A method for preparing a high-barrier multilayer co-extruded film for food packaging

Abstract

The present invention discloses a method for preparing a high-barrier multilayer co-extruded film for food packaging, which belongs to the technical field of food packaging. The present invention co-extrudes an antibacterial layer, an adhesive layer, and a barrier layer, and uses one-time extrusion molding to prepare a co-extruded film, which is simple to operate; the barrier layer is polyvinyl alcohol, which can improve the barrier performance of the co-extruded film to gas and is environmentally friendly; the antibacterial layer is modified polylactic acid, which has good heat resistance and mechanical properties, and is environmentally friendly and degradable; the prepared antibacterial agent molecules contain various functional groups, which can greatly enhance the antibacterial properties and a certain degree of waterproofness and mechanical properties of the co-extruded film, and the performance is stable and not easy to fall off; therefore, the co-extruded film prepared by the present invention has good barrier, waterproof and mechanical properties, and also has stable and efficient antibacterial properties, and is environmentally friendly and degradable, and has important application value in the field of food packaging technology.

Description

Technical Field

[0001] The present invention belongs to the technical field of food packaging, and in particular relates to a method for preparing a high-barrier multi-layer co-extruded film for food packaging. Background Art

[0002] With the rapid development of the plastic packaging industry in recent years, functionality, particularly high barrier properties, has become a key research focus to meet diverse and increasingly functional requirements. Food packaging, as a barrier film material, is widely used primarily to prevent oxygen and water vapor from entering the packaging, potentially spoiling the food and reducing its shelf life. However, food is sensitive to small molecules like oxygen and water vapor. To better protect food from spoilage, packaging film materials are now subject to higher requirements.

[0003] Traditional packaging materials include polyethylene, polypropylene, polyvinyl chloride and polystyrene. These materials have good barrier properties against oxygen and water vapor. Most packages will be discarded after opening. However, these plastic materials cannot be degraded in the natural environment. After being discarded, they are likely to cause white pollution, which is not conducive to the sustainable development of the environment.

[0004] Biodegradable materials are those that can be degraded into small molecules such as carbon dioxide and water by microorganisms or natural factors under natural conditions, such as soil, water, and sunlight, or under specific conditions such as composting or anaerobic digestion. These materials, due to their environmentally friendly nature, are considered an important solution to addressing plastic pollution. Among them, polylactic acid (PLA), a bio-based biodegradable plastic with excellent biocompatibility and heat resistance, has been used to produce biaxially oriented polylactic acid (BOPLA) film. As a new green film packaging material, this film has great potential for application in food packaging. However, PLA's gas and water barrier properties are inferior to those of traditional packaging materials, and due to its use in food packaging, it also has certain antimicrobial properties. These issues hinder its application in food packaging. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the prior art and provide a method for preparing a high-barrier multi-layer co-extruded film for food packaging.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A method for preparing a high-barrier multi-layer co-extruded film for food packaging comprises the following steps:

[0008] The antibacterial layer material, adhesive layer material and barrier layer material are added to three extruders for melt processing respectively, and then co-extruded into a three-layer co-extrusion structure through a die head, cooled and formed, and then longitudinally stretched, transversely stretched, heat-set and rolled to obtain a high-barrier multi-layer co-extrusion film for food packaging.

[0009] Furthermore, the antibacterial layer is made of modified polylactic acid and has a thickness of 7-15 μm.

[0010] Furthermore, the adhesive layer material components include polylactic acid resin and adhesive; the mass ratio of the components is: 75% polylactic acid resin, 25% adhesive; the thickness is 5-12 μm.

[0011] Furthermore, the barrier layer material is polyvinyl alcohol and has a thickness of 8-13 μm.

[0012] Furthermore, the temperature in the molten state is set to 200°C-220°C, and the temperature of the die head is set to 220°C-240°C.

[0013] Furthermore, the longitudinal stretching ratio is 2.5-5 times, and the longitudinal stretching temperature is 60°C-65°C.

