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

Abstract

The application 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 comprises, from inside to outside, a barrier layer, a heat-sealing layer and a reinforcing layer in sequence, raw materials of the barrier layer, the heat-sealing layer and the reinforcing layer are respectively added into three hoppers of a three-layer co-extruder to perform co-extrusion casting, the order of the barrier layer, the heat-sealing layer and the reinforcing layer is reasonably arranged, food-grade materials are selected, the problem that the barrier layer is easily weakened in a traditional structure can be effectively solved, and multiple requirements of food packaging on safety, weather resistance and processing performance can be met. The interlayer structure is adjusted, high-efficiency blocking of oxygen and water vapor is realized under the premise that the cost is not obviously increased, a longer-effect preservation solution is provided for prepared food, meat, baked food and the like, and the application has important practical application value.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of plastic products, in particular to a food-grade high-barrier packaging bag and a preparation method thereof. BACKGROUND

[0002] In the food industry, the barrier property of packaging materials directly affects the shelf life, flavor and nutrient retention of food. With the improvement of consumers' requirements for food freshness and safety, the research and application of high-barrier packaging materials have become the focus of the industry. High-barrier packaging realizes effective barrier to oxygen, water vapor, odor and microorganisms through multi-layer composite structure design, thereby delaying food oxidation, mold and flavor loss. Early high-barrier packaging bags mainly use polyethylene film combined with metal foil to achieve protection of food by taking advantage of the excellent barrier property of metal foil, but have 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 process, which improves the processing adaptability and economy of the material while ensuring the barrier property. For example, PVDC-coated film uses polypropylene (OPP) or polyethylene terephthalate (PET) as the substrate, and after coating, the oxygen transmission rate can be reduced to 1 / 1000 of the original substrate, and it is widely used in perishable food packaging such as meat and cheese; EVOH has excellent barrier property to gas and good processing performance, and becomes the core material of 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 structural design and material combination. On the one hand, the barrier ability of single-layer film is limited and needs to be realized through multi-layer composite to achieve synergistic effect. On the other hand, some materials have environmental adaptability defects, such as the barrier property of nylon materials being easily affected by humidity, the narrow processing temperature range of PVDC and the difficulty in recycling, which limits its application in a wider range of scenarios. In addition, with the tightening of environmental regulations, the development of degradable and recyclable high-barrier materials (such as PLA / PBAT composite film) has become a new direction, but their barrier performance still needs to be improved compared with traditional materials.

[0004] Therefore, according to the related technology in the above, it is urgent to develop a food-grade high-barrier packaging bag and a preparation method thereof. SUMMARY

[0005] In view of the above, the purpose of the present application 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 synergies and higher barrier efficiency. By reasonably arranging the order of the barrier layer, heat-sealing layer and reinforcing layer, and combining the selection of food-grade materials, the problem of weakening of the barrier layer in the traditional structure can be effectively solved, while meeting the multiple requirements of safety, weather resistance and processing performance of food packaging. The present application adjusts the interlayer structure, realizes efficient barrier to oxygen and water vapor under the premise of not significantly increasing the cost, provides a longer-lasting preservation solution for prepared foods, meat, baked foods and the like, and has important practical application value.

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

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

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

[0009] The barrier layer comprises 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 comprises the following raw materials: linear low-density polyethylene, metallocene polyethylene and slip agent.

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

[0014] The biaxially oriented polypropylene is a process of realizing high performance by biaxial stretching and orientation of polypropylene resin, and the specific process is as follows:

[0015] The homopolymer polypropylene with a melt flow rate (MFR) of 2-6 g / 10 min (to ensure stretchability), ash content of ≤0.01% (to avoid impurities affecting transparency) is selected, dried at 80-90°C for 4-6 h to reduce the water content to ≤0.01%, to avoid bubbles during extrusion, 0.1-0.3% of a slip agent and 0.2-0.5% of an antistatic agent are added to improve the surface properties of the film, a single screw extruder (length-diameter ratio L / D = 30-35) is used, the extrusion temperature is 220-260°C to ensure that the resin is completely melted and not decomposed; 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 chromium-plated cooling roller (temperature 50-70°C), the cooling rate is ≥100°C / s to inhibit the crystallization of polypropylene, forming amorphous transparent sheet (crystallinity ≤5%), the product with a surface roughness Ra of ≤0.2 μm, a light transmittance of ≥90% and a haze of ≤1% is selected, bidirectional stretching is performed, a roll-type longitudinal stretching machine is used, the stretching roller group temperature is 90-120°C, the stretching ratio is 3 times-5 times, the stretching speed is 50-100 m / min, the polypropylene molecular chain is orderly arranged along the longitudinal direction, a tenter is used for transverse stretching, the preheating zone temperature is 120-140°C, the stretching zone temperature is 140-160°C, the stretching ratio is 6-10 times, the sheet is stretched in the transverse direction by a chain gripper device to form a "cross-shaped" molecular orientation; during the stretching process, the polypropylene is transformed from amorphous to β crystal form, the impact strength and transparency are improved, the stretching speed is matched: MD / TD speed ratio 1:1.2-1:1.5, to avoid uneven stress leading to film warping, 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 setting, the film is cooled by multiple-stage cooling rollers (temperature from 100°C to 30°C) to avoid internal stress concentration, the surface temperature is ≤40°C before winding, and a biaxially stretched polypropylene is obtained.

