Note: Descriptions are shown in the official language in which they were submitted.
9892
~ 69 ~
This invention relates to vulcanizable semi-
conducting compositions of ethylene-vinyl acetate co-
polymers which provide strippable semi-conducting insulation
shielding compositions for crosslinked polyethylene in-
sulated conductors.
The construction of insulated electrical conductors,
i.e. wires and cable, designed for medium to high voltage
applications is well known in the art and commonly com-
prises a core conductor which comprises one or more strands
or a conducting metal or alloy such as copper or aluminum,
a layer of semi-conducting conductor shielding, a layer of
; insulation, such as crosslinked polyethylene, and a layer
of semi-conducting insulation shielding overlying said in-
sulation. A plurality of neutral wires which are usually
made of copper or aluminum may be embedded in or wrapped
around the layer of semi-conducting insulation shielding,
if desired, in the form o~ a concentric ring around the
`~ insulated cable.
The insulation layer and its overlying semi-con-
ducting shielding layer are usually formed by what is known ~ ~ ;
in the art as tandem extrusion whereby these layers are
formed in sequence employing tandem extruders and cured
simultaneously in a single operation to minimize manufactur-
ing steps, However, the simultaneous curing of the two
layers by heat and presRure results in apparent mixing at _
the interface and formation of crossl-,nking bonds across
the interface. This appears true even in what is known in
the art as a two pass operation where the insulation i5 cured
in a previous operation and the semi-conducting shielding layer
afterwards extruded and cured onto the insulation.
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9892
~01~4~i~6
The formation of these crosslinking bonds between
the insulation and shielding makes subsequent separation
of the two layers (insula~ion and semi-conducting shielding)
such as occurs in making splices or terminal connections,
very difficult and time consuming. Such a strong bond
also makes the semi-conductive layer prone to leave
carbon residue on the insulation even when it is finally
pealed off. A Strippable semi-conducting shielding which
can be easily and cleanly stripped from the insulation of
an insulated conductor is therefore very desirable in
this art.
In order to achieve strippability between the
insulation layer and its semi-conducting shielding layer
several methods have been herétofore attempted with varying
degrees of success such as sulfonating the insulation or -
Coating it with release agents prior to applying the semi- "
conducting shielding layer. Other methods include using
a synthetic rubber made of chlorosulfonated polyethylene
sold under the tradename of Hypalonas the base resin
for the insulation shield or using a semi-conducting shield-
ing composition such as qeen disclosed by U.S. Patents
3,719,769, and 3,769,085.
As considered herein an easily strippable semi-
conducting insulation shielding composition is one in which
the adhesion between the insulation and said shielding
composition of an insulated electrical conductor is not
greater than 16 pounds per half inch strip.
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It has now been discovered ~hat such types of easily
strippable aemi-con~ucting shielding compositions for cross-
linked polyethylene insulation can be obtained from the
w lcanizable semi-conducting insulation shielding com- -
positions of this invention as described more fully below.
Thus, it is an object of this invention to provide
a vulcanizable semi-conducting insulation shielding com-
position which is particularly useful for providing a
strippable shielding for insulated electrical conductors,
e.g. wires and cables, that contain, as the prlmary in-
sula~ion, crosslinked polyethylene. Another object of
this invention is to provide insulated electrical con-
ductors, e.g. wires and cables, comprising, as the primary
insulation, a crosslinked polyethylene, and as the shielding
material for said insulation an easily strippable crosslinked
semi-conducting shielding composition. Other objects and
advantages of this invention will become rea~ily appar~nt
from the following description and appended claims.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
More specifically, the instant invention may be
described as a strippable w lcanizable semi-conducting
insulation shielding composition comprising, based on the
total weight of said compo~ltion, (A) about 55 to about 90
weight percent of an ethylene-vinyl acetate copolymer con- ;
taining from about 27 to 45 weight percent of vinyl acetate
based on the total weight of said copolymer, (B) about
10 to 45 weight percent of conducting carbon black and
(C) as the only crosslinking agent in said composition
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~892
~ 69 ~
from 0.2 to 5 weight percent of an agent selected from the
group consisting of o~ , ~< ' bis(tertiary-butylperoxy)
diisopropyl benzene and 2,5-dimethyl-2', 5'-di(tertiary-
butylperoxy) hexane and mixtures thereof.
The vulcanizable ethylene-vinyl acetate copolymers
and/or their methods of preparation, which can be employed
in this invention are well known in the art. However, it
has been discovered that in order to obtain a semi-conduct-
ing insulation shielding that is easily strippable from
crosslinked polyethylene insulated electrical conductors
it is important to employ an ethylene-vinyl acetate co-
polymer that is incompatible with the crosslinked poly-
ethylene insulation. The term incompatible is used herein
to mean that there is a lack of physiochemieal affinity
between the polyethylene and ethylene-vinyl acetate resins
during extrusion and curing conditions. This incompatibility
, results in a mutual repulsion between said resins which in
; turn prevents mixing and bond formation between them.
