High temperature retortable printable polyethylene blown film and method of making same

Abstract

The application discloses a high-temperature-resistant and printable polyethylene blown film and a preparation method thereof, and belongs to the technical field of plastic packaging materials, and aims to solve the technical problem that traditional polyethylene films cannot simultaneously consider high-temperature-resistant and printing adaptability. The blown film comprises a printing layer, an intermediate layer and a heat-sealing layer which are stacked in sequence, a dynamic covalent cross-linking network is constructed by using an epoxy modifier to improve heat resistance, a three-layer differential formula design is used to ensure printability, and a three-section temperature control extrusion and a two-way stretching blowing process are combined to optimize the microstructure. The film prepared by the application can withstand 121 DEG C high-temperature cooking for 30 min, has excellent printing adhesion, balanced mechanical properties and optical properties, and has the advantages of simple process, low cost, and can be widely applied to high-temperature cooking packaging in the fields of food, medicine and the like.

Description

Technical Field

[0001] This invention relates to the field of plastic packaging materials technology, and in particular to a high-temperature resistant, retortable, printable polyethylene blown film and its preparation method. Background Technology

[0002] With the rapid development of the food and pharmaceutical industries, high-temperature retort sterilization technology has been widely used in the packaging of cooked food, meat products, dairy products, and pharmaceutical consumables due to its advantages such as thorough sterilization, long shelf life, and no chemical residues. Polyethylene (PE) film, with its excellent flexibility, heat-sealing properties, chemical resistance, and low cost, has become one of the most widely used plastic film materials in the packaging industry. However, conventional polyethylene film has a low glass transition temperature, and its molecular chains are prone to relaxation and movement at high temperatures. This makes it unable to withstand the 121°C high-temperature retort sterilization conditions, resulting in severe heat shrinkage, deformation, cracking, and even delamination after retort sterilization. This seriously affects the sealing of the packaging and the safety of the product, greatly limiting its application in the field of high-temperature retort packaging.

[0003] To address the poor high-temperature retortability of polyethylene films, existing technologies primarily employ multilayer composite structures or chemical cross-linking modifications. Multilayer composite structures typically combine polyethylene with high-temperature resistant resins such as polyamide (PA) and polypropylene (PP), utilizing the resin for structural support. While this can improve the film's heat resistance to some extent, it suffers from drawbacks including complex processes, long production cycles, high costs, easy delamination at the composite interface, and difficulties in recycling. Furthermore, the composite process requires large amounts of organic solvents, posing serious environmental problems. Peroxide chemical cross-linking modification improves heat resistance by initiating free radical cross-linking reactions in the polyethylene molecular chains with peroxides, forming a three-dimensional network structure. However, this method suffers from difficulties in controlling the degree of cross-linking, increased film brittleness, and decreased processing performance. Moreover, peroxide residues can affect the safety of food and pharmaceuticals.

[0004] Furthermore, existing high-temperature resistant polyethylene films generally suffer from insufficient printability. To improve the film's opening properties and smoothness, traditional formulations typically add low-molecular-weight additives such as erucamide and silica to all layers. These additives gradually migrate to the surface after film formation, forming a low-energy lubricating layer. This leads to decreased surface tension, poor ink adhesion, and problems such as ink smudging, bleed-through, and pattern distortion, failing to meet the demands of modern packaging for high-quality printing. Currently, the industry lacks a single-layer polyethylene blown film technology that simultaneously achieves excellent high-temperature retortability and high-precision printability. Developing a simple, low-cost, and balanced high-temperature retortable printable polyethylene blown film has become a pressing technical challenge in the packaging materials field. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a high-temperature resistant, retortable, printable polyethylene blown film and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a high-temperature resistant, retortable, printable polyethylene blown film, comprising, from top to bottom, a printing layer, an intermediate layer, and a heat-sealing layer stacked sequentially.

[0007] Preferably, the printed layer comprises, by weight parts: 75-85 parts of high-density metallocene polyethylene, 15-25 parts of high-density polyethylene, 0.5-1.5 parts of epoxy modifier, and 0.2-0.5 parts of composite heat stabilizer; Preferably, the intermediate layer comprises, by weight parts: 35-45 parts of high-density metallocene polyethylene, 55-65 parts of high-density polyethylene, 1.0-2.0 parts of epoxy modifier, and 0.3-0.6 parts of composite heat stabilizer; Preferably, the heat-sealing layer comprises, by weight, 97-99 parts of high-density metallocene polyethylene, 0.6-1.0 parts of erucamide, 0.6-1.0 parts of silica, 0.3-0.8 parts of epoxy modifier, and 0.2-0.4 parts of composite heat stabilizer.

