KR100283985B1 - Polylefin film and method for manufacturing the same - Google Patents

Abstract

The present invention discloses a polyolefin film used to prepare a separator of a lithium ion secondary battery and a method of manufacturing the same. (a) An unstretched sheet is prepared by kneading an organic liquid body in an ethylene-propylene copolymer having a melt index of 1 to 25 gr / min, a melting temperature of 117 to 125 ° C, and an ethylene content of 5 to 50 mol%, followed by melting and extruding. (B) stretching the unstretched sheet 3 to 7 times in the longitudinal and transverse directions, respectively, and (c) dipping the stretched film in a volatile solvent to remove the organic liquid body and thus having an average diameter of 0.1. Polyolefin film according to the invention prepared according to the method comprising the step of forming a number of micropores of 1 ~ 1㎛ has a melt index of 1 ~ 25gr / min, a melting temperature of 117 ~ 125 ℃ and an ethylene content of 5 It is made of ethylene-propylene copolymer of ~ 50% by weight, characterized in that a plurality of micropores having an average diameter of 0.1 ~ 1㎛ is formed, the polyolefin film according to the present invention is a conventional polyethylene Excellent in mechanical strength, and shutdown (shutdown) characteristics compared to rimak to be suitable for use as a battery separator.

Description

Polyolefin film and method for manufacturing same

The present invention relates to a polyolefin film and a method for manufacturing the same, and more particularly, to a polyolefin film suitable for use as a separator for a battery because of its excellent mechanical strength and excellent shutdown properties, and a method for manufacturing the same.

In general, in a separator for a secondary battery, particularly a lithium ion secondary battery, shutdown characteristics and melt integrity characteristics are important characteristics that directly influence the stability of the battery. In other words, when the electrolyte is heated by the external short circuit during use of the battery, the temperature inside the battery rises rapidly. At this time, the membrane is partially melted so that the micropores of the microporous membrane inside the battery are blocked so that the micropores are shut down. This is because the battery can be rapidly increased to block the flow of current to maintain battery stability. In addition, the deformation of the separator must be prevented, that is, the membrane is partially melted to block the micropores, but the overall shape of the separator must be maintained intact (melt integrity) to maintain the stability of the battery.

In addition, mechanical strength is also one of the important physical properties in the separator, because the low mechanical strength is lowered in the stability and productivity of the separator. For example, in the case of a battery separator, the puncture strength is particularly important because an internal short circuit occurs when the separator is damaged by a fine protrusion on the surface of the electrode plate, causing a fatal defect in the battery. In addition, when the mechanical strength of the separator is excellent, the assembly speed can be improved during battery assembly can greatly contribute to productivity.

Conventionally, microporous separators made of polyethylene alone have been used as disclosed in Japanese Patent Laid-Open Nos. 3-64334 and 6-212006 as separators for lithium ion secondary batteries. However, such a separator has a disadvantage in that it is difficult to maintain the stability of the battery because the resistance to the separator deformation is weak when the current rapidly increases due to an external short circuit or an internal short circuit and the battery internal temperature rapidly increases.

In addition, Japanese Patent Laid-Open No. 7-60084, Japanese Patent Laid-Open No. 6-96753 and the like disclose a technique for preparing a microporous separator by blending polymethylpentine or polypropylene with polyethylene in order to solve the above problems. However, when manufacturing the microporous separator in this manner, there is a problem that the compounding process is further required because the resin is not sufficiently mixed in the conventional dry blending method.

In addition, Japanese Laid-Open Patent Publication No. 6-234181 discloses a laminated film prepared by thermally laminating a nonwoven fabric on a conventional polyethylene film in order to improve the heat resistance of the film. However, even in this case, there is a need for a process of coating or laminating an adhesive or other resin during the manufacturing process to impart thermal adhesiveness. In this regard, it is difficult to maintain the adhesion improving effect of the final film only by coating with adhesives or other resins, and in the case of lamination, an additional complicated process requiring multiple steps is required, and thus economic loss Of course, there is a problem that the inherent properties of the polyethylene film as the base film is weakened.

The technical problem to be achieved by the present invention is to provide a polyolefin film suitable for use as a battery separator because of excellent mechanical strength and shutdown characteristics compared to the conventional polyethylene separator.

Another technical problem to be achieved by the present invention is to provide a manufacturing method that can easily produce the polyolefin film.

