A polypropylene microporous membrane, its preparation method, and a lithium-ion battery

Abstract

This invention provides a polypropylene microporous membrane, its preparation method, and a lithium-ion battery. The preparation method includes: adding a polypropylene raw material composition to a first extruder for melt blending to obtain a polypropylene blend raw material, wherein the polypropylene raw material composition includes isotactic polypropylene, olefin block copolymers, and ultra-high molecular weight polypropylene; melting and extruding the polypropylene blend raw material through a second extruder, followed by melt stretching between the die and a casting roller assembly of the second extruder, wherein the casting roller assembly includes a mirror casting roller and a rubber pressure roller arranged opposite each other, and the melt is subjected to opposing rolling pressure between the two to obtain a precursor membrane; and sequentially subjecting the precursor membrane to heat treatment, stretching to form pores, and heat setting to obtain the polypropylene microporous membrane. The polypropylene microporous membrane provided by this invention has high puncture resistance and transverse yield strength, which can solve the defects of poor transverse strength and insufficient puncture resistance in existing dry-process lithium battery separators.

Description

Technical Field

[0001] This invention relates to a polypropylene microporous membrane, its preparation method, and a lithium-ion battery, belonging to the technical field of porous membrane materials, particularly lithium-ion battery separator membranes. Background Technology

[0002] Microporous membranes, as a key component of liquid lithium-ion batteries, play a crucial role. Their primary functions include isolating the positive and negative electrodes, preventing direct contact and short circuits, storing and maintaining sufficient electrolyte, and utilizing their microporous structure to allow lithium ions to pass freely, enabling rapid lithium ion transport between the positive and negative electrodes. Microporous membranes need to be as thin as possible to reduce the distance between the electrodes and lower the battery's internal resistance, while simultaneously possessing sufficient mechanical strength to ensure safety.

[0003] Polypropylene microporous membranes are among the most widely used commercial lithium-ion battery separators, offering advantages such as low cost, uniform pore size, and good high-temperature dimensional stability. Their preparation primarily employs the melt stretching method (dry method). The melt stretching method (dry method) for preparing microporous membranes mainly involves three steps: (a) forming a precursor film with a parallel lamellar structure under the induction of melt stretching stress after exiting the die; (b) heat-treating the precursor film to eliminate defects and increase lamellar thickness; and (c) two-step stretching (or direct hot stretching) and heat setting to obtain the microporous separator. The initial crystalline structure of the precursor obtained from melt stretching directly affects the subsequent pore formation of the separator, and the initially formed lamellar structure also determines the mechanical properties of the microporous separator. Due to the parallel distribution of the lamellar structure and the low amorphous strength between the lamellars, the transverse strength of the polypropylene microporous separator is reduced to some extent, leading to decreased puncture resistance and posing a certain safety hazard to the processing and service of lithium-ion batteries.

[0004] The following will briefly introduce some prior art that is closely related to this invention, so that those skilled in the art can better understand the situation of the prior art and the drawbacks of the prior art.

[0005] CN114784457A discloses a lithium-ion battery composite separator, its preparation method, and its application, relating to the field of multilayer composite separators. The composite separator includes a base membrane and a coating applied to one or both sides of the separator; the coating is made of a core-shell structured polymer particle emulsion; the core-shell structured polymer particles include a polyacrylate core and a polyurethane shell covering the polyacrylate core; the polyacrylate is polymerized from a first monomer and a second monomer, the first monomer containing one alkenyl group and the second monomer containing at least two alkenyl groups; the polyurethane contains polyethylene glycol segments. This prior art uses special core-shell structured polymer particles as the coating material, enabling the composite separator to possess good electrolyte wettability, ion conductivity, mechanical properties, puncture resistance, and thermal stability; however, it relies on the coating layer to improve impact resistance, which differs from the technical method of this invention.

