structural beams, trim and similar applications and, as a result of being well impregnated, can be machined. The densified wood product offers improved properties of wood and wood-composite materials related to dimensional stability, color stability, environmental impact, acoustics and fungal resistance.

[0019] In another aspect, the invention relates to the use of said aqueous phenol-formaldehyde resin composition for wood impregnation for the manufacture of densified wood products and to the use of densified wood products in various high-end applications such as trim, flooring and structural beams, as well as for mechanical processing of profiled products.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 shows a schematic representation of the process of the invention,

[0021] Figure 2 shows density profiles of various densified wood samples, showing density as a function of depth along the thickness direction of densified wood samples.

[0022] Figure 3 shows the shrinkage recovery of several densified wood samples as a function of repeated moisture cycles for a control thermoformed but not resin-impregnated wood sample and densified wood samples 1-4 according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] As described above, a method for producing a densified wood product includes, as a first step, producing a wood product. The wood product can, in principle, have a variety of shapes, but taking into account the preferred method of pressing using a hot press, said wood product is preferably a flat product having at least two substantially parallel surface planes and an initial thickness TH1, defined as the distance between essentially parallel surface planes. The wood product can be a monolithic wood slab, but the method according to the invention also works in principle for laminated wood (veneer).

[0024] The initial thickness of TH1 is not limited in principle, but for practical purposes it is preferably at least 5 mm. Since the invention has proven to be very suitable for compacting thicker solid wood slabs, the thickness TH1 of the wood product, in particular the solid wood slab, is preferably at least 10, 20 or even at least 30 mm. The maximum thickness is in principle not limited, but in view of achieving uniform impregnation and low shrinkage recovery is typically less than 300 mm, preferably less than 200 mm and more preferably less than 100 mm.

[0025] The type of wood is low density porous wood having a density of less than 700 kg/m ³ , preferably 100 to 600, more preferably 200 to 500 kg/m ³ , and is preferably hardwood or softwood. Suitable examples of low density wood are known in the art, typically using untreated wood, sapwood, poplar, alder, maple, birch, aspen, cypress, pine, spruce. However, an advantage of the method of the invention compared to other known impregnation-based methods is that good results can also be obtained with wood that is not extremely porous and has a density of more than 300, 400, 450 or even more than 500 kg/m ^3, but still preferably less than 700 kg/m ³ . This provides greater flexibility in the choice of starting materials. It is believed that the combination of the properties of the resin and the high vacuum impregnation method according to the invention results in good penetration also into less porous wood species.

[0026] Figure 1 shows a schematic representation of the process according to the invention, including, from left to right, the step of hot plate production and wood conditioning, vacuum impregnation of wood (phase 1), air drying (phase 2A) and oven drying (phase 2B), then in phase 3 from A to F, setting up the hot press, inserting the wood sample and pressing the wood sample (B and C), heating the slabs to a higher temperature (D), cooling the slabs (E), releasing the pressure, opening the mold to remove the compacted wood sample (F), post-processing of wood (phase 4).

[0027] It has been found that it is important that the provided wood product be dampened prior to impregnation to ensure equilibrium moisture content. The conditioning step is optional if the wood provided is already at equilibrium content. The advantage lies in better diffusion of the FF resin throughout the wooden product at the impregnation stage. Without being bound by any theory, it is believed that this conditioning prevents the aqueous composition of the FF resin from absorbing water too quickly, which can destabilize the aqueous composition of the FF resin, may prevent the diffusion of the FF resin throughout the wood product, and possibly even cause precipitation of the FF resin in the surface zone . Preferably, the resulting wood product is conditioned at a temperature of 15 to 25° C. and a relative humidity (r.h.) of 55 to 80%, most preferably about 20° C. and 65% r.h. ow. If the wood product has the desired moisture content, the conditioning step can be omitted.

[0028] For the impregnation step, an aqueous composition of phenol-formaldehyde (FF) resin with a solids content of 20 - 50 wt. is obtained. %, preferably 30 - 40 wt. %, where wt. % is defined as the ratio of the weight of dry resin solids to the total weight of water and resin in the composition. If the content of solid resin particles in the proposed resin composition is too high, it is more difficult to impregnate the wood with it, and the resin composition should be diluted with water until the content of solid resin particles is at the level of 20 - 50 wt. %, preferably 30 - 40 wt. %.

