Cable and method for using a cable-sheathing composition including an ethylene polymer mixture

A cable-sheating composition and its use as outer sheath for a power cable or a communication cable are disclosed. The cable-sheating composition is a multimodal, preferably bimodal, mixture of olefin polymers, preferable ethylene plastics, having a density of about 0.915-0.955 g/cm.sup.3 and a melt flow rate of about 0.1-0.3 g/10 min, said olefin polymer mixture comprising at least a first and a second olefin polymer, of which the first has a density and a melt flow rate selected from (a) about 0.930-0.975 g/cm.sup.3 and about 50-2000 g/10 min and (b) about 0.88-0.93 g/cm.sup.3 and about 0.1-0.8 g/10 min. Preferably, the olefin polymer mixture has been obtained by coordination-catalyzed polymerization of at least one .alpha.-olefin in several stages, preferably two stages consisting of a loop reactor/a gas-phase reactor or a gas-phase reactor/a gas-phase reactor, through polymerization or copolymerization of ethylene in the first stage and copolymerization of ethylene with butene, 4-methyl-1-pentene, 1-hexene or 1-octene in the second stage.

The present invention relates to a cable-sheathing composition, as well as
 the use thereof as outer sheathing for a power cable or a communication
 cable.
 Cables, by which is meant power cables for high voltage, medium voltage or
 low voltage, and communication cables, such as optical cables, coaxial
 cables and pair cables, generally comprise a core surrounded by a sheath
 consisting of one or more layers. The outermost layer is referred to as
 outer sheath or sheathing layer and is nowadays made of polymer material,
 preferably ethylene plastic. The highly diverse fields of application for
 various sorts of cables, such as telecommunication cables, including
 conventional copper cables and fibre-optical cables, as well as power
 cables, entail that the sheathing material has to meet a number of
 property requirements which in some respects are contradictory. Thus,
 important properties of cable-sheathing materials are good processability,
 i.e. it should be easy to process the material within a broad temperature
 range, low shrinkage, high mechanical strength, high surface finish as
 well as high environmental stress cracking resistance (ESCR). Since it has
 hitherto been difficult or even impossible to meet all these property
 requirements, prior-art sheathing materials have been the result of
 compromise, such that that good properties in one respect have been
 obtained at the cost of poorer properties in some other respect.
 Thus, it would be highly advantageous if this compromise as regards the
 properties of cable-sheathing materials could be reduced or even
 eliminated. In particular, it would be advantageous if one were able to
 improve the ESCR of the material and reduce the shrinkage at a given
 processability.
 The present invention achieves this goal by a cable-sheathing composition
 which, instead of the unimodal polyethylene plastic used in conventional
 cable-sheathing compositions, consists of a multimodal olefin polymer
 mixture having certain given values of density and melt flow rate, both as
 regards the polymer mixture and as regards the polymers forming part
 thereof.
 The present invention thus provides a cable-sheathing composition, which is
 characterised in that it consists of a multimodal olefin polymer mixture
 having a density of about 0.915-0.955 g/cm.sup.3 and a melt flow rate of
 about 0.1-0.3 g/10 min, said olefin polymer mixture comprising at least a
 first and a second olefin polymer, of which the first has a density and a
 melt flow rate selected from (a) about 0.930-0.975 g/cm.sup.3 and about
 50-2000 g/10 min and (b) about 0.88-0.93 g/cm.sup.3 and about 0.1-0.8 g/10
 min.
 The invention further concerns the use of this cable-sheathing composition
 as outer sheath for a power cable or a communication cable.
 Further distinctive features and advantages of the invention will appear
 from the following description and the appended claims.
 However, before the invention is described in more detail, a few key
 expressions will be defined.
 By the "modality" of a polymer is meant the structure of the
 molecular-weight distribution of the polymer, i.e. the appearance of the
 curve indicating the number of molecules as a function of the molecular
 weight. If the curve exhibits one maximum, the polymer is referred to as
 "unimodal", whereas if the curve exhibits a very broad maximum or two or
 more maxima and the polymer consists of two or more fractions, the polymer
 is referred to as "bimodal", "multimodal" etc. In the following, all
 polymers whose molecular-weight-distribution curve is very broad or has
 more than one maximum are jointly referred to as "multimodal".
