CN111883724B - Multilayer polypropylene microporous membrane and preparation method and application thereof - Google Patents
Multilayer polypropylene microporous membrane and preparation method and application thereof Download PDFInfo
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- CN111883724B CN111883724B CN202010786045.1A CN202010786045A CN111883724B CN 111883724 B CN111883724 B CN 111883724B CN 202010786045 A CN202010786045 A CN 202010786045A CN 111883724 B CN111883724 B CN 111883724B
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
The application provides a multilayer polypropylene microporous membrane, a preparation method and an application thereof, wherein the preparation method comprises the following steps: s1, melting a polypropylene raw material, forming a multilayer melt through a multilayer coextrusion die head, and crystallizing the multilayer melt under acting force to obtain a precursor of the multilayer polypropylene microporous membrane; s2, carrying out post-treatment on the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane; wherein, in step S1, the melt draw ratio and the shear rate satisfy the following relationship: the epsilon/eta is more than 0.0004 and less than or equal to 0.16. The process parameter control is easy to realize, products with excellent product consistency and comprehensive performance can be obtained only by forming effective control on the die head outlet of the traditional process equipment, and compared with the traditional dry method single-layer diaphragm, the obtained multilayer polypropylene microporous membrane has 10% -30% improvement on physical indexes related to safety performances such as mechanical strength, voltage resistance and the like, obviously improves the integral stability of the diaphragm product, has stable battery cycle performance, and can be used as the diaphragm of a lithium electronic battery.
Description
Technical Field
The application relates to the field of microporous membranes, in particular to a multilayer polypropylene microporous membrane and a preparation method and application thereof.
Background
The lithium ion battery diaphragm is one of four main materials of the lithium ion battery, and is very mature after decades of development, and the global capacity scale exceeds 50 hundred million square meters.
However, with the demand of increasing the energy density of lithium ion batteries, the thickness of the diaphragm is required to be thinner and thinner from 32 micrometers to 25 micrometers and 20 micrometers, and then to 16 micrometers and 12 micrometers which are commonly used at present, the mechanical strength of the diaphragm, especially the puncture strength, the breakdown voltage resistance and other performances are weakened to a great extent with the thinning of the thickness, and a great risk is formed for the safety of the lithium ion batteries.
Meanwhile, with the demand of thinning the lithium ion battery diaphragm, the single-layer polypropylene type diaphragm product cannot meet the use demand of a thinned high-density energy battery. The traditional dry-method multilayer composite diaphragm is low in production efficiency and high in cost by extruding a single-layer base material, compounding multiple layers later and stretching. For the reasons, the prior art mostly adopts a melt coextrusion process to prepare a multilayer polypropylene diaphragm, and in order to obtain a product with better comprehensive performance, two or more polyolefins with different characteristics are generally adopted for coextrusion molding. However, when polyolefin materials having different characteristics are used, the phenomenon of non-uniform film thickness is easily caused, and also delamination is caused due to low interlayer peeling strength of the product, bubble formation and swelling are generated, the cycle performance of the battery is affected, and the commercialization of the prepared product is limited.
Prior art patent CN101536221B discloses a coextruded multilayer battery separator that discloses a process with a minimum shear rate of 4/sec and a flow rate per layer of 8.2 to 45.4 kg/hr, the product having a uniform thickness defined by a standard deviation of <0.80 microns. Although the prior art discloses that the product thickness is consistent through the improvement of the process form, the obtained product has poor mechanical properties such as heat shrinkage, tensile strength, puncture strength and the like, and the heat stability of the product in the long-term use process is restricted.
Disclosure of Invention
In view of the problems in the prior art, the present application aims to provide a method for preparing a multi-layer polypropylene microporous membrane, so as to solve the problems of non-uniform thickness, low interlayer peel strength and poor mechanical properties in the process of forming a multi-layer microporous membrane by coextrusion.
Another object of the present invention is to provide a multi-layer polypropylene microporous membrane having characteristics of uniform thickness and excellent mechanical strength and electrical properties.
Another object of the present application is to provide a lithium ion battery separator, which includes the multilayer polypropylene microporous membrane prepared by the above method or the above multilayer polypropylene microporous membrane.
