Preparation method of polypropylene antibacterial film

Abstract

The invention discloses a preparation method of a polypropylene antibacterial film. Comprising the following steps: step 1, treating the surface of polypropylene by using plasmas, then immersing the polypropylene into a glycidyl methacrylate solution in a protective atmosphere, raising the temperature to 40-50 DEG C, reacting for 1-2 hours, cooling, and respectively washing and drying by using deionized water and methanol to obtain epoxidized polypropylene; step 2, mixing epoxidized polypropylene, a modifier, dilaurate thiodipropionate, erucyl amide, dicumyl peroxide and modified graphene, heating and melting, extruding through a casting machine head, and stretching to obtain the polypropylene film. The preparation method has the beneficial effects that the polypropylene film with excellent mechanical property, antibacterial property and ultraviolet resistance is successfully prepared by designing and synthesizing the multifunctional modifier and combining plasma treatment, chemical grafting and compounding technologies.

Description

Technical Field

[0001] The invention belongs to the technical field of packaging films, and in particular relates to a method for preparing an antibacterial polypropylene film. Background Art

[0002] As a multifunctional material, polypropylene antibacterial film has broad application prospects in the fields of architectural films and automotive window films. Architectural films need to have excellent UV resistance to extend service life, and also need to have antibacterial properties to inhibit the growth of microorganisms and keep the environment clean; automotive window films require high transparency, UV resistance and antibacterial functions to improve driving comfort and safety.

[0003] In the prior art, the antibacterial and anti-ultraviolet properties of polypropylene films are usually achieved by directly adding antibacterial agents and anti-ultraviolet agents, which has obvious defects. First, these additives have poor compatibility with the polypropylene matrix and are prone to migration or shedding during processing and use, resulting in a gradual decline in performance; second, the uneven dispersion of additives will affect the mechanical and optical properties of the film, and may even cause problems such as embrittlement or discoloration of the material. In addition, traditional antibacterial agents and anti-ultraviolet agents are not stable enough and are prone to failure under long-term exposure to light, humidity and heat.

[0004] Therefore, in order to solve the above problems, the present invention develops a polypropylene film that can stably combine antibacterial and anti-ultraviolet functions. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the prior art and provide a method for preparing an antibacterial polypropylene film.

[0006] The purpose of the present invention can be achieved through the following technical solutions: A method for preparing a polypropylene antibacterial film comprises the following steps: Step 1: Treat the surface of polypropylene with plasma, then immerse it in a glycidyl methacrylate solution under a protective atmosphere, raise the temperature to 40-50°C, react for 1-2 hours, cool, wash with deionized water and methanol, respectively, and dry to obtain epoxidized polypropylene; Step 2: Mix the modifier, epoxidized polypropylene, dilauric acid thiodipropionate, erucic acid amide, diisopropylbenzene peroxide, and modified graphene, heat and melt, extrude through a casting die, and stretch to obtain a polypropylene film.

[0007] In the scheme, free radicals are generated by plasma treatment, so that glycidyl methacrylate is grafted onto the surface of polypropylene through free radical polymerization, thereby forming epoxidized polypropylene, which can then undergo a ring-opening reaction with the secondary amine nitrogen at the end of the modifier in a molten state, ultimately obtaining a functionalized polypropylene film.

[0008] Optimally, the working parameters of the plasma are: pressure 0.2-0.5 mbar, power 70-80 W, time 120-150 s; the concentration of the glycidyl methacrylate solution is 4-8 wt %.

[0009] More optimally, the polypropylene film raw material includes the following components: by weight, 22-30 parts of modifier, 70-80 parts of epoxidized polypropylene, 0.1-0.2 parts of dilauric acid thiodipropionate, 0.3-0.4 parts of erucamide, 0.1-0.2 parts of diisopropyl peroxide, and 2-3 parts of modified graphene.

