A formaldehyde-free adhesive system and a method for preparing plywood using the same

Abstract

This invention relates to the field of adhesive technology, and discloses a formaldehyde-free adhesive system and a method for preparing plywood using the same. The formaldehyde-free adhesive system includes component A and component B, which are applied to the surface of a substrate, respectively. Component A is an isocyanate adhesive, and component B includes an acrylic copolymer emulsion with a glass transition temperature (Tg) of 100℃ ≤ Tg ≤ 125℃. Applying components A and B to the substrate surface separately improves the ease of application and the synergistic effect of the two components at the bonding interface. The high-Tg acrylic copolymer emulsion in component B has good adhesion and water resistance, effectively covering and filling defects on the substrate surface, especially burrs and grooves, and synergistically improving bond strength and water resistance with the isocyanate adhesive. Plywood prepared using the adhesive system of this invention has low formaldehyde release, high bond strength, and excellent resistance to immersion peeling.

Description

Technical Field

[0001] This invention relates to the field of adhesive technology, specifically to a formaldehyde-free adhesive system and a method for preparing plywood using the same. Background Technology

[0002] In recent years, with increasingly stringent environmental protection requirements and growing consumer attention to green home furnishing products, formaldehyde-free engineered wood products have experienced rapid development, and formaldehyde-free technology has become one of the important development directions for the engineered wood product industry. Currently, the adhesives used in formaldehyde-free engineered wood products mainly include isocyanate adhesives and biomass adhesives. Among them, isocyanate adhesives are more widely used in actual production due to their superior overall performance.

[0003] Isocyanate adhesives themselves do not contain free formaldehyde, are generally odorless, and release relatively few volatile organic compounds, exhibiting good environmental performance. Furthermore, the polyurea structure formed after isocyanate curing possesses high strength and good chemical stability; therefore, engineered wood products made with isocyanate adhesives typically exhibit good mechanical and performance properties. Currently, polymethylene polyphenyl polyisocyanate is the most widely used isocyanate adhesive in the field of veneer engineered wood products.

[0004] In existing technologies, isocyanate adhesives are typically applied to the surface of veneers in the production of engineered wood panels using atomized spraying. This can be achieved through air spraying, airless spraying, or by using ultrasonic or cyclone-assisted atomization methods. The veneer is then assembled and hot-pressed to produce formaldehyde-free engineered wood panels. This process improves the uniformity of adhesive application and reduces the amount of adhesive used, thus possessing certain application value.

[0005] However, the aforementioned existing processes typically rely on veneer surfaces with good condition and few defects. Due to the relatively high cost of isocyanate adhesives, lower application rates are often preferred in actual production. With lower application rates, a relatively smooth veneer surface can achieve good bonding; however, when the veneer surface has many burrs, loose fibers, or is rough, the adhesive is prone to insufficient distribution or difficulty in forming effective bonds at key bonding interfaces. This leads to a high defect rate in mass production, further manifesting as insufficient bond strength, unstable dip-peel performance, and even a low pass rate.

[0006] To address the aforementioned issues, existing technologies include pretreatment of veneers to improve their surface condition. For example, specific solvent systems can soften the veneer surface fibers, smoothing out any burrs and improving subsequent bonding. While these methods can improve the adhesiveness of the veneer surface to some extent, the high boiling points of the treatment agents can leave residues after the board is formed. Furthermore, the softening effect on the wood fibers may persist after board formation, potentially affecting the long-term stability and bond strength of the adhesive interface. In other words, while these methods aid in veneer pressing, they are insufficient for improving the high-strength bonding performance of the boards, especially in terms of impregnation peel resistance. Summary of the Invention

[0007] This invention provides a formaldehyde-free adhesive system and a method for preparing plywood using the same system, in order to solve the problem in the prior art that when using isocyanate adhesives to prepare plywood, there is insufficient adhesive distribution and poor bonding effect at the bonding interface for veneers with many burrs on the surface, which makes it difficult to guarantee the bonding strength and impregnation peel performance of the plywood.

[0008] In a first aspect, the present invention provides a formaldehyde-free adhesive system comprising component A and component B for application to the surface of a substrate, wherein component A is an isocyanate adhesive and component B comprises an acrylic copolymer emulsion with a glass transition temperature of 100°C ≤ Tg ≤ 125°C.

[0009] It should be noted that "applying components A and B separately" means that components A and B are not pre-mixed before being applied to the veneer surface, but are applied separately as independent components to the veneer surface. The separate application can be either applying component A first and then applying component B, or applying component B first and then applying component A; components A and B can be applied to the same veneer surface, or they can be applied separately to the opposite adhesive surfaces of adjacent veneers according to the assembly requirements.

