Moisture-curing polyurethane and polyvinyl alcohol mixed adhesive as well as preparation method and application thereof

Abstract

The invention discloses a moisture-curable polyurethane and polyvinyl alcohol mixed adhesive and a preparation method and application thereof, and belongs to the field of adhesives, the adhesive is prepared from the following raw materials by mass: 20-80 parts of a polyvinyl alcohol solution, 20-80 parts of a polyurethane adhesive, 15 parts of a dispersant, 9 parts of a plasticizer, and 5 parts of a cross-linking agent. Slowly adding the weighed polyurethane adhesive into the polyvinyl alcohol solution, and then sequentially adding the dispersing agent, the plasticizer and the cross-linking agent; and uniformly stirring for 15-30 minutes to form a uniform mixed adhesive. And transferring the uniformly stirred mixed adhesive into a closed container, sealing, standing for 24 hours in a normal-temperature environment, and fully reacting to obtain the moisture-cured polyurethane and polyvinyl alcohol mixed adhesive.

Description

Technical Field

[0001] The present invention belongs to the field of adhesives, and in particular relates to a moisture-curing polyurethane and polyvinyl alcohol mixed adhesive, and a preparation method and application thereof. Background Art

[0002] Thinly sliced ​​bamboo, an important deep-processed bamboo product, has attracted increasing attention due to its unique physical and chemical properties and is widely used in furniture manufacturing, interior decoration, and construction. During the slicing process for bamboo laminated timber, controlling the moisture content of the bamboo is a key step. Traditional processing methods require reducing the moisture content of the bamboo to a certain level before gluing, and the moisture content must be adjusted again before the subsequent slicing process. This process is not only time-consuming and energy-intensive, but can also lead to waste of bamboo and is associated with high labor costs. To address these issues, CN202410391567 proposes a method for treating high-moisture bamboo strips. This method fine-planes high-moisture bamboo strips and then radially slices them. The resulting thinly sliced ​​bamboo veneer is toughened by atmospheric pressure impregnation and then dried to a final moisture content of 12%. Although this method simplifies the steps for controlling the moisture content of the bamboo and improves the processing performance of the bamboo strips, it does not effectively address the need for gluing high-moisture bamboo strips during the gluing process.

[0003] Polyurethane (PU) adhesives excel at bonding high-moisture substrates, but they also present numerous challenges. First, they contain large amounts of isocyanate monomers, which have a pungent odor and are harmful to human health. Second, the high price of PU adhesives limits their large-scale application. Furthermore, their curing requirements are stringent, and improper handling can easily lead to incomplete or premature curing, compromising bonding quality.

[0004] Polyvinyl alcohol (PVA) has high moisture content adaptability and strong hydrogen bond formation ability, which makes it well suited for gluing high moisture content substrates. PVA can still maintain stable performance in a humid environment and will not lose bonding strength due to water absorption. The large number of hydroxyl groups on its molecular chain can form strong hydrogen bonds with the surface of the substrate, enhancing the bonding effect. Based on the above research background and needs, the applicant directly selected polyvinyl alcohol (PVA) as an adhesive to conduct bamboo gluing experiments. The experimental results show that the bonding strength of bamboo using PVA adhesive can reach 0.7MPa, showing relatively ideal bonding performance. However, in actual applications, PVA adhesives also expose some shortcomings, mainly reflected in their relatively poor water resistance and long curing time. These problems have, to a certain extent, limited their widespread application in gluing high moisture content bamboo. Summary of the Invention

[0005] To address the problem that bamboo materials are difficult to bond due to their high moisture content during bonding, the present invention provides a moisture-curing polyurethane and polyvinyl alcohol mixed adhesive, as well as its preparation method and application. The mixed adhesive can effectively bond bamboo materials under high moisture content conditions, while reducing the cost of the adhesive and improving production efficiency.

[0006] A moisture-curing polyurethane and polyvinyl alcohol mixed adhesive is prepared from the following raw materials in parts by weight: 20-80 parts of polyvinyl alcohol solution, 20-80 parts of polyurethane adhesive, 15 parts of dispersant, 9 parts of plasticizer, and 5 parts of cross-linking agent.

