Patent Description:
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 cable is then also provided with a cable jacket.

The cables are commonly produced by extruding the layers on a conductor. Flexibility and abrasion resistance are two of the key properties of a cable jacket material. Flexibility allows the cable to be more easily handled during installation and better abrasion resistance makes cable more robust during installation in the ground. There is a general need therefore for a flexible and abrasion resistant cable. A more robust cable may allow the use of a thinner jacket making the cable lighter, more flexible and cheaper.

<CIT> describes certain LLDPEs for use in cable jackets. The examples describe multimodal metallocene produced LLDPEs with a particular Mz/Mw value. The claimed materials offer cable jackets with enhanced surface smoothness and processability.

<CIT> describes multimodal metallocene LLDPEs for cable jacket applications that have very low flex modulus and good processability.

<CIT> describes an ethylene copolymer composition with a high flexibility. The ethylene copolymer composition has a density of <NUM> and <NUM>/m<NUM> and can be used in a cable jacket, such as a power cable jacket.

It would be useful to prepare a polymer for a cable jacket that has low flex modulus and good abrasion resistance. Lower density polyethylenes tend to offer poor abrasion resistance but improved flex modulus. Maximising both of these parameters is challenging. The present inventors have found that certain medium/high density multimodal polyethylene compositions can provide a flexible material with higher abrasion resistance.

The multimodal polyethylene composition of the invention is produced using a split loop configuration, which enables a broadening of the molecular weight distribution thus improving the processability of the material.

Viewed from one aspect the invention provides a multimodal polyethylene composition having a lower molecular weight (LMW) ethylene homo or copolymer component (A) and a higher molecular weight ethylene copolymer component (B);.

Viewed from another aspect the invention provides a cable comprising a conductor surrounded by at least one layer, such as a jacketing layer, comprising a multimodal polyethylene composition as hereinbefore defined.

Viewed from another aspect the invention provides a process for the preparation of a multimodal polyethylene composition comprising lower molecular weight (LMW) ethylene homo or copolymer component (A) and a higher molecular weight ethylene copolymer component (B), said process comprising:.

wherein said multimodal polyethylene composition has a density of <NUM>/m<NUM> or more, an MFR<NUM> (ISO1133 at <NUM> and <NUM> load) in the range of <NUM> to <NUM>/<NUM>, a flex modulus of up to <NUM> MPa (ISO <NUM>:<NUM>) and a taber abrasion resistance of <NUM> to <NUM>/<NUM> cycle (ASTM D <NUM>: <NUM>).

It is particularly preferred if fractions (ai) and (aii) are prepared in first and second slurry reactors and that component (B) is prepared in a gas phase reactor, all three reactors being connected in series.

In one embodiment, the invention provides a process for polymerizing ethylene in at least two slurry reactors and at least one a gas phase reactor connected in series to prepare a multimodal polyethylene composition, wherein said multimodal polyethylene composition comprises.

The present invention relates to a multimodal polyethylene composition which is ideally suited for use in the jacketing layer of a cable. Unexpectedly, the multimodal polyethylene composition of the present invention has advantageous mechanical properties, e.g. the flex modulus is low and abrasion resistance is high. This is achieved without any crosslinking by means of a crosslinking agent, such as peroxide. The multimodal polyethylene composition is also processable.

The multimodal polyethylene composition of the invention is a medium or high density multimodal polyethylene composition having a density of <NUM>/m<NUM> or more, preferably <NUM>/m<NUM> or more, such as <NUM> to <NUM>/m<NUM> (measured using ISO <NUM>).

A preferred range is <NUM> to <NUM>/m<NUM>, such as <NUM> to <NUM>/m<NUM>, more preferably <NUM>/m<NUM> to <NUM>/m<NUM>, especially <NUM> to <NUM>/m<NUM>.

The term multimodal means 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 polymerisation 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.

The term "multimodal" therefore means herein, unless otherwise stated, multimodality at least with respect to molecular weight distribution (MWD= Mw/Mn) and includes also bimodal polymer.

It is preferred if the multimodal polyethylene composition of the invention is trimodal.

It is preferred if the multimodal polyethylene composition of the invention consists of the lower and higher molecular weight components.

It is preferred if the lower molecular weight component consists of the fractions (ai) and (aii) (and optionally the prepolymer component) and hence the multimodal polyethylene composition of the invention preferably consists of fractions (ai) and (aii) and the higher molecular weight components (and optionally the prepolymer component).

The multimodal polyethylene composition may have a Mw/Mn (MWD) of at least <NUM>, for example, <NUM> to <NUM>, preferably <NUM> to <NUM> (measured by GPC).

The multimodal composition may have an MFR<NUM> of <NUM> to <NUM>/<NUM>, preferably <NUM> to <NUM>/<NUM>, more preferably <NUM> to <NUM>/<NUM> (ISO1133, at <NUM> and <NUM> load).

