Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
2002-11-07
2004-07-27
Nguyen, Chau N. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
C174S1130AS, C174S12000C
Reexamination Certificate
active
06768060
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a coating for cables which is capable of protecting the cable from accidental impacts.
DESCRIPTION OF THE RELATED ART
Accidental impacts on a cable, which may occur, for example, during their transportation, laying etc., may cause a series of structural damage to the cable, including deformation of the insulating layer, detachment of the insulating layer from the semiconductive layer, and the like; this damage may cause variations in the electrical gradient of the insulating coating, with a consequent decrease in the insulating capacity of this coating.
In the cables which are currently commercially available, for example in those for low- or medium-tension power transmission or distribution, metal armor capable of withstanding such impacts is usually applied in order to protect cables from possible damages caused by accidental impacts. This armor may be in the form of tapes or wires (generally made of steel), or alternatively in the form of a metal sheath (generally made of lead or aluminum); this armor is, in turn, usually clad with an outer polymer sheath. An example of such a cable structure is described in U.S. Pat. No. 5,153,381.
The Applicant has observed that the presence of the abovementioned metal armor has a certain number of drawbacks. For example, the application of the said armor includes one or more additional phases in the processing of the cable. Moreover, the presence of the metal armor increases the weight of the cable considerably, in addition to posing environmental problems since, if it needs to be replaced, a cable constructed in this way is not easy to dispose of.
The Japanese patent published under the number (Kokai) 7-320550 describes a domestic cable with an impact-resistant coating 0.2-1.4 mm in thickness, placed between the insulator and the outer sheath. This impact-resistant coating is a non-expanded polymer material containing a polyurethane resin as main component.
On the other hand, use of expanded polymeric materials in cables' construction is known for a variety of purposes.
For instance, German patent application no. P 15 15 709 discloses the use of an intermediate layer between the outer plastic sheath and the inner metallic sheath of a cable, in order to increase the resistance of the outer plastic sheath to low temperatures. No mention is made in such document about protecting the inner structure of the cable with said intermediate layer. As a mattter of fact, such intermediate layer should compensate for elastic tensions generated in the outer plastic sheath due to temperature's lowering and may consist of loosely disposed glass fibers or of a material which may either be expanded or incorporating hollow glass spheres.
Another document, German utility model no. G 81 03 947.6, discloses an electric cable for use in connections inside apparatuses and machines, having particular mechanical resistance and flexibility. Said cable is specifically designed for passing on a pulley and is sufficiently flexible in order to recover its straight structure after the passage on said pulley. Accordingly, this kind of cable is specifically aimed to resist to mechanical loads of the static type (such as those generated during the passage onto a pulley), and its main feature is the flexibility. It is readily apparent to those skilled in the art that this kind of cable substantially differs from low- or medium-tension power transmission or distribution having a metal armor which, rather to be flexible, should be capable of withstanding dynamic loads due to impacts of a certain strength onto the cable.
In addition, in signal transmission cables of the coaxial or twisted pair type, it is known to use expanded materials in order to insulate a conductive metal. Coaxial cables are usually intended to carry high-frequency signals, such as coaxial cables for TV (CATV) (10-100 MHz), satellite cables (up to 2 GHz), coaxial cables for computers (above 1 MHz); traditional telephone cables usually carry signals with frequencies of about 800 Hz.
The purpose of using an expanded insulator in such cables is to increase the transmission speed of the electrical signals, in order to approach the ideal speed of signal transmission in an aerial conductive metal (which is close to the speed of light). The reason for this is that, compared with non-expanded polymer materials, expanded materials generally have a lower dielectric constant (K), which is proportionately closer to that of air (K=1) the higher the degree of expansion of the polymer.
For example, U.S. Pat. No. 4,711,811 describes a signal transmission cable having an expanded fluoropolymer as insulator (thickness of 0.05-0.76 mm) clad with a film of ethylene/tetrafluoroethylene or ethylene/chlorotrifluoroethylene copolymer (thickness of 0.013-0.254 mm). As described in that patent, the purpose of the expanded polymer is to insulate the conductor, while the purpose of the film of non-expanded polymer which clads the expanded polymer is to improve the mechanical properties of the insulation, in particular by imparting the necessary compression strength when two insulated conductors are twisted to form the so-called “twisted pair”.
Patent EP 442,346 describes a signal transmission cable with an insulating layer based on expanded polymer, placed directly around the conductor; this expanded polymer has an ultramicrocellular structure with a void volume of greater than 75% (corresponding to a degree of expansion of greater than 300%). The ultramicrocellular structure of this polymer should be such that it is compressed by at least 10% under a load of 6.89×10
4
Pa and recovers at least 50% of its original volume after removal of the load; these values correspond approximately to the typical compression strength values which the material needs to have in order to withstand the compression during twisting of the cables.
In International patent application WO 93/15512, which also relates to a signal transmission cable with an expanded insulating coating, it is stated that by coating the expanded insulator with a layer of non-expanded insulating thermoplastic polymer (as described, for example, in the abovementioned U.S. Pat. No. 4,711,811) the required compression strength is obtained, this however reducing the speed of propagation of the signal. The said patent application WO 93/15512 describes a coaxial cable with a double layer of insulating coating, where both the layers consist of an expanded polymer material, the inner layer consisting of microporous polytetrafluoroethylene (PTFE) and the outer layer consisting of a closed-cell expanded polymer, in particular perfluoroalkoxytetrafluoroethylene (PFA) polymers. The insulating coating based on expanded polymer is obtained by extruding the PFA polymer over the inner layer of PTFE insulator, injecting Freon 113 gas as expanding agent. According to the details given in the description, this closed-cell expanded insulator makes it possible to maintain a high speed of signal transmission. It is moreover defined in that patent application as being resistant to compression, although no numerical data regarding this compression strength are given. The description emphasizes the fact that conductors clad with such a double-layer insulator can be twisted. Moreover, according to that patent application, the increase in void volume in the outer expanded layer makes it possible to obtain an increase in the speed of transmission, thereby giving rise to small variations in the capacity of this coating to oppose the compression of the inner expanded layer.
As is seen from the abovementioned documents, the main purpose of using “open cell” expanded polymer materials as insulating coatings for signal transmission cables is to increase the speed of transmission of the electrical signal; however, these expanded coatings have the drawback of having an insufficient compression strength. A few expanded materials are also generically defined as “resistant to compression”, since they have to ensure not only a high speed of
Balconi Luca
Bareggi Alberto
Belli Sergio
Caimi Luigi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Nguyen Chau N.
Pirelli Cavi e Sistemi S.p.A.
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