Microporous polyolefin film

Abstract

A microporous polyolefin film which comprises 5 to 95 wt. % polyethylene (A) having a viscosity-average molecular weight (Mv) of 2,000,000 or higher, a first-melting-peak signal height as determined by DSC (differential scanning calorimetry) of 3.0 mW / mg or higher, a specific surface area of 0.7 m2 / g or larger, and an average particle diameter of 1 to 150 μm and 95 to 5 wt. % polyethylene (B) having an Mw of 10,000 to 200,000, excluding 10,000 and 200,000, wherein the ratio of the Mv of the compound (A) to that of the compound (B), (A) / (B), is 10 or higher, the film as a whole has a molecular weight of 300,000 to 1,500,000, and the film has a fuse temperature of 120 to 140° C., a breaking temperature of 150° C. or higher, and a ratio of the piercing strength at 25° C. to the piercing strength at 140° C. of from 0.01 to 0.25.

Description

TECHNICAL FIELD [0001] The present invention relates to a separator used for electrochemical reaction devices such as a battery and a capacitor. BACKGROUND ART [0002] A microporous polyolefin film is used for a microfiltration membrane, a separator for a battery, a separator for a capacitor, fuel cell material, or a basic material for a functional membrane in which a functional material is filled in the pores to give rise to a new function. When the microporous polyolefin film is used, among these applications, as a separator for a battery, particularly a separator for a lithium ion battery, it is required to have a high piercing strength and a high elongation modulus so as to prevent internal short circuit due to foreign bodies and the like within the battery and to prevent the separator from elongating in the flow direction when wound up to form a battery. [0003] Furthermore, “fuse effect” and “heat resistance” securing the safety of a battery are demanded as well as general physi...

AI Technical Summary

Benefits of technology

[0017] The microporous film consisting of a specific ultra-high-molecular-weight polyethylene and a low molecular weight polyethylene in a specific molecular weight range of the present invention enables to provide a separator having excellent low fuse and high short-circuit properties as well as a low heat-shrinkability at a high temperature.

Problems solved by technology

High strength and low heat shrinkability and high film breakage resistance and low heat shrinkability are, however, incompatible characteristics.

However, such a simple high molecular orientation may cause a high shrinkage stress in a fused state, and, as a result, lowers heat resistance.

For example, Patent Document 1 proposed a microporous film having a high film breakage resistance by extrusion molding polyolefin and a polymer cross-linkable therewith followed by cross-linking treatment, but, to the contrary, relaxation at a high temperature was slow and accordingly it was inappropriate due to remaining strong shrinkage stress.

In addition, there was a problem in productivity because a thermal cross-linking treatment step, which incurs a long period of time, was included.

Meanwhile, Patent Document 2 realized high strength by using a polyolefin having a sharp molecular weight distribution, but it was difficult to have low temperature fuse and high short-circuit ability because the contents of ultra-high-molecular-weight ingredients and low molecular weight ingredients are decreased.

However, there is no description in Patent Document 3 about necessity of a step for imparting strength to the microporous film to be heat drawn, that is, a step of drawing the sheet under heating before removing a good solvent, and therefore, it is difficult to obtain a highly strong film.

Besides, a step of fusing polyethylenes having a large difference in molecular weight needs a significant period of time and lacks in productivity.

However, thermal shrinkage was not sufficiently prevented only by the heat-treatment step as described for a microporous film comprising a polyethylene having a molecular weight of 200,000 or less.

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