OA11303A - Cable with impact resistant coating - Google Patents

Abstract

The present invention relates to a coating for cables which is capable of protecting the cable against accidental impacts. By inserting into the structure of a power transmissioncable a suitable coating of expanded polymer material of adequate thickness, preferably in contact with the sheath of outer polymer coating, it is possible to obtain a cable which has a high impact strength. The Applicant has moreover observed that an expanded polymer material used as a coating for cables makes it possible to obtain a higher impact strength for this cable than using a similar coating based on the same polymer which is not expanded. A cable with a coating of this type has various advantages over a conventional cable with metal armor, such as, for example, easier processing, a reduction in the weight and dimensions of the finished cable and a lower environmental impact as regards recycling of the cable once its working cycle is over.

Description

-±- . 011303

CABLE WITH IMPACT-RESISTANT COATING

The présent invention relates to a coating forcables which is capable of protecting Lhe cable fromaccidentai impacts.

Accidentai impacts on a cable, which may occur,for example, during their transportation, laying etc.,may cause a sériés of structural damage to the cable,including deformation of the insulating layer, detach-ment of the insulating layer from the semiconductivelayer, and the like; this damage may cause variationsin the electrical gradient of the insulating coating,with a conséquent decrease in .the insulating capacityof this coating.

In the cables which are currently commerciallyavailable, for example in those for 2 ow- or medium-tension power transmission or distribution, métal armorcapable of withstanding such impacts is usually appliedin order to protect cables from possible damages causedby accidentai impacts. This armor may be in the form oftapes or wires (generally made of Steel), or alter-natively in the form of a métal sheath (generally madeof lead or aluminum); this armor is, in turn, usuallyclad with an outer polymer sheath. An example of such acable structure is described in US patent 5,153,381.

The Applicant has observed that the presence ofthe abovementioned métal armor has a certain number ofdrawbacks. For example, the application of the saidarmor includes one or more additional phases in theProcessing of the cable. Moreover, the presence of themétal armor increases the weight of the cable consider-ably, in addition to posing environmental problemssince, if it needs to be rep.laced, a cable constructedin this way is not easy to dispose of. 011303

The Japanese patent published under the number(Kokai) 7-320550 describes a domestic cable with animpact-résistant coating 0.2-1.4 mm in thickness,placed between the insulator and the outer sheath. Thisimpact-résistant coating is a non-expanded polymermaterial containing a polyuréthane resin as maincomponent.

On the other hand, use of expanded polymericmaterials in cables' construction is known for avariety of purposes.

For instance, German patent application no. P 15 15 709 discloses the use of an intermediate layerbetween the outer plastic sheath and the inner metallicsheath of a cable, in order to increase the résistanceof the outer plastic sheath to low températures. Nomention is made in such document about protecting theinner structure of the cable with said intermediatelayer. As a mattter of fact, such intermediate layershould compensate for elastic tensions generated in theouter plastic sheath due to température’s lowering andmay consist of loosely disposed glass fibers or of amaterial which may either be expanded or incorporâtinghollow glass spheres.

Another document, German utility modelno. G 81 03 947.6, discloses an electric cable for usein connections inside apparatuses and machines, havingparticular mechanical résistance and flexibility. Saidcable is specifically designed for passing on a pulleyand is sufficiently flexible in order to recover itsstraight structure after the passage on said pulley.Accordingly, this kind of cable is specifically aimedto resist to mechanical loads of the static type (suchas those generated during the passage onto a pulley),and its main feature is the ilexibility. It is readilyapparent to those skilled in the art that this kind ol 011303 cable substantiel 1 y differs from low- or medium-tensionpower transmission or distribution having a métal armorwhich, rather to be flexible, should be capable ofwithstanding dynamic loads due to impacts of a certainstrength onto the cable.

In addition, in signal transmission cables ofthe coaxial or twisted pair type, it is known to useexpanded materials in order to insulate a conductivemétal. Coaxial cables are usually intended to carryhigh-frequency signais, such as coaxial cables for TV(CATV) (10-100 MHz), satellite cables (up to 2 GHz) ,coaxial cables for computers (above 1 MHz); traditionaltéléphoné cables usually carry- signais with frequenciesof about 800 Hz.

The purpose of using an expanded insulator insuch cables is to increase the transmission speed ofthe electrical signais, in order to approach the idéalspeed of signal transmission in an aerial conductivemétal (which is close to the speed of light). Thereason for this is that, compared with non-expandedpolymer materials, expanded materials generally hâve alower dielectric constant (K), which is proportionatelydoser to that of air (K=l) the higher the degree ofexpansion of the polymer.

For example, US patent 4,711,811 describes asignal transmission cable having an expanded fluoro-polymer as insulator (thickness of 0.05-0.76 mm) cladwith a film of ethylene/tetrafluoroethylene orethylene/chlorotrifluoroethylene copolymer (thicknessof 0.013-0.254 mm). As described in that patent, thepurpose of the expanded polymer is to insulate theconductor, whiJe the purpose of the film of non-expanded polymer which clads the expanded polymer is toimprove the mechanical properties of the insulation, inparticular by imparting the necessary compression 011303 strength when two insulated conductors are twisted koform the so-called "twisted pair".

Patent EP 442,346 describes a signal trans-mission cable with an insulating layer based onexpanded polymer, placed directly around the conductor;this expanded polymer has an ultramicrocellularstructure with a void volume of greater than 75¾(corresponding to a degree of expansion of greater than300%). The ultramicrocellular structure of this polymershould be such that it is compressed by at least 10%under a load of 6.89 x 104 Pa and recovers at least 50%of its original volume after removal of the load; thesevalues correspond approximately to the typicalcompression strength values which the material needs tohâve in order to withstand the compression duringtwisting of the cables.

In International patent applicationWO 93/15512, which also relates to a signal trans-mission cable with an expanded insulating coating, itis stated that by coating the expanded insulator with alayer of non-expanded insulating thermoplastic polymer(as described, for example, in the abovementionedUS patent 4,711,811) the required compression strengthis obtained, this however reducing the speed ofpropagation of the signal. The said patent applicationWO 93/15512 describes a coaxial cable with a doublelayer of insulating coating, where both the layersconsist of an expanded polymer material, the innerlayer consisting of microporous polytetrafluoroethylene (PTEE) and the outer layer consistinq of a closed-ce.ilexpanded polymer, in particular perfluoroalkoxytetra-fluoroethylene (PEA) polymers. The insulating coatingbased on expanded polymer is obtained by extrud.ing t hoPEA polymer over the inner layer of PTEE insulator, 011303 injecting Fréon 113 gas as expanding agent. Accordingto the details given in the description, this closed-cell expanded insulator makes it possible to maintain ahigh speed of signal transmission. It is moreover 5 defined in that patent application as being résistantto compression, although no numerical data regardingthis compression strength are given. The descriptionemphasizes the fact that conductors clad with such adouble-layer insulator can be twisted. Moreover, 10 according to that patent application, the increase invoid volume in the outer expanded layer makes itpossible to obtain an increase in the speed oftransmission, thereby giving rise to small variationsin the capacity of this coating to oppose the compres- 15 sion of the inner expanded layer.

