Flame retardant for plywood and preparation method thereof

Abstract

According to the flame retardant for the plywood and the preparation method of the flame retardant, the phosphorus-nitrogen-magnesium synergistic flame retardant effect of guanidine phosphate and nano magnesium hydroxide is utilized, the limit oxygen index is increased, dripping is avoided during vertical combustion, open fire spreading is avoided, the combustion time is shortened, a chain reaction is blocked, the flame retardant efficiency is nearly doubled, and the problem that the combustion performance of the plywood is poor is solved; the preparation method has the advantages of no need of newly adding complex equipment, good adaptability with an existing production line, only additional arrangement of an auxiliary device, strong compatibility, low modification difficulty, controllable cost, reduction of preparation energy consumption, no emission of harmful wastes, conformity with environmental protection requirements, simple preparation method, small environmental influence, suitability for mass production, simple and convenient plywood treatment operation, easy parameter control, and wide application prospect. And a scientific basis is provided for research and development of green flame-retardant wood materials.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of artificial board, in particular to a fire retardant for plywood and a preparation method thereof. BACKGROUND

[0002] In recent years, with the implementation of the "double carbon" strategy and the concept of sustainable development, the construction industry is accelerating the transformation towards green and low-carbon. Wood, as a natural, renewable and biodegradable green building material, has become an important part of the building material system due to its excellent environmental performance and high structural efficiency. Especially modern engineered wood structure buildings, due to their short construction period, low energy consumption and strong recyclability, have been rapidly promoted worldwide. However, the flammability of wood itself makes it prone to cause chain reactions in fires, posing a serious threat to building safety.

[0003] Fire is one of the most common and most dangerous urban disasters. According to statistics, the economic losses and casualties caused by fires worldwide continue to rise every year. In Europe, with the increasing proportion of wood structure buildings, the frequency of fire incidents has also increased significantly. For example, the Swedish Fire Service reported that in 2022, wood structure building fires accounted for 13% of all building fires, an increase of nearly 3% from 2019; in Finland and Germany, with the widespread use of cross-laminated timber (CLT) and glued laminated timber (GLT), fire incidents in the exterior facade and balcony area have also shown a growing trend. According to the National Fire Protection Association (NFPA), wood structure building fires accounted for 9% in 2023, showing the prominence of their fire safety problems.

[0004] Compared with reinforced concrete structures, wood structure buildings have a heat release rate that is about 2-3 times higher, and once they spread, they tend to cause catastrophic consequences. Frequent fire incidents have exposed the shortcomings of current wood structure buildings in terms of fire prevention design, material fire retardation and fire resistance. Therefore, how to improve the fire resistance of wood structures from the material level has become a key scientific problem for building safety and sustainable development of the industry.

[0005] In the green building material system, plywood, as a high-performance engineered wood, has become an important material for modern wood structure buildings due to its high resource utilization rate, superior mechanical properties, and good structural uniformity. LVL is prepared by stacking, gluing and hot pressing rotary cut veneers along the grain direction, which not only retains the natural properties of wood, but also overcomes the defects of natural wood such as knots, cracks, uneven strength, etc. Its research began in the 1940s, and after the 1960s, it was widely used in the fields of construction, transportation and furniture manufacturing. The bending, compression and shear strength of LVL are significantly higher than that of ordinary solid wood, and it has excellent dimensional stability, which can meet the strength and deformation requirements of large structural components such as beams, columns and trusses through precise processing.

[0006] Furthermore, plywood can be customized in thickness and number of layers to meet specific needs. The manufacturing process involves drying and controlled adhesive application, giving it excellent moisture resistance and durability. Its production process is environmentally friendly, significantly improving wood utilization and reducing waste emissions. Currently, market demand for this type of board continues to grow in North America, Europe, and the Asia-Pacific region, showing broad application prospects in construction, bridges, rail transportation, and high-end furniture.

[0007] Despite plywood's excellent structural properties and environmental friendliness, its combustion performance remains a key bottleneck restricting its further widespread application. Its combustion behavior mainly includes three stages: pyrolysis, carbonization, and combustion. The interlayer adhesives, wood species, coating characteristics, and the number and thickness of layers all significantly influence the combustion rate and smoke production characteristics. During combustion, the carbonized layer formed on the surface of plywood, especially laminated veneer lumber (LVL), can impede heat transfer to some extent, but the toxic gases released during the combustion of adhesives and coatings still pose a safety hazard.

[0008] Given the increasingly stringent requirements for fire safety in buildings, the development of high-performance, low-toxicity, and sustainable new flame-retardant plywood materials is of significant practical importance. On the one hand, by introducing green flame retardants or constructing multifunctional flame-retardant systems during the plywood preparation process, the fire resistance limit and smoke suppression performance can be significantly improved while maintaining its mechanical and processing properties. On the other hand, the widespread application of flame-retardant plywood will effectively improve the fire safety level of timber-framed buildings, providing technical support for the sustainable development of my country's timber-framed building industry. Summary of the Invention

[0009] In order to solve the technical problems existing in the prior art, the present invention provides a flame retardant for plywood and its preparation method.

