A method for preparing a water-resistant formaldehyde-free plywood using water-based polyvinyl alcohol

Abstract

The application discloses a preparation method of a water-resistant formaldehyde-free plywood by using water-based polyvinyl alcohol, and belongs to the technical field of deep processing of wood materials and adhesives. The application aims at solving the problem of poor water resistance of the existing polyvinyl alcohol adhesive plywood. The preparation method comprises the following steps: one, preparation of water-based polyvinyl alcohol adhesive; two, gluing; three, pre-pressing and storage; four, curing; and five, post-processing. The application is used for preparing the water-resistant formaldehyde-free plywood by using water-based polyvinyl alcohol.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of deep processing of wood materials and adhesives. BACKGROUND

[0002] The problem of indoor decoration and renovation formaldehyde pollution and volatile organic pollutants caused by plywood has been plaguing people. The reason is that the current market for decorative plywood generally uses low-cost, moderate-performance formaldehyde-based adhesives. Formaldehyde-based plywood products have problems of releasing free formaldehyde during manufacturing, storage and use, which poses a hidden danger to the physical and mental health of producers and consumers.

[0003] In the field of plywood technology, developing a plywood bonding technology that is water-based, environmentally friendly, free of formaldehyde addition, low-cost, performance compliant, suitable for multiple tree species, low equipment requirements, and simple process has always been the direction of people's efforts. Polyvinyl alcohol is one of the few and largest water-soluble polymers in the world today, with an annual output of up to 1.2 billion kilograms, abundant raw materials, and large-scale production through non-petroleum routes (bioethanol-ethylene-vinyl acetate-polyvinyl alcohol), making it a sustainable and environmentally friendly material. In addition, polyvinyl alcohol also has the advantages of colorless and odorless, good compatibility, low price (market price about 11200 yuan / ton), good water-soluble adhesive, stable chemical properties, and excellent comprehensive performance, and can be used to prepare water-based environmentally friendly adhesives. It is a sustainable and degradable commercial polymer with great application prospects. If a simple water-based polyvinyl alcohol plywood bonding technology can be developed, its environmental, economic and social benefits will be very great. However, the artificial boards prepared by polyvinyl alcohol adhesives currently have dry strength that meets national standards, but poor water resistance, which will open the glue in cold water. This is because polyvinyl alcohol is a polyhydroxyl compound, and the hydroxyl group is a highly hydrophilic functional group, and the cross-linking structure is a single hydrogen bond cross-linking. Although existing chemical modification, physical filler and other methods can solve the above problems, the process complexity and cost increase brought by modification weaken the cost performance of polyvinyl alcohol wood adhesives. SUMMARY

[0004] The present application solves the problem of poor water resistance of plywood prepared by existing polyvinyl alcohol adhesives, and further provides a preparation method of water-based polyvinyl alcohol plywood with water resistance and no formaldehyde addition.

[0005] A preparation method of water-based polyvinyl alcohol plywood with water resistance and no formaldehyde addition, which is carried out according to the following steps:

[0006] I. Preparation of water-based polyvinyl alcohol adhesive:

[0007] Polyvinyl alcohol is added to water, then dissolved by gradient heating to obtain a water-based polyvinyl alcohol adhesive;

[0008] II. Gluing:

[0009] applying the water-based polyvinyl alcohol adhesive on both sides of the core board to obtain a coated core board, and then stacking the coated core board and the veneer in a cross-grain manner to obtain a board blank;

[0010] III. Pre-pressing and aging:

[0011] cold-pressing the board blank, and then aging at room temperature to obtain a pre-pressed and aged board blank;

[0012] IV. Curing:

[0013] hot-pressing or microwave curing the pre-pressed and aged board blank to obtain a cured board blank;

[0014] The hot-pressing curing is specifically performed at a hot-pressing temperature of 180℃ to 230℃ and a hot-pressing pressure of 0.5MPa to 5MPa for 1min to 40min.

[0015] The microwave curing is specifically performed at a microwave output power of 500W to 1000W and a microwave frequency of 2400MHz to 2500MHz for 10s to 100s.

[0016] V. Post-treatment:

[0017] placing the cured board blank at room temperature, thereby completing the preparation method of the water-resistant and formaldehyde-free water-based polyvinyl alcohol plywood.

