Patent Application
20080230464
The present invention generally relates to filtration media that exhibit conductive properties. More particularly, the invention relates to filtration media having two sided conductive patterns in registration.
Filtration media can take many forms, including knit textiles, woven textiles, nonwovens, papers, films and the like. The media can be used in a number of physical orientations within the filtration device. Recent trends in the filtration industry include the use of these media in folded and pleated orientations. As a general definition, the filtration media is the part of the filter device that actually separates the filtered material from the air or liquid matrix.
There are numerous industries and applications requiring filters. These include home, industrial, transportation, and many other application areas. These areas include two general classifications of filtration, those being air filtration and liquid filtration. In some of the applications requiring air filtration, the passage of the air through the filtration media and the collection of the filtrate material may generate a static charge. In severe circumstances this static charge may build up and discharge in an explosive event.
There is a need to have air filtration media capable of dissipating a static charge, while preserving the air permeability of the filtration media. By proper installation of such a media in a filter, the static charges can be dissipated to avoid static build-up which could cause an explosive event. The static dissipative media can be subsequently grounded at specific locations on the assembled filter, or used to dissipate the static charges to a larger surface area, diminishing the possibility of these charges building up to the point of causing an explosive event.
The present invention provides advantages and/or alternatives over the prior art by providing a conductive filtration media having a textile substrate (with a defined first side and a second side and a machine and cross-machine direction), where the conductive pattern on the first side is in registration with the conductive pattern on the second side of the textile substrate. The conductive pattern has a plurality of continuous conductive pathways across the textile substrate, the resistivity of the conductive pattern is less than 100 mega ohms when measured on a 2″ by 12″ sample taken in the machine and cross-machine direction of the textile substrate in accordance with test DIN 54 345, and the air permeability of the conductive filtration media is between 1 and 100 cc/cm2/sec (as measured by ASTM D737). A conductive air filter made from the conductive filtration media and the method of making the conductive filtration media is also disclosed.
The present invention will now be described by way of example only, with reference to the accompanying drawings which constitute a part of the specification herein and in which:
Referring now to
The textile substrate 100 may be of any stitch construction suitable to the end use, including by not limited to woven, knitted, non-woven, and tufted textiles, or the like. Woven textiles can include, but are not limited to, satin, twill, basket-weave, poplin, and crepe weave textiles. Jacquard woven structures may be useful for creating more complex electrical patterns. Knit textiles can include, but are not limited to, circular knit, reverse plaited circular knit, double knit, single jersey knit, two-end fleece knit, three-end fleece knit, terry knit or double loop knit, warp knit, and warp knit with or without a microdenier face. The textile substrate 100 may be flat or may exhibit a pile. Nonwoven fabrics or substrates can be formed from many processes such as, for example, meltblowing processes, spunbonding processes, air laying processes, needle punched, and bonded carded web processes. In one embodiment, spunbond nonwovens are tend to have a low cost of manufacture.
The textile substrate 100 is formed of fibers. As used herein fibers shall include continuous strand of textile fibers, spun or twisted textile fibers, textile filaments, or material in a form suitable for knitting, weaving, or otherwise intertwining to form a textile. The term fiber includes, but is not limited to, monofilament fibers, multifilament fibers, staple fibers, or a combination thereof.
The fiber of the textile substrate 100 may be any natural or man-made fiber (mixtures thereof including but not limited to man-made fibers such as polyethylene, polypropylene, polyesters (polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polylactic acid, and the like, including copolymers thereof), nylons (including nylon 6 and nylon 6,6), regenerated cellulosics (such as rayon or Tencel™), elastomeric materials such as Lycra™, high-performance fibers such as the polyaramids, polyimides, PEI, PBO, PBI, PEEK, liquid-crystalline, thermosetting polymers such as melamine-formaldehyde (Basofil™) or phenol-formaldehyde (Kynol™), basalt, glass, ceramic, cotton, coir, bast fibers, proteinaceous materials such as silk, wool, other animal hairs such as angora, alpaca, or vicuna, and blends thereof. In one embodiment, the fibers are polyester continuous filament fibers. It has been found that polyester continuous filament fibers are able to provide the required strength, stability and other performance requirements at a reasonable manufacturing cost.
Referring still to
The continuous conductive pathways may form at least one intersection, and they may form a plurality of intersections. As used herein, an intersection is defined as one point having at least 3 lines or pathways radiating from it. The lines of the conductive patterns 210 and 220 typically define an opening not greater than about 3 inches square, and preferably not greater than about 1 inch square. “About three inches square” typically represents a square area with approximately three inches on each side, and “about one inch square” typically represents a square area with approximately 1 inch on each side. For example, when a three inch square is placed on the conductive filtration media 10, it should make contact with the conductive pattern 210 or 220 in at least one location. However, it is foreseeable in some instances that the edges of the fabric may have areas free from the conductive patterns 210, 220 greater than about 3 inches square.
