A method for the in-situ curing of fire-retardant wood-based composites

Abstract

The present application relates to the technical field of wood modification and flame-retardant material, and particularly relates to a preparation method of in-situ cured flame-retardant wood-based composite material. The raw wood is treated by alkali, then vacuum impregnation is carried out by using an impregnation solution compounded by phytic acid, epoxy resin, boron nitride and silicon dioxide, and slow hot pressing is carried out by using a hot press. The phytic acid promotes the in-situ curing of the epoxy resin during hot pressing, and no additional curing agent is needed. The wood-based composite material obtained has both flame-retardant performance and mechanical performance. Compared with untreated wood, the limiting oxygen index of the composite material obtained by the present application is increased from 20.9% to 36.0%, the ignition time is extended from 29s to 55s, the two heat release rate peaks are reduced by about 39.5% and 11.6% respectively, the smoke generation rate peak and the total smoke release amount are reduced by 30.8% and 39.6% respectively. The present application has simple process, mild reaction, and smooth material surface, and can be widely applied in the fields of building and furniture, provides a new idea for the high-value utilization and fire safety of wood, and meets the requirements of green and sustainable development.

Description

Technical Field

[0001] This invention relates to the field of wood modification and flame retardant materials technology, and in particular to a method for preparing an in-situ cured flame retardant wood-based composite material. Background Technology

[0002] Against the backdrop of green and low-carbon development, wood, with its natural advantages such as being renewable, biodegradable, and environmentally friendly, is increasingly being used in fields such as construction, furniture, and decoration, and is regarded as an important biomass material for achieving the "dual carbon" goal. However, the flammability of wood means that its application always faces serious fire safety hazards. How to improve its flame-retardant properties while maintaining the natural advantages of wood has become a key issue that urgently needs to be addressed for the high-value utilization of wood.

[0003] Wood, as a natural porous material, has advantages such as light weight, high strength-to-weight ratio, easy processing, and renewability, and is widely used in construction, furniture and other fields. However, wood itself is a combustible material with a low ignition point. It is extremely easy to ignite and spread rapidly when exposed to fire, and releases a large amount of heat and produces toxic fumes during combustion, posing a serious fire safety hazard. At the same time, the mechanical strength of natural wood is limited, while flame retardant treatment is often accompanied by a decrease in mechanical properties. It is difficult to achieve both flame retardant performance and mechanical properties, which greatly limits the application of wood in the field of structural materials.

[0004] To improve the flame retardant properties of wood, existing technologies (CN103817755A, this patent document uses vacuum-pressure impregnation and drying of salts (amidine urea phosphate / boric acid / borax), without resin, in-situ curing, or hot-press densification; the flame retardant relies on physical adsorption and is easily lost; CN112442895A, this patent document first synthesizes chelated flame retardants and then uses them as additives / coatings; phytic acid is "consumed" in the synthesis of flame retardants and does not participate in epoxy curing) typically employ flame retardant impregnation or coating treatments. Commonly used flame retardants include phosphorus-based, nitrogen-based, boron-based, and inorganic fillers. However, single flame retardants often suffer from low flame retardant efficiency, large addition amounts, weak bonding with the wood matrix, easy migration and loss, and poor durability. Furthermore, some flame retardant treatments can damage the microstructure of wood, leading to a significant decrease in mechanical properties. In recent years, some studies have attempted to combine resins with flame retardant components and impregnate them into the wood, then prepare flame-retardant composite materials through hot-press densification. However, existing resin systems usually require the addition of curing agents and need to be cured at high temperatures, resulting in complex processes and high energy consumption. At the same time, the pores of natural wood are limited, making it difficult for resin and flame-retardant components to fully penetrate and distribute evenly. During hot pressing, flame-retardant components may be released, resin may be unevenly distributed, and problems such as surface cracking and poor smoothness may occur. It is still difficult to achieve both flame-retardant performance and mechanical properties.

