U.S. patent application number 17/011613 was filed with the patent office on 2021-01-28 for polymer composition for w&c application with advantageous electrical properties.
The applicant listed for this patent is Borealis AG. Invention is credited to Jan-Ove Bostrom, Gustavo Dominguez, Villgot ENGLUND, Andreas Farkas, Per-Ola Hagstrand, Ulf Nilsson, Annika Smedberg.
Application Number | 20210027912 17/011613 |
Document ID | / |
Family ID | 1000005146904 |
Filed Date | 2021-01-28 |
United States Patent
Application |
20210027912 |
Kind Code |
A1 |
ENGLUND; Villgot ; et
al. |
January 28, 2021 |
POLYMER COMPOSITION FOR W&C APPLICATION WITH ADVANTAGEOUS
ELECTRICAL PROPERTIES
Abstract
The invention relates to a use of a polymer composition with
improved DC electrical properties in a power cable layer and to a
cable surrounded by at least one layer comprising the polymer
composition.
Inventors: |
ENGLUND; Villgot; (Goteborg,
SE) ; Hagstrand; Per-Ola; (Stenungsund, SE) ;
Nilsson; Ulf; (Stenungsund, SE) ; Smedberg;
Annika; (Myggenas, SE) ; Bostrom; Jan-Ove;
(Odsmal, SE) ; Farkas; Andreas; (Stenungsund,
SE) ; Dominguez; Gustavo; (Vasteras, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Borealis AG |
Vienna |
|
AT |
|
|
Family ID: |
1000005146904 |
Appl. No.: |
17/011613 |
Filed: |
September 3, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13635517 |
Dec 18, 2012 |
10811164 |
|
|
PCT/EP2011/052988 |
Mar 1, 2011 |
|
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17011613 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01B 9/00 20130101; H01B
9/02 20130101; C08L 23/06 20130101; H01B 3/441 20130101; H01B 9/027
20130101; H01B 5/14 20130101; C08L 23/0815 20130101; H01B 13/0026
20130101; H01B 3/44 20130101; H01B 3/30 20130101 |
International
Class: |
H01B 3/44 20060101
H01B003/44; H01B 9/00 20060101 H01B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 2010 |
EP |
10156722.0 |
Claims
1. A direct current (DC) power cable comprising a conductor
surrounded by at least an inner semiconductive layer, an insulation
layer, and an outer semiconductive layer, in that order, wherein at
least one layer comprises a polymer composition comprising: (a) a
polyolefin selected from the group consisting of a propylene
homopolymer, a random copolymer of propylene with one or more
comonomer(s), and a heterophasic copolymer of propylene with one or
more comonomer(s), and (b) optionally a second polyolefin which is
different from the polyolefin (a).
2. The direct current (DC) power cable according to claim 1,
wherein the inner semiconductive layer comprises a first
semiconductive composition, the insulation layer comprises the
polymer composition, and the outer semiconductive layer comprises a
second semiconductive composition.
3. The direct current (DC) power cable according to claim 1 wherein
the outer semiconductive layer comprises the outer semiconductive
layer comprises a non-crosslinked or crosslinked second
semiconductive composition, and the inner semiconductive layer
comprises a non-crosslinked first semiconductive composition.
4. The direct current (DC) power cable according to claim 1,
wherein the polymer composition has an electrical conductivity of
0.05 to 40 fS/m, when measured according to DC conductivity method
as described under "Determination Methods".
5. The direct current (DC) power cable according to claim 1,
wherein the amount of the polyolefin (a) is 95 to 100 wt % based on
the combined weight of the polyolefin (a) and the optional second
polyolefin (b).
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. The direct current (DC) power cable according to claim 1,
wherein the optional second polyolefin (b) is present and is an
optionally unsaturated LDPE homopolymer or an optionally
unsaturated LDPE copolymer of ethylene with one or more
comonomer(s).
11. The direct current (DC) power cable according to claim 10,
wherein the polymer composition comprises no polyolefin (b).
12. (canceled)
13. The direct current (DC) power cable according to claim 1,
wherein the polymer composition comprises a peroxide.
14. (canceled)
15. (canceled)
16. (canceled)
17. A process for producing the direct current (DC) power cable
according to claim 1, wherein the process comprises the steps of
applying on the conductor, the inner semiconductive layer
comprising a first semiconductive composition, the insulation layer
comprising an insulation composition, and the outer semiconductive
layer comprising a second semiconductive composition, in that
order, wherein at least one layer comprises the polymer
composition, and optionally crosslinking one or more layers of the
obtained cable in the presence of a crosslinking agent.
18. The direct current (DC) power cable according to claim 13,
wherein the polymer composition comprises up to 30 mmol --O--O--/kg
polymer composition.
19. The direct current (DC) power cable according to claim 1,
wherein the layer comprising the polymer composition is not
crosslinked.
20. The direct current (DC) power cable according to claim 1,
wherein the polyolefin (a) is a propylene homopolymer.
21. The direct current (DC) power cable according to claim 1,
wherein the polyolefin (a) is a random-heterophasic copolymer
comprising a matrix phase and an elastomeric phase, the matrix
phase comprising a propylene homopolymer or a propylene copolymer,
and the elastomeric phase comprising a propylene copolymer.
Description
FIELD OF INVENTION
[0001] The invention relates to a use of a polymer composition for
producing a layer of a direct current (DC) power cable, which is
optionally crosslinkable and subsequently crosslinked, to a direct
current (DC) power cable, which is optionally crosslinkable and
subsequently crosslinked, as well as to a preparation process of
the cable.
BACKGROUND ART
[0002] Polyolefins produced in a high pressure (HP) process are
widely used in demanding polymer applications wherein the polymers
must meet high mechanical and/or electrical requirements. For
instance in power cable applications, particularly in medium
voltage (MV) and especially in high voltage (HV) and extra high
voltage (EHV) cable applications the electrical properties of the
polymer composition has a significant importance. Furthermore, the
electrical properties of importance may differ in different cable
applications, as is the case between alternating current (AC) and
direct current (DC) cable applications.
[0003] A typical power cable comprises a conductor surrounded, at
least, by an inner semiconductive layer, an insulation layer and an
outer semiconductive layer, in that order. The cables are commonly
produced by extruding the layers on a conductor.
Crosslinking of Cables
[0004] The polymer material in one or more of said layers is often
crosslinked to improve e.g. heat and deformation resistance, creep
properties, mechanical strength, chemical resistance and abrasion
resistance of the polymer in the layer(s) of the cable. In
crosslinking reaction of a polymer interpolymer crosslinks
(bridges) are primarily formed. Crosslinking can be effected using
e.g. a free radical generating compound. Free radical generating
agent is typically incorporated to the layer material prior to the
extrusion of the layer(s) on a conductor. After formation of the
layered cable, the cable is then subjected to a crosslinking step
to initiate the radical formation and thereby crosslinking
reaction.
[0005] Peroxides are very commonly used as free radical generating
compounds. The resulting decomposition products of peroxides may
include volatile by-products which are often undesired, since e.g.
may have an negative influence on the electrical properties of the
cable. Therefore the volatile decomposition products such as
methane are conventionally reduced to a minimum or removed after
crosslinking and cooling step. Such removal step, generally known
as a degassing step, is time and energy consuming causing extra
costs.
Electrical Conductivity
[0006] The DC electrical conductivity is an important material
property e.g. for insulating materials for high voltage direct
current (HV DC) cables. First of all, the strong temperature and
electric field dependence of this property will influence the
electric field. The second issue is the fact that heat will be
generated inside the insulation by the electric leakage current
flowing between the inner and outer semiconductive layers. This
leakage current depends on the electric field and the electrical
conductivity of the insulation. High conductivity of the insulating
material can even lead to thermal runaway under high stress/high
temperature conditions. The conductivity must therefore be
sufficiently low to avoid thermal runaway.
[0007] Accordingly, in HV DC cables, the insulation is heated by
the leakage current. For a specific cable design the heating is
proportional to the insulation conductivity x (electrical
field).sup.2. Thus, if the voltage is increased, far more heat will
be generated.
[0008] JP2018811A discloses an insulation layer for a DC cable
which contains a blend of 2-20wt % of a high density polyethylene
with a low density polyethylene. It is stated that blend provides
improved DC breakdown and an impulse property. The blend is mixed
with 2-3wt % of a crosslinking agent. The type and layer structure
of the cable has not been specified.
[0009] WO0137289 discloses a specific thermoplastic propylene homo-
or copolymer for a cable layer material. The applicability of the
material to DC applications is not discussed and the focus is in
LV, MV and HV AC cables, as well as in telecommunication
cables.
[0010] There are high demands to increase the voltage of a power
cable, and particularly of a direct current (DC) power cable, and
thus a continuous need to find alternative polymer compositions
with reduced conductivity. Such polymer compositions should
preferably also have good mechanical properties required for
demanding power cable embodiments. SE0920PCT
OBJECTS OF THE INVENTION
[0011] One of the objects of the present invention is to provide a
use of a further polymer composition with advantageous electrical
properties, i.a. low electrical conductivity, for producing a
direct current (DC) power cable layer.
[0012] Another object of the invention is to provide a direct
current (DC) power cable, wherein at least one layer comprises said
polymer composition with advantageous electrical properties, i.a.
low electrical conductivity. Also a preparation process of the
power cable is provided.
[0013] The invention and further objects and benefits thereof are
described and defined in details below.
DESCRIPTION OF THE INVENTION
[0014] The present invention provides a use of a polymer
composition for producing a layer, preferably an insulation layer,
of a direct current (DC) power cable which comprises [0015] (a) a
polyolefin provided that, when the polyolefin is a low density
polyethylene (LDPE), then it is non-crosslinked, and optionally
[0016] (b) a second polyolefin which is different from the
polyolefin (a).
[0017] Unexpectedly, the polymer composition has advantageous
electrical properties. Namely, the polymer composition of the
invention has reduced, i.e. low, electrical conductivity. "Reduced"
or "low" electrical conductivity as used herein interchangeably
means that the value obtained from the DC conductivity measurement
as defined below under "Determination methods" is low, i.e.
reduced. The low electrical conductivity is beneficial for
minimising the undesired heat formation, e.g. in an insulation
layer of a power cable. Moreover and unexpectedly, the polymer
composition has low electrical conductivity without crosslinking
with a crosslinking agent, such as peroxide. Further unexpectedly,
the non-crosslinked polymer composition can still meet e.g. the
mechanical properties desired for a layer, preferably an insulation
layer, of a power cable, preferably a DC power cable. "Low density
polyethylene", LDPE, is a polyethylene produced in a high pressure
process.
[0018] Typically the polymerization of ethylene and optional
further comonomer(s) in the high pressure process is carried out in
the presence of an initiator(s). The meaning of LDPE polymer is
well known and documented in the literature. Although the term LDPE
is an abbreviation for low density polyethylene, the term is
understood not to limit the density range, but covers the LDPE-like
HP polyethylenes with low, medium and higher densities. The term
LDPE describes and distinguishes only the nature of HP polyethylene
with typical features, such as different branching architecture,
compared to the PE produced in the presence of an olefin
polymerisation catalyst. A "non-crosslinked" low density
polyethylene (LDPE) means that the LDPE present in a layer of a
final DC cable (in use) is not crosslinked and is thus
thermoplastic.
[0019] Accordingly, the low electrical conductivity makes the
polymer composition very desirable for DC power cable applications.
The voltage applied to the power cable is direct (DC). A DC power
cable is defined to be a DC cable transferring energy operating at
any voltage level, typically operating at voltages higher than 1
kV. Moreover, the polymer composition is very advantageous layer
material for a DC power cable, which can be e.g. a low voltage
(LV), a medium voltage (MV), a high voltage (HV) or an extra high
voltage (EHV) DC cable, which terms, as well known, indicate the
level of operating voltage. The polymer composition is even more
preferable layer material for a DC power cable operating at
voltages higher than 36 kV, such as a HV DC cable. For HV DC cables
the operating voltage is defined herein as the electric voltage
between ground and the conductor of the high voltage cable.
[0020] The present invention is further directed to a direct
current (DC) power cable, comprising a conductor surrounded by at
least an inner semiconductive layer, an insulation layer and an
outer semiconductive layer, in that order, wherein at least one
layer, preferably at least an insulation layer, comprises a polymer
composition comprising [0021] (a) a polyolefin provided that, when
the polyolefin is a low density polyethylene (LDPE), then it is
non-crosslinked, and [0022] (b) optionally a second polyolefin
which is different from the polyolefin (a).
[0023] In a first preferable embodiment of the DC cable, or for
producing a DC cable, the inner semiconductive layer comprises a
first semiconductive composition, the insulation layer comprises an
insulation composition and an outer semiconductive layer comprises
a second semiconductive composition, in that order, and wherein the
insulation composition of the insulation layer comprises,
preferably consists, of said polymer composition comprising [0024]
(a) a polyolefin which is a non-crosslinked LDPE, preferably
selected from a LDPE homopolymer or a LDPE copolymer of ethylene
with one or more comonomer(s), and [0025] (b) optionally a second
polyolefin which is different from the polyolefin (a), and wherein
the outer semiconductive layer comprises, preferably consists of, a
crosslinked second semiconductive composition. More preferably, the
inner semicoductive layer comprises, preferably consists of, a
non-crosslinked first semiconductive composition. In this first
embodiment of the DC cable it is thus preferred that the first
semiconductive composition of the inner semiconductive layer and
the polymer composition of the invention of the insulation layer
are non-crosslinked, and the second semiconductive composition of
the outer semiconductive layer is crosslinked.
[0026] Unexpectedly, conductivity properties and, additionally,
mechanical properties of the final cable of this first embodiment
are very suitable for the DC cable applications and even for HV DC
applications including EHV DC applications.
