High-strength and high-toughness heat-resistant synchronous-stretching polypropylene film and preparation method thereof

Abstract

This invention discloses a high-strength, high-toughness, and heat-resistant synchronously stretched polypropylene film and its preparation method, belonging to the field of polymer film material technology. The film is a multilayer co-extruded film obtained through a synchronous biaxial stretching process. The layer structure includes at least a core layer, a sub-layer, and a surface layer. The core layer contains homopolymer polypropylene with an isotacticity ≥96% and an α-crystal nucleating agent. The sub-layer contains isotactic polypropylene, ethylene propylene rubber, and an ethylene propylene block copolymer as a compatibilizer. The surface layer contains ternary copolymer polypropylene. The longitudinal and transverse stretching ratios of the film are both 5:1 to 7:1, the stretching temperature is 100-160℃, and the heat setting temperature is 160-170℃. This invention achieves spatial decoupling of rigidity and toughness by concentrating rigidity and heat resistance in the core layer and toughening in the sub-surface layer containing a built-in compatibilizer. Combined with simultaneous biaxial equal-multiplication stretching and tension heat setting processes, the resulting film exhibits a longitudinal tensile strength ≥180MPa, a transverse tensile strength ≥170MPa, an anisotropy ratio ≤1.06, a heat shrinkage rate ≤1% at 150℃, and a crazing density ≤1 crazing / cm². 2 It combines high strength, high toughness, high heat resistance and quasi-isotropy, and is suitable for high-end electrical insulation, high-capacity capacitors and high-performance packaging materials.

Description

Technical Field

[0001] This invention belongs to the field of polymer film material technology, specifically relating to a modification and preparation method of polypropylene film, and in particular to a polypropylene film with high strength, high toughness and heat resistance properties achieved through raw material compounding and simultaneous biaxial stretching process optimization, which is suitable for high-end electrical insulation, new energy capacitors and food and pharmaceutical packaging. Background Technology

[0002] Biaxially oriented polypropylene (BOPP) film, with its lightweight, excellent transparency, superior insulation properties, good chemical stability, and excellent machinability, has become a core functional material in packaging, electrical insulation, and film capacitors. Compared to ordinary polyethylene and polyester films, BOPP film combines good mechanical properties with weather resistance, making it suitable for various industrial applications. The simultaneous biaxial stretching process enables simultaneous orientation and shaping of the film in both the longitudinal and transverse directions. Compared to the traditional step-stretching process, it effectively optimizes film thickness uniformity and reduces differences in anisotropic properties. In high-precision, high-reliability applications such as high-end electrical insulation and high-capacity capacitors, BOPP is gradually replacing traditional step-stretched BOPP film, becoming the mainstream direction for the industry's high-end development.

[0003] Currently, the industry has conducted research on high-strength, high-toughness, and heat-resistant modifications for synchronously stretched polypropylene (SPPP) films. Mainstream technologies often employ multi-layer co-extrusion structures combined with elastomer modification schemes. This involves improving film toughness by adding flexible components such as ethylene propylene rubber to the film matrix, while simultaneously using conventional nucleating agents to regulate the polypropylene crystal structure. Combined with fixed-range stretching and heat-setting processes, this enhances the film's basic mechanical strength and heat resistance. Existing technologies, through optimizing layer structure and basic process parameters, have to some extent improved the problems of insufficient strength, poor toughness, and easy deformation at high temperatures in ordinary polypropylene films. They can basically meet the needs of conventional packaging and general insulation materials, and are currently the main preparation scheme for low- to mid-range synchronously stretched polypropylene films.

