Preparation method of polyolefin porous membrane and polyolefin porous membrane

Abstract

The invention provides a preparation method of a polyolefin porous membrane and the polyolefin porous membrane, the preparation method of the polyolefin porous membrane comprises the following steps: S20: carrying out multi-stage stretching treatment on a polyolefin wet membrane, each stage of stretching is longitudinal and transverse two-way stretching, and S21: at room temperature, carrying out first-stage stretching treatment on the polyolefin wet membrane to obtain a first stretched membrane; s22, at least one stage of stretching treatment is conducted on the first stretching membrane in the heating environment, the temperature of the heating environment is higher than the first temperature, and in the heating environment, the temperature of the heating environment corresponding to each stage of stretching treatment is lower than the melting point of the membrane obtained after the last stage of stretching treatment by 5-20 DEG C; wherein in the multi-stage stretching treatment, except the first-stage stretching treatment, the stretching multiplying power and the stretching temperature of each stage of stretching treatment are respectively higher than the stretching multiplying power and the stretching temperature of the previous-stage stretching treatment. The environmental pollution caused by the halogen-containing solvent is avoided, and meanwhile, the polyolefin porous membrane is ensured to have a proper pore structure and material strength.

Description

[0001] Technology

[0002] This application belongs to the field of thin film material preparation technology, and particularly relates to a method for preparing polyolefin porous membranes and polyolefin porous membranes. Background Technology

[0003] Polyolefin porous membranes have wide applications in lithium battery separators, air filter membranes, microfiltration membranes, clothing membranes, and proton exchange substrate membranes for fuel cells. Methods for preparing polyolefin porous membranes, especially ultra-high molecular weight polyethylene (UHMWPE) porous membranes, include the TIPS method, which first prepares a membrane sheet and then biaxially stretches the sheet to prepare a thin film, resulting in a self-supporting membrane with higher strength and porosity.

[0004] Biaxial stretching is a crucial step in improving the porosity of polyolefin (UHMWPE) membranes. Current technology for UHMWPE membranes involves biaxially stretching a UHMWPE membrane containing white oil to a certain ratio, followed by extraction with dichloromethane to remove the white oil and its pore-forming properties. This extraction process increases the number of steps required and introduces waste liquid and waste gas recovery processes, raising production costs. Preparing UHMWPE porous membranes using volatile solvents can avoid the use of halogen-containing extractants, as these can be removed before stretching or directly during the stretching process. However, volatile solvents often soften the pore walls of the membrane, and the capillary pressure generated during evaporation further contributes to the problem. Therefore, directly stretching a membrane containing volatile solvents often results in a collapsed pore structure in the prepared UHMWPE porous membrane, leading to poor permeability and low porosity, failing to meet the requirements for high porosity. Summary of the Invention

[0005] The first aspect of this application provides a method for preparing a polyolefin porous membrane, comprising:

[0006] S10: Polyolefin resin and volatile organic solvent are mixed into a homogeneous casting solution. The casting solution is melt-extruded into a cast film by an extruder. The cast film is cooled to allow phase separation and obtain a solvent-containing polyolefin wet film sheet.

[0007] S20: Multi-stage stretching treatment of polyolefin wet films, each stage of stretching being bidirectional stretching in both longitudinal and transverse directions. The multi-stage stretching treatment includes:

[0008] S21: At a first temperature, a first-stage stretching treatment is performed on a polyolefin wet film to obtain a first stretched film. The first temperature ranges from 0℃ to 40℃.

[0009] S22: The first stretched film is subjected to at least one more stretching process in a heating environment. The temperature of the heating environment is higher than the first temperature. In the heating environment, the temperature of the heating environment corresponding to each stretching process is 5℃~20℃ lower than the melting point of the film obtained after the previous stretching process. The temperature of the heating environment is the stretching temperature.

[0010] In the multi-stage stretching process, the stretching ratio and the stretching temperature of each stage of stretching, except the first stage of stretching, are greater than the stretching ratio and the stretching temperature of the previous stage of stretching, respectively.

[0011] In some optional embodiments of the first aspect of the present application, the multi-stage stretching process is a two-stage stretching process,

[0012] In the step S21, the first stage of stretching is stretched by using a synchronous stretching method, and the longitudinal and transverse stretching ratio is 1.5x1.5 to 3x3.

[0013] In some optional embodiments of the first aspect of the present application, in the step S21, the stretching rate of the first stage of stretching is 50mm / min to 1200mm / min.

[0014] In some optional embodiments of the first aspect of the present application, the stretching rate of the first stage of stretching is 400mm / min to 1200mm / min.

[0015] In some optional embodiments of the first aspect of the present application, in the step S22, the first stretched film sheet is further subjected to a second stage of stretching, and the second stage of stretching is stretched by using a synchronous stretching method, and the longitudinal and transverse stretching ratio of the second stage of stretching is 6x6 to 8x8.

[0016] In some optional embodiments of the first aspect of the present application, the stretching temperature of the second stage of stretching is 90℃ to 135℃.

[0017] In some optional embodiments of the first aspect of the present application, the stretching rate of the second stage of stretching is less than or equal to the stretching rate of the first stage of stretching.

[0018] In some optional embodiments of the first aspect of the present application, the stretching rate of the second stage of stretching is 50mm / min to 800mm / min.

[0019] In some optional embodiments of the first aspect of the present application, the stretching rate of the second stage of stretching is 50mm / min to 400mm / min.

[0020] In some optional embodiments of the first aspect of the present application, the multi-stage stretching process is a three-stage stretching process or a four-stage stretching process, wherein the step S22 comprises a second stage of stretching, and the stretching rate of the second stage of stretching is the lowest in the multi-stage stretching process.

[0021] In some optional embodiments of the first aspect of the present application, the stretching rate of the second stage of stretching is 50mm / min to 400mm / min.