[0010] In one optional embodiment, the acrylic copolymer emulsion is copolymerized from hard monomers and soft monomers. In this invention, the specific types and ratios of hard and soft monomers are not particularly limited, as long as the resulting acrylic copolymer emulsion has a glass transition temperature of 100℃ to 125℃ and can be stably dispersed and applied to the surface of wood veneer. Those skilled in the art can adjust the mass ratio of hard and soft monomers according to the Fox equation or conventional emulsion polymerization experience to obtain an acrylic copolymer emulsion with a target Tg range. In this invention, the glass transition temperature (Tg) of the acrylic copolymer emulsion can be determined by differential scanning calorimetry (DSC) or by using product testing data provided by the supplier. When using DSC, the acrylic copolymer emulsion can be dried into a film and then sampled for testing, with the midpoint temperature of the glass transition region in the heat flow curve taken as Tg.

[0011] Preferably, the hard monomer is selected from one or more of methyl methacrylate, styrene, acrylic acid, methacrylic acid, and acrylonitrile; Preferably, the soft monomer is selected from one or more of isooctyl acrylate, n-butyl acrylate, ethyl acrylate, and methyl acrylate.

[0012] In this invention, component B includes an acrylic copolymer emulsion with a glass transition temperature of 100℃≤Tg≤125℃. The acrylic copolymer emulsion is copolymerized from hard monomers and soft monomers. By adjusting the types and ratios of hard and soft monomers, the glass transition temperature of the copolymer can be controlled.

[0013] The acrylic copolymer emulsion of this invention exhibits good wettability and adhesion to wood surfaces. When applied to the surface of wood veneer, it effectively covers and fills defects, especially burrs and grooves. Due to the weak molecular chain mobility, dense molecular arrangement, low swelling degree, and high cohesive force of the high-Tg acrylic copolymer, the resulting film or granules possess good water resistance and structural stability. Simultaneously, after the high-Tg acrylic copolymer emulsion is applied to the wood surface, it quickly forms a film or granules adhering to the wood surface as moisture evaporates, thereby reducing emulsion migration and improving the coverage and filling effect on the wood surface, especially defective areas.

[0014] Furthermore, the formed acrylic film or granules can improve the integrity of the adhesive interface, as well as the bonding strength and water resistance of the adhesive layer. In particular, when the initial acrylic copolymer emulsion Tg is greater than or equal to 100°C, the formed film or granules can withstand boiling water treatment for a longer period of time, which is beneficial for maintaining effective bonding between wood and improving the anti-immersion peeling properties of the board.

[0015] In one optional embodiment, component B further includes a hydroxyl-terminated compound; Preferably, the mass ratio of the acrylic copolymer emulsion to the hydroxyl-terminated compound is 100:(5-20). For example, the mass ratio of the acrylic copolymer emulsion to the hydroxyl-terminated compound is 100:5, 100:10, 100:15, or 100:20.

[0016] Hydroxyl-terminated compounds can react and cure rapidly with isocyanates, thus promoting more complete curing of isocyanates under the same hot-pressing time and improving adhesive strength. Meanwhile, acrylic copolymer emulsions can effectively cover and fill defects on the substrate surface, especially burrs, grooves, etc. Therefore, when acrylic copolymer emulsions are used in combination with hydroxyl-terminated compounds, a synergistic effect can be achieved between surface coverage, defect filling, and accelerated curing, further improving the bonding effect.

[0017] In one optional embodiment, the hydroxyl-terminated compound includes one or more of polyether polyols and small molecule polyols; Preferably, the polyether polyol has a functionality of 2-4 and a number-average molecular weight of 500-6000; more preferably, the number-average molecular weight is 3000-5000; for example, the polyether polyol has a functionality of 2, 3 or 4 and a number-average molecular weight of 500, 1000, 2000, 3000, 4000, 5000 or 6000.

[0018] Preferably, the small molecule polyol refers to an organic polyhydroxy compound containing two or more hydroxyl groups and having a relative molecular mass not exceeding 500. The small molecule polyol includes one or more of ethylene glycol, 1,4-butanediol, triethanolamine, glycerol, pentaerythritol, trimethylolpropane, and trimethylolethane.

[0019] In one optional embodiment, the isocyanate adhesive includes one or more of aromatic polyisocyanates and their modified products, and aliphatic polyisocyanates and their modified products. Preferably, the isocyanate adhesive comprises one or more of toluene diisocyanate and its modified products, diphenylmethane diisocyanate and its modified products, and polymethylene polyphenyl polyisocyanate and its modified products. Preferably, the modified product is a polyether modified product or a polyester modified product; Preferably, the isocyanate adhesive contains 20wt%-34wt% of -NCO, more preferably 30.5wt%-32wt%. For example, the isocyanate adhesive contains 20wt%, 25wt%, 30wt%, 30.5wt%, 31wt%, 32wt%, and 34wt% of -NCO.

