Electrical line

An improved electrical line comprising an electrical conductor (5) and a layer of thermoplastic material wherein the layer consists of at least one inner layer (6) and one outer layer (7) and the outer layer (7) comprises a lubricant which has migrated from the inner layer (6) into the outer layer (7).

RELATED APPLICATION

The present application is related to and claims the benefit of priority from European Patent Application No. 06290431.3, filed on Mar. 15, 2006, the entirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an electrical line, in particular an electrical line to be laid in cable ducts.

BACKGROUND

In order to reduce the time of installation of electrical lines in ducts it is very important, to reduce friction between the electrical line and the inner surface of the cable duct.

From WO 02/39560 A1 it is known to apply a friction-reducing layer to the inner surface of the duct. The friction-reducing layer is a lubricating particulate solid phase, which includes carbon black and a lubricating plastic material.

WO 2004/114489 A1 discloses a tube for installation of flexible cables. The tube comprises an outer layer of low-density polyethylene and an inner layer of high-density polyethylene containing a blend of polyamide and polyether block amides with a silicone slip agent.

On the other hand it is known to reduce the friction by providing the outer surface of the outermost layer (the sheath) with longitudinal or helical ribs. Such a solution is described in DE 8901210 U1.

JP 01089210A proposes a cable to be pulled into a duct wherein a lubricant layer for reducing a friction coefficient of the cable and a rip cord for tearing an outer sheath are interposed between an inner sheath and the outer sheath. When tearing off the outer sheath with the rip cord the friction coefficient between the inner sheath and the duct line inner wall is lowered by the lubricant layer. Thus, the cable can be introduced smoothly into the duct line without adding surplus tension to it.

Still another solution of the described problem is object of JP 02123622. To improve the self-lubricating ability a resin film layer is obtained by adding a predetermined amount of a specific lubricant to the insulating layer on a conductor. For instance 0.2-5.0 weight part of alkylphenoxypoly (ethylene oxide) ethanole are added to 100 weight part of the insulating resin.

One main problem of this solution is the compatibily between the slipperiness and the coil packaging film. It is not possible to move the coil of the electrical line provided with the lubricant without rigid boxes. This is a main drawback for the producer of such electrical lines.

OBJECTS AND SUMMARY

The object of the present invention is therefore that of providing an electrical line (or cable) with a layer, which is not slippery immediately after the production but which has a lubricating layer before it is drawn into ducts.

This object is achieved by the fact that the line comprises an inner layer and an outer layer and that the outer layer comprises a lubricant which has migrated from the inner layer into the outer layer.

Preferably the said lubricant is a wax of the group of fatty acid amides.

By way of example one can cite behenamide, oleamide, stearamide, erucamide, oleyl palmitamide, stearyl or ethylene bis-stearamide as a wax of the group of fatty acid amides.

According to a further embodiment of the invention the thermoplastic material of the inner layer is a halogene free, flame retardant compound based on copolymers of ethylene, as for instance Ethylene vinyl acetate (EVA).

The thermoplastic material of the outer layer may be the same material as for the inner layer. A polypropylene compound can be used by preference, too.

In a further preferred embodiment the ratio of the wall thickness of the inner and the outer layer is between 50:50 and 95:5.

The outer layer may be coloured in order to distinguish the lines, when more than one lines are positioned in one duct.

In a first embodiment the inner layer of thermoplastic material is extruded to the electrical conductor, whereby this material contains a lubricant, the outer layer is extruded to the inner layer, whereby the lubricant migrates at least partly from the inner layer to the outer layer over a period of at least two days, preferably at least one week.

The thermoplastic material of the inner layer is compounded with the lubricant and granulated.

Preferably the inner and the outer layer are coextruded.

In a further possibility of producing an electrical line according to the invention the inner layer of thermoplastic material is extruded to the conductor, the lubricant is applied to the inner layer and the outer layer is extruded to the inner layer coated with the lubricant. The lubricant will migrate at least partly from the surface of the inner layer into the outer layer in a period of time between two days and one week. The lubricant is applied to the inner layer preferably by spraying.

DETAILED DESCRIPTION

InFIG. 1three electrical lines1,2and3are pulled into a cable duct4. Each of the electrical lines1,2and3has a layer8of lubricant on their outer surface in order to reduce friction between the inner surface of the cable duct4and the outer surface of the electrical lines1,2and3during the installation of the electrical lines1,2and3in the cable duct4.

One embodiment of the invention is show inFIG. 2.

The electrical line comprises an electrical conductor5consisting of a plurality of twisted wires for example of copper or aluminium. Proceeding outwardly, the electrical conductor5is successively surrounded by a first inner layer6and a second outer layer7.

The inner layer6and the outer layer7are produced by coextrusion.

Alternatively the inner layer6may be extruded first to the electrical conductor5, then a wax of the group of fatty acid amides is applied to the outer surface of the inner layer6and at last the outer layer7is extruded to the inner layer6.

Preferably layers6and7consist of a halogene free, flame retardant compound based on copolymers of ethylene. Alternatively the outer layer7may consist of a polypropylene compound.

A mixture used for the inner layer6and preferably for the layer7has for example the following formulation:

The preferred materials for the EVA-Copolymer and the PE-LL are available from Exxon Chemical Co designated as “Escorene UL”, and “Escorene LL”. PW-MAH-grafted is available from Dupont and designated as “Fusabond”.

A mixture used for the outer layer7has the same formulation but without Searyl bis-stearamide.

The Searyl bis-stearamide will migrate from the inner layer6or from the coating of the inner layer6into the outer layer7and will form a lubricating layer8on the surface of the outer layer7.

The lubricant should not become effective on the surface of the layer7before one week and should be finished after four weeks.

The migration speed depends on the temperature in the layers6and7, the concentration of the lubricant in the layers6and7and the wall thickness of the layers6and7, especially of the layer7. Therefore the ratio of the wall thicknesses of the layers6and7should be in the range of 50:50 and 95:5.

When more than one electrical lines1,2or3have been installed in the duct4it is necessary to make it possible to identity the electrical lines. Therefore the outer layer7of the electrical line is coloured.