Note: Descriptions are shown in the official language in which they were submitted.
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HIGH TEMPERATURE PAPER
CONTAINING ARAMID COMPONENT
This application claims the benefit of U.S. Provisional Application No.
S 60/409,230, filed on September 10, 2002, and is incorporated herein by
reference
in its entirety .
The present invention relates to high temperature papers, and in particular
to high temperature papers comprising an aramid component. Such high
temperature papers can comprise a single layer or multiple layers.
High temperature E-board is used in transformers and performs two
functions. First, the E-board provides electrical insulation. This keeps the
coils in
the transformer from short circuiting. Secondly, the board provides mechanical
strength. When there is a large passage of current through the transformer,
there
is force on the layers of the coil to move the board, which is glued to the
coils.
The glueing of the board to the coil keeps the various coils from telescoping.
Each coil acts like a solenoid and tries to move. It is the E-board which
prevents
this telescoping.
Improving the mechanical strength of the E-board would aid in avoiding
problems with telescoping coils. Having a reinforced E-board to strengthen the
paper would help to provide the necessary mechanical strength. The paper,
however, would have to be made in an efficient and effective manner.
There is also interest in increasing the temperature resistance of E-board
for use in transformers so that a less expensive transformer could be
designed. By
reducing the diameters of the wires in a transformer, the coils would become
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smaller. Smaller coils require smaller cores and smaller metal containers.
Smaller containers hold less oil, and this means that less copper for the
wire, steel
for the cores and oil for the insulation are needed. Because of the thinner
wire,
however, the transformer would have more electrical resistance and would run
hotter. Thus, the E-board would have to exhibit enhanced thermal resistance
before such a transformer would be practical.
A paper which exhibits such enhanced thermal resistance, as well as
enhanced mechanical strength would allow the industry to design transformers
which can recognize the economic benefits and performance benefits discussed
above.
Accordingly, it is an object of the present invention to provide a paper
structure which exhibits enhanced thermal resistance.
Yet another object of the present invention is to provide a paper structure
which exhibits enhanced mechanical strength.
Still another object of the present invention is to provide a high temperature
paper suitable for use in transformers.
These and other objects of the present invention will become apparent to
the skilled artisan upon a review of the following description, and the claims
appended hereto.
SUMMARY OF THE INVENTION
In accordance with the present invention, provided is a paper structure
comprised of an aramid component. The paper structure comprises an aramid
fiber and/or fibrid, a polymeric binder, such as polyvinyl alcohol, and
cellulosic
pulp fiber.
In a preferred embodiment, the paper structure is comprised of two outside
layers and at least one inside layer. The two outside layers are preferably
comprised of substantially cellulosic (wood) pulp fiber. The inside layer is
comprised of cellulosic pulp fiber, the aramid fiber and/or fibrid and a
polymeric
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binder. In a preferred embodiment, the structure comprises at least three
inside
layers, all comprised of cellulosic pulp fiber, the aramid component and a
polymeric binder.
The resulting paper structure provides a paper quite useful as E-board in
transformers due to its enhanced thermal resistance. Moreover, the aramid
fiber
also helps to reinforce the paper to avoid the problems in telescoping coils.
DETAILED DESCJfLIPTION OF THE PI~F,FEIZRFD EMBODIMENTS
The paper structure of the present invention is quite useful as a high
temperature E-board. The paper exhibits enhanced thermal resistance as well as
good mechanical strength. The good thermal resistance would allow the use of
the
paper in a transformer with coils of smaller size as it would allow the coils
to run
hotter. Furthermore, the paper is also reinforced so that when glued to the
coils,
it would keep the various coils from telescoping. In actual use in a
transformer,
the paper is coated with an adhesive, such as an epoxy adhesive, and heat
bonded
to the wire coil surface. It is this bonding to the coils that permits the
board to
keep the various coils from telescoping. The mechanical strength of the paper
structure of the present invention, including its aramid component in
combination
with the polymeric binder, permits the E-board to act efficiently and
effectively
while preventing the coils from telescoping.