[0004] Existing synchronous stretching polypropylene film modification technologies still have significant technical shortcomings, making them unsuitable for the demanding working conditions in high-end applications. Conventional modification systems use only a single elastomer for toughening, lacking synergistic modifying components. This leads to stress concentration during high-ratio stretching, resulting in numerous silver streaks on the film surface and low yield rates. Simultaneously, conventional polypropylene matrices have low isotacticity and poor crystallinity control. Combined with inappropriate biaxial stretching ratios and tension-free heat setting processes, this results in an imbalance of longitudinal and transverse mechanical properties, significant anisotropy, and high thermal shrinkage at 150°C, leading to insufficient dimensional stability. Existing technologies generally suffer from the technical contradiction of incompatibility between high strength, high toughness, and high heat resistance, making it difficult to meet the stringent requirements of high-end electrical insulation and high-capacity capacitors for films with high balanced strength, low defects, high temperature resistance, and low shrinkage. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention provides a high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film and its preparation method.

[0006] The objective of this invention can be achieved through the following technical solutions: A high-strength, high-toughness, and heat-resistant synchronously stretched polypropylene film, wherein the film is a multilayer co-extruded film obtained by synchronous biaxial stretching process, and its layer structure includes at least: The core layer comprises homopolymer polypropylene, an α-crystal nucleating agent, and an antioxidant, wherein the isotacticity of the homopolymer polypropylene is ≥96%. The sublayer, located on at least one side of the core layer, comprises high-impact block copolymer polypropylene having an isotactic polypropylene continuous phase, an ethylene propylene rubber dispersion phase dispersed in the continuous phase, and an ethylene propylene block copolymer as a compatibilizer. The outer layer, located outside the sub-layer, comprises ternary copolymer polypropylene and an anti-blocking agent; The longitudinal and transverse stretch ratios are both 5:1 to 7:1, the stretching temperature is 100-160℃, and the heat setting temperature is 160-170℃.

[0007] As a preferred technical solution, the core layer accounts for 70%-90% of the total film thickness, ensuring that the core layer provides the main mechanical support and heat-resistant skeleton for the film. Furthermore, the isotacticity of the homopolymer polypropylene is preferably ≥98%, and the melt index at 230℃ and 2.16kg is 2-5g / 10min, to balance molecular chain regularity and film-forming properties.

[0008] The α-crystalline nucleating agent is preferably a sorbitol dibenzylide derivative or a substituted aryl phosphate. The nucleating agent added to the core layer can promote the formation of a large number of fine and uniform spherulites in homopolymer polypropylene, which makes it easier to form a highly oriented lamellar structure during subsequent synchronous stretching. Together with the highly isotactic matrix, it endows the film with extremely high rigidity, strength and heat resistance.

[0009] The subsurface layer is a key functional layer that imparts high toughness to the film. Ethylene-propylene rubber (EPR) serves as the dispersed phase, providing the toughening core, while the EPR block copolymer acts as a compatibilizer. Its molecular chain contains both crystallizable polypropylene segments and rubber segments, effectively reducing the interfacial tension between the EPR and the isotactic polypropylene matrix. This results in finer, more uniformly distributed rubber phase particles, achieving efficient toughening without producing excessive streaks in a synchronous biaxial stretching flow field. Preferably, in the high-impact block copolymer polypropylene, the EPR dispersed phase content is 15-30 wt%, and the EPR block copolymer content is 5-15 wt%.

[0010] To further enrich the functional gradient or processing characteristics of the subsurface layer, the material of the subsurface layer may also include at least one of polyolefin elastomer, propylene-based elastomer, metallocene polyethylene, and polybutene, and be blended with the high-impact block copolymer polypropylene.

[0011] The surface layer is made of ternary copolymer polypropylene, which has high surface energy after corona treatment, and can meet the requirements of post-processing such as printing, coating, and heat sealing; the anti-blocking agent contained therein prevents the film from sticking together during winding and use.

[0012] Furthermore, the layer structure is preferably an asymmetrical structure, meaning that the sub-layers on both sides of the core layer have different thicknesses or materials. This concept, which intentionally deviates from the conventional symmetrical design of biaxially oriented films, allows for the creation of a more flexible mechanical property gradient in the film thickness direction, further pushing the limits of performance balance.

