Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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POWER CABLE COMPOSITIONS FOR STRIPPABLE ADHESION
This invention relates to power cable compositions. Specifically, it relates
to
semiconductive power cable compositions, articles (such as a semiconductive
cable
layer and power cable constructions) prepared from the semiconductive
compositions,
and processes for preparing the semiconductive compositions and related
articles.
Power cables, rated for a conductor operating temperature of 90-degree
Centigrade or higher, axe commonly prepared by extruding chemically-
crosslinkable
polymer materials around the conductor. Following extrusion, the chemically-
crosslinkable polymeric materials are crosslinked to resist material
deformation at the
rated cable operating temperature and related overload conditions.
to For medium- and high-voltage cable designs, the chemically-crosslinkable
polymeric materials commonly contain conductive fillers to render the
resulting cable
layer semiconductive. The chemically-crosslinlcable polymeric materials are
extruded
to prepare an electrical stress control layer between the metallic conductor
and a
polymer-dielectric insulation layer and may also be used as an electrical
stress control
layer between the polymer dielectric layer and grounding wires or tapes. The
various
layers are typically co-extruded and subsequently, simultaneously crosslinked.
In
addition, some cable constructions can include protective sheaths, moisture
barriers,
or protective jackets.
Co-extrusion and simultaneous crosslinking are generally desirable because
the resulting cable layers axe closely bonded. Close bonding prevents partial
delamination of the layers and precludes void forming between the layers,
thereby
preventing premature cable failure. Delamination and void formation can result
from
flexure and/or heat during the normal use of the cable.
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Unfortunately, close bonding as a result of co-extrusion and simultaneous
crosslinking is not free of disadvantages. Notably, the method of manufacture
presents problems for applications in which stripping the outermost electrical
stress
control layer (or semiconductive layer) from the polymer-dielectric insulation
layer is
desirable. (It is believed that crosslinking bond occur across the interface
between the
electrical stress control layer and the polymer-dielectric insulation layer.
Those bonds
must be broken to strip the layers apart.) Stripping the semiconductive layer
away
from the insulation layer damages the insulation layer when the force to
separate the
layers is excessive.
to It is desirable for the semiconductive layer to adhere to the insulation
layer
under normal operating conditions while being easily strippable from the
insulation
layer on demand. These features promote utilisation of the cable for its
normal life
and ease of installation of such accessories as joints, splices, and
terminations.
Currently, the chemically-crosslinkable polymeric materials often contain
polar polymers to reduce their melt miscibility with insulation materials,
which
generally contain non-polar polyolefinic polymers. 1'~ost commonly, the
chemically-
crosslinkable polymer materials are based upon ethylene vinyl acetate
copolymers,
having a vinyl acetate comonomer content of greater than 28°/~ by
weight. A
disadvantage of these high-polarity copolymers is that they tend to yield
compounds
2o prone to agglomeration. It is desirable to avoid the problem of
agglomeration.
The present invention is a semiconductive power cable composition
comprising (a) a mixture of a high-temperature polymer and a soft polymer, and
(b) a
conductive filler, wherein a semiconductive cable layer prepared from the
composition strippably adheres to a second cable layer. The invention also
includes a
semiconductive cable layer prepared from the semiconductive power cable
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composition as well as a power cable construction prepared by applying the
semiconductive cable layer over a wire or cable.
Moreover, the present invention includes a process for preparing the
semiconductive power cable composition comprising the step of blending a
mixture
of a high-temperature polymer, a soft polymer, and a conductive filler.
Alternatively,
the process comprises the steps of (a) reactively-coupling a mixture of a high-
temperature polymer, a soft polymer, and a first coupling agent, in the
presence of a
conductive filler and wherein the resulting mixture having a reduced curative
level,
and (b) admixing a second coupling agent, wherein the second coupling agent
does
to not substantially affect the curative level of the resulting mixture.
The present invention also includes a process for preparing a power cahle
comprising the steps of (a) extruding a power cable semiconductive composition
over
a metallic conductor to yield a semiconductive cable layer over the metallic
conductor, (b) extruding a chemically-crosslinkable insulation composition
over the
semiconductive cable layer, (c) extruding a second semiconductive power cable
composition over the polymer-dielectric insulation to yield a second
semiconductive
cable layer, and (d) crosslinking the chemically-crosslinkable insulation
composition
to yield a crosslinked, polymer-dielectric insulation.
