JP2011246539A - Method for producing biaxially stretched polyolefin-based porous film, and biaxially stretched polyolefin-based porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a porous film, capable of producing the porous film in high productivity.SOLUTION: The method for producing a biaxially stretched polyolefin-based porous film stretched by a dry method is provided by comprising an MD (machine direction) cold stretching step of MD-cold stretching the film constituted by the polyolefin-based resin and a TD (transverse direction) cold stretching step of TD-cold stretching the film at a stretching temperature satisfying a condition expressed by formula (1): (Tg+20)≤T≤(Tm-30) [wherein, T is the stretching temperature (unit:°C), Tg is the glass transition temperature (unit:°C) of the film, and Tm is the melting point (unit:°C) of the film].

Description

  The present invention relates to a method for producing a polyolefin biaxially stretched porous membrane and a polyolefin biaxially stretched porous membrane.

  Resin porous membranes typified by polyolefin-based porous membranes are excellent in mechanical properties, chemical resistance, electrical properties and the like, and are used for packaging material applications, medical applications, electrical material applications, and the like. Among them, separators used in lithium ion batteries, etc., utilize the property of shutting down pores at a specific temperature (shutdown function), and resin porous membranes are widely used for consumer devices such as personal computers and mobile phones. It came to be. In the future, lithium-ion batteries are expected to be widely used as power sources for hybrid vehicles (HEV) and electric vehicles (EV), but cost reduction is a major issue.

  The method for producing a resin porous membrane typified by a polyolefin-based porous membrane can be broadly classified into two methods: a wet method for making a porous layer in an extraction step and a dry method for making a porous layer in a stretching step. . Examples of the former method include the method described in Patent Document 1. Patent Document 1 discloses a manufacturing method in which a plasticizer or the like is kneaded with a resin and melt-extruded, and then the plasticizer or the like is extracted and made porous in an extraction tank.

  On the other hand, examples of the latter method include the method described in Patent Document 2. Patent Document 2 discloses a production method in which a lamellar crystal is formed on a melt-extruded original fabric, and the lamellar crystal is cleaved by longitudinal uniaxial stretching to make it porous. According to this method, unlike the wet method, an extraction step is unnecessary, and the process can be simplified. Moreover, the method of patent document 3 is mentioned as a manufacturing method of the biaxially-stretched porous membrane using a dry method. Patent Document 3 discloses a technique in which a porous film obtained by known longitudinal uniaxial stretching is laterally stretched while relaxing in the longitudinal direction with heat.

JP 58-59072 A JP-A-62-1121737 International Publication No. 2007/098339

  However, the method described in Patent Document 1 has drawbacks such as an increase in the cost of the solvent used in the extraction process, and the time required for extraction and poor productivity. In addition, the method described in Patent Document 2 realizes a high speed because the main characteristics such as air permeability of the porous film are reduced as the speed of the longitudinal uniaxial stretching is increased (that is, the strain rate is increased). Is difficult. Furthermore, since the method described in Patent Document 3 simultaneously performs longitudinal relaxation (shrinkage) at the time of transverse stretching, the productivity is further reduced, and a special simultaneous biaxial stretching machine is required for transverse stretching. It has drawbacks such as equipment costs.

  As described above, since the techniques described in Patent Documents 1 to 3 are all inferior in productivity, it is difficult to obtain a cheaper porous film. When used as a separator provided, a new production technique that can be manufactured at a lower cost is desired.

  Accordingly, the present invention has been made in view of the above circumstances, and provides a method for producing a polyolefin biaxially stretched porous membrane and a polyolefin biaxially stretched porous membrane capable of producing a porous membrane with high productivity. For the purpose.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that there is room for further study although it is necessary to obtain a resin porous membrane by at least biaxial stretching. Specifically, a film obtained by longitudinal uniaxial stretching by a dry method has a large anisotropy and a high crystallinity. It has been found that when such a film is stretched in the transverse direction by adopting heat stretching that is usually considered, there are problems such as breakage and stretching unevenness. Therefore, it was further examined how the lateral stretching can be performed uniformly and stably. As a result, it was ascertained that the above breakage and stretching unevenness occur due to melting of the crystal phase. And it discovered that the above-mentioned purpose was achieved by performing transverse stretching at a temperature lower than the temperature at which the melting of the crystal phase starts (rubbery region), and completed the present invention.

