ES2178223T5 - CABLE WITH IMPACT RESISTANT COATING. - Google Patents

Abstract

The invention relates to a cable sheath that is capable of protecting the cable from accidental impacts. By inserting into the structure of an energy transmission cable a suitable coating of expanded polymeric material of a suitable thickness, preferably in contact with the polymeric coating cover, it is possible to obtain a cable having a high impact resistance. It has also been observed that an expanded polymeric material used as a cable coating makes it possible to obtain greater impact resistance for this cable than using a similar coating based on the same polymer that is not expanded. A cable with a coating of this type has different advantages over a traditional cable with metal reinforcement, such as, for example, easier processing, a reduction in the weight and dimensions of the finished cable and a lower environmental impact in which refers to the recycling of the cable once it has finished its work cycle.

Description

Cable with resistant coating impact.

The present invention relates to a cable sheath that is able to protect the cable from accidental impacts.

Accidental impacts on a cable, which they can happen, for example, during transport, laying, etc., they can cause a series of structural damage to the cable, which comprise the deformation of the insulating layer, separation of the insulating layer of the semiconductor layer, and the like; this damage may cause variations in the electrical gradient of the insulating coating, with a decrease in consequence of the insulating capacity of this coating.

In the cables that are currently commercially available, for example those used for medium or low voltage power transmission or distribution, it normally apply a layer of metal armor capable of resisting such impacts to protect cables from possible damage caused by accidental impacts. This armor can be in shape of ribbons or threads (usually of steel), or alternatively in the form of a metal cover (usually of lead or aluminum); this armor is, in turn, coated with a polymer outer shell. An example of said structure of Cable is described in US Patent 5,153,381.

The applicant has observed that the presence of the aforementioned metal armor has a certain number of inconveniences For example, the application of such armor comprises one or more additional phases in the elaboration of the cable. In addition, the presence of metal armor increases considerably the weight of the cable, besides presenting problems environmental since, if it is necessary to replace it, a cable Built in this way is not easy to remove.

The Japanese patent published under the number (Kokai) 7-320550 describes a household cable with a 0.2-1.4 mm impact resistant coating thickness, arranged between the insulator and the outer cover. This Impact resistant coating is a non-polymeric material expanded containing a polyurethane resin as a component principal.

On the other hand, the use of materials expanded polymers in cable construction is known For a variety of purposes.

For example, the German patent application number P 15 15 709 shows the use of an intermediate layer between the outer plastic cover and the metal cover internal cable, to increase the resistance of the cover Plastic exterior at low temperatures. None is done mention in said document on the protection of the structure internal cable with said intermediate layer. In fact, that layer intermediate should compensate for elastic stresses generated in the plastic outer sheath due to temperature drop and may consist of glass fibers arranged loosely or in a material that can expand or incorporates spheres of hollow glass

Another document, the German utility model number G 81 03 947.6, shows an electric cable for use in connections within devices and machines, which presents a particular mechanical strength and flexibility. Said cable is specifically designed to go through a pulley and is flexible enough to recover its straight structure after going through said pulley. Therefore, this type of cable is specifically aimed at resisting mechanical loads of static type (such as those generated during the passage through a pulley), and its main feature is flexibility. It does immediately apparent to persons skilled in the art that this type of cable differs substantially from the transmission or medium or low voltage power distribution that has a metal armor that, before being flexible, should be able to withstand dynamic loads due to impacts of a certain force over the cable

In addition, in signal transmission cables of coaxial or twisted pair type, the use of expanded materials to insulate a conductive metal. The cables Coaxial should normally carry high frequency signals, as in coaxial TV cables (CATV) (10-100 MHz), satellite signal cables (up to 2 GHz), coaxial cables for computers (above 1 MHz); telephone wires traditional normally carry signals with frequencies of approximately 800 Hz.

The purpose of using an expanded insulator in these cables is to increase the transmission speed of the electrical signals, to get close to the ideal speed of signal transmission in a metal conductor antenna (which is approaching At the speed of light). The reason for this is that, in comparison with non-expanded polymeric materials, expanded materials they generally have a lower dielectric constant (K), which is closer proportional to that of air (K = 1) the higher It is the degree of polymer expansion.

For example, the United States patent 4,711,811 describes a signal transmission cable that has as an insulator an expanded fluoropolymer (thickness of 0.05-0.76 mm) coated with a film of ethylene / tetrafluoroethylene copolymer or ethylene / chlorotrifluoroe-tylene (thickness of 0.013-0.254 mm). As described in that patent, The purpose of the expanded polymer is to insulate the conductor, while that the purpose of the unexpanded polymer film that coated expanded polymer is to improve mechanical properties of insulation, in particular by imparting resistance to the necessary compression when two insulated conductors are twisted to form the so-called "twisted pair".

EP 442,346 describes a cable of signal transmission with a polymer based insulating layer expanded, arranged directly around the conductor; East expanded polymer has an ultramicrocellular structure with a empty volume greater than 75% (corresponding to a degree of expansion greater than 300%). The ultramicrocellular structure of this polymer it should be such that at least 10% is compressed under a load 6.89 x 10 4 Pa and recover at least 50% of its volume initial after removing the load; these values correspond approximately the typical compressive strength you need have the material to resist compression during braiding of the cables.