[0013] The film prepared using the above-mentioned specific multilayer structure, after synchronous biaxial stretching and specific heat setting, achieves the following performance indicators: longitudinal tensile strength ≥180MPa, transverse tensile strength ≥170MPa, anisotropy ratio ≤1.06; longitudinal and transverse thermal shrinkage rates ≤1% at 150℃, and silver crazing density ≤1 crazing / cm², thus achieving a unity of quasi-isotropy, high and low temperature toughness and excellent high temperature dimensional stability.

[0014] The present invention also provides a method for preparing the above-mentioned high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film, comprising the following steps: S1. Prepare core layer material, sub-surface layer material and surface layer material separately, melt each layer material separately, and co-extrude them through a multi-layer co-extrusion die to obtain multi-layer polypropylene sheet; S2. The multilayer polypropylene sheet is heated to 100-160℃ and subjected to synchronous biaxial stretching, with longitudinal and transverse stretching ratios of 5:1 to 7:1 and stretching rate of 10-30mm / s. S3. Heat-set the stretched film at 160-170℃ for 1-5 seconds, maintaining 30-50% of the film's initial tension during the heat-setting process; S4. Cool and wind up to obtain the film.

[0015] In this method, each layer is separately prepared and melted, then cast into a multi-layer sheet in one pass through a co-extrusion die, resulting in a tight bond between the layers. The simultaneous biaxial stretching process ensures that each layer achieves equivalent orientation in two directions within the plane, achieving quasi-isotropy while preventing warping or delamination of asymmetric structures due to asynchronous stretching. High-temperature heat setting under tension fully releases internal stress, locking the oriented molecular chains within the crystal lattice, a key process guarantee for achieving an extremely low thermal shrinkage rate at 150°C.

[0016] The beneficial effects of this invention are as follows: (1) This invention abandons the traditional approach of simultaneously achieving toughening and reinforcement through blending within a single film layer. Instead, it adopts a multi-layer asymmetric structure of core layer-subsurface layer-surface layer, concentrating high rigidity and high heat resistance in the high isotacticity homopolymer polypropylene core layer and high toughness in the subsurface layer containing a built-in compatibilizer, thus achieving spatial separation of rigidity and toughness. The core layer does not need to sacrifice rigidity for toughness, and the subsurface layer does not need to sacrifice toughness for rigidity. This breaks the problem that toughening inevitably reduces strength and reinforcement inevitably leads to embrittlement in single-layer blending systems. As a result, the overall rigidity of the film is close to that of pure homopolymer polypropylene, while its low-temperature toughness significantly surpasses that of traditional toughened films.

[0017] (2) The α-crystal nucleating agent added to the core layer of this invention promotes the formation of a fine and uniform spherulitic structure in homopolymer polypropylene, which not only improves the crystallinity and heat resistance of the core layer itself, but also increases the interfacial contact area between the core layer and the subsurface layer. In the subsurface layer, the ethylene-propylene block copolymer acts as a built-in compatibilizer, and its molecular chain contains both crystallizable polypropylene segments and amorphous rubber segments, which effectively reduces the interfacial tension between the ethylene-propylene rubber dispersed phase and the isotactic polypropylene continuous phase. This allows the rubber phase particles to maintain a fine and uniform dispersion morphology in the high-ratio flow field of synchronous biaxial stretching, and significantly inhibits the initiation and expansion of crazes during stretching. This cross-layer synergy from the crystal region scale to the rubber phase scale enables contradictory properties such as high strength, high toughness, and low craze density to be achieved simultaneously on the same film.

[0018] (3) This invention organically combines a multilayer structure with a synchronous biaxial stretching process. By simultaneously stretching the longitudinal and transverse layers at equal ratios, the molecular chains of each layer achieve almost equivalent orientation within the film plane, eliminating anisotropy at its source. The resulting film has an anisotropy ratio ≤1.06. Simultaneously, synchronous stretching effectively avoids the interlayer shear failure and warping deformation problems that are easily generated in step-by-step stretching of asymmetric structures. After stretching, heat setting treatment at 160-170℃ while maintaining 30-50% of the initial tension allows the oriented molecular chains to fully crystallize and lock under controlled tension, fully releasing internal stress and giving the film excellent high-temperature dimensional stability with longitudinal and transverse thermal shrinkage rates ≤1% at 150℃. Detailed Implementation

[0019] To further illustrate the technical means and effects of the present invention in achieving the intended purpose, the following detailed description of the specific implementation methods, structures, features and effects of the present invention, in conjunction with preferred embodiments, is provided below.