That is, the present invention is as follows.
[1] A method for producing a porous membrane stretched by a dry method, which comprises a MD cold stretching step in which a membrane composed of a polyolefin-based resin is cold-stretched into MD, and a condition represented by the following formula (1) A method for producing a polyolefin-based biaxially stretched porous membrane, comprising a TD cold stretching step of cold stretching the membrane to its TD at a stretching temperature satisfying
(Tg + 20) ≦ T ≦ (Tm−30) (1)
(In the formula, T represents the stretching temperature (unit: ° C), Tg represents the glass transition temperature (unit: ° C) of the film, and Tm represents the melting point (unit: ° C) of the film.)
[2] The method for producing a polyolefin biaxially stretched porous membrane according to [1], wherein a draw ratio in the TD cold drawing step is 1.01 to 3.0 times.
[3] The method for producing a porous membrane according to [1] or [2], wherein the polyolefin resin is at least one selected from the group consisting of a polyethylene resin and a polypropylene resin.
[4] A polyolefin-based biaxially stretched porous membrane obtained by the production method of any one of [1] to [3].

  According to the method for producing a polyolefin-based biaxially stretched porous membrane of the present invention, the porous membrane can be produced with high productivity. The polyolefin biaxially stretched porous membrane of the present invention can be applied to a separator and is inexpensive.

It is a chart which shows a modification about the order of each process in the present invention.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail.

The method for producing a polyolefin-based biaxially stretched porous membrane (hereinafter simply referred to as “porous membrane”) of the present embodiment is a method for producing a porous membrane stretched by a dry method, and includes a polyolefin-based resin. The MD cold-stretching step of cold-stretching the formed film into MD and the TD cold-stretching step of cold-stretching the film to its TD at a stretching temperature satisfying the condition represented by the following formula (1) .
(Tg + 20) ≦ T ≦ (Tm−30) (1)
Here, in the formula (1), T represents a stretching temperature (unit: ° C.) in the TD cold stretching step. Tg represents the glass transition temperature (unit: ° C.) of the film, and is a temperature determined from the peak temperature of the loss modulus when the viscoelastic properties are measured. Tm represents the melting point (unit: ° C.) of the film, and is a melting peak top temperature obtained by heating at 10 ° C./min using a DSC (differential scanning calorimeter), and is a temperature higher by 50 ° C. than Tg. It is. In addition, “TD” indicates a direction (Transverse Direction) perpendicular to the flow direction of the film when a film before being provided with a hole (hereinafter also referred to as “non-porous original fabric”) is obtained. On the other hand, “MD” to be described later indicates the machine direction of the non-porous raw fabric.

  As the resin used in the production method of the present embodiment, a polyolefin-based resin is more preferable from the viewpoint that the crystallization speed and the crystallization degree can be appropriately adjusted.

(Polyolefin resin)
The “polyolefin resin” according to this embodiment means a resin having 50% by mass or more of polyethylene or polypropylene as a component constituting the porous membrane. Hereinafter, the polyethylene resin and the polypropylene resin will be described.

(Polyethylene resin)
The polyethylene resin is a resin containing 50% by mass or more of polyethylene as a main component, more preferably 80% by mass or more, and still more preferably 90% by mass or more. The blending amount of polyethylene in the polyethylene-based resin can be adjusted within the above-mentioned range depending on the air permeability and the required characteristics required for its use.
The polyethylene used as the main component has a density of preferably 0.940 to 0.970 g / cm ³ , more preferably 0 from the viewpoint of obtaining a non-porous raw fabric having a higher degree of crystallinity and stretchability. A high density polyethylene of 950 to 0.967 g / cm ³ , more preferably 0.960 to 0.964 g / cm ³ is particularly preferably used. The high density polyethylene may be a homopolymer polyethylene or a copolymer polyethylene. In the case of copolymer polyethylene, examples of the comonomer component include propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, and 1-decene. The content of the comonomer component is preferably 2 mol% or less from the viewpoint of crystallinity, and the copolymer polyethylene may be a random or block copolymer.

  The weight average molecular weight (Mw) of the high-density polyethylene is preferably 50,000 to 500,000, more preferably 100,000 to 450,000, and still more preferably 200,000 to 400,000 in consideration of mechanical strength and moldability. It is. Furthermore, the molecular weight distribution (Mw / Mn) determined by Mw and the number average molecular weight (Mn) is not particularly limited, and may be, for example, in the range of about 3 to 15. In the present specification, the weight average molecular weight (Mw) and the number average molecular weight (Mn) are obtained by a GPC (gel permeation chromatography) method using polystyrene as a standard substance (the same applies hereinafter).

