High-strength plastic film and method for producing the same

Abstract

The application belongs to the technical field of polymer materials, and particularly relates to a high-strength plastic film and a preparation method thereof. The high-strength plastic film comprises the following raw materials in parts by weight: 80-90 parts of polylactic acid, 5-10 parts of polyurethane resin, 2-6 parts of flame-retardant modified halloysite, 2-6 parts of antioxidant modified halloysite, 0.5-2 parts of silane coupling agent and 1-3 parts of plasticizer. The halloysite is subjected to flame-retardant modification and antioxidant modification, and is added to a PLA matrix, so that the halloysite can not only strengthen the rigidity and strength of the PLA matrix as an inorganic filler, but also has the functions of flame retardation and oxidation resistance; meanwhile, the tubular structure of the halloysite nanotube can form a tortuous path in the PLA matrix, delay the diffusion of water vapor and oxygen, reduce the water vapor transmission amount and the oxygen transmission coefficient, and prolong the food preservation period when the high-strength plastic film is applied to food packaging.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a high-strength plastic film and its preparation method. Background Technology

[0002] Polylactic acid (PLA), as a biodegradable polymer, has broad application prospects in environmentally friendly packaging fields such as food packaging and seafood preservation due to its good biocompatibility and degradability. However, PLA's severe brittleness and poor flame retardancy, as well as its susceptibility to oxidative degradation, limit its further application in high-performance packaging. Especially in the packaging of easily oxidized foods such as seafood, PLA's insufficient antioxidant properties can lead to a shortened shelf life and affect product quality.

[0003] Pure polylactic acid (PLA) is difficult to process directly into packaging films and bags. It must be blended with other polymers, starch, and other additives for modification before being blown film, vacuum-formed, injection-molded, or cast. However, while the addition of toughening agents and plasticizers improves the toughness of PLA, it also affects its strength and rigidity. Blending with other polymers such as polyurethane (PU) can improve PLA's toughness to some extent, but simple physical blending often leads to poor interfacial compatibility, resulting in a decrease in mechanical properties. Furthermore, existing research has explored improving PLA's mechanical properties by adding inorganic fillers such as nanoclay and carbon nanotubes, or by introducing flame retardants and antioxidants to enhance its functional properties. However, these modification methods often suffer from poor additive dispersibility, limited functionality, and poor compatibility with the matrix, resulting in limited modification effects and even affecting the material's processing and final performance.

[0004] Halloysite nanotubes (HNTs), as a naturally occurring nanoclay material, possess a hollow tubular structure, high specific surface area, and good interfacial compatibility, making them ideal nanofillers for PLA modification. However, unmodified halloysite has limited flame-retardant and antioxidant properties, failing to meet the comprehensive requirements of high-performance packaging materials. Therefore, developing a composite modified material that can significantly improve the mechanical properties of PLA while also possessing highly efficient flame-retardant and antioxidant functions has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the primary objective of this invention is to provide a high-strength plastic film that has excellent mechanical properties, flame retardant properties, oxidation resistance, and good barrier properties.

[0006] Another object of the present invention is to provide a method for preparing the above-mentioned high-strength plastic film.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A high-strength plastic film comprises the following raw materials in parts by weight: 80-90 parts polylactic acid, 5-10 parts polyurethane resin, 2-6 parts flame-retardant modified halloysite, 2-6 parts antioxidant modified halloysite, 0.5-2 parts silane coupling agent, and 1-3 parts plasticizer.

[0009] The preparation processes of the flame-retardant modified halloysite and the antioxidant modified halloysite are as follows:

[0010] (1) Halloysite was dispersed in deionized water and then phenylphosphonic acid was added for reflux reaction; after cooling to room temperature, it was filtered, washed and dried to obtain phenylphosphonic acid modified halloysite;

[0011] (2) Disperse the phenylphosphonic acid modified halloysite from step (1) into dichloromethane, add N,N-dimethylformamide, heat to 40-50℃ and then add oxalyl chloride to react; after the reaction is completed, remove the solvent to obtain phenylphosphonic acid modified halloysite;

[0012] (3) Melamine and triethylamine are dissolved in N,N-dimethylformamide to obtain a mixed solution; then the phenylphosphonic chloride modified halloysite from step (2) is dispersed in N,N-dimethylformamide, the mixed solution is added dropwise at -5~0℃, and then the reaction is carried out at room temperature; the solid product is collected by filtration, washed and dried to obtain the flame-retardant modified halloysite.