[0006] CN112993490A discloses a lithium battery separator, its preparation method, and its application, relating to the technical field of lithium batteries. The lithium battery separator includes a base film and an organic elastic material coating covering the base film; the separator has a tensile strength of 200-300 MPa in both the transverse and longitudinal directions, an elongation at break of 80%-150%, and a puncture resistance of over 950 GF. The organic elastic material in the lithium battery separator includes at least one of acrylic resin, hydrogenated styrene-butadiene block copolymer, and thermoplastic polyurethane elastomer rubber. This lithium battery separator has high mechanical strength, improving the tensile strength and puncture resistance in the mechanical directions (MD and TD). The organic elastic material coating is resistant to electrolyte corrosion; when the battery is impacted by external force, the coating will ensure that the positive and negative electrode materials do not short-circuit, improving the safety of lithium-ion batteries. However, coating the surface of the lithium battery separator with an elastomer has poor technical feasibility.

[0007] CN102263220A discloses a method for preparing a battery separator, comprising: unidirectionally stretching a first polypropylene film and a second polypropylene film at -3 to -7°C, and then holding them at 45-60°C to obtain a first polypropylene microporous film and a second polypropylene microporous film, respectively; vertically compositing the first polypropylene microporous film and the second polypropylene microporous film in the stretching direction to obtain a polypropylene microporous composite film; and hydrophilizing the polypropylene microporous composite film to obtain the battery separator. This prior art, by vertically compositing the first polypropylene microporous film and the second polypropylene microporous film in the stretching direction, causes the straight micropores in the first polypropylene microporous film to be misaligned with those in the second polypropylene microporous film, forming tortuous pores. This significantly reduces the pore size, effectively blocking the passage of zinc microparticles and zincates without affecting the passage of organic ions, thus preventing dendrite penetration. However, the pore size is at the nanometer scale, which cannot be achieved by this technical method.

[0008] Therefore, providing a novel polypropylene microporous membrane with high puncture resistance and transverse strength, its preparation method, and a lithium-ion battery have become urgent technical problems to be solved in this field. Summary of the Invention

[0009] To address the aforementioned shortcomings and deficiencies, the present invention aims to provide a polypropylene microporous membrane, its preparation method, and a lithium-ion battery. The polypropylene microporous membrane provided by this invention possesses high puncture resistance and transverse yield strength. Using it as a separator in a lithium-ion battery can overcome the defects of poor transverse strength and insufficient puncture resistance in existing dry-process lithium battery separators.

[0010] To achieve the above objectives, in one aspect, the present invention provides a method for preparing a polypropylene microporous membrane, wherein the preparation method includes:

[0011] Step (1): Add the polypropylene raw material composition to the first extruder for melt blending to obtain polypropylene blend raw material, wherein the polypropylene raw material composition includes isotactic polypropylene, olefin block copolymer and ultra-high molecular weight polypropylene;

[0012] Step (2): After the polypropylene blend raw material is melt-extruded through the second extruder, it is melt-stretched between the die and the casting roll group of the second extruder. The casting roll group includes a mirror casting roll and a rubber pressure roll arranged opposite to each other, and the melt is rolled against each other between the two to obtain a precursor film. The distance between the mirror casting roll and the rubber pressure roll is 0.85-0.95 times the thickness of the melt.

[0013] Step (3): The precursor membrane is subjected to heat treatment, stretching to form pores and heat setting in sequence to obtain the polypropylene microporous membrane.

[0014] In the preparation method described above, the polypropylene raw material composition further includes ultra-high molecular weight polypropylene. The use of ultra-high molecular weight polypropylene can significantly improve the melt strength of the polypropylene microporous membrane and increase the wafer thickness during crystallization, thereby improving puncture resistance.

[0015] In one specific embodiment of the preparation method described above in this invention, based on the total weight of the polypropylene raw material composition as 100%, the amount of isotactic polypropylene is 80-93 wt%, preferably 88-92.5%, the amount of ultra-high molecular weight polypropylene is 1-2 wt%, preferably 1-1.5%, and the balance is the olefin block copolymer. For example, in some embodiments of this invention, the amount of olefin block copolymer may be 7-18%, preferably 7.5-10%.

[0016] In one specific embodiment of the preparation method described above in this invention, the ultra-high molecular weight polypropylene has a molecular weight of 100 × 10⁻⁶. 4-150×10 4 kg / mol.

[0017] In a specific embodiment of the preparation method described above in this invention, the melt index of the isotactic polypropylene is 0.5-10 g / 10 min, preferably 2.4-3.6 g / 10 min.