  When high-density polyethylene is used as the main component of the polyolefin-based resin, other polyolefin-based resins and various elastomers can be blended in addition to the high-density polyethylene. Examples of other polyolefin resins include linear low density polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, and ethylene-α polyolefin copolymers. Moreover, as various elastomers, ethylene-alpha polyolefin copolymer elastomer other than styrene-type elastomers, such as SEBS and SEPS, is mentioned, for example. These may be used alone or in combination of two or more. In this case, the addition amount of each resin may be an addition amount suitable for the purpose as long as air permeability is not impaired.

(Polypropylene resin)
The polypropylene-based resin is a resin containing 50% by mass or more of polypropylene, and preferably contains 80% by mass or more, more preferably 90% by mass or more of polypropylene from the viewpoint of obtaining a non-porous raw material having a higher crystallinity. .

Polypropylene as a main component, the density is preferably 0.905 g / cm ³ or more, more preferably 0.910 g / cm ³ or more, further preferably 0.915 g / cm ³ or more. The polypropylene resin may be a homopolymer polypropylene or a copolymer polypropylene.

  In the case of copolymer polypropylene, examples of the comonomer component include α-olefins such as ethylene, 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, and 1-decene. The content of the comonomer component is preferably 5 mol% or less, and the copolymer polypropylene may be either a block or a random copolymer. Among these, homopolypropylene can be preferably used from the viewpoint of obtaining higher air permeability.

  The weight-average molecular weight (Mw) of the polypropylene resin is preferably 50,000 to 700,000, more preferably 100,000 to 600,000, still more preferably 200,000 to 500,000, taking into account mechanical strength and moldability. It is. Furthermore, the molecular weight distribution (Mw / Mn) determined by Mw and the number average molecular weight (Mn) is not particularly limited, and may be, for example, in the range of about 3 to 15.

  When homopolypropylene is used as the main component, in addition to the homopolypropylene, other polyolefin resins and various elastomers can be blended. Examples of other polyolefin resins include high-density polyethylene, linear low-density polyethylene, poly-1-butene, poly-4-methyl-1-pentene, and ethylene-α polyolefin copolymer. Examples of the various elastomers include styrene elastomers such as SEBS and SEPS. These may be used alone or in combination of two or more. In this case, the addition amount of each resin may be an addition amount suitable for the purpose within the range not impairing the air permeability, as in the case of adding in addition to the high density polyethylene.

  Examples of known additives that can be contained in the porous resin membrane include, for example, lubricants, antiblocking agents, antistatic agents, antioxidants, light stabilizers, crystal nucleating agents, fillers, and the like. It is also possible to add.

[Production method]
The porous membrane manufacturing method of the present embodiment includes the TD cold stretching step in addition to the MD cold stretching step described above, whereby a porous membrane having excellent air permeability can be manufactured at a higher speed. Moreover, the manufacturing method of the porous film of this embodiment includes a raw fabric manufacturing process for manufacturing a non-porous raw fabric, an annealing process for annealing the non-porous raw fabric, an MD thermal stretching process for thermally stretching the film into MD, and a film. You may have 1 or more types of processes chosen from the group which consists of MD heat setting process heat-fixed to MD. Moreover, the manufacturing method of the porous membrane of this embodiment may have a TD heat setting process which heat-sets a film | membrane to TD, and / or a surface treatment process which surface-treats to a film | membrane.

(Raw fabric manufacturing process)
A raw fabric manufacturing process will not be specifically limited if it is a process of obtaining a non-porous raw fabric. For example, in a raw fabric manufacturing process, a polyolefin resin as a raw material is melted with an extruder, and a die such as a T die or a circular die is used, and the molten resin flowing out from the die is cast using a roll such as a cast roll or a roll take-up machine. Take over. At this time, it is desirable to rapidly cool and solidify the resin in the melting stage using an air knife or an air ring device. Thereby, lamellar crystals important for pore formation can be regularly and densely arranged in the polyolefin resin.