[0007] Furthermore, the polyvinyl alcohol solution is an aqueous solution containing 10% by mass of polyvinyl alcohol.

[0008] The polyvinyl alcohol is industrial grade polyvinyl alcohol with a degree of polymerization of 500-2500 and a degree of alcoholysis of 87%-99%. This polyvinyl alcohol has good water solubility and film-forming properties, and can provide excellent matrix properties for the mixed adhesive.

[0009] Furthermore, the polyurethane adhesive is polyester or polyether polyurethane with a viscosity ranging from 1000 to 5000 mPa·s and a solid content of 40% to 60%.

[0010] Furthermore, the dispersant is triolein, the plasticizer is tributyl citrate (TBC), and the cross-linking agent is dimer diisocyanate (DDI).

[0011] The dispersant triolein is a naturally derived surfactant. Its main component is the esterification product of glycerol and oleic acid. It has good surface activity and dispersibility, can effectively reduce the surface tension of the system, promote uniform dispersion between the components, and thus improve the stability and bonding performance of the mixed adhesive.

[0012] Plasticizer tributyl citrate (TBC) is an environmentally friendly plasticizer. Its main component is the esterification product of citric acid and n-butanol. It has good compatibility and plasticizing efficiency, and can significantly improve the flexibility of the adhesive, making it more adaptable to different substrates.

[0013] The crosslinking agent dimer acid diisocyanate (DDI) is mainly composed of the reaction product of dimer acid and diisocyanate, has good crosslinking activity, and can react with the active groups in polyurethane and polyvinyl alcohol to form a stable crosslinking network, thereby improving the mechanical properties and water resistance and solvent resistance of the adhesive.

[0014] The preparation method of the moisture-curing polyurethane and polyvinyl alcohol mixed adhesive comprises the following steps:

[0015] Accurately weigh all ingredients according to the above formula. Slowly add the weighed polyurethane adhesive to the polyvinyl alcohol solution, followed by the dispersant, plasticizer, and crosslinker. Stir at 600-1200 rpm for 15-30 minutes until all ingredients are evenly mixed and a uniform adhesive mixture is formed. Transfer the mixed adhesive to a sealed container, seal it, and let it sit at room temperature for 24 hours to allow for full reaction. This will yield a moisture-curing polyurethane and polyvinyl alcohol mixed adhesive.

[0016] Use a multi-stage stirring method, first stir at a low speed (300-600 rpm) for 5-10 minutes to initially mix the components, then increase the speed to 800-1200 rpm and stir for 10-20 minutes to ensure uniform mixing.

[0017] The moisture-curing polyurethane and polyvinyl alcohol mixed adhesive can be used to bond materials with high moisture content. Bonding can be achieved with materials with a moisture content of 30-90%. The bonding pressure is 0.5-1.5 MPa and the bonding time is 20-30 minutes.

[0018] Furthermore, when the moisture-curing polyurethane and polyvinyl alcohol mixed adhesive is used to prepare bamboo square materials, the steps are as follows:

[0019] Step 1: Using freshly cut round bamboo as raw material, the moisture content of the bamboo is about 30-90%; splitting the round bamboo into bamboo strips, and then processing the bamboo strips into bamboo strips with a length of 2000-3000 mm, a width of 10-60 mm, and a thickness of 5-20 mm;

[0020] Step 2: Splice the standard bamboo strips radially along the thickness direction and apply mixed adhesive on the splicing surface;

[0021] Step 3: Place the glued bamboo strips into a press and press them at a pressure of 0.5-1.5 MPa for 20-30 minutes at room temperature to obtain bamboo square materials.

[0022] Beneficial Effects: The moisture-curing polyurethane and polyvinyl alcohol mixed adhesive prepared in this invention can be used to bond high-moisture bamboo to produce bamboo laminated timber. This adhesive effectively bonds materials even at high moisture contents, eliminating the strict moisture control required in traditional processing. This optimized process improves the efficiency of slicing and thinning bamboo, reducing factory energy consumption and adhesive, labor, and equipment costs. Experimental results demonstrate the effectiveness of this adhesive in bamboo laminated timber, providing the industry with a more environmentally friendly and efficient processing technology solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The infrared spectrum analysis diagram of the adhesive in Example 1, Example 3, Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0024] The technical solutions of the present invention are described in detail below through examples, but the protection scope of the present invention is not limited to the examples.

