Preparation method of aperture-adjustable wet-process polyolefin microporous membrane

Abstract

The invention relates to a preparation method of an aperture-adjustable wet-process polyolefin microporous membrane, which comprises the following steps: blending polyolefin resin and a diluent to form a uniform suspension, carrying out melt extrusion on the suspension through a screw extruder to form a transparent melt sheet, annealing the transparent melt sheet, cooling to room temperature, and carrying out hot stretching to obtain the polyolefin microporous membrane. Compared with the prior art, the method has the advantages that liquid-liquid phase separation is carried out in the annealing process, the pore diameter is increased by utilizing mutual fusion of polymer lean phases, and the pore diameter is regulated and controlled by adjusting the phase separation time, so that the purpose of adjusting the pore diameter is achieved. Meanwhile, the crystallization time is prolonged, the cast sheet crystal structure is improved, and the mechanical strength of the membrane is also improved.

Description

Technical Field

[0001] The invention belongs to the technical field of polyolefin microporous membrane preparation, and relates to a method for preparing a wet-process polyolefin microporous membrane with adjustable pore size. Background Art

[0002] Microporous membrane is a thin film composed of tiny pores, and the diameter of the micropores is usually less than 10 microns. The pore size specifications of the microporous membrane can be changed according to the intended application. The pore size of the membrane can be changed by adjusting the manufacturing process or using different materials. The wet process uses the principle of thermally induced phase separation to extrude the polyolefin resin and the diluent to obtain a jelly, which is then cooled and solidified by direct casting to form a cast sheet, and then the cast sheet is stretched, extracted, dried, heat-set, etc. to obtain a polyolefin microporous membrane. A common use of microporous membranes is filtration. For example, some membranes can be designed to have a specific pore size, such as 0.2 microns, which is usually used for filtering pharmaceutical and biotechnology products. Secondly, it can also be used as a lithium battery separator in the field of new energy. As one of the key parameters, the membrane pore size has an important impact on the performance and safety of lithium batteries.

[0003] In the existing technology, it is difficult to adjust the pore size of polyolefin membranes and the adjustment process is complicated. The small pore size in lithium battery separators leads to low energy transmission efficiency, which has also become an important step restricting the development of lithium batteries.

[0004] Chinese patent CN200910109633.5 provides a method for preparing a polyolefin microporous membrane, comprising the following steps: 1) mixing a polyolefin resin with a diluent, wherein the polyolefin resin and the diluent are calculated in a weight percentage of 10-60%: 40-90%; 2) extruding and blowing the mixture to form a film bubble by extrusion blow molding and then cooling; 3) shaping the film after lamination and traction; 4) extracting the diluent and drying; 5) transverse stretching and shaping; 6) winding. This patent only prepares a polyolefin membrane with a relatively uniform structure, and cannot adjust the pore size. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a wet-process polyolefin microporous membrane with adjustable pore size, so as to solve the problem that the pore size of the existing polyolefin microporous membrane is difficult to adjust.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] In the first aspect, the present invention provides a method for preparing a wet-process polyolefin microporous membrane with adjustable pore size, wherein a polyolefin resin and a diluent are first blended into a uniform suspension, and the suspension is then melt-extruded into a transparent melt sheet through a screw extruder. Subsequently, the transparent melt sheet is annealed, subsequently cooled to room temperature, and heat-stretched to obtain a polyolefin microporous membrane.

[0008] Furthermore, the polyolefin resin is one or a combination of ultra-high molecular weight polyethylene polymerized by conventional magnesium-titanium based Ziegler-Natta catalyst, ultra-high molecular weight polyethylene with narrow molecular weight distribution synthesized by metallocene catalyst, ultra-high molecular weight polyethylene with bimodal molecular weight distribution, or ultra-high molecular weight polyethylene copolymerized with α-olefin.