[0022] In some optional embodiments of the first aspect of this application, the multi-stage stretching process is a three-stage stretching process, and the total stretching ratio of the multi-stage stretching process is 8×8 times to 10×10 times.

[0023] In some optional embodiments of the first aspect of this application, in step S21, the first-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 1.5×1.5 times to 2×2 times, and the stretching rate of the first-stage stretching process is 50mm / min to 1200mm / min.

[0024] In some optional embodiments of the first aspect of this application, in step S22, the second-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 4×4 times to 6×6 times, and the stretching temperature of the second-stage stretching process is 90℃ to 125℃.

[0025] In some optional embodiments of the first aspect of this application, in step S22, the third-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 8×8 times to 10×10 times, and the stretching temperature of the third-stage stretching process is 125℃ to 135℃; the stretching rate of the third-stage stretching process is 600mm / min to 1200mm / min.

[0026] In some optional embodiments of the first aspect of this application, the multi-stage stretching process is a four-stage stretching process, and the total stretching ratio of the multi-stage stretching process is 8×8 times to 10×10 times.

[0027] In some optional embodiments of the first aspect of this application, in step S21, the first-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 1.5×1.5 times to 2×2 times, and the stretching rate of the first-stage stretching process is 50mm / min to 1200mm / min.

[0028] In some optional embodiments of the first aspect of this application, in step S22, the second-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 4×4 times to 5×5 times, and the stretching temperature of the second-stage stretching process is 90℃ to 115℃.

[0029] In some optional embodiments of the first aspect of this application, in step S22, the third-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 6×6 to 7×7 times, and the stretching temperature of the third-stage stretching process is 115℃ to 125℃; the stretching rate of the third-stage stretching process is 600mm / min to 1200mm / min.

[0030] In some optional embodiments of the first aspect of this application, in step S22, the fourth-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 8×8 to 10×10 times, and the stretching temperature of the fourth-stage stretching process is 125℃ to 135℃; the stretching rate of the fourth-stage stretching process is 600mm / min to 1200mm / min.

[0031] The second aspect of this application provides a polyolefin porous membrane, which is prepared using the polyolefin porous membrane preparation method of the first aspect of this application.

[0032] Beneficial effects:

[0033] The method for preparing a polyolefin porous membrane provided in the first aspect of this application uses a volatile organic solvent to prepare the casting solution. In the early stage of stretching, stretching at a lower initial temperature and lower ratio expands the evaporation area of ​​the wet membrane, utilizing the cavitation effect to form pores on the membrane surface, facilitating the subsequent formation of a high-porosity structure and optimizing air permeability. In the second stage of stretching, the stretching temperature is increased, accelerating solvent evaporation and causing molecular chain orientation in the wet membrane. This enhances the material strength of the polyolefin porous membrane. Simultaneously, the rapid evaporation and removal of the solvent from the material's interior can lead to the formation of micropores or cavities, enhancing the cavitation effect and further expanding the pore size during the second stage of stretching, forming a rich pore structure and simultaneously improving material strength. The third and fourth stages of stretching further orient the molecular chains, further enhancing the material's strength and mechanical properties.

[0034] The method for preparing polyolefin porous membranes provided in this application eliminates the need for an extraction step using halogen-containing extractants, thus avoiding environmental pollution risks. Furthermore, the method removes the solvent before stretching, allowing for continued stretching to expand pores and improve strength. The method also allows for adjustment of the stretching conditions at each stage of the stretching process according to specific material performance requirements.

[0035] The polyolefin porous membrane provided in the second aspect of this application has superior porosity and air permeability, as well as good material strength. Attached Figure Description

[0036] Figure 1 These are SEM images of Embodiment 1 of this application;

[0037] Figure 2 These are SEM images of Embodiment 6 of this application;

[0038] Figure 3 This is a scanning electron microscope (SEM) image of Embodiment 10 of this application. Detailed Implementation

[0039] The present application will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

[0040] The first aspect of this application provides a method for preparing a polyolefin porous membrane, comprising:

[0041] S10: Polyolefin resin and volatile organic solvent are mixed into a homogeneous casting solution. The casting solution is melt-extruded into a cast film by an extruder. The cast film is cooled to allow phase separation and obtain a solvent-containing polyolefin wet film sheet.

[0042] S20: Multi-stage stretching treatment of polyolefin wet films, each stage of stretching being bidirectional stretching in both longitudinal and transverse directions. The multi-stage stretching treatment includes:

[0043] S21: At a first temperature, a first-stage stretching treatment is performed on a polyolefin wet film to obtain a first stretched film. The first temperature ranges from 0℃ to 40℃.

[0044] S22: The first stretched film is subjected to at least one more stretching treatment in a heating environment. The temperature of the heating environment is higher than the first temperature. In the heating environment, the temperature of the heating environment corresponding to each stretching treatment is 5℃~20℃ lower than the melting point of the film obtained after the previous stretching treatment. The temperature of the heating environment is the stretching temperature. Among them, in the multi-stage stretching treatment, except for the first stage stretching treatment, the stretching ratio and stretching temperature of each stage stretching treatment are both greater than the stretching ratio and stretching temperature of the previous stage stretching treatment.

[0045] It should be noted that the melting point of the wet film gradually increases after each stretching stage, and the melting point temperature is lowest before it is completely unstretched.

[0046] The stretching ratio for each stretching treatment is the product of the transverse stretching ratio and the longitudinal stretching ratio of the diaphragm.

[0047] In some optional embodiments of the first aspect of this application, the first temperature is room temperature, specifically ranging from 15°C to 25°C.

[0048] In some optional embodiments of the first aspect of this application, the temperature of the heating environment is 90°C to 135°C.

