High-barrier EVOH heat-shrinkable cover film and preparation method thereof

Abstract

The invention is suitable for the technical field of heat-shrinkable cover films, and provides a high-barrier EVOH heat-shrinkable cover film and a preparation method thereof.The heat-shrinkable cover film comprises an outer surface layer, a heat-sealing layer and at least one barrier layer arranged between the outer surface layer and the heat-sealing layer, and the adjacent film layers are bonded through bonding layers; the barrier layer is prepared from the following raw materials: ethylene-vinyl alcohol copolymer, polyolefin, modified medical polyvinyl alcohol, modified zein, modified nano zeolite, epoxidized soybean oil and zinc stearate, and the ethylene-vinyl alcohol copolymer is used as a main raw material of the barrier layer, so that the barrier layer has excellent processability and has an excellent blocking effect on air and water; by adding modified polyvinyl alcohol, the mechanical property and barrier property of the barrier layer are remarkably improved, and by adding modified zein, the barrier layer has a film-forming property, so that the barrier layer has good moisture-barrier and oxygen-barrier properties, and the mechanical property of the barrier layer is improved; and by adding the modified nano zeolite, the mechanical property and compactness of the film layer are improved, and the oxygen resistance and water resistance are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of heat shrinkable cover films, and in particular relates to a high-barrier EVOH heat shrinkable cover film and a preparation method thereof. Background Art

[0002] In recent years, my country's new retail model has emerged, and the fresh food e-commerce industry (such as Hema Fresh, Dingdong Maicai, etc.) has developed rapidly. There is a strong demand for various barrier packaging. Among them, relying on the exquisite shelf appearance and excellent fresh-keeping performance, tray-type barrier packaging has emerged in meat retail. The tray packaging consists of an upper cover film and a lower tray. The upper cover film is an EVOH multi-layer co-extruded film; The heat shrink film products currently circulating in the market generally use a single PVDC or EVOH as a barrier layer, which has the following problems: first, the water barrier performance needs to be improved, especially for the packaging of fresh cold meat with a high water content, which requires higher water barrier properties; second, the physical and mechanical properties of existing heat shrink films still need to be improved. Summary of the invention

[0003] The present invention provides a high-barrier EVOH heat-shrinkable cover film, aiming to solve the above-mentioned problems.

[0004] The present invention is achieved in this way: a high-barrier EVOH heat-shrinkable cover film comprises an outer layer, a heat-sealing layer and at least one barrier layer arranged therebetween, and adjacent film layers are bonded by an adhesive layer, and the barrier layer comprises the following raw materials in parts by weight: 100-120 parts of ethylene-vinyl alcohol copolymer, 30-40 parts of polyolefin, 15-20 parts of modified medical polyvinyl alcohol, 5-10 parts of modified corn protein, 6-10 parts of modified nano zeolite, 4-8 parts of epoxidized soybean oil, and 3-5 parts of zinc stearate.

[0005] Preferably, the barrier layer comprises the following raw materials in parts by weight: 105-115 parts of ethylene-vinyl alcohol copolymer, 33-37 parts of polyolefin, 16-19 parts of modified medical polyvinyl alcohol, 6-9 parts of modified corn protein, 7-9 parts of modified nano zeolite, 5-7 parts of epoxidized soybean oil, and 3.5-4.5 parts of zinc stearate.

[0006] Preferably, the barrier layer comprises the following raw materials in parts by weight: 110 parts of ethylene-vinyl alcohol copolymer, 35 parts of polyolefin, 17.5 parts of modified medical polyvinyl alcohol, 7.5 parts of modified corn protein, 8 parts of modified nano zeolite, 6 parts of epoxidized soybean oil, and 4 parts of zinc stearate.

[0007] Preferably, the thickness of the outer layer accounts for 8-12% of the total thickness; the barrier layer accounts for 20-30% of the total thickness; the heat-sealing layer accounts for 20-25% of the total thickness; and the rest is the thickness of the adhesive layer.

[0008] Preferably, the outer layer is made of one or more of polypropylene, low-density polyethylene, and linear low-density polyethylene, the heat-sealing layer is made of PE, and the adhesive layer is made of one or two of EVA and maleic anhydride-grafted PE.

