阅读说明：本技术 聚乙烯微孔膜及其制备方法和应用 (Polyethylene microporous membrane and preparation method and application thereof ) 是由 庞博 杜泽学 于 2019-09-26 设计创作，主要内容包括：本发明提供一种聚乙烯微孔膜的制备方法,包括将聚乙烯与复合稀释剂混合得预混料,其中聚乙烯占预混料的20wt％～60wt％,复合稀释剂占预混料的40wt％～80wt％；预混料经熔融挤出、冷却成型后得初生膜片；采用萃取剂除去初生膜片中的复合稀释剂,然后进行热处理得聚乙烯微孔膜；复合稀释剂包括高温溶剂和非溶剂,高温溶剂选自十四酸、十六酸、十八酸、二十酸、二十二酸、十四醇、十六醇、十八醇和二十醇中的一种或多种,非溶剂选自二苯甲酮、碳酸二苯酯和二甲基砜中的一种或多种,高温溶剂占复合稀释剂不小于60wt％。通过采用特定复合稀释剂使得在相分离过程中聚合物浓度较高时仍可发生液液相分离,所得聚乙烯微孔膜具有优异透气性能和力学强度,适用于作为锂电池隔膜材料。(The invention provides a preparation method of a polyethylene microporous membrane, which comprises the steps of mixing polyethylene and a composite diluent to obtain a premix, wherein the polyethylene accounts for 20-60 wt% of the premix, and the composite diluent accounts for 40-80 wt% of the premix; the premix is subjected to melt extrusion and cooling molding to obtain a primary membrane; removing the composite diluent in the primary membrane by using an extracting agent, and then carrying out heat treatment to obtain a polyethylene microporous membrane; the composite diluent comprises a high-temperature solvent and a non-solvent, wherein the high-temperature solvent is selected from one or more of myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, tetradecanol, hexadecanol, octadecanol and eicosanol, the non-solvent is selected from one or more of benzophenone, diphenyl carbonate and dimethyl sulfone, and the high-temperature solvent accounts for not less than 60 wt% of the composite diluent. By adopting the specific composite diluent, liquid-liquid phase separation can still occur when the concentration of the polymer is higher in the phase separation process, and the obtained polyethylene microporous membrane has excellent air permeability and mechanical strength and is suitable for being used as a lithium battery diaphragm material.)

1. A preparation method of a polyethylene microporous membrane is characterized by comprising the following steps:

mixing polyethylene and a compound diluent to obtain a premix, wherein the polyethylene accounts for 20-60% of the mass of the premix, and the compound diluent accounts for 40-80% of the mass of the premix;

the premix is subjected to melt extrusion and cooling molding to obtain a primary membrane;

removing the composite diluent in the primary membrane by using an extracting agent, and then carrying out heat treatment to obtain the polyethylene microporous membrane;

wherein the composite diluent comprises a high-temperature solvent and a non-solvent, the high-temperature solvent is selected from one or more of myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, tetradecanol, hexadecanol, octadecanol and eicosanol, the non-solvent is selected from one or more of benzophenone, diphenyl carbonate and dimethyl sulfone, and the high-temperature solvent accounts for not less than 60% of the mass of the composite diluent.

2. The preparation method according to claim 1, wherein during the cooling and forming, the polyethylene and the composite diluent are subjected to liquid-liquid phase separation, wherein the maximum mass percentage concentration of the polyethylene in the liquid-liquid phase separation zone is 50-80%.

3. The preparation method according to claim 1, wherein the composite diluent further comprises an additive, the additive accounts for not more than 10% by mass of the composite diluent, and the non-solvent accounts for not more than 30% by mass of the composite diluent.

4. The method according to claim 3, wherein the additive is one or more selected from the group consisting of maleic acid, maleic anhydride, lactic acid, oxalic acid, malic acid, tartaric acid, salicylic acid, triisopropanolamine, triethanolamine, monoethanolamine, diisopropanolamine, porous zeolite, carbon nanotubes, calcium carbonate, lithium chloride, and titanium dioxide.

