WO2007020156A1

WO2007020156A1 - A polymer impregnated steel cord

Info

Publication number: WO2007020156A1
Application number: PCT/EP2006/064555
Authority: WO
Inventors: Daniël MAUER
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2005-08-19
Filing date: 2006-07-24
Publication date: 2007-02-22
Anticipated expiration: 2008-02-19
Also published as: CN101243225B; CN101243225A

Abstract

A polymer impregnated steel cord (100) is presented .The steel cord discriminates itself from other prior art impregnated steel cords in that the polymer also fills the voids that exist in e.g. compact cord types .The polymer is carried into the voids by means of a water based micro-emulsion ,out of wich the water is driven out by heating , leaving behind the condensed polymer. A method to improve the ingress of the micro-emulsion into the voids is also described.

Description

A POLYMER IMPREGNATED STEEL CORD

Field of the invention.

The invention relates to steel cords used to reinforce elastomer products such as tires, hoses, belts and the like. The inventive steel cord comprises a polymer material that is applied by immersion in a micro-emulsion that ingresses the hollows of the cord prior to the curing of the polymer.

Background of the invention.

Steel cord is the preferred reinforcement for a lot of elastomer articles due to its unique combination of properties: it is strong yet flexible, it does not creep and has designable tension-bending- torsion moduli, it has a predictable dynamic fatigue limit and survives compression without damage. When provided with the proper adhesive enabling coating it forms with the elastomer a true composite structure. Yet steel cord has also a major disadvantage in that it corrodes when not properly protected. Filaments that repeatedly rub against one another in a corrosive environment will erode each other - a phenomenon called 'fretting' - possibly leading to an earlier than anticipated failure.

The predominant use of round steel wires to make a steel cord is not helpful in this respect: when more than two round wires are bunched or twisted together into a cord or rope, elongated cavities or 'channels' will form alongside the filaments. If moisture can get into these channels, the repeated bending of the steel cord - as e.g. exerted during the use of a tire or the running of a rope over a sheave - acts like a peristaltic pump driving the moisture over the entire length of the cord. The reason why round steel wires are preferred is that they can be cold drawn to high tensile strengths something which is difficult with other shapes. Over the years tensile strength has risen from 3200 N/mm² in the past to currently 3700 N/mm² while 4000 N/mm² is becoming available for small diameters. Many solutions have been put forward in order to counter this corrosion disadvantage. A first class of solutions uses the elastomer itself to seal the steel from the outside environment. Solutions in this class are: • Use of one single or two filaments. Then no channels form inside the cord and all filaments are completely surrounded by elastomer. The 2χd -two filaments with a diameter ^λd' in mm, twisted around each other with a certain lay length and direction - has become very popular in this respect (IT 1 042 293). Single, waved filaments have also been described many times (e.g. US 4235274). The difficulty with these solutions is that the degrees of freedom to design the steel cord moduli (particularly the bending and elongation moduli) are greatly reduced even when introducing waved filaments, limiting the use of such steel cord to a particular application for example to the belt of a passenger tire. • Making the cord ^λopen'. With ^λopen' is meant that filaments are separated sufficiently from one another so that elastomer can fill the channels during fabrication of the composite. Separation of filaments can be obtained in a number of ways o By preforming the filaments such that they remain open during fabrication of the composite (e.g. US 4 258 543). The solution is particularly used for cords of type n x d (n>2). o By introducing 'unsaturated layers' in layered cords. Layered cords are cords with a centre filament or strand on which layer by layer groups of filaments are added surrounding the centre strand. Unsaturated layers are layers in the cord with less than the theoretical maximum of filaments present. The space thus freed up is redistributed over the circumference of the layer leading to gaps between the filaments. An example is the 3+8+12 x d (the outer two layers are unsaturated) construction that compares to the 3+9+15 x d (all layers are saturated). Further examples are described in US 4 158 946. o By adapting the filament sizes in respective layers (e.g. by making core filaments thicker) so that gaps between filaments occur in the layers. Examples are described in US 5 461 850. o By introducing localised gaps i.e. gaps that intermittently occur over a short length of the cord. This can be achieved by using wavy filaments like e.g. described in EP 0734468. The above class of solutions suffers from two drawbacks. First the ratio of strength to the circumscribed circle area is less than optimal. Secondly, the degree of elastomer penetration depends on the production conditions of the composite. If e.g. not enough pressure is used in the formation of the composite, the channels will not be filled. A second class of solutions introduces an elastomer to fill the channels in the cord during the fabrication of the cord:

• Channels can e.g. be filled with a monofilament stranded concurrently with the steel filaments, by preference made of the same elastomer as the elastomer the cord is intended to be used in. Examples are described in EP 1 033 435.