[0013] Further, the preparation process of the antioxidant modified halloysite is as follows: curcumin and triethylamine are dissolved in ethyl acetate to obtain a mixture; then the phenylphosphonic chloride modified halloysite from step (2) above is dispersed in ethyl acetate, the mixture is added dropwise at -5~0℃ and kept warm for 20-30 min, and then the temperature is raised to room temperature for reaction; the solid product is collected by filtration, washed and dried to obtain the antioxidant modified halloysite.

[0014] Further, in step (1), the mass ratio of halloysite to phenylphosphonic acid is 1:(2-3); the reflux reaction time is 40-48 h.

[0015] Further, in step (2), the mass ratio of phenylphosphonic acid-modified halloysite, oxaloyl chloride, and N,N-dimethylformamide is 1:(7-8):(1.4-1.6); the reaction time is 3-4 h.

[0016] Further, in step (3), the mass ratio of phenylphosphonic chloride-modified halloysite, melamine, and triethylamine is 1:(0.5-0.6):(0.5-0.6); the reaction time is 20-24 h.

[0017] Further, the mass ratio of the phenylphosphonic chloride-modified halloysite, curcumin, and triethylamine is 1:(0.5-0.8):(0.5-0.8); the reaction time is 40-48 h.

[0018] Further, the polylactic acid has a weight-average molecular weight of 10,000-100,000 g / mol; the polyurethane resin is a polyester-type polyurethane; the silane coupling agent is γ-aminopropyltriethoxysilane; and the plasticizer is tributyl citrate or PEG-400.

[0019] A method for preparing the above-mentioned high-strength plastic film includes the following steps:

[0020] According to the stated weight proportions, polylactic acid, polyurethane resin and silane coupling agent are mixed to obtain a premix; then the premix, flame-retardant modified halloysite, antioxidant modified halloysite and plasticizer are fed into a twin-screw extruder for melt extrusion and granulation to obtain a masterbatch; then the masterbatch is blow-molded to obtain the high-strength plastic film.

[0021] Furthermore, the temperatures of each zone of the twin-screw extruder are as follows: Zone 1 150℃, Zone 2 155℃, Zone 3 165℃, Zone 4 170℃, Zone 5 170℃, and Zone 6 175℃, with a rotation speed of 50-70 rpm.

[0022] Furthermore, during the blow molding process, the blown film temperature is 155-170℃, the screw speed is 50-100 rpm, the traction ratio is 5-8, and the blow-up ratio is 3-4.

[0023] The present invention has the following advantages over the prior art:

[0024] 1. This invention modifies halloysite to be flame-retardant and antioxidant and adds it to a PLA matrix. This not only strengthens the rigidity and strength of the PLA matrix as an inorganic filler, but also has the dual functions of flame retardancy and antioxidant. At the same time, the tubular structure of halloysite nanotubes can form tortuous paths in the PLA matrix, delaying the diffusion of water vapor and oxygen, reducing water vapor permeability and oxygen permeability coefficient. When applied to food packaging, it can extend the shelf life of food.

[0025] 2. The melamine structure introduced into the flame-retardant modified halloysite in this invention can decompose at high temperatures to generate non-flammable gases such as nitrogen, diluting flammable gases; and the phenylphosphonic acid structure can promote char formation, forming a dense char layer that isolates oxygen and heat, achieving synergistic flame retardancy between the gas phase and condensed phase, thereby improving the flame-retardant performance of the film. Curcumin, as a natural polyphenolic antioxidant, can capture free radicals through its phenolic hydroxyl groups, interrupting the oxidation chain reaction; this invention grafts it onto the surface of halloysite, avoiding migration loss caused by direct addition, and endowing the film with long-lasting antioxidant capabilities.

