Retort pouch inner layer composite film and preparation method thereof

Abstract

The invention relates to the field of food packaging bags, and particularly discloses an inner-layer composite film for a retort pouch and a preparation method of the inner-layer composite film for the retort pouch. The material is prepared from 20-40 parts of PET, 10-30 parts of polyamide, 40-50 parts of modified polypropylene, 2-4 parts of compatilizer, 1-2 parts of plasticizer and 1-3 parts of filler. The modified polypropylene is prepared from polypropylene, nano cellulose and polyvinylpyrrolidone according to the mass ratio of 100 to (2 to 4) to (1 to 3); the preparation method comprises the following steps: weighing the PET, the polyamide, the modified polypropylene, the compatilizer, the plasticizer and the filler, and uniformly mixing and stirring to obtain a mixture; carrying out melt co-extrusion on the mixture, and then cooling and rolling to obtain a composite film; the sealing device has the advantage that film separation is not easy to occur during high-temperature cooking, and food safety is not easy to influence while the sealing effect is ensured.

Description

Technical Field

[0001] The present application relates to the field of food packaging bags, and more specifically, to a composite film for the inner layer of a retort bag and a preparation method thereof. Background Art

[0002] As an important part of food packaging, the performance of the inner composite film of the retort pouch directly affects the quality and safety of the packaged food. At present, the inner composite film of the retort pouch on the market is mainly composed of polyester (PET), aluminum foil (AL), polyamide (PA), polypropylene (CPP) and other materials. Although these materials meet the basic packaging needs to a certain extent, they are insufficient in heat resistance, sealing and protection of food flavor during high-temperature retorting.

[0003] After high-temperature cooking, the existing inner layer composite film of the cooking bag is prone to delamination between the film layers, resulting in a decrease in sealing performance. At the same time, the heat resistance of the film layer is insufficient, affecting the packaging effect and food safety. Therefore, in order to avoid film layer separation, the performance of the composite film is generally improved by increasing the amount of adhesive used and improving the composite process. However, excessive use of adhesives can easily pose a food safety hazard.

[0004] Therefore, how to prepare a new composite film for the inner layer of the cooking bag so that it has the advantages of not easily separating the film layers during high-temperature cooking, ensuring the sealing effect while not affecting food safety, is an urgent problem to be solved. Summary of the Invention

[0005] In order to prepare a new inner layer composite film for a cooking bag, which has the advantages of not easily causing film separation during high-temperature cooking, ensuring the sealing effect while not easily affecting food safety, the present application provides an inner layer composite film for a cooking bag and a preparation method thereof.

[0006] In a first aspect, the present application provides a composite film for the inner layer of a retort pouch, which adopts the following technical solution: A composite film for the inner layer of a retort pouch, comprising the following raw materials in parts by weight: 20-40 parts of PET, 10-30 parts of polyamide, 40-50 parts of modified polypropylene, 2-4 parts of a compatibilizer, 1-2 parts of a plasticizer, and 1-3 parts of a filler; the modified polypropylene comprises polypropylene, nanocellulose, and polyvinyl pyrrolidone in a mass ratio of 100:2-4:1-3.

[0007] By adopting the above technical solution, PET has high transparency and good mechanical strength, which can extend the service life of the composite film, while polyamide has excellent barrier properties and can block the penetration of oxygen and moisture. Polypropylene provides good heat sealing properties, so that the prepared composite film can have a good sealing barrier effect, meet the requirements of the inner layer composite film of the cooking bag, and is not easy to affect the safety of the food inside the cooking bag.

[0008] The use of nanocellulose and polyvinyl pyrrolidone to modify polypropylene can reduce the melt index of the modified polypropylene, reduce the melt fluidity, and increase the melt viscosity; in the process of preparing a composite film by mixing the modified polypropylene with PET and polyamide, the modified polypropylene has good thermal stability, is less likely to have the problem of high-temperature layer peeling, and extends the service life of the composite film.

