The present invention relates to a rope comprising steel wires. It also relates to a use of such a rope as, for example, a fishing rope.

Background of the invention.

Ropes comprising steel wires are widely known in the art.

Users of such ropes are generally confronted with two types of problems.

A first problem is a mechanical problem. During use the various steel wires of the rope rub against each other and cause damage. This is called fretting.

A second problem is a chemical problem, namely corrosion. Particularly in aggressive environments such as a marine environment, the steel wires of the rope are subject to corrosion, which considerably reduces the lifetime. Even when not in active use, i. e. when the rope is not immersed in seawater, the corrosion continues due to water rests and salt particles still present between steel wires of a same rope. Steel wires have been conventionally galvanized, i. e. subject to a zinc plating in order to withstand better the corrosion attack.

Although the zinc plating has provided a substantially improved corrosion resistance in comparison with steel wires, which are not plated with zinc, the market is still demanding for higher life times.

Summary of the invention.

It is an object of the present invention to avoid the disadvantages of the prior art.

It is also an object of the present invention to provide a rope which comprises steel wires and which has an improved resistance against corrosion.

It is a further object of the present invention to increase the lifetime of a rope.

According to the invention, there is provided a rope, which comprises steel wires. At least one of the steel wires is a double-coated steel wire.

This double-coated steel wire comprises a steel core which is individually coated with a zinc or zinc alloy plating, as the first coating.

The zinc or zinc alloy plating is further coated with an extruded material selected from the group consisting of polyethylene terephtalate ("PET"), polybutylene terephtalate ("PBT"), polyethylene naphtenate ("PNT") or any copolymer comprising polyethylene terephtalate, polybutylene terephtalate or polyethylene naphtenate. This is the second coating.

The rope may further comprise grease in order to reduce the fretting between adjacent steel wires.

Within the context of the present invention, the terms"polyethylene terephtalate"denote not only homopolymers of ethylene terephtalate but also copolymers of ethylene terephtalate containing not more than 20 % of other copolymerized units, e. g. derived from acids other than terephtalic acid, such as isophtalic acid or from other glycols than ethylene glycol. The polymer may also contain mixtures of polymers in order to modify certain of the properties thereof.

The polyethylene terephtalate gives the coated steel wire a good resistance against corrosion.

Surprisingly the individual polyethylene terephtalate coating on the double-coated steel wires is not damaged seriously during the twisting operation. Indeed polyethylene terephtalate exhibits a good resistance against abrasion. Furthermore, polyethylene terephtalate has a relatively low coefficient of friction.

In addition to these advantages, polyethylene terephtalate has still more advantages.

A polyethylene terephtalate has a good adhesion and adhesion retention, has a high resistance against ultra-violet light (= better weatherability) and has a lower absorption of water or moisture. A

polyethylene terephtalate coating absorbs only one tenth of the amount of moisture absorbed by a nylon-6 coating in the same circumstances.

This means that a steel wire with a polyethylene terephtalate coating does not swell up to the same degree as a steel cable with a nylon-6 coating. Moreover, application of a polyethylene terephtalate coating can be done in an environment-friendly way, i. e. with a much simpler pre-treatment and without the use of primers.

Some prior art embodiments, such as US-A-5,303,498, disclose a rope of zinc coated steel wires which are twisted together to form the rope and which are thereafter, as a whole, coated with a synthetic coating such as polyethylene terephtalate. While providing also a double protection against corrosion, these embodiments have the drawback that the interstices between the steel wires are not completely penetrated and filled with the polyethylene terephtalate so that the double protection does not work along the whole length of the steel wires.

According to a typical embodiment of the present invention, the rope is a multi-strand rope, i. e. the rope comprises more than one strand.

For example, the rope may comprise a core strand and layer strands, where the layer strands are twisted around the core strand. All steel wires present in the rope may be double-coated steel wires. Preferably, however, the double-coated steel wires are present in the interior of the rope, i. e. the steel wires which are radially outermost are not double- coated steel wires.

More particularly, the core strand and the layer strands may comprise inner steel wires and outer steel wires surrounding the inner steel wires.

Preferably none of the outer steel wires of the layer strands are double- coated steel wires. The reason is that the polyethylene terephtalate coating is damaged relatively easily if present on the outer steel wires of the layer strands. Once damaged, the double protection against corrosion is no longer present.

Having regard to the double corrosion protection, the invention rope is not particularly designed for reinforcement of elastomers or rubbers, where the elastomer or the rubber already protect the steel reinforcement partially against corrosion. The invention rope is particularly suitable for use in aggressive environments: an example is its use as a fishing rope, e. g. to drag or trawl fishing nets.

The steel used for the steel wires of the rope is a plain carbon steel and has a perlite structure.

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein -FIGURE 1 is a cross-section-of a double-coated steel wire ; - FIGURE 2 is a cross-section of a first embodiment of a fishing rope; - FIGURE 3 is a cross-section of a second embodiment of a fishing rope; - FIGURE 4 is a cross-section of a third embodiment of a fishing rope; -FIGURE 5 is a cross-section of a fourth embodiment of a fishing rope.

Description of the preferred embodiments of the invention.

A double coated steel wire is made starting from a wire rod with following steel composition: a carbon content ranging from 0.30 % to 0.70 % (in some cases this may be more than 0.80 %), a manganese content ranging from 0.30 % to 0.80 %, a silicon content ranging from 0.10 % to 0.50 %; a maximum sulfur content of 0.05 %, a maximum phosphorous content of 0.05 %, the remainder being iron and possible traces of copper, chromium, nickel, vanadium, molybdenum or boron.

