HK1029215A - Cable with impact-resistant coating - Google Patents

Description

Cable with impact-resistant coating

The present invention relates to a coating for cables, which is capable of protecting the cable from accidental impacts.

Accidental impacts, such as those occurring during cable transportation, laying, etc., may cause a series of damages to the structure of the cable, including deformation of the insulating layer, separation of the insulating and semiconductive layers, etc.; such damage may result in a change in the electrical gradient of the insulating layer, with the result that the insulating capacity of the layer is reduced.

In the case of cables currently commercially available, such as those used for low or medium voltage transmission or distribution, it is common to coat the cables with a metal armor capable of withstanding accidental impacts in order to protect them from possible damage caused by such impacts. Such armor may be in the form of a tape or wire (typically made of steel), or in the form of a metal sheath (typically made of lead or aluminum); this armor, in turn, is surrounded by an outer polymer jacket. An example of a cable of this construction is described in US patent 5,153,381.

The presence of the above-mentioned metal armor has been observed by the applicant to have a number of drawbacks. For example, the coating of the armor may be performed in one or more additional stages of cable processing. Furthermore, the presence of metal armor adds significant weight to the cable in addition to creating environmental problems, and it is not easy to dispose of a cable constructed in this manner if it is necessary to change the cable.

JP patent publication No. 7-320550 describes a household electric cable having an impact-resistant coating 0.2-1.4mm thick, which is placed between an insulator and an outer sheath. The impact resistant coating is a non-foamed polymer containing a polyurethane resin as a main component.

On the other hand, it is known to use foamed polymer materials in the construction of cables for various purposes.

For example, DE patent application p 1515709 discloses the use of an intermediate layer between the outer plastic sheath and the inner metal sheath in order to increase the low temperature resistance of the outer plastic sheath. The document is silent about protecting the internal structure of the cable with said intermediate layer. In fact, such an intermediate layer should compensate for the elastic tension in the outer plastic sheath due to the temperature reduction and may consist of loosely arranged glass fibers or of a material that may be foamed or incorporated into hollow glass spheres.

Another DE utility model G8103947.6 discloses a cable for internal connection of equipment and machines, which has particular mechanical resistance and flexibility. The cable is specifically designed to pass over the pulley and is flexible enough to return to its straight configuration after passing over the pulley. This type of cable is therefore dedicated to mechanical loads of the antistatic type (as generated when passing over pulleys) and is mainly characterized by flexibility. It is readily apparent to those skilled in the art that this type of cable is substantially different from low or medium voltage transmission or distribution cables with metal armor, which are not flexible but should be able to withstand dynamic loads caused by impacts of certain strengths on the cable.

In addition, in a coaxial or twisted pair type signal transmission cable, it is known that a foamed material may be used for insulating conductive metal. Coaxial cables are typically used to carry high frequency signals, such as coaxial cable for TV (CATV) (10-100MHz), satellite cable (up to 2GHz), computer coaxial cable (above 1 MHz); conventional telephone cables typically carry signals at a frequency of about 800 Hz.

The purpose of using foamed insulation in such cables is to increase the speed of transmission of electrical signals in order to achieve the desired signal transmission speed (approaching the speed of light) in air-conducting metal. The reason for this is that foamed materials generally have a lower dielectric constant (K) than non-foamed polymer materials, and the greater the degree of foaming of the polymer, the more proportionately the dielectric constant approaches that of air (K ═ 1).

For example, US patent 4,711,811 describes a signal transmission cable having a foamed fluoropolymer as the insulation (0.05 to 0.76mm in thickness) coated with a film of ethylene/tetrafluoroethylene or ethylene/chlorotrifluoroethylene copolymer (0.013 to 0.254mm in thickness). As described in this patent, the purpose of the foamed polymer is to insulate the conductors, and the purpose of the unfoamed polymer film, which is coated with the foamed polymer, is to improve the mechanical properties of the insulation, in particular by imparting the necessary compressive strength when twisting the two insulated conductors into a so-called "twisted pair".

EP patent 442,346 describes a signal transmission cable having an insulating layer based on foamed polymer, said insulating layer being placed directly around the conductor; the foamed polymer has an ultramicropore structure with a void volume of more than 75% (corresponding to a degree of foaming of more than 300%). The polymer should have an ultramicropore structure of 6.89X 10⁴Compressed by at least 10% under Pa load and capable of recovering at least 50% of the original volume after removal of the load; these values correspond approximately to the typical compressive strength values that the material must have in order to resist compression when twisting the cable.

International patent application WO93/15512 also relates to a signal transmission cable having a foamed polymer insulation layer, and it is stated that the required compressive strength is obtained by coating the foamed insulation (as described in US4,711,811 above) with a non-foamed insulating thermoplastic polymer, which however reduces the propagation speed of the signal. Said patent application WO93/15512 describes a coaxial cable with a double insulating layer, in which both layers consist of expanded polymer material, the inner layer consisting of microporous Polytetrafluoroethylene (PTFE) and the outer layer consisting of a closed-cell expanded polymer, in particular perfluoroalkoxy tetrafluoroethylene (PFA) polymer. This expanded polymer based insulation coating is obtained by extruding PFA polymer out of the inner layer of PTFE insulation, injecting Freon 113 gas as a blowing agent. According to the detailed description given in the specification, the closed-cell foamed insulator makes it possible to maintain a high-speed transmission signal. It is also clear in this patent application that it is compression resistant, although no numerical data on the compressive strength is given. It is emphasized in the description that the conductor surrounded by such a double insulation can be twisted and that an increase in the porosity in the outer foamed layer can lead to an increase in the speed of conveyance, whereby the resistance of the covering against the compression of the inner foamed layer is not greatly changed.

