A biaxially stretched composite material and its use

Abstract

The application discloses a kind of bidirectional stretching composite material and its application, the composite material is with composite grafting polypropylene as master batch, with aluminium oxide nanosheet as inorganic phase. Through the action between composite grafting polypropylene and aluminium oxide nanosheet, the problem that inorganic filler of prior art causes the decrease of polypropylene processability is improved, improves bidirectional stretching processing performance, compared with polypropylene monomer material, with higher energy storage density and breakdown field strength, and can keep higher charge-discharge efficiency.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite materials technology, and relates to a biaxially oriented composite material and its applications. Background Technology

[0002] Numerous studies have shown that introducing inorganic nanofillers into a polypropylene matrix can significantly improve the dielectric constant and breakdown field strength of the composite material, thereby increasing energy storage density. However, for the modification of biaxially oriented polypropylene, the introduction of inorganic particles can lead to problems such as severe filler agglomeration, decreased electrical strength, increased leakage current, and increased dielectric loss. Furthermore, the poor compatibility between rigid inorganic particles and polymer materials results in interfacial discontinuities and the inability to stretch and form films, thus limiting the further application of nanocomposite modification methods in polypropylene matrices.

[0003] Therefore, it is urgent to solve the problem of synergistic modulation of film-forming properties and energy storage density in polypropylene matrix composites. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a polypropylene-based biaxially oriented composite material with high breakdown field strength and excellent film-forming properties, as well as its applications.

[0005] Specifically, the object of the present invention is to provide the following aspects:

[0006] On the one hand, a biaxially oriented composite material is provided, wherein the composite material uses grafted polypropylene as the masterbatch and alumina nanosheets as the inorganic phase.

[0007] The composite grafted polypropylene uses polypropylene as the matrix.

[0008] The composite grafted polypropylene uses acrylic acid grafted polypropylene as the organic composite phase.

[0009] The composite material contains a grafted carbonyl group.

[0010] In a second aspect, a method for preparing the composite material described in the first aspect is provided, the method comprising:

[0011] Step 1: Prepare alumina nanosheets and acrylic acid-grafted polypropylene, respectively;

[0012] Step 2: React alumina nanosheets, acrylic acid-grafted polypropylene, and polypropylene granules to obtain the composite material.

[0013] In step 2, the reaction includes:

[0014] Step 2-1: The acrylic-grafted polypropylene and polypropylene granules are melt-blended once to obtain composite grafted polypropylene.

[0015] Step 2-2: The composite grafted polypropylene and alumina nanosheets are melt-blended a second time to obtain the composite material.

[0016] Thirdly, a biaxially oriented composite film is provided, wherein the composite film employs the biaxially oriented composite material described in the first aspect.

[0017] The composite film uses a biaxially oriented composite material as a filler.

[0018] The dielectric constant of the composite film is between 2.35 and 2.41.

[0019] Fourthly, a method for preparing the composite film described in the third aspect is provided, the method comprising: using a biaxially oriented composite material as a filler and a polyimide film as a mold, and obtaining the composite film by hot pressing and biaxial stretching.

[0020] The beneficial effects of this invention include:

[0021] (1) The biaxially oriented composite material provided by the present invention uses polypropylene as the matrix, acrylic acid-grafted polypropylene as the organic composite phase, and alumina nanosheets as the inorganic phase. The composite of the three takes into account the biaxially oriented characteristics. The introduction of acrylic acid-grafted polypropylene and the oriented alumina nanosheet structure significantly improve the breakdown field strength, resulting in a higher energy storage density than polypropylene monomer materials, while maintaining a lower dielectric loss.

[0022] (2) The biaxially oriented composite material provided by the present invention uses a stepped sintering method for alumina nanosheets to prevent the intermediate products from breaking during rapid heating, thereby obtaining a complete nanosheet structure.

[0023] (3) The preparation method of the biaxially stretched composite material provided by the present invention is simple and easy to implement, and is easy to promote and apply in industry. It is of great significance in capacitors.

[0024] (4) The biaxially oriented composite film provided by the present invention has uniform thickness and uniform film formation; the dielectric constant is between 2.35 and 2.41, the dielectric loss is maintained on the order of 1E-3, the breakdown field strength reaches 860~870 kV / mm, and the energy storage density reaches 7.8~8.0 J / cm². 3 . Attached Figure Description

[0025] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0026] In the attached diagram:

[0027] Figure 1(a) shows the TEM characterization of the γ-alumina nanosheets prepared in Example 1;

[0028] Figure 1(b) shows the TEM characterization of the γ-alumina nanosheets prepared in Comparative Example 1;

[0029] Figure 2 The dielectric constant and material loss spectrum of the biaxially stretched composite films prepared in Example 1, Comparative Example 2 and Comparative Example 3 are shown in the comparison diagram.

[0030] Figure 3 The diagram shows a comparison of the DC breakdown Weibull distribution of the biaxially stretched composite films prepared in Example 1, Comparative Example 2, and Comparative Example 3.