[0010] More optimally, the preparation process of the modifier is: S1: 3-hydroxyflavone, N,N-dimethylformamide, potassium carbonate and epibromopropane are mixed, heated to 60-65°C, reacted for 3-4 hours, cooled to room temperature, filtered, washed and dried, and the obtained solid is transferred to isopropanol, 1-tert-butyloxycarbonylpiperazine and triethylamine are added, heated to 50-60°C, reacted for 2-3 hours, the solvent is removed by rotary evaporation, and purified by column chromatography to separate piperazine flavone; S2: Toluene diisocyanate, dimethylaminoethanol, dibutyltin dilaurate and tetrahydrofuran are mixed, and stirred at a constant speed for 12-15 hours at room temperature, and then 1,3-propane sultone solution is added dropwise, and the stirring reaction is continued for 8-10 hours, centrifuged, washed and dried to obtain zwitterions; S3: Mix zwitterion, piperazine flavonoids, tetrahydrofuran and dibutyltin dilaurate, heat to 60-65°C, react for 1-2h, cool to room temperature, filter, wash, dry, then transfer to ethyl acetate, add concentrated hydrochloric acid, stir at room temperature for 1-2h, purify and separate to obtain the modifier.

[0011] In the scheme, the hydroxyl group of 3-hydroxyflavone is deprotonated under alkaline conditions, and then acts as a nucleophilic reagent to attack the bromine atom of epibromohydrin. The epoxy group contained in the obtained solid undergoes a nucleophilic ring-opening reaction with the nitrogen atom of 1-tert-butyloxycarbonylpiperazine to obtain piperazine flavone. The specific process is shown in the figure below:

[0012] In the scheme, the isocyanate group of toluene diisocyanate reacts with the hydroxyl group of dimethylaminoethanol to form a carbamate bond, and the tertiary amine contained in the intermediate undergoes a ring-opening reaction with the sultone ring of 1,3-propane sultone to obtain a zwitterion; the isocyanate group contained at the other end of the zwitterion continues to react with the hydroxyl group in piperazine flavonoid, and the protective group is removed to obtain a modifier; the structure of the modifier is shown in the figure below:

[0013] More optimally, the piperazine flavonoid raw material includes the following components: by weight, 2-4 parts of 3-hydroxyflavonoid, 20-25 parts of N,N-dimethylformamide, 2-3 parts of potassium carbonate, 2-3 parts of epibromopropane, 50-55 parts of isopropanol, 3-4 parts of 4-(1-methylpiperazine)benzoic acid, and 1-2 parts of triethylamine.

[0014] More optimally, the zwitterion raw material comprises the following components: by weight, 4-6 parts of toluene diisocyanate, 2-3 parts of dimethylaminoethanol, 0.1-0.2 parts of dibutyltin dilaurate, 70-80 parts of tetrahydrofuran, and 2-3 parts of 1,3-propane sultone solution; wherein the mass fraction of the 1,3-propane sultone solution is 30-35wt%.

[0015] More optimally, the modifier raw material includes the following components: by weight, 2-3 parts of zwitterions, 7-9 parts of piperazine flavonoids, 70-80 parts of tetrahydrofuran, 0.1-0.2 parts of dibutyltin dilaurate, 9-10 parts of ethyl acetate, and 5-8 parts of concentrated hydrochloric acid.

[0016] More optimally, the preparation process of the modified graphene is: (1) Add the modifier and 2-amino-4-pentenoic acid to tetrahydrofuran, heat to 40-50°C, stir thoroughly to dissolve, then add triethylamine, continue to heat to 80-90°C, react for 3-4h to obtain an olefinic modifier; (2) Mix the olefinic modifier, graphene oxide, and N,N-dimethylformamide, add N,N'-dicyclohexylcarbodiimide, react at 60-70°C for 3-4 hours, filter, wash, and dry to obtain modified graphene.

[0017] More optimally, the alkenyl modifier comprises the following components: by weight, 10-12 parts of modifier, 15-18 parts of 2-amino-4-pentenoic acid, 70-80 parts of tetrahydrofuran, and 0.1-0.2 parts of triethylamine; the modified graphene comprises the following components: by weight, 10-12 parts of alkenyl modifier, 8-10 parts of graphene oxide, 80-90 parts of N,N-dimethylformamide, and 0.1-0.2 parts of N,N'-dicyclohexylcarbodiimide.

