A biaxially stretched polypropylene film for composite copper foil and a method for manufacturing the same

Abstract

This invention discloses a biaxially oriented polypropylene film for composite copper foil and its manufacturing method, relating to the field of polymer materials technology. The method includes the following steps: S1, raw material preparation; S2, mixing and feeding extrusion; S3, casting; S4, biaxial stretching process; S5, heat setting surface treatment; S6, final performance indicators. This invention improves melt flowability and film uniformity by selecting polypropylene raw materials with a melt index of 0.8–1.5 g / 10 min, adding surface-modified nano-inorganic fillers with silane coupling agents and / or titanate coupling agents, and combining this with a three-stage mixing and feeding extrusion process. The nano-inorganic fillers are used to modify the bulk phase of the film. This solves the problems of uneven film formation, insufficient tensile strength, and poor adhesion of the copper plating layer in traditional processes, ultimately obtaining a composite copper foil film with a thickness of 1.6–8.0 μm, high strength, surface tension of 38–45 mN / m, and low thermal shrinkage.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a biaxially oriented polypropylene film for composite copper foil and its manufacturing method. Background Technology

[0002] With the widespread application of lithium-ion batteries in new energy vehicles, energy storage systems, and portable electronic devices, the performance requirements for battery current collector materials are increasingly stringent. While traditional copper foil offers good conductivity, it is heavy, expensive, and prone to deformation under high-intensity applications. In recent years, polypropylene films have been extensively studied for their excellent insulation, low density, and high strength potential, and are used in composites with copper foil to prepare lightweight, high-strength current collector films. However, existing polypropylene films still have shortcomings in film-forming properties, mechanical strength, and adhesion between copper plating layers, especially during biaxial stretching, where it is difficult to simultaneously achieve high strength in both the longitudinal and transverse directions while maintaining good film-forming properties and dimensional stability.

[0003] In existing polypropylene film forming technologies, the tensile strength of polypropylene films is typically 150-200 MPa in the MD direction and 180-250 MPa in the TD direction, which is insufficient to meet the tensile strength requirements of high-performance composite copper foil. Furthermore, existing manufacturing processes are prone to film tearing or uneven film formation under high longitudinal stretch ratios. While low melt index polypropylene materials possess the potential for high molecular weight and high tensile strength, their melt flowability is poor during high-temperature casting, requiring optimization of extrusion process parameters and screw configuration to improve film uniformity. Additionally, polypropylene itself is a non-polar polymer with low surface free energy, making it difficult to form effective chemical bonds or physical anchors with metal coatings without surface modification. Copper coatings adhere to the polypropylene surface only through weak physical adsorption, and are prone to peeling, blistering, or detachment during battery charge-discharge cycles due to thermal stress, mechanical stress, or electrolyte erosion.

[0004] In existing technologies, corona treatment is a conventional method to improve the surface tension of polypropylene films. However, the effect of corona treatment is subject to aging; the surface tension of films treated with corona will decrease significantly after several days to weeks of storage, making it difficult to ensure the consistency of interfacial adhesion during subsequent magnetron sputtering or copper plating. In addition, the depth of polar groups introduced into the film surface by corona treatment alone is limited, and they are easily destroyed during subsequent processing and use.

[0005] There are already relevant invention patents concerning the manufacture of polypropylene films for lithium batteries, as detailed below:

[0006] Chinese Patent Application No. CN202411180712.6, entitled "A Polypropylene Film for Lithium-ion Battery Current Collectors and Its Preparation Method," belongs to the field of polymer film technology, specifically relating to a polypropylene film for lithium-ion battery current collectors and its preparation method. The polypropylene film comprises an upper layer, an intermediate layer, and a lower layer. By weight, the upper and lower layers each contain 70-80 parts of isotactic polypropylene and 20-25 parts of modified polystyrene; the intermediate layer contains 80-85 parts of isotactic polypropylene and 15-20 parts of cyclic olefin copolymer. The components of the upper, intermediate, and lower layers are first mixed evenly, then melt-extruded using the main and auxiliary machines of a three-layer co-extrusion casting machine to obtain a polypropylene film sheet. The polypropylene film sheet is then biaxially stretched to obtain a polypropylene film. The resulting composite polypropylene film exhibits excellent mechanical and dielectric properties, high thermal stability, and is more suitable for lithium-ion battery current collectors.

[0007] However, although the existing patents mentioned above can obtain polypropylene films through biaxial stretching, they still have the following shortcomings: (1) The surface of the polypropylene film is not effectively modified, making it difficult to guarantee the adhesion of the copper plating layer. (2) Nano-inorganic fillers are not introduced into the polypropylene matrix to achieve bulk phase modification, making it impossible to sustainably improve surface energy and surface roughness through the surface micro-nano structures formed by the fillers inside the film during the stretching process. (3) Specific parameters for low thermal shrinkage rate are not given, making it difficult to assess the dimensional stability and safety under high-temperature battery conditions. Summary of the Invention

[0008] The purpose of this application is to provide a biaxially oriented polypropylene film for composite copper foil and a method for manufacturing the same, in order to solve the problem of insufficient bonding between existing polypropylene films and copper plating, resulting in poor plating adhesion and poor adhesion stability over time, while providing clear low heat shrinkage parameters to ensure battery safety.

[0009] This invention provides a high-strength biaxially oriented polypropylene film for composite copper foil and its manufacturing method. It employs a dual modification strategy of bulk phase modification and surface treatment, introducing nano-inorganic fillers with surface modification via coupling agents into the polypropylene matrix. The intrinsic improvement of the film's surface properties is achieved through the following triple effects of the nano-inorganic fillers:

[0010] First, the surface micro-nano roughening effect: During the biaxial stretching process, the rigid nano-inorganic particles and the polypropylene matrix undergo differential deformation due to the difference in modulus. Micro-nano-scale protrusions and micropores are formed around the nanoparticles, which increases the surface roughness Ra of the film to 0.05–0.5 μm, providing physical anchoring sites for the copper coating and significantly enhancing the mechanical interlocking effect.

