Polyurethane super-strong adhesive as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of adhesives, and particularly relates to a polyurethane super-strong adhesive as well as a preparation method and application thereof. The preparation method comprises the following steps: mixing branched-chain multi-mercapto mercaptan with an epoxy compound, adding a catalyst for reaction, and then adding a neutralizer for terminating the reaction to prepare an epoxy prepolymer; the preparation method comprises the following steps: mixing an epoxy prepolymer with branched chain multi-mercapto mercaptan to prepare a mixture; and mixing the mixture with isocyanate, reacting to obtain a product, and defoaming the product to obtain the polyurethane super-strong adhesive. The sulfur element is introduced into a polymer molecular chain, so that the bonding performance between the base cloth (base cloth) and the coating is remarkably improved, meanwhile, the water resistance, solvent resistance and low temperature resistance are also greatly improved, the method is suitable for humid, low-temperature and organic solvent environments, no solvent participates in the preparation process, and the environment-friendly requirement is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of adhesives, and particularly relates to a polyurethane super adhesive, a preparation method thereof and an application thereof. Background Art

[0002] Polyurethane adhesives are widely used in many fields due to their excellent adhesion performance, flexibility and chemical resistance. Oxygen and nitrogen atoms in polyurethane adhesives act together on adhesion in various ways, such as forming polar groups, enhancing cohesive energy, promoting chemical reactions and affecting thermal stability. These action mechanisms enable polyurethane adhesives to firmly bond various materials and maintain stable performance.

[0003] Synthetic leather is widely used in fields such as shoe materials, luggage, furniture, etc., and its performance depends to a large extent on the quality of the adhesive. However, traditional polyurethane adhesives are prone to problems such as debonding and peeling with poor adhesion performance in humid or organic solvent-contact environments. Especially in polar or non-polar organic solvents such as toluene, xylene, ethanol, etc., they are easily swollen or dissolved, resulting in the softening of the adhesive and loss of strength, and especially accelerating degradation at high temperatures.

[0004] In addition, most of the polyurethane adhesives used in the prior art rely on the preparation with organic solvents, which causes environmental pollution and health hazards; and although there are some literature reports that by using disulfide bonds as cross-linking agents or thiourethane for preparing sulfur-containing polyurethane adhesives, although their adhesion is improved, their stability and mechanical properties at high temperatures are poor, and the preparation method of sulfur-containing polyurethane adhesives is complex, and their water resistance and solvent resistance still need to be improved. Summary of the Invention

[0005] The purpose of the present invention is to provide a polyurethane super adhesive, a preparation method thereof and an application thereof, so as to overcome the deficiencies of the prior art. By introducing sulfur elements into the polymer molecular chain, the adhesion performance between the base fabric (bottom fabric) and the coating is significantly improved, and at the same time, the water resistance and solvent resistance are also greatly improved, and the preparation process does not involve solvents and meets environmental protection requirements.

[0006] The overall inventive concept adopted by the present invention is: The present invention aims to solve the problems of the decline in bonding performance and delamination and failure between layers of existing polyurethane adhesives in high-humidity, high-temperature and organic solvent environments. Most traditional polyurethane adhesives use hydroxyl polyols and isocyanates as the main reaction system, with a flexible structure but insufficient crosslinking density, and are easily eroded by water vapor or polar solvents, resulting in the failure of the bonding interface, especially performing poorly in structural bonding such as synthetic leather, photovoltaic, and metal composites.

[0007] In order to overcome the above technical problems, the present invention proposes a ternary structure polyurethane adhesive system based on a thiol-containing compound and an epoxy compound. The system is prepared by the following steps: reacting a branched polythiol mercaptan with an epoxy compound to generate an epoxy prepolymer with a hydroxyl end, and using a catalyst and a neutralizer to accurately control the degree of reaction to ensure that the prepolymer has appropriate activity; by compounding the above prepolymer with an additional thiol compound, a high-density cross-linking site is introduced; and then polycondensing with a multifunctional isocyanate to form a polyurethane adhesive with both a flexible hydroxyl segment and a highly reactive thiol segment in the main chain structure, and after the reaction, vacuum degassing treatment is performed to obtain a solvent-free adhesive with a uniform structure.

