Food-grade high-barrier packaging bag and preparation method thereof

Abstract

The invention relates to the field of plastic products, in particular to a food-grade high-barrier packaging bag and a preparation method thereof.The packaging bag sequentially comprises a barrier layer, a heat sealing layer and a reinforcing layer from inside to outside, and the packaging bag is prepared by adding raw materials of the barrier layer, the heat sealing layer and the reinforcing layer into three hoppers of a three-layer co-extruder for co-extrusion casting, by reasonably arranging the sequence of the barrier layer, the heat sealing layer and the reinforcing layer and combining with selection of food-grade materials, the problem that a barrier layer in a traditional structure is prone to being weakened can be effectively solved, and meanwhile the multiple requirements of food packaging for safety, weather resistance and processability are met. By adjusting the interlayer structure, on the premise that the cost is not remarkably increased, oxygen and water vapor are efficiently blocked, a long-acting fresh-keeping solution is provided for prefabricated food, meat, baked food and the like, and important practical application value is achieved.

Description

Technical Field

[0001] The present invention relates to the field of plastic products, and particularly to a food-grade high-barrier packaging bag and a preparation method thereof. Background Art

[0002] In the food industry, the barrier performance of packaging materials directly affects the shelf life, flavor, and nutritional retention of food. With the increasing requirements of consumers for food freshness and safety, the research and application of high-barrier packaging materials have become the focus of the industry. High-barrier packaging achieves effective barrier against oxygen, water vapor, odor, and microorganisms through multi-layer composite structure design, thereby delaying food oxidation, mildew, and flavor loss. In the early days, high-barrier packaging bags mainly consisted of polyethylene film laminated with metal foil, using the excellent barrier property of the metal foil to protect food, but there were limitations such as high cost and poor flexibility. Polymer-based high-barrier materials such as polyvinylidene chloride (PVDC), ethylene / vinyl alcohol copolymer (EVOH), and polyamide (PA) form composite films through multi-layer co-extrusion or coating processes, improving the processing adaptability and economy of the materials while ensuring barrier performance. For example, PVDC-coated films with polypropylene (OPP) or polyethylene terephthalate (PET) as the substrate can reduce the oxygen transmission rate to 1 / 1000 of the original substrate after coating and are widely used in the packaging of perishable foods such as meat and cheese; EVOH, due to its excellent gas barrier property and good processing performance, has become the core material for aseptic packaging and retort pouches.

[0003] Although significant progress has been made in high-barrier materials in the prior art, their performance is still limited by the structural design and material combination. On the one hand, the barrier ability of a single-layer film is limited, and multi-layer composites are required to achieve synergistic effects. On the other hand, some materials have environmental adaptability defects. For example, the barrier property of nylon-based materials is easily affected by humidity, and PVDC has a narrow processing temperature range and high recycling difficulty, restricting its application in a wider range of scenarios. In addition, with the tightening of environmental protection regulations, the development of degradable and easily recyclable high-barrier materials (such as PLA / PBAT composite films) has become a new direction, but there is still room for improvement in their barrier performance compared with traditional materials.

[0004] Therefore, according to the above related technologies, it is urgent to develop a food-grade high-barrier packaging bag and a preparation method thereof. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide a food-grade high-barrier packaging bag and a preparation method thereof, so as to provide a food-grade packaging bag with stronger interlayer synergistic effect and higher barrier efficiency. By reasonably arranging the order of the barrier layer, heat-sealing layer and reinforcing layer, combined with the selection of food-grade materials, the problem that the barrier layer in the traditional structure is easily weakened can be effectively solved, and at the same time, the multiple requirements of food packaging for safety, weather resistance and processing performance can be met. By adjusting the interlayer structure, the present invention can achieve efficient barrier to oxygen and water vapor without significantly increasing the cost, providing a more long-term fresh-keeping solution for prefabricated foods, meats, baked foods, etc., and has important practical application value.

[0006] Based on the above object, the present invention provides a food-grade high-barrier packaging bag and a preparation method thereof.

[0007] A food-grade high-barrier packaging bag, which sequentially includes a barrier layer, a heat-sealing layer and a reinforcing layer from outside to inside;

[0008] The thickness ratio of the barrier layer, the heat-sealing layer and the reinforcing layer is 10-12 μm: 22-25 μm: 13-15 μm.

[0009] The barrier layer includes the following raw materials: ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, montmorillonite and antioxidant;

[0010] Preferably, the antioxidant is antioxidant 1010.

[0011] Preferably, the slip agent is any one of oleic acid amide and erucic acid amide.

[0012] The heat-sealing layer includes the following raw materials: linear low-density polyethylene, metallocene polyethylene and slip agent;

[0013] The reinforcing layer includes the following raw materials: polyglycolic acid and biaxially oriented polypropylene;

[0014] The biaxially oriented polypropylene realizes the high-performance process through the biaxial stretching and orientation of polypropylene resin, and the specific process is as follows:

[0015] Select homopolypropylene with a melt flow rate (MFR) of 2 - 6 g / 10 min (to ensure stretchability) and an ash content of ≤0.01% (to avoid impurities affecting transparency). Dry it at 80 - 90°C for 4 - 6 h to reduce the moisture content to ≤0.01% and avoid generating bubbles during extrusion. Add 0.1 - 0.3% of a slip agent and 0.2 - 0.5% of an antistatic agent to improve the surface properties of the film. Use a single-screw extruder (length-diameter ratio L / D = 30 - 35), with an extrusion temperature of 220 - 260°C to ensure complete melting of the resin without decomposition; the melt pressure is 8 - 15 MPa to ensure uniform and stable delivery of the melt to the die head. The melt passes through a slot die head (lip gap 0.8 - 1.5 mm) to form a sheet blank with a thickness of 0.3 - 0.5 mm. The die head temperature control accuracy is ±1°C to ensure the thickness uniformity error of the sheet blank is ≤1%. The extruded sheet blank is immediately adhered to a chrome-plated cooling roll (temperature 50 - 70°C), with a cooling rate of ≥100°C / s to inhibit the crystallization of polypropylene and form an amorphous transparent sheet (crystallinity ≤5%). Select a product with a surface roughness Ra ≤0.2 μm, a light transmittance of ≥90%, and a haze of ≤1%, and perform biaxial stretching. Use a roller-type longitudinal stretching machine, with the temperature of the stretching roller group being 90 - 120°C, a stretching ratio of 3 - 5 times, and a stretching rate of 50 - 100 m / min, so that the polypropylene molecular chains are arranged orderly along the longitudinal direction. Use a tenter frame for transverse stretching, with the preheating zone temperature being 120 - 140°C, the stretching zone temperature being 140 - 160°C, and a stretching ratio of 6 - 10 times. Stretch the sheet blank horizontally through a chain clamping device to form a "cross-shaped" molecular orientation; during the stretching process, polypropylene transforms from an amorphous state to a β-crystalline form, improving the impact strength and transparency. The stretching speed is matched: the MD / TD speed ratio is 1:1.2 - 1:1.5 to avoid film warping caused by uneven stress. The stretched film is heat-set at 150 - 180°C for 10 - 30 seconds to fix the molecular orientation structure and reduce the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%). After heat-setting, the film passes through multiple cooling rolls (temperature decreasing from 100°C to 30°C) to avoid internal stress concentration and is wound up after the surface temperature ≤40°C to obtain biaxially stretched polypropylene.

[0016] Preferably, the mass ratio of the ethylene-vinyl alcohol copolymer, maleic anhydride-grafted polyethylene, nano-montmorillonite, and antioxidant is 40 - 60:15 - 25:3 - 8:0.5 - 1.

[0017] Preferably, the mass ratio of the linear low-density polyethylene, metallocene polyethylene, and slip agent is 60 - 70:20 - 30:0.3 - 0.8.

[0018] Preferably, the metallocene polyethylene is prepared by solution polymerization. Using cyclohexane as a solvent, under the conditions of 100 - 150 °C and a pressure of 3 - 5 MPa, a metallocene compound such as zirconocene dichloride and a cocatalyst methylaluminoxane are activated in an inert solvent such as cyclohexane or toluene to form an active center. Then, ethylene monomer is introduced, and polymerization occurs under stirring to generate a homogeneous solution. Finally, the solvent is removed and dried to obtain metallocene polyethylene.

[0019] Preferably, the mass ratio of polyglycolic acid to biaxially oriented polypropylene is 5 - 15:10 - 20.

[0020] A preparation method of a food-grade high-barrier packaging bag includes the following steps:

[0021] Step S1. Coextrusion casting: The raw materials of the prepared barrier layer, heat-sealing layer, and reinforcing layer are respectively added into three hoppers of a three-layer coextrusion machine for coextrusion casting. Control the temperature of the coextrusion machine at 180 - 220 °C and the screw speed at 60 - 90 rpm. During the extrusion process, the raw materials of each layer converge at the die head and form a three-layer composite film through the casting process. During the casting process, by adjusting parameters such as the screw speed of the extruder, the gap and temperature of the die head, control the thickness and uniformity of the composite film. During the coextrusion process, due to the presence of maleic anhydride-grafted polyethylene, a gradient compatible interface is formed between the barrier layer and the heat-sealing layer. The maleic anhydride groups therein chemically react with the hydroxyl groups in ethylene-vinyl alcohol copolymer, reducing the interfacial tension between the barrier layer and the heat-sealing layer and improving the interlayer bonding strength;

[0022] Step S2. Perform solvent-free corona treatment on the extruded composite film, control the power of the corona treatment at 3 - 5 kW, and the treatment speed at 10 - 15 m / min. The corona treatment can increase the surface energy of the composite film surface and enhance the adhesion of printing ink and adhesives;

[0023] Step S3. Slitting and bag making: The corona-treated composite film is slit into the required width and length by a slitter, and a heat-sealing machine is used to make the slit composite film into a packaging bag. Control the temperature of the heat-sealing machine at 160 - 180 °C, the pressure at 0.3 - 0.5 MPa, and the heat-sealing time at 1 - 3 s. Through the heat-sealing process, the edges of the composite film are sealed to form a packaging bag with a certain shape and size.

[0024] The beneficial effects of the present invention:

[0025] The present invention provides a food-grade high-barrier packaging bag and a preparation method thereof. By separately adding the raw materials of the barrier layer, heat-sealing layer and reinforcing layer into three hoppers of a three-layer co-extrusion machine for co-extrusion casting, and through reasonable arrangement of the order of the barrier layer, heat-sealing layer and reinforcing layer, combined with the selection of food-grade materials, the problem that the barrier layer in the traditional structure is easily weakened can be effectively solved, and at the same time, multiple requirements of food packaging for safety, weather resistance and processing performance can be met. By adjusting the interlayer structure, the present invention can achieve efficient barrier to oxygen and water vapor without significantly increasing the cost, providing a more long-term fresh-keeping solution for prefabricated foods, meats, baked foods, etc., and having important practical application value. Detailed Embodiments

[0026] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further describes the present invention in detail with reference to specific embodiments.

[0027] The sources and properties of some raw materials used in the present invention are as follows:

[0028] The grade of low-density polyethylene is 2420H, the density is 0.924 g / cm 3 , and the melt flow index (10 min) is 1.9 g (190 °C, 2.16 kg), purchased from CNOOC and Shell Petrochemical Company Limited.

[0029] The grade of ethylene-vinyl alcohol copolymer is EVAL TM H17B, and the ethylene content is 32%, purchased from Kuraray Co., Ltd. of Japan.

[0030] Antioxidant 1010 is of analytical purity, purchased from Sinopharm Chemical Reagent Co., Ltd.

[0031] The molecular weight of maleic anhydride grafted polyethylene is 3000 - 4000, the acid value is 25 - 30, and the grafting rate is 4%, purchased from Qingdao Sainuo Company.

