Method for preparing double-sided uniform dry-process polyolefin membrane

Abstract

The present invention relates to a method for preparing a double-sided uniform dry-process polyolefin membrane, characterized in that a polyolefin resin and a volatile organic solvent are first mixed and then fed into a twin-screw extruder and are subjected to melt extrusion and casting to form a membrane, both sides of the obtained membrane are simultaneously cooled to induce thermally induced phase separation, and then the obtained membrane is stretched to obtain a finished polyolefin membrane, i.e., a target product. Compared with the prior art, the membrane prepared in the present invention has the same morphology on both sides, thereby facilitating the preparation of finished membranes having the same morphology on both sides, and effectively ensuring the performance of the finished membranes.

Description

Preparation method of double-sided uniform dry method polyolefin film TECHNICAL FIELD

[0001] The present application belongs to the technical field of polyolefin film preparation, and relates to a preparation method of double-sided uniform dry method polyolefin film. BACKGROUND

[0002] At present, the lithium battery separator is prepared by a dry method using isotactic polypropylene as raw material and a wet method using ultra-high molecular weight polyethylene (UHMWPE) as raw material. The UHMWPE separator has a high temperature melting without collapse, uniform pore size and high mechanical strength, and accounts for more than 70% of the market share in lithium batteries.

[0003] At present, the wet method is mainly used to prepare the UHMWPE separator, such as the patent CN113904059A "A high-pore uniformity microporous membrane and a preparation method thereof and a battery" of Shanghai Enjie, which proposes to dissolve the UHMWPE resin with non-volatile white oil as a solvent, to stretch synchronously or asynchronously in two directions, and to remove the non-volatile white oil with dichloromethane as an extractant to form pores. The melt of the scheme undergoes thermal phase separation through single-roller cooling, and the film is only in contact with one side of the cooling roller. The UHMWPE / white oil film prepared by the scheme has obvious differences in morphology and performance on both sides, and the side first contacting the roller is commonly called a glossy surface, and the side not contacting the roller is called a matte surface. SUMMARY

[0004] The present application aims to overcome the defects of the prior art and provides a preparation method of double-sided uniform dry method polyolefin film to solve the problems of the difference in surface morphology and performance between the two sides of the existing cast UHMWPE porous film.

[0005] The object of the present application can be achieved by the following technical solutions:

[0006] The preparation method of the double-sided uniform dry method polyolefin film comprises the following steps: mixing polyolefin resin and volatile organic solvent, feeding the mixture into a double-screw extruder, melt extruding and casting a film, cooling both sides of the obtained film to cause thermal phase separation, and then stretching the obtained film to obtain a polyolefin finished film, which is the target product.

[0007] Further, the cooling of both sides of the film is achieved by double-sided roller cooling, double-sided liquid cooling or double-sided air cooling.

[0008] Further, when double-sided roller cooling is used, the distance between the film cast by the die and the two cooling rollers is set as an air gap section g, g<30mm, the temperature of the two cooling rollers is set to 5-30℃, and the diameter of one of the cooling rollers is 200-800mm and the diameter of the other is 100-500mm.

[0009] More preferably, the die and the cooling roll are arranged in a negative pressure cover, and the volatile solvent vapor is collected and recovered by negative pressure.

[0010] Further, when using double-sided liquid cooling, the film cast by the die enters a coagulation bath device, and a solvent immiscible with the volatile organic solvent is also arranged in the coagulation bath device, and the distance between the film and the liquid surface of the solvent is set as an air gap g, g < 30 mm. Exemplarily, the solvent used in the coagulation bath is water or the like. The solvent of the coagulation bath is maintained at a temperature of 10-30℃ by a circulating refrigerator, and the length of the coagulation bath is ≥1.5 meters.

[0011] More preferably, a liquid blocking plate surrounding the die is arranged in the coagulation bath device, and the bottom end of the liquid blocking plate extends below the liquid surface of the coagulation bath device to prevent the spread of the condensed liquid to the entire coagulation bath.

