A method for preparing a biaxially stretched microporous membrane

Abstract

The present application provides a method for preparing a biaxially oriented microporous membrane, comprising: uniformly mixing a filled polypropylene composite material with a polypropylene resin, placing the mixture into an extrusion system, and casting the mixture to obtain a cast membrane; and annealing the cast membrane, performing a heat treatment composite, stretching, and heat setting the cast membrane to obtain the polypropylene microporous membrane; the filled polypropylene composite material comprises the following raw materials in parts by weight: 60-70 parts by weight of polypropylene resin, 10-20 parts by weight of cobalt-aluminum hydrotalcite, 10-20 parts by weight of a compatibilizer, and 1-3 parts by weight of a beta-nucleating agent. The polypropylene microporous membrane obtained by this preparation method has improved elastic recovery, tensile strength, porosity, and thermal shrinkage.

Description

Technical Field

[0001] The present application relates to the technical field of polymer material plastic films, and in particular to a method for preparing a biaxially oriented microporous membrane. Background Art

[0002] Polypropylene (PP) microporous membranes, due to their unique microporous structure and excellent overall performance, have shown broad application potential in numerous fields, such as battery separators, filter media, and medical supplies. Among current energy storage technologies, lithium-ion batteries have become the preferred power source for portable electronic devices and electric vehicles due to their high energy density, low self-discharge rate, and good cycle performance. As one of the key components of lithium-ion batteries, the performance of polypropylene (PP) microporous membranes directly affects the safety, energy density, and cycle life of the batteries. Therefore, improving the key performance indicators of PP microporous membranes, such as elastic recovery, tensile strength, porosity, and thermal shrinkage, is of great significance for improving the overall performance of lithium-ion batteries. Summary of the Invention

[0003] In order to solve at least one of the above technical problems, in a first aspect of the present application, the present application provides a method for preparing a biaxially oriented microporous membrane, comprising:

[0004] The filled polypropylene composite material and the polypropylene resin are uniformly mixed, put into an extrusion system, and cast into a film to obtain a cast film; the cast film is annealed, heat-treated, composited, stretched, and heat-set to obtain the polypropylene microporous membrane;

[0005] The filled polypropylene composite material comprises the following raw materials in parts by weight: 60-70 parts by weight of polypropylene resin, 10-20 parts by weight of cobalt aluminum hydrotalcite, 10-20 parts by weight of a compatibilizer, and 1-3 parts by weight of a beta nucleating agent.

[0006] Through the above technical solution, the addition of cobalt-aluminum hydrotalcite may improve the interactions between polypropylene molecular chains and the local stress distribution in the polypropylene matrix, thereby enhancing the elastic recovery ability of the polypropylene microporous membrane. The layered structure of cobalt-aluminum hydrotalcite may hinder the stacking of polypropylene molecular chains, thereby increasing the porosity. The thermal stability of cobalt-aluminum hydrotalcite may help reduce the shrinkage of polypropylene during heating. The addition of cobalt-aluminum hydrotalcite may also promote the crystallization of polypropylene molecular chains, thereby increasing the tensile strength of the polypropylene microporous membrane.

[0007] Preferably, in the raw materials of the filled polypropylene composite material, the weight ratio of the polypropylene resin to the cobalt-aluminum hydrotalcite is (3-7):1.

[0008] By adopting the above technical solution, the weight ratio of the polypropylene resin to the cobalt-aluminum hydrotalcite can be controlled to obtain a polypropylene microporous membrane with better performance.

[0009] Preferably, the mass ratio of the filled polypropylene composite material to the polypropylene resin is 1:(9-11).

[0010] Through the above technical solution, the mass ratio of the filled polypropylene composite material to the polypropylene resin is limited, which facilitates the formation of effective stress transfer. At the same time, it can avoid polypropylene aggregation, reduce internal defects, and thus improve the performance of the polypropylene microporous membrane.

[0011] Preferably, the filled polypropylene composite material is prepared by the following method: uniformly mixing polypropylene resin, cobalt aluminum hydrotalcite, a compatibilizer, and a β-nucleating agent, extruding, granulating, and drying to obtain the filled polypropylene composite material.

