A wide-temperature-range-curable flame-retardant foamed structural adhesive film and a preparation method thereof

Abstract

This invention belongs to the field of foaming materials and relates to a flame-retardant foamed structural adhesive film that can be cured over a wide temperature range and its preparation method. It is composed of the following components in parts by weight: 85-115 parts epoxy resin, 25-40 parts thermoplastic resin, 20-35 parts flame retardant, 5-10 parts inorganic filler, 13-18 parts curing agent, 2-5 parts foaming agent, and 1-5 parts silane coupling agent. The flame-retardant epoxy foam structural adhesive film prepared by this invention can be cured in a wide temperature range, from 107 to 177°C, and has a wide range of curing process adaptability. It has excellent initial surface tack, and the performance after curing should meet GJB 1480A-2013. Specific performance indicators include tube shear strength, tube shear strength at high temperature, and tube shear strength after boiling in water for 3 days. The flame retardant properties should meet the requirements of the 60s vertical burning test of IMO FTPC Part 7 (B1 level) for ships and aerospace composites. This invention solves the disadvantages of the current epoxy foam adhesive film's narrow curing process adaptability and lack of flame retardancy, thereby expanding its application in the field of composite materials.

Description

Technical Field

[0001] This invention belongs to the field of foaming materials and relates to a flame-retardant foamed structural adhesive film that can be cured over a wide temperature range and its preparation method. Background Technology

[0002] Expanding foam is an adhesive that, after application, expands through physical or chemical means to form a honeycomb or porous structure, thereby achieving functions such as filling, sealing, bonding, cushioning, or thermal insulation. In structural adhesives used in the shipbuilding and aerospace industries, expanding foam serves as a functional adhesive for splicing aluminum honeycomb cores and Nomex honeycomb cores, mesh filling, and edge reinforcement, and is an important component of honeycomb sandwich structure composite materials in these fields. Expanding foam is generally divided into two types: medium-temperature curing and high-temperature curing, applied to the operating temperature environments of low-speed and high-speed aircraft, respectively. Domestic research and development has yielded a series of medium-temperature expanding foams, such as the medium-temperature curing J-47D, J-97, and SY-P9, and the high-temperature curing expanding foams J-118 and J-275. To meet the requirements of secondary curing of honeycomb structure materials and curing conditions after assembly of different honeycomb components, foreign countries have developed epoxy foam films that can be cured at medium and high temperatures, such as 3M's AF3024 and Cytec's FM490A, which have a wide range of curing process adaptability.

[0003] Currently, domestically produced foamed adhesive films can be used at high temperatures up to 177℃. For example, Chinese invention patent publication number CN112662357A, entitled "An epoxy foamed structural adhesive film resistant to 177℃ heat and oxygen conditions for 3000h and its preparation method," describes an epoxy foamed structural adhesive film that uses a special epoxy and aromatic amine combination system. The curing system used is mainly aromatic amine, which only takes into account the use in the high-temperature curing field. However, it cannot guarantee complete curing at medium and low temperatures, which limits its use. At the same time, the curing speed is slow and the cycle is long, which greatly increases the production cost. In addition, the preparation of the aforementioned epoxy foam structural adhesive film requires the thermoplastic resin to be added to the epoxy resin in three separate additions at a high temperature of 220-230℃, and the temperature must be maintained for 60-70 minutes. This places extremely high demands on the temperature control accuracy and operational stability of the equipment. In actual production, problems such as poor batch stability and resin thermal degradation are prone to occur. The structural adhesive film involves multiple functional components (epoxy resin, thermoplastic resin, core-shell rubber, inorganic filler, antioxidant, foaming agent, compound curing agent, etc.), and requires multiple processes such as coarse mixing, three-roll mill fine mixing, kneading machine kneading, and two-roll calendering. The process is lengthy and requires high process control, which is not conducive to large-scale and continuous production. Furthermore, the main resins include various specialty epoxy resins such as bifunctional Cardo epoxy resin, biphenolic epoxy resin, and trifunctional epoxy resin, all of which are non-general-purpose products with limited supply chains and high costs. The curing agent is a ternary compound of DDS, ether-ketone structured aromatic diamine (BADK), and "imide ring carborane diamine." Among these, carborane aromatic imides are difficult to synthesize, costly, and have limited sources, hindering industrial-scale promotion. Most importantly, the aforementioned epoxy foam structural film cannot achieve flame-retardant effects, thus limiting its application in complementary composite materials.

