Cast polypropylene film for lithium battery aluminum plastic film and preparation method thereof

Abstract

This invention provides a cast polypropylene film for lithium-ion battery aluminum-plastic film and its preparation method, relating to the field of aluminum-plastic film technology. The cast polypropylene film for lithium-ion battery aluminum-plastic film has a three-layer co-extrusion structure, consisting of a heat-sealing layer, a core layer, and a corona layer from the inside out. The heat-sealing layer is mainly composed of heat-sealing high-temperature resistant block copolymer polypropylene resin, propylene-ethylene copolymer, and hyperbranched polyamide resin; the core layer is mainly composed of high-melting-point block copolymer polypropylene resin, silica-doped propylene-ethylene copolymer, and maleic anhydride-grafted polypropylene; the corona layer is mainly composed of high-melting-point binary copolymer polypropylene resin, maleic anhydride-grafted propylene-ethylene copolymer, and silica micropowder. This structural design enables the film to possess excellent high-temperature resistance, electrolyte resistance, heat-sealing properties, and mechanical properties under both high-temperature heat-sealing and long-term high-temperature conditions, effectively solving the technical problem that existing lithium-ion battery aluminum-plastic films struggle to achieve multiple key performance characteristics simultaneously.

Description

Technical Field

[0001] This invention relates to the field of aluminum-plastic film technology, and in particular to a cast polypropylene film for lithium battery aluminum-plastic film and its preparation method. Background Technology

[0002] Aluminum-plastic film is a key packaging material for pouch lithium batteries, and its performance directly affects the battery's safety, lifespan, and reliability. A typical aluminum-plastic film consists of an outer nylon layer, a middle aluminum foil layer, and an inner cast polypropylene (CPP) film layer. The inner CPP film plays a crucial role in ensuring the battery's sealing and maintaining the stability of the internal environment.

[0003] Currently, the mainstream CPP film used in aluminum-plastic composite films on the market has a significant shortcoming in high-temperature resistance. In the heat-sealing process of lithium battery production, the heat-sealing temperature is typically 170~210℃, and some high-end power lithium batteries even have higher heat-sealing temperatures. Under these temperature conditions, existing CPP films are prone to thermal deformation and decreased heat-sealing strength, resulting in poor battery encapsulation sealing and an increased risk of electrolyte leakage.

[0004] During long-term use of batteries, especially in the high-temperature working environment faced by power lithium batteries, the aging of CPP film will be accelerated, causing its electrolyte resistance and insulation performance to deteriorate, thereby affecting the cycle life and safety performance of the battery.

[0005] To improve the high-temperature resistance of CPP films, existing technologies mainly rely on adding heat-resistant additives or changing the type of polypropylene resin. However, this often results in problems such as poor compatibility, decreased mechanical properties, and significantly increased production costs. For example, some technologies use the addition of large amounts of inorganic heat-resistant fillers, which can improve heat resistance but leads to reduced film toughness and poorer deep-drawing performance, failing to meet the encapsulation requirements of complex battery designs. Furthermore, some modification technologies for CPP films fail to balance high-temperature resistance with other key properties such as electrolyte resistance and heat-sealing properties, making them unsuitable for the stringent requirements of pouch lithium batteries.

[0006] Therefore, it is both necessary and urgent to research and develop a cast polypropylene film for lithium battery aluminum-plastic films that combines high temperature resistance, electrolyte resistance, heat sealing properties, and mechanical properties to meet the high-performance requirements of soft-pack lithium batteries, especially power and energy storage lithium batteries, for packaging materials.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] The primary objective of this invention is to provide a cast polypropylene film for use in lithium battery aluminum-plastic films, which exhibits excellent high-temperature resistance, electrolyte resistance, heat-sealing properties, and mechanical properties under conditions of high-temperature heat sealing and long-term high-temperature use.