As can be seen from the above documents, the main purpose of using "open-cell" foamed polymer materials as insulating layers for signal transmission cables is to increase the transmission speed of electrical signals; however, these foamed coatings have the disadvantage of having an insufficient compressive strength. A few foamed materials are also defined "compression-resistant" in the upper part, because they not only ensure high-speed signal transmission, but also are sufficiently resistant to the compressive forces typically generated when two conductors coated with the above-mentioned foamed insulating layer are twisted together; thus, also in this case, the applied load is of a substantially static type.

Thus, on the one hand, these insulating layers for signal transmission cables made of foamed polymer material must have the characteristic that they are able to withstand relatively moderate compressive loads (such as those which occur when two cables are twisted together), and on the other hand, nothing in any document known to the applicant is mentioned of providing any type of impact strength by means of a foamed polymer coating. Furthermore, while such foamed insulating coatings promote higher speed signal transmission, as is believed in the above-mentioned patent application WO93/15512, they are less advantageous in compressive strength than coatings made from similar non-foamed materials.

It has now been found by the applicant that by inserting in the structure of an electric transmission cable a suitable coating made of foamed polymeric material, having a suitable thickness and flexural modulus, preferably in contact with a sheath of outer polymeric coating, it is possible to obtain a cable having a high impact strength, whereby the use of the above-mentioned protective metal armor in the cable structure can be avoided. In particular, the applicant has noticed that the polymeric material should be chosen so that it has a sufficiently high flexural modulus (measured before its foaming) to achieve the desired impact resistance and to avoid possible damages of the internal structure of the cable due to undesired impacts on its external surface. In this specification, the term "impact" is meant to include all dynamic loads of a certain energy that can cause substantial damage to a conventional unarmored cable structure, but have negligible effect on a conventional armored cable structure. By way of illustration, such an impact can be considered to be an impact of about 20-30 joules on the cable jacket produced by a V-shaped round edge punch (with a radius of curvature of about 1 mm).

The applicant has also noted, surprisingly, that the use of a foamed polymeric material as a coating for the cables according to the invention gives rise to impact strengths which are better than those obtained with a similar coating based on the same polymer, not foamed.

Various advantages of cables having this type of coating over conventional cables with metal armor are, for example, easier processing, reduced weight and size of the final finished cable, and reduced environmental impact on cable recycling once the work cycle is over.

One aspect of the invention relates to an electrical power transmission cable, the cable comprising:

a) a conductor;

b) at least one dense insulating coating;

c) a coating layer made of a foamed polymer material, wherein said polymer material has predetermined mechanical strength properties and a predetermined degree of foaming, so as to provide said cable with impact resistance properties.

According to a preferred aspect of the invention, the foamed polymeric material is obtained from a polymeric material having a flexural modulus at room temperature, measured according to ASTM standard D790, of more than 200MPa, preferably from 400MPa to 1500MPa, particularly preferably from 600MPa to 1300MPa, before foaming.

According to a preferred aspect of the invention, the polymeric material has a degree of foaming of from about 20% to about 3000%, preferably from about 30% to about 500%, and particularly preferably a degree of foaming of from about 50% to about 200%.

According to a preferred embodiment of the invention, the thickness of the coating of foamed polymeric material is at least 0.5mm, preferably from 1 to 6mm, in particular from 2 to 4 mm. According to a preferred aspect of the invention, the foamed polymeric material is selected from Polyethylene (PE), low density PE (ldpe), medium density PE (mdpe), high density PE (hdpe), and linear low 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) copolymers; polystyrene; ethylene/acrylate copolymers, ethylene/methyl acrylate (EMA) copolymers, ethylene/ethyl acrylate (EEA) copolymers, ethylene/butyl acrylate (EBA) copolymers; ethylene/alpha-olefin copolymers; acrylonitrile Butadiene Styrene (ABS) resin; halogenated polymers, polyvinyl chloride (PVC); a polyurethane; a polyamide; aromatic polyesters, polyethylene terephthalate (PET), polybutylene terephthalate (PBT); and copolymers or mechanical mixtures thereof.

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

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

According to another preferred aspect, the cable further comprises an outer polymeric sheath, preferably in contact with the foamed polymeric coating, preferably the sheath having a thickness of at least 0.5mm, preferably 1-5 mm.

In another aspect, the invention relates to a method of providing impact strength to a cable comprising coating said cable with a coating made of a foamed polymeric material.

According to a preferred aspect, the method of providing impact strength to a cable further comprises coating said foamed coating with an outer protective sheath.

Another aspect of the invention relates to the use of a foamed polymer material for providing impact strength to an electrical transmission cable.

Another aspect of the invention relates to a method for evaluating the impact strength of a cable comprising at least one insulating layer, consisting of the following steps:

a) measuring the average peel strength of said insulation layer;

b) subjecting the cable to a predetermined energy;

c) measuring the peel strength of said insulation layer at the point of impact;

d) the difference between the average peel strength and the peel strength measured at the point of impact is checked to be less than a predetermined value for the average peel strength of the cable.

According to a preferred aspect, the peel strength is measured between the insulating coating and the outer semiconductive coating.

In the present invention, the term "degree of foaming of the polymer" is understood to mean the degree of foaming of the polymer measured according to the following formula:

g (degree of foaming) ═ d₀/d_e-1)·100

Wherein d is₀Denotes the density, d, of the non-foamed polymer (i.e., meaning a polymer whose structure has essentially no void volume)_eRefers to the measured apparent density of the foamed polymer.

In the present invention, the term "foamed" polymer is understood to mean a polymer whose structure has a percentage of void volume (i.e. the space not occupied by the polymer but occupied by gas or air) generally greater than 10% of the total polymer volume.

In the present invention, the term "peel" strength is understood to mean the force required to separate (peel) one layer of the coating from the other layer of the conductor layer or coating; in the case of separating two layers of the coating from each other, these two layers are generally an insulating layer and an outer semiconducting layer.