[0031] Figure 4 The cross-section of the biaxially stretched composite film prepared in Example 1 is shown by TEM characterization.

[0032] Figure 5 The infrared spectrum of the biaxially oriented composite material prepared in Example 1 is shown.

[0033] Figure 6 The thickness distribution of the biaxially stretched composite films prepared in Example 1, Comparative Example 2, and Comparative Example 3 is shown in the comparison diagram.

[0034] Figure 7 The frequency domain spectrum comparison diagrams of the dielectric constant and dielectric loss tangent of the biaxially stretched composite films prepared in Examples 1-4 are shown.

[0035] Figure 8 The diagram shows a comparison of the breakdown field strength distribution of the biaxially stretched composite films prepared in Examples 1-4.

[0036] Figure 9(a) shows the breakdown field strength diagram of the biaxially stretched composite film prepared in Example 1;

[0037] Figure 9(b) shows the breakdown field strength diagram of the biaxially stretched composite film prepared in Comparative Example 1. Detailed Implementation

[0038] The following will refer to the appendix. Figures 1(a) to 9(b)Specific embodiments of the invention will be described in more detail below. While specific embodiments of the invention are shown in the accompanying drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0039] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0040] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship in the working state of this invention, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0041] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0042] On the one hand, according to the biaxially oriented composite material provided by the present invention, the composite material uses composite grafted polypropylene as the masterbatch and alumina nanosheets as the inorganic phase.

[0043] Furthermore, the alumina nanosheets are γ-alumina nanosheets.

[0044] Among them, γ-alumina is a transitional state of alumina with an amorphous to microcrystalline structure, and usually has a higher specific surface area.

[0045] Furthermore, the composite grafted polypropylene uses polypropylene as the matrix and acrylic acid grafted polypropylene as the organic composite phase.

[0046] According to a preferred embodiment, the composite material comprises the following components in parts by weight:

[0047] 90-110 parts of polypropylene;

[0048] 90-100 parts of acrylic acid grafted polypropylene;

[0049] 0.5 to 2 parts of alumina nanosheets.

[0050] In this invention, the introduction of acrylic acid-grafted polypropylene and the oriented alumina nanosheet structure significantly improve the breakdown field strength.

[0051] In a further preferred embodiment, the composite material comprises the following components in parts by weight:

[0052] 100-105 parts of polypropylene;

[0053] 95-100 parts of acrylic acid grafted polypropylene;

[0054] 1 to 1.5 parts of alumina nanosheets.

[0055] In a further preferred embodiment, the composite material comprises the following components in parts by weight:

[0056] 100 parts of polypropylene;

[0057] 100 parts of acrylic acid grafted polypropylene;

[0058] One part of alumina nanosheets.

[0059] According to the present invention, the composite material is at 1712 cm⁻¹ -1 The presence of a grafted carbonyl group indicates that acrylic acid grafted polypropylene has been successfully introduced into the composite material.

[0060] In this invention, the alumina nanosheets are prepared by a stepped sintering method to prevent the intermediate products from breaking during rapid heating, thereby obtaining a complete nanosheet structure. Preferably, the alumina nanosheets are prepared by the following steps:

[0061] Step 1-1: Pretreatment of soluble aluminum salts yields a weakly acidic solution;

[0062] Steps 1-2 involve heat-treating the weakly acidic solution, followed by washing and drying to obtain the product to be treated.

[0063] Steps 1-3 involve step sintering the product to be processed to obtain the alumina nanosheets.

[0064] In step 1-1, the pretreatment includes: dissolving the soluble aluminum salt in water, and then adjusting the pH with a weak alkali.

[0065] In step 1-1, the soluble aluminum salt is selected from aluminum carbonate, aluminum acetate, and aluminum sulfate octadeca water powder, preferably aluminum sulfate octadeca water powder.

[0066] The soluble aluminum salt can easily form a solution in water, which is beneficial for subsequent steps. Aluminum sulfate octadeca water powder typically has high purity and contains few impurities, which helps in the preparation of high-purity alumina nanosheets. More importantly, the water of crystallization in aluminum sulfate octadeca water powder can be released during heat treatment, which helps in the formation of porous alumina nanosheets.

[0067] In step 1-1, in order to uniformly dissolve the soluble aluminum salt in water, the mixture is stirred at a rate of 1000~2000 r / min for 1~5 h; preferably, it is stirred at a rate of 1400~1600 r / min for 2~4 h; for example, it is stirred at a rate of 1500 r / min for 3 h.

[0068] In step 1-1, since soluble aluminum salts are usually highly acidic, a weak base is typically used to adjust the pH to 4.8-5.1, such as 5.0. At this point, the soluble aluminum salt forms an aluminum hydroxide precursor suspended in the aqueous liquid, resulting in a weakly acidic solution that is a suspension. If the pH is greater than 7, rod-shaped materials will be obtained.

