Heat-resistant polypropylene composite current collector base film and application thereof

Abstract

This invention relates to the field of current collector base films, specifically to a heat-resistant polypropylene composite current collector base film and its applications. The heat-resistant polypropylene composite current collector base film, by weight, comprises: 100 parts homopolymer polypropylene resin, 12-18 parts flame retardant reinforcing agent, 4-7 parts dopamine-modified nano-montmorillonite, 7-11 parts compatibilizer, 0.6-1.2 parts composite antioxidant, 0.3-0.6 parts light stabilizer, and 0.4-0.8 parts composite lubricant. This invention solves the defects of traditional polypropylene base films as composite current collectors, such as poor flame retardancy, insufficient mechanical strength, high thermal shrinkage, and weak adhesion to metal coatings. The resulting base film simultaneously possesses excellent mechanical properties, high flame retardancy, low thermal shrinkage, and strong interfacial adhesion, fully meeting the stringent requirements for high-safety, high-energy-density lithium-ion battery composite current collectors.

Description

Technical Field

[0001] This invention relates to the field of current collector base films, and more specifically to a heat-resistant polypropylene composite current collector base film and its applications. Background Technology

[0002] With the rapid development of new energy vehicles and the energy storage industry, the requirements for energy density, cycle life, and safety performance of lithium-ion batteries are constantly increasing. Composite current collectors adopt a "polymer base film-metal layer" sandwich structure, which can reduce weight by more than 50% compared with traditional pure metal foil, significantly improving battery energy density, while also possessing self-melting safety characteristics. It has become one of the core development directions of next-generation battery technology.

[0003] Polypropylene (PP) is the preferred material for composite current collector base films due to its low cost, low density, resistance to electrolyte corrosion, and excellent processing performance. However, existing polypropylene base films have the following problems: (1) Insufficient mechanical properties: When the thickness is reduced to below 12μm, the tensile strength and puncture resistance drop sharply, which cannot meet the requirements of magnetron sputtering and battery assembly processes; (2) Weak interfacial bonding: The non-polar surface has poor bonding with the copper / aluminum metal layer, and the metal layer is prone to peeling during cycling; (3) Poor thermal stability: The thermal shrinkage rate exceeds 2% at 150℃, which can easily lead to short circuits between the positive and negative electrodes at high temperatures; (4) Lack of flame retardant properties: The limiting oxygen index is only about 21%, which will increase the risk of combustion when the battery is thermally runaway.

[0004] Existing technologies often employ modification methods involving the addition of single inorganic fillers or small-molecule flame retardants, which suffer from problems such as uneven filler dispersion, weak interfacial bonding, and contradictions between flame retardancy and mechanical properties. Furthermore, existing modification technologies often only address a single problem and struggle to simultaneously meet multiple requirements, including mechanical properties, flame retardancy, thermal stability, and strong adhesion to metals.

[0005] Therefore, developing a polypropylene composite current collector base film that simultaneously possesses high mechanical strength, strong interfacial bonding, excellent heat resistance, and flame retardancy is of great significance for promoting the industrial application of composite current collectors. Summary of the Invention

[0006] To address the problems existing in the prior art, the purpose of this invention is to provide a heat-resistant polypropylene composite current collector film and its applications.

[0007] The objective of this invention is achieved through the following technical solution: In a first aspect, the present invention provides a heat-resistant polypropylene composite current collector base film, comprising, by weight parts: 100 parts homopolymer polypropylene resin, 12-18 parts flame retardant reinforcing agent, 4-7 parts dopamine-modified nano-montmorillonite, 7-11 parts compatibilizer, 0.6-1.2 parts composite antioxidant, 0.3-0.6 parts light stabilizer and 0.4-0.8 parts composite lubricant.

[0008] Preferably, the homopolymer polypropylene resin has a crystallinity ≥72% and a melt index of 3.0-6.0 g / 10 min (230℃, 2.16 kg).

[0009] More preferably, the homopolymer polypropylene resin is Sinopec PPH-Y40, with a crystallinity of 76% and a melt index of 4.2 g / 10 min.

[0010] Preferably, the compatibilizer is maleic anhydride-grafted polypropylene (PP-g-MAH) with a grafting rate of 1.2%-1.8% and a melt index of 7-11 g / 10 min (230℃, 2.16 kg).

[0011] Preferably, the composite antioxidant is composed of antioxidant 1076 and antioxidant 618 in a mass ratio of 1-2:2-3.

[0012] Preferably, the light stabilizer is at least one of light stabilizer 622, light stabilizer 770, and light stabilizer 944.

[0013] Preferably, the composite lubricant is composed of ethylene bis-stearamide and pentaerythritol stearate in a mass ratio of 1-2:1-2.