  The draft ratio ((roll take-up speed) / (flow rate of the resin flowing out from the die lip converted from the density)) when the molten resin flowing out from the die is taken up with a roll is preferably 10 to 600 times, more preferably 20 to 500 times, more preferably 30 to 400 times. The draft ratio is preferably in the above range from the viewpoint of air permeability and moldability. That is, when the draft ratio is 10 times or more, the air permeability is excellent, and when it is 600 times or less, it becomes easy to stably take out the molten resin flowing out from the die.

(Annealing process)
Prior to the MD stretching step, the non-porous raw fabric is annealed in the annealing step, whereby the crystallinity of the non-porous raw fabric can be increased and a porous film having better air permeability can be obtained. Therefore, it is preferable that the manufacturing method of this embodiment includes an annealing step. The temperature range of the annealing treatment is preferably (Tm-3 ° C) to (Tm-30 ° C), more preferably (Tm-5 ° C) to (Tm-3 ° C) from the viewpoint of further increasing the crystallinity of the non-porous raw fabric. Tm-20 ° C).

  The annealing treatment may be a continuous method or a batch method. When performing continuously, from the viewpoint of effectively exhibiting the effect of the annealing treatment, the treatment time is preferably 0.5 minutes or more, more preferably 1 minute or more, and further preferably 2 minutes or more. In addition, in the case of a batch method, when the annealing treatment is performed in a state in which the non-porous raw fabric is wound on a roll, it is preferable to appropriately change the processing time depending on the diameter at the time of winding. Finally, it is preferable to perform an annealing treatment by obtaining a treatment time such that main characteristics such as air permeability on both sides in the thickness direction of the wound non-porous raw roll are close.

(MD cold drawing process)
The MD cold drawing step is not particularly limited as long as it is a known one, but in the MD cold drawing step, the MD is preferably cold drawn with respect to the non-porous raw fabric that has undergone the annealing step. Thereby, since the crack used as a trigger of opening can be formed in a non-porous original fabric, it is preferable that the manufacturing method of this embodiment has MD cold drawing process. It is preferable to use a plurality of rolls for the cold drawing. The draw ratio at this time is preferably 1.05 to 3 times, more preferably 1.1 to 2.5 times, and still more preferably 1.2 to 2 times. By setting the draw ratio within this range, the air permeability, the appearance of the obtained porous film (pinholes, etc.), and the stability of the drawing are improved.

  The stretching temperature in the MD cold stretching step is preferably -20 ° C to (Tm-50 ° C), more preferably 0 ° C to (Tm-60 ° C), and further preferably 10 ° C to (Tm-70 ° C). When the stretching temperature is −20 ° C. or higher, the membrane can be more effectively prevented from being broken, and when it is (Tm−50 ° C.) or lower, a porous membrane having further excellent air permeability can be obtained.

When cold-drawing MD using a plurality of rolls, the strain rate between rolls (maximum value in the case of multi-stage stretching) is preferably 20 to 10,000% / min, more preferably 50 to 5000% / min, More preferably, it is in the range of 80 to 3000% / min. When the strain rate is 20% / min or more, the productivity of the porous film is further improved, and when it is 10,000% / min or less, a porous film having further excellent air permeability can be obtained. In the present specification, the strain rate is obtained by the following formula (2).
Strain rate (stretching rate) = S / {2 · L / (V1 + V2)} (2)
Here, S represents the draw ratio (%), V1 represents the low-speed roll speed (m / min), V2 represents the high-speed roll speed (m / min), and L represents the stretch distance between the rolls. (M) is shown.

(MD heat stretching process)
In the MD hot stretching step, preferably, after the MD cold stretching step, the pore diameter of the MD can be expanded and fixed by subjecting the non-porous raw fabric having cracks to hot stretching. Therefore, it is preferable that the manufacturing method of this embodiment has a MD hot drawing process. The draw ratio at this time is preferably 1.1 to 4 times, more preferably 1.2 to 3.5 times, and still more preferably 1.3 to 3 times, based on the speed before MD heat drawing. is there. By setting the thermal draw ratio to 1.1 times or more, the air permeability can be improved, and by making it 4 times or less, the occurrence of pinholes and breakage can be further suppressed.

  The stretching temperature in the MD thermal stretching step is preferably (Tm-30 ° C) to (Tm-5 ° C), more preferably (Tm-25 ° C) to (Tm-5 ° C), and still more preferably (Tm- 20 ° C.) to (Tm−5 ° C.). If the stretching temperature is (Tm-30 ° C.) or higher, the air permeability is further improved. Moreover, generation | occurrence | production of problems, such as a pinhole, a fracture | rupture, and roll melt | fusion, can be suppressed more because the extending | stretching temperature is (Tm-5 degreeC) or less.