[0020] In a second aspect, the present invention provides a method for preparing plywood using any of the formaldehyde-free adhesive systems described in the above-mentioned technical solutions, comprising the following steps: Component A and component B are applied to the surface of the veneer to obtain an adhesive veneer; wherein, component A and component B can be applied to the surface of the veneer by spraying, roller coating, brushing, scraping or conventional adhesive application methods in the art.

[0021] The sizing sheet is combined with other veneers to form a slab blank; Plywood is obtained by hot pressing the slab blank.

[0022] The plywood preparation method provided by the present invention applies component A and component B to the surface of the veneer, allowing component B to act independently on the defective areas of the veneer surface, covering, filling and adjusting defects such as burrs and grooves. Then, the isocyanate adhesive in component A forms an effective cured adhesive structure during the assembly and hot pressing process, thereby improving the integrity and bonding strength of the adhesive interface.

[0023] In one optional embodiment, the amount of component B applied to the veneer surface is 5 g / m². 2 -30g / m 2 ; In one optional embodiment, the amount of component A applied to the veneer surface is 25 g / m². 2 -50g / m 2 The preferred value is 40g / m 2 -50g / m 2 .

[0024] In one alternative embodiment, the veneer comprises a wood veneer; More preferably, it is a wood veneer with burrs, grooves, or loosely fibrous areas on its surface; Preferably, the wood veneer includes eucalyptus veneer, pine veneer, poplar veneer, birch veneer or bamboo veneer; Preferably, the moisture content of the veneer is 8%-25%; more preferably, it is 8%-15%.

[0025] Preferably, the area ratio of burrs and grooves on the surface of the veneer is ≥35%, wherein the area ratio of burrs and grooves on the surface of the veneer refers to the proportion of the area of ​​surface defects such as burrs, grooves, and loose fibers in the total area of ​​the observation area when several observation areas are randomly selected on the surface of the veneer to be glued, and the area is statistically analyzed by visual marking, photo recognition or image analysis.

[0026] Wood veneers contain a certain amount of moisture, which can react with isocyanates to form a polyurea structure, thus facilitating strong bonding. At the same time, considering the characteristics of wood veneer surfaces that are prone to defects such as burrs and grooves, acrylic copolymer emulsions with a glass transition temperature (Tg) ≥ 100℃ can effectively cover and fill the defective areas, thereby improving the bonding strength and water resistance.

[0027] Furthermore, controlling the moisture content of the veneer within the range of 8%-25% helps to balance the requirements of the isocyanate curing reaction with the process adaptability caused by fluctuations in the moisture content of raw materials in actual production, thereby improving the industrial feasibility of this solution.

[0028] In one optional embodiment, the temperature conditions for hot pressing are 120℃-200℃, the pressure conditions are 0MPa-1.5MPa, and the hot pressing factor is 10s / mm-30s / mm. It should be noted that the hot pressing factor in this invention refers to the hot pressing time corresponding to a unit of finished plate thickness, and the unit is s / mm. The finished plate thickness is the thickness of the plywood obtained after the blank is hot-pressed.

[0029] Preferably, the pressure condition is 0.3MPa-1.5MPa.

[0030] In one optional embodiment, the hot pressing is performed using constant pressure hot pressing or segmented pressure hot pressing. The segmented pressure hot pressing includes a high-pressure segment and a low-pressure segment, wherein the hot pressing pressure of the high-pressure segment is greater than that of the low-pressure segment; the high-pressure segment is used to compact the slab and promote full adhesion between adjacent veneers, while the low-pressure segment is used to continue to cure the adhesive while maintaining the interlayer adhesion of the slab.

[0031] In one optional embodiment, the high-pressure section hot-pressing pressure is 1.0MPa to 1.5MPa, and the low-pressure section hot-pressing pressure is 0.3MPa to 0.5MPa; preferably, the high-pressure section hot-pressing time is 170s to 400s, and the low-pressure section hot-pressing time is 56s to 170s.

[0032] In one optional embodiment, the slab is preheated before hot pressing, preferably by microwave preheating; after preheating, the center temperature of the slab is 60℃~100℃, preferably 80℃. Preheating improves the internal temperature uniformity of the slab, allowing it to reach the required adhesive curing temperature more quickly during subsequent hot pressing, thereby shortening the hot pressing time and improving hot pressing efficiency.