[0009] The dispersibility of polyvinyl pyrrolidone and the hydrophilic groups on its surface facilitate interconnection with the hydroxyl groups on the surface of nanocellulose, thereby improving the dispersion effect of nanocellulose and forming a connection network inside the polypropylene. The long chain structure of polyvinyl pyrrolidone facilitates interpenetration between polypropylene molecular chains, hindering the slippage of polypropylene molecular chains and increasing flow resistance. The intermolecular forces between polyvinyl pyrrolidone and polypropylene restrict the free movement of polypropylene chain segments, thereby reducing the melt index. At the same time, the nanostructure of nanocellulose has a high specific surface area and can form a three-dimensional network structure when dispersed in polypropylene. By entanglement, it hinders the flow of polypropylene molecular chains in a molten state, increases melt viscosity, and reduces the melt index. Nanocellulose and polyvinyl pyrrolidone can also act as nucleating agents to block the movement of polypropylene chain segments and reduce the melt index. The combination of nanocellulose and polyvinyl pyrrolidone connects the network, making the modified polypropylene with a low melt index uniformly distributed in the composite film, thereby making the composite film have better stability during high-temperature cooking, less prone to layer peeling problems, and extending the service life of the composite film in the inner layer of the cooking bag.

[0010] The modification of polypropylene with nanocellulose and polyvinyl pyrrolidone can increase the strength and thermal stability of the modified polypropylene, and can also improve the interface compatibility between the modified polypropylene and other materials, and improve the bonding force between the raw materials, thereby extending the service life of the composite membrane.

[0011] Preferably, the average particle size of the nanocellulose is 40-80 nm.

[0012] By adopting the above technical solution, the melt index of polypropylene can be further reduced.

[0013] Preferably, the compatibilizer is composed of maleic anhydride grafted polypropylene and polyether block polyamide in a mass ratio of 1:0.1-0.3.

[0014] By adopting the above technical solution, maleic anhydride grafted polypropylene and polyether block polyamide are matched, and the anhydride groups in the maleic anhydride grafted polypropylene can react and connect with the terminal hydroxyl groups of PET and the terminal amino groups of polyamide, forming chemically bonded interfaces between polypropylene and PET and between polypropylene and polyamide, significantly reducing the phase separation phenomenon and improving the interlayer adhesion; and the polyamide hard segment of the polyether block polyamide is compatible with polyamide, and the polyether segment is partially compatible with polypropylene, forming nanoscale microphase separation, which not only strengthens the interface but also retains elasticity, thereby improving the mechanical properties and durability of the composite film and extending the service life of the composite film.

[0015] Preferably, the plasticizer consists of epoxy soybean oil and acetyl tributyl citrate in a mass ratio of 1:0.2-0.5.

[0016] By adopting the above technical solution, epoxy soybean oil and acetyl tributyl citrate are combined, and the epoxy group in the epoxy soybean oil is easy to form hydrogen bonds with the amide group of polyamide, thereby improving flexibility; the acetyl group and ester bond in acetyl tributyl citrate are matched with the polarity of polyamide, further improving flexibility; and the connection effect between PET, polyamide and polypropylene is improved, thereby improving the flexibility and strength of the composite film and extending the service life of the composite film.

[0017] Preferably, the filler is composed of epoxy silicone oil-modified nano-silica and citric acid-modified hydroxyapatite microspheres in a mass ratio of 1:0.5-1.

[0018] By adopting the above technical solution, epoxy silicone oil-modified nano-silica and citric acid-modified hydroxyapatite microspheres are combined, the epoxy group in the epoxy silicone oil is used to further improve the bonding effect between the nano-silica and PET, polyamide and polypropylene, and the carboxyl group in the citric acid and the hydroxyl group in the hydroxyapatite microspheres are combined to improve the bonding effect between the hydroxyapatite microspheres and PET and polyamide, so that the filler is stably adhered to the composite film, and the strength of the nano-silica and hydroxyapatite microspheres is combined to improve the strength and thermal stability of the composite film.

[0019] Preferably, the epoxy silicone oil-modified nano-silica is prepared from epoxy silicone oil and nano-silica in a mass ratio of 1:1-3.