[0014] Furthermore, the transverse stretching ratio is 3.5-6.5 times, and the transverse stretching temperature is 80° C.-85° C.

[0015] The antibacterial layer, adhesive layer and barrier layer are co-extruded and formed in one step. The method is simple and effectively reduces costs. The antibacterial layer is made of polylactic acid, which is a new type of biodegradable material with high transparency and good heat resistance. The barrier layer is made of polyvinyl alcohol, which has excellent barrier properties to oxygen, nitrogen, hydrogen and carbon dioxide, thereby improving the barrier performance of the co-extruded film to gases. Polyvinyl alcohol is a degradable material and is environmentally friendly.

[0016] Furthermore, the modified polylactic acid is prepared by the following steps:

[0017] The polylactic acid, cellulose, antimicrobial agent and processing aid are mixed in a mixer to obtain a mixture; the mixture is then added into a screw extruder, melt-blended and extruded to obtain modified polylactic acid.

[0018] Furthermore, the raw materials are calculated in parts by weight as follows: 60-70 parts of polylactic acid, 10-16 parts of cellulose, 6-18 parts of antibacterial agent, and 4-6 parts of processing aid.

[0019] Furthermore, the cellulose is one of wood cellulose, fruit cellulose and hemp cellulose.

[0020] Furthermore, the processing aid is one of epoxidized soybean oil and olive oil.

[0021] Adding plant cellulose to the raw materials can enhance the mechanical properties of the polylactic acid matrix and make it biodegradable.

[0022] Furthermore, the antibacterial agent is prepared by the following steps:

[0023] S1. In a three-necked flask equipped with a thermometer and a stirring device, add triphenylphosphine (PPh3) to toluene, raise the temperature to 35°C, and stir continuously until the triphenylphosphine is completely dissolved. Then, slowly add 4-chlorobutyric acid dropwise, raise the temperature to 55°C, and stir for 6 hours. After the reaction is completed, remove the solvent by distillation under reduced pressure to obtain intermediate 1; the ratio of triphenylphosphine, toluene, and 4-chlorobutyric acid is 26.2g:100mL:12.2g;

[0024] Triphenylphosphine reacts with the chlorine group of 4-chlorobutyric acid to obtain the quaternary phosphonium product intermediate 1; the specific reaction process is as follows:

[0025]

[0026] S2. In a three-necked flask equipped with a thermometer and a stirring device, 4-aminostyrene, intermediate 1, and toluene were stirred and mixed uniformly. After adding DCC (dicyclohexylcarbodiimide, a dehydrating agent), the mixture was stirred again. The device was placed in a 50°C water bath and stirred for 2 hours. After the reaction, the solvent was partially removed by rotary evaporation, and then purified by column chromatography (eluent: a mixed solvent of benzene / ethyl acetate in a volume ratio of 4:1). The eluent was removed by rotary evaporation to obtain intermediate 2. The ratio of 4-aminostyrene, intermediate 1, toluene, and dicyclohexylcarbodiimide used was 11.9 g:34.9 g:150 mL:20.6 g.

[0027] Under the action of DCC, the amino group of 4-aminostyrene undergoes amidation reaction with the carboxyl group of intermediate 1 to obtain intermediate 2. The specific reaction process is as follows:

[0028]

[0029] S3. In a three-necked flask equipped with a thermometer and a stirring device, maleic acid, guanidine hydrochloride and anhydrous ethanol were mixed and stirred continuously until the solid was completely dissolved. Nitrogen was introduced into the device as a protective gas, and the device was placed in a water bath. The reaction temperature was controlled to 70°C and the reaction was kept warm for 5 hours. After the reaction was completed, the solvent was removed by distillation under reduced pressure, washed with deionized water, and dried in vacuo to obtain intermediate 3; the ratio of maleic acid, guanidine hydrochloride and anhydrous ethanol was 9.9g:20.7g:100mL;