The application is realized by adopting the following technical scheme:
in a first aspect, the present application provides a method of making a multilayer polypropylene microporous membrane, comprising:
s1, melting a polypropylene raw material, forming a multilayer melt through a multilayer coextrusion die head, and crystallizing the multilayer melt under acting force to obtain a precursor of the multilayer polypropylene microporous membrane;
s2, carrying out post-treatment on the precursor of the multi-layer polypropylene microporous membrane to obtain the multi-layer polypropylene microporous membrane;
wherein, in step S1, the melt draw ratio and the shear rate satisfy the following relationship:
0.0004＜ε/η＜0.16， (1)
in formula (1), η represents a melt draw ratio at the die outlet; ε represents the shear rate in s during extrusion -1 。
Further, in the preferred embodiment of the present application, in step S1, the melt draw ratio and the shear rate satisfy the following relationship:
0.002≤ε/η＜0.032。
further, in the preferred embodiment of the present application, in step S1, ε < 8/sec, and/or η is 50-250, preferably 80-200.
Further, in the preferred embodiment of the present application, the polypropylene raw material comprises 90-100 wt.% polypropylene and 0-10 wt.% additive by mass fraction.
Further, in the preferred embodiment of the present application, the polypropylene is a polypropylene having an isotacticity of greater than 95%.
Further, in the preferred embodiment of the present application, the additives include one or more of nucleating agents, polypropylene waxes, inorganic particles, lubricants.
Further, in a preferred embodiment of the present application, the multilayer polypropylene microporous membrane precursor comprises at least an outer membrane layer portion and an inner membrane layer portion, wherein the polypropylene of the outer membrane layer portion has a melt flow index of 0.8 to 3.0g/10min and the polypropylene of the inner membrane layer portion has a melt flow index of 0.8 to 2.0g/10 min.
Further, in the preferred embodiment of the present application, the melt flow index of the polypropylene of the outer film layer portion and the inner film layer portion are different.
Further, in the preferred embodiment of the present application, in step S2, the post-processing includes: and carrying out thermal annealing and stretching treatment on the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane.
Further, in a preferred embodiment of the present application, the preparation method further includes stacking a plurality of multi-layer microporous membranes in a cold-state compounding manner or a hot-state compounding manner to obtain a multi-layer polypropylene microporous membrane with a target number of layers; the temperature of cold-state compounding is preferably 20-30 ℃, and/or the temperature of hot-state compounding is preferably 80-150 ℃.
In a second aspect, the present application provides a multilayer polypropylene microporous membrane having a thickness of from 5 to 100 microns, preferably from 5 to 50 microns, more preferably from 10 to 25 microns; wherein the range of the thickness of the polypropylene microporous membrane is less than or equal to 2 microns, preferably less than or equal to 1.2 microns, more preferably less than or equal to 1.0 micron, the puncture strength of the polypropylene microporous membrane is more than 300g, preferably more than or equal to 350MPa, more preferably more than or equal to 400MPa, the interlaminar peel strength of the polypropylene microporous membrane is more than 100g/m, preferably more than 150g/m, more preferably more than 200g/m, the longitudinal tensile strength of the polypropylene microporous membrane is more than 150MPa, preferably more than or equal to 200MPa, more preferably more than or equal to 230MPa, and the transverse tensile strength of the polypropylene microporous membrane is more than or equal to 10MPa, preferably more than or equal to 15 MPa.
In a third aspect, the present application provides a lithium ion battery separator, which includes the multilayer polypropylene microporous membrane prepared according to the above preparation method or the above multilayer polypropylene microporous membrane.
Compared with the prior art, this application beneficial effect includes:
the application provides a multilayer polypropylene microporous membrane, compares with traditional dry process individual layer diaphragm, and the multilayer polypropylene microporous membrane thickness of this application is even, has 10% ~ 30% promotion in the rerum natura index that security performance such as mechanical strength, withstand voltage is relevant, is showing to have promoted diaphragm product holistic stability, battery cycle performance stability, can be used as lithium ion battery's diaphragm. Compared with the prior art, the preparation method of the multilayer polypropylene microporous membrane is easy to realize in process parameter control, products with excellent product consistency and comprehensive performance can be obtained only by forming effective control on the die head outlet of the traditional equipment, and the method is simple and convenient to operate and convenient for industrial application.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application are described below.
In the present context "melt draw ratio", which generally refers to the ratio of the opening of the die lip to the thickness of the film after cooling, the adjustable process conditions are: t-die lip width, casting roll speed, and roll diameter.
