High-barrier polyolefin film extrusion film blowing process based on in-situ layering and gradient construction

Abstract

The invention relates to a high-performance polyolefin film extrusion blowing process constructed based on in-situ layering and a gradient structure, and the core of the process is as follows: precisely premixing matrix resin, an in-situ reaction type compatilizer, a nanoscale lamellar barrier agent and an endogenous reinforced fiber phase precursor according to a specific proportion; carrying out melt blending in a specific temperature gradient and shear field through a double-screw extruder, and extruding through an annular die head; then, in a strong two-way stretching flow field in which the blow-up ratio, the traction ratio and the product of the blow-up ratio, the traction ratio and the product are strictly controlled, a nanosheet layer is induced to migrate to the surface layer of the bubble and is highly oriented, meanwhile, an incompatible precursor is subjected to in-situ fibrillation to form a core layer reinforced network, and interface chemical anchoring of the nanosheet is achieved through a compatilizer; and finally, instantly freezing the gradient structure by using an asymmetric strong cooling field. The defects that a traditional single-layer film is insufficient in performance, high in multi-layer co-extrusion cost and difficult to recycle are overcome, and the prepared film has the microscopic characteristics of high barrier of the surface layer and high strength of the core layer, has ultrahigh oxygen barrier property (smaller than 50 cm / (m * day * atm)), high mechanical strength (larger than or equal to 40 MPa) and good transparency, and is high in process compatibility and environmentally friendly.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, and in particular to an innovative method for preparing polyolefin films with ultra-high gas barrier properties, high mechanical strength, and excellent transparency by precisely controlling the raw material formulation and processing flow field to induce the spontaneous formation of multi-scale microstructures with vertical gradient distribution inside the film during the single-layer extrusion blown film process. Background Technology

[0002] To improve the barrier properties of thin films, existing technologies mainly employ the following approaches: Multilayer co-extrusion composite technology: This involves co-extruding polyolefins with high-barrier resins (such as ethylene-vinyl alcohol copolymer EVOH and polyamide PA) through a multi-layer die with complex flow channels. While this method effectively combines the advantages of different materials, it has inherent drawbacks such as huge equipment investment (especially for five-layer or higher co-extrusion systems), complex production process control, increased costs due to the need for interlayer bonding resins, and poor recyclability.

[0003] Surface coating / plating technology: This involves coating the surface of a formed polyolefin film with polyvinylidene chloride (PVDC) or depositing inorganic layers such as silicon oxide or alumina through physical / chemical vapor deposition (PVD / CVD). This method has drawbacks, including potential coating peeling, expensive plating equipment, limited production speed, and environmental controversies surrounding some coating materials (such as PVDC).

[0004] Nanocomposite blending technology involves melt blending nanoscale sheet fillers (such as montmorillonite) with polyolefins. Theoretically, well-dispersed and parallel-oriented nanosheets in the matrix can create a "zigzag path" effect, improving barrier properties. However, in practice, nanofillers are prone to agglomeration, have weak interfacial bonding with the matrix, and their distribution and orientation are difficult to control during processing, resulting in limited improvement in barrier properties and often accompanied by a decrease in toughness. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction. The core of this process lies in: designing a precisely proportioned raw material system containing specific components (matrix resin, in-situ reactive compatibilizer, nanoscale sheet barrier agent, and endogenous reinforcing fiber phase precursor), and coupling it with a complex process flow that can precisely control thermal history, shear field, tensile field, and cooling kinetics. During the standard single-layer extrusion blown film process, this actively guides and solidifies a microstructure with compositional and structural gradients, thereby endowing the film with superior comprehensive performance.