[0012] Preferably, the extrusion is carried out in an extruder, and the temperatures of zones 1-5 of the extruder are controlled to be 200° C., 210° C., 220° C., 210° C. and 200° C., respectively.

[0013] Through the above technical solution, the filled polypropylene composite material is easy to prepare and the raw materials are easily available.

[0014] Preferably, the preparation method of cobalt aluminum hydrotalcite comprises: dissolving cobalt nitrate, aluminum nitrate and urea in water, reacting at 140-160° C. for 10-15 hours, cooling, filtering, washing to neutrality, drying and grinding to obtain the cobalt aluminum hydrotalcite.

[0015] Preferably, among the raw materials of the filled polypropylene composite material, the polypropylene resin is a modified polypropylene resin, and the preparation method of the modified polypropylene resin comprises: step a: uniformly stirring the polypropylene resin, initiator and acrylic acid to obtain a blend; step b: melting, plasticizing, shearing and granulating the blend obtained in step a to obtain acrylic acid-grafted polypropylene.

[0016] Preferably, the preparation method of the modified polypropylene resin further comprises: step c: soaking the acrylic acid grafted polypropylene obtained in step b in a cyclodextrin aqueous solution, filtering, washing, and drying to obtain the modified polypropylene resin.

[0017] Preferably, the cyclodextrin is at least one selected from α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, methylated cyclodextrin, and hydroxypropylated cyclodextrin.

[0018] The above technical solution improves the compatibility and dispersibility of polypropylene resin, helping to form a more uniform pore structure and thus increasing porosity. At the same time, the modification may increase the thermal stability of polypropylene and reduce shrinkage during heat treatment. Furthermore, the modification may increase hydrogen bonding or other polar interactions between polypropylene molecular chains, which helps to improve the material's elastic recovery and tensile strength.

[0019] Preferably, the cyclodextrin is β-cyclodextrin and methylated cyclodextrin, and the concentrations of β-cyclodextrin and methylated cyclodextrin are both 1 wt%.

[0020] Preferably, the compatibilizer is POEgMA.

[0021] In the second aspect of the present application, the present application provides a polypropylene microporous membrane prepared by the method for preparing a biaxially oriented microporous membrane described in the first aspect of the present application.

[0022] In summary, the present invention includes at least one of the following beneficial technical effects:

[0023] 1. The present application provides a method for preparing a biaxially oriented microporous membrane, wherein a filled polypropylene composite material is mixed with a polypropylene resin, and the filled polypropylene composite material uses cobalt-aluminum hydrotalcite as an inorganic filler. After the polypropylene resin is modified, the performance of the obtained polypropylene microporous membrane is further improved;

[0024] 2. The present application provides a polypropylene microporous membrane obtained by the above-mentioned method for preparing a biaxially oriented microporous membrane. The elastic recovery rate, tensile strength, porosity and thermal shrinkage rate of the obtained polypropylene microporous membrane are better than those of the prior art. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with specific examples, but the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. In the following examples, if no specific conditions are specified, the conditions according to normal conditions or manufacturer's recommendations are carried out, and the method used, if not otherwise specified, is a conventional method well known in the art, and the consumable materials and reagents used, if not otherwise specified, are commercially available. Unless otherwise indicated, the professional and scientific terms used herein are identical in meaning to those skilled in the art. In addition, any method or material similar or equivalent to the described content can also be applied to the present invention. Specific embodiments

[0027] In the embodiment, the β nucleating agent is a lithium battery separator nucleating agent purchased from Shenzhen Texuan Technology Co., Ltd., and the compatibilizer is POE-g-MA purchased from Dongguan Xingyuan Chemical Co., Ltd.