[0032] The grade of polyglycolic acid is Purasorb@PGS, purchased from Corbion of the Netherlands.

[0033] Example 1: A preparation method of a food-grade high-barrier packaging bag, comprising the following steps:

[0034] S1. Select a homopolypropylene with a melt flow rate (MFR) of 2 - 6 g / 10 min (to ensure stretchability) and an ash content of ≤0.01% (to avoid impurities affecting transparency). Dry it at 80 - 90°C for 4 - 6 h to reduce the moisture content to ≤0.01% and prevent bubbles from forming during extrusion. Add 0.1 - 0.3% of a slip agent and 0.2 - 0.5% of an antistatic agent to improve the surface properties of the film. Use a single-screw extruder (length-diameter ratio L / D = 30 - 35), with an extrusion temperature of 220 - 260°C to ensure complete melting of the resin without decomposition; the melt pressure is 8 - 15 MPa to ensure uniform and stable delivery of the melt to the die. The melt passes through a slit die (lip gap 0.8 - 1.5 mm) to form a sheet blank with a thickness of 0.3 - 0.5 mm. The die temperature control accuracy is ±1°C to ensure the thickness uniformity error of the sheet blank is ≤1%. The extruded sheet blank is immediately adhered to a chrome-plated cooling roll (temperature 50 - 70°C), with a cooling rate of ≥100°C / s to inhibit polypropylene crystallization and form an amorphous transparent sheet (crystallinity ≤5%). Select a product with a surface roughness Ra ≤0.2 μm, a light transmittance ≥90%, and a haze ≤1% for biaxial stretching. Use a roller-type longitudinal stretching machine, with the stretching roller group temperature at 90 - 120°C, a stretching ratio of 3 - 5 times, and a stretching rate of 50 - 100 m / min. The polypropylene molecular chains are arranged orderly in the longitudinal direction. Use a tenter frame for transverse stretching, with the preheating zone temperature at 120 - 140°C and the stretching zone temperature at 140 - 160°C, a stretching ratio of 6 - 10 times. The sheet is transversely stretched through a chain clamp device to form a "cross-shaped" molecular orientation; during the stretching process, the polypropylene transforms from amorphous to β-crystalline form, improving the impact strength and transparency. The stretching speed is matched: the MD / TD speed ratio is 1:1.2 - 1:1.5 to avoid film warping caused by uneven stress. The stretched film is heat-set at 150 - 180°C for 10 - 30 seconds to fix the molecular orientation structure and reduce the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%). After heat-setting, the film passes through multiple cooling rolls (temperature decreasing from 100°C to 30°C) to avoid internal stress concentration and is wound up after the surface temperature ≤40°C to obtain biaxially stretched polypropylene;

[0035] S2. Raw material preparation: Prepare ethylene-vinyl alcohol copolymer, maleic anhydride-grafted polyethylene, nano-montmorillonite, and an antioxidant according to weight percentages. The nano-montmorillonite is pretreated with a silane coupling agent before use to improve its compatibility with the polymer. Add 3 g of the pretreated nano-montmorillonite, 40 g of ethylene-vinyl alcohol copolymer, and 15 g of maleic anhydride-grafted polyethylene to a high-speed mixer, and stir and mix at 60°C for 15 min to fully disperse each component. Then add 0.5 g of antioxidant 1010 and continue stirring for 5 min to ensure the antioxidant is evenly distributed in the mixer;

[0036] S3. Prepare linear low-density polyethylene, metallocene polyethylene, and slip agent by weight percentage. Add 60 g of linear low-density polyethylene and 20 g of metallocene polyethylene into a mixing device, stir and mix at room temperature for 10 min, then add 0.3 g of oleamide, and continue stirring for 5 min to evenly disperse the slip agent in the polymer, so as to reduce the surface friction coefficient of the heat-sealing layer and improve the slipperiness during the bag-making process;

[0037] S4. Prepare polyglycolic acid and biaxially oriented polypropylene by weight percentage. Add 5 g of polyglycolic acid and 10 g of biaxially oriented polypropylene into the corresponding extruder hoppers respectively;

[0038] S5. Coextrusion casting: Add the prepared raw materials of the barrier layer, heat-sealing layer, and reinforcement layer into the three hoppers of a three-layer coextrusion machine for coextrusion casting. Control the temperature of the coextrusion machine at 180 °C and the screw speed at 60 rpm. During the extrusion process, the raw materials of each layer converge at the die head and form a three-layer composite film through the casting process. During the casting process, control the thickness and uniformity of the composite film by adjusting parameters such as the screw speed of the extruder, the gap of the die head, and the temperature. During the coextrusion process, due to the presence of maleic anhydride grafted polyethylene, a gradient compatible interface is formed between the barrier layer and the heat-sealing layer. The maleic anhydride groups therein chemically react with the hydroxyl groups in the ethylene-vinyl alcohol copolymer, reducing the interfacial tension between the barrier layer and the heat-sealing layer and improving the interlayer bonding strength. The thickness ratio of the barrier layer, heat-sealing layer, and reinforcement layer is 10 μm:22 μm:13 μm;

[0039] S6. Perform solvent-free corona treatment on the extruded composite film. Control the power of the corona treatment at 3 kW and the treatment speed at 10 m / min. The corona treatment can increase the surface energy of the composite film surface and enhance the adhesion of printing ink and adhesives;

[0040] S7. Slitting and bag-making: Slit the corona-treated composite film into the required width and length by a slitter, and use a heat-sealing machine to make the slit composite film into a packaging bag. Control the temperature of the heat-sealing machine at 160 °C, the pressure at 0.3 MPa, and the heat-sealing time at 1 s. Through the heat-sealing process, seal the edges of the composite film to form a packaging bag with a certain shape and size.