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

[0013] Further, when using double-sided air cooling, the die is arranged inside an air cooling box, and the medium used for air cooling is one or a mixture of several of air, nitrogen or inert gas, and the temperature is -10℃-130℃. Preferably, the air cooling medium is nitrogen. In addition, the die and the air cooling box are arranged in a negative pressure cover, and the volatile solvent vapor is collected and recovered by negative pressure.

[0014] Further, the polyolefin resin is one or a combination of several of ultrahigh molecular weight polyethylene polymerized by a conventional magnesium-titanium Ziegler-Natta catalyst, narrow molecular weight distribution ultrahigh molecular weight polyethylene synthesized by a metallocene catalyst, bimodal molecular weight distribution ultrahigh molecular weight polyethylene, or ultrahigh 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, decahydronaphthalene, tetrahydronaphthalene.

[0016] Further, the stretching is synchronous or asynchronous bidirectional stretching, the stretching ratio is 5-10 times, the stretching rate is 50-800 mm / min, the stretching temperature is 1-20℃ lower than the melting point of the film, and the stretching rate in the transverse direction and the longitudinal direction is the same or different.

[0017] The prior art adopts single-roller cooling, that is, the film sheet is only in contact with a cold roller on one side. The side in contact with the cold roller is rapidly cooled, and the side not in contact with the cold roller is slowly cooled, thereby causing the film sheet to have different appearances. The key point of the method is that the two sides of the film sheet need to be cooled at the same time, the film sheet prepared has consistent appearances on the two sides, which is beneficial to the preparation of a finished film with consistent appearances on the two sides and effectively ensures the performance of the finished film. BRIEF DESCRIPTION OF DRAWINGS

[0018] Fig. 1 is a schematic diagram of the cooling of the film sheet cast out by the present application, wherein a is double-side roller cooling, b is double-side liquid cooling, and c is double-side air cooling;

[0019] Fig. 2 is a schematic diagram of the appearance of the film sheet obtained by casting and cooling after drying;

[0020] Fig. 3 is a schematic diagram of the appearance of the film sheet obtained by casting and cooling after drying and stretching;

[0021] Explanation of reference numerals in the drawing: 1 - die, 2 - cooling roller, 3 - coagulation bath device, 4 - refrigeration cycle machine, 5 - liquid blocking plate, 6 - air uniformizing plate, and 7 - air cooling box. DETAILED DESCRIPTION

[0022] The present application will be described in detail below in combination with the drawings and specific examples. The present embodiment is implemented on the premise of the technical solution of the present application, and a detailed implementation and specific operation process are given, but the protection scope of the present application is not limited to the following examples.

[0023] In each of the following examples, the ultra-high molecular weight polyethylene (UHMWPE) used is purchased from Shanghai Lianle Chemical Technology Co., Ltd., and the resin model number is LL-LB-1080.

[0024] The tensile strength test method is tested according to GB / T 1040.3-2006. The sample type is selected as type II sample, the sample length is 150 mm, the sample width is 15 mm, the distance between the clamps is (100±5) mm, and the tensile rate is (250±10) mm / min.

[0025] The porosity determination method is the national standard GB / T 36363-2018, and the porosity is calculated by the following two formulas:

[0026] wherein, ρ1(g / cm 3 ) is the surface density of the diaphragm, m (g) is the mass of the diaphragm, L (cm) is the length of the diaphragm, b (cm) is the width of the diaphragm, ε (%) is the porosity of the diaphragm, d (μm) is the thickness of the diaphragm, and ρ0(g / cm 3 ) is the density of the raw material.

[0027] The pore size test method is the national standard GB / T 32361-2015, and the pore size is calculated by the following formula:

[0028] Wherein, D (pm) is the pore diameter, γ (mN / m) is the liquid surface tension, θ is the liquid contact angle, P (Psi) is the gas pressure.

[0029] The rest of the raw materials or processing techniques, if not specified, indicate that they are all conventional commercially available raw materials or conventional processing techniques in the art.