[0018] The present application has the following advantages:

[0019] 1. The raw material of the water-based polyvinyl alcohol adhesive is only polyvinyl alcohol, which is non-toxic, and the solvent is water, which is green and environmentally friendly without any formaldehyde addition. It can fundamentally solve the indoor formaldehyde pollution hazard that people have been facing for a long time, avoid the use and emission of a large amount of organic solvents, and realize non-toxic and harmless to consumers, industrial producers and the environment, which is a truly healthy and environmentally friendly glue in the sense of health and environmental protection, and conforms to the principle of atomic economy.

[0020] 2. The water-based polyvinyl alcohol adhesive is the simplest wood adhesive so far, which has only a single component and can be dissolved by heating only, without technical threshold, simple equipment required, mild reaction conditions, and easy to scale up the operation process and realize industrialized production.

[0021] 3. The polyvinyl alcohol adhesive of this invention has an extremely long shelf life. Only commercial polyvinyl alcohol needs to be purchased. Solid polyvinyl alcohol powder on the market has a shelf life of more than 5 years. Since commercial polyvinyl alcohol is solid, it is easy to transport and store. The polyvinyl alcohol adhesive has a long trial period, and the prepared adhesive solution has a usable period of more than one month.

[0022] 4. No curing agent needs to be added during the polyvinyl alcohol bonding process of the present invention, and the curing conditions are simple;

[0023] 5. The polyvinyl alcohol-bonded plywood technology of this invention can completely adopt the traditional liquid adhesive process, with the same process conditions and is simpler (no need to add flour, hardener, etc.); it does not require new equipment or modifications and is simpler (it does not require the mixing equipment used when mixing flour and hardener in the traditional urea-formaldehyde resin process); the curing time is short, the curing temperature is moderate, and the hot pressing pressure is low; it is universally applicable to different tree species (typical fast-growing poplar, eucalyptus; typical coniferous pine; typical broadleaf birch) and bamboo, and is fully suitable for large-scale industrial production;

[0024] 6. The finished plywood products demonstrate that the plywood's properties not only fully meet the requirements of the latest national standard (GB / T 9846-2015) for indoor panel materials (Table 2), but also fully comply with US standards for indoor panel materials. Figure 1 The overall performance of this polyvinyl alcohol adhesive is comparable to that of existing urea-formaldehyde resin adhesives. The cost of this adhesive is low, with an adhesive cost of only about 1,500 yuan / ton. The amount of adhesive applied per cubic meter of plywood is about 95.4 kg / m³, corresponding to a cost of about 143 yuan / m³, which is lower than the cheapest urea-formaldehyde adhesive on the market (≥1,800 yuan / ton). The amount of adhesive applied per cubic meter of plywood is about 85.5 kg / m³, with a cost of about 153 yuan / m³ (Table 3).

[0025] 7. In principle, polyvinyl alcohol adhesives undergo cross-linking after curing, transforming from a single hydrogen bond network into a double cross-linked network composed of hydrogen bonds and covalent bonds (ether bonds / carbon-carbon double bonds). Infrared spectral images show that cured polyvinyl alcohol exhibits cross-linking at 960-970 cm⁻¹. -1 1060-1150cm -1 Stronger absorption peaks were generated at these locations, which are attributed to the stretching vibrations of carbon-carbon double bonds (C=C) in olefins and the absorption peaks of ether bonds (COC), respectively; additionally, absorption peaks were observed at 3000-3500 cm⁻¹. -1The hydroxyl absorption vibration of the cured polyvinyl alcohol is reduced, indicating that the hydrogen bonding of the hydroxyl group of the cured polyvinyl alcohol is reduced, and a condensation reaction occurs to generate an unsaturated carbon-carbon double bond (C=C) and an ether bond (C-O-C). Further, the XRD image shows that the diffraction intensity of the cured polyvinyl alcohol is significantly increased, indicating that the crystallinity is improved. This is because the cured polyvinyl alcohol adhesive generates ether bonds and double bonds, and the molecular chains are arranged more closely and compactly, and the cohesion is improved, thereby facilitating the improvement of the bonding strength and water resistance. In summary, the cured polyvinyl alcohol adhesive undergoes a crosslinking reaction, and the single hydrogen bond crosslinking is converted into a double network crosslinking structure composed of chemical bonds and hydrogen bonds.

