Seven-layer co-extrusion sealing strip resistant to high-low temperature cycle, and preparation method and application thereof

Abstract

This invention relates to the field of high-temperature cold chain liquid packaging technology, specifically to a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, its preparation method, and its application. It comprises an outer layer A, a second outer layer B, a first intermediate adhesive layer C, a core layer D, a second intermediate adhesive layer c, a second inner layer b, and an inner layer a. It employs a mirrored seven-layer co-extruded structure (A / B / C / D / c / b / a) with the ethylene-vinyl alcohol copolymer core layer as the center of symmetry. This ensures that the outer and inner layers, the second outer and inner layers, and the first and second intermediate adhesive layers are completely symmetrically distributed relative to the core layer in the thickness direction, forming a self-balancing system for thermal shrinkage stress. This improves the interlayer bonding stability of the seven-layer structure under high and low temperature cycling impacts, not only preventing interlayer delamination induced by temperature cycling during long-term storage but also avoiding micro-cracks penetrating the channel due to the accumulation of shrinkage strain at a single interface, thereby improving the overall sealing reliability of the sealing strip under freeze-thaw cycle conditions.

Description

Technical Field

[0001] This invention relates to the field of high-temperature cold chain liquid packaging technology, specifically to a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, its preparation method, and its application. Background Technology

[0002] Currently, the performance requirements for sealing strips in the high-temperature cold chain liquid packaging field focus on resistance to high and low temperature cycling shocks, resistance to brittleness during long-term storage, and stability of interlayer bonding. Liquid foods such as yogurt and milk undergo high-temperature filling, low-temperature cold chain transportation, and freeze-thaw cycles during packaging. Under repeated temperature fluctuations, the sealing strip is prone to heat-sealing layer hardening and loss of elasticity. Traditional multi-layer co-extruded sealing strips often employ asymmetrical structural designs with uneven thickness distribution among functional layers. During heat sealing, differences in thermal shrinkage generate internal stress, leading to warping or localized leakage in the sealing area.

[0003] For example, Chinese Patent Publication No. CN106183294A discloses a composite film material and its preparation method for making liquid packaging sealing strips. This method involves sequentially composited a double-sided corona-electrode biaxially stretched polyester film layer with multiple layers of metallocene polyethylene and low-density polyethylene resin film layers to form a seven-layer structure. The introduction of metallocene polyethylene enhances low-temperature heat-sealing performance, while the polyester film layer strengthens tensile strength. However, for scenarios involving room temperature or short-term cold chains, the method fails to consider the differences in crystallization behavior of different polyethylene materials under high and low temperature cycling conditions. During repeated heating and cooling, the volume shrinkage of each polyethylene layer is uneven due to changes in crystallinity, causing microcracks on the surface of the heat-sealing layer. These microcracks can penetrate the sealing area and form capillary leakage channels. Simultaneously, the polarity difference between the polyester film layer and the polyethylene layer causes the interfacial bonding strength to decrease with increasing temperature cycling, resulting in localized peeling voids between the layers. These voids expand into continuous separation surfaces under pressure, weakening the overall barrier performance of the sealing strip and thus reducing the overall sealing reliability of the seven-layer co-extruded sealing strip.

[0004] Therefore, there is an urgent need for a seven-layer co-extruded sealing strip that can withstand high and low temperature cycles, as well as its preparation method and application. Summary of the Invention

[0005] The purpose of this invention is to provide a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, its preparation method, and its application, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, firstly, this invention provides a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, comprising an outer layer A, a second outer layer B, a first intermediate adhesive layer C, a core layer D, a second intermediate adhesive layer c, a second inner layer b, and an inner layer a, wherein: The outer layer A is made of metallocene polyethylene resin; the second outer layer B is made of low-density polyethylene resin; the first intermediate adhesive layer C is made of modified ethylene-vinyl acetate copolymer (modified EVA); the core layer D is made of ethylene-vinyl alcohol copolymer (EVOH); the second intermediate adhesive layer c is made of modified ethylene-vinyl acetate copolymer (modified EVA); the second inner layer b is made of low-density polyethylene resin; and the inner layer a is made of metallocene polyethylene resin. The aforementioned seven layers form a symmetrical co-extrusion structure A / B / C / D / c / b / a with the core layer D as the center of symmetry. This ensures that the outer layer A and inner layer a, the second outer layer B and second inner layer b, and the first intermediate adhesive layer C and second intermediate adhesive layer c are mirror images of the core layer D in the thickness direction, forming a thermal shrinkage stress balance system. Specifically, the hexene comonomer branches in the metallocene polyethylene resin of the outer layer A and the vinyl acetate segments in the modified ethylene-vinyl acetate copolymer (modified EVA) undergo molecular chain entanglement at the co-extrusion melt interface, forming an interpenetrating transition network structure that eliminates the thermal shrinkage stress difference during heat sealing. Furthermore, the maleic anhydride graft groups in the modified ethylene-vinyl acetate copolymer (modified EVA) and the hydroxyl groups in the ethylene-vinyl alcohol copolymer (EVOH) of the core layer D undergo esterification under high-temperature extrusion conditions, generating a covalent bond interface to prevent cavity expansion caused by interlayer slippage during high and low temperature cycling. Both the outer layer A and the inner layer a have a textured heat-sealing structure on the side away from the core layer D, which can discretize the heat-sealing pressure and suppress the continuous transmission of shrinkage stress, forming a heat shrinkage stress discretization buffer interface.

[0007] Furthermore, the textured heat-sealing structure is composed of continuously distributed textured pits and textured ridges arranged alternately. The depth of the textured pits is 8-15 μm, the width of the textured ridges is 20-40 μm, and the center distance between adjacent textured pits is 150-250 μm. This ensures that the textured ridges provide sufficient heat-sealing pressure during heat sealing without puncturing the substrate. The depth of the textured pits ensures that the molten resin does not completely fill the pits under pressure, leaving the bottom of the pits as a buffer zone for releasing the shrinkage stress of the heat-sealing layer. Moreover, the ratio of the center distance between adjacent textured pits to the width of the textured ridges is 3.75 to 12.5, which enables the textured heat-sealing structure to discretize the thermal shrinkage stress into multiple independent units in the width direction of the sealing strip. The shrinkage deformation of each unit is restricted within the buffer zone of a single textured pit, preventing the stress from continuously accumulating in the length direction of the sealing strip.

[0008] Furthermore, the textured heat-sealing structure employs regularly arranged rhomboid or hexagonal recesses. The acute angle of the rhomboid recesses is 45-60°, and the side length of the hexagonal recesses is 30-50 μm. The acute vertices of the rhomboid recesses are arranged along the length of the sealing strip, so that the shrinkage stress of the heat-sealing layer in the length direction is dispersed along the diagonal of the rhombus. The hexagonal recesses are arranged in a honeycomb topology, with adjacent hexagonal recesses sharing a convex ridge edge, so that the shrinkage strain of the heat-sealing layer in any direction is uniformly constrained by the equilateral convex ridge structure of the hexagonal array, preventing the heat-sealing layer from exhibiting anisotropic microcrack propagation paths.

[0009] Furthermore, the amount of the metallocene polyethylene resin is 12-18 parts; the amount of the low-density polyethylene resin is 10-16 parts; the amount of the modified ethylene-vinyl acetate copolymer (modified EVA) is 4-10 parts; and the amount of the ethylene-vinyl alcohol copolymer (EVOH) is 8-15 parts.

[0010] Furthermore, the outer layer A and the inner layer a each have a thickness of 10-25 μm, the second outer layer B and the second inner layer b each have a thickness of 15-35 μm, the first intermediate adhesive layer C and the second intermediate adhesive layer c each have a thickness of 5-15 μm, the core layer D has a thickness of 8-20 μm, and the total thickness of the sealing strip is 80-150 μm.

[0011] Furthermore, the modified ethylene-vinyl acetate copolymer (modified EVA) includes the following preparation steps: S1.1. Ethylene-vinyl acetate copolymer and maleic anhydride are fed into a twin-screw extruder at a mass ratio of 100:3-100:8. Dicumyl peroxide is added as an initiator at a mass ratio of 0.1-0.5% of the ethylene-vinyl acetate copolymer. The reaction is carried out at 150-180℃ for 2-5 minutes. After extrusion and granulation, maleic anhydride graft-modified ethylene-vinyl acetate copolymer with a grafting rate of 0.8-1.5% is obtained, which is used to provide active anhydride groups for esterification reaction with the ethylene-vinyl alcohol copolymer layer. S1.2. Maleic anhydride-grafted modified ethylene-vinyl acetate copolymer and unmodified ethylene-vinyl acetate copolymer are premixed in a high-speed mixer at a mass ratio of 1:2-1:4 for 10-20 min. Then, antioxidant 1010 and calcium stearate are added at a mass ratio of 0.3-0.8% and 0.2-0.5% respectively. The mixture is stirred at 80-100℃ for 10-20 min to form a blended powder in which the high-graft-rate modified ethylene-vinyl acetate copolymer region and the low-graft-rate unmodified ethylene-vinyl acetate copolymer region are distributed in an island-type phase. The island-type phase allows the high-graft-rate region to preferentially undergo esterification reaction with the ethylene-vinyl alcohol copolymer during co-extrusion, while the low-graft-rate region delays the reaction initiation time. This disperses the total heat release of the esterification reaction across multiple temperature ranges, preventing localized excessive cross-linking caused by instantaneous overshoot of the interface temperature. S1.3. The blended powder is fed into a single-screw extruder and melt-plasticized at 130-160℃ for 1-3 minutes. Then, 0.5-1.2% vinyltrimethoxysilane by mass of the blended powder is added to the melt. After dispersion at a shear rate of 300-600 r / min through a dynamic mixing head, the siloxane groups of vinyltrimethoxysilane preferentially migrate to the surface of the melt, forming a silane-rich layer with a thickness of 20-50 nm on the surface of the modified ethylene-vinyl acetate copolymer. This allows it to form silane-C-H bonds with the hexene comonomer branches of metallocene polyethylene during coextrusion, improving the compatibility of the melt interface between the modified EVA and the metallocene polyethylene layer, and obtaining a surface-functionalized modified ethylene-vinyl acetate copolymer melt. S1.4 The modified ethylene-vinyl acetate copolymer melt is granulated through an underwater granulation system at a granulation water temperature of 30-50℃. After centrifugal drying, it is passed through a 20-mesh sieve to obtain columnar modified ethylene-vinyl acetate copolymer particles with an average particle size of 2-4mm.

