Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
2000-12-01
2003-05-20
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C264S210400, C264S235800, C264S041000, C210S500420, C521S079000
Reexamination Certificate
active
06566012
ABSTRACT:
TECHNICAL FIELD
This invention relates to a microporous membrane comprising an ultra-high-molecular-weight polyolefin or composition containing an ultra-high-molecular-weight polyolefin, more particularly to a microporous polyolefin membrane for battery separators, and also relates to a method for producing the same.
BACKGROUND OF THE INVENTION
Microporous polyolefin membranes are widely used in various applications such as battery separators (in particular, separators for lithium ion primary and secondary batteries, and large-sized batteries for electric vehicles or the like), capacitor separators, various separation membranes (e.g., for water treatment, ultrafiltration, microfiltration and reverse osmosis), various filters, and moisture-permeable/waterproof clothes or base materials therefor, because they are insoluble in organic solvents, and resistant to electrolytes and electrode-active materials.
One of the known methods for producing microporous polyolefin membranes is extraction process comprising the steps of mixing a polyolefin with an organic medium and inorganic powder (e.g., finely powdered silica), melting and molding the mixture, and extracting the organic solvent and inorganic powder. This method needs a process of extracting the inorganic powder, and permeability of the membrane produced depends largely on particle size of the inorganic powder used and is difficult to control.
Recently, various processes have been proposed to produce high-strength, microporous membranes which contain an ultra-high-molecular-weight polyolefin. For example, Japanese Patent Application Laid-Open Nos. 60-242035, 61-195132, 61-195133, 63-39602, 63-273651, 3-64334 and 3-105851 disclose processes to produce microporous membranes by forming a gel-like sheet from a heated solution of a polyolefin composition containing an ultra-high-molecular-weight polyolefin dissolved in a solvent, stretching it while heating, and removing the solvent by extraction. These processes give the microporous polyolefin membranes characterized by a sharp pore size distribution and small pore size, which have been used for battery separators.
Recently, lithium ion batteries have been increasingly required to have higher battery characteristics, safety and productivity. The functions these batteries are required to have include battery capacity at low temperature for the improved battery characteristics, and permeability of the lithium ions and a function of controlling effects of clogging for the recycling characteristics. In order to improve battery characteristics, it is generally essential to reduce energy loss by increasing separator permeability and decreasing ion migration resistance in a range that safety of the battery is securely kept. It should be noted that excessively increasing pore size helps accelerate formation of dendrite, possibly deteriorating safety of the battery. It is necessary, when the battery is subjected to a destructive test, the electrodes are securely kept apart from each other for safety reasons, even when the separator is deformed. Development of the separator having a high pin puncture strength, adequate pore diameter and high porosity (permeability) has been increasingly demanded.
It is an object of the present invention to provide a microporous polyolefin membrane, high in pin puncture strength, adequate pore diameter and high porosity (permeability), and excellent in battery characteristics and high in safety when used for battery separators.
DISCLOSURE OF THE INVENTION
The inventors of the present invention have found, after having extensively studied to solve the above-described problems, that a microporous polyolefin membrane well-balanced between high pin puncture strength and porosity can be produced by biaxially stretching a gel-like formed product of solution comprising a specific content of an ultra-high-molecular-weight polyolefin or composition containing an ultra-high-molecular-weight polyolefin dissolved in a solvent under optimized stretching conditions (e.g., stretching ratio) and then heat-setting the stretched product also under optimized conditions, to reach the present invention.
Thus, the present invention provides a microporous polyolefin membrane comprising (A) an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5×10
5
or more, or (B) a composition containing an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5×10
5
or more, and having a porosity of 30 to 95%, bubble point exceeding 980 KPa and pin puncture strength of 4900 mN/25 &mgr;m or more.
The present invention also provides a method for producing a microporous polyolefin membrane, which comprises the steps of (a) preparing a solution comprising 10 to 40 wt. % of (A) an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5×10
5
or more, or (B) a composition containing an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5×10
5
or more, and 90 to 60 wt. % of a solvent, (b) extruding the solution, (c) quenching the extruded solution to have a gel-like extrudate, (d) biaxially stretching the gel-like extrudate at 110 to 120° C. at a stretching ratio of 5 or more in both MD and TD directions, (e) removing the solvent and drying the extrudate, and (f) heat-setting the extrudate at 115 to 125° C.
PREFERRED EMBODIMENTS OF THE INVENTION
The microporous polyolefin membrane of the present invention is described in more detail by constituent components, properties and method of production.
1. Polyolefin
The ultra-high-molecular-weight polyolefin (A) for the microporous polyolefin membrane of the present invention has a weight-average molecular weight of 5×10
5
or more, preferably in a range from 1×10
6
to 15×10
6
. A polyolefin having a weight-average molecular weight less than 5×10
5
is undesirable for the present invention, because of anticipated deterioration of membrane strength.
The composition (B) containing an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5×10
5
or more is not limited. One of the preferred examples of the composition (B) comprises an ultra-high-molecular-weight polyolefin (B-1) having a weight-average molecular weight of 1×10
6
or more, preferably 1.5×10
6
or more, more preferably 1.5×10
6
to 15×10
6
and a polyolefin (B-2) having a weight-average molecular weight of 1×10
4
or more but less than 1×10
6
, preferably 1×10
5
or more but less than 5×10
5
. It is necessary for the composition (B) to contain the ultra-high-molecular-weight polyolefin (B-1) at 1 wt. % or more. The composition containing the ultra-high-molecular-weight polyolefin (B-1) at less than 1 wt. % will not give the microporous membrane of sufficient strength, because the molecular chains of the polyolefin (B-1) are scarcely entwined with each other. When the polyolefin (B-2) has a weight-average molecular weight less than 1×10
4
, it cannot give a desired microporous membrane, because it tends to be broken.
Examples of the above-described polyolefins include crystalline homopolymers, two-stage polymers or copolymers of ethylene, propylene, 1-butene, 4-methyl-pentene-1,1-hexene, 1-octene, vinyl acetate, methyl methacrylate or styrene, or blends thereof. Preferable among them are polypropylene, polyethylene and compositions thereof. Polyethylenes may be of high density, low density or medium density. Of these, the preferable ones are the compositions comprising an ultra-high-molecular-weight polyolefin (B-1) having a weight-average molecular weight of 1×10
6
or more and high-density polyethylene (B-2) having a weight-average molecular weight of 1×10
4
or more but less than 1×10
6
. Particularly preferable compositions are those comprising an ultra-high-molecular-weight polyolefin (B-1) having a weight-average molecular weight of 1×10
6
or more and high-density polyethylene (B-2) having a weight-average mo
Funaoka Hidehiko
Kaimai Norimitsu
Kobayashi Shigeaki
Kono Koichi
Takita Kotaro
Armstrong Westerman & Hattori, LLP
Maples John S.
Tonen Chemical Corporation
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