 The "melt flow rate" (MRF) of a polymer is determined in accordance with
 ISO 1133, condition 4, at a temperature of 190.degree. C. and a nominal
 load of 2,160 kg and is equivalent to the term "melt index" previously
 used. The melt flow rate, which is indicated in g/10 min, 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.
 By the term "environmental stress cracking resistance" (ESCR) is meant the
 resistance of the polymer to crack formation under the action of
 mechanical stress and a reagent in the form of a surfactant. The ESCR is
 determined in accordance with ASTM D 1693 A, the reagent employed being
 Igepal CO-630.
 By the term "ethylene plastic" is meant a plastic based on polyethylene or
 on copolymers of ethylene, the ethylene monomer making up most of the
 mass.
 As indicated in the foregoing, the cable-sheathing composition according to
 the invention is distinguished by the fact that it consists of a
 multimodal olefin polymer mixture of specified density and melt flow rate.
 It is previously known to produce multimodal, in particular bimodal, olefin
 polymers, preferably multimodal ethylene plastics, in two or more reactors
 connected in series. As instances of this prior art, mention may be made
 of EP 040 992, EP 041 796, EP 022 376 and WO 92/12182, which are hereby
 incorporated by way of reference as regards the production of multimodal
 polymers. According to these references, each and every one of the
 polymerisation stages can be carried out in liquid phase, slurry or gas
 phase.
 According to the present invention, the main polymerisation stages are
 preferably carried out as a combination of slurry polymerisation/gas-phase
 polymerisation or gas-phase polymerisation/gas-phase polymerisation. The
 slurry polymerisation is preferably performed in a so-called loop reactor.
 The use of slurry polymerisation in a stirred-tank reactor is not
 preferred in the present invention, since such a method is not
 sufficiently flexible for the production of the inventive composition and
 involves solubility problems. In order to produce the inventive
 composition of improved properties, a flexible method is required. For
 this reason, it is preferred that the composition is produced in two main
 polymerisation stages in a combination of loop reactor/gas-phase reactor
 or gas-phase reactor/gas-phase reactor. It is especially preferred that
 the composition is produced in two main polymerisation stages, in which
 case the first stage is performed as slurry polymerisation in a loop
 reactor and the second stage is performed as gas-phase polymerisation in a
 gas-phase reactor. Optionally, the main polymerisation stages may be
 preceded by a prepolymerisation, in which case up to 20% by weight,
 preferably 1-10% by weight, of the total amount of polymers is produced.
 Generally, this technique results in a multimodal polymer mixture through
 polymerisation with the aid of a chromium, metallocene or Ziegler-Natta
 catalyst in several successive polymerisation reactors. In the production
 of, say, a bimodal ethylene plastic, which according to the invention is
 the preferred polymer, a first ethylene polymer is produced in a first
 reactor under certain conditions with respect to monomer composition,
 hydrogen-gas pressure, temperature, pressure, and so forth. After the
 polymerisation in the first reactor, the reaction mixture including the
 polymer produced is fed to a second reactor, where further polymerisation
 takes place under other conditions. Usually, a first polymer of high melt
 flow rate (low molecular weight) and with a moderate or small addition of
 comonomer, or no such addition at all, is produced in the first reactor,
 whereas a second polymer of low melt flow rate (high molecular weight) and
 with a greater addition of comonomer is produced in the second reactor. As
 comonomer, use is commonly made of other olefins having up to 12 carbon
 atoms, such as .alpha.-olefins having 3-12 carbon atoms, e.g. propene,
 butene, 4-methyl-1-pentene, hexene, octene, decene, etc., in the
 copolymerisation of ethylene. The resulting end product consists of an
 intimate mixture of the polymers from the two reactors, the different
 molecular-weight-distribution curves of these polymers together forming a
 molecular-weight-distribution curve having a broad maximum or two maxima,
 i.e. the end product is a bimodal polymer mixture. Since multimodal, and
 especially bimodal, polymers, preferably ethylene polymers, and the
 production thereof belong to the prior art, no detailed description is
 called for here, but reference is had to the above specifications.
