Preparation method of high-strength formaldehyde-free plywood and plywood

Abstract

The invention relates to the technical field of plywood, and discloses a preparation method of high-strength formaldehyde-free plywood and the plywood, the preparation method comprises the following steps: (1) an adhesive is adopted to apply glue to a veneer, and the adhesive comprises a styrene-vinyl acetate-maleic anhydride terpolymer, a biomass raw material and citric acid; (2) the veneers are assembled in a criss-cross mode in the wood grain direction, and a plate blank is obtained; and (3) the plate blank is subjected to cold pressing and hot pressing in sequence, the hot pressing temperature ranges from 125 DEG C to 150 DEG C, the pressure ranges from 0.6 MPa to 0.8 MPa, and the time ranges from 30 min to 90 min. The novel adhesive free of formaldehyde components is used for sizing, and the plywood is prepared by cooperatively adopting an improved production process, so that formaldehyde-free addition is realized, the environmental protection performance of the plywood is improved, and meanwhile, the plywood has excellent physical and chemical properties such as bonding strength, static bending strength and elastic modulus.

Description

Technical Field

[0001] The present invention relates to the technical field of plywood, in particular to a preparation method of high-strength formaldehyde-free plywood and the plywood. Background Art

[0002] With the improvement of environmental awareness, people are paying more and more attention to the environmental performance of indoor products, especially "formaldehyde", which has always attracted much attention from the audience. It can be said that "people are afraid of formaldehyde". Formaldehyde is a common indoor air pollutant. Long-term exposure will have negative effects on human health, such as irritation to the eyes, nose and throat, and cause symptoms such as headaches and dizziness.

[0003] The existing plywood on the market is still currently made of urea-formaldehyde glue, melamine-formaldehyde glue (also called melamine resin) and other glues containing formaldehyde, and during the use of plywood in the later stage, the free formaldehyde that is not completely reacted in the glue and the decomposition of the resin will also produce free formaldehyde, which is released into the indoor home environment and endangers health. The invention patent with publication number CN106078947A discloses a method for preparing modified urea-formaldehyde resin plywood, which adds an appropriate amount of melamine during the resin synthesis process, modified hydrolyzed protein and oxidized starch as modifiers to obtain a modified urea-formaldehyde resin adhesive, which improves the bonding strength of urea-formaldehyde resin by controlling the methylol content in urea-formaldehyde resin and reduces the free formaldehyde emission in plywood. However, the formaldehyde emission of the plywood is still higher (formaldehyde emission is 1.5mg / L), which can not meet the higher environmental protection level requirements and still endangers human health. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing plywood production process that still uses traditional formaldehyde-containing adhesives, the present invention provides a method for preparing high-strength formaldehyde-free plywood and plywood. A new adhesive without formaldehyde components is used for gluing, and the plywood is prepared in conjunction with an improved production process, achieving formaldehyde-free addition and improving the environmental performance of the plywood. At the same time, the plywood has excellent physical and chemical properties such as bonding strength, static bending strength and elastic modulus.

[0005] The purpose of the present invention is achieved through the following technical solutions: In a first aspect, the present invention provides a method for preparing high-strength formaldehyde-free plywood, comprising the following steps: (1) gluing the veneer with an adhesive, wherein the adhesive comprises a styrene-vinyl acetate-maleic anhydride terpolymer, a biomass raw material, and citric acid; (2) assembling the veneers in a crisscross pattern along the wood grain direction to obtain slabs; (3) The slab is cold pressed and hot pressed in sequence, with the hot pressing temperature being 125-150°C, the pressure being 0.6-0.8 MPa, and the time being 30-90 min.

[0006] Styrene-maleic anhydride copolymers contain amic acid groups, which can simultaneously generate anhydride and imide groups during the hot pressing process. The formation of imide groups transforms the polymer from hydrophilic to hydrophobic, imparting excellent water resistance to the board. The anhydride groups formed can undergo an esterification reaction with the hydroxyl groups on the wood surface during the hot pressing process, imparting excellent bonding strength to the board. Because the reaction between the glue and the wood is an esterification reaction, the hot pressing curing temperature is higher and the curing time is longer than that of conventional hot pressing processes.