[0008] In the above polymerization pathway, thiol (–SH) and hydroxyl groups undergo polycondensation with -NCO or -NCS to form structurally stable thiocarbamates and carbamates, while the hydroxyl segments give the molecular chains good flexibility and interfacial adaptability. The synergistic effect of epoxy prepolymers and thiols with isocyanates can regulate the internal and external non-covalent interactions of the polymer, thereby enhancing the cohesion and chain crosslinking between hydrogen bonds. By controlling the R value (NCO / active hydrogen) between 0.8-1.3, especially around 1.05, the relationship between crosslinking density and cohesive strength can be adjusted to construct a dense and tough polyurethane network; its branched topology can conveniently encode various rigid cores (such as diphenylmethane) and flexible lateral units (such as thioether chains) to coordinate the flexibility and rigidity of the molecular network. Taking full advantage of its unique structural advantages, it achieves super strong adhesion strength and effectively solves the problems of bonding performance degradation and interlayer debonding failure in high humidity, high temperature, and organic solvent environments.

[0009] In order to achieve the above object, the technical solution of the present invention is: In a first aspect, the present invention provides a method for preparing a polyurethane super strong adhesive, comprising the following steps: (1) After mixing the branched polythiothiol and the epoxy compound, a catalyst is added to react, and then a neutralizing agent is added to terminate the reaction to obtain an epoxy prepolymer; (2) mixing the epoxy prepolymer with the branched polythiol mercaptan to prepare a mixture; (3) mixing the mixture with isocyanate and reacting them to obtain a product, and degassing the product to obtain a polyurethane super strong adhesive; In step (1), the branched polythiothiol and the epoxy compound are mixed in a molar ratio of thiol to epoxy of 1: (0.1-25). In this step, the mechanism of the ring-opening reaction between the thiol in the branched polythiothiol and the epoxy compound and the epoxy is as follows: ① The catalyst activates the thiol (-SH) in the branched polythiol thiol and removes the proton (H + ), generating thiol anions (S- ), and the protonated catalyst. Since the mercapto anion is a very strong nucleophile, its reactivity is higher than that of the neutral mercapto group; ② The mercapto anion attacks the less substituted carbon atom (i.e., the primary carbon) in the epoxide, and an SN2-type ring-opening reaction occurs, causing the epoxy ring in the epoxide to open and form an intermediate with a β-hydroxy thioether structure; ③ The intermediate with a β-hydroxy thioether structure undergoes a proton transfer with the protonated catalyst to form a stable product, namely the epoxy prepolymer.

[0010] It can be seen from the above mechanism of the ring-opening reaction between thiol and epoxy that during the reaction between the branched poly(mercapto) thiol and the epoxide, the actually participating mercapto groups of the branched poly(mercapto) thiol react with the epoxy groups in the epoxide to undergo a ring-opening reaction. And when the branched poly(mercapto) thiol and the epoxide are mixed according to the molar ratio of mercapto to epoxy of 1:(0.1 - 25), it can ensure the production of a polyurethane super adhesive during the subsequent reaction process. The specific analysis is as follows: When the branched poly(mercapto) thiol and the epoxide are mixed according to the molar ratio of mercapto to epoxy of 1:0.1, that is, when the mercapto is in excess of the epoxy, 0.9 moles of mercapto will be in excess and do not participate in the reaction, resulting in a decrease in the hydroxyl content in the synthesized epoxy prepolymer. Since the reactivity of the hydroxyl group is higher than that of the mercapto group, it can react with isocyanate to obtain a polyurethane adhesive with better strength; When the molar ratio of mercapto to epoxy is 1:25, the hydroxyl group will continue to react with the epoxy to open the ring, increasing the molecular chain length, decreasing the sulfur content, and having a relatively high viscosity. In a solvent-free polyurethane adhesive, the viscosity is too high. Although there will be small bubbles during bonding, it still has good adhesion strength.

