EP4680664A1

EP4680664A1 - A rubber composite and a method for making the rubber composite

Info

Publication number: EP4680664A1
Application number: EP24704855.6A
Authority: EP
Inventors: Degui XIA; Baoxing WANG; Hans Vandeputte
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2023-03-14
Filing date: 2024-02-19
Publication date: 2026-01-21
Also published as: WO2024187381A1; WO2024188594A1; CN120958075A

Abstract

The invention provides a rubber composite, the rubber composite comprises the rubber compound and at least one steel cord embedded in the rubber compound, the steel cord comprises at least one steel filament, the steel filament has a coating, the coating comprises copper, zinc and a metal M being selected from the group consisting of nickel, tin, indium, manganese, iron, bismuth, cobalt and molybdenum, when the rubber composite is in humid aged condition, with the oxidation reaction of said coating, ZnO layer is present on the remaining unoxidized part of the coating at the contact portion of the steel filament and the rubber compound, and at at least one section (A) in a direction perpendicular to the surface of the steel cord, the ZnO layer has a thickness of at least 50nm and the remaining unoxidized part of said coating has a thickness of at least 50nm, wherein the metal M is present in the ZnO layer of the section (A) with more than 5% expressed as weight percent related to the total of C, O, S, Cu, Zn and metal M as determined by line scanning of Energy Dispersive X-ray Spectroscopy. The invention rubber composite has improved adhesion performance.

Description

Title A rubber composite and a method for making the rubber composite Description

Technical Field

[1] The invention relates to a rubber composite. The invention also relates to a method for making the rubber composite.

Background Art

[2] Steel cord for reinforcing rubber articles, such as tires, is normally composed of at least one steel filament with brass coating. The brass coating is contributing to facilitating the adhesion performance between the steel substate and the rubber compound, and the adhesion is attained by the combination of sulphur from the rubber compound with copper from the brass coating of the steel filament.

[3] Adhesion performance is very important for a tire. To stabilize the adhesion between the steel cord and the rubber compound, cobalt based organic salts are added into the rubber compound, such as cobalt naphthenate, cobalt stearates or cobalt boron decanoate complexes. The cobalt based organic salts firstly promote the formation of copper sulphide, and secondly improve the retention of the steel cord to rubber bond in hot and humid conditions by suppressing the oxidation of the coating and thereby overgrowth of the adhesion layer.

[4] However, the adding of cobalt would lead to a problem of accelerating rubber aging. Furthermore, cobalt becomes a strategic material, so that the price of cobalt becomes higher. Therefore, the reduction or the elimination of the use of cobalt is expected, and cobalt free rubber compound is desired.

[5] One solution is to incorporate a third metal into brass coating of the steel filament, such as nickel, iron, or cobalt. Instead of adding the cobalt into the whole rubber compound which leads to a waste of cobalt since it only acts at the brass surface of the steel filament, the adding of the third metal into brass coating concentrates the third metal only on the surface, as thus, the use of cobalt in rubber compound can be reduced or avoided. WO201 3/117248 and WO2013/117249 disclose a steel filament coated with brass coating incorporated with cobalt, nickel, tin, indium, manganese, iron, bismuth and molybdenum. US4446198 discloses a steel wire coated with brass coating incorporated with iron.

[6] A coating for steel filament with improved adhesion performance and allowing the reduction or elimination of cobalt in rubber compound is still desired.

Disclosure of Invention

[7] The primary object of the invention is to provide a rubber composite with improved adhesion performance between the steel cord and rubber compound.

[8] A second object of the invention is to provide a rubber article comprising a rubber composite with a longer lifetime.

[9] A third object of the invention is to provide a method for making a rubber composite with improved adhesion performance between the steel cord and rubber compound.

[10] According to a first aspect of the invention, a rubber composite is provided, the rubber composite comprises the rubber compound and at least one steel cord embedded in the rubber compound, the steel cord comprises at least one steel filament, the steel filament has a coating, the coating comprises copper, zinc and a metal M being selected from the group consisting of nickel, tin, indium, manganese, iron, bismuth, cobalt and molybdenum, when the rubber composite is in humid aged condition, with the oxidation reaction of the coating, ZnO layer is present on the remaining unoxidized part of the coating at the contact portion of the steel filament and the rubber compound, and at at least one section (A) in a direction perpendicular to the surface of the steel cord, the ZnO layer has a thickness of at least 50nm and the remaining unoxidized part of the coating has a thickness of at least 50nm, wherein the metal M is present in the ZnO layer of the section (A) with more than 5% expressed as weight percent related to the total of C, 0, S, Cu, Zn and metal M as determined by line scanning of Energy Dispersive X-ray Spectroscopy.

