A polyimide-polyurethane copolymer modified epoxy resin adhesive and preparation method thereof

Abstract

A polyimide-polyurethane copolymer-modified epoxy resin adhesive and a preparation method thereof. The present invention relates to a copolymer-modified epoxy resin adhesive and a preparation method thereof. The present invention solves the problems of existing epoxy resin adhesives being brittle after cross-linking and curing, having poor heat resistance, and having poor bonding performance under hot and humid conditions. The adhesive is prepared from a polyamic acid prepolymer, a polyurethane prepolymer, and an epoxy resin; the preparation method includes the following steps: 1. Preparation of a polyamic acid prepolymer; 2. Preparation of a polyurethane prepolymer; 3. Preparation of a polyamic acid-blocked polyurethane prepolymer; 4. Thermal imidization; 5. Blending of a polyimide-blocked polyurethane with an epoxy resin. The present invention is used for a polyimide-polyurethane copolymer-modified epoxy resin adhesive and its preparation.

Description

Technical Field

[0001] The invention relates to a copolymer modified epoxy resin adhesive and a preparation method thereof. Background Art

[0002] Under hot and humid conditions, adhesives undergo physical changes such as expansion and softening, gradually decreasing their mechanical properties and initial thermal decomposition temperature, and weakening their bonding performance. Hot and humid conditions can trigger chemical reactions within the adhesive, such as hydrolysis and oxidation, which disrupt the adhesive's molecular structure. Epoxy resin aging in hot and humid conditions can be caused by degradation of segments at sites such as CN bonds in the backbone chain, as well as oxidation reactions at hydroxyl groups and methylene groups attached to nitrogen atoms. Boiling and hot and humid conditions can also lead to interface failure between the adhesive and the adherend, severely impacting bond strength and mechanical properties.

[0003] Research on adhesives under humid and hot conditions has primarily focused on their aging behavior and environmental damage mechanisms. Studies have shown that humid and hot environments significantly affect adhesive performance, particularly shear strength and initial thermal decomposition temperature. These performance indicators gradually decline with continued aging.

[0004] Moisture-resistant adhesives exhibit excellent performance in high-temperature and high-humidity environments. They are self-leveling, resulting in a smooth, glossy, and bubble-free surface after curing. In addition to excellent waterproofing, they also offer excellent sealing properties. However, these adhesives do have some limitations.

[0005] While moisture-resistant adhesives perform well in high-temperature and high-humidity environments, their performance may be affected under extreme moisture and heat conditions. For example, prolonged exposure to extremely high temperatures and humidity can cause the adhesive's bond strength to decrease, or the adhesive itself to degrade or deform.

[0006] Because moisture-heat-resistant adhesives require specialized properties, their production costs can be high. This can limit their use in cost-sensitive applications. To achieve excellent moisture-heat resistance, the preparation process can be complex. This can increase the difficulty of controlling the production process, impacting product quality and stability.

[0007] Epoxy resins offer excellent comprehensive mechanical properties, strong adhesion, minimal shrinkage, good stability, and superior electrical insulation. They are widely used in a variety of fields, including machinery, electronics, electrical appliances, aerospace, aviation, and coating bonding, as coatings, adhesives, composite resin matrices, and electronic packaging materials. However, due to their high crosslink density, cured epoxy resins are brittle, with poor toughness and impact resistance. Therefore, toughening and modification of epoxy resins has been a hot topic for researchers both in China and abroad.

[0008] Regardless of the material used to toughen epoxy resin, it can be divided into three types: chemical toughening method (the toughening agent and epoxy resin undergo chemical cross-linking), physical toughening method (the toughening agent and epoxy resin do not undergo chemical cross-linking) and chemical-physical synergistic method.

[0009] Polyurethane molecular chains contain a large number of ether or ester bonds, which have a low internal rotation barrier. This allows for easy rotation of the molecular segments, resulting in good flexibility. Therefore, modifying them with polyurethane can effectively improve the flexibility of epoxy resins. Epoxy resins can be blended with polyurethane to form cross-linked, interpenetrating copolymers.

[0010] However, polyurethane materials have poor heat resistance. The operating temperature range of polyurethane materials is -40 to 120°C. When the temperature exceeds 100°C, it is easy to soften and deform, resulting in a significant decrease in the mechanical properties of the material. In particular, for polyurethane materials with only physical crosslinking, the long-term operating temperature cannot exceed 80°C. Therefore, to a certain extent, the application range of polyurethane-modified epoxy resins at high temperatures is limited.

[0011] Polyimide (PI), an aromatic, rigid, nitrogen-containing heterocyclic polymer, exhibits excellent heat resistance and mechanical properties. As a functional material, polyimide (PI) is the highest-rated polymer material used in industrial applications to date. Due to its excellent mechanical properties and good heat resistance, electrical properties, and mechanical strength, it plays a vital role in industries such as aerospace, electrical appliances, and communications. Introducing polyimide into the polyurethane backbone can improve the copolymer's heat resistance.

[0012] Currently, physical blending and chemical modification are the primary methods for modifying polyurethane and polyimide. Blending modification involves forming an alloy of polyimide and polyurethane to modify the polyurethane material. However, due to the insolubility and poor processability of polyimide, the soft segments of the alloy formed by polyurethane and polyimide with different molecular weights undergo phase separation, resulting in a three-phase structure. Consequently, products modified using this method have poor performance in subsequent applications. Summary of the Invention

[0013] The present invention aims to solve the problems of existing epoxy resin adhesives being brittle after cross-linking and curing, having poor heat resistance, and having poor bonding performance under hot and humid conditions, and further provides a polyimide-polyurethane copolymer modified epoxy resin adhesive and a preparation method thereof.