[0033] The technical solution of this invention has the following advantages: In the formaldehyde-free adhesive system provided by this invention, component A is an isocyanate adhesive, and component B is an acrylic copolymer emulsion with a glass transition temperature of 100℃≤Tg≤125℃, both applied to the substrate surface. On one hand, the isocyanate adhesive itself does not contain free formaldehyde, which helps reduce formaldehyde release from the board and achieves formaldehyde-free addition. On the other hand, the high glass transition temperature acrylic copolymer emulsion has good adhesion, high cohesive strength, and good water resistance, effectively covering and filling the substrate surface, especially defective areas, and forming a synergistic effect with the isocyanate curing system, thereby improving adhesive strength and water resistance. It is particularly suitable for substrates with rough surfaces or burr defects, effectively improving the overall performance of the final product.

[0034] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed implementation mode

[0035] To better understand the present invention, the following embodiments are provided. However, the following embodiments do not limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with those of other prior arts falls within the protection scope of the present invention.

[0036] For those embodiments where specific experimental steps or conditions are not indicated, they are all operated or carried out according to the conventional experimental steps or conditions in the art. For reagents or instruments whose manufacturers are not indicated, they are all conventional reagent products or instruments that can be obtained through commercial purchase.

[0037] The raw materials involved in the embodiments and comparative examples of the present invention are as follows: Polymethylene polyphenyl polyisocyanate (PMDI), model WANNATEC W20, with -NCO mass content of 30.5% - 32.0%, purchased from Wanhua Chemical.

[0038] Modified isocyanate, model WANNATE PM-300E, with -NCO mass content of 30.5% - 30.6%, purchased from Wanhua Chemical.

[0039] Acrylic copolymer emulsion, model Crysol 6319, which is a copolymer of styrene, methyl methacrylate, butyl acrylate and isooctyl acrylate, with a glass transition temperature (Tg) of 105 °C, purchased from Wanhua Chemical.

[0040] Acrylic copolymer emulsion, model Wantipro 0839, which is a copolymer of styrene, acrylic acid and isooctyl acrylate, with a glass transition temperature (Tg) of 100 °C, purchased from Wanhua Chemical.

[0041] Acrylic copolymer emulsion, model Adwel 1318S, which is a copolymer of styrene, butyl acrylate and isooctyl acrylate, with a glass transition temperature (Tg) of 17 °C, purchased from Wanhua Chemical.

[0042] Acrylic copolymer emulsion, model Lacper 4316, which is a copolymer emulsion of styrene and acrylate, with a glass transition temperature (Tg) of 90 °C, purchased from Wanhua Chemical.

[0043] Acrylic copolymer emulsion, model Crysol 6359, which is a copolymer emulsion of methacrylic acid, methyl methacrylate and butyl acrylate, with a glass transition temperature (Tg) of 125 °C, purchased from Wanhua Chemical.

[0044] Polyether polyol, model WANALYST KC373, with a number average molecular weight of 4000 and a functionality of 3, purchased from Wanhua Chemical.

[0045] Glycerin, commercially available.

[0046] 1,4-Butanediol, commercially available.

[0047] Eucalyptus veneer, moisture content 15%, thickness 2.2mm, surface burrs and grooves ≥35%; Eucalyptus veneer, moisture content 10%, thickness 2.2mm, surface burrs and grooves ≥35%; Eucalyptus veneer, with a moisture content of 8% and a thickness of 2.2mm, has a surface burr and groove area ratio of ≥35%.

[0048] Example 1 This embodiment provides a formaldehyde-free adhesive system and a method for preparing plywood using this formaldehyde-free adhesive system, as detailed below: The adhesive system used consists of component A and component B. Component A is polymethylene polyphenyl polyisocyanate (PMDI), and component B is acrylic copolymer emulsion Crysol 6319.

[0049] The preparation method is as follows: Eucalyptus veneer with a moisture content of 8% is used as the base material. The veneer thickness is 2.2 mm, and the surface area of ​​burrs and grooves is ≥35%. Component A is first applied to the veneer surface at an application rate of 40 g / m², and then component B is applied to the veneer surface at an application rate of 25 g / m².

[0050] After gluing, the veneers are stacked in an odd number of alternating layers to form a blank, so that the fiber directions of adjacent veneers are perpendicular to each other, thus producing a 9-layer plywood blank.

[0051] The slab is then placed in a hot press for hot pressing at a temperature of 170°C. The hot pressing pressure in the high-pressure section is 1.2 MPa, and the hot pressing pressure in the low-pressure section is 0.5 MPa. The hot pressing time in the high-pressure section is 400 s, and the hot pressing time in the low-pressure section is 165 s. The hot pressing factor is 30 s / mm. After hot pressing, formaldehyde-free plywood is obtained.