Whether or not a particular w lcanizable semi-
conducting composition will exhibit such incompatibility
and will furnish a crosslinked polyethylene insulated
electrical conductor with an easily strippable semi-
conducting insulatlon shielding may be determined by
, measuring the adhesion between a laminate of crosslinked
polyethylene and the crosslinked product of the w lcanizable
semi-conducting composition according to ASTM-D903. In
order to be considered an easily strippable insulation
shielding composition the laminate adhesion level of
the semi-conducting composition for the crosslinked
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~ 6~36 9~92
polyethylene should not be more than 16 pounds per half
inch strip when measured accoxding to said test method.
Thus, the ethylene-vinyl acetatP copolymer employed
herein should contain from 27 to 45, preferably from 29
to 35 weight percent of vinyl acetate based on the total
weight of said copolymer for it is considered that copolymers
containing less than 27 weight percent vinyl acetate will
result in semi-conductive shieldings that are bonded too ~-
strongly to an insulation of crosslinked polyethylene
to be considered easily strippable; while copolymers having
more than 45 weight percent of vinyl acetate may provide
too weak an adhesion to crosslinked polyethylene. The
amount of ethylene-vinyl acetate copolymer present in the
^ vulcanizable semi-conducting insulation shielding com-
positions of this invention can range from about 55 to 90
weight percent, preferably from about 60 to 75 weight
percent, based on the total weight of the w lcanizable
composition. Such ethylene-vinyl acetate copolymers and/or ~ -
methods for their preparation are well known in the art.
The employment o conducting carbon black in
semi-conducting compositions is well ~nown in the art
and ~ny conducting carbon black ln any suitable form can
be employed in this lnvention including channel blacks,
oil furnace blacks or ~cetylene blacks, providing they
are conducting. The amount o~ conducting carbon black -~
present in the w lcanizable semi-conducting insulation~;
shielding compositions of this invention can range from
about 10 to 45 welght percent, preferably ~rom about 30
to 40 weight percent, based on the total weight o the
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~ 9892
; vulcanizable composition.
The only crosslinking agents employed in the
;emi-conducting compositions of this invention include
:~ose selected from the group consisting of 9C ,o~ '
bis-(tertiary-butylperoxy) diisopropyl benzen~, (Vulcup*),
~ 2,5-dimethyl-2', 5'-di(tertiary-butylperoxy) hexane,
; (Varox*),and mixtures thereof. While the preferred amount
of crosslinking agent employed may vary depending upon
the particular e~hylene-vinyl acetate copolymer employed
and other such obvious conditions, in general, it is ~
considered that said amount of crosslinking agent will ~ -
;~ norn,ally fall within the range of about 0.2 to 5
preferably about 0.6 to 2, weight percent ~ased on the
total weight of the w lcanizable semi-conducting composition.
Of course, it is to be understaod, that said amount
` ranges may not be suitable for every po~sible semi-
conduc~ing composition of this invention and that for any
given w lcanizable semi-conducting composition the use of
amounts of crosslinking agents that may provide a cross-
linked semi-conducting product that would have a~ adhesion
of greater than 16 pounds per half inch when measured as
defined above for crosslinked polyethylene should be
avoided, and that such can be determined by routine
experimentation.
0~ course, it is to be also understood that the
vulcanizable semi-conducting insulation shielding
compositions of this invention, if desired, can contain
other conventional additives in the conventional used
quantities commonly employed in semi-conducting compositions.
* tradename
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Examples of such additives include e.g. age resistors
proces~ing aids, stabilizers, antioxidants, crosslinking
boosters and retarders, pigments, fillers, lubricants,
ultraviolet stabilizers, anti-block agents and the like.
The total amount of such additives which are commonly
used normally amounts to no more than about 0.05 to 3
percent by weight based on the total weight of the insu-
lation shielding compositionA
As pointed out above, another aspect of this
invention may be more specifically described as an in-
sulated electrical conductor, e.g. electrical wire, elec-
trical cable, etc., containing as the primary insulation,
crosslinked polyethylene and as the external semi-conductins
shielding for said insulation, a crosslinked ethylene-vinyl
acetate copolymer obtained upon crosslinking the vulcanizabl-~
semi-conducting insulation shielding composition of this ;
invention which has been already previously defined above. -~
; Of course, it is to be understood that the term
"crosslinked polyethylene" as used herein includes and
encompasses insulation compositions derived from a cross-
linkable polyethylene homopolymer or a crosslinkable
polyethylene copolymer having a comonomer content that
will not adversely affect the desired result of the in-
~tant invention. Normally the preferred crosslinked poly-
ethylene insulation is derived from a crosslinkable
polyethylene homopol~mer. The use of polyethylene
i~sulation compositions and semi-conducting compositions,
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~ 696 9892
the manner of cheir preparation, and the preparation of
insulated conductors are so well known that no further
discussion is required here to enable one skilled in the
art to understand how the polymer components are produced
and used in the preparation of insulated conductors.