[0006] A high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction, the process comprising the following sequential steps: Step 1: Precise formulation and pretreatment of the raw material system: a. Provide a matrix resin, which is linear low-density polyethylene or homopolymer polypropylene with a melt flow rate in the range of 0.5 to 5.0 g / 10 min, in an amount of 75 to 92 parts by weight. b. Provide an in-situ reactive compatibilizer, which is maleic anhydride-grafted polyethylene with a maleic anhydride grafting rate of 0.8% to 1.5% or glycidyl methacrylate-grafted polyolefin with a glycidyl methacrylate grafting rate of 0.5% to 1.2%, in an amount of 4 to 12 parts by weight. c. Provide nanoscale sheet barrier agents, which are nano-montmorillonite modified with organic double long-chain alkyl quaternary ammonium salts and with interlayer spacing d001 values ​​between 2.8 and 3.5 nm as determined by X-ray diffraction, or hydrotalcite modified with hydrogenated tallow-based quaternary ammonium salts, with an average sheet aspect ratio greater than 150, and an amount of 2.5 to 8 parts by weight. d. Provide an endogenous reinforcing fiber phase precursor, which is polyethylene terephthalate with an intrinsic viscosity in the range of 0.7 to 0.9 dL / g, or polyamide 66 with a melting point of 215 to 225°C, having a melt viscosity ratio of 0.4 to 0.7 at the blending temperature with the matrix resin, and in an amount of 4 to 10 parts by weight; e. Place the four components a, b, c, and d together in a high-speed hot mixer, and mix them for 8 to 15 minutes at a temperature range of 45°C to 65°C and a speed of 400 to 600 rpm under inert gas protection. Then cool the mixture to below room temperature and discharge it to obtain a homogeneous premix. Step 2: Controlled melt blending and extrusion of nascent membrane bubbles: The premix obtained in step one is fed into a twin-screw extruder with a barrier-type mixing screw. The screw extruder is divided into five temperature control zones from the feed port to the die, with the temperatures set sequentially as follows: Zone 1 155~165℃, Zone 2 170~180℃, Zone 3 185~195℃, Zone 4 195~205℃, and Zone 5 (die) 200~210℃. The screw speed is precisely controlled at 40~70 rpm, and the torque is maintained within the range of 60%~85%. The molten and plasticized material passes through a flow channel with a perforated plate and a melt pump, and is extruded under stable pressure through a mandrel-type annular die. The die gap of the annular die is 0.9~1.3 mm, and the ratio of die diameter to die gap is 40:1 to 60:1, forming a primary tube blank. Step 3: Dynamic formation and online solidification of gradient structures: a. Structural Evolution Stage: Filtered and temperature-controlled compressed air is injected into the nascent tube blank to inflate it, while simultaneously stretching it longitudinally under constant tension via an upper traction roller. The inflation ratio is precisely controlled at 2.2~3.5, and the traction ratio at 4.0~7.0, allowing the membrane bubble to form in a bidirectional stretching flow field. During this stage, the nanoscale sheet barrier agent undergoes directional migration to the inner and outer surfaces of the membrane bubble and achieves highly preferred orientation under the combined influence of strong planar stretching flow, temperature gradients generated by asymmetric cooling of the inner and outer surfaces of the membrane bubble, and interfacial tension differences between components. Simultaneously, the endogenous reinforcing fiber precursor is stretched in situ and refined into microfibers with an average diameter of 0.5~2.0 μm and an aspect ratio greater than 30 under high tensile deformation in the melt, forming an interlocking network within the film plane. Under high temperature and shear conditions throughout the process, the active functional groups of the in-situ reactive compatibilizer undergo a full chemical reaction with the polar sites on the surface of the nanoscale sheet barrier agent, forming covalent bonds. b. Structural Freezing Stage: A dual-outlet air ring with an independent closed-loop control system is used to force-cool the forming membrane bubble; the air velocity of the upper air outlet is controlled at 8~12 m / s, and the temperature is set at 12~18℃; the air velocity of the lower air outlet is controlled at 12~18 m / s, and the temperature is set at 8~12℃, and the axial distance between the lower air outlet and the membrane bubble is smaller than that of the upper air outlet, thereby forming a high-intensity, asymmetric cooling field, so that the frost line height of the membrane bubble is stabilized at 250~350 mm above the die opening; this rapid cooling process instantly freezes and fixes the nanosheet surface enriched orientation structure and the core microfiber network reinforcement structure formed in step a; Step 4: Online processing and collection of products: The cooled and shaped film bubble is smoothly clamped by a herringbone plate and fed into the traction roller; then the film passes through a corona treatment roller with a power density of 300~500 W·min / m² to modify at least one surface; finally, the film is wound up under constant winding tension and slit according to specifications.

