Preparation method of polyvinyl alcohol solvent-free adhesive and bonding method thereof

Abstract

The application discloses a preparation method and a bonding method of a polyvinyl alcohol solvent-free adhesive, and belongs to the technical field of deep processing / adhesive of wood materials. The application aims at solving the problems of poor process usability caused by high viscosity and low solid content of existing green formaldehyde-free wood adhesive and low bonding strength and poor water resistance of polyvinyl alcohol wood adhesive. The preparation method comprises the following steps: firstly, polyvinyl alcohol is added into water and heated to be dissolved; and secondly, the water-based polyvinyl alcohol solution is formed into a film and dried. The bonding method comprises the following steps: firstly, rapid water activation; secondly, lapping; thirdly, pre-pressing; and fourthly, hot-pressing and curing. The application is used for the preparation method and the bonding of the polyvinyl alcohol solvent-free adhesive.

Description

Technical Field

[0001] This invention belongs to the field of wood material deep processing / adhesive technology. Background Technology

[0002] The widespread use of sustainable materials is a key means for the world to address climate change and reduce emissions of greenhouse gases such as carbon dioxide. Biomass resources, represented by wood, have unique advantages in terms of renewability and carbon sequestration, attracting widespread attention. Processing wood into engineered wood products requires adhesives. However, existing commercial adhesives are mainly formaldehyde-based (such as urea-formaldehyde resin adhesives, phenolic resin adhesives, and melamine-formaldehyde resin adhesives), which release free formaldehyde, posing health risks. Therefore, to reduce formaldehyde release, various green adhesives have been developed, such as soybean adhesives, starch adhesives, and polyvinyl alcohol solution adhesives. However, these adhesives are mainly solvent-based adhesives (using water as the solvent). Because their solid components are mainly large polymer molecules, they are characterized by high viscosity and low solids content, making them prone to causing engineered wood products to crack during hot pressing. Although viscosity can be reduced by adding fillers or increasing the temperature, this increases the complexity of use and raises costs. In addition, the short shelf life and easy spoilage of existing solvent-based adhesives further limit their widespread application.

[0003] Preparing high-viscosity, low-solids solvent-based adhesives into dry, solvent-free adhesives can significantly improve workability, reduce transportation costs, prevent pre-curing and deterioration of solvent-based adhesives, and extend shelf life. Existing solvent-free adhesives mainly include isocyanate adhesives and thermoplastic polymers. However, isocyanate adhesives are highly toxic before curing and are flammable and explosive during transportation, posing safety risks. Thermoplastic polymers lack viscosity and surface activity, requiring physical (e.g., plasma treatment) or chemical (e.g., introducing coupling agents and their groups) activation, leading to a significant increase in usage costs. Furthermore, the water resistance and durability of thermoplastic polymers as adhesives require careful evaluation.

[0004] Polyvinyl alcohol (PVA) is the world's most produced water-soluble polymer, with an annual output of up to 12 billion kilograms. It can be produced on a large scale through non-petroleum routes (bioethanol-ethylene-vinyl acetate-PVA), making it a sustainable and environmentally friendly raw material. PVA's chemical properties are similar to other polyols, rich in hydroxyl functional groups, allowing it to undergo etherification, esterification, and acetalization reactions, showing great potential in wood adhesives. However, due to the large number of hydrophilic hydroxyl groups in its structure, pure PVA suffers from relatively weak bonding strength and poor water resistance as a wood adhesive. Further modification of PVA, such as chemical crosslinking, inorganic nanofiller modification, and polymer blending, is an effective way to address the low bonding strength and poor water resistance of PVA wood adhesives. However, this complicates the preparation process and increases costs, reducing its cost-effectiveness. Summary of the Invention

[0005] This invention aims to address the problems of poor processability caused by the high viscosity and low solid content of existing green formaldehyde-free wood adhesives, as well as the low bonding strength and poor water resistance of polyvinyl alcohol wood adhesives. Therefore, it provides a method for preparing a solvent-free polyvinyl alcohol adhesive and its bonding method.

[0006] A method for preparing a solvent-free polyvinyl alcohol adhesive, comprising the following steps:

[0007] I. Preparation of aqueous polyvinyl alcohol solution:

[0008] Polyvinyl alcohol is added to water and then heated to dissolve, resulting in an aqueous polyvinyl alcohol solution.