[0010] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a flame retardant for plywood includes the following steps: Step Sa: Add an appropriate amount of preheated deionized water at 50°C to a water bath constant temperature reaction vessel, keep stirring, add an appropriate amount of guanidine phosphate, and then stir until completely dissolved to obtain solution A; Step Sb: Add an appropriate amount of silane-modified nano-magnesium hydroxide to solution A and stir to obtain a stable suspension B; Preparation of flame retardant for plywood. Step Sc: Lower the water bath temperature, continue stirring, and add an appropriate amount of KH-550 silane coupling agent and low molecular weight phenolic resin to the stable suspension B in sequence. Stir for a period of time to obtain solution C. Step Sd: Add an appropriate amount of citric acid to solution C, adjust the pH to be stable at 4.5-5.5, and let it stand at room temperature to obtain a flame retardant for plywood.

[0011] Furthermore, in step Sa, when preheating deionized water in a water bath constant temperature reactor, the stirring speed should be maintained at 150 r / min, and guanidine phosphate should be added slowly in three equal portions at 5-min intervals. After all the guanidine phosphate has been added, the stirring speed should be increased to 200 r / min and stirred continuously until completely dissolved to obtain solution A. In step Sb, when adding silane-modified nano-magnesium hydroxide to solution A, an ultrasonic auxiliary device with a power of 200W needs to be turned on and stirred at a speed of 800r / min for 1h to obtain a stable suspension B. In step Sc, first turn off the ultrasonic auxiliary device, then lower the water bath temperature to 40℃, and stir continuously at a speed of 250r / min. First add KH-550 silane coupling agent to the stable suspension B, stir for 15min, and then add low molecular weight phenolic resin at a dropping rate of 5mL / min. After stirring for 30min, solution C is obtained. In step Sd, when adding citric acid to solution C, the mixture should be continuously stirred at a speed of 200 r / min. The pH value should be monitored with high precision. For each drop of citric acid added, the mixture should be stirred for 15 seconds. After the pH is adjusted to be stable at 4.5-5.5, the mixture should be allowed to stand at room temperature for 10 minutes and then passed through a 200-mesh filter to obtain the flame retardant for plywood.

[0012] Furthermore, in step Sd, the pH is adjusted to be stable at 5.0-5.5.

[0013] A flame retardant for plywood comprises the following components: 25-30 parts of guanidine phosphate 3-5 parts of silane-modified nano magnesium hydroxide 1-2 parts of KH-550 silane coupling agent 8-12 parts of low molecular weight phenolic resin Citric acid 0.5-1 part 100 portions of deionized water.

[0014] Furthermore, the flame retardant for plywood, after preparation, needs to be sealed and stored in a cool, ventilated place at 25-30℃. Before use, it needs to be stirred at low speed for 10 minutes to re-mix.

[0015] Furthermore, the preparation method for flame-retardant plywood using a plywood flame retardant includes the following steps: Step S1: Single-board preprocessing Step S1.1: The logs are peeled, cut, steamed and softened in sequence, and finally rotary cut into veneers, which are then dried to obtain veneers with a moisture content of 10%. Step S1.2: Immerse the veneer in a 2.4-3.0 mol / L solution of NaOH and sodium sulfite for a period of time, then wash it with water until neutral, and then dry it until the water content is 8-10%. Step S2, Flame Retardant Treatment Step S2.1: Immerse the veneer that has been pretreated in step S1 completely in the plywood flame retardant, impregnate it under vacuum conditions, and then dry it for later use. Step S2.2: Coat both sides of the veneer treated in step S2.1 with adhesive, stack them in the direction of grain, and add flame-retardant fiber paper between adjacent layers to obtain the assembled plywood. Step S3, Hot pressing curing Step S3.1: Perform the first hot pressing on the assembled plywood to obtain the pre-pressed plywood; Step S3.2: Then, the pre-pressed plywood is subjected to a second hot press to obtain medium-pressed plywood. Step S3.3: Then, the plywood after medium pressure is subjected to a third hot press to obtain plywood after high pressure. Step S3.4: Then, the plywood under high pressure is slowly cooled to room temperature while maintaining pressure to obtain the hot-pressed and cured plywood. Step S4, Post-processing The plywood obtained by hot pressing and curing in step S3 is cured at room temperature for 72 hours and dried to a moisture content of 8-10%. After edge trimming and inspection, flame-retardant plywood based on guanidine phosphate is obtained.

[0016] Furthermore, in step S1.2, when the single board is immersed in the mixture of NaOH and sodium sulfite, it needs to be treated at a temperature of 60-80℃ for 2-4 hours before being washed with water.

[0017] Furthermore, in step S2.1, when the pretreated veneer is completely immersed in the flame retardant for plywood, it is specifically immersed for 4-6 hours under vacuum conditions of 0.08-0.1MPa, and then dried at 80°C for later use. In step S2.2, the thickness of the flame-retardant fiber paper placed between adjacent layers is 0.1 mm.

[0018] Furthermore, in step S3.1, during the first hot pressing, the temperature is 60-80℃, the pressure is 1-2MPa, and the hot pressing time is 30min; In step S3.2, during the second hot pressing, the temperature is 120-140℃, the pressure is 2-3MPa, and the hot pressing time is 60min, to obtain plywood under medium pressure; In step S3.3, during the third hot pressing, the temperature is 160℃, the pressure is 3-4MPa, and the hot pressing time is 30min, resulting in a medium-pressure plywood. In step S3.4, the pressure holding and slow cooling to room temperature takes at least 2 hours.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The flame retardant for plywood provided by this invention improves the limiting oxygen index of plywood by utilizing the synergistic flame retardant effect of guanidine phosphate and nano-magnesium hydroxide (phosphorus-nitrogen-magnesium). In vertical burning tests, there was no dripping or open flame spread, and the burning time was significantly shortened, effectively blocking the chain reaction. Compared with ordinary plywood, its flame retardant efficiency is nearly doubled, solving the problem of poor plywood combustion performance, greatly improving flame retardant performance, and breaking through the bottleneck of combustion performance.