"range" in this context generally refers to the difference between the maximum and minimum values.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of this application belong. The terminology used herein in the description of the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the embodiments of the present application.
Currently, the existing multilayer polyolefin microporous membrane mainly adopts two technologies, namely a dry method (melt stretching, MSCS) and a wet method (solvent extraction, phase inversion, thermally induced phase separation or gel extraction), however, the wet method involves the use of organic solvents, and the production environment is affected, so that the dry method is mostly adopted to prepare the multilayer polyolefin microporous membrane. During coextrusion, the various polymers are simultaneously collected in the extrusion die and exit from the extrusion die outlet in the form of a multilayer melt; wherein the multilayer melt is a planar structure having at least two melt layers bonded together by intermixing of the melts at the interface between the two melt layers. However, when polyolefin materials having different characteristics are used, the resulting film has problems of non-uniform thickness, low interlayer peel strength, etc., thereby affecting the performance of the battery and limiting the commercialization of the manufactured product.
In order to solve the problems existing in the prior art, the inventor finds that a multilayer polypropylene microporous membrane product with consistent thickness can be obtained by controlling the melt draw ratio of an outlet of an extrusion die head in the extrusion process and the shearing rate of an extruder, and the obtained multilayer polypropylene microporous membrane has excellent performances such as mechanical strength, thermal shrinkage, peeling strength, breakdown voltage resistance and the like.
Specifically, the application provides a preparation method of a multilayer polypropylene microporous membrane, which comprises the following steps:
s1, melting a polypropylene raw material, forming a multilayer melt through a multilayer coextrusion die head, and crystallizing the multilayer melt under acting force to obtain a precursor of the multilayer polypropylene microporous membrane;
s2, carrying out post-treatment on the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane;
wherein, in step S1, the melt draw ratio and the shear rate satisfy the following relationship:
0.0004＜ε/η＜0.16 (1)
in formula (1), η represents a melt draw ratio at the die outlet; ε represents the shear rate in s during extrusion -1 。
In some embodiments, the ratio of melt draw ratio to shear rate may be 0.001, 0.0015, 0.0020, 0.0025, 0.0030, 0.0035, 0.0040, 0.0045, 0.0050, 0.0055, 0.0060, 0.0065, 0.0070, 0.0075, 0.0080, 0.0090, 0.0100, 0.0200, 0.0300, 0.0315, 0.0320, 0.0350, 0.040, 0.045, 0.050, 0.055, 0.060, 0.065, 0.070, 0.075, 0.080, 0.090, 0.095, 0.1, 0.105, 0.110, 0.120, 0.130, 0.140, 0.150, 0.155, 0.159, and any value therebetween, provided that the above values for mechanical strength and mechanical strength resistance of the microporous film are consistent with a shear strength (e.g., < 0.15).
In the melt-draw process for making films, the polymer melt is drawn off the die at a certain draw ratio in the melt state and cooled to form a nonporous precursor film of a certain thickness that is moderately drawn at a suitable temperature to form sufficiently dense micropores therein. It is important to note that the case of industrial success generally uses semi-crystalline polymers such as polypropylene and polyethylene. These semi-crystalline polymers crystallize from the melt upon cooling under stretching conditions, forming microscopically parallel platelet structures that are uniaxially oriented. Therefore, the excessively small melt traction ratio can cause insufficient orientation and low crystallinity in the melt casting process, so that the mechanical strength of a final product is excessively low, and the use safety of the battery is influenced; the higher melt traction ratio is beneficial to the orientation of polymer molecular chains, and the more the polymer molecular chains are oriented, the higher the crystallization degree of the polymer is, and the better the mechanical property of the product is. However, in the actual operation, the inventors found that the melt draft ratio is too large, and the melt casting process is shaken, and the orientation of the casting film surface is not uniform, resulting in poor uniformity of the overall properties of the final product.
Meanwhile, the inventor also finds that the shear rate in the extrusion process also affects the performance of the final product, and too high shear rate can lead the molecular chain to be largely broken into short chains, which can seriously affect the processing and the subsequent product performance. This application makes the polymer chain slip through shearing unwrapping through the relation between control shear rate and the fuse-element traction ratio, and the flexibility increases, and the polymer fuse-element is controlled at suitable processing viscosity scope, combines certain fuse-element traction ratio, can be so that polymer molecular chain fully orients, crystallization, has improved multilayer crowded polypropylene diaphragm mechanical strength altogether greatly, puncture strength and interlayer bonding strength, and the product property ability that obtains satisfies battery safety in utilization.