[0011] Second, the surface enrichment effect of polar groups: After the coupling agent modifies the surface of the nano-inorganic filler, the filler surface has polar functional groups such as methacryloyl, amino, epoxy, and ester groups. During biaxial stretching, some nanoparticles are stretched to the surface layer of the film, exposing their surface polar groups. This increases the intrinsic surface free energy of the film from 29–31 mN / m for pure polypropylene to 34–38 mN / m, providing a higher starting point for subsequent corona treatment. Moreover, this polarity increase is permanent and does not decay with storage time.

[0012] Third, the synergistic effect of filler types: different types of nano-inorganic fillers can produce synergistic effects - nano-silica provides high specific surface area and excellent dispersibility; nano-alumina improves the surface hardness and wear resistance of the film and improves the scratch resistance of the copper coating; nano-barium sulfate improves the dimensional stability and thermal stability of the film and reduces the thermal shrinkage rate; nano-barium titanate improves the dielectric properties and surface polarity of the film, which is beneficial to the insulation reliability in battery applications.

[0013] Based on bulk phase modification, the surface tension is further increased to 38–45 mN / m through double-sided corona treatment, forming a long-lasting high surface energy system with bulk phase modification as the main component and corona treatment as the auxiliary component. This fundamentally solves the problem of surface tension decay over time in traditional pure corona treatment.

[0014] A biaxially oriented polypropylene film for composite copper foil and its manufacturing method thereof, specifically including the following steps:

[0015] S1, Raw material preparation: Select polypropylene resin with a melt index of 0.8–1.5 g / 10 min and isotacticity ≥95%. The purpose of selecting low melt index polypropylene resin is that it corresponds to a higher molecular weight and a longer molecular chain, which can obtain higher tensile strength and film toughness after biaxial stretching and orientation, while ensuring the mechanical strength brought by high crystallinity.

[0016] S2, Mixed Feeding Extrusion: Polypropylene resin, surface-modified nano-inorganic filler treated with coupling agent, and β-nucleating agent are mixed and melt-extruded using a twin-screw extruder at 180–280℃. The nano-inorganic filler is selected from one or more combinations of nano-silica, nano-alumina, nano-barium sulfate, and nano-barium titanate. The coupling agent is selected from one or more combinations of KH-570 silane coupling agent, KH-550 silane coupling agent, KH-560 silane coupling agent, and titanate coupling agent to achieve uniform dispersion of filler and β-crystal induction to improve toughness and interfacial compatibility. After modification with coupling agent, the polar groups carried on the surface of the nano-inorganic filler are exposed along with the nanoparticles on the film surface during biaxial stretching, thereby intrinsically improving the surface polarity and surface free energy of the film and improving the wettability and adhesion of the film surface.

[0017] S3, Casting: Controlling the melt extrusion temperature gradient and casting roll cooling parameters to form 80–200μm thick sheets;

[0018] S4, Biaxial stretching process: The film is biaxially stretched with a longitudinal stretch ratio of 4.0–7.0 and a transverse stretch ratio of 5.0–9.0. The production line speed is 150–300 m / min to optimize the mechanical properties of the film.

[0019] S5, Heat-setting surface treatment: Double-sided corona treatment is performed on the heat-set polymer materials to ensure dimensional stability and enhance the adhesion of the copper plating.

[0020] S6, final performance indicators: Ensure film thickness of 1.6–8.0 μm, tensile strength MD≥260 MPa, TD≥300 MPa, surface tension 38–45 mN / m, surface roughness Ra0.05–0.5 μm, thermal shrinkage rate ≤2.5% in the MD direction and ≤1.5% in the TD direction after holding at 120℃ for 15 min, thermal shrinkage rate ≤5.0% in the MD direction and ≤3.5% in the TD direction after holding at 150℃ for 30 min, and copper plating peel strength ≥0.8 N / cm.

[0021] As a further improvement of the present invention, in step S2, the particle size of the nano-inorganic filler is 10–50 nm, the specific surface area of ​​the nano-inorganic filler is 50–400 m² / g, the amount of coupling agent is 0.5–3.0 wt% of the mass of the nano-inorganic filler, and the percentage of the nano-inorganic filler in the mass of polypropylene resin is 0.5–5.0%.

[0022] When the nano-inorganic filler is selected as nano-silica, its particle size is 10–30 nm, its specific surface area is 200–400 m² / g, and its usage percentage is 1.5–3.0%.

[0023] When the nano-inorganic filler is selected as nano-alumina, its particle size is 15–50 nm, its specific surface area is 50–200 m² / g, and its usage percentage is 1.0–3.0%.

[0024] When the nano-inorganic filler is selected as nano-barium sulfate, its particle size is 20–50 nm, and its specific surface area is 30–100 m². 2 / g, the percentage of the nano-barium sulfate used is 1.0–4.0%;

[0025] When the nano-inorganic filler is selected as barium titanate, its particle size is 20–50 nm, and the specific surface area of ​​the barium titanate is 30–80 m². 2 / g, the percentage of the nano-barium titanate used is 0.5–3.0%;

[0026] When two or more nano-inorganic fillers are used in combination, the total percentage of the nano-inorganic fillers used shall not exceed 5.0%.

[0027] When the nano-inorganic filler is nano-silica or nano-alumina, the coupling agent is KH-570 silane coupling agent or KH-550 silane coupling agent for surface modification.

