The present invention relates to a cable, in particular a cable for electric power transmission or distribution or for telecommunications. More particularly, the present invention relates to a cable as defined above comprising at least one coating layer and having self-repairing protection which is capable of restoring the continuity of the coating layer after it has been broken.
Electrical cables, in particular low- or medium-voltage cables for the distribution of electric energy for domestic or industrial use, generally consist of one or more conductors individually insulated by a polymeric material and coated with a protective sheath, which is also made of a polymeric material. These cables, particularly when installed underground, in tunnels or inside buried pipes, are subject to damage on these layers caused by various types of mechanical abuses, for example accidental impact with sharp tools such as shovels or picks, which exert on the cable both cutting and compression action. This can lead to partial or total rupture of the outer sheath and possibly also of the insulating layer, with consequent infiltration of moisture and generation of leakage currents. If the rupture of the coating layers reaches the conductor, the combined effect of leakage currents and moisture leads to a gradual corrosion of the conductor until, at the utmost, to a complete breakage of the conductor itself.
To obtain effective protection against such mechanical abuses, the cable can be provided with an outer structure capable of withstanding both cutting and compression, this outer structure consisting, for example, of a sheath made of a metal or of a plastic material combined with metal armouring. Besides being expensive, this solution leads to a considerable increase in cable dimensions and rigidity, thus making this solution unsuitable for cables which require easiness of installation and low costs, such as, in particular, in the case of low-voltage cables.
In patent application DE-1,590,958 a telecommunications or high-current cable is described which is protected from mechanical damage by means of an outer sheath having, on its inside, microcapsules containing a liquid which is capable of solidifying rapidly once the microcapsule has been broken. To this purpose, it is mentioned as preferred the use of the two components commonly used for manufacturing expanded polyurethane, these components being microencapsulated separately so that they react together at the moment the microcapsules are broken, forming an expanded material which closes the accidental cut. Alternatively, it is possible to use liquids which solidify when placed in contact with external agents, for example with moisture.
According to the Applicant, the solution proposed in the above-mentioned patent application is difficult to implement in practice and has many drawbacks. Firstly, it is to be observed that the possibility of self-repairing is limited to the outer sheath, and no indications regarding the possibility of restoring integrity of the insulating layer are provided. Moreover, to obtain an effective self-repairing effect, it is necessary to introduce a large amount of microencapsulated material during sheath extrusion, and this operation can result to be extremely difficult, besides being expensive. Lastly, it is to be pointed out that the mechanism of action of the microcapsules is irreversible, consequently the self-repairing effect can be carried out only once, namely at the moment the microcapsules are broken. Actually, during the various stages of the cable life (manufacturing, storage, installation, use), the coating layers are inevitably subjected to external mechanical actions of compression and bending and to thermal cycles of expansion and compression, which can lead to rupture of the microcapsules with consequent expansion and/or solidification of the material contained therein. Therefore, this material will no longer be able to effect the desired self-repairing action when the sheath should actually be damaged. It is also to be noted that, even when microcapsules are used containing a liquid material which solidifies on contact with moisture, accidental rupture of the microcapsules without any actual damage to the outer sheath nonetheless leads to solidification of the material because residual moisture is always present inside the cable.
The Applicant has now found that, in consequence of a mechanical damage which creates a discontinuity in at least one of the cable coating layers, it is possible to obtain effective self-repairing of the coating by virtue of the presence of an inner layer, placed, for example, between the insulating layer and the outer sheath, this inner layer comprising a material having a predetermined cohesiveness and at the same time a controlled flowability, which is capable of repairing the damage by restoring the continuity of the coating layer. After creation of a discontinuity in the coating, the material “moves” towards the point of damage and fills up, at least partly, the discontinuity by forming a substantially continuous layer which is capable of maintaining the functionality of the cable under the expected working conditions. The action of the self-repairing material, which occurs with a revesible mechanism, prevents, among other things, moisture infiltration and establishment of leakage currents, and thus a quick corrosion of the conductor.
