A polypropylene composite film for packaging and a method for preparing the same

Abstract

The application discloses a polypropylene composite film for packaging, characterized by outer, middle and inner layers which are respectively prepared by polypropylene films with different raw material ratios and preparation methods. The three-layer film is differentiated and comprises 70-90% of PP, 3-15% of maleic anhydride modified polypropylene, 5-10% of UHMWPP, 0.5-3% of a flow promoter and 0.5-3% of a nucleating agent. The UHMWPP is pretreated and post-treated to improve its processing performance in the composite film. The three-layer film is prepared by bidirectional, unidirectional stretching and flow casting. In addition, the polypropylene composite film prepared by the application can achieve a higher recycling rate through specific flow promoters and nucleating agents.

Description

Technical Field

[0001] This invention relates to the field of composite film technology, and specifically to a polypropylene composite film for packaging and its preparation method. Background Technology

[0002] Traditional composite films often use non-single-material structures to achieve multiple functions and properties, which is not conducive to recycling and reuse. In contrast, environmentally friendly film materials are usually made of single-component thermoplastic materials, and they also need to have the printability, barrier properties, heat-sealing properties of traditional composite films, while maintaining a single component accounting for more than 80% of the total weight of the composite film.

[0003] The invention patent application with publication number CN114851666A discloses a composite membrane made of a single polyolefin material, which consists of an outer printing layer, an intermediate barrier layer and an inner sealing layer. The three layers are bonded together with a two-component polyurethane. Before aluminizing the membrane material, polyvinyl alcohol is coated to increase the barrier properties, homopolymer polypropylene is added to the formula, and a heat-resistant coating is applied to improve the temperature resistance of the composite membrane.

[0004] Chinese patent CN114714723B discloses a polypropylene-based recyclable packaging film and its preparation method, comprising a CPP inner film layer, a MOPP intermediate film layer, and a BOPP surface film layer connected in sequence. The key technical points are the addition of modified carbon black and modified zinc oxide to the BOPP surface layer; the addition of modified calcium carbonate to the MOPP intermediate film layer; and the inclusion of binary copolymer polypropylene and ternary copolymer polypropylene in the CPP inner layer to improve the photo-aging resistance of the PP film and reduce its breakage rate. However, this method requires multiple modifications in the pretreatment process, making the process extremely complex and introducing various modifiers, which inconveniences the subsequent recycling of packaging materials. Therefore, it is necessary to develop a polypropylene composite film with a simpler process that improves its mechanical strength while also facilitating environmental recycling. Summary of the Invention

[0005] The purpose of this invention is to overcome the problem of complex production processes for modified polypropylene composite membranes in the prior art, and to develop a polypropylene composite membrane with a simpler production process, better mechanical properties, and a simpler composition for easy recycling.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0007] A polypropylene composite film for packaging, comprising, from the outside to the inside:

[0008] The outer layer, by weight percentage, comprises 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent.

[0009] The middle layer, by weight percentage, comprises 75-90% PP, 3-8% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent.

[0010] The inner layer, by weight percentage, comprises 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent.

[0011] Furthermore, at least one of the outer layer, middle layer, and inner layer, the UHMWPP is pretreated, and the pretreatment process includes: melting the foaming agent and UHMWPP powder together in a sealed container at 180~230°C, and then extruding and granulating, wherein the mass ratio of the foaming agent to the UHMWPP powder is (1~2):10.

[0012] Furthermore, the UHMWPP undergoes post-processing, which includes annealing the pre-treated UHMWPP. The annealing temperature is 80~140℃, and the annealing rate is 0.5~2℃ / min.

[0013] Furthermore, the outer layer is also made into BOPP through a biaxial stretching process;

[0014] The middle layer is also made of MOPP through a unidirectional stretching process;

[0015] The inner layer is also made of CPP through a casting process.

[0016] Furthermore, the flow promoter is a polypropylene-based flow promoter, preferably polypropylene wax, polyacrylonitrile, polyacryl alcohol, or polyacrylamide.

[0017] Furthermore, the nucleating agent is a polymer nucleating agent, preferably a vinyl cycloalkane polymer and a vinyl alkane polymer.

[0018] Furthermore, the foaming agent is an azo compound or a peroxide compound, wherein the azo compound is preferably azodicarbonamide, azobisisobutyronitrile, or azodimethylurea; and the peroxide compound is preferably hydrogen peroxide.