[0052] Example 2 This embodiment provides a formaldehyde-free adhesive system and a method for preparing plywood using this formaldehyde-free adhesive system, as detailed below: The adhesive system used includes component A and component B. Component A is polymethylene polyphenyl polyisocyanate (PMDI); component B is obtained by mixing acrylic copolymer emulsion Crysol 6319 and polyether polyol (model WANALYSTKC373) in a mass ratio of 100:15.

[0053] The preparation method is as follows: Eucalyptus veneer with a moisture content of 10% is used as the base material. The veneer thickness is 2.2 mm, and the surface area of ​​burrs and grooves is ≥35%. Component A is first applied to the veneer surface at an application rate of 50 g / m², and then component B is applied to the veneer surface at an application rate of 15 g / m².

[0054] After gluing, the veneers are stacked in an odd-numbered, alternating pattern, ensuring that the fiber directions of adjacent veneers are perpendicular to each other, resulting in a 9-layer plywood blank. The blank is preheated using microwaves to a center temperature of 80°C. The preheated blank is then placed in a hot press for hot pressing at 200°C, a pressure of 1 MPa, a pressing time of 188 seconds, and a pressing factor of 10 s / mm, yielding formaldehyde-free plywood.

[0055] Example 3 This embodiment provides a formaldehyde-free adhesive system and a method for preparing plywood using this formaldehyde-free adhesive system, as detailed below: The adhesive system used includes component A and component B. Component A is an isocyanate adhesive, which is obtained by mixing polymethylene polyphenyl polyisocyanate (PMDI) and modified isocyanate (model WANNATEPM-300E) in a mass ratio of 1:1. Component B is obtained by mixing acrylic copolymer emulsion Crysol6319 and 1,4-butanediol in a mass ratio of 100:10.

[0056] The preparation method is as follows: Using eucalyptus veneer with a moisture content of 15% as the base material, the veneer thickness is 2.2 mm, and the surface area ratio of burrs and grooves is ≥35%. First, component B is applied to the veneer surface at an application rate of 30 g / m²; then component A is applied to the veneer surface at an application rate of 40 g / m².

[0057] After gluing, the veneers are stacked in an odd-numbered, alternating pattern to form a 9-layer plywood blank, with the fiber directions of adjacent veneers perpendicular to each other. The blank is then placed in a hot press for hot pressing at 120°C. The high-pressure section has a pressing pressure of 1 MPa, the low-pressure section has a pressing pressure of 0.4 MPa, the high-pressure section has a pressing time of 300 s, the low-pressure section has a pressing time of 170 s, and the hot pressing factor is 25 s / mm. After hot pressing, formaldehyde-free plywood is obtained.

[0058] Example 4 This embodiment provides a formaldehyde-free adhesive system and a method for preparing plywood using this formaldehyde-free adhesive system, as detailed below: The adhesive system used includes component A and component B. Component A is polymethylene polyphenyl polyisocyanate (PMDI); component B is obtained by mixing acrylic copolymer emulsion Wantipro0839 and glycerol in a mass ratio of 100:5.

[0059] The preparation method is as follows: Eucalyptus veneer with a moisture content of 10% is used as the base material. The veneer thickness is 2.2 mm, and the surface area ratio of burrs and grooves is ≥35%. Component A is first applied to the veneer surface at an application rate of 50 g / m²; then component B is applied to the veneer surface at an application rate of 5 g / m².

[0060] After gluing, the veneers are stacked in an odd number of alternating layers to form a 9-layer plywood blank, with the fiber directions of adjacent veneers perpendicular to each other. The blank is then placed in a hot press for hot pressing at 200°C. The high-pressure section has a pressing pressure of 1.5 MPa, and the low-pressure section has a pressing pressure of 0.3 MPa. The high-pressure section pressing time is 250 s, and the low-pressure section pressing time is 88 s. The hot pressing factor is 18 s / mm. After hot pressing, formaldehyde-free plywood is obtained.

[0061] Example 5 The only difference between this embodiment and Embodiment 1 is that component B is applied first, followed by component A.

[0062] Example 6 The only difference between this embodiment and Example 1 is that a hydroxyl-terminated compound is added to component B. Component B consists of acrylic acid copolymer emulsion Crysol 6319 and polyether polyol (model WANALYSTKC373) in a mass ratio of 100:15. Since the introduction of the hydroxyl-terminated compound is beneficial for improving the system's reactivity and interfacial crosslinking efficiency, the hot-pressing factor in this embodiment is adjusted to 15 s / mm, the pressures in the high-pressure and low-pressure sections remain unchanged, the high-pressure section time is adjusted to 200 s, and the low-pressure section time is adjusted to 82 s. All other raw materials, amounts, and preparation conditions are the same as in Example 1.

