Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
Electroconductive Aramid Paper
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electroconductive aramid paper suitable for
electrostatic discharge interference and/or electromagnetic interference
shielding.
2. Description of Related Art
NOMEX Type 843 Conductive Carbon Blend aramid paper
consists of NOMEX brand floc and fibrids blended with conductive
carbon fibers. This paper has been available in both hot calendered and
uncalendered versions. The uncalendered version of this paper has a
basis weight of about 40 g/mZ, a density of about 0.29 g/cm3, and a tensile
strength of about 16 N/cm, which corresponds to tensile index of 40
N*m/g, and can be easily saturated with polymer resins. However, it has
been found that this paper does not have adequate tensile strength for
automated tape winding of conductors, resulting in breakage and tearing
of aramid tapes when wrapped using the under the tensions normally used
by automatic winding devices. The hot calendered version of this paper
has an improved tensile strength of about 35 N/cm (a tensile index about
90 N*m/g) and is strong enough for the automated tape winding; however,
this calendered paper is less saturable and less formable, because after
calendering the resulting paper is denser (about 0.64 g/cm3). The
saturability of the paper is important for paper used as electrical insulation
because in many applications the insulation is wrapped around a part, and
the wrapped part is then impregnated with a polymer resin to substantially
eliminate any air voids in the wrapping and to reduce the non-uniformity of
electrical field and subsequent premature failure of the insulation. After
the paper is wrapped around a part or another wrapping, the paper must
be porous enough to allow polymeric resins to pass through the paper to
fully impregnate both the paper and any other wrappings that might be
present.
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It is also des'ired that the conductive paper have a certain level of
surface resistivity to avoid buildup of charge and provide an optimum
electrical shielding in the particular application. Thus, a preferable surface
resistivity of conductive tapes for the outside layers of the main wall
insulation of coils in stators of high voltage motors is in the about 100 to
400 ohms/in2 range. Also, it is very important to have a manufacturing
process which allows a good control of surface resistivity of the final
paper. The surface resistivity of the hot calendered lightweight NOMEX@
paper type 843 (about 700 ohms/in2 in the machine direction and about
1800 ohms/in 2 in the cross direction) is about seven times that of the
uncalendered paper (95 ohms/in2 in the machine direction and 250
ohms/in2 in the cross direction).
U.S. Patent Nos. 2,999,788 to Morgan; 3,756,908 to Gross; and
4,481,060 to Hayes disclose papers based on fibrids from synthetic
polymers including papers from aromatic polyamide (aramid) fibrids and
their combination with different fibers.
U.S. Patent No. 5,233,094 to Kirayoglu et al. discloses a process
for making strong paper comprising 45-97% by weight of p-aramid fiber,
3-30% by weight of m-aramid fibrids and 0-30% by weight of quartz fiber.
The paper is produced by forming, calendering, and additional high
temperature heat treatment at least at 510 F (266 C).
U.S. Patent No. 5,126,012 to Hendren et al. discloses high strength
aramid paper from floc and fibrids, and carbon fiber is among the possible
types of the floc. Necessary mechanical properties are achieved after hot
compression of the paper in the press at a temperature of 279 C.
U.S. Patent No. 5,316,839 to Kato et al. discloses multilayered
aramid paper with conductive fibers in the conductive layer of the
structure. The paper is prepared by forming followed by hot compression
or hot calendering at or above the glass transition temperature of
polymetaphenylene isophthalamide (275 C).
Previously, aramid papers with conductive fillers required hot
calendering or hot compression to make the paper stronger and thereby
suitable for automated tape winding. At the same time, calendering or hot
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compressPon significantly changes the electrical properties of the paper, as
well as reducing its free volume and ability to be saturated and
impregnated by a resin. What is needed therefore is a conductive aramid
paper that has the desired electrical properties, is saturable by resins, and
is also strong enough to be processed in automated tape winding
machines.
BRIEF SUMMARY OF THE INVENTION
This invention relates to aramid paper comprising 5 to 65 parts by
weight aramid fiber, 30-90 parts by weight aramid fibrids, and 1-20 parts
by weight of conductive filler, based on the total weight of the aramid fiber,
fibrids, and filler; the paper having an apparent density of not more than
0.43 g/cm3 and a tensile index not less than 60 Nm/g.
