Highly water-resistant, cookable polyolefin composite film and method for producing the same

Abstract

The application provides a high-water-resistance cookable polyolefin composite film and a preparation method thereof, and aims to solve the problems of insufficient water resistance of existing polyolefin films and cost increase and environmental protection problems caused by adding a water resistance coating. The composite film comprises, from outside to inside, an outer surface layer (a biaxially stretched polyolefin film), a water resistance layer (an interpenetrating network structure formed by polypropylene-polypropylene acid block copolymer and polyvinyl alcohol grafted polyacrylamide), a heat insulation layer (a casting film containing a polyolefin elastomer) and a heat sealing layer (a polyolefin film). Through a specific preparation process, such as a multilayer co-extrusion film blowing method, the effective combination of the materials of the layers is realized. The application has the advantages of significantly improving the water resistance of the film, reducing the production cost, meeting the environmental protection requirements and being beneficial to the recycling of resources.

Description

Technical Field

[0001] This invention relates to the field of composite membrane technology, specifically to a polyolefin thin composite membrane, more specifically to a high water-resistant, boilable polyolefin thin composite membrane, and also to a method for preparing the composite membrane. Background Technology

[0002] With increasingly stringent requirements for food quality and safety, packaging materials need to possess excellent barrier properties and high-temperature resistance to extend shelf life. Traditional packaging materials such as polyethylene and polypropylene films are widely used due to their low cost and superior processing performance. However, they have shortcomings in high-temperature retort resistance. Specifically, polyethylene or polypropylene is prone to softening and deformation when exposed to high temperatures for extended periods. Especially during retort processes, water vapor can penetrate the polyethylene film into the packaging. To maintain high water resistance at high temperatures, materials such as aluminum foil or retortable coatings need to be added to the packaging material.

[0003] Patent application CN113043688A discloses a high-barrier retortable stretchable environmentally friendly composite film, which is made by laminating two uniaxially stretched polyolefin films, a polyvinylidene chloride (PVDC) film, and a polyurethane adhesive after two online corona treatments on both sides of the PVDC film. However, the PVDC film contains halogens, and halogen groups may dissociate into liquid food, which is detrimental to health. In addition, similar packaging materials with added water-blocking coatings are also expensive, limiting their application. Furthermore, packaging materials that are not made of a single material are not conducive to environmental recycling.

[0004] Therefore, it is necessary to develop a high water-resistant, boilable polyolefin film to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the technical problems of insufficient water-blocking performance of polyolefin films, and the increased cost and unfavorable recycling caused by adding water-blocking coatings.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0007] A high water-resistant, retortable polyolefin composite membrane, comprising, arranged sequentially from the outside in:

[0008] Outer layer: made of biaxially oriented polyolefin film, preferably BOPE or BOPP film;

[0009] Water-blocking layer: Its raw material composition includes polypropylene-polyacrylic acid block copolymer and polyvinyl alcohol grafted polyacrylamide. The two polymers form an interpenetrating network structure. The term "interpenetrating network structure" refers to a polymer blend composed of two or more polymer networks that are cross-linked and interpenetratingly bonded.

[0010] Thermal insulation layer: a cast film containing polyolefin elastomer; the term "polyolefin elastomer" refers to a thermoplastic elastomer copolymerized from ethylene and propylene or other α-olefins, and the term "cast film" refers to a film layer made by a casting process, such as CPP (cast polypropylene) film.

[0011] Heat-sealing layer: As a preferred embodiment, a low-temperature heat-sealing polyolefin film material is used. The low-temperature heat-sealing function can be achieved by adding a plasticizer to the heat-sealing layer raw material. The heat-sealing layer is used to achieve a rapid sealing effect for the packaging.

[0012] Furthermore, the preparation method of the polypropylene-polyacrylic acid block copolymer includes: in a synthesis reactor, at 85°C, 1 MPa pressure, and in an inert gas atmosphere, propylene gas is introduced into an acrylic acid solution, an initiator (A) and an emulsifier are added and mixed, the mixture is kept warm until the reaction is complete, and then the product is taken out of the solution, washed, and dried to constant weight to obtain the polypropylene-polyacrylic acid block copolymer.

