ITMI972839A1 - ELECTRIC CABLE HAVING AN EXPANDED SEMICONDUCTIVE LAYER - Google Patents

Description

ELECTRICAL CABLE HAVING AN EXPANDED SEMICONDUCTIVE LAYER

The present invention relates to electrical, in particular for the transport or distribution of medium or high voltage energy, equipped with an expanded semiconductive layer which is able to elastically and uniformly absorb the radial forces of expansion and contraction of the layers of cable coating, due to the thermal cycles to which the cable is subjected during use. In the present description, the term "medium voltage" means a voltage between about 1 kV and about 30 kV, while the term "high voltage" includes voltages above 30 kV.

The cables for the transport or distribution of medium or high voltage energy generally consist of a metal conductor coated with a first internal semiconductive layer, an insulating layer, and an external semiconductive layer. For some applications, in particular when a watertight seal towards the outside is required, the cable is enclosed inside a metal screen, usually made of aluminum or lead, consisting of a continuous tube or a metal band shaped in a tubular shape and welded or sealed to ensure watertight integrity. In these embodiments, the metal screen, in addition to constituting a barrier in the radial direction with respect to the penetration of water, performs important functions of the electrical type and must be in contact with the external semiconductive layer. In particular, a first function of the metal screen is to create a uniform radial electric field inside the cable and at the same time to cancel the electric field outside the cable itself. A further function is to withstand short-circuit currents.

During use, the cable is subject to thermal cycles due to daily variations in the intensity of the transported current, with corresponding cable temperature variations between the ambient temperature and the maximum operating temperature (for example between 20 ° C and 90 ° ° C). These thermal cycles cause expansion and subsequent contraction of the cable coating layers, with consequent radial stresses on the metal shield. The latter can therefore undergo mechanical deformations with the formation of empty spaces between the screen and the external semiconductive layer and possible generation of non-uniformity in the electric field. In the limit, these deformations can lead to the breakage of the screen, particularly in the case in which this is welded or joined by means of a sealant, and therefore to complete loss of the functionality of the screen itself.

To try to overcome these drawbacks, it is possible to make a taping of the outer semiconductive layer with a plastic material having semiconductive properties, for example polyester pre-impregnated with a solution containing carbon black, which acts as a "buffer" between the cable and metal screen. From the production point of view, a solution of this type involves a considerable increase in costs and a decrease in productivity, since it is necessary to use expensive pre-impregnated tapes and to add a taping stage to the cable production process.

Another possible solution to try to dampen the effect of thermal expansions on the metal screen is to create an external semiconductive layer equipped with longitudinal V-profile channels. A geometry of this type on one side guarantees the electrical contact between the semiconductive layer. and metal shield, on the other hand it should help the elastic recovery of the thermal expansions by the material that constitutes the semiconductive layer. Furthermore, the channels themselves can be used to contain powders of hygro-expanding materials, which prevent the longitudinal propagation of water that can possibly infiltrate under the metal screen.

However, the realization of these longitudinal channels involves the use of a semiconductive layer having a high thickness (around 2 mm or more), with a consequent increase in the cost and overall weight of the cable. Furthermore, the desired geometry of the semiconductive layer is generally achieved through an accurate extrusion process in which suitably designed molds are used. On the basis of the Applicant's experience, the formation of channels with irregular geometry is practically inevitable during this extrusion process. These geometric irregularities can give rise to a non-uniform distribution of the pressure exerted on the metal screen and therefore prevent the semiconductive layer from correctly carrying out the function of elastic absorption of the radial stresses.

The Applicant has found that by inserting a layer of expanded polymeric material having semiconductive properties under the metal shield, the latter is able to elastically and uniformly absorb the radial forces of expansion and contraction of the cable coating layers generated by thermal cycles. the cable itself is subjected to during use. On the one hand, this layer guarantees the necessary electrical continuity between the cable and the metal screen, and on the other it avoids the deformation of the metal screen caused by the above radial forces.

In a first aspect, the present invention therefore relates to an electric cable comprising a conductor, at least one layer of insulating coating and an external metal shield, characterized in that said cable further comprises a layer of expanded polymeric material having semiconductive properties arranged underneath. of said metal screen.

In the following, the "layer of expanded polymeric material having semiconductive properties 11 will be briefly referred to as" expanded semiconductive layer ".