• Using filaments or strands with an extruded coating. Just enough coating is provided so that the coating flows into the gaps upon stranding (WO 03/031716, US 5279695).

The first solution again suffers from a loss in maximal attainable breaking load. The second solution performs very well but is too expensive.

Overall the second class of solutions is more preferred from the standpoint of a cord manufacturer because he then has complete control of the final quality of the product. This is not the case for the first class of solutions where elastomer ingress is a shared responsibility between the cord producer and the producer of the composite.

Summary of the invention. The inventors therefore focused to do away with the drawbacks of the second class of solutions: the primary object of their invention. A second object was to devise a steel cord with the maximum attainable ratio of strength over circumscribed circle area while having the inside channels blocked, in other words to allow closed cords to be used without the drawback of moisture pumping. A third object of the invention was to do away with the cost drawback of the previous art. A fourth object of the invention is to provide a cord that blocks corrosion propagation irrespective of the production circumstances of the composite article.

According to a first aspect of the invention, a steel cord is provided. The inventive steel cord (independent claim 1) comprises steel filaments of substantially round cross section. The filaments in the steel cord are twisted around each other to form a strand. Strands on their turn can be twisted together to form a steel cable. In what follows the wording ^λsteel cord' can designate a strand made up of steel filaments or a cable made up of such strands.

When looking now upon such a steel cord as a geometrical arrangement, a series of cross-sections with a plane perpendicular to the axis of the steel cord can be envisaged. Within such a cross- section sub-structures of neighbouring wires that surround a void can be discerned. The filaments remain neighbours to one another as the plane progresses along the axis of the steel cord. These substructures rotate one revolution for each lay length the perpendicular plane progresses along the axis of the steel cord. The presence of such substructures depends on the way the filaments are added into the steel cord in an arrangement that is commonly called the 'construction' of the steel cord. With 'neighbouring' is meant that the outer surfaces of the wires remain within 30 μm from one another, the touching to one another of course not being excluded. When such touching occurs it will occur over a substantial length of the steel cord and is therefore called a "line contact'.

Besides the steel filaments, the steel cord also comprises a polymer material. Polymer materials are known in the art of steel cord manufacturing (cfr. super), but this polymer material is the result of the application of a micro-emulsion that is subsequently dried. For the purpose of this application, a micro-emulsion is understood to be an aqueous emulsion comprising particles of functionalised polymers or copolymers. Examples of polymers comprise acrylic or methacrylic homo and copolymers, ethylene acrylic copolymers and maleic anhydride or epoxy grafted polypropylene or polyethylene, polyurethane polymers, silane grafted polyurethane polymers. The particles have preferably an average particle size between 20 and 120 nm. Additives such as surfactants, pigments, crosslinking agents and/or catalysts can be added to the emulsion. Examples of pigments comprise calcium carbonate, titanium dioxide or carbon black. Examples of crosslinking agents comprise polyamines, epoxies, phenol- formaldehyde resins, urea-formaldehyde resins, zinc or zirconium complexes, melamine-formaldehyde resins and trifunctional azinidines.

The filaments are preferably made from plain carbon steel. Such a steel generally comprises a minimum carbon content of 0.40wt% C or at least 0.70 wt% C but most preferably at least 0.80wt% C with a maximum of 1.1 wt% C, a manganese content ranging from 0.10 to 0.90 wt% Mn, the sulphur and phosphorus contents are each preferably kept below 0.030 wt%; additional micro-alloying elements such a chromium (up to 0.20 to 0.4 wt%), boron, cobalt, nickel, vanadium -a non-exhaustive enumeration- may also be added. Also preferred are stainless steels. Stainless steels contain a minimum of 12wt%Cr and a substantial amount of nickel. More preferred are austenitic stainless steels, which lend themselves more to cold forming . The most preferred compositions are known in the art as AISI (American Iron and Steel Institute) 302, AISI 301, AISI 304 and AISI 316.