[0013] Furthermore, the preparation method of the polyvinyl alcohol-grafted polyacrylamide includes dissolving polyvinyl alcohol in deionized water, heating to dissolve, adding acrylamide monomer and N,N'-methylenebisacrylamide crosslinking agent, mixing to obtain a prepolymer solution; then adding initiator (B) to the prepolymer solution, keeping it heated until the reaction is complete, then taking the product out of the solution, washing, and drying to constant weight to obtain polyvinyl alcohol-grafted polyacrylamide.

[0014] Furthermore, the grafting rate of the polyvinyl alcohol-grafted polyacrylamide is ≥6%, preferably 6-9%.

[0015] Furthermore, the method for preparing the water-blocking layer includes adding polypropylene-polyacrylic acid block copolymer and polyvinyl alcohol-grafted polyacrylamide to a mixing device at a mass ratio of 1-3:1, adding initiator C, wherein the mass ratio of polypropylene-polyacrylic acid block copolymer to initiator C is 100:1-3.

[0016] Initiators are used in polypropylene-polyacrylic acid block copolymer, polyvinyl alcohol grafted polyacrylamide and water-blocking layer. Since all three reactions can be achieved by free radical polymerization, initiators (A), (B) and (C) can be free radical initiators such as azobisisobutyronitrile and benzoyl peroxide.

[0017] Furthermore, the insulation layer comprises, by mass percentage, 65-75% polyolefin matrix, 20-30% polyolefin elastomer, and 2-5% processing aids.

[0018] Furthermore, the insulation layer and / or the heat-sealing layer are made of high-density polyethylene (HDPE) or high-density polypropylene (HDPP).

[0019] Furthermore, 1,3-butadiene is added to the heat insulation layer and / or the heat sealing layer, wherein the mass ratio of 1,3-butadiene to the polyolefin matrix is ​​2-5:100.

[0020] A method for preparing a high water-barrier, retortable polyolefin film (multilayer co-extrusion blown film method) includes the following steps:

[0021] A method for preparing a high water-barrier, retortable polyolefin film includes the following steps:

[0022] S1, apply 5-12 mg / ㎡ of polyurethane adhesive between multiple thin films, and then dry-laminate them together under 3-15 bar pressure;

[0023] S2, the composite membrane is aged for more than 48 hours.

[0024] Furthermore, the curing temperature of the composite membrane in S2 is 25-60℃.

[0025] The advantages and beneficial effects of this invention are as follows:

[0026] 1. By introducing polypropylene-polyacrylic acid block copolymer and polyvinyl alcohol-grafted polyacrylamide, two materials with excellent water-blocking properties, into the polyolefin film and forming them into an interpenetrating network structure, the water-blocking performance of the film is significantly improved. This structure can effectively prevent the penetration of water molecules, ensuring that the product inside the packaging remains dry.

[0027] 2. Compared to traditional methods of adding water-blocking coatings, this invention directly adds water-blocking materials during the film preparation process, avoiding additional coating steps and thus reducing production costs. Furthermore, the use of recyclable polyolefin materials meets environmental protection requirements and promotes resource recycling. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the composite membrane structure provided by the present invention;

[0029] In the diagram: 1-outer layer, 2-water barrier layer, 3-heat insulation layer, 4-heat seal layer. Detailed Implementation

[0030] This patent application provides a high water-barrier, boilable polyolefin composite membrane, the structure of which is as follows.

[0031] Outer layer: Biaxially oriented polyolefin film is used. This material has good mechanical strength, temperature resistance, and dimensional stability, providing basic physical performance support for the composite film. The biaxial stretching process orients the molecular chains in both longitudinal and transverse directions, increasing the intermolecular forces and improving the barrier properties and stiffness of the film.

