A strong alkali-resistant underwater adhesive, its preparation method and application

Abstract

This invention belongs to the technical field of combining polyurethane preparation and materials for use in flow battery electrolyte environments. Specifically, it relates to a strong alkali-resistant underwater adhesive, its preparation method, and its application in flow battery electrolytes. The strong alkali-resistant underwater adhesive provided by this invention comprises the following raw materials: polyurethane prepolymer, epoxy resin, and a fluorinated amine curing agent. The polyurethane prepolymer includes polycaprolactone polyol and isocyanate. This invention aims to solve the technical problems of existing flow battery sealants, which are prone to hydrolytic cracking and swelling in strongly alkaline electrolyte environments, and are difficult to firmly bond to low surface energy substrates such as polypropylene underwater, leading to electrolyte leakage and battery failure. This adhesive can maintain an extremely low swelling rate and excellent mechanical properties in high-concentration alkaline solutions, and achieve long-term stable underwater sealing.

Description

Technical Field

[0001] This invention belongs to the technical field of combining polyurethane preparation and materials for environmental applications in flow battery electrolytes. Specifically, it relates to an adhesive resistant to strong alkali underwater and its preparation method, as well as the application of the adhesive in flow battery electrolytes. Background Technology

[0002] Flow batteries (such as alkaline zinc-iron flow batteries), as a new type of large-scale energy storage device with advantages such as independently designable capacity and power, long lifespan, and high safety, rely heavily on the reliable encapsulation of the stack components for long-term stable operation. During stack assembly, components such as electrodes, ion-conducting membranes, and polypropylene flow channels are typically bonded and encapsulated with sealant to prevent electrolyte leakage. Since the electrolytes in these batteries are often strongly alkaline systems (e.g., 1-3M potassium hydroxide or sodium hydroxide solutions), and the batteries operate in a complex electrochemical environment during long-term charging and discharging, extremely high requirements are placed on the chemical corrosion resistance and underwater bonding stability of the sealing materials.

[0003] Currently, polyurethane adhesives are commonly used in structural adhesives and sealant systems due to their excellent designability, flexibility, and universal adhesion to various substrates. However, traditional polyurethane adhesives face severe challenges in chemical stability in highly alkaline underwater environments.

[0004] From a microstructural perspective, traditional polyurethanes generally rely on urethane bonds (-NH-COO-) and urea bonds formed by the reaction of polyols and isocyanates to form the main chain structure. These chemical bonds are highly susceptible to hydrolysis and alkaline reactions in strongly alkaline environments. High concentrations of hydroxide ions (OH-) attack these polar bonds, leading to polymer chain segment breakage and destruction of the cross-linking network, causing the material to rapidly swell, soften, or even dissolve and fail. Therefore, conventional polyurethanes struggle to maintain structural integrity in scenarios such as flow batteries that are immersed in strongly alkaline solutions for extended periods. Furthermore, traditional polyurethanes have inherent limitations in their ability to adhere to low surface energy substrates such as polypropylene in underwater environments. Underwater adhesion relies on interfacial forces between the material and the substrate, as well as the interfacial reaction of a small amount of residual isocyanate groups (-NCO). However, the hydration layer on the substrate surface can prevent effective contact between the polyurethane and the substrate, and the highly active -NCO groups are rapidly consumed by the ambient water underwater, resulting in insufficient establishment of interfacial chemical bonds. At the same time, the hydrophilic segments introduced into conventional polyurethanes to improve underwater wettability can cause the material to absorb too much water and swell during long-term immersion, resulting in a significant decrease in cohesive strength and ultimately leading to delamination of the sealed interface.

[0005] In summary, there is currently a lack of a sealing material that can maintain structural integrity in high-concentration alkaline solutions for a long time, significantly suppress swelling behavior, and achieve high-strength and long-term stable adhesion to difficult-to-adhere substrates such as polypropylene in underwater environments. Summary of the Invention

[0006] This invention aims to solve the technical problems of existing flow battery sealants (especially conventional polyurethane-based sealants) being prone to hydrolytic cracking and swelling under strongly alkaline electrolyte environments, and failing to achieve strong adhesion to low surface energy substrates such as polypropylene underwater, leading to electrolyte leakage and battery failure. This invention provides a strong alkali-resistant underwater adhesive, its preparation method, and its application. This adhesive maintains an extremely low swelling rate and excellent mechanical properties in high-concentration alkaline solutions, achieving long-term stable underwater sealing.

