A high-barrier retort film capable of quickly recovering a flat state after high-temperature retorting, a preparation method therefor, and use thereof

Abstract

The present application belongs to the field of packaging materials and packaging products, and relates to a high-barrier retort film capable of quickly recovering a flat state after high-temperature retorting, a preparation method and application thereof. Specifically, the present application relates to a composite film which comprises a printing layer, an intermediate layer and a heat-sealing layer from top to bottom, and a material with similar transverse and longitudinal cold shrinkage forces is selected as the intermediate layer through testing and screening of the shrinkage characteristics of each layer of the original film, so that the transverse and longitudinal shrinkage characteristics of the intermediate layer are more stable; the printing layer and the heat-sealing layer are matched with similar cold shrinkage forces, so that the composite film has similar cold shrinkage forces of the inner and outer layers after compounding, and the time required for recovering from the upward warping to the flat state during the cooling process is shortened.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of packaging materials and packaging methods, in particular to a high-barrier retort film capable of quickly recovering flat state after high-temperature retort, a preparation method and use thereof. BACKGROUND

[0002] With the development of economy, the pace of life of people is also accelerating with the development of economy, and people's requirements for food are also developing towards safety and health, fast cooking and convenient eating. Therefore, the speed of product innovation of instant food in the market is also accelerating.

[0003] Rice, as one of the staple foods of Chinese people, is deeply loved by the Chinese people. However, compared with the relatively developed instant rice industry and rich instant rice products in Japan and Korea, the domestic microwave instant rice products are relatively lacking. Therefore, developing a safe and convenient instant rice has a good market prospect.

[0004] Currently, the instant rice products in Japan and Korea generally adopt the packaging form of a thermoformed blister container plus a sealing film. The main material of the thermoformed blister container is polypropylene (PP), and the sealing film is mainly a multi-layer plastic composite film. The production process mainly includes a one-time sealing film process and a two-time sealing film process. The one-time sealing film process is to fill rice into the packaging container, seal the sealing film, and then sterilize at 121℃ for 30 minutes. The two-time sealing film process is to sterilize the container and the sealing film by high-temperature / ultraviolet irradiation, then fill rice, seal the film, and reserve an exhaust port between the film and the container. The rice and the packaging are put into a retort machine for high-temperature retort. Compared with the one-time sealing film process, the two-time sealing film process has a very high requirement for the flatness of the sealing film after high-temperature retort.

[0005] However, the existing multi-layer plastic composite film is difficult to meet the above requirements. In particular, after the instant rice product is retorted, the bending of the sealing film makes the exhaust port between the sealing film and the lower container larger, so that oxygen and bacteria in the external air are easily introduced into the product, affecting the product quality. Moreover, if the bending angle of the sealing film exceeds 90°, the heat-sealing layer will contact the pressed heat-sealing plate, the heat-sealing layer will be adhered to the heat-sealing plate, the product will be lifted by the heat-sealing plate when the heat-sealing plate is lifted, resulting in poor heat sealing, and even abnormal stop, affecting the production efficiency. SUMMARY

[0006] The inventors have found through a large amount of research work that the reason for the bending of the existing packaging sealing film after high-temperature retort is that the existing packaging sealing film is a plastic composite film composed of multiple different plastic materials. The expansion coefficients of different plastics are different. When the sealing film is heated, the composite film will bend towards the side with a smaller expansion coefficient due to the influence of the expansion coefficient.

[0007] Taking a high-temperature cooking sealing film commonly seen on the market as an example, the material structure is PET / NY / RCPP (as shown in Figure 1 Table 1 shows the linear thermal expansion coefficients of different layers of materials.

[0008] Table 1 shows the linear thermal expansion coefficients of different layers of materials.

[0009]

[0010] As can be seen from Table 1, when heated, the thermal expansion coefficient of the heat-sealing layer is the largest, and the thermal expansion coefficient of the printing layer is the smallest. Therefore, when heated, the sealing film of the material structure will be warped towards the printing layer side.

