Hydrophobic / hydrophilic electrolyte and low-ash porous polypropylene membrane, preparation method thereof and application of membrane as diaphragm

Abstract

The invention belongs to the technical field of porous diaphragms, and discloses a hydrophobic / hydrophilic electrolyte and low-ash porous polypropylene membrane, a preparation method thereof and application of the membrane as a diaphragm. The film is prepared by the following steps: preparing a film from raw materials including polypropylene resin, a beta nucleating agent, an antioxidant and an optional film preparation auxiliary agent, and selecting alpha-homo-polypropylene with a specific melt flow rate, specific isotacticity and low ash content as a raw material by controlling annealing conditions after casting and post-treatment annealing conditions after directional stretching; according to the present invention, the porous polypropylene film has characteristics of uniform pore structure, suitable air permeability and pore structure stabilization through the cooperation of the beta nucleating agent, the antioxidant and the like, further, the porous polypropylene film has characteristics of hydrophobicity, electrolyte affinity and low ash content, and can improve the electrolyte wettability of the diaphragm when the porous polypropylene film is used as the diaphragm. The porous polypropylene film can be used in the fields of lithium ion batteries, supercapacitors and the like, and is widely applied.

Description

Technical Field

[0001] The present invention belongs to the technical field of porous diaphragms, and more specifically, relates to a hydrophobic / electrophilic electrolyte, low-ash porous polypropylene membrane, a preparation method thereof, and an application thereof as a diaphragm. Background Art

[0002] With the vigorous promotion of new energy vehicles, lithium-ion batteries, supercapacitors and their constituent materials have also received extensive attention. As one of the key components of lithium-ion batteries and supercapacitors, the main functions of the diaphragm are (1) isolating the positive and negative electrodes to prevent the positive and negative electrodes from directly contacting and short-circuiting; (2) conducting the ions in the electrolyte, providing a channel for the ions to move freely between the positive and negative electrodes to form a conductive loop. Therefore, the diaphragm plays an important role in the capacity, cycle stability, rate performance and safety of lithium-ion batteries and supercapacitors. At present, polyolefin diaphragms (including PP, PE and PP / PE composite diaphragms, etc.) are the most widely used in the market. However, with the rapid development of the new energy vehicle industry, traditional polyolefin diaphragms can no longer meet the needs of high-performance lithium-ion batteries and supercapacitors due to their poor electrolyte wettability, high impurities and ash content. Therefore, it is very valuable to develop diaphragms with good electrolyte wettability and low ash content.

[0003] In recent years, based on the many advantages of polyolefin diaphragms, the scientific and industrial communities have adopted chemical or physical modification methods to modify polyolefin diaphragms, thereby improving the wettability of the diaphragm's electrolyte, and have made some progress. For example, ceramic nanopowders such as alumina (CN106299204A, CN202888277U, CN105347778A, CN104269509A, CN205335329U), silicon dioxide (CN106340604A), magnesium hydroxide (CN106654124A) and hydrated alumina (CN106531941A, CN106531941) are used as coating materials to improve the wettability of the diaphragm. The above patented technology has achieved certain results in improving the wettability of the diaphragm electrolyte, but it will cause the diaphragm to become very hydrophilic, increasing the risk of water and moisture absorption during storage and use of the diaphragm, especially in coastal areas with high humidity. Studies have shown that trace moisture in the diaphragm can have serious adverse effects on the performance of lithium-ion batteries and supercapacitors (Adv Mater 2018, 30, 1706375; J. Power Sources 2019, 434, 226734). At the same time, the addition of coating materials will greatly increase the cost, thickness and weight of the diaphragm, and also introduce more impurities and ash. Studies have shown that impurities and ash in the diaphragm can also have serious adverse effects on the performance of lithium-ion batteries and supercapacitors, especially affecting the cycle stability and reducing the service life (Electrochim Acta 2017, 228, 214-225).

[0004] Therefore, the use of a low-cost, simple and scalable method to prepare hydrophobic / electrophilic electrolyte and low-ash porous polypropylene membrane is of great significance to promoting the rapid and healthy development of the new energy industry and is one of the key technologies urgently needed in this field. Summary of the invention

[0005] The purpose of the present invention is to provide a porous polypropylene film and a preparation method thereof and an application as a diaphragm, wherein the porous polypropylene film has the characteristics of being hydrophobic, electrolyte-friendly and low in ash content; the present invention can regulate the pore size of the micropores of the porous polypropylene film by controlling the annealing conditions of the initial film sheet after casting and the post-treatment annealing conditions, and realize the stabilization of the pore structure, and the preparation method is simple and environmentally friendly, without toxic and harmful waste gas and waste liquid discharge, and is easy to industrialize. The porous polypropylene film can be used in the fields of lithium ion batteries, supercapacitors, etc., and is widely used.

