Multifunctional layer structure composite film as well as preparation method and application thereof

Abstract

The invention discloses a multifunctional layer structure composite film and a preparation method and application thereof, and belongs to the technical field of polymer dielectric materials and preparation thereof. According to the invention, the problems of poor insulating property and degraded mechanical property of the existing polyethylene glycol terephthalate film in a high-temperature environment are solved. The preparation method comprises the following steps: firstly, adsorbing a certain amount of photoinitiator on the surface of the film by utilizing the swelling adsorbability of the PET film, grafting a functional monomer on the surface of the PET film by virtue of an ultraviolet radiation method to form a compact grafted functional layer, and finally compounding the PET film with the grafted functional layer with a PC film with a relatively high glass transition temperature, so as to obtain the high-glass-transition-temperature PC / PET composite film. When the grafting functional layer is used as the outermost layer of the composite film and the grafting functional layer exists between the PET film and the PC film, the obtained composite film with the multifunctional layer structure obtains excellent breakdown field strength and lower dielectric loss under the conditions of high temperature, high frequency and high field strength.

Description

Technical Field

[0001] The present invention relates to a multifunctional layer structure composite film and its preparation method and application, belonging to the technical field of polymer dielectric materials and their preparation. Background Art

[0002] The intrinsic properties of polymer dielectric materials will directly affect the efficiency, reliability, and service life of electrical equipment such as motors, reactors, and transformers. At present, due to the high dielectric strength, low dielectric loss, and wide temperature range adaptability of polyethylene terephthalate (PET) film (the long-term allowable working temperature is -40 to 120 °C), it has become an ideal material for insulation requirements such as insulation between multi-layer windings of electrical equipment, magnetic core coating, or skeleton support. At the same time, polyethylene terephthalate film has the advantages of light weight, easy processing, and low price, which further promotes its wide application in electrical equipment. However, with the development of electrical equipment towards high power density and high frequency, higher requirements are put forward for the heat resistance and insulation performance of insulating materials. The insulation performance of traditional polyethylene terephthalate film deteriorates rapidly under high temperature, high frequency, and high electric field, which will lead to a significant decrease in the stability, reliability, and service life of the insulation system of electrical equipment.

[0003] Therefore, it is expected to further promote the application of polyethylene terephthalate film in electrical equipment under complex working conditions by optimizing the electro-thermal-mechanical properties of existing commercial polyethylene terephthalate film through a simple and effective modification method. Summary of the Invention

[0004] The present invention aims to solve the problems of poor insulation performance and deteriorated mechanical properties of existing polyethylene terephthalate film in high-temperature environments, and provides a multifunctional layer structure composite film and its preparation method and application.

[0005] The technical solution of the present invention:

[0006] One of the objectives of the present invention is to provide a preparation method of a multifunctional layer structure composite film, which includes the following steps:

[0007] (1) Place the PET film in a photoinitiator solution, take it out after standing, and wash the film;

[0008] (2) Immerse the PET film treated in (1) in a functional monomer solution, and perform ultraviolet light irradiation treatment to obtain a PET film with a grafted functional layer on the surface;

[0009] (3) Through hot pressing and annealing treatment, laminate multiple PET films with grafted functional layers on the surface and multiple polycarbonate PC films, and the grafted functional layer is located between the PET film and the PC film to obtain a multifunctional layer structure composite film.

[0010] Further limitation: the photoinitiator solution in (1) is formed by mixing benzophenone and N,N-dimethylformamide at a mass-volume ratio of 1 g: 18 mL.

[0011] Further limitation: in (1), ethanol and deionized water are used to alternately clean the PET film multiple times.

[0012] Further limitation: in (1), the PET film is placed on the surface of the photoinitiator solution or in the photoinitiator solution.

[0013] Further limitation: in (1), the standing time is 2 h.

[0014] Further limitation: the functional monomer solution in (2) is formed by mixing the functional monomer and N,N-dimethylformamide at a mass-volume ratio of 1 g: 9 mL, and the functional monomer is acrylic acid or acrylamide.

