Abstract:
A fiber tape for producing fiber reinforced parts is provided. The fiber tape includes a fiber layer and a binder material securing the fibers of the fiber layer to each other. The binder material is provided as a binder mesh covering at least an upper surface and/or a lower surface of the fiber layer.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 U.S.C. §119 to European Patent Application No. 15175028, filed Jul. 2, 2015, entitled “A Fiber Tape for Producing Fiber Reinforced Parts and Process for Manufacturing such a Fiber Tape.” The contents of this application are incorporated herein by reference in their entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a fiber tape for producing fiber reinforced parts, comprising a layer of fibers and a binder material securing the fibers to each other. 
       BACKGROUND 
       [0003]    FR 3 005 065 A1 discloses a method for providing a layer of fibers which may be used in the fiber tape according to the present disclosure. Usually, a binder powder is added to the layer of fibers in order to secure the fibers to each other. The particles of the binder powder penetrate into the volume of the layer of fibers and punctually connect the fibers to each other. In practice, it has been found that the surface of the so-obtained fiber tapes has an irregular appearance, as free fiber ends may project from the fiber tape surface. As a consequence, additional measures have to be taken in order to ensure that prepregs or fiber reinforced parts produced using the fiber tape will have a smooth surface. 
         [0004]    Therefore, it is desirable to provide a fiber tape having improved surface properties. 
       SUMMARY 
       [0005]    According to the present disclosure, there is provided a fiber tape for producing fiber reinforced parts, the fiber tape comprising a fiber layer and a binder material securing the fibers of the fiber layer to each other, wherein the binder material is provided as a binder mesh covering at least an upper surface and/or a lower surface of the fiber layer. 
         [0006]    According to the present disclosure, the most important property of the binder mesh is that the binder material forms a continuous structure such that each section of binder material is supported by other sections of binder material surrounding it. It should be noted that, in the context of the present application, each continuous structure providing such an inherent support is considered to be a “binder mesh”. In particular, the binder mesh may have a regular structure, e.g. similar to that of a fishing list, a fabric or the like, as well as an irregular structure, e.g. similar to a veil, an omentum or the like. 
         [0007]    Due to the support provided by the continuous structure of the binder mesh, the fibers, and in particular their free ends, are held more effectively inside the plane of the fiber layer, resulting in a smoother surface of the fiber tape. 
         [0008]    In order to provide a lateral confinement for the fiber layer, the binder mesh may be provided as a binder mesh strip laterally folded around the side edges of the fiber layer. According to this embodiment, the fiber tape may easily be produced with a pre-defined width. 
         [0009]    When the two side edges of the binder mesh strip are overlapping each other, the fiber layer may be completely wrapped by the binder mesh, thus enclosing the fibers of the fiber layer securely inside the envelope formed by the binder mesh. 
         [0010]    According to a further embodiment, the binder mesh may be made of a hot-melt or hot-softening glue material. In particular, the hot-melt glue material may be selected from the group of materials including polyamide, co-polyamide, polyester, polyvinyl alcohol, and duromers having thermoplastic properties. Polyvinyl alcohol, for example, has the property to be solvable in thermoset matrix materials. As such it may be preferred in applications where it is advantageous that the binder material doesn&#39;t form a separate structure in the fiber reinforced part. For other applications, however, it may be preferable to have such a separate structure in the fiber reinforced part, e.g. for preventing or at least restraining crack propagation through the fiber reinforced part. A typical melting temperature of the afore-mentioned hot-melt glue materials is between about 80° C. and about 220° C. As a hot-softening glue material, for example, an amorphous plastic material, e.g. an epoxy based material, may be used. 
         [0011]    During the further process of manufacturing fiber reinforced parts, the fiber tape according to the present disclosure may be cut into pieces which then may be arranged in layers for forming fiber preforms which do not yet include a matrix material. In order to facilitate the handling of such fiber preforms before they are impregnated with the matrix material, it is suggested that the binder mesh material is not or at least not fully activated. According to this embodiment, the binder mesh material may be activated or fully activated after the fiber preforms have been formed by arranging pieces of fiber tape in layers. As the binder mesh is arranged along the outer surface of the fiber layer, and as such at the outer surface of the fiber tape, the fiber tape pieces may be connected to each other by this activation or full activation. 
