Cover film with high flexibility and high heat resistance and preparation method thereof

Abstract

The invention discloses a high-flexibility and high-heat-resistance cover film which is characterized by comprising a base material, a hot melt adhesive layer I and a hot melt adhesive layer II which are sequentially connected from bottom to top, the hot melt adhesive layer I is obtained by heating and curing a hot melt adhesive I coated on the base material, and the hot melt adhesive layer II is obtained by heating and curing a hot melt adhesive II coated on the base material. The hot melt adhesive layer II is obtained by heating and curing a hot melt adhesive II coated on the hot melt adhesive layer I; the hot melt adhesive I is prepared from nitrile rubber modified epoxy resin, epoxy polysiloxane, dicyandiamide, filler, a flame retardant and a solvent; the hot melt adhesive II is prepared from nitrile rubber modified epoxy resin, dicyandiamide, filler, a flame retardant and a solvent. According to the high-flexibility and high-heat-resistance covering film, on the premise that it is ensured that the covering film has proper initial adhesion, high flexibility and high heat resistance can be achieved at the same time, and the defects in the prior art are overcome.

Description

Technical Field

[0001] The invention relates to the technical field of covering films, and in particular to a covering film with high flexibility and high heat resistance and a preparation method thereof. Background Art

[0002] FPC (Flexible Printed Circuit Board) is a product obtained by coating copper foil on a flexible insulating substrate, using photolithography, etching and electroplating technology to form a conductive circuit with the copper foil, and then covering the conductive circuit with a layer of covering film. It has the advantages of softness, bendability, thinness and small size, and is widely used in mobile devices, vehicle systems, medical equipment and other fields.

[0003] As a key layer to protect the conductive circuit from damage by the external environment, the covering film is not only required to have excellent adhesion to the conductive circuit, but also to have excellent flexibility, heat resistance, moisture resistance, chemical corrosion resistance and flame retardancy. Common covering films usually include a substrate and a hot melt adhesive layer obtained by applying hot melt adhesive on the substrate. Epoxy resin is one of the preferred materials for hot melt adhesive due to its excellent adhesion, good heat resistance and chemical corrosion resistance. However, the brittleness of epoxy resin is relatively large, which greatly limits its application in occasions with high flexibility requirements. In order to improve this shortcoming, the traditional practice is to add a rubber with higher flexibility to the hot melt adhesive to improve the flexibility of the covering film. However, although the above approach improves the flexibility of the covering film to a certain extent, it also reduces its heat resistance, making the covering film easy to fail in a high temperature environment.

[0004] In order to take into account both flexibility and heat resistance, the industry has tried to use epoxy resins with four or higher functional groups. Although the above method is conducive to increasing the degree of cross-linking, thereby limiting the movement of molecular chains, making the material less likely to deform and decompose at high temperatures, thereby improving heat resistance to a certain extent. However, the increase in the degree of cross-linking will increase the rigidity of the system, resulting in a significant decrease in its flexibility, making it difficult for the covering film to take into account both high flexibility and high heat resistance. Summary of the invention

[0005] The purpose of the present invention is to provide a covering film with high flexibility and high heat resistance, which can simultaneously take into account high flexibility and high heat resistance while ensuring that it has suitable initial adhesion, so as to overcome the shortcomings of the prior art.

[0006] The second purpose of the present invention is to provide a method for preparing a covering film with high flexibility and high heat resistance. The preparation method is simple and easy to operate, ensuring that the obtained covering film has both high flexibility and high heat resistance while having suitable initial adhesion, thereby meeting actual usage requirements.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] A covering film with high flexibility and high heat resistance, comprising a substrate, a hot melt adhesive layer I and a hot melt adhesive layer II connected in sequence from bottom to top, wherein the hot melt adhesive layer I is obtained by heating and curing the hot melt adhesive I coated on the substrate, and the hot melt adhesive layer II is obtained by heating and curing the hot melt adhesive II coated on the hot melt adhesive layer I;

[0009] The raw materials of the hot melt adhesive I include nitrile rubber modified epoxy resin, epoxy polysiloxane, dicyandiamide, filler, flame retardant and solvent;

[0010] The raw materials of the hot melt adhesive II include nitrile rubber modified epoxy resin, dicyandiamide, filler, flame retardant and solvent.

[0011] Furthermore, the molecular weight of the epoxy polysiloxane is 250 to 2500, and the epoxy equivalent is 100 to 1000 g / mol.

[0012] Furthermore, the raw materials of the nitrile rubber modified epoxy resin include 20 to 50 parts of epoxy resin, 50 to 80 parts of carboxyl-terminated nitrile rubber, 7 to 13 parts of catalyst and 70 to 130 parts of solvent, calculated by weight.

[0013] The epoxy resin includes a low-softening-point bisphenol A epoxy resin, a high-softening-point bisphenol A epoxy resin and a multifunctional heat-resistant epoxy resin, and the softening point of the high-softening-point bisphenol A epoxy resin is greater than the softening point of the low-softening-point bisphenol A epoxy resin;

[0014] Calculated by mass ratio, the mixing ratio of the low softening point bisphenol A epoxy resin, the high softening point bisphenol A epoxy resin and the multifunctional heat-resistant epoxy resin is (1-3):1:(2-4).

[0015] Furthermore, the low softening point bisphenol A epoxy resin has a softening point of 60 to 80° C. and an epoxy equivalent of 400 to 600 g / mol;

[0016] The high softening point bisphenol A epoxy resin has a softening point of 100 to 120° C. and an epoxy equivalent of 800 to 1200 g / mol;

[0017] The multifunctional heat-resistant epoxy resin has a softening point of 70 to 90° C. and an epoxy equivalent of 200 to 300 g / mol.

[0018] Furthermore, the multifunctional heat-resistant epoxy resin includes any one of o-cresol epoxy resin, dicyclopentadienol type epoxy resin, biphenyl epoxy resin and polyphenyl ring type epoxy resin.

[0019] Furthermore, the molecular weight of the carboxyl-terminated nitrile rubber is 200,000 to 800,000, and the nitrile content of the carboxyl-terminated nitrile rubber is 13 to 35% calculated in terms of mass percentage.