[0009] Exemplarily, the molecular weight of the ultra-high molecular weight polyethylene is 800,000 to 4 million, and the molecular weight distribution is 2 to 8; the molecular weight of the narrow molecular weight distribution ultra-high molecular weight polyethylene is 800,000 to 4 million, and the molecular weight distribution is 1 to 4; the α-olefin is 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, etc.

[0010] Further, the diluent is one or a combination of white oil, paraffin wax, decalin, tetralin, xylene, phthalate, etc. More preferably, the diluent is white oil or decalin.

[0011] Furthermore, the mass ratio of the polyolefin resin to the diluent is 1:3-5.

[0012] Furthermore, the temperature of the annealing treatment is between the melting point and the crystallization point of the transparent melt sheet, and the annealing time is 10 minutes to 1 hour.

[0013] Furthermore, the transparent melt sheet has a thickness range of 0-100 mm, a width range of 0-10 m, and a length range of 0-100 m.

[0014] Furthermore, the heating method for annealing the melt sheet is one or a combination of hot air heating, hot nitrogen heating, hot roller heating, microwave heating, infrared heating, and steam heating.

[0015] Furthermore, the oil film obtained after heat stretching is further subjected to post-treatment including extraction, drying and / or heat setting.

[0016] Furthermore, the extraction agent used for extraction is a combination of one or more of gasoline, toluene, xylene, hydrocarbon cleaning agent, tetrachloroethylene, and dichloromethane. More preferably, the extraction agent is dichloromethane.

[0017] In a second aspect, the present invention provides a method for adjusting the pore size of a wet-process polyolefin microporous membrane, wherein a polyolefin resin and a diluent are mixed into a uniform suspension, the suspension is melt-extruded into a transparent melt sheet through a screw extruder, and then the transparent melt sheet is annealed and the annealing time is adjusted, and then cooled to room temperature and heat-stretched to obtain a polyolefin microporous membrane with adjustable pore size.

[0018] Furthermore, the annealing temperature (Ta) is a temperature between the melting point (Tm) and the crystallization point (Tc) of the transparent melt sheet, and the annealing time is 10 minutes to 1 hour.

[0019] More preferably, when the pore diameter is 20-30 nm, the annealing time is 5-10 min; when the pore diameter is 30-40 nm, the annealing time is 10-20 min; when the pore diameter is 40-50 nm, the annealing time is 20-30 min; when the pore diameter is 50-60 nm, the annealing time is 30-40 min.

[0020] Compared with the prior art, the present invention firstly separates liquid-liquid phases during the annealing process, utilizes the mutual fusion of polymer-poor phases to increase the pore size, and adjusts the pore size by adjusting the phase separation time, thereby achieving the purpose of adjustable pore size. At the same time, the crystallization time is extended to improve the crystal structure of the casting sheet and also improve the mechanical strength of the membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 DSC curve of 20wt% UHMWPE / white oil jelly;

[0022] Figure 2 DSC curve of 18wt% UHMWPE / decalin jelly;

[0023] Figure 3 It is a schematic diagram of preparing the melt sheet of the present invention;

[0024] Figure 4 This is the pore size distribution diagram of Comparative Example 1;

[0025] Figure 5 This is the pore size distribution diagram of Example 3. DETAILED DESCRIPTION

[0026] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0027] In the following examples, the ultra-high molecular weight polyethylene (UHMWPE) used was purchased from Shanghai Lianle Chemical Technology Co., Ltd., product model LL-LB-1080.

[0028] T m With T c The measurement is carried out by differential scanning calorimetry (DSC). 5-10 mg of sample is sealed in an aluminum crucible, and the temperature is raised from room temperature to 150°C at 10°C / min, kept constant for 5 minutes, and then cooled to room temperature at the same rate. The temperature corresponding to the highest peak of the scanning curve is determined as T m (Tempering curve) or T c (Temperature reduction curve).

[0029] The tensile strength test method is in accordance with GB / T 1040.3-2006. The specimen type is type II specimen, the specimen length is 150mm, the width is 15mm, the distance between the clamps is (100±5mm), and the tensile rate is (250±10)mm / min.