[0015] Further limitation: the ultraviolet light irradiation treatment conditions in (2) are as follows: the light source power is 1000 W, the wavelength is 265 nm, the irradiation time is 30 - 90 s, and the distance from the ultraviolet light source to the film surface is 10 - 15 cm.

[0016] Further limitation: in (3), the hot pressing temperature is 210 °C, the pressure is 15 MPa, the preheating time is 10 min, and the hot pressing time is 20 min.

[0017] Further limitation: in (3), the annealing temperature is 10 °C and the time is 10 min.

[0018] Further limitation: the preparation method of the PC film in (3) is as follows: PC particles are added to the organic solvent tetrahydrofuran, and a magnetic stirrer is used to stir until the polycarbonate particles are completely dissolved to obtain a polycarbonate solution. Then the polycarbonate solution is coated on a substrate and finally dried in an oven and a vacuum oven successively to obtain a polycarbonate film.

[0019] Even further limitation: the mass-volume ratio of PC particles to tetrahydrofuran is 17 g: 100 mL.

[0020] Even further limitation: the stirring speed of the magnetic stirrer is 500 r / min, the time is 8 h, and the temperature is 20 °C.

[0021] Even further limitation: the oven temperature is 60 °C and the treatment time is 8 h.

[0022] Even further limitation: the vacuum oven temperature is 60 °C and the treatment time is 8 h.

[0023] The second object of the present invention is to provide a multi-functional layer structure composite film prepared by the above method. The composite film is composed of a PET film, a grafted functional layer, and a PC film.

[0024] Further limitation: The graft functional layer is located on one or both sides of the PET film.

[0025] Further limitation: Two PET films with graft functional layers on their surfaces are respectively located on both sides of the PC film, or two PC films are respectively located on both sides of the PET film with a graft functional layer.

[0026] Further limitation: The composite film has a thickness of 14 - 16 μm.

[0027] The third object of the present invention is to provide an application of the above-mentioned composite film with a multi-functional layer structure. The composite film is used as a polymer dielectric material for electrical equipment.

[0028] Advantages of the present invention:

[0029] Firstly, the present invention utilizes the swelling and adsorption property of the PET film to adsorb a certain amount of photoinitiator on the surface of the film, and through ultraviolet irradiation, grafts functional monomers onto the surface of the PET film to form a dense graft functional layer. Finally, the PET film with the graft functional layer is compounded with a PC film having a relatively high glass transition temperature, and by adjusting the compounding method of the PET film, the graft functional layer, and the PC film, the graft functional layer is used as the outermost layer of the composite film, and when there is a graft functional layer between the PET film and the PC film, a multi-functional layer structure composite film with graft functional layers at different positions and a high-temperature layer is obtained. Compared with the prior art, the multi-functional layer structure composite film prepared by the present invention has the following advantages:

[0030] (1) In the multi-functional layer structure composite film prepared by the present invention, the graft functional layer is the outermost layer. Under high-temperature and high-electric-field conditions, due to its relatively high LUMO energy level and bandgap width, the graft functional layer can significantly increase the potential barrier at the electrode and reduce the injection of carriers, thereby improving the insulation performance of the composite dielectric. At the same time, electron-withdrawing groups such as carboxyl (-COOH) and amide (-CONH2) in the functional monomers can significantly reduce the electron density of the PET main chain, making the electrons of the PET main chain highly localized, thereby hindering the migration of carriers on the surface PET main chain and further weakening the entry of carriers into the material to participate in conduction; the potential barrier of the graft functional layer located between the PET film and the PC film and the electron-withdrawing ability within the graft chain play a hindering role in carrier migration, which can cause the breakdown path to develop along the direction parallel to the trap layer, weaken the free acceleration distance of carriers, and consume the energy of the breakdown path, effectively inhibiting the overall transport behavior of carriers. As a result, under high temperature, high frequency, and high electric field strength, the multi-functional layer structure composite film obtains excellent breakdown field strength and low dielectric loss. Specifically, the leakage current density of the multi-functional layer structure composite film at a relatively high temperature (120 °C) is 3.29×10 -9 A / cm2 , which is two orders of magnitude lower than that of the PET film, and the DC breakdown field strength reaches 761.9 kV / mm, which is 30% higher than that of the PET film; at high frequencies (10 6 Hz), the relative dielectric constant is stable at about 3.2, and the dielectric loss is reduced from 0.056 to 0.011.