[0020] Furthermore, calculated by weight, the raw materials of the hot melt adhesive I include 50-70 parts of nitrile rubber modified epoxy resin, 2-10 parts of epoxy polysiloxane, 5-10 parts of dicyandiamide, 5-15 parts of filler, 5-15 parts of flame retardant and 5-15 parts of solvent;

[0021] Calculated by weight, the raw materials of the hot melt adhesive II include 55-75 parts of nitrile rubber modified epoxy resin, 5-10 parts of dicyandiamide, 5-15 parts of filler, 5-15 parts of flame retardant and 5-15 parts of solvent.

[0022] Furthermore, the thickness of the hot melt adhesive layer I is 50 to 90% of the sum of the thicknesses of the hot melt adhesive layer I and the hot melt adhesive layer II.

[0023] Furthermore, the substrate is a polyimide film.

[0024] A method for preparing a covering film with high flexibility and high heat resistance, which is used to prepare the above-mentioned covering film with high flexibility and high heat resistance, comprises the following steps:

[0025] A. According to the formula of the nitrile rubber modified epoxy resin, epoxy resin, carboxyl-terminated nitrile rubber and solvent are mixed to obtain a first mixture; a catalyst is added to the first mixture, heated to 60-65° C. under an inert atmosphere, and a second mixture is obtained after the first mixture and the catalyst are completely dissolved; the temperature of the second mixture is increased to 122-126° C., and heating is stopped when the acid value is ≤0.05 mgKOH / g to obtain the nitrile rubber modified epoxy resin;

[0026] B. According to the formula of the hot melt adhesive I, the nitrile rubber modified epoxy resin, the solvent, the epoxy polysiloxane and the dicyandiamide are uniformly mixed to obtain a third mixture; the filler and the flame retardant are added to the third mixture, and the hot melt adhesive I is obtained after stirring, grinding and filtering;

[0027] C. According to the formula of the hot melt adhesive II, the nitrile rubber modified epoxy resin, the solvent and the dicyandiamide are uniformly mixed to obtain a fourth mixture; the filler and the flame retardant are added to the fourth mixture, and the hot melt adhesive II is obtained after stirring, grinding and filtering;

[0028] D. Coating hot melt adhesive I on the substrate, and forming hot melt adhesive layer I after heating and curing; coating hot melt adhesive II on hot melt adhesive layer I, and forming hot melt adhesive layer II after heating and curing, thereby obtaining a covering film with high flexibility and high heat resistance.

[0029] The technical solution provided by the present invention may include the following beneficial effects:

[0030] 1. Nitrile rubber modified epoxy resin is obtained by the reaction of epoxy resin and nitrile rubber, and the molecular chain constituting nitrile rubber is usually composed of thousands of carbon-carbon bonds, and the above chain segments can rotate internally, so that the nitrile rubber modified epoxy resin has higher flexibility. At the same time, the nitrile rubber molecular chain and the epoxy resin molecular chain in the nitrile rubber modified epoxy resin form an interpenetrating network structure, so that the nitrile rubber modified epoxy resin can absorb energy through deformation when subjected to external force, thereby improving the flexibility of the nitrile rubber modified epoxy resin.

[0031] 2. Nitrile rubber modified epoxy resin has the heat resistance of epoxy resin. At the same time, the interpenetrating network structure formed by the nitrile rubber molecular chain and the epoxy resin in the nitrile rubber modified epoxy resin is conducive to improving the heat resistance of the nitrile rubber modified epoxy resin. In addition, in a high temperature environment, nitrile rubber can maintain good stability and is not prone to thermal decomposition or thermal oxidation, which is also conducive to improving the heat resistance of the nitrile rubber modified epoxy resin.

[0032] 3. This technical solution also adds epoxy polysiloxane to the hot melt adhesive I formula. The siloxane chain in the epoxy polysiloxane has a flexible "-Si-O-Si-" bond, which gives the epoxy polysiloxane higher flexibility. At the same time, the silicon atom in the siloxane segment is large in volume and has a high steric hindrance effect. The above effect can interfere with the arrangement of the polymer chain, increase the spacing between the molecular chains, increase the bending and stretching ability of the epoxy polysiloxane, and thus improve its flexibility. In addition, the siloxane segment can exist as a dispersed phase in the epoxy polysiloxane matrix. The above dispersed phase can absorb and disperse stress when loaded, thereby preventing the expansion and propagation of cracks and improving the flexibility of the epoxy polysiloxane. The above multi-faceted effects jointly enhance the flexibility of the covering film. In addition, the silicon oxygen bond in the epoxy polysiloxane has a high bond energy, which makes the epoxy polysiloxane stable at high temperatures, making it have excellent heat resistance and can be used for a long time in a high temperature environment without performance degradation, which is conducive to improving the heat resistance of the covering film.

[0033] 4. The amine groups in dicyandiamide can undergo a ring-opening reaction with the rubber-modified epoxy resin in the hot melt adhesive I and the epoxy groups in the epoxy polysiloxane, and the amine groups in dicyandiamide can undergo a ring-opening reaction with the epoxy groups in the rubber-modified epoxy resin in the hot melt adhesive I, so that the hot melt adhesive layer I formed after the hot melt adhesive I is heated and cured and the hot melt adhesive layer II formed after the hot melt adhesive II is heated and cured have a cross-linked network structure. The cross-linked network structure not only increases the intermolecular force, making the hot melt adhesive layer I and the hot melt adhesive layer II more resistant to the thermal motion of molecules when heated, but also makes the molecular chains of the hot melt adhesive layer I and the hot melt adhesive layer II more stable and not easy to break or rearrange when heated, thereby improving the heat resistance of the covering film. In addition, the cross-linked network structure reduces the free movement and entanglement of the molecular chains when heated, reduces the heat loss and deformation caused by the movement of the molecular chains, and is also conducive to improving the heat resistance of the covering film. DETAILED DESCRIPTION

[0034] The technical solution provides a covering film with high flexibility and high heat resistance, comprising a substrate, a hot melt adhesive layer I and a hot melt adhesive layer II connected in sequence from bottom to top, wherein the hot melt adhesive layer I is obtained by heating and curing the hot melt adhesive I coated on the substrate, and the hot melt adhesive layer II is obtained by heating and curing the hot melt adhesive II coated on the hot melt adhesive layer I;

[0035] The raw materials of the hot melt adhesive I include nitrile rubber modified epoxy resin, epoxy polysiloxane, dicyandiamide, filler, flame retardant and solvent;

[0036] The raw materials of the hot melt adhesive II include nitrile rubber modified epoxy resin, dicyandiamide, filler, flame retardant and solvent.