[0020] The sources of the raw materials used in the following examples and comparative examples are as follows: Homopolymer polypropylene: Sinopec, grade T30S, isotacticity 98.5%, melt index 3.2 g / 10 min (230℃, 2.16 kg). α-Crystal nucleating agent: Shanxi Chemical Research Institute, brand name TMP-5, sorbitol dibenzylide derivative; Antioxidant: BASF Irganox 1010 and Irgafos 168 are compounded in a 1:1 mass ratio; Ternary copolymer polypropylene (surface layer): Singapore TPC FS5612; Anti-blocking agent: Constance Engineering Plastics AB-06; High-impact block copolymer polypropylene: Yanshan Petrochemical F8403, wherein the content of ethylene propylene rubber phase is about 22wt%, the content of ethylene propylene block copolymer is about 8wt%, and the balance is isotactic polypropylene; Polyolefin elastomer: ExxonMobil 6102FL; Propylene-based elastomer: ExxonMobil 3980FL; Metallocene polyethylene: Mitsui Chemicals SP1520; Polybutene: LyondellBasell PB 8220M.

[0021] Example 1 This embodiment provides a five-layer asymmetric structure of high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film. The layer structure is as follows: surface layer / upper surface layer / core layer / lower surface layer / surface layer. The upper and lower surface layers have the same thickness, while the upper and lower surface layers have different thicknesses, forming an asymmetric structure.

[0022] The formulation and thickness ratio of each layer are as follows: Core layer (accounting for 75% of the total film thickness): 99.5 parts homopolymer polypropylene, 0.3 parts α-crystal nucleating agent, and 0.2 parts antioxidant; Top layer (accounting for 10% of the total film thickness): 100 parts of high-impact block copolymer polypropylene; Next surface layer (5% of the total film thickness): 100 parts of high-impact block copolymer polypropylene; Upper and lower surface layers (accounting for 10% of the total film thickness, 5% each): 99.5 parts of ternary copolymer polypropylene, 0.5 parts of antiblocking agent.

[0023] Preparation method: S1. Prepare each layer material separately. According to the above formula, mix the core layer material, the upper surface layer material, the lower surface layer material, and the surface layer material separately in a high-speed mixer at 80°C and 600 rpm for 8 minutes to obtain each layer mixture. Add each layer mixture to the corresponding single-screw extruder for melt plasticization. The core layer extrusion temperature is 220°C, the lower surface layer extrusion temperature is 215°C, and the surface layer extrusion temperature is 215°C. Co-extrude through a five-layer co-extrusion die (die temperature 240°C) and cool on a casting roller at 40°C to obtain a five-layer asymmetric polypropylene sheet.

[0024] S2. Heat the above sheet to 135°C and feed it into a synchronous biaxial stretching machine for simultaneous longitudinal and transverse stretching, with a stretching ratio of 6 times and a stretching rate of 20 mm / s.

[0025] S3. Heat set the stretched film at 165°C for 3 seconds, and maintain 40% of the initial tension of the film during the heat setting process.

[0026] S4. Cool to 35°C, rewind, and obtain a high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film.

[0027] Example 2 The difference between this embodiment and Embodiment 1 is that: The core layer accounts for 80% of the total film thickness, the upper surface layer 8%, the lower surface layer 7%, and the surface layers a total of 5%. The core layer homopolymer polypropylene uses Maoming Petrochemical FD03 (96% isotacticity). The subsurface material is a blend of 80 parts high-impact block copolymer polypropylene and 20 parts polyolefin elastomer.

[0028] The remaining formulations, layer structures, and preparation process steps are the same as in Example 1.