[0009] The second objective of this invention is to provide a method for preparing cast polypropylene film for lithium battery aluminum-plastic film.

[0010] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: This invention provides a cast polypropylene film for use in lithium battery aluminum-plastic films. The film has a three-layer co-extruded structure, consisting of a heat-sealing layer, a core layer, and a corona layer from the inside out. The heat-sealing layer is mainly composed of heat-sealing high-temperature resistant block copolymer polypropylene resin, propylene-ethylene copolymer, and hyperbranched polyamide resin. The core layer is mainly composed of high-melting-point block copolymer polypropylene resin, silica-doped propylene-ethylene copolymer, and maleic anhydride-grafted polypropylene. The corona layer is mainly composed of high-melting-point binary copolymer polypropylene resin, maleic anhydride-grafted propylene-ethylene copolymer, and silica micropowder.

[0011] Furthermore, the melting point of the heat-sealing high-temperature resistant block copolymer polypropylene resin is 160~165℃; Preferably, the heat-sealing layer further comprises a high-temperature resistant additive and a slip agent; Preferably, the high-temperature resistant additive is obtained by compounding hindered phenolic high-temperature resistant additives and phosphite high-temperature resistant additives, and the compounding mass ratio of hindered phenolic high-temperature resistant additives to phosphite high-temperature resistant additives is 70~30:30~70. Preferably, the hindered phenolic high-temperature resistant additive is obtained by compounding antioxidant 1010 and phosphite-based high-temperature resistant antioxidant 168, and the mass ratio of antioxidant 1010 to antioxidant 168 is 70~30:30~70. Preferably, the slip agent is one of erucamide, behenamide, and oleamide. Further, by weight, the heat-sealing layer comprises: 50-70 parts of heat-sealing high-temperature resistant block copolymer polypropylene resin, 15-25 parts of propylene-ethylene copolymer, 5-10 parts of hyperbranched polyamide resin, 1-2 parts of high-temperature resistant additives, 0.5-1 part of antioxidant, and 0.5-1 part of slip agent.

[0012] Furthermore, the high-melting-point block copolymer polypropylene resin has a melting point of 160-180°C; Preferably, the core layer further includes a nucleating agent; Preferably, the nucleating agent is selected from one of sorbitol derivatives, organophosphates, and rosin-based nucleating agents, and is preferably a sorbitol-based nucleating agent.

[0013] Furthermore, by weight, the core layer comprises: 60–80 parts of high-melting-point block copolymer polypropylene resin, 10–20 parts of silica-doped POE resin, 5–15 parts of maleic anhydride-grafted polypropylene, and 1–2 parts of nucleating agent.

[0014] Furthermore, the high-melting-point binary copolymer polypropylene resin has a melting point of 150-160℃.

[0015] Furthermore, by weight, the corona layer comprises: 40–60 parts of high-melting-point binary copolymer polypropylene resin, 15–25 parts of maleic anhydride-grafted POE, and 5–10 parts of silica micropowder.

[0016] The present invention provides a method for preparing the above-mentioned cast polypropylene film, the method comprising: (a) Mix the components of the heat-sealing layer, core layer, and corona layer thoroughly; (b) The mixed raw materials are fed into the corresponding hoppers of the three-layer co-extrusion casting machine, and after being melted and plasticized, they are extruded through the multi-layer co-extrusion die. (c) Cast the composite melt onto a cooling roller to cool and shape it; (d) Perform corona treatment on the corona layer of the cooled and formed film; (e) Trim and wind up the corona-treated film.

[0017] Furthermore, in step (a), the mixing of each layer component is carried out at 50–70°C for 10–15 min. In step (b), the melting and plasticizing temperature of the heat-sealing layer material is 220–240℃; In step (b), the melting and plasticizing temperature of the core layer raw material is 230–250°C; In step (b), the melting and plasticizing temperature of the corona layer raw material is 210–230°C; In step (b), the temperature of the multi-layer co-extrusion die is 230–240°C.

