PVC (polyvinyl chloride) multi-layer co-extrusion film and preparation method of multi-layer co-extrusion film

Abstract

The invention relates to a PVC multilayer co-extrusion film and a preparation method thereof, the PVC multilayer co-extrusion film comprises five PVC resin layers A, B, C, D and E. The A layer is a puncture-resistant support layer, and the puncture-resistant support layer comprises PVC-SG5 type resin; the layer B is a hard barrier layer, and the hard barrier layer comprises PVC-SG5 type resin and chlorinated polyethylene; the layer C is a toughening buffer layer, and the toughening buffer layer comprises PVC-SG3 type resin, a plasticizer and nano calcium carbonate; the layer D is a flexible airtight layer, and the flexible airtight layer comprises PVC-SG3 type resin, a plasticizer and a heat stabilizer; the layer E is a low-temperature heat sealing layer, and the low-temperature heat sealing layer comprises PVC-SG3 type resin, a plasticizer and polycaprolactone. And on the premise of ensuring the recoverability, the comprehensive performance of the multi-layer composite co-extrusion film is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer composite materials, and particularly relates to a PVC multi-layer co-extrusion film and a preparation method of the multi-layer co-extrusion film. BACKGROUND

[0002] Air column bag, also known as buffer air column bag or inflatable cushion packaging, is a new type of packaging material which forms independent air columns by sealing the film and provides cushioning protection by relying on the internal air pressure after inflation. Because it has the advantages of small space occupation, excellent cushioning performance, and less material consumption, it is widely used in the logistics transportation protection of fragile products such as electronic products, cosmetics, wine, and precision instruments.

[0003] At present, the mainstream air column bag film on the market generally adopts a multi-layer co-extrusion composite film structure, which aims to combine the advantages of multiple materials with different properties to meet the comprehensive requirements of air column bag for gas barrier property, heat sealing property, flexibility, and strength. Common existing technology structures are usually three layers or five layers.

[0004] The typical structure is PA / adhesive resin / PE. Among them, the nylon layer (PA) mainly provides excellent gas barrier property (especially oxygen barrier property) to prevent slow penetration of gas in the air column, ensuring that the air column can maintain an upright position for a long time during transportation; and the polyethylene layer (PE) provides excellent heat sealing performance to ensure that the heat sealing bag process of the air column bag during manufacturing is reliable and has strong sealing performance.

[0005] Since PA (or EVOH) and PE belong to different types of high molecular polymers, they have great differences in molecular structure, polarity, and melting temperature, resulting in complete incompatibility after melting. When this composite film enters the plastic recycling system, it cannot be directly melt blended and regranulated. If forced to recycle, it will cause structural defects due to phase separation, resulting in a sharp decrease in the mechanical properties of the regenerated material and losing its use value. This forces most air column bags to be incinerated or landfilled as non-recyclable mixed garbage, causing serious resource waste and environmental pollution, which is contrary to the increasingly stringent global environmental protection policies and sustainable development requirements.

[0006] Attempting to use only one of these materials to create a single-material air column bag film presents insurmountable performance defects. While PE material offers good heat-sealing performance and low cost, its gas barrier properties are extremely poor. Air column bags made of pure PE will quickly collapse due to the rapid permeation of gases (especially nitrogen and oxygen) through the film, failing to provide durable and effective cushioning protection, thus rendering them completely impractical. PA material, while offering good gas barrier properties, suffers from extremely poor heat-sealing performance, requiring extremely high temperatures to seal, and has a narrow heat-sealing window, making air column bags prone to leakage at the heat-sealed point, resulting in a low yield rate. Furthermore, pure PA film is expensive, and its balance between flexibility and puncture resistance is inferior to composite structures. Existing technologies have attempted to use single-layer PVC film to make air column bags. However, ordinary single-layer PVC film, in order to achieve a balance of various performance aspects, often suffers from deficiencies in one of the following areas: barrier properties, strength, or heat-sealing properties. For example, adding a large amount of plasticizer to improve flexibility may sacrifice its mechanical strength and airtightness; while improving airtightness may make the film harder, affecting its cushioning performance and impact resistance.

[0007] To achieve functionality, a multi-layered composite structure must be used, but this inevitably leads to non-recyclable products and causes environmental problems; on the other hand, using a single material to pursue recyclability cannot achieve the comprehensive performance required for air column bags through existing technology. Summary of the Invention

[0008] In view of the shortcomings of the existing technology, the technical problem to be solved by the present invention is that multi-layer composite co-extruded films are not recyclable and a single material cannot achieve the comprehensive performance required for air column bags.

