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

[0044] The film is preferably formed using a conventionally known film-forming method such as a solvent casting method, a melt extrusion method, and a heat press method. After the film is formed by such the methods, if desired, uniaxial or biaxial drawing and heat setting are preferably performed. If drawing orientation and crystallization treatment are performed, the charge stability against moisture and temperature can be increased even in the same composition. The thickness of the film should be from 1 to 50 μm. In the lower range, the thickness of the film is preferably from 1.5 to 10 μm. If the thickness is more than 50 μm, the electret forming effect can be saturated.

[0045] Any known spinning method such as a melt extrusion method and a dry or wet type solvent spinning method may preferably be used to form the fibers. After the fibers are formed by such the methods, if desired, drawing treatment and heat setting are preferably performed. If drawing orientation and crystallization treatment are performed, the charge stability can further be increased. It is also preferred that a uniaxially oriented film is divided into fibrillated film yarns or film split yarns as conventionally known.

[0046] When the nonwoven fabric is formed and used, the fibers produced by the above method are cut into an appropriate length and then subjected to a dry type web-forming method in a carding machine, a me

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.

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