The term "expanded polymeric material" means a polymeric material with a predetermined percentage of "free" space inside the material, ie not occupied by the polymer but by a gas or air.

In general, the percentage of free space in an expanded polymer is expressed by the degree of expansion (G), defined by the following formula:

G = (d0 / de - 1) -100

where d0 indicates the density of the unexpanded polymer and d the apparent density measured on the expanded polymer.

The degree of expansion of the semiconductive layer according to the present invention can vary within wide limits, both as a function of the specific polymeric material used and as a function of the thickness of the coating to be obtained. The degree of expansion is predetermined in such a way as to guarantee the elastic absorption of the radial forces of thermal expansion and contraction of the cable and, at the same time, to maintain the semiconductive properties. In general, the degree of expansion can vary from 5% to 500%, preferably from 10% to 200%.

As regards the thickness of the expanded semiconductive layer according to the present invention, this is equal to at least 0.1 mm, preferably between 0.2 and 2 mm, even more preferably between 0.3 and 1 mm. In practice, thicknesses of less than 0.1 mm are difficult to produce and in any case allow a limited compensation of the deformation, while thicknesses greater than 2 mm, while not presenting in principle functional drawbacks, can be created in the event that specific needs justify their higher cost.

In a further aspect, the present invention relates to a method for controlling the electric field inside a cable in the presence of thermal cycles, said cable comprising a conductor, at least one layer of insulating coating and an external metal screen, characterized by the fact that said method comprises elastically deforming a polymeric layer placed under said metal screen by a value lower than that which causes a detachment between said metal screen and a layer underlying it.

According to a preferred aspect, this method comprises elastically deforming an expanded polymeric layer, said expanded layer preferably having semiconductive properties.

According to a preferred aspect, the electric cable according to the present invention further comprises a compact semiconductive layer arranged between the insulating coating and the expanded semiconductive layer.

The term "compact semiconductive layer" means a layer of non-expanded semiconductive material, that is, with a zero degree of expansion.

According to the Applicant's perception, this compact semiconductive layer can advantageously perform the function of avoiding partial discharges, and therefore damage to the cable, due to any irregularities in the interface surface between the insulating coating and the expanded semiconductive layer. This function can be performed by a semiconductive layer, even very thin, around 0.1 mm or even lower. However, from the point of view of practical embodiment, a thickness of between 0.2 and 1 mm, even more preferably between 0.2 and 0.5 mm, is preferable.

According to a further preferred aspect, the expanded semiconductive layer comprises a hygro-expanding material in the form of a powder.

The Applicant has in fact found that it is possible to incorporate within the expanded polymeric material a hygro-expanding material in the form of powder, which is capable of expanding in contact with water, preventing the longitudinal propagation of water which may possibly infiltrate below. of the metal screen. This solution has numerous advantages with respect to the known art in which the hygro-expanding powder is simply injected between the metal screen and the external semiconductive layer, using for example the longitudinal channels formed on the semiconductive layer as described above. In fact, the expanded layer is able to incorporate the hygro-expanding material in high quantities, avoiding the presence of free dust that can be dispersed into the environment during the installation of the cable. On the other hand, the hygro-expanding powder, even though it is incorporated in the expanded material, is in any case capable of expanding when put into contact with water and therefore effectively performs its function of blocking the water.

The hygro-expanding material generally consists of a homopolymer or copolymer having hydrophilic groups along the polymer chain, for example: cross-linked and at least partially salified polyacrylic acid (for example the Cabloc® products of C. F. Stockhausen GmbH, or Waterlock® of Sanyo); starch or its derivatives mixed with copolymers between acrylamide and sodium acrylate (for example the SGP Absorbent Polymer® products from Henkel AG); sodium carboxymethylcellulose (for example the Blanose® products of Hercules Ine.).

Figure 1 shows in section an example of embodiment of an electric cable according to the present invention, of the unipolar type, for the transport of medium voltage energy.

Such cable comprises a conductor (1), an internal semiconductive layer (2), an insulating layer (3), a compact semiconductive layer (4), an expanded semiconductive layer (5), a metal shield (6), and an outer sheath (7).

The conductor (1) generally consists of metal wires, preferably copper or aluminum, strung together according to conventional techniques. The metal screen (6), usually in aluminum or lead, is formed by a continuous metal tube, or by a metal band shaped in a tubular shape and welded or sealed with an adhesive material in such a way as to guarantee watertight integrity. The metal screen (6) is usually coated with an outer sheath (7), consisting of a cross-linked or non-cross-linked polymeric material, for example polyvinyl chloride (PVC) or polyethylene (PE).