[0004] With the increasing proportion of composite materials used in domestic shipbuilding and aerospace industries, corresponding material standards are becoming more comprehensive. To align with IMO FTPC Part 7 (B1 level) for marine applications and Boeing's BMS 590G material specification, new requirements have been placed on adhesives that offer wide-temperature-range curing and flame-retardant properties. Therefore, rationally designing flame-retardant and foaming systems within existing medium- and high-temperature curing systems to narrow the gap with foreign products is of great significance. Some application areas, especially civil aviation, require adhesives with excellent flame-retardant properties. Traditional halogenated flame retardants, while highly efficient and effective, limit their application due to the toxicity and corrosiveness of the smoke released during combustion. Additive flame retardants for epoxy resins offer simpler processes, a wider variety of suitable flame retardants, and lower costs. However, achieving a certain flame-retardant effect often requires the addition of large amounts of additive flame retardants, significantly impacting the mechanical and processing properties of the epoxy resin. Summary of the Invention

[0005] The purpose of this invention is to provide a flame-retardant foamed structural adhesive film that can be cured over a wide temperature range, in order to solve the problem that existing foamed adhesive films in China lack flame-retardant properties and cannot meet the bonding requirements of parts with high fire protection requirements, thus achieving a combination of wide temperature range curing and flame-retardant properties.

[0006] The present invention also aims to provide a method for preparing the wide-temperature-range curable flame-retardant foamed structural adhesive film.

[0007] To achieve the above objectives, the technical solution of this invention employs a wide-temperature-range curable flame-retardant foamed structural adhesive film, composed of the following components in parts by weight: 85-115 parts epoxy resin, 25-40 parts thermoplastic resin, 20-35 parts flame retardant, 5-10 parts inorganic filler, 13-18 parts curing agent, 2-5 parts foaming agent, and 1-5 parts silane coupling agent; the epoxy resin is composed of one or more of bisphenol A epoxy resin, phenolic modified epoxy resin, and core-shell epoxy resin; the thermoplastic resin is composed of one or more of phenolphthalein-based polyarylene ether sulfone (PES-C), phenolphthalein-based polyarylene ether ketone (PEK-C), thermoplastic polyimide (TPI), hydroxyl-terminated polyarylene ether sulfone (PES-OH), polyetherimide (PEI), and ultra-high molecular weight polyphenolic resin; the flame retardant is composed of liquid flame retardant and powdered flame retardant.

[0008] The wide temperature range curing flame retardant foamed structural adhesive film is prepared from the following raw materials in parts by weight: 90-105 parts epoxy resin, 28-35 parts thermoplastic resin, 25-32 parts flame retardant, 7-9 parts inorganic filler, 14-16 parts curing agent, 3-4 parts foaming agent, and 2-3 parts silane coupling agent.

[0009] The curing agent is composed of modified dicyandiamide, urea accelerator and latent imidazole, and the mass ratio of the three is modified dicyandiamide: urea accelerator: latent imidazole = (4~4.8): (0.8~1.2): (1.6~2.3).

[0010] The epoxy resin is composed of bisphenol A epoxy resin, phenolic modified epoxy resin and core-shell epoxy resin, and the mass ratio of the three is bisphenol A epoxy resin: phenolic modified epoxy resin: core-shell epoxy resin = (10~13): (4~6): (2~5).

[0011] The bisphenol A epoxy resin is any one of E-51, AG80, or E-44, the phenolic modified epoxy resin is F-51 or F-44, and the core-shell epoxy resin is any one of EPX-140, EPX-125, or EPX-152.

[0012] The mass ratio of the liquid flame retardant to the powdered flame retardant is (1.2~1.5):1.