[0063] Example 7 The only difference between this embodiment and Example 6 is that the mass ratio of acrylic copolymer emulsion Crysol 6319 to polyether polyol (model WANALYSTKC373) in component B is 100:5. Since the introduction of the terminal hydroxyl compound is beneficial for improving the system's reactivity and interfacial crosslinking efficiency, the hot-pressing factor in this embodiment is adjusted to 20 s / mm, the pressures in the high-pressure and low-pressure sections remain unchanged, the high-pressure section time is adjusted to 280 s, and the low-pressure section time is adjusted to 96 s. All other raw materials, amounts, and preparation conditions are the same as in Example 6.

[0064] Example 8 The only difference between this embodiment and Example 6 is that the mass ratio of acrylic acid copolymer emulsion Crysol 6319 to polyether polyol (model WANALYSTKC373) in component B is 100:10. Since the introduction of the terminal hydroxyl compound is beneficial for improving the system's reactivity and interfacial crosslinking efficiency, the hot-pressing factor in this embodiment is adjusted to 17 s / mm, the pressures in the high-pressure and low-pressure sections remain unchanged, the high-pressure section time is adjusted to 250 s, and the low-pressure section time is adjusted to 70 s. All other raw materials, amounts, and preparation conditions are the same as in Example 6.

[0065] Example 9 The only difference between this embodiment and Example 6 is that the mass ratio of acrylic acid copolymer emulsion Crysol 6319 to polyether polyol (model WANALYSTKC373) in component B is 100:20. Since the introduction of the terminal hydroxyl compound is beneficial for improving the system's reactivity and interfacial crosslinking efficiency, the hot-pressing factor in this embodiment is adjusted to 12 s / mm, the pressures in the high-pressure and low-pressure sections remain unchanged, the high-pressure section time is adjusted to 170 s, and the low-pressure section time is adjusted to 56 s. All other raw materials, amounts, and preparation conditions are the same as in Example 6.

[0066] Example 10 The only difference between this embodiment and Example 1 is that the acrylic copolymer emulsion in component B is Crysol 6359, which has a glass transition temperature of 125°C; the other raw materials, amounts, and preparation conditions are the same as in Example 1.

[0067] Comparative Example 1 The only difference between this comparative example and Example 1 is that only component A, polymethylene polyphenyl polyisocyanate (PMDI, WANNATECW20), is applied, while component B is not applied. All other raw materials, amounts, and preparation conditions are the same as in Example 1.

[0068] Comparative Example 2 The only difference between this comparative example and Example 1 is that the acrylic copolymer emulsion Crysol 6319 used in component B of Example 1 is replaced with acrylic copolymer emulsion Adwel 1318S. All other raw materials, amounts and preparation conditions are the same as in Example 1.

[0069] Comparative Example 3 The only difference between this comparative example and Example 1 is that the acrylic copolymer emulsion Crysol 6319 used in component B of Example 1 is replaced with acrylic copolymer emulsion Lacper 4316. All other raw materials, amounts and preparation conditions are the same as in Example 1.

[0070] Comparative Example 4 Plywood was prepared according to the method of Example 1, except that: component A and component B were premixed evenly at a mass ratio of 40:25 and then applied to the surface of the veneer, with a total application amount of 65 g / m², instead of applying component A and component B separately; the other raw materials and preparation conditions were the same as in Example 1.

[0071] Test Example 1 This test example tests and compares the performance of the plywood prepared according to the embodiments and comparative examples of the present invention. At the same time, the formaldehyde emission of eucalyptus veneer raw material is also tested as a background control for formaldehyde emission.

[0072] The test items included Class I bond strength, Class I impregnation peel strength, and formaldehyde emission of the plywood. The test methods for each property are as follows: Class I bond strength was determined according to the method specified in GB / T17657—2022, section 4.17.4.2.c); Class I impregnation peel strength was determined according to the method specified in GB / T17657—2022, section 4.19.4.1.a); and formaldehyde emission was determined according to the method specified in GB18580. The results are shown in Table 1 below. Table 1: Performance test results of plywood prepared in each embodiment and comparative example

[0073] (Note: "--" indicates that the corresponding project was not tested.) As shown in Table 1, the plywood prepared in Examples 1-10 all exhibited high Class I bond strength, and the Class I immersion peel test results were all negative. This indicates that the formaldehyde-free adhesive system composed of isocyanate adhesive and acrylic copolymer emulsion with a glass transition temperature of 100℃≤Tg≤125℃ can effectively improve the bond strength and water-resistant bond stability of the plywood, thus producing plywood with both high bond strength and excellent immersion peel performance. Meanwhile, the formaldehyde emission of the plywood obtained in Examples 1-10 was undetectable, which is basically consistent with the formaldehyde emission test results of eucalyptus veneer raw materials. This shows that the technical solution of the present invention improves the bonding performance of the board without introducing additional formaldehyde emission risks, maintaining good environmental performance.