[0011] To solve the above problems, the inventors combine films with different shrinkage properties. By testing the shrinkage properties of each layer of the original film, the inventors select materials with similar transverse and longitudinal cold shrinkage forces as the middle layer, so that the shrinkage properties of the middle layer in the transverse and longitudinal directions are more stable. The printing layer and the heat-sealing layer are matched to have similar cold shrinkage forces, so that the cold shrinkage forces of the inner and outer layers of the composite sealing film are similar, and the time required for the sealing film to recover to a flat state from the warped state during cooling is shortened.

[0012] Further, the inventors also improve the preparation process of the composite film, especially the control of the composite tension. In the composite film of the present application, the printing layer and the middle layer have relatively large rigidity and are not easily stretched during the composite process, while the heat-sealing layer has relatively small rigidity and is more easily stretched during the composite process, resulting in that the composite lid film is more difficult to recover to a flat state after heating and cooling. Therefore, while ensuring the flatness of the composite lid film, the composite tension of the heat-sealing layer is reduced to avoid stretching of the heat-sealing layer as much as possible, which is beneficial to the rapid recovery of the lid film to a flat state during cooling.

[0013] The present application provides the following inventions:

[0014] Composite film

[0015] In one aspect, the present application provides a composite film, which comprises a printing layer, a middle layer and a heat-sealing layer from top to bottom.

[0016] The printing layer is a biaxially oriented polyester film (BOPET film), a PET film evaporated with aluminum oxide or silicon oxide (AlO x -PET film or SiO x -PET film), a nylon film (NY film), a uniaxially oriented polypropylene film (OPP film) or a biaxially oriented polypropylene film (BOPP).

[0017] The intermediate layer is a biaxially oriented polyester film (BOPET film), a PET film evaporated with aluminum oxide or silicon oxide (AlO x -PET film or SiO x -PET film), or a nylon film (NY film);

[0018] The heat-sealing layer is a retort grade cast polypropylene film (RCPP film) or a cast polyethylene film (CPE film).

[0019] The present application uses a material with similar transverse and longitudinal cold shrinkage (such as NY) as the intermediate layer, so that the shrinkage characteristics of the intermediate layer in the transverse and longitudinal directions are more stable; and the printing layer (such as PET / OPP) and the heat-sealing layer (such as RCPP or CPE) have similar cold shrinkage, so that the inner and outer layers of the sealing film have similar cold shrinkage after compounding, and the time required for the sealing film to recover from the upward warping to a flat state during cooling is shortened.

[0020] In some embodiments, the printing layer is an AlO x -PET / SiO x -PET film. AlO x -PET / SiO x The PET film is a composite film with barrier properties, which includes a substrate layer PET and a barrier material aluminum oxide or silicon oxide. The barrier material can be compounded with the PET substrate by evaporation.

[0021] Evaporated films or vacuum evaporated films are formed by depositing aluminum, aluminum oxide or silicon dioxide onto various plastic film substrates using various physical vapor deposition methods. Vacuum aluminum oxide film is formed by introducing a reaction gas (such as oxygen) during aluminum evaporation, so that the aluminum oxide (Al2O3) coating formed and deposited on the film substrate during the coating process is not pure aluminum oxide (Al2O3), but a mixture of aluminum oxide (Al2O3) and aluminum (Al) (AlO x ). Vacuum silicon oxide film uses silicon dioxide as the raw material for coating, and the mixture of silicon oxide (SiO2) and SiO (SiO x ) formed and deposited on the film substrate.

[0022] In some embodiments, the printing layer is an OPP film. OPP film is a uniaxially oriented polypropylene film, which can be made by co-extrusion and directional stretching, and only has some orientation in the longitudinal direction. OPP film has good transparency and high tensile strength, impact strength and rigidity, and strong toughness, and can be used for adhesive labels, wound labels, transparent tapes, etc., and can also be printed with text, patterns, trademarks, etc.

[0023] In some embodiments, the printed layer is a BOPP film, i.e., a biaxially oriented polypropylene film, which can be made by co-extrusion biaxial orientation. The BOPP film has high mechanical strength, air tightness, toughness, transparency, etc., and also has good printing adaptability.

