Composition and product with improved adhesion between a metal member and a contiguous cured rubber skim stock

This invention is directed to a rubber skim stock and a product containing the skim stock having improved adhesion between a brassed metal member and contiguous rubber skim stock. The invention lies in the discovery that improved rubber-to-metal adhesion, and adhesion retention, can be obtained by adding to an otherwise conventional rubber skim stock composition appropriate amounts of sulfides of copper. In the practice of this invention, a sulfide of copper is mixed into a rubber skim stock composition, which composition is brought into contiguous relationship with a brassed metal member in the unvulcanized composition followed by vulcanization to yield the end product.

BACKGROUND OF THE INVENTION AND PRIOR ART 
In the production of rubber articles such as hose, pneumatic tires or power 
transmission belts such as V-belts, toothed positive drive belts, etc., it 
is generally necessary to reinforce the rubber or elastomeric product. In 
the past, textile materials have been employed for this purpose. However, 
wire cord has been found to be more desirable under certain conditions of 
use, for example, in pneumatic tires of the radial ply type. Maximum 
reinforcement of the rubber is obtained when maximum adhesion is produced 
and retained between the laminate of a contiguous rubber skim stock and 
the metal reinforcing element as used to form a unitary structure. Of 
equal importance is the requirement that, for example, the laminate of the 
reinforcing metal element and contiguous rubber remain in a bonded 
relationship with each other throughout the useful life of the reinforced 
structure in which the laminate is used. 
To achieve satisfactory rubber-to-metal bonding, numerous methods have been 
developed. For example, U.S. Pat. No. 2,720,479 describes a system wherein 
a phenolic resin and a brominated isoolefin-polyolefin interpolymer are 
dissolved in a suitable liquid carrier and the resulting adhesive 
composition is spread on rubber which is to be bonded to metal. The rubber 
and metal are subsequently pressed together and vulcanized. U.S. Pat. No. 
2,581,920 also describes the use of halogenated polymers to bond rubber to 
metal. 
In U.S. Pat. No. 3,517,722 to Endter et al., a rubber-metal adhesion system 
is described which involves formation of a resorcinol-formaldehyde resin 
at the interface between the rubber and the metal, thereby bonding these 
members together. In forming the resin, compounds capable of liberating 
the methylene precursor are added to resorcinol in the rubber stock. Upon 
vulcanization, the methylene and resorcinol react to form the 
resorcinol-formaldehyde resin. 
According to U.S. Pat. No. 3,846,160, the adhesion force between steel cord 
and rubber is enhanced by applying a mineral oil solution containing, for 
example, an organic acid salt of a higher aliphatic amine to zinc plated 
or brass plated steel cord. 
The adhesion of rubber to metal such as wire tire cord is improved 
according to U.S. Pat. No. 3,847,727 by incorporating a halogenated 
quinone and the condensation product of resorcinol and acetaldehyde into 
the rubber prior to application to the metal and vulcanization of the 
composite. 
The disclosure of U.S. Pat. No. 3,903,026 teaches the preparation of a 
rubber composition having improved metal adhesion properties even after 
thermal aging; this is disclosed to be achieved by compounding cobalt 
carboxylate and magnesium oxide (0.1 to 4 phr) into rubber. 
U.S. Pat. No. 3,738,948 is directed to a fiber reinforced rubber 
composition that can be utilized in tire construction. The fiber can be 
glass, nylon, rayon or metal wire. The composition is disclosed to contain 
a finely divided silica, hexamethylene tetramine, resorcinol and a 
compatible metal soap such as calcium stearate. Similar compositions 
containing discontinuous fiber filaments are disclosed in U.S. Pat. No. 
3,746,669. 
Pages 17 and 25 of the Vanderbilt Rubber Handbook, published by R. T. 
Vanderbilt, 1968, indicate that the presence of copper in rubber is 
harmful to said rubber in that it functions as a rubber oxidation 
catalyst. 
Rubber Age, October 1975, pages 31 to 36, "Evaluation of Resotropin" by 
Cunningham and Hart. Resotropin, the reaction product of equimolar 
quantities of resorcinol and hexamethylene tetramine, is disclosed to be a 
rubber adhesion promoter for use with brassed steel wire cord. Improved 
cure, including higher degrees of cure are alleged to be achieved with 
equal scorch resistance. 