[0014] A polyimide-polyurethane copolymer-modified epoxy resin adhesive is prepared from a polyamic acid prepolymer, a polyurethane prepolymer, and an epoxy resin. Specifically, the polyamic acid prepolymer is added to a polyurethane prepolymer for copolymerization to obtain a polyamic acid-blocked polyurethane prepolymer, the polyamic acid-blocked polyurethane prepolymer is thermally imidized to obtain a polyimide-blocked polyurethane, and finally the polyimide-blocked polyurethane is added to the epoxy resin for physical crosslinking. The molar ratio of the polyurethane prepolymer to the polyamic acid prepolymer is (2.01-3):1; the mass ratio of the polyimide-blocked polyurethane to the epoxy resin is (0.1-0.4):1.

[0015] The polyamic acid prepolymer is synthesized from aromatic diamine and aromatic dianhydride; the aromatic diamine is m-phenylenediamine; the aromatic dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 4,4'-biphenyl ether dianhydride or 3,3',4,4'-dibenzophenone tetracarboxylic dianhydride;

[0016] The polyurethane prepolymer is synthesized from dehydrated polyol and dehydrated isocyanate; the dehydrated isocyanate is toluene-2,4-diisocyanate; the dehydrated polyol is polybutylene glycol;

[0017] The general structural formula of the polyimide-blocked polyurethane is:

[0018] Where n = 5 to 7;

[0019] The R1 is

[0020] The R2 is Where m=5~13.

[0021] A method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive is carried out according to the following steps:

[0022] 1. Preparation of polyamic acid prepolymer:

[0023] Weigh aromatic diamine, aromatic dianhydride, and an organic solvent in proportion, add the aromatic diamine to the organic solvent and stir to dissolve at room temperature, then add the aromatic dianhydride and stir to dissolve at room temperature, then react at a temperature of 20°C to 40°C for 1 to 2 hours, add deionized water under high-speed stirring after the reaction, stop stirring when the system becomes a milky white suspension, filter and wash, and finally dry to obtain a polyamic acid prepolymer;

[0024] 2. Preparation of polyurethane prepolymer:

[0025] Dehydrating the isocyanate and the polyol separately, then weighing the dehydrated polyol and the dehydrated isocyanate in proportion, and then dripping the dehydrated isocyanate into the dehydrated polyol under a nitrogen atmosphere at a temperature of 60°C to 80°C for 2h to 3h to obtain a polyurethane prepolymer;

[0026] 3. Preparation of polyurethane prepolymer with polyamic acid block:

[0027] Under nitrogen atmosphere and temperature of 60° C. to 80° C., adding polyamic acid prepolymer to polyurethane prepolymer and reacting for 0.5 h to 1 h to obtain polyamic acid-blocked polyurethane prepolymer;

[0028] The molar ratio of the polyurethane prepolymer to the polyamic acid prepolymer is (2.01-3):1;

[0029] 4. Thermal imidization:

[0030] Under a nitrogen atmosphere, the polyurethane prepolymer having polyamic acid segments is thermally imidized to obtain a polyurethane having polyimide segments;

[0031] 5. Blending of polyimide-blocked polyurethane and epoxy resin:

[0032] Under nitrogen atmosphere and temperature of 60°C to 80°C, polyimide-blocked polyurethane is added to epoxy resin for reaction for 1 to 3 hours to obtain a polyimide-polyurethane copolymer-modified epoxy resin adhesive;

[0033] The mass ratio of the polyurethane of the polyimide block to the epoxy resin is (0.1-0.4):1.

[0034] The beneficial effects of the present invention are:

[0035] The present invention provides a polyimide-polyurethane copolymer modified epoxy resin adhesive. Unlike traditional polyurethane-modified epoxy resins, the introduction of imide groups not only enhances the toughness of the epoxy resin but also improves the heat resistance and bonding performance of the system.

[0036] The present invention utilizes a copolymer-modified epoxy resin adhesive—a polyimide-blocked polyurethane polymer. The copolymer and epoxy resin interpenetrate each other, forming an interpenetrating network (IPN) structure. The IPN structure can disperse tensile stress. When subjected to external tensile stress, the tensile force is transmitted from one network to another, making the molecular structure less susceptible to damage and improving the toughness of the system. The epoxy-modified polyimide polyurethane polymer is divided into a hard segment structure and a soft segment structure. The hard segment is mainly the PI molecular chain structure, and the soft segment is mainly the flexible PU chain segment. The PI segment has a high-strength rigid structure and forms the overall skeleton structure of the IPN system. The imide ring and aromatic ring in the molecular chain are conjugated, and the electron cloud distribution in the conjugated system is relatively uniform, the internal energy of the molecule is low, and the molecular structure is not easily destroyed. The conjugation effect also enhances the rigidity and modulus of the system, giving the system good support. The benzene ring structure and imide ring structure on the PI molecular main chain increase the polarity and hydrogen bonding degree between the hard segments, thereby increasing the interaction force between the hard segments and hindering the movement of the molecular chain. The imide group is also a polar group that can react with the polar hydroxyl and epoxy groups in the epoxy resin to form chemical bonds, increasing the cohesive energy density of the system. The molecular chain is difficult to move, and the cohesive energy density of the system increases, which increases the modulus of the system and enhances the mechanical properties and heat resistance. PU, as a soft segment, can undergo microphase separation with the hard segment and form a co-continuous phase or a particle dispersed phase with the epoxy resin, thereby increasing the toughness of the epoxy resin and improving its mechanical properties. Polyimide-block polyurethane copolymer modified epoxy resin not only improves the temperature resistance of the system, but also improves the mechanical properties of the system, so the adhesive has good high-temperature bonding properties.