The multimodal composition may have an MFR<NUM> of <NUM> to <NUM>/<NUM>, preferably <NUM> to <NUM>/<NUM> (ISO1133, at <NUM> and <NUM> load).

The multimodal polyethylene composition comprises at least one C3-<NUM> alpha olefin comonomer(s), preferably at least one C3-<NUM> alpha olefin comonomer(s). In some embodiments, the multimodal polyethylene composition comprises at least two C3-<NUM> alpha olefin comonomers, preferably at least two C3-<NUM> alpha olefin comonomers. The term comonomer as used herein means monomer units other than ethylene, which are copolymerisable with ethylene.

The olefin comonomer(s) is preferably a C<NUM>-<NUM> alpha-olefins, e.g. <NUM>-butene, <NUM>-hexene or <NUM>-octene. A particularly preferred multimodal polyethylene composition comprises <NUM>-butene and <NUM>-hexene as comonomers.

The amount of comonomer(s) present in the multimodal polyethylene composition of the invention may be <NUM> to <NUM> mol%, such as <NUM> to <NUM> mol%, preferably <NUM> to <NUM> mol%. This can be determined using NMR.

The multimodal composition of the invention may have a flex modulus of up to <NUM> MPa, such as <NUM> to <NUM> MPa, more preferably <NUM> to <NUM> MPa, such as <NUM> to <NUM> MPa.

The multimodal polyethylene composition of the invention may also have an eta300 value of less than <NUM> Pa. s, such as <NUM> to <NUM> Pa.

The multimodal polyethylene composition of the invention may also have an eta0. <NUM> value of more than <NUM> Pa. s, such as <NUM> to <NUM> Pa.

The melting point of the multimodal polyethylene composition is preferably, at least <NUM>, such as in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>, especially <NUM> to <NUM>. In one embodiment, the ratio between flexural modulus and melting point is between <NUM> and <NUM> (flex mod. in MPa/melting point in celsius), preferably between <NUM> and <NUM>. This is a key feature that defines a low flexural modulus with retained high melting point.

The multimodal polyethylene composition of the invention may have a tensile strain at break of <NUM> to <NUM>%.

The multimodal polyethylene composition of the invention may have a Shore D hardness after <NUM> seconds of <NUM> to <NUM>.

The multimodal polyethylene composition of the invention may have a tensile stress at break <NUM> to <NUM> MPa, especially <NUM> to <NUM> MPa.

The multimodal polyethylene composition of the invention may have a taber resistance of <NUM> to <NUM>/<NUM> cycles, such as <NUM> to <NUM>/<NUM> cycles.

The multimodal polyethylene composition of the invention may have a relaxation vs time of <NUM> to <NUM> seconds.

The multimodal polyethylene composition of the invention comprises a LMW component which is an ethylene homo or copolymer and a HMW component which is an ethylene copolymer. The expression 'ethylene homopolymer' according to the present invention relates to an ethylene polymer that consists substantially of ethylene and thus is an ethylene polymer which only includes ethylene monomer units.

The comonomer(s) present in the LMW and HMW components may be the same or different, preferably different. The comonomer present in the fractions (ai) and (aii) of the LMW component may be the same or different, preferably the same. For example <NUM>-butene may be used for the fractions of the LMW component and <NUM>-hexene may be employed in the HMW component. Where there are at least two different comonomers present, the multimodal polyethylene composition of the invention is regarded as a terpolymer herein.

Any component or fraction of the multimodal polyethylene composition may also be a terpolymer which means that at least one component (A) or (B) or fraction (ai) or (aii) comprises ethylene and at least two different C3-<NUM> alpha olefin comonomers.

The LMW component may be a homopolymer or an ethylene copolymer with at least one C4-<NUM> alpha olefin, especially an ethylene <NUM>-butene copolymer.

Preferably the HMW component is an ethylene copolymer with at least one C4-<NUM> alpha olefin, especially an ethylene <NUM>-hexene copolymer or ethylene <NUM>-octene copolymer.

In a further embodiment, the HMW component is an ethylene terpolymer with at least two C4-<NUM> alpha olefins, especially an ethylene, <NUM>-butene and <NUM>-hexene terpolymer.

The multimodal polyethylene composition as defined herein 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, e.g. by at least <NUM> mass units. Alternatively viewed, said LMW component has a higher MFR<NUM> than the HMW component, e.g. by at least <NUM>/<NUM>.

It is preferred if the multimodal polyethylene composition comprises:.

More preferably, the multimodal polyethylene composition comprises.

The split between components (A) and (B) can be controlled. A multimodal polyethylene composition of the invention may have <NUM> to <NUM> wt%, preferably <NUM> to <NUM> wt% of said lower molecular weight component (A) and <NUM> to <NUM> wt%, preferably <NUM> to <NUM> wt% of said higher molecular weight component (B), preferably <NUM> to <NUM> wt% of said lower molecular weight component (A) and <NUM> to <NUM> wt% of said higher molecular weight component (B).