[0009] II. Preparation of solvent-free polyvinyl alcohol adhesives:

[0010] An aqueous polyvinyl alcohol solution is poured onto a substrate and coated onto a flat plate using a casting process to form a film. Finally, the film is dried to obtain a solvent-free polyvinyl alcohol adhesive.

[0011] A bonding method using a solvent-free polyvinyl alcohol adhesive, comprising the following steps:

[0012] I. Rapid water activation:

[0013] Water was sprayed onto both sides of the polyvinyl alcohol solvent-free adhesive to activate it, resulting in activated polyvinyl alcohol solvent-free adhesive.

[0014] II. Overlap:

[0015] The activated polyvinyl alcohol solvent-free adhesive is placed between adjacent veneers to obtain overlapping veneers;

[0016] III. Pre-compression:

[0017] The overlapped veneers are cold-pressed to obtain a pre-pressed sample;

[0018] IV. Hot pressing curing:

[0019] Under the conditions of hot pressing temperature of 180℃~230℃ and hot pressing pressure of 0.2MPa~2.5MPa, the pre-pressed sample is hot-pressed for 60s~600s, which completes the bonding method of polyvinyl alcohol solvent-free adhesive.

[0020] The beneficial effects of this invention are:

[0021] 1. In terms of composition, polyvinyl alcohol solvent-free adhesive contains only polyvinyl alcohol. The raw materials are non-toxic, very simple, green and environmentally friendly, and contain no formaldehyde.

[0022] 2. In terms of preparation, polyvinyl alcohol solvent-free adhesive can be prepared by a very simple dissolution-casting film-forming method, which produces green and non-toxic substances, does not rely on complex equipment conditions, and is very easy to scale up and industrialize.

[0023] 3. In terms of application, polyvinyl alcohol (PVA) solvent-free adhesives can achieve adhesive activation through rapid water activation treatment. Subsequently, hot pressing enhances the cohesive strength of the PVA, enabling high-performance bonding of wood / bamboo materials (Table 1) and allowing the preparation of high-performance wood / bamboo plywood (Table 2). Furthermore, PVA solvent-free adhesives exhibit excellent durability, maintaining good bonding performance without cracking during a "63°C hot water immersion-drying" cycle (Table 3). In contrast, traditional solvent-free adhesives based on thermoplastic polymers generally have poorer performance. Compared to existing commercial adhesives, PVA solvent-free adhesives also offer advantages in cost (Table 4), transportation (Table 5), and storage time (Table 6), effectively addressing the problems of inconvenience, short shelf life, and easy deterioration associated with existing high-viscosity, low-solids adhesives.

[0024] 4. In principle, water is sprayed onto both sides of the polyvinyl alcohol (PVA) solvent-free adhesive to activate it. This is because PVA is rich in hydroxyl groups and is a highly hydrophilic adhesive. After water activation, the PVA hydrogen bond network opens, and the PVA becomes activated through adhesion. Then, under heat treatment, a cross-linking and curing reaction occurs, transforming the single hydrogen bond network into a double network structure composed of carbon-carbon double bonds (C=C) / ether bonds (COC) and covalent bonds, with hydrogen bonds forming a synergistic cross-linking effect. Before heat treatment, the endothermic peak of the PVA solvent-free adhesive is located around 107℃. After heat treatment, the endothermic peak disappears, indicating that the PVA solvent-free adhesive has undergone curing and cross-linking after heat treatment. The specific reasons are as follows: Before heat treatment, the PVA solvent-free adhesive is rich in hydroxyl groups. During heat treatment, the hydroxyl groups undergo a condensation reaction, generating chemical bonds and producing water vapor as a byproduct. Water vapor evaporation is an endothermic process, thus forming an endothermic peak. After heat treatment, the PVA solvent-free adhesive has completed cross-linking and curing, therefore the corresponding endothermic peak disappears. To further confirm the chemical bond changes in the solvent-free polyvinyl alcohol (PVA) adhesive before and after heat treatment, X-ray photoelectron spectroscopy (XPS) was performed. Before heat treatment, the C1s spectrum of the PVA adhesive showed signal peaks at 284.8 eV, 286.2 eV, and 287.9 ​​eV, corresponding to C-C, C-OH, and OCO bonds, respectively. After heat treatment, the peak intensity at C-OH decreased, while the peak at 284.8 eV, corresponding to the C-C / C=C bond, increased, indicating that the C-OH bond intensity decreased and a C=C bond was formed. Furthermore, the FTIR spectra of the PVA adhesive before and after heat treatment also showed that after heat treatment, the signal intensity at 1140 cm⁻¹ increased. -1 The peak intensity attributable to COC bonds increased significantly, indicating that ether bonds were formed in the solvent-free polyvinyl alcohol adhesive after heat treatment. In summary, the solvent-free polyvinyl alcohol adhesive can undergo curing and crosslinking after heat treatment, transforming from a single hydrogen-bonded crosslinking network into a dual-network structure with the cooperation of chemical bonds (COC bonds / C=C bonds) and hydrogen bonds.