[0074] Comparing Example 1 and Comparative Example 1, it can be seen that Comparative Example 1 only applied isocyanate adhesive, without applying Component B containing high-Tg acrylic copolymer emulsion. Under the same conditions of other raw materials, dosages, and preparation conditions, the Type I bond strength of the board obtained in Comparative Example 1 was significantly lower than that in Example 1, and peeling occurred in the Type I immersion peel test. This result indicates that for veneers with surface defects such as burrs and grooves, isocyanate adhesive alone is insufficient to adequately improve the interfacial bonding effect at the defective areas; introducing an acrylic copolymer emulsion with a glass transition temperature (Tg) ≥ 100℃ is beneficial for covering and filling the defective areas on the veneer surface, and improving the bonding strength and water resistance of the adhesive interface.

[0075] Comparing Example 1 with Comparative Examples 2 and 3, it can be seen that Comparative Examples 2 and 3, which used acrylic copolymer emulsions with lower glass transition temperatures to replace the acrylic copolymer emulsion with a Tg ≥ 100℃ in Example 1, showed that, under the same conditions, the resulting boards exhibited significantly lower Class I bond strength and Class I immersion peel performance compared to Example 1. This demonstrates that not all acrylic copolymer emulsions can achieve the performance improvement effects of this invention. For veneers with numerous surface defects such as burrs and grooves, acrylic copolymer emulsions with a Tg ≥ 100℃ can form an interfacial covering structure with higher cohesive strength and better water resistance, and their effect on improving the adhesive and immersion peel performance of the boards is significantly better than that of low-Tg acrylic copolymer emulsions.

[0076] Comparing Example 1 and Example 10, it can be seen that Example 1 used an acrylic copolymer emulsion with a Tg of 105℃, while Example 10 used an acrylic copolymer emulsion with a Tg of 125℃. Both resulted in plywood with high Class I bond strength, and both showed no peeling in the Class I immersion peel test. This demonstrates that within the Tg range defined by this invention, the acrylic copolymer emulsion can form a good synergistic effect with the isocyanate adhesive, thereby effectively improving the bonding interface of burr-filled veneers and maintaining good bond strength and water-resistant bonding stability.

[0077] Comparing Examples 1 and 6 to 9, Example 1 did not add a terminal hydroxyl compound to component B, and its hot-pressing factor was 30 s / mm. The resulting plywood had a Class I bond strength of 1.97 MPa and a Class I immersion peel test result of no peeling. Examples 6 to 9 further introduced polyether polyols into component B. Even when the hot-pressing factor was reduced to 15 s / mm, 20 s / mm, 17 s / mm and 12 s / mm, respectively, the resulting plywood still achieved Class I bond strengths of 2.05 MPa, 2.03 MPa, 2.04 MPa and 2.07 MPa, respectively, and the Class I immersion peel test result was no peeling in all cases.

[0078] It is evident that, after further adding hydroxyl-terminated compounds to the high-Tg acrylic copolymer emulsion system, although the Class I bond strength of the resulting plywood is not significantly different from that of Example 1 without the addition of hydroxyl-terminated compounds, it still maintains high bond strength and excellent impregnation peel performance even under conditions of significantly shortened hot-pressing factor. This indicates that the introduction of hydroxyl-terminated compounds is beneficial to improving the reaction efficiency and interfacial curing efficiency of the adhesive system during hot pressing, thereby helping to shorten the hot-pressing time, improve production efficiency, and maintain good bonding performance of the board.

[0079] Comparing Example 1 and Comparative Example 4, both used polymethylene polyphenyl isocyanate and acrylic copolymer emulsion with a Tg ≥ 100℃, and the types and application amounts of components A and B were the same. The only difference was that in Example 1, components A and B were applied separately, while in Comparative Example 4, components A and B were premixed before being applied to the veneer surface. The plywood obtained in Example 1 had a Type I bond strength of 1.97 MPa and a Type I immersion peel test result of no peeling; while the plywood obtained in Comparative Example 4 had a Type I bond strength of only 1.68 MPa and a Type I immersion peel test result of 10% peeling. This shows that even when using the same type and amount of isocyanate adhesive and high-Tg acrylic copolymer emulsion, it is difficult to obtain a bonding effect comparable to that achieved by applying them separately if they are premixed and applied uniformly.