[0009] Preferably, the preparation method of the modified medical polyvinyl alcohol is as follows: Weigh 40-50 parts by weight of a 20-30% aqueous solution of medical polyvinyl alcohol; Heat the medical polyvinyl alcohol aqueous solution to 80-100°C, keep warm and stir until it is completely dissolved, add 20-25wt% sodium hydroxide solution, adjust the pH value to 10-11, and stir for 0.5-1h; Add 1-3 parts of silane coupling agent, heat to 100-120°C at a heating rate of 1-3°C / min, and stir to react for 50-60min; Cool to room temperature, add acid to adjust the pH to 7-8, and distill under reduced pressure to obtain modified polyvinyl alcohol; Through the reaction of polyvinyl alcohol and silane coupling agent, the silane coupling agent generates silanol in hydrolysis reaction, which can undergo condensation reaction at a certain temperature to generate siloxane, producing long flexible chains with active groups, which can generate modified polyvinyl alcohol with long flexible chains through condensation reaction with the hydroxyl groups on polyvinyl alcohol, thereby improving the processing performance of polyvinyl alcohol and forming a denser cross-linking structure, thereby significantly improving the mechanical properties and barrier properties of the barrier layer.

[0010] Preferably, the preparation method of the modified zein is as follows: By weight, take 20-25 parts of 80-90% purity corn protein, then add 60-80 parts of 80-92% ethanol solution, and stir for 10-15 minutes; Add 6-10 parts of polyfructose with a concentration of 0.2 g / mL and a purity of 95%, heat to 70-80°C, and stir magnetically while ultrasonically treating at 300-400 W for 30-50 min to prepare a mixed solution; The mixed solution was freeze-dried in vacuum, and after drying, it was crushed and sieved through a 100-mesh sieve to obtain dry powder; The dry powder is placed at 70-80° C. and 60-80% relative humidity for reaction for 24-36 hours, washed repeatedly with distilled water, centrifuged and freeze-dried to obtain modified zein; Corn gluten has film-forming properties, which makes the barrier layer have good moisture and oxygen barrier properties. By adding polyfructose to corn gluten, glycoside components can be introduced to form a three-dimensional network structure, thereby improving the mechanical properties of the barrier layer and the mechanical properties of the shrink film.

[0011] Preferably, the preparation method of the modified nano zeolite is as follows: Weigh 12-16 parts of zeolite powder by weight, then add 6-10 times the weight of 25-35% magnesium chloride solution, and stir at 400-500 r / min for 30-50 min; After filtering, the material is rinsed with deionized water for 2-3 times, then mixed with 30-40 parts of hexadecyltrimethylammonium bromide, heated to 60-80°C, and stirred at a speed of 400-500 r / min for 30-50 min; After filtering, rinse with deionized water 2-3 times, and dry at 80-100°C to obtain modified nano zeolite; First, the zeolite is exfoliated and dispersed into smaller crystal structures using magnesium chloride to make it uniformly dispersed at the nanoscale. The zeolite is then treated with hexadecyltrimethylammonium bromide to cause a cation exchange reaction, so that organic groups cover the surface of the zeolite or are inserted between its layers, thereby changing its surface energy and increasing the interlayer spacing, changing it from hydrophilic to lipophilic, thereby improving the water-blocking effect. At the same time, the modified zeolite dispersed at the nanoscale has a strong ability to bind to the system to form a network structure, which is beneficial to improving the mechanical properties and density of the membrane layer, and improving the oxygen and water barrier properties.

[0012] The present invention also provides a method for preparing the above-mentioned high-barrier EVOH heat-shrinkable cover film, comprising the following steps: After pretreatment, the raw materials of each layer are added to the extruder of each layer for melting and plasticization, and then extruded through a multi-layer die to obtain a film blank; It is cooled by a vacuum cylinder water jacket, then enters a hot water bath or a heating box, heated to above the glass transition temperature for longitudinal stretching, and then inflated transversely with compressed air to obtain a film; The film is cooled and shaped, and then an electron accelerator is used to perform online radiation cross-linking treatment, and the film is rolled up to obtain a heat shrinkable cover film.

[0013] Preferably, the temperature of the compressed air is 30-50° C., the blowing ratio and the stretching ratio are both 2-4:1, and the radiation dose of the radiation cross-linking treatment is 3-8 MR.