The flowability of the material is predetermined so as to have sufficient fluidity at the working temperature of the cable, and at the same time so as to prevent the material from draining from the cable extremities or leaking in an uncontrolled manner from the coating rupture point.
In a first aspect, the present invention thus relates to a cable comprising a conductor and at least one coating layer, characterized in that the said cable comprises an inner layer comprising a self-repairing material having a predetermined cohesiveness and a controlled flowability.
According to a preferred aspect, the cable according to the present invention comprises an insulating coating layer and an outer sheath, and is characterized in that the inner layer is placed between the insulating layer and the outer sheath.
According to another embodiment of the present invention, the inner layer is placed between the conductor and the insulating layer.
According to a further embodiment, the cable according to the present invention comprises at least two insulating coating layers and is characterized in that the inner layer is placed between two of the said insulating layers.
According to a further aspect, the present invention relates to a method for imparting to a cable comprising a conductor and at least one coating layer a capacity of self-repairing the coating layer, characterized in that the said method comprises providing the cable with an inner layer comprising a material having the capacity, upon creation of a discontinuity in the coating layer, of re-establishing the continuity in the coating layer in a reversible manner.
In the description hereinbelow and in the claims, the material which constitutes the inner layer will be referred to, for simplicity, as the “self-repairing material”.
The term “inner layer” is understood herein to refer to a layer placed in any position between the conductor and the outermost coating layer, for example between the conductor and the insulating layer or, preferably, between the insulating layer and the outer sheath. Alternatively, when at least two insulating layers are present, the self-repairing layer can be placed between two of the said insulating layers.
The expression “discontinuity in at least one of the coating layers” is understood herein to refer to a partial or complete rupture of that layer. In the case of partial rupture only part of the thickness of the coating layer has been damaged, whereas there is complete rupture when the layer has been cut throughout its thickness. Needless to say, a partial rupture may become complete over time, for example following tractional or flexural mechanical stresses or alternatively as a result of thermal cycles of expansion and contraction to which the cable is subjected during use.
The expression “re-establishing the continuity” is understood herein to mean refill, at least partially, a point of rupture which has been created in the cable coating, so as to maintain the functionality of the cable at least for a predetermined period of time, and preferably for the entire period of the life of the cable, at least under the normal conditions of use. In other words, the self-repairing material is capable of preventing or at least slowing down the degradation of the materials constituting the cable, and in particular of the conductor, due to the infiltration of external agents through the point of discontinuity.
The Applicant has noted that, for the purposes of the present invention, the desired self-repairing of the cable is obtained by using a material having a predetermined cohesiveness and a controlled flowability. Although high cohesiveness values are considered desirable for the purposes of self-repairing, it is clear that these high values may conflict with flowability. A person skilled in the art will be capable of selecting the most suitable material, in which the desired compromise between cohesion and fluidity is achieved as a function of the specific cable which it is desired to manufacture and, above all, as a function of the conditions of installation and use envisaged for this cable, in particular in terms of temperature and pressure.
In a preferred embodiment, the self-repairing material is a dielectric material which is capable of re-establishing the electrical insulation of the cable. This property is particularly important in the case where mechanical damage is such as to cause in the insulating layer a partial or complete rupture, i.e. up to reaching the conductor. In general, dielectric rigidity values, under alternating current, of greater than 15 kV/mm, preferably greater than 20 kV/mm, and resistivity values of greater than 1014Ω·cm, preferably greater than 1016Ω·cm, are sufficient.
As mentioned above, the self-repairing material has predetermined cohesiveness which is such that, following the creation of a discontinuity in this material, for example by the action of a cutting tool, and once the cause of the discontinuity has been removed, the molecules which constitute the self-repairing material are capable of spontaneously recreating intermolecular bonds that are sufficient to restore continuity of the material. This phenomenon is of a reversible kind, i.e. the self-repairing material is capable of effectively carrying out its function an indefinite number of times.
