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Title:
PROCESS FOR PREPARING A METAL WIRE WITH A NON-ISOCYANATE POLYURETHANE COATING
Document Type and Number:
WIPO Patent Application WO/2007/062812
Kind Code:
A1
Abstract:
The present invention relates a method of applying a coating layer comprising a non-isocyanate polyurethane (NIPU) on an elongated metal wire. The invention allows to achieve a thin coating on a wire in an environment-friendly way and results in an increased resistance against corrosion and in an increased fatigue resistance.

Inventors:
MAUER DANIEL (BE)
SOMERS ALBERT (BE)
Application Number:
EP2006/011432
Publication Date:
June 07, 2007
Filing Date:
November 29, 2006
Export Citation:
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Assignee:
BEKAERT SA NV (BE)
MAUER DANIEL (BE)
SOMERS ALBERT (BE)
International Classes:
C08G71/04; C09D175/04; H01B3/30
Domestic Patent References:
WO2005016993A12005-02-24
Foreign References:
US6120905A2000-09-19
US4544725A1985-10-01
US3526678A1970-09-01
Attorney, Agent or Firm:
HEYDE, Katrien (Bekaertstraat 2, Zwevegem, BE)
Download PDF:
Claims:
CLAIMS

1. A method of applying a coating layer on an elongated metal wire with a composition, said method comprising the steps of - providing an elongated metal wire; passing said elongated metal wire into and through a bath of non-isocyanate polyurethane composition whereby said non-isocyanate polyurethane is formed by cross-linking a cyclocarbonate oligomer and an amine oligomer, wherein the cyclocarbonate oligomer has an average functionality towards primary amines from about 2.0 to about 5.44, wherein the cyclocarbonate oligomer comprises at least one cyclocarbonate-terminated oligomer terminated with a plurality of cyclocarbonate groups, wherein at least one cyclocarbonate-terminated oligomer further comprises from about 4% to about 12% by weight of terminal epoxy groups based on the weight of terminal cyclocarbonate groups present, wherein the amine oligomer has an average functionality towards cyclocarbonate groups of from about 3.0 to about 3.8, wherein the amine oligomer comprises at least one primary amine- terminated oligomer terminated with a plurality of primary amine groups, wherein the amine oligomer is present in an amount from about 0.93 to about 0.99 of the amount of the amine oligomer that would be required to achieve a stoichiometric ratio of the primary amine groups of the amine oligomer to the cyclocarbonate groups of the cyclocarbonate oligomer, and wherein the network polymer formed has a gel fraction of not less than about 0.96 by weight.

2. A method according to claim 1, further characterized in that the average functionality of the cyclocarbonate oligomer towards primary amines ranges from about 2.6 to about 5.3.

3. A method according to claim 1 or 2, further characterized in that the gel fraction of the non-isocyanate polyurethane polymer is not less than about 0.975.

4. A method according to any one of the preceding claims, further characterized in that the cyclocarbonate-terminated oligomer has a number average molecular weight of from about 350 g/mol to about 3,200 g/mol.

5. A method according to claim 4, wherein the cyclocarbonate-terminated oligomer has a number average molecular weight of from about 700 g/mol to about 1400 g/mol.

6. A method according to any one of the preceding claims wherein the cyclocarbonate-terminated oligomer has a viscosity at 25 0 C of from about

150 mPa s to about 8,800 mPa s.

7. A method according to any one of the preceding claims, wherein the cyclocarbonate-terminated oligomer has a viscosity at 25 0 C of from about 350 mPa s to about 1,500 mPa s.

8. A method according to any one of the preceding claims, wherein the cyclocarbonate-terminated oligomer comprises at least one material selected from the group consisting of di-carbonate, tri-carbonate, tetra-carbonate and penta-carbonate ester, ether or amine derivatives of aromatic or aliphatic compounds comprising from 2 to 5 terminal functional groups selected from the group consisting of hydroxy groups, amine groups, and mixtures thereof.