5. The process according to claim 1, wherein the polyethylene has a viscosity average relative molecular weight of 1,000,000Da to 1,500,000 Da.

6. The method of claim 1, wherein the melt extrusion temperature is 130 ℃ to 180 ℃.

7. The production method according to claim 1, wherein the melt-extruded premix is prepared in a flat plate shape and subjected to the cooling molding in a cooling medium at a temperature of 0 ℃ to 60 ℃.

8. The method according to claim 7, wherein the cooling medium is selected from one or more of liquid paraffin, glycerol, air, and water.

9. The method of claim 1, wherein the extractant is selected from one or more of methanol, ethanol, propanol, butanol, petroleum ether, water, and n-hexane.

10. The method according to claim 1, wherein the heat treatment temperature is 40 ℃ to 90 ℃ and the heat treatment time is 30min to 120 min.

11. The method according to claim 1, wherein the heat treatment medium used in the heat treatment is at least one selected from the group consisting of air, methanol, ethanol, propanol, butanol, petroleum ether, water and n-hexane.

12. The method of claim 1, further comprising stretching the primary membrane sheet and removing the composite diluent from the primary membrane sheet with an extractant.

13. The polyethylene microporous membrane obtained by the preparation method according to claims 1-12.

14. Use of the polyethylene microporous membrane according to claim 13 as a lithium battery separator.

Technical Field

The invention belongs to the technical field of high polymer material preparation, and particularly relates to a polyethylene microporous membrane and a preparation method and application thereof.

Background

The lithium battery diaphragm is one of core materials in the lithium ion battery, and has the main function of separating a positive electrode and a negative electrode of the battery and simultaneously ensuring that lithium ions can freely pass between the positive electrode and the negative electrode.

Currently, lithium ion battery separators on the market mainly comprise dry-method unidirectional tensile separators, dry-method bidirectional tensile separators and wet-method separators. The main difference between these several membrane preparation methods is the pore-forming mechanism of the micropores. In China, the dry-process diaphragm almost realizes self-sufficiency. The dry process is to mix the polymer and additive material to form homogeneous melt at high temperature, to form lamellar structure under the action of tensile force during extrusion, to obtain hard elastic polymer film through heat treatment, to form slit micropores through stretching at certain temperature, and to obtain microporous film through heat setting. The method has the main advantages that no solvent is used in the production process, and the process is environment-friendly and pollution-free. However, the dry-method diaphragm is difficult to control in production, high in precision requirement, complex in used equipment, difficult to control in aperture and porosity, easy to cause micro short circuit in the battery, and low in safety and reliability of the battery.

As the demand for energy density of batteries is gradually increased, the safety of batteries is receiving more and more attention. The wet diaphragm can be very thin, the porosity is higher than that of a dry diaphragm, the pore size distribution is more uniform, and the diaphragm is ensured to have better energy density, mechanical property and product uniformity. The wet process is also called thermal phase separation process, and the process includes high temperature phase separation and low temperature phase separation, and through regulating temperature field to induce the polymer and diluent to produce solid-liquid phase separation or liquid-liquid phase separation, the diluent is eliminated and the polymer film is prepared. Wet-process diaphragms have a very high technical access threshold, and current high-end wet-process diaphragms mainly rely on imports. Technical pain points of the wet diaphragm mainly include the aspects of limited polymer sources, difficult structure regulation, difficult formula selection of casting solution, complex process and the like.

One of the core technologies for preparing the polymer diaphragm by a wet method is the selection of a polymer diluent. The diluent is generally a high temperature solvent for the polymer, i.e., a non-solvent for the polymer at low or room temperature, and a good solvent for the polymer at high temperature. It should have the basic characteristics of high boiling point and low volatility. In order to ensure that the diaphragm has a good diaphragm pore structure, the ideal liquid-liquid phase separation zone should be arranged between the polymer and the diluent in combination with the basic principle of a wet method. In addition, the toxicity, recyclability, flammability and explosiveness of the diluent should be considered sufficiently.

