Polypropylene composite film for packaging and preparation method thereof

Abstract

The invention discloses a polypropylene composite film for packaging, which is characterized in that an outer layer, a middle layer and an inner layer are respectively prepared from polypropylene films with different raw material proportions and manufacturing methods. The polypropylene film is prepared from 70 to 90 percent of PP (Polypropylene), 3 to 15 percent of maleic anhydride modified polypropylene, 5 to 10 percent of UHMWPP (Ultra High Molecular Weight Polypropylene), 0.5 to 3 percent of a flow promoter and 0.5 to 3 percent of a nucleating agent. The UHMWPP is subjected to pretreatment and post-treatment, so that the processability of the UHMWPP in the composite film is improved. The three layers of films are prepared by adopting two-way and one-way stretching and tape casting characteristics respectively. In addition, by means of the specific flow promoter and the nucleating agent, the polypropylene composite film prepared through the method can achieve higher recovery rate.

Description

Technical Field

[0001] The invention relates to the technical field of composite films, and in particular to a polypropylene composite film for packaging and a preparation method thereof. Background Art

[0002] In order to obtain multiple functions and characteristics, traditional composite films are often made of non-single material structures, which is not conducive to recycling and reuse. Environmentally friendly film materials are usually made of single-component thermoplastic materials. They also need to have the printability, barrier properties, heat sealing properties and other properties of traditional composite films, and keep the 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 single polyolefin material composite film, including an outer printing layer, a middle barrier layer and an inner sealing layer. The three layers of material are bonded by two-component polyurethane. Polyvinyl alcohol is coated on the film before aluminum plating to increase the barrier properties, homopolymer polypropylene is added to the formula, and a temperature-resistant coating is applied to improve the temperature resistance of the composite film.

[0004] The invention patent with the authorization number CN114714723B discloses a polypropylene type recyclable packaging film and its preparation method, including a CPP film inner layer, a MOPP film middle layer and a BOPP film surface layer connected in sequence. The technical points are that modified carbon black and modified zinc oxide are added to the BOPP surface layer; modified calcium carbonate is added to the MOPP film middle layer; the CPP film inner layer includes binary copolymer polypropylene and ternary copolymer polypropylene, which are used to improve the light aging resistance of the PP film and reduce its rupture rate. However, the pre-treatment process of this solution requires multiple modifications, the process is extremely complicated and introduces a variety of modifiers, which brings inconvenience to the subsequent recycling of packaging materials. Therefore, it is necessary to develop a polypropylene composite film with a simpler process, which can improve its mechanical strength while being conducive to environmentally friendly recycling. Summary of the invention

[0005] The purpose of the present invention is to overcome the problem of complicated production process of modified polypropylene composite film in the prior art and develop a polypropylene composite film with simpler production process, good mechanical properties and relatively simple composition for easy recycling.

[0006] To achieve the above purpose, the technical solution provided by the present invention is as follows: A polypropylene composite film for packaging, which is arranged from outside to inside in sequence and comprises: The outer layer includes, 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; The middle layer includes, 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; The inner layer includes, 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.

[0007] Furthermore, in at least one of the outer layer, the middle layer and the inner layer, the UHMWPP is pretreated, and the pretreatment process comprises: melting the foaming agent and the UHMWPP powder at 180-230° C. in a closed container, and then extruding and granulating, and the mass ratio of the foaming agent to the UHMWPP powder is (1-2):10.

[0008] Furthermore, the UHMWPP is post-treated, and the post-treatment process includes: annealing the UHMWPP obtained by the pre-treatment. The annealing temperature is 80-140°C, and the annealing rate is 0.5-2°C / min.

[0009] Furthermore, the outer layer is also made into BOPP by a biaxial stretching process; The middle layer is also made of MOPP through a uniaxial stretching process; The inner layer is also made of CPP through a casting process.

[0010] Furthermore, the flow promoter is a polypropylene-based flow promoter, preferably polypropylene wax, polyacrylonitrile, polypropylene alcohol, or polyacrylamide.

[0011] Furthermore, the nucleating agent is a polymer nucleating agent, preferably a vinyl cycloalkane polymer and a vinyl alkane polymer.

