Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 R~CKG~OUND OF THE INVENTION
Field o~ the Invention
The present inventlon relates to electric wires or
cables.
Description of the Prior Art
Electric wires or cables having dielectric layers of
polyolefins such as polyethylene, cross-linked polyethylene
and the like are in wide use. Such polymers have in recent
10 years been improved so that they are able to withstand high ~ -
applied voltages. One important problem at present with such
electric wires and cables is that they undergo a gradual de-
gradation with time during under continuous applied voltages,
resulting in a reduction in their capability to withstand applied
voltages. Since the service life of electric wires or cables
per se is as long as thirty ~ears, the initial thickness of
dielectric layers therefor must be increased over initial re-
quirements with present designs so as to oompensate for the above ~ --
reduction in their ability to withstand applied voltages with ~ -
i 2C the passage of time. On the other hand, recent urban develop- ~ -
ments have required increased power transmission capacity for
electric wires or cables while keeping volume as small as
possible. This has led to the difficult problem, with electric
wires or cables having a dielectric layer essentially consisting
of a polyolefin, of increasing the transmission voltage
- without increasing the thickness of the dielectric layer.
Various approaches to overcome the above problem have
been considered, includlng preventlng the occurrence of the
above described degradation, whereby the decrease in capability ~-
to withstand applied voltages during a thlrty year service life
need not be taken into account, and the requirements of urban
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1 areas regardin~ high transmission capabillty and low volume
can be met.
SU~ARY ~P T~E I~VENTION
The inventors, takIng note of the above points,
performed extensive studies on t~e aging of electric wires or
cables of the foregoing type, and found that aging is primarily
caused by trees ~or electrochemical trees, as they are sometimes
termed) generated in the polyolefin dielectric from voids
` 10 which develop in the polyolefin dielectric material, and that
the degradation described above can be avoided by preventing
the generation of trees from the voids in dielectric polyolefins
material while significantly decreasing the thickness of the
dielectric layer.
The primary object of the present invention is thus
to provide electric wires or cables in which the development
, of trees is prevented and longer service life is obtained.
The present invention provides electric wires or cables
having a dielectric layer or layers of an uncross-linked or
e` 20 cross-linked mixture of ~1) a polyolefin and (2a) polystyrene
or (2b) a styrene copolymer. The polystyrene copolymer is
' present in such an amount as to provide a 10 to 30% styrene
'f content on the basis of the entire mixture weight, while if
the styrene copolymer is used it is present in such an amount
as to provide a 3 to 30~ styrene content on the basis of the
entire mixture weight. The mixture may comprise, alternatively,
both polystyrene and a styrene copolymer in an amount so as
to provide a 10 to 30% and a 5 to 20~ styrene content, due to
the former and the latter, respectively, based on the entire
r 30 weight of the polymers in the mixture.
By the use of the electric wires or cables of the
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1 present invention, the disadvantages of the prior ar-~ are
overcome in that the development of the trees is prevented to
thereby attain service life several to several tens of times
as long as that of a conventional polyolefin dielectric insulated
wire or cable. As a result, we have succeeded in significantly
decreasing the initial thickness of the polyolefin dielectric
layer in such wires on cables.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The term styrene content ~) as is used herein means
the percent by weight of styrene on the weight of the entire - -
polymer mixture. Polystyrene, for example, has a 100% styrene
content, while a polyolefin blended with 30~ by weight of
polystyrene has a 30% styrene content. As a further example,
assuming that 70 parts by weight of polyolefin is blended with
30 parts by weight of SBR which contains 50~ by weight styrene,
in this instance the styrene content would be 15~
,
The term "polyolefin" as is used herein refers to
a polyolefin which is predominantly polyethylene, which can
contain, if desired, a small amount of a component such as
propylene, butene, butadiene, isobutylene, etc. Preferred
of such polyolefins are those which have a melt index of 0.1 or
higher and a density of 0.90 to 1.0, and include both low
density and high density ethylene homopolymers.
The term "polyolefin" also includes copolymers of
ethylene and another comonomer other than styrene, e.g., an
ethylene-vinyl acetate copolymer, an ethylene-propylene copolymer,
and an ethylene-acrylate copolymer (e.g., an ethylene-methyl-
acrylate copolymer, an ethylene-ethylacrylate copolymer), etc. ~ "
Preferred copolymers of ethylene and another comonomer have
a melt index of 0.6 or higher and a density of 0.90 to 1.2. If
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1~4~39
1 desired, of course, a blend of polyolefins and olefin copolymers
can be used in the present invention. An example of a system
based on such is a polyethylene: styrene-butadiene rubber:
ethylene-propylene rubber = 95:4:1.
