A biaxially oriented stretched polypropylene composite film and a preparation method thereof

Abstract

The present application relates to a kind of biaxial stretching flow polypropylene composite film and its preparation method, belong to packaging material technical field;The present application is prepared with homopolymerization polypropylene, copolymerization polypropylene, slip agent, nucleating agent, antioxidant, modified nanosilica as raw material a kind of biaxial stretching flow polypropylene composite film of high transparency and with good antistatic property and ultraviolet resistance performance, which can be used in food packaging field, the present application is modified to nanosilica, ultraviolet absorber is grafted on the surface of nanosilica, not only solve the problem of nanosilica easy to agglomerate, also solve the problem of ultraviolet absorber easy to migrate, by quaternary ammonium salt structure conjugation, ultraviolet absorber has better ultraviolet absorption performance and antistatic property.

Description

A biaxially stretched cast polypropylene composite film and its preparation method Technical Field

[0001] This invention belongs to the field of packaging materials technology, specifically, it relates to a biaxially oriented cast polypropylene composite film and its preparation method, and more specifically, it relates to a high-transparency biaxially oriented cast polypropylene composite film for food packaging and its preparation method. Background Technology

[0002] Biaxially oriented polypropylene (BOPP) film is widely used in the food packaging industry due to its excellent transparency, mechanical strength, barrier properties, and printability. However, higher transparency also means higher UV transmittance. UV rays not only cause photo-oxidative degradation of polypropylene in BOPP raw materials but also accelerate the spoilage of packaged food. Adding UV absorbers can improve the UV absorption performance of the packaging film. Small molecule UV absorbers have good UV absorption performance, but they are prone to migration in polypropylene materials, affecting the safety of packaging materials. Although large molecule UV absorbers are less prone to migration, their UV absorption performance is weaker than that of small molecule UV absorbers, thus requiring higher addition amounts, which may affect the transparency of BOPP.

[0003] BOPP film has high insulation properties, but this also leads to the accumulation of static electricity due to friction during processing. Static electricity not only attracts dust and affects the cleanliness of the film's appearance, but also affects the adhesion of electronic tags and inks, as well as interlayer bonding. Furthermore, antistatic agents also have the disadvantage of being prone to migration. To solve the above technical defects, this invention provides a biaxially oriented cast polypropylene composite film and its preparation method. Summary of the Invention

[0004] The purpose of this invention is to provide a biaxially oriented cast polypropylene composite film and its preparation method, in order to solve the problems mentioned in the background art.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A biaxially oriented cast polypropylene composite film comprises the following raw materials in parts by weight: 60-80 parts homopolymer polypropylene, 20-40 parts copolymer polypropylene, 0.05-0.15 parts slip agent, 0.1-0.5 parts nucleating agent, 0.1-0.3 parts antioxidant, and 2-3 parts modified nano silica.

[0007] Furthermore, the copolymerized polypropylene is a propylene-ethylene copolymer.

[0008] Furthermore, the slip agent is one or more of erucamide and oleamide.

[0009] Furthermore, the nucleating agent is a sorbitol derivative, including at least one of EDBS (p-methylbenzyl sorbitol), DMDBS (di(3,4-dimethylbenzyl)sorbitol), and DBS (dibenzyl sorbitol).

[0010] Furthermore, the antioxidant is one or more of BHT (butylated hydroxytoluene), TBHQ (tert-butylhydroquinone), and PG (propyl gallate).

[0011] Furthermore, the modified nano-silica is prepared by the following steps:

[0012] S1. N,N-Diethyl-3-aminopropyltrimethoxysilane, ethanol solution, and nano-silica were mixed in a three-necked flask, a condenser and a thermometer were installed, and a magnetic stirrer was turned on. Glacial acetic acid was added to the three-necked flask to adjust the pH of the system to 4-5. The reaction was then carried out at 40-60℃ for 3-4 hours. After the reaction was completed, the solid was filtered out and washed with anhydrous ethanol and deionized water and then dried to obtain tertiary ammonium nano-silica.

[0013] S2. Mix UV absorber UV-P, N-bromosuccinimide, initiator and acetonitrile in a three-necked flask, attach a condenser and thermometer, turn on magnetic stirring, and react at 50-80℃ for 4-8 hours. After the reaction is complete, remove the solvent by rotary evaporation, wash the remaining solid with deionized water and dry to obtain the brominated UV absorber.