[0008] Preferably, the high-density metallocene polyethylene has a melt index of 0.8-1.5 g / 10 min at 190°C and 2.16 kg, and a density of 0.940-0.950 g / cm³. 3 The weight-average molecular weight is 150,000-250,000, and the molecular weight distribution index is 2.0-3.0.

[0009] Preferably, the high-density polyethylene has a melt index of 0.5-1.2 g / 10 min at 190°C and 2.16 kg, and a density of 0.950-0.960 g / cm³. 3 The weight-average molecular weight is 200,000-300,000, and the molecular weight distribution index is 3.0-4.0.

[0010] Preferably, the epoxy modifier is selected from at least one of bisphenol A type epoxy resin E-44, bisphenol A type epoxy resin E-51, or epoxidized soybean oil.

[0011] Preferably, the composite heat stabilizer is composed of calcium stearate, zinc stearate and triphenyl phosphite in a mass ratio of 2:1:1.

[0012] Preferably, the purity of the erucamide is ≥98%.

[0013] Preferably, the silicon dioxide is fumed silicon dioxide with an average particle size of 10-20 nm.

[0014] Preferably, the thickness of the high-temperature resistant, retortable, printable polyethylene blown film is 50-80 μm.

[0015] Furthermore, the present invention also provides a method for preparing a high-temperature resistant, retortable, printable polyethylene blown film, comprising the following steps: S1 Raw Material Weighing: Weigh the raw materials for the printing layer, intermediate layer and heat-sealing layer according to the formula; S2 Mixing: Add each layer of raw materials to the mixer separately, and mix at 300-500 r / min for 5-10 min to obtain the mixture of each layer; S3 Melt Plasticization: The mixture of each layer is added to the corresponding screw of the three-layer co-extrusion blown film machine, and melt plasticization is carried out using a three-stage temperature-controlled extrusion process; S4 die forming: The molten material from each layer merges in the three-layer co-extrusion die to form a three-layer melt; S5 gradient blow molding: The three-layer melt is inflated and stretched using biaxial stretch blow molding technology, while being cooled and shaped simultaneously; S6 Surface Treatment: Perform surface corona treatment on the cooled and shaped film; S7 Cutting and Rewinding: The processed film is cut and rewound to obtain a high-temperature resistant, retortable, printable polyethylene blown film.

[0016] Preferably, the temperature parameters of the three-stage temperature-controlled extrusion process in S3 are: feeding stage 150-160℃, melting stage 170-180℃, and homogenization stage 185-195℃.

[0017] Preferably, the temperature of the three-layer co-extrusion die in S43 is 190-200℃.

[0018] Preferably, the parameters of the biaxial stretch blow molding technology in S5 are: longitudinal stretch ratio 3.0-4.0, and transverse blow ratio 2.5-3.5.

[0019] Preferably, in step S5, the cooling and shaping process uses a cooling air ring with a temperature of 15-25℃ and a cooling air velocity of 8-12m / s.

[0020] Preferably, the power density of the surface corona treatment in S6 is 30-40 W·min / m². 2 The surface tension of the treated film is ≥38mN / m.

[0021] Preferably, the mechanism of action of the high-temperature resistant, retortable, printable polyethylene blown film of the present invention is explained as follows: This invention systematically solves the shortcomings of traditional polyethylene blown films in simultaneously achieving high-temperature cooking stability and high-precision printing adaptability by synergistically regulating resin matrix compounding, dynamic covalent cross-linking network construction, three-layer functional differentiation design, and gradient blow molding process, from molecular chain movement and microstructure evolution to macroscopic performance matching. It achieves a unity of heat resistance, mechanical properties, printability, and processability.