[0007] However, when styrene-maleic anhydride copolymer is used as an adhesive alone, its wettability and fluidity on the wood surface are poor. By adding vinyl acetate to copolymerize and obtain a terpolymer, the segment mobility of the system can be improved, the wettability and fluidity of the glue system are improved, and it is conducive to the diffusion of glue on the surface of the wood veneer, thereby better improving the bonding strength of plywood. In addition, the addition of biomass raw materials in the adhesive can improve the initial viscosity of the glue, making it conducive to the adhesion of glue on the surface of the wood veneer. Citric acid can form hydrogen bonds with the hydroxyl (-OH) of the biomass raw material, interfere with the formation of hydrogen bonds between biomass raw material molecules, and reduce the interaction force between molecules. It is easier to separate the biomass raw material molecules, making the glue mixing smoother, and can promote the uniform dispersion of biomass raw materials in the terpolymer.

[0008] Preferably, the preparation method of the styrene-vinyl acetate-maleic anhydride terpolymer comprises the following steps: (a) adding styrene (ST), vinyl acetate (VAc), maleic anhydride (MAH) and an initiator into a solvent for copolymerization to obtain a primary polymer; (b) Aqueous ammonia is added to the primary polymer to continue the reaction to obtain a styrene-vinyl acetate-maleic anhydride terpolymer.

[0009] Preferably, in step (a), the mass ratio of styrene, vinyl acetate and maleic anhydride is 2:0.8-1.2:2, more preferably, the mass ratio of styrene, vinyl acetate and maleic anhydride is 2:1.0-1.2:2; and the copolymerization reaction is carried out at 70-80°C for 4-6h.

[0010] Preferably, in step (a), the initiator is benzoyl peroxide (BPO); the amount of the initiator added is 3-6% of the mass of maleic anhydride.

[0011] Preferably, in step (a), the ratio of the added amount of maleic anhydride to the added amount of the solvent is 1 g:40-50 mL; and the solvent is toluene.

[0012] Preferably, in step (b), the amount of ammonia water added is 35-45% of the mass of the primary polymer; the mass concentration of ammonia water is 20-30%; and the continued reaction is carried out at 80-90° C. for 6-8 hours.

[0013] Preferably, in the adhesive, the mass ratio of styrene-vinyl acetate-maleic anhydride terpolymer, biomass raw material and citric acid is 100:20-30:2-4.

[0014] The more biomass raw materials are added, the better their dispersion in the adhesive. However, if too much is added, the bonding strength of the plywood will decrease. The optimal quality ratio of biomass raw materials relative to styrene-vinyl acetate-maleic anhydride terpolymer is 25-30%.

[0015] Preferably, the biomass raw material includes flour and / or soybean meal.

[0016] As a preference, the adhesive coating weight on one side is 200-260 g / m 2 More preferably, the adhesive coating amount on one side is 240-260g / m 2 .

[0017] The adhesive used in this invention requires a lower application rate than conventional urea-formaldehyde resin adhesives, and due to its superior bonding properties, it can achieve higher bond strengths at lower application rates. Furthermore, excessive application of the adhesive, excessively high hot-pressing temperatures, or prolonged hot-pressing times can lead to excessive moisture generation during the hot-pressing process, making it difficult to expel moisture and thus compromising the plywood's bond strength.

[0018] Preferably, the cold pressing pressure is 0.7-0.9 MPa and the time is 40-60 min.

[0019] The cold pressing process gives the slab its initial shape and prevents the core veneer from falling apart.

[0020] Preferably, the moisture content of the surface layer of the slab after cold pressing is controlled at 20-30%.

[0021] Since the glue contains moisture, the moisture content of the slab will increase compared to the single board. After cold pressing, if the moisture content of the slab is still high, it can be reduced by aging.

[0022] Preferably, the rough-edged boards obtained after hot pressing are post-processed to obtain plywood; the post-processing includes trimming, turning over and cooling, aging and curing, and surface sanding.