[0028] When the nano-inorganic filler is nano-barium sulfate or nano-barium titanate, the coupling agent is a titanate coupling agent or KH-560 silane coupling agent for surface modification. When the coupling agents are used in combination, their total amount is 0.5–3.0 wt% of the mass of the nano-inorganic filler. Film uniformity and mechanical strength are ensured by precisely limiting the melt index and isotacticity of the polypropylene resin, while using particles with a diameter of 10-30 nm and a specific surface area of ​​200-400 m². 2 The combination of nano-silica ( / g) and differentiated β-nucleating agents, along with a twin-screw extruder with a production speed of 150-300 m / min and a three-stage mixing process, achieves uniform dispersion of fillers, efficient induction of β-crystal form, and stable material properties. This effectively solves the problems of vague raw material indicators, poor filler dispersion, low nucleation efficiency, and uneven mixing in existing technologies, and significantly improves the toughness and processing reliability of the film.

[0029] As a further improvement of the present invention, in step S2, the β nucleating agent is TMB-5 or WBG-II, and the amount of TMB-5 or WBG-II used is 0.01%-0.5% of the mass of polypropylene resin. The 180–280°C is divided into feeding section temperature, compression section temperature, metering section temperature, and die temperature. The feeding section temperature is 180–220°C, the compression section temperature is 230–260°C, the metering section temperature is 245–280°C, and the die temperature is 230–250°C. The L / D ratio of the twin-screw extruder is 28:1 to 32:1, and the compression ratio of the twin-screw extruder is 3.5:1 to 4.5:1, where the maximum depth of the feeding section screw groove is equal to the depth of the metering section screw groove. By precisely dividing the temperature range of the twin-screw extruder into the feeding section, compression section, metering section, and die, and with optimized L / D ratio and compression ratio, gradient heating and precise temperature control of the melt during the extrusion process are achieved. This effectively improves the plasticizing effect and mixing uniformity of polypropylene resin, while avoiding material degradation or uneven crystallization caused by temperature fluctuations, and significantly enhances the mechanical properties and processing stability of the film.

[0030] As a further improvement of the present invention, in step S3, the temperature of the casting roller is controlled at 80–100°C, and the cooling is assisted by an air knife. The air pressure of the air knife is 0.3–0.6 MPa, the angle of the air knife to the film surface is 15–30 degrees, and the distance between the air knife and the film is 5–10 mm. By precisely controlling the temperature of the casting roller at 80–100°C and using an air knife-assisted cooling system, rapid and uniform cooling of the film is achieved, effectively reducing the accumulation of internal stress and significantly improving the flatness and dimensional stability of the film. At the same time, the uniformity of the ultrathin film thickness of 1.6–8.0 μm is ensured by dynamically adjusting the air knife parameters, solving the technical problems of low efficiency and uneven film thickness in traditional cooling methods.

[0031] As a further improvement of the present invention, in step S4, the bidirectional stretching is divided into longitudinal stretching and transverse stretching. The longitudinal stretching adopts a multi-segment stretching process, in which the stretching ratio of each segment gradually increases. The temperature of the longitudinal stretching preheating zone is 115–130℃, the preheating time of the longitudinal stretching preheating zone is 3–5s, the temperature of the longitudinal stretching stretching zone is 130–150℃, and the longitudinal stretching stretching zone adopts a three-segment stretching process. The stretching ratios of the longitudinal stretching stretching zone are 1.4–1.8, 1.5–2.0, and 1.8–2.2, respectively. The product of the three stretching ratios is the total longitudinal stretching ratio of 4.0–7.0. The temperature of the longitudinal stretching shaping zone is 140–150℃, and the preheating time of the longitudinal stretching shaping zone is 2–4s. By using a longitudinal multi-segment stretching process design, the temperature-time parameters of the preheating zone, stretching zone, and shaping zone are precisely matched. Combined with a progressively increasing stretching ratio, the stress concentration and film breakage risk during the stretching process are effectively reduced. At the same time, the molecular chain orientation is optimized, significantly improving the longitudinal tensile strength and dimensional stability of the film. This solves the technical problems of uneven mechanical properties and poor film quality in traditional single-segment stretching processes.

[0032] As a further improvement of the present invention, the transverse stretching adopts a chain clamp type stretching device and a multi-segment stretching process. The stretching ratio of each segment gradually increases, and the product of the stretching ratios of the multiple segments is the total transverse stretching ratio of 5.0–9.0. The temperature of the transverse stretching preheating zone is 160–170℃, and the preheating time of the transverse stretching preheating zone is 4–12s. The temperature of the transverse stretching stretching zone is 150–170℃, and the stretching time of the transverse stretching stretching zone is 6–12s. The temperature of the transverse stretching shaping zone is 160–180℃, and the temperature of the transverse stretching shaping zone is 4–8s. By precisely controlling the temperature-time parameters of the preheating zone, stretching zone, and shaping zone, the problem of large performance differences between the edge and center in traditional transverse stretching is effectively solved, significantly improving the transverse tensile strength and stretching uniformity of the film. At the same time, it avoids the risk of film breakage caused by tensile stress concentration, providing more stable mechanical property support for high-performance composite copper foil.

[0033] As a further improvement of the present invention, in step S5, the heat setting temperature is 140–180°C, the heat setting time is 5–15s, the power of the double-sided corona treatment is 2–4kW, and the surface tension of the film needs to be increased to 38–45mN / m. The surface tension of the film before corona treatment has been increased from 29–31mN / m of pure polypropylene to 34–38mN / m. The corona treatment further increases it to 38–45mN / m on this basis. The aging stability of the surface tension of the corona treatment is better than that of the pure corona-treated film without the addition of nano-inorganic fillers. By precisely controlling the heat setting temperature at 140–180℃, residual stress inside the film is effectively eliminated and the molecular chain structure is stabilized. Combined with double-sided corona treatment using 2–4kW power, the surface tension of the film is increased to 38–39mN / m, which significantly enhances the interfacial bonding force between the copper plating and the substrate. This solves the battery bulging problem caused by excessive heat shrinkage in traditional processes, while ensuring the long-term adhesion stability of the copper plating under complex working conditions. This provides key technical support for the high-safety application of composite copper foil in the field of power batteries.

