Multi-layer co-extrusion heat shrinkage film and preparation method thereof

Abstract

The invention belongs to the field of multi-layer co-extrusion thermal shrinkage films, and particularly relates to a preparation method of a multi-layer co-extrusion thermal shrinkage film, which comprises the following steps: taking an inner layer and an outer layer made of LLDPE (Linear Low Density Polyethylene) resin, taking a temperature-variable master batch formed by mixing a temperature-variable microcapsule and HDPE (High-Density Polyethylene) according to a mass ratio of 1: 4 as a middle layer, extruding and plasticizing, overlapping a die head through multi-layer co-extrusion, blowing and forming, and cooling and shaping. The anti-counterfeiting effect formed by utilizing the color change mechanism of the temperature change material and the synergistic effect of the multi-layer co-extrusion process is not easily copied by illegal merchants, consumers can trigger the color change reaction of the heat shrink film only through the simple behavior of heating or warm water spraying on the surface of the heat shrink film without the help of additional tools or complex processes, and compared with a traditional anti-counterfeiting means, the anti-counterfeiting effect is better. The method has the advantages that the difficulty in distinguishing the authenticity of the product is reduced for consumers, and the consumers can quickly and accurately judge whether the product is counterfeit or not on the purchase site.

Description

Technical Field

[0001] The present invention relates to the field of multi-layer co-extruded heat-shrinkable films, and specifically to a multi-layer co-extruded heat-shrinkable film and a preparation method thereof. Background Art

[0002] In today's commodity market, packaging plays a crucial role in protecting products, displaying them, and establishing brand images. Especially in industries such as liquor and cosmetics, heat-shrinkable films are widely used for packaging box encapsulation. Heat-shrinkable films tightly wrap the packaging boxes through the heat-shrinkage principle, not only providing physical protection for products to prevent damage such as collision and scratching during transportation and storage, but also enhancing the display effect of products due to their transparent characteristics, enabling consumers to directly see the product appearance. With the development of packaging technology, multi-layer co-extruded heat-shrinkable films have gradually become the mainstream due to their excellent performance. They combine the characteristics of multiple materials and have better flexibility, strength, barrier properties, etc., which can effectively extend the shelf life of products and improve the overall quality of packaging;

[0003] For the currently widely used transparent multi-layer co-extruded heat-shrinkable films on liquor and cosmetics packaging boxes, their anti-counterfeiting methods mainly rely on printed anti-counterfeiting marks on product labels or packaging, such as laser anti-counterfeiting labels, two-dimensional code anti-counterfeiting, etc. However, these anti-counterfeiting means are extremely easy to be copied and counterfeited by lawbreakers. Bad merchants can recycle genuine product packaging and re-encapsulate counterfeit products using ordinary heat-shrinkable films. Consumers are difficult to directly distinguish the authenticity through the appearance. Therefore, a multi-layer co-extruded heat-shrinkable film and a preparation method thereof are proposed to solve the above problems. Summary of the Invention

[0004] In order to make up for the problems of the existing transparent multi-layer co-extruded heat-shrinkable film, whose anti-counterfeiting method mainly relies on printed anti-counterfeiting marks on product labels or packaging, is easy to be copied and counterfeited by lawbreakers, bad merchants can recycle genuine product packaging and re-encapsulate counterfeit products using ordinary heat-shrinkable films, and consumers are difficult to directly distinguish the authenticity through the appearance, the present invention proposes a multi-layer co-extruded heat-shrinkable film and a preparation method thereof.

[0005] The technical solution adopted by the present invention to solve its technical problems is: a preparation method of a multi-layer co-extruded heat-shrinkable film, including an inner layer and an outer layer made of LLDPE resin, a temperature-changing masterbatch obtained by mixing temperature-changing microcapsules and HDPE in a mass ratio of 1:4 as the middle layer, extrusion plasticization, multi-layer co-extrusion and superposition die head, blow molding and cooling and shaping.

[0006] The processing steps of the temperature-changing microcapsules include:

[0007] S1: Raw material preparation

[0008] Core material: 30% of bisphenol A leuco dye (crystal violet lactone), 65% of dodecanol (solvent), 5% of initiator (benzoyl peroxide).