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

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

[0018] Preferably, the metallocene polyethylene is prepared by a solution polymerization method, in which a metallocene compound such as dicyclopentadienyl zirconium dichloride and a cocatalyst methylaluminoxane are activated in an inert solvent such as cyclohexane or toluene at a temperature of 100-150 DEG C and a pressure of 3-5 MPa to form active centers, then ethylene monomers are introduced and polymerized under stirring to form a homogeneous solution, and finally the solvent is removed and dried to obtain the metallocene polyethylene.

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

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

[0021] Step S1. Co-extrusion casting: the prepared raw materials of the barrier layer, the heat-sealing layer and the reinforcing layer are respectively added to three hoppers of a three-layer co-extruder for co-extrusion casting, the temperature of the co-extruder is controlled at 180-220 DEG C, and the screw rotation speed is 60-90 rpm; in the extrusion process, the raw materials of the layers are combined at the die head, and a three-layer composite film is formed through the casting process; in the casting process, the thickness and uniformity of the composite film are controlled by adjusting the screw rotation speed, the gap of the die head and the temperature of the extruder; in the co-extrusion process, a gradient compatible interface is formed between the barrier layer and the heat-sealing layer due to the presence of the maleic anhydride grafted polyethylene, the maleic anhydride groups in the interface chemically react with the hydroxyl groups in the ethylene-vinyl alcohol copolymer, the interfacial tension between the barrier layer and the heat-sealing layer is reduced, and the bonding strength between the layers is improved;

[0022] Step S2. Solvent-free corona treatment is performed on the extruded composite film, the power of the corona treatment is controlled at 3-5 kW, and the treatment speed is 10-15 m / min; the corona treatment can improve the surface energy of the surface of the composite film and enhance the adhesion of the printing ink and the adhesive;

[0023] Step S3. Cutting and bag making: the composite film treated by the corona treatment is cut into a desired width and length by a cutting machine, and a heat-sealing machine is used to make the cut composite film into a packaging bag; the temperature of the heat-sealing machine is controlled at 160-180 DEG C, the pressure is 0.3-0.5 MPa, and the heat-sealing time is 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 application are as follows:

[0025] The application provides a food-grade high-barrier packaging bag and a preparation method thereof. The raw materials of a barrier layer, a heat-sealing layer and a reinforcing layer are respectively added into three hoppers of a three-layer co-extruder to perform co-extrusion casting. By reasonably arranging the sequence of the barrier layer, the heat-sealing layer and the reinforcing layer, and by combining the selection of food-grade materials, the problem that the barrier layer is easily weakened in a traditional structure can be effectively solved, and multiple requirements of food packaging on safety, weather resistance and processing performance can be met. By adjusting the interlayer structure, the application realizes efficient blocking of oxygen and water vapor under the premise of not significantly increasing the cost, provides a longer-lasting preservation solution for prepared food, meat, baked food and the like, and has important practical application value. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the application clearer, the application is further described in detail below with reference to specific examples.

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

[0028] The brand of the low-density polyethylene is 2420H, the density is 0.924 g / cm 3 , the melt flow index (10 min) is 1.9 g (190℃, 2.16 kg), and it is purchased from China Petroleum and Chemical Corporation.

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

[0030] The antioxidant 1010 is analytical pure, and it is purchased from China Reagent Co., Ltd.

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

[0032] The brand of the polyglycolic acid is Purasorb@PGS, and it is purchased from the Netherlands Ceben Company.