[0014] S3. Mix the brominated ultraviolet absorber, tertiary ammonium nano-silica, and acetonitrile in a three-necked flask, attach a condenser and a thermometer, turn on the magnetic stirrer, and react at a temperature of 40-60℃ for 8-12 hours. After the reaction is complete, filter out the solid, wash it with anhydrous ethanol and deionized water, and then dry it to obtain modified nano-silica.

[0015] Furthermore, the ethanol solution is an aqueous solution of ethanol with a volume fraction of 40-80%.

[0016] Furthermore, the particle size of the nano-silica is 1–50 nm.

[0017] Furthermore, the ultraviolet absorber UV-P is 2-(2-hydroxy-5-methylphenyl)benzotriazole.

[0018] Furthermore, the initiator is one of benzoyl peroxide and azobisisobutyronitrile.

[0019] Furthermore, the mass ratio of N,N-diethyl-3-aminopropyltrimethoxysilane, ethanol solution, and nano-silica in S1 is 3–6:80–160:6–8.

[0020] Furthermore, the mass ratio of UV absorber UV-P, N-bromosuccinimide, initiator, and acetonitrile in S2 is 4.4–6.8: 3.5–5.5: 0.9–1.5: 40–60.

[0021] Furthermore, the mass ratio of brominated ultraviolet absorber, tertiary ammonium-modified nano-silica, and acetonitrile in S3 is 3.5–7:6–8:80–160.

[0022] The present invention also discloses a method for preparing the biaxially stretched cast polypropylene composite film.

[0023] A method for preparing a biaxially oriented cast polypropylene composite film includes the following steps:

[0024] Weigh each raw material according to the mass fraction, dry the raw materials and add them to a twin-screw extruder. Then, after melt extrusion, casting, longitudinal stretching, transverse stretching, heat setting, cooling, corona treatment and traction winding, the biaxially stretched cast polypropylene composite film is obtained.

[0025] Furthermore, the temperature settings for the twin-screw extruder are: Zone 1 180–190°C, Zone 2 200–210°C, Zone 3 210–220°C, and Zone 4 215–225°C.

[0026] Furthermore, the casting process involves extruding the melt and casting it onto a cooling roller at 25-35°C to form a homogeneous casting sheet with a thickness of 0.3-0.5 mm.

[0027] Furthermore, the longitudinal stretching temperature is 125–135℃, the longitudinal stretching ratio is 3–5:1, the transverse stretching temperature is 155–165℃, the transverse stretching ratio is 8–10:1, and the heat setting temperature is 160–170℃.

[0028] The beneficial effects of this invention are:

[0029] 1) This invention obtains tertiary amination nano-silica with a tertiary amine structure by grafting N,N-diethyl-3-aminopropyltrimethoxysilane onto the surface of nano-silica. Then, using N-bromosuccinimide as a brominating agent, hydrogen atoms at the benzyl position of UV-P are directionally substituted under the action of an initiator to obtain a brominated ultraviolet absorber. Taking advantage of the characteristic that bromine atoms at the benzyl position readily undergo quaternary ammonium salt reactions with the tertiary amine structure, the brominated ultraviolet absorber is grafted onto the surface of a tertiary amination silane coupling agent to obtain a modified nano-silica. The modified nano-silica of this invention has a long organic chain with large steric hindrance on its surface, and it carries a positively charged quaternary ammonium salt structure. It can effectively improve the dispersion performance of nano-silica through the synergistic effect of steric hindrance and electrostatic repulsion, and inhibit the formation of nano-silica agglomerates. The modified nano-silica can be uniformly distributed in polypropylene substrates and will not cause visible light dispersion or reduce the transparency of polypropylene films due to agglomeration.

[0030] 2) UV-P is a commonly used small-molecule ultraviolet absorber with good ultraviolet absorption effect, but it is easy to migrate in macromolecular polymers and is not suitable for use in the food packaging field. This invention modifies it by grafting the modified UV-P onto the surface of nano-silica using a quaternary ammonium salt reaction, which effectively improves the dispersibility and stability of UV-P in polypropylene substrate and reduces the potential contamination risk to food caused by the migration of ultraviolet absorbers. Moreover, the quaternary ammonium salt structure in the modified UV-P molecule can also have a synergistic effect with the benzene ring structure in the UV-P molecule, forming additional electron transition channels and thus improving ultraviolet absorption performance.