[0009] Wall material: Polyurethane prepolymer (NC0 content 18%) and epoxy resin (EP-828) are mixed in a ratio of 3:1.

[0010] S2: Mixing and dissolving

[0011] Add the core material into the reaction kettle to fully dissolve and mix the raw materials to form a uniform solution.

[0012] S3: Emulsification treatment

[0013] Wait for the solution to cool to 60°C, add an emulsifier (such as sorbitan monooleate, i.e., Span-80) accounting for 8% of the total mass of the raw materials and deionized water, and stir at a speed of 1000 r / min for 60 min to form a stable emulsion.

[0014] S4: Drying treatment

[0015] Dry the emulsion through a spray drying device to quickly evaporate the water in the emulsion and obtain thermochromic microcapsules with a particle size of 5 - 10 μm.

[0016] The processing steps of the thermochromic masterbatch include:

[0017] S5: Put the prepared thermochromic microcapsules and high-density polyethylene into a high-speed mixer according to a mass ratio of 1:8.

[0018] S6: Add a dispersant accounting for 3% of the total mass, and stir and mix at 80°C at a speed of 500 r / min for 20 min to uniformly disperse the thermochromic material microcapsules in HDPE.

[0019] S7: Granulate the mixed materials through a twin-screw extruder. The temperature settings of each section of the extruder are as follows: zone 1 at 170°C, zone 2 at 180°C, zone 3 at 190°C, zone 4 at 200°C, zone 5 at 200°C, the die head at 200°C, and the screw speed is controlled at 250 r / min to obtain the thermochromic masterbatch.

[0020] Preferably, in the mixing and dissolving of step S2, the reaction kettle is a reaction kettle with a stirring device and a heating device.

[0021] Preferably, in the emulsification treatment of step S3, ultrasonic-assisted treatment is carried out. By using an ultrasonic generating device with a frequency of 40 kHz, intermittent ultrasound (working for 5 s / interval for 2 s) is applied during the emulsification stirring process, and the total action time is 15 min.

[0022] Preferably, the spray drying uses a centrifugal atomizer, with an inlet temperature of 180 - 200°C, an outlet temperature of 80 - 100°C, and an atomization pressure of 0.2 - 0.4 MPa.

[0023] Preferably, in the preparation of the core material raw materials in step S1, aminated silica nanoparticles equivalent to 5 wt% of the total mass of the core material are additionally added.

[0024] Preferably, the particle size of the aminated silica nanoparticles is 30 nm.

[0025] Preferably, on the basis of the core material raw materials, the aminated silica nanoparticles and the core material raw materials are added to the reaction kettle synchronously, the stirring speed is 400 - 500 r / min, and after heating to 90 °C, stirring is maintained for 60 min to make the nanoparticles uniformly dispersed.

[0026] Preferably, the dispersant in step S6 is ethylene-vinyl acetate copolymer.

[0027] A multilayer co-extruded heat-shrinkable film is prepared by a preparation method of a multilayer co-extruded heat-shrinkable film.

[0028] The beneficial effects of the present invention are as follows:

[0029] 1. The present invention utilizes the discoloration mechanism of the temperature-changing material, and the anti-counterfeiting effect formed by its synergistic effect with the multilayer co-extrusion process is not easily replicated by illegal merchants. Consumers do not need to rely on additional tools or complex processes. Only by simply breathing hot air or sprinkling warm water on the surface of the heat-shrinkable film can trigger the discoloration reaction of the heat-shrinkable film. Compared with traditional anti-counterfeiting means, such as scanning codes for querying and comparing complex laser anti-counterfeiting labels, etc., it reduces the difficulty for consumers to distinguish the authenticity of products, enables consumers to quickly and accurately judge whether the product is fake or shoddy at the point of purchase, effectively protects the legitimate rights and interests of consumers, effectively curbs the spread of fake and shoddy products, and purifies the market environment;

[0030] 2. Through the design of the polyurethane / epoxy composite wall material, the present invention raises the upper temperature limit of the temperature-changing microcapsules from 120 °C to 180 °C, meeting the requirements of blown film processing.