[0027] The polymer composition of the invention of this embodiment
is referred below also shortly as a polymer composition of the
first embodiment.
[0028] In a second, more preferable embodiment of the DC cable, or
for producing a DC cable, of the invention the inner semiconductive
layer comprises a first semiconductive composition, the insulation
layer comprises an insulation composition and an outer
semiconductive layer comprises a second semiconductive composition,
in that order, and wherein the insulation composition of the
insulation layer comprises, preferably consists, of said polymer
composition comprising [0029] (a) a polyolefin which is other than
LDPE, and [0030] (b) optionally a second polyolefin which is
different from the polyolefin (a).
[0031] In this more preferable second embodiment the polyolefin (a)
other than LDPE is optionally, and preferably, non-crosslinked. In
this embodiment the outer semiconductive layer comprises,
preferably consists of, a non-crosslinked or crosslinked second
semiconductive composition. Preferably, the outer semiconductive
layer comprises, preferably consists of, a crosslinked second
semiconductive composition. Optionally, and preferably, the inner
semiconductive layer comprises, preferably consists of, a
non-crosslinked first semiconductive composition. In this second
embodiment of the DC cable it is thus preferred that the first
semiconductive composition of the inner semiconductive layer and
the polymer composition of the invention of the insulation layer
are non-crosslinked, and the second semiconductive composition of
the outer semiconductive layer is non-crosslinked or crosslinked,
preferably crosslinked. The polymer composition of this embodiment
is referred below also shortly as a polymer composition of the
second embodiment.
[0032] Preferably the polymer composition is used in a layer of a
HV DC power cable operating at voltages of 40 kV or higher, even at
voltages of 50 kV or higher. More preferably, the polymer
composition is used in a layer of a HV DC power cable operating at
voltages of 60 kV or higher. The invention is also highly feasible
in very demanding cable applications and can be used in a layer of
a HV DC power cable operating at voltages higher than 70 kV. The
upper limit is not limited. The practical upper limit can be up to
900 kV. The invention is advantageous for use in HV DC power cable
applications operating from 75 to 400 kV, preferably 75 to 350 kV.
The invention is also found to be advantageous even in demanding
extra HV DC power cable applications operating 400 to 850 kV.
[0033] HV DC power cable as used below or in claims means herein
either HV DC power cable, preferably with operating at voltages as
defined above, or extra HV DC power cable, preferably with
operating at voltages as defined above. Thus the term covers
independently the operating areas for both the HV DC cable also EHV
DC cable applications.
[0034] The polymer composition of the invention is referred herein
below also shortly as "polymer composition". The polymer components
thereof as defined above are also shortly referred herein as
"polyolefin (a)" and, respectively, "second polyolefin (b)"
[0035] The polymer composition has preferably an electrical
conductivity of 160 fS/m or less, preferably of 150 fS/m or less,
more preferably of 140 fS/m or less, more preferably of 130 fS/m or
less, more preferably of 120 fS/m or less, more preferably of 110
fS/m or less, more preferably of 100 fS/m or less, more preferably
of 90 fS/m or less, more preferably of 0.01 to 80 fS/m, more
preferably of 0.01 to 70 fS/m, more preferably of 0.05 to 60 fS/m,
more preferably of 0.05 to 50 fS/m, more preferably of 0.05 to 40
fS/m, more preferably of 0.05 to 30 fS/m, more preferably of 0.05
to 20.0 fS/m, more preferably of 0.05 to 10.0 fS/m, most preferably
of 0.05 to 5.0 fS/m, even most preferably of 0.05 to 4.0 fS/m, when
measured according to DC conductivity method as described under
"Determination Methods".
[0036] Accordingly, the invention is also directed to a method for
reducing, i.e. for providing a low, electrical conductivity of a
polymer composition of a DC power cable, by producing at least one
layer, preferably an insulation layer using the polymer composition
of the invention of the second embodiment comprising (a) a
polyolefin which is other than low density polyethylene (LDPE) and
[0037] (b) optionally a second polyolefin which is different from
the polyolefin (a).
[0038] Preferably, the polymer composition comprises the polyolefin
(a) in an amount of 0.1 to 100wt %, preferably of 0.5 to 100wt %,
more preferably of 1.0 to 100wt %, more preferably of 5.0 to 100wt
%, more preferably of 10.0 to 100wt %, more preferably of 20 to
100wt %, more preferably of 30 to 100 wt %, even more preferably of
40 to 100wt %, even more preferably of 50 to 100 wt %, even more
preferably of 60 to 100wt %, even more preferably of 70 to 100wt %,
most preferably of 80 to 100 wt %, even most preferably of 90 to
100wt %, even most preferably of 95 to 100wt % based on the
combined weight of the polyolefin (a) and the optional second
polyolefin (b).
[0039] The polyolefin (a) is preferably a polyethylene polymerised
in the presence of an olefin polymerisation catalyst and selected
from an ethylene homopolymer or a copolymer of ethylene with one or
more comonomer(s); or a homo- or copolymer of C3-20 alpha-olefin
which is preferably selected from a propylene homopolymer, a random
copolymer of propylene with one or more comonomer(s) or
heterophasic copolymer of propylene with one or more comonomer(s),
or from homo- or copolymers of butene. "Polyethylene polymerised in
the presence of an olefin polymerisation catalyst" is also often
called as "low pressure polyethylene" to distinguish it clearly
from LDPE. Both expressions are well known in the polyolefin
field.
[0040] According to one preferred embodiment, the polyolefin (a) is
polyethylene selected from very low density polyethylene (VLDPE)
copolymers, linear low density polyethylene (LLDPE) copolymers,
medium density polyethylene (MDPE) copolymers or high density
polyethylene (HDPE) homopolymers or copolymers. The low pressure
polyethylene can be unimodal or multimodal with respect to
molecular weight distribution. Preferably the polyolefin (a) is
selected from a LLDPE copolymer, a MDPE copolymer or a HDPE
polymer, more preferably the polyolefin (a) is selected from a
LLDPE copolymer, a MDPE copolymer or a HDPE polymer which is
unimodal or multimodal with respect to molecular weight
distribution. Preferably such LLDPE, MDPE or HDPE polymer is
multimodal with respect to molecular weight distribution.
[0041] According to another equally preferred embodiment, the
polyolefin (a) is a propylene homopolymer, a random copolymer of
propylene with one or more comonomer(s) or heterophasic copolymer
of propylene with one or more comonomer(s).
[0042] In case of the low pressure PE or PP, more preferably a low
pressure PE, even more preferably a low pressure HDPE, as the
preferred polyolefin (a), it is believed, however without binding
to any theory, that the lamella thickness of crystals and the
weight fraction of such crystals present in the low pressure PE or
PP, more preferably in the low pressure PE, even more preferably in
the low pressure HDPE, can further contribute to the reduced
(=improved) electrical conductivity (determined according to said
DC conductivity method) of the polymer composition. Moreover, even
when such preferred polyolefin (a) is used in small amounts in a
polymer composition, it can contribute to the improved electrical
conductivity property of the polymer composition. Accordingly, in
one preferable embodiment the polymer composition comprises at
least 3 wt %, preferably at least 5 wt %, more preferably from 10
to 100 wt %, even more preferably from 15 to 95 wt %, of crystals
having a lamella thickness of at least 10 nm, when measured
according to DSC method as described below under "Determination
Methods". More preferably in this embodiment the polymer
composition has a weight fraction of crystals with lamella
thickness >10 nm of at least 1 wt %, more preferably of at least
3 wt %, even more preferably from 5 to 100 wt %, and most
preferably from 10 to 95 wt %", when measured according to DSC
method as described below under "Determination Methods. More
preferably in this embodiment, the polymer composition comprises a
polyolefin (a) which is a low pressure PE or PP, more preferably a
low pressure PE, even more preferably a low pressure HDPE, and
which polyolefin (a) comprises at least 3 wt %, preferably at least
5 wt %, more preferably from 10 to 100 wt %, even more preferably
from 15 to 95 wt %, of crystals with a lamella thickness of at
least 10 nm, when measured according to DSC method as described
below under "Determination Methods". Even more preferably in this
embodiment, the polymer composition comprises a polyolefin (a)
which is a low pressure PE or PP, more preferably a low pressure
PE, even more preferably a low pressure HDPE, and which polyolefin
(a) has a weight fraction of crystals with lamella thickness >10
nm of at least 1 wt %, more preferably of at least 3 wt %, even
more preferably from 5 to 100 wt %, and most preferably from 10 to
95 wt %", when measured according to DSC method as described below
under "Determination Methods. Such polymers are e.g. commercial and
supplied for instance by Borealis, Ineos, Total Petrochemicals,
Exxonmobil, Dow etc. In this context the above used definitions
have the following meanings: [0043] "Lamella thickness"=Thickness
of crystal lamellas in the material (fractions*<0.1 wt % are
ignored). [0044] * Refer to crystal fractions of one degree Celsius
intervals.
[0045] "Crystal fraction with lamella thickness>10 nm"=Fraction
of the crystals which have a thickness above 10 nm based on the
amount of the crystallised part of the polymer "Crystallinity"=wt %
of the polymer that is crystalline
[0046] "Weight fraction of crystals with lamella thickness>10 nm
[wt %] "=Crystal fraction with lamella thickness>10
nm".times."Crystallinity".
[0047] The defined properties are measured according to DSC method
as described below under "Determination Methods".
[0048] Further preferably, the polymer composition comprises the
optional polyolefin (b) in an amount of 0 to 99.9wt %, preferably 0
to 99.5wt %, preferably 0 to 99.0wt %, prefereably 0 to 95wt %,
preferably 0 to 90wt %, 0 to 80wt %, more preferably 0 to 70wt %,
more preferably 0 to 60wt %, more preferably 0 to 50wt % , more
preferably 0 to 40wt %, even more preferably 0 to 30wt %, more
preferably 0 to 20wt %, more preferably 0 to 10.0wt %, even more
preferably of 5.0wt % or less, based on the combined weight of the
polyolefin (a) and the optional second polyolefin (b).
[0049] Preferably, the optional second polyolefin (b) is a
polyolefin as defined above or later below for the polyolefin (a)
of the second embodiment and is different from polyolefin (a), or
is a low density polyethylene (LDPE) polymer selected from an
optionally unsaturated LDPE homopolymer or an optionally
unsaturated LDPE copolymer of ethylene with one or more
comonomer(s). In the preferred second embodiment of the polyolefin
(b), if present, the polyolefin (b) is an LDPE selected from an
optionally unsaturated LDPE homopolymer or an optionally
unsaturated LDPE copolymer of ethylene with one or more
comonomer(s).
[0050] The combined amount of polyolefin (a) and the optional
second polyolefin (b) in the polymer composition of the invention
is typically of at least 50 wt %, preferably at least 60 wt %, more
preferably at least 70 wt %, more preferably at least 75 wt %, more
preferably from 80 to 100 wt % and more preferably from 85 to 100
wt %, of the total weight of the polymer component(s) present in
the polymer composition. The preferred polymer composition consists
of polyolefin (a) and the optional second polyolefin (b) as the
only polymer components. The expression means that the polymer
composition does not contain further polymer components, but the
polyolefin (a) and the optional second polyolefin (b) as the sole
polymer component(s). However, it is to be understood herein that
the polymer composition may comprise further components other than
the polyolefin (a) and the optional second polyolefin (b)
components, such as additives which may optionally be added in a
mixture with a carrier polymer, i.e. in so called master batch.
[0051] Most preferably the polymer composition comprises no
polyolefin (b) (0 wt % of polyolefin (b)), based on the combined
weight of the polyolefin (a) and the optional second polyolefin
(b).
[0052] Accordingly, it is preferred that the polymer composition
comprises 100 wt % of the polyolefin (a) as defined above, below or
in claims, based on the combined weight of the polyolefin (a) and
the optional second polyolefin (b). More preferably the polymer
composition consists of the polyolefin (a) as the sole polymer
component.
[0053] The polyolefin (a) and the optional second polyolefin (b)
and the further properties and preferable embodiments thereof are
further described later below.
[0054] The Polymer composition of the second embodiment can have a
beneficial low electrical conductivity also when it is crosslinked
with a crosslinking agent. The polymer composition of the second
embodiment of the invention can thus optionally be
crosslinkable.
[0055] "Crosslinkable" means that the polymer composition can be
crosslinked using a crosslinking agent(s) before the use in the end
application thereof. Crosslinkable polymer composition further
comprises a crosslinking agent. In case of crosslinking, it is
preferred that the polyolefin (a) and the optional second
polyolefin (b) of the polymer composition are crosslinked.
Moreover, the crosslinked polymer composition or, respectively, the
crosslinked polyolefin (a) and the optional second polyolefin (b),
is most preferably crosslinked via radical reaction with a free
radical generating agent. The crosslinked polymer composition has a
typical network, i.a. interpolymer crosslinks (bridges), as well
known in the field. As evident for a skilled person, the
crosslinked polymer composition can be and is defined herein with
features that are present in the polymer composition, polyolefin
(a) or the optional second polyolefin (b) before or after the
crosslinking, as stated or evident from the context. For instance
the amount of the crosslinking agent in the polymer composition or
a compositional property, such as MFR, density and/or unsaturation
degree, of the polyolefin (a) or the second polyolefin (b) are
defined, unless otherwise stated, before crosslinking.
"Crosslinked" means that the crosslinking step provides a further
technical feature to the crosslinked polymer composition (product
by process) which makes a further difference over prior art. For
instance the crosslinking may contribute to the mechanical
properties and the heat and deformation resistance of the polymer
composition.
