Note: Descriptions are shown in the official language in which they were submitted.
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SEMICONDUCTIVE JACKET FOR CABLE AND
CABLE JACKETED THEREWITH
CROSS REFERENCE TO RELATED APPLICATION
This application is related to United States Patent No. 6,514,608.
FIELD OF INVENTION
The present invention relates generally to cables, and more particularly to
compositions
suitable for semiconductive jackets especially for medium and high voltage
power cables and
cables jacketed therewith.
BACKGROUND OF THE INVENTION
Electric power cables for medium and high voltages typically include a core
electrical
conductor, an overlaying semiconductive shield, an insulation layer formed
over the
semiconductive shield, an outermost insulation shield, and some type of
metallic component.
The metallic component may include, for example, a lead sheath, a
longitudinally applied
corrugated copper tape with overlapped seam, or helically applied wires,
tapes, or flat strips.
U.S. Patent No. 5,281,757 assigned to the current assignee, and U.S. Patent
No. 5,246,783,
disclose examples of electric power cables and methods of making the same.
Electric power cables for medium and high voltage applications also typically
include an
overall extruded plastic jacket which physically protects the cable thereby
extending the useful
life of the cable. The afore-described overall jacket may be insulating or
semiconducting. If the
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overall jacket is insulating, it may overlay or encapsulate the metallic
component of the cable as
discussed in the September/October 1995 Vol. 11, No. 5, IEEE Electrical
Insulation Magazine
article, entitled, Insulating and Semiconductive Jackets for Medium and High
Voltage
Underground Power Cable Applications.
According to the National Electrical Safety Code, power cables employing
insulating
jackets must be grounded every 0.125 to 0.25 mile depending on the
application, or at every
splice for cable in duct (at every manhole). Such grounding reduces or
eliminates the losses in a
cable system. Furthermore, as the neutral to ground voltage may be very high,
such grounding is
also required for safety purposes.
In contrast to insulating jackets, semiconductive jackets are advantageously
grounded
throughout the length of the cable and therefore do not need periodic
grounding previously
described. Accordingly, semiconductive jackets are only grounded at the
transformer and at the
termination.
Although semiconductive jackets are advantageous for the foregoing reasons,
they are not
widely employed in the power cable industry. Prior art semiconductive jacket
materials were
usually not developed for jacketing applications, and as such, often do not
meet performance
criteria for long-life cable protection.
The Insulated Cable Engineers Association (ICEA) specifies in ICEA S-94-649-
1997,
"Semiconducting Jacket Type 1", mechanical properties for semiconductive
electrical cable,
jackets and references American Society for Testing and Materials (ASTM) test
methods to test
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materials suitable for these applications.
Prior art semiconductive jackets, even if they do meet performance criteria
for long-life
cable protection, are often cost prohibitive for widespread industry
employment. This high cost
is primarily due to the high weight percentage of conductive additive
necessary in the jacket
material to make the jacket semiconductive. Typically this weight percentage
is greater than 15
to 30 weight percent to achieve the required conductivity or volume
resistivity for the jacket.
See, for example, U.S. Patent No. 3,735,025, which discloses an electric cable
jacketed
with a thermoplastic semiconducting composition comprising chlorinated
polyethylene,
ethylene ethyl acrylate, and 30 to 75 or 40 to 60 parts by weight of
semiconducting carbon
black.
Prior art polymer compounds used in the role of a semiconductive jackets are
normally
thermoplastic and get their conductivity by use of a large weight percentage
of a conductive filler
material, usually conductive grades of carbon black, to incur a high level of
conductivity (or low
level of resistivity), to the compound. The National Electrical Safety Code
(Section 354D2-c)
requires a radial resistivity of the semiconducting jacket to be not more than
100 0=m and shall
remain essentially stable in service. Prior art compositions required loadings
of conductive filler
material of at least about 15% to 60% by weight to achieve this criteria.
These high levels of
conductive filler material inherently add significantly to the cost of such
compositions, inhibit
the ease of extrusion of the jacketing composition, and decrease the
mechanical flexibility of the
resultant cable.
Percolation theory is relatively successful in modeling the general
conductivity
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characteristics of conducting polymer composite (CPC) materials by predicting
the convergence
of conducting particles to distances at which the transfer of charge carriers
between them
becomes probable. The percolation threshold (pc), which is the level at which
a minor phase
material is just sufficiently incorporated volumetrically into a major phase
material resulting in
both phases being co-continuous, that is, the lowest concentration of
conducting particles needed
to form continuous conducting chains when incorporated into another material,
can be
determined from the experimentally determined dependence of conductivity of
the CPC material
on the filler concentration. For a general discussion on percolation theory,
see the October 1973
Vol. 45, No. 4, Review of Modem Physics article, entitled, Percolation and
Conduction.
Much work has been done on determining the parameters influencing the
percolation
threshold with regard to conductive filler material. See for example, Models
Proposed to
Explain the Electrical Conductivity of Mixtures Made of Conductive and
Insulating
Materials, 1993 Journal of Materials Science, Vol. 28; Resistivity of Filled
Electrically
Conductive Crosslinked Polyethylene, 1984 Journal of Applied Polymer Science,
Vol 29;
and Electron Transport Processes in Conductor-Filled Polymers, 1983 Polymer
Engineering and Science Vol. 23, No. 1. See also, Multiple Percolation in
Conducting
Polymer Blends, 1993 Macromolecules Vol. 26, which discusses "double
percolation".
Attempts for the reduction of conductive filler content in CPC materials have
been
reported for polyethylene/polystyrene and for polypropylene/polyamide, both
employing carbon
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black as the conductive filler. See Design of Electrical Conductive
Composites: Key role of the
Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends,
1995
Macromolecules, Vol. 28 No. 5 and Conductive Polymer Blends with Low Carbon
Black
Loading: Polypropylene/Polyamide, 1996 Polymer Engineering and Science, Vol.
36, No. 10.
However, none of the prior art concerned with minimizing the conductive filler
content
has addressed materials suitable for use as a semiconductive jacket material
for cables which
must meet not only the electrical requirements, but also stringent mechanical
requirements as
discussed heretofore.
What is needed, and apparently lacking in the art is a semiconductive jacket
material
which has a significant reduction of conductive filler mater ial, thereby
decreasing the cost of the
material and the processing by increasing the ease of extrusion and mechanical
flexibility of the
jacketed cable, while maintaining industry performance criteria for
resistivity and mechanical
properties.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide a conductive polymer composition
(CPC) material for semiconductive jackets for cables which has a significant
reduction in
conductive filler content while maintaining the required conductivity and
mechanical
properties specified by industry by selecting materials and processing
approaches to reduce the
percolation threshold of the conductive filler in the composite, while
balancing the material
selection with the industry required mechanical properties of the
semiconductive jacket.
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The present inventive semiconductive jackets for cables share certain
attributes
with U.S. Patent No. 6,277,303, entitled, Conductive Polymer Composite
Materials
And Methods of Making Same, filed on an even date with the parent of this
application,
July 10, 1998, by the same applicant. That is, the semiconductive jacket
materials of
embodiments of the present invention are based on immiscible polymer blends
wherein
the immiscibility is exploited to create semiconductive compounds with low
content
conductive filler through a multiple percolation approach to network
formation. The conductive
filler material content can be reduced to about 10% by weight of the total
composite or less,
depending on the conductive filler material itself and the selection of major
and minor phase
materials, without a corresponding loss in the conductivity performance of the
compound.