[0011] In some other embodiments, in step (1), the branched poly(mercapto) thiol and the epoxide are mixed according to the molar ratio of mercapto to epoxy of 1:(0.8 - 1.5); In some other embodiments, in step (1), the branched poly(mercapto) thiol and the epoxide are mixed according to the molar ratio of mercapto to epoxy of 1:(1.0 - 1.1); In some other embodiments, in step (1), when the branched poly(mercapto) thiol and the epoxide are mixed according to the molar ratio of mercapto to epoxy of 1:1.05, the hydroxyl content in the synthesized epoxy prepolymer is relatively high, and the viscosity is appropriate. There will be no small bubbles generated during the bonding of the adhesive, and the adhesion strength is relatively high.

[0012] The branched poly(mercapto) thiol is one of 2,3 - dithio(2 - mercapto)-1 - propane thiol, trimethylolpropane tris(3 - mercaptopropionate), tris(3 - mercaptopropyl) silane, and 1,3,5 - tris(mercaptomethyl) benzene; The epoxide is one of ethylene oxide, propylene oxide and pentylene oxide.

[0013] The branched-chain polythiol is 2,3 - dithio(2 - mercapto)-1 - propane thiol; The epoxide is propylene oxide. This is because propylene oxide is a three-membered epoxide with a relatively large ring strain, and its ring-opening reaction activity is higher than that of pentylene oxide with a five-membered ring. The high reaction activity makes the ring-opening reaction of propylene oxide with thiol more complete, generating more hydroxyl groups. When these hydroxyl groups react with isocyanate subsequently, a higher-density crosslinked network is formed, enhancing the mechanical strength and interfacial bonding ability of the polyurethane. The chain segment derived from propylene oxide is shorter and the molecular chain has stronger rigidity, which is beneficial to form a tighter mechanical interlock at the interface.

[0014] In some other embodiments, in step (1), the catalyst is one of triethylenediamine, triethylamine, N-N-dimethylethanolamine, trimethylbenzylamine, N-N-dimethylcyclohexylamine and benzyltrimethylammonium hydroxide; The neutralizer is one of hydrochloric acid, phosphoric acid, formic acid, acetic acid and glycine; The addition amount of the catalyst is 2‰ - 6‰ of the total mass of the branched-chain polythiol and the epoxide; The addition amount of the neutralizer is 2‰ - 6‰ of the total mass of the branched-chain polythiol and the epoxide.

[0015] In some other embodiments, in step (1), the catalyst is benzyltrimethylammonium hydroxide; the neutralizer is phosphoric acid; The addition amount of the catalyst is 5‰ of the total mass of the branched-chain polythiol and the epoxide; The addition amount of the neutralizer is 5‰ of the total mass of the branched-chain polythiol and the epoxide.

[0016] In some other embodiments, in step (1), the temperature of the reaction is 5 - 20 °C and the time is 2 - 6 h; After the termination of the reaction, it further includes continuous stirring for 5 - 10 min.

[0017] In some other embodiments, in step (2), the epoxy prepolymer and the branched-chain polythiol are mixed at a molar ratio of (2 - 6):10 of the hydroxyl groups in the epoxy prepolymer and the mercapto groups in the branched-chain polythiol; The temperature of the mixing is 45 - 55 °C and the mixing time is 5 - 10 min.

[0018] In some other embodiments, in step (3), the mixture and the isocyanate are mixed at a molar ratio of 0.8 - 1.3 of the -N=C=O groups in the isocyanate and the active hydrogen groups in the mixture; The active hydrogen groups are -SH and -OH.

[0019] In some other embodiments, in step (3), the reaction temperature is 60 - 90 °C, and the reaction time is 2 - 4 hours; The defoaming treatment is carried out at 65 - 75 °C under vacuum conditions for 30 - 60 min.

[0020] In a second aspect, the present invention provides a polyurethane super adhesive prepared by the preparation method of the polyurethane super adhesive described in the first aspect.

[0021] In a third aspect, the present invention provides the application of the polyurethane super adhesive described in the second aspect in leather, building materials, photovoltaic, automotive and electronic product encapsulation.

