Marc, Van Raemdonck
Dewitte, Marc Van Raemdonck Walther
|1.||A substrate for reinforcing elastomeric polymers charac¬ terized in that at least part of the substrate is made of steel, said part being covered at least in part by a layer of an alloy consisting of, apart from impurities, between 4.2 and 6.5 % wght of aluminium, possibly less than 0.1 % of at least one element stimulating the wetting ability of the liquid alloy to the substrate and the balance zinc.|
|2.||A substrate element according to claim 1, wherein the weight of said layer is between 10 and 60 g per m2 of the covered surface of the substrate.|
|3.||A steel wire according to claim 1 having a carbon content of at least 0,4 % wght.|
|4.||A steel wire according to claim 3 having a carbon content between 0.7 and 1 % wght of carbon.|
|5.||A steel wire according to claim 3 having a tensile strength Rm of at least 2100 N/mm2.|
|6.||A steel wire according to claim 5 having a tensile strength of at least 3100 N/mm2.|
|7.||A steel wire according to claim 5 wherein the tensile strength Rm is larger than 2250 1130 log d wherein d is the diameter of the wire.|
|8.||A steel wire substrate according to claim 1 having a rectangular cross section.|
|9.||A substrate according to claim 1 comprising a number of filaments bundled together wherein at least one of the filaments is a steel wire with a diameter between 0.08 mm and 0.50 mm.|
|10.||A substrate according to claim 9 wherein the filaments are bundled together by twisting.|
|11.||A substrate according to claim 10 wherein at least a part of the centrally disposed filaments in the twisted bundle are steel wires.|
|12.||A substrate according to claim 10 wherein at least a part of the circumferentially disposed filaments in the twisted bundle are steel wires.|
|13.||A substrate according to claim 10 wherein at least a part of the filaments disposed between the central and circum¬ ferential filaments are steel wires.|
|14.||A substrate according to claim 10 wherein all the fila¬ ments are steel wires.|
|15.||A substrate according to claim 10 or 14 wherein a number of the wires have a diameter which is different from the diameter of any other wire or filament in the twisted bundle.|
|16.||A substrate according to claim 10 or 14 wherein a number of the wires have a tensile strength which is different from the tensile of any other wire or filament in the twisted bundle.|
|17.||A substrate according to claim 16 wherein said number of wires have a tensile strength Rm > 2250 1130 log d.|
|18.||A substate according to claim 1 wherein said alloy layer is covered at least in part with another layer promoting the adhesion to elastomeric polymers.|
|19.||A substrate according to claim 18 wherein said other layer comprises Cu, Zn, Ni and/or Co.|
|20.||A substrate according to claim 19 wherein said other layer comprises brass.|
|21.||A substrate according to claim 1 or 18 wherein said alloy layer is deposited on an intermediate layer comprising Zn and/or Ni.|
|22.||A elastomeric polymer product reinforced with a substrate according to claim 1.|
|23.||A elastomeric polymer product according to claim 22 in the form of a conveyor belt.|
The present invention relates to a substrate for reinforce¬ ment of elastomeric polymers wherein at least part of the sub- strate is made of steel. Steel wires and cords comprising steel wires twisted together (possibly together with other synthetic filaments such as aramid fibers) are often used for reinforcing rubber products such as tires, belts and hoses. In view of securing a proper and durable adhesion to the rub- ber, the wire surfaces are generally coated with an alloy layer such as brass or zinc.
Besides a proper adhesion capacity, the coating layer should preferably also protect the wires against corrosion attack. Indeed, corrosion of the reinforcing steel structure should always be avoided as the reinforcing effect decreases as a consequence of corrosion. Besides exposure of the steel ele¬ ments to atmospheric corrosion before their embedment into rubber, corrosion attack is also possible after such e bed- ment, especially when incisions in the rubber, which reach the wire surfaces, are produced.
Numerous efforts have been made up to now to design specific coating layers for steel wires which offer a good adhesion capacity (also after ageing of the reinforced composite) in combination with a proper corrosion resistance. Unfortunate¬ ly, the application of those coating layers requires quite complicated processes which generally raise the production cost of the coated reinforcing material. Further, the coating process often becomes quite critical when steel wires are involved with elevated tensile strength eg. over 3000 N/mm 2 , as those wires often require specific manufacturing processes.
It is now a primary object of the invention to provide a rela¬ tively simple coating composition and process for a rein¬ forcing steel substrate which offers adequate adhesion strength (and adhesion retention after aging) to the sur- rounding elastomeric matrix combined with an improved resis¬ tance against static and dynamic corrosion attack.