[0014] Compared with the prior art, the embodiments of the present application have the following beneficial effects: The high-barrier EVOH heat-shrinkable cover film provided by the present invention uses ethylene-vinyl alcohol copolymer as the main raw material of the barrier layer, has excellent processing performance, and exhibits excellent blocking effect on air and water. By adding modified polyvinyl alcohol, a denser cross-linking structure is formed, which significantly improves the mechanical properties and barrier properties of the barrier layer. By adding modified corn protein, the film-forming property is achieved, so that the barrier layer has good moisture and oxygen barrier properties. By adding polyfructose, a glycosyl component is introduced into the corn protein to form a three-dimensional network structure, which improves the mechanical properties of the barrier layer and the mechanical properties of the shrink film. By adding modified nano zeolite, it is converted into lipophilicity to improve the water-blocking effect. At the same time, it has a strong ability to combine with the system to form a network structure, which is beneficial to improving the mechanical properties and density of the film layer, and improving the oxygen and water barrier properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The present invention provides a preparation flow chart of a high-barrier EVOH heat-shrinkable cover film. DETAILED DESCRIPTION

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of this application; the terms used in the specification of the application herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned drawings are intended to cover non-exclusive inclusions. The terms "first", "second", etc. in the specification and claims of this application or the above-mentioned drawings are used to distinguish different objects, not to describe a specific order.

[0017] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0018] Example 1 The embodiment of the present invention provides a high barrier EVOH heat shrinkable cover film, such as Figure 1 As shown, it includes an outer layer, a heat-sealing layer and a barrier layer arranged therebetween, and adjacent film layers are bonded by an adhesive layer. The barrier layer includes the following raw materials in parts by weight: 100 parts of ethylene vinyl alcohol copolymer, 30 parts of polyolefin, 15 parts of modified medical polyvinyl alcohol, 5 parts of modified corn protein, 6 parts of modified nano zeolite, 4 parts of epoxy soybean oil, and 3 parts of zinc stearate. The preparation method of the high-barrier EVOH heat-shrinkable cover film includes the following steps: After pretreatment, the raw materials of each layer are added to the extruder of each layer for melting and plasticization, and then extruded through a multi-layer die to obtain a film blank; The film is cooled by a water jacket of a vacuum cylinder, and then enters a hot water bath or a heating box, heated to above the glass transition temperature for longitudinal stretching, and then inflated transversely by compressed air to obtain a film, wherein the compressed air temperature is 30°C, and the inflation ratio and the stretching ratio are both 2:1; The film is cooled and shaped, and then subjected to online radiation cross-linking treatment using an electron accelerator, and rolled up to obtain a heat shrinkable cover film, wherein the radiation dose of the radiation cross-linking treatment is 3MR.

[0019] The thickness of the outer layer accounts for 8% of the total thickness; the barrier layer accounts for 20% of the total thickness; the heat-sealing layer accounts for 20% of the total thickness; and the rest is the thickness of the adhesive layer.

[0020] Specifically, the outer layer is made of one or more of polypropylene, low-density polyethylene, and linear low-density polyethylene, the heat-sealing layer is made of PE, and the adhesive layer is made of one or two of EVA and maleic anhydride grafted PE.

[0021] In this embodiment, the preparation method of the modified medical polyvinyl alcohol is as follows: By weight, weigh 40 parts of a 20% aqueous solution of medical polyvinyl alcohol; Heat the medical polyvinyl alcohol aqueous solution to 80°C, keep it warm and stir until it is completely dissolved, add 20wt% sodium hydroxide solution, adjust the pH value to 10, and stir for 0.5h; Add 1 part of silane coupling agent, heat to 100°C at a heating rate of 1°C / min, and stir to react for 50min; The mixture was cooled to room temperature, acid was added to adjust the pH to 7, and vacuum distillation was performed to obtain modified polyvinyl alcohol; Furthermore, the preparation method of the modified zein is as follows: By weight, take 20 parts of 80% pure corn protein, then add 60 parts of 80% ethanol solution, and stir for 10 minutes; Add 6 parts of polyfructose with a concentration of 0.2 g / mL and a purity of 95%, heat to 70°C, and prepare a mixed solution by magnetic stirring and ultrasonic treatment at 300 W for 30 min; The mixed solution was freeze-dried in vacuum, and after drying, it was crushed and sieved through a 100-mesh sieve to obtain dry powder; The dry powder was placed at 70°C and 60% relative humidity for 24 hours, washed repeatedly with distilled water, centrifuged and freeze-dried to obtain modified zein; Furthermore, the preparation method of the modified nano zeolite is as follows: Weigh 12 parts of zeolite powder by weight, then add 6 times the weight of 25% magnesium chloride solution, and stir at 400 r / min for 30 min; After filtration, the material was rinsed twice with deionized water, then mixed with 30 parts of hexadecyltrimethylammonium bromide, heated to 60°C, and stirred at 400 r / min for 30 min; After filtering, the precipitate was rinsed twice with deionized water and dried at 80° C. to obtain modified nano zeolite.