[0007] The technical solution of the present invention is as follows: An adhesive resistant to strong alkali for underwater use, comprising the following raw materials: polyurethane prepolymer, epoxy resin, and fluorinated amine curing agent; The raw materials for the polyurethane prepolymer include polycaprolactone polyol and isocyanate. The mass ratio of the epoxy resin, polycaprolactone polyol and isocyanate is 1:(1.60-2.90):(1.10-2.40).

[0008] This invention provides a strong alkali-resistant underwater adhesive. The adhesive uses epoxy resin and flexible polyurethane as its matrix, wherein the polyurethane is formed by reacting polycaprolactone polyol with isocyanate, and a fluorinated amine curing agent is introduced. This is prepared through a two-step method to form a network cross-linked structure. The strong alkali-resistant underwater adhesive achieves its alkali resistance in electrolyte environments through the cross-linking effect of the epoxy resin and the fluorinated amine curing agent, as well as the constructed highly cross-linked network structure. Furthermore, by adjusting the ratio of epoxy resin to polyurethane, the adhesive's resistance to decomposition and its underwater adhesion strength in alkaline solutions such as sodium hydroxide can be further controlled.

[0009] Further, the fluorinated amine curing agent is one or more of 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl, 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline], and 2,2'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl.

[0010] The strong alkali resistant underwater adhesive provided by this invention utilizes fluorine atoms and rigid biphenyl structures in fluorine-containing amine curing agents to construct a hydrophobic shielding layer at the microscopic level, which can effectively protect the internal urethane bonds from hydroxyl ion attack, and use residual isocyanate groups to consume the hydration layer on the substrate surface in situ to achieve chemical anchoring; furthermore, by adjusting the ratio of epoxy resin to polyurethane prepolymer, the flexibility and alkali resistance and anti-swelling properties of the material can be further balanced.

[0011] Furthermore, the mass ratio of the epoxy resin to the fluorinated amine curing agent is 1:(1.50 - 1.80).

[0012] Furthermore, the mass ratio of the epoxy resin to the fluorinated amine curing agent is 1:(1.60 - 1.70).

[0013] Further, the mass ratio of the epoxy resin, polycaprolactone polyol and isocyanate is 1:(1.73-2.74):(1.2-2.16).

[0014] Further, the epoxy resin includes epoxy resin E51, bisphenol A epoxy resin E44, bisphenol F type epoxy resin EPON 862 or phenolic epoxy resin DEN 438. The isocyanate includes hexamethylene diisocyanate, isophorone diisocyanate or 4,4'-dicyclohexylmethane diisocyanate. Using aliphatic HDI can effectively avoid the problems of light yellowing and poor weather resistance of aromatic isocyanates. The molecular weight (Mn) of the polycaprolactone polyol is 1000-4000 g / mol; preferably, a polycaprolactone polyol with a molecular weight (Mn) of 2000 g / mol is used to provide excellent hydrophobicity and flexible segments.

[0015] Furthermore, the present invention also provides a method for preparing the strong alkali resistant underwater adhesive as described above, comprising the following steps: Step (1): After dehydration treatment, polycaprolactone polyol and epoxy resin are reacted with isocyanate to obtain the reaction product, forming component A; Step (2): Use the dissolved fluorinated amine curing agent as component B; Step (3): Mix and stir the components A and B to obtain an adhesive resistant to strong alkali underwater.

[0016] Furthermore, it includes the following steps: Step (1): Polycaprolactone polyol and epoxy resin are dehydrated under reduced pressure at 110-130℃ for 3-5 hours, then cooled to 75-85℃ and isocyanate is added to react and obtain the reaction product. Then, the product is cooled to room temperature and diluted with acetone to form component A. Step (2): Dissolve the fluorinated amine curing agent in N,N-dimethylformamide as component B; Step (3): Mix and stir the components A and B to obtain an adhesive resistant to strong alkali underwater.