The LMW component (A) may have an MFR<NUM> of <NUM> to <NUM>/<NUM>, such as <NUM> to <NUM>/<NUM>. As the HMW component tends to be produced in the presence of the LMW component in a multistage process, its properties often cannot be directly measured but they can be estimated using the Hagström equation.

The LMW component (A) may have a density of <NUM> to <NUM>/m<NUM>.

The LMW component (A) of the multimodal polyethylene composition is itself made up of two fractions (ai) and (aii). Hence the lower molecular weight component may comprise:.

Preferably, the lower molecular weight component comprises.

In a preferred embodiment there can be <NUM> to <NUM> wt% of (ai) and <NUM> to <NUM> wt% of (aii).

Each fraction (ai) and (aii) of the LMW component forms at least <NUM> wt% of the multimodal polyethylene composition, such as at least <NUM> wt% of the multimodal polyethylene composition.

Each fraction (ai) and (aii) of the LMW component ideally forms <NUM> to <NUM> wt% of the multimodal polyethylene composition, such as at least <NUM> to <NUM> wt% of the multimodal polyethylene composition.

The weight ratio between fractions (ai) and (aii) of the LMW component is preferably in the range of <NUM>:<NUM> to <NUM>:<NUM>, such as <NUM>:<NUM> to <NUM>:<NUM>. Each fraction (ai) and (aii) preferably forms at least <NUM> wt% of the LMW component, such as at least <NUM> wt% of the LMW component.

It is preferred if the comonomer used in both fractions of the LMW component is the same and it is more preferred if this is <NUM>-butene. Alternatively, the LMW component (A) can be a homopolymer, i.e. both fractions (ai) and (aii) are homopolymers. These fractions still need to differ in terms of MFR.

The MFR<NUM> of the fraction (ai) is lower than fraction (aii), e.g. by at least <NUM>/<NUM>.

Fraction (ai) preferably has an MFR<NUM> of <NUM> to <NUM>/<NUM>.

Fraction (ai) preferably has a density of <NUM> to <NUM>/m<NUM>.

The properties of the lower molecular weight component discussed herein can be regarded as those of the combination of fractions (ai) and (aii) (and optionally the prepolymer component).

More preferably, the multimodal polyethylene composition comprises a lower molecular weight ethylene <NUM>-butene copolymer component; and a higher molecular weight ethylene <NUM>-hexene copolymer component, especially wherein the lower molecular weight component comprises:.

More preferably, the multimodal polyethylene composition comprises a lower molecular weight ethylene homopolymer component; and a higher molecular weight ethylene <NUM>-hexene copolymer component, especially wherein the lower molecular weight component comprises:.

Without wishing to be limited by theory, it is envisaged that the use of this split lower molecular weight component structure leads to certain reductions in flex modulus.

In a highly preferred embodiment, the invention defines a multimodal polyethylene composition comprising.

wherein the polymer composition has a density of <NUM>/m<NUM> or more (ISO1183), such as <NUM> to <NUM>/m<NUM>, an MFR<NUM> (ISO1133 at <NUM> and <NUM> load) in the range of <NUM> to <NUM>/<NUM>, a flex modulus of up to <NUM> MPa (ISO <NUM>:<NUM>), such as <NUM> to <NUM> MPa (ISO <NUM>:<NUM>) and a taber abrasion resistance of <NUM> to <NUM>/<NUM> cycle (ASTM D <NUM>: <NUM>).

In a highly preferred embodiment, the invention defines a multimodal polyethylene composition comprising at least two C3-<NUM> alpha olefin comonomers, said composition comprising.

Alternatively viewed, the invention defines a multimodal polyethylene composition comprising.

It is preferred if the multimodal polyethylene composition comprises.

In all embodiments of the invention, the multimodal polyethylene composition is preferably non-crosslinked. Alternatively viewed, the multimodal polyethylene composition is thermoplastic.

It is a further feature of the invention that the thermal conductivity of the polymer is improved. High thermal conductivity leads to higher power transmission capacity or lower losses.

All the properties above can be determined in the presence or absence of standard additives.

In a most preferred embodiment therefore the invention provides a multimodal polyethylene composition having a density of <NUM>/m<NUM> or more, preferably <NUM>/m<NUM> or more, an MFR<NUM> in the range of <NUM> to <NUM>/<NUM>, a flexural modulus of up to <NUM> MPa (ISO <NUM>:<NUM>), such as <NUM> to <NUM> MPa (ISO <NUM>:<NUM>) and a taber abrasion resistance of <NUM> to <NUM>/<NUM> cycle (ASTM D <NUM>: <NUM>), comprising:.

The multimodal polyethylene composition of the invention is one that is preferably prepared using a single site catalyst, preferably a metallocene catalyst, especially one with two cyclopentadienyl type ligands. The same catalyst is preferably used to prepare all components.

