A high-barrier aluminum-free cooking-resistant PET film and its preparation method and application

Abstract

The present invention discloses a high-barrier aluminum-free cooking-resistant PET film and its preparation method and application, belonging to the technical field of packaging films. The PET film includes a substrate and a coating on the surface of the substrate, a glue layer is arranged between the coating and the substrate, and the coating is formed by coating the cooking-resistant polyvinylidene chloride latex obtained by modification on the surface of the glue layer and drying; the cooking-resistant polyvinylidene chloride latex includes the following raw materials: vinylidene chloride, ethylene-vinyl alcohol copolymer, acrylonitrile, isocyanate, acrylamide, modified inorganic filler, initiator, softened water and slip agent; the modified inorganic filler is an oxide wrapped by epoxy phenolic resin, and the oxide includes aluminum oxide, silicon dioxide and zirconium oxide according to the mass ratio (3-5): (8-10): 1. The PET film prepared by the present invention not only has excellent cooking resistance and high barrier performance, but also has good mechanical strength.

Description

Technical Field

[0001] The invention belongs to the technical field of packaging films, and in particular relates to a high-barrier aluminum-free cooking-resistant PET film and a preparation method and application thereof. Background Art

[0002] Plastic packaging bags are a widely used form of packaging, which are used in various fields due to their advantages of lightness, durability and low cost. Commonly used plastic packaging bag materials include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyvinylidene chloride (PVDC), etc. These plastic materials have formed a variety of plastic bags due to their different characteristics and processing methods to meet the packaging needs of different industries.

[0003] With the globalization of food processing and logistics systems, the selection of packaging materials has become crucial. They must be able to effectively resist external environmental factors such as humidity, oxygen and light during the transportation and storage of food. At the same time, consumers are increasingly demanding convenient, fast and easy-to-operate food packaging. In response to these challenges, a large number of food packaging products with high barrier properties, high impact resistance, high temperature cooking resistance and medium resistance have appeared on the market. These products ensure the freshness and nutritional value of convenience foods and effectively extend their shelf life. High-temperature cooking packaging film is a composite plastic film bag that can be heated. Food can be kept intact in the bag and sterilized at high temperature (generally 120-135°C) and can be taken out and eaten. If high-temperature sterilized food is packaged with high-barrier materials, it will not only have a long shelf life at room temperature, but also be convenient for transportation, storage and sales, which is very suitable for the consumer needs of modern society.

[0004] High barrier retort packaging bags can be divided into many types according to different material combinations and structural designs. The following are some common types of high barrier retort packaging bags:

[0005] (1) Multilayer composite film bags: This type of packaging bag is usually made of a variety of materials, such as PET / AL / RCPP: polyethylene terephthalate (PET) as the outer layer provides strength and transparency, aluminum foil (AL) as the middle layer provides excellent barrier properties, and modified polypropylene (RCPP) as the inner layer provides heat sealing properties.

[0006] (2) Aluminum-plastic composite bags: These are composite bags made of aluminum foil and other plastic films. These bags have excellent barrier properties and are suitable for foods that need to be stored for a long time.

[0007] (3) Co-extruded high barrier bags:

[0008] Multilayer plastic films produced by coextrusion technology can form a multilayer structure in one production step, such as multilayer coextruded films containing EVOH or MXD6 nylon, which have good barrier properties.

[0009] (4) PEN composite bag: PEN (polyethylene naphthalate) is a high-performance plastic with high heat resistance and barrier properties, and can be composited with other materials.

[0010] It can be seen from the current high-barrier retort packaging bags that most of them contain aluminum foil. Using aluminum foil (AL) as a barrier layer does have many advantages, such as excellent barrier properties, which can effectively block oxygen, moisture, light and odor, thereby extending the shelf life of food. However, there are also some potential disadvantages to using aluminum foil: aluminum foil is a material that is not easy to degrade. If the packaging bags cannot be properly recycled, it will have a long-term impact on the environment. After aluminum foil is compounded with plastic or other materials, it is difficult to separate, which increases the difficulty of recycling and reuse. Aluminum foil is heavier than most plastic films, which will increase the overall weight of the package, thereby affecting transportation efficiency and cost.

