STEEL CORD WITH A BRASS COATING ENRICHED WITH IRON PARTICLES

Abstract

A steel cord that is suitable for reinforcing rubber articles such as tires. The inventive steel cord enables to completely eliminate the presence of cobalt in a tire when combined with the proper cobalt free compound. Advantageously the steel cord adheres equally well to rubbers containing organic cobalt salts. The inventive wire is different from prior art steel cords in that the brass coating now comprises iron particles. The iron particles have a size between 10 nm and 10 000 nm. The presence of iron mitigates the adhesion retention loss of the rubber to steel cord bond in a hot and humid environment. It is a further advantage that the inventive steel cord does not contain any intentionally added cobalt thereby contributing to the elimination of harmful substances in the production area as well as the environment.

Claims

1. A steel cord comprising one or more filaments, said filaments comprising a steel filamentary substrate and a coating partly or totally covering said steel filamentary substrate, said coating comprising brass consisting of copper and zinc, said coating being enriched with iron, wherein said iron is present as particles in said brass, said particles having a size between 10 and 10 000 nanometer.

2. The steel cord according to claim 1 wherein said particles have a size between 20 and 5000 nanometer.

3. The steel cord according to claim 1 wherein said brass comprises at least 63% of copper by mass, the remainder being zinc.

4. The steel cord according to claim 1 wherein the amount of iron in said coating is greater than or equal to 1% in mass and is smaller than 10% in mass compared to the total mass of brass and iron.

5. The steel cord according to claim 4 wherein the amount of iron in said coating is greater than or equal to 3% in mass and is smaller than 9% in mass compared to the total mass of brass and iron.

6. The steel cord according to claim 1 wherein said coating is substantially free of zinc iron alloy.

7. The steel cord according to claim 1 wherein the amount of phosphorous present at the surface of said filament is P.sub.s and wherein the amount of iron present at the surface of said filament is Fe.sub.s, said P.sub.s and Fe.sub.s being determined by the P.sub.s and Fe.sub.s Procedure 3 as defined in the description and expressed in milligram per square meter, said P.sub.s being smaller than or equal to 4 mg/m.sup.2 and larger than zero.

8. The steel cord according to claim 7 wherein the amount of iron present at the surface Fe.sub.s is larger than or equal to 30 mg/m.sup.2.

9. The steel cord according to claim 7 wherein the ratio of Fe.sub.s over P.sub.s is larger than 27.

10. The steel cord according to claim 7 wherein the surface coating weight SCW is the sum of the mass of brass and iron present in said coating per unit of surface area, said coating weight being expressed in grams per square meter wherein the ratio of Fe.sub.s over the product of P.sub.s and coating weight SCW is larger than 13.

11. The steel cord according to claim 1 wherein said steel cord consists of a single filament.

12. A rubber product comprising vulcanized rubber reinforced with a steel cord according to claim 1 wherein said rubber product is one out of the group consisting of tire, passenger car tire, truck tire, van tire, off-the-road tire, hose, hydraulic hose, belt, synchronous belt, conveyor belt, elevator belt.

13. The rubber product according to claim 12 wherein said vulcanized rubber is substantially free of cobalt.

14. The use of the steel cord according to claim 1 for the reinforcement of a rubber product.

15. A method to produce a filament of a steel cord according to claim 1 comprising the following steps: a. Providing an intermediate steel wire having an intermediate diameter; b. Electrolytically coating said intermediate steel wire with copper, iron and zinc; c. Subjecting said copper-iron-zinc coated intermediate steel wire to a heat treatment to diffuse the zinc into the copper at a temperature of at least 420° C. and below 530° C. resulting in an intermediate steel wire with a brass coating enriched with iron particles; d. Optionally removing the zinc oxide and iron oxide from the surface of said intermediate steel wire with a brass coating enriched with iron particles by immersion in an acid bath; e. Subjecting the intermediate steel wire with a brass coating enriched with iron particles to a wet wire drawing operation thereby obtaining said filament; wherein by the wet wire drawing operation the iron particles as present on the intermediate steel wire with a brass coating enriched with iron particles are diminished to a size of below 10 000 nm and larger than 10 nm.