In the international patent application WO 93/15512, which also refers to a signal transmission cable with an expanded insulating layer, it is claimed that by means of coat the expanded insulation with a layer of polymeric insulation non-expanded thermoplastic (as described, for example, in the U.S. Patent 4,711,811 mentioned above) is obtains the required compressive strength, reducing this however the speed of propagation of the signal. That request WO 93/15512 describes a coaxial cable with a double layer insulating coating, where both layers consist of a expanded polymeric material, the inner layer consisting of microporous polytetrafluoroethylene (PTFE) and consisting of the layer exterior in an expanded closed cell polymer, specifically perfluoroalkyl oxytetra-fluoroethylene polymers (PFA). The expanded polymer based insulating coating is obtained by extrusion of the PFA polymer on the inner layer of PTFE insulator, injecting Freon gas 113 as an expanding agent. According the details given in the description, this expanded insulator closed cell makes it possible to maintain a high speed of signal transmission Also in that patent application it is defined which is resistant to compression, although no numerical data is given referring to this compressive strength. The description highlights the fact that conductors coated with an insulator Double layer of this type can be braided. Also, according to that patent application, the increase in empty volume in the layer expanded exterior makes it possible to obtain a speed increase of transmission, resulting in the appearance of small variations of the capacity of this coating that oppose compression of the expanded inner layer.

As seen from the documents mentioned above, the main purpose when using "open cell" expanded polymer materials such as insulating coatings for signal transmission cables is increase the transmission speed of the electrical signal; without However, these expanded coatings have the drawback of having insufficient compressive strength. Some expanded materials are also defined generically as "compression resistant", since they must guarantee no only a high signal transmission rate but also sufficient resistance to compression forces that typically generated when two conductors are braided together coated with the aforementioned expanded insulation; therefore, also in this case, the applied load is substantially static type.

Therefore, while on the one hand it is necessary that these insulating materials made with polymeric material expanded for signal transmission cables have characteristics that allow them to withstand a relatively compressive load modest (like the one that appears when two wires are braided together), on the other hand it is not mentioned in any document known by the applicant any kind of impact resistance that can provide expanded polymer coating. In addition, though an expanded insulating coating of this type favors a higher signal transmission rate, this is considered less advantageous that a coating made of a similar material does not expanded in terms of compressive strength, as indicated in the aforementioned patent application WO 93/15512.

A signal transmission cable is mentioned in DE 7122512. A buffer layer made of polymer material foamed, such as high intensity polyethylene, low density polyethylene, protects the core of said cable against 30 impacts. The polymer, material that expands from 30 to 40%

The applicant has found that through insert into the structure of a transmission cable of enhances a suitable coating made of polymeric material expanded thickness and flexural modulus appropriate, preferably in contact with the polymer outer shell cover, it is possible to obtain a cable with high impact resistance, making it possible to avoid the use of armor metal previously mentioned in the structure of this cable. In specifically, the applicant has observed that the polymeric material should be selected to have a flex module high enough, measured before its expansion, to achieve desired properties of impact resistance and avoid possible damage to the internal cable structure due to impacts not desired on the outer surface thereof. In the description present, the term "impact" is used to encompass all the dynamic loads of a certain energy capable of producing substantial damage to the structure of conventional cables without armor, while having negligible effects on the Cable structure with conventional reinforcement. As an indication, such an impact can be considered an impact of approximately 20-30 joules produced by a V-shaped punch rounded edge, with a radius of curvature of approximately 1 mm, on the outer cover of the cable.

The applicant has also observed that, surprisingly, an expanded polymeric material used as cable sheath according to the invention makes it possible to obtain an impact resistance that is better than the one obtained using a similar coating based on the same polymer not expanded.

A cable with such a coating It has numerous advantages over a conventional cable with metal armor, for example, easier manufacturing, reduction of the weight and dimensions of the finished cable and a reduced environmental impact in terms of recycling of cable once its useful life is over.

The present invention relates to a cable power transmission as set forth in the claim one.

The expanded polymeric material is obtained at from a polymeric material that presents, before the expansion, a flexural modulus at room temperature, measured according to ASTM D790 standard, greater than 200 MPa, preferably between 400 MPa and 1500 MPa, with values between 600 MPa being particularly preferred and 1300 MPa.

Said polymeric material has a degree of expanded polymeric material expansion, the layer consisting internal microporous polytetrafluoroethylene (PTFE) and the external from about 30% to about 500%, with preference particularly a degree of expansion between approximately 50% up to about 200%

According to a preferred embodiment of the present invention, the coating of expanded polymeric material has a thickness of at least 0.5 mm, preferably between 1 and 6 mm, specifically between 2 and 4 mm. According to a preferred aspect of the present invention, this expanded polymeric material is chosen between polyethylene (PE), low density PE (LDPE), density PE medium (MDPE), high density PE (HDPE) and linear low PE density (LLDPE); polypropylene (PP); gum of ethylene-propylene (EPR), copolymer of ethylene-propylene (EPM), terpolymer of ethylene-propylene-diene (EPDM); rubber natural; butyl rubber; ethylene / vinyl acetate copolymer (EVE); polystyrene; ethylene / acrylate copolymer, copolymer of ethylene / methyl acrylate (EMA), acrylate copolymer of ethylene / ethyl (EEA), ethylene / butyl acrylate copolymer (EBA); ethylene / α-olefin copolymer; resins of Acrylonitrile Butadiene Stirene (ABS); halogenated polymer, polyvinyl chloride (PVC); polyurethane (PUR); polyamide; aromatic polyester, terephthalate polyethylene (PET), polybutylene terephthalate (PBT); and copolymers or mechanical mixtures thereof.

According to another preferred aspect, this material polymeric is a PE based polyolefin polymer or copolymer and / or PP, preferably modified with rubber ethylene-propylene, in which the weight ratio PP / EPR is between 90/10 and 50/50, preferably between 85/15 and 60/40, specifically about 70/30.

According to another preferred aspect, this polymer or PE and / or PP based polyolefin copolymer contains an amount Default vulcanized rubber in powder form, preferably between 10% and 60% of the polymer weight.

According to another preferred aspect, this cable it also comprises an outer polymer shell, which is located preferably in contact with the polymer coating expanded, this cover preferably having a thickness of at least 0.5 mm, preferably between 1 and 5 mm.