In one embodiment, one or both of the conductive patterns 210, 220 comprise a conductive pattern of a series of lines which intersect each other to form a grid pattern. The grid pattern may be diagonal relative to the machine 111 and cross-machine 112 directions of the textile substrate 100. This is shown, for example, in
During the application of these patterns it is further important to insure that the two conductive patterns 210, 220 are applied in registration such that both patterns line up directly across from one another on the two sides of the conductive filtration media 10.
The registered application of these two patterns is important for several reasons.
1) Having conductive patterns 210 and 220 in registration produce a more aesthetically appealing media.
2) When conductive patterns 210 and 220 are applied in registration, the conductive material as applied to the media in an aqueous or liquid state will penetrate into the media from both sides. This penetration of the conductive material into the filtration media will enhance conductivity of the media from one side to the other and actually provides an enhanced reduction in the measured resistance of an electrical charge across the media in either the machine 111 or 112 direction. The first conductive pattern 210 is electrically connected to the second conductive pattern 200 through the textile substrate 100.
3) By placing the conductive grid in registration, improved air permeability through the media is achieved by reducing the surface area of the media containing the conductive particles. This allows for the highest possible conductivity while still maintaining the highest level of air permeability through the filtration media.
It is desirous that both the first conductive pattern 210 and the second conductive pattern 220 have exactly the same (meaning identical) pattern in registration with one another.
In one embodiment, the conductive patterns 210 and 220 comprise a conductive material applied as a paste. The conductive paste generally includes a conducting agent and a binding agent. Preferably, the conducting agent is carbon. The conductive paste may also optionally include a dispersing agent and/or a thickening agent. Additional details on conductive patterns and materials may be found in US Patent Publication 2004/0053552 (Child et al.), incorporated herein in its entirety.
One potentially preferred, non-limiting conducting agent is graphite, such as, for example, Timrex® SFG available from Timcal Ltd. of Switzerland. Other conducting agents include, for example, Zelec® (available from Milliken Chemical of Spartanburg, S.C.); carbon particles; intrinsically conductive polymers; metal; metal oxides; metal shavings; fibers or beads coated with graphite, carbon particles, intrinsically conductive polymers, metal, metal oxides, or metal shavings; and the like; and combinations thereof. The conducting agent may be comprised of particles of various shapes, such as spheres, rods, flakes, and the like, and combinations thereof. The conducting agent may be comprised of conducting particles having a size between about 0.1 and about 100 microns, or more preferably having a size between about 1 and about 5 microns. Conducting agents may be characterized by having an aspect ratio number which is the ratio of a conducting particle's length divided by its width. For example, a perfect sphere has an aspect ratio of one. The longer the particle (i.e., the more rod-like the particle), the higher the aspect ratio. Generally, for a conducting agent having a high aspect ratio, less conducting agent is needed to provide the same electrical conductivity in an object, such as the present invention, when compared to a conducting agent made of a similar conducting agent but having a lower aspect ratio.
The conductive paste may include a binding agent which typically provides a non-conducting matrix which holds the conducting particles together and helps them bond to the textile substrate 100. Binding agents include water-borne latexes, solvent-borne polymer systems, liquid rubbers, thermoplastic hot melts, thermoset hot melts, multi-component reactive polymers, and the like, and combinations thereof. More specifically, binding agents may be acrylic latex, polyurethane, silicone, polyvinyl chloride latex, and the like, or combinations thereof. Binders generally vary, for example, in elongation and flex modulus properties which may affect the hand, drape, and stretch properties of the coated fabric. Thus, the selection of a particular binder for the conductive paste may depend on the end-use application of the conductive filtration media 10. It may be preferable that the binding agent has an elongation at break equal to or greater than about 80 percent of the elongation at break of the fabric. It may be preferable that the binding agent has a glass transition temperature equal to or less than about 0° C. and a melting temperature equal to or greater than about 100° C. Acrylic binders are preferred for their commercial availability and flexibility.
Preferably, the conductive paste has a viscosity of between 10,000 and 40,000 centipoise as measured by an LVF viscometer with a #4 spindle at 6 rpm. This viscosity range has been found to produce a conductive paste that is easily printed onto the textile substrate without excessive bleeding. By selecting the appropriate paste viscosity, penetration through the filter media can be controlled, as well as pattern definition (or amount of bleed of the printed pattern). In one embodiment, the conductive patterns 210 and 220 cover about 15 to 25 percent of the surface area of the first side 101 and second side 102 of the textile substrate 100. It has been found that this range of coverage produces conductive patterns with an appropriate amount of conductivity and a conductive filtration media 10 with adequate air permeability.
In one embodiment, the conductive pastes applied as the conductive pattern 210 and 220 absorb through the textile substrate 100 and are electrically connected through the textile substrate 100. This increases the conductivity of conductive filtration media 10. This penetration of the conductive material into the filtration media will enhance conductivity of the media from one side to the other and actually provides an enhanced reduction in the measured resistance of an electrical charge across the media in either the machine 111 or 112 direction.