Further, in step S1, the melt draw ratio and the shear rate satisfy the following relationship:
0.002≤ε/η＜0.032。
optionally, in step S1, ε < 8/sec, and/or η is 50-250, preferably 80-200.
In some specific embodiments herein, the resulting multilayer polypropylene microporous membrane precursor is a base membrane of non-porous structure, comprising at least an outer membrane layer portion and an inner membrane layer portion. Wherein the outer film layer part refers to the surface layer part of the microporous film contacting the atmosphere; the inner film layer part refers to a film layer between the outer film layer parts, and can be of a one-layer structure or a multi-layer structure, and can be designed correspondingly according to the actual operation requirement. For example, where the coextruded polypropylene microporous membrane precursor is a three layer coextrusion, the outer layer (e.g., A and C as described above) where the three layer structure can be in the form of an A-B-A or A-B-C structure can be a polypropylene material of the same or different properties.
Further, the polypropylene raw material comprises 90-100 wt.% of polypropylene and 0-10 wt.% of additives in terms of mass fraction.
Optionally, the polypropylene raw material used in the present application is polypropylene with isotacticity greater than 95%, and may be homo-polypropylene or co-polypropylene. Typical copolymerized polypropylenes may be copolymers of other olefins and propylene, and the other olefins may be commonly used olefins, including but not limited to: ethylene, 1-butene, 1-pentene, 1-hexene, 1-decene, 1-octene, etc., as long as the isotacticity of the resulting copolymerized polypropylene is greater than 95%, the requirements of the raw materials of the present application can be satisfied.
Meanwhile, the melt flow index of the polypropylene material is an important parameter for measuring the processability of the polypropylene, and particularly, in the application, the melt index of the polypropylene resin is preferably 0.8-3.0g/10min, the melt index is too low, the viscosity of the polypropylene is high, the polypropylene is difficult to extrude, the polypropylene is difficult to mix uniformly, the melt index is too high, and the tensile strength of the microporous membrane is low. With polypropylene materials in the above range, such resins do not require the use of plasticizers during extrusion and can produce precursor films having highly platelet-oriented internal microstructures.
As an example, in a multi-layer polypropylene microporous membrane precursor, each of the membrane layers may be a membrane layer of polypropylene raw material having a different melt flow index, for example, the melt flow index of the outer membrane layer portion of the polypropylene microporous membrane precursor is 0.8 to 3.0g/10min and the melt flow index of the inner membrane layer portion is 0.8 to 2.0g/10 min.
Optionally, in the embodiment of the present application, the film layer formed by selecting polypropylene raw materials with different melt flow indexes can obtain a microporous film with more excellent and controllable performance by matching the polypropylene raw materials with different melt fingers, for example, the polypropylene with lower melt fingers can provide higher strength for the microporous film, and the processing difficulty can be increased by using lower melt fingers, so that the processing difficulty can be reduced by matching the polypropylene with higher melt fingers, and the pore structures obtained by the polypropylene layers with different melt fingers after the pore-forming process are different, so that the pore structure of the microporous film can be more controllable by matching the polypropylene layers with different melt fingers.
Also, to enhance the performance of the polypropylene material, various additives may be added to the polymer that can adjust or enhance the performance or characteristics of the individual film layers or the integral microporous film. These materials may include, but are not limited to:
additives for reducing the closed cell temperature can be selectively added, for the battery separator, the automatic shutdown protection performance is an important performance to be considered for the lithium ion battery separator, therefore, a layer of shutdown layer, namely a layer for closing the pore diameter to prevent ions from flowing between the electrodes of the battery when a preset temperature is reached, is considered for designing the multilayer microporous membrane, and in order to reduce the closed cell temperature of the shutdown layer, the additives for reducing the closed cell temperature can be added into the polymer, wherein the additives comprise but are not limited to metallocene, polypropylene wax and the like, and the addition amount is not more than 10 wt.% of the total mass of raw materials;
optionally adding a material for improving the nucleation performance of the polymer, wherein the material comprises a nucleating agent, the nucleating agent is preferably a beta nucleating agent, including but not limited to sodium benzoate and magnesium phthalate, and the adding amount is not more than 10 wt.% of the total mass of the raw materials;
additives that improve the heat resistance of the polymer may be optionally added, including but not limited to various inorganic particles such as alumina, boehmite, etc., in amounts not exceeding 10 wt.% of the total mass of the starting materials;
an additive for improving the surface smoothness of the microporous membrane, which includes silicone resin and the like, may also be optionally added. The addition amount is not more than 10 wt.% of the total mass of the raw materials.