[0026] Therefore, the present application realizes the water-based polyvinyl alcohol bonding technology using only a single component, which is expected to break through the technical bottleneck of the current organic solvent formaldehyde bonding mode. A water-based formaldehyde-free polyvinyl alcohol plywood technology with great application prospect is provided.

[0027] DRAWINGS

[0028] Figure 1 Figure 1 is a real picture of the cold water resistance performance of the plywood bonded by the water-based polyvinyl alcohol adhesive prepared in Example 1 using the water-based polyvinyl alcohol adhesive prepared in Example 2; PY@0, PY@1, PY@2 and PY@3 represent the poplar plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively; PH@0, PH@1, PH@2 and PH@3 represent the birch plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively; PA@0, PA@1, PA@2 and PA@3 represent the eucalyptus plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively; PS@0, PS@1, PS@2 and PS@3 represent the pine plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively;

[0029] Figure 2 Figure 2 is the infrared spectrum images of the water-based polyvinyl alcohol adhesive prepared in Example 2 before and after curing;

[0030] Figure 3 Figure 3 is the XRD images of the water-based polyvinyl alcohol adhesive prepared in Example 2 before and after curing. DETAILED DESCRIPTION

[0031] Embodiment I: The present embodiment is a preparation method of water-based polyvinyl alcohol plywood with water resistance and formaldehyde-free addition, which is carried out according to the following steps:

[0032] I. Preparation of water-based polyvinyl alcohol adhesive:

[0033] The polyvinyl alcohol is added to water, and then is dissolved by gradient heating to obtain the water-based polyvinyl alcohol adhesive;

[0034] II. Gluing:

[0035] The water-based polyvinyl alcohol adhesive is coated on both sides of the core board to obtain the coated core board, and then the coated core board is stacked with the veneer in a cross-grain manner to obtain a board blank;

[0036] III. Pre-pressing and standing:

[0037] The board blank is cold-pressed and then is closed and stood at room temperature to obtain a pre-pressed and stood board blank;

[0038] IV. Curing:

[0039] The pre-pressed and stood board blank is hot-pressed or microwaved to obtain a cured board blank;

[0040] The hot-pressing is specifically carried out at a hot-pressing temperature of 180-230 DEG C and a hot-pressing pressure of 0.5-5 MPa for 1-40 min;

[0041] The microwave curing is specifically carried out at a microwave output power of 500-1000 W and a microwave frequency of 2400-2500 MHz for 10-100 s;

[0042] V. Post-treatment:

[0043] The cured board blank is left to stand at room temperature, and the preparation method of the water-resistant formaldehyde-free water-based polyvinyl alcohol plywood is completed.

[0044] The standing in step V is to eliminate internal stress.

[0045] The beneficial effects of the embodiment are:

[0046] 1. The raw material of the water-based polyvinyl alcohol adhesive in the embodiment is only polyvinyl alcohol, which is non-toxic, and the solvent is water, which is green and environmentally friendly and does not contain any formaldehyde. The indoor formaldehyde pollution hazard that people have long faced can be fundamentally solved, and the use and discharge of a large amount of organic solvents can be avoided. In the whole production process, the polyvinyl alcohol bonded plywood is non-toxic and harmless to consumers, industrial producers and the environment, and is a truly healthy and environmentally friendly glue, which meets the principle of atomic economy;

[0047] 2. The water-based polyvinyl alcohol adhesive in the embodiment is the simplest wood adhesive so far, which has only a single component and can be dissolved by heating only, without technical threshold, simple equipment required, mild reaction conditions, and easy operation process amplification and industrial production;

[0048] 3. The polyvinyl alcohol adhesive in this embodiment has an extremely long shelf life. You only need to purchase commercial polyvinyl alcohol. Solid polyvinyl alcohol powder on the market has a shelf life of more than 5 years. Since commercial polyvinyl alcohol is solid, it is easy to transport and store. The trial period of polyvinyl alcohol adhesive is very long. The usable period of the prepared adhesive solution is more than one month.