[0056] Accordingly, in embodiments, wherein the polymer composition
comprises no crosslinking agent, the electrical conductivity as
described under the "Determination method" is measured from a
sample of said polymer composition which is non-crosslinked (i.e.
does not contain a crosslinking agent and has not been crosslinked
with a crosslinking agent). In embodiments, wherein the polymer
composition is crosslinkable and comprises a crosslinking agent,
then the electrical conductivity is measured from a sample of the
crosslinked polymer composition (i.e. a sample of the polymer
composition is first crosslinked with the crosslinking agent
initially present is the polymer composition and then the
electrical conductivity is measured from the obtained crosslinked
sample). The conductivity measurement from a non-crosslinked or a
crosslinked polymer composition sample is described under
"Determination Methods". The amount of the crosslinking agent, if
present, can vary, preferably within the ranges given below.
[0057] Accordingly, the polymer composition of the second
embodiment optionally comprises a crosslinking agent, which is then
preferably a peroxide in an amount of 0 to 110 mmol --O--O-/kg
polymer composition, preferably 0 to 90 mmol --O--O--/kg polymer
composition (corresponds 0 to 2.4 wt % of dicumyl peroxide based on
the polymer composition), 0 to 75 mmol --O--O--/kg polymer
composition, preferably of 0 to 50 mmol --O--O--/kg polymer
composition, preferably of 0 to 40 mmol --O--O--/kg polymer
composition, preferably of 0 to 37 mmol --O--O--/kg polymer
composition, preferably of 0 to 35 mmol --O--O--/kg polymer
composition, preferably of 0 to 34 mmol --O--O--/kg polymer
composition, preferably of 0 to 33 mmol --O--O--/kg polymer
composition, more preferably from 0 to 30 mmol --O--O--/kg polymer
composition, more preferably from 0 to 20 mmol --O--O--/kg polymer
composition, more preferably from 0 to 10.0 mmol --O--O--/kg
polymer composition, more preferably from 0 to 7.0 mmol --O--O--/kg
polymer composition, more preferably less than 5.0 mmol --O--O--/kg
polymer composition, most preferably the polymer composition
comprises no crosslinking agent (=0wt % of added crosslinking
agent). The lower limit of the crosslinking agent, if present, is
not limited and can be at least 0.1 mmol --O--O--/kg polymer
composition, preferably at least 0.5 mmol --O--O--/kg polymer
composition, more preferably at least 5.0 mmol --O--O--/kg polymer
composition. The lower peroxide content can shorten the required
degassing step of the produced and crosslinked cable, if
desired.
[0058] The unit "mmol --O--O--/kg polymer composition" means herein
the content (mmol) of peroxide functional groups per kg polymer
composition, when measured from the polymer composition prior to
crosslinking. For instance the 35 mmol --O--O--/kg polymer
composition corresponds to 0.95.0 wt % of the well known dicumyl
peroxide based on the total amount (100 wt %) of the polymer
composition.
[0059] Such polymer composition may comprise one type of peroxide
or two or more different types of peroxide, in which case the
amount (in mmol) of --O--O--/kg polymer composition, as defined
above, below or in claims, is the sum of the amount of --O--O--/kg
polymer composition of each peroxide type. As non-limiting examples
of suitable organic peroxides, di-tert-amylperoxide,
2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne,
2,5-di(tert-butylperoxy)-2,5-dimethylhexane,
tert-butylcumylperoxide, di(tert-butyl)peroxide, dicumylperoxide,
butyl-4,4-bis(tert-butylperoxy)-valerate,
1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane,
tert-butylperoxybenzoate, dibenzoylperoxide, bis(tert
butylperoxyisopropyl)benzene,
2,5-dimethyl-2,5-di(benzoylperoxy)hexane,
1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert
amylperoxy)cyclohexane, or any mixtures thereof, can be mentioned.
Preferably, the peroxideis selected from
2,5-di(tert-butylperoxy)-2,5-dimethylhexane,
di(tert-butylperoxyisopropyl)benzene, dicumylperoxide,
tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures
thereof. Most preferably, the peroxide is dicumylperoxide.
[0060] However, as mentioned above, the electrical conductivity of
a non-crosslinked polymer composition is surprisingly low.
Accordingly, in a preferred embodiment the polymer composition is
not crosslinked. Thus it is preferred that the polymer composition
comprises no crosslinking agent. In this embodiment the
non-crosslinked polymer composition has very advantageous low
electrical conductivity and need not to be crosslinked for use in a
layer, preferably in an insulation layer, of a DC power cable. In
this embodiment the prior art drawbacks relating to the use of a
crosslinking agent in cable layer can be avoided. Naturally, the
embodiment enables to simplify the cable production process. The
preferred non-crosslinked polymer composition is the polymer
composition according to the second embodiment.
[0061] "Without crosslinking", "not crosslinked" or
"non-crosslinked" as used herein above, below or in claims means
that no crosslinking agent is added to the polymer composition for
crosslinking the composition. Similarly, "comprises no crosslinking
agent" means herein the polymer composition does not comprise any
crosslinking agent which would have been added to crosslink the
composition.
[0062] Additionally, the polymer composition of the invention may
contain, in addition to the polyolefin (a), optional second
polyolefin (b) and the optional peroxide, further component(s) such
as polymer component(s) and/or additive(s), preferably additive(s),
such as any of antioxidant(s), scorch retarder(s) (SR),
crosslinking booster(s), stabiliser(s), processing aid(s), flame
retardant additive(s), water tree retardant additive(s), acid or
ion scavenger(s), inorganic filler(s) and voltage stabilizer(s), as
known in the polymer field. The polymer composition comprises
preferably conventionally used additive(s) for W&C
applications, such as one or more antioxidant(s) and optionally one
or more scorch retarder(s), preferably at least one or more
antioxidant(s). The used amounts of additives are conventional and
well known to a skilled person.
[0063] As non-limiting examples of antioxidants e.g. sterically
hindered or semi-hindered phenols, aromatic amines, aliphatic
sterically hindered amines, organic phosphites or phosphonites,
thio compounds, and mixtures thereof, can be mentioned.
[0064] It is preferred that the polymer composition and the
subgroup thereof are used for producing an insulation layer.
Preferably, the polymer composition is avoid of, i.e. does not
comprise, a carbon black. Also preferably, the polymer composition
is avoid of, does not comprise, flame retarding additive(s) in such
amounts conventionally used for acting as "flame retardants", e.g.
a metal hydroxide containing additives in flame retarding
amounts.
[0065] The following preferable embodiments, properties and
subgroups of the polyolefin (a) and the optional second polyolefin
(b) components suitable for the polymer composition are
independently generalisable so that they can be used in any order
or combination to further define the preferable embodiments of the
polymer composition and the cable produced using the polymer
composition. Moreover, it is evident that the given polyolefin (a)
and the optional second (b) descriptions apply to the polyolefin
prior optional crosslinking.
[0066] Polyolefin (a)
[0067] In case the polyolefin (a) is a non-crosslinked LDPE, then
the suitable LDPE is according to the LDPE as described and defined
under the second optional polyolefin (b).
[0068] Preferably the polyolefin (a) is a low pressure
polyethylene, i.e. polyethylene polymerised in the presence of an
olefin polymerisation catalyst; or a homo- or copolymer of C3-20
alpha-olefin which is preferably a polypropylene or a homo- or
copolymers of butene. Most preferred polyolefin (a) is a low
pressure polyethylene or polypropylene.
[0069] "Olefin polymerisation catalyst" means herein a conventional
coordination catalyst. It is preferably selected from a
Ziegler-Natta catalyst, single site catalyst which term comprises a
metallocene and a non-metallocene catalyst, or a chromium catalyst,
or any mixture thereof.
[0070] Term "Polyethylene" (PE) means homopolymer of ethylene or a
copolymer of ethylene with one or more comonomer(s).
"Polypropylene" (PP) means propylene homopolymer, a random
copolymer of propylene with one or more comonomer(s) or
heterophasic copolymer of propylene with one or more
comonomer(s).
[0071] Low pressure PE or PP can be unimodal or multimodal with
respect to molecular weight distribution (MWD=Mw/Mn). Generally, a
polymer comprising at least two polymer fractions, which have been
produced under different polymerization conditions resulting in
different (weight average) molecular weights and molecular weight
distributions for the fractions, is referred to as "multimodal".
The prefix "multi" relates to the number of different polymer
fractions present in the polymer. Thus, for example, multimodal
polymer includes so called "bimodal" polymer consisting of two
fractions. The form of the molecular weight distribution curve,
i.e. the appearance of the graph of the polymer weight fraction as
a function of its molecular weight, of a multimodal polymer will
show two or more maxima or is typically distinctly broadened in
comparison with the curves for the individual fractions. For
example, if a polymer is produced in a sequential multistage
process, utilizing reactors coupled in series and using different
conditions in each reactor, the polymer fractions produced in the
different reactors will each have their own molecular weight
distribution and weight average molecular weight. When the
molecular weight distribution curve of such a polymer is recorded,
the individual curves from these fractions form typically together
a broadened molecular weight distribution curve for the total
resulting polymer product.
[0072] The term "multimodal" means herein, unless otherwise stated,
multimodality at least with respect to molecular weight
distribution (MWD=Mw/Mn) and includes also bimodal polymer.
[0073] A multimodal low pressure PE or PP usable in the present
invention comprises a lower weight average molecular weight (LMW)
component (A) and a higher weight average molecular weight (HMW)
component (B). Said LMW component has a lower molecular weight than
the HMW component.
[0074] Naturally, the multimodal low pressure PE or PP may in
addition or alternatively to multimodality with respect to MWD be
multimodal with respect to density and comonomer content. I.e. the
LMW and HMW components may have different comonomer content or
density, or both.
[0075] Preferably the low pressure PE and PP independently have a
MWD of at least 2.0, preferably of at least 2.5, preferably of at
least 2.9, preferably from 3 to 30, more preferably from 3.3 to 25,
even more preferably from 3.5 to 20, preferably 3.5 to 15. A
unimodal PE or PP has typically a MWD of 3.0 to 10.0.
[0076] The low pressure PE or PP can be a copolymer of ethylene or,
respectively, of propylene (random or heterophasic), with one or
more comonomer(s). Comonomer as used herein means monomer units
other than ethylene or, respectively propylene, which are
copolymerisable with ethylene or, respectively with propylene.
[0077] The low pressure PE copolymer is preferably a copolymer of
ethylene with one or more olefin comonomer(s), preferably with at
least C3-20 alpha olefin, more preferably with at least one C4-12
alpha-olefin, more preferably with at least one C4-8 alpha-olefin,
e.g. with 1-butene, 1-hexene or 1-octene. The amount of
comonomer(s) present in a PE copolymer is from 0.1 to 15 mol%,
typically 0.25 to 10 mol-%.
[0078] The PP copolymer is preferably a copolymer of propylene with
one or more olefin comonomer(s), preferably with at least one of
ethylene or C4-20 alpha olefin, more preferably with at least one
of ethylene or C4-12 alpha-olefin, more preferably with at least
one of ethylene or C4-8 alpha-olefin, e.g. with ethylene, 1-butene,
1-hexene or 1-octene.
[0079] Preferably, the low pressure PE or PP copolymer can be a
binary copolymer, i.e. e.g. the PE copolymer contains ethylene and
one comonomer, or a terpolymer, i.e. e.g. the PE copolymer contains
ethylene and two or three comonomers.
[0080] In one preferable embodiment the polyolefin (a) is a low
pressure PE selected from a very low density ethylene copolymer
(VLDPE), a linear low density ethylene copolymer (LLDPE), a medium
density ethylene copolymer (MDPE) or a high density ethylene
homopolymer or copolymer (HDPE). These well known types are named
according to their density area. The term VLDPE includes herein PEs
which are also known as plastomers and elastomers and covers the
density range of from 850 to 909 kg/m.sup.3. The LLDPE has a
density of from 909 to 930 kg/m.sup.3, preferably of from 910 to
929 kg/m.sup.3, more preferably of from 915 to 929 kg/m.sup.3. The
MDPE has a density of from 930 to 945 kg/m.sup.3, preferably 931 to
945 kg/m.sup.3The HDPE has a density of more than 945 kg/m.sup.3,
preferably of more than 946 kg/m.sup.3, preferably form 946 to 977
kg/m.sup.3, more preferably form 946 to 965 kg/m.sup.3.
[0081] LLDPE, MDPE or HDPE are preferable types of low pressure PE
for use as the polyolefin (a) of the present invention. Such LLDPE,
MDPE or HDPE can be unimodal or multimodal. Preferably, the low
pressure PE, more preferably the LLDPE, MDPE or HDPE, is
multimodal, such as bimodal. The multimodality contributes to the
mechanical and processing properties, such as thermal stress
cracking (TSCR).
[0082] The low pressure PE has preferably an MFR2 of up to 1200
g/10 min, such as of up to 1000 g/10 min, preferably of up to 500
g/10 min, preferably of up to 400 g/10 min, preferably of up to 300
g/10 min, preferably of up to 200 g/10 min, preferably of up to 150
g/10 min, preferably from 0.01 to 100, preferably from 0.01 to 50
g/10 min, preferably from 0.01 to 40.0 g/10 min, preferably of from
0.05 to 30.0 g/10 min, preferably of from 0.1 to 20.0 g/10 min,
more preferably of from 0.2 to 15.0 g/10 min.
[0083] In another equally preferable embodiment the polyolefin (a)
is a propylene homopolymer, a random copolymer of propylene with
one or more comonomer(s) or heterophasic copolymer of propylene
with one or more comonomer(s). The type of polypropylenes are well
known in the field.
[0084] In "random copolymer" the comonomer(s) in said copolymer is
distributed randomly, i.e. by statistical insertion of the
comonomer units, within the copolymer chain. Said "heterophasic
copolymer of propylene" comprises a matrix phase which can be a
propylene homopolymer or a propylene copolymer, and an elastomeric
phase of propylene copolymer, also known as rubber part, which is
dispersed in said matrix phase.