[0023] In a second aspect, the present invention provides a high-strength formaldehyde-free plywood produced by the above-mentioned preparation method.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) Adding vinyl acetate to the styrene-maleic anhydride copolymer for copolymerization to obtain a terpolymer can improve the chain segment mobility of the system, enhance the wettability and fluidity of the glue system, and facilitate the diffusion of the glue on the surface of the wood veneer; during the hot pressing process, the amic acid groups contained in the copolymer can simultaneously generate anhydride groups and imide groups. The formation of the imide group realizes the transformation of the polymer from hydrophilic to hydrophobic, giving the board good water resistance, and the formed anhydride group can undergo esterification reaction with the hydroxyl groups on the wood surface during the hot pressing process, giving the board excellent bonding strength; (2) Use a new adhesive without formaldehyde components for gluing, and adopt an improved production process to prepare plywood, so as to achieve formaldehyde-free production and improve the environmental performance of plywood. At the same time, the plywood has excellent physical and chemical properties such as bonding strength, static bending strength and elastic modulus. DETAILED DESCRIPTION

[0025] The technical solutions of the present invention are described below with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0026] Example 1 (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0027] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer and blended. The mass ratio of flour, citric acid and terpolymer was 25:2:100 to obtain an adhesive.

[0028] Wherein, the preparation of styrene-vinyl acetate-maleic anhydride terpolymer includes: (a) Styrene, vinyl acetate, and maleic anhydride were added to toluene and stirred. The ratio of styrene, vinyl acetate, maleic anhydride, and toluene was 2 g:1 g:2 g:80 mL. The mixture was then heated to 70° C., and benzoyl peroxide (5% by weight of maleic anhydride) was added and reacted for 4 h to obtain a primary polymer. (b) adding 40% of the mass of the primary polymer to ammonia water (mass concentration is 25%), raising the temperature to 80° C., and reacting for 7 hours to obtain a styrene-vinyl acetate-maleic anhydride terpolymer.

[0029] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The amount of adhesive applied on one side is 200g / m 2 .

[0030] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0031] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0032] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 145°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 60 min.

[0033] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0034] Example 2 The difference from Example 1 is that the adhesive coating amount on one side is 225g / m 2 ; The hot pressing temperature is 135℃ and the hot pressing time is 30min.

[0035] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0036] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer (the same as in Example 1) and blended. The mass ratio of flour, citric acid and terpolymer was 25:2:100 to obtain an adhesive.

[0037] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The single-sided glue coating amount of the adhesive is 225g / m 2 .

[0038] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0039] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0040] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 135°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 30 min.

[0041] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0042] Example 3 The difference from Example 1 is that the adhesive coating amount on one side is 225g / m 2 ; The hot pressing temperature is 125℃ and the hot pressing time is 90min.

[0043] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0044] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer (the same as in Example 1) and blended. The mass ratio of flour, citric acid and terpolymer was 25:2:100 to obtain an adhesive.

[0045] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The single-sided glue coating amount of the adhesive is 225g / m 2 .

[0046] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0047] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0048] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 125°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 90 min.

[0049] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0050] Example 4 The difference from Example 1 is that the adhesive coating amount on one side is 250g / m 2 ; The hot pressing temperature is 145℃ and the hot pressing time is 30min.

[0051] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0052] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer (the same as in Example 1) and blended. The mass ratio of flour, citric acid and terpolymer was 25:2:100 to obtain an adhesive.

[0053] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The amount of adhesive applied on one side is 250g / m 2 .

[0054] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0055] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0056] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 145°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 30 min.

[0057] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0058] Example 5 The difference from Example 1 is that the adhesive coating amount on one side is 250g / m 2 ; The hot pressing temperature is 135℃ and the hot pressing time is 60min.

[0059] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0060] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer (the same as in Example 1) and blended. The mass ratio of flour, citric acid and terpolymer was 25:2:100 to obtain an adhesive.

[0061] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The amount of adhesive applied on one side is 250g / m 2 .

[0062] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0063] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0064] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 135°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 60 min.

[0065] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0066] Example 6 The difference from Example 1 is that the adhesive is not prepared using the styrene-vinyl acetate-maleic anhydride terpolymer in Example 1, but a styrene-vinyl acetate-maleic anhydride terpolymer prepared by the following preparation method.