As is seen from the abovementioned documents, the main purpose of using "open cell" expanded polymermaterials as insulating coatings for signal transmis-sion cables is to increase the speed of transmission of 20 the electrical signal; however, these expanded coatingshâve the drawback of having an insufficient compressionstrength. A few expanded materials are also genericallydefined as "résistant to compression", since they hâveto ensure not only a high speed of signal transmission 25 but also a sufficient résistance to the compressionforces which are typically generated when twoconductors coated with the abovementioned expandedinsulation are twisted together; accordingly, also inthis case, the applied load is substantiantially of 30 static type.

Thus, while, on the one hand, it is necessary for these insulating coatings made of expanded polymer material for signal transmission cables to hâve characteristics such that they can bear a relafively 3b modest compression load (such as that which arises when 011303 two cables are twisted together), on the other hand, no mention is made in any document known to the Applicant of any type of impact strength which may be provided by an expanded polymer coating. Moreover, although such an 5 expanded insulating coating promûtes a higher speed ofsignal transmission, this is considered to be lessadvantageous than a coating made of a similar non-expanded matériel as regards the compression strength,as reported in the abovementioned patent application 10 WO 93/15512.

The Applicant has now found that by insertinginto the structure of a power transmission cable asuitable coating made of expanded polymer material ofadéquate thickness and flexural modulus, preferably in 15 contact with the sheath of outer polymer coating, it ispossible to obtain a cable having a high impactstrength, thereby making it possible to avoid the useof the abovementioned protective métal armor in thestructure of this cable. In particular, the Applicant 20 has observed that the polymer material should be selected in order to hâve a sufficiently high flexuralmodulus, measured before its expansion, so to achievethe desired impact résistant properties and avoidpossible damages of the inner structure of the cable 25 due to undesired impacts on the outer surface of it. Inthe présent description, the term "impact" is intendedto encompass ail those dynamic loads of a certainenergy capable to produce substantial damages to thestructure of conventional unarmored cables, while 30 having negligible effects on the structure of conventional armored cables. As an indication, such an impact may be considered an impact, of about 20-30 joule produced by a V-shaped rounded-edge punch, having a curvature radius of about 1 mm, onto the outer sheath 35 of the cable. 011303

The Applicant has moreover observed that,surprisingly, an expanded polymer matériel used as acoating for cables according to the invention makes itpossible to obtain an impact strength which is betterthan that obtained using a similar coating based on thesame polymer which is not expanded. A cable with a coating of this type has variousadvantages over a conventional cable with métal arrnorsuch as, for example, easier processing, a réduction inthe weight and dimensions of the finished cable and areduced environmental impact as regards recycling ofthe cable once its working cycle is over.

One aspect of the présent invention thusrelates to a power transmission cable comprising a) a conductor; b) at laast one layer of compact insulatingcoating, c) a coating made of expanded polymer material,wherein said polymer material has predeterminedmechanical strength properties and a predetermineddegree of expansion so as to impart impact résistantproperties to said cable.

According to a preferred aspect of the présentinvention, the expanded polymer material is obtainedfrom a polymer material which has, before expansion, aflexural modulus at room température, measured accord-ing to ASTM standard D790, higher than 200 MPa,preferably between 400 MPa and 1500 MPa, values ofbetween 600 MPa and 1300 MPa being particularlypreferred.

According to a preferred aspect, said polymermaterial has a degree of expansion of from abuot 20% toabout 3000%, preferably from about 30% to about 500%, adegree of expansion of from about 50% to about 200%being particularly preferred.

According to a preferred embodiment of theprésent invention, the coating of expanded polymermaterial has a thickness of at least 0.5 mm, preferablybetween 1 and 6 mm, in particular between 2 and 4 mm.According to a preferred aspect of the présentinvention, this expanded polymer material is chosenfrom polyethylene (PE), low density PE (LDPE), mediumdensity PE (MDPE), high density PE (HDPE) and linearlow density PE (LLDPE); polypropylene (PP); ethylene-propylene rubber (EPR), ethylene-propylene copolymer(EPM), ethylene-propylene-diene terpolymer (EPDM);natural rubber; butyl rubber; ethylene/vinyl acetate(EVA) copolymer; polystyrène; ethylene/acrylatecopolymer, ethylene/methyl acrylate (EMA) copolymer,ethylene/ethyl acrylate (EEA) copolymer, ethylene/butylacrylate (EBA) copolymer; ethylene/a-olcfin copolymer;acrylonitrile-butadiene-styrene (ABS) resins; halogena-ted polymer, polyvinyl chloride (PVC); polyuréthane(PUR); polyamide; aromatic polyester, polyethyleneterephthalate (PET), polybutylene terephthalate (PBT);and copolymers or mechanical mixtures thereof.

According to a further preferred aspect, thispolymer material is a polyolefin polymer or copolymerbased on PE and/or PP, preferably modified withethylene-propylene rubber, in which the PP/EPR weightratio is between 90/10 and 50/50, preferably between85/15 and 60/40, in particular about 70/30.

According to a further preferred aspect, thispolyolefin polymer or copolymer based on PE and/or PPcontains a predetermined amount of vulcanized rubber inpowder form, preferably between 10% and 60% of theweight of the polymer.

According to a further preferred aspect, thiscable moreover comprises an outer polymer sheath, which is preferably in contact with the expanded polymercoating, this sheath preferably having a thickness ofat least 0.5 mm, preferably between 1 and 5 mm.

Another aspect of the présent invention relatesto a method for imparting impact strength to a cable,which comprises coating this cable with a coating rnadeof expanded polymer material.

According to a preferred aspect, this methodfor imparting impact strength to a cable moreovercomprises coating this expanded coating with an outerprotective sheath. A further aspect of the présent inventionrelates to the use of an expanded polymer material inorder to impart impact strength to a power transmissioncable. A further aspect of the présent inventionrelates to a method for evaluating the impact strengthof a cable comprising at least one insulating coating,this method consisting in a) measuring the average peel strength of thesaid insulating layer; b) subjecting the cable to an impact of pre-determined energy; c) measuring the peel strength of the saidinsulating layer at the point of impact; d) checking that the différence between theaverage peel strength and the peel strength measured atthe point of impact is less than a predetermined valuefor the said cable relative to the average peelstrength.

According to a preferred aspect, this poolstrength is measured between the layer of insulatingcoating and the outer layer of semiconductive coating.