[0037] When the adhesive is immersed in water, the support provided by its rigid structure prevents the system from absorbing water and expanding. Even in high-temperature environments, the system does not shrink due to heat, making the adhesive resistant to high-temperature boiling. While the hard PI molecular segments possess rigidity and heat resistance, acting as a scaffold to support the material's structure, pure PI structures are susceptible to water penetration in humid environments, degrading mechanical properties. The PU structure, the flexible segments within the IPN structure, contains numerous flexible groups. The ether bonds have low cohesive energy, allowing for easy rotation and molecular movement, allowing the segments to fill gaps in the system. In addition to providing toughness, the flexible PU segments also prevent water from penetrating the system, reducing water absorption and maintaining excellent bonding properties even in boiling and damp-heat conditions. The modified polyimide-polyurethane block copolymer has more complex segments, resulting in increased steric hindrance, making water penetration more difficult and reducing water absorption, enabling the adhesive to maintain excellent bonding properties in boiling and damp-heat conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the FTIR spectrum of the polyimide-segmented polyurethane prepared in step 4 of Example 1. DETAILED DESCRIPTION

[0039] Specific embodiment 1: A polyimide-polyurethane copolymer modified epoxy resin adhesive of this embodiment is prepared from a polyamic acid prepolymer, a polyurethane prepolymer and an epoxy resin. Specifically, the polyamic acid prepolymer is added to the polyurethane prepolymer for copolymerization to obtain a polyamic acid-blocked polyurethane prepolymer, the polyamic acid-blocked polyurethane prepolymer is thermally imidized to obtain a polyimide-blocked polyurethane, and finally the polyimide-blocked polyurethane is added to the epoxy resin for physical crosslinking; the molar ratio of the polyurethane prepolymer to the polyamic acid prepolymer is (2.01-3):1; the mass ratio of the polyimide-blocked polyurethane to the epoxy resin is (0.1-0.4):1;

[0040] The polyamic acid prepolymer is synthesized from aromatic diamine and aromatic dianhydride; the aromatic diamine is m-phenylenediamine; the aromatic dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 4,4'-biphenyl ether dianhydride or 3,3',4,4'-dibenzophenone tetracarboxylic dianhydride;

[0041] The polyurethane prepolymer is synthesized from dehydrated polyol and dehydrated isocyanate; the dehydrated isocyanate is toluene-2,4-diisocyanate; the dehydrated polyol is polybutylene glycol;

[0042] The general structural formula of the polyimide-blocked polyurethane is:

[0043] Where n = 5 to 7;

[0044] The R1 is

[0045] The R2 is Where m=5~13.

[0046] This specific embodiment adopts the prepolymer method, wherein the oligomer polyamic acid is synthesized in the first step, the -NCO-terminated PU prepolymer is synthesized in the second step, and then the PU prepolymer and polyamic acid (precursor of polyimide) are used to block to prepare a polyurethaneimide copolymer. The obtained product has a ternary network structure.

[0047] Taking advantage of the good compatibility between polyimide precursor polyamic acid and polyurethane, solution polymerization was used to introduce polyamic acid into the polyurethane main chain to synthesize linear polyamic acid-polyurethane block copolymers, which were then thermally imidized to prepare cross-linked polyimide-polyurethane block copolymers.

[0048] Compared to polyurethane, imidized polyimide-polyurethane block copolymers blended with epoxy resins not only improve the toughness of the epoxy resin but also further enhance its heat resistance. The modified adhesive can maintain its excellent bonding properties even under damp heat and water boiling conditions. The modified epoxy resin adhesive not only solves the problems of brittleness and heat resistance, improving toughness and heat resistance, but also enhances water resistance and reduces water absorption. The imide rings in the system are rigid structures and do not easily swell when absorbing water. The polyurethane segments, on the other hand, fill the gaps in the system, making it difficult for water molecules to penetrate. Epoxy resin adhesives modified with polyimide-polyurethane block copolymers do not easily swell when absorbing water even under damp heat and water boiling conditions, while maintaining excellent bonding properties.

[0049] The beneficial effects of this specific embodiment are:

[0050] This specific embodiment provides a polyimide-polyurethane copolymer modified epoxy resin adhesive, which is different from traditional polyurethane modified epoxy resin. While enhancing the toughness of the epoxy resin, the introduction of the imide group also improves the heat resistance and bonding performance of the system.

[0051] This specific embodiment uses a copolymer-modified epoxy resin adhesive - a polyimide-blocked polyurethane polymer. The copolymer and the epoxy resin penetrate each other to form an interpenetrating network (IPN) structure. The IPN structure can disperse tensile stress. When subjected to external tensile stress, the tensile force will be transmitted from one network to another, the molecular structure is not easily destroyed, and the toughness of the system is improved. The epoxy-modified polyimide polyurethane polymer is divided into a hard segment structure and a soft segment structure. The hard segment is mainly the PI molecular chain structure, and the soft segment is mainly the flexible chain segment of PU. The PI segment has a high-strength rigid structure and becomes the overall skeleton structure of the IPN system. The imide ring and aromatic ring in the molecular chain have a conjugated effect. The electron cloud distribution in the conjugated system is relatively uniform, the internal energy of the molecule is low, and the molecular structure is not easily destroyed. The conjugated effect also enhances the rigidity and modulus of the system, giving the system a good supporting effect. The main chain of the PI molecule contains a benzene ring structure and an imide ring structure, which increases the polarity and hydrogen bonding degree between the hard segments, increases the force of the hard segments, and hinders the movement of the molecular chain. The imide group is also a polar group that can react with the polar hydroxyl and epoxy groups in the epoxy resin to form chemical bonds, increasing the cohesive energy density of the system. The molecular chain is difficult to move, and the cohesive energy density of the system increases, which increases the modulus of the system and enhances the mechanical properties and heat resistance. PU, as a soft segment, can undergo microphase separation with the hard segment and form a co-continuous phase or a particle dispersed phase with the epoxy resin, thereby increasing the toughness of the epoxy resin and improving its mechanical properties. Polyimide-block polyurethane copolymer modified epoxy resin not only improves the temperature resistance of the system, but also improves the mechanical properties of the system, so the adhesive has good high-temperature bonding properties.