9. A method according to any one of the preceding claims, wherein the at least one terminal epoxy group comprising cyclocarbonate-terminated oligomer consists essentially of a remainder and an epoxy group, wherein the epoxy group is bonded to the remainder by at least one primary carbon atom adjacent to the epoxy group.

10. A method according to any one of the preceding claims, wherein said coating layer includes a non-isocyanate polyurethane produced from a carbonated vegetable oil.

11. A method according to any one of the preceding claims, wherein the coating layer includes non-isocyanate-based polyurethane-and polyurethane-epoxy network nanocomposite polymeric composition.

12. A method according to any one of the preceding claims, whereby said metal wire is coated with a metal or metal alloy coating.

13. A method according to claim 12, whereby said metal or metal alloy comprises zinc or a zinc alloy.

14. A method according to any one of the preceding claims, whereby said elongated metal wire is passed in a horizontal or vertical direction into and through said bath.

15. A method according to any one of the preceding claims, whereby said elongated metal wire is heated before and/or after entry into said bath.

16. A method according to any one of the preceding claims, whereby said elongated metal element is passed axially through a rotatable tube situated with its lower end below the surface level of said bath.

17. A method according to claim 16, whereby said tube is rotated at a speed to remove excess coating composition from said elongated metal element to obtain a coating layer having the desired thickness.

Description:

PROCESS FOR PREPARING A METAL WIRE WITH A NON-ISOCYANATE POLYURETHANE COATING

Field of the Invention

The present invention relates to a method of applying a corrosion-resistant coating 5 layer on a metal wire.

Background of the Invention

It is generally known that, thermodynamically, metal wires are stable only under reducing conditions and corrode upon exposure to an oxidizing ambient. 10 The corrosion resistance of metal wires such as steel wires can be increased by applying a galvanizing treatment where a plating layer such as a zinc coating is applied to the metal wire core.

Alternatively or additionally and depending upon the eventual application, polymer coatings such as polyvinylchloride or polyamide can be applied to the metal wire. 15 Organic corrosion inhibitors may also be applied to the metal wire by immersion into a water or other organic solvent containing the inhibitor or by vapor treatment. All these procedures require additional equipment and processing time. Therefore, there exists a need for a method of treating metal wires which protects the bare metallic surface from corrosion. 20

Summary of the Invention

Accordingly, it is a primary objective of the present invention to provide for a coating which is resistant against corrosion.

It is another object of the present invention to provide for a coating which gives a 25 low friction resistance to the metal wire.

It is still another object of the present invention to provide for a coating which gives the metal wire a good weatherability especially under dynamically loaded conditions.

It is yet another object of the present invention to provide for a coating which gives the metal wire a good resistance against fatigue. 30 It is also another object of the present invention to provide for a coating which is acceptable from an environment point of view.

A further objective of the present invention is to provide a metal wire, in particular an elongated metal wire, coated with a material which is especially suitable to be mechanically treated in machines for the manufacture of metallic wire products, such as fencing wire, brush wire, spring wire, wire for rock protection, gabion wire, cable armoring wire, brassiere wire, rope wire such as fishing rope wire, wire for fish farming, spoke wire, catheter wire, wire for bedding and seating. Such a wire must not only be coated with a weather and corrosion resistant material, but it must also be flexible enough to withstand any bending operations necessary in shaping the wire to form desired articles. The coating must also be hard enough to be caught without damage in machines, e.g. in a twisting head or between two toothed crimping wheels etc. the coating must demonstrate accordingly, a good corrosion resistance in combination with a high fatigue resistance.

It is yet another objective of the present invention to provide, for providing a dense and closed coating to a metal wire surface, a method by which the formation of voids at the interface between the two materials may be reduced or prevented.

We have now found that the above stated objectives may be accomplished by the following invention wherein a coating composition, suitable for application to metal wire comprises non-isocyanate polyurethanes.