[0013] The liquid flame retardant is a liquid halogen-free organophosphorus compound, which is mainly composed of any one or more of resorcinol bis(diphenyl) phosphate (RDP) or bisphenol AP bis(diphenyl) phosphate.

[0014] The powdered flame retardant is any one or a combination of powdered antimony trioxide, aluminum hydroxide, or magnesium hydroxide.

[0015] The inorganic filler is any one or a combination of several of fumed silica, silica powder, or nano-calcium carbonate.

[0016] The foaming agent is composed of p-toluenesulfonyl hydrazine and azodicarbonamide in a mass ratio of (2-3):1.

[0017] The silane coupling agent mainly comprises any one or more of γ-(methacryloyloxy)propyltrimethoxysilane (KH570), γ-aminopropyltriethoxysilane (KH550), or γ-glycidoxypropyltrimethoxysilane (KH560).

[0018] The technical solution of this invention also employs a method for preparing a flame-retardant foamed structural adhesive film that can be cured over a wide temperature range, which is carried out by the following steps: (1) Take 85-115 parts by weight of epoxy resin, 25-40 parts of thermoplastic resin, 20-35 parts of flame retardant, 5-10 parts of inorganic filler, 13-18 parts of curing agent, 2-5 parts of foaming agent, and 1-5 parts of silane coupling agent. (2) Mix the epoxy resin and liquid organophosphorus flame retardant from step (1), heat to 120-150℃, stir thoroughly, heat to 200-230℃ and add thermoplastic resin, vacuum stir at high speed for 15-20 min, cool to 110-130℃, add powdered flame retardant and inorganic filler, vacuum stir at high speed for 10-15 min, cool to 70-80℃, and obtain the adhesive. (3) After softening the adhesive obtained in step (2) at 70-80℃, add curing agent, foaming agent and silane coupling agent. After feeding the material 3-5 times with a large roller gap and fine mixing 3-5 times with a small roller gap, take out the adhesive for later use. (4) The adhesive material in step (1) is formed into a film by double-roll calendering, and a wide temperature range curing flame retardant foamed structure adhesive film can be obtained.

[0019] In step (4), the calendering temperature of the adhesive film is 75-85℃.

[0020] In step (4), the calendering thickness of the adhesive film is 1.0 to 1.3 mm.

[0021] The epoxy resin in this invention is a compound of bisphenol A epoxy resin, phenolic modified epoxy resin, and core-shell epoxy resin in a specific ratio. The phenolic modified epoxy phase has a higher glass transition temperature (Tg), while the addition of the core-shell epoxy resin, compared to traditional rubber toughening, provides high toughness to the system without reducing the overall glass transition temperature (Tg), ensuring the cured film meets the high-temperature resistance requirements of 177°C for aerospace applications. The selection of high-strength, high-toughness thermoplastic resins such as phenolphthalein-based polyarylene ether sulfone (PES-C), phenolphthalein-based polyarylene ether ketone (PEK-C), thermoplastic polyimide (TPI), hydroxyl-terminated polyarylene ether sulfone (PES-OH), polyetherimide (PEI), and ultra-high molecular weight polyphenolic resin makes film formation feasible. Furthermore, the high heat resistance (Tg ≥ 200°C) of the thermoplastic resins themselves ensures the film's application in high-temperature fields.

[0022] The flame retardant in this invention is a compound of liquid halogen-free organophosphorus, powdered antimony trioxide, aluminum hydroxide, and magnesium hydroxide. This ensures that the entire system meets the national requirements for halogen-free flame retardancy. The combination of liquid and powdered flame retardants results in good initial tack of the entire system. It also avoids the problems of adding a large amount of powdered flame retardant, which would cause the adhesive to be too hard and have local lumps, while using only liquid flame retardant would result in a sticky system and poor workability of the adhesive film.