[0014] Preferably, the preparation method of the dopamine-modified nano-montmorillonite includes: Sodium-based montmorillonite was dispersed in Tris-HCl buffer solution at pH 8-9 to prepare a suspension with a mass fraction of 3%-5%, and ultrasonically dispersed until uniform. Dopamine hydrochloride at 20%-30% of the mass of montmorillonite was added, and the mixture was stirred and reacted at room temperature in the dark for 15-20 hours. The mixture was then centrifuged, washed until neutral, vacuum dried, and heat-treated in a nitrogen atmosphere at 150-160℃ for 1-2 hours to obtain dopamine-modified nano-montmorillonite.

[0015] Preferably, the method for preparing the flame retardant enhancer includes: S1. Under nitrogen protection, octachlorocyclotetraphosphazene, triethylamine, and polymerization inhibitor are dissolved in anhydrous tetrahydrofuran, cooled to 0-5°C in an ice bath, and hydroxyethyl acrylate is slowly added dropwise. After the addition is complete, the temperature is raised to 40°C and reacted for 15-20 hours. The solution is filtered to remove salt, concentrated, dissolved in dichloromethane, washed with water, dried, concentrated, recrystallized, and dried again to obtain octachlorocyclotetraphosphazene. S2. Octaacryloyloxy-functionalized cyclotetraphosphazene and 2-mercaptoimidazole are dissolved in N,N-dimethylformamide, a photoinitiator is added, and the mixture is irradiated with ultraviolet light under a nitrogen atmosphere at room temperature for 2-4 hours. After the reaction is completed, the mixture is concentrated to dryness under reduced pressure, recrystallized, and the precipitate is collected and dried under vacuum to obtain imidazole thioether-functionalized cyclotetraphosphazene.

[0016] Preferably, in S1, the mass ratio of octachlorocyclotetraphosphazene, hydroxyethyl acrylate, and triethylamine is 4.63-4.64:9.29-9.52:8.1-8.6.

[0017] More preferably, in S1, the mass ratio of octachlorocyclotetraphosphonium, hydroxyethyl acrylate and triethylamine is 4.63:9.34:8.2.

[0018] Preferably, in S1, the polymerization inhibitor is p-hydroxyanisole (MEHQ), and the amount added is 0.2%-0.4% of the mass of octachlorocyclotetraphosphazene.

[0019] Preferably, in S2, the mass ratio of octaacryloyloxyfunctionalized cyclotetraphosphazene to 2-mercaptoimidazole is 13.8-14.2:8.0-8.3.

[0020] Preferably, in S2, the photoinitiator is photoinitiator 1173, and the amount added is 1.3%-2.6% of the mass of octaacryloyloxyfunctionalized cyclotetraphosphonium nitrile.

[0021] Secondly, the present invention provides a method for preparing a heat-resistant polypropylene composite current collector film, comprising the following steps: Step 1: Take the homopolymer polypropylene resin, flame retardant reinforcing agent, dopamine-modified nano-montmorillonite, maleic anhydride-grafted polypropylene, composite antioxidant, light stabilizer and composite lubricant according to the weight parts, dry them separately and then mix them at high speed until uniform. Step 2: The mixture is melt-extruded and granulated using a twin-screw extruder to obtain polypropylene composite granules; Step 3: After drying the composite granules, melt and plasticize them through a single-screw extruder, and then cast them to obtain a casting sheet; Step 4: The casting sheet is stretched longitudinally and transversely in sequence, and then heat-set, cooled, trimmed, and wound up to obtain a heat-resistant polypropylene composite current collector base film.

[0022] The beneficial effects of this invention are as follows: 1. This invention uses highly crystalline homopolymer polypropylene resin as the matrix, and combines it with dopamine-modified nano-montmorillonite, imidazole sulfide-functionalized cyclotetraphosphazene flame retardant enhancer, maleic anhydride-grafted polypropylene compatibilizer, and an optimized composite antioxidant, composite lubricant, and light stabilizer system to construct a uniformly dispersed, strongly interfacially bonded multiphase composite structure in the polypropylene matrix. At the same time, the heterogeneous nucleation effect significantly refines the spherulite size, solving the defects of traditional polypropylene-based films as composite current collectors, such as poor flame retardancy, insufficient mechanical strength, high thermal shrinkage, and weak adhesion to metal coatings. The resulting film simultaneously possesses excellent mechanical properties, high flame retardancy, low thermal shrinkage, and strong interfacial bonding, fully meeting the stringent requirements for high-safety, high-energy-density lithium-ion battery composite current collectors.

[0023] 2. The imidazole sulfide-functionalized tetraphosphazene flame retardant enhancer synthesized in this invention uses a rigid eight-membered tetraphosphazene ring as its core. Multiple imidazole sulfide functional groups are grafted through a highly efficient mercapto-olefin click chemistry reaction, forming a phosphorus-nitrogen-sulfur ternary synergistic flame retardant system. The imidazole groups in the molecular structure can also form strong hydrogen bonds with the hydroxyl and amino groups on the surface of dopamine-modified nano-montmorillonite, significantly enhancing the interfacial bonding between the functional components and the polypropylene matrix. Furthermore, it can form coordination bonds with metal atoms such as copper and aluminum, greatly improving the adhesion between the metal coating and the base film. In addition, this flame retardant enhancer is an organic small molecule compound with good compatibility with the polypropylene matrix. Unlike inorganic flame retardants, it does not agglomerate, and while exerting a highly efficient flame retardant effect, it can also synergistically improve the mechanical properties and thermal stability of the matrix.