[0009] A PVC multilayer co-extruded film comprising five PVC resin layers A, B, C, D, and E, wherein:

[0010] Layer A is a puncture-resistant support layer, which includes PVC-SG5 type resin;

[0011] Layer B is a rigid barrier layer, which includes PVC-SG5 type resin and chlorinated polyethylene.

[0012] Layer C is a toughening and buffering layer, which includes PVC-SG3 type resin, plasticizer, and nano calcium carbonate.

[0013] Layer D is a flexible airtight layer, which includes PVC-SG3 type resin, plasticizer, and heat stabilizer.

[0014] Layer E is a low-temperature heat-sealing layer, which includes PVC-SG3 type resin, plasticizer, and polycaprolactone.

[0015] It is worth mentioning that layer A in this invention, as the surface layer directly bearing external impact and wear, has core design requirements of high rigidity, high strength, and excellent puncture resistance. PVC-SG5 resin is a high-polymerization-degree polyvinyl chloride with longer molecular chains and more chain entanglements, which gives its virgin material higher tensile strength, modulus, and hardness. Compared to the low-polymerization-degree SG3 resin, SG5 resin provides a more robust mechanical framework for this layer. To maximize the rigidity of this layer, this invention intentionally does not add plasticizers to layer A. Plasticizers weaken the intermolecular forces of PVC, which, while increasing flexibility, significantly reduces the material's modulus and strength.

[0016] The design goal of layer B is to improve the barrier properties against gases (such as nitrogen and oxygen) while maintaining rigidity. Chlorinated polyethylene (CPE), as an elastomer, has segments in its molecular chain similar to PVC, allowing it to be well-compatible with the PVC matrix, while the chlorine-containing polar portions interact strongly with other PVC molecular chains. This structure acts as a "bridge" and "filler" within the PVC matrix, reducing microscopic phase interface defects and making the layer B structure more compact. The uniformly dispersed CPE phase domains, together with the PVC matrix, form a complex "maze" structure. When gas molecules permeate through the film, they must bypass these impermeable phase regions, making the path more tortuous and lengthy, thus significantly reducing the gas permeation rate and achieving effective barrier properties.

[0017] The C-layer is the primary energy-absorbing layer, requiring both good flexibility and toughness to absorb and disperse impact energy. A low-polymerization-degree (SG3) PVC resin is selected, along with a large amount of plasticizer, to create a highly flexible and easily deformable matrix. Plasticizer molecules insert into the PVC molecular chains, widening the interchain distance and lowering the energy barrier for chain segment movement, thus making the material very soft and prone to large deformation to cushion impacts. Introducing nano-calcium carbonate into this flexible matrix is ​​key to achieving "toughening." These nanoscale rigid particles act as stress concentration points; when the material is impacted, the surrounding matrix generates numerous crazing and shear bands, absorbing a large amount of impact energy. Simultaneously, the nanoparticles can prevent the further propagation of microcracks. This "rigid particle toughening" mechanism allows the C-layer to achieve extremely high impact toughness while remaining flexible, avoiding the insufficient strength problem of purely soft PVC.

[0018] Layer D, located adjacent to the interior of the gas column, requires extremely high flexibility to ensure a tight seal when in contact with gas, while also guaranteeing processing stability. Layer D contains a high proportion of plasticizers, which can exacerbate the degradation risk of PVC during heat processing. The addition of a heat stabilizer is crucial; it effectively captures hydrogen chloride (HCl) produced during PVC degradation, terminating the chain reaction and ensuring that this layer does not decompose or yellow during processing, maintaining its chemical stability and long-term airtightness. A stable matrix is ​​a prerequisite for achieving durable airtightness.

[0019] The E layer requires reliable heat sealing between films, demanding low heat sealing temperature and high strength. While PVC itself has a relatively low initial heat sealing temperature, its thermal stability is poor. The decomposition temperature of PVC (approximately 200°C) is very close to its melt processing temperature. During heat sealing, if the temperature is too high or the heating is uneven, thermal decomposition easily occurs, releasing hydrogen chloride (HCl) gas, leading to yellowing, brittleness, and reduced strength of the sealed edges. PCL has good compatibility with PVC. As a polymeric plasticizer, it effectively reduces the viscosity of the blend melt, allowing for melt flow at lower temperatures, meeting the "low-temperature heat sealing" requirement. PCL is a semi-crystalline polymer, with a melting point (58-62°C) far lower than the processing temperature of PVC. During the cooling process after heat sealing, PCL segments can rapidly crystallize. These PCL microcrystals, acting as physical cross-linking points, greatly enhance the cohesive strength and sealing strength at the heat-sealing point, avoiding the disadvantage of low heat-sealing strength of simple soft PVC. Due to the distinct melt-crystallization behavior of PCL, it provides a wider effective temperature range for the heat-sealing process, making the production process more stable and the yield rate higher.