[0023] The curing agent of this invention is a compound of modified dicyandiamide, urea accelerator, and latent imidazole accelerator. This ensures the shelf life of the adhesive film and extends its service life. Simultaneously, it achieves complete curing in 1-1.5 hours at 107-177°C and exhibits excellent resistance to boiling water aging. Furthermore, it avoids the delayed reaction or incomplete curing often observed between organophosphorus flame retardants and dicyandiamide. This invention, by combining urea, imidazole accelerators, and modified dicyandiamide, achieves rapid and complete curing of the entire system.

[0024] This invention uses a two-roll mill to mix the rubber compound. The high shear force of the two rolls achieves good dispersion of the curing agent and the foaming agent. At the same time, the foaming agent powder will be pulverized multiple times in terms of particle size under the high shear force of the two rolls, which greatly helps to achieve uniform and effective foaming of the entire system.

[0025] The flame-retardant epoxy foam structural adhesive film prepared by this invention can be cured in a wide temperature range, from 107-177℃, and has a wide curing process adaptability. It has excellent initial surface tack, and the performance after curing should meet GJB 1480A-2013. Specific performance indicators include tube shear strength, tube shear strength at high temperature, and tube shear strength after boiling in water for 3 days. The flame retardant properties should meet the requirements of the 60s vertical burning test of IMO FTPC Part 7 (B1 level) for ships and aerospace composites. This invention solves the disadvantages of the current epoxy foam adhesive film curing process being narrow and not flame-retardant, and thus expands its application in the field of composite materials. Detailed Implementation

[0026] The wide-temperature-range curable flame-retardant foamed structural adhesive film of this embodiment is composed of the following components in parts by weight: 85-115 parts epoxy resin, 25-40 parts thermoplastic resin, 20-35 parts flame retardant, 5-10 parts inorganic filler, 13-18 parts curing agent, 2-5 parts foaming agent, and 1-5 parts silane coupling agent; the epoxy resin is composed of one or more of bisphenol A epoxy resin, phenolic modified epoxy resin, and core-shell epoxy resin; the thermoplastic resin is composed of one or more of phenolphthalein polyarylene ether sulfone (PES-C), phenolphthalein polyarylene ether ketone (PEK-C), thermoplastic polyimide (TPI), hydroxyl-terminated polyarylene ether sulfone (PES-OH), polyetherimide (PEI), and ultra-high molecular weight polyphenolic resin; the flame retardant is composed of liquid flame retardant and powdered flame retardant.

[0027] A method for preparing a wide-temperature-range curable flame-retardant foamed structural adhesive film, comprising the following steps: (1) Take 85-115 parts by weight of epoxy resin, 25-40 parts of thermoplastic resin, 20-35 parts of flame retardant, 5-10 parts of inorganic filler, 13-18 parts of curing agent, 2-5 parts of foaming agent, and 1-5 parts of silane coupling agent. (2) Add the epoxy resin from step (1) to a kneader and add liquid organophosphorus flame retardant to mix. Heat to 120-150℃ and stir thoroughly. Then heat to 200-230℃ and add thermoplastic resin. Stir at high speed under vacuum for 15-20 minutes. Then cool to 110-130℃, add powdered flame retardant and inorganic filler. Stir at high speed under vacuum for 10-15 minutes. Then cool to 70-80℃ to obtain the adhesive. (3) After softening the rubber compound obtained in step (2) at a temperature of 70-80℃, weigh it according to the mass and put it into a two-roll mill. During the process, cold water at 0-10℃ is passed through. Then, the curing agent, foaming agent and silane coupling agent are added. The material is fed 3-5 times with the large roller gap and finely mixed 3-5 times with the small roller gap. The rubber compound is then taken out for use. During the two-roll milling process, the temperature of the rubber compound should be controlled at 60-70℃. If the temperature exceeds the limit, the mixing should be stopped and the rubber compound should be cooled down in time. (4) The adhesive material in step (1) is formed into a film by double-roll calendering, and a wide temperature range curing flame retardant foamed structure adhesive film can be obtained.

[0028] Example 1: Wide-temperature-range curable flame-retardant foamed structural adhesive film and its preparation method The wide-temperature-range curable flame-retardant foamed structural adhesive film of this embodiment is composed of the following raw materials in parts by weight: 100 parts epoxy resin, 32 parts thermoplastic resin, 25 parts flame retardant, 5 parts inorganic filler, 17 parts curing agent, 3 parts foaming agent, and 1.5 parts silane coupling agent.