[0022] At the molecular structure design level, the gradient blending of high-density metallocene polyethylene (MLLDPE) and high-density polyethylene (HDPE) lays the foundation for the film's performance. MLLDPE, with its narrow molecular weight distribution and uniform short-branched structure resulting from single-active-center catalytic synthesis, exhibits high molecular chain entanglement density and few crystal defects, endowing the film with excellent low-temperature toughness, puncture resistance, and heat-sealing interfacial adhesion. HDPE, with its high linearity and high crystallinity molecular chain characteristics, provides the film with the required stiffness and heat deformation resistance. The two are distributed in different proportions in the three-layer structure, achieving a precise division of functions for each layer: the high proportion of MLLDPE in the printing layer ensures surface uniformity, the high proportion of HDPE in the intermediate layer provides structural support, and the high proportion of MLLDPE in the heat-sealing layer ensures heat-sealing performance. Furthermore, the introduction of epoxy modifiers is the core breakthrough in achieving high-temperature retort resistance: during melt processing at 180-200℃, epoxy groups undergo ring-opening addition reactions with trace amounts of hydroxyl and carboxyl groups at the ends of the polyethylene molecular chains, as well as oxidative active sites generated during processing, forming a dynamic covalent cross-linked network with COC bonds as the connecting units. This cross-linked network exhibits unique temperature response characteristics: at processing temperatures, the cross-links can undergo reversible breakage and recombination, preventing excessively high melt viscosity that could affect blow molding; while under cooking conditions, the cross-links remain thermodynamically stable, limiting the Brownian motion and deorientation behavior of polyethylene molecular chains, and simultaneously inhibiting secondary crystallization and spherulite coarsening at high temperatures, eliminating the thermal shrinkage, deformation, and cracking problems caused by molecular chain relaxation after cooking of traditional PE films. The composite heat stabilizer, through the synergistic effect of calcium stearate / zinc, blocks the thermal oxidative degradation chain reaction of polyethylene, protecting the integrity of the dynamic cross-linked network and further extending the film's high-temperature resistance time.

[0023] In terms of interface performance control, the three-layer differentiated formulation design achieves compatibility between high-temperature resistance and printability. The printing layer strictly eliminates low-molecular-weight additives such as erucamide and silica, avoiding the migration, accumulation, and blooming of additives on the film surface, ensuring the uniformity of the chemical composition and the smoothness of the microstructure of the printing layer surface. Simultaneously, the high-purity MLLDPE / HDPE composite system has moderate surface energy. After corona treatment, a large number of polar functional groups such as hydroxyl and carbonyl groups can be introduced onto the surface. These functional groups can chemically bond with acrylates, polyurethanes, and other resins in the ink, forming a mechanical interlocking effect with the appropriate micro-roughness of the film surface. This is suitable for gravure and flexographic printing, without causing ink smudging, bleeding, or pattern distortion. The intermediate layer, as the structural support layer, increases the amount of HDPE and epoxy modifier, forming a network structure with a higher cross-linking density. This gives the film excellent overall rigidity and creep resistance, enabling it to withstand the internal and external pressure differences and thermal stress during high-temperature cooking, preventing wrinkles, delamination, and cracking. The heat-sealing layer is mainly composed of MLLDPE, with the addition of erucamide and fumed silica controlled at around 800ppm. This results in a uniform, smooth film with a single-layer thickness and nanoscale anti-adhesion protrusions on the film surface, ensuring smooth operation of the film on high-speed packaging production lines and avoiding the decrease in heat-sealing strength and printing contamination caused by excessive migration of additives.

[0024] At the microstructure forming level, the gradient process of three-stage temperature-controlled extrusion and biaxial stretch blow molding achieves the control of the condensed-state structure of the film. The three-stage temperature-controlled extrusion process, through a stepped heating curve, ensures that the resin is fully preheated and transported in the feeding section, avoiding raw material agglomeration and screw slippage; in the melting section, it gradually melts and plasticizes, ensuring complete melting of the resin crystalline regions; and in the homogenization section, it achieves stable melt viscosity and a uniform temperature field, while providing sufficient reaction time and energy for the crosslinking reaction between the epoxy modifier and the resin, ensuring that crosslinking points are evenly distributed throughout the matrix and avoiding increased film brittleness caused by excessive local crosslinking. The biaxial stretch blow molding technology causes the polyethylene molecular chains to orient simultaneously in both the longitudinal and transverse directions, forming a highly ordered interpenetrating network structure of transverse and lamellar crystals. This orientation structure not only improves the tensile strength and impact resistance of the film but also further restricts the mobility of molecular chains at high temperatures, reducing the heat shrinkage rate during cooking. Precise temperature control and wind speed adjustment of the cooling air ring enable rapid and uniform cooling of the film, freezing the orientation structure of the molecular chains and preventing excessive crystal growth and coarsening of spherulites. This ensures that the film has low haze and high light transmittance, while keeping the surface roughness within the optimal range required for printing.