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

[0034] S1. Selecting homopolymer polypropylene with melt flow rate (MFR) 2-6 g / 10 min (ensuring stretchability), ash content ≤0.01% (avoiding impurities affecting transparency), drying at 80-90℃ for 4-6h to reduce the moisture content to ≤0.01%, avoiding bubbles during extrusion, adding slip agent 0.1-0.3%, antistatic agent 0.2-0.5% to improve the surface properties of the film, using a single screw extruder (length-diameter ratio L / D = 30-35), extrusion temperature 220-260℃ 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 with a thickness of 0.3-0.5 mm, the die temperature control accuracy is ±1℃ to ensure that the thickness uniformity error of the sheet is ≤1%, the extruded sheet is immediately attached to a chrome-coated cooling roller (temperature 50-70℃), the cooling rate is ≥100℃ / s to inhibit the crystallization of polypropylene, forming amorphous transparent sheet (crystallinity ≤5%), selecting the product with surface roughness Ra ≤0.2μm, light transmittance ≥90%, haze ≤1%, carrying out two-way stretching, using a roll-type longitudinal stretching machine, the stretching roller group temperature is 90-120℃, the stretching ratio is 3-5 times, the stretching rate is 50-100 m / min, the polypropylene molecular chain is orderly arranged along the longitudinal direction, using a tenter frame for transverse stretching, the preheating zone temperature is 120-140℃, the stretching zone temperature is 140-160℃, the stretching ratio is 6-10 times, the sheet is stretched in the transverse direction by a chain gripper device to form a "cross-shaped" molecular orientation; during the stretching process, the polypropylene changes from amorphous to β crystal form, improving the impact strength and transparency, the stretching speed is matched: MD / TD speed ratio 1:1.2-1:1.5 to avoid uneven stress leading to film warping, the stretched film is heat set at 150-180℃ for 10-30 seconds to fix the molecular orientation structure and reduce the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%), after setting, the film is cooled by multiple cooling rollers (temperature from 100℃ to 30℃) to avoid stress concentration, and the surface temperature is ≤40℃ before winding to obtain biaxially oriented polypropylene;

[0035] S2. Raw material preparation: prepare ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, nano-montmorillonite and antioxidant according to weight percentage, the nano-montmorillonite is pretreated with silane coupling agent before use to improve its compatibility with the polymer, 3g of pretreated nano-montmorillonite is added to 40g of ethylene-vinyl alcohol copolymer, 15g of maleic anhydride grafted polyethylene in a high-speed mixer, stirring and mixing at 60℃ for 15min to make the components fully dispersed, adding 0.5g of antioxidant 1010, continuing to stir for 5min to ensure uniform distribution of the antioxidant in the mixer;

[0036] S3 prepared by weight percentage of linear low-density polyethylene, metallocene polyethylene and a slip agent, 60g of linear low-density polyethylene and 20g of metallocene polyethylene were added to a mixing device, stirred at room temperature for 10min, 0.3g of oleamide was added, and stirring was continued for 5min to uniformly disperse the slip agent in the polymer to reduce the surface friction coefficient of the heat seal layer and improve the slipperiness during the bag making process;

[0037] S4 prepared by weight percentage of polyglycolic acid, biaxially oriented polypropylene, 5g of polyglycolic acid and 10g of biaxially oriented polypropylene were added to the corresponding extruder hopper;

[0038] S5. Co-extrusion casting: The prepared raw materials of the barrier layer, heat-sealing layer and reinforcing layer are respectively added to the three hoppers of a three-layer co-extruder for co-extrusion casting. The temperature of the co-extruder is controlled at 180°C and the screw speed is 60 rpm. During the extrusion process, the raw materials of each layer converge at the die head and a three-layer composite film is formed by a casting process. During the casting process, the thickness and uniformity of the composite film are controlled by adjusting parameters such as the screw speed of the extruder, the gap and temperature of the die head. 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 react chemically with the hydroxyl groups in the ethylene-vinyl alcohol copolymer, thereby reducing the interfacial tension between the barrier layer and the heat-sealing layer and improving the bonding strength between the layers. The thickness ratio of the barrier layer, the heat-sealing layer and the reinforcing layer is 10 μm:22 μm:13 μm;

[0039] S6. The extruded composite film was subjected to solvent-free corona treatment, with the power of the corona treatment controlled at 3 kW and the processing speed at 10 m / min. The corona treatment can increase the surface energy of the composite film surface and enhance the adhesion of printing inks and adhesives.

[0040] S7. Slitting and bagging: The corona-treated composite film is cut into the required width and length by a slitting machine, and the slit composite film is made into packaging bags using a heat sealing machine. The temperature of the heat sealing machine is controlled at 160°C, the pressure is 0.3 MPa, and the heat sealing time is 1 second. Through the heat sealing process, the edges of the composite film are sealed to form packaging bags with a certain shape and size.

