A polyimide-polytetrafluoroethylene composite film and preparation method thereof

Abstract

The present invention belongs to the technical field of composite materials, and in particular to a polyimide-polytetrafluoroethylene composite film and a preparation method thereof. The composite film is obtained by high-temperature curing after super-hydrophobic polytetrafluoroethylene coating is applied on the surface of polyimide film, and the super-hydrophobic polytetrafluoroethylene water-based coating is made of the following mass fraction raw materials: 50-90 parts of polytetrafluoroethylene aqueous solution with a solid content of 25-60%, 5-30 parts of water, 0.4-3 parts of surfactants, 0.3-3 parts of emulsifiers, 2.5-40 parts of silicone oils, 1-5 parts of film-forming aids, 0.5-2 parts of thickeners, and 0.3-2 parts of silane coupling agents. The coating can form a rough structure with orderly distribution of nanopores and micropores on the surface of polyimide film, and the water contact angle is up to 142°, and the wear resistance is good. The surface of the PI / PTFE composite film prepared by the method has super-hydrophobicity and self-cleaning ability, and the water absorption rate of the composite film is greatly reduced, and the volume resistivity in a humid environment is maintained at 10 16 Ω.mm.

Description

Technical Field

[0001] The invention relates to the technical field of composite materials, in particular to a polyimide-polytetrafluoroethylene composite film and a preparation method thereof. Background Art

[0002] Polyimide (PI) film is widely used in the fields of wire and cable insulation layer, traction motor and transformer coil insulation layer, insulation slot lining and flexible circuit board because of its wide temperature resistance range, high mechanical strength and excellent electrical insulation. However, the imide ring group in the polyimide molecular chain segment causes its own high water absorption rate, which is generally 0.5-2%, seriously affecting the insulation performance of the film in a humid environment. The above defects can be effectively overcome by coating the surface of the polyimide film with a high temperature resistant super hydrophobic coating. Polytetrafluoroethylene (PTFE) is widely used in the preparation of hydrophobic coatings due to its symmetrical CF bond and special helical molecular chain structure, which has high non-adhesion, excellent heat resistance, cold resistance and outstanding chemical stability. However, the adhesion between polytetrafluoroethylene and polyimide is poor, the coating is easy to crack, and the peel strength is low. In addition, PTFE is very soft, and the loss is large after multiple frictions, and the hydrophobic performance decays sharply. These defects restrict the application of polytetrafluoroethylene coatings in the field of polyimide insulating films.

[0003] At present, the wear resistance of polytetrafluoroethylene coating is mainly improved by adding a large amount of inorganic ceramic particles, such as the Chinese invention patent "a kind of inorganic particles / polytetrafluoroethylene composite hydrophobic wear-resistant coating and preparation method thereof" with publication number CN113248986A, using tetraethyl orthosilicate and polyethylene glycol to modify nano silicon carbide, using silane coupling agent KH570 to modify micron zinc oxide, the modified particles are added to the polytetrafluoroethylene aqueous solution and stirred evenly, and finally sprayed evenly on the primer by air spraying method to obtain inorganic particles / polytetrafluoroethylene composite hydrophobic wear-resistant coating. The addition of nano inorganic particles improves the wear resistance of the coating, and the contact angle of SiC / ZnO / PTFE composite coating is about 110 °. However, the method is complicated to operate, and the rigid particles used all need to be surface modified to achieve better compatibility; and the spraying method is not suitable for the construction of flexible substrate surface coating, and the contact angle is still difficult to meet the needs of super hydrophobicity.