[0049] 4. In this embodiment, no curing agent needs to be added during the process of bonding plywood with polyvinyl alcohol, and the curing conditions are simple;

[0050] 5. The polyvinyl alcohol-bonded plywood technology of this embodiment can completely adopt the traditional liquid adhesive process, with the same process conditions and is simpler (no need to add flour, hardener, etc.); it does not require new equipment or upgrades and is simpler (it does not require the mixing equipment used when mixing flour and hardener in the traditional urea-formaldehyde resin process); the curing time is short, the curing temperature is moderate, and the hot pressing pressure is low; it is universally applicable to different tree species (typical fast-growing poplar, eucalyptus; typical coniferous pine; typical broadleaf birch) and bamboo, and is fully suitable for large-scale industrial production;

[0051] 6. The finished plywood products demonstrate that the plywood's properties not only fully meet the requirements of the latest national standard (GB / T 9846-2015) for indoor panel materials (Table 2), but also fully comply with US standards for indoor panel materials. Figure 1 The overall performance of this polyvinyl alcohol adhesive is comparable to that of existing urea-formaldehyde resin adhesives. The cost of this adhesive is low, with an adhesive cost of only about 1,500 yuan / ton. The amount of adhesive applied per cubic meter of plywood is about 95.4 kg / m³, corresponding to a cost of about 143 yuan / m³, which is lower than the cheapest urea-formaldehyde adhesive on the market (≥1,800 yuan / ton). The amount of adhesive applied per cubic meter of plywood is about 85.5 kg / m³, with a cost of about 153 yuan / m³ (Table 3).

[0052] 7. In principle, polyvinyl alcohol adhesives undergo cross-linking after curing, transforming from a single hydrogen bond network into a double cross-linked network composed of hydrogen bonds and covalent bonds (ether bonds / carbon-carbon double bonds). Infrared spectral images show that cured polyvinyl alcohol exhibits cross-linking at 960-970 cm⁻¹. -1 1060-1150cm -1 Stronger absorption peaks were generated at these locations, which are attributed to the stretching vibrations of carbon-carbon double bonds (C=C) in olefins and the absorption peaks of ether bonds (COC), respectively; additionally, absorption peaks were observed at 3000-3500 cm⁻¹. -1The hydroxyl absorption vibration of the cured polyvinyl alcohol is reduced, indicating that the hydrogen bonding of the hydroxyl groups of the cured polyvinyl alcohol is reduced, and a condensation reaction occurs to generate unsaturated carbon-carbon double bonds (C=C) and ether bonds (C-O-C). Further, the XRD image shows that the diffraction intensity of the cured polyvinyl alcohol is significantly increased, indicating that the crystallinity is improved. This is because the cured polyvinyl alcohol adhesive generates ether bonds and double bonds, and the molecular chains are arranged more closely and compactly, and the cohesion is improved, thereby facilitating the improvement of the bonding strength and water resistance. In summary, the cured polyvinyl alcohol adhesive undergoes a crosslinking reaction, and the single hydrogen bond crosslinking is converted into a double network crosslinking structure composed of chemical bonds and hydrogen bonds.

[0053] Therefore, the present embodiment realizes a water-based polyvinyl alcohol bonding plywood technology using only a single component, which is expected to break through the technical bottleneck of the current organic solvent formaldehyde-based bonding mode. An extremely promising water-based formaldehyde-free polyvinyl alcohol plywood technology is provided.

[0054] Specific embodiment two: The difference between the present embodiment and the first specific embodiment is that the polyvinyl alcohol in step one has a molecular weight of 25,000 to 300,000 and an alcoholysis degree of 70% to 99%. The rest is the same as the first specific embodiment.

[0055] Specific embodiment three: The difference between the present embodiment and the first or second specific embodiment is that the gradient heating dissolution in step one is specifically carried out by stirring at a temperature of 20°C to 40°C, a temperature of 50°C to 70°C, and a temperature of 80°C to 100°C, respectively, for 0.5h to 5h, until the solution is transparent. The rest is the same as the first or second specific embodiment.