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

[0042] S1. Selecting homopolymer polypropylene with melt flow rate (MFR) 2-6 g / 10 min (ensuring stretchability), ash content ≤0.01% (avoiding impurities affecting transparency), drying at 80-90℃ for 4-6h to reduce the moisture content to ≤0.01%, avoiding bubbles during extrusion, adding 0.1-0.3% slip agent, 0.2-0.5% antistatic agent to improve the surface properties of the film, using a single screw extruder (length-diameter ratio L / D = 30-35), the extrusion temperature is 220-260℃, ensuring that the resin is completely melted and not decomposed; the melt pressure is 8-15 MPa, ensuring that the melt is uniformly and stably delivered to the die, the melt passes through a slit die (lip gap 0.8-1.5 mm), forming a sheet with a thickness of 0.3-0.5 mm, the die temperature control accuracy is ±1℃, ensuring that the sheet thickness uniformity error is ≤1%, the extruded sheet is immediately attached to a chrome-coated cooling roller (temperature 50-70℃), the cooling rate is ≥100℃ / s, inhibiting the crystallization of polypropylene, forming amorphous transparent sheet (crystallinity ≤5%), selecting the product with surface roughness Ra ≤0.2μm, light transmittance ≥90%, haze ≤1%, carrying out two-way stretching, using a roll-type longitudinal stretching machine, the stretching roller group temperature is 90-120℃, the stretching ratio is 3-5 times, the stretching rate is 50-100 m / min, the polypropylene molecular chain is orderly arranged along the longitudinal direction, using a tenter frame to carry out transverse stretching, the preheating zone temperature is 120-140℃, the stretching zone temperature is 140-160℃, the stretching ratio is 6-10 times, the sheet is stretched in the transverse direction by a chain device, forming a "cross-shaped" molecular orientation; during the stretching process, the polypropylene changes from amorphous to β crystal form, improving the impact strength and transparency, the stretching speed matching is MD / TD speed ratio 1:1.2-1:1.5, avoiding uneven stress leading to film warping, the stretched film is heat set at 150-180℃ for 10-30 seconds, fixing the molecular orientation structure, reducing the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%), after setting, the film passes through multiple cooling rollers (temperature from 100℃ to 30℃), avoiding stress concentration, the surface temperature is ≤40℃ before winding, obtaining a biaxially stretched polypropylene;

[0043] S2. Raw material preparation: prepare ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, nano-montmorillonite and antioxidant according to weight percentage, the nano-montmorillonite is pretreated with silane coupling agent before use to improve its compatibility with the polymer, 5g pretreated nano-montmorillonite, 43g ethylene-vinyl alcohol copolymer, 18g maleic anhydride grafted polyethylene are added to a high-speed mixer, stirring and mixing at 65℃ for 20min to make the components fully dispersed, adding 0.7g antioxidant 1010, continuing to stir for 7min to ensure uniform distribution of the antioxidant 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 the mixing device, stir and mix at room temperature for 13 min, add 0.5 g of erucic acid amide, continue to stir for 8 min, so that the slip agent is uniformly dispersed in the polymer, to reduce the surface friction coefficient of the heat seal layer and improve the slip property during bag making;

[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 hopper of the corresponding extruder

[0046] S5. Co-extrusion casting: add the prepared barrier layer, heat seal layer and reinforcing layer materials into the three hoppers of the 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, and a three-layer composite film is formed through the casting process. During the casting process, the thickness and uniformity of the composite film are controlled by adjusting the screw speed of the extruder, the gap of the die and the temperature, etc. During the co-extrusion process, a gradient compatible interface is formed between the barrier layer and the heat seal layer due to the presence of maleic anhydride grafted polyethylene. The maleic anhydride groups in it react with the hydroxyl groups in the ethylene-vinyl alcohol copolymer, reducing the interfacial tension between the barrier layer and the heat seal layer and improving the interlayer bonding strength. The thickness ratio of the barrier layer, heat seal layer and reinforcing layer is 11 μm:23 μm:14 μm;

[0047] S6. Perform 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. Corona treatment can increase 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 slitting machine, 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 sealing time at 2 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.