[0025] Instruction manual illustrations

[0026] Figure 1 The following are the DSC spectra of the polyvinyl alcohol solvent-free adhesive prepared in Example 2 before and after heat treatment;

[0027] Figure 2 XPS spectra of the polyvinyl alcohol solvent-free adhesive prepared in Example 2 before and after heat treatment;

[0028] Figure 3 The images show the FTIR spectra of the alcohol-based solvent-free adhesive prepared in Example 2 before and after heat treatment. Detailed Implementation

[0029] Specific Implementation Method 1: This implementation method provides a method for preparing a solvent-free polyvinyl alcohol adhesive, which is carried out according to the following steps:

[0030] I. Preparation of aqueous polyvinyl alcohol solution:

[0031] Polyvinyl alcohol is added to water and then heated to dissolve, resulting in an aqueous polyvinyl alcohol solution.

[0032] II. Preparation of solvent-free polyvinyl alcohol adhesives:

[0033] An aqueous polyvinyl alcohol solution is poured onto a substrate and coated onto a flat plate using a casting process to form a film. Finally, the film is dried to obtain a solvent-free polyvinyl alcohol adhesive.

[0034] In this specific embodiment, polyvinyl alcohol is dissolved in water in one step, and then a solvent-free polyvinyl alcohol adhesive is prepared by casting film formation, which is very simple. After the solvent-free polyvinyl alcohol adhesive is rapidly activated by water, it is bonded to the veneer. Then, the solvent-free polyvinyl alcohol adhesive is cured by hot pressing to enhance the cohesive force and achieve high-performance bonding.

[0035] The beneficial effects of this embodiment are:

[0036] 1. In terms of composition, polyvinyl alcohol solvent-free adhesive contains only polyvinyl alcohol. The raw materials are non-toxic, very simple, green and environmentally friendly, and contain no formaldehyde.

[0037] 2. In terms of preparation, polyvinyl alcohol solvent-free adhesive can be prepared by a very simple dissolution-casting film-forming method, which produces green and non-toxic substances, does not rely on complex equipment conditions, and is very easy to scale up and industrialize.

[0038] 3. In terms of application, polyvinyl alcohol (PVA) solvent-free adhesives can achieve adhesive activation through rapid water activation treatment. Subsequently, hot pressing enhances the cohesive strength of the PVA, enabling high-performance bonding of wood / bamboo materials (Table 1) and allowing the preparation of high-performance wood / bamboo plywood (Table 2). Furthermore, PVA solvent-free adhesives exhibit excellent durability, maintaining good bonding performance without cracking during a "63°C hot water immersion-drying" cycle (Table 3). In contrast, traditional solvent-free adhesives based on thermoplastic polymers generally have poorer performance. Compared to existing commercial adhesives, PVA solvent-free adhesives also offer advantages in cost (Table 4), transportation (Table 5), and storage time (Table 6), effectively addressing the problems of inconvenience, short shelf life, and easy deterioration associated with existing high-viscosity, low-solids adhesives.