[0024] 3. This invention utilizes dopamine to surface-modify nano-montmorillonite. The polydopamine layer is tightly bonded to the nano-montmorillonite through covalent and hydrogen bonds. Simultaneously, the benzene rings and alkyl chains in its molecular structure exhibit good compatibility with the polypropylene matrix, solving the problem of easy agglomeration of inorganic nanofillers. This allows the nano-montmorillonite to be uniformly dispersed at the nanoscale within the matrix, fully leveraging its two-dimensional lamellar reinforcement effect. The two-dimensional lamellar structure of dopamine-modified nano-montmorillonite forms a physical barrier network, hindering the thermal motion and heat conduction of the polypropylene molecular chains. Synergistically, with the imidazole sulfide-functionalized cyclotetraphosphazene flame retardant reinforcing agent, a three-dimensional heat-resistant skeleton is constructed within the matrix, significantly reducing the high-temperature thermal shrinkage rate of the base film. This ensures that the base film maintains good flatness and mechanical integrity even under the localized high temperatures generated during vacuum coating processes and battery charging and discharging, fundamentally avoiding the risk of short circuits at the positive and negative electrode contacts caused by base film thermal shrinkage.

[0025] 4. This invention selects maleic anhydride-grafted polypropylene with a suitable grafting rate as a compatibilizer, which can effectively improve the compatibility between polar functional components and non-polar polypropylene matrix without adversely affecting melt flowability, further optimizing the overall performance of the base film. In the composite antioxidants used, antioxidant 1076 is the primary antioxidant, and antioxidant 618 is the secondary antioxidant. Their synergistic effect effectively inhibits the thermal oxidative degradation of polypropylene during high-temperature processing and long-term use. Simultaneously, a light stabilizer is used, which can absorb ultraviolet light and capture free radicals generated by photodegradation, improving the photoaging resistance of the base film. A composite lubricant composed of ethylene bis-stearamide and pentaerythritol stearate is used, providing both internal and external lubrication. This improves the flowability of the polypropylene melt and reduces friction between the melt and processing equipment, ensuring the stability of the production process and the consistency of product quality. Detailed Implementation

[0026] The technical solution of the present invention is illustrated below through specific examples. It should be understood that the one or more method steps mentioned in the present invention do not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps; it should also be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or defining the scope of the present invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the present invention.

[0027] To better understand the above technical solutions, exemplary embodiments of the present invention are described in more detail below. While exemplary embodiments of the present invention are shown, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present invention and to fully convey the scope of the invention to those skilled in the art.

[0028] The present invention will be further described below with reference to the following embodiments.

[0029] Example 1

[0030] A heat-resistant polypropylene composite current collector base film, comprising, by weight parts: 100 parts homopolymer polypropylene resin (Sinopec PPH-Y40, crystallinity 76%, melt index 4.2 g / 10 min), 12 parts flame retardant reinforcing agent, 5 parts dopamine-modified nano-montmorillonite, 9 parts maleic anhydride grafted polypropylene (PP-g-MAH, grafting rate 1.5%, melt index 9 g / 10 min), 0.9 parts composite antioxidant (antioxidant 1076: antioxidant 618 = 1:2), 0.45 parts light stabilizer 622, and 0.6 parts composite lubricant (ethylene bis-stearamide: pentaerythritol stearate = 1:1).

[0031] The preparation methods for dopamine-modified nano-montmorillonite include: 10g of sodium-based montmorillonite (particle size 100±10nm) was dispersed in Tris-HCl buffer solution at pH=8.5 to prepare a 4% (w / w) suspension. The suspension was ultrasonically dispersed for 25 min at 350W until homogeneous. 25% (w / w) of dopamine hydrochloride was added to the suspension, and the mixture was stirred at 350 rpm for 18 h at room temperature in the dark. After the reaction, the mixture was centrifuged at 9000 rpm for 12 min, and the precipitate was collected. The precipitate was washed with deionized water until the washing solution was neutral. The washed precipitate was dried in a vacuum drying oven at 65℃ for 14 h, and then transferred to a tube furnace for heat treatment in a nitrogen atmosphere at 155℃ for 1.5 h. After cooling to room temperature, the precipitate was removed to obtain dopamine-modified nano-montmorillonite for later use.

