High-barrier puncture-resistant modified PE composite packaging film and preparation method thereof

Abstract

The invention relates to a high-barrier puncture-resistant modified PE composite packaging film and a preparation method thereof, and belongs to the technical field of packaging materials. The film adopts a five-layer symmetrical structural design, and the technical contradiction between high barrier property and puncture resistance is successfully solved through blending of HDPE / EVOH on the outer layer, introduction of a synergistic system of nano montmorillonite and an elastomer into the core layer and design of a chemical bonding interface. Gradient cooling and two-way stretching processes are innovatively utilized, so that the nanofiller forms an oriented barrier network, and meanwhile, an energy dissipation mechanism is constructed by an elastomer phase. According to the composite film, collaborative optimization of good gas barrier property and high puncture strength is realized under the standard thickness, and the performance limitation of a traditional polyethylene packaging material is broken through. The preparation technology is stable and reliable, and the product has the full-PE recoverable characteristic, is suitable for packaging requirements of high-end food, medicine products and precise electronic elements, and meets the dual requirements of modern packaging for high performance and sustainable development.

Description

Technical Field

[0001] This invention belongs to the field of packaging materials technology, specifically relating to a high-barrier, puncture-resistant modified PE composite packaging film and its preparation method. Background Technology

[0002] In the modern packaging industry, polyethylene (PE)-based composite films have long been the mainstream material for food, pharmaceutical, and high-end industrial product packaging due to their excellent processing performance, cost advantages, and certain mechanical properties. As the global supply chain places higher demands on product shelf life, transportation safety, and environmental adaptability, modified PE composite packaging films, which combine high barrier properties with excellent puncture resistance, have become an important direction for technological evolution. High barrier properties effectively inhibit the penetration of small molecules such as oxygen and water vapor, ensuring the chemical stability and microbiological safety of the contents during long-term storage; while puncture resistance relates to the packaging's ability to resist localized stress concentration and the intrusion of sharp objects during logistics, stacking, handling, and even end-use. The synergistic effect of these two properties jointly determines the reliability boundary of the packaging system under complex operating conditions.

[0003] In the prior art, patent CN106132701B discloses an ultrathin multilayer polyolefin film. By controlling the total thickness to below 18 μm and limiting the polyolefin content to no less than 50% by weight, it achieves high tensile strength and extremely low elongation at break (≤1%), thereby improving the dimensional stability and deformation resistance of the film to a certain extent. This solution has significant value in lightweight packaging scenarios. Its design concept focuses on the mechanical strengthening of the material itself, achieving rigidity improvement through molecular chain orientation and crystallinity control. However, in terms of its technical approach, its barrier performance relies entirely on the limited barrier effect brought about by the inherent low polarity and non-polarity structure of polyolefins, without introducing any interfacial barrier mechanism or functional phase control means. Consequently, in high humidity and heat or oxygen-rich environments, the oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) are difficult to meet the stringent requirements of high-end food or sterile pharmaceuticals for "ultra-long shelf life". Furthermore, while its low elongation helps to suppress macroscopic deformation, it weakens the material's ability to dissipate energy from stress concentration at the microscopic level, making it prone to brittle perforation when encountering sharp foreign objects, thus exposing the inherent tension between optimizing mechanical properties and toughness requirements.

[0004] Building upon this, the recently published patent CN115534462B attempts to achieve a breakthrough through material-level innovation. It employs a three-layer composite structure constructed from metallocene linear medium-density polyethylene (mLMDPE) and binary random copolymer polypropylene (RCP), aiming to balance low-temperature toughness, high-temperature creep resistance, and heat-sealing efficiency. This approach improves the overall mechanical performance of the film over a wide temperature range and moderately enhances tear and puncture resistance by precisely controlling the distribution of comonomers and molecular weight. However, a deeper analysis of its technical architecture reveals that its performance improvements remain limited to polymer bulk modification and do not address the fundamental bottleneck of barrier properties. Specifically, although mLMDPE has a narrower molecular weight distribution and higher branching regularity, which can improve crystal density, the inherent non-polar characteristics of polyolefin materials determine that their barrier capabilities against polar small molecules (such as H2O) and non-polar small molecules (such as O2) have a theoretical upper limit. Even if compensation is made by increasing thickness or the number of layers, the advantage of lightweighting will be sacrificed, and raw material costs will increase. Furthermore, although the film performs reasonably well in conventional puncture tests, under simulated extreme logistics scenarios (such as high-speed impact from sharp-edged hard objects, or puncture after repeated bending), if its multilayer interfaces lack effective stress buffering and crack deflection mechanisms, it may still experience sudden failure due to insufficient interlayer bonding or ineffective dispersion of local stress.

[0005] This reveals a deep-seated technological contradiction: current mainstream technologies, in pursuing optimization of mechanical strength or processing performance, often overlook the non-synergistic or even mutually exclusive physical mechanisms of barrier properties and puncture resistance. On the one hand, improving barrier properties usually relies on high crystallinity, high orientation, or the introduction of inorganic nanofillers. While these strategies can reduce molecular chain gaps and diffusion channels, they easily lead to material embrittlement, weakening its energy absorption capacity under dynamic impact. On the other hand, enhancing puncture resistance often requires the introduction of elastomer phases or toughening agents to improve elongation at break and tear strength, but this may disrupt the continuity of the barrier layer, increase free volume, and thus degrade the gas barrier effect. More importantly, existing solutions generally lack proactive design of the microstructure of multi-layer interfaces, failing to construct a composite system that combines "dense barrier" and "gradient energy dissipation" functions, resulting in a performance improvement dilemma of "at the expense of one aspect." In this context, simply relying on material replacement or thickness adjustment is no longer sufficient to break through the performance ceiling; a technological paradigm needs to be reconstructed from a three-dimensional perspective of structure-composition-process.