In the prior art, there is a method for preparing a polymer film by a wet method, for example, patent CN1104458C discloses a method for preparing a high molecular weight polyolefin microporous film by a blow molding method, and a polyethylene membrane prepared by the method forms a vein-like fiber structure. Although the method does not need to add low molecular weight compounds, the prepared diaphragm has larger aperture and wide aperture distribution, and the mechanical strength is obviously not ideal; methods for producing porous films of high molecular weight polyolefins have also been previously reported. For example, JP-B-6-53826 and JP-B-6-2841 report diluents comprising low molecular weight compounds such as decane, dodecane, decalin, paraffin oil, mineral oil, diphenyl ether, paraffin wax, diphenyl ether, dioctyl phthalate, and di-n-butyl sebacate added to polyolefins. However, the system formed by mixing the above diluent and polyethylene cannot obtain a wider liquid-liquid phase separation zone, that is, in the phase separation process, with the decrease of temperature, the high molecular weight polyethylene in the system can only undergo liquid-liquid phase separation when the polymer concentration is low, and a microporous membrane with a through bicontinuous structure and good permeability cannot be obtained, so that the membrane cannot be guaranteed to have good mechanical strength and porosity at the same time, and the improvement of the membrane performance is limited. In addition, the diluent is usually liquid or viscous at normal temperature, and part of the diluent has pungent smell, so the diluent is not environment-friendly. Some oil diluents are not easily dissolved in the common extractant ethanol, and toxic n-hexane is used as the extractant, which is not suitable for industrial application.

It is noted that the information disclosed in the foregoing background section is only for enhancement of background understanding of the invention and therefore it may contain information that does not constitute prior art that is already known to a person of ordinary skill in the art.

Disclosure of Invention

The invention mainly aims to overcome at least one defect of the prior art and provides a polyethylene microporous membrane, a preparation method and application thereof.

In order to achieve the purpose, the invention adopts the following technical scheme:

the invention provides a preparation method of a polyethylene microporous membrane, which comprises the following steps: mixing polyethylene and a compound diluent to obtain a premix, wherein the polyethylene accounts for 20-60% of the mass of the premix, and the compound diluent accounts for 40-80% of the mass of the premix; the premix is subjected to melt extrusion and cooling molding to obtain a primary membrane; removing the composite diluent in the primary membrane by using an extracting agent, and then carrying out heat treatment to obtain a polyethylene microporous membrane; the composite diluent comprises a high-temperature solvent and a non-solvent, wherein the high-temperature solvent is selected from one or more of myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, tetradecanol, hexadecanol, octadecanol and eicosanol, the non-solvent is selected from one or more of benzophenone, diphenyl carbonate and dimethyl sulfone, and the high-temperature solvent accounts for not less than 60% of the mass of the composite diluent.

According to one embodiment of the invention, during the cooling forming process, the polyethylene and the composite diluent are subjected to liquid-liquid phase separation, wherein the maximum mass percentage concentration of the polyethylene corresponding to the liquid-liquid phase separation zone is 50-80%.

According to one embodiment of the invention, the compound diluent further comprises an additive, wherein the additive accounts for not more than 10% of the compound diluent by mass, and the non-solvent accounts for not more than 30% of the compound diluent by mass.

According to one embodiment of the invention, the additive is selected from one or more of maleic acid, maleic anhydride, lactic acid, oxalic acid, malic acid, tartaric acid, salicylic acid, triisopropanolamine, triethanolamine, monoethanolamine, diisopropanolamine, porous zeolite, carbon nanotubes, calcium carbonate, lithium chloride and titanium dioxide.

According to one embodiment of the invention, the polyethylene has a viscosity average relative molecular weight of 1,000,000Da to 1,500,000 Da.

According to one embodiment of the invention, the melt extrusion temperature is 130 ℃ to 180 ℃.