Correspondingly, the rheology of the melt phase of the inventive material will
more closely
follow an unfilled system due to the reduction of the reinforcing conductive
filler content thereby
increasing the ease of processing the material.
Semiconductive jackets for power cables must have a conductive network
throughout the
material. The physics of network formation of a minor second phase material in
a differing
major phase is effectively described by percolation theory as discussed
heretofore. The
"percolation threshold" (p.) is the level at which a minor phase material is
just sufficiently
incorporated volumetrically into a major phase material resulting in both
phases being co-
continuous, that is, the lowest concentration of conducting particles needed
to form continuous
conducting chains when incorporated into another material. A minor second
phase material in
the form of nonassociating spheres, when dispersed in a major phase material,
must often be in
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excess of approximately 16% by volume to generate an infinite network. This 16
volume %
threshold, which is exemplary for spheres, is dependent on the geometry of the
conductive filler
particles, (i.e. the surface area to volume ratio of the particle) and may
vary with the type of
filler. The addition of a single dispersion of conductor filler particles to a
single major phase is
termed "single percolation". It has been found that by altering the morphology
of the
minor/major phase a significant reduction in percolation threshold can be
realized. The present
invention exploits these aspects of percolation theory in developing very low
conductive filler
content semiconductive jacket materials for cables.
In accordance with an embodiment of the present invention, a method requiring
an immiscible
1D blend of at least two polymers that phase separate into two continuous
morphologies is employed.
By requiring the conductive filler to reside in the minor polymer phase, the
concentration of
conductive filler can be concentrated above the percolation threshold required
to generate a
continuous conductive network in the minor polymer phase while the total
concentration of
conductive filler in the volume of the combined polymers is far below the
threshold if the filler
was dispersed uniformly throughout both phases. In addition, since the minor
polymer phase is
co-continuous within the major polymer phase, the concentration is conductive.
This approach
employs multiple percolation due to the two levels of percolation that are
required: percolation
of conductive dispersion in a minor phase and percolation of a minor phase in
a major phase.
In a binary mixture of a semicrystalline polymer and a conductive filler, the
filler
particles are rejected from the crystalline regions into the amorphous regions
upon
recrystallization, which accordingly decreases the percolation threshold.
Similarly, using a
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polymer blend with immiscible polymers which results in dual phases as the
matrix in CPC
materials promotes phase inhomogeneities and lowers the percolation threshold.
The conductive
filler is heterogeneously distributed within the polymers in this latter
example. In one alternative
of this approach, either one of the two polymer phases is continuous and
conductive filler
S particles are localized in the continuous phase. In a second alternative,
the two phases are co-
continuous and the filler is preferably in the minor phase or at the
interface.
Some embodiments of the present invention concentrate primarily on two aspects
of percolation phenomenon: the interaction of the conductive dispersion in the
minor
phase, and the interaction of the minor phase with the major phase. Further,
the foregoing
approach as disclosed in the afore-referenced U.S. Patent No. 6,277,303,
entitled,
Conductive Polymer Composite Materials and Methods of Making Same may be
employed
and has been optimized and balanced for semiconductive jacket applications.
In accordance with one aspect of the present invention, a semiconductive
jacket material
for jacketing a cable comprises: a minor phase material comprising a
semicrystalline polymer; a
conductive filler material dispersed in said minor phase material in an amount
sufficient to be
equal to or greater than an amount required to generate a continuous
conductive network in said
minor phase material; and a major phase material, said major phase material
being a polymer
which when mixed with said minor phase material will not engage in
electrostatic interactions
that promote miscibility, said major phase material having said minor phase
material dispersed
therein in an amount sufficient to be equal to or greater than an amount
required to generate a
continuous conductive network in said major phase material, forming a
semiconductive jacket
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material of a ternary composite having distinct co-continuous phases.
In accordance with another aspect of the present invention, the ternary
composite has a
volume resistivity of about s 100 A-m, an unaged tensile strength of at least
about 1200 psi, a
tensile strength of at least about 75% of said unaged tensile strength after
aging in an air oven at
100 C for 48 hours, an aged and unaged elongation at break of at least about
100%, a heat
distortion at 90 C of at least about -25%, and a brittleness temperature of
about s-10 C.
In accordance with another aspect of the present invention, the conductive
filler material
comprises about s 10 percent by weight of total conducting polymer composite
weight.
In accordance with yet another aspect of the present invention, the
semiconductive jacket
material further comprises a second major phase material, wherein said ternary
composite is
dispersed in an amount sufficient for said ternary composite to be continuous
within said second
major phase material, said second major phase material being selected from a
group of polymers
which when mixed with said ternary composite will not engage in electrostatic
interactions that
promote miscibility with said minor phase material or said major phase
material, forming a
semiconductive jacket material of a quatemary composite having distinct co-
continuous phases.
In accordance with a further aspect of the present invention, a method of
producing a
semiconductive jacket material for jacketing a cable comprises: mixing a
semicrystalline
polymer having a melting temperature in a mixer, said mixer preheated to above
the melting
temperature of said semicrystalline polymer; adding a conductive filler
material to said
semicrystalline polymer in said mixer in an amount z an amount required to
generate a
continuous conductive network in said semicrystalline polymer; mixing said
conductive filler
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material and said semicrystalline for a time and at a speed sufficient to
insure a uniform
distribution of said conductive filler in said semicrystalline polymer,
thereby forming a binary
composite; and mixing a major phase material having a melting temperature with
said binary
composite in said mixer preheated to above the melting temperature of said
major phase material,
for a time and at a speed sufficient to insure a uniform distribution of said
binary composite in
said major phase material, such that a weight ratio of said binary composite
to said major phase
material is sufficient for said binary composite to be ~ an amount required to
generate a
continuous conductive network in said major phase material, said major phase
material being
selected from a group of polymers which when mixed with said binary composite
will not
engage in electrostatic interactions that promote miscibility, such that a
semiconductive jacket
material of a ternary composite with distinct co-continuous phases is formed.
In accordance with yet a further aspect of the present invention, a method of
producing a
semiconductive jacket material for jacketing a cable comprises: mixing a
semicrystalline minor
phase polymer material with a conductive filler material, the conductive
filler material being in
l5 an amount sufficient to be equal to or greater than an amount required to
generate a continuous
conductive network within the minor phase polymer material, thereby forming a
binary
composite; mixing the binary composite with a major phase polymer material to
form a
semiconductive jacket material of a ternary composite having distinct phases;
and annealing the
ternary composite to coarsen the morphology and thereby further increase
conductivity of the
jacket material, said major phase polymer material being selected from a group
of polymers
which when mixed with said binary composite will not engage in electrostatic
interactions that
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promote miscibility, such that a semiconductive tecnary composite with
distinct co-continuous
phases is formed.
In accordance with yet a further aspect of the present invention, a method of
producing a
semiconductive jacket material for jacketing a cable comprises: mixing a
semicrystalline minor
phase polymer material having a melting temperature with a conductive filler
material, the
conductive filler material being in an amount sufficient to be equal to or
greater than an amount
required to generate a continuous conductive network within the minor phase
polymer material,
thereby forming a binary composite; annealing the binary composite; and mixing
the binary
composite with a major phase material at a temperature below the melting
temperature of the
binary composite, said major phase polymer material being selected from a
group of polymers
which when mixed with said binary composite will not engage in electrostatic
interactions that
promote miscibility, thereby forming a semiconductive jacket material of a
ternary composite
having distinct co-continuous phases.
