A high puncture resistant polyethylene based film having microfoamed stress isolation structure and a method of making the same

Abstract

The application discloses a high-puncture-resistance polyethylene base film with a micro-foaming stress isolation structure and a preparation method thereof. The base film has a three-layer structure, wherein outer and inner surface layers are formed by compounding metallocene polyethylene (mPE), high-density polyethylene (HDPE) and low-density polyethylene (LDPE); and a micro-foaming core layer is formed by blending mPE, polyolefin elastomer (POE), hydrophobic nano-silicon dioxide and LDPE and then physically foaming by supercritical carbon dioxide, and a three-layer co-extrusion film blowing process is adopted in the preparation. Tests show that the puncture resistance of the film reaches 14.7-17.4 N, and the film is suitable for vacuum packaging of bone and meat and parts with edges and corners.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, and in particular to a highly puncture-resistant polyethylene film with a micro-foamed stress isolation structure and its preparation method. Background Technology

[0002] In the vacuum packaging of bone-in meat and shellfish, as well as the packaging of industrial parts with sharp edges, packaging films need to possess excellent puncture and tear resistance. The irregular edges of these items are highly susceptible to puncture or tearing of the packaging film during vacuuming or transportation, leading to seal failure. Currently, the industry commonly uses solid high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE) films. However, when these solid films are punctured by sharp objects, the resulting stress is highly concentrated at the puncture point, causing the material to whiten due to stress and tear along the stress concentration direction, failing to effectively prevent crack propagation. Simply increasing the thickness goes against the trend of lightweighting and is costly; while using large amounts of metallocene polyethylene can improve toughness, it leads to insufficient film stiffness, easily causing jamming and wrinkles on automated packaging lines. Furthermore, traditional foaming technology, due to its large and unevenly distributed pores, severely degrades the mechanical strength of the film, making it unsuitable for packaging scenarios with high defensive requirements.

[0003] Therefore, this invention is proposed to solve the above-mentioned technical problems. Summary of the Invention

[0004] The primary objective of this invention is to provide a highly puncture-resistant polyethylene film with a micro-foamed stress-isolation structure. The film structure comprises an outer surface layer, a micro-foamed core layer, and an inner surface layer. Both the outer and inner surface layers are composed of metallocene polyethylene resin, high-density polyethylene resin, and low-density polyethylene resin. The micro-foamed core layer is formed by physical foaming of a blend of metallocene polyethylene resin, polyolefin elastomer (POE), an inorganic nucleating agent, and low-density polyethylene resin. The thickness ratio of the outer surface layer, the micro-foamed core layer, and the inner surface layer is 1:(1.33~2):1.

[0005] Preferably, in the outer and inner outer layers, the mass percentage of metallocene polyethylene resin is 30-40%, the mass percentage of high-density polyethylene resin is 10-20%, and the mass percentage of low-density polyethylene resin is 40-50%.

[0006] Preferably, in the blend forming the microfoamed core layer, the mass percentage of metallocene polyethylene resin is 30%~35%, the mass percentage of polyolefin elastomer POE is 25%~30%, the mass percentage of inorganic nucleating agent is 1%~5%, and the mass percentage of low-density polyethylene resin is 30%~35%.

[0007] Preferably, the inorganic nucleating agent is hydrophobically modified nano-silica.

[0008] This invention also provides a method for preparing a highly puncture-resistant polyethylene film with a micro-foamed stress-isolation structure, comprising the following steps: S1. Extrusion: The raw materials of the outer surface layer, the micro-foamed core layer and the inner surface layer are fed into the three barrels of the three-layer co-extrusion blown film machine according to the above component ratio requirements. The raw materials in the barrels are heated into melts in sequence through the trough barrel, barrel 1 zone, barrel 2 zone, barrel 3 zone, barrel 4 zone, screen changing zone and flange zone, and then extruded into tubular materials through the extruder die. S2. Foaming molding: Before the tubular material leaves the die head, supercritical carbon dioxide is injected into the micro-foamed core layer, and the pressure drops sharply when it leaves the die head lip to form a micro-foamed structure; at the same time, compressed air is injected from the central hole of the die head to inflate it into a bubble film; blown film parameters: blow ratio is 2.5~3.0, draw ratio is 3.0~5.0; S3. Cooling and Shaping: The bubble membrane is rapidly cooled and shaped under the cooling action of the water ring shaping device and the air ring, and the position of the sizing cage is adjusted to stabilize the membrane bubble. S4. Thickness Measurement and Corona Treatment: After cooling and shaping, the thickness of the film is measured by an automatic thickness measuring device, then pulled up by a traction device and flattened by a herringbone plate bubble extinguishing clamp, and then clamped together by a traction roller to form a double-layer film. The outer surface of the film is corona treated by a corona device. S5. Edge trimming and winding: Finally, the edges are trimmed and sliced ​​by a winding device, and the trimmed edge material is returned to step S1 for reuse to obtain the high puncture-resistant polyethylene film with micro-foamed stress isolation structure.