[0020] (2) The flame retardant for plywood provided by the present invention has a main component, guanidine phosphate, which decomposes at high temperature to produce polyphosphoric acid, which can catalyze the wood to form char. The MgO generated by the decomposition of nano-magnesium hydroxide can fill the micropores and form a double protective structure. The char layer is dense and stable, which can effectively block flames and combustible gases, and greatly improve the fire resistance limit of plywood, thus meeting the high-level fire protection requirements of buildings.

[0021] (3) The flame retardant for plywood provided by the present invention has nano magnesium hydroxide as its component, which has the functions of heat absorption and cooling and adsorption of smoke particles. When combined with a halogen-free system, it significantly reduces the total smoke production (TSP) of plywood and the smoke density rating (SDR) also decreases, thereby improving visibility during a fire and solving the hidden danger of "excessive smoke and choking".

[0022] (4) The flame retardant for plywood provided by the present invention adopts a halogen-free system. Through cross-linking anchoring, it reduces the pyrolysis volatilization of the adhesive, reduces the release of toxic gases such as carbon monoxide and formaldehyde, avoids the generation of highly corrosive toxic gases such as hydrogen halides, and solves the hidden danger of "releasing toxic gases".

[0023] (5) The plywood prepared by the present invention based on flame retardant for plywood has less loss of mechanical properties, can meet the load-bearing requirements, has strong stability and good environmental performance.

[0024] (6) The method for preparing flame retardant for plywood provided by this invention does not require additional complex equipment and has good compatibility with existing plywood production lines, requiring only the addition of auxiliary devices. This solution has strong compatibility, low modification difficulty, and controllable cost. At the same time, the energy consumption of preparation is reduced through optimized design, and there is no harmful waste emission, which meets environmental protection requirements. In addition, the preparation method is simple, less affected by the environment, suitable for mass production of various scales, and its process for treating plywood is simple to operate and the parameters are easy to control.

[0025] (7) The flame retardant for plywood provided by this invention has excellent flame retardant properties, mechanical properties, and environmental protection properties, thereby expanding the application scenarios of plywood and contributing to the upgrading of wood-structured buildings. This flame retardant can break through the application limitations of traditional plywood in high-rise buildings, public buildings, and other scenarios with strict fire protection requirements, providing core material support for the large-scale and high-end development of wood-structured buildings. Its promotion and application can promote the popularization of wood-structured buildings in residential buildings, stadiums, cultural and tourism facilities, and other fields.

[0026] (8) The technical solution provided by this invention provides a scientific basis for the research and development of green flame-retardant wood materials by systematically studying the combustion behavior, thermal decomposition characteristics and mechanical property change law of flame-retardant LVL, and promotes technological innovation and industrial upgrading in the field of fireproof materials for wood structure buildings. Attached Figure Description

[0027] The disclosure of this invention is illustrated with reference to the accompanying drawings. It should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. In the drawings, the same reference numerals are used to refer to the same parts. Wherein: Figure 1 Thermogravimetric analysis curves of a 5% flame retardant concentration in the comparative example; Figure 2 Thermogravimetric analysis curves of the flame retardant concentration at 10% in the comparative example; Figure 3 This is the thermogravimetric analysis curve of the flame retardant concentration of 15% in the comparative example. Detailed Implementation

[0028] It is readily understood that, based on the technical solution of this invention, those skilled in the art can propose various interchangeable structural methods and implementations without altering the essential spirit of the invention. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this invention and should not be considered as the entirety of the invention or as limitations or restrictions on the technical solution of this invention.

[0029] The technical solution of this application will now be described in detail with reference to the accompanying drawings. The description of exemplary embodiments is merely for illustrative purposes and is by no means a limitation on the invention or its application or use.

[0030] Example 1 The preparation method of the flame retardant for plywood used in this embodiment includes the following steps: Step Sa: Add 100 parts of preheated deionized water at 50°C to a water bath constant temperature reactor, and stir at 150 r / min. Slowly add 25 parts of guanidine phosphate in three equal portions, with an interval of 5 min. After all the guanidine phosphate has been added, increase the speed to 200 r / min and continue stirring until completely dissolved to obtain solution A, which is transparent. Step Sb: Add 5 parts of silane-modified nano magnesium hydroxide to the solution A, turn on the ultrasonic-assisted device with a power of 200W, and stir at a speed of 800r / min for 1h to obtain a stable suspension B, which is white. Step Sc: Turn off the ultrasonic auxiliary device, lower the water bath temperature to 40℃ (strict control required), and stir continuously at a speed of 250r / min. First, add 1 part of KH-550 silane coupling agent to the stable suspension B, stir for 15min, and then add 12 parts of low molecular weight phenolic resin at a dropping rate of 5mL / min. After stirring for 30min, solution C is obtained, which is homogeneous and viscous. Step Sd: Add 0.5 parts of citric acid dropwise to solution C. During the dropwise addition process, stir continuously at a speed of 200 r / min. Use high-precision pH monitoring to monitor the pH value. Stir for 15 seconds after each drop of citric acid is added. After adjusting the pH to stabilize at 5.0, let it stand at room temperature for 10 minutes, and then pass it through a 200-mesh sieve to obtain the flame retardant for plywood. (Store in a cool, ventilated place at 25℃. Stir at low speed for 10 minutes to re-mix before use).