[0032] The preparation method of the dopamine-modified nano-montmorillonite includes: S1. Under nitrogen protection, 46.3 g of octachlorocyclotetraphosphazene, 81 g of triethylamine, and 0.093 g of polymerization inhibitor p-hydroxyanisole (MEHQ, added at 0.2% of the mass of octachlorocyclotetraphosphazene) were added to 500 mL of anhydrous tetrahydrofuran and stirred to dissolve. The reaction system was cooled to 0 °C in an ice bath, and 92.9 g of hydroxyethyl acrylate was slowly added dropwise at a rate of 1 mL / min. After the addition was complete, the temperature was raised to 40 °C and the reaction was maintained for 15 h. After the reaction was completed, the generated triethylamine hydrochloride was removed by filtration, and the filtrate was concentrated under reduced pressure to 1 / 4 of its original volume. 300 mL of dichloromethane was added to the concentrate to dissolve it, and the solution was washed three times with 200 mL of deionized water each time to separate the organic phase. 50 g of anhydrous sodium sulfate was added and the solution was dried for 6 h. The desiccant was removed by filtration, and the filtrate was concentrated to dryness under reduced pressure. The solution was recrystallized by hexane precipitation and dried to obtain a white solid, octaacryloyloxyfunctionalized cyclotetraphosphazene.

[0033] S2. Dissolve 13.8 g of octaacryloyloxy-functionalized cyclotetraphosphazene and 8.0 g of 2-mercaptoimidazole in 150 mL of N,N-dimethylformamide and stir until dissolved. Add 0.179 g of photoinitiator 1173 (1.3% of the mass of octaacryloyloxy-functionalized cyclotetraphosphazene) to the above solution. After purging the air in the reaction system with nitrogen three times, irradiate with 365 nm ultraviolet light under a nitrogen atmosphere and react at room temperature for 2 h. After the reaction is completed, concentrate the reaction solution to dryness under reduced pressure, add 100 mL of dichloromethane to dissolve the residue, and slowly add the solution dropwise to 1000 mL of cold acetone (10 times the volume of the dichloromethane solution), and a white precipitate will precipitate. Filter and collect the precipitate, wash twice with 50 mL of cold acetone each time, and dry in a vacuum drying oven at 60 °C for 12 h to obtain the imidazole thioether-functionalized cyclotetraphosphazene flame retardant enhancer.

[0034] The preparation method of the above-mentioned heat-resistant polypropylene composite current collector film includes the following steps: Step 1, raw material pretreatment: Place the homopolymer polypropylene resin in an 85℃ vacuum drying oven and dry for 5 hours; place the dopamine-modified nano-montmorillonite, flame retardant reinforcing agent, maleic anhydride grafted polypropylene, composite antioxidant, light stabilizer and composite lubricant in a 65℃ vacuum drying oven and dry for 2.5 hours respectively, for later use.

[0035] Step 2, high-speed mixing: Add the dried components to a high-speed mixer according to the above weight proportions, and mix at 1000 r / min for 7 min to ensure uniform dispersion of the components and obtain a premix.

[0036] Step 3, melt extrusion granulation: The premixed material is added to a twin-screw extruder for melt extrusion granulation; the temperature of each zone of the twin-screw extruder is set as follows: Zone 1 175℃, Zone 2 190℃, Zone 3 200℃, Zone 4 205℃, Zone 5 210℃, Zone 6 205℃, and the die head temperature is 210℃; the screw speed is controlled at 250 r / min, and the feeding speed is controlled at 20 kg / h; the extruded strip is cooled by water at 23℃, air-dried, and then granulated to obtain polypropylene composite granules.

[0037] Step 4, Casting: After drying the polypropylene composite granules in an 85℃ vacuum drying oven for 5 hours, they are added to a single-screw extruder for melt plasticization. The temperatures of each zone of the single-screw extruder are set as follows: Zone 1 185℃, Zone 2 195℃, Zone 3 205℃, Zone 4 210℃, and the die head temperature 215℃. The screw speed is controlled at 80 r / min. The molten material is extruded and cast through a coat hanger die onto a chrome-plated cooling roller. The temperature of the cooling roller is controlled at 25℃. After cooling and shaping, a casting sheet with a thickness of 220μm is obtained.

[0038] Step 5, Biaxial stretching: The cast film is fed into the longitudinal stretching machine and longitudinally stretched at a preheating temperature of 115°C, with a longitudinal stretching ratio of 4.5 times; the longitudinally stretched film is immediately fed into the transverse stretching machine and transversely stretched at a preheating temperature of 125°C, with a transverse stretching ratio of 7 times.

[0039] Step 6, heat setting and winding: The biaxially stretched film is sent into a heat setting box and heat-set at 135°C for 4 seconds to eliminate internal stress in the film; then it is cooled to room temperature by cooling rollers, the irregular parts at the edges are trimmed, and finally it is wound up to obtain a heat-resistant polypropylene composite current collector base film with a thickness of 10μm.

[0040] Example 2

[0041] A heat-resistant polypropylene composite current collector base film, comprising, by weight parts: 100 parts homopolymer polypropylene resin (Sinopec PPH-Y40, crystallinity 76%, melt index 4.2 g / 10 min), 15 parts flame retardant reinforcing agent, 6 parts dopamine-modified nano-montmorillonite, 10 parts maleic anhydride grafted polypropylene (PP-g-MAH, grafting rate 1.5%, melt index 9 g / 10 min), 1.0 part composite antioxidant (antioxidant 1076: antioxidant 618 = 1.5:2.5), 0.5 parts light stabilizer 770, and 0.7 parts composite lubricant (ethylene bis-stearamide: pentaerythritol stearate = 1.5:1).