The conductive filtration media 10 has a surface resistance in a range less than 100 mega ohms (108 ohms) when measured on a 2″ by 12″ sample taken in the machine and cross-machine direction of the conductive filtration media 10 in accordance with test DIN 54 345. It has been found that this range provides good discharge of static build up on air filters. The air permeability of the conductive filtration media 10 is between 1 and 100 cc/cm2/sec as measured by ASTM D737. This air permeability is effective for use in filtration applications.
The process for forming the conductive filtration media 10 includes forming textile substrate 100 having a first side 101 and a second side 102 and printing a first conductive pattern 210 on the first side 101 and a second conductive pattern 220 on the second side 102 of the textile substrate. The first conductive pattern 210 and the second conductive pattern 220 are in registration with one another and the conductive patterns 210, 220 include a plurality of continuous conductive pathways across the textile substrate.
The printing of the conductive patterns 210 and 220 onto the textile substrate 100 may be of any known method such as transfer printing, lithographic printing, ink jet printing, digital printing, and the like. One potentially preferred non-limiting method for applying the conductive coating to the fabric is to apply the coating as a paste onto the fabric through screen printing. Screen printing techniques have been available for many years as a way of selectively producing a pattern on a fabric by forcing a paste through holes in a screen. For example, U.S. Pat. No. 4,365,551 to Horton; U.S. Pat. No. 4,854,230 to Niki et al.; U.S. Pat. No. 5,168,805 to Kasanami et al.; U.S. Pat. No. 5,493,969 to Takahashi et al.; and U.S. Pat. No. 6,237,490 to Takahashi et al. each describe various screen printing methods and apparatus, and are herein incorporated by reference. For purposes of the present invention, a conductive paste may be forced through a specially prepared screen onto a substrate such as a fabric. The screen typically has areas in which the mesh has been blocked. These areas, which remain impervious to the conductive paste, correspond to patterned areas on the fabric in which no conductive coating is desired.
In one embodiment, the textile substrate is fed into a nip formed by two screen printing rollers and the first conductive pattern 210 and second conductive pattern 220 are printed in registration simultaneously. Certain conductive patterns, such as the skewed grid-like pattern shown in
The conductive filtration media 10 next typically moves in a continuous fashion to a drying oven where the conductive coating is dried. Drying can be accomplished by any technique typically used in manufacturing operations, such as dry heat from a tenter frame, microwave energy, infrared heating, steam, superheated steam, autoclaving, or the like, or any combination thereof. Typically, the fabric may be dried and/or cured for between about 30 seconds and about 5 minutes at a temperature of between about 250 and about 375 degrees F. Drying typically removes the water or solvent from the binder formulation in the conductive coating. The amount of conductive coating required depends generally on the pattern chosen for the fabric, and this is typically determined by the fabric's end-use. The drying temperatures may vary depending on the exact chemistry and/or viscosity of the conductive coating employed in the application process. In one embodiment, the textile substrate is heated to a temperature of between 270 and 210° F. prior to printing on the textile substrate 100. This serves to prepare the substrate for paste application and allows the printing material to flow more easily onto the media without clogging the print screens.
The conductive filtration media 10 of the invention may be used in many applications where static charge needs to be dissipated and air needs to be filtered. One application of the conductive filtration media 10 is in a conductive air filter cartridge 500 as shown in
the textile substrate with an identical conductive pattern, but the patterns were not in registration as can be seen, for example, in
The spunbond nonwoven textile was then screen printed on both sides of the textile substrate using a diamond pattern. The conductive pattern was formed using a conductive paste of graphite carbon and an acrylic binder to have a viscosity of 50,000 centipoise as measured by an LVF viscometer with a #4 spindle at 6 rpm. The conductive patterns were not in registration, but off-set from one another by approximately 10 millimeters. The conductive pattern had line widths of approximately 1 millimeter and the conductive paste covered approximately 20% of the surface area of the first and second surfaces of the filtration media.
Example 2 was a conductive filtration media printed on both sides of the textile substrate with the conductive patterns in registration. Example 2 was formed using the same materials and process as Example 1, except that the patterns on the two sides were identical and in registration as shown, for example, in
Examples 1 and 2 were tested for a variety of physical parameters as shown in the table below.
As can be seen from the results in the table above, when the two conductive patterns on the first and second side of the textile substrate are in registration, the air permeability increases and the electrical resistance remained essentially constant. This decrease in resistivity can be used to create a more conductive filtration media or may be used to be able to reduce the coverage (or line width) of the conductive pattern and still maintain adequate electrical resistance.
While the present invention has been illustrated and described in relation to certain potentially preferred embodiments and practices, it is to be understood that the illustrated and described embodiments and practices are illustrative only and that the present invention is in no event to be limited thereto. Rather, it is fully contemplated that modifications and variations to the present invention will no doubt occur to those of skill in the art upon reading the above description and/or through practice of the invention. It is therefore intended that the present invention shall extend to all such modifications and variations as may incorporate the broad aspects of the present invention within the full spirit and scope of the invention.