DESCRIPTION OF THE INVENTION 
This invention relates to a rubber skim stock and a product containing the 
skim stock wherein the invention yields improved adhesion between a 
contiguous rubber skim stock and a brassed metal member. The invention 
lies in the discovery that improved rubber-to-metal adhesion, and adhesion 
retention, can be obtained by adding to an otherwise conventional rubber 
skim stock composition appropriate amounts of a sulfide of copper. 
In the practice of this invention, a sulfide of copper is mixed into a 
rubber skim stock composition, as described below, this composition is 
then brought into contiguous relationship with at least one brassed metal 
member in the unvulcanized composition, followed by vulcanization to yield 
the end product. 
The rubber compound is described with all ingredients based on the total 
rubber component in the compound being 100 parts by weight. 
DETAILED DESCRIPTION OF THE INVENTION 
A skim stock material for use in wire belts of tires was mixed in an 1100 
cc Banbury mixer. Mix times were about 7 minutes with a final Banbury 
temperature of 325.degree. F. This stock was then final mixed on a mill at 
a temperature below 220.degree. F. The addition of the copper sulfides was 
done on the final mill. 
Typical properties of a reinforcing belt skim stock include hardness values 
of over 60 shore A, 300% modulus values of at least 1200 PSI, and 
accompanying elongations at break of at least 250%. The known uses for 
such a stock would include preparation of belt skims for belted tires such 
as radials or bias belted. In addition this stock could be used in wire 
reinforced continuous belts or any other application where the rubber 
would come in contact with the brass plated steel wire. 
The level of copper sulfides which were found to give acceptable 
improvements in adhesion and adhesion retention between rubber skim stock 
and brass coated wire was preferably between 2 and 8 parts of both the Cus 
and Cu.sub.2 S per 100 parts of rubber. The levels of copper sulfides 
necessary to maintain adhesion in the stock appear to be directly related 
to the nature of the particular rubber stock, and can vary from about 0.5 
to about 15 parts per 100 parts of rubber. 
All testing of the wire-rubber composites was done in T-adhesion pads 
prepared by placing 60 gage slabs of uncured skim stock on 50 gage fiber 
reinforced backing. Sample width was 1/2 inch. Clean, brass wires were 
placed between two pads of the reinforced skim stocks with the wires in 
contact with the uncured skim at 1/2 inch intervals. The pads were placed 
in a mold and were cured for 30 minutes at 300.degree. F. Testing was done 
on an Instron Universal tester at 10 inches per minute at 230.degree. F. 
Steam bomb aging was done in a pressure vessel at 1 atmosphere, 
300.degree. F., and in a saturated steam atmosphere. Humidity chamber 
aging was done at 90% relative humidity and 95.degree. F. 
The following Examples are representative. The composition component parts 
are expressed in parts per hundred rubber, phr, unless otherwise 
specified. These components are broadly within the ranges set out below: 
Masterbatch 
100 parts rubber 
20 to 90 phr carbon black 
0 to 20 phr extender oil 
1 to 12 phr zinc oxide 
0.1 to 5 phr antioxidant 
0 to 5 phr stearic acid 
0.1 to 5 phr resorcinol 
Mill Mix 
0 to 10 phr sulfur 
0.1 to 3 phr N-(cyclohexylthio) phthalimide 
1 to 5 phr 95/5 hexamethylene tetramine/stearic acid preblend 
0.2 to 5 phr n-oxydiethylene benzothiazole 2-sulfenamide 
0.5 to 15 phr of, for example, Cu.sub.2 S or CuS as copper sulfide, and 
0.3 to 2.0 phr extender oil. 
Polymeric reinforcing resins that can be incorporated into our skim stock 
composition to improve the modulus properties of our cured stock can be 
added together with an extender oil, preferably medium process, if needed 
to maintain processability. Specific polymeric reinforcing resins that can 
be utilized in the practice of our invention include the 
phenol-formaldehyde resins, hexamethylenetetramine-formaldehyde, 
urea-formaldehyde and the like as illustrated in, for example, J. K. 
Stille, Introduction to Polymer Chemistry, John Wiley & Sons, Inc., 1962, 
Section 6.3, pages 102 to 106. Our preferred polymeric resin is the 1:1 
molar reaction product of resorcinol and hexamethylenetetramine known also 
as "resotropin". Selected mixtures of more than one of such polymeric 
resins can be utilized. 
The cupric sulfide (CuS) and cuprous sulfide (Cu.sub.2 S) adhesion and 
adhesion retention promoter selected in the practice of the present 
invention is not critical; it can be any of those commercially available, 
for example, such as those provided by Glidden Metals, a division of SCM 
Corporation; these compounds are described in detail in Bulletins Nos. 