[0004] Publication number CN112552783A Chinese invention patent "A kind of multi-dimensional composite super hydrophobic coating and production method" polytetrafluoroethylene, water-based epoxy resin emulsion, composite inorganic nanoparticles (preparing nano silicon dioxide or nano silicon carbide coated graphite or graphene particles by sol-gel method) and a variety of additives are mixed to obtain a multi-dimensional composite super hydrophobic coating. Composite inorganic particles can greatly increase the irregularity of the surface, so as to be more conducive to constructing more rough structures on the coating surface, and the low surface energy of polytetrafluoroethylene makes the coating contact angle reach 153 °. The addition of water-based epoxy resin also greatly improves the adhesion of coating and substrate. However, water-based epoxy resin has low heat resistance and cannot be applied to high temperature resistant (>300 ° C) occasions, and the graphite particles used also affect the electrical insulation performance of the coating. At present, there is still a lack of polytetrafluoroethylene wear-resistant coating solutions dedicated to super hydrophobic and high temperature resistant polyimide films. Summary of the invention

[0005] One of the purposes of the present invention is to provide a method for preparing a polyimide-polytetrafluoroethylene composite film, which has low cost and simple operation. The method reduces the water absorption rate of the polyimide film in a humid environment and improves its service life through the super hydrophobicity and wear resistance of the coating.

[0006] To achieve the above object, the present invention adopts the following technical scheme: a method for preparing a polyimide-polytetrafluoroethylene composite film, comprising the following steps:

[0007] S1. Weigh the following raw materials in parts by mass: 50-90 parts of a polytetrafluoroethylene aqueous solution with a solid content of 25-60%, 5-30 parts of water, 0.4-3 parts of a surfactant, 0.3-3 parts of an emulsifier, 2.5-40 parts of silicone oil, 1-5 parts of a film-forming aid, 0.5-2 parts of a thickener, and 0.3-2 parts of a silane coupling agent. Mix the above raw materials and emulsify them at a shear rate of 2000-10000 rpm for 2-10 minutes to obtain a super-hydrophobic polytetrafluoroethylene water-based coating;

[0008] S2. Apply polytetrafluoroethylene water-based coating on one or both sides of the surface of a 15-50 μm thick polyimide film, and then place it in a high-temperature oven at 280-420° C. for curing for 2-15 minutes to obtain a polyimide-polytetrafluoroethylene composite film.

[0009] As a further improvement of the preparation method of polyimide-polytetrafluoroethylene composite membrane:

[0010] Preferably, the polytetrafluoroethylene aqueous solution is a dispersion of polytetrafluoroethylene particles in water, and the particle size of the polytetrafluoroethylene particles is 50-200 nm.

[0011] Preferably, the surfactant is one or a combination of two or more of sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and sodium ethoxylated fatty acid methyl ester sulfonate (FMES).

[0012] Preferably, the emulsifier is one or a combination of two or more of diethylhexyl sodium sulfosuccinate (OT-75), sorbitan fatty acid ester (S-80), alkylphenol polyoxyethylene ether (OP-10), and fatty alcohol polyoxyethylene ether (AEO-9).

[0013] Preferably, the silicone oil is one or a combination of two or more of dimethyl silicone oil, amino silicone oil, trifluoropropyl silicone oil, hydroxy silicone oil, and methylphenyl silicone oil.

[0014] Preferably, the film-forming aid is one or a combination of two or more of dipropylene glycol butyl ether, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, ethylene glycol butyl ether, and dipropylene glycol methyl ether.

[0015] Preferably, the thickener is one or a combination of two or more of polyurethanes C4, C5, C8, C77 or acrylics R1004, R1005, R1006; or, the silane coupling agent is one or a combination of two or more of KH550, KH560, KH570, KH792.

[0016] Preferably, the amount of silicone oil in the super-hydrophobic polytetrafluoroethylene water-based coating in step S1 accounts for 25% of the total mass.

[0017] Preferably, the coating method used is wire rod coating, slot coating or micro-concave coating.

[0018] A second object of the present invention is to provide a polyimide-polytetrafluoroethylene composite membrane prepared by any one of the above preparation methods.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1) The present invention obtains a super-hydrophobic polytetrafluoroethylene water-based coating by subjecting raw materials including a silicone oil component to high-speed shear emulsification; then the coating is coated on one side or both sides of a polyimide film surface, and cured at high temperature to obtain a polyimide-polytetrafluoroethylene composite film.