[0030] The carboxyl group in the maleic acid molecule undergoes an amidation reaction with the amino group in the guanidine hydrochloride molecule. By controlling the molar ratio of the two to be close to 1:2 and a slight excess of guanidine hydrochloride, the two carboxyl groups in the maleic acid participate in the reaction to obtain intermediate 3. The structure of intermediate 3 is shown below:

[0031]

[0032] S4. In a three-necked flask equipped with a stirring device, intermediate 2, intermediate 3 and N,N-dimethylformamide (DMF) were mixed and stirred evenly. Azobisisobutyronitrile (AIBN) was added and copolymerized in a constant temperature water bath at 85°C with constant temperature stirring. After the reaction was carried out for 8 hours, heating was stopped, and the solid was precipitated with distilled water. After that, the obtained solid was placed in a vacuum drying oven and dried for 6 hours to obtain an antibacterial agent. The ratio of intermediate 2, intermediate 3, N,N-dimethylformamide and azobisisobutyronitrile was 21.8g:58.5g:200mL:0.8g.

[0033] Under the action of azobisisobutyronitrile, the unsaturated carbon-carbon double bonds of intermediate 2 and intermediate 3 molecules undergo polymerization reaction to obtain an antibacterial agent; the specific reaction is as follows:

[0034]

[0035] The antimicrobial agent prepared through a series of reactions has the efficacy of guanidine antimicrobial agents. Guanidine antimicrobial agents can affect the growth and division of bacteria, spore germination and produce abnormal phenomena such as respiratory inhibition, cell swelling, cytoplasmic disintegration and cell wall destruction. Thus, the purpose of inhibiting or killing organisms is achieved, and the antibacterial properties of the polylactic acid matrix can be improved. Moreover, the antibacterial agent molecule also contains a quaternary phosphonium salt functional group. The positively charged phosphonium ions in the quaternary phosphonium salt are adsorbed on the surface of the bacteria, can interact with the negatively charged cell membrane of the bacteria, penetrate the cell wall, bind to the cell membrane, destroy the surface structure of the cell, cause the intracellular substances to flow out, stop the respiratory function of the bacteria and cause cell death. Moreover, compared with quaternary ammonium salts, the antibacterial activity is better, and it can synergize with guanidine antibacterial agents, greatly enhancing the antibacterial properties of the matrix. In addition, the antibacterial agent molecule also contains a large number of benzene rings. As a rigid group and a hydrophobic group, the benzene ring can enhance the mechanical properties and water resistance of the matrix to a certain extent. Finally, because the antibacterial agent is a polymer macromolecule, it is not easy to migrate and seep out compared with small molecule antibacterial agents, and can exert its performance stably for a long time.

[0036] Beneficial effects of the present invention:

[0037] 1. The present invention co-extrudes the antibacterial layer, the adhesive layer and the barrier layer and uses one-time extrusion molding to prepare a co-extruded film, which is simple to operate;

[0038] 2. The barrier layer is polyvinyl alcohol, which can improve the gas barrier performance of the co-extruded film and is environmentally friendly;

[0039] 3. The antibacterial layer is made of modified polylactic acid, which has good heat resistance and mechanical properties, and is environmentally friendly and degradable;

[0040] 4. The antibacterial agent molecules contain various functional groups, which can greatly enhance the antibacterial properties and waterproof and mechanical properties of the co-extruded film to a certain extent, and the performance is stable and not easy to fall off;

[0041] Therefore, the co-extruded film prepared by the present invention has good barrier properties, waterproof properties and mechanical properties, and also has stable and efficient antibacterial properties, and is environmentally friendly and degradable, and has important application value in the field of food packaging technology. DETAILED DESCRIPTION

[0042] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0043] Example 1

[0044] Preparation of antimicrobial agent:

[0045] S1. In a three-necked flask equipped with a thermometer and a stirring device, 26.2 g of triphenylphosphine was added to 100 mL of toluene, the temperature was raised to 35°C, and stirring was continued until the triphenylphosphine was completely dissolved. Then, 12.2 g of 4-chlorobutyric acid was slowly added dropwise, and the temperature was raised to 55°C. The reaction was stirred for 6 hours. After the reaction was completed, the solvent was distilled off under reduced pressure to obtain Intermediate 1;