According to an embodiment of the present invention, there is provided a method for use in providing elongated metal wire with a non-isocyanate polyurethane coating material, which method uses a coating application unit such as a dipping tank, means for feeding the metal wire to be coated into and through the coating application unit in a horizontal or vertical direction; means for heating the metal wire to be coated before and / or after exiting the coating application unit whereby in use the coating material composition can be formed to a coating on the metal wire; and a coating control device in the container comprising a rotatable tube situated so that in use the article with the coating composition thereon passes axially through the tube while the tube is rotating.

Various organic and inorganic coatings to protect metal wires have been described in the art. Some coatings are designed to control the electrochemistry of corrosion, while others such as conversion and organic coatings create physical barriers to retard the corrosion rate in an oxidizing environment. Examples of prior art are U.S. patents No. 5,108,793 and No. 5,200,275, disclosing insoluble silicate coating. U.S. patents No. 5,292,549, No. 5,433,976 and No. 5,750,197 also disclosed a method of using both a cross-linking silane and a functionalized silane to treat a metallic substrate. Non-isocyanate polyurethanes are described in US patent No 5,340,889 ; US patent No 6,120,905, US 2004192803, WO 2005016993 and US 2004230009.

None of the prior art documents describe the technical features underlying the present invention nor do these disclose or suggest the benefits associated with the present invention.

These and other benefits and aspects of the present invention are described in more detail below and will be appreciated from the detailed description and figures.

Description of Preferred Embodiments of the Invention

The present invention is directed to a metal wire coated with a coating layer comprising a non-isocyanate polyurethane (NIPU).

Non-isocyanate polyurethanes are formed from the reaction between a cyclocarbonate reactant, which typically is an oligomer or a mixture of oligomers comprising terminal cyclocarbonate groups, and at least one primary diamine and/or polyamine, which typically is an oligomer or a mixture of oligomers comprising terminal primary amine groups. Within this structure, an intramolecular hydrogen bond is thought to form which is able to raise the hydrolytic stability of the non- isocyanate polyurethane. Generally, materials containing intramolecular hydrogen bonds have chemical resistance from 1.5 to 2 times greater than materials of similar chemical structure but without such bonds. Examples of said non-isocyanate polyurethanes are described in WO 9965969 and WO 03028644.

Non-isocyanate polyurethanes exhibit superior resistance properties to chemical degradation, from 30% to 50% greater than conventional polyurethanes. Non- isocyanate polyurethanes also have significantly reduced permeability, from 3 to 4 times less than conventional polyurethanes. Unlike conventional polyurethanes that have a porous structure, non-isocyanate polyurethane form a material substantially free of pores because, during their formation, they are not sensitive to moisture on surfaces or fillers. Since they are not formed from highly toxic isocyanate compounds, non-isocyanate polyurethanes can be easily and safely synthesized with material hardening commonly occurring at room temperature in an environment friendly way.

A mechanism by which the hydrolytic stability is raised is thought to involve hydrogen bond formation through the introduction, into the non-isocyanate polyurethane network, of hydroxy groups adjacent to the urethane carbonyl groups. Non-isocyanate polyurethanes are formed from the reaction of a cyclocarbonate group and a primary amine group to form a urethane link.

The preparation and properties of linear non-isocyanate polyurethanes is disclosed by W. J. Blank["Non-Isocyanate Routes to Polyurethanes", Proceedings of the 17th Water-Borne and Higher Solids Coatings Symposium, New Orleans, LA, February 21- 23,1990, pp.279-291].

Preferred non-isocyanate polyurethane polymer are formed by cross-linking at least one cyclocarbonate oligomer and at least one amine oligomer. The cyclocarbonate oligomer contains a plurality of terminal cyclocarbonate groups.