 It should here be pointed out that, in the production of two or more
 polymer components in a corresponding number of reactors connected in
 series, it is only in the case of the component produced in the first
 reactor stage and in the case of the end product that the melt flow rate,
 the density and the other properties can be measured directly on the
 material removed. The corresponding properties of the polymer components
 produced in reactor stages following the first stage can only be
 indirectly determined on the basis of the corresponding values of the
 materials introduced into and discharged from the respective reactor
 stages.
 Even though multimodal polymers and their production are known per se, it
 is not, however, previously known to use such multimodal polymer mixtures
 in cable-sheathing compositions. Above all, it is not previously known to
 use in this context multimodal polymer mixtures having the specific values
 of density and melt flow rate as are required in the present invention.
 As hinted at above, it is preferred that the multimodal olefin polymer
 mixture in the cable-sheathing composition according to the invention is a
 bimodal polymer mixture. It is also preferred that this bimodal polymer
 mixture has been produced by polymerisation as above under different
 polymerisation conditions in two or more polymerisation reactors connected
 in series. Owing to the flexibility with respect to reaction conditions
 thus obtained, it is most preferred that the polymerisation is carried out
 in a loop reactor/a gas-phase reactor, a gas-phase reactor/a gas-phase
 reactor or a loop reactor/a loop reactor as the polymerisation of one, two
 or more olefin monomers, the different polymerisation stages having
 varying comonomer contents. Preferably, the polymerisation conditions in
 the preferred two-stage method are so chosen that a comparatively
 low-molecular polymer having a moderate, low or, which is preferred, no
 content of comonomer is produced in one stage, preferably the first stage,
 owing to a high content of chain-transfer agent (hydrogen gas), whereas a
 high-molecular polymer having a higher content of comonomer is produced in
 another stage, preferably the second stage. The order of these stages may,
 however, be reversed.
 Preferably, the multimodal olefin polymer mixture in accordance with the
 invention is a mixture of propylene plastics or, which is most preferred,
 ethylene plastics. The comonomer or comonomers in the present invention
 are chosen from the group consisting of .alpha.-olefins having up to 12
 carbon atoms, which in the case of ethylene plastic means that the
 comonomer or comonomers are chosen from .alpha.-olefins having 3-12 carbon
 atoms. Especially preferred comonomers are butene, 4-methyl-1-pentene,
 1-hexene and 1-octene.
 In view of the above, a preferred ethylene-plastic mixture according to the
 invention consists of a low-molecular ethylene homopolymer mixed with a
 high-molecular copolymer of ethylene and butene, 4-methyl-1-pentene,
 1-hexene or 1-octene.
 The properties of the individual polymers in the olefin polymer mixture
 according to the invention should be so chosen that the final olefin
 polymer mixture has a density of about 0.915-0.955 g/cm.sup.3, preferably
 about 0.920-0.950 g/cm.sup.3, and a melt flow rate of about 0.1-3.0 g/10
 min, preferably about 0.2-2.0 g/10 min. According to the invention, this
 is preferably achieved by the olefin polymer mixture comprising a first
 olefin polymer having a density of about 0.930-0.975 g/cm.sup.3,
 preferably about 0.955-0.975 g/cm.sup.3, and a melt flow rate of about
 50-2000 g/10 min, preferably about 100-1000 g/10 min, and most preferred
 about 200-600 g/10 min, and at least a second olefin polymer having such a
 density and such a melt flow rate that the olefin polymer mixture obtains
 the density and the melt flow rate indicated above.