[0012] Furthermore, the metallocene polyethylene resin is an ethylene-hexene copolymer with a density of 0.912-0.925 g / cm³ and a melt index of 1.5-3.0 g / 10 min, wherein the mass content of the hexene comonomer is 6-12%, used to construct branching sites for silane-C-H bonding with the silane-rich layer on the surface of the modified ethylene-vinyl acetate copolymer; the low-density polyethylene resin is a long-chain branched polyethylene with a density of 0.918-0.930 g / cm³ and a melt index of 2.0-5.0 g / 10 min; the ethylene-vinyl alcohol copolymer is a barrier resin with an ethylene molar content of 32-44% and a density of 1.12-1.20 g / cm³, the difference between its melt index and that of the low-density polyethylene resin not exceeding 1.5 g / 10 min, used to match the interlayer melt viscosity during multilayer co-extrusion.

[0013] In this invention, firstly, the hexene comonomer branches in the metallocene polyethylene resin of outer layer A undergo silane-C-H bonding with the silane-rich layer on the surface of the modified ethylene-vinyl acetate copolymer at the co-extrusion melt interface, forming a molecular-level anchoring structure. This not only eliminates the internal stress concentration caused by the difference in thermal shrinkage coefficients of different materials during heat sealing, but also allows the maleic anhydride graft groups in the modified EVA to undergo esterification with the hydroxyl groups in the ethylene-vinyl alcohol copolymer of core layer D under high-temperature extrusion conditions, generating a covalently bonded interface. The reaction formula is as follows: Modified EVA - g-MAH + EVOH - OH → Modified The esterification reaction forms an irreversible chemical bond, replacing the traditional physical bond that relies on polar adsorption. It can also effectively improve the interlayer anti-slip peeling ability during high and low temperature cycling, prevent the expansion of interface voids caused by repeated temperature rises and falls, and thus maintain the overall sealing integrity of the seven-layer symmetrical structure under temperature shocks from -20℃ to 121℃.

[0014] Secondly, according to Figure 1 As shown, this invention provides a method for preparing a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, comprising the following steps: S2.1 Seven-layer symmetrical co-extrusion melt distribution: Outer layer A, second outer layer B, first intermediate adhesive layer C, core layer D, second intermediate adhesive layer c, second inner layer b and inner layer a are respectively fed into seven extruders. The melt is distributed by a distributor in a symmetrical stacked flow channel with core layer D as the center, so that outer layer A and inner layer a, second outer layer B and second inner layer b, first intermediate adhesive layer C and second intermediate adhesive layer c are mirrored in the thickness direction relative to core layer D, forming a thermal shrinkage stress balance system, and obtaining a composite melt in which the seven-layer melt has been symmetrically stacked at the die inlet; S2.2 Melt Interface Reaction and Interpenetrating Network Construction: The composite melt is transferred to a T-shaped flat die and held in the flow channel at a die temperature of 190-210℃ for 2-4 seconds. This allows the hexene comonomer branches in the metallocene polyethylene of the outer layer A to undergo silane-hydrogen bonding with the silane-rich layer on the surface of the modified ethylene-vinyl acetate copolymer of the first intermediate adhesive layer C. Simultaneously, the maleic anhydride graft groups of the first intermediate adhesive layer C undergo esterification with the hydroxyl groups of the ethylene-vinyl alcohol copolymer of the core layer D, generating a covalently bonded interface. The low-density polyethylene of the second outer layer B, with its long-chain branched structure, forms a bond between the A layer and the interpenetrating network. The C layers form an elastic stress buffer transition and do not participate in interfacial chemical bonding; the innermost layer b and the second intermediate adhesive layer c, which are symmetrically distributed on the other side of the core layer D, react synchronously with the inner layer a through the same mechanism. The modified ethylene-vinyl acetate copolymer of the second intermediate adhesive layer c forms an esterification bond with the core layer D and a silane-hydrogen bond with the metallocene polyethylene of the inner layer a, respectively. The low-density polyethylene of the innermost layer b also serves as a stress buffer layer between the inner layer a and the second intermediate adhesive layer c. After the seven layers of melt are extruded through the die slit, a sheet-like molten preform with symmetrical stacking direction from the outer layer A to the inner layer a is formed. S2.3, Reticulated Heat-Sealing Structure Imprinting: The sheet-like molten preform is transferred to a three-roll calender. The surface of the upper calender roller is pre-engraved with a laser to form a regularly arranged diamond or hexagonal pit pattern. Under a temperature of 40-60℃ and a linear pressure of 0.3-0.6MPa, the outer layer A and inner layer a of the preform are simultaneously imprinted, so that the heat-sealing layers on both sides form a mirror-symmetrical discrete buffer interface. The shrinkage deformation of each unit is limited to the range of a single reticulated pit, preventing stress from continuously accumulating in the length direction of the sealing strip, resulting in a seven-layer symmetrical co-extruded sheet with double-sided reticulation. S2.4 Gradient Cooling and Shaping: The seven-layer symmetrical co-extruded sheet is sequentially fed into a three-section cooling roller group for cooling and shaping, so that the sheet forms a crystallinity gradient distribution in the thickness direction, decreasing from the highly crystalline area of ​​the textured surface layer to the amorphous area of ​​the core layer. The crystallinity gradient direction intersects perpendicularly with the direction of thermal shrinkage stress transmission, forcing the volume shrinkage behavior of each layer during high and low temperature cycling to be released in stages along the thickness direction, avoiding a single interface from bearing all the shrinkage strain, and finally obtaining a seven-layer co-extruded sealing strip with a surface layer resistant to brittle cracking, a core layer with high barrier properties, and no void expansion channels between layers.

[0015] Furthermore, in S2.4, the three-section cooling roller assembly includes the following three sections: The temperature of the first cooling roller group is 65-75℃, and the cooling residence time is 3-5s. This allows the surface of the outer layer A and the inner layer a to cool down to below the crystallization initiation temperature of metallocene polyethylene first, forming a high crystallinity hardened layer in the ridge area. The bottom of the ridge pit retains a low crystallinity elastic area due to the low contact pressure. The temperature of the second cooling roller is 45-55℃, and the cooling residence time is 6-10s, so that the melt temperature of the outermost layer B, the innermost layer b, the first intermediate adhesive layer C, and the second intermediate adhesive layer c simultaneously passes through the crystallization range of low-density polyethylene and modified ethylene-vinyl acetate copolymer, forming a stress buffer transition layer with medium crystallinity. The temperature of the third cooling roller is 20-30℃, and the cooling residence time is 6-15s, so that the core layer D is cooled to below the glass transition temperature of ethylene-vinyl alcohol copolymer, forming a low-crystallinity, high-barrier amorphous core layer.

[0016] Thirdly, the present invention provides a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, prepared by any one of the above preparation methods, which is applied to high-temperature filling and cold chain packaging of liquid food.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This seven-layer co-extruded sealing strip resistant to high and low temperature cycles, its preparation method, and its application employ a mirror-image seven-layer co-extruded structure (A / B / C / D / c / b / a) with the ethylene-vinyl alcohol copolymer core layer as the center of symmetry. This ensures that the outer and inner layers, the second outer and second inner layers, and the first and second intermediate adhesive layers are completely symmetrically distributed relative to the core layer in the thickness direction, forming a self-balancing system for thermal shrinkage stress. Under high-temperature extrusion conditions, the maleic anhydride graft groups in the modified ethylene-vinyl acetate copolymer on both sides of the core layer undergo esterification with the hydroxyl groups in the ethylene-vinyl alcohol copolymer of the core layer, generating a covalently bonded interface. This prevents voids caused by interlayer slippage during high and low temperature cycles. The cavity expands; the hexene comonomer branches in the outer metallocene polyethylene and the silane-rich layer on the surface of the modified ethylene-vinyl acetate copolymer undergo silane-hydrogen bonding at the co-extrusion melt interface to form a transition layer interpenetrating network structure, eliminating the thermal shrinkage stress difference during heat sealing; while eliminating the internal stress concentration problem during co-extrusion, it effectively improves the interlayer bonding stability of the seven-layer structure under high and low temperature cycling impact, not only preventing interlayer delamination induced by temperature cycling during long-term storage of the sealing strip, but also avoiding microcracks penetrating the channel caused by the accumulation of shrinkage strain at a single interface, thereby improving the overall sealing reliability of the high-temperature cold chain liquid packaging sealing strip under freeze-thaw cycle conditions.