[0018] In step (c), the temperature of the cooling roller is 20–30°C, and the casting speed of the composite melt is 30–50 m / min.

[0019] Furthermore, the surface dyn value of the corona layer after the corona treatment in step (d) is 44–50 dyn / cm.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: The cast polypropylene film for lithium battery aluminum-plastic film provided by this invention, through the synergistic design of the heat-sealing layer, core layer and corona layer in the three-layer co-extrusion structure, and the matching combination of specific components in each layer, enables the film to have stable heat-sealing strength and good electrolyte resistance during high-temperature heat sealing. At the same time, it has good dimensional stability, mechanical strength and deep-drawing performance under high-temperature service environment, and can achieve excellent corona treatment performance. Thus, it achieves synergistic improvement in high temperature resistance, electrolyte resistance, heat sealing performance, mechanical properties, deep-drawing performance and interlayer bonding, solving the technical problem that existing technologies cannot simultaneously achieve multiple key performances.

[0021] The method for preparing the cast polypropylene film provided by this invention involves uniformly mixing the heat-sealing layer, core layer, and corona layer components, then melting and plasticizing them in a three-layer co-extrusion casting machine and extruding them through a die to achieve precise forming of the three-layer structure. Subsequently, the film is cast, cooled, and shaped to ensure dimensional stability. Next, the corona layer is corona treated to increase its surface energy and enhance its adhesion to the aluminum foil layer. Finally, the edges are trimmed and the film is wound up to obtain a finished film with regular edges and stable roll state. The method described in this application adopts a mature three-layer co-extrusion casting process, which does not require large-scale modification of existing production equipment and has a good foundation for industrial application. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] According to one aspect of the present invention, a cast polypropylene film for use in lithium battery aluminum-plastic films, the film having a three-layer co-extruded structure, comprising, from the inside out, a heat-sealing layer, a core layer, and a corona layer, wherein: The heat-sealing layer is mainly composed of heat-sealing high-temperature resistant block copolymer polypropylene resin, propylene-ethylene copolymer, and hyperbranched polyamide resin. The core layer is mainly composed of high-melting-point block copolymer polypropylene resin, silica-doped propylene-ethylene copolymer, and maleic anhydride-grafted polypropylene. The corona layer is mainly composed of high-melting-point binary copolymer polypropylene resin, maleic anhydride-grafted propylene-ethylene copolymer, and silica micropowder.

[0024] The cast polypropylene film for lithium battery aluminum-plastic film provided by this invention achieves a synergistic improvement in high-temperature resistance, electrolyte resistance, heat sealing performance, mechanical properties, deep-drawing performance, and interlayer bonding strength through a three-layer co-extrusion structure design and the synergistic effect of each layer component. This solves the technical problem of difficulty in simultaneously achieving multiple key performance characteristics in existing technologies. The combination of heat-sealable high-temperature resistant block copolymer polypropylene resin, propylene-ethylene copolymer, and hyperbranched polyamide resin in the heat-sealable layer ensures stable heat-seal strength and good electrolyte resistance during high-temperature heat sealing. The combination of high-melting-point block copolymer polypropylene resin, silica-doped propylene-ethylene copolymer, and maleic anhydride-grafted polypropylene in the core layer provides excellent dimensional stability, mechanical strength, and deep-drawing performance at high temperatures. The combination of high-melting-point binary copolymer polypropylene resin, maleic anhydride-grafted propylene-ethylene copolymer, and silica micropowder in the corona layer provides excellent corona treatment performance and aluminum foil adhesion, while maintaining interfacial bonding stability under high-temperature conditions.

[0025] As an optional implementation, the melting point of the heat-sealing high-temperature resistant block copolymer polypropylene resin is 160–165°C, for example, it can be 160°C, 163°C or 165°C, or any value within the range of 160–165°C.