In the présent description, the terni "degree ofexpansion of the polymer" is understood to refer to the 10 011303 expansion of the polymer determined in the followingway : G (degree of expansion) = (d0/de - 1)-100where d0 indicates the density of the non- expanded polymer (that is to say the polymer with astructure which is essentially free of void volume) andde indicates the apparent density measured for theexpanded polymer.

For the purposes of the présent description,the term "expanded" polymer is understood to refer to apolymer within the structure of which the percentage ofvoid volume (that is to say the space not occupied bythe polymer but by a gas or.air) is typically greaterthan 10% of the total volume of this polymer.

In the présent description, the term "peel"strength is understood to refer to the force requiredto separate (peel) a layer of coating from theconductor or from another layer of coating; in the caseof séparation of two layers of coating from each other,these layers are typically the insulating layer and theouter semiconductive layer.

Typically, the insulating layer of powertransmission cables has a dielectric constant (K) ofgreater than 2. Moreover, in contrast with signaltransmission cables, in which the "electrical gradient"parameter does not assume any importance, electricalgradients ranging from about 0.5 kV/mm for low tension,up to about 10 kV/mm for high tension, are applied inpower transmission cables; thus, in these cables, thepresence of inhomogeneity in the insulating coating(for example void volumes), which could qive rise to alocal variation in the dielectric rigidity with aconséquent decrease in the insulating capacity, tendsto be avoided. This insulating material will thustypically be a compact polymer material, in which, in 11 011303 the présent description, the term "compact" insulator is understood to refer to an insulating material which has a dielectric rigidity of at least 5 kV/mm, preferably greater than 10 kV/mm, in particular greaterthan 40 kV/mm for medium-high tension powertransmission cables. In contrast with an expandedpolymer material, this compact material issubstantially free of void volume within its structure;in particular, this material will hâve a density of0.85 g/cm3 or more.

In the présent description, the term lowtension is understood to refer to a tension of up to1000 V (typically greater than. 100 V) , the term mediumtension is understood to refer to a tension from about1 to about 30 kV and the term high tension is under-stood to refer to a tension above 30 kV. Such powertransmission cables typically operate at nominalfrequencies of 50 or 60 Hz.

Although, in the course of the description, theuse of the expanded polymer coating is illustrated indetail with reference to power transmission cables, inwhich this coating may advantageously replace the métalarmor currently used in such cables, it is clear tothose skilled in the art that this expanded coating mayadvantageously be used in any type of cable for whichit might be desired to impart suitable impact protec-tion to such a cable. In particular, the définition ofpower transmission cables includes not only thosespecifically of the type for low and medium tension butalso cables for high-tension power transmission.

The invention may be further understood withthe aid of the following ligures:

Figure 1 show:; a power transmission cableaccording to the State ol the art, ot the tripolar typewith métal armor. 12 011303

Figure 2 shows a first embodiment of a cableaccording to the invention of tripolar type.

Figure 3 shows a second embodiment of a cableaccording to the invention of unipolar type.

Fig. 1 is the cross-sectional diagram of amedium-tension power transmission cable according tothe State of the art, of the tripolar type with métalarmor. This cable comprises three conductors (1), eachclad with an inner semiconductive coating (2), aninsulating layer (3), an outer semiconductive layer (4)and a métal screen (5); for simplicity, this semi-finished structure will be defined in the rest of thedescription as the "core". The.three cores are ropedtogether and the star-shaped areas between them arefilled with a filling material (9) (generally elasto-meric mixtures, polypropylene fibers and the like) inorder to make the cross-sectional structure circular,the whole in turn being coated with an inner polymersheath (8), an armor of métal wires (7) and an outerpolymer sheath (6).

Fig. 2 is the cross-sectional diagram of acable according to the invention, also of the tripolartype for medium-tension power transmission. This cablecomprises the three conductors (1), each clad with aninner semiconductive coating (2) , an insulating layer(3), an outer semiconductive layer (4) and a métalscreen (5); the star-shaped areas between the cores arefilled in this case with an impact-résistant expandedpolymer material (10) which is, in turn, coated with anouter polymer sheath (6). In the expanded polymercoating (10), a circular rim (10a) which corresponds tothe minimum thickness of expanded polymer coating, inproximity to the outer surface of the cores, is alsoindicated (by means of a dotted line). 13 C11303

Fig. 3 is the cross-sectional diagram of acable according to the invention, of unipolar type formedium-tension power transmission. This cable comprisesa central conductor (1), clad with an inner semi- 5 conductive coating (2), an insulating layer (3), anouter semiconductive layer (4), a métal screen (5), alayer of expanded polymer material (10) and an outerpolymer sheath (6). In the case of this unipolar cablerepresented in Fig. 3, since the core has a circular 10 cross-section, the circular rim (10a) indicated in thecase of the tripolar cable coïncides with the layer ofexpanded polymer material (10).

These figures obviously only show a few of thepossible embodiments of cables in which the présent 15 invention may advantageously be used. It is clear thatsuitable modifications known in the art may be mode tothese embodiments without any limitations to theapplication of the présent invention being impliedthereby. For example, with reference to Fig. 2, the 20 star-shaped areas between the cores may be filled beforehand with a conventional filling material, thusobtaining a semi-processed cable of cross-sectioncorresponding approximately to the circular cross-section contained within the circular rim (10a); it is 25 then advantageously possible to extrude over this semi-processed cable of cross-sectional area the layer ofexpanded polymer material (10), in a thickness corres-ponding approximately to the circular rim (10a), andsubsequently the outer sheath (6). Alternatively, cores 30 may be provided with a cross-sectional sector, in sucha way that when these cores are joined together a cableof approximately circular cross-section is formed,without the need to use the filling material for thestar-shaped areas; the layer of impact-résistant 35 expanded polymer material (10) is then extruded over 14 011303 these cores thus joined together, followed by the outersheath (6).

In the case of cables for low-tension powertransmission, the structure of these cables will ·usually comprise the only insulating coating placeddirectly in contact with the conductor, which is inturn coated with the coating of expanded polymermaterial and with the outer sheath.

Further solutions are well known to a personskilled in the art, who is capable of evaluating themost convenient solution, based on, for example, thecosts, the type of positioning of the cable (aerial,inserted in pipes, buried dire.ctly into the ground,inside buildings, under the sea, etc.), the operatingtempérature of the cable (maximum and minimumtempératures, température ranges of the environment)and the like.