[0031] 3. By adding 5 wt% of aminated silica nanoparticles, the present invention improves the heat transfer efficiency by 40% by increasing the thermal conductivity of the material, and reduces the lower limit of the discoloration temperature range to 40 °C, thus providing convenience for users to test the discoloration effect of the heat-shrinkable film. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0033] Figure 1 It is a comparison chart for the color rendering performance test;

[0034] Figure 2 It is the curve of the color difference of the thermochromic film changing with temperature Figure 1 ;

[0035] Figure 3 It is the curve of the color difference of the thermochromic film changing with temperature Figure 2 . Specific implementation manners

[0036] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] The following further elaborates on this application Figures 1-3 in further detail.

[0038] Embodiment 1:

[0039] A preparation method of a multi-layer co-extruded heat-shrinkable film includes an inner layer and an outer layer made of LLDPE resin, a thermochromic masterbatch obtained by mixing thermochromic microcapsules and HDPE in a mass ratio of 1:4 as the middle layer, extrusion plasticization, multi-layer co-extrusion and superposition die head, blow molding and cooling and shaping.

[0040] The processing steps of the thermochromic microcapsules include:

[0041] S1: Raw material preparation

[0042] Core material: 30% of bisphenol A leuco dye (crystal violet lactone), 65% of dodecanol (solvent), 5% of initiator (benzoyl peroxide).

[0043] Among them, crystal violet lactone is a commonly used thermosensitive color-changing dye that can change color under specific temperature stimulation and is the key component for the thermochromic microcapsules to achieve the thermochromic function. Dodecanol is used as a solvent, and its function is to dissolve crystal violet lactone, make it evenly dispersed in the system, and provide a suitable environment for the color-changing reaction. The initiator is used to initiate and promote the thermochromic reaction.

[0044] Wall material: Polyurethane prepolymer (NCO content 18%) and epoxy resin (EP-828) are mixed in a ratio of 3:1. Among them, the polyurethane prepolymer has good flexibility and adhesiveness, and the epoxy resin has high strength and chemical corrosion resistance. The wall material formed by mixing the two can effectively encapsulate the core material, prevent the core material from leaking, and at the same time endow the microcapsules with good mechanical properties and chemical stability.

[0045] S2: Mixing and Dissolving

[0046] Add the core material into a reaction kettle equipped with a stirring device and a heating device. Under the condition of a stirring speed of 300 r / min, raise the temperature to 90 °C at a heating rate of 5 °C / min and maintain it for 30 min to fully dissolve and mix the raw materials to form a uniform solution.

[0047] S3: Emulsification Treatment

[0048] Wait for the solution to cool to 60 °C, add an emulsifier (such as sorbitan monooleate, i.e., Span-80) accounting for 8% of the total mass of the raw materials and deionized water, and stir at a speed of 1000 r / min for 60 min to form a stable emulsion with the emulsion particle size controlled within 1 - 3 μm to ensure the performance and dispersibility of the thermochromic material.

[0049] S4: Drying Treatment

[0050] Dry the emulsion through a spray drying device to rapidly evaporate the water in the emulsion and obtain thermochromic microcapsules with a particle size of 5 - 10 μm. The spray drying uses a centrifugal atomizer with an inlet temperature of 180 - 200 °C, an outlet temperature of 80 - 100 °C, and an atomization pressure of 0.2 - 0.4 MPa.

[0051] The specific operation steps of the drying treatment are as follows:

[0052] Equipment Preheating: Before starting the spray drying device, first start the hot air system to preheat the drying chamber. Raise the temperature of the drying chamber to the set inlet air temperature. For example, when preparing thermochromic material microcapsules, the inlet air temperature is usually set to 200 °C. The purpose of preheating is to make the drying chamber reach a suitable working temperature to ensure that the droplets can rapidly evaporate water after entering the drying chamber.

[0053] Among them, the hot air system includes an air heater, a blower, and a temperature control system. The temperature control system includes installing multiple high-precision temperature sensors, such as K-type thermocouples, in the drying chamber and on the hot air pipeline. These sensors feed back the real-time temperature data to the temperature controller, and the controller adjusts the power of the air heater through the PID algorithm according to the deviation between the preset temperature and the actual temperature.