[0034] As a further improvement of the present invention, the mixing and feeding extrusion adopts a co-rotating twin-screw extruder. The twin-screw extruder is equipped with a three-stage mixing strategy with a modular screw configuration. In the first stage of the twin-screw extruder, a high-shear kneading block element is used to pre-disperse the nano-inorganic filler modified with coupling agent to achieve preliminary homogenization. In the middle stage of the twin-screw extruder, a β-nucleating agent is added through a side-feeding device, and a low-shear conveying screw element is used to promote good interfacial bonding between the nucleating agent and the polypropylene matrix. In the final stage of the twin-screw extruder, trace amounts of low-molecular-weight volatiles generated during the mixing process are removed through a vacuum devolatilization port. The vacuum degree of the vacuum devolatilization port is -0.08 to -0.1 MPa. A real-time monitoring system for the mixing torque and power curves is established at the vacuum devolatilization port, and the mixing uniformity is verified by the torque fluctuation amplitude at the vacuum devolatilization port. The twin-screw extrusion process was optimized using a three-stage mixing strategy. The first stage employed a high-shear screw element to pre-disperse nano-silica and polypropylene resin. The middle stage utilized a low-shear screw element to promote the interfacial bonding between the β-nucleating agent and the matrix. The final stage used a vacuum de-volatile port to precisely remove low-molecular-weight volatiles. Combined with a real-time monitoring system for mixing torque and power curves, the mixing uniformity was quantitatively verified. This effectively solved the performance fluctuation problems caused by filler agglomeration, uneven nucleating agent dispersion, and volatile residues in traditional processes. It significantly improved the toughness, crystallization uniformity, and batch stability of the film, providing a reliable quality control solution for the industrial production of high-performance composite copper foil.

[0035] As a further improvement of this invention, a temperature sensor is introduced into the melt extrusion temperature gradient. This temperature sensor is used to monitor temperature fluctuations in each zone in real time. It also works in conjunction with the closed-loop control of the casting roll cooling system. By dynamically adjusting the air knife pressure, angle, and distance, a uniform film with a thickness of 80–200 μm is formed. Introducing a temperature sensor into the melt extrusion temperature gradient enables real-time and accurate monitoring of temperature fluctuations in each zone. This, combined with the closed-loop control of the casting roll cooling system, allows for dynamic adjustment of the air knife pressure, angle, and distance parameters based on temperature changes. This ensures highly uniform cooling within the 1.6–8 μm ultrathin film thickness range, effectively solving problems such as uneven film thickness and internal stress concentration caused by temperature fluctuations in traditional processes. It significantly improves the film thickness uniformity and surface smoothness, providing a key technological guarantee for the stable production of high-performance composite copper foil.

[0036] A biaxially oriented polypropylene film for composite copper foil, wherein the film has a thickness of 1.6–8.0 μm, a longitudinal tensile strength (MD) ≥ 260 MPa, a transverse tensile strength (TD) ≥ 300 MPa, a surface tension of 38–45 mN / m, and a surface roughness (Ra) of 0.05–0.5 μm; when held at 120°C for 15 min, the film exhibits a thermal shrinkage rate ≤ 2.5% in the MD direction and ≤ 1.5% in the TD direction; when held at 150°C for 30 min, the film exhibits a thermal shrinkage rate ≤ 5.0% in the MD direction and ≤ 3.5% in the TD direction; the film surface is modified by nano-inorganic filler bulk phase and subjected to double-sided corona treatment, resulting in a copper plating peel strength ≥ 0.8 N / cm.

[0037] Compared with the prior art, the beneficial effects of this invention are as follows:

[0038] 1. High molecular weight raw materials ensure high tensile strength: By selecting polypropylene resin with a melt index of 0.8–1.5 g / 10 min and isotacticity ≥95%, and utilizing the high molecular weight and long molecular chain characteristics corresponding to the low melt index, excellent tensile strength is obtained after biaxial stretching and orientation, providing a high-strength substrate for ultra-thin films for composite copper foil.

[0039] 2. Multi-component nanofillers achieve bulk phase modification and sustained high surface energy: Innovatively, nano-inorganic fillers modified with coupling agents are used to modify the polypropylene matrix in bulk phase. By utilizing the surface micro-nano protrusion structure formed by the nanofillers during biaxial stretching and the surface enrichment effect of the polar groups of the coupling agent, the surface energy of the film is intrinsically and permanently improved. After corona treatment, it reaches 38–45 mN / m, and the aging stability of the surface tension is significantly better than that of traditional pure corona-treated films, fundamentally solving the problem of insufficient adhesion between the non-polar surface of polypropylene and the copper plating layer.

[0040] 3. Multiple coupling agents to suit different fillers and application requirements: Four coupling agent options are provided, namely KH-570, KH-550, KH-560, and titanate coupling agents, as well as optimized matching schemes with different nanofillers. Among them, the amino group of KH-550 can enhance the chemical affinity with metal coatings, and the titanate coupling agent has excellent modification effect on barium-containing fillers, meeting the differentiated needs of different application scenarios.

[0041] 4. Clear low heat shrinkage parameters ensure battery safety: By optimizing the heat setting process, clear and quantifiable heat shrinkage parameters are provided—MD≤2.5% and TD≤1.5% at 120℃ / 15min, and MD≤5.0% and TD≤3.5% at 150℃ / 30min. This effectively avoids safety hazards such as current collector deformation, copper layer peeling, and battery bulging caused by film heat shrinkage under high-temperature conditions during battery charge and discharge cycles.