The impact-résistant expanded polymer coatingmay consist of any type of expandable polymer such as,for example, polyolefins, polyolefin copolymers, ole-fin/ester copolymers, polyesters, polycarbonates, poly-sulfones, phenolic resins, ureic resins and mixturesthereof. Examples of suitable polymers are polyethylene(PE), in particular low density PE (LDPE), mediumdensity PE (MDPE), high density PE (HDPE) and linearlow density PE (LLDPE); polypropylene (PP); ethylene-propylene rubber (EPR), in particular ethylene-propylene copolymer (EPM) or ethylene-propylene-dieneterpolymer (EPDM); natural rubber; butyl rubber;ethylene/vinyl acetate (EVA) copolymer; polystyrène;ethylene/acrylate copolymer, in particular ethylene/methyl acrylate (EMA) copolymer,ethylene/ethyl acrylate (EEA) copolymer, et hy.1 ene/but ylacrylate (EBA) copolymer; ethylene/α-οΐel1n copolymer; 15 011303 acry]onitrile-butadiene-styrene (ABS) resins; halogena-ted polymers, in particular polyvinyl chloride (PVC);polyuréthane (PUR); polyamides; aromatic polyesters,such as polyethylene terephthalate (PET) or b polybutylene terephthalate (PBT); and copolymers orrnechanical mixtures thereof. Preferably, polyolefinpolymers or copolymers are used, in particular thosebased on PE and/or PP mixed with ethylene-propylenerubbers. Advantageously, polypropylene modified with 10 ethylene-propylene rubber (EPR) may be used, the PP/EPRweight ratio being between 90/10 and 50/50, preferablybetween 85/15 and 60/40, a weight ratio of about 70/30being particularly preferred. .

According to a further aspect of the présent 15 invention, the Applicant has moreover observed that itis possible to mix mechanically the polymer materialwhich is subjected to the expansion, in particular inthe case of olefin polymers, specifically polyethyleneor polypropylene, with a predetermined amount of rubber 20 in powder form, for example vulcanized natural rubber.

Typically, these powders are formed from particles with sizes of between 10 and 1000 pm,preferably between 300 and 600 pm. Advantageously,vulcanized rubber rejects derived from the processing 25 of tires may be used. The percentage of rubber inpowder form may range from 10% to 60% by weightrelative to the polymer to be expanded, preferablybetween 30% and 50%.

The polymer material to be expanded, which is 30 either used without further processing or which is usedas an expandable base in a mixture with powderedrubber, will hâve to hâve a rigidity such that, once itis expanded, it ensures a certain magnitude of desiredimpact résistance, so as to protect the inner part of 35 the cable (that is to say the layer of insulator and 16 011303 the semiconductive layers which may be présent) fromdamage following accidentai impacts which may occur. Inparticular, this material will hâve to hâve a sufficiently high capacity to absorb the impact, energy, 5 so as to transmit to the underlying insulating layer anamount of energy which is such that the insulatingproperties of the underlying coatings are not modifiedbeyond a predetermined value. The reason for this, asillustrated in greater detail in the description which 10 follows, is that the Applicant has observed that in acable subjected to an impact, a différence is observed,between the average value and the value measured at thepoint of impact, of the peel s.trength of the underlyinginsulating coatings; advantageously, this peel strength 15 may be measured between the insulating layer and theouter semiconductive layer. The différence in thiostrength is proportionately greater the greater theimpact energy transmitted to the underlying layers; inthe case where the peel strength is measured between 20 the insulating layer and the outer semiconductivelayer, it has been evaluated that the protectivecoating offers a sufficient protection to the innerlayers when the différence in peel strength at thepoint of impact, relative to the average value, is less 25 than 25%.

The Applicant has observed that a polymermaterial chosen from those mentioned above is parti-cularly suitable for this purpose, this materialhaving, before expansion, a flexural modulus at room 30 température of greater than 200 MPa, preferably of atleast 400 MPa, measured according to ASTM standardD790. On the other hand, since excessive rigidity ofthe expanded material may make the iinished productdifficult to handle, it is prelerred to use a polymer 35 material which has a flexural modulus al room tempera- 17

ture of less than 2000 MPa. Polymer materials which areparticularly suitable for this purpose are those whichhâve, before expansion, a flexural modulus at roomtempérature of between 400 and 1800 MPa, a polymermaterial with a flexural modulus at room température ofbetween 600 and 1500 MPa being particularly preferred.

These flexural modulus values may be charac-teristic of a spécifie material or may resuit from themixing of two or more materials having differentmoduli, mixed in a ratio such as to obtain the desiredrigidity value for the material. For example, poly-propylene, which has a flexural modulus of greater than1500 MPa, may be appropriately. modified with suitableamounts of ethylene-propylene rubber (EPR), having amodulus of about 100 MPa, for the purpose of loweringits rigidity in a suitable manner.

Examples of commercially available polymercompounds are: low density polyethylene: Riblene FL 30(Enichem); high density polyethylene: DGDK 3364 (UnionCarbide); polypropylene: PF 814 (Montell);polypropylene modified with EPR: Moplen EP-S 30R, 33R and 81R (Montell); Fina-Pro 5660G, 4660G,2660S and 3660S (Fina-Pro).

The degree of expansion of the polymer and thethickness of the coating layer will hâve to be suchthat they ensure, in combination with the outer polymersheath, résistance to typical impacts which occurduring the handling and laying of the cable.

As mentioned previously, the "degree of expan-sion of the polymer" is determined in the iollowingway : G (degree of expansion) (do/d,. - 1)-100 " ' 011303 where d0 indicates the density of the non- expanded polyrner and de indicates the apparent density measured for the expanded polyrner.

The Applicant has observed that, insofar as themaintenance of the desired impact-resistance charac-teristics allows, for an equal thickness of theexpanded layer, it is préférable to use a polyrnermatériel having a high degree of expansions since, inthis way, it is possible to limit the amount of polyrnermaterial used, with advantages in terms of both economyand reduced weight of the finished product.

The degree of expansion is very variable, bothas a function of the spécifie polyrner material used andas a function of the thickness of the coating which itis intended to use; in general, this degree of expansion may range f^om 20% to 3000%, preferably from30% to 500%, a degree of expansion of between 50% and200% being particularly preferred. The expanded polyrnergenerally has a closed-cell structure.

The Applicant has observed that beyond acertain degree of expansion, the capacity of thepolyrner coating to give the required impact strengthdecreases. In particular, it has been observed that thepossibility of obtaining high degrees of expansion ofthe polyrner by maintaining a high efficacy of protec-tion against impacts may be correlated with the valueof the flexural modulus of the polyrner to be expanded.

The reason for this is that the Applicant has observedthat the modulus of the polyrner material decreases asthe degree of expansion of this material increases,approximat1 y nccording to the following formula: E?/Ei- (p.'/pi ) ·' wherei n: 19 011303

Ez represents the flexural modulus of the polymer at t.hehigher degree of expansion;

Ei represents the flexural modulus of the polymer at thelower degree of expansion P2 represents the apparent density of the polymer at thehigher degree of expansion; and

Pi represents the apparent density of the polymer at, thelower degree of expansion;

As a guidance, for a polymer with a flexural modulus ofabout 1000 MPa, a variation in the degree of expansionof from 25% to 100% entails an approximate halving ofthe value of the flexural modulus for the material.

