Coating processes – Electrical product produced – Cellulosic or fibrous base
Reexamination Certificate
2000-04-13
2004-06-01
Beck, Shrive P. (Department: 1762)
Coating processes
Electrical product produced
Cellulosic or fibrous base
C427S058000, C427S255240, C427S372200, C427S377000, C307S400000
Reexamination Certificate
active
06743464
ABSTRACT:
BACKGROUND
The invention pertains to a method of making an electret article by condensing vapor onto a dielectric article followed by drying.
Electrets are dielectric articles that exhibit a lasting charge. This unique property allows electrets to be used in a variety of applications including air, furnace, and respiratory filters, face masks, and electro-acoustic devices such as microphones, headphones, and electrostatic recorders. The charged nature of the electret enhances the article's ability to attract and retain particles such as dust, dirt, and fibers that are suspended in the air.
A variety of methods have been developed for producing electrets. The methods include contact electrification, thermal charging, charge-deposition, liquid contact charging, and impinging jets of water on the surface of the article. Examples of these methods are disclosed in the following documents: P. W. Chudleigh,
Mechanism of Charge Transfer to a Polymer Surface by a Conducting Liquid Contact
, 21 A
PPL
. P
HYS
. L
ETT
., 547-48 (Dec. 1, 1972); P. W. Cudleigh,
Charging of Polymer Foils Using Liquid Contacts
, 47 J. A
PPL
. P
HYS
., 4475-83 (October 1976); U.S. Pat. No. 4,215,682 to Kubic and Davis; U.S. Pat. No. 4,588,537 to Klaase et al.; U.S. Pat. Nos. Re. 30,782, Re. 31,285, and Re. 32,171 to van Turnhout; U.S. Pat. No. 4,798,850 to Brown; U.S. Pat. No. 5,280,406, Coufal et al.; and U.S. Pat. No. 5,496,507 to Angadjivand et al.
SUMMARY OF THE INVENTION
The present invention provides a new method of making an electret that involves condensing a vapor onto a dielectric article followed by drying.
In one aspect, the invention features a method of making an electret that may suitably comprise or consist essentially of: condensing vapor from the atmosphere of a controlled environment onto an article that includes a nonconductive polymeric material; and drying the article to remove the condensate. In other aspects, the invention features a filter or respirator that includes an electret produced according to the method of this invention.
The method is particularly useful for forming an electret from a porous material such as a nonwoven fibrous web. The method can enable individual fibers in the web to exhibit at least quasi-permanent electrical charge. The method may advantageously be used to form an electret without altering the physical structure of the polymer article. That is, the process can be employed without damaging, for example, breaking or eroding, the individual fibers or the bulk structure of the article, or unduly compressing a porous fibrous web. The method also may be suitable for charging articles that have a variety of shapes and constructions including, for example, articles that exhibit a contoured shape, multi-layer articles, flat articles and combinations thereof. The inventive method is also advantageous in that less liquid may be used to charge the article. Although electret articles can be produced in this invention through saturation with the condensate, the invention does allow electrets to be produced without complete saturation, and thus allows less liquid to be used to charge an electret.
Also, the liquid used in the method can be water, which is not an environmental pollutant, is readily available, and has a relatively low cost.
GLOSSARY
In reference to the invention, these terms have the meanings set forth below:
“atmosphere” means a gaseous medium;
“condensate” means the product that results from condensing;
“condensing” means altering to another and denser form, e.g., reducing gas or vapor to a liquid;
“controlled environment” means a surrounding whose volume, pressure, temperature, or a combination thereof, can be regulated and/or altered in a predetermined manner;
“dielectric material” means a material in which an electric field gives rise to no net flow of electric charge but only to a displacement of charge;
“drying” means removing condensate from the surface of the article;
“electret” means a dielectric material that exhibits at least a quasi-permanent electrical charge;
“nonconductive” means having a volume resistivity of greater than 10
14
ohm-cm;
“persistent electric charge” means that the electric charge resides in the article for at least the commonly-accepted useful life of the device in which the electret is employed;
“polymeric” means containing a polymer and possibly other ingredients;
“quasi-permanent” means that the electric charge resides in the electret under standard atmospheric conditions (22° C., 101,300 Pascals atmospheric pressure, and 50% humidity) for a time period long enough to be significantly measurable; and
“vapor” means a gaseous system such as air, which contains molecules that can be condensed to form a liquid.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
An electret can be prepared according to the invention, for example, by placing an article in a controlled environment, altering at least one property of the environment such that the atmosphere surrounding the article becomes saturated with vapor, altering the same or different property of the environment such that the vapor condenses on the article, and then drying the article. The properties of the environment that may be altered to condense the vapor include pressure, volume, and temperature.
In one embodiment, an electret may be prepared by altering the pressure of the atmosphere in a controlled environment that includes the atmosphere and a liquid. An article is submerged in the liquid in the controlled environment. Although submerged, there is an atmosphere of gas, vapor, or a combination thereof, around the article. For those articles that include interstitial spacing, the atmosphere permeates throughout the interstices. The method may further include reducing the pressure (P) on the atmosphere, for example, to a pressure P
1
, to allow at least a portion of the liquid to evaporate into the atmosphere to increase the vapor present in the atmosphere. The pressure may be further reduced to pressure P
2
to the vapor pressure of the liquid, to cause the liquid to boil. The resulting vapor displaces the gas molecules in the atmosphere. The pressure may then be increased to ambient pressure to cause the vapor to condense on the surface of the article, including, when present, the surfaces that define the interstitial spaces of the article, so as to wet the article surface. The article is then dried to create an electret.
An electret also may be prepared by (i) placing the article in a controlled environment that includes a vapor-saturated volume and (ii) increasing the pressure on the volume to cause the vapor to condense on the article. The pressure can be increased by placing the article in a sealed chamber that has a first sealed volume V
1
and reducing the volume of the chamber to a second sealed volume V
2
such that at least a portion of the vapor condenses from the sealed chamber's atmosphere onto the article. The reduction in sealed volume can be accomplished, for example, through the actuation of a piston that reduces the sealed volume of the chamber without releasing the atmosphere.
In other embodiments, an electret may be prepared by: (i) placing an article in a controlled environment that is saturated with vapor; (ii) rapidly decreasing the pressure, to cause an adiabatic expansion to occur, which, in turn, causes vapor to condense on the surface of the article; and (iii) drying the article.
In yet another embodiment, the electret may be prepared by (i) placing an article in a controlled environment that includes a vapor-saturated atmosphere, which article has been conditioned at a temperature T
1
and the controlled environment is conditioned at a temperature T
2
(the saturation temperature), where T
1
is sufficiently less than T
2
so as to cause the vapor to condense on the article, and then (ii) drying the article.
The controlled environment in which the electret may be produced is one where the properties of the environment—such as volume, temperature, pressure and combinations thereof—are capable of being regulated and/or
Insley Thomas I.
Knoll Randall L.
3M Innovative Properties Company
Beck Shrive P.
Hanson Karl G.
Johnson Allison
Markham Wesley D
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