The filaments have a diameter that can vary between 0.04 mm and 1.20 mm depending on the application. For tire cord the range between 0.15 and 0.38 mm is most preferred. When the diameters of the filaments are between 0.04 and 0.175 mm the cords are referred to as ^λfine steel cord' and are mainly used as reinforcement of synchronous belts, hoisting belts, control cables and similar goods. Larger diameter filaments (above 0.35 mm to 1.20 mm) are used in steel cords for heavy-duty applications such as bonded flexible pipes, rubber tracks, hoisting ropes and conveyor belts. Note that it is not necessary that the filaments of the substructure have the same diameter! It is even more preferred that they have different diameters so that the strength over circumference area is as high as possible. However, from a manufacturing point of view it is preferred that all filaments have the same diameter.

The filaments can be uncoated. Or they can be coated with a suitable coating. Preferred are:

• electrolytically applied brass having a composition of between 62.5 and 75 wt% Cu, the remainder being zinc. The total coating mass is between 0 to 10 g/kg.

• or the wires can be coated with zinc with a coating mass ranging from 0 to 300 g of zinc per kg of wire. The zinc can be applied onto the wire by means of an electrolytic process or by means of a hot dip process, followed or not followed by a wiping operation in order to reduce the total weight of the zinc.

• or the wires can be coated with a primer selected from organo functional silanes, organo functional titanates and organo functional zirconates which are known in the art for said purpose.

Preferably the micro-emulsion is present inside said voids

(dependent claim 2). In order to overcome the corrosion problem the dried micro-emulsion must be capable of blocking the moisture from penetrating the cord further. Hence a lump of micro-emulsion must block the void at intermittent distances. Taking into account the dimensions of the final applications, the moisture must preferably be blocked within an average distance of five centimetres (dependent claim 3). I.e. the average length of unblocked voids, must be less than 5 cm, preferably 2 cm or even more preferred 1 cm. Most preferred is if the voids are completely blocked. The polymer can also be found back on the periphery of the steel cord. There it is present in the form of a thin film that conformally follows the outer metallic surface of the steel cord (dependent claim 4). The thickness of the film can vary widely along the periphery of the steel cord as it is the result of the drying of a watery micro-emulsion. While initially the watery micro-emulsion will be more or less circularly present around the cord, the polymer material will condense on the cord while the water is being evaporated. The thickness of the film will therefore be larger there where the watery emulsion was pulled by capillary forces into the concave hollows of the steel cord and less at the outer convex curvatures where the water film is drawn thin. Therefore the thickness of the polymer film is largely non-uniform and varies between 5 to 500 μm (dependent claim 5). In this respect the film is easily discriminated from polymer jackets applied by means of extrusion that show a more uniform coating thickness.

There are a number of ways in which the sub-structures of claim 1 may appear in the cord. In any case the sub-structure will appear when at least three steel filaments - not necessarily of equal diameter - are twisted together with the same lay direction and lay length (dependent claim 6). A first preferred embodiment in this respect is when just three filaments are twisted together without giving them a mechanical preforming or bending i.e. a 3x1 construction. In this embodiment, the filaments pairwise remain in line contact with one another over substantially the entire length of the steel cord. A void forms in between the three filaments. Likewise a 4x1 embodiment will show one or two voids depending on whether the filament centres are arranged substantially square (one void) or diamond (two voids) like. Likewise, a 5x1 embodiment will show one, two or three voids and the 6x1 embodiment will show from one to four voids, depending on how the filaments are arranged.

When progressing to seven filaments the most stable and preferred arrangement is when one filament is centrally positioned, while the other filaments surround this centre filament. In principle, only at infinite lay length and perfectly equal diameters of the filaments, the voids will be entirely closed and full line contacts will form. Twisting these filaments in a finite lay will result in separation from the outer filaments from the centre filament leading to distances that can easily be held below 30 μm. Likewise it can be beneficial to make the centre filament thicker than the six surrounding filaments to even the load distribution on the different filaments. Again this incremented diameter can be kept low enough such that the gaps formed between the outer filaments remain below 30 μm. When the number of filaments is further increased, some numbers will stand out as being particularly stable to manufacture:

- 12 filaments of substantially equal diameter twisted together in one operation with one lay length and direction with a triplet in the middle and surrounded by 9 filaments, forming 13 voids in between them.