[0026] 3. This invention, by introducing polyurethane resin and synergistically combining it with a specific ratio of silane coupling agent, effectively improves the toughness and fracture resistance of the film while maintaining the rigidity of PLA. The silane coupling agent also improves the interfacial compatibility between halloysite and the PLA matrix. The surfaces of the two modified halloysites are still rich in active groups such as amino or hydroxyl groups, which can covalently bond with the hydrolysis products of the silane coupling agent. Meanwhile, the amino group at the other end of the coupling agent can react with the terminal groups of the PLA molecular chain, thereby promoting the uniform dispersion of the nanofiller, effectively transferring stress, and improving the tensile strength and elongation at break of the film. Attached Figure Description

[0027] Figure 1 Infrared spectra of the flame-retardant modified halloysite and the antioxidant modified halloysite of this invention. Detailed Implementation

[0028] The technical solution of the present invention will be further described below with reference to specific embodiments. However, those skilled in the art should understand that the following embodiments are only for illustrating the present invention and should not be regarded as limiting the present invention. Specific conditions not specified in the embodiments are performed according to conventional conditions or conditions recommended by the manufacturer. Unless otherwise specified, the reagents or instruments used are all conventional products obtained through commercial channels. In the following embodiments or comparative examples, the polylactic acid has a weight-average molecular weight of 50,000 g / mol, and the polyurethane resin is a polyester-type polyurethane.

[0029] Example 1

[0030] Example 1 provides a high-strength plastic film comprising the following raw materials in parts by weight: 85 parts polylactic acid, 8 parts polyurethane resin, 4 parts flame-retardant modified halloysite, 5 parts antioxidant modified halloysite, 1 part γ-aminopropyltriethoxysilane, and 2 parts tributyl citrate.

[0031] The preparation processes for flame-retardant modified halloysite and antioxidant modified halloysite are as follows:

[0032]

[0033] (1) According to the ratio of halloysite, phenylphosphonic acid and deionized water, 1 g: 2.5 g: 25 mL, halloysite was dispersed in deionized water, and then phenylphosphonic acid was added for reflux reaction for 45 h; the reaction solution was cooled to room temperature, the solid material was filtered out, washed with distilled water, and dried to obtain phenylphosphonic acid modified halloysite.

[0034] (2) According to the ratio of phenylphosphonic acid modified halloysite, oxalyl chloride, dichloromethane, and N,N-dimethylformamide, 1 g: 7.5 g: 25 mL: 1.5 g, the phenylphosphonic acid modified halloysite obtained in step (1) was dispersed in dichloromethane, and N,N-dimethylformamide was added as a catalyst. The mixture was heated to 45°C, and oxalyl chloride was added dropwise over 25 min. After the addition was completed, the reaction was stirred for 3.5 h. After the reaction was completed, the solvent and unreacted oxalyl chloride were removed under vacuum to obtain phenylphosphonic acid modified halloysite.

[0035] (3) According to the ratio of phenylphosphonic chloride-modified halloysite, melamine, triethylamine and N,N-dimethylformamide 1g:0.5g:0.6g:65mL, melamine and triethylamine are dissolved in an appropriate amount of N,N-dimethylformamide to form a mixed solution. Then, the phenylphosphonic chloride-modified halloysite obtained in step (2) is added to the remaining N,N-dimethylformamide. The above mixed solution is added dropwise at -5℃. After the addition is completed, the reaction solution is raised to room temperature and stirred continuously for 21 h. The solid product is collected by filtration, washed with N,N-dimethylformamide and water in sequence, and dried to obtain flame-retardant modified halloysite.

[0036] (4) According to the ratio of phenylphosphonic chloride-modified halloysite, curcumin, triethylamine and ethyl acetate 1 g: 0.6 g: 0.6 g: 70 mL, curcumin and triethylamine were dissolved in an appropriate amount of ethyl acetate to form a mixed solution. The phenylphosphonic chloride-modified halloysite obtained in step (2) was added to the remaining ethyl acetate. The above mixed solution was added dropwise at -5℃. After the addition was completed, the temperature was kept for 25 min. Then the reaction solution was raised to room temperature and stirred continuously for 44 h. The solid product was collected by filtration and washed with ethyl acetate, acetonitrile and water in sequence. After drying, antioxidant modified halloysite was obtained.