The polymeric material which constitutes the expanded semiconductive layer can be any type of expandable polymer such as: polyolefins, copolymers of different olefins, olefin copolymers / unsaturated esters, polyesters, polycarbonates, polysulfones, phenolic resins, urea resins, and their mixtures. Examples of suitable polymers are: polyethylene (PE), in particular low density PE (LDPE), medium density PE (MDPE), high density PE (HDPE) and linear low density PE (LLDPE); polypropylene (PP); ethylene-propylene elastomeric copolymers (EPM) or ethylene-propylene-diene terpolymers (EPDM); natural rubber; butyl rubber; ethylene / vinyl ester copolymers, for example ethylene / vinyl acetate (ÈVA); ethylene / acrylate copolymers, in particular ethylene / methyl acrylate (EMA), ethylene / ethyl acrylate (EEA), ethylene / butyl acrylate (EBA); ethylene / a-olefin thermoplastic copolymers; polystyrene; acrylonitrile butadiene styrene resins (ABS); halogenated polymers, in particular polyvinyl chloride (PVC); polyurethane (PUR); polyamides; aromatic polyesters, such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT); and their copolymers or mechanical mixtures.

Preferably, the polymeric material is a polyolefin polymer or copolymer based on ethylene and / or propylene, and is in particular selected from:

(a) copolymers of ethylene with an ethylenically unsaturated ester, for example vinyl acetate or butyl acetate, in which the amount of unsaturated ester is generally comprised between 5 and 80% by weight, preferably between 10 and 50% by weight;

(b) ethylene-propylene (EPR) or ethylene-propylene-diene (EPDM) elastomeric copolymers, preferably having the following composition: 35-90% by moles of ethylene, 10-65% by moles of propylene, 0-10% by moles of diene (for example 1,4-hexadiene or 5-ethylidene-2-norbornene);

(c) polypropylene modified with EPR or EPDM elastomeric copolymers, where the weight ratio PP / EPR or PP / EPDM is comprised between 90/10 and 30/70, preferably between 80/20 and 40/60.

For example, the commercial products Elvax® (Du Pont), Levapren® (Bayer), Lotryl® (Elf-Atochera), in class (b) the products Dutral® (Enichem) or Nordel® (Dow) are included in class (a) -Du Pont), while PP modified with EPR or EPDM are available on the market under the brands Moplen® or Hifax® (Montell), or Fina-Pro® (Fina), and the like.

To impart semiconductive properties to the polymeric material, products known in the art for the preparation of semiconductive polymeric compositions can be used. In particular, an electroconductive carbon black, for example acetylene black or electroconductive furnace black, and the like, can be used. The surface area of carbon black is generally greater than 20 m2 / g, usually between 40 and 500 m2 / g. It is also possible to use a carbon black with high conductivity, with a surface area of at least 900 m2 / g, such as for example the furnace carbon black known commercially under the brand name Ketjenblack® EC (Akzo Chemie NV).

The quantity of carbon black to be added to the polymer matrix can vary according to the type of polymer and carbon black used, the degree of expansion to be obtained, the expanding agent, etc. The quantity of carbon black must in any case be such as to impart sufficient semiconductive properties to the foam material, in particular such as to obtain a volumetric resistivity value of the foam material, at room temperature, lower than 500 Ω-m, preferably lower than 20 Ω- m. Typically, the amount of carbon black can vary between 5 and 80%, preferably between 10 and 70%, by weight with respect to the weight of the polymer.

The compact semiconductive layer possibly present between the insulating coating and the expanded semiconductive layer, as well as the internal semiconductive layer, also of the compact type, are made according to known techniques, in particular by extrusion, choosing a polymeric material and a carbon black from those indicated above for the expanded semiconductive layer.

The insulating layer is preferably made by extrusion of a polyolefin selected from those indicated above for the expanded semiconductive layer, in particular polyethylene, polypropylene, ethylene propylene copolymers, and the like. After extrusion, the material is preferably cross-linked according to known techniques, for example by means of peroxides or via silanes.