[0020] By adopting the above technical solution, epoxy silicone oil is loaded on the surface of nano-silica, which improves the dispersion effect of nano-silica and the bonding effect between nano-silica and PET, polyamide and modified polypropylene, thereby improving the high-temperature thermal stability and mechanical strength of the composite film.

[0021] Preferably, the citric acid-modified hydroxyapatite microspheres are prepared by placing hydroxyapatite microspheres in a citric acid solution, ultrasonically dispersing the microspheres, and then filtering the dispersed microspheres.

[0022] By adopting the above technical solution, hydroxyapatite microspheres are ultrasonically dispersed in a citric acid solution, which facilitates uniform contact between the hydroxyapatite microspheres and the citric acid solution, so that citric acid is present in the pores on the surface of the hydroxyapatite microspheres. The carboxyl groups of the citric acid and the hydroxyl groups of the hydroxyapatite microspheres are combined to further improve the connection effect between the hydroxyapatite microspheres and the citric acid. The carboxyl groups of the citric acid and the hydroxyl groups on the surface of the hydroxyapatite microspheres are used to further improve the bonding effect between PET, polyamide and modified polypropylene, thereby improving the thermal stability and strength of the composite film and extending the service life of the composite film.

[0023] In a second aspect, the present application provides a method for preparing a composite film for an inner layer of a retort pouch, which adopts the following technical solution: A method for preparing a composite film for an inner layer of a retort pouch, comprising the following steps: S1. Weigh PET, polyamide, modified polypropylene, compatibilizer, plasticizer, and filler, mix and stir evenly to obtain a mixture; S2. Melt co-extrude the mixture, and then cool and roll it up to obtain a composite film.

[0024] By adopting the above technical solution, the prepared composite film has the advantage that it is not easy for the film layer to separate during high-temperature cooking, ensuring the sealing effect while not easily affecting food safety.

[0025] Preferably, the preheating temperature of the melt coextrusion is 190-210°C, and the extrusion temperature is 240-260°C.

[0026] By adopting the above technical solution, the preheating temperature and extrusion temperature are limited to ensure the bonding effect between the layers, so that the finished composite film is less likely to peel off during the high-temperature cooking process, thereby extending the service life of the composite film inner layer of the cooking bag.

[0027] Preferably, the temperature of the cooling roller during the cooling process is 30-50°C.

[0028] By adopting the above technical solution, the temperature of the cooling roller is limited to ensure the forming effect of the composite film.

[0029] In summary, this application has the following beneficial effects: 1. PET has high transparency and good mechanical strength, which can extend the service life of the composite film, while polyamide has excellent barrier properties and can block the penetration of oxygen and moisture. Polypropylene provides good heat sealing properties, so that the prepared composite film can have a good sealing barrier effect, meeting the requirements of the inner composite film of the cooking bag, and is not easy to affect the safety of the food inside the cooking bag.

[0030] 2. Polyvinylpyrrolidone and nanocellulose act as nucleating agents. Polyvinylpyrrolidone can refine the crystal size of polypropylene, increase the interface area between the crystalline and amorphous regions, hinder the movement of amorphous chain segments, and increase the melt viscosity. Nanocellulose can promote the formation of smaller spherulites in polypropylene, thereby further hindering the movement of amorphous chain segments, increasing the melt viscosity, and making the modified polypropylene have a lower melt index.

[0031] 3. Epoxy silicone oil-modified nano-silica and citric acid-modified hydroxyapatite microspheres are combined to further improve the bonding effect between nano-silica and PET, polyamide and polypropylene by utilizing the epoxy group in epoxy silicone oil. The carboxyl group in citric acid and the hydroxyl group in hydroxyapatite microspheres are combined to improve the bonding effect between hydroxyapatite microspheres and PET and polyamide, so that the filler can be stably adhered to the composite film. The strength and heat resistance of nano-silica and hydroxyapatite microspheres are combined to improve the mechanical strength and thermal stability of the composite film.