[0029] Example 3 The difference between this embodiment and Embodiment 1 is that: The subsurface material is a blend of 70 parts high-impact block copolymer polypropylene, 15 parts propylene-based elastomer, and 15 parts metallocene polyethylene. The stretching temperature was adjusted to 145℃. Heat setting temperature 170℃, maintaining 50% of the initial tension.

[0030] The remaining formulations, layer structures, and preparation process steps are the same as in Example 1.

[0031] Example 4 This embodiment provides a seven-layer asymmetric structure of high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film. The layer structure is as follows: surface layer / upper surface layer 1 / upper surface layer 2 / core layer / lower surface layer 2 / lower surface layer 1 / surface layer. The upper and lower surface layers are symmetrical. The upper surface layer 1 and the lower surface layer 1 have different thicknesses, while the upper surface layer 2 and the lower surface layer 2 have the same thickness.

[0032] The formulation and thickness ratio of each layer are as follows: Core layer (accounting for 70% of the film thickness): 99.4 parts homopolymer polypropylene, 0.4 parts α-crystal nucleating agent, and 0.2 parts antioxidant; Previous surface layer 2 (5% of film thickness): 100 parts of high-impact block copolymer polypropylene; Next surface layer 2 (5% of film thickness): 100 parts of high-impact block copolymer polypropylene; Previous surface layer 1 (7% of film thickness): 60 parts high-impact block copolymer polypropylene, 40 parts polybutene; Next surface layer 1 (3% of film thickness): 60 parts high-impact block copolymer polypropylene, 40 parts polybutene; Surface layer (accounting for 10% of the total film thickness, 5% each): 99.5 parts of ternary copolymer polypropylene, 0.5 parts of anti-blocking agent.

[0033] Preparation method: S1. Prepare each layer of material according to the above formula, and co-extrude the casting sheet through a seven-layer co-extrusion die (die temperature 240℃), with the casting roll temperature 40℃, to obtain a seven-layer asymmetric sheet.

[0034] S2. Heat the sheet to 150℃ and perform simultaneous biaxial stretching with a longitudinal and transverse stretching ratio of 5.5 times and a stretching rate of 15mm / s.

[0035] S3. Heat set the stretched film at 162°C for 5 seconds, maintaining 30% of the initial tension.

[0036] S4. Cool and rewind to obtain the finished film.

[0037] Comparative Example 1 A single-layer structure is adopted. The formula, calculated based on the overall film composition, is as follows: 90 parts homopolymer polypropylene, 10 parts high-impact block copolymer polypropylene, 0.3 parts α-crystal nucleating agent, 0.2 parts antioxidant, and 0.5 parts anti-blocking agent. All raw materials are mixed evenly in a high-speed mixer, then melt-extruded at 240°C using a single-screw extruder, and cooled by a 40°C casting roller to obtain a single-layer sheet. This sheet is then simultaneously biaxially stretched 6×6 times at 135°C with a stretching rate of 20 mm / s; heat-set at 165°C for 3 seconds, maintaining 40% tension. After cooling, it is wound up to obtain a single-layer film.

[0038] Comparative Example 2 A symmetrical three-layer structure is adopted: top layer / sub-top layer / top layer. The sub-top layer formulation is the same as the core layer of Example 1 (99.5 parts homopolymer PP, 0.3 parts nucleating agent, and 0.2 parts antioxidant), with a thickness of 90%; the top and bottom top layers are the same as in Example 1, each accounting for 5%. After co-extrusion casting, synchronous stretching and heat setting are carried out according to the process conditions of Example 1.

[0039] Comparative Example 3 The structure and formulation are the same as in Example 1. The difference lies in the stretching method: the sheet is first stretched longitudinally by 6 times at 120°C, and then stretched transversely by 6 times at 135°C; the heat setting temperature is 155°C, the time is 3 seconds, and it is held without tension. The rest is the same as in Example 1.