[0018] As a specific embodiment of the preparation method described above in this invention, the olefin block copolymer includes one or more of the following: ethylene / propylene block copolymer, ethylene / propylene / 1-octene polyblock copolymer, propylene / hexene block copolymer, and propylene / butene block copolymer, preferably a block copolymer containing propylene copolymer units, such as ethylene / propylene block copolymer, propylene / hexene block copolymer, etc.

[0019] As a specific embodiment of the preparation method described above in this invention, in step (1), the first extruder includes, but is not limited to, a single-screw extruder, a twin-screw extruder, or a multi-screw extruder, preferably a twin-screw extruder.

[0020] As a specific embodiment of the preparation method described above in this invention, in step (2), the stretching ratio between the die and the casting roller group is 60-110, preferably 85-95.

[0021] As a specific embodiment of the preparation method described above in this invention, in step (2), the mirror casting roller is a heatable roller with a temperature of 80-125°C, preferably 110-120°C.

[0022] In a specific embodiment of the preparation method described above in this invention, in step (2), the distance between the mirror casting roller and the rubber pressure roller is adjustable, and is 0.85-0.95 times the melt thickness, preferably 0.88-0.92 times, that is, the roller compression ratio is 0.85-0.95, preferably 0.88-0.92. The melt thickness refers to the thickness of the product before stretching after extrusion.

[0023] In step (2) of the preparation method described above, the melt is stretched between the die and the casting roller group of the second extruder, and the melt is subjected to opposing rolling pressure between the mirror casting roller and the rubber pressure roller arranged oppositely. The melt stretching is achieved by utilizing the speed difference between the extrusion speed and the casting roller speed.

[0024] The precursor film obtained by step (2) of the preparation method described above has an oriented lamellar structure.

[0025] As a specific embodiment of the preparation method described above in this invention, in step (2), the second extruder may include, but is not limited to, a single-screw extruder, a twin-screw extruder, or a multi-screw extruder, and is preferably a single-screw extruder.

[0026] This invention does not impose specific requirements on the temperature, time, and stretching ratio of the heat treatment, stretching pore forming, and heat setting in step (3) of the preparation method described above. These parameters can be reasonably adjusted as needed, as long as the purpose of obtaining the polypropylene microporous membrane can be achieved. Furthermore, this invention does not impose specific requirements on the stretching pore forming operation. These parameters can be reasonably selected and adjusted as needed, as long as the purpose of stretching pore forming can be achieved. For example, in some embodiments of this invention, the stretching pore forming can be cold-hot stretching, or it can be directly hot stretching.

[0027] In this invention, the core of controlling the transverse strength of polypropylene microporous membranes lies in controlling the crystalline arrangement structure of the precursor membrane and the strength of the amorphous regions between the lamellae. Olefin block copolymers are linear block copolymers; the thermodynamic incompatibility of the soft and hard segments leads to the formation of heterogeneous spherical or layered aggregate structures. The preparation method provided by this invention blends olefin block copolymers with isotactic polypropylene. The hard segments entering the crystalline region increase the connection points between the lamellae, improving mechanical strength; while the amorphous portion of the block copolymer is displaced into the amorphous regions between the lamellae, providing higher elasticity and strength to these regions. This, in turn, increases the deformation stress on both the crystalline and amorphous regions during transverse deformation of the polypropylene membrane, improving the transverse strength and puncture resistance of the membrane.

[0028] Meanwhile, the alteration of the crystalline structure arrangement depends on the optimization of processing technology. Considering that melt stretching only occurs along the casting direction, forming parallel lamellar crystals perpendicular to the extrusion direction, the preparation method provided by this invention, without altering the original melt stretching, combines opposing rollers with rubber pressure rollers on a mirror-finish casting roller. This increases extrusion in the melt thickness direction, causing the melt to extend to both sides, thus controlling the crystalline arrangement in the thickness direction to a certain extent. In this way, the transverse properties of the precursor film are optimized without changing the pore-forming performance, achieving an improvement in the transverse strength of the polypropylene microporous membrane.

[0029] On the other hand, the present invention also provides a polypropylene microporous membrane, which is prepared by the polypropylene microporous membrane preparation method described above.

[0030] As a specific embodiment of the polypropylene microporous membrane described above in this invention, the puncture strength of the polypropylene microporous membrane is 450-500 kgf, and the transverse yield strength is 9.5-15 MPa.