For instance, the use of low densi~y polye~hylene com-
positions, which if desired, may contain conventional
additives such as fillers, age resistors, talc, clay,
calcium carbonate and other processing aides, along with
a conventional crosslinking agent is well known in the
art as are the conventional semi-conducting conductor
shielding compositions. The insulated electrical con- - -
ductors o~ this invention can also be prepared by con- --
ventional procedures, e.g. such as by tandem extrusion
whereby thè insulating layer is extruded over the con-
ductor, which has been previously covered with an ordinary
extruded semi-conductive conductor s~ielding, followed
by extruding the vulcanizable layer and then simultaneously
curing (crosslinking) both the insulation and insulation
shielding layer under pressure. Another conventional
; method involves curing the insulation layer prior to
contact with the vulcanizable semi-conducting insulation
shielding composition which is then itsel~ cured while in
contact under pressure with said cured insulation layer.
However, it is considered desirable to prevent any pre-
mixing o~ the ins~lation cornposition and vulcanizable
semi-conducting insulation ~hielding composition prior to
curing said compositions slnce such can allow the cross-
linking agents employed to assert their in1uence on
adhesion between the two layers through intercrosslinking
.
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~ 6~ 6 9892
across the interface of the two layers. The other particular
attributes of the insulated electrical conductors of this
- invention may also conform to con~entional insulated
electrical conductors and are not- ,~ritical for they depend
for the most part merely upon the desired end use of the
insulated electrical conductors.
The insulated electrical conductors of the
instant invention are indeed unique in view of the fact
that the crosslinked insulation shi~lding composition can
be easily and cleanly stripped> generally in one piece,
; from the crosslinked polyethylene insulation. -
The following examples are illustrative of the
present invention and are not to be regarded as limitative.
~`~ It is to be understood that all parts> percentages and
proportions referred to herein and in the appended claims
are by wei.ght unless otherwise indicated.
GLOSSARY
EVA = ethylene vinyl acetate copolymer
VA = weight percent of vinyl acetate in copolymer
MI s melt index
Dicup* -di-~C-cumyl perox~de
~ Lupersol*-130 =2,5-dimethyl-2', 5'-di(tertiary-butyl-
; peroxy)-hexyne-3
Vulcup*= c~ '-bis-(tertiary butylperoxy~ diisopropyl-
benzene
Varox* ~2,5-dimethyl, 2',5'-di(tertiary-butylperoxy)-
hexane
EXAMPLES 1 to 16
A series of w lcanizable semi-conducting com-
positions were prepared wherein the weight percent of
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vinyl acetate in the ethylene-vinyl acetate copolymer was
varied as were the crosslinking agents employed. The
components of each composition are listed below in Table
I and each composition contained in addition to the com-
ponen~s listed 40 weight percent of conducting carbon
black and 0.4 weight percent of polymerized 1,2-dihydro-2,
2,4-trimethyl quinoline, an antioxidant, the amounts of
all the ingredients in each composition being based on
the total weight of each composition unless otherwise
indica~ed.
The compositions were formed by uniformly ad-
mixing the components thereof in a laboratory size Banbury
mixer and about 1300 grams of each composition were pre-
pared.
In order to evaluate the strippability properties
of these compositions as semi-conducting insulation sheild-
ings, each composition was respectively used to prepare
a polyethylene/ethylene-vinyl ace~ate laminate. Said
~` laminates were prepared from laboratory test plaques, the
polyethylene plaque in each instance was derived from a
crosslinkable polyethylene homopolymer composition con-
sisting of polyethylene homopolymer (98%), di-~-cumyl
peroxide (2a/o) and bis(2-methyl,5-tertiary butyl,4-hydroxy-
phenyl) sulfide (0.2%), an antloxidant.