[0012] Furthermore, the foaming agent is an azo compound or a peroxide compound, the azo compound is preferably azodicarbonamide, azobisisobutyronitrile, azodimethylurea; the peroxide compound is preferably hydrogen peroxide.

[0013] A composite method for polypropylene composite film for packaging: S1: According to the weight percentage, accurately weigh 55-82% PP, 3-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent in a reactor, mix well at 3-8 MPa, 190-240°C, and react for 4-6 hours; S2: taking out the polypropylene resin obtained in S1, cooling and granulating it, and then using a biaxial stretching process, a uniaxial stretching process, and a casting process to prepare an outer layer film, a middle layer film, and an inner layer film; S3: The outer film, the middle film and the inner film are compounded into one by dry process; S4: Curing the composite film obtained in S3 at 40-80° C. for more than 48 hours.

[0014] Furthermore, the dry lamination includes placing the outer layer film, the middle layer film, and the inner layer film into three winding frames of the dry laminating machine in sequence, applying 2.5~5.2g / ㎡ adhesive between the outer layer film and the middle layer film, and between the middle layer film and the inner layer film, and then laminating them into one with a laminating roller at a pressure of 5~9bar at unwinding tensions of 3~6kgf, 4~8kgf, and 3~6kgf, respectively.

[0015] Furthermore, the adhesive is a thermoplastic acrylic adhesive, preferably a solvent-free acrylic adhesive, such as cyanoacrylate adhesive.

[0016] The advantages and beneficial effects of the present invention are: 1. By adding different proportions of PP, maleic anhydride modified polypropylene, UHMWPP, flow promoter and nucleating agent into the outer layer, middle layer and inner layer and optimizing the ratio, the composite film significantly improves the mechanical strength while maintaining flexibility.

[0017] 2. UHMWPP undergoes pretreatment and post-treatment. The foaming agent in the pretreatment is fused with the UHMWPP powder to improve the melt fluidity of UHMWPP, making it easier to form during the processing, and thus having better dispersibility and stability in the composite film.

[0018] 3. The outer film, middle film and inner film are made through biaxial stretching process, uniaxial stretching process and casting process respectively, so that each layer of film has different properties to meet the packaging requirements. And through optimization, various components can use polymers or green reagents, so that the packaging can achieve 100% recycling rate.

[0019] 4. The dry composite process enables each layer of the film to be firmly combined together, improving the overall performance of the composite film. The aging process makes the performance of the composite film more stable and extends the service life of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the structure of the present invention; In the figure: 1-outer layer, 2-middle layer, 3-inner layer. DETAILED DESCRIPTION

[0021] The present 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 components, wherein the main resin component in the three-layer structure is composed of PP (polypropylene), which is used to provide good mechanical strength and chemical resistance and ensure the basic structure of the composite film. One of the secondary components is maleic anhydride-modified polypropylene, which has the functions of enhancing the compatibility of PP with other materials through chemical bonding and improving the interfacial bonding of the composite material, and an addition amount of 10-15% is helpful to improve the interlayer bonding of the composite film.

[0022] UHMWPP (ultra-high molecular weight polypropylene) is a thermoplastic engineering plastic with a viscosity-average molecular weight of more than one million. Its molecular weight is much higher than that of ordinary polypropylene, so UHMWPP exhibits extraordinary strength and can withstand large external forces without being easily damaged. However, this property also brings inconvenience to its production and processing. Specifically, this extremely high molecular weight causes the melt viscosity of UHMWPP to increase significantly. This means that during processing, UHMWPP has to use higher temperatures and pressures to flow. In order to improve fluidity, it is introduced in small amounts and blended with low molecular weight polypropylene for extrusion for processing.

[0023] In order to achieve the introduction of UHMWPP, the method adopted by the present invention also includes adding maleic anhydride modified polypropylene. On the one hand, the maleic anhydride polar group of maleic anhydride modified polypropylene is used to 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, which helps to effectively disperse stress in the composite material, thereby improving the mechanical properties of the material.

[0024] It is not difficult to understand that in order to enhance the above effects, the technical solution provided by the present invention also includes: adding a flow promoter and a nucleating agent. The flow promoter is mainly used to reduce the viscosity of the polypropylene melt and UHMWPP, so that the blending process of the two is smoother. And the flow promoter helps to improve the dispersibility of UHMWPP in PP, thereby making the two mixed more uniform and easy to process into thin films. The introduction of the nucleating agent is to increase the crystallinity of the UHMWPP and PP blend, which helps to refine the grain size and improve the mechanical properties of the material.