[0022] Example 2 The embodiment of the present invention provides a high barrier EVOH heat shrinkable cover film, such as Figure 1 As shown, it includes an outer layer, a heat-sealing layer and a barrier layer arranged therebetween, and adjacent film layers are bonded by an adhesive layer. The barrier layer includes the following raw materials in parts by weight: 105 parts of ethylene vinyl alcohol copolymer, 33 parts of polyolefin, 16 parts of modified medical polyvinyl alcohol, 6 parts of modified corn protein, 7 parts of modified nano zeolite, 5 parts of epoxidized soybean oil, and 3.5 parts of zinc stearate. The preparation method of the high-barrier EVOH heat-shrinkable cover film includes the following steps: After pretreatment, the raw materials of each layer are added to the extruder of each layer for melting and plasticization, and then extruded through a multi-layer die to obtain a film blank; The film is cooled by a water jacket of a vacuum cylinder, and then enters a hot water bath or a heating box, heated to above the glass transition temperature for longitudinal stretching, and then inflated transversely by compressed air to obtain a film, wherein the compressed air temperature is 30°C, and the inflation ratio and the stretching ratio are both 2:1; The film is cooled and shaped, and then subjected to online radiation cross-linking treatment using an electron accelerator, and rolled up to obtain a heat shrinkable cover film, wherein the radiation dose of the radiation cross-linking treatment is 3MR.

[0023] The thickness of the outer layer accounts for 8% of the total thickness; the barrier layer accounts for 20% of the total thickness; the heat-sealing layer accounts for 20% of the total thickness; and the rest is the thickness of the adhesive layer.

[0024] Specifically, the outer layer is made of one or more of polypropylene, low-density polyethylene, and linear low-density polyethylene, the heat-sealing layer is made of PE, and the adhesive layer is made of one or two of EVA and maleic anhydride grafted PE.

[0025] In this embodiment, the preparation method of the modified medical polyvinyl alcohol is as follows: By weight, weigh 40 parts of a 20% aqueous solution of medical polyvinyl alcohol; Heat the medical polyvinyl alcohol aqueous solution to 80°C, keep it warm and stir until it is completely dissolved, add 20wt% sodium hydroxide solution, adjust the pH value to 10, and stir for 0.5h; Add 1 part of silane coupling agent, heat to 100°C at a heating rate of 1°C / min, and stir to react for 50min; The mixture was cooled to room temperature, acid was added to adjust the pH to 7, and vacuum distillation was performed to obtain modified polyvinyl alcohol; Furthermore, the preparation method of the modified zein is as follows: By weight, take 20 parts of 80% pure corn protein, then add 60 parts of 80% ethanol solution, and stir for 10 minutes; Add 6 parts of polyfructose with a concentration of 0.2 g / mL and a purity of 95%, heat to 70°C, and prepare a mixed solution by magnetic stirring and ultrasonic treatment at 300 W for 30 min; The mixed solution was freeze-dried in vacuum, and after drying, it was crushed and sieved through a 100-mesh sieve to obtain dry powder; The dry powder was placed at 70°C and 60% relative humidity for 24 hours, washed repeatedly with distilled water, centrifuged and freeze-dried to obtain modified zein; Furthermore, the preparation method of the modified nano zeolite is as follows: Weigh 12 parts of zeolite powder by weight, then add 6 times the weight of 25% magnesium chloride solution, and stir at 400 r / min for 30 min; After filtration, the material was rinsed twice with deionized water, then mixed with 30 parts of hexadecyltrimethylammonium bromide, heated to 60°C, and stirred at 400 r / min for 30 min; After filtering, the precipitate was rinsed twice with deionized water and dried at 80° C. to obtain modified nano zeolite.

[0026] Example 3 The embodiment of the present invention provides a high barrier EVOH heat shrinkable cover film, such as Figure 1 As shown, it includes an outer layer, a heat-sealing layer and a barrier layer arranged therebetween, and adjacent film layers are bonded by an adhesive layer. The barrier layer includes the following raw materials in parts by weight: 110 parts of ethylene-vinyl alcohol copolymer, 35 parts of polyolefin, 17.5 parts of modified medical polyvinyl alcohol, 7.5 parts of modified corn protein, 8 parts of modified nano zeolite, 6 parts of epoxy soybean oil, and 4 parts of zinc stearate. The preparation method of the high-barrier EVOH heat-shrinkable cover film includes the following steps: After pretreatment, the raw materials of each layer are added to the extruder of each layer for melting and plasticization, and then extruded through a multi-layer die to obtain a film blank; The film is cooled by a vacuum cylinder water jacket, then enters a hot water bath or a heating box, is heated to above the glass transition temperature, is longitudinally stretched, and then is transversely inflated by compressed air to obtain a film, wherein the compressed air temperature is 40°C, and the inflation ratio and the stretching ratio are both 3:1; The film is cooled and shaped, and then subjected to online radiation cross-linking treatment using an electron accelerator, and rolled up to obtain a heat shrinkable cover film, wherein the radiation dose of the radiation cross-linking treatment is 5.5 MR.