Generally, the dielectric constant (K) of the insulation layer of the power transmission cable is greater than 2. Furthermore, unlike signal transmission cables where the "elevator level" parameter is not of any importance, elevator levels in the range of about 0.5KV/mm for low voltages up to about 10KV/mm for high voltages are used in the transmission cables; in these cables, therefore, the presence of inhomogeneities (for example void volumes) in the insulating coating, which can lead to local variations in the dielectric strength, is intended to be avoided, with a consequent reduction in the insulating capacity. Such insulating materials are therefore typically dense polymeric materials, among others, in the present invention. The term "dense" insulator is understood to mean an insulating material having a dielectric strength of at least 5KV/mm, preferably more than 10KV/mm, in particular more than 40KV/mm, for medium-high voltage transmission cables. Unlike foamed polymeric materials, such dense materials have a structure that is substantially free of void volume; in particular, the density of this material is 0.85g/cm³Or higher.

In the present invention, the term "low voltage" is understood to mean a voltage of up to 1000V (typically more than 100V), the term "medium voltage" is understood to mean a voltage of about 1 to about 30KV, and the term "high voltage" is understood to mean a voltage of more than 30 KV. Such power transmission cables typically operate at a nominal frequency of 50 or 60 Hz.

Although the use of foamed polymer coatings in the description has been exemplified in detail with reference to power transmission cables, where such coatings may advantageously replace metal armor currently used in such cables, it will be clear to those skilled in the art that such foamed coatings may advantageously be used in any type of cable, and thus desirably provide suitable impact protection to such cables. In particular, the definition of transmission cable includes not only types dedicated to low and medium voltage but also cables for high voltage transmission.

The following drawings will assist in further understanding the invention:

fig. 1 shows a transmission cable of the tripolar type with metallic armor known in the art.

Fig. 2 shows a first embodiment of the invention, a three-pole type cable.

Fig. 3 shows a second embodiment of the invention, a cable of the unipolar type.

Fig. 1 is a cross-sectional view of a medium voltage transmission cable of the prior art, a three-pole type cable with metal armor. The cable comprises three conductors (1), wherein an inner semiconductor coating (2), an insulating layer (3), an outer semiconductor layer (4) and a metal shielding layer (5) are respectively coated outside the conductors; for simplicity, the finished semiconductor structure is defined as the "core" in the following portions of the specification. The three cores are bundled together and the star-shaped space region between them is filled with a filler material (9) (typically an elastomer compound, polypropylene fibers, etc.) so as to make the cross-sectional structure circular and the whole is coated with an inner polymer sheath (8), a metal wire armor (7) and an outer polymer sheath (6).

Fig. 2 is a cross-sectional view of the cable of the invention, also of the three-pole type for medium voltage transmission. The cable comprises three conductors (1), wherein an inner semiconductor coating (2), an insulating layer (3), an outer semiconductor layer (4) and a metal shielding layer (5) are respectively coated outside the conductors; the star-shaped spaces between the cores are then filled with an impact-resistant foamed polymer material (10), said foamed polymer material (10) in turn being coated with an outer polymer jacket (6). In the foamed polymer coating (10), a rounded edge (10a) corresponding to the minimum thickness of the foamed polymer coating is also shown (by a dotted line) close to the outer surface of the core.

Fig. 3 is a cross-sectional view of a cable according to the invention, of the monopole type for medium voltage transmission. The cable comprises a central conductor (1) which is externally wrapped with an inner semiconductive coating (2), an insulating layer (3), an outer semiconductive layer (4), a metal shielding layer (5), a foamed polymer material layer (10) and an outer polymer jacket (6). In the unipolar cable illustrated in fig. 3, the rounded edges (10a) illustrated in the three-pole cable coincide with the layer (10) of foamed polymer material, since the core has a circular cross-section.

These figures clearly show only a few possible embodiments of the cable in which the invention can be advantageously utilized. It will be apparent that suitable modifications known in the art can be made to these embodiments without limiting the scope of the invention. For example, with reference to fig. 2, the star-shaped zone between the cores can be pre-filled with a conventional filler material, obtaining a semi-finished cable whose cross section corresponds substantially to the circular cross section contained in the circular edge (10a), then a layer of foamed polymer material (10) can be advantageously extruded over the cross-sectional zone of this semi-finished cable, whose thickness corresponds substantially to the circular edge (10a), followed by the extrusion of the outer sheath (6). Alternatively, the cores may be fitted with a cross-sectional quadrant in such a way that when the cores are joined together they form a cable of substantially circular cross-section, without the use of filler material for the star-shaped spatial region; a layer of impact resistant foamed polymer material (10) is then extruded over these cores thus bonded together, followed by an outer sheath (6).

For cables for low voltage transmission, the construction of the cable generally comprises only an insulating coating in direct contact with the conductor, which in turn is coated with a coating of expanded polymer material and an outer sheath.

Other solutions are known to those skilled in the art, who will be able to judge the most convenient solution based on factors such as cost, type of cable erection (aerial, in-pipe, directly buried, in-building, subsea, etc.), cable operating temperature (maximum and minimum temperature, ambient temperature range), etc.

The impact resistant foamable polymer coating can be comprised of any type of foamable polymer, such as polyolefins, polyolefin copolymers, olefin/ester copolymers, polyesters, polycarbonates, polysulfones, phenolic resins, urea-formaldehyde resins, and mixtures thereof. Examples of suitable polymers are Polyethylene (PE), low density PE (ldpe), medium density PE (mdpe), high density PE (hdpe), and linear low density PE (lldpe); polypropylene (PP); ethylene Propylene Rubber (EPR), in particular ethylene propylene copolymer (EPM), or ethylene propylene diene terpolymer (EPDM); natural rubber; butyl rubber; ethylene/vinyl acetate (EVA) copolymers; polystyrene; ethylene/acrylate copolymers, in particular ethylene/methyl acrylate (EMA) copolymers, ethylene/ethyl acrylate (EEA) copolymers, ethylene/butyl acrylate (EBA) copolymers; ethylene/alpha-olefin copolymers; acrylonitrile Butadiene Styrene (ABS) resin; halogenated polymers, in particular polyvinyl chloride (PVC); polyurethane (PUR); a polyamide; aromatic polyesters, such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT); and copolymers or mechanical mixtures thereof. Preference is given to using polyolefin polymers or copolymers, in particular those based on PE and/or PP mixed with ethylene propylene rubber. Advantageously, an Ethylene Propylene Rubber (EPR) modified polypropylene may be used, with a PP/EPR weight ratio of from 90/10 to 50/50, preferably from 85/15 to 60/40, particularly preferably about 70/30.