[0046] S2. In a three-necked flask equipped with a thermometer and a stirring device, 11.9 g of 4-aminostyrene, 34.9 g of intermediate 1, and 150 mL of toluene were stirred and mixed uniformly. After adding 20.6 g of DCC, the mixture was stirred again. The device was placed in a 50° C. water bath and stirred for 2 h. After the reaction, part of the solvent was removed by rotary evaporation, and then purified by column chromatography (eluent: a mixed solvent of benzene / ethyl acetate in a volume ratio of 4:1). The eluent was removed by rotary evaporation to obtain intermediate 2.

[0047] S3. In a three-necked flask equipped with a thermometer and a stirring device, 9.9 g of maleic acid, 20.7 g of guanidine hydrochloride and 100 mL of anhydrous ethanol were mixed and stirred continuously until the solid was completely dissolved. Nitrogen was introduced into the device as a protective gas, and the device was placed in a water bath. The reaction temperature was controlled to 70°C and the reaction was kept warm for 5 hours. After the reaction was completed, the solvent was removed by distillation under reduced pressure, the mixture was washed with deionized water, and vacuum dried to obtain intermediate 3;

[0048] S4. In a three-necked flask equipped with a stirring device, 21.8 g of intermediate 2, 58.5 g of intermediate 3 and 200 mL of N,N-dimethylformamide were mixed and stirred evenly. Then, 0.8 g of azobisisobutyronitrile was added and copolymerized under constant temperature stirring in a constant temperature water bath at 85°C. After the reaction was carried out for 8 hours, heating was stopped, and the solid was precipitated with distilled water. After that, the obtained solid was placed in a vacuum drying oven and dried for 6 hours to obtain an antibacterial agent.

[0049] Example 2

[0050] Preparation of antimicrobial agent:

[0051] S1. In a three-necked flask equipped with a thermometer and a stirring device, 52.4 g of triphenylphosphine was added to 200 mL of toluene, the temperature was raised to 35°C, and stirring was continued until the triphenylphosphine was completely dissolved. Then, 24.4 g of 4-chlorobutyric acid was slowly added dropwise, and the temperature was raised to 55°C. The reaction was stirred for 6 hours. After the reaction was completed, the solvent was removed by distillation under reduced pressure to obtain Intermediate 1;

[0052] S2. In a three-necked flask equipped with a thermometer and a stirring device, 23.8 g of 4-aminostyrene, 69.8 g of intermediate 1, and 300 mL of toluene were stirred and mixed uniformly. After adding 41.2 g of DCC, the mixture was stirred again. The apparatus was placed in a 50° C. water bath and stirred for 2 h. After the reaction was completed, part of the solvent was first removed by rotary evaporation, and then purified by column chromatography (eluent: a mixed solvent of benzene / ethyl acetate, the volume ratio of the two being 4:1). The eluent was removed by rotary evaporation to obtain intermediate 2;

[0053] S3. In a three-necked flask equipped with a thermometer and a stirring device, 19.8 g of maleic acid, 41.4 g of guanidine hydrochloride and 200 mL of anhydrous ethanol were mixed and stirred continuously until the solid was completely dissolved. Nitrogen was introduced into the device as a protective gas, and the device was placed in a water bath. The reaction temperature was controlled to 70°C and the reaction was kept warm for 5 hours. After the reaction was completed, the solvent was removed by distillation under reduced pressure, the mixture was washed with deionized water, and vacuum dried to obtain intermediate 3;

[0054] S4. In a three-necked flask equipped with a stirring device, 43.6 g of intermediate 2, 117.0 g of intermediate 3 and 400 mL of N,N-dimethylformamide were mixed and stirred evenly. Then, 1.6 g of azobisisobutyronitrile was added and copolymerized under constant temperature stirring in a constant temperature water bath at 85°C. After the reaction was carried out for 8 hours, heating was stopped, and the solid was precipitated with distilled water. After that, the obtained solid was placed in a vacuum drying oven and dried for 6 hours to obtain an antibacterial agent.