[0009] Preferably, the extrusion parameters in step S1 are as follows: the temperature of the trough barrel is 50±5℃, the temperature of barrel zone 1 is 125±5℃, the temperatures of barrel zones 2, 3, and 4, the screen changing zone, and the flange zone are all 150±5℃, and the die temperature is 180±5℃; the filter screen structure of the micro-foamed core layer adopts 80 mesh + 300 mesh + 80 mesh, and the back pressure is 15~30MPa.

[0010] Preferably, the parameters for foaming molding in step S2 are as follows: the injection pressure of supercritical carbon dioxide is 10~20MPa, and the injection amount is 1.5%~2.0% of the total mass of the micro-foamed core layer resin; the melt pressure inside the die head is maintained at 8~15MPa, and the pressure drops instantly to 0.1MPa after leaving the die head.

[0011] Preferably, the cooling and shaping parameters in step S3 are as follows: the water surface of the water ring shaping device is 5cm away from the die lip, and the cooling water temperature is 10~15℃; the cooling air temperature of the air ring is 15~25℃.

[0012] Preferably, the parameters of the thickness measuring corona in step S4 are as follows: corona power 7±1KW, traction speed 25±5m / min; traction tension 10±2N, winding roller tension 60±10N.

[0013] Preferably, the parameters for edge trimming and winding in step S5 are as follows: the winding mode is surface center curling, and the winding tension on both sides is 100±10N.

[0014] The beneficial effects of this invention are: The outer and inner surface layers of this invention form a rigid skeleton using HDPE, improving surface hardness and stiffness to resist punctures at the edges of items. mPE and LDPE ensure interlayer bonding and flexibility, avoiding the problems of poor stiffness and easy deformation of pure mPE films or insufficient toughness of pure HDPE films. This allows the base film of this invention to maintain high puncture resistance while possessing good adaptability to packaging machinery. The microfoamed core layer contains a large number of closed-cell microbubbles, which can convert puncture stress into deformation energy of the surrounding area through tensile deformation of the pore walls, thus efficiently dissipating energy. The microfoamed core layer is formed by physical foaming of a blend of metallocene polyethylene resin, polyolefin elastomer (POE), inorganic nucleating agent, and low-density polyethylene resin. The metallocene polyethylene resin has a narrow molecular weight distribution and extremely high molecular weight, which endows it with excellent melt strength and elongation at break. The long branched structure in POE can effectively resist the rupture of the cell walls under biaxial tension, prevent gas from breaking through the partition walls and causing cell merging, and ensure the integrity and closed-cell rate of the microfoamed structure. LDPE has good melt flowability and viscosity compatibility, making the foaming process more stable and controllable. The inorganic nucleating agent has a low surface energy, providing a large number of low-energy attachment sites and nucleation interfaces for supercritical CO2. In addition, the rigidity of nanoparticles can also play a certain role in reinforcing the cell walls. Attached Figure Description

[0015] Figure 1 This is a scanning electron microscope (SEM) image of the cross-section of the membrane core layer prepared in Example 1 of the present invention; Figure 2 This is a scanning electron microscope (SEM) image of the cross-section of the membrane core layer prepared in Comparative Example 1 of the present invention. Detailed Implementation

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0017] The main raw materials used in the following examples and comparative examples are described below: Metallocene polyethylene (mPE): Dow Chemical, grade Elite 5400G, density 0.918 g / cm³, melt index 1.0 g / 10 min (190℃, 2.16 kg). High-density polyethylene (HDPE): Borealis, grade FB5600, density 0.960 g / cm³, melt index 0.7 g / 10 min (190℃, 2.16 kg). Low-density polyethylene (LDPE): ExxonMobil, grade LD150BW, density 0.923 g / cm³, melt index 0.78 g / 10 min (190℃, 2.16 kg). Polyolefin elastomer (POE): Exxon Corporation, grade 1100, density 0.862 g / cm³, melt index 2.0 g / 10 min (190 °C, 2.16 kg). Hydrophobically modified nano-silica: Hubei Huifu Nanomaterials Co., Ltd., grade HB-151, nitrogen adsorption specific surface area 130±30m² 2 / g, silica content ≥99.8%.

