Polyolefin resin composition for porous film

Abstract

Disclosed is a polyolefin resin composition for use in the production of a single-layer dry porous membrane having excellent melting and closing properties, and a porous membrane using the same. The present invention provides a polyolefin resin composition for a porous membrane, comprising: 30 to 70 wt% of a high-density polyethylene having a melt flow index (190°C, 2.16 kg load) of 0.1 to 5 g / 10 min, a crystallization temperature of 115 to 125°C, and a crystallinity of 75% or more; and 30 to 70 wt% of a high-crystalline polypropylene having a melt flow index (230°C, 2.16 kg load) of 5 to 20 g / 10 min.

Description

Technical Field

[0001] This invention relates to a polyolefin resin composition for porous membranes, and more specifically to a polyolefin resin composition for porous membranes with excellent melting and closed-cell properties.

[0002] This application claims priority and benefit to Korean Patent Application No. 10-2020-0154902, filed on November 18, 2020, the entire contents of which are incorporated herein by reference. Background Technology

[0003] Porous membranes are widely used in applications requiring the separation of specific components, such as separators and filters for secondary batteries. Recently, the demand for porous membranes in thin-film form has been increasing, particularly for polyolefins. As demand grows, the amount of raw materials used is also gradually increasing. Among these, polyethylene, which can meet the required physical properties even with relatively thin thicknesses, shows a significant upward trend in demand.

[0004] Recently, with the increasing capacity and power of secondary batteries, battery safety issues are becoming more prominent. Polyolefin porous membranes are not used alone due to their melting and closed-cell characteristics; they are generally prepared by stacking polypropylene microporous membranes and polyethylene microporous membranes. Batteries preferably have a relatively low closing temperature and a relatively high melting temperature. Polyethylene resins are used to achieve the lower closing temperature, while polypropylene resins are used to achieve the higher melting temperature (membrane damage temperature).

[0005] Thus, the membranes used in batteries in the past were constructed with a two-layer structure of polypropylene porous layer / polyethylene porous layer or a three-layer structure of polypropylene porous layer / polyethylene porous layer / polypropylene porous layer, which resulted in complex production processes, more control variables, and increased preparation costs.

[0006] Korean Patent No. 1305264 only mentions conventional multilayer membranes, which are polyolefin multilayer microporous membranes formed by stacking a porous layer A made of polypropylene and a porous layer B made of polyethylene resin in the order of porous layer B / porous layer A / porous layer B. Summary of the Invention

[0007] The present invention provides a polyolefin resin composition and a porous membrane thereof, the polyolefin resin composition being used in the preparation of a single-layer dry porous membrane with excellent melting and closing properties.

[0008] To address the aforementioned technical problems, the present invention provides a polyolefin resin composition for porous membranes, comprising: 20-70% by weight of high-density polyethylene, wherein the high-density polyethylene has a melt flow index (190°C, 2.16 kg load) of 0.1-5 g / 10 min, a crystallization temperature of 115-125°C, and a crystallinity of 75% or higher; and 30-80% by weight of highly crystalline polypropylene, wherein the highly crystalline polypropylene has a melt flow index (230°C, 2.16 kg load) of 5-20 g / 10 min.

[0009] In addition, a polyolefin resin composition for porous membranes is provided, characterized in that the closed-cell temperature of the resin composition, measured by the following method, is below 140°C, the melting temperature is above 160°C, and the puncture strength is above 520 gf.

[0010] [Measurement Method]

[0011] After forming a sheet by extruding the resin composition at 220–250°C using a biaxial extruder via a T-die, a single-layer porous film with a thickness of 15 μm is prepared by successively stretching along the MD and TD directions inside a stretching machine. A 50 × 50 mm sample of the porous film is placed on a plate with holes of 10 mm in diameter. The force required to penetrate the film when pressed by a 1 mm probe is defined as the puncture strength. While heating a lithium-ion secondary battery prepared using the porous film in an oven at a rate of 2°C / min, the resistance is measured in real time. The temperature at which the resistance exceeds 10,000 Ω is defined as the pore-closing temperature, and the temperature at which the resistance drops sharply after the pore-closing temperature is defined as the melting temperature.

[0012] In addition, a porous membrane is provided, wherein the porous membrane is a single-layer membrane formed using the above-mentioned resin composition.