The invention is also directed to processes for making aramid
paper comprising the steps of forming an aqueous dispersion of 5 to 65
parts by weight aramid fiber, 30-90 parts by weight aramid fibrids, and
1-20 parts by weight of conductive filler, based on the total weight of the
aramid fiber, fibrids, and filler; blending the dispersion to form a slurry;
draining the aqueous liquid from the slurry to yield a wet paper
composition; drying the wet paper composition; and heat treating the
paper at or above the glass transition temperature of the polymer in the
aramid fibrids without consolidation of the paper.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to aramid paper comprising 5 to 65 parts by
weight aramid fiber, 30-90 parts by weight aramid fibrids, and 1-20 parts
by weight of conductive filler, based on the total weight of the aramid fiber,
fibrids, and filler, the paper having an apparent density of not more than
0.43 g/cm3 and a tensile index not less than 60 Nm/g. Surprisingly, the
inventors have found that a strong paper with no significant changes in the
paper free volume or surface resistivity can be made by heat-treating the
formed paper at a temperature of about or above the glass transition
temperature of the aramid polymer of the fibrids but without applying
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subs'tantial pressure to the sheet in the heated state to consolidate or
compress the paper.
The papers of this invention include fiber and fibrids made from
aramid polymers. Aramid polymers are polyamides wherein at least 85%
of the amide (-CO-NH-) linkages are attached directly to two aromatic
rings. Additives can be used with the aramid and it has been found that
up to as much as 10 percent, by weight, of other polymeric material can be
blended with the aramid. Copolymers can be used having as much as 10
percent of other diamines substituted for the diamine of the aramid or as
much as 10 percent of other diacid chlorides substituted for the diacid
chloride of the aramid. Methods for making aramid polymers and fibers
are disclosed in United States Patent Nos. 3,063,966; 3,133,138;
3,287,324; 3,767,756; and 3,869,430. In some preferred embodiments of
this invention the aramid polymers are meta- and para-oriented aramids,
with poly (metaphenylene isophthalamide) and poly (paraphenylene
terephthalamide) being the preferred aramid polymers.
The papers of this invention comprise aramid fiber. In many
embodiments of this invention, the aramid fiber can be in the form of floc
or pulp. By "floc" is meant fibers having a length of about 2 to 25
millimeters, preferably 3 to 7 millimeters; the fibers preferably have a
diameter of about 3 to 20 micrometers, preferably 5 to 14 micrometers. If
the floc length is less than about 2 millimeters it is difficult to make
strong
papers and if the length is more than about 25 millimeters, it is difficult to
form a uniform web by a wet-laid method. If the floc diameter is less than
about 3 micrometers, it can be difficult to produce it with adequate
uniformity and reproducibility, and if it is more than about 25 micrometers,
it is difficult to form a uniform paper having a low to medium basis weight.
Floc is generally made by cutting continuous spun filaments or tows into
specific-length pieces using conventional fiber cutting equipment.
The term "pulp", as used herein, means particles of aramid material
having a stalk and fibrils extending generally therefrom, wherein the stalk
is generally columnar and about 10 to 50 micrometers in diameter and the
fibrils are fine, hair-like members generally attached to the stalk measuring
only a fraction of a micrometer or a few micrometers in diameter and about
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to 100 micrometers long. One possible illustrative process for making
aramid pulp is generally disclosed in United States Patent No. 5,084,136.
The papers of this invention comprise 5 to 65 parts by weight
5 aramid fiber, and in some embodiments 30 to 50 parts by weight are
preferred. It is believed that less that 5 parts by weight results in a paper
that is too brittle and does not have sufficient tear properties, while papers
having more than 65 parts by weight of aramid fibers results in a
corresponding reduction in the amount of fibrids available in the
10 composition to help bind the composition together, which results in an
unacceptable reduction in paper tensile strength. In some embodiments
of this invention, the preferred types of the fiber useful in this invention
are
poly (metaphenylene isophthalamide) floc, poly (paraphenylene
terephthalamide) pulp, and poly (paraphenylene terephthalamide) floc,
with poly (metaphenylene isophthalamide) floc being the most preferred
fiber.