[0039] 4. In principle, water is sprayed onto both sides of the polyvinyl alcohol (PVA) solvent-free adhesive to activate it. This is because PVA is rich in hydroxyl groups and is a highly hydrophilic adhesive. After water activation, the PVA hydrogen bond network opens, and the PVA becomes activated through adhesion. Then, under heat treatment, a cross-linking and curing reaction occurs, transforming the single hydrogen bond network into a double network structure composed of carbon-carbon double bonds (C=C) / ether bonds (COC) and covalent bonds, with hydrogen bonds forming a synergistic cross-linking effect. Before heat treatment, the endothermic peak of the PVA solvent-free adhesive is located around 107℃. After heat treatment, the endothermic peak disappears, indicating that the PVA solvent-free adhesive has undergone curing and cross-linking after heat treatment. The specific reasons are as follows: Before heat treatment, the PVA solvent-free adhesive is rich in hydroxyl groups. During heat treatment, the hydroxyl groups undergo a condensation reaction, generating chemical bonds and producing water vapor as a byproduct. Water vapor evaporation is an endothermic process, thus forming an endothermic peak. After heat treatment, the PVA solvent-free adhesive has completed cross-linking and curing, therefore the corresponding endothermic peak disappears. To further confirm the chemical bond changes in the solvent-free polyvinyl alcohol (PVA) adhesive before and after heat treatment, X-ray photoelectron spectroscopy (XPS) was performed. Before heat treatment, the C1s spectrum of the PVA adhesive showed signal peaks at 284.8 eV, 286.2 eV, and 287.9 ​​eV, corresponding to C-C, C-OH, and OCO bonds, respectively. After heat treatment, the peak intensity at C-OH decreased, while the peak at 284.8 eV, corresponding to the C-C / C=C bond, increased, indicating that the C-OH bond intensity decreased and a C=C bond was formed. Furthermore, the FTIR spectra of the PVA adhesive before and after heat treatment also showed that after heat treatment, the signal intensity at 1140 cm⁻¹ increased. -1 The peak intensity attributable to COC bonds increased significantly, indicating that ether bonds were formed in the solvent-free polyvinyl alcohol adhesive after heat treatment. In summary, the solvent-free polyvinyl alcohol adhesive can undergo curing and crosslinking after heat treatment, transforming from a single hydrogen-bonded crosslinking network into a dual-network structure with the cooperation of chemical bonds (COC bonds / C=C bonds) and hydrogen bonds.

[0040] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the molecular weight of the polyvinyl alcohol mentioned in step one is 10,000 to 300,000, and the degree of alcoholysis is 70% to 99%. Everything else is the same as in Specific Implementation Method One.

[0041] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the heating and dissolving process in step one is specifically carried out at a temperature of 10℃ to 100℃, with stirring for 0.5h to 72h until the solution becomes transparent. Everything else is the same as in Specific Implementation Method One or Two.

[0042] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the mass ratio of polyvinyl alcohol to water in step one is 1:(2-20). Everything else is the same as in Specific Implementation Methods One to Three.

[0043] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the drying process in step two is specifically carried out at a temperature of 10℃ to 100℃ for 1 hour to 72 hours. Everything else is the same as in Specific Implementation Methods One to Four.

[0044] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the polyvinyl alcohol solvent-free adhesive mentioned in step two is a film-like adhesive with a thickness of 0.01 mm to 0.1 mm. Everything else is the same as in Specific Implementation Methods One to Five.

[0045] Specific Implementation Method Seven: This implementation method provides a bonding method using a solvent-free polyvinyl alcohol adhesive, which is carried out according to the following steps:

[0046] I. Rapid water activation:

[0047] Water was sprayed onto both sides of the polyvinyl alcohol solvent-free adhesive to activate it, resulting in activated polyvinyl alcohol solvent-free adhesive.

[0048] II. Overlap:

[0049] The activated polyvinyl alcohol solvent-free adhesive is placed between adjacent veneers to obtain overlapping veneers;

[0050] III. Pre-compression:

[0051] The overlapped veneers are cold-pressed to obtain a pre-pressed sample;

[0052] IV. Hot pressing curing:

[0053] Under the conditions of hot pressing temperature of 180℃~230℃ and hot pressing pressure of 0.2MPa~2.5MPa, the pre-pressed sample is hot-pressed for 60s~600s, which completes the bonding method of polyvinyl alcohol solvent-free adhesive.

[0054] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Seven in that the mass ratio of the polyvinyl alcohol solvent-free adhesive mentioned in step one to the total mass ratio of the water sprayed on both sides is 1:(1-5). Everything else is the same as in Specific Implementation Method Seven.