[0085] A propylene homopolymer as the polyolefin (a) has typically
a xylene solubles content (XS, in wt %) e.g. of below 5 wt %, such
as 0.1 to 3 wt %, based on the amount of said propylene
homopolymer.
[0086] A random propylene copolymer as the polyolefin (a) contains
preferably conventionally used amounts of comonomer, for example up
to 30 wt % of the amount of said random propylene copolymer, e.g.
between 0.5 to 20wt %, preferably 1.0 to 10.0 wt %, more preferably
between 2.0 to 7.0 wt % based on the amount of said random
propylene copolymer. The xylene solubles content (wt %) of said
random propylene copolymer is preferably of up to 20 wt %,
preferably of up to 15 wt %, e.g. 0.5 to 10.0 wt %, based on the
amount of said random propylene copolymer.
[0087] A heterophasic propylene copolymer as the polyolefin (a)
comprises the matrix phase of propylene homo- or copolymer of up to
95.0 wt %, preferably of from 20 to 90 wt %, the elastomeric
propylene copolymer phase of up to 80 wt %, preferably of from 10.0
to 40wt %, based on the amount of said heterophasic propylene
copolymer. In case said matrix phase of said heterophasic propylene
copolymer is a random propylene copolymer, then the comonomer
content and XS (wt %) content of said matrix phase is preferably as
defined above for said random copolymer as said polyolefin
component (a). In case said matrix phase is a propylene
homopolymer, then said XS (wt %) content is preferably as defined
above for said propylene homopolymer component as said polyolefin
component (a). The heterophasic propylene copolymer has typically a
total xylene solubles (wt %) of up to 50 wt %, preferably of up to
30 wt %, based on the amount of the heterophasic propylene
copolymer.
[0088] PP as the polyolefin (a) has preferably an MFR.sub.2 of up
to 1200 g/10 min, such as of up to 1000 g/10 min, preferably of up
to 500 g/10 min, preferably of up to 400 g/10 min, preferably of up
to 300 g/10 min, preferably of up to 200 g/10 min, preferably of up
to 150 g/10 min, preferably from 0.01 to 100, preferably from 0.01
to 50 g/10 min, preferably from 0.01 to 40.0 g/10 min, preferably
of from 0.05 to 30.0 g/10 min, preferably of from 0.1 to 20.0 g/10
min, more preferably of from 0.2 to 15.0 g/10 min
[0089] Suitable low pressure PE and PP as the polyolefin (a) are as
such well known and can be e.g. commercially available or,
alternatively, can be produced according to or analogously to
conventional polymerisation processes which are well documented in
the literature.
[0090] The catalyst can be selected from well known coordination
catalysts, preferably from Ziegler Natta, single site, which term
comprises well known metallocene and non-metallocene catalyst, or
Chromium catalyst, or any mixtures thereof. It is evident for a
skilled person that the catalyst system comprises a co-catalyst.
Suitable Ziegler Natta catalysts for low pressure PE are described
e.g. in EP0810235 or EP0688794 which are all incorporated by
reference herein. Suitable Ziegler Natta catalysts for PP are
described e.g. in WO03000754 or EP 1 484 345, which are all
incorporated by reference herein. As known PP catalysts typically
may contain internal or external donors. As well known the
catalytically active catalyst component(s), such as the
catalytically active component of the Ziegler Natta catalyst, is
normally combined with an activator. Moreover the catalyst system
can be non-supported or supported on a carrier, such as external
carrier, like silica-based or Mg-based carrier.
[0091] The unimodal low pressure PE and PP can be produced by a
single stage polymerisation in a single reactor in a well known and
documented manner The multimodal (e.g. bimodal) low pressure PE or
PP can be produced e.g. by blending mechanically together two or
more separate polymer components or, preferably, by in-situ
blending during the polymerisation process of the components. Both
mechanical and in-situ blending are well known in the field.
Accordingly, the preferable in-situ blending means the
polymerisation of the polymer components under different
polymerisation conditions, e.g. in a multistage, i.e. two or more
stage, polymerization or by the use of two or more different
polymerization catalysts, including multi- or dual site catalysts,
in a one stage polymerization, or by use a combination of
multistage polymerisation and two or more different polymerisation
catalysts. In the multistage polymerisation process the polymer is
polymerised in a process comprising at least two polymerisation
stages. Each polymerisation stage may be conducted in at least two
distinct polymerisation zones in one reactor or in at least two
separate reactors. Preferably, the multistage polymerisation
process is conducted in at least two cascaded polymerisation zones.
Polymerisation zones may be connected in parallel, or preferably
the polymerisation zones operate in cascaded mode. The
polymerisation zones may operate in bulk, slurry, solution, or gas
phase conditions or in any combinations thereof. In the preferred
multistage process a first polymerisation step is carried out in at
least one slurry, e.g. loop, reactor and the second polymerisation
step in one or more gas phase reactors. One preferable multistage
process is described in EP517868. For suitable polypropylenes as
said polyolefin (a) the preparation processes thereof, reference is
also made to e.g. Nello Pasquini (Ed.) Polypropylene Handbook,
Hanser, Munich, 2005, pages 15-141.
[0092] In general, the temperature in the low pressure PE and PP
polymerisation is typically from 50 to 115.degree. C., preferably
from 60 to 110.degree. C. The pressure is from 1 to 150 bar,
preferably from 10 to 100 bar. The precise control of
polymerisation conditions can be performed by using different types
of catalyst and using different comonomer and/or hydrogen
feeds.
[0093] Prepolymerisation may precede the actual polymerisation
step(s), as well known in the field.
[0094] In case of heterophasic copolymer of propylene the matrix of
propylene homopolymer or random copolymer can be produced e.g. in a
single stage or as a multistage process described above and the
elastomeric (rubber) part of the propylene copolymer can be
produced as an in-situ polymerisation e.g. in a separate reactor,
e.g. gas phase reactor in the presence of the matrix polymer
produced in the previous stage. Alternatively the elastomeric
copolymer of propylene part can be mechanically compounded to the
matrix phase material, as well known in the art.
[0095] The obtained low pressure PE or PP polymerisation product
may be compounded in a known manner and optionally with additive(s)
and pelletised for further use.
[0096] Optional Second Polyolefin (b)
[0097] The optional second polyolefin (b) (referred herein also
shortly as "the second polyolefin") can be any polyolefin as
defined for polyolefin (a) or a low density polyethylene (LDPE)
polymer. The below description of the suitable LDPE applies also to
the non-crosslinked LDPE of the polyolefin (a).
[0098] A suitable polyolefin as the second polyolefin (b) can be
any polyolefin, such as any conventional polyolefin, which can be
used in a cable layer, preferably in an insulating layer, of a
cable.
[0099] Suitable polyolefins as the second polyolefin (b) are e.g.
as such well known and can be e.g. commercially available or can be
prepared according to or analogously to known polymerization
processes described in the chemical literature.
[0100] The low pressure polyolefin as described for polyolefin (a)
above and LDPE polymer are equally preferable as the optional
second polyolefin (b).
[0101] For the description of low pressure polyolefin as the second
polyolefin (b), reference is made to above polyolefin (a).
[0102] As to LDPE as the optional second polyolefin (b), the LDPE
polymer may be a low density homopolymer of ethylene (referred
herein as LDPE homopolymer) or a low density copolymer of ethylene
with one or more comonomer(s) (referred herein as LDPE copolymer).
The one or more comonomers of LDPE copolymer are preferably
selected from the polar comonomer(s), non-polar comonomer(s) or
from a mixture of the polar comonomer(s) and non-polar
comonomer(s), as defined above or below. Moreover, said LDPE
homopolymer or LDPE copolymer as said second polyolefin (b) may
optionally be unsaturated.
[0103] As well known "comonomer" refers to copolymerisable
comonomer units. As a polar comonomer for the LDPE copolymer as
said second polyolefin (b), comonomer(s) containing hydroxyl
group(s), alkoxy group(s), carbonyl group(s), carboxyl group(s),
ether group(s) or ester group(s), or a mixture thereof, can be
used. More preferably, comonomer(s) containing carboxyl and/or
ester group(s) are used as said polar comonomer. Still more
preferably, the polar comonomer(s) of LDPE copolymer is selected
from the groups of acrylate(s), methacrylate(s) or acetate(s), or
any mixtures thereof. If present in said LDPE copolymer, the polar
comonomer(s) is preferably selected from the group of alkyl
acrylates, alkyl methacrylates or vinyl acetate, or a mixture
thereof. Further preferably, said polar comonomers are selected
from C.sub.1- to C6-alkyl acrylates, C.sub.1- to C6-alkyl
methacrylates or vinyl acetate. Still more preferably, said polar
LDPE copolymer is a copolymer of ethylene with C.sub.1- to
C.sub.4-alkyl acrylate, such as methyl, ethyl, propyl or butyl
acrylate, or vinyl acetate, or any mixture thereof.
[0104] As the non-polar comonomer(s) for the LDPE copolymer as said
second polyolefin (b), comonomer(s) other than the above defined
polar comonomers can be used. Preferably, the non-polar comonomers
are other than comonomer(s) containing hydroxyl group(s), alkoxy
group(s), carbonyl group(s), carboxyl group(s), ether group(s) or
ester group(s). One group of preferable non-polar comonomer(s)
comprise, preferably consist of, monounsaturated (=one double bond)
comonomer(s), preferably olefins, preferably alpha-olefins, more
preferably C.sub.3 to C.sub.10 alpha-olefins, such as propylene,
1-butene, 1-hexene, 4-methyl-1-pentene, styrene, 1-octene,
1-nonene; polyunsaturated (=more than one double bond)
comonomer(s); a silane group containing comonomer(s); or any
mixtures thereof. The polyunsaturated comonomer(s) are further
described below in relation to unsaturated LDPE copolymers.
[0105] If the LDPE polymer is a copolymer, it preferably comprises
0.001 to 50 wt.-%, more preferably 0.05 to 40 wt.-%, still more
preferably less than 35 wt.-%, still more preferably less than 30
wt.-%, more preferably less than 25 wt.-%, of one or more
comonomer(s).
[0106] The polymer composition, preferably at least the optional
second polyolefin (b) component thereof, more preferably the LDPE
polymer, may optionally be unsaturated, i.e. the polymer
composition, preferably at least the second polyolefin (b),
preferably the LDPE polymer, may comprise carbon-carbon double
bonds (--C.dbd.C--). The "unsaturated" means herein that the
polymer composition, preferably the second polyolefin (b), contains
carbon-carbon double bonds/1000 carbon atoms in a total amount of
at least 0.4/1000 carbon atoms. If the non-crosslinked LDPE is used
in the final cable, then the LDPE is typically not unsaturated as
defined above.
[0107] As well known, the unsaturation can be provided to the
polymer composition i.a. by means of the polyolefin component(s), a
low molecular weight (Mw) compound(s), such as crosslinking
booster(s) or scorch retarder additive(s), or any combinations
thereof. The total amount of double bonds means herein double bonds
determined from the source(s) that are known and deliberately added
to contribute to the unsaturation. If two or more above sources of
double bonds are chosen to be used for providing the unsaturation,
then the total amount of double bonds in the polymer composition
means the sum of the double bonds present in the double-bond
sources. It is evident that a characteristic model compound for
calibration is used for each chosen source to enable the
quantitative infrared (FTIR) determination.
[0108] Any double bond measurements are carried out prior to
optional crosslinking.
[0109] If the polymer composition is unsaturated (prior to optional
crosslinking), then it is preferred that the unsaturation
originates at least from an unsaturated optional second polyolefin
(b) component, if present. More preferably, if present, then the
unsaturated second polyolefin (b) is an unsaturated polyethylene,
more preferably an unsaturated LDPE polymer, even more preferably
an unsaturated LDPE homopolymer or an unsaturated LDPE copolymer.
When polyunsaturated comonomer(s) are present in the LDPE polymer
as said unsaturated polyolefin, then the LDPE polymer is an
unsaturated
[0110] LDPE copolymer.
[0111] In an unsaturated embodiment the term "total amount of
carbon-carbon double bonds" is defined from the polymer
composition, preferably from the unsaturated second polyolefin (b),
if present, and refers, if not otherwise specified, to the combined
amount of double bonds which originate from vinyl groups,
vinylidene groups and trans-vinylene groups, if present. Naturally
the second polyolefin (b) does not necessarily contain all the
above three types of double bonds. However, any of the three types,
when present, is calculated to the "total amount of carbon-carbon
double bonds". The amount of each type of double bond is measured
as indicated under "Determination methods".
[0112] If an LDPE homopolymer as the second polyolefin (b) is
unsaturated, then the unsaturation can be provided e.g. by a chain
transfer agent (CTA), such as propylene, and/or by polymerization
conditions. If an LDPE copolymer as the second polyolefin (b) is
unsaturated, then the unsaturation can be provided by one or more
of the following means: by a chain transfer agent (CTA), by one or
more polyunsaturated comonomer(s) or by polymerisation conditions.
It is well known that selected polymerisation conditions such as
peak temperatures and pressure, can have an influence on the
unsaturation level. In case of an unsaturated LDPE copolymer, it is
preferably an unsaturated LDPE copolymer of ethylene with at least
one polyunsaturated comonomer, and optionally with other
comonomer(s), such as polar comonomer(s) which is preferably
selected from acrylate or acetate comonomer(s). More preferably an
unsaturated LDPE copolymer is an unsaturated LDPE copolymer of
ethylene with at least polyunsaturated comonomer(s).