[0025] UHMWPP is also preferably pre-treated and post-treated. The pre-treatment process is to extrude and granulate the UHMWPP powder after co-melting with the foaming agent. Specifically, the UHMWPP powder is mixed with the selected foaming agent in a closed container in a specific ratio. Ensure that in the subsequent co-melting process, the foaming agent can be effectively dispersed in the UHMWPP matrix and produce enough gas under appropriate conditions to achieve the foaming effect. The above mixture is added to the extruder and heated and mixed at a temperature range of 180~230℃. In this temperature range, the UHMWPP powder gradually melts to form a continuous melt phase. At the same time, the foaming agent begins to decompose at high temperature and releases a large amount of gas. These gases are quickly captured by the UHMWPP melt, causing the melt volume to expand and form a microporous structure. The melt after being fully mixed and foamed is extruded into strips or granules in a conventional extruder at a high temperature of about 250℃ and a relatively low pressure (about 2MPa), maintaining its porosity and microporous structure unchanged, and then cooled and solidified. These particles or strips are pretreated UHMWPP materials, which have lighter weight, certain porosity and better processing characteristics. The pretreated UHMWPP is annealed at 80~140℃, with an annealing rate of 0.5~2℃ / min. Annealing helps to eliminate stress inside the material, stabilize the microporous structure, and improve dimensional stability and mechanical strength.

[0026] After a series of treatment processes, UHMWPP forms a composite material with polypropylene, and the processing characteristics are close to those of pure polypropylene materials, so that each layer can be further processed, including biaxial stretching (longitudinal stretching ratio of 3, transverse stretching ratio of 2), uniaxial stretching (longitudinal stretching ratio of 1.5) and casting process for the outer layer, middle layer and inner layer, respectively, to obtain BOPP, MOPP (also known as MDOPP) and CPP. The biaxial stretching process stretches the film in both the longitudinal and transverse directions, resulting in the orientation of the molecular chains 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, which enables the items in the package to be clearly displayed, improving the aesthetics and shelf appeal of the product. The uniaxial stretching process stretches the film only in one direction (usually, that is, in the embodiment to which the present invention relates, in the longitudinal direction), resulting in the orientation of the molecular chains in this direction, thereby significantly improving the mechanical properties of the film in this direction, such as tensile strength and modulus. The casting process forms a film by extruding the molten polymer through a slit die and then cooling and solidifying it on a cooling roller. The cast film process gives the film good heat-sealing properties and surface smoothness, and can be used as an inner layer to achieve the technical effect of heat sealing.

[0027] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0028] In each embodiment, unless otherwise specified, the nucleating agent is vinyl cyclohexane and the adhesive is cyanoacrylate.

[0029] Example 1 A polypropylene composite film for packaging, the preparation method of which is as follows: Accurately weigh PP, maleic anhydride modified polypropylene UHMWPP, flow promoter, and nucleating agent in a reactor, mix and react at 6MPa, 190℃ for 6h to obtain inner, middle, and outer monolayer films. Then put the outer, middle, and inner films into the three winding racks of the dry laminating machine in turn, apply 6.2g / ㎡ adhesive between the outer and middle films, and between the middle and inner films, and then laminate them into one with unwinding tensions of 6kgf, 8kgf, and 6kgf, respectively, through a laminating roller with a pressure of 6bar. The composite film is aged at 40℃ for 48h.

[0030] The components of the prepared polypropylene composite film for packaging are as follows: The outer layer includes 74% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent; The middle layer includes 82% PP, 8% maleic anhydride modified polypropylene, 8% UHMWPP, 1% flow promoter, and 1% nucleating agent; The inner layer includes 74% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0031] Wherein: the flow promoter is polypropylene wax, and the foaming agent is azodicarbonamide.