[0042] The preparation method of dopamine-modified nano-montmorillonite is the same as that in Example 1.

[0043] The preparation methods of flame retardant reinforcing agents include: S1. Under nitrogen protection, 46.3 g of octachlorocyclotetraphosphazene, 82 g of triethylamine, and 0.14 g of the polymerization inhibitor p-hydroxyanisole (MEHQ) were added to 500 mL of anhydrous tetrahydrofuran and stirred to dissolve. The reaction system was cooled to 0-5 °C in an ice bath, and 93.4 g of hydroxyethyl acrylate was slowly added dropwise at a rate of 1.5 mL / min. After the addition was complete, the temperature was raised to 40 °C and the reaction was maintained for 18 h. After the reaction was completed, the generated triethylamine hydrochloride was removed by filtration, and the filtrate was concentrated under reduced pressure to 1 / 3 of its original volume. 300 mL of dichloromethane was added to the concentrate to dissolve it, and the solution was washed three times with 200 mL of deionized water each time to separate the organic phase. 50 g of anhydrous sodium sulfate was added and the solution was dried for 7 h. The desiccant was removed by filtration, and the filtrate was concentrated to dryness under reduced pressure. The solution was recrystallized by hexane precipitation and dried to obtain a white solid, octachlorooxyfunctionalized cyclotetraphosphazene.

[0044] S2. Dissolve 14.0 g of octaacryloyloxyfunctionalized cyclotetraphosphazene and 8.15 g of 2-mercaptoimidazole in 150 mL of N,N-dimethylformamide and stir until dissolved. Add 0.25 g of photoinitiator 1173 to the above solution. After purging the air in the reaction system with nitrogen four times, irradiate with 365 nm ultraviolet light under a nitrogen atmosphere and react at room temperature for 3 h. After the reaction is completed, concentrate the reaction solution to dryness under reduced pressure, add 100 mL of dichloromethane to dissolve the residue, and slowly add the solution dropwise to 1200 mL of cold acetone, precipitating a white precipitate. Filter and collect the precipitate, wash twice with 50 mL of cold acetone each time, and dry in a vacuum drying oven at 65 °C for 14 h to obtain the imidazole thioether-functionalized cyclotetraphosphazene flame retardant enhancer.

[0045] The preparation method of the above-mentioned heat-resistant polypropylene composite current collector film includes the following steps: Step 1, raw material pretreatment: Place the homopolymer polypropylene resin in an 80℃ vacuum drying oven and dry for 4 hours; place the dopamine-modified nano-montmorillonite, flame retardant reinforcing agent, maleic anhydride grafted polypropylene, composite antioxidant, light stabilizer and composite lubricant in a 60℃ vacuum drying oven and dry for 2 hours respectively, for later use.

[0046] Step 2, high-speed mixing: Add the dried components to a high-speed mixer according to the above weight proportions, and mix at 800 r / min for 5 min to ensure uniform dispersion of the components and obtain a premix.

[0047] Step 3, melt extrusion granulation: The premixed material is added to a twin-screw extruder for melt extrusion granulation; the temperature of each zone of the twin-screw extruder is set as follows: Zone 1 170℃, Zone 2 185℃, Zone 3 195℃, Zone 4 200℃, Zone 5 205℃, Zone 6 200℃, and the die head temperature is 205℃; the screw speed is controlled at 200 r / min, and the feeding speed is controlled at 15 kg / h; the extruded strip is cooled by water at 20℃, air-dried, and then granulated to obtain polypropylene composite granules.

[0048] Step 4, Casting: After drying the polypropylene composite granules in an 80℃ vacuum drying oven for 4 hours, they are added to a single-screw extruder for melt plasticization. The temperatures of each zone of the single-screw extruder are set as follows: Zone 1 180℃, Zone 2 190℃, Zone 3 200℃, Zone 4 205℃, and the die head temperature 210℃. The screw speed is controlled at 60 r / min. The molten material is extruded and cast through a coat hanger die onto a chrome-plated cooling roller. The temperature of the cooling roller is controlled at 20℃. After cooling and shaping, a casting sheet with a thickness of 200μm is obtained.

[0049] Step 5, biaxial stretching: The cast film is fed into the longitudinal stretching machine and longitudinally stretched at a preheating temperature of 110°C, with a longitudinal stretching ratio of 4 times; the longitudinally stretched film is immediately fed into the transverse stretching machine and transversely stretched at a preheating temperature of 120°C, with a transverse stretching ratio of 6 times.

[0050] Step 6, heat setting and winding: The biaxially stretched film is sent into a heat setting box and heat-set at 130°C for 3 seconds to eliminate internal stress in the film; then it is cooled to room temperature by cooling rollers, the irregular parts at the edges are trimmed, and finally it is wound up to obtain a heat-resistant polypropylene composite current collector base film with a thickness of 8μm.