  When MD is stretched using a plurality of rolls, the strain rate between rolls (maximum value in the case of multi-stage stretching) is preferably 20 to 10,000% / min, more preferably 50 to 5000% / min. Min, more preferably in the range of 80-3000% / min. Similar to the MD cold drawing process, the strain rate is 20% / min or more, so that the productivity of the porous membrane is further improved, and the porosity is 10000% / min or less. A membrane is obtained.

(MD heat setting process)
The manufacturing method of the present embodiment is not less than the stretching temperature in the MD hot stretching step for the purpose of reducing the thermal shrinkage of MD and the bowing phenomenon (distortion occurring during stretching) that occurs in the tenter in the TD cold stretching step described later. It is desirable to have the MD heat setting process which heat-processes and heat-sets without substantially extending | stretching with respect to the film | membrane which passed MD heat-stretching process at this temperature. In this MD heat setting step, a plurality of rolls are used, a speed difference is provided between them, and the film can be relaxed (shrinked) to MD within a range where appearance defects such as wrinkles do not occur. This is effective from the viewpoint of suppressing heat shrinkage.

(TD cold drawing process)
The production method of the present embodiment preferably includes a TD cold stretching process in which the film that has undergone each of the above-described processes is cold-stretched using stretching equipment to TD such as a tenter. Thereby, the pore diameter of TD of a film | membrane expands and it becomes possible to make an average pore diameter larger than the porous film obtained by vertical uniaxial stretching. As a result, excellent air permeability can be imparted to the porous membrane and the puncture strength is also improved.

  The draw ratio to TD in the TD cold drawing step is preferably 1.01 to 3.0 times, more preferably 1.05 to 2 times, and still more preferably 1.1 to 1.5 times. By making the draw ratio 1.01 times or more, the air permeability of the porous membrane is further improved, and by setting the draw ratio to 3.0 times or less, the puncture strength is further improved. A porous membrane having an excellent balance of these characteristics can be obtained.

  In general, if the draw ratio to TD is increased, the anisotropy decreases as the degree of orientation of TD increases, so that it is considered that the mechanical strength such as the puncture strength of the film shows an increasing behavior. However, the porous membrane shows a general behavior up to a specific draw ratio to TD, but the porosity (or porosity) increases when it exceeds the specific draw ratio to TD. The effect results in a decrease in the mechanical strength as a result of a decrease in the network that supports the mechanical strength. Therefore, when determining the draw ratio to TD, it is desirable to determine the permeability and mechanical strength required for the application, and the above numerical range is preferable.

  Moreover, in order to perform the extending | stretching to TD stably, it is important to perform a extending | stretching process in cold, From the viewpoint of performing the extending | stretching stably, the extending | stretching temperature in a TD cold extending process is the said Formula ( The temperature T satisfies 1). The stretching temperature T is preferably (Tg + 40 ° C.) to (Tm−40 ° C.).

  In the present embodiment, the stretching speed when cold stretching is performed on the TD does not particularly affect the air permeability, and is preferably faster from the viewpoint of emphasizing productivity. However, if the stretching speed is too high, film breakage may occur near the tenter clip. Therefore, the stretching speed is preferably 10 to 5000% / min, more preferably 50 to 4000% / min, and still more preferably 100 to 3000% / min.

(TD heat setting process)
In the manufacturing method of the present embodiment, in order to suppress thermal shrinkage of TD, TD heat is preferably applied to the film that has undergone the TD cold stretching process by performing heat treatment without substantially stretching the film. It is preferable to have a fixing step. The treatment temperature at this time is preferably in the range of not less than the stretching temperature in the TD cold stretching step and not more than (Tm−5 ° C.) from the viewpoint of suppression of thermal shrinkage of TD and suppression of film breakage. In this TD heat setting step, the film can be relaxed to TD from the viewpoint of more effectively suppressing the thermal shrinkage of TD.

(Surface treatment process)
The production method of the present embodiment uses a known technique such as a corona treatment machine, a plasma treatment machine, an ozone treatment machine, a flame treatment machine, etc., for the purpose of improving the affinity with a coating agent or a solvent for the porous membrane. It is also possible to subject the membrane that has undergone the TD heat setting step to surface treatment.