In addition to containing a plurality of terminal cyclocarbonate groups, at least one cyclocarbonate oligomer further comprises from about4% to aboutl2% by weight (wt.%) of terminal epoxy groups based on the weight of terminal cyclocarbonate groups present. The cyclocarbonate oligomer or oligomers have an average functionality towards primary amines of from about 2.0 to about 5.44. Determination of the average functionality of the reactants which form the non-isocyanate polyurethane is discussed in detail in WO 99/65969. The amine oligomer comprises

at least one primary amine terminated oligomer terminated with a plurality of primary amine groups and has an average functionality towards cyclocarbonate groups of from about 3.0 to about 3.8. The amine oligomer is present in an amount from about 0.93 to about 0.99 of the amount of the amine oligomer that would be required to achieve a stoichiometric ratio between the primary amine groups of the amine oligomer and the cyclocarbonate groups of the cyclocarbonate oligomer.

Because at least one cyclocarbonate oligomer comprises both cyclocarbonate and epoxy reactive groups, the network formed there from is referred to as a hybrid non- isocyanate polyurethane (HNIPU). The hybrid non-isocyanate polyurethane polymer formed has a gel fraction, i. e., the weight fraction of insoluble material, of not less than about 0.96.

Highly preferred non-isocyanate polyurethane polymer comprises: (a) selecting least one oligomer terminated with a plurality of cyclocarbonate groups, the cyclocarbonateterminated oligomer further comprising from about 4% to about 12% by weight of terminal epoxy groups based on the weight of terminal cyclocarbonate groups present, where the oligomer has an average functionality towards primary amines of from about 2.0 to about 5.44; (b) selecting at least one other oligomer terminated with a plurality of primary amine groups, where the amine oligomer has an average functionality towards cyclocarbonate groups of from about 3.0 to about 3.8;

(c) mixing the oligomers in an amount to form a mixture with a pot life such that the amount of the amine oligomer (s) present is from about 0.93 to about 0.99 of the amount of the amine oligomer (s) that would be required to achieve a stoichiometric ratio between the primary amine groups of the amine oligomer (s) and the cyclocarbonate groups of the cyclocarbonate-terminated oligomer (s); and

(d) curing the mixture at a temperature of from aboutlO C to-about 140 C to form a hybrid non-isocyanate polyurethane polymer with a gel fraction of not less than about 0.96 by weight.

By using mixtures of different cyclocarbonate oligomers, it is possible to prepare cyclocarbonate oligomer compositions with the desired average functionality toward primary amines, i. e., over the range of from about 2.0 to about 5.44 and, preferably, from about 2.6 to about 5.3. When a mixture of cyclocarbonate oligomers is present, any or all of the components of such a mixture may have a functionality toward primary amine groups less than about 2.0 or greater than about 5.44, so long as the average functionality of the mixture falls within the range of from about 2.0 to about 5.44 and, preferably, from about 2.6 to about 5.3. Each cyclocarbonate oligomer of the present invention, whether used alone or in a mixture of such oligomers, typically has a number average molecular weight of from about 350 g/mol to about 3,200 g/mol and, preferably, from about 700 g/mol to about 1400 g/mol. Each cyclocarbonate oligomer of the present invention, whether used alone or in a mixture of such oligomers, typically has a viscosity at 25 0 C of from about 150 mPa s to about 8,800 mPa s and, preferably, from about 350 mPa s to about 1,500 mPa s. Preferred cyclocarbonate oligomers include but are not limited to the di-, tri-, tetra- and penta-carbonate ester, ether or amine derivatives of aromatic or aliphatic compounds comprising from 2 to 5 terminal hydroxy and/or amine functional groups and mixtures thereof.

For the purpose of the present invention, it will be understood that the term "cyclocarbonate oligomer" as used herein includes molecules comprising only cyclocarbonate terminal groups and molecules comprising both terminal cyclocarbonate groups and a terminal epoxy group or groups. Thus, polymers formed from such epoxy comprising oligomers are sometimes referred to as hybrid non- isocyanate polyurethane polymers to distinguish them from non-isocyanate polyurethane formed only by the reaction of cyclocarbonate and amine terminal groups. As used herein, the terms hybrid non-isocyanate polyurethane and non- isocyanate polyurethane are synonymous.

