KR101420042B1 - The production methods of separators for secondary cell - Google Patents

Abstract

In a production method of a separator for a secondary cell, the present invention provides a method of producing a separator for a secondary cell with excellent permeability by controlling the temperature of a casting roll to 100-130°C, controlling the temperature of a rear face and a surface of a precursor film to 100-130°C, and controlling a time that the precursor film discharged from a T-die is cooled and hardened in the casting roll in a range of 5-30 seconds when the precursor film is manufactured in a casting process.

Description

[0001] The present invention relates to a separator for secondary batteries,

The present invention relates to a process for producing a separator for a secondary battery, and more particularly, to a process for producing a polyolefin precursor film, which comprises melt extruding a polyolefin resin, casting the polyolefin resin to prepare a polyolefin precursor film, The present invention relates to a method for producing a separator having excellent air permeability by regulating the temperature range of the surface and back surface of the precursor film in the casting process and controlling the residence time of the casting roll.

As a generally known biaxial stretching membrane production method, Japanese Patent Application Laid-Open No. 2008-111134 discloses a method of extruding a polypropylene resin containing a beta-nucleating agent into a T-DIE and casting it into a casting roll A longitudinal direction drawing of a disk sheet obtained by cooling, and a longitudinal direction stretching in the transverse direction. However, the Japanese Patent Laid-open Publication (Kokai) discloses a case in which it is not possible to produce a separator having excellent breathability even if the K value (relative ratio of beta crystals) is high. That is, when the elongation speed of the precursor film is high, a separator having excellent air permeability can not be produced, and in this case, the productivity is lowered. Therefore, in the above Japanese Unexamined Patent Publication . That is, when the amide compound as the beta nucleating agent is formed into acicular crystals and the polypropylene resin composition containing the acicular crystals is extruded from the tee die, the resin temperature is set to be equal to or higher than the melting point of the polypropylene, , And the polypropylene resin composition is extruded from the tie die while the amide compound is present as acicular crystals.

However, the present inventors have found that it is not possible to produce a separator having excellent air permeability even when the temperature condition of the extruder proposed in the method of Japanese Patent Laid-open No. 2000-005960 is applied. That is, when the molten polypropylene resin is extruded from a TeDe sheet and then cooled in a casting roll, the temperature of the cooling roll is not sufficiently transmitted to the surface of the extruded sheet (the sheet surface contacting the air) It is impossible to produce a separator having excellent air permeability because it is not uniform. In the Japanese Laid-Open Patent Publication, the temperature of the roll is maintained at 120 캜 for cooling for 12 seconds in the casting roll, and the casting method is not described in detail.

Japanese Laid-Open Patent Publication No. 2010-120217 discloses that the casting roll is cooled and solidified at a casting roll of 125 ° C. Japanese Laid-Open Patent Application No. 2009-43485 discloses that the contact time between a molten sheet and a casting roll . Japanese Unexamined Patent Publication No. 2009-26499 discloses that the contact time is set to 12 seconds when the molten sheet is cooled and solidified by casting on a casting roll (125 DEG C).

As described above, most of the prior art describes only the contact time of the molten sheet in the casting roll. However, the contact time in such a casting roll was also a matter which was not mentioned at the time of general film production.

The inventors of the present invention found that controlling the cooling rate of the front and back surfaces of the film is an important parameter in order to form a uniform beta crystal in the casting process from the viewpoint of producing a polypropylene-based separation membrane containing a beta nucleating agent having excellent permeability. That is, the control of the heat transfer related to the cooling of the front and back surfaces of the film is very important. Specifically, when the molten sheet is formed from a tie die and cooled in the casting roll, the temperature of the casting roll is set in the range of 100 to 130 占 폚. It means that the sheet should be cooled to 100 to 130 캜, not to the normal temperature. That is, when the molten sheet is cooled to 100 to 130 ° C, the beta nucleus starts to generate beta crystals. Incidentally, the surface of the precursor film comes into contact with air, and a difference in the cooling temperature occurs as shown in Fig.

In Fig. 1, when the cross-section of the film is taken in the process of cooling and solidifying the precursor film in the molten state on the casting roll, a temperature distribution occurs in the thickness direction. That is, the point (a) is almost the same as the temperature of the casting roll, and the point (b) is close to the temperature of the air. As a result, a temperature gradient of the casting roll side (back side) and the air side (surface side) occurs in the precursor film layer in the molten state. That is, when the molten sheet comes out of the tie die and is placed on the casting roll, the temperature of the surface (back surface) contacting the casting roll is the same as that of the casting roll, I get more influence. Thus, when the temperature of the casting roll is set at 100 to 130 ° C, beta crystals can be formed on the back side of the precursor film, but beta crystals are not formed on the surface side (surface contacting the air). Due to such a problem, beta-crystals are not formed on the surface of the precursor film, so that even when the biaxial oriented separator is produced, the air permeability may not be ensured.