The expression "a single site polyethylene (SSPE)" means that the polyethylene is polymerised in the presence of a single site catalyst which is a conventional coordination catalyst. The single site catalyst may suitably be a metallocene catalyst. Such catalysts comprise a transition metal compound which contains a cyclopentadienyl, indenyl or fluorenyl ligand. The catalyst preferably contains two cyclopentadienyl, indenyl or fluorenyl ligands, which may be bridged by a group preferably containing silicon and/or carbon atom(s). Further, the ligands may have substituents, such as alkyl groups, aryl groups, arylalkyl groups, alkylaryl groups, silyl groups, siloxy groups, alkoxy groups and like. Suitable metallocene compounds are known in the art and are disclosed, among others, in<CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

Especially, the metallocene compound must be capable of producing polyethylene having sufficiently high molecular weight, which is needed in order to guarantee good mechanical properties of, for example, a cable jacket.

One example of suitable metallocene compounds is the group of metallocene compounds having zirconium, titanium or hafnium as the transition metal and one or more ligands having indenyl structure bearing a siloxy substituent, such as [ethylenebis(<NUM>,<NUM>-di(tri-isopropylsiloxy)inden-<NUM>-yl)]zirconium dichloride (both rac and meso), [ethylenebis(<NUM>,<NUM>-di(tri-isopropylsiloxy)inden-<NUM>-yl)]zirconium dichloride (both rac and meso), [ethylenebis(<NUM>-tert-butyldimethylsiloxy)inden-<NUM>-yl)]zirconium dichloride (both rac and meso), bis(<NUM>-tert-butyldimethylsiloxy)inden-<NUM>-yl)zirconium dichloride, [dimethylsilylenenebis(<NUM>-tert-butyldimethylsiloxy)inden-<NUM>-yl)]zirconium dichloride (both rac and meso), (N-tert-butylamido)(dimethyl)(η<NUM>-inden-<NUM>-yloxy)silanetitanium dichloride and [ethylenebis(<NUM>- (tert-butydimethylsiloxy)inden-<NUM>-yl)]zirconium dichloride (both rac and meso).

Another example is the group of metallocene compounds having zirconium or hafnium as the transition metal atom and bearing a cyclopentadienyl type ligand, such as bis(n-butylcyclopentadienyl)hafnium/Zr dichloride, bis(n-butylcyclopentadienyl) dibenzyl hafnium/Zr, dimethylsilylenenebis(n-butylcyclopentadienyl)hafnium/Zr dichloride (both rac and meso) and bis[<NUM>,<NUM>,<NUM>-tri(ethyl)cyclopentadienyl]hafnium/Zr dichloride.

Still another example is the group of metallocene compounds bearing a tetrahydroindenyl ligand such as bis(<NUM>,<NUM>,<NUM>,<NUM>-tetrahydroindenyl)zirconium dichloride, bis(<NUM>,<NUM>,<NUM>,<NUM>-tetrahydroindenyl)hafnium dichloride, ethylenebis(<NUM>,<NUM>,<NUM>,<NUM>-tetrahydroindenyl)zirconium dichloride, dimethylsilylenebis(<NUM>,<NUM>,<NUM>,<NUM>-tetrahydroindenyl)zirconium dichloride. The use of bis(<NUM>-methyl-<NUM>-n-butylcyclopentadienyl) zirconium (IV) chloride is especially preferred.

A suitable single site catalyst may thereby especially be alumoxane containing, supported catalyst containing metallocene bis(<NUM>-methyl-<NUM>-n-butylcyclopentadienyl) zirconium (IV) chloride and with enhanced ActivCat® activator technology from Albemarle (Grace) Corporation.

The single site catalyst typically also comprises an activator. Generally used activators are aluminoxane compounds, such as methylalumoxane (MAO), tetraisobutylalumoxane (TIBAO) or hexaisobutylalumoxane (HIBAO). Also boron activators, such as those disclosed in <CIT> may be used. The activators mentioned above may be used alone or they may be combined with, for instance, aluminium alkyls, such as triethylaluminium or tri-isobutylaluminium.

Depending on the polymerisation process, the single site catalyst may be supported. The support may be any particulate support, including inorganic oxide support, for example, silica, alumina or titanium, or a polymeric support, for example, a polymeric support comprising styrene or divinylbenzene.

The catalyst may also be prepared according to emulsion solidification technology. Such catalysts are disclosed, among others, in <CIT> or <CIT>.

The multimodal polyethylene composition can be produced by blending mechanically together two or more separate polymer components or, preferably, by in-situ blending during a polymerisation process. Both mechanical and in-situ blending are well known in the field. The multimodal polyethylene composition of the invention is preferably prepared in two or more reactors or zones connected in series as described in <CIT>.

The polymerisation may be effected in bulk, slurry, solution, or gas phase conditions or in any combinations thereof. In one embodiment, the multistage process involves a first polymerisation step (LMW component) carried out in at least one slurry, e.g. loop, reactor, preferably two slurry reactors, and a second polymerisation step (HMW component) in a gas phase reactor.