[0032] Water-blocking layer: This layer consists of an interpenetrating network structure formed by two copolymers: a polypropylene-polyacrylic acid block copolymer and a polyvinyl alcohol-grafted polyacrylamide copolymer. During synthesis, propylene gas is passed into an acrylic acid solution, and an initiator and emulsifier are added. The reaction proceeds under specific temperature and pressure to generate the polypropylene-polyacrylic acid block copolymer. Its molecular chains contain carboxyl groups from polyacrylic acid. Through block copolymerization, the molecular chains are arranged more tightly and orderly, forming a dense water-blocking structure. Polyvinyl alcohol is dissolved in deionized water, and acrylamide monomer and N,N'-methylenebisacrylamide crosslinking agent are added and mixed to obtain a prepolymer solution. An initiator is then added to react and produce polyvinyl alcohol-grafted polyacrylamide. In the subsequent reaction to prepare the water-blocking layer, the hydroxyl and amide groups in this copolymer can undergo esterification and nucleophilic addition reactions with the carboxyl groups (-COOH) of polyacrylic acid, respectively. Esterification forms ester bonds connecting the two polymers, while nucleophilic addition involves the reaction of the carboxyl group with the amino group in the amide group, forming an unstable intermediate. Following carbon chain rearrangement and elimination processes, an amyl acid is generated connecting the two polymers, significantly increasing the intermolecular forces. The interpenetrating network structure formed by these two copolymers enhances the barrier to water molecules, hindering their penetration. Simultaneously, the grafting of polyvinyl alcohol with polyacrylamide facilitates subsequent lamination processes and improves the processing characteristics during lamination.

[0033] Insulation Layer: A cast film containing polyolefin elastomers. Polyolefin elastomers have good flexibility and elasticity, which can absorb and buffer external heat to a certain extent, thus playing a role in insulation and reducing the direct impact of heat on the packaged product. Specifically, the addition of an appropriate amount of polyolefin elastomers to the insulation layer enables the composite film to better resist deformation and breakage when subjected to external impact or stretching, maintaining the integrity of the film. The molecular structure of polyolefin elastomers is relatively compact, and the arrangement between molecular chains is relatively regular, which can hinder the penetration of small molecules such as water vapor to a certain extent. When it is compounded with other materials such as polyvinyl alcohol grafted polyacrylamide, the barrier performance of the composite film can be further optimized, reducing water vapor permeability and better achieving waterproof and moisture-proof effects, extending the shelf life of the packaged goods. In addition, some polyolefin elastomers (such as ethylene-octene copolymers, styrene thermoplastic elastomers, etc.) have excellent low-temperature resistance, maintaining good elasticity and flexibility even in low-temperature environments. This allows the high water-resistant, boilable polyolefin composite film to still effectively perform its waterproof and moisture-proof functions under low-temperature storage or transportation conditions, without becoming brittle or cracking due to temperature drops, thus expanding the application range of the composite film.

[0034] Heat-sealing layer: Polyolefin film, as a heat-sealing layer, has good heat-sealing performance. It can melt rapidly under heating conditions and seal with the packaging container or other materials to ensure the airtightness of the packaging.

[0035] The composite membrane of this invention combines layered materials with different functions, requiring water molecules to pass through multiple layers to penetrate it. By controlling the grafting rate of polyvinyl alcohol grafted with polyacrylamide and adjusting the composition ratio of the insulation layer, the permeation path is extended. Each layer of material provides resistance to water molecule penetration, and the combined effect of the multi-layer structure significantly improves water-blocking performance. The layers work together to form a unified barrier system. For example, the high water-blocking properties of the water-blocking layer combined with the physical barrier effect of the outer layer, the insulation performance of the insulation layer reduces the risk of increased water molecule penetration due to temperature changes, and the heat-sealing layer ensures the integrity of the packaging seal, preventing moisture from entering from the packaging edges. The insulation layer and / or heat-sealing layer are made of high-density polyethylene or high-density polypropylene, which have high melting points and good thermal stability, maintaining good sealing performance at high temperatures and ensuring that the contents of the package will not deteriorate due to the penetration of moisture or gas.