[0069] Furthermore, the weak base is beneficial for adjusting the pH and, in order to avoid introducing impurity ions (i.e., removing them in subsequent steps), the weak base is preferably ammonia.

[0070] In step 1-2, the aluminum hydroxide precursor formed in step 1-1 is converted into aluminum oxide through heat treatment.

[0071] In steps 1-2, the heat treatment temperature is 180~200℃ and the time is 18~36h; preferably, the heat treatment temperature is 185~195℃ and the time is 24~38h; for example, the heat treatment temperature is 190℃ and the time is 24h.

[0072] Under these conditions, alumina nanosheets with a sheet-like structure can be obtained.

[0073] In steps 1-2, it is preferable to wash the heat-treated material with water until it is neutral to remove impurities, unreacted weak bases such as ammonia, and water-soluble impurities.

[0074] In steps 1-2, freeze drying is preferred to remove moisture, and the product obtained by freeze drying is purer.

[0075] Furthermore, there are no strict requirements regarding the freeze-drying time; the goal is simply to remove the water. Freeze-drying typically takes 12-24 hours, with 12 hours usually sufficient.

[0076] In steps 1-2, the heat treatment is preferably carried out in a high-pressure reactor.

[0077] In steps 1-3, the stepped sintering includes: heating from room temperature to 140-160°C at a heating rate of 5-20°C / min and holding at that temperature for 1-4 h; then heating to 500-700°C at a heating rate of 5-20°C / min and holding at that temperature for 2-3.5 h; and then naturally cooling to room temperature.

[0078] The inventors have discovered that stepped sintering can prevent product breakage caused by excessively rapid heating.

[0079] In a further preferred embodiment, the stepped sintering includes: heating from room temperature to 145-155°C at a heating rate of 8-12°C / min and holding at that temperature for 2-3 hours; then heating to 550-650°C at a heating rate of 8-12°C / min and holding at that temperature for 2.5-3 hours; and then naturally cooling to room temperature.

[0080] In a further preferred embodiment, the stepped sintering includes: heating from room temperature to 150°C at a heating rate of 10°C / min and holding at that temperature for 3 hours; then heating to 600°C at a heating rate of 10°C / min and holding at that temperature for 3 hours; and then naturally cooling to room temperature.

[0081] In steps 1-3, the stepped sintering is preferably carried out in an inert gas atmosphere such as nitrogen.

[0082] In this invention, the acrylic acid-grafted polypropylene is obtained by modifying polypropylene with acrylic acid. Preferably, the acrylic acid-grafted polypropylene is prepared through the following steps:

[0083] Step 1-1': Dissolve the polypropylene granules in an organic solvent to obtain the first solution;

[0084] Step 1-2': Add acrylic acid to the first solvent to obtain the second solution;

[0085] Steps 1-3': Initiator solution is added to the second solvent, and acrylic acid-grafted polypropylene is obtained by sequential filtration and extraction.

[0086] In step 1-1', the organic solvent is preferably selected from one or more of decahydronaphthalene, toluene, and xylene, with xylene being the most preferred.

[0087] The organic solvent is used to dissolve the polypropylene granules to facilitate the grafting reaction. Xylene can also be used as a solvent for the initiator, helping to ensure the initiator is uniformly dispersed in the reaction system.

[0088] In step 1-1', the mass ratio (g:mL) of polypropylene granules to organic solvent is 3:(90~150), preferably 3:(95~110), for example 3:100.

[0089] Polypropylene granules have limited solubility in organic solvents, so a large amount of organic solvent is needed to ensure that the polypropylene granules can be fully dissolved. Excessive organic solvent may reduce the reaction rate of the reaction system.

[0090] In step 1-1', the polypropylene granules are dissolved in an organic solvent and fully dissolved at 90~110℃ to 100℃.

[0091] Polypropylene, in particular, is a non-polar polymer, and its solubility in organic solvents increases with increasing temperature. A temperature range of 90–110°C is sufficient for polypropylene to dissolve completely in organic solvents.

[0092] In steps 1-2', acrylic acid is added to the first solvent at 70~80℃, such as 80℃.

[0093] Acrylic acid exhibits high reactivity within a temperature range of 70-80℃, enabling it to effectively undergo free radical graft polymerization with polypropylene and avoid adverse consequences such as uncontrolled polymerization caused by excessively rapid reaction.

[0094] In steps 1-2', the ratio (g:mL) of polypropylene granules to acrylic acid solution is 1:0.05~0.5, preferably 1:0.1~0.2, for example 1:0.17. Under these conditions, acrylic acid readily polymerizes with polypropylene.

[0095] In steps 1-3', the initiator solution is obtained by dissolving the initiator in an organic solvent to obtain a mixture with a concentration of 20-30%, which is then added to the second solution; for example, obtaining an initiator solution with a concentration of 25%, and then adding the initiator solution to the second solution; the organic solvent at this time is preferably the same as the organic solvent selected when dissolving the polypropylene granules in an organic solvent in step 1-1', for example, xylene.