[0032] 4. The combination of compatibilizer, plasticizer and filler can further promote the cross-linking between PET, polyamide and modified polypropylene, increase the flexibility and strength of the composite film, and extend the service life of the composite film.

[0033] 5. Citric acid can also be used as a plasticizer to further improve the flexibility and plasticity of the composite film, and cooperate with the strength of hydroxyapatite microspheres to improve the mechanical strength and service life of the composite film. DETAILED DESCRIPTION

[0034] The present application is further described in detail below with reference to the embodiments.

[0035] The following raw materials are all commercially available.

[0036] Preparation example of modified polypropylene Preparation Example 1: Modified polypropylene was prepared by the following method: 2 kg of polyvinyl pyrrolidone was weighed and dissolved in 98 kg of 60°C hot ethanol, with an ethanol mass fraction of 99%, to prepare a solution. 3 kg of nanocellulose was added to the solution and ultrasonically dispersed at 20 kHz for 10 minutes. The average particle size of the nanocellulose was 60 nm. The composite was then spray-dried to obtain a composite material. The compound material is added to 100 kg of polypropylene, mixed and stirred evenly, placed in an extruder for melt extrusion, and pelletized after cooling to obtain modified polypropylene; the melt index of the modified polypropylene is 5 g / 10 min.

[0037] Preparation Example 2: This preparation example differs from Preparation Example 1 in that: 3 kg of polyvinyl pyrrolidone was weighed and dissolved in 97 kg of 60°C hot ethanol with stirring to prepare a solution, 2 kg of nanocellulose was added to the solution, and ultrasonic dispersion was performed at 20 kHz for 10 minutes. The average particle size of the nanocellulose was 80 nm, and then the composite was obtained by spray drying. The compound material is added to 100 kg of polypropylene, mixed and stirred evenly, placed in an extruder for melt extrusion, and pelletized after cooling to obtain modified polypropylene; the melt index of the modified polypropylene is greater than 5 g / 10 min.

[0038] Preparation Example 3: This preparation example differs from Preparation Example 1 in that: 1 kg of polyvinyl pyrrolidone was weighed and dissolved in 98 kg of 60°C hot ethanol with stirring to prepare a solution, 4 kg of nanocellulose was added to the solution, and ultrasonic dispersion was performed at 20 kHz for 10 minutes. The average particle size of the nanocellulose was 40 nm, and then the composite was obtained by spray drying. The compound material is added to 100 kg of polypropylene, mixed and stirred evenly, placed in an extruder for melt extrusion, and pelletized after cooling to obtain modified polypropylene; the melt index of the modified polypropylene is less than 5 g / 10 min.

[0039] Preparation example of epoxy silicone oil modified nano-silica Preparation Example 4: Epoxy silicone oil-modified nano-silica was prepared by the following method: 1 kg of epoxy silicone oil is evenly sprayed on the surface of 2 kg of nano-silicon dioxide. The average particle size of the nano-silicon dioxide is 50 nm and it is evenly dispersed to obtain a finished product. The particle size of the finished product is less than 100 nm.

[0040] Preparation Example 5: This preparation example differs from Preparation Example 4 in that: 1 kg of epoxy silicone oil was evenly sprayed on the surface of 1 kg of nano-silicon dioxide and dispersed evenly to obtain a finished product.

[0041] Preparation Example 6: This preparation example differs from Preparation Example 4 in that: 1 kg of epoxy silicone oil was evenly sprayed on the surface of 3 kg of nano-silicon dioxide and dispersed evenly to obtain a finished product.