[0019] A method for laminating a polypropylene composite film for packaging:

[0020] S1: Accurately weigh the corresponding raw materials for the outer membrane, middle membrane, and inner membrane into the reactor according to weight percentage, mix them at 3~8MPa and 190~240℃, and react for 4~6 hours;

[0021] S2: Take out the raw materials corresponding to the outer layer membrane, middle layer membrane and inner layer membrane obtained in S1, cool and granulate them respectively, and then obtain the outer layer membrane, middle layer membrane and inner layer membrane through biaxial stretching process, uniaxial stretching process and casting process.

[0022] S3: The outer membrane, middle membrane, and inner membrane are combined into one unit through a dry lamination process;

[0023] S4: The composite membrane obtained in S3 is aged at 40~80℃ for more than 48 hours.

[0024] Furthermore, the dry lamination process includes sequentially placing the outer film, middle film, and inner film into three winding racks of a dry laminating machine, applying 2.5~5.2 g / m² of adhesive between the outer film and the middle film, and between the middle film and the inner film, and then laminating them together with a laminating roller at a pressure of 5~9 bar using unwinding tensions of 3~6 kgf, 4~8 kgf, and 3~6 kgf, respectively.

[0025] Furthermore, the adhesive is a thermoplastic acrylate adhesive, preferably a solvent-free acrylate adhesive, such as cyanoacrylate adhesive.

[0026] The advantages and beneficial effects of this invention are as follows:

[0027] 1. By adding different proportions of PP, maleic anhydride modified polypropylene, UHMWPP, flow promoter and nucleating agent to the outer, middle and inner layers and optimizing the ratio, the composite membrane can significantly improve its mechanical strength while maintaining its flexibility.

[0028] 2. UHMWPP undergoes pretreatment and posttreatment. The foaming agent in the pretreatment is eutectic with UHMWPP powder to improve the melt flowability of UHMWPP, making it easier to form during processing, and thus having better dispersibility and stability in the composite film.

[0029] 3. The outer, middle, and inner films are fabricated using biaxial stretching, uniaxial stretching, and casting processes, respectively, giving each film different properties to meet packaging requirements. Furthermore, through optimization, all components can be made from polymers or green reagents, enabling 100% recyclability of the packaging.

[0030] 4. The dry lamination process ensures that each membrane layer is firmly bonded together, improving the overall performance of the composite membrane. The curing process further stabilizes the composite membrane's performance and extends the product's lifespan. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the present invention;

[0032] In the diagram: 1-outer layer, 2-middle layer, 3-inner layer. Detailed Implementation

[0033] This invention provides a polypropylene composite film for packaging, comprising a three-layer structure of an outer layer, a middle layer, and an inner layer with similar compositions. The main resin component in the three-layer structure is PP (polypropylene), which provides good mechanical strength and chemical resistance, ensuring the basic structure of the composite film. One of the minor components is maleic anhydride-modified polypropylene, which enhances the compatibility of PP with other materials through chemical bonding, improves the interfacial adhesion of the composite material, and, with an addition of 10-15%, helps improve the interlayer bonding of the composite film.

[0034] UHMWPP (ultra-high molecular weight polypropylene) is a thermoplastic engineering plastic with a viscosity-average molecular weight of over one million. Its molecular weight far exceeds that of ordinary polypropylene, giving UHMWPP extraordinary strength and making it resistant to damage from significant external forces. However, this property also presents challenges in its production and processing. Specifically, this extremely high molecular weight leads to a significant increase in the melt viscosity of UHMWPP. This means that higher temperatures and pressures are required for UHMWPP to flow during processing. To improve flowability, it is often introduced in small quantities and co-extruded with lower molecular weight polypropylene for processing.

[0035] To achieve the introduction of UHMWPP, the method adopted in this invention also includes adding maleic anhydride-modified polypropylene. On the one hand, the maleic anhydride polar groups of the modified polypropylene improve the interfacial compatibility and adhesion between PP and UHMWPP. On the other hand, the addition of maleic anhydride-modified polypropylene can increase the crystallinity of the PP component in the composite material, so as to help PP form a stable phase structure under high temperature and high pressure conditions. This helps to effectively disperse stress in the composite material, thereby improving the mechanical properties of the material.

[0036] It is easy to understand that, to enhance the above effects, the technical solution provided by this invention further includes the addition of a flow promoter and a nucleating agent. The flow promoter primarily reduces the viscosity of the polypropylene melt and UHMWPP, making the blending process smoother. Furthermore, the flow promoter helps improve the dispersibility of UHMWPP in PP, resulting in a more uniform mixture and easier film processing. The introduction of the nucleating agent increases the crystallinity of the UHMWPP / PP blend, helping to refine the grain size and improve the mechanical properties of the material.