[0017] Preferably, the method for preparing the adhesive includes the following steps: Step (1): Dehydrate polycaprolactone polyol and epoxy resin under reduced pressure at 120°C for 4 hours, cool down to 80°C, add hexamethylene diisocyanate and react for 4 hours, cool down to room temperature, add acetone to dilute as component A. Step (2): Dissolve 2,2'-bis(trifluoromethyl)diaminobiphenyl in a solvent to prepare a curing agent as component B; Step (3): Mix components A and B and stir for 30 minutes to prepare an adhesive that can be used for bonding.

[0018] Furthermore, the present invention also provides an application of the strong alkali resistant underwater adhesive as described above, the strong alkali resistant underwater adhesive being used for bonding or sealing in an aqueous medium environment, the aqueous medium environment including a water-containing environment under neutral or alkaline conditions.

[0019] Furthermore, the aqueous medium environment is an alkaline electrolyte environment.

[0020] Furthermore, the alkaline electrolyte is the electrolyte of an alkaline flow battery.

[0021] This invention designs and synthesizes a polyurethane-based adhesive that achieves highly efficient underwater adhesion in alkaline environments. The adhesive incorporates epoxy resin segments to enhance the overall structural stability of the material and employs a fluorinated amine curing agent, thereby significantly improving the material's corrosion resistance and adhesion retention in high-concentration alkaline solutions. The polyurethane portion can form various intermolecular interactions at the adhesion interface, including hydrogen bonds and van der Waals forces. Simultaneously, the residual isocyanate groups can interact with the hydration layer and further react with water molecules, thus constructing a robust network structure and providing a reliable structural basis for underwater adhesion. Therefore, the adhesive prepared in this invention exhibits excellent underwater adhesion performance in alkaline environments of varying concentrations, unaffected by significant fluctuations in alkaline concentration. Particularly under 3 M high-concentration alkaline conditions, the adhesive still exhibits significant anti-swelling properties and high adhesion strength, thus significantly expanding the application range of underwater adhesives in complex alkaline environments.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) Excellent "alkali-resistant strengthening" effect: Existing polyurethane adhesives usually experience a significant decrease in strength after immersion in alkaline solutions, while the adhesive prepared in this invention exhibits an anomalous performance enhancement. Test results show that the tensile strength of the strong alkali-resistant underwater adhesive of this invention can be increased from 7.46 MPa in pure water to 8.91 MPa after immersion in a 3M NaOH strong alkali solution (test data from Example 6). This indicates that the fluorinated crosslinked network constructed in this invention undergoes densification rather than degradation in an alkaline environment, demonstrating excellent chemical stability.

[0023] (2) Unique hydrophobic shielding and anti-swelling mechanism: This invention specifically selects fluorinated diamines such as 2,2'-bis(trifluoromethyl)diaminobiphenyl as curing agents. By utilizing their fluorine atoms and rigid biphenyl structure, a hydrophobic shielding layer is constructed at the microscopic level while providing rigid steric hindrance, thereby effectively blocking water molecules and OH groups. - The penetration of ions results in an extremely low swelling rate (below 2.4%) in alkaline solutions such as 1-3M sodium hydroxide, which is far superior to conventional hydrophilic modified polyurethane, fundamentally solving the softening problem after long-term immersion.

[0024] (3) Excellent underwater in-situ anchoring capability: This invention utilizes a large number of active isocyanate groups in component A, which can penetrate and react to consume the hydration layer on the surface of the substrate (such as polypropylene, glass, metal, etc.) during underwater construction, generating urea bonds and releasing carbon dioxide, thereby eliminating the weak boundary layer and achieving chemical anchoring. Tests have shown that the strong alkali resistant underwater adhesive of this invention maintains an adhesion strength of about 3.3 MPa after being adhered and soaked underwater for 180 days, and exhibits excellent adhesion to low surface energy polypropylene substrates.

[0025] (4) Wide temperature range adaptability: The adhesive for underwater use in this invention is resistant to strong alkali. Thanks to the rigid network of epoxy resin and the high bond energy of fluorine-containing segments, the adhesive still has an adhesion strength of 9.1 MPa at a high temperature of 120°C, which fully meets the operating requirements of flow batteries under extreme conditions. Attached Figure Description

[0026] Figure 1 The swelling curves are of the adhesives obtained in Examples 1-7 and Comparative Examples 1-5 of the present invention.