[0054] Emulsion Preparation: Ensure that the thermochromic material emulsion is thoroughly stirred and evenly mixed. The temperature of the emulsion is generally controlled between room temperature and 60°C to ensure good fluidity and atomization effect. At the same time, check whether there are impurities or agglomerates in the emulsion. If necessary, perform filtration treatment. That is, at the starting end of the emulsion delivery pipeline, the installed filter is a basket filter, and the filter mesh uses multiple layers of stainless steel woven mesh. The innermost layer is a coarse filter mesh with a pore size of about 500μm for intercepting larger particle impurities; the outer layer is a fine filter mesh with a pore size between 10 - 50μm, which can effectively filter out tiny particle impurities and agglomerates. Pressure sensors are installed at the inlet and outlet of the filter to monitor the pressure difference across the filter mesh in real time.

[0055] Start the Feeding System: When the temperature of the drying chamber reaches the set value and stabilizes, start the feeding system, and pump the emulsion into the atomizer through a delivery pump. Adjust the flow rate of the delivery pump so that the emulsion enters the atomizer at an appropriate speed. The delivery pump uses a metering pump, and the pump head is made of high-strength stainless steel or ceramic material, which has good wear resistance and corrosion resistance and can adapt to the chemical properties of the thermochromic material emulsion.

[0056] Atomization Process: After the emulsion enters the atomizer, it is dispersed into tiny droplets under the action of a high-speed rotating disk or high-pressure air flow. The droplets come into full contact with hot air in the drying chamber and start heat exchange and water evaporation. The atomization disk is made of high-strength aluminum alloy 7075 or stainless steel 316L material, which has been precision machined and surface treated, and the number of blades is generally 8 - 16.

[0057] Drying Process: After the hot air contacts the droplets, it quickly transfers heat to the droplets, causing the water in the droplets to evaporate. During the drying process, the droplets gradually become smaller and finally form solid particles. The outlet air temperature of the drying chamber is controlled at about 90°C to ensure sufficient evaporation of water and at the same time avoid the influence of too high temperature on the performance of the thermochromic material.

[0058] Gas-Solid Separation: The dried solid particles and waste gas enter the gas-solid separation system together. First, they are preliminarily separated by a cyclone separator, and most of the solid particles are separated and collected at the bottom of the cyclone separator. Then, the waste gas passes through a bag filter for further filtration to separate the remaining fine particles.

[0059] The processing steps of the thermochromic masterbatch include:

[0060] S5: Put the prepared thermochromic microcapsules and high-density polyethylene into a high-speed mixer according to a mass ratio of 1:8.

[0061] S6: Add a dispersant accounting for 3% of the total mass, and stir and mix at 80°C at a speed of 500r / min for 20min to evenly disperse the thermochromic material microcapsules in HDPE.

[0062] S7: The mixed material is pelletized through a twin-screw extruder. The temperature of each section of the extruder is set as follows: zone 1 170°C, zone 2 180°C, zone 3 190°C, zone 4 200°C, zone 5 200°C, die head 200°C, and the screw speed is controlled at 250r / min to obtain temperature-variable masterbatch.

[0063] In this embodiment, the dispersant in step S6 is ethylene-vinyl acetate copolymer.

[0064] As a further illustration of the above embodiment:

[0065] Extrusion plasticization: The three materials are added to three extruders for plasticization. The barrel temperature of the inner and outer layer extruders is set to 190-210℃, and the barrel temperature of the middle layer extruder is set to 190-220℃ to ensure that the materials are fully plasticized.

[0066] Multi-layer co-extrusion superposition die head: The melts from three extruders enter the multi-layer co-extrusion superposition die head through their respective flow channels. In the die head, the melts of each layer are superimposed in sequence to form a tube blank with a specific structure. The die head temperature is controlled at 190-200℃.

[0067] Blow molding: The tube billet extruded from the die head is inflated by compressed air, and the inflation ratio is controlled between 2-3. At the same time, it is pulled at a certain speed by a traction roller to stretch the tube billet in the transverse and longitudinal directions to form a film. The pulling speed is 15-25m / min.

[0068] Cooling and shaping: The inflated film is cooled and shaped by a combination of air cooling and water cooling. The air cooling speed is 8-12m / s, and the water cooling temperature is controlled at 15-25℃.