[0007] Preferably, in step one, the nanoscale sheet barrier agent needs to be dried in a vacuum oven at 80°C for more than 4 hours before being added to remove adsorbed water.

[0008] Preferably, in step two, the screw configuration of the twin-screw extruder includes at least three sets of kneading block elements, and the last kneading block element is a reverse meshing block to enhance the dispersion and mixing effect and establish back pressure.

[0009] Preferably, in the structural evolution stage of step three, an infrared thermal imager is used to monitor the temperature distribution on the surface of the membrane bubble in real time, and the wind speed and temperature of the cooling air ring are finely adjusted through a feedback system to ensure that the fluctuation range of the frost line height does not exceed ±15 mm.

[0010] Preferably, in step three, the product of the inflation ratio and the traction ratio is controlled within the range of 10 to 20 to obtain the optimal planar bidirectional orientation balance.

[0011] Preferably, when the endogenous reinforcing fiber phase precursor is polyethylene terephthalate, its moisture content before blending must be strictly controlled below 50 ppm.

[0012] Preferably, in the structural freezing stage of step three, the lower air outlet angle of the dual-air-vent cooling air ring is tilted forward by 5~15° relative to the vertical axis of the membrane bubble to generate a downward "traction" airflow to help stabilize the membrane bubble morphology.

[0013] Preferably, in step four, the corona treatment is carried out in an atmosphere containing a mixture of nitrogen and carbon dioxide with a volume fraction of 0.5% to 2.0% to improve the durability of the treatment effect.

[0014] Preferably, the thin film finally obtained by the process has a cross-section in which a clear compositional gradient can be observed under a scanning electron microscope: in the surface region 0-5 μm away from the film surface, the areal density of the nanosheets is more than 3 times that of the core region; and by wide-angle X-ray diffraction, the Hermann orientation factor of the nanosheets in the film plane is not less than 0.7.

[0015] The beneficial effects of this invention are: This invention constructs a stable gradient structure of "high surface barrier - high core strength," resulting in an oxygen barrier performance that is more than an order of magnitude higher than that of ordinary polyolefin films. Simultaneously, it significantly enhances mechanical strength, breaking through the bottleneck of performance imbalance in traditional technologies. This invention is not a simple blending of components, but rather a deep integration of materials science and precision machining. By designing a multi-component reaction system and coupling precise synergistic control of multiple physical fields such as temperature, shear, tension, and cooling, it achieves active design and controllable fabrication of the film's microstructure, presenting a high technological barrier. Detailed Implementation

[0016] The embodiments of the present invention are described in detail below. Examples of the embodiments are provided, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0018] The term "connection" as used in the specification can be understood as a fixed connection, a detachable connection, or an integral connection; it can be a direct connection or a connection through an intermediate medium. Those skilled in the art can understand the connection relationship according to the specific circumstances and derive different implementation methods such as screwing, riveting, welding, snap-fitting, or embedding to suitably replace the connection.

[0019] The directional terms such as up, down, left, right, top, and bottom mentioned in the instruction manual refer to components that can directly contact each other or contact each other through other features; for example, "up" can mean directly above or diagonally above, or it simply means above other objects; other directions can be understood by analogy.

[0020] The materials used for manufacturing solid-shaped parts as indicated in the specification may be metallic, non-metallic, or other synthetic materials; the machining processes used for solid-shaped parts may include stamping, forging, casting, wire cutting, laser cutting, injection molding, CNC milling, 3D printing, machining, etc.; those skilled in the art may adapt or combine the above materials and manufacturing processes according to different processing conditions, costs, and precision requirements.