In order to achieve the above technical problem, the present invention is made of an ethylene-propylene copolymer having a melt index of 1 to 25 gr / min, a melting temperature of 117 to 125 ℃ and an ethylene content of 5 to 50 mol% and an average diameter of 0.1 to 1 Provided is a polyolefin film characterized in that a plurality of micropores are formed.

In the present invention, the porosity of the olefin film is preferably 30 to 70%.

In the present invention, the polyolefin film has a tensile strength of 1,400 ~ 1,550kg / cm ² in the longitudinal direction and the transverse direction, it is preferable that the puncture strength is 600 ~ 770Gf.

In this invention, it is preferable that the thickness of the said polyolefin film is 15-50 micrometers.

In order to achieve the above technical problem, the present invention melts after kneading the organic liquid in an ethylene-propylene copolymer having a melt index of 1 to 25 gr / min, a melting temperature of 117 to 125 ° C, and an ethylene content of 5 to 50 mol%. And extruding the unstretched sheet; Stretching the unstretched sheet 3 to 7 times in the longitudinal and transverse directions, respectively; And removing the organic liquid by depositing the stretched film in a volatile solvent to form a plurality of micropores having an average diameter of 0.1 to 1 μm.

In this invention, it is preferable that the mixing ratio of the said organic liquid body is 30-70 weight part with respect to 100 weight part of ethylene-propylene copolymers.

In the present invention, the organic liquid body is preferably at least one compound selected from the group consisting of ethylene bissteamide wax, liquid paraffin, paraffin wax, process oil, stearyl alcohol and phthalyl dioctyl.

In the present invention, the volatile solvent is at least one selected from the group consisting of isopropyl alcohol, methylene chloride, trichloroethylene, acetone, 1,1,1-trichloroethane, methanol, ethanol, methyl ethyl ketone, heptane and hexane It is preferable that it is a compound.

In the present invention, the porosity of the olefin film is preferably 30 to 70%.

In the present invention, the polyolefin film has a tensile strength of 1,400 ~ 1,550kg / cm ² in the longitudinal direction and the transverse direction, it is preferable that the puncture strength is 600 ~ 770Gf.

In this invention, it is preferable that the thickness of the said polyolefin film is 15-50 micrometers.

Polyolefin film according to the present invention is excellent in shutdown characteristics (melt integrity) and mechanical strength compared to the conventional polyethylene separator is suitable for use as a battery separator.

Hereinafter, the polyolefin film and the manufacturing method thereof according to the present invention will be described in detail.

Conventional separators have been prepared in a multilayer or blended form of polyethylene and polypropylene in order to improve thermal properties and mechanical strength. However, this type of membrane shutdown start temperature is 130 ℃ or more close to the sublimation temperature of lithium ion 150 ℃ to ensure stability, the melting temperature of the material is at least 10 ~ 15 than the above temperature (130 ℃) It is preferable that ° C is low.

However, since copolymerization of ethylene and propylene (hereinafter referred to as ″ ethylene-propylene copolymer ″) has a lower degree of crystallinity than polyethylene, the melting temperature is also 10 to 15 ° C on average than 135 ° C of general polyethylene, depending on the content of ethylene. About low In addition, in the case of a certain amount of ethylene content, an eutectic point is formed between the two materials, in which case the degree of crystallinity is minimal and therefore the melting temperature is also minimal. Therefore, in the present invention, a copolymer of ethylene and propylene is used to prepare a separator having excellent mechanical strength while greatly improving the shutdown characteristics of the conventional separator.

The ethylene-propylene copolymer is prepared by the slurry polymerization method as follows.

First, aluminum halides and silicon compounds are reacted, and magnesium compounds are reacted thereon, followed by catalytic treatment with titanium halide compounds to prepare a catalyst solid component, which is suspended in an inert solvent such as toluene or n-heptane to obtain a slurry. An alkylaluminum or halogen alkylaluminum compound, an organosilicon compound, and a liquid propylene monomer and an ethylene monomer, which are organoaluminum compounds, are added thereto in a predetermined ratio and prepolymerized at a low temperature of about 10 ° C. Subsequently, a high temperature polymerization at about 50 to 60 ° C. followed by drying under reduced pressure yields a white ethylene-propylene copolymer.