[0025] Materials in the following examples: polyester polyol Mn=2000, Sinopharm Chemical Co., Ltd.; polyether polyol (PPG-2000), industrial grade, Shanghai Beko Chemical Co., Ltd.; PM-200, Wanhua Chemical Group Co., Ltd.; trimethylolpropane (TMP), analytical grade, Sinopharm Chemical Co., Ltd.; silane coupling agent (KH-550), analytical grade, Sinopharm Chemical Co., Ltd.; carbodiimide (Stabaxol.P), industrial grade, Shanghai Puzhan Co., Ltd. Dibutyltin dilaurate (DBTDL), analytical grade, Sinopharm Chemical Co., Ltd.; ethyl acetate, anhydrous grade, Sinopharm Chemical Co., Ltd.; polyvinyl alcohol, analytical grade, Sinopharm Chemical Co., Ltd.; bamboo, provided by Fujian Xinlongda Bamboo Industry Co., Ltd.; triolein, analytical grade, Sinopharm Chemical Co., Ltd.; tributyl citrate, analytical grade, Sinopharm Chemical Co., Ltd.; dimer acid diisocyanate, analytical grade, Sinopharm Chemical Co., Ltd.

[0026] In the following embodiments, the polyurethane adhesive is prepared as follows:

[0027] Add 100 parts of polyester polyol and 20 parts of polyether polyol (by weight) to a reactor, stir at 100-150 rpm, and heat to 120°C. Evacuate to ≤0.1 kPa and maintain for 2 hours. During this period, introduce nitrogen every 30 minutes to break the vacuum and remove moisture (moisture content must be ≤0.05%). Cool to 60°C, turn off the vacuum pump, and introduce nitrogen to maintain a slight positive pressure (0.02-0.05 MPa). Simultaneously, weigh 3 parts of trimethylolpropane (TMP) and dissolve it in 5 parts of anhydrous ethyl acetate in a beaker heated to 60°C. After cooling to room temperature, slowly add dropwise to the reactor.

[0028] Under nitrogen protection, 70 parts of PM-200 were slowly added dropwise to the reactor (about 30 minutes to complete the addition), and the reaction temperature was controlled at 60-70°C. After the addition was complete, the temperature was raised to 80°C and the stirring was maintained (200-250rpm) for 2 hours. Samples were taken every 30 minutes and the -NCO content was titrated using the di-n-butylamine method. When the -NCO content was close to 8%-10%, the temperature was lowered to 50°C. 0.06 parts of DBTDL were diluted with 5 parts of anhydrous ethyl acetate and slowly added to the reactor, and stirring was continued for 30 minutes. 2 parts of silane coupling agent (KH-550) and 6 parts of carbodiimide (Stabaxol.P) were added in sequence and stirred for 1 hour to ensure uniform dispersion. After the reaction was completed, the adhesive solution was transferred to a sealed container that had been dried and allowed to stand for 24 hours at room temperature (25±2°C) in a dark environment to allow the system to be fully stabilized for use.

[0029] In the following examples, the polyvinyl alcohol solution was prepared by weighing a certain amount of polyvinyl alcohol powder and preparing a 10% aqueous solution (by mass). The polyvinyl alcohol powder was placed in deionized water and stirred continuously for 20-30 minutes at 1800-2200 rpm using a magnetic stirrer at 60-80°C. After the polyvinyl alcohol powder was completely dissolved, a uniform polyvinyl alcohol solution was formed. The solution was then transferred to a sealed container, sealed, and allowed to stand at room temperature for 24 hours to allow the solution to fully mature and stabilize before use.

[0030] Example 1

[0031] Accurately weigh each raw material according to the formula ratio, slowly add 80 parts of polyurethane adhesive to 20 parts of polyvinyl alcohol solution, and at the same time, add 15 parts of triolein, 9 parts of tributyl citrate and 5 parts of dimer acid diisocyanate in sequence. Turn on the stirring device and stir at 300 rpm for 5 minutes to initially mix the components, then increase the speed to 800 rpm and stir for 15 minutes to ensure uniform mixing. Transfer the evenly stirred mixed adhesive to a sealed container, seal it well, and let it stand at room temperature for 24 hours to allow it to fully react, and you can get a moisture-curing polyurethane (PU) and polyvinyl alcohol (PVA) mixed adhesive.