According to other aspects of the invention, the applicant has also observed that it is possible to mechanically mix the polymeric material to be subjected to a foaming treatment (in particular an olefin polymer, in particular polyethylene or polypropylene) with a predetermined amount of rubber in powder form, for example vulcanized natural rubber.

In general, the powder may be formed from particles having a particle size of 10-1000. mu.m, preferably 300-600. mu.m. Advantageously, vulcanized rubber waste from tire processing can be used. The percentage of rubber in powder form can be from 10 to 60% by weight, preferably from 30 to 50% by weight, relative to the polymer to be foamed.

The polymeric material to be foamed, whether used without further processing or as a foamable base in a mixture with a powdered rubber, should have such a rigidity as to ensure a certain degree of desired impact resistance when it is foamed, so as to protect the interior of the cable (i.e. with reference to the insulator and semiconductive layers that may be present) from damage by accidental impacts that may subsequently occur. In particular, the material should have a sufficiently high capacity to absorb impact energy in order to transmit a certain amount of energy to the underlying insulating layer, so that the insulating properties of the underlying coating are not improved beyond a predetermined value. The reason for this is, as illustrated in the more detailed description below, that, according to the observation of the applicant, in cables subjected to impacts, a difference is observed between the average value of the peel strength of the underlying insulating coating and the value measured at the point of impact; advantageously, the peel strength is measured between the insulating layer and the outer semiconducting layer. The greater the impact energy transmitted to the underlying layer, the greater this difference in strength; where peel strength is measured between the insulating layer and the outer semiconductive layer, it is judged that the protective coating can provide adequate protection for the inner layer when the difference in peel strength at the point of impact is less than 25% from the average.

The applicant has found that a polymeric material selected from the above, having a flexural modulus at room temperature, measured according to ASTM standard D790, before foaming, of greater than 200MPa, preferably at least 400MPa, is particularly suitable for this purpose. On the other hand, polymer materials having a room temperature flexural modulus of less than 2000MPa are preferably used because excessive rigidity of the foamed material makes the finished product difficult to handle. Particularly suitable polymer materials for this purpose are those having a flexural modulus of 400-1800MPa at room temperature before foaming, particularly preferably 600-1500MPa at room temperature.

These flexural modulus values may be characteristic of a particular material or may result from blending two or more materials having different moduli in proportions that are capable of achieving the desired stiffness values for the material. For example, for the purpose of reducing stiffness in a suitable manner, polypropylene having a flexural modulus of greater than 1500MPa may be suitably modified with a suitable amount of an Ethylene Propylene Rubber (EPR) having a modulus of about 100 MPa.

Examples of compounds of commercially available polymers are: low density polyethylene: rible FL 30 (Enichem); high density polyethylene: DGDK 3364(Union Carbide); polypropylene: PF 814 (Montell); polypropylene modified with EPR: moplen EP-S30R, 33R and 81R (Montell); Fina-Pro 5660G, 4660G, 2660S and 3660S (Fina-Pro).

The degree of foaming of the polymer and the thickness of the coating should be such that they ensure, together with the outer polymer sheath, resistance to the typical impacts that may occur during cable handling and laying.

As previously mentioned, the "degree of foaming of the polymer" is measured according to the following formula:

g (degree of foaming) ═ d₀/d_e-1)·100

Wherein d is₀Denotes the density of the non-foamed polymer, d_eRefers to the measured apparent density of the foamed polymer.

The applicant has found that for foamed layers of equal thickness, it is preferable to use a polymer material with a high degree of foaming, within the limits allowed for maintaining the desired impact resistance, since this makes it possible to limit the amount of polymer material used, which is advantageous both for saving the finished product and for reducing the weight.

The degree of foaming is very variable, both as a function of the particular polymer material used and as a function of the thickness of the coating to be used; in general, the degree of foaming may be from 20% to 3000%, preferably from 30% to 500%, particularly preferably from 50% to 200%. Foamed polymers generally have a closed cell structure.

According to the applicationIt has been observed that beyond a certain degree of foaming, the ability of the polymer coating to provide the desired impact strength will be reduced. In particular, it has been observed that the possibility of obtaining polymers with a high degree of foaming that maintain a high degree of protection against impacts is linked to the flexural modulus value of the polymer to be foamed. The reason for this is observed by the applicant that the modulus of a polymeric material decreases with increasing degree of foaming of the material, roughly corresponding to the following formula: e₂/E₁＝(ρ₂/ρ₁)²Wherein: e₂Flexural modulus of the polymer indicating a higher degree of foaming; e₁Flexural modulus of the polymer indicating a lower degree of foaming; rho₂Apparent density of the polymer indicating a higher degree of foaming; rho₁Apparent density of the polymer indicating a lower degree of foaming;

as a guide, for polymers having a flexural modulus of about 1000MPa, a change in the 25% -100% degree of foaming requires approximately a halving of the flexural modulus value of the material. Thus, a polymeric material with a higher flexural modulus can be foamed to a greater extent than a polymeric material with a lower flexural modulus value without compromising the impact resistance of the coating.