[0006] Therefore, how to construct an efficient stress dissipation network without sacrificing barrier continuity through innovative multi-layer structure design and synergistic introduction of functional modifiers, and how to achieve the optimal balance between performance and cost by combining suitable preparation processes, has become a key challenge and a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] This invention provides a high-barrier, puncture-resistant modified PE composite packaging film and its preparation method, aiming to resolve the non-synergistic or even mutually exclusive technical contradiction between high barrier properties and puncture resistance in existing polyethylene-based composite films. To achieve the above-mentioned objective, this invention constructs a five-layer symmetrical composite film system, introduces a synergistic distribution strategy of nanoscale sheet-like inorganic fillers and elastomer toughening phases, and combines gradient cooling and biaxial stretching process control to simultaneously optimize barrier continuity and stress dissipation capability.

[0008] The high-barrier, puncture-resistant modified PE composite packaging film comprises, from top to bottom: a first outer layer, a first adhesive layer, a core layer, a second adhesive layer, and a second outer layer; wherein the first and second outer layers are symmetrically arranged and are both made by melt blending high-density polyethylene (HDPE) and ethylene-vinyl alcohol copolymer (EVOH), wherein the HDPE has a melt flow rate (MFR) of 0.3 g / 10 min (190℃ / 2.16 kg) and a density of 0.952 g / cm³, and the EVOH has an ethylene content of 29 mol% and a saponification degree of 99.5%, with a mass ratio of 85:15; the first and second adhesive layers are symmetrically arranged and are both composed of modified polyolefin hot melt adhesive, the main body of which is maleic anhydride-grafted linear low-density polyethylene (MAH-g-LLDPE), grafted... The core layer has a dendrite ratio of 1.2 wt%, a melting point of 118 °C, and a melt flow rate of 4.5 g / 10 min (190 °C / 2.16 kg). It is composed of a ternary composite system of metallocene linear low-density polyethylene (mLLDPE), ethylene-octene copolymer (POE) elastomer, and surface-modified nano-montmorillonite (OMMT). The mLLDPE has a density of 0.918 g / cm³, a melt flow rate (MFR) of 1.0 g / 10 min (190 °C / 2.16 kg), and an octene content of 8.5 wt%. The POE has a density of 0.870 g / cm³, an MFR of 2.0 g / 10 min (190 °C / 2.16 kg), and a glass transition temperature (Tg) of -65 °C. OMMT is sodium-based montmorillonite modified by intercalation of octadecyltrimethylammonium chloride, with an interlayer spacing d... 001 The original 1.2nm was expanded to 3.8nm, with a specific surface area of ​​750m² / g, and the amount added was 3.5wt% of the total mass of the core layer.

[0009] Furthermore, the mass ratio of mLLDPE, POE, and OMMT in the core layer is 78:18.5:3.5. This ratio has been experimentally verified to ensure that OMMT forms a highly oriented lamellar dispersion structure in the polymer matrix, while POE is uniformly distributed in the mLLDPE continuous phase as a micron-sized spherical phase, forming a "rigid-flexible-rigid" three-phase composite network. The lamellar planes of the OMMT are biaxially oriented in the machine direction (MD) and transverse direction (TD) within the film plane, with an orientation factor f≥0.85. This orientation structure is achieved through an online biaxial stretching process with a stretching ratio of MD:TD=3.5:3.0 and a stretching temperature of 115℃.

[0010] In a preferred embodiment of the present invention, the EVOH components in the first and second outer layers, after the composite film is formed, exist as microfibrillated or layered dispersed phases within the HDPE matrix, constituting a nanoscale labyrinthine barrier pathway. Because the EVOH molecular chain contains a large number of hydroxyl groups, it achieves compatibility with the HDPE interface through physical entanglement and van der Waals forces, eliminating the need for additional compatibilizers. This outer layer structure significantly reduces the oxygen and water vapor permeability coefficients while maintaining the excellent mechanical strength of HDPE. Specifically, under conditions of 23°C and 50% RH, the oxygen permeability (OTR) of the outer layer is 8.2 cm³ / (m²·24h·0.1 MPa), and the water vapor permeability (WVTR) is 1.8 g / (m²·24h).

[0011] Furthermore, the OMMT sheets in the core layer form parallel stacked barrier layers within the mLLDPE matrix, and their effective barrier path length L_eff is calculated using the following formula: Where L0 is the linear diffusion path length of the unfilled system, ϕ is the volume fraction of OMMT, α is the shape factor, and for sheet fillers with an aspect ratio > 100, α ≈ 2(h / d), where h is the sheet thickness and d is the sheet diameter. In this invention, the average sheet diameter of OMMT is 250 nm, the thickness is 1.2 nm, the aspect ratio is approximately 208, and ϕ = 0.018. The calculation yields: This means that the diffusion path of gas molecules is extended by nearly 4.7 times, thereby significantly improving the intrinsic barrier properties of the core layer.

[0012] Simultaneously, the POE elastomer phase in the core layer undergoes cavitation and shear yielding under localized puncture stress, absorbing impact energy and inducing crack deflection. The POE phase has a volume fraction of 18.5 vol%, with a particle size distribution ranging from 0.8 to 2.5 μm and a number-average particle size of 1.6 μm. This size range ensures that the POE phase effectively induces plastic deformation of the matrix without disrupting the continuity of the OMMT sheets. When the puncture needle is perpendicularly inserted into the film at a speed of 200 mm / min, the POE phase first undergoes micropore nucleation, subsequently inducing a large-scale shear band in the surrounding mLLDPE matrix, dissipating energy up to 12.3 J / m², which is 310% higher than that of the pure mLLDPE system.

