Abstract:
A splice and a method of making the same. The splice is formed to join together first and second pieces of mica tape having a mica layer bonded to a first surface of a carrier layer. The mica layer from a portion of the first piece is removed so as to expose the first surface of the carrier layer. A thermoplastic film is disposed on the exposed first surface of the carrier layer of the first piece. A second surface of the carrier layer of the second piece is disposed on the bonding film so as to form an overlap area of the first and second pieces. Pressure and heat are applied to the overlap area to form the splice therein.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to mica tape and in particular to a splice for joining together lengths of mica tape.  
           [0002]    Mica tape is typically used to insulate electric conductors in applications where the conductors may generate or be exposed to high temperatures. Conventionally, mica tape is very thin, having a thickness between 3 mils (0.003 inches) and 5 mils (0.005 inches), and is formed by bonding a layer of mica to a carrier such as an open-weave fiberglass fabric. This construction provides the mica tape with flexibility and good tensile strength.  
           [0003]    A manufacturer of mica tape often produces mica tape in large mill rolls having a width between three and four feet. The manufacturer or an intermediate processor cuts the mica tape to a narrower width and winds the mica tape onto narrow rolls or traverse spools. Often, the manufacturer or the intermediate processor is required to join together pieces of mica tape to meet a particular customer&#39;s needs.  
           [0004]    Conventionally, pieces of mica tape are joined together using a butt splice, wherein ends of the pieces are butted together to form a seam, and adhesive tape is wrapped around the seam. Typically, the adhesive tape is disposed along 3-4 inches of the length of the joined-together pieces, thereby making the butt splice 3-4 inches long. In addition, the wrapped adhesive tape is typically 3-5 mils thick, thereby making the butt splice 3-5 mils thicker than the rest of the joined-together pieces. Thus, the butt splice extends along a substantial portion of the joined-together pieces and substantially increases the thickness of the joined-together pieces. As a result, the butt splice may produce lumps when the joined-together pieces are wrapped around a spool or an electrical conductor, which is undesirable. Moreover, the butt splice is typically weaker than the rest of the joined-together pieces.  
           [0005]    Based upon the foregoing, there is a need in the art for a splice for joining together mica tape and a method of making the same, wherein the splice is strong and does not substantially increase the thickness of the mica tape at the splice. The present invention is directed to such a splice and a method of making the same.  
         SUMMARY OF THE INVENTION  
         [0006]    It therefore would be desirable, and is an advantage of the present invention, to provide a method of forming a length of mica tape. Pursuant to the method, first and second pieces of mica tape are selected. The mica tape has a mica layer and a carrier layer. The carrier layer has opposing first and second surfaces. The first surface is bonded to the mica layer. The mica layer is removed from a portion of the first piece so as to expose the first surface of the carrier layer. A bonding film is disposed on the exposed first surface of the carrier layer of the first piece. The second surface of the carrier layer of the second piece is disposed on the bonding film so as to form an overlap area of the first and second pieces. In one embodiment, a treatment pressure is applied to the overlap area. In another embodiment, the bonding film is a thermoplastic film and the overlap area is heated to a treatment temperature so as to soften the thermoplastic film.  
           [0007]    Also provided in accordance with the present invention is a length of mica tape comprised of first and second pieces of mica tape. The mica tape has a mica layer and a carrier layer. The carrier layer has opposing first and second surfaces. The first surface is bonded to the mica layer. A thermoplastic film is disposed between the carrier layers of the first and second pieces of mica tape. The thermoplastic film is comprised of a terpolymer of tetrafluorethylene, hexafluoropropylene, and vinylidene fluoride.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
         [0009]    [0009]FIG. 1 shows a schematic view of portions of first and second pieces of mica tape;  
         [0010]    [0010]FIG. 2 shows a schematic view of a portion of a mica layer being removed from the first piece;  
         [0011]    [0011]FIG. 3 shows a schematic view of the first piece with the mica layer removed so as to expose a carrier layer;  
         [0012]    [0012]FIG. 4 shows a schematic view of the second piece spaced above a bonding film disposed on the exposed carrier layer of the first piece;  
         [0013]    [0013]FIG. 5 shows a schematic side view of an overlap area of the first and second pieces in a seeming device; and  
         [0014]    [0014]FIG. 6 shows a schematic side view of the first and second pieces joined together by a splice in the overlap area.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    It should be noted that in the detailed description which follows, identical components have the same reference numerals, regardless of whether they are shown in different embodiments of the present invention. It should also be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form.  