The papers of this invention also comprise aramid fibrids. The term
"fibrids" as used herein, means a very finely-divided polymer product of
small, filmy, essentially two-dimensional, particles known having a length
and width on the order of 100 to 1000 micrometers and a thickness only
on the order of 0.1 to 1 micrometer. Fibrids are made by streaming a
polymer solution into a coagulating bath of liquid that is immiscible with the
solvent of the solution. The stream of polymer solution is subjected to
strenuous shearing forces and turbulence as the polymer is coagulated.
Aramid fibrids can be prepared using a fibridating apparatus where a
polymer solution is precipitated and sheared in a single step as described
in United States Patent No. 3,756,908 or 3,018,091.
The papers of this invention comprise 30 to 90 parts by weight
aramid fibrids. It is believed that papers having less that 30 parts by
weight fibrids do not have adequate tensile strength for most preferred
applications, while papers having more than 90 parts by weight are not
only typically too brittle and do not have sufficient tear properties for many
processing steps, but also such high fibrid content papers have very
limited resin impregnability even at low density. In some embodiments,
the papers of this invention preferably have an aramid fibrid content of
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about 35 to 60 parts by weight. In some embodiments of this invention,
the preferred aramid fibrids of this invention are made from meta-aramid
polymer, with the most preferred meta-aramid being poly (metaphenylene
isophthalamide).
The aramid fiber and fibrids used in the paper of this invention can
be the natural color of the spun filament or can be colored by dyes or
pigments. The fiber can also be treated by materials that alter its surface
characteristics so long as such treatment does not adversely affect the
ability of binders to contact and hold to the fiber surfaces.
The papers of this invention further include a conductive filler. By
"conductive filler" it is meant any fibrous or particulate (such as a powder
or a flake) form having a conductivity over a wide range, such as a
conductivity typical for conductors of greater than about 102
siemens/meter, to a conductivity typical for semiconductors of from about
10"$ to102 siemens/meter). The structure of the conductive filler can be
chosen based on the particular application requirements and the
conductive filler can be relatively homogenous, where substantially all the
volume of the material can conduct electricity (such as metal fibers,
carbon fibers, carbon black, etc.) or the material can be heterogeneous,
where conductive and dielectric parts co-exist in the volume of the material
(such as metal coated fibers or particles, or fibers or particles filled with
conductive ingredients).
The papers of this invention comprise 1 to 20 parts by weight
conductive filler. It is believed that less that 1 part by weight results in a
paper that does not provide an adequate amount of conduction for many
applications, while having more than 20 parts by weight usually results in
noticeable reduction of the paper mechanical properties. In some
preferred embodiments the conductive filler is carbon fiber, and in other
preferred embodiments the conductive filler is carbon black. The most
preferred conductive filler that is useful in many versions of the inventive
paper is carbon fiber.
The papers of this invention have an apparent density of not more
than 0.43 g/cm3 and a tensile index of not less than 60 Nm/g. Such papers
can be used in any interference discharge or shielding application and can
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be easily taped and impregnated with a resin. The apparent density
describes the weight-to-volume ratio of the paper and is determined in
accordance with ASTM D202. The tensile index describes the tensile
strength-to-basis weight (grammage) ratio and is determined in
accordance with ASTM D828. In some embodiments of this invention, the
papers of this invention have a final basis weight of about 30 to 60 g/m2
and have a final thickness of about 0.08 to 0.16 mm.
The papers of this invention are generally impregnated with resins
either prior to or after they are installed in/on an electrical device or
conductor. Such resins include epoxy resins, polyesterimide resins, and
other resin systems. It has been found that it is critical that the papers of
this invention have an apparent density of not more than about 0.43 g/cm3
to be formable and to allow fast impregnation with typical resins. A higher
density provides a structure that is too consolidated to be formable or to
allow fast resin impregnation. Further, it is thought the apparent density of
the paper can be as low as 0.15 g/cm3 or lower, depending on the
application, the resin used, and the amount of resin used.