2315 (9-9-75) and 2313 (7-6-75) respectively. These copper sulfides can be 
routinely prepared by known procedures.

Specific detailed examples of useful compositions within the ranges set out 
above follow: 
EXAMPLE 1 
The following composition was blended in a 1100 cc. Banbury mixer for about 
7 minutes with a final Banbury temperature of 325.degree. F. 
(1) 100 parts of natural rubber 
(2) 50 phr carbon black 
(3) 7.5 phr zinc oxide 
(4) 3 phr antioxidant (1,3-dimethyl butyl)-N'-phenyl-p-phenylenediamine 
(5) 0.5 phr stearic acid 
(6) 3.3 phr extender oil, and 
(7) 2 phr resorcinol. 
The masterbatch resulting from the preceding blending was then roll mill 
mixed with the following composition at a temperature below 220.degree. F. 
(a) 3.5 phr of an 80/20 sulfur/oil preblend 
(b) 1.0 phr N-(cyclohexylthio)-phthalimide as retarder 
(c) 0.6 phr n-oxydiethylene-benzothiazole-2-sulfenamide as accelerator 
(d) 2.2 phr 95/5 hexamethylenetetramine/stearic acid preblend. 
T-adhesions to brass coated wire for the resulting mill mix were determined 
at 230.degree. F. The samples were cured 30' at 300.degree. F. Percent 
coverage was determined visually. The control of Table I was the above 
mix. 
TABLE I 
______________________________________ 
Control + Control + 
Control 2 phr Cu.sub.2 S 
8 phr Cu.sub.2 S 
______________________________________ 
Monsanto Rheometer - 300.degree. F. 
Time to TS (2) 7.0 6.8 6.9 
Time to TC (90) 
21.2 20.0 18.4 
Ultimate Torque, dN M 
47.8 45.1 39.9 
R. T. Ring Tensile - 23' Cure 
300% Modulus, (PSI) 
2610 2570 2260 
Tensile Strength, (PSI) 
2950 2880 2420 
Elongation, (%) 
340 330 320 
R. T. Ring Tensile - 23' Cure - Aged 4 Days at 212.degree. F. 
Tensile Strength, (PSI) 
1480 1550 1510 
Elongation, (%) 
160 170 220 
Firestone Flexometer ASTM D 623/Method B 
Initial Shore A 
72 70 67 
Hot Shore A 70 66 61 
% Deflection 11.3 10.7 13.3 
Running Temp., (.degree.F.) 
220 220 240 
T-Adhesion to W152 Brass at 230.degree. F. 
Unconditioned 145(100) 142(100) 140(100) 
Steam Bomb - 1 Hr., 
300.degree. F. 
119(70) 110(80) 83(60) 
Humidity Chamber - 
14 Days 75(20) 69(40) 132(90) 
60 Days 20(0) 46(0) 95(50) 
90 Days 22(0) 47(0) 81(70) 
120 Days 23(0) 46(0) 75(40) 
150 Days 32(0) 43(0) 66(40) 
______________________________________ 
This Example I composition components can be varied within the following 
ranges: 
EXAMPLE II 
A masterbatch was prepared by mixing, in an 1100 cc. Banbury mixer the 
following formulation for about 7 minutes with a final Banbury temperature 
of 325.degree. F. This stock was then final milled on a mill at a 
temperature below 220.degree. F. The addition of the resotropin and 
adhesion promoter was done on the final mill. 
Masterbatch Formulation: 
(1) 100 parts natural rubber 
(2) 50 phr carbon black as reinforcing agent 
(3) 4 phr extender oil as processing aid 
(4) 7.5 phr zinc oxide as activator and reinforcing pigment 
(5) 0.5 phr stearic acid as internal lubricant stabilizer 
(6) 3 phr antioxidant (1,3-dimethylbutyl)-N'-p-phenylenediamine) 
The masterbatch resulting from this Banbury mixing was then roll mill mixed 
with 2.8 phr sulfur as curative, 0.7 phr extender oil, 0.9 phr 
N-oxy-diethylene benzothiazole 2-sulfenamide as accelerator, 2.2 phr 
resotropin, 8 phr Cu.sub.2 S as adhesion retention promoter for brass 
coated wire and as retarder, 0.3 phr N-(cyclohexylthio)-phthalimide; the 
sulfur and extender oil were mill mixed as an 80/20 sulfur/oil preblend. 