  The porous film of this embodiment is obtained through the above-described steps.

(thickness)
The thickness of the polyolefin-based porous membrane thus obtained is not particularly limited and may be any thickness required for its use, but is generally 5 to 50 μm, more preferably 8 to 40 μm, still more preferably. 10-30 μm.

(Air permeability)
The air permeability of the polyolefin-based porous membrane of the present embodiment is not particularly limited, but it is preferably 10 to 600 sec / 100 cc in terms of a membrane thickness of 20 μm in consideration of the balance between air permeability and puncture strength. It is more preferably 30 to 500 sec / 100 cc, and further preferably 50 to 400 sec / 100 cc. The air permeability is measured according to the method described in the following examples.

(Puncture strength)
The puncture strength of the polyolefin-based porous membrane of the present embodiment is not particularly limited and may be any as long as it is required for its use, but in general, the membrane thickness is preferably 1 mN or more in terms of 20 μm, It is more preferably 2 mN or more, and further preferably 3 mN or more. The puncture strength is measured according to the method described in the following examples.

  By the porous membrane manufacturing method of the present embodiment, productivity of the porous membrane higher than the conventional known technology can be obtained, and the air permeability and strength characteristics of the porous membrane are also improved. Further, according to the present embodiment, the pore diameter can be increased by obtaining a porous film by stretching to TD. As a result, the air permeability of the porous membrane required for each application can be ensured, and the porous membrane can be produced at a higher speed and more stably than the known longitudinal uniaxial stretching method. Moreover, the porous membrane of this embodiment turns into an inexpensive porous membrane excellent in balance with air permeability and an intensity | strength characteristic.

  As mentioned above, although the form for implementing this invention was demonstrated, this invention is not limited to the said this embodiment. For example, in another embodiment of the present invention, the order of the steps may be as shown in FIG. In the figure, the steps surrounded by single lines may be omitted. Also by these, a porous membrane can be manufactured with high productivity.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this embodiment is described more concretely, this embodiment is not limited to a following example, unless the summary is exceeded. In addition, the measuring method of various physical properties and the method of producing a film by longitudinal uniaxial stretching are as follows.

(thickness)
The thickness of the porous membrane was measured with a dial gauge in accordance with JIS K 7130 (1992) A-2 method.

(Glass transition temperature Tg)
The glass transition temperature Tg of the film was measured using a viscoelastic spectrometer (product name “EXSTAR DMS6100”, manufactured by SII Nanotechnology). TD is measured for the film stretched in the longitudinal uniaxial direction (stretched to MD) in the tensile mode (temperature increase rate = 2 ° C./min, frequency = 1 Hz), and the peak top temperature of the loss elastic modulus E ″ is obtained. The glass transition temperature Tg was used.

(Melting point Tm)
The melting point of the non-porous raw fabric subjected to the annealing treatment was measured using a differential scanning calorimeter (product name “EXSTAR6000”, manufactured by SII Nanotechnology). The top temperature of the melting peak when heated at a rate of temperature increase of 10 ° C./min was measured and determined as the melting point Tm.

(Air permeability)
Based on JIS P-8117, the air permeability of the porous membrane was measured using a Gurley air permeability meter (manufactured by Toyo Seiki Co., Ltd.), and the air permeability in terms of 20 μm thickness of the membrane was determined. From the measurement result of the air permeability, the air permeability of the porous membrane was evaluated according to the following criteria.
300 sec / 100 cc or less: ◎
301-600sec / 100cc: ○
601-800sec / 100cc: △
801 sec / 100cc or more: ×

(Puncture strength)
Using a handy compression tester (product name “KES-G5”) manufactured by Kato Tech Co., Ltd., the puncture strength of the porous membrane was measured under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed = 2 mm / sec. The puncture strength in terms of 20 μm thickness of the film was determined. From the measurement results of the puncture strength, the mechanical strength characteristics of the porous membrane were evaluated according to the following criteria.
4.00mN or more: ◎
3.00 to 3.99 mN: ○
2.00-2.99mN: △
1.99 mN or less: ×

(Productivity evaluation)
From the results of the stretching rate (strain rate) in the MD hot stretching step, the productivity of the porous membrane was evaluated according to the following criteria.
300% / min or more: ◎
200-299% / min: ○
100-199% / min: △
99% / min or less: ×

(Evaluation of TD stretchability)
The appearance of the film after being stretched to TD was visually confirmed to evaluate TD stretchability. The case where no film breakage or stretched spots was observed was evaluated as “◯”, the case where only stretched spots were observed but no film breakage was evaluated as “Δ”, and the case where film breakage was observed was evaluated as “x”.

