Preparation method of double-sided uniform dry-process polyolefin film

Abstract

The invention relates to a preparation method of a double-sided uniform dry-process polyolefin film, which is characterized by comprising the following steps: mixing polyolefin resin and a volatile organic solvent, feeding the mixture into a double-screw extruder, carrying out melt extrusion and curtain coating to form a film sheet, cooling two sides of the film sheet at the same time to generate thermally induced phase separation, and stretching the film sheet to obtain a polyolefin finished product film, a target product is obtained. Compared with the prior art, the shapes of the two sides of the prepared membrane are consistent, the finished membrane with the shapes of the two sides consistent is prepared, and the performance of the finished membrane is effectively guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of polyolefin film preparation, and relates to a method for preparing a double-sided uniform dry-process polyolefin film. Background Art

[0002] At present, lithium battery separators are prepared by dry process using isotactic polypropylene as raw material and wet process using ultra-high molecular weight polyethylene (UHMWPE) as raw material. UHMWPE separators melt without collapse at high temperature, have uniform pore size and high mechanical strength, and occupy more than 70% of the market share in lithium batteries.

[0003] At present, the mainstream of UHMWPE diaphragms is prepared by wet process technology. For example, Shanghai Enjie's patent CN113904059A "A high-pore uniform microporous membrane and its preparation method and battery" proposes to dissolve UHMWPE resin with non-volatile white oil as solvent, and then remove the non-volatile white oil by synchronous or asynchronous biaxial stretching, and then use dichloromethane as an extractant to form pores. The melt of this scheme undergoes thermally induced phase separation through single-roll cooling, and only one side of the membrane contacts the cold roller. The morphology and performance of the two sides of the prepared UHMWPE / white oil membrane are significantly different. The side of the membrane that first contacts the roller surface is often called the glossy side, and the non-roller surface is called the matte side. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing a double-sided uniform dry-process polyolefin membrane in order to overcome the defects of the above-mentioned prior art, so as to solve the problems of the surface morphology and performance differences on both sides of the existing cast UHMWPE porous membrane.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for preparing a double-sided uniform dry-process polyolefin film comprises the following steps: firstly, a polyolefin resin and a volatile organic solvent are mixed and then fed into a twin-screw extruder, melt-extruded and cast into a film sheet, both sides of the obtained film sheet are cooled simultaneously to cause thermally induced phase separation, and then the obtained film sheet is stretched to obtain a polyolefin finished film, which is the target product.

[0007] Furthermore, the two sides of the membrane are cooled simultaneously by double-sided roller cooling, double-sided liquid cooling or double-sided air cooling.

[0008] Furthermore, when double-sided roller cooling is adopted, the distance between the film cast through the die head and the two cooling rollers is defined as the air gap section g, g<30mm, and the temperature of the two cooling rollers is set to 5-30°C, wherein the diameter of one cooling roller is 200-800mm and the other is 100-500mm.

[0009] More preferably, the die head and the cooling roller are arranged in a negative pressure hood, and the volatile solvent vapor is centrally recovered by the negative pressure.

[0010] Further, when double-sided liquid cooling is adopted, the film sheet extruded from the die head enters the coagulation bath device, and a solvent immiscible with the volatile organic solvent is also installed in the coagulation bath device. The distance between the film sheet and the liquid level of the solvent is defined as the air gap section g, and g < 30 mm. Exemplarily, the solvent used in the coagulation bath is water or the like. The solvent in the coagulation bath is maintained at a temperature of 10 - 30°C by a circulating refrigerator, and the length of the coagulation bath ≥ 1.5 meters.

[0011] More preferably, a liquid blocking plate surrounding the die head is further provided in the coagulation bath device, and the bottom end of the liquid blocking plate extends below the liquid level of the coagulation bath device to prevent the condensed liquid from spreading to the entire coagulation bath.

[0012] More preferably, the solvent volatilized at the die head condenses on the liquid cooling surface, and the thickness of the condensed solvent is t, 0 < t < 10 mm. By providing an overflow port in the coagulation bath device, the condensed solvent flows out through the overflow port to maintain the solvent height t.

[0013] Furthermore, when double-sided air cooling is adopted, the die head is placed inside the air cooling box body, and the medium used for air cooling is one or a mixture of several of air, nitrogen, or inert gas, and its temperature is -10°C to 130°C. Preferably, the air cooling medium is nitrogen. In addition, the die head and the air cooling box are in a negative pressure hood, and the volatile solvent vapor is centrally recovered through negative pressure.

[0014] Further, the polyolefin resin is one or a combination of several of ultra-high molecular weight polyethylene polymerized by a conventional magnesium-titanium-based Ziegler-Natta catalyst, ultra-high molecular weight polyethylene with a narrow molecular weight distribution synthesized by a metallocene catalyst, ultra-high molecular weight polyethylene with a bimodal molecular weight distribution, or ultra-high molecular weight polyethylene copolymerized with an α-olefin.