[0052] When the adhesive is immersed in water, the support provided by its rigid structure prevents the system from absorbing water and expanding. Even in high-temperature environments, the system does not shrink due to heat, making the adhesive resistant to high-temperature boiling. While the hard PI molecular segments possess rigidity and heat resistance, acting as a scaffold to support the material's structure, pure PI structures are susceptible to water penetration in humid environments, degrading mechanical properties. The PU structure, the flexible segments within the IPN structure, contains numerous flexible groups. The ether bonds have low cohesive energy, allowing for easy rotation and molecular movement, allowing the segments to fill gaps in the system. In addition to providing toughness, the flexible PU segments also prevent water from penetrating the system, reducing water absorption and maintaining excellent bonding properties even in boiling and damp-heat conditions. The modified polyimide-polyurethane block copolymer has more complex segments, resulting in increased steric hindrance, making water penetration more difficult and reducing water absorption, enabling the adhesive to maintain excellent bonding properties in boiling and damp-heat conditions.

[0053] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the molar ratio of the aromatic diamine to the aromatic dianhydride is (1.01-2):1. Other aspects are the same as those of specific embodiment 1.

[0054] Specific embodiment 3: This embodiment differs from specific embodiment 1 or 2 in that the molar ratio of the dehydrated isocyanate to the dehydrated polyol is (1.01-3):1. Other aspects are the same as specific embodiment 1 or 2.

[0055] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the epoxy resin is E-51 epoxy resin and the molecular weight of the polybutylene glycol is 1000 g / mol to 3000 g / mol. Other aspects are the same as specific embodiments 1 to 3.

[0056] Specific embodiment 5: This embodiment is a method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive, which is carried out according to the following steps:

[0057] 1. Preparation of polyamic acid prepolymer:

[0058] Weigh aromatic diamine, aromatic dianhydride, and an organic solvent in proportion, add the aromatic diamine to the organic solvent and stir to dissolve at room temperature, then add the aromatic dianhydride and stir to dissolve at room temperature, then react at a temperature of 20°C to 40°C for 1 to 2 hours, add deionized water under high-speed stirring after the reaction, stop stirring when the system becomes a milky white suspension, filter and wash, and finally dry to obtain a polyamic acid prepolymer;

[0059] 2. Preparation of polyurethane prepolymer:

[0060] Dehydrating the isocyanate and the polyol separately, then weighing the dehydrated polyol and the dehydrated isocyanate in proportion, and then dripping the dehydrated isocyanate into the dehydrated polyol under a nitrogen atmosphere at a temperature of 60°C to 80°C for 2h to 3h to obtain a polyurethane prepolymer;

[0061] 3. Preparation of polyurethane prepolymer with polyamic acid block:

[0062] Under nitrogen atmosphere and temperature of 60° C. to 80° C., adding polyamic acid prepolymer to polyurethane prepolymer and reacting for 0.5 h to 1 h to obtain polyamic acid-blocked polyurethane prepolymer;

[0063] The molar ratio of the polyurethane prepolymer to the polyamic acid prepolymer is (2.01-3):1;

[0064] 4. Thermal imidization:

[0065] Under a nitrogen atmosphere, the polyurethane prepolymer having polyamic acid segments is thermally imidized to obtain a polyurethane having polyimide segments;

[0066] 5. Blending of polyimide-blocked polyurethane and epoxy resin:

[0067] Under nitrogen atmosphere and temperature of 60°C to 80°C, polyimide-blocked polyurethane is added to epoxy resin for reaction for 1 to 3 hours to obtain a polyimide-polyurethane copolymer-modified epoxy resin adhesive;

[0068] The mass ratio of the polyurethane of the polyimide block to the epoxy resin is (0.1-0.4):1.

[0069] Specific embodiment 6: This embodiment differs from specific embodiment 5 in that the organic solvent in step 1 is N,N-dimethylacetamide, and the mass ratio of the total mass of the aromatic diamine and aromatic dianhydride to the organic solvent in step 1 is 1:(2-5). Other aspects are the same as specific embodiment 5.

[0070] Specific embodiment seven: This embodiment differs from specific embodiment five or six in that: in step one, at room temperature and a stirring speed of 200r / min~400r / min, an aromatic diamine is added to an organic solvent and stirred to dissolve, then at room temperature, a stirring speed of 200r / min~400r / min and an addition rate of 1mL / min~2mL / min, an aromatic dianhydride is added and stirred to dissolve, and then at a temperature of 20℃~40℃ and a stirring speed of 400r / min~600r / min, the reaction is carried out for 1h~2h to obtain a reaction system, and after the reaction, deionized water is added to the reaction system at a stirring speed of 1000r / min~1200r / min and an addition rate of 3mL / min~5mL / min, the stirring is stopped when the system becomes a milky white suspension, and then filtered to obtain a filter cake, the filter cake is washed 2~4 times with acetone and deionized water, and finally dried; the volume ratio of the reaction system to deionized water is 1:(1.5~2.5). The rest is the same as the fifth or sixth embodiment.