[0029] In this embodiment, the epoxy resin is composed of bisphenol A epoxy resin, phenolic modified epoxy resin, and core-shell epoxy resin in a weight ratio of 11:5:4. The flame retardant is composed of liquid organophosphorus flame retardant and powdered flame retardant in a weight ratio of 3:2. The curing agent is composed of modified dicyandiamide, urea accelerator, and latent imidazole in a weight ratio of 10:1:3. The foaming agent is composed of p-toluenesulfonyl hydrazine and azodicarbonamide in a weight ratio of 5:2. The thermoplastic resin is phenolphthalein-based polyaryletherketone resin (PEK-C). The inorganic filler is composed of fumed silica and silica powder in a weight ratio of 1:4. The silane coupling agent is γ-aminopropyltriethoxysilane (KH550).

[0030] The preparation method of the wide-temperature-range curable flame-retardant foamed structural adhesive film in this embodiment is carried out according to the following steps: (1) Weigh the epoxy resin, thermoplastic resin, flame retardant, inorganic filler, curing agent, foaming agent, and silane coupling agent by weight; (2) Add the weighed epoxy resin and liquid organophosphorus flame retardant to the kneader, and heat it to 130°C while stirring. Continue stirring at this temperature for 10 minutes, then continue to heat it to 210°C, add thermoplastic resin, and vacuum stir at high speed for 15 minutes at this temperature. Then cool it down to 120°C, add powdered flame retardant and inorganic filler, and vacuum stir at high speed for 10 minutes. Then cool it down to 70°C and take out the rubber material for later use. (3) After softening the rubber compound in step (2) at 70°C in an oven, weigh it according to the mass and put it into a two-roll mill. During this process, 5°C cold water is passed through, and weighed curing agent, foaming agent and silane coupling agent are added. The large roller gap is mixed 3 times and the small roller gap is mixed 3 times before discharge. The rubber temperature is controlled below 70°C throughout the process. (4) The rubber material obtained in step (3) is calendered into a film on two rollers, wherein the film thickness is (1.0±0.2) mm and the calendering roller temperature is 80℃.

[0031] Example 2: Wide-temperature-range curable flame-retardant foamed structural adhesive film and its preparation method The wide-temperature-range curable flame-retardant foamed structural adhesive film of this embodiment is prepared from 95 parts epoxy resin, 35 parts thermoplastic resin, 32 parts flame retardant, 6 parts inorganic filler, 15 parts curing agent, 2.5 parts foaming agent, and 1 part silane coupling agent.

[0032] In this embodiment, the epoxy resin is composed of bisphenol A epoxy resin, phenolic modified epoxy resin, and core-shell epoxy resin in a weight ratio of 6:3:2; the flame retardant is composed of liquid organophosphorus flame retardant and powdered flame retardant in a weight ratio of 7:5; the curing agent is composed of modified dicyandiamide, urea accelerator, and latent imidazole in a weight ratio of 9:2:4; the foaming agent is composed of p-toluenesulfonyl hydrazine and azodicarbonamide in a weight ratio of 5:3; the thermoplastic resin is phenolphthalein-based polyarylene ether sulfone resin (PES-C); the inorganic filler is composed of fumed silica and nano-calcium carbonate in a weight ratio of 2:5; and the silane coupling agent is γ-glycidoxypropyltrimethoxysilane (KH560).