[0027] The thickness of the outer layer accounts for 10% of the total thickness; the thickness of the barrier layer accounts for 25% of the total thickness; the thickness of the heat-sealing layer accounts for 23% of the total thickness; and the rest is the thickness of the adhesive layer.

[0028] Specifically, the outer layer is made of one or more of polypropylene, low-density polyethylene, and linear low-density polyethylene, the heat-sealing layer is made of PE, and the adhesive layer is made of one or two of EVA and maleic anhydride grafted PE.

[0029] In this embodiment, the preparation method of the modified medical polyvinyl alcohol is as follows: By weight, weigh 45 parts of a 25% aqueous solution of medical polyvinyl alcohol; Heat the medical polyvinyl alcohol aqueous solution to 90°C, keep it warm and stir until it is completely dissolved, add 23wt% sodium hydroxide solution, adjust the pH value to 10, and stir for 0.7h; Add 2 parts of silane coupling agent, raise the temperature to 110°C at a heating rate of 2°C / min, and stir the reaction for 55min; The mixture was cooled to room temperature, acid was added to adjust the pH to 7, and vacuum distillation was performed to obtain modified polyvinyl alcohol; Furthermore, the preparation method of the modified zein is as follows: By weight, 22.5 parts of 85% pure corn protein were taken, and then 70 parts of 86% ethanol solution were added and stirred for 12 minutes; 8 parts of polyfructose with a concentration of 0.2 g / mL and a purity of 95% were added, heated to 75°C, and subjected to magnetic stirring and ultrasonic treatment at 350 W for 40 min to prepare a mixed solution; The mixed solution was freeze-dried in vacuum, and after drying, it was crushed and sieved through a 100-mesh sieve to obtain dry powder; The dry powder was placed at 75°C and 70% relative humidity for 30 hours, washed repeatedly with distilled water, centrifuged and freeze-dried to obtain modified zein; Furthermore, the preparation method of the modified nano zeolite is as follows: Weigh 14 parts of zeolite powder by weight, then add 8 times the weight of 30% magnesium chloride solution, and stir at 450r / min for 40min; After filtration, the material was rinsed twice with deionized water, then mixed with 35 parts of hexadecyltrimethylammonium bromide, heated to 70°C, and stirred at 450 r / min for 40 min; After filtering, the mixture was rinsed twice with deionized water and dried at 90° C. to obtain modified nano zeolite.

[0030] Example 4 The embodiment of the present invention provides a high barrier EVOH heat shrinkable cover film, such as Figure 1 As shown, it includes an outer layer, a heat-sealing layer and a barrier layer arranged between the two, and the adjacent film layers are bonded by an adhesive layer. The barrier layer includes the following raw materials in parts by weight: 115 parts of ethylene vinyl alcohol copolymer, 37 parts of polyolefin, 19 parts of modified medical polyvinyl alcohol, 9 parts of modified corn protein, 9 parts of modified nano zeolite, 7 parts of epoxidized soybean oil, and 4.5 parts of zinc stearate. The preparation method of the high-barrier EVOH heat-shrinkable cover film includes the following steps: After pretreatment, the raw materials of each layer are added to the extruder of each layer for melting and plasticization, and then extruded through a multi-layer die to obtain a film blank; The film is cooled by a vacuum cylinder water jacket, then enters a hot water bath or a heating box, is heated to above the glass transition temperature, is longitudinally stretched, and then is transversely inflated by compressed air to obtain a film, wherein the compressed air temperature is 50°C, and the inflation ratio and the stretching ratio are both 4:1; The film is cooled and shaped, and then subjected to online radiation cross-linking treatment using an electron accelerator, and rolled up to obtain a heat shrinkable cover film, wherein the radiation dose of the radiation cross-linking treatment is 8 MR.

[0031] The thickness of the outer layer accounts for 12% of the total thickness; the barrier layer accounts for 30% of the total thickness; the heat-sealing layer accounts for 25% of the total thickness; and the rest is the thickness of the adhesive layer.

[0032] Specifically, the outer layer is made of one or more of polypropylene, low-density polyethylene, and linear low-density polyethylene, the heat-sealing layer is made of PE, and the adhesive layer is made of one or two of EVA and maleic anhydride grafted PE.