[0027] Figure 2 The graph shows a comparison of the stress test results of the adhesives obtained in Examples 1-7 and Comparative Examples 1-5 of this invention after swelling in pure water and 3M alkaline solution.

[0028] Figure 3 The figures show a comparison of the strain test results of the adhesives obtained in Examples 1-7 and Comparative Examples 1-5 of this invention after swelling in pure water and 3M alkaline solution.

[0029] Figure 4 The graph shows the adhesion performance test results of the adhesives obtained in Examples 1-7 and Comparative Examples 1-5 of this invention in pure water.

[0030] Figure 5 The graph shows the adhesion performance test results of the adhesive obtained in Example 6 of this invention in NaOH solutions of different concentrations.

[0031] Figure 6 The images show underwater adhesion tests of the adhesive obtained in Example 6 of this invention on different substrates.

[0032] Figure 7 The figure shows the underwater adhesion performance test results of the adhesive obtained in Example 6 of the present invention on different substrates.

[0033] Figure 8 The graph shows the adhesion performance test results of the adhesives obtained in Example 6, Comparative Example 3 and Comparative Example 4 of the present invention at different temperatures. Detailed Implementation

[0034] The technical solutions of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings. The embodiments described herein are specific implementations of the present invention, used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary, and should not be construed as limiting the implementation methods or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can employ other obvious technical solutions based on the content disclosed in the claims and specification of this application. These technical solutions include those that make any obvious substitutions and modifications to the embodiments described herein.

[0035] Unless otherwise specified, all reagents and equipment used in the following examples were purchased from commercial sources.

[0036] Example 1 A method for preparing an alkali-resistant underwater adhesive includes the following steps: Step (1): 10.25 g of polycaprolactone polyol and 3.75 g of epoxy resin E51 were dehydrated under reduced pressure at 120℃ for 4 hours. After cooling to 80℃, 4.5 g of hexamethylene diisocyanate was added and reacted for 4 hours. After cooling to room temperature, 3.5 ml of acetone was added to dilute it as component A. The molecular weight of the polycaprolactone polyol is 2000 g / mol. Step (2): Dissolve 6.23 g of curing agent 2,2'-bis(trifluoromethyl)diaminobiphenyl in 2 ml of N,N-dimethylformamide as component B; Step (3): Mix components A and B and stir for 30 minutes to prepare an adhesive resistant to strong alkali underwater, which is used for bonding in the electrolyte environment of alkaline flow batteries.

[0037] Example 2 The difference between Example 2 and Example 1 is that the epoxy resin E51 in step (1) is replaced with bisphenol A epoxy resin E44, while the rest is the same as in Example 1.

[0038] Example 3 The difference between Example 3 and Example 1 is that the hexamethylene diisocyanate in step (1) is replaced with isophorone diisocyanate, while the rest is the same as in Example 1.

[0039] Example 4 The difference between Example 4 and Example 1 is that the molecular weight of the polycaprolactone polyol in step (1) is 1000 g / mol, while the rest is the same as in Example 1.

[0040] Example 5 The difference between Example 5 and Example 1 is that the 2,2'-bis(trifluoromethyl)diaminobiphenyl in step (2) is replaced with 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl, while the rest is the same as in Example 1.

[0041] Example 6: The difference between Example 6 and Example 1 is that the content of hexamethylene diisocyanate in step (1) is 6.3g.

[0042] Example 7: The difference between Example 7 and Example 1 is that the content of hexamethylene diisocyanate in step (1) is 8.1g.

[0043] Example 8 The difference between Example 8 and Example 1 is that the epoxy resin E51 in step (1) is replaced with bisphenol F type epoxy resin EPON 862, and the rest is the same as in Example 1.

[0044] Example 9 The difference between Example 9 and Example 1 is that the epoxy resin E51 in step (1) is replaced with phenolic epoxy resin DEN 438, CAS No.: 28064-14-4, and the rest is the same as in Example 1.