[0055] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Seven or Eight in that the veneer mentioned in step two is a wood veneer or a bamboo veneer; the wood veneer is made of poplar, birch, ash, oak, fir, pine, or beech. Everything else is the same as in Specific Implementation Method Seven or Eight.

[0056] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods Seven to Nine in that the cold pressing described in step two is specifically performed under a cold pressing pressure of 0.2 MPa to 2.5 MPa for 2 to 20 minutes. Everything else is the same as in Specific Implementation Methods Seven to Nine.

[0057] The beneficial effects of the present invention are verified using the following embodiments:

[0058] Example 1:

[0059] A method for preparing a solvent-free polyvinyl alcohol adhesive, comprising the following steps:

[0060] I. Preparation of aqueous polyvinyl alcohol solution:

[0061] Polyvinyl alcohol is added to water and then heated to dissolve, resulting in an aqueous polyvinyl alcohol solution.

[0062] II. Preparation of solvent-free polyvinyl alcohol adhesives:

[0063] A water-based polyvinyl alcohol solution is poured onto a glass substrate and coated onto a flat plate using a casting process to form a film. Finally, the film is dried to obtain a solvent-free polyvinyl alcohol adhesive.

[0064] The polyvinyl alcohol mentioned in step one has a molecular weight of 13,000 to 23,000 and a degree of alcoholysis of 87% to 89%.

[0065] The heating and dissolving process described in step one specifically involves stirring at 50°C for 24 hours until the solution becomes transparent.

[0066] The mass ratio of polyvinyl alcohol to water mentioned in step one is 12:100.

[0067] The drying process described in step two specifically involves drying at a temperature of 30°C for 48 hours.

[0068] The polyvinyl alcohol solvent-free adhesive mentioned in step two is a film adhesive with a thickness of 0.05 mm.

[0069] Example 2:

[0070] A method for preparing a solvent-free polyvinyl alcohol adhesive, comprising the following steps:

[0071] I. Preparation of aqueous polyvinyl alcohol solution:

[0072] Polyvinyl alcohol is added to water and then heated to dissolve, resulting in an aqueous polyvinyl alcohol solution.

[0073] II. Preparation of solvent-free polyvinyl alcohol adhesives:

[0074] A water-based polyvinyl alcohol solution is poured onto a glass substrate and coated onto a flat plate using a casting process to form a film. Finally, the film is dried to obtain a solvent-free polyvinyl alcohol adhesive.

[0075] The polyvinyl alcohol mentioned in step one has a molecular weight of 31,000 to 50,000 and a degree of alcoholysis of 98% to 99%.

[0076] The heating and dissolving process described in step one specifically involves stirring at a temperature of 95°C for 6 hours until the solution becomes transparent.

[0077] The mass ratio of polyvinyl alcohol to water mentioned in step one is 12:100.

[0078] The drying process described in step two specifically involves drying at a temperature of 20°C for 72 hours.

[0079] The polyvinyl alcohol solvent-free adhesive mentioned in step two is a film adhesive with a thickness of 0.06 mm.

[0080] Example 3:

[0081] A method for preparing a solvent-free polyvinyl alcohol adhesive, comprising the following steps:

[0082] I. Preparation of aqueous polyvinyl alcohol solution:

[0083] Polyvinyl alcohol is added to water and then heated to dissolve, resulting in an aqueous polyvinyl alcohol solution.

[0084] II. Preparation of solvent-free polyvinyl alcohol adhesives:

[0085] A water-based polyvinyl alcohol solution is poured onto a glass substrate and coated onto a flat plate using a casting process to form a film. Finally, the film is dried to obtain a solvent-free polyvinyl alcohol adhesive.

[0086] The polyvinyl alcohol mentioned in step one has a molecular weight of 30,000 to 70,000 and a degree of alcoholysis of 87% to 89%.

[0087] The heating and dissolving process described in step one specifically involves stirring at 70°C for 48 hours until the solution becomes transparent.

[0088] The mass ratio of polyvinyl alcohol to water mentioned in step one is 12:100.

[0089] The drying process described in step two specifically involves drying at a temperature of 50°C for 12 hours.

[0090] The polyvinyl alcohol solvent-free adhesive mentioned in step two is a film adhesive with a thickness of 0.08 mm.