[0042] Preparation example of citric acid modified hydroxyapatite microspheres Preparation Example 7: Citric acid-modified hydroxyapatite microspheres were prepared by the following method: 1 kg of hydroxyapatite microspheres are placed in 10 kg of citric acid solution, ultrasonically dispersed at 20 kHz for 10 minutes, and then the hydroxyapatite microspheres are filtered and separated, and dried to obtain a finished product; the average particle size of the hydroxyapatite microspheres is 40 μm, the average porosity is 40%, the citric acid solution is a 5% citric acid aqueous solution, and the particle size of the finished product is less than 60 μm. Example

[0043] Example 1: A composite film for the inner layer of a retort bag: 30kg of PET, 20kg of polyamide, 45kg of modified polypropylene, 3kg of compatibilizer, 1.5kg of plasticizer, and 2kg of filler; the modified polypropylene is the modified polypropylene prepared in Preparation Example 1; the compatibilizer is composed of maleic anhydride grafted polypropylene and polyether block polyamide in a mass ratio of 1:0.2; the plasticizer is composed of epoxy soybean oil and acetyl tributyl citrate in a mass ratio of 1:0.4; the filler is composed of epoxy silicone oil-modified nano-silica prepared in Preparation Example 4 and citric acid-modified hydroxyapatite microspheres prepared in Preparation Example 7 in a mass ratio of 1:0.8; The preparation method is as follows: S1. Weigh PET, polyamide, modified polypropylene, compatibilizer, plasticizer, and filler, mix and stir evenly to obtain a mixture; S2. Place the mixture in an extruder, melt co-extrude under the conditions of a preheating temperature of 200°C and an extrusion temperature of 250°C, then cool it through a 40°C cooling roller, and wind it up to obtain a composite film.

[0044] Example 2: This example differs from Example 1 in that: 20 kg of PET, 10 kg of polyamide, 40 kg of modified polypropylene, 2 kg of compatibilizer, 1 kg of plasticizer, and 1 kg of filler; the modified polypropylene is the modified polypropylene prepared in Preparation Example 2; the compatibilizer is composed of maleic anhydride grafted polypropylene and polyether block polyamide in a mass ratio of 1:0.1; the plasticizer is composed of epoxy soybean oil and acetyl tributyl citrate in a mass ratio of 1:0.2; the filler is composed of epoxy silicone oil-modified nano-silica prepared in Preparation Example 5 and citric acid-modified hydroxyapatite microspheres prepared in Preparation Example 7 in a mass ratio of 1:0.5; The preparation method is as follows: S1. Weigh PET, polyamide, modified polypropylene, compatibilizer, plasticizer, and filler, mix and stir evenly to obtain a mixture; S2. Place the mixture in an extruder, melt co-extrude under the conditions of a preheating temperature of 190°C and an extrusion temperature of 240°C, then cool it through a 30°C cooling roller, and wind it up to obtain a composite film.

[0045] Example 3: This example differs from Example 1 in that: 40 kg of PET, 30 kg of polyamide, 50 kg of modified polypropylene, 4 kg of compatibilizer, 2 kg of plasticizer, and 3 kg of filler; the modified polypropylene is the modified polypropylene prepared in Preparation Example 3; the compatibilizer is composed of maleic anhydride grafted polypropylene and polyether block polyamide in a mass ratio of 1:0.3; the plasticizer is composed of epoxy soybean oil and acetyl tributyl citrate in a mass ratio of 1:0.5; the filler is composed of epoxy silicone oil-modified nano-silica prepared in Preparation Example 6 and citric acid-modified hydroxyapatite microspheres prepared in Preparation Example 7 in a mass ratio of 1:1; The preparation method is as follows: S1. Weigh PET, polyamide, modified polypropylene, compatibilizer, plasticizer, and filler, mix and stir evenly to obtain a mixture; S2. Place the mixture in an extruder, melt co-extrude under the conditions of a preheating temperature of 210°C and an extrusion temperature of 260°C, then cool it through a 50°C cooling roller, and wind it up to obtain a composite film.

[0046] Example 4: This example differs from Example 1 in that: No polyether block polyamide is added to the compatibilizer raw materials.

[0047] Example 5: This example differs from Example 1 in that: The filler is polypropylene particles.

[0048] Example 6: This example differs from Example 1 in that: In the filler, the epoxy silicone oil modified nano-silica was replaced by the same mass of nano-silica, and the citric acid modified hydroxyapatite microspheres were replaced by the same mass of hydroxyapatite microspheres.

[0049] Comparative Example Comparative Example 1: The difference between this comparative example and Example 1 is that: No nanocellulose was added during the preparation of modified polypropylene.

