Note: Descriptions are shown in the official language in which they were submitted.
CA 02583386 2007-04-05
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Title: Polyethylene molding composition for external
sheathing of electric cables
The present invention relates to a polyethylene (PE) molding
composition which has a multimodal molar mass distribution and
is particularly suitable for producing cable sheathing, and a
process for preparing this molding composition in the presence
of a catalytic system comprising a Ziegler catalyst and a
cocatalyst via a multistage reaction sequence comprising
successive polymerization steps.
Polyethylene is widely used for industrial applications in
which a material having a high mechanical strength and a high
resistance to thermooxidative degradation is required in order
to ensure a long life even at elevated use temperatures. In
addition, polyethylene has the particular advantages that it
has good chemical resistance, it has a low intrinsic weight
and it is a material which can easily be processed in the
melt.
PE-molding compositions for cable sheathing should accordingly
possess the following important properties:
- Easy processability.
- Good resistance to weathering influences.
- Good thermal aging resistance.
- Ability to withstand high mechanical stresses and good
abrasion resistance.
- Low permeability to water vapor and oxygen so as to avoid
corrosion of the metallic conductor or conductors.
WO 97/03124 describes a coating composition based on
polyethylene which has a bimodal molar mass distribution. This
coating composition is very suitable for producing external
sheathing for energy and information transmission cables,
CONFIRMATION COPY
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which give the cables sheathed therewith improved durability
in respect of corrosion, in respect of oxidative aging, in
respect of weathering influences of all types and in respect
of mechanical stresses.
Known polyethylene molding compositions having a unimodal
molar mass distribution have disadvantages in terms of their
processability, their environmental stress cracking resistance
and their mechanical toughness. Compared to these, molding
compositions having a bimodal molar mass distribution
represent a technical improvement. They are easier to process
and at the same density have an improved environmental stress
cracking resistance and a higher mechanical strength.
It was thus an object of the present invention to develop a
polyethylene molding composition which retains good
processability but when used as cable sheathing displays
significant advantages in respect of environmental stress
cracking resistance, resistance to mechanical stresses and
improved abrasion behavior for simpler laying and relatively
low water vapor permeation and oxygen permeation.
This object is achieved by a molding composition of the
generic type mentioned at the outset whose distinguishing
features are that it comprises from 45 to 55% by weight of a
low molecular weight ethylene homopolymer A, from 30 to 40% by
weight of a high molecular weight copolymer B of ethylene and
another olefin having from 4 to 8 carbon atoms and from 10 to
20% by weight of an ultra high molecular weight ethylene
copolymer C, where all percentages are based on the total
weight of the molding composition.
The invention further provides a process for preparing this
molding composition in a cascaded suspension polymerization
and a defect-free external sheathing of energy and information
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transmission cables comprising this molding composition and
having excellent mechanical strength properties combined with
high stiffness.
The polyethylene molding composition of the invention has a
density at a temperature of 23 C in the range from 0.94 to
0.950 g/cm3 and a trimodal molar mass distribution. The high
molecular weight copolymer B contains a proportion of further
olefin monomer units having from 4 to 8 carbon atoms, namely
from 1 to 8% by weight. Examples of such comonomers are
1-butene, 1-pentene, 1-hexene, 1-octene and 4-methyl-l-
pentene. The ultra high molecular weight ethylene copolymer C
likewise contains one or more of the abovementioned comonomers
in an amount in the range from 1 to 8% by weight.
Furthermore, the molding composition of the invention has a
melt flow index in accordance.with ISO 1133, expressed as
MFI190/5r in the range from 0.5 to 2.1 dg/min and a viscosity
number VNoverallr measured in accordance with ISO/R 1191 in
decalin at a temperature of 135 C, in the range from 260 to
340 cm3/g, in particular from 280 to 320 cm3/g.
The trimodality as a measure of the position of the centers of
gravity of the three individual molar mass distributions_can
be described with the aid of the viscosity numbers VN in
accordance with ISO/R 1191 of the polymers formed in the
successive polymerization stages. Here, the band widths of the
polymers formed in the individual reaction stages are as
follows:
The viscosity number VN1 measured on the polymer after the
first polymerization stage is identical to the viscosity
number VNA of the low molecular weight polyethylene A and is,
according to the invention, in the range from 50 to 90 cm3/g,
in particular in the range from 60 to 80 cm3/g.