[0037] In view of the shortcomings of the existing covering films that it is difficult to have both high flexibility and high heat resistance, the present technical solution proposes a covering film with high flexibility and high heat resistance, including a substrate, a hot melt adhesive layer I and a hot melt adhesive layer II connected in sequence from bottom to top. By optimizing the formulas of hot melt adhesive I and hot melt adhesive II, it is ensured that the obtained covering film can have both high flexibility and high heat resistance while having suitable initial adhesion, so as to meet actual use requirements.

[0038] Specifically, the raw materials of hot melt adhesive I include nitrile rubber modified epoxy resin, epoxy polysiloxane, dicyandiamide, filler, flame retardant and solvent, and the raw materials of hot melt adhesive II include nitrile rubber modified epoxy resin, dicyandiamide, filler, flame retardant and solvent. Among them, nitrile rubber modified epoxy resin is obtained by the reaction of epoxy resin and nitrile rubber, and the molecular chain constituting nitrile rubber is usually composed of thousands of carbon-carbon bonds, and the above-mentioned chain segments can undergo internal rotation, so that the nitrile rubber modified epoxy resin has higher flexibility. At the same time, the nitrile rubber molecular chain and the epoxy resin molecular chain in the nitrile rubber modified epoxy resin form an interpenetrating network structure, so that the nitrile rubber modified epoxy resin can absorb energy through deformation when subjected to external force, thereby improving the flexibility of the nitrile rubber modified epoxy resin.

[0039] Furthermore, the epoxy resin modified by nitrile rubber has the heat resistance of epoxy resin, and the interpenetrating network structure formed by the nitrile rubber molecular chain and the epoxy resin in the epoxy resin modified by nitrile rubber is conducive to improving the heat resistance of the epoxy resin modified by nitrile rubber. In addition, in a high temperature environment, nitrile rubber can maintain good stability and is not prone to thermal decomposition or thermal oxidation, which is also conducive to improving the heat resistance of the epoxy resin modified by nitrile rubber.

[0040] In summary, the nitrile rubber modified epoxy resin is obtained by the reaction of epoxy resin with nitrile rubber (such as terminal carboxyl nitrile rubber and terminal hydroxyl nitrile rubber, etc.). With its multiple advantages, it exhibits higher flexibility and heat resistance, so that the hot melt adhesive I and hot melt adhesive II added with nitrile rubber modified epoxy resin can both improve flexibility and heat resistance, which is beneficial to simultaneously improve the flexibility and heat resistance of the covering film.

[0041] Secondly, although adding nitrile rubber modified epoxy resin to the formula of hot melt adhesive I and hot melt adhesive II is beneficial to improve the flexibility and heat resistance of the covering film at the same time, the improvement effect is limited and is still not enough to fully meet the use requirements of the covering film with high flexibility and heat resistance. Therefore, the technical solution also adds epoxy polysiloxane to the formula of hot melt adhesive I. The siloxane chain in the epoxy polysiloxane has a flexible "-Si-O-Si-" bond, thereby giving the epoxy polysiloxane higher flexibility. At the same time, the silicon atom in the siloxane segment is large in volume and has a high steric hindrance effect. The above effect can interfere with the arrangement of the polymer chain, increase the spacing between the molecular chains, increase the bending ability and stretching ability of the epoxy polysiloxane, thereby improving its flexibility. In addition, the siloxane segment can exist as a dispersed phase in the epoxy polysiloxane matrix. The above dispersed phase can absorb and disperse stress when loaded, thereby preventing the expansion and propagation of cracks and improving the flexibility of the epoxy polysiloxane. The above-mentioned multi-faceted effects jointly enhance the flexibility of the covering film. In addition, the silicon-oxygen bonds in epoxy polysiloxane have a relatively high bond energy, which enables epoxy polysiloxane to remain stable at high temperatures, giving it excellent heat resistance and allowing it to be used for a long time in a high temperature environment without performance degradation, thereby helping to improve the heat resistance of the covering film.

[0042] Again, although the heat resistance and flexibility of the covering film can be effectively improved by introducing nitrile rubber modified epoxy resin and epoxy polysiloxane into hot melt adhesive I and introducing nitrile rubber modified epoxy resin into hot melt adhesive II, the above improvements are still not sufficient to fully meet the use requirements of the covering film with higher heat resistance. Therefore, the technical solution once again adds dicyandiamide to the formula of hot melt adhesive I and hot melt adhesive II, and the amine group in dicyandiamide can react with the epoxy group in the rubber modified epoxy resin and epoxy polysiloxane in hot melt adhesive I to undergo a ring-opening reaction, and the amine group in dicyandiamide can react with the epoxy group in the rubber modified epoxy resin in hot melt adhesive I to undergo a ring-opening reaction, so that the hot melt adhesive layer I formed after the hot melt adhesive I is heated and cured and the hot melt adhesive layer II formed after the hot melt adhesive II is heated and cured have a cross-linked network structure. The cross-linked network structure not only increases the intermolecular force, so that the hot melt adhesive layer I and the hot melt adhesive layer II are more resistant to the thermal motion of molecules when heated, but also makes the molecular chains of the hot melt adhesive layer I and the hot melt adhesive layer II more stable and not easy to break or rearrange when heated, thereby improving the heat resistance of the covering film. In addition, the cross-linked network structure reduces the free movement and entanglement of the molecular chain when heated, reduces the heat loss and deformation caused by the movement of the molecular chain, and is also conducive to improving the heat resistance of the covering film.

[0043] It should be noted that dicyandiamide, as a latent curing agent, is relatively stable at room temperature, and can trigger the curing reaction of nitrile rubber-modified epoxy resin and epoxy polysiloxane only under heating conditions. This makes it possible to apply hot melt adhesive I and hot melt adhesive II on the substrate in sequence without having to apply them immediately, which is beneficial to production convenience.