[0049] The method for preparing a polyolefin porous membrane provided in the first aspect of this application uses a volatile organic solvent to prepare the casting solution. In the early stage of stretching, stretching at a low ratio at room temperature expands the evaporation area of ​​the wet membrane, utilizing the cavitation effect to form pores on the membrane surface, facilitating the subsequent formation of a high-porosity structure and optimizing air permeability. In the second stage of stretching, the stretching temperature is increased, accelerating solvent evaporation and causing molecular chains in the wet membrane to orient, enhancing the material strength of the polyolefin porous membrane. Simultaneously, the rapid evaporation and removal of the solvent from the material's interior can also lead to the formation of micropores or cavities locally, enhancing the cavitation effect and further expanding the pore size during the second stage of stretching, forming a rich pore structure and simultaneously improving material strength. The third and fourth stages of stretching further orient the molecular chains, further enhancing the material's strength and mechanical properties.

[0050] The method for preparing polyolefin porous membranes provided in this application eliminates the need for an extraction step using halogen-containing extractants, thus avoiding environmental pollution risks. Furthermore, the method removes the solvent before stretching, allowing for continued stretching to expand pores and improve strength. The method also allows for adjustment of the stretching conditions at each stage of the stretching process according to specific material performance requirements.

[0051] In some optional embodiments of the first aspect of this application, each stretching process can be a synchronous stretching method or an asynchronous stretching method. The stretching temperature of each stage is 5°C to 20°C below the melting point of the film.

[0052] In some optional embodiments of the first aspect of this application, the multi-stage stretching process is a two-stage stretching process.

[0053] In step S21, the first-stage stretching process uses a synchronous stretching method, with the longitudinal and transverse stretching ratios reaching 1.5×1.5 times to 3×3 times.

[0054] In some optional embodiments of the first aspect of this application, in step S21, the stretching rate of the first-stage stretching treatment is 50 mm / min to 1200 mm / min.

[0055] In some optional embodiments of the first aspect of this application, in step S22, the first stretching diaphragm is subjected to a second stretching process. The second stretching process is performed by a synchronous stretching method, and the longitudinal and transverse stretching ratio of the second stretching process is 6×6 times to 8×8 times.

[0056] In some optional embodiments of the first aspect of this application, the stretching temperature of the second-stage stretching treatment is 90°C to 135°C.

[0057] In some optional embodiments of the first aspect of this application, the stretching rate of the second-stage stretching process is less than or equal to the stretching rate of the first-stage stretching process.

[0058] In some optional embodiments of the first aspect of this application, the stretching rate of the second-stage stretching process is 50 mm / min to 800 mm / min.

[0059] In some optional embodiments of the first aspect of this application, the stretching rate of the second-stage stretching process is 50 mm / min to 400 mm / min.

[0060] In some optional embodiments of the first aspect of this application, the multi-stage stretching process is a three-stage stretching process or a four-stage stretching process, wherein step S22 includes a second-stage stretching process, and the stretching rate of the second-stage stretching process is the lowest in the multi-stage stretching process.

[0061] In some optional embodiments of the first aspect of this application, the stretching rate of the second-stage stretching process is 50 mm / min to 400 mm / min.

[0062] In some optional embodiments of the first aspect of this application, the multi-stage stretching process is a three-stage stretching process, and the total stretching ratio of the multi-stage stretching process is 8×8 times to 10×10 times.

[0063] In some optional embodiments of the first aspect of this application, in step S21, the first-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 1.5×1.5 times to 2×2 times, and the stretching rate of the first-stage stretching process is 50mm / min to 1200mm / min.

[0064] In some optional embodiments of the first aspect of this application, in step S22, the second-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 4×4 times to 6×6 times, and the stretching temperature of the second-stage stretching process is 90℃ to 125℃.

[0065] In some optional embodiments of the first aspect of this application, in step S22, the third-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 8×8 times to 10×10 times, and the stretching temperature of the third-stage stretching process is 125℃ to 135℃; the stretching rate of the third-stage stretching process is 600mm / min to 1200mm / min.

[0066] In some optional embodiments of the first aspect of this application, the multi-stage stretching process is a four-stage stretching process, and the total stretching ratio of the multi-stage stretching process is 8×8 times to 10×10 times.

[0067] In some optional embodiments of the first aspect of this application, in step S21, the first-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 1.5×1.5 times to 2×2 times, and the stretching rate of the first-stage stretching process is 50mm / min to 1200mm / min.

[0068] In some optional embodiments of the first aspect of this application, in step S22, the second-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 4×4 times to 5×5 times, and the stretching temperature of the second-stage stretching process is 90℃ to 115℃.

[0069] In some optional embodiments of the first aspect of this application, in step S22, the third-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 6×6 to 7×7 times, and the stretching temperature of the third-stage stretching process is 115℃ to 125℃; the stretching rate of the third-stage stretching process is 600mm / min to 1200mm / min.

[0070] In some optional embodiments of the first aspect of this application, in step S22, the fourth-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 8×8 to 10×10 times, and the stretching temperature of the fourth-stage stretching process is 125℃ to 135℃; the stretching rate of the fourth-stage stretching process is 600mm / min to 1200mm / min.

[0071] In some optional embodiments of the first aspect of this application, the second-stage stretching process begins with stretching in an oven. The medium used for the hot air in the stretching oven is one or a mixture of air, nitrogen, or an inert gas. The oven operating temperature is room temperature to 150°C. The air-cooling medium is nitrogen. Additionally, the oven is located within a negative pressure hood, where the volatile solvent vapors are collected and recovered through negative pressure.

[0072] In some optional embodiments of the first aspect of this application, the uniformity of the finished film is related to the film cooling method. Both sides of the cast film should be cooled simultaneously, and the cooling method is double-sided roller cooling, double-sided liquid cooling, or double-sided air cooling.