[0005] Therefore, how to enable flame-retardant resin to fully penetrate and cure in situ within wood without adding additional curing agents, thereby achieving effective bonding between the flame-retardant components and the wood matrix, and thus balancing flame-retardant performance and mechanical properties, while simplifying the process and ensuring a smooth surface of the resulting material, remains a pressing technical problem to be solved in this field. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, such as the flammability of existing wood, the complexity of flame-retardant treatment processes, and the difficulty in simultaneously achieving both flame-retardant and mechanical properties, this invention utilizes phytic acid, which possesses both flame-retardant and curing functions. This allows epoxy resin to be cured in situ within the wood without the need for additional curing agents. Combined with vacuum impregnation and hot pressing, the epoxy resin, phytic acid, silica, boron nitride, and other flame-retardant components are firmly fixed within the wood matrix, improving both the flame-retardant and mechanical properties of the wood. Furthermore, the preparation process of this invention is simple, providing a new approach to the high-value utilization of wood and fire safety, and meeting the requirements of green and sustainable development.

[0007] This invention provides a method for preparing in-situ cured flame-retardant wood-based composite materials, specifically including the following steps: (1) The logs are treated with alkali to obtain alkali-treated wood; (2) The alkali-treated wood is impregnated to obtain impregnated wood; (3) The impregnated wood is hot-pressed and cured to obtain an in-situ cured flame-retardant wood-based composite material; The impregnation solution is a phytic acid composite impregnation solution.

[0008] Preferably, in the preparation method of the in-situ cured flame-retardant wood-based composite material, step (1) satisfies at least one of the following conditions: The logs include poplar, balsa wood, linden, fir, or birch; The solvent used for alkali treatment includes an aqueous solution of sodium hydroxide. The sodium hydroxide aqueous solution has a mass fraction of 5-10%; The temperature for the alkali treatment is 70~80℃; The alkali treatment time is 8-12 hours.

[0009] Preferably, in the method for preparing the in-situ cured flame-retardant wood-based composite material, step (2) satisfies at least one of the following conditions: The number of immersions is 3 to 6; The immersion time is 30-50 minutes; The impregnation includes vacuum impregnation.

[0010] Preferably, in the method for preparing the in-situ cured flame-retardant wood-based composite material, the pressure of the vacuum impregnation is -0.1 to -0.6 MPa; When vacuum impregnation is used for the impregnation, the number of impregnations is 3 to 6, and the impregnation time is 30 to 50 minutes.

[0011] Preferably, in the preparation method of the in-situ cured flame-retardant wood-based composite material, step (3) satisfies at least one of the following conditions: The temperature for hot pressing and curing is 80~100℃; The pressure for hot pressing and curing is 15~25MPa; The hot-press curing time is 8~12 hours; During the hot-press curing process, the compression rate is 65%~85%.

[0012] Preferably, in the method for preparing the in-situ cured flame-retardant wood-based composite material, the composite impregnation solution includes phytic acid, boron nitride, silicon dioxide and epoxy resin, and ethanol is used as a diluent for the epoxy resin.

[0013] Preferably, the preparation method of the in-situ cured flame-retardant wood-based composite material, The resin is an epoxy resin, and the mass ratio of the epoxy resin to the ethanol is 1:(1.5~2.5), preferably 1:2; the mass of the silica is 3%~8% of the mass of the ethanol, preferably 5%.

[0014] Preferably, in the method for preparing the in-situ cured flame-retardant wood-based composite material, the mass of the ethanol is used as a basis. The phytic acid has a mass fraction of 6% to 10%, preferably 8%; The boron nitride has a mass fraction of 3% to 5%, preferably 5%. The present invention also provides a method for preparing an in-situ cured flame-retardant wood-based composite material, which is prepared by the above-mentioned method for preparing an in-situ cured flame-retardant wood-based composite material.

[0015] The present invention also provides a method for preparing an in-situ cured flame-retardant wood-based composite material, which is applied to enhance the flame-retardant effect of plant fiber raw materials.

