FIELD OF THE INVENTION

This invention relates to filtration media, and particularly to a fibrous filtration media formed from a nonwoven web of cellulose pulp fibers and polyolefin fibers.

BACKGROUND OF THE INVENTION

Various particulate filtering media may be formed from diverse materials, such as glass fibers, asbestos fibers, synthetic polymer fibers, e.g., polyolefins, polyamides, polyesters and the like, and natural fibers, such as wood pulp and the like. Desirably, a particulate filter medium should possess a high particulate filtration efficiency, but should also possess a high filtered fluid (e.g., gas or liquid) permeability. However, these performance attributes tend to be inversely related. For example, in some instances, increasing the particulate filtration efficiency of a filter media may tend to increase the pressure "differential at the filter media between the filtered fluid and the unfiltered fluid.

As is known in the filtration art, filtration efficiency is improved by improving the ability of the filter media to mechanically entrap contaminates. In some instances, the filter media's ability to mechanically entrap contaminates, such as air born particulates, is improved by increasing the loft or thickness of the filter media without increasing the density of the filter media.

However, increasing the filter media's thickness has several disadvantages. In some instances, existing filter receiving structures may not be large enough to receive such thickened filters. In other instances, and particularly in

those instances when the filter media is formed from a coform of wood pulp and polymer fibers, such increased thickness is generally achieved by incorporating increased quantities of the coformed materials. Increasing the quantities of these materials not only results in increased material costs and shipping costs but also reduces the filter material's fluid throughput by increasing the pressure differential across the filter media.

Therefore, there exists a need for a filter media, and particularly for filter media formed form a coform of wood pulp and polymer fibers, and methods of making the same which provides improved filtration efficiencies over conventional filter media formed from similar materials and which avoids the problems associated with conventional methods of improving the filtration efficiencies of filter media.

SUMMARY OF THE INVENTION

In response to the above problems encountered by those skilled in the art, the present invention provides a web, formed from dielectric fibers and non-dielectric fibers, which has been subjected to corona discharge. The dielectric and non- dielectric fibers may be intermingled within the web such that the distribution of the dielectric and non-dielectric fibers within the web is generally uniform. In one embodiment, the dielectric fibers may be formed from a thermoplastic polymer, and particularly polyolefin and the non-dielectric fibers may be formed from cellulose, and particularly wood pulp.

The percent weight of dielectric fibers present in the web may range from about 1% to 99% weight of the web and particularly from about 5% to 95% weight of the web, and particularly from about 10% to 90% weight of the web, and more particularly from about 20% to 80% weight of the web, and still more particularly from about 30% to 70% weight of the web, and still more particularly from about 40% to 60% weight of the web. Generally, the percent weight of non-dielectric fibers present in the web may range from about 1% to 99% weight of the

web, and particularly from about 5% to 95% weight of the web, and particularly from about 10% to 90% weight of the web, and more particularly from about 20% to 80% weight of the web, and still more particularly from about 30% to 70% weight of the web, and still more particularly from about 40% to 60% weight of the web.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term "dielectric" means, according to

McGraw-Hill Encyclopedia of Science & Technology. 7th Edition, Copyright 1992, a material, such as a polymer, which is an electrical insulator or in which an electric field can be sustained with a minimum dissipation of power. A solid material is a dielectric if its valence band is full and is separated from the conduction band by at least 3 eV.

As used herein, the term "nonwoven web" refers to a web that has a structure of individual fibers or filaments which are interlaid, but not in an identifiable repeating manner. As used herein the term "spunbond fibers" refers to fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine, usually circular capillaries of a spinnerette with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Patent no. 4,340,563 to Appel et al., and U.S. Patent no. 3,692,618 to Dorschner et al., U.S. Patent no. 3,802,817 to Matsuki et al., U.S. Patent nos. 3,338,992 and 3,341,394 to Kinney, U.S. Patent nos. 3,502,763 and 3,909,009 to Levy, and U.S. Patent no. 3,542,615 to Dobo et al which are all herein incorporated by reference.