[0022] Advantages of the present invention: The polyurethane super adhesive prepared by the present invention has the following beneficial technical effects: (1) It can still maintain a high shear strength (>50 MPa) in a humid, strongly polar or non-polar solvent environment; it can be applied to the bonding between various materials such as leather, metal, and plastic, especially the high-strength bonding between the surface layer and the bottom cloth in the dry composite structure of synthetic leather; (2) It has good water boiling resistance, acid and alkali resistance, organic solvent resistance and low temperature adaptability (such as maintaining stable adhesion in a liquid nitrogen environment); (3) The whole system does not contain solvents, is suitable for environmentally friendly manufacturing processes, and meets the development trend of green bonding materials. Combining with the data of the examples, the adhesive prepared under the condition of appropriate ratio (hydroxyl / thiol = 2 - 6:10, R = 1.05) still maintains a stable bonding structure after being soaked in various polar / non-polar media such as ethanol, xylene, acetone and DMF for several weeks, showing extremely strong chemical stability and long-term service ability.

[0023] In summary, the present invention introduces a "thiol + epoxy + isocyanate" synergistic strategy in the molecular structure design, constructs a multi-level cross-linked polyurethane structure network, and provides a stronger, more stable and more environmentally friendly solution in structural bonding, composite material bonding and special application scenarios (such as wet / corrosive / swelling environments). Description of the Drawings

[0024] The specification drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0025] Figure 1 It is the DTG diagram of the polyurethane super adhesive in Example 1 of the present invention; Figure 2Schematic diagram of the test process for the macroscopic adhesion behavior of the polyurethane super adhesive in liquid nitrogen in Example 1 of the present invention; Figure 3 Shear strength tests with different R values in Example 1 and Example 5 of the present invention; Figure 4 Shear data of the polyurethane super adhesive in different solvents for different soaking times in Example 1, Example 2 and Comparative Example 1 of the present invention; where A is deionized water, B is 3.5% NaCl solution, C is xylene, D is ethanol, E is acetone, and F is N,N-dimethylformamide; Figure 5 Acid and alkali resistance test of the polyurethane super adhesive in Example 1 of the present invention; Figure 6 Lap shear strength of the polyurethane super adhesive soaked in potassium permanganate solution in Example 1 of the present invention; Figure 7 High-temperature boiling water test of the polyurethane super adhesive in Example 1 of the present invention. Detailed implementation mode

[0026] Those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. The specific conditions are not specified in the examples and are carried out according to the conventional conditions or the conditions recommended by the manufacturer. Those components whose producers are not specified are all conventional products available commercially.

[0027] Example 1 A preparation method of a polyurethane super adhesive specifically includes the following steps: After mixing 37.2 g of 2,3 - dithio(2 - mercapto)-1 - propane thiol and 26.16 g of propylene oxide (with a molar ratio of mercapto to epoxy of 1:1.05), 0.3168 g of benzyltrimethylammonium hydroxide is added as a catalyst (accounting for 5‰ of the total mass of the mixture of 2,3 - dithio(2 - mercapto)-1 - propane thiol and propylene oxide). After stirring and reacting at 10°C for 5 hours, 0.3168 g of phosphoric acid (accounting for 5‰ of the total mass of the mixture of 2,3 - dithio(2 - mercapto)-1 - propane thiol and propylene oxide) is added as a neutralizing agent, and stirring is continued for 10 min to obtain an epoxy prepolymer; (2) After mixing 5.72 g of the epoxy prepolymer and 26.76 g of 2,3 - dithio(2 - mercapto)-1 - propane thiol (with a molar ratio of hydroxyl in the epoxy prepolymer to mercapto in 2,3 - dithio(2 - mercapto)-1 - propane thiol of 2:10), stirring and reacting at 50°C for 10 min to obtain a mixture; (3) Mix the mixture with isocyanate at R = 1.05 (where R is the molar ratio of the -N=C=O group in the isocyanate to the active hydrogen groups in the mixture, and the active hydrogen groups are -SH and -OH), react at 75 °C for 4 hours to obtain a product; defoam the product under vacuum conditions at 70 °C for 30 min to obtain a super-strong polyurethane adhesive.