[0016] The present invention further discovered in the research that: phytic acid acts as a curing agent and crosslinks with epoxy resin through ring opening; phosphorus is covalently anchored to the resin network and distributed in the cell wall—the flame retardant element is also a structural element, so it is not lost; during pyrolysis, it generates phosphoric acid / polyphosphoric acid in situ and catalyzes carbonization near the anchoring point. Boron nitride lamellars are dispersed in the carbon layer, acting as a "lamellar skeleton" (brick-mud structure), which improves the density and crack resistance of the carbon layer, delays the second exothermic peak, and further enhances the carbon layer.

[0017] Silica reacts with phosphoric acid / polyphosphoric acid to form phosphosilicate and Si-OC structure, making the carbon layer "ceramic". The stable retention of B and Si also comes from the high thermal stability of boron nitride and silica itself. Gas phase assistance: Phosphorus-containing free radicals capture H· / OH·, further inhibiting combustion (condensed phase is dominant), making the carbon layer more resistant to high temperature.

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes a multi-component synergistic flame retardant approach combining phytic acid, boron nitride, silica, and epoxy resin, along with in-situ curing, to increase the limiting oxygen index (LOI) of modified wood to 36%, which is 15.1% higher than that of natural wood. Simultaneously, the time to ignition (TTI) is increased by approximately 89.7% compared to natural wood, nearly twice that of natural wood, significantly enhancing the flame retardant properties of the wood.

[0019] The alkali treatment of this invention improves the permeability of wood, and the epoxy resin is cured in situ inside the wood and combined with the wood fibers. After hot pressing and densification, the tensile strength of the product is as high as 134.12 MPa, and its mechanical properties far exceed those of untreated wood.

[0020] This invention utilizes phytic acid, which possesses both flame-retardant and curing functions. It eliminates the need for additional curing agents and high-temperature, long-duration curing steps required by traditional epoxy resin systems, simplifying the process, reducing energy consumption, and employing a slow pressurization process to effectively prevent the release of flame-retardant components and surface cracking. The resulting material has a smooth surface and preferentially uses renewable biomass flame-retardant components such as phytic acid. The process is simple, the reaction conditions are mild, and it meets the requirements of green and sustainable development. While simplifying the process, it ensures the smoothness of the resulting material surface, promoting the development of "green flame-retardant" technology and aligning with the national "dual-carbon" strategic goals. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0022] Figure 1 The limiting oxygen index test results are shown in Figure 1 for Example 1 and Comparative Example 1. Figure 2 The graphs show the heat release rates of Example 1 and Comparative Example 1. Figure 3 This is a graph showing the total smoke release rate of Example 1 and Comparative Example 1; Figure 4 The ignition time diagrams are for Example 1 and Comparative Example 1. Figure 5 The graph shows the tensile property test results of Example 1 and Comparative Example 1; Figure 6 Here is a SEM image of the carbon residue after combustion in Example 1; Figure 7 The image shows the EDS diagram of the carbon residue after combustion in Example 1. Figure 8 Thermogravimetric analysis diagram of Example 1; Figure 9 This is a graph showing the rate of weightlessness in Example 1. Detailed Implementation

[0023] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0024] Unless otherwise specified, the raw materials used in the embodiments of the present invention are all purchased commercially and used directly without any special treatment.

[0025] Unless otherwise specified, the analytical methods in the embodiments all adopt conventional instrument or equipment settings and conventional analytical methods.

[0026] This invention has been demonstrated through extensive experiments and research, and it has been found that: During alkali treatment, the optimal mass fraction of NaOH is 5-10%.

[0027] During alkaline treatment, a low concentration of alkaline solution (below 5%) is insufficient to effectively swell the cell wall, open pits and micropores for permeation, resulting in inadequate activation. At higher concentrations (above 10%), excessive removal of hemicellulose and damage to the cell wall skeleton leads to a loss of strength. Therefore, this invention has experimentally confirmed that 5-10% is the balance window between "activation and permeation and maintaining structure," and 70°C for 10 hours is the optimal condition for mild alkaline treatment. In the components of the phytic acid composite impregnation solution, the optimal dosage of phytic acid is 8%.