[0037] Figure 1 FT-IR spectra of halloysite, flame-retardant modified halloysite, and antioxidant modified halloysite, from... Figure 1 It can be seen that halloysite is at 3695 cm. -1 and 3620 cm -1 A stretching vibration peak of -OH appears at 1640 cm⁻¹. -1 The characteristic peak for adsorbed water is located at 1040-1050 cm⁻¹. -1 Stretching vibrations attributed to Si-O-Si and Si-O-Al, 910 cm⁻¹ -1 This is the bending vibration of Al-OH. Compared to halloysite, flame-retardant modified halloysite exhibits better performance at 3100-3300 cm⁻¹. -1 1670cm -1 The characteristic NH2 peak of melamine appears at 1510 cm⁻¹. -1 and 1405cm -1A characteristic peak of the triazine ring skeleton was observed at 1450 cm⁻¹. -1 690 cm -1 The characteristic peak of the aromatic ring (phenylphosphonic acid) appears, and at 3620 cm⁻¹ -1 910 cm -1 The hydroxyl peak was significantly weakened, proving the successful preparation of flame-retardant modified halloysite. Antioxidant modified halloysite showed a peak at 1630 cm⁻¹. -1 A characteristic C=O peak appears at 1500 cm⁻¹. -1 Fanghuan Peak, 960cm, appears nearby. -1 An olefin CH vibration peak appears at 3620 cm⁻¹. -1 910cm -1 The hydroxyl peak was significantly weakened, proving the successful preparation of antioxidant modified halloysite.

[0038] This embodiment also provides a method for preparing the above-mentioned high-strength plastic film, including the following steps:

[0039] According to the stated weight proportions, polylactic acid, polyurethane resin, flame-retardant modified halloysite, and antioxidant modified halloysite were vacuum dried at 80 °C for 4 h to remove adsorbed moisture. Then, polylactic acid, polyurethane resin, and silane coupling agent were mixed in a high-speed mixer for 5-10 min to obtain a premix. The premix, along with the flame-retardant modified halloysite, antioxidant modified halloysite, and tributyl citrate, were fed into a twin-screw extruder for melt extrusion and granulation to obtain a masterbatch. The temperatures of each zone of the twin-screw extruder were: zone 1 150 °C, zone 2 155 °C, zone 3 165 °C, zone 4 170 °C, zone 5 170 °C, and zone 6 175 °C, with a rotation speed of 60 rpm. The masterbatch was then blown into a film using a blown film extruder to obtain the high-strength plastic film. The blown film extruder settings were: zone 1 155 °C, zone 2 160 °C, zone 3 165 °C, zone 4 170 °C, and the main screw rotation speed was 80 rpm. rpm, traction ratio of 6, inflation ratio of 3.

[0040] Example 2

[0041] Example 2 provides a high-strength plastic film comprising the following raw materials in parts by weight: 80 parts polylactic acid, 5 parts polyurethane resin, 2 parts flame-retardant modified halloysite, 2 parts antioxidant modified halloysite, 0.5 parts γ-aminopropyltriethoxysilane, and 1 part PEG-400.

[0042] The preparation processes for flame-retardant modified halloysite and antioxidant modified halloysite are as follows:

[0043] (1) According to the ratio of halloysite, phenylphosphonic acid and deionized water, 1 g: 2 g: 20 mL, halloysite was dispersed in deionized water, and then phenylphosphonic acid was added and refluxed for 40 h. The reaction solution was cooled to room temperature, the solid material was filtered out, washed with distilled water and dried to obtain phenylphosphonic acid modified halloysite.

[0044] (2) According to the ratio of phenylphosphonic acid modified halloysite, oxalyl chloride, dichloromethane, and N,N-dimethylformamide, 1 g: 7 g: 20 mL: 1.4 g, the phenylphosphonic acid modified halloysite obtained in step (1) was dispersed in dichloromethane, and N,N-dimethylformamide was added as a catalyst. The mixture was heated to 40°C, and oxalyl chloride was added dropwise over 30 min. After the addition was completed, the reaction was stirred for 4 h. After the reaction was completed, the solvent and unreacted oxalyl chloride were removed under vacuum to obtain phenylphosphonic acid modified halloysite.