As used herein the term "meltblown fibers" means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into a high velocity, usually heated gas (e.g. air) stream which attenuates the filaments of molten thermoplastic material to reduce their diameter. Thereafter, the meltblown fibers are carried by the high velocity gas

stream and are deposited on a collecting surface to form a web of randomly- disbursed meltblown fibers. Meltblowing is described, for example, in U.S. Patent no. 3,849,241 to Buntin, U.S. Patent no. 4,307,143 to Meitner et al., and U.S. Patent 4,707,398 to Wisneski et al which are all herein incorporated by reference.

The present invention provides a web, formed from dielectric fibers and non-dielectric fibers, which has been subjected to corona discharge. The dielectric and non- dielectric fibers may be intermingled within the web such that the distribution of the dielectric and non-dielectric fibers within the web is generally uniform. The web formed from intermingled dielectric and non-dielectric fibers may be in the form of a woven web or nonwoven web. Nonwoven webs formed from intermingled dielectric and non-dielectric fibers can be made from a variety of processes including, but not limited to, coforming processes and hydroentangling processes.

It has been found that webs formed from thermoplastic based dielectric fibers and particularly polyolefin based dielectric fibers are well-suited for use in the present invention. Generally, dielectric materials suitable for forming dielectric fibers of the present invention include, but are not limited to, polyesters, polyolefins, nylon and copolymers, polymer blends and bi-component polymers of these materials. Generally, these fibers may be melt extruded fibers such as, for example, meltblown fibers or spunbond fibers. It has been found that nonwoven webs formed from polyolefin-based fibers and particularly polypropylene-based fibers are particularly well-suited for use in the present invention. It has also been found that cellulose based non-dielectric fibers and particularly wood pulp based fibers are particularly well-suited for use in the present invention. Generally, non- dielectric materials suitable for forming non-dielectric fibers include, but are not limited to, cellulose, such as wood pulp, glass, wool and protein polymers.

Generally, the percent weight of dielectric fibers present in the web may range from about 1% to 99% weight of the web and

all percent weight ranges of the web therebetween. Desirably, the percent weight of dielectric fibers present in the web may range about 5% to 95% weight of the web, and particularly from about 10% to 90% weight of the web, and more particularly from about 20% to 80% weight of the web, and still more particularly from about 30% to 70% weight of the web, and still more particularly from about 40% to 60% weight of the web.

Generally, the percent weight of non-dielectric fibers present in the web may range from about 1% to 99% and all ranges of the web therebetween. Desirably, the percent weight of non-dielectric fibers present in the web may range from about 5% to 95% weight of the web, and particularly from about 10% to 90% weight of the web, and more particularly from about 20% to 80% weight of the web, and still more particularly from about 30% to 70% weight of the web, and still more particularly from about 40% to 60% weight of the web.

Methods of subjecting the web to corona discharge, are well known by those skilled in the art. Briefly, corona discharge is achieved by the application of sufficient direct current (DC) voltage to an electric field initiating structure (EFIS) in the proximity of an electric field receiving structure (EFRS) . The voltage should be sufficiently high such that ions are generated at the EFIS and flow from the EFIS to the EFRS. Both the EFIS and the EFRS are desirably formed from conductive materials. Suitable conductive materials include conductive metals such as, copper, tungsten, stainless steel and aluminum.

One particular technique of subjecting the web to corona discharge is the technique disclosed in U.S. Patent No. 5,401,446, which is assigned to the University of Tennessee, and is herein incorporated by reference. This technique involves subjecting the web to a pair of electrical fields wherein the electrical fields have opposite polarities. Each electrical field forms a corona discharge.

EXAMPLES

The following examples demonstrate several embodiments of the present invention. Such examples, however, are not to be construed as limiting in any way either the spirit or scope of the present invention. Two sets of web samples, SAMPLE 1 and SAMPLE 2, were evaluated. Each set of web samples was formed from dielectric fibers and non-dielectric fibers.