[0011] However, the use of a single PET film for high-barrier retort-resistant packaging film has the problems of poor barrier properties, poor retort-resistant properties and poor mechanical strength. Therefore, there is an urgent need for a PET film with better barrier properties, better retort-resistant properties and higher mechanical strength. Summary of the invention

[0012] The purpose of the present invention is to provide a high-barrier aluminum-free cooking-resistant PET film and its preparation method and application, so as to solve the problems of poor barrier performance, poor cooking resistance and low mechanical properties of the above-mentioned single PET material as a high-barrier cooking-resistant packaging film. The present invention uses modified EVOH (ethylene-vinyl alcohol copolymer) and modified inorganic filler in the preparation process of the PET film, thereby improving various properties of the PET film. The modified EVOH has better hydrophobicity and can reduce the influence of moisture on its structure, thereby improving the barrier performance of the material in a humid environment; the thermal stability of the modified EVOH is also improved, so that it can still maintain good physical properties during high-temperature cooking, and is not easy to degrade or deteriorate; the modified EVOH can also enhance the mechanical strength and toughness of the material, so that it is not easy to break when subjected to external forces, thereby improving the durability of the overall packaging material. The modified inorganic filler can be better dispersed in the PET matrix to form a uniform distribution, thereby enhancing the rigidity and tensile strength of the film; the modified inorganic filler can fill the tiny pores in the PET matrix, reduce the chance of gas and water penetration, and further improve the barrier performance; the inorganic filler itself has good thermal stability, which can improve the heat resistance of the composite material, so that it can still maintain good physical properties under high temperature conditions; through the coating of epoxy phenolic resin, the interface compatibility between the inorganic filler and the PET matrix is ​​improved, so that the filler can be better combined with the matrix to form a more stable composite system, and the coating treatment can also prevent the agglomeration of the inorganic filler in the matrix, ensure the uniform distribution of the filler, and avoid the performance degradation caused by local stress concentration. Therefore, the PET film prepared by the present invention not only has excellent boiling resistance and high barrier performance, but also has good mechanical strength.

[0013] To achieve the above object, the first aspect of the present invention provides a high-barrier aluminum-free cooking-resistant PET film, comprising a substrate and a coating on the surface of the substrate, an adhesive layer is arranged between the coating and the substrate, and the coating is formed by coating the cooking-resistant polyvinylidene chloride latex obtained by modification on the surface of the adhesive layer and drying; the cooking-resistant polyvinylidene chloride latex comprises the following raw materials in parts by weight:

[0014]

[0015]

[0016] The modified inorganic filler is an oxide wrapped by epoxy phenolic resin, and the oxide includes aluminum oxide, silicon dioxide and zirconium oxide according to the mass ratio of (3-5): (8-10): 1.

[0017] The retort-resistant packaging films currently used mostly use aluminum foil. Although the use of aluminum foil can improve the retort-resistant performance and high barrier performance of the packaging film, it is not easy to be recycled during use, has a high weight, and increases transportation costs. In view of the above problems, the prior art tends to use polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyvinylidene chloride (PVDC), especially PET film, but the above packaging films are plastic films, and their barrier properties, retort-resistant performance and mechanical properties are all poor, and cannot meet the high requirements for packaging films. In view of the above problems, the present invention specifically applies a coating on the surface of the PET film, thereby improving the comprehensive performance of the PET film as a packaging film. The retort-resistant polyvinylidene chloride latex in the present invention is added with ethylene-vinyl alcohol copolymer (EVOH). EVOH is a well-known high-performance barrier material, which is particularly good at blocking oxygen, carbon dioxide and other odor molecules, and can effectively prevent food from oxidizing and deteriorating, maintain the freshness and flavor of food, and thus extend the shelf life of food. The present invention has further studied and found that although the addition of EVOH can improve the barrier of PET film to gas molecules, the barrier performance to water vapor is still relatively weak. To address this problem, the present invention uses acrylonitrile, isocyanate and acrylamide to modify EVOH, which significantly improves the water vapor resistance of PET film, and comprehensively improves the resistance to boiling and high barrier performance of PET film. Analysis of the reasons: Monomers such as acrylonitrile and acrylamide are introduced into EVOH through copolymerization, which can increase the hydrophobicity of the polymer chain, thereby reducing the affinity of the material to water, so that even in a humid environment, the water absorption rate of EVOH will decrease, thereby improving the moisture resistance of the overall material; in addition, acrylonitrile and acrylamide also have good thermal stability, and their introduction helps to improve the heat resistance of the modified EVOH, making the material less likely to decompose or deteriorate during high-temperature boiling. The introduction of isocyanate can not only promote cross-linking, but also improve the chemical stability of the material, so that the modified EVOH can still maintain good barrier properties under high-temperature and high-pressure boiling conditions.