[0045] (3) According to the ratio of phenylphosphonic chloride modified halloysite, melamine, triethylamine and N,N-dimethylformamide 1g:0.6g:0.5g:60mL, melamine and triethylamine were dissolved in an appropriate amount of N,N-dimethylformamide to form a mixed solution. Then, the phenylphosphonic chloride modified halloysite obtained in step (2) was added to the remaining N,N-dimethylformamide. The above mixed solution was added dropwise at 0℃. After the addition was completed, the reaction solution was raised to room temperature and stirred continuously for 24 h. The solid product was collected by filtration, washed with N,N-dimethylformamide and water in sequence, and dried to obtain flame-retardant modified halloysite.

[0046] (4) According to the ratio of phenylphosphonic chloride-modified halloysite, curcumin, triethylamine and ethyl acetate 1g:0.5g:0.5g:60mL, curcumin and triethylamine were dissolved in an appropriate amount of ethyl acetate to form a mixed solution. The phenylphosphonic chloride-modified halloysite obtained in step (2) was added to the remaining ethyl acetate. The above mixed solution was added dropwise at 0℃. After the addition was completed, the temperature was kept for 30 min. Then the reaction solution was raised to room temperature and stirred continuously for 48 h. The solid product was collected by filtration and washed with ethyl acetate, acetonitrile and water in sequence. After drying, antioxidant modified halloysite was obtained.

[0047] This embodiment also provides a method for preparing the above-mentioned high-strength plastic film, including the following steps:

[0048] According to the stated weight proportions, polylactic acid, polyurethane resin, flame-retardant modified halloysite, and antioxidant modified halloysite were vacuum dried at 80 °C for 4 h to remove adsorbed moisture. Then, polylactic acid, polyurethane resin, and silane coupling agent were mixed in a high-speed mixer for 5-10 min to obtain a premix. The premix, along with the flame-retardant modified halloysite, antioxidant modified halloysite, and PEG 400, were fed into a twin-screw extruder for melt extrusion and granulation to obtain masterbatch. The temperatures of each zone of the twin-screw extruder were: zone 1 150 °C, zone 2 155 °C, zone 3 165 °C, zone 4 170 °C, zone 5 170 °C, and zone 6 175 °C, with a rotation speed of 50 rpm. The masterbatch was then blown into film using a blown film extruder to obtain the high-strength plastic film. The blown film extruder settings were: zone 1 155 °C, zone 2 160 °C, zone 3 165 °C, zone 4 170 °C, and the main screw rotation speed was 50 rpm. rpm, traction ratio of 5, inflation ratio of 3.

[0049] Example 3

[0050] Example 3 provides a high-strength plastic film comprising the following raw materials in parts by weight: 90 parts polylactic acid, 10 parts polyurethane resin, 6 parts flame-retardant modified halloysite, 6 parts antioxidant modified halloysite, 2 parts γ-aminopropyltriethoxysilane, and 3 parts tributyl citrate.

[0051] The preparation processes for flame-retardant modified halloysite and antioxidant modified halloysite are as follows:

[0052] (1) According to the ratio of halloysite, phenylphosphonic acid and deionized water, 1 g: 3 g: 30 mL, halloysite was dispersed in deionized water, and then phenylphosphonic acid was added for reflux reaction for 48 h; the reaction solution was cooled to room temperature, the solid material was filtered out, washed with distilled water, and dried to obtain phenylphosphonic acid modified halloysite.

[0053] (2) According to the ratio of phenylphosphonic acid modified halloysite, oxalyl chloride, dichloromethane, and N,N-dimethylformamide, 1 g: 8 g: 30 mL: 1.6 g, the phenylphosphonic acid modified halloysite obtained in step (1) was dispersed in dichloromethane, and N,N-dimethylformamide was added as a catalyst. The mixture was heated to 50°C, and oxalyl chloride was added dropwise over 20 min. After the addition was completed, the mixture was stirred for 3 h. After the reaction was completed, the solvent and unreacted oxalyl chloride were removed under vacuum to obtain phenylphosphonic acid modified halloysite.