[0045] Example 10 The difference between Example 10 and Example 1 is that the hexamethylene diisocyanate in step (1) is replaced with 4,4'-dicyclohexylmethane diisocyanate, while the rest is the same as in Example 1.

[0046] Example 11 The difference between Example 11 and Example 1 is that the molecular weight of the polycaprolactone polyol in step (1) is 3000 g / mol, and the rest is the same as in Example 1.

[0047] Example 12 The difference between Example 12 and Example 1 is that the molecular weight of the polycaprolactone polyol in step (1) is 4000 g / mol, while the rest is the same as in Example 1.

[0048] Example 13 The difference between Example 13 and Example 1 is that the 2,2'-bis(trifluoromethyl)diaminobiphenyl in step (2) is replaced with 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline], while the rest is the same as in Example 1.

[0049] Example 14 The difference between Example 14 and Example 1 is that the 2,2'-bis(trifluoromethyl)diaminobiphenyl in step (2) is replaced with 2,2'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl, while the rest is the same as in Example 1.

[0050] Example 15 The difference between Example 15 and Example 1 is that the polycaprolactone polyol in step (1) is 6.5g.

[0051] Comparative Example 1 This comparative example is used to verify the effect of excess isocyanate on performance: The difference between this comparative example and Example 1 is that the content of hexamethylene diisocyanate in step (1) is 9.9 g.

[0052] Comparative Example 2 This comparative example is used to verify the effect of excess isocyanate on performance: The difference between this comparative example and Example 1 is that the content of hexamethylene diisocyanate in step (1) is 11.7g.

[0053] Comparative Example 3 This comparative example is used to verify the effect of fluorine-free curing agent on performance: The difference between this comparative example and Example 1 is that the curing agent in step (1) is isophorone diamine.

[0054] Comparative Example 4 This comparative example is used to verify the effect of other types of epoxy resins on performance: The difference between this comparative example and Example 1 is that the epoxy resin in step (1) is diglycidyl hexahydrophthalic acid.

[0055] Comparative Example 5 This comparative example is used to verify the effect of other types of isocyanates on performance: The difference between this comparative example and Example 1 is that the isocyanate in step (1) is lysine diisocyanate.

[0056] Test Example: Performance analysis of the adhesives obtained in the above examples and comparative examples. 1. Swelling test The samples prepared in Examples 1-7 and Comparative Examples 1-5 were immersed in 3M alkaline solution and pure water for 7 days. The samples were weighed using an analytical balance, and the mass of the adhesive was recorded and statistically analyzed. The swelling ratio was calculated using the formula: , After weighing the initial sample, immerse it in pure water or alkaline solution for 7 days. Change the immersion solution every 24 hours. After 7 days, remove the sample, wipe off the surface moisture, and weigh it after immersion. Repeat the test three times for each group.

[0057] The test results are shown in the table below: Table 1. Results of the swelling test like Figure 1 As shown in Table 1, data analysis revealed that the swelling rates of the adhesives prepared in Examples 1-7 and Comparative Examples 1-5 in alkaline solutions were 1.4%, 1.5%, 1.3%, 1.7%, 1.6%, 2.0%, 2.3%, 4.8%, 5.0%, 12.8%, 6.3%, and 19.5%, respectively. The reason for the relatively low swelling rates of the adhesives obtained in this invention is that epoxy resin has good chemical stability, especially the three-dimensional network structure formed after curing, which effectively blocks the erosion of alkaline solutions. This network is rich in hydrolysis-resistant COC ether bonds and stable benzene ring structures, making it difficult to be hydrolyzed and saponified by OH- ions, thus ensuring the integrity of the skeleton. The introduction of carbon-fluorine bonds (CF bonds) into the cured network—one of the most stable chemical bonds in organic chemistry with extremely high bond energy and high inertness to strong bases and acids—further enhances the chemical inertness of the entire cross-linked network. The highly cross-linked network structure reduces chain segment mobility and enhances overall permeability resistance.

[0058] 2. Tensile test According to the national standards GB / T1040.1-2018 and GB / T1040.2-2022, the tensile strength and elongation at break of the samples prepared in Examples 1-7 and Comparative Examples 1-5 were determined. The mechanical tensile speed was 10 mm / min. The tensile results are shown in Tables 2 and 3. Figure 2 , Figure 3 As shown.