The invented semiconductive power cable composition comprises (a) a
2o mixture of a high-temperature polymer and a soft polymer and (b) a
conductive filler,
wherein a semiconductive cable layer prepared from the composition strippably
adheres to a second cable layer. Preferably, the resulting semiconductive
cable layer
with have a heat resistance of less than 100% as measured by a Hot Creep test
at a
testing temperature of 150 degrees Centigrade (Test Method described in ICEA T-
28-
2s 562, and referenced in ANSI/ICEA Standards S-94-649 and S-97-682).
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As the term is used herein, "strippably adheres" means that the
semiconductive layer adheres to a second layer (usually, an insulation layer)
under
normal operating conditions of the power cable while having the property of
being
easily strippable from the second layer (i.e., delaminating/separating the
semiconductive layer from the second layer without substantially damaging the
second layer) on demand. With reference to strip tension, the term "strippably
adheres" means a strip tension between 3 and 24 pounds per 0.5 inch wide strip
(1.3
to 10.9 kilograms per 13 millimeter wide strip). This test method is also is
also
referenced in ANSI/ICEA Standards S-94-649 and S-97-682.
to A high-temperature polymer, as that term is used herein, means a polymer
having suitable heat resistance for the semiconductive cable layer but lacking
other
desirable properties. For e:~ample, the high-temperature polymer may not have
desirable processing characteristics or other material properties. Suitable
high-
temperature polymers include polypropylenes, polyesters, nylons, polysulfones,
and
polyaxamides. Preferred high-temperature polymers are polypropylenes. The high
temperature polymer is preferably in the c~mposition in an amount less than 50
weight percent. More preferably, the high temperature polymer is present in an
amount between 10 and 40 weight percent. Most preferably, it is present in an
amount between 20 and 30 weight percent.
A soft polymer, as that term is used herein, means a polymer that enhances the
processing characteristics of the high-temperature polymer and provides a
networking
source as the soft polymer is coupled to the high-temperature polymer for
additional
heat resistance. Suitable soft polymers include polyethylenes, polypropylenes,
polyesters, and rubbers. Preferred soft polymers are polyethylenes.
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Polyethylenes include homopolymers of ethylene and copolymers of ethylene
and one or more alpha-olefins, and, optionally, a dime. The polyethylene can
also be
a copolymer of ethylene and an unsaturated ester such as a vinyl ester (e.g.,
vinyl
acetate or an acrylic or methacrylic acid ester), a copolymer of ethylene and
an
unsaturated acid such as acrylic or methacrylic acid, or a copolymer of
ethylene and a
vinyl silane (e.g., vinyltrimethoxysilane and vinyltriethoxysilane) as well as
interpolymers of any of these comonomers. Post-modified polyethylenes of any
other
of the above are considered within the scope of this invention as well as
blends
thereof. Preferred polyethylenes are homopolymers of ethylene, copolymers of
to ethylene and one or more alpha-olefins, and a copolymer of ethylene and an
unsaturated ester. More preferred polyethylenes for soft polymers are
copolymers of
a polar monomer and a nonpolar comonomer. Most preferred polyethylenes are
copolymers of ethylene and an unsaturated ester.
Suitable polypropylenes include homopolymers of propylene, copolymers of
propylene and other olefins, and terpolymers of propylene, ethylene, and
dimes.
Suitable polyesters include thermoplastic resins comprising a saturated
dicarboxylic acid and a saturated difunctional alcohol. Specific examples
include
polyethylene terephthalate, polypropylene terephthalate (or trimethylene
terephthalate), polybutylene terephthalate, polytetramethylene terephthalate,
2o polyhexamethylene terephthalate, polycyclohexane-1,4-dimethylol
terephthalate, and
polyneopentyl terephthalate. Preferred polyesters are polyethylene
terephthalate,
polypropylene terephthalate (or trimethylene terephthalate), and polybutylene
terephthalate.
Suitable nylons include nylon 6, nylon 6,6, and nylons based upon longer
chain-length diamines. Preferred nylons are nylon 6 and nylon 6,6.
