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• the present invention
  relates to a sheet structure for electrical insulation or flame resistance comprising a barrier ply having a mica-rich face and a mica-poor face and a reinforcing ply containing a saturable backing layer attached to the mica-poor face of the barrier ply.
  
• mica-based sheets and tapes
  as an electrical insulating and flame barrier material is well known in the art.
  
• Such sheets and tapes
  typically consist of mica paper glued to a backing of reinforcing material.
  
• U.S. Pat. Nos. 4,286,010, 4,769,276, 6,103,382, 6,153,301, and European Patent Application 0 373 137 A2
  disclose the preparation and use of mica sheets and tapes in electrical insulation and flame protection applications.
  
• mica tapes produced by gluing mica paper to a backing reinforcement
  have limitations because the glue must be compatible with saturating resins that are commonly applied to the tapes in a subsequent step.
  
• the glue
  must also be thermally stable for extended periods of time.
  
• a widely-used combination of backing glass cloth and epoxy resin as a glue
  limits the end-use temperature of the insulating tape to 180° C., as well as to the possible resins that may be used in the impregnation step (one normally must use the same epoxy resin).
  
• the process of gluing of many resins
  usually involves continuous solvent-based processes and all the technological and safety complications related to dealing with volatile organics (ventilation, recuperation of the solvent and etc.).
  
• An aramid-mica blended paper
  made from a homogeneous blend of m-aramid fiber, m-aramid fibrids, and mica is manufactured by DuPont under trade name NOMEX® paper type 418.
  
• NOMEX® paper type 418
  Such papers are described, for example, in U.S. Pat. No. 6,312,561. Because of superior mechanical properties vs. mica paper, tapes from NOMEX® 418 paper can be used without any backing reinforcement. However, 30–60 weight percent of organic (aramid) components in this barrier material limit its possible corona resistance or voltage endurance.
  
• Another limitation
  is that during vacuum-pressure impregnation of an item insulated with a tape based on this paper, it is necessary to rotate this item until the resin is completely cured because the tape is not readily saturable, that is the tape has no component that can hold uncured resin.
  
• a two-ply paper
  has also been disclosed in which one ply consists of mica and another ply can consists of aramid, glass, polyester or other fibrous components (Chinan Pat. 2051434, 1996). These papers do not have the necessary reinforcement and are relatively weak in sheet form and do not make acceptable narrow-width tapes.
  
• What is needed
  is a sheet structure containing a high concentration of mica that can be made into tapes and easily saturated with a resin and that has sufficient mechanical properties for use in electrical insulation and flame retardant structures.
  
• This invention
  relates to a sheet structure comprising a barrier ply containing a mica-rich face and a mica-poor face, and a reinforcing ply attached to the mica-poor face of the barrier ply, with the reinforcing ply comprising a saturable backing layer.
  
• the mica content in the mica-rich face of the barrier ply
  is at least 60 weight percent based on the total components in the mica-rich face; the mica-poor face contains less than 40 weight percent mica, based on the total components in the mica-poor face; and the saturable backing layer of the reinforcing ply is a spunlaced aramid material.
  
• the mica-rich face of the barrier ply
  has greater than 85 weight percent mica by weight based on the total components in the mica-rich face and the mica-poor face has less than 10 weight percent mica based on the total components in the mica-poor face layer.
  
• the sheet structure of this invention
  may also include aramid material in the barrier ply in addition to mica. The amount of mica in the mica-rich and mica-poor faces can be measured in the sheet structure by carefully microtoming or microcutting the various integrated layers from the sheet structure and performing standard analytical techniques to analyze the composition of each layer.
  
• the sheet structure of this invention
  can be used to make mica-based tapes and can be combined with matrix resin to make prepregs.
  
• This invention
  further relates to a process for making a sheet structure comprising the steps of forming of a barrier ply containing a mica-rich face and a mica-poor face and attaching a reinforcing ply containing a saturable backing layer to the mica-poor face of the barrier ply.
  
• the barrier ply
  may be formed on a paper machine, and the mica-rich and mica-poor faces of the barrier ply may be formed by supplying separate mica-rich and mica-poor slurries to the paper machine. Further, the reinforcing ply may be attached to the barrier ply by laminating or calendaring the plies.
  