[0018] The present invention also found that the mechanical strength of the PET film during use is poor, which is not conducive to long-term use and is prone to cracking. In view of this problem, the present invention adds inorganic fillers composed of alumina, silica and zirconium oxide to the boiling-resistant polyvinylidene chloride latex. Inorganic fillers such as alumina, silica and zirconium oxide, etc., because of their high hardness, can significantly improve the rigidity of the film when added to the PET film. This makes the film less likely to deform when subjected to external pressure or impact. Inorganic fillers usually have high thermal stability, which can help the PET film maintain its physical properties in a high temperature environment and prevent the film from deforming or softening during heating, which is also very important for boiling-resistant packaging. The present invention continues to study and find that although the addition of inorganic fillers can improve the mechanical strength of the PET film, once it is boiled at high temperature, the mechanical strength of the PET film will decrease. This is mainly because the interface bonding between the inorganic filler and other organic polymers is poor. Once heated and boiled, the interface bonding force between the two will be aggravated, resulting in the deterioration of the mechanical strength of the PET film. The present invention continues to modify the inorganic filler, uses the prepared epoxy phenolic resin to coat the inorganic filler, increases the interface bonding force between the inorganic filler and the organic polymer, and then improves the mechanical properties of the PET film after high-temperature cooking, as well as the cooking resistance and high barrier properties. Specific analysis of the reasons: the inorganic filler coated with epoxy phenolic resin can effectively disperse stress and prevent the expansion of cracks through good bonding with the polymer matrix, thereby improving the tensile strength of the film, which means that the film can withstand greater forces and is not easy to break during the stretching process. Modifying the inorganic filler can make it better dispersed in the polymer matrix and avoid the performance degradation caused by filler agglomeration. In addition, epoxy phenolic resin, as an organic coating layer, can improve the interface compatibility between the inorganic filler and the PET matrix, further improving the comprehensive performance of the composite material.

[0019] Preferably, the melt index of the ethylene-vinyl alcohol copolymer at 190° C. and 5.0 kg is 1.5 to 2.5 g / 10 min, and the ethylene content is 36.0 to 40.0 mol%.

[0020] Preferably, the isocyanate comprises diphenylmethane diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate in a mass ratio of (1-3):(6-8):1.

[0021] Preferably, the preparation method of the modified inorganic filler is:

[0022] (1) mixing bisphenol A epoxy resin and phenolic resin in a mass ratio of (1-2):1 at 100-120° C. for 3-5 hours to obtain epoxy phenolic resin;

[0023] (2) mixing alumina, silica and zirconium oxide to obtain a mixture, adding the mixture into a ball mill and adding a sulfuric acid aqueous solution with a mass concentration of 5% to 8%, the mass volume ratio of the mixture to the sulfuric acid aqueous solution being 1 g:10 to 20 mL, grinding at a speed of 500 to 800 r / min for 1 to 3 hours to obtain a slurry, separating, washing and drying the slurry to obtain a modified mixture;

[0024] (3) The modified mixed material, epoxy phenolic resin and solvent are mixed, reacted at 100-120° C. for 6-8 hours, filtered, washed and dried to obtain a modified inorganic filler.

[0025] Preferably, the viscosity of the bisphenol A epoxy resin is 2500-4500 mPa·s, and the free phenol content of the phenolic resin is 3%-4.5%.

[0026] Preferably, the initiator is sodium persulfate.

[0027] Preferably, the slip agent is palm wax slip agent.

[0028] Preferably, the substrate is a PET transparent sheet layer with a thickness of 0.3 to 0.4 mm, and the adhesive layer is a polyurethane adhesive.

[0029] The second aspect of the present invention provides a method for preparing a high-barrier aluminum-free cooking-resistant PET film, comprising the following steps:

[0030] a) uniformly mixing acrylonitrile, isocyanate and acrylamide to obtain a monomer mixture, dissolving an ethylene-vinyl alcohol copolymer in tetrahydrofuran, then adding the monomer mixture and azobisisobutyronitrile, reacting at 80-120° C. for 5-8 hours, filtering, washing and drying to obtain a modified ethylene-vinyl alcohol copolymer;