[0071] Specific embodiment 8: This embodiment differs from specific embodiments 5 to 7 in that the dehydration in step 2 is carried out at a temperature of 100° C. to 110° C. for 2 to 3 hours. Other steps are the same as specific embodiments 5 to 7.

[0072] Specific embodiment 9: This embodiment differs from Specific embodiments 5 to 8 in that, in step 2, the dehydrated isocyanate is added dropwise to the dehydrated polyol under a nitrogen atmosphere at a temperature of 60°C to 80°C, a stirring speed of 400 to 600 r / min, and a dropwise addition rate of 0.25 to 1 mL / min for a reaction of 2 to 3 hours. Otherwise, this embodiment is the same as Specific embodiments 5 to 8.

[0073] Specific embodiment 10: This embodiment differs from specific embodiments 5 to 9 in that the thermal imidization described in step 4 is specifically carried out under a nitrogen atmosphere, first at a temperature of 60° C. to 65° C. for 1 hour to 1.5 hours, then at a temperature of 70° C. to 80° C. for 1 hour to 1.5 hours, then at a temperature of 85° C. to 90° C. for 1 hour to 1.5 hours, then at a temperature of 95° C. to 100° C. for 1 hour to 1.5 hours, and finally at a temperature of 110° C. to 120° C. for 1 hour to 1.5 hours. Other aspects are the same as specific embodiments 5 to 9.

[0074] The following examples are used to verify the beneficial effects of the present invention:

[0075] Example 1:

[0076] A method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive is carried out according to the following steps:

[0077] 1. Preparation of polyamic acid prepolymer:

[0078] Weigh aromatic diamine, aromatic dianhydride and organic solvent in proportion, add aromatic diamine to organic solvent and stir to dissolve at room temperature and stirring at 400 r / min, then add aromatic dianhydride and stir to dissolve at room temperature, stirring at 400 r / min and adding at a rate of 1.5 mL / min, then react for 2 h at a temperature of 30° C. and stirring at 400 r / min to obtain a reaction system, after which deionized water is added to the reaction system at a stirring rate of 1200 r / min and an adding rate of 4 mL / min, stop stirring when the system becomes a milky white suspension, then filter to obtain a filter cake, wash the filter cake with acetone and deionized water 3 times, and finally vacuum dry at a temperature of 60° C. for 6 h to obtain a polyamic acid prepolymer; the volume ratio of the reaction system to deionized water is 1:1.5;

[0079] The molar ratio of the aromatic diamine to the aromatic dianhydride is 2:1; the mass ratio of the total mass of the aromatic diamine and the aromatic dianhydride to the organic solvent is 1:3; the aromatic diamine is m-phenylenediamine; the aromatic dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride; and the organic solvent is N,N-dimethylacetamide;

[0080] 2. Preparation of polyurethane prepolymer:

[0081] The isocyanate and the polyol were dehydrated separately, and then the dehydrated polyol and the dehydrated isocyanate were weighed in proportion. Then, under a nitrogen atmosphere, a temperature of 70° C., a stirring speed of 600 r / min, and a dropping speed of 0.5 mL / min, the dehydrated isocyanate was dropped into the dehydrated polyol for reaction for 2 h to obtain a polyurethane prepolymer;

[0082] The molar ratio of the dehydrated isocyanate to the dehydrated polyol is 2:1; the dehydrated isocyanate is toluene-2,4-diisocyanate; the dehydrated polyol is polybutylene glycol; and the molecular weight of the polybutylene glycol is 3000 g / mol;

[0083] 3. Preparation of polyurethane prepolymer with polyamic acid block:

[0084] Under nitrogen atmosphere and temperature of 70° C., the polyamic acid prepolymer was added to the polyurethane prepolymer and reacted for 1 hour to obtain a polyamic acid-blocked polyurethane prepolymer;

[0085] The molar ratio of the polyurethane prepolymer to the polyamic acid prepolymer is 2.2:1;

[0086] 4. Thermal imidization:

[0087] Under a nitrogen atmosphere, the polyurethane prepolymer having polyamic acid segments is thermally imidized to obtain a polyurethane having polyimide segments;

[0088] The structural formula of the polyimide-blocked polyurethane is:

[0089]

[0090] n=5~7;

[0091] The R1 is

[0092] The R2 is m=5~13;

[0093] 5. Blending of polyimide-blocked polyurethane and epoxy resin:

[0094] Under nitrogen atmosphere and temperature of 80°C, polyimide-blocked polyurethane was added to epoxy resin for reaction for 2 hours to obtain polyimide-polyurethane copolymer-modified epoxy resin adhesive;

[0095] The mass ratio of the polyimide-blocked polyurethane to the epoxy resin is 0.3:1; the epoxy resin is E-51 epoxy resin;

[0096] The dehydration in step 2 is specifically performed at a temperature of 110° C. for 3 hours;

[0097] The thermal imidization described in step 4 is specifically carried out under a nitrogen atmosphere, first at a temperature of 60°C for 1 hour, then at a temperature of 80°C for 1 hour, then at a temperature of 90°C for 1 hour, then at a temperature of 100°C for 1 hour, and finally at a temperature of 110°C for 1 hour.