[0042] 5. A three-stage mixing and feeding extrusion process is adopted, and a real-time monitoring system for torque and power curves is established at the devolatilization port. This achieves efficient and uniform dispersion of nanofillers and β nucleating agents and effective induction of β crystal form, significantly improving film toughness and batch stability, and overcoming the defects of uneven filler dispersion and lack of quantitative monitoring in existing technologies.

[0043] 6. During the casting stage, a temperature measuring instrument is installed at the die exit and casting area to form a closed-loop control with the air knife cooling system, which accurately stabilizes the surface temperature of the melt and achieves high uniformity of the thickness of 80–200μm thick sheets. This effectively solves the problems of uneven film thickness and internal stress concentration caused by temperature fluctuations in traditional processes, and provides a high-quality substrate for subsequent high-ratio stretching.

[0044] 7. Optimize the biaxial stretching process to achieve ultra-thin high-strength films: By using a high-ratio biaxial stretching process with a longitudinal stretching ratio of 4–7 and a transverse stretching ratio of 5–9, along with optimized and matched zoned temperature-time parameters and a three-stage progressive stretching strategy, the orientation degree of the film molecular chains is significantly improved. This results in high-strength films with a thickness of 1.6–8.0 μm, a tensile strength of MD ≥260 MPa, and a tensile strength of TD ≥300 MPa. The longitudinal stretching ratio is controlled within a reasonable range to avoid the risk of film tearing caused by overstretching, thus meeting the high mechanical performance requirements of composite copper foil. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a flowchart of the steps of the present invention. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0048] A biaxially oriented polypropylene film for composite copper foil and its manufacturing method, such as Figure 1 As shown, it includes the following steps:

[0049] S1, Raw material preparation: Select polypropylene resin with a melt index of 0.8–1.5 g / 10 min and isotacticity ≥95%. The purpose of selecting low melt index polypropylene resin is that it corresponds to a higher molecular weight and a longer molecular chain, which can obtain higher tensile strength and film toughness after biaxial stretching and orientation, while ensuring the mechanical strength brought by high crystallinity.

[0050] S2, Mixed Feeding Extrusion: Polypropylene resin, surface-modified nano-inorganic fillers treated with coupling agents, and β-nucleating agents are mixed and melt-extruded using a twin-screw extruder at 180–280℃. The nano-inorganic fillers are selected from one or more combinations of nano-silica, nano-alumina, nano-barium sulfate, and nano-barium titanate. The coupling agents are selected from one or more combinations of KH-570 silane coupling agent, KH-550 silane coupling agent, KH-560 silane coupling agent, and titanate coupling agent. This achieves uniform dispersion of the fillers and β-crystal induction to improve toughness and interfacial compatibility. After modification with coupling agents, the polar groups carried on the surface of the nano-inorganic fillers are exposed along with the nanoparticles on the film surface during biaxial stretching, intrinsically improving the surface polarity and surface free energy of the film, thus improving the wettability and adhesion of the film surface.

[0051] The particle size of the nano-inorganic filler is 10–50 nm, and the specific surface area of ​​the nano-inorganic filler is 50–400 m². 2 / g, the coupling agent dosage is 0.5–3.0 wt% of the mass of the nano-inorganic filler, and the percentage of the nano-inorganic filler in the mass of the polypropylene resin is 0.5–5.0%.

[0052] When the nano-inorganic filler is nano-silica, its particle size is 10–30 nm and its specific surface area is 200–400 m². 2 / g, with a dosage percentage of 1.5–3.0%;

[0053] When nano-alumina is selected as the inorganic filler, its particle size is 15–50 nm, and its specific surface area is 50–200 m². 2 / g, the percentage of nano-alumina used is 1.0–3.0%.

[0054] When using nano-barium sulfate as a nano-inorganic filler, the particle size is 20–50 nm, and the specific surface area is 30–100 m². 2 / g, the percentage of nano barium sulfate used is 1.0–4.0%.

[0055] When using nano-barium titanate as the inorganic filler, the particle size is 20–50 nm, and the specific surface area is 30–80 m². 2 / g, the percentage of barium titanate used is 0.5–3.0%.

[0056] When two or more nano-inorganic fillers are used in combination, the total percentage of nano-inorganic fillers used shall not exceed 5.0%.

[0057] When the nano-inorganic filler is nano-silica or nano-alumina, the coupling agent is KH-570 silane coupling agent or KH-550 silane coupling agent for surface modification.

[0058] When the nano-inorganic filler is nano-barium sulfate or nano-barium titanate, the coupling agent is a titanate coupling agent or KH-560 silane coupling agent for surface modification. When the coupling agents are used in combination, their total dosage is 0.5–3.0 wt% of the nano-inorganic filler mass. The β-nucleating agent is TMB-5 or WBG-II, and the dosage of TMB-5 or WBG-II is 0.01%–0.5% of the polypropylene resin mass. The temperature range of 180–280°C is divided into the feeding section temperature, compression section temperature, metering section temperature, and die head temperature. The feeding section temperature is 180–220°C, the compression section temperature is 230–260°C, the metering section temperature is 245–280°C, and the die head temperature is 230–250°C. The L / D ratio of the twin-screw extruder is 28:1 to 32:1, and the compression ratio of the twin-screw extruder is 3.5:1 to 4.5:1, which is the maximum depth of the screw channel in the feeding section to the depth of the screw channel in the metering section. The dual-mixing feeding extrusion adopts a co-rotating twin-screw extruder. The twin-screw extruder is equipped with a three-stage mixing strategy with a modular screw configuration. In the first stage of the twin-screw extruder, a high-shear kneading block element is used to pre-disperse the nano-inorganic filler modified with coupling agent to achieve initial homogenization. In the middle stage of the twin-screw extruder, a β-nucleating agent is added through a side-feeding device, and a low-shear conveying screw element is used to promote good interfacial bonding between the nucleating agent and the polypropylene matrix. In the last stage of the twin-screw extruder, a vacuum devolatilization port is used to remove trace amounts of low molecular weight volatiles generated during the mixing process. The vacuum degree of the vacuum devolatilization port is -0.08 to -0.1 MPa. A real-time monitoring system for the mixing torque and power curves is established at the vacuum devolatilization port, and the mixing uniformity is verified by the torque fluctuation amplitude at the vacuum devolatilization port.