[0044] It should be further explained that the low surface tension and good wetting properties of epoxy polysiloxane are conducive to enhancing the contact and wetting with the surface of the adherend, thereby improving the interaction between it and the adherend, and then enhancing the initial adhesion. At the same time, the amine group in dicyandiamide can undergo a ring-opening reaction with the epoxy group in epoxy polysiloxane. The above reaction will increase the intermolecular force, which is also conducive to enhancing the initial adhesion. And the initial adhesion mainly depends on the hot melt adhesive layer II, that is, the initial adhesion mainly depends on the hot melt adhesive II. If epoxy polysiloxane is added to the hot melt adhesive II, the initial adhesion of the cover film will be significantly improved. In the assembly or welding process of FPC, if the cover film covers the conductive circuit to be connected, the cover film of these parts must be torn off to ensure the circuit connection. However, too high initial adhesion means that the peeling force required to tear off the cover film will also increase accordingly, which increases the difficulty of removing the cover film, and then has an adverse effect on production. At the same time, too large initial adhesion means that the cover film can generate strong adhesion when it just contacts the adherend, making it difficult to adjust the position or perform precise alignment during the production process. Therefore, epoxy polysiloxane cannot be added to the hot melt adhesive II of the present technical solution to ensure that the initial adhesion of the covering film is maintained at an appropriate level, thereby improving the convenience of using the covering film and improving production efficiency. It should be clarified that the initial adhesion refers to the adhesion generated when objects first come into contact.

[0045] Finally, the raw materials of hot melt adhesive I and hot melt adhesive II also include fillers, flame retardants and solvents. The fillers are used to reduce the shrinkage and water absorption of the covering film. The flame retardants give the covering film a higher flame retardancy, making the covering film meet the flame retardant usage requirements. The solvents are used to adjust the viscosity of hot melt adhesive I and hot melt adhesive II, making them easy to coat and ensuring production efficiency.

[0046] Therefore, based on the formula design of hot melt adhesive I and hot melt adhesive II, the covering film obtained by the present technical solution can not only utilize hot melt adhesive I and hot melt adhesive II to simultaneously improve the performance of flexibility and heat resistance, but can also utilize the performance of hot melt adhesive II without adding epoxy polysiloxane and having suitable initial adhesion, so that the covering film can have both high flexibility and high heat resistance while having suitable initial adhesion.

[0047] It should be noted that the fillers in hot melt adhesive I and hot melt adhesive II can be fumed silica, silicon powder, etc., and the specific types are not limited here; the flame retardant can be triphenyl phosphate, ethylphenyl phenyl phosphate, tert-butylbenzene diyl phosphate, tetraarylene arylene bisphosphate, resorcinol phosphate and tetraphenyl bisphenol A-diphosphate, etc., and the specific types are not limited here; the solvent can be butanone, methyl acetate, ethyl acetate, phenyl acetate, methyl benzoate, butyl acetate and octyl acrylate, etc., and the specific types are not limited here.

[0048] It is further described that the molecular weight of the epoxy polysiloxane is 250 to 2500, and the epoxy equivalent is 100 to 1000 g / mol.

[0049] If the molecular weight of epoxy polysiloxane is too low, its ability to improve flexibility is limited, and it is not easy to meet the higher flexibility requirements of the covering film; if the molecular weight of epoxy polysiloxane is too high, the compatibility between epoxy polysiloxane and epoxy resin will be affected, causing the epoxy polysiloxane to easily precipitate from the system, affecting the overall performance and appearance of the covering film.

[0050] Furthermore, if the epoxy equivalent of epoxy polysiloxane is too high, it means that the number of epoxy groups in epoxy polysiloxane is small, the reaction ability is poor, and the effect of improving the heat resistance of the covering film is poor; if the epoxy equivalent of epoxy polysiloxane is too low, it means that the number of epoxy groups in epoxy polysiloxane is too large and the reaction is too fast, which can easily lead to defects such as bubbles and cracks in the final product.

[0051] Further description, calculated by weight, the raw materials of the nitrile rubber modified epoxy resin include 20 to 50 parts of epoxy resin, 50 to 80 parts of carboxyl-terminated nitrile rubber, 7 to 13 parts of catalyst and 70 to 130 parts of solvent;

[0052] The epoxy resin includes a low-softening-point bisphenol A epoxy resin, a high-softening-point bisphenol A epoxy resin and a multifunctional heat-resistant epoxy resin, and the softening point of the high-softening-point bisphenol A epoxy resin is greater than the softening point of the low-softening-point bisphenol A epoxy resin;

[0053] Calculated by mass ratio, the mixing ratio of the low softening point bisphenol A epoxy resin, the high softening point bisphenol A epoxy resin and the multifunctional heat-resistant epoxy resin is (1-3):1:(2-4).

[0054] The raw materials of nitrile rubber modified epoxy resin include epoxy resin, carboxyl-terminated nitrile rubber, catalyst and solvent. Under the action of the catalyst, the epoxy group in the epoxy resin can react with the carboxyl groups at both ends of the carboxyl-terminated nitrile rubber, so that the nitrile rubber molecular chain is introduced into the epoxy resin network in the form of chemical bonds, thereby realizing the modification of the epoxy resin to form nitrile rubber modified epoxy resin. The reaction method is simple and efficient, which is conducive to improving production efficiency.

[0055] Furthermore, the epoxy resin includes low softening point bisphenol A epoxy resin, high softening point bisphenol A epoxy resin and multifunctional heat-resistant epoxy resin, and the low softening point bisphenol A epoxy resin has high fluidity, so that the epoxy resin formed by mixing the softening point bisphenol A epoxy resin, the high softening point bisphenol A epoxy resin and the multifunctional heat-resistant epoxy resin has good permeability and wettability, thereby increasing its contact area and reaction efficiency with the carboxyl-terminated nitrile rubber, making the reaction between the two more complete, which is conducive to ensuring the performance of the nitrile rubber modified epoxy resin. The high softening point bisphenol A epoxy resin cooperates with the softening point bisphenol A epoxy resin to facilitate the regulation of the fluidity of the hot melt adhesive layer I and the hot melt adhesive layer II, thereby reducing the defects such as bubbles and holes that appear during the hot pressing process of the cover film to fit the conductive circuit, and ensuring the appearance of the obtained flexible printed circuit board; the multifunctional heat-resistant epoxy resin can provide more chemical crosslinking points, which is conducive to improving the density of the crosslinking network structure, and then to improving the heat resistance of the cover film.

[0056] Furthermore, if the proportion of low-softening-point bisphenol A epoxy resin is too high, the fluidity of hot-melt adhesive layer I and hot-melt adhesive layer II will be too high, and the cover film will easily overflow when hot-pressing the conductive circuit; if the proportion of low-softening-point bisphenol A epoxy resin is too low, the fluidity of hot-melt adhesive layer I and hot-melt adhesive layer II will deteriorate, and holes will easily form when hot-pressing the conductive circuit; if the proportion of multifunctional heat-resistant epoxy resin is too high, the shrinkage rate of hot-melt adhesive layer I and hot-melt adhesive layer II will be large and hot-melt adhesive layer I and hot-melt adhesive layer II will become brittle; if the proportion of multifunctional heat-resistant epoxy resin is too low, it will easily reduce the heat resistance of hot-melt adhesive layer I and hot-melt adhesive layer II. Therefore, the technical solution limits the mixing ratio of low-softening-point bisphenol A epoxy resin, high-softening-point bisphenol A epoxy resin and multifunctional heat-resistant epoxy resin, which is conducive to ensuring the hot-pressing performance and heat resistance of the cover film.