         [0016]    Referring now to FIG. 1, there is schematically shown first and second pieces  10 ,  12  of mica tape that are to be joined together at a splice  14  (shown in FIG. 6) to form an elongated piece  16  (shown in FIG. 6) of mica tape. The elongated piece  16  of mica tape may be used as an insulator for electric conductors.  
         [0017]    The first piece  10  has opposing side edges  18 , and the second piece  12  has opposing side edges  20 . The first and second pieces  10 ,  12  have substantially the same width and the same thickness. End users of mica tape commonly request widths between 0.125 inches and 2.0 inches. Accordingly, for purpose of example, but without limitation, the first and second pieces  10 ,  12  will be presumed to have a width between 0.125 inches and 2.0 inches. Each of the first and second pieces  10 ,  12  may have any length.  
         [0018]    The mica tape includes a mica layer  22 , a carrier layer  24  and a binder  26 . The carrier layer  24  has a first surface  24   a  and a second surface  24   b.  The first surface  24   a  of the carrier layer  24  is joined to the mica layer  22  by the binder  26 .  
         [0019]    An example of mica tape suitable for the present invention is sold under the name GEMAX 77635 Mica Cable Tape (the “GEMAX Tape”) by Insulating Materials Incorporated (“IMI”) of Schenectady, N.Y., which is a subsidiary of Von Roll Isola. The GEMAX Tape has an average thickness of 4.5 mils (0.114 millimeters), a weight of 0.235 lb/yd 2  (127 g/m 2 ), and an average tensile strength of 90 lb/in (157 N/cm). The GEMAX Tape is comprised of calcined muscovite mica paper bonded to woven fiberglass fabric by a silicone resin.  
         [0020]    Another example of mica tape suitable for the present invention is sold under the name CABLOSAM ART. 366.22-80 Mica Cable Tape (the “CABLOSAM Tape”) by IMI. The CABLOSAM Tape has an average thickness of 3.0 mils (0.075 millimeters), a weight of 0.160 lb/yd 2  (87 g/m 2 ), and an average tensile strength of 45 lb/in (80 N/cm). The CABLOSAM Tape is comprised of Phlogopite Samica paper bonded to woven fiberglass fabric by a silicone resin.  
         [0021]    For purpose of example and without limitation, the mica tape of the first and second pieces  10 ,  12  will be assumed to be the GEMAX Tape. With this composition, the first and second pieces  10 ,  12  each have an average thickness of 4.5 mils. In addition, the mica layer  22  is comprised of calcined muscovite mica paper, the carrier layer  24  is comprised of woven fiberglass fabric, and the binder  26  is a silicone resin.  
         [0022]    The splice  14  for joining together the first and second pieces  10 ,  12  of mica tape is formed by first cutting the first and second pieces  10 ,  12  to provide the first and second pieces  10 ,  12  with ends  28 ,  30 . As shown in FIG. 1, the first piece  10  is preferably cut such that the end  28  of the first piece  10  is disposed substantially perpendicular to the side edges  18 . Similarly, the second piece  12  is preferably cut such that the end  30  is disposed substantially perpendicular to the side edges  20 .  
         [0023]    A cut  32  is made through the mica layer  22  and the binder  26  of the first piece  10  at a selected distance from the end  28  of the first piece  10  so as to divide the first portion  10  into a main region  34  and a splice region  36 . The cut  32  is made so as to form an end edge  38  (shown in FIG. 3) in the mica layer  22  of the main region  34  that is complementary to the end  30  of the second piece  12 , i.e., the end edge  38  can contiguously abut the end  30  when the side edges  18  of the first piece  10  are collinear with the side edges  20  of the second piece  12 . Since the end  30  is disposed substantially perpendicular to the side edges  20 , the cut  32  is made substantially perpendicular to the side edges  18 .  
         [0024]    The splice region  36  has a length that extends from the end edge  38  of the mica layer  22  in the main region  34  to the end  28  of the first piece  10 . Preferably, the length of the splice region  36  is from about 0.10 inches to about 5.0 inches. More preferably, the length is from about 0.25 inches to about 2.0 inches, and more preferably from about 0.5 inches to about 1.0 inch. Still more preferably, the length is about 0.5 inches.  