[0013] According to the present invention, in the preparation of porous membranes using polyolefin resins, by mixing high-density polyethylene with specific flowability and crystallization characteristics and highly crystalline polypropylene with specific flowability in a specific ratio, the complex multilayer structure of the past is eliminated, thereby enabling the provision of a porous membrane polyolefin resin composition and a porous membrane using the same through single-layer molding, which is both simple to process and achieves excellent closed-cell and melting characteristics. Detailed Implementation

[0014] The present invention will now be described in detail through preferred embodiments. Before proceeding, it should be noted that the terms and vocabulary used in this specification and claims should not be limited to their general or dictionary meanings. Based on the principle that "inventors may appropriately define the concepts of terms in order to best illustrate their invention," the aforementioned terms and vocabulary should be interpreted as conforming to the meaning and concept of the technical concept of the present invention. Therefore, the structures of the embodiments described in this specification are merely one preferred embodiment of the present invention and do not represent all the technical concepts of the present invention. It should be understood that at the time of this application, various equivalents and modifications could be used to replace them.

[0015] The inventors recognized that in the preparation of battery films that utilize polyolefin resins to enhance safety, multilayer films using polyethylene resins for lower closed-cell characteristics and polypropylene resins for higher melting-breaking characteristics have complex structures, leading to process and cost issues. Through repeated research on porous films using polyolefin resin compositions that achieve excellent closed-cell and melting-breaking characteristics even with a single-layer structure, it was confirmed that when high-density polyethylene with specific flowability and crystallinity characteristics and highly crystalline polypropylene with specific flowability are mixed in a specific ratio, excellent closed-cell and melting-breaking characteristics can also be achieved through single-layer molding, thus leading to the present invention.

[0016] Therefore, the present invention discloses a polyolefin resin composition for porous membranes, comprising: 20-70% by weight of high-density polyethylene, wherein the high-density polyethylene has a melt flow index (190°C, 2.16 kg load) of 0.1-5 g / 10 min, a crystallization temperature of 115-125°C, and a crystallinity of 75% or more; and 30-80% by weight of highly crystalline polypropylene, wherein the highly crystalline polypropylene has a melt flow index (230°C, 2.16 kg load) of 5-20 g / 10 min.

[0017] The polyolefin resin composition of the present invention imparts closed-cell properties to the membrane to improve the stability of the secondary battery. In the present invention, "closed-cell" refers to the property that the membrane partially melts and closes the pores at a temperature above a certain temperature. At high temperatures, the short circuit caused by the thermal deformation of the membrane, which directly connects the positive and negative electrodes of the secondary battery, is the main cause of explosions and fires. However, such accidents can be prevented by closing the pores.

[0018] Furthermore, in this invention, the term "meltdown" (or "film breakage") refers to the phenomenon where the film melts at a temperature above a certain temperature, resulting in damage to a portion of the film. When this phenomenon occurs, accidents such as fires or explosions may occur due to a short circuit caused by the connection of the positive and negative electrodes of the secondary battery.

[0019] In this invention, the high-density polyethylene is generally given a low closed-cell characteristic when it is made into a single-layer film, but in order to maintain the low closed-cell characteristic while giving full play to the high melting characteristics inherent in the mixed high-crystalline polypropylene by melt mixing with the high-crystalline polypropylene described later, a high-density polyethylene with specified flowability and crystallinity characteristics is selected.

[0020] That is, in this invention, the melt flow index (190°C, 2.16 kg load) of high-density polyethylene is 0.1–5 g / 10 min, the crystallization temperature is 115–125°C, and the crystallinity is 75% or higher; preferably, the melt flow index can be 0.5–1.5 g / 10 min, the crystallization temperature can be 117–122°C, and the crystallinity can be 78% or higher. When the melt flow index, crystallization temperature, or crystallinity range is deviated from the above-defined range, even if a molten mixture with highly crystalline polypropylene is used to prepare a single-layer film, it is difficult to achieve the desired melting and closed-cell characteristics.

[0021] The density of the high-density polyethylene can be 0.945–0.985 g / cm³. 3 The preferred concentration is 0.955–0.975 g / cm³. 3 .

[0022] In the overall resin composition, the content of high-density polyethylene can be 20-70% by weight, preferably 30-70% by weight. When the content is less than 20% by weight, it leads to a decrease in closed-cell properties when forming a film, and when the content exceeds 70% by weight, it leads to a decrease in melting properties.

[0023] In this invention, the highly crystalline polypropylene is made into a single-layer film and is given high melting characteristics. However, in order to maintain the high melting characteristics while taking advantage of the inherent low closed-cell characteristics of the mixed high-density polyethylene through melt mixing with the high-density polyethylene, a highly crystalline polypropylene with specified flowability is selected.

[0024] That is, in this invention, the melt flow index (230°C, 2.16 kg load) of high crystallinity polypropylene can be 5 to 20 g / 10 min, preferably 6 to 10 g / 10 min. When the melt flow index is deviated from the above-defined limit, even if a single-layer film is prepared by using a melt mixture with high-density polyethylene, it is difficult to achieve the desired melting and closed-cell characteristics.