[0051] Example 3

[0052] A heat-resistant polypropylene composite current collector base film, comprising, by weight parts: 100 parts homopolymer polypropylene resin (Sinopec PPH-Y40, crystallinity 76%, melt index 4.2 g / 10 min), 18 parts flame retardant reinforcing agent, 7 parts dopamine-modified nano-montmorillonite, 11 parts maleic anhydride grafted polypropylene (PP-g-MAH, grafting rate 1.5%, melt index 9 g / 10 min), 1.2 parts composite antioxidant (antioxidant 1076: antioxidant 618 = 2:3), 0.6 parts light stabilizer 944, and 0.8 parts composite lubricant (ethylene bis-stearamide: pentaerythritol stearate = 2:1).

[0053] The preparation method of dopamine-modified nano-montmorillonite is the same as that in Example 1.

[0054] The preparation methods of flame retardant reinforcing agents include: S1. Under nitrogen protection, 46.4 g of octachlorocyclotetraphosphazene, 86 g of triethylamine, and 0.186 g of polymerization inhibitor p-hydroxyanisole (MEHQ, added at 0.4% of the mass of octachlorocyclotetraphosphazene) were added to 500 mL of anhydrous tetrahydrofuran and stirred to dissolve. The reaction system was cooled to 5 °C in an ice bath, and 95.2 g of hydroxyethyl acrylate was slowly added dropwise at a rate of 2 mL / min. After the addition was complete, the temperature was raised to 40 °C and the reaction was maintained for 20 h. After the reaction was completed, the generated triethylamine hydrochloride was removed by filtration, and the filtrate was concentrated under reduced pressure to 1 / 3 of its original volume. 300 mL of dichloromethane was added to the concentrate to dissolve it, and the solution was washed four times with 200 mL of deionized water each time to separate the organic phase. 50 g of anhydrous sodium sulfate was added and the solution was dried for 8 h. The desiccant was removed by filtration, and the filtrate was concentrated to dryness under reduced pressure. The solution was recrystallized by hexane precipitation and dried to obtain a white solid, octaacryloyloxyfunctionalized cyclotetraphosphazene.

[0055] S2. Dissolve 14.2g of octaacryloyloxy-functionalized cyclotetraphosphazene and 8.3g of 2-mercaptoimidazole in 150mL of N,N-dimethylformamide and stir until dissolved. Add 0.369g of photoinitiator 1173 (2.6% of the mass of octaacryloyloxy-functionalized cyclotetraphosphazene) to the above solution. After purging the air in the reaction system with nitrogen five times, irradiate with 365nm ultraviolet light under a nitrogen atmosphere and react at room temperature for 4h. After the reaction is completed, concentrate the reaction solution to dryness under reduced pressure, add 100mL of dichloromethane to dissolve the residue, and slowly add the solution dropwise to 1500mL of cold acetone (15 times the volume of the dichloromethane solution), and a white precipitate will precipitate. Filter and collect the precipitate, wash it three times with 50mL of cold acetone each time, and dry it in a vacuum drying oven at 70℃ for 16h to obtain the imidazole thioether-functionalized cyclotetraphosphazene flame retardant enhancer.

[0056] The preparation method of the above-mentioned heat-resistant polypropylene composite current collector film includes the following steps: Step 1, raw material pretreatment: Place the homopolymer polypropylene resin in a 90℃ vacuum drying oven and dry for 6 hours; place the dopamine-modified nano-montmorillonite, flame retardant reinforcing agent, maleic anhydride grafted polypropylene, composite antioxidant, light stabilizer and composite lubricant in a 70℃ vacuum drying oven and dry for 3 hours respectively, for later use.

[0057] Step 2, high-speed mixing: Add the dried components to a high-speed mixer according to the above weight proportions, and mix at 1200 r / min for 10 min to ensure uniform dispersion of the components and obtain a premix.

[0058] Step 3, melt extrusion granulation: The premixed material is added to a twin-screw extruder for melt extrusion granulation; the temperature of each zone of the twin-screw extruder is set as follows: Zone 1 180℃, Zone 2 195℃, Zone 3 205℃, Zone 4 210℃, Zone 5 215℃, Zone 6 210℃, and the die head temperature is 215℃; the screw speed is controlled at 300 r / min, and the feeding speed is controlled at 25 kg / h; the extruded strip is cooled by water at 25℃, air-dried, and then granulated to obtain polypropylene composite granules.

[0059] Step 4, Casting: After drying the polypropylene composite granules in a 90℃ vacuum drying oven for 6 hours, they are added to a single-screw extruder for melt plasticization. The temperatures of each zone of the single-screw extruder are set as follows: Zone 1 190℃, Zone 2 200℃, Zone 3 210℃, Zone 4 215℃, and the die head temperature 220℃. The screw speed is controlled at 100 r / min. The molten material is extruded and cast through a coat hanger die onto a chrome-plated cooling roller. The temperature of the cooling roller is controlled at 30℃. After cooling and shaping, a casting sheet with a thickness of 250μm is obtained.