For preventing electrical discharges in electrical machines, tapes
made from the papers of this invention are generally applied on the
conductor coils using automated tape winding machines, and it has been
found that a tensile index of not less than 60 Nm/g is necessary to avoid
excessive breakout or tearing of the papers in these machines.
Additional ingredients, such as other fillers for the adjustment of
paper corona resistance and other properties, or pigments or antioxidants,
etc., in powder, flake or fibrous form can be added to the paper
composition of this invention, provided they do not affect increase the
apparent density nor reduce the tensile index to unacceptable levels.
This invention also relates to a process for making aramid paper,
comprising the steps of:
a) forming an aqueous dispersion qof 5 to 65 parts by
weight aramid fiber, 30-90 parts by weight aramid fibrids, and 1-20
parts by weight of conductive filler, based on the total weight of the
aramid fiber, fibrids, and filler,
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b) blending the dispersion to form a slurry,
c) draining the water from the slurry to yield a wet paper
composition,
d) drying the wet paper composition, and
e) heat treating the paper at or above the glass transition
temperature of the polymer in the aramid fibrids without
consolidation of the paper.
The first step of this invention involves forming a dispersion of
aramid fiber, aramid fibrids and conductive filler in an aqueous liquid such
as water. The dispersion can be made either by dispersing the fibers and
then adding the fibrids and other materials or by dispersing the fibrids and
then adding the fibers and other materials. The dispersion can also be
made by combining a first dispersion of fibers with a second dispersion of
the fibrids and other materials. Any number of possibilities of combining
fiber, fibrids, and other materials is possible however in one preferred
embodiment the concentration of fibers in the final dispersion is about 0.01
to 1.0 weight percent based on the total weight of the dispersion. In other
preferred embodiments, the concentration of the fibrids in the dispersion is
up to about 95 weight percent based on the total weight of solids.
The aqueous liquid of the dispersion is generally water, but may
include various other materials such as pH-adjusting materials, forming
aids, surfactants, defoamers and the like.
The second step in the process for making the papers of this
invention is blending the dispersion to form a slurry. The dispersion can
be blended in a totally separate step or vessel or the dispersion can be
blended essentially simultaneously while being formed, and the blending
may be accomplished in the same vessel that forms the dispersion.
Blending can be accomplished by any known means, such as by agitation
of the dispersion by, say, a stirring device, or by refining the dispersion in
a refiner, or in some embodiments blending can be accomplished by
pumping the dispersion at a rate to provide adequate turbulence to blend
the materials.
The third step in the process for making the paper of this invention
involves draining the aqueous liquid from the second slurry to yield a wet
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paper composition. In some embodiments, the aqueous liquid is drained
from the dispersion by conducting the dispersion onto a screen or other
perforated support, retaining the dispersed solids and then passing the
liquid to yield a wet paper composition. For example, the papers of this
invention can be formed on equipment of any scale from laboratory
screens to commercial-sized papermaking machinery, such as a
Fourdrinier or inclined wire machines.
The next step in the process for making the paper of this invention
involves drying the wet paper composition. In many embodiments of the
process of this invention the wet paper composition, once formed on the
support or screen, is further dewatered by vacuum or other pressure
forces and further dried by evaporating the remaining liquid using a dryer,
oven, or similar device known in the art for drying webs and papers.
The final step in the process for making the paper of this invention
involves heat treating the paper at or above the glass transition
temperature of the polymer in the fibrids without consolidation of the
paper. For poly (m-phenylene isophthalamide) glass transition is about
275 C.
The heat-treatment can be conducted in line with forming or as a
separate processing step. Surprisingly, the inventors have found that a
strong paper with no significant changes in the paper free volume or
surface resistivity can be made by heat-treating the formed paper at a
temperature of about or above the glass transition temperature of the
aramid polymer of the fibrids but without applying substantial pressure to
the sheet in the heated state to consolidate or compress the paper.