The coating layer of the present invention also includes a non-isocyanate polyurethane including those produced from a carbonated vegetable oil. US 2004230009 discloses a method of converting an epoxide ring to a five-membered cyclic carbonate ring, comprising a step of: reacting a starting material that contains

an epoxide ring with carbon dioxide, wherein the epoxide ring is converted to a five- membered cyclic carbonate ring. Such a starting material containing an epoxide ring(s) is particularly preferred for use when contained in a natural resource, especially a non-hazardous natural resource, of which epoxidized vegetable oils are preferred examples, with epoxidized soybean oil (ESBO) being a particularly preferred example. Thus, in another embodiment, the present invention provides a method of making a monomeric functionalized oil, comprising the step of: carbonating an epoxidized vegetable oil, wherein a carbonated vegetable oil is produced. As the vegetable oil, there may be used, e.g., soybean oil (SBO), linseed oil, palm oil, sunflower oil, or other vegetable oils, of which soybean oil is a particularly preferred example. Vegetable oils are commercially available, and may even be purchased at a grocery store. Using such a non-hazardous starting material is beneficial. Epoxidization of a vegetable oil may be accomplished by appropriate chemical derivatization. Alternately, vegetable oils may be purchased in epoxidized form. Among the carbonated vegetable oils that may be produced according to the present invention are vegetable oils containing cyclic carbonate groups, of which carbonated soybean oil (CSBO) is mentioned as a preferred example of a novel carbonated vegetable oil.

A particularly preferred use of these novel carbonated products is as a reaction product for forming a non-isocyanate polyurethane, such as by mixing (1) a carbonated vegetable oil (such as CSBO, etc.,) and (2) an amine having functionality of at least two. Most preferably, the carbonated vegetable oil and amine are mixed stoichiometrically at or within nearly balanced stoichiometry, preferably within +- 15% of balanced stoichiometry. Preferred examples of amines having functionality of at least two are, e.g., ethylenediamine (ED), hexamethylenediamine (HMD), and tris(2-aminoethyl) amine (TA). Other non-mono-amines may be used. Most preferably, the method includes a viscous solution being produced from the mixing of the carbonated vegetable oil and the amine having functionality of at least two, and the viscous solution is transferred into a mold, followed by curing.

The coating layer of the present invention also includes non-isocyanate-based polyurethane-and polyurethane-epoxy network nanocomposite polymeric composition

such as disclosed in WO 2005016993. Preferred compositions include composition for forming a polyurethane polymeric compositions comprising (a) a polymerisable organic material that bears at least one cyclocarbonate group or a mixture thereof; (b) a natural or synthetic, modified or unmodified nano-clay [ionic phyllosilicate] with a platelet thickness of less than 25A (-2. 5 nm), more preferable less than 10A(~l nm), and most preferably between 4-8A (-0. 5-0.8 nm) and an aspect ratio(length/thickness) higher than 10, more preferably higher than 50 and most preferably higher than 100) or a mixture thereof or a nanocomposite formed from such a nano-clay, preferably the nano-clay is a natural or modified montmorillonite ; and (c) at least one hardener for component (a), which preferably is a primary and/or secondary amine or a mixture thereof.

Metal wire

As elongated metal wire, a steel wire, a metal strip, a metal tape or ribbon can be considered. The metal wire may have any cross-section such as a circular, oval or flat (rectangular) cross-section. It should be understood that, in accordance with the present invention, the concept underlying the present invention is not restricted to wire materials having a circular cross-section. Other shapes such as a strip, a flat laminate or polygonal forms can be used also and the expression "wire" is not to be limiting. The tensile strength of a metal wire can be preferably higher than 600 N/mm 2 , e.g. higher than 1500 N/mm 2 . The range of the tensile strength is for example between 1500 and 4000 N/mm 2 and higher. Any metal or metal alloy can be used to provide the metal wires of the composite article according to the invention. Preferably, the metals or metal alloys are selected from iron, titanium, aluminium, copper and alloys thereof. Preferred alloys comprise low and high carbon steels or stainless steel alloys. The metal wire can be coated with one or more metal or metal alloy coating before the coating layer according to the present invention is applied. Preferred metal or metal alloy coatings comprise zinc and zinc alloy coatings such as zinc-aluminum, zinc-manganese, zinc-aluminum-manganese, zinc- cobalt alloy, zinc-nickel alloy, zinc iron alloy or zinc-tin alloy coatings. A preferred zinc- aluminum coating comprises a zinc coating comprising 2 to 10 % Al and possibly 0.1 to 0.4 % or a rare earth wire such as La and/or Ce.