In addition to actual polymerization steps, the process can contain a prepolymerization step. Optionally and advantageously, the main polymerisation stages may be preceded by a pre-polymerisation, in which case a prepolymer is produced, most preferably in an amount of for example <NUM> to <NUM>% or <NUM> to <NUM> % by weight of the total amount of polymers is produced. The pre-polymer may be an ethylene homo- or copolymer. If a pre-polymerisation takes place, in this case all of the catalyst is preferably charged into the first prepolymerisation reactor and the prepolymerisation is performed as slurry polymerisation. Thus, the prepolymerization step may be conducted in a loop reactor. Such a polymerisation leads to less fine particles being produced in the following reactors and to a more homogeneous product being obtained in the end.

The prepolymerization is preferably conducted in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from <NUM> to <NUM> carbon atoms or a mixture of such hydrocarbons.

In the present invention it is preferred that the pre-polymerisation operates at a temperature between <NUM> to <NUM>, more preferred between <NUM> to <NUM> and preferably at a pressure of <NUM> to <NUM> bar, more preferably of <NUM> to <NUM> bar.

The molecular weight of the prepolymer may be controlled by hydrogen as it is known in the art. Note that where a prepolymerisation step is used, any weight percentage of product produced in such a step should be considered part of component (A) and should be taken into account when identifying the weight percentage of component (A). Preferably, any prepolymer can be regarded as part of the first fraction (ai) and its weight can be considered part of the weight of fraction (ai).

After prepolymerisation, the main polymerisation takes place preferably in two slurry reactors then a gas phase reactor.

The LMW component is ideally produced in a first polymerization stage which is preferably a slurry polymerization. The slurry polymerization usually takes place in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from <NUM> to <NUM> carbon atoms or a mixture of such hydrocarbons. An especially preferred diluent is propane, possibly containing minor amount of methane, ethane and/or butane.

The ethylene content in the fluid phase of the slurry may be from <NUM> to about <NUM> % by mole, preferably from about <NUM> to about <NUM> % by mole and in particular from about <NUM> to about <NUM> % by mole.

The temperature in the first slurry reactor is typically from <NUM> to <NUM>, preferably from <NUM> to <NUM>. An excessively high temperature should be avoided to prevent partial dissolution of the polymer into the diluent and the fouling of the reactor. The pressure may range from <NUM> to <NUM> bar, preferably from <NUM> to <NUM> bar.

The temperature in the second slurry reactor is typically from <NUM> to <NUM>, preferably from <NUM> to <NUM>. The pressure may range from <NUM> to <NUM> bar, preferably from <NUM> to <NUM> bar.

The slurry polymerization may be conducted in any known reactor used for slurry polymerization. Such reactors include a continuous stirred tank reactor and a loop reactor. It is especially preferred to conduct the polymerization in a loop reactor.

Hydrogen can be introduced to control the MFR<NUM> of the LMW component. The amount of hydrogen needed to reach the desired MFR depends on the catalyst used and the polymerization conditions.

The comonomer can be introduced into the first polymerization stage. The amount of comonomer needed to reach the desired density depends on the comonomer type, the catalyst used and the polymerization conditions.

The average residence time in the first polymerization stage is typically from <NUM> to <NUM> minutes, preferably from <NUM> to <NUM> minutes.

It is preferred if the LMW component is produced in two loop reactors in series. The first loop reactor forms a first fraction and the second loop reactor forms a second fraction of the LMW component.

The MFR<NUM> after loop2 can be up to double or <NUM> times the MFR<NUM> after loop1. Thus the MFR<NUM> of the first fraction (ai) is lower than the MFR<NUM> of the LMW component as a whole. Ideally, the MFR increases from first to second fraction.

Generally the quantity of catalyst used will depend upon the nature of the catalyst, the reactor types and conditions and the properties desired for the polymer product.

Gas phase polymerisation can be carried out using known conditions. In general, the temperature in gas phase polymerisation is typically from <NUM> to <NUM>, e.g., <NUM> to <NUM>. The pressure is from <NUM> to <NUM> bar, for example, <NUM> to <NUM> bar.

The obtained polymerisation product, may be compounded in a known manner and optionally with additive(s) and pelletised for further use.

The resulting end product consists of an intimate mixture of the polymers from the three main reactors, the different molecular-weight-distribution curves of these polymers together forming a molecular-weight-distribution curve having a broad maximum or three maxima, i.e. the end product is a trimodal polymer mixture.

It is most preferred that the polymerization is carried out in a prepolymerization reactor/two slurry loop reactors/ a gas-phase reactor. Preferably, the polymerization conditions in the preferred four-step method are chosen so that fraction (ai) is produced in one step in a first slurry loop reactor, fraction (aii) is produced in a second step in a second slurry loop reactor and fraction (B) is produced in further step, preferably the third reactor. The order of these steps may, however, be reversed.