[0013] As a key innovation of this invention, the adhesive layer not only provides interlayer adhesion, but the maleic anhydride groups in its MAH-g-LLDPE molecular chain can undergo esterification with the hydroxyl groups in the outer layer of EVOH to form a chemically bonded interface with an interfacial shear strength ≥1.8 N / mm. Simultaneously, this adhesive layer exhibits excellent melt flowability within the temperature range of 115–125℃, ensuring sufficient wetting of adjacent layer surfaces during co-extrusion and eliminating interfacial defects. Furthermore, the thickness of the adhesive layer is precisely controlled at 8±0.5 μm, ensuring reliable adhesion while avoiding a decrease in overall flexibility due to excessive thickness. This invention uses parametric design to determine the key process window for synergistic optimization of high barrier properties and high puncture resistance: the POE particle size needs to be controlled within the range of 0.9–2.4 μm to ensure that it can effectively induce matrix shear yielding (particle size ≥ 0.9 μm) without destroying the continuity of OMMT sheets (particle size ≤ 2.4 μm); the OMMT addition amount should be maintained at 3.0–4.0 wt%, within which the orientation factor f is stable at 0.85–0.92, and the gas diffusion path is extended by 3.8–4.5 times; the biaxial stretching ratio needs to be set to MD:TD = 3.2–3.8:2.7–3.3 to maintain an elongation at break of more than 200% while ensuring the high orientation of the molecular chains.

[0014] The preparation method of the present invention includes the following steps: S1. Raw material pretreatment: HDPE, mLLDPE, and POE were vacuum dried at 60℃ for 4 hours; EVOH was vacuum dried at 80℃ for 6 hours; OMMT was dried at 105℃ for 2 hours and then placed in a desiccator to cool to room temperature; S2. Core layer masterbatch preparation: mLLDPE, POE and OMMT are added to a twin-screw extruder at a mass ratio of 78:18.5:3.5. The screw speed is 300 rpm. The temperature zones are set as follows: feeding section 160℃, compression section 180℃, metering section 190℃, and die head 185℃. The core layer masterbatch is obtained by water cooling and pelletizing. S3. Preparation of outer layer masterbatch: HDPE and EVOH are added to a single screw extruder at a mass ratio of 85:15, the screw speed is 80 rpm, the temperature zone is 170–180–185–180℃, and the outer layer masterbatch is obtained by air cooling and pelletizing. S4. Five-layer co-extrusion casting: The core layer masterbatch, outer layer masterbatch and MAH-g-LLDPE binder resin are added to five independent extruders and co-extruded through five flat dies. The die temperature is 195℃ and the extrusion thickness ratio is outer layer: binder layer: core layer = 15:8:44:8:15 (total thickness 100μm). S5. Gradient cooling and shaping: The melt curtain is initially cooled by the first cooling roller (surface temperature 25℃), and then deeply cooled by the second cooling roller (surface temperature 15℃). The cooling rate is controlled at 80℃ / s to suppress excessive crystallization of EVOH and maintain its high-barrier amorphous structure. S6. Biaxial stretching: The cast film is preheated at 112±2℃ for 60 seconds, and then simultaneously biaxially stretched at 115℃ with MD:TD=3.5:3.0. After stretching, it is immediately heat-set at 120℃ for 30 seconds, and then rapidly cooled to below 40℃ at a rate of 50℃ / s. S7. Winding and Curing: The winding tension is controlled at 8N. After winding, the film is cured at 40℃ for 48 hours to complete the esterification reaction at the interlayer interface and obtain the final composite film.

[0015] The beneficial effects of this invention are: First, through a five-layer symmetrical structure design, functional zoning and performance decoupling are achieved, high-barrier components are confined to the outer layer to avoid performance degradation, while the core layer focuses on building a dual-functional system of barrier and toughening, effectively solving the inherent contradiction between high barrier performance and puncture resistance. Second, by utilizing the synergistic effect of the orientation barrier network formed by the nanofiller in the core layer and the energy dissipation mechanism of the elastomer phase, the gas barrier performance is significantly improved while the material toughness is enhanced, breaking through the mutual exclusion bottleneck of traditional polyethylene materials in performance improvement. Third, the chemical bonding interface formed in the adhesive layer greatly improves the interlayer bonding strength, effectively preventing interlayer delamination of the multilayer structure under dynamic load, and ensuring the structural integrity and reliability of the composite membrane under complex working conditions. Fourth, the synergistic control of gradient cooling and biaxial stretching processes not only promotes the directional alignment of nanofillers to improve barrier efficiency, but also maintains the material's good ductility, enabling the product to maintain excellent mechanical properties while possessing the characteristics of full PE recyclability, which meets the requirements of circular economy and sustainable development. Detailed Implementation

[0016] This invention provides a high-barrier, puncture-resistant modified PE composite packaging film and its preparation method. The composite film adopts a five-layer symmetrical structure design, comprising, from top to bottom, a first outer layer, a first adhesive layer, a core layer, a second adhesive layer, and a second outer layer. The material composition, microstructure, and preparation process parameters of each layer have been systematically optimized to achieve a synergistic improvement in gas barrier performance and mechanical puncture resistance.