[11 ] With the use or the running of the tire, ZnO is generated more and more due to the oxidation of zinc of the coating, this is so-called dezincification. Such dezincification causes the consumption of the coating, so that the coating is consumed partly or even completely. When the coating is consumed in large quantities and thereby the ZnO layer is too thick, the adhesion interphase between the steel substrate and the rubber compound become fragile and thereby has the risk of failure, and then the steel cord is not able to adhere with the rubber compound, and this leads to the failure of the rubber composite or the failure of the rubber article comprising the rubber composite. The invention provides a rubber composite that has sufficient metallic element M content in ZnO layer even when the ZnO layer has a thickness of more than 50nm, the sufficient metallic element M content can delay the dezincification and the consumption of the coating, the adhesion performance of the steel cord with the rubber compound is improved, thereby the lifetime of the rubber composite or the rubber article comprising the rubber composite is prolonged.

[12] When the rubber composite is in humid aged condition, the zinc in the coating is oxidized at the coating surface contacting the rubber compound, and then the ZnO layer which is present at the contact portion of the steel filament and the rubber compound becomes thicker. In the meanwhile, Cu_xS layer becomes thicker due to the further sulfidization of copper from coating during the aging of the rubber composite. Due to the growth of ZnO layer with the aging of the rubber composite, the thickness of coating is reduced, in other words, the remaining unoxidized part of the coating excludes the ZnO layer.

[13] According to the invention, every cross-section of the aged rubber composite has at least one section (A) which has a high content of the metal M. The section (A) is one part of the cross-section of the rubber composite in the direction perpendicular to the surface of the steel cord, and it at least includes the area starting from the steel substrate and extending into the rubber compound, and the section (A) includes a part of coating (the unoxidized) and a part of ZnO. There could be two or more sections (A) present on one cross-section of the rubber composite, and this provides a better adhesion performance of steel cord with the rubber compound. Preferably the at least one section (A), i.e. , the total of the sections (A), covers at least 30% of the circumference length of the steel substrate, for example 30%-70% of the circumference length of the steel substrate. Due to the rough surface of the steel substate or the noncontacting between the surface of the steel filament and the rubber compound in the case of incomplete rubber penetration, it is impossible to have the at least one section (A) covering 100% of the circumference length of the steel substrate for an aged rubber composite.

[14] Preferably, the content of metal M present in the ZnO layer of the section (A) is more than 7% expressed as weight percent related to the total of C, 0, S, Cu, Zn and metal M as determined by line scanning of Energy Dispersive X-ray Spectroscopy. More preferably, metal M present in the ZnO layer of the section (A) accounts for more than 10% and less than 30% expressed as weight percent related to the total of C, O, S, Cu, Zn and metal M as determined by line scanning of Energy Dispersive X-ray Spectroscopy.

[15] The metal M is iron preferably. The more presence of iron in the ZnO layer is beneficial for the suppression of dezincification.

[16] The copper content inside the coating is within the range from 58 weight percent to 75 weight percent, the content of metal M inside the coating is within the range from 0.5 weight percent to 10 weight percent, the remainder being zinc and unavoidable impurities.

[17] Preferably, the mass of the coating is 2.0-8.0 g/kg for the steel filament prior to the aging of the rubber composite.

[18] According to the invention, the steel cord could be any one of existing steel cord construction comprising at least one steel filament, such as monofilament, 1 +n, 2+n, nx1 , nxm, nxmxl, m+n, m+n+l and etc..

[19] The invention is developed for reducing the use of cobalt in the rubber compound. However, the invention is also applicable for the cobalt- containing rubber compound. The rubber compound of the invention could be cobalt-containing rubber compound or cobalt-free rubber compound. ‘Cobalt-free rubber compound’ means the rubber compound, as a raw material for making the rubber composite, has no cobalt. The rubber compound could be any one of existing rubber compound for rubber articles such as tire, conveyor belt or hose. [20] According to a second object of the invention, a rubber article is provided, the rubber article comprises at least one invention rubber composite. The rubber article comprising the invention rubber composite has a prolonged lifetime. The rubber article could be any one of existing rubber product, such as a tire, a hose or a conveyor belt. For a tire, the rubber composite is in the belt layer, the carcass layer, the chafer layer and/or the bead of the tire, and the tire could have one or more invention rubber composite.