[0113] The polyunsaturated comonomers suitable for the unsaturated
second polyolefin (b) preferably consist of a straight carbon chain
with at least 8 carbon atoms and at least 4 carbons between the
non-conjugated double bonds, of which at least one is terminal,
more preferably, said polyunsaturated comonomer is a diene,
preferably a diene which comprises at least eight carbon atoms, the
first carbon-carbon double bond being terminal and the second
carbon-carbon double bond being non-conjugated to the first one.
Preferred dienes are selected from C.sub.8 to C.sub.14
non-conjugated dienes or mixtures thereof, more preferably selected
from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene,
1,13-tetradecadiene, 7-methyl-1,6-octadiene,
9-methyl-1,8-decadiene, or mixtures thereof. Even more preferably,
the diene is selected from 1,7-octadiene, 1,9-decadiene,
1,11-dodecadiene, 1,13-tetradecadiene, or any mixture thereof,
however, without limiting to above dienes.
[0114] It is well known that e.g. propylene can be used as a
comonomer or as a chain transfer agent (CTA), or both, whereby it
can contribute to the total amount of the carbon-carbon double
bonds, preferably to the total amount of the vinyl groups. Herein,
when a compound which can also act as comonomer, such as propylene,
is used as CTA for providing double bonds, then said
copolymerisable comonomer is not calculated to the comonomer
content.
[0115] If LDPE polymer as the second polyolefin (b) is unsaturated,
then it has preferably a total amount of carbon-carbon double
bonds, which originate from vinyl groups, vinylidene groups and
trans-vinylene groups, if present, of more than 0.4/1000 carbon
atoms, preferably of more than 0.5/1000 carbon atoms. The upper
limit of the amount of carbon-carbon double bonds present in the
polyolefin is not limited and may preferably be less than 5.0/1000
carbon atoms, preferably less than 3.0/1000 carbon atoms.
[0116] In some embodiments, e.g. wherein higher crosslinking level
with the low peroxide content is desired, the total amount of
carbon-carbon double bonds, which originate from vinyl groups,
vinylidene groups and trans-vinylene groups, if present, in the
unsaturated LDPE, is preferably higher than 0.40/1000 carbon atoms,
preferably higher than 0.50/1000 carbon atoms, preferably higher
than 0.60/1000 carbon atoms.
[0117] If the LDPE as the second polyolefin (b) is unsaturated LDPE
as defined above, it contains preferably at least vinyl groups and
the total amount of vinyl groups is preferably higher than
0.05/1000 carbon atoms, still more preferably higher than 0.08/1000
carbon atoms, and most preferably of higher than 0.11/1000 carbon
atoms. Preferably, the total amount of vinyl groups is of lower
than 4.0/1000 carbon atoms. More preferably, the second polyolefin
(b), prior to crosslinking, contains vinyl groups in total amount
of more than 0.20/1000 carbon atoms, still more preferably of more
than 0.30/1000 carbon atoms.
[0118] Typically, and preferably in wire and cable (W&C)
applications, the density of LDPE polymer as the optional second
polyolefin (b), is higher than 860 kg/m.sup.3. Preferably the
density of the LDPE homopolymer or copolymer as the optional second
polyolefin (b), is not higher than 960 kg/m.sup.3, and preferably
is from 900 to 945 kg/m.sup.3. The MFR.sub.2 (2.16 kg, 190.degree.
C.) of the second polyolefin (b), preferably of the LDPE polymer,
is preferably from 0.01 to 50 g/10min, preferably of from 0.05 to
30.0 g/10 min, more preferably is from 0.1 to 20 g/10min, and most
preferably is from 0.2 to 10 g/10min.
[0119] Accordingly, a LDPE polymer as the optional second
polyolefin (b) is preferably produced at high pressure by free
radical initiated polymerisation (referred to as high pressure (HP)
radical polymerization). The HP reactor can be e.g. a well known
tubular or autoclave reactor or a mixture thereof, preferably a
tubular reactor. The high pressure (HP) polymerisation and the
adjustment of process conditions for further tailoring the other
properties of the polyolefin depending on the desired end
application are well known and described in the literature, and can
readily be used by a skilled person. Suitable polymerisation
temperatures range up to 400.degree. C., preferably from 80 to
350.degree. C. and pressure from 70 MPa, preferably 100 to 400 MPa,
more preferably from 100 to 350 MPa. Pressure can be measured at
least after compression stage and/or after the tubular reactor.
Temperature can be measured at several points during all steps.
[0120] After the separation the obtained LDPE is typically in a
form of a polymer melt which is normally mixed and pelletized in a
pelletising section, such as pelletising extruder, arranged in
connection to the HP reactor system. Optionally, additive(s), such
as antioxidant(s), can be added in this mixer in a known manner
[0121] Further details of the production of ethylene (co)polymers
by high pressure radical polymerization can be found i.a. in the
Encyclopedia of Polymer Science and Engineering, Vol. 6 (1986), pp
383-410 and Encyclopedia of Materials: Science and Technology, 2001
Elsevier Science Ltd.: "Polyethylene: High-pressure, R.Klimesch,
D.Littmann and F.-O. Mailing pp. 7181-7184.
[0122] When an unsaturated LDPE copolymer of ethylene is prepared,
then, as well known, the carbon-carbon double bond content can be
adjusted by polymerising the ethylene e.g. in the presence of one
or more polyunsaturated comonomer(s), chain transfer agent(s), or
both, using the desired feed ratio between monomer, preferably
ethylene, and polyunsaturated comonomer and/or chain transfer
agent, depending on the nature and amount of C-C double bonds
desired for the unsaturated LDPE copolymer. I.a. WO 9308222
describes a high pressure radical polymerisation of ethylene with
polyunsaturated monomers. As a result the unsaturation can be
uniformly distributed along the polymer chain in random
copolymerisation manner
[0123] End Uses and End Applications of the Polymer Composition of
Invention
[0124] The polymer composition of the invention can be used for
producing a layer of a direct current (DC) power cable, as defined
above, below or in claims.
[0125] The invention further provides a direct current (DC) power
cable, comprising a conductor which is surrounded at least by an
inner semiconductive layer, an insulation layer and an outer
semiconductive layer, in that order, wherein at least one layer,
preferably at least the insulation layer, comprises, preferably
consists of, a polymer composition as defined above, below or in
claims, comprising [0126] (a) a polyolefin provided that, when the
polyolefin is a low density polyethylene (LDPE), then it is
non-crosslinked, and [0127] (b) optionally a second polyolefin
which is different from the polyolefin (a).
[0128] Accordingly, the inner semiconductive layer of the power
cable comprises, preferably consists of, a first semiconductive
composition, the insulation layer comprises, preferably consists
of, an insulation composition, and the outer semiconductive layer
comprises, preferably consists of, a second semiconductive
composition. Thus one of the compositions, preferably at least the
insulation composition comprises, more preferably, consists of the
polymer composition of the invention.
[0129] The first and the second semiconductive compositions can be
different or identical and comprise a polymer(s) which is
preferably a polyolefin or a mixture of polyolefins and a
conductive filler, preferably carbon black. Suitable polyolefin(s)
are e.g. polyethylene produced in a low pressure process or a
polyethylene produced in a HP process (LDPE). The general polymer
description as given above in relation to the polyolefin (a) and,
respectively, in relation to the second optional polyolefin (b)
apply also for the suitable polymers for semiconductive layers. The
carbon black can be any conventional carbon black used in the
semiconductive layers of a DC power cable, preferably in the
semiconductive layer of a DC power cable. Preferably the carbon
black has one or more of the following properties: a) a primary
particle size of at least 5 nm which is defined as the number
average particle diameter according ASTM D3849-95a, dispersion
procedure D b) iodine number of at least 30 mg/g according to ASTM
D1510, c) oil absorption number of at least 30 ml/100g which is
measured according to ASTM D2414. Non-limiting examples of carbon
blacks are e.g. acetylene carbon black, furnace carbon black and
Ketjen carbon black, preferably furnace carbon black and acetylene
carbon black. Preferably, the polymer composition comprises 10 to
50 wt % carbon black, based on the weight of the Semiconductive
composition.
[0130] In a first preferable embodiment of the DC cable the inner
semiconductive layer comprises a first semiconductive composition,
the insulation layer comprises an insulation composition and an
outer semiconductive layer comprises a second semiconductive
composition, in that order, and wherein the insulation composition
of the insulation layer comprises, preferably consists, of said
polymer composition comprising [0131] (a) a polyolefin which is a
non-crosslinked LDPE which is preferably selected from a LDPE
homopolymer or a LDPE copolymer of ethylene with one or more
comonomer(s), and [0132] (b) optionally a second polyolefin which
is different from the polyolefin (a), as defined above, below or in
claims, and wherein the outer semiconductive layer comprises,
preferably consists of, a crosslinked second semiconductive
composition. Furthermore, optionally, and preferably, the inner
semicoductive layer comprises, preferably consists of, a
non-crosslinked first semiconductive composition. It is thus
preferred that the first semiconductive composition of the inner
semiconductive layer is non-crosslinked, the insulation layer
comprises the polymer composition with a non-crosslinked LDPE
polyolefin (a) and the second semiconductive composition of the
outer semiconductive layer is crosslinked.
[0133] In a more preferable second embodiment of the DC cable the
inner semiconductive layer comprises a first semiconductive
composition, the insulation layer comprises an insulation
composition and an outer semiconductive layer comprises a second
semiconductive composition, in that order, and wherein the
insulation composition of the insulation layer comprises,
preferably consists, of said polymer composition comprising [0134]
(a) a polyolefin which is other than LDPE, and which is optionally,
and preferably, non-crosslinked, and [0135] (b) optionally a second
polyolefin which is different from the polyolefin (a), as defined
above, below or in claims.
[0136] In this embodiment the polyolefin (a) which is other than
LDPE can be non-crosslinked and crosslinked.
[0137] The DC power cable of the more preferable second embodiment
of the invention may thus optionally be crosslinkable. Accordingly,
if the DC power cable is crosslinkable, then at least one layer,
preferably at least the insulation layer, comprises, preferably
consists of, the polymer composition as defined above, below or in
claims comprising [0138] (a) a polyolefin which is other than low
density polyethylene (LDPE), [0139] (b) optionally a second
polyolefin, which is different from the polyolefin (a), and a
crosslinking agent, which is preferably a peroxide in an amount of
up to 110 mmol --O--O--/kg polymer composition, preferably of up to
90 mmol --O--O--/kg polymer composition, more preferably of 1.0 to
75 mmol --O--O--/kg polymer composition, preferably of less than 50
mmol --O--O--/kg polymer composition, preferably of less than 40
mmol --O--O--/kg polymer composition, preferably of less than 37
mmol --O--O--/kg polymer composition, preferably of less than 35
mmol --O--O--/kg polymer composition, preferably of 0.1 to 34 mmol
--O--O--/kg polymer composition or less, preferably of 0.5 to 33
mmol --O--O--/kg polymer composition or less, more preferably from
5.0 to 30 mmol --O--O--/kg polymer composition, more preferably
from 7.0 to 30 mmol --O--O--/kg polymer composition, more
preferably from 10.0 to 30 mmol --O--O--/kg polymer
composition.
[0140] However, it is preferred that the polymer composition of the
second embodiment of the invention comprises no crosslinking agent.
Accordingly, in the more preferable DC cable of this embodiment,
the inner semiconductive layer comprises a first semiconductive
composition, the insulation layer comprises an insulation
composition and an outer semiconductive layer comprises a second
semiconductive composition, in that order, and wherein the
insulation composition of the insulation layer comprises,
preferably consists, of said polymer composition comprising [0141]
(a) a polyolefin which is other than LDPE, and which is
non-crosslinked, and [0142] (b) optionally a second polyolefin
which is different from the polyolefin (a), as defined above, below
or in claims.
[0143] In this second embodiment, optionally, and preferably, the
outer semiconductive layer comprises, preferably consists of,
non-crosslinked or crosslinked, preferably a crosslinked second
semiconductive composition. Furthermore, optionally, and
preferably, the inner semiconductive layer comprises, preferably
consists of, a non-crosslinked first semiconductive composition. It
is thus preferred that the first semiconductive composition of the
inner semiconductive layer is non-crosslinked, the insulation layer
comprises the polymer composition with a non-crosslinked polyolefin
(a) other than LDPE and the second semiconductive composition of
the outer semiconductive layer is non-crosslinked or crosslinked,
preferably crosslinked.
[0144] The expressions "without crosslinking", "not crosslinked" or
"non-crosslinked", as used herein above and below, mean that no
crosslinking agent is added to the polymer composition for the
purpose of crosslinking the composition. Similarly, the expression
"no crosslinking agent" means herein that the polymer composition
does not comprise any crosslinking agent which had been added to
the polymer composition for the purpose of crosslinking the polymer
composition. E.g. a non-crosslinked LDPE or a non-crosslinked
polyolefin (a) other than LDPE comprises no crosslinking agent.
[0145] Naturally, the further preferable subgroups of the above
properties, further properties, variants and embodiments as defined
above or below for the polymer composition or for the polyolefin
(a) and the optional second polyolefin (b) components thereof apply
similarly to the DC power cable, of the invention.
[0146] The term "conductor" means herein above and below that the
conductor comprises one or more wires. Moreover, the cable may
comprise one or more such conductors. Preferably the conductor is
an electrical conductor and comprises one or more metal wires.
[0147] It is preferred that the polymer composition of the layer,
preferably of the insulation layer, consists of the polyolefin (a),
which is preferably other than LDPE, based on the combined weight
of the polyolefin (a) and the optional second polyolefin (b).
Accordingly, the polymer composition of the invention comprises no
polyolefin (b) or other polymer component(s).
[0148] As well known the cable can optionally comprise further
layers, e.g. layers surrounding the insulation layer or, if
present, the outer semiconductive layers, such as screen(s), a
jacketing layer(s), other protective layer(s) or any combinations
thereof.