The wire rod is cold drawn from a starting diameter ranging of about 5.5 mm to 6.5 mm to a final diameter ranging between 0.80 mm and 2.50 mm. This cold drawing is done through one or more series of cold drawing steps. Before the last series, the steel wire may be plated with a zinc or with a zinc alloy. This plating is preferably done by means of a hot dip coating.

The zinc alloy coating is preferably an alloy where the aluminium content ranges from 2 to 12 %. For example, in the metallic fraction of the zinc alloy coating, the aluminium content ranges from 4 to 6.5 %, and is preferably about the eutectic value of 5 %.

A wetting agent may be present in an amount sufficient to have wetting of the substrate steel by the alloy. Amounts smaller than 0.1 % are usually sufficient. The wetting agent can be cerium in an amount ranging from 0.01 % to 0.05 % and/or lanthanum in an amount ranging from 0.01 % and 0.06 %. All percentages mentioned here are percentages by weight of the zinc alloy coating.

The weight of the zinc aluminium alloy on the final steel wire may range from 15 g/m2 to 120 g/m2, e. g. from 20 g/m2 to 60 g/m2 for wire diameters up to 1.25 mm and from 30 g/m2 to 80 g/m2 for wire diameters greater than 1.25 mm. The minimum values are imposed by reason of providing a sufficient corrosion protection. The maximum values are imposed by reason of cost of by reason of fatigue resistance. Too thick a zinc aluminium coating may cause decrease in fatigue resistance.

After the final drawing step the plated steel wire is coated with a polyethylene terephtalate by means of an extrusion process.

The thickness of the polyethylene terephtalate coating on the final steel wire may range from 20 micrometer (urn) to 150 um. The minimum values are imposed by reason of providing a sufficient abrasion resistance. The maximum values are imposed by reasons of cost.

However, more important than the actual value of the thickness of a polyethylene terephtalate coating is the absence of damages or discontinuities in the polyethylene terephtalate coating.

FIGURE 1 shows the cross-section of a double-coated steel wire 10.

Steel wire 10 comprises a steel core 12, an intermediate coating 14 of a zinc aluminium (5%) alloy and an external coating 16 of polyethylene terephtalate.

FIGURE 2 shows the cross-section of a first embodiment of a fishing rope 20. Fishing rope 20 comprises a core strand 22 and six layer strands 24 twisted around the core strand 22.

Core strand 22 comprises a textile core 26. This textile core 26 may have natural or synthetic fibers, but natural fibers are preferred since they absorb better grease and fat. Examples of natural fibers are jute fibers and hemp fibers. Before its use, textile core 26 is saturated with a suitable grease. Double-coated steel wires 10 are twisted around the textile core 26. Conventional twisting apparatus such as a tubular strander or a double-twister may do this twisting. In order to avoid as much as possible damage to the external polyethylene terephtalate coating 16, the guidings of the steel filaments and of the twisted rope may be carried out in a suitable synthetic material or may be coated with a suitable synthetic material.

Each layer strand 24 also comprises a textile core 26. Zinc coated steel wires 28 are twisted around the textile core 26 of each layer strand 24.

Double-coated steel wires are not excluded here, but less desired and needed than in the core strand 22. The reason is that a polyethylene terephtalate coating on the zinc coated steel wires 28 could be more easily damaged than such a polyethylene terephtalate coating on steel wires 10 inside the rope 20.

The layer strands 24 are then twisted around the core strand 22.

Typical fishing rope diameters range from 16 mm to 20 mm.

FIGURE 3 shows a cross-section of a second embodiment of a fishing rope 30. Fishing rope 30 comprises a textile core 32 saturated with a suitable grease. Six outer strands 34 surround textile core 32. Each outer strand 34 comprises seven double-protected steel wires 10,12, 14,16 in a (1 + 6)-configuration. Each (1 + 6)-configuration is further

coated by means of a polypropylene or polyethylene coating 36. Fishing rope 30 may have rope diameters ranging from 10 mm to 32 mm.

FIGURE 4 shows a cross-section of a third embodiment of a fishing rope 40. Fishing rope 40 comprises a textile core 42 saturated with a suitable grease. Six outer strands 44 have been twisted around textile core 42. The six outer strands 44 have a (1 + 6 + 12)-configuration, i. e. one steel wire is surrounded by six intermediate steel wires and the six intermediate steel wires are surrounded by twelve outer steel wires.

Only the core steel wire and the six intermediate steel wires are double coated steel wires 10,12,14,16. The outer steel wires are only coated with a zinc coating or with a zinc alloy coating 14. The diameter of rope 40 may range from 10 mm to 32 mm.

FIGURE 5 shows yet another cross-section of a fourth embodiment of a fishing rope 50. Fishing rope 50 comprises a core strand 52 and six layer strands 54 twisted around core strand 52. The core strand 52 may have a (7 x 7)-configuration, i. e. [ (1 + 6) + 6 x (1 + 6)]. The six layer strands 54 each have a (1 + 9 + 9)-configuration, i. e. a single core steel wire, surrounded by nine intermediate steel wires, which are in their turn surrounded by nine outer steel wires 56. The nine outer steel wires are profiled steel wires, i. e. they have a flattened cross-section. Except for the nine profiled outer steel wires 56 for each of the layer strands 54, all the other steel wires of the layer strands 54 and all the steel wires of the core strand 52 are preferably double-coated steel wires 10,12,14,16.