[0073] In some optional embodiments of the first aspect of this application, the polyolefin resin is one or a combination of several of the following: ultra-high molecular weight polyethylene polymerized with conventional magnesium-titanium Ziegler-Natta catalysts, ultra-high molecular weight polyethylene with narrow molecular weight distribution synthesized with metallocene catalysts, ultra-high molecular weight polyethylene with bimodal molecular weight distribution, or ultra-high molecular weight polyethylene copolymerized with α-olefins.

[0074] In some optional embodiments of the first aspect of this application, the volatile organic solvent is one or a combination of several of toluene, xylene, n-nonane, n-decane, undecane, dodecane, kerosene, decahydronaphthalene, and tetrahydronaphthalene.

[0075] The second aspect of this application provides a polyolefin porous membrane, which is prepared using the polyolefin porous membrane preparation method of the first aspect of this application.

[0076] The polyolefin porous membrane provided in the second aspect of this application has superior porosity and air permeability, as well as good material strength.

[0077] In some optional embodiments of the second aspect of this application, the polyolefin porous membrane can be used in various application fields such as lithium battery separators, waterproof and breathable fabrics, radiation-cooled fabrics, and proton exchange substrate membranes for fuel cells. Specifically, in the lithium battery separator field, the membrane strength is higher than 110 MPa, and the porosity is not less than 30%; in the waterproof and breathable fabric and radiation-cooled fabric field, the membrane strength is higher than 30 MPa, and the porosity is not less than 50%; in the fuel cell proton exchange substrate membrane field, the membrane strength is higher than 40 MPa, the porosity is not less than 50%, and the proton conductivity after coating with perfluorosulfonic acid resin is not less than 0.1 S / cm at 80°C / 80% air humidity. [Specific Implementation Examples]

[0079] The following specific embodiments and comparative examples illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0080] Ultra-high molecular weight polyethylene (UHMWPE) membranes possess outstanding comprehensive properties such as chemical resistance, abrasion resistance, aging resistance, biocompatibility, excellent mechanical strength, and low cost. They are generally made from polyethylene resin with a molecular weight of over 1 million.

[0081] I. Preparation of Polyolefin Porous Membranes

[0082] Comparative Example 1

[0083] UHMWPE (molecular weight 1.2 million, 20wt%) and 50# white oil (80wt%) were precisely metered and then fed into a mixing tank at 30°C. The overflow mixture (a homogeneous casting solution) entered a twin-screw extruder at 200°C, 180°C, and 60 rpm. The homogeneous casting solution was melt-extruded into a cast film. The cast film was cooled in a water bath at 15°C at a winding rate of 1.0 m / min. The resulting film was extracted with dichloromethane and then dried in a 50°C oven to obtain a dry film with a thickness of 0.59 mm. The dry film was simultaneously stretched at a ratio of 8×8 times at 120°C and a stretching rate of 800 mm / min.

[0084] Comparative Example 2

[0085] The difference from the comparative example is that, based on the dry film prepared in comparative example 1, the dry film was simultaneously stretched by a ratio of 6×6, the stretching temperature was 120℃, and the stretching rate was 800mm / min.

[0086] Comparative Example 3

[0087] The difference from the comparative example is that, based on the dry film prepared in comparative example 1, the dry film was simultaneously stretched by a ratio of 10×10, the stretching temperature was 120℃, and the stretching rate was 800mm / min.

[0088] Comparative Example 4

[0089] UHMWPE (molecular weight 1.2 million, 20 wt%) and decahydronaphthalene (boiling point ~187℃, 80 wt%) were precisely metered and then introduced into a mixing tank for stirring. The overflow mixture (a homogeneous casting solution) entered a twin-screw extruder. The screw temperature was 180℃, the die temperature was 150℃, and the screw speed was 60 rpm. The homogeneous casting solution was melt-extruded into a cast film. The cast film was cooled in a water bath at 15℃, and the film winding rate was 1.0 m / min to obtain a solvent-containing wet film. The solvent-containing wet film was directly subjected to a single synchronous stretching with a stretch ratio of 8×8 times, a stretching temperature of 120℃, and a stretching rate of 800 mm / min.

[0090]

Example 1

[0091] UHMWPE (molecular weight 1.2 million, 20 wt%) and decahydronaphthalene (boiling point ~187℃, 80 wt%) are precisely metered and then introduced into a mixing tank for stirring. The overflow mixture (homogeneous casting solution) enters a twin-screw extruder with a screw temperature of 180℃, a die temperature of 150℃, and a screw speed of 60 rpm. The cast film is cooled by a water bath at 15℃, with a film winding rate of 1.0 m / min, resulting in a solvent-containing wet film.

[0092] The wet film undergoes two stages of stretching, namely, a first-stage synchronous stretching and a second-stage synchronous stretching. The first-stage synchronous stretching ratio is 2×2, the stretching temperature is room temperature, and the stretching rate is 800 mm / min; the second-stage synchronous stretching ratio is 6×6, the stretching temperature is 120℃, and the stretching rate is 200 mm / min.

[0093] The secondary synchronous stretching in Example 1 was carried out in an oven.

[0094]

Example 2

[0095] Example 2: The preparation process of the wet film is the same as that of Example 1, except that:

[0096] The wet film undergoes two stages of stretching. The first stage involves a simultaneous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min. The second stage involves a simultaneous stretching ratio of 8×8 times, a stretching temperature of 120℃, and a stretching rate of 200 mm / min.

[0097]

Example 3

[0098] Example 2: The preparation process of the wet film is the same as that of Example 1, except that:

[0099] The wet film undergoes two stages of stretching. The first stage involves simultaneous stretching with a ratio of 3×3 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min. The second stage involves simultaneous stretching with a ratio of 8×8 times, a stretching temperature of 120℃, and a stretching rate of 200 mm / min.