• the present invention
  provides a mica-based sheet structure, tape, and prepreg that is useful, for example, as an electrical insulation. Further, the present invention may be used as a flame barrier due to a significant content of mica in the composition.
  
• a sheet structure or tape of the present invention
  comprises at least 30 weight percent of mica on a total weight basis and is comprised of at least one barrier ply and one reinforcing ply.
  
• the mica
  is concentrated in one face or side of the barrier ply with the other face or side of the barrier ply having no or a low concentration of mica.
  
• mica-rich
  it is meant that the face has a mica content of at least 60 percent by weight, based on the total weight of the components in the mica-rich face.
  
• mica-poor
  it is meant the face has a mica content of less than 40 percent by weight, based on the total weight of the components in the mica-rich face.
  
• the barrier ply of the present invention
  may be made on a conventional paper-making machine equipped with two headboxes for forming a layered paper.
  
• Two different slurries having the desired proportions of mica and/or other material solids
  are provided to the two headboxes and the two slurries are then wet-laid as layers onto a papermaking wire with, for example, the mica-poor face being formed on the papermaking wire and the mica-rich face being formed on top of the mica-poor face. This creates a single sheet having an intermingling of the fibrous components at the interface of the mica-rich and mica-poor faces.
  
• the barrier ply
  may then be combined with a separate reinforcing ply, with the reinforcing ply facing the mica-poor face of the barrier ply.
  
• the two plies
  are then attached to each other and preferred methods are to attach the two plies together without adhesive resins or glues by laminating the two plies in the wet press of the papermaking machine or in the nip of a hot roll calender.
  
• the sheet structure or tape of the present invention
  can have a basis weight from about 30 g/m 2 to about 300 g/m 2 and a thickness from about 0.02 mm to about 1.0 mm.
  
• the mica-rich face of the barrier ply in mica-based sheet or tape of the current invention
  contains at least 60 weight percent, and preferably at least 85 weight percent of mica, based on total weight of components in the mica-rich face.
  
• Other components of the mica-rich face
  may include meta-aramid fibrids and other binder materials; and aramid, glass and other fibers, which can be used for the reduction of dusting from mica tape and adjustment of the mechanical properties of the mica-rich ply.
  
• the mica-poor face of the barrier ply
  can have the same components as the mica-rich face and preferably contains meta-aramid fibrids as a binder along with aramid fiber or other thermostable floc or fiber or mixtures of such flocs or fibers.
  
• thermostable
  it is meant that the given floc or fiber can withstand a long exposure to the end-use temperature without significant degradation (usually, to remain at least 50% of its initial properties after exposure for 100,000 hours to the necessary temperature.
  
• Other thermostable floc or fibers
  can include, but are not limited to, such fibers as polyesters and polybenzimdiazoles, and fibers and flocs based on fluoropolymers, glass, and silica.
  
• the reinforcing ply of the sheet structure of this invention
  contains a saturable backing layer which may be a woven, knitted or nonwoven fabric prepared from any thermally stable organic or inorganic fibers or their mixtures of those fibers. Possible fibers include, but not limited to, aramid, polyester, and glass fibers. It is preferred that the reinforcing ply be a spunlaced nonwoven fabric due to the good cushioning properties such fabrics exhibit during winding of the sheet structure, along with the fast and uniform resin pick-up that these nonwoven exhibit during resin impregnation.
  
• the reinforcing ply
  may include other plies or layers to impart other functionality as long as the saturability of the saturable backing layer is not compromised.
  
• the amount of mica in the faces of the barrier ply
  can be different depending on the particular end-use with the mica-rich face having at least 60 weight percent mica based on the total weight of components in that face, and the mica-poor face portion having less than 40 weight percent of mica based on the total weight of components in that face.
  
• the barrier ply
  Prior to impregnation by a resin, the barrier ply may be between 50 and 95 weight percent based on the total weight of the sheet structure, and the reinforcing ply may be between 5 and 50 weight percent, based on the total weight of sheet structure.
  
• Mica of various types
  can be used in the present invention.
  
• the optimum type for any particular sheet structure
  generally depends on the end-use application. Typically, muscovite is preferred for electrical insulation and phlogopite is preferred for flame barriers.
  
