Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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STATE OF THE ART
The use of area insulatiny materials based on fine mica
for the electrical insulation of coils and conductor rods of
heavy-duty electrical machines is known and the said insulating
materials are usually made of a support material such as glass
fiber cloths or thermostable plastic foils bonded to a fine mica
paper impregnated with a thermosetting resin which acts as a
binder. The thermosetting resin in the said insula-ting material
is in the B-state. The said area electrical insulating materials
are applied to the element to be insula-ted such as a conductor rod
as a tape or as a wide material and the coated element is heated in
a coil press to cure the insulation and form the final product.
The preparation of the known area insulating materials is
relatively expensive as the fine mica paper has to be prepared
from mica in a suitable manner and then the support material and
the fine mica paper are coated with the thermosetting resin and
finally bonded together by calendering.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an inexpensive
method of producing an electrically insulating foil made of a hard-
enable synthetic resin and electrical grade mica and the foil pro-
duced thereby.
It is another object of the invention to provide a novel
method of electrically insulating elements and the elements pro-
duced thereby.
These and other objects and advantages of the invention
will become obvious from the following detai]ed description.
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THE INVENTION
The invention provides a process for the production of
an electrical insulating foil comprising substantially a thin-
layer of electrical grade mica powder and a hardenable syn-thetic
resin with a thickness of 0.1 to 0.5 mm comprising the steps oE
forming a pas-te of electrical grade mlca powder and a hardenable
substantially solvent free synthetic resin and optional fillers
and/or reinforcement material, forming a strand of said paste and
evenly distributing the pasty strand over the entrance of a
calender or a ribbon press to reduce the strand thickness into a
thin-layer 0.1 to 0.5 mm thick. The foil preferably has a thick-
ness of about 0.2 mm.
The expression "electrical grade mica" means mica mater-
ia] free of all electrically conductive compounds or materials
such as metals and the preferred mica is muscovite or phlogopite
based powder with a grain range of 0.03 to 2.8 mm. The pasty
strand preferably contains 35 to 70% by weight, more preferably
45 to 65% by weight, of the mica powder and up to 20% by weight of
fillers and/or reinforcing materials.
The hardenable synthetic resin mixture may be based on
an epoxy resin, a polyester resin or a silicone resin. Preferred
are mixtures of epoxy resins with epoxy equivalents of 150 to 800.
The hardenable resins will contain a hardener such as an amine
dissolved in a reactive thinner such as diphenyl glycidyl e-ther,
butanediol diglycidyl ether or more preferably a low viscosity
cycloaliphatic epoxy resin. The hardenable synthetic resin may be
used in the liquid state.
In a preferred embodiment of the invention, a mixture of
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resin and mica and optional filler and/or reinforcement materials
is added to an extruder and the hardener is separately added to the
extruder whereby mixing of the pasty material takes place in the
extruder itself.
In a preferred embodiment of the invention, the pasty
strand is supplied to the calender nip or the nip of the ribbon
press between two incoming webs of sheet type ma-terial and prefer-
ably the thin layer produced in the calender or ribbon press is
permanently bonded to at leas-t one of the sheet type webs or
advantageously a fiber fabric web is placed between the pasty
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strand and one of the webs of sheet material before entry into the calender
nip or nip of the ribbon press. The fiber fabric web is permanently bonded to
the thin foil produced thereby.
While the production of the resin foils using an extruder followed
by calendering is known, one skilled in the art would not expect the calender-
ing of a strand of a mixture of mica powder and resin would produce a foil or
thin layer in which the mica powder particles are oriented essentially parallel
to the foil surfaces by the thickness reduction of the strand in the calender.
The particle orientation of the mica par~icles parallel to the layer surfaces
is similar to tha-t of mica in known mica paper which is important to obtain
high electrical strength values.
Referring now to the drawing:
The Figure is a schematic view of an apparatus for carrying out the
process of the invention.
