Abstract:
A composite, bicomponent, sheath-core fiber ( 8 ) and methods of making (FIGS.  1  and  2 ) and using such fiber by it self or in a multifilament form are described in which the sheath comprises E-CTFE and the core is any spinnable polymer similara to Nylon, PET and copolymers thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage filing of International Application No. PCT/US97/16750, filed Sep. 12, 1997, which was co-pending with and claims the benefit of U.S. Provisional Application No. 60/025,256 filed Sep. 13, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to composite bicomponent fibers having a sheath-core structure. The advantages of the composite bicomponent fiber are achieved principally by the cooperation of the characteristics of the core component, such as high tensile strength and low cost, with the enhanced surface properties of the sheath component, particularly resistance to staining, water, chemicals, and high temperatures, along with low electrical conductivity. 
     2. Prior Art 
     Composite bicomponent sheath-core fibers and production processes therefor are known. Typically, nylon fibers, nylon 6, nylon 6,6, or copolymers thereof, are used as a core component (see for example U.S. Pat. No. 5,447,794-Lin). The sheath component is typically a variation of the same material as the core material, as shown by Lin, or a polymer such as a polyester or polyolefin (see Hoyt and Wilson European Patent Application No. 574,772). Composite, bicomponent, sheath-core fibers are generally made by delivery of the two component materials through a common spinnerette or die-plate adapted for forming such composite, bicomponent, sheath-core fibers. 
     Generally, composite bicomponent sheath-core fibers have been used in the manufacture of non-woven webs, wherein a subsequent heat and pressure treatment to the non-woven web causes point-to-point bonding of the sheath components within the web matrix to enhance strength or other such desirable properties in the finished web or fabric product. Other uses of composite bicomponent sheath-core fibers include the production of smaller denier filaments, using a technology generally referred to as “islands-in-the-sea”, to produce velour-like woven fabrics typically used for apparel. 
     Such technology is typically employed in the production of relatively large diameter, monofilament, composite, bicomponent sheath-core fibers for specialized end uses. Typically, many individual monofilaments are grouped into a multifilament yarn. However, the spinning of a small denier multifilament yarn bundle, e.g. less than 100 denier comprised of many (e.g. ten or more) individual sheath-core continuous filaments, is generally commercially unavailable because of the complexities associated with the process and materials used for the sheath and core components. 
     In order to successfully spin a small denier multifilament yarn bundle comprised of a plurality of individual, composite, bicomponent, sheath-core fibers, the limitations imposed by the known production processes and the materials used as the core and sheath components must be overcome. The demanding requirements of the final composite yarn would be met by simultaneously extruding two different materials in a common process, which requires a degree of Theological, thermal and viscoelastic similarity between the two materials. Additionally, the complexity of quality extrusion increases as the diameter of the individually extruded composite bicomponent sheath-core fibers decreases. Further, once the extruded filaments exit the spin-plate of the spinnerette or die-plate, the filaments must be drawn, typically employing an annealing process done at high speed and under tension, to align the crystal structure and develop strength in the overall composite. 
     A similarity in stress/strain behavior of the materials used for the core component and the sheath component is required to avoid premature overstretching and breaking (% elongation) during the drawing process. Additionally, sufficient elongation, and tensile strength (tenacity) must be achieved in the final composite yarn to withstand the physical rigors of weaving. Further, the generally thin sheath component should withstand high abrasion while maintaining its integrity and encapsulation of the core component. 
     The choice of materials used for the sheath-core components is limited by both the rigors of the manufacturing process and the requirements of the final composite yarn. The prior art includes at least the following combinations of materials for sheath-core fibers: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 sheath 
                 core 
               
               
                   
                   
               
             
             
               
                   
                 polyethylene terephtalate 
                 polyethylene (PE) 
               
               
                   
                 (polyester, PET) 
               
               
                   
                 PET 
                 polypropylene (PP) 
               
               
                   
                 PP 
                 PET 
               
               
                   
                 nylon 6 
                 nylon 6,6 
               
               
                   