The expanded semiconductive layer can be made by extruding the semiconductive polymeric material on the core of the cable, that is the set of conductor (1), internal semiconductive layer (2), insulating layer (3) and possible compact semiconductive layer (6). The core of the cable can also be made by extrusion, in particular coextrusion of the three layers according to known techniques.

The expansion of the polymer is normally carried out during the extrusion step. This expansion can take place either chemically, by adding a suitable expanding agent, that is capable of developing a gas under certain conditions of pressure and temperature, or physically, by injecting high pressure gas directly into the extruder cylinder. .

Examples of suitable blowing agents are: azodicarbamide, paratoluensulfonylhydrazide, mixtures of organic acids (for example citric acid) with carbonates and / or bicarbonates (for example sodium bicarbonate), and the like.

Examples of gases to be injected at high pressure into the extruder cylinder are: nitrogen, carbon dioxide, air, low-boiling hydrocarbons, for example propane or butane, halogenated hydrocarbons, for example methylene chloride, trichlorofluoromethane, 1-chloro-1, 1-difluoroethane, and the like, or mixtures thereof.

Preferably, the mold of the extruder head will have a diameter slightly smaller than the final diameter of the cable with expanded coating to be obtained, so that the expansion of the polymer outside the extruder leads to the achievement of the desired diameter.

It has been observed that, under the same extrusion conditions (such as rotation speed of the screw, speed of the extrusion line, diameter of the extruder head, and so on) one of the process variables that most influences the degree of expansion is the extrusion temperature. In general, for extrusion temperatures below 130 ° C it is difficult to obtain a sufficient degree of expansion; the extrusion temperature is preferably at least 140 ° C, in particular about 180 ° C. Normally, an increase in the extrusion temperature corresponds to a greater degree of expansion.

Furthermore, it is possible to control to some extent the degree of expansion of the polymer by acting on the cooling rate. In fact, by suitably delaying or anticipating the cooling of the polymer which forms the expanded coating at the outlet of the extruder, the degree of expansion of said polymer can be increased or decreased.

The expanded polymeric material can be cross-linked or non-cross-linked. The crosslinking can be carried out, after the extrusion and expansion step, according to known techniques, in particular by heating in the presence of a radical initiator, for example an organic peroxide such as dicumyl peroxide. Alternatively, crosslinking via silanes can be carried out, which involves the use of a polymer such as those indicated above, in particular a polyolefin, to which silane units comprising at least one hydrolyzable group, for example trialkoxysilane groups, have been covalently bonded. particular trimethoxysilane. The grafting of the silane units can occur by radical reaction with silane compounds, for example methyl triethoxysilane, dimethyldietoxy silane, vinyl dimethoxysilane, and the like. The crosslinking is carried out in the presence of water and a crosslinking catalyst, for example an organic titanate or a metal carboxylate. Particularly preferred is dibutyltin dilaurate (DBTL).

Once the expanded semiconductive layer has been made, the cable is enclosed within the metal shield. According to a preferred embodiment, the diameter of the expanded semiconductive layer in the absence of applied forces is greater than the internal diameter of the metal screen, so as to obtain, after application of the metal screen, a predetermined degree of prestressing of the expanded semiconductive layer. This prestressing allows to make an optimal contact between the expanded semiconductive layer and the metal shield and can allow to recover a possible residual deformation of the expanded semiconductive layer, or a certain degree of plastic deformation of the metal shield, during the thermal contraction phase of the insulating layer.

Finally, the metal screen can be coated with a protective sheath, obtainable for example by extrusion of a polymeric material, usually polyvinyl chloride or polyethylene.

To further describe the invention, some illustrative examples are given below.

EXAMPLES 1-5

Some blends suitable for forming the expanded semiconductive layer according to the present invention have been prepared. The compositions are reported in Table 1 (in parts by weight per 100 parts by weight of base polymer, phr). The various components of the compound were mixed in a closed Banbury-type mixer (1.2 1 of useful volume), first loading the base polymer, then, after a short processing, the carbon black, any hygro-expanding powder and the other additives. . The mixing was carried out for about 6 min at 150 ° C.

At the end of the mixing, the expanding agent was added to the mixture after cooling the material up to about 100 ° C in order to avoid the premature decomposition of the expanding agent which would cause an uncontrolled expansion of the polymer. The mixture was then molded by compression at 160 ° C using a frame with dimensions of 200x200 mm and a height of 3 mm. The mixture was added in such a quantity as to obtain an initial layer with a height of 1 mm, in order to leave sufficient space for the expansion of the polymer.