[21 ] According to the third object of the invention, a method for making a rubber composite is provided, the method comprises the following steps: a. providing an intermediate steel wire with a controlled circumferential roughness; b. coating the intermediate steel wire with copper, zinc and a metal M being selected from the group consisting of nickel, tin, indium, manganese, iron, bismuth, cobalt and molybdenum by electroplating, while the current density of electroplating of the metal M is 2 - 4 A/dm²; c. diffusing the copper, zinc and metal M in a diffusion heat treatment; d. drawing the coated intermediate steel wire in wet wire drawing process, with the resulting steel filament having a coating; e. assembling one or more steel filaments into a steel cord; f. providing a rubber compound; g. embedding at least one of the steel cord into the rubber compound, thereby forming a green rubber composite, vulcanizing the green rubber composite to obtain the final rubber composite.

[22] Preferably, in step a, the circumferential roughness Ra of the intermediate steel wire is controlled to be between 0.15 and 0.30 micrometer.

[23] The intermediate steel wire normally has a diameter of 0.9-2.5mm. The intermediate steel wire is an intermediate product in the production of steel cord and prepared for the electroplating of the coating. The surface of the intermediate wire has a certain roughness mainly due to the previous drawing processing for diameter reduction from wire rod, for example with a diameter of 5.5mm, to the intermediate steel wire, so that a lot of valleys exist on the surface of the intermediate steel wire. If the valleys are too big or too deep, the particles of metal M will be deposited inside the valleys and stay there, hence those metal M particles won’t be present in the surface layer of the coating after the wet wire drawing process, and this leads to less metal M in the surface layer of the coating of the steel filament, as thus there is no sufficient metal M in the ZnO layer after the aging of rubber composite. To increase the amount of metal M in the surface layer of coating of the steel filament, the invention method controls the roughness of the intermediate steel wire, which results in a relatively more smooth surface of the intermediate steel wire for electroplating, and the presence of big and deep valley on the surface of intermediate steel wire is reduced; furthermore, the current density of the electroplating of metal M is set to be 2 - 4 A/dm², which decreases the amount of the metal M in the deep valley on the surface of intermediate steel wire and improves the uniformity of electroplating of M as well, as thus the presence of metal M is more and evenly distributed in the surface layer of the coating after wet wire drawing, which ensures sufficient metal M in the ZnO layer after the aging of rubber composite.

[24] The roughness of the intermediate steel wire can be controlled by any of existing technologies, for example adjusting the lubrication conditions during the wire drawing process from wire rod to intermediate steel wire, or reducing the damage from the acid to the surface of the intermediate steel wire in the acid pickling processing which is for removing the oxide prior to electroplating, via the use of weak acid or the use of corrosion inhibitor.

[25] The mass of the coating on intermediate steel wire is preferably 2.0-8.0 g/kg in step b.

Brief Description of Figures in the Drawings

[26] Figure 1 describes a cross-section of the steel cord out of the aged invention rubber composite by Scanning Electron Microscope (SEM).

[27] Figure 2 describes section (A) at the cross-section of the steel cord out of the aged invention rubber composite by SEM.

[28] Figure 3 describes a line scanning of Energy Dispersive X-ray Spectroscopy of the invention.

Mode(s) for Carrying Out the Invention

[29] An intermediate steel wire is provided from a wire rod. [30] The wire rod, with a diameter of 5.0-5.5mm, is firstly cleaned by mechanical descaling and I or by chemical pickling in a H2SO4 or HCI solution in order to remove the oxides present on the surface. The wire rod is then rinsed in water and is dried. The dried wire rod is then subjected to a first series of dry drawing operations in order to reduce the diameter until a first diameter.

[31] At this first diameter D1 , e.g., at about 3.0 to 3.5 mm, the dry drawn steel wire is subjected to a first intermediate heat treatment, called patenting. Patenting means first austenitizing until a temperature of about 1000 °C followed by a transformation phase from austenite to pearlite at a temperature of about 600 - 650 °C. The steel wire is then ready for further mechanical deformation.

[32] Thereafter the steel wire is further dry drawn from the first diameter until a second diameter in a second number of diameter reduction steps. The lubrication conditions during the dry drawing is adjusted for improving the roughness of the intermediate steel wire. The second diameter typically ranges from 0.9 mm to 2.5 mm.

[33] At this second diameter, the steel wire is subjected to a second patenting treatment, i.e. , austenitizing again at a temperature of about 1000 °C and thereafter quenching at a temperature of 600 to 650 °C to allow for transformation to pearlite.