The apparatus of the Figure comprises a heatable double screw extruder
1 with only one of the two screws 2 being illustrated in the drawing. The
extruder housing is connected to feed hopper 4 by feed inlet 3 into which mi.ca
powder 6 is metered from supply tank 5 by metering spiral 7 and inlet feed 3
is also connected via line 8 and proportioning pump 9 to heatable storage
vessel 10 for the hardenable resin. A second feed inlet 12 offset relative to
inlet 3 in the direction of arrow 10 is connected via line 13 and proportion-
ing pump 14 to storage vessel 15 holding hardener solution 31. The extruder
1 is provided with orifice 16 for extrusion of a round strand and can pivot
in oscillation at the point of orifice 16 as indicated by 17 in a direction
perpendicular to the drawing plane.
Following the extruder is a calendar with a pair of heatable calendar
rolls 18 which enclose nip 19 and provided with a draw-off device for foil
product 20 comprising two guide rollers 21 and 22, a pair of draw-off rollers
23 and a winding device 24. The ca.lender is also provided with unrolling
devices 25, 26 and 27 to feed two separate foils 28 in tape form and glass
fiber fabric 29 to ni~ 19.
To produce an area electrical insulating material in ribbon form with
a thickness of 0.2 mm consisting of a foil of a thermosetting epoxy resin and
mica powder on one surface and a fiber glass fabric web on the other surface,
a muscovite-based mica powder with about 50% by weight having a grain size
o 0.08 to 0.10 mm and about 50% by weight having a grain size of 1.5 to 1.8 mm
is placed in storage tank 5 and an epoxy resin mixture 30 consisting of 7
parts by weight of epoxy resin with an epoxy equivalent of 200 and 3 parts by
weight of epoxy resin with an epoxy equivalent o 420 is placed in heatable
storage vessel lO. A latent amine hardener in solution in a reactive thinner
31 is placed in storage vessel 15 and the fiber glass fabric 29 has a thickness
of 0.04 mm. The separating foils 28 made of polyethylene terephthalate have
a thickness of 0.03 mm.
Mica powder 6 and liquid epoxy resin 30 are continuously fed together
in a wei.ght ratio of 58:41 respectively through inlet 3 and are intimately
mixed in first mixing ~one 32 of extruder l. At inlet 12, l part by weight of
hardener solution 31 is added to the mica-resin mixture whereby extruder screws
2 transports a uniform mixture of resin-mica-hardener solution to orifice 16.
The temperature of the mixture in the extruder is maintained at about
100C over the entire screw length and is lowered to 85 to 90C at orifice 16.
By oscillation of the extruder in the area of orifice 16 in a direction per-
pendicular to the drawing plane, the pa.sty strand 33 continuously issuing from
orifice 16 is supplied to nip 19 with even distribution over the total width
thereof between two separating foils 28 fed from unwind.ing devices 25 and 27.
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Fiber glass web 29 is supplied from unwinding device 26 between one of separat-
ing foils 28 and pasty strand 33.
The material of the pasty strand 33 is continuously transformed
between heated calender rollers 18 maintained at 75C into a thin foil which
becomes permanently bonded to fiber glass fabric web 29. The flaky particles of
mica powder in the foil are predominantly oriented parallel to the layer plane
due to the calendering effest which is desired. The laminated material 20
emerging from the nip is drawn-off via guide rollers 21 and 22 with the aid of
draw-off rollers 23 and is then wound at 24.
The separating foil 28 applied to fiber glass fabric 29 is removed
from laminate 20 by a special rewinding device and the laminate is then ready
for use as an area electrical insulating material after cutting into narrower
strips, if desired.
The temperature contro] in the treatment of the mixture of mica
powder, epoxy resin and hardener during the method is such that the resin is
brought into the B state. Instead of being bonded with the glass fabric web 29,
the ~hin -foil produced from the mixture of mica powder, epoxy resin and
hardener can be bonded in the calendar also with other sheet type materials,
in particular support materials such as plastic foils, synthetic fiber fabrics
and the like. The method of the invention can advan~ageously be employed also
quite generally for the production of foils which are to contain besides a
hardenable resin mixture, instead of the mica powder or a part thereof, other
Eillers and/or reinforcement substances such as carbon black, iron powder,
reinforcement fibers and the like.
Various modifications of the products and processes of the invention
may be made without departing from the spirit or scope thereof and it is to be
understood that the invention is intended to be limited only as defined in the
appended claims.