And then, cooling and crystallizing the multilayer melt under the action of force to obtain a precursor of the multilayer polypropylene microporous membrane. In this process, the crystallization is carried out by cooling and solidifying the multilayer melt extruded from the outlet of the coextrusion die. The cooling method may be air cooling, medium cooling, cooling in contact with a cooling roll in which a refrigerant circulates, or the like, and from the viewpoint of facilitating the thickness control of the microporous membrane precursor, it is preferable to perform cooling by cooling in contact with a cooling roll in which a refrigerant circulates.
Subsequently, the multilayer polypropylene microporous membrane precursor is subjected to post-treatment, which mainly comprises: and carrying out thermal annealing and stretching treatment on the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane. In the post-treatment process, the annealing treatment is mainly used for further perfecting the crystal structure and improving the pore forming property of the microporous membrane, so that the mechanical property of the membrane is improved; the stretching process mainly comprises cold drawing and hot drawing along the MD and TD directions respectively to form a final microporous membrane, wherein the cold drawing at low temperature aims to induce the microporous structure of the membrane, and the stretching at higher temperature aims to expand the microporous structure induced by the cold drawing process to a proper size.
In order to further obtain the multilayer polypropylene microporous membrane precursor with the target number of layers, the preparation method further comprises the step of superposing the multiple multilayer microporous membranes in a cold-state compounding or hot-state compounding manner to obtain the multilayer polypropylene microporous membrane precursor with the target number of layers.
Specifically, in the cold-state compounding process, the guide roller is compounded at the temperature of 20-30 ℃, and the multiple multilayer film layers are low in interlayer peeling strength after compounding, so that subsequent peeling is facilitated. In the thermal state compounding process, the guide roller is used for compounding a plurality of multi-layer microporous films at the temperature of 80-150 ℃, and the peeling strength among a plurality of multi-layer film layers and among all layers of the microporous films is improved after compounding.
As an example, when the coextruded multilayer polypropylene microporous membrane precursor is a three-layer multiple structure, a plurality of three-layer microporous membranes can be stacked by cold or hot compounding to form a structure of 6, 9 or more 3-fold layers.
The target multilayer polypropylene microporous membrane can be obtained by the method, the thickness of the multilayer polypropylene microporous membrane is 5-100 microns, the range of the thickness is less than or equal to 2 microns, the puncture strength of the polypropylene microporous membrane is more than 300g, the interlayer peeling strength is more than 100g/m, the longitudinal tensile strength is more than 150MPa, and the transverse tensile strength is more than or equal to 10MPa, preferably more than or equal to 15 MPa. The multi-layer polypropylene microporous membrane has uniform thickness, excellent mechanical strength and interlayer peeling strength, and can be used as a lithium ion battery diaphragm.
The features and properties of the present application are described in further detail below with reference to examples.
The operations and treatments referred to in this application are conventional in the art, unless otherwise indicated.
The apparatus used in this application is conventional in the art, unless otherwise specified.
The detection method related in the specific embodiment of the application is as follows:
1. thickness: reference is made to the specification of GB/T6672-2001, wherein the resolution instrument of the thickness gauge should be no more than 0.1um, no less than 3 points are tested at equal distance along the width direction, and the average value of the readings is taken.
2. Ventilating: according to the specification of GB/T36363-2018, the 100ml air passing area is 6.45cm under the pressure of 1.21KPa 2 The time required for the membrane.