The multimodal polyethylene composition of the invention is ideally used in a layer in a cable such as the jacketing layer of a cable. Typically, a polymer composition comprising the multimodal polyethylene composition of the invention and one or more additional components, such as additives, is prepared and can be used in the jacketing layer of a cable. Such a jacketing layer may comprise at least <NUM> wt% of the multimodal polyethylene composition, such as at least <NUM> wt% of the multimodal polyethylene composition.

In some embodiments, the multimodal polyethylene composition is the only polymer component present in the jacketing layer (other than any masterbatch carrier polymers that may be present).

The jacketing layer may comprise further components other than the multimodal polyethylene composition, such as additives which may optionally be added in a mixture with a carrier polymer, i.e. in so called master batch.

The jacketing layer in the cable may contain, in addition to the multimodal polyethylene of the invention further component(s) such as antioxidant(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.

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.

The multimodal composition of the invention is suitable for use in the jacketing layer of a power cable or communication cable, in particular a power cable.

Suitable power cables may be AC or DC and may operate at a low voltage (LV), medium voltage (MV), high voltage (HV) and/or extra-high voltage (EHV) power cables.

High voltage direct current (HV DC) is usually considered to be operating at voltages higher than <NUM> kV and up to <NUM> kV DC, extra high voltage direct current (EHV DC) is usually considered to be above <NUM> kV DC, high voltage alternating current (HV AC) is usually considered to be up to <NUM> kV AC, and extra high voltage alternating current (EHV AC) is usually considered to be above <NUM> kV AC. Typically a high voltage direct current (HV DC) power cable and extra high voltage direct current (EHV DC) power cable operate at voltages of <NUM> kV or higher, even at voltages of <NUM> kV or higher. A power cable operating at very high voltages is known in the art as extra high voltage direct current (EHV DC) power cable which in practice can operate as high as <NUM> kV, or possibly even higher.

In a most preferred embodiment, the power cable is a high voltage AC or DC (HV) and/or an extra high voltage AC or DC (EHV) power cable.

The present invention is further directed to a cable comprising a conductor surrounded by at least an inner semiconductive layer, an insulation layer, an outer semiconductive layer and a jacketing layer, in that order, wherein at least the jacketing layer comprises the multimodal composition of the present invention.

The outer semiconductive layer preferably comprises, for example, consists of, a non-crosslinked second semiconductive composition. The inner semiconductive layer, for example, comprises, e.g., consists of, a non-crosslinked first semiconductive composition. The insulation layer may comprise any known insulation material such as an XLPE or polypropylene. In one embodiment, the insulation layer may comprise a multimodal polyethylene composition of the invention.

The first and the second semiconductive compositions can be different or identical and comprise a polymer(s) which is, for example, a polyolefin or a mixture of polyolefins and a conductive filler, e.g., 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 term "conductor" means herein that the conductor comprises one or more wires. Moreover, the cable may comprise one or more such conductors. Further, the conductor may be a electrical conductor and comprise one or more metal wires.

The thickness of the jacketing layer of the cable is typically <NUM> or more, for example, at least <NUM>, for example, at least <NUM> to <NUM>, for example, from <NUM> to <NUM>, and conventionally <NUM> to <NUM>, e.g. <NUM> to <NUM>, when measured from a cross section of the jacketing layer of the cable.

The invention also provides a process for producing a cable wherein the process comprises the steps of.

Unless otherwise stated in the description or experimental part the following methods were used for the property determinations.

The melt flow rate (MFR) is determined according to ISO <NUM> and is indicated in g/<NUM>. 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 <NUM> for polyethylene. MFR may be determined at different loadings such as <NUM> (MFR<NUM>) or <NUM> (MFR<NUM>).

The density was measured according to ISO <NUM>-<NUM>. The sample preparation was executed according to ISO <NUM>-<NUM> Table <NUM> Q (compression moulding).

Molecular weight averages (Mz, Mw and Mn), Molecular weight distribution (MWD) and its broadness, described by polydispersity index, PDI= Mw/Mn (wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) were determined by Gel Permeation Chromatography (GPC) according to ISO <NUM>-<NUM>:<NUM>, ISO <NUM>-<NUM>:<NUM>, ISO <NUM>-<NUM>:<NUM> and ASTM D <NUM>-<NUM> using the following formulas: <MAT> <MAT> <MAT>.

For a constant elution volume interval ΔVi, where Ai, and Mi are the chromatographic peak slice area and polyolefin molecular weight (MW), respectively associated with the elution volume, Vi, where N is equal to the number of data points obtained from the chromatogram between the integration limits.

A high temperature GPC instrument, equipped with either infrared (IR) detector (IR4 or IR5 from PolymerChar (Valencia, Spain), equipped with <NUM> x Agilent-PLgel Olexis and 1x Agilent-PLgel Olexis Guard columns was used. As the solvent and mobile phase <NUM>,<NUM>,<NUM>-trichlorobenzene (TCB) stabilized with <NUM>/L <NUM>,<NUM>-Di tert butyl-<NUM>-methyl-phenol) was used. The chromatographic system was operated at <NUM> and at a constant flow rate of <NUM>/min. <NUM>µL of sample solution was injected per analysis. Data collection was performed using either Agilent Cirrus software version <NUM> or PolymerChar GPC-IR control software.