[0018] 2. In the high and low temperature cycling resistant seven-layer co-extruded sealing strip, its preparation method, and its application, a textured heat-sealing structure is set on the side of the outer and inner layers away from the core layer. This structure consists of continuously distributed textured pits and textured ridges arranged alternately. The depth of the textured pits ensures that the molten resin does not completely fill the pits under heat-sealing pressure, leaving the bottom of the pits as a buffer zone for shrinkage stress release. The ratio of the center distance between adjacent textured pits to the width of the textured ridges is controlled within 3.75 to 12.5, so that the heat-sealing pressure is discretized and transmitted through the textured ridges. At the same time, the thermal shrinkage stress in the width direction of the sealing strip is divided into multiple independent units, and the shrinkage deformation of each unit is restricted. Within the buffer zone of a single textured pit, stress is prevented from continuously accumulating along the length of the sealing strip. The textured pits are either rhomboid or hexagonal. The acute vertices of the rhomboids are arranged along the length of the sealing strip, which disperses the shrinkage stress along the diagonal direction. The hexagonal pits are arranged in a honeycomb topology, which uniformly constrains the shrinkage strain in any direction by the equilateral ridge structure. This textured heat-sealing structure maintains the heat-sealing strength while increasing the discrete buffer interface of the shrinkage stress on the surface of the heat-sealing layer. This prevents the sealing strip from developing penetrating microcracks due to the hardening of the heat-sealing layer during high and low temperature cycles, thereby improving the sealing strip's resistance to brittle cracking under repeated freeze-thaw conditions. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the preparation method of the seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to the present invention. Figure 2 This is a line graph showing the heat seal strength of the present invention; Figure 3 This is a line graph showing the heat seal strength retention rate of the present invention. Figure 4 This is a line graph showing the interlayer peel strength of the present invention; Figure 5 This is a line graph showing the attenuation rate of interlayer peel strength in this invention. Figure 6 This is a bar chart showing the density of microcracks that resist brittle fracture according to the present invention. Figure 7 This is a bar chart showing the leakage rate of the liquid encapsulation in this invention. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] This invention provides a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, comprising an outer layer A, a second outer layer B, a first intermediate adhesive layer C, a core layer D, a second intermediate adhesive layer c, a second inner layer b, and an inner layer a, wherein: The outer layer A is made of metallocene polyethylene resin; the second outer layer B is made of low-density polyethylene resin; the first intermediate adhesive layer C is made of modified ethylene-vinyl acetate copolymer; the core layer D is made of ethylene-vinyl alcohol copolymer; the second intermediate adhesive layer c is made of modified ethylene-vinyl acetate copolymer; the second inner layer b is made of low-density polyethylene resin; and the inner layer a is made of metallocene polyethylene resin.

[0022] The aforementioned seven layers form a symmetrical co-extrusion structure A / B / C / D / c / b / a with the core layer D as the center of symmetry. This ensures that the outer layer A and inner layer a, the second outer layer B and second inner layer b, and the first intermediate adhesive layer C and second intermediate adhesive layer c are mirror images of the core layer D in the thickness direction, forming a thermal shrinkage stress balance system. Specifically, the hexene comonomer branches in the metallocene polyethylene resin of the outer layer A and the vinyl acetate segments in the modified ethylene-vinyl acetate copolymer undergo molecular chain entanglement at the co-extrusion melt interface, forming an interpenetrating transition layer network structure that eliminates the thermal shrinkage stress difference during heat sealing. Furthermore, the maleic anhydride graft groups in the modified ethylene-vinyl acetate copolymer and the hydroxyl groups in the ethylene-vinyl alcohol copolymer of the core layer D undergo esterification under high-temperature extrusion conditions, generating a covalently bonded interface to prevent cavity expansion caused by interlayer slippage during high and low temperature cycling. The outer layer A and inner layer a are both 10-25 μm thick, the second outer layer B and second inner layer b are both 15-35 μm thick, the first intermediate adhesive layer C and the second intermediate adhesive layer c are both 4-10 μm thick, the core layer D is 8-15 μm thick, and the total thickness of the sealing strip is 80-150 μm.

[0023] The amount of the metallocene polyethylene resin is 12-18 parts; the amount of the low-density polyethylene resin is 10-16 parts; the amount of the modified ethylene-vinyl acetate copolymer is 4-10 parts; and the amount of the ethylene-vinyl alcohol copolymer is 8-15 parts.

[0024] Furthermore, the modified ethylene-vinyl acetate copolymer includes the following preparation steps: S1.1. Ethylene-vinyl acetate copolymer and maleic anhydride are fed into a twin-screw extruder at a mass ratio of 100:3-100:8. Dicumyl peroxide is added as an initiator at a mass ratio of 0.1-0.5% of the ethylene-vinyl acetate copolymer. The reaction is carried out at 150-180℃ for 2-5 minutes. After extrusion and granulation, maleic anhydride graft-modified ethylene-vinyl acetate copolymer with a grafting rate of 0.8-1.5% is obtained, which is used to provide active anhydride groups for esterification reaction with the ethylene-vinyl alcohol copolymer layer. S1.2. Maleic anhydride-grafted modified ethylene-vinyl acetate copolymer and unmodified ethylene-vinyl acetate copolymer are premixed in a high-speed mixer at a mass ratio of 1:2-1:4 for 10-20 min. Then, antioxidant 1010 and calcium stearate are added at a mass ratio of 0.3-0.8% and 0.2-0.5% respectively. The mixture is stirred at 80-100℃ for 10-20 min to form a blended powder in which the high-graft-rate modified ethylene-vinyl acetate copolymer region and the low-graft-rate unmodified ethylene-vinyl acetate copolymer region are distributed in an island-type phase. The island-type phase allows the high-graft-rate region to preferentially undergo esterification reaction with the ethylene-vinyl alcohol copolymer during co-extrusion, while the low-graft-rate region delays the reaction initiation time. This disperses the total heat release of the esterification reaction across multiple temperature ranges, preventing localized excessive cross-linking caused by instantaneous overshoot of the interface temperature. S1.3. The blended powder is fed into a single-screw extruder and melt-plasticized at 130-160℃ for 1-3 minutes. Then, 0.5-1.2% vinyltrimethoxysilane by mass of the blended powder is added to the melt. After dispersion at a shear rate of 300-600 r / min through a dynamic mixing head, the siloxane groups of vinyltrimethoxysilane preferentially migrate to the surface of the melt, forming a silane-rich layer with a thickness of 20-50 nm on the surface of the modified ethylene-vinyl acetate copolymer. This allows it to form silane-C-H bonds with the hexene comonomer branches of metallocene polyethylene during coextrusion, improving the compatibility of the melt interface between the modified EVA and the metallocene polyethylene layer, and obtaining a surface-functionalized modified ethylene-vinyl acetate copolymer melt. S1.4 The modified ethylene-vinyl acetate copolymer melt is granulated through an underwater granulation system at a granulation water temperature of 30-50℃. After centrifugal drying, it is passed through a 20-mesh sieve to obtain columnar modified ethylene-vinyl acetate copolymer particles with an average particle size of 2-4mm.

[0025] The metallocene polyethylene resin is an ethylene-hexene copolymer with a density of 0.912-0.925 g / cm³ and a melt index of 1.5-3.0 g / 10 min, wherein the mass content of the hexene comonomer is 6-12%, used to construct branching sites for silane-C-H bonding with the silane-rich layer on the surface of the modified ethylene-vinyl acetate copolymer; the low-density polyethylene resin is a long-chain branched polyethylene with a density of 0.918-0.930 g / cm³ and a melt index of 2.0-5.0 g / 10 min; the ethylene-vinyl alcohol copolymer is a barrier resin with an ethylene molar content of 32-44% and a density of 1.12-1.20 g / cm³, the difference between its melt index and that of the low-density polyethylene resin not exceeding 1.5 g / 10 min, used to match the interlayer melt viscosity during multilayer co-extrusion.

[0026] Both the outer layer A and the inner layer a have a textured heat-sealing structure on the side away from the core layer D, which can discretize the heat-sealing pressure and suppress the continuous transmission of shrinkage stress, forming a buffer interface for discretized heat shrinkage stress. The textured heat-sealing structure is composed of continuously distributed textured pits and textured ridges arranged alternately. The depth of the textured pits is 8-15μm, the width of the textured ridges is 20-40μm, and the center distance between adjacent textured pits is 150-250μm. This ensures that the textured ridges provide sufficient heat-sealing pressure during heat sealing without piercing the substrate. The depth of the textured pits ensures that the molten resin does not completely fill the pits under pressure, leaving the bottom of the pits as a buffer zone for releasing the shrinkage stress of the heat-sealing layer. Furthermore, the ratio of the center distance between adjacent textured pits to the width of the textured ridges is 3.75 to 12.5, which enables the textured heat-sealing structure to achieve the desired effect. The heat-shrinkage stress is discretized into multiple independent units along the width of the sealing strip. The shrinkage deformation of each unit is confined within the buffer zone of a single textured pit, preventing stress from continuously accumulating along the length of the sealing strip. The textured heat-sealing structure uses regularly arranged rhomboid or hexagonal pits. The acute angle of the rhomboid pits is 45-60°, and the side length of the hexagonal pits is 30-50μm. The acute vertices of the rhomboid pits are arranged along the length of the sealing strip, so that the shrinkage stress of the heat-sealing layer in the length direction is dispersed along the diagonal of the rhomboid. The hexagonal pits are arranged in a honeycomb topology, with adjacent hexagonal pits sharing a convex ridge edge, so that the shrinkage strain of the heat-sealing layer in any direction is uniformly constrained by the equilateral convex ridge structure of the hexagonal array, preventing anisotropic microcrack propagation paths from appearing in the heat-sealing layer.