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

[0050] S1. Selecting homopolymer polypropylene with melt flow rate (MFR) 2-6 g / 10 min (ensuring stretchability), ash content ≤0.01% (avoiding impurities affecting transparency), drying at 80-90℃ for 4-6h to reduce the moisture content to ≤0.01%, avoiding bubbles during extrusion, adding slip agent 0.1-0.3%, antistatic agent 0.2-0.5% to improve the surface properties of the film, using a single screw extruder (length-diameter ratio L / D = 30-35), extrusion temperature 220-260℃ 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 with a thickness of 0.3-0.5 mm, the die temperature control accuracy is ±1℃ to ensure that the sheet thickness uniformity error is ≤1%, the extruded sheet is immediately attached to a chrome-coated cooling roller (temperature 50-70℃), the cooling rate is ≥100℃ / s to inhibit polypropylene crystallization, forming amorphous transparent sheet (crystallinity ≤5%), selecting products with surface roughness Ra ≤0.2μm, light transmittance ≥90%, haze ≤1% for two-way stretching, using a roll-type longitudinal stretching machine, the stretching roller group temperature is 90-120℃, the stretching ratio is 3-5 times, the stretching rate is 50-100 m / min, the polypropylene molecular chain is orderly arranged along the longitudinal direction, using a tenter frame for transverse stretching, the preheating zone temperature is 120-140℃, the stretching zone temperature is 140-160℃, the stretching ratio is 6-10 times, the sheet is stretched in the transverse direction by a chain device to form a "cross-shaped" molecular orientation; during stretching, polypropylene changes from amorphous to β crystal form, improving impact strength and transparency, the stretching speed is matched: MD / TD speed ratio 1:1.2-1:1.5 to avoid uneven stress leading to film warping, the stretched film is heat set at 150-180℃ for 10-30 seconds to fix the molecular orientation structure and reduce the shrinkage rate (longitudinal / transverse shrinkage rate ≤3%), after setting, the film passes through multiple cooling rollers (temperature from 100℃ to 30℃) to avoid internal stress concentration, the surface temperature is ≤40℃ before winding, obtaining biaxially oriented polypropylene;

[0051] S2. Raw material preparation: prepare ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, nano-montmorillonite and antioxidant according to weight percentage, the nano-montmorillonite is pretreated with silane coupling agent before use to improve its compatibility with polymers, 7g pretreated nano-montmorillonite, 46g ethylene-vinyl alcohol copolymer and 21g maleic anhydride grafted polyethylene are added to a high-speed mixer, stirred and mixed at 70℃ for 25min to make the components fully dispersed, 0.9g antioxidant 1010 is added and stirred for another 9min 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 into the mixing device, stir and mix at room temperature for 16 min, add 0.7 g of oleic acid amide, continue to stir for 9 min, and make the slip agent uniformly dispersed in the polymer to reduce the surface friction coefficient of the heat seal layer and improve the slip property during bag making;

[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 into the corresponding hopper of the extruder, respectively;

[0054] S5. Co-extrusion casting: add the prepared raw materials of the barrier layer, heat seal layer and reinforcing layer into the three hoppers of the three-layer co-extruder for co-extrusion casting, control the temperature of the co-extruder at 200°C, and the screw rotation speed at 80 rpm. During the extrusion process, the raw materials of each layer converge at the die head, and a three-layer composite film is formed through the casting process. During the casting process, the thickness and uniformity of the composite film are controlled by adjusting the screw rotation speed of the extruder, the gap of the die head and the temperature and other parameters. During the co-extrusion process, a gradient compatible interface is formed between the barrier layer and the heat seal layer due to the presence of maleic anhydride grafted polyethylene. The maleic anhydride groups in the interface react with the hydroxyl groups in the 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 reinforcing 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. 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 slitting machine, and use a heat sealer to make the cut composite film into a packaging bag. Control the temperature of the heat sealer at 175°C, the pressure at 0.5 MPa, and the heat sealing time at 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.

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

[0058] S1. Select homopolypropylene with a melt flow rate (MFR) of 2-6g / 10min (to ensure stretchability) and an ash content of ≤0.01% (to avoid impurities affecting transparency), dry it at 80-90℃ for 4-6h to reduce the moisture content to ≤0.01% to avoid 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-to-diameter ratio L / D = 30-35), and extrusion temperature of 220-260℃ to ensure the resin Complete melting without decomposition; melt pressure is 8-15MPa, ensuring that the melt is evenly and stably transported to the die head, the melt passes through the slit die head (lip gap 0.8-1.5mm), forming a sheet with a thickness of 0.3-0.5mm, the die head temperature control accuracy is ±1℃, ensuring that the sheet thickness uniformity error is ≤1%, the extruded sheet is immediately attached to the chrome-plated cooling roller (temperature 50-70℃), the cooling rate is ≥100℃ / s, inhibiting the crystallization of polypropylene to form an amorphous transparent sheet (crystallization ≤5%), and selecting the surface roughness. The product with Ra≤0.2μm, transmittance≥90%, and haze≤1% is subjected to biaxial stretching, using a roller longitudinal stretching machine, a stretching roller group temperature of 90-120℃, a stretching ratio of 3-5 times, a stretching rate of 50-100m / min, and a polypropylene molecular chain arranged in order along the longitudinal direction. A tenter is used for transverse stretching, a preheating zone temperature of 120-140℃, a stretching zone temperature of 140-160℃, a stretching ratio of 6-10 times, and a clamp chain device to stretch the sheet transversely to form a "cross-shaped" molecular orientation; the stretching process During the process, polypropylene transforms from amorphous to β-crystal, improving impact strength and transparency. The stretching speed is matched: the MD / TD speed ratio is 1:1.2-1:1.5 to avoid uneven stress and film warping. The stretched film is heat-set at 150-180°C for 10-30 seconds to fix the molecular orientation structure and reduce shrinkage (longitudinal / transverse shrinkage ≤ 3%). After setting, the film passes through multi-stage cooling rollers (the temperature is reduced from 100°C to 30°C) to avoid internal stress concentration. After the surface temperature is ≤40°C, it is rolled up to obtain biaxially oriented polypropylene.