The ethylene-propylene copolymer used in the present invention is an ethylene-propylene copolymer having a melt index of 1 to 25 gr / min, a melting temperature of 117 to 125 ° C, and an ethylene content of 5 to 50 mol%. If the ethylene content is less than 5 mol%, the melting point lowering effect of the resin is insignificant, and if the ethylene content exceeds 50 mol%, there is a problem that the mechanical strength is sharply lowered. In addition, if the melt index is less than 1gr / min, there is a problem in the discharge during melt extrusion, if the melt index exceeds 25gr / min, there is a problem of fracture during the film forming process.

As said aluminum halide, aluminum trichloride is most preferable. Examples of the silicon compound include tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, methyltriethoxysilane, and dimethyldiethoxysilane. Of these, tetramethoxysilane and methyltriethoxysilane are most preferred. .

As the magnesium compound, any one or more selected from the group consisting of ethyl magnesium chloride, propyl magnesium chloride, butyl magnesium chloride, hexyl magnesium chloride, octyl magnesium chloride, and ethyl magnesium bromide can be used.

Titanium tetrachloride, titanium tetrabromide, or the like may be used as the titanium halide compound used for the catalytic treatment.

As the organoaluminum compound, one or more compounds selected from the group consisting of trimethylaluminum, triethylaluminum, and triisobutylaluminum can be used.

As said organosilicon compound, dipiperidino methoxysilane (DPIPDMS), dipyrrolidinodimethoxysilane (DPYRDMS), isopropyl piperidino dimethoxysilane (IPPIPDMS), etc. are preferable.

In the polyolefin film according to the present invention, micropores having an average diameter of 0.1 to 1 µm are formed by the method described below. If the average pore size of the micropores is less than 0.1 μm, lithium ions may not move smoothly. If the average pore size of the micropores exceeds 1 μm, there is a problem of short circuit due to contact of the positive electrode in the battery.

In the polyolefin film according to the present invention, the porosity of the micropores is preferably 30 to 70%. If the porosity is less than 30%, there is a problem that the smooth movement of lithium ions in the lithium battery is reduced and the battery efficiency is lowered. If the porosity is greater than 70%, the strength of the separator rapidly decreases.

In the polyolefin film according to the present invention, the tensile strength in the longitudinal and transverse directions is preferably 1,400 to 1,550 kg / cm ² , and the puncture strength is 600 to 770 Gf. If the tensile strength in the longitudinal and transverse directions is less than 1,400 g / cm ^2, there is a problem that the productivity is lowered due to the breakage of the separator during battery assembly. If the tensile strength exceeds 1,550 kg / cm ² , the propylene content in the film increases and shutdown starts. There is a problem that the temperature rises. If the pore strength is less than 600Gf, there is a problem in that the separator is easily damaged by the fine protrusions in the battery. If the pore strength exceeds 770Gf, the propylene content is increased in the film, causing the shutdown start temperature to increase.

It is preferable that the thickness of the polyolefin film which concerns on this invention is 15-50 micrometers. If the thickness of the film is less than 15㎛ there is a problem that the tensile strength falls to 1,400 kg / cm ² or less, if it exceeds 50㎛ there is a problem that causes the capacity of the lithium battery to decrease.

Next, the method for producing the polyolefin film according to the present invention will be described in detail.

First, the melt index is 1 to 25 gr / min, the melting temperature is 117 ~ 125 ℃ and the ethylene content of 100 to 100 parts by weight of ethylene-propylene copolymer of 5 to 50 mol%, preferably 40 to 50 parts by weight An organic liquid mixture is kneaded in the compounding method with the ethylene-propylene copolymer in a proportion by weight to prepare a blending chip. The unstretched sheet is prepared by melting and extruding the blending chip. Subsequently, the unstretched sheet is stretched 3 to 7 times, preferably 4 to 5.5 times, respectively in the longitudinal direction and the transverse direction. If the draw ratio is less than 3 times, there is a problem that the tensile strength is lowered to 1,400 kg / cm ² or less, and if it exceeds 7 times, there is a problem that the fracture of the film increases.

Subsequently, the stretched film is passed through a container containing volatile solvent for about 10 to 50 seconds to extract and remove the organic liquid contained in the film, thereby forming a plurality of micropores having an average diameter of 0.1 to 1 μm in the film. This completes the production of the polyolefin film according to the present invention.

The polyolefin film prepared in this way is excellent in melt integrity and mechanical strength, even though the shutdown is started at a temperature lower than the sublimation temperature of lithium ions, compared to a separator made of a polyethylene-based resin. It is suitable as a separator.