In the above prior art documents, the biaxially stretched film was prepared by setting the temperature of the casting roll at 100 to 130 ° C. and the contact time at 10 to 25 seconds, It has been found that air permeability can not be ensured even when it is manufactured by the method of the prior art as described above and it is found that the reason is the temperature on the surface side of the precursor film and therefore the temperature of the casting roll and the contact time And that the temperature on the surface side of the molten sheet should be controlled.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a biaxially stretched polyolefin film having a biaxially stretched polyolefin film and a biaxially stretched polyolefin film in which the temperature of the surface of the precursor film is lowered to 100 to 130 ° C or less when the molten sheet discharged from the tie die is cooled in the casting roll, The present invention provides a method for manufacturing a separation membrane having excellent air permeability by controlling the surface temperature of a precursor film in a casting process.

In order to accomplish the above object, the present invention provides a method for producing a separator, comprising the steps of: melt extruding a polyolefin resin to prepare a molten sheet; and stretching the polyolefin precursor film produced by casting the same to form a separator, In the casting process for producing the precursor film, the temperature of the polyolefin precursor film on the surface (surface contacting the air) and the back surface (surface contacting the casting roll) is controlled within the range of 100 to 130 ° C, and the residence time is set in the range of 5 to 30 seconds .

According to a particularly preferred embodiment of the present invention, the separation membrane production method of the present invention comprises the following steps:

(1) compounding a beta-nucleating agent and a polyolefin-based resin;

(2) melt extruding the result of step (1) in an extruder in a temperature range of (melting point of polyolefin resin + 50 ° C) to (melting point of polyolefin resin + 90 ° C)

(3) passing the result of step (2) through a first filter and then transferring the resultant to a gear pumpper;

(4) passing the result of the transfer to the gear pump through the gear pump to the second filter;

(5) transferring the resultant having passed through the second filter to the tie die to form a molten sheet on the tie die;

(6) casting the molten sheet to a casting roll to obtain a polyolefin-based precursor film having a beta crystal formed therein;

(D) stretching the polyolefin-based precursor film in the machine direction at a temperature of 70 to 130 ° C by 1.5 to 5 times; And

Stretching the longitudinally stretched film in the transverse direction by a factor of 1.5 to 5 at a temperature of 70 to 160 ° C.

In the present invention, the polyolefin-based resin is not particularly limited as long as it is used in a separation membrane, but polypropylene, a polypropylene-based copolymer, and a polyethylene propylene copolymer are preferably used.

When the polyolefin-based resin is melt-extruded, it is preferable to compound the polyolefin-based resin with a beta nucleating agent and, if necessary, other additives such as an antioxidant and a neutralizing agent.

Examples of the beta-nucleating agent that can be used in the present invention include amide-based compounds, alkali metal salts or alkaline earth metal salts of carboxylic acids, aromatic sulfonic acid compounds, tetraoxaspiro compounds, phthalocyanine compounds, quinacridone compounds, nanoscale- A composition comprising a phosphoric acid compound and a magnesium compound, and the like.

Examples of the amide-based beta-nucleating agent include N, N'-diphenylhexanamide, N, N'-dicyclohexylterephthalamide, N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide, N, N'-dibenzoyl-1,4-diaminocyclohexane, N, N'-dibenzoyl-1,5-diaminonaphthalene, dicyclohexanecarbonyl- (N-cyclohexanecarbonylamino) benzamide, N-phenyl-5- (N-benzoylamino) pentaimide, N-cyclohexanecarbonyl- 3,9-bis [4- (N-cyclohexylcarbamoyl) phenyl] -2,4,8,10-tetraoxaspiro [5,5] undecane.

Examples of the alkali metal salt or alkaline earth metal salt nucleating agent of the carboxylic acid include a 1,2-hydroxystearic acid potassium salt, magnesium succinate, and magnesium phthalate.

Examples of the aromatic sulfonic acid-based nucleating agent include sodium benzenesulfonate and sodium naphthalenesulfonate.

Specific commercial names of commercially available beta nucleating agents include TMB-5 of Shan Xin Institute of Chemical Engineering and NU-100 of NJC.