[0098] Example 2: This example is different from Example 1 in that the aromatic dianhydride used in step 1 is 4,4'-(hexafluoroisopropylene) diphthalic anhydride, and the R1 The rest is the same as that of the first embodiment.

[0099] Example 3: This example is different from Example 1 in that the aromatic dianhydride used in step 1 is 4,4'-biphenyl ether dianhydride, and R1 is The rest is the same as that of the first embodiment.

[0100] Example 4: This example is different from Example 1 in that the aromatic dianhydride used in step 1 is 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and R1 is The rest is the same as that of the first embodiment.

[0101] Example 5: This example differs from Example 1 in that the molar ratio of the aromatic diamine to the aromatic dianhydride in step 1 is 3:2. Other steps are the same as those in Example 1.

[0102] Example 6: This example differs from Example 1 in that the molar ratio of the aromatic diamine to the aromatic dianhydride in step 1 is 5:4. Other steps are the same as those in Example 1.

[0103] Example 7: This example differs from Example 1 in that the molar ratio of the aromatic diamine to the aromatic dianhydride in step 1 is 6:5. Other steps are the same as those in Example 1.

[0104] Example 8: This example differs from Example 1 in that the molar ratio of the aromatic diamine to the aromatic dianhydride in step 1 is 4:3. Other steps are the same as those in Example 1.

[0105] Example 9: This example differs from Example 1 in that the synthesis temperature of the isocyanate and polybutylene glycol in step 2 is 60° C. The rest is the same as Example 1.

[0106] Example 10: This example differs from Example 1 in that the synthesis temperature of the isocyanate and polybutylene glycol in step 2 is 80° C. The rest is the same as Example 1.

[0107] Comparative Example 1: This comparative example differs from Example 1 in that epoxy resin is used as the adhesive, and the addition of other substances and the preparation process are omitted; the epoxy resin structural formula is:

[0108] n=0.49~0.51. Other aspects are the same as those in the first embodiment.

[0109] Comparative Example 2: This comparative example differs from Example 1 in that steps 1, 3, and 4 are omitted; the structural formula of the polyurethane prepolymer prepared in step 2 is:

[0110] n=13-15; in step 5, under nitrogen atmosphere and temperature of 80°C, the polyurethane prepolymer is added to the epoxy resin and reacted for 2 hours; the mass ratio of the polyurethane prepolymer to the epoxy resin is 0.3:1. Other steps are the same as those in Example 1.

[0111] Comparative Example 3: This comparative example differs from Example 1 in that steps 2 and 3 are omitted; in step 1, aromatic diamine, aromatic dianhydride and organic solvent are weighed in proportion, and the aromatic diamine is added to the organic solvent and stirred to dissolve at room temperature and a stirring speed of 400 r / min, and then the aromatic dianhydride is added and stirred to dissolve at room temperature, a stirring speed of 400 r / min and an addition rate of 1.5 mL / min, and then the reaction is carried out at a temperature of 30° C. and a stirring speed of 400 r / min for 2 h to obtain a polyamic acid prepolymer solution; in step 4, the polyamic acid prepolymer solution is thermally imidized under a nitrogen atmosphere to obtain a polyimide; the structural formula of the polyimide is:

[0112] The R1 is n=5~7;

[0113] In step 5, polyimide was added to epoxy resin under nitrogen atmosphere and temperature of 80° C. for reaction for 2 hours; the mass ratio of polyimide to epoxy resin was 0.3:1. Other steps were the same as those in Example 1.

[0114] The materials and process parameters of Examples 1 to 10 and the comparative example are shown in Table 1:

[0115] Table 1

[0116]

[0117]

[0118]

[0119] Performance testing:

[0120] 1. GPC: The relative molecular mass and relative molecular mass distribution were analyzed using a LC-20AD gel permeation chromatography instrument from Shimadzu Corporation of Japan, using N,N-dimethylformamide (DMF) as the mobile phase of the chromatographic column and DMF as the solvent for the sample.

[0121] 2. Shear: A polyimide-polyurethane copolymer-modified epoxy resin adhesive and a dicyandiamide curing agent (all dicyandiamide curing agents are from Guangzhou Xinxi Metallurgical Chemical Co., Ltd., product model: SH-500) were mixed in a mass ratio of 100:10. The mixture was then applied to the bonding surfaces of two aluminum alloy specimens (6061-T6) and laminated. The adhesive was then heated at 110°C for 1 hour and then at 180°C for 1 hour to produce the bonded components. Shear strength was tested using an Instron 5969 universal testing machine (INSTRON, USA). Room temperature shear strength was measured according to GB / T 7124-2008; 150°C shear strength was measured according to GJB 444-1988.

[0122] 3. Peeling: The 90° peel strength was tested according to GJB 446-1988. The polyimide-polyurethane block copolymer modified epoxy was cured (mixed with dicyandiamide curing agent at a mass ratio of 100:10, then heated at 110°C for 1 hour and then at 180°C for 1 hour). The peel strength was tested using an Instron 5969 universal testing machine (INSTRON, USA).

[0123] 4. Impact: After curing, the polyimide-polyurethane block copolymer modified epoxy was mixed with dicyandiamide curing agent in a mass ratio of 100:10, and then heated at 110°C for 1 hour and then at 180°C for 1 hour. The impact strength test was carried out on a pendulum impact testing machine in accordance with the national standard GB / T6328-1999 "Test method for shear impact strength of adhesives".

[0124] 5. Rheological Test: The viscosity-temperature curve of the sample was measured using a rheometer in the temperature range from room temperature to 120°C at a test speed of 10 rad / s. The data in the table is the complex viscosity at 120°C.