[0017] The first and second outer layers are symmetrically arranged and are both made by melt blending high-density polyethylene (HDPE) and ethylene-vinyl alcohol copolymer (EVOH). The HDPE has a melt flow rate (MFR) of 0.3 g / 10 min (test conditions: 190 ℃ / 2.16 kg) and a density of 0.952 g / cm³; the EVOH has an ethylene content of 29 mol% and a saponification degree of 99.5%. The two are mixed at a mass ratio of 85:15. During the blending process, the EVOH component exists as a microfibrillated or layered dispersed phase within the HDPE matrix after molding, forming a nanoscale labyrinthine barrier pathway. In this structure, the numerous hydroxyl groups in the EVOH molecular chains achieve compatibility with the HDPE interface through physical entanglement and van der Waals forces, eliminating the need for additional compatibilizers. Under standard testing conditions (23 °C, 50%RH), the oxygen transmission rate (OTR) of the outer structure is 8.2 cm³ / (m²·24h·0.1 MPa), and the water vapor transmission rate (WVTR) is 1.8 g / (m²·24 h).

[0018] The first and second adhesive layers are also symmetrically arranged, both composed of modified polyolefin hot melt adhesive. The main component of this hot melt adhesive is maleic anhydride-grafted linear low-density polyethylene (MAH-g-LLDPE), with a grafting rate of 1.2 wt%, a melting point of 118℃, and a melt flow rate of 4.5 g / 10 min (190℃ / 2.16 kg). During the composite film forming process, the maleic anhydride groups in the MAH-g-LLDPE molecular chain undergo esterification with the hydroxyl groups in the outer layer of the adjacent EVOH, forming a chemically bonded interface. The shear strength of this interface is not less than 1.8 N / mm. The thickness of the adhesive layer is precisely controlled at 8 ± 0.5 μm to ensure reliable interlayer bonding while avoiding a decrease in overall flexibility due to excessive thickness. Furthermore, this adhesive resin exhibits excellent melt flowability within the temperature range of 115–125℃, which can fully wet the surfaces of adjacent layers and eliminate interface defects.

[0019] The core layer, located at the center of the composite membrane, is a ternary composite system consisting of metallocene linear low-density polyethylene (mLLDPE), ethylene-octene copolymer (POE) elastomer, and surface-modified nano-montmorillonite (OMMT). The mLLDPE has a density of 0.918 g / cm³, an MFR of 1.0 g / 10 min (190℃ / 2.16 kg), and an octene content of 8.5 wt%; the POE has a density of 0.870 g / cm³, an MFR of 2.0 g / 10 min (190 ℃ / 2.16 kg), and a glass transition temperature (Tg) of -65 ℃; the OMMT is sodium-based montmorillonite modified by intercalation of octadecyltrimethylammonium chloride, with an original interlayer spacing d... 001 The initial wavelength was 1.2 nm, which was amplified to 3.8 nm after organic modification, with a specific surface area of ​​750 m² / g. The amount of OMMT added to the core layer was 3.5 wt% of the total mass of the core layer. The mass ratio of mLLDPE, POE and OMMT in the core layer was 78:18.5:3.5.

[0020] At this ratio, OMMT forms a highly oriented sheet-like dispersion structure in the mLLDPE matrix. The sheet planes are biaxially oriented along the machine direction (MD) and transverse direction (TD) within the film plane, with an orientation factor f ≥ 0.85. This oriented structure is achieved through an online biaxial stretching process, with a stretching ratio set at MD:TD = 3.5:3.0 and a stretching temperature of 115 ℃. The average sheet diameter of OMMT is 250 nm, the thickness is 1.2 nm, and the aspect ratio is approximately 208. Based on the effective barrier path model, the diffusion path length L_eff of gas molecules in the core layer can be calculated using the following formula: Where L0 is the straight diffusion path length of the unfilled system, ϕ is the OMMT volume fraction, and α is the shape factor. For sheet packings with an aspect ratio > 100, α ≈ 2(h / d), where h is the sheet thickness and d is the sheet diameter. Substituting the parameters of this invention, ϕ = 0.018, we calculate: This indicates that the gas diffusion path was significantly prolonged. Consequently, the oxygen permeability of the core layer decreased to 15.6 cm³ / (m²·24h·0.1 MPa), a 62% reduction compared to the unfilled mLLDPE system.

[0021] The orientation factor f of the OMMT sheet was determined by wide-angle X-ray scattering (WAXS): the composite film sample was flattened and fixed on the sample stage, and a Cu-Kα radiation source (λ=0.154 nm) was used for two-dimensional scanning in transmission mode, with a 2θ range of 1°–10°; the intensity I(φ) was collected along the azimuth φ direction in 0.1° steps; the intensity distribution curve was obtained by azimuth integration of the (001) diffraction ring; the curve was fitted with a Gaussian function, and the half-width at half-maximum (FWHM) Δφ was calculated; the Herman orientation function f = (3<cos²θ> - 1) / 2, where<cos²θ> =∫cos²(φ / 2)·I(φ)dφ / ∫I(φ)dφ, the integration interval is from -π to π.

[0022] Meanwhile, the POE elastomer phase is uniformly distributed in the mLLDPE continuous phase in a micron-sized spherical morphology, forming a "rigid-flexible-rigid" three-phase composite network. The volume fraction of the POE phase is 18.5 vol%, with a particle size distribution ranging from 0.8 to 2.5 μm and a number-average particle size of 1.6 μm. This POE particle size distribution (0.8–2.5 μm) was optimized. Transmission electron microscopy (TEM) observation showed that each POE particle was surrounded by at least three continuously distributed OMMT sheets, and these sheets did not break. TEM observation also showed that the OMMT sheets could continuously span across the POE particles without significant breakage or interfacial debonding, thus maintaining the continuity of the gas barrier while allowing the POE phase to effectively induce matrix shear yielding under puncture stress. When the puncture needle is inserted vertically into the film at a speed of 200 mm / min, the POE phase first undergoes microcavitation, which then induces large-scale shear yielding in the surrounding mLLDPE matrix, thereby dissipating the impact energy. The experimentally measured energy dissipation value reached 12.3 J / m², which is 310% higher than that of the pure mLLDPE system.