[0029] The phenol-formaldehyde resin in the aqueous composition of the FF resin is a phenol-formaldehyde oligomeric resin having a low weight average molecular weight (Mm), measured by size exclusion chromatography, MEC, by size, and being 150-700, preferably 200-500 g mol ^-1 , The relatively low molecular weight combined with the relatively high solids content is believed to provide good impregnation at the cellular level and even within the wood cells and provide a high resin content to achieve low moisture shrinkage recovery. On the other hand, despite the low molecular weight of the resin, the free monomer content is low, preferably the aqueous composition of the FF resin contains free phenol and free formaldehyde at a level of less than 0.7 wt. % each, preferably less than 0.5 wt. % and more preferably with a total content of free phenol and free formaldehyde of less than 0.7 wt. % relative to the total weight of the FF resin composition. The aqueous composition of FF resin provides good impregnation properties combined with low risk for health, safety, environmental protection and product quality assurance.

[0030] Preferably, the FF resin composition does not contain urea, urea derivatives or other formaldehyde scavengers. The amount of urea, urea derivatives or other formaldehyde scavengers is preferably less than 5 wt. %, more preferably less than 3, even more preferably less than 2 wt. % relative to the mass of solid particles in the composition of the FF resin.

[0031] In addition, it is desirable that the aqueous FF resin formulation have an indefinite dilution period with water greater than 4, preferably 6 weeks of storage at 25°C or 8 weeks at 20°C. Infinitely dilutable with water means, as is known in the art, that an infinite amount of water can be added without resin settling. The advantage of the resin formulation's ability to be infinitely diluted with water is not only that it is very stable, but also the ability to recycle excess resin formulation used in the process without the risk of the resin formulation being destabilized by the recycled resin. High shelf stability is expressed as the period of time during which the aqueous formulation of the FF resin maintains infinite dilutibility with water, which is preferably at least 4, preferably 6 weeks of storage at 25°C, or 8 weeks at 20°C.

[0032] A suitable resin composition with the characteristics described above has a viscosity of from 50 to 250 mPa⋅s, more preferably from 90 to 130, most preferably about 115 mPa⋅s (determined at 20°C using an MCR 51 viscometer from Anton Paar, with cone CP50-1 and a shear rate of 1/200), but before impregnation it is diluted with water to a solids content of 20 - 50 wt. %, preferably 30 - 40 wt. %, at which the viscosity of the resin composition is reduced to a viscosity of approximately 10 to 100 or 10 to 50 mPa⋅s.

[0033] The composition includes a catalyst, preferably an alkaline catalyst, for catalyzing the curing of a resin in a wood product. It is preferable that the pH of the composition be in the range of 7.5 to 11, preferably about 10.

[0034] The next step is vacuum impregnation of the wood product with the resin composition at a pressure Pi below 0.01 bar and for a time ti from 10 to 240 minutes, preferably P is about 0.001 bar, and ti is about 60 minutes. The resin content after impregnation is preferably from 25% to 45%, most preferably from 30% to 40% by weight of unimpregnated wood. Vacuum impregnation is a known procedure and can be performed, for example, by immersing a wooden product in an aqueous FF resin composition and then applying a vacuum.

[0035] It is important that after impregnation, the impregnated wood product is dried to remove at least some of the water. A drying step comprising an oven drying step and, more preferably, a preceding air drying step. The oven drying step is preferably carried out at a temperature Td of from 50 to 80°C, preferably about 60°C, for a time of 48 to 60 hours, and the optional previous air drying step is carried out for at least 24 hours, preferably in ambient air. environment that is inexpensive. It is preferable that some water remains in the wood to maintain heat transfer in the wood product during thermohydromechanical (THM) treatment, so drying is preferably carried out to a residual water content of 5 to 15, preferably 8 to 12 wt. % relative to the total mass.

