Abstract:
The present invention relates to a fiber which is produced by means of a melt spinning method, including a first component including a melt processable, fully fluorinated first polymer, and a second component including a thermoplastic second polymer.

Description:
[0001]    This application is a continuation of International application no. PCT/EP2013/057433 filed on Apr. 10, 2013 and claims the benefit of German application no. 10 2012 103 301.3 filed on Apr. 17, 2012, which are incorporated herein by reference in their entirety and for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a novel fiber which is produced by means of a melt spinning method. 
         [0003]    It has long been known that synthetic fibers made from a multiplicity of thermoplastic polymers can be produced by melt spinning. These fibers may be monofilaments, that is single filaments, or multifilaments formed from a plurality of filaments, and the fibers may be produced continuously to a virtually unlimited length, that is they may take the form of endless fibers. A fundamental prerequisite for the applicability of the melt spinning method is that the polymer concerned is melt processable, that is it has a sufficiently high melt viscosity. 
         [0004]    For various technical applications, there is great interest in fibers made from fully fluorinated polymers such as PTFE, in order that its particular properties, such as chemical and heat resistance and low coefficient of friction, may be utilised for the production of corresponding textile materials. However, the use of the melt spinning method is not possible for homopolymer PTFE because, as a result of its extremely high melt viscosity, it cannot undergo thermoplastic processing. 
         [0005]    For this reason, according to the prior art PTFE fibers are produced by splicing and stretching films, wherein these films may be formed from expanded PTFE or a PTFE suspension. However, fibers of this kind have a number of disadvantages, in particular the limited fiber length as a result of the discontinuous production, the pronounced roughness of the fiber surface, and the fact that the fiber cross section cannot be given a specific profile. One result of this is that the fibers are not easily woven. Furthermore, the fact that PTFE is not melt processable results in the disadvantage that the fibers or textile materials made therefrom cannot be welded. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the present invention is therefore to provide a fiber produced by means of a melt spinning method that has improved properties. 
         [0007]    This object is achieved according to the invention with the fiber of the type mentioned in the introduction in that the fiber includes a first component including a melt processable, fully fluorinated first polymer, and a second component including a thermoplastic second polymer. Here, the invention includes both the case of the two components or polymers being present as a compound, that is as a homogeneous mixture, and the case of the two components forming regions of the fiber that are spatially separated from one another. In the latter case, the components may be arranged in the greatest variety of ways, as will be described below in detail. 
         [0008]    In the fiber according to the invention, the advantages of the melt spinning method can be combined with the advantageous properties of a fully fluorinated polymer, since in the case of the first polymer a fluoropolymer is used which, unlike normal PTFE, is melt processable. In addition, the properties of the fiber may be further influenced by the second component which is provided, of a thermoplastic second polymer, and may be adapted to the respective requirements. In particular, this second component may serve to impart greater mechanical strength to the fiber than PTFE or other fully fluorinated polymers. 
         [0009]    It is favourable if the first polymer is a copolymer of tetrafluoroethylene and at least one fully fluorinated comonomer, wherein the comonomer content amounts to approximately 1 mole % or less, preferably from approximately 0.1 mole % to approximately 1 mole %. A comonomer content even in this low range is sufficient to impart suitability for melt processing to the fluoropolymer, while retaining the advantageous properties of PTFE such as heat and chemical resistance. This effect is attributable to the low molecular weight of the comonomers, as a result of which the melt flow rate is significantly increased by comparison with normal PTFE. 
         [0010]    Particularly preferably, the comonomer of the first polymer is selected from hexafluoropropylene, perfluoroalkyl vinyl ethers (in particular perfluoroethyl vinyl ether and perfluoropropyl vinyl ether), perfluoro-(2,2-dimethyl-1,3-dioxole) and mixtures thereof. Copolymers of this kind are for example described in EP 1 263 877 B1. 
         [0011]    A melt processable fully fluorinated polymer which may advantageously be used as the first polymer in the context of the invention is sold by ElringKlinger Kunststofftechnik GmbH in Bietigheim-Bissingen under the trade name Moldflon®. 
         [0012]    It is favourable if the first polymer has an amorphous content of at least 50%, typically in the region of approximately 60%. The high amorphous content is the result of the irregularities in the molecular structure brought about by the comonomer. 
         [0013]    For the second polymer of the second portion of the fiber according to the invention, in principle any thermoplastic polymers which are also melt processable are possible and may be selected in dependence on the desired property profile of the fiber. Preferably, the second polymer may be selected from polyethylene terephthalate, polybutylene terephthalate, polyamides, polyimides (in particular polyether imide), polyether ketones (in particular polyether ether ketone), polyphenylene sulfide or mixtures thereof. 
         [0014]    It is particularly favourable to use as the second polymer in particular high-temperature plastics such as polyimides, polyether ketones or polyphenylene sulfide, whereof the melting points are in some cases above that of the fully fluorinated first polymer. As an alternative, a further fluoropolymer may also be used as the second polymer of the fiber according to the invention. 