[0025] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention introduces an epoxy modifier into the polyethylene matrix to construct a dynamic covalent cross-linked network with temperature response characteristics during melt processing. This network can undergo reversible fracture and recombination at the processing temperature without affecting the blow molding performance of the film. It maintains thermodynamic stability under high-temperature cooking conditions, effectively restricting the Brownian motion and deorientation behavior of polyethylene molecular chains, inhibiting high-temperature secondary crystallization and spherulite coarsening, fundamentally solving the problems of thermal shrinkage, deformation, and cracking of traditional polyethylene films after cooking, and significantly improving the film's high-temperature cooking stability.

[0026] 2. This invention employs a three-layer functional differentiated formulation design. The printing layer strictly eliminates low molecular weight additives such as erucamide and silica, avoiding surface energy reduction and printing defects caused by additive migration. This ensures the uniformity of the chemical composition and the smoothness of the microstructure of the printing layer surface. After corona treatment, it can form a strong chemical bond and mechanical bond with the ink resin, giving the film excellent printability. It can perfectly adapt to gravure and flexographic printing processes, resulting in high pattern clarity, strong adhesion, and no ink smudging or blurring.

[0027] 3. This invention combines a three-stage temperature-controlled extrusion and biaxial stretch blow molding gradient forming process to precisely control the condensed state structure of the film, causing the polyethylene molecular chains to undergo uniform orientation in both the longitudinal and transverse directions, forming a highly regular interpenetrating network structure of tandem crystals and lamellar crystals. With the synergistic effect of the dynamic covalent cross-linked network, the tensile strength, impact strength, and elongation at break of the film are significantly improved. At the same time, by controlling the cooling process parameters, the excessive growth of spherulites is suppressed, giving the film low haze and high light transmittance, achieving the best balance between mechanical and optical properties.

[0028] 4. This invention employs a three-layer co-extruded single-layer film structure, eliminating the need for multi-layer lamination with other high-temperature resistant resins. This simplifies the production process, shortens the production cycle, reduces production costs, and avoids the use of organic solvents during the lamination process, making it more environmentally friendly. Furthermore, through optimized additive formulation, this invention balances the film's heat resistance, flexibility, printability, and processability. The film thickness is controllable, and its performance is stable, making it widely applicable to high-temperature retort packaging in various fields such as cooked food, meat products, and pharmaceutical consumables, with broad market application prospects. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0030] Example 1: A specific method for preparing a high-temperature resistant, retortable, printable polyethylene blown film, comprising the following steps: Raw material formula Printing layer: MLLDPE: 750g, HDPE: 250g, bisphenol A epoxy resin E-44: 5g, composite heat stabilizer: 2g Intermediate layer: HDPE: 550g, MLLDPE: 450g, bisphenol A type epoxy resin E-44: 10g, composite heat stabilizer: 3g Heat-sealing layer: MLLDPE: 990g, erucamide: 6g, fumed silica: 6g, bisphenol A epoxy resin E-44: 3g, composite heat stabilizer: 2g Raw material parameters: MLLDPE melt index 0.8 g / 10 min (190℃, 2.16 kg), density 0.940 g / cm³ 3 The weight-average molecular weight is 150,000, and the molecular weight distribution index is 2.0; the HDPE melt index is 0.5 g / 10 min (190℃, 2.16 kg), and the density is 0.950 g / cm³. 3 Weight-average molecular weight 200,000, molecular weight distribution index 3.0; erucamide purity 98%; fumed silica specific surface area 150 m². 2 / g, average particle size 10nm; the composite heat stabilizer is calcium stearate: zinc stearate: triphenyl phosphite = 2:1:1.

[0031] Preparation process S1. Accurately weigh the raw materials for each layer according to the formula; S2 raw materials are added to a high-speed mixer and stirred at 300 rpm for 10 minutes until they are evenly mixed. S3 three-layer co-extrusion blown film mill screw temperatures: feeding section 150℃, melting section 170℃, homogenization section 185℃, die head temperature 190℃; S4 biaxial stretch blow molding: longitudinal stretch ratio 3.0, transverse blow ratio 2.5, cooling air ring temperature 15℃, cooling air velocity 8m / s; S5 corona treatment: power density 30W·min / m 2 The surface tension after treatment is 38 mN / m; S6 cutting and winding: winding tension 10N, winding speed 80m / min, to obtain a high-temperature resistant, retortable, printable polyethylene blown film with a total thickness of 50μm.