[0037] UHMWPP is also preferably pretreated and post-treated. The pretreatment process involves co-melting UHMWPP powder with a foaming agent and then extruding and granulating it. Specifically, UHMWPP powder and a selected foaming agent are mixed in a sealed container at a specific ratio. This ensures that the foaming agent is effectively dispersed in the UHMWPP matrix during the subsequent co-melting process and generates sufficient gas under appropriate conditions to achieve the foaming effect. The mixture is then added to an extruder and heated and mixed within a temperature range of 180–230°C. Within this temperature range, the UHMWPP powder gradually melts, forming a continuous melt phase. Simultaneously, the foaming agent begins to decompose at high temperatures, releasing a large amount of gas. This gas is rapidly captured by the UHMWPP melt, causing the melt volume to expand and form a microporous structure. The fully mixed and foamed melt is then extruded into strips or granules using a conventional extruder at approximately 250°C and relatively low pressure (approximately 2 MPa), maintaining its porosity and microporous structure, and then cooled and solidified. These particles or strips are the pretreated UHMWPP material, which has a lighter weight, a certain porosity, and better processing characteristics. The pretreated UHMWPP is annealed at 80–140°C at a rate of 0.5–2°C / min. Annealing helps to eliminate internal stress, stabilize the microporous structure, and improve dimensional stability and mechanical strength.

[0038] After undergoing a series of processing steps, UHMWPP, when combined with polypropylene to form a composite material, exhibits processing characteristics closely similar to pure polypropylene. This allows for further processing of each layer, including biaxial stretching (longitudinal stretch ratio of 3, transverse stretch ratio of 2), uniaxial stretching (longitudinal stretch ratio of 1.5), and casting processes for the outer, middle, and inner layers, respectively, to produce BOPP, MOPP (also known as MDOPP), and CPP. Biaxial stretching stretches the film in both longitudinal and transverse directions, causing the molecular chains to align along the stretching direction. After biaxial stretching, the tensile strength, tear strength, and heat-sealing strength of the film are improved, while maintaining good optical properties. This allows for clear display of items within the packaging, enhancing product aesthetics and shelf appeal. Uniaxial stretching stretches the film in only one direction (typically, in the embodiments described herein, longitudinal), causing the molecular chains to align in that direction, thereby significantly improving the film's mechanical properties, such as tensile strength and modulus. Casting involves extruding molten polymer through a slit die and then cooling and solidifying it on cooling rollers to form a film. The casting process gives the film good heat-sealing properties and surface smoothness, and it can be used as an inner layer to achieve the technical effect of heat sealing.

[0039] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0040] In each embodiment, unless otherwise specified, the nucleating agent is vinylcyclohexane and the adhesive is cyanoacrylate.

[0041] Example 1

[0042] A polypropylene composite film for packaging is prepared by the following method:

[0043] Accurately weigh PP, maleic anhydride-modified polypropylene (UHMWPP), flow promoter, and nucleating agent into a reactor. Mix and react at 6 MPa and 190℃ for 6 hours to obtain a single-layer film consisting of an inner, middle, and outer layer. Then, place the outer, middle, and inner layers sequentially into the three winding racks of a dry laminator. Apply 6.2 g / m² adhesive between the outer and middle layers, and between the middle and inner layers. Then, laminate them together using a laminating roller with a pressure of 6 bar at unwinding tensions of 6 kgf, 8 kgf, and 6 kgf, respectively. Cure the composite film at 40℃ for 48 hours.

[0044] The composition of the polypropylene composite film for packaging is as follows:

[0045] The outer layer consists of 74% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0046] The middle layer consists of 82% PP, 8% maleic anhydride modified polypropylene, 8% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0047] The inner layer consists of 74% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0048] The flow promoter is polypropylene wax, and the foaming agent is azodicarbonamide.

[0049] Example 2

[0050] A polypropylene composite film for packaging is prepared by the following method:

[0051] Accurately weigh PP, maleic anhydride-modified polypropylene (UHMWPP), flow promoter, and nucleating agent into a reactor. Mix and react at 8 MPa and 230℃ for 4 hours to obtain a single-layer film consisting of an inner, middle, and outer layer. Then, place the outer, middle, and inner layers sequentially into the three winding racks of a dry laminator. Apply 7.8 g / m² adhesive between the outer and middle layers, and between the middle and inner layers. Then, laminate them together using a laminating roller with a pressure of 7 bar at unwinding tensions of 6 kgf, 7 kgf, and 6 kgf, respectively. Cure the composite film at 50℃ for 48 hours.