[0031] b) adding an initiator and vinylidene chloride to a reaction kettle in sequence, stirring the mixture at 60 to 80° C. to obtain a mixed solution, adding a modified ethylene-vinyl alcohol copolymer to the mixed solution, heating the mixture to 90 to 110° C. to react for 0.5 to 1 h, and cooling the mixture to room temperature to obtain a blended emulsion;

[0032] c) adding the modified inorganic filler to the blended emulsion, mixing and reacting at 55-60° C. for 2-3 hours, then adding a slip agent and soft water, mixing evenly, to obtain a cooking-resistant polyvinylidene chloride latex;

[0033] d) adding the polyurethane adhesive into the solvent to mix, dilute and dissolve, and stirring evenly to form a primer;

[0034] e) corona treating the substrate film, uniformly coating the primer on the surface of the corona treated substrate film by a coating device and drying it to obtain a glue layer, wherein the coating amount of the primer is 0.1-1.5 g / m, and then uniformly coating the cooking-resistant polyvinylidene chloride latex on the glue layer by a coating device, wherein the coating amount of the cooking-resistant polyvinylidene chloride latex is 0.8-4.0 g / m, and then drying the coated film, wherein the coating speed is 50-250 m / min and the drying temperature is 80-120°C.

[0035] The third aspect of the present invention provides an application of a high-barrier aluminum-free cooking-resistant PET film in a food packaging film.

[0036] Therefore, the present invention adopts the above-mentioned high-barrier aluminum-free cooking-resistant PET film and its preparation method and application, which have the following beneficial effects:

[0037] 1. The PET film of the present invention uses modified cooking-resistant polyvinylidene chloride latex as a coating material and contains ingredients such as ethylene-vinyl alcohol copolymer. This combination can provide excellent oxygen and water vapor barrier properties, thereby extending the shelf life of food.

[0038] 2. Compared with the traditional packaging material using aluminum foil as a barrier layer, the PET film of the present invention does not contain aluminum, which reduces the amount of aluminum used, is more environmentally friendly, and avoids the risk of aluminum migrating into food.

[0039] 3. The polyvinylidene chloride latex used in the PET film of the present invention has been modified and can maintain good physical properties and chemical stability under high-temperature cooking conditions, and is suitable for food packaging that requires high-temperature sterilization.

[0040] 4. The modified inorganic filler is introduced into the PET film of the present invention, which not only improves the barrier properties of the film, but also enhances the mechanical strength and toughness of the film, making the film less likely to be damaged during processing and use.

[0041] 5. The present invention makes the production process of PET film more convenient through specific process flow, such as corona treatment, coating and drying steps, which is conducive to industrial large-scale production.

[0042] The technical solution of the present invention is further described in detail below through embodiments. DETAILED DESCRIPTION

[0043] The present invention will be further described below. It should be noted that this embodiment is based on the technical solution and provides a detailed implementation method and a specific operation process, but the present invention is not limited to this embodiment.

[0044] Example 1

[0045] This embodiment provides a high-barrier aluminum-free cooking-resistant PET film, including a substrate and a coating on the surface of the substrate, a glue layer is arranged between the coating and the substrate, and the coating is formed by coating the modified cooking-resistant polyvinylidene chloride latex on the surface of the glue layer and drying it; the substrate is a PET transparent sheet layer with a thickness of 0.3 mm, and the PET transparent sheet layer is purchased from Shanghai Fuzhong Industrial Co., Ltd., model FZ-TMM. The glue layer is a polyurethane adhesive purchased from Dongguan Yantai Chemical Technology Co., Ltd., model TS-8810.

[0046] The cooking-resistant polyvinylidene chloride latex comprises the following raw materials in parts by weight:

[0047] 60 parts of vinylidene chloride;

[0048] 25 parts of ethylene-vinyl alcohol copolymer;

[0049] The melt index of the ethylene-vinyl alcohol copolymer at 190° C. and 5.0 kg is 1.5-2.5 g / 10 min, and the ethylene content is 36.0-40.0 mol %. The ethylene-vinyl alcohol copolymer was purchased from Dongjiu Plastic Technology (Dongguan) Co., Ltd., model EW-3801.