[0031] 3) After modifying UV-P and grafting it onto the surface of nano-silica, the present invention forms a positively charged quaternary ammonium salt structure on the surface of nano-silica. The quaternary ammonium salt structure can form a conductive path in the polymer material, reduce the surface resistance of the material, accelerate the leakage rate of static charge, and improve the antistatic properties of the material.

[0032] 4) The raw materials used in this invention are food antioxidants that meet my country's national food safety standards, and the ultraviolet absorbers and antistatic components are fixed on the surface of nano-silica by grafting, so there is no risk of migration and good safety performance. Detailed Implementation

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0034] In this article, the technical features described in an open-ended manner include both closed technical solutions composed of the listed features and open technical solutions that include the listed features.

[0035] The term “and / or” as used herein includes any and all combinations of one or more of the related listed items.

[0036] In this document, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0037] In this article, when referring to units for data ranges, if the unit is only followed by the right endpoint, it indicates that the units for the left and right endpoints are the same. For example, 40~60℃ means that the units for the left endpoint "40" and the right endpoint "60" are both in degrees Celsius (℃).

[0038] This document only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0039] Example 1

[0040] A biaxially oriented cast polypropylene composite film comprises the following raw materials in parts by weight: 60 parts homopolymer polypropylene, 40 parts copolymer polypropylene, 0.05 parts slip agent, 0.1 parts nucleating agent, 0.1 parts antioxidant, and 2 parts modified nano silica.

[0041] Wherein, the copolymer polypropylene is a propylene-ethylene copolymer, the slip agent is erucamide, the nucleating agent is p-methylbenzyl sorbitol, the antioxidant is butylated hydroxytoluene, and the modified nano-silica is prepared by the following steps:

[0042] S1. By mass fraction, 3 parts of N,N-diethyl-3-aminopropyltrimethoxysilane, 80 parts of 80% ethanol aqueous solution, and 6 parts of nano-silica with a particle size of 1-50 nm were mixed in a three-necked flask, fitted with a condenser and a thermometer, and the magnetic stirrer was turned on. Glacial acetic acid was added to the three-necked flask to adjust the pH of the system to 4. The reaction was then carried out at 60°C for 3 hours. After the reaction was completed, the solid was filtered out and washed with anhydrous ethanol and deionized water and dried to obtain tertiary ammonium nano-silica.

[0043] S2. By mass, 4.4 parts of UV absorber UV-P, 3.5 parts of N-bromosuccinimide, 0.9 parts of azobisisobutyronitrile, and 40 parts of acetonitrile were mixed in a three-necked flask, fitted with a condenser and a thermometer, and the magnetic stirrer was turned on. The mixture was reacted at 50°C for 8 hours. After the reaction was completed, the solvent was removed by rotary evaporation. The remaining solid was washed with deionized water and dried to obtain the brominated UV absorber.

[0044] S3. By mass, 3.5 parts of brominated ultraviolet absorber, 6 parts of tertiary ammonium nano-silica, and 80 parts of acetonitrile are mixed in a three-necked flask, a condenser and a thermometer are attached, and a magnetic stirrer is turned on. The mixture is reacted at 40°C for 12 hours. After the reaction is completed, the solid is filtered out and washed with anhydrous ethanol and deionized water and then dried to obtain modified nano-silica.

[0045] A method for preparing a biaxially oriented cast polypropylene composite film includes the following steps:

[0046] Weigh each raw material according to the mass fraction, dry the raw materials and add them to a twin-screw extruder. Then, after melt extrusion, casting, longitudinal stretching, transverse stretching, heat setting, cooling, corona treatment and traction winding, the biaxially stretched cast polypropylene composite film is obtained.

[0047] The temperature settings for the twin-screw extruder are 180℃ in zone 1, 200℃ in zone 2, 210℃ in zone 3, and 215℃ in zone 4.

[0048] The casting process involves extruding the melt and casting it onto a 25°C cooling roller to form a homogeneous casting sheet with a thickness of 0.3 mm.

[0049] The longitudinal stretching temperature is 125℃, the longitudinal stretching ratio is 3:1, the transverse stretching temperature is 155℃, the transverse stretching ratio is 8:1, and the heat setting temperature is 160℃.