[0028] The adhesion method of the super-strong polyurethane adhesive to the substrate is as follows: Uniformly coat the super-strong polyurethane adhesive on the surface of the substrate (such as steel substrate, leather, synthetic leather) (coating amount: 70 g / m²), coat the polyurethane PU resin on the surface of the release paper (coating amount: 100 g / m²), and then hot press the two (hot press temperature is 100 °C, 500 g weight, hot press for 2 hours) for adhesive curing. After the adhesive is cured, peel off the release paper to complete the adhesion of the super-strong polyurethane adhesive to the substrate.

[0029] Example 2 Different from Example 1, in step (2), mix the epoxy prepolymer and 2,3 - dithio(2 - mercapto)-1 - propanethiol according to the molar ratio of the hydroxyl group in the epoxy prepolymer to the mercapto group in 2,3 - dithio(2 - mercapto)-1 - propanethiol of 4:10; Other preparation steps are the same as those in Example 1.

[0030] Example 3 Different from Example 1, in step (2), mix the epoxy prepolymer and 2,3 - dithio(2 - mercapto)-1 - propanethiol according to the molar ratio of the hydroxyl group in the epoxy prepolymer to the mercapto group in 2,3 - dithio(2 - mercapto)-1 - propanethiol of 6:10; Other preparation steps are the same as those in Example 1.

[0031] Example 4 Different from Example 1, in step (2), mix the epoxy prepolymer and 2,3 - dithio(2 - mercapto)-1 - propanethiol according to the molar ratio of the hydroxyl group in the epoxy prepolymer to the mercapto group in 2,3 - dithio(2 - mercapto)-1 - propanethiol of 10:0; Other preparation steps are the same as those in Example 1.

[0032] Example 5 Different from Example 1, in step (3), mix the mixture and isocyanate at R = 0.85, 0.95, 1.15, and 1.25 respectively; Other preparation steps are the same as those in Example 1. Example 6 Different from Example 1, in steps (1) and (3), trimethylolpropane tris(3-mercaptopropionate) was used to replace 2,3-dithio(2-mercapto)-1-propanethiol, and other preparation steps were the same as those in Example 1.

[0033] Example 7 Different from Example 1, in steps (1) and (3), tris(3-mercaptopropyl)silane was used to replace 2,3-dithio(2-mercapto)-1-propanethiol, and other preparation steps were the same as those in Example 1.

[0034] Example 8 Different from Example 1, in steps (1) and (3), 1,3,5-tris(mercaptomethyl)benzene was used to replace 2,3-dithio(2-mercapto)-1-propanethiol, and other preparation steps were the same as those in Example 1.

[0035] Example 9 Different from Example 1, in step (1), the molar ratios of mercapto and epoxy were mixed at 1:0.1 and 1:25 respectively; Other preparation steps were the same as those in Example 1.

[0036] Example 10 Different from Example 1, in step (1), epoxy pentane was used to replace epoxy propane, and other preparation steps were the same as those in Example 1.

[0037] Comparative Example 1 Different from Example 1, in step (2), the epoxy prepolymer and 2,3-dithio(2-mercapto)-1-propanethiol were mixed at a molar ratio of 0:10 for the hydroxyl group in the epoxy prepolymer and the mercapto group in 2,3-dithio(2-mercapto)-1-propanethiol; Other preparation steps were the same as those in Example 1.

[0038] Comparative Example 2 Different from Example 1, in steps (1) and (3), 1,3-propanedithiol was used to replace 2,3-dithio(2-mercapto)-1-propanethiol, and other preparation steps were the same as those in Example 1.

[0039] Performance Test 1. Thermogravimetric DTG The thermal stability of the sample in Example 1 was measured using a thermogravimetric analyzer (TGA) in a nitrogen atmosphere, and the nitrogen flow rate was controlled at 50 ml∙min -1 . The sample was cut into solid small particles of 5-10 mg, placed in an alumina thermal analysis crucible, and the heating / cooling rate was 10 °C∙min -1, the sample was heated from 30 °C to 800 °C. Figure 1 This is the DTG diagram of the polyurethane super adhesive in Example 1 of the present invention.