         [0012]    A partial activation of the binder mesh material may be achieved, for example, by heating the binder mesh material to a temperature which is below its melting temperature or softening temperature range, but high enough that the surface of the binder mesh becomes adherent, e.g. to a temperature which is about 5 to 10° C. below the melting temperature or softening temperature range of the binder mesh material. In this case, the binder mesh only adheres to the fiber layer, but is not fully glued to it. In contrast, a full activation of the binder mesh material may be achieved by heating the binder mesh material to a temperature which is above its melting temperature or softening temperature range. 
         [0013]    It is, however, also conceivable to only mechanically connect the binder mesh to the fiber layer, e.g. by stitching the binder mesh to the fiber layer. In this case, the binder mesh material needs not to be activated at all. 
         [0014]    The sewing thread used for stitching may, for example, be made from a thermoplastic material. 
         [0015]    In view of applications, in which a connection of the fiber tape pieces is not required, the binder mesh material may be fully activated in the fiber tape, e.g. by heating the binder mesh material to a temperature which is above its melting temperature or softening temperature range, such that it glues the fibers of the fiber layer together, thus providing optimum support for the fibers. 
         [0016]    The heating of the binder mesh, be it the heating for partial or full activation, may be made using hot air, infrared radiation, electricity or any other suitable heating medium. 
         [0017]    In particular in the case of full activation of the binder mesh material, the fiber tape may be pressed, in order to make the binder mesh material penetrate into the volume of the fiber layer. 
         [0018]    According to a further embodiment, the binder mesh may have a weight amounting to between about 3 g/m 2  and about 30 g/m 2 , preferably between about 6 g/m 2  and about 20 g/m 2 . 
         [0019]    The fiber layer may include all types of fibers which may be used for producing fiber reinforced parts. For example, the fiber layer may comprise, carbon fibers and/or glass fibers and/or polyester fibers and/or aramid fibers and/or basalt fibers. In other words, the fiber layer may be made from fibers of only one type of material or from fiber blends. 
         [0020]    Preferably, the fibers may be recycled fibers. When recuperating the fibers, all types of waste material produced during the production of fiber reinforced parts may be used. In particular, so-called “dry” waste material, i.e. fiber material which has not yet been brought into contact with matrix material, e.g. waste material generated during the production of fiber preforms, as well as so-called “wet” waste material, i.e. fiber material brought into contact with matrix material, e.g. waste material generated during the production of fiber reinforced parts from fiber pre-pregs, may be used. During the recuperation of the fibers from “wet” waste material, the matrix material, which may be either thermoset material or thermoplastic material, may be stripped off the fibers by using pyrolysis, solvolysis or any other suitable process. It is to be noted that it is acceptable and sometimes unavoidable that the stripped fibers still have a remainder of matrix material thereon (&lt;1% by weight). 
         [0021]    The recycled fibers may be supplied in every suitable form, e.g. as strands or slivers or staple fibers. In the case of slivers, it may be necessary to roll the sliver before supplying it to the binder mesh in order to obtain a flat fiber layer suitable for the manufacturing of the fiber tape according to the present disclosure. 
         [0022]    In order to ensure that fiber reinforced parts made from the fiber tape according to the present disclosure have high-quality mechanical properties, such as tensile strength, E modulus, and the like, it is advantageous if the fibers are long fibers, i.e. if more than 50% of the fibers have a length of at least 60 mm, preferably a length of at least 100 mm, and/or if the fibers are realigned fibers. The mechanical properties of fiber tapes made according to the present disclosure made using such discontinuous recycled fibers are comparable to fiber tapes according to the present disclosure made from newly produced continuous fibers. 
         [0023]    If the fibers are recycled fibers, the waste material supplied to the recuperation process may be pre-treated as to their fiber length. For example, too small pieces or pieces including too short fibers, may be removed from the supply, whereas too large pieces or pieces including too long fibers, which may be difficult to handle during the process of manufacturing the fiber tape according to the present disclosure, may be cut to the desired length. 
         [0024]    For realigning the fibers, for example the process and the device known, from FR 3 005 065 A1 may be used. 