         [0025]    Referring now to FIG. 2, the mica layer  22  and the binder  26  in the splice region  36  are stripped off the first surface  24   a  of the carrier layer  24 . Preferably, the mica layer  22  and the binder  26  are stripped away using a stripping tape  40  having a layer  42  comprised of a very aggressive adhesive, such as a rubber-based industrial adhesive. A portion of the layer  42  is placed on the mica layer  22  so that the mica layer  22  adheres thereto. The layer  42  is then pulled away to remove a portion  22   a  of the mica layer  22 . This process is repeated with different portions of the layer  42  until substantially all of the mica layer  22  and the binder  26  are removed. Once the mica layer  22  and the binder  26  are removed, the first surface  24   a  of the carrier layer  24  is exposed in the splice region  36 , as shown in FIG. 3.  
         [0026]    Referring now to FIG. 4, a bonding film  44  comprised of thermoplastic film is disposed on the exposed first surface  24   a  of the carrier layer  24  of the splice region  36 . Any thermoplastic film may be used for the bonding film  44 . Preferably, however, one of the following types of thermoplastic film is used as the bonding film  44 : (i) polyethylene, (ii) thermoplastic polyester, such as polyethylene terephthalate (PET), and (iii) fluorocarbon. These types of thermoplastic film are preferred because they have good dielectric properties and good resistance to chemicals. Having good dielectric properties is important because the elongated piece  16  of mica tape may be used as an insulator.  
         [0027]    A fluorocarbon film is especially suitable for use as the bonding film  44  because fluorocarbon films have good thermal and chemical resistance, a low dissipation factor, and a low dielectric constant. Examples of suitable fluorocarbon films include: (i) a polytetrafluoroethylene (PTFE) film, (ii) a perfluoroalkoxy (PFA) resin film, (iii) a fluorinated ethylene propylene (FEP) film, and (iv) a terpolymer of tetrafluorethylene, hexafluoropropylene, and vinylidene fluoride (a “THV terpolymer”).  
         [0028]    A THV terploymer is most preferred for use as the bonding film  44 . As with the other fluorocarbon films, a THV terpolymer has good thermal and chemical resistance, a low dissipation factor, and a low dielectric constant. A THV terpolymer also has good adhesive and elastomeric properties, which will make the splice  14  strong and flexible. In addition, a THV terpolymer has a moderately low melt point compared to other thermoplastics, which permits the splice  14  to be made quickly and with less sophisticated heating equipment. Moreover, when a THV terpolymer is completely degraded under extreme temperatures, no conductive residue is left to provide an electrical path to ground.  
         [0029]    A suitable THV terpolymer that may used for the bonding film  44  is sold under the name FLUOROLIN film THV-830 by Dewal Industries, Inc. of Saunderstown, R.I. The FLUOROLIN film THV-830 has a tensile strength of 2,500-4,000 PSI, an elongation of 400-500%, a dielectric strength of 1,300-4,000 Volts/Mil, a specific gravity of 1.98-2.01, and a melting point of 356° F. (180° C.). Other THV terpolymers are available from companies such as the 3M Company. The FLUOROLIN film THV-830 is provided in rolls as a dry film, and is available in a plurality of thicknesses, ranging between 0.5 mils and 20 mils, in increments of 0.1 mil.  
         [0030]    In accordance with the preferred embodiment of the invention, a dry film of THV terpolymer is used as the bonding layer  44 . Preferably, the bonding layer  44  has a thickness from about 0.5 mils to about 5 mils, more preferably from about 1.0 mil to about 4 mils, and still more preferably from about 1.5 mils to about 2.5 mils. Still more preferably, the bonding  44  layer has a thickness of about 2 mils.  
         [0031]    The bonding layer  44  is cut to match the splice region  36  and is deposited on the first surface  24   a  of the carrier layer  24  of the splice region  36  so as to be aligned therewith. The second surface  24   b  of the carrier layer  24  of the second piece  12  is then placed over the bonding layer  44 . The second piece  12  is positioned such that the end  30  of the second piece  12  contiguously abuts the end edge  38  of the mica layer  22  in the main region  34  of the first piece  10 , and such that the side edges  18  of the first piece  10  are collinear with the side edges  20  of the second piece  12 . This positioning of the first and second pieces  10 ,  12  forms an overlap area  46  (shown in FIGS. 5, 6) of the first and second pieces  10 ,  12 .  