On the specimens thus obtained, the following were measured:

- the bulk density, and therefore, knowing the density of the non-expanded material, the degree of expansion was calculated according to the formula reported above;

- the volume resistivity at room temperature.

The data are reported in Table 1.

The samples of Examples 4 and 5, containing the hygro-expanding powder, were placed in water: an immediate expansion of the hygro-expanding powder was observed up to a volume about three times the initial volume.

Elvax® 470 (Du Pont): ethylene / vinyl acetate copolymer (ÈVA) (18% VA, Melt Index 0.7);

Elvax® 265 (Du Pont): ÈVA copolymer (28% VA, Melt Index 3.0);

Dutral® TER 40-54 (Enichem): elastomeric terpolymer ethylene / propylene / 5-ethylidene-2-norbornene;

Black N472: furnace carbon black;

Ketjenblack® EC (Akzo Chemie): high conductivity furnace carbon black;

Hydrocerol® CF70: carboxylic acid / sodium bicarbonate blowing agent (Boeheringer Ingelheim).

EXAMPLE 6

Using the polymeric composition of Example 4, a medium voltage cable was made according to the construction scheme shown in Fig. 1.

The core of the cable on which to carry out the deposition of the expanded semiconductive layer was constituted by an aluminum conductor with a section of 70 mm2, coated with the following layers cross-linked with peroxide on the catenary line:

- an internal semiconductive EPR layer (0.5 mm thick);

- an EPR insulating layer (5.5 mm thick);

- an external (compact) semiconductive layer in ÈVA containing 35% by weight of carbon black N472 (thickness 0.5 mm).

On this core (having an external diameter of about 23 mm) we proceeded to the deposition of the expanded semiconductive layer containing the expanding powder, having the composition reported in Table 1 for Example 4. A single screw extruder 80 mm in configuration 25 was used D. The extruder was equipped with an initial stroke of the cylinder with longitudinal grooves, a box-type nozzle and a 25-length transfer thread screw D. The depth of the screw channel was equal to 9.6 mm in the feeding area and equal to 7.2 mm in the final section, for an overall compression ratio of the screw of approximately 1: 1.33.

Downstream of the extruder, an electrically heated orthogonal extrusion head was used, equipped with a double suture line conveyor. The following assembly of the molds was used: male with 24 mm diameter, compression female with 24 mm diameter. The male mold was chosen with the aim of allowing an easy passage for the core to be coated, with a diameter greater than about 1 mm compared to the diameter of the core to be coated. The female mold was instead chosen with a diameter slightly smaller than the final diameter to be obtained, in such a way as to prevent the expansion of the material from taking place inside the extrusion head.

The following thermal profile (° C) was used for the extruder and the extrusion head:

The advancement speed of the core to be coated was set according to the desired thickness of foam material. In our case, a line speed of 1.2 m / min was used. Under these conditions, the following extrusion parameters were recorded:

Extruder rotation speed: 1.2 rpm;

Hot semi-finished diameter: 25.0 mm;

Semi-finished cold-worked diameter: 24.8 mm.

The cooling of the semi-finished product was carried out in air. Direct contact with the cooling water was avoided to avoid accidental swelling problems of the buffering powder. Subsequently, the semi-finished product obtained was wound on a reel.

The material was deposited on the core with a thickness of about 1 mm. The expansion of the same was obtained chemically, by adding approximately 2% of the expanding agent Hydrocerol® CF 70 (carboxylic acid sodium bicarbonate) from Boehringer Ingelheim to the hopper.

On samples of the expanded semiconductive layer thus obtained, measurements of electrical conductivity and the degree of expansion were carried out. The degree of expansion measured is approximately 20%.

Furthermore, expansion tests were carried out on the material in the presence of water (buffering effect): the material swelled, thanks to the presence of the buffering powder, up to a volume of approximately 3 times the initial volume.