[0100]

Example 4

[0101] Example 2: The preparation process of the wet film is the same as that of Example 1, except that:

[0102] The wet film undergoes two stages of stretching. The first stage involves a simultaneous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min. The second stage involves a simultaneous stretching ratio of 8×8 times, a stretching temperature of 120℃, and a stretching rate of 800 mm / min.

[0103]

Example 5

[0104] Example 2: The preparation process of the wet film is the same as that of Example 1, except that:

[0105] The wet film undergoes three stages of stretching: the first stage involves a synchronous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a synchronous stretching ratio of 5×5 times, a stretching temperature of 90℃, and a stretching rate of 200 mm / min; and the third stage involves a synchronous stretching ratio of 10×10 times, a stretching temperature of 125℃, and a stretching rate of 800 mm / min.

[0106]

Example 6

[0107] The difference from Example 1 is as follows:

[0108] The wet film undergoes three stages of stretching: the first stage involves a synchronous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a synchronous stretching ratio of 5×5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; and the third stage involves a synchronous stretching ratio of 10×10 times, a stretching temperature of 125℃, and a stretching rate of 800 mm / min.

[0109]

Example 7

[0110] The difference from Example 1 is as follows:

[0111] The wet film undergoes three stages of stretching: the first stage involves a synchronous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a synchronous stretching ratio of 5×5 times, a stretching temperature of 110℃, and a stretching rate of 200 mm / min; and the third stage involves a synchronous stretching ratio of 10×10 times, a stretching temperature of 125℃, and a stretching rate of 800 mm / min.

[0112]

Example 8

[0113] The difference from Example 1 is as follows:

[0114] The wet film undergoes three stages of stretching: the first stage involves a synchronous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a synchronous stretching ratio of 5×5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; and the third stage involves a synchronous stretching ratio of 10×10 times, a stretching temperature of 135℃, and a stretching rate of 800 mm / min.

[0115]

Example 9

[0116] The difference from Example 1 is as follows:

[0117] The wet film undergoes three stages of stretching: the first stage involves a simultaneous stretching ratio of 1.5 × 1.5 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a simultaneous stretching ratio of 5 × 5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; and the third stage involves a simultaneous stretching ratio of 10 × 10 times, a stretching temperature of 125℃, and a stretching rate of 800 mm / min.

[0118]

Example 10

[0119] The difference from Example 1 is as follows:

[0120] The wet film undergoes three stages of stretching: the first stage involves a synchronous stretching ratio of 3×3 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a synchronous stretching ratio of 5×5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; and the third stage involves a synchronous stretching ratio of 10×10 times, a stretching temperature of 125℃, and a stretching rate of 800 mm / min.

[0121]

Example 11

[0122] The difference from Example 1 is as follows:

[0123] The wet film undergoes four stages of stretching: the first stage involves a simultaneous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a simultaneous stretching ratio of 5×5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; the third stage involves a simultaneous stretching ratio of 7×7 times, a stretching temperature of 115℃, and a stretching rate of 800 mm / min; and the fourth stage involves a simultaneous stretching ratio of 10×10 times, a stretching temperature of 125℃, and a stretching rate of 800 mm / min.

[0124]

Example 12

[0125] The difference from Example 1 is as follows:

[0126] The wet film undergoes four stages of stretching: the first stage involves a simultaneous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a simultaneous stretching ratio of 5×5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; the third stage involves a simultaneous stretching ratio of 7×7 times, a stretching temperature of 115℃, and a stretching rate of 800 mm / min; and the fourth stage involves a simultaneous stretching ratio of 10×10 times, a stretching temperature of 130℃, and a stretching rate of 800 mm / min.

[0127]

Example 13

[0128] The difference from Example 1 is as follows:

[0129] The wet film undergoes four stages of stretching: the first stage involves a simultaneous stretching ratio of 2×2 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a simultaneous stretching ratio of 5×5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; the third stage involves a simultaneous stretching ratio of 7×7 times, a stretching temperature of 115℃, and a stretching rate of 800 mm / min; and the fourth stage involves a simultaneous stretching ratio of 10×10 times, a stretching temperature of 135℃, and a stretching rate of 800 mm / min.

[0130]

Example 14

[0131] The difference from Example 1 is as follows:

[0132] The wet film undergoes four stages of stretching: the first stage involves a simultaneous stretching ratio of 3×3 times, a stretching temperature of room temperature, and a stretching rate of 800 mm / min; the second stage involves a simultaneous stretching ratio of 5×5 times, a stretching temperature of 100℃, and a stretching rate of 200 mm / min; the third stage involves a simultaneous stretching ratio of 7×7 times, a stretching temperature of 115℃, and a stretching rate of 800 mm / min; and the fourth stage involves a simultaneous stretching ratio of 10×10 times, a stretching temperature of 125℃, and a stretching rate of 800 mm / min.

[0133] Comparative analysis of experimental conditions between the examples and comparative examples:

[0134] 1. The wet film undergoes two stages of simultaneous stretching. The first stage of stretching is at room temperature, and the second stage of stretching is heated.

[0135] Example 1: The first-stage stretching temperature is lower than the second-stage stretching temperature, and the first-stage stretching ratio is also lower than the second-stage stretching ratio, but the first-stage stretching rate is higher than the second-stage stretching rate.

[0136] Example 2: The second-stage stretching ratio of Example 2 is greater than that of Example 1.

[0137] Example 3: The first-stage stretching ratio is increased compared to Example 1.

[0138] Example 4: The stretching rate of the second stage is greater than that of the second stage in Example 1. The stretching rates of the first and second stages in the example are equal.

[0139] 2. The wet film sheet underwent three stages of stretching, all of which were simultaneous stretching.

[0140] Example 5: The synchronous stretching ratio of the first, second and third stages gradually increases, and the stretching temperature of the first, second and third stages also gradually increases. The stretching rate follows the pattern of large-small-large.