[0031] The components of the flame retardant for plywood prepared in this embodiment are: 25 parts of guanidine phosphate 5 parts of silane-modified nano magnesium hydroxide 1 part of KH-550 silane coupling agent 12 parts of low molecular weight phenolic resin Citric acid 0.5 parts 100 parts deionized water The method for preparing flame-retardant plywood based on guanidine phosphate used in this embodiment includes the following steps: Step S1: Single-board preprocessing Step S1.1: The logs are peeled, cut, steamed and softened in sequence, and finally rotary cut into 1.5mm thick veneers, which are then dried to obtain veneers with a moisture content of 10%. Step S1.2: Immerse the veneer in a 2.4 mol / L NaOH and sodium sulfite mixture at 80°C for 2 hours to partially remove lignin and hemicellulose, enlarge cell wall micropores, wash with water until neutral, and then dry to a moisture content of 10%. Step S2, Flame Retardant Treatment Step S2.1: Immerse the veneer that has been pretreated in step S1 completely in a 15% flame retardant for plywood under a vacuum of 0.08 MPa for 6 hours, and then dry it at 80°C for later use. Step S2.2: Coat both sides of the veneer processed in step S2.1 with adhesive, stack them in the direction of grain, and place 0.1mm thick flame-retardant fiber paper between adjacent layers to obtain the assembled plywood. Step S3, Hot pressing curing Step S3.1: The plywood after assembly is hot-pressed for 30 minutes at a temperature of 60°C and a pressure of 2MPa to obtain pre-pressed plywood. Step S3.2: Then, the pre-pressed plywood is hot-pressed for 60 minutes at a temperature of 120°C and a pressure of 3MPa to obtain medium-pressed plywood. Step S3.3: Then, the medium-pressure plywood is hot-pressed for 30 minutes at a temperature of 160℃ and a pressure of 3MPa to obtain high-pressure plywood. Step S3.4: Then, the plywood under high pressure is slowly cooled to room temperature for 2 hours to reduce internal stress. Step S4, Post-processing After the plywood is hot-pressed and cured in step S3, it is cured at room temperature for 72 hours and dried until the moisture content is 10%. After the edges are cut and inspected, flame-retardant plywood is obtained.

[0032] Example 2 The preparation method of the flame retardant for plywood used in this embodiment includes the following steps: Step Sa: Add 100 parts of preheated deionized water at 50°C to a water bath constant temperature reactor, and stir at 150 r / min. Slowly add 30 parts of guanidine phosphate in three equal portions with 5 min intervals. After all the guanidine phosphate has been added, increase the speed to 200 r / min and continue stirring until completely dissolved to obtain solution A, which is transparent. Step Sb: Add 3 parts of silane-modified nano magnesium hydroxide to the solution A, turn on the ultrasonic-assisted device with a power of 200W, and stir at a speed of 800r / min for 1h to obtain a stable suspension B, which is white. Step Sc: Turn off the ultrasonic auxiliary device, lower the water bath temperature to 40℃ (strict control required), and stir continuously at a speed of 250r / min. First, add 2 parts of KH-550 silane coupling agent to the stable suspension B, stir for 15min, and then add 8 parts of low molecular weight phenolic resin at a dropping rate of 5ml / min. After stirring for 30min, solution C is obtained, which is homogeneous and viscous. Step Sd: Add 1 part citric acid to solution C. During the addition process, stir continuously at 200 r / min, monitor the pH value with high precision, and stir for 15 seconds after each drop of citric acid is added. After adjusting the pH to stabilize at 5.5, let it stand at room temperature for 10 minutes, and then pass it through a 200-mesh sieve to obtain the flame retardant for plywood. (Store the finished product in a cool, ventilated place at 30℃. Stir at low speed for 10 minutes to re-mix before use).

[0033] 30 parts of guanidine phosphate 3 parts of silane-modified nano magnesium hydroxide 2 parts of KH-550 silane coupling agent 8 parts of low molecular weight phenolic resin 1 part citric acid 100 portions of deionized water.

[0034] The method for preparing flame-retardant plywood based on guanidine phosphate used in this embodiment includes the following steps: Step S1: Single-board preprocessing Step S1.1: The logs are peeled, cut, steamed and softened in sequence, and finally rotary cut into 1.8mm veneers, which are then dried to obtain veneers with a moisture content of 10%. Step S1.2: Immerse the veneer in a 3.0 mol / L NaOH and sodium sulfite mixture at 60°C for 4 hours to partially remove lignin and hemicellulose, enlarge cell wall micropores, wash with water until neutral, and then dry to a moisture content of 8%. Step S2, Flame Retardant Treatment Step S2.1: Immerse the veneer that has been pretreated in step S1 completely in a 5% flame retardant for plywood, and immerse it in a vacuum of 0.1 MPa for 4 hours, and then dry it at 80°C for later use. Step S2.2: Coat both sides of the veneer processed in step S2.1 with adhesive, stack them in the direction of grain, and place 0.1mm thick flame-retardant fiber paper between adjacent layers to obtain the assembled plywood. Step S3, Hot pressing curing Step S3.1: The plywood after assembly is hot-pressed for 30 minutes at a temperature of 80°C and a pressure of 1MPa to obtain pre-pressed plywood. Step S3.2: Then, the pre-pressed plywood is hot-pressed for 60 minutes at a temperature of 140℃ and a pressure of 2MPa to obtain medium-pressed plywood. Step S3.3: Then, the medium-pressure plywood is hot-pressed for 30 minutes at a temperature of 160℃ and a pressure of 4MPa to obtain high-pressure plywood. Step S3.4: Then, the plywood under high pressure is slowly cooled to room temperature for more than 2 hours to reduce internal stress. Step S4, Post-processing After the plywood was hot-pressed and cured in step S3, it was cured at room temperature for 72 hours and dried to a moisture content of 8%. After the edges were cut and inspected, flame-retardant plywood was obtained.

