Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Producing or treating porous product
Reexamination Certificate
2000-05-17
2003-04-01
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Producing or treating porous product
C264S083000, C264S210300, C264S210400, C264S210500, C264S210700, C264S211180, C264S211200, C264S235000, C264S235600, C264S470000, C264S473000
Reexamination Certificate
active
06540953
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a national phase application of International Application No. PCT/KR98/00365, which was filed on Nov. 16, 1998 and which published in English on May 27, 1999, which in turn claims priority from Korean Application No. KR 1997/60661, which was filed on Nov. 17, 1997.
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a microporous membrane and a method for providing the same, and more particularly to a method for providing a microporous membrane having hydrophilic/hydrophobic properties and pores with uniform size and shape by irradiating energized ion particles to a polymer film under vacuum.
(b) Description of the Related Art
Currently, there are various types of microporous membranes being used as a separator in lithium battery. Conventional methods for producing these microporous membranes are classified into a wet method and dry method. These methods utilize fillers or wax with a solvent as in wet method, or without the solvent as in dry method, to produce a precursor film. Then a resulting microporous membrane is obtained by forming micro-pores in the precursor film.
There are numerous methods of forming micro-pores, such as in cold and hot stretching methods the precursor film is subjected to a stretching process, and in an extraction method low molecular weight particles are extracted from the precursor film which has been subjected to a biaxial stretching (alternatively, biaxial stretching process can be implemented after the extraction method) to form micro-pores on the precursor film. Further, the precursor film can be subjected to a corona discharge method followed by a stretching, or it can be etched after being irradiated with high-energy ion-beams as in a track-etching method to obtain microporous membrane. The method utilizing cold or hot stretching process is referred to as a dry process. U.S. Pat. Nos. 3,679,538;1 3,801,692 3,843,761; 4,238,459; and 5,013,439 disclose the dry process, while U.S. Pat. Nos. 3,471,597 and 3,880,966 disclose corona discharge process for obtaining a precursor film with pores.
The dry process has an advantage in that it does not utilize environmental hazardous solvents, and hence the method is referred to as a clean process and is widely used in the industry. However, microporous membranes produced by the dry process have pores with undesirable small sizes, and presents the difficulties of adjusting and increasing shape and size of the pores. Further, there is a drawback in that during stretching, maintaining shape of the pores becomes difficult as stretch ratio increases.
The conventional methods for producing microporous mebranes to be used as a separator in lithium battery utilize polyolefin resin because of its cost and chemical and physical property. However, due to the hydrophobicity of the polyolefin resin, there is a low wettability of electrolytes for the separator. Currently, there are numerous researches being carried out to incorporate hydrophilic property to polyolefin resin membranes. The method described by Hoechst Celenese processes the surface of the polyolefin resin membrane with surfactants, and other methods described by U.S. Pat. Nos. 3,231,530; 3,853,601; 3,951,815; 4,039,440; and 4,340,482 integrates monomers having high hydrophillic property or processes the polyolefin resin membranes with chemicals. However, because of simultaneously occuring chemical reactions, the molecular weight of polymer decreases and the structural integrity of the polyolefin membrane weakens. Further, due to the complexity of the processes involved, it is difficult to mass produce the polyolefin membranes having hydrophilic property.
Other methods for integrating hydrophilic property to the polyolefin membranes are further described by U.S. Pat. Nos. 4,346,142; 5,085,775; and 5,294,346. These methods use monomers of acrylic acid having hydrophilic property and polymers of polyethylene oxide by grafting them on to the surface of polymer membranes utilizing corona or plasma method. JP-A-8-31399 (unexamined published Japanese application) discloses a method of integrating both the hydrophilic and hydrophobic property to the polyolefin film surface by oxygen and carbon tetrafluoride gas utilizing plasma or sputter etching method. However, due to the plasma's unique properties characterized by having a wide range of energy distribution and a high degree of environmental susceptibility, it is difficult to obtain an uniformed porosity. Further, obtaining a polyolefin membrane having excellent physical properties is made difficult by the degradation of its mechanical property due to the damage to the surface of the film caused by the reactions accompanying the method.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a method for producing a microporous membrane having hydrophilic/hydrophobic properties and pores with uniform size and shape by irradiating energized ion particles to a polymer film under vacuum.
It is another object of the present invention to provide a method for producing a microporous membrane having high-density of pores.
It is yet another object of the present invention to provide a simple process method for producing a microporous membrane having hydrophilic property.
It is further object of the present invention to provide a method for producing a microporous membrane having hydrophilic property and excellent physical characteristics.
It is further object of the present invention to provide a microporous membrane prepared by the method.
According to the above methods of the present invention, a microporous membrane having excellent physical characteristics can also be obtained by irradiating ion particles of the microporous membrane produced from conventional methods.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below.
The present invention utilizes the principles for decreasing contact angle of hydrophilic solvents onto the surface of a polymer surface and increasing adhesion of the same by utilizing ion-beam irradiation.
Preparation of Precursor Film
A polymer film is obtained by using an extruder having a T-die or a tubular die, and is made from a polyolefin group consisting of polypropylene, high-density polyethylene, low-density polyethylene, and low-density linear polyethylene, because of cost and its low reactivity. Although an extrusion process can be carried out in a conventional extruding temperature, it is more preferable to carry out the process in temperature range of (polymer film melting point +10° C.)~(polymer melting point +100° C.). Extruding the polymer beyond this temperature range can lead to polymer degradation and consequently weaken its physical property.
The extruded polymer is drawn by using cast roll at 5~120 m/min in 10~150° C. to obtain a precursor film, at draw down ratio of 10~400 and the freezing temperature of 10~120° C.
Annealing
The precursor film is annealed at temperature range of (polymer film melting point −10° C.)~(polymer melting point −100° C.) for 10 sec. to 1 hour in order to obtain an elastic recovery over 40% at 25° C. This annealing process increases both the elastic recovery and crystallinity of the precursor polymer film. Annealing at a temperature higher than this range may melt the polymer film, and annealing at a temperature lower than the range restricts the polymer movement and any significant increase in both the elastic recovery and crstallinity is very marginal.
Irradiation
The annealed precursor film is placed in a vacuum chamber under 10
−2
~10
−8
torr, then both surfaces of the precursor film were irradiated with an ion-gun. The ion-gun was prepared by injecting a gas for generating energized ion particles to be used in irradiation by changing electrical current of ion-beam. Although, an irradiating distance from ion-gun to the surface of the precursor film of 5~100 cm is adequate, irradiating distance should be adjusted according t
Kim Myung-Man
Lee Sang-Young
Song Heon-Sik
Baker & Botts L.L.P.
LG Chemical Ltd.
Tentoni Leo B.
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