[0031] In another aspect, the present invention also provides a lithium-ion battery, including a separator, wherein the separator is the polypropylene microporous membrane described above.

[0032] In summary, the technical solution of the present invention has at least the following beneficial effects:

[0033] 1) From the perspective of raw material formulation, the preparation method provided by this invention blends olefin block copolymers, isotactic polypropylene, and ultra-high molecular weight polypropylene. During the cooling crystallization process, the olefin block copolymers enter the amorphous region between the lamellar crystals, providing stronger ductility and improved processability in the subsequent rolling stage. Simultaneously, during the subsequent lateral stretching and puncture testing of the polypropylene microporous membrane, it provides ductility and strength to the amorphous region, thereby improving the lateral strength, toughness, and impact resistance of the polypropylene microporous membrane. The use of ultra-high molecular weight polypropylene significantly increases the melt strength of the polypropylene microporous membrane and increases the wafer thickness during crystallization, thereby improving puncture resistance. In other words, the preparation method provided by this invention, using isotactic polypropylene, olefin block copolymers, and ultra-high molecular weight polypropylene as raw materials, can significantly improve the lateral yield strength and puncture strength of the polypropylene microporous membrane, thereby enhancing the safety of the polypropylene microporous membrane.

[0034] 2) From the perspective of processing method, the preparation method provided by the present invention applies rolling pressure at the same time as the melt stretching. That is, the preparation method combines melt stretching and rolling pressure. Rolling pressure can effectively make the melt have a certain orientation in the transverse direction, thereby controlling the crystal arrangement in the thickness direction to a certain extent and forming an inclined crystal interlocking structure in the thickness direction of the film, which can further effectively improve the transverse yield strength of the polypropylene microporous membrane.

[0035] 3) Using the polypropylene microporous membrane provided by this invention as a separator for lithium-ion batteries can solve the defects of poor transverse strength and insufficient puncture resistance of existing dry-process lithium battery separators, thereby significantly improving the safety performance of lithium-ion batteries. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a simplified structural diagram of the equipment used for melt stretching in a specific embodiment of the present invention.

[0038] Explanation of main icon numbers:

[0039] 1. Single-screw extruder;

[0040] 11. T-shaped die;

[0041] 12. Wind blade;

[0042] 2. Casting roller assembly;

[0043] 21. Mirror-finish casting roller;

[0044] 22. Rubber pressure roller;

[0045] 3. Traction roller. Detailed Implementation

[0046] It should be noted that the term "comprising" and any variations thereof in the specification, claims, and accompanying drawings of this invention are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices.

[0047] The "range" disclosed in this invention is given in the form of a lower limit and an upper limit. It can be one or more lower limits and one or more upper limits, respectively. A given range is defined by selecting a lower limit and an upper limit. The selected lower and upper limits define the boundaries of the particular range. All ranges defined in this way are composable, meaning that any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for specific parameters, it is also expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if the listed minimum range values ​​are 1 and 2, and the listed maximum range values ​​are 3, 4, and 5, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5.

[0048] In this invention, unless otherwise specified, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this invention, and "0-5" is simply a shortened representation of these numerical combinations.

[0049] In this invention, unless otherwise specified, all embodiments and preferred embodiments mentioned in this invention can be combined with each other to form new technical solutions.

[0050] In this invention, unless otherwise specified, all technical features and preferred features mentioned in this invention can be combined with each other to form new technical solutions.

[0051] In this invention, unless otherwise specified, all steps mentioned herein may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying tables, drawings, and embodiments. The embodiments described below are some, but not all, embodiments of this invention, and are only used to illustrate the invention, and should not be considered as limiting the scope of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0053] Example 1

[0054] This embodiment provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0055] Step (1): Isotactic polypropylene with a melt index of 0.5 g / 10 min, olefin block copolymer (ethylene / propylene / 1-octene multiblock copolymer), and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol are added to a twin-screw extruder at a mass ratio of 88:10:2 to prepare a composite polypropylene composition, namely polypropylene blend raw material, by melt blending.

[0056] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretching ratio between the die 11 and the casting roller group 2 is 110. The roller temperature of the mirror casting roller 21 is 125°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.95 times the thickness of the melt, that is, the inter-roller compression ratio is 0.95.