The term "high carbon steel" is intended to include carbon steel, also called ordinary steel, straight or plain carbon steel such as American Iron and Steel Institute Grade 1070 or 1080 high carbon steel. This steel owes its properties chiefly to the presence of carbon without substantial amounts of other alloying wires. In this respect see Metals Handbook, The American Society for Metals, Metals Park, Cleveland, Oh.

The metal wires coated with the non-isocyanate polyurethane of the present invention are excellent in both wet corrosion resistance such as salt spray resistance and hot-salt-water immersion resistance and dry corrosion resistance such as exposure corrosion resistance and filiform corrosion resistance. Due to the higher corrosion resistance given by the coating layer of the present invention to the metal wire such as ropes or wires, plain carbon steels or high-carbon micro-alloyed steels (from 0.70 % to 1.10% C with additions of Cr, V, Cu, Ni, B or Nb ... up to maximum 0.50 %) can be used instead of stainless steels so that much higher tensile strengths (up to 4000 N/mm 2 and higher) can be obtained. This results in ropes which are lighter and more flexible.

Prior art extrusion coatings may have a thickness starting from 10 μm on in order to provide a good barrier against corrosion. In contrast herewith, the invention allows to have thicknesses down to 1 μm and yet provide a sufficient barrier against corrosion. Suitable thicknesses are 1 μm, 2 μm, 5 μm up to 50 μm a 60 μm and higher. This thin coating is favorable to the flexibility of, e.g. a rope.

The steel wire may be provided with a lubricant oil, such as a silicon oil, on top of the coating or incorporated in the coating in order to further reduce the friction resistance.

In order to increase the adhesion between the non-isocyanate coating and the metal wire a primer layer or a silane may be applied on the metal wire before its coating with the non-isocyanate coating.

Alternatively or additionally, fillers may be provided in the non-isocyanate coating. These fillers may have various functions such as increasing the UV resistance or giving a color to the coated wire.

Method of the Present Invention

According to another embodiment of the present invention there is provided a method of coating elongate metal wire of essentially circular cross-section with a coating composition which comprises passing the metal wire in a horizontal or vertical direction into and through a bath of non-isocyanate polyurethane composition, the metal being heated before and / or after entry into the bath, whereby the non-isocyanate polyurethane adheres as a coating, the metal being passed axially through a rotatable tube situated with its lower end below the surface level of the bath, the tube having a diameter and being rotated at a speed whereby excess coating composition is removed from the metal and a coating of the desired thickness is obtained.

It is well known, for example, that moving a wire through a liquid material can cause the wire to drag along a layer of the liquid material. This method is used, for instance, in the galvanizing of steel wires. It is also known that increasing the speed of the moving wire causes the quantity of the dragged material or the thickness of the layer to increase very rapidly. Typically, when this method is applied to the coating of a wire with a liquid viscous composition, two big problems arise which are very disadvantageous and noticeable at high working speeds, viz. (a) the moving wire drags along a layer which is irregular and too thick and (b) the coating is not applied concentrically around the wire.

It has been found that the coating compositions of the present invention makes it possible to obtain high working speeds and to provide layers or coatings of normal thickness which are formed concentrically around an elongated cylindrical or substantially cylindrical object.

Brief Description of the Drawings

The invention will now be further described with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic view of the method according to the present invention;

Figure 2 and Figure 3 are diagrams showing the embodiments specific to the immersion drip coating unit.