According to one embodiment of the present invention, the melt-extruded premix is formed into a flat plate shape and is subjected to cooling molding in a cooling medium, wherein the cooling temperature is 0 ℃ to 60 ℃.

According to one embodiment of the invention, the cooling medium is selected from one or more of liquid paraffin, glycerol, air and water.

According to one embodiment of the invention, the extractant is selected from one or more of methanol, ethanol, propanol, butanol, petroleum ether, water and n-hexane.

According to one embodiment of the present invention, the temperature of the heat treatment is 40 to 90 ℃ and the time of the heat treatment is 30 to 120 min.

According to an embodiment of the present invention, the heat treatment medium used for the heat treatment is at least one selected from the group consisting of air, methanol, ethanol, propanol, butanol, petroleum ether, water and n-hexane.

According to an embodiment of the invention, the method further comprises the step of stretching the primary membrane, and then removing the composite diluent in the primary membrane by using an extracting agent.

The invention also provides the polyethylene microporous membrane prepared by the preparation method.

The invention also provides the application of the polyethylene microporous membrane as a lithium battery diaphragm.

According to the technical scheme, the polyethylene microporous membrane and the preparation method thereof have the advantages and positive effects that:

the polyethylene microporous membrane is prepared by a thermal phase separation method, and a specific composite diluent is compounded with polyethylene to obtain a polyethylene/composite diluent system with a wide liquid-liquid phase separation zone, so that the system can still perform liquid-liquid phase separation when the concentration of a polymer is higher in the phase separation process; in addition, the composite diluent adopted by the invention is a solid environment-friendly diluent at normal temperature, has no pungent smell, is suitable for large-scale industrial production, and has good industrial application prospect.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention.

FIG. 1 shows an equilibrium phase diagram of a polymer-diluent system;

fig. 2 is a scanning electron micrograph of the polyethylene microporous membrane of example 1.

Detailed Description

The following presents various embodiments or examples in order to enable those skilled in the art to practice the invention with reference to the description herein. These are, of course, merely examples and are not intended to limit the invention. The endpoints of the ranges and any values disclosed in the present application are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to yield one or more new ranges of values, which ranges of values should be considered as specifically disclosed herein.

The invention provides a preparation method of a polyethylene microporous membrane, which comprises the following steps: mixing polyethylene and a compound diluent to obtain a premix, wherein the polyethylene accounts for 20-60% of the mass of the premix, and the compound diluent accounts for 40-80% of the mass of the premix; the premix is subjected to melt extrusion and cooling molding to obtain a primary membrane; removing the diluent in the primary membrane by using an extracting agent, and then carrying out heat treatment to obtain a polyethylene microporous membrane; wherein the composite diluent comprises a high-temperature solvent and a non-solvent, the high-temperature solvent is selected from one or more of myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, myristic alcohol, cetyl alcohol, stearyl alcohol and arachidyl alcohol, and preferably, the high-temperature solvent is myristic alcohol, cetyl alcohol, stearyl alcohol or arachidyl alcohol; the non-solvent is selected from one or more of benzophenone, diphenyl carbonate and dimethyl sulfone; the high-temperature solvent accounts for not less than 60% of the composite diluent by mass.

In accordance with the present invention, the thermal phase separation (TIPS) method for preparing polymer films essentially utilizes a specific polymer solvent, also known as a diluent, which is a solvent at high temperatures and a non-solvent at low temperatures, and the incompatibility that loses its solvent ability is due to the loss of thermal energy (i.e., heat as the driving force for phase separation). Since the diluent is non-volatile, it needs to be extracted from the finished product with an extractant that is a solvent for the diluent but a non-solvent for the polymer to form a microporous structure.