[0033] In this embodiment, the preparation method of the modified medical polyvinyl alcohol is as follows: By weight, weigh 50 parts of a 30% aqueous solution of medical polyvinyl alcohol; Heat the medical polyvinyl alcohol aqueous solution to 100°C, keep it warm and stir until it is completely dissolved, add 25wt% sodium hydroxide solution, adjust the pH value to 11, and stir for 0.1h; Add 3 parts of silane coupling agent, heat to 120°C at a heating rate of 3°C / min, and stir to react for 60min; The mixture was cooled to room temperature, acid was added to adjust the pH to 8, and vacuum distillation was performed to obtain modified polyvinyl alcohol; Furthermore, the preparation method of the modified zein is as follows: By weight, take 25 parts of 90% pure corn protein, then add 80 parts of 92% ethanol solution, and stir for 15 minutes; 10 parts of polyfructose with a concentration of 0.2 g / mL and a purity of 95% were added, heated to 80°C, and ultrasonically treated at 400 W for 50 min under magnetic stirring to prepare a mixed solution; The mixed solution was freeze-dried in vacuum, and after drying, it was crushed and sieved through a 100-mesh sieve to obtain dry powder; The dry powder was placed at 80°C and 80% relative humidity for 36 hours, washed repeatedly with distilled water, centrifuged and freeze-dried to obtain modified zein; Furthermore, the preparation method of the modified nano zeolite is as follows: Weigh 16 parts of zeolite powder by weight, then add 10 times the weight of 35% magnesium chloride solution, and stir at 500 r / min for 50 min; After filtration, the material was rinsed with deionized water three times, then mixed with 40 parts of hexadecyltrimethylammonium bromide, heated to 80°C, and stirred at a speed of 500 r / min for 50 min; After filtering, the product was rinsed with deionized water for three times and dried at 100° C. to obtain modified nano zeolite.

[0034] Example 5 The embodiment of the present invention provides a high barrier EVOH heat shrinkable cover film, such as Figure 1 As shown, it includes an outer layer, a heat-sealing layer and a barrier layer arranged between the two, and the adjacent film layers are bonded by an adhesive layer. The barrier layer includes the following raw materials in parts by weight: 120 parts of ethylene vinyl alcohol copolymer, 40 parts of polyolefin, 20 parts of modified medical polyvinyl alcohol, 10 parts of modified corn protein, 10 parts of modified nano zeolite, 8 parts of epoxy soybean oil, and 5 parts of zinc stearate. The preparation method of the high-barrier EVOH heat-shrinkable cover film includes the following steps: After pretreatment, the raw materials of each layer are added to the extruder of each layer for melting and plasticization, and then extruded through a multi-layer die to obtain a film blank; The film is cooled by a vacuum cylinder water jacket, then enters a hot water bath or a heating box, is heated to above the glass transition temperature, is longitudinally stretched, and then is transversely inflated by compressed air to obtain a film, wherein the compressed air temperature is 50°C, and the inflation ratio and the stretching ratio are both 4:1; The film is cooled and shaped, and then subjected to online radiation cross-linking treatment using an electron accelerator, and rolled up to obtain a heat shrinkable cover film, wherein the radiation dose of the radiation cross-linking treatment is 8 MR.

[0035] The thickness of the outer layer accounts for 12% of the total thickness; the barrier layer accounts for 30% of the total thickness; the heat-sealing layer accounts for 25% of the total thickness; and the rest is the thickness of the adhesive layer.

[0036] Specifically, the outer layer is made of one or more of polypropylene, low-density polyethylene, and linear low-density polyethylene, the heat-sealing layer is made of PE, and the adhesive layer is made of one or two of EVA and maleic anhydride grafted PE.

[0037] In this embodiment, the preparation method of the modified medical polyvinyl alcohol is as follows: By weight, weigh 50 parts of a 30% aqueous solution of medical polyvinyl alcohol; Heat the medical polyvinyl alcohol aqueous solution to 100°C, keep it warm and stir until it is completely dissolved, add 25wt% sodium hydroxide solution, adjust the pH value to 11, and stir for 0.1h; Add 3 parts of silane coupling agent, heat to 120°C at a heating rate of 3°C / min, and stir to react for 60min; The mixture was cooled to room temperature, acid was added to adjust the pH to 8, and vacuum distillation was performed to obtain modified polyvinyl alcohol; Furthermore, the preparation method of the modified zein is as follows: By weight, take 25 parts of 90% pure corn protein, then add 80 parts of 92% ethanol solution, and stir for 15 minutes; 10 parts of polyfructose with a concentration of 0.2 g / mL and a purity of 95% were added, heated to 80°C, and ultrasonically treated at 400 W for 50 min under magnetic stirring to prepare a mixed solution; The mixed solution was freeze-dried in vacuum, and after drying, it was crushed and sieved through a 100-mesh sieve to obtain dry powder; The dry powder was placed at 80°C and 80% relative humidity for 36 hours, washed repeatedly with distilled water, centrifuged and freeze-dried to obtain modified zein; Furthermore, the preparation method of the modified nano zeolite is as follows: Weigh 16 parts of zeolite powder by weight, then add 10 times the weight of 35% magnesium chloride solution, and stir at 500 r / min for 50 min; After filtration, the material was rinsed with deionized water three times, then mixed with 40 parts of hexadecyltrimethylammonium bromide, heated to 80°C, and stirred at a speed of 500 r / min for 50 min; After filtering, the product was rinsed with deionized water for three times and dried at 100° C. to obtain modified nano zeolite.

