A polypropylene microporous separator and a method for preparing the same

Abstract

The application discloses a kind of polypropylene microporous diaphragm and its preparation method, wherein preparation method includes the following steps: (1) melt extrusion: polypropylene raw material is heated and melted after extrusion to form melt;(2) casting: the melt is drawn by chilling roller, longitudinal casting is carried out while using extension roller on the melt upper surface to roll the melt and make it transverse extension to obtain film;(3) annealing: the film is annealed to obtain single-layer film;(4) stretching: the single-layer film is stacked to obtain multilayer composite film, then the multilayer composite film is stretched, and after shrinkage setting, delamination, the polypropylene microporous diaphragm is obtained.The polypropylene microporous diaphragm prepared by the preparation method has excellent transverse tensile strength and puncture strength, which can effectively improve the safety of battery.

Description

Technical Field

[0001] This invention belongs to the field of battery separator technology, and specifically relates to a polypropylene microporous separator and its preparation method. Background Technology

[0002] The separator is one of the key internal components of a lithium-ion battery. Its main function is to isolate the positive and negative electrodes and prevent electrons from passing through, while allowing ions to pass through, thus enabling the rapid transport of lithium ions between the positive and negative electrodes during charging and discharging. Among the four key materials of lithium batteries, the separator material plays a crucial role in ensuring battery safety. The thinner the separator and the higher its porosity, the lower the battery's internal resistance and the better its high-rate discharge performance. However, this may also reduce battery safety. Therefore, striking a balance between performance and safety has always been a core issue in the research and development of separator industrialization technology.

[0003] Existing battery separators are mainly produced using a dry single-stretch process. The principle behind the current dry single-stretch process for preparing polypropylene microporous separators is to form a lamellar structure under tensile stress during extrusion, and then utilize crystal growth and stretching to form pores during subsequent annealing and stretching processes. However, the existing dry single-stretch process only stretches the polypropylene material longitudinally, resulting in polypropylene microporous separators with very low transverse tensile strength and puncture strength, which is detrimental to the safe use of batteries. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a polypropylene microporous membrane and its preparation method. The polypropylene microporous membrane prepared by this method exhibits excellent transverse tensile strength and puncture strength, effectively improving battery safety.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0006] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt; (2) casting: drawing the melt through a chilling roller and casting it longitudinally while rolling the melt on the upper surface of the melt with an extension roller to make it extend laterally to obtain a membrane; (3) annealing: annealing the membrane to obtain a single-layer membrane; (4) stretching: stacking the single-layer membrane to obtain a multi-layer composite membrane, stretching the multi-layer composite membrane, shrinking and shaping it, and peeling it off layer by layer to obtain the polypropylene microporous membrane.

[0007] Preferably, in step (1), the melt index of the polypropylene raw material is 0.9-1.0 g / 10 min.

[0008] Preferably, in step (1), the heating and melting temperature is 200-220°C.

[0009] Preferably, in step (1), the extrusion rate is 80-100 kg / h.

[0010] Preferably, in step (2), the temperature of the chilling roller is 65-75°C, and the traction ratio of the chilling roller is 8000%-11000%.

[0011] Preferably, in step (2), the rolling pressure of the extended roller is 5-6.5 MPa.

[0012] Preferably, in step (2), the surface of the extension roller is provided with a threaded groove.

[0013] Preferably, in step (3), the annealing temperature is 120-128°C, the annealing time is 12-18h, and the annealing method is to heat the film with circulating hot air in an oven.

[0014] Preferably, in step (4), the stretching includes cold stretching and hot stretching. The temperature of the cold stretching is 40-50℃ and the ratio is 110%-125%. The temperature of the hot stretching is 136℃-145℃ and the ratio is 280%-320%.

[0015] Preferably, in step (4), the temperature for shrinkage and shaping is 150-155°C, and the longitudinal shrinkage ratio is 22%-35%.