It is a second object of the invention to provide such coatings on steel wire substrates with an elevated tensile strength.
According to another object of the invention, a bundle, eg. a twisted cord or cable is provided comprising a number of said steel wires, possibly combined with filaments of other mate- rial .
Another object of the invention deals with the combination of steel wires of different kinds in said bundle or cord, eg. wires with different diameter and/or strength.
Yet another object of the invention relates to the combina¬ tion of the simple coating composition and/or process with the deposition of a specific sublayer and/or top layer of another material in view of meeting specific requirements for adhesion and/or corrosion resistance.
A further object of the invent 4 resides in methods and means for manufacturing and using said steel substrates and said combinations of substrates.
An additional object of the invention concerns the elastomic products reinforced with said substrates such as conveyor belts, transmission belts, (high pressure) hoses, tires etc.
According to the invention, the relatively simple coating layer composition for the reinforcing substrate comprises an alloy which, apart from impurities, consists of between 4,2 and 6,5 % wght of aluminium, possibly less than 0,1 % of at least one element stimulating the wetting ability of the liquid alloy to the substrate and the balance zinc. At least a part of the substrate is made of steel, and the above coating layer composition is applied to at least some portions of said part.
The weight of said layer is between 10 and 60 g per m 2 of the covered surface of the substrate. Steel wire is a suitable reinforcing substrate. The steel thereby has a carbon content of at least 0.4 % wght and preferably between 0.7 and 1 % wght. Further, the steel wire has a tensile strength Rm of at least 2100 N/mm 2 . However wires with a tensile strength of at least 3100 N/mm 2 are also contemplated. In particular wires with Rm > 2250 - 1130 log d are envisaged wherein d is the diameter of the wire. The wire may have a round, square or rectangular cross section.
The reinforcing substrate according to the invention can con¬ sist of a number of single wires, however it can also com¬ prise a number of filaments bundled together wherein at least one of the filaments is a steel wire with a diameter between 0.08 mm and 0.50 mm. The filaments are preferably bundled together by twisting. Steel wires can then be disposed either in the center of the bundle, in the circumference and/or in an intermediate layer between core and outer layer of the bundle. If desirable, only part of the filaments in either core, circumferential or intermediate layer may be of steel. Often however, all filaments in the twisted bundle will be steel wires.
Further, not all wires in the twisted substrate should have the same diameter or the same tensile strength. A number of wires can have a diameter and/or tensile strength which is different from the diameter or strength of any other wire or filament in the twisted bundle. In particular, a number of wires can have a tensile strength Rm > 2250 - 1130 log d.
In cases where adhesion and adhesion retention is required to specific rubber compounds, it may be desirable to further cover the steel wire, already provided with the Zn/Al-alloy layer according to the invention, with an additional layer promoting said adhesion to the specific elastomeric polymers. The additional layer may be a metal layer comprising Cu, Zn, Ni and/or Co. In particular said metal layer may comprise brass.
In other instances it may be contemplated to deposit an inter¬ mediate or subcoating on the wire substrate before applying the Zn/Al-alloy coating according to the invention. Such a subcoating may comprise Zn and/or Ni.
The invention covers also elastomeric products, reinforced with substrates having the specific Zn/Al-alloy-coating layer at their surface. Hose reinforcement steel wires, hose wire cords, respectively conveyor belt cord with said Zn/Al-alloy coating as well as the so reinforced hoses, particularly high pressure hoses, resp. conveyor and driving or transmission belts are contemplated.
A steel cord according to the invention (specimen 2 in the table below) and for the reinforcement of a rubber conveyor belt was prepared with the following characteristics : the
cord comprised 7 strands twisted together. Each strand con¬ sisted of 7 steel wires twisted together. Each wire had a diameter of 0.42 mm, a carbon content of 0.86 % wght and a Zn-Al-alloy layer with a weight of 42 g per m 2 of wire sur- face. The Zn-Al-alloy comprised about 5 % wght of Al and about 0,02 % La and about 0,02 % of Ce as a wetting agent to steel. Besides other impurities the balance of Zn amounted to about 95 % wght.
The same cord (7x7x0.42 - specimen 1 -) was prepared ; how¬ ever each wire had a coating of zinc (hot dip) of about 50 g per 2 of wire surface. As explained above, the eutetic Zn-Al-coating has an excellent corrosion resistance wich is generally at least three times the corrosion resistance of conventionally galvanised (hot dip Zn-coated) wire when sub¬ mitted to a salt spray test. This is the reason why corrosion tests were not repeated here.