[0006] The first aspect of the present invention is to provide a method for preparing a porous polypropylene film, the preparation method comprising the following steps:

[0007] (1) melt-mixing raw materials including a polypropylene resin, a β-nucleating agent, an antioxidant, and an optional film-forming aid to obtain a mixed melt;

[0008] (2) subjecting the mixed melt to casting, annealing, and rolling in sequence to obtain an initial film; wherein the annealing temperature is 80 to 140° C., and the annealing time is 0.5 to 20 min;

[0009] (3) subjecting the initial membrane sheet to directional stretching and post-treatment annealing to obtain the porous polypropylene membrane; wherein the temperature of the post-treatment annealing is 90 to 160° C.; and the time of the post-treatment annealing is 0.1 to 10 min;

[0010] The polypropylene resin is selected from α-homopolypropylene; according to GB / T9345.1-2008, the ash content of the polypropylene resin is less than 0.1wt‰; according to GB / T3682.1-2018, the melt flow rate of the polypropylene resin at 230°C and 2.16kg load is 0.5-20g / 10min; according to GB / T2412-2008, the isotacticity index of the polypropylene resin is 95-99wt%.

[0011] In a preferred embodiment of the present invention, the step (1) comprises: firstly mixing raw materials including polypropylene resin, β-nucleating agent, antioxidant and optional film-forming aid to obtain a mixture, and then melt-mixing the mixture;

[0012] More preferably, the melt mixing is carried out by melt extrusion in a twin-screw extruder;

[0013] Further more preferably, the melt extrusion temperature is 180 to 270°C, preferably 190 to 260°C, and further preferably 200 to 250°C.

[0014] According to the present invention, the conditions of step (2) can be selected within a wide range. In a preferred embodiment of the present invention, in step (2):

[0015] The annealing temperature is 100-140° C., and / or the annealing time is 1-15 min.

[0016] The casting temperature is 100 to 140° C., preferably 110 to 140° C., and / or the casting time is 0.5 to 20 min, preferably 1 to 15 min; and / or,

[0017] The rolling temperature is 40 to 90° C., preferably 50 to 90° C.; and / or,

[0018] The step (2) is performed in the following manner: the mixed melt is sequentially cast by a casting roller, annealed by an annealing roller, rolled by a rubber roller, and then rolled by a winding roller to obtain an initial film sheet.

[0019] According to the present invention, the conditions of step (3) can be selected within a wide range. In a preferred embodiment of the present invention, in step (3):

[0020] The temperature of the post-treatment annealing is 100-160° C., preferably 100-150° C.; and / or, the time of the post-treatment annealing is 0.2-10 min, preferably 0.5-5 min; and / or,

[0021] The temperature of the directional stretching is 60 to 140° C., preferably 80 to 140° C., and more preferably 100 to 140° C.; and / or,

[0022] The directional stretching is unidirectional stretching or bidirectional stretching, preferably one of simultaneous bidirectional stretching or stepwise bidirectional stretching; preferably,

[0023] The direction of uniaxial stretching is MD (Machine Direction), and the directions of biaxial stretching are MD and TD (Transverse Direction); more preferably, the stretching ratio in the MD direction of the uniaxial stretching is 0.5 to 5 times;

[0024] The synchronous biaxial stretching has a stretching ratio of 0.5 to 5 times in the MD direction and a stretching ratio of 0.5 to 5 times in the TD direction;

[0025] The stepwise biaxial stretching has a stretching ratio of 0.5 to 5 times in the MD direction and a stretching ratio of 0.5 to 5 times in the TD direction.

[0026] According to the present invention, the polypropylene resin is selected from α-homopolypropylene; the ash content of the polypropylene resin is less than 0.1wt‰ according to GB / T9345.1-2008; the melt flow rate of the polypropylene resin at 230°C and 2.16kg load is 0.5-20g / 10min according to GB / T3682.1-2018; the isotacticity index of the polypropylene resin is 95-99wt% according to GB / T2412-2008. In a preferred embodiment of the present invention, the polypropylene resin has at least one of the following characteristics:

[0027] According to GB / T9345.1-2008 test, the ash content of the polypropylene resin is less than 0.05wt‰; and / or,

[0028] According to GB / T3682.1-2018, the melt flow rate of the polypropylene resin at 230°C and 2.16 kg load is 0.5 to 10 g / 10 min, preferably 1 to 10 g / 10 min; and / or,

[0029] According to GB 2412-2008 test, the isotacticity index of the polypropylene resin is 97-99 wt %.

[0030] The polypropylene resin of the present invention can be purchased, for example, from Beijing Research Institute of Chemical Industry of China Petrochemical Corporation, or can be prepared by the prior art disclosed in CN104558814A and CN104558813A. The main catalyst of the polypropylene is prepared by HA-DQ and oil ester, and the co-catalyst is triethyl aluminum.