[0057] It should be noted that the catalyst may be triphenylphosphine, and the specific type is not limited here. The solvent may be butanone, methyl acetate, ethyl acetate, phenyl acetate, methyl benzoate, butyl acetate, octyl acrylate, etc., and the specific type is not limited here.

[0058] It is further described that the low softening point bisphenol A epoxy resin has a softening point of 60 to 80° C. and an epoxy equivalent of 400 to 600 g / mol;

[0059] The high softening point bisphenol A epoxy resin has a softening point of 100 to 120° C. and an epoxy equivalent of 800 to 1200 g / mol;

[0060] The multifunctional heat-resistant epoxy resin has a softening point of 70 to 90° C. and an epoxy equivalent of 200 to 300 g / mol.

[0061] The technical solution optimizes the performance parameters of low softening point bisphenol A epoxy resin, high softening point bisphenol A epoxy resin and multifunctional heat-resistant epoxy resin, which is beneficial to ensure the performance of the epoxy resin formed by mixing the softening point bisphenol A epoxy resin, high softening point bisphenol A epoxy resin and multifunctional heat-resistant epoxy resin, and further ensure the performance of the covering film.

[0062] To further illustrate, the multifunctional heat-resistant epoxy resin includes any one of o-cresol epoxy resin, dicyclopentadienol type epoxy resin, biphenyl epoxy resin and polyphenyl ring type epoxy resin.

[0063] O-cresol epoxy resin, dicyclopentadienol epoxy resin, biphenyl epoxy resin and polyphenyl ring epoxy resin all have good heat resistance, so the technical solution preferably uses any one of the above-mentioned resins as the multifunctional heat-resistant epoxy resin, which is conducive to ensuring the performance of the covering film. In addition, the preferred multifunctional heat-resistant epoxy resin has a high cost performance, which is not only conducive to selecting suitable raw materials according to actual needs and improving the flexibility of the formula, but also conducive to saving production costs under the premise of ensuring the performance of the covering film.

[0064] It is further described that the molecular weight of the carboxyl-terminated nitrile rubber is 200,000 to 800,000, and the nitrile content of the carboxyl-terminated nitrile rubber is 13 to 35% calculated by mass percentage.

[0065] If the molecular weight of the carboxyl-terminated nitrile rubber is too low, the viscosity of the hot melt adhesive I and II will be significantly reduced, and sagging will easily occur during the coating process, making the thickness uniformity of the hot melt adhesive layer I and the hot melt adhesive layer II worse. At the same time, if the molecular weight of the carboxyl-terminated nitrile rubber is too large, it will easily weaken the cohesion of the hot melt adhesive layer I and the hot melt adhesive layer II, making the hot melt adhesive layer I and the hot melt adhesive layer II easy to fall off, affecting the durability of the covering film; if the molecular weight of the carboxyl-terminated nitrile rubber is too high, it will not only cause the solubility of the obtained nitrile rubber modified epoxy resin to deteriorate, but also easily increase the difficulty of coating the hot melt adhesive I and the hot melt adhesive II, affecting the production efficiency.

[0066] Furthermore, if the nitrile content in the terminal carboxyl nitrile rubber is too low, it will affect the heat resistance of the terminal carboxyl nitrile rubber, thereby affecting the heat resistance of the covering film, making it easy to fail at high temperatures; if the nitrile content in the terminal carboxyl nitrile rubber is too high, the terminal carboxyl nitrile rubber is prone to stiffness, brittleness, cracking or breakage, etc., affecting its flexibility, thereby affecting the flexibility of the covering film.

[0067] Further, calculated by weight, the raw materials of the hot melt adhesive I include 50-70 parts of nitrile rubber modified epoxy resin, 2-10 parts of epoxy polysiloxane, 5-10 parts of dicyandiamide, 5-15 parts of filler, 5-15 parts of flame retardant and 5-15 parts of solvent;

[0068] Calculated by weight, the raw materials of the hot melt adhesive II include 55-75 parts of nitrile rubber modified epoxy resin, 5-10 parts of dicyandiamide, 5-15 parts of filler, 5-15 parts of flame retardant and 5-15 parts of solvent.

[0069] This scheme also optimizes the raw material ratio in hot melt adhesive I and hot melt adhesive II. Among them, the addition amount of nitrile rubber modified epoxy resin, epoxy polysiloxane and dicyandiamide in hot melt adhesive I is optimized, and the addition amount of nitrile rubber modified epoxy resin and dicyandiamide in hot melt adhesive II is optimized, which is conducive to ensuring the heat resistance and flexibility of the covering film at a lower cost.

[0070] It is further explained that the thickness of the hot melt adhesive layer I is 50 to 90% of the sum of the thicknesses of the hot melt adhesive layer I and the hot melt adhesive layer II.

[0071] Compared with the hot melt adhesive layer II, the hot melt adhesive layer I has higher flexibility and heat resistance. Therefore, in the technical solution, the thickness of the hot melt adhesive layer I is limited to 50-80% of the sum of the thicknesses of the hot melt adhesive layer I and the hot melt adhesive layer II to ensure the flexibility and heat resistance of the covering film.

[0072] It is further specified that the substrate is a polyimide film.

[0073] The polyimide film (PI film) is an aromatic heterocyclic polymer compound containing an imide group in its molecular structure, which has extremely high heat resistance, thereby facilitating the heat resistance of the covering film.