[0096] Furthermore, the initiator is benzoyl peroxide or dicumyl peroxide, preferably benzoyl peroxide.

[0097] Among them, benzoyl peroxide has a moderate effective catalytic cycle as an initiator and performs well in the polypropylene grafting reaction. The reaction conditions are milder than those of dicumyl peroxide, and the cleavage effect on polypropylene molecular chains is weak within a reaction time of 2 hours.

[0098] In steps 1-3', the temperature when adding the initiator solution should not exceed the boiling point of acrylic acid, such as 70~110℃ or 80℃.

[0099] In steps 1-3', the mass ratio of polypropylene to initiator is 1:0.005~0.25, for example, 1:0.13. Excessive initiator content will cause severe polypropylene cracking, while insufficient initiator content will lead to incomplete reaction and a low grafting rate.

[0100] In steps 1-3', an initiator solution is added to the second solvent and reacted for 6-10 hours, for example, 8 hours, to ensure the reaction proceeds fully.

[0101] In steps 1-3', acetone is preferably used for filtration. Acetone has relatively high volatility, which is beneficial for rapid removal; and acetone has high solubility, which can effectively remove residual organic solvents and other organic impurities from polypropylene.

[0102] On the other hand, according to the method for preparing the composite material according to the first aspect of the present invention, the method includes:

[0103] Step 1: Prepare alumina nanosheets and acrylic acid-grafted polypropylene, respectively;

[0104] Step 2: React alumina nanosheets, acrylic acid-grafted polypropylene, and polypropylene granules to obtain the composite material.

[0105] In step 2, the reaction includes:

[0106] Step 2-1: The acrylic-grafted polypropylene and polypropylene granules are melt-blended once to obtain composite grafted polypropylene.

[0107] Step 2-2: The composite grafted polypropylene and alumina nanosheets are melt-blended a second time to obtain the composite material.

[0108] In step 2-1, the initial melting temperature is 170℃~220℃, preferably 185℃~200℃, for example, 200℃. Within this temperature range, polypropylene melts fully, exhibits strong fluidity, and does not undergo thermal degradation.

[0109] In step 2-1, the melting time for one melting cycle is 10-20 minutes, preferably 15 minutes. One melting cycle is sufficient to ensure the polypropylene is fully melted.

[0110] In step 2-2, the secondary melting temperature is 185℃~195℃, for example, 185℃. This temperature should be lower than the primary melting temperature, but higher than its melting point.

[0111] In step 2-2, the secondary melting time is 10-20 minutes, preferably 15 minutes, to ensure that the composite material is completely mixed.

[0112] Thirdly, according to the present invention, a biaxially oriented composite film is provided, wherein the composite film uses a biaxially oriented composite material as a filler and a polyimide film as a mold; the composite film has uniform thickness and uniform film formation; its dielectric constant is between 2.35 and 2.41, its dielectric loss is maintained on the order of 1E-3, its breakdown field strength reaches 860-870 kV / mm, and its energy storage density reaches 7.8-8.0 J / cm². 3 The dielectric constant is between 2.35 and 2.41.

[0113] In one embodiment, the dielectric constant reaches a maximum of 2.41, the dielectric loss is maintained on the order of 1E-3, the breakdown field strength is 864 kV / mm, and the energy storage density reaches 7.96 J / cm². 3 .

[0114] Fourthly, according to the method for preparing the composite film according to the third aspect of the present invention, the method includes: using a biaxially oriented composite material as a filler and a polyimide film as a mold, the composite film is obtained by hot pressing and biaxial stretching.

[0115] In this invention, the polyimide film should have an appropriate thickness and size so that it can completely encapsulate the biaxially oriented composite material during the subsequent hot pressing process. Typically, a thickness of 100~150μm is selected.

[0116] In this invention, the hot-pressing temperature is 190~230℃, the time is 15~20min, and the pressure is 20~30MPa; for example, the temperature is 200℃, the time is 18min, and the pressure is 25MPa. Under the above hot-pressing parameters, the biaxially oriented composite film can be fully melted, formed, and shaped.

[0117] In this invention, the stretching temperature is 150~170℃ and the stretching rate is 90~110 mm / s; for example, the stretching temperature is 160℃ and the stretching rate is 100 mm / s. Under these stretching parameters, the biaxially oriented composite film has moderate flowability, which is beneficial for stretching.

[0118] The present invention is further described below through specific examples; however, these examples are merely exemplary and do not constitute any limitation on the scope of protection of the present invention.

[0119] Example 1

[0120] (1) Preparation of γ-alumina nanosheets:

[0121] 0.05 mol aluminum sulfate octahydrate powder was dissolved in 50 mL of deionized water at room temperature and stirred at 1500 r / min for 3 h; then the pH of the solution was adjusted to 5 with ammonia and stirred with a stir bar for 30 min to obtain a suspension.