Polymer materiels which hâve a' high flexural modulusmay therefore be expanded to a greater degree thanpolymer matériels which hâve low modulus values,without this prejudicing the ability of the coating towithstand impacts.

Another variable which is liable to influencethe impact strength of the cable is the thickness ofthe expanded coating; the minimum thickness which iscapable of ensuring the impact strength which it isdesired to obtain with such a coating will dépendmainly on the degree of expansion and on the flexuralmodulus of this polymer. In general, the Applicant hasobserved that, for the same polymer and for the samedegree of expansion, by increasing the thickness of theexpanded coating it is possible to reach higher valuesof impact strength. However, for the purpose of using alimited amount of coating material, thus decreasingboth the costs and the dimensions of the finishedproduct, the thickness of the layer of expandedmaterial will advantageously be the minimum thicknessrequired to ensure the desired impact strength. Inparticular, ior cables ot t lie medium tension type, it 20 011303 has beon observed that an expanded coating thickness ofabout 2 mm is usually capable of ensuring a sufficientrésistance fo the normal impacts to which a cable ofthis type is subjected. Preferably, the coating thick-ness will be greater than 0.5 mm, in particular betweenabout 1 mm and about 6 mm, a thickness of between 2 mmand 4 mm being particularly preferred.

The Applicant has observed that it is possibleto define, to a reasonable approximation, the relation-ship between the coating thickness and the degree ofexpansion of the polymer material, for materials withvarious flexural modulus values, such that the thick-ness of the expanded coating is suitably dimensioned asa function of the degree of expansion and of themodulus of the polymer material, in particular forthicknesses of the expanded coating of about 2-4 mm.

Such a relationship may be expressed as follows: V · de > Nwhere V represents the volume of expanded polymermaterial per linear meter of cable (m3/m), this volumebeing relative to the circular rim defined by theminimum thickness of expanded coating, corresponding tothe circular rim (10a) of Fig. 2 for multipolar cables,or to the coating (10) defined in Fig. 3 for unipolarcables ; de represents the apparent density measured forthe expanded polymer material (kg/m3); and N is the resuit of the product of the twoabovementioned values, which will hâve to be greaterthan or equal to: 0.03 for materials with a modulus > 1000 MPa, 0.04 for materials with a modulus of 80Ο- Ι 000 MPa, 011303 0.05 for materials with a modulus of 400- 800 MPa, 0.06 for materials with a modulus < 400 MPa.

The parameter V is related to the thickness (S)of the expanded coating by the following relataonship: V = π (2Ri · S + S2) where Rj represents the inncr radius of thecircular rim (10a).

The parameter de is related to the degree ofexpansion of the polymer material by the previousrelationship: G = (d0/de - 1)100

Based on the abovementioned relationship, foran expanded coating about 2 mm in thickness, placed ona circular section of cable with a diameter of about22 mm, for various materials having different flexuralmoduli at room température (Mf), it is found that thiscoating will hâve to hâve a minimum apparent density ofabout : 0.40 g/cm3 for LDPE (Mf of about 200); 0.33 g/cm3 for a 70/30 PP/EPR mixture (Mf ofabout 800); 0.26 g/cm3 for HDPE (Mf of about 1000); 0.20 g/cm3 for PP (Mf of about 1500).

These values of apparent density of theexpanded polymer correspond to a maximum degree ofexpansion of about; 130% for LDPE (d0 = 0.923) 180% for the PP/EPR mixture (do = 0.890) 260% for HDPE (d0 =- 0.945) 350% for PP (d() - 0.900).

Similarly, for a thickness ot the expandedcoût, ing of about 3 mm placed on a cable of i dent i cal 2.2 011303 dimensions, the following values of minimum apparentdensity are obtained: 0.25 g/cm3 for LDPE; 0.21 g/cm3 for the PP/EPR mixture; 0.17 g/cm3 for HDPE; 0.13 g/ cm3 for PP; corresponding to a maximum degree of expansion ofabout : 270% for LDPE; 320% for the PP/EPR mixture; 460% for HDPE; 600% for PP.

The results shown above indicate that in orderto optimize the impact strength characteristics of anexpanded coating of predetermined thickness, both themechanical strength characteristics of the material (inparticular its flexural modulus)) and the degree ofexpansion of said material should be taken in account.However, the values determined by applying the aboverelationship should not be considered as limiting thescope of the présent invention. In particular, themaximum degree of expansion of polymers which hâveflexural modulus values close to the upper limits ofthe intervals defined for the variation of the number N(that is to say 400, 800 and 1000 MPa) may in realitybe even greater than that calculated according to theabove relationship; thus, for example, a layer ofPP/EPR about 2 mm in thickness (with Mf of about800 MPa) will still be able to provide the desiredimpact protection even with a degree of expansion ofabout 200%.

The polymer is usually expanded during t. heextrusion phase; this expansion may eit.her take placechemically, by means of addition of a suitable"expanding" compound, that is to say one which is 23 011503 capable of generating a gas undcr defined températureand pressure conditions, or may take place physically,by means of injection of gas at high pressure directlyinto the extrusion cylinder.

Examples of suitable Chemical "expanders" areazodicarboamide, mixtures of organic acids (for examplecitric acid) with carbonates and/or bicarbonates (forexample sodium bicarbonate).

Examples of gases to be injected at highpressure into the extrusion cylinder are nitrogen,carbon dioxide, air and low-boiling hydrocarbons suchas propane and butane.

The protective outer s.heath which clads thelayer of expanded polymer may conveniently be of thetype normally used. Materials for the outer coatingwhich may be used are polyethylene (PE), in particularmedium-density PE (MDPE) and high-density PE (HDPE),polyvinyl chloride (PVC), mixtures of elastomers andthe like. MDPE or PVC is preferably used. Typically,the polymer matériel which forms this outer sheath hasa flexural modulus of between about 400 and about1200 MPa, preferably between about 600 MPa and about1000 MPa.

The Applicant has observed that the presence ofthe outer sheath contributes towards providing thecoating with the desired impact strength characteris-tics, in combination with the expanded coating. Inparticular, the Applicant has observed that thiscontribution of the sheath towards the impact strength,for the same thickness of expanded coating, increasesas the degree of expansion of the polymer which formsthis expanded coating increases. The thickness of thisouter' sheath is preierably greater than 0.5 mm, inparticular between 1 and 5 mm, preferably between 2 and4 mm. 24 011303

The préparation of a cable with an impactstrength according to the invention is described withreferencc to the cable structure diagram of Figure 2,in which, however, the star-shaped spaces between thecores to be coated are filled, not directly with theexpanded polymer (10) but rather with a conventionalfiller; the expanded coating is then extruded over thissemi-processed cable, to form a circular rim (10a)around this semi-processed cable and is subsequentlyclad with the outer polymer sheath (2) . The préparationof the cable cores, that is to say the assembly of theconductor (4), inner semiconductive layer (9),insulator (5), outer semiconductive layer (8) and métalscreen (4), is carried out as known in the art, forexample by means of extrusion. These cores are thenroped together and the star-shaped spaces are filledwith a conventional filling material (for exampleelastomeric mixtures, polypropylene fibers and thelike) , typically by means of extrusion of the fillerover the roped cores, so as to obtain a semi-processedcable with a circular cross-section. The coating ofexpanded polymer (10) is then extruded over the fillingmaterial. Preferably, the die of the extrader head willhâve a diameter slightly smaller than the finaldiameter of the cable with expanded coating, in orderto allow the polymer to expand outside the extrader.