[0149] The invention also provides a process for producing a DC
power cable, wherein the process comprises the steps of [0150]
applying on a conductor, preferably by (co)extrusion, an inner
semiconductive layer comprising a first semiconductive composition,
an insulation layer comprising an insulation composition and an
outer semiconductive layer comprising a second semiconductive
composition, in that order, wherein at least the composition of the
one layer, preferably the insulation composition of the insulation
layer, comprises, preferably consists of, a polymer composition of
the invention comprising [0151] (a) a polyolefin provided that,
when the polyolefin is a low density polyethylene (LDPE), then it
is non-crosslinked, more preferably, (a) a polyolefin which is
other than a low density polyethylene (LDPE), and [0152] (b)
optionally a second polyolefin which is different from the
polyolefin (a), as defined above, below or in claims, and [0153]
optionally crosslinking one or more layers of the obtained cable in
the presence of a crosslinking agent and at crosslinking
conditions.
[0154] Preferably in this embodiment, at least the layer of the
polymer composition the invention is not crosslinked. In this
embodiment the process comprises the step of optionally
crosslinking one or both of the first semiconductive composition of
the inner semiconductive layer and the second semiconductive
composition of the outer semiconductive layer of the obtained
cable, without crosslinking the polymer composition of the
insulation layer. Preferably, the second semiconductive composition
of the outer semiconductive layer is non-crosslinked or
crosslinked, more preferably crosslinked, without crosslinking the
polymer composition of the insulation layer. Also preferably, the
first semiconductive composition of the inner semiconductive layer
is not crosslinked. More preferably, the insulation composition of
the invention of the insulation layer is the polymer composition of
the invention, where a polyolefin (a) is according to the second
embodiment, i.e. the polyolefin (a) is other than low density
polyethylene (LDPE), and preferably, comprises no crosslinking
agent.
[0155] More preferably, an optionally crosslinkable DC power cable
is produced, wherein the process comprises the steps of
(a) [0156] providing and mixing, preferably meltmixing in an
extruder, an optionally crosslinkable first semiconductive
composition comprising a polymer, a carbon black and optionally
further component(s) for the inner semiconductive layer, [0157]
providing and mixing, preferably meltmixing in an extruder,
optionally crosslinkable polymer composition of the invention,
comprising [0158] (a) a polyolefin provided that, when the
polyolefin is a low density polyethylene (LDPE), then it comprises
no crosslinking agent, preferably a polyolefin which is other than
a low density polyethylene (LDPE) and [0159] (b) optionally a
second polyolefin which is different from the polyolefin (a), for
the insulation layer, [0160] providing and mixing, preferably
meltmixing in an extruder, a second semiconductive composition
which is optionally crosslinkable and comprises a polymer, a carbon
black and optionally further component(s) for the outer
semiconductive layer, (b) applying on a conductor, preferably by
coextrusion, [0161] a meltmix of the first semiconductive
composition obtained from step (a) to form the inner semiconductive
layer, [0162] a meltmix of polymer composition of the invention
obtained from step (a) to form the insulation layer, and [0163] a
meltmix of the second semiconductive composition obtained from step
(a) to form the outer semiconductive layer, and (c) optionally
crosslinking at crosslinking conditions one or both of the first
semiconductive composition of the inner semiconductive layer and
the second semiconductive composition of the outer semiconductive
layer, of the obtained cable, and optionally crosslinking the
polymer composition of the insulation layer, more preferably at
least without crosslinking the polymer composition of the
insulation layer. Preferably in step (c) the second semiconductive
polymer composition of the outer semiconductive layer is
non-crosslinked or crosslinked, more preferably crosslinked without
crosslinking the polymer composition of the insulation layer. Also
preferably, in step (c) the second semiconductive polymer
composition of the outer semiconductive layer is non-crosslinked or
crosslinked, more preferably crosslinked without crosslinking the
polymer composition of the insulation layer and the first
semiconductive composition of the inner semiconductive layer.
[0164] Melt mixing means mixing above the melting point of at least
the major polymer component(s) of the obtained mixture and is
carried out for example, without limiting to, in a temperature of
at least 15.degree. C. above the melting or softening point of
polymer component(s).
[0165] The term "(co)extrusion" means herein that in case of two or
more layers, said layers can be extruded in separate steps, or at
least two or all of said layers can be coextruded in a same
extrusion step, as well known in the art. The term "(co)extrusion"
means herein also that all or part of the layer(s) are formed
simultaneously using one or more extrusion heads. For instance a
triple extrusion can be used for forming three layers. In case a
layer is formed using more than one extrusion heads, then for
instance, the layers can be extruded using two extrusion heads, the
first one for forming the inner semiconductive layer and the inner
part of the insulation layer, and the second head for forming the
outer insulation layer and the outer semiconductive layer.
[0166] As well known, the polymer composition of the invention and
the optional and preferred first and second semiconductive
compositions can be produced before or during the cable production
process. Moreover the polymer composition of the invention and the
optional and preferred first and second semiconductive composition
can each independently comprise part or all of the component(s) of
the final composition, before introducing to the (melt)mixing step
a) of the cable production process.
[0167] Preferably, the polymer composition of the invention and,
optionally, the optional first and second semiconductive
composition are provided to the cable production process in form of
powder, grain or pellets. Pellets mean herein generally any polymer
product which is formed from reactor-made polymer (obtained
directly from the reactor) by post-reactor modification to a solid
polymer particles. A well-known post-reactor modification is
pelletising a meltmix of a polymer product and optional additive(s)
in a pelletising equipment to solid pellets. Pellets can be of any
size and shape. Moreover, the polyolefin (a) and the optional
second polyolefin (b), if present, can be combined in a same
powder, grain or pellet product, which thus contains a solid
polymer mixture of the polyolefin (a) and the second polyolefin
(b). Alternatively and preferably, the polyolefin (a) and the
optional second polyolefin (b), if present, are provided
separately, e.g. as two separate pellet products, to the cable
production process.
[0168] Accordingly, the polyolefin (a) and the optional second
polyolefin (b), if present, of the polymer composition can be
premixed, e.g. meltmixed together and pelletised, before providing
to the mixing step (a). Alternatively, and preferably, these
components can be provided e.g. in separate pellets to the
(melt)mixing step (a), where the pellets are blended together.
[0169] The (melt)mixing step (a) of the provided polymer
composition of the invention and of the preferable first and second
semiconductive compositions is preferably carried out in a cable
extruder. The step a) of the cable production process may
optionally comprise a separate mixing step, e.g. in a mixer
arranged in connection and preceding the cable extruder of the
cable production line. Mixing in the preceding separate mixer can
be carried out by mixing with or without external heating (heating
with an external source) of the component(s). In case one of the
polyolefin (a) or the optional second polyolefin (b), or the
optional further component(s), such as peroxide or further
additive(s), of the polymer composition of the invention and,
respectively, part or all of the component(s) of the first or
second semiconductive compositions, are added to the polyolefin
during the cable production process, then the addition(s) can take
place at any stage during the mixing step (a), e.g at the optional
separate mixer preceding the cable extruder or at any point(s) of
the cable extruder. The addition of the optional peroxide and
optional additive(s) can be made simultaneously or separately as
such, preferably in liquid form, or in a well known master batch,
and at any stage during the mixing step (a).
[0170] As already mentioned, the polymer composition of the second
embodiment of the invention, comprises optionally a crosslinking
agent, which, if present, is preferably peroxide. The crosslinking
agent can be added before the cable production process or during
the (melt)mixing step(a). For instance, and preferably, the
crosslinking agent and also the optional further component(s), such
as additive(s), can already be present in at least one of the
polyolefin (a) or the optional second polyolefin (b) before the use
in the production line of the cable production process. The
crosslinking agent can be e.g.
[0171] meltmixed together with the polyolefin (a) or the optional
second polyolefin (b), or both, or a mixture thereof, and optional
further component(s), and then the meltmix is pelletised.
Alternatively and preferably, the crosslinking agent is added,
preferably impregnated, onto the solid polymer particles,
preferably pellets, of the polyolefin component or of the polymer
composition.
[0172] It is preferred that the meltmix of the polymer composition
obtained from (melt)mixing step (a) consists of the polyolefin (a)
and optionally the second polyolefin (b), if present, of the
invention as the sole polymer component(s). The optional and
preferable additive(s) can be added to polymer composition as such
or as a mixture with a carrier polymer, i.e. in a form of so-called
master batch.
[0173] In one preferred embodiment of the cable production process,
an optionally crosslinkable DC power cable is produced, wherein the
insulation layer comprises, preferably consists of, a polymer
composition of the invention. Preferably the insulation layer
comprises no crosslinking agent.
[0174] The optional crosslinking agent(s) can already be present in
the optional first and second semiconductive composition before
introducing to the crosslinking step c) or introduced during the
cros slinking step.
[0175] The optional crosslinking can be carried out at increased
temperature which is chosen, as well known, depending on the type
of crosslinking agent. For instance temperatures above 150.degree.
C., such as from 160 to 350.degree. C., are typical, however
without limiting thereto.
[0176] The processing temperatures and devices are well known in
the art, e.g. conventional mixers and extruders, such as single or
twin screw extruders, are suitable for the process of the
invention.
[0177] The invention further provides an optionally crosslinked DC
power cable, preferably a crosslinked HV DC power cable, comprising
a conductor surrounded by one or more layers, preferably at least
by an insulation layer, more preferably at least by an inner
semiconductive layer, insulation layer and an outer semiconductive
layer, in that order, wherein at least the insulation layer
comprises a non-crosslinked polymer composition of the invention as
defined above or in claims, and wherein one or both of the inner
semiconductive composition and the optional and preferred outer
semiconductive composition are optionally crosslinked. Preferably,
the second semiconductive polymer composition of the outer
semiconductive layer is non-crosslinked or crosslinked, preferably
crosslinked and the polymer composition of the invention,
preferably the polymer composition of the second embodiment, is
non-crosslinked. More preferably the first semiconductive
composition of the inner semiconductive layer is
non-crosslinked.
[0178] The advantages of the most preferred embodiment of having
the inner semiconductive layer and the insulation layer comprising,
preferably consisting of a non-crosslinked polymeric composition in
combination with an outer semiconductive layer comprising,
preferably consisting of a crosslinked polymeric composition, as
defined above, below or in claims are as follows: [0179] Optimal
electrical performance of the insulation system of the HV DC cable,
[0180] The connection of cables is very feasible due to
non-crosslinked thermoplastic insulation composition, [0181] No
need to wait and allow the heat to transfer through the insulation
and inner semiconductive layers, since not crosslinked. The overall
production efficiency is improved, especially in HV applications
with thick insulation layer, since the inner semiconductive layer
and the insulation layer need not to be crosslinked. Crosslinking
of inner and outer semiconductive and insulation layers require
typically at least 1 hour, while crosslinking of only the outer
semiconductive layer takes less than 8 minutes, [0182] Robust high
speed extrusion possible leading to longer stable production
periods at higher extrusion speed and quality due to no risk to
scorching (undesired premature crosslinking) in the inner
semiconductive and insulation layers, [0183] Degassing step can be
reduced, and thus accelerate the overall cable production process,
since any undesired by-products, i.e. decomposition products,
formed from the crosslinking agent, are easier to remove, i.e.
degas, only from the outer layer, [0184] The crosslinked outer
semiconductive layer is mechanically unexpectedly sufficient to
protect the insulation layer from mechanical and thermal crack
initiation, expressed as TSCR (Thermal stress cracking).
[0185] The preferred DC power cable of the invention is a HV DC
power cable. Preferably the HV DC power cable operates at voltages
as defined above for HV DC cable or extra HV DC cable, depending on
the desired end cable application.
[0186] The thickness of the insulation layer of the DC power cable,
more preferably of the HV DC power cable, is typically 2 mm or
more, preferably at least 3 mm, preferably of at least 5 to 100 mm,
more preferably from 5 to 50 mm, and conventionally 5 to 40 mm,
e.g. 5 to 35 mm, when measured from a cross section of the
insulation layer of the cable. The thickness of the inner and outer
semiconductive layers is typically less than that of the insulation
layer, and in HV DC power cables can be e.g. more than 0.1 mm, such
as from 0.3 up to 20 mm, 0.3 to 10 of inner semiconductive and
outer semiconductive layer. The thickness of the inner
semiconductive layer is preferably 0.3-5.0 mm, preferably 0.5-3.0
mm, preferably 0.8-2.0 mm The thickness of the outer semiconductive
layer is preferably from 0.3 to 10 mm, such as 0.3 to 5 mm,
preferably 0.5 to 3.0 mm, preferably 0.8-3.0 mm It is evident for
and within the skills of a skilled person that the thickness of the
layers of the DC cable depends on the intended voltage level of the
end application cable and can be chosen accordingly.
[0187] Determination Methods
[0188] Unless otherwise stated in the description or experimental
part the following methods were used for the property
determinations.
[0189] Wt%: % by Weight
[0190] Melt Flow Rate
[0191] The melt flow rate (MFR) is determined according to ISO 1133
and is indicated in g/10 min. The MFR is an indication of the
flowability, and hence the processability, of the polymer. The
higher the melt flow rate, the lower the viscosity of the polymer.
The MFR is determined at 190.degree. C. for polyethylene and at
230.degree. C. for polypropylene. MFR may be determined at
different loadings such as 2.16 kg (MFR.sub.2) or 21.6 kg
(MFR.sub.21).