[0027] according to Figure 1 As shown, this embodiment of the invention also provides a method for preparing the above-mentioned seven-layer co-extruded sealing strip resistant to high and low temperature cycles, the specific steps of which are as follows: S2.1 Seven-layer symmetrical co-extrusion melt distribution: Outer layer A, second outer layer B, first intermediate adhesive layer C, core layer D, second intermediate adhesive layer c, second inner layer b and inner layer a are respectively fed into seven extruders. The melt is distributed by a distributor in a symmetrical stacked flow channel with core layer D as the center, so that outer layer A and inner layer a, second outer layer B and second inner layer b, first intermediate adhesive layer C and second intermediate adhesive layer c are mirrored in the thickness direction relative to core layer D, forming a thermal shrinkage stress balance system, and obtaining a composite melt in which the seven-layer melt has been symmetrically stacked at the die inlet; S2.2 Melt Interface Reaction and Interpenetrating Network Construction: The composite melt is transferred to a T-shaped flat die and held in the flow channel at a die temperature of 190-210℃ for 2-4 seconds. This allows the hexene comonomer branches in the metallocene polyethylene of the outer layer A to undergo silane-hydrogen bonding with the silane-rich layer on the surface of the modified ethylene-vinyl acetate copolymer of the first intermediate adhesive layer C. Simultaneously, the maleic anhydride graft groups of the first intermediate adhesive layer C undergo esterification with the hydroxyl groups of the ethylene-vinyl alcohol copolymer of the core layer D, generating a covalently bonded interface. The low-density polyethylene of the second outer layer B, with its long-chain branched structure, forms a bond between the A layer and the interpenetrating network. The C layers form an elastic stress buffer transition and do not participate in interfacial chemical bonding; the innermost layer b and the second intermediate adhesive layer c, which are symmetrically distributed on the other side of the core layer D, react synchronously with the inner layer a through the same mechanism. The modified ethylene-vinyl acetate copolymer of the second intermediate adhesive layer c forms an esterification bond with the core layer D and a silane-hydrogen bond with the metallocene polyethylene of the inner layer a, respectively. The low-density polyethylene of the innermost layer b also serves as a stress buffer layer between the inner layer a and the second intermediate adhesive layer c. After the seven layers of melt are extruded through the die slit, a sheet-like molten preform with symmetrical stacking direction from the outer layer A to the inner layer a is formed. S2.3, Reticulated Heat-Sealing Structure Imprinting: The sheet-like molten preform is transferred to a three-roll calender. The surface of the upper calender roller is pre-engraved with a laser to form a regularly arranged diamond or hexagonal pit pattern. Under a temperature of 40-60℃ and a linear pressure of 0.3-0.6MPa, the outer layer A and inner layer a of the preform are simultaneously imprinted, so that the heat-sealing layers on both sides form a mirror-symmetrical discrete buffer interface. The shrinkage deformation of each unit is limited to the range of a single reticulated pit, preventing stress from continuously accumulating in the length direction of the sealing strip, resulting in a seven-layer symmetrical co-extruded sheet with double-sided reticulation. S2.4 Gradient Cooling and Shaping: The seven-layer symmetrical co-extruded sheet is sequentially fed into a three-section cooling roller group for cooling and shaping, so that the sheet forms a crystallinity gradient distribution in the thickness direction, decreasing from the highly crystalline area of ​​the textured surface layer to the amorphous area of ​​the core layer. The crystallinity gradient direction intersects perpendicularly with the direction of thermal shrinkage stress transmission, forcing the volume shrinkage behavior of each layer during high and low temperature cycling to be released in stages along the thickness direction, avoiding a single interface from bearing all the shrinkage strain, and finally obtaining a seven-layer co-extruded sealing strip with a surface layer resistant to brittle cracking, a core layer with high barrier properties, and no void expansion channels between layers.

[0028] Wherein: In S2.4, the three-section cooling roller assembly includes the following three sections: The temperature of the first cooling roller group is 65-75℃, and the cooling residence time is 3-5s. This allows the surface of the outer layer A and the inner layer a to cool down to below the crystallization initiation temperature of metallocene polyethylene first, forming a high crystallinity hardened layer in the ridge area. The bottom of the ridge pit retains a low crystallinity elastic area due to the low contact pressure. The temperature of the second cooling roller is 45-55℃, and the cooling residence time is 6-10s, so that the melt temperature of the outermost layer B, the innermost layer b, the first intermediate adhesive layer C, and the second intermediate adhesive layer c simultaneously passes through the crystallization range of low-density polyethylene and modified ethylene-vinyl acetate copolymer, forming a stress buffer transition layer with medium crystallinity. The temperature of the third cooling roller is 20-30℃, and the cooling residence time is 6-15s, so that the core layer D is cooled to below the glass transition temperature of ethylene-vinyl alcohol copolymer, forming a low-crystallinity, high-barrier amorphous core layer.

[0029] The following detailed description of the seven-layer co-extruded sealing strip resistant to high and low temperature cycles provided by the present invention is based on several specific embodiments, combined with different raw material dosages, layer thicknesses, and mesh structure parameters. Example

[0030] A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: its composition by weight is as follows: metallocene polyethylene resin (17 parts each of outer layer A and inner layer a, totaling 34 parts); low-density polyethylene resin (14 parts each of the second outer layer B and the second inner layer b, totaling 28 parts); modified ethylene-vinyl acetate copolymer (7 parts each of the first intermediate adhesive layer C and the second intermediate adhesive layer c, totaling 14 parts); ethylene-vinyl alcohol copolymer (core layer D) 24 parts. The thickness of outer layer A and inner layer a is 15 μm, the thickness of second outer layer B and second inner layer b is 25 μm, the thickness of the first intermediate adhesive layer C and the second intermediate adhesive layer c is 10 μm, the thickness of core layer D is 12 μm, and the total thickness of the sealing strip is 112 μm. The outer layer A and the inner layer a are provided with a textured heat-sealing structure on the side away from the core layer D. The textured pits are diamond-shaped pits with a depth of 12μm, a ridge width of 30μm, a center distance of 200μm between adjacent pits, and an acute angle of 50° for the diamond-shaped pits. The vertices of the acute angles are arranged along the length of the sealing strip.

[0031] Preparation method of a seven-layer co-extruded sealing strip resistant to high and low temperature cycles: Weigh 34 parts by weight of metallocene polyethylene resin, 28 parts by weight of low-density polyethylene resin, 14 parts by weight of modified ethylene-vinyl acetate copolymer, and 24 parts by weight of ethylene-vinyl alcohol copolymer. Feed the raw materials for outer layer A, second outer layer B, first intermediate adhesive layer C, core layer D, second intermediate adhesive layer c, second inner layer b, and inner layer a into seven extruders. Set the temperatures of the seven extruders as follows: outer layer A and inner layer a 190℃, second outer layer B and second inner layer b 185℃, first intermediate adhesive layer C and second intermediate adhesive layer c 175℃, core layer D 200℃. The melt is distributed through a distributor, forming symmetrically stacked flow channels centered on core layer D. The composite melt enters a T-shaped flat die at a die temperature of 195℃, with a residence time of 3 seconds within the flow channel. In the outer layer A, the hexene comonomer branches of the metallocene polyethylene undergo silane-hydrogen bonding with the silane-rich layer on the modified EVA surface of the first intermediate adhesive layer C. Simultaneously, the maleic anhydride graft groups of the first intermediate adhesive layer C undergo esterification with the EVOH hydroxyl groups of the core layer D. The seven-layer melt is extruded through a die slit to form a sheet-like molten preform. The preform is transferred to a three-roll calender, where a diamond-shaped recessed pattern is pre-engraved on the surface of the upper calender roller. The outer layer A and inner layer a of the preform are simultaneously imprinted at a temperature of 50°C and a linear pressure of 0.45 MPa. The double-sided textured sheet is then sequentially fed into a three-stage cooling roller assembly: the first cooling roller is at 70°C with a residence time of 4 seconds; the second cooling roller is at 50°C with a residence time of 8 seconds; and the third cooling roller is at 25°C with a residence time of 10 seconds, resulting in a seven-layer co-extruded sealing strip. Example

[0032] A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: its composition by weight is as follows: metallocene polyethylene resin (21 parts each of outer layer A and inner layer a, totaling 42 parts); low-density polyethylene resin (12 parts each of the second outer layer B and the second inner layer b, totaling 24 parts); modified ethylene-vinyl acetate copolymer (6 parts each of the first intermediate adhesive layer C and the second intermediate adhesive layer c, totaling 12 parts); ethylene-vinyl alcohol copolymer (core layer D) 22 parts. The thickness of outer layer A and inner layer a is 22 μm, the thickness of second outer layer B and second inner layer b is 18 μm, the thickness of the first intermediate adhesive layer C and the second intermediate adhesive layer c is 8 μm, the thickness of core layer D is 10 μm, and the total thickness of the sealing strip is 104 μm. The outer layer A and the inner layer a are provided with a textured heat-sealing structure on the side away from the core layer D. The textured pits are hexagonal pits with a depth of 10μm, a side length of 40μm, and a center distance of 180μm between adjacent pits. The hexagonal pits are arranged in a honeycomb topology.

