Article of manufacture having a metallic surface coated with an elastomer and an intermediate cobalt-copper alloy coating to improve the adhesion of the elastomer

The adhesion of an elastomer such as rubber to a metal surface is improved by coating the metal surface first with an alloy containing cobalt and copper, applying the elastomer to the coated metal surface and vulcanizing the elastomer.

This invention relates to an article of manufacture having a metal surface 
coated with a metallic alloy to which an elastomeric material is adhered 
by vulcanization. The invention also provides a process for the 
manufacture of such an article. 
An article of the type described above and processes for the manufacture 
thereof are known. In the automobile industry, for example, steel cords 
are used extensively for the reinforcement of rubber such as in vehicle 
tires. The steel cord is composed of a number of steel wires coated with a 
thin layer of brass. The cords composed of brass-coated steel wires or 
filaments are first contacted with unvulcanized rubber. Subsequently, the 
rubber is vulcanized, resulting in a bond between the rubber and the brass 
coating. The steel cords coated with brass generally provide a reasonable 
degree of adhesion, provided that the various process conditions, 
especially during vulcanization, and the properties and/or the state of 
the starting rubber are accurately controlled. 
It is generally known that the tires of automobiles are subjected to high 
loads, particularly during cornering and at high speeds, and that they 
must have a long useful life. For the steel cord embedded in the 
automobile tire to function properly as a reinforcement there must be a 
firm bond between the cord and the rubber. In other words, the adhesion 
between steelcord and rubber plays an essential role. In this connection 
both the manufacturers of automobile tires and the manufacturers of 
steelcord have for many years carried out a great deal of research work to 
improve the adhesion between cord and rubber. 
It is an object of this invention to provide metal articles having an 
adherent elastomeric coating. Another object of the invention is to 
provide a process for making improved metal articles having an elastomeric 
coating which is securely adhered to the surface of the metallic article. 
A further object of the invention is to provide a method for improving the 
adhesion of an elastomer to a metal base such as a steel wire.

According to the invention it has surprisingly been found that the adhesion 
of an elastomer such as rubber to a metal surface such as steel can be 
improved if instead of the abovementioned coating of brass the metal 
surface is coated with a cobalt-copper alloy before the elastomer is 
applied. In other words, the invention provides an article having a metal 
surface coated with an alloy containing cobalt and copper to which an 
elastomeric material is bonded by vulcanization. According to the 
invention favorable results are obtained if the cobalt-copper alloy 
contains 5 to 50 percent by weight of copper, preferably 10 to 30 percent 
by weight of copper and the balance cobalt. The coating thickness of the 
cobalt-copper alloy is 30 to 750 nm (nanometer). 
Although the material for the base surface of the article to be coated with 
an alloy may be any suitable metal such as one selected from a group of 
various metals resistant to vulcanization temperatures, such as aluminum, 
copper and the like, the most favorable results are obtained with a steel 
surface. 
Although all aritcles having a metal surface coated with an elastomer are 
contemplated broadly, a preferred embodiment of the invention is an 
article in the form of a steel or other metal wire or filament having a 
diameter in the range of from 0.05 to 0.75 mm. According to the invention 
a number of these filaments may be advantageously formed into a cord or 
bundle. 
It should be added that by cobalt-copper alloys are to be understood here 
one- or multi-phase, practically solid solutions of copper in cobalt or 
intermetallic compounds, with the copper atoms mainly occupying crystal 
lattice positions of the cobalt. It has been found that the cobalt-copper 
alloy according to the invention has a face-centered cubic structure. 