         [0025]    As the realigned fibers may show a straight as well as a curved shape, it may be difficult to determine their degree of alignment based on their shape. As a consequence, it is suggested to manufacture a test plate of predetermined dimensions using sections of the fiber tape according to the present disclosure having predetermined length and all sections extending in the same nominal fiber direction (unidirectional test plate), to cut a first test sample having rectangular shape from the test plate, the longer rectangle side of the first test sample extending parallel to the nominal fiber direction, to cut a second test sample having the same rectangular shape as the first test sample from the test plate, the longer rectangle side of the second test sample extending perpendicular to the nominal fiber direction, and to determine the tensile strength (TS) of the first test sample (TS(0°)) and of the second test sample (TS(90°)). If the tensile strengths (TS) of the two test samples differ from each other by more than 10% ((TS(0°)−TS(90°))/TS(0°)&gt;10%), the fibers are deemed to be realigned. 
         [0026]    The fiber tape according to the present disclosure may in particular be formed using fiber layers including realigned long recycled carbon fibers (LrCF). 
         [0027]    In view of most of the practical applications of the fiber tape according to the present disclosure, it is advantageous if the fiber layer has a weight amounting to between about 10 g/ml and about 50 g/m, and/or if the fiber layer has a width of between about 10 mm and about 50 mm, preferably of about 25 mm. 
         [0028]    According to a further aspect, the present disclosure also relates to a process for manufacturing a fiber tape according to the present disclosure, wherein the binder mesh and the fiber layer are connected to each other, after the binder mesh is brought into contact with the upper surface and/or the lower surface of the fiber layer. 
         [0029]    With respect to further details of this process, it is referred to the above discussion of the fiber tape according to the present disclosure. 
         [0030]    By the manufacturing process according to the present disclosure, the fiber tape may be produced as a so-called endless fiber tape which may be coiled up on a spool or coil for being transferred to a manufacturing site for manufacturing fiber reinforced parts, where it may be cut to length and combined with other fiber tape sections to form a preform (still lacking matrix material) or a prepreg (impregnated with matrix material). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    In the following, the present disclosure will be explained in more detail referring to the accompanying drawings, in which 
           [0032]      FIG. 1  shows a schematic perspective view of a fiber tape according to a first embodiment of the present disclosure; 
           [0033]      FIG. 2  shows a schematic side view of a device for manufacturing the fiber tape of  FIG. 1 ; 
           [0034]      FIG. 3  shows an enlarged perspective view of the wrapping unit of the device of  FIG. 2 ; and 
           [0035]      FIG. 4  shows a test plate used in a check procedure for checking the quality of the fiber tape of  FIG. 1  produced using the manufacturing device of  FIG. 2 ; 
           [0036]      FIG. 5  shows a schematic perspective view of a fiber tape according to a second embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  shows a fiber tape  10  according to a first embodiment of the present disclosure. 
         [0038]    The fiber tape  10  comprises a fiber layer  13  formed of a plurality of fibers  14  extending substantially parallel to the longitudinal direction L of the fiber tape  10 . The fiber layer  12  has a substantially flat shape, in other words, its width W is much larger than its height H. The fiber tape  18  further comprises a binder mesh  16 , which is wrapped around the fiber layer  12  and secures the fibers  14  of the fiber layer  12  to each other. In particular, the binder mesh  16  covers an upper surface  12   a  and a lower surface  12   b  of the fiber layer  12 , and also extends around the side surfaces  12   c  and  12   d  of the fiber layer  12 . 
         [0039]    According to a preferred, but not limiting embodiment, the fibers  14  may be long recycled carbon fibers (LrCF) realigned in order to extend substantially parallel to each other. The realignment may, for example, be achieved by the process and device known from FR 3 005 065 A1. It should be noted that, in the context of the present disclosure, the term “long fibers” refers to the fact that at least 50% of the fibers  14  have a length of at least 60 mm, preferably a length of at least 100 mm. 
         [0040]    Furthermore, the binder mesh  16  may be made from a hot-melt or hot-softening glue material and may have a weight amounting to between about 3 g/m 2  and about 30 g/m 2 , preferably between about 6 g/m 2  and about 20 g/m 2 . 
         [0041]    In order to form a closed envelope for the fiber layer  12 , the two lateral edge portions  16   a  and  16   b  of the binder mesh  16  are overlapping each other. Furthermore, the gluing property of the hot-melt or hot-softening glue material of the binder mesh  16  may be at least partially activated in order to connect the two overlapping edge portions  16   a  and  16   b  of the binder mesh  16  to each other and to connect the entire binder mesh  16  to the fibers  14  of the fiber layer  12 . A partial activation of the hot-melt glue material of the binder mesh  16  may, for example, be obtained by heating the hot-melt or hot-softening glue material to a temperature which is about 5 to 10° C. below its melting temperature or softening temperature range. 