[0016] Preferably, in step (4), during the retraction and shaping process, the multilayer composite film is further rolled by a main drive extension roller with a threaded groove on its surface to stretch the multilayer composite film laterally, with a lateral stretching ratio of 10%-15%. There are 20-30 extension rollers in the retraction and shaping process, and each extension roller can achieve a lateral stretching effect of 0.5%.

[0017] Preferably, in step (4), the threaded expansion roller used for lateral stretching is a main drive structure. The positive speed difference between the main drive expansion roller and the steel roller is used to roll and expand the film to achieve the purpose of lateral stretching. The positive speed difference makes the original film wider and longer. The longer film is simultaneously retracted and shaped, eliminating the problem of loose film and film sagging between adjacent retraction and shaping points.

[0018] A polypropylene microporous membrane is prepared by the preparation method described above.

[0019] The beneficial effects of this invention are:

[0020] (1) This invention uses an original combination of technology and process to select polypropylene raw materials with ultra-low melt index and improves the transverse strength of polypropylene microporous membrane through unique casting process and stretching process, so that the manufactured polypropylene microporous membrane has excellent transverse tensile strength and puncture strength, thereby effectively improving the safety of battery.

[0021] (2) This invention represents a creative breakthrough in the dry single-stretch separator process, making up for some of the defects of the dry single-stretch separator and improving the safety of the dry single-stretch separator in battery applications.

[0022] (3) The pore structure of the polypropylene microporous membrane prepared by the existing dry single-stretch process is slit-like, while the pore structure of the polypropylene microporous membrane prepared by the present invention is elliptical. The preparation method of the present invention changes the pore structure of the polypropylene microporous membrane, which can further improve the safety of the battery. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the extended roller in Embodiment 1 of the present invention;

[0024] Figure 2 This is a cross-sectional view of the extended roller in Embodiment 1 of the present invention.

[0025] Figure label:

[0026] 100. Groove. Detailed Implementation

[0027] The present invention will be further described below with reference to specific embodiments.

[0028] Example 1:

[0029] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 0.9 g / 10 min, the heating and melting temperature is 200 °C, and the extrusion rate is 80 kg / h;

[0030] (2) Casting: The melt is drawn longitudinally through a chilling roller while an expanding roller rolls on the upper surface to roll the melt laterally, thus forming a film. The temperature of the chilling roller is 65°C, the traction ratio of the chilling roller is 8000%, and the rolling pressure of the expanding roller is 5 MPa. A schematic diagram of the expanding roller is shown below. Figure 1 and Figure 2 As shown, the surface of the extension roller is provided with threaded grooves, and the grooves on both sides of the extension roller are symmetrically arranged along the extension roller.

[0031] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 120℃ and the annealing time is 12h. The annealing method is to heat the film with circulating hot air in an oven.

[0032] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 40℃ and the ratio is 110%. The hot-stretching temperature is 136℃ and the ratio is 280%. The shrunk and shaped temperature is 150℃ and the longitudinal shrunk ratio is 22%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 20 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 10%.

[0033] Example 2:

[0034] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.0 g / 10 min, the heating and melting temperature is 220 °C, and the extrusion rate is 100 kg / h;

[0035] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 75℃, the traction ratio of the chilling roller is 11000%, the rolling pressure of the spreading roller is 6.5MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0036] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 128℃ and the annealing time is 18h. The annealing method is to heat the film with circulating hot air in an oven.

[0037] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 50℃ and the ratio is 125%. The hot-stretching temperature is 145℃ and the ratio is 320%. The shrunk and shaped temperature is 155℃ and the longitudinal shrunk ratio is 35%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 30 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 15%.

[0038] Example 3:

[0039] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.0 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0040] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 70℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the spreading roller is 6MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0041] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0042] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrunk ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 26 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 13%.