[0035] Example 3 The preparation method of the flame retardant for plywood used in this embodiment includes the following steps: Step Sa: Add 100 parts of preheated deionized water at 50°C to a water bath constant temperature reactor, and stir at 150 r / min. Slowly add 25 parts of guanidine phosphate in three equal portions, with an interval of 5 min. After all the guanidine phosphate has been added, increase the speed to 200 r / min and continue stirring until completely dissolved to obtain solution A, which is transparent. Step Sb: Add 5 parts of silane-modified nano magnesium hydroxide to the solution A, turn on the ultrasonic-assisted device with a power of 200W, and stir at a speed of 800r / min for 1h to obtain a stable suspension B, which is white. Step Sc: Turn off the ultrasonic auxiliary device, lower the water bath temperature to 40℃ (strict control required), and stir continuously at a speed of 250r / min. First, add 1 part of KH-550 silane coupling agent to the stable suspension B, stir for 15min, and then add 12 parts of low molecular weight phenolic resin at a dropping rate of 5mL / min. After stirring for 30min, solution C is obtained, which is homogeneous and viscous. Step Sd: Add 0.5 parts of citric acid dropwise to the solution C. During the dropwise addition process, stir continuously at a speed of 200 r / min. Use high-precision pH monitoring to monitor the pH value. Stir for 15 seconds after each drop of citric acid is added. After adjusting the pH to stabilize at 4.5, let it stand at room temperature for 10 minutes, and then pass it through a 200-mesh sieve to obtain the flame retardant for plywood. (The prepared flame retardant for plywood should be stored in a cool, ventilated place at 26℃. Before use, it needs to be stirred at low speed for 10 minutes to re-mix.)

[0036] The components of the flame retardant for plywood prepared in this embodiment are: 25 parts of guanidine phosphate 5 parts of silane-modified nano magnesium hydroxide 1 part of KH-550 silane coupling agent 12 parts of low molecular weight phenolic resin Citric acid 0.5 parts 100 portions of deionized water.

[0037] The method for preparing flame-retardant plywood based on guanidine phosphate used in this embodiment includes the following steps: Step S1: Single-board preprocessing Step S1.1: The logs are peeled, cut, steamed and softened in sequence, and finally rotary cut into 2mm veneers, which are then dried to obtain veneers with a moisture content of 10%. Step S1.2: Immerse the veneer in a 2.4 mol / L NaOH and sodium sulfite mixture at 80°C for 2 hours to partially remove lignin and hemicellulose, enlarge cell wall micropores, wash with water until neutral, and then dry to a moisture content of 10%. Step S2, Flame Retardant Treatment Step S2.1: Immerse the veneer that has been pretreated in step S1 completely in a 10% flame retardant for plywood under a vacuum of 0.08 MPa for 6 hours, and then dry it at 80°C for later use. Step S2.2: Coat both sides of the veneer processed in step S2.1 with adhesive, stack them in the direction of grain, and place 0.1mm thick flame-retardant fiber paper between adjacent layers to obtain the assembled plywood. Step S3, Hot pressing curing Step S3.1: The plywood after assembly is hot-pressed for 30 minutes at a temperature of 60°C and a pressure of 2MPa to obtain pre-pressed plywood. Step S3.2: Then, the pre-pressed plywood is hot-pressed for 60 minutes at a temperature of 120°C and a pressure of 3MPa to obtain medium-pressed plywood. Step S3.3: Then, the medium-pressure plywood is hot-pressed for 30 minutes at a temperature of 160℃ and a pressure of 3MPa to obtain high-pressure plywood. Step S3.4: Then, the plywood under high pressure is slowly cooled to room temperature for 2 hours to reduce internal stress. Step S4, Post-processing After the plywood is hot-pressed and cured in step S3, it is cured at room temperature for 72 hours and dried until the moisture content is 10%. After the edges are cut and inspected, flame-retardant plywood is obtained.

[0038] Comparative Example 1 The flame-retardant treated veneers obtained in step S2.1 of Examples 1-3 were compared for performance using flame retardants based on ammonium dihydrogen phosphate (DAP system) and borax-boric acid (PP system). A three-factor, three-level orthogonal test was designed. The other two factors were T (veneer thickness), namely 1.5 mm (T1), 1.8 mm (T2), and 2.0 mm (T3); flame retardant concentrations were 5% (B1), 10% (B2), and 15% (B3). At the same time, blank control groups 10, 11, and 12 without flame retardant were set up, for a total of 12 groups of samples, as shown in Table 1.

[0039] It should be noted that the flame retardant for plywood used in Examples 1-3 of the present invention is referred to as the GUP system because its core component is guanidine phosphate. In Table 1, Group 3 represents the flame-retardant treated veneers obtained in step S2.1 of Example 1, Group 6 represents Example 2, and Group 9 represents Example 3.