[0055] Example 3

[0056] Preparation of modified polylactic acid:

[0057] 60g of polylactic acid, 10g of fruit cellulose, 6g of the antibacterial agent prepared in Example 1 and 4g of epoxy soybean oil were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder, melt-blended and extruded to obtain modified polylactic acid.

[0058] Example 4

[0059] Preparation of modified polylactic acid:

[0060] 65 g of polylactic acid, 13 g of lignocellulose, 12 g of the antibacterial agent prepared in Example 2 and 5 g of olive oil were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder, melt-blended and extruded to obtain modified polylactic acid.

[0061] Example 5

[0062] Preparation of modified polylactic acid:

[0063] 70 g of polylactic acid, 16 g of lignocellulose, 18 g of the antibacterial agent prepared in Example 2 and 6 g of olive oil were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder, melt-blended and extruded to obtain modified polylactic acid.

[0064] Example 6

[0065] An antibacterial layer material: the modified polylactic acid prepared in Example 5; an adhesive layer material: 75% polylactic acid resin, 25% adhesive (produced by Guangzhou Synthetic Company, trade name HS3-405H3); and a barrier layer material: polyvinyl alcohol were added separately into three extruders for melt processing at a melting temperature of 200°C. The films were co-extruded through a die head at 220°C into a three-layer co-extruded structure, cooled and formed, and then longitudinally stretched (at a ratio of 2.5 times and a temperature of 60°C), transversely stretched (at a ratio of 3.5 times and a temperature of 80°C), heat-set, and wound to obtain a high-barrier multi-layer co-extruded film for food packaging, wherein the antibacterial layer had a thickness of 7 μm, the adhesive layer had a thickness of 5 μm, and the barrier layer had a thickness of 8 μm.

[0066] Comparative Example 1

[0067] The antibacterial agent in Example 5 was replaced by a commercially available quaternary ammonium salt antibacterial agent of equal quality, and the remaining steps were the same as those in Example 5.

[0068] Comparative Example 2

[0069] Commercially available polylactic acid was used.

[0070] Comparative Example 3

[0071] Commercially available polylactic acid biodegradable food wrap was used.

[0072] Examples 3, 4, 5 and Comparative Examples 1 and 2 were made into corresponding shapes according to different test standards and subjected to the following performance tests:

[0073] The degradation rate was determined using the national standard GB / T 19811-2005;

[0074] The water resistance of the sample was determined using the national standard GB / T 1034-1998 "Test method for water absorption of plastics";

[0075] The tensile strength is measured according to the national standard GB / T 1040-2006 “Determination of tensile properties of plastics”;

[0076] The antibacterial rate of the sample was measured under the conditions of 20°C ± 5°C and 10% ± 5% RH using QB / T 2591-2003 "Antibacterial Plastics—Test Method for Antibacterial Performance and Antibacterial Effect". The antibacterial rate of Examples 3, 4, 5 and Comparative Example 1 was measured after they were left at room temperature for 200 days.

[0077] The measured results are shown in Table 1:

[0078] Table 1

[0079]

[0080] The oxygen permeation rates of Example 6 and Comparative Example 3 were measured using the national standard GB / T 1038.1-2022 "Plastic Film and Sheeting Gas Permeability Test Method Part 1: Pressure Difference Method";

[0081] The measured results are shown in Table 2:

[0082] Table 2

[0083]

[0084] It can be seen from the above two tables that the co-extruded film prepared in the embodiment of the present invention has better waterproof, mechanical, barrier and antibacterial properties than the comparative example, and has stable performance, is environmentally friendly and degradable, and has important application value in the field of food packaging technology.