In the present description, the term "degree polymer expansion "is understood to refer to the polymer expansion determined as follows:

G (degree of expansion) = (d_ {0} / d_ {e} -1) \ cdot 100

where d_ {0} indicates the density of the unexpanded polymer (i.e. the polymer with a structure essentially free of empty volume) and d e indicates the density apparent measure for the polymer expanded.

For the purpose of this description, the term "expanded" polymer is understood to refer to a polymer within whose structure the percentage of empty volume (or be the space not occupied by the polymer but by a gas or air) is typically greater than 10% of the total volume of this polymer.

In the present description, the term drag resistance is understood to refer to the force required to separate (peel) a layer of conductor coating or other coating layer; at case of separation of two layers of coating from each other, these layers are typically the insulating layer and the layer external semiconductor.

Typically, the insulating layer of the cables Power transmission has a dielectric constant (K) greater than 2. In addition, in contrast to the transmission cables of signal, in which the "electrical gradient" parameter does not assumes no importance, in power transmission cables electrical gradients between approximately 0.5 apply kV / mm for low voltage, up to approximately 10 kV / mm for high tension; therefore, in these cables, the presence of inhomogeneity in the insulating coating (eg empty volumes), which could cause a local variation of dielectric strength with a consequent decrease in insulation capacity tends to avoid. This insulating material will therefore typically be a compact polymeric material, where, in the present description, the "compact" insulating term is understood to refer to a insulating material that has a dielectric strength of at least less 5 kV / mm, preferably greater than 10 kV / mm, specifically greater that 40 kV / mm for power transmission cables of medium-high voltage In contrast to a material expanded polymer, this compact material is substantially free of empty volume inside its structure; specifically, this material will have a density of 0.85 g / cm3 or greater.

In the present description, the term falls voltage is understood to refer to a voltage up to 1000 V (typically greater than 100 V), the term medium voltage is understood as which refers to a voltage from about 1 to approximately 30 kV and the term high voltage is understood to be refers to a voltage above 30 kV. These cables power transmission typically work at frequencies 50 or 60 Hz nominal.

Although, in the course of the description, the use of the coating is illustrated in detail of expanded polymer with reference to transmission cables of power, in which this coating can replace advantageously the metal armor that is currently used in said cables, it is clear to those skilled in the art that this expanded coating can be used advantageously in any type of cable to which protection is desired adequate against impacts. Specifically, the definition of cables power transmission comprises not only those that are specifically of the low and medium voltage type but also cables for high voltage power transmission.

The present invention can be better understood. With the help of the following figures:

Figure 1 shows a transmission cable of power according to the state of the art, of the three-pole type with metal armor

Figure 2 shows a first embodiment of a tripolar type cable according to the present invention.

Figure 3 shows a second embodiment of a unipolar type cable according to the present invention.

Figure 1 is the sectional diagram cross section of a medium voltage power transmission cable according to the state of the art, of the tripolar type with armor metallic This cable comprises three conductors (1), each Coated with an internal semiconductor coating (2), one layer insulator (3), an external semiconductor layer (4) and a screen metallic (5); for reasons of simplicity, this structure semi-finished will be referred to in the rest of the description as the "nucleus". The three cores are wired together and the zones in Star shape between them are filled with a filler material (9) (usually elastomeric mixtures, polypropylene fibers and similar) to make the circular transverse structure, covering the set in turn with an internal cover of polymer (8), a metal wire armor (7) and a cover of external polymer (6).

Figure 2 is the section diagram cross section of a cable according to the present invention, also of the 3-pole type for medium voltage power transmission. This cable comprises the three conductors (1), each coated with a internal semiconductor coating (2), an insulating layer (3), a external semiconductor layer (4), and a metal screen (5); the Star-shaped areas between the nuclei are filled in this case with an expanded polymeric material (10) impact resistant which, in turn, is coated with a polymer outer shell (6). In the expanded polymer coating (10) it is also indicates (by means of a dotted line) a circular border (10a) corresponding to the minimum thickness of the polymer coating expanded, near the outer surface of the nuclei.

Figure 3 is the section diagram cross section of a cable according to the present invention, of type unipolar for medium voltage power transmission. This cable comprises a central conductor (1), coated with a coating internal semiconductor (2), an insulating layer (3), an outer layer semiconductor (4), a metal screen (5), a layer of material expanded polymer (10) and an outer polymer shell (6). In the case of this unipolar cable shown in Figure 3, since the core has a circular cross section, the circular edge (10a) indicated in the case of the three-pole cable coincides  with the expanded polymeric material layer (10).

These figures only obviously show some few of the possible embodiments of cables in which You can advantageously use the present invention. It is clear that they can make suitable modifications known in the art to these embodiments without limiting the application of this invention involved. For example, in reference to Figure 2, the Star-shaped areas between the nuclei can be filled with beforehand with a conventional filling material, obtaining from this forms a semi-finished cable of corresponding cross section approximately to the cross section contained within the edge circular (10a); then it is possible advantageously to extrude the layer of expanded polymeric material (10) on this cable semi-finished, with a thickness corresponding to approximately circular edge (10a), and subsequently the outer cover (6). Alternatively, cores can be provided with a sector of cross section such that when these cores come together it forms a cable of approximately circular cross section, without need to use the filling material for shaped areas of star; then it is extruded over these cores put together the layer of expanded polymeric material (10) impact resistant, followed by the outer cover (6).

In the case of cables for the transmission of low voltage power, the structure of these cables will comprise normally the only insulating coating directly arranged in contact with the driver, who in turn dresses with the coating of expanded polymeric material and with the cover Exterior.

Other solutions are well known for the person skilled in the art, who can evaluate the solution more convenient, based on, for example, costs, type of laying of the cable (aerial, inserted in pipes, buried directly in the ground, inside buildings and under the sea), the temperature of cable operation (maximum and minimum temperature, and margins of ambient temperature).