The present invention is to be considered of particular advantage for 
articles of elastomeric material containing reinforcing elements which are 
characterized according to the invention in that they are formed by metal 
wires and/or cords coated with a cobalt-copper alloy. According to the 
invention a preferred embodiment for such an elastomeric object is a 
vehicle tire. The elastomeric object according to the invention may be 
also in the form of a conveyor belt or flexible tube. Favorable results 
are also obtained with an article coated with a cobalt-copper alloy in the 
form of a plate, pipe or beam. The invention also comprises a process for 
the manufacture of an object which is coated with a metallic alloy which 
is subsequently bonded to an elastomeric material by vulcanization, which 
process is characterized in that the alloy is a cobalt-copper alloy 
containing 5 to 50 percent by weight of copper, and preferably 10 to 40 
percent by weight of copper, having a coating thickness of 30 to 750 mm. A 
simple process for the manufacture of the above object is, according to 
the invention, characterized in that the cobalt-copper alloy is deposited 
electrolytically, preferably from a pyrophosphate bath or from a cyanide 
bath. A preferred embodiment of the process of the invention is 
characterized in that the coating is applied to a metallic base surface at 
a current density of at least 15 amperes/dm.sup.2, with the current 
efficiency being at least 50%. By current efficiency is to be understood 
the proportion of the total current, expressed as a percentage, that is 
effective in the depositing of metal ions. It is preferred that the 
coating of steel wires according to the invention should be carried out at 
a current density of 20-25 amps/dm.sup.2. 
Another process of manufacturing the above-mentioned object, which is of 
particular advantage for non-electrically-conductive surfaces, is 
according to the invention characterized in that the cobalt-copper alloy 
coating is applied by a vacuum vapor deposition process. The elastomeric 
material to be used in the various objects according to the invention may 
be composed of various substances or combinations of substances, provided 
that the elastomeric material has such properties which make it display a 
more or less elastic deformation behavior and in any case permit it to be 
vulcanized with sulfur. The component substances of the elastomeric 
material may be formed by synthetic and natural polymers displaying 
plastic deformation behavior before vulcanization and rubber-elastic 
behavior after vulcanization, as is more particularly the case with 
synthetic and natural rubber. As examples of such polymers may be 
mentioned polypentadiene, polybutadiene, poly(olefins), polyisoprene, 
poly(butadiene styrene), poly(butadiene-acrylonitrile), 
poly(ethylene-propylene), poly(isobutene-isoprene), polychloroprene. 
Moreover, to the elastomeric material there may have been added the usual 
fillers, additives and vulcanizing agents such as carbon black, particular 
substances or oils for the processing or softening of rubber, 
antioxidants, sulfur, zinc oxide, stearic acid, and accelerators. 
It has been found that the adhesion between rubber and the surface coated 
according to the invention with a cobalt-copper alloy is not adversely 
affected by the presence of moisture in the elastomer. As the presence of 
moisture in the rubbers to be used in the vulcanization process is often 
inevitable and moreover considerable variations in the moisture content of 
the rubber and in the air may occur, the use of a less moisture-sensitive 
adhesive alloy forms a great improvement over the prior art. 
It has further been found that the adhesion between rubber and the surface 
coated according to the invention with a cobalt-copper alloy is hardly or 
little impaired by overvulcanization, i.e., a vulcanization process 
carried out at too high a temperature, for instance 180.degree. C. instead 
of 150.degree. C., and/or lasting a too long time. As such 
overvulcanization is often difficult to avoid, the present invention 
offers a considerable advantage also in this respect. Moreover, the aging 
behavior or in other words the deterioration with time of the bond between 
the cobalt-copper alloy and rubber is more favorable than that of the bond 
between brass and rubber. Also the corrosion resistance of steel coated 
with a cobalt-copper alloy is better than that of steel coated with brass, 
which is a considerable advantage particularly in the use of the steel in 
automobile tires. 
To illustrate the present invention various experiments were carried out; 
their results are reported in the following Tables I, II and III. 
Table I 
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Adhesion to rubber of steel sample plates coated with a cobalt- 
copper alloy. 
Dry adhesion.sup.(1) 
Wet adhesion.sup.(2) 
Thickness of Vulcanization Vulcanization 
the alloy temperature temperature 
coat (nm) 
% Cu 150.degree. C. 
180.degree. C. 
150.degree. C. 
180.degree. C. 
______________________________________ 
2 470 660 310 325 
17 665 675.sup.(3) 
695 535 
24 785 800 540 455 
35 35 &gt;790 &gt;795 600 540 
52 780 780 480 275 
70 &gt;640 &gt;640 345 370 
2 &gt;670 &gt;655 575 540 
11 &gt;670 &gt;655 &gt;660 590 
110 20 &gt;800 &gt;805 &gt;795 710.sup.(4) 
28 &gt;710 &gt;655 &gt;707 710.sup.(4) 
2 &gt;675 &gt;660 505 500 
11 &gt;785 &gt;780 &gt;782 735.sup.(4) 
220 18 560 785 790 780.sup.(4) 
26 750 720 &gt;740 725.sup.(4) 
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Table II 
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Adhesion to rubber of steel sample plates coated with a copper- 
zinc alloy. 