The content of the beta nucleating agent used in the present invention is preferably 0.001 to 1 part by weight based on 100 parts by weight of the polyolefin-based resin. When the amount of the beta nucleating agent is less than 0.001 parts by weight, a sufficient amount of beta-crystals can not be formed in the polyolefin-based precursor film. When the amount of the beta-nucleating agent is more than 1 part by weight, beta-crystals in the precursor film become saturated, Resulting in excessive use of the material.

When the beta nucleating agent is compounded with the polyolefin-based resin in the present invention, the compounding temperature must be higher than the temperature (dissolution temperature) at which the beta-nucleating agent can be sufficiently melted in the polyolefin-based melt, And adjustment are possible in the known art to those skilled in the art.

For example, when 0.05 parts by weight of NU-100 manufactured by NJC Corporation is used as a nucleating agent in polypropylene, it is necessary to dissolve the polypropylene at 240 ° C or higher before the beta nucleating agent dissolves in polypropylene. When 0.10 parts by weight is used, The nucleating agent is well dissolved, and when 0.20 parts by weight is used, the nucleating agent is dissolved well by 280 DEG C or more. That is, the dissolution temperature of the beta nucleating agent to the polypropylene melt increases as the amount of the beta nucleating agent added increases.

A method for confirming that the polyolefin resin and the beta-nucleating agent are well compounded is a method of compounding a beta-nucleating agent and a polyolefin-based resin, and then, when the strand of the molten image is discharged through the strand die, Is well compounded. In other words, it can be seen that the beta-nucleating agent is well dissolved in the polyolefin-based resin.

 The polyolefin-based resin containing the beta-nucleating agent thus prepared is melt-extruded through an extruder equipped with a tee-type die or a T-die to form a molten sheet. The extruder may be a single screw extruder, a twin screw extruder, a multi screw extruder, or the like. In the extruder, the polyolefin resin containing the beta nucleating agent is uniformly melted and then passed through the first filter to remove foreign substances in the molten resin as it exits through the tip of the extruder.

The temperature from the extruder to the first filter is subjected to melt extrusion, filtering (removal of impurities) at a temperature at which the polyolefin resin containing the beta nucleating agent can be sufficiently melted, that is, in the range of melting point (Tm) + 50 ° C to melting point ). The resin in the molten state through the first filter passes through the gear pump, and the gear pump serves to quantitatively extrude the tea die. The molten resin passing through the gear pump is once again passed through the filter (second filter), which causes pressure increase and pulsation when pushing the molten resin through the gear pump to the tee die. . The pore size of the second filter is larger than that of the first filter. When using a filter with a too large pore size, a second filter with a large hole at the level of no pressure increase or pulsation should be used because it is practically impossible to suppress the pressure increase phenomenon or the pulsation from the gear pump. The temperature from the gear pump to the second filter is preferably set in the temperature range of [dissolution temperature (Ts) of beta nucleating agent - 5 [deg.] C] to [Ts + 20 [deg.] C]. If the temperature from the gear pump to the second filter is set below (Ts - 5 ℃), there is a problem that the pressure of the second filter may rise due to recrystallization of the beta nucleating agent. (Ts + 20 DEG C), the polyolefin-based resin melt containing the beta-nucleating agent may be too hot to be cooled to the target temperature in the next step. That is, the polyolefin-based resin melt containing the beta-nucleating agent must induce recrystallization of the beta-nucleating agent in the section to the tee after passing through the second filter. In order to induce recrystallization, .

The molten resin that has passed through the second filter reaches the Ti dies again and forms a molten sheet shape. At this time, it is preferable to set the temperature from immediately after the second filter to the tee die to be lower than the crystal formation starting temperature of the beta nucleating agent so that the beta- nucleus agent can be recrystallized. That is, it is preferable to set the temperature in the range of the crystallization start temperature (Tci) to (Tci - 40 ° C) of the beta nucleating agent for the polyolefin-based melt. If the temperature from the second filter to the tee die is set to a temperature lower than " Tci - 40 캜 ", crystals of the beta nucleating agent become very large and defects can be caused by foreign matter. In addition, This can be difficult.

The molten sheet containing the beta nucleating agent discharged from the tee dies is placed on a casting roll to form a white opaque precursor film containing beta crystals by the cooling and solidifying casting method of Figs.

At this time, a method of mounting can be used, such as an electrostatic application method (pinning method), an air knife method, a vacuum knife method, or the like. If an air knife is used, the temperature of the air must be maintained in the range of 100 to 130 ° C. If the temperature is lower than 100 ° C, the surface of the molten sheet is cooled to less than 100 ° C to prevent formation of beta crystals, and even if the temperature is higher than 130 ° C, beta crystals are difficult to form on the surface of the molten sheet.