[34] If the total reduction in the first and second dry drawing step is not too big a direct drawing operation can be done from wire rod till second diameter.

[35] The wire is then subjected to the pickling in acid for removing the oxide prepared for electroplating, and the roughness of the wire is improved with the use of weak acid or the adding of corrosion inhibitor in the acid.

[36] As thus the intermediate steel wire with a circumferential roughness Ra of 0.15-0.3 micrometre is provided. The measurement of Ra is according to ISO 4287.

[37] Then the intermediate steel wire is coated with copper, zinc and a metal M being selected from the group consisting of nickel, tin, indium, manganese, iron, bismuth, cobalt and molybdenum by electroplating. The current density of the electroplating of metal M is set to be 2 - 4 A/dm² The copper, zinc and the metal M can be coated in any order. Preferably, the copper, zinc and the metal M is coated in the order of Cu, M and then Zn. The mass of the coating is 2.0-8.0 g/kg.

[38] And then the coated intermediate steel wire is subjected to a diffusion heat treatment to diffuse the copper, zinc and metal M.

[39] The coated intermediate steel wire is drawn in wet wire drawing, this results in a steel filament having a coating, and the steel filament has a carbon content higher than 0.60 percent by weight, or no less than 0.70 percent by weight, or even higher than 0.80 or 0.90 percent by weight, with a tensile strength (TS) typically above 2800 MPa and adapted for the reinforcement of rubber articles. Steel filament has a final diameter D ranging from 0.05 mm to 0.60 mm, e.g. from 0.10 mm to 0.50 mm. Examples of steel filament diameters are 0.10 mm, 0.12 mm, 0.15 mm, 0.175 mm, 0.18 mm, 0.20 mm, 0.22 mm, 0.245 mm, 0.28 mm, 0.30 mm, 0.32 mm, 0.35 mm, 0.38 mm, 0.40 mm. Better that the diameter the steel filament D is in the range of 0.10mm-0.50mm.

[40] Assemble one or more steel filaments into a steel cord.

[41] Rubber compound is provided and embedded with at least one steel cord thereby to form a green rubber composite, and then the green rubber composite is vulcanized to obtain the final rubber composite. For the fresh rubber composite, there is only a few and very thin ZnO on the coating.

[42] The presence of metal M is measured by line scanning of Energy Dispersive X-ray Spectroscopy,

- Specimen preparation:

1 ) Aging of rubber composite: the rubber composite is aged in humid aged condition, the humid aging condition and time are controlled, for example, 93°C&95%RH for 14days, to obtain a desired thickness of ZnO layer and the remaining unoxidized part of the coating for observing the invention feature, i.e. , the thickness of ZnO layer is at least 50nm, and the thickness of remaining unoxidized part of the coating is at least 50nm.

2) Specimen making: take out the steel cord from the aged rubber composite, make the rubber remaining on the surface of the steel cord as thin as possible, cut the steel cord in the direction perpendicular to the surface of the steel cord to expose the crosssection of the steel cord which has a thin rubber remained, grind the cross-section of the steel cord with sandpaper and polish the crosssection of the steel cord with ion beam milling to obtain a specimen with a smooth cross-section;

- Measurement:

1 ) Determination of the area to be measured: obverse the crosssection of the steel cord by scanning electron microscope (SEM) and measure the thickness of ZnO and the thickness of the remaining unoxidized part of the coating thereby to determine the section to be measured,

2) Measurement: select one flat section of the section which is determined to be measured to detect the presence of metal M in ZnO by Energy Dispersive X-ray Spectroscopy (EDS), set the voltage of the electron beam of EDS at 10kV, do the line scanning on the selected flat section, and then the weight percentage of metal M related to the total of C, 0, S, Cu, Zn and metal M is obtained from the EDS line scanning result, make sure the line scanning is in the direction substantially perpendicular to the boundary between the coating and ZnO layer and the boundary between ZnO layer and Cu_xS layer. The ZnO layer is the portion starting from the first intersection of the Cu line and Zn line to the second intersection of the Cu line and Zn line in the direction from the steel substrate towards rubber compound. For a section wherein the weight percentage of metal M is higher than 5%, this is the section (A).