In accordance with a further aspect of the present invention, a method of
producing a
semiconductive jacket material for jacketing a cable comprises: mixing a
semicrystalline minor
phase polymer material with a conductive filler material, the conductive
filler material being in
an amount sufficient to be equal to or greater than an amount required to
generate a continuous
conductive network within the minor phase polymer material, thereby forming a
binary
composite; mixing the binary composite with a major phase polymer material to
form a ternary
composite; mixing the ternary composite with a second major phase polymer
material to form a
semiconductive jacket material of a quaternary composite having distinct
phases; and annealing
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the quaternary composite to coarsen the morphology and thereby further
increase the
conductivity of the jacket material, said major phase polymer material being
selected from a
group of polymers which when mixed with said binary composite will not engage
in electrostatic
interactions that promote miscibility, such that a semiconductive ternary
composite with distinct
co-continuous phases is formed.
In accordance with a fu.rther aspect of the present invention, a cable
comprises at least
one transmission medium and a semiconductive jacket surrounding said
transmission medium,
said semiconductive jacket comprising: a minor phase material comprising a
semicrystalline
polymer; a conductive filler material dispersed in said minor phase material
in an amount
sufficient to be equal to or greater than an amount required to generate a
continuous conductive
network in said minor phase material; and a major phase material, said major
phase material
being a polymer which when mixed with said minor phase material will not
engage in
electrostatic interactions that promote miscibility, said major phase material
having said minor
phase material dispersed therein in an amount sufficient to be equal to or
greater than an amount
required to generate a continuous conductive network in said major phase
material, forming a
semiconductive jacket material of a ternary composite having distinct co-
continuous phases.
In general, the superior results of embodiments of the present invention may
be achieved
by allowing the conductive filler material to reside in a minor phase of the
immiscible blend; the
minor phase being a semicrystalline polymer having a relatively high
crystallinity, such as between
about 30% and about 80%, and illustratively about _70%, thereby causing the
conductive filler
aggregates to concentrate in amorphous regions of the minor phase or at the
interface of the
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continuous minor and major phases. Annealing processes of the composite at
different points in
the mixing process or modifying the morphology of the minor phase can further
increase the
crystalline phase or further coarsen the morphology of the blend and thereby
improve the
conductive network.
In accordance with another aspect of the present invention, in order that a
favorable phase
morphology, that is, phase separation, develops between minor and major phase
materials, the minor
and major phase materials must be such that when mixed, the minor and major
phase polymeric
materials do not engage in electrostatic interactions that promote miscibility
resulting in a negative
enthalpy of mixing. Thus, hydrogen bonding does not occur between any of the
phases and there
is phase separation between all of the phases. Furthermore, the solubility
parameter difference
(S,, - SB ) of the minor and major phase materials in the ternary composites
of the present
invention meet the following criteria for immiscibility:
ULz(S,-SB)2z0
Where,
UL = 7, more preferably 5;
Sõ = the solubility parameter of the minor phase material; and
8g = the solubility parameter of the major phase material.
The Hoftyzer-Van Krevelen definition of solubility parameter has been adopted.
See,
D.W. Van Krevelen, "Properties of Polymers", Third Edition, Elsevier Science
B.V.,
Amsterdam, 1990.
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According to another aspect of the invention,
there is provided a semiconductive jacket material for
jacketing a cable, comprising: a minor phase material
comprising a semicrystalline polymer; a conductive filler
material dispersed in and residing in said minor phase
material in an amount of about 6% by weight or less to form
a binary composite; and a major phase material, said major
phase material being a polymer which when mixed with said
binary composite forms a ternary composite of an immiscible
polymer blend, said major phase material having said binary
composite dispersed therein in an amount sufficient to be
equal to or greater than an amount required to generate a
continuous conductive network in said major phase material,
said ternary composite being free of hydrogen bonding and
having co-continuous distinct phases, a volume resistivity
of about <_ 100 S2=m, an unaged tensile strength of at least
about 1200 psi, a tensile strength of at least about 75%
of said unaged tensile strength after aging in an air oven
at 100 C. for 48 hours, an aged and unaged elongation at
break of at least about 100%, a heat distortion at 90 C. of
at least about -25%, and a brittleness temperature of
about _ -10 C .
A further aspect of the invention provides a
semiconductive jacket material for jacketing a cable,
comprising: a minor phase material comprising a
semicrystalline polymer selected from the group consisting
of high density polyethylene, polypropylene, polypropene,
poly-l-butene, poly(styrene), polycarbonate, poly(ethylene
terephthalate), polyethylene, nylon 66 and nylon 6; a
conductive filler material selected from the group
consisting of carbon black, polyacetylene, polyaniline,
polypyrrole, graphite and carbon fibers, dispersed in and
residing in said minor phase material in an amount of
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about 10% by weight or less to form a binary composite; and
a major phase material selected from the group consisting of
poly(ethylene-co-vinyl acetate), polybutylene terephthalate,
poly(styrene), poly(methyl methacrylate), polyethylene,
polypropylene, polyisobutylene, poly(vinyl chloride),
poly(vinylidene chloride), poly(tetrafluoroethylene),
poly(vinyl acetate), poly(methyl acrylate),
polyacrylonitrile, polybutadiene, poly(ethylene
terephthalate), poly(8-aminocaprylic acid) and
poly(hexamethylene adipamide), said major phase material
which when mixed with said binary composite forms a ternary
composite of an immiscible polymer blend, said major phase
material having said binary composite dispersed therein in
an amount sufficient to be equal to or greater than an
amount required to generate a continuous conductive network
in said major phase material, said ternary composite being
free of hydrogen bonding and having co-continuous distinct
phases, a volume resistivity of about < 100 Q=m, an unaged
tensile strength of at least about 1200 psi, a tensile
strength of at least about 75% of said unaged tensile
strength after aging in an air oven at 100 C. for 48 hours,
an aged and unaged elongation at break of at least 100%, a
heat distortion at 90 C. of at least about -25%, and a
brittleness temperature of about <_ -10 C.
There is also provided a cable comprising at least
one transmission medium and a semiconductive jacket
surrounding said transmission medium, said semiconductive
jacket comprising: a minor phase material comprising a
semicrystalline polymer; a conductive filler material
dispersed in and residing in said minor phase material in an
amount of about 6% by weight or less to form a binary
composite; and a major phase material, said major phase
material being a polymer which when mixed with said binary
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composite forms a ternary composite of an immiscible polymer
blend, said major phase material having said binary
composite dispersed therein in an amount sufficient to be
equal to or greater than an amount required to generate a
continuous conductive network in said major phase material,
said ternary composite being free of hydrogen bonding and
having distinct co-continuous phases, a volume resisitivity
of about <_ 100 S2=m, an unaged tensile strength of at least
about 1200 psi, a tensile strength of at least about 75% of
said unaged tensile strength after aging in an air oven
at 100 C. for 48 hours, an aged and unaged elongation at
break of at least about 100%, a heat distortion at 90 C. of
at least about -25%, and a brittleness temperature of
about < -10 C.