SAMPLE 1

SAMPLE 1 included 7 nonwoven webs each having a basis weight of 150 grams per square meter (gsm) . The percent weight range of dielectric and non-dielectric fibers in each of the SAMPLE 1 nonwoven webs was varied to achieve the respective distributions of the dielectric and non-dielectric fibers reported in TABLE II. The SAMPLE 1 nonwoven webs were formed by the process described in U.S. Patent No. 4,818,464, which is herein incorporated by reference. These nonwoven webs were generally prepared using the conditions listed in TABLE I. The dielectric fibers were formed from Himont PF015 polypropylene. The non-dielectric fibers were formed from Weyerhauser NF405 cellulose pulp.

Table I Conditions Used To Prepare SAMPLE 1 Webs

Code 1

Extr #1 Extr #2

PP Pump Rate (RPM) 12 12

Zone 1 Temp 300°F 300°F

Zone 2 Temp 370°F 370°F

Zone 3 Temp 420°F 420°F

Zone 4 Temp 480°F 480°F

Zone 5 Temp 500°F 500°F

Zone 6 Temp 500°F 500°F

Extruder Melt Temp 517°F 510°F

Hose Temp 500°F 500°F

Adapter Temp 500°F 500°F

Spin Pump Body Temp 500°F 500°F

Die Zone 1 500°F 500°F

Die Zone 2 500°F 500°F

Die Zone 3 500°F 500°F

Die Zone 4 500°F 500°F

Die Tip Melt Temp 505°F 508°F

Primary Air Temp

Extruder Pressure 300 150

Spin Pump Pressure 147 139

Adapter Pressure 300 300

Melt Pressure 110 320

Primary Air Pressure 7 7

Prim Air Htr 20" line 570

Primary Air Heater

Primary Air Flow 2 470

CET Feed rpm 7

Line Speed fpm 213

Die Angles 48° 49°

Tip to Tip Distance 6 3/4" 6 3/4"

Tip to Wire Distance 12 3/4" 11 1/2"

Forming Height

CET Duct to Wire Dist 18 1/2"

Under Wire Zone 1 0

Under Wire Zone 2 -4

Under Wire Zone 3 -16

Under Wire Zone 4 -15

Under Wire Zone 5 -3

Under Wire Zone 6 -6

Note : All Pressures are in pounds per square inch (psi) .

Several of the SAMPLE 1 nonwoven webs were subjected to corona discharge. The corona discharge was produced by using a Model No. P/N 25A - 120volt, 50/60 Hz reversible polarity power unit (Simco Corp., Hatfield, PA.), which was connected to the EFIS, and a Model No. P16V 120V, .25A 50/60 Hz power unit (Simco Corp., Hatfield, PA.) which was connected to the EFRS. The EFIS was a RC-3 Charge Master charge bar (Simco. Corp.) and the EFRS was a solid, three inch diameter, aluminum roller. The corona discharge environment was 70° F and 53% relative humidity. As described in the above U.S. Patent 5,401,446, two sets of EFIS/EFRS are used. The voltage applied to the first set of EFIS/EFRS was 15 KV/0.0 KV, respectively. The voltage applied to the second set of EFIS/EFRS was 25 KV/7.5 KV, respectively. The gap between the EFIS and the EFRS for each set was one inch.

The particulate filtration properties of the SAMPLE 1 nonwoven webs exposed to corona discharge (Post Corona - TABLE II) were compared to the SAMPLE 1 nonwoven webs which were not exposed to corona discharge (Pre Corona - TABLE II) . The particulate filtration test used to evaluate the particulate filtration properties of these SAMPLES is generally known as the NaCl Filter Efficiency Test (hereinafter the "NaCl Test") . The NaCl Test was conducted on an automated filter tester, Certitest™ Model # 8110, which is available from TSI Inc., St. Paul, MN. The particulate filtration efficiency of the test fabric is reported as "% penetration". The "% penetration" is calculated by the following formula: 100 x (downstream particles/upstream particles) . The upstream particles represent the total quantity of approximately 0.1 μm NaCl aerosol particles which are introduced into the tester. The downstream particles are those particles which have been introduced into the tester and which have passed through the bulk of the test fabric. Therefore, the "% penetration" value reported in TABLE II is a percentage of the total quantity of particles introduced into a controlled air flow within the tester which pass through the bulk of the test fabric. The size of the test fabric was 4.5" in diameter.