[0032] Example 2 A polypropylene composite film for packaging, the preparation method of which is as follows: Accurately weigh PP, maleic anhydride modified polypropylene UHMWPP, flow promoter, and nucleating agent in a reactor, mix and react at 8MPa, 230℃ for 4h to obtain inner, middle, and outer monolayer films. Then put the outer, middle, and inner films into the three winding racks of the dry laminating machine in turn, apply 7.8g / ㎡ adhesive between the outer and middle films, and between the middle and inner films, and then laminate them into one with unwinding tensions of 6kgf, 7kgf, and 6kgf, respectively, through a laminating roller with a pressure of 7bar. The composite film is aged at 50℃ for 48h.

[0033] The components of the prepared polypropylene composite film for packaging are as follows: The outer layer includes 82% PP, 10% maleic anhydride modified polypropylene, 6% UHMWPP, 1.5% flow promoter, and 0.5% nucleating agent; The middle layer includes 82% PP, 8% maleic anhydride modified polypropylene, 8% UHMWPP, 1% flow promoter, and 1% nucleating agent; The inner layer includes 74% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent.

[0034] The UHMWPP of each layer is pretreated, and the foaming agent and the UHMWPP powder are co-melted at 180° C. in a closed container, and then extruded and granulated, and the mass ratio of the foaming agent to the UHMWPP powder is 1:10.

[0035] Wherein: the flow promoter is polyacrylonitrile, and the foaming agent is azodicarbonamide.

[0036] Example 3 A polypropylene composite film for packaging, the preparation method of which is as follows: Accurately weigh PP, maleic anhydride modified polypropylene UHMWPP, flow promoter, and nucleating agent in a reactor, mix and react at 7MPa, 40℃ for 4h to obtain inner, middle, and outer monolayer films. Then put the outer, middle, and inner films into three winding racks of the dry laminating machine in turn, apply 6.8g / ㎡ adhesive between the outer and middle films, and between the middle and inner films, and then laminate them into one with unwinding tensions of 5kgf, 8kgf, and 4kgf, respectively, through a laminating roller with a pressure of 5bar. The composite film is aged at 50℃ for 72h.

[0037] The components of the prepared polypropylene composite film for packaging are as follows: The outer layer includes 82% PP, 10% maleic anhydride modified polypropylene, 7% UHMWPP, 1.5% flow promoter, and 1.5% nucleating agent; The middle layer includes 77% PP, 8% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 3% nucleating agent; The inner layer includes 74% PP, 15% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 3% nucleating agent.

[0038] The UHMWPP of each layer is pretreated, and the foaming agent and the UHMWPP powder are co-melted at 190° C. in a closed container, and then extruded and granulated, and the mass ratio of the foaming agent to the UHMWPP powder is 1:10.

[0039] Wherein: the flow promoter is polypropylene alcohol, and the foaming agent is azodimethyl urea.

[0040] Example 4 A polypropylene composite film for packaging, the preparation method of which is as follows: Accurately weigh PP, maleic anhydride modified polypropylene UHMWPP, flow promoter, and nucleating agent in a reactor, mix and react at 3MPa, 200℃ for 5h to obtain inner, middle, and outer monolayer films. Then put the outer, middle, and inner films into the three winding racks of the dry laminating machine in turn, apply 8.8g / ㎡ adhesive between the outer and middle films, and between the middle and inner films, and then laminate them into one with unwinding tensions of 6kgf, 4kgf, and 6kgf, respectively, through a laminating roller with a pressure of 9bar. The composite film is aged at 45℃ for 72h.

[0041] The components of the prepared polypropylene composite film for packaging are as follows: The outer layer includes 70% PP, 15% maleic anhydride modified polypropylene, 9% UHMWPP, 3% flow promoter, and 3% nucleating agent; The middle layer includes 90% PP, 3% maleic anhydride modified polypropylene, 5% UHMWPP, 1.5% flow promoter, and 0.5% nucleating agent; The inner layer includes 82% PP, 10% maleic anhydride modified polypropylene, 7% UHMWPP, 1.5% flow promoter, and 1.5% nucleating agent.

[0042] The UHMWPP of each layer is pre-treated and post-treated, and the foaming agent and the UHMWPP powder are co-melted at 220°C in a closed container, and then extruded and granulated, and the mass ratio of the foaming agent to the UHMWPP powder is 2:10. After cooling to room temperature, the UHMWPP is annealed. The room temperature UHMWPP particles are heated to 140°C and cooled to room temperature at a retreat rate of 2°C / min.