[0122] The above suspension was transferred to a 100 mL stainless steel high-pressure reactor with a Teflon liner and hydrothermally treated at 190 °C for 24 h. The resulting product was then filtered and washed with deionized water until neutral, and freeze-dried for 12 h to obtain a white product.

[0123] The obtained white product was placed in a muffle furnace under a nitrogen atmosphere and heated from room temperature to 150°C at a heating rate of 10 °C / min, and held at that temperature for 3 h; then heated to 600°C at a heating rate of 10 °C / min, and held at that temperature for 3 h; then naturally cooled to room temperature to obtain γ-alumina nanosheets.

[0124] The obtained γ-alumina is shown in Figure 1(a), and it can be seen that its structure is relatively regular and its edges are clear.

[0125] (2) Preparation of acrylic acid grafted polypropylene:

[0126] 60g of polypropylene granules were dissolved in 2000 mL of xylene at 100°C to obtain the first solution; the temperature was lowered to 80°C, and 10 mL of acrylic acid was added dropwise, and the mixture was stirred until homogeneous to obtain the second solution; benzoyl peroxide was added to xylene to obtain a mixed solution with a benzoyl peroxide concentration of 25%; 32 mL of the mixed solution was added to the second solution, and the reaction was carried out for 8 h with stirring and a nitrogen atmosphere throughout the reaction. After the reaction was completed, the product was poured into acetone and filtered, and then extracted in deionized water using a Soxhlet extractor for 24 h to obtain 56g of acrylic acid-grafted polypropylene.

[0127] (3) 15g of acrylic acid-grafted polypropylene and 15g of polypropylene were melt-blended once at 200℃ for 15 min to obtain 30g of composite grafted polypropylene; the obtained 30g of composite grafted polypropylene and 0.15g of γ-alumina nanosheets were melt-blended twice at 185℃ for 15 min to obtain biaxially oriented composite material.

[0128] (4) Using a 50mm diameter circular cutter, cut several 5cm diameter holes in a 100μm thick first polyimide film. Place a 100μm thick second polyimide film at the bottom of the first polyimide film. Place 0.2g of biaxially oriented composite material in the holes of the first polyimide film. Place a 100μm thick third polyimide film on top of the first polyimide film to seal it. Then, use a flat vulcanizing apparatus for hot pressing treatment. The temperature is set to 200℃, the pressure is set to 25MPa, the treatment time is set to 18mins, the number of venting times is set to 15, and the venting time for each time is set to 10s to obtain a composite polypropylene casting sheet.

[0129] Biaxially stretched composite films were prepared by biaxially stretching composite polypropylene cast sheets at a temperature of 160℃ and a speed of 100 mm / s.

[0130] Figure 4 The cross-section of the prepared biaxially stretched composite film is shown by TEM characterization. It can be seen that the effective biaxial stretching caused the nanosheets to be oriented, and the oriented nanosheets effectively suppressed the electric field distortion and hindered the development of the electrical breakdown path, thereby leading to the improvement of the breakdown field strength.

[0131] Figure 5 The infrared spectrum of the prepared biaxially oriented composite material is shown, indicating that at 1712 cm⁻¹... -1 The presence of grafted groups (carbonyl groups, derived from grafted acrylic acid groups) proves that the acrylic acid-grafted polypropylene proposed in this invention has been successfully introduced into the composite system.

[0132] Example 2

[0133] (1) γ-alumina nanosheets were prepared in the same manner as in Example 1;

[0134] (2) Prepare acrylic acid-grafted polypropylene in the same manner as in Example 1;

[0135] (3) 15g of acrylic acid-grafted polypropylene and 15g of polypropylene were melt-blended at 200℃ for 15 min to obtain 30g of composite grafted polypropylene; the obtained 30g of composite grafted polypropylene and 0.3g of γ-alumina nanosheets were melt-blended at 185℃ for 15 min to obtain biaxially oriented composite material.

[0136] (4) Using a 50mm diameter circular cutter, cut several 5cm diameter holes in a 100μm thick first polyimide film. Place a 100μm thick second polyimide film at the bottom of the first polyimide film. Place 0.2g of biaxially oriented composite material in the holes of the first polyimide film. Place a 100μm thick third polyimide film on top of the first polyimide film to seal it. Then, use a flat vulcanizing apparatus for hot pressing treatment. The temperature is set to 200℃, the pressure is set to 25MPa, the treatment time is set to 18mins, the number of venting times is set to 15, and the venting time for each time is set to 10s to obtain a composite polypropylene casting sheet.

[0137] Biaxially stretched composite films were prepared by biaxially stretching composite polypropylene castings at a temperature of 160℃ and a speed of 100 mm / s.