The expanded polymer coating impact resistant can consist of any type of polymer expandable such as polyolefins, copolymers of polyolefin, olefin / ester copolymers, polyesters, polycarbonates, polysulfones, phenolic resins, ureic resins and mixtures thereof. Examples of suitable polymers are polyethylene (PE), specifically low density PE (LDPE), PE medium density (MDPE), high density PE (HDPE) and linear PE low density (LLDPE); polypropylene (PP); gum of ethylene-propylene (EPR), specifically copolymer of ethylene-propylene (EPM) or terpolymer of ethylene-propylene-diene (EPDM); natural rubber; butyl rubber; ethylene / vinyl acetate copolymer (EVE); polystyrene; ethylene / acrylate copolymer, specifically ethylene / methyl acrylate copolymer (EMA), copolymer of ethylene / ethyl acrylate (EEA), acrylate copolymer of ethylene / butyl (EBA); copolymer of ethylene / α-olefin; resins of Acrylonitrile Butadiene Stirene (ABS); halogenated polymers, in particular polyvinyl chloride (PVC); polyurethane (PUR); polyamides; aromatic polyesters, such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT); and copolymers or mechanical mixtures thereof. Preferably, polyolefin polymers or copolymers are used, specifically those based on PE and / or PP mixed with rubber ethylene-propylene. Advantageously, it can be used  rubber modified polypropylene ethylene-propylene (EPR), being the weight ratio PP / EPR between 90/10 and 50/50, preferably between 85/15 and 60/4, particularly preferred a weight ratio of approximately 70/30

According to another aspect of the present invention, the applicant has also observed that it is possible to mix mechanically the polymeric material that undergoes expansion, specifically in the case of olefin polymers, especially polyethylene or polypropylene, with a predetermined amount of rubber in powder form, for example vulcanized natural rubber.

Typically, these powders are formed from particles with sizes between 10 and 1000 µm, preferably between 300 and 600 µm. Advantageously, waste can be used of vulcanized rubber derived from the manufacture of tires. He percentage of rubber in powder form can vary from 10% to 60% by weight relative to the polymer to be expanded, preferably between 30% and 50%.

The polymeric material to expand, which is used without further processes or used as an expandable base in a mixture with powdered rubber, it should have a stiffness such that, once it has expanded, ensure a certain magnitude of desired impact resistance, to protect the inside of the cable (that is the insulating layer and the semiconductor layers that may exist) of damage subsequent to accidental impacts That may occur. Specifically, this material must have a ability to absorb sufficiently high impact energy as to transmit to the lower insulating layer a quantity of energy such that the insulating properties of the Interior linings beyond a predetermined value. The reason for this, as illustrated in greater detail in the description Next, is that the applicant has observed that in a cable subject to an impact, a difference is observed, between the average value and the value measured at the point of impact of the resistance to entrainment of the underlying insulating coatings; advantageously, this drag resistance can be measured between the insulating layer and the outer semiconductor layer. The difference in this resistance is proportionally greater the greater the impact energy transmitted to the underlying layers; if in which the drag resistance is measured between the insulating layer and the external semiconductor layer, it has been evaluated that the protective coating offers sufficient protection to internal layers when the difference in drag resistance in the point of impact, relative to the average value, is less than the 25%

The applicant has observed that a material polymeric chosen among those mentioned above is particularly suitable for this purpose, presenting this material, before expansion, a temperature flexural module environment greater than 200 MPa, preferably at least 400 MPa, measured according to ASTM D790 standard. On the other hand, since the Excessive stiffness of expanded material can make it difficult to handle the finished product, it is preferred to use a material polymeric that presents a flexural modulus at room temperature less than 2000 MPa. The polymeric materials that are particularly suitable for this purpose are those that present, before expansion, a temperature flexural module environment between 400 and 1800 MPa, specifically preferring a polymeric material with a flexural modulus at room temperature between 600 and 1500 MPa.

These flexural modulus values can be characteristic of a specific material or may result from the mixing of two or more materials with different modules, mixed in such a relationship that the desired stiffness value is obtained for the material. For example, polypropylene, which has a module of Flexion greater than 1500 MPa, can be appropriately modified with adequate amounts of ethylene-propylene rubber (EPR), with a module of approximately 100 MPa, for the purpose of Decrease its rigidity properly.

For example, polypropylene, which has a bending module greater than 1500 MPa, can be modified properly with adequate amounts of rubber ethylene-propylene (EPR), with a module approximately 100 MPa, in order to reduce its stiffness Correctly.

Examples of polymeric compounds available commercially they are:

Low density polyethylene: Riblene FL 30 (Enichem);

high density polyethylene: DGDK 3364 (Union Carbide);

polypropylene: PF 814 (Montell);

EPR modified polypropylene: Moplen EP-S 30R, 33R and 81R (Montell); Fina-Pro 5660G, 4660G, 2660S and 3660S (Fina-Pro).

The degree of polymer expansion and thickness of the coating layer should be such that it ensures, in combination with polymer outer shell, resistance to typical impacts that occur during handling and laying of cable.

As mentioned earlier, the "grade polymer expansion "is determined as follows shape:

G (degree of expansion) = (d_ {0} / d_ {e} - 1) \ cdot 100

where d_ {0} indicates the density of the unexpanded polymer and d e indicates the bulk density polymer measure expanded.

The applicant has observed that while maintain the desired impact resistance characteristic, to an equal thickness of the expanded layer, it is preferable to use a polymeric material with a high degree of expansion since, of In this way, it is possible to limit the amount of polymeric material used, with advantages in both economic and economic terms weight reduction of the finished product.