16. The method according to claim 15 wherein the step: b. Electrolytically coating said intermediate steel wire with copper, iron and zinc; is performed by the following substeps of: b1. Electrolytically coating said intermediate wire with copper; b2. Electrolytically coating the copper coated intermediate wire with iron; b3. Electrolytically coating the copper-iron coated intermediate wire with zinc;

17. The method according to claim 16 wherein the step: b2. Electrolytic coating said copper coated intermediate steel wire with iron; is performed in any one out of the group consisting of following electrolytic plating solutions: Ferrous chloride solutions; Ferrous sulfate solutions; Ferrous ammonium sulfate solutions; Ferrous fluoroborate solutions; Ferrous sulfamate solutions; Mixed sulfate-chloride baths

18. The method according to claim 15 where in the step of: e. Subjecting the intermediate steel wire with a brass coating enriched with iron particles to a wet wire drawing operation; wet wire drawing is performed to a true elongation of at least 3.5.

19. The method according to claim 15 wherein the step of: e. Subjecting the intermediates steel wire with a brass coating enriched with iron particles to a wet wire drawing operation; wet wire drawing is performed by means of one or more dies comprising diamond.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0095] FIG. 1a, shows iron particles originating from the coating that are pressed onto, against the steel substrate.

[0096] FIG. 1b shows an iron particle as present in the brass coating as detected by HAADF-STEM.

[0097] FIG. 2a shows the pull out force adhesion results as obtained in Family I compounds in under cure vulcanisation conditions.

[0098] FIG. 2b shows the pull out force adhesion results as obtained in Family I compounds in regular cure vulcanisation conditions.

[0099] FIG. 2c shows the pull out force adhesion results as obtained in Family I compounds in over cure vulcanisation conditions.

[0100] FIG. 2d shows the pull out force adhesion results as obtained in Family I compounds after cured humidity aging.

[0101] FIG. 2e shows the pull out force adhesion results as obtained in Family I compounds after steam aging.

[0102] FIG. 3a shows the pull out force adhesion results as obtained in Family II compounds in under cure vulcanisation conditions.

[0103] FIG. 3b shows the pull out force adhesion results as obtained in Family II compounds in regular cure vulcanisation conditions.

[0104] FIG. 3c shows the pull out force adhesion results as obtained in Family II compounds in over cure vulcanisation conditions.

[0105] FIG. 3d shows the pull out force adhesion results as obtained in Family II compounds after cured humidity aging.

[0106] FIG. 3e shows the pull out force adhesion results as obtained in Family II compounds after steam aging.

[0107] Family I compounds are five different compounds that contain an organic cobalt salt as is currently used in the industry. Family II compounds are five different compounds that are free of added cobalt.

[0108] Each point in FIGS. 2a to 2e, FIGS. 3a to 3e represents the average of five different compounds within the respective Family according the different vulcanisation conditions (a to c) or aging conditions (d to e).

[0109] In FIGS. 2a to 2e, and 3a to 3e the reference value ‘0’ is the average of regular brass coated steel cord of which the filaments are drawn in Set W dies as obtained on Family I compounds.

MODE(S) FOR CARRYING OUT THE INVENTION

[0110] The invention has been implemented on a 3×0.28 Super Tensile construction. With ‘Super Tensile’ is meant that the tensile strength of a single filament is at least 3265 N/mm.sup.2 with a target value of 3440 N/mm.sup.2.