• floc
  means fibers that are cut to a short length and which are customarily used in the preparation of wet-laid sheets.
  
• floc
  has a length of from about 3 to about 20 millimeters.
  
• a preferred length
  is from about 3 to about 7 millimeters.
  
• Aramid floc
  is a preferred floc used in the mica-rich and mica-poor faces of the barrier ply. Floc is normally produced by cutting continuous fibers into the required lengths using well-known methods in the art.
  
• aramid
  means aromatic polyamide, wherein at least 85% of the amide (—CONH—) linkages are attached directly to two aromatic rings.
  
• additives
  can be used with the aramid and may be dispersed throughout the polymer structure. It has been found that up to as much as about 10 percent by weight of other polymeric material can be blended with the aramid. It has also been found that copolymers can be used having as much as about 10 percent of other diamines substituted for the diamine of the aramid or as much as about 10 percent of other diacid chlorides substituted for the diacid chloride of the aramid.
  
• fibrids
  means very small, nongranular, fibrous or film-like particles with at least one of their three dimensions being of minor magnitude relative to the largest dimension. These particles are prepared by precipitation of a solution of polymeric material using a non-solvent under high shear.
  
• aramid fibrids
  means non-granular film-like particles of aromatic polyamide having a melting point or decomposition point above 320° C. The fibrids generally have an largest dimension length in the range of about 0.2 mm to about 1 mm with a length-to-width aspect ratio of about 5:1 to about 10:1.
  
• the thickness dimension
  is on the order of a fraction of a micron, for example, about 0.1 microns to about 1.0 micron. While not required, it is preferred to incorporate aramid fibrids into barrier plies while the fibrids are still in a never-dried state. In addition to aromatic polyamide, aramid fibrids can optionally comprise dyes, pigments, and other additives such as antistatic agents, surfactants, or fillers such as carbon black, silica and titanium dioxide.
  
• Either ply of the sheet structure and/or tape of this invention
  may contain special additives for further improvement of voltage endurance behavior.
  
• additives
  can include but are not limited to particles based on silicone dioxide, aluminum oxide, titanium dioxide, zirconium dioxide or their combinations.
  
• the sheet structure and/or tape of this invention
  preferably does not contain any adhering resin between the plies, which allows the use of a broad range of resins during impregnation of the sheet or tape to make the final insulation material.
  
• the first common method
  comprises impregnating the tape or sheet with resin after it has been inserted into, or wound around, the item to be insulated and then curing the resin.
  
• the second common method
  comprises impregnating the tape or sheet with resin prior to it being inserted into or wound around the item to be insulated and then curing the resin.
  
• the mica-poor face of the barrier ply
  can provide good bonding between the barrier ply and the reinforcing ply without using any additional adhering resin. It is known in the art that one ply of aramid paper or board can be laminated in a hot calender nip or in the hot press with another ply of aramid paper or board, if both of the plies contain fibrids, moldable fibers or some other bonding agents, as it described, for example, in U.S. Pat. Nos. 4,481,060 and 4,752,355.
  
• prepreg
  includes a mica-based sheet or tape impregnated by a resin and having enough formability to be compressed and/or shaped into a final composite.
  
• the resin
  is usually in a curing stage B (partially soluble) in the prepreg and can be cured additionally to stage C (cross-linked, not soluble) later.
  
• a prepreg based on the mica-based sheet or tape of the present invention
  can be made by any known technique by the use of a matrix resin solution, dispersion, or melt which will flow into the structure of the present invention.
  
• the viscosity of the matrix resin
  is not especially critical to this invention. If the matrix resin has a very high viscosity it should be dissolved in a solvent to form a solution and then the solution can be used to impregnate the sheet. If a low viscosity matrix resin is utilized, the solution or dispersion is not necessary.
  
• low-viscosity epoxy compositions developed for vacuum-pressure impregnation (VPI)
  are in use in the industry now and can be used with the materials of present invention without any addition of solvents.
  
• the following test methods
  were used in the Examples provided below.
  
• the tensile properties of mica-based sheet and tape of the present invention
  were measured on an Instron-type testing machine using test specimens 2.54 cm wide and a gage length of 18 cm, in accordance with ASTM D 828-93.
  