[0030] The puncture strength test method is in accordance with GB / T 6672-2001. The thickness of the diaphragm is 20 μm, the diameter of the puncture needle is 1.0 mm, the radius of the tip sphere is 0.5 mm, the inner diameter of the sample fixture is 10 mm, and the puncture rate is (100 ± 10) mm / min.

[0031] The air permeability test method is based on the Gurley method in GB / T 458-2008, which is defined as 100 mL of gas passing through a 25.4 mm diameter tube at a pressure of 275.79 kPa. 2 (1in 2 ) area of ​​the diaphragm.

[0032] The porosity determination method is the national standard GB / T 36363-2018, and the porosity is calculated by the following two formulas:

[0033]

[0034] Among them, 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, ρ0 (g / cm 3 ) is the raw material density.

[0035] The aperture test method is the national standard GB / T 32361-2015, and the aperture is calculated by the following formula:

[0036]

[0037] Where D (μm) is the pore diameter, γ (mN / m) is the liquid surface tension, θ is the liquid contact angle, and p (Psi) is the gas pressure.

[0038] The ionic conductivity test method is the national standard GB / T 36363-2018. The ionic conductivity is calculated by the following formula:

[0039]

[0040] Wherein, σ is the ionic conductivity of the membrane, in Siemens per centimeter (S / cm), d is the thickness of one membrane layer, in micrometers (μm), R is the resistance value of one membrane layer, in ohms (Ω), and S is the area of ​​the membrane cut during the test, in square centimeters (cm 2 ).

[0041] The rest of the raw materials or processing techniques, unless otherwise specified, are conventional commercially available raw materials or conventional processing techniques in the art.

[0042] Comparative Example 1

[0043] UHMWPE (20wt%) and 50# white oil (80wt%) were mixed evenly to form a uniform suspension. The suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder. The extrusion temperature was 180°C, and the sheet was directly cooled to obtain a cast sheet. The cast sheet was synchronously stretched 8×8 times at 130°C to obtain an oil film. The oil film was extracted in dichloromethane. After the film was extracted, it was dried at 60°C in a blast oven. The residual dichloromethane was taken out within 20 minutes by hot air, and then it was heat-set at 135°C in a blast oven for 3 minutes to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0044] Comparative Example 2

[0045] UHMWPE (20wt%) and decahydronaphthalene (80wt%) were mixed evenly to form a uniform suspension, and the suspension was melt-extruded into a transparent melt sheet with a thickness of about 1 mm through a twin-screw extruder at an extrusion temperature of 180°C, and directly cooled to obtain a cast sheet, and then synchronously stretched 8×8 times at 100°C to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0046] Comparative Example 3

[0047] UHMWPE (20wt%) and decahydronaphthalene (80wt%) were mixed uniformly to form a uniform suspension, and the suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder, and the extrusion temperature was 180°C. The melt sheet was directly cooled to 100°C for annealing for 1min, and then the cast sheet was cooled and stretched 8×8 times at 100°C, and then dried at 60°C in a blast oven, and then heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0048] Example 1

[0049] UHMWPE (20wt%) and 50# white oil (80wt%) were mixed evenly to form a uniform suspension. The suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder. The extrusion temperature was 180°C. The melt sheet was directly cooled to 105°C for annealing for 40min, and then the cast sheet (i.e., melt sheet) was cooled and synchronously stretched 8×8 times at 130°C to obtain an oil film. The oil film was tightened and fixed on the frame and extracted in dichloromethane. After the film was extracted, it was dried in a blast oven at 60°C, and the residual dichloromethane was taken out within 20min by hot air, and then heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0050] Example 2

[0051] UHMWPE (20wt%) and 50# white oil (80wt%) were mixed evenly to form a uniform suspension. The suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder. The extrusion temperature was 180°C. The melt sheet was directly cooled to 110°C for annealing for 40min, and then the cast sheet was cooled and stretched 8×8 times at 130°C to obtain an oil film. The oil film was extracted in dichloromethane. After the film was extracted, it was dried at 60°C in a blast oven, and the residual dichloromethane was taken out within 20min by hot air. Then, it was heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0052] Example 3