[0043] Example 4: (The only difference from Example 3 is that the melt index of the polypropylene raw material is 0.85 g / 10 min)

[0044] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 0.85 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0045] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 70℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the spreading roller is 6MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0046] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0047] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrunk ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 26 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 13%.

[0048] Example 5: (The only difference from Example 3 is that the melt index of the polypropylene raw material is 1.05 g / 10 min)

[0049] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.05 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0050] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 70℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the spreading roller is 6MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0051] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0052] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrunk ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 26 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 13%.

[0053] Example 6: (The only difference from Example 3 is that the temperature of the chilling roller during casting is 60°C)

[0054] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.0 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0055] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 60℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the spreading roller is 6MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0056] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0057] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrunk ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 26 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 13%.

[0058] Example 7: (The only difference from Example 3 is that the temperature of the chilling roller during casting is 80°C)

[0059] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.0 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0060] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 80℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the spreading roller is 6MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0061] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0062] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrunk ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 26 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 13%.

[0063] Example 8: (The only difference from Example 3 is that the rolling pressure of the spreading roller during casting is 4.5 MPa)

[0064] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.0 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0065] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 70℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the spreading roller is 4.5MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0066] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0067] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrunk ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 26 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 13%.

[0068] Example 9: (The only difference from Example 3 is that the rolling pressure of the spreading roller during casting is 7 MPa)

[0069] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.0 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0070] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the spreading roller to extend it laterally to obtain a film. The temperature of the chilling roller is 70℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the spreading roller is 7MPa, and the surface of the spreading roller is provided with a threaded groove. The grooves on both sides of the spreading roller are symmetrically arranged along the spreading roller.

[0071] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0072] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrunk ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same extension roller as in step (2) to stretch the multi-layer composite film laterally. There are 26 extension rollers in the shrunk and shaped process. Each extension roller has a 0.5% lateral stretching effect and the lateral stretching ratio is 13%.

[0073] Example 10: (The only difference from Example 3 is that the surface of the extension roller used in the casting process is a plain, non-grooved pressure roller.)

[0074] A method for preparing a polypropylene microporous membrane includes the following steps: (1) melt extrusion: heating and melting polypropylene raw material and then extruding it to form a melt, wherein the melt index of polypropylene raw material is 1.0 g / 10 min, the heating and melting temperature is 210 °C, and the extrusion rate is 95 kg / h;

[0075] (2) Casting: The melt is drawn through the chilling roller and cast longitudinally. At the same time, the melt is rolled on the upper surface by the pressure roller to obtain a film. The temperature of the chilling roller is 70℃, the traction ratio of the chilling roller is 10000%, the rolling pressure of the pressure roller is 6MPa, and the surface of the pressure roller is smooth without grooves.

[0076] (3) Annealing: The film is annealed to obtain a single-layer film. The annealing temperature is 122℃ and the annealing time is 16h. The annealing method is to heat the film with circulating hot air in an oven.

[0077] (4) Stretching: The single-layer film is stacked to obtain a multi-layer composite film. The multi-layer composite film is then cold-stretched and hot-stretched, then shrunk and shaped, and peeled off in layers to obtain a polypropylene microporous membrane. The cold-stretching temperature is 42℃ and the ratio is 120%. The hot-stretching temperature is 143℃ and the ratio is 300%. The shrunk and shaped temperature is 153℃ and the longitudinal shrinkage ratio is 28%. During the shrunk and shaped process, the multi-layer composite film is rolled by the same pressure rollers as in step (2). In the shrunk and shaped process, 26 pressure rollers roll the multi-layer composite film.

[0078] Comparative Example 1:

[0079] Commercially available polypropylene microporous membranes are prepared by a dry single-stretching process that involves longitudinal stretching of the polypropylene material.