Meanwhile, in the preparation of the polyolefin film according to the present invention, the organic liquid body may include at least one compound selected from the group consisting of ethylenebissteamide-based wax, liquid paraffin, paraffin wax, process oil, stearyl alcohol, and phthalyl dioctyl. It is preferable to use.

The volatile solvent is slightly different depending on the type of organic liquid, but isopropyl alcohol, methylene chloride, trichloroethylene, acetone, 1,1,1-trichloroethane, methanol, ethanol, methyl ethyl ketone, heptane and hexane Preference is given to using one or more compounds selected from the group consisting of.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the scope of the present invention is not limited to the following Examples. Performance evaluation of the film produced in the following Examples was carried out by the following method.

(1) Average diameter and film thickness

The cross section was observed using an electron scanning microscope manufactured by Jeol, Japan, and measured using an image analyzer.

(2) porosity

Melt the sample at a temperature increase rate of 5 ° C / min using a differential thermal analyzer (Model: DSC-7, Perkin Elmer, USA) to measure the heat of fusion and calculate the degree of crystallization therefrom to calculate theoretical density from the relationship between density and crystallinity. Was calculated. Porosity was calculated by the difference between the theoretical density and the apparent density.

(3) sting strength

Measurement was made using UTM of Instron, USA. The needle diameter was 1 mm.

(5) tensile strength

The tensile strength of the film is measured according to ASTM D 882 using UTM of Instron, USA.

(6) kneading fairness

In the same process / cooling condition at the time of compounding, the discharge state and the chip cutting property were measured to evaluate the relative characteristics as follows.

A: Both discharge / chip cutting are good

B: One of the discharge / chip cutting conditions is bad

C: Bad discharge / chip cutting

(7) Filmmaking Fairness

The film was relatively evaluated as follows by measuring the incidence of breakage of the film for 12 hours.

A: 0 to 1 break occurs B: 2 to 3 breaks C: 4 or more breaks

(8) Shutdown start temperature

In the shutdown area, the shutdown start temperature was measured by measuring the impedance using a device developed by SKC.

(9) melting temperature

The sample was measured by melting a sample at a temperature rising rate of 10 ° C./min using a differential thermal analyzer (model name: DSC-7, Perkin Elmer, USA).

(10) ethylene content

The content of ethylene in the ethylene-propylene copolymer was measured from the peak area of the methylene group appearing at 730 cm ⁻¹ using a FT-IR spectrometer under Digi-Lab's model name FTS-60.

<Example 1>

Aluminum trichloride, tetramethoxysilane, and ethylmagnesium chloride were mixed in an equivalence ratio of 1: 1: 2, respectively, and the resulting reaction was catalyzed with titanium tetrachloride and suspended in n-heptane to slurry the solid catalyst component. Trimethylaluminum and dipiperidinodimethoxysilane were mixed and reacted in the slurry in an equivalence ratio of 1: 1, and ethylene gas and liquid propylene were respectively supplied at an equivalence ratio of 25:75 at 10 ° C. and prepolymerized for 1 hour, followed by 60 ° C. The polymerization was further conducted for 1 hour to obtain an ethylene-propylene copolymer having a melt index of 4.5, a melting temperature of 117 ° C, and an ethylene content of 25% by weight.

40 parts by weight of paraffin wax was mixed as an organic liquid with respect to 100 parts by weight of the copolymer, a mixed resin was prepared by a compounding method, and the resultant was vacuum dried at 80 ° C. for 12 hours. Subsequently, the above-mentioned mixed resin was melted and extruded using an extruder and brought into close contact with a roll having a temperature of 60 ° C to obtain an unstretched sheet.

The unstretched film thus obtained was stretched 5 times in the longitudinal direction at 105 ° C. and 5 times in the transverse direction at 105 ° C. using a conventional biaxial stretching apparatus to obtain a film having a thickness of 25 μm. Subsequently, the stretched film was immersed in methylene chloride for 30 seconds to extract and remove paraffin wax, which is an organic liquid, to obtain a polyolefin film having a large amount of fine pores having an average diameter of 0.1 µm. At this time, the porosity of the micropores was 40%. The physical properties of the film were evaluated in the same manner as described above for the film, and the results are shown in Table 1.

Referring to Table 1, it can be seen that the polyolefin film of this embodiment has both good processability and good shutdown properties and tensile strength of the film.