- 15 filaments, with a small filament in the middle, surrounded by 5 nearest neighbours, on its turn surrounded by an outer shell of 10 and twisted together in one single step thus forming 20 voids in between them. The cord has an envelope of roughly pentagonal shape.

- 19 filaments, all of substantially equal diameter twisted together in one single step with identical lay direction and lay length, with a single filament in the middle surrounded by a first shell of six filaments that on its turn is surrounded by a shell of 12 filaments forming 24 voids in between them. The envelope subscribing the outer periphery of such a cord is a substantially regular hexagon.

- 27 filaments, all of substantially equal diameter twisted together in one single step with identical lay direction and lay length, with a 3x1 in the centre that is surrounded by a first shell of 9 filaments, that on its turn is surrounded with a shell of 15 filaments. There are 36 voids in between the filaments. The envelope subscribing the outer periphery is a hexagon, the sides of which alternatively count 4 and 3 filaments. Such constructions are generically known as compact cords. They are characterised by their parallel lay (all filaments in the same direction and with the same lay direction) and their filament diameters that are equal. When allowing different diameters but keeping the parallel lay, other industrial important configurations emerge that are characterised by a very high metallic density (reference is made to the page numbers in "Drahtseile" of Prof. Dr.- Ing. D. G. Shitkow and Ing. LT. Pospechow, V.E.B. Verlag Technik Berlin, 1957):

- Warrington type where a central core is surrounded by two layers, where the outer layer consists of twice the number of filaments of the first layer and the outer layer diameters are alternatively small and large (page 251 to 263)

- Seale type wherein a central core is surrounded by two layers having an equal number of filaments, the filament diameters within one layer being substantially equal and the filament diameters of the outer layer are larger than those of the inner layer (page 229 to 237).

Filler type where a central core is surrounded by two layers, where the filament diameter within one layer are substantially equal and the number of filaments in the second layer is twice the number of filaments in the first layer, and wherein the position of the filaments in the layers is stabilised by the presence of thin filler wires (page 241 to 251). Combinations of the above types such as Warrington-Seale are equally well possible (dependent claim 7). The above mentioned sub-structures can also be used as intermediate products in the further production of the steel cord. Either they can be used as e.g. a core around which other layers of steel filaments can be twisted (with a different lay length or lay direction) as in a 3+9+15 or 1+6+15 (underlining indicates where voids can form) type of cord. Due to the presence of the dried polymer the cord does not longer have to be of the open type as described in US 4158946. On the contrary: it is preferred that the free space in at least one of the layers is lower than 14% because such limited free space allows to increase the breaking load of the cord. With ^λfree space' is meant that proportion of the circumference of the circle containing the centres of the filaments in a layer that is not occupied by the filaments i.e. consists of spaces between the filaments (dependent claim 8).

The steel cord can also be a steel cord comprising at least two strands, wherein inside the strands sub-structures built up of at least three filaments are present (dependent claim 9). The most notable structures in this respect are cords of the type nxm consisting of ^λn' strands each containing ^λm' filaments wherein the filaments of one strand have the same lay direction and length. The following configurations are particularly important: 3x3, 7x3, 7x4,

7x7, 7x19. In this respect the configurations 12x3, 19x3, as described in EP 0770726 are also steel cords on which the inventive principles of the current application can be applied. Also steel cords with a core strand that is different from the outer strands are of interest such as e.g. 1x3+5x7, 19+8x7. The core can on its turn be a cable such as in 7x7+6x19.

According to a second aspect of the invention a method is provided to impregnate a steel cord in its entirety (independent claim 10). First of all the steel cord is unwound from a spool. In the steel cord, the aforementioned sub-structures are already present.

One also needs a water based micro-emulsion wherein the polymer material is present.