[0020] By designing five functionally distinct PVC resin layers, the non-recyclability problem caused by the uneven material composition of traditional PE / PA composite films is fundamentally solved. Furthermore, by selecting different types of PVC resin (SG5 and SG3) and specific additives, each layer achieves a precise division of function: layer A provides mechanical protection, layer B provides gas barrier, layer C absorbs impact energy, layer D ensures overall flexibility and airtightness, and layer E provides reliable sealing. This ensures comprehensive performance of the multi-layered co-extruded film while maintaining recyclability.

[0021] Preferably, the puncture-resistant support layer is composed of PVC-SG5 resin with a degree of polymerization of 1000-1300, and the toughening buffer layer is composed of 60-70 wt% PVC-SG3 resin with a degree of polymerization of 700-900, 20-35 wt% plasticizer, and 5-15 wt% nano-calcium carbonate. The high degree of polymerization of SG5 resin ensures that the A layer has extremely high molecular chain entanglement, thereby obtaining excellent puncture resistance; while the SG3 resin in the C layer, along with a specific ratio of plasticizer and nano-calcium carbonate, ensures toughness, and the nano-calcium carbonate, as rigid particles, can also induce crazing, absorbing more impact energy and improving the buffering performance of the film.

[0022] Preferably, the puncture-resistant support layer accounts for 15-25% of the thickness, the toughening buffer layer accounts for 25-35%, the rigid barrier layer accounts for 10-15%, and the combined thickness of the rigid barrier layer and the flexible airtight layer accounts for 25-45%. This ensures the overall mechanical strength and impact resistance of the film. By controlling the high-cost barrier layer to a smaller thickness (10-15%), costs are saved while maintaining the barrier effect. Furthermore, by considering the thickness of the rigid barrier layer and the flexible airtight layer as a whole (totaling 25-45%), the rigid-flexible properties of the film can be precisely controlled as needed to adapt to different application scenarios.

[0023] Preferably, the rigid barrier layer consists of 75-85 wt% PVC-SG5 resin and 15-25 wt% chlorinated polyethylene, the flexible airtight layer consists of 65-75 wt% PVC-SG3 resin and 25-35 wt% plasticizer, and the low-temperature heat-sealing layer consists of 45-55 wt% PVC-SG3 resin, 30-40 wt% plasticizer, and 10-20 wt% polycaprolactone. In layer B, the addition of chlorinated polyethylene (CPE) not only acts as an impact modifier, but its polar molecular chains also intertwine with PVC, significantly increasing the tortuosity of the gas molecule path through the film, thereby enhancing the barrier properties. In layer E, polycaprolactone (PCL), as a high-molecular plasticizer and crystalline polymer, has good compatibility with PVC, effectively reducing the heat-sealing temperature, widening the heat-sealing window, and improving the strength and toughness of the heat-sealing layer.

[0024] Preferably, the plasticizer is dioctyl terephthalate, the nano-calcium carbonate is calcium carbonate with a particle size of 50-200 nm, and the melting point of polycaprolactone is 58-62℃. Using a plasticizer with a relatively high molecular weight, such as dioctyl terephthalate, can effectively reduce the risk of migration and volatilization, ensuring the long-lasting flexibility of the film. Controlling the particle size of the nano-calcium carbonate (50-200 nm) allows it to achieve optimal dispersion in the PVC matrix, realizing effective stress transfer and toughening. The selection of polycaprolactone with a melting point of 58-62℃ perfectly matches the processing and heat-sealing temperature range of PVC, ensuring both fluidity during processing and rapid formation of a strong seal after cooling, thus ensuring high stability and reproducibility of product performance.