[0050] Example 2

[0051] A biaxially oriented cast polypropylene composite film comprises the following raw materials in parts by weight: 70 parts homopolymer polypropylene, 30 parts copolymer polypropylene, 0.1 parts slip agent, 0.3 parts nucleating agent, 0.2 parts antioxidant, and 2.5 parts modified nano silica.

[0052] Wherein, the copolymer polypropylene is a propylene-ethylene copolymer, the slip agent is erucamide, the nucleating agent is bis(3,4-dimethylbenzyl)sorbitol, the antioxidant is tert-butylhydroquinone, and the modified nano-silica is prepared by the following steps:

[0053] S1. By mass, 4.5 parts of N,N-diethyl-3-aminopropyltrimethoxysilane, 120 parts of 60% ethanol aqueous solution, and 7 parts of nano-silica with a particle size of 1-50 nm were mixed in a three-necked flask. A condenser and thermometer were installed, and a magnetic stirrer was turned on. Glacial acetic acid was added to the three-necked flask to adjust the pH of the system to 4.5. The reaction was then carried out at 50°C for 3.5 h. After the reaction was completed, the solid was filtered out and washed with anhydrous ethanol and deionized water and dried to obtain tertiary ammonium nano-silica.

[0054] S2. By mass, 5.6 parts of UV absorber UV-P, 4.5 parts of N-bromosuccinimide, 1.2 parts of azobisisobutyronitrile, and 50 parts of acetonitrile were mixed in a three-necked flask, fitted with a condenser and a thermometer, and the magnetic stirrer was turned on. The mixture was reacted at 65°C for 6 hours. After the reaction was completed, the solvent was removed by rotary evaporation. The remaining solid was washed with deionized water and dried to obtain the brominated UV absorber.

[0055] S3. By mass, 5.25 parts of brominated ultraviolet absorber, 7 parts of tertiary ammonium-modified nano silica, and 120 parts of acetonitrile are mixed in a three-necked flask, a condenser and a thermometer are attached, and a magnetic stirrer is turned on. The mixture is reacted at 50°C for 10 hours. After the reaction is completed, the solid is filtered out and washed with anhydrous ethanol and deionized water and then dried to obtain modified nano silica.

[0056] A method for preparing a biaxially oriented cast polypropylene composite film includes the following steps:

[0057] Weigh each raw material according to the mass fraction, dry the raw materials and add them to a twin-screw extruder. Then, after melt extrusion, casting, longitudinal stretching, transverse stretching, heat setting, cooling, corona treatment and traction winding, the biaxially stretched cast polypropylene composite film is obtained.

[0058] The temperature settings for the twin-screw extruder are 185℃ in zone 1, 205℃ in zone 2, 215℃ in zone 3, and 220℃ in zone 4.

[0059] The casting process involves extruding the melt and casting it onto a 30°C cooling roller to form a homogeneous casting sheet with a thickness of 0.4 mm.

[0060] The longitudinal stretching temperature is 130℃, the longitudinal stretching ratio is 4:1, the transverse stretching temperature is 160℃, the transverse stretching ratio is 9:1, and the heat setting temperature is 165℃.

[0061] Example 3

[0062] A biaxially oriented cast polypropylene composite film comprises the following raw materials in parts by weight: 80 parts homopolymer polypropylene, 20 parts copolymer polypropylene, 0.15 parts slip agent, 0.5 parts nucleating agent, 0.3 parts antioxidant, and 3 parts modified nano silica.

[0063] Wherein, the copolymer polypropylene is a propylene-ethylene copolymer, the slip agent is oleamide, the nucleating agent is dibenzyl sorbitol, the antioxidant is propyl gallate, and the modified nano silica is prepared by the following steps:

[0064] S1. By mass fraction, 6 parts of N,N-diethyl-3-aminopropyltrimethoxysilane, 160 parts of 40% ethanol aqueous solution, and 8 parts of nano-silica with a particle size of 1-50 nm were mixed in a three-necked flask, fitted with a condenser and a thermometer, and the magnetic stirrer was turned on. Glacial acetic acid was added to the three-necked flask to adjust the pH of the system to 5. The reaction was then carried out at 40°C for 4 hours. After the reaction was completed, the solid was filtered out and washed with anhydrous ethanol and deionized water and dried to obtain tertiary ammonium nano-silica.