[0111] The filament was prepared as follows: [0112] Steel wire rod of class 0.80C was selected meaning that the steel has a minimum carbon content of 0.80 wt % carbon and a maximum carbon content of 0.85 wt %. Other elements were present according the specifications in paragraphs [0019] to [0020], this patent application (plain carbon steel composition). The steel wire was dry drawn to a diameter of 1.98 mm; [0113] This steel wire was duly patented by first heating the wire to above 950° C. in order to reach full austenisation. Subsequently the wire was cooled down in a water-air-water patenting installation as known in the art. This is the ‘intermediate steel wire having an intermediate diameter’ as per the method claims; [0114] This intermediate steel wire was electroplated with a copper layer by guiding the wire through a copper pyrophosphate bath containing complexes of Cu.sup.2+ cations and P.sub.2O.sub.7.sup.4− anions in an aqueous alkaline bath consisting of Cu.sup.2+ with concentration in the range of 22 to 38 g/L, pyrophosphate (P.sub.2O.sub.7.sup.4−) in a concentration range of 150 to 250 g/L, nitrate NO.sub.3.sup.− in a concentration range of 5 to 10 g/L and ammonia NH.sub.3 in a concentration of 1 to 3 g/L. The bath is run at a pH of 8.0 to 9.0 and the current density is held between 1 to 9 A/dm.sup.2. The amount of copper deposited is adjusted in function of the desired final coating composition; [0115] The copper coated intermediate wire is subsequently guided through a ferrous sulfamate (Fe(OSO.sub.2NH.sub.2).sub.2) solution with a composition along following lines: 75 g/L of iron(II), ammonium sulfamate in a concentration range of 30 to 38 g/L, sodium chloride 37 to 45 g/L at a pH of between 2.7 to 3.0, a temperature of 50 to 60° C. and a current density of 5 to 6 A/dm.sup.2. The amount of iron deposited is adjusted in function of the desired final coating composition [0116] The use of a ferrous sulfamate electrolyte solution results in a stable and well controllable bath; [0117] The copper-iron coated intermediate wire is subsequently guided through an aqueous zinc sulphate (ZnSO.sub.4.7H.sub.2O) bath containing between 40 to 90 g/L of zinc at a pH of between 3 to 3.7. The zinc layer is deposited with a current density of between 20 to 30 A/dm.sup.2; [0118] The copper-iron-zinc coated intermediate wire is subsequently subjected to heat by means of a mid frequent heating stage, followed by a temperature insulation zone. Care has been taken that the temperature does not rise above 530° C. in order to prevent the formation of hard iron-zinc alloys. The resulting wire is an intermediate steel wire with a brass coating enriched with iron particles; [0119] In a next stage the zinc oxide and iron oxide formed during the heat treatment is removed by means of a phosphoric acid dip. Depending on the immersion time and cleaning the amount of phosphorous present on the surface can be modulated;

[0120] In a first design of experiments a coating composition of 62, 64, 66, 68 wt % Cu in combination with iron contents of 1, 2, 3, 4, 5 wt % Fe were combined, the remainder being zinc. Weight fractions are relative to the total coating amount. The results showed that best adhesion results were obtained with higher Fe concentrations. Therefore a second design of experiments was launched with even higher iron contents.

[0121] In a second design of experiments the following compositions and coat weights were obtained on the intermediate steel wire with a brass coating enriched with iron particles (Table I):

TABLE-US-00001 TABLE I Cu/ Copper Iron Zinc Total (Cu + Zn) Sample (wt %) (wt %) (wt %) (g/kg) (wt %) Ref. 62.5 — 37.5 4.28 62.5 S64-1 64.1 1.2 34.6 4.12 64.9 S64-2 63.9 2.4 33.8 4.17 65.4 S64-4 64.5 4.0 31.4 4.135 67.2 S64-6 64.2 6.6 29.3 4.17 68.7 S64-8 63.1 9.4 27.5 4.14 69.7 S64-10 62.5 11.7 25.7 4.21 70.8

[0122] ‘Ref’ is the reference that is a brass coated wire without intended addition of iron. Inventive wires are indicated with leading ‘S’.