[0031] According to the present invention, the β-nucleating agent can be selected in a wide range. In a preferred embodiment of the present invention, the β-nucleating agent is at least one of a single-component organic small molecule, a multi-component inorganic salt and a composite nano-powder rubber;

[0032] Preferably, the single-component organic small molecule is at least one of N,N'-diphenyl adipamide, N,N'-dicyclohexyl terephthalamide and N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide; and / or,

[0033] Preferably, the multi-component inorganic salt is at least two of a hydrazine salt of adipic acid, a hydrazine salt of suberic acid, a calcium salt of pimelic acid, and a calcium salt of suberic acid.

[0034] In a preferred embodiment of the present invention, the composite nano powder rubber contains nano powder rubber and N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide; more preferably, the mass ratio of nano powder rubber to N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide is 1:(1-9); and / or,

[0035] More preferably, the nano powder rubber is vulcanized powder rubber and / or non-vulcanized powder rubber; the vulcanized powder rubber is preferably at least one of vulcanized natural rubber, vulcanized styrene butadiene rubber, vulcanized nitrile rubber, vulcanized chloroprene rubber, vulcanized polybutadiene rubber, vulcanized polyacrylate rubber, vulcanized styrene butadiene rubber, vulcanized isoprene rubber, vulcanized ethylene propylene rubber and vulcanized polyurethane rubber; the non-vulcanized powder rubber is preferably at least one of cross-linked styrene butadiene powder rubber, cross-linked polybutadiene powder rubber, cross-linked nitrile powder rubber, cross-linked chloroprene powder rubber and cross-linked acrylate powder rubber.

[0036] According to the present invention, the nano powder rubber can be purchased from the market (preferably including but not limited to VP-101B, VP-101T, VP-108, VP-108 (5) etc. produced by Beijing Research Institute of Chemical Industry of Sinopec), or can be prepared by methods in the prior art. Preferably, the nano powder rubber can be prepared by referring to the methods in US6423760: Fully vulcanized powder rubber with controllable particle size, preparation method and use thereof, and US6838490: Silicone rubber in finely dispersed powder form, preparation method and use thereof.

[0037] According to the present invention, the antioxidant can be selected from a wide range. In a preferred embodiment of the present invention, the antioxidant is at least one of phenolic antioxidants, and the phenolic antioxidant is preferably at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1010) and n-octadecylβ-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1076).

[0038] According to the present invention, the ratio of each component in the raw material can be selected within a wide range. In a preferred embodiment of the present invention, the amount of the β-nucleating agent is 0.3 to 4 parts by weight, preferably 0.3 to 3 parts; more preferably 0.3 to 2 parts, based on the total amount of the raw materials as 100 parts. The amount of the antioxidant is 0.1 to 2 parts; preferably 0.1 to 1.5 parts; more preferably 0.1 to 1 part.

[0039] According to the present invention, the porous polypropylene film may optionally contain a film-forming aid. In a preferred embodiment of the present invention, the film-forming aid includes an antistatic agent and an optional opening agent;

[0040] More preferably, based on 100 parts of the total amount of the raw materials, the amount of the antistatic agent is 0.1 to 2 parts, preferably 0.1 to 1.5 parts, and more preferably 0.1 to 1 part; the amount of the opening agent is 0 to 2 parts, preferably 0.1 to 2 parts; and more preferably 0.1 to 1 part.

[0041] In the above technical solution, preferably, the antistatic agent is at least one of nonionic antistatic agents, preferably, the nonionic antistatic agent is at least one of ethoxylated aliphatic alkylamine, ethoxylated alkyl acid amine and glycerol monostearate; and / or,

[0042] Preferably, the opening agent is at least one of an inorganic and an organic opening agent; preferably, the inorganic opening agent is at least one of talc, diatomaceous earth and synthetic silica, and / or the organic opening agent is at least one of oleamide, erucamide and an EBS derivative (vinyl bisstearamide).

[0043] The second aspect of the present invention is to provide a porous polypropylene film, preferably the porous polypropylene film is prepared by the preparation method described in the first aspect.

[0044] The porous polypropylene membrane is distributed with through micropores, and the average diameter of the narrow side of the micropores is 0.05-1 μm; using GB / T1038-2000 to detect, the air permeability of the porous polypropylene membrane is 200-700s / 100ml, preferably 300-500s / 100ml; using GB / T30693-2014 to detect, the contact angle of the porous polypropylene membrane with water is not less than 100°, and the contact angle of the porous polypropylene membrane with the electrolyte is not more than 15°.