[0028] Example 1

[0029] The method for preparing a biaxially oriented microporous membrane comprises the following steps:

[0030] Step 1): Preparation of a filled polypropylene composite material: Weigh 65 parts by weight of polypropylene resin, 15 parts by weight of cobalt aluminum hydrotalcite, 15 parts by weight of a compatibilizer, and 2 parts by weight of a β-nucleating agent; the compatibilizer is POEgMA;

[0031] Step 2): The above raw materials in parts by weight are mixed uniformly in a high-speed mixer, added to a twin-screw extruder for extrusion, granulation, and drying. The aspect ratio of the twin-screw extruder is 40:1, the temperatures of zones 1-5 of the extruder are 200° C., 210° C., 220° C., 210° C., and 200° C., respectively, and the screw speed is 300 r / min to obtain a filled polypropylene composite material;

[0032] Step 3): The filled polypropylene composite material obtained in step 2) was mixed with a polypropylene resin in a mass ratio of 1:10, and the mixture was put into an extrusion system at an extrusion temperature of 240° C. and a casting temperature of 100° C. at a production speed of 70 m / min to obtain a cast film. The cast film had a thickness of 8.9 μm.

[0033] Step 4): annealing the cast film obtained in step 3) at 140° C. for 10 h;

[0034] Step 5): The annealed cast films are stacked in two layers and placed in a composite device for heat treatment and composite. The heat treatment temperature is 150°C, the time is 10 minutes, and the composite speed is 50 m / min.

[0035] Step 6): The cast film compounded in step 5) is stretched transversely at 145° C. with a transverse stretch ratio of 3 times, stretched longitudinally at 155° C. with a longitudinal stretch ratio of 3 times, and heat-set at 160° C. to obtain a polypropylene microporous membrane.

[0036] The obtained polypropylene microporous membrane had a thickness of 16 μm.

[0037] The weight average molecular weight of polypropylene resin is 5×10 5 The isotactic resin has a melt index of 0.5g / 10min.

[0038] The preparation method of cobalt aluminum hydrotalcite comprises the following steps:

[0039] The preparation method of cobalt aluminum hydrotalcite comprises the following steps: dissolving cobalt nitrate, aluminum nitrate (0.8 g) and urea in 80 mL of deionized water at 25° C., wherein the molar ratio of cobalt nitrate to aluminum nitrate is 2:1, and the amount of urea is 3.3 times the total amount of metal ions; transferring the obtained solution into a reactor, placing the reactor in a 150° C. constant temperature box for reaction; taking out the reactor after 12 hours; filtering the product after cooling, washing it to neutrality, drying it, and grinding it to obtain the cobalt aluminum hydrotalcite.

[0040] Example 2

[0041] The only difference between Example 2 and Example 1 is that in step 1), the weight parts of the raw materials for filling the polypropylene composite material are different. Example 2 is 60 parts by weight of polypropylene resin, 20 parts by weight of cobalt aluminum hydrotalcite, 15 parts by weight of compatibilizer, and 2 parts by weight of β-nucleating agent.

[0042] Example 3

[0043] The only difference between Example 3 and Example 1 is that in step 1), the weight parts of the raw materials for filling the polypropylene composite material are different. Example 2 is 70 parts by weight of polypropylene resin, 10 parts by weight of cobalt aluminum hydrotalcite, 15 parts by weight of compatibilizer, and 2 parts by weight of β-nucleating agent.

[0044] Example 4

[0045] The only difference between Example 4 and Example 1 is that in step 3), the mass ratio of the filled polypropylene composite material to the polypropylene resin is 1:9.

[0046] Example 5

[0047] The only difference between Example 5 and Example 1 is that in step 3), the mass ratio of the filled polypropylene composite material to the polypropylene resin is 1:11.

[0048] Example 6

[0049] The only difference between Example 6 and Example 1 is that when preparing the filled polypropylene composite material in step 1) and step 2), the raw polypropylene resin is replaced by modified polypropylene resin in equal mass.

[0050] The preparation method of modified polypropylene resin is as follows:

[0051] Step a: 100 parts of polypropylene resin, 0.4 parts of initiator DCP and 15 parts of acrylic acid are stirred uniformly to obtain a blend;

[0052] Step b: feeding the blend obtained in step a into a twin-screw extruder, and subjecting the mixture to melting, plasticizing, shearing, and granulation to obtain acrylic acid-grafted polypropylene;

[0053] Step c: soaking the acrylic acid grafted polypropylene obtained in step b in a 2 wt% β-cyclodextrin aqueous solution for 3 hours, filtering, washing, and drying to obtain a modified polypropylene resin.