[0036] The next step is to perform a thermohydromechanical (THM) treatment on the dried impregnated wood product to obtain a densified wood product. Known THM processing uses only heat, water and mechanical pressure to compress and densify wood. This leads to an increase in density, hardness, abrasion resistance and an improvement in some strength properties. During the THM process, wood softens under the influence of heat and moisture and contracts, resulting in compaction without destroying the cell walls of the wood, the most important component of wood that provides its mechanical properties. Densified wood will retain its shape when cooled under pressure. However, with this known method, dimensional stability is poor and long processing times, typically several hours, are required. These disadvantages have been overcome in the present invention, in which the THM treatment involves compressing a dried FF resin-impregnated wood product at an elevated temperature, while a compressive force is applied to the dried impregnated wood product to reduce the thickness to a final thickness TH2 of 25% to 80% of the initial thickness. thickness TN1. The temperature T _wa of the dried impregnated wood product is raised to cure the impregnated phenol-formaldehyde resin, preferably to a T _wa of 125 to 250° C., preferably 150 to 200° C., more preferably about 170° C., followed by cooling of the resulting densified wood product.

[0037] Curing of the FF resin at elevated temperatures occurs after compression, but heating is not necessarily a separate subsequent step since the temperature can also be increased by heating during compression, and some limited pre-curing may also occur during compression. The densified wood product is maintained in a compressed state until the curing of the resin substantially locks the shape of the densified wood product. It has been found that this thermoforming process allows the curing reaction of the resin in the wood to occur at the desired time at elevated temperatures to prevent the buildup of mechanical stress in the wood. Pressing the dried impregnated wood product in a press at elevated temperature is carried out for a period of time from 1 to 20 minutes, preferably from 1 to 15 minutes, more preferably from 1 to 10 minutes, and most preferably from 1 to 5 minutes. A particular advantage of the method of the invention is that the thermal pressing step can be so short. After the hot pressing step, the densified wood product can be cooled, preferably also maintaining a compressed state. The cooled compacted wood product can then be removed from the press.

[0038] Preferably, the resulting densified wood product is then post-processed under ambient conditions, preferably at 20° C. and 65% relative humidity to ensure dimensional stability.

[0039] The resulting densified wood product has shrinkage recovery, defined as the percentage increase in TH3 thickness after 3 cycles of soaking and drying in water for 24 hours / 20°C ((TH3-TH2)/TH2×100%), less 7%, preferably less than 5% and more preferably less than 3 or 2%.

[0040] In a preferred embodiment, in the method according to the invention, thermal hydromechanical treatment includes pressing the dried impregnated wood product at an elevated temperature between the platens of a mechanical press, including

a. Phase a; heating the press plates preferably to a temperature Tp1 of 150 to 200°C, more preferably about 170°C,

b. Phase b; placing the dried impregnated wood product in a press and pressing to the specified thickness TH2, preferably at an applied pressure P, preferably from 6.5 kPa to 34.0 kPa, more preferably from 13.0 kPa to 27.0 kPa, even more preferably about 20 kPa,

c. Phase c; when the desired thickness of TH2 is reached, maintain pressure for a period of 1 to 5 minutes, preferably 2 to 3 minutes, to reduce stress in the product, which can lead to a reverse spring effect, and to ensure curing of the resin,

d. Phase d; raising the temperature of the plate to a higher temperature Tr2 in the range of 170 to 250°C, preferably to about 200°C, and when the temperature Tr2 is reached, maintaining at the temperature Tr2 for a time of 0 to 5 minutes, preferably 0 to 2 minutes, to further reduce the stresses in the product and allow further curing of the resin, while the temperature of the wood product is raised to a temperature of preferably from 200 to 220°C,

e. Phase e; cooling the boards to a temperature Tre while maintaining the densified wood product under pressure, preferably to Tre from 20 to 80°C, preferably from 30 to 70°C and more preferably about 60°C, and at a cooling rate Spc from 5 to 20, preferably about 15°C/min.,

f. Phase f; releasing the applied pressure, opening the press and removing the resulting product from the compacted wood.

[0041] In another embodiment of the method according to the invention, a relief structure is pressed into one or both planes of the surface of the densified wood product, preferably by placing a mold having a relief structure between the boards and the dried impregnated wood product or using press plates having a relief structure .

[0042] The invention also relates to a densified wood product produced by the method of the invention described above. Compacted wood product preferred

a) is a monolithic wooden slab impregnated with cured phenol-formaldehyde resin,

b) has a thickness of at least 5 mm, preferably at least 10 mm and preferably less than 100, 60, 40 or 30 mm,

c) has a density of at least 800 kg/m ³ , preferably at least 900 kg/m ³ or more preferably at least 1000 kg/m ³ ,

d) has a shrinkage recovery below 5 or even 3%.

[0043] Compared to typical shrinkage recovery of 60 to 80% hardwood treated with known THMs, this invention represents a significant advance over the state of the art and offers high commercialization potential.