The degree of expansion is very variable, both in function of the specific polymeric material used as in function of the thickness of the coating to be used; in In general, this degree of expansion can be found from 30% to 500%, with a particular degree of expansion being preferred between 50% and 200%. The expanded polymer generally has a structure of closed cell

The applicant has observed that beyond one certain degree of expansion, decreases coating capacity Polymer provide the desired impact resistance. In specifically, it has been observed that the possibility of obtaining high degrees of polymer expansion while maintaining high efficiency in the impact protection can be correlated with the value of flexural modulus of the polymer to expand. The reason for this is that the applicant has observed that the material module polymer decreases when the degree of expansion of this increases material, approximately according to the following formula:

E_ {2} / E_ {1} = (\ rho_ {2} / \ rho_ {1}) 2

where:

E_ {2} represents the flexural modulus of the polymer in the high degree of expansion;

E_ {1} represents the flexural modulus of the polymer in the low degree of expansion;

\ rho_ {2} represents the apparent density of polymer in the high degree of expansion;

\ rho_ {1} represents the apparent density of polymer in the low expansion degree.

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As a guide, for a polymer with a module flexion of approximately 1000 MPa, a variation in the degree of expansion from approximately 20% to 100% entails approximately the decrease in half of the module value of flexion for the material. The polymeric materials they present a high flex module can therefore expand up to a greater degree than polymeric materials that have some low module values, without this representing a detriment to The coating's ability to withstand impacts.

Another variable that can influence the cable impact resistance is the thickness of the coating expanded; the minimum thickness that is able to ensure resistance the impact that is desired to obtain with said coating will depend mainly of the degree of expansion and the flexural modulus of this polymer. In general, the applicant has observed that, for the same polymer and for the same degree of expansion, it is possible achieve higher values of impact resistance through increase the thickness of the expanded coating. However, for the purpose of using a limited amount of material from coating, thus reducing both costs and dimensions of the finished product, the thickness of the layer of expanded material will advantageously be the minimum thickness required to ensure the desired impact resistance. Specifically for medium voltage cables, it has been observed that a thickness of expanded coating of approximately 2 mm is normally able to ensure sufficient impact resistance normal to which a cable of this type is exposed. Preferably, the thickness of the coating will be greater than 0.5 mm, specifically between about 1 mm and about 6 mm, with a thickness of between 2 mm and 4 mm being specifically preferred.

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The applicant has observed that it is possible define, up to a reasonable approximation, the relationship between the coating thickness and the degree of expansion of the material polymeric, for materials with various module values of bending, so that the thickness of the expanded coating is dimension properly as a function of the degree of expansion and module of the polymeric material, specifically for thicknesses of expanded coating of approximately 2-4 mm. This relationship can be expressed as follows:

V \ cdot d_ {e} \ geq N

where

V represents the volume of polymeric material expanded by linear meter of cable (m3 / m), this being volume relative to the circular edge defined by the minimum thickness of the expanded coating, corresponding to the circular edge (10a) of Figure 2 for multipolar cables, or to the sheathing (10) defined in figure 3 for unipolar cables;

d_ {e} represents the measured bulk density for the expanded polymeric material (kg / m3); Y

N is the result of the product of the two values mentioned above, which must be greater than or equal to:

0.03 for materials with a module> 1000 MPa,

0.04 for materials with a module 800-1000 MPa,

0.05 for materials with a module 400-800 MPa,

0.06 for materials with a module <400 MPa.

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Parameter V is related to thickness (S) of the expanded coating by means of the following relationship:

V = \ pi (2R_ {i} \ cdot S + S2)

where R_ {i} represents the radius inside the circular edge (10a).

The parameter d_ {e} is related to the degree of expansion of the polymeric material by means of the relation previous:

G = (d_ {0} / d_ {e} - 1) \ cdot 100

Based on the aforementioned relationship, for an expanded coating approximately 2 mm thick, arranged on a circular section of cable with a diameter of approximately 22 mm, for various materials with different bending modules at room temperature (Mf), it is found that This coating should have a minimum apparent density of approximately:

0.40 g / cm3 for LDPE (Mf of approximately 200);

0.33 g / cm3 for a 70/30 mixture of PP / EPR (Mf of about 800);

0.26 g / cm3 for HDPE (Mf of approximately 1000);

0.20 g / cm3 for PP (Mf of approximately 1500).

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These polymer bulk density values expanded correspond to a maximum degree of expansion of approximately:

130% for LDPE (d_ {0} = 0.923)

180% for the PP / EPR mixture (d_ {0} = 0.890)

260% for HDPE (d_ {0} = 0.945)

350% for PP (d_ {0} = 0.900)

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Similarly, for a coating thickness expanded approximately 3 mm arranged on a cable identical dimensions, the following values are obtained for the minimum apparent density:

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;

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corresponding to a maximum degree of expansion approximately:

270% for LDPE;

320% for the PP / EPR mixture;

460% for HDPE;

600% for PP.

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The results shown above indicate than to optimize the impact resistance characteristics of an expanded coating of a predetermined thickness, both characteristics of mechanical strength of the material (specifically its flexural modulus) as the degree of expansion of said material They should be taken into account. However, the values determined by applying the above relationship should not be considered as limiting the scope of the present invention. In concrete, the maximum degree of expansion of polymers that present flexural modulus values close to the maximum limit of the defined intervals for the variation of the number N (ie 400, 800 and 1000 MPa) may actually be even higher than calculated according to the previous relationship; therefore, for example, a PP / EPR layer approximately 2 mm thick (with Mf of approximately 800 MPa) can still provide protection to desired impact even with a degree of expansion of approximately 200%

The polymer normally expands during extrusion phase; This expansion can take place chemically, by adding a suitable "expander" compound, it is say a compound that is capable of generating a gas under conditions of defined temperature and pressure, or it can take place physically, by means of high pressure gas injection directly inside of the extrusion cylinder.

Examples of suitable chemical "expanders" they are azodicarboamide, mixtures of organic acids (for example acid citric) with carbonates and / or bicarbonates (for example bicarbonate sodium)

Examples of gases for injecting at high pressure inside the extrusion cylinder are nitrogen, carbon dioxide, low boiling air and hydrocarbons such as propane and butane.