[0031] (2) The present invention selects a PC film with a relatively high glass transition temperature to be compounded with a PET film, which provides support for the overall structure and mechanical properties of the composite film, and avoids the deterioration of insulation performance and the reduction of service stability caused by the thermal deformation of the composite film at high temperatures. In addition, the graft functional layer between the PC film and the PET film has polar groups such as carboxyl (-COOH) and amide (-CONH2), which can form hydrogen bonds with the carbonyl group (C=O) in the PC to improve the interfacial bonding force and enhance the mechanical properties of the composite film.

[0032] (3) The preparation process of the multi-functional layer structure composite film provided by the present invention is simple and low-cost, suitable for industrial production, and provides a reliable solution for the development of high-performance insulating films for electrical equipment. Description of the Drawings

[0033] Figure 1 SEM cross-sectional view of the *TCT* composite film prepared in Example 1;

[0034] Figure 2 SEM cross-sectional view of the *TCT* composite film prepared in Example 2;

[0035] Figure 3 SEM cross-sectional view of the TCT composite film prepared in Comparative Example 1;

[0036] Figure 4 Morphology diagrams of the films prepared in Examples 1 and 2 and Comparative Examples 1 and 4 after aging at 150 °C for 30 min;

[0037] Figure 5 Infrared spectra of the films prepared in Examples 1 and 2 and Comparative Examples 1-4;

[0038] Figure 6 XRD diagrams of the films prepared in Examples 1 and 2 and Comparative Examples 1-4;

[0039] Figure 7 Dielectric constant diagrams of the films prepared in Examples 1 and 2 and Comparative Examples 1-4;

[0040] Figure 8 Dielectric loss diagrams of the films prepared in Examples 1 and 2 and Comparative Examples 1-4;

[0041] Figure 9 Leakage current density diagrams of the thin films prepared in Example 1 and 2 and Comparative Examples 1-4 at 120 °C.

[0042] Figure 10 DC breakdown Weibull distribution diagrams of the thin films prepared in Example 1 and 2 and Comparative Examples 1-4 at 120 °C. Detailed implementation manners

[0043] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following describes the detailed implementation manners of the present invention in conjunction with the embodiments of the specification.

[0044] In the following description, many specific details are set forth to facilitate a thorough understanding of the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar generalizations without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0045] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that can be included in at least one implementation manner of the present invention. The phrase "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment that is mutually exclusive with other embodiments.

[0046] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified. The materials, reagents, methods, and instruments used, unless otherwise specified, are all conventional materials, reagents, methods, and instruments in this field, and those skilled in the art can obtain them through commercial channels.

[0047] Example 1

[0048] Step 1: Add 2 g of benzophenone (BP) to 36 ml of N,N-dimethylformamide to prepare a photoinitiator solution. Immerse the cleaned PET film with a thickness of 5 μm in the photoinitiator solution for 2 h. After taking it out, wash it successively with ethanol, deionized water, and ethanol to obtain a swollen PET film. Add 3 ml of acrylic acid to 27 ml of the organic solvent N,N-dimethylformamide to obtain an acrylic acid solution. Immerse the swollen PET film in the acrylic acid solution, and irradiate both sides of the film with a UV lamp with a power of 1000 W and a wavelength of 265 nm for 60 s respectively. After taking it out, wash it quickly with ethanol, deionized water, and ethanol successively to obtain a PET film with acrylic acid functional layers grafted on both sides of the surface, denoted as *PET*;

[0049] Step 2, in a 50 ml beaker, dissolve 1.7 g of PC particles in 10 ml of tetrahydrofuran, stir at 20 °C using a magnetic stirrer, set the rotation speed to 500 r / min, and stir for 8 h to obtain a PC solution. After degassing, coat the PC solution on a glass substrate by a doctor blade process, dry it in an oven at 60 °C for 8 h, and then transfer it to a vacuum oven to continue drying for 8 h to obtain a PC film with a thickness of about 5 μm;

[0050] Step 3, place two *PET* films on both sides of the PC film, place the three-layer film between two steel plates and put it into a hot press, preheat at 210 °C for 10 min, and then hot press at 15 MPa and 210 °C for 20 min. After hot pressing, perform an annealing operation for 10 min to obtain a multi-functional layer structure composite film, denoted as *T*C*T*, with a thickness of 15 μm.