[0024] In some embodiments, the intermediate layer is a NY film. Polyamide (PA), also known as nylon (NY), is a thermoplastic resin containing repeating amide groups [-NHCO-] in the molecular backbone. Nylon obtained by ring-opening polymerization of lactam is referred to as nylon n, abbreviated as Pan, such as PA6, which is obtained by ring-opening polymerization of caprolactam. Polymers obtained by condensation polymerization of dibasic acid and dibasic amine are referred to as nylon mn, where m represents the number of carbon atoms in the dibasic amine that constitutes the main chain portion, and n represents the number of carbon atoms in the dibasic acid that constitutes the main chain portion, such as PA610, which is obtained by condensation polymerization of sebacic acid and hexamethylene diamine. The nomenclature of nylon can also use the abbreviations of repeating diamines or diacids, such as m-xylylenediamine, which is abbreviated as MXDA, so the polymer of m-xylylenediamine and adipic acid is referred to as nylon MXD6. Nylons that can be used for film materials include, but are not limited to, PA4, PA410, PA46, PA54, PA510, PA512, PA516, PA6, PA66, PA69, PA610, PA612, PA613, PA11, PA1010, PA1012, PA12, PA10T, etc. According to the stretching condition, PA films are classified into: biaxially oriented film, uniaxially oriented film, and un-stretched film. The type of nylon can be selected according to actual needs. In this application, PA and NY have the same meaning and can be used interchangeably.

[0025] In some embodiments, the heat-seal layer is a RCPP film. CPP film, i.e., cast polypropylene, is also referred to as un-stretched polypropylene film, and can be divided into general CPP (General CPP, abbreviated as GCPP) film, metalized CPP (Metalize CPP, abbreviated as MCPP) film, and retort CPP (Retort CPP, abbreviated as RCPP) film, etc. according to different uses. RCPP can be used as a composite base film for high-temperature retort sterilization. This kind of film mainly uses copolymerized polypropylene raw materials. When used for high-temperature retort (resistant to retort and sterilization above 120°C), block copolymerized polypropylene is used, and when used for general retort (resistant to 100-120°C below), random copolymerized polypropylene is used. Polyethylene (PE) can be added to PP for modification to form easy-to-peel RCPP.

[0026] In some embodiments, the heat-sealing layer is a CPE film. CPE film is a polyethylene film processed by a casting method, typically using LDPE resin. Compared to blown polyethylene film, it has advantages such as better flexibility and higher transparency; as a non-stretched film, its strength is lower than that of stretched film; and due to its low orientation, it has better heat-sealing performance.

[0027] Furthermore, the shrinkage force of each film layer can be selected. Shrinkage force is one of the indicators characterizing the shrinkage performance of plastic films; it is defined as the shrinkage force generated by the film during cooling and can be expressed as the maximum shrinkage force that occurs during the sample's cooling process. The following method can be used with a film heat shrinkage performance tester (e.g., the Jinan Langguang FST-02 film heat shrinkage performance tester). Figure 2 The shrinkage force of the plastic film was measured (as shown).

[0028] Preheat the machine for 30 minutes. Select a flat and uniform sample and cut it into strips 15mm wide and 150mm long (in the shrinkage direction). Clamp the strips flat on the sample holding device. Simulate the actual application temperature of the cover film and set the heating chamber temperature to 100℃. When the set temperature is reached, start the test. The sample is sent into the heating chamber and balanced at 100℃ for 60 seconds. After the time is up, the sample is removed from the heating chamber. Record the maximum cooling shrinkage force value detected by the instrument during the natural cooling process of the sample from 100℃ to room temperature. This value is the cold shrinkage force of the sample. Perform 5 horizontal tests and take the arithmetic mean.

[0029] In some embodiments, the shrinkage force of the printed layer is 0.05N-0.2N (e.g., 0.05N-0.1N, 0.1N-0.15N, 0.15N-0.17N, or 0.17N-0.2N).

[0030] In some embodiments, the cold shrinkage force of the intermediate layer is 0.15N-0.55N (e.g., 0.15N-0.2N, 0.2N-0.25N, 0.25N-0.28N, 0.28N-0.3N, 0.3N-0.4N, 0.4N-0.5N, or 0.5N-0.55N).