[0041] Example 2: A preparation method of a food-grade high-barrier packaging bag, comprising the following steps:

[0042] S1. Select a homopolypropylene with a melt flow rate (MFR) of 2 - 6 g / 10 min (to ensure stretchability) and an ash content of ≤0.01% (to avoid impurities affecting transparency). Dry it at 80 - 90°C for 4 - 6 h to reduce the moisture content to ≤0.01% and avoid generating bubbles during extrusion. Add 0.1 - 0.3% of a slip agent and 0.2 - 0.5% of an antistatic agent to improve the surface properties of the film. Use a single-screw extruder (length-diameter ratio L / D = 30 - 35), with an extrusion temperature of 220 - 260°C to ensure complete melting of the resin without decomposition; the melt pressure is 8 - 15 MPa to ensure uniform and stable delivery of the melt to the die head. The melt passes through a slit die head (lip gap 0.8 - 1.5 mm) to form a sheet blank with a thickness of 0.3 - 0.5 mm. The die head temperature control accuracy is ±1°C to ensure that the thickness uniformity error of the sheet blank is ≤1%. The extruded sheet blank is immediately attached to a chrome-plated cooling roll (temperature 50 - 70°C), and the cooling rate is ≥100°C / s to inhibit the crystallization of polypropylene and form an amorphous transparent sheet (crystallinity ≤5%). Select a product with a surface roughness Ra ≤0.2 μm, a light transmittance ≥90%, and a haze ≤1% for biaxial stretching. Use a roller-type longitudinal stretching machine, with the stretching roller group temperature at 90 - 120°C, a stretching ratio of 3 - 5 times, and a stretching rate of 50 - 100 m / min, so that the polypropylene molecular chains are arranged orderly along the longitudinal direction. Use a tenter frame for transverse stretching, with the preheating zone temperature at 120 - 140°C and the stretching zone temperature at 140 - 160°C, a stretching ratio of 6 - 10 times. The sheet is transversely stretched through a chain clamp device to form a "cross-shaped" molecular orientation; during the stretching process, polypropylene transforms from amorphous to β-crystalline form, improving the impact strength and transparency. The stretching speed is matched: the MD / TD speed ratio is 1:1.2 - 1:1.5 to avoid film warping caused by uneven stress. The stretched film is heat-set at 150 - 180°C for 10 - 30 seconds to fix the molecular orientation structure and reduce the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%). After heat-setting, the film passes through multiple cooling rolls (temperature decreasing from 100°C to 30°C) to avoid internal stress concentration and is wound up after the surface temperature ≤40°C to obtain biaxially stretched polypropylene;

[0043] S2. Raw material preparation: Prepare ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, nanoclay, and antioxidant according to weight percentages. The nanoclay is pretreated with a silane coupling agent before use to improve its compatibility with the polymer. Add 5 g of pretreated nanoclay, 43 g of ethylene-vinyl alcohol copolymer, and 18 g of maleic anhydride grafted polyethylene to a high-speed mixer, stir and mix at 65°C for 20 min to fully disperse each component, then add 0.7 g of antioxidant 1010 and continue stirring for 7 min to ensure that the antioxidant is evenly distributed in the mixer;

[0044] S3. Prepare linear low density polyethylene, metallocene polyethylene and slip agent by weight percentage. Add 63 g of linear low density polyethylene and 23 g of metallocene polyethylene into a mixing device, stir and mix at room temperature for 13 min, add 0.5 g of erucamide, and continue stirring for 8 min to evenly disperse the slip agent in the polymer, so as to reduce the surface friction coefficient of the heat seal layer and improve the slipperiness during the bag making process;

[0045] S4. Prepare polyglycolic acid and biaxially oriented polypropylene by weight percentage. Add 9 g of polyglycolic acid and 13 g of biaxially oriented polypropylene into the corresponding extruder hoppers respectively.

[0046] S5. Co - extrusion casting: Add the prepared raw materials of the barrier layer, heat seal layer and reinforcement layer into the three hoppers of a three - layer co - extruder for co - extrusion casting. Control the temperature of the co - extruder at 190 °C and the screw speed at 70 rpm. During the extrusion process, the raw materials of each layer converge at the die head and form a three - layer composite film through the casting process. During the casting process, by adjusting parameters such as the screw speed of the extruder, the gap and temperature of the die head, control the thickness and uniformity of the composite film. During the co - extrusion process, due to the presence of maleic anhydride grafted polyethylene, a gradient compatible interface is formed between the barrier layer and the heat seal layer. The maleic anhydride groups therein chemically react with the hydroxyl groups in ethylene - vinyl alcohol copolymer, reducing the interfacial tension between the barrier layer and the heat seal layer and improving the interfacial bonding strength. The thickness ratio of the barrier layer, heat seal layer and reinforcement layer is 11 μm:23 μm:14 μm;

[0047] S6. Carry out solvent - free corona treatment on the extruded composite film. Control the power of the corona treatment at 4 kW and the treatment speed at 12 m / min. The corona treatment can improve the surface energy of the composite film surface and enhance the adhesion of printing ink and adhesive;

[0048] S7. Slitting and bag making: Cut the corona - treated composite film into the required width and length by a slitter, and use a heat sealer to make the cut composite film into a packaging bag. Control the temperature of the heat sealer at 170 °C, the pressure at 0.4 MPa, and the heat seal time at 2 s. Through the heat seal process, seal the edges of the composite film to form a packaging bag with a certain shape and size.