[0067] Wherein, the preparation of styrene-vinyl acetate-maleic anhydride terpolymer includes: (a) Styrene, vinyl acetate, and maleic anhydride were added to toluene and stirred. The ratio of styrene, vinyl acetate, maleic anhydride, and toluene was 2 g:0.8 g:2 g:80 mL. The mixture was then heated to 70° C., and benzoyl peroxide (5% by weight of maleic anhydride) was added and reacted for 4 h to obtain a primary polymer. (b) adding 40% of the mass of the primary polymer to ammonia water (mass concentration is 25%), raising the temperature to 80° C., and reacting for 7 hours to obtain a styrene-vinyl acetate-maleic anhydride terpolymer.

[0068] Example 7 The difference from Example 1 is that during the preparation of the adhesive, the mass ratio of flour, citric acid and terpolymer is 20:2:100.

[0069] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0070] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer (the same as in Example 1) and blended. The mass ratio of flour, citric acid and terpolymer was 20:2:100 to obtain an adhesive.

[0071] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The amount of adhesive applied on one side is 200g / m 2 .

[0072] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0073] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0074] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 145°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 60 min.

[0075] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0076] Comparative Example 1 The difference from Example 4 is that the glue used is urea-formaldehyde glue purchased from Jiangshan Bolan Co., Ltd., the main component of which is melamine-modified urea-formaldehyde resin (MUF) with a solid content of 45%.

[0077] Comparative Example 2 The difference from Example 4 is that the glue coating amount is 275g / m 2 .

[0078] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0079] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer (the same as in Example 1) and blended. The mass ratio of flour, citric acid and terpolymer was 25:2:100 to obtain an adhesive.

[0080] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The single-sided glue coating amount of the adhesive is 275g / m 2 .

[0081] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0082] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0083] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 145°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 30 min.

[0084] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0085] Comparative Example 3 The difference from Example 4 is that the hot pressing temperature is 155° C. and the hot pressing time is 105 min.

[0086] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0087] (2) Preparation of adhesive: flour and citric acid were added to styrene-vinyl acetate-maleic anhydride terpolymer (the same as in Example 1) and blended. The mass ratio of flour, citric acid and terpolymer was 25:2:100 to obtain an adhesive.

[0088] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The amount of adhesive applied on one side is 250g / m 2 .

[0089] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0090] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0091] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 155°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 105 min.

[0092] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0093] Comparative Example 4 The difference from Example 4 is that the adhesive is not prepared using the styrene-vinyl acetate-maleic anhydride terpolymer in Example 1, but a styrene-maleic anhydride binary copolymer prepared by the following preparation method.

[0094] Wherein, the preparation of styrene-maleic anhydride binary copolymer comprises: (a) Styrene and maleic anhydride were added to toluene and stirred in a ratio of 2 g styrene, 2 g maleic anhydride, and 80 mL toluene. The mixture was then heated to 70° C., and benzoyl peroxide (5% by weight of maleic anhydride) was added and reacted for 4 h to obtain a primary polymer. (b) adding ammonia water (mass concentration is 25%) accounting for 40% of the mass of the primary polymer to the primary polymer, raising the temperature to 80° C., and reacting for 7 hours to obtain a styrene-maleic anhydride binary copolymer.

[0095] Comparative Example 5 The difference from Example 4 is that citric acid is not added to the adhesive.

[0096] (1) Log processing: First, cut the logs into appropriate lengths and then peel them; put the peeled wood sections into a peeling machine for peeling; put the peeled veneer into a drying kiln for drying, and control the moisture content at 15-18%.

[0097] (2) Preparation of adhesive: flour was added to styrene-vinyl acetate-maleic anhydride terpolymer and blended, with the mass ratio of flour to terpolymer being 25:100, to obtain an adhesive.

[0098] (3) Gluing: Use a double-roller glue coating machine to apply glue on the surface of the veneer. The amount of adhesive applied on one side is 250g / m 2 .

[0099] (4) Assembly: The veneers are stacked and assembled in a crisscross pattern along the wood grain direction to obtain 11 layers of slabs.

[0100] (5) Cold pressing and shaping: Place the slab into a cold press and cold press for 60 minutes to give it a preliminary shape. The moisture content of the slab surface is controlled at 20-30%.

[0101] (6) Hot pressing: Place the cold-pressed slab into a multi-layer hot press for hot pressing at a temperature of 145°C, a pressure of 0.6-0.8 MPa, and a hot pressing time of 30 min.