[0026] In a preferred embodiment of the present invention, the heat-sealing layer further comprises a high-temperature resistant additive and a slip agent; wherein the high-temperature resistant additive is used to inhibit the oxidative degradation of the film under high-temperature conditions and improve the thermo-oxidative stability of the film, and the slip agent is used to improve the surface friction properties and processing properties of the film.

[0027] Preferably, the high-temperature resistant additive is obtained by compounding hindered phenolic high-temperature resistant additives and phosphite high-temperature resistant additives, and the mass ratio of hindered phenolic high-temperature resistant additives to phosphite high-temperature resistant additives is 70~30:30~70. Preferably, the antioxidant is obtained by compounding antioxidant 1010 and antioxidant 168, and the mass ratio of antioxidant 1010 to antioxidant 168 is 70~30:30~70, more preferably 50~60:40~50; Preferably, the slip agent is one of erucamide, behenamide, and oleamide.

[0028] In a preferred embodiment of the present invention, the heat-sealing layer comprises, by weight, 50–70 parts of heat-sealing high-temperature resistant block copolymer polypropylene resin, 15–25 parts of propylene-ethylene copolymer, 5–10 parts of hyperbranched polyamide resin, 1–2 parts of high-temperature resistant additives, 0.5–1 part of antioxidants, and 0.5–1 part of slip agent; wherein, the heat-sealing high-temperature resistant block copolymer polypropylene resin provides high-temperature stability and heat-sealing performance, the propylene-ethylene copolymer enhances flexibility and impact resistance, the hyperbranched polyamide resin strengthens the interfacial bonding between the heat-sealing layer and the core layer and improves electrolyte resistance, the high-temperature resistant additives inhibit oxidative degradation of the film under high-temperature conditions, the antioxidants improve the thermo-oxidative stability of the film, and the slip agent reduces the surface friction coefficient of the film and improves processing performance.

[0029] As an optional implementation, by weight, the heat-sealable high-temperature resistant block copolymer polypropylene resin is 50–70 parts, for example, 50 parts, 55 parts, 60 parts, 65 parts, or 70 parts, or any value between 50 and 70 parts; the propylene-ethylene copolymer is 15–25 parts, for example, 15 parts, 18 parts, 20 parts, 22 parts, or 25 parts, or any value between 15 and 25 parts; the hyperbranched polyamide resin is 5–10 parts, for example, […]. The amounts can be 5, 6, 7, 8, 9, or 10 parts, or any value between 5 and 10 parts; high-temperature resistant additives: 1-2 parts, for example, 1 part, 1.2 parts, 1.5 parts, 1.8 parts, or 2 parts, or any value between 1 and 2 parts; antioxidants: 0.5-1 part, for example, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, or 1 part, or any value between 0.5 and 1 part; slip agent: 0.5 to 1 part.

[0030] In a preferred embodiment of the present invention, the high-melting-point block copolymer polypropylene resin has a melting point of 160–180°C, which provides good high-temperature resistant skeleton support in the film core layer and maintains the dimensional stability and mechanical property stability of the core layer in a high-temperature environment.

[0031] In a preferred embodiment of the present invention, the core layer further includes a nucleating agent; the nucleating agent is used to refine polypropylene grains and improve the crystallinity and mechanical properties of the film.

[0032] Preferably, the nucleating agent is selected from one of sorbitol derivatives, organophosphates, and rosin-based nucleating agents, and is preferably a sorbitol-based nucleating agent.

[0033] In a preferred embodiment of the present invention, the core layer comprises, by weight, 60-80 parts of high-melting-point block copolymer polypropylene resin, 10-20 parts of silica-doped POE resin, 5-15 parts of maleic anhydride-grafted polypropylene, and 1-2 parts of nucleating agent; wherein, the high-melting-point block copolymer polypropylene resin provides good high-temperature resistant skeletal support for the film, the silica-doped POE resin enhances the toughness, high-temperature resistance, and dimensional stability of the core layer, the maleic anhydride-grafted polypropylene improves the compatibility of the core layer with the heat-sealing layer and the corona layer, and the nucleating agent refines the polypropylene grains and improves the crystallinity and mechanical properties of the film.