[0021] Example 1 Raw material formula (parts by weight): Matrix resin: LLDPE (MFR=2.0 g / 10min), 85 parts; In-situ reactive compatibilizer: PE-g-MAH (grafting rate 1.0%), 7 parts; Nanoscale sheet barrier agent: 4 parts of OMMT modified with bis(octadecyl) quaternary ammonium salt (d001=3.2 nm) (vacuum dried at 80℃ for 6 hours before use); Endogenous reinforcing fiber phase precursor: PET (IV=0.8 dL / g, moisture content <30 ppm), 6 parts.

[0022] Process: Raw material premixing: Mix each component according to the formula at 55°C and 500 rpm for 10 minutes under nitrogen protection, and then discharge after cooling.

[0023] Melt co-extrusion: The premixed material is fed into a twin-screw extruder. Temperature settings: Zone 1 160℃, Zone 2 175℃, Zone 3 190℃, Zone 4 200℃, Zone 5 205℃. Screw speed 60 rpm, torque approximately 75%. After pressure stabilization by the melt pump, the material is extruded through an annular die.

[0024] Gradient structure formation and curing: The blow-up ratio (BUR) was controlled at 2.8, and the traction ratio (DDR) at 5.0 (BUR*DDR=14). A dual-outlet air ring cooling system was used: upper outlet: air velocity 10 m / s, temperature 15℃; lower outlet: air velocity 15 m / s, temperature 10℃, with the lower outlet tilted forward by 10°. The frost line height was controlled at approximately 300 mm (monitored using an infrared thermal imager).

[0025] Post-processing: After passing through the herringbone plate and traction roller, the film bubble is subjected to corona treatment in a nitrogen atmosphere containing 1% CO2 (power density 400 W·min / m²), and finally wound up and slit.

[0026] Example 2 Raw material formula (parts by weight): Matrix resin: Homopolymer PP (MFR=3.0 g / 10min), 80 parts; In-situ reactive compatibilizer: GMA-g-PP (grafting rate 0.8%), 10 parts; Nanoscale sheet barrier agent: HTAB modified OHT, 5 parts; Endogenous reinforcing fiber phase precursor: PA66 (melting point 220℃), 8 parts.

[0027] Process: The premixing conditions are the same as in Example 1.

[0028] Extruder temperature settings: Zone 1 165℃, Zone 2 180℃, Zone 3 195℃, Zone 4 205℃, Zone 5 210℃. Screw speed 55 rpm.

[0029] BUR is controlled at 3.2, and DDR at 5.5 (BUR*DDR=17.6). Dual air vent rings: upper vent air velocity 9 m / s, temperature 17℃; lower vent air velocity 16 m / s, temperature 12℃, lower vent tilted forward 8°. Frost line height is controlled at approximately 320 mm.

[0030] The subsequent steps are the same as in Example 1.

[0031] Comparative Example 1 The same matrix resin and nanoscale sheet barrier agent as in Example 1 were used, but the in-situ reactive compatibilizer and endogenous reinforcing fiber phase precursor were omitted. The formulation was: 96 parts LLDPE and 4 parts OMMT. The process parameters were basically the same as in Example 1, but due to the change in the system, the actual processing torque was lower.

[0032] Comparative Example 2 The same raw material formulation as in Example 1 was used, but the key process parameters were changed: the blow-up ratio was 1.8, the traction ratio was 3.0 (BUR*DDR=5.4), and a common single-outlet cooling ring was used for slow cooling (frost line height approximately 500 mm).

[0033] Performance Testing and Result Analysis The films obtained in Examples 1 and 2, and Comparative Examples 1 and 2, were subjected to performance tests. Oxygen permeability was tested according to ASTM D3985, tensile strength according to ASTM D882, and haze according to ASTM D1003. Microstructure: The morphology of the brittle fracture section of the film was observed using a scanning electron microscope. The orientation factor of the nanosheets was calculated by wide-angle X-ray diffraction.

[0034] Table 1. Thin Film Performance Test Results

[0035] Results analysis: Performance comparison: The films of Examples 1 and 2 are far superior to the comparative example and ordinary films in terms of oxygen barrier properties and tensile strength, and maintain a low haze.