[0138] Example 3

[0139] (1) γ-alumina nanosheets were prepared in the same manner as in Example 1;

[0140] (2) Prepare acrylic acid-grafted polypropylene in the same manner as in Example 1;

[0141] (3) 15g of acrylic acid-grafted polypropylene and 15g of polypropylene were melt-blended at 200℃ for 15 min to obtain 30g of composite grafted polypropylene; the obtained 30g of composite grafted polypropylene and 0.9g of γ-alumina nanosheets were melt-blended at 185℃ for 15 min to obtain biaxially oriented composite material.

[0142] (4) Using a 50mm diameter circular cutter, cut several 5cm diameter holes in a 100μm thick first polyimide film. Place a 100μm thick second polyimide film at the bottom of the first polyimide film. Place 0.2g of biaxially oriented composite material in the holes of the first polyimide film. Place a 100μm thick third polyimide film on top of the first polyimide film to seal it. Then, use a flat vulcanizing apparatus for hot pressing treatment. The temperature is set to 200℃, the pressure is set to 25MPa, the treatment time is set to 18mins, the number of venting times is set to 15, and the venting time for each time is set to 10s to obtain a composite polypropylene casting sheet.

[0143] Biaxially stretched composite films were prepared by biaxially stretching composite polypropylene castings at a temperature of 160℃ and a speed of 100 mm / s.

[0144] Example 4

[0145] (1) γ-alumina nanosheets were prepared in the same manner as in Example 1;

[0146] (2) Prepare acrylic acid-grafted polypropylene in the same manner as in Example 1;

[0147] (3) 15g of acrylic acid-grafted polypropylene and 15g of polypropylene were melt-blended at 200℃ for 15 min to obtain 30g of composite grafted polypropylene; the obtained 30g of composite grafted polypropylene and 1.5g of γ-alumina nanosheets were melt-blended at 185℃ for 15 min to obtain biaxially oriented composite material.

[0148] (4) Using a 50mm diameter circular cutter, cut several 5cm diameter holes in a 100μm thick first polyimide film. Place a 100μm thick second polyimide film at the bottom of the first polyimide film. Place 0.2g of biaxially oriented composite material in the holes of the first polyimide film. Place a 100μm thick third polyimide film on top of the first polyimide film to seal it. Then, use a flat vulcanizing apparatus for hot pressing treatment. The temperature is set to 200℃, the pressure is set to 25MPa, the treatment time is set to 18mins, the number of venting times is set to 15, and the venting time for each time is set to 10s to obtain a composite polypropylene casting sheet.

[0149] Biaxially stretched composite films were prepared by biaxially stretching composite polypropylene castings at a temperature of 160℃ and a speed of 100 mm / s.

[0150] Comparative Example 1

[0151] Comparative Example 1 is an unoptimized method for preparing γ-alumina nanosheets. The preparation of its biaxially stretched composite film is the same as in Example 1, and the specific technical solution is as follows:

[0152] (1) Preparation of γ-alumina nanosheets:

[0153] 0.05 mol aluminum sulfate octahydrate powder was dissolved in 50 mL of deionized water at room temperature and stirred at 1500 r / min for 3 h; then the pH of the solution was adjusted to 5 with ammonia and stirred with a stir bar for 30 min to obtain a suspension.

[0154] The above suspension was transferred to a 100 mL stainless steel high-pressure reactor with a Teflon liner and hydrothermally treated at 190 °C for 24 h. The resulting product was then filtered and washed with deionized water until neutral, and freeze-dried for 12 h to obtain a white product.

[0155] The obtained white product was placed in a muffle furnace under a nitrogen atmosphere and heated from room temperature to 600°C at a heating rate of 10 °C / min, and held at that temperature for 3 h; then it was naturally cooled to room temperature to obtain γ-alumina nanosheets.

[0156] (2) Preparation of acrylic acid grafted polypropylene:

[0157] 60g of polypropylene granules were dissolved in 2000 mL of xylene at 100°C to obtain the first solution; the temperature was lowered to 80°C, and 10 mL of acrylic acid was added dropwise, and the mixture was stirred until homogeneous to obtain the second solution; benzoyl peroxide was added to xylene to obtain a mixed solution with a benzoyl peroxide concentration of 25%; 32 mL of the mixed solution was added to the second solution, and the reaction was carried out for 8 h with stirring and a nitrogen atmosphere throughout the reaction. After the reaction was completed, the product was poured into acetone and filtered, and then extracted in deionized water using a Soxhlet extractor for 24 h to obtain 56g of acrylic acid-grafted polypropylene.

[0158] (3) 15g of acrylic acid-grafted polypropylene and 15g of polypropylene were melt-blended once at 200℃ for 15 min to obtain 30g of composite grafted polypropylene; the obtained 30g of composite grafted polypropylene and 0.15g of γ-alumina nanosheets were melt-blended twice at 185℃ for 15 min to obtain biaxially oriented composite material.

