Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-06-30
2002-04-09
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C526S348000, C526S348200
Reexamination Certificate
active
06369129
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an insulating composition for an electric power cable which comprises a crosslinkable ethylene polymer. The present invention also relates to an electric power cable comprising a conductor surrounded by an inner semiconducting layer, an insulating layer, and an outer semiconducting layer.
BACKGROUND OF THE INVENTION
Electric power cables for medium voltages (6-69 kV) and high voltages (>69 kV) normally include one or more metal conductors surrounded by an insulating material like a polymer material, such as an ethylene polymer. In power cables the electric conductor is usually coated first with an inner semiconducting layer followed by an insulating layer, then an outer semiconducting layer followed by water barrier layers, if any, and on the outside a sheath layer. The layers of the cable are based on different types of ethylene polymers which usually are crosslinked.
LDPE (low density polyethylene), i.e. polyethylene prepared by radical polymerisation at a high pressure and crosslinked by adding a peroxide in connection with the extrusion of the cable, is today the predominant cable insulating material. Radical polymerization results in long chain branched polymers having a relatively broad molecular weight distribution (MWD). This in turn results in desirable rheological properties with regard to their application as insulating materials for electric power cables.
A limitation with LDPE lies in the fact that it is made by radial polymerisation. Radical polymerisation of ethylene is carried out at high temperatures of up to about 300° C. and at high pressures of about 100-300 MPa. To generate the high pressures needed energy consuming compressors are required. Considerable investment costs are also required for the polymerisation apparatus which must be able to resist the high pressures and temperatures of radical initiated high pressure polymerisation.
With regard to insulating compositions for electric power cables it would be desirable both from a technical and an economical point of view if it was possible to make an ethylene polymer with the advantageous properties of LDPE, but which was not made by radical polymerisation. This would mean that insulation for electric cables could be made not only at plants for high pressure polymerisation of ethylene, but also at the many existing plants for low pressure polymerisation of ethylene. In order to be a satisfactory replacement for LDPE such a low pressure material would have to fulfil a number of requirements for insulating materials, such as good processability, high dielectric strength and good crosslinking properties. It has turned out, though, that for various reasons existing low pressure materials are not suitable as replacement for LDPE as insulating material for electric cables.
Thus, conventional high density polyethylene (HDPE) produced by polymerisation with a coordination catalyst of Ziegler-Natta type at low pressure has a melting point of about 130-133° C. When a HDPE is processed in an extruder the temperature should lie above the melting point of 130-135° C. to achieve good processing. This temperature lies above the decomposition temperature of the peroxides used for the crosslinking of insulating ethylene polymer compositions. Dicumyl peroxide e.g. which is the most frequently used crosslinking peroxide starts to decompose at a temperature of about 135° C. Therefore, when HDPE is processed above its melting temperature in an extruder the crosslinking peroxide decomposes and prematurely crosslinks the polymer composition, a phenomenon referred to as “scorching”. If, on the other hand the temperature is kept below the decomposition temperature of the peroxide then the HDPE will not melt adequately and unsatisfactorily processing will result.
Further, ethylene copolymers made by polymerisation with a coordination catalyst at low pressure, like linear low density polyethylene (LLDPE) are unsuitable due to poor processability. The processability may be improved by polymerising the LLDPE in two or more steps (bimodal or multimodal LLDPE), but such LLDPE includes high melting HDPE fractions or components, particularly when the polymerisation is carried out with conventional Ziegler-Natta catalysts, which makes LLDPE unsuitable for the same reason as conventional HDPE.
In this connection WO 93/04486 discloses an electrically conductive device having an electrically conductive member comprising at least one electrically insulating member. The insulating member comprises an ethylene copolymer with a density of 0.86-0.96 g/cm
3
, a melt index of 0.2-100 dg/min, a molecular weight distribution of 1.5-30, and a composition distribution breadth index (CDBI) greater than 45%. The copolymer of this reference is unimodal as opposed to multimodal.
WO 97/50093 relates to a cable comprising one or more electrical conductors surrounded by an insulating layer of a multimodal copolymer of ethylene and one or more C
3
-C
8
alpha-olefins. The copolymer has a broad comonomer distribution as measured by Temperatures Rising Eluation Fractionation (TREF) with a value for the percent of copolymer, which elutes out at a temperature of grater than 90° C., of greater than about 5%; a Water Tree Growth Rate (WTGR) value of less than abut 20%; a melt index in the range of about 0.1 to about 30 g/10 min; and a density in the range of 0.880-0.950 g/cm
3
, and is prepared by a low pressure process.
In view of the above it would be an advantage if it was possible to replace crosslinkable LDPE made by radical initiated polymerisation as a material for the insulating layer of electric power cables by an ethylene polymer made by coordination catalysed low pressure polymerisation. Such a replacement polymer should have rheological properties, including processability similar to those of LDPE. Further, it should have a low enough melting temperature to be completely melted at 125° C. in order to avoid “scorch” due to premature decomposition of the crosslinked peroxide.
SUMMARY OF THE INVENTION
It has now been discovered that LDPE may be replaced as a crosslinkable material for the insulation type layer of electric cables by a crosslinkable ethylene copolymer made by coordination catalysed low pressure polymerisation with ethylene copolymer is a multimodal ethylene copolymer with specified density and viscosity and with melting temperature of at most 125° C.
More particularly the present invention provides an insulating composition for an electric power cable which comprises a crosslinkable, multimodal ethylene copolymer obtained by coordination catalysed polymerisation of ethylene and at least one other alpha-olefin, said multimodal ethylene copolymer having a density of 0.890-0.940 g/cm
3
and an MFR
2
of 0.1-10 g/10 min, characterised in that the ethylene polymer has an MWD of 3-12, a melting temperature of at most 125° C., and a viscosity of
2500-7000 Pa.s at 135° C. and a shear rate of 10 s
−1
,
1000-1800 Pa.s at 135° C. and a shear rate of 100 s
−1
, and
250-400 Pa.s at 135° C. and a shear rate of 1000 s
−1
,
said multimodal ethylene copolymer including an ethylene copolymer fraction selected from (a) a low molecular weight ethylene copolymer having a density of 0.900-0.950 g/cm
3
and an MFR
2
of 25-300 g/10 min, and (b) a high molecular weight ethylene copolymer having a density of 0.870-0.940 g/cm
3
and an MFR
2
of 0.01-3 g/10 min.
A density in the lower part of the range, i.e. 0.890-0.910 g/cm
3
is aimed at when a very flexible cable is desired. Such cables are suitable for applications in cars, mines and the building industry. These low densities are only possible to reach by using a single-site type catalyst, at least for the higher molecular weight fraction. When densities in the range 0.910-0.940 g/cm
3
are chosen, the resulting cables are stiffer, but have better mechanical strength values, and are therefore more suitable for non-flexible power supply cables.
The present invention also provides an electric power cable comprising a conductor surrounded
Mårtensson Hans
Nymark Anders
Poikela Merja
Borealis Technology Oy
Cain Edward J.
Merchant & Gould P.C.
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