[0059] Table 2 Tensile Test Stress Test Results Table 3. Tensile Test Strain Test Results The test results above show that the tensile strengths of the adhesives prepared in Examples 1-7 and Comparative Examples 1-5 after immersion in pure water are 3.09, 3.15, 3.42, 3.22, 3.71, 7.46, 5.98, 5.23, 3.28, 1.02, 2.97 and 3.25 MPa, respectively, and the tensile strengths after immersion in alkaline solution are 3.47, 3.88, 4.37, 4.21, 4.56, 8.91, 5.22, 4.1, 1.89, 0.21, 0.56 and 0.31 MPa, respectively. The adhesives prepared in Examples 1-6 of this invention do not show a decrease in mechanical properties when soaked in pure water and alkaline solution; in fact, their mechanical properties are significantly improved after soaking in alkaline solution. The adhesives prepared in Examples 1-7 and Comparative Examples 1-5 showed strains of 764%, 752%, 725%, 740%, 650%, 85%, 142%, 99%, 37%, 120%, 620%, and 680% after soaking in pure water, respectively, and strains of 712%, 698%, 665%, 680%, 580%, 67%, 160%, 134%, 67%, 15%, 45%, and 30% after soaking in alkaline solution, respectively. From their strain... Figure 3As can be seen, the adhesive strain decreased after immersion in alkaline solution, possibly due to micro-shrinkage caused by the high concentration of alkaline solution, while no hydrolysis behavior was observed. Examples 1-5 exhibited a clear flexibility-dominated characteristic (high strain, relatively low stress), which is due to the low isocyanate content allowing the polyurethane soft segments to dominate the system, resulting in a high degree of freedom of molecular chain movement. Example 6 achieved the best balance between rigidity and toughness, with a tensile strength as high as 8.91 MPa after immersion in alkaline solution. This is because an appropriate amount of isocyanate formed a dense polymer network with epoxy resin and fluorinated curing agent. This high crosslinking density structure restricts chain slippage, significantly improving the material's load-bearing capacity and resistance to chemical attack, giving it the characteristics of "high modulus and high strength". For Example 7, its mechanical properties decreased compared to Example 6, and the strain further decreased. This is not due to hydrolytic degradation of the material, but rather attributed to the network hardening effect caused by the high crosslinking density. This also illustrates that excessive hard segments make the polymer network too rigid, limiting the relaxation ability of internal molecular chains. During stress application, localized stress is difficult to dissipate effectively, leading to stress concentration and brittle fracture at relatively low deformation. For example, in Comparative Examples 1-2, the isocyanate content was significantly higher than in the examples, resulting in excessive cross-linking and side reactions between isocyanate and water. This led to uneven cross-linking and introduced microscopic defects, reducing the material's load-bearing capacity and durability in alkaline media. In Comparative Example 3, the use of a fluorine-free alicyclic diamine curing agent failed to construct an effective hydrophobic shielding structure at the molecular level, allowing hydroxide ions to easily penetrate the polyurethane backbone, significantly reducing alkali resistance. In Comparative Example 4, the insufficient rigidity of the epoxy resin resulted in an epoxy network that could not provide effective support under alkaline and high-temperature conditions, weakening the synergistic effect of the interpenetrating network. In Comparative Example 5, the introduction of hydrophilic groups into the isocyanate molecule increased the material's water absorption tendency and weakened the stability of the cross-linked network, leading to decreased mechanical properties and alkali resistance.

[0060] In summary, the adhesives prepared in Examples 1-7 all exhibited relatively excellent mechanical properties in alkaline environments and all possessed excellent alkali resistance. In particular, Example 6 achieved a dual breakthrough in mechanical strength and environmental resistance by precisely controlling the ratio of soft and hard segments and the crosslinking density.

[0061] 3. Underwater adhesion test The underwater and alkaline shear adhesion strength of the adhesives prepared in Examples 1-7 and Comparative Examples 1-5 was determined according to the method specified in the national standard GB / T7124-2008, with the following adjustments made for the underwater / strong alkaline environment: after the sample was completely immersed in (pure water / NaOH solution) for a specified time, it was immediately taken out and tested, and the tensile speed was set to 30 mm / min.