 If the multimodal olefin polymer mixture is bimodal, i.e. is a mixture of
 two olefin polymers (a first olefin polymer and a second olefin polymer),
 the first olefin polymer being produced in the first reactor and having
 the density and the melt flow rate indicated above, the density and the
 melt flow rate of the second olefin polymer, which is produced in the
 second reactor stage, may, as indicated in the foregoing, be indirectly
 determined on the basis of the values of the materials supplied to and
 discharged from the second reactor stage.
 In the event that the olefin polymer mixture and the first olefin polymer
 have the above values of density and melt flow rate, a calculation
 indicates that the second olefin polymer produced in the second stage
 should have a density in the order of about 0.88-0.93 g/cm.sup.3,
 preferably 0.91-0.93 g/cm.sup.3, and a melt flow rate in the order of
 about 0.01-0.8 g/10 min, preferably about 0.05-0.3 g/10 min.
 As indicated in the foregoing, the order of the stages may be reversed,
 which would mean that, if the final olefin polymer mixture has a density
 of about 0.915-0.955 g/cm.sup.3, preferably about 0.920-0.950 g/cm.sup.3,
 and a melt flow rate of about 0.1-3.0 g/10 min, preferably about 0.2-2.0
 g/10 min, and the first olefin polymer produced in the first stage has a
 density of about 0.88-0.93 g/cm.sup.3, preferably about 0.91-0.93
 g/cm.sup.3, and a melt flow rate of 0.01-0.8 g/10 min, preferably about
 0.05-0.3 g/10 min, then the second olefin polymer produced in the second
 stage of a two-stage method should, according to calculations as above,
 have a density in the order of about 0.93-0.975 g/cm.sup.3, preferably
 about 0.955-0.975 g/cm.sup.3, and a melt flow rate of 50-2000 g/10 min,
 preferably about 100-1000 g/10 min, and most preferred about 200-600 g/10
 min. This order of the stages in the production of the olefin polymer
 mixture according to the invention is, however, less preferred.
 In order to optimise the properties of the cable-sheathing composition
 according to the invention, the individual polymers in the olefin polymer
 mixture should be present in such a weight ratio that the aimed-at
 properties contributed by the individual polymers are also achieved in the
 final olefin polymer mixture. As a result, the individual polymers should
 not be present in such small amounts, such as about 10% by weight or
 below, that they do not affect the properties of the olefin polymer
 mixture. To be more specific, it is preferred that the amount of olefin
 polymer having a high melt flow rate (low-molecular weight) makes up at
 least 25% by weight but no more than 75% by weight of the total polymer,
 preferably 35-55% by weight of the total polymer, thereby to optimise the
 properties of the end product.
 The use of multimodal olefin polymer mixtures of the type described above
 results in inventive cable-sheathing compositions having much better
 properties than conventional cable-sheathing compositions, especially as
 regards shrinkage, ESCR and processability. In particular the reduced
 shrinkage of the inventive cable-sheathing composition is a great
 advantage.
 As indicated in the foregoing, the cable-sheathing composition according to
 the invention can be used for producing outer sheaths for cables,
 including power cables as well as communication cables. Amongst power
 cables, whose outer sheaths may advantageously be produced from the
 cable-sheathing composition according to the invention, mention may be
 made of high-voltage cables, medium-voltage cables and low-voltage cables.
 Amongst communication cables, whose outer sheaths may advantageously be
 made from the cable-sheathing composition according to the invention,
 mention may be made of pair cables, coaxial cables and optical cables.
 Here follows a few non-restricting Examples intended to further elucidate
 the invention and its advantages.

EXAMPLE 1
 In a polymerisation plant consisting of a loop reactor connected in series
 to a gas-phase reactor and involving the utilisation of a Ziegler-Natta
 catalyst, a bimodal ethylene plastic was polymerised under the following
 conditions.
 The First Reactor (Loop Reactor)
 In this reactor, a first polymer (Polymer 1) was produced by the
 polymerisation of ethylene in the presence of hydrogen (molar ratio of
 hydrogen to ethylene=0.38:1). The resulting ethylene homopolymer had an
 MFR value of 492 g/10 min and a density of 0.975 g/cm.sup.3.