[0031] In some embodiments, the shrinkage force of the heat-sealing layer is 0.05N-0.3N (e.g., 0.05N-0.1N, 0.1N-0.12N, 0.12N-0.15N, 0.15N-0.17N, 0.17N-0.2N, 0.2N-0.25N, or 0.55N-0.3N).

[0032] The composite film of the present application is a composite material made of multiple layers, and the thickness thereof is not particularly limited. In some embodiments, the thickness of the composite film can be 70 μm - 200 μm, such as 70 - 90 μm, 90 - 100 μm, 100 - 103 μm, 103 - 105 μm, 105 - 120 μm, 120 - 150 μm, or 150 - 200 μm.

[0033] In some embodiments, the thickness of the printing layer is 12 μm - 40 μm (such as 12 μm - 15 μm, 15 μm - 20 μm, 20 μm - 30 μm, 30 μm - 38 μm, or 38 μm - 40 μm).

[0034] In some embodiments, the thickness of the intermediate layer is 9 μm - 20 μm (such as 9 μm - 12 μm, 12 μm - 15 μm, or 15 μm - 20 μm).

[0035] In some embodiments, the thickness of the heat-seal layer is 50 μm - 120 μm (such as 50 μm - 70 μm, 70 μm - 100 μm, or 100 μm - 120 μm).

[0036] In some embodiments, the printing layer is a SiO x In some embodiments, the printing layer, the intermediate layer, and the heat-seal layer have a cold shrinkage force of 0.15 N - 0.17 N, 0.25 N - 0.28 N, and 0.12 N - 0.15 N, respectively. In some embodiments, the printing layer, the intermediate layer, and the heat-seal layer have a thickness of 12 μm - 15 μm, 12 μm - 15 μm, and 70 μm - 100 μm, respectively.

[0037] In some embodiments, the printing layer is an OPP film, the intermediate layer is a NY film, and the heat-seal layer is a CPE film. In some embodiments, the printing layer, the intermediate layer, and the heat-seal layer have a cold shrinkage force of 0.17 N - 0.2 N, 0.28 N - 0.3 N, and 0.15 N - 0.17 N, respectively. In some embodiments, the printing layer, the intermediate layer, and the heat-seal layer have a thickness of 12 μm - 15 μm, 12 μm - 15 μm, and 70 μm - 100 μm, respectively.

[0038] Method of preparing a composite film

[0039] In one aspect, the present application provides a method for preparing the composite film of the present application.

[0040] The composite film of the present application can be prepared using a dry lamination process. In some embodiments, the method comprises the following steps:

[0041] Step 1: obtaining a printing layer film, an intermediate layer film and a heat-sealing layer film, which are used to form a printing layer, an intermediate layer and a heat-sealing layer, respectively;

[0042] Step 2: printing the printing layer film;

[0043] Step 3: compounding the printed printing layer film and the intermediate layer film by a dry compounding process to obtain a double-layer film;

[0044] Step 4: dry-compounding the double-layer film obtained in Step 3 and the heat-sealing layer film to obtain a triple-layer film;

[0045] Step 5: placing the triple-layer film obtained in Step 4 into a maturation chamber for maturation to obtain the composite film of the present application;

[0046] Optionally, the method further comprises Step 6: cutting the composite film.

[0047] In the composite film of the present application, the printing layer and the intermediate layer material have relatively large rigidity and are not easily stretched during compounding, while the heat-sealing layer has relatively small rigidity, especially the modified easy-to-peel RCPP, which is more easily stretched during compounding due to the addition of part of PE, resulting in that the compounded cover film is more difficult to recover to a flat state after heating and cooling. Therefore, while ensuring the flatness of the cover film after compounding, the compounding tension of the heat-sealing layer is reduced, and the stretching of the heat-sealing layer is avoided as much as possible, which is beneficial to the rapid recovery of the cover film to a flat state during cooling.

[0048] The compounding tension can be adjusted by controlling the unwinding tension.

[0049] In some embodiments, in Step 3, the unwinding tension of the printing layer film is 100N-130N (for example, 120N).

[0050] In some embodiments, in Step 3, the unwinding tension of the intermediate layer film is 90N-110N (for example, 100N).