[0045] In the present invention, the through micropores may be connected from one side surface of the diaphragm to the other side surface, and the shape thereof is generally spindle-shaped.

[0046] In a preferred embodiment of the present invention, the thickness of the porous polypropylene membrane is 10 to 100 μm, preferably 10 to 40 μm, and more preferably 14 to 30 μm.

[0047] In a preferred embodiment of the present invention, the ash content of the porous polypropylene membrane is less than 0.1 wt‰, preferably less than 0.05 wt‰, as tested in accordance with GB / T9345.1-2008.

[0048] The third aspect of the present invention is to provide a use of the porous polypropylene film described in the first aspect as a separator, preferably as a separator for batteries and / or capacitors.

[0049] When the porous polypropylene membrane is used as a diaphragm, it can improve the electrolyte wettability of the diaphragm and avoid water and moisture absorption during storage and use. The low ash content prevents the ash content of the diaphragm from having a serious adverse effect on the performance of lithium-ion batteries and supercapacitors, and can improve cycle stability and increase service life. The porous polypropylene membrane can be used in the fields of lithium-ion batteries, supercapacitors, etc. and has a wide range of applications.

[0050] The present invention has the following advantages:

[0051] The present invention selects α-homopolymer polypropylene with specific melt flow rate, specific isotacticity and low ash as raw material, cooperates with β nucleating agent, antioxidant, etc., and controls the annealing conditions after casting and the post-treatment annealing conditions after directional stretching. The obtained porous polypropylene film has a uniform pore structure, showing suitable air permeability, and realizing the stabilization of the pore structure. In addition, the porous polypropylene film of the present invention has the characteristics of hydrophobic, electrophilic electrolyte and low ash. When used as a diaphragm, it can improve the electrolyte wettability of the diaphragm, and can avoid water absorption and moisture absorption during storage and use. The low ash feature makes it possible to avoid the ash content of the diaphragm from having a serious adverse effect on the performance of lithium ion batteries and supercapacitors, and can improve cycle stability and increase service life. The porous polypropylene film can be used in the fields of lithium ion batteries, supercapacitors, and is widely used.

[0052] The method for preparing the porous polypropylene film of the invention is simple and environmentally friendly, has no emission of toxic and harmful waste gas and waste liquid, and is easy for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

[0054] Figure 1 The scanning electron micrograph of the porous polypropylene membrane without post-treatment annealing in Comparative Example 1 of the present invention is shown.

[0055] Figure 2 The scanning electron micrograph of the porous polypropylene membrane after post-treatment annealing in Example 1 of the present invention is shown.

[0056] Depend on Figure 1 , Figure 2 It can be seen that the porous polypropylene membrane after post-treatment annealing has uniform micropores with similar pore sizes, and the opening ratio is higher than that of the membrane without post-treatment annealing, indicating that the pore structure of the porous polypropylene membrane is improved after the post-treatment annealing process, the stability of the pores is improved, which is beneficial to improve the air permeability of the membrane.

[0057] Figure 3 The hydrophobic properties of the porous polypropylene membrane in Example 1 of the present invention are shown.

[0058] Figure 4 The electrolyte affinity characteristics of the porous polypropylene membrane in Example 1 of the present invention are shown. DETAILED DESCRIPTION

[0059] The present invention is described in detail below in conjunction with specific embodiments. It is necessary to point out that the following embodiments are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the contents of the present invention still fall within the scope of protection of the present invention.

[0060] The present invention will be further described below in conjunction with embodiments, but the scope of the present invention is not limited to these embodiments.

[0061] The stretching processes involved in the preparation process in the examples were all carried out on a KARO IV testing machine produced by Brückner, Germany.

[0062] The pore structure of the porous polypropylene membrane prepared in the example was observed by a scanning electron microscope of model NanoSEM450 produced by FEI Company of the United States. The sample needed to be gold-sprayed before testing, and the average pore size of the micropores was measured by NanoMeasured software on the micropores observed in the scanning electron microscope photograph.

[0063] The narrow side diameter of the micropore refers to: the open micropores on the sample can be approximately regarded as elliptical or long strip cracks, and the diameter in the narrower direction is the narrow side diameter of the micropore; the specific detection method is to select different areas of the sample, take 3 to 5 scanning electron microscope photos of the sample, and the magnification is 20,000 times. 10 to 20 open micropores are randomly selected in each photo, and the narrow side diameter of the micropore is measured using Nano Measured software. The average value of all measured narrow side diameters is calculated to obtain the average narrow side diameter of the micropore.

[0064] The ash content of the porous polypropylene film in the embodiment is determined according to the direct calcination method specified in GB / T9345.1-2008. The sample to be tested with a mass of m0 is placed in a crucible, and the temperature is raised to 600°C at a rate of 10°C / min in a muffle furnace, and the temperature is kept for 1 hour. After the sample is naturally cooled to room temperature, the ash weight m1 is weighed with an analytical balance. The sample ash content is calculated by the formula m1 / m0×1000, and the unit is wt‰.