[0052] The composition of the polypropylene composite film for packaging is as follows:

[0053] The outer layer consists of 82% PP, 10% maleic anhydride modified polypropylene, 6% UHMWPP, 1.5% flow promoter, and 0.5% nucleating agent.

[0054] The middle layer consists of 82% PP, 8% maleic anhydride modified polypropylene, 8% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0055] The inner layer consists of 74% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0056] In this process, each layer of UHMWPP undergoes pretreatment, and the foaming agent and UHMWPP powder are co-melted at 180°C in a sealed container, followed by extrusion granulation. The mass ratio of the foaming agent to the UHMWPP powder is 1:10.

[0057] The flow promoter is polyacrylonitrile, and the foaming agent is azodicarbonamide.

[0058] Example 3

[0059] A polypropylene composite film for packaging is prepared by the following method:

[0060] Accurately weigh PP, maleic anhydride-modified polypropylene (UHMWPP), flow promoter, and nucleating agent into a reactor. Mix and react at 7 MPa and 40°C for 4 hours to obtain a single-layer film consisting of an inner, middle, and outer layer. Then, place the outer, middle, and inner layers sequentially into the three winding racks of a dry laminator. Apply 6.8 g / m² of adhesive between the outer and middle layers, and between the middle and inner layers. Then, laminate them together using a laminating roller with a pressure of 5 bar at unwinding tensions of 5 kgf, 8 kgf, and 4 kgf, respectively. Cure the composite film at 50°C for 72 hours.

[0061] The composition of the polypropylene composite film for packaging is as follows:

[0062] The outer layer consists of 82% PP, 10% maleic anhydride modified polypropylene, 7% UHMWPP, 1.5% flow promoter, and 1.5% nucleating agent.

[0063] The middle layer consists of 77% PP, 8% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 3% nucleating agent.

[0064] The inner layer consists of 74% PP, 15% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 3% nucleating agent.

[0065] In this process, each layer of UHMWPP undergoes pretreatment, and the foaming agent and UHMWPP powder are co-melted at 190°C in a sealed container, followed by extrusion granulation. The mass ratio of the foaming agent to the UHMWPP powder is 1:10.

[0066] The flow promoter is polyacrylol, and the foaming agent is azodimethylurea.

[0067] Example 4

[0068] A polypropylene composite film for packaging is prepared by the following method:

[0069] Accurately weigh PP, maleic anhydride-modified polypropylene (UHMWPP), flow promoter, and nucleating agent into a reactor. Mix and react at 3 MPa and 200℃ for 5 hours to obtain a single-layer film consisting of an inner, middle, and outer layer. Then, place the outer, middle, and inner layers sequentially into the three winding racks of a dry laminator. Apply 8.8 g / m² adhesive between the outer and middle layers, and between the middle and inner layers. Then, laminate them together using a laminating roller with a pressure of 9 bar at unwinding tensions of 6 kgf, 4 kgf, and 6 kgf, respectively. Cure the composite film at 45℃ for 72 hours.

[0070] The composition of the polypropylene composite film for packaging is as follows:

[0071] The outer layer consists of 70% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 3% flow promoter, and 3% nucleating agent.

[0072] The middle layer consists of 90% PP, 3% maleic anhydride modified polypropylene, 5% UHMWPP, 1.5% flow promoter, and 0.5% nucleating agent.

[0073] The inner layer consists of 82% PP, 10% maleic anhydride modified polypropylene, 7% UHMWPP, 1.5% flow promoter, and 1.5% nucleating agent.

[0074] In this process, each layer of UHMWPP undergoes pretreatment and post-treatment. The foaming agent and UHMWPP powder are co-melted at 220°C in a sealed container, followed by extrusion granulation. The mass ratio of the foaming agent to the UHMWPP powder is 2:10. After cooling to room temperature, the UHMWPP undergoes annealing. The room-temperature UHMWPP particles are heated to 140°C and then cooled to room temperature at an annealing rate of 2°C / min.

[0075] The flow promoter is polyacrylamide, and the foaming agent is azodicarbonamide.