[0060] Step 5, Biaxial stretching: The cast film is fed into the longitudinal stretching machine and longitudinally stretched at a preheating temperature of 120°C with a longitudinal stretching ratio of 5 times; the longitudinally stretched film is immediately fed into the transverse stretching machine and transversely stretched at a preheating temperature of 130°C with a transverse stretching ratio of 8 times.

[0061] Step 6, heat setting and winding: The biaxially stretched film is sent into a heat setting box and heat-set at 140°C for 5 seconds to eliminate internal stress in the film; then it is cooled to room temperature by cooling rollers, the irregular parts at the edges are trimmed, and finally it is wound up to obtain a heat-resistant polypropylene composite current collector base film with a thickness of 12μm.

[0062] Comparative Example 1 A heat-resistant polypropylene composite current collector film, differing from Example 2 in that it does not contain a flame-retardant reinforcing agent. It comprises, by weight parts: 100 parts homopolymer polypropylene resin (Sinopec PPH-Y40, crystallinity 76%, melt index 4.2 g / 10 min), 0 parts flame retardant reinforcing agent, 6 parts dopamine-modified nano-montmorillonite, 10 parts maleic anhydride grafted polypropylene (PP-g-MAH, grafting rate 1.5%, melt index 9 g / 10 min), 1.0 part composite antioxidant (antioxidant 1076: antioxidant 618 = 1.5:2.5), 0.5 parts light stabilizer 770, and 0.7 parts composite lubricant (ethylene bis-stearamide: pentaerythritol stearate = 1.5:1).

[0063] The preparation of the polypropylene composite current collector base film is exactly the same as in Example 2, except that no flame retardant reinforcing agent is added.

[0064] Comparative Example 2 A heat-resistant polypropylene composite current collector film, differing from Example 2 in that it incorporates commercial ammonium polyphosphate (APP) instead of the flame-retardant reinforcing agent. It comprises, by weight parts: 100 parts homopolymer polypropylene resin (Sinopec PPH-Y40, crystallinity 76%, melt index 4.2 g / 10 min), 15 parts commercial ammonium polyphosphate (APP, degree of polymerization n≥1000), 6 parts dopamine-modified nano-montmorillonite, 10 parts maleic anhydride grafted polypropylene (PP-g-MAH, grafting rate 1.5%, melt index 9 g / 10 min), 1.0 part composite antioxidant (antioxidant 1076: antioxidant 618 = 1.5: 2.5), 0.5 parts light stabilizer 770, and 0.7 parts composite lubricant (ethylene bis-stearamide: pentaerythritol stearate = 1.5: 1).

[0065] The preparation of the polypropylene composite current collector base film is exactly the same as in Example 2, except that 15 parts of commercial ammonium polyphosphate are used to replace the flame retardant reinforcing agent.

[0066] Comparative Example 3 A heat-resistant polypropylene composite current collector film differs from Example 2 in that it incorporates unmodified octaacryloyloxyfunctionalized tetraphosphazene (the product of step S1 in Example 2) instead of the flame retardant reinforcing agent. It comprises, by weight parts: 100 parts homopolymer polypropylene resin (Sinopec PPH-Y40, crystallinity 76%, melt index 4.2 g / 10 min), 15 parts unmodified octaacryloyloxyfunctionalized tetraphosphazene, 6 parts dopamine-modified nano-montmorillonite, 10 parts maleic anhydride grafted polypropylene (PP-g-MAH, grafting rate 1.5%, melt index 9 g / 10 min), 1.0 part composite antioxidant (antioxidant 1076: antioxidant 618 = 1.5: 2.5), 0.5 parts light stabilizer 770, and 0.7 parts composite lubricant (ethylene bis-stearamide: pentaerythritol stearate = 1.5: 1).

[0067] The preparation of the polypropylene composite current collector base film is exactly the same as in Example 2, except that 15 parts of unmodified octaacryloyloxyfunctionalized tetraphosphazene are used to replace the flame retardant reinforcing agent.

[0068] Experimental testing The performance of the composite current collector base film prepared in Example 2 and Comparative Examples 1-3 was tested.

[0069] (1) Mechanical property test: The test was conducted in accordance with GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", with a tensile speed of 50 mm / min and a test temperature of 23℃.

[0070] (2) Flame retardant performance test: The limiting oxygen index (LOI) was tested in accordance with GB / T 2406.2-2009 "Determination of burning behavior by oxygen index method for plastics - Part 2: Room temperature test".

[0071] (3) Interfacial bonding strength test (copper foil peel strength): A pure copper foil layer with a thickness of 1μm was deposited on the surface of the base film by vacuum evaporation. The 180° peel strength test was carried out in accordance with GB / T 2792-2014 "Test method for peel strength of adhesive tape", with a tensile speed of 300mm / min and a test temperature of 23℃.

[0072] (4) Heat shrinkage rate test: The test was conducted in accordance with GB / T 13519-2016 "Polyethylene heat shrink film". The sample was cut into a square sample of 100mm×100mm, the longitudinal and transverse dimensions were marked, and the sample was placed in a 130℃ drying oven for 30min. After cooling to room temperature, the longitudinal and transverse dimensions were measured and the heat shrinkage rate was calculated.