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Suitable rubbers include thermoplastic rubbers, ethylene propylene dime
rubber, styrene-butadiene block copolymers, styrene-butadiene rubber,
polybutadiene
rubbers, isoprene rubbers, nitrite rubbers, polychloroprene rubbers,
hydrogenated
styrene-butadiene block copolymers, methacrylate butadiene styrene rubber,
acrylic
elastomers (such ethylene methylacrylate), fluoroelastomers, and thermoplastic
elastomers (such as thermoplastic urethanes, polyamids, and polyester ethers).
Suitable conductive fillers include carbon blacks, carbon fibers, carbon
nanotubes, graphite particles, metals, and metal-coated particles. Preferred
conductive fillers are carbon blacks. Preferably, the conductive filler will
be present
to in the composition in an amount sufficient to impart a volume resistivity
of less than
50,000 ohm-cm for a semiconductive cable layer prepared therefrom, as measured
by
the methods described in ICEA S-66-524.
In addition, a curing agent may be present in the semiconductive composition.
Suitable curing agents include organic peroxides, azides, organofunctional
silanes,
maleated polyolefms, phenols, and sulfur vulcanizing agents. Suitable organic
peroxides include aromatic diacyl peroxides, aliphatic diacyl peroxides,
dibasic acid
peroxides, ketone peroxides, alkyl peroxyesters, and alkyl hydroperoxides.
Suitable
azide curing agents include alkyl azide, aryl azides, aryl azides,
azidoformates,
phosphoryl azides, phosphinic azides, silyl azides, and polyfunctional azides.
2o Suitable silanes include unsaturated silanes that comprise an ethylenically
unsaturated
hydrocarbyl group, such as a vinyl, allyl, isopropenyl, butenyl, cyclohexenyl
or y-
(meth)acryloxy allyl group, and a hydrolyzable group, such as, for example, a
hydrocarbyloxy, hydrocarbonyloxy, or hydrocarbylamino group. Preferred curing
agents are organic peroxides.
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In addition, the semiconductive power cable composition may further
comprise a coupling agent. The term "coupling agent," as used herein, means a
compound or mixture of compounds used for the purposes of coupling or grafting
a
polymer or polymer blend. The coupling agent may be present in an amount
sufficient to reduce the amount of a curing agent required to impart heat
resistance to
the semiconductive cable layer. The coupling agent may be the same compound as
the curing agent..
Suitable coupling agents include organic peroxides, azides, organofunctional
silanes, maleated polyolefms, phenols, and sulfur vulcanizing agents. Suitable
to organic peroxides include aromatic diacyl peroxides, aliphatic diacyl
peroxides,
dibasic acid peroxides, ketone peroxides, alkyl peroxyesters, and alkyl
hydroperoxides. Suitable azide coupling agents include alkyl azide, aryl
azides, aryl
azides, azidoformates, phosphoryl azides, phosphinic azides, silyl azides, and
polyfunctional azides. Suitable silanes include unsaturated silanes that
comprise an
ethylenically unsaturated hydrocarbyl group, such as a vinyl, allyl,
isopropenyl,
butenyl, cycloheacenyl or ~-(meth)acryloxy allyl group, and a hydrolyzable
group,
such as, for example, a hydrocarbyloxy, hydrocarbonyloxy, or hydrocarbylamino
group. Preferred coupling agents are organic peroxides.
In addition, the semiconductive power cable composition may further
comprise a compatibilizing polymer. As used herein, the term "compatibilizing
polymers" includes those polymers having an affinity for both the high-
temperature
polymer and the soft polymer. Preferred compatibilizing polymer are copolymers
(such as ethylene-alpha-olefin copolymers) and functionalized polymers (such
as
maleated polyolefins and glycidil-functional polyolefins). Based on the
selection of
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the high-temperature and soft polymers, a person skilled in the art can
readily identify
other suitable compatibilizing polymers.
In addition, the composition may contain other additives such as antioxidants,
stabilizers, blowing agents, pigments, processing aids, and cure boosters.
In a preferred embodiment, the present invention is a semiconductive power
cable composition comprising (a) a mixture of a high-temperature polymer and a
soft
polymer and (b) a conductive filler, wherein a semiconductive cable layer
prepared
from the composition strippably adheres to a second cable layer. The high-
temperature polymer and the soft polymer may have different heat resistance.
In a
to more preferred embodiment, the semiconductive cable layer has a heat
resistance of
less than 100% as measured by a Hot Creep test at a testing temperature of 150
degrees Centigrade. Also, in a more preferred embodiment, the second cable
layer is
a chemically-crosslinked layer.