Another variable that can easily affect the impact strength of a cable is the thickness of the foamed coating; the minimum thickness that can ensure the desired impact strength with such a coating depends on the degree of foaming and the flexural modulus of the polymer. Generally speaking, it is observed by the applicant that for the same polymer and the same degree of foaming, a higher value of impact strength can be achieved by increasing the thickness of the foamed coating. However, for the purpose of using a limited amount of coating material to reduce the cost and size of the finished product, it is advantageous for the thickness of the foam layer to be the minimum thickness required to ensure the desired impact strength. In particular, for cables of the medium voltage type, it has been observed that a foamed coating having a thickness of about 2mm will generally be able to guarantee a sufficient resistance to the normal impacts to which this type of cable is subjected. Preferably, the coating thickness is greater than 0.5mm, in particular from about 1mm to about 6mm, particularly preferably from 2mm to 4 mm.

The applicant has found that the relationship between the thickness of the coating and the degree of foaming of the polymeric material can be defined with reasonable approximation for materials having various values of flexural modulus, so that the thickness of the foamed coating can be suitably dimensioned as a function of the degree of foaming and the modulus of the polymeric material, in particular for a thickness of the foamed coating of about 2-4 mm.

This relationship can be expressed as follows: v.d_eN wherein V represents the volume (m) of the foamed polymer material per linear meter of the cable³/m) which volume is related to the circular edge defined by the minimum thickness of the foamed coating, corresponding to the circular edge (10a) of the multipolar cable of fig. 2, or to the coating (10) defined in the unipolar cable of fig. 3. de represents the apparent density (kg/m) of the foamed polymeric material³) (ii) a N represents the result of the product of the two values, which should be greater than or equal to: 0.03, for materials with modulus > 1000 Mpa; 0.04 for a material with modulus of 800-; 0.05 for a material with modulus of 400-; 0.06 for a material with a modulus < 400 MPa.

The parameter V is related to the thickness (S) of the foamed coating by the following relationship: n (2R)_i·S+S²) Wherein R is_iThe inner radius of the circular edge (10a) is shown.

Parameter d_eThe degree of foaming of the polymer material is related by the aforementioned relationship:

G＝(d₀/d_e-1)·100

based on the above relationship, it was found that for various materials having different room temperature flexural moduli (Mf), placing a foamed coating of about 2mm thickness on a circular portion of a cable of about 22mm diameter should have a minimum apparent density of about the following values: 0.40g/cm³For LDPE (Mf about 200); 0.33g/cm³For the 70/30 PP/EPR mixture (Mf about 800); 0.26g/cm³For HDPE (Mf about 1000); 0.20g/cm³For PP (Mf about 1500).

These apparent density values of the expanded polymer correspond to a maximum degree of expansion of about the following values: 130% for LDPE (d)₀＝0.923) (ii) a 180% for PP/EPR mixtures (d)₀0.890); 260% for HDPE (d)₀0.945); 350% for PP (d)₀＝0.900)。

Also, placing a foamed coating layer of about 3mm thickness on the same size cable, the following minimum apparent densities were obtained: 0.25g/cm³For LDPE; 0.21g/cm³For PP/EPR blends; 0.17g/cm³For HDPE; 0.13g/cm³For PP; maximum foaming corresponding to about the following values: 270%, for LDPE; 320% for PP/EPR blends; 460% for HDPE; 600% for PP.

The above results demonstrate that in order to optimize the impact strength properties of a foamed coating of predetermined thickness, both the mechanical strength properties of the material (in particular its flexural modulus) and the degree of foaming of said material should be taken into account. However, the numerical values measured by applying the above-described relationship should not be construed as limiting the scope of the present invention. In particular, the maximum degree of foaming of the polymer having flexural modulus values close to the upper limit of the interval values at which the defined number N varies (i.e. 400, 800 and 1000MPa), in fact may even be greater than the values calculated according to the above relation; thus, for example, a PP/EPR layer having a thickness of about 2mm (Mf about 800MPa) may still provide the desired impact protection even with a degree of foaming of about 200%.

The polymer is typically foamed during extrusion; this foaming can take place either chemically by the addition of suitable "foaming" compounds (i.e. compounds capable of generating gas under the specified conditions of temperature and pressure) or physically by direct injection of the gas under high pressure into the extruder barrel.

Suitable examples of chemical "blowing agents" are azodicarbonamide, mixtures of organic acids (e.g., citric acid) with carbonates and/or bicarbonates (e.g., sodium bicarbonate).

Examples of gases for high pressure injection into the extruder barrel are nitrogen, carbon dioxide, air and low boiling hydrocarbons such as propane and butane.

The protective outer sheath over which the foamed polymer layer is wrapped may conveniently be of the type commonly used. Materials for the outer coating which can be used are Polyethylene (PE), in particular medium density PE (mdpe) and high density PE (hdpe), polyvinyl chloride (PVC), mixtures of elastomers, etc. MDPE or PVC is preferably used. Generally, the polymeric material forming the outer jacket has a flexural modulus of from about 400 to about 1200MPa, preferably from about 600 to about 1000 MPa.

The applicant has found that the presence of the outer sheath helps to provide the desired impact strength properties to the coating together with the foamed coating. In particular, it has been observed by the applicant that, for a foamed coating of the same thickness, the contribution of such a sheath to the impact strength increases with the degree of foaming of the polymer forming the foamed coating. The thickness of the outer sheath is preferably greater than 0.5mm, in particular 1-5mm, preferably 2-4 mm.

The preparation of the cable with impact strength according to the invention is described with reference to the cable construction diagram of fig. 2, but in which the star-shaped spaces between the cores to be coated are not filled directly with foamed polymer (10), but with conventional fillers; the foamed coating is then extruded onto this semi-finished cable, forming a rounded edge (10a) around the semi-finished cable, and then coated with an outer polymer jacket (2). The preparation of the cable core, i.e. the assembly of the conductor (4), the inner semiconductive layer (9), the insulation (5), the outer semiconductive layer (8) and the metallic shielding layer (4), is done by methods known in the art, for example by extrusion. The cores are then bundled together and filled with conventional filler materials (e.g., elastomer compounds, polypropylene fibers, etc.), typically by extruding the filler over the bundled cores to obtain a semi-finished cable of circular cross-section. The foamed polymeric coating (10) is then extruded onto the filler material. Preferably, the diameter of the extruder head die is slightly smaller than the final diameter of the cable with the foamed coating in order to allow the polymer to foam outside the extruder.