[0062] Table 4. Test Results of Underwater Adhesion Test In an aquatic environment, the underwater adhesion strengths of the prepared adhesives were 3.64, 3.82, 4.15, 3.58, 4.05, 10.8, 1.29, 1.01, 0.89, 0.45, 1.12, and 1.35 MPa, respectively. Figure 4 Of the samples, the adhesive prepared in Example 6 exhibited the highest adhesion strength. This result demonstrates that by controlling the crosslinking density of the polyurethane-epoxy interpenetrating network, a synergistic improvement in interfacial anchoring capacity and material load-bearing capacity can be achieved in underwater and strongly alkaline environments. An appropriate amount of isocyanate eliminates the water film in situ and forms chemical bonds, while the optimized interpenetrating network structure effectively balances cohesive strength and flexibility. Although Example 7 experienced a decrease in adhesion strength due to the higher crosslinking density limiting surface physical wetting ability, it maintained structural integrity and showed no swelling failure in a strongly alkaline environment, verifying the chemical stability of the high-hardness formulation of this invention.

[0063] Table 5. Adhesion test results of the adhesive obtained in Example 6 under different concentrations of alkaline solution. Table 6. Adhesion test results of the adhesive obtained in Example 6 on different substrates and underwater. The adhesive obtained in Example 6 was placed in alkaline environments of different concentrations (1 M, 1.5 M, 2 M, 2.5 M, and 3 M), and the adhesion strengths were measured to be 10.5, 9.6, 9.4, 8.9, and 8.7 MPa, respectively. Figure 5 It exhibits excellent alkali resistance. More importantly, Example 6 maintains adhesion performance above 8.7 MPa in alkaline solutions of different concentrations, indicating that the adhesive has adaptability to various alkaline environments and provides possibilities for future practical applications. Therefore, the introduction of epoxy resin and fluorinated amine curing agent in this invention significantly improves the alkali-resistant adhesion performance of the adhesive.

[0064] like Figure 6 , Figure 7As shown, when the substrates to which the adhesive is applied are aluminum, stainless steel, polypropylene, ceramic, glass, and polyvinyl chloride, the underwater adhesion strengths obtained are 6.17, 8.7, 0.91, 2.05, 0.77, and 1.19 MPa, respectively. The adhesive of this invention can form various interfacial interactions with different substrates, including hydrogen bonding, electrostatic interactions, dipole-dipole interactions, ion-π interactions, and van der Waals interactions. Glass, aluminum, and stainless steel are hydrophilic substrates, while polyvinyl chloride and polypropylene are hydrophobic surfaces. The adhesive exhibits good underwater adhesion on both hydrophilic and hydrophobic surfaces.

[0065] Table 7. Adhesion test results of the adhesive obtained in Example 6 under different temperatures and alkaline conditions. In Example 6 of the present invention, the adhesion strength was 7.53, 9.94, 9.92, and 8.67 MPa at alkaline solution temperatures of 30, 60, 90, and 120 °C, respectively; in Comparative Example 3, the adhesion strength was 0.18, 0.02, 0, and 0 MPa at alkaline solution temperatures of 30, 60, 90, and 120 °C, respectively; and in Comparative Example 4, the adhesion strength was 0.39, 0.11, 0.03, and 0 MPa at alkaline solution temperatures of 30, 60, 90, and 120 °C, respectively. Figure 8 ).

[0066] This invention benefits from the unique "hydrophobic-rigid" dual shielding mechanism and interpenetrating network structure of the resulting adhesive. By introducing corrosion-resistant epoxy resin and a fluorinated aromatic diamine curing agent, the alkali resistance and mechanical strength of the polyurethane matrix are perfectly controlled. The strongly hydrophobic trifluoromethyl and sterically hindered biphenyl structures in the fluorinated curing agent construct a dense hydrophobic shielding layer around the molecular chain, effectively blocking hydroxide ions (OH-). - The attack on the urethane bonds results in an extremely low swelling rate (<2.4%) in strongly alkaline environments (such as 3M NaOH). Simultaneously, the high crosslinking density of the epoxy resin restricts chain segment movement, further enhancing the structural integrity of the material under long-term immersion, enabling it to maintain excellent adhesion strength in both room temperature and high-temperature strongly alkaline environments.