[0091] Example 4:

[0092] A method for preparing a solvent-free polyvinyl alcohol adhesive, comprising the following steps:

[0093] I. Preparation of aqueous polyvinyl alcohol solution:

[0094] Polyvinyl alcohol is added to water and then heated to dissolve, resulting in an aqueous polyvinyl alcohol solution.

[0095] II. Preparation of solvent-free polyvinyl alcohol adhesives:

[0096] An aqueous polyvinyl alcohol solution is poured onto a glass substrate and coated onto a flat plate using a casting process to form a film. Finally, the film is dried to obtain a solvent-free polyvinyl alcohol adhesive.

[0097] The polyvinyl alcohol mentioned in step one has a molecular weight of 85,000 to 124,000 and a degree of alcoholysis of 99%.

[0098] The heating and dissolving process described in step one specifically involves stirring at a temperature of 95°C for 72 hours until the solution becomes transparent.

[0099] The mass ratio of polyvinyl alcohol to water mentioned in step one is 5:100.

[0100] The drying process described in step two specifically involves drying at a temperature of 60°C for 72 hours.

[0101] The polyvinyl alcohol solvent-free adhesive mentioned in step two is a film adhesive with a thickness of 0.10 mm.

[0102] Example 1:

[0103] A bonding method using a solvent-free polyvinyl alcohol adhesive, comprising the following steps:

[0104] I. Rapid water activation:

[0105] At room temperature, water was sprayed onto both sides of the polyvinyl alcohol solvent-free adhesive prepared in Examples 1, 2, 3 or 4, and activated for 5 seconds to obtain the activated polyvinyl alcohol solvent-free adhesive.

[0106] The mass ratio of the solvent-free polyvinyl alcohol adhesive to the total mass of water sprayed on both sides is 1:1.

[0107] II. Overlap:

[0108] The activated polyvinyl alcohol solvent-free adhesive is placed between adjacent veneers to obtain overlapping veneers;

[0109] III. Pre-compression:

[0110] Under a cold pressing pressure of 1 MPa, the overlapping veneers are cold-pressed for 7 minutes to obtain a pre-pressed sample.

[0111] IV. Hot pressing curing:

[0112] Under the conditions of hot pressing temperature of 200℃ and hot pressing pressure of 1MPa, the pre-pressed sample is hot-pressed for 180s to complete the bonding method of polyvinyl alcohol solvent-free adhesive.

[0113] The veneer mentioned in step two is a poplar veneer that has been rotary-cut.

[0114] Example 2: This example differs from Example 1 in that the veneer mentioned in step two is rotary-cut birch veneer. Everything else is the same as in Example 1.

[0115] Example 3: This example differs from Example 1 in that the veneer mentioned in step two is a rotary-cut beech veneer. Everything else is the same as in Example 1.

[0116] Example 4: This example differs from Example 1 in that the veneer mentioned in step two is bamboo veneer. Everything else is the same as in Example 1.

[0117] The polyvinyl alcohol solvent-free adhesive prepared in Example 2 was subjected to DSC, XPS, and FTIR spectra before and after heat treatment; the heat treatment was specifically carried out at a heat treatment temperature of 200°C for 3 minutes.

[0118] Figure 1 The images show the DSC spectra of the polyvinyl alcohol solvent-free adhesive prepared in Example 2 before and after heat treatment. Before heat treatment, the endothermic peak of the polyvinyl alcohol solvent-free adhesive was located near 107°C, while after heat treatment, the endothermic peak disappeared, indicating that the polyvinyl alcohol solvent-free adhesive underwent curing and crosslinking after heat treatment.

[0119] Figure 2 The XPS spectra of the solventless polyvinyl alcohol adhesive prepared in Example 2 before and after heat treatment are shown. Before heat treatment, the C1s spectrum of the solventless polyvinyl alcohol adhesive showed signal peaks at 284.8 eV, 286.2 eV, and 287.9 ​​eV, corresponding to C-C bonds, C-OH bonds, and OCO bonds, respectively. After heat treatment, the peak intensity at C-OH decreased, while the peak at 284.8 eV corresponding to the C / C=C bond increased, indicating that the C-OH bonds decreased and C=C bonds were formed.