[0059] S2. Raw material preparation: Prepare ethylene-vinyl alcohol copolymer, maleic anhydride grafted polyethylene, nano-montmorillonite and antioxidant by weight percentage. The nano-montmorillonite is pretreated with a silane coupling agent before use to improve its compatibility with the polymer. 8g of pretreated nano-montmorillonite, 50g of ethylene-vinyl alcohol copolymer and 25g of maleic anhydride grafted polyethylene are added to a high-speed mixer and stirred at 80°C for 30min to fully disperse the components. 1g of antioxidant 1010 is added and stirring is continued for 10min 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 the mixing device, stir and mix at room temperature for 20 min, add 0.8 g of erucic acid amide, continue to stir for 10 min, and make the slip agent uniformly dispersed in the polymer to reduce the surface friction coefficient of the heat sealing layer and improve the slip property during bag making;

[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 hopper of the extruder respectively, make them meet at the die of the co-extrusion device, and co-extrude to form the reinforcing layer;

[0062] S5. Co-extrusion casting: add the prepared barrier layer, heat sealing layer and reinforcing layer into the three hoppers of the three-layer co-extruder respectively for co-extrusion casting, control the temperature of the co-extruder at 220°C, and the screw rotation speed at 90 rpm, in the extrusion process, the raw materials of each layer meet at the die, and a three-layer composite film is formed by the casting process, in the casting process, the thickness and uniformity of the composite film are controlled by adjusting the screw rotation speed of the extruder, the gap of the die and the temperature and other parameters, in 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 in it 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, wherein the thickness ratio of the barrier layer, the heat sealing layer and the 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 improve the surface energy of the composite film surface, and enhance the adhesion of printing ink and adhesive;

[0064] S7. Slitting and bag making: cut the corona treated composite film into the required width and length by a slitting machine, use a heat sealer to make the cut composite film into a packaging bag, control the temperature of the heat sealer at 180°C, the pressure at 0.5 MPa, and the heat sealing time at 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.

[0065] Comparative Example 1:

[0066] This comparative example does not add maleic anhydride grafted polyethylene in the preparation process of the barrier layer compared with Example 1, the rest of the steps and parameters are the same, and this comparative example will not be repeated. The final packaging bag is obtained.

[0067] Comparative Example 2:

[0068] The comparative example is the same as example 1 except that the "metallocene polyethylene" is replaced by "commercial traditional polyethylene", and the rest of the steps and parameters are the same, which will not be repeated. Finally, a packaging bag is obtained.

[0069] Comparative Example 3:

[0070] The comparative example is the same as example 1 except that the thickness ratio of the barrier layer, heat-seal layer and reinforcing layer is adjusted from 10 μm:22 μm:13 μm to 15 μm:20 μm:10 μm, and the rest of the steps and parameters are the same, which will not be repeated. Finally, a packaging bag is obtained.

[0071] Comparative Example 4:

[0072] The comparative example is the same as example 1 except that the order of the barrier layer, heat-seal layer and reinforcing layer is adjusted to heat-seal layer, barrier layer and reinforcing layer, and the rest of the steps and parameters are the same, which will not be repeated. Finally, a packaging bag is obtained.