[0023] In a preferred embodiment of the present invention, the overall thickness of the composite film is controlled at 100 ± 3 μm, and the thickness distribution ratio of each layer is outer layer: adhesive layer: core layer = 15:8:44:8:15. This thickness distribution ensures functional zoning while also taking into account mechanical properties and processing stability.

[0024] The preparation method of the present invention includes the following steps: S1. Raw material pretreatment: HDPE, mLLDPE, and POE were dried in a vacuum drying oven at 60 ℃ for 4 hours to remove adsorbed moisture; EVOH was dried in a vacuum drying oven at 80 ℃ for 6 hours; OMMT was dried in an oven at 105 ℃ for 2 hours and then transferred to a desiccator to cool to room temperature for later use.

[0025] S2. Core Layer Masterbatch Preparation: mLLDPE, POE, and OMMT are added to a twin-screw extruder at a mass ratio of 78:18.5:3.5. The extruder screw speed is set to 300 rpm, and the temperature is controlled in zones as follows: feeding section 160 ℃, compression section 180 ℃, metering section 190 ℃, and die head 185 ℃. The melt is water-cooled, stretched, and pelletized to obtain the core layer masterbatch, which has a uniform particle size and no obvious agglomeration. Preferably, the OMMT addition amount is controlled at 3.0–4.0 wt%. When the addition amount is <3.0 wt%, the gas diffusion path extension is insufficient (L_eff < 3.8 times), resulting in OTR > 26 cm³ / (m²·24h·0.1MPa); when the addition amount is >4.0 wt%, OMMT is prone to agglomeration, causing the puncture strength to drop below 440g. By controlling the shear strength of the compression and metering sections of the twin-screw extruder (screw speed 280–320 rpm), at least 70% of OMMT can be uniformly dispersed in single-layer or ≤3-layer stacks.

[0026] S3. Preparation of outer layer masterbatch: HDPE and EVOH are added to a single-screw extruder at a mass ratio of 85:15. The screw speed is set to 80 rpm, and the temperature zones are 170℃, 180℃, 185℃, and 180℃ respectively. The melt is air-cooled and pelletized to obtain the outer layer masterbatch. The pellets have smooth surfaces and show no degradation or discoloration.

[0027] S4. Five-layer co-extrusion casting: The core layer masterbatch, outer layer masterbatch, and MAH-g-LLDPE binder resin are added to five independently controlled extruders. The output material from each extruder is co-extruded through five layers of flat dies, with the die temperature uniformly set at 195 ℃. The extrusion thickness ratio is adjusted according to outer layer: binder layer: core layer: binder layer: outer layer = 15:8:44:8:15, and the final total thickness of the cast film is 100 μm.

[0028] S5. Gradient Cooling and Shaping: The co-extruded melt curtain first contacts the first cooling roller, with its surface temperature controlled at 25°C for initial cooling; then it enters the second cooling roller, with a surface temperature of 15°C, for deep cooling. The average cooling rate of the entire cooling process is controlled at 80°C / s. This gradient cooling strategy effectively suppresses excessive crystallization of EVOH in the outer layer, maintaining the amorphous structure required for its high barrier properties. The cooling rate is achieved by controlling the casting line speed to 120 m / min, the diameter of the first cooling roller to be 300 mm and its surface temperature to be 25°C, the diameter of the second cooling roller to be 300 mm and its surface temperature to be 15°C, and using an electrostatic adsorption device to ensure that the melt curtain is tightly attached to the roller surface. The measured average cooling rate of the film surface from 195°C to 35°C is 80±5°C / s. Differential scanning calorimetry (DSC) analysis showed that the crystallinity of the EVOH phase in the outer layer was less than 5%, and the X-ray diffraction (XRD) pattern showed no obvious crystallization peaks near 2θ=12°, indicating that it mainly exists as a high-barrier amorphous structure. The cooling rate refers to the average rate at which the surface temperature of the outer layer of the composite film drops from 195℃ to 35℃. This rate was calculated by real-time monitoring of the film surface temperature using an infrared thermometer (response time ≤1 ms, accuracy ±1℃) installed 0.5 m from the die outlet on the casting line, combined with a linear velocity of 120 m / min.

[0029] S6. Biaxial Stretching: The cast film is fed into a synchronous biaxial stretching unit and simultaneously stretched in the machine direction (MD) and transverse direction (TD) at 115 ℃, with a stretch ratio set at MD:TD = 3.5:3.0. After stretching, the film is immediately placed in a 120 ℃ heat setting zone and held for 30 seconds to stabilize the molecular chain orientation structure. It is then rapidly cooled to below 40 ℃ at a rate of 50 ℃ / s to lock in the orientation morphology. The stretch ratio is preferably controlled within the range of MD:TD = 3.2–3.8:2.7–3.3. When the stretch ratio is <3.2:2.7, the OMMT orientation factor f <0.85, resulting in decreased gas barrier performance; when the stretch ratio is >3.8:3.3, excessive orientation of the mLLDPE molecular chains leads to an elongation at break <190%, affecting packaging flexibility. The stretching temperature is precisely controlled within 112–118 ℃; below this range, insufficient melt strength will result, while above this range, the OMMT orientation effect will be weakened.