It should be clear from the above discussion that the
copolymers of ethylene and another comonomer include, for
example, a copolymer of a polyolefin which is predominantly
ethylene, e.g., polyethylene with a small amount of polypropy-
lene with, e.g., vinyl acetate. While technically a terpolymer,
such material still equivalent in this invention to a poly-
ethylene homopolymer.
When cross-linking is desired for the dielectric
layer, it can be effected through electron beam irradiation,
for example, at 3 Mrad to 40 Mrad, after extrusion coating of
the dielectric layer or, alternatively, through heating to
activate a conventional cross-linking agent, for example, --
using about 2% dicumyl peroxide, with heating at 200C to
300C or by like means after extrusion coating.
As well be apparent to one skilled in the art, the
dielectric layer or layers of the electric wires or cables of
~ the present invention can have added thereto, if desired,
`~ conventional additives such as anti-aging agents and the like
in conventionally used amounts, for example, 4,4'-thiobis(6-
tert-butyl-m-cresol) and N',N-di-~-naphthyl-p-phenylenediamine
in an amount of 0.1 to 0.5~.
When either polystyrene or a styrene copolymer is
blended alone with the polyolefin, no remarkable increase in
service life can be obtained with a less than 10% styrene
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content, while on the other hand, with a more than 30% styrene
~; 30 content, the physical properties of the composition are signi--
ficantly degraded, primarily failing the bending characteristics
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1 required in electric wires or ca~les.
When both polystyrene and one or more styrene copolymers
are used, if the styrene content attributable to the polystyrene
is less than 10~, and the styrene content attributable to the
styrene copolymer is less than 5~, based on the entire polymer
weight, a remarkable increase in service life is not achieved.
On the other hand, with a more than 30~ styrene content
attributable to the polystyrene or with a more than 20~ ;
styrene content attributable to the styrene copolymer, the
physical properties of the composition are significantly de-
graded, primarily the bending characteristic required in
electric wires or cables. Preferred styrene copolymers are
styrene-butadiene copolymers containing 20 to 80% styrene.
Preferred polystyrenes are those having a low melting point;
most styrene copolymers used contain at least about 10~ styrene. -
~
Preferred cross-linked materials used in the present
invention-have a degree of gelation of from about 70% to about
95%.
The present invention will now be described with
reference to preferred embodiments and several comparison
examples.
EXAMPLES
:
Onto twisted conductors 100 mm2 in cross section, an
inner semiconductor layer of a thickness of about 0.8 mm was
extruded at 120C and then compositions comprising various
compounds as shown in Tables 1 and 2 were extruded at 120C (2
mm thickness) thereover to provide electric wires or cables. -~
As is shown in Tables 1 and 2, where cross-linking by electron
beam irradiation is desired, it is eff~cted in a conventional
manner at 30 Mrad after extrusion. When chemical cross-linking
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1 is desired, a cross-linking agent or agents is/are is blended
with the composition prior to extrusion and cross-linking
efrected by heating at 200C after extrusion coating.
The electric wires or cables thus obtained were
immersed in water at 70C, a 20 KV AC voltage applied thereto, -
and kept under these conditions for a certain period of time. -~
The development of trees was examined and the time required for
dielectric bxeakdown in the insulating layers determined relative
to the standard of a polyethylene layer (melt index: l.0,
density: 0.92) set as "l". The results are shown in Tables l
and 2.
In Table l, polystyrene or a styrene copolymer was
used, as ind;cated therein, whereas ;n Table 2, polystyrene plus
a styrene copolymer were used, as ;ndicated therein.
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t *l The test specimens were remo~ed from the water after
the application of the applied voltage and micro-
scopically examined to observe the development of trees.
X: Considerable development of the trees
O: No substantial development of the trees
~: No development of the trees
*2 Determined by tensile testing using dumbbells according
to JIS-3 (modified)
O: With more than 200% elongation
X: With less than 200~ elongation
1 0
*3 Low density polyethylene: M.I. = 1.2 and p= 0.92
; *4 SBR : styrene-butadiene copolymer; styrene content: 23%
*5 SBR : styrene-butadiene copolymer; styrene content: 70%
*6 Et-St : graft copolymer of polyethylene and polystyrene
*7 ABS : acrylonitrile-butadiene-styrene copolymer
*8 ACS : acrylonitrile-chlorinated ethylene-styrene copolymer
- *9 HDPE: high density polyethylene with M.I. = 0.2 and
p = 0.96 ~ -
*10 EVA : ethylene-vinyl acetate copolymer
*11 Cross-linked by electron beam irradiation
*12 Chemîcally cross-linked by heating blend of polymers
~with 2 parts by weight of dicumyl peroxide therein
as a cross-linking agent per 1~0 parts by weight of
the polymer mixture.
- While the invention has been described in detail and
with reference to specific embodiments thereof, it will be
apparent to one skilled in the art that various changes and
modifications can be made therein without departing from the
~ spirit and scope thereof.
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