It was observed that under the same extrusion conditions (e.g. screw rotation speed, extrusion line speed, extruder head diameter, etc.), the extrusion temperature is a process variable that has a considerable influence on the degree of foaming. Generally, it is difficult to obtain a sufficient degree of foaming at extrusion temperatures below 160 ℃; the extrusion temperature is preferably at least 180 ℃ and in particular about 200 ℃. Generally, an increase in extrusion temperature corresponds to a higher degree of foaming.

In addition, the degree of foaming of the polymer can be controlled to some extent by acting on the cooling rate, since by suitably slowing down or accelerating the cooling (if the polymer for forming the foamed coating is at the extruder outlet) the degree of foaming of said polymer can be increased or decreased.

As the applicant has found, the impact effect on the cable coating can be quantitatively determined by measuring the peel strength of the cable coating, the average value of this peel strength and the difference between the measured values at the evaluation impact point. In particular, for medium voltage cables having a structure comprising an inner semiconductive layer, an insulating layer and an outer semiconductive layer, it is advantageous to measure the peel strength (and the associated difference) between the outer semiconductive material layer and the insulating layer.

The applicant has observed that the particularly severe impact effects which a cable may be subjected to, in particular an armoured medium voltage cable, can be reproduced by means of an impact test based on the French standard HN33-S-52, which relates to armoured cables for high voltage transmission, HN33-S-52, which allows a cable impact energy of about 72 joules (J).

The peel strength of the coating can be determined according to French standard HN33-S-52, which is the measure of the force required to separate the outer semiconductive layer from the insulating layer. The applicant has found that by continuously measuring said force at the point of impact occurrence, a peak value of the force can be measured which is indicative of the change in adhesion between the two layers. It has been observed that these changes are generally associated with a reduction in the insulating ability of the coating. The smaller the impact strength provided by the outer cover (in the present invention, the outer cover is composed of the foamed coating and the outer jacket), the larger the variation will be proportionately. Thus, the magnitude of the change in force measured at the point of impact (relative to the average measured along the cable) is indicative of the degree of protection provided by the protective coating. Generally, variations in peel strength of up to 20-25% from the average are considered acceptable.

The properties of the foamed coating (material, degree of foaming, thickness) may be suitably selected in accordance with the impact protection to be provided to the underlying cable structure, wherein the foamed coating is advantageously used together with a suitable protective outer polymer jacket, and may also depend on the properties of the particular material used as insulator and/or semiconductor, such as the hardness, density, etc. of the material.

As will be appreciated from the description of the invention, the cable of the invention is particularly suitable for replacing conventional armored cables due to the advantageous properties of the foamed polymer coating compared to metal armor. However, its use should not be limited to a specific application. In fact, the cables of the present invention are advantageous for use in all applications where it is desirable for the cable to have enhanced impact resistance. In particular, in applications where the use of armored cables has been advantageous until now but hindered by the lack of metal armor, the impact-resistant cables of the present invention can replace conventional unarmored cables in all of these applications.

To further describe the invention in detail, some illustrative examples are given below. Example 1

Preparation of a Cable with a foamed coating

To evaluate the impact strength of the foamed polymer coatings of the invention, various test pieces were prepared by extruding several polymers of various degrees of foaming in varying thicknesses onto a core consisting of an approximately 14mm thick multi-wire conductor coated with a 0.5mm layer of semiconductive material, a 3mm layer of an insulating compound based on EPR, and a 0.5mm layer of a "peel-off" semiconductive material based on EVA reinforced with carbon black to a total core thickness of approximately 22 mm.

70/30 weight ratio mechanical mixture of Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE), polypropylene (PP), LDPE and fine powder vulcanized natural rubber (particle size 300-(PE-powder), ERP rubber modified PP (70/30 weight ratio PP-ERP of blend) as polymer material to be foamed; these materials are identified by letters a through E in the following and are described in detail in the following table:

	material	Brand and manufacturer	Modulus (MPa)
				ABCDE	LDPEHDPEPPPP-EPRPE/powder	Riblene FL 30-EnichemDGDK 3364-Union CarbidePF 814-MontellFINA-PRO 3660SRiblene FL 30	260100016001250

The polymer is chemically foamed or two different foaming Compounds (CE) are used, as illustrated below:

	compound (I)	Brand and manufacturer
			CE1	Azodicarbonamide	Sarmapor PO-Sarma
CE2	Carboxylic acid-bicarbonate salt	Hydrocerol CF 70-BoehringerIngelheim

The polymer to be foamed and the foaming compound are loaded (in the proportions used, see Table 2) into an 80mm-25D single-screw extruder (Bandra); the extruder was equipped with a threaded conveyor screw characterized by a final zone depth of 9.6 mm. The extrusion system consists of a male die (whose diameter is generally about 0.5mm greater than that of the core to be coated) capable of providing a smooth extrusion of the core to be coated, and a female die (whose diameter is chosen so as to be of a size about 2mm smaller than that of the cable with the foamed coating); in this way, the extruded material foams at the outlet of the extrusion head, rather than inside the head or inside the extruder. The extrusion speed (extrusion line speed) of the core to be coated is set as a function of the desired thickness of the foamed material (see table 2). At a distance of about 500mm from the extrusion head is a cooling tube (containing cooling water) to stop foaming and to cool the extruded material. The cable is then wound on a spool.