[0015] Further, the volatile organic solvent is one or a combination of several of toluene, xylene, n-nonane, n-decane, undecane, dodecane, kerosene, decalin, and tetralin.

[0016] Further, the stretching is synchronous or asynchronous biaxial stretching, the stretching ratio is 5 - 10 times, the stretching rate is 50 - 800 mm / min, the stretching temperature is 1 - 20°C lower than the melting point of the film sheet, and the stretching rates in the transverse and longitudinal directions are the same or different.

[0017] The prior art adopts single-roll cooling, that is, only one side of the film sheet contacts the cold roll. The side in contact with the cold roll is rapidly cooled, and the side not in contact with the cold roll is cooled more slowly, resulting in differences in the film sheet morphology. The key point of this method is that both sides of the film sheet need to be cooled simultaneously, and the two sides of the prepared film sheet have the same morphology, which is beneficial to preparing a finished film with the same morphology on both sides and effectively guarantees the performance of the finished film. Description of the Drawings

[0018] Figure 1 Schematic diagram of cooling of the film cast by the present invention, wherein a is double-sided roller cooling, b is double-sided liquid cooling, and c is double-sided air cooling;

[0019] Figure 2 This is a schematic diagram of the front and back morphologies of the film obtained by casting and cooling after drying;

[0020] Figure 3 This is a schematic diagram of the front and back morphologies of the film obtained by casting and cooling after drying and stretching;

[0021] Description of the numbers in the figure:

[0022] 1-die head, 2-cooling roller, 3-coagulation bath device, 4-refrigeration cycle machine, 5-liquid blocking plate, 6-air uniforming plate, 7-air cooling box. DETAILED DESCRIPTION

[0023] 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.

[0024] In the following examples, the ultra-high molecular weight polyethylene (UHMWPE) used was purchased from Shanghai Lianle Chemical Technology Co., Ltd., and the resin grade was LL-LB-1080.

[0025] 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.

[0026] The porosity determination method is the national standard GB / T 36363-2018, and the porosity is calculated by the following two formulas:

[0027]

[0028] 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.

[0029] The aperture test method is the national standard GB / T 32361-2015, and the aperture is calculated by the following formula:

[0030]

[0031] 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.

[0032] 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.

[0033] Comparative Example 1

[0034] UHMWPE (molecular weight 1.2 million, 18 wt%) resin and decalin (82 wt%) were accurately metered by a metering pump and then entered into a mixing kettle for uniform mixing. They were then entered into a twin-screw extruder with a screw temperature of 210°C, a die temperature of 180°C, and a screw speed of 60 rpm. The cast film extruded by the screw was cooled by a single roller with a cooling roller temperature of 15°C, an air gap height g of 5 mm, and a film winding rate of 1.0 m / min. The morphology of the obtained film after drying was as follows: Figure 2 a. The dried film was subjected to synchronous biaxial stretching to obtain a finished film at a stretching temperature of 130°C, a stretching speed of 800 mm / min, and a stretching ratio of 8×8. The morphology of the finished film is shown in Figure 3 a, The properties of the finished membranes are summarized in Table 1.

[0035] Example 1

[0036] like Figure 1 As shown in Figure a, UHMWPE (molecular weight 1.2 million, 18 wt%) resin and decalin (82 wt%) are accurately metered by a metering pump and then enter a mixing kettle for uniform mixing, and then enter a twin-screw extruder, with a screw temperature of 210°C, a die temperature of 180°C, and a screw speed of 60 rpm. The cast film extruded from die 1 is cooled by a double-sided roller, the temperature of cooling roller 2 is 15°C, the air gap section height g is 5 mm, and the film winding rate is 1.0 m / min. The morphology of the obtained film after drying is as follows Figure 2 b. The dried film was subjected to synchronous biaxial stretching to obtain a finished film at a stretching temperature of 130°C, a stretching speed of 800 mm / min, and a stretching ratio of 8×8. The morphology of the finished film is shown in Figure 3 b. The properties of the finished membranes are summarized in Table 1.

[0037] Example 2

[0038] like Figure 1As shown in Figure b, UHMWPE (molecular weight 1.2 million, 18wt%) resin and decahydronaphthalene (82wt%) are accurately measured by a metering pump and then enter a mixing kettle for uniform stirring, and then enter a twin-screw extruder with a screw temperature of 210°C, a die temperature of 180°C, and a screw speed of 60rpm. The cast film extruded through the die 1 is cooled by water and specifically sent to a coagulation bath device 3. In the condensation chamber surrounded by the liquid blocking plate 5, etc., the thickness t of the condensed organic solvent layer is 0mm, the coagulation bath is controlled to be 15°C by a refrigeration cycler 4, the air gap section height g is 5mm, the film winding rate is 1.0m / min, and the dried film is subjected to synchronous biaxial stretching to obtain a finished film, the stretching temperature is 130°C, the stretching speed is 800mm / min, and the stretching ratio is 8×8 times. The properties of the finished film are summarized in Table 1.

[0039] Example 3

[0040] On the basis of Example 2, the thickness of the organic solvent layer t was changed to 1 mm, and the other preparation parameters were the same. The properties of the finished film are summarized in Table 1.