[0044] Shrinkage recovery is already significantly improved by reducing the sensitivity of surface areas to moisture through compaction and the presence of cured FF resin on the surface where the wood product is exposed to moisture or moisture. However, with the method of the invention a much better established restoration is achieved, since the result is a very good impregnation throughout the entire thickness of the wood product. Preferably, the densified wood product has a density profile characterized by a density at 50% of the thickness of the product Rc of at least 60%, preferably at least 70, 80 or even 90% of the density Rs at the surface of the densified wood product and preferably also after conditioning at 20 °C and 65% rel. ow.

[0045] A densified wood product according to the invention, preferably further characterized by one or more features a) to c);

a) having an elastic modulus MoE greater than 5, preferably greater than 10 GPa,

b) a modulus of rupture MoR of at least 75, preferably at least 100 MPa, MoE and MoR determined by three-point bending tests in accordance with DIN 52186,

c) rot resistance, defined as the weight loss of specimens before and after exposure to the white rot fungus Trametes versicolor for 10 weeks, and being less than 20, preferably less than 15 or even less than 10 or 5%. The method for determining these features is described in more detail below.

[0046] The invention also relates to the use of an aqueous FF resin composition for impregnating wood in the production of a densified wood product, preferably by the method of the invention as described above. An alkaline catalyst, preferably not mixed with the formulation, but provided as a separate part to be mixed with the formulation shortly before use to provide a resulting pH value of 7.5 to 11, preferably about 10. The aqueous FF resin formulation includes an alkaline catalyst shortly before use of the formulation . As described, the composition preferably does not contain urea, urea derivatives or other formaldehyde scavengers because this may result in poor storage stability of the resin, poor impregnation and compression properties of the resin, and poor properties of the final product.

[0047] Low molecular weight FF resin allows thicker wood blocks to be compacted, preferably having an initial thickness of at least 10, 25 or even more than 50 mm. Penetration of the FF resin composition into the cell walls leads to plasticization of the cell walls and consolidation of compressive deformation. Compared to commercially available products, higher densification rates (even up to 300% increase in density) can be achieved with a small amount of FF resin, typically 1 to 15 wt. %, preferably from 2 to 10 wt. %, more preferably from 3 to 8 wt. % FF resin in a densified wood product.

[0048] The FF resin composition must have sufficient shelf stability to ensure complete impregnation over the period of time required for the industrial process. The resin formulation is preferably formulated such that the period of unrestricted dilution with water exceeds 6 weeks of storage at 25°C or 8 weeks at 20°C.

[0049] An aqueous phenol-formaldehyde resin composition can be prepared by a method comprising

a) Contacting phenol and formaldehyde, preferably in amounts close to stoichiometric, where stoichiometric amounts mean the molar amounts of reactive sites of the phenolic ring with respect to formaldehyde molecules, in aqueous solution, preferably with a total amount of phenol and formaldehyde from 30 to 60 wt. %, preferably about 50 wt. % relative to the total mass of the aqueous solution and,

b) Addition of an alkaline catalyst to ensure a pH value between 7.5 and 11, preferably around 10,

c) Raising the temperature and carrying out the condensation reaction at a temperature of 40 to 100°C, preferably 45 to 80°C, more preferably 60 to 80°C, most preferably 65 to 70°C,

d) stopping the condensation reaction by cooling when the water resistance of the resulting phenol-formaldehyde resin composition in an aqueous solution of sodium chloride with a concentration of 10% is from 1:10 to 1:30, preferably from 1:15 to 1:20.

[0050] The advantage of this method, in addition to producing an aqueous FF resin formulation which is preferred for wood impregnation as described above, is that it results in low free formaldehyde and free phenol content and thus avoids the need for scavengers formaldehyde in the composition.

[0051] The invention therefore also relates to the use of the aqueous FF resin composition of the invention in a process for making densified wood products and to the use of a densified wood product or part of a densified wood product of the invention in value-added applications requiring high strength/stiffness such as trim, flooring and structural beams, usually to replace tropical wood, such as chair, table, pot, crockery, structural beam, door, baseboard, floor, countertop, stair tread, stair balustrade, shelf, cabinet, cupboard, tools , jewelry.