[0065] The contact angle between the porous polypropylene film and water or electrolyte in the examples is tested according to the standard specified in GB / T30693-2014.

[0066] Air permeability: Also known as the Gurley number, it is the time required for a certain volume of gas to pass through a 1 square inch membrane under certain pressure conditions. It is determined by the pore size, pore size distribution, porosity and open porosity, etc. The size of the Gurley value is negatively correlated with the permeability of the gas. Test standard GB / T1038-2000, test equipment: American Gurley 4110N air permeability tester.

[0067] In the following examples, α-homopolymer polypropylene resin was purchased from Beijing Research Institute of Chemical Industry of Sinopec.

[0068] Sulphurized styrene butadiene nanopowder rubber was purchased from Beijing Research Institute of Chemical Industry, Sinopec, model VP-101B.

[0069] Example 1

[0070] An alpha-homopolymerized polypropylene resin with a low ash content (less than 0.02 wt‰) and an isotacticity index of 98 wt% and a melt flow rate of 3.0 g / 10 min at a load of 2.16 kg at 230° C., N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide, glycerol monostearate and antioxidant 1010 are uniformly mixed in a low-speed mixer according to a weight ratio of 97.5:2:0.2:0.3, and then the uniformly mixed materials are added to a twin-screw extruder with a T-die for melt extrusion at a temperature of 210° C. to obtain a polypropylene mixed melt, and the polypropylene mixed melt is attached to a casting roll after flowing out of the T-die for casting and shaping, and then enters an annealing roll for annealing, and then is rolled by a rubber roll, and finally rolled to obtain a polypropylene initial film; wherein the casting roll temperature is 130° C., the casting time is 1 min; the annealing roll temperature is 135° C., the annealing time is 1 min, and the rubber roll temperature is 70° C.

[0071] The initial polypropylene membrane was cut into a 94mm×94mm square, and then subjected to synchronous biaxial stretching on a KARO IV testing machine produced by Brückner of Germany. The stretching temperature was 135°C, the stretching ratio in the MD direction was 3 times, and the stretching ratio in the TD direction was 4 times. Finally, the sample was post-treated and annealed at 140°C for 1 minute to obtain a porous polypropylene membrane.

[0072] Example 2

[0073] A porous polypropylene membrane was prepared according to the method of Example 1, except that the post-treatment annealing temperature was 130° C. and the time was 3 min.

[0074] Example 3

[0075] A porous polypropylene membrane was prepared according to the method of Example 1, except that the post-treatment annealing temperature was 110° C. and the time was 5 min.

[0076] Example 4

[0077] A porous polypropylene membrane was prepared according to the method of Example 1, except that the post-treatment annealing temperature was 150° C. and the time was 0.5 min.

[0078] Example 5

[0079] A porous polypropylene membrane was prepared according to the method of Example 1, except that the temperature of the annealing roller was 100°C.

[0080] Example 6

[0081] A porous polypropylene membrane was prepared according to the method of Example 1, except that the temperature of the annealing roller was 140°C.

[0082] Example 7

[0083] A porous polypropylene membrane was prepared according to the method of Example 1, except that the formula was different from that of Example 1: the ratio of polypropylene resin, N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide, glycerol monostearate and antioxidant 1010 was 96.5:3:0.2:0.3. After testing, the micropore diameter, air permeability and contact angle with water / electrolyte parameters were similar to those of Example 1.

[0084] Example 8

[0085] A porous polypropylene membrane was prepared according to the method of Example 1. The difference from Example 1 is that the formulation is different: the polypropylene resin is an α-homopolymer polypropylene resin with a low ash content (<0.02wt‰) and an isotacticity index of 97wt% and a melt flow rate of 3.0g / 10min at 230°C and a load of 2.16kg. After testing, the micropore diameter, air permeability and contact angle with water / electrolyte parameters are similar to those of Example 1.

[0086] Example 9

[0087] A porous polypropylene membrane was prepared according to the method of Example 1, and the difference from Example 1 was that the formula was different: the β-nucleating agent was a composite nano-powder rubber, and the composite nano-powder rubber was a mixture of sulphurized butadiene styrene nano-powder rubber and N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide in a weight ratio of 1:4. After testing, the micropore diameter, air permeability and contact angle with water / electrolyte parameters were similar to those in Example 1.

[0088] Comparative Example 1

[0089] A porous polypropylene membrane was prepared according to the method of Example 1. The difference from Example 1 is that commercially available ordinary α-homopolypropylene was used. The polypropylene had an isotacticity index of 98 wt %, a melt flow rate of 3.0 g / 10 min at 230° C. and a load of 2.16 kg, and an ash content of 0.5 wt‰.