As described above, the polymer microporous membrane is prepared by the TIPS method mainly using a phase separation process. FIG. 1 shows an equilibrium phase diagram of a polymer-diluent system. As shown in FIG. 1, when the temperature is high, the polymer and the solvent form a uniform phase, and the polymer is in a high concentration region (. phi.) with the decrease in temperature_mRight CD line) shows a polymer melting point depression curve in a polymer low concentration region (phi)_mLine ABC on the left) is a liquid-liquid phase separation curve, and the region below the horizontal dashed line forms a two-phase region of polymer-rich (solid) and solvent-rich (liquid) phases. Phi is a_mThe point is called the monotectic point, at which the polymer crystalline state and the two-phase achieve a three-phase equilibrium, phi_mThe dots also indicate the demarcation points between the solid-liquid phase separation and the liquid-liquid phase separation regions.

It is known to those skilled in the art that different phase separation processes directly affect the microporous structure of the final material. For preparing a polyethylene microporous membrane for a lithium battery material, a through bicontinuous microporous structure can be formed by a final material through liquid-liquid phase separation, and meanwhile, the higher the polymer concentration in a system is, the higher the porosity of the microporous membrane is, the smaller the pore diameter is, the pore diameter distribution is uniform, so that the mechanical property of the membrane is favorably improved, and the method is a phase separation process expected to occur. However, existing polymer-diluent systems tend to have a relatively narrow liquid-liquid phase separation zone, i.e., φ_mThe maximum polymer mass concentration in the liquid-liquid phase separation zone corresponding to the point is small. In the phase separation process, once the polymer concentration in the system is higher, liquid-liquid phase separation cannot occur, so that a microporous membrane which has a through bicontinuous structure and good permeability cannot be obtained, and meanwhile, the microporous membrane cannot have good mechanical strength and porosity, and the improvement of the membrane performance is limited.

The inventor of the invention finds that by adopting the composite diluent of the invention, utilizing the combination of a specific high-temperature solvent and a non-solvent and adjusting the component ratio, the polyethylene-composite diluent system can have a wider liquid-liquid phase separation zone, namely, the liquid-liquid phase separation can still occur under the condition of high polymer concentration. This is because by adjusting the combination and ratio of the solvent and the non-solvent in the diluent, the interaction parameters with the polymer can be adjusted, i.e., the compatibility and the interaction parameters can be qualitatively and quantitatively adjusted, so that the liquid-liquid phase separation zone can be regulated and controlled, thereby obtaining a wider liquid-liquid phase separation zone.

In some embodiments, the liquid-liquid phase separation zone φ_mThe maximum mass percentage concentration of the corresponding polyethylene is 50-80%. The whole section structure of the polyethylene microporous membrane obtained by the invention is through bicontinuous, the porosity is high, and the obtained microporous membrane has excellent air permeability and mechanical property and is particularly suitable for being used as a lithium battery diaphragm material.

In some embodiments, the composite diluent of the present invention further comprises additives including, but not limited to, maleic acid, maleic anhydride, lactic acid, oxalic acid, malic acid, tartaric acid, salicylic acid, triisopropanolamine, triethanolamine, monoethanolamine, diisopropanolamine, porous zeolites, carbon nanotubes, calcium carbonate, lithium chloride, titanium dioxide, and the like. The porosity and mechanical property of the microporous membrane can be further improved by adding a proper amount of additive, preferably, the additive accounts for not more than 10% of the composite diluent by mass, and the non-solvent accounts for not more than 30% of the composite diluent by mass.

According to the present invention, in some embodiments, the polyethylene has a viscosity average relative molecular weight of 1,000,000Da to 1,500,000 Da. Too low a molecular weight of the polyethylene will not result in the aforementioned microporous structure of the present invention. The polyethylene material generally has an average particle size of 150 to 250 μm.

According to the invention, adjusting the specific preparation conditions and parameters of the polyethylene microporous membrane also has a certain influence on the final properties of the product.

Specifically, in some embodiments, the premix obtained after mixing the polyethylene and the composite diluent is melt-extruded through a screw extruder, and the melt-extrusion temperature is controlled to be 130 ℃ to 180 ℃. Then the melt-extruded premix is made into a flat plate shape by a spinneret plate, and is immersed into a cooling medium for solidification and molding to obtain the nascent membrane.