[0052] Densified wood products can be used as a base material in a process for manufacturing wood containing products, for example by machine molding a densified monolithic wood slab according to the invention, preferably having a density profile characterized by a density at 50% of the product thickness Rc equal to at least 80%, preferably at least 90% of the density Rs on the surface of the compacted monolithic wood slab.

[0053] Thus, the invention has been described with reference to some of the embodiments discussed above. It should be recognized that these embodiments are subject to various modifications and alternative forms well known to those skilled in the art.

[0054] Further modifications in addition to those described above may be made to the structures and methods described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, they are examples only and do not limit the scope of the invention. The invention is further illustrated by the following examples.

Experimental methods

Density

[0055] The density of wood samples was measured by immersion method. Density profiles were determined by measuring the density of samples at various depths, with depth expressed as a percentage of the thickness of the product.

Recovery after shrinkage

[0056] Shrinkage recovery is defined as the percentage increase in thickness due to moisture. The thickness of the samples was measured at the beginning (Tstart) and after soaking (Ts). Several soaking and drying cycles can be performed, so that there will be several thickness values after soaking (Ts,1, Ts,2, Ts,3, ..., Ts,n). Thus, the shrinkage recovery after the nth cycle is given by [(Ts,n - Tstart) / (Tstart)] × 100%. Wood samples were soaked in water at 20°C for 24 hours, after which the increase in thickness was measured. Several cycles of subsequent drying and soaking were carried out. Drying is performed as described above for the drying step.

Modulus of Elasticity (MoE), Modulus of Rupture (MoR)

[0057] The elastic and tensile moduli were determined using three-point bending tests. Tests were carried out based on DIN 52186. Samples measuring 10 mm (thickness) × 20 mm (width) × 180 mm (length) were prepared. A span of 150 mm was used to maintain a span-to-depth ratio of 15. The specimens were tested at 5 mm/min until failure within 90 s as prescribed.

Fungal decomposition

[0058] Fungal decay was determined by exposing samples to the white rot fungus Trametes versicolor. Fungal degradation was assessed based on mass loss after exposure.

[0059] 4.2% Potato Dextrose Agar (PDA) medium was sterilized in an autoclave (A-21 CA Kambic) at 121°C for 20 minutes before transferring to pre-sterilized Petri dishes. Petri dishes were inoculated with Trametes versicolor. After inoculation of the fungi, the Petri dishes were kept at 26°C in a Kambic growth chamber until the fungi had completely colonized the surface of the medium.

[0060] Wood samples measuring 20 mm x 20 mm x 9.8 mm were dried at 103°C for 24 hours and weighed. After sterilizing the samples at 134°C for 30 minutes in an autoclave (A-21 CA Kambic), they were placed on the surface of the inoculated test fungi in Petri dishes. Samples were exposed to fungi for 2, 4, 6, 8 and 10 weeks in an incubator chamber at 26°C.

[0061] After 2, 4, 6, 8 and 10 weeks of exposure, 10 samples of each group were removed, cleaned (surface mycelium removed) and dried at 103°C for 24 hours. Weight loss was determined by the difference between the dry weight of each sample before and after the fungal degradation test.

Example 1: Preparation of resin composition

[0062] An FF resin was synthesized that has low molecular weight, low monomer content, and is stable during long-term storage at ambient temperatures. A mixture of phenol, water, aqueous 50 wt. % solution of formaldehyde and alkaline catalyst was prepared with relative amounts of 30 wt. % water, 29.5 wt. % phenol, 22.5 wt. % formaldehyde, 2 wt. % NaOH. The condensation reaction was carried out in a temperature range of 65 to 70°C until the tolerance value of the resin solution with 10% sodium chloride aqueous solution was 1:15 to 1:20. The reaction was stopped by cooling to room temperature.

[0063] The finished resin had the following characteristics:

Solid content (2 hours / 130°C / 0.6 g): 58.4%,

Viscosity (20°C): 115 mPa,

pH value: 10.3,

free formaldehyde: 0.40%,

free phenol: 0.31%,

Water resistance (20°C)>1:50,

Average molecular weight Mm: 486 g/mol,

Stability at 25°C until water resistance is no longer infinite: 47 days.

Example 2: Preparation of densified wood products

[0064] Aqueous solutions of phenolic resin with target solids content were prepared by mixing the resin composition obtained in Example 1 with water. The target resin solids content was 30 and 40 wt. %.