Therefore, this process does not involve any of the preliminary
compression or subsequent calendering steps to consolidate the sheet
structure as is typical in prior art processes. If desired, the paper can be
restrained while heat treated to help reduce shrinkage.
Heat-treatment can be accomplished by any known method of
heating including, but not limited to contact heating with paper touching
hot surface of metal rolls or other hot surfaces, by conventional heating
such as by infrared or hot-air heating in an oven.
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The paper of this invention is useful as a, as a conductive material
with tailored level of electrical properties for electrostatic discharge
interference and/or electromagnetic interference shielding. For example, it
can be used as a conductive tape for electrostatic discharge in the slots of
the stators of high voltage rotating machines.
TEST METHODS
Thickness and Basis Weight (Grammage) were determined for
papers of this invention in accordance with ASTM D 374 and ASTM D 646
correspondingly. At thickness measurements, method E with pressure on
specimen of about 172 kPa was used.
Density (Apparent Density) of papers was determined in
accordance with ASTM D 202.
Tensile Index was determined based on the tensile test on an
Instron-type testing machine using test specimens 2.54 cm wide and a
gage length of 12.7 cm in accordance with ASTM D 828.
Surface Resistivity was measured in accordance with ASTM D 257
on about 2.54 cm wide strips of the paper.
EXAMPLES
Physical properties of all the paper samples made in the examples
are shown in the Table.
Example I
An aqueous dispersion was made of never-dried poly
(metaphenylene isophthalamide) (MPD-1) fibrids at a 0.5% consistency
(0.5 weight percent solid materials in water). Carbon fiber was added to
this dispersion. After about ten minutes of continued agitation, additional
water and meta-aramid floc were added with additional agitation of about
ten minutes to completely blend the materials and to yield a slurry having
a final consistency of 0.35%. The final slurry was comprised of the
following solids by weight: 39% MPD-I floc, 50% MPD-1 fibrids, and 11 %
carbon fiber.
The MPD-1 fibrids were made using the general method as
disclosed and described in U.S. Pat No. 3,756,908. The MPD-1 floc had a
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linear density of'0.22 tex (2.0 denier), a cut length of 0.64 cm, and an
initial modulus of about 800 cN/tex (sold by DuPont under the trade name
NOMEXO). The carbon fiber was FORTAFIL fiber type 150 (length of 0.32
cm), available from FORTAFIL Inc.
The slurry was pumped to a supply chest and fed from there to a
Fourdrinier machine to make paper having a basis weight of about 30.9
g/m2. The paper was then heat treated by surface contact on heated
metal rolls having a surface temperature of about 320 C and a contact
residence time of about 7 seconds. A 2 cm wide tape made from this
paper was successfully wrapped without breakage or tearing on a coil
using an automated winding process.
Example 2
A slurry was prepared as in Example 1, however the final slurry was
comprised of the following solids by weight: 40% MPD-1 floc, 50% MPD-1
fibrids, and 10% carbon fiber. A paper with a basis weight of 50.2 g/m2
was formed on a Fourdrinier and additionally heat-treated as in Example
1. A 2 cm wide tape from this paper was successfully wrapped without
breakage or tearing on a coil using an automated winding process.
Example 3
A slurry was prepared as in Example 1, however the final slurry was
comprised of the following solids by weight: 44% MPD-1 floc, 50% MPD-1
fibrids, and 6% carbon fiber. A paper with a basis weight of 53.9 g/m2 was
formed on a Fourdrinier and additionally heat-treated as in Example 1. A
2 cm wide tape from this paper was successfully wrapped without
breakage or tearing on a coil using an automated winding process.
Example 4
A slurry was prepared as in Example 1, however the final slurry was
comprised of the following solids by weight: 60% MPD-1 floc, 40% MPD-1
fibrids, and 10% carbon fiber. A paper with a basis weight of 45.8 g/m2
was formed on a Deltaformer inclined wire machine and additionally heat-
treated as in Example 1. A 2 cm wide tape from this paper was
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successfully wrapped without breakage or tearing on a coil using an
automated winding process.