[0038] Comparative Example 1: Compared with Example 3, the modified medical polyvinyl alcohol is replaced by ordinary medical polyvinyl alcohol.

[0039] Comparative Example 2: Compared with Example 3, the modified corn gluten was replaced by ordinary corn gluten.

[0040] Comparative Example 3: Compared with Example 3, the modified nano zeolite is replaced by ordinary nano zeolite.

[0041] Comparative Example 4: Compared with Example 3, the modified medical polyvinyl alcohol is replaced by ordinary medical polyvinyl alcohol, the modified corn protein is replaced by ordinary corn protein, and the modified nano zeolite is replaced by ordinary nano zeolite.

[0042] Comparative Example 5: A commercially available EVOH heat shrinkable cover film.

[0043] Performance Testing The tensile strength and barrier properties of the heat shrinkable cover films of Examples 1-5 and Comparative Examples 1-4 were tested, and the results are shown in Table 1: Table 1: Test results of tensile strength and barrier properties of heat shrinkable cover films Test Group <![CDATA[Water vapor transmission rate (g / m 2 .atm.day.38 °C)]]> <![CDATA[Oxygen transmission rate (ml / m 2 .atm.day.23°C)]]> Tensile strength (MPa) Example 1 1.15 3.82 50.3 Example 2 1.11 3.75 50.5 Example 3 1.05 3.66 50.8 Example 4 1.09 3.71 50.7 Example 5 1.12 3.78 50.5 Comparative Example 1 2.05 5.56 39.5 Comparative Example 2 1.98 5.51 39.8 Comparative Example 3 1.86 5.45 40.5 Comparative Example 4 3.26 8.58 28.3 Comparative Example 5 4.35 12.82 26.5 From the above results, it can be seen that the heat shrinkable cover film prepared by the present invention has excellent barrier properties and mechanical properties, especially by adding modified medical polyvinyl alcohol, modified corn protein and modified nano zeolite, the three have a synergistic effect, which can further improve the barrier properties and mechanical properties.

[0044] It should be noted that, for the above-mentioned embodiments, for the sake of simplicity, they are all described as a series of action combinations, but those skilled in the art should know that the present invention is not limited by the described order of actions, because according to the present invention, some steps may be performed in other orders or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily required by the present invention.

[0045] In the several embodiments provided in the present application, it should be understood that the disclosed device can be implemented in other ways. For example, the device embodiments described above are only schematic, such as the division of the above-mentioned units, which is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or communication connection shown or discussed can be through some interfaces, and the indirect coupling or communication connection between devices or units can be in the form of telecommunication or other forms.

[0046] The units described above as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0047] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on these embodiments, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in this field can still combine, add, delete or make other adjustments to the features in the various embodiments of the present invention according to the circumstances without conflict, without making creative work, so as to obtain different other technical solutions that do not deviate from the concept of the present invention in essence, and these technical solutions also belong to the scope of protection of the present invention.

Claims

1. A high barrier EVOH heat shrinkable cover film, comprising an outer layer, a heat sealing layer and at least one barrier layer arranged therebetween, wherein adjacent film layers are bonded by an adhesive layer, characterized in that: The barrier layer comprises the following raw materials in parts by weight: 100-120 parts of ethylene-vinyl alcohol copolymer, 30-40 parts of polyolefin, 15-20 parts of modified medical polyvinyl alcohol, 5-10 parts of modified corn protein, 6-10 parts of modified nano zeolite, 4-8 parts of epoxidized soybean oil, and 3-5 parts of zinc stearate.