[0080] This invention applies components A and B separately, which facilitates the application of a high-Tg acrylic copolymer emulsion to defective areas on the surface of wood veneer, either pre- or independently. This covers, fills, and modulates the interface of defects such as burrs and grooves, and, combined with isocyanate adhesives, forms an effective bonding interface, thereby significantly improving bond strength and impregnation peeling performance. This invention, through the combined use of isocyanate adhesives and acrylic copolymer emulsions with a glass transition temperature (Tg) of 100℃≤Tg≤125℃, and by applying components A and B separately, effectively solves the problems of insufficient bond strength and poor water-resistant bonding stability in veneers with burrs on the surface, and has significant industrial application value.

[0081] As shown in the table above, this invention utilizes isocyanate and acrylic acid copolymer emulsion with a glass transition temperature of 100℃≤Tg≤125℃ to produce formaldehyde-free veneer-type engineered wood panels that possess high bonding strength, good water resistance, and excellent environmental performance. Furthermore, a comparison of the formaldehyde emission results of the panels obtained in Examples 1-10 with those of eucalyptus veneer reveals that the formaldehyde emission of the panels prepared using the technical solution of this invention is essentially equivalent to that of the raw material veneer, indicating that this invention can achieve good environmental performance while ensuring the bonding performance of the panels.

[0082] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A formaldehyde-free adhesive system, characterized in that, It includes component A and component B for application to the surface of a substrate, wherein component A is an isocyanate adhesive and component B comprises an acrylic copolymer emulsion with a glass transition temperature of 100℃≤Tg≤125℃.

2. The formaldehyde-free adhesive system according to claim 1, characterized in that, The acrylic copolymer emulsion is formed by copolymerizing hard monomers and soft monomers; Preferably, the hard monomer is selected from one or more of methyl methacrylate, styrene, acrylic acid, methacrylic acid, and acrylonitrile; Preferably, the soft monomer is selected from one or more of isooctyl acrylate, n-butyl acrylate, ethyl acrylate, and methyl acrylate.

3. The formaldehyde-free adhesive system according to claim 1 or 2, characterized in that, Component B also includes hydroxyl-terminated compounds; Preferably, the mass ratio of the acrylic copolymer emulsion to the hydroxyl-terminated compound is 100:(5-20).

4. The formaldehyde-free adhesive system according to claim 3, characterized in that, The terminal hydroxyl compound includes one or more of polyether polyols and small molecule polyols; Preferably, the polyether polyol has a functionality of 2-4 and a number-average molecular weight of 500-6000. Preferably, the small molecule polyol includes one or more of ethylene glycol, 1,4-butanediol, triethanolamine, glycerol, pentaerythritol, trimethylolpropane, and trimethylolethane.

5. The formaldehyde-free adhesive system according to any one of claims 1-4, characterized in that, The isocyanate adhesive includes one or more of aromatic polyisocyanates and their modified products, and aliphatic polyisocyanates and their modified products. Preferably, the isocyanate adhesive comprises one or more of toluene diisocyanate and its modified products, diphenylmethane diisocyanate and its modified products, and polymethylene polyphenyl polyisocyanate and its modified products. Preferably, the modified product is a polyether modified product or a polyester modified product; Preferably, the isocyanate adhesive contains 20wt%-34wt% -NCO, more preferably 30.5wt%-32wt%.

6. A method for preparing plywood using the formaldehyde-free adhesive system as described in any one of claims 1-5, characterized in that, Includes the following steps: Component A and component B are applied to the surface of the veneer to obtain the sizing plate; The sizing sheet is combined with other veneers to form a slab blank; Plywood is obtained by hot pressing the slab.

7. The method for preparing plywood according to claim 6, characterized in that, The application rate of component B on the veneer surface is 5 g / m². 2 -30g / m 2 ; And / or, the application rate of component A on the veneer surface is 25 g / m². 2 -50g / m 2 Preferably 40g / m 2 -50g / m 2 .

8. The method for preparing plywood according to claim 6 or 7, characterized in that, The veneer includes wood veneer; Preferably, the moisture content of the veneer is 8%-25%; more preferably, it is 8%-15%.

9. The method for preparing plywood according to any one of claims 6-8, characterized in that, The temperature conditions for hot pressing are 120℃-200℃, the pressure conditions are 0MPa-1.5MPa, and the hot pressing factor is 10s / mm-30s / mm. Preferably, the pressure condition is 0.3MPa-1.5MPa.

10. The method for preparing plywood according to any one of claims 6-9, characterized in that, The slab is preheated before hot pressing, preferably by microwave preheating.

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