The composition of the polymeric material/blowing agent mixture and the extrusion conditions (speed, temperature) may be suitably varied as shown in table 2 below. Table 2: foaming mixture and extrusion conditions

Cable number	Material +% and blowing agent type	Extruder speed(1)Extruder temperature production line speed (rpm) (° c) degrees (m/min)
			1234567891011	A+2％CE1A+2％CE1A+2％CE1A+2％CE1A+2％CE1A+0.8％CE2C+0.8％CE2C+0.8％CE2E+0.8％CE2B+1.2％CE2D+2％CE2	6.4 165 311.8 190-180 25.5 190-180 36.8 190-180 26.4 165 1.55.7 225-200 23.7 200 26.3 200 24.9 225-200 1.88.2 225-200 28 225-200 2

⁽¹⁾: the extruder temperature is relative to the barrel and the extrusion head. When only one value is given, these temperatures are the same. In the initial zone of the extruder, the temperature was about 150 ℃.

Sample 1 did not experience foaming, presumably because the temperature of the extruder was too low (165 ℃), for which reason sample 5 experienced limited foaming (only 5%).

The cables with the foamed coatings were subsequently coated with conventional MDPE sheaths (CE 90-material plastics brewer) of different thicknesses (see table 3) by a conventional extrusion method to obtain cable samples with the characteristics defined in table 3; cable No. 1, in which the polymer did not undergo foaming, was taken as a comparative non-foamed polymer coating. For comparison purposes, table 3 also gives the characteristics of a cable lacking a foamed filler and coated with an outer sheath only (cable No. 0). Table 3: coating characteristics

Cable number	Foaming degree of Filler (%) Filler thickness (mm) sheath thickness (mm)
		01234567891011	- 0 30 1 331 4.3 361 1 348 2.5 35 3 335 2 252 2 229 3 2.223 2.5 278 4 282 4 2

Another batch of 6 cable samples was prepared in the same manner as described above using a foamed polymer coating consisting of polypropylene rubber modified with about 30% ERP, with a flexural modulus of about 600MPa, see table 4 (examples 12-17); table 4 also gives comparative examples of two cables, wherein the cables have a foamed coating but lack an outer sheath (examples 16a and 17 a). Table 4: coating characteristics:

cable number	Foaming degree of Filler (%) Filler thickness (mm) sheath thickness (mm)
		121314151616a1717a	71 3 1.922 2 2167 3 1.8124 2 256 2 256 2 -84 2 284 2 -

Example 2

Impact strength test

To evaluate the impact strength of the cable prepared according to example 1, the cable was subjected to an impact test followed by evaluation of the degree of damage. The impact effect was evaluated both visually by means of an analysis of the cable and by means of a determination of the change in the peel strength of the layer of semiconducting material at the point of impact. The impact test was based on the French standard HN33-S-52, which is provided by a cable impact energy of about 72 joules (J) obtained by dropping a 27kg weight from a height of 27 cm. For the present test, this impact energy was generated by dropping an 8kg weight from a height of 97 cm. The impact end of the weight is provided with a V-shaped circular edge (with a curvature radius of 1mm) punching head. For the purposes of the present invention, the impact strength of a single impact is evaluated. For samples 6-12, the second test was repeated from the first at a distance of about 100 mm.

The peel strength was measured according to French standard HN33-S-52, according to which the force required to separate the outer semiconducting layer from the insulating layer was measured. By continuously measuring the force, the peak of the force is measured at the point where the impact occurs. For each test piece, at the point of impact, a "positive" force peak (corresponding to the increase in force required to separate the two layers (relative average)) and a "negative" force peak (decrease in relative average) were measured. The maximum change value of the peel strength at the impact point is obtained from the difference between the maximum (Fmax) and minimum (Fmin) values of the measured force peak.

Calculating the rate of change of the peel strength by determining the percentage between the above-mentioned difference (Fmax-Fmin) and the measured cable average peel strength value (F < >) according to the following relationship:

the% Change rate is 100(Fmax-Fmin)/F < >

The magnitude of the change in said force measured at the point of impact (relative to the average value measured along the cable) is therefore indicative of the degree of protection provided by the foamed coating. Generally, variations in peel strength of up to 20-25% from the average are considered acceptable. Table 5 shows the change in peel strength of samples 0-17 a. Table 5: percent change in peel Strength

Cable with a protective layer	First test second test
		01234567891011121314151616a1717a	62 7840 -18 -27 -13 -21 -17 239 124 519 159.8 12.54.3 2.57 1416 1714 1210 1016 1830 5515.5 13116 103

As can be seen from table 3, the percent change in peel strength of sample 1 (no foaming obtained) was extremely high; this indicates that the impact absorbing ability of the non-foamed polymer is significantly lower than that of the same polymer layer of the same thickness foamed (see sample 3, with 61% foamed coating). Sample 3 showed a slightly higher change in peel strength than the 25% limit; the limited impact strength of this sample is mainly due to the thickness of the foamed coating, which is only 1mm relative to the 2-3mm thickness of the other samples.

Sample 5 (with a 3mm thick foamed coating) has a high value of peel strength due to the low degree of polymer foaming (5%), thus demonstrating that the low degree of foaming coating provides a relatively limited impact strength. While sample 4 (although the foam thickness was thinner than that of sample 5) had a higher impact strength, the change in peel strength was 13% compared to sample 5 at a rate of change of 21%, thus demonstrating that higher degrees of foaming provide higher impact strength.

By comparing sample 13 and sample 15, it is seen how the increase in polymer foaming (22-124%) is required for the increase in impact strength of the coating (the peel strength varies from 16-17% to 10%) for the same thickness of foam layer and outer jacket layer. This trend is confirmed by comparing samples 16 and 17. However, by comparing samples 16a and 17a (without the outer sheath) with each of samples 16 and 17, it can be seen how the contribution of the outer sheath to impact protection increases with increasing degree of foaming. Example 3

Comparative test of impact strength of armored cable

To verify the impact strength efficacy of the foamed coating, cable No. 10 was tested against a conventional armored cable.

The armored cable had the same core as cable No. 10 (i.e. an about 14mm thick multi-wire conductor coated with a layer of 0.5mm semiconductive material, a layer of 3mm insulating compound (this material is based on EPR), and a layer of 0.5mm "strippable" semiconductive material (this material is based on EVA reinforced with carbon black), with a total core thickness of up to about 22 mm). According to the following steps:

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

b) a PVC jacket about 0.66mm thick;

c) 2 armor steel strips each about 0.5mm thick;

d) an MDPE outer sheath with the thickness of about 2mm sequentially wraps the cable core from inside to outside.