In accordance with a still further aspect of the
invention, there is provided a semiconductive jacket
material for jacketing a cable, comprising: a minor phase
material comprising a semicrystalline polymer; a conductive
filler material dispersed in and residing in said minor
phase material in an amount of about 10% by weight or less
to form a binary composite; and a major phase material, said
major phase material being a polymer which when mixed with
said binary composite forms a ternary composite of an
immiscible polymer blend, said major phase material having
said binary composite dispersed therein in an amount
sufficient to be equal to or greater than an amount required
to generate a continuous conductive network in said major
phase material, said ternary composite being free of
hydrogen bonding and having co-continuous distinct phases, a
volume resistivity of about <- 10052=m, an unaged tensile
strength of at least about 1200 psi, a tensile strength of
at least about 75% of said unaged tensile strength after
aging in an air oven at 100 C. for 48 hours, an aged and
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unaged elongation at break of at least about 100%, a heat
distortion at 90 C. of at least about -25%, and a
brittleness temperature of about - -10 C.
According to another aspect of the invention,
there is provided a semiconductive jacket material for
jacketing a cable, comprising: a minor phase material
comprising a semicrystalline polymer having a crystallinity
from about 30% to about 80%; a conductive filler material
dispersed in and residing in said minor phase material in an
amount of about 10% by weight or less to form a binary
composite; and a major phase material, said major phase
material being a polymer which when mixed with said binary
composite forms a ternary composite of an immiscible polymer
blend, said major phase material having said binary
composite dispersed therein in an amount sufficient to be
equal to or greater than an amount required to generate a
continuous conductive network in said major phase material,
said ternary composite being free of hydrogen bonding and
having co-continuous distinct phases.
A further aspect of the invention provides a
semiconductive jacket material for jacketing a cable,
comprising: a minor phase material comprising a
semicrystalline polymer; a conductive filler material
dispersed in and residing in said minor phase material in an
amount of about 10% by weight or less to form a binary
composite; a major phase material, said major phase material
being a polymer which when mixed with said binary composite
forms a ternary composite of an immiscible polymer blend,
said major phase material having said binary composite
dispersed therein in an amount sufficient to be equal to or
greater than an amount required to generate a continuous
conductive network in said major phase material, said
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ternary composite being free of hydrogen bonding and having
co-continuous distinct phases; and a second major phase
material, wherein said ternary composite is dispersed in an
amount sufficient for said ternary composite to be
continuous within said second major phase material; said
second major phase material being selected from a group of
polymers which when mixed with said ternary composite forms
a quaternary composite of an immiscible polymer blend having
co-continuous distinct phases.
There is also provided a semiconductive jacket
material for jacketing a cable, comprising: a minor phase
material comprising a semicrystalline polymer; a conductive
filler material dispersed in and residing in said minor
phase material in an amount of about 10% by weight or less
to form a binary composite; and a major phase material, said
major phase material being a polymer which when mixed with
said binary composite forms a ternary composite of an
immiscible polymer blend, said major phase material having
said binary composite dispersed therein in an amount
sufficient to be equal to or greater than an amount required
to generate a continuous conductive network in said major
phase material, said ternary composite being free of
hydrogen bonding and having co-continuous distinct phases
wherein said minor phase material has a solubility parameter
bA, said major phase material has a solubility parameter bB,
and said ternary composite meets the following criteria for
immiscibility, 7 - (bA - bB) 2>- 0.
In accordance with a still further aspect of the
invention, there is provided a semiconductive jacket
material for jacketing a cable, comprising: a minor phase
material comprising a semicrystalline polymer; metallic
particles dispersed in and residing in said minor phase
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material in an amount of about 85% by weight or greater to
form a binary composite; and a major phase material, said
major phase material being a polymer which when mixed with
said binary composite forms a ternary composite of an
immiscible polymer blend, said major phase material having
said binary composite dispersed therein in an amount
sufficient to be equal to or greater than an amount required
to generate a continuous conductive network in said major
phase material, said ternary composite being free of
hydrogen bonding and having co-continuous distinct phases, a
volume resistivity of about <- 100Q=m, an unaged tensile
strength of at least about 1200 psi, a tensile strength of
at least about 75% of said unaged tensile strength after
aging in an air oven at 100 C. for 48 hours, an aged and
unaged elongation at break of at least about 100%, a heat
distortion at 90 C. of at least about -25%, and a
brittleness temperature of about :~-100 C.
According to another aspect of the invention,
there is provided a cable comprising at least one
transmission medium and a semiconductive jacket surrounding
said transmission medium, said semiconductive jacket
comprising: a minor phase material comprising a
semicrystalline polymer; a conductive filler material
dispersed in and residing in said minor phase material in an
amount of about 10% by weight or less to form a binary
composite; and a major phase material, said major phase
material being a polymer which when mixed with said binary
composite forms a ternary composite of an immiscible polymer
blend, said major phase material having said binary
composite dispersed therein in an amount sufficient to be
equal to or greater than an amount required to generate a
continuous conductive network in said major phase material,
said ternary composite being free of hydrogen bonding and
13f
CA 02277704 2008-01-31
77909-85
having distinct co-continuous phases, a volume resistivity
of about <- 100Q=m, an unaged tensile strength of at least
about 1200 psi, a tensile strength of at least about 75% of
said unaged tensile strength after aging in an air oven
at 100 C. for 48 hours, an aged and unaged elongation at
break of at least about 100%, a heat distortion at 90 C. of
at least about -25%, and a brittleness temperature of
about :!~-10 C.
A further aspect of the invention provides a cable
comprising at least one transmission medium and a
semiconductive jacket surrounding said transmission medium,
said semiconductive jacket comprising: a minor-phase
material comprising a semicrystalline polymer; a conductive
filler material dispersed in and residing in said minor
phase material in an amount of about 10% by weight or less
to form a binary composite; and a major phase material, said
major phase material being a polymer which when mixed with
said binary composite forms a ternary composite of an
immiscible polymer blend, said major phase material having
said binary composite dispersed therein in an amount
sufficient to be equal to or greater than an amount required
to generate a continuous conductive network in said major
phase material, said ternary composite being free of
hydrogen bonding and having distinct co-continuous phases, a
solubility parameter 5A, said major phase material has a
solubility parameter bB, and said ternary composite meets the
following criteria for immiscibility, 7 _ 5A - 6B)2 ? 0.
An advantage of some embodiments of the present
invention includes the reduction of conductive filler
material
13g
CA 02277704 2008-01-31
77909-85
content in a semiconductive cable jacket to less than about 6 weight percent
of total composite
weight without a corresponding loss in the conductivity performance of the
jacket.
Yet another advantage of some embodiments of the present invention is the
ability
to produce a semiconductive cable jacket which satisfies the ICEA A-94-649-
1997
"Semiconducting Jacket Type 1" specification requirements.
Yet another advantage is the cost reduction due to the reduced conductive
filler content
and ease of processing over conventional semiconducting jackets. =
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of embodiments of the present invention
will be apparent from the following detailed description in conjunction with
the
accompanying drawings in which:
FIG. 1 depicts a portion of an electrical cable jacketed with the
semiconductive jacket of
the invention; and
FIG. 2 depicts a portion of an optical fiber cable jacketed with the
semiconductive jacket
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Semiconductive jacket material for cables having good conductivity with
significant
reduction of conductive filler content of the present invention are based on a
conductive filler
dispersed in a minor phase material, forming a binary composite; the binary
composite being
mixed with at least one major phase polymeric material. More specifically, the
present invention
may be achieved by adhering to the hereinafter discussed four general
principles and alternate
14
CA 02277704 1999-07-09
hereinafter described embodiments. (1) The conductive filler content is
preferably at or just
greater than the percolation threshold in the minor phase material (i.e. the
lowest concentration of
conductive filler content required to generate a continuous conductive network
in the minor
phase material); (2) the minor phase content is at or just greater than the
percolation threshold in
the major phase material (i.e. the lowest concentration of minor phase
material required to
generate a continuous conductive network in the major phase material); (3) the
minor phase
material is semicrystalline; and (4) the major/minor phase blend is immiscible
having distinct
phases.