[0073] The test results are shown in the table below:

[0074] Test results show that the polypropylene composite current collector film prepared in Example 2 has significantly better overall performance than the comparative example. Its longitudinal tensile strength is 142 MPa, transverse tensile strength is 155 MPa, limiting oxygen index is ≥30%, longitudinal thermal shrinkage rate at 130℃×30 min is ≤1%, transverse thermal shrinkage rate is ≤0.8%, and the 180° peel strength of the copper plating layer is 7.2 N / cm. This invention uses imidazole sulfide-functionalized tetraphosphazene as a flame retardant reinforcing agent, forming a synergistic effect with dopamine-modified nano-montmorillonite. This solves the problems of poor flame retardancy, insufficient mechanical strength, high thermal shrinkage rate, and weak adhesion to metal layers in traditional polypropylene films, fully meeting the stringent requirements for lithium-ion battery composite current collectors.

[0075] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. The illustrative expressions of the above terms in this specification should not be construed as necessarily referring to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0076] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A heat-resistant polypropylene composite current collector film, characterized in that, Calculated by weight, including: 100 parts homopolymer polypropylene resin, 12-18 parts flame retardant reinforcing agent, 4-7 parts dopamine-modified nano-montmorillonite, 7-11 parts compatibilizer, 0.6-1.2 parts composite antioxidant, 0.3-0.6 parts light stabilizer and 0.4-0.8 parts composite lubricant.

2. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, The melt index of the homopolymer polypropylene resin is 3.0-6.0 g / 10 min at 230℃ and 2.16 kg; the compatibilizer is maleic anhydride-grafted polypropylene with a grafting rate of 1.2%-1.8% and a melt index of 7-11 g / 10 min at 230℃ and 2.16 kg.

3. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, The composite antioxidant is composed of antioxidant 1076 and antioxidant 618 in a mass ratio of 1-2:2-3.

4. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, The light stabilizer is at least one of light stabilizer 622, light stabilizer 770 and light stabilizer 944.

5. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, The composite lubricant is composed of ethylene bis-stearamide and pentaerythritol stearate in a mass ratio of 1-2:1-2.

6. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, The dopamine-modified nano-montmorillonite is obtained by first modifying sodium-based montmorillonite in situ with dopamine, and then heat-treating it at 150-160℃; wherein the amount of dopamine hydrochloride added is 20%-30% of the mass of montmorillonite.

7. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, The preparation method of the flame retardant enhancer includes: S1. Under nitrogen protection, octachlorocyclotetraphosphazene, triethylamine, and polymerization inhibitor are dissolved in tetrahydrofuran, cooled to 0-5°C in an ice bath, and hydroxyethyl acrylate is slowly added dropwise. After the addition is complete, the temperature is raised to 40°C and reacted for 15-20 hours. The solution is filtered to remove salt, concentrated, dissolved in dichloromethane, washed with water, dried, concentrated, recrystallized, and dried again to obtain octachlorooxy-functionalized cyclotetraphosphazene. S2. Octaacryloyloxy-functionalized cyclotetraphosphazene and 2-mercaptoimidazole are dissolved in N,N-dimethylformamide, a photoinitiator is added, and the mixture is irradiated with ultraviolet light under a nitrogen atmosphere at room temperature for 2-4 hours. After the reaction is completed, the mixture is concentrated to dryness under reduced pressure, recrystallized, and the precipitate is collected and dried under vacuum to obtain imidazole thioether-functionalized cyclotetraphosphazene.

8. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, In S1, the mass ratio of octachlorocyclotetraphosphazene, hydroxyethyl acrylate, and triethylamine is 4.63-4.64:9.29-9.52:8.1-8.6; the polymerization inhibitor is p-hydroxyanisole, and the amount added is 0.2%-0.4% of the mass of octachlorocyclotetraphosphazene.

9. The heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, In S2, the mass ratio of octaacryloyloxyfunctionalized cyclotetraphosphazene to 2-mercaptoimidazole is 13.8-14.2:8.0-8.

3.

10. A method for preparing the heat-resistant polypropylene composite current collector film according to claim 1, characterized in that, Includes the following steps: Step 1: Take the homopolymer polypropylene resin, flame retardant reinforcing agent, dopamine-modified nano-montmorillonite, maleic anhydride-grafted polypropylene, composite antioxidant, light stabilizer and composite lubricant according to the weight parts, dry them separately and then mix them at high speed until uniform. Step 2: The mixture is melt-extruded and granulated using a twin-screw extruder to obtain polypropylene composite granules; Step 3: After drying the composite granules, melt and plasticize them through a single-screw extruder, and then cast them to obtain a casting sheet; Step 4: The casting sheet is stretched longitudinally and transversely in sequence, and then heat-set, cooled, trimmed, and wound up to obtain a heat-resistant polypropylene composite current collector base film.

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