[0025] The highly crystalline polypropylene has a crystallinity of over 53% and a stereoregularity of over 95%, which are very high. Therefore, it has a melting temperature of over 165°C, exhibiting high crystallization temperature characteristics. It also possesses excellent mechanical and physical properties such as high rigidity, high heat resistance, high impact resistance, scratch resistance, and high dimensional stability.

[0026] In the overall resin composition, the content of the highly crystalline polypropylene can be 30-80% by weight, preferably 30-70% by weight. When the content is less than 30% by weight, it leads to a decrease in melt-breaking properties when the film is formed, and when the content exceeds 80% by weight, it leads to a decrease in closed-cell properties.

[0027] The porous membrane polyolefin resin composition of the present invention can be prepared by mixing and extruding the components according to conventional methods known in the art. For example, a particulate porous membrane polyolefin resin composition can be prepared by melting and mixing the components in a biaxial extruder and then cooling and solidifying them.

[0028] The polyolefin resin composition for porous membranes of the present invention has excellent melting and closing characteristics, and can be used in the preparation of single-layer dry porous membranes with excellent strength. Specifically, the closing temperature measured by the following method can be below 140°C, preferably below 136°C, the melting temperature can be above 160°C, preferably above 163°C, and the perforation strength can be above 520gf, preferably above 550gf.

[0029] [Measurement Method]

[0030] After forming a sheet by extruding the resin composition at 220–250°C using a biaxial extruder via a T-die, a single-layer porous film with a thickness of 15 μm is prepared by successively stretching along the MD and TD directions inside a stretching machine. A 50 × 50 mm sample of the porous film is placed on a plate with holes of 10 mm diameter. The force required to penetrate the film when pressed by a 1 mm probe is defined as the puncture strength. While heating a lithium-ion secondary battery prepared using the porous film in an oven at a rate of 2°C / min, the resistance is measured in real time. The temperature at which the resistance exceeds 10,000 Ω is defined as the pore-closing temperature, and the temperature at which the resistance drops sharply after the pore-closing temperature is defined as the melting temperature. During sheet forming, the cooling roller temperature is preferably set to 0–150°C, and the winding speed is set to 5–100 m / min.

[0031] Alternatively, the present invention provides a porous membrane formed by molding the polyolefin resin composition into a single layer.

[0032] The porous membrane can be prepared using a dry method on the polyolefin resin composition. For example, the polymer crystal portion can be oriented in a predetermined direction, and then cold-stretched to break the relatively weak amorphous portion to form pores. In one embodiment, the porous membrane can be prepared by extruding the polyolefin resin composition using a biaxial extruder at a temperature of 200–300°C through a T-die to form a sheet, and then simultaneously and successively stretching it along the MD and TD directions inside a stretching machine to form a porous film.

[0033] The present invention will now be described in more detail through specific embodiments and comparative examples. In the embodiments and comparative examples, density and melt flow index were measured by the following methods.

[0034] [Measurement Method]

[0035] (1) Density

[0036] Measured according to ASTM D1505.

[0037] (2) Melt flow index (MI)

[0038] Measurements were performed according to ASTM D1238 under a 2.16 kg load condition, with measurements taken at 190°C for high-density polyethylene and at 230°C for highly crystalline polypropylene.

[0039] Example 1

[0040] 70% by weight of high-density polyethylene (density 0.967 g / cm³) 3 A granular polyolefin resin composition for porous membranes was prepared by physically mixing 30% by weight of highly crystalline polypropylene (MI of 0.7 g / 10 min, crystallization temperature of 119 °C, and crystallinity of 75%) and 30% by weight of highly crystalline polypropylene (MI of 8 g / 10 min, crystallinity of 55%, and stereoregularity of 96%), followed by melt mixing using a biaxial extruder and then cooling and solidification.

[0041] Examples 2, 3, 6 and Comparative Examples 1-3

[0042] Except that the composition of high-density polyethylene and highly crystalline polypropylene in Example 1 is set to the conditions in Table 1 below, a polyolefin resin composition for porous membranes is prepared by the same method as in Example 1.

[0043] Example 4

[0044] 50% by weight of high-density polyethylene (density 0.954 g / cm³) 3A granular polyolefin resin composition for porous membranes was prepared by physically mixing 50% by weight of highly crystalline polypropylene (MI of 0.95 g / 10 min, crystallization temperature of 116 °C, and crystallinity of 67%) and 50% by weight of highly crystalline polypropylene (MI of 8 g / 10 min, crystallinity of 55%, and stereoregularity of 96%), followed by melt mixing using a biaxial extruder and then cooling and solidification.