3. Porosity: the length, width and thickness of the diaphragm were measured as specified in GB/T6673-2001 and GB/T6672-2001, the mass of the sample was weighed with an analytical balance having a resolution of 0.0001g, and the porosity was calculated according to the following formula
P＝(1-m/(L*b*d*ρ))*100％
In the formula: p is the porosity of the separator in%; m is the mass of the diaphragm and is given in g; l is the length of the diaphragm in cm; b is the width of the diaphragm in cm; d is the thickness of the diaphragm in um; rho is the density of the raw material and has the unit of g/cm 2 。
4. Tensile strength: the test was carried out as specified in GB/T1040.3-2006 using a type 2 specimen having a width of (15. + -. 0.1) mm, an initial distance between the grips (100. + -.5) mm and a test speed of (250. + -.10) mm/min.
5. Puncture strength: reference is made to the provisions of GB/T6672-2001, with a load cell resolution of 0.1N, a puncture needle diameter of 1.0mm, a sample holding jig internal diameter of 10mm, a septum flattened and clamped in the jig, the puncture is performed at a rate of (100. + -. 10) mm/min, no less than 3 points are measured, and the values are averaged.
6. Heat shrinkage ratio: the method is carried out according to the specification of GB/T36363-2018, a square diaphragm with the thickness of 100mm is cut out, the square diaphragm is placed in a blast type constant temperature box and is kept for 1 hour at the temperature of 105 ℃, and then the square diaphragm is taken out, and the contraction ratio of the diaphragm is measured.
△L＝(L1-L0)/L0*100％
In the formula: Δ L is the thermal shrinkage of the separator in%; l1 is the length of the heated diaphragm in mm; l0 is the length of the diaphragm before heating in mm.
7. Average pore diameter: mean pore size data were obtained using PMI instrument measurements, pore size expressed in um or nm.
8. Breakdown voltage resistance: and (3) placing the diaphragm between two electrodes, applying voltage between the electrodes, increasing the voltage until the diaphragm is broken down, and taking the voltage value when the diaphragm is broken down.
9. Closed pore temperature & rupture temperature: the diaphragm is placed in a conductive container filled with electrolyte, two ends of the container are connected with an electrode and a resistance tester, the container is heated together with a power supply, the numerical value of the resistance tester is recorded, the corresponding temperature is the closed pore temperature of the diaphragm when the numerical value of the battery tester suddenly rises greatly, the temperature continues to rise, and the corresponding temperature is the film breaking temperature of the diaphragm when the numerical value of the resistance tester drops greatly.
10. Surface friction coefficient: the diaphragm is placed on a flat table, the pressing block is placed on the diaphragm and is pulled to move in one direction by force, and the ratio of the force value to the weight of the pressing block is the surface friction coefficient of the diaphragm.
Example 1
Adding polypropylene A (with isotacticity of more than or equal to 95 percent and melt flow index of 0.8g/10min) and polypropylene B (with isotacticity of more than or equal to 95 percent and melt flow index of 1.4g/10min) into an extruder for melting, converging the two polypropylene melts through a distributor to form an A + B + A structure, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a multilayer polypropylene microporous membrane precursor (a non-porous structure); subsequently, the multilayer polypropylene microporous membrane precursor is annealed and stretched to obtain the multilayer polypropylene microporous membrane 1. Wherein the extruder shear rate was 3/sec, the melt draw ratio was 172.4, and ε/η was 0.017. The resulting multilayer polypropylene microporous membrane 1 was tested and the results are detailed in table 1.
Example 2
Adding polypropylene A (with isotacticity of not less than 95%, melt flow index of 2.0g/10min), polypropylene B (with isotacticity of not less than 95%, melt flow index of 0.8g/10min) and polypropylene C (with isotacticity of not less than 95%, melt flow index of 1.4g/10min) into an extruder for melting, converging the three polypropylene melts through a distributor to form an A + B + C structure, extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a multi-layer polypropylene microporous membrane precursor with a non-porous structure; subsequently, the precursor of the multilayer polypropylene microporous membrane is annealed and stretched to obtain the multilayer polypropylene microporous membrane 2. Wherein the extruder shear rate is 8/sec, the melt draw ratio is 250, and ε/η is 0.032. The resulting multilayer polypropylene microporous membrane 2 was tested and the results are detailed in table 1.
Example 3
Adding polypropylene A (with isotacticity being more than or equal to 95 percent and melt flow index being 2.0g/10min) and polypropylene B (with isotacticity being more than or equal to 95 percent and melt flow index being 1.4g/10min) into an extruder for melting, converging the two polypropylene melts through a distributor to form an A + B + A structure, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a porous-structure-free multilayer polypropylene microporous membrane precursor; subsequently, the multilayer polypropylene microporous membrane precursor is annealed and stretched to obtain the multilayer polypropylene microporous membrane 3. Wherein the extruder shear rate was 4/sec, the melt draw ratio was 200, and ε/η was 0.02. The resulting multilayer polypropylene microporous membrane 3 was tested and the results are detailed in table 1.