[0028] During the impregnation process, the impregnation solution determines the direct flame retardant effect. In the impregnation system of this invention, phytic acid serves as both a curing agent and a flame retardant acid source. Its content needs to reach the threshold where it can fully crosslink with epoxy and the phosphorus content of the system is sufficient to catalyze char formation. If it is too low (below 5%), the degree of crosslinking and phosphorus content will be insufficient, resulting in weak flame retardant and fixing effects. If it is too high (above 15%), the system will be too acidic, which is not conducive to operation and the wood matrix.

[0029] The optimal amount of boron nitride is 5%. Boron nitride needs to form a continuous and diffuse lamellar network in the carbon layer to play a role in skeleton reinforcement and heat insulation and smoke suppression. Too low a content (below 3%) is not enough to strengthen the carbon layer. Too high a content (above 8%) will result in poor dispersion stability of the impregnation solution.

[0030] The optimal amount of silica is 5% of the ethanol mass. Silica can react with phosphoric acid produced by the decomposition of phytic acid to form a phosphosilicate structure, and together with boron nitride, it forms an inorganic barrier. During the impregnation process, the optimal conditions are vacuum impregnation at -0.1 to -0.6 MPa, 3 to 6 times, and 3 to 50 minutes each time.

[0031] Negative pressure is used to remove air from the cell cavity and promote the penetration of the impregnation solution; too few times (less than 3 times) will result in insufficient penetration, while too many times (more than 6 times) will reduce efficiency. During the hot-press curing process, the optimal experimental conditions are hot pressing at 80~100℃, 15~25MPa, for 8~12h, and with a compression rate of 65%~85%.

[0032] Phytic acid-epoxy ring-opening curing is incomplete below 80℃, and above 100℃ it is easy to cause premature resin gelation and thermal degradation of wood. Therefore, the temperature window and curing kinetics must be matched; the pressure and compressibility window ensure sufficient densification while avoiding excessive cell wall crushing; the holding time ensures the full formation of the cross-linked network.

[0033] Example 1 A method for preparing an in-situ cured flame-retardant wood-based composite material includes the following steps: (1) Place natural poplar wood in a 10% sodium hydroxide aqueous solution and boil it at 70°C for 10 hours. After taking it out, wash it with deionized water until neutral to obtain alkali-treated wood. Store it in anhydrous ethanol for later use. (2) First, the phytic acid solution is vacuum impregnated into the wood, and then the wood is vacuum impregnated with an impregnation solution made of epoxy resin, boron nitride and silica, wherein the mass fraction of phytic acid is 8%, the mass fraction of boron nitride is 5%, and the mass fraction of silica aqueous solution is 8%.

[0034] Epoxy resin was dissolved in ethanol at twice its mass to obtain a resin dilution. Phytic acid, boron nitride, and silicon dioxide were then added and mixed evenly to obtain a phytic acid composite impregnation solution. The mass fractions of phytic acid, boron nitride, and silicon dioxide were 8%, 5%, and 5%, respectively, based on the mass of ethanol.

[0035] (3) Place the alkali-treated wood in the impregnation solution and vacuum impregnate it 3 to 6 times under the conditions of -0.1 to -0.6 MPa, each time for 30 to 50 minutes, so that the impregnation solution is introduced into the wood in situ.

[0036] (4) Place the impregnated wood in a hot press and hot press at 100°C for more than 10 hours to ensure that it is completely compacted. During the hot pressing process, the pressure slowly rises to 25MPa. After the hot pressing is completed, an in-situ cured flame-retardant wood-based composite material with excellent performance is obtained.

[0037] The wood-based composite material obtained in Example 1 is designated as SP-Wood.

[0038] Comparative Example 1 To verify that the in-situ cured flame-retardant wood-based composite material prepared in this invention has high-efficiency flame-retardant and mechanical properties, untreated wood was used as a comparison.