 The Second Reactor (Gas-Phase Reactor)
 In this reactor, a second polymer (Polymer 2) was produced by the
 polymerisation of ethylene and butene (molar ratio in the gas phase of
 butene to ethylene=0.22:1, of hydrogen to ethylene=0.03:1). The resulting
 copolymer of ethylene and butene was present in the form of an intimate
 mixture with the ethylene homopolymer from the first reactor, the weight
 ratio of Polymer 1 to Polymer 2 being 45:55.
 The bimodal mixture of Polymer 1 and Polymer 2 had a density of 0.941
 g/cm.sup.3 and an MFR value of 0.4 g/10 min. After compounding with carbon
 black, one obtained an end product containing 2.5% by weight thereof,
 resulting in a final density of 0.951 g/cm.sup.3. This end product will in
 the following be referred to as Bimodal Ethylene Plastic 1.
 Bimodal Ethylene Plastic 1 was used as cable-sheathing composition, and the
 properties of this composition were determined and compared with those of
 a conventional cable-sheathing composition of unimodal ethylene plastic
 (Reference 1). Reference 1 had a density of 0.941 g/cm.sup.3 (after
 compounding to a carbon-black content of 2.5% by weight and a density of
 0.951 g/cm.sup.3) and an MFR value of 0.24 g/10 min.
 In this Example, as well as the following Examples, the shrinkage of the
 composition produced was determined in accordance with a method (in the
 following referred to as UNI-5079) which had been developed in order to
 evaluate the shrinkage tendency of sheathing materials. The shrinkage is
 determined in the following manner.
 Cable samples for the evaluation are extruded as follows.

Conductor: 3.0 mm solid, Al conductor
 Wall thickness: 1.0 mm
 Temperature, die: +210.degree. C. or +180.degree. C.
 Distance between die 35 cm
 and water bath:
 Temperature, water bath: +23.degree. C.
 Line velocity: 75 m/min
 Die type: Semi-tube
 Nipple: 3.65 mm
 Die: 5.9 mm
 Screw design: Elise
 Breaking plate
 The shrinkage in per cent is measured after 24 h in a room with constant
 temperature (+23.degree. C.) as well as after 24 h at a temperature of
 +100.degree. C.
 Cable samples measuring approximately 40 cm are measured. Conveniently, the
 cable sample is so marked that measurement after the conditionings can be
 carried out at the same point on the cable sample.
 Should the sample be found to shrink during measurement, marks of about 40
 cm first have to be made. Then, the length is cut and remeasured. Double
 samples are taken of each cable that is to be analysed. The samples are
 placed in the room with constant temperature for 24 h, whereupon they are
 measured, and the shrinkage value in percent is calculated.
 All the samples are then placed on a talcum bed at +100.degree. C. for 24
 h. The samples are then measured, and the total shrinkage value in percent
 is calculated on the basis of the initial length.
 The measurement results are indicated in Table 1 below.
 TABLE 1
 Material properties Bimodal 1 Reference 1
 Tensile break strength 34 38
 (MPa).sup.1
 Elongation at break (%).sup.1 800 900
 ESCR.sup.2 0/2000 h F20/550 h
 Shrinkage (%) at
 23.degree. C./24 h.sup.3 0.0 0.7
 23.degree. C./24 h.sup.4 0.0 0.7
 Shrinkage (%) at
 100.degree. C./24 h.sup.3 1.0 2.0
 100.degree. C./24 h.sup.4 0.9 2.3
 Surface finish.sup.5
 After extrusion at
 180.degree. C. at
 15 m/min 0-1 0
 35 m/min 0-1 0
 75 m/min 0 0
 140 m/min 0 1
 After extrusion at
 210.degree. C. at
 15 m/min -- 0
 35 m/min 0-1 0
 75 m/min 0-1 0
 140 m/min 0 0-1
 .sup.1 Determined in accordance with ISO 527-2 1993/5A on cable samples.