[0032] Application: The raw material is freshly cut round bamboo with a moisture content of about 60%. The round bamboo is split and processed into bamboo strips, which are then processed into strips with a length of 2500mm, a width of 30mm, and a thickness of 10mm. The strips are coated with adhesive, with a double-sided adhesive coating of 300g / m 2 , under the conditions of unit pressure 1.0MPa and pressing time at room temperature for 30min, it is processed into side-pressed bamboo board.

[0033] The resulting moisture-curing polyurethane (PU) and polyvinyl alcohol (PVA) hybrid adhesive achieves bonding requirements, exhibits improved colloid fluidity during application, boasts a relatively high solids content, and cures in 20-30 minutes. Tested according to national standard GB / T17657-2022, the adhesive strength reached 4.2 MPa, the wood breakage rate exceeded 60%, and no delamination was observed in any of the two types of impregnation-stripped samples. A water spray prior to planing ensured the subsequent planing process was successful without cracking.

[0034] Example 2

[0035] Accurately weigh all raw materials according to the formula ratio. Slowly add 60 parts of polyurethane adhesive to 40 parts of polyvinyl alcohol solution. Simultaneously, add 15 parts of triolein, 9 parts of tributyl citrate, and 5 parts of dimer acid diisocyanate. Turn on the stirring device and stir at 300 rpm for 5 minutes to initially mix the components. Then increase the speed to 800 rpm and stir for 15 minutes to ensure uniform mixing. Transfer the evenly stirred mixed adhesive to a sealed container, seal it, and let it stand at room temperature for 24 hours to allow it to fully react. This will produce a moisture-curing polyurethane and polyvinyl alcohol mixed adhesive.

[0036] The material application and operation methods are as described in Example 1. The resulting moisture-curing polyurethane and polyvinyl alcohol mixed adhesive can meet gluing requirements. The adhesive exhibits good fluidity during application, facilitating the application process and lasting 30-40 minutes. Testing in accordance with national standard GB / T 17657-2022 demonstrated a bond strength of 3.8 MPa and a wood breakage rate exceeding 60%. No delamination was observed in either of the two types of impregnation-stripped samples. Water spraying prior to planing allowed for subsequent planing without cracking.

[0037] Example 3

[0038] Accurately weigh all raw materials according to the formula ratio. Slowly add 40 parts of polyurethane adhesive to 60 parts of polyvinyl alcohol solution. Simultaneously, add 15 parts of triolein, 9 parts of tributyl citrate, and 5 parts of dimer acid diisocyanate. Turn on the stirring device and stir at 300 rpm for 5 minutes to initially mix the components. Then increase the speed to 800 rpm and stir for 15 minutes to ensure uniform mixing. Transfer the evenly stirred mixed adhesive to a sealed container, seal it, and let it stand at room temperature for 24 hours to allow it to fully react. This will produce a moisture-curing polyurethane and polyvinyl alcohol mixed adhesive.

[0039] The material application and operation methods are as described in Example 1. The resulting moisture-curing polyurethane and polyvinyl alcohol mixed adhesive can achieve bonding requirements, has good colloid fluidity for easy application, and cures in 40-50 minutes. Testing according to the national standard GB / T17657-2022 showed a bonding strength of 2.7 MPa, and no delamination was observed in either type II immersion peeling sample. Spraying the surface with water before planing allowed for subsequent planing without cracking.

[0040] Example 4

[0041] Accurately weigh all raw materials according to the formula ratio. Slowly add 20 parts of polyurethane adhesive to 80 parts of polyvinyl alcohol solution. Simultaneously, add 15 parts of triolein, 9 parts of tributyl citrate, and 5 parts of dimer acid diisocyanate. Turn on the stirring device and stir at 300 rpm for 5 minutes to initially mix the components. Then increase the speed to 800 rpm and stir for 15 minutes to ensure uniform mixing. Transfer the evenly stirred mixed adhesive to a sealed container, seal it, and let it stand at room temperature for 24 hours to allow it to fully react. This will produce a moisture-curing polyurethane and polyvinyl alcohol mixed adhesive.