[0069] A multi-layer co-extruded heat shrinkable film is prepared by the preparation method of a multi-layer co-extruded heat shrinkable film in this embodiment.

[0070] Embodiment 2:

[0071] A method for preparing a multi-layer co-extruded heat shrinkable film comprises an inner layer and an outer layer made of LLDPE resin, a middle layer made of a temperature-variable masterbatch obtained by mixing temperature-variable microcapsules and HDPE in a mass ratio of 1:4, extrusion plasticization, multi-layer co-extrusion superimposed die heads, inflation molding and cooling and shaping.

[0072] In this embodiment, in the preparation of the core material in step S1, additional amino-silica nanoparticles equivalent to 5 wt % of the total mass of the core material are added, and the particle size of the amino-silica nanoparticles is 30 nm.

[0073] Based on the core material raw materials, the aminated silica nanoparticles and the core material raw materials are added to the reaction kettle synchronously. The stirring speed is 400 - 500 r / min, and after heating to 90 °C, stirring is maintained for 60 min to make the nanoparticles disperse evenly.

[0074] In this embodiment, in the emulsification treatment of step S3, ultrasonic wave-assisted treatment is carried out. By using an ultrasonic wave generating device with a frequency of 40 kHz, intermittent ultrasonic waves (working for 5 s / intermittent for 2 s) are applied during the emulsification stirring process, and the total action time is 15 min. The action of ultrasonic waves can further refine the emulsion particles, make their particle size distribution more uniform, and improve the stability of the emulsion. After testing, the average particle size of the emulsion obtained by using this emulsification treatment method is between 2 - 3 μm, which is much smaller than the particle size of the emulsion without ultrasonic wave-assisted treatment, and is beneficial to the subsequent formation of high-quality temperature-changing microcapsules.

[0075] A multilayer co-extruded heat-shrinkable film is prepared by the preparation method of a multilayer co-extruded heat-shrinkable film in this embodiment.

[0076] Comprehensive performance comparison between Example 1 and Example 2:

[0077]

[0078]

[0079] Data annotation indicates the color difference value (ΔE)

[0080] Test standard: CIE Lab color difference formula:

[0081] Test conditions: 30 °C (initial state) → 35 °C (trigger state), measured using a spectrophotometer (model X-Rite Ci64).

[0082] Error range: ±0.3.

[0083] Color development time

[0084] Definition: The time required from when exhaled breath touches the film surface to when ΔE reaches 90% of the peak value.

[0085] Test equipment: High-speed camera (1000 fps) synchronized with a temperature recorder.

[0086] Error range: ±0.2 seconds.

[0087] Among them, the calculation formula for the microcapsule breakage rate is:

[0088] Breakage rate = N(total) / N(broken) × 100%.

[0089] N (broken): The number of microcapsules penetrated by the stain.

[0090] N (total): The total number of microcapsules within the observed field of view.

[0091] Combined with the above embodiments, the standardized process of the detection method includes:

[0092] 1. Sample preparation

[0093] Take the masterbatch or film sample and prepare a cross-section (thickness 5 - 10 μm) using the liquid nitrogen brittle fracture method;

[0094] Toluene etching treatment: Immerse the sample in toluene at 60°C for 30 minutes to dissolve the HDPE matrix (retaining intact microcapsules);

[0095] Rinse 3 times with deionized water and vacuum dry at 60°C for 2 hours.

[0096] 2. Stain penetration

[0097] Prepare a 0.1% methylene blue solution (pH = 7.4 PBS buffer).

[0098] Immerse the sample in the staining solution and incubate at 60°C with constant shaking (150 rpm) for 30 minutes.

[0099] After removal, dehydrate with a gradient of ethanol (50% → 75% → 100%) to terminate the staining reaction.

[0100] 3. Microscopic observation

[0101] Use a laser confocal microscope (such as ZEISS LSM 880), excitation wavelength 638 nm;

[0102] Randomly select 10 fields of view (each field of view ≥ 100 microcapsules) and record:

[0103] Intact microcapsules: The wall material is intact and there is no blue penetration;

[0104] Broken microcapsules: The wall material is ruptured and the internal core material is stained.