[0043] Wherein: the flow promoter is polyacrylamide, and the foaming agent is azodicarbonamide.

[0044] Example 5 A polypropylene composite film for packaging, the preparation method of which is as follows: Accurately weigh PP, maleic anhydride modified polypropylene UHMWPP, flow promoter, and nucleating agent in a reactor, mix and react at 6MPa, 230℃ for 5h to obtain inner, middle, and outer monolayer films. Then put the outer, middle, and inner films into three winding racks of the dry laminating machine in turn, apply 5.2g / ㎡ adhesive between the outer and middle films, and between the middle and inner films, and then laminate them into one with unwinding tensions of 6kgf, 8kgf, and 6kgf, respectively, through a laminating roller with a pressure of 9bar. The composite film is aged at 50℃ for 48h.

[0045] The components of the prepared polypropylene composite film for packaging are as follows: The outer layer includes 76% PP, 10% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 2% nucleating agent; The middle layer includes 78% PP, 11% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent; The inner layer includes 78% PP, 12% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 2% nucleating agent.

[0046] The UHMWPP of each layer is pre-treated and post-treated, and the foaming agent and the UHMWPP powder are co-melted at 190°C in a closed container, and then extruded and granulated, and the mass ratio of the foaming agent to the UHMWPP powder is 1.5:10. After cooling to room temperature, the UHMWPP is annealed. The room temperature UHMWPP particles are heated to 90°C and cooled to room temperature at a retreat rate of 1°C / min.

[0047] Wherein: the flow promoter is polypropylene wax, and the foaming agent is azobisisobutyronitrile.

[0048] Example 6 A polypropylene composite film for packaging, the preparation method of which is as follows: Accurately weigh PP, maleic anhydride modified polypropylene UHMWPP, flow promoter, and nucleating agent in a reactor, mix and react at 3MPa, 230℃ for 5h to obtain inner, middle, and outer monolayer films. Then put the outer, middle, and inner films into three winding racks of the dry laminating machine in turn, apply 8.8g / ㎡ adhesive between the outer and middle films, and between the middle and inner films, and then laminate them into one with unwinding tensions of 4kgf, 5kgf, and 6kgf, respectively, through a laminating roller with a pressure of 4bar. The composite film is aged at 40℃ for 48h.

[0049] The components of the prepared polypropylene composite film for packaging are as follows: The outer layer includes 76% PP, 10% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 2% nucleating agent; The middle layer includes 82% PP, 8% maleic anhydride modified polypropylene, 8% UHMWPP, 1% flow promoter, and 1% nucleating agent; The inner layer includes 74% PP, 15% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 3% nucleating agent.

[0050] The UHMWPP of each layer is pre-treated and post-treated, and the foaming agent and the UHMWPP powder are co-melted at 230°C in a closed container, and then extruded and granulated, and the mass ratio of the foaming agent to the UHMWPP powder is 2:10. After cooling to room temperature, the UHMWPP is annealed. The room temperature UHMWPP particles are heated to 110°C and cooled to room temperature at a retreat rate of 0.5°C / min.

[0051] Wherein: the flow promoter is polypropylene wax, and the foaming agent is azodicarbonamide.

[0052] Example 7 A polypropylene composite film for packaging, the preparation method of which is as follows: Accurately weigh PP, maleic anhydride modified polypropylene UHMWPP, flow promoter, and nucleating agent in a reactor, mix and react at 5MPa, 240℃ for 6h to obtain inner, middle, and outer monolayer films. The outer film is made into BOPP by biaxial stretching process, the middle film is made into MOPP by uniaxial stretching process, and the inner film is made into CPP by casting process. Then put the outer film, middle film, and inner film into three winding racks of the dry laminating machine in turn, apply 6.4g / ㎡ adhesive between the outer film and the middle film, and between the middle film and the inner film, and then compound them into one with unwinding tensions of 5kgf, 8kgf, and 4kgf respectively through a composite roller with a pressure of 9bar. The composite film is aged at 50℃ for 48h.

[0053] The components of the prepared polypropylene composite film for packaging are as follows: The outer layer includes 74% PP, 15% maleic anhydride modified polypropylene, 5% UHMWPP, 3% flow promoter, and 3% nucleating agent.