As a next step, the steel cord is immersed in the micro- emulsion and while being immersed, the sub-structures are opened and subsequently closed in order to allow the emulsion to enter the voids. Immersion can be done by leading the steel cord through a dipping tank containing the micro-emulsion. Or it can be done e.g. by leading the steel cord in a vertical direction through a funnel that is continuously fed with emulsion. Preferably the cord is then lead upward i.e. in the direction opposite to the flow of the micro- emulsion. It is advisable to wipe off excess micro-emulsion after the steel cord leaves the immersion area. This can e.g. be done by using rubber dies with a hole slightly larger than the diameter of the cable. Now, the micro-emulsion is caught in the voids of the steel cord and the water needs to be dried out to make the polymer condense inside the voids. Drying can be done by any means known in the art: by conduction, by convection or by radiation. Most preferred are by means of inductive heating of the steel cord or by hot gasses such as heated air or by means of infrared heaters such as IR lamps or by combinations of some of the methods such as infrared emitting gas burners. After this the cord is wound onto a spool. The procedure of dipping and drying can be repeated in order to increase the overall amount of polymer material on the cord.

In a first preferred embodiment of the method (dependent claim 11), the opening and closing of the sub-structures is obtained by repeatedly bending the cord over wheels. The wheels must have a sufficiently low diameter e.g. 1 to 50 times or more preferred 10 to 40 times the diameter of the cord so that due to the bending the sub-structures are stretched open and the micro emulsion can penetrate the voids. Although one wheel could provide sufficient opening, it is more preferred if 2 to 10 wheels mounted one after the other are used. The wheels can be mounted such that all of them lay in the same plane. Or the wheels can be mounted in planes that are under an angle to one another. The latter is more preferred because a more uniform treatment over the circumference of the cord is obtained.

According a second preferred embodiment of the method (dependent claim 12), the opening and closing of the sub-structures is obtained by continuously twisting the sub-structure open to allow the micro-emulsion to enter the voids. This can be done continuously by feeding the cord through a rotationally restraining device that rotates i.e. a false twister. After the device the substructures are closed as there is no net rotation between the unwinding and winding spool. In practice such a false twister is implemented by three pairs of wheels around which the steel cord is laced in a figure 8 form. The centers of the figure 8's are on one line. The pair of wheels through which the steel cord enters is held stationary. The second pair of wheels is rotatably driven. The axes of these wheels circumferential Iy rotate around the centre line of the three pairs of wheels. The steel cord pursues its track through the dip tank. After the dip tank the third pair of wheels is again held stationary. It is preferred that the rotation of the first wheel pair and the third wheel pair is coupled to one another. The coupling (mechanical or electrical or by any other means) is such that the translational speed through both wheel pairs is equal. The figure 8 lacing of the wheel pairs torsionally restrains the steel cord from rolling, while allowing longitudinal movement. In this application, the second, rotatable pair of wheels is driven such that the steel cord filaments are opened between the first and second pair of wheels. Between the second and third pair of wheels, the steel cord closes again. During the closing of the filaments, the micro-emulsion can enter the cord. Although false twisters are known in the art, they are always used to first twist the cord in the closing direction giving the steel filaments a plastic deformation, followed by an equal twisting in the opposite direction.

Additional means for improving the ingress of the micro- emulsion can further be used such as agitation of the bath by e.g. ultrasonic transducers or vibration of the cord itself.

Of course it is not excluded that the steel cord is produced from filaments that have been coated by means of a micro-emulsion prior to assembling them together.

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein

- FIGURE 1 shows a picture of a cross section of a 0.34+18x0.30 Brass coating construction with polymer

FIGURE 2 shows a picture of a cross section 7x3x0.15 Zinc coated construction with polymer - FIGURE 3 shows a picture of a cross section 19+8x7 Zinc coated construction with polymer

All pictures have been enhanced in order to make the salient features of the invention more visible.

- - FIGURE 4 shows a schematic drawing of a false twister for opening the filaments in order to facilitate the ingress of the micro-emulsion

Description of the preferred embodiments of the invention.

In a first lab trial a 0.34+18x0.30 brass coated compact cord was used of which a picture 100 of a cross section is shown in FIGURE 1. Around the central somewhat thicker filament 110 of 0.34 mm diameter, 18 filaments of diameter 0.30 have been cabled in one operation with a lay length of 21 mm in the Z direction. The untreated cable has many voids. For example the void formed by the filaments 121,131 and 132 forms a closed substructure of three filaments as 131, 132 and 121, 131 contact one another and 131 and 121 are less than 30 μm apart from one another. Likewise the four filaments 140, 142, 110 and 143 form a sub-structure because all neighbouring filaments in the substructure are less than 30μm apart. On the other hand the triplet 121, 122, 123 does not form a substructure.