[0033] Preparation method of a seven-layer co-extruded sealing strip resistant to high and low temperature cycles: 42 parts by weight of metallocene polyethylene resin, 24 parts by weight of low-density polyethylene resin, 12 parts by weight of modified ethylene-vinyl acetate copolymer, and 22 parts by weight of ethylene-vinyl alcohol copolymer. Temperature settings for seven extruders: outer layer A and inner layer a 195℃, second outer layer B and second inner layer b 180℃, first intermediate adhesive layer C and second intermediate adhesive layer c 178℃, core layer D 205℃. Die temperature 200℃, residence time in the runner 3.5s. A hexagonal recessed pattern is pre-engraved on the surface of the upper pressure roller, with an imprinting temperature of 55℃ and a linear pressure of 0.5MPa. Three-stage cooling: first stage 68℃ / 4.5s, second stage 48℃ / 9s, third stage 22℃ / 12s. The remaining steps are the same as in Example 1. Example

[0034] A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: its composition by weight is as follows: metallocene polyethylene resin (18 parts each of outer layer A and inner layer a, totaling 36 parts); low-density polyethylene resin (17 parts each of the second outer layer B and the second inner layer b, totaling 34 parts); modified ethylene-vinyl acetate copolymer (5 parts each of the first intermediate adhesive layer C and the second intermediate adhesive layer c, totaling 10 parts); ethylene-vinyl alcohol copolymer (core layer D) 20 parts. The thickness of outer layer A and inner layer a is 18 μm, the thickness of second outer layer B and second inner layer b is 30 μm, the thickness of the first intermediate adhesive layer C and the second intermediate adhesive layer c is 12 μm, the thickness of core layer D is 15 μm, and the total thickness of the sealing strip is 135 μm. The outer layer A and the inner layer a are provided with a textured heat-sealing structure on the side away from the core layer D. The textured pits are diamond-shaped pits with a depth of 14μm, a ridge width of 25μm, a center distance of 220μm between adjacent pits, and an acute angle of 55°.

[0035] Preparation method of a seven-layer co-extruded sealing strip resistant to high and low temperature cycles: Weigh out 36 parts by weight of metallocene polyethylene resin, 34 parts by weight of low-density polyethylene resin, 10 parts by weight of modified ethylene-vinyl acetate copolymer, and 20 parts by weight of ethylene-vinyl alcohol copolymer. Temperature settings for seven extruders: outer layer A and inner layer a 188℃, second outer layer B and second inner layer b 182℃, first intermediate adhesive layer C and second intermediate adhesive layer c 172℃, core layer D 198℃. Die temperature 192℃, residence time in the flow channel 2.5s. Pre-engraving a diamond-shaped recessed pattern on the surface of the upper pressure roller, imprinting temperature 45℃, linear pressure 0.4MPa. Three-stage cooling: first stage 72℃ / 3.5s, second stage 52℃ / 7s, third stage 28℃ / 8s. The remaining steps are the same as in Example 1. Example

[0036] A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: its composition by weight is as follows: metallocene polyethylene resin (14 parts each of outer layer A and inner layer a, totaling 28 parts); low-density polyethylene resin (19 parts each of the second outer layer B and the second inner layer b, totaling 38 parts); modified ethylene-vinyl acetate copolymer (8 parts each of the first intermediate adhesive layer C and the second intermediate adhesive layer c, totaling 16 parts); ethylene-vinyl alcohol copolymer (core layer D) 18 parts. The thickness of outer layer A and inner layer a is 12 μm, the thickness of second outer layer B and second inner layer b is 32 μm, the thickness of the first intermediate adhesive layer C and the second intermediate adhesive layer c is 14 μm, the thickness of core layer D is 18 μm, and the total thickness of the sealing strip is 148 μm. The outer layer A and the inner layer a are provided with a textured heat-sealing structure on the side away from the core layer D. The textured pits are hexagonal pits with a depth of 9μm, a side length of 35μm, and a center distance of 160μm between adjacent pits. The hexagonal pits are arranged in a honeycomb topology.

[0037] Preparation method of a seven-layer co-extruded sealing strip resistant to high and low temperature cycles: 28 parts by weight of metallocene polyethylene resin, 38 parts by weight of low-density polyethylene resin, 16 parts by weight of modified ethylene-vinyl acetate copolymer, and 18 parts by weight of ethylene-vinyl alcohol copolymer. Temperature settings for seven extruders: outer layer A and inner layer a 192℃, second outer layer B and second inner layer b 184℃, first intermediate adhesive layer C and second intermediate adhesive layer c 176℃, core layer D 202℃. Die temperature 198℃, residence time in the runner 3.8s. A hexagonal recessed pattern is pre-engraved on the surface of the upper pressure roller, with an imprinting temperature of 58℃ and a linear pressure of 0.55MPa. Three-stage cooling: first stage 66℃ / 5s, second stage 46℃ / 10s, third stage 26℃ / 14s. The remaining steps are the same as in Example 1.

[0038] To verify that the seven-layer co-extruded sealing strip prepared in the embodiments of the present invention has good resistance to high and low temperature cycling, anti-brittleness, and interlayer anti-void propagation properties, the following test examples are used to illustrate the seven-layer co-extruded sealing strip with high and low temperature cycling resistance provided in the embodiments of the present invention.

[0039] Test case The purpose of this experimental group is to investigate the effect of different component ratios on the seven-layer co-extruded sealing strip resistant to high and low temperature cycles, and to test the heat seal strength retention rate, interlayer peel strength attenuation rate, anti-brittle microcrack density, and liquid encapsulation leakage rate of the seven-layer co-extruded sealing strip of the present invention after temperature cycling, so as to verify its anti-brittle cracking performance and interlayer anti-cavitation propagation performance under high and low temperature cycles.

[0040] Experimental Objective: Experimental groups A, B, C, and D adopted the component ratios of the seven-layer co-extruded sealing strips resistant to high and low temperature cycles provided in Examples 1-4, respectively; the control group consisted of control groups A, B, C, D, E, F, G, H, and I, wherein: Control group A A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: Unmodified ethylene-vinyl acetate copolymer is used instead of modified ethylene-vinyl acetate copolymer as the first intermediate adhesive layer C and the second intermediate adhesive layer c, and the rest of the composition is the same as in Example 1.

[0041] The difference between the preparation method and Example 1 is that the first intermediate adhesive layer C and the second intermediate adhesive layer c directly use commercially available ethylene-vinyl acetate copolymer, without maleic anhydride graft modification, and without adding vinyltrimethoxysilane surface treatment.

[0042] Control group B A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: The composition by weight is the same as in Example 1. The thickness of each layer is adjusted as follows: outer layer A and inner layer a are 35 μm thick; second outer layer B and second inner layer b are 50 μm thick; first intermediate adhesive layer C and second intermediate adhesive layer c are 25 μm thick; core layer D is 30 μm thick; and the total thickness of the sealing strip is 230 μm. The parameters of the textured heat-sealing structure are the same as in Example 1.

[0043] The preparation method differs from that in Example 1 in that: the seven layers of raw materials are fed into seven extruders respectively, the melt is symmetrically stacked and merged through a distributor, the die temperature is 200℃, and the residence time is 3s; when imprinting the anilox pattern, the temperature of the upper pressure roller is 50℃, and the linear pressure is 0.45MPa; the parameters of the three-stage cooling roller group are: the first stage is 70℃ and the residence time is 4s, the second stage is 50℃ and the residence time is 8s, and the third stage is 25℃ and the residence time is 10s.

[0044] Control group C A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: The mesh heat-sealing structure uses ordinary square grid pits instead of rhomboid or hexagonal pits: the square pit has a side length of 40μm, a pit depth of 12μm, a ridge width of 30μm, and a center distance of 200μm between adjacent pits. The remaining composition is the same as in Example 1: 34 parts of metallocene polyethylene resin, 28 parts of low-density polyethylene resin, 14 parts of modified EVA, and 24 parts of EVOH. The thickness of each layer is the same as in Example 1.

[0045] The difference between the preparation method and Example 1 is that: seven-layer symmetrical co-extrusion melt distribution, die temperature 200℃, imprinting square mesh, and three-section cooling roller group cooling and shaping at 70℃ / 4s, 50℃ / 9s, and 25℃ / 10s; the right-angle vertices of the square mesh form stress concentration points when the heat-sealing layer shrinks, and anisotropic microcrack propagation paths are generated along the right-angle side, which cannot achieve the diagonal dispersion function of the rhomboid pit or the honeycomb uniform constraint function of the hexagonal pit.

[0046] Control group D A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: metallocene polyethylene resin (30 parts each of outer layer A and inner layer a, totaling 60 parts); low-density polyethylene resin (5 parts each of the second outer layer B and the second inner layer b, totaling 10 parts); modified EVA (2 parts each of layers C and c, totaling 4 parts); EVOH core layer D (6 parts). The outer layer A and inner layer a have a thickness of 28 μm, the second outer layer B and the second inner layer b have a thickness of 8 μm, the C and c layers have a thickness of 3 μm, and the core layer D has a thickness of 5 μm, for a total thickness of 85 μm. The textured heat-sealing structure has a diamond-shaped pit depth of 8 μm, a ridge width of 20 μm, and a center-to-center distance of 150 μm.