[0040] Comparative Example 4 A symmetrical five-layer structure is adopted: surface layer / core layer / surface layer, with no sub-surface layer. The core layer formulation is the same as that of Example 1, accounting for 90%; the surface layer is the same as that of Example 1, each accounting for 5%. The rest is the same as that of Example 1.

[0041] Comparative Example 5 The structure is the same as in Example 1. The difference is that the sub-layer uses a simple blend of 80 parts isotactic polypropylene and 20 parts ethylene propylene rubber (ethylene content 50wt%), without ethylene propylene block copolymer. The rest is the same as in Example 1.

[0042] Comparative Example 6 The structure is the same as in Example 1. The difference is that the synchronous stretching ratio is adjusted to 9 times in the longitudinal direction and 4 times in the transverse direction. Everything else is the same as in Example 1.

[0043] Comparative Example 7 The structure is the same as in Example 1. The difference is that the stretching temperature is reduced to 95°C. Everything else is the same as in Example 1.

[0044] Comparative Example 8 This comparative example was prepared according to the technical solution of Example 1 in Chinese Patent CN119348120A.

[0045] Performance testing: Tensile strength: According to GB / T 1040.3-2006 standard, dumbbell-shaped specimens (type 5) were used, and specimens were cut along the longitudinal direction (MD) and transverse direction (TD) respectively to determine the tensile strength (tensile speed 50 mm / min).

[0046] Calculate the anisotropy ratio as: longitudinal tensile strength / transverse tensile strength.

[0047] Impact resistance: Tested at -20℃ according to GB / T 9639.1-2008 standard. The specimen is a circular single-layer film with a diameter of 100 mm. The specimen and fixture are pretreated in a -20℃ low temperature chamber for 1 hour. The impact fracture mass of the dart is determined by the stepwise method using Method A (dart head diameter 38 mm, release height 660 mm), and the result is expressed in grams (g).

[0048] Heat resistance: According to GB / T 12027-2004 standard, cut 100mm×100mm samples, mark the longitudinal and transverse directions, hang them freely in an oven at 150℃ for 30min, take them out and cool them to room temperature, and measure the dimensional changes in each direction; heat shrinkage rate (%) = [(original size - shrunken size) / original size] ×100%.

[0049] Silver crazing density: Scanning electron microscopy (SEM) was used. A 10mm × 10mm sample was cut from the film, and after gold sputtering, 10 fields of view were randomly selected. The number of silver crazings was counted, and the silver crazing density per unit area (crazings / cm²) was calculated. 2 ).

[0050]

[0051] The test results show that the embodiments of the present invention, through the synergy of a multi-layer asymmetric structure and synchronous biaxial stretching process, comprehensively achieve a unity of high strength, high toughness, and high heat resistance: the longitudinal tensile strength of embodiments 1-5 is ≥180MPa, the transverse tensile strength is ≥175MPa, the anisotropy ratio is ≤1.03, exhibiting quasi-isotropy; the breaking mass of the film after dart impact at -20℃ is ≥760g, demonstrating excellent low-temperature toughness; the longitudinal and transverse thermal shrinkage rates at 150℃ are ≤1.0%, demonstrating excellent high-temperature dimensional stability; the silver streak density is 0, indicating excellent film surface quality. In contrast, Comparative Example 1 uses a single-layer blend structure, where the components mutually restrict each other within the same layer, resulting in a prominent contradiction between rigidity and toughness, a significant decrease in tensile strength and low-temperature toughness, and the appearance of silver streak defects. Comparative Example 2 lacks a core layer support, achieving adequate rigidity but severely insufficient toughness, with the breaking mass plummeting to 280g and a silver streak density as high as 10 streaks / cm². 2 Comparative Example 3 employed stepwise biaxial stretching. Although the raw materials and structure were the same as in Example 1, the transverse tensile strength decreased to 128 MPa, the anisotropy ratio reached 1.48, and the thermal shrinkage and toughness also deteriorated across the board, demonstrating the crucial role of simultaneous equal-multiplication stretching in achieving balanced performance. Comparative Example 4, without a toughened subsurface layer, exhibited extreme embrittlement of the film, with a low-temperature breakage mass of only 190 g and a silver crazing density of 12 crazing points / cm². 2 This indicates that an independent toughening layer is indispensable. In Comparative Example 5, the subsurface layer lacked an ethylene-propylene block copolymer compatibilizer, resulting in poor rubber phase dispersion, compromised strength and thermal shrinkage, and increased streaking density. This demonstrates that an internal compatibilizer is crucial for achieving efficient toughening and suppressing streaking. In Comparative Example 6, the simultaneous stretching ratios were severely uneven (9×4), causing a surge in longitudinal strength to 298 MPa while the transverse strength plummeted to 82 MPa, resulting in a complete performance imbalance. In Comparative Example 7, the stretching temperature was too low (95℃), leading to an explosive increase in streaking density to 25 streaks / cm². 2 The impact toughness and film-forming properties are severely deteriorated. Comparative Example 8, referring to the existing technical solution, has an anisotropy ratio of 1.44, high thermal shrinkage rate, and insufficient toughness, and its overall performance is far inferior to that of the present invention.