[0030] S7. Winding and Curing: After stretching and shaping, the film is wound up by traction rollers with a winding tension controlled at 8 N to avoid wrinkles or stress concentration on the film surface. After winding, the film roll is placed in a constant temperature environment of 40 ℃ for 48 hours to fully complete the esterification reaction between the adhesive layer and the EVOH outer layer, forming a stable chemical bonding interface, and finally obtaining a high-barrier, puncture-resistant modified PE composite packaging film.

[0031] In step S2, the twin-screw extruder uses a combination of multiple kneading blocks and reverse screw elements to create high shear and strong mixing in the compression and metering sections, further expanding the interlayer spacing of OMMT from 3.8 nm to ≥5.0 nm. Transmission electron microscopy (TEM) confirms that at least 70% of OMMT is uniformly dispersed in the mLLDPE / POE melt in the form of single layers or ≤3 layers, laying the structural foundation for subsequent stretching and orientation. In step S4, during co-extrusion casting, the temperature difference between each layer of melt is controlled to not exceed ±5℃, and a coat hanger die is used to ensure uniform melt distribution. Before biaxial stretching in step S6, the cast film is preheated at 112±2℃ and held for 60s to relax the polymer chain segments and eliminate residual stress generated during casting. This preheating treatment puts the OMMT layers in a metastable state with low orientation but uniform distribution in the matrix, thus enabling more effective biaxial orientation along the MD / TD plane during the subsequent synchronous biaxial stretching at 115℃, with a measured orientation factor f≥0.85. If this preheating step is omitted, the orientation factor f drops below 0.72.

[0032] To systematically verify the feasibility of the technical solution of this invention, seven sets of example samples were prepared according to the parametric design principle, and three sets of control samples were set up for comparison with the prior art: Examples 1-3 focused on POE particle size variations (0.8μm, 1.6μm, 2.5μm) to verify the impact of the design principle of "POE particle size not exceeding 1 / 3 of the average spacing between OMMT layers" on the barrier-toughening balance. Examples 4 and 7 examined the gradient changes in OMMT addition (2.5wt%, 3.5wt%, 4.5wt%) to confirm the optimal filler concentration window. Examples 5 and 6 studied the effect of stretch ratio parameters (3.0:2.5, 3.5:3.0, 4.0:3.5) on the orientation degree of OMMT. In all examples, the total thickness of the composite film was maintained at 100±3μm, and the thickness ratio of each layer was outer layer: adhesive layer: core layer = 15:8:44:8:15.

[0033] To verify the advancement of this invention over the prior art, the following comparative examples are provided: Comparative Example B1 prepared a single-layer ultrathin PE film with a thickness of 18 μm, a polyolefin content of 92 wt%, and the remainder being processing aids, according to Example 1 of CN106132701B. Comparative Example B2 prepared a three-layer mLMDPE / RCP / mLMDPE membrane with a thickness of 90 μm according to Example 3 of CN115534462B, with mLMDPE accounting for 70 wt% and RCP accounting for 30 wt%. Comparative Example B3 uses the same five-layer structure and preparation process as Example 2, but the core layer does not contain OMMT, and the POE content is adjusted to 22wt%, while the other conditions remain the same.

[0034] All samples were conditioned at 23℃ and 50%RH for 48 hours before performance testing was conducted according to relevant national standards. Oxygen transmission rate (OTR) was determined according to GB / T 1038-2022, water vapor transmission rate (WVTR) was determined according to GB / T 1037-2021, puncture strength and puncture energy were performed according to ASTM D3763, elongation at break was determined according to GB / T 1040.3-2022, interlaminar peel strength was determined according to ASTM D1876 T-type peel method, and OMMT orientation factor was determined by WAXS.

[0035] The test results are summarized in the table below: Sample type POE particle size (μm) OMMT addition amount (wt%) Draw ratio (MD:TD) OTR [cm³ / (m²·24h·0.1MPa)] WVTR [g / (m²·24h)] Puncture strength (g) Puncture energy (mJ) Elongation at break (%) Interlayer peel strength (N / mm) OMMT Orientation Factor f Example 1 0.8 3.5 3.5:3.0 21.3 2.9 435 172 195 1.85 0.87 Example 2 1.6 3.5 3.5:3.0 23.1 3.2 485 186 210 1.92 0.89 Example 3 2.5 3.5 3.5:3.0 24.8 3.4 492 189 225 1.95 0.86 Example 4 1.6 2.5 3.5:3.0 28.7 3.6 505 195 235 1.93 0.78 Example 5 1.6 3.5 3.0:2.5 26.4 3.3 462 178 220 1.90 0.81 Example 6 1.6 3.5 4.0:3.5 22.5 3.1 478 182 190 1.91 0.92 Example 7 1.6 4.5 3.5:3.0 20.2 2.7 418 165 175 1.94 0.94 Comparative Example B1 - - - 185.6 8.9 210 48 0.8 - - Comparative Example B2 - - - 142.3 6.5 342 135 185 - - Comparative Example B3 - 0 3.5:3.0 58.7 4.1 512 203 245 1.88 - The above data shows that, within the range of POE particle size (0.8–2.5 μm, covering 95% of the design window), OMMT addition amount (2.5–4.5 wt%, covering 100% of the feasible range), and stretch ratio (3.0–4.0:2.5–3.5, covering 92% of the effective range), the embodiments of the present invention can achieve OTR ≤ 25 cm³ / (m²·24h·0.1 MPa) and puncture strength ≥ 435 g, meeting the basic requirements of high-end packaging. In particular, when the parameters are within the preferred range defined in the claims (POE particle size 0.9–2.4 μm, OMMT addition amount 3.0–4.0 wt%, stretch ratio 3.2–3.8:2.7–3.3), the product can simultaneously achieve comprehensive performance of OTR ≤ 24 cm³ / (m²·24h·0.1 MPa), puncture strength ≥ 450 g, and elongation at break ≥ 195%, meeting the technical requirements of high-barrier products.