In an alternate embodiment, a semiconductive cable layer is prepared from the
semiconductive power cable composition. In a yet another embodiment, a power
cable constuuction is prepared by applying the semiconductive cable layer over
a wire
or cable.
In another alternate embodiment, the present invention is a process for
preparing a semiconductive power cable composition comprising the step of
blending
a mixture of a high-temperature polymer, a soft polymer, and a conductive
filler,
wherein a semiconductive cable layer prepared from the composition strippably
adheres to a second cable layer. In this embodiment, the mixture may further
comprise a coupling agent. Preferably, the coupling agent reduces the amount
of a
curing agent required to impart heat resistance to a semiconductive cable
layer
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prepared from a mixture of the high-temperature polymer, the soft polymer, and
the
conductive filler in the absence of the coupling agent.
In yet another embodiment, the invention is a process for preparing a
semiconductive power cable composition comprising the steps of (a) reactively-
coupling a mixture of a high-temperature polymer, a soft polymer, and a
coupling
agent, in the presence of a conductive filler, and (b) admixing a curing
agent, wherein
a semiconductive cable layer prepared from the composition strippably adheres
to a
second cable layer. Preferably, the coupling agent reduces the amount of the
curing
agent required to impart heat resistance to a semiconductive cable layer
prepared from
1 o a mixture of the high-temperature polymer, the soft polymer, and the
conductive filler
in the absence of the coupling agent.
In another embodiment of the present invention, the invention is a process for
preparing a power cable comprising the steps of (a) extruding a semiconductive
power
cable composition comprising a mixture of a high-temperature polymer, a soft
polymer, and a conductive filler, over a metallic conductor to yield a
semiconductive
cable layer over the metallic conductor, and (b) extruding a polymer-
dielectric
insulation over the semiconductive cable layer. This embodiment may further
comprise the step of (c) extruding a second semiconductive power cable
composition
over the polymer-dielectric insulation to yield a second semiconductive cable
layer.
2o In an alternate aspect of this embodiment, the invention is a process
comprising the steps of (a) extruding a power cable semiconductive composition
comprising a mixture of a high-temperature polymer, a soft polymer, and a
conductive
filler, over a metallic conductor to yield a semiconductive cable layer over
the
metallic conductor, (b) extruding a chemically-crosslinkable insulation
composition
over the semiconductive cable layer, (c) extruding a second semiconductive
power
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cable composition over the polymer-dielectric insulation to yield a second
semiconductive cable layer, and (d) crosslinking the chemically-crosslinkable
insulation composition to yield a crosslinked, polymer-dielectric insulation.
EXAMPLES
The following non-limiting examples illustrate the invention.
Exam les 1-7
In Comparative Examples 1, 6, and 7 and Examples 2, 4, and 5, the mixtures
were combined in a lab-scale compounder to achieve a melt temperature of 190
degrees Centigrade for 5 minutes. In Example 3, the mixture of the high-
temperature
to polymer, the soft polymer, and the conductive filler were combined in a lab-
scale
compounder to achieve a melt temperature of 190 degrees Centigrade for 5
minutes
and then allowed to cool9 then the peroxide was added at 120 degrees
Centigrade.
Each exemplified formulation was evaluated for Hot Creep performance and
adhesion to a polyethylene-insulation substrate. The Hot Creep test specimens
were
evaluated for their resistance to thermal deformation under load conditions of
20
N/sq. cm, tensile stress for 15 minutes at 150 degrees Centigrade. Elongation
and
residual deformation were measured. Residual deformation is reported in Table
I as
Hot Set.
For the adhesion measurement, 30 mil plaques of the exemplified formulations
2o were prepared. A polyethylene-insulation substrate was prepared from The
Dow
Chemical Company's commercially available HFDB-4202 crosslinkable polyethylene
insulation at 120 degrees Centigrade. Subsequently, the test plaques and the
polyethylene-insulation substrate were molded together under pressure at a
temperature in excess of 1 ~0 degrees Centigrade for a length time sufficient
for the
?5 substrate to cure. Next, the dual-layer specimens were conditioned at
ambient
to
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WO 2004/088674 PCT/US2004/009075
temperature overnight. A one-half inch wide strip was scored from the dual-
layer
specimen. A 90-degree peel test was performed in an INSTRONTM tensile machine
at
a peel rate of 20inches per minute.