[0125] 6. 5% thermal weight loss temperature (T 5％ After curing, a polyimide-polyurethane block copolymer-modified epoxy resin (mixed with a dicyandiamide curing agent at a mass ratio of 100:10, then heated at 110°C for 1 hour and then at 180°C for 1 hour) was analyzed using a TA Instruments Q50 TGA. The heating rate was 10°C / min and the test atmosphere was air.

[0126] 7. DMA Testing: Dynamic mechanical testing was performed on cured polyimide-polyurethane block copolymer-modified epoxy resins (mixed with dicyandiamide curing agent at a mass ratio of 100:10, then heated at 110°C for 1 hour and then at 180°C for 1 hour). Dynamic mechanical testing was performed using a DMAQ800 dynamic mechanical analyzer from TA Instruments (USA). Sample dimensions were (20-40) mm × (5-10) mm × (1-2) mm (length × width × height), with a drive frequency of 1 Hz and a heating rate of 5°C / min. The test temperature range was from room temperature to 300°C.

[0127] 8. Water Absorption Test: After curing, the polyimide-polyurethane block copolymer modified epoxy (mixed with dicyandiamide curing agent at a mass ratio of 100:10, then heated at 110°C for 1 hour and then at 180°C for 1 hour) was tested for water absorption. At room temperature, a sample with a mass of m1 was completely immersed in deionized water for 24 hours. The sample was then removed and its surface moisture was wiped dry. The sample mass was weighed and recorded as m3. The water absorption rate W was calculated according to the following formula:

[0128]

[0129] 9. Moisture treatment: According to the standard GB / T 30779-2014, the adhesive parts in Test 2 were placed in an HS-50 constant temperature and humidity chamber for moisture treatment. First, the sample joints were placed in a constant temperature and humidity chamber at a relative humidity of 50% and a temperature of 23°C for 48 hours. Then, the sample joints were placed in a constant temperature and humidity chamber at a relative humidity of 85% and a temperature of 85°C for moisture treatment for 24 hours. Finally, the sample joints were placed in a constant temperature and humidity chamber at a relative humidity of 50% and a temperature of 23°C for 24 hours. Then, shear tests were performed according to the standard GB / T 7124-2008 and DMA tests were performed.

[0130] 10. Boiling cycle wet strength test: Place the adhesive parts in test 2 in boiling water and boil for 4 hours. Place them in a 63±3℃ forced air drying oven while they are still hot. Dry them in boiling water for 20 hours. Continue to boil them in boiling water for 4 hours. Then soak them in cold water below 30℃ and cool them for 1 hour. Then perform shear test according to GB / T7124-2008 standard and DMA test.

[0131] Table 2 Specific test results

[0132]

[0133]

[0134] Thermal performance tests of Examples 1 to 10 and the comparative example show that after the introduction of the polyimide block, the 5% thermal weight loss temperature of the modified epoxy resins of Examples 1 to 10 all exceeded 100°C, which is higher than the thermal weight loss temperature of the comparative example. The glass transition temperature of the modified epoxy resins of Examples 1 to 10 all exceeded 60°C, which is higher than the 63°C of the pure epoxy resin. This is due to the increase in the molecular weight of the introduced polyimide and the presence of intramolecular imide rings and intermolecular cross-linking imide bonds in the system. The hard segment content of the polyimide-polyurethane increases, hindering the movement of the molecular chain, causing the glass transition temperature of the hard segment to shift toward higher temperatures, resulting in a significant increase in the glass transition temperature. The main chain of the polyimide molecule generally contains benzene ring structures and imide ring structures, resulting in strong interactions between its molecular chains and dense molecular chain stacking, resulting in a higher melting point and melt viscosity.

[0135] Mechanical property testing of Examples 1 to 10 and the comparative example demonstrates that the adhesive properties of the polyimide-polyurethane block copolymer-modified epoxy resin are significantly improved. The shear strength, peel strength, impact strength, and complex viscosity of Examples 1 to 10 are all higher than those of the epoxy resin in the comparative example. This is because the polyurethane and epoxy resin form a co-continuous phase or a dispersed particle phase, which improves the toughness of the epoxy resin.

[0136] The shear strength and glass transition temperature (Tg) of the modified epoxy resin under both wet heat and water boiling conditions indicate that both are higher than those of pure epoxy resin. This is because the introduced imide groups are rigid and do not expand or deform under water boiling conditions. Furthermore, the polyurethane segments fill the gaps in the system, making it difficult for water molecules to penetrate. This allows the modified epoxy resin to maintain excellent bonding properties under both wet heat and water boiling conditions.

[0137] Figure 1 This is the FTIR spectrum of the polyimide-blocked polyurethane prepared in step 4 of Example 1. As can be seen from the figure, 1723 cm -1 ~1768cm -1 and 1656cm -1 ~1678cm -1 The characteristic peaks of carbamate and amide groups are at 3200 cm -1 ~3500cm -1 The stretching vibration peak of -OH in -COOH disappears at 1777cm -1 ~1789cm -1 and 1713cm -1 ~1721cm -1 The characteristic peaks of imide indicate that -COOH and NH-CO have been successfully imidized to synthesize PI-PU block copolymer. The synthesis of polyimide was confirmed by infrared spectrum.