[0141] Example 6: Compared with Example 5, the stretching temperature of the second stage is increased, while the other operating rules are the same.

[0142] Example 7: Compared to Example 5, the tensile temperature of the second stage is increased, while the other operating principles remain the same. The tensile temperature of the second stage in Example 7 is further increased compared to the tensile temperature of the second stage in Example 6.

[0143] Example 8: Compared with Example 6, the third-stage tensile temperature is increased.

[0144] Example 9: Compared to Example 6, the first-stage stretching ratio is reduced.

[0145] Example 10: Compared to Example 6, the first-stage stretching ratio is increased.

[0146] 3. The wet film sheet underwent four stages of tensioning, all of which were simultaneous tensioning.

[0147] Example 11: The synchronous stretching ratios of the first, second, third and fourth stages gradually increase, and the stretching temperatures of the first, second, third and fourth stages also gradually increase. The stretching rate follows a large-large-large pattern.

[0148] Example 12: Compared to Example 11, the fourth-stage tensile temperature is increased.

[0149] Example 13: Compared with Examples 11 and 12, the fourth-stage tensile temperature is increased.

[0150] Example 14: Compared to Example 11, the stretching ratio of the first stage of synchronous stretching is increased.

[0151] 4. Comparative Examples 1 to 3: non-volatile white oil was extracted with dichloromethane, dried, and then stretched once simultaneously with a dry film.

[0152] 5. Comparative Example 4: A volatile solvent, decahydronaphthalene, was used, and then a single synchronous stretching was performed directly using a wet film.

[0153] II. Performance Tests of Polyolefin Porous Membranes in Each Example and Comparative Example

[0154] In the above examples and comparative examples, the UHMWPE used in the following examples was purchased from Shanghai Lianle Chemical Technology Co., Ltd. as LL-LB-1080. The UHMWPE had a molecular weight of 1.2 million, a molecular weight distribution of 4.5, a particle size D50 of 150 μm, and a particle size distribution (D90-D10) / D50 of 0.68. The solvents, white oil and decahydronaphthalene, were both commercially available products and were not treated before use.

[0155] 1) Air permeability test: Air permeability reflects the permeability of the diaphragm. It is tested according to the Gurley method in GB / T 458-2008, which is the time required for a certain volume (100mL) of gas to pass through a diaphragm with a surface area of ​​1 square inch under a certain pressure (40psi).

[0156] 2) Thickness test: The test shall be conducted in accordance with the provisions of GB / T6672-2001. At least 3 points shall be tested at equal intervals along the TD direction as a group, and a group shall be tested every 200mm along the MD direction, for a total of 5 groups; MD refers to longitudinal direction and TD refers to transverse direction.

[0157] 3) Tensile strength test: The test was conducted according to GB / T 1040.3-2006 Determination of tensile properties of plastics - Part 3: Test conditions for thin plastics and sheets. Type II specimens were selected, with a tensile rate of 250±100 mm / min, a specimen length of 150 mm, a width of 15 mm, and a test fixture of 100±5 mm. The breaking strength, elongation at break, initial modulus, and elongation at break were read from the testing instrument. Each sample was tested 5 times, and the average value and coefficient of variation were recorded. A coefficient of variation less than 10% was considered reliable.

[0158] 4) Porosity testing: Porosity is calculated according to the national standard GB / T 36363-2018 using the following two formulas:

[0159]

[0160] Wherein, ρ1(g / cm 3) is the membrane surface density, m (g) is the membrane mass, L (cm) is the membrane length, b (cm) is the membrane width, ε (%) is the membrane porosity, d (μm) is the membrane thickness, and ρ0 (g / cm²) is the membrane thickness. 3 () is the density of the raw material.

[0161] 5) Electron microscopy characterization equipment and procedures

[0162] The morphology and structure of the samples were observed using a scanning electron microscope (Sigma 300, ZEISS, Germany). The prepared samples were fixed to the stage using conductive double-sided adhesive tape, sputtered with gold for 30 seconds, and then observed and photographed using a scanning electron microscope.

[0163] Table 1 Summary of tensile parameters of the wet film in each stage of stretching in the embodiments.

[0164]

[0165] In Examples 1 to 14, room temperature refers to 25°C.

[0166] Table 2 Summary of performance test results of polyolefin porous membranes in each embodiment and comparative example

[0167]

[0168]

[0169] Combining Table 1 and Table 2 and Figures 1 to 2 Analyze the data:

[0170] Comparative Examples 1 to 3 are polyolefin membranes prepared using white oil as a solvent. Their permeability (the lower the permeability value, the better the permeability, because a smaller value means lower resistance to gas passing through the material per unit time, resulting in stronger permeability) and strength increase with increasing stretch ratio. However, membrane rupture occurs after the stretch ratio reaches 10 × 10. The preparation process requires extraction with dichloromethane, a halogenated and highly volatile extractant that is environmentally unfriendly.

[0171] Comparative Example 4 is a polyolefin membrane prepared by directly stretching a wet membrane sheet containing a volatile solvent (decahydronaphthalene) to 8×8 times in a single stage. The solvent was not pre-evaporated, so the volatile solvent softened the pore walls of the membrane during the stretching process. In addition, the volatile solvent also evaporated during the stretching process, generating capillary pressure during the evaporation process. The pores on the membrane surface collapsed and became dense and non-porous, resulting in poor air permeability, low porosity, and a transparent appearance of the membrane, which also indicates that there are fewer pore structures.

[0172] It is understandable that the cavitation effect originates from the stress concentration caused by intercrystalline deformation during stretching. When the stretching exceeds the critical strain, the interface between the crystalline and amorphous regions slips, forming pores. The cavitation effect is mainly manifested as the localization of voids in the material under tensile load. Low-ratio stretching at room temperature not only expands the evaporation area but also utilizes the cavitation effect to form pores on the surface of the wet film, thus creating an initial microporous structure and increasing the film porosity.