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The viscosity number VN2 measured on the polymer after the
second polymerization stage does not correspond to VNB of the
relatively high molecular weight polyethylene B formed in the
second polymerization stage, but is instead the viscosity
number of the mixture of polymer A plus polymer B. According
to the invention, VN2 is in the range from 260 to 320 cm3/g.
The viscosity number VN3 measured on the polymer after the
third polymerization stage does not correspond to VNc of the
ultra high molecular weight copolymer C formed in the third
polymerization sta.ge, which can likewise be determined only
mathematically, but is instead the viscosity number of the
mixture of the polymer A, polymer B plus polymer C. According
to the invention, VN3 is in the range from 260 to 340 cm3/g, in
particular from 280 to 320 cm3/g.
The polyethylene is obtained by polymerization of the monomers
in suspension at temperatures in the range from 70 to 90 C,
preferably from 75 to 90 C, a pressure in the range from 2 to
10 bar and in the presence of a highly active Ziegler catalyst
composed of a transition metal compound and an organoaluminum
compound. The polymerization is a three-stage polymerization,
i.e. it is carried out in three successive stages, with the
molar mass being regulated in each stage by means of added
hydrogen.
Apart from the polyethylene, the polyethylene molding
composition of the invention can further comprise additional
additives. Such additives are, for example, heat stabilizers,
antioxidants, UV absorbers, light stabilizers, metal
deactivators, peroxide-decomposing compounds, basic
costabilizers, in amounts of from 0 to 10o by weight,
preferably from 0 to 5% by weight, and also carbon black,
fillers, pigments, flame retardants, or combinations of these
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in total amounts of from 0 to 50% by weight, based on the
total weight of the mixture.
The molding composition of the invention is particularly
5 useful for producing external sheathing of electric cables of
all types, e.g. cables for the transmission of information or
energy. Such cables usually comprise one or more metallic or
nonmetallic conductors which can each be coated with an
insulating layer. The cable sheath has the task of protecting
the cable against damage by external influences, e.g. during
laying, and is preferably applied by extrusion by firstly
plasticizing the polyethylene molding composition in an
extruder at temperatures in the range from 200 to 250 C and
then extruding it through a suitable nozzle onto the cable
surface and cooling it there.
The molding composition of the invention can be processed
particularly well by the extrusion process to produce coatings
and has a notched impact toughness (ISO) in the range from 8
to 14 kJ/m2 and an environmental stress cracking resistance
(ESCR) in the range > 200 h.
The notched impact toughnesslSO is measured at -30 C in
accordance with ISO 179-1/leA / DIN 53453. The dimensions of
the specimen are 10 x 4 x 80 mm, and it is provided with a V-
notch having an angle of 45 , a depth of 2 mm and a radius at
the bottom of the notch of 0.25 mm.
The environmental stress cracking resistance (ESCR) of the
molding composition of the invention is determined by an
internal measurement method and is reported in h. This
laboratory method is described by M. Flei3ner in Kunststoffe
77 (1987), p. 45 ff, and corresponds to ISO/CD 16770 which is
now valid. The publication shows that there is a relationship
between the determination of slow crack growth in a creep test
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on test rods having a circumferential notch and the brittle
branch of the internal pressure test in accordance with
ISO 1167. A shortening of the time to failure is achieved by
shortening the crack initiation time by means of the notch
(1.6 mm/razor blade) in ethylene glycol as medium inducing
environmental stress cracking at a temperature of 80 C and a
tensile stress of 3.5 MPa. The production of the specimens is
carried out by sawing three test specimens having dimensions
of 10 x 10 x 90 mm from a 10 mm thick pressed plate. The test
specimens are in turn notched in the middle by means of a
razor blade in a notching apparatus made in-house for this
purpose (cf. figure 5 in the publication). The notch depth is
1.6 mm.