[0057] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0058] Example 2

[0059] This embodiment provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0060] Step (1): Isotactic polypropylene with a melt index of 10 g / 10 min, olefin block copolymer (which includes ethylene / propylene block copolymer, propylene / hexene block copolymer and propylene / butene block copolymer, and the mass ratio of the three is 3:4:3) and ultra-high molecular weight polypropylene with a molecular weight of 1.5 million kg / mol are added to a twin-screw extruder at a mass ratio of 88:10:2 to prepare a composite polypropylene composition, namely polypropylene blend raw material, by melt blending.

[0061] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretching ratio between the die 11 and the casting roller group 2 is 110. The roller temperature of the mirror casting roller 21 is 125°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.95 times the thickness of the melt, that is, the inter-roller compression ratio is 0.95.

[0062] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0063] Example 3

[0064] This embodiment provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0065] Step (1): Isotactic polypropylene with a melt index of 3 g / 10 min, olefin block copolymer (which includes ethylene / propylene block copolymer and propylene / hexene block copolymer, and the mass ratio of the two is 4:6) and ultra-high molecular weight polypropylene with a molecular weight of 1.1 million kg / mol are added to a twin-screw extruder at a mass ratio of 90:9:1 to prepare a composite polypropylene composition, namely polypropylene blend raw material, by melt blending.

[0066] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretching ratio between the die 11 and the casting roller group 2 is 110. The roller temperature of the mirror casting roller 21 is 125°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.95 times the thickness of the melt, that is, the inter-roller compression ratio is 0.95.

[0067] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0068] Example 4

[0069] This embodiment provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0070] Step (1): Add isotactic polypropylene and olefin block copolymer (propylene / hexene block copolymer) with a melt index of 3g / 10min to a twin-screw extruder at a mass ratio of 94:6 and melt blend to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0071] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretching ratio between the die 11 and the casting roller group 2 is 110. The roller temperature of the mirror casting roller 21 is 125°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.95 times the thickness of the melt, that is, the inter-roller compression ratio is 0.95.

[0072] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0073] Example 5

[0074] This embodiment provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0075] Step (1): Add isotactic polypropylene and olefin block copolymer (propylene / hexene block copolymer) with a melt index of 3g / 10min to a twin-screw extruder at a mass ratio of 94:6 and melt blend to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0076] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretching ratio between the die 11 and the casting roller group 2 is 60. The roller temperature of the mirror casting roller 21 is 80°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.85 times the thickness of the melt, that is, the inter-roller compression ratio is 0.85.

[0077] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0078] Example 6

[0079] This embodiment provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0080] Step (1): Add isotactic polypropylene and olefin block copolymer (propylene / hexene block copolymer) with a melt index of 3g / 10min to a twin-screw extruder at a mass ratio of 94:6 and melt blend to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0081] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretch ratio between the die 11 and the casting roller group 2 is 90. The roller temperature of the mirror casting roller 21 is 115°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.95 times the thickness of the melt, that is, the inter-roller compression ratio is 0.95.

[0082] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0083] Example 7

[0084] This embodiment provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0085] Step (1): Isotactic polypropylene with a melt index of 3 g / 10 min, propylene / hexene block copolymer and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol are added to a twin-screw extruder at a mass ratio of 89:9:2 and melt-blended to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0086] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretch ratio between the die 11 and the casting roller group 2 is 90. The roller temperature of the mirror casting roller 21 is 115°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.9 times the thickness of the melt, that is, the inter-roller compression ratio is 0.9.

[0087] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0088] Comparative Example 1

[0089] This comparative example provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0090] Step (1): Isotactic polypropylene with a melt index of 3 g / 10 min is melt-extruded and then melt-stretched, wherein the stretching ratio is 90, the roller temperature is 115℃, the roller compression ratio is 1, and after stretching, a polypropylene precursor film is obtained.

[0091] Step (2): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain the polypropylene microporous membrane.

[0092] Comparative Example 2

[0093] This comparative example provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0094] Step (1): Isotactic polypropylene with a melt index of 3 g / 10 min is melt-extruded and then subjected to melt stretching and roller pressing. The stretching ratio is 90, the roller temperature is 115℃, and the roller compression ratio is 0.9 times. After the process, a polypropylene precursor film is obtained.