[0076] Example 5

[0077] A polypropylene composite film for packaging is prepared by the following method:

[0078] Accurately weigh PP, maleic anhydride-modified polypropylene (UHMWPP), flow promoter, and nucleating agent into a reactor. Mix and react at 6 MPa and 230℃ for 5 hours to obtain a single-layer film consisting of an inner, middle, and outer layer. Then, place the outer, middle, and inner layers sequentially into the three winding racks of a dry laminator. Apply 5.2 g / m² adhesive between the outer and middle layers, and between the middle and inner layers. Then, laminate them together using a laminating roller with a pressure of 9 bar at unwinding tensions of 6 kgf, 8 kgf, and 6 kgf, respectively. Cure the composite film at 50℃ for 48 hours.

[0079] The composition of the polypropylene composite film for packaging is as follows:

[0080] The outer layer consists of 76% PP, 10% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 2% nucleating agent.

[0081] The middle layer consists of 78% PP, 11% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0082] The inner layer consists of 78% PP, 12% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 2% nucleating agent.

[0083] In this process, each layer of UHMWPP undergoes pretreatment and post-treatment. The foaming agent and UHMWPP powder are co-melted at 190°C in a sealed container, followed by extrusion granulation. The mass ratio of the foaming agent to the UHMWPP powder is 1.5:10. After cooling to room temperature, the UHMWPP undergoes annealing. The room-temperature UHMWPP particles are heated to 90°C and then cooled to room temperature at an annealing rate of 1°C / min.

[0084] The flow promoter is polypropylene wax, and the foaming agent is azobisisobutyronitrile.

[0085] Example 6

[0086] A polypropylene composite film for packaging is prepared by the following method:

[0087] Accurately weigh PP, maleic anhydride-modified polypropylene (UHMWPP), flow promoter, and nucleating agent into a reactor. Mix and react at 3 MPa and 230℃ for 5 hours to obtain a single-layer film consisting of an inner, middle, and outer layer. Then, place the outer, middle, and inner layers sequentially into the three winding racks of a dry laminator. Apply 8.8 g / m² adhesive between the outer and middle layers, and between the middle and inner layers. Then, laminate them together using a laminating roller with a pressure of 4 bar at unwinding tensions of 4 kgf, 5 kgf, and 6 kgf, respectively. Cure the composite film at 40℃ for 48 hours.

[0088] The composition of the polypropylene composite film for packaging is as follows:

[0089] The outer layer consists of 76% PP, 10% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 2% nucleating agent.

[0090] The middle layer consists of 82% PP, 8% maleic anhydride modified polypropylene, 8% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0091] The inner layer consists of 74% PP, 15% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 3% nucleating agent.

[0092] In this process, each layer of UHMWPP undergoes pretreatment and post-treatment. The foaming agent and UHMWPP powder are co-melted at 230°C in a sealed container, followed by extrusion granulation. The mass ratio of the foaming agent to the UHMWPP powder is 2:10. After cooling to room temperature, the UHMWPP undergoes annealing. The room-temperature UHMWPP particles are heated to 110°C and then cooled to room temperature at an annealing rate of 0.5°C / min.

[0093] The flow promoter is polypropylene wax, and the foaming agent is azodicarbonamide.

[0094] Example 7

[0095] A polypropylene composite film for packaging is prepared by the following method:

[0096] Accurately weigh PP, maleic anhydride-modified polypropylene (UHMWPP), flow promoter, and nucleating agent into a reactor. Mix and react at 5 MPa and 240℃ for 6 hours to obtain a single-layer film consisting of an inner, middle, and outer layer. The outer layer film is biaxially stretched to form BOPP, the middle layer film is uniaxially stretched to form MOPP, and the inner layer film is cast to form CPP. Then, the outer, middle, and inner layers are sequentially placed into the three winding racks of a dry laminator. Apply 6.4 g / m² adhesive between the outer and middle layers, and between the middle and inner layers. Then, laminate them together using a 9 bar pressure laminating roller at unwinding tensions of 5 kgf, 8 kgf, and 4 kgf, respectively. Cure the composite film at 50℃ for 48 hours.

[0097] The composition of the polypropylene composite film for packaging is as follows:

[0098] The outer layer consists of 74% PP, 15% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 3% nucleating agent.

[0099] The middle layer consists of 78% PP, 11% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0100] The inner layer consists of 76% PP, 10% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 2% nucleating agent.