The column set was calibrated using universal calibration (according to ISO <NUM>-<NUM>:<NUM>) with <NUM> narrow MWD polystyrene (PS) standards in the range of <NUM>,<NUM>/mol to <NUM><NUM>/mol. The PS standards were dissolved at room temperature over several hours. The conversion of the polystyrene peak molecular weight to polyolefin molecular weights is accomplished by using the Mark Houwink equation and the following Mark Houwink constants: <MAT> <MAT> <MAT>.

All samples were prepared in the concentration range of <NUM>,<NUM> -<NUM>/ml and dissolved at <NUM> for <NUM> hours for PP or <NUM> hours for PE under continuous gentle shaking.

Flexural modulus was determined according to ISO <NUM>, which describes the procedure for a <NUM> point bending test. The test specimens were prepared by milling samples <NUM>*<NUM>*<NUM> from <NUM> thick compression moulded plaques prepared according to EN ISO <NUM>-<NUM>:<NUM> and tested at a cross-head speed of <NUM>/min using a 100N load cell.

Melting points were determined using Instrument: DSC Q2000 from TA Instruments.

Dynamic rheological properties of the polymer, here the polymer composition may be determined using a controlled stress rheometer, using a parallel-plate geometry (<NUM> diameter) and a gap of <NUM> between upper and bottom plates. Previous to test, samples need to be stabilized by dry blending pellets together with <NUM>-<NUM>% Irganox B225. Irganox B <NUM> is a blend of <NUM>% Irganox <NUM>, Pentaerythritol tetrakis(<NUM>-(<NUM>,<NUM>-ditert-butyl-<NUM>-hydroxyphenyl)propionate), CAS-no. <NUM>-<NUM>-<NUM> and <NUM>% lrgafos <NUM>, Tris(<NUM>,<NUM>-di-tert-butylphenyl) phosphite, <NPL>. Note that to add an antioxidant, here Irganox B225, is normally not the standard procedure of method <NUM> ISO <NUM>-<NUM>. Frequency sweep test, i.e. the "Rheology, dynamic (Viscosity) method", was performed according to the ISO standard method, ISO <NUM>-<NUM> with an angular frequency range from <NUM>-<NUM> rad/s. All experiments were conducted under nitrogen atmosphere at a constant temperature of <NUM> and strain within the linear viscoelastic region. During analysis, storage modulus (G'), loss modulus (G"), complex modulus (G*) and complex viscosity (η*) were recorded and plotted versus frequency (ω). The measured values of complex viscosity (η*) at angular frequency of <NUM> and <NUM> rad/s are taken from test. The abbreviation of these parameters <NUM> are η*<NUM> and η*<NUM> respectively. The zero viscosity η*<NUM> value is calculated using Carreau-Yasuda model. For cases when the use of this model for the estimation of the Zero shear viscosity is not recommendable a rotational shear test at low shear rate is performed. This test is limited to a shear rate range of <NUM> to <NUM>-<NUM> and a temperature of <NUM>.

Abrasion resistance was tested on <NUM> thick compression moulded plaques prepared by <NUM> compression moulding at <NUM> with cooling rate <NUM>/min. The testing was performed on a Taber abraser according to ASTM D <NUM> with the abrasive wheel CS17. Two specimens are tested for each material, and the wear index of the materials is determined after <NUM> cycles of abrasion. The wear index is defined as the weight loss in mg per <NUM> cycles of abrasion.

For tensile testing (stress and strain at break), specimens were prepared and measured according to ISO <NUM>-<NUM>/5A by die cutting from compression moulded plaques of <NUM> thickness tested at <NUM> and <NUM>% relative humidity with <NUM> kN load cell a tensile testing speed of <NUM>/min, a grip distance of <NUM> and a gauge length of <NUM>.

Shore D (<NUM>) is determined acc. ISO868 on moulded specimen with a thickness of <NUM>. The shore hardness is determined after <NUM> seconds after the pressure foot is in firm contact with the test specimen. The specimen was moulded according to EN ISO <NUM>-<NUM>.