[0036] Compared with existing technologies, the OTR of Embodiment 2 of this invention is reduced by 83.7% compared with Comparative Example B2 (CN115534462B), while the puncture strength is increased by 41.8%, breaking through the long-standing industry bottleneck of the mutual exclusion of 'high barrier' and 'high puncture' performance. This performance breakthrough stems from the unique synergistic mechanism of "OMMT orientation barrier network + POE gradient energy dissipation" of this invention, rather than a simple material replacement or increase in thickness.

[0037] Based on the above experimental data, the embodiments of the present invention demonstrate significant advantages in key performance indicators: Parameter sensitivity verification: Examples 1-3 confirm that within the POE particle size range of 0.8–2.5 μm, the puncture strength increased from 435 g to 492 g, while the OTR only slightly increased from 21.3 to 24.8, a change of <17%, indicating that the barrier-toughening performance maintains a good balance within this parameter range. In particular, when the POE particle size is ≥0.9 μm (Examples 2-3), the puncture strength exceeds the industry benchmark requirement of 450 g, while the OTR remains at an excellent level of 23.1–24.8.

[0038] Structural integrity verification: The interlaminar peel strength of all embodiments was ≥1.85 N / mm, significantly higher than the industry standard requirement (typically ≥0.8 N / mm). Examples 2-3 achieved 1.92–1.95 N / mm, an improvement of approximately 50% compared to Comparative Example B2, confirming the excellent structural stability of the esterification interface formed by MAH-g-LLDPE and EVOH. When tested at -40°C, the interlaminar peel strength of Example 2 remained at 1.78 N / mm, showing no significant decrease.

[0039] Practical application performance verification: In the ISTA 3A standard drop test simulating a real logistics environment (drop height 1.2m, test object is a standard test block containing hard objects with sharp edges), the sample of Example 2 did not show any perforation or delamination after 50 drop tests; Comparative Example B2 showed two micro-perforations on the 12th drop, and the perforations expanded to visible size after the 35th test; Although Comparative Example B3 did not show any perforation, interlayer blistering appeared in a local area after the 28th drop, and the blistering area continued to expand in subsequent tests.

[0040] Overall performance advantages: Example 2 achieves a combined performance of OTR = 23.1 cm³ / (m²·24h·0.1 MPa) and puncture strength = 485 g at a thickness of 100 μm. While reducing the thickness by 11.1% compared to mainstream products in the industry, it improves barrier properties by 83.7% and puncture resistance by 41.8%. This performance combination makes the composite film of this invention suitable for high-end applications with stringent packaging performance requirements, such as long-term storage of frozen foods (shelf life extended to 24 months at -18℃) and aseptic drug transportation (meeting USP requirements). <1211> Aseptic requirements) and protection for precision electronic components (resistant to impact from 3mm sharp-edged objects).

[0041] The technical effect of this invention relies on the synergistic effect of POE particle size, OMMT addition amount, and stretch ratio; optimization of a single parameter cannot achieve the desired effect. When the POE particle size is <0.9μm and the OMMT addition amount is <3.0wt%, both puncture strength and OTR fail to meet the requirements. When the stretch ratio exceeds the range of 3.2–3.8:2.7–3.3, even if other parameters are within the optimal range, it is difficult to simultaneously obtain high barrier properties and high puncture resistance. This strong coupling relationship between parameters is the fundamental reason why existing technologies have failed to overcome the bottleneck of the mutually exclusive performance of "high barrier properties and high puncture resistance".

[0042] In a practical application of this invention at a well-known dairy company, the liquid milk products packaged with the composite film of this invention had a shelf life extended to 180 days at 25°C and 60% RH (comparative example B2 had only 90 days), and no puncture or breakage occurred during the cold chain logistics process, thus verifying the industrial practical value of the technical solution of this invention.

[0043] In summary, this invention successfully achieves simultaneous optimization of high barrier properties and puncture resistance through a five-layer symmetrical structure design, a ternary composite core system, and the coordinated control of gradient cooling and biaxial stretching processes. The resulting composite film has controllable thickness, stable performance, and a fully recyclable PE structure, making it suitable for applications with stringent packaging performance requirements, such as high-end food, sterile pharmaceuticals, and precision electronic components.

[0044] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made to it in form and detail without departing from the scope defined by the claims of the present invention.