(Method for producing a polyethylene-based longitudinally uniaxially stretched porous membrane)
The product name “Suntech HD S160S” (MI = 0.8 g / 10 min, density = 0.960 g / cm ³ , manufactured by Asahi Kasei Chemicals Corporation) was used as the polyethylene resin. The polyethylene resin was melted at a set temperature of 200 ° C. with a φ40 mm single screw extruder (P / D = 32, manufactured by Plastics Engineering Laboratory) and extruded from a 190 ° C. T die (lip clearance = 7 mm). . The molten resin was taken up with a cast roll set at 115 ° C. while being cooled with an air knife. The draft ratio at this time was 300 times, and a non-porous raw fabric having a thickness of 23 μm was obtained.

  Next, this non-porous raw fabric is annealed for 3 hours with a hot air dryer at 120 ° C., then cold stretched to MD, hot stretched to MD, and heat fixed to MD (thermal relaxation) Were further obtained to obtain three types of porous polyethylene-based longitudinally uniaxially stretched porous films PE-1, PE-2 and PE-3. The production conditions, thickness, Tg, and Tm of each film were as shown in Table 1.

(Method for producing a polypropylene-based longitudinally uniaxially stretched porous membrane)
The product name “Prime Polypro E111G” (MI = 0.5 g / 10 min, density = 0.910 g / cm ³ , manufactured by Prime Polymer Co., Ltd.) was used as the polypropylene resin. The polypropylene resin was melted at a set temperature of 260 ° C. with a φ40 mm single-screw extruder (manufactured by Plastics Engineering Laboratory, L / D = 32) and extruded from a 260 ° C. T-die (lip clearance = 5 mm). . The molten resin was taken up with a cast roll set at 130 ° C. while being cooled with an air knife. The draft ratio at this time was 150 times, and a non-porous original fabric having a thickness of 23 μm was obtained.

  Next, this non-porous raw fabric was annealed for 3 hours in a hot air dryer at 150 ° C., then cold-drawn to MD in one step, and then hot-drawn into MD in three steps (equal strain) Speed). Thereafter, heat setting (thermal relaxation) on the MD was further performed to obtain one type of polypropylene-based longitudinally uniaxially stretched porous membrane PP-1. In addition, the manufacturing conditions, thickness, Tg, and Tm of the film were as shown in Table 1.

Example 1
The polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 is cold-drawn to TD at a temperature of 60 ° C. and a condition of 1.03 times with a tenter stretching machine, and is further heat-set (heat relaxed) to TD. A biaxially stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Example 2)
The polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 is cold-stretched to TD at a temperature of 60 ° C. and a condition of 1.1 times with a tenter stretching machine, and further heat-fixed (thermal relaxation) to TD, A polyethylene biaxially stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Example 3)
The polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 is cold-drawn to TD at a temperature of 60 ° C. and a condition of 1.18 times with a tenter stretcher, and is further heat-set (heat relaxed) to TD, and then polyethylene A biaxially stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Example 4)
A polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 is cold-drawn to TD at a temperature of 60 ° C. and a condition of 1.38 times with a tenter stretching machine, and further heat-set (heat relaxed) to TD, and then polyethylene A biaxially stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Example 5)
The polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 is cold-drawn to TD at a temperature of 25 ° C. and a condition of 1.18 times with a tenter stretching machine, and is further heat-set (heat relaxed) to TD to obtain polyethylene. A biaxially stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Example 6)
A polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 is cold-drawn to TD at a temperature of 100 ° C. and a condition of 1.18 times with a tenter stretching machine, and is further heat-set (heat relaxed) to TD, and then polyethylene A biaxially stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Example 7)
Polyethylene biaxial in the same manner as in Example 4 except that the stretching speed in each step from cold stretching to MD to heat fixation to TD was increased 2.5 times that in Example 4. A stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Example 8)
Polypropylene-based longitudinally uniaxially stretched porous membrane PP-1 is cold-stretched to TD at a temperature of 90 ° C. and 1.2 times in a tenter stretching machine, and further heat-set (heat relaxed) to TD, A biaxially stretched porous membrane was obtained. Table 2 shows the conditions for cold stretching to TD and thermal relaxation, and the results of various evaluations.

