Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Reexamination Certificate
2000-12-01
2001-06-12
Foelak, Morton (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C264S041000, C264S235800, C264S290200, C210S500420, C521S079000, C521S143000, C429S254000
Reexamination Certificate
active
06245272
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 medium 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. Each battery system is required to have an optimum pore size, porosity and permeability for improved battery 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 microporous membrane satisfying the above conditions has been increasingly demanded.
It is an object of the present invention to provide a microporous polyolefin membrane, high in elongation and porosity, low in shrinkage, and capable of improving battery characteristics, safety and productivity, 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 in properties (e.g., high in elongation and porosity and low in shrinkage) can be produced by extruding a solution of an ultra-high-molecular-weight polyolefin or composition containing an ultra-high-molecular-weight polyolefin dissolved in a solvent, and stretching the resultant gel-like extrudate, removing the solvent therefrom and thermally setting it under controlled conditions for the stretching and heat-setting processes, 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 45 to 95%, tensile elongation of 300% or more, bubble point exceeding 980 KPa and thermal shrinkage in the TD direction of 3% or less.
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 50 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 50 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 116 to 125° C. at a stretching ratio of 2 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 120° 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 in a range from 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
4
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, preferably 15 wt. % or more, more preferably in a range from 15 to 40 wt. % . 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
, the microporous membrane produced tends to be broken.
Examples of the above-described polyolefins include crystalline homopolymers, two-stage polymers or copolymers of ethylene, propylene, 1-butene, 4methyl-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 5×10
5
.
Low-density polyethylene may be incorporated as the polyolefin which imparts a shut-down function to the membrane, to improve its characteristics for battery separators. The low-density polyethylene useful for the present invention includes linear low-density polyethylene (LLDPE) produced by the medium pressure method, low-density
Funaoka Hidehiko
Kaimai Norimitsu
Kobayashi Shigeaki
Kono Koichi
Takita Kotaro
Armstrong, Westerman Hattori, McLeland, and Naughton, LLP
Foelak Morton
Tonen Chemical Corporation
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