[0065] Samples of poplar wood (Populus sp.) with a specified initial thickness of 20 mm were provided. The samples were conditioned at 20°C and 65% relative humidity, followed by vacuum impregnation at 0.001 bar for 60 minutes. After impregnation, the samples were air dried overnight followed by drying at 60°C in an oven for 48 or 60 hours.

[0066] The samples were then subjected to THM treatment in a hot press equipped with a water cooling system (Langzauner Perfect, type LZT-UK-30-L). The target thickness was 9.8 mm. The samples entered the press at a platen temperature of 170°C. Mechanical force was applied until the specified thickness was achieved and held for 2-3 minutes. The plate temperature was increased to 200°C, and the samples were kept for another 0-2 minutes. The slabs were then cooled to 60°C at a rate of 15°C/min, after which the press was opened and the samples were removed. The samples were then post-conditioned at 20°C and 65% relative humidity. Control samples were prepared by compacting samples that were not soaked in the same manner. Table 1 shows the initial thickness TH1 in mm, the initial (wet) density D1 in g/cm ³ , the compressed thickness TH2 in mm and the compressed density D2 in g/cm ³ of poplar samples with different solid resin content RS and time drying DT in hours. WG is the weight gain of the unimpregnated wood product in %, which is the weight of the resin-impregnated wood after the vacuum impregnation step (iv) and the drying step (v) minus the weight of the wood before impregnation as provided (in step i) after conditioning at 20°C and rel. ow. 65% (stage ii). RU is the absorption of phenol-formaldehyde resin after vacuum impregnation in the resin, wt. % relative to the total mass.

[0067] Density profiles of the samples were also determined. They are shown in Figure 2. The numbers correspond to the sample numbers, and C - the control sample. As can be seen, the density in the center of the impregnated samples is higher than that of the control sample.

[0068] Shrinkage recovery of the samples was determined over 3 cycles. These are shown in Figure 3. The impregnated samples gave much lower shrinkage recovery values (between 2 and 5%) than the controls (approximately 72%).

[0069] The MoE and MoR of the samples were determined. Higher density of THM samples resulted in higher MoE values (Table 2). The same relationships were found for MoR of THM-treated samples. Since densities of compacted samples do not differ significantly, MoE values also do not differ significantly among THM-treated samples. However, samples D-30-60 (sample 2), with a density lower than other compacted samples, had lower MoE and MoR. The same relationships were found for MoR of THM-treated samples. The control samples, although having a lower density after compaction, had comparable MoE and MoR to the resin-impregnated samples. SR1 - Shrinkage recovery after the first soaking/drying cycle in water [%]. SR3 - Shrinkage recovery after the third cycle of soaking/drying in water [%].

[0070]

Example 3: Fungal Decay Test

[0071] Sample (No. 5) was prepared in accordance with the method described above, in which the impregnating resin had a solids content of 30%, and the sample was dried in an oven for 72 hours. Two control samples were provided: one compacted without resin impregnation and an untreated control sample.

[0072] These samples were subjected to fungal degradation testing. After 4 weeks of exposure, sample 5 lost about 5% in weight. In control samples, the weight loss was about 12-13%. After 6 weeks of exposure, Sample 5 had a weight loss of approximately 5%, the compacted control sample had a weight loss of approximately 12%, and the untreated control sample had a weight loss of approximately 15%. The weight loss of the untreated control sample continued, while the weight loss of sample 5 and the compacted control sample stabilized at about 13%.

Claims (21)