Applicant however had very much doubts as to the adhesion capacity and adhesion retention after aging of the new Zn-Al-coatings, when compared to Zn-coatings. Therefor the Zn-Al-coated cords described above were embedded and vulca¬ nised in two rubber compounds for conveyor belts. The pull-out force (N/mm) was determined as per AISI/ASTM test.No. 2630 as well as the appearance rating (APR) which is a visual estimation of the degree of rubber coverage after peeling the rubber from the cord layer.
The table 1 below represents the values obtained for each of two compounds A and B, for the Zn-coated cord (specimen 1) and for the Zn-Al-coated cord (specimen 2).
The results obtained indicate that values for initial adhe¬ sion (freshly vulcanised composite rubber/cord) are quite com¬ parable for both specimens. This means that the adhesion capacity for Zn-Al-coated cords according to the invention is generally not worse than for conventionally Zn-coated cords. Surprisingly however, the adhesion retention after aging is also excellent for the cords according to the invention and overall even slightly better than for conventionally Zn-coated steel cords. From the above data can thus be con¬ cluded that the Zn/Al-coated substrates according to the invention offer at the same time a better corrosion resis¬ tance and an adhesion strength to rubber which is in general at least equal to that of conventionally Zn-coated sub¬ strates, even after aging. The better corrosion resistance
does not only relate to circumstances of static corrosion but also to those of dynamic corrosion which then results in a better corrosion fatigue resistance.
As a proof thereof wet and dry fatigue tests were carried out as set out in example 2 below.
Steel wire filaments with substantial residual compressive stresses at their surface were coated with the Zn/Al-alloy coating described in example 1. They had a diameter of 0.19 mm resp. 0.21 mm and a tensile strength of between 3600 and 3850 N/mm 2 resp. between 3400 and 3600 N/mm 2 . Three different coating amounts were present on the filaments. The heaviest coating had a weight of about 35 g/m 2 of filament surface whereas the coating with the lowest weight was about 11 g/m 2 . An intermediate coating amount of about 25 g/m 2 was tested also.
Conventional fatigue tests were carried out (540.000 cycles) in dry (35 % relative humidity) and wet (demineralised water) conditions as described e.g. at the bottom of page 4 of the published European patent application No. 220.766. The results are summarized in the table 2 below :
Tabl e 2
Professionals in the field will certainly recognise that the values in table 2 are very high.
Example 3 A tire cord was prepared of the construction 3x0.21 + 9x0.19 with a cable pitch of 12.5 mm. The filaments (used in example 1) with a diameter of 0.19 mm and with the Zn/Al-alloy coating weight of 13 g/m2 were unwound from the cord and sub¬ mitted to the same corrosion fatigue test (wet conditions) as described in example 2. The corrosion fatigue limit value was about 825 N/mm2 which is still considered satisfactory. In fact, due to the listing operation, corrosion fatigue limits decreased from 925 N/mm2 (example 2) only by about 10 %. The filaments with a diameter of 0.21 mm had a Zn/Al-alloy coating weight of 11 g/m2.
The cords (1) according to the invention and described in example 3 were embedded in a rubber compound comprising as
quantitatively most important ingredients per 100 parts of rubber: 45 parts of C.B. Regal 300; 12.5 parts of Ultrasil VN 3; 8 parts of ZnO; 6 parts of Dutrex 729; 6 parts of sulfur; 5 parts of Cofill 11; 4 parts of Cyrez 963; 2 parts of Santoflex 13 and 1.5 parts of Manobond C 16. The composite was vulcanised for about 25 in. at 150'C.
Adhesion (expressed in N) was determined according to the conventional pull-out test and the appearance rating (APR in %) was noted. The same tests were carried out for comparison on similar cords (2), (3), (4) (same construction and similar tensile strengths). Cords (2) had on top of the Zn/Al-alloy coating a very thin Co- coating (1000 nm) applied by physical vapor deposition. Cords (3) were conventional brass coated cords (about 63% Cu and 37% Zn) and cords (4) were the same brass coated cords with again a thin Co-layer (of about 1000 nm in thickness) applied by physical vapor deposition. Table 3 summarises the results. Adhesion is somewhat lower for the cords (1) and (2) compared to the brass coated cords (3) and (4) but much better than normally would have been expected by persons skilled in the art. The influence of Co is not very significant for the rubber compound used in these experi¬ ments.