Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-12-09
2001-04-24
Edwards, Newton (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S085000, C428S372000
Reexamination Certificate
active
06221486
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of plastic fibers, more specifically to the fields of expandable, fusible, and hollow plastic fibers or microtubes, and composite materials produced therefrom.
BACKGROUND OF THE INVENTION
In the current state of the art, the production of hollow fibers is greatly hindered by the required extrusion process using an annular die, which makes the production of fibers having a small cross-sectional area very difficult. Such processes are also sensitive to the polymer composition being extruded, limiting the variety of compositions available for practice. Hollow fiber production is further limited because hollow fibers are especially prone to breakage, rupture, or other defects during any draw-down (spinning) process, which further limits the size, geometries, and other physical properties typically achieved during a conventional spinning process.
Another limitation of the current art is the control (or lack thereof) of the polymer molecule orientation in the final hollow fiber. In conventional hollow fiber production, the polymer molecules become oriented, at least partially, in the longitudinal direction by the extrusion and/or spinning process. While this may benefit the strength of the fiber in the longitudinal direction, it actually can degrade other properties such as its resistance to collapse, crushing, fraying, crimping or other failure modes of the fiber. Thus, a technique capable of providing or imparting some degree of radial orientation to the polymer molecules in the shell wall of the fiber is desirable. Such radial orientation of the polymer chains is not currently achievable in the current art, yet would provide the hollow fibers with heretofore unattainable properties, even if only partial radial orientation could be achieved.
SUMMARY OF THE INVENTION
The present invention is directed to thermo-expandable, thermoplastic or thermosettable fibers, their method of production, and the microtubes or hollow fibers that result upon heating said thermo-expandable fibers. The thermo-expandable fibers of the present invention are characterized by having a polymeric wall surrounding a core of liquid or solid blowing agent or propellant within the fiber. The fibers may be cylindrical in shape, having a circular cross-section. Or they may have other various geometries, such as oval, star-shaped, or triangular cross-sections. The fibers may even possess a cross-section that has a large aspect ratio so that they resemble a sheet. The fibers may additionally be designed to possess multiple, parallel cores instead of a single core. The fibers themselves can be singular and independent from one another, or they may be agglomerated together and pressed flat to form non-woven sheets or membranes consisting of many fibers. In one embodiment of the invention, the expandable fibers of the present invention are also fusible or crosslinkable with each other and/or with a surrounding matrix material in which the fibers are mixed.
The terms “thermo-expandable fiber” and “expandable fiber” as used herein and in the appended claims, mean a strand that is many times longer than it is wide and is capable of increasing in size upon heating due to the formation of one or more continuous or segmented voids or bubbles in the interior of the fiber. The terms “hollow fiber” and “microtube” as used herein and in the appended claims, mean a hollow fiber defined by having a polymeric shell wall surrounding one or more continuous or segmented internal, gaseous voids. The term “fusible”, as used herein and in the appended claims, means able to fuse together into an agglomerated or connected mass.
The present invention discloses a unique approach that overcomes the drawbacks of commercially established processes for the production of microtubes or hollow fibers. It is unique in that it uses only physical processes and solution thermodynamics to create novel expandable fibers. The expandable fibers in turn form the hollow fibers or microtubes upon heating and/or depressurization. Polymerization is an optional but not requisite step in the fiber formation process of this invention. The process is also unique in that any pre-polymerized material having a suitable solvent may be used to form the fiber shell walls, irrespective of the polymerization technique used to synthesize the polymer. By selecting functional polymers that have reactive sites within the polymer chain, and/or by incorporating crosslinking agents into the polymer walls, expandable fibers may be readily produced which are fusible or crosslinkable with adjacent fibers or with a matrix material in which the fibers have been incorporated. For these reasons and others that will become clear, the present invention is an extremely economical process suitable for mass production.
In one embodiment of the process of the invention, a polymer, co-polymer, or polymer blend is solvated by an appropriate solvent, and combined with an inert liquid, which serves as a blowing agent or propellant. Optionally, crosslinkers, catalysts, plasticizers, stabilizers, pigments, and other desirable additives may be added to the mixture. Fiber spinning proceeds by extruding or ejecting the mixture through an orifice into air or a second immiscible liquid to produce strands. Either by evaporation or liquid-liquid extraction, the solvent is then removed from the strands, precipitating the polymer from solution and effectively solidifying the strands into hard-walled expandable polymeric fibers containing liquid blowing agent cores and other optional additives.
In another embodiment of the process of the invention, a polymer, co-polymer, or polymer blend is solvated by an appropriate solvent, and is combined with a solid that produces a gas upon heating. Optionally, crosslinkers, catalysts, plasticizers, stabilizers, pigments, and other desirable additives may be added to the mixture. Fiber spinning proceeds by extruding or ejecting the mixture through an orifice into air or a second immiscible liquid to produce strands. Either by evaporation or liquid-liquid extraction, the solvent is then removed from the strands, precipitating the polymer from solution and effectively solidifying the strands into rigid-walled expandable polymeric fibers containing blowing agent cores that are solid at room temperature.
Utilizing either method, the final product is a fiber that is dispersible, residue-free, thermoplastic or thermosetting, and expandable. The polymer, co-polymer, or polymer blend may be chosen from any existing polymers, provided there exists a suitable solvent capable of dissolving said polymer. The fibers may also beneficially possess reactive functionalities, either built into the polymer chains or added to the formulation in the form of crosslinking or other reactive groups, that allow the fibers to fuse to each other or to a surrounding matrix upon expansion into hollow fibers or microtubes.
Expansion of the fibers of the invention into hollow microtubes occurs with the reduction in pressure, application of heat, or some other triggering energy such that the pressure differential between the inside and outside walls of the fiber(s) is great enough to expand the fiber walls to form a hollow fiber. For example, in one preferred embodiment of this invention, a liquid propellant trapped inside a fiber may be heated sufficiently to generate a vapor pressure capable of expanding the fiber walls outward, thereby producing a hollow fiber.
This invention makes possible a broad selection of fiber compositions. It may be used to obtain hollow fibers made from conventional thermoplastics, thermosets, elastomers, naturally occurring polymers, engineering thermoplastics, or mixtures of these or other polymers. The polymer and propellant may further be chosen to give a wide range of blowing temperatures at which the polymer softens and the fibers expand to produce hollow fibers.
These and other benefits of the invention will be made apparent in the detailed description of the invention that follows.
DETAILED DESCRIPTION OF
Houston Michael R.
Soane David S.
Edwards Newton
Larson Jacqueline S.
ZMS LLC
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