[0018] Beneficial effects of the present invention: The present invention successfully prepared a polypropylene film with excellent mechanical properties, antibacterial properties and UV resistance by designing and synthesizing a multifunctional modifier and combining plasma treatment, chemical grafting and composite technology. The details are as follows: First, the modifier prepared in the scheme contains flavonoid groups, whose conjugated π-electron system can effectively absorb ultraviolet light and convert the absorbed energy into heat energy or low-energy fluorescence release, thereby reducing the damage of ultraviolet light to the material; in addition, the flavonoid group also has certain antibacterial activity, which can further enhance the antibacterial properties of the material by destroying the integrity of bacterial cell membranes or inhibiting the activity of bacterial enzymes; at the same time, the zwitterions in the modifier carry both positive and negative charges, and can combine with the negatively charged areas on the bacterial cell membrane through electrostatic action, destroying the integrity of the cell membrane, thereby enhancing the antibacterial effect of the material; in addition, the introduction of zwitterions also improves the thermal stability and chemical stability of the modifier, making it more stable and not easy to decompose during processing and use; Second: In the scheme, in order to improve the dispersibility and compatibility of graphene oxide in the material, olefinic groups are introduced into the modifier and the modifier is grafted onto the surface of graphene oxide, thereby effectively improving the interface bonding between graphene oxide and the matrix material; due to the active groups contained in the modifier, under the action of the initiator, it can chemically react with the matrix polypropylene during the processing, further enhancing the bonding force between graphene oxide and polypropylene; this synergistic effect enables the modified graphene oxide to be evenly dispersed in the matrix material to form a stable composite material, thereby comprehensively improving the comprehensive properties of the material such as mechanical properties, UV resistance and antibacterial properties. DETAILED DESCRIPTION

[0019] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] It should be noted that the following parts are parts by weight, and the purchase manufacturers of all raw materials involved in the present invention are not subject to any special restrictions, and exemplary examples include: the graphene oxide is purchased from Zhongke Yueda Material Technology Co., Ltd., and the average flake size is ≤300nm; the polypropylene is polypropylene powder purchased from Dow, USA, with a brand number of 227; the 2-hydroxy-4-n-octyloxybenzophenone CAS number is 1843-05-6, purchased from Zhengzhou Alpha Chemical Co., Ltd.

[0021] The polyguanidine / polysilicate composite antibacterial agent is obtained by mixing an aqueous solution of a water-soluble polyguanidine inorganic acid salt or an organic acid salt and an aqueous solution of a water-soluble silicate, and then adding an aqueous solution of a water-soluble metal salt; wherein the molar ratio of the water-soluble polyguanidine inorganic acid salt or an organic acid salt to the water-soluble silicate is 10:1, and the molar ratio of the water-soluble silicate to the water-soluble metal salt is 5:1.

[0022] Embodiment 1: A method for preparing a polypropylene antibacterial film, comprising the following steps: Step 1: The surface of polypropylene was treated with plasma, and then immersed in a glycidyl methacrylate aqueous solution (concentration of 4 wt%) under a protective atmosphere, the temperature was raised to 40°C, the reaction was performed for 1 hour, and after cooling, the surface was washed with deionized water and methanol respectively, and dried to obtain epoxidized polypropylene; Step 2: 22 parts of a modifier, 70 parts of epoxidized polypropylene, 0.1 parts of dilauric acid thiodipropionate, 0.3 parts of erucamide, 0.1 parts of diisopropylbenzene peroxide, and 2 parts of modified graphene are mixed, heated to melt, extruded through a casting die, and stretched to obtain a polypropylene film; Wherein, the preparation process of the modifier is: S1: 2 parts of 3-hydroxyflavone, 20 parts of N,N-dimethylformamide, 2 parts of potassium carbonate, and 2 parts of epibromopropane were mixed, heated to 60°C, reacted for 3 hours, cooled to room temperature, filtered, washed, and dried, and the obtained solid was transferred to 50 parts of isopropanol, 3 parts of 1-tert-butyloxycarbonylpiperazine and 1 part of triethylamine were added, heated to 50°C, reacted for 2 hours, the solvent was removed by rotary evaporation, and purified by column chromatography to separate piperazine flavone; S2: 4 parts of toluene diisocyanate, 2 parts of dimethylaminoethanol, 0.1 parts of dibutyltin dilaurate, and 70 parts of tetrahydrofuran were mixed, and stirred at a constant speed for 12 hours at room temperature, and then 2 parts of 1,3-propane sultone tetrahydrofuran solution (concentration of 15wt%) were added dropwise, and the stirring reaction was continued for 8 hours, centrifuged, washed, and dried to obtain zwitterions; S3: 2 parts of zwitterion, 7 parts of piperazine flavonoids, 70 parts of tetrahydrofuran, and 0.1 parts of dibutyltin dilaurate were mixed, heated to 60°C, reacted for 1 hour, cooled to room temperature, filtered, washed, dried, and then transferred to 9 parts of ethyl acetate, 5 parts of concentrated hydrochloric acid were added, stirred at room temperature for 1 hour, purified, and separated to obtain a modifier; The preparation process of modified graphene is: (1) Add 10 parts of the modifier and 15 parts of 2-amino-4-pentenoic acid to 70 parts of tetrahydrofuran, heat to 40°C, stir thoroughly to dissolve, then add 0.1 parts of triethylamine, continue to heat to 80°C, react for 3 hours, and obtain an olefinic modifier; (2) Mix 10 parts of olefinic modifier, 8 parts of graphene oxide, and 80 parts of N,N-dimethylformamide, add 0.1 parts of N,N'-dicyclohexylcarbodiimide, react at 60°C for 3 hours, filter, wash, and dry to obtain modified graphene.

