Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TREE RESISTANT COMPOSITIONS
Technical Field
This invention relates to compositions
which are useful in low to high voltage insulation
because of their resistance to water trees.
Background Art
Power cables insulated with extruded
dielectrics are known to suffer from shortened life
when installed underground where water contact is
likely. The shortened life has been attributed to
the formation of water trees, which occur when an
organic polymeric material is subjected to an
electrical field over a long period of time in the
presence of water in liquid or vapor form. The net
result is a reduction in the dielectric strength of
the insulation.
Many solutions have been proposed for
increasing the resistance of organic insulating
materials to degradation by water treeing. These
include, for example, the addition to polyethylene
of (i) a polar copolymer such as a copolymer of
ethylene and vinyl acetate; (ii) a voltage
stabilizer such as dodecanol; and (iii) a filler,
e.g., clay. These solutions all have shortcomings
of some kind such as an increase in dielectric loss,
i.e., the power factor, volatility, or cost.
Disclosure of the Invention
An object of this invention, therefore, is
to provide a water tree resistant composition
adapted for use in low to high voltage insulation,
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which does not depend on the use of modifying
additives such as polar copolymers, voltage
stabilizers, or fillers to achieve treeing
resistance, and which will take advantage of the
desirable electrical characteristics of "pure"
polyethylene, for example, its low dissipation
f actor .
Other objects and advantages will become
apparent hereinafter.
According to the present invention, the
above object is met by a water tree resistant
insulation composition comprising:
(a) an ethylene copolymer having a
density of not greater than about 0.920 gram per
cubic centimeter produced by polymerizing a mixture
of ethylene and 4-methyl-1-pentene and, optionally,
one or more other alpha-olefin comonomers in the
presence of a polymerization catalyst; or
(b) an ethylene copolymer having a
density of not greater than about 0.920 gram per
cubic centimeter, produced by polymerizing a mixture
of ethylene and 1-octene and, optionally, one or
more other alpha-olefin comonomers in the presence
of a polymerization catalyst; or
(c) an ethylene copolymer according
to paragraphs (a) or (b) grafted with a hydrolyzable
vinyl silane compound: or
(d) a mixture of any two or more of
the copolymers described in paragraphs (a), (b), and
(c) in any proportion,
wherein said polymerization catalyst is comprised of
(A) magnesium, titanium, halogen, and
an electron donor together with one or more aluminum
containing compounds; or
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(B) vanadium, halogen, and an
electron donor together with one or more aluminum
containing compounds and, optionally, a lower
haloalkane compound.
Detailed Description
In this specification, the term "copolymer"
is considered to mean a polymer based on two or more
comonomers. Either of the above-mentioned
copolymers, i.e., the ethylene/4-methyl-1-pentene
copolymer and the ethylene/1-octene copolymer, can
include additional comonomers such as alpha-olefins
having 3 to 12 carbon atoms or dienes. The dienes
can be conjugated or non-conjugated dienes
containing 5 to 25 carbon atoms such as
1,4-pentadiene, 1,4-hexadiene, 1,5-hexadiene,
dicyclopentadiene, 4-vinyl cyclohexene, 1-vinyl-1-
cyclopentene, and the alkylbicyclononadienes,
indenes, and norbornenes. Ethylidene norbornene is
an example of the latter. The non-conjugated dienes
are preferred.
The ethylene/4-methyl-1-pentene or 1-octene
copolymers can be advantageously blended with one or
more of the following:
(i) polyethylenes having densities in
the range of about 0.91 to 0.93 prepared by
conventional high pressure techniques;
(ii) ethylene copolymers wherein at
least one comonomer is a vinyl acid, a vinyl acid
ester, or a vinyl ester of an organic acid;
(iii) ethylene terpolymers based on at
least two comonomers referred to in items (ii) and
(viii);
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(iv) ethylene terpolymers based on
alpha-olefins having 3 to 8 carbon atoms;
(v) ethylene/propylene rubbers;
(vi) ethylene/propylene/diene monomer
rubbers;
(vii) hydrolyzable graft polymers
produced by grafting silane to any of items (i) to
(vi); or
(viii) ethylene/hydrolyzable silane
' copolymers.