[0049] Example 3: A preparation method of a food - grade high - barrier packaging bag, comprising the following steps:

[0050] S1. Select a homopolypropylene with a melt flow rate (MFR) of 2 - 6 g / 10 min (to ensure stretchability) and an ash content of ≤0.01% (to avoid impurities affecting transparency). Dry it at 80 - 90°C for 4 - 6 h to reduce the moisture content to ≤0.01% and avoid generating bubbles during extrusion. Add 0.1 - 0.3% of a slip agent and 0.2 - 0.5% of an antistatic agent to improve the surface properties of the film. Use a single-screw extruder (length-diameter ratio L / D = 30 - 35), with an extrusion temperature of 220 - 260°C to ensure complete melting of the resin without decomposition; the melt pressure is 8 - 15 MPa to ensure uniform and stable delivery of the melt to the die head. The melt passes through a slot die head (lip gap 0.8 - 1.5 mm) to form a sheet blank with a thickness of 0.3 - 0.5 mm. The die head temperature control accuracy is ±1°C to ensure the thickness uniformity error of the sheet blank is ≤1%. The extruded sheet blank is immediately adhered to a chrome-plated cooling roll (temperature 50 - 70°C), with a cooling rate of ≥100°C / s to inhibit polypropylene crystallization and form an amorphous transparent sheet (crystallinity ≤5%). Select a product with a surface roughness Ra ≤0.2 μm, a light transmittance ≥90%, and a haze ≤1% for biaxial stretching. Use a roller-type longitudinal stretching machine, with the stretching roller group temperature at 90 - 120°C, a stretching ratio of 3 - 5 times, and a stretching rate of 50 - 100 m / min, so that the polypropylene molecular chains are arranged orderly longitudinally. Use a tenter frame for transverse stretching, with the preheating zone temperature at 120 - 140°C and the stretching zone temperature at 140 - 160°C, a stretching ratio of 6 - 10 times. The sheet is transversely stretched through a chain clamp device to form a "cross-shaped" molecular orientation; during the stretching process, the polypropylene transforms from amorphous to β-crystalline form, improving the impact strength and transparency. The stretching speed is matched: the MD / TD speed ratio is 1:1.2 - 1:1.5 to avoid film warping caused by uneven stress. The stretched film is heat-set at 150 - 180°C for 10 - 30 seconds to fix the molecular orientation structure and reduce the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%). After heat-setting, the film passes through multiple cooling rolls (temperature decreasing from 100°C to 30°C) to avoid internal stress concentration and is wound up after the surface temperature ≤40°C to obtain biaxially stretched polypropylene;

[0051] S2. Raw material preparation: Prepare ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, nano-montmorillonite, and an antioxidant by weight percentage. The nano-montmorillonite is pretreated with a silane coupling agent before use to improve its compatibility with the polymer. Add 7 g of the pretreated nano-montmorillonite, 46 g of ethylene-vinyl alcohol copolymer, and 21 g of maleic anhydride grafted polyethylene to a high-speed mixer, stir and mix at 70°C for 25 min to fully disperse each component, add 0.9 g of antioxidant 1010, and continue stirring for 9 min to ensure uniform distribution of the antioxidant in the mixer;

[0052] S3. Prepare linear low-density polyethylene, metallocene polyethylene, and slip agent by weight percentage. Add 66 g of linear low-density polyethylene and 26 g of metallocene polyethylene to a mixing device, stir and mix at room temperature for 16 min, add 0.7 g of oleic acid amide, and continue stirring for 9 min to uniformly disperse the slip agent in the polymer, thereby reducing the surface friction coefficient of the heat-sealing layer and improving the slipperiness during the bag-making process;

[0053] S4. Prepare polyglycolic acid and biaxially oriented polypropylene by weight percentage. Add 10 g of polyglycolic acid and 16 g of biaxially oriented polypropylene to the corresponding extruder hoppers respectively;

[0054] S5. Co-extrusion casting: Add the prepared raw materials of the barrier layer, heat-sealing layer, and reinforcement layer to the three hoppers of a three-layer co-extrusion machine for co-extrusion casting. Control the temperature of the co-extrusion machine at 200 °C and the screw speed at 80 rpm. During the extrusion process, the raw materials of each layer converge at the die head and form a three-layer composite film through the casting process. During the casting process, control the thickness and uniformity of the composite film by adjusting parameters such as the screw speed of the extruder, the gap of the die head, and the temperature. During the co-extrusion process, due to the presence of maleic anhydride-grafted polyethylene, a gradient compatible interface is formed between the barrier layer and the heat-sealing layer. The maleic anhydride groups therein chemically react with the hydroxyl groups in ethylene-vinyl alcohol copolymer, reducing the interfacial tension between the barrier layer and the heat-sealing layer and improving the interlayer bonding strength. The thickness ratio of the barrier layer, heat-sealing layer, and reinforcement layer is 12 μm:24 μm:15 μm;

[0055] S6. Perform solvent-free corona treatment on the extruded composite film. Control the power of the corona treatment at 5 kW and the treatment speed at 13 m / min. The corona treatment can increase the surface energy of the composite film surface and enhance the adhesion of printing ink and adhesive;

[0056] S7. Slitting and bag-making: Cut the corona-treated composite film into the required width and length by a slitter, and use a heat-sealing machine to make the cut composite film into a packaging bag. Control the temperature of the heat-sealing machine at 175 °C, the pressure at 0.5 MPa, and the heat-sealing time at 3 s. Through the heat-sealing process, seal the edges of the composite film to form a packaging bag with a certain shape and size.

[0057] Example 4: A preparation method of a food-grade high-barrier packaging bag, comprising the following steps:

[0058] S1. Select a homopolypropylene with a melt flow rate (MFR) of 2 - 6 g / 10 min (to ensure stretchability) and an ash content of ≤0.01% (to avoid impurities affecting transparency). Dry it at 80 - 90°C for 4 - 6 h to reduce the moisture content to ≤0.01% and avoid generating bubbles during extrusion. Add 0.1 - 0.3% of a slip agent and 0.2 - 0.5% of an antistatic agent to improve the surface properties of the film. Use a single-screw extruder (length-diameter ratio L / D = 30 - 35), with an extrusion temperature of 220 - 260°C to ensure complete melting of the resin without decomposition; the melt pressure is 8 - 15 MPa to ensure uniform and stable delivery of the melt to the die. The melt passes through a slit die (lip gap 0.8 - 1.5 mm) to form a sheet blank with a thickness of 0.3 - 0.5 mm. The die temperature control accuracy is ±1°C to ensure that the thickness uniformity error of the sheet blank is ≤1%. The extruded sheet blank is immediately attached to a chrome-plated cooling roll (temperature 50 - 70°C), with a cooling rate of ≥100°C / s to inhibit the crystallization of polypropylene and form an amorphous transparent sheet (crystallinity ≤5%). Select a product with a surface roughness Ra ≤0.2 μm, a light transmittance ≥90%, and a haze ≤1% for biaxial stretching. Use a roller-type longitudinal stretching machine, with the temperature of the stretching roller group at 90 - 120°C, a stretching ratio of 3 - 5 times, and a stretching speed of 50 - 100 m / min, so that the polypropylene molecular chains are arranged orderly in the longitudinal direction. Use a tenter frame for transverse stretching, with the preheating zone temperature at 120 - 140°C and the stretching zone temperature at 140 - 160°C, a stretching ratio of 6 - 10 times. The sheet is transversely stretched through a chain clamp device to form a "cross-shaped" molecular orientation; during the stretching process, polypropylene transforms from amorphous to β-crystalline form, improving the impact strength and transparency. The stretching speed is matched: the MD / TD speed ratio is 1:1.2 - 1:1.5 to avoid film warping caused by uneven stress. The stretched film is heat-set at 150 - 180°C for 10 - 30 seconds to fix the molecular orientation structure and reduce the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%). After heat-setting, the film passes through multiple cooling rolls (temperature decreasing from 100°C to 30°C) to avoid internal stress concentration and is wound up after the surface temperature ≤40°C to obtain biaxially stretched polypropylene;

[0059] S2. Raw material preparation: Prepare ethylene-vinyl alcohol copolymer, maleic anhydride-grafted polyethylene, nano-montmorillonite, and antioxidant according to weight percentages. The nano-montmorillonite is pretreated with a silane coupling agent before use to improve its compatibility with the polymer. Add 8 g of the pretreated nano-montmorillonite, 50 g of ethylene-vinyl alcohol copolymer, and 25 g of maleic anhydride-grafted polyethylene to a high-speed mixer, and stir and mix at 80°C for 30 min to fully disperse each component. Then add 1 g of antioxidant 1010 and continue stirring for 10 min to ensure that the antioxidant is evenly distributed in the mixer;

[0060] S3. Prepare linear low density polyethylene, metallocene polyethylene and slip agent by weight percentage. Add 70 g of linear low density polyethylene and 30 g of metallocene polyethylene into a mixing device, stir and mix at room temperature for 20 min, add 0.8 g of erucamide, and continue stirring for 10 min to evenly disperse the slip agent in the polymer, so as to reduce the surface friction coefficient of the heat-sealing layer and improve the slipperiness during the bag-making process;

[0061] S4. Prepare polyglycolic acid and biaxially oriented polypropylene by weight percentage. Add 15 g of polyglycolic acid and 20 g of biaxially oriented polypropylene into the corresponding extruder hoppers respectively, and make them converge at the die head through a co-extrusion device to co-extrude and form a reinforcing layer;

[0062] S5. Co-extrusion casting: Add the prepared raw materials of the barrier layer, heat-sealing layer and reinforcing layer into the three hoppers of a three-layer co-extrusion machine for co-extrusion casting. Control the temperature of the co-extrusion machine at 220 °C and the screw speed at 90 rpm. During the extrusion process, the raw materials of each layer converge at the die head and form a three-layer composite film through the casting process. During the casting process, control the thickness and uniformity of the composite film by adjusting parameters such as the screw speed of the extruder, the gap of the die head and the temperature. During the co-extrusion process, due to the presence of maleic anhydride grafted polyethylene, a gradient compatible interface is formed between the barrier layer and the heat-sealing layer. The maleic anhydride groups therein chemically react with the hydroxyl groups in the ethylene-vinyl alcohol copolymer, reducing the interfacial tension between the barrier layer and the heat-sealing layer and improving the interfacial bonding strength. The thickness ratio of the barrier layer, heat-sealing layer and reinforcing layer is 12 μm:25 μm:15 μm;

[0063] S6. Perform solvent-free corona treatment on the extruded composite film. Control the power of the corona treatment at 5 kW and the treatment speed at 15 m / min. The corona treatment can increase the surface energy of the composite film surface and enhance the adhesion of printing ink and adhesive;

[0064] S7. Slitting and bag-making: Slit the corona-treated composite film into the required width and length through a slitter, and use a heat-sealing machine to make the slit composite film into a packaging bag. Control the temperature of the heat-sealing machine at 180 °C, the pressure at 0.5 MPa, and the heat-sealing time at 3 s. Through the heat-sealing process, seal the edges of the composite film to form a packaging bag with a certain shape and size.

[0065] Comparative Example 1:

[0066] In this comparative example, maleic anhydride grafted polyethylene was not added during the preparation process of the barrier layer compared with Example 1. The remaining steps and parameters are the same. This comparative example will not be repeated here, and finally a packaging bag was obtained.

[0067] Comparative Example 2:

[0068] In this comparative example, compared with Example 1, only "metallocene polyethylene" was replaced with "commercially available traditional polyethylene", and the remaining steps and parameters were the same. This comparative example will not be repeated here, and finally a packaging bag was obtained.

[0069] Comparative Example 3:

[0070] In this comparative example, compared with Example 1, only the thickness ratio of the barrier layer, heat-sealing layer and reinforcing layer was adjusted from 10μm: 22μm: 13μm to 15μm: 20μm: 10μm, and the remaining steps and parameters were the same. This comparative example will not be repeated here, and finally a packaging bag was obtained.

[0071] Comparative Example 4:

[0072] In this comparative example, compared with Example 1, only the order of the barrier layer, heat-sealing layer and reinforcing layer was adjusted to heat-sealing layer, barrier layer and reinforcing layer, and the remaining steps and parameters were the same. This comparative example will not be repeated here, and finally a packaging bag was obtained.