         [0042]      FIG. 2  shows a device  20  for manufacturing the fiber tape  10 . The realigned fibers  14  are supplied onto a manufacturing platform  22  by a fiber supply unit  24 , e.g. the fiber supply unit known from FR 3 005 065 A1. To be precise, the fibers  14  are supplied onto the binder mesh  16 , which is supplied from a binder mesh coil  26 . By means of a wrapping unit  28 , which is shown in more detail in  FIG. 3 , the two lateral edge portions  16   a  and  16   b  of the binder mesh  16  are folded around the fiber layer  12  so that they overlap each other. 
         [0043]    By means of a heat source  32 , e.g. a hot air blowing unit  32  electing hot air  30  (see  FIG. 2 ), the binder mesh  16  is heated, e.g. to a temperature, which is about 5 to 10° C. below the melting temperature or softening temperature range of the material of the binder mesh  16 , in order to make the binder mesh material adherent (partial activation). It should be noted that instead of the hot air blowing unit  32  any other suitable type of heat source may be used, e.g. an infrared heater. 
         [0044]    A pressure roller  34  presses the binder mesh  16  with its overlapping edge portions  16   a  and  16   b  against the fiber layer  12 , in order to connect the overlapping edge portions  16   a  and  16   b  to each other and the overall binder mesh  16  to the fibers  14  of the fiber layer  12 , or even to make the binder mesh material penetrate into the volume of the fiber layer  12 . A cooling unit  36  cools the so-produced fiber tape  10  in order to maintain the achieved connected state of fiber layer  12  and binder mesh  16 . Finally, the fiber tape  10  is coiled to a spool or coil  38  pulling the binder mesh  16  through the manufacturing device  20 . 
         [0045]    The degree of alignment of the fibers  14  supplied by the fiber supply unit  24  may be checked during the manufacturing process by a quality check unit  40 , which, for example, may work optically on a pattern recognition basis. 
         [0046]    In addition or alternatively, the quality of the fiber tape  10  produced by the manufacturing device  20  may be checked using a test plate  42  (see  FIG. 4 ). The test plate  42  is manufactured by placing pieces of fiber tape  10  in several layers above each other such that the fibers  14  of all pieces of fiber tape  10  are extending parallel to each other, impregnating the so-obtained unidirectional preform with matrix material, e.g. thermoplastic or thermoset plastics material, and curing the matrix material. Then, two test samples  44  and  46  are cut from this test plate  42 , both test samples  44 ,  46  having identical width w and identical length l, but, while the length direction, of test sample  44  extends parallel to the fiber direction L (0°), length direction of test sample  46  extends orthogonal thereto (90°). These two test samples  44 ,  46  are placed in a tensile strength testing equipment, in order to determine the tensile strength (TS) of the parallel test, sample  44  (TS(0°)) and of the orthogonal test sample (TS(90°)). And, if the tensile strengths (TS) of the two test samples  44 ,  46  differ from each other by more than 10% ((TS(0°)−TS(90°))/TS(0°)&gt;10%), the fibers are deemed, to be aligned. 
         [0047]      FIG. 5  shows an alternative embodiment of a fiber tape according to the present disclosure substantially corresponding to the fiber tape  10  of  FIG. 1 . As a consequence, in  FIG. 5 , identical elements are designated by the same reference numerals as in  FIG. 1 , but increased by 100. Moreover, fiber tape  110  shown in  FIG. 5  is described only insofar it differs from fiber tape  10  of  FIG. 1  to the description of which it is hereby explicitly referred. 
         [0048]    Fiber tape  110  of  FIG. 5  differs from fiber tape  10  of  FIG. 1  by the fact that the overlapping edge portions  116   a  and  116   b  of the binder mesh  116 , which are wrapped around the fibers  114  of the fiber layer  112 , are connected to each other and to the fiber layer  112  by stitches  150 , e.g. made from a thermoplastic thread. According to this embodiment, the hot-melt glue material of the binder mesh  116  needs not to be activated at all, as the stitches  150  are connecting not only the edge portions  116   a  and  116   b  to each other, but also the binder mesh  116  to the fibers  114  of the fiber layer  112  without the binder mesh material having to be adherent (partial activation) or tacky (full activation).