[0074] A method for preparing a covering film with high flexibility and high heat resistance, which is used to prepare the above-mentioned covering film with high flexibility and high heat resistance, comprises the following steps:

[0075] A. According to the formula of the nitrile rubber modified epoxy resin, epoxy resin, carboxyl-terminated nitrile rubber and solvent are mixed to obtain a first mixture; a catalyst is added to the first mixture, heated to 60-65° C. under an inert atmosphere, and a second mixture is obtained after the first mixture and the catalyst are completely dissolved; the temperature of the second mixture is increased to 122-126° C., and heating is stopped when the acid value is ≤0.05 mgKOH / g to obtain the nitrile rubber modified epoxy resin;

[0076] B. According to the formula of the hot melt adhesive I, the nitrile rubber modified epoxy resin, the solvent, the epoxy polysiloxane and the dicyandiamide are uniformly mixed to obtain a third mixture; the filler and the flame retardant are added to the third mixture, and the hot melt adhesive I is obtained after stirring, grinding and filtering;

[0077] C. According to the formula of the hot melt adhesive II, the nitrile rubber modified epoxy resin, the solvent and the dicyandiamide are uniformly mixed to obtain a fourth mixture; the filler and the flame retardant are added to the fourth mixture, and the hot melt adhesive II is obtained after stirring, grinding and filtering;

[0078] D. Coating hot melt adhesive I on the substrate, and forming hot melt adhesive layer I after heating and curing; coating hot melt adhesive II on hot melt adhesive layer I, and forming hot melt adhesive layer II after heating and curing, thereby obtaining a covering film with high flexibility and high heat resistance.

[0079] The technical solution also proposes a method for preparing a covering film with high flexibility and high heat resistance. The preparation method is simple and easy to operate, ensuring that the obtained covering film has both high flexibility and high heat resistance while having suitable initial adhesion, thereby meeting actual usage requirements.

[0080] Specifically, in step A, when the acid value is ≤0.05 mgKOH / g, the epoxy resin and the carboxyl-terminated nitrile rubber have reacted completely under the action of the catalyst, so stopping heating at this time is beneficial to improving production efficiency and reducing production costs while ensuring the performance of the nitrile rubber-modified epoxy resin.

[0081] Preferably, in step B and step C, the stirring speed of the stirring is 1000-3000 r / min, the stirring time is 10-30 min, the grinding time is 30-50 min, and the mesh size of the filtering is 150-200 mesh.

[0082] By limiting the stirring speed and stirring time in step B and step C, it is beneficial to promote the mixing uniformity of the raw materials and ensure the performance of hot melt adhesive I and hot melt adhesive II. In addition, grinding is beneficial to further refine the particles, improve the fineness and uniformity of the raw materials, and thus improve the appearance and performance of the final product. Limiting the grinding time to 30 to 50 minutes can ensure that the grinding process will not be damaged or deteriorated due to excessive grinding. Furthermore, filtering with a 150-200 mesh filter can remove large particle impurities in hot melt adhesive I and hot melt adhesive II, ensuring the purity and uniformity of hot melt adhesive I and hot melt adhesive II.

[0083] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0084] Performance Test:

[0085] Initial adhesion: Roll a 14# steel ball down from the top of an inclined plate with a height of 6.51cm and an angle of 21°30ˊ with the horizontal line, and measure the length L of the 14# steel ball rolling down on the covering film. If the length L is greater than 20cm, the initial adhesion is qualified. The smaller the length L, the greater the initial adhesion.

[0086] Flexibility: Take two covering films from the same batch, use different diameter shafts to perform a bending test on one of the covering films, and record the minimum shaft diameter at which one covering film can be bent smoothly without obvious deformation or damage. The larger the minimum shaft diameter, the worse the flexibility. In addition, bond the other covering film to the conductive circuit, bend the bonded covering film into an M shape, and place it in an oven at 158°C for 7 days, and observe and record whether there are cracks, deformation or damage at the bend. If the minimum curling diameter of one covering film on the shaft is less than or equal to 1.5mm, and the other covering film has no cracks when bent after being placed in an oven at 158°C for 7 days, the flexibility test is qualified.

[0087] Heat resistance: Lay the covering film on the conductive circuit, and cut the covered film into samples with a width of 2 cm and a length of 6 cm; bend the cut sample into an M shape, fix the bent sample with a clamp, insert the fixed sample vertically into a tin bath preheated to the set temperature, keep it for 30 seconds and then take it out to observe whether there is cracking, deformation or damage at the bend. If there is no obvious cracking, deformation or damage at the bend, the heat resistance is qualified.

[0088] Example 1

[0089] A. According to the formula of nitrile rubber modified epoxy resin, 20 parts of epoxy resin, 50 parts of carboxyl-terminated nitrile rubber and 70 parts of butanone are mixed to obtain a first mixture; 8 parts of triphenylphosphine are added to the first mixture, heated to 60°C in a nitrogen atmosphere, and a second mixture is obtained after the first mixture and triphenylphosphine are completely dissolved; the temperature of the second mixture is increased to 125°C, and heating is stopped when the acid value is ≤0.05mgKOH / g to obtain nitrile rubber modified epoxy resin; according to the mass ratio, the low softening point bisphenol A epoxy resin, high The mixing ratio of the softening point bisphenol A epoxy resin and the dicyclopentadienol type epoxy resin is 3:1:2; the softening point of the low softening point bisphenol A epoxy resin is 60°C, and the epoxy equivalent is 480g / mol; the softening point of the high softening point bisphenol A epoxy resin is 110°C, and the epoxy equivalent is 800g / mol; the softening point of the dicyclopentadienol type epoxy resin is 70°C, and the epoxy equivalent is 230g / mol; the molecular weight of the carboxyl-terminated nitrile rubber is 250000, and the nitrile content in the carboxyl-terminated nitrile rubber is 25% calculated by mass percentage;

[0090] B. According to the formula of hot melt adhesive I, 60 parts of nitrile rubber modified epoxy resin, 10 parts of butanone, 4 parts of epoxy polysiloxane and 6 parts of dicyandiamide are mixed evenly to obtain a third mixture; 8 parts of silicon micropowder and 7 parts of tert-butylbenzene diyl phosphate are added to the third mixture, and hot melt adhesive I is obtained after stirring, grinding and filtering; wherein the molecular weight of epoxy polysiloxane is 1000, and the epoxy equivalent is 300 g / mol;

[0091] C. According to the formula of hot melt adhesive II, 60 parts of nitrile rubber modified epoxy resin, 10 parts of butanone and 8 parts of dicyandiamide are mixed evenly to obtain a fourth mixture; 6 parts of silicon micropowder and 6 parts of tert-butylbenzene diyl phosphate are added to the fourth mixture, and hot melt adhesive II is obtained after stirring, grinding and filtering;

[0092] D. Coating hot melt adhesive I on the polyimide film, and forming a hot melt adhesive layer I with a thickness of 25 μm after heating and curing; coating hot melt adhesive II on the hot melt adhesive layer I, and forming a hot melt adhesive layer II with a thickness of 25 μm after heating and curing, thereby obtaining a covering film with high flexibility and high heat resistance.