         [0032]    Referring now to FIG. 5, a seeming process is performed on the overlap area  46  in a seeming device  48 . The seeming device  48  includes an upper bar  50  disposed above a stationary lower plate  52 . Preferably, the upper bar  50  has substantially the same shape and size as the overlap area  46 . The overlap area  46  is disposed between the upper bar  50  and the lower plate  52  when the upper bar  50  is in an upper position (shown in FIG. 5). The upper bar  50  is vertically movable between the upper position, wherein the upper bar  50  is spaced from the overlap area  46  and the lower plate  52 , and a lower position, wherein the upper bar  50  presses the overlap area  46  against the lower plate  52 . When the upper bar  50  is in the lower position, the upper bar  50  exerts a treatment pressure on the overlap area  46 .  
         [0033]    During the seeming process, the upper bar  50  is moved to the lower position so as to exert the treatment pressure on the overlap area  46 . The upper bar  50  is heated to raise the temperature of the overlap area  46  to an elevated treatment temperature. The treatment pressure and the treatment temperature are maintained for a treatment period of time.  
         [0034]    Preferably, the treatment temperature is from about 400° F. to about 600° F., more preferably from about 500° F. to about 600° F., and still more preferably from about 500° F. to about 550° F. Still more preferably, the treatment temperature is about 530° F. The treatment pressure exerted by the upper bar  50  is preferably from about 5 psi to about 40 psi, more preferably from about 10 psi to about 30 psi, and still more preferably from about 14 psi to about 20 psi. Still more preferably, the treatment pressure is about 14 psi. The treatment period is preferably from about 10 seconds to about 2 minutes, more preferably from about 20 seconds to about 60 seconds, and still more preferably from about 30 seconds to about 35 seconds. Still more preferably, the treatment period is about 30 seconds. In summary, during the seeming process, the overlap area is preferably heated to a temperature of about 530° F. and preferably subjected to a pressure of about 14 psi for preferably about 30 seconds.  
         [0035]    During the seeming process, the bonding film  44  softens and fuses together the carrier layers  24  of the first and second pieces  10 ,  12  in the overlap area  46 . After the treatment period of time has expired, the overlap area  46  is allowed to cool for at least 15 seconds. The upper bar  50  is then moved to the upper position and the overlap area  46  is removed from the seeming device  48 . The cooling of the overlap area  46  causes the bonding film  44  to harden and thereby fixedly secure together the carrier layers  24  of the first and second pieces  10 ,  12 . At this point, the formation of the splice  14  in the overlap area  46  is complete.  
         [0036]    Referring now to FIG. 6, the first and second pieces  10 ,  12  of mica tape are shown joined together by the splice  14  so as to form the elongated piece  16  of mica tape. The splice  14  is about 0.5 inches long and has an average thickness in a range from about 5.5 mils to about 6.0 mils. As set forth above, the average thickness of the mica tape is about 4.5 mils. Thus, the splice  14  is between 1.0 mil and 1.5 mils thicker than the rest of the elongated piece  16 .  
         [0037]    It should be appreciated that the splice  14  provides numerous benefits over conventional splices. The splice  14  is stronger than prior art splices. In one comparison test, the splice  14  was tested against a butt splice that joined together substantially similar pieces of the same mica tape. The results of the comparison test showed the splice  14  to have almost double the tensile strength of the butt splice.  
         [0038]    In addition to being stronger than conventional splices, the splice  14  is shorter than conventional splices, having a length of only 0.5 inches, as compared to a length of 3-4 inches for a conventional butt splice. Moreover, the splice  14  is thinner than conventional splices. The splice  14  only increases the thickness of mica tape by 1.0 mil to 1.5 mils, whereas a conventional butt splice increases the thickness of mica tape by 3 mils to 5 mils.  
         [0039]    It should be understood that the present invention is not limited to the preferred embodiments shown and described herein, but covers and includes any and all modifications thereof which fall within the purview of the present invention. For example, the present invention can be practiced with any type of mica tape. In addition, it is contemplated that the materials disclosed for use as the bonding film  44  of the present invention could be replaced by materials having similar structural, chemical and thermal features without departing from the scope of the invention. Therefore, the specific materials disclosed herein are merely provided to illustrate and describe the preferred embodiments of the present invention presently contemplated by the inventor, and are not intended to limit the scope of the present invention, which is defined by the claims appended hereto.