[0056] The gradient heating dissolution process according to the present embodiment is as follows:

[0057] The polyvinyl alcohol is added to water and stirred at a temperature of 20°C to 40°C for 0.5h to 5h. If the temperature is significantly dissolved at this time, continue stirring at this temperature until complete dissolution to form a transparent glue solution. If it cannot be effectively dissolved, then heat the system to a temperature of 50°C to 70°C, and stir at this temperature for 0.5h to 5h. If the temperature is significantly dissolved at this time, continue stirring at this temperature until complete dissolution to form a transparent glue solution. If it cannot be effectively dissolved, continue heating to a temperature of 80°C to 100°C and keep stirring until the polyvinyl alcohol is completely dissolved to form a uniform transparent glue solution.

[0058] Specific embodiment four: The difference between the present embodiment and any one of the first to third specific embodiments is that the mass ratio of polyvinyl alcohol to water in step one is 1:(3-20). The rest is the same as the first to third specific embodiments.

[0059] Specific embodiment five: the difference between this embodiment and one of the specific embodiments one to four is that: the glue amount in step two is 90.4 kg / m 3 ~100.4kg / m 3 The water-based polyvinyl alcohol adhesive is coated on both sides of the core board. The others are the same as specific embodiments one to four.

[0060] Specific embodiment six: the difference between this embodiment and one of the specific embodiments one to five is that: the core board and the veneer in step two are wood or bamboo; the wood is poplar, birch, water willow, oak, cedar, pine or beech. The others are the same as specific embodiments one to five.

[0061] Specific embodiment seven: the difference between this embodiment and one of the specific embodiments one to six is that: the core board in step two is a veneer, a spliced board composed of wood strips or a wood lattice structure board. The others are the same as specific embodiments one to six.

[0062] When the wood unit is a common veneer, the prepared plywood is called a common plywood; when the core is a spliced board composed of wood strips or a wood lattice structure board, the prepared plywood is called a carpentry board (a plywood with a solid wood core).

[0063] Specific embodiment eight: the difference between this embodiment and one of the specific embodiments one to seven is that: the cold pressing in step three is specifically under the condition that the cold pressing pressure is 0.1 MPa~1.5 MPa, and the cold pressing time is 10 min~24 h. The others are the same as specific embodiments one to seven.

[0064] Specific embodiment nine: the difference between this embodiment and one of the specific embodiments one to eight is that: the room temperature closed storage in step three is until the moisture content is 8%~30%. The others are the same as specific embodiments one to eight.

[0065] Specific embodiment ten: the difference between this embodiment and one of the specific embodiments one to nine is that: the solidified board blank is placed at room temperature for 1 day~5 days in step five. The others are the same as specific embodiments one to nine.

[0066] The following examples are used to verify the beneficial effects of the present application:

[0067] Example one:

[0068] The polyvinyl alcohol is added to water, and then dissolved by gradient heating to obtain a water-based polyvinyl alcohol adhesive;

[0069] The polyvinyl alcohol is PVA1795, and the alcoholysis degree is 91%~98%.

[0070] The gradient heating dissolving specifically stirs for 1h under the condition of 40℃, then stirs for 1h under the condition of 60℃, and then stirs until the solution is transparent under the condition of 90℃.

[0071] The mass ratio of the polyvinyl alcohol to water is 15:100.

[0072] Example Two:

[0073] The polyvinyl alcohol is added into water, and then gradient heating dissolving is performed to obtain the water-based polyvinyl alcohol adhesive.

[0074] The polyvinyl alcohol is PVA1799, and the alcoholysis degree is 98%~99%.

[0075] The gradient heating dissolving specifically stirs for 1h under the condition of 40℃, then stirs for 1h under the condition of 60℃, and then stirs until the solution is transparent under the condition of 95℃.

[0076] The mass ratio of the polyvinyl alcohol to water is 15:100.

[0077] Example Three:

[0078] The polyvinyl alcohol is added into water, and then gradient heating dissolving is performed to obtain the water-based polyvinyl alcohol adhesive.

[0079] The polyvinyl alcohol is PVA2499, and the alcoholysis degree is 98%~99%.

[0080] The gradient heating dissolving specifically stirs for 1h under the condition of 40℃, then stirs for 2h under the condition of 60℃, and then stirs until the solution is transparent under the condition of 95℃.