[0039] The wood in Comparative Example 1 is labeled Nature Wood.

[0040] Effective Examples (Flame Retardancy and Mechanical Property Testing) The limiting oxygen index, cone calorimetry, and mechanical properties of Example 1 and Comparative Example 1 were tested.

[0041] (1) Limiting oxygen index test The combustion performance of wood specimens (100mm × 6.5mm) was evaluated using an oxygen index tester. A high oxygen index indicates that the material is not easily combustible, while a low oxygen index indicates that the material is easily combustible.

[0042] The results are as follows Figure 1 As shown, the limiting oxygen index of Nature Wood is only 20.9%, while the limiting oxygen index of SP-Wood after flame retardant treatment is 36.0%, which is significantly higher than that of Nature Wood, indicating that the wood-based composite material prepared by this invention has excellent flame retardant properties.

[0043] (2) Cone calorimetry test Using a cone calorimeter at 50 kW / m³ in air 2 The flame retardant properties of wood specimens (100 mm × 100 mm) were evaluated under an external heat flux density. The heat release rate (HRR), total smoke release (TSP), and ignition time (TTI) of Example 1 and Comparative Example 1 were measured.

[0044] The results are as follows Figure 2 As shown, under these heat flux conditions, Nature Wood exhibits its first HRR peak at approximately 50 seconds, with a Peak1-HRR of 245.05 kW / m²; followed by a second HRR peak at approximately 205 seconds, with a Peak2-HRR reaching 380.11 kW / m². In contrast, SP-Wood's first HRR peak is delayed to approximately 81 seconds, with the Peak1-HRR decreasing to 148.34 kW / m²; the second peak is delayed to approximately 231 seconds, with the Peak2-HRR decreasing to 336.00 kW / m². The two HRR peaks of SP-Wood are reduced by approximately 39.5% and 11.6%, respectively, and both peak occurrence times are delayed, with the first peak delayed by approximately 31 seconds.

[0045] visible Figure 2The results show that the method for preparing wood-based composite materials in this invention can not only reduce the heat release intensity during the combustion process of wood, but also delay its rapid combustion process, thereby effectively reducing the combustion hazard of the material.

[0046] like Figure 3 As shown, the total smoke emission (TSP) decreased from 3.13 m² in Nature Wood to 1.89 m² in SP-Wood, a reduction of approximately 39.6%, indicating that the flame retardant treatment of the present invention has an excellent smoke suppression effect.

[0047] like Figure 4 As shown, the ignition time (TTI) of Nature Wood is only 29s, while that of SP-Wood is extended to 55s, an increase of approximately 89.7%, nearly twice that of natural wood. This indicates that the flame-retardant treatment of this invention can form a flame-retardant protective layer on the surface of the wood, reducing heat transfer to the interior of the wood and delaying the production of combustible volatiles from wood pyrolysis, thereby improving the flame-retardant ability of the material.

[0048] (3) Mechanical property testing The mechanical properties of Example 1 and Comparative Example 1 were tested using a universal testing machine.

[0049] The results are as follows Figure 5 As shown, SP-Wood has a tensile strength of 134.12 MPa, while Nature Wood has a tensile strength of 43.24 MPa. Therefore, it can be seen that the product prepared in Example 1 of the present invention has a strength several times higher than that of Nature Wood.

[0050] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

[0051] The research of this invention has revealed that the flame-retardant components of this invention are centered on promoting the formation of char from the condensed phase and constructing a stable organic-inorganic barrier layer, which is directly confirmed by the characterization of the residual char structure.

[0052] Depend on Figure 6 The SEM images show that natural wood fragments and collapses after carbonization, while SP-Wood basically retains the original structure of the wood.

[0053] Depend on Figure 7 The EDS diagram shows that a large number of Si, P and B elements are evenly distributed on the surface of SP-Wood char residue, indicating that the flame retardant components are effectively retained in the wood during pyrolysis and work together with carbon residue to form a protective layer.