The outer protective cover that covers the expanded polymer layer may conveniently be of the type normally used Materials that can be used for outer shell are polyethylene (PE), specifically PE density medium (MDPE) and high density PE (HDPE), polyvinyl chloride (PVC), mixtures of elastomers and the like. Preferably it Use MDPE or PVC. Typically, the polymeric material that forms This outer cover has a flexural modulus between about 400 and about 1200 MPa, preferably between about 600 MPa and about 1000 MPa.

The applicant has observed that the presence of the outer cover contributes to providing the liner with desired impact resistance characteristic, in combination with The expanded coating. Specifically, the applicant has observed that this contribution of the cover to the resistance to impact, for the same expanded coating thickness, increases by increasing the degree of expansion of the polymer that forms this expanded coating. The thickness of this outer cover is preferably greater than 0.5 mm, specifically between 1 and 5 mm, preferably between 2 and 4 mm.

Preparing a cable with a resistor the impact according to the present invention is described in reference to the cable structure diagram of figure 2, in which, without However, the star-shaped spaces between the nuclei to coating is filled, not directly with the expanded polymer (10) but with a conventional filling; then the lining expanded is extruded over this semi-finished cable, to form a circular edge (10a) around this semi-finished cable and it subsequently coated with the outer polymer cover (2). The preparation of the cable cores, that is the assembly of the conductor (4), inner semiconductor layer (9), insulator (5), layer external semiconductor (8) and metal screen (4) is made of the way as is known within the art, for example by means Extrusion These cores are then wired together and the Star-shaped spaces are filled with a filler material conventional (for example elastomeric mixtures, fibers of polypropylene and the like), typically by extrusion of the padding over the wired cores, to obtain a cable semi-finished with a circular cross section. He expanded polymer coating (10) is extruded to continuation on the filling material. Preferably, the extrusion head nozzle will have a diameter slightly smaller than the final diameter of the expanded coated cable, to allow the polymer to expand outside the extruder.

It has been observed that, under conditions of identical extrusion (such as screw rotation speed, extrusion line speed, extruder head diameter and the like) extrusion temperature is one of the variables of the process that has a considerable influence on the degree of expansion. In general, for extrusion temperatures below 160 ° C, it is difficult to obtain a sufficient degree of expansion; the extrusion temperature is preferably at least 180 ° C, in concrete approximately 200 ° C. Normally, an increase in extrusion temperature corresponds to a degree of expansion higher.

In addition, it is possible to control to some extent the degree of expansion of the polymer by acting on the cooling rate since, doing properly more slow or faster the cooling of the polymer that forms the expanded coating at the extruder outlet, it is possible increase or decrease the degree of expansion of said polymer.

As mentioned, the applicant has observed that it is possible to quantitatively determine the effects of an impact on a cable sheath by measuring the drag resistance of the cable sheathing layers, evaluating the differences between the average value of this resistance  to drag and value at the point of impact. Specifically for medium voltage type cables, with a structure comprising an internal semiconductor layer, an insulating layer and a layer External semiconductor, drag resistance (and the difference relative) can be advantageously measured between the material layer External semiconductor and insulating layer.

The applicant has observed that the effects of the particularly severe impacts to which you may be subjected the cable, specifically a medium voltage cable with armor, is they can reproduce by means of an impact test based on the French standard HN 33-S-52, which is refers to cables with armor for high power transmission voltage, which provides an impact energy on the cable approximately 72 joules (J).

The drag resistance of the layer lining can be measured according to the French standard HN 33-S-52, according to which the force that needs to be applied to separate the semiconductor layer external insulating layer. The applicant has observed that by means of continuously measuring this force, at the points where the impact takes place, force peaks are measured that indicate a variation of the cohesive force between the two layers. It was observed that these variations are generally associated with the decreased insulating capacity of the coating. The variation will be proportionately greater the lower the impact force provided by the outer sheath (which, in the case of this invention, consists of expanded lining and cover Exterior). The magnitude of the variation of this force measured in the impact points, in relation to the average value measured along of the cable, therefore provides an indication of the degree of protection provided by the protective coating. In general, variations in drag resistance of up to 20-25% relative to the average value.

The characteristics of the expanded coating (material, degree of expansion, thickness), which can be used advantageously together with a protective outer cover of polymer, can be appropriately selected according to strength to the impact that is intended to provide the cable structure underlying, and also depending on the characteristics of the specific material that is used as an insulator and / or semiconductor, such as material hardness and density.

As can be seen through this description, the cable of the present invention is suitable particularly to replace cables with conventional reinforcement, due to the advantageous properties of the polymer coating expanded with respect to metal armor. However, their use should not be limited to such a specific application. In fact, the cable of the present invention can be used advantageously in all those applications in which it would be desirable a cable with impact resistance properties improved. Specifically, the impact-resistant cable of the present invention can replace conventional cables without armor in all those applications in which, until now, it would be advantageous to use armored cables but it has been dismissed due to the inconvenience of the metal armor.

Next, there are some examples illustrative to describe the invention in more detail.

Example one

Preparing the cable with expanded coating

To assess the impact resistance of a expanded polymer coating according to the present invention, is they prepared several test pieces extruding variable thicknesses of a few polymers with various degrees of expansion over a core composed of a multifilar conductor of approximately 14 mm thick coated with a 0.5 mm layer of material semiconductor, a 3 mm layer of an insulating mixture based on EPR and another 0.5 mm layer of semiconductor material "separation Easy "based on EVA supplemented with carbon black, for a total core thickness of approximately 22 mm.