 .sup.2 Determined in accordance with ASTM D 1693/A, 10% Igepal. The results
 are indicated as the percentage of cracked sample rods at a given time.
 F20 means that 20% of the sample rods were cracked after the time
 indicated.
 .sup.3 Determined in accordance with UNI-5079 after extrusion at
 180.degree. C.
 .sup.4 Determined in accordance with UNI-5079 after extrusion at
 210.degree. C.
 .sup.5 Classification: 0 = excellent to 4 = very uneven.
 It is evident from the values indicated in Table 1 that the inventive
 sheathing material exhibits improved properties as regards shrinkage,
 especially at room temperature, and environmental stress cracking
 resistance (ESCR). The tensile-strength properties of the sheathing
 material according to the invention are on a level with those of Reference
 1. Also the processability, which can be deduced from the MFR value, of
 the sheathing material according to the invention is as good as that of
 Reference 1. It should be emphasised that, whereas the sheathing material
 of Reference 1 has good processing properties obtained at the cost of poor
 shrinkage properties, especially at room temperature, the sheathing
 material according to the invention has good processing properties as well
 as good (low) shrinkage properties. This is a considerable advantage,
 which is enhanced by the improved ESCR properties of the sheathing
 material according to the invention.
 EXAMPLE 2
 In the polymerisation plant of Example 1, a bimodal ethylene plastic was
 produced under the following conditions.
 The First Reactor (Loop Reactor)
 In this reactor, a first polymer (Polymer 1) was produced by the
 polymerisation of ethylene in the presence of hydrogen (molar ratio of
 hydrogen to ethylene=0.38:1). The resulting ethylene homopolymer had an
 MFR value of 444 g/10 min and a density of 0.975 g/cm.sup.3.
 The Second Reactor (Gas-Phase Reactor)
 In this reactor, a second polymer (Polymer 2) was produced by the
 polymerisation of ethylene and butene (molar ratio of butene to
 ethylene=0.23:1; molar ratio of hydrogen to ethylene=0.09:1). The
 resulting copolymer of ethylene and butene was present in the form of an
 intimate mixture with the ethylene homopolymer from the first reactor, the
 weight ratio of Polymer 1 to Polymer 2 being 40:60.
 The bimodal mixture of Polymer 1 and Polymer 2, which constituted the end
 product, had a density of 0.941 g/cm.sup.3 (after an addition of 2.5% by
 weight of carbon black, 0.951 g/cm.sup.3) and an MFR value of 1.4 g/10
 min. In the following, this end product will be referred to as Bimodal
 Ethylene Plastic 2.
 In similar fashion, yet another bimodal ethylene plastic was produced (in
 the following referred to as Bimodal Ethylene Plastic 3), the molar ratio
 of hydrogen to ethylene in the first reactor being 0.39:1, and the
 resulting ethylene homopolymer (Polymer 1) in the first reactor having an
 MFR value of 468 g/10 min and a density of 0.962 g/cm.sup.3. In the second
 reactor, a copolymer of ethylene and butene (Polymer 2) was produced, the
 molar ratio of butene to ethylene being 0.24:1, and the molar ratio of
 hydrogen to ethylene being 0.07:1. The weight ratio of Polymer 1 to
 Polymer 2 was 45:55. The end product (Bimodal Ethylene Plastic 3) had a
 density of 0.941 g/cm.sup.3 (after compounding with 2.5% by weight of
 carbon black, 0.951 g/cm.sup.3) and an MFR value of 1.3 g/10 min.
 Bimodal Ethylene Plastic 2 and Bimodal Ethylene Plastic 3 were used as
 cable-sheathing compositions, and the properties of these compositions
 were determined and compared with those of a prior-art sheathing
 composition (Reference 2). Reference 2 was a special composition intended
 for use in cases where particularly low shrinkage is required, such as
 fibre-optical applications, and this composition consisted of a melt blend
 of a polyethylene fraction having a density of 0.960 g/cm.sup.3 and an MFR
 value of 3.0 g/10 min, and another polyethylene fraction having a density
 of 0.920 g/cm.sup.3 and an MFR value of 1.0 g/10 min. This resulted in an
 end product having a density of 0.943 g/cm.sup.3 (after an addition of
 2.5% by weight of carbon black, 0.953 g/cm.sup.3) and an MFR value of 1.7
 g/10 min.