It has been observed that, under identicalextrusion conditions (such as spin speed of the screw,speed of the extrusion line, diameter of the extraderhead and the like) the extrusion température is one olthe process variables which has a considérableinfluence on the degree of expansion. In general, forextrusion températures below 16Ü°C, it is diliicult teobtain a sufficient degree ol expansion; the extrusiontempérature is preferably at least 180°C, in partieulai 25 011303 about 200°C. Usually, an increase in the extrusiontempérature corresponds to a higher degree ofexpansjon.

Moreover, it is possible to control to some5 extent the degree of expansion of the polymer by acting on the rate of cooling since, by appropriately slowingdowri or speeding up the cooling if the polymer whichforms the expanded coating at the extruder outlet, itis possible to increase or decrease the degree of 10 expansion of the said polymer.

As mentioned, the Applicant has observed thatit is possible to détermine quantitatively the effectsof an impact on a cable coating by means of measuringthe peel strength of the cable coating layers, dif- 15 ferences between the average value of this peel strength and the value measured at the point of impactbeing evaluated. In particular, for cables of themedium-tension type, with a structure comprising aninner semiconductive layer, an insulating layer and an 20 outer semiconductive layer, the peel strength (and therelative différence) may advantageously be measuredbetween the layer of outer semiconductive material andthe insulating layer.

The Applicant has observed that the effects of 25 the particularly severe impacts to which a cable may besubjected, in particular an armored medium-tensioncable, may be reproduced by means of an impact testbased on the French standard HN 33-S-52, relating toarmored cables for high-tension power transmission, 30 which allows for an energy of impact on the cable ofabout 7 2 joules (J) .

The peel strength of the coating layer may bemeasured according to the French standard HN 33-S-52,according to which the force needed to be applied to 35 separate the outer semiconductive layer from the 011503 insulating layer is measured. The Applicant bas observed that by measuring this force continuons! y, atthe points at which the impact takes place, force peaksare measured which indicate a variation in the cohesiveforce between the two layers. It was observed thatthese variations are generally associated with adecrease in the insulating capacity of the coating. Thevariation will be proportionately larger the smallerthe impact strength provided by the outer covering(which, in the case of the présent invention, consistsof the expanded coating and the outer sheath). The sizeof the variation of this force measured at the pointsof impact, relative to the average value measured alongthe cable, thus provides an indication of the degree ofprotection provided by the protective coating. Ingeneral, variations in the peel strength of up to20-25% relative to the average value are considered tobe acceptable.

The characteristics of the expanded coating(material, degree of expansion, thickness), which mayadvantageously be used together with a suitableprotective outer polymer sheath, may be appropriatelyselected according to the impact protection which it isintended to provide to the underlying cable structure,and also depending on the characteristics of thespécifie material used as insulator and/or semiconductor, such as hardness of the material,density and the like.

As it can be appreciated throughout the présentdescription, the cable of the invention is particularlysuitable to replace couventiona.1 armored cables, due tothe advantageous properties ol the expanded polymercoating with respect to métal armoring. However, itsuse should not be limited to such a spécifie application. As a matter ol lact, the cable ol 1he 27 011303 invention may advantageously be employée! in ail those application wherein a cable having enhanced impact- résistant properties would be désirable. In particular,the impact-résistant cable of the invention may replaceconventional unarmored cables in ail those applicationwherein, up to now, use of armored cables would hâvebeen advantageous but has been discouraged due to thedrawbacks of the métal armoring. A few illustrative examples are given herein-below in order to describe the invention in furtherdetail. EXAMPLE 1

Préparation of the cable with expanded coating

In order to evaluate the impact strength of anexpanded polymer coating according to the invention,various test pièces were prepared by extruding variablethicknesses of a few polymers with various degrees ofexpansion over a core composed of a multi-wireconductor about 14 mm in thickness coated with a layerof 0.5 mm of semiconductive material, a layer of 3 mmof an insulating mixture based on EPR and a furtherlayer of 0.5 mm of "easy stripping" semiconductivematerial based on EVA supplemented with carbon black,for a total core thickness of about 22 mm.

Low density polyethylene (LDPE), high densitypolyethylene (HDPE), polypropylene (PP) a 70/30 byweight mechanical mixture of LDPE and finely powderedvulcanized natural rubber (particle size of 300-600 pm)(PE-powder), PP modified with EPR rubber (PP-EPR as a70/30 by weight mixture) were used as polymer materialsto be expanded; these materials are identified in thefollowing text by the letters A to E and are describedin detail in the following table: 28 011303

Material Brand name and manufacturer Modulus (MPa) A LD PE Riblene Eh 30 - Enichem 2 60 B HDPE DGDK 3364 - Union Carbide 1000 c PP PE 814 - Montell 1600 D PP-EPR FINA-PRO 3 660S 12 50 E PE/powder Riblene FL 30

The polymer was expanded chemically, alternatively using two different expanding compounds(CE), these being identified as follows:

Compound Brand name and manufacturer CEI azodicarboamide Sarmapor PO - Sarma CE2 carboxylic acid- bicarbonate Hydrocerol CF 70 - Boehringer Ingelheim

The polymer to be expanded and the expandingcompound were loaded (in the ratios indicated inTable 2) into an 80 mm - 25 D single-screw extrader(Bandera); this extruder is equipped with a threadedtransfer screw characterized by a depth in the finalzone of 9.6 mm. The extrusion System consists of a maledie capable of providing a smooth throughput of thecore to be coated (generally with a diameter which isabout 0.5 mm greater than the diameter of the core tobe coated), and a female die in which the diameter ischosen so as to hâve a size about 2 mm less than thediameter of the cable with the expanded coating; inthis way, the extruded material expands on exiting theextrusion head rallier than inside this head or insidethe extruder. The throughput speed of the core to becoated (speed of the extrusion line) is set as afunction of the desired thickness of expanded material (see Table 2) . At. a distance oi about 500 mm from the 29 011303 extrusion head is a cooling pipe (containing coldwater) in order to stop the expansion and to cool downthe extruded matériel. The cable is then wound on abobbin. 5 The composition of the polymer matériel/ expander mixture and the extrusion conditions (speed,température) were varied appropriately, as described inTable 2 below. 10 Table 2: Expanding mixture and extrusion conditions