[0054] (3) According to the ratio of phenylphosphonic chloride modified halloysite, melamine, triethylamine and N,N-dimethylformamide 1g:0.6g:0.6g:70mL, melamine and triethylamine are dissolved in an appropriate amount of N,N-dimethylformamide to form a mixed solution. Then, the phenylphosphonic chloride modified halloysite obtained in step (2) is added to the remaining N,N-dimethylformamide. The above mixed solution is added dropwise at 0℃. After the addition is completed, the reaction solution is raised to room temperature and stirred continuously for 24 h. The solid product is collected by filtration, washed with N,N-dimethylformamide and water in sequence, and dried to obtain flame-retardant modified halloysite.

[0055] (4) According to the ratio of phenylphosphonic chloride-modified halloysite, curcumin, triethylamine and ethyl acetate 1 g: 0.8 g: 0.8 g: 80 mL, curcumin and triethylamine were dissolved in an appropriate amount of ethyl acetate to form a mixed solution. The phenylphosphonic chloride-modified halloysite obtained in step (2) was added to the remaining ethyl acetate. The above mixed solution was added dropwise at 0℃. After the addition was completed, the temperature was kept for 20 min. Then the reaction solution was raised to room temperature and stirred continuously for 48 h. The solid product was collected by filtration and washed with ethyl acetate, acetonitrile and water in sequence. After drying, antioxidant modified halloysite was obtained.

[0056] This embodiment also provides a method for preparing the above-mentioned high-strength plastic film, including the following steps:

[0057] According to the stated weight proportions, polylactic acid, polyurethane resin, flame-retardant modified halloysite, and antioxidant modified halloysite were vacuum dried at 80 °C for 4 h to remove adsorbed moisture. Then, polylactic acid, polyurethane resin, and silane coupling agent were mixed in a high-speed mixer for 5-10 min to obtain a premix. The premix, along with the flame-retardant modified halloysite, antioxidant modified halloysite, and tributyl citrate, were fed into a twin-screw extruder for melt extrusion and granulation to obtain a masterbatch. The temperatures of each zone of the twin-screw extruder were: zone 1 150 °C, zone 2 155 °C, zone 3 165 °C, zone 4 170 °C, zone 5 170 °C, and zone 6 175 °C, with a rotation speed of 70 rpm. The masterbatch was then blown into a film using a blown film extruder to obtain the high-strength plastic film. The blown film extruder settings were: zone 1 155 °C, zone 2 160 °C, zone 3 165 °C, zone 4 170 °C, and the main screw rotation speed was 100 rpm. rpm, traction ratio is 8, inflation ratio is 4.

[0058] Comparative Example 1

[0059] Comparative Example 1 is basically the same as Example 1, except that the flame-retardant modified halloysite and antioxidant modified halloysite in Example 1 are replaced with halloysite.

[0060] Test case

[0061] The films prepared in Examples 1-3 and Comparative Example 1 were subjected to the following performance tests.

[0062] 1. Mechanical property testing

[0063] According to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", the sample films prepared in Examples 1-3 and Comparative Example 1 were cut into 150 mm × 15 mm strips. The initial distance between the clamps was set to 100 mm, and the tensile speed was 10 mm / min. The tensile strength and elongation at break of each sample film were measured. Five parallel samples were tested in each group, and the results were recorded as average values, as shown in Table 1.

[0064] 2. Flame retardant performance test

[0065] The limiting oxygen index of the sample films prepared in Examples 1-3 and Comparative Example 1 was tested according to GB / T 2406.2-2009 "Determination of Combustion Behavior by Oxygen Index Method for Plastics - Part 2: Room Temperature Test". The limiting oxygen index of each sample film was tested 5 times and the average value was taken. The results are shown in Table 1.

[0066] 3. Antioxidant performance test

[0067] Weigh 500 mg of the film material prepared in Examples 1-3 and Comparative Example 1 respectively, add it to 15 mL of anhydrous ethanol, sonicate for 2 h, and centrifuge at 6000 rpm for 30 min to mix thoroughly, and use it as the sample to be tested.