The high pressure technique referred to in
item (i) is described in Introduction to Polymer
Chemistry, Stille, Wiley and Sons, New York, 1982,
at pages 149 to 153. The ethylene/hydrolyzable
silane copolymer can be prepared by the process
described in United States patent 3,225,018 and the
terpolymer by the process described in United States
patent 4,291,136.
The item (i) high pressure polymers, which
are blended with the base ethylene/alpha-olefin
copolymer, are preferably blended to provide an
average density for the blend of no higher than
about 0.920 gram per cubic centimeter. The weight
ratio of base copolymer to added copolymer is
usually in the range of about 3:1 to about 1:3. If
polar copolymers such as ethylene/vinyl acetate or
ethylene/ethyl acrylate are used in the blend, the
concentration of the polar comonomer should be kept
at low levels to avoid high dielectric losses, e.g.,
less than about 10 percent by weight of the blend.
The ethylene/4-methyl-1-pentene or 1-octene
copolymers are produced using either a titanium or
vanadium containing catalyst system
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With respect to the titanium containing
catalyst system, the respective comonomers are
contacted with a catalyst system containing a
catalyst precursor comprising magnesium, titanium, a
halogen, and an electron donor as well as one or
more aluminum containing compounds cocatalyst such
as triethylaluminum and triisobutylaluminum. The
catalyst system and the preparation of the copolymer
are described in United States patent 4,302,565.
The copolymer produced in the presence of
the titanium containing catalyst system can be
prepared as follows: Into a flask equipped with a
mechanical stirrer are placed anhydrous MgCl2 and
tetrahydrofuran (THF). To this mixture, TiCl4 is
added. Porous dehydrated silica is added to the
solution and stirred. The mixture is dried to
provide a dry, free-flowing powder having the
particle size of the silica. The desired weight of
impregnated precursor composition and activator
compound, e.g., triethylaluminum, is added to a
mixing tank with a sufficient amount of anhydrous
aliphatic hydrocarbon diluent such as isopentane to
provide a slurry system. The activator compound and
precursor compound are used in such amounts as to
provide a partially activated precursor composition
which has an A1/Ti ratio of up to 10:1. The
contents of the slurry system are then mixed and
dried. The resulting catalyst is in the form of a
partially activated precursor composition which is
impregnated within the pores of the silica. It is
injected into, and fully activated within, the
polymerization reactor. Activator compound is added
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to the polymerization reactor so as to maintain the
A1/Ti ratio in the reactor at a level of about 10:1
to 400:1. Ethylene is then copolymerized with
4-methyl-1-pentene, preferably, in the gas phase in
a fluidized bed. The reaction is conducted, after
equilibrium is reached, for 1 hour at 85°C and under
a pressure of 300 psig, a gas velocity of about 3 to
6 times Gmf, and a space time yield of about 4.4 to
6.3 in a fluid bed reactor system.
With regard to the vanadium containing
catalyst system, the respective comonomers are
preferably contacted with a supported catalyst
system containing a catalyst precursor comprising a
vanadium trihalide, an electron donor, and a
hydrocarbyl aluminum halide together with a
hydrocarbyl aluminum cocatalyst and a halogen
substituted lower alkane promoter, the lower alkane
promoter having 1 to 7 carbon atoms. The catalyst
system and a process for preparing the copolymer are
described in European Patent Application 0 120 501
published on October 3, 1984.
The copolymer produced in the presence of
the vanadium containing catalyst system can be
prepared as follows: To a flask containing
anhydrous tetrahydrofuran is added VC13. The
mixture is stirred until the VC13 is dissolved.
To this solution is added dehydrated silica and
stirring is continued. The flask is vented and the
solution is dried to the mud stage. The impregnated
silica is a free-flowing solid which has 0.25
millimole of vanadium per gram. The solid is
removed from the flask and stored under nitrogen.