[0120] Figure 3 The images show the FTIR spectra of the solvent-free alcohol adhesive prepared in Example 2 before and after heat treatment; after heat treatment, the spectra are at 1140 cm⁻¹. -1 The peak intensity attributable to the COC bond increased significantly, indicating that the solvent-free polyvinyl alcohol adhesive formed ether bonds after heat treatment.

[0121] In summary, after heat treatment, the solventless polyvinyl alcohol adhesive can undergo curing and cross-linking, transforming from a single hydrogen bond cross-linking network into a dual cross-linking network structure with the cooperation of chemical bonds (COC bonds / C=C bonds) and hydrogen bonds.

[0122] Performance index measurement:

[0123] (1) Overlap Shear Strength Test: Plywood samples with a width of 25 mm were cut, with an overlap area of ​​25 × 25 mm, and the overlap direction was parallel to the grain. Dry shear strength was tested directly on a universal testing machine, and wet shear strength was tested after soaking in 63℃ hot water for 3 hours. The universal testing machine (AGS-X, Shimadzu, Japan) was used, with a tensile speed of 5 mm / min and a mechanical sensor of 1 kN.

[0124] (2) Adhesive shear strength test: Specimens were prepared according to national standard GB / T 17657-2013. The wood grain direction of the core board between the adhesive layers should be perpendicular to the length direction of the specimen. The groove width and depth of the specimen were determined according to the dimensions and requirements shown in the national standard drawings. Wet shear strength: According to GB / T17657-2013, the prepared standard specimens were immersed in hot water at 63℃ for 3 hours, and then tested for wet shear strength.

[0125] (3) Wet-dry cycle shear strength test: The lapped sample was immersed in hot water at 63℃ for 3 hours, and then dried at 63℃ for 16 hours. This was recorded as one wet-dry cycle test, and the wet and dry strengths were recorded respectively. The second cycle test was conducted by repeating the same wet-dry cycle test after the first cycle, and the wet and dry strengths were recorded respectively.

[0126] Table 1. Comparison of lap shear strength test results between polyvinyl alcohol solvent-free adhesive and other adhesives in Example 1.

[0127]

[0128]

[0129] Compared with existing urea-formaldehyde resin (Hebei Qingjun Chemical Co., Ltd.) and existing solvent-free adhesive polyvinyl chloride plastic (Nantong Nanya Plastics Industry Co., Ltd.), the solvent-free polyvinyl alcohol adhesive has significantly better dry and wet strength.

[0130] Table 2. Shear strength test of adhesives prepared using polyvinyl alcohol solvent-free adhesives in Examples 1 to 4.

[0131]

[0132] Poplar plywood, beech plywood, birch plywood, and bamboo plywood prepared using polyvinyl alcohol solvent-free adhesives exhibit wet shear properties far exceeding national standards, demonstrating the high-performance bonding strength of polyvinyl alcohol solvent-free adhesives.

[0133] Table 3 Comparison of durability performance of polyvinyl alcohol solvent-free adhesive and polyvinyl chloride plastic under cyclic testing in Example 1

[0134]

[0135] Compared with existing solvent-free adhesives, the lap strength of polyvinyl chloride (PVC) plastics under cyclic testing is significantly lower than that of polyvinyl alcohol (PVA) solvent-free adhesives. Moreover, PVA solvent-free adhesives still have high dry strength after two wet-dry cycles, indicating that PVA solvent-free adhesives have excellent durability.

[0136] Table 4. Cost Comparison of Polyvinyl Alcohol Solvent-Free Adhesive with Existing Commercial Adhesives in Example 2

[0137] adhesives Unit price (yuan / ton) <![CDATA[Dosage (g / m 2 )]]> <![CDATA[Unit sizing cost (yuan / m 2 )]]> Polyvinyl alcohol solvent-free adhesive 10000 62 0.62 Urea-formaldehyde resin 2300 320 0.74 Isocyanate adhesives 18000 200 3.6 Soy adhesive 4000 350 1.4

[0138] In Example 2, the thickness of the polyvinyl alcohol solvent-free adhesive was 0.06 mm, which, after conversion, yielded a coating weight of 62 g / m². 2 Compared to existing commercial adhesives, the application cost of polyvinyl alcohol solvent-free adhesive is only 0.62 yuan / m². 2 It is comparable to urea-formaldehyde resin and far cheaper than isocyanate adhesives and soybean adhesives, giving it a significant cost advantage.