[0095] Step (2): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain the polypropylene microporous membrane.

[0096] Comparative Example 3

[0097] This comparative example provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0098] Step (1): Add propylene / hexene block copolymer and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol to a twin-screw extruder at a mass ratio of 98:2 and melt blend to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0099] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretch ratio between the die 11 and the casting roller group 2 is 90. The roller temperature of the mirror casting roller 21 is 115°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.9 times the thickness of the melt, that is, the inter-roller compression ratio is 0.9.

[0100] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain the polypropylene microporous membrane.

[0101] Comparative Example 4

[0102] This comparative example provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0103] Step (1): Add isotactic polypropylene with a melt index of 3 g / 10 min and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol to a twin-screw extruder at a mass ratio of 98:2 to prepare a composite polypropylene composition, namely polypropylene blend raw material, by melt blending.

[0104] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretch ratio between the die 11 and the casting roller group 2 is 90. The roller temperature of the mirror casting roller 21 is 115°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.9 times the thickness of the melt, that is, the inter-roller compression ratio is 0.9.

[0105] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0106] Comparative Example 5

[0107] This comparative example provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0108] Step (1): Isotactic polypropylene with a melt index of 3 g / 10 min and propylene / hexene block copolymer are added to a twin-screw extruder at a mass ratio of 91:9 and melt-blended to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0109] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretch ratio between the die 11 and the casting roller group 2 is 90. The roller temperature of the mirror casting roller 21 is 115°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.9 times the thickness of the melt, that is, the inter-roller compression ratio is 0.9.

[0110] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0111] Comparative Example 6

[0112] This comparative example provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0113] Step (1): Isotactic polypropylene with a melt index of 3 g / 10 min, propylene / hexene block copolymer and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol are added to a twin-screw extruder at a mass ratio of 89:9:2 and melt-blended to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0114] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1 It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretching ratio between the die 11 and the casting roller group 2 is 90. The roller temperature of the mirror casting roller 21 is 115°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 1 times the thickness of the melt, that is, the inter-roller compression ratio is 1.

[0115] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0116] Comparative Example 7

[0117] This comparative example provides a polypropylene microporous membrane, which is prepared by a method including the following specific steps:

[0118] Step (1): Isotactic polypropylene with a melt index of 3 g / 10 min, propylene / hexene block copolymer and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol are added to a twin-screw extruder at a mass ratio of 89:9:2 and melt-blended to prepare a composite polypropylene composition, namely polypropylene blend raw material.

[0119] Step (2): The composite polypropylene composition is melt-extruded using a single-screw extruder and then melt-stretched to obtain a polypropylene precursor film; wherein, a simplified structural diagram of the equipment used for melt stretching is shown below. Figure 1 As shown, from Figure 1It is known that the equipment includes a casting roller group 2 disposed between the traction roller 3 and the die 11 of the single screw extruder 1. The casting roller group 2 includes a mirror casting roller 21 and a rubber pressure roller 22 disposed opposite to each other. The melt is stretched between the die 11 of the single screw extruder 1 (an air knife 12 is also disposed below the die 11) and the casting roller group 2, and the melt is subjected to opposing rolling pressure between the mirror casting roller 21 and the rubber pressure roller 22. The stretch ratio between the die 11 and the casting roller group 2 is 90. The roller temperature of the mirror casting roller 21 is 115°C. The distance between the mirror casting roller 21 and the rubber pressure roller 22 is 0.8 times the thickness of the melt, that is, the inter-roller compression ratio is 0.8.

[0120] Step (3): First, heat-treat the polypropylene precursor membrane at 145℃ for 30 minutes, then cold-stretch it at 25℃ by 20% to form the initial pore core, then hot-stretch it at 145℃ and control the hot stretching ratio to be 100%, and finally heat-set it at 150℃ for 10 minutes to obtain a polypropylene microporous membrane with high puncture strength and high transverse strength.

[0121] Test Example 1

[0122] This test example performs tensile property tests, puncture strength tests, and porosity tests on the polypropylene microporous membranes provided in the above examples and comparative examples. The tensile property tests are conducted using an electronic universal tensile testing machine in accordance with GB / T1040.1-2018 and following the operating procedures of the electronic universal tensile testing machine. Specifically, the samples are cut perpendicular to the extrusion direction to make the sample width 15 mm and the middle length of the clamp 50 mm. Five samples are tested as a group.