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Then, the modifier is introduced using the following
procedure. To a flask containing anhydrous
isopentane is added the impregnated silica described
above. To this mixture is added with stirring,
diethylaluminum chloride, as modifier, in anhydrous
hexane. The amount of modifier is employed in an
amount sufficient to provide 1.7 mole of modifier
per mole of tetrahydrofuran. This mixture is heated
until the product is a free-flowing powder. The
vanadium precursor is then removed from the flask
and stored under nitrogen. The ethylene copolymer
is produced in the gas phase in a fluidized bed
reactor under the following operating conditions:
promoter/cocatalyst molar ratio: 1.0;
aluminum/vanadium atomic ratio: 40; temperature:
70°C: gas velocity: 1.5 feet per second; nitrogen
pressure (mole percent): 50; comonomer/ethylene
molar ratio: 0.24; hydrogen/ethylene molar ratio:
0.007; and pounds/hour-cubic feet: 4.7.
Trisobutylaluminum cocatalyst is added during
polymerization. Chloroform, CHC13, is added as
the promoter as a 5 Weight percent solution in
isopentane. The polymerization is conducted for
more than one hour after equilibrium is reached
under a pressure of about 300 psig.
While the gas phase polymerization,
particularly one carried out in one or more
fluidized beds, is preferred, the copolymers can be
produced in a conventional solution phase process
such as the process described in the Introduction to
Polymer Chemistry, referred to above.
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The amount of 4-methyl-1-pentene or
1-octene, in the case where the comonomers are the
primary comonomers along with ethylene, is
preferably in the range of about 5 to about 50
percent by weight based on the weight of the
copolymer and is most preferably in the range of
about 15 to about 40 percent by weight. The amount
of ethylene is preferably greater than about 50
percent by weight and is preferably more than 60
percent by weight. Where additional comonomers are
used, the amount of these comonomers is preferably
in the range of about 1 to about 15 percent by
weight.
The density of the ethylene/4-methyl-1-
pentene copolymer is no greater than about 0.920
gram per cubic centimeter and is preferably in the
range of about 0.88 to about 0.920 gram per cubic
centimeter. The melt index is preferably in the
range of about 0.5 to about l0 grams per ten
minutes. Melt index is determined in accordance
with ASTM D-1238, Condition E, and is measured at
190°C. The density of the ethylene/1-octene
copolymer is no greater than about 0.920 gram per
cubic centimeter and is preferably in the range of
about 0.88 to about 0.920 gram per cubic
centimeter. The melt index is preferably in the
range of about 0.5 to about 20 grams per ten minutes
and is most preferably in the range of about 0.5 to
about 10 grams per ten minutes.
Various processes for preparing silane
grafted polyethylene and ethylene/silane copolymers
and numerous unsaturated silanes suitable for use in
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preparing these polymers and bearing hydrolyzable
groups such as alkoxy, oxy aryl, oxyaliphatic, and
halogen are mentioned in United States patents
3.075.948: 3.225.018; 4.412,042; 4.413.066;
4,574,133; and 4,593,071. In the silane grafted
copolymer, the amount of incorporated silane monomer
is preferably from about 0.5 percent to about l0
percent by weight based on the total weight of the
copolymer. The silane monomer is most preferably
incorporated into the copolymer in an amount of
about 0.5 to about 10 percent by weight. The silane
grafted to the copolymer can be, among others, a
vinyl trialkoxy silane such as vinyl trimethoxy
silane or vinyl triethoxy silane. Generally
speaking, any unsaturated monomeric silane
containing at least one hydrolyzable group can be
used. If slower water cure or better shelf
stability are desired, vinyl triisobutoxy silane,
vinyl tris-(2-ethyl-hexoxy) silane or vinyl
trisopropoxy silane can be used.
A free radical generator or catalyst is
used in the preparation of the silane grafted
polymer. Among the most useful free radical
generators are dicumyl peroxide, lauroyl peroxide,
azobisisobutyronitrile, benzoyl peroxide, tertiary
butyl perbenzoate, di(tertiary-butyl) peroxide.
cumene hydroperoxide, 2,5-dimethyl-2,5-di(t-butyl-
peroxy) hexyne. 2,5-dimethyl-2,5-di(t-butylperoxy)-
hexane tertiary butyl hydroperoxide, and isopropyl
percarbonate. The organic peroxides are preferred.