[0067] This invention employs a two-step curing strategy involving the blending of a hydrophobic polyurethane prepolymer with epoxy resin. The adhesive utilizes residual highly reactive isocyanate groups (-NCO) in the system as "chemical anchoring points." During underwater application, the -NCO groups preferentially react with the hydration layer and adsorbed water on the substrate surface, generating urea bonds and releasing carbon dioxide, thereby disrupting the water film barrier and achieving chemical bonding at the interface. Subsequently, a fluorinated curing agent initiates a crosslinking reaction, forming a dense three-dimensional network that locks the adhesion interface. This synergistic mechanism of "in-situ water removal-chemical anchoring-hydrophobic shielding" overcomes the shortcomings of traditional polyurethanes, such as easy water absorption and swelling underwater and interface failure, demonstrating the material's excellent long-term sealing stability in complex, strongly alkaline electrolyte environments such as flow batteries.

[0068] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. An adhesive resistant to strong alkali for underwater use, characterized in that, The raw materials include: polyurethane prepolymer, epoxy resin, and fluorinated amine curing agent; The raw materials for the polyurethane prepolymer include polycaprolactone polyol and isocyanate. The mass ratio of the epoxy resin, polycaprolactone polyol and isocyanate is 1:(1.60 - 2.90):(1.10 - 2.40).

2. The adhesive for underwater use resistant to strong alkali as described in claim 1, characterized in that, The fluorinated amine curing agent is one or more of 2,2'-bis(trifluoromethyl)diaminobiphenyl, 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl, 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline], and 2,2'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl.

3. The adhesive for underwater use resistant to strong alkali as described in claim 1, characterized in that, The mass ratio of the epoxy resin to the fluorinated amine curing agent is 1:(1.50 - 1.80).

4. The adhesive for underwater use resistant to strong alkali as described in claim 1, characterized in that, The mass ratio of the epoxy resin, polycaprolactone polyol and isocyanate is 1:(1.73 - 2.74):(1.2 - 2.16).

5. The adhesive for underwater use resistant to strong alkali as described in claim 1, characterized in that, The epoxy resin includes epoxy resin E51, bisphenol A epoxy resin E44, bisphenol F type epoxy resin EPON 862 or phenolic epoxy resin DEN 438. The isocyanate includes hexamethylene diisocyanate, isophorone diisocyanate or 4,4'-dicyclohexylmethane diisocyanate. The molecular weight (Mn) of the polycaprolactone polyol is 1000-4000 g / mol.

6. The method for preparing the strong alkali-resistant underwater adhesive according to any one of claims 1-5, characterized in that, Includes the following steps: Step (1): After dehydration treatment, polycaprolactone polyol and epoxy resin are reacted with isocyanate to obtain the reaction product, forming component A; Step (2): Use the dissolved fluorinated amine curing agent as component B; Step (3): Mix and stir the components A and B to obtain an adhesive resistant to strong alkali underwater.

7. The method for preparing the strong alkali-resistant underwater adhesive as described in claim 6, characterized in that, Includes the following steps: Step (1): Polycaprolactone polyol and epoxy resin are dehydrated under reduced pressure at 110-130℃ for 3-5 hours, then cooled to 75-85℃ and isocyanate is added to react and obtain the reaction product. Then, the product is cooled to room temperature and diluted with acetone to form component A. Step (2): Dissolve the fluorinated amine curing agent in N,N-dimethylformamide as component B; Step (3): Mix and stir the components A and B to obtain an adhesive resistant to strong alkali underwater.

8. The application of an adhesive for underwater use resistant to strong alkalis as described in any one of claims 1-5, characterized in that, The strong alkali resistant underwater adhesive is used for bonding or sealing in aqueous media environments, including water-containing environments under neutral or alkaline conditions.

9. The application of the strong alkali-resistant underwater adhesive as described in claim 8, characterized in that, The aqueous medium environment is an alkaline electrolyte environment.

10. The application of the strong alkali-resistant underwater adhesive as described in claim 9, characterized in that, The alkaline electrolyte is the electrolyte of an alkaline flow battery.

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