Claims

1. A polyimide-polyurethane copolymer modified epoxy resin adhesive, characterized in that The invention is prepared from a polyamic acid prepolymer, a polyurethane prepolymer and an epoxy resin. Specifically, the polyamic acid prepolymer is added to the polyurethane prepolymer for copolymerization to obtain a polyamic acid-blocked polyurethane prepolymer, the polyamic acid-blocked polyurethane prepolymer is thermally imidized to obtain a polyimide-blocked polyurethane, and finally the polyimide-blocked polyurethane is added to the epoxy resin for physical crosslinking. The molar ratio of the polyurethane prepolymer to the polyamic acid prepolymer is (2.01-3):1; the mass ratio of the polyimide-blocked polyurethane to the epoxy resin is (0.1-0.4):

1. The polyamic acid prepolymer is synthesized from aromatic diamine and aromatic dianhydride; the aromatic diamine is m-phenylenediamine; the aromatic dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 4,4'-biphenyl ether dianhydride or 3,3',4,4'-dibenzophenone tetracarboxylic dianhydride; The polyurethane prepolymer is synthesized from dehydrated polyol and dehydrated isocyanate; the dehydrated isocyanate is toluene-2,4-diisocyanate; the dehydrated polyol is polybutylene glycol; The general structural formula of the polyimide-blocked polyurethane is: Where n = 5 to 7; The R1 is -, The R2 is Where m=5~13.

2. The polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 1, characterized in that The molar ratio of the aromatic diamine to the aromatic dianhydride is (1.01-2):

1.

3. The polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 1, characterized in that The molar ratio of the dehydrated isocyanate to the dehydrated polyol is (1.01-3):

1.

4. The polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 1, characterized in that The epoxy resin is E-51 epoxy resin; the molecular weight of the polybutylene glycol is 1000 g / mol to 3000 g / mol.

5. The method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 1, wherein It is carried out in the following steps:

1. Preparation of polyamic acid prepolymer: Weigh aromatic diamine, aromatic dianhydride, and an organic solvent in proportion, add the aromatic diamine to the organic solvent and stir to dissolve at room temperature, then add the aromatic dianhydride and stir to dissolve at room temperature, then react at a temperature of 20°C to 40°C for 1 to 2 hours, add deionized water under high-speed stirring after the reaction, stop stirring when the system becomes a milky white suspension, filter and wash, and finally dry to obtain a polyamic acid prepolymer; 2. Preparation of polyurethane prepolymer: Dehydrating the isocyanate and the polyol separately, then weighing the dehydrated polyol and the dehydrated isocyanate in proportion, and then dripping the dehydrated isocyanate into the dehydrated polyol under a nitrogen atmosphere at a temperature of 60°C to 80°C for 2h to 3h to obtain a polyurethane prepolymer; 3. Preparation of polyurethane prepolymer with polyamic acid block: Under nitrogen atmosphere and temperature of 60° C. to 80° C., adding polyamic acid prepolymer to polyurethane prepolymer and reacting for 0.5 h to 1 h to obtain polyamic acid-blocked polyurethane prepolymer; The molar ratio of the polyurethane prepolymer to the polyamic acid prepolymer is (2.01-3):1; 4. Thermal imidization: Under a nitrogen atmosphere, the polyurethane prepolymer having polyamic acid segments is thermally imidized to obtain a polyurethane having polyimide segments; 5. Blending of polyimide-blocked polyurethane and epoxy resin: Under nitrogen atmosphere and temperature of 60°C to 80°C, polyimide-blocked polyurethane is added to epoxy resin for reaction for 1 to 3 hours to obtain a polyimide-polyurethane copolymer-modified epoxy resin adhesive; The mass ratio of the polyurethane of the polyimide block to the epoxy resin is (0.1-0.4):

1.

6. The method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 5, characterized in that The organic solvent described in step 1 is N,N-dimethylacetamide; the mass ratio of the total mass of the aromatic diamine and aromatic dianhydride described in step 1 to the organic solvent is 1:(2-5).

7. The method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 5, characterized in that In the first step, aromatic diamine is added to an organic solvent and stirred to dissolve at room temperature and a stirring speed of 200 r / min to 400 r / min, and then aromatic dianhydride is added and stirred to dissolve at room temperature, a stirring speed of 200 r / min to 400 r / min and a adding speed of 1 mL / min to 2 mL / min, and then reacted for 1 h to 2 h at a temperature of 20 ° C to 40 ° C and a stirring speed of 400 r / min to 600 r / min to obtain a reaction system, and after the reaction, deionized water is added to the reaction system at a stirring speed of 1000 r / min to 1200 r / min and a adding speed of 3 mL / min to 5 mL / min. Stirring is stopped when the system becomes a milky white suspension, and then filtered to obtain a filter cake, which is washed 2 to 4 times with acetone and deionized water, and finally dried; the volume ratio of the reaction system to deionized water is 1: (1.5 to 2.5).

8. The method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 5, characterized in that The dehydration in step 2 is specifically carried out at a temperature of 100° C. to 110° C. for 2 to 3 hours.

9. The method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 5, characterized in that In step 2, under the conditions of nitrogen atmosphere, temperature of 60° C. to 80° C., stirring speed of 400 r / min to 600 r / min and dropping speed of 0.25 mL / min to 1 mL / min, the dehydrated isocyanate is added dropwise to the dehydrated polyol for reaction for 2 h to 3 h.

10. The method for preparing a polyimide-polyurethane copolymer modified epoxy resin adhesive according to claim 5, characterized in that The thermal imidization described in step 4 is specifically carried out under a nitrogen atmosphere, first at a temperature of 60°C to 65°C for 1h to 1.5h, then at a temperature of 70°C to 80°C for 1h to 1.5h, then at a temperature of 85°C to 90°C for 1h to 1.5h, then at a temperature of 95°C to 100°C for 1h to 1.5h, and finally at a temperature of 110°C to 120°C for 1h to 1.5h.

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