[0159] (4) Using a 50mm diameter circular cutter, cut several 5cm diameter holes in a 100μm thick first polyimide film. Place a 100μm thick second polyimide film at the bottom of the first polyimide film. Place 0.2g of biaxially oriented composite material in the holes of the first polyimide film. Place a 100μm thick third polyimide film on top of the first polyimide film to seal it. Then, use a flat vulcanizing apparatus for hot pressing treatment. The temperature is set to 200℃, the pressure is set to 25MPa, the treatment time is set to 18mins, the number of venting times is set to 15, and the venting time for each time is set to 10s to obtain a composite polypropylene casting sheet.

[0160] Biaxially stretched composite films were prepared by biaxially stretching composite polypropylene castings at a temperature of 160℃ and a speed of 100 mm / s.

[0161] Figure 1(a) shows the TEM characterization of the γ-alumina nanosheets prepared in Example 1, and Figure 1(b) shows the TEM characterization of the γ-alumina nanosheets prepared in Comparative Example 1. It is clear that the γ-alumina nanosheets prepared in Example 1 have a complete structure. This is due to the step sintering method preventing the intermediate products from breaking during the rapid heating process, thus obtaining a complete nanosheet structure.

[0162] Comparative Example 2

[0163] In Comparative Example 2, biaxially oriented composite films were prepared using unmodified polypropylene granules. The specific technical solution is as follows:

[0164] Using a 50mm diameter circular cutter, several 5cm diameter holes were cut into a 100μm thick first polyimide film. A 100μm thick second polyimide film was placed at the bottom of the first polyimide film. 0.2g of polypropylene granules were placed in the holes of the first polyimide film. A 100μm thick third polyimide film was placed on top of the first polyimide film to seal it. Then, a flat vulcanizing apparatus was used for hot pressing treatment. The temperature was set to 200℃, the pressure was set to 25MPa, the treatment time was set to 18min, the number of venting times was set to 15, and the venting time was set to 10s each time to obtain a polypropylene casting sheet.

[0165] Biaxially stretched polypropylene sheets were subjected to biaxial stretching at a temperature of 160℃ and a speed of 100 mm / s to obtain a biaxially stretched composite film.

[0166] Comparative Example 3

[0167] Comparative Example 3 uses pure polypropylene as the organic phase and γ-alumina nanosheets as the inorganic phase to prepare a biaxially oriented composite film. The specific technical solution is as follows:

[0168] (1) Preparation of γ-alumina nanosheets:

[0169] 0.05 mol aluminum sulfate octahydrate powder was dissolved in 50 mL of deionized water at room temperature and stirred at 1500 r / min for 3 h; then the pH of the solution was adjusted to 5 with ammonia and stirred with a stir bar for 30 min to obtain a suspension.

[0170] The above suspension was transferred to a 100 mL stainless steel high-pressure reactor with a Teflon liner and hydrothermally treated at 190 °C for 24 h. The resulting product was then filtered and washed with deionized water until neutral, and freeze-dried for 12 h to obtain a white product.

[0171] The obtained white product was placed in a muffle furnace under a nitrogen atmosphere and heated from room temperature to 150°C at a heating rate of 10 °C / min, and held at that temperature for 3 h; then heated to 600°C at a heating rate of 10 °C / min, and held at that temperature for 3 h; then naturally cooled to room temperature to obtain γ-alumina nanosheets.

[0172] (2) 15g of polypropylene granules and 0.15g of γ-alumina nanosheets were melt-blended at 180℃ for 15 min to obtain a composite material.

[0173] (3) Using a 50mm diameter circular cutter, cut several 5cm diameter holes in a 100μm thick first polyimide film. Place a 100μm thick second polyimide film at the bottom of the first polyimide film. Place 0.2g of composite material in the holes of the first polyimide film. Place a 100μm thick third polyimide film on top of the first polyimide film to seal it. Then, use a flat vulcanizing apparatus for hot pressing treatment. The temperature is set to 200℃, the pressure is set to 25MPa, the treatment time is set to 18mins, the number of venting times is set to 15, and the venting time for each time is set to 10s to obtain a composite polypropylene casting sheet.

[0174] Biaxially stretched composite films were prepared by biaxially stretching composite polypropylene castings at a temperature of 160℃ and a speed of 100 mm / s.

[0175] Figure 2 The frequency domain spectra of the dielectric constant and dielectric loss tangent of the biaxially stretched composite films prepared in Example 1, Comparative Example 2, and Comparative Example 3 are shown. It can be seen that the dielectric constant of the biaxially stretched composite film prepared in Example 1 is 2.41, the dielectric constant of the biaxially stretched composite film prepared in Comparative Example 2 is 2.2, and the dielectric constant of the biaxially stretched composite film prepared in Comparative Example 3 is 2.3. Clearly, the dielectric constant of the biaxially stretched composite film prepared in Example 1 is superior, representing a 9.5% improvement compared to the biaxially stretched composite film prepared in Comparative Example 2. The dielectric loss of the biaxially stretched composite film prepared in Example 1 is maintained at the order of 1E⁻³.