[0054] Depend on Figure 8 It can be seen that the char residue rate of [TGA] at 600℃ under nitrogen atmosphere increased from 24.6% of natural wood to 33.6% of SP-Wood.

[0055] and Figure 9 The DTG curves show a significant change in the main pyrolysis stages and weight loss rates, with the pyrolysis process shifting from rapid formation of volatile products to condensed phase char formation. This indicates that the dense and stable protective layer can hinder the transfer of external heat to the interior of the wood, while simultaneously inhibiting oxygen diffusion and the escape of combustible volatiles, thereby slowing down further pyrolysis and combustion of the wood. This result is consistent with the findings in the cone calorimetry test, which showed a decrease in heat release peak, an extension of ignition time, and a reduction in smoke release, indicating that SP-Wood primarily achieves flame retardancy by promoting condensed phase char formation and constructing an organic-inorganic barrier layer.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing an in-situ cured flame-retardant wood-based composite material, characterized in that, Specifically, the steps include the following: (1) The logs are treated with alkali to obtain alkali-treated wood; (2) The alkali-treated wood is impregnated to obtain impregnated wood; (3) The impregnated wood is hot-pressed and cured to obtain an in-situ cured flame-retardant wood-based composite material; The impregnation solution is a phytic acid composite impregnation solution.

2. The preparation method of the in-situ cured flame-retardant wood-based composite material as described in claim 1, characterized in that, Step (1) satisfies at least one of the following conditions: The logs include poplar, balsa wood, linden, fir, or birch; The solvent used for alkali treatment includes an aqueous solution of sodium hydroxide. The sodium hydroxide aqueous solution has a mass fraction of 5-10%; The temperature for the alkali treatment is 70~80℃; The alkali treatment time is 8-12 hours.

3. The preparation method of the in-situ cured flame-retardant wood-based composite material as described in claim 1, characterized in that, Step (2) satisfies at least one of the following conditions: The immersion is repeated 3 to 6 times; The immersion time is 30-50 minutes; The impregnation includes vacuum impregnation.

4. The preparation method of the in-situ cured flame-retardant wood-based composite material as described in claim 3, characterized in that, The pressure of the vacuum impregnation is -0.1 to -0.6 MPa; When vacuum impregnation is used for the impregnation, the number of vacuum impregnations is 3 to 6, and the vacuum impregnation time is 30 to 50 minutes.

5. The preparation method of the in-situ cured flame-retardant wood-based composite material as described in claim 1, characterized in that, Step (3) satisfies at least one of the following conditions: The temperature for hot pressing and curing is 80~100℃; The pressure for hot pressing and curing is 15~25MPa; The hot-press curing time is 8~12 hours; During the hot-press curing process, the compression rate is 65%~85%.

6. The preparation method of the in-situ cured flame-retardant wood-based composite material as described in claim 1, characterized in that, The phytic acid composite impregnation solution includes phytic acid, boron nitride, silicon dioxide, epoxy resin, and ethanol.

7. The preparation method of the in-situ cured flame-retardant wood-based composite material as described in claim 6, characterized in that, Based on the mass of the ethanol: The mass ratio of the epoxy resin to the ethanol is 1:(1.5~2.5), for example 1:2; The mass of the silicon dioxide is 3% to 8% of the mass of the ethanol, for example, 5%.

8. The preparation method of the in-situ cured flame-retardant wood-based composite material as described in claim 6, characterized in that, Based on the mass of the ethanol: The phytic acid has a mass fraction of 6% to 10%, for example, 8%; The boron nitride has a mass fraction of 3% to 5%, for example, 5%.

9. A method for preparing an in-situ cured flame-retardant wood-based composite material, characterized in that, It is prepared by the preparation method of in-situ cured flame-retardant wood-based composite material according to any one of claims 1 to 8.

10. A method for preparing an in-situ cured flame-retardant wood-based composite material as described in claim 9, characterized in that, It is used to enhance the flame retardant effect of plant fiber raw materials.

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