[0021] 2) After adding silicone oil components to the components, PTFE changes from an aqueous emulsion to an oil-in-water emulsion. With the help of various additives, silicone oil is dispersed to the micron or even nanometer level and stably exists in the system. After coating and high-temperature curing, a polytetrafluoroethylene coating with a hierarchical pore structure is formed on the surface of the membrane, and a rough structure with orderly distribution of nanopores and micropores is formed on the surface of the polyimide film. Under the dual effects of appropriate roughness and low surface energy of polytetrafluoroethylene, the water contact angle on the surface of the composite membrane increases significantly, producing a super-hydrophobic effect similar to the surface of a lotus leaf, improving hydrophobicity, reducing water absorption, and the water contact angle reaches up to 142°. The volume resistivity of the composite membrane is maintained in a humid environment. The residual silicone component greatly improves the wear resistance of the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a contact angle measurement photograph of the polyimide of Comparative Example 1 that has not been treated in any way.

[0023] Figure 2 This is a scanning electron microscope photograph of the polyimide film of Comparative Example 2 after coating with pure PTFE emulsion.

[0024] Figure 3-7 The following are scanning electron microscope photos of super-hydrophobic composite films 1-5 prepared in Examples 1-5, respectively.

[0025] Figure 8 This is a contact angle measurement photograph of the super-hydrophobic composite film 3 prepared in Example 3. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0027] Example 1

[0028] This embodiment provides a method for preparing a polyimide-polytetrafluoroethylene composite film, comprising the following steps:

[0029] S1, 80 parts by mass of polytetrafluoroethylene aqueous solution (solid content 30%), 10 parts by mass of water, 1.2 parts by mass of surfactant (SDS), 0.8 parts by mass of emulsifier (sodium diethyl sulfosuccinate, model OT-75), 5 parts by mass of silicone oil (dimethyl silicone oil), 1.5 parts by mass of film-forming aid (ethylene glycol butyl ether), 1 part by mass of thickener (acrylic acid R1004), 0.5 parts by mass of silane coupling agent (KH550) were uniformly mixed, and then emulsified at 2000rpm for 10 minutes using a high-speed shear emulsifier to obtain a super-hydrophobic polytetrafluoroethylene water-based coating;

[0030] S2, then the super-hydrophobic polytetrafluoroethylene water-based coating is double-sidedly coated on the surface of the 25 μm thick polyimide film using a 10# wire rod, and the coating thickness is 4 μm. Finally, the composite film is placed in an oven and treated at 330°C for 10 minutes to obtain a super-hydrophobic composite film 1. The scanning electron microscope photo is shown in Figure 3 shown.

[0031] Example 2

[0032] This embodiment provides a method for preparing a polyimide-polytetrafluoroethylene composite film, comprising the following steps:

[0033] S1. Evenly mix 70 parts by mass of polytetrafluoroethylene aqueous solution (solid content 25%), 8 parts by mass of water, 1.9 parts by mass of surfactant (SDS), 1 part by mass of emulsifier (alkylphenol polyoxyethylene ether, model OP-10), 15 parts by mass of silicone oil (dimethyl silicone oil), 2 parts by mass of film-forming aid (dipropylene glycol methyl ether), 1.3 parts by mass of thickener (acrylic R1006), and 0.8 parts by mass of silane coupling agent (KH550), and then emulsify at 5000 rpm for 6 min using a high-speed shear emulsifier to obtain a super hydrophobic polytetrafluoroethylene water-based coating.

[0034] S2, then the super-hydrophobic polytetrafluoroethylene water-based coating is double-sidedly coated on the surface of the 25 μm thick polyimide film using a 10# wire rod, and the coating thickness is 6 μm. Finally, the composite film is placed in an oven and treated at 330°C for 10 minutes to obtain a super-hydrophobic composite film 2. The scanning electron microscope photo is shown in Figure 4 shown.