[0176] Figure 3 The diagram shows a comparison of the DC breakdown Weibull distribution of the biaxially oriented composite films prepared in Example 1, Comparative Example 2, and Comparative Example 3. It can be seen that the biaxially oriented composite film prepared in Example 1 exhibits the best breakdown field strength, reaching 864 kV / mm, which is 1.5 times that of the biaxially oriented composite film prepared in Comparative Example 2, representing a significant improvement in breakdown field strength. Compared with Comparative Example 3, the breakdown field strength is increased by 240 kV / mm. The characteristic energy storage density of the biaxially oriented composite film prepared in Example 1 was measured to be 7.96 J / cm³. 3 .

[0177] Figure 6 The thickness distribution of the biaxially oriented composite films prepared in Example 1, Comparative Example 2, and Comparative Example 3 is shown in the comparison diagram. The thickness of the three different samples was measured 15 times at different locations to characterize the thickness uniformity of the materials. It can be seen that the biaxially oriented composite film prepared in Example 1 has a thickness of 10 micrometers, which is close to that of the biaxially oriented composite film prepared in Comparative Example 2; while the biaxially oriented composite film prepared in Comparative Example 3 has poor thickness dispersion, uneven film formation, and poor film-forming properties.

[0178] Figure 7 The frequency domain spectra of the dielectric constant and dielectric loss tangent of the biaxially stretched composite films prepared in Examples 1-4 are shown. The comparison reveals that the dielectric constant of Examples 1, 2, 3, and 4 increases significantly with increasing content of inorganic γ-alumina nanosheets. On the other hand, the introduction of inorganic particles leads to a significant increase in low-frequency conductivity loss, reaching a maximum order of magnitude of 0.04, which exceeds the usable range of thin-film capacitors.

[0179] Figure 8 The diagram shows a comparison of the breakdown field strength distribution of the biaxially stretched composite films prepared in Examples 1-4. The comparison reveals that the breakdown field strength decreases with increasing content of inorganic γ-alumina nanosheets. In Example 1, the introduction of the inorganic phase of low-content (0.5 wt%) γ-alumina nanosheets significantly improved the breakdown field strength.

[0180] Figure 9(a) shows the breakdown field strength diagram of the biaxially stretched composite film prepared in Example 1; Figure 9(b) shows the breakdown field strength diagram of the biaxially stretched composite film prepared in Comparative Example 1. By comparison, it can be seen that the biaxially stretched composite film prepared in Example 1 has a relatively significant improvement in breakdown field strength, which is due to the regular nanosheet structure.

[0181] The present invention has been described in detail above with reference to preferred embodiments and exemplary examples. However, it should be noted that these specific embodiments are merely illustrative explanations of the invention and do not constitute any limitation on the scope of protection of the invention. Various improvements, equivalent substitutions, or modifications can be made to the technical content and embodiments of the present invention without departing from the spirit and scope of protection of the invention, and all such modifications fall within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing a biaxially oriented composite material, characterized in that, The composite material uses composite grafted polypropylene as the masterbatch and alumina nanosheets as the inorganic phase. The composite grafted polypropylene uses polypropylene as the matrix and acrylic acid grafted polypropylene as the organic composite phase. Step 1: Prepare alumina nanosheets and acrylic acid-grafted polypropylene, respectively; Step 2: React alumina nanosheets, acrylic acid-grafted polypropylene, and polypropylene granules to obtain the composite material; In step 1, the alumina nanosheets are prepared through the following steps: Step 1-1: The soluble aluminum salt is pretreated to obtain a weakly acidic solution; the pretreatment includes: dissolving the soluble aluminum salt in water, and then adjusting the pH to 4.8-5.1 with a weak alkali; Steps 1-2 involve heat-treating the weakly acidic solution, followed by washing and drying to obtain the product to be treated. Steps 1-3: The product to be processed is subjected to step sintering to obtain the alumina nanosheets; In step 1, the acrylic acid-grafted polypropylene is prepared through the following steps: Step 1-1': Dissolve the polypropylene granules in an organic solvent to obtain the first solution; Steps 1-2': At 70~80℃, acrylic acid is added to the first solution to obtain the second solution; Steps 1-3': At 70~110℃, an initiator solution is added to the second solution, and the reaction is carried out for 6~10h. The acrylic acid-grafted polypropylene is obtained by sequential filtration and extraction. In step 2, the reaction includes: Step 2-1: The acrylic-grafted polypropylene and polypropylene granules are melt-blended once to obtain composite grafted polypropylene. Step 2-2: The composite grafted polypropylene and alumina nanosheets are melt-blended a second time to obtain the composite material.

2. The method according to claim 1, characterized in that, The composite material contains a grafted carbonyl group.

3. A biaxially oriented composite film, characterized in that, The composite film is prepared using the biaxially oriented composite material prepared by the method described in claim 1.

4. The composite film according to claim 3, characterized in that, The composite film uses a biaxially oriented composite material as a filler.

5. The composite film according to claim 3, characterized in that, The dielectric constant of the composite film is between 2.35 and 2.41.

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