[0081] The mass ratio of the polyvinyl alcohol to water is 12.5:100.

[0082] Example Four:

[0083] The polyvinyl alcohol is added into water, and then gradient heating dissolving is performed to obtain the water-based polyvinyl alcohol adhesive.

[0084] The polyvinyl alcohol is PVA2299, and the alcoholysis degree is 98%~99%.

[0085] The gradient heating dissolving specifically stirs for 1h under the condition of 40℃, then stirs for 2h under the condition of 60℃, and then stirs until the solution is transparent under the condition of 95℃.

[0086] The mass ratio of the polyvinyl alcohol to water is 12.5:100.

[0087] Embodiment 1:

[0088] A preparation method of water-resistant formaldehyde-free added water-based polyvinyl alcohol plywood is carried out according to the following steps:

[0089] I. Gluing:

[0090] According to the glue application amount of 95.4 kg / m³, the water-based polyvinyl alcohol adhesive prepared in embodiment one, two, three or four is coated on both sides of the core board to obtain the coated core board, and then the coated core board is stacked with the veneer in a cross-grain manner to obtain a board blank;

[0091] II. Pre-pressing and standing:

[0092] The board blank is cold-pressed, and then closed and stood at room temperature for 6 h to obtain a pre-pressed and stood board blank;

[0093] III. Curing:

[0094] The pre-pressed and stood board blank is hot-pressed and cured to obtain a cured board blank;

[0095] The hot-pressing and curing is specifically carried out at a hot-pressing temperature of 170℃ and a hot-pressing pressure of 1 MPa for 7 min;

[0096] IV. Post-treatment:

[0097] The cured board blank is placed at room temperature for 2 days to obtain a plywood, i.e. the preparation method of water-resistant formaldehyde-free added water-based polyvinyl alcohol plywood is completed.

[0098] The core board and the veneer in step one are both poplar veneers cut by rotary cutting, which are seasoned at room temperature to a moisture content of 6% to 15%.

[0099] The cold-pressing in step two is specifically carried out at a cold-pressing pressure of 0.5 MPa for 6 h.

[0100] The closed standing in step two is to a moisture content of 8%.

[0101] Embodiment 2: Different from embodiment 1, the veneer and the core board in step one are both birch veneers cut by rotary cutting. The others are the same as embodiment 1.

[0102] Embodiment 3: Different from embodiment 1, the veneer and the core board in step one are both eucalyptus veneers cut by rotary cutting. The others are the same as embodiment 1.

[0103] Embodiment 4: Different from embodiment 1, the veneer and the core board in step one are both pine veneers cut by rotary cutting. The others are the same as embodiment 1.

[0104] Example 5: The difference between this example and Example 1 is that the veneer and core board in Step 1 are both bamboo veneer. The others are the same as Example 1.

[0105] Example 6: The difference between this example and Example 1 is that the pre-pressed and stored board is subjected to microwave curing in Step 3; the microwave curing is specifically under the conditions of microwave output power of 700 W and microwave frequency of 2450 MHz, and the microwave treatment time is 10 s. The others are the same as Example 1.

[0106] The determination of each performance index of the water-based polyvinyl alcohol adhesive and plywood is as follows:

[0107] (1) pH value test: The pH value is measured accurately according to the requirements of the national standard GBT14074-2006.

[0108] (2) Solid content test: The required data is recorded after the treatment by using an electric heating constant temperature air drying oven (DCG-9070 type) and weighing according to the requirements of the national standard GBT14074-2006.

[0109] (3) Viscosity test: The water-based polyvinyl alcohol adhesive is subjected to air bubble removal treatment, the temperature of the water-based polyvinyl alcohol adhesive is room temperature, and the test volume is about 15 mL. The test is carried out according to the standard GB / T2794-1995, and a rotary viscometer (Shanghai Pingxuan Scientific Instrument Co., Ltd., NDJ-79) is used for the test.

[0110] Through the test, the pH of the water-based polyvinyl alcohol adhesive prepared in Example 2 is about 7, neutral; the solid content is about 13%; and the viscosity is 1200 mP·s.

[0111] (4) Cold water resistance: The test sample is prepared according to the American standard, that is, cut into a rectangular test piece with a length of 127 mm and a width of 50.8 mm, and at least 6 test pieces are taken from each board, and stored in a dryer for 2 days.