[0059] Using raw material preparation, mixing, feeding, and extrusion processes as comparative criteria, the advantages of this invention compared to existing technologies are analyzed in detail. A detailed comparison table is shown below:

[0060] Table 1 Comparison of Raw Material Preparation and Mixing & Feeding Extrusion Processes

[0061]

[0062] Based on Table 1 above, it can be concluded that by precisely limiting the melt index and isotacticity of polypropylene resin, a solid material foundation for film uniformity and mechanical strength is established. Optimizing the particle size, specific surface area, and dosage of nano-silica, combined with a differentiated and precise ratio of two β-nucleating agents, efficiently induces β-crystals and enhances toughness. An innovative three-stage mixing strategy and a real-time torque power monitoring system, combined with optimized extrusion equipment parameters, solve the pain points of uneven mixing, lack of quantity monitoring, and poor plasticizing effect in existing technologies, achieving a synergistic improvement in filler dispersion, crystal induction, and material stability.

[0063] S3, Casting: Controlling the melt extrusion temperature gradient and casting roll cooling parameters to form a sheet with a thickness of 80–200 μm. The casting roll temperature is controlled at 80–100°C, and cooling is assisted by air knives. The air pressure of the air knives is 0.3–0.6 MPa, the angle of the air knives to the film surface is 15–30 degrees, and the distance between the air knives and the film is 5–10 mm. A thermometer is introduced into the melt extrusion temperature gradient to monitor temperature fluctuations in each zone in real time. The thermometer is used to cooperate with the closed-loop control of the casting roll cooling system. By adjusting the air knife air pressure, angle, and distance, a uniform film with a thickness of 80–200 μm is formed.

[0064] Based on the comparison conditions of the casting process, the advantages of the present invention compared with the prior art are analyzed in detail, and the specific comparison table is as follows:

[0065] Table 2 Comparison of Cast Film Molding Processes

[0066]

[0067] Based on Table 2 above, it can be concluded that the combination of graded temperature control and real-time monitoring by a temperature measuring instrument avoids melt defects caused by temperature fluctuations. A composite cooling system using casting rollers and air knife-assisted cooling, coupled with precisely adjustable air knife parameters, achieves rapid and uniform cooling of the film, reducing internal stress and improving flatness. Relying on the closed-loop linkage control of infrared temperature measurement and the cooling system, the film thickness is precisely controlled within the range of 1.6-8 μm, meeting the core requirements of ultra-thin composite copper foil and high uniformity, far exceeding the film thickness control level of existing technologies.

[0068] S4, Biaxial Stretching Process: The film is biaxially stretched with a longitudinal stretch ratio of 4.0–7.0 and a transverse stretch ratio of 5.0–9.0. The production line speed is 150–300 m / min, optimizing the film's mechanical properties. Biaxial stretching is divided into longitudinal and transverse stretching. The longitudinal stretching process uses a multi-stage stretching process, with the stretch ratio gradually increasing in each stage. The temperature of the longitudinal stretching preheating zone is 115–130℃, and the preheating time is 3–5 seconds. The temperature of the longitudinal stretching stretching zone is 130–150℃. The longitudinal stretching stretching zone uses a three-stage stretching process with stretch ratios of 1.4–1.8, 1.5–2.0, and 1.8–2.2, respectively. The product of the three stretch ratios is the total longitudinal stretch ratio of 4.0–7.0. The temperature of the longitudinal stretching setting zone is 140–150℃, and the preheating time of the longitudinal stretching setting zone is 2–4 seconds. The transverse stretching process uses a chain clamp type stretching equipment and a multi-stage stretching process. The stretching ratio of each stage gradually increases. The temperature of the transverse stretching preheating zone is 160~170℃, and the preheating time is 4~12s. The temperature of the transverse stretching stretching zone is 150~170℃, and the stretching time is 6~12s. The temperature of the transverse stretching setting zone is 160~180℃, and the setting time is 4~8s.

[0069] Using biaxial stretching as a comparative condition, the advantages of this invention compared to existing technologies are analyzed in detail, as shown in the following comparison table:

[0070] Table 3 Comparison of Biaxial Stretching Processes

[0071]

[0072] Based on Table 3 above, it can be concluded that refined design surpasses the simplistic processes of existing technologies. By scientifically setting the longitudinal and transverse stretching ratios, coupled with a multi-stage incremental stretching strategy and precisely controlling the temperature and time parameters at each stage, tensile stress concentration is effectively reduced, film breakage is avoided, and molecular chain orientation is optimized. The use of a chain-clamp type stretching device in the transverse direction ensures uniformity in transverse stretching, solving the problems of uneven stretching, unbalanced mechanical properties, and significant differences in properties between the edges and the center in existing technologies, thus significantly improving the balance and stability of the film's longitudinal and transverse mechanical properties.

[0073] S5, Heat-Setting Surface Treatment: This involves double-sided corona treatment of the heat-set polymer materials to ensure dimensional stability and enhance the adhesion of the copper plating. The heat-setting temperature is 140-180℃, the heat-setting time is 5-15s, and the power of the double-sided corona treatment is 2-4kW. The surface tension of the film needs to be increased to 38-45mN / m. Before corona treatment, the surface tension of the film has already increased from 29-31mN / m in pure polypropylene to 34-38mN / m. The corona treatment further increases it to 38-45mN / m. The aging stability of the surface tension after corona treatment is superior to that of a pure corona-treated film without added nano-inorganic fillers.