Example 4
Adding polypropylene A (with isotacticity being more than or equal to 95%, melt flow index being 3.0g/10min) and polypropylene B (with isotacticity being more than or equal to 95%, melt flow index being 2.0g/10min) into an extruder for melting, converging the two polypropylene melts through a distributor to form an A + B + A structure, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a porous-structure-free multilayer polypropylene microporous membrane precursor; subsequently, the multilayer polypropylene microporous membrane precursor is annealed and stretched to obtain the multilayer polypropylene microporous membrane 4. Wherein the extruder shear rate was 7.6/sec, the melt draw ratio was 50, and ε/η was 0.152. The resulting multilayer polypropylene microporous membrane 4 was tested and the results are detailed in table 1.
Example 5
Adding polypropylene A (with isotacticity being more than or equal to 95%, melt flow index being 3.0g/10min) and polypropylene B (with isotacticity being more than or equal to 95%, melt flow index being 2.0g/10min) into an extruder for melting, converging the two polypropylene melts through a distributor to form an A + B + A structure, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a porous-structure-free multilayer polypropylene microporous membrane precursor; and then annealing and stretching the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane 5. Wherein the extruder shear rate was 0.5/sec, the melt draw ratio was 250, and ε/η was 0.002. The resulting multilayer polypropylene microporous membrane 5 was tested and the results are detailed in table 1.
Comparative example 1
Adding polypropylene A (with isotacticity of more than or equal to 95 percent and melt flow index of 0.8g/10min) and polypropylene B (with isotacticity of more than or equal to 95 percent and melt flow index of 1.4g/10min) into an extruder for melting, converging the two polypropylene melts through a distributor to form an A + B + A structure, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a porous-structure-free multilayer polypropylene microporous membrane precursor; and then annealing and stretching the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane 6. Wherein the extruder shear rate was 10/sec, the melt draw ratio was 300, and ε/η was 0.033. The resulting multilayer polypropylene microporous membrane 6 was tested and the results are detailed in table 1.
Comparative example 2
Adding polypropylene A (with isotacticity being more than or equal to 95%, melt flow index being 3.0g/10min) and polypropylene B (with isotacticity being more than or equal to 95%, melt flow index being 2.0g/10min) into an extruder for melting, converging the two polypropylene melts through a distributor to form an A + B + A structure, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a porous-structure-free multilayer polypropylene microporous membrane precursor; subsequently, the multilayer polypropylene microporous membrane precursor is annealed and stretched to obtain the multilayer polypropylene microporous membrane 7. Wherein the extruder shear rate is 4/sec, the melt draw ratio is 40, and ε/η is 0.028. The resulting multilayer polypropylene microporous membrane 7 was tested and the results are detailed in table 1.
Comparative example 3
Adding polypropylene A (with isotacticity not less than 95% and melt flow index of 3.0g/10min) into an extruder for melting, converging polypropylene melts through a distributor, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a porous-structure-free multilayer polypropylene microporous membrane precursor; subsequently, the multilayer polypropylene microporous membrane precursor is annealed and stretched to obtain the multilayer polypropylene microporous membrane 8. Wherein the extruder shear rate is 2/sec, the melt draw ratio is 70, and ε/η is 0.028. The resulting multilayer polypropylene microporous membrane 8 was tested and the results are detailed in table 1.
Comparative example 4
Adding polypropylene A (with isotacticity of more than or equal to 95% and melt flow index of 1.4g/10min) into an extruder for melting, converging polypropylene melts through a distributor, then extruding from a slit-shaped die head, cooling through a cooling roller, and drawing and stretching to form a porous-structure-free multilayer polypropylene microporous membrane precursor; subsequently, the precursor of the multilayer polypropylene microporous membrane is annealed and stretched to obtain the multilayer polypropylene microporous membrane 9. Wherein the extruder shear rate was 8/sec, the melt draw ratio was 250, and ε/η was 0.032. The resulting multilayer polypropylene microporous membrane 9 was tested and the results are detailed in table 1.