Claims (27)

CLAIMS 1. Electric cable comprising a conductor, at least one layer of insulating coating and an external metal screen, characterized in that said cable further comprises a layer of expanded polymeric material having semiconductive properties arranged below said metal screen. Cable according to claim 1, wherein the expanded semiconductive layer has a predetermined degree of expansion so as to ensure the elastic absorption of the radial forces of thermal expansion and contraction of the cable and to maintain the semiconductive properties. Cable according to claim 2, wherein the degree of expansion of the expanded semiconductive layer is comprised between 5% and 500%. 4. Cable according to claim 3, wherein the degree of expansion of the expanded semiconductive layer is comprised between 10% and 200%. Cable according to any one of the preceding claims, wherein the thickness of the expanded semiconductive layer is equal to at least 0.1 mm. 6. Cable according to claim 5, wherein the thickness of the expanded semiconductive layer is comprised between 0.2 and 2 mm, preferably between 0.3 and 1 mm. Cable according to any one of the preceding claims, further comprising a compact semiconductive layer disposed between the insulating coating and the expanded semiconductive layer. A cable according to claim 7, wherein the compact semiconductive layer has a thickness of between 0.1 and 1 mm. A cable according to claim 8, wherein the compact semiconductive layer has a thickness of between 0.2 and 0.5 mm. Cable according to any of the preceding claims, wherein the expanded semiconductive layer comprises a moisture-expanding material in the form of a powder. A cable according to claim 10, wherein the hygro-expanding material is a homopolymer or copolymer having hydrophilic groups along the polymer chain. Cable according to any one of the preceding claims, wherein the polymeric material which constitutes the expanded semiconductive layer is an expandable polymer selected from: polyolefins, copolymers of different olefins, olefin copolymers / unsaturated esters, polyesters, polycarbonates, polysulfones, phenolic resins, urea resins, and their mixtures. A cable according to claim 12, wherein the polymeric material is a polyolefin polymer or copolymer based on ethylene and / or propylene. 14. Cable according to claim 13, wherein the polymeric material is selected from: (a) copolymers of ethylene with an ethylenically unsaturated ester, in which the amount of unsaturated ester is generally comprised between 5 and 80% by weight, preferably between 10 and 50% by weight; (b) ethylene-propylene (EPR) or ethylene-propylene-diene (EPDM) elastomeric copolymers, having the following composition: 35-90 mol% ethylene, 10-65 mol% propylene, 0-10 mol% diene (e.g. 1,4-hexadiene or 5-ethylidene-2-norbornene); (c) polypropylene modified with EPR or EPDM elastomeric copolymers, where the weight ratio PP / EPR or PP / EPDM is comprised between 90/10 and 30/70, preferably between 80/20 and 40/60. Cable according to any one of the preceding claims, wherein the expanded semiconductive layer has a volumetric resistivity value of the expanded material at ambient temperature lower than 500 Ω-m, preferably lower than 20 Ω-m. A cable according to any one of the preceding claims, wherein the expanded semiconductive layer comprises a predetermined amount of electroconductive carbon black. A cable according to claim 16, wherein the electroconductive carbon black has a surface area of at least 20 m2 / g. A cable according to claim 17, wherein the carbon black has a surface area of at least 900 m2 / g. A cable according to any one of claims 16 to 18, wherein the carbon black is present in amounts ranging from 5 to 80%, preferably from 10 to 70%, by weight with respect to the weight of the polymer. Cable according to any one of the preceding claims, wherein the expanded semiconductive layer is obtained by extrusion. A cable according to claim 20, wherein expansion of the semiconductive layer is achieved during extrusion by adding a blowing agent. A cable according to claim 20, wherein expansion of the semiconductive layer is achieved during extrusion by high pressure gas injection. 23. Cable according to any one of the preceding claims, in which the diameter of the expanded semiconductive layer in the absence of applied forces is greater than the internal diameter of the metal screen, in such a way as to obtain, after application of the metal screen, a predetermined degree of pre-compression of the expanded semiconductive layer. 24. Method for controlling the electric field inside a cable in the presence of thermal cycles, said cable comprising a conductor, at least one layer of insulating coating and an external metal shield, characterized in that said method comprises elastically deforming a polymeric layer placed below said metal screen by a value lower than that which determines a detachment between said metal screen and a layer below it. A method according to claim 24, wherein the polymeric layer is an expanded polymeric layer. A method according to claim 25, wherein the expanded polymeric layer has semiconductive properties. 27. Method according to claim 25 or 26, in which the polymeric layer is in a state of elastic pre-compression such as to determine the absence of detachment between said metal screen and a layer underlying it in the presence of a predetermined plastic deformation of at least one layer underneath the metal screen or of said metal screen.