To determine the relaxation behaviour, stress relaxation tests were conducted using a Paar Physica MCR <NUM> rotational rheometer. A parallel-plate geometry with a diameter of <NUM> and a gap of <NUM> was chosen as measuring system. The tests were conducted at a set temperature of <NUM> using a strain step of <NUM>%. The test specimens can be prepared in a disk shape by compression moulding with a thickness of about <NUM>, directly on a frame mould or by stamping out from a plaque using a cutting die, with the required diameter. The specimen was loaded between the plates of the pre-heated rheometer and the heating chamber was closed to allow for the sample to melt. Before the application of the strain step, and after loading the sample onto the plates, a waiting time for thermal equilibration inside the heating chamber of about <NUM> to <NUM> minutes was applied. The heating chamber was continuously purged with nitrogen during the tests to avoid degradation of the sample. After the step strain is applied, the test geometry was kept on a fixed angular position and the decaying (relaxation) stress (in Pascal, Pa) was determined as a function of time. The relaxation modulus (in Pascal, Pa) as a function of time is then determined by dividing the stress by the applied strain (in dimensionless units). The relaxation behaviour is characterized by the parameter Time (G(t)=<NUM> Pa), which is the time (in seconds) at which the relaxation modulus attains an arbitrary value of <NUM> Pascal (Pa). Lower values is an indication of lower shrinkage. Materials with high elastic share have taken longer time to relax and consequently most of its elastic stresses would be frozen into the resultant jacket after the drawing operation (Shrinkage).

Quantification of comonomer content in polymers by NMR spectroscopy The comonomer content was determined by quantitative nuclear magnetic resonance (NMR) spectroscopy after basic assignment (e.g. "<NPL>). Experimental parameters were adjusted to ensure measurement of quantitative spectra for this specific task (e. Quantities were calculated using simple corrected ratios of the signal integrals of representative sites in a manner known in the art.

As catalyst was used alumoxane containing, supported catalyst containing metallocene bis(<NUM>-methyl-<NUM>-n-butylcyclopentadienyl) zirconium (IV) chloride and with enhanced ActivCat® activator technology from Albemarle (Grace) Corporation.

The multimodal polyethylene composition was prepared using a prepolymerisation reactor, a first and second slurry-loop reactor as well as a gas phase reactor. The prepolymerisation stage was carried out in slurry in a <NUM> dm<NUM> loop reactor under conditions and using feeds of catalyst (as prepared above), monomers, antistatic agent and diluent (propane (C3)) as disclosed in Table <NUM>.

The obtained slurry together with the prepolymerised catalyst was continuously introduced into the <NUM> dm<NUM> first loop reactor. The polymer slurry was continuously withdrawn from the first loop reactor and transferred into the <NUM> dm<NUM> second loop reactor. The slurry was continuously withdrawn from the second loop reactor to a flash stage where hydrocarbons were removed from the polymer. The polymer was then transferred into a gas phase reactor where the polymerisation was continued. The conditions and feeds/feed ratio in loop and gas phase polymerisation steps are disclosed in Table <NUM> and <NUM>.

A comparative example <NUM> is also prepared using the same catalyst.

The polymer obtained from the gas phase reactor is pelletised with <NUM> wt% of <NUM>:<NUM> mixture of Pentaerythrityl-tetrakis(<NUM>-(<NUM>',<NUM>'-di-tert. butyl-<NUM>-hydroxyphenyl)-propionate cas nr <NUM>-<NUM>-<NUM> and Tris (<NUM>,<NUM>-di-t-butylphenyl) phosphite <NPL>, <NUM> wt% of Calcium stearate <NPL> and <NUM> wt% of1:<NUM> mixture of Dimethyl succinate polymer with <NUM>-hydroxy-<NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-piperidine ethanol<NPL> and Poly((<NUM>-((<NUM>,<NUM>,<NUM>,<NUM>-tetramethylbutyl)amino)-<NUM>,<NUM>,<NUM>-triazine-<NUM>,<NUM>-diyl)(<NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-piperidyl)imino)-<NUM>,<NUM>-hexanediyl ((<NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-piperidyl)imino)) <NPL>.

The properties are determined on the pellets (other than * marked properties of CE2 - determined on powder). The inventive examples are compared to a commercial HDPE copolymer (CE1) of density <NUM>/m<NUM> and MFR2 of <NUM>/<NUM> or a bimodal HDPE (CE2).

Lower values of relaxation vs time is an indication of lower shrinkage.

Materials with high elastic share have taken longer time to relax and consequently most of its elastic stresses would be frozen into the resultant jacket after the drawing operation (Shrinkage).

Claim 1:
A multimodal polyethylene composition having a lower molecular weight (LMW) ethylene homo or copolymer component (A) and a higher molecular weight ethylene copolymer component (B);
wherein the LMW component comprises two fractions (ai) and (aii) wherein the MFR<NUM> of the fraction (ai) is lower than fraction (aii) and wherein each fraction (ai) and (aii) of the LMW component forms at least <NUM> wt% of the multimodal polyethylene composition;
wherein the polymer composition has a density of <NUM>/m<NUM> or more (ISO1183), such as <NUM> to <NUM>/m<NUM>, an MFR<NUM> (ISO1133 at <NUM> and <NUM> load) in the range of <NUM> to <NUM>/<NUM>, a flex modulus of up to <NUM> MPa (ISO <NUM>:<NUM>), such as <NUM> to <NUM> MPa (ISO <NUM>:<NUM>) and a taber abrasion resistance of <NUM> to <NUM>/<NUM> cycle (ASTM D <NUM>: <NUM>).