[0051] Example 2

[0052] Step 1, add 2 g of benzophenone (BP) to 36 ml of N,N-dimethylformamide to prepare a photoinitiator solution. Place a cleaned PET film with a thickness of 5 μm on the surface of the photoinitiator solution and soak it for 2 h. After taking it out, wash it successively with ethanol, deionized water, and ethanol to obtain a swollen PET film. Add 3 ml of acrylic acid to 27 ml of the organic solvent N,N-dimethylformamide to obtain an acrylic acid solution. Immerse the swollen PET film into the acrylic acid solution, irradiate the side of the film adsorbed with the photoinitiator with a UV lamp with a power of 1000 W and a wavelength of 265 nm for 60 s, and then quickly wash it successively with ethanol, deionized water, and ethanol to obtain a PET film with a grafted acrylic acid functional layer on one side of the surface, denoted as *PET;

[0053] Step 2, in a 50 ml beaker, dissolve 1.7 g of PC particles in 10 ml of tetrahydrofuran, stir at 20 °C using a magnetic stirrer, set the rotation speed of the magnetic stirrer rotor to 500 r / min, and stir for 8 h to obtain a PC solution. After degassing, coat the PC solution on a glass substrate by a doctor blade process, dry it in an oven at 60 °C for 8 h and then transfer it to a vacuum oven to continue drying for 8 h to obtain a PC film with a thickness of about 5 μm;

[0054] Step 3, place two *PET* films on both sides of the PC film (the grafted functional layer side is located outside the composite film), place the three-layer film between two steel plates and put it into a hot press, preheat at 210 °C for 10 min, and then hot press at 15 MPa and 210 °C for 20 min. After hot pressing, perform an annealing operation for 10 min to obtain a multi-functional layer structure composite film, denoted as *TCT*, with a thickness of 15 μm.

[0055] Comparative Example 1

[0056] Step 1, in a 50 ml beaker, dissolve 1.7 g of PC particles in 10 ml of tetrahydrofuran, stir at 20 °C using a magnetic stirrer, set the rotational speed of the magnetic stirrer rotor to 500 r / min, and after stirring for 8 h, obtain a PC solution. After defoaming treatment, coat the PC solution on a glass substrate by a doctor blade process, dry it in an oven at 60 °C for 8 h, and then transfer it to a vacuum oven for continued drying for 8 h to obtain a PC film with a thickness of about 5 μm;

[0057] Step 2, place two PET films on the outer layer and the PC film on the middle layer, place the three-layer film between two steel plates and put it into a hot press, preheat at 210 °C for 10 min, and then hot press at 15 MPa and 210 °C for 20 min. After hot pressing, perform an annealing operation for 10 min to obtain a three-layer composite film, denoted as TCT, with a thickness of 15 μm.

[0058] Comparative Example 2

[0059] Add 2 g of benzophenone (BP) to 36 ml of N,N-dimethylformamide to prepare a photoinitiator solution. Immerse a cleaned PET film with a thickness of 15 μm in the photoinitiator solution for 2 h, take it out and wash it clean with ethanol, deionized water, and ethanol in sequence to obtain a swollen PET film. Add 3 ml of acrylic acid to 27 ml of the organic solvent N,N-dimethylformamide to obtain an acrylic acid solution. Immerse the swollen PET film in the acrylic acid solution, irradiate both sides of the film with a UV lamp with a power of 1000 W and a wavelength of 265 nm for 60 s respectively, take it out and quickly wash it with ethanol, deionized water, and ethanol in sequence to obtain a PET film with acrylic acid functional layers grafted on both surfaces, denoted as *PET*, with a thickness of 15 μm.