[0065] S2. By mass, 6.8 parts of UV absorber UV-P, 5.5 parts of N-bromosuccinimide, 1.5 parts of benzoyl peroxide, and 60 parts of acetonitrile were mixed in a three-necked flask, fitted with a condenser and a thermometer, and the magnetic stirrer was turned on. The mixture was reacted at 80°C for 4 hours. After the reaction was completed, the solvent was removed by rotary evaporation. The remaining solid was washed with deionized water and dried to obtain the brominated UV absorber.

[0066] S3. By mass, 7 parts of brominated ultraviolet absorber, 8 parts of tertiary ammonium nano-silica, and 160 parts of acetonitrile were mixed in a three-necked flask, a condenser and a thermometer were attached, and a magnetic stirrer was turned on. The mixture was reacted at 60°C for 8 hours. After the reaction was completed, the solid was filtered out and washed with anhydrous ethanol and deionized water and then dried to obtain modified nano-silica.

[0067] A method for preparing a biaxially oriented cast polypropylene composite film includes the following steps:

[0068] Weigh each raw material according to the mass fraction, dry the raw materials and add them to a twin-screw extruder. Then, after melt extrusion, casting, longitudinal stretching, transverse stretching, heat setting, cooling, corona treatment and traction winding, the biaxially stretched cast polypropylene composite film is obtained.

[0069] The temperature settings for the twin-screw extruder are 190℃ in zone 1, 210℃ in zone 2, 220℃ in zone 3, and 225℃ in zone 4.

[0070] The casting process involves extruding the melt and casting it onto a 35°C cooling roller to form a homogeneous casting sheet with a thickness of 0.5 mm.

[0071] The longitudinal stretching temperature is 135℃, the longitudinal stretching ratio is 5:1, the transverse stretching temperature is 165℃, the transverse stretching ratio is 10:1, and the heat setting temperature is 170℃.

[0072] Comparative Example 1

[0073] A biaxially oriented cast polypropylene composite film comprises the following raw materials in parts by weight: 70 parts homopolymer polypropylene, 30 parts copolymer polypropylene, 0.1 parts slip agent, 0.3 parts nucleating agent, 0.2 parts antioxidant, 0.3 parts ultraviolet absorber, and 2.5 parts nano silica.

[0074] Wherein, the copolymer polypropylene is a propylene-ethylene copolymer, the slip agent is erucamide, the nucleating agent is di(3,4-dimethylbenzyl)sorbitol, the antioxidant is tert-butylhydroquinone, the ultraviolet absorber is ultraviolet absorber UV-3030, and the particle size of the nano silica is 1-50 nm.

[0075] A method for preparing a biaxially oriented cast polypropylene composite film includes the following steps:

[0076] Weigh each raw material according to the mass fraction, dry the raw materials and add them to a twin-screw extruder. Then, after melt extrusion, casting, longitudinal stretching, transverse stretching, heat setting, cooling, corona treatment and traction winding, the biaxially stretched cast polypropylene composite film is obtained.

[0077] The temperature settings for the twin-screw extruder are 185℃ in zone 1, 205℃ in zone 2, 215℃ in zone 3, and 220℃ in zone 4.

[0078] The casting process involves extruding the melt and casting it onto a 30°C cooling roller to form a homogeneous casting sheet with a thickness of 0.4 mm.

[0079] The longitudinal stretching temperature is 130℃, the longitudinal stretching ratio is 4:1, the transverse stretching temperature is 160℃, the transverse stretching ratio is 9:1, and the heat setting temperature is 165℃.

[0080] Experimental Example

[0081] The performance of the biaxially oriented cast polypropylene composite films in Examples 1-3 and Comparative Example 1 was tested. The average absorbance at wavelengths of 280-400 nm was measured using a UV-Vis spectrophotometer; higher absorbance indicates stronger UV absorption performance. Referring to the national standard GB / T 1410-2006 "Test Methods for Volume Resistivity and Surface Resistivity of Solid Insulating Materials," lower surface resistance indicates stronger antistatic ability. The transparency of the composite film was tested according to the national standard GB / T 2410-2008 "Determination of Transmittance and Haze of Transparent Plastics." The test results are shown in Table 1.