[0042] The material application and operation methods described in Example 1 demonstrate that the resulting moisture-curing polyurethane and polyvinyl alcohol hybrid adhesive meets bonding requirements, exhibits good fluidity, is easy to apply, and cures in 50-60 minutes. Testing in accordance with national standard GB / T 17657-2022 demonstrated a bond strength of 1.7 MPa, with no delamination observed in either type II immersion peeling sample. Water spraying prior to planing ensured subsequent planing without cracking.

[0043] Comparative Example 1

[0044] Using polyvinyl alcohol (PVA) adhesive (the homemade polyvinyl alcohol solution mentioned above), freshly cut round bamboo of Moso bamboo was used as raw material with a moisture content of about 60%. The round bamboo was split and processed into bamboo strips, which were then processed into bamboo strips with a length of 2500mm, a width of 30mm, and a thickness of 10mm. The bamboo strips were coated with polyvinyl alcohol (PVA) adhesive with a double-sided adhesive coating of 300g / m 2 Under conditions of a unit pressure of 1.0 MPa and a pressing time of 30 minutes at room temperature, side-pressed bamboo panels were processed. Pure polyvinyl alcohol adhesives are sufficient for gluing, exhibiting high colloid fluidity during application, low solids content, and a curing time of 12 hours. Testing in accordance with national standard GB / T 17657-2022 demonstrated a bond strength of 0.7 MPa and a Class II impregnation peel strength of 10 mm. Spraying the surface with water before planing allows for subsequent planing, but cracking may occur.

[0045] Comparative Example 2

[0046] Using a homemade polyurethane adhesive, freshly cut round bamboo (Moso bamboo) with a moisture content of approximately 60% was used as the raw material. The round bamboo was split and processed into strips, which were then processed into strips with a length of 2500mm, a width of 30mm, and a thickness of 10mm. The strips were then coated with a polyurethane moisture-curing adhesive, with a double-sided adhesive coating of 300g / m. 2 , processed into side-pressed bamboo boards under a unit pressure of 1.0 MPa and a room temperature pressing time of 30 minutes. Polyurethane moisture-curing adhesives can meet gluing requirements. During the gluing process, the colloid has weak fluidity, a high solid content, and rapid foaming. Its curing time is 10-15 minutes, and the gluing and pressing time is very demanding, requiring quick operation to avoid premature curing. Tested in accordance with the national standard GB / T 17657-2022, the bonding strength reached 4.3 MPa, and no peeling occurred in the second-class immersion peeling samples. The surface was sprayed with water before the planing process, allowing for subsequent planing operations without cracking.

[0047] The adhesives prepared in Example 1 and Example 3, the polyurethane (PU) adhesive in Comparative Example 2, and the polyvinyl alcohol (PVA) adhesive in Comparative Example 1 were analyzed by infrared spectroscopy. The results are as follows: Figure 1 As shown in the figure, the green line is the polyvinyl alcohol adhesive in comparative example 1; the blue line is the polyurethane adhesive in comparative example 2; the red line is the mixed adhesive in example 3; and the gray line is the mixed adhesive in example 1. -1 and 1510cm -1 The absorption peak intensity at 1720 cm-1 is significantly weakened, and the carbonyl (C=O) stretching vibration of the carbamate group (—NHCOO—) in the polyurethane molecule is at 1720 cm-1. -1 The characteristic absorption peak at 1510cm , whose intensity weakens, indicates the formation of hydrogen bonds between polyurethane and polyvinyl alcohol. The abundant hydroxyl (—OH) groups in polyvinyl alcohol molecules can form hydrogen bonds with the carbamate carbonyl groups in polyurethane. This interaction not only changes the electron cloud distribution of the C=O bond, but also leads to a significant decrease in its vibration frequency, thus shifting the absorption peak to a lower wave number and weakening it. -1The weakening of the absorption peak at 1510 cm⁻¹ further reveals the complexity of intermolecular interactions. This absorption peak is associated with the N-H bending vibration of the urethane group or the stretching vibration of the C-O bond in the polyurethane molecule. In a polyurethane / polyvinyl alcohol mixture, hydrogen bond formation not only affects the vibration of the C=O bond but also influences the vibrational modes of the N-H bending vibration or the C-O bond by altering the molecular conformation or vibrational coupling. This multi-dimensional intermolecular interaction leads to a significant weakening of the absorption peak at 1510 cm⁻¹, further confirming the complex chemical interaction between polyurethane and polyvinyl alcohol. From the perspective of chemical reaction principles, the interaction between polyurethane and polyvinyl alcohol is not a simple physical mixing, but involves a chemical reaction at the molecular level. This reaction, through the formation of hydrogen bonds and chemical crosslinking, alters the vibrational modes of the polyurethane molecules, manifesting as a weakening of the characteristic absorption peak in the infrared spectrum.