[0074] S6, final performance indicators: ensure film thickness of 1.6–8.0 μm, tensile strength MD≥260MPa, TD≥300MPa, surface tension 38–45mN / m, surface roughness Ra0.05–0.5μm, thermal shrinkage rate ≤2.5% in the MD direction and ≤1.5% in the TD direction when held at 120℃ for 15 min, thermal shrinkage rate ≤5.0% in the MD direction and ≤3.5% in the TD direction when held at 150℃ for 30 min, and copper plating peel strength ≥0.8N / cm.

[0075] Based on the comparison of heat-setting surface treatment and final performance, a detailed analysis of the advantages of this invention compared to existing technologies is presented in the following table:

[0076] Table 4 Comparison of Heat-Setting Surface Treatment and Final Performance

[0077]

[0078] Based on Table 4 above, it can be concluded that the wide-range adjustable heat setting temperature effectively eliminates internal stress. Combined with double-sided corona treatment and precise power control, the surface tension is stably increased to the target range, completely solving the problems of poor dimensional stability and insufficient coating adhesion in existing technologies. In terms of final performance indicators, this solution achieves precise tensile strength compliance, eliminates copper plating peeling, and mitigates battery safety risks with its low thermal shrinkage rate. Compared to existing technologies, this represents a qualitative leap in interface bonding reliability, mechanical properties, and safety performance.

[0079] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A biaxially oriented polypropylene film for composite copper foil and its manufacturing method, characterized in that, Specifically, the steps include the following: S1, Raw material preparation: Select polypropylene resin with a melt index of 0.8–1.5 g / 10 min and isotacticity ≥95%. The purpose of selecting low melt index polypropylene resin is that it corresponds to a higher molecular weight and a longer molecular chain, which can obtain higher tensile strength and film toughness after biaxial stretching and orientation, while ensuring the mechanical strength brought by high crystallinity. S2, Mixed Feeding Extrusion: Polypropylene resin, surface-modified nano-inorganic filler treated with coupling agent, and β-nucleating agent are mixed and melt-extruded using a twin-screw extruder at 180–280℃. The nano-inorganic filler is selected from one or more combinations of nano-silica, nano-alumina, nano-barium sulfate, and nano-barium titanate. The coupling agent is selected from one or more combinations of KH-570 silane coupling agent, KH-550 silane coupling agent, KH-560 silane coupling agent, and titanate coupling agent to achieve uniform dispersion of filler and β-crystal induction to improve toughness and interfacial compatibility. After modification with coupling agent, the polar groups carried on the surface of the nano-inorganic filler are exposed along with the nanoparticles on the film surface during biaxial stretching, thereby intrinsically improving the surface polarity and surface free energy of the film and improving the wettability and adhesion of the film surface. S3, Casting: Controlling the melt extrusion temperature gradient and casting roll cooling parameters to form 80–200μm thick sheets; S4, Biaxial stretching process: The film is biaxially stretched with a longitudinal stretch ratio of 4.0–7.0 and a transverse stretch ratio of 5.0–9.

0. The production line speed is 150–300 m / min to optimize the mechanical properties of the film. S5, Heat-setting surface treatment: Double-sided corona treatment is performed on the heat-set polymer materials to ensure dimensional stability and enhance the adhesion of the copper plating. S6, final performance indicators: ensure film thickness of 1.6–8.0 μm, tensile strength MD≥260MPa, TD≥300MPa, surface tension 38–45mN / m, surface roughness Ra0.05–0.5μm, thermal shrinkage rate ≤2.5% in the MD direction and ≤1.5% in the TD direction when held at 120℃ for 15 min, thermal shrinkage rate ≤5.0% in the MD direction and ≤3.5% in the TD direction when held at 150℃ for 30 min, and copper plating peel strength ≥0.8N / cm.

2. The biaxially oriented polypropylene film for composite copper foil and its manufacturing method as described in claim 1, characterized in that: In step S2, the particle size of the nano-inorganic filler is 10–50 nm, and the specific surface area of ​​the nano-inorganic filler is 50–400 m². 2 / g, wherein the coupling agent is used at a rate of 0.5–3.0 wt% of the mass of the nano-inorganic filler, and the nano-inorganic filler accounts for 0.5–5.0% of the mass of the polypropylene resin; When the nano-inorganic filler is selected as nano-silica, its particle size is 10–30 nm, and the specific surface area of ​​the nano-silica is 200–400 m². 2 / g, the percentage of the nano-silica used is 1.5–3.0%; When the nano-inorganic filler is selected as nano-alumina, its particle size is 15–50 nm, and the specific surface area of ​​the nano-alumina is 50–200 m². 2 / g, the percentage of nano-alumina used is 1.0–3.0%; When the nano-inorganic filler is selected as nano-barium sulfate, its particle size is 20–50 nm, and its specific surface area is 30–100 m². 2 / g, the percentage of the nano-barium sulfate used is 1.0–4.0%; When the nano-inorganic filler is selected as barium titanate, its particle size is 20–50 nm, and the specific surface area of ​​the barium titanate is 30–80 m². 2 / g, the percentage of the nano-barium titanate used is 0.5–3.0%; When two or more nano-inorganic fillers are used in combination, the total percentage of the nano-inorganic fillers used shall not exceed 5.0%. When the nano-inorganic filler is nano-silica or nano-alumina, the coupling agent is KH-570 silane coupling agent or KH-550 silane coupling agent for surface modification. When the nano-inorganic filler is nano-barium sulfate or nano-barium titanate, the coupling agent is a titanate coupling agent or KH-560 silane coupling agent for surface modification. When the coupling agents are used in combination, the sum of their amounts is 0.5–3.0 wt% of the mass of the nano-inorganic filler.