In accordance with one embodiment of the present invention, a semiconductive
jacket
material for jacketing a cable, said semiconductive jacket material comprises:
a minor phase
material comprising a semicrystalline polymer; a conductive filler material
dispersed in said
minor phase material in an amount sufficient to be equal to or greater than an
amount required to
generate a continuous conductive network in said minor phase material; and a
major phase
material, said major phase material being a polymer which when mixed with said
minor phase
material will not engage in electrostatic interactions that promote
miscibility, said major phase
material having said minor phase material dispersed therein in an amount
sufficient to be equal to
or greater than an amount required to generate a continuous conductive network
in said major
phase material, forming a semiconductive jacket material of a ternary
composite having distinct
co-continuous phases.
The material chosen for the conductive filler in any of the embodiments of the
present
invention influences the amount of conductive filler required to meet or
exceed the percolation
CA 02277704 2008-01-31
77909-85
threshold to form a conductive network. The conductive filler material may be
any suitable
material exhibiting conductivity and should have a chemical structure which
results in an
inherently high conductivity and an affinity to develop a strong network. The
conductive filler
material may, for example, be selected from the group consisting of carbon
black (CB), graphite,
metallic particles, intrinsically conductive polymers, carbon fibers, and
mixtures thereof. In
particular, the CB may be an "acetylene black" or a"furnace black" or any
commercial grade of
conductive CB, the acetylene blacks being superior in producing conductive
blends. Exemplary
CBs are also disclosed in U.S. Patent No. 5,556,697. "Furnace blacks" are
lower
quality CBs and are inferior in their ability to produce conductive blends
when
compared to "acetylene blacks", which are fabricated from the pyrolysis of
acetylene. Therefore "acetylene blacks" are most preferred in the present
invention over other CB types. Intrinsically conductive polymers, such as
polyacetylene,
polyaniline, polypyrrole, mixtures thereof and the like, are also preferable
for optimizing the
reduction of conductive filler in the present invention and thus may also be
employed as the
conductive filler material. These polymers generally have conductivity's
higher than that of even
acetylene blacks, but are more costly. Also, carbon fibers or "whiskers" may
be employed and
will have a lower weight percent content than that of CB or intrinsically
conductive polymers to
exceed percolation threshold.
An important feature of the present invention is the low amount of conductive
filler
material employed while still maintaining a desired level of conductivity. The
particular weight
percent of conductive filler material employed is dependent upon the type of
conductive filler
16
CA 02277704 1999-07-09
material, and the type of minor phase material and major phase material. For
non-metallic
conductive filler materials, the conductive filler content can be as high as
10-12 percent by
weight of the total composite. When metallic particles are employed as the
conductive filler
material, the weight percent may be quite high (85% or higher by weight of the
total composite),
while the volume fraction would be very low (< 10%). One skilled in the art
would recognize
that such values may be determined experimentally for each set of chosen
materials. An
important criteria, however, is that it is an amount sufficient to meet or
exceed the percolation
threshold which varies depending upon the selected materials. For example, in
the working
example set forth hereinafter it may be seen that the minor phase material may
be about 44% by
weight high density polyethylene (HDPE); the conductive filler may be about
<6% by weight
furnace grade CB; and the major phase material may be about 50% by weight
poly(ethylene-co-
vinyl-acetate ( EVA), the EVA having a vinyl acetate (VA) content of from
about 12% to about
45% by weight. If an acetylene black or an intrinsically conductive polymer or
carbon fiber is
used as the conductive filler in this example, the conductive filler content
may be < 6% and
preferably < about 4% by weight. Based on the foregoing and for example, the
minor phase
material may be from about 30% to about 50% by weight HDPE and the EVA may be
from
about 65% to about 50% by weight EVA depending on the VA content in the EVA.
As can be seen from the foregoing, material selection in each embodiment of
the present
invention is important in meeting the ICEA S-94-649-1997, "Semiconducting
Jacket Type 1"
specification for mechanical properties. For example, the minor phase material
for each
embodiment of the present invention must be semicrystalline in nature and the
crystallinity may
17
CA 02277704 1999-07-09
range from about 30% to about 80% and preferably > about 70% based on the heat
of fusion of a
perfect crystal. Suitable minor phase materials include any semicrystalline
polymer such as
HDPE, polypropylene, polypropene, poly-l-butene, poly(styrene) (PS),
polycarbonate,
poly(ethylene terephthalate), nylon 66, nylon 6 and mixtures thereof.
One skilled in the art would recognize that the level of minor phase material
content
required to meet or exceed the percolation threshold in the major phase
material and to meet the
required mechanical properties for semiconducting cable jackets is dependent
on the constituents
of the system, such as the conductive filler material and major phase
material(s), and the
description and the examples set forth herein should serve as a guide. For
example, it has been
found that for an HDPE/EVA/CB system with a VA content of 40% that the minor
phase
HDPE/CB should be about >45% and preferably 50% to meet the mechanical
properties required
in a suitable jacket material, although less is needed to meet the electrical
properties.
Suitable materials for the major phase material may be any polymeric material
which
meets the afore-described criteria for not engaging in electrostatic
interactions that promote
miscibility in relation to the heretofore described suitable minor phase
materials. It should be
noted that minor electrostatic interactions may be permissible within the
above criteria as long as
miscibility is not promoted. That is, the blend must be immiscible.
Furthermore, the solubility
parameter difference (Sõ - SB ) of the minor and major phase materials in the
tennary composites
of the present invention meet the following criteria for immiscibility:
UL 2(8" - SB )2 Z 0
18
CA 02277704 1999-07-09
Where,
UL = 7, more preferably 5;
S., = the solubility parameter of the minor phase material; and
SB = the solubility parameter of the major phase material.
Suitable materials for the major phase material may include, for example, EVA,
polybutylene terephthalate, PS, poly(methyl methacrylate) (PMMA),
polyethylene,
polypropylene, polyisobutylene, poly(vinyl chloride), poly(vinylidene
chloride),
poly(tetrafluoroethylene), poly(vinyl acetate), poly(methyl acrylate),
polyacrylonitrile,
polybutadiene, poly(ethylene terephthalate), poly(8-aminocaprylic acid),
poly(hexamethylene
adipamide) and mixtures thereof.
As indicated above, one skilled in the art will recognized that the selection
and amount of
major phase material employed is also dependent upon the constituents of the
system, and the
description and examples set forth herein should serve as a guide.
In furtherance to the above, exemplary major/minor pairs may include the
following.
That is, minor phase materials polyethylene, polypropene and poly-l-butene may
be paired with
major phase materials PS, poly(vinyl chloride), poly(vinylidene chloride),
poly(tetrafluoroethylene), poly(vinyl acetate), poly(methyl acrylate),
poly(methyl methacrylate),
polyacrylonitrile, polybutadiene, poly(ethylene terephthalate), poly(8-
aminocaprylic acid),
poly(hexamethylene adipamide). Similarly, minor phase materials PS,
polycarbonate,
poly(ethylene terephthalate), nylon 66, nylon 6 may be paired with major phase
materials
polyethylene, polypropylene and polyisobutylene.