[0053] UHMWPE (20wt%) and 50# white oil (80wt%) were mixed evenly to form a uniform suspension. The suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder. The extrusion temperature was 180°C. The melt sheet was directly cooled to 115°C for annealing for 40min, and then the cast sheet was cooled and stretched 8×8 times at 130°C to obtain an oil film. The oil film was extracted in dichloromethane. After the film was extracted, it was dried in a blast oven at 60°C, and the residual dichloromethane was taken out within 20min by hot air, and then heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0054] Example 4

[0055] UHMWPE (20wt%) and 50# white oil (80wt%) were mixed evenly to form a uniform suspension. The suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder. The extrusion temperature was 180°C. The melt sheet was directly cooled to 120°C for annealing for 40min, and then the cast sheet was cooled and stretched 8×8 times at 130°C to obtain an oil film. The oil film was extracted in dichloromethane. After the film was extracted, it was dried at 60°C in a blast oven, and the residual dichloromethane was taken out within 20min by hot air. Then, it was heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0056] Example 5

[0057] UHMWPE (20wt%) and decahydronaphthalene (80wt%) were mixed uniformly to form a uniform suspension, and the suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder, and the extrusion temperature was 180°C. The melt sheet was directly cooled to 100°C for annealing for 20min, and then the cast sheet was cooled and stretched 8×8 times at 100°C, and then dried at 60°C in a blast oven, and then heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0058] Example 6

[0059] UHMWPE (20wt%) and decahydronaphthalene (80wt%) were mixed to form a uniform suspension, and the suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder, and the extrusion temperature was 180°C. The melt sheet was directly cooled to 100°C for annealing for 40min, and then the cast sheet was cooled and stretched 8×8 times at 100°C, and then dried at 60°C in a blast oven, and then heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0060] Example 7

[0061] UHMWPE (20wt%) and decahydronaphthalene (80wt%) were mixed to form a uniform suspension, and the suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder, and the extrusion temperature was 180°C. The melt sheet was directly cooled to 100°C for annealing for 1h, and then the cast sheet was cooled and stretched 8×8 times at 100°C, and then dried at 60°C in a blast oven, and then heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0062] Example 8

[0063] UHMWPE (20wt%) and decahydronaphthalene (80wt%) were mixed uniformly to form a uniform suspension, and the suspension was melt-extruded into a transparent melt sheet with a thickness of about 1mm through a twin-screw extruder, and the extrusion temperature was 180°C. The melt sheet was directly cooled to 100°C for annealing for 10min, and then the cast sheet was cooled and stretched 8×8 times at 100°C, and then dried at 60°C in a blast oven, and then heat-set at 135°C in a blast oven to obtain a polyolefin microporous membrane. The product properties are shown in Table 1.

[0064] Table 1 Properties of polyolefin microporous membranes in various embodiments and comparative examples

[0065]

[0066] Figure 1 The DSC curves of the heating and cooling of 20wt% UHMWPE / white oil gel show that its T m With T c They are 121.75℃ and 108.53℃ respectively. Figure 2 The DSC curves of the heating and cooling of 18wt% UHMWPE / decalin gel show that its T m With T c They are 103.19℃ and 96.57℃ respectively. c After annealing, the system is in the solid-liquid phase separation zone, and the polymer-poor phases will not merge with each other; m After heat treatment, the system is in a homogeneous state and phase separation has not yet occurred. Therefore, the annealing temperature should be between T m With T c Comparative Examples 1 and 2 reflect the performance of the diaphragm obtained by the prior art. Examples 1-4 explore the effect of annealing temperature on the pore size of the finished membrane. In the temperature range between the crystallization point and the melting point, the pore size increases as the temperature rises. Examples 5-7 explore the effect of annealing time on the finished diaphragm. The longer the annealing time, the larger the pore size. At the same time, the ionic conductivity increases with the pore size. Comparative Example 3 is annealed for 1 minute, and Example 8 is annealed for 10 minutes. The annealing time of this method is more than 10 minutes to increase the pore size. The pore size of Examples 5-7 does not change much because the solvent used in Examples 5-7 is decahydronaphthalene, and there is no obvious liquid-liquid phase separation, which does not have much effect on the pore size, but it also conforms to the law of the present invention.