Thickness of 
the alloy coat Dry adhesion.sup.(1) 
Wet adhesion.sup.(2) 
(nm) % Cu 150.degree. C. 
180.degree. C. 
150.degree. C. 
180.degree. C. 
______________________________________ 
120 68 &gt;810 114 245 420 
75 &gt;810 &gt;810 505 305 
53 650 235 610 290 
250 71 &gt;810 180 515 470 
82 510 405 570 215 
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The reference numerals in the Tables I and II have the following meaning: 
(1) in N/64 mm.sup.2 ; vulcanization times 25 minutes; 
(2) rubber stored at 90% relative humidity for not less than 10 days; 
(3) &lt;=rupture of the metal plate; 
(4) 100% Rubber Coverage, i.e., the adhesive strength between alloy and 
rubber was higher than the strength of rubber. 
The coatings mentioned in Tables I and II were deposited electrolytically 
from a pyrophosphate bath on steel plates according to FIG. 1. Each steel 
plate consisted of a square end measuring 8.times.8 mm and an elongated 
clamping portion measuring 20.times.8 mm. Between the square 8.times.8 mm 
ends of two steel plates (see FIG. 2) there was placed a layer of rubber 
and the two plates were subsequently placed in a vulcanizing press and 
subjected to a high mechanical pressure. 
Prior to vulcanication the layer thickness of the piece of rubber was about 
2 mm, and after vulcanization about 0.5 mm. The metal plates vulcanized 
pairwise to each other were then at their free ends clamped in an Instron 
tensile tester, and the adhesion in N/64 mm.sup.2 given in the tables was 
determined. 
Table III 
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Adhesion to rubber.sup.(1) of steel plates coated with a 
cobalt-copper alloy and of a brass plate under different conditions 
of the pyrophosphate bath containing cobalt and copper. 
Dry adhesion.sup.(2) 
Wet adhesion.sup.(3) 
vulcanization vulcanization 
Plating.sup.(4) 
temperature temperature 
Material 
conditions 
150.degree. C. 
180.degree. C. 
150.degree. C. 
180.degree. C. 
______________________________________ 
CoCu Flat 960 1030 560 730 
cathode 
60.degree. C.; 
no AC.sup.(5) 
CoCu Rotating 900 not not 530 
cathode; deter- deter- 
50.degree. C.; mined mined 
no AC 
CoCu Rotating 780 not not 720 
cathode; deter- deter- 
60.degree. C.; mined mined 
no AC 
CoCu Rotating 890 not not 610 
cathode; deter- deter- 
50.degree. C.; mined mined 
10 g/l AC 
CoCu Rotating 960 not not 570 
cathode; deter- deter- 
60.degree. C.; mined mined 
10 g/l AC 
brass 
plate.sup.(6) 
-- 890-970 730-740 
370-470 
30-180 
______________________________________ 
The reference numerals in Table III have the following meaning: 
(1) rubber sample of over 3 months old; 
(2) adhesive strength measured with the aid of improved test plates 
according to FIG. 3; 
(3) rubber stored for 3 months at 90% relative humidity; 
(4) best observed adhesion levels for various samples containing 13.6-67.5 
percent by weight of copper in a coating thickness range of 45-668 nm and 
prepared by means of a rotating cathode (35-500 r.p.m); 
(5) AC = ammonium citrate; 
(6) lowest and highest values found for brass plate. 
From Tables I and III and from a comparison of these tables with Table II 
it appears that the adhesion between rubber and a surface plated according 
to the invention with a cobalt-copper alloy is better than in the case of 
a brass-plated surface. Particularly improved are the adhesion to moist 
rubber and the adhesion obtained during overvulcanization. The adhesion 
tests, the results of which are listed in the Tables I, II and III, were 
carried out with a particular commercially available type of rubber used 
for the manufacture of automobile tires. Tests were also carried out on 
various types of rubber by varying the sulfur content from 1-11 phr. These 
variations in the sulfur content dit not lead to any considerable change 
in adhesion. It should be added that 1 phr = the number of parts by weight 
of sulfur per 100 parts by weight of rubber without any additives. 