[0036] Structural verification: SEM images show that the cross-section of the film in Example 1 exhibits dense, parallel nanosheets in the near-surface region, while a PET microfiber network is visible in the core. In contrast, the nanosheets in the film of Comparative Example 1 are randomly distributed and show aggregates. Although the film of Comparative Example 2 has a certain orientation, the gradient distribution is far less pronounced than that of Example 1. The orientation factor data measured by WAXD are consistent with the SEM observations.

[0037] Key Factor Demonstration: Comparative Example 1's poor performance demonstrates that the lack of interfacial anchoring with a compatibilizer and a reinforcing fiber network limits the effectiveness of simple physical blending. Comparative Example 2's performance falls in the middle, demonstrating that even with a complete raw material system, without following the specific and complex process window of this invention (sufficient BUR, DDR and their product, and strong asymmetric cooling), it is impossible to fully induce and fix the optimal gradient structure, thus failing to achieve the ultimate effect of this invention.

[0038] In summary, this invention, through the synergistic effect of a specific multi-component raw material system and a complex and precise extrusion blown film process, successfully achieved the gradient design and controllable preparation of the microstructure of polyolefin films, resulting in high-barrier and high-strength film products with excellent performance.

[0039] Although the present invention has been described in detail with reference to the above embodiments, it will be apparent to those skilled in the art that various changes or modifications can be made to the invention without departing from the principles and spirit of the invention as defined by the claims. Therefore, the detailed description of the embodiments in this disclosure is for illustrative purposes only and is not intended to limit the invention; rather, the scope of protection is defined by the content of the claims.

Claims

1. A high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction, characterized in that, The process includes the following steps performed sequentially: Step 1: Precise formulation and pretreatment of the raw material system: a. Provide a matrix resin, which is linear low-density polyethylene or homopolymer polypropylene with a melt flow rate in the range of 0.5 to 5.0 g / 10 min, in an amount of 75 to 92 parts by weight. b. Provide an in-situ reactive compatibilizer, which is maleic anhydride-grafted polyethylene with a maleic anhydride grafting rate of 0.8% to 1.5% or glycidyl methacrylate-grafted polyolefin with a glycidyl methacrylate grafting rate of 0.5% to 1.2%, in an amount of 4 to 12 parts by weight. c. Provide nanoscale sheet barrier agents, which are nano-montmorillonite modified with organic double long-chain alkyl quaternary ammonium salts and with interlayer spacing d001 values ​​between 2.8 and 3.5 nm as determined by X-ray diffraction, or hydrotalcite modified with hydrogenated tallow-based quaternary ammonium salts, with an average sheet aspect ratio greater than 150, and an amount of 2.5 to 8 parts by weight. d. Provide an endogenous reinforcing fiber phase precursor, which is polyethylene terephthalate with an intrinsic viscosity in the range of 0.7 to 0.9 dL / g, or polyamide 66 with a melting point of 215 to 225°C, having a melt viscosity ratio of 0.4 to 0.7 at the blending temperature with the matrix resin, and in an amount of 4 to 10 parts by weight; e. Place the four components a, b, c, and d together in a high-speed hot mixer, and mix them for 8 to 15 minutes at a temperature range of 45°C to 65°C and a speed of 400 to 600 rpm under inert gas protection. Then cool the mixture to below room temperature and discharge it to obtain a homogeneous premix. Step 2: Controlled melt blending and extrusion of nascent membrane bubbles: The premix obtained in step one is fed into a twin-screw extruder with a barrier-type mixing screw. The screw extruder is divided into five temperature control zones from the feed port to the die, with the temperatures set sequentially as follows: Zone 1 155~165℃, Zone 2 170~180℃, Zone 3 185~195℃, Zone 4 195~205℃, and Zone 5 (die) 200~210℃. The screw speed is precisely controlled at 40~70 rpm, and the torque is maintained within the range of 60%~85%. The molten and plasticized material passes through a flow channel with a perforated plate and a melt pump, and is extruded under stable pressure through a mandrel-type annular die. The die gap of the annular die is 0.9~1.3 mm, and the ratio of die diameter to die gap is 40:1 to 60:1, forming a primary tube blank. Step 3: Dynamic formation and online solidification of gradient structures: a. Structural Evolution Stage: Filtered and temperature-controlled compressed air is injected into the nascent tube blank to inflate it, while simultaneously stretching it longitudinally under constant tension via an upper traction roller; the inflation ratio is controlled at 2.2~3.5, and the traction ratio at 4.0~7.0, allowing the membrane bubble to form in a bidirectional stretching flow field; during this stage, the nanoscale sheet barrier agent undergoes directional migration to the inner and outer surfaces of the membrane bubble and achieves highly preferred orientation under the combined driving forces of strong planar stretching flow, temperature gradients generated by asymmetric cooling of the inner and outer surfaces of the membrane bubble, and interfacial tension differences between components; simultaneously, the endogenous reinforcing fiber precursor is stretched in situ and refined into microfibers with an average diameter of 0.5~2.0 μm and an aspect ratio greater than 30 under high tensile deformation in the melt, forming an interlocking network within the film plane; under high temperature and shear conditions throughout the process, the active functional groups of the in-situ reactive compatibilizer undergo sufficient chemical reaction with the polar sites on the surface of the nanoscale sheet barrier agent, forming covalent bonds; b. Structural Freezing Stage: A dual-outlet air ring with an independent closed-loop control system is used to force-cool the forming membrane bubble; the air velocity of the upper air outlet is controlled at 8~12 m / s, and the temperature is set at 12~18℃; the air velocity of the lower air outlet is controlled at 12~18 m / s, and the temperature is set at 8~12℃, and the axial distance between the lower air outlet and the membrane bubble is smaller than that of the upper air outlet, thereby forming a high-intensity, asymmetric cooling field, so that the frost line height of the membrane bubble is stabilized at 250~350 mm above the die opening; this rapid cooling process instantly freezes and fixes the nanosheet surface enriched orientation structure and the core microfiber network reinforcement structure formed in step a; Step 4: Online processing and collection of products: The cooled and shaped film bubble is smoothly clamped by a herringbone plate and fed into the traction roller; then the film passes through a corona treatment roller with a power density of 300~500 W·min / m² to modify at least one surface; finally, the film is wound up under constant winding tension and slit according to specifications.

2. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, In step one, the nanoscale sheet barrier agent needs to be dried in a vacuum oven at 80°C for more than 4 hours before being added to remove adsorbed water.

3. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, In step two, the screw configuration of the twin-screw extruder includes at least three sets of kneading block elements, and the last kneading block element is a reverse meshing block to enhance the dispersion and mixing effect and establish back pressure.

4. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, In the structural evolution stage of step three, an infrared thermal imager is used to monitor the temperature distribution on the surface of the membrane bubble in real time, and the wind speed and temperature of the cooling air ring are finely adjusted through a feedback system to ensure that the fluctuation range of the frost line height does not exceed ±15 mm.

5. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, In step three, the product of the inflation ratio and the traction ratio is controlled within the range of 10 to 20 to obtain the optimal planar bidirectional orientation balance.

6. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, When the intrinsic reinforcing fiber phase precursor is polyethylene terephthalate, its moisture content before blending must be strictly controlled below 50 ppm.

7. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, In the structural freezing stage of step three, the air outlet angle of the lower air outlet of the dual-air-vent cooling air ring is tilted forward by 5~15° relative to the vertical axis of the membrane bubble to generate a downward "traction" airflow to help stabilize the membrane bubble shape.

8. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, In step four, the corona treatment is carried out in an atmosphere containing a mixture of nitrogen and carbon dioxide with a volume fraction of 0.5% to 2.0% to improve the durability of the treatment effect.

9. The high-performance polyolefin film extrusion blown film process based on in-situ layering and gradient structure construction according to claim 1, characterized in that, The thin film finally produced by the process has a clear compositional gradient that can be observed in its cross-section under a scanning electron microscope: in the surface region 0-5 μm away from the film surface, the areal density of the nanosheets is more than 3 times that of the core region; and by wide-angle X-ray diffraction, the Hermann orientation factor of the nanosheets in the film plane is not less than 0.7.

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