         [0015]    In one of the embodiments of the invention, the fiber is formed from a compound which includes approximately 20 to approximately 80 weight % of the first polymer and approximately 20 to approximately 80 weight % of the second polymer. The fiber, which may be a monofilament or a multifilament, in this case comprises a homogeneous material whereof the properties may be varied over a wide range by the type and proportion of the second polymer. In addition to the two polymers, the compound may also include further constituents, such as different types of fillers or further polymers. 
         [0016]    According to a further preferred embodiment, the first and the second component form regions of the fiber that are spatially separated from one another. In this case, particularly advantageous properties of the fiber may be achieved, as described below. 
         [0017]    In this case, it may be provided for the first component of the fiber to consist substantially entirely of the first polymer and to contain no or only insubstantial further constituents. As an alternative, the first component may comprise a compound of the first polymer with one or more further polymers and/or one or more fillers. A mixture of this kind allows the properties of the first component of the fiber to be varied and/or optimised. Possible fillers which may be mentioned are in particular molybdenum sulfide and graphite, which enable the wear resistance of the first polymer to be increased. 
         [0018]    If the first and the second component of the fiber according to the invention form regions that are spatially separated from one another, they may be arranged and/or connected to one another in any suitable way. Some preferred variants of such arrangements are described below. 
         [0019]    According to a preferred embodiment of the invention, the fiber is a multifilament including filaments of the first polymer, which form the first component, and filaments of the second polymer, which form the second component. Multifilaments of this kind, comprising different filaments, may be produced by a melt spinning method, using spinning jets having a plurality of apertures which are known per se from the prior art. 
         [0020]    Preferably, a fiber according to the invention in the form of a multifilament includes a total of approximately 10 to approximately 150 filaments, wherein approximately 20% to approximately 80% of the filaments are formed form the first polymer and approximately 20% to approximately 80% of the filaments are formed from the second polymer. The ratio between the first component and the second component may be selected in dependence on whether the properties of the first polymer or those of the second polymer are to dominate in the fiber according to the invention. 
         [0021]    In a further preferred embodiment of the invention, the fiber is a monofilament in which the first and the second component respectively extend in the direction of the fiber and are connected to one another by an adhesive bond. In this case, the two components of the fiber are thus present within a monofilament, but are not in the form of a compound but are spatially separated from one another. The fiber according to the invention may furthermore also be a multifilament having a plurality of filaments which are constructed in the manner of the monofilament described above. 
         [0022]    Within the monofilament, the first and the second component of the fiber according to the invention may be arranged next to one another, as seen in cross section. A fiber of this kind may be produced by means of a melt spinning method in which two spinning jets, respectively for the first and the second polymer, are arranged directly next to one another. 
         [0023]    In a further advantageous embodiment of the invention, the first or the second component forms the core of the monofilament and the other component forms a jacket surrounding the core. A core/jacket structure of the fiber of this kind enables particularly advantageous properties to be achieved. 
         [0024]    If for example the jacket is formed by the first component, using the first polymer, which has a very high chemical resistance, the core may be formed from a second polymer having a low chemical resistance (but having for example high strength), such that because the core is shielded by the jacket the overall result is a fiber having high strength and high chemical resistance. 
         [0025]    In the embodiments described above, the proportions of the first and the second polymer may be varied over a wide range in order to adapt the properties of the fiber to respective demands. For example, the first component, using the first polymer, may form approximately 5 to approximately 95 weight % of the monofilament, and the second component, using the second polymer, may accordingly form approximately 95 to approximately 5 weight %. 
         [0026]    The fiber according to the invention, or the filaments thereof, preferably have a regular cross-sectional profile, in particular a circular, oval or polygonal cross-sectional profile. The preferred cross-sectional profile may be determined, depending on the intended use of the fiber, by the geometry of the spinning jet, which represents a substantial advantage of the fibers according to the invention over PTFE fibers produced by splitting. In contrast to the fibers last mentioned, and as a result of their production, the fibers according to the invention, or the filaments thereof, also have a low level of surface roughness, which is particularly advantageous if they are woven or otherwise further processed to produce textile materials. 
         [0027]    Depending on the type of polymers used and depending on the purpose of use of the fiber, it is possible to vary the fineness of the fiber according to the invention, or the filaments thereof, over a wide range, wherein the fineness is preferably in a range of approximately 1 to approximately 1000 dtex, in particular in the range of approximately 2 to approximately 100 dtex. Typically, the fineness of the individual filaments will also depend predominantly on whether the fiber according to the invention is a monofilament or a multifilament. 
         [0028]    When the fiber according to the invention is produced by means of a melt spinning method, once the fiber has come out of the aperture or apertures of the spinning jet it can be stretched, as known in principle in the melt spinning of thermoplastic polymers. As a result of the stretching, the polymer molecules are oriented at least to a certain extent in alignment with the direction of the fiber, as a result of which the mechanical strength of the fiber can be increased. 