[0051] In some embodiments, in Step 4, the unwinding tension of the double-layer film is 100N-130N (for example, 120N).

[0052] In some embodiments, in Step 4, the unwinding tension of the heat-sealing layer film is 90N-110N (for example, 100N).

[0053] In some embodiments, in Step 5, the maturation is carried out at a temperature of 40°C-50°C (for example, 45°C).

[0054] In some embodiments, in Step 5, the maturation is carried out for 48-96 hours.

[0055] Use and packaging container

[0056] The present application also relates to the use of the composite film of any one of the above for preparing a packaging container.

[0057] The present application also relates to a packaging container comprising the composite film of any one of the above.

[0058] In some embodiments, the packaging container of the above is a bowl-, cup-, tub- or box-shaped packaging container comprising a peelable lid film, which comprises the composite film of the present application.

[0059] The packaging container of the present application is particularly suitable for packaging instant food, particularly instant food that needs to be cooked at high temperature.

[0060] In some embodiments, the instant food is instant rice. Instant rice refers to a staple food that is produced in an industrialized large scale, and only needs to be simply cooked or directly edible before consumption, with a flavor, taste and appearance comparable to ordinary rice. Instant rice has the characteristics of convenient consumption and convenient carrying.

[0061] The present application also relates to an instant food (e.g. instant rice) comprising the packaging container of the present application.

[0062] Method of preparing a convenience food

[0063] The present application also relates to a method for preparing an instant food, which comprises using the composite film of the present application as a sealing film of a packaging container.

[0064] In some embodiments, the method for preparing is a two-step sealing method.

[0065] In some embodiments, the method for preparing comprises the following steps:

[0066] (1) filling food materials into a bowl (box) -shaped packaging container;

[0067] (2) first sealing: using the composite film of the present application cut into a suitable shape to heat seal a part of the bowl (box) opening, and leaving two outlets;

[0068] (3) cooking (e.g. at 100℃ for 30min) the packaging container filled with food materials after the first sealing, and then cooling;

[0069] (4) filling nitrogen into the packaging container through one of the two outlets left in step (2), and discharging the air in the packaging container from the other outlet;

[0070] (5) second sealing: heat sealing the two outlets, completing the sealing of the packaging container.

[0071] Definitions of terms

[0072] In the present application, the scientific and technical terms used herein have the meanings commonly understood by one of ordinary skill in the art, unless otherwise indicated. Also, the laboratory operations steps referred to herein are conventional steps widely used in the corresponding field. Meanwhile, in order to better understand the present application, the definitions and explanations of the related terms are provided as follows.

[0073] In the present application, the terms "film" and "thin film" can be used interchangeably, referring to a thin and soft sheet made of a polymer, with a thickness from about 1 micrometer to several hundred micrometers (e.g., about 1 micrometer to about 250 micrometers), including a plated film formed by plating a metal or non-metallic element or oxide on a polymer film by evaporation or the like.

[0074] In the present application, the term "about" should be understood by one of ordinary skill in the art and will vary to some degree depending on the context in which it is used. If the use of the term "about" is not clear to one of ordinary skill in the art from the context in which it is used, then "about" means plus or minus 10% of the particular numerical value or range recited.

[0075] In the description of the present application, it should be noted that the terms "upper", "lower", and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the objects (such as devices or elements) referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0076] Advantages of the Invention

[0077] The composite film of the present application has good high-temperature cooking resistance and high barrier properties, and is suitable for use as a sealing film for packaging containers of instant food such as bowls and boxes. In particular, during the cooling process after high-temperature cooking, the composite film of the present application can quickly recover to a flat state, and can meet the extremely high requirements for flatness of the sealing film after cooking in the secondary sealing film process. BRIEF DESCRIPTION OF DRAWINGS

[0078] Figure 1 An exemplary material structure of a prior art high-temperature cooking sealing film is shown.

[0079] Figure 2 An instrument for measuring cold shrinkage force is exemplarily shown.

[0080] Figure 3 A material structure of the composite film of Example 1 is shown. DETAILED DESCRIPTION

[0081] The embodiments of the present application will be described in detail below with examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. The specific conditions not noted in the examples are carried out according to the conventional conditions or the conditions suggested by the manufacturer. The reagents or instruments used are not noted by the manufacturer, which are all conventional products available in the market.