[0123] Dissolution of the coating on the intermediate steel wire according Procedure 1 revealed the presence of iron particles. An X-Ray Diffractogram revealed that no beta (β)—brass peak was present at a two theta (2θ) angle of 43.3° where one would expect a peak when beta—brass would be present and this for all inventive samples.

[0124] Thereafter the intermediate wire with a brass coating enriched with iron particles is drawn to a final diameter of 0.28 mm by wet wire drawing the wire through subsequently smaller dies in a lubricant. The lubricant contains high pressure additives that generally comprise phosphorous in organic compounds. Two types of dies were compared during wet wire drawing: [0125] Set W: all drawing dies are tungsten carbide dies; inclusive the last three dies of which the last one is the head die; [0126] Set D: At least the head die is a sintered diamond die, the remainder of the dies being tungsten carbide dies.

[0127] The total true elongation applied to the intermediate steel wire with a brass coating enriched with iron particle is thus 3.91.

[0128] When considering an iron particle in the brass coating of the intermediate wire, this particle is subjected to an elongation of (D/d).sup.2 in the direction of the wire i.e. the longitudinal direction. At the same time the particle is compressed in radial and circumferential direction, both with a factor of (d/D). This under the assumption that iron is not compressible. This implies that iron particles present in the intermediate wire of 1.98 mm are elongated by a factor of about 50 when drawn to a diameter of 0.28 mm. As iron cannot sustain such a high elongation the larger particles of the intermediate wire are milled, minced, broken up into particles with size between 10 nm and 10 000 nm as could be verified by the Procedure 1 describe above.

[0129] FIG. 1a shows the surface of the filament 564-8-D after removal of the brass in a Scanning Electron Microscope (SEM). Various iron particles 102 (7.5 μm), 102′ (6.8 μm), 102″ (8.5 μm), 104 (1.0 μm), 104′ (1.0 μm) are detectable that have been pushed onto the filamentary substrate. The size of the particles—thereby taking the points that are furthest away from one another is at the most 8.5 μm.

[0130] The smaller iron particles (104, 104′) may even be smaller than the noted size of 1.0 μm when other techniques such as high angle annular dark field, scanning transmission electron microscopy (HAADF-STEM) are used. Particles with a size of 120 nm can be detected inside the brass coating: see FIG. 1b that shows the concentration of iron in the coating. An iron particle—indicated by the arrow—is visible. The dotted line has been added to make the outer border of the coating better visible.

[0131] There appears to be a correlation with the size of the largest particles with the amount of iron incorporated into the brass coating of the intermediates steel wire: the more iron is incorporated, the larger the particles appear to be.

[0132] Three steel filaments were twisted into a 3×0.28 ST cord and the surface residues of phosphorous and iron were measured of the resulting cord as per Procedure 3. The results are represented in Table II:

TABLE-US-00002 TABLE II P.sub.s Fe.sub.s SCW Fe.sub.s/ Sample (mg/m.sup.2) (mg/m.sup.2) Fe.sub.s/P.sub.s (g/m.sup.2) (SCW × P.sub.s) Ref-W 1.5 36.2 24.0 2.15 11.2 S64-1-W 1.5 32.2 21.1 2.13 9.9 S64-2-W 1.7 38.4 22.6 2.09 10.8 S64-4-W 1.9 43.2 22.4 2.01 10.8 S64-6-W 2.6 52.3 20.3 2.11 9.6 S64-1-D 0.8 34.3 41.9 2.00 21.0 S64-2-D 0.8 34.5 42.1 1.98 21.2 S64-6-D 1.0 35.7 35.8 1.96 18.3 S64-8-D 1.5 46.9 31.3 1.98 15.8 S64-10-D 1.9 55.5 29.5 1.91 15.4

[0133] Ref-W is the reference that is a brass coated wire without iron particles drawn in Set W dies.