[0173] In summary, Examples 1 to 14 all used multi-stage stretching of wet membrane sheets containing volatile solvent (decahydronaphthalene) to prepare polyolefin porous membranes, which is environmentally friendly.

[0174] Examples 1 to 4 describe the preparation of polyolefin porous membranes using a two-stage simultaneous stretching process on a wet film containing a volatile solvent (decahydronaphthalene). The first stage, at room temperature, expands the evaporation area, while the second stage, conducted in an oven, involves simultaneous stretching and solvent evaporation. During this second stage, molecular chain orientation also occurs, enhancing material strength and causing the molecular chains to align orderly along the direction of force, forming an anisotropic structure. Decahydronaphthalene has a high boiling point (187°C), and excessively high stretching rates prevent complete solvent evaporation. However, slow stretching rates result in a higher deorientation rate than orientation rate, leading to poor mechanical properties of the membrane. Controlling the stretching ratio and stretching rate in the second stage is crucial for maintaining the pore structure, enhancing porosity, and optimizing the strength of the polyolefin porous membrane. Figure 2 As shown, the polyolefin porous membrane of Example 1 is mainly composed of strip-shaped coarse fibers and has a large number of pore structures.

[0175] From Examples 1 and 2, the first-stage stretch ratios are equal and at a low level. The second-stage stretch ratio of Example 2 is greater than that of Example 1. The porosity of Examples 1 and 2 is high and basically similar. In terms of air permeability and material strength, Example 2 is better than Example 1, indicating that appropriately increasing the second-stage stretch ratio can improve the strength of the polyolefin porous membrane.

[0176] Compared with Example 2, Example 3 has a higher first-stage stretching ratio than Example 1, resulting in better material strength. However, its porosity and air permeability are inferior to Example 2. The material strength of Example 3 is significantly improved compared to Example 2. This is because a higher first-stage stretching ratio reduces the microporous structure initially formed during the stretching process at room temperature in the polyolefin porous membrane, leading to decreased air permeability and reduced porosity. However, the increased first-stage stretching ratio in Example 3 further enhances the material's strength.

[0177] Compared to Example 2, Example 4 showed a significantly higher stretching rate during the second-stage stretching process compared to Example 2. This resulted in a marked increase in material strength, while porosity and air permeability decreased. This may be because the excessively rapid second-stage stretching rate affected the complete volatilization of decahydronaphthalene, thus partially inhibiting the cavitation effect during stretching and impacting the pore structure and air permeability of the polyolefin porous membrane. However, the material strength of Example 4 was further improved compared to both Examples 2 and 3, indicating that the second-stage stretching rate had a significant impact on the improvement in material strength.

[0178] Examples 5 to 10 are all examples of three-stage stretching of wet membrane sheets. Analyzing Examples 5 to 7 together, in these three examples, except that the second-stage stretching temperature gradually increases from Example 5 to Example 7, other conditions remain unchanged. The air permeability gradually increases, and the air permeability gradually decreases as the second-stage stretching temperature increases. The material strength (including transverse and longitudinal strength) gradually increases. The porosity of all three examples is between 40% and 70%, which is at a relatively optimal level. In the three-stage stretching of the wet membrane sheet, the second-stage stretching temperature affects the air permeability. Regarding the process of simultaneous three-stage stretching of the wet membrane sheet: the first-stage stretching mainly increases the evaporation area, which is beneficial for solvent evaporation; the second-stage stretching, with the stretching temperature increased and stretching in an oven, mainly serves to ensure sufficient solvent evaporation and molecular chain orientation; the third-stage stretching further orients the molecular chains, enhancing the material strength. The final stretching ratio of the wet membrane sheet can be increased to 10 × 10 times. Figure 2 As shown, Figure 2 As can be seen in Example 6, the polyolefin porous membrane structure is composed of a microfiber network at the microscopic level.

[0179] The difference between Example 8 and Example 7 lies in the stretching temperature of the third stage of tensioning; the third stage stretching temperature of Example 8 is higher than that of Example 7. Material properties show that the air permeability of Example 8 is better than that of Example 7, while the material strength of Example 8 is slightly lower than that of Example 7, and the porosity of Example 7 is slightly lower than that of Example 8. This is likely because increasing the third stage stretching temperature may melt part of the crystal structure, causing some membrane pores to collapse, thus affecting air permeability and porosity.

[0180] The difference between Examples 9 and 10 and Example 7 lies in the first-stage stretching ratio. The first-stage stretching ratio of Example 9 is smaller than that of Example 7. The air permeability and porosity of Example 9 are better than those of Example 7, but the material strength of Example 9 is slightly lower than that of Example 7. The first-stage stretching ratio of Example 10 is greater than that of Example 7, reaching 3×3 times. In Example 10, the pores on the membrane surface collapse and become very dense, the membrane appears transparent, the air permeability is undetectable, and the porosity is low. However, the material strength of Example 10 is significantly enhanced compared to Examples 7 and 8. Analysis: An excessively large first-stage stretching ratio is detrimental to the subsequent formation of the pore structure, resulting in a transparent membrane. Figure 3 These are SEM images of Embodiment 10 of this application. Figure 3 As shown, the pores in Example 10 are extremely few, essentially indicating that the pore structure has collapsed and disappeared. During the preparation process of Example 10, the observed phenomenon was that the membrane initially turned white upon low-ratio stretching at room temperature, and gradually became translucent with continued stretching. It is speculated that low-ratio stretching at room temperature initially forms micropores, and with further stretching, the original solvent is squeezed into the formed micropores. The solvent then softens the pore walls and, due to capillary pressure, the micropores reclose.