[0030] Comparative Example 1

[0031] UHMWPE (molecular weight 1.2 million, 18wt%) resin and decalin (82wt%) are accurately metered by a metering pump into a mixing kettle and stirred uniformly, then into a twin-screw extruder, screw temperature 210℃, die temperature 180℃, screw speed 60rpm, the cast film extruded from the die is cooled by a single roll, the cooling roll temperature is 15℃, the air gap section height g is 5mm, the film winding rate is 1.0m / min, the morphology of the obtained film after drying is shown in Figure 2a, the dried film is synchronously bidirectionally stretched to obtain the finished film, the stretching temperature is 130℃, the stretching speed is 800mm / min, the stretching ratio is 8x8 times, the morphology of the finished film is shown in Figure 3a, and the performance of the obtained finished film is summarized in Table 1.

[0032] Example 1

[0033] As shown in Figure 1a, UHMWPE (molecular weight 1.2 million, 18wt%) resin and decalin (82wt%) are accurately metered by a metering pump into a mixing kettle and stirred uniformly, then into a twin-screw extruder, screw temperature 210℃, die temperature 180℃, screw speed 60rpm, the cast film extruded from the die is cooled by a double-sided roll, the cooling roll 2 temperature is 15℃, the air gap section height g is 5mm, the film winding rate is 1.0m / min, the morphology of the obtained film after drying is shown in Figure 2b, the dried film is synchronously bidirectionally stretched to obtain the finished film, the stretching temperature is 130℃, the stretching speed is 800mm / min, the stretching ratio is 8x8 times, the morphology of the finished film is shown in Figure 3b, and the performance of the obtained finished film is summarized in Table 1.

[0034] Example 2

[0035] As shown in Figure 1b, the UHMWPE (molecular weight 1.2 million, 18wt%) resin and decalin (82wt%) are precisely metered by a metering pump, then enter a mixing kettle for stirring, and then enter a twin-screw extruder, with screw temperature 210°C, die temperature 180°C, screw speed 60 rpm, the cast film extruded through die 1 is cooled by water, and is fed into a solidification bath device 3, in which the thickness t of the condensed organic solvent layer is 0 mm, the temperature of the solidification bath is controlled by a refrigeration cycle machine 4 to be 15°C, the height g of the air gap section is 5 mm, the film winding speed is 1.0 m / min, and the finished film is obtained after the dried film is synchronously bidirectionally stretched, with stretching temperature 130°C, stretching speed 800 mm / min, and stretching ratio 8x8 times. The performance of the finished film is summarized in Table 1.

[0036] Example 3

[0037] On the basis of Example 2, the thickness t of the organic solvent layer is changed to 1 mm, and the other preparation parameters are the same, and the performance of the finished film obtained is summarized in Table 1.

[0038] Example 4

[0039] On the basis of Example 3, the thickness t of the organic solvent layer is changed to 2 mm, and the other preparation parameters are the same, and the performance of the finished film obtained is summarized in Table 1.

[0040] Example 5

[0041] On the basis of Example 3, the thickness t of the organic solvent layer is changed to 4 mm, and the other preparation parameters are the same, and the performance of the finished film obtained is summarized in Table 1.

[0042] Example 6

[0043] As shown in Figure 1c, the UHMWPE (molecular weight 1.2 million, 18wt%) resin and decalin (82wt%) are precisely metered by a metering pump, then enter a mixing kettle for stirring, and then enter a twin-screw extruder, with screw temperature 210°C, die temperature 180°C, screw speed 60 rpm, the cast film extruded through die 1 is cooled by 0°C nitrogen gas distributed by a uniform air distribution plate 6, the air inlet amount in the air cooling box 7 is 300 Nm 3 / h, the film winding speed is 1.0 m / min, and the finished film is obtained after the dried film is synchronously bidirectionally stretched, with stretching temperature 130°C, stretching speed 800 mm / min, and stretching ratio 8x8 times. The performance of the finished film is summarized in Table 1.