[0090] Comparative Example 2

[0091] A porous polypropylene membrane was prepared according to the method of Example 1, except that no post-treatment annealing was performed.

[0092] Comparative Example 3

[0093] A porous polypropylene membrane was prepared according to the method of Example 1, except that the post-treatment annealing time was 20 min.

[0094] Comparative Example 4

[0095] A porous polypropylene membrane was prepared according to the method of Example 1, except that the membrane was not subjected to the annealing roller and the rubber roller.

[0096] Comparative Example 5

[0097] A porous polypropylene membrane was prepared according to the method of Example 1, except that the membrane was not subjected to the annealing roller.

[0098] Comparative Example 6

[0099] A porous polypropylene membrane was prepared according to the method of Example 1, except that the temperature of the annealing roller was 90°C.

[0100] Comparative Example 7

[0101] A porous polypropylene membrane was prepared according to the method of Example 1, except that the temperature of the annealing roller was 150°C.

[0102] Comparative Example 8

[0103] A porous polypropylene membrane was prepared according to the method of Example 1. The difference from Example 1 is that commercially available ordinary α-homopolymer polypropylene was used. The polypropylene has an isotacticity index of 92 wt %, a melt flow rate of 3.0 g / 10 min at 230° C. and a load of 2.16 kg, and an ash content of 0.5 wt‰.

[0104] Comparative Example 9

[0105] A porous polypropylene membrane was prepared according to the method of Example 1, except that no β-nucleating agent was added.

[0106] The porous polypropylene membranes prepared in Examples 1 to 9 and Comparative Examples 1 to 9 were tested, and the results of some of the Examples and Comparative Examples are shown in Table 1.

[0107] Table 1

[0108]

[0109]

[0110] From the comparison between Examples 1 to 6 and Comparative Examples 1 to 9, it can be seen that the porous polypropylene membrane provided by the present invention has the characteristics of being hydrophobic and electrolyte-friendly, having low ash content and having large micropore diameters. By selecting the specific low-ash α-homopolypropylene of the present invention as a raw material, combining with appropriate additives, and controlling the annealing conditions of the film after casting and the post-treatment annealing conditions, the hydrophobic, electrolyte-friendly and low-ash properties of the diaphragm can be achieved, and the pore size of the micropores of the porous polypropylene membrane can be regulated, and the pore structure can be stabilized to adapt to applications in the fields of lithium-ion batteries and supercapacitors.

[0111] As shown in Table 1, from Example 1 and Comparative Example 1, it can be seen that the use of the specific α-homopolypropylene of the present invention as a raw material can effectively reduce the ash content of the diaphragm, improve the hydrophobicity of the diaphragm, and improve the electrolyte affinity of the diaphragm. Better hydrophobicity makes the diaphragm more conducive to long-term storage in an environment with high humidity, and better wettability with electrolytes is conducive to improving the liquid retention capacity of the diaphragm and reducing leakage of batteries or supercapacitors.

[0112] It can be seen from Example 1, Example 2, Example 3, Example 4 and Comparative Example 2, Comparative Example 3 that post-treatment annealing can change the pore size of the diaphragm and its wettability with the electrolyte, and the post-treatment annealing temperature and annealing time can be adjusted in coordination. If a higher temperature is used, the time can be appropriately reduced. However, if the annealing time is too long or the temperature is too high, it will cause melting and recrystallization of the α crystals in the diaphragm, thereby destroying the pore structure and reducing the wettability of the diaphragm with the electrolyte.

[0113] It can be seen from Example 1, Example 5, Example 6 and Comparative Examples 4, Comparative Example 5, Comparative Example 6 and Comparative Example 7 that annealing can improve the perfection of β crystals, but when the annealing temperature is lower than 90°C, it is not conducive to the melt recrystallization of β crystals, and when the annealing temperature is higher than 140°C, it is not conducive to the formation of β crystals during recrystallization.

[0114] It can be seen from Example 1 and Comparative Examples 8 and 9 that the target material of the present invention can only be obtained by suitable formula and process coordination. When low-isotactic α-homopolypropylene is used, due to the low crystallinity of low-isotactic polypropylene, the number of α crystals available for conversion into β crystals is small, which is not conducive to the formation of β crystals during annealing and post-treatment annealing, resulting in fewer micropores and smaller pore sizes, and thus poor air permeability. When no β nucleating agent is added, even if the same process is used, β crystals cannot be produced, resulting in the final polypropylene film having no microporous structure.