[0067] Figure 3 is a schematic diagram of the preparation process of the present invention, Figure 4 This is the pore size distribution diagram of comparative example 1 before annealing, the pore size distribution is around 0.029 microns, Figure 5 This is the pore size distribution diagram of Example 3 after annealing for 40 minutes at an annealing temperature of 115°C. The pore size distribution is around 0.035 microns, and the pore size increases after annealing. Figure 4 and Figure 5 It reflects that the pore size of the diaphragm can be significantly increased through the annealing process, thereby achieving the purpose of adjustable pore size. The parameter table of pore size range changing with annealing temperature and annealing time is shown in Table 2.

[0068] Table 2 Parameters of pore size range changing with annealing temperature and annealing time

[0069] serial number Pore ​​diameter / nm Annealing temperature (Ta) / ℃ Annealing time / min 1 20-30 Tc<Ta<Tm-10 5-10 2 30-40 Tc<Ta<Tm-8 10-30 3 40-50 Tc<Ta<Tm-6 30-50 4 50-60 Tc<Ta<Tm-4 50-70

[0070] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for preparing a wet-process polyolefin microporous membrane with adjustable pore size, characterized in that: Firstly, the polyolefin resin and the diluent are mixed into a uniform suspension, and then the suspension is melt-extruded into a transparent melt sheet through a screw extruder. Then, the transparent melt sheet is annealed, then cooled to room temperature, and heat-stretched to obtain a polyolefin microporous membrane.

2. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The polyolefin resin is one or a combination of ultra-high molecular weight polyethylene polymerized by conventional magnesium-titanium series Ziegler-Natta catalyst, ultra-high molecular weight polyethylene with narrow molecular weight distribution synthesized by metallocene catalyst, ultra-high molecular weight polyethylene with bimodal molecular weight distribution, or ultra-high molecular weight polyethylene copolymerized with α-olefin.

3. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The diluent is one or a combination of white oil, solid paraffin, decahydronaphthalene, tetrahydronaphthalene, xylene, and phthalate.

4. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The mass ratio of the polyolefin resin to the diluent is 1:3-5.

5. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The temperature of the annealing treatment is between the melting point and the crystallization point of the transparent melt sheet, and the annealing time is 10 minutes to 1 hour.

6. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The transparent melt sheet has a thickness range of 0-100 mm, a width range of 0-10 m, and a length range of 0-100 m, and the thickness, width, and length are not zero.

7. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The heating method for annealing the transparent melt sheet is one or a combination of hot air heating, hot nitrogen heating, hot roller heating, microwave heating, infrared heating, and steam heating.

8. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The annealing temperature Ta is a temperature between the melting point Tm and the crystallization point Tc of the transparent melt sheet, and the annealing time is 10 minutes to 1 hour.

9. The method for preparing a wet-process polyolefin microporous membrane with adjustable pore size according to claim 1, characterized in that: The oil film obtained after heat stretching is further subjected to post-treatment including extraction, drying and / or heat setting; the extraction agent used for extraction is a combination of one or more of gasoline, toluene, xylene, hydrocarbon cleaning agent, tetrachloroethylene, and dichloromethane.

10. A method for adjusting the pore size of a wet-process polyolefin microporous membrane, characterized in that: The polyolefin resin and the diluent are mixed into a uniform suspension, and the suspension is melt-extruded into a transparent melt sheet through a screw extruder. Then, the transparent melt sheet is annealed and the annealing time is adjusted, and then cooled to room temperature and heat-stretched to obtain a polyolefin microporous membrane with adjustable pore size.

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