When placed on a casting roll, the temperature of the casting roll is in the range of 100 to 130 占 폚. If the temperature is less than 100 ° C, it is difficult to form beta crystals on the back side of the sheet (the side contacting the casting roll) in the process of cooling and solidifying the molten sheet state. Even when the temperature of the casting roll exceeds 130 캜, beta crystals are hardly formed on the back side of the sheet, and the cooling and solidifying process of the precursor film in the casting roll is unstable, so that the film may be wrinkled.

The surface side of the sheet (film) should be adjusted to a temperature range of 100 to 130 캜 after the molten sheet is seated on the casting roll. If the temperature is lower than 100 ° C, beta crystals may be less on the surface side of the precursor film, and even if the temperature exceeds 130 ° C, it may be difficult to form beta crystals on the surface side of the precursor film. At this time, as a specific method for controlling the surface temperature of the precursor film in the range of 100 to 130 ° C, a method using a hot air chamber and a method using an infrared heating chamber can be applied.

In the present invention, it is preferable to prepare the precursor film in the same manner as in FIGS. 1 to 4 so as to uniformly produce beta crystals from the surface to the back surface and the inner layer of the precursor film in the casting process.

In the casting process of the present invention, it is preferable to control the time for cooling and solidifying the precursor film within a range of 5 to 30 seconds. Below 5 seconds, the beta crystal is too short to be produced, and when it exceeds 30 seconds, the membrane productivity may be a problem, but there is a facility burden that the casting roll size must be very large.

After the precursor film is formed, the film is stretched 1.5 to 5 times in the longitudinal direction at a temperature of 70 to 130 캜, and further stretched 1.5 to 5 times in the transverse direction at a temperature of 70 to 160 캜 to prepare a separator.

If the stretching temperature is lower than 70 캜, it is difficult to stretch at a high magnification of at least 3 times. When the stretching temperature exceeds 130 캜 at the longitudinal stretching, the air permeability is lowered. There is a risk that the pores generated in the separator melt. When the film is stretched to less than 1.5 times, the pore size of the separating film is small, which may be unsuitable for use as a lithium ion secondary battery. If the film is stretched more than 5 times, breakage of the film may occur. The stretching method may be a sequential biaxial stretching in which the stretching is first in the longitudinal direction and then in the transverse direction, or the simultaneous biaxial stretching in which the longitudinal direction and the transverse direction are simultaneously stretched may be applied.

According to the present invention, when the precursor film for a biaxial oriented film is produced, the temperature of the casting roll is controlled at 100 to 130 ° C, the back surface and the surface temperature of the precursor film are controlled at 100 to 130 ° C, The precursor film can be cooled and solidified in the casting roll to a range of 5 to 30 seconds to provide a separator having excellent air permeability.

FIG. 1 is a view for explaining the generation of a temperature distribution of a precursor film in a process in which a precursor film in a molten state is cooled and solidified in a casting roll in the prior art.
2 shows a state in which the temperature of the surface of the precursor film 3 is set to 100 ° C. in the hot air chamber 4 when the molten sheet is discharged from the tie die and is seated on the casting roll 5 by the electrostatic induction line 2, To 130 < 0 > C.
3 is a schematic view showing a state in which a hot air knife 7 is used to place the precursor film melt sheet discharged from the tie die 1 onto the casting roll 5 and a hot air knife 7 is used to adjust the surface temperature of the precursor film 3. [ (4) is used.
4 is a view showing that the temperature of the precursor film 3 is controlled in the range of 100 to 130 占 폚 by using the electrostatic induction line 2 when the precursor film is placed on the casting roll and the infrared heater 9 is used .
Figure 5 shows that when placing the precursor film on a casting roll, the vacuum knife 10 is placed on the back of the film as well as the hot air knife 7 to ensure that the precursor film 3 is seated on the casting roll, 4) to adjust the temperature of the precursor film 3 in the range of 100 to 130 占 폚.
6 is a view showing the production of a precursor film using an air knife 11 at room temperature without using a hot air knife or a hot air chamber.

Hereinafter, the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited to the following examples.

In the following Examples and Comparative Examples, the method of measuring the air permeability is as follows.

Air permeability ( Gurley )

According to the Japanese Industrial Standard (JIS-P8117) gauze measurement method, the time (in seconds) it takes for 100 mL of air to pass through a 1 square inch (2 inch) microporous membrane at a constant pressure of 4.8 inches of H2O at room temperature was measured Unit sec / 100 ml).