[0082] Table 1

[0083] Project Average Absorbance Surface Resistance / Ω Transparency / % Example 12.13 1.8×10 11 97.2 Example 22.121.4×10 11 98.3 Example 32.201.2×10 11 95.8 Comparative Example 11.264.2×10 16 92.4 surface

[0084] As can be seen from Table 1, the biaxially oriented cast polypropylene composite films of the present invention in Examples 1 to 3 have good transparency and better ultraviolet absorption performance because the nano-silica is modified and is not prone to agglomeration, and the ultraviolet absorber used is a small molecule ultraviolet absorber. In addition, the quaternary ammonium salt structure on the surface of the nano-silica can also reduce the surface resistance of the biaxially oriented cast polypropylene composite film and improve its antistatic properties, which can be widely used in the food packaging field.

[0085] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of these embodiments are merely to aid in understanding the method and core ideas of the present invention, including the best mode, and to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims. The scope of protection of this patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to those expressed in the claims, or if they include equivalent structural elements that are not substantially different from those expressed in the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A biaxially oriented cast polypropylene composite film, characterized in that, The raw materials include: homopolymer polypropylene, copolymer polypropylene, slip agent, nucleating agent, antioxidant, and modified nano-silica; wherein, the modified nano-silica is prepared by the following steps: grafting N,N-diethyl-3-aminopropyltrimethoxysilane onto the surface of nano-silica to obtain tertiary ammonium-modified nano-silica for later use; then dissolving ultraviolet absorber UV-P, N-bromosuccinimide, and initiator in acetonitrile and reacting at a controlled temperature of 50-80℃ to obtain a brominated ultraviolet absorber; and then reacting the brominated ultraviolet absorber with tertiary ammonium-modified nano-silica. Modified nano-silica was obtained by reacting a mixture of nano-silica and acetonitrile at a controlled temperature of 40–60 °C. The mass ratio of N,N-diethyl-3-aminopropyltrimethoxysilane to nano-silica was 3–6:6–8. The mass ratio of UV absorber UV-P, N-bromosuccinimide, and initiator was 4.4–6.8:3.5–5.5:0.9–1.

5. The mass ratio of brominated UV absorber, tertiary ammonium-modified nano-silica, and acetonitrile was 3.5–7:6–8:80–160.

2. The biaxially oriented cast polypropylene composite film according to claim 1, characterized in that, It contains the following raw materials in parts by weight: 60-80 parts homopolymer polypropylene, 20-40 parts copolymer polypropylene, 0.05-0.15 parts slip agent, 0.1-0.5 parts nucleating agent, 0.1-0.3 parts antioxidant, and 2-3 parts modified nano silica.

3. The biaxially oriented cast polypropylene composite film according to claim 1, characterized in that, The copolymer polypropylene is a propylene-ethylene copolymer, and the slip agent is one or a mixture of erucamide and oleamide.

4. The biaxially oriented cast polypropylene composite film according to claim 1, characterized in that, The nucleating agent is a sorbitol derivative, and the antioxidant is one or more of butylated hydroxytoluene, tert-butylhydroquinone, and propyl gallate.

5. The biaxially oriented cast polypropylene composite film according to claim 1, characterized in that, The initiator is one of benzoyl peroxide and azobisisobutyronitrile, the grafting solution is an aqueous solution of ethanol with a volume fraction of 40-80%, and the particle size of the nano-silica is 1-50 nm.

6. A method for preparing a biaxially oriented cast polypropylene composite film as described in any one of claims 1 to 5, characterized in that, The process includes the following steps: weighing each raw material according to the mass fraction, drying the raw materials and adding them into a twin-screw extruder, then melting and extruding, casting, longitudinal stretching, transverse stretching, heat setting, cooling, corona treatment, and traction winding to obtain the biaxially stretched cast polypropylene composite film.

7. The method for preparing a biaxially oriented cast polypropylene composite film according to claim 6, characterized in that, The temperature settings for the twin-screw extruder are: Zone 1 180-190℃, Zone 2 200-210℃, Zone 3 210-220℃, and Zone 4 215-225℃.

8. The method for preparing a biaxially oriented cast polypropylene composite film according to claim 6, characterized in that, The casting process involves extruding the melt and casting it onto a cooling roller at 25-35°C to form a homogeneous casting sheet with a thickness of 0.3-0.5 mm.

9. The method for preparing a biaxially oriented cast polypropylene composite film according to claim 6, characterized in that, The longitudinal stretching temperature is 125–135℃, the longitudinal stretching ratio is 3–5:1, the transverse stretching temperature is 155–165℃, the transverse stretching ratio is 8–10:1, and the heat setting temperature is 160–170℃.