For the purposes of the present invention, the expression “cohesiveness of the self-repairing material” refers both to the actual cohesive force up to detachment (referred to hereinbelow more simply as “cohesive force”), i.e. the force per surface area unit required to cause within the mass of a sample of material a complete detachment of one part of the material from the remaining part, and to the force of re-cohesion (or of auto-adhesion), that is the force required to recreate a complete detachment within the material once two portions of this material have been placed in contact for a predetermined time and under predetermined pressure and temperature conditions. In other words, the cohesiveness of the self-repairing material must be assessed both as regards the strength of intermolecular forces which hold the material together thereby ensuring its integrity, and as regards its capacity to recreate these intermolecular bonds spontaneously once they have been broken by the intervention of an external force.
The cohesive force can be measured according to the method given in the examples hereinafter. It has been found that cohesive force values, measured at room temperature, of at least 0.05 kg/cm2 ensure a sufficient cohesiveness of the self-repairing material, although values of between 0.1 and 4 kg/cm2 are preferred, and even more preferably between 0.2 and 2 kg/cm2.
The force of re-cohesion can be evaluated empirically by placing, one on top of the other, two disks of material of predetermined dimensions and leaving the two disks in contact for a predetermined time at room temperature. At the end of this period, the force required to separate the two disks is measured. The closer this force is to the intrinsic cohesive force value of the material as such, the more the material is capable of re-unifying spontaneously after damage, thus reforming a continuous material. In practice, the Applicant has found that in the self-repairing materials according to the present invention, the force of re-cohesion is preferably substantially identical to the cohesive force as defined above, and at least has a value not less than 80%, preferably not less than 90%, relative to the cohesive force measured on the material as such.
In the Applicant's perception, another property of the self-repairing material according to the present invention is its controlled flowability, i.e. the self-repairing material must be capable of “moving” so as to migrate towards the point of rupture of the coating in an amount which is sufficient to repair the damage.
On the other hand, as already mentioned above, the flowability of the self-repairing material must be controlled in such a way as to avoid loss of material either by drainage from the extremities of the cable or by leaking from the point of rupture of the coating. This control of the flowability must be ensured not only at ambient temperature but also at higher temperatures, for example at the maximum working temperature envisaged for the cable (usually 75-90° C.).
It might be thought that the flowability of the self-repairing material could be evaluated on the basis of viscosity measurements. Actually, for the purposes of the present invention, the Applicant believes that a viscosity measurement is not significant per se, besides being not easy to carry out, in particular for materials with semi-solid properties. The Applicant has therefore found that it is more convenient to evaluate empirically the flowability of the self-repairing material by means of a test in which the displacement of a predetermined amount of material placed on an inclined plane at a predetermined temperature and for a predetermined period is measured. This test is described in the technical specification ST/LAB/QFE/06, § 5.5, established by France Telecom/CNET (published: January 1994). In particular, this evaluation can be carried out as follows. About 3 grams of self-repairing material are placed on a smooth aluminium plate inclined at 60° relative to the horizontal plane. The material constituting the plate is selected so as to ensure high adhesion of the test material to the plate itself, thereby preventing the material from sliding down the plate without undergoing a substantial deformation. The plate is placed in an oven thermostatically adjusted to 60° C.; after 24 hours the material is checked for any demixing of the various components present therein, and displacement of the material front down the inclined plane, relative to its initial position, is measured, for example by means of a gauge, checking that there has been essentially no sliding of the entire mass.
In practice, the Applicant has found that the desired control of the flowability is obtained when the self-repairing material, subjected to the flow test on an inclined plane at 60° C. for 24 hours described above, shows a displacement of the front of the material sample on the inclined plane of between 0.5 and 400 mm, preferably between 1 and 200 mm and even more preferably between 50 and 100 mm.