[0068] First, prepare a 0.03 g / L DPPH working solution using 95% ethanol, store it in the dark, and prepare it fresh each time. Then, add 0.1 mL of 95% ethanol solution to 5 mL of the 0.03 g / L DPPH working solution, mix well, and react in the dark for 30 min. After stabilization, use 95% ethanol as a reference and measure the absorbance at 518 nm. Repeat the measurement three times and take the average value, which is recorded as A0. Add 0.1 mL of each test sample to 5 mL of the 0.03 g / L DPPH working solution, mix well, and react in the dark for 30 min. After stabilization, use 95% ethanol as a reference and measure the absorbance at 518 nm. Repeat the measurement three times and take the average value, which is recorded as A1. Add 0.1 mL of each test sample to 5 mL of 95% ethanol solution, mix well, and react in the dark for 30 min. After stabilization, use 95% ethanol as a reference and measure the absorbance at 518 nm. Repeat the measurement three times and take the average value, which is recorded as A2.

[0069] The DPPH radical scavenging rate is calculated using the formula: DPPH radical scavenging rate = {1 - [(A1 - A2) / A0]} × 100%. The results of the DPPH radical scavenging rate are shown in Table 1.

[0070] 4. Barrier performance test

[0071] The water vapor transmission capacity (WVT) of each film sample was determined according to GB / T 1037-2021 "Determination of Water Vapor Transmission Performance of Plastic Films and Sheets - Cup Weight Gain and Weight Loss Method". The film diameter was 74 mm, the test temperature was 38℃, the relative humidity was 10%, the humidity in the cup was 100%, and the test time was 48 h. Three parallel tests were set up for each sample and the average value was taken. The results are shown in Table 1.

[0072] The oxygen permeability coefficient of each film sample was determined according to GB / T 1038.1-2022 "Test method for gas permeability of plastic films and sheets - Part 1: Differential pressure method". Three parallel tests were set up for each sample and the average value was taken. The results are shown in Table 1.

[0073] Table 1

[0074]

[0075] As shown in Table 1, Examples 1-3 have excellent mechanical properties, flame retardant properties, and antioxidant properties, and also have good barrier properties.

[0076] Compared to Example 1, Comparative Example 1 replaced both the flame-retardant modified halloysite and the antioxidant modified halloysite with unmodified halloysite. The mechanical properties, flame-retardant properties, and antioxidant properties all decreased significantly. This indicates that the present invention, through flame-retardant and antioxidant modification of halloysite, can achieve a multifunctional integrated PLA film with high strength, high flame retardancy, and high antioxidant properties. Specifically, in terms of mechanical properties, halloysite, as an inorganic filler, can strengthen the rigidity and strength of the PLA matrix. The addition of a silane coupling agent can improve the interfacial compatibility between halloysite and the PLA matrix, promoting its uniform dispersion, thereby effectively transferring stress and improving the tensile strength and elongation at break of the film. In terms of flame retardancy, the present invention introduces phenylphosphonic acid and melamine structures into halloysite to improve the flame-retardant properties of the film. The melamine structure can decompose at high temperatures to generate non-combustible gases such as nitrogen, diluting combustible gases, while the phenylphosphonic acid structure can promote char formation, forming a dense char layer that isolates oxygen and heat, thereby achieving gas-phase and condensed-phase cross-linking flame retardancy. In terms of antioxidant properties, this invention grafts curcumin onto the surface of halloysite, utilizing its phenolic hydroxyl groups to capture free radicals, interrupting the oxidation chain reaction, and also avoiding migration loss caused by direct addition, thus endowing the film with long-lasting antioxidant capabilities.

[0077] In summary, this invention, by modifying halloysite for flame retardancy and oxidation and adding it to a PLA matrix, not only strengthens the rigidity and strength of the PLA matrix as an inorganic filler, but also possesses both flame retardant and oxidation-resistant functions. Simultaneously, the tubular structure of halloysite nanotubes forms tortuous paths within the PLA matrix, slowing the diffusion of water vapor and oxygen, reducing water vapor permeability and oxygen permeability coefficients. When applied to food packaging, this can extend the shelf life of food.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. The basic principles and main features of the present invention have been described above with specific implementation schemes. Based on the present invention, some modifications or substitutions can be made, but these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of protection claimed by the present invention.