[0054] The middle layer includes 78% PP, 11% maleic anhydride modified polypropylene, 9% UHMWPP, 1% flow promoter, and 1% nucleating agent; The inner layer includes 76% PP, 10% maleic anhydride modified polypropylene, 10% UHMWPP, 2% flow promoter, and 2% nucleating agent.

[0055] The UHMWPP of each layer is pre-treated and post-treated, and the foaming agent and the UHMWPP powder are co-melted at 230°C in a closed container, and then extruded and granulated, and the mass ratio of the foaming agent to the UHMWPP powder is 2:10. After cooling to room temperature, the UHMWPP is annealed. The room temperature UHMWPP particles are heated to 120°C and cooled to room temperature at a retreat rate of 1°C / min.

[0056] Wherein: the flow promoter is polypropylene wax, and the foaming agent is hydrogen peroxide.

[0057] According to GB / T 13022-1991 “Test method for tensile properties of plastic films”, the packaging films prepared in each embodiment were tested for tensile strength (MPa), Young's modulus (MPa), tensile yield stress (MPa) and elongation at break (%) of the structure with three layers of 20 μm thickness. The results are shown in Table 1.

[0058] Table 1

[0059] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A polypropylene composite film for packaging, which is arranged from outside to inside in sequence: The outer layer comprises, 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; The middle layer comprises, 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; The inner layer comprises, by weight percentage, 70-82% of PP, 10-15% of maleic anhydride modified polypropylene, 5-10% of UHMWPP, 0.5-3% of flow promoter, and 0.5-3% of nucleating agent.

2. The polypropylene composite film according to claim 1, characterized in that: The UHMWPP in at least one of the outer layer, the middle layer and the inner layer is pretreated, and the pretreatment process includes: melting the foaming agent and the UHMWPP powder at 180-230° C. in a closed container, and then extruding and granulating, and the mass ratio of the foaming agent to the UHMWPP powder is 1-2:

10.

3. The polypropylene composite film according to claim 2, characterized in that: The UHMWPP is also post-treated, and the post-treatment process includes: annealing the UHMWPP obtained by pre-treatment, the annealing temperature is 80-140° C., and the annealing rate is 0.5-2° C. / min.

4. The polypropylene composite film according to any one of claims 1 to 3, characterized in that: The outer layer is also made into BOPP through a biaxial stretching process; The middle layer is also made of MOPP through a uniaxial stretching process; The inner layer is also made of CPP through a casting process.

5. The polypropylene composite film according to any one of claims 1 to 3, characterized in that: The flow promoter is a polypropylene-based flow promoter.

6. The polypropylene composite film according to any one of claims 1 to 3, characterized in that: The nucleating agent is a polymer nucleating agent.

7. The polypropylene composite film according to any one of claims 1 to 3, characterized in that: The foaming agent is an azo compound or a peroxide compound.

8. A method for preparing a polypropylene composite film as claimed in claim 4, characterized in that: include: S1: According to the weight percentage, accurately weigh 55-82% PP, 3-15% maleic anhydride modified polypropylene, 5-10% UHMWPP, 0.5-3% flow promoter, and 0.5-3% nucleating agent in a reactor, mix well at 3-8 MPa, 190-240°C, and react for 4-6 hours; S2: taking out the polypropylene resin obtained in S1, cooling and granulating it, and then using a biaxial stretching process, a uniaxial stretching process, and a casting process to prepare an outer layer film, a middle layer film, and an inner layer film; S3: The outer film, the middle film and the inner film are compounded into one by dry process; S4: Curing the composite film obtained in S3 at 40-80° C. for more than 48 hours.

9. The method for preparing a polypropylene composite film according to claim 8, characterized in that: The dry lamination includes placing the outer layer film, the middle layer film, and the inner layer film in turn into three winding frames of the dry laminating machine, applying 2.5-5.2 g / ㎡ adhesive between the outer layer film and the middle layer film, and between the middle layer film and the inner layer film, and then laminating them into one with a laminating roller at a pressure of 5-9 bar at unwinding tensions of 3-6 kgf, 4-8 kgf, and 3-6 kgf, respectively.

10. The method for preparing a polypropylene composite film according to claim 9, characterized in that: The adhesive is a thermoplastic acrylic adhesive.

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