Moreover, the Applicant believes that the “movement”0 of the self-repairing material towards the point of rupture is promoted by the action of radial compression exerted on the self-repairing layer by the other layers constituting the cable, in particular by the outer sheath. Indeed, the specific volume of plastics decreases as the temperature decreases, thus during the cooling process following extrusion the outer sheath contracts to produce a radial compressing action on underlying layers, with an estimated pressure of the order of a few bar. In the case of rupture of the cable coating, this pressure forces the self-repairing material towards the point of rupture, thereby assisting the self-repairing. Moreover, the Applicant has observed that the leakage of self-repairing material from the point of rupture stops rapidly by virtue of the cohesive properties of this material.
Another advantageous property of the self-repairing material is its capacity to exert an effective blocking action against external moisture which tends to infiltrate the cable through the point of rupture of the coating. For this purpose, it is appropriate for the self-repairing material to have a low saturation water content, with values, measured at room temperature by Karl-Fisher titration, generally of less than 400 ppm, preferably less than 200 ppm.
Moreover, in the case where the self-repairing layer is placed outside the insulating layer and the latter consists of a material which is crosslinkable via silanes, it is convenient for the self-repairing material, although absorbing small amounts of moisture, to have a sufficient permeability to water vapour since, as is known, crosslinking via silanes takes place in the presence of water. Preferred values of permeability to water vapour, measured at room temperature according to ASTM E96, are generally between 1.2×10−7 and 8.0×10−6 g/(cm·hour·mmHg).
A further preferred characteristic of the self-repairing material is a substantial physico-chemical inertness with respect to the plastic materials with which it is placed in contact. The reason for this is that it is desirable that the self-repairing material does not interact, under the working conditions, with the materials which constitute adjacent layers (generally polyolefins such as polyethylene and ethylene copolymers, which may or may not be crosslinked), thereby avoiding swelling phenomena for these materials with a consequent worsening in their mechanical properties.
A first class of materials suitable for making the self-repairing layer according to the present invention consists of amorphous polymers having properties of high-viscosity liquids or of semi-solids, these polymers being selected, for example, from the following classes of products:
(a) polyisobutene or isobutene copolymers with minor amounts of different C4-C12 α-olefins;
(b) atactic propylene homopolymers;
(c) silicone rubbers, consisting of linear chains of monomer units of formula —O—SiR1R2-, in which R1 and R2 are optionally substituted aliphatic or aromatic radicals such as, for example: dimethylsilicone, methylphenylsilicone, methylvinylsilicone, silicones containing cyanoacrylic or fluoroalkyl groups, and the like.
Among the products mentioned above, it is particularly preferred to use polyisobutene having a viscosimetric (Staudinger) average molecular weight of between 2,000 and 50,000, preferably between 5,000 and 20,000, known commercially under the trademarks Vistanex® (Esso Chemical), Hycar® (Goodrich), Oppanol® (BASF), and the like.
The amorphous polymers mentioned above can be used as such or dissolved in a suitable solvent, for example a mineral oil or a synthetic oil, in particular a paraffin oil or a naphthenic oil such as, for example, the oils known by the abbreviations ASTM 103, 104A and 104B. Preferably, low molecular weight products that are homologues of the amorphous polymer can be used as solvents.
For example, in the case of polyisobutene, a polybutene oil with an osmometric average molecular weight of between 400 and 1,300, preferably between 500 and 1,000, which can be obtained by polymerization of C4 olefin mixtures containing mainly isobutene, can advantageously be used as solvent. Products corresponding to these characteristics can be found on the market under the trademarks Napvis® (BP Chemicals) and Indopol® (Amoco).
In the case of silicone rubbers, it is possible to use a silicone oil with a viscosity generally between 100 and 5,000 mm2/sec at 25° C. as solvent.
In general, the amount of solvent is between 5 and 95% by weight, preferably between 50 and 90% by weight, relative to the total weight of the mixture.