[0093] Example 2

[0094] A. According to the formula of nitrile rubber modified epoxy resin, 30 parts of epoxy resin, 60 parts of carboxyl-terminated nitrile rubber and 80 parts of methyl acetate are mixed to obtain a first mixture; 9 parts of triphenylphosphine are added to the first mixture, heated to 60°C under a nitrogen atmosphere, and a second mixture is obtained after the first mixture and triphenylphosphine are completely dissolved; the temperature of the second mixture is increased to 126°C, and heating is stopped when the acid value is ≤0.05mgKOH / g to obtain the nitrile rubber modified epoxy resin; according to the mass ratio, the low softening point bisphenol A epoxy resin in the epoxy resin , the mixing ratio of high softening point bisphenol A epoxy resin and multifunctional heat-resistant epoxy resin is 1:1:2; the softening point of low softening point bisphenol A epoxy resin is 70°C, and the epoxy equivalent is 500g / mol; the softening point of high softening point bisphenol A epoxy resin is 100°C, and the epoxy equivalent is 900g / mol; the softening point of dicyclopentadienol type epoxy resin is 80°C, and the epoxy equivalent is 240g / mol; the molecular weight of carboxyl-terminated nitrile rubber is 350000, and the nitrile content in the carboxyl-terminated nitrile rubber is 20% calculated by mass percentage;

[0095] B. According to the formula of hot melt adhesive I, 60 parts of nitrile rubber modified epoxy resin, 8 parts of methyl acetate, 5 parts of epoxy polysiloxane and 8 parts of dicyandiamide are uniformly mixed to obtain a third mixture; 9 parts of fumed silica and 8 parts of triphenyl phosphate are added to the third mixture, and hot melt adhesive I is obtained after stirring, grinding and filtering; wherein the molecular weight of epoxy polysiloxane is 1000, and the epoxy equivalent is 700 g / mol;

[0096] C. According to the formula of hot melt adhesive II, 65 parts of nitrile rubber modified epoxy resin, 7 parts of methyl acetate and 8 parts of dicyandiamide are mixed evenly to obtain a fourth mixture; 7 parts of fumed silica and 8 parts of triphenyl phosphate are added to the fourth mixture, and hot melt adhesive II is obtained after stirring, grinding and filtering;

[0097] D. Coat hot melt adhesive I on the polyimide film, and form a hot melt adhesive layer I with a thickness of 35 μm after heating and curing; coat hot melt adhesive II on the hot melt adhesive layer I, and form a hot melt adhesive layer II with a thickness of 15 μm after heating and curing, thereby obtaining a covering film with high flexibility and high heat resistance.

[0098] Example 3

[0099] A. According to the formula of nitrile rubber modified epoxy resin, 30 parts of epoxy resin, 60 parts of carboxyl-terminated nitrile rubber and 110 parts of ethyl acetate are mixed to obtain a first mixture; 8 parts of triphenylphosphine are added to the first mixture, heated to 62°C in a nitrogen atmosphere, and a second mixture is obtained after the first mixture and triphenylphosphine are completely dissolved; the temperature of the second mixture is increased to 124°C, and heating is stopped when the acid value is ≤0.05mgKOH / g to obtain the nitrile rubber modified epoxy resin; according to the mass ratio, the low softening point bisphenol A ring in the epoxy resin is 1.25wt%; the acid value is ≤0.05mgKOH / g ... The mixing ratio of epoxy resin, high softening point bisphenol A epoxy resin and biphenyl epoxy resin is 2:1:3; the softening point of low softening point bisphenol A epoxy resin is 70°C, and the epoxy equivalent is 600g / mol; the softening point of high softening point bisphenol A epoxy resin is 120°C, and the epoxy equivalent is 1100g / mol; the softening point of biphenyl epoxy resin is 80°C, and the epoxy equivalent is 250g / mol; the molecular weight of terminal carboxyl nitrile rubber is 320000, and the nitrile content in the terminal carboxyl nitrile rubber is 23% calculated by mass percentage;

[0100] B. According to the formula of hot melt adhesive I, 70 parts of nitrile rubber modified epoxy resin, 15 parts of ethyl acetate, 10 parts of epoxy polysiloxane and 8 parts of dicyandiamide are uniformly mixed to obtain a third mixture; 13 parts of silicon micropowder and 12 parts of ethylphenyl phenyl phosphate are added to the third mixture, and hot melt adhesive I is obtained after stirring, grinding and filtering; wherein the molecular weight of epoxy polysiloxane is 1800, and the epoxy equivalent is 700 g / mol;

[0101] C. According to the formula of hot melt adhesive II, 60 parts of nitrile rubber modified epoxy resin, 10 parts of ethyl acetate and 8 parts of dicyandiamide are mixed evenly to obtain a fourth mixture; 8 parts of silicon micropowder and 6 parts of ethylphenyl phenyl phosphate are added to the fourth mixture, and hot melt adhesive II is obtained after stirring, grinding and filtering;

[0102] D. Coating hot melt adhesive I on the polyimide film, and forming a hot melt adhesive layer I with a thickness of 40 μm after heating and curing; coating hot melt adhesive II on the hot melt adhesive layer I, and forming a hot melt adhesive layer II with a thickness of 10 μm after heating and curing, thereby obtaining a covering film with high flexibility and high heat resistance.

[0103] Comparative Example 1

[0104] The preparation method and raw materials of Comparative Example 1 are the same as those of Example 2, except that no hot melt adhesive layer II is provided in Comparative Example 1.

[0105] Comparative Example 2

[0106] The preparation method and raw materials of Comparative Example 2 are the same as those of Example 2, except that no hot melt adhesive layer I is provided in Comparative Example 2.

[0107] The covering films were prepared by the preparation methods in the above-mentioned embodiment and comparative example respectively, and the performance of the covering films was tested. The test results are shown in Table 1 below.

[0108] Table 1 Test results of relevant properties of covering film

[0109]

[0110]

[0111] From the test results in Table 1, it can be seen that the initial adhesion, flexibility and heat resistance of the covering film prepared by the present technical solution are all qualified. While ensuring appropriate initial adhesion, it can also take into account high flexibility and high heat resistance to meet actual use requirements.