In some embodiments, the cooling medium includes, but is not limited to, a cooling liquid, such as liquid paraffin, glycerol, water, and the like, preferably water, and may also be cooled directly in air. Wherein the cooling temperature is controlled between 0 ℃ and 60 ℃.

In some embodiments, the method further comprises performing extraction treatment after the primary membrane is subjected to stretching treatment. Wherein the stretching process comprises transverse stretching and longitudinal stretching.

In some embodiments, the extractant includes, but is not limited to, one or more of methanol, ethanol, propanol, butanol, petroleum ether, water, and n-hexane.

In some embodiments, the temperature of the heat treatment is 40 ℃ to 90 ℃ and the time of the heat treatment is 30min to 120 min. The heat treatment medium may be air, methanol, ethanol, propanol, butanol, petroleum ether, water and n-hexane, etc. And carrying out heat treatment to obtain the polyethylene microporous membrane.

The polyethylene microporous membrane obtained by the method is an asymmetric structure diaphragm with a through bicontinuous cross-section structure and gradient pores near the outer surface. The microporous membrane has excellent air permeability and mechanical properties, the gradient pore structure enables the outer pore diameter of the microporous membrane to be small, the inner pore diameter to be large, the mechanical properties are further improved, and the microporous membrane is particularly suitable for being used as a lithium battery diaphragm material. In addition, the polyethylene microporous membrane can be continuously and stably prepared in a screw extrusion mode, the preparation period is short, the process is simple and convenient, and the cost is low.

The compound diluent adopted by the invention has low toxicity, and in some embodiments, the compound diluent can be recycled in a simple manner, for example, the extracted compound diluent can be re-extracted by distillation, filtration and the like, so as to be further recycled. And the composite diluent has small environmental pollution and is suitable for the actual industrial production process.

The invention will be further illustrated by the following examples, but is not to be construed as being limited thereto. Unless otherwise specified, all reagents used in the invention are analytically pure.

The mechanical property test of the invention is carried out according to GB/T1040.3-2006 and ASTM D3763-10 standard.

The air permeability test of the invention is carried out according to the GB/T458-2008 standard.

The porosity of the invention is tested by a liquid absorption method. The method comprises the steps of selecting absolute ethyl alcohol as absorption liquid, weighing the mass of a dry film, completely soaking a diaphragm in the absolute ethyl alcohol for a certain time, then quickly taking out the diaphragm, slightly wiping off the absolute ethyl alcohol on the surface of the diaphragm by using filter paper, weighing the mass of a wet film, and calculating to obtain the porosity of the diaphragm.

The film pore structure of the film of the invention is observed by a scanning electron microscope.

Example 1

Polyethylene (viscosity average molecular weight of 1,500,000Da, average particle diameter of 150 μm) is mixed with composite diluent hexadecanol and benzophenone uniformly to form premix, wherein the mass percent of the polyethylene is 35%, the mass percent of the hexadecanol is 50%, and the mass percent of the benzophenone is 15%. Adding the premix into a screw extruder, and melting and mixing uniformly to form a membrane making solution, wherein the melting temperature is 130 ℃. And (3) molding the membrane-making solution by an extruder, and then injecting the molded membrane-making solution into cooling water for solidification to obtain a primary membrane, wherein the cooling temperature is 25 ℃. The obtained primary membrane is stretched transversely and longitudinally, and then immersed in ethanol for extraction to remove hexadecanol and benzophenone. And (3) carrying out heat treatment on the extracted film in water at 90 ℃ for 60min to obtain the polyethylene microporous film.

Fig. 2 is a scanning electron micrograph of the polyethylene microporous membrane of example 1. As shown in FIG. 2, the polyethylene microporous membrane has a through bicontinuous membrane pore structure with uniform pore size distribution. The system has a liquid-liquid phase separation phenomenon through microscopic observation, and the liquid-liquid phase separation zone corresponds to the maximum polymer mass concentration of 65 wt%. The microporous membrane was subjected to mechanical property test, air permeability test and porosity test, respectively, and the results are shown in table 1.