[0040] Table 1

[0041] (1) Combustion performance analysis Thermogravimetric analysis (TGA) curves of various flame-retardant veneers and untreated veneers, such as Figure 1 The thermogravimetric analysis curves for a 5% flame retardant concentration are shown below. Figure 2 The thermogravimetric analysis curves for a 10% flame retardant concentration are shown below, as well as... Figure 3 The thermogravimetric analysis (TGA) curves for flame retardant concentrations of 15% show that each group of samples exhibits a typical three-stage thermal decomposition trend. In the first stage (approximately 30-150 °C), mass loss is relatively small, mainly corresponding to the removal of adsorbed moisture and a small amount of low-molecular-weight volatiles. The second stage (approximately 20-380 °C) is the main pyrolysis stage, with a steeper curve drop, corresponding to the rapid decomposition of hemicellulose and cellulose in the wood. In the third stage (approximately 380-600 °C), the curve drop becomes gentler, indicating that the slow decomposition of lignin and carbonization products mainly occurs in this stage. The curves show that DAP and PP can slow down the mass loss of veneer in the first and second heating stages, while GUP, although it can accelerate the mass loss of veneer in the early stages, can effectively form a carbonized layer, protecting the veneer from further thermal decomposition, thereby increasing its char content.

[0042] The above analysis and summary of the characteristic points of each group of thermogravimetric curves are shown in Table 2.

[0043] Table 2

[0044] It should be noted that "T" 5% "T" represents the temperature at which the mass loss is 5%; that is, the temperature at which thermal decomposition begins. 50% "The temperature at which the mass fraction is 50%;" T Vmax "This is the temperature at which the mass loss rate is at its maximum."

[0045] Thermogravimetric analysis results showed that the thermal stability and high-temperature resistance of all samples were significantly improved after flame retardant treatment. Compared with the untreated sample (T... 5% At 50℃, T 50% The temperature is 354℃, T Vmax Compared to the initial decomposition temperature (T0.05) of 342℃ and 18.1% char residue, the initial decomposition temperatures (T0.05) of each flame-retardant system sample were significantly different. 5% The temperature generally increased to 59-72℃, indicating that the flame retardant effectively delayed the initiation of thermal decomposition. Among them, the PP system showed the most significant performance, with T... 5% The maximum temperature can reach 72℃ (PP-10%), which is about 22℃ higher than that of the untreated sample; DAP system sample T 5% The temperature was 66-68℃, which was significantly higher than the original sample. Although the GUP system sample was slightly lower than other flame retardant groups in the range of 59-67℃, it was still nearly 10-17℃ higher than the untreated sample, indicating that its initial stability was also improved.

[0046] During the main decomposition stage, sample T after flame retardant treatment 50% The temperatures were concentrated between 303-348 °C, showing little change compared to the untreated sample (354 °C), but each system exhibited different pyrolysis characteristics. The Ta of the DAP system... 50% Maintain at 337–348 ℃, T Vmax The corresponding temperature is 338-354℃, indicating that it mainly delays thermal decomposition by improving thermal stability; the T of the PP system 50% The temperature range is 327-345℃, T Vmax The temperature ranged from 326 to 352℃, slightly earlier than expected, indicating that phosphate flame retardants can catalyze the char formation of wood at high temperatures, thus slowing down the pyrolysis rate; the T of the GUP system... 50% Temperatures dropped to 303-335℃, T Vmax The corresponding temperature is 285-294℃, indicating that it can promote the dehydration and carbonization of materials at relatively low temperatures, forming a dense carbon layer to protect the internal structure.

[0047] At high temperatures (600-800℃), the char residue of the flame-retardant samples significantly increased. The residual mass of the untreated samples was only 18.1%, while the flame-retardant treated samples all showed higher charring capacity, with the DAP system at 19.2-20.9%, the PP system at 18.3-23.1%, and the GUP system reaching a maximum of 26.9-29.3%. In particular, the GUP-15% sample achieved a residual rate of 29.3%, which was about 11.2 percentage points higher than the untreated sample, demonstrating a very strong charring protection effect.

[0048] In summary, flame retardant treatment significantly improves the thermal decomposition characteristics of the veneer. 5% The temperature is increased by approximately 10-22 °C, the char residue is increased by 5-11%, and the overall thermal stability is enhanced. Among them, the GUP system performs the best, not only can it rapidly generate a protective char layer at a lower temperature, but it also retains nearly 30% of the residual mass at 800 °C, demonstrating a significant phosphorus-nitrogen synergistic flame retardant effect, making it the flame retardant modification system with the best overall performance.

[0049] (2) Oxygen index The results of the oxygen index test are shown in Table 3.

[0050] Table 3

[0051] Oxygen index testing results showed that flame retardant treatment significantly improved the flame resistance of LVL materials. The average oxygen indices of the untreated control group samples (numbered 10-12) were 26.9%, 26.5%, and 26.4%, respectively, all within the B2 grade, classifying them as combustible materials. However, after modification with different flame retardant systems, the average oxygen indices of the samples significantly increased to 29.0%–48.0%, with some samples successfully upgraded from combustible to flame-retardant (B1 grade), demonstrating the significant superiority of the flame-retardant treatment.