[0018] Processing equipment: a three-layer co-extrusion blown film unit, with the B-layer extruder screw having a length-to-diameter ratio of 32:1, and the screw mixing section is equipped with static mixing elements, as well as a supercritical carbon dioxide injection port and a high-precision metering injection system. The filter screen structure of the micro-foamed core layer adopts 80 mesh + 300 mesh + 80 mesh. Example 1

[0019] In this embodiment, a thin film with a total thickness of approximately 100 μm was prepared, with a thickness ratio of 1:1.6:1.

[0020] Outer and inner layer formulation (by weight): 37% mPE, 18% HDPE, 45% LDPE.

[0021] Microfoamed core layer formulation: 34% mPE, 29% POE, 3% nano silica, 34% LDPE.

[0022] After mixing the raw materials of each layer in the specified proportions, they are fed into the three barrels of a three-layer co-extrusion blown film extruder. The temperatures of each section of the extruder are set as follows: trough barrel temperature 50±5℃, barrel zone 1 temperature 125±5℃, barrel zones 2, 3, and 4, screen changing zone, and flange zone temperatures are all 150±5℃, and the die temperature is 180±5℃. Supercritical carbon dioxide is injected into the mixing section of the B-layer extruder at an injection pressure of 10~20MPa, with an injection amount of 1.8% of the total mass of the B-layer resin. The melt pressure inside the die is maintained at approximately 8~15MPa. After the melts of each layer converge at the die, they are extruded into a tubular shape. Simultaneously, compressed air is injected through the central hole of the die to blow up the tubular film into a bubble-like film, with a blow-up ratio of 2.8 and a draw ratio of 4.0. When the tubular film leaves the die lip, the pressure drops sharply to approximately 0.1MPa. The supercritical carbon dioxide expands with the inorganic nucleating agent as the core, forming a closed-cell micro-foamed structure in the B-layer. The membrane bubble leaves the die head and enters a water ring shaping device 5cm above the water surface. The cooling water temperature is 10-15℃, while the air ring provides auxiliary cooling at 15-25℃. After rapid cooling and shaping, the sizing cage is adjusted to stabilize the membrane bubble. The cooled and shaped film is then measured by an automatic thickness measuring device and pulled upwards at a traction speed of 25±5m / min. It is flattened by a herringbone plate bubble extinguishing clamp and then clamped by a traction roller to form a double-layer film. The outer surface of the film is corona treated with a corona device at a power of 7±1KW. The traction tension is 10±2N, and the winding roller tension is 60±10N. Finally, it is wound in a center-curling mode with a winding tension of 100±10N on both sides. The edges are trimmed and sliced ​​to obtain the finished product. The finished product of Example 1 is subjected to liquid nitrogen cryogenic fracture sampling, and its cross-sectional core layer SEM image is shown below. Figure 1 As shown. Example 2

[0023] In this embodiment, a thin film with a total thickness of approximately 100 μm was prepared, with a thickness ratio of 1:1.33:1.

[0024] Outer and inner layer formulation: 30% mPE, 20% HDPE, 50% LDPE.

[0025] Microfoamed core layer formulation: 30% mPE, 30% POE, 5% nano silica, 35% LDPE.

[0026] The preparation process is the same as in Example 1, with supercritical carbon dioxide injection amount of 1.5% of the total mass of B layer resin, blow-up ratio of 2.5, and draw ratio of 3.0. Example 3

[0027] In this embodiment, a thin film with a total thickness of approximately 100 μm was prepared, with a thickness ratio of 1:2:1.

[0028] Outer and inner layer formulation: 40% mPE, 10% HDPE, 50% LDPE.

[0029] Microfoamed core layer formulation: 35% mPE, 30% POE, 5% nano silica, 30% LDPE.

[0030] The preparation process is the same as in Example 1, with supercritical carbon dioxide injection amount of 2.0% of the total mass of the microfoamed core layer resin, blow-up ratio of 3.0, and stretch ratio of 5.0. Example 4

[0031] In this embodiment, a thin film with a total thickness of approximately 100 μm was prepared, with a thickness ratio of 1:1.6:1.