[0050] Acrylonitrile 9 parts;

[0051] 2 parts of isocyanate;

[0052] Isocyanates include diphenylmethane diisocyanate (CAS: 101-68-8), hexamethylene diisocyanate (CAS: 822-06-0) and isophorone diisocyanate (CAS: 4098-71-9) in a mass ratio of 2:7:1;

[0053] 10 parts of acrylamide;

[0054] 8 parts of modified inorganic filler;

[0055] The modified inorganic filler is an oxide wrapped by epoxy phenolic resin, and the oxide includes alumina, silica and zirconia in a mass ratio of 4:9:1; the particle size of zirconia is 20-40nm, purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd., model XFI01; the particle size of silica is 100-150nm, purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd., model XFF31-3; the particle size of alumina is 150-200nm, purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd., model XFI09;

[0056] 0.8 parts of initiator; the initiator is sodium persulfate;

[0057] 10 parts of softened water;

[0058] 3 parts of lubricant; the lubricant is palm wax lubricant, CAS: 8015-86-9.

[0059] The preparation method of the modified inorganic filler is:

[0060] (1) Bisphenol A epoxy resin and phenolic resin were mixed at a mass ratio of 2:1 at 100° C. for 4 h to obtain epoxy phenolic resin; the viscosity of the bisphenol A epoxy resin was 2500-4500 mPa·s, purchased from Guangzhou Xingchuangzhi New Materials Co., Ltd., model number YYHEP-3000; the free phenol content of the phenolic resin was 3%-4.5%, purchased from Hebei Zetian Chemical Co., Ltd., model number 9186;

[0061] (2) mixing alumina, silica and zirconium oxide to obtain a mixture, adding the mixture into a ball mill and adding a sulfuric acid aqueous solution with a mass concentration of 6%, the mass volume ratio of the mixture to the sulfuric acid aqueous solution being 1 g:15 mL, grinding at a speed of 650 r / min for 2 h to obtain a slurry, separating, washing and drying the slurry to obtain a modified mixture;

[0062] (3) The modified mixed material, epoxy phenolic resin and solvent are mixed, the mass ratio of the modified mixed material to the epoxy phenolic resin is 10:1, the solvent is ethyl acetate, and the concentration of the epoxy phenolic resin in the solvent is 10 g / L. After reacting at 100° C. for 7 hours, the mixture is filtered, washed and dried to obtain a modified inorganic filler.

[0063] A method for preparing a high-barrier aluminum-free cooking-resistant PET film comprises the following steps:

[0064] a) acrylonitrile (CAS: 107-13-1), isocyanate and acrylamide (CAS: 79-06-1) are uniformly mixed to obtain a monomer mixture, an ethylene-vinyl alcohol copolymer is dissolved in tetrahydrofuran, and then the monomer mixture and azobisisobutyronitrile are added, reacted at 85° C. for 6 hours, filtered, washed and dried to obtain a modified ethylene-vinyl alcohol copolymer; the amount of azobisisobutyronitrile added is 5% of the mass of acrylonitrile;

[0065] b) adding an initiator and vinylidene chloride to a reaction kettle in sequence, stirring the mixture at 65° C. for 2 h to obtain a mixed solution, adding a modified ethylene-vinyl alcohol copolymer to the mixed solution, heating the mixture to 100° C. for reaction for 0.5 h, and cooling the mixture to room temperature to obtain a blended emulsion;

[0066] c) adding the modified inorganic filler to the blended emulsion, mixing and reacting at 55° C. for 2.5 hours, then adding a slip agent and soft water, mixing evenly, to obtain a cooking-resistant polyvinylidene chloride latex;

[0067] d) adding the polyurethane adhesive to the solvent to mix, dilute and dissolve, the solvent is ethyl acetate, the mass ratio of the polyurethane adhesive to the solvent is 1:2, and stirring evenly to form a primer;

[0068] e) corona treating the substrate film, uniformly coating the primer on the surface of the corona treated substrate film with a coating device and drying it to obtain a glue layer, the coating amount of the primer is 1 g / m, and then uniformly coating the cooking resistant polyvinylidene chloride latex on the glue layer with a coating device, the coating amount of the cooking resistant polyvinylidene chloride latex is 2.5 g / m, and then drying the coated film, the coating speed is 150 m / min, and the drying temperature is 90°C.

[0069] Example 2

[0070] The difference between this embodiment and embodiment 1 is that the formula of the cooking-resistant polyvinylidene chloride latex is different.

[0071] The boiling-resistant polyvinylidene chloride latex in this embodiment includes the following raw materials in parts by weight:

[0072]

[0073] The isocyanate includes diphenylmethane diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate in a mass ratio of 1:6:1;

[0074] 8 parts of acrylamide;

[0075] 8 parts of modified inorganic filler;

[0076] The modified inorganic filler is an oxide wrapped by epoxy phenolic resin, and the oxide includes aluminum oxide, silicon dioxide and zirconium oxide in a mass ratio of 3:8:1;

[0077] 0.5 part of initiator;

[0078] 15 parts of softened water;

[0079] 5 parts of lubricant.