The cable was degreased with isopropylalcohol and after drying dipped for one minute in an organofunctional silane to improve the adhesion of the polymer. The silane solution contained 1.5 vol% of N-(2-amino ethyl)-3-amino propyl tri methoxy silane dissolved in a mixture of isopropanol and water. Thereafter it was dried for an hour at HO⁰C. Subsequently the cord was dipped in Permanol 703 and excess material was wiped off. The cord was then dried for 15 minutes at 15O⁰C. Permanol 703 is a maleϊc-anhydride- grafted polypropylene water-based micro-emulsion that is obtainable from Solvay.

The cross section reveals that the polymer has condensed not only at the outside of the steel cord, but also in the voids. Some bubbles like e.g. 170 remain as they are probably the result of water vapour pressure. It is also apparent that the polypropylene shows a more solid structure at the outer skin of the coating 150 than inside the voids e.g. 160. This is due to microscopic bubbles that are trapped inside the condensed polypropylene and scatter the light differently. A thin layer of polymer 170 remains visible at the periphery of the coated steel cord and forms a conformal coating on the steel cord. Note that such a conformal coating is very difficult - if not impossible - to obtain on a steel cord by means of extrusion.

FIGURE 2 shows a picture 200 of a second preferred embodiment of the inventive steel cord, but now a cord of type 7x3x0.15 has been coated according exactly the same procedure as described for the first embodiment. Such a cord is particularly usefull to reinforce timing belts as e.g. described in WO 2005/043003. Each of the 7 strands that consist of 3 wires forms a substructure: the three wires are twisted together with the same lay direction and lay length. Within the substructure neighbouring wires are not further than 30 μm apart. Within each of the strand, a single void is formed. The salient features are:

- the solid coating 230

- the presence of highly reflective area's such as 210, 210', 210" - the presence of air bubbles such as 220

- the presence of a conformal film 240, a result from the capillary forces that pull the micro-emulsion into the voids of the steel cord prior to drying.

In a third preferred embodiment, a semi-industrial continuous coating method has been established. A steel cord of type 19+8x7 was coated. The steel cord is built up from core strand assembled out of a single core wire 0.19 mm thick around which 6 filaments of diameter 0.17 mm are wound with a lay length of 3.8 mm in ^λZ' direction around which 12 filaments of a diameter 0.17 mm are twisted with a lay length of 8 mm in ^λZ'. Around this core strand, 8 equal strands again built-up out of one 0.17 mm core wire surrounded by 6 outer wires of diameter 0.135 mm at a lay of 8.8 mm in the ^λS' direction are cabled at a lay of 12 mm in lay direction ^λZ'. The total diameter of the cord is 1.83 mm. It will be clear to the person skilled in the art that the 1+6 configurations in the core strand and in the outer strand will form longitudinal voids that can be filled-up with polymer. The impregnation method of the cord starts with the unwinding of the cord - that has already been treated with an organosilane - from a spool. The cord is then led through a dipping tank of 12 cm length (although it could be varied between 10 and 25 cm). The dipping tank contains the same solution of Permolan 703 as used for the lab trials. While the steel cord is still soaked with the water based micro emulsion, it was led through a wheel block with 8 wheels of diameter 25 mm, all in one single plane. After that excess polymer was wiped off by means of two rubbery dies of which the first one had a diameter of 1.72 mm and the second one 1.66 mm. Thereafter there was an infrared heating zone, followed by a hot air drying zone. The results of the coating process are shown on photograph 300 of FIGURE 3. Again a conformal film 310 has formed around the cord. Inside the voids 320, 340 enough material is present in order to block the void within an average distance of 5 mm. The interstices between the strands 330 remain largely empty, also because they do not form closed voids (the surface of the strands is not smooth) and the micro- emulsion is expelled from there during drying.

The impregnation of the steel cord by micro-emulsion can further be improved by the use of a false twister device 400 as described in FIGURE 4. There the cord 402 is led through a first pair of wheels 404 that is stationary. The steel cord has an ^λS' lay direction. The figure 8 lacing 406 of the steel cord prevents the cord from rolling on the wheels while still allowing a longitudinal movement of the steel cord. The second wheel pair 408 is rotatably mounted such that the axes of the wheels circumference the centre line formed by the figure 8 centres. The arrow 410 depicts the rotational direction of the wheel pair. In this method, the false twister rotates such that filaments are opened prior to entering the second wheel pair. After exiting the second wheel pair, the opened steel cord enters the overflow dipping tank 412 wherein the micro- emulsion 416 is recycled after being caught in an overflow tank 414.