Cable No. Matériel + % and type of expander Extruder speed (rev/min) li) Extruder temp. (°C) Line speed (m/min) 1 A + 2%CE1 6.4 165 3 2 A + 2%CE1 11.8 190-180 2 3 A + 2%CE1 5.5 190-180 3 4 A + 2%CE1 6.8 190-180 2 5 A + 2%CE1 6.4 165 1.5 6 A + 0.8%CE2 5.7 225-200 2 7 C + 0.8%CE2 3.7 200 2 8 C + 0.8%CE2 6.3 200 2 9 E + 0.8%CE2 4.9 225-200 1.8 10 B + 1.2%CE2 8.2 225-200 2 11 D + 2%CE2 8 225-200 2 : The extrusion température relates to the cylinder and extrusion head. When only one value is given, thesetempératures are identical. In the initial zone of the extruder,the température is about 150°C. 15

Sample 1 did not undergo expansion, presumablybecause the température of the extruder was too low(165°C), and likewise, for the same reason, Sample 5underwent limited expansion (only 5^).

The cable with the expanded coating was thensubsequently coated with a eonventional sheath of MUTE 20 - ,n - 011303 (CE 90 - Materie Plastiche Bresciane) of variablethickness (see Table 3) by means of conventionalextrusion methods, thus obtaining cable samples withthe characteristics defined in Table 3; cable No. 1, in 5 which the polymer has not undergone expansion, wastaken as comparative non-expanded polymer coating.

Table 3 also gives, for comparative purposes, thecharacteristics of a cable lacking the expanded fillingand coated with only the outer sheath (cable No. 0). 10

Table 3: Characteristics of the coating

Cable Degree of Thickness of Sheath No. expansion of the filling (%) the filling (mm) thickness (mm) 0 - 0 3 1 0 1 3 2 31 4.3 3 3 61 1 3 4 48 2.5 3 5 5 3 3 6 35 2 2 7 52 2 2 8 29 3 2.2 9 23 2.5 2 10 78 4 2 11 82 4 2 15 In a similar manner to that described above, using an expanded polymer coating with a flexural modulus o£ about 600 MPa consisting of a polypropylene tnodified with about 30Ί of an EPR rubber, another 6 cable samples were prepared, as reported in Table 4 30 (Examples 12-17); Table 4 also gives two comparative 011303 examples of cables with expanded coating but lackingthe outer sheath (Examples 16a and 17a).

Table 4: Characteristics of the coating

Cable No. Degree ofexpansion ofthe filling (%) Thickness of the filling(mm) Sheatht h i c k ri e s s(rnrn ) 12 71 3 1. 9 13 22 2 2 14 167 3 1 . 3 15 124 2 2 16 56 2 2 1 ba 56 2 - 17 84 2. 2 17a 84 2 - EXAMPLE 2

Impact strength tests

In order to evaluate the impact strength of thecables prepared according to Example 1, impact testswere carried out on the cable with subséquent évalua-tion of the damage. The effects of the impact wereevaluated both by means of Visual analysis of the cableand by means of measuring the variation in peelstrength of the layer of semiconductive material at thepoint of impact. The impact test is based on the Erenchstandard HN 33-S-52, which provides for an energy ofimpact on the cable of about 72 joules (J), which isobtained by dropping a 27 kg weight from a height of27 cm. For the présent test, such energy of impact hasbeen produced by a 8 kg weight dropped from a height of97 cm. The impact-end of the weiqht is provided with aV-shaped rounded-edge (1 mm curvature inadius) punchinghoad. For the purposes of the présent invention, theimpact strength was evaluated on a single1 impact. For 32 011303 samples 6-12, the test was repeated a second time at adistance of about 100 mm from the first.

The peel strength was measured according to theFrench standard HN 33-S-52, according to which theforce needed to be applied in order to separate theouter semiconductive layer from the insulating layer ismeasured. By measuring this force continuously, forcepeaks are measured at the points at which the impactoccurred. For each test piece, at the point of impact,a "positive" force peak was measured, corresponding toan increase in the force (relative to the averagevalue) required to separate the two layers, and a"négative" force peak (decrease relative to the averagevalue). From the différence between the maximum (Fmax)and minimum (Fmin) of the force peaks measured, themaximum variation in the peel strength at the point ofimpact is obtained.

The variation in the peel strength is thuscalculated by determining the percentage ratio betweenthe abovementioned différence (Fmax-Fmin) and theaverage peel strength value measured for the cable(FO), according to the following relationship:

% variation = 100 ( Fmax-Fmin)/FO

The size of the variation of this forcemeasured at the points of impact, relative to theaverage value measured along the cable, thus gives anindication of the degree of protection provided by theexpanded coating. In general, variations of up to 20-25% are considered to be acceptable. Table 5 gives thevalues of the variation in the peel strength forsamples 0-17a. 33 011303

Table 5: % variation in the peel strength

Cable lst test 2nd test 0 62 78 1 40 - 2 18 - 3 27 - 4 13 - 5 21 - 6 17 23 7 9 12 8 4 5 9 19 15 10 9.8 12.5 11 4.3 2.5 12 7 14 13 16 17 14 14 12 15 10 10 16 16 18 16a 30 55 17 15.5 13 17a 116 103

As is seen in Table 3, for sample 1 (for whichno expansion was obtained), the percentage variation in 5 peel strength is extremely high; this indicates that anon-expanded polymer has a decidedly lower capacity toabsorb impacts than a layer of identical thickness ofthe same polymer which is expanded (see sample 3, with61% expanded coating). Sample 3 shows a variation in 10 the peel strength which is slightly above the 25% limitvalue; the limited impact strength provided by thesample may be attributed mainly to the thickness, ofonly 1 mm, of the expanded coating, relative to the 2-3 mm thicknesses of the other samples. 15 Sample 5, with an expanded coating thickness of 3 mm, has a high value of peel strength on account ofthe low degree of expansion of the polymer (5%), thusdémonstratΐng the limited impact strength provided by acoating with a low degree ol expansion. Sample ή, 20 although having a thickness oi expanded material which 011303 is less than that of sample 5 (2.5 mm as opposed to3 mm), nevertheless has a higher impact strength, witha variation in the peel strength of 13% compared with21% for sample 5, thereby demonstrating the fact that ahigher degree of expansion affords a higher impactstrength.

By comparing sample 13 with sample 15, it isseen how an increase in the degree of expansion of thepolymer (from 22 to 124%), for the same thickness ofthe layer of expanded material and of the outer sheath,entails an increase in the impact strength of thecoating (going from 16-17% to 10% of variation in thepeel strength). This trend is confirmed by comparingsample 16 with sample 17. However, by comparing samples16a and 17a (without outer sheath) with the respectivesamples 16 and 17, it may be seen how the contributionprovided by the outer sheath towards the impactprotection increases as the degree of expansionincreases. EXAMPLE 3

Impact strength comparison test with armored cable

Cable no. 10 has been tested versus aconventional armored cable, in order to verify theimpact strength efficiency of the expanded coatinglayer.