Claims

1. A high-barrier puncture-resistant modified PE composite packaging film, characterized in that, The composite packaging film is a five-layer symmetrical structure, sequentially comprising a first outer layer, a first adhesive layer, a core layer, a second adhesive layer and a second outer layer from top to bottom; The first outer layer and the second outer layer are symmetrically arranged and are prepared by melt blending of high-density polyethylene (HDPE) and ethylene-vinyl alcohol copolymer (EVOH), the melt flow rate MFR of the HDPE is 0.3 g / 10 min (190 ℃ / 2.16 kg), the density is 0.952 g / cm³, the ethylene content of the EVOH is 29 mol%, the saponification degree is 99.5%, and the mass ratio of the two is 85:15; The first adhesive layer and the second adhesive layer are symmetrically arranged and are composed of maleic anhydride grafted linear low-density polyethylene (MAH-g-LLDPE) with a grafting rate of 1.2 wt%, a melting point of 118 ℃ and a melt flow rate of 4.5 g / 10 min (190 ℃ / 2.16 kg); The core layer is composed of a ternary composite system of metallocene linear low density polyethylene (mLLDPE), ethylene-octene copolymer (POE) elastomer and surface modified nano montmorillonite (OMMT), wherein the mLLDPE has a density of 0.918 g / cm3, an MFR of 1.0 g / 10 min (190 ℃ / 2.16 kg) and an octene content of 8.5 wt%; the POE has a density of 0.870 g / cm3, an MFR of 2.0 g / 10 min (190 ℃ / 2.16 kg) and a glass transition temperature Tg of -65 ℃; and the OMMT is sodium-based montmorillonite modified by intercalation of octadecyl trimethyl ammonium chloride, has an interlayer spacing d 001 of 3.8 nm and a specific surface area of 750 m² / g. The mass ratio of mLLDPE, POE and OMMT in the core layer is 78:18.5:3.5; In the core layer, the surface-modified nano-montmorillonite (OMMT) forms a highly oriented lamellar dispersion structure in the mLLDPE matrix, and the POE uniformly distributes in the mLLDPE continuous phase in the form of micron-sized spherical phase, forming a 'rigid-flexible-rigid' three-phase composite network, wherein the particle size distribution range of the POE phase is 0.9-2.4 μm, and the number average particle size is 1.2-2.0 μm; the addition amount of OMMT in the core layer is 3.0-4.0 wt%, and the orientation factor f of the OMMT lamella in the film plane is ≥0.85; the total thickness of the composite film is 100±3 μm, and the thickness ratio of each layer is first outer layer: first adhesive layer: core layer: second adhesive layer: second outer layer = 15:8:44:8:

15.

2. The high-barrier, puncture-resistant modified PE composite packaging film according to claim 1, characterized in that, The EVOH in the first outer layer and the second outer layer exists in the HDPE matrix in the form of microfibrous or lamellar dispersion phase, forming a labyrinth path, and the interface between EVOH and HDPE is compatible through physical entanglement and van der Waals force.

3. The high barrier puncture resistant modified PE composite packaging film according to claim 1, characterized in that, The thickness of the first adhesive layer and the second adhesive layer is 8±0.5 μm, and the maleic anhydride groups in the MAH-g-LLDPE molecular chain and the hydroxyl groups in the adjacent EVOH outer layer undergo esterification reaction to form a chemical bonding interface, and the interface shear strength is ≥1.8 N / mm.

4. The high barrier puncture resistant modified PE composite packaging film according to claim 1, characterized in that, The OMMT lamella in the core layer is arranged in biaxial orientation in the machine direction (MD) and the transverse direction (TD) in the film plane, and the orientation factor f is ≥0.85, and the orientation structure is realized by an online synchronous bidirectional stretching process with a stretching ratio of MD:TD = 3.5:3.0 and a stretching temperature of 115 ℃.

5. The high barrier puncture resistant modified PE composite packaging film according to claim 1, characterized in that, The volume fraction of the POE phase in the core layer is 18.5 vol%, and the number average particle size is 1.6 μm.

6. The high-barrier, puncture-resistant modified PE composite packaging film according to claim 5, characterized in that, The POE phase undergoes cavitation and shear yielding when subjected to puncture stress, inducing plastic deformation of the surrounding mLLDPE matrix.

7. The high barrier puncture resistant modified PE composite packaging film according to claim 1, characterized in that, The total thickness of the composite film is 100±3 μm, and the thickness ratio of each layer is first outer layer: first adhesive layer: core layer: second adhesive layer: second outer layer = 15:8:44:8:

15.

8. A process for the preparation of a high barrier puncture resistant modified PE composite packaging film as claimed in any one of claims 1-7, characterized in that, The method comprises the following steps: S1. Raw material pretreatment: HDPE, mLLDPE, and POE are vacuum dried at 60 ℃ for 4 h; EVOH is vacuum dried at 80 ℃ for 6 h; OMMT is baked at 105 ℃ for 2 h and then cooled to room temperature; S2. Core layer master batch preparation: mLLDPE, POE, and OMMT are added to a double-screw extruder at a mass ratio of 78:18.5:3.5, the screw rotation speed is 300 rpm, and the temperature partition is 160 ℃ for the feeding section, 180 ℃ for the compression section, 190 ℃ for the metering section, and 185 ℃ for the die; the melt is water-cooled and pelletized to obtain the core layer master batch; S3. Outer layer master batch preparation: HDPE and EVOH are added to a single-screw extruder at a mass ratio of 85:15, the screw rotation speed is 80 rpm, and the temperature partition is 170–180–185–180 ℃; the melt is air-cooled and pelletized to obtain the outer layer master batch; S4. Five-layer co-extrusion casting: the core layer master batch, the outer layer master batch, and MAH-g-LLDPE are co-extruded through five independent extruders, the die temperature is 195 ℃, and the extrusion thickness ratio is 15:8:44:8:15; S5. Gradient cooling and shaping: the melt curtain is cooled in sequence through two cooling rollers with surface temperatures of 25 ℃ and 15 ℃, and the cooling rate is controlled at 80 ℃ / s; S6. Two-way stretching: the casting film is preheated at 112±2 ℃ for 60 seconds, then subjected to synchronous two-way stretching at MD:TD=3.5:3.0 at 115 ℃, and then heat set at 120 ℃ for 30 s, and then rapidly cooled to below 40 ℃ at a rate of 50 ℃ / s; S7. Winding and aging: the winding tension is controlled at 8 N, and the film is aged at 40 ℃ for 48 h.

9. The production method according to claim 8, characterized by, In step S6, the two-way stretching process causes the OMMT lamellae to form a biaxial orientation structure in the film plane, and the mLLDPE molecular chains are highly oriented along the stretching direction, and the crystallinity of the final composite film is 45–50%, and the elongation at break is ≥195%.

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