[0041] Example 4

[0042] On the basis of Example 3, the thickness t of the organic solvent layer was changed to 2 mm, and the other preparation parameters were the same. The properties of the finished film are summarized in Table 1.

[0043] Example 5

[0044] On the basis of Example 3, the thickness t of the organic solvent layer was changed to 4 mm, and the other preparation parameters were the same. The properties of the finished film are summarized in Table 1.

[0045] Example 6

[0046] like Figure 1 As shown in Figure c, UHMWPE (molecular weight 1.2 million, 18wt%) resin and decalin (82wt%) are accurately metered by a metering pump and then enter a mixing kettle for uniform mixing, and then enter a twin-screw extruder with a screw temperature of 210°C, a die temperature of 180°C, and a screw speed of 60rpm. The 0°C nitrogen distributed by the air-distributing plate 6 is used to blow and cool the two sides of the cast film extruded through the die head 1, and the air intake volume in the air cooling box 7 is 300Nm 3 / h, the film winding rate is 1.0m / min, and the obtained film is dried and then subjected to synchronous biaxial stretching to obtain a finished film. The stretching temperature is 130℃, the stretching speed is 800mm / min, and the stretching ratio is 8×8 times. The properties of the obtained finished film are summarized in Table 1.

[0047] Example 7

[0048] On the basis of Example 6, the film was cooled by blowing air at 0°C, and the other preparation parameters were the same. The properties of the finished film are summarized in Table 1.

[0049] Table 1

[0050] serial number Air permeability (s / 100mL) MD strength (MPa) TD strength (MPa) Porosity (%) Comparative Example 1 63.7 160.4 155.9 68.8 Example 1 57.3 157.8 156.3 79.8 Example 2 56.5 155.4 152.5 81.3 Example 3 49.1 154.2 153.9 84.3 Example 4 20.2 155.4 151.5 86.5 Example 5 10.6 156.1 152.9 83.1 Example 6 15.3 154.2 149.6 82.5 Example 7 19.5 132.5 125.9 79.9

[0051] Comparative Example 1 and Example 1 reflect the difference between single-roll cooling and double-roll cooling. Figure 2 and Figure 3 This shows that double-roll cooling can significantly improve the uniformity of both sides of the membrane, and the air permeability of the finished membrane is also improved to a certain extent. Water cooling can further improve the uniformity of both sides of the membrane, and the air permeability of the membrane can be further reduced. In addition, the thickness of the solvent liquid seal layer has the greatest impact on the membrane permeability. Within a certain range, the thicker the liquid seal, the better the air permeability of the finished membrane. Air cooling can achieve similar effects to water cooling, but the air cooling medium needs to be an inert gas, otherwise it will cause molecular chain degradation and affect the strength of the finished membrane.

[0052] 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 double-sided uniform dry-process polyolefin film, It is characterized in that First, a polyolefin resin and a volatile organic solvent are mixed and then fed into a twin-screw extruder, melt-extruded and cast into a film, and both sides of the obtained film are cooled simultaneously to cause thermally induced phase separation, and then the obtained film is stretched to obtain a polyolefin finished film, which is the target product.

2. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 1, It is characterized in that The two sides of the membrane are cooled simultaneously by double-sided roller cooling, double-sided liquid cooling or double-sided air cooling.

3. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 2, It is characterized in that When double-sided roller cooling is adopted, the distance between the film sheet cast through the die head and the two cooling rollers is less than 30 mm, the temperature of the two cooling rollers is set to 5-30°C, the diameter of one cooling roller is 200-800 mm, and the other is 100-500 mm.

4. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 3, It is characterized in that The die head and the cooling roller are arranged in a negative pressure cover.

5. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 2, It is characterized in that When double-sided liquid cooling is adopted, the film cast from the die head enters a coagulation bath device, in which a solvent immiscible with the volatile organic solvent is also contained, and the distance between the film and the liquid surface of the solvent is less than 30 mm.

6. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 5, It is characterized in that The coagulation bath device is also provided with a liquid blocking plate surrounding the die head, and the bottom end of the liquid blocking plate extends below the liquid surface of the coagulation bath device.

7. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 2, It is characterized in that When double-sided air cooling is used, the die head is placed inside the air cooling box, and the air cooling medium is one or a mixture of air, nitrogen or inert gas, and the temperature is -10°C to 130°C.

8. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 1, It is 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.

9. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 1, It is characterized in that The volatile organic solvent is one or a combination of toluene, xylene, n-nonane, n-decane, undecane, dodecane, kerosene, decahydronaphthalene and tetrahydronaphthalene.

10. The method for preparing a double-sided uniform dry-process polyolefin film according to claim 1, It is characterized in that The stretching is synchronous or asynchronous biaxial stretching, the stretching ratio is 5 to 10 times, the stretching rate is 50 to 800 mm / min, the stretching temperature is 1 to 20° C. lower than the melting point of the film, and the transverse and longitudinal stretching rates are the same or different.

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