[0134] From Table II it is clear that Set W dies result in more phosphorous as well as more iron at the surface. Set D dies result in a lower phosphorous and iron concentration for the same iron added to the coating. The ratio Fe.sub.s/P.sub.s is always above 27 for filaments drawn with Set-D drawing dies. When using the ratio (Fe.sub.s/SCW)/P.sub.s the difference is even more clearcut and the relative range of the values is more reduced. All the values of Set D drawn wires have this ratio above 14 and even 15 while the Set W drawn wires have this ratio below 13 and even below 11.

[0135] In the series of FIGS. 2a to 2e, 3a to 3e the adhesion results of the different samples of Table II in five compounds of Family I (that contain organic cobalt salts) and Family II (that are free of intentionally added cobalt) are represented. Adhesion results are pull-out forces as determined according to the ASTM D2229-04 standard, as further detailed in the BISFA (“The International Bureau for Standardisation of Man-made fibres”) brochure ‘Internationally agreed methods for testing of steel tyre cord’ 1995 Edition, “D12 Determination of static adhesion to rubber compounds” according to the conditions given (under cure, regular cure, over cure). In this test steel cords are embedded in a block shaped rubber and pulled out of the rubber along the axial direction after vulcanisation. The maximum force (in N) attained is noted. The average of 24 individual maximum forces (in N) is noted as the ‘Pull-Out Force’ (POF).

[0136] For each one of the ten compounds the conditions for regular cure (RC) were set as the TC90 time plus 5 minutes, TC90 being that time where the particular rubber reaches 90% of its maximum torque on a rheometer curve taken at the vulcanisation temperature. ‘Over cure’ condition (OC) occurs when the rubber is vulcanised well over its normal cure time in this application twice as long as the regular cure time. Under cure (UC) vulcanisation is done by vulcanising the rubber only half of the regular cure time.

[0137] In order to establish the adhesion retention the following aging conditions are applied to RC cured samples: [0138] After Cured Humidity (CH): RC samples are held at 93° C. in a 95% relative humidity environment for 14 days [0139] After Steam Aging (SA): in which RC samples are steam cooked at 120° C. for 2 days.

[0140] In what follows any one of the vulcanisation conditions UC, RC or OC or any one of the aging conditions CH or SA will be referred to as a ‘Condition’.

[0141] The results of the adhesion tests are represented as a Z-score deviation from the Reference Average (RA′) in the FIGS. 2a to 2e and 3a to 3e. The Reference Average RA—indicated with ‘0’ in all figures—is equal to the weighted average of the Ref-W sample in all cobalt containing compounds of Family I and this for the particular Condition as per that figure. The statistical standard deviation of all results obtained on the Ref-W sample in the Family I compounds in the particular Condition is calculated and called Reference Standard Deviaton (‘RSTD’) for that condition. In short: the reference is the known brass (Ref-W sample)—cobalt containing rubber system (Family I) in each of the Condition mentioned in the figure's caption.

[0142] For each of the Families I and II and for each of the samples (‘Samples’) of Table II the Pull-Out Force has been determined for each Condition. The Pull-out Forces are weight averaged to a Sample Average (‘SA’) and the statistical standard deviation calculated, referred to as the Sample Standard Deviation, (‘SSTD’) for that Family and Condition.

[0143] The Z-score of a Sample in a Family of compounds for a certain Condition is then equal to the difference between the Sample Average for that Family and Condition minus the Reference Average for that Condition divided by the pooled standard deviation of the Reference Standard Deviation and Sample Standard Deviation. In short:

[00001]$Z=\frac{SA-RA}{\sqrt{\frac{\left(\left(N_S-1\right)SST{D}^2+\left(N_R-1\right)RST{D}^2\right)}{N_S+N_R-2}}}$

[0144] Wherein N.sub.S is the number of results pooled to obtain SA and SSTD and N.sub.R is the number of results pooled to obtain RA and RSTD.