[0040] The results are as Figure 1 shown. The maximum thermal decomposition temperatures of the samples in Example 1 were 308 °C and 313 °C, which were the maximum decomposition temperatures of thiocarbamate and carbamate, respectively. The larger atomic radius and lower electronegativity of sulfur atoms make the C–S bond easier to break than the C–O bond, resulting in a lower decomposition temperature.

[0041] 2. Macroscopic adhesion behavior in liquid nitrogen The steel substrate adhered well in Example 1 with a 500 g weight was placed in liquid nitrogen for 1 hour, during which the adhesion area was kept below the liquid nitrogen. After one hour, it was taken out and suspended for ablation, and it completely melted after 1 hour. From Figure 2 it can be seen that the adhered steel substrate still remained in good condition.

[0042] 3. Shear strength test Using an XWN-20 microcomputer-controlled electronic universal testing machine, single-lap shear testing was carried out at a tensile speed of 10 mm·min -1 , and the adhesion strength was obtained by dividing the force generated during the test by the adhesion area of the substrate. Three parallel samples were used for each measurement.

[0043] Figure 3 This is the shear strength test of different R values in Example 1 and Example 5 of the present invention; from Figure 3 it can be seen that through the lap shear test results of different R values, the shear strengths at R = 0.85, 0.95, 1.05, 1.15, and 1.25 were 39.17 ± 3.35, 36.57 ± 3.27, 50.31 ± 1.22, 43.105 ± 3.54, and 37.155 ± 3.43 MPa, respectively. When the NCO group is capped and the molar ratio of the NCO group to the active hydrogen group is close to one, and the reaction is sufficient, the hydrogen bond interaction between carbamate and thiocarbamate in the molecular chain is enhanced. Therefore, when R = 1.05, the lap shear strength is the highest at 50.31 ± 1.22 MPa.

[0044] 4. Shear strength test after soaking in different solvents for different times The steel substrates adhered well in Example 1, Example 2, and Comparative Example 1 were respectively placed in deionized water, ethanol, xylene, 3.5% NaCl solution, acetone (at room temperature, soaked for 0 - 28 days), and N,N-dimethylformamide (DMF) (at room temperature, soaked for 0 - 60 days). Using an XWN-20 microcomputer-controlled electronic universal testing machine, single-lap shear testing was carried out at a tensile speed of 10 mm·min -1, the adhesion strength is obtained by dividing the force generated during the test by the adhesion area of the substrate, and three parallel samples are used for each measurement. Measure the lap shear strength at different times, and the results are as Figure 4 shown, where A is deionized water, B is 3.5% NaCl solution, C is xylene, D is ethanol, E is acetone, and F is N,N-dimethylformamide.

[0045] From Figure 4 it can be seen that the samples of Example 1, Example 2 and Comparative Example 1 all have good water and solvent resistance, but the lap shear strength and stability of Example 1 are better. The samples of Example 1 are immersed in deionized water, ethanol, xylene, 3.5% NaCl solution and acetone for 28 days, and their lap shear strengths are 38.15±2.08 MPa, 47.49±1.81 MPa, 35.97 ±5.62 MPa, 27.04±0.55 MPa and 39.18±1.48 MPa respectively. Among them, the sample of Example 1 was not disconnected after being immersed in DMF solvent for 60 days. Measure its lap shear strength after being immersed in DMF (N,N-dimethylformamide) solvent for 32 days, and it still remains about 71% (35.85±3.25 MPa) compared with the initial lap shear strength.

[0046] 5. Acid and alkali resistance test Figure 5 This is the acid and alkali resistance test of the polyurethane super adhesive in Example 1 of the present invention. From Figure 5 it can be seen that the adhesive prepared in Example 1 is used for lap bonding of steel substrates, and the samples are respectively immersed in sulfuric acid solution with pH = 1 and sodium hydroxide solution with pH = 14 to evaluate their acid and alkali resistance. The experimental results show that in the sulfuric acid solution, the samples maintain a stable adhesion strength from 1 h to 72 h. After 72 h, its adhesion strength can still reach 31.87±2.56 MPa, and the retention rate reaches 63.35% compared with the initial lap shear strength. In addition, the samples also show good tolerance in an alkaline environment. After being immersed in sodium hydroxide solution for 24 h, its adhesion strength is 31.66±4.82 MPa, slightly higher than the corresponding value in the sulfuric acid solution; after 72 h, its adhesion strength still remains at 22.41±5.21 MPa.