[0034] In a preferred embodiment of the present invention, the high-melting-point binary copolymer polypropylene resin has a melting point of 150–160°C, which gives it good corona treatment performance and improves the adhesion strength between the film and the aluminum foil layer, while maintaining the stability of the interface bonding between the corona layer and the aluminum foil under high temperature conditions.

[0035] In a preferred embodiment of the present invention, the corona layer comprises, by weight, 40–60 parts of high-melting-point binary copolymer polypropylene resin, 15–25 parts of maleic anhydride-grafted POE, and 5–10 parts of silica micropowder; wherein, the high-melting-point binary copolymer polypropylene resin has good corona treatment performance, the maleic anhydride-grafted POE enhances the flexibility and electrolyte resistance of the corona layer, and the silica micropowder improves the high-temperature resistance of the corona layer.

[0036] According to one aspect of the present invention, a method for preparing the above-described cast polypropylene film, the method comprising: (a) Mix the components of the heat-sealing layer, core layer, and corona layer thoroughly; (b) The mixed raw materials are fed into the corresponding hoppers of the three-layer co-extrusion casting machine, and after being melted and plasticized, they are extruded through the multi-layer co-extrusion die. (c) Cast the composite melt onto a cooling roller to cool and shape it; (d) Perform corona treatment on the corona layer of the cooled and formed film; (e) Trim and wind up the corona-treated film.

[0037] The method for preparing the cast polypropylene film provided by this invention involves uniformly mixing the heat-sealing layer, core layer, and corona layer components, then melting and plasticizing them in a three-layer co-extrusion casting machine and extruding them through a die to achieve precise forming of the three-layer structure. Subsequently, the film is cast, cooled, and shaped to ensure dimensional stability. Next, the corona layer is corona treated to increase its surface energy and enhance its adhesion to the aluminum foil layer. Finally, the edges are trimmed and the film is wound up to obtain a finished film with regular edges and stable roll state. The method described in this application adopts a mature three-layer co-extrusion casting process, which does not require large-scale modification of existing production equipment and has a good foundation for industrial application.

[0038] In a preferred embodiment of the present invention, in step (a), the mixing of each layer component is carried out at 50–70°C for 10–15 min. In step (b), the melting and plasticizing temperature of the heat-sealing layer material is 220–240℃; In step (b), the melting and plasticizing temperature of the core layer raw material is 230–250°C; In step (b), the melting and plasticizing temperature of the corona layer raw material is 210–230°C; In step (b), the temperature of the multi-layer co-extrusion die is 230–240°C.

[0039] In step (c), the temperature of the cooling roller is 20–30°C, and the casting speed of the composite melt is 30–50 m / min.

[0040] In a preferred embodiment of the present invention, the surface dyn value of the corona layer after the corona treatment in step (d) is 44–50 dyn / cm, which gives the corona layer good surface energy, thereby improving its adhesion performance to the aluminum foil layer and maintaining the stability of the interface bonding under high temperature conditions.

[0041] The technical solution of the present invention will be further described below with reference to the embodiments.

[0042] Note: The amounts of each raw material used in the following examples and comparative examples are all by weight. The manufacturers and models of the raw materials used are shown in Table 1. Raw materials not listed in Table 1 were all purchased from commercially available sources.

[0043] Table 1:

[0044] Examples 1-3 A method for preparing a cast polypropylene film, the method comprising: (1) Prepare the raw materials for each layer according to the following weight proportions, see Table 2 for details.