TABLE II SAMPLE 1

% Penetration-0.1 μm NaCl

% wt Pulp %wt Pol Pre Corona Post Corona

90 10 49.3 37.1

80 20 55.4 38.4

70 30 56.3 23.7

60 40 53.4 13.6

50 50 42.7 9.72

30 70 43.8 3.43

20 80 35.7 2.53

0 100 43.5 0.4

SAMPLE 2

SAMPLE 2 included 2 nonwoven webs having different basis weights (See TABLE III) . Each of the nonwoven webs were formed from a combination of dielectric and non-dielectric fibers. The percent weight range of dielectric and non-dielectric fibers for each nonwoven web was also varied. The dielectric fibers were formed from Shell DX5E65 polypropylene. The non- dielectric fibers were formed primarily from Northern Softwood Kraft pulp.

The SAMPLE 2 nonwoven webs were formed by hydroentangling as described in U.S. Patent No.s 5,284,703 and 5,389,202, which are herein incorporated by reference.

A portion of the SAMPLE 2 nonwoven webs were subjected to corona discharge. The corona discharge was produced by using a Model No. P/N 25A - 120volt, 50/60 Hz reversible polarity power unit (Simco Corp., Hatfield, PA.), which was connected to the EFIS, and a Model No. P16V 120V,.25A 50/60 Hz power unit (Simco Corp., Hatfield, PA.) which was connected to the EFRS. The

EFIS was a RC-3 Charge Master charge bar (Simco. Corp.) and the EFRS was a solid, three inch diameter, aluminum roller. The corona discharge environment was 70° F and 53% relative humidity. As described in the above U.S. Patent Application No. 07/958,958, two sets of EFIS/EFRS are used. The voltage applied to the first set of EFIS/EFRS was 15 KV/0.0 KV, respectively. The voltage applied to the second set of EFIS/EFRS was 25 KV/7.5 KV, respectively. The gap between the EFIS and the EFRS for each set was one inch. The particulate filtration properties of the SAMPLE 2 nonwoven webs which were exposed to corona discharge (Post Corona - TABLE III) were compared to the SAMPLE 2 nonwoven webs which were not exposed to corona discharge (Pre Corona - TABLE III) . The particulate filtration test used to evaluate the particulate filtration properties of these SAMPLES is generally known as the NaCl Filter Efficiency Test (hereinafter the "NaCl Test") . The NaCl Test was conducted on an automated filter tester, Certitest™ Model # 8110, which is available from TSI Inc., St. Paul, MN. The particulate filtration efficiency of the test fabric is reported as "% penetration". The "% penetration" is calculated by the following formula: 100 x (downstream particles/upstream particles) . The upstream particles represent the total quantity of approximately 0.1 μm NaCl aerosol particles which are introduced into the tester. The downstream particles are those particles which have been introduced into the tester and which have passed through the bulk of the test fabric. Therefore, the "% penetration" value reported in TABLE III is a percentage of the total quantity of particles introduced into a controlled air flow within the tester which pass through the bulk of the test fabric. The size of the test fabric was 4.5" in diameter.

TABLE III SAMPLE 2

Basis % Penetration-O.1 μm NaCl Wt. (gsm) % Pulp % polypropylene Pre Corona Post Corona

86 gsm 80 20 76.5 60.4

60 gsm 77 23 86.5 69.2

It is clear from the data contained in TABLES II and III, that the particulate filtration efficiency of a web formed from dielectric and non-dielectric materials is significantly improved by subjecting such webs to corona discharge.

While the invention has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of and equivalents to these embodiments. Accordingly, the scope of the present invention should be assessed as that of the appended claims and any equivalents thereto.