[0032] Example 2: A specific method for preparing a high-temperature resistant, retortable, printable polyethylene blown film, comprising the following steps: Raw material formula Printing layer: MLLDPE: 800g, HDPE: 200g, bisphenol A epoxy resin E-51: 10g, composite heat stabilizer: 3.5g Intermediate layer: HDPE: 600g, MLLDPE: 400g, bisphenol A epoxy resin E-51: 15g, composite heat stabilizer: 4.5g Heat-sealing layer: MLLDPE: 984g, erucamide: 8g, fumed silica: 8g, bisphenol A epoxy resin E-51: 5.5g, composite heat stabilizer: 3g Raw material parameters: MLLDPE melt index 1.15 g / 10 min (190℃, 2.16 kg), density 0.945 g / cm³ 3 The weight-average molecular weight is 200,000, and the molecular weight distribution index is 2.5; the HDPE melt index is 0.85 g / 10 min (190℃, 2.16 kg), and the density is 0.955 g / cm³. 3 Weight-average molecular weight 250,000, molecular weight distribution index 3.5; erucamide purity 98%; fumed silica specific surface area 175 m². 2 / g, average particle size 15nm; the composite heat stabilizer is calcium stearate: zinc stearate: triphenyl phosphite = 2:1:1.

[0033] Preparation process S1. Accurately weigh the raw materials for each layer according to the formula; S2 raw materials are added to a high-speed mixer and stirred at 400 rpm for 8 minutes until they are evenly mixed. S3 three-layer co-extrusion blown film mill screw temperatures: feeding section 155℃, melting section 175℃, homogenization section 190℃, die head temperature 195℃; S4 biaxial stretch blow molding: longitudinal stretch ratio 3.5, transverse blow ratio 3.0, cooling air ring temperature 20℃, cooling air velocity 10m / s; S5 corona treatment: power density 35W·min / m 2 The surface tension after treatment is 40 mN / m; S6 cutting and winding: winding tension 12N, winding speed 100m / min, to obtain a high-temperature resistant, retortable, printable polyethylene blown film with a total thickness of 65μm.

[0034] Example 3: A specific method for preparing a high-temperature resistant, retortable, printable polyethylene blown film, comprising the following steps: Raw material formula Printed layer: MLLDPE: 850g, HDPE: 150g, Epoxidized soybean oil: 15g, Composite heat stabilizer: 5g Intermediate layer: HDPE: 650g, MLLDPE: 350g, epoxidized soybean oil: 20g, composite heat stabilizer: 6g Heat-sealing layer: MLLDPE: 970g, erucamide: 10g, fumed silica: 10g, epoxidized soybean oil: 8g, composite heat stabilizer: 4g Raw material parameters: MLLDPE melt index 1.5 g / 10 min (190℃, 2.16 kg), density 0.950 g / cm³ 3 The weight-average molecular weight is 250,000, and the molecular weight distribution index is 3.0; the HDPE melt index is 1.2 g / 10 min (190℃, 2.16 kg), and the density is 0.960 g / cm³. 3 Weight-average molecular weight 300,000, molecular weight distribution index 4.0; erucamide purity 98%; fumed silica specific surface area 200 m². 2 / g, average particle size 20nm; the composite heat stabilizer is calcium stearate: zinc stearate: triphenyl phosphite = 2:1:1.

[0035] Preparation process S1. Accurately weigh the raw materials for each layer according to the formula; Add each layer of raw materials to a high-speed mixer and mix at 500 rpm for 5 minutes until homogeneous. S3 three-layer co-extrusion blown film mill screw temperatures: feeding section 160℃, melting section 180℃, homogenization section 195℃, die head temperature 200℃; S4 biaxial stretch blow molding: longitudinal stretch ratio 4.0, transverse blow ratio 3.5, cooling air ring temperature 25℃, cooling air velocity 12m / s; S5 corona treatment: power density 40W·min / m 2 The surface tension after treatment is 41 mN / m; S6 cutting and winding: winding tension 15N, winding speed 120m / min, to obtain a high-temperature resistant, retortable, printable polyethylene blown film with a total thickness of 80μm.

[0036] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that 0.8 parts of erucamide and 0.8 parts of fumed silica were added to the printing layer formulation, while the other raw material formulations and preparation processes were exactly the same.