[0115] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0116] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0117] It should be noted that the embodiments described above are only used to explain the present invention and do not constitute any limitation to the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than restrictive words. The present invention may be modified as specified within the scope of the claims of the present invention, and the present invention may be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein, on the contrary, the present invention can be extended to all other methods and applications with the same functions.

[0118] All publications, patent applications, patents and other references mentioned in this specification are incorporated herein by reference. Unless otherwise defined, all technical and scientific terms used in this specification have the meanings conventionally understood by those skilled in the art. In the event of a conflict, the definition in this specification shall prevail.

[0119] When this specification uses the prefix "well-known to those skilled in the art", "prior art" or similar terms to introduce materials, substances, methods, steps, devices or components, etc., the objects introduced by the prefix cover those commonly used in the art when this application is filed, but also include those that are not commonly used at present but will become recognized in the art as being suitable for similar purposes.

[0120] The endpoints and any values ​​of the scope disclosed in the present application document are not limited to the precise scope or value, and these scopes or values ​​should be understood to include values ​​close to these scopes or values. For numerical ranges, between the endpoint values ​​of each scope, between the endpoint values ​​of each scope and a separate point value, and between separate point values, one or more new numerical ranges can be combined with each other, and these numerical ranges should be considered as specifically disclosed in this article. Hereinafter, in principle, each technical solution can be combined with each other to obtain a new technical solution, which should also be considered as specifically disclosed in this article.

[0121] In the context of the present specification, any matters or items not mentioned are directly applicable to those known in the art without any changes except those explicitly stated.

[0122] Moreover, any embodiment described in this document may be freely combined with one or more other embodiments described in this document, and the technical solutions or technical ideas formed thereby are deemed to be part of the original disclosure or original record of the present invention, and should not be regarded as new content that has not been disclosed or anticipated in this document, unless a person skilled in the art considers that the combination is obviously unreasonable.

Claims

1. A method for preparing a porous polypropylene film, the method comprising the following steps: (1) melt-mixing raw materials including a polypropylene resin, a β-nucleating agent, an antioxidant, and an optional film-forming aid to obtain a mixed melt; (2) subjecting the mixed melt to casting, annealing, and rolling in sequence to obtain an initial film; wherein the annealing temperature is 80 to 140° C., and the annealing time is 0.5 to 20 min; (3) subjecting the initial membrane sheet to directional stretching and post-treatment annealing to obtain the porous polypropylene membrane; wherein the temperature of the post-treatment annealing is 90 to 160° C.; and the time of the post-treatment annealing is 0.1 to 10 min; The polypropylene resin is selected from α-homopolypropylene; according to GB / T9345.1-2008, the ash content of the polypropylene resin is less than 0.1wt‰; according to GB / T3682.1-2018, the melt flow rate of the polypropylene resin at 230°C and 2.16kg load is 0.5-20g / 10min; according to GB / T2412-2008, the isotacticity index of the polypropylene resin is 95-99wt%.

2. The preparation method according to claim 1, characterized in that: The step (1) comprises: firstly mixing raw materials including polypropylene resin, β-nucleating agent, antioxidant and optional film-forming aid to obtain a mixture, and then melt-mixing the mixture; Preferably, the melt mixing is carried out by melt extrusion in a twin-screw extruder; More preferably, the melt extrusion temperature is 180 to 270°C, preferably 190 to 260°C, and further preferably 200 to 250°C.

3. The preparation method according to claim 1, characterized in that: In the step (2): The annealing temperature is 100-140° C., and / or the annealing time is 1-15 min. The casting temperature is 100 to 140° C., preferably 110 to 140° C., and / or the casting time is 0.5 to 20 min, preferably 1 to 15 min; and / or, The rolling temperature is 40 to 90° C., preferably 50 to 90° C.; and / or, The step (2) is performed in the following manner: the mixed melt is sequentially cast by a casting roller, annealed by an annealing roller, rolled by a rubber roller, and then rolled by a winding roller to obtain an initial film sheet.

4. The preparation method according to claim 1, characterized in that: In the step (3): The temperature of the post-treatment annealing is 100-160° C., preferably 100-150° C.; and / or, the time of the post-treatment annealing is 0.2-10 min, preferably 0.5-5 min; and / or, The temperature of the directional stretching is 60 to 140° C., preferably 80 to 140° C., and more preferably 100 to 140° C.; and / or, The directional stretching is unidirectional stretching or bidirectional stretching, preferably one of simultaneous bidirectional stretching or stepwise bidirectional stretching; preferably: The direction of uniaxial stretching is the MD direction, and the directions of biaxial stretching are the MD direction and the TD direction; more preferably, the stretching ratio in the MD direction of the uniaxial stretching is 0.5 to 5 times; The synchronous biaxial stretching has a stretching ratio of 0.5 to 5 times in the MD direction and a stretching ratio of 0.5 to 5 times in the TD direction; The stepwise biaxial stretching has a stretching ratio of 0.5 to 5 times in the MD direction and a stretching ratio of 0.5 to 5 times in the TD direction.