[0073] Performance test: the packaging bags prepared in examples 1-4 and comparative examples 1-4 are subjected to the following performance tests:

[0074] Oxygen barrier coefficient test, according to GB / T 1038-2000 "Plastics - Determination of gas transmission rate - Pressure difference method", using a pressure difference type gas permeation instrument, at a temperature of 23℃, a relative humidity of 0%, a pressure difference of 0.1 MPa, an effective area of the sample of 50 cm 2 , a test time of 24 h, by monitoring the volume of oxygen permeating through the sample, the oxygen barrier coefficient is calculated (unit: cm 3 / (m 2 ·24h·0.1MPa));

[0075] Water vapor transmission rate test, according to GB / T 1037-1988 "Plastics - Determination of water vapour transmission rate - Cup method", using a water vapor transmission rate tester, at a temperature of 23℃, a relative humidity of 50%, a sample thickness of 50±5 μm, a test time of 48 h, by weighing the mass change of the moisture cup, the water vapor transmission rate is calculated (unit: g / (m 2 ·24h).

[0076] Tensile strength and elongation at break, according to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: test conditions for films and sheets", using an electronic universal testing machine, sample size 150 mm x 15 mm, clamp distance 100 mm, tensile speed 50 mm / min, test 5 times and take the average value.

[0077] Heat seal strength test, according to the standard of ASTM D3354 "Heat seal strength test method of heat-sealed plastic film", using heat sealing machine (set temperature 170℃, pressure 0.4MPa, time 2s), peel tester, after heat sealing the sample, peel at 180°, speed 100mm / min, record the average peel force (unit: N / 15mm).

[0078] Interlayer peel strength, according to the standard of ASTM D3359 "Adhesive peel strength test method by tape test", using peel tester, peel along the interlayer interface, speed 100mm / min, record the average peel force (unit: N / 15mm), 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 interface properties, while in Comparative Example 1, maleic anhydride grafted polyethylene is not added, the interface compatibility is poor, and the oxygen barrier coefficient rises to 0.45, which shows that the compatibilizer has a key effect on the densification between layers; in Comparative Example 4, the adjustment of the layer sequence leads to the weakening of the support effect of the reinforcing layer, and the tensile strength and elongation at break decrease, which proves the direct influence of structural design on mechanical properties.

[0083] The present application provides a food-grade high-barrier packaging bag and a preparation method thereof. The present application solves the problem that the barrier layer is easily weakened in the traditional structure by reasonably arranging the order of the barrier layer, the heat-sealing layer and the reinforcing layer, and combining the selection of food-grade materials, while meeting the multiple requirements of safety, weather resistance and processing performance of food packaging. The present application realizes efficient barrier to oxygen and water vapor under the premise of not significantly increasing the cost, provides a longer-lasting preservation solution for pre-prepared food, meat, baked food, etc., and has important practical application value.

[0084] In the three-layer co-extrusion process, the materials of the barrier layer, the heat-sealing layer and the reinforcing layer can produce a synergistic effect of performance complementation or enhancement through interfacial chemical reaction, compatibility optimization, crystallization induction and nano synergistic effect. The specific reaction and promotion mechanism are as follows:

[0085] Interfacial chemical reaction: form chemical bond connection:

[0086] Esterification between barrier layer and heat-seal layer: Maleic anhydride grafted polyethylene (PE-g-MAH) in the barrier layer contains polar maleic anhydride groups (-COOH), which can react with the hydroxyl groups (-OH) of EVOH in the barrier layer to form ester bonds when it contacts with the non-polar LDPE / mPE of the heat-seal layer. Performance promotion: interfacial tension is reduced, interlayer peeling strength is improved (better than pure physical compounding); a "gradient compatible transition zone" is formed, reducing interlayer defects and avoiding gas penetration from the interface.

[0087] Hydrogen bonding between reinforcing layer and barrier layer: PGA (polyglycolic acid) in the reinforcing layer contains hydroxyl groups (-OH) and ester groups (-COO-), which form intermolecular association with the hydroxyl groups of EVOH in the barrier layer through hydrogen bonding (-OH…O-). Performance promotion: PGA induces EVOH to form more regular alpha crystal form, improving crystallinity and oxygen barrier properties; a "rigidity-toughness" transition layer is formed at the interface, improving tensile strength.

[0088] Optimization of compatibility: reduction of phase separation

[0089] Bridge effect of PE-g-MAH: As an amphiphilic compatibilizer, the polyethylene segment of PE-g-MAH is compatible with LDPE / mPE of the heat-seal layer, and the maleic anhydride group is compatible with EVOH of the barrier layer, forming a "core-shell" structure wrapping nano-montmorillonite. Performance promotion: the particle size of nano-montmorillonite in the barrier layer is reduced, the specific surface area is increased, and the gas diffusion path is lengthened; the melt viscosity matching degree of the heat-seal layer and the barrier layer is improved, and the layer thickness uniformity error is small during co-extrusion.