[0082] Example 1

[0083] 1. Preparation of composite film

[0084] The material of the printing layer is SiO x -PET, thickness 12 μm, SiO x The cold shrinkage force of the PET base film is 0.15 N, and after the printing is completed, the composite film is compounded with the intermediate layer NY film using a dry compound machine, the thickness of the NY film is 15 μm, and the cold shrinkage force is 0.25 N. During the compounding, the unwinding tension of the printing layer SiO x The unwinding tension of the PET film is 120 N, and the unwinding tension of the intermediate layer NY film is 100 N. After the compounding is completed, the obtained double-layer film is compounded with the heat-sealing layer RCPP film, the thickness of the RCPP is 70 μm, and the cold shrinkage force of the base film is 0.12 N. The unwinding tension of the double-layer film is 120 N, and the unwinding tension of the heat-sealing layer RCPP film is 100 N. The obtained three-layer film after the compounding is completed is placed in a curing chamber at 45°C for 96 hours for curing, and a composite film is obtained, and the material structure thereof is as shown in Figure 3

[0085] 2. Test of flatness of the composite film after high-temperature cooking

[0086] The composite film cut into a suitable shape is used to heat-seal the bowl-shaped packaging container, and an exhaust port is reserved. After being cooked at 100°C for 30 min, the sealing film at the exhaust port position is obviously upwarping, the angle is less than 60°, and the time required for the sealing film at the exhaust port position to return to the flat state at room temperature is not more than 5 seconds.

[0087] Example 2

[0088] 1. Preparation of composite film

[0089] The material of the printing layer is OPP, the thickness is 38 μm, and the cold shrinkage force of the OPP base film is 0.17 N. After the printing is completed, the composite film is compounded with the intermediate layer NY film using a dry compound machine, the thickness of the NY film is 15 μm, and the cold shrinkage force is 0.28 N. During the compounding, the unwinding tension of the printing layer OPP film is 120 N, and the unwinding tension of the intermediate layer NY film is 100 N. After the compounding is completed, the obtained double-layer film is compounded with the heat-sealing layer CPE film, the thickness of the CPE is 70 μm, and the cold shrinkage force of the base film is 0.15 N. The unwinding tension of the double-layer film is 120 N, and the unwinding tension of the heat-sealing layer CPE film is 100 N. The obtained three-layer film after the compounding is completed is placed in a curing chamber at 45°C for 96 hours for curing.​

[0090] 2. Test the flatness of the composite film after high temperature retort

[0091] The composite film cut into appropriate shape was used to heat seal the bowl-shaped packaging container, and the vent was reserved. After retort at 100°C for 30 min, the sealing film at the vent position was obviously upwarping, the angle was less than 60°, and the sealing film at the vent position returned to flat state within 30 s after cooling at room temperature.

[0092] Comparative Example

[0093] 1. Preparation of the composite film

[0094] The printing layer material was ordinary PET film with thickness of 12 μm, and the cold shrinkage force of the PET original film was 0.19 N. After printing, the printing layer PET film was compounded with the intermediate layer NY film using a dry compounding machine, the NY film had a thickness of 15 μm and a cold shrinkage force of 0.24 N. The unwinding tension of the printing layer PET film during compounding was 80 N, and the unwinding tension of the intermediate layer NY film was 100 N. The double-layer film obtained after compounding was compounded with the heat-sealing layer RCPP, the RCPP had a thickness of 70 μm and a cold shrinkage force of 0.02 N. The unwinding tension of the double-layer film was 80 N, and the unwinding tension of the heat-sealing layer RCPP film was 100 N. The three-layer film obtained after compounding was placed in a curing chamber for curing at 45°C for 96 hours.

[0095] 2. Test the flatness of the composite film after high temperature retort

[0096] The composite film cut into appropriate shape was used to heat seal the bowl-shaped packaging container, and the vent was reserved. After retort at 100°C for 30 min, the sealing film at the vent position was obviously upwarping, the angle was less than 60°, and the sealing film at the vent position returned to flat state within 30 s after cooling at room temperature.