                 PET, PP, nylon 6 
                 water soluble components 
               
               
                   
                   
               
             
          
         
       
     
     The rheological and viscoelastic properties of thermoplastic fluoropolymers such as polytrifluoroethylene (PTFE), are very dissimilar to the above listed materials. Consequently few such fluoropolymers have been made as one component fibers, particularly in a multifilament format. For example, PTFE has not been known to be melt processible and has only been described as extruded in a proprietary wet spinning process wherein the PTFE latex is mixed and coextruded with a cellulosic dope. 
     SUMMARY OF THE INVENTION 
     HALAR® (ethylenemonochlorotrifluoroethylene, E-CTFE), which is supplied by Ausimont USA, Inc., possesses certain enhanced surface properties which are desirable in a sheath component. However, ordinary E-CTFE also has several properties which are adverse to its use as a sheath component. E-CTFE exhibits high viscosity in the melted state and also requires stabilization against thermal degradation by inclusion of volatile additives which may off-gas and interfere with extrusion. Standard E-CTFE also rapidly crystallizes, cools and sets before the drawing process and other necessary fiber making parameters can be applied. Experimental composite bicomponent sheath-core fibers made with standard E-CTFE as a sheath component typically have exhibited low elongation capability, exhibit fracture even when not under tension, and exhibit discontinuities in the sheath component and strength too low to successfully weave into a fabric comprised of small denier yarn bundles. 
     While different ones of the prior composite bicomponent sheath-core fibers have certain desirable properties, there has been a continuing need and a desire in the art to develop a bicomponent sheath-core fiber having a material such as E-CTFE as the sheath component, while possessing the advantages of the cooperation of the desirable characteristics of a strong core component and the enhanced surface properties of a sheath component. 
     Accordingly, it is an object of the present invention to provide an E-CTFE coating (sheath) material which overcomes the physical and manufacturing disadvantages of prior E-CTFE components when used as the sheath component in a composite, bicomponent sheath-core fiber. 
     It is another object of the present invention to provide a composite bicomponent fiber having a sheath-core structure where the core component is any spinnable polymer with fiber properties similar to nylon 6, nylon 6,6, polyethylene terephtalate and copolymers thereof and a sheath component of the fluoroploymer ethylenemonochlorotrifluoroethylene having a range of volume crystallinity between about 10% and 49%, and extending at the lower end of the range to about 1%. 
     It is another object of the present invention to provide composite bicomponent fiber having a sheath-core structure where the sheath component is ethylenemonochlorotrifluoroethylene having a non 1:1 molar ratio of ethylene to monochlorotrifluoroethylene. 
     It is another object of the present invention to provide composite bicomponent fiber having a sheath-core structure where the sheath component is ethylenemonochlorotrifluoroethylene having a volume crystallinity between about 20% and 30%. 
     It is another object of the present invention to provide a composite, bicomponent, sheath-core fiber using E-CTFE as the sheath component which ensures better utilization of the properties of the sheath-core bicomponent fiber without deterioration in the properties of the sheath component. 
     It is another object of the present invention to provide new and better performing, small denier continuous yarns comprised of a plurality of sheath-core fibers having E-CTFE as the sheath component without a deterioration of the properties of the yarns. 
     It is another object of the present invention to provide a process for producing such an E-CTFE component and a composite, bicomponent sheath-core fiber and a process for producing such a yarn. 
     In accordance with one aspect of the present invention, a method of producing composite bicomponent fiber having a sheath-core structure includes the steps of formulating ethylenemonochlorotrifluoroethylene having a low volume crystallinity by the alteration of the molar ratio of ethylene and monochlorotrifluoroethylene or by the addition of another fluoropolymer monomer, and feeding a core component of any spinnable polymer with fiber properties similar to nylon 6, nylon 6,6, polyethylene terephtalate and copolymers thereof, and sheath components via a first spinnerette plate to a second spinnerette plate in a plurality of individual streams and, between the first and second spinnerette plates each individual stream of core material is enveloped by the sheath material being fed onto the core component, the two components being commonly spun, drawn and wound. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG.  1  and FIG. 2 are schematic representations of a process for melt spinning composite bicomponent fibers suitable to make the sheath-core filaments of this invention. 