In Examples 1 to 9 the polyethylene/ethylene-
vinyl acetate laminates were made by first molding the
- polyethylene plaques (measuring 8" by 8" and 250 mils
thick) at 175C. for 15 minutes and crosslinking same,
then the w lcanizable ethylene-vinyl acetate plaques
(measuring 8" by 8" and 125 mils thick) were separately
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9892
~ 6
molded, but not crosslinked, and the laminates made by
pressing each vulcanizable ethyl~ne-vinyl acetate plaque
together with one of the crosslinked polyethylene plaques
at 200C. and 200 psi. pressure for 20 minutes during
which time the w lcanizable ethylene-vinyl acetate com-
position became crosslinked.
In Examples 10 to 16 the polyethylene/ethylene-
vinyl acetate laminates were prepared in the same manner
as described above except that the polyethylene plaques
were molded at 110C. so as not to crosslink them. Cross~
linking of both types of plaques took place simultaneously
when the laminate was made.
The adhesion between the test laminates (cut to
8" by 1"~ were then determined according to ASTM Test
Method D903 which measures the peel strength between the
two plaques of the laminate in terms of pounds per half-
inch strip and which is employed herein as a measure of
thestriPpability of a semi-conducting ethylene-vinyl
acetate insulation sheilding from a crosslinked poly-
ethylene insulation. The test results of each laminate
of polyethylene/ethylene-vinyl acetate sheilding prepared
as di~cussed above are also listed in said Table I.
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Examples 5, 7, 9 and 12 to 15 which represent
the present inven~ion demonstrate the excellent stripp-
ability of the insulation shielding compositions, the
insulation shielding layer in each example having been
stripped clean and in one piece from the insulation.
EXAMPLE 17
This example illustrates the preparation of an
insulated electrical cable.
A standard aluminum conductor was se~uentially
covered with an ordinary semiconducting strand shielding
layer (0.025" thick), a polyethylene insulating layer
(0.267" thick), and a semiconducting insulation shielding
layer (0.055" thick) consisting of polyethylene/vinyl
acetate (2~/o vinyl acetate, MI = 20) and 0.6% Vulcup-
In preparing the cable the extruded strand
shielding layer and insulation layer were cured in a
steam w lcanization tube t250 lbs./sq. in. of steam) prior
to extruding the insulation shielding layer over the
insulation, said insulation shielding layer then being
cured during a second pass through the steam vulcanization
tube. This process procedure is conventionally known in
the art as a two pass extrusion.
Iwo parallel incisions in the insulation shield-
ing of the insulated cable so prepared were mad~ one-half
inch apart running in the axial direction of the cable -
; and said insulation shielding was subjected to a tensile
; peeling test to determine the adhesion of the insulation.
- The insulation shielding wa~ stripped clean and in one
. .
piece from the insulation and exhibited an adhesion level
of 6-8 pounds per half-inch strip, thus demonstrating the
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~ 6 9892
excellent strippability of the insulation shielding
composition of this invention.
A comparative insulated electrical cable pre-
pared and tested in the same manner, but using an insu-
lation shielding composition consisting of vinyl acetate
and 2% Dicup resulted in a cable having an insulation
shielding that exhibited an adhesion level of 20-25
pounds per half-inch strip and which did not strip clean
butbroke off in pieces from the insulation.
EXAMPLE 18
This example illustrates the preparation of
an insulated electrical cable.
A standard aluminum conductor was sequentially
covered with an ordinary semiconducting strand shielding
; layer (0.025" thick), a polyethylene insulating layer
. .
(0.267" thick), and a semi~onducting insulation
shielding layer (0.055" thick) consisting of polyethylene/
vinyl acetate (29% vinyl acatate, MI = 20) and 0.6%
Vulcup.
In preparing the cable, all three layers of
strand shielding, insulation and insulation shielding
were extruded sequentially and simultaneously cured in a
steam vulcanization tube (250 lbs./sq. in. of steam).
This process procedure is conventionally known in the
art as a single pass triple extrusion.
The adhesion o~ the insulation shielding to
the insulation of the insulated cable was determined by
the same method described in Example 17. The insulation
shielding was stripped clean and in one piece from the
insulation and exhibi.ted an adhesion level o 14-16
.
9892
~ 69 6
poun~ pcr h~lf-inch strlp, thus demonstratlng the
excellent strippabllity of the insulation sheilding com-
position of ~his invention.
A comparative insulated electrical cable pre-
pared and tested in the same manner, but using the insu-
lation shielding composition consisting of vinyl acetate
and 2% Dicup resulted in a cable having an insulation
shielding that exhibited such a strong adhesion to the
insulation that its strippability could not be measured
by the peel test, the two layers being intimately fused
at the interface.
Various modifications and variations of this
invention will be obvious to a worker skilled in the art
and it is to be understood that such modifications and -- :
variations are to be included within the purview of this
appllcation and the spirit and scope of the appended
, . .
claims.
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