[0047] 6. Lap shear strength after immersion in potassium permanganate solution Immerse in potassium permanganate solution, use the XWN-20 microcomputer-controlled electronic universal testing machine, adopt single lap shear test, and the tensile speed is 10 mm·min -1 , the adhesion strength is obtained by dividing the force generated during the test by the adhesion area of the substrate, and three parallel samples are used for each measurement.

[0048] Figure 6 The lap shear strength of the polyurethane super adhesive in the potassium permanganate solution in Example 1 of the present invention is as follows. From Figure 6 It can be seen that it has good adhesion strength when soaked in 2 wt% KMnO4 solution for 1 - 72 hours. After soaking for 72 hours, its adhesion strength is 33.44 MPa. Further tests show that after soaking in 2 wt% KMnO4 solution for 120 h, the adhesion strength of the sample still remains above 50%.

[0049] 7. High-temperature decomposition resistance The bonded stainless steel in Example 1 was boiled at high temperature. The samples were placed in distilled water at 100 °C and boiled for 1 hour, 3 hours, 8 hours, 10 hours, 12 hours, 18 hours, and 24 hours respectively. After stopping heating, their lap shear strength was tested. As shown in Figure 7, the measured tensile shear strengths were 37.56 ± 2.22, 38.23 ± 2.27, 33.12 ± 1.81, 30.19 ± 2.14, 31.27 ± 2.81, 30.81 ± 1.82, and 32.63 ± 2.98 MPa respectively. The results show that the samples still maintain good mechanical properties after high-temperature boiling.

[0050] 8. The water resistance, adhesion, corrosion resistance, and high-temperature resistance of the samples prepared in the examples and comparative examples are shown in Table 1. Among them, the test method for water resistance is: soak the sample in deionized water for 7 days and then test the lap shear strength; the test method for corrosion resistance is: soak the sample in 2 wt% potassium permanganate for 72 hours and then test the lap shear strength; the test method for high-temperature resistance is: heat the sample in a muffle furnace at 265 °C for 1 hour and then test the lap shear strength.

[0051] Table 1 Water resistance, adhesion, corrosion resistance, and high-temperature resistance of the samples prepared in the examples and comparative examples

[0052] As can be seen from Table 1, introducing S element into the polymer, the appropriate ratio of topological structure, thiol and epoxy prepolymer can improve its adhesion strength, high temperature resistance, water resistance and corrosion resistance. Among them, in Example 9, when the molar ratio of branched-chain polythiol to epoxide is 1:0.1, that is, when the thiol is greater than the epoxy, 0.9 moles of thiol will be in excess and not participate in the reaction, resulting in a decrease in the hydroxyl content in the epoxy prepolymer synthesized by ring-opening. Since the reaction activity of hydroxyl is higher than that of thiol, reacting with isocyanate can obtain a polyurethane adhesive with better strength; when the molar ratio of thiol to epoxy is 1:25, the hydroxyl will continue to react with epoxy by ring-opening, increasing the molecular chain length, reducing the sulfur content, and having a strong viscosity. In a solvent-free polyurethane adhesive, the viscosity is too high, and there will be small bubbles during bonding, resulting in a slightly lower adhesion strength than in Example 1 (the molar ratio of thiol to epoxy is 1:1.05). In Example 10, using pentene oxide as the epoxide compared with propylene oxide used in Example 1, the long-chain structure of pentene oxide leads to an increase in molecular flexibility, reduces polar interactions, and weakens the adhesion force.

[0053] Based on the above tests, it was found that compared with traditional polyurethane adhesives, which have poor acid and alkali resistance and are easily affected by oxidation degradation by potassium permanganate, the adhesive of the present invention has the characteristics of epoxy resin adhesive by introducing epoxy prepolymer, thus significantly improving the acid, alkali and oxidation resistance. This study shows that the adhesive still has excellent bonding stability in extreme chemical environments, showing broad engineering application prospects.