[0033] The preparation method of the wide-temperature-range curable flame-retardant foamed structural adhesive film in this embodiment is carried out according to the following steps: (1) Weigh the epoxy resin, thermoplastic resin, flame retardant, inorganic filler, curing agent, foaming agent, and silane coupling agent by weight; (2) Add the weighed epoxy resin and liquid organophosphorus flame retardant to the kneader, and heat it to 140°C while stirring. Continue stirring at this temperature for 15 minutes, then continue heating to 230°C, add thermoplastic resin, and vacuum stir at high speed for 10 minutes at this temperature. Then cool down to 130°C, add powdered flame retardant and inorganic filler, and vacuum stir at high speed for 10 minutes. Then cool down to 70°C and take it out for later use. (3) After softening the rubber material in step two at 80°C in an oven, weigh it according to the mass and put it into a two-roll mill. During this process, 2°C cold water is passed through, and weighed curing agent, foaming agent and silane coupling agent are added. The large roller gap is mixed 3 times and the small roller gap is mixed 3 times before discharge. The rubber temperature is controlled below 70°C throughout the process. (4) The rubber material obtained in step (3) is calendered into a film on two rollers, wherein the film thickness is (1.0±0.2) mm and the calendering roller temperature is 85℃.

[0034] Example 3: Wide-temperature-range curable flame-retardant foamed structural adhesive film and its preparation method The difference between the wide-temperature-range curable flame-retardant foamed structural adhesive film of this embodiment and that of Embodiment 2 is that the adhesive film of this embodiment is made of 110 parts epoxy resin, 40 parts thermoplastic resin, 22 parts flame retardant, 10 parts inorganic filler, 18 parts curing agent, 4 parts foaming agent, and 2 parts silane coupling agent.

[0035] Table 1 shows the mechanical properties and aging properties of wide-temperature-range curable flame-retardant foamed structural adhesive films from Examples 1, 2, and 3, as well as commercially available J-188 epoxy foamed adhesive film, after curing at 107℃, 121℃, and 177℃. As shown in Table 1, Examples 1, 2, and 3 all exhibit rapid curing characteristics within the temperature range of 107–177°C, achieving the goal of wide-temperature-range curing. In contrast, the commercially available J-118 epoxy foam film failed to fully cure at both 107°C and 121°C. In these examples, the wide-temperature-range curing flame-retardant foam structural adhesive film achieved a tube shear strength of 8.98–9.62 MPa at 23°C, and its tube shear strength at 107°C was comparable to that at 177°C. This demonstrates that the adhesive film of this invention can achieve complete curing at 107°C, while the commercially available J-118 epoxy foam film exhibited a tube shear strength of 3.87 MPa at 107°C and 8.87 MPa at 177°C, with a curing degree of only about 44% at 107°C. Meanwhile, the tube shear strength of the wide-temperature-range curing flame-retardant foamed structural adhesive film of this invention reaches (1.29~1.89) MPa at 177℃, proving that it also has good temperature resistance.

[0036] Table 2 shows that the wide-temperature-range curable flame-retardant foamed structural adhesive film of this invention, after curing and boiling in water for 3 days, retains over 80% of its 23°C pipe shear strength, exhibiting excellent resistance to media and aging. This wide-temperature-range curable flame-retardant foamed structural adhesive film can meet the requirements of splicing honeycomb cores in sandwich structures using metals and composite materials, as well as structural bonding and filling between honeycomb cores and beams, ribs, and partitions, and possesses good mechanical properties.

[0037] Table 1. Performance Comparison of the Wide Temperature Range Curing Flame Retardant Foamed Structural Adhesive Film of the Present Invention and J-188 Epoxy Foamed Adhesive Film

[0038] Table 2. Performance test results of the adhesive film of the present invention after boiling in water for 3 days.

[0039]

[0040] Example 4: Wide-temperature-range curable flame-retardant foamed structural adhesive film and its preparation method The wide-temperature-range curable flame-retardant foamed structural adhesive film of this embodiment is composed of 105 parts epoxy resin, 30 parts thermoplastic resin, 28 parts flame retardant, 7 parts inorganic filler, 14 parts curing agent, 3.5 parts foaming agent, and 1 part silane coupling agent.