[0060] Comparative Example 3

[0061] In a 50 ml beaker, dissolve 1.7 g of PC particles in 10 ml of tetrahydrofuran, stir at 20 °C using a magnetic stirrer, set the rotational speed of the magnetic stirrer rotor to 500 r / min, and after stirring for 8 h, obtain a PC solution. After defoaming treatment, coat the PC solution on a glass substrate by a doctor blade process, dry it in an oven at 60 °C for 8 h, and then transfer it to a vacuum oven for continued drying for 8 h to obtain a PC film with a thickness of 15 μm.

[0062] Comparative Example 4

[0063] PET film with a thickness of 15 μm.

[0064] Effect Example

[0065] Perform structural characterization and performance testing on the composite films obtained in Examples 1 and 2 and Comparative Examples 1-4. The specific test results and analysis are as follows:

[0066] (1) Figure 1-3 SEM cross-sectional images of the three composite films provided in Example 1 and 2 and Comparative Example 1. It can be found that the thickness of the three composite films is about 15 μm and the thickness is relatively uniform. Among them, the cross-section of the *T*C*T* composite film is relatively flat and basically free of defects such as holes. This is because the grafted functional layer has a carboxyl (-COOH) polar group, which can form hydrogen bonds with the carbonyl group (C=O) in PC, thereby enhancing the interfacial bonding force and the compactness of the composite film. However, there are multiple hole defects at the cross-section of the *TCT* and TCT composite films (see the red circles in the figure). This is because the interfacial bonding force between PET and PC is relatively weak, resulting in poor compactness of the composite film.

[0067] (2) Figure 4 Morphology diagrams of the films prepared in Example 1 and 2 and Comparative Examples 1 and 4 after aging at 150 °C for 30 min. It can be found that the PC film with a high glass transition temperature can significantly enhance the high-temperature resistance and mechanical properties of composite films such as *T*C*T*, and reduce their morphological changes at high temperatures.

[0068] (3) Figure 5 Infrared spectra comparison diagrams of the six composite films provided in Example 1, 2 and Comparative Examples 1-4. From Figure 5 it can be seen that due to the stretching vibration of hydroxyl groups in the PET film grafted with acrylic acid, a stronger absorption peak appears at 1430 cm -1 ; at the same time, due to the higher degree of hydrogen bonding of carboxyl groups in the grafted film, at 3500 - 3700 cm -1 , the infrared characteristic peak of the PET film grafted with acrylic acid shows a wider characteristic, indicating that the PET film grafted with acrylic acid has been successfully prepared. No new infrared characteristic peaks appear in the TCT film after hot pressing and annealing compared with the PET film and the PC film. Similarly processed *TCT* films and *T*C*T* films also do not show new infrared characteristic peaks compared with *PET* films and PC films, indicating that hot pressing and annealing do not change the chemical structure of the composite film.

[0069] (4) Figure 6 XRD patterns of the six composite films provided in Example 1, 2 and Comparative Examples 1, 2. From Figure 6 it can be seen that ultraviolet irradiation grafting reduces the crystallinity of the PET film. This is because ultraviolet irradiation breaks the main chain of PET, resulting in a decrease in molecular weight and the destruction of the long-range order of molecular chains.

[0070] (5) Figure 7 Diagrams showing the variation of the relative dielectric constant with frequency of the six composite films provided in Example 1, 2 and Comparative Examples 1, 2. From Figure 7It can be seen that the relative dielectric constant of the *PET* film is higher than that of the original PET film, which is due to the influence of polar groups in acrylic acid. The relative dielectric constant of the *TCT* composite film is lower than that of the *PET* film, which is because the relative dielectric constant of PC in the composite film is relatively low. However, compared with the original PET film, the change in the relative dielectric constant of the composite film is not significant, which is caused by the combined effect of interfacial polarization and polar groups. In addition, the relative dielectric constant of the *TCT* composite film changes little with frequency, improving its stability in the application of a wide frequency range.

[0071] (6) Figure 8 Graphs showing the variation of dielectric loss with frequency for the six composite films provided in Examples 1 and 2 and Comparative Examples 1 and 2. From Figure 8 It can be seen that the dielectric loss of the *PET* film is lower than that of the original PET film, and the reduction of dielectric loss at high frequencies is more significant. At the same time, the dielectric loss of the *TCT* composite film is further reduced compared with the *PET* film, effectively reducing the loss and heat generation of the insulation of electrical equipment under high-frequency conditions.