 The results of the measurements of the properties of the three
 cable-sheathing compositions are indicated in Table 2 below.
 TABLE 2
 Bimodal ethylene
 plastic
 Material properties 2 3 Reference 2
 Tensile break strength 32 30 32
 (MPa).sup.1
 Elongation at break (%).sup.1 900 890 1150
 ESCR.sup.2 0/2000 h 0/2000 h F20/190 h
 Shrinkage (%) at 0.0 0.0 0.1
 23.degree. C./24 h.sup.4
 Shrinkage (%) at 0.8 1.0 0.8
 100.degree. C./24 h.sup.4
 Surface finish.sup.5
 After extrusion at
 210.degree. C. at
 15 m/min 2 2 3
 35 m/min 1-2 1 4
 75 m/min 0-1 0 4
 140 m/min 0-1 0 4
 .sup.1 Determined in accordance with ISO 527-2 1993/5A.
 .sup.2 Determined in accordance with ASTM D 1693/A, 10% Igepal. The results
 are indicated as the percentage of cracked sample rods at a given time.
 F20 means that 20% of the sample rods were cracked after the given time.
 .sup.4 Determined in accordance with UNI-5079 after extrusion at
 210.degree. C.
 .sup.5 Classification: 0 = excellent to 4 = very uneven.
 As is evident from Table 2, the prior-art special sheathing material
 (Reference 2) has good shrinkage properties at room temperature. However,
 the shrinkage properties of Reference 2 have been achieved at the cost of
 poor processing properties, as appears from, inter alia, the poor values
 of surface finish. Generally, the sheathing material of Reference 2 can
 only be processed within a narrow "process window", i.e. within narrow
 ranges as regards the processing parameters. In contrast to Reference 2,
 the sheathing materials according to the invention (Bimodal Ethylene
 Plastic 2 and 3) exhibit as good shrinkage properties as Reference 2 while
 presenting better processing properties (broader process window) involving
 a better surface finish of the cable sheath. Furthermore, the sheathing
 materials according to the invention exhibit much better environmental
 stress cracking resistance (ESCR) and also present good tensile break
 strength.
 EXAMPLE 3
 In the polymerisation plant used in Examples 1 and 2, a bimodal
 polyethylene plastic (Ethylene Plastic 4) was produced under the following
 conditions.
 The First Reactor (Loop Reactor)
 In this reactor, a first polymer (Polymer 1) was produced by the
 polymerisation of ethylene in the presence of 1-butene and hydrogen gas
 (molar ratio 1-butene:hydrogen gas:ethylene=1.74:0.22:1). Polymer 1 had an
 MFR value of 310 g/10 min and a density of 0.939 g/cm.sup.3.
 The Second Reactor (Gas-Phase Reactor)
 The polymer from the loop reactor was transferred to the gas-phase reactor,
 where further polymerisation of ethylene with 1-butene in the presence of
 hydrogen gas was carried out (molar ratio 1-butene:hydrogen
 gas:ethylene=0.80:0.02:1), resulting in a new polymer component (Polymer
 2). The weight ratio of Polymer 1 to Polymer 2 was 42:58. The MFR value of
 the resulting end product was 0.3 g/10 min, and the density was 0.922
 g/cm.sup.3.
 Excellent mechanical properties, good ESCR as well as good shrinkage
 properties were achieved also in this case, where both polymer components
 contain 1-butene as comonomer, as is evident from Table 3 below.
 TABLE 3
 Material properties Ethylene plastic 4
 Tensile break strength 25.9 MPa
 Elongation at break 905%
 ESCR 0/2000 h
 Shrinkage % 23.degree. C./24 h 0%
 100.degree. C./24 h 0%