[0073] Performance test: The following performance tests were carried out on the packaging bags prepared in Examples 1-4 and Comparative Examples 1-4:

[0074] Oxygen barrier coefficient test: Referring to the method of GB / T 1038-2000 "Test Method for Gas Permeability of Plastics - Differential Pressure Method", a differential pressure type gas permeation instrument was used. At a temperature of 23°C, a relative humidity of 0%, a differential pressure of 0.1 MPa, and an effective area of the specimen of 50 cm 2 , and the test time was 24 h. By monitoring the volume of oxygen permeating through the specimen, the oxygen barrier coefficient was calculated (unit: cm 3 / (m 2 ·24 h·0.1 MPa));

[0075] Water vapor transmission rate test: Referring to the standard of GB / T 1037-1988 "Test Method for Water Vapor Transmission of Plastic Films and Sheets - Cup Method", a water vapor transmission rate tester was used. At a temperature of 23°C, a relative humidity of 50%, a specimen thickness of 50 ± 5μm, and a test time of 48 h. By weighing the change in the mass of the moisture permeation cup, the water vapor transmission rate was calculated (unit: g / (m 2 ·24 h)).

[0076] Tensile strength and elongation at break: Referring to the standard of GB / T 1040.3-2006 "Determination of Tensile Properties of Plastics - Part 3: Test Conditions for Films and Sheets", an electronic universal testing machine was used. The specimen size was 150 mm × 15 mm, the clamp spacing was 100 mm, and the tensile speed was 50 mm / min. The test was carried out 5 times and the average value was taken.

[0077] For the heat seal strength test, referring to the standard ASTM D3354 "Test Method for Heat Seal Strength of Heat Sealable Plastic Films", a heat sealer (set temperature 170°C, pressure 0.4 MPa, time 2 s) and a peel tester were used. After heat-sealing the specimens, they were peeled at 180° at a speed of 100 mm / min, and the average peel force (unit: N / 15 mm) was recorded.

[0078] For the interlayer peel strength, referring to the standard ASTM D3359 "Determination of Adhesive Peel Strength by Tape Test Method", a peel tester was used to peel along the interlayer interface at a speed of 100 mm / min, and the average peel force (unit: N / 15 mm) was recorded. The results are shown in Table 1 below:

[0079] Table 1

[0080]

[0081] Data analysis:

[0082] As can be seen from Table 1, the packaging bags prepared in Examples 1 - 4 have better barrier properties, better mechanical properties, and better processing and interfacial properties. In Comparative Example 1, maleic anhydride grafted polyethylene was not added, resulting in poor interfacial compatibility and an increase in the oxygen barrier coefficient to 0.45, indicating the key influence of the compatibilizer on interlayer densification; in Comparative Example 4, the adjustment of the layer sequence led to a weakening of the support effect of the reinforcing layer, a decrease in tensile strength, and a decrease in elongation at break, proving the direct influence of the structure design on mechanical properties.

[0083] The present invention provides a food-grade high-barrier packaging bag and its preparation method. By adding the raw materials of the barrier layer, heat-seal layer, and reinforcing layer into three hoppers of a three-layer co-extrusion machine for co-extrusion casting respectively, and through reasonable arrangement of the sequence of the barrier layer, heat-seal layer, and reinforcing layer, combined with the selection of food-grade materials, the problem that the barrier layer in the traditional structure is easily weakened can be effectively solved, while meeting the multiple requirements of food packaging for safety, weather resistance, and processing performance. By adjusting the interlayer structure, the present invention can achieve efficient barrier against oxygen and water vapor without significantly increasing costs, providing a more long-term fresh-keeping solution for prefabricated foods, meats, baked foods, etc., and having important practical application value.

[0084] During the three-layer co-extrusion process of the present invention, the materials of the barrier layer, heat-seal layer, and reinforcing layer can produce a synergistic effect of performance complementation or enhancement through interfacial chemical reactions, compatibility optimization, crystallization induction, and nano-synergistic effects. The specific reactions and promotion mechanisms are as follows:

[0085] Interfacial chemical reaction: Formation of chemical bond connections:

[0086] Esterification reaction between the barrier layer and the heat-sealing layer: Maleic anhydride grafted polyethylene (PE-g-MAH) in the barrier layer contains polar maleic anhydride groups (-COOH). When it comes into contact with the LDPE / mPE (non-polar) of the heat-sealing layer, its polar groups can react with the hydroxyl groups (-OH) of EVOH in the barrier layer to form an ester bond connection. Performance promotion: The interfacial tension is reduced, and the interlayer peel strength is improved (superior to simple physical lamination); a "gradient compatible transition zone" is formed, reducing interlayer defects and preventing gas from permeating through the interface.

[0087] Hydrogen bond interaction between the reinforcement layer and the barrier layer: Poly(glycolic acid) (PGA) in the reinforcement layer contains hydroxyl groups (-OH) and ester groups (-COO-). It forms intermolecular association with the hydroxyl groups of EVOH in the barrier layer through hydrogen bond interaction (-OH…O-). Performance promotion: PGA induces EVOH to form a more regular α crystal form, increasing the crystallinity and improving the oxygen barrier property; a "rigid-tough" transition layer is formed at the interface, enhancing the tensile strength.

[0088] Compatibility optimization: Reducing phase separation

[0089] The bridging effect of PE-g-MAH: As an amphiphilic compatibilizer, the polyethylene chain segments of PE-g-MAH are compatible with LDPE / mPE in the heat-sealing layer, and the maleic anhydride groups are compatible with EVOH in the barrier layer, forming a "core-shell" structure to encapsulate nanoclay. Performance promotion: The dispersed particle size of nanoclay in the barrier layer is reduced, the specific surface area is increased, and the gas diffusion path is extended; the melt viscosity matching degree between the heat-sealing layer and the barrier layer is improved, and the layer thickness uniformity error is small during coextrusion.

