Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Production of continuous or running length
Reexamination Certificate
2000-03-22
2002-10-29
Theisen, Mary Lynn (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Production of continuous or running length
C156S167000, C425S072200, C264S172170
Reexamination Certificate
active
06471910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improved nonwoven fabrics and melt spinning apparatus and processes for producing such nonwoven fabrics. In particular, the present invention involves the extrusion of ribbon-shaped fibers and the formation of nonwoven fabrics made from synthetic ribbon-shaped fibers having superior coverage, filtration, strength and elongation properties.
2. Description of the Related Art
The most common synthetic textile fibers used in nonwoven fabrics are made from materials such as nylon, polyester or polypropylene polymers. All of these polymers are melt spinnable. Some nonwoven fabrics made from carded or air-laid
25
webs comprise rayon or acrylic fibers. Many of the nonwoven fabrics made from melt-spinnable polymers are produced using a spunbond process. The term “spunbond” refers to a process of forming a nonwoven fabric or web from thin fibers or filaments produced by extruding molten polymers from orifices of a spinneret. The filaments are drawn as they cool (e.g., by an aspirator, positioned below the spinneret, which longitudinally stretches and transversely attenuates the fibers) and are randomly laid on a forming surface, such that the filaments form a nonwoven web. The web is subsequently bonded using one of several known techniques to form the nonwoven fabric. Carded or air-laid webs can also be formed from these polymers.
Fibers having a round (i.e., circular) transverse cross-sectional shape, as shown in
FIG. 1
, are the most common and least expensive melt spun fibers used to form nonwoven fabrics. Such fibers have a number of limitations, however. For example, fibers having a circular transverse cross-sectional shape tend to be relatively stiff and do not bend as readily as fibers of other cross-sectional shapes; consequently, these round fibers tend to produce fabrics having a texture that is less soft.
For a given fabric basis weight, the shape and stiffness of round fibers produce fabrics having limited surface area coverage, i.e., a significant amount of open area is present between the fibers of the fabric relative to fibers having other cross-sectional shapes. This limited coverage results in a limited ability of the fabric to serve as a filter or barrier material, since gasses, fluids and particulate matter can pass through the gaps or holes between fibers with relative ease.
Further, round fibers inherently have a limited fiber surface area, which has a number of implications for the spunbond process for forming nonwoven fabric as well as for the properties of the fabric itself. Specifically, round fibers extruded in a molten state quench first at the fiber surface and more slowly in the center of the fiber. Significant molecular orientation cannot take place while the polymer is still molten; hence, only the fiber surface is well oriented. This makes the fiber less strong than if it were equally well oriented throughout its cross-section.
The use of round fibers also limits the efficiency of the aspirator. The aspirator or other fiber-drawing mechanism, is designed to longitudinally stretch and transversely attenuate the fibers as they travel substantially vertically downward from the spinneret. This drawing of the fibers is achieved by applying a downward air drag on the fibers, which air drag is produced by air pressure creating a generally-downward, high-velocity air flow. Because of the limited surface area of round fibers, a limited downward air drag on the fibers is induce by the downward air flow, thereby limiting the amount of fiber stretching and attenuation for a given aspirator air pressure and as well as limiting the energy efficiency of the aspirator.
With limited surface area, circular fibers quench relatively slowly and remain in a molten or soft state for a relatively long time after extrusion; consequently, the aspirator used to draw the fibers must be located a significant distance away from the spinneret from which the fibers are extruded to allow sufficient quenching time in order to prevent the fibers from sticking to components of the aspirator. The requirement for this distance between the spinneret and the aspirator causes significant unwanted air drag associated with length of the fiber between the spinneret and aspirator (to be distinguished from the desired downward air drag produced by the aspirator), and reduces the efficiency of the aspirator by requiring more aspirator drawing force (i.e., air pressure) to overcome this pre-aspirator air drag (with less of the drawing force contributing to stretching and attenuating the fibers).
The relatively low surface area of round fibers also limits the usefulness of fabrics made from such fibers in filtration and barrier material applications. Specifically, the round fibers present a limited surface area for collecting or blocking dirt, gasses or fluids. Further, it is more difficult to apply finish oil or other topical treatments to fabrics formed from round fibers.
Fibers having other transverse cross-sectional shapes, such as a delta shape (
FIG. 2
) or a Y shape tend to give a slightly greater fiber stiffness relative to fibers having a round transverse cross section, and may also add sparkle to the fiber appearance. Hollow fibers can conserve polymer and reflect light in a desirable manner.
FIG. 4
illustrates a conventional hollow fiber having a circular transverse cross-sectional shape and a single, concentric longitudinal cavity. A plural cavity hollow fiber is shown in
FIG. 5
in which four circular longitudinal cavities arranged transversely in a square pattern extend through a fiber having a substantially square transverse cross-sectional shape. Thus, fibers whose transverse cross sections are other than round can provide certain advantages. However, none of these fibers overcomes all of the aforementioned limitations of round fibers.
While some use has been made of fibers with flat or ribbon-shaped cross sections, there have been no known attempts to form nonwoven fabrics from fibers extruded with a ribbon-shaped cross section. Nor has there been any significant investigation into the possible advantages of using such extruded ribbon-shaped fibers in nonwoven fabrics.
U.S. Pat. No. 5,498,468 to Blaney, the disclosure of which is incorporated herein by reference in its entirety, discloses a process of extruding sheath-core conjugate filaments having a substantially circular transverse cross-sectional shape, and applying a flattening force to the filaments with a calendar roll arrangement to flatten the circular filaments into ribbon-like filaments. The method requires that the core polymer have a lower softening point than the sheath polymer such that, when the filaments are heated to the softening point of the core polymer, the flattening force causes the filaments to deform in accordance with deformation of the core without causing the sheath to soften, thereby preventing adjacent fibers from fusing in the flattening process. While fabrics formed from the flattened fibers purportedly exhibit superior coverage properties (i.e., reduced open area in the fabric), the fibers are extruded with circular cross-sectional shapes; consequently, any potential advantages of extruding ribbon-shaped fibers are not suggested by and cannot be realized in this system. Moreover, the process is limited to sheath-core conjugate fibers wherein the core component has a lower softening point than the sheath component; thus, the process does not have general applicability to fibers other than those with this specific conjugate fiber configuration.
U.S. Pat. No. 5,593,768 to Gessner, the disclosure of which is incorporated herein by reference in its entirety, discloses a process for forming a nonwoven fabric laminate from a thermally bonded multiconstituent fiber nonwoven web. Specifically, the fibers of the web are formed of two highly intermixed polymer components, one of which has a lower softening point which facilitates bonding of the fibers in the web. In order to enhance bonding, it is desirable to maximize t
Haggard Jeffrey S.
Harris W. Scott
Hills William H.
Lu Fumin
Wilkie Arnold E.
Edell, Shapiro, Finnan & Lytle LLC
Finnan Patrick J.
Hills, Inc.
Theisen Mary Lynn
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