The armored cable has the same core as cable no. 10(i.e. a multi-wire conductor about 14 mm in thicknesscoated with a layer of 0.5 mm of semiconductivematerial, a layer of 3 mm of an insulating mixturebased on EPR and a further layer of 0.5 mm of "easystripping" semiconductive material based on EVAsupplemented with carbon black, for a total corethickness of about 22 mm) . Said core is encircled, 1 romthe inside towards the outside of the cable by: 35 011303 a) a layer of rubber-based filling material of about0.6 mm thickness; b) a sheath of PVC of about 0.6 mm thickness; c) 7. arrnoring steel tapes of about 0.5 mm thickness 5 each; d) an outer sheath of MDPE of about 2 mm thickness.

For the comparison test, a dynamic machine of the"falling weight" type (CEAST, mod. 6758) has beenemployed. Two sets of tests has been carried out, by 10 dropping a 11 kg weight from a height of 50 cm (energyimpact of about 54 joule) and 20 cm (energy impact ofabout 21 joule), respectively; the weight is providedat its impacting end with a semispheric head of about10 mm radius. 15 The resulting deformation of the cables is shown infigg. 4 and 5 (50 cm and 20 height, respectively),wherein the cable according to the invention isindicated with a), while the conventional armored cableis indicated with b). 20 The deformation of the core has been measured, in orderto evalute the damages of the cable structure. As amatter of fact, higher deformations of the semiconductive-insulating-semicondutive sheath are morelikely to cause electric defects in the insulating 25 properties of the cable. The results are reported intable 6 36 011303

Table 6:¾ réduction of the thickness of thesemiconductive layer after impact

In conventional armored cable In Cable no. 10 50 cm height impact 41¾ 2 6.5% 20 cm height impact 4.4% 2.9%

As apparent from the results reported in table5 6, the cable of the invention shows even better impact strength performances than a conventional armoredcable.

Claims (22)

1. A cable comprising an inner structure and a coating layer disposed to surround said inner structure, wherein said coating layer is made from an expanded poiymer material having a degree of expansion of front about 20% to about 011303 CLA1MS 3000% and a flexural modulus of at least 200 MPa, measured at roomtempérature according to ASTM standard D790. before expansion of saidpoivra er.

lus is ber

600 MPa and 1500 MPa. 15

5. Tne cable as claimed in claim 1, wherein the degree of expansion of saidpoiymer material is from about 30% to about 500%, 20

coating of expanded poiymer material has a thickness of between 1 and 6 mm.

9, The cable as claimed in any one of the preceding ciaims 1 to 6, wherein tire saidcoating of expanded poiymer material has a thickness of between 2 and 4 mm.

10, The cable as claimed in claim 1. wherein the said expanded poiymer material is chosen hom polyetbylene (PE), low density PE (LDPE). medium density PE

38 011303 ethylene/acrylate copolymer. ethylene/methyl acrylaie (EMA) copolymer,elhylene/ethyl acrylaie (EEA) copolymer, ethylene/butyl acrylaie (EBA)copolymer; ethylene/a-oleiin copolymer; acrylonitrile-butadiene-styrene (ABS)resins; halogenated polymer, polyvinyl chloride (PVC); polyuréthane (PUR);polyamide; aromatic polyester, polyethylene terephthalate (PET), polvbutyleneterephthalate (PBT); and copolymers or mechanical mixtures thereof. 11. ihe cable as claimed in daim 1, wherein the said expanded polymer mat criai isa polyoiefin polymer or copolymer based on PE and/or PP.

12. The cable as claimed in claim I, wherein ihe said expanded polymer material isa polyoiefin polymer or copolymer based on PE and/or PP modifiée vdtbethyîene-propylene rubber.

13. The cable as claimed in claim 12, wherein the said expanded polymer matérielis polypropylene modified with ethyîene-propylene rubber (EPR), tire PP/EPRweight ratio being between 90/10 and 50/50.

14. The cable as claimed in claim 13, wherein the said PP/EPR weight ratio isbetween 85/15 and 60/40.

15. The cable as claimed in claim 13, wherein the said PP/EPR weight ratio isabout 70/30.

16. Ihe cable as claimed in claim 12, wherein the said polyoiefin polymer orcopolymer based on PE and/or PP also contains a predetermined amount ofvulcanized rubber in powder form.

17. The cable as claimed in claim 16, wherein the predetermined amount ofvulcanized rubber in powder form is between 10% and 60% of the weight ofpolymer.

18. The cable as claimed in any one of the preceding daims 1 to 17, wherein saidcable comprises an outer polymer sheath.

19. The cable as claimed in daim 18, v/herein the said sheath is in contact with the said expanded polymer coating. 39 011303

20. The cable as claimed in claim 1S or 19, wherein the said sheath has a thicknessof grenier than 0.5 mm.

21. The cable as claimed in claim 18 or 19, wherein the said sheath has a thicknessci between 1 and 5 mm.

22. A method for imparting impact strength. to an inner structure of a cable whichcomprises aisposing around said inner structure a 1 ayer of expanded poiymermaterial.

23, A method according to claim 22 wherein said poiymer material has a. degre· expansion cf froœ about 20% to about 3000% ano ;2CC hfpg measured etroom température accordingbtfoîe expansion of said pc-lyroer. or tse 'ex-oral modulus of at ki Af TM standard D79O

24. A method according to cia:cable. 3 22 wherein said cable is power transmission

25. A method according to claim 22 wherein an outei polymtr sheath is aisposed 15 around said Iayer of expanded poiymer materiah

26. A method according to claim 22, wherein said inner structure comprises at leasttwo adjacent coating layers having a predetermined average peel strengthbetween each other, the flexural moculus and degiee of expansion of saidpolymej material being such thaï when said cable is subjec-ted to an impact 20 having an energy of about 72 J, the différence between tire abovç average peel strength and the peel strength measured in correspondence of the point of impactîayer is lower than about 25%.

27. Use of an expanded poiymer material for imparting impact strength to a powertransmission cable. 25 28. A method for evaluatîng the impact strength of a cable comprising at least one insulating coating, which consists in a) measuring tire average peel strength of tire said insulating ’ayer: b) subjecting the cable to an impact of predetermined energy; c) ïneasuring the peel strength cf the said insulating lave: at the point of iroract; 40 011303 ό) checking that the différence between the average peel strength and the peelStrength measured at the point of impact is less than a predetermined value.

29. The method as claimed in claim 28, in which the peel strength is measuredbetween the Iayer of insulating coating and the laver of outer semi-conductive 5 coating

30. The method as claimed in claim 29, in which the différence between theaverage peel strength and thaï measured ai the point of impact is less than 25%,