5. The preparation method according to claim 1, characterized in that: The polypropylene resin has at least one of the following characteristics: According to GB / T9345.1-2008 test, the ash content of the polypropylene resin is less than 0.05wt‰; and / or, According to GB / T3682.1-2018, the melt flow rate of the polypropylene resin at 230°C and 2.16 kg load is 0.5 to 10 g / 10 min, preferably 1 to 10 g / 10 min; and / or, According to GB 2412-2008 test, the isotacticity index of the polypropylene resin is 97-99 wt %.

6. The preparation method according to claim 1, characterized in that: The β-nucleating agent is at least one of a single-component organic small molecule, a multi-component inorganic salt and a composite nano-powder rubber; Preferably, the single-component organic small molecule is at least one of N,N'-diphenyl adipamide, N,N'-dicyclohexyl terephthalamide and N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide; and / or, Preferably, the multi-component inorganic salt is at least two of hydrazine salt of adipic acid, hydrazine salt of suberic acid, calcium salt of pimelic acid, and calcium salt of suberic acid.

7. The preparation method according to claim 6, characterized in that: The composite nanometer powder rubber contains nanometer powder rubber and N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide; Preferably, the mass ratio of the nano-powder rubber to N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide is 1:(1-9); and / or, Preferably, the nano powder rubber is vulcanized powder rubber and / or non-vulcanized powder rubber; the vulcanized powder rubber is preferably at least one of vulcanized natural rubber, vulcanized styrene butadiene rubber, vulcanized nitrile rubber, vulcanized chloroprene rubber, vulcanized polybutadiene rubber, vulcanized polyacrylate rubber, vulcanized styrene butadiene rubber, vulcanized isoprene rubber, vulcanized ethylene propylene rubber and vulcanized polyurethane rubber; the non-vulcanized powder rubber is preferably at least one of cross-linked styrene butadiene powder rubber, cross-linked polybutadiene powder rubber, cross-linked nitrile powder rubber, cross-linked chloroprene powder rubber and cross-linked acrylate powder rubber.

8. The preparation method according to claim 1, characterized in that: The antioxidant is at least one of phenolic antioxidants, and the phenolic antioxidant is preferably at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and n-octadecylβ-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate.

9. The preparation method according to any one of claims 1 to 8, characterized in that: In parts by weight, based on 100 parts of the total amount of the raw materials, the amount of the β-nucleating agent is 0.3 to 4 parts, preferably 0.3 to 3 parts; more preferably 0.3 to 2 parts, and the amount of the antioxidant is 0.1 to 2 parts; preferably 0.1 to 1.5 parts; more preferably 0.1 to 1 part.

10. The preparation method according to any one of claims 1 to 8, characterized in that: The film-making aid includes an antistatic agent and an optional opening agent; Preferably, based on 100 parts of the total amount of the raw materials, the amount of the antistatic agent is 0.1 to 2 parts, preferably 0.1 to 1.5 parts; the amount of the opening agent is 0 to 2 parts, preferably 0.1 to 2 parts; and / or, Preferably, the antistatic agent is at least one of nonionic antistatic agents, preferably, the nonionic antistatic agent is at least one of ethoxylated fatty alkylamine, ethoxylated alkyl acid amine and glycerol monostearate; and / or, Preferably, the opening agent is at least one of an inorganic and an organic opening agent; preferably, the inorganic opening agent is at least one of talc, diatomaceous earth and synthetic silica, and / or the organic opening agent is at least one of oleamide, erucamide and an EBS derivative.

11. A porous polypropylene film, preferably the porous polypropylene film is prepared by the preparation method according to any one of claims 1 to 10, The porous polypropylene membrane is distributed with through micropores, and the average diameter of the narrow side of the micropores is 0.05-1 μm; the air permeability of the porous polypropylene membrane is 200-700s / 100ml, preferably 300-500s / 100ml, according to GB / T1038-2000 test; According to GB / T30693-2014 test, the contact angle between the porous polypropylene membrane and water is not less than 100°, and the contact angle between the porous polypropylene membrane and the electrolyte is not more than 15°.

12. The porous polypropylene membrane according to claim 11, characterized in that: The thickness of the porous polypropylene film is 10 to 100 μm, preferably 10 to 40 μm, and more preferably 14 to 30 μm; and / or, According to GB / T9345.1-2008 testing, the ash content of the porous polypropylene membrane is less than 0.1wt‰, preferably less than 0.05wt‰.

13. Use of the porous polypropylene film according to claim 11 or 12 as a separator, preferably as a separator for batteries and / or capacitors.

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