[0035] Example 3

[0036] This embodiment provides a method for preparing a polyimide-polytetrafluoroethylene composite film, comprising the following steps:

[0037] S1, 60 parts by mass of polytetrafluoroethylene aqueous solution (solid content 40%), 8 parts by mass of water, 0.7 parts by mass of surfactant (SDBS), 1.3 parts by mass of emulsifier (fatty alcohol polyoxyethylene ether, model AEO-9), 25 parts by mass of silicone oil (hydroxy silicone oil), 2 parts by mass of film-forming aid (ethylene glycol butyl ether), 2 parts by mass of thickener (polyurethane C77), 1 part by mass of silane coupling agent (KH570) were uniformly mixed, and then emulsified at 5000rpm for 5min using a high-speed shear emulsifier to obtain a super-hydrophobic polytetrafluoroethylene water-based coating;

[0038] S2, then the super-hydrophobic polytetrafluoroethylene water-based coating is double-sidedly coated on the surface of the 25 μm thick polyimide film using a 20# wire rod, and the coating thickness is 8 μm. Finally, the composite film is placed in an oven and treated at 330°C for 10 minutes to obtain a super-hydrophobic composite film 3, as shown in the scanning electron microscope photo Figure 5As shown, the contact angle picture is Figure 8 shown.

[0039] Example 4

[0040] This embodiment provides a method for preparing a polyimide-polytetrafluoroethylene composite film, comprising the following steps:

[0041] S1. 50 parts by mass of polytetrafluoroethylene aqueous solution (solid content 40%), 5 parts by mass of water, 3 parts by mass of surfactant (FMES), 2 parts by mass of emulsifier (sorbitan fatty acid ester, model S-80), 35 parts by mass of silicone oil (amino silicone oil), 2 parts by mass of film-forming aid (dipropylene glycol butyl ether, model Dow DPnB), 1.5 parts by mass of thickener (acrylic acid R1004), and 1.5 parts by mass of silane coupling agent (KH550) were uniformly mixed, and then emulsified at 8000 rpm for 3 min using a high-speed shear emulsifier to obtain a super-hydrophobic polytetrafluoroethylene water-based coating;

[0042] S2, then the super-hydrophobic polytetrafluoroethylene water-based coating is double-sidedly coated on the surface of the 25 μm thick polyimide film using a 20# wire rod, and the coating thickness is 6 μm. Finally, the composite film is placed in an oven and treated at 330°C for 10 minutes to obtain a super-hydrophobic composite film 4, as shown in the scanning electron microscope photo Figure 6 shown.

[0043] Example 5

[0044] This embodiment provides a method for preparing a polyimide-polytetrafluoroethylene composite film, comprising the following steps:

[0045] S1. 70 parts by mass of polytetrafluoroethylene aqueous solution (solid content 25%), 8 parts by mass of water, 1.9 parts by mass of surfactant (SDS), 1 part by mass of emulsifier (OP-10), 15 parts by mass of silicone oil (dimethyl silicone oil), 2 parts by mass of film-forming aid (dipropylene glycol methyl ether), 1.3 parts by mass of thickener (acrylic acid R1006), and 0.8 parts by mass of silane coupling agent (KH550) were uniformly mixed, and then emulsified at 5000 rpm for 6 min using a high-speed shear emulsifier to obtain a super-hydrophobic polytetrafluoroethylene water-based coating;

[0046] S2, then the super-hydrophobic polytetrafluoroethylene water-based coating is double-sidedly coated on the surface of the 25 μm thick polyimide film using a 10# wire rod, and the coating thickness is 6 μm. Finally, the composite film is placed in an oven and treated at 380°C for 5 minutes to obtain a super-hydrophobic composite film 5. The scanning electron microscope photo is shown in Figure 7 shown.

