Electret treatment of high loft and low density nonwoven webs

Details Electret treatment of high loft and low density nonwoven webs Electret treatment of high loft and low density nonwoven webs

1999-06-24

2002-04-02

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of electrical or wave energy to work

Molecular aligning or molecular orientating

C246S438000, C246S484000, C425S17480E

Reexamination Certificate

active

06365088

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to electret treated filtration materials and processes for electret treating such materials.
BACKGROUND
It is frequently desirable to provide a filtration material with enhanced filtration efficiency. Such filtration materials may be utilized in a wide variety of areas, such as, for example, HVAC filter applications, medical applications, and protective garments. Materials such as, for example, nonwoven webs and porous films may be subjected to electret treatment to enhance the filtration efficiencies of these materials.
In some electreting processes, an electric field is established between a corona producing apparatus such as a charge bar or ionizer (the anode) and the bias roll (the cathode). A web or film of dielectric material is passed through the electric field and a charge is established on the web or film.
The electret treatment conditions such as, for example, current, field strength, air temperature and relative humidity, and the resistance between the charge bar (or ionizer) and bias roll, are selected so that the electrical discharge is just below the arcing point or ignition voltage. The arcing point is defined as the point where current increases with no associated increase in applied voltage.
The resistance between the charge bar and the bias roll is dependent on a variety of factors, including upon the distance between the charge bar and the bias roll (air gap), the temperature and relative humidity of the air within the air gap, and the characteristics of the web disposed between the charge bar and bias roll. In particular, variations in the basis weight and formation consistency of the web may cause selected areas of the web or film to have a lower resistance than other portions of the web. Thus, as the web or film is moved between the charge bar and bias roll, the resistance presented by the web or film varies. When the resistance presented by the combination of the web or film and the air gap declines sufficiently during the electret treatment process, a conductive pathway may form and arcing may occur between the charge bar and the bias roll.
Additionally, the relative humidity and temperature at which the electret treatment is applied should be controlled in many applications, as increasing amounts of water vapor increases the average dielectric constant of the gas in the gap. As the dielectric constant of the gas changes, the slope of the current-voltage curve is altered because the resistance of the gap changes. Altering the resistance of the gap changes the conditions at which arcing will occur. Also, the presence of gas phase water in the gap decreases the free electron population, thereby reducing the ability to form an electret.
To enhance the filtration efficiency of a web, the electret treatment must be uniformly applied to the web and the electret treatment process should form no undesired apertures in the web.
When charge bars which are configured as series devices are used in the electret treatment process, the portion of the web or film passing through the charge bars during an arcing event is not electret treated, resulting in a non-uniformly electret treated web. In such a device, all the current applied to the charge bar flows through the arc, thereby preventing a charge from being formed on other portions of the web or film moving past the charge bar during the arcing event. An example of an ionizer configured as a series device is disclosed in U.S. Pat. No. 5,627,376 to Jaisinghani et al., the entirety of which is hereby incorporated by reference. As an inconsistent application of an electrical charge to the web or film is undesirable in many applications, the use of ionizers or charge bars which are configured as series devices presents a particular problem for webs which have a non-uniform density, such as high loft webs.
If arcing occurs, an undesirable aperture may be formed in the web or film, preventing the application of a uniform electric charge to portions of the web or film moving between the charge bar and bias roll. Various nonwoven webs, such as those described below, are particularly well suited for use as filtration materials.
SUMMARY
The present invention is directed to a process of forming an electret nonwoven web, the process including the step of providing a nonwoven web. The nonwoven web provided may, in selected embodiments of the step of providing a nonwoven web, include the step of providing a high loft nonwoven web. Such a high loft nonwoven web may have an Ames Bulk of greater than about 0.10 inches (3 mm). The high loft nonwoven web may have a thickness greater than about 0.15 inches (about 4 mm). The high loft nonwoven web may also have a bulk density which is less than about 0.1 grams per cubic centimeter.
The present invention may further include the step of generating a DC corona discharge from an apparatus which includes an anode electrically configured as a parallel device. In such a device, the anode may be configured as a charge bar. In some embodiments of the process of the present invention, the anode may include a plurality of sets of pins, each pin within a set being spaced apart from the next nearest pin by at least 0.125 inches (0.3175 cm), each set of pins being spaced apart from the next nearest pin by at least 0.125 inches (0.3175 cm). In other embodiments, the pins may be spaced apart by 0.25 inches (0.635 cm). Each set of pins may include at least four pins. The pins within each set of pins may be electrically connected in series to a resistor extending from a power bus, each set of pins and the associated resistor being electrically connected in parallel to the power bus.
The process of the present invention may further include the step of subjecting the web to DC corona discharge to form an electret.
In certain embodiments of the process, the ambient atmosphere may be controlled so that the process is conducted in an environment having a relative humidity between about 60% at 60 degrees F. and about 50% at 90 degrees F. Other embodiments of the process may include controlling the ambient atmosphere so that a relative humidity between about 40% at 60 degrees F. to about 30% at 90 degrees F. is desirable. In particular processes, a range of about 30% at 60 degrees F. to about 20% at 90 degrees F. may be desirable.


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