[0054] Example 7

[0055] The difference between this embodiment and embodiment 6 is that the mass of β-cyclodextrin is replaced by α-cyclodextrin.

[0056] Example 8

[0057] The difference between this embodiment and embodiment 6 is that the mass of β-cyclodextrin is replaced by γ-cyclodextrin.

[0058] Example 9

[0059] The difference between this embodiment and embodiment 6 is that the mass of β-cyclodextrin is replaced by methylated cyclodextrin.

[0060] Example 10

[0061] The difference between this embodiment and embodiment 6 is that the mass of β-cyclodextrin is replaced by hydroxypropylated cyclodextrin.

[0062] Example 11

[0063] The difference between this embodiment and embodiment 5 is that in step c: the acrylic acid grafted polypropylene obtained in step b is immersed in an aqueous solution containing 1 wt% β-cyclodextrin and 1 wt% methylated cyclodextrin for 3 hours, filtered, washed, and dried to obtain a modified polypropylene resin.

[0064] Example 12

[0065] The only difference between Example 12 and Example 1 is that when preparing the filled polypropylene composite material in step 1) and step 2), the raw material polypropylene resin is replaced by acrylic acid grafted polypropylene in mass.

[0066] The preparation method of acrylic acid grafted polypropylene is as follows:

[0067] Step a: 100 parts of polypropylene resin, 0.4 parts of initiator DCP and 15 parts of acrylic acid are stirred evenly to obtain a blend; Step b: the blend obtained in step a is put into a twin-screw extruder, and after melting, plasticizing, shearing and granulating, acrylic acid-grafted polypropylene is obtained.

[0068] Comparative Example 1

[0069] The only difference between Comparative Example 1 and Example 1 is that the cobalt aluminum hydrotalcite of equal mass is replaced by commercially available magnesium aluminum hydrotalcite (purchased from Nantong Aidewang Chemical Co., Ltd., model: HT-33).

[0070] Experimental testing

[0071] The polypropylene microporous membranes prepared in Examples 1-12 and Comparative Examples 1-4 were tested for elastic recovery according to GB / T 17794-2008, tensile strength and porosity according to GB / T 36363-2018, and thermal shrinkage according to GB / T 10003-2008.

[0072] The results are shown in Table 1.

[0073] Table 1 Test results of polypropylene microporous membranes prepared in Examples and Comparative Examples

[0074]

[0075]

[0076] From the test results of Comparative Example 1 and Example 1 combined with Table 1, it can be seen that when the inorganic filler cobalt aluminum hydrotalcite is replaced with commercially available magnesium aluminum hydrotalcite, the elastic recovery rate, tensile strength, porosity and thermal shrinkage rate of the polypropylene microporous membrane are all deteriorated.

[0077] From the test results of Examples 2 and 3 combined with Example 1 in Table 1, it can be seen that when the ratio of polypropylene resin to cobalt-aluminum hydrotalcite in the raw materials for filling the polypropylene composite material changes, the elastic recovery rate, tensile strength, porosity, and thermal shrinkage rate of the polypropylene microporous membrane change. The weight ratio of the polypropylene resin to the cobalt-aluminum hydrotalcite is preferably (3-7):1.

[0078] From the test results of Examples 4 and 5 combined with Example 1 in Table 1, it can be seen that when preparing a polypropylene microporous membrane, when the ratio of the filled polypropylene composite material to the polypropylene resin changes, the elastic recovery rate, tensile strength, porosity, and thermal shrinkage rate of the polypropylene microporous membrane change. The mass ratio of the filled polypropylene composite material to the polypropylene resin is preferably 1:(9-11).

