Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-12-08
2002-09-03
Edwards, Newton (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S373000, C428S374000
Reexamination Certificate
active
06444312
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to fine denier polyacrylonitrile fibers, and in particular, fine denier polyacrylonitrile fibers obtained by splitting multicomponent fibers and to fabrics made from such fine fibers.
BACKGROUND OF THE INVENTION
Filtration processes are used to separate compounds of one phase from a fluid stream of another phase by passing the fluid stream through filtration media, or septum, which traps the entrained or suspended matter. The fluid stream may be either a liquid stream containing a solid particulate or a gas stream containing a liquid or solid aerosol.
In recent years, particular emphasis has been placed on air filtration, in specific the filtration of respirable dust from air streams. It is now widely recognized that inhaled particulates, particularly particles in the sub-10 micron range, have adverse health effects. In 1970, the U.S. Environmental Protection Agency (EPA) set forth a National Ambient Quality standard for particulate matter directed at the reduction of respirable particles contained in emissions. Filters are widely used to control the particulate matter released in emissions because filters are reliable, efficient, and economical. For example, high efficiency particulate air (HEPA) filters and ultra efficiency particulate air (ULPA) filters have been developed which specifically target the removal of fine respirable particulates.
Fabrics are widely used as filtration media. Conventional filters remove particulates by physically obstructing the flow of particles of a given size or larger; i.e., by mechanical action. A fundamental dilemma in the use of fabrics in small particulate filtration is that conventional textile fibers, having fiber diameters of 20 microns or more, are relatively coarse in comparison to the particulates to be removed. These relatively thick fibers produce filter media having large interfiber pores. Such open, porous structures do not provide suitable interstitial configurations for efficiently trapping fine contaminant particles.
Extremely fine fibers are known to be beneficial in the filtration of extremely small particulates. These fine denier fibers may be used to produce fabrics having smaller pore sizes, thus allowing smaller particulates to be filtered from a fluid stream. In addition, fine denier fibers can provide a greater surface area per unit weight of fiber, also considered beneficial in filtration applications.
Meltblown technology is one avenue by which to produce such fine denier filaments. Fine denier meltblown webs have been widely employed as filter media because the densely packed fibers of these webs are conducive for providing high filter efficiency. However, meltblown webs typically do not have good physical strength, primarily because less orientation is imparted to the polymer during processing and lower molecular weight resins are employed. Thus, in general, meltblown filter media are laminated to at least one separate, self-supporting layer, which adds cost and complexity to the manufacturing process. Although the physical integrity of the meltblown web can be improved by increasing the thickness of the web, this in turn increases the pressure drop required to force air through the filter media. In addition to producing fine denier filaments, meltblown technology also typically yields shorter fibers than other fiber formation techniques. This is problematic because these short fibers cannot easily be entangled using conventional nonwoven web formation processes, such as hydroentangling and needlepunching.
Conventional melt extrusion processes can provide higher strength fibers than meltblown fibers. However, it is difficult to produce fine denier fibers, in particular fibers of 2 denier or less, using conventional melt extrusion processes. Therefore, while filter media produced from nonwoven webs of conventional textile fibers, such as spunbond and staple fiber webs, have been used in filtration applications such as stove hood filters, there is room for improvement in their use as filter media for fine particulates.
One avenue by which to produce fine denier fibers using conventional melt extrusion is to split multicomponent continuous filament or staple fiber into fine denier filaments, or microfilaments, in which each fine denier filament has only one polymer component. Multicomponent fibers, also referred to as composite fibers, may be split into fine fibers comprised of the respective components, if the composite fiber is formed from polymers which are incompatible in some respect. The single composite filament thus becomes a bundle of individual component microfilaments following splitting. See, for example, U.S. Pat. Nos. 5,783,503 and 5,759,926, reporting splittable multicomponent fibers containing polypropylene, such as splittable polyester/polypropylene and nylon/polypropylene fibers.
A number of processes are known for separating multicomponent fibers into fine denier filaments. The particular process employed depends upon the specific combination of components comprising the fiber, as well as their configuration. One common process by which to divide a multicomponent fiber involves mechanically working the fiber, by means such as drawing on godet rolls, needle punching or hydroentangling. The production of mechanically splittable multicomponent fibers presents challenges not encountered in the production of other types of composite fibers. In particular, when mechanical action is used to separate multicomponent fibers, the fiber components must be selected carefully to provide an adequate balance between adhesive and dissociative properties. In particular, poor bonding is known to facilitate the separation process. Conversely, the components should remain bonded during at least a portion of the downstream processing incurred in fabric formation. To add to this difficulty, many conventional textile processes, such as carding, impart significant stress to the fiber, thus promoting premature splitting. Premature splitting is highly undesirable because conventional textile equipment is frequently not designed to process extremely fine filaments, and quickly becomes fouled by them. In addition to their adhesive properties, the melt rheologies of the polymers comprising the multicomponent fiber also strongly influence the splitting process. For example, the melt rheologies of the two components must be such that one component does not totally encapsulate the other during melt spinning, thus precluding later splitting.
As an alternative to the use of fine denier fibers, the efficiency of filters may also be increased by utilizing electrets, generally defined as electrically non-conductive materials capable of storing an applied charge for a relatively long period of time. In particular, electret filters are known to have a higher filtration efficiency than a comparable neutral filter, with no greater resistance to air flow. This increase in efficiency is due to the fact that substantially all industrial processes produce both positively and negatively charged particulate matter. For example, energy intensive operations, such as grinding, are known to produce particles with extremely high levels of charge. It is generally accepted that these charged particles are electrostatically attracted to oppositely charged surfaces within an electret filter. Further, in contrast to traditional mechanical filtration, which occurs primarily at the surface of the filtration media, electret filters contain charged fiber surfaces throughout the filter thickness, thus providing a greater total surface area for filtration.
Many conventional polymers, such as those used in textile fibers, develop and retain charges on their surface for an extended period of time, thus forming electrets. A wide variety of polymers may be used to produce electret fibers, including polypropylene, polyethylene, nylon, acrylic, modacrylic and polytetraflouroethylene. In particular, nonwoven fabrics formed from polypropylene fiber are known for use in electret filters. Such filters are disclosed
Edwards Newton
Fiber Innovation Technology, Inc.
LandOfFree
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