[0041] The epoxy resin is composed of bisphenol A epoxy resin, phenolic modified epoxy resin, and core-shell epoxy resin in a weight ratio of 7:3:1, wherein the core-shell epoxy resin is composed of EPX-140 and EPX-152 in a weight ratio of 3:2; the flame retardant is composed of liquid organophosphorus flame retardant and powdered flame retardant in a weight ratio of 6:5; the curing agent is composed of modified dicyandiamide, urea accelerator, and latent imidazole in a weight ratio of 10:3:4; the foaming agent is composed of p-toluenesulfonyl hydrazine and azodicarbonamide in a weight ratio of 3:1; the thermoplastic resin is composed of phenolphthalein-based polyarylene ether sulfone resin (PES-C) and phenolphthalein-based polyarylene ether sulfone resin in a weight ratio of 2:1; the inorganic filler is composed of fumed silica, silica powder, and nano-calcium carbonate in a weight ratio of 2:4:3; and the silane coupling agent is γ-glycidoxypropyltrimethoxysilane (KH560).

[0042] The preparation method of the wide-temperature-range curable flame-retardant foamed structural adhesive film in this embodiment is carried out according to the following steps: (1) Weigh the epoxy resin, thermoplastic resin, flame retardant, inorganic filler, curing agent, foaming agent, and silane coupling agent by weight; (2) Add the weighed epoxy resin and liquid organophosphorus flame retardant to the kneader, and heat it to 135°C while stirring. Continue stirring at this temperature for 10 minutes, then continue heating to 220°C, add thermoplastic resin, and vacuum stir at high speed for 13 minutes at this temperature. Then cool down to 110°C, add powdered flame retardant and inorganic filler, and vacuum stir at high speed for 15 minutes. Then cool down to 70°C and take it out for later use. (3) After softening the rubber material in step (2) at 80°C in an oven, weigh it according to the mass and put it into a two-roll mill. During this process, 0°C cold water is passed through, and weighed curing agent, foaming agent and silane coupling agent are added. The large roller gap is mixed 3 times and the small roller gap is mixed 3 times before discharge. The rubber temperature is controlled below 70°C throughout the process. (4) The rubber material obtained in step (3) is calendered into a film on two rollers, wherein the film thickness is (1.3±0.2)mm and the calendering roller temperature is 85℃.

[0043] The wide-temperature-range curing flame-retardant foamed structural adhesive film of Example 4 was prepared and tested as a vertical combustion sample according to the requirements of Appendix F of CCAR25 China Civil Aviation Airworthiness Standard. Its performance is shown in Table 3. As can be seen from the data in Table 4, the wide-temperature-range curing flame-retardant foamed structural adhesive film of the present invention exhibits excellent flame-retardant properties at temperatures ranging from 107 to 177°C, meeting the relevant requirements of Appendix F of CCAR25 China Civil Aviation Airworthiness Standard. It can be used in conjunction with aviation composite materials to meet the corresponding aviation flame-retardant requirements.

[0044] Table 3 Results of the 60s vertical burning experiment of the adhesive film in Example 4 of the present invention

[0045] Comparative example: Using commercially available J-275 medium-temperature curing foam that can cure over a wide temperature range, the same proportion of flame retardant was added to it as in Example 4. Following the preparation method of Example 4, a film with a diameter of (1.3 ± 0.2) mm was prepared as a comparative example. The mechanical properties and flame retardant properties of the film prepared in Example 4 were compared with those of Example 4. As can be seen from Table 4, the performance of Example 4 of the present invention is higher than that of the comparative example. This is mainly because the addition of flame retardant reduces the overall crosslinking density after curing. Furthermore, the liquid and powdered flame retardants are difficult to adjust the overall viscosity of the adhesive by external addition, resulting in uneven dispersion and uneven cell ratio after foaming, leading to poor mechanical and flame-retardant properties. In this example, the liquid organophosphorus flame retardant is pre-melted with the epoxy resin at high temperature to achieve good dispersion, while also adjusting the overall viscosity of the adhesive. This facilitates the dispersion of subsequent inorganic fillers, curing agents, and foaming agents, thereby achieving the preparation of a high-strength foamed adhesive film with wide temperature range curing and flame-retardant properties.

[0046] Table 4. Comparison of film performance between Example 4 and the comparative example.