[43] Figure 1 illustrates a SEM picture of the cross-section of the steel cord out of the aged invention rubber composite. The steel cord has a construction of 3x0.28, and each steel filament of the steel cord has a coating of Cu-Fe- Zn with a mass of 3.9 g/kg prior to the aging of the rubber composite. Figure 2 illustrates a SEM picture of the section (A) at the cross-section of the steel cord as illustrated in Figure 1 , that the area in the highlighted white box is the section (A). Figure 3 is a graph illustrating the EDS line scanning result of the section (A) of invention rubber composite. The graph includes the scanning lines of C, 0, S, Cu, Zn and Fe.

[44] Table below shows the comparison of the invention rubber composite and the reference rubber composite.

[45] Table 1

[46] POF, pull out force in accordance with the test method described in ASTM D2229, is well known in the art to characterize the adhesion performance of the steel cord with the rubber compound by measuring the force of pulling the steel cord out of the rubber composite.

Claims

1 . A rubber composite, comprising the rubber compound and at least one steel cord embedded in said rubber compound, said steel cord comprising at least one steel filament, said steel filament having a coating, said coating comprising copper, zinc and a metal M being selected from the group consisting of nickel, tin, indium, manganese, iron, bismuth, cobalt and molybdenum, characterized in that when said rubber composite is in humid aged condition, with the oxidation reaction of said coating, ZnO layer is present on the remaining unoxidized part of said coating at the contact portion of said steel filament and said rubber compound, and at at least one section (A) in a direction perpendicular to the surface of said steel cord, said ZnO layer has a thickness of at least 50nm and said remaining unoxidized part of said coating has a thickness of at least 50nm, wherein said metal M is present in said ZnO layer of said section (A) with more than 5% expressed as weight percent related to the total of C, 0, S, Cu, Zn and metal M as determined by line scanning of Energy Dispersive X-ray Spectroscopy.

2. A rubber composite as claimed in claim 1 , characterized in that said metal M in said ZnO layer of said section (A) accounts for more than 7% expressed as weight percent related to the total of C, 0, S, Cu, Zn and metal M as determined by line scanning of Energy Dispersive X-ray Spectroscopy.

3. A rubber composite as claimed in claim 2, characterized in that said metal M in said ZnO layer of said section (A) accounts for more than 10% and less than 30% expressed as weight percent related to the total of C, 0, S, Cu, Zn and metal M as determined by line scanning of Energy Dispersive X-ray Spectroscopy.

4. A rubber composite as claimed any one of claims 1 to 3, characterized in that said metal M is iron.

5. A rubber composite as claimed in any one of claims 1 to 4, characterized in that the copper content inside said coating is within the range from 58 weight percent to 75 weight percent, the content of metal M inside said coating is within the range from 0.5 weight percent to 10 weight percent, the remainder being zinc and unavoidable impurities.

6. A rubber composite as claimed in any one of claims 1 to 5, characterized in that the mass of said coating is 2.0-8.0 g/kg.

7. A rubber composite as claimed in any one of claims 1 to 6, characterized in that said rubber compound is cobalt-containing rubber compound or cobalt-free rubber compound.

8. A rubber composite as claimed in any one of claims 1 to 7, characterized in that said at least one section (A) covers 30%-70% of the circumference length of the steel substrate of said steel filament.

9. A rubber article comprising at least one rubber composite as claimed in any one of claims 1 to 8.

10. A rubber article as claimed in claim 9 is a tire, a hose or a conveyor belt.

11 .A tire as claimed in claim 10, characterized in that said at least one rubber composite is in the belt layer, the carcass layer, the chafer layer and/or the bead of said tire.

12. A method for making a rubber composite as claimed in any one of claims 1 to 8, comprising the following steps: a. providing an intermediate steel wire, said intermediate steel wire having a controlled circumferential roughness; b. coating said intermediate steel wire with copper, zinc and a metal M being selected from the group consisting of nickel, tin, indium, manganese, iron, bismuth, cobalt and molybdenum by electroplating, while the current density of electroplating of the metal M is 2 - 4 A/dm²; c. diffusing said copper, zinc and metal M in a diffusion heat treatment; d. drawing the coated intermediate steel wire in wet wire drawing process, with the resulting steel filament having a coating; e. assembling one or more steel filaments into a steel cord; f. providing a rubber compound; g. embedding at least one of said steel cord into said rubber compound, thereby forming a green rubber composite, vulcanizing the green rubber composite to obtain the final rubber composite.

13. A method for making a rubber composite as claimed in claim 12, characterized in that the circumferential roughness Ra of said intermediate steel wire is controlled to be between 0.15-0.30 micrometer.

14. A method for making a rubber composite as claimed in claim 12 or 13, characterized in that the mass of said coating is 2.0-8.0 g/kg.