Example 5
172 g of an aqueous, never-dried, meta-aramid fibrid slurry (0.58%
consistency and freeness 330 ml of Shopper-Riegler), 0.34 g of carbon
black and 0.66 g of meta-aramid floc were placed together in a laboratory
mixer (British pulp evaluation apparatus) with about 1600 g of water and
agitated for 1 min. The final slurry was comprised of the following solids
by weight: 33% MPD-1 floc, 50% MPD-1 fibrids, and 17% carbon black.
The MPD-1 floc and MPD-I fibrids were the same as described in
Example 1. The carbon black was KetjenblackOEC300J produced by
Akzo Nobel Co. The dispersion was poured, with 8 liters of water, into an
approximately 21 x 21 cm handsheet mold and a wet-laid sheet was
formed. The sheet was placed between two pieces of blotting paper, hand
couched with a rolling pin and dried in a handsheet dryer at 190 C. After
drying, the sheet was heat treated in a restrained position (fixed by metal
clips to a metal plate) in an oven at 300 C for 20 min.
Comparative Example A
A paper was prepared as in Example 5, but without additional heat
treatment after drying. As a result, tensile index of the paper was
significantly lower than necessary for the automated taping operation.
Comparative Example B
A paper was prepared as in Example 5, but instead of additional
heat treatment after drying, the sheet was passed through the nip of a
metal-metal calender with a roll diameter of about 20 cm at a temperature
of about 300 C and a linear pressure of about 3000 N/cm.
Comparative Examples C-F
Papers were formed as described in Examples 1-4 correspondingly,
but additional heat-treatment was not conducted. During automated
taping of 2 cm wide tapes from these papers, breaks occurred.
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Comparative Example G
The paper from Example 1 was passed through the nip of a metal-
metal calender with a roll diameter of about 20 cm at a temperature of
about 300 C and a linear pressure of about 1200 N/cm.
Comparative Example H
A paper was formed as described in Example 2, calendered in the
soft nip calender at ambient temperature and linear pressure of 870 N/cm,
and heat treated at the same conditions as described in Example 1.
As can be seen from Table 1, the tensile index of the inventive
papers (Examples 1-5) ranges from 61 to 87 N/cm, which is close to
tensile index for calendered paper of the same composition (Examples B,
G, & H) which range from 68-85; however, apparent density values for the
inventive papers (Examples 1-5) ranges between 0.28 to 0.41 g/cm3 are
almost the same as for the formed precursor paper represented by
Examples A & C-F, which range between 0.27 to 0.40 g/cm3.
Surface resistivity of the inventive papers is also very close to
surface resistivity of the formed precursors (compare Examples 1 and C, 2
and D, 3 and E, 4 and F, 5 and A). The biggest difference in resistivity for
formed and heat treated papers versus formed papers is for the pair of
Examples 3 and E (the change in about 2.4 times), but it is still much lower
than after calendering (described below).
Examples G and H illustrate that the surface resistivity of
calendered papers with carbon fiber is much higher than the resistivity of
the formed precursors represented by Examples C and D or formed and
heat treated paper represented by Examples 1 and 2.
Examples A and B illustrate that the surface resistivity of
calendered paper with carbon black (Example B) is 10 times lower that the
resistivity of the corresponding formed precursor (Example A). This
reaction, which is different from that of papers made with carbon fibers, is
believed to be due to the brittleness of carbon fiber and there is significant
crushing and length reduction of these fibers when they are compressed in
the nip of the calender, resulting in a corresponding increase in the
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surface resistivity. Such effect can be less pronounced for heavier papers,
but for practically important lightweight papers (60 g/m2 and less) this is
very negative factor. Also, more uniform paper formation can reduce the
scale of the effect; however, the economics of the paper manufacturing
always limits such opportunity. In the case of such conductive powder
filler as a carbon black, it is believed that there is significant reduction
in
the paper resistivity after calendering due to the higher volume
concentration of the conductive elements of the structure (i.e., the
particles) without any change to their individual size. The main problem
with calendering of the papers with both types of conductive fillers (carbon
fiber and carbon black), as shown in the examples, is the dramatic change
in surface resistivity after calendering.
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