[0047] The difference between the preparation method and Example 1 is that the seven layers of raw materials are fed into seven extruders respectively. The extrusion temperature of the outer layer A and the inner layer a is 195°C, the second outer layer B and the second inner layer b is 180°C, the C and c layers are 170°C, the core layer D is 205°C, the die temperature is 200°C and the dwell time is 2.5s, the imprinting temperature is 45°C and the linear pressure is 0.35MPa, and the three-stage cooling is 65°C / 3s in the first stage, 45°C / 6s in the second stage, and 20°C / 6s in the third stage.

[0048] Control group E A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: metallocene polyethylene resin (5 parts each of outer layer A and inner layer a, totaling 10 parts); low-density polyethylene resin (25 parts each of the second outer layer B and the second inner layer b, totaling 50 parts); modified EVA (15 parts each of layers C and c, totaling 30 parts); EVOH core layer D (30 parts). The outer layer A and inner layer a have a thickness of 8 μm, the second outer layer B and the second inner layer b have a thickness of 40 μm, layers C and c have a thickness of 18 μm, and core layer D has a thickness of 25 μm, for a total thickness of 155 μm. The textured heat-sealing structure has a diamond-shaped pit depth of 18 μm, a ridge width of 50 μm, and a center-to-center distance of 280 μm.

[0049] The difference between the preparation method and Example 1 is that the seven layers of raw materials are fed into seven extruders respectively. The extrusion temperature of the outer layer A and the inner layer a is 185°C, the second outer layer B and the second inner layer b is 190°C, the C and c layers are 180°C, the core layer D is 210°C, the die temperature is 210°C and held for 4 seconds, the imprinting temperature is 65°C and the linear pressure is 0.7MPa, and the three-stage cooling is 80°C / 5s in the first stage, 60°C / 12s in the second stage and 35°C / 15s in the third stage.

[0050] control group F A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: 34 parts metallocene polyethylene resin, 28 parts low-density polyethylene resin, 14 parts modified EVA, and 24 parts EVOH, with the same thickness for each layer as in Example 1.

[0051] The difference between the preparation method and Example 1 is that vinyltrimethoxysilane is not added in step S1.3 of the modified EVA preparation, that is, granulation is performed directly after melt plasticization, and no silane enrichment layer is formed.

[0052] control group G A seven-layer co-extruded sealing strip resistant to high and low temperature cycles: 34 parts metallocene polyethylene resin, 28 parts low-density polyethylene resin, 14 parts modified EVA, and 24 parts EVOH, with the same thickness for each layer as in Example 1.

[0053] The difference between the preparation method and Example 1 is that in the gradient cooling and shaping step, instead of using a three-section cooling roller group, a single-section cooling roller is used, the temperature of the cooling roller is uniformly 25°C, and the cooling residence time is 20s.

[0054] control group H A double-sided corona-electroplated biaxially oriented polyester film layer with a thickness of 12 μm is used as the core layer. On one side of the core layer, a first metallocene polyethylene resin film layer with a thickness of 15 μm, a first low-density polyethylene resin film layer with a thickness of 25 μm, and a second metallocene polyethylene resin film layer with a thickness of 10 μm are sequentially laminated. On the other side of the core layer, a third metallocene polyethylene resin film layer with a thickness of 15 μm, a second low-density polyethylene resin film layer with a thickness of 25 μm, and a fourth metallocene polyethylene resin film layer with a thickness of 10 μm are sequentially laminated, for a total thickness of 112 μm.

[0055] Preparation method: Each layer is melted separately and then extruded together via a distributor. The die temperature is 195℃, and the material is cooled and shaped using a three-roll cooling system. No textured heat-sealing structure is used, and no gradient cooling is employed; a single-stage cooling roller at 50℃ is used. This comparative example has no EVOH barrier layer, no modified EVA adhesive layer, no symmetrical co-extrusion structure, no textured discrete buffer interface, and no silane-C-H bonding and esterification reaction interface.

[0056] Control group I It adopts a five-layer asymmetric structure: the outer layer is metallocene polyethylene with a thickness of 15μm, the second layer is low-density polyethylene with a thickness of 25μm, the third layer is unmodified EVA (VA content 18%) with a thickness of 10μm, the fourth layer is ethylene-vinyl alcohol copolymer with a thickness of 12μm, the inner layer is metallocene polyethylene with a thickness of 50μm, and the total thickness is 112μm.

[0057] Preparation method: The material was melted separately by five extruders and then combined and extruded via a distributor. The die temperature was 195℃, and the material was cooled and shaped using a three-roll cooling system. No textured structure was set on the outer and inner surfaces. A single-stage cooling roller at 50℃ was used for cooling. This comparative example does not have a symmetrical mirror distribution, the adhesive layer thicknesses on both sides of the core layer are unequal, there is no esterification reaction covalent interface, and no surface silane functionalization treatment.

[0058] Test methods: The heat seal strength retention rate, interlayer peel strength attenuation rate, microcrack density, and liquid encapsulation leakage rate of the seven-layer co-extruded sealing strip according to the present invention after temperature cycling were tested respectively. The specific test methods are as follows: Heat seal strength retention rate: The seven-layer co-extruded sealing strips prepared by the experimental group AD and the control group AI were cut into specimens with a width of 15 mm and a length of 200 mm, and 10 specimens of each type of sealing strip were taken. The specimens were placed in a high and low temperature cycling test chamber. Each cycle included: holding at a high temperature of 121℃ for 30 min, then cooling to -20℃ within 30 s and holding at that temperature for 30 min, and then heating back to 121℃ within 30 s, for a total of 50 cycles. After completing 50 cycles, the specimens were removed and placed in an environment with a temperature of 23±2℃ and a relative humidity of 50±5% for 24 h. Using a heat sealing tester, the heat sealing layers (outer layer A and inner layer a) of each specimen were placed opposite each other, and double-sided heat sealing was performed under the conditions of heat sealing temperature of 135℃, heat sealing pressure of 0.3 MPa, and heat sealing time of 1.0 s, with a heat sealing width of 10 mm. The heat-sealed specimens were subjected to a T-peel test on a universal testing machine at a tensile speed of 300 mm / min, and the maximum peel force was recorded; where: Formula for calculating heat seal strength: Where: P is the heat seal strength, in N / 15mm; F ​​is the maximum peel force, in N; W is the sample width, in 15mm. Each sample is tested 5 times, and the arithmetic mean is taken. The formula for calculating the heat seal strength retention rate is: Where: R is the heat seal strength retention rate; The heat seal strength of the sample after 50 high and low temperature cycles is expressed in N / 15mm. The heat seal strength of the original sample without high and low temperature cycling is expressed in N / 15mm.

[0059] Interlayer peel strength attenuation rate: Seven-layer co-extruded sealing strips prepared by the experimental group AD and the control group AI were cut into samples with a width of 15 mm and a length of 200 mm, with 10 samples of each type of sealing strip. The samples were placed in a high and low temperature cycling test chamber. Each cycle consisted of: holding at a high temperature of 121℃ for 30 min, then cooling to -20℃ within 30 s and holding at that temperature for 30 min, and then heating back to 121℃ within 30 s, for a total of 50 cycles. After completing 50 cycles, the samples were removed and placed in an environment with a temperature of 23±2℃ and a relative humidity of 50±5% for 24 h. Along the length of the sample, a sharp blade was used to separate the outer layer A and the second outermost layer B at the end of the sample, creating an initial peel segment of 30 mm in length. The separated ends were clamped in the upper and lower fixtures of a universal testing machine with a fixture spacing of 50 mm. A 180° peel test was performed at a tensile speed of 200 mm / min, with a peel distance of 80 mm. The average stable peel force during the peeling process was recorded. Formula for calculating interlayer peel strength: Where: S is the interlayer peel strength, in N / 15mm; The average peel force is expressed in N; W is the sample width, expressed in 15 mm; each sample is tested 5 times, and the arithmetic mean is taken. The formula for calculating the interlayer peel strength attenuation rate is: D represents the interlayer peel strength attenuation rate. The interlayer peel strength of the original sample without high and low temperature cycling is expressed in N / 15mm. The interlayer peel strength of the sample after 50 high and low temperature cycles is expressed in N / 15mm.

[0060] Microcrack density resistance: Seven-layer co-extruded sealing strips prepared in the experimental group AD and the control group AI were cut into samples with a width of 20 mm and a length of 50 mm, with 5 samples of each type of sealing strip. The samples were placed in a high and low temperature cycling test chamber. Each cycle consisted of: holding at 121℃ for 30 min, then cooling to -20℃ within 30 s and holding for 30 min, and then heating back to 121℃ within 30 s, for a total of 50 cycles. After completing 50 cycles, the samples were removed and placed in an environment with a temperature of 23±2℃ and a relative humidity of 50±5% for 24 h. The samples were laid flat on a glass slide with the outer layer A facing up and observed under an optical microscope (magnification 200x). A continuous region with a length of 10 mm was selected in the center area of ​​the sample. Observation lines were taken every 1 mm along the direction perpendicular to the length (width) of the sealing strip, for a total of 10 observation lines. The number of microcracks penetrating the heat-sealing layer and exceeding 50% of its thickness was counted on each observation line. The average value of the 10 observation lines was taken as the number of microcracks in the sample; where: Formula for calculating the density of brittle microcracks: Where: ρ is the density of microcracks resisting brittle fracture, in units of cracks / mm; N is the total number of microcracks in a 10mm length region, in units of cracks; L is the length of the observation area, in units of 10mm; each sample is measured 3 times, and the arithmetic mean is taken.