Claims

1. A high-strength plastic film, characterized in that, The raw materials include the following parts by weight: 80-90 parts polylactic acid, 5-10 parts polyurethane resin, 2-6 parts flame retardant modified halloysite, 2-6 parts antioxidant modified halloysite, 0.5-2 parts silane coupling agent, and 1-3 parts plasticizer. The preparation process of the flame-retardant modified halloysite is as follows: (1) Halloysite was dispersed in deionized water and then phenylphosphonic acid was added for reflux reaction; after cooling to room temperature, it was filtered, washed and dried to obtain phenylphosphonic acid modified halloysite; (2) Disperse the phenylphosphonic acid modified halloysite from step (1) into dichloromethane, add N,N-dimethylformamide, heat to 40-50℃ and then add oxalyl chloride to react; after the reaction is completed, remove the solvent to obtain phenylphosphonic acid modified halloysite; (3) Melamine and triethylamine were dissolved in N,N-dimethylformamide to obtain a mixed solution; then the phenylphosphonic chloride-modified halloysite from step (2) was dispersed in N,N-dimethylformamide, and the mixed solution was added dropwise at -5~0℃, and then the reaction was carried out at room temperature; the solid product was collected by filtration, washed and dried to obtain the flame-retardant modified halloysite; The preparation process of the antioxidant modified halloysite is as follows: curcumin and triethylamine are dissolved in ethyl acetate to obtain a mixture; then the phenylphosphonic chloride modified halloysite from step (2) is dispersed in ethyl acetate, the mixture is added dropwise at -5~0℃ and kept warm for 20-30 min, and then the temperature is raised to room temperature for reaction; the solid product is collected by filtration, washed and dried to obtain the antioxidant modified halloysite.

2. The high-strength plastic film according to claim 1, characterized in that, The mass ratio of halloysite to phenylphosphonic acid in step (1) is 1:(2-3); the reflux reaction time is 40-48 h.

3. The high-strength plastic film according to claim 1, characterized in that, In step (2), the mass ratio of phenylphosphonic acid-modified halloysite, oxaloyl chloride, and N,N-dimethylformamide is 1:(7-8):(1.4-1.6); the reaction time is 3-4 h.

4. The high-strength plastic film according to claim 1, characterized in that, In step (3), the mass ratio of phenylphosphonic chloride-modified halloysite, melamine, and triethylamine is 1:(0.5-0.6):(0.5-0.6); the reaction time is 20-24 h.

5. The high-strength plastic film according to claim 1, characterized in that, In the preparation of antioxidant modified halloysite, the mass ratio of phenylphosphonic chloride modified halloysite, curcumin, and triethylamine is 1:(0.5-0.8):(0.5-0.8); the reaction time is 40-48 h.

6. The high-strength plastic film according to claim 1, characterized in that, The polylactic acid has a weight-average molecular weight of 10,000-100,000 g / mol; the polyurethane resin is a polyester-type polyurethane; the silane coupling agent is γ-aminopropyltriethoxysilane; and the plasticizer is tributyl citrate or PEG-400.

7. A method for preparing a high-strength plastic film according to any one of claims 1-6, characterized in that, Includes the following steps: According to the stated weight proportions, polylactic acid, polyurethane resin and silane coupling agent are mixed to obtain a premix; then the premix, flame-retardant modified halloysite, antioxidant modified halloysite and plasticizer are fed into a twin-screw extruder for melt extrusion and granulation to obtain a masterbatch; then the masterbatch is blow-molded to obtain the high-strength plastic film.

8. The method for preparing a high-strength plastic film according to claim 7, characterized in that, The temperatures of each zone of the twin-screw extruder are as follows: Zone 1 150℃, Zone 2 155℃, Zone 3 165℃, Zone 4 170℃, Zone 5 170℃, and Zone 6 175℃, with a rotation speed of 50-70 rpm.

9. The method for preparing a high-strength plastic film according to claim 7, characterized in that, During the blow molding process, the blown film temperature is 155-170℃, the screw speed is 50-100 rpm, the traction ratio is 5-8, and the blow-up ratio is 3-4.

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