Note: Descriptions are shown in the official language in which they were submitted.
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COMPOSITION FOR ELECTRIC CABLES
Field of the Invention
The present invention relates to a composition for
electric cables, more particularly an ethylene polymer
composition for the insulating layer of an electric
cable, preferably a medium, or high or very high voltage
electric power cable. The composition comprises an
ethylene polymer and additives, including a peroxide
cross-linking agent and stabilising agents.
Background of the Invention
Electric cables and particularly electric power
cables for medium voltage (MV; 1-35 kV), high voltage
(HV; 35-500 kV) and extra high voltage (EHV; >500 kV) may
be composed of a plurality of polymer layers extruded
around the electric conductor. In power cables the elect-
ric conductor is usually coated first with an inner semi-
conductor layer followed by an insulating layer, then an
outer semiconductor layer followed by water barrier
layers, if any, and on the outside a sheath layer. In
addition, some HV and EHV cables are enclosed in a tube,
usually of aluminium. The layers of the cable are based
on different types of ethylene polymers, which usually
are crosslinked.
Crosslinked ethylene polymers are used for the
insulating layer of electric cables. By the expression
"ethylene polymer" is meant, generally and in connection
with the present invention, a polymer based on polyethy-
lene or a copolymer of ethylene, wherein the ethylene
monomer constitutes the major part of the mass. Thus,
ethylene polymers may be composed of homopolymers or
copolymers of ethylene, wherein the copolymers may be
copolymers of ethylene and one or more monomers which are
copolymerisable with ethylene or graft copolymers. LDPE
(low-density polyethylene, i.e. polyethylene prepared by
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radical polymerisation at a high pressure) is today the
predominant cable insulating material. As mentioned above
the ethylene polymer may be an ethylene copolymer, and in
that case it includes from 0 to about 25% by weight, pre-
y erably about 1-20% by weight of one or more comonomers
which are copolymerisable with ethylene. Such monomers
are well known to those skilled in the art and no exten-
sive enumeration will be required, but as examples, men-
tion can be made of vinylically unsaturated monomers,
such as C3-Ce alpha olefins, for instance propene, butene;
dienes, for instance 1,7-octadiene, 1,9-decadiene;
vinylically unsaturated monomers containing functional
groups, such as hydroxyl groups, alkoxy groups, carbonyl
groups, carboxyl groups and ester groups. Such monomers
may consist of, for instance, (meth)acrylic acid and
alkyl esters thereof, such as methyl-, ethyl- and butyl-
(meth}acrylate; vinylically unsaturated, hydrolysable
silane compounds, such as vinyl trimethoxysilane; vinyl
acetate etc. However, if the ethylene polymer is an
ethylene copolymer the amount of polar comonomer should
be kept low, such that the polar comonomer comprises at
most 10% by weight of the ethylene polymer in order not
to increase the dissipation factor too much. Besides the
additives described in more detail below, the remainder
of the composition according to the present invention is
made up of the ethylene polymer specified above. This
means that the amount of ethylene polymer in the compo-
sition should lie in the range from about 95% by weight
to about 99.7%, preferably about 96-99% by weight of the
composition.
In order to improve the physical properties of the
insulating layer of the electric cable and to increase
its resistance to the influence of different conditions,
the ethylene polymer contains additives the total amount
of which usually is about 0.3-5% by weight, preferably
about 1-4% by weight. These additives include stabilising
additives such as antioxidants to counteract degradation
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due to oxidation, radiation, etc.; lubricating additives,
such as stearic acid; additives for water-tree resist-
ance, such as polyethylene glycol, silicones etc.; and
crosslinking additives such as peroxides which decompose
upon heating and initiate crosslinking of the ethylene
plastic of the insulating composition, optional-ly used
in combination with unsaturated compounds having the
ability to form crosslinks when initiated by radical
forming agents.
In electric cables of the type described above the
presence of water or moisture should be avoided, parti-
cularly in the insulating layer, because of its detri-
mental effect on the properties of the cable. Moisture
leads to the formation of dendritically branched defects,
so-called water trees, which in turn can lead to break-
down and possible electric failure. The risk of formation
of water trees is higher the higher the voltage of the
cable. It is therefore a strong desire to minimise and if
possible eliminate moisture from electric cables, espe-
cially electric power cables (MV, HV and EHV cables).