[0101] In this process, each layer of UHMWPP undergoes pretreatment and post-treatment. The foaming agent and UHMWPP powder are co-melted at 230°C in a sealed container, followed by extrusion granulation. The mass ratio of the foaming agent to the UHMWPP powder is 2:10. After cooling to room temperature, the UHMWPP undergoes annealing. The room-temperature UHMWPP particles are heated to 120°C and then cooled to room temperature at an annealing rate of 1°C / min.

[0102] The flow promoter is polypropylene wax, and the foaming agent is hydrogen peroxide.

[0103] According to GB / T 13022-1991 "Test Method for Tensile Properties of Plastic Films", the tensile strength (MPa), Young's modulus (MPa), tensile yield stress (MPa) and elongation at break (%) of the packaging films prepared in each embodiment with a three-layer thickness of 20μm were tested, and the results are shown in Table 1.

[0104] Table 1

[0105]

[0106] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1.A polypropylene composite film for packaging, comprising, from outside to inside: an outer layer comprising, by weight percentage, 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; a middle layer comprising, by weight percentage, 75-90% PP, 3-8% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; and an inner layer comprising, by weight percentage, 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; wherein the UHMWPP in at least one of the outer layer, the middle layer and the inner layer is pretreated by a process comprising: co-melting a foaming agent and UHMWPP powder in a closed container at 180-230℃, and then extruding and granulating, the mass ratio of the foaming agent to the UHMWPP powder being 1-2:10; and the UHMWPP is further post-treated by a process comprising: annealing the pretreated UHMWPP at a temperature of 80-140℃ at an annealing rate of 0.5-2℃ / min; the outer layer is made into BOPP by a biaxial stretching process; the middle layer is made into MOPP by a uniaxial stretching process; and the inner layer is made into CPP by a casting process; the flow promoter is a polypropylene-based flow promoter; the nucleating agent is a polymeric nucleating agent; and the foaming agent is an azo compound or a peroxide compound. 1.A polypropylene composite film for packaging, comprising, from outside to inside: an outer layer comprising, by weight percentage, 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; a middle layer comprising, by weight percentage, 75-90% PP, 3-8% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; and an inner layer comprising, by weight percentage, 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; wherein the UHMWPP in at least one of the outer layer, the middle layer and the inner layer is pretreated by a process comprising: co-melting a foaming agent and UHMWPP powder in a closed container at 180-230℃, and then extruding and granulating, the mass ratio of the foaming agent to the UHMWPP powder being 1-2:10; and the UHMWPP is further post-treated by a process comprising: annealing the pretreated UHMWPP at a temperature of 80-140℃ at an annealing rate of 0.5-2℃ / min; the outer layer is made into BOPP by a biaxial stretching process; the middle layer is made into MOPP by a uniaxial stretching process; and the inner layer is made into CPP by a casting process; the flow promoter is a polypropylene-based flow promoter; the nucleating agent is a polymeric nucleating agent; and the foaming agent is an azo compound or a peroxide compound. 1.A polypropylene composite film for packaging, comprising, from outside to inside: an outer layer comprising, by weight percentage, 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; a middle layer comprising, by weight percentage, 75-90% PP, 3-8% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; and an inner layer comprising, by weight percentage, 70-82% PP, 10-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent; wherein the UHMWPP in at least one of the outer layer, the middle layer and the inner layer is pretreated by a process comprising: co-melting a foaming agent and UHMWPP powder in a closed container at 180-230℃, and then extruding and granulating, the mass ratio of the foaming agent to the UHMWPP powder being 1-2:10; and the UHMWPP is further post-treated by a process comprising: annealing the pretreated UHMWPP at a temperature of 80-140℃ at an annealing rate of 0.5-2℃ / min; the outer layer is made into BOPP by a biaxial stretching process; the middle layer is made into MOPP by a uniaxial stretching process; and the inner layer is made into CPP by a casting process; the flow promoter is a polypropylene-based flow promoter; the nucleating agent is a polymeric nucleating agent; and the foaming agent is an azo compound or a peroxide compound. ​ ​ ​ ​ ​ ​ 2. The polypropylene composite film according to claim 1, characterized in that: ​ 3. The polypropylene composite film according to any one of claim 1, characterized in that: ​ 4. The polypropylene composite film according to any one of claim 1, characterized in that: ​ 5. A method for producing the polypropylene composite film according to any one of claims 1 to 4, characterized by, ​ ​ ​ ​ ​ 6. The method for preparing the polypropylene composite membrane according to claim 5, characterized in that: ​ 7. The method for preparing the polypropylene composite membrane according to claim 6, characterized in that: ​

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