[0080] Comparative Example 1

[0081] The difference between this comparative example and Example 1 is that the ethylene content of EVOH is 30.0-34.0 mol%, which is purchased from Dongjiu Plastic Technology (Dongguan) Co., Ltd., model EW-3201.

[0082] Comparative Example 2

[0083] The difference between this comparative example and Example 1 is that no isocyanate is added to the cooking-resistant polyvinylidene chloride latex, and an equal mass of acrylonitrile is used instead.

[0084] Comparative Example 3

[0085] The difference between this comparative example and Example 1 is that acrylamide is not added to the cooking-resistant polyvinylidene chloride latex, and acrylonitrile of equal mass is used instead.

[0086] Comparative Example 4

[0087] The difference between this comparative example and Example 1 is that no acrylonitrile, isocyanate and acrylamide are added to the cooking-resistant polyvinylidene chloride latex, that is, the EVOH is not modified.

[0088] Comparative Example 5

[0089] The difference between this comparative example and Example 1 is that the viscosity of the bisphenol A epoxy resin is 11000-14000 mPa·s, purchased from Guangzhou Zhuolilian New Materials Co., Ltd., model E-44.

[0090] Comparative Example 6

[0091] The difference between this comparative example and Example 1 is that the free phenol content in the phenolic resin is 1.5% to 3.5%, and the phenolic resin is purchased from Hebei Zetian Chemical Co., Ltd., and the model number is 9182-2.

[0092] Comparative Example 7

[0093] The difference between this comparative example and Example 1 is that the mixed material in the modified inorganic filler is not modified by epoxy phenolic resin.

[0094] Comparative Example 8

[0095] The difference between this comparative example and Example 1 is that the mixture formed by mixing aluminum oxide, silicon dioxide and zirconium oxide is not ball-milled.

[0096] Comparative Example 9

[0097] The difference between this comparative example and Example 1 is that the silica particle size in the modified inorganic filler is 3-5 μm, purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd., model number XFF31-3.

[0098] Comparative Example 10

[0099] The difference between this comparative example and Example 1 is that zirconium oxide is not added to the modified inorganic filler, and an equal mass of silicon dioxide is used to replace zirconium oxide.

[0100] Performance Testing

[0101] Retort resistance test method: Cut the packaging film into 20cm×20cm size, then heat-seal the upper and lower layers to make a packaging bag, put distilled water into the bag, seal it after removing the air, and then perform pressure retort treatment at 120℃ for 30 minutes. After cooling down, take out the packaging bag and observe its appearance, and test the oxygen permeability and water permeability of the packaging film after retort.

[0102] The test and evaluation of the boiling resistance of the packaging film in the embodiments and comparative examples mainly tests the oxygen permeability and water permeability before and after boiling, as well as the apparent changes of the packaging film before and after boiling. The apparent changes of the packaging film after boiling are divided into two categories:

[0103] After cooking, the packaging film has appearance problems such as wrinkles, delamination, bubbles, etc., which are indicated by "×".

[0104] There is no change in the packaging film after steaming compared with before steaming, which is represented by "√".

[0105] Table 1 Test results of boiling resistance and high barrier properties

[0106]

[0107] The mechanical strength test was performed on the PET films prepared in Examples 1-2 and Comparative Examples 1-10, and specifically the tensile property test was performed using the film tensile property test method specified in GB / T1040.3-2006.

[0108] Table 2 Mechanical strength test results

[0109]

[0110]

[0111] It can be seen from the above performance test results that the PET films prepared in Examples 1-2 not only have excellent boiling resistance and high barrier properties, but also have good mechanical strength. In particular, the comprehensive performance of Example 1 is the most outstanding. This is mainly because the present invention uses modified EVOH (ethylene-vinyl alcohol copolymer) and modified inorganic filler in the preparation process of the PET film, which improves the various properties of the PET film.