[0192] Molecular Weight
[0193] Mz, Mw, Mn, and MWD are measured by Gel Permeation
Chromatography (GPC) according to the following method:
[0194] The weight average molecular weight Mw and the molecular
weight distribution (MWD =
[0195] Mw/Mn wherein Mn is the number average molecular weight and
Mw is the weight average molecular weight; Mz is the z-average
molecular weight) is measured according to ISO 16014-4:2003 and
ASTM D 6474-99. A Waters GPCV2000 instrument, equipped with
refractive index detector and online viscosimeter was used with
2.times.GMHXL-HT and 1.times. G7000HXL-HT TSK-gel columns from
Tosoh Bioscience and 1,2,4-trichlorobenzene (TCB, stabilized with
250 mg/L 2,6-Di tert-butyl-4-methyl-phenol) as solvent at
140.degree. C. and at a constant flow rate of 1 mL/min. 209.5 .mu.L
of sample solution were injected per analysis. The column set was
calibrated using universal calibration (according to ISO
16014-2:2003) with at least 15 narrow MWD polystyrene (PS)
standards in the range of 1 kg/mol to 12 000 kg/mol. Mark Houwink
constants were used as given in ASTM D 6474-99. All samples were
prepared by dissolving 0.5-4.0 mg of polymer in 4 mL (at
140.degree. C.) of stabilized TCB (same as mobile phase) and
keeping for max. 3 hours at a maximum temperature of 160.degree. C.
with continuous gentle shaking prior sampling in into the GPC
instrument.
[0196] Comonomer Contents
[0197] a) Comonomer Content in Random Copolymer of
Polypropylene:
[0198] Quantitative Fourier transform infrared (FTIR) spectroscopy
was used to quantify the amount of comonomer. Calibration was
achieved by correlation to comonomer contents determined by
quantitative nuclear magnetic resonance (NMR) spectroscopy.
[0199] The calibration procedure based on results obtained from
quantitative .sup.13C-NMR spectroscopy was undertaken in the
conventional manner well documented in the literature. The amount
of comonomer (N) was determined as weight percent (wt %) via:
N=k1(A/R)+k2
wherein A is the maximum absorbance defined of the comonomer band,
R the maximum absorbance defined as peak height of the reference
peak and with k1 and k2 the linear constants obtained by
calibration. The band used for ethylene content quantification is
selected depending if the ethylene content is random (730
cm.sup.-1) or block-like (as in heterophasic PP copolymer) (720
cm.sup.-1). The absorbance at 4324 cm.sup.-1 was used as a
reference band.
[0200] b) Quantification of Alpha-Olefin Content in Linear Low
Density Polyethylenes and Low Density Polyethylenes by NMR
Spectroscopy:
[0201] The comonomer content was determined by quantitative 13C
nuclear magnetic resonance (NMR) spectroscopy after basic
assignment (J. Randall JMS--Rev. Macromol. Chem. Phys.,
C29(2&3), 201-317 (1989). Experimental parameters were adjusted
to ensure measurement of quantitative spectra for this specific
task.
[0202] Specifically solution-state NMR spectroscopy was employed
using a Bruker Avancelll 400 spectrometer. Homogeneous samples were
prepared by dissolving approximately 0.200 g of polymer in 2.5 ml
of deuterated-tetrachloroethene in 10 mm sample tubes utilising a
heat block and rotating tube oven at 140 C. Proton decoupled 13C
single pulse NMR spectra with NOE (powergated) were recorded using
the following acquisition parameters: a flip-angle of 90 degrees, 4
dummy scans, 4096 transients an acquisition time of 1.6 s, a
spectral width of 20 kHz, a temperature of 125 C, a bilevel WALTZ
proton decoupling scheme and a relaxation delay of 3.0 s. The
resulting FID was processed using the following processing
parameters: zero-filling to 32k data points and apodisation using a
gaussian window function; automatic zeroth and first order phase
correction and automatic baseline correction using a fifth order
polynomial restricted to the region of interest.
[0203] Quantities were calculated using simple corrected ratios of
the signal integrals of representative sites based upon methods
well known in the art.
[0204] c) Comonomer Content of Polar Comonomers in Low Density
Polyethylene [0205] (1) Polymers containing >6 wt % polar
comonomer units
[0206] Comonomer content (wt %) was determined in a known manner
based on Fourier transform infrared spectroscopy (FTIR)
determination calibrated with quantitative nuclear magnetic
resonance (NMR) spectroscopy. Below is exemplified the
determination of the polar comonomer content of ethylene ethyl
acrylate, ethylene butyl acrylate and ethylene methyl acrylate.
Film samples of the polymers were prepared for the FTIR
measurement: 0.5-0.7 mm thickness was used for ethylene butyl
acrylate and ethylene ethyl acrylate and 0.10 mm film thickness for
ethylene methyl acrylate in amount of >6wt %. Films were pressed
using a Specac film press at 150.degree. C., approximately at 5
tons, 1-2 minutes, and then cooled with cold water in a not
controlled manner The accurate thickness of the obtained film
samples was measured.
[0207] After the analysis with FTIR, base lines in absorbance mode
were drawn for the peaks to be analysed. The absorbance peak for
the comonomer was normalised with the absorbance peak of
polyethylene (e.g. the peak height for butyl acrylate or ethyl
acrylate at 3450 cm.sup.-1 was divided with the peak height of
polyethylene at 2020 cm.sup.-1). The NMR spectroscopy calibration
procedure was undertaken in the conventional manner which is well
documented in the literature, explained below.
[0208] For the determination of the content of methyl acrylate a
0.10 mm thick film sample was prepared. After the analysis the
maximum absorbance for the peak for the methylacrylate at 3455
cm.sup.-1 was subtracted with the absorbance value for the base
line at 2475 cm.sup.-1 (A.sub.methylacrylate-A.sub.2475). Then the
maximum absorbance peak for the polyethylene peak at 2660 cm.sup.-1
was subtracted with the absorbance value for the base line at 2475
cm.sup.-1 (A.sub.2660-A.sub.2475). The ratio between
(A.sub.methylacrylate-A.sub.2475) and (A.sub.266-A.sub.247) was
then calculated in the conventional manner which is well documented
in the literature.
[0209] The weight-% can be converted to mol-% by calculation. It is
well documented in the literature.
[0210] Quantification of Copolymer Content in Polymers by NMR
Spectroscopy
[0211] The comonomer content was determined by quantitative nuclear
magnetic resonance (NMR) spectroscopy after basic assignment (e.g.
"NMR Spectra of Polymers and Polymer Additives", A. J. Brandolini
and D. D. Hills, 2000, Marcel Dekker, Inc. New York). Experimental
parameters were adjusted to ensure measurement of quantitative
spectra for this specific task (e.g "200 and More NMR Experiments:
A Practical Course", S. Berger and S. Braun, 2004, Wiley-VCH,
Weinheim). Quantities were calculated using simple corrected ratios
of the signal integrals of representative sites in a manner known
in the art.
[0212] (2) Polymers Containing 6 wt. % or less polar comonomer
units Comonomer content (wt. %) was determined in a known manner
based on Fourier transform infrared spectroscopy (FTIR)
determination calibrated with quantitative nuclear magnetic
resonance (NMR) spectroscopy. Below is exemplified the
determination of the polar comonomer content of ethylene butyl
acrylate and ethylene methyl acrylate. For the FT-IR measurement a
film samples of 0.05 to 0.12 mm thickness were prepared as
described above under method 1). The accurate thickness of the
obtained film samples was measured.
[0213] After the analysis with FT-IR base lines in absorbance mode
were drawn for the peaks to be analysed. The maximum absorbance for
the peak for the comonomer (e.g. for methylacrylate at 1164
cm.sup.-1 and butylacrylate at 1165 cm.sup.-1) was subtracted with
the absorbance value for the base line at 1850 cm.sup.-1
(A.sub.polar comonomer-A.sub.1850). Then the maximum absorbance
peak for polyethylene peak at 2660 cm.sup.-1 was subtracted with
the absorbance value for the base line at 1850 cm.sup.-1
(A.sub.2660-A.sub.1850). The ratio between
(A.sub.comonomer-A.sub.1850) and (A.sub.2660-A.sub.1850) was then
calculated. The NMR spectroscopy calibration procedure was
undertaken in the conventional manner which is well documented in
the literature, as described above under method 1).
[0214] The weight-% can be converted to mol-% by calculation. It is
well documented in the literature.
[0215] Below is exemplified how polar comonomer content obtained
from the above method (1) or (2), depending on the amount thereof,
can be converted to micromol or mmol per g polar comonomer as used
in the definitions in the text and claims:
[0216] The millimoles (mmol) and the micro mole calculations have
been done as described below.
[0217] For example, if 1 g of the poly(ethylene-co-butylacrylate)
polymer, which contains 20 wt % butylacrylate, then this material
contains 0.20/M.sub.butylacrylate (128 g/mol)=1.56.times.10.sup.-3
mol. (=1563 micromoles).
[0218] The content of polar comonomer units in the polar copolymer
C.sub.polar comonomer is expressed in mmol/g (copolymer). For
example, a polar poly(ethylene-co-butylacrylate) polymer which
contains 20 wt. % butyl acrylate comonomer units has a C.sub.polar
comonomer of 1.56 mmol/g. The used molecular weights are:
M.sub.butylacrylate=128 g/mole, M.sub.ethylacrylate=100 g/mole,
M.sub.methylacrylate=86 g/mole).
[0219] Density
[0220] Low density polyethylene (LDPE): The density was measured
according to ISO 1183-2. The sample preparation was executed
according to ISO 1872-2 Table 3 Q (compression moulding).
[0221] Low pressure process polyethylene: Density of the polymer
was measured according to ISO 1183/1872-2B.
[0222] Xylene Solubles (XS)
[0223] Xylene solubles were determined at 23.degree. C. according
ISO 6427.
[0224] Method for Determination of the Amount of Double Bonds in
the Polymer Composition or in the Polymer
[0225] A) Quantification of the Amount of Carbon-Carbon Double
Bonds by IR Spectroscopy
[0226] Quantitative infrared (IR) spectroscopy was used to quantify
the amount of carbon-carbon doubles (C.dbd.C). Calibration was
achieved by prior determination of the molar extinction coefficient
of the C.dbd.C functional groups in representative low molecular
weight model compounds of known structure.
[0227] The amount of each of these groups (N) was determined as
number of carbon-carbon double bonds per thousand total carbon
atoms (C.dbd.C/1000C) via:
N=(A.times.14)/(E.times.L.times.D)
were A is the maximum absorbance defined as peak height, E the
molar extinction coefficient of the group in question
(1mol.sup.-1mm.sup.-1), L the film thickness (mm) and D the density
of the material (gcm.sup.-1).
[0228] The total amount of C.dbd.C bonds per thousand total carbon
atoms can be calculated through summation of N for the individual
C.dbd.C containing components.
[0229] For polyethylene samples solid-state infrared spectra were
recorded using a FTIR spectrometer (Perkin Elmer 2000) on
compression moulded thin (0.5-1.0 mm) films at a resolution of 4
cm.sup.-1 and analysed in absorption mode.
[0230] 1) Polymer Compositions Comprising Polyethylene Homopolymers
and Copolymers, Except Polyethylene Copolymers with >0.4 Wt %
Polar Comonomer
[0231] For polyethylenes three types of C.dbd.C containing
functional groups were quantified, each with a characteristic
absorption and each calibrated to a different model compound
resulting in individual extinction coefficients: [0232] vinyl
(R--CH.dbd.CH2) via 910 cm.sup.-1 based on 1-decene [dec-1-ene]
giving E=13.131mol.sup.-1mm.sup.-1 [0233] vinylidene (RR'C.dbd.CH2)
via 888 cm.sup.-1 based on 2-methyl-1-heptene [2-methyhept-1-ene]
giving E=18.24 lmol.sup.-1mm.sup.-1 [0234] trans-vinylene
(R--CH.dbd.CH--R') via 965 cm.sup.-1 based on trans-4-decene
[(E)-dec-4-ene] giving E=15.14 lmol.sup.-1mm.sup.-1
[0235] For polyethylene homopolymers or copolymers with <0.4 wt
% of polar comonomer linear baseline correction was applied between
approximately 980 and 840 cm.sup.-1.
[0236] 2) Polymer Compositions Comprising Polyethylene Copolymers
with >0.4 wt % Polar Comonomer
[0237] For polyethylene copolymers with >0.4 wt % of polar
comonomer two types of C.dbd.C containing functional groups were
quantified, each with a characteristic absorption and each
calibrated to a different model compound resulting in individual
extinction coefficients: [0238] vinyl (R--CH.dbd.CH2) via 910
cm.sup.-1 based on 1-decene [dec-1-ene] giving
E=13.131lmol.sup.-1mm.sup.-1 [0239] vinylidene (RR'C.dbd.CH2) via
888 cm.sup.-1 based on 2-methyl-1-heptene [2-methyl-hept-1-end]
giving E=18.24 lmol.sup.-1mm .sup.-1
[0240] EBA:
[0241] For poly(ethylene-co-butylacrylate) (EBA) systems linear
baseline correction was applied between approximately 920 and 870
cm.sup.-1.
[0242] EMA:
[0243] For poly(ethylene-co-methylacrylate) (EMA) systems linear
baseline correction was applied between approximately 930 and 870
cm.sup.-1.
[0244] 3) Polymer Compositions Comprising Unsaturated Low Molecular
Weight Molecules
[0245] For systems containing low molecular weight C.dbd.C
containing species direct calibration using the molar extinction
coefficient of the C.dbd.C absorption in the low molecular weight
species itself was undertaken.
[0246] B) Quantification of Molar Extinction Coefficients by IR
Spectroscopy
[0247] The molar extinction coefficients were determined according
to the procedure given in ASTM D3124-98 and ASTM D6248-98.
Solution-state infrared spectra were recorded using a FTIR
spectrometer (Perkin Elmer 2000) equipped with a 0.1 mm path length
liquid cell at a resolution of 4 cm.sup.-1.