[0090] Synergistic crystallization of BOPP and PGA: BOPP (non-polar) in the reinforcement layer interacts with PGA (polar) through van der Waals forces. The crystallization behavior of PGA is induced by the oriented structure of BOPP, forming a shish-kebab structure. Performance promotion: The elastic modulus of the reinforcement layer is increased, the impact strength is improved, and the breakage during packaging and transportation is reduced; the high transparency of BOPP combined with the crystallinity of PGA results in a high light transmittance of the composite film, meeting the requirements of transparent packaging.

[0091] Synergistic effect of nano and crystallization:

[0092] Interlayer diffusion of nanoclay: Under the action of the coextrusion shear force, the nanoclay platelets in the barrier layer partially migrate to the interface between the heat-sealing layer and the reinforcement layer, forming a "nano-bridging" structure. Performance promotion: The puncture resistance of the heat-sealing layer is improved (the clay platelets hinder crack propagation); the water vapor transmission rate is reduced (the clay platelets increase the diffusion path of water molecules).

[0093] Crystallization induction of EVOH and PGA: The high crystallinity of PGA (melting point 220 °C) acts as a "nucleating agent" during the co-extrusion cooling process, promoting the formation of a denser lamellar crystal of EVOH. Performance promotion: The crystallinity of EVOH increases, and the oxygen permeability decreases; the orderly arrangement in the crystal region reduces the movement of polymer segments, and the high-temperature resistance is improved (the glass transition temperature increases).

[0094] Dynamic coordination in the processing technology:

[0095] Synergy mechanism of melt viscosity matching and layer thickness control: The melt viscosity gradient design (viscosity ratio 1:1.5:2) of the heat-sealing layer (LDPE / mPE, low viscosity), barrier layer (EVOH / modified PE, medium viscosity), and reinforcement layer (PGA / BOPP, high viscosity) forms a stable laminar flow distribution in the co-extrusion die. Performance promotion: Reducing the "melt fracture" defect between layers, the surface roughness of the composite film is reduced by 20%, and the printing adaptability is improved; the thickness uniformity error of each layer is small, avoiding barrier failure caused by local thinning.

[0096] Cross-layer migration protection mechanism of antioxidants: Antioxidants (such as 1010) in the barrier layer partially migrate to the heat-sealing layer and reinforcement layer during high-temperature co-extrusion, forming a "gradient antioxidant protection". Performance promotion: The oxidation induction time (OIT) of the overall film is extended, suitable for packaging of oil-containing foods; the mPE in the heat-sealing layer is protected by antioxidants, and the heat-sealing strength retention rate is relatively high during long-term storage.

[0097] The "synergistic enhancement effect" of three-layer co-extrusion, through mechanisms such as chemical bonding, compatibility optimization, crystallization induction, and nano-bridging, produces the following core performance promotions among the three-layer materials:

[0098] Barrier property: Interface densification + crystallization optimization, oxygen / water vapor barrier property is improved;

[0099] Mechanical properties: Interlayer chemical bond + nano-reinforcement, tensile strength increases, puncture resistance increases;

[0100] Processability: Viscosity matching + compatibilizer, layer thickness uniformity increases, defect rate decreases;

[0101] Durability: Antioxidant migration + crystallization stability, shelf life is extended.

[0102] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features between the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity.

[0103] The present invention aims to cover all such substitutions, modifications and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A food-grade high-barrier packaging bag, characterized in that, The food-grade high-barrier packaging bag sequentially comprises a barrier layer, a heat-sealing layer and a reinforcing layer from the inside to the outside; The barrier layer comprises the following raw materials: ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, montmorillonite and antioxidant; The heat-sealing layer comprises the following raw materials: linear low-density polyethylene, metallocene polyethylene and slip agent; The reinforcing layer comprises the following raw materials: polyglycolic acid and biaxially oriented polypropylene.

2. The food-grade high-barrier packaging bag according to claim 1, wherein The thickness ratio of the barrier layer, the heat-sealing layer and the reinforcing layer is 10-12μm: 22-25μm: 13-15μm.

3. The food-grade high-barrier packaging bag according to claim 1, characterized in that The mass ratio of the ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, nano-montmorillonite and antioxidant is 40-60: 15-25: 3-8: 0.5-1.

4. The food-grade high-barrier packaging bag according to claim 1, wherein The antioxidant is antioxidant 1010.

5. The food-grade high-barrier packaging bag according to claim 1, wherein, The mass ratio of the linear low-density polyethylene, metallocene polyethylene and slip agent is 60-70: 20-30: 0.3-0.

8.

6. The food-grade high-barrier packaging bag according to claim 1, characterized in that The slip agent is any one of oleic acid amide and erucic acid amide.

7. The food-grade high-barrier packaging bag according to claim 1, characterized in that, The mass ratio of the polyglycolic acid and biaxially oriented polypropylene is 5-15: 10-20.

8. A preparation method of a food-grade high-barrier packaging bag, characterized in that, It includes the following steps: Step S1. Co-extrusion casting: Add the prepared raw materials of the barrier layer, the heat-sealing layer and the reinforcing layer into three hoppers of a three-layer co-extrusion machine respectively for co-extrusion casting. During the extrusion process, the raw materials of each layer converge at the die head and form a three-layer composite film through the casting process; Step S2. Perform solvent-free corona treatment on the extruded composite film; Step S3. Slitting and bag making: Slit the corona-treated composite film into the required width and length by a slitter, and use a heat-sealing machine to make the slit composite film into a packaging bag. Through the heat-sealing process, seal the edges of the composite film to form a packaging bag with a certain shape and size.

9. The preparation method of the food-grade high-barrier packaging bag according to claim 8, wherein, In step S1, the temperature of the co-extrusion machine is 180-220°C and the screw speed is 60-90 rpm.

10. The preparation method of the food-grade high-barrier packaging bag according to claim 8, characterized in that, In step S2, the power of the corona treatment is 3-5 kW and the treatment speed is 10-15 m / min; In step S3, the temperature of the heat-sealing machine is 160-180°C, the pressure is 0.3-0.5 MPa, and the heat-sealing time is 1-3 s.

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