• the thickness and basis weight of mica-based sheet of present invention
  were determined by measuring the thickness and the weight of an area of a sample of the test nonwoven sheet in accordance with ASTM D 645/D 645-M-96 and ASTM D 646-96, respectively.
  
• Resin pick-up
  was determined based on measuring the weight of a mica-based sheet before impregnation and after impregnation and complete curing of a resin. Before each weighing, the material was dried in the oven at 110° C. for 24 hours. Resin pick-up in weight percent was calculated based on the equation: ⁇ (Wt. After ⁇ Wt. Before)/(Wt. After) ⁇ 100%, Where Wt. Before and Wt. After is weight of the material before and after impregnation, respectively.
  
• aqueous dispersion containing a mixture of meta-aramid floc and meta-aramid fibrids
  was made having a relative content of non-aqueous components (percents by weight) as follows:
  
• the dispersions
  were pumped through primary and secondary headboxes of a Fourdrinier-type papermaking machine and a layered wet-laid ply was formed with a mica-rich layer on the top and a mica-poor layer containing predominantly aramid components on the bottom.
  
• the basis weight of the top layer
  was about 84.8 g/m2 and basis weight of the bottom layer was about 50.9 g/m2.
  
• the mica used
  was muscovite type, Electrical Samica Flake (sold by US SAMICA Incorporated, Rutland, Vt., USA).
  
• the meta-aramid fibrids
  were made from poly(metaphenylene isophthalamide) in a manner generally described in U.S. Pat. No. 3,756,908.
  
• the meta-aramid floc
  was poly(metaphenylene isophthalamide) floc of linear density 0.22 tex and length of 0.64 cm (sold by E. I. du Pont de Nemours and Company under the trade name NOMEX®).
  
• the layered wet-laid ply
  formed the barrier ply and was laminated with a reinforcing ply of nonwoven meta-aramid material in the hot nip of a calender at nip pressure of about 3000 N/cm.
  
• the temperature of the bottom role
  was about 350° C.
  
• the temperature of the top roll
  was about 180° C.
  
• the nonwoven meta-aramid material used as the reinforcing ply
  was poly(metaphenylene isophthalamide) spunlaced fabric having a basis weight of 68 g/m2 (sold by E. I. du Pont de Nemours and Company as NOMEX® spunlaced fabric type E88, style 320).
  
• the resulting mica-based sheet structure
  had a basis weight about 205 g/m2 and a thickness about 0.208 mm. About 66.1 weight percent of the total weight of the sheet was contained in the barrier ply with a mica content of 95 weight percent in the mica-rich face based on mica-rich face components, and essentially no mica in the mica-poor face. About 33.9 weight percent of the total weight of the sheet was contained in the reinforcing layer. The total mica content of the sheet on a total weigh basis was about 39.2 weight percent. Other properties of the final mica-based sheet are shown in Table 1. A part of the prepared mica-based sheet was then slit into mica-based tapes with width about 12.7 mm.
  
• mica-based sheet
  Another part of the mica-based sheet was cut into 20 cm ⁇ 20 cm pieces and impregnated with a solvent-free low-viscosity epoxy composition type E-833 (sold by P. D. George Co., St. Louis, Mo., USA).
  
• a piece of the sheet
  was dried in the oven at 110° C. for 24 hours, then impregnated with the resin in a vacuum oven at room temperature with residue pressure of about 5 kPa and, finally, heat treated in a platen press in a multi-step process at constant pressure of about 650 kPa and temperature about 135 C for 60 min., 156° C. for 120 min., 177° C. for 60 min., and 93° C. for 1 min.
  
• the barrier ply of the previous example
  was calendered without the reinforcing material at the same conditions as were used in that example.
  
• the calendered paper
  had thickness of about 0.105 mm.
  
• the reinforcing ply of Example 1
  provided a significant increase in tensile and tear properties of the mica-based barrier sheet, as well as an increase in resin pick-up. This confirmed that bonding through the mica-poor layer of the barrier ply was adequate, and that the saturable surface of the reinforcing ply was still saturable by resins after the lamination process.
  

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• 2002
  • 2002-12-13
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