1. A method for manufacturing a densified wood product, comprising i. providing a wood product, said wood product: a) has at least two substantially parallel surface planes having an initial thickness TH1, defined as the distance between the substantially parallel surface planes, b) said initial thickness TH1 is preferably at least 5 mm, c) is a low density porous wood having a density of less than 700 kg/m ³ , preferably from 100 to 600, more preferably from 200 to 500 kg/m ³ , ii. optionally conditioning the provided wood product to ensure equilibrium moisture content, preferably at a temperature of 15 to 25°C and a relative humidity (RH) of 55 to 80%, most preferably about 20°C and 65% RH. ow., iii. providing an aqueous composition of phenol-formaldehyde (FF) resin, said aqueous composition of phenol-formaldehyde resin: a) has a solids content of 20-50 wt. %, preferably 30-40 wt. %, where wt. % is defined as the ratio of the weight of dry resin solids to the total weight of water and resin in the composition, b) contains phenol-formaldehyde resin oligomers having a low weight average molecular weight (Mm), measured by size exclusion chromatography, MEC, by size, and is 150-500 g ⋅mol ^-1 , c) has a period of unlimited dilubility with water exceeding 4, preferably 6 weeks of storage at 25°C or 8 weeks at 20°C, d) with a low content of free monomer, preferably with a content of free phenol and free formaldehyde less than 0 .7 wt. % each, preferably less than 0.5 wt. % and more preferably with a total content of free phenol and free formaldehyde of less than 0.7 wt. % based on the total weight of the FF resin composition, e) contains an alkaline catalyst, f) preferably has a pH value from 7.5 to 11, iv. vacuum impregnation of a wooden product with a resin composition, at a pressure Pi below 0.01 bar and for a time ti from 10 to 240 minutes, preferably P is about 0.001 bar, and ti is about 60 minutes, preferably at room temperature, with a resin content of wooden product after vacuum impregnation is preferably from 25 to 45, more preferably from 30 to 40% of the weight of the unimpregnated wooden product, v. drying the resulting impregnated wood product, drying comprising an oven drying step and, preferably, a preceding air drying step, where the oven drying step is carried out at a temperature Td of 50 to 80°C, preferably about 60°C, for a time of 48 to 60 hours, and wherein the optional preceding air drying step is carried out for at least 24 hours, preferably in ambient air, vi. performing a thermo-hydro-mechanical treatment of the dried impregnated wood product to obtain a densified wood product, said treatment comprising pressing the dried impregnated wood product in a press at an elevated temperature, wherein a compressive force is applied to the dried impregnated wood product to reduce the thickness to a final thickness TH2 in the range of 25% up to 80% of the initial thickness TH1, wherein the temperature of the dried impregnated wood product T _WA is increased to cure the impregnated phenol-formaldehyde resin to T _wa from 150 to 200°C, more preferably to about 170°C, cooling the resulting densified wood product, vii. optional post-conditioning of the densified wood product under ambient conditions, preferably 20°C and 65% relative humidity. 2. The method according to claim 1, characterized in that the low-density porous wood has a density of 100 to 600, more preferably 200 to 500 kg/m ³ . 3. The method according to claim 1 or 2, characterized in that the porous low-density wood has a density below 700 kg/m ³ and above 300, 400, 450 or even 500 kg/m ³ . 4. The method in accordance with any of paragraphs. 1-3, comprising a conditioning step ii) prior to an impregnation step for conditioning the provided wood product to equilibrium moisture content, preferably at a temperature of 15 to 25° C. and a relative humidity (RH) of 55 to 80%, most preferably about 20 °C and 65% rel. ow. 5. Method according to any one of paragraphs. 1-4, characterized in that the composition of the FF resin includes a total of less than 5, preferably less than 3, and more preferably less than 2 wt. % of urea, urea derivatives or other formaldehyde absorbents relative to the weight of solid FF resin in the composition. 6. The method in accordance with any of paragraphs. 1-5, characterized in that the amount of resin composition in the wooden product after vacuum impregnation is from 25 to 45%, more preferably from 30 to 40% by weight of the impregnated wooden product. 7. Method according to any one of paragraphs. 1-6, characterized in that the impregnated wooden product is dried in step v) until the residual water content is from 5 to 15, preferably from 8 to 12 wt. %. 8. The method in accordance with any of paragraphs. 1-7, characterized in that the pressing of the dried impregnated wooden product in a press at elevated temperature is carried out for a period of time from 1 to 20 minutes, preferably from 1 to 15 minutes, more preferably from 1 to 10 minutes, and most preferably from 1 to 5 minutes. 9. The method in accordance with any of paragraphs. 1-8, characterized in that the compacted wood product obtained at the stage of thermohydromechanical processing, stage vi), is cooled in a press, maintaining a compressed state. 