[0045] Table 2:

[0046] (2) Place the raw materials of each layer into a high-speed mixer and mix at 60°C for 12 minutes to ensure that the components are fully and evenly dispersed; The mixed raw materials are fed into the corresponding hoppers of the three-layer co-extrusion casting machine. The extruder temperature for the heat-sealing layer is set to 230℃, the core layer extruder temperature is set to 240℃, the corona layer extruder temperature is set to 220℃, and the die temperature is controlled at 235℃. The molten material is then extruded through the multi-layer co-extrusion die.

[0047] The composite melt is cast onto a cooling roller to cool and form a shape. The temperature of the cooling roller is controlled at 25℃, and the casting speed is 40 m / min. (3) The corona layer of the cooled and formed film is subjected to corona treatment so that the surface dyn value reaches 46 dyn / cm; The corona-treated film is trimmed to remove irregular edges, and then wound up to obtain a finished cast polypropylene film. The total thickness of the cast polypropylene film is 80µm, of which: the heat-sealing layer is 12µm thick, the core layer is 56µm thick, and the corona layer is 12µm thick.

[0048] Comparative Example 1 This comparative example is the same as Example 1, except that the heat-sealing high-temperature resistant block copolymer polypropylene resin in the heat-sealing layer of Example 1 is replaced with Shanghai Petrochemical F800E.

[0049] Comparative Example 2 This comparative example is the same as Example 1, except that the high melting point block copolymer polypropylene resin in the core layer of Example 1 is replaced with Shanghai Petrochemical F780R.

[0050] Comparative Example 3 This comparative example is the same as Example 1, except that the high melting point binary copolymer polypropylene resin in the corona layer of Example 1 is replaced with Wuhan Petrochemical FCP80.

[0051] Comparative Example 4 This comparative example is the same as Example 1 except that the core layer of Example 1 does not contain a nucleating agent.

[0052] Comparative Example 5 This comparative example uses commercially available cast polypropylene film 1257 from Zhangjiagang Kangdexin Composite Materials Co., Ltd.

[0053] Experimental Example 1 In this experiment, aluminum-plastic film samples were prepared by dry lamination of the cast polypropylene films obtained in Examples 1-3 and Comparative Examples 1-5. The aluminum-plastic film structure was CPP (cast polypropylene film) / AL (aluminum foil, 40µm thick) / PA (nylon, 15µm thick) / PET (polyester film, 6µm thick). Performance comparison tests were then conducted, and the specific test methods are as follows: Heat seal strength: Take a sample piece with a length of more than 200 mm and a width of more than 100 mm, fold it in half along the length direction and heat seal it (temperature 210±5℃, pressure 0.3MPa, time 3s). After heat sealing, cut 3 strips with a width of 15 mm along the direction perpendicular to the heat seal line and perform a 180° peel test at room temperature at a speed of 200 mm / min.

[0054] High temperature resistance: Take a sample piece with a length of more than 200 mm and a width of more than 100 mm, fold it in half along the length direction and heat seal it (temperature 210±5℃, pressure 0.3MPa, time 3s). After heat sealing, cut 3 strips with a width of 15 mm along the direction perpendicular to the heat sealing line. Then place the strips in a 130℃ constant temperature tensile tester for 30 min. Afterwards, peel them 180° at a speed of 200 mm / min in a 130℃ constant temperature environment and record the peeling force.

[0055] Electrolyte resistance: Take one sample piece with a length of more than 200 mm and a width of more than 100 mm, immerse it in electrolyte (1 mol / L LiPF6 in EC:DMC:EMC=1:1:1 volume ratio) and soak it in a constant temperature oven at 25℃ for 72 h; then fold it in half along the length direction and heat seal it (temperature 210±5℃, pressure 0.3MPa, time 3s). After heat sealing, cut three strips with a width of 15 mm along the direction perpendicular to the heat seal line and perform a 180° peel test at room temperature at a speed of 200 mm / min.