[0112] The comparative examples, however, are significantly inferior to the examples in terms of corresponding performance tests because they do not adopt the necessary technical solutions. In comparative example 1, the ethylene content in EVOH is changed, and the ethylene content is lower than that in the examples. From the results, it can be seen that the boiling resistance and mechanical strength of the PET film are reduced, proving that the ethylene content of EVOH has an important influence on the improvement of the comprehensive performance of the PET film. In comparative examples 2 and 3, EVOH is not modified with isocyanate and acrylamide, respectively. From the results, it can be seen that the reduction of the modified substances further leads to a decrease in the comprehensive effect of the PET film, indicating the necessity of using acrylonitrile, isocyanate and acrylamide to modify EVOH at the same time in the present invention. The three monomers work synergistically to jointly improve the corrosion resistance and boiling resistance of EVOH, thereby improving the comprehensive performance of the PET film. In comparative example 4, EVOH is not modified and is directly added to the boiling-resistant polyvinylidene chloride latex. From the results, it can be seen that the comprehensive performance of the PET film is further reduced compared with comparative examples 1 to 3, indicating that modifying EVOH in the present invention is the key to improving the comprehensive performance of the PET film. Comparative Examples 5 and 6 respectively changed the viscosity of the epoxy resin and the phenol content of the phenolic resin used in the preparation process of the modified inorganic filler. From the results, it can be seen that the comprehensive properties of the PET films prepared in Comparative Examples 5 and 6 are slightly reduced compared with Examples 1-2, indicating that the use of specific types of bisphenol A epoxy resin and phenolic resin in the present invention has a certain influence on the comprehensive properties of the PET film. In Comparative Example 7, the surfaces of the three inorganic fillers are not coated with epoxy phenolic resin. From the results, it can be seen that the cooking resistance, high barrier performance and mechanical strength of the PET film prepared therefrom are further reduced. This is mainly because inorganic fillers (such as alumina, silica and zirconium oxide) are usually hydrophilic, while the organic polymer matrix is ​​hydrophobic. If the inorganic filler is directly added to the organic polymer, the interface compatibility between the two is poor, which may lead to a weak bond between the filler and the matrix, thereby affecting the overall mechanical properties of the composite material. If the bond between the filler and the matrix is ​​not strong, micropores or cracks may also be formed. These defects will become paths for gas or water penetration, thereby reducing the barrier properties of the material. Therefore, it is very important to wrap and modify the inorganic filler with epoxy phenolic resin to improve the comprehensive properties of the PET film. In Comparative Example 8, the inorganic filler was not subjected to acid ball milling, and the surface of the inorganic filler was not activated. From the results, it can be seen that the comprehensive properties of the PET film prepared in Comparative Example 8 also decreased significantly compared with Examples 1-2, indicating that activating the surface of the inorganic filler has a very important influence on improving the comprehensive properties of the PET film.Comparative Example 9 changes the particle size of silica, and the particle size becomes larger than that of Examples 1 to 2. It can be seen from the results that the comprehensive performance of the PET film prepared in Comparative Example 9 decreases to a certain extent. This is mainly because silica with larger particle size is difficult to disperse evenly in the polymer matrix and easily forms agglomerates, which not only affects the uniformity of the material but also may lead to a decrease in mechanical properties. Fillers with large particle sizes may become stress concentration points inside the material. After long-term use or under repeated stress, these parts are more likely to crack or break, thereby reducing the durability of the material. Therefore, the particle size of silica plays an important role in improving the comprehensive performance of the PET film. In Comparative Example 10, the addition of one inorganic filler is reduced, and zirconium oxide is not added. It can be seen from the results that the comprehensive performance of the PET film prepared in Comparative Example 10 decreases to a certain extent. This is mainly because different inorganic fillers have their own characteristics and advantages. Through reasonable proportions, the complementary and synergistic effects in performance can be achieved, and the mechanical strength, thermal stability, barrier properties, etc. of the PET film can be jointly improved. In addition, zirconium oxide has an extremely high melting point (about 2700°C) and has the best thermal stability among the three fillers. If zirconium oxide is not added, the heat resistance of the PET film may decrease, and deformation or performance degradation may occur easily during high-temperature cooking. Therefore, the specific addition of three inorganic fillers including zirconium oxide in the present invention has an important influence on improving the comprehensive performance of the PET film. The above experimental results further prove the importance of the technical solution defined in the present invention for its technical effect.