[0248] The molar extinction coefficient (E) was determined as
lmol.sup.-1mm.sup.-1 via:
E=A/(C.times.L)
where A is the maximum absorbance defined as peak height, C the
concentration (moll.sup.-1) and L the cell thickness (mm).
[0249] At least three 0.18 moll.sup.-1 solutions in
carbondisulphide (CS.sub.2) were used and the mean value of the
molar extinction coefficient determined.
[0250] DSC Method: The Lamella Thickness and the Crystallinity
Determinations.
[0251] The lamella thickness distribution is analysed according to
the description below. The melting curves and temperatures from a
DSC (Differential Scanning Caliometry) analysis of 5 mg of sample
obtained are used for calculating the lamella thickness
distribution. For thermoplastic materials the DSC analysis cycle is
as follows: a first heating from 30.degree. C. to 180.degree. C. at
a rate of 10.degree. C./min, then the sample is kept for 2 minutes
at 180.degree. C., before cooling from 180.degree. C. to
-30.degree. C. at a rate of 10.degree. C./min and then the sample
is kept at -30.degree. C. for 2 min before the second heating from
-30.degree. C. to 220.degree. C. at a rate of 10.degree. C./min is
done. For peroxide containing materials the DSC analysis cycle is
as follows: a first heating from 30.degree. C. to 130.degree. C. at
a rate of 10.degree. C./min, then the sample is kept for 2 minutes
at 130.degree. C., before cooling from 130.degree. C. to
-30.degree. C. at a rate of 10.degree. C./min and then the sample
is kept at -30.degree. C. for 2 min before the second heating from
-30.degree. C. to 220.degree. C. at a rate of 10.degree. C./min is
done.
[0252] The lamella thickness for each melting temperature is
calculated according to the Thompson-Gibbs equation:
T.sub.m=T.degree..sub.m(1-2.sigma..sub.e/.DELTA.H.degree..sub.mL.sub.c)
[0253] Where T.degree..sub.m is equilibirum melting point for an
infinite crystal, .sigma..sub.e is the specific surface free energy
of the basal plane, and .DELTA.H.degree..sub.m is the enthalpy of
melting per mass unit and they are all constants, L.sub.c is the
lamella thickness and T.sub.m is the melting temperature of the
lamella.
[0254] Parameters for lamella thickness calculations of
Polyethylene
T.degree..sub.m[m/K] 415
.sigma..sub.e[J/m.sup.2] 93.times.10.sup.-3
.DELTA.H.degree..sub.m[J/m.sup.3] 300.times.10.sup.6
[0255] Reference J. A. Parker, D. C. Bassett, R. H. Olley, P.
Jaaskelainen; On high pressure crystallization and the
characterization of linear low-density polyethylenes; Polymer 1994,
35 (19), 4140-4145.
[0256] Using the values above, the equation for determining lamella
thickness for PE using the Thompson-Gibbs equation will be:
L.sub.c=0.62.times.10.sup.-9.times.415/(415-T.sub.m)
[0257] The melting temperature is given in Kelvin and the unit for
lamella thickness is nm.
[0258] The lamella thickness distribution is calculated using the
second heating cycle in the DSC analysis to ensure that the thermal
history of the sample is taken away. The lamella thickness
distribution is calculated in intervalls of 1.degree. C. from
-20.degree. C. to 220.degree. C. For each temperature intervall,
which relates to certain lamella thickness intervall according to
the Thompson-Gibbs equation, the corresponding specific heat input
.DELTA.H.sub.i is calculated from the DSC analysis. The weight
fraction of crystals melting in a certain lamella thickness
intervall is calculated by dividing the .DELTA.H.sub.i with the
total specific heat input for the melting of all crystals
.DELTA.H.sub.total.
[0259] From .DELTA.H.sub.total, the crystallinity of the material
can be determined: crystallinity
[%]=100.times..DELTA.H.sub.total/.DELTA.H.sub.100% where
.DELTA.H.sub.100% (J/g) is 290.0 for PE (L. Mandelkem,
Macromolecular Physics, Vol. 1-3,Academic Press, New York 1973,
1976 &1980).
[0260] The result from this type of analysis is usually presented
as a plot displaying the crystal fraction as a function of lamella
thickness. This data can easily be used to determine the "crystal
fraction with lamella thickness >10 nm". Multiplying this
fraction with the overall crystallinity of the material results in
the overall "weight fraction crystals with lamella thickness >10
nm".
[0261] The used properties determined with the DSC method:
[0262] In this context the above used definitions have the
following meanings: "Lamella thickness"=Thickness of crystal
lamellas in the material (fractions*<0.1 wt % are ignored).
[0263] *Refer to crystal fractions of one degree Celsius intervals
.
[0264] "Crystal fraction with lamella thickness>10 nm"=Fraction
of the crystals which have a thickness above 10 nm based on the
amount of the crystallised part of the polymer
[0265] "Crystallinity"=wt % of the polymer that is crystalline
[0266] "Weight fraction of crystals with lamella thickness >10
nm [wt %]"=Crystal fraction with lamella thickness>10
nm".times."Crystallinity".
[0267] DC Conductivity Method
[0268] The plaques are compression moulded from pellets of the test
polymer composition. The final plaques consist of the test polymer
composition and have a thickness of 1 mm and a diameter of 330
mm
[0269] The conductivity measurement can be performed using a test
polymer composition which does not comprise or comprises the
optional crosslinking agent. In case of no crosslinking agent, the
conductivity is measured from a non-crosslinked plaque sample using
the below procedure. If the test polymer composition comprises the
crosslinking agent, then the crosslinking occurs during the
preparation of the plaque samples, whereby the conductivity is then
measured according to the below procedure from the resulting
crosslinked plaque sample. Crosslinking agent, if present in the
polymer composition prior to crosslinking, is preferably a
peroxide, as herein.
[0270] The plaques are press-moulded at 130.degree. C. for 12 min
while the pressure is gradually increased from 2 to 20 MPa.
Thereafter the temperature is increased and reaches 180.degree. C.
after 5 min. The temperature is then kept constant at 180.degree.
C. for 15 min during which the plaque becomes fully crosslinked by
means of the peroxide, if present in the test polymer composition.
Finally the temperature is decreased using the cooling rate
15.degree. C./min until room temperature is reached when the
pressure is released. The plaques are immediately after the
pressure release wrapped in metallic foil in order to prevent loss
of volatile substances.
[0271] A high voltage source is connected to the upper electrode,
to apply voltage over the test sample. The resulting current
through the sample is measured with an electrometer. The
measurement cell is a three electrodes system with brass
electrodes. The brass electrodes are equipped with heating pipes
connected to a heating circulator, to facilitate measurements at
elevated temperature and provide uniform temperature of the test
sample. The diameter of the measurement electrode is 100 mm
Silicone rubber skirts are placed between the brass electrode edges
and the test sample, to avoid flashovers from the round edges of
the electrodes.
[0272] The applied voltage was 30 kV DC meaning a mean electric
field of 30 kV/mm The temperature was 70.degree. C. The current
through the plaque was logged throughout the whole experiments
lasting for 24 hours. The current after 24 hours was used to
calculate the conductivity of the insulation.
[0273] This method and a schematic picture of the measurement setup
for the conductivity measurements has been thoroughly described in
a publication presented at the Nordic Insulation Symposium 2009
(Nord-IS 09), Gothenburg, Sweden, June 15-17, 2009, page 55-58:
Olsson et al, "Experimental determination of DC conductivity for
XLPE insulation".
[0274] Experimental Part
[0275] Components of the Polymer Compositions of the Invention:
[0276] HDPE: A commercially available Bormed HE9621-PH (supplier
Borealis) which is a high density polyethylene (1-butene as the
comonomer), has an MFR.sub.2 of 12 g/10 min (190.degree. C./2,16
kg) and a density of 962 kg/m.sup.3.
[0277] Bimodal HDPE: A commercially available Borstar HE6068
(supplier Borealis) which is a high density polyethylene (1-butene
as the comonomer), has an MFR.sub.2 of 10 g/10 min (190.degree.
C./2,16 kg) and a density of 944 kg/m.sup.3.
[0278] LLDPE: A commercially available product FG5190 (supplier
Borealis) which is a linear low density polyethylene with an
MFR.sub.2 (190.degree. C./2,16 kg) of 1.2 g/10min, a density of 919
kg/m.sup.3, a molecular weight (Mw) of 133000 GPC, and an MWD
(Mw/Mn) of 4.5.
[0279] PP1: A commercially available product Borclean HB311BF
(supplier Borealis) which is homopolymer of propylene with
MFR.sub.2 (230.degree. C./2,16 kg) of 2.2 g/10min, MFR.sub.5
(230.degree. C./5 kg) of 9.5 g/10 min, melting temperature (DSC)
161-165.degree. C. (according to ISO 3146). PP2: A commercially
available product Borsoft SA233CF (Borealis) which is a
random-heterophasic copolymer with an MFR.sub.2 of 0.8 g/10min,
Melting temperature (DSC) 138-142.degree. C. (according to ISO
3146) and a density of 900 kg/m.sup.3.
[0280] Inventive and Reference Example LDPE: Low Density
Polyethylene
[0281] The low density polyethylenes produced in a high pressure
reactor.
[0282] Purified ethylene was liquefied by compression and cooling
to a pressure of 90 bars and a temperature of -30.degree. C. and
split up into to two equal streams of roughly 14 tons/hour each.
The CTA (methyl ethyl ketone (MEK)), air and a commercial peroxide
radical initiator dissolved in a solvent were added to the two
liquid ethylene streams in individual amounts. Here also
1,7-octadiene was added to the reactor in amount of 40 kg/h. The
two mixtures were separately pumped through an array of 4
intensifiers to reach pressures of 2200-2300 bars and exit
temperatures of around 40.degree. C. These two streams were
respectively fed to the front (zone 1) (50%) and side (zone 2)
(50%) of a split-feed two-zone tubular reactor. The inner diameters
and lengths of the two reactor zones were 32 mm and 200 m for zone
1 and 38 mm and 400 m for zone 2. MEK was added in amounts of 190
kg/h to the front stream to maintain a MFR.sub.2 of around 2 g/10
min. The front feed stream was passed through a heating section to
reach a temperature sufficient for the exothermal polymerization
reaction to start. The reaction reached peak temperatures were
251.degree. C. and 290.degree. C. in the first and second zones,
respectively. The side feed stream cooled the reaction to an
initiation temperature of the second zone of 162.degree. C. Air and
peroxide solution was added to the two streams in enough amounts to
reach the target peak temperatures. The reaction mixture was
depressurized by product valve, cooled and polymer was separated
from unreacted gas.
TABLE-US-00001 TABLE 1 Polymer properties of LDPE Base Resin
Properties LDPE MFR.sub.2, 190.degree. C. [g/10 min] 1.90 Density
[kg/m.sup.3] 922 Vinyl [C = C/1000 C] 0.33
TABLE-US-00002 TABLE 2 Polymer compositions of the invention and
reference compositions and the electrical conductivity results:
Inv. Inv. Inv. Inv. Inv. Inv. Components comp 1 comp 2 comp 3 comp
4 comp 5 comp 6 Ref LDPE, wt %* 100 100 LLDPE, wt %* 100 HDPE, wt
%* 100 Bimodal HDPE 100 wt %* PP1, wt %* 100 PP2, wt %* 100
Crosslinking -- -- -- -- -- -- 50 (1.35) agent, mmol - O--O--/kg
polymer composition (wt %**) AO, wt %** 0.08 0.08 SR, wt %** 0.35
Lamella thickness 2.1-16.2 4.8-66.8 2.7-27 -- -- 1.7-9.7 1.8-8.6
[nm] Crystal fraction 34 90 69 -- -- 0 0 with lamella thickness
>10 nm [wt %] Crystallinity 46.7 79.8 63 -- -- 43 40 [wt %]
Weight fraction 15.9 71.8 43.5 -- -- 0 0 of crystals with lamella
thickness >10 nm [wt %] DC 2.2 1.5 3.2 2.8 0.6 152 122
conductivity, fS/m Crosslinking agent: Dicumylperoxide (CAS no.
80-43-3) AO: Antioxidant: 4,4'-thiobis (2-tertbutyl-5-methylphenol)
(CAS no. 96-69-5) SR: Scorch retardant:
2,4-Diphenyl-4-methyl-1-pentene (CAS 6362-80-7) Ref is crosslinked
LDPE Ref contained crosslinking agent and was crosslinked as
disclosed in DC conductivity method under "Determination methods"
and the conductivity was measured from the crosslinked plaques.
*The amounts of polymer components in table are based on the
combined amount of the used polymer components. The amount 100 wt %
of polymer component in table 1 means that the polymer is the sole
polymer component. **The amounts of peroxide (wt %), AO and SR are
based on the final composition.
[0283] In this context the above used definitions have the
following meanings:
[0284] "Lamella thickness"=Thickness of crystal lamellas in the
material (fractions*<0.1 wt % are ignored). [0285] * Refer to
crystal fractions of one degree Celsius intervals .
[0286] "Crystal fraction with lamella thickness >10 nm"=Fraction
of the crystals which have a thickness above 10 nm based on the
amount of the crystallised part of the polymer
[0287] "Crystallinity"=wt % of the polymer that is crystalline
[0288] "Weight fraction of crystals with lamella thickness >10
nm [wt %]"=Crystal fraction with lamella thickness>10
nm".times."Crystallinity".
[0289] As can be seen form table 2 non-crosslinked low pressure
polymers of inventive examples 1-5 show excellent low DC
conductivity. Furthermore, a crosslinked LDPE produced in high
pressure has conventionally been used in the power cable
insulations. also the non-crosslinked LDPE of inventive example 6
has unexpectedly an industrially feasible DC conductivity. The
non-crosslinked polymers of the invention are particularly
preferable in DC power cables, preferably in HV DC power
cables.
* * * * *