[0097] Table 2 shows the film material structure and compounding process parameters of Examples 1, 2 and Comparative Example.

[0098] Table 2

[0099]

[0100] Although the specific embodiments of the present application have been described in detail, those skilled in the art will understand that various modifications and changes can be made to the details without departing from the spirit and scope of the application as disclosed in the above teachings. The scope of the application is defined by the appended claims and any equivalents thereto.

Claims

1. A composite film comprising, in order from top to bottom, a printing layer, an intermediate layer, and a heat-seal layer. wherein The printing layer is a biaxially-stretched polyester film, a PET film vapor-deposited with aluminum oxide or silicon oxide, a nylon film, a uniaxially-stretched polypropylene film, or a biaxially-stretched polypropylene film. The intermediate layer is a biaxially-stretched polyester film, a PET film vapor-deposited with aluminum oxide or silicon oxide, or a nylon film. The heat-seal layer is a retort-grade cast polypropylene film or a cast polyethylene film. The cold shrinkage of the printing layer is 0.05 N to 0.2 N. The cold shrinkage of the intermediate layer is 0.15 N to 0.55 N. The cold shrinkage of the heat-seal layer is 0.05 N to 0.3 N.

2. The composite film of claim 1, wherein, The thickness of the printing layer is 12 μm to 40 μm.

3. The composite film of claim 1, wherein, The thickness of the intermediate layer is 9 μm to 20 μm.

4. The composite film of claim 1, wherein, The thickness of the heat-seal layer is 50 μm to 120 μm. 5.A method of producing the composite film according to any one of claims 1 to 4. The method is a dry lamination process. 6.The method according to claim 5, comprising the following steps: Step 1: obtaining a printing layer film, an intermediate layer film, and a heat-seal layer film, which are used to form the printing layer, the intermediate layer, and the heat-seal layer, respectively; Step 2: printing the printing layer film; Step 3: laminating the printed printing layer film and the intermediate layer film by a dry lamination process to obtain a double-layer film; Step 4: laminating the double-layer film obtained in Step 3 and the heat-seal layer film by a dry lamination process to obtain a triple-layer film; Step 5: placing the triple-layer film obtained in Step 4 into a maturation chamber to mature to obtain the composite film. 7.The method according to claim 6, further comprising Step 6: cutting the composite film. 8.The method according to claim 6, wherein in Step 3, the unwinding tension of the printing layer film is 100 N to 130 N. 9.The method according to claim 6, wherein in Step 3, the unwinding tension of the intermediate layer film is 90 N to 110 N. 10.The method according to claim 6, wherein in Step 4, the unwinding tension of the double-layer film is 100 N to 130 N. 11.The method according to claim 6, wherein in Step 4, the unwinding tension of the heat-seal layer film is 90 N to 110 N. 12.Use of the composite film according to any one of claims 1 to 4 for producing a packaging container. 13.A packaging container comprising the composite film according to any one of claims 1 to 4. 14.The packaging container according to claim 13, which is a bowl-shaped, cup-shaped, tub-shaped, or box-shaped packaging container comprising a peelable lid film comprising the composite film according to any one of claims 1 to 4. 15.A convenience food comprising the packaging container according to claim 13 or 14. 16.The convenience food according to claim 15, which is a convenience rice. 17.A method of producing a convenience food, the method comprising using the composite film according to any one of claims 1 to 4 as a sealing film of a packaging container. 18.The method according to claim 17, which is a secondary sealing film method.

19. The production method according to claim 17, comprising the steps of: (1) filling a bowl-shaped or box-shaped packaging container with food materials; (2) first heat-sealing: heat-sealing a portion of the bowl opening or box opening using the composite film according to any one of claims 1 to 4 cut into a suitable shape, and leaving two outlets; (3) cooking the packaging container filled with food materials after the first heat-sealing, and then cooling; (4) filling the packaging container with nitrogen gas through one of the two outlets left in step (2), and discharging the air in the packaging container from the other outlet; (5) second heat-sealing: heat-sealing the two outlets, and completing the sealing of the packaging container.

Images