[0023] Embodiment 2: A method for preparing a polypropylene antibacterial film, comprising the following steps: Step 1: The surface of polypropylene was treated with plasma, and then immersed in a glycidyl methacrylate aqueous solution (concentration of 4 wt%) under a protective atmosphere, the temperature was raised to 50°C, the reaction was performed for 2 hours, and after cooling, the surface was washed with deionized water and methanol, respectively, and dried to obtain epoxidized polypropylene; Step 2: 30 parts of a modifier, 80 parts of epoxidized polypropylene, 0.2 parts of dilauric acid thiodipropionate, 0.4 parts of erucamide, 0.2 parts of diisopropylbenzene peroxide, and 3 parts of modified graphene are mixed, heated to melt, extruded through a casting die, and stretched to obtain a polypropylene film; Wherein, the preparation process of the modifier is: S1: 4 parts of 3-hydroxyflavone, 25 parts of N,N-dimethylformamide, 3 parts of potassium carbonate, and 3 parts of epibromopropane were mixed, heated to 65°C, reacted for 4 hours, cooled to room temperature, filtered, washed, and dried, and the obtained solid was transferred to 55 parts of isopropanol, 4 parts of 1-tert-butyloxycarbonylpiperazine and 2 parts of triethylamine were added, heated to 60°C, reacted for 3 hours, the solvent was removed by rotary evaporation, and purified by column chromatography to separate piperazine flavone; S2: 6 parts of toluene diisocyanate, 3 parts of dimethylaminoethanol, 0.2 parts of dibutyltin dilaurate, and 80 parts of tetrahydrofuran were mixed, and stirred at a constant speed for 15 hours at room temperature, and then 3 parts of 1,3-propane sultone tetrahydrofuran solution (concentration of 15wt%) were added dropwise, and the reaction was continued for 10 hours by stirring, centrifuged, washed, and dried to obtain zwitterions; S3: 3 parts of zwitterion, 9 parts of piperazine flavonoids, 80 parts of tetrahydrofuran, and 0.2 parts of dibutyltin dilaurate were mixed, heated to 65°C, reacted for 2 hours, cooled to room temperature, filtered, washed, dried, and then transferred to 10 parts of ethyl acetate, 8 parts of concentrated hydrochloric acid were added, stirred at room temperature for 2 hours, purified, and separated to obtain a modifier; The preparation process of modified graphene is: (1) Add 12 parts of the modifier and 18 parts of 2-amino-4-pentenoic acid to 80 parts of tetrahydrofuran, heat to 50°C, stir thoroughly to dissolve, then add 0.2 parts of triethylamine, continue to heat to 90°C, react for 4 hours, and obtain an olefinic modifier; (2) Mix 12 parts of olefinic modifier, 10 parts of graphene oxide, and 90 parts of N,N-dimethylformamide, add 0.2 parts of N,N'-dicyclohexylcarbodiimide, react at 70°C for 4 hours, filter, wash, and dry to obtain modified graphene.