[0047] Comparative Example 1

[0048] The contact angle test was performed on a pure 25 μm thick polyimide film without any treatment. The results are as follows: Figure 1 shown.

[0049] Comparative Example 2

[0050] A 25 μm thick polyimide film was coated on both sides with a pure polytetrafluoroethylene aqueous solution (solid content 40%), coated with a 10# wire rod, with a coating thickness of 4 μm, and treated at 330°C for 10 minutes. The scanning electron microscope photo of the obtained ordinary composite film is shown in the figure below: Figure 1 shown.

[0051] Comparative Examples 3 to 7

[0052] The preparation methods were respectively referred to Examples 1 to 5, except that no silicone oil component was added, and a common composite membrane was obtained.

[0053] The films of Examples 1-5 and Comparative Examples 1-5 were tested, and the test results are shown in Table 1 below. The test method is as follows:

[0054] 1. Wear resistance: Use 180-grit sandpaper to rub the coating 20 times under a force of 100g, and measure its contact angle;

[0055] 2. Water absorption rate: Soak in water for 24 hours, take out the sample, wipe the surface dry and measure the weight gain

[0056] 3. Resistivity in a humid environment: Place the sample in a constant humidity chamber at 90% humidity for 120 minutes, take it out, shake off the water droplets on the surface, wipe off the residual moisture with filter paper, and measure its volume resistivity.

[0057] Table 1 Performance test results of the films of various embodiments and comparative examples

[0058]

[0059] As can be seen from the table, when silicone oil is not added, the surface contact angle of comparative examples 3-7 is not much different. This is due to the comparative example without silicone oil, and the surface hydrophobic effect depends entirely on the film-forming property and sintering degree of polytetrafluoroethylene (PTFE) itself. The thicker the polytetrafluoroethylene (PTFE) film, the less cracking, the higher the sintering degree, and the better the waterproof effect. The auxiliary agent combination in the present invention needs to be used in conjunction with silicone oil. When there is no silicone oil, a large number of cracks are still distributed in the polytetrafluoroethylene (PTFE) coating. Therefore, the water-resistant effect is not significantly improved compared to the comparative example. The technical scheme of the present invention benefits from the decomposition product of silicone oil at high temperature, and the wear resistance of polytetrafluoroethylene (PTFE) coating is significantly improved. After friction, the contact angle of the coating does not significantly decay, which improves the service life of the composite membrane to a certain extent.

[0060] Analysis: Pass Figure 1It can be found that the contact angle of polyimide (PI) itself with water is less than 90°, showing a wet state. This is because water molecules combine with the carboxyl groups (C=O) on the polyimide molecules through hydrogen bonds, resulting in a high water absorption rate of the polyimide film and a significant decrease in the volume resistivity in a humid environment. Although simply coating the polyimide film with polytetrafluoroethylene can increase the contact angle to a certain extent, the coating is prone to cracking due to the poor bonding between polytetrafluoroethylene and polyimide (see Figure 2 ), resulting in the exposure of the underlying polyimide film, so the water absorption rate of the composite film has not been significantly improved. Figure 3-7 It can be seen that the silicone oil component added in the present application forms small droplets of oil-in-water in the aqueous environment of polytetrafluoroethylene. Under the synergistic effect of multiple additives, silicone oil and polytetrafluoroethylene are evenly distributed on the surface of the polyimide film. After high-temperature curing, the surface silicone oil decomposition products and polytetrafluoroethylene form a double continuous network, presenting a rough surface with orderly distribution of nanopores and micropores. Comparative Examples 1-3, the higher the proportion of silicone oil components, the more obvious the rough structure. When the silicone oil mass fraction reaches 25%, relying on the rough structure and the hydrophobicity of polytetrafluoroethylene (PTFE) itself, the water contact angle of the composite membrane can reach up to 142°, and the water absorption rate is significantly reduced. Further increasing the silicone oil content will reduce the uniformity of the coating, and the contact angle will decrease instead. Further increasing the curing temperature in Example 5 has little effect on the surface morphology of the coating. It is worth mentioning that after the super-hydrophobic polytetrafluoroethylene water-based coating with added silicone oil is applied to the surface of the polyimide film, the silicone oil tends to float up, so the multi-level pore structure only exists on the surface of the coating, and the bottom layer is still dense. Therefore, the resistivity remains at 10 in a humid environment. 16 Order of magnitude.