10. The method in accordance with any of paragraphs. 1-9, characterized in that the thermohydromechanical processing step vi) includes pressing a dried impregnated wooden product at an elevated temperature between the plates of a mechanical press, including a. Phase a; heating the press plates to a temperature Tp1 from 150 to 200°C, preferably about 170°C, b. Phase b; placing the dried impregnated wood product in a press and pressing to the specified thickness TH2 under an applied pressure P of 6.5 kPa to 34.0 kPa, preferably 13.0 kPa to 27.0 kPa, more preferably about 20 kPa, c. Phase c; upon reaching the specified thickness TN2, maintain pressure for a period of time from 1 to 5 minutes, preferably from 2 to 3 minutes, d. Phase d; raising the temperature of the plate to a higher temperature Tr2 in the range of 170 to 250°C, preferably to about 200°C, and when the temperature Tr2 is reached, maintaining at the temperature Tr2 for a time of 0 to 5 minutes, preferably 0 to 2 minutes, e. Phase e; cooling the slabs to a temperature Tre while maintaining the compacted wood product under pressure to Tre from 20 to 80°C, preferably from 30 to 70°C and more preferably about 60°C, and at a cooling rate Spc from 5 to 20, preferably about 15° S/min, f. Phase f; releasing the applied pressure, opening the press and removing the resulting product from the compacted wood. 11. The method in accordance with any of paragraphs. 1-10, characterized in that a relief structure is pressed into one or both planes of the surface of the densified wood product, preferably by placing a mold having a relief structure between the boards and the dried impregnated wood product or using press plates having a relief structure. 12. Product made of compacted wood, obtained by the method according to any one of paragraphs. 1-11, characterized in that the densified wood product a) is a monolithic wooden slab impregnated with cured phenol-formaldehyde resin, b) has a thickness of at least 5 mm, preferably at least 10 mm, and preferably less than 100, 60, 40 or 30 mm , c) has a density of at least 800 kg/m ³ , preferably at least 900 kg/m ³ or more preferably at least 1000 kg/m ³ , d) has a specified recovery after 3 times soaking in water for 24 hours / 20°C and drying, defined as a percentage increase in thickness TH3 ((TH3-TH2) / TH2 × 100%) below 7%, preferably below 5 or even 3% and e) preferably with a density profile characterized by density at 50% of the product thickness Rc of at least 60%, preferably at least 70, 80 or even 90% of the density Rs on the surface of the densified wood product and preferably also after conditioning at 20°C and 65% rel. ow. 13. A product made of compacted wood according to claim 12, additionally characterized by one or more features a) - c); a) having a modulus of elasticity MoE greater than 5, preferably greater than 10 GPa, b) modulus of rupture MoR not less than 75, preferably not less than 100 MPa, MoE and MoR determined by three-point bending tests in accordance with DIN 52186, c) rot resistance , defined as the weight loss of samples before and after exposure to the white rot fungus Trametes versicolor for 10 weeks, and being less than 20, preferably less than 15 or even less than 10 or 5%. 14. The use of an aqueous composition of phenol-formaldehyde resin in the process of wood impregnation for the manufacture of products from compacted wood, while the aqueous composition of phenol-formaldehyde resin is characterized by: a. with a solids content of 20-50 wt. %, preferably 30-40 wt. %, where wt. % is defined as the weight of dry resin solids relative to the total weight of water and resin in the composition, or a solids content greater than 50 wt. % for dilution to specified solids range, b. contains oligomers of phenol-formaldehyde resin having a low weight average molecular weight (Mm), measured by size exclusion chromatography, MEC, and amounting to 150-500 g mol ^-1 , c. has a period of unlimited dilution with water exceeding 4, preferably 6 weeks of storage at 25°C or 8 weeks at 20°C, d. with a low content of free monomer, preferably with a content of free phenol and free formaldehyde of less than 0.7 wt. % each, preferably less than 0.5 wt. % and more preferably with a total content of free phenol and free formaldehyde of less than 0.7 wt. % relative to the total weight of the FF resin composition, e. contains an alkaline catalyst, either mixed with the formulation or as a separate part to be mixed with the formulation shortly before use, to provide a resulting pH value of 7.5 to 11, preferably about 10, f. preferably does not contain urea, urea derivatives or other formaldehyde scavengers, this process involves vacuum impregnation of a wood product with a resin composition at a pressure Pi below 0.01 bar and for a time ti from 10 to 240 minutes. 15. Use of a densified wood product or part of a densified wood product according to claims. 12, 13 in decoration, flooring and building beams, preferably chair, table, pot, utensils, building beam, door, baseboard, floor, countertop, stair tread, stair balustrade, shelf, sideboard, cabinet, tools or decorations or product use densified wood for machining to shape densified wood products or as a substitute for tropical hardwood.