[0123] The puncture strength was tested using an electronic universal tensile testing machine (load range 0-500N); puncture needle: Crew cut.

[0124] Porosity was tested in accordance with GB / T 36363-2018.

[0125] The tensile properties, puncture strength, and porosity data obtained from this test example are shown in Table 1 below.

[0126] Table 1. Tensile properties, puncture strength, and porosity data of the polypropylene microporous membranes provided in the examples and comparative examples.

[0127] Porosity % Puncture strength (kgf) Transverse yield strength (MPa) Transverse elongation at break (%) Example 1 40 466 10.3 132 Example 2 41 463 10.3 133 Example 3 43 454 9.7 95 Example 4 43 462 10.2 105 Example 5 39 475 11 110 Example 6 43 472 10.9 108 Example 7 43 483 11.6 107 Comparative Example 1 43 400 8 73 Comparative Example 2 43 428 9.2 78 Comparative Example 3 5 249 4.5 220 Comparative Example 4 32 430 9.8 102 Comparative Example 5 41 370 10.3 104 Comparative Example 6 43 460 9.2 110 Comparative Example 7 46 403 4.3 88

[0128] As can be seen from Table 1 above, the polypropylene microporous membranes provided in Examples 1-7 of this invention all have relatively high porosity, puncture strength, transverse yield strength, and transverse elongation at break. Among them, compared to Example 7, Comparative Example 5 did not use ultra-high molecular weight polypropylene. As shown in Table 1, the porosity, puncture strength, transverse yield strength, and transverse elongation at break of the polypropylene microporous membrane provided in Comparative Example 5 are all inferior to those of the polypropylene microporous membrane provided in Example 7. Compared to Example 7, the inter-roller compression ratios used in Comparative Examples 6 and 7 are either too high or too low, and neither is within the range of 0.85-0.95. As shown in Table 1, the puncture strength and transverse yield strength of the polypropylene microporous membrane provided in Comparative Example 6 are inferior to those of the polypropylene microporous membrane provided in Example 7, while the puncture strength, transverse yield strength, and transverse elongation at break of the polypropylene microporous membrane provided in Comparative Example 7 are inferior to those of the polypropylene microporous membrane provided in Example 7.

[0129] Both Comparative Example 1 and Comparative Example 2 used only isotactic polypropylene as raw material. As can be seen from Table 1, the overall performance of the polypropylene microporous membranes provided by Comparative Example 1 and Comparative Example 2 is relatively poor.

[0130] Compared to Example 7, Comparative Example 3 used propylene / hexene block copolymer and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol as raw materials, instead of isotactic polypropylene. As shown in Table 1, although the transverse elongation at break of the polypropylene microporous membrane provided in Comparative Example 3 was better than that of the polypropylene microporous membrane provided in Example 7, its porosity, puncture strength, and transverse yield strength were significantly worse than those of the polypropylene microporous membrane provided in Example 7. This is because the polypropylene microporous membrane prepared by the uniaxial stretching method mainly relies on the stretching and separation of the lamellar skeleton formed after isotactic polypropylene crystallization to obtain micropores. In contrast, Comparative Example 3 only used flexible propylene / hexene block copolymer and ultra-high molecular weight polypropylene with a slower crystallization rate, resulting in low crystallization perfection and poor lamellar arrangement during the casting process. This makes it difficult to form micropores during the post-stretching process, leading to extremely low porosity. However, the flexible propylene / hexene block copolymer gives the stretched membrane a lower transverse yield strength, excellent transverse elongation at break, and lower puncture strength.

[0131] Compared to Example 7, Comparative Example 4 used isotactic polypropylene with a melt index of 3 g / 10 min and ultra-high molecular weight polypropylene with a molecular weight of 1 million kg / mol as raw materials, but did not use olefin block copolymers. As shown in Table 1, the porosity, puncture strength, transverse yield strength and transverse elongation at break of the polypropylene microporous membrane provided by Comparative Example 4 were all lower than those of Example 7.