The paper structure of the present invention comprises a polymeric binder,
an aramid component which can be an aramid fiber, a fibrid, or a combination
thereof, together with cellulosic pulp fiber.
The aramid fiber can be any commercially available polyaramid fiber, such
as that under the trademark NOMEX~. Generally, the fiber is about 1/4 inch in
length and has about a 2 denier. The fibrid is a small irregularly shaped
piece of
aramid polymer that is much larger in two dimensions than it is in the third
dimension. It is like a microscopic corn flake in shape. The large dimensions
are
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on the order of 5 to 25 micrometers while the third and smaller dimension is
about
0.01 to 1 micrometer.
In the paper structure, the amount of cellulosic wood pulp fiber generally
ranges from 50 to 80 wt % , while the amount of aramid component generally
ranges from about 5 to 25 wt % . The amount of polymeric binder, preferably
polyvinyl alcohol, generally ranges from about 10 to 25 wt % . The aramid
component, from 5 to 25 wt % , can be comprised of solely aramid fiber, aramid
fibrid, or preferably a combination of the two. When a combination is used, it
is
preferred that about a 60/40 wt % ratio of fiber/fibrid is employed. When
aramid
fibrid is present, the amount of polymeric binder can be reduced as the fibrid
can
also function as binder for the web. For this reason, it is preferred that
some
aramid fibrid is present. It performs a dual role and can reduce the amount of
aramid fiber and binder needed.
A minor amount of other synthetic fiber might also be present. Such
synthetic fiber can be, for example, polyester or nylon fiber.
The paper structure of the present invention can comprise a single layer or
multiple layers. When a single layer is employed, the paper structure contains
the
cellulosic pulp fiber, aramid component and polymeric binder as discussed
above.
Multiple layers of such combinations can also be employed, wherein the amounts
of each component of a particular layer may change. Alternatively, a veiled
structure can also be used. In such a veiled structure, the two outside layers
are
comprised of substantially wood pulp fiber, preferably without any polymeric
binder, while the inside layers, whether one or more, comprise the components
of
cellulosic wood pulp fiber, aramid component and polymeric binder. A minor
amount of synthetic fiber, such as polyester or nylon fiber, can be present in
the
outside cellulosic pulp fiber layers.
When multiple layers are to be employed, it is preferred to make the paper
structure using a cylinder machine, as is known in the art, with at least
three
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different cylinders. Different stock compositions can be fed to each of the
cylinders, which correspond to a particular layer of the paper structure.
In a preferred embodiment, the paper structure is comprised of five
different layers. The two outside layers are comprised substantially of
cellulosic,
preferably wood, pulp fiber. The three internal layers are all comprised of
cellulosic pulp fiber, aramid component and a polymeric binder. Optionally,
the
internal layers can be of different compositions. For example, they can
contain
different relative amounts of the cellulosic pulp fiber, aramid component and
polymeric binder, since different stock compositions can be fed to the various
corresponding cylinders to make the various layers. As well, it may be desired
to
have only one layer which contains the aramid component and the polymeric
binder. The remaining layers would then be comprised primarily of cellulosic
pulp fibers, or the relative amounts of fiber/fibrid within the aramid
component
can be changed.
In another embodiment, the paper structure comprises two outer layers
comprised substantially of cellulosic pulp fiber, and the inner layer is
comprised of
the aramid component and polymeric binder. The presence of the aramid
component and polymeric binder together is important, in at least one inner
layer
of the paper structure. The remaining layers may differ in composition, as
long as
the two outside layers do not contain the polymeric binder.
The preferred polymeric binder is polyvinyl alcohol, but other polymeric
binders such as acrylics can also be used. It can be added in the form of a
synthetic fiber or as a dry powder. If the binder is added as a fiber, it is
important
that the fiber has the proper chemical characteristics. Polyvinyl alcohol
fiber is
available with a wide range of water solubilization temperatures. The
temperature
at which the polymer becomes soluble depends on the properties of the polymer
like the degree of polymerization, degree of hydrolysis, and crystallinity.