[0032] Outer and inner layer formulation: 35% mPE, 20% HDPE, 45% LDPE.

[0033] Microfoamed core layer formulation: 35% mPE, 25% POE, 5% nano silica, 35% LDPE.

[0034] The preparation process is the same as in Example 1, with supercritical carbon dioxide injection amount of 1.8% of the total mass of the microfoamed core layer resin, blow-up ratio of 2.8, and stretch ratio of 4.0. Example 5

[0035] In this embodiment, a thin film with a total thickness of approximately 100 μm was prepared, with a thickness ratio of 1:1.6:1.

[0036] Outer and inner layer formulation: 35% mPE, 15% HDPE, 50% LDPE.

[0037] Microfoamed core layer formulation: 34% mPE, 30% POE, 1% nano silica, 35% LDPE.

[0038] The preparation process is the same as in Example 1, with supercritical carbon dioxide injection amount of 1.8% of the total mass of the microfoamed core layer resin, blow-up ratio of 2.8, and stretch ratio of 4.0.

[0039] Comparative Example 1 The formulation and process of this comparative example are basically the same as those of Example 1, except that: no POE is added to the micro-foamed core layer, and the formulation of the micro-foamed core layer is 64% mPE, 1% nano silica, and 35% LDPE.

[0040] The thickness ratio of the three layers was maintained at 1:1.6:1, and the preparation method was basically the same as in Example 1. The finished product of Comparative Example 1 was subjected to liquid nitrogen cryogenic fracturing to obtain a sample, and SEM images of its cross-sectional core layer were taken as follows. Figure 2 As shown.

[0041] Comparative Example 2 The micro-foamed core layer formulation, supercritical carbon dioxide injection amount, and all process parameters in this comparative example are exactly the same as those in Example 1. The difference is that the formulations of the outer and inner surface layers are 50% mPE and 50% LDPE, with no HDPE added.

[0042] Actual test After the thin film samples prepared in each embodiment and comparative example were conditioned for 24 hours at 23℃±2℃ and 50%±5% relative humidity, their mechanical properties were tested according to the following standards: Puncture resistance: Refer to GB / T 10004-2008, puncture needle diameter 1.0mm, spherical tip radius 0.5mm, puncture speed 50mm / min.

[0043] Dart impact damage quality: Refer to GB / T 9639.1-2008, Method A.

[0044] Right-angle tear strength: Refer to QB / T 1130-1991. Test results are shown in Table 1.

[0045] Table 1 Example 1 14.7 780 19 Example 2 17.4 840 22 Example 3 16.1 830 21 Example 4 16.5 820 20 Example 5 16.7 825 22 Comparative Example 1 6.2 240 5 Comparative Example 2 8.7 400 10 As can be seen from the table, compared with the comparative example, the puncture resistance, dart impact damage quality, and right-angle tear strength of the product in the embodiment are all improved. This indicates that the micro-foamed core layer effectively plays a role in stress isolation and energy dissipation. When the film is punctured or impacted, the uniformly distributed micropores in the core layer absorb a large amount of external energy through the stretching, deformation of the pore walls, and deflection of the tear path, thus inhibiting the penetration and propagation of cracks. In the core layer of Comparative Example 1 without POE, during the foaming process, due to the lack of melt strength support provided by the long-chain structure, CO2 bubbles cannot be effectively encapsulated, leading to cell collapse and merging, forming large-sized openings or ruptures. These irregular large pores actually constitute stress concentration points and weak defects inside the material.

[0046] Compared with Comparative Example 2, the performance of Comparative Example 2 decreased because the lack of HDPE in the surface layer resulted in insufficient surface stiffness of the film. In the initial stage of the puncture test, the needle tip could easily penetrate and mainly relied on the very soft surface material for support, which could not effectively transfer and disperse stress to the core foam structure. Only when the rigid skeleton of the surface HDPE and the core micro-foamed stress isolation structure work together can the puncture resistance effect be achieved.

[0047] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A highly puncture-resistant polyethylene film with a micro-foamed stress-isolation structure, characterized in that, The high puncture-resistant polyethylene-based membrane has a membrane structure consisting of an outer layer, a micro-foamed core layer, and an inner layer. The outer and inner layers are composed of metallocene polyethylene resin, high-density polyethylene resin, and low-density polyethylene resin. The micro-foamed core layer is formed by physical foaming of a blend of metallocene polyethylene resin, polyolefin elastomer (POE), inorganic nucleating agent, and low-density polyethylene resin. The thickness ratio of the outer layer, micro-foamed core layer, and inner layer is 1:(1.33~2):

1.