[0061] Those skilled in the art should understand that the above are only some specific embodiments of the present invention, rather than all embodiments. It should be noted that for those of ordinary skill in the art, many modifications and improvements can be made, and all modifications or improvements that do not exceed the scope of protection of the present invention should be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide-polytetrafluoroethylene composite film, characterized in that: The steps include: S1. Weigh the following raw materials in parts by mass: 50-90 parts of a polytetrafluoroethylene aqueous solution with a solid content of 25-60%, 5-30 parts of water, 0.4-3 parts of a surfactant, 0.3-3 parts of an emulsifier, 2.5-40 parts of silicone oil, 1-5 parts of a film-forming aid, 0.5-2 parts of a thickener, and 0.3-2 parts of a silane coupling agent. Mix the above raw materials and emulsify them at a shear rate of 2000-10000 rpm for 2-10 minutes to obtain a super-hydrophobic polytetrafluoroethylene water-based coating; The polytetrafluoroethylene aqueous solution is a dispersion of polytetrafluoroethylene particles in water, and the particle size of the polytetrafluoroethylene particles is 50-200nm; S2. Apply polytetrafluoroethylene water-based coating on one or both sides of a 15-50 μm thick polyimide film to a coating thickness of 2-50 μm, and then place in a high-temperature oven at 280-420° C. for curing for 2-15 minutes to obtain a polyimide-polytetrafluoroethylene composite film.

2. The method for preparing a polyimide-polytetrafluoroethylene composite film according to claim 1, characterized in that: The surfactant is one or a combination of two or more of sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and sodium ethoxylated fatty acid methyl ester sulfonate (FMES).

3. The method for preparing a polyimide-polytetrafluoroethylene composite film according to claim 1, characterized in that: The emulsifier is one or a combination of two or more of dioctyl sodium sulfosuccinate, sorbitan fatty acid ester, alkylphenol polyoxyethylene ether, and fatty alcohol polyoxyethylene ether.

4. The method for preparing a polyimide-polytetrafluoroethylene composite film according to claim 1, characterized in that: The silicone oil is one or a combination of two or more of dimethyl silicone oil, amino silicone oil, trifluoropropyl silicone oil, hydroxy silicone oil, and methylphenyl silicone oil.

5. The method for preparing a polyimide-polytetrafluoroethylene composite film according to claim 1, characterized in that: The film-forming aid is one of dipropylene glycol butyl ether, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, ethylene glycol butyl ether, and dipropylene glycol methyl ether, or a combination of two or more thereof.

6. The method for preparing a polyimide-polytetrafluoroethylene composite film according to claim 1, characterized in that: The thickener is one or a combination of two or more of polyurethane C4, C5, C8, C77 or acrylic R1004, R1005, R1006; or, the silane coupling agent is one or a combination of two or more of KH550, KH560, KH570, KH792.

7. The method for preparing a polyimide-polytetrafluoroethylene composite film according to claim 1, characterized in that: In step S1, the amount of silicone oil in the super-hydrophobic polytetrafluoroethylene water-based coating accounts for 25% of the total mass.

8. The method for preparing a polyimide-polytetrafluoroethylene composite film according to claim 1, characterized in that: The coating method used is wire rod coating, slot coating or micro-concave coating.

9. A polyimide-polytetrafluoroethylene composite membrane obtained by the preparation method according to any one of claims 1 to 8.

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