Electret filter media and process for producing the same

Abstract

It is intended to provide an electret filter which has a large surface charge density, sustains stable electret properties over a long time even in a high-temperature atmosphere and yet degrades to reduce its volume when buried for disposal; and a process for producing the same. The molar ratio of an L-lactic acid monomer to a D-lactic acid monomer ranges from 100 to 85:0 to 15 or 0 to 15:85 to 100. An electret filter having a surface charge density of 1.2×10−9 C / cm2 or more can be obtained by crystallizing by heating at a temperature from the glass transition temperature to the melting point and then, while heating to 60 to 140° C., applying a direct current corona electrical field to thereby cool to 40° C. or lower under.

Description

TECHNICAL FIELD [0001] The invention relates to electret filter media having both biodegradability and good electret properties at the same time and to a process for producing the same. Specifically, the invention relates to electret filter media that are mainly composed of a biodegradable lactic acid polymer, are for use in capture of fine particles in a gas, have large surface charge density and thus high collection efficiency, and can have high particle-collection efficiency initially and even after exposed to a high temperature atmosphere, and relates to a process for producing the same. BACKGROUND ART [0002] Conventionally, dielectric materials having relatively long-life electrostatic charges are called “electret” and generally used for separation materials such as air filters, sanitary materials such as a mask, and electronic materials such as microphone materials. Nonwoven fabrics of polyolefin fibers electrically polarized by high-voltage corona treatment or the like have m...

AI Technical Summary

Benefits of technology

[0022] The lactic acid polymer having the composition as stated above is capable of having crystallinity and has a glass transition temperature of room temperature or higher so that it can form an electret filter medium having excellent charge stability.

[0049] The electret filter media produced as described above according to the invention can be more resistant to the influence of temperature or moisture than the conventionally known degradable electret filter media and can be superior in practical applications.

Problems solved by technology

However, such electret media are characterized by being almost undegradable or having very low rate of degradation in the natural environment, although they are effective in producing electrets.

Thus, the products produced from such materials maintain their form over a long time period when dumped and buried, and thus problems such as environmental destruction have come to the surface.

In addition, the conventional nonwoven fabrics of polyolefin fibers still have insufficient electret performance.

In the document, however, neither the technique for high performance nor the charge stability over time is taken into account.

However, this publication only discloses the stability of the electrostatic force during high-temperature use and is silent on improvement in surface charge density or improvement in collection efficiency.

According to the inventors' finding, only the application of high voltage at the above atmospheric temperature is insufficient for an increase in surface charge density.

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