Rbout 0.01 to about 5 percent by weight of free
radical generator based on the weight of the
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copolymer is used, and preferably about 0.01 to
about 0.1 percent by weight.
The organic peroxides are also used to
cross-link or cure the ethylene/4-methyl-1-pentene
and ethylene/1-octene copolymers. The hydrolyzable
silane grafted copolymer can be cross-linked using
an organic peroxide or with moisture in the presence
of a conventional silanol condensation catalyst such
as dibutyl tin dilaurate. The amount of organic
peroxide used for cross-linking is in the range of
about 0.5 to about 5 percent by weight based on the
weight of the copolymer.
Various conventional additives can be added
in conventional amounts to the insulation
compositions. Typical additives are antioxidants,
ultraviolet absorbers, antistatic agents, pigments,
fillers, slip agents, fire retardants, stabilizers,
cross-linking agents, halogen scavengers, smoke
inhibitors, cross-linking boosters, processing aids,
lubricants, plasticizers, and viscosity control
agents.
Wire and cable is generally constructed of
one or more metal conductors insulated with a
polymeric material. In cable, these elements form a
core and are protected by another polymeric sheath
or jacket material. In certain cases, added
protection is afforded by inserting a wrap between
the core and the sheath. Subject composition is
typically used as the insulating or jacketing layer,
and is coated on or extruded about the electrical
conductor. The hydrolyzable composition is
generally cross-linked after it is in place on the
wire or cable.
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The invention is illustrated by the
following examples.
Examples 1 to 7
The resistance of insulating compositions
to water treeing is determined by the method
described in United States patent 4.144,202. This
measurement leads to a value for water treeing
resistance relative to a standard polyethylene
insulating material. The term used for the value is
"water tree growth rate" (WTGR). From experience in
laboratory tests of materials and for accelerated
tests of cables, it has been established that the
value for WTGR should be equal to or less than about
percent of the standard to provide a useful
improvement in cable performance, i.e., in the life
of a cable which is in service and in contact with
water during the period of service.
The compositions of examples 1 to 3 and 5
to 7 are based on ethylene/1-octene copolymers and
are prepared by a gas phase process similar to the
typical procedure for titanium based catalysts,
mentioned above. The compositions of examples 4 and
6 are also based on ethylene/1-octene copolymers and
are prepared by a gas phase process similar to the
typical procedure for vanadium based catalysts,
mentioned above. The copolymers contain
antioxidant, calcium stearate, and dicumyl peroxide
as the cross-linking agent. All specimens are
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compression molded, cured, i.e., cross-linked, in
the press, and vacuum treated for seven days at 80°C
prior to testing. Variables and results are set
forth below.
Exampla
density 0.89950.90180.90500.90600.91050.91100.9165
(g/cc)
melt index0.73 0.40 1.08 0.28 0.93 0.42 1.2
WTGR (%) 0.20 0.05 3.0 0.60 0.2 3.5 3.9
Example 8
The procedure of example 3 is repeated
except that it is not cross-linked. The density is
0.905 gram per cubic centimeter; the melt index is
1.08 grams per 10 minutes; and the WTGR (%) is 6.4.
Examples 9 and 10
The procedure of example 1 is repeated
except that the copolymer is prepared in the
solution phase.
Example 1 2
density (g/cc) 0.905 0.912
melt index 0.9 1.0
WTGR (%) 16.0 g,g
Examples 11 to 14
In these examples, the effect of density on
WTGR of cross-linked ethylene/4-methyl-1-pentene
copolymer produced using a catalyst prepared by a
procedure similar to the typical procedure for
titanium based catalysts, mentioned above. The test
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specimens are example 1, above.Density
made as for
and WTGR are set forth
below.
Example li 12 13 14
density (g/cc) 0.895 0.902 0.910 0.917
WTGR ($) 0.1 0.4 0.9 8.2
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