         [0029]    Favourably, the fiber according to the invention has a tear strength of approximately 4 to approximately 200 cN/tex. The elongation of the fiber at break is preferably in the range of approximately 10% to approximately 50%. 
         [0030]    The fibers according to the invention may be used for a multiplicity of technical applications, in particular for producing textile materials such as woven fabrics or nonwovens, which because the polymers used are melt processable may be manufactured by means of welding. Both the fibers themselves and corresponding textile materials may be used in particular for producing filter elements in which a high chemical resistance is required. 
         [0031]    A further area of application is the production of water-repellent textiles, because of the high level of hydrophobicity of the fully fluorinated polymers used. 
         [0032]    The hydrophobic properties of the fibers according to the invention may furthermore also be utilised for producing electrochemical elements such as gas diffusion electrodes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    These and other advantages of the invention will be explained in more detail by way of the exemplary embodiments below, with reference to the Figures. 
           [0034]      FIG. 1  shows a schematic cross-sectional illustration of a first exemplary embodiment of a fiber according to the invention; 
           [0035]      FIG. 2  shows a schematic illustration of a spinning jet device for producing the fiber according to  FIG. 1 ; and 
           [0036]      FIG. 3  shows a schematic cross-sectional illustration of a second exemplary embodiment of a fiber according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  shows a first exemplary embodiment of a fiber  10  according to the invention, having a circular cross-sectional profile and a core/jacket structure. The jacket  12  forms the first component of the fiber  10  and includes a melt processable, fully fluorinated first polymer. The core  14  forms the second component of the fiber  10  and includes a thermoplastic second polymer. The fiber  10  is a monofilament, wherein a plurality of such monofilaments may also be grouped together to form a multifilament. 
         [0038]    The fiber  10  includes approximately the same proportions of the first and the second polymer, that is the core  14  and the jacket  12  each form approximately 50% of the mass of the fiber  10 . As an alternative, however, these proportions may be varied over a wide range; for example, the proportion of the first polymer may be reduced, so that the jacket  12  becomes thinner. 
         [0039]    In the exemplary embodiment described here, the first polymer is a melt processable copolymer of tetrafluoroethylene with a content of 0.1 to 1 mole % perfluoropropyl vinyl ether, which has a melting point in the range of 314 to 320° C., and the second polymer is a polyether ether ketone (PEEK) having a melting point of at least 335° C. PEEK has very high mechanical strength and so also imparts high strength to the fiber  10  (e.g. up to 200 cN/tex), while the TFE copolymer in the jacket  12 , because of its low coefficient of friction, chemical resistance and stability to UV, determines the corresponding properties in the fiber  10 . The jacket  12  made from the TFE copolymer also allows the fiber  10  to be given a slight coloration, which is significantly more difficult with pure PEEK fibers. 
         [0040]    Both the core  14  and the jacket  12  of the fiber  10  may include further constituents in addition to the second and the first polymer respectively, in order to modify the properties of the fiber  10  accordingly, for example fillers or reinforcing agents. Furthermore, it is possible to modify the core  14  or the jacket  12  electroconductively. 
         [0041]      FIG. 2  shows schematically a spinning jet device  20  which is suitable for producing the fiber  10  according to  FIG. 1 . Here, a melt  22  of the first polymer (TFE copolymer) is pressed through a bath  24  of the melt  26  of the second polymer (PEEK), wherein this production method enables the viscosity of the second melt  26  in the bath  24  to be lower than the viscosity of the first melt  22 . This fulfils the prerequisite for combining the TFE copolymer with PEEK. 
         [0042]    From the device  20 , the fiber  10  comes out of two spinning apertures  28 , it also being possible for a larger number of spinning apertures (for example in the region of  50 ) to be provided. The fibers  10  coming out of the spinning apertures  28  may either be taken up individually as monofilaments or combined as filaments to give a multifilament. 
         [0043]    If the melt spinning method is performed using a TFE copolymer and PEEK, processing temperatures of up to approximately 400° C. may be used, and this also applies to further high-temperature plastics such as polyimides (PI), polyether ketone ketone (PEKK) or polyphenylene sulfide (PES). For example, the temperature at three zones of the extruder may be 355° C., 375° C. and 380° C., and the temperature at the spinning jet may be 390° C. 
         [0044]    As an alternative to the core/jacket structure of the fiber  10 , it is also possible, in the case of a monofilament, for the two components of the fiber according to the invention to be arranged in another way, for example next to one another as seen in the cross section of the fiber. This variant is illustrated schematically in  FIG. 3 . In the case of the fiber  30  according to this second exemplary embodiment, the first component  32 , using the first polymer, forms the left-hand half of the fiber cross section, and the second component  34 , using the second polymer, forms the right-hand half. In order to improve the adhesive bond of the two components  32  and  34  along the contact face  36 , additives that are appropriate for the first and/or the second polymer and enable a chemical bond to be made between the materials may be added.