[0181] Examples 11 to 14 are all examples of four-stage stretching of wet membrane sheets. Combined analysis of Examples 11 to 13 revealed that as the fourth-stage stretching temperature was gradually increased to 135°C, the material's air permeability and porosity gradually decreased. When the fourth-stage stretching temperature reached 135°C, the material experienced pore collapse, resulting in a reduced pore structure, lower porosity, and undetectable air permeability; the membrane as a whole became transparent. Analysis of Examples 11 and 14 together showed that in Example 14, increasing the first stretching ratio reduced the microporous structure initially formed during room-temperature stretching of the polyolefin porous membrane, leading to decreased air permeability and affected porosity. Therefore, the prepared membrane also appeared transparent with low porosity and high material strength.

[0182] In summary, the technical method involved in this invention relates to the preparation of polyolefin porous membranes using volatile solvents and different stretching methods.

[0183] It should be noted that, in this document, "comprising," "including," or any other variation thereof is intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0184] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0185] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the principles of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing a polyolefin porous membrane, characterized in that, include: S10: A polyolefin resin and a volatile organic solvent are mixed to form a homogeneous casting solution. The casting solution is melt-extruded into a cast film using an extruder. The cast film is cooled to allow phase separation and obtain a solvent-containing polyolefin wet film sheet. S20: The polyolefin wet film is subjected to a multi-stage stretching process, each stage of which is a bidirectional stretching process, including: S21: At a first temperature, the polyolefin wet film is subjected to a first-stage stretching treatment to obtain a first stretched film, wherein the first temperature ranges from 0°C to 40°C. S22: The first stretched film is subjected to at least one more stretching process in a heating environment, wherein the temperature of the heating environment is greater than the first temperature, and in the heating environment, the temperature of the heating environment corresponding to each stretching process is 5°C to 20°C lower than the melting point of the film obtained after the previous stretching process, wherein the temperature of the heating environment is the stretching temperature. In the multi-stage stretching process, except for the first stage stretching process, the stretching ratio and stretching temperature of each stage stretching process are greater than those of the previous stage stretching process.

2. The method for preparing a polyolefin porous membrane according to claim 1, characterized in that, The multi-stage stretching process is a two-stage stretching process. In step S21, the first-stage stretching process adopts a synchronous stretching method, with the longitudinal and transverse stretching ratios reaching 1.5×1.5 times to 3×3 times. Preferably, in step S21, the stretching rate of the first-stage stretching treatment is 50 mm / min to 1200 mm / min, and more preferably, the stretching rate of the first-stage stretching treatment is 400 mm / min to 1200 mm / min.

3. The method for preparing a polyolefin porous membrane according to claim 2, characterized in that, In step S22, the first stretching diaphragm is subjected to a second-stage stretching process. The second-stage stretching process is performed using a synchronous stretching method, and the longitudinal and transverse stretching ratio of the second-stage stretching process is 6×6 times to 8×8 times. Preferably, the stretching temperature of the second-stage stretching treatment is 90℃~135℃; Preferably, the stretching rate of the second-stage stretching process is less than or equal to the stretching rate of the first-stage stretching process. Preferably, the stretching rate of the second-stage stretching process is 50 mm / min to 800 mm / min. Preferably, the stretching rate of the second-stage stretching process is 50 mm / min to 400 mm / min.

4. The method for preparing a polyolefin porous membrane according to claim 1, characterized in that, The multi-stage stretching process is a three-stage stretching process or a four-stage stretching process, wherein step S22 includes a second-stage stretching process, and the stretching rate of the second-stage stretching process is the lowest in the multi-stage stretching process. Preferably, the stretching rate of the second-stage stretching treatment is 50 mm / min to 400 mm / min.

5. The method for preparing a polyolefin porous membrane according to claim 4, characterized in that, The multi-stage stretching process is a three-stage stretching process, and the total stretching ratio of the multi-stage stretching process is 8×8 times to 10×10 times.

6. The method for preparing a polyolefin porous membrane according to claim 5, characterized in that, In step S21, the first-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 1.5×1.5 times to 2×2 times, and the stretching rate of the first-stage stretching process is 50mm / min to 1200mm / min. Preferably, in step S22, the second-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 4×4 to 6×6, and the stretching temperature of the second-stage stretching process is 90℃ to 125℃. Preferably, in step S22, the third-stage stretching treatment adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 8×8 to 10×10 times, a stretching temperature of 125℃ to 135℃, and a stretching rate of 600mm / min to 1200mm / min.

7. The method for preparing a polyolefin porous membrane according to claim 4, characterized in that, The multi-stage stretching process is a four-stage stretching process, and the total stretching ratio of the multi-stage stretching process is 8×8 times to 10×10 times.

8. The method for preparing a polyolefin porous membrane according to claim 7, characterized in that, In step S21, the first-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 1.5×1.5 times to 2×2 times, and a stretching rate of 50mm / min to 1200mm / min.

9. The method for preparing a polyolefin porous membrane according to claim 7, characterized in that, In step S22, the second-stage stretching process adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 4×4 to 5×5, and the stretching temperature of the second-stage stretching process is 90℃ to 115℃. Preferably, in step S22, the third-stage stretching treatment adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 6×6 to 7×7 times, a stretching temperature of 115℃ to 125℃, and a stretching rate of 600mm / min to 1200mm / min. Preferably, in step S22, the fourth-stage stretching treatment adopts a synchronous stretching method, with a longitudinal and transverse stretching ratio of 8×8 to 10×10 times, a stretching temperature of 125℃ to 135℃, and a stretching rate of 600mm / min to 1200mm / min.

10. A polyolefin porous membrane, characterized in that, The polyolefin porous membrane is prepared by the polyolefin porous membrane preparation method according to any one of claims 1 to 9.

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