[0079] From the test results of Example 6 and Example 1 combined with Table 1, it can be seen that when preparing the filled polypropylene composite material, the raw material polypropylene resin is replaced with modified polypropylene resin, and the elastic recovery rate, tensile strength, porosity and thermal shrinkage rate of the polypropylene microporous membrane are improved.

[0080] From the test results of Examples 7, 8, 9, and 10 combined with Example 6 in Table 1, it can be seen that when different types of cyclodextrin are used to prepare the modified polypropylene resin, the elastic recovery rate, tensile strength, porosity, and thermal shrinkage rate of the polypropylene microporous membrane will also be different, among which β-cyclodextrin and methylated cyclodextrin have better effects.

[0081] From the test results of Example 11, Examples 6 and 9 combined with Table 1, it can be seen that the simultaneous use of β-cyclodextrin and methylated cyclodextrin has a better effect.

[0082] From the test results of Example 12 and Example 6 combined with Table 1, it can be seen that acrylic acid grafted onto polypropylene and then further modified with cyclodextrin has a better effect.

[0083] The above are all preferred embodiments of the present application, and are not intended to limit the scope of protection of the present application. Therefore, any equivalent changes made based on the structure, shape, and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A method for preparing a biaxially oriented microporous membrane, characterized in that: include: The filled polypropylene composite material is mixed evenly with the polypropylene resin, and the mixture is put into an extrusion system and cast into a film to obtain a cast film; The cast film is subjected to annealing treatment, heat treatment compounding, stretching, and heat setting to obtain the biaxially oriented microporous film; The filled polypropylene composite material comprises the following raw materials in parts by weight: 60-70 parts by weight of polypropylene resin, 10-20 parts by weight of cobalt aluminum hydrotalcite, 10-20 parts by weight of compatibilizer, and 1-3 parts by weight of beta nucleating agent; In the raw materials of the filled polypropylene composite material, the weight ratio of the polypropylene resin to the cobalt-aluminum hydrotalcite is (3-7):1; The preparation method of the cobalt aluminum hydrotalcite comprises: dissolving cobalt nitrate, aluminum nitrate and urea in water, reacting at 140-160° C. for 10-15 hours, cooling, filtering, washing to neutrality, drying and grinding to obtain the cobalt aluminum hydrotalcite.

2. The method for preparing a biaxially oriented microporous membrane according to claim 1, wherein The mass ratio of the filled polypropylene composite material to the polypropylene resin is 1:(9-11).

3. The method for preparing a biaxially oriented microporous membrane according to claim 1, wherein The filled polypropylene composite material is prepared by the following method: uniformly mixing polypropylene resin, cobalt aluminum hydrotalcite, a compatibilizer, and a beta nucleating agent, and then extruding, granulating, and drying to obtain the filled polypropylene composite material.

4. The method for preparing a biaxially oriented microporous membrane according to claim 1, wherein Among the raw materials of the filled polypropylene composite material, the polypropylene resin is a modified polypropylene resin. The preparation method of the modified polypropylene resin includes: step a: uniformly stirring the polypropylene resin, an initiator and acrylic acid to obtain a blend; step b: melting, plasticizing, shearing and granulating the blend obtained in step a to obtain acrylic acid-grafted polypropylene.

5. The method for preparing a biaxially oriented microporous membrane according to claim 4, wherein: The preparation method of the modified polypropylene resin further comprises: step c: soaking the acrylic acid grafted polypropylene obtained in step b in a cyclodextrin aqueous solution, filtering, washing, and drying to obtain the modified polypropylene resin.

6. The method for preparing a biaxially oriented microporous membrane according to claim 5, wherein: The cyclodextrin is selected from at least one of α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, methylated cyclodextrin, and hydroxypropylated cyclodextrin.

7. The method for preparing a biaxially oriented microporous membrane according to claim 6, wherein The cyclodextrins are β-cyclodextrin and methylated cyclodextrin, and the concentrations of β-cyclodextrin and methylated cyclodextrin are both 1 wt %.

8. A biaxially oriented microporous membrane prepared by the method for preparing a biaxially oriented microporous membrane according to any one of claims 1 to 7.

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