[0061] Liquid encapsulation leakage rate: The seven-layer co-extruded sealing strips prepared by the experimental group AD and the control group AI were cut into strips with a width of 10 mm and a length of 100 mm. One hundred 250 mL paper-aluminum-plastic composite bags for milk packaging were taken, with the filling port size of each composite bag matching the width of the sealing strip. The sealing strip was heat-sealed to the inner wall of the filling port of the composite bag using a heat-sealing machine at a temperature of 140℃, a heat-sealing pressure of 0.35 MPa, and a heat-sealing time of 1.2 s. Each composite bag was filled with 200 mL of a 0.9% sodium chloride aqueous solution (simulating liquid food medium) at a filling temperature of 85℃. After filling, the filling port of the composite bag was folded and secured with a sealing clip. The composite bags were placed in a high-low temperature cycling test chamber. Each cycle included: holding at 121℃ for 30 min, then cooling to -20℃ within 30 s and holding for 30 min, then heating back to 121℃ within 30 s, for a total of 50 cycles. After 50 cycles, the composite bag was removed and hung vertically for 72 hours in an environment with a temperature of 23±2℃ and a relative humidity of 50±5%. After 72 hours, the area where the sealing strip meets the inner wall of the composite bag was gently wiped with filter paper to observe for any liquid seepage; among which: Formula for calculating liquid encapsulation leakage rate: ;in: For liquid encapsulation leakage rate; The number of composite bags that leaked, expressed in units of individual bags; The total number of composite bags tested is 100.

[0062] Table 1 Detection indicators of each sample Experimental group A 42.6 94.8 8.7 6.9 0.04 0 Experimental group B 40.3 93.2 8.2 8.5 0.05 0 Experimental group C 41.5 93.9 8.5 7.8 0.04 0 Experimental group D 39.8 92.5 7.9 9.2 0.06 1 Control group A 28.4 62.3 4.2 38.1 0.52 12 Control group B 35.2 78.6 6.1 22.4 0.31 8 Control group C 36.8 81.2 6.5 19.7 0.28 6 Control group D 31.5 70.4 5.3 31.2 0.43 10 Control group E 30.2 67.8 4.9 35.6 0.48 11 control group F 33.6 75.1 5.8 27.5 0.35 9 control group G 34.1 76.8 6.0 25.3 0.33 7 control group H 22.5 48.6 3.1 52.4 0.78 24 Control group I 24.8 53.2 3.6 47.9 0.69 18 according to Figure 2-7As shown in Table 1, the heat seal strength retention rate of experimental group AD was in the range of 92.5-94.8%, the interlayer peel strength attenuation rate was controlled between 6.9-9.2%, the microcrack density was 0.04-0.06 cracks / mm, and the liquid encapsulation leakage rate was 0% except for experimental group D, which was 1%. In contrast, control example H, which used a double-sided corona-electroplated biaxially oriented polyester film core layer without a modified EVA adhesive layer, showed a heat seal strength retention rate of 48.6% after 50 high and low temperature cycles, an interlayer peel strength attenuation rate of 52.4%, a microcrack density of 0.78 cracks / mm, and a leakage rate of 24%. These data indicate that... The polarity difference between the polyester film and the polyethylene layer causes the interfacial bonding strength to decrease with increasing temperature cycling, resulting in continuous cavitation expansion channels between the layers. Without a textured, discretized buffer interface, the shrinkage stress of the heat-sealing layer is continuously transmitted, forming penetrating microcracks. In Control Example I, a five-layer asymmetric structure without an esterification reaction interface was used, resulting in a heat-sealing strength retention rate of 53.2%, a peel strength attenuation rate of 47.9%, and a leakage rate of 18%. The uneven thickness of the adhesive layers on both sides of the core layer caused an imbalance in the distribution of thermal shrinkage stress. The unmodified EVA and EVOH rely only on polar adsorption without covalent bonding, and the accumulation of interfacial sliding during temperature cycling leads to interfacial separation.

[0063] In control example A, unmodified EVA was used instead of modified EVA. The heat seal strength retention rate was 62.3%, the peel strength decay rate was 38.1%, the microcrack density was 0.52 cracks / mm, and the leakage rate was 12%. Unmodified EVA lacked maleic anhydride grafting groups and could not undergo esterification with the EVOH core layer to form a covalent bond interface. At the same time, it lacked a surface silane enrichment layer and only formed physical entanglement with the metallocene polyethylene layer. During high and low temperature cycling, the lack of esterification reaction resulted in zero chemical anchoring points at the interface, and the interlayer sliding could not be constrained by covalent bonds. Compared with Example B, the thickness of each layer was increased to a total thickness of 230 μm. The heat seal strength retention rate was 78.6%, the peel strength decay rate was 22.4%, the microcrack density was 0.31 cracks / mm, and the leakage rate was 8%. The increase in thickness increased the temperature gradient from the surface of the heat seal layer to the core layer. During gradient cooling, the uneven distribution of crystallinity was aggravated, the proportion of the low crystallinity elastic zone at the bottom of the network pit decreased, and the shrinkage stress buffer capacity was insufficient.

[0064] Compared with Example C, square grid pits were used instead of rhomboid or hexagonal pits. The heat seal strength retention rate was 81.2%, the peel strength attenuation rate was 19.7%, the microcrack density was 0.28 cracks / mm, and the leakage rate was 6%. The right-angle vertices of the square grid form stress concentration points in four directions when the heat seal layer shrinks, and anisotropic microcrack propagation paths are generated along the right-angle sides. It cannot achieve the diagonal dispersion function of rhomboid pits or the honeycomb-like uniform constraint of hexagonal pits. In control example D, the amount of metallocene polyethylene was too high (60 parts) while the amount of modified EVA was only 4 parts. The heat seal strength retention rate was 70.4%, the peel strength decay rate was 31.2%, the microcrack density was 0.43 cracks / mm, and the leakage rate was 10%. The insufficient amount of modified EVA resulted in a low density of silane-hydrogen bonding sites, a decrease in the coverage of the esterification reaction interface, and a rapid decay of interlayer bonding strength during temperature cycling.

[0065] In Comparative Example E, the amount of low-density polyethylene was too high (50 parts) and modified EVA reached 30 parts. The heat seal strength retention rate was 67.8%, the peel strength decay rate was 35.6%, the microcrack density was 0.48 cracks / mm, and the leakage rate was 11%. The excessive modified EVA formed an excessively thick esterification reaction layer at the co-extrusion interface. The concentrated release of exothermic esterification reaction caused local temperature overshoot, leading to excessive cross-linking and embrittlement.

[0066] In control example F, no vinyltrimethoxysilane was added during the modified EVA preparation step. The heat seal strength retention rate was 75.1%, the peel strength decay rate was 27.5%, the microcrack density was 0.35 cracks / mm, and the leakage rate was 9%. Without the silane enrichment layer, silane-hydrogen bonds could not be formed between the modified EVA and the metallocene polyethylene. The interpenetrating network structure of the transition layer was incomplete, and the thermal shrinkage stress difference during heat sealing could not be completely eliminated. Microslippage occurred at the interface during high and low temperature cycling.

[0067] Comparative Example G, which uses a single-stage cooling roller with uniform cooling at 25°C, exhibits a heat seal strength retention rate of 76.8%, a peel strength attenuation rate of 25.3%, a microcrack density of 0.33 cracks / mm, and a leakage rate of 7%. Single-stage cooling prevents the formation of a crystallinity gradient from the highly crystalline surface layer to the amorphous core layer along the sheet thickness direction. Consequently, the volumetric shrinkage behavior of each layer cannot be released in stages along the thickness direction, and the shrinkage strain is concentrated at a single interface.

[0068] Comparing the data of test group A with control examples H and I, it can be seen that the symmetrical seven-layer structure, combined with the covalent bonding interface of the esterification reaction of modified EVA and the silane-C-H bonding effect of the surface silane-enriched layer, reduced the interlayer peel strength attenuation rate from over 47.9% to below 9.2%. Comparing the data of test group A with control example C, it can be seen that the rhomboid or hexagonal pit network structure reduced the density of brittle microcracks from 0.28 cracks / mm to 0.04 cracks / mm, while the square grid could not achieve the same effect due to stress concentration at the right angle vertices. Comparing the data of test group A with control example G, it can be seen that the three-stage gradient cooling increased the heat seal strength retention rate from 76.8% to 94.8%, and the design of the crystallinity gradient direction perpendicular to the thermal shrinkage stress transmission direction forced the volume shrinkage to be released in stages.

[0069] In summary, the synergistic effect of the symmetrical seven-layer mirror structure, the covalent bonding interface of the esterification reaction, the silane-C-H interpenetrating network, the rhombic or hexagonal textured discretized buffer interface, and the three-stage gradient cooling crystallinity distribution in this invention has been verified. This combined technical solution achieves the technical indicators of heat seal strength retention of no less than 92.5%, interlayer peel strength attenuation rate of no more than 9.2%, and brittle fracture resistance microcrack density of no more than 0.06 cracks / mm under high and low temperature cycling conditions. This indicates that the present invention achieves the technical indicators of heat seal strength retention of no less than 92.5%, interlayer peel strength attenuation rate of no more than 9.2%, and brittle fracture resistance microcrack density of no more than 0.06 cracks / mm through the interaction between the core layer EVOH and the modified EVA. The esterification reaction generates irreversible covalent bonding interfaces to prevent the expansion of interlayer voids. The silane-hydrogen bonds between the outer metallocene polyethylene and the silane-rich layer on the modified EVA surface form a transition layer interpenetrating network to eliminate the stress difference during heat sealing. The thermal shrinkage stress is discretized into independent units by rhombic or hexagonal network pits to prevent microcrack penetration. The crystallinity is released in stages along the thickness direction through three-stage gradient cooling to avoid a single interface bearing all the shrinkage strain. Finally, the overall sealing reliability of the sealing strip is improved under high temperature cold chain cycle conditions.