[0132] In summary, from the perspective of raw material formulation, the embodiments of this invention blend olefin block copolymers with isotactic polypropylene, or further add ultra-high molecular weight polypropylene as a raw material to prepare polypropylene microporous membranes. Specifically, the olefin block copolymers enter the amorphous region between the lamellar crystals during the cooling crystallization process, providing stronger ductility and improved processability in the subsequent rolling stage; simultaneously, they provide ductility and strength to the amorphous region during the subsequent transverse stretching and puncture of the polypropylene microporous membrane, thereby improving the transverse strength, toughness, and impact resistance of the polypropylene microporous membrane. That is, the preparation method provided by this invention, using isotactic polypropylene and olefin block copolymers as raw materials, can significantly improve the transverse yield strength and puncture strength of polypropylene microporous membranes, thereby improving the safety of polypropylene microporous membranes. Furthermore, the addition of ultra-high molecular weight polypropylene can significantly increase the melt strength of the polypropylene microporous membrane and increase the lamellar thickness during crystallization, thereby improving puncture resistance.

[0133] From the perspective of processing methods, in the preparation of polypropylene microporous membranes, the embodiments of the present invention apply rolling pressure simultaneously during melt stretching. That is, the preparation method combines melt stretching and rolling pressure. Rolling pressure can effectively cause the melt to have a certain orientation in the transverse direction, thereby controlling the crystallization arrangement in the thickness direction to a certain extent and forming an inclined crystallization interlocking structure in the thickness direction of the film, which can further effectively improve the transverse yield strength of the polypropylene microporous membrane.

[0134] The above description is merely a specific embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any substitution of equivalent components or equivalent changes and modifications made within the scope of protection of this patent should still fall within the scope of this patent. Furthermore, the technical features, technical features and technical inventions, and technical inventions in this invention can be freely combined and used.

Claims

1. A method for preparing a polypropylene microporous membrane, characterized in that, The preparation method includes: Step (1): Add the polypropylene raw material composition to the first extruder for melt blending to obtain polypropylene blend raw material, wherein the polypropylene raw material composition includes isotactic polypropylene, olefin block copolymer and ultra-high molecular weight polypropylene; Step (2): After the polypropylene blend raw material is melt-extruded through the second extruder, it is melt-stretched between the die and the casting roll group of the second extruder. The casting roll group includes a mirror casting roll and a rubber pressure roll arranged opposite to each other, and the melt is rolled against each other between the two to obtain a precursor film. The distance between the mirror casting roll and the rubber pressure roll is 0.85-0.95 times the thickness of the melt. Step (3): The precursor membrane is subjected to heat treatment, stretching to form pores and heat setting in sequence to obtain the polypropylene microporous membrane.

2. The preparation method according to claim 1, characterized in that, Based on the total weight of the polypropylene raw material composition as 100%, the amount of isotactic polypropylene is 80-93 wt%, the amount of ultra-high molecular weight polypropylene is 1-2 wt%, and the balance is olefin block copolymer.

3. The preparation method according to claim 1 or 2, characterized in that, The ultra-high molecular weight polypropylene has a molecular weight of 100 × 10⁻⁶. 4 -150×10 4 kg / mol.

4. The preparation method according to claim 1 or 2, characterized in that, The melt index of the isotactic polypropylene is 0.5-10 g / 10 min.

5. The preparation method according to claim 1 or 2, characterized in that, The olefin block copolymer includes one or a combination of several of the following: ethylene / propylene block copolymer, ethylene / propylene / 1-octene polyblock copolymer, propylene / hexene block copolymer, and propylene / butene block copolymer.

6. The preparation method according to claim 1, characterized in that, In step (2), the stretching ratio between the die and the casting roller group is 60-110.

7. The preparation method according to claim 1 or 6, characterized in that, In step (2), the temperature of the mirror casting roller is 80-125℃.

8. A polypropylene microporous membrane, which is prepared by the method of preparing the polypropylene microporous membrane according to any one of claims 1-7.

9. The polypropylene microporous membrane according to claim 8, characterized in that, The puncture strength of the polypropylene microporous membrane is 450-500 kgf, and the transverse yield strength is 9.5-15 MPa.

10. A lithium-ion battery, comprising a separator, characterized in that, The diaphragm is the polypropylene microporous membrane as described in claim 8 or 9.

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