[0052] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film, characterized in that, The film is a multilayer co-extruded film produced by a simultaneous biaxial stretching process, and its layer structure includes at least: The core layer comprises homopolymer polypropylene, an α-crystal nucleating agent, and an antioxidant, wherein the isotacticity of the homopolymer polypropylene is ≥96%. The sublayer, located on at least one side of the core layer, comprises high-impact block copolymer polypropylene having an isotactic polypropylene continuous phase, an ethylene propylene rubber dispersion phase dispersed in the continuous phase, and an ethylene propylene block copolymer as a compatibilizer. The outer layer, located outside the sub-layer, comprises ternary copolymer polypropylene and an anti-blocking agent; The longitudinal and transverse stretch ratios are both 5:1 to 7:1, the stretching temperature is 100-160℃, and the heat setting temperature is 160-170℃.

2. The high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to claim 1, characterized in that, The core layer accounts for 70%-90% of the total thickness of the film.

3. The high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to claim 1, characterized in that, The homopolymer polypropylene has a melt index of 2-5 g / 10 min at 230℃ and 2.16 kg.

4. The high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to claim 1, characterized in that, The α-crystal nucleating agent is a sorbitol dibenzylide or a substituted aryl phosphate.

5. The high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to claim 1, characterized in that, In the high-impact block copolymer polypropylene, the content of ethylene propylene rubber dispersion phase is 15-30 wt%, and the content of ethylene propylene block copolymer is 5-15 wt%.

6. The high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to claim 1 or 5, characterized in that, The material of the subsurface layer also includes at least one of polyolefin elastomer, propylene-based elastomer, metallocene polyethylene, and polybutene, and is used in blend with the high-impact block copolymer polypropylene.

7. The high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to claim 1, characterized in that, The film has a longitudinal tensile strength ≥180MPa, a transverse tensile strength ≥170MPa, and an anisotropy ratio ≤1.

06.

8. The high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to any one of claims 1-7, characterized in that, The film exhibits a longitudinal and transverse thermal shrinkage rate of ≤1% at 150°C, and a silver crazing density of ≤1 crazing point / cm². 2 .

9. A method for preparing the high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Prepare the core layer material, sub-surface layer material and surface layer material separately, melt each layer material separately, and co-extrude them through a multi-layer co-extrusion die to obtain a multi-layer polypropylene sheet; S2. The multilayer polypropylene sheet is heated to 100-160℃ and subjected to synchronous biaxial stretching, with longitudinal and transverse stretching ratios of 5:1 to 7:1 and stretching rate of 10-30mm / s. S3. Heat-set the stretched film at 160-170℃ for 1-5 seconds, maintaining 30-50% of the film's initial tension during the heat-setting process; S4. Cool and wind up to obtain the film.

10. The application of the high-strength, high-toughness, heat-resistant, synchronously stretched polypropylene film according to any one of claims 1-8 in high-end electrical insulation materials, high-capacity capacitors, or high-performance packaging materials.

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