[0024] Embodiment 3: A method for preparing a polypropylene antibacterial film, comprising the following steps: Step 1: The surface of polypropylene was treated with plasma, and then immersed in a glycidyl methacrylate aqueous solution (concentration of 4 wt%) under a protective atmosphere, the temperature was raised to 45°C, the reaction was performed for 1.5 hours, and after cooling, the surface was washed with deionized water and methanol, respectively, and dried to obtain epoxidized polypropylene; Step 2: 25 parts of a modifier, 75 parts of epoxidized polypropylene, 0.15 parts of dilauric acid thiodipropionate, 0.35 parts of erucamide, 0.15 parts of diisopropylbenzene peroxide, and 2.5 parts of modified graphene are mixed, heated to melt, extruded through a casting die, and stretched to obtain a polypropylene film; Wherein, the preparation process of the modifier is: S1: 3 parts of 3-hydroxyflavone, 22.5 parts of N,N-dimethylformamide, 2.5 parts of potassium carbonate and 2.5 parts of epibromopropane were mixed, heated to 62.5°C, reacted for 3.5 hours, cooled to room temperature, filtered, washed and dried, and then the obtained solid was transferred to 52.5 parts of isopropanol, 3.5 parts of 1-tert-butyloxycarbonylpiperazine and 1.5 parts of triethylamine were added, heated to 55°C, reacted for 2.5 hours, the solvent was removed by rotary evaporation, and purified by column chromatography to separate piperazine flavone; S2: 5 parts of toluene diisocyanate, 2.5 parts of dimethylaminoethanol, 0.15 parts of dibutyltin dilaurate, and 75 parts of tetrahydrofuran were mixed, and stirred at a constant speed for 13.5 hours at room temperature, and then 2.5 parts of 1,3-propane sultone tetrahydrofuran solution (concentration of 15wt%) were added dropwise, and the stirring reaction was continued for 9 hours, centrifuged, washed, and dried to obtain zwitterions; S3: 2.5 parts of zwitterion, 8 parts of piperazine flavonoids, 75 parts of tetrahydrofuran, and 0.15 parts of dibutyltin dilaurate were mixed, heated to 62.5°C, reacted for 1.5 hours, cooled to room temperature, filtered, washed, dried, and then transferred to 9.5 parts of ethyl acetate, 6.5 parts of concentrated hydrochloric acid were added, stirred at room temperature for 1.5 hours, purified, and separated to obtain a modifier; The preparation process of modified graphene is: (1) 11 parts of the modifier and 16.5 parts of 2-amino-4-pentenoic acid were added to 75 parts of tetrahydrofuran, heated to 45°C, stirred thoroughly to dissolve, and then 0.15 parts of triethylamine were added, and the temperature was continued to be raised to 85°C, and the reaction was carried out for 3.5 hours to obtain an olefinic modifier; (2) 11 parts of olefinic modifier, 9 parts of graphene oxide, and 85 parts of N,N-dimethylformamide were mixed, and 0.15 parts of N,N'-dicyclohexylcarbodiimide were added. The mixture was reacted at 65°C for 3.5 hours, filtered, washed, and dried to obtain modified graphene.

[0025] Comparative Example 1: No modification is performed on polypropylene and graphene, and an antibacterial agent (polyguanidine / polysilicate composite antibacterial agent) and an ultraviolet absorber (2-hydroxy-4-n-octyloxybenzophenone) are added. The rest is the same as in Example 3, as follows: A method for preparing an antibacterial polypropylene film comprises the following steps: mixing 75 parts of polypropylene, 0.15 parts of dilauric acid thiodipropionate, 0.35 parts of erucamide, 0.15 parts of dicumyl peroxide, 2.5 parts of graphene oxide, 0.2-0.3 parts of an antibacterial agent, and 0.2-0.3 parts of an ultraviolet absorber, heating and melting, extruding through a casting die, and stretching to obtain a polypropylene film.