2. The high puncture-resistant polyethylene-based film according to claim 1, characterized in that, In the outer and inner outer layers, the mass percentage of metallocene polyethylene resin is 30-40%, the mass percentage of high-density polyethylene resin is 10-20%, and the mass percentage of low-density polyethylene resin is 40-50%.

3. The high puncture-resistant polyethylene-based film according to claim 1, characterized in that, In the blend that forms the microfoamed core layer, the mass percentage of metallocene polyethylene resin is 30%~35%, the mass percentage of polyolefin elastomer POE is 25%~30%, the mass percentage of inorganic nucleating agent is 1%~5%, and the mass percentage of low-density polyethylene resin is 30%~35%.

4. The high puncture-resistant polyethylene-based film according to claim 3, characterized in that, The inorganic nucleating agent is hydrophobically modified nano-silica.

5. A method for preparing a highly puncture-resistant polyethylene film with a micro-foamed stress-isolation structure as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Extrusion: The raw materials of the outer surface layer, the micro-foamed core layer and the inner surface layer are fed into the three barrels of the three-layer co-extrusion blown film machine according to the above component ratio requirements. The raw materials in the barrels are heated into melts in sequence through the trough barrel, barrel 1 zone, barrel 2 zone, barrel 3 zone, barrel 4 zone, screen changing zone and flange zone, and then extruded into tubular materials through the extruder die. S2. Foaming molding: Before the tubular material leaves the die head, supercritical carbon dioxide is injected into the micro-foamed core layer, and the pressure drops sharply when it leaves the die head lip to form a micro-foamed structure; at the same time, compressed air is injected from the central hole of the die head to inflate it into a bubble film; blown film parameters: blow ratio is 2.5~3.0, draw ratio is 3.0~5.0; S3. Cooling and Shaping: The bubble membrane is rapidly cooled and shaped under the cooling action of the water ring shaping device and the air ring, and the position of the sizing cage is adjusted to stabilize the membrane bubble. S4. Thickness Measurement and Corona Treatment: After cooling and shaping, the thickness of the film is measured by an automatic thickness measuring device, then pulled up by a traction device and flattened by a herringbone plate bubble extinguishing clamp, and then clamped together by a traction roller to form a double-layer film. The outer surface of the film is corona treated by a corona device. S5. Edge trimming and winding: Finally, the edges are trimmed and sliced ​​by a winding device, and the trimmed edge material is returned to step S1 for reuse to obtain the high puncture-resistant polyethylene film with micro-foamed stress isolation structure.

6. The preparation method according to claim 5, characterized in that, The extrusion parameters in step S1 are as follows: the temperature of the trough barrel is 50±5℃, the temperature of barrel zone 1 is 125±5℃, the temperatures of barrel zones 2, 3, and 4, the screen changing zone, and the flange zone are all 150±5℃, and the die head temperature is 180±5℃; the filter screen structure of the micro-foamed core layer adopts 80 mesh + 300 mesh + 80 mesh, and the back pressure is 15~30MPa.

7. The preparation method according to claim 5, characterized in that, The parameters for foaming molding in step S2 are as follows: the injection pressure of supercritical carbon dioxide is 10~20MPa, and the injection amount is 1.5%~2.0% of the total mass of the micro-foamed core layer resin; the melt pressure inside the mold head is maintained at 8~15MPa, and the pressure drops instantly to 0.1MPa after leaving the mold head.

8. The preparation method according to claim 5, characterized in that, The parameters for cooling and shaping in step S3 are as follows: the water surface of the water ring shaping device is 5cm away from the lip of the mold head, and the cooling water temperature is 10~15℃; the cooling air temperature of the air ring is 15~25℃.

9. The preparation method according to claim 5, characterized in that, The parameters of the thickness measurement corona in step S4 are as follows: corona power 7±1KW, traction speed 25±5m / min; traction tension 10±2N, winding roller tension 60±10N.

10. The preparation method according to claim 5, characterized in that, The parameters for edge trimming and winding in step S5 are as follows: the winding mode is surface center curling, and the winding tension on both sides is 100±10N.

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