[0054] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a polyurethane super strong adhesive, characterized in that: The following steps are involved: (1) After mixing the branched polythiothiol and the epoxy compound, a catalyst is added to react, and then a neutralizing agent is added to terminate the reaction to obtain an epoxy prepolymer; (2) mixing the epoxy prepolymer with the branched polythiol mercaptan to prepare a mixture; (3) mixing the mixture with isocyanate and reacting them to obtain a product, and degassing the product to obtain a polyurethane super strong adhesive; In step (1), the branched polythiol mercaptan and the epoxy compound are mixed in a molar ratio of mercapto to epoxy of 1:(0.1-25).

2. The method for preparing the polyurethane super strong adhesive according to claim 1, characterized in that: In step (1), the branched polymer mercaptan and epoxy compound are mixed in a molar ratio of mercapto to epoxy of 1:(0.8-1.5); The branched polymer mercaptan is one of 2,3-dithio(2-mercapto)-1-propanethiol, trimethylolpropane tris(3-mercaptopropionate), tris(3-mercaptopropyl)silane and 1,3,5-tris(mercaptomethyl)benzene; The epoxy compound is one of ethylene oxide, propylene oxide and pentyl oxide.

3. The method for preparing the polyurethane super strong adhesive according to claim 1, characterized in that: In step (1), the branched polymer mercaptan and epoxy compound are mixed in a molar ratio of mercapto to epoxy of 1:1.05; The branched polythiol mercaptan is 2,3-dithio(2-mercapto)-1-propanethiol; The epoxy compound is propylene oxide.

4. The method for preparing the polyurethane super strong adhesive according to claim 1, characterized in that: In step (1), the catalyst is one of triethylenediamine, triethylamine, NN-dimethylethanolamine, trimethylbenzylamine, NN-dimethylcyclohexylamine and benzyltrimethylammonium hydroxide; The neutralizing agent is one of hydrochloric acid, phosphoric acid, formic acid, acetic acid and aminoacetic acid; The amount of the catalyst added is 2‰-6‰ of the total mass of the branched polymercaptan and the epoxy compound; The amount of the neutralizer added is 2‰-6‰ of the total mass of the branched polythiothiol and the epoxy compound.

5. The method for preparing the polyurethane super strong adhesive according to claim 4, characterized in that: In step (1), the catalyst is benzyltrimethylammonium hydroxide; The neutralizing agent is phosphoric acid; The amount of the catalyst added is 5‰ of the total mass of the branched polymercaptan and the epoxy compound; The amount of the neutralizer added is 5‰ of the total mass of the branched polythiothiol and the epoxy compound.

6. The method for preparing the polyurethane super strong adhesive according to claim 1, characterized in that: In step (1), the reaction temperature is 5-20°C and the reaction time is 2-6 h; After the reaction is terminated, stirring is continued for 5-10 minutes.

7. The method for preparing the polyurethane super strong adhesive according to claim 1, characterized in that: In step (2), the epoxy prepolymer and the branched polythiothiol mercaptan are mixed in a molar ratio of (2-6):10 for the hydroxyl group in the epoxy prepolymer and the thiol group in the branched polythiothiol mercaptan; The mixing temperature is 45-55° C., and the mixing time is 5-10 min.

8. The method for preparing the polyurethane super strong adhesive according to claim 1, characterized in that: In step (3), the mixture is mixed with isocyanate at a molar ratio of -N=C=O groups in the isocyanate to active hydrogen groups in the mixture of 0.8-1.3; The active hydrogen groups are -SH and -OH; The reaction temperature is 60-90°C and the reaction time is 2-4 hours; The degassing treatment is performed at 65-75° C. under vacuum conditions for 30-60 min.

9. A super strong polyurethane adhesive prepared by the preparation method of the super strong polyurethane adhesive according to any one of claims 1 to 8.

10. Use of the polyurethane super strong adhesive according to claim 9 in leather, building materials, photovoltaics, automobiles and electronic products.

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