Example 1
The polymerization of ethylene was carried out in a continuous
process in three reactors connected in series. A Ziegler
catalyst which had been prepared by the method of WO 91/18934,
example 2, and has the operations number 2.2 in the WO in an
amount of 0.08 mmol/h and also sufficient suspension medium
(hexane), triethylaluminum as cocatalyst in an amount of
0.08 mmol/h, ethylene and hydrogen were fed into the first
reactor. The amount of ethylene (= 65 kg/h) and the amount of
hydrogen (= 68 g/h) were set so that a proportion of from 25
to 26% by volume ethylene and a proportion of 65% by volume of
hydrogen were measured in the gas space of the first reactor;
the remainder was a mixture of nitrogen and vaporized
suspension medium.
The polymerization in the first reactor was carried out at a
temperature of 84 C.
The suspension from the first reactor was then passed to a
second reactor in which the proportion of hydrogen in the gas
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space had been reduced to 7 - 9% by volume and into which an
amount of 48.1 kg/h of ethylene plus an amount of 2940 g/h of
1-butene were introduced. The reduction in the amount of
hydrogen was achieved by means of H2 intermediate
depressurization. 73% by volume of ethylene, 8% by volume of
hydrogen and 0.82% by volume of 1-butene were measured in the
gas space of the second reactor; the remainder was a mixture
of nitrogen and vaporized suspension medium.
The polymerization in the second reactor was carried out at a
temperature of 80 C.
The suspension from the second reactor was passed via a
further H2 intermediate depressurization, by means of which the
amount of hydrogen in the gas space in the third reactor was
set to 2.5% by volume, into the third reactor.
An amount of 16.9 kg/h of ethylene plus an amount of 1500 g/h
of 1-butene were introduced into the third reactor. A
proportion of ethylene of 87% by volume, a proportion of
hydrogen of 2.5% by volume and a proportion of 1-butene of
1.2% by volume were measured in the gas space of the third
reactor; the remainder was a mixture of nitrogen and vaporized
suspension medium.
The polymerization in-the third reactor was carried out at a
temperature of 80 C.
The long-term activity of the polymerization catalyst
necessary for the above-described cascaded mode of operation
was ensured by a specially developed Ziegler catalyst having
the composition reported in the abovementioned WO document. A
measure of the usability of this catalyst is its extremely
high response to hydrogen and its high activity which remains
constant over a long period of from 1 to 8 hours.
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The suspension medium is separated off from the polymer
suspension leaving the third reactor, the powder is dried and
the powder is passed to pelletization.
The viscosity numbers and the proportions WA, WB and WC of
polymers A, B and C for the PE molding composition prepared as
described in example 1 are shown in table 1 below:
Table 1
Example 1
WA [% by weight] 50
WB [% by weight] 37
WC [% by weight] 13
VN1 [cm3/g] 80
VN2 [cm3/g] 280
VNoveral.i [cm3/g] 304
FNCT [h] 220
AFM (-30 C) 3.8 kJ/m2
ACN (+23 C) 13 kJ/m2
The abbreviations for the physical properties in table 1 have
the following meaning:
- FNCT = environmental stress cracking resistance (Full
Notch Creep Test) measured by the internal measurement
method described by M. Fleil3ner in [h], conditions: 95 C,
3.5 MPa, water / 2% of Arkopal.
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- AFM (-300C) = notched impact toughness, measured in
accordance with ISO 179-1/leA / DIN 53453 in [kJ/m2] at
-30 C.
- ACN (+23 C) = notched impact toughness, measured in
accordance with ISO 179-1/leA / DIN 53453 in [kJ/m2] at
+23 C.
An energy cable having a diameter of 5 cm was sheathed
externally with the polyethylene molding composition prepared
in this way by plasticizing the molding composition at a
temperature of 220 C in an extruder and then extruding it
through an annular nozzle onto the electric cable and cooling
it there. The sheath produced in this way had a thickness of
0.5 cm.
The resulting surface on the electric cable was smooth and
displayed no visible damage.
The electric cable produced in this way was, for test
= purposes, packed in a steel mesh basket and stored in the
River Main in Frankfurt at a depth of 2 m below the water
surface. The duration of the storage was 1 year, and the water
temperature varied, depending on the time of year, from + 3 to
+ 27 C.
After a storage time of one year, the cable was taken out
again and examined visually. After mechanical removal of
adhering mud and algae, the appearance of the external
sheathing of the cable was no different from the state
immediately after it had been produced.
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