[0050] Comparative Example 2: This comparative example differs from Example 1 in that: No polyvinyl pyrrolidone was added during the preparation of the modified polypropylene.

[0051] Performance testing 1. High temperature stability test Composite films were prepared using the methods of Examples 1-6 and Comparative Examples 1-2, respectively. The peel strength was tested with reference to GB / T 8808 and recorded as the initial peel strength. The films were then boiled in boiling water at 100°C for 30 minutes and the peel strength was tested again, recorded as the peel strength after high-temperature treatment. The difference in peel strength was recorded as initial peel strength minus peel strength after high-temperature treatment. The smaller the difference in peel strength, the better the high-temperature boiling resistance and stability of the composite film, and the less likely it is to experience layer peeling.

[0052] 2. Sealing effect detection Composite films were prepared using the methods of Examples 1-6 and Comparative Examples 1-2, respectively. The oxygen transmission rates of the composite films were tested in accordance with GB / T 1038-2000, and recorded as the initial oxygen transmission rate. The composite films were then steamed in 100°C water for 30 minutes, and the oxygen transmission rates were tested again, and recorded as the oxygen transmission rate after high-temperature treatment. The difference in oxygen transmission rate was calculated as the oxygen transmission rate after high-temperature treatment minus the initial oxygen transmission rate. The smaller the difference, the better the sealing effect of the composite film.

[0053] 3. Mechanical properties testing Composite films were prepared using the methods of Examples 1-3 and 5, respectively. The tensile strength of the composite films was tested with reference to GB / T 1040.3, which was recorded as the initial tensile strength. The films were then placed in 100°C water and steamed for 60 minutes. The tensile strength was tested again, which was recorded as the tensile strength after high-temperature treatment. The difference in tensile strength was recorded as initial tensile strength minus tensile strength after high-temperature treatment. The smaller the difference, the better the high-temperature mechanical properties.

[0054] Table 1 Performance Test Table (“ / ” in the table indicates that the corresponding embodiment or comparative example was not tested for the item, so there is no data) From Examples 1-3 and Table 1, it can be seen that the composite film prepared in the present application has a good sealing effect, is not prone to layer peeling during high-temperature cooking, has high tensile strength, and has good mechanical properties at high temperatures.

[0055] From Example 1 and Examples 4-6 and Table 1, it can be seen that no polyether block polyamide is added to the compatibilizer raw material in Example 4. Compared with Example 1, the composite film prepared in Example 4 has a greater difference in peel strength than that in Example 1, and a greater difference in oxygen permeability than that in Example 1; this indicates that the addition of polyether block polyamide can increase the bonding compatibility between the components in the composite film, thereby improving the bonding strength of the composite film, making it less likely for layer peeling to occur under high temperature conditions, and extending the service life of the composite film.

[0056] The filler in Example 5 is polypropylene particles. Compared with Example 1, the composite film prepared in Example 5 has a greater difference in peel strength than that in Example 1, a greater difference in oxygen permeability than that in Example 1, and a greater difference in tensile strength than that in Example 1. This indicates that the combination of epoxy silicone oil-modified nano-silica and citric acid-modified hydroxyapatite microspheres can improve the tensile strength of the composite film, improve the high-temperature stability and high-temperature sealing effect of the composite film, and extend the service life of the composite film during high-temperature cooking.

[0057] In Example 6, the nano-silica in the filler is replaced by epoxy silicone oil modified nano-silica in the same mass, and the hydroxyapatite microspheres are replaced by citric acid modified hydroxyapatite microspheres in the same mass. Compared with Example 1, the difference in the peeling strength of the composite film prepared in Example 6 is greater than that of Example 1, and the difference in the oxygen permeability is greater than that of Example 1. It is shown that the treatment of nano-silica and hydroxyapatite microspheres with epoxy silicone oil and citric acid can not only improve the bonding stability of the filler in the composite film, but also improve the bonding effect between PET, polyamide and modified polypropylene, and further improve the peeling strength and sealing effect of the composite film in cooperation with the good high temperature resistance of nano-silica and hydroxyapatite microspheres.