[0052] In terms of flame retardant types, the GUP system performed best. The sample with 1.5mm + GUP + 15% had an oxygen index as high as 48.0%, an improvement of about 21% compared to the untreated sample, reaching a high-grade B1 level, indicating its significant effect in promoting charring and inhibiting combustion reactions. Other GUP group samples also had oxygen indices exceeding 33%, also reaching B1 level, demonstrating that the synergistic effect of phosphorus and nitrogen effectively improves the flame resistance and stability of the material. The PP system samples had an oxygen index between 29.0% and 32.5%, an improvement of about 3-6% compared to the untreated sample. The 1.5mm + PP + 10% sample reached 32.5%, reaching B1 level, indicating that PP has a good flame retardant effect, but the overall improvement was lower than that of the GUP system. The DAP system samples had an oxygen index of 29.2% to 32.2%, also significantly higher than the control group, showing some flame retardant performance, but the improvement was relatively limited.

[0053] In terms of the effect of thickness, the oxygen index generally decreased slightly as the sample thickness increased from 1.5 mm to 2.0 mm. This is mainly because the flame retardant penetration in the thicker samples was insufficient, and the distribution of the flame retardant components was uneven, resulting in a slight weakening of the flame retardant effect. For example, the 1.5 mm samples generally reached the B1 rating, while some of the 2.0 mm samples were still at the B2 rating.

[0054] Overall, the oxygen index of the veneer improved by an average of 3-21% after flame retardant modification, significantly enhancing its flame retardant performance. Among them, the GUP system showed the best overall performance, especially the 1.5 mm + GUP + 15% sample, which not only had the highest oxygen index but also achieved a qualitative leap from flammable grade (B2) to flame-retardant grade (B1). This fully demonstrates that flame retardant treatment can significantly improve the flame resistance and thermal stability of the veneer, providing strong support for its application in high-safety fields.

[0055] (3) Vertical combustion test The results of vertical combustion are shown in Table 4.

[0056] Table 4

[0057] It should be noted that "t1, t2" are the afterflame times of a single sample, and "t2+t3" are the afterflame times of a single sample after the second application of flame.

[0058] Vertical burning test results show that flame retardant treatment significantly improves the flame resistance of the veneer material. The control group samples (numbered 10-12) that were not treated with flame retardant had burning durations of 60s, 67s, and 75s, respectively, and all of them were unable to self-extinguish and burned violently, thus belonging to unrated samples (no flame retardant properties), demonstrating the high flammability of the veneer substrate.

[0059] In contrast, the flame-retardant modified samples exhibited extremely superior flame-retardant effects. The GUP system samples showed the most outstanding flame-retardant performance; the 1.5mm + GUP + 15% (sample 3), 1.8mm + GUP + 5% (sample 6), and 2.0mm + GUP + 10% (sample 9) samples showed almost no afterflame during testing, with burning times of 0-4 seconds, ultimately achieving a V-0 rating, the highest flame-retardant level. This demonstrates that the GUP system can rapidly form a dense carbon layer, effectively isolating oxygen and heat, thus achieving rapid self-extinguishing and exhibiting extremely strong flame-retardant stability.

[0060] Overall, the burning time of the veneer after flame retardant treatment was generally reduced sharply from 60-75 seconds to 0-26 seconds, and most samples met the UL-94 V-0 or V-1 standard, indicating that the flame retardant system significantly inhibited the combustion process and improved the safety of the material. Comparing the three flame retardant systems, the GUP system performed best, achieving the fastest extinguishing speed, the lowest combustion loss, and the most stable flame retardant effect. Flame retardant modification transformed the veneer from a flammable material into a highly flame-retardant material, with some samples reaching the highest UL-94 V-0 rating. In particular, the 2mm + GUP + 10% sample completely self-extinguished in just 0 seconds, fully demonstrating the significant performance advantages and engineering application value of the flame retardant treatment.

[0061] (4) Moisture content test The moisture content test results are shown in Table 5.

[0062] Table 5

[0063] The moisture content test results showed that all samples had a moisture content between 9.8% and 10.6%, which falls within the 6%-14% range required by the standard GB / T20241-2021 "Laminated Veneer Materials". This indicates that the samples were properly controlled during drying and curing, and no moisture absorption or cracking occurred, demonstrating good overall stability. The control group (numbers 10-12) maintained a moisture content of 9.8%-10.0%, indicating stable substrate condition. The DAP system samples (numbers 1, 4, and 7) had slightly higher moisture content but still met the standard, showing that although phosphate flame retardants have some hydrophilicity, they did not significantly affect the moisture balance of the material. The PP system samples (numbers 2, 5, and 8) showed the smallest fluctuation in moisture content, indicating good compatibility with the substrate. The GUP system samples (numbers 3, 6, and 9) had relatively low moisture content, indicating that their phosphorus-nitrogen composite structure has good moisture-proof properties after curing. Overall, the moisture content distribution of each flame retardant system sample was concentrated and stable. In particular, the GUP system maintained excellent moisture content control while taking into account high flame retardant performance, which ensured the dimensional stability and long-term service performance of the material.

[0064] Based on the above performance data, it can be seen that the GUP system samples exhibit significant improvements in all aspects, with groups 6 and 9 showing the most outstanding performance. Therefore, the flame retardant for plywood provided by this invention has excellent flame retardant modification effects.

[0065] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A method for preparing a flame retardant for plywood, characterized in that, Includes the following steps: Step Sa: Add an appropriate amount of preheated deionized water at 50°C to a water bath constant temperature reaction vessel, keep stirring, add an appropriate amount of guanidine phosphate, and then stir until completely dissolved to obtain solution A; Step Sb: Add an appropriate amount of silane-modified nano-magnesium hydroxide to solution A and stir to obtain a stable suspension B; Preparation of flame retardant for plywood Step Sc: Lower the water bath temperature, continue stirring, and add an appropriate amount of KH-550 silane coupling agent and low molecular weight phenolic resin to the stable suspension B in sequence. Stir for a period of time to obtain solution C. Step Sd: Add an appropriate amount of citric acid to the solution C, adjust the pH to be stable at 4.5-5.5, and let it stand at room temperature to obtain a flame retardant for plywood.