[0037] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that no epoxy modifiers are added to the printing layer, intermediate layer and heat-sealing layer, while the other raw material formulations and preparation processes are exactly the same.

[0038] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the longitudinal stretching step is omitted in the preparation process, and only transverse blowing is used. The blowing ratio is kept at 3.0, and the other raw material formulations and process parameters are exactly the same.

[0039] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that Comparative Example 4 uses commercially available mainstream 121℃ retort grade packaging film, with a structure of PE / PA / PE layer dry composite, a total thickness of 65μm, and the printing layer is ordinary low-density polyethylene film.

[0040] Performance testing: All samples were placed in a standard environment of 23℃ and 50% relative humidity for 24 hours before testing. For the boiling performance test, a high-pressure steam sterilizer was used. Samples were cut into 100mm × 100mm squares, marked with longitudinal and transverse dimensions, and placed in the sterilizer. They were boiled at 121℃ and 0.1MPa for 30 minutes. After cooling to room temperature, the dimensions were measured again, and the heat shrinkage rate and appearance were calculated. For the printing performance test, polyurethane gravure ink was used to print a full-page pattern on the sample's printing layer. After drying for 24 hours, a 1mm × 1mm grid was drawn on the printed surface using a cross-cutting tool. 3M 600 tape was tightly adhered to the grid and quickly peeled off. The adhesion level was assessed based on the amount of ink residue in the grid. For the heat seal strength test, a heat sealer was used. The samples were heat-sealed for 1 second at 150℃ and 0.3MPa pressure. After cooling, they were cut into 15mm wide specimens and subjected to tensile testing at 300mm / min on a universal testing machine. The maximum force at the heat seal point was recorded. For mechanical property testing, the samples were cut into dumbbell-shaped specimens according to standard requirements and subjected to tensile testing at a speed of 500 mm / min on a universal testing machine. Tensile strength and elongation at break were recorded. For haze testing, a haze meter was used, and five points were randomly selected on the sample for testing, with the average value taken. The experimental results are shown in Table 1.

[0041] Table 1 Performance Test Results

[0042] Data Analysis: As can be seen from the performance test data in Table 1, all three embodiments of the present invention exhibit excellent comprehensive performance, can stably withstand high-temperature cooking at 121°C for 30 minutes without significant deformation or cracking, and possess excellent printability and balanced mechanical and optical properties; among them, the performance indicators of Embodiment 2 achieve the best balance and have the best comprehensive performance, making it the preferred embodiment of the present invention with the greatest industrial application value.

[0043] Regarding heat shrinkage and cooking stability, Example 2 constructed a dynamic covalent cross-linked network with moderate density and uniform distribution by controlling the amount of epoxy modifier added and the biaxial stretching process parameters. At the same time, it enabled the polyethylene molecular chains to form the most regular orientation structure in the longitudinal and transverse directions. The synergistic effect of the two maximized the restriction of the Brownian motion and deorientation behavior of the molecular chains at high temperature. Therefore, the heat shrinkage rate was the lowest after cooking and there were no abnormalities in appearance. Although Comparative Example 1 added slip agents and anti-blocking agents to the printing layer, these additives do not participate in the crosslinking reaction or affect the molecular chain orientation. Therefore, its heat shrinkage rate is similar to that of Example 2, but other properties have obvious defects. Comparative Example 2, due to the complete absence of epoxy modifiers, cannot form an effective dynamic crosslinking network. At high temperatures, the polyethylene molecular chains can move freely and undergo secondary crystallization, resulting in a significant increase in heat shrinkage rate after cooking and severe deformation and wrinkling. Comparative Example 3 eliminated the longitudinal stretching step, and the molecular chains only oriented in the transverse direction, lacking sufficient structural constraints in the longitudinal direction. Therefore, its heat shrinkage rate was significantly higher than that of Example 2, and slight wrinkling occurred. Comparative Example 4, as a commercially available conventional composite film, neither used epoxy crosslinking modification nor underwent biaxial stretching. The high-temperature movement of the molecular chains could not be effectively restricted, so its heat shrinkage rate was much higher than that of Example 2, and slight deformation problems existed.