[0025] A method for preparing a multilayer co-extruded film, used to prepare the PVC multilayer co-extruded film according to any one of claims 1-5, comprising the following steps:

[0026] (1) Mix the five layers of raw materials separately in different mixers at 80-100℃ for 5-10 minutes;

[0027] (2) The mixed materials are fed into five single-screw extruders for melting, and the melts are compounded into films through multi-layer co-extrusion dies;

[0028] (3) After being cooled and shaped by a three-roll calender, it is finally rolled up.

[0029] This ensures that the formulations of each functional layer do not interfere with each other, thereby precisely realizing the complex layered structure and properties designed in claims 1-5. A mixing temperature of 80-100℃ is beneficial for the initial swelling and dispersion of the additives, laying the foundation for the subsequent melt extrusion preparation of a homogeneous melt; the use of a three-roll calender for cooling, compared with ordinary air cooling, can obtain films with a smoother surface, higher gloss, and more stable dimensions, which is suitable for subsequent printing and bag making of high-quality air column bags.

[0030] As a preferred design, the extrusion temperatures of the five single-screw extruders form a temperature gradient along the film structure: 170-190℃ for the puncture-resistant support layer, 165-180℃ for the rigid barrier layer and toughening buffer layer, 155-170℃ for the flexible airtight layer, and 150-165℃ for the low-temperature heat-sealing layer. The design fully considers the differences in resin polymerization degree and plasticizer content in each layer's formulation. High temperatures are used for layer A, which has a high polymerization degree and low plasticization, to ensure sufficient plasticization; relatively low temperatures are used for layer E, which has a low polymerization degree and high plasticization, to effectively prevent the decomposition and volatilization of heat-sensitive additives (such as plasticizers). This gradient temperature control is key to obtaining high-quality co-extruded films with clear interfaces, strong interlayer bonding, and no drooling or deformation.

[0031] Furthermore, the temperatures of the three rollers in the three-roll calender decrease sequentially along the film exit direction: upper roller 60-80℃, middle roller 40-60℃, and lower roller 20-40℃; the traction speed to roller speed ratio is 1.05-1.10. This achieves progressive cooling and micro-stretching of the film. This process effectively eliminates internal stress and prevents film shrinkage and deformation during subsequent winding. At the same time, micro-stretching can induce appropriate molecular chain orientation, slightly improving the longitudinal strength of the film. This parameter combination is the core to ensuring the balance of film flatness, dimensional stability, and mechanical properties.

[0032] Preferably, the multilayer co-extrusion die has at least five independent temperature control zones within its flow channel, with each layer's temperature decreasing by 5°C-15°C from the temperature control zone of the puncture-resistant support layer to the temperature control zone of the low-temperature heat-sealing layer. This achieves precise matching and coordination of the melt viscosity of each layer, ensuring that the melts of each layer have similar flow rates and shear stresses when they converge. This fundamentally avoids defects such as uneven layer thickness, unstable interfaces (e.g., wavy edges), and even interlayer delamination caused by differences in flowability, and is a necessary condition for preparing a five-layer composite film with a uniform structure.

[0033] Furthermore, all three rollers of the three-roll calender are coated with a polytetrafluoroethylene (PTFE) coating with a thickness of 0.3-0.5 mm. The PTFE coating provides excellent anti-stick properties, effectively preventing the high-temperature, soft PVC film from adhering to the rollers, thereby avoiding surface damage and ensuring a very high yield.

[0034] In summary, this structural design enables the comprehensive performance of multilayer composite co-extruded films while ensuring recyclability. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0036] Example 1: Five-layer PVC co-extruded film and its preparation

[0037] 1. Raw materials and formulation (by weight percentage, wt%)

[0038] Layer A (puncture-resistant support layer): 100% PVC-SG5 type resin with a polymerization degree of 1200.

[0039] Layer B (rigid barrier layer): 80wt% PVC-SG5 resin, 20wt% chlorinated polyethylene (CPE).

[0040] Layer C (toughening and buffering layer): 65wt% PVC-SG3 type resin (degree of polymerization 800), 28wt% dioctyl terephthalate (DOTP), and 7wt% nano-calcium carbonate with a particle size of 100nm.

[0041] Layer D (flexible airtight layer): 70wt% PVC-SG3 type resin, 30wt% dioctyl terephthalate (DOTP).

[0042] E layer (low-temperature heat-sealing layer): 50wt% PVC-SG3 type resin, 35wt% dioctyl terephthalate (DOTP), and 15wt% polycaprolactone (PCL) with a melting point of 60℃.

[0043] 2. Thickness Design

[0044] The total thickness of the co-extruded film is 80 μm. Among them:

[0045] Layer A has a thickness of 20% (16μm).