19
CA 02277704 1999-07-09
Another embodiment of the present invention employs a minor phase material of
HDPE
with a crystallinity of greater than about 70%, conductive filler of furnace
grade CB and a major
phase material of EVA. If the VA in the EVA is greater than about 40% by
weight, the
HDPE/CB minor phase material with a 12% by weight conductive filler content in
the minor
phase material (which is about 6% by weight of total composite), should be
equal to or in excess
of about 50% by weight of the total composite to meet both conductivity and
mechanical
property criteria for semiconductive cable jackets. However, if the VA of the
EVA is less than
about 40% by weight, the EVA is more crystalline, and the level of HDPE/CB
minor phase
material may be less than about 50% by weight of the total composite provided
that the
HDPE/CB content is sufficient to exceed the percolation threshold required to
generate a
continuous conductive network in the EVA. Whether or not the HDPE/CB content
is sufficient
to exceed the percolation threshold required to generate a continuous
conductive network in the
EVA and meet the requirements for a semiconductive cable jacket may be
verified
experimentally by measuring the volume resistivity of the material. For
example, a volume
resistivity of s 100 0=m is required.
In accordance with another embodiment of the present invention, the
semiconductive
jacket material further comprises a second major phase material, wherein said
ternary composite
is dispersed in an amount sufficient for said ternary composite to be
continuous within said
second major phase material, said second major phase material being selected
from a group of
polymers which when mixed with said ternary composite will not engage in
electrostatic
interactions that promote miscibility with said minor phase material or said
major phase material,
CA 02277704 1999-07-09
forming a semiconductive jacket material of a quaternary composite having
distinct co-
continuous phases.
The second major phase material may be selected as described above for the
previously
discussed major phase material.
One skilled in the art would recognize that the amount of ternary composite
sufficient for
the ternary composite to be continuous within the second major phase material
is dependent upon
the constituents of the system and may be determined experimentally by
measuring the volume
resistivity as a function of ternary composite content to ensure that
semiconductivity results.
It also should be noted that for quaternary blends, all four constituents
(i.e. conductive
filler, minor phase, and two major phases) must be mutually insoluble for the
temperature and
conditions of the material use.
In accordance with a further embodiment of the present invention, a method of
producing
a semiconductive jacket material for jacketing cables is disclosed. In this
embodiment, a
semicrystalline polymer having a melting temperature is mixed in a mixer,
wherein the mixer is
preheated to above the melting temperature of the semicrystalline polymer.
A conductive filler material is added to the semicrystalline polymer in the
mixer in an
amount z an amount required to generate a continuous conductive network in the
semicrystalline
polymer. For example, the conductive filler material may be added in an amount
between about
0.1 weight percent and about 12 weight percent for a HDPE/EVA/CB system.
However, one
skilled in the art would recognize that the amount of conductive filler
material employed is
dependent upon the conductive filler material and the other particular
constituents of the system.
21
CA 02277704 1999-07-09
The conductive filler material and semicrystalline polymer are conventionally
mixed for a
time and at a speed sufficient to insure a uniform distribution of the
conductive filler in the
semicrystalline polymer, thereby forming a binary composite.
A major phase material having a melting temperature is conventionally mixed
with the
binary composite in a mixer preheated to above the melting temperature of the
major phase
material, for a time and at a speed sufficient to insure a uniform
distribution of said binary
composite in the major phase material. The weight ratio of the binary
composite to the major
phase material is sufficient for the binary composite to be Z an amount
required to generate a
continuous conductive network in the major phase material, the major phase
material being
selected from a group of polymers which when mixed with the binary composite
will not engage
in electrostatic interactions that promote miscibility, such that a
semiconductive jacket material
of a ternary composite with distinct co-continuous phases is formed.
For example, the following non-limiting parameters may be particularly
employed: from
about 0.1 % by weight to about 10% by weight conductive filler; from about
49.9% by weight to
about 44% by weight HDPE; and about 50% by weight EVA if VA is about 40% by
weight.
The semicrystalline polymer may be selected from the afore-described minor
phase
materials and may be present in the amounts described therefor.
In accordance with a further embodiment of the present invention, a second
major phase
material having a melting temperature is conventionally mixed with the afore-
described ternary
composite in a mixer preheated to above the melting temperature of the second
major phase
material, for a time and at a speed sufficient to insure a uniform
distribution of said ternary
22
CA 02277704 1999-07-09
composite in the second major phase material. The weight ratio of the ternary
composite to the
second major phase material is sufficient for the ternary composite to be z
the percolation
threshold required to generate a continuous conductive network in the second
major phase
material, the second major phase material being selected from a group of
polymers which when
mixed with the temary composite will not engage in electrostatic interactions
that promote
miscibility, such that a semiconductive jacket material of a quaternary
composite with distinct
co-continuous phases is formed. The second major phase material may be as
previously
described for the major phase material.
Thus, it can be seen that in accordance with the present invention, more than
two phases
can be blended to further reduce the conductive filler content by weight
percent of the final
composite. For example, preferably, the conductive filler content is just
above percolation
threshold in a minor phase material forming a binary composite. The binary
composite is mixed
just above the percolation threshold with a major phase material, forming a
ternary composite.
The temary composite is mixed with a second major phase material just above
the percolation
threshold. A quaternary composite results which preferably has less than about
3% by weight
conductive filler content with respect to the total quatemary composite
weight, yet which forms a
continuous conductive network in the composite. A requirement for this
embodiment is that the
resultant composite is an immiscible blend with distinct phases, and that the
conductive filler is
in the continuous minor phase. For example, a quaternary composite of the
present invention
could be formed with a minor phase of "furnace grade" CB in HDPE; the CB
comprising about
3.6% by weight of the quaternary composite and about 26.4% by weight HDPE, the
major phase
23
CA 02277704 1999-07-09
material being about 30% by weight EVA and about 40% by weight PS. Of course
other
combinations meeting the requirements of the present invention will be
apparent to those skilled
in the art.
In a like manner, semiconductive jacket materials of the invention can be
formed with
more than two major phase materials. For example, the heretofore described
quaternary
composite may be mixed in an amount sufficient to exceed the amount required
to generate a
continuous conductive network with a third major phase material, said third
major phase material
being such that it will not engage in electrostatic interactions that promote
miscibility with the
second, first or minor phase materials. Thus the resultant composite is an
immiscible blend with
distinct phases. In accordance with the present invention, semiconductive
composite materials
may be formed by repeating the heretofore described mixing procedure with any
number of
further major phase materials which meet the requirements for major phase
materials set forth
heretofore such that the resultant semiconductive composite material is an
immiscible polymer
blend having distinct co-continuous phases.
l5 The resulting composites of the present invention can be further enhanced
by
conventional annealing processes. That is, in accordance with a further
embodiment of the
present invention, the afore-described ternary composite, binary composite
and/or quaternary
composite may be annealed thereby coarsening the morphology of the respective
composite. For
example, the percolation threshold of the minor phase in the major phase may
be further reduced
by preferably annealing the final CPC composite from approximately just above
the melting
temperature of both the minor phase material and the major phase material.
This results in
24
CA 02277704 1999-07-09
reinforcing the phase separation between the major and minor phase materials
by coarsening the
morphology of the composite, and thus resulting in the formation of a CPC
material with reduced
conductive filler content which maintains good conductivity.
Alternately, according to the present invention, the percolation threshold of
the
conductive filler in the minor phase material can be reduced by annealing the
minor
phase/conductive filler composite before mixing in the major phase material.