For comparative testing, a "drop-weight" dynamic machine (type CEAST 6758) was used. Two sets of tests were carried out by dropping a weight of 11kg from a height of 50cm (about 54 joules of impact energy) and a height of 20cm (about 21 joules of impact energy), respectively; a hemispherical head of about 10mm radius was fitted to the impact end of the weight.

The resulting cable deformation results are shown in fig. 4 and 5 (50 cm and 20cm height, respectively), where the cable of the invention is denoted by a) and the cable of the conventional armor by b).

The core deformation was measured to evaluate the damage level of the cable structure. In fact, the semiconductor-insulation-semiconductor sheath deforms more, more likely resulting in an electrical loss of insulation of the cable. The results are summarized in Table 6. Table 6: percent thickness reduction of semiconductor layer after impact

	Conventionally armored cable	No. 10 cable
			50cm high impact	41％	26.5％
20cm high impact	4.4％	2.9％

From the results summarized in table 6, it is seen that the impact strength properties of the cable of the invention are better than those of conventional armored cables.

Claims (28)

1. An electrical transmission cable comprising:

a) a conductor;

b) at least one dense insulating coating disposed about said conductor; and

c) a coating layer made of a foamed polymer material disposed about said dense insulating coating layer, wherein said polymer material has predetermined mechanical strength characteristics and a predetermined degree of foaming to provide impact resistance characteristics to said cable.

2. A cable as claimed in claim 1, wherein the foamed polymeric material is obtained from a polymeric material having a flexural modulus at room temperature, before foaming, of at least 200MPa, determined according to ASTM standard D790.

3. A cable as claimed in claim 1, wherein said flexural modulus is from 400MPa to 1800 MPa.

4. A cable as claimed in claim 1, wherein said flexural modulus is from 600MPa to 1500 MPa.

5. A cable as claimed in claim 1, wherein said polymeric material has a degree of foaming of from about 20% to about 3000%.

6. A cable as claimed in claim 1, wherein said polymeric material has a degree of foaming of from about 30% to about 500%.

7. A cable as claimed in claim 1, wherein said polymeric material has a degree of foaming of from about 50% to about 200%.

8. A cable as claimed in any one of the preceding claims 1 to 7, wherein the coating of said foamed polymeric material has a thickness of 0.5 mm.

9. A cable as claimed in any one of the preceding claims 1 to 7, wherein the thickness of the coating of said foamed polymeric material is from 1 to 6 mm.

10. A cable as claimed in any one of the preceding claims 1 to 7, wherein the thickness of the coating of said foamed polymeric material is from 2 to 4 mm.

11. A cable as claimed in claim 1 wherein said foamed polymeric material is selected from the group consisting of Polyethylene (PE), low density PE (ldpe), medium density PE (mdpe), high density PE (hdpe), and linear low 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) copolymers; polystyrene; ethylene/acrylate copolymers, ethylene/methyl acrylate (EMA) copolymers, ethylene/ethyl acrylate (EEA) copolymers, ethylene/butyl acrylate (EBA) copolymers; ethylene/alpha-olefin copolymers; acrylonitrile Butadiene Styrene (ABS) resin; halogenated polymers, polyvinyl chloride (PVC); a polyurethane; a polyamide; aromatic polyesters, polyethylene terephthalate (PET), polybutylene terephthalate (PBT); and copolymers or mechanical mixtures thereof.

12. A cable as claimed in claim 1, wherein said foamed polymeric material is a polyolefin polymer or copolymer based on PE and/or PP.

13. A cable as claimed in claim 1, wherein said foamed polymeric material is a PE and/or PP based polyolefin polymer or copolymer modified with an ethylene propylene rubber.

14. A cable as claimed in claim 13, wherein said foamed polymeric material is polypropylene modified with an Ethylene Propylene Rubber (EPR) in a PP/EPR weight ratio of from 90/10 to 50/50.

15. A cable as claimed in claim 14, wherein said PP/EPR weight ratio is from 85/15 to 60/40.

16. A cable as claimed in claim 14, wherein said PP/ERP weight ratio is about 70/30.

17. Cable as claimed in claim 12, wherein said polyolefin polymer or copolymer based on PE and/or PP further comprises a predetermined amount of a vulcanized rubber in powder form.

18. A cable as claimed in claim 17, wherein the predetermined amount of vulcanized rubber in powder form is between 10 and 60% by weight of the polymer.

19. A cable as claimed in any one of claims 1 to 18, wherein said cable comprises an outer polymer jacket.

20. A cable as claimed in claim 19, wherein said jacket is in contact with said foamed polymer coating.

21. A cable as claimed in claim 19 or 20, wherein said sheath has a thickness of greater than 0.5 mm.

22. A cable as claimed in claim 19 or 20, wherein said sheath has a thickness of 1 to 5 mm.

23. A method for providing impact strength to an electrical power transmission cable comprising coating said cable with a coating made of a foamed polymeric material.

24. A method as claimed in claim 23 further comprising coating said foamed coating with an outer polymeric sheath.

25. Use of a foamed polymer material to provide impact strength to a power transmission cable.

26. A method for evaluating the impact strength of a cable comprising at least one insulating coating, the method consisting of the following steps:

a) measuring the average peel strength of said insulation layer;

b) subjecting the cable to a predetermined energy;

c) measuring the peel strength of said insulation layer at the point of impact;

d) the difference between the average peel strength and the peel strength measured at the impact point was checked to be less than a predetermined value.

27. A method as claimed in claim 26, wherein the peel strength is measured between the insulating coating and the outer semiconductive coating.

28. A method as claimed in claim 27, wherein the difference between the average peel strength and the peel strength measured at the point of impact is less than 25%.