(Comparative Example 1)
The polyethylene-based longitudinally uniaxially stretched porous membrane PE-1 was used as the membrane of Comparative Example 1. Table 2 shows the results of various evaluations.

(Comparative Example 2)
The polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 was used as the membrane of Comparative Example 2. Table 2 shows the results of various evaluations.

(Comparative Example 3)
A polyethylene-based longitudinally uniaxially stretched porous membrane PE-3 was used as the membrane of Comparative Example 3. Table 2 shows the results of various evaluations.

(Comparative Example 4)
The polyethylene-based longitudinally uniaxially stretched porous membrane PE-2 is heat-stretched to TD at 120 ° C., which is a temperature in the vicinity of the melting point Tm, and further heat-set (heat relaxed) to TD. A membrane was obtained. Table 2 shows the conditions of thermal stretching to TD and thermal relaxation, and the results of various evaluations.

(Comparative Example 5)
The polypropylene-based longitudinally uniaxially stretched porous membrane PP-1 was used as the membrane of Comparative Example 5. Table 2 shows the results of various evaluations.

(Comparative Example 6)
The polypropylene-based longitudinally uniaxially stretched porous membrane PP-1 is heat-stretched to TD at 140 ° C., which is a temperature in the vicinity of the melting point Tm, and further heat-set (heat relaxed) to TD, so that a porous polypropylene-based membrane Got. Table 2 shows the conditions of thermal stretching to TD and thermal relaxation, and the results of various evaluations. In Comparative Example 6, various physical properties varied widely, and reliable data could not be collected.

  As is clear from the results shown in Table 2, when the film is stretched to TD in the vicinity of the melting point, film breakage or uneven stretching occurs, and a good biaxially stretched porous film cannot be obtained. It was. On the other hand, a uniform biaxially stretched porous membrane could be obtained by cold stretching to TD at a temperature satisfying the above formula (1).

  Further, as shown in Comparative Examples 1, 2, and 3 in Table 2, as the MD stretching rate (strain rate) is increased to increase productivity, the porous longitudinally uniaxially stretched membrane has an air permeability. In the comparative example 3, it deteriorated to 1914 sec / 100 cc. However, it was possible to lower the air permeability to 231 sec / 100 cc as shown in Example 7 by further subjecting this membrane to cold stretching to TD. Thus, the air permeability of the porous longitudinal uniaxially stretched film deteriorated due to high speed can be improved by cold stretching to TD, and as a result, it was difficult to reach by conventional longitudinal uniaxial stretching, good It was possible to produce a porous membrane with excellent characteristics in a high-speed region.

  Further, when comparing the porous longitudinal uniaxially stretched membrane and the biaxially stretched porous membrane according to the present invention with respect to their puncture strength, Comparative Example 1 (179 sec / 100 cc) and Example 3 (170 sec / 100 cc) are equivalent. Although it has air permeability, the puncture strength in Comparative Example 1 is 2.77 mN, whereas it is 3.84 mN in Example 3, and the porous membrane of the present invention has superior puncture strength, and air permeability. The physical property balance was good.

  Since the present invention can provide a porous membrane excellent in air permeability and strength balance with high productivity, it can be used for medical separation membranes, food packaging, batteries, sanitary products, optical reflection films, etc. It can be suitably used in the field.

Claims (4)

A method for producing a porous membrane stretched by a dry method,
In the MD cold-stretching step of cold-stretching a film composed of a polyolefin-based resin into MD, and in the stretching temperature satisfying the condition represented by the following formula (1), the film is cold-stretched in TD to the TD A method for producing a polyolefin-based biaxially stretched porous membrane, comprising a step.
(Tg + 20) ≦ T ≦ (Tm−30) (1)
(In the formula, T represents the stretching temperature (unit: ° C), Tg represents the glass transition temperature (unit: ° C) of the film, and Tm represents the melting point (unit: ° C) of the film.)   The method for producing a polyolefin-based biaxially stretched porous membrane according to claim 1, wherein a draw ratio in the TD cold drawing step is 1.01 to 3.0 times.   The method for producing a porous membrane according to claim 1 or 2, wherein the polyolefin resin is at least one selected from the group consisting of a polyethylene resin and a polypropylene resin.   A polyolefin-based biaxially stretched porous membrane obtained by the production method according to claim 1.

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