3. The biaxially oriented polypropylene film for composite copper foil and its manufacturing method as described in claim 2, characterized in that: In step S2, the β-nucleating agent is TMB-5 or WBG-II, and the amount of TMB-5 or WBG-II used is 0.01%-0.5% of the mass of polypropylene resin. The 180–280°C is divided into feeding section temperature, compression section temperature, metering section temperature, and die temperature. The feeding section temperature is 180–220°C, the compression section temperature is 230–260°C, the metering section temperature is 245–280°C, and the die temperature is 230–250°C. The L / D ratio of the twin-screw extruder is 28:1 to 32:1, and the compression ratio of the twin-screw extruder is 3.5:1 to 4.5:1, where the maximum depth of the feeding section screw groove is equal to the depth of the metering section screw groove.

4. The biaxially oriented polypropylene film for composite copper foil and its manufacturing method as described in claim 1, characterized in that: In step S3, the temperature of the casting roller is controlled at 80–100°C, and the cooling is assisted by an air knife. The air pressure of the air knife is 0.3–0.6 MPa, the angle of the air knife to the film surface is 15–30 degrees, and the distance between the air knife and the film is 5–10 mm.

5. The biaxially oriented polypropylene film for composite copper foil and its manufacturing method as described in claim 1, characterized in that: In step S4, the biaxial stretching is divided into longitudinal stretching and transverse stretching. The longitudinal stretching adopts a multi-segment stretching process, in which the stretching ratio of each segment gradually increases. The temperature of the longitudinal stretching preheating zone is 115–130℃, and the preheating time of the longitudinal stretching preheating zone is 3–5s. The temperature of the longitudinal stretching stretching zone is 130–150℃, and the longitudinal stretching stretching zone adopts a three-segment stretching process. The stretching ratios of the longitudinal stretching stretching zone are 1.4–1.8, 1.5–2.0, and 1.8–2.2, respectively. The product of the three stretching ratios is the total longitudinal stretching ratio of 4.0–7.

0. The temperature of the longitudinal stretching shaping zone is 140–150℃, and the preheating time of the longitudinal stretching shaping zone is 2–4s.

6. The biaxially oriented polypropylene film for composite copper foil and its manufacturing method as described in claim 5, characterized in that: The transverse stretching is performed using a chain clamp type stretching device and a multi-segment stretching process. The stretching ratio of each segment gradually increases, and the product of the stretching ratios of the multiple segments is the total transverse stretching ratio of 5.0–9.

0. The temperature of the transverse stretching preheating zone is 160–170℃, and the preheating time of the transverse stretching preheating zone is 4–12s. The temperature of the transverse stretching stretching zone is 150–170℃, and the stretching time of the transverse stretching stretching zone is 6–12s. The temperature of the transverse stretching setting zone is 160–180℃, and the setting time of the transverse stretching setting zone is 4–8s.

7. The biaxially oriented polypropylene film for composite copper foil and its manufacturing method as described in claim 6, characterized in that: In step S5, the heat setting temperature is 140–180℃, the heat setting time is 5–15s, the power of the double-sided corona treatment is 2–4kW, and the surface tension of the film needs to be increased to 38–45mN / m. The surface tension of the film before corona treatment has been increased from 29–31mN / m of pure polypropylene to 34–38mN / m. The corona treatment further increases it to 38–45mN / m. The aging stability of the surface tension of the corona-treated film is better than that of the pure corona-treated film without the addition of nano-inorganic fillers.

8. The biaxially oriented polypropylene film for composite copper foil and its manufacturing method as described in claim 2, characterized in that: The mixing and feeding extrusion adopts a co-rotating twin-screw extruder. The twin-screw extruder is equipped with a three-stage mixing strategy with a modular screw configuration. In the first stage of the twin-screw extruder, a high-shear kneading block element is used to pre-disperse the surface-modified nano-inorganic filler with coupling agent to achieve initial homogenization. In the middle stage of the twin-screw extruder, a β-nucleating agent is added through a side-feeding device, and a low-shear conveying screw element is used to promote good interfacial bonding between the nucleating agent and the polypropylene matrix. In the final stage of the twin-screw extruder, a vacuum devolatilization port is used to remove trace amounts of low molecular weight volatiles generated during the mixing process. The vacuum degree of the vacuum devolatilization port is -0.08 to -0.1 MPa. A real-time monitoring system for the mixing torque and power curves is established at the vacuum devolatilization port, and the mixing uniformity is verified by the torque fluctuation amplitude at the vacuum devolatilization port.

9. The biaxially oriented polypropylene film for composite copper foil as described in claim 4, and its manufacturing method, characterized in that: A temperature measuring instrument is introduced into the melt extrusion temperature gradient. The temperature measuring instrument is used to monitor the temperature fluctuations in each zone in real time. The temperature measuring instrument is used to cooperate with the closed-loop control of the casting roll cooling system. By adjusting the air knife pressure, angle and distance, a uniform film in the range of 80–200 μm is formed.

10. A biaxially oriented polypropylene film for composite copper foil manufactured using the manufacturing method according to any one of claims 1 to 9, characterized in that, The film has a thickness of 1.6–8.0 μm, a longitudinal tensile strength (MD) ≥ 260 MPa, a transverse tensile strength (TD) ≥ 300 MPa, a surface tension of 38–45 mN / m, and a surface roughness (Ra) of 0.05–0.5 μm. When held at 120°C for 15 min, the film exhibits a thermal shrinkage rate ≤ 2.5% in the MD direction and ≤ 1.5% in the TD direction. When held at 150°C for 30 min, the film exhibits a thermal shrinkage rate ≤ 5.0% in the MD direction and ≤ 3.5% in the TD direction. The film surface undergoes both bulk phase modification with nano-inorganic fillers and double-sided corona treatment, resulting in a copper plating peel strength ≥ 0.8 N / cm.

Images