[0139] Table 5 compares the transportation costs and safety of the polyvinyl alcohol solvent-free adhesives in Examples 1-4 with existing commercial adhesives.

[0140]

[0141]

[0142] The method for calculating the relative price of transportation costs: Assuming the solid content of the adhesive is A%, and the remaining solvent portion is 1-A%, then the relative cost of transporting a unit mass of adhesive is 1 / A%. This means that the extra portion is all transportation solvent, but it is not the effective component of the adhesive. Therefore, the higher the solid content of the adhesive, the more beneficial it is to reduce transportation costs.

[0143] Compared to existing commercial adhesives, polyvinyl alcohol (PVA) solvent-free adhesives have lower transportation costs due to their 100% solids content. The costs of other commercial adhesives are significantly higher than those of PVA solvent-free adhesives. Furthermore, PVA solvent-free adhesives are safe and convenient to transport. Although isocyanates also have a 100% solids content, they are hazardous chemicals and therefore less safe to transport than PVA solvent-free adhesives.

[0144] Table 6 Comparison of storage time between the polyvinyl alcohol solvent-free adhesives of Examples 1-4 and existing commercial adhesives

[0145] adhesives Storage time Polyvinyl alcohol solvent-free adhesive >1 year Urea-formaldehyde resin 10-30 days Isocyanate adhesives 120-180 days Soy adhesive <30 days

[0146] Compared with existing commercial adhesives, polyvinyl alcohol solvent-free adhesives have a storage time of more than one year under storage conditions of 20℃~30℃ and relative humidity of 45%~75%, which is a significant advantage in terms of storage. In contrast, the storage period of urea-formaldehyde resin adhesives, isocyanate adhesives and soybean adhesives is significantly shorter than that of polyvinyl alcohol solvent-free adhesives.

Claims

1. A bonding method using a solvent-free polyvinyl alcohol adhesive, characterized in that... It is done in the following steps: I. Rapid water activation: Water was sprayed onto both sides of the polyvinyl alcohol solvent-free adhesive and activated for 5 seconds to obtain the activated polyvinyl alcohol solvent-free adhesive; the mass ratio of the polyvinyl alcohol solvent-free adhesive to the total mass of water sprayed on both sides was 1:(1~5). The solvent-free polyvinyl alcohol adhesive is prepared according to the following steps: ① Polyvinyl alcohol is added to water and then heated to dissolve, resulting in an aqueous polyvinyl alcohol solution; the mass ratio of polyvinyl alcohol to water is 1:(3~20); the molecular weight of the polyvinyl alcohol is 10,000~300,000, and the degree of alcoholysis is 70%~99%; ② Pour the aqueous polyvinyl alcohol solution onto the substrate and perform flat scraping-casting to form a film, and finally dry it to obtain a solvent-free polyvinyl alcohol adhesive; The solvent-free polyvinyl alcohol adhesive is a film adhesive with a thickness of 0.01 mm to 0.1 mm; II. Overlap: The activated polyvinyl alcohol solvent-free adhesive is placed between adjacent veneers to obtain overlapping veneers; The veneer is either wood veneer or bamboo veneer; III. Pre-compression: Under a cold pressing pressure of 0.2MPa to 2.5MPa, the overlapping veneers are cold-pressed for 2 to 20 minutes to obtain a pre-pressed sample. IV. Hot pressing curing: Under the conditions of hot pressing temperature of 180℃~230℃ and hot pressing pressure of 0.2MPa~2.5MPa, the pre-pressed sample is hot-pressed for 60s~600s to complete the bonding method of polyvinyl alcohol solvent-free adhesive.

2. The bonding method of the solvent-free polyvinyl alcohol adhesive according to claim 1, characterized in that... The heating and dissolving process described in step 1① specifically involves stirring at a temperature of 10℃~100℃ for 0.5h~72h until the solution becomes transparent.

3. The bonding method of the solvent-free polyvinyl alcohol adhesive according to claim 1, characterized in that... The drying process described in step 1② specifically involves drying at a temperature of 10℃~100℃ for 1h~72h.

4. The bonding method of the solvent-free polyvinyl alcohol adhesive according to claim 1, characterized in that... The wood veneer mentioned in step two is poplar, birch, ash, oak, fir, pine, or beech.

Images