[0036] A small amount of 1,3-butadiene, a conjugated diene with two double bonds in its molecular structure, is also added to the insulation layer and / or heat-sealing layer. During polymerization, it can copolymerize with polyolefins to form copolymers with a special elastic structure. Unsaturated double bonds are introduced into the main chain of this copolymer, forming side chains containing double bonds, increasing the rotational freedom and mobility of the molecular chains. The interactions between these side chains and with the main chain increase the flexibility of the molecular chains. When the insulation and heat-sealing layers are in high-temperature environments, such as during high-temperature treatments like sterilization, polyolefins without 1,3-butadiene may experience a slight decrease in their sterilization performance due to the vigorous movement of the molecular chains. However, with the addition of 1,3-butadiene, the stability of the molecular chains is enhanced, allowing them to maintain better physical properties at high temperatures, thus ensuring that the insulation and sealing functions of the insulation and heat-sealing layers are almost unaffected. Furthermore, adding an appropriate amount of 1,3-butadiene can alter the melt viscosity of the polyolefin, allowing it to flow and diffuse better at heat-sealing temperatures.

[0037] This invention also provides a method for preparing the aforementioned composite film. The water-blocking layer is prepared by melting and extruding the raw material in an extruder using a blown film method with a blow-up ratio of 3. After downblowing, the film structure is cooled to room temperature and cured. A dry lamination method is used to coat multiple layers of film material with adhesive and then roll-press them together. This method allows for precise control of the thickness and ratio of each layer, ensuring uniform lamination between the materials and avoiding problems such as delamination, bubbles, cracking, or wrinkles. Simultaneously, during the extrusion process, the material in the water-blocking layer is subjected to shear and tensile forces, further aligning the molecular chains and improving the film's performance. The preform of the water-blocking layer is blown up using a downblowing method and then cooled to form the composite film. The blow-up process further stretches the molecular chains of the film laterally, increasing the film's lateral strength and barrier properties. The cooling process fixes the film's morphology and structure, maintaining its good performance.

[0038] The preparation method of polypropylene-polyacrylic acid block copolymer involves introducing propylene gas into 500 mL of acrylic acid solution at a rate of 2 mL / s in a synthesis reactor at 85 °C, 1 MPa, and a nitrogen atmosphere. 2 g of initiator (A) and 5 g of emulsifier (sodium dodecylbenzenesulfonate) are added and mixed. The mixture is kept at this temperature until the reaction is complete. The product is then removed from the solution, washed, and dried to constant weight.

[0039] The preparation method of polyvinyl alcohol grafted polyacrylamide involves dissolving 20g of polyvinyl alcohol in 500mL of deionized water, heating to 80℃ and mechanically stirring to assist dissolution, then adding 20g of acrylamide monomer and 0.5g of N,N'-methylenebisacrylamide crosslinking agent, and mixing to obtain a prepolymer solution; then adding 0.5g of initiator (B) to the prepolymer solution, maintaining the temperature until the reaction is complete, and then removing the product from the solution, washing, and drying to constant weight.

[0040] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0041] Example 1

[0042] A high water-resistant, retortable polyolefin composite membrane, comprising, from the outside in:

[0043] Outer layer: BOPE film;

[0044] Water-blocking layer: By weight, its composition is as follows: 100 parts polypropylene-polyacrylic acid block copolymer, 100 parts polyvinyl alcohol-grafted polyacrylamide, 1 part initiator C, and the grafting rate of polyvinyl alcohol-grafted polyacrylamide is 6%.

[0045] Insulation layer: By mass percentage, its composition is: 65% HDPE, 30% EPM (ethylene propylene diene monomer rubber), 5% processing aids (including 3% slip agent and 2% opening agent), with a casting ratio of 1:2;

[0046] Heat-sealing layer: HDPE film;

[0047] Preparation methods include:

[0048] S1, apply 5mg / ㎡ of polyurethane adhesive between each layer of the multilayer film, and then dry-laminate them together under 3bar pressure;

[0049] S2, the composite membrane is aged at 25°C for 72 hours.

[0050] Example 2

[0051] A high water-resistant, retortable polyolefin composite membrane, comprising, from the outside in:

[0052] Outer layer: BOPP film;

[0053] Water-blocking layer: By weight, its composition is as follows: 200 parts polypropylene-polyacrylic acid block copolymer, 100 parts polyvinyl alcohol-grafted polyacrylamide, 3 parts initiator C, and the grafting rate of polyvinyl alcohol-grafted polyacrylamide is 7%.