[0112] In Comparative Example 1, since the hot melt adhesive layer II is not provided, it is impossible to utilize the characteristics of the hot melt adhesive layer II to improve the flexibility and heat resistance of the covering film, resulting in the deterioration of the flexibility and heat resistance of the covering film. At the same time, since the initial adhesion of the hot melt adhesive layer I is relatively large, when the hot melt adhesive layer II is not provided, it is impossible to utilize the hot melt adhesive layer II to reduce the initial adhesion, resulting in the initial adhesion of the covering film being too large.

[0113] In Comparative Example 2, since the hot melt adhesive layer I is not provided, the properties of the hot melt adhesive layer I for improving the flexibility and heat resistance of the covering film cannot be utilized, resulting in poor flexibility and heat resistance of the covering film.

[0114] The technical principle of the present invention is described above in conjunction with specific embodiments. These descriptions are only for explaining the principle of the present invention and cannot be interpreted as limiting the scope of protection of the present invention in any way. Based on the explanations herein, those skilled in the art can associate other specific implementations of the present invention without paying creative labor, and these methods will fall within the scope of protection of the present invention.

Claims

1. A covering film with high flexibility and high heat resistance, characterized in that: It comprises a substrate, a hot melt adhesive layer I and a hot melt adhesive layer II which are sequentially connected from bottom to top, wherein the hot melt adhesive layer I is obtained by heating and curing the hot melt adhesive I coated on the substrate, and the hot melt adhesive layer II is obtained by heating and curing the hot melt adhesive II coated on the hot melt adhesive layer I; The raw materials of the hot melt adhesive I include nitrile rubber modified epoxy resin, epoxy polysiloxane, dicyandiamide, filler, flame retardant and solvent; The raw materials of the hot melt adhesive II include nitrile rubber modified epoxy resin, dicyandiamide, filler, flame retardant and solvent.

2. A covering film with high flexibility and high heat resistance according to claim 1, characterized in that: The molecular weight of the epoxy polysiloxane is 250 to 2500, and the epoxy equivalent is 100 to 1000 g / mol.

3. A covering film with high flexibility and high heat resistance according to claim 1, characterized in that: Calculated by weight, the raw materials of the nitrile rubber modified epoxy resin include 20 to 50 parts of epoxy resin, 50 to 80 parts of carboxyl-terminated nitrile rubber, 7 to 13 parts of catalyst and 70 to 130 parts of solvent; The epoxy resin includes a low-softening-point bisphenol A epoxy resin, a high-softening-point bisphenol A epoxy resin and a multifunctional heat-resistant epoxy resin, and the softening point of the high-softening-point bisphenol A epoxy resin is greater than the softening point of the low-softening-point bisphenol A epoxy resin; Calculated by mass ratio, the mixing ratio of the low softening point bisphenol A epoxy resin, the high softening point bisphenol A epoxy resin and the multifunctional heat-resistant epoxy resin is (1-3):1:(2-4).

4. A covering film with high flexibility and high heat resistance according to claim 3, characterized in that: The low softening point bisphenol A epoxy resin has a softening point of 60 to 80° C. and an epoxy equivalent of 400 to 600 g / mol; The high softening point bisphenol A epoxy resin has a softening point of 100 to 120° C. and an epoxy equivalent of 800 to 1200 g / mol; The multifunctional heat-resistant epoxy resin has a softening point of 70 to 90° C. and an epoxy equivalent of 200 to 300 g / mol.

5. A covering film with high flexibility and high heat resistance according to claim 3, characterized in that: The multifunctional heat-resistant epoxy resin includes any one of o-cresol epoxy resin, dicyclopentadienol type epoxy resin, biphenyl epoxy resin and polyphenyl ring type epoxy resin.

6. A covering film with high flexibility and high heat resistance according to claim 3, characterized in that: The molecular weight of the carboxyl-terminated nitrile rubber is 200,000-800,000, and the nitrile content of the carboxyl-terminated nitrile rubber is 13-35% calculated by mass percentage.

7. A covering film with high flexibility and high heat resistance according to claim 1, characterized in that: Calculated by weight, the raw materials of the hot melt adhesive I include 50-70 parts of nitrile rubber modified epoxy resin, 2-10 parts of epoxy polysiloxane, 5-10 parts of dicyandiamide, 5-15 parts of filler, 5-15 parts of flame retardant and 5-15 parts of solvent; Calculated by weight, the raw materials of the hot melt adhesive II include 55-75 parts of nitrile rubber modified epoxy resin, 5-10 parts of dicyandiamide, 5-15 parts of filler, 5-15 parts of flame retardant and 5-15 parts of solvent.

8. A covering film with high flexibility and high heat resistance according to claim 1, characterized in that: The thickness of the hot melt adhesive layer I is 50-90% of the sum of the thicknesses of the hot melt adhesive layer I and the hot melt adhesive layer II.

9. A covering film with high flexibility and high heat resistance according to claim 1, characterized in that: The substrate is a polyimide film.

10. A method for preparing a covering film with high flexibility and high heat resistance, characterized in that: The method for preparing a covering film with high flexibility and high heat resistance as claimed in any one of claims 3 to 9 comprises the following steps: A. According to the formula of the nitrile rubber modified epoxy resin, epoxy resin, carboxyl-terminated nitrile rubber and solvent are mixed to obtain a first mixture; a catalyst is added to the first mixture, heated to 60-65° C. under an inert atmosphere, and a second mixture is obtained after the first mixture and the catalyst are completely dissolved; the temperature of the second mixture is increased to 122-126° C., and heating is stopped when the acid value is ≤0.05 mgKOH / g to obtain the nitrile rubber modified epoxy resin; B. According to the formula of the hot melt adhesive I, the nitrile rubber modified epoxy resin, the solvent, the epoxy polysiloxane and the dicyandiamide are uniformly mixed to obtain a third mixture; the filler and the flame retardant are added to the third mixture, and the hot melt adhesive I is obtained after stirring, grinding and filtering; C. According to the formula of the hot melt adhesive II, the nitrile rubber modified epoxy resin, the solvent and the dicyandiamide are uniformly mixed to obtain a fourth mixture; the filler and the flame retardant are added to the fourth mixture, and the hot melt adhesive II is obtained after stirring, grinding and filtering; D. Coating hot melt adhesive I on the substrate, and forming hot melt adhesive layer I after heating and curing; coating hot melt adhesive II on hot melt adhesive layer I, and forming hot melt adhesive layer II after heating and curing, thereby obtaining a covering film with high flexibility and high heat resistance.

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