[0044] Regarding printing adhesion, Example 2 strictly adhered to the design principle of no additives in the printing layer, avoiding the formation of a low-energy lubricating layer due to the migration of low molecular weight additives to the surface. This ensured the uniformity of the chemical composition and the smoothness of the microstructure of the printing layer surface. After corona treatment, the surface energy was significantly increased and a large number of polar functional groups were introduced, enabling strong chemical and mechanical bonding with the ink resin. Therefore, the printing adhesion reached the highest level. Comparative Example 1 added erucamide and silica in the same amount as the heat-sealing layer to the printing layer. These additives migrated rapidly to the surface after film formation, disrupting the interface between the ink and the substrate, resulting in a significant decrease in printing adhesion and obvious ink stripping. Although Comparative Examples 2 and 3 did not add additives to the printing layer, their printing adhesion was comparable to that of Example 2, but both had defects in heat resistance and mechanical properties. Comparative Example 4 also added a high content of slip agent and anti-blocking agent to the printing layer, resulting in a lower surface energy. Therefore, its printing adhesion was far inferior to that of Example 2 and could not meet the requirements of high-precision printing.

[0045] Regarding heat-sealing strength, the heat-sealing layer of Example 2 uses an optimal ratio of high-density metallocene polyethylene and epoxy modifiers, which ensures that the molecular chains can fully diffuse and fuse to form a continuous heat-sealing interface during the heat-sealing process, and also improves the mechanical strength of the heat-sealing interface through a moderate cross-linking network. Therefore, the heat-sealing strength is at a high level. Comparative Example 1 contains erucamide in both the heat-sealing layer and the printing layer. This type of additive forms an isolation layer at the heat-sealing interface, hindering the mutual diffusion of molecular chains, resulting in a heat-sealing strength slightly lower than that of Example 2. Comparative Example 2 does not contain epoxy modifiers, and the heat-sealing interface lacks the support of a cross-linking network, resulting in weak molecular chain entanglement and thus the lowest heat-sealing strength. Comparative Example 3 does not undergo longitudinal stretching, resulting in a decrease in the overall mechanical properties of the film and a corresponding decrease in the tensile strength of the heat-sealing interface, with a heat-sealing strength slightly lower than that of Example 2. Comparative Example 4 contains a polyamide barrier layer, and its heat-sealing strength is slightly higher than that of Example 2. However, this is an inherent structural advantage of composite films, and it also has many problems such as complex processes, high costs, and poor environmental performance.

[0046] In terms of mechanical properties, Example 2 employed the optimal longitudinal stretch ratio and transverse blow-up ratio, causing the polyethylene molecular chains to undergo uniform orientation in both directions simultaneously, forming a highly regular interpenetrating crystal-lamellar network structure. Combined with the additional mechanical support of the dynamic covalent cross-linked network, both tensile strength and elongation at break reached the highest levels. The small amount of additives added in Comparative Example 1 had almost no effect on the mechanical properties of the polyethylene matrix, therefore its mechanical properties were similar to those of Example 2. Comparative Example 2 lacked the constraint of the cross-linked network, making the molecular chains prone to slippage and breakage under stress, resulting in a significant decrease in both tensile strength and elongation at break. Comparative Example 3 only underwent transverse stretching, resulting in extremely low longitudinal orientation of the molecular chains, leading to a significant reduction in longitudinal tensile strength, and its overall mechanical properties were far inferior to those of Example 2. The polyethylene layer in Comparative Example 4 was not biaxially stretched, and the three-layer composite structure suffered from interfacial stress concentration, thus its mechanical properties were the worst among all samples.

[0047] In terms of optical performance, Example 2 achieved rapid and uniform cooling of the film by controlling the temperature and wind speed of the cooling ring, freezing the orientation structure of the molecular chains and inhibiting the excessive growth of spherulites. This resulted in a fine and uniformly distributed crystalline structure within the film, significantly reducing light scattering and thus achieving the lowest haze and best light transmittance. Comparative Example 1 had a smaller amount of additives, which had little impact on the crystalline structure and optical properties of the film, so its haze was similar to that of Example 2. Comparative Example 2 did not form a cross-linked network, making it prone to secondary crystallization during processing and use, leading to coarsening of the spherulites, resulting in significantly higher haze than Example 2. Comparative Example 3, lacking longitudinal stretching, had a low degree of molecular chain orientation, making it easier to form large spherulites during crystallization, thus further increasing its haze. The three-layer composite structure of Comparative Example 4 had two distinct interfaces, generating additional light reflection and scattering, resulting in the highest haze and worst light transmittance among all samples.