[0070] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A seven-layer co-extruded sealing strip resistant to high and low temperature cycles, characterized in that, It includes outer layer A, second outer layer B, first intermediate adhesive layer C, core layer D, second intermediate adhesive layer c, second inner layer b, and inner layer a, wherein: The outer layer A is made of metallocene polyethylene resin; the second outer layer B is made of low-density polyethylene resin; the first intermediate adhesive layer C is made of modified ethylene-vinyl acetate copolymer; the core layer D is made of ethylene-vinyl alcohol copolymer; the second intermediate adhesive layer c is made of modified ethylene-vinyl acetate copolymer; the second inner layer b is made of low-density polyethylene resin; and the inner layer a is made of metallocene polyethylene resin. The above seven layers are arranged in a symmetrical co-extrusion structure A / B / C / D / c / b / a with the core layer D as the center of symmetry. The outer layer A and the inner layer a, the second outer layer B and the second inner layer b, the first intermediate adhesive layer C and the second intermediate adhesive layer c are mirrored in the thickness direction relative to the core layer D, forming a thermal shrinkage stress balance system. Both the outer layer A and the inner layer a have a textured heat-sealing structure on the side away from the core layer D, which can discretize the heat-sealing pressure and suppress the continuous transmission of shrinkage stress, forming a heat shrinkage stress discretization buffer interface.

2. The seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to claim 1, characterized in that, The textured heat-sealing structure is composed of continuously distributed textured pits and textured ridges arranged alternately. The depth of the textured pits is 8-15μm, the width of the textured ridges is 20-40μm, the center distance between adjacent textured pits is 150-250μm, and the ratio of the center distance between adjacent textured pits to the width of the textured ridges is 3.75 to 12.

5.

3. The seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to claim 1, characterized in that, The textured heat-sealing structure employs regularly arranged rhomboid or hexagonal recesses, wherein: The acute angle of the rhomboid recess is 45-60°, and the side length of the hexagonal recess is 30-50μm. The acute vertices of the rhomboid recess are arranged along the length of the sealing strip, so that the shrinkage stress of the heat seal layer in the length direction is dispersed along the diagonal of the rhombus. The hexagonal pits are arranged in a honeycomb topology, with adjacent hexagonal pits sharing a convex ridge, so that the shrinkage strain of the heat seal layer in any direction is uniformly constrained by the equilateral convex ridge structure of the hexagonal array.

4. The seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to claim 1, characterized in that, The amount of the metallocene polyethylene resin is 12-18 parts; the amount of the low-density polyethylene resin is 10-16 parts. The amount of the modified ethylene-vinyl acetate copolymer is 4-10 parts; the amount of the ethylene-vinyl alcohol copolymer is 8-15 parts.

5. The seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to claim 1, characterized in that, The outer layer A and inner layer a each have a thickness of 10-25 μm, the second outer layer B and second inner layer b each have a thickness of 15-35 μm, the first intermediate adhesive layer C and the second intermediate adhesive layer c each have a thickness of 5-15 μm, the core layer D has a thickness of 8-20 μm, and the total thickness of the sealing strip is 80-150 μm.

6. The seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to claim 1, characterized in that, The modified ethylene-vinyl acetate copolymer comprises the following preparation steps: S1.

1. Ethylene-vinyl acetate copolymer and maleic anhydride are fed into a twin-screw extruder at a mass ratio of 100:3-100:

8. Dicumyl peroxide is added as an initiator at a mass ratio of 0.1-0.5% of the ethylene-vinyl acetate copolymer. The reaction is carried out at 150-180℃ for 2-5 minutes. After extrusion and granulation, maleic anhydride grafted modified ethylene-vinyl acetate copolymer is obtained. S1.

2. Maleic anhydride-grafted modified ethylene-vinyl acetate copolymer and unmodified ethylene-vinyl acetate copolymer are premixed in a high-speed mixer at a mass ratio of 1:2-1:4 for 10-20 min. Then, antioxidant 1010 and calcium stearate accounting for 0.3-0.8% of the total mass of the mixture are added, and the mixture is stirred at 80-100℃ for 10-20 min to form a blended powder. S1.

3. The blended powder is fed into a single-screw extruder and melt-plasticized at 130-160℃ for 1-3 minutes. Then, 0.5-1.2% of vinyltrimethoxysilane by mass of the blended powder is added to the melt. After dispersion at a shear rate of 300-600 r / min through a dynamic mixing head, a silane enrichment layer with a thickness of 20-50 nm is formed on the surface of the modified ethylene-vinyl acetate copolymer, thus obtaining the modified ethylene-vinyl acetate copolymer melt. S1.4 The modified ethylene-vinyl acetate copolymer melt is granulated through an underwater granulation system at a granulation water temperature of 30-50℃. After centrifugal drying, it is passed through a 20-mesh sieve to obtain columnar modified ethylene-vinyl acetate copolymer particles with an average particle size of 2-4mm.

7. The seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to claim 1, characterized in that, The metallocene polyethylene resin is an ethylene-hexene copolymer with a density of 0.912-0.925 g / cm³ and a melt index of 1.5-3.0 g / 10 min, wherein the mass content of hexene comonomer is 6-12%; the low-density polyethylene resin is a long-chain branched polyethylene with a density of 0.918-0.930 g / cm³ and a melt index of 2.0-5.0 g / 10 min; the ethylene-vinyl alcohol copolymer is a barrier resin with an ethylene molar content of 32-44% and a density of 1.12-1.20 g / cm³, the difference between its melt index and that of the low-density polyethylene resin not exceeding 1.5 g / 10 min, used to match the interlayer melt viscosity during multilayer co-extrusion.

8. A method for preparing a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, comprising using the seven-layer co-extruded sealing strip resistant to high and low temperature cycles as described in any one of claims 1-7, characterized in that, Includes the following steps: S2.1, outer layer A, second outer layer B, first intermediate adhesive layer C, core layer D, second intermediate adhesive layer c, second inner layer b and inner layer a are fed into seven extruders respectively. The melt is distributed by a distributor and converges in a symmetrical stacked flow channel with core layer D as the center to obtain composite melt. S2.2 Transfer the composite melt to a T-shaped flat die head and hold it in the flow channel at a die head temperature of 190-210℃ for 2-4 seconds, so that the outer layer A and the first intermediate adhesive layer C undergo silane-hydrogen bonding, and the first intermediate adhesive layer C and the core layer D undergo esterification reaction to generate a covalent bonding interface; the second outer layer B forms an elastic stress buffer transition between the A layer and the C layer; the second inner layer b and the second intermediate adhesive layer c, which are symmetrically distributed on the other side of the core layer D, react synchronously with the inner layer a in the same mechanism to form a sheet-like molten preform with symmetrical stacking direction from the outer layer A to the inner layer a; S2.3 Transfer the sheet-like molten preform to a three-roll calender. The surface of the upper calender roller is pre-engraved with a laser to form a regularly arranged diamond-shaped or hexagonal pit pattern. The outer layer A and inner layer a of the preform are simultaneously imprinted at a temperature of 40-60℃ and a linear pressure of 0.3-0.6MPa to obtain a seven-layer symmetrical co-extruded sheet with double-sided textured pattern. S2.

4. The seven-layer symmetrical co-extruded sheet is sequentially fed into a three-section cooling roller group for cooling and shaping to obtain a seven-layer co-extruded sealing strip.

9. The method for preparing the seven-layer co-extruded sealing strip resistant to high and low temperature cycles according to claim 8, characterized in that, In S2.4, the three-section cooling roller assembly includes the following three sections: The temperature of the first cooling roller group is 65-75℃, and the cooling residence time is 3-5s. This allows the surface of the outer layer A and the inner layer a to cool down to below the crystallization initiation temperature of metallocene polyethylene first, forming a high crystallinity hardened layer in the ridge area. The bottom of the ridge pit retains a low crystallinity elastic area due to the low contact pressure. The temperature of the second cooling roller is 45-55℃, and the cooling residence time is 6-10s, so that the melt temperature of the outermost layer B, the innermost layer b, the first intermediate adhesive layer C, and the second intermediate adhesive layer c simultaneously passes through the crystallization range of low-density polyethylene and modified ethylene-vinyl acetate copolymer, forming a stress buffer transition layer with medium crystallinity. The temperature of the third cooling roller is 20-30℃, and the cooling residence time is 6-15s, so that the core layer D is cooled to below the glass transition temperature of ethylene-vinyl alcohol copolymer, forming a low-crystallinity, high-barrier amorphous core layer.

10. The application of a seven-layer co-extruded sealing strip resistant to high and low temperature cycles, characterized in that, The seven-layer co-extruded sealing strip resistant to high and low temperature cycles as described in claims 1-7 is applied to high-temperature filling and cold chain packaging of liquid foods.

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