[0072] (7) Figure 9 and 10 Graphs of leakage current density and Weibull distribution of DC breakdown field strength at 120 °C for the six composite films provided in Examples 1 and 2 and Comparative Examples 1 and 2. From Figure 9 and 10It can be seen that the introduction of the graft functional layer and the construction of the multi-functional layer structure can significantly reduce the leakage current density of the composite film and increase the breakdown field strength. At 120 °C, the leakage current density of the *T*C*T* film prepared in Example 1 was reduced by two orders of magnitude compared with the PET film, and the DC breakdown field strength reached 761.9 kV / mm, which was 30% higher than that of the PET film. This is because the outermost graft functional layer has a high LUMO energy level and band gap, which can significantly increase the potential barrier at the electrode and reduce the injection of carriers, thereby improving the insulation performance of the composite dielectric. At the same time, electron-withdrawing groups such as carboxyl (-COOH) and amide (-CONH2) in the functional monomer can significantly reduce the electron density of the PET main chain, localize the electrons of the PET main chain, and thus hinder the migration of carriers on the surface PET main chain, further weakening the entry of carriers into the material for conduction; the barrier of the graft functional layer between the PET film and the PC film and the electron-withdrawing ability within the graft chain play a hindering role in carrier migration, enabling the breakdown path to develop along the direction parallel to the trap layer, weakening the free acceleration distance of carriers and consuming the energy of the breakdown path, effectively suppressing the overall transport behavior of carriers. In addition, the PC layer with a high glass transition temperature provides support for the overall structure and high-temperature performance of the composite film, improving the structural damage, electrode detachment, and electric field distortion caused by the thermal deformation of the composite film at high temperature, effectively avoiding the deterioration of its insulation performance and the reduction of stability.

[0073] As described above, the above are only the preferred specific embodiments of the present invention. These specific embodiments are different implementation manners based on the overall concept of the present invention, and the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for preparing a multi-functional layer structure composite film, characterized in that, Including: (1) Place the PET film in the photoinitiator solution, take it out after standing still, and wash the film; (2) Immerse the PET film treated in (1) in the functional monomer solution, and perform ultraviolet irradiation treatment to obtain a PET film with a grafted functional layer on the surface; (3) Through hot pressing and annealing treatment, laminate multiple PET films with grafted functional layers on the surface and multiple PC films, and the grafted functional layer is located between the PET film and the PC film to obtain a multi-functional layer structure composite film.

2. The preparation method according to claim 1, wherein, The photoinitiator solution in (1) is composed of benzophenone and N,N-dimethylformamide mixed according to a mass-volume ratio of 1 g:18 mL.

3. The preparation method according to claim 1, characterized in that, The standing time in (1) is 2 h.

4. The preparation method according to claim 1, characterized in that, The functional monomer solution in (2) is composed of a functional monomer and N,N-dimethylformamide mixed according to a mass-volume ratio of 1 g:9 mL, and the functional monomer is acrylic acid or acrylamide.

5. The preparation method according to claim 1, characterized in that, The conditions for ultraviolet irradiation treatment in (2) are: light source power is 1000 W, wavelength is 265 nm, irradiation time is 30 - 90 s, and the distance from the ultraviolet light source to the film surface is 10 - 15 cm.

6. The preparation method according to claim 1, characterized in that, The hot pressing temperature in (3) is 210 °C, the pressure is 15 MPa, the preheating time is 10 min, and the hot pressing time is 20 min.

7. The preparation method according to claim 1, characterized in that, The annealing temperature in (3) is 10 °C and the time is 10 min.

8. A multi-functional layer structure composite film prepared by the method according to any one of claims 1-7, characterized in that, The composite film is a layer structure composed of a PET film, a grafted functional layer, and a PC film.

9. The multi-functional layer structure composite film according to claim 8, wherein, The thickness is 14 - 16 μm.

10. Use of the multi-functional layer structure composite film according to claim 8 or 9, characterized in that, It is used as a polymer dielectric material for electrical equipment.

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