[0044] Example 7

[0045] On the basis of Example 6, the cast film extruded through die 1 is cooled by 0°C air, and the other preparation parameters are the same, and the performance of the finished film obtained is summarized in Table 1.

[0046] Table 1

[0047] Comparative Example 1 and Example 1 reflect the difference between single roll cooling and double roll cooling. Figures 2 and 3 show that double roll cooling can significantly improve the uniformity of both sides of the film and to some extent improve the air permeability of the finished film. Water cooling can further improve the uniformity of both sides of the film and further reduce the air permeability of the film. In addition, the thickness of the solvent liquid seal layer has the greatest impact on the air permeability of the film. Within a certain range, the thicker the liquid seal, the better the air permeability of the finished film. Wind cooling can achieve similar results to water cooling, but the wind cooling medium needs to be inert gas, otherwise it will cause molecular chain degradation and affect the strength of the finished film.

[0048] The above description of the examples is to facilitate the understanding and use of the invention by those of ordinary skill in the art. Those skilled in the art can easily make various modifications to these examples and apply the general principles described herein to other examples without having to undergo creative labor. Therefore, the present application is not limited to the above examples, and any improvements and modifications made by those skilled in the art based on the disclosure of the present application without departing from the scope of the present application should be within the scope of protection of the present application.

Claims

1. A process for the production of a double-sided uniform dry polyolefin film, characterized by, The polyolefin resin and the volatile organic solvent are mixed and then fed into a twin-screw extruder, melted, extruded and cast into a film sheet, the film sheet is cooled on both sides to cause thermal-induced phase separation, and then the film sheet is stretched to obtain a polyolefin finished film, which is the target product.

2. The process for preparing a uniform dry polyolefin film on both sides according to claim 1, characterized in that, The film sheet is cooled on both sides by double-sided roller cooling, double-sided liquid cooling or double-sided air cooling.

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

4. The method of claim 3, wherein the method is characterized by, The die and the cooling rollers are arranged in a negative pressure cover.

5. The process for making a biaxially uniform dry polyolefin film according to claim 2, characterized in that, When the double-sided liquid cooling is used, the film sheet cast out of the die is fed into a solidification bath device, and a solvent immiscible with the volatile organic solvent is also provided in the solidification bath device, the distance between the film sheet and the liquid surface of the solvent is less than 30 mm.

6. The process for preparing a dual-sided uniform dry polyolefin film according to claim 5, characterized in that, A liquid blocking plate is also provided in the solidification bath device to surround the die, and the bottom end of the liquid blocking plate extends below the liquid surface of the solidification bath device.

7. The process for making a dual-sided uniform dry polyolefin film according to claim 2, wherein, When the double-sided air cooling is used, the die is arranged in an air cooling box, and the medium for air cooling is air, nitrogen or a mixture of one or more of inert gases, and the temperature of the medium is -10℃-130℃.

8. The process for preparing a dual-sided uniform dry polyolefin film according to claim 1, characterized in that, The polyolefin resin is one or a combination of ultrahigh molecular weight polyethylene polymerized by a conventional magnesium-titanium Ziegler-Natta catalyst, narrow molecular weight distribution ultrahigh molecular weight polyethylene synthesized by a metallocene catalyst, bimodal molecular weight distribution ultrahigh molecular weight polyethylene or ultrahigh molecular weight polyethylene copolymerized with an α-olefin.

9. The method of claim 1, wherein the method is characterized by, The volatile organic solvent is one or a combination of toluene, xylene, n-nonane, n-decane, undecane, dodecane, kerosene, decalin, tetralin.

10. The method of claim 1, wherein the method is characterized by, The stretching is synchronous or asynchronous bidirectional stretching, the stretching ratio is 5-10 times, the stretching rate is 50-800 mm / min, the stretching temperature is 1-20℃ lower than the melting point of the film sheet, and the stretching rate in the transverse direction is the same as or different from that in the longitudinal direction.

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