[0054] Insulation layer: By mass percentage, its composition is: PE 75%, POE (ethylene-octene copolymer elastomer) 23%, 2% processing aids (including 1% slip agent and 1% opening agent), with a casting ratio of 1:3;

[0055] Heat-sealing layer: PE film;

[0056] Preparation methods include:

[0057] S1, apply 8mg / ㎡ of polyurethane adhesive between each layer of the multilayer film, and then dry-laminate them together under 5bar pressure.

[0058] S2, the composite membrane is aged at 50°C for 48 hours.

[0059] Example 3

[0060] A high water-resistant, retortable polyolefin composite membrane, comprising, from the outside in:

[0061] Outer layer: BOPE film;

[0062] Water-blocking layer: By weight, its composition is as follows: 300 parts polypropylene-polyacrylic acid block copolymer, 100 parts polyvinyl alcohol-grafted polyacrylamide, 2 parts initiator C, and the grafting rate of polyvinyl alcohol-grafted polyacrylamide is 8%.

[0063] Insulation layer: By mass percentage, its composition is: 70% HDPE, 27% TPV (thermoplastic dynamic vulcanizate), 3% processing aids (including 2% slip agent and 1% opening agent), with a casting ratio of 1:5;

[0064] Heat-sealing layer: PE film;

[0065] Preparation methods include:

[0066] S1, apply 7mg / ㎡ of polyurethane adhesive between each layer of the multilayer film, and then dry-laminate them together under 10bar pressure;

[0067] S2, the composite membrane is aged at 50°C for 48 hours.

[0068] Example 4

[0069] A high water-resistant, retortable polyolefin composite membrane, comprising, from the outside in:

[0070] Outer layer: BOPP film;

[0071] Water-blocking layer: By weight, its composition is as follows: 150 parts polypropylene-polyacrylic acid block copolymer, 100 parts polyvinyl alcohol-grafted polyacrylamide, 3 parts initiator C, and the grafting rate of polyvinyl alcohol-grafted polyacrylamide is 8%.

[0072] Insulation layer: By mass percentage, its composition is: 65% HDPE, 30% EPM (ethylene propylene diene monomer rubber), 5% processing aids (including 1% slip agent and 4% opening agent), with a casting ratio of 1:5;

[0073] Heat-sealing layer: 1,3-butadiene and PE are co-extruded into a film at a mass ratio of 5:100.

[0074] Preparation methods include:

[0075] S1, apply 12mg / ㎡ of polyurethane adhesive between each layer of the multilayer film, and then dry-laminate them together under 12bar pressure;

[0076] S2, the composite membrane is aged at 60°C for 48 hours.

[0077] Example 5

[0078] A high water-resistant, retortable polyolefin composite membrane, comprising, from the outside in:

[0079] Outer layer: BOPE film;

[0080] Water-blocking layer: By weight, its composition is as follows: 250 parts polypropylene-polyacrylic acid block copolymer, 100 parts polyvinyl alcohol-grafted polyacrylamide, 5 parts initiator C, and the grafting rate of polyvinyl alcohol-grafted polyacrylamide is 8%.

[0081] Insulation layer: By mass percentage, its composition is: PE 73.5%, 1,3-butadiene 1.5%, EPM (ethylene propylene diene monomer rubber) 20%, and 5% processing aids (including 2% slip agent and 3% opening agent), with a casting ratio of 1:3;

[0082] Heat-sealing layer: 1,3-butadiene and HDPE are co-extruded into a film at a mass ratio of 2:100.

[0083] Preparation methods include:

[0084] S1, apply 10mg / ㎡ of polyurethane adhesive between each layer of the multilayer film, and then dry-laminate them together under 15bar pressure.

[0085] S2, the composite membrane is aged at 55°C for 48 hours.

[0086] Example 6

[0087] A high water-resistant, retortable polyolefin composite membrane, comprising, from the outside in:

[0088] Outer layer: BOPP film;

[0089] Water-blocking layer: By weight, its composition is as follows: 300 parts polypropylene-polyacrylic acid block copolymer, 100 parts polyvinyl alcohol-grafted polyacrylamide, 2 parts initiator C, and the grafting rate of polyvinyl alcohol-grafted polyacrylamide is 7%.