Moisture in electric cables may either be derived
from moisture in the ambient atmosphere that migrates
into the cable or moisture that is generated in situ in
the cable due to chemical reactions.
In electric cables with peroxide-crosslinked poly-
mers, such as peroxide-crosslinked ethylene polymer
insulating layers, moisture is generated due to decompo-
sition of the peroxide and interaction with additives in
the polymer. The prevailing peroxide-crosslinking agent
is dicumyl peroxide, which during crosslinking inter alia
gives rise to cumyl alcohol, which in turn is prone to
decompose to a-methylstyrene and water. This reaction is
strongly catalysed by acids, i.e. the decomposition and
formation of water is strongly increased if the polymer
composition of the insulating layer contains acidic
substances. Antioxidant additives in polymer compositions
of electric cables are usually sulphur containing com-
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pounds that due to oxidation and decomposition form
acids, such as sulphenic acids, and these acidic sub-
stances strongly influence the decomposition of peroxide
to formation of water and decomposition products such as
a-methylstyrene.
In order to minimise or inhibit moisture in per-
oxide-crosslinked polymers of electric cables, such as an
peroxide-crosslinked ethylene polymer of the insulating
layer of an electric cable, it is therefore essential
that the generation of moisture due to peroxide decompo-
sition should be decreased as much as possible.
Summary of the invention
It has now been found that generation of moisture
due to peroxide decomposition can be substantially
reduced with retention of excellent ageing resistance by
using certain hindered amine light stabilising (HALS)
agents as a combined antioxidant and light stabilising
agent while excluding any conventional antioxidants, such
as phenolic antioxidants, sulphur containing antioxidants
and organic phosphate antioxidants. Surprisingly, the
HALS compound acts not only as an effective light
stabilising agent, but also as an effective antioxidant
making it possible for the composition to pass stringent
requirements for thermo-oxidative stability in spite of
the fact that the composition contains little or no
conventional antioxidants.
More particularly, the present invention provides a
peroxide-crosslinkable ethylene polymer composition for
an insulating layer of an electric cable, which composi-
tion contains up to about 5% by weight of additives in-
cluding a peroxide crosslinking agent and stabilising
agents, characterised in that the stabilising agents
comprise an N-substitued 2,2,6,6-tetramethylpiperidine
compound as an antioxidant and light stabilising agent;
and that the composition after 21 days at 135°C has a re-
tained ultimate tensile strength of at least 75o and a
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retained ultimate elongation of at least 75% when tested
in accordance with IEC 811.
Other distinguishing features and advantages of the
invention will appear from the following specification
5 and the appended claims.
Detailed description of the invention
While as indicated above sulphur containing anti-
oxidants are prone to form acidic substances on oxidation
and decomposition which accelerate moisture formation by
peroxide decomposition, it has been found that certain
N-substitued hindered amine stabilisers comprised of
2,2,6,6-tetramethylpiperidine compounds can be used as
antioxidants that do not form acidic substances and thus
do not contribute to moisture generation but at the same
time give excellent ageing resistance. The 2,2,6,6-tetra-
methylpiperidine compounds are preferably used alone as
antioxidants. Different 2,2,6,6-tetrametylpiperidine
compounds may be used singly or in combination with each
other as stabilising agents in the composition according
to the present invention. Preferably, the composition
includes little or no conventional antioxidants. This
means that the combined amounts of conventional anti-
oxidants, such as phenolic antioxidants, organic
phosphite antioxidants and sulphur containing antioxi-
dams are at most 0.15% by weight of the composition,
preferably at most 0.10% by weight of the composition.
Most preferably the composition does not contain any such
conventional antioxidant at all.
The 2,2,6,6-tetramethylpiperidine compounds can be
incorporated in the ethylene polymer composition by
compounding together with other additives, such as
peroxide crosslinking agent, lubricating additives,
additives for water tree resistance, etc. Generally, the
total amount of antioxidants) should lie in a range of
about 0.1-1.0% by weight, preferably about 0.1-0.5% by
weight.