2. The method for preparing a flame retardant for plywood according to claim 1, characterized in that, In step Sa, when preheating deionized water in a water bath constant temperature reactor, the stirring speed should be maintained at 150 r / min, and guanidine phosphate should be added slowly in three equal portions at 5-minute intervals. After all the guanidine phosphate has been added, the stirring speed should be increased to 200 r / min and stirred continuously until completely dissolved to obtain solution A. In step Sb, when adding silane-modified nano-magnesium hydroxide to solution A, an ultrasonic auxiliary device with a power of 200W needs to be turned on and stirred at a speed of 800r / min for 1h to obtain a stable suspension B. In step Sc, the ultrasonic auxiliary device is first turned off, then the water bath temperature is lowered to 40°C, and the mixture is continuously stirred at a speed of 250 r / min. KH-550 silane coupling agent is first added to the stable suspension B, and after stirring for 15 min, low molecular weight phenolic resin is added at a dropping rate of 5 mL / min. After stirring for 30 min, solution C is obtained. In step Sd, when adding citric acid to solution C, the mixture should be continuously stirred at a speed of 200 r / min, and the pH value should be monitored with high precision. For each drop of citric acid added, the mixture should be stirred for 15 seconds. After adjusting the pH to be stable at 4.5-5.5, the mixture should be allowed to stand at room temperature for 10 minutes, and then passed through a 200-mesh filter to obtain the flame retardant for plywood.

3. The method for preparing a flame retardant for plywood according to claim 2, characterized in that, In step Sd, the pH is adjusted to be stable at 5.0-5.

5.

4. The flame retardant for plywood prepared according to the method for preparing a flame retardant for plywood as described in claim 3, characterized in that, Includes the following components: 25-30 parts of guanidine phosphate 3-5 parts of silane-modified nano magnesium hydroxide 1-2 parts of KH-550 silane coupling agent 8-12 parts of low molecular weight phenolic resin Citric acid 0.5-1 part 100 portions of deionized water.

5. The flame retardant for plywood according to claim 4, characterized in that, The flame retardant for plywood, after preparation, needs to be sealed and stored in a cool, ventilated place at 25-30℃. Before use, it needs to be stirred at low speed for 10 minutes to re-mix.

6. The flame retardant for plywood according to claim 4, characterized in that, The preparation method of flame-retardant plywood using the aforementioned flame retardant includes the following steps: Step S1: Single-board preprocessing Step S1.1: The logs are peeled, cut, steamed and softened in sequence, and finally rotary cut into veneers with a thickness of 1.5-2mm. Then they are dried to obtain veneers with a moisture content of 10%. Step S1.2: Immerse the single board in a 2.4-3.0 mol / L NaOH and sodium sulfite mixture for a period of time, then wash it with water until neutral, and then dry it until the water content is 8-10%. Step S2, Flame Retardant Treatment Step S2.1: Immerse the veneer pretreated in step S1 completely in the plywood flame retardant with a concentration of 5-15% under vacuum conditions, and then dry it for later use. Step S2.2: Coat both sides of the veneer processed in step S2.1 with adhesive, stack them in the direction of grain, and add flame-retardant fiber paper between adjacent layers to obtain the assembled plywood. Step S3, Hot pressing curing Step S3.1: Perform a first hot press on the assembled plywood to obtain a pre-pressed plywood; Step S3.2: The pre-pressed plywood is then subjected to a second hot press to obtain a medium-pressed plywood; Step S3.3: Then, the plywood after medium pressure is subjected to a third hot press to obtain plywood after high pressure. Step S3.4: Then, the plywood under high pressure is slowly cooled to room temperature while maintaining pressure to obtain the hot-pressed and cured plywood. Step S4, Post-processing The plywood obtained after hot pressing and curing in step S3 is cured at room temperature for 72 hours and dried to a moisture content of 8-10%. After edge trimming and inspection, flame-retardant plywood is obtained.

7. The flame retardant for plywood according to claim 5, characterized in that, In step S1.2, when the single board is immersed in a mixture of NaOH and sodium sulfite, it needs to be treated at a temperature of 60-80℃ for 2-4 hours before being washed with water.

8. The method for preparing guanidine phosphate composite flame-retardant plywood according to claim 1, characterized in that, In step S2.1, when the pretreated veneer is completely immersed in the flame retardant for plywood, it is specifically immersed for 4-6 hours under vacuum conditions of 0.08-0.1MPa, and then dried at 80°C for later use. In step S2.2, the thickness of the flame-retardant fiber paper placed between adjacent layers is 0.1 mm.

9. The method for preparing guanidine phosphate composite flame-retardant plywood according to claim 1, characterized in that, In step S3.1, during the first hot pressing, the temperature is 60-80℃, the pressure is 1-2MPa, and the hot pressing time is 30min. In step S3.2, during the second hot pressing, the temperature is 120-140℃, the pressure is 2-3MPa, and the hot pressing time is 60min, to obtain plywood under medium pressure. In step S3.3, during the third hot pressing, the temperature is 160℃, the pressure is 3-4MPa, and the hot pressing time is 30min, to obtain plywood under medium pressure. In step S3.4, the pressure holding and slow cooling to room temperature takes at least 2 hours.

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