The annealing will
result in the threshold concentration for forming conductive networks in the
binary composite to
be lower when employing semicrystalline polymers as the minor phase material,
such as HDPE
or isostatic polypropylene. During the crystallization process a major part of
the conductive
filler particles are rejected into interspherulitic boundaries and the
remaining, non-rejected
conductive filler particles may be located in amorphous regions within the
spherulites, resulting
in the heretofore described reduction in percolation threshold. Thus annealing
of the foregoing
minor phase/conductive filler composite refines and increases the crystalline
phase. The afore-
described binary composite may be annealed to below the binary composite's
melting
temperature prior to mixing the afore-described major phase material with the
binary composite,
wherein the second polymer has a melting temperature less than the binary
composite's melting
temperature. The major phase material and the binary composite being mixed at
a temperature
below the melting temperature of the binary composite.
In a fiirther embodiment of the present invention, a reduction of the
percolation threshold
of the minor phase material in the major phase material may be achieved by
modifying the
surface area to volume ratio of the minor phase material, thereby increasing
the minor phase's
CA 02277704 1999-07-09
affinity to create a conductive network, before mixing the minor phase with
the major phase
material. This can be accomplished by pulverizing (i.e. crushing) the binary
composite of minor
phase material with conductive filler dispersed therein, or more preferably by
extruding thread-
like structures of binary composite as described below. The pulverized or
thread-like structures
of binary composite are then mixed with the major phase material below the
melting temperature
of the minor phase material. It is noted that one skilled in the art would
readily know how to
pulverize the afore-described material.
In further accordance with another embodiment of the present invention, the
afore-
described binary composite may be extruded into threadlike structures prior to
mixing the major
phase material with the binary composite, the major phase material having a
melting temperature
less than the binary composite's melting temperature, wherein the major phase
material and the
extruded threadlike structures of the binary composite are mixed at a
temperature below the
melting temperature of the binary composite. The threadlike structures may be,
for example,
about 2 mm long and about 0.25 mm in diameter and one skilled in the art would
readily
understand how to extrude the binary composite.
Referring now to Figure 1, a semiconductive jacket of the invention, which may
be a
thermoplastic compound, is shown on an electrical cable 10. The electrical
cable 10 comprises a
central core conductor 12, an overlaying semiconductive conductor shield 14,
at least one
polymeric insulation layer 16 formed over the semiconductive conductor shield,
a
semiconductive insulation shield 18 formed over the insulation layer 16, and a
metal component
20 which may be embedded in the semiconductive insulation shield 18 as shown,
or may overlay
26
CA 02277704 1999-07-09
the semiconductive insulation shield 18. A semiconductive jacket 22 is
preferably extruded over
the semiconductive insulation shield 18 by methods readily known to those
skilled in the art.
The semiconductive jacket may also be applied over an optical fiber cable as
shown in
Figure 2 or a hybrid cable. Optical fiber cables and hybrid cables, (i.e.
cables carrying electrical
conductors and optical fiber), are often grounded periodically if they contain
metallic elements,
especially for lightening protection. Further, some optical fiber cables are
installed by blowing
them into ducts. Often in this process, depending on the duct material, a
static charge builds up
on the surface of the cable which opposes the charge on the duct, hindering
installation. A
semiconductive jacket of the present invention on these cables would
advantageously dissipate
the static charge and ease installation. In Figure 2, the optical fiber cable
30 comprises a metallic
central strength member 32, at least one tube 34 containing optical fibers 36,
and a
semiconductive jacket 38 formed around the tubes 34 preferably by extruding
the
semiconductive jacket 38 by methods readily known to those skilled in the art.
A layer of armor,
waterblocking material, and additional strength member material may be
optionally included in
the optical cable 30.
The principles of the invention can be further illustrated by the following
non-limiting
examples.
EXAMPLE I
Suitable semiconductive jacket materials of the present invention were made
using
commercial grades of a random copolymer of EVA, HDPE, and furnace grade CB. In
this
example, the semiconductive jacket material is 6% by weight CB, 44% by weight
HDPE, and
27
CA 02277704 1999-07-09
50% by weight EVA. The characteristics of the materials used in this example
are set forth in
Table 1. In particular, the EVA was selected to have a high concentration, 45%
by weight, of VA
in order to reinforce the phase separation between the minor phase material
(HDPE/CB) and the
major phase (EVA). EVA's with lower weight % VA are less preferable for
increased
conductivity, but could be substituted without departing from the general
principles of the
invention. The composite was mixed at 170 C in a Brabender internal mixer with
a 300 cm3
cavity using a 40 rpm mixing rate. The mixing procedure for the semiconductive
jacket material
of the invention comprises adding the HDPE into the preheated rotating mixer
and allowing the
polymer to mix for 6 minutes. The CB is added to the mixing HDPE and is
allowed to mix for
an additional 9 minutes, which insures a uniform distribution of CB within the
HDPE. The EVA
is added and the mixture allowed to mix for an additional 10 minutes. The
semiconductive
jacket material, thus formed was then molded at a pressure of about 6 MPa for
12 minutes at
170 C into a plaque of about 0.75 mm in thickness for testing.
TABLE 1
Constituent Tradename Characteristics Producer
EVA Levapren 450 45 weight % VA content Bayer
Corporation
HDPE Petrolene LS6081-00 Density = 0.963 g/cm3 Millennium
Chemical
CB Vulcan XC72 N2 Surface Area = 254 m2/g Cabot
DBP oil absorption = 174 cm3/100g Corporation
mean particle diameter = 300 A
The electrical conductivity of the resultant composite was measured by cutting
101.6 mm
28
CA 02277704 1999-07-09
x 6.35 mm x 1.8 mm strips from the molded plaque, and colloidal silver paint
was used to
fabricate electrodes 50 mm apart along the strips in order to remove the
contact resistance. A
Fluke 75 Series II digital multimeter and a 2 point technique was used to
measure the electrical
resistance of the strips.
Mechanical properties of the semiconductive jacket material were tested in
accordance
with ASTM D-638. Unaged and 2 day/100 C aged mechanical properties (i.e.
tensile strength
and elongation at break) were determined for the semiconductive jacket
material using dogbones
cut from ASTM D-470 - ASTM D-412 Die C. The draw rate on the Instron Mode14206
tensile
testing machine was at 2 inch/minute, and all measurements were conducted at
23 C unless
otherwise indicated.
In addition, the heat deformation for the semiconductive jacket material at 90
C was
tested in accordance with UL 1581 Section 560; this temperature is required
for the ICEA S-94-
649-1997 "Semiconducting Jacket Type 1" specification. This procedure
calculates the
deformation that a 2000 gram weight with a defmed loading area imparts to a 24
mm x 14 mm x
1.52 mm specimen at a prescribed temperature.
The results of the electrical and mechanical property tests for the
semiconductive jacket
material of the invention for this example are set forth in Table 2.
29
CA 02277704 1999-07-09
TABLE 2
Industry Semiconductive Jacket
Property Requirement Material of the Invention
unaged tensile strength (minimum) 1200 psi 1172 t 76b psi
aged8 tensile strength (minimum) 75% of unaged 1090 51b psi
unaged elongation at break (minimum) 100% 140% 24b%
ageda elongation at break (minimum) 100% 109% 19b%
heat distortion at 90 C (minimum) -25% 1%
volume resistivity (maximum) 100 0-m 4.36 f.48b Q=m
brittleness temperature s -10 C < -10 C
aaged in an air oven at 100 C for 48 hours.
b68% confidence levels.