[0080] Experimental example:

[0081] The transverse tensile strength and puncture strength of the polypropylene microporous membranes prepared in Examples 1-10 and the polypropylene microporous membrane of Comparative Example 1 were tested respectively. The testing methods for transverse tensile strength and puncture strength were as follows:

[0082] 1. Tensile strength

[0083] 1.1 Testing Instruments

[0084] XLW(PC) Intelligent Electronic Tensile Testing Machine

[0085] ruler

[0086] 1.2 Sample

[0087] Cut the sample to a width of 15mm.

[0088] The clamp has a center length of 100mm. Five clamps are sold as a set.

[0089] 1.3 Test according to the operating procedures of the tensile testing machine.

[0090] 2. Puncture strength

[0091] 2.1 Instruments

[0092] Electronic tensile testing machine (load range 0~500N)

[0093] ruler, blade

[0094] Puncture needle: buzz cut

[0095] Ring clamp

[0096] 2.2 Test according to the operating procedures for the puncture test on the tensile testing machine.

[0097] The test results are shown in Table 1.

[0098] Table 1:

[0099]

[0100]

[0101] As shown in Table 1, the polypropylene microporous membrane prepared by the method of the present invention has a transverse tensile strength of 216 N and a puncture strength of 443 N. Under the same conditions, by controlling the melt index of the polypropylene raw material to 0.9-1.0 g / 10 min, the temperature of the chilling roller during casting to 65-75 °C, and the rolling pressure of the spreading roller during casting to 5-6.5 MPa, and by using a spreading roller with threaded grooves on its surface, the porosity of the prepared polypropylene microporous membrane can reach more than 40%, the transverse tensile strength can reach 290 N, and the puncture strength can reach 550 N. In contrast, the transverse tensile strength of commercially available polypropylene microporous membranes is only 203 N and the puncture strength is only 405 N. This shows that the performance of the polypropylene microporous membrane prepared by the method of the present invention is far superior to that of commercially available polypropylene microporous membranes.

[0102] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a polypropylene microporous membrane, characterized in that: Includes the following steps: (1) Melt extrusion: Polypropylene raw material is heated and melted, and then extruded to form a melt; (2) Casting: The melt is drawn by a chilling roller and cast longitudinally while an extension roller is used to roll the melt on the upper surface of the melt to extend it laterally to obtain a film. (3) Annealing: The membrane is annealed to obtain a single-layer membrane; (4) Stretching: The single-layer membrane is stacked to obtain a multi-layer composite membrane, and then the multi-layer composite membrane is stretched, shrunk and shaped, and peeled off in layers to obtain the polypropylene microporous membrane; In step (1), the melt index of the polypropylene raw material is 0.9-1.0 g / 10 min; In step (2), the temperature of the chilling roller is 65-75℃, the traction ratio of the chilling roller is 8000%-11000%, the rolling pressure of the extension roller is 5-6.5 MPa, and the surface of the extension roller is provided with a threaded groove; In step (4), during the retraction and shaping process, the multilayer composite film is rolled by an extension roller with a threaded groove on its surface to stretch the multilayer composite film laterally, with a lateral stretching ratio of 10%-15%. In the shrinkage and shaping process, there are 20-30 of the aforementioned extension rollers, each of which can achieve a 0.5% lateral stretching effect.

2. The method for preparing a polypropylene microporous membrane according to claim 1, characterized in that: In step (3), the annealing temperature is 120-128℃ and the annealing time is 12-18h.

3. The method for preparing a polypropylene microporous membrane according to claim 1, characterized in that: In step (4), the stretching includes cold stretching and hot stretching. The temperature of cold stretching is 40-50℃ and the ratio is 110%-125%. The temperature of hot stretching is 136℃-145℃ and the ratio is 280%-320%.

4. The method for preparing a polypropylene microporous membrane according to claim 1, characterized in that: In step (4), the temperature for shrinkage and shaping is 150-155℃, and the longitudinal shrinkage ratio is 22%-35%.

5. A polypropylene microporous membrane, characterized in that: It is prepared by the preparation method according to any one of claims 1-4.

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