[0085] Throughout the specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0086] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in similar ways. As long as they do not deviate from the invention or exceed the scope defined by the claims, they should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a high barrier multi-layer co-extruded film for food packaging, characterized in that: The following steps are involved: The antibacterial layer material, the adhesive layer material and the barrier layer material are respectively added to three extruders for melt processing, co-extruded through a die head into a three-layer co-extrusion structure, cooled and formed, and then longitudinally stretched, transversely stretched, heat-set, and rolled to obtain a high-barrier multi-layer co-extruded film for food packaging; Wherein, the antibacterial layer material is modified polylactic acid, and the modified polylactic acid is prepared by the following steps: The polylactic acid, cellulose, antimicrobial agent and processing aid are mixed in a mixer to obtain a mixture; the mixture is then added to a screw extruder, melt-blended and extruded to obtain modified polylactic acid; Wherein, the antibacterial agent is prepared by the following steps: S1. Add triphenylphosphine to toluene, raise the temperature to 35°C, and stir continuously until the triphenylphosphine is completely dissolved. Then slowly add 4-chlorobutyric acid dropwise, raise the temperature to 55°C, and stir for 6 hours. After the reaction is completed, remove the solvent by distillation under reduced pressure to obtain intermediate 1; S2. 4-aminostyrene, intermediate 1, and toluene were stirred and mixed evenly, dicyclohexylcarbodiimide was added, and the mixture was stirred again. The mixture was placed in a water bath at 50°C and stirred for 2 hours. After the reaction was completed, the mixture was rotary evaporated, purified by column chromatography, and rotary evaporated to obtain intermediate 2. S3, maleic acid, guanidine hydrochloride and anhydrous ethanol were mixed and stirred continuously until the solid was completely dissolved, nitrogen was introduced into the apparatus, and the apparatus was placed in a water bath and kept in a temperature of 70°C for 5 h. After the reaction was complete, the mixture was evaporated under reduced pressure, washed and dried under vacuum to obtain intermediate 3; S4. Mix intermediate 2, intermediate 3 and N,N-dimethylformamide, stir evenly, add azobisisobutyronitrile, and carry out copolymerization in a constant temperature water bath at 85°C with constant temperature stirring. After the reaction has been carried out for 8 hours, stop heating, precipitate with distilled water, and dry the obtained solid to obtain an antibacterial agent.

2. The method for preparing a high barrier multi-layer co-extruded film for food packaging according to claim 1, characterized in that: The raw materials are calculated as follows in parts by weight: 60-70 parts of polylactic acid, 10-16 parts of cellulose, 6-18 parts of antibacterial agent, and 4-6 parts of processing aid.

3. The method for preparing a high barrier multi-layer co-extruded film for food packaging according to claim 1, characterized in that: In step S1, the ratio of triphenylphosphine, toluene, and 4-chlorobutyric acid is 26.2 g:100 mL:12.2 g.

4. The method for preparing a high barrier multi-layer co-extruded film for food packaging according to claim 1, characterized in that: In step S2, the ratio of the amount of 4-aminostyrene, intermediate 1, toluene, and dicyclohexylcarbodiimide used is 11.9 g:34.9 g:150 mL:20.6 g.

5. The method for preparing a high barrier multi-layer co-extruded film for food packaging according to claim 1, characterized in that: In step S3, the ratio of maleic acid, guanidine hydrochloride, and anhydrous ethanol is 9.9 g:20.7 g:100 mL.

6. The method for preparing a high barrier multi-layer co-extruded film for food packaging according to claim 1, characterized in that: In step S4, the ratio of the amount of intermediate 2, intermediate 3, N,N-dimethylformamide, and azobisisobutyronitrile is 21.8 g:58.5 g:200 mL:0.8 g.

7. The method for preparing a high barrier multi-layer co-extruded film for food packaging according to claim 1, characterized in that: The longitudinal stretching ratio is 2.5-5 times, and the longitudinal stretching temperature is 60°C-65°C.

8. The method for preparing a high barrier multi-layer co-extruded film for food packaging according to claim 1, characterized in that: The transverse stretching ratio is 3.5-6.5 times, and the transverse stretching temperature is 80° C.-85° C.

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