Examples 2-5

Except for changing the content of ethylene in the ethylene-propylene copolymer within the claims of the present invention as shown in Table 1, after producing the film in the same manner as in Example 1, the physical properties of the film was evaluated The results are shown in Table 1.

Referring to Table 1, it can be seen that the polyolefin films of Examples 2 to 5 have both good processability and good shutdown properties and tensile strength of the film.

Comparative Examples 1 to 3

Except for changing the content of ethylene in the ethylene-propylene copolymer so as not to fall within the claims of the present invention as shown in Table 1, after producing the film in the same manner as in Example 1, the physical properties were evaluated The results are shown in Table 1.

Referring to Table 1, it can be seen that the polyolefin films of Comparative Examples 1 to 3 have lower film forming processability than those of Examples 1 to 5, and both the shutdown characteristics and the tensile strength of the film are poor.

Comparative Examples 4 to 5

Except for changing the content of paraffin wax added to the mixed resin as shown in Table 1 when manufacturing a polyolefin film, the film was prepared in the same manner as in Example 1, and then the physical properties thereof were evaluated. Shown in

Referring to Table 1, it can be seen that the polyolefin films of Comparative Examples 4 to 5 are poor in both kneading processability and film forming processability, and poor in both shot-down properties and tensile strength of the film.

Ethylene-propylene Copolymer Paraffin Wax Content Polyolefin film Micropores Fairness Final film properties Ethylene content Melting temperature Porosity Average diameter Kneading Fairness Production Process Tensile strength (longitudinal / transverse) Stinging strength Shot down start temperature unit weight% ℃ Parts by weight % Μm - - g / cm ² Gf ℃ Example One 25 117 40 40 0.1 A A 1550/1440 770 117 2 30 119 40 40 0.3 A A 1530/1350 710 119 3 35 121 40 40 0.4 A A 1520/1400 700 121 4 40 122 40 40 0.2 A A 1510/1410 670 122 5 50 123 40 40 0.4 A A 1500/1430 650 123 Comparative example One 60 126 40 40 0.5 A B 1450/1390 590 126 2 70 127 40 40 0.5 A B 1440/1360 575 127 3 80 126 40 40 0.5 A B 1440/1400 560 126 4 25 117 75 55 1.2 C C 1480/1420 740 117 5 25 117 80 60 1.5 C C 1470/1380 710 117

As described above, the polyolefin film according to the present invention is excellent in mechanical strength and shutdown characteristics as compared to the conventional polyethylene separator is suitable for use as a battery separator.

Claims (5)

It is composed of an ethylene-propylene copolymer with a melt index of 1 to 25 gr / min, a melting temperature of 117 to 125 ° C, and an ethylene content of 5 to 50 mol%, and micropores having an average diameter of 0.1 to 1 μm with a porosity of 30 to 70%. It is formed, The polyolefin film characterized by the above-mentioned. The polyolefin film of claim 1, wherein the polyolefin film has a tensile strength of 1,400 to 1,550 kg / cm ² in the longitudinal direction and a transverse direction, and a puncture strength of 600 to 770 Gf. The polyolefin film according to claim 1, wherein the polyolefin film has a thickness of 15 to 50 µm. (a) Ethylene bissteamide wax, liquid paraffin, paraffin wax and process oil in an ethylene-propylene copolymer having a melt index of 1 to 25 gr / min, a melting temperature of 117 to 125 ° C., and an ethylene content of 5 to 50 mol%. Preparing an unstretched sheet by kneading, melting and extruding an organic liquid selected from stearyl alcohol, phthalate dioctyl, and mixtures thereof; (b) stretching the unstretched sheet 3 to 7 times in the longitudinal and transverse directions, respectively; And (c) the stretched film is volatile selected from isopropyl alcohol, methylene chloride, trichloroethylene, acetone, 1,1,1-trichloroethane, methanol, ethanol, methyl ethyl ketone, heptane, hexane and mixtures thereof A method for producing a polyolefin film, comprising the step of forming an organic pore having a mean diameter of 0.1 to 1 占 퐉 by a porosity of 30 to 70% by removing the organic liquid by dipping in a solvent. The method of claim 4, wherein the mixing ratio of the organic liquid phase in the step (a) is 30 to 70 parts by weight with respect to 100 parts by weight of the ethylene-propylene copolymer.