[0112] Cold water resistance performance test was determined according to American national standard ANSI / HPV HP-1-2009-4.6, using 3 cycles of immersion test, immersed in (24±3) ℃ water for 4h, then placed in an oven, dried at 49~52 ℃ for 19h until the moisture content was between 4%~12%. The above cycle was repeated until all test pieces were destroyed or completed 3 cycles directly. When the continuous length of any delamination between the glue layers of the plywood was more than 50.8mm, the delamination depth of any point was more than 6.4mm and the width was 0.08mm, the test piece was unqualified. In 90% of the test panels, 5 / 6 test pieces should pass 1 cycle, 4 / 6 test pieces should pass the 3rd cycle; in all test panels, 95% of the test pieces should pass the first cycle, 85% of the test pieces should pass the 3rd cycle.

[0113] Figure 1 The actual figure of the cold water resistance performance of the plywood bonded with the water-based polyvinyl alcohol adhesive prepared in Example 2 in Example 1; PY@0, PY@1, PY@2 and PY@3 represent the poplar plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively; PH@0, PH@1, PH@2 and PH@3 represent the birch plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively; PA@0, PA@1, PA@2 and PA@3 represent the eucalyptus plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively; PS@0, PS@1, PS@2 and PS@3 represent the pine plywood without cold water immersion, one-time cold water immersion, twice cold water immersion and three-time cold water immersion, respectively. It can be seen from Figure 1 that the plywood panels did not crack, meeting the cold water resistance performance requirements.

[0114] (5) Bonding shear strength test: the test piece was prepared according to the national standard GB / T17657-2013, and when the test piece was prepared, the wood grain direction of the measured core board between the glue layers should be perpendicular to the length direction of the test piece, and the slot width and depth of the test piece should be according to the size and requirements of the drawing according to the national standard.

[0115] Dry state shear strength: according to GB / T17657-2013 standard, the prepared standard test piece was stored in a dryer for 2 days, and the test was carried out using a universal mechanical testing machine (RGT-20 polyvinyl alcohol), the force sensor was 50KN, and the tensile speed was 5mm / min.

[0116] 24h cold water immersion wet state shear strength: according to GB / T17657-2013 standard, the prepared standard test piece was immersed in cold water for 24h, and then immediately subjected to tensile shear test.

[0117] (6) Free formaldehyde test: According to GB / T 17657-2013.4.59 standard, the prepared plywood was tested for free formaldehyde content by using the desiccator method. Mainly including two steps: (1) Collecting formaldehyde - placing a crystallizing dish containing distilled water at the bottom of the desiccator, and placing the sample on the metal bracket fixed above it, the released formaldehyde is absorbed by the distilled water as the sample solution; (2) Determining the concentration of formaldehyde - the absorbance of the solution in the sample was determined by spectrophotometer, and the concentration of formaldehyde was obtained from the standard curve drawn in advance.

[0118] Table 1 Storage time comparison of water-based polyvinyl alcohol adhesive prepared in examples one to four and other wood adhesives

[0119]

[0120] Under the conditions of storage temperature 20℃~30℃ and environmental relative humidity 45%~75%, compared with other wood adhesives, the storage time of polyvinyl alcohol adhesive is greater than one month, while the storage time of urea-formaldehyde resin and soybean adhesive is less than 30 days.

[0121] Table 2 Tensile shear strength and free formaldehyde release of plywood prepared in examples 1 to 6

[0122]

[0123]

[0124] The prepared plywood tensile shear strength can completely pass the 20℃±3℃ cold water immersion test, meeting the requirements of the latest ordinary plywood standard GB / T 9846-2015 indoor plywood; in addition, it can also pass the three times immersion cold water cycle test determined by the American standard polyvinyl alcohol NSI / HPV polyvinyl alcohol HP-1-2009-4.6, meeting the American standard indoor plywood (Plywood for use in interior Figure 1 ). The free formaldehyde release is almost 0, and the trace amount of formaldehyde content is the release amount of wood itself, so the plywood product prepared by this method is water-resistant and formaldehyde-free polyvinyl alcohol adhesive.