[0045] Example 5

[0046] 50% by weight of high-density polyethylene (density 0.967 g / cm³) 3 A granular polyolefin resin composition for porous membranes was prepared by physically mixing 50% by weight of highly crystalline polypropylene (MI of 0.7 g / 10 min, crystallization temperature of 119 °C, and crystallinity of 75%) and 50% by weight of highly crystalline polypropylene (MI of 11 g / 10 min, crystallinity of 52%, and stereoregularity of 95%), followed by melt mixing using a biaxial extruder and then cooling and solidification.

[0047] Test case

[0048] The prepared resin composition was extruded into a sheet using a biaxial extruder at 220–250°C through a T-die. Then, it was stretched sequentially along the MD and TD directions inside a stretching machine to prepare a single-layer porous film with a thickness of 15 μm. The perforation strength, closure temperature, and melting temperature were measured according to the following method, and the results are shown in Table 1 below.

[0049] [Methods for measuring puncture strength]

[0050] A 50×50mm sample of the prepared porous film was placed on a plate with holes of 10mm in diameter. The force required to penetrate the film when pressed by a 1mm probe was measured and evaluated as the perforation strength.

[0051] [Methods for measuring closed-hole temperature and melting temperature]

[0052] The resistance of the lithium-ion secondary battery prepared using the porous film is measured in real time while being heated in an oven at a rate of 2°C / min. The temperature at which the resistance exceeds 10,000Ω is evaluated as the pore-closing temperature, and the temperature at which the resistance drops sharply after the pore-closing temperature is evaluated as the melting temperature.

[0053] [Table 1]

[0054]

[0055] Referring to Table 1, it can be confirmed that when high-density polyethylene with specific flowability and crystallinity properties and highly crystalline polypropylene with specific flowability are mixed in a specific ratio according to the present invention (Examples 1-3), the closed-cell temperature is below 136°C, the melting point is above 165°C, and the puncture strength is above 550 gf. Even when molded into a single-layer film, a porous membrane with excellent closed-cell and melting characteristics and high strength can be prepared. However, when the crystallinity of high-density polyethylene deviates from the preferred range (Example 4) or the crystallinity and flowability of highly crystalline polypropylene deviate from the preferred range (Example 5), the closed-cell characteristics, melting characteristics, and puncture strength decrease. Therefore, it can be seen that there is an ideal range of flowability and crystallinity characteristics for high-density polyethylene and highly crystalline polypropylene to achieve a porous membrane with excellent closed-cell and melting characteristics and high strength.

[0056] In contrast, it can be seen that when the content of high-crystallinity polypropylene is lower than the specified level (Comparative Example 1 and Comparative Example 3), the melting characteristics and strength characteristics are significantly reduced, and when the content of high-density polyethylene is lower than the specified level (Comparative Example 2), the closed-cell characteristics are significantly reduced.

[0057] The preferred embodiments of the present invention have been described in detail above. The description of the present invention is for illustrative purposes, and those skilled in the art should understand that other specific forms can be readily derived without altering the technical concept or essential features of the invention.

[0058] Therefore, the scope of this invention is defined less by the content of the above detailed description and more by the content of the claims. All modifications or variations derived from the meaning, scope and equivalent concepts of the claims should be interpreted as being included within the scope of this invention.

Claims

1. A polyolefin resin composition for porous membranes, comprising: 20-70% by weight of high-density polyethylene, wherein the high-density polyethylene has a melt flow index of 0.5-1.5 g / 10 min at 190°C and 2.16 kg load, a crystallization temperature of 115-125°C, and a crystallinity of ≥75%; and 30-80% by weight of highly crystalline polypropylene, wherein the highly crystalline polypropylene has a melt flow index of 6-10 g / 10 min at 230°C and 2.16 kg load, a crystallinity of 53% or more, and a stereoregularity of 95% or more.

2. The polyolefin resin composition for porous membranes according to claim 1, characterized in that, The resin composition, measured according to the following methods, has a closed-cell temperature below 140°C, a melting temperature above 160°C, and a puncture strength above 520 gf. [Measurement Method] After forming a sheet by extruding the resin composition at 220–250°C using a biaxial extruder via a T-die, a single-layer porous film with a thickness of 15µm is prepared by successively stretching along the MD and TD directions inside a stretching machine. A 50×50mm sample of the porous film is placed on a plate with holes of 10mm diameter. The force required to penetrate the film when pressed by a 1mm probe is defined as the puncture strength. While heating a lithium-ion secondary battery prepared using the porous film in an oven at a rate of 2°C / min, the resistance is measured in real time. The temperature at which the resistance exceeds 10,000Ω is defined as the pore-closing temperature, and the temperature at which the resistance drops sharply after the pore-closing temperature is defined as the melting temperature.

3. A porous membrane, said porous membrane being a single-layer membrane formed using the resin composition of claim 1 or 2.

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