In the case where the amorphous polymer is dissolved in a suitable solvent as mentioned above, a thickener can advantageously be added to the composition, the main function of this thickener being to control flowability, thereby reducing the risk of the self-repairing material uncontrollably leaking from the cable.
Inorganic products such as pyrogenic silica, bentonite and the like, or mixtures thereof, can for example be used as thickeners. The amount of thickener is generally between 1 and 20 parts by weight, preferably between 2 and 10 parts by weight, relative to the total weight of the mixture.
The self-repairing material as described above can be prepared according to standard techniques, for example by dissolving the amorphous polymer and any additives in the oily solvent by heating. If a thickener is used, it can be dispersed by vigorous stirring under heating.
Another category of materials which are suitable for forming the self-repairing inner layer according to the present invention consists of solid polymeric materials dispersed in an oily phase.
The oily phase can consist, for example, of:
(a) paraffinic oils or naphthenic oils, for example the oils ASTM 103, 104A or 104B;
(b) polybutene oils with an osmometric average molecular weight of between 400 and 1,300, preferably between 500 and 1,000, which can be obtained by polymerization of C4 olefin mixtures containing mainly isobutene, for example the commercial products Napvis® (BP Chemicals) and Indopol® (Amoco);
(c) polypropylene oils;
(d) low molecular weight polyesters, for example acrylic acid polyesters, such as the product ECA 7955 from Exxon Chemical Co.;
or mixtures thereof.
The solid polymeric material is generally a high molecular weight polymer with elastomeric properties, selected, for example, from:
(i) styrene block copolymers or terpolymers with different olefins and/or with dienes, for example with butene, ethylene, propylene, isoprene, butadiene and the like, and in particular: styrene-butadiene-styrene (S-B-S), styrene-isoprene-styrene (S-I-S) and styrene-ethylene/butene-styrene (S-EB-S) triblock polymers; styrene-ethylene/propylene (S-EP) and styrene-ethylene/butene (S-EB) diblock polymers; styrene-butadiene or styrene-isoprene branched polymers; such products are commercially available, for example, under the trademark Kraton® (Shell Chemical);
(ii) polyisobutene or isobutene copolymers with minor amounts of different C4-C12 α-olefins, having a viscosimetric (Staudinger) average molecular weight generally of greater than 40,000, preferably between 50,000 and 200,000;
(iii) copolymers of propylene with ethylene and/or with C4-C12 α-olefins (for example 1-butene, isobutene, 1-hexene, and the like), or with C4-C20 dienes (for example 1,3-butadiene, 1,4-hexadiene, 5-ethylidene-2-norbornene, and the like), known commercially, for example, under the trademarks Dutral® (Enichem) or Nordel® (Dow-Du Pont);
(iv) polyisoprene or natural rubber;
(v) nitrile rubbers;
(vi) butyl rubbers;
(vii) amorphous ethylene copolymers, for example copolymers of ethylene with esters having ethylenic unsaturation, such as ethylene/vinyl acetate (EVA), ethylene/methyl acrylate (EMA), ethylene/ethyl acrylate (EEA), ethylene/butyl acrylate (EBA) copolymers, and the like;
or mixtures thereof.
The solid polymeric material can be dispersed in the oily base in a subdivided form, for example in the form of granules or powder, in amounts generally of between 5 and 70% by weight, preferably between 10 and 60% by weight, relative to the total weight of the mixture. A homogeneous dispersion can be obtained by suitable mixing according to the standard techniques, for example using an internal mixer of the type with tangential rotors (Banbury) or interlocking rotors, or alternatively in continuous mixers of the Ko-Kneader type (Buss) or co-rotating or counter-rotating twin-screw mixers.
To avoid an unacceptable reduction in the mobility of the self-repairing material at low temperatures, the optionally present oily products generally have a pour point, determined according to ASTM D97-57, of less than 0° C., preferably less than −10° C. and even more preferably less than −20° C.