[0046] Layer B accounts for 12% of the total thickness (9.6 μm).

[0047] Layer C accounts for 30% of the total thickness (24μm).

[0048] Layer D accounts for 18% of the total thickness (14.4 μm).

[0049] The E layer thickness accounts for 20% (16μm).

[0050] 3. Preparation process

[0051] (1) Mixing: The above five layers of raw materials are put into five high-speed mixers and mixed at 90°C for 8 minutes to fully mix and pre-plasticize the additives and PVC resin.

[0052] (2) Melt co-extrusion: The mixed materials are fed into five single-screw extruders. The extruder temperature gradient is set as follows:

[0053] A-layer extruder: 180℃

[0054] B-layer extruder: 175℃

[0055] C-layer extruder: 170℃

[0056] D-layer extruder: 160℃

[0057] E-layer extruder: 155℃

[0058] (3) Combining and molding: Five melts are combined and extruded through a multi-layer co-extrusion die with five independent temperature control zones. The die temperature is set to 178℃, 173℃, 168℃, 158℃ and 153℃ from the A layer flow channel to the E layer flow channel.

[0059] (4) Cooling and shaping: The extruded film immediately enters the three-roll calender. The surface of each of the three rolls is coated with 0.4 mm thick polytetrafluoroethylene. The temperature of the three rolls is set as follows: upper roll 70℃, middle roll 50℃, and lower roll 30℃. The ratio of traction speed to lower roll linear speed (draw ratio) is controlled at 1.08.

[0060] (5) Winding: The cooled and shaped film is wound up at a constant speed by a winding machine to obtain the five-layer PVC co-extruded film.

[0061] Example 2: Adjustment of Formula and Process Parameters

[0062] The main difference between this embodiment and Embodiment 1 lies in the optimization of the formula and thickness to meet the requirements of higher strength.

[0063] Layer C (toughening and buffering layer): 60wt% PVC-SG3 type resin, 35wt% DOTP, 5wt% nano calcium carbonate.

[0064] Thickness design: Total thickness 100μm. Layer A accounts for 25% (25μm) of the thickness, Layer C accounts for 25% (25μm) of the thickness, Layers B and D together account for 30% (30μm) of the thickness, and Layer E accounts for 20% (20μm) of the thickness.

[0065] Process: The extrusion temperature of the E layer is adjusted to 150°C, the flow channel temperature of the E layer in the die head is adjusted to 148°C, the draw ratio is adjusted to 1.06, and the rest is the same as in Example 1.

[0066] Comparative Example 1: Traditional PE / PA Composite Film

[0067] It uses a commercially available three-layer co-extruded air column bag film with a structure of polyethylene / adhesive resin / nylon (PE / Tie / PA) and a total thickness of 80μm. It has excellent heat-sealing and gas barrier properties and is a mainstream product in the market.

[0068] Comparative Example 2: Single-layer PVC film

[0069] It is made of ordinary single-layer PVC calendered film, containing plasticizer DOP40phr, with a thickness of 80μm. It has good flexibility, but its strength and airtightness are average.

[0070] The five-layer PVC co-extruded films obtained in Examples 1 and 2 were compared with those in Comparative Examples 1 and 2 to conduct key performance tests. The results are shown in the table below:

[0071] Test item Test standard Example 1 Example 2 Comparative Example 1 (PE / PA) Comparative Example 2 (single layer PVC) Tensile strength (machine direction, MPa) GB / T 1040.3 42 45 35 25 Elongation at break (%) GB / T 1040.3 380 350 500 300 Oxygen transmission rate (cm³ / m²·24h·0.1MPa) GB / T 1038 95 105 80 ＞600 Heat seal strength (N / 15mm) GB / T 2358 12 11 15 8 Dart impact performance (g) GB / T 2912 850 900 650 400 Recycle compatibility Subjective evaluation Good Good Poor phase separation Good

[0072] *Recycling compatibility evaluation method: After crushing the films of each sample, they were melt-blended and granulated using a twin-screw extruder at 180°C, and the appearance of the granules was observed. The recycled granules of Examples 1, 2, and Comparative Example 2 had smooth surfaces and uniform textures; while the recycled granules of Comparative Example 1 had rough surfaces and obvious phase separation striations, indicating that it was not recyclable.