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As indicated above, the 2,2,6,6-tetramethylpiperi-
dine compounds of the present invention not only act as
effective light stabilising agents, but surprisingly also
as very effective antioxidants providing thermo-oxidative
stability to the composition. The thermo-oxidative
stability provided by the N-substituted 2,2,6,6-tetra-
methylpiperidine compound is usually sufficient for the
requirement of an electric cable insulating layer
composition, so that no other antioxidants are required
for therrno-oxidative stability. That the 2,2,6,6-
tetramethylpiperidine compound alone is able to provide
sufficient thermo-oxidative stability is particularly
surprising in view of the fact that the requirement for
thermo-oxidative stability is very rigourus for electric
cables which have a service life of about 30-40 years.
The thermo-oxidative stability is determined accord-
ing to the International Standard IEC 811. According to
IEC 811 dumbbell test pieces are made of the composition
to be evaluated and are tested for thermo-oxidative
ageing. Normal test temperature is 135°C but the testing
has been performed also at 150°C. The ultimate tensile
strength at break and the ultimate elongation at break of
the composition are determined before the testing is
started and thereafter at predetermined time intervals.
The results are expressed as percent retained ultimate
tensile strength at break (RUTS) and percent retained
ultimate elongation at break (RUE), the initial values
(ageing time 0 days) being given as 1000. The requirement
according to IEC 811 is that after 21 days at 135°C the
retained ultimate tensile strength at break (RUTS) should
be least 75% and that the retained ultimate elongation at
break (RUE) should be at least 750. An increasingly
common request i the cable industry is, however, that 75%
RUTS and RUE should be kept also after 10 days at 150°C.
It is a requisite that the 2,2,6,6-tetramethyl-
piperidine compound is N-substituted. The substituent is
preferably a C1-C8 alkyl, C6-Clz cycloalkyl, C1-Clo acyl or
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acyloxy group or a C1-Ce alkoxy group. Among these
substituents are preferably Ci-C8 alkyl or C1-C8 alkoxy
groups axe preferred. Particularly preferred are C1-CS
alkyl groups, such as methyl, ethyl, propyl or butyl, or
S C1~C~ alkoxy groups, such as methoxy, ethoxy, propoxy or
butoxy.
By way of example 2,2,6,6-tetramethylpiperidine
compounds for use as antioxidants in accordance with the
present invention may be selected from the following:
Structure Trade name
R R
R-NH(CH=),-N{CH=)~-N(CH=),NH-R CHIMASSORB 119
C,H
Irr~Hv
_ N
R ~ON
\N-~
N-( _h-CH,
C, H.~,
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R R
N N'~N~ N CGL--116
H R
to
N/~N
C4Hs CsHs
R = N N N
I
rr~
N \'
O 0
Major component
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N-CHzCHi-O ~--CHiCHz ~ TINUVIN 622
(rIW 3100-4000)
O O
d--C(CH~)aC--O
TINUVIN 765
CH, CH,
For comparison purposes also this compound has been
evaluated:
~ H, CH,
HN- ~---CHI=-C-CHI
CHIMASSORB 944
CHI CH, (MW 2500-4000)
NON
N~--N-(CH,)6 N
N . N
H H
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Among the above mentioned compounds Chimassorb 119
is particularly preferred at present as an antioxidant
according to the present invention.
Preferably the N-substituted 2,2,6,6-tetrametyl-
5 piperidine compound should be compatible with the
ethylene polymer resin of the composition. By "compat-
ible" in this connection is meant that it should be
possible to homogeneously blend the 2,2,6,6-tetramethyl-
piperidine compound with the ethylene polymer resin
10 without migration or exudation of the 2,2,6,6-tetra-
methylpiperidine compound. The N-substituted 2,2,6,6-
-tetramethylpiperidine compound is preferably
incorporated in the ethylene polymer composition by
compounding together c,:ith the other additives of the
composition.
To further facilitate the understanding of the
invention, some illustrative, non-restrictive examples
will be given below. X11 parts and percentages refer to
weight, unless other4:ise stated.
Example 1
Compositions for insulating layers of electric
cables were made by compounding an ethylene polymer resin
consisting of low density polyethylene (LDPE) (density
922 kg/m3, MFRZ 0.9 g/IO min) with various additives
listed in Table 1.
Three compositions according to the present
invention (A, B and C) and two comparative compositions
(D and E) were made. The additives were compounded with
the ethylene polymer resin at a temperature of 220°C. The
contents of the polymer compositions A-E are shown in
Table 1.