Moreover, test plates were prepared using 5 different kinds of commercially 
available rubber mixtures A, B, C, D and F used by different firms for the 
manufacture of automobile tires. The results of these tests, too, 
confirmed the above-mentioned favorable adhesion properties as a result of 
the use of the cobalt-copper coating according to the invention. 
The results of these tests with 5 different kinds of rubber are given in 
the FIGS. 4, 5, 6 and 7. On the horizontal axes therein are plotted the 
rubbers A, B, C, D and E, and on the vertical axes the adhesion to rubber 
in N/64 mm.sup.2 after vulcanization. 
The FIGS. 4 and 5 refer to tests with steel plates of the type indicated 
above electrolytically plated with a cobalt-copper alloy to a coating 
thickness in the range of 45-380 nm and a copper content in the range of 
13.6 to 36.9 percent by weight. The FIGS. 4 and 5 mention both the 
adhesion to dry and to moist rubber. FIG. 4 gives the results obtained 
after vulcanization for 25 minutes at 150.degree. C., and FIG. 5 shows the 
results obtained after 25 minutes vulcanization at 180.degree. C. The 
FIGS. 6 and 7 refer to tests with solid brass plates to which rubber had 
been bonded by vulcanization. 
The FIGS. 6 and 7 both show the results for the adhesion to dry and to 
moist rubber. FIG. 6 mentions the results obtained after vulcanization for 
25 minutes at 150.degree. C., and FIG. 7 gives the results obtained after 
25 minutes vulcanization at 180.degree. C. Comparison of the FIGS. 4, 5, 6 
and 7 teaches that the cobalt-copper coating according to the invention 
provides such adhesion to different kinds of rubber and is distinctly less 
likely to be affected by the moisture condition of the rubber and 
overvulcanization. 
Tests were also carried out to find out to what extent adhesion is affected 
by temperature. For that purpose adhesion was measured at temperatures 
between 0.degree. and 200.degree. C. It was found that in this respect the 
level of adhesion to rubber of objects plated with a cobalt-copper alloy 
is approximately equivalent to the adhesion level of objects plated with 
brass. It was further found that steel filaments having a diameter of 
about 2 mm and plated with said thin coating of cobalt-copper alloy can in 
a number of drawing stages be reduced to a diameter of a few tenths of a 
millimeter without the cobalt-copper alloy being segregated and without a 
great loss of alloy in the dies. Favorable results are therefore expected 
in adhesion tests. 
It should be added that in U.S. Pat. No. 2,939,207 it has been proposed 
that a ferrous core should be provided with a corrosion resistant coating 
of zinc, cadmium or tin, to which there is applied a coating of nickel, 
cobalt or antimony, which is finally provided with a metallic coating of 
copper, copper-zinc, copper-cadmium or copper-tin which adheres to rubber. 
In U.S. Pat. No. 2,323,890 it has been suggested that the zinc of a coating 
of zinc and copper on drawn metal wire should be replaced by tin, lead or 
cadmium. 
Further, U.S. Pat. No. 2,240,805 proposes that to improve the adhesion of 
rubber to a metal object the metal should be plated with a cobalt coating. 
It also states that the cobalt surface may, moreover, contain other 
metals, such as iron, copper and zinc. Such metals, however, are not 
contained in an alloy along with cobalt. 
With the scope of the present invention various modifications may be made. 
In the case of steel filaments the cobalt-copper alloy may be applied to 
filaments drawn to a diameter of, say, 1.0-1.5 mm, as is practiced with 
brass. 
The filaments coated with the alloy are then further drawn in various 
stages until they have the desired final diameter of, say, 0.25 mm. 
Although the invention has been described in detail for the purpose of 
illustration, it is to be understood that such detail is solely for that 
purpose and that variations can be made therein by those skilled in the 
art without departing from the spirit and scope of the invention except as 
it may be limited by the claims.