[0048] The above results indicate that the hydrogen bonds formed between polyurethane and polyvinyl alcohol restrict the free movement of the polyurethane molecular chains, increasing the number of physical crosslinks between the chains, thereby improving the adhesive's cohesive strength. The chemical crosslinking between the two further stabilizes the molecular chain conformation, reduces segmental motion in vibrational modes, limits the vibrational freedom of the polyurethane molecular chains, and increases the number of physical crosslinks and chemical covalent bonds between the chains, thereby enhancing the adhesive's cohesive strength, bonding strength, and water resistance, ultimately improving the mechanical properties, interfacial bonding, and environmental stability of the adhesive layer.

[0049] As described above, although the present invention has been shown and described with reference to specific preferred embodiments, it should not be construed as limiting the present invention itself. Various changes may be made to the form and details without departing from the spirit and scope of the present invention.

Claims

1. A moisture-curing polyurethane and polyvinyl alcohol mixed adhesive, characterized in that: The adhesive is prepared from the following raw materials in parts by mass: 20-80 parts of polyvinyl alcohol solution, 20-80 parts of polyurethane adhesive, 15 parts of dispersant, 9 parts of plasticizer, and 5 parts of crosslinking agent.

2. The adhesive according to claim 1, characterized in that The polyvinyl alcohol solution is an aqueous solution containing 10% by mass of polyvinyl alcohol.

3. The adhesive according to claim 2, characterized in that The polyvinyl alcohol is industrial grade polyvinyl alcohol with a polymerization degree of 500-2500 and an alcoholysis degree of 87%-99%.

4. The adhesive according to claim 1, characterized in that The polyurethane adhesive is polyester or polyether polyurethane with a viscosity range of 1000-5000 mPa·s and a solid content of 40%-60%.

5. The adhesive according to claim 1, characterized in that The dispersant is triolein, the plasticizer is tributyl citrate, and the cross-linking agent is dimer acid diisocyanate.

6. The method for preparing the moisture-curing polyurethane and polyvinyl alcohol mixed adhesive according to any one of claims 1 to 5, characterized in that: The steps include: Adding polyurethane adhesive to polyvinyl alcohol solution, followed by adding dispersant, plasticizer and cross-linking agent in sequence; Turn on the stirring device and stir at a speed of 600-1200 rpm for 15-30 minutes until all ingredients are evenly mixed to form a uniform mixed adhesive; The evenly stirred mixed adhesive is transferred to a sealed container, sealed, and placed at room temperature for 24 hours to allow it to fully react, thereby obtaining a moisture-curing polyurethane and polyvinyl alcohol mixed adhesive.

7. The preparation method according to claim 6, characterized in that Use a multi-stage stirring method when stirring. First, stir at 300-600 rpm for 5-10 minutes to initially mix the components. Then increase the speed to 800-1200 rpm and stir for 10-20 minutes to ensure uniform mixing.

8. Use of the moisture-curing polyurethane and polyvinyl alcohol mixed adhesive according to any one of claims 1 to 5, characterized in that: Used for gluing materials with a moisture content of 30-90%.

9. The use according to claim 8, characterized in that The pressing pressure during gluing is 0.5-1.5 MPa, and the pressing time is 20-30 minutes.

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