     Referring to FIG. 1, composite bicomponent fibers having a sheath-core structure of this invention are produced by a process wherein a core component and sheath component are measured and extruded by means of their respective metering pump drive  9 ,  11 , metering pump  10 ,  12 , and extruder  1 ,  2  and are fed via a first spinnerette plate to a second spinnerette plate contained within a spinnerette pack  3 , wherein each individual stream of core component is enveloped by the sheath component being fed into it. The resulting sheath-core filaments pass through a quench cabinet  13  where a cooling gas is blown past the filaments. The two components pass over a finish roll  4 , are taken up on godet cans  5 , 6 , 7  and winder  8 . The rate of revolution of the godet cans determines the wind up speed. Typically, the godet cans run at approximately the same rate. The foregoing equipment is generally conventional for making sheath-core filaments. 
     Referring to FIG. 2, godet cans  15 ,  16 , and  17  are run at different speeds in a drawing process. Can  16  runs faster than can  15 , and can  17  runs faster than can  16 . The ratio of the speed of can  17  to can  15  is the draw ratio, typically around 3 to 5. Cans  15 ,  16 , and  17  typically are heated to make the component materials draw more easily and to a greater extent, with the temperature determined by the type of components used. Generally, cans  15  and  16  are heated to near the glass transition of the component materials. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Table 1 shows, in the first line thereof, the results of making and testing a composite bicomponent sheath-core fiber having an inner nylon core and an outer sheath of a 50:50 molar ratio of E-CTFE (Standard E-CTFE). The resulting fiber was tested and examined and was found to exhibit undesirable characteristics as listed and as explained above. It was subsequently discovered that, by adjusting the molar ratio of CTFE and ethylene to a 55:45 molar ratio E-CTFE (CTFE-rich E-CTFE) for the sheath component, a particularly advantageous and useful result was unexpectedly obtained. Thus, as indicated in the succeeding lines of data shown in Table 1, for two different core filaments (PET and Nylon 6) having a coating thickness of the CTFE-rich E-CTFE polymer between 1% to 99% by weight of the finished fiber with 10% to 50% by weight being preferred, a strong, compatible, continuous sheath fiber was obtained which is suitable for making continuous fine denier fiber. Lower crystallinity at the present time is attributed to be a factor in the desired results obtained. The CTFE-rich E-CTFE has less volume crystallinity, a lower melting point allowing for faster quenching and greater undrawn elongation than the bicomponent fiber utilizing Standard E-CTFE as the sheath component. A lower volume crystallinity E-CTFE is achieved by making E-CTFE rich in one monomer, CTFE. Another method to lower crystallinity is the inclusion of an additional monomer in E-CTFE. The additional monomer is selected from those copolymerizable olefinic fluorinated and non-fluorinated monomers which when incorporated into E-CTFE will reduce the crystallinity. 
     The lower volume crystallinity sheath-core fiber E-CTFE can be drawn more than such sheath-core fiber utilizing Standard E-CTFE without the sheath cracking. The greater draw allows the core material to develop superior strength (drawn tenacity) and extension after drawing (drawn elong. at break), desired properties for easy weaving and use in continuous yarns. While the modified E-CTFE with 55:45 molar ratio was successful, it is anticipated that other similar ratios in the vicinity of that ratio also may be expected to exhibit similar desirable and advantageous characteristics in such applications. E-CTFE with such desired and advantageous characteristics can also be obtained by incorporation of appropriate modifying monomer during polymerization. 
     While the various aspects of the present invention have been described in terms of preferred embodiments, it will readily be apparent to persons skilled in this art that various modifications may be made without departing from the scope of the invention which is set forth in the following claims. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 CTFE 
                   