[0105] 4. Data statistics

[0106] Use Image Pro Plus software to automatically identify the stained area

[0107] Calculation formula correction term (to reduce human error):

[0108]

[0109] 5. Example calculation

[0110]

[0111] 6. Method Verification

[0112] Accuracy Verification:

[0113] Use standard samples with known breakage rates (prepare control groups with 5% and 10% breakage rates by mechanical grinding method).

[0114] The measured values are 4.92±0.35% and 10.15±0.47% respectively, and the relative error < 5%.

[0115] Repeatability Verification:

[0116] Results of 3 independent tests on the same sample: 4.7%, 4.9%, 4.5%.

[0117] RSD (Relative Standard Deviation) = 4.3% < 5%, meeting the requirements of IS05725.

[0118] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed.

Claims

1. A method for preparing a multi-layer co-extruded heat shrinkable film, characterized in that: include: LLDPE resin is used as the inner and outer layers, and the middle layer is made of a mixture of temperature-sensitive microcapsules and HDPE in a mass ratio of 1:

4. The processing steps of the temperature-changing microcapsules include: S1: Raw material preparation Core material: bisphenol A type leuco dye (crystal violet lactone) 30%, dodecanol (solvent) 65%, initiator (benzoyl peroxide) 5%; Wall material: polyurethane prepolymer (NCO content 18%) and epoxy resin (EP-828) mixed in a ratio of 3:1; S2: Mixed dissolution Add the core material into the reactor to fully dissolve and mix the raw materials to form a uniform solution; S3: Emulsification When the solution is cooled to 60°C, add 8% of the total mass of the raw materials into an emulsifier (such as sorbitan monooleate, i.e. Span-80) and deionized water, and stir at a speed of 1000 r / min for 60 min to form a stable emulsion; S4: Drying The emulsion is dried by a spray drying device to evaporate the water in the emulsion quickly, thereby obtaining temperature-dependent microcapsules with a particle size of 5-10 μm; The processing steps of the temperature-changing masterbatch include: S5: putting the prepared temperature-variable microcapsules and high-density polyethylene into a high-speed mixer at a mass ratio of 1:8; S6: Add 3% of the total mass of dispersant, stir and mix at 80°C at a speed of 500 r / min for 20 min to evenly disperse the temperature-variable material microcapsules in the HDPE; S7: The mixed material is pelletized through a twin-screw extruder. The temperature of each section of the extruder is set as follows: zone 1 170°C, zone 2 180°C, zone 3 190°C, zone 4 200°C, zone 5 200°C, die head 200°C, and the screw speed is controlled at 250r / min to obtain temperature-variable masterbatch.

2. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 1, characterized in that: In the mixed dissolution in S2, the reactor is a reactor equipped with a stirring device and a heating device.

3. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 1, characterized in that: In the emulsification treatment in S3, ultrasonic-assisted treatment is performed by using an ultrasonic generator with a frequency of 40 kHz to apply intermittent ultrasound (working 5 seconds / interval 2 seconds) during the emulsification stirring process, with a total action time of 15 minutes.

4. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 1, characterized in that: The spray drying adopts a centrifugal atomizer with an inlet temperature of 180-200°C, an outlet temperature of 80-100°C and an atomization pressure of 0.2-0.4MPa.

5. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 1, characterized in that: In the preparation of the core material in step S1, additional amino-containing silica nanoparticles are added in an amount equivalent to 5 wt % of the total mass of the core material.

6. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 5, characterized in that: The particle size of the amino-silica nanoparticles is 30 nm.

7. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 5, characterized in that: On the basis of the core material, the amino silicon dioxide nanoparticles and the core material are added into the reaction kettle simultaneously, with a stirring speed of 400-500 r / min, and after heating to 90° C., stirring is maintained for 60 min to make the nanoparticles evenly dispersed.

8. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 1, characterized in that: The dispersant in step S6 is ethylene-vinyl acetate copolymer.

9. The method for preparing a multi-layer co-extruded heat shrinkable film according to claim 1, characterized in that: It also includes extrusion plasticization, multi-layer co-extrusion superposition die head, inflation molding and cooling shaping.

10. A multi-layer co-extruded heat shrinkable film, prepared by the method for preparing a multi-layer co-extruded heat shrinkable film according to any one of claims 1 to 8.

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