TABLE 1
The above description is only a few examples of the present application and is not intended to limit the present application, and various modifications and changes may occur to those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims (23)
1. A preparation method of a multilayer polypropylene microporous membrane is characterized by comprising the following steps:
s1, melting a polypropylene raw material, forming a multilayer melt through a multilayer coextrusion die head, and crystallizing the multilayer melt under acting force to obtain a multilayer polypropylene microporous membrane precursor;
s2, carrying out post-treatment on the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane;
wherein, in step S1, the melt draw ratio and the shear rate satisfy the following relationship:
0.002＜ε/η＜0.032， (1)
in formula (1), η represents a melt draw ratio at a die outlet, and ε < 8/sec; ε represents the shear rate in s during extrusion -1 And eta is 50-250.
2. The method of claim 1, wherein η is between 80 and 200.
3. The method of claim 1, wherein the polypropylene feedstock comprises 90 to 100 wt.% polypropylene and 0 to 10 wt.% additives, in terms of mass fraction.
4. The method of claim 3, wherein the polypropylene material is polypropylene having an isotacticity of greater than 95%.
5. The method of claim 3, wherein the additives comprise one or more of nucleating agents, polypropylene waxes, inorganic particles, lubricants.
6. The method of claim 1, wherein the multilayer polypropylene microporous membrane precursor comprises at least an outer membrane layer portion and an inner membrane layer portion, wherein the polypropylene of the outer membrane layer portion has a melt flow index of 0.8 to 3.0g/10min and the polypropylene of the inner membrane layer portion has a melt flow index of 0.8 to 2.0g/10 min.
7. The method of claim 6, wherein the polypropylenes of the outer film layer portion and the inner film layer portion have different melt flow indices.
8. The method of claim 1, wherein in step S2, the post-treatment comprises: and carrying out thermal annealing and stretching treatment on the precursor of the multilayer polypropylene microporous membrane to obtain the multilayer polypropylene microporous membrane.
9. The method of claim 1, further comprising stacking a plurality of the multi-layer microporous membranes in a cold or hot state to obtain a desired number of layers.
10. The preparation method of the multilayer polypropylene microporous membrane according to claim 9, wherein the temperature of the cold compounding is 20-30 ℃ and/or the temperature of the hot compounding is 80-150 ℃.
11. The multilayer polypropylene microporous membrane is characterized by being prepared by the preparation method of the multilayer polypropylene microporous membrane according to any one of claims 1 to 10, wherein the thickness of the multilayer polypropylene microporous membrane is 5-100 micrometers, the range of the thickness of the polypropylene microporous membrane is less than or equal to 2 micrometers, the puncture strength of the polypropylene microporous membrane is more than 300g, the interlayer peeling strength of the polypropylene microporous membrane is more than 100g/m, the longitudinal tensile strength of the polypropylene microporous membrane is more than 150MPa, and the transverse tensile strength of the polypropylene microporous membrane is more than or equal to 10 MPa.
12. The multilayer polypropylene microporous membrane according to claim 11, wherein the multilayer polypropylene microporous membrane has a thickness of 5 to 50 microns.
13. The multilayer polypropylene microporous membrane according to claim 11, wherein the multilayer polypropylene microporous membrane has a thickness of 10 to 25 microns.
14. The multilayer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has a thickness variation of 1.2 μm or less.
15. The multilayer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has a thickness variation of 1.0 μm or less.
16. The multi-layer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has a puncture strength of 350g or more.
17. The multilayer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has a puncture strength of 400g or more.
18. The multilayer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has an interlayer peel strength of greater than 150 g/m.
19. The multilayer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has an interlayer peel strength greater than 200 g/m.
20. The multilayer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has a longitudinal tensile strength of 200MPa or more.
21. The multilayer polypropylene microporous membrane according to claim 11, wherein the polypropylene microporous membrane has a longitudinal tensile strength of 230MPa or more.
22. The multilayer polypropylene microporous membrane according to claim 11, wherein the tensile strength of the polypropylene microporous membrane in the transverse direction is not less than 15 MPa.
23. A lithium ion battery separator is characterized by comprising the multilayer polypropylene microporous membrane prepared by the preparation method of the multilayer polypropylene microporous membrane according to any one of claims 1 to 10 or the multilayer polypropylene microporous membrane according to claims 11 to 22.
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