The polymeric materials for the exemplified formulations were added in the
concentrations shown in Table I and include:
(1) duPont Elvax 265TM ethylene vinylacetate copolymer (EVA-1), having
a vinylacetate content of 28% by weight and a melt index of 3
g/1 Ominutes;
(2) DXM-451TM ethylene vinylacetate copolymer (EVA-2), having a
to vinylacetate content of 18% by weight and a melt index of 3
g/lOminutes and commercially available from The Dow Chemical
Company;
(3) 5D45TM polypropylene (PP-1), which was a fractional homopolymer
having a melt flow rate of 0.8 and commercially available from The
Dow Chemical Company; and
(4~) a homopolymer of polypropylene (PP-2) having a melt flow rate of 20.
The carbon black was CSX614 and commercially available from Cabot
Corporation. Each formulation contained 55 parts per hundred polymer (pphr) of
carbon black. The peroxide used was TRIGANOX 101, and commercially available
2o from Akzo Nobel. For Comparative Example 1 and Examples 2-5, the peroxide
was
added in the amount of 0.4 pphr during the compounding. For Comparative
Example
6, the peroxide was added in the amount of 0.4 pphr following the compounding.
The
formulation for Comparative Example 7 did not contain any peroxide.
While the peroxide in the formulation exemplified as Comparative Example 1
was fully reacted during compounding, the test specimens were unable to
withstand
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WO 2004/088674 PCT/US2004/009075
the Hot Creep load. Without any residual unreacted peroxide, the test specimen
for
Comparative Example 1 was fully bonded to the polyethylene-insulation
substrate.
This bonding also demonstrates that, if the formulation had not contained any
peroxide during the compounding, the resulting test specimen would have fully
bond
itself to the polyethylene-insulation substrate and that current practices
using EVA-2
would result in a fully-bonded test specimen.
Table I shows that formulation exemplified as Example 2 had desirable Hot
Creep and strippability from the polyethylene-insulation substrate. While the
Example 3 formulation showed improved heat resistance, its test specimens
became
to fully bonded to the polyethylene-insulation substrate. Examples 4 and 5
demonstrated
formulations with improvements over the Example 2 formulation in heat
resistance
and strippability.
While Comparative Example 6 demonstrated excellent heat resistance, its
unreacted peroxide ~i.e., peroxide not consumed during compounding) did not
sufficiently reduce the curative level of the mixture to prevent the test
specimens from
fully bonding to the polyethylene-insulation substrate. Comparative Example 7,
which is a blend with no coupling agent, yielded test specimens with
relatively poor
heat resistance.
12
CA 02520362 2005-09-26
WO 2004/088674 PCT/US2004/009075
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CA 02520362 2005-09-26
WO 2004/088674 PCT/US2004/009075
Example 8
In Example 8, a mixture was combined in a lab-scale compounder to achieve a
melt temperature of 190 degrees Centigrade for 5 minutes. The mixture
contained 65
parts of EVA-1, 35 parts of PP-l, 55 parts per hundred polymer (pphr) of Cabot
Corporation CSX614 carbon black, and 0.4 pphr Akzo Nobel TRIGANOX 101
peroxide.
As part of a 15-kilovolt power cable design, the mixture was extruded as a
semiconductive layer over a peroxide crosslinkable polyethylene insulation
(HFDB-
4202). The HFDB-4202 crosslinkable polyethylene insulation is available
commercially from The Dow Chemical Company.
The 15-kilovolt power cable design used a 1/0 AWG aluminum conductor, 15
mils of a crosslinkable semiconductive power cable compound, 175 mils of the
crosslinkable polyethylene insulation, and 40 mils of the semiconductive power
cable
composition of Example 8. The extruded cable was passed through a hot, dry
nitrogen tube (continuous vulcanization tube) wherein the thermal
decomposition of
organic peroxide initiates polymer crosslinking. The cured cable was then
passed
through a cooling water trough.
The outer semiconductive power cable composition of Example 8 was found
to strippably adhere to the crosslinked polyethylene cable insulation, having
a strip
tension of 11-12 pounds per 0.5 inches. It also had a Hot Creep elongation of
19%
when tested at 150 degrees Centigrade and 0.2 MPa of applied tensile stress.
14