Example 2

A microporous polyethylene membrane was prepared as in example 1, except that the composite diluent was hexadecanol and diphenyl carbonate, in which the mass percent of polyethylene was 35%, the mass percent of hexadecanol was 50%, and the mass percent of diphenyl carbonate was 15%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Example 3

A microporous polyethylene membrane was prepared according to the method of example 1, except that the composite diluent was hexadecanol and diphenyl carbonate and calcium carbonate, wherein the mass percent of polyethylene was 35%, the mass percent of hexadecanol was 30%, the mass percent of diphenyl carbonate was 28.5%, and the mass percent of calcium carbonate was 6.5%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Example 4

A microporous polyethylene membrane was prepared as in example 1, except that the composite diluent was hexadecanol and diphenyl carbonate, the mass percent of polyethylene was 40%, the mass percent of hexadecanol was 42%, and the mass percent of diphenyl carbonate was 18%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Example 5

A microporous polyethylene membrane was prepared as in example 1, except that the composite diluent was 60% by mass of cetyl alcohol, 28% by mass of cetyl alcohol and 12% by mass of diphenyl carbonate. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Example 6

A microporous polyethylene membrane was prepared as in example 1, except that the composite diluent was behenyl alcohol and diphenyl carbonate, the mass percent of polyethylene was 60%, the mass percent of behenyl alcohol was 28%, and the mass percent of diphenyl carbonate was 12%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Example 7

A microporous polyethylene membrane was prepared as in example 1, except that the composite diluent was stearic acid and diphenyl carbonate, wherein the polyethylene was 35% by mass, the stearic acid was 50% by mass, and the diphenyl carbonate was 15% by mass. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Comparative example 1

A polyethylene microporous membrane was prepared according to the method of example 1, except that the diluent was cetyl alcohol, wherein the mass percentage of polyethylene was 35% and the mass percentage of cetyl alcohol was 65%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Comparative example 2

A microporous polyethylene membrane was prepared according to the method of example 1, except that the diluent was diphenyl ether in which the mass percentage of polyethylene was 35% and the mass percentage of diphenyl ether was 65%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Comparative example 3

A polyethylene microporous film was prepared according to the method of example 1, except that the diluent was di-n-octyl phthalate, in which the mass percentage of polyethylene was 35% and the mass percentage of di-n-octyl phthalate was 65%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

Comparative example 4

A polyethylene microporous film was prepared according to the method of example 1, except that the mass percentage of polyethylene was 10% and the mass percentage of benzophenone was 90%. The system has a liquid-liquid phase separation phenomenon, the microporous membrane is respectively subjected to a mechanical property test, an air permeability test and a porosity test, and the results and the maximum polymer mass concentration corresponding to a liquid-liquid phase separation zone are shown in table 1.

TABLE 1

As can be seen from table 1 above, by using the composite diluent system, compared with the diluent of a single component, the maximum mass percentage concentration of the liquid-liquid phase separation region can reach more than 60%, and the prepared polyethylene microporous membrane has excellent air permeability and mechanical strength, and is suitable for being used as a lithium battery diaphragm material. In addition, the content and the proportion of the composite diluent also reflect certain influence on the performance of the material.

In conclusion, the invention provides a novel polyethylene microporous membrane and a preparation method thereof, the method adopts the compounding of specific composite diluent and polyethylene to obtain a polyethylene/composite diluent system with a wide liquid-liquid separation zone, so that the system can still carry out liquid-liquid separation when the concentration of a polymer is higher in the phase separation process, and the prepared polyethylene microporous membrane has excellent air permeability and mechanical strength and is suitable for being used as a lithium battery diaphragm material; in addition, the method has the advantages of short preparation period, simple and convenient process, low cost, environment-friendly raw materials and good industrial application prospect.

It should be noted by those skilled in the art that the described embodiments of the present invention are merely exemplary and that various other substitutions, alterations, and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the above-described embodiments, but is only limited by the claims.