[0073] Based on the above test results, the following five-layer PVC co-extruded film is described in the embodiments of this application:

[0074] It achieves recyclability of a single material: solving the industry problem of Comparative Example 1 (traditional composite membrane) being unable to be recycled due to material inhomogeneity.

[0075] Excellent overall performance: While maintaining similar gas barrier properties and good heat seal strength as Comparative Example 1, its mechanical strength (tensile strength) and impact resistance (dart impact) are significantly better than Comparative Example 1 and Comparative Example 2 (single-layer PVC film).

[0076] Successful functional design: Through the precise design of the five-layer structure and formula, based on Comparative Example 2 (single-layer PVC), the professional division of functions of each layer was achieved, making up for the shortcomings of single-layer film in terms of barrier properties and high-strength heat sealing.

[0077] In summary, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A PVC multilayer co-extruded film, characterized in that, The five-layer PVC resin layer includes A, B, C, D, E, wherein: A layer is a puncture-resistant support layer, which includes PVC-SG5 resin; B layer is a hard barrier layer, which includes PVC-SG5 resin and chlorinated polyethylene; C layer is a toughening buffer layer, which includes PVC-SG3 resin, plasticizer and nano calcium carbonate; D layer is a flexible airtight layer, which includes PVC-SG3 resin, plasticizer and heat stabilizer; E layer is a low-temperature heat sealing layer, which includes PVC-SG3 resin, plasticizer and polycaprolactone.

2. The PVC multilayer co-extruded film according to claim 1, characterized by: The puncture-resistant support layer is composed of PVC-SG5 resin with a polymerization degree of 1000-1300, and the toughening buffer layer is composed of PVC-SG3 resin with a polymerization degree of 700-900, 60-70wt% plasticizer and 5-15wt% nano calcium carbonate.

3. The PVC multilayer co-extruded film according to claim 2, characterized by: The thickness ratio of the puncture-resistant support layer is 15-25%, the thickness ratio of the toughening buffer layer is 25-35%, the thickness ratio of the hard barrier layer is 10-15%, and the thickness ratio of the hard barrier layer and the flexible airtight layer is 25-45%.

4. The PVC multilayer co-extruded film according to claim 3, characterized by: The hard barrier layer is composed of 75-85wt% PVC-SG5 resin and 15-25wt% chlorinated polyethylene, the flexible airtight layer is composed of 65-75wt% PVC-SG3 resin and 25-35wt% plasticizer, and the low-temperature heat sealing layer is composed of 45-55wt% PVC-SG3 resin, 30-40wt% plasticizer and 10-20wt% polycaprolactone.

5. The PVC multilayer co-extruded film according to claim 1, characterized in that: The plasticizer is dioctyl terephthalate, the nano calcium carbonate has a particle size of 50-200nm, and the polycaprolactone has a melting point of 58-62℃.

6. A process for the preparation of a multilayer co-extruded film for the preparation of a PVC multilayer co-extruded film according to any one of claims 1 to 5, characterized in that, The method comprises the following steps: (1) mixing the five-layer raw materials in a mixer at 80-100℃ for 5-10 minutes; (2) melting the mixed materials in five single-screw extruders respectively, and then combining the melts into a film through a multi-layer co-extrusion die; (3) cooling and shaping through a three-roller calender, and finally winding.

7. The method for producing a multilayer co-extruded film according to claim 6, characterized in that: The extrusion temperature of the five single-screw extruders forms a temperature gradient along the film layer structure direction: the extrusion temperature of the puncture-resistant support layer is 170-190℃, the extrusion temperature of the hard barrier layer and the toughening buffer layer is 165-180℃, the extrusion temperature of the flexible airtight layer is 155-170℃, and the extrusion temperature of the low-temperature heat sealing layer is 150-165℃.

8. The process for the preparation of PVC multilayer co-extruded film according to claim 6, characterized by the fact that: The three-roller calender has three rollers with decreasing temperatures along the film direction: the upper roller is 60-80℃, the middle roller is 40-60℃, and the lower roller is 20-40℃; the pulling speed is 1.05-1.10 times the roller speed.

9. The method of claim 6, wherein: The multi-layer co-extrusion die is provided with at least five independent temperature control zones, and the temperature of each layer decreases by 5-15℃ from the temperature control zone of the puncture-resistant support layer to the temperature control zone of the low-temperature heat sealing layer.

10. The method of claim 8, wherein: The surfaces of the three rollers of the three-roller calender are coated with a polytetrafluoroethylene coating with a thickness of 0.3-0.5mm.