[0102] (7) Post-processing: The rough-edged boards obtained by hot pressing are trimmed, turned over for cooling, left to cure, and surface sanded to prepare plywood that meets the requirements in terms of size, appearance, etc.

[0103] Table 1 Note: Formaldehyde emission is tested according to the desiccant method.

[0104] As shown in Table 1, the present invention uses a novel formaldehyde-free adhesive for gluing, resulting in plywood with high bond strength, static flexural strength, and elastic modulus, and a formaldehyde emission of ≤0.05 mg / L, meeting the national ENF grade standard. In contrast, in Comparative Example 1, which used conventional urea-formaldehyde glue for gluing, the resulting plywood not only had a higher formaldehyde emission but also exhibited lower bond strength, static flexural strength, and elastic modulus.

[0105] In addition, Comparative Examples 2-5 are compared with Example 4, showing that too much glue application, too high hot pressing temperature, and too long a time are all unfavorable for obtaining good bonding strength. This is because the adhesive in the present invention not only contains moisture itself, but also reacts to produce water vapor during the hot pressing process. The hot pressing process is subject to pressure, and the difficulty in discharging water vapor can cause the bonding strength of the plywood to decrease. Without the addition of vinyl acetate to the copolymer for copolymerization, the wettability and fluidity of the styrene-maleic anhydride copolymer on the wood surface when used alone as an adhesive are poor, and the diffusivity of the glue on the wood veneer surface is poor, resulting in a lower bonding strength of the plywood. Without the addition of citric acid to the adhesive, the dispersibility of flour in the terpolymer is poor, which also affects the bonding strength.

[0106] The above are only preferred embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the description of the present invention, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing high-strength formaldehyde-free plywood, characterized in that: The steps include: (1) gluing the veneer with an adhesive, wherein the adhesive comprises a styrene-vinyl acetate-maleic anhydride terpolymer, a biomass raw material, and citric acid; (2) Assembling the veneers in a crisscross pattern along the wood grain direction to obtain slabs; (3) The slab is cold pressed and hot pressed in sequence. The hot pressing temperature is 125-150°C, the pressure is 0.6-0.8 MPa, and the time is 30-90 min.

2. The method for preparing high-strength formaldehyde-free plywood according to claim 1, characterized in that: The preparation method of the styrene-vinyl acetate-maleic anhydride terpolymer comprises the following steps: (a) adding styrene, vinyl acetate, maleic anhydride and an initiator into a solvent to carry out copolymerization reaction to obtain a primary polymer; (b) Aqueous ammonia is added to the primary polymer to continue the reaction to obtain a styrene-vinyl acetate-maleic anhydride terpolymer.

3. The method for preparing high-strength formaldehyde-free plywood according to claim 2, characterized in that: In step (a), the mass ratio of styrene, vinyl acetate and maleic anhydride is 2:0.8-1.2:2; and the copolymerization reaction is carried out at 70-80° C. for 4-6 hours.

4. The method for preparing high-strength formaldehyde-free plywood according to claim 2 or 3, characterized in that: In step (a), the initiator is benzoyl peroxide; the amount of the initiator added is 3-6% of the mass of maleic anhydride.

5. The method for preparing high-strength formaldehyde-free plywood according to claim 2 or 3, characterized in that: In step (b), the amount of ammonia added is 35-45% of the mass of the initial polymer; and the continued reaction is carried out at 80-90° C. for 6-8 hours.

6. The method for preparing high-strength formaldehyde-free plywood according to claim 1, characterized in that: In the adhesive, the mass ratio of styrene-vinyl acetate-maleic anhydride terpolymer, biomass raw material and citric acid is 100:20-30:2-4.

7. The method for preparing high-strength formaldehyde-free plywood according to claim 1, characterized in that: The biomass raw material includes flour and / or soybean meal.

8. The method for preparing high-strength formaldehyde-free plywood according to claim 1, 6 or 7, characterized in that: The adhesive coating amount on one side is 200-260g / m².

9. The method for preparing high-strength formaldehyde-free plywood according to claim 1, 6 or 7, characterized in that: The cold pressing pressure is 0.7-0.9 MPa and the time is 40-60 min.

10. A high-strength formaldehyde-free plywood produced by the method according to any one of claims 1 to 9.

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