[0090] Insulation layer: By mass percentage, its composition is: HDPE 62.7%, 1;3-butadiene 3.3%, POE (ethylene-octene copolymer elastomer) 30%, 4% processing aids (including slip agent 2% and opening agent 2%), with a casting ratio of 1:2;

[0091] Heat-sealing layer: HDPE film;

[0092] Preparation methods include:

[0093] S1, apply 12mg / ㎡ of polyurethane adhesive between each layer of the multilayer film, and then dry-laminate them together under 10bar pressure;

[0094] S2, the composite membrane is aged at 55°C for 48 hours.

[0095] For each of the above embodiments, the water vapor transmission rate was tested according to ASTM F 1249-13; the heat seal strength was tested according to GB / T19787-2005; after the ends of the composite film in each embodiment were heat-sealed and enclosed to form a packaging bag, the bag was filled with water containing pigment, and the packaging bag was placed in a pot and boiled for 20 minutes. It was considered qualified if no leakage occurred.

[0096] The test results are shown in the table below:

[0097]

[0098] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A high water-resistant, cookable polyolefin composite film, comprising, from outside to inside, sequentially arranged: an outer layer, which is made of a biaxially stretched polyethylene film or a biaxially stretched polypropylene film; a water-resistant layer, which comprises a polypropylene-polyacrylic block copolymer and a polyvinyl alcohol grafted polyacrylamide, the two polymers forming an interpenetrating network structure; a heat-insulating layer, which is a cast film containing a polyolefin matrix and a polyolefin elastomer; a heat-sealing layer, which is made of polyethylene or polypropylene; a preparation method of the polypropylene-polyacrylic block copolymer, comprising, in a synthesis kettle, passing propylene gas into an acrylic acid solution at 85°C under 1 MPa gas pressure in an inert gas atmosphere, adding an initiator and an emulsifier thereto and mixing uniformly, keeping warm until the reaction is complete, then taking the product out of the solution, washing, and drying to constant weight to obtain the polypropylene-polyacrylic block copolymer; a preparation method of the polyvinyl alcohol grafted polyacrylamide, comprising heating and dissolving polyvinyl alcohol into deionized water, adding acrylamide monomer and N,N'-methylene bisacrylamide crosslinking agent, mixing uniformly to obtain a prepolymer solution; then adding an initiator to the prepolymer solution, keeping warm until the reaction is complete, then taking the product out of the solution, washing, and drying to constant weight to obtain the polyvinyl alcohol grafted polyacrylamide with a grafting rate of 6%-9%; a preparation method of the water-resistant layer, comprising adding the polypropylene-polyacrylic block copolymer and the polyvinyl alcohol grafted polyacrylamide in a mass ratio of 1-3:1 to a mixing device, adding an initiator, and the mass ratio of the polyvinyl alcohol grafted polyacrylamide to the initiator being 100:1-3; the heat-insulating layer comprises, by mass percentage, 65-75% of a polyolefin matrix, 20-30% of a polyolefin elastomer, and 2-5% of a processing aid; the heat-insulating layer and / or the heat-sealing layer is made of high-density polyethylene or high-density polypropylene; the heat-insulating layer and / or the heat-sealing layer further comprises 1,3-butadiene, and the mass ratio of the 1,3-butadiene to the polyolefin matrix is 2-5:100; comprising the following steps: S1. coating 5-12 mg / ㎡ of polyurethane glue between the multilayer films, and then dry compounding into one body under a pressure of 3-15 bar; S2. aging the compounded film for more than 48 h; the aging temperature of the compounded film in S2 is 25-60°C. ​ ​ ​ ​ wherein ​ ​ ​ 2. The high-water-barrier, cookable polyolefin composite film according to claim 1, characterized by: ​ 3. The high-water-barrier, cookable polyolefin composite film according to claim 1 or 2, characterized in that: ​ 4. The high-water-barrier, cookable polyolefin composite film according to claim 1 or 2, characterized in that: ​ 5. A process for the production of a high water barrier, cookable polyolefin film according to claim 1, characterized in that, ​ ​ ​ 6. The method of producing a high-water-barrier, cookable polyolefin composite film according to claim 5, characterized by: ​

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