It was used as a polymeric material to expand low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), a mechanical mixture at 70/30 by weight of LDPE and finely powdered vulcanized natural rubber (size of 300-600 µm particle (PE powder), PP modified with EPR rubber (PP-EPR mixed at 70/30 in weigh); These materials are identified in the following text by middle of letters A to E and are described in detail in the following table:

one

The polymer expanded chemically, using alternatively two different expansion compounds (CE), which They are identified as follows:

2

The polymer to expand and the expander compound they were loaded (in the relationships indicated in table 2) in a 80 mm - 25D single screw extruder (Flag); is extruder is equipped with a skewered transfer screw characterized by a depth in the final area of 9.6 mm. He extrusion system consists of a male nozzle capable of provide a smooth transit of the core to be coated (usually with a diameter that is approximately 0.5 mm larger than the diameter of the core to be coated), and a female nozzle in which the diameter It is chosen to have a size of approximately 2 mm less than the cable diameter with expanded coating; in this way, the Extruded material expands when exiting the extrusion head more than inside this head or inside the extruder. The transit speed of the core to be coated (line speed extrusion) is established as a function of the desired thickness of expanded material (see table 2). At a range of approximately 500 mm of the extrusion head is a cooling conduit (containing cold water) to stop the expansion and cool the extruded material. Then you Wind the cable over a coil.

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The composition of the material mixture polymeric / expander and extrusion conditions (speed, temperature) were varied appropriately, as described at continued in table 2.

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TABLE 2 Mix of expansion and conditions of extrusion

3

100

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Sample 1 did not undergo expansion, presumably because the extruder temperature was too much sample (165 ° C), and similarly, for the same reason, sample 5 It suffered a limited expansion (only 5%).

Subsequently the cable with the coating expanded was coated with a conventional MDPE cover (CE 90 - Materie Plastiche Bresciane) of variable thickness (see table 3) by means of conventional extrusion procedures, obtaining in this way cable samples with the characteristics that are defined in table 3; the number 1 cable, in which the polymer does not underwent expansion, it has been taken as a polymer coating not comparative expanded. Table 3 also gives, for reasons of comparison, the characteristics of a cable that lacks padding expanded and coated only with the cover external (cable number 0).

TABLE 3 Coating characteristics

4

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Similar to the one described above, it they prepared 6 more samples using a polymer coating expanded with a flexural module of approximately 600 MPa consisting of modified polypropylene with approximately 30% of an EPR gum, as presented in table 4 (examples 12-17); table 4 also gives two examples comparatives of cables with expanded coating but not they have the outer cover (examples 16a and 17a).

TABLE 4 Coating characteristics

6

Example 2

Impact resistance tests

To assess the impact resistance of cables prepared according to example 1, tests of impact on the cable with subsequent damage assessment. The Damage effects were assessed both by visual analysis of the cable as by measuring the variation in resistance to drag of the layer of semiconductor material at the point of impact. Impact tests are based on the standard French HN 33-S-52, which provides an impact energy on the cable of approximately 72 Joules (J), which is obtained by dropping a weight of 27 kg from a height 27 cm For the present test, said impact energy has been produced by a weight of 8 kg dropped from a height of 97 cm. The impact end of the weight has a percussive head with a rounded V-shaped edge (1 mm radius of curvature). For the purposes of the present invention, the impact resistance in a single impact. For samples 6-12, the test was repeated a second time at a distance of approximately 100 mm from the first.

Drag resistance was measured according to French standard HN 33-S-52, according which measures the force that needs to be applied to separate the external semiconductor layer of the insulating layer. By measuring continuously this force, the peaks of force in the points where the impact has taken place. For each piece of test, at the point of impact, a peak of force was measured "positive", corresponding to an increase in strength (in relationship with the average value) that is required to separate the two layers, and a peak of "negative" force (decrease relative to middle value). From the difference between the minimum (Fmin) and the maximum (Fmax) of the measured force peaks, the maximum variation of drag resistance at the point of impact.

The variation of drag resistance is calculates therefore determining the percentage ratio between the difference mentioned above (Fmax-Fmin) and the value medium drag resistance measured for the cable (F <>), according to the following relation:

% variation = 100 (Fmax-Fmin) / F <>

The magnitude of the variation of this resistance measured at the points of impact, relative to the average value measured at along the cable, therefore gives an indication of the degree of protection provided by expanded coating. In Overall, variations of up to 20-25% are considered acceptable. Table 5 gives the values of the variation of the drag resistance for samples 0-17a.

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TABLE 5 % variation of resistance to drag

7

8

As seen in table 3, for sample 1 (for which no expansion was obtained), the variation in percentage of drag resistance is extremely high; this indicates that an unexpanded polymer has a capacity decidedly less of absorbing impacts than an identical layer thickness of the same expanded polymer (see example 3, with 61% expanded coating). Sample 3 presents a slightly varying drag resistance above the limit value of 25%; impact resistance Limited provided by the sample can be attributed mainly to the thickness of only 1 mm of the coating expanded, in relation to the thicknesses of 2-3 mm of The other samples.

Sample 5, with a coating thickness expanded by 3 mm, it has a high resistance value to drag due to the low degree of polymer expansion (5%), thus demonstrating the limited impact resistance it provides a coating with a low degree of expansion. Sample 4, although it has an expanded material thickness less than that of the Sample 5 (2.5 mm as opposed to 3 mm), has despite all greater impact resistance, with a variation of the 13% drag resistance compared to 21% of the sample 5, demonstrating in this way the fact that a degree of expansion Higher provides greater impact resistance.

Comparing sample 13 with sample 15, observe how an increase in the degree of polymer expansion (from 22 to 124%), for the same thickness of the layer of expanded material and of the outer cover, leads to an increase in resistance to coating impact (from 16-17% to 10% of variation of drag resistance). This trend looks confirmed by comparing sample 16 with sample 17. However, comparing samples 16a and 17a (without external cover) with the respective samples 16 and 17, you can see how the contribution provided by the outer cover to protection in the face of the impact it increases as the degree of expansion increases.

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Example 3

Impact resistance comparison test with cable with armor

Cable number 10 has been tested against a cable with conventional reinforcement, to verify the efficiency of the impact resistance of the expanded coating layer.