[0048] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-temperature resistant, retortable, printable polyethylene blown film, characterized in that, From top to bottom, it includes a printed layer, an intermediate layer, and a heat-sealing layer stacked sequentially. The printed layer comprises, by weight, 75-85 parts of high-density metallocene polyethylene, 15-25 parts of high-density polyethylene, 0.5-1.5 parts of epoxy modifier, and 0.2-0.5 parts of composite heat stabilizer. The intermediate layer comprises, by weight parts: 35-45 parts of high-density metallocene polyethylene, 55-65 parts of high-density polyethylene, 1.0-2.0 parts of epoxy modifier, and 0.3-0.6 parts of composite heat stabilizer; The heat-sealing layer comprises, by weight, 97-99 parts of high-density metallocene polyethylene, 0.6-1.0 parts of erucamide, 0.6-1.0 parts of silica, 0.3-0.8 parts of epoxy modifier, and 0.2-0.4 parts of composite heat stabilizer.

2. The high-temperature resistant, retortable, printable polyethylene blown film according to claim 1, characterized in that, The high-density metallocene polyethylene has a melt index of 0.8-1.5 g / 10 min at 190℃ and 2.16 kg, and a density of 0.940-0.950 g / cm³. 3 The weight-average molecular weight is 150,000-250,000, and the molecular weight distribution index is 2.0-3.

0.

3. The high-temperature resistant, retortable, printable polyethylene blown film according to claim 1, characterized in that, The high-density polyethylene has a melt index of 0.5-1.2 g / 10 min at 190℃ and 2.16 kg, and a density of 0.950-0.960 g / cm³. 3 The weight-average molecular weight is 200,000-300,000, and the molecular weight distribution index is 3.0-4.

0.

4. The high-temperature resistant, retortable, printable polyethylene blown film according to claim 1, characterized in that, The epoxy modifier is selected from at least one of bisphenol A type epoxy resin E-44, bisphenol A type epoxy resin E-51, or epoxy soybean oil; the composite heat stabilizer is composed of calcium stearate, zinc stearate, and triphenyl phosphite in a mass ratio of 2:1:

1.

5. The high-temperature resistant, retortable, printable polyethylene blown film according to claim 1, characterized in that, The purity of the erucamide is ≥98%; the silica is fumed silica with an average particle size of 10-20 nm.

6. The method for preparing a high-temperature resistant, retortable, printable polyethylene blown film according to any one of claims 1-5, characterized in that, Includes the following steps: S1 Raw Material Weighing: Weigh the raw materials for the printing layer, intermediate layer and heat-sealing layer according to the formula; S2 Mixing: Add each layer of raw materials to the mixer separately, and mix at 300-500 r / min for 5-10 min to obtain the mixture of each layer; S3 Melt Plasticization: The mixture of each layer is added to the corresponding screw of the three-layer co-extrusion blown film machine, and melt plasticization is carried out using a three-stage temperature-controlled extrusion process; S4 die forming: The molten material from each layer merges in the three-layer co-extrusion die to form a three-layer melt; S5 gradient blow molding: The three-layer melt is inflated and stretched using biaxial stretch blow molding technology, while being cooled and shaped simultaneously; S6 Surface Treatment: Perform surface corona treatment on the cooled and shaped film; S7 Cutting and Rewinding: The processed film is cut and rewound to obtain a high-temperature resistant, retortable, printable polyethylene blown film.

7. The method for preparing a high-temperature resistant, retortable, printable polyethylene blown film according to claim 6, characterized in that, The temperature parameters of the three-stage temperature-controlled extrusion process in S3 are: feeding stage 150-160℃, melting stage 170-180℃, and homogenization stage 185-195℃.

8. The method for preparing a high-temperature resistant, retortable, printable polyethylene blown film according to claim 6, characterized in that, The temperature of the three-layer co-extrusion die in S43 is 190-200℃.

9. The method for preparing a high-temperature resistant, retortable, printable polyethylene blown film according to claim 6, characterized in that, The parameters of the biaxial stretch blow molding technology in S5 are: longitudinal stretch ratio 3.0-4.0, transverse blow ratio 2.5-3.5; cooling and shaping adopts a cooling air ring with a temperature of 15-25℃ and a cooling air velocity of 8-12m / s.

10. The method for preparing a high-temperature resistant, retortable, printable polyethylene blown film according to claim 6, characterized in that, The power density of the surface corona treatment in S6 is 30-40 W·min / m². 2 The surface tension of the treated film is ≥38mN / m.

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