[0056] Drawing depth: 4.0mm is used as the initial depth, with a drawing depth gradient of 0.5mm until the membrane shell ruptures. Ten pieces are tested for each drawing depth gradient. If all 10 pieces are undamaged, the next gradient is tested. The ultimate drawing depth is the drawing depth of the previous gradient at which rupture occurs. For example, if the membrane shell ruptures at 8.0mm, the ultimate drawing depth is 7.5mm.

[0057] Interlayer peel strength: Take three strips with a length of 150 mm or more and a width of 15 mm along the MD direction (longitudinal direction). Then, roll the strips and place them in a 20*15cm aluminum-plastic film bag (maximum 25 strips per bag). Add 40g of electrolyte (1mol / L LiPF6 in EC:DMC:EMC = 1:1:1 volume ratio) and 0.2% water relative to the weight of the electrolyte. Then seal the bag. Place the bag in an 85℃ oven for 24 hours. Remove the bag and rinse each strip under running water. After rubbing the strip back and forth twice by hand, immediately dry the surface of the strip with absorbent paper. The process from rinsing to drying should be completed within 5 seconds. Measure the peel force of CPP within 10 minutes of drying.

[0058] The specific performance test results are shown in Table 3.

[0059] Table 3:

[0060] As shown in Table 3 above, the three-layer co-extruded cast polypropylene films obtained in Examples 1 to 3 of this application all show a stable and synergistic improvement trend in five core properties: heat seal strength is 123-132 N / 15mm, high temperature resistance (peel strength at 130℃) is 79-80 N / 15mm, electrolyte resistance is 125-128 N / 15mm, deep drawing depth is 7.5-8.0 mm, and interlayer peel strength (after aging at 85℃ / 24h) is 17.5-18.0 N / 15mm. In Comparative Example 1, the heat-sealing high-temperature resistant block copolymer polypropylene resin in the heat-sealing layer was replaced with conventional F800E, resulting in a decrease in heat-sealing strength to 85 N / 15mm and a drop in high-temperature resistance to 9 N / 15mm, which confirms that the special resin of this application is a key component to ensure the stability of the high-temperature heat-sealing interface. Comparative Example 2 replaced the high melting point block copolymer polypropylene resin in the core layer with F780R, which reduced the deep drawing depth from 7.5 mm to 5.5 mm, and the interlayer peel strength also decreased to 14 N / 15 mm. This shows that the high melting point matrix of the core layer of this application plays an irreplaceable role in maintaining mechanical integrity and interfacial bonding durability at high temperatures. Comparative Example 3 replaced the high melting point binary copolymer polypropylene resin in the corona layer with FCP80, which caused the interlayer peel strength to drop to 12 N / 15 mm, and the heat seal strength to drop to 110 N / 15 mm. This shows that the special resin for the corona layer in this application not only determines the initial aluminum / PP bond, but also affects the overall reliability of the heat seal area through the transfer of interfacial stress. Comparative Example 4 omitted the nucleating agent in the core layer, resulting in a decrease in the deep drawing depth and electrolyte resistance to 5.5 mm and 90 N / 15 mm, respectively. This confirms that the nucleating agent in this application has the effect of synergistically improving the film's resistance to plastic deformation and electrolyte barrier performance by regulating crystallization behavior. Comparative Example 5 uses commercially available Kangdexin 1257, whose heat sealing strength, high temperature resistance, electrolyte resistance, drawing depth and interlayer peel strength are significantly lower than those of the embodiments in this application.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A cast polypropylene film for lithium battery aluminum laminate, characterized by, The film has a three-layer co-extruded structure, consisting of a heat-sealing layer, a core layer, and a corona layer from the inside out, wherein: The heat-sealing layer is mainly composed of heat-sealing high-temperature resistant block copolymer polypropylene resin, propylene-ethylene copolymer, and hyperbranched polyamide resin. The core layer is mainly composed of high-melting-point block copolymer polypropylene resin, silica-doped propylene-ethylene copolymer, and maleic anhydride-grafted polypropylene. The corona layer is mainly composed of high-melting-point binary copolymer polypropylene resin, maleic anhydride-grafted propylene-ethylene copolymer, and silica micropowder.