[0113] Therefore, the present invention adopts a high-barrier aluminum-free cooking-resistant PET film of the above structure and its preparation method and application. The prepared PET film not only has excellent cooking resistance and high barrier properties, but also has good mechanical strength.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A high barrier aluminum-free cooking-resistant PET film, comprising a substrate and a coating on the surface of the substrate, wherein a glue layer is provided between the coating and the substrate, characterized in that: The coating is formed by coating the modified cooking-resistant polyvinylidene chloride latex on the surface of the adhesive layer and drying it; the cooking-resistant polyvinylidene chloride latex includes the following raw materials in parts by weight: Vinylidene chloride 50~70 parts Ethylene-vinyl alcohol copolymer 20~30 parts Acrylonitrile 8~10 parts Isocyanate 1~3 parts Acrylamide 5~15 parts Modified inorganic filler 5~10 parts Initiator 0.5~2 parts 5~30 parts of softened water 1~8 parts of lubricant; The modified inorganic filler is an oxide wrapped by epoxy phenolic resin, and the oxide includes aluminum oxide, silicon dioxide and zirconium oxide in a mass ratio of (3-5): (8-10): 1; The melt index of ethylene-vinyl alcohol copolymer at 190°C and 5.0kg is 1.5-2.5g / 10min, and the ethylene content is 36.0-40.0mol%; Isocyanates include diphenylmethane diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate in a mass ratio of (1-3):(6-8):1; The preparation method of the modified inorganic filler is: (1) mixing bisphenol A epoxy resin and phenolic resin in a mass ratio of (1-2):1 at 100-120° C. for 3-5 hours to obtain epoxy phenolic resin; (2) Alumina, silica and zirconium oxide are mixed to obtain a mixture, the mixture is added into a ball mill and a sulfuric acid aqueous solution having a mass concentration of 5-8% is added, the mass volume ratio of the mixture to the sulfuric acid aqueous solution is 1 g:10-20 mL, the mixture is ground at a speed of 500-800 r / min for 1-3 h to obtain a slurry, the slurry is separated, washed and dried to obtain a modified mixture; (3) mixing the modified mixed material, epoxy phenolic resin and solvent, reacting at 100-120° C. for 6-8 hours, filtering, washing and drying to obtain a modified inorganic filler; The viscosity of bisphenol A epoxy resin is 2500~4500mPa•s, and the free phenol content of phenolic resin is 3~4.5%.

2. A high barrier aluminum-free cooking-resistant PET film according to claim 1, characterized in that: The initiator is sodium persulfate.

3. The high barrier aluminum-free cooking-resistant PET film according to claim 1, characterized in that: The lubricant is palm wax lubricant.

4. The high barrier aluminum-free cooking-resistant PET film according to claim 1, characterized in that: The substrate is a PET transparent sheet layer with a thickness of 0.3~0.4mm, and the adhesive layer is a polyurethane adhesive.

5. The method for preparing a high barrier aluminum-free cooking-resistant PET film according to any one of claims 1 to 4, characterized in that: The following steps are involved: a) uniformly mixing acrylonitrile, isocyanate and acrylamide to obtain a monomer mixture, dissolving ethylene-vinyl alcohol copolymer in tetrahydrofuran, then adding the monomer mixture and azobisisobutyronitrile, reacting at 80-120° C. for 5-8 hours, filtering, washing and drying to obtain a modified ethylene-vinyl alcohol copolymer; b) adding an initiator and vinylidene chloride to a reaction kettle in sequence, stirring the mixture at 60-80° C. to obtain a mixed solution, adding a modified ethylene-vinyl alcohol copolymer to the mixed solution, heating the mixture to 90-110° C. to react for 0.5-1 h, and cooling the mixture to room temperature to obtain a blended emulsion; c) adding modified inorganic filler to the blended emulsion, mixing and reacting at 55-60° C. for 2-3 hours, then adding a slip agent and soft water, mixing evenly, to obtain a cooking-resistant polyvinylidene chloride latex; d) Add the polyurethane adhesive into the solvent, mix, dilute and dissolve, and stir evenly to form a primer; e) The substrate film is subjected to corona treatment, and a primer is evenly coated on the surface of the corona treated substrate film by a coating device and dried to obtain a glue layer, and the coating amount of the primer is 0.1-1.5 g / m; then, cooking-resistant polyvinylidene chloride latex is evenly coated on the glue layer by a coating device, and the coating amount of the cooking-resistant polyvinylidene chloride latex is 0.8-4.0 g / m; then, the coated film is dried, and the coating speed is 50-250 m / min, and the drying temperature is 80-120°C.

6. Use of the high-barrier aluminum-free cooking-resistant PET film according to any one of claims 1 to 4 in food packaging films.

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