[0145] The Z-score indicates in how for the deviations from the averages are statistically significant from the Reference Average i.e. the current state of the art in the particular Family, Condition the Sample has been tested: [0146] Z-scores that are below ‘−2’ indicate statistically significant deterioration compared to the Reference Average; [0147] Z-scores between −2 and −1 are indicative for a possible deterioration but are not statistically significant; [0148] Z-scores between ‘−1’ and ‘+1’ indicate that no statistical significant deterioration or improvement to the Reference Average can be inferred; [0149] Z-scores between +1 and +2 are indicative for a possible improvement but that is not statistically significant; [0150] Z-scores above +2 represent a statistically significant improvement to the current state of the art.

[0151] With regard to the Family I i.e. cobalt containing compounds the following conclusions can be drawn:

[0152] FIG. 2a: in under cure condition the presence of iron particles in the brass coating . . . [0153] . . . does not result in a statistically significant improvement or deterioration compared to the Reference Average when Set-W dies are used; [0154] . . . may lead to an improvement compared to the Reference Average when Set-D dies are used;

[0155] Best results in UC conditions are obtained when a lower amount of iron is incorporated into the coating.

[0156] FIG. 2b: in regular cure conditions the presence of iron particles in the brass coating . . . [0157] . . . does result in an improvement that is not significant when Set-W dies are used. [0158] . . . does result in an improvement when Set-D dies are used.

[0159] However, the improvement is not statistically significant.

The amount of iron particles incorporated does not have a significant influence.

[0160] FIG. 2c: in overcure conditions the presence of iron particles in the brass coating does not lead to statistically improved results compared to the current state of the art. However, there are no indications that the invention would lead to a deterioration: all Z-scores are positive throughout.

[0161] FIG. 2d: After cured humidity conditioning the use of the invention results in a statistically significant improvement when a higher amount of iron (8 wt % to 10 wt %) is incorporated into the brass coating while using Set-D dies. On other samples there is no significant improvement. In general the use of the invention does not result in deteriorated results.

[0162] FIG. 2e: the invention results in a high, statistically significant improvement of the adhesion retention results after steam aging when Set-D dies are used. The results even further improve with increased iron content in the coating. Use of Set-W dies does lead to an improvement but it is not statistically significant.

[0163] The inventors conclude that their invention can be used in exchange with currently used steel cord in currently used cobalt containing compounds without facing any risk of possibly inferior adhesion results or increased adhesion retention problems. On the contrary: when using Set-D dies adhesion retention results after steam aging are highly and statistically significantly improved.

[0164] With regard to the Family II i.e. compounds that are free of intentionally added cobalt the following conclusions can be drawn:

[0165] FIG. 3a: in under cure the invention results in no significant improvement or deterioration compared with the state of the art (that is: brass coated Set-W drawn steel cord in cobalt containing compounds). In general there is slight tendency that increased iron content may lead to lower under cure results. The trend is less outspoken when Set-D dies are used.

[0166] FIG. 3b: in regular cure condition the Z-scores are all positive indicating that no adverse effect of the invention is to be expected. The improvements are not statistically significant.

[0167] The same conclusions can be drawn for over cure results as represented in FIG. 3c: the inventive steel cord is better but the improvement is not statistically significant.

[0168] FIG. 3d: the invention shows a statistically significant improvement in the cured humidity results for higher iron contents (6 wt %, 8 wt % and 10 wt %) and when Set-D drawing dies are used. The other results remain statistically not significant.

[0169] FIG. 3e: the invention shows a marked and statistically significant improvement over the prior art for both Set-W and Set-D dies in after steam aging condition. There is a clear indication that an increased content of iron in the coating leads to increased results, but only up to 8 wt % of iron.

[0170] In FIGS. 3a to 3e: ‘Brass’ refers to the results obtained with Set-W reference wires when tested in Family II compounds.

[0171] In conclusion it has been demonstrated that the incorporation of iron particles in a brass coating leads to improved adhesion retention in compounds that are free of intentionally added cobalt as well as in compounds that do contain cobalt.

[0172] The invention has been particularly made for reinforcing rubber products such as tires, hoses or belts, for totally eliminating the presence of cobalt in the rubber as well as in the steel cord coating.