                   
                   
                   
                   
                 (2) 
                 Undrawn 
               
               
                 Ethylene 
                   
                 (1) 
                   
                 Sheath 
                 Wind up 
                 Undrawn 
                 Elong 
               
               
                 Molar 
                 Melting 
                 Volume % 
                 Core 
                 Core 
                 Speed 
                 Total 
                 at Break 
               
               
                 Ratio 
                 Point, C. 
                 Crystalinity 
                 Material 
                 Ratio 
                 (m/min) 
                 Denier 
                 % 
               
               
                   
               
               
                 50:50 
                 240 
                 50 
                 Nylon 6 
                 50/50 
                  500 
                   
                 40-60 
               
               
                 55:45 
                 207 
                 20 to 30 
                 PET 
                 50/50 
                  150 
                 12,441  
                 250 
               
               
                 55:45 
                 207 
                 20 to 30 
                 PET 
                 50/50 
                 2000 
                 2,190 
                  35 
               
               
                 55:45 
                 207 
                 20 to 30 
                 PET 
                 50/50 
                 1000 
                 1,166 
                 300 
               
               
                 55:45 
                 207 
                 20 to 30 
                 PET 
                 50/50 
                 1000 
                 1,166 
                 150 
               
               
                 55:45 
                 207 
                 20 to 30 
                 Nylon 6 
                 40/60 
                 1000 
                 1,166 
                 250 
               
               
                 55:45 
                 207 
                 20 to 30 
                 Nylon 6 
                 45/60 
                 1000 
                 1,166 
                 200 
               
               
                   
               
             
          
           
               
                   
                   
                   
                 (6) 
                   
                   
               
               
                 Undrawn 
                   
                 Drawn 
                 Drawn Elong. 
                 Drawn 
               
               
                 Tenacity 
                 Draw 
                 Denier/ 
                 at Sheath Break 
                 Tenacity 
                 Sheath 
               
               
                 (gm/denier) 
                 Ratio 
                 Filament 
                 % 
                 (gm/denier) 
                 Continuity 
               
               
                   
               
               
                 0-7 
                 4.0 
                 7 
                  0 
                 1.99 
                 No (3, 4, 5) 
               
               
                   
                   
                   
                 (7) 
               
               
                   
                 4.0 
                 9.8 
                   
                 2.0 
                 Yes (4) 
               
               
                 0.7 
                 2.0 
                 2.0 
                 15 
                 2.4 
                 Yes (4) 
               
               
                 0.8 
                 3.0 
                 1.2 
                 20 
                 2.9 
                 Yes (4) 
               
               
                   
                 3.0 
                 1.2 
                 18 
                 3.1 
                 Yes (4, 5) 
               
               
                 0.9 
                 3.0 
                 1.1 
                 15 
                 3.7 
                 Yes (3, 4, 5) 
               
               
                 0.7 
                 2.5 
                 2.0 
                 50 
                 2.7 
                 Yes, (3, 4) 
               
               
                   
               
               
                 (1) Approximate, based on heats of melting determined by differential scanning calorimeter.  
               
               
                 (2) All tests were done with a 288 hole spinnerette.  
               
               
                 (3) A one meter length of composite yarn was placed into a heated solution of material known to be strong dyestuff for nylon and polyester. Only the “cross-section” ends of the filament bundles were not exposed to the solution. The dye solution was agitated for approx. 30 minutes and the yarn was then removed and thoroughly rinsed with water. The yarn was then examined against a white background for observance of color.  
               
               
                 (5) At the wind-up position wherein the package of accumulating yarn is being wound at very high speed, a minor break in the sheath covering would give the “package, or cone” the visual appearance of “grayness” during the winding and one could actually “feel” the wisp-like broken sheath component “whipping” against the skin if the hand was placed in very close contact with the accumulating, moving package.  
               
               
                 (5) Scanning Electron Microscope photographs at very high magnification (200× to 1000×) were taken of both the cross-section of the bi-component yarn as well as along the length of the bicomponent yarn. Thus, one could readily examine the integrity of the sheath covering for splits, cracks or voids.  
               
               
                 (6) The point of breakage was determined by when the first sheaths were observed to break.  
               
               
                 (7) Significant numbers of sheaths were already broken during the drawing process. No additional elongation was possible.