[0047] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still make modifications to the technical solutions described in the foregoing embodiments without creative effort, or make equivalent substitutions for some of the technical features, or organically combine different types of specific implementation methods. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A flame-retardant foamed structural adhesive film that can be cured over a wide temperature range, characterized in that: It is composed of the following components in parts by weight: 85-115 parts epoxy resin, 25-40 parts thermoplastic resin, 20-35 parts flame retardant, 5-10 parts inorganic filler, 13-18 parts curing agent, 2-5 parts foaming agent, and 1-5 parts silane coupling agent; wherein the epoxy resin is composed of one or more of bisphenol A epoxy resin, phenolic modified epoxy resin, and core-shell epoxy resin; wherein the thermoplastic resin is composed of one or more of phenolphthalein polyarylene ether sulfone (PES-C), phenolphthalein polyarylene ether ketone (PEK-C), thermoplastic polyimide (TPI), hydroxyl-terminated polyarylene ether sulfone (PES-OH), polyetherimide (PEI), and ultra-high molecular weight polyphenolic resin; wherein the flame retardant is composed of liquid flame retardant and powdered flame retardant.

2. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 1, characterized in that: The curing agent is composed of modified dicyandiamide, urea accelerator and latent imidazole, and the mass ratio of the three is modified dicyandiamide: urea accelerator: latent imidazole = (4~4.8): (0.8~1.2): (1.6~2.3).

3. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 1, characterized in that: The epoxy resin is composed of bisphenol A epoxy resin, phenolic modified epoxy resin and core-shell epoxy resin, and the mass ratio of the three is bisphenol A epoxy resin: phenolic modified epoxy resin: core-shell epoxy resin = (10~13): (4~6): (2~5).

4. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 3, characterized in that: The bisphenol A epoxy resin is any one of E-51, AG80, or E-44, the phenolic modified epoxy resin is F-51 or F-44, and the core-shell epoxy resin is any one of EPX-140, EPX-125, or EPX-152.

5. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 1, characterized in that: The mass ratio of the liquid flame retardant to the powdered flame retardant is (1.2~1.5):

1.

6. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 5, characterized in that: The liquid flame retardant is a liquid halogen-free organophosphorus compound, which is any one or a combination of several of resorcinol bis(diphenyl) phosphate (RDP) or bisphenol AP bis(diphenyl) phosphate.

7. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 1, characterized in that: The inorganic filler is any one or a combination of several of fumed silica, silica powder, or nano-calcium carbonate.

8. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 1, characterized in that: The foaming agent is composed of p-toluenesulfonyl hydrazine and azodicarbonamide in a mass ratio of (2-3):

1.

9. The wide-temperature-range curable flame-retardant foamed structural adhesive film according to claim 1, characterized in that: The silane coupling agent mainly comprises any one or more of γ-(methacryloyloxy)propyltrimethoxysilane (KH570), γ-aminopropyltriethoxysilane (KH550), or γ-glycidoxypropyltrimethoxysilane (KH560).

10. A method for preparing a flame-retardant foamed structural adhesive film that can be cured over a wide temperature range, characterized in that: Prepared by the following steps: (1) Take 85-115 parts by weight of epoxy resin, 25-40 parts of thermoplastic resin, 20-35 parts of flame retardant, 5-10 parts of inorganic filler, 13-18 parts of curing agent, 2-5 parts of foaming agent, and 1-5 parts of silane coupling agent. (2) Mix the epoxy resin and liquid organophosphorus flame retardant from step (1), heat to 120-150℃, stir thoroughly, heat to 200-230℃ and add thermoplastic resin, vacuum stir at high speed for 15-20 min, cool to 110-130℃, add powdered flame retardant and inorganic filler, vacuum stir at high speed for 10-15 min, cool to 70-80℃, and obtain the adhesive. (3) After softening the adhesive obtained in step (2) at 70-80℃, add curing agent, foaming agent and silane coupling agent. After feeding the material 3-5 times with a large roller gap and fine mixing 3-5 times with a small roller gap, take out the adhesive for later use. (4) The adhesive material in step (1) is formed into a film by double-roll calendering, and a wide temperature range curing flame retardant foamed structure adhesive film can be obtained.

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