The armored cable has the same core as cable number 10 (that is a multi-conductor conductor of approximately  14 mm thick coated with a 0.5 mm layer of material semiconductor, a 3 mm layer of an insulating mixture based on EPR and another 0.5 mm layer of semiconductor material "separation Easy "based on EVA supplemented with carbon black, for a total core thickness of approximately 22 mm). Said core is Wrap, from the inside to the outside of the cable with:

a) a layer of filler material based on rubber about 0.6 mm thick;

b) a PVC cover of approximately 0.6 mm thick;

c) two steel bands of armor approximately 0.5 mm thick each;

d) an outer cover of MDPE of approximately 2 mm thick.

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For the comparison test it has been used a dynamic machine of the "weight loss" type (CEAST, model  6758). Two sets of tests have been performed, dropping a 11 kg weight from a height of 50 cm (impact energy of approximately 54 joules) and 20 cm (impact energy of approximately 21 joules), respectively; the weight has in its impact end of a hemispherical head of approximately 10 mm radius

The resulting cable deformation is shown in figures 4 and 5 (50 cm and 20 cm high, respectively), in which the cable according to the present invention indicated by means of a), while the cable with armor Conventional is indicated by b).

Core deformation has been measured, to evaluate the damage to the cable structure. In fact, great cover deformations semiconductor-insulator-semiconductor most likely cause electrical defects in the properties cable insulators. The results are presented in table 6.

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TABLE 6 % reduction of the thickness of the semiconductor layer after impact

9

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As seen from the results that presented in table 6, the cable of the present invention shows even better impact resistance behavior than a cable with conventional armor.

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References cited in the description

This list of references cited by the applicant is intended solely to help the reader and not It is part of the European patent document. Although it has been put maximum care in its realization, errors cannot be excluded or omissions and the EPO declines any responsibility in this respect.

Patent documents cited in the description

US 5153381 A [0003]

DE 1515709 [0007]

DE 8103947 [0008]

US 4711811 A [0011] [0013]

EP 442346 A [0012]

WO 9315512 A [0013] [0013] [0015]

DE 7122512 [0016]

Claims (19)

1. A power transmission cable that understands: a) a driver (1); b) at least one layer of insulating coating compact (3) with an electric gradient from approximately 0.5kV / mm to approximately 10kV / mm placed around said (1); Y c) a coating (10, 10a) made of material polymer expanded around said insulating coating compact (3), wherein said polymeric material, before the expansion, it has a modulus of flexion at higher room temperature than 200 MPa, measured according to the ASTM D790 standard, and a degree of expansion from approximately 30% to 500%. 2. Cable as claimed in the claim 1, wherein said flexural module is between 400 MPa and 1800 MPa 3. Cable as claimed in the claim 1, wherein said flexural module is between 600 MPa and 1500 MPa 4. Cable as claimed in the claim 1, in which the degree of expansion of said polymeric material is It is between about 50% and about 200%. 5. Cable as claimed in any of the preceding claims 1 to 4, wherein said coating of Expanded polymeric material has a thickness of 0.5 mm. 6. Cable as claimed in any of the preceding claims 1 to 4, wherein said coating of expanded polymeric material have a thickness between 1 and 6 mm 7. Cable as claimed in any of the preceding claims 1 to 4, wherein said coating of Expanded polymeric material has a thickness between 2 and 4 mm. 8. Cable as claimed in the claim 1, wherein said expanded polymeric material is Choose between polyethylene (PE), low density PE (LDPE), PE medium density (MDPE), high density PE (HDPE) and linear PE low density (LLDPE); polypropylene (PP); gum of ethylene-propylene (EPR), copolymer of ethylene-propylene (EPM), terpolymer of ethylene-propylene-diene (EPDM); rubber natural; butyl rubber; ethylene / vinyl acetate copolymer (EVE); polystyrene; ethylene / acrylate copolymer, copolymer of ethylene / methyl acrylate (EMA), acrylate copolymer of ethylene / ethyl (EEA), ethylene / butyl acrylate copolymer (EBA); ethylene / α-olefin copolymer; resins of Acrylonitrile Butadiene Stirene (ABS); halogenated polymer, polyvinyl chloride (PVC); polyurethane (PUR); polyamide; aromatic polyester, terephthalate polyethylene (PET), polybutylene terephthalate (PBT); and copolymers or mechanical mixtures thereof. 9. Cable as claimed in the claim 1, wherein said expanded polymeric material is a polymer of polyolefin or copolymer based on PE and / or PP. 10. Cable as claimed in the claim 1, wherein said expanded polymeric material is a polyolefin polymer or compolymer based on PE and / or PE modified with ethylene-propylene rubber. 11. Cable as claimed in the claim 10, wherein said expanded polymeric material is rubber modified polypropylene ethylene-propylene (EPR), the weight ratio being PP / EPR between 90/10 and 50/50. 12. Cable as claimed in the claim 11, wherein said weight ratio of PP / EPR is It is between 85/15 and 60/40. 13. Cable as claimed in the claim 11, wherein said weight ratio of PP / EPR is approximately 70/30. 14. Cable as claimed in the claim 11, wherein said polyolefin polymer or PE and / or PP based copolymer also contains an amount Default rubber vulcanized powder. 15. Cable as claimed in the claim 14, wherein the predetermined amount of rubber Vulcanized in powder form is between 10% and 60% of the weight of polymer. 16. Cable as claimed in any of the preceding claims 1 to 15, wherein said cable It comprises an outer polymer shell. 17. Cable as claimed in the claim 16, wherein said cover is in contact with said expanded polymer coating. 18. Cable as claimed in claim 16 or 17, wherein said cover has a thickness greater than
0.5 mm 19. Cable as claimed in claim 16 or 17, wherein said cover has a thickness of between 1 and
5 mm