Inorganic fillers of various types can be added to the self-repairing materials described above, these fillers having the function of improving processability and of controlling flowability, for example: kaolin, calcium carbonate, aluminium hydroxide, magnesium hydroxide, talc, precipitated silica, and the like, or mixtures thereof. The amount of inorganic fillers can vary within a wide range, generally between 5 and 50 parts by weight, preferably between 10 and 30 parts by weight, relative to the total weight of the mixture. The self-repairing material can also contain additives of various types, such as: stabilizers, antioxidants, anti-copper products, glass microspheres, and the like.
To give the anti-repair material greater cohesiveness, tackifying agents can optionally be added such as: natural or synthetic rosins (for example the products Polypale® from Hercules, or Escorez® from Esso Chemicals), or derivatives thereof; esterified polyalcohols (for example the products Oulupale® from Veitsiluotooy), or mixtures thereof. The amount of tackifying agent is generally between 1 and 20% by weight, preferably between 5 and 10% by weight, relative to the total weight of the mixture.
The thickness of the self-repairing material layer according to the present invention must be sufficient to ensure self-repairing of the cable, therefore this thickness is selected mainly as a function of the dimensions of the cable and of the type of damage which the latter might sustain. In general, thicknesses of not less than 0.1 mm, preferably between 0.2 and 2 mm and more preferably between 0.3 and 1 mm, are sufficient.
For the purpose of giving the electrical cable according to the present invention impact-strength properties, an expanded polymer coating can be added, as described in European patent application No. 97107969.4 filed on 15.05.97 in the name of the Applicant. This coating is preferably placed in direct contact with the outer protective sheath. Although the expanded polymer coating per se does not have any particular resistance to cutting, it has the capacity of absorbing, at least partly, the energy transmitted by impact with a cutting tool and thus of reducing the risk of damage to the layers of cable coating.
The expanded polymer coating can consist of any type of expandible polymer such as, for example: polyolefins, olefinic copolymers, olefin/unsaturated ester copolymers, polyesters, polycarbonates, polysulphones, phenolic resins, ureic resins, and mixtures thereof. Preferably, olefinic polymers or copolymers, in particular based on polyethylene (PE) and/or polypropylene (PP), mixed with ethylene-propylene rubbers can be used. PP modified with ethylene-propylene rubbers (EPR), with a PP/EPR weight ratio of between 90/10 and 50/50, preferably between 85/15 and 60/40, can advantageously be used. It is also possible to mix prior to expansion the polymeric material with a predetermined amount of rubber in powder form, for example vulcanized natural rubber. In particular, the Applicant has found that a polymer material which has, prior to expansion, a flexural modulus at room temperature of greater than 200 MPa, preferably of at least 400 MPa (measured according to ASTM D790), but not greater than 2,000 MPa, so as not to excessively increase the rigidity of the finished product, is particularly suitable for this purpose. The degree of expansion of the polymer is extremely variable depending on the specific polymer used and on the thickness of the coating which it is intended to obtain. In general, the degree of expansion can range between 20% and 3,000%, preferably between 30% and 500%. The minimum thickness of the expanded layer which is capable of ensuring the desired impact strength depends mainly on the degree of expansion and on the flexural modulus of the polymer. In particular, for medium voltage cables an expanded-coating thickness of at least 0.5 mm, preferably of between 1 and 6 mm is sufficient. Further details regarding the characteristics of this expanded polymer layer are given in the above-mentioned European patent application No. 97107969.4, whose text constitutes an integral part of the present description.
For the purpose of promoting dispersion of the leakage microcurrents which may be present directly after the cable has been damaged and before it has been fully self-repaired, this conductor can advantageously be coated with a layer of polymeric material having semiconductive properties. By favouring dispersion of the leakage microcurrents, this material reduces the risk of triggering of corrosion points onto the conductor. The semiconductive layer, which is applied to the conductor by, for example, taping or, preferably, extrusion, generally has a thickness of at least 0.05 mm, preferably between 0.1 and 0.5 mm.