[0125] Table 3 Comparison of various indexes of water-based polyvinyl alcohol adhesive prepared in example two and urea-formaldehyde adhesive

[0126]

[0127] The water-based polyvinyl alcohol adhesive prepared in example two was dried into a film, and then the infrared spectrum images and XRD images before and after curing were tested, and the curing was specifically at a temperature of 170℃ for 7min.

[0128] Figure 2The infrared spectrum images of the waterborne polyvinyl alcohol adhesive prepared in Example 2 before and after curing; show that after curing, polyvinyl alcohol produces stronger absorption peaks at 960-970 cm -1 and 1060-1150 cm -1 , respectively, which are attributed to the stretching vibration of olefin carbon-carbon double bond (C=C) and ether bond (C-O-C) absorption peaks. In addition, the hydroxyl absorption vibration at 3000-3500 cm -1 is reduced, indicating that the hydrogen bonding of hydroxyl groups in the cured polyvinyl alcohol is reduced, and condensation reaction occurs to generate unsaturated carbon-carbon double bond (C=C) and ether bond (C-O-C).

[0129] Figure 3 The XRD images of the waterborne polyvinyl alcohol adhesive prepared in Example 2 before and after curing; show that after curing, the diffraction intensity of polyvinyl alcohol increases significantly, indicating that the crystallinity is improved. This is because the cured polyvinyl alcohol adhesive generates ether bond and double bond, and the molecular chains are arranged more closely and compactly, and the cohesion is improved, thereby facilitating the improvement of its bonding strength and water resistance.

[0130] In summary, the cured polyvinyl alcohol adhesive undergoes crosslinking reaction, and the single hydrogen bond crosslinking is transformed into a double network crosslinking structure composed of carbon-carbon double bond and ether bond.

Claims

1. A method for preparing water-resistant, formaldehyde-free plywood using water-based polyvinyl alcohol, characterized in that... It is done in the following steps: I. Preparation of water-based polyvinyl alcohol adhesive: Polyvinyl alcohol is added to water and then heated in a gradient to dissolve it, thus obtaining an aqueous polyvinyl alcohol adhesive. The polyvinyl alcohol has a molecular weight of 25,000 to 300,000 and a degree of alcoholysis of 70% to 99%. The gradient heating dissolution specifically involves stirring at temperatures of 20℃~40℃, 50℃~70℃, and 80℃~100℃ for 0.5h~5h respectively, until the solution becomes transparent; The mass ratio of polyvinyl alcohol to water is 1:(3~20); II. Applying adhesive: The application rate is 90.4 kg / m² 3 ~100.4kg / m 3 Water-based polyvinyl alcohol adhesive is applied to both sides of the core board to obtain the coated core board. Then, the coated core board is stacked with the veneer in a cross-grain pattern to obtain the board blank. III. Pre-compression and aging: The slab is cold-pressed and then aged at room temperature until the moisture content is 8%~30%, resulting in a pre-pressed and aged slab. The cold pressing specifically refers to cold pressing for 6 hours to 24 hours under a cold pressing pressure of 0.1 MPa to 0.5 MPa. IV. Curing: The pre-pressed and aged slabs are then subjected to hot pressing or microwave curing to obtain cured slabs. The hot-press curing process specifically involves hot-pressing at a temperature of 180℃~230℃ and a pressure of 0.5MPa~5MPa for 7 minutes. The microwave curing is specifically performed under the conditions of microwave output power of 500W~1000W and microwave frequency of 2400MHz~2500MHz, with microwave treatment for 10s~100s. V. Post-processing: The cured board blank is left to stand at room temperature for 2 to 5 days to complete the preparation method of water-resistant, formaldehyde-free waterborne polyvinyl alcohol plywood.

2. The method for preparing water-resistant, formaldehyde-free plywood using water-based polyvinyl alcohol according to claim 1, characterized in that... The core board and veneer mentioned in step two are both made of wood or bamboo; the wood is poplar, birch, ash, oak, fir, pine or beech.

3. The method for preparing water-resistant, formaldehyde-free plywood using water-based polyvinyl alcohol according to claim 1, characterized in that... The core board mentioned in step two is a single veneer or a spliced ​​board composed of wooden strips.

Images