Between the rotating wheel pair 408 and the third stationary wheel pair 418 the filaments of the cord close again, thereby catching the micro-emulsion in the voids between the filament.

In another preferred embodiment, the longitudinal speed of the steel cord at the first and third wheel pair is coupled to one another through coupling 420. The coupling is such that the linear speed of the steel cord before and after the first and third pair is identical. The coupling can be achieved through mechanical means (chains, timing belts, gear wheels). By preference the wheel diameters of first and third wheel pair are identical such that the coupling can be made in a 1: 1 gear ratio. Another possibility is to couple the wheels electrically to one another. The coupling has a positive effect on the opening of the cord in that as it forces the filaments to open (the lay length decreases by the action of the second wheel pair, while the held length between first and third wheel pair is fixed by the coupling).

In an alternative, not depicted embodiment, the dip tank 412 and the overflow tank is situated between the first wheel pair 404 and the rotating wheel pair 408. The advantage of this arrangement is that the micro-emulsion is sucked into the voids when the filaments are opened by the false twister. The disadvantage is that the rotating wheel pair 408 will centrifuge part of the micro- emulsion out of the voids.

Claims

1. A steel cord comprising steel filaments twisted together to form said cord, said steel cord showing cross-sections where three or more of said steel filaments form a closed sub-structure so that three or more steel filaments either contact neighboring steel filaments or are maximum 30 μm remote from neighboring steel filaments in order to form a void in the middle of said three or more steel filaments, said steel cord further comprising polymer material, said polymer material resulting from a dried micro- emulsion.

2. The steel cord according to claim 1, wherein said polymer material is present in said void.

3. The steel cord according to claim 2, wherein said polymer material blocks said void within an average distance of five centimeter along said cord's length.

4. The steel cord according to any one of the preceding claims, wherein said polymer is present in the form of a thin film on the periphery of said steel cord.

5. The steel cord according to any one of the preceding claims, wherein said thin film has a thickness ranging from 5 μm to 500 μm.

6. The steel cord according to any one of the preceding claims wherein said steel cord comprises at least three or more filaments that are twisted together with the same lay length and lay direction.

7. The steel cord according claim 6 wherein said steel cord is one of the types out of the group comprising Warrington type, Seale type, Warrington-Seale type, filler type, compact cord type.

8. The steel cord according to claim 6, wherein said steel cord is a layered cord, said layered cord comprising a core and one or more layers of steel filaments twisted around said core, at least one of said layers having a free space available, said free space ranging from 0 % to 14 %.

9. A steel cord according to any one of claims 1 to 5, wherein said steel cord is a multi-strand cord, said multi-strand cord comprising two or more strands, each strand comprising three or more steel filaments.

10. A method to impregnate a steel cord comprising the steps of:

- continuously spooling a steel cord comprising steel filaments twisted together from a spool, said steel cords showing cross- sections where three or more of said steel filaments form a closed sub-structure so that three or more steel filaments either contact neighboring steel filaments or are maximum 30 μm remote from neighboring steel filaments in order to form a void in the middle of said three or more steel filaments

- providing a water based micro-emulsion comprising polymer material

- while continuously immersing said steel cord in said micro- emulsion, opening said sub-structure to allow said micro- emulsion to ingress said void followed by closing said substructure to enclose said micro-emulsion

- continuously drying said steel cord so as to remove the water from said water based micro-emulsion

- winding said steel cord onto a spool 11. The method according claim 10 wherein said continuously opening and closing of said sub-structure is obtained by bending said steel cord over wheels. 12. The method according claim 10 wherein said continuously opening and closing of said sub-structure is obtained by continuously twisting the steel cord in the direction opposite to the lay direction followed by twisting the steel cord in the direction of the lay direction. 13. The method according to claim 12 wherein the opening and closing of said steel cord is obtained by guiding said steel cord through a false twister, comprising one rotatable wheel pair, preceded and followed by a stationary wheel pair. 14. The method according to claim 13 wherein said first and third stationary wheel pairs are coupled to one another such that the linear speed of said steel cord at the entrance and at the exit of said false twister is equal.