2. The cast polypropylene film according to claim 1, characterized in that, The melting point of the heat-sealable high-temperature resistant block copolymer polypropylene resin is 160~165℃; Preferably, the heat-sealing layer further comprises a high-temperature resistant additive and a slip agent; Preferably, the high-temperature resistant additive is obtained by compounding hindered phenolic high-temperature resistant additives and phosphite high-temperature resistant additives, and the compounding mass ratio of hindered phenolic high-temperature resistant additives to phosphite high-temperature resistant additives is 70~30:30~70. Preferably, the hindered phenolic high-temperature resistant additive is obtained by compounding antioxidant 1010 and phosphite-based high-temperature resistant antioxidant 168, and the mass ratio of antioxidant 1010 to antioxidant 168 is 70~30:30~70. Preferably, the slip agent is one of erucamide, behenamide, and oleamide.

3. The cast polypropylene film according to claim 2, characterized in that, By weight, the heat-sealing layer comprises: 50-70 parts of heat-sealing high-temperature resistant block copolymer polypropylene resin, 15-25 parts of propylene-ethylene copolymer, 5-10 parts of hyperbranched polyamide resin, 1-2 parts of high-temperature resistant additives, and 0.5-1 parts of slip agent.

4. The cast polypropylene film according to claim 1, characterized in that, The high-melting-point block copolymer polypropylene resin has a melting point of 160~180℃; Preferably, the core layer further includes a nucleating agent; Preferably, the nucleating agent is selected from one of sorbitol derivatives, organophosphates, and rosin-based nucleating agents, and is preferably a sorbitol-based nucleating agent.

5. The cast polypropylene film according to claim 4, characterized in that, By weight, the core layer comprises: 60-80 parts of high-melting-point block copolymer polypropylene resin, 10-20 parts of silica-doped POE resin, 5-15 parts of maleic anhydride-grafted polypropylene, and 1-2 parts of nucleating agent.

6. The cast polypropylene film according to claim 1, characterized in that, The high-melting-point binary copolymer polypropylene resin has a melting point of 150~160℃.

7. The cast polypropylene film according to claim 6, characterized in that, By weight, the corona layer comprises: 40-60 parts of high-melting-point binary copolymer polypropylene resin, 15-25 parts of maleic anhydride-grafted POE, and 5-10 parts of silica micropowder.

8. A method for preparing a cast polypropylene film according to any one of claims 1 to 7, characterized in that, The preparation method includes: (a) Mix the components of the heat-sealing layer, core layer, and corona layer thoroughly; (b) The mixed raw materials are fed into the corresponding hoppers of the three-layer co-extrusion casting machine, and after being melted and plasticized, they are extruded through the multi-layer co-extrusion die. (c) Cast the composite melt onto a cooling roller to cool and shape it; (d) Perform corona treatment on the corona layer of the cooled and formed film; (e) Trim and wind up the corona-treated film.

9. The method for preparing cast polypropylene film according to claim 8, characterized in that, In step (a), the mixing of each layer of components is carried out at 50~70°C for 10~15 min. In step (b), the melting and plasticizing temperature of the heat-sealing layer material is 220~240℃. In step (b), the melting and plasticizing temperature of the core layer raw material is 230~250℃; In step (b), the melting and plasticizing temperature of the corona layer raw material is 210~230℃; In step (b), the temperature of the multi-layer co-extrusion die is 230~240℃; In step (c), the temperature of the cooling roller is 20~30℃, and the casting speed of the composite melt is 30~50 m / min.

10. The method for preparing cast polypropylene film according to claim 8, characterized in that, After the corona treatment in step (d), the surface dyn value of the corona layer is 44~50 dyn / cm.

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