As evident from Table 2, this example demonstrates the ability to make a
semiconducting
jacket with low conductive filler content that meets, within the margin of
error, the
"Semiconducting Jacket Type 1 ".
It would be expected that the use of an acetylene black or an intrinsically
conductive
polymer or carbon fiber in place of the fumace grade black used in the present
example would
result in a similar properties with < 6 weight % and preferably < 4 weight %
conductive filler
loading of the semiconductive jacket material.
EXAMPLE 2
Suitable semiconductive jacket materials of the present invention may be made
using
commercial grades of a random copolymer of EVA, HDPE, and furn.ace grade CB.
In this
example, the semiconductive jacket material is 6% by weight CB, 44% by weight
HDPE, and
50% by weight EVA. The characteristics of the materials which may be used in
this example are
set forth in Table 3. In particular, the EVA is selected to have a lower
concentration, 25% by
CA 02277704 1999-07-09
weight, of VA than that of Example 1. While the higher VA content in Example 1
reinforces the
phase separation between the minor phase material (HDPE/CB) and the major
phase (EVA),
which results in better conductivity of the resultant composite material.
EVA's with lower
weight % VA will have increased crystallinity which will enhance the
mechanical properties of
the resultant semiconductive material without a significant loss in
conductivity. It is expected
that the resistivity of the semiconductive jacket material of this example
will be within industry
specifications, that is s 100 S2=m with improved tensile strength and
elongation properties.
The composite is mixed at 170 C in a Brabender internal mixer with a 300 cm3
cavity
using a 40 rpm mixing rate. The mixing procedure for the semiconductive jacket
material of the
invention comprises adding the HDPE into the preheated rotating mixer and
allowing the
polymer to mix for 6 minutes. The CB is added to the mixing HDPE and is
allowed to mix for
an additional 9 minutes, which insures a uniform distribution of CB within the
HDPE. The EVA
is added and the mixture allowed to mix for an additional 10 minutes. The
semiconductive
jacket material, thus formed is then molded at a pressure of about 6 MPa for
12 minutes at 170 C
into a plaque of about 0.75 mm in thickness.
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TABLE 3
Constituent Tradename Characteristics Producer
EVA Elvax 360 25 weight % VA content DuPont
Company
HDPE Petrolene LS6081-00 Density = 0.963 g/cm3 Millennium
Chemical
CB Vulcan XC72 N2 Surface Area = 254 mZ/g Cabot Corp.
DBP oil absorption = 174 cm3/l00g
mean particle diameter = 300 A
This example further demonstrates the ability to produce a CPC material having
low conductive
filler content, as well as enhanced physical properties.
EXAMPLE 3
In a further embodiment of the present invention, a quaternary immiscible
blend may be
formed using the constituents: PS, EVA, HDPE, and CB by the method comprising
the steps set
forth hereinafter.
The PS is added to the Brabender internal rotating mixer preheated to 170 C
and allowed
to mix for about 6 minutes at 40 rpm, prior to the addition of the EVA/HDPE/CB
ternary
composite already prepared as, for instance, set forth in the foregoing
examples. This blend is
allowed to mix for an additional 9 minutes. The final quaternary composite is
then molded at a
pressure of about 6 MPa for 12 minutes at 170 C in plaques of about 0.75 mm in
thickness. In
this example, the follow constituents may be employed: 3.6% by weight CB;
26.4% by weight
HDPE; 30% by weight EVA; 40% by weight PS; and 40% by weight VA in the EVA.
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In a multiple percolation like this heretofore described, it is important that
the quaternary
composite is an immiscible blend with distinct phases, and that the conductive
filler is in the
continuous phases. Thus, a CPC composite with less than about 4% by weight of
CB of the total
PS/EVA/HDPE/CB may be formed.
Thus, in accordance with the present invention and view of the examples and
disclosures
set forth herein, a CPC material having less than or equal to about 6% by
weight conductive
dispersion content of CB residing in a minor phase of HDPE is mixed with EVA.
By modifying
the level of HDPE in the EVA, crystallinity of HDPE, level of VA in the EVA
copolymer, and
CB content in the HDPE, a highly conductive compound may be generated with a
resistivity of
less than about 100 0-m. In addition, due to the low levels of required CB to
impart a high
conductivity to the CPC material, the rheology of the compound is more
analogous to an unfilled
compound in terms of extrusion properties and processability. The CPC can be
further tailored
to meet the mechanical properties required for semiconductive cable jackets by
modifying the
level of VA in the EVA in further accordance with the present invention, as
demonstrated in
1S Example 2.
In further accordance with the present invention and in view of the foregoing,
it can be
seen that the afore-described advantages and superior results may be achieved
by the selection of
a conductive filler with a chemical structure which results in an inherently
high conductivity and
an affinity to develop a strong network, such as CB, and by the modification
of the
thermodynamic stability of the conductive filler and the minor polymer phases
to encourage
coarsening of the filler/minor phase morphology, such as by the afore-
described annealing
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CA 02277704 1999-07-09
technique.
The advantages are also realized by selecting a minor phase polymer with a
high level of
crystallinity such that the conductive filler and minor phase material
preferentially phase separate
in order to increase the concentration of the conductive filler in the
amorphous phase, as well as
by reducing the percolation threshold of the minor phase/conductive filler
material in the major
phase material through a processing approach, such as the afore-described
extruding, annealing
and pulverizing means, to changing the morphology of the minor
phase/conductive filler
material.
The advantages are also realized by coarsening the morphology of the
major/minor phase
through modifying the thermodynamic stability of the polymer phases to promote
immiscibility
by selecting suitable minor/major pair materials.
As also described above, advantages of the present invention are achieved by
post-
annealing of the CPC material to coarsen the morphology of the major/minor
phase, as well as by
increasing the crystalline component of the major phase polymer; for example,
modifying the
VA content in the EVA as heretofore described or by incorporating 0.0 1% by
weight to about
2% by weight of a nucleating agent in the major phase material to promote
crystallinity; in order
to increase the concentration of the minor phase in the amorphous major phase.
It is to be understood that conventional additives such as nucleating agents
and
antioxidants may be added into the composite material or in the major phase of
minor phase
materials in the amount of about 0.01% by weight to about 5% by weight without
departing from
the spirit and scope of the invention. Exemplary nucleating agents are talc,
silica, mica, and
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kaolin. Examples of antioxidants are: hindered phenols such as
tetrakis[methylene (3,5-di-tert-
butyl-4-hydroxyhydrocinnamate)]-methane, bis[(beta-(3,5-diter-butyl-4-
hydroxybenzyl)methylcarboxyethyl)]sulphide, 4,4-thiobis(2-methyl-6-tert-
butylphenol), 4,4-
thiobis(2-tert-butyl-5-methylphenol), 2,2-thiobis(4-methyl-6-tert-
butylphenol), and thiodethylene
S bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate; phosphites and phosphonites
such as tris(2,4-di-
tert-butylphenyl)phosphite and di-tert-butylphenylphosphonitie; thio compounds
such as
dilaurylthiodipropionte, dimyristylthiodipropionate, and
disterylthiodipropionate; various
siloxanes; and various amines such as polymerized 2,2,4-trimethyl-l,2-
dihydroquinoline.
While various embodiments of the invention have been shown and described, it
is to be
understood that the above-described embodiments are merely illustrative of the
invention and
other embodiments may be devised by those skilled in the art which will embody
the principles
of the invention and fall within the spirit and scope thereof.