Example  One

0.05 parts by weight of NJC NU-100 as a beta-nucleating agent, 0.05 parts by weight of an antioxidant IRGANOX-1010, 100 parts by weight of an antioxidant IRFOS-168 (trade name: 0.05 part by weight, a neutralizing agent and 0.05 part by weight of calcium stearate as an electrostatic application improving agent were mixed and compounded at 250 ° C. At this time, the state of the molten resin coming out through the detachment of the strand die was transparent.

The pellet thus compounded was poured into a hopper of an extruder equipped with a tie die. The temperature of the extruder was set at 230 DEG C to dissolve the pellets. The pellets were passed through a first filter (pore size 20um) set at 230 DEG C, The molten polypropylene sheet was prepared by setting the temperature from the gear pump to the second filter (hole size 70um) to 215 ° C and setting the temperature from the second filter to the tee die to 200 ° C.

1, the temperature of the casting roll was set to 120 ° C, the temperature of the hot air chamber was set to 120 ° C, and the time to be cooled and solidified in the casting roll was set to 25 seconds. A precursor film was prepared, and then stretched at 80 DEG C in the machine direction in the longitudinal direction and further stretched 3 times in the transverse direction at 80 DEG C. The air permeability of the elongated separation membrane was 240 sec / 100 ml. This is a level suitable for application to a lithium ion secondary battery.

Example  2

 The same procedure as in Example 1 was carried out except that the method of FIG. 2 (hot air knife temperature: 120 DEG C) was used for seating on a casting roll. The prepared separator had an air permeability of 210 sec / 100 ml.

Example  3

The same procedure as in Example 1 was carried out except that the method of FIG. 3 (infrared heater temperature: 120 ° C.) was used for placing on a casting roll. The prepared separator had an air permeability of 190 sec / 100 ml.

Example  4

The same procedure as in Example 1 was carried out except that the method of FIG. 4 (hot air knife: 120 DEG C) was used for seating on a casting roll. The prepared separator had an air permeability of 250 sec / 100 ml.

Comparative Example  One

The method of placing on a casting roll was as shown in Fig. 5, and the same procedure as in Example 1 was carried out except that the temperature of the casting roll was 120 占 폚 and the cooling and solidifying time was 12 seconds on the casting roll. The prepared membrane had an air permeability of 2600 sec / 100 ml. This is a separation membrane unsuitable for use in a lithium ion secondary battery.

1: T-die, 2: pinning wire, 3: precursor film, 4: hot air chamber, 5: casting roll, 6: 7: hot air knife, 8: IR heater chamber, 9: infrared heater, 10: vacuum knife, 11: air knife,

Claims (5)

A method for producing a separator by stretching a polyolefin precursor film prepared by melt-extruding a polyolefin-based resin to produce a molten sheet and casting the polyolefin-based precursor film, wherein in the casting step of producing the polyolefin-based precursor film, And the temperature of the surface of the precursor film is set at 130 DEG C and at least one of infrared heating and hot air heating is applied so that the temperature of the surface of the polyolefin precursor film (surface contacting the air) and the back surface (surface contacting the casting roll) Is adjusted to a range of 100 to 130 ° C, and a retention time is adjusted to a range of 5 to 30 seconds. delete The method according to claim 1,
Wherein at least one of an electrostatic applying method, a hot air knife, and a vacuum knife is applied as a method of placing the film on the casting roll in the casting step of the precursor film. The method according to claim 1,
A method of manufacturing a separation membrane for a secondary battery, comprising the steps of:
(1) compounding a beta-nucleating agent and a polyolefin-based resin;
(2) melt extruding the result of step (1) in an extruder in a temperature range of (melting point of polyolefin resin + 50 ° C) to (melting point of polyolefin resin + 90 ° C)
(3) passing the result of step (2) through a first filter and then transferring the resultant to a gear pumpper;
(4) passing the result of the transfer to the gear pump through the gear pump to the second filter;
(5) transferring the resultant having passed through the second filter to the tie die to form a molten sheet on the tie die;
(6) casting the molten sheet to a casting roll to obtain a polyolefin-based precursor film having a beta crystal formed therein;
(D) stretching the polyolefin-based precursor film in the machine direction at a temperature of 70 to 130 ° C by 1.5 to 5 times; And
Stretching the longitudinally stretched film in the transverse direction by a factor of 1.5 to 5 at a temperature of 70 to 160 ° C. 5. The method of claim 4,
The hole size of the first filter is smaller than the hole size of the second filter and the temperature of the first filter is set to (the melting point of the polyolefin resin + 50 ° C) to (the melting point of the polyolefin resin + 90 ° C) Temperature is set to be in the range of [the dissolution temperature (Ts) of the beta nucleating agent-5 ° C] to [Ts + 20 ° C] (Tci - 40 占 폚)].

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