Detailed Description of Preferred Embodiments

It needs to be understood that typical systems for two-component polyurethane can be applied: a dipping tank or other systems such as hot melt guns may be used. Said and other coating processes and systems are described in www.teonline.com/coatinq-process.htm.

The basis elements of a coating line according to the present invention are :

1. A wire cleaning unit (for optimal conditions);

2. Delivery and coating units for NIPU can be any system that can supply on a continuous and uniform manner fluids of the same range of viscosities. Examples are a dip system ; hot melt gun;

3. A heating system which delivers the required amount of energy to ensure complete reaction between the cyclocarbonate and the amine. Examples are hot air, IR and induction.

Referring to Figure 1, an embodiment of a method for the coating of a wire with the coating material composition consists of a take off unit 10, a wire cleaning unit 12, a deposition unit 14, a curing unit 16 such as drying unit or infrared oven or hot air unti or usual furnace and a take up unit 18.

Referring to Figure 2 and Figure 3, different embodiments of the deposition system are illustrated. A wire 20 is moved at high speed through a dipping tank 38 filled with a coating material composition of the present invention. The dipping tank 38 is followed by a sizing unit 40. As a matter of example, a rotatable tube 30 preceded by a stopper 22 and followed by a wet drawing die 34 are rotated with a normal speed, a coating control effect is obtained: excess coating liquid is removed, and a smooth concentric coating is left on the wire 20. A rubber ring 32 acts as a seal between tube 30 and wet drawing die 34. In this way, coatings of a desired thickness for many purposes can be obtained on the coated wire 36. Supply

container 26 delivers the cyclocarbonate and supply container 28 delivers the primary amine oligomers. Both components are mixed in a mixing tank 24. It is preferable for the coating of a wire 20 with a coating material composition, that the wire be heated after its passage through the dipping tank so that a coating layer is cured and formed on the wire. Using the method according to the invention, it is possible to control the thickness of the coating by varying the reactivity of the coating material composition, the heating of the wire, the speed of the wire and/or the speed of rotation of the rotatable tube. As previously stated, one of the advantages of the method according to the invention is that it is possible to obtain very high working speeds and more without dragging along too thick a layer of coating material. However, it is also necessary that a coating layer be formed on the wire during its passage through the dipping tank 38. As the working speed of the wire may be high, the immersion or dipping time may be very short (for instance, less than two seconds). Therefore, it is necessary that the coating material composition be very reactive in order that a coating layer of sufficient thickness may be obtained during the immersion. Moreover, to obtain a coating of good quality it is necessary to obtain a coating with good adhesion, without air bubbles and with good weathering properties. It is therefore necessary that no evaporation of the coating takes place during the curing of the coating. The coating must also be flexible enough to undergo sharp bending without cracking, i.e. a coating which is too brittle must be avoided.

Measurement and Tests

Some examples of steel wires with a coating layer according to the present invention are tested and are compared with a non-treated steel wire.

Table I illustrates the influence of a coating layer according to the present invention on the adhesion, fatigue and corrosion resistance of a steel wire. The steel wires are manufactured as follows. Starting from a rod wire, the wire is drawn in one or more steps until the desired diameter is obtained. Subsequently, the steel wires are coated with a coating layer according to the present invention by a method as shown in Figure 2 and 3. The adhesion test has been carried out in accordance with ASTM D 2229.

The fatigue test carried out is a low cycle fatigue test and grips the test wire of a limited length at both ends, bends the test wire over a test pulley of a specified radius and carries out a cycling under a specified axial load over a certain stroke until break. The wire sample is running over a 18 mm diameter pulley, with a frequency of 260 bendings/minute and with a load of 12000 g.

The salt spray test has been carried out in accordance with DIN SS 50021 / ASTM B 117 (100% relative humidity, 35 0 C, 5% NaCI).

The steel wires are 0.80 %C high-carbon steel wires. The coating material NIPU (1) and (2) is a non-isocyanate polyurethane as described and exemplified in US 6 120 905.

Table I

DBR = dark brown rust