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

[0091] Nanometer and crystallization synergistic effect:

[0092] Cross-layer diffusion of nano-montmorillonite: Under the action of shear force during co-extrusion, part of the nano-montmorillonite layers in the barrier layer migrate to the interface between the heat-seal layer and the reinforcing layer, forming a "nano-bridging" structure. Performance promotion: the puncture resistance of the heat-seal layer is improved (the montmorillonite layers hinder crack propagation); the water vapor transmission rate is reduced (the montmorillonite layers increase the diffusion path of water molecules).

[0093] Crystallization induction of EVOH by PGA: The high crystallinity of PGA (melting point 220℃) acts as a "nucleating agent" during the co-extrusion cooling process, promoting EVOH to form more compact lamellar crystals. Performance promotion: EVOH crystallinity is improved, oxygen permeability is reduced; the ordered arrangement of crystalline regions reduces polymer chain segment movement, and the high temperature resistance is improved (glass transition temperature is increased).

[0094] Dynamic synergy in processing technology:

[0095] Melt viscosity matching and layer thickness control synergy mechanism: The melt viscosity gradient design (viscosity ratio 1:1.5:2) of the heat-sealing layer (LDPE / mPE, low viscosity) and the barrier layer (EVOH / modified PE, medium viscosity), the reinforcing layer (PGA / BOPP, high viscosity) forms a stable laminar flow distribution in the co-extrusion die. Performance promotion: Reduce interlayer "melt fracture" defects, reduce the surface roughness of the composite film by 20%, improve the printing adaptability; the thickness uniformity error of each layer is small, which avoids the barrier failure caused by local thinness.

[0096] Antioxidant cross-layer migration protection mechanism: The antioxidant (such as 1010) in the barrier layer partially migrates to the heat-sealing layer and the reinforcing layer during high-temperature co-extrusion, forming a "gradient antioxidant protection". Performance promotion: The oxidation induction time (OIT) of the whole film is prolonged, which is suitable for packaging of food containing oil; The mPE of the heat-sealing layer is protected by the antioxidant, and the heat-sealing strength retention rate is higher during long-term storage.

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

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

[0099] Mechanical property: 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 skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to suggest that the scope of the present application is limited to these examples; under the idea of the present application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the present application as described above. In order to be brief, they are not provided in detail.

[0103] The present application is intended to cover all such alternatives, modifications, and variations as fall within the broad scope of the appended claims. Accordingly, any and all such alternatives, modifications, equivalents, improvements and the like are intended to be encompassed by the present application.

Claims

1. A food-grade high barrier packaging bag, characterized in that: The food-grade high-barrier packaging bag comprises a barrier layer, a heat-sealing layer and a reinforcement 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 seal layer comprises the following raw materials: linear low density polyethylene, metallocene polyethylene and a slip agent; The reinforcement layer comprises the following materials: polyglycolic acid and biaxially oriented polypropylene; The thickness ratio of the barrier layer, heat seal layer and reinforcement layer is 10-12 μm: 22-25 μm: 13-15 μm; 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; The mass ratio of the linear low-density polyethylene, metallocene polyethylene and lubricant is 60-70:20-30:0.3-0.8; The mass ratio of the polyglycolic acid to the biaxially oriented polypropylene is 5-15:10-20.

2. The food-grade high-barrier packaging bag according to claim 1, characterized in that: The antioxidant is antioxidant 1010.

3. The food-grade high-barrier packaging bag according to claim 1, characterized in that: The lubricant is any one of oleamide and erucamide.

4. The method for preparing a food-grade high-barrier packaging bag according to any one of claims 1 to 3, characterized in that: The following steps are involved: Step S1. Co-extrusion casting: The prepared raw materials for the barrier layer, heat seal layer, and reinforcement layer are added to the three hoppers of a three-layer co-extruder for co-extrusion casting. During the extrusion process, the raw materials of each layer converge at the die head to form a three-layer composite film through a casting process; Step S2. performing a solvent-free corona treatment on the extruded composite film; Step S3. Slitting and bagging: the corona-treated composite film is cut into the required width and length by a slitting machine, and the cut composite film is made into packaging bags using a heat sealing machine. The edges of the composite film are sealed by a heat sealing process to form packaging bags with a certain shape and size.

5. The method for preparing a food-grade high-barrier packaging bag according to claim 4, characterized in that: The temperature of the co-extruder in step S1 is 180-220° C., and the screw speed is 60-90 rpm.

6. The method for preparing a food-grade high-barrier packaging bag according to claim 4, characterized in that: The power of the corona treatment in step S2 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 seconds.

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