[0058] As can be seen from Example 1 and Comparative Examples 1-2 in combination with Table 1, no nano-cellulose is added in the preparation of the modified polypropylene in Comparative Example 1. Compared with Example 1, the difference in the peeling strength of the composite film prepared in Comparative Example 1 is greater than that of Example 1, and the difference in the oxygen permeability is greater than that of Example 1. It is shown that the addition of nano-cellulose can reduce the melt index of the modified polypropylene, increase the melt viscosity, and reduce the melt flowability. In cooperation with PET and polyamide, the bonding effect of each component of the composite film can be improved, thereby improving the stability of the composite film during the cooking process, reducing the layer peeling problem, and prolonging the service life of the composite film.

[0059] In Comparative Example 2, no polyvinylpyrrolidone is added in the preparation of the modified polypropylene. Compared with Example 1, the difference in the peeling strength of the composite film prepared in Comparative Example 2 is greater than that of Example 1, and the difference in the oxygen permeability is greater than that of Example 1. It is shown that the addition of polyvinylpyrrolidone can improve the dispersibility of nano-cellulose while preventing molecular chain migration, thereby reducing the melt index of the modified polypropylene, improving the melt viscosity of the modified polypropylene, improving the bonding stability of the composite film, reducing the layer peeling problem, and prolonging the service life of the composite film.

[0060] The specific embodiments are only an explanation of the present application, and are not a limitation of the present application. Those skilled in the art can make modifications to the embodiments without creative contribution after reading the specification, as long as the modifications are within the scope of the claims of the present application.

Claims

1. A composite film for the inner layer of a retort bag, characterized in that: The composite film comprises the following raw materials in parts by weight: 20-40 parts of PET, 10-30 parts of polyamide, 40-50 parts of modified polypropylene, 2-4 parts of compatibilizer, 1-2 parts of plasticizer, and 1-3 parts of filler; The modified polypropylene consists of polypropylene, nanocellulose and polyvinyl pyrrolidone in a mass ratio of 100:2-4:1-3.

2. The inner layer composite film of a retort pouch according to claim 1, characterized in that: The average particle size of the nanocellulose is 40-80 nm.

3. The inner layer composite film of a retort pouch according to claim 1, characterized in that: The compatibilizer consists of maleic anhydride grafted polypropylene and polyether block polyamide in a mass ratio of 1:0.1-0.

3.

4. The inner layer composite film of a retort pouch according to claim 1, characterized in that: The plasticizer consists of epoxy soybean oil and acetyl tributyl citrate in a mass ratio of 1:0.2-0.

5.

5. The inner layer composite film of a retort pouch according to claim 1, characterized in that: The filler consists of epoxy silicone oil-modified nano-silica and citric acid-modified hydroxyapatite microspheres in a mass ratio of 1:0.5-1.

6. The inner layer composite film of a retort pouch according to claim 5, characterized in that: The epoxy silicone oil modified nano-silicon dioxide is prepared from epoxy silicone oil and nano-silicon dioxide in a mass ratio of 1:1-3.

7. The inner layer composite film of a retort pouch according to claim 5, characterized in that: The citric acid modified hydroxyapatite microspheres are prepared by placing the hydroxyapatite microspheres in a citric acid solution, ultrasonically dispersing the microspheres, and then filtering the solution.

8. The method for preparing the inner layer composite film of a retort pouch according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1. Weigh PET, polyamide, modified polypropylene, compatibilizer, plasticizer, and filler, mix and stir evenly to obtain a mixture; S2. The mixed material is melt co-extruded, cooled, and rolled to obtain a composite film.

9. The method for preparing the inner layer composite film of a retort pouch according to claim 8, characterized in that: The preheating temperature of the melt coextrusion is 190-210°C, and the extrusion temperature is 240-260°C.

10. The method for preparing the inner layer composite film of a retort pouch according to claim 8, characterized in that: The temperature of the cooling roller during the cooling process is 30-50°C.

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