2. The high barrier EVOH heat shrinkable cover film according to claim 1, characterized in that: The barrier layer comprises the following raw materials in parts by weight: 105-115 parts of ethylene-vinyl alcohol copolymer, 33-37 parts of polyolefin, 16-19 parts of modified medical polyvinyl alcohol, 6-9 parts of modified corn protein, 7-9 parts of modified nano zeolite, 5-7 parts of epoxidized soybean oil, and 3.5-4.5 parts of zinc stearate.

3. The high barrier EVOH heat shrinkable cover film according to claim 2, characterized in that: The barrier layer comprises the following raw materials in parts by weight: 110 parts of ethylene-vinyl alcohol copolymer, 35 parts of polyolefin, 17.5 parts of modified medical polyvinyl alcohol, 7.5 parts of modified corn protein, 8 parts of modified nano zeolite, 6 parts of epoxidized soybean oil, and 4 parts of zinc stearate.

4. The high barrier EVOH heat shrinkable cover film according to claim 1, characterized in that: The thickness of the outer layer accounts for 8-12% of the total thickness; the barrier layer accounts for 20-30% of the total thickness; the heat-sealing layer accounts for 20-25% of the total thickness; and the rest is the thickness of the adhesive layer.

5. The high barrier EVOH heat shrinkable cover film according to claim 1, characterized in that: The outer layer is made of one or more of polypropylene, low-density polyethylene, and linear low-density polyethylene, the heat-sealing layer is made of PE, and the adhesive layer is made of one or two of EVA and maleic anhydride grafted PE.

6. The high barrier EVOH heat shrinkable cover film according to claim 1, characterized in that: The preparation method of the modified medical polyvinyl alcohol is as follows: Weigh 40-50 parts by weight of a 20-30% aqueous solution of medical polyvinyl alcohol; Heat the medical polyvinyl alcohol aqueous solution to 80-100°C, keep warm and stir until it is completely dissolved, add 20-25wt% sodium hydroxide solution, adjust the pH value to 10-11, and stir for 0.5-1h; Add 1-3 parts of silane coupling agent, heat to 100-120°C at a heating rate of 1-3°C / min, and stir to react for 50-60min; The mixture was cooled to room temperature, acid was added to adjust the pH to 7-8, and vacuum distillation was performed to obtain modified polyvinyl alcohol.

7. The high barrier EVOH heat shrinkable cover film according to claim 1, characterized in that: The preparation method of the modified zein is as follows: By weight, take 20-25 parts of 80-90% purity corn protein, then add 60-80 parts of 80-92% ethanol solution, and stir for 10-15 minutes; Add 6-10 parts of polyfructose with a concentration of 0.2 g / mL and a purity of 95%, heat to 70-80°C, and stir magnetically while ultrasonically treating at 300-400 W for 30-50 min to prepare a mixed solution; The mixed solution was freeze-dried in vacuum, and after drying, it was crushed and sieved through a 100-mesh sieve to obtain dry powder; The dry powder is placed at 70-80° C. and 60-80% relative humidity for reaction for 24-36 hours, washed repeatedly with distilled water, centrifuged and freeze-dried to obtain modified corn protein.

8. The high barrier EVOH heat shrinkable cover film according to claim 1, characterized in that: The preparation method of the modified nano zeolite is as follows: Weigh 12-16 parts of zeolite powder by weight, then add 6-10 times the weight of 25-35% magnesium chloride solution, and stir at 400-500 r / min for 30-50 min; After filtering, the material is rinsed with deionized water for 2-3 times, then mixed with 30-40 parts of hexadecyltrimethylammonium bromide, heated to 60-80°C, and stirred at a speed of 400-500 r / min for 30-50 min; After filtering, the product is rinsed with deionized water for 2-3 times and dried at 80-100° C. to obtain modified nano zeolite.

9. The method for preparing the high barrier EVOH heat shrinkable cover film according to any one of claims 1 to 8, characterized in that: The steps include: After pretreatment, the raw materials of each layer are added to the extruder of each layer for melting and plasticization, and then extruded through a multi-layer die to obtain a film blank; It is cooled by a vacuum cylinder water jacket, then enters a hot water bath or a heating box, heated to above the glass transition temperature for longitudinal stretching, and then inflated transversely with compressed air to obtain a film; The film is cooled and shaped, and then an electron accelerator is used to perform online radiation cross-linking treatment, and the film is rolled up to obtain a heat shrinkable cover film.

10. The method for preparing a high barrier EVOH heat shrinkable cover film according to claim 9, characterized in that: The temperature of the compressed air is 30-50° C., the blowing ratio and the stretching ratio are both 2-4:1, and the radiation dose of the radiation cross-linking treatment is 3-8 MR.

Images