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Table 1
Composition in o by weight
Component A B _C _D _E
LDPE 97.9 97.7 97.7 97.7 97.7
Chimassorb 119 0.2 0.4
CGL-II6 0.4
Chimassorb 944 0.4
Irganox~ 1035 0.2
Irganox PS 802 0.2
Methylstyrene dimer
0.4 0.9 0.4 0.4
0.4
Dicumylperoxide 1.5 I.5 1.5 1.5 I.5
The following properties of the compositions B-E
were evaluated: the peroxide response determined as the
change in Gottfert elastograph-value in Nm after 10 min
at 180C; and the
a-methylstyrene
content after 40
min at
220C and 250C, respectively
(which is a measure
of
moisture generation originating from the decomposition
of
the peroxide), dete rmined by HPLC analysis. The results
are shown in Table 2.
Table 2
Properties of compositions B-E
Test B _C _D _E
Elastograph, 180C, 0.81 0.81 0.8I 0.66
10 min
a-Methyl- 100 130 90 3500
styrene 220C,
40 min, (ppm)
a-methyl- 200 320 190 4100
styrene 250C,
40 min, (ppm)
It is evident from Table 2 that the peroxide res-
ponse of the compositions B-C according to the invention,
and also of composition D was clearly better than that of
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the comparative composition E, both in terms of peroxide
response and low water formation.
With regard to the level of a-methylstyrene it is
noted from Table 2 that all HALS-based compositions B-D,
which included the 2,2,6,6-tetramethylpiperidine compound
Chimassorb 119, CGL-116 and Chimassorb 944, respectively,
instead of conventional sulphur-containing antioxidant
additives, gave a substantially reduced level of a-
methylstyrene and thus a substantially reduced moisture
generation.
Example 2
Thermo-oxidative ageing properties
The compositions A-D in Example 1 were also tested in a
thermo-oxidative ageing test.
In this example the heat ageing properties were
determined. Dumbbell test pieces were punched out from
crosslinked, compression moulded plaques made of the
compositions and tested for thermo-oxidative ageing at
135°C (Compositions C and D) and at 150°C (Compositions
A-D) for various periods of time. The ultimate tensile
strength and the ultimate elongation at break of the
compositions were determined before the testing started
and subsequently at predetermined time intervals. In
Table 2 the values are expressed as percent retained
ultimate tensile strength at break (RUTS) and percent
retained ultimate elongation at break (RUE) The initial
values, ageing time 0 days, being given as 100%. The
requirement is that RUTS and RUE after 21 days at 135°C
should not be lower than 750. As stated before, new,
coming requirements may prescribe that RUTS and RUE not
decrease below 75% after 10 days at 150°C. The testing
was carried out in accordance with the International
Standard IEC $11. The results are shown in Table 3.
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Table 3
Composition Ageing time at RUTS (o) RUE (%)
135C (days)
C 0 100 100
14 97 91
21 92 84
D 0 100 100
14 96 82
21 85 72
Composition Ageing time at RUTS (o) RUE (%)
150C (days)
A 0 100 100
85 86
14 75 78
B 0 100 100
10 89 93
C 0 100 100
5 86 78
15 84 76
D 0 100 100
86 69
10 79 62
From the results it can be seen that the innovative
compositions A-C all pass both requirements while the
non-N-substituted Chimassorb 944 does not confer suffi-
cient RUE to compound D.
Example 3
Scorch properties
The scorch properties were evaluated at 135°C in a
Brabender Plasticorder PL 2000-6. The oil-heated kneader
350, 287 cm3 with walzenkneaders W 7646 was used. The
torque was measured as a function of time and the
reported value, T10, is the time when an 10 Nm increase
in torque, using the minimum value as a reference point,
was observed. Composition B was tested with and without
the methylstyrene dimer present in a scorch test. The
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scorch retardant effect of the methylstyrene dimer is
easily seen from the tests since a T10 value of 33 min
was measured in the composition without the methylstyrene
dimer compared to a T10 value of 55 min for the composi-
tion containing the methylstyrene dimer.
Another potential scorch additive, Irganox HP-136,
was also tested replacing the methylstyrene dimer in
composition A, with everything else in composition A
remaining unchanged. It was found to lead to somewhat
inferior crosslinking and a shorter T10-value but still
offers an alternative to methylstyrene dimer.