[0026] Comparative Example 2: Graphene oxide is not modified, and the rest is the same as Example 3, specifically as follows: Step 1: The surface of polypropylene was treated with plasma, and then immersed in a glycidyl methacrylate aqueous solution (concentration of 4 wt%) under a protective atmosphere, the temperature was raised to 45°C, the reaction was performed for 1.5 hours, and after cooling, the surface was washed with deionized water and methanol, respectively, and dried to obtain epoxidized polypropylene; Step 2: 25 parts of a modifier, 75 parts of epoxidized polypropylene, 0.15 parts of dilauric acid thiodipropionate, 0.35 parts of erucamide, 0.15 parts of diisopropylbenzene peroxide, and 2.5 parts of graphene oxide are mixed, heated to melt, extruded through a casting die, and stretched to obtain a polypropylene film; Wherein, the preparation process of the modifier is: S1: 3 parts of 3-hydroxyflavone, 22.5 parts of N,N-dimethylformamide, 2.5 parts of potassium carbonate and 2.5 parts of epibromopropane were mixed, heated to 62.5°C, reacted for 3.5 hours, cooled to room temperature, filtered, washed and dried, and then the obtained solid was transferred to 52.5 parts of isopropanol, 3.5 parts of 1-tert-butyloxycarbonylpiperazine and 1.5 parts of triethylamine were added, heated to 55°C, reacted for 2.5 hours, the solvent was removed by rotary evaporation, and purified by column chromatography to separate piperazine flavone; S2: 5 parts of toluene diisocyanate, 2.5 parts of dimethylaminoethanol, 0.15 parts of dibutyltin dilaurate, and 75 parts of tetrahydrofuran were mixed, and stirred at a constant speed for 13.5 hours at room temperature, and then 2.5 parts of 1,3-propane sultone tetrahydrofuran solution (concentration of 15wt%) were added dropwise, and the stirring reaction was continued for 9 hours, centrifuged, washed, and dried to obtain zwitterions; S3: Mix 2.5 parts of zwitterion, 8 parts of piperazine flavonoids, 75 parts of tetrahydrofuran, and 0.15 parts of dibutyltin dilaurate, heat to 62.5°C, react for 1.5 hours, cool to room temperature, filter, wash, dry, then transfer to 9.5 parts of ethyl acetate, add 6.5 parts of concentrated hydrochloric acid, stir at room temperature for 1.5 hours, purify, and separate to obtain the modifier.

[0027] Testing: (1) The ultraviolet absorption rate of the polypropylene film obtained in the embodiment and the comparative example was tested according to GB / T38245 standard; (2) The polypropylene film obtained in the embodiment and the comparative example was cut into corresponding samples and placed in a bacterial solution, cultured at 37°C for 5 hours. After the culture, the sample was taken out and the surface was continuously washed with PBS solution. The washed PBS solution was then placed in a solid culture medium and cultured at 37°C for 24 hours. The colony count was performed to calculate the antibacterial rate. The bacteria tested were Escherichia coli and Staphylococcus aureus; (3) The tensile strength of the polypropylene film obtained in the embodiment and the comparative example was tested according to GB / T1040.3-2006 standard; the obtained data are shown in the following table:

[0028] Conclusion: The present invention provides a method for preparing an antibacterial polypropylene film, and successfully prepares a polypropylene film with excellent mechanical properties, antibacterial properties and anti-ultraviolet properties through plasma treatment, chemical grafting and composite technology. Among them, the polypropylene films prepared in Examples 1, 2 and 3 are superior to those in Comparative Example 1 (unmodified polypropylene and graphene) and Comparative Example 2 (unmodified graphene oxide) in terms of tensile strength, anti-ultraviolet rate and antibacterial rate, indicating that the modification treatment has a significant effect on improving the performance of the film.

[0029] In the description of the specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0030] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the invention or exceed the scope defined by the claims, they shall all fall within the protection scope of the present invention.