This
solubilization temperature can range from about 60°C to over
100°C. It is
important to match this solubilization temperature to the paper making
process.
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To be most effective the polyvinyl alcohol fiber should behave as a binder
while it
is in the fiber form. It should not be allowed to fully dissolve. The
strongest
binding occurs when the surface of the fiber just starts to dissolve. Then
upon
drying, the polyvinyl alcohol fiber will bond to all of the other fibers, both
synthetic and natural, that it contacts.
This means that a polyvinyl alcohol fiber with a low solubilization
temperature should be used with a low to medium basis weight paper (roughly 25
to 120 pounds per 3000 square feet) that is typically run at high machine
speeds.
Because of the higher machine speed and low sheet mass, evaporation will cool
the
paper. It will dry before it gets very hot. The maximum temperature that the
paper will reach is likely to be less than 70°C.
With high basis weight papers (200 pounds per 3000 square feet and above)
a polyvinyl alcohol fiber with a higher solubilizafion temperature can be
used.
These papers are typically run at slower machine speeds so that the sheet
temperature is much higher.
When the powder form of the polyvinyl alcohol binder is used, the polymer
should be fully hydrolyzed (99 % or higher) and the polymer should be ground
to a
particle size of 100 mesh or smaller. The powder can be added to the wood
fiber
prior to refining or it can be added to the system after refining. It is
important that
the powdered polymer be allowed to swell after it is added to the paper making
system. Swelling time depends on the water temperature. Cold water (0-
14°C)
requires a swelling period of about one hour. Warm water (40-50°C) will
swell
the particles in about 20 minutes. It is essential that the process water used
with
either polyvinyl alcohol fibers or powder not be over 60°C, as hot
water will
dissolve the polymer and most of the bonding characteristics will be lost.
It is advantageous to use a steam shower with the powder form of the
polyvinyl alcohol binder. This shower should hit the paper prior to the dryer
section. The steam shower is particularly useful with low basis weight papers.
It
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will heat the sheet while it is still wet thus allowing the outside of the
swollen
polymer particles to begin to dissolve.
In preparing the "veiled" embodiment of the present invention, a cylinder
machine, as is well known in the art, can be and is preferably employed. The
cylinder machine allows for the creation of different layers using different
stock
compositions, as discussed above, thus allowing the paper structure to be
tailored
as needed within the present invention.
The process for making a veiled paper structure comprises feeding a stock
composition comprised substantially of wood pulp fiber to the cylinders
corresponding to the outer layers. Thus, the two outside layers of the
resulting
paper structure comprises substantially cellulosic, preferably wood, pulp
fibers. A
minor amount of synthetic fibers can be included in the stock compositions.
A cylinder corresponding to the inner layer is then fed with a stock solution
comprised of cellulosic pulp fiber, the chosen aramid component, whether
fiber,
fibrid or a mixture thereof, and a polymeric binder. Thus, the inner layer of
the
paper structure is comprised of the cellulosic pulp fiber, aramid component
and
polymeric binder. The resulting paper structure is such that only the inner
layer
contains the polymeric binder, whereas the outside layers do not, and thus
potential sticking problems are avoided when the paper structure is dried,
preferably on drier cans, and the polymeric binder is activated due to the
high
temperature. Upon activation of the polymeric binder, the binder acts to bind
the
aramid component together with the wood pulp fiber, and since it is on the
inside
layer it will not cause sticking problems.
The paper structure of the present invention, whether of a single layer or
multiple layers, provides one with a paper quite useful as a high temperature
paper
for transformers. The paper exhibits enhanced thermal resistance, as well as
excellent mechanical strength to perform all of the necessary functions of a
transformer E-board.
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While the invention has been described with preferred embodiments, it is to
be understood that variations and modifications may be resorted to as will be
apparent to those skilled in the art. Such variations and modifications are to
be
considered within the purview and the scope of the claims appended hereto.