Claims

1. A method for preparing a polypropylene antibacterial film, characterized in that: The following steps are involved: Step 1: Treat the surface of polypropylene with plasma, then immerse it in a glycidyl methacrylate solution under a protective atmosphere, raise the temperature to 40-50°C, react for 1-2 hours, cool, wash with deionized water and methanol, respectively, and dry to obtain epoxidized polypropylene; Step 2: mixing epoxidized polypropylene, a modifier, dilauric acid thiodipropionate, erucic acid amide, diisopropylbenzene peroxide, and modified graphene, heating and melting, extruding through a casting die, and stretching to obtain a polypropylene film; The preparation process of the modifier is: S1: 3-hydroxyflavone, N,N-dimethylformamide, potassium carbonate and epibromopropane are mixed, heated to 60-65°C, reacted for 3-4 hours, cooled to room temperature, filtered, washed and dried, and the obtained solid is transferred to isopropanol, 1-tert-butyloxycarbonylpiperazine and triethylamine are added, heated to 50-60°C, reacted for 2-3 hours, the solvent is removed by rotary evaporation, and purified by column chromatography to separate piperazine flavone; S2: Toluene diisocyanate, dimethylaminoethanol, dibutyltin dilaurate and tetrahydrofuran are mixed, and stirred at a constant speed for 12-15 hours at room temperature, and then 1,3-propane sultone solution is added dropwise, and the stirring reaction is continued for 8-10 hours, centrifuged, washed and dried to obtain zwitterions; S3: Mix zwitterion, piperazine flavonoids, tetrahydrofuran and dibutyltin dilaurate, heat to 60-65°C, react for 1-2h, cool to room temperature, filter, wash, dry, then transfer to ethyl acetate, add concentrated hydrochloric acid, stir at room temperature for 1-2h, purify and separate to obtain the modifier.

2. The method for preparing a polypropylene antibacterial film according to claim 1, characterized in that: The working parameters of the plasma are: pressure 0.2-0.5 mbar, power 70-80 W, time 120-150 s; the concentration of the glycidyl methacrylate solution is 4-8 wt %.

3. The method for preparing a polypropylene antibacterial film according to claim 1, characterized in that: The polypropylene film comprises the following components: by weight, 22-30 parts of a modifier, 70-80 parts of epoxidized polypropylene, 0.1-0.2 parts of an antioxidant, 0.3-0.4 parts of a lubricant, 0.1-0.2 parts of an initiator, and 2-3 parts of modified graphene.

4. The method for preparing a polypropylene antibacterial film according to claim 1, characterized in that: The piperazine flavonoid raw material comprises the following components: by weight, 2-4 parts of 3-hydroxyflavonoid, 20-25 parts of N,N-dimethylformamide, 2-3 parts of potassium carbonate, 2-3 parts of epibromopropane, 50-55 parts of isopropanol, 3-4 parts of 4-(1-methylpiperazine)benzoic acid, and 1-2 parts of triethylamine.

5. The method for preparing a polypropylene antibacterial film according to claim 1, characterized in that: The zwitterion raw material comprises the following components: by weight, 4-6 parts of toluene diisocyanate, 2-3 parts of dimethylaminoethanol, 0.1-0.2 parts of dibutyltin dilaurate, 70-80 parts of tetrahydrofuran, and 2-3 parts of 1,3-propane sultone solution; wherein the mass fraction of the 1,3-propane sultone solution is 30-35wt%.

6. The method for preparing a polypropylene antibacterial film according to claim 1, characterized in that: The modifier raw material comprises the following components: by weight, 2-3 parts of zwitterion, 7-9 parts of piperazine flavonoids, 70-80 parts of tetrahydrofuran, 0.1-0.2 parts of dibutyltin dilaurate, 9-10 parts of ethyl acetate, and 5-8 parts of concentrated hydrochloric acid.

7. The method for preparing a polypropylene antibacterial film according to claim 1, characterized in that: The preparation process of the modified graphene is: (1) Add the modifier and 2-amino-4-pentenoic acid to tetrahydrofuran, heat to 40-50°C, stir thoroughly to dissolve, then add triethylamine, continue to heat to 80-90°C, react for 3-4h to obtain an olefinic modifier; (2) Mix the olefinic modifier, graphene oxide, and N,N-dimethylformamide, add N,N'-dicyclohexylcarbodiimide, react at 60-70°C for 3-4 hours, filter, wash, and dry to obtain modified graphene.

8. The method for preparing a polypropylene antibacterial film according to claim 7, characterized in that: The alkenyl modifier comprises the following components: by weight, 10-12 parts of modifier, 15-18 parts of 2-amino-4-pentenoic acid, 70-80 parts of tetrahydrofuran, and 0.1-0.2 parts of triethylamine; the modified graphene comprises the following components: by weight, 10-12 parts of alkenyl modifier, 8-10 parts of graphene oxide, 80-90 parts of N,N-dimethylformamide, and 0.1-0.2 parts of N,N'-dicyclohexylcarbodiimide.

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