In Table II which follows, the masterbatch was present at 161 parts for 
each experiment; the mill mix components were as above designated. 
TABLE II 
______________________________________ 
Stock 
Aging Time Control + 8 PHR 
Control + 8 PHR 
(Days) Control Cuprous Sulfide 
Cupric Sulfide 
______________________________________ 
0 136(90) 32(100) 140(100) 
14 135(90) 140(100) 151(100) 
60 57(30) 93(40) 80(60) 
90 42(0) 75(60) 71(40) 
120 46(0) 90(70) 79(50) 
______________________________________ 
TABLE III 
______________________________________ 
Control Control 
+ 8 PHR + 8 PHR 
Control Cu.sub.2 S CuS 
______________________________________ 
Masterbatch of 
Example II 161 161 161 
Resotropin 2.2 2.2 2.2 
80/20 Sulfur/oil 
3.5 3.5 3.5 
preblend 
Accelerator of 
.9 .9 .9 
Example III 
Cupric Sulfide 
-- -- 8.0 
Cuprous Sulfide 
-- 8.0 -- 
Total Parts 167.6 175.6 175.6 
Monsanto Rheometer 300.degree. F. rpm 1.degree. Arc 
Scorch time (min. 
6.1 7.8 7.3 
to TS (2) ) 
Min. to opt. cure 
15.3 16.4 21.5 
Min. to YD (90) 
Max. Torque DN-M 
43.9 36.0 41.0 
Room Temp. Ring Tensile Slabs Cured 23' at 300.degree. F. 
Modulus at 300% 
2530 1930 2160 
Elongation (PSI) 
Tensile at Break 
2700 2300 2630 
(psi) 
Ultimate Elong- 
320 340 360 
ation (%) 
23' At 300.degree. F. Slabs After Aging 4 Days in 212.degree. F. Oven 
Tensile at Break 
690 610 680 
(psi) 
Ultimate Elong- 
80 120 130 
ation 
T-Adhesion to Brass Plated 1 .times. 5 (w 152) Wire @ 230.degree. F., 
Test Pieces 
Unconditioned 
136(90) 132(100) 140(100) 
After 1 Hr. in 
84(50) 96(60) 104(50) 
300.degree. F. steam 
After Conditioning in the Cured State in 90% R.H., 95.degree. F. 
14 Days 135(90)* 140(10)* 151(10)* 
2 months 57(30) 93(40) 80(60) 
3 months 42(0) 65(60) 71(40) 
4 months 46(0) 90(70) 79(50) 
5 months 43(0) 74(60) 63(20) 
______________________________________ 
*% Coverage as visually determined 
This Example II can be varied within the following practical composition 
component ranges: 
Masterbatch 
100 parts rubber 
20-90 phr carbon black 
0-20 phr extender oil 
3-12 phr zinc oxide 
0.1 to 5 phr antioxidant 
0 to 5 phr stearic acid 
Mill Mix 
1.0 to 10 phr curative 
0 to 3 phr retarder 
0.2 to 5 phr accelerator 
0.5 to 15 phr of copper sulfide 
0.5 to 12 phr polymeric reinforcing resin 
Mill mixing was carried out into the masterbatch on a two roll 20" mill 
followed by sheeting out the product at a thickness of 60 gage for 
adhesion testing. The product was cured for 23 minutes at 300.degree. F. 
EXAMPLE III 
The following composition was blended in a Banbury mixer: 
(1) 100 phr natural rubber 
(2) 55 phr carbon black 
(3) 7.5 phr zinc oxide 
(4) 2.0 phr antioxidant 
(5) 0.5 phr stearic acid 
(6) 2.0 phr pine tar 
The masterbatch resulting from the preceding was then roll mill mixed with 
each of the following compositions A (control) and B (control) plus 2 phr 
cuprous sulfide: 
______________________________________ 
A B 
______________________________________ 
(1) 7.5 phr of an 80/20 sulfur/oil 
Same 
preblend 
(2) 0.6 phr NOBS Special 
Same 
accelerator 
(3) 0.4 phr PVI retarder 
Same 
(4) 1 phr Santoflex 13 Same 
antioxidant 
(5) 2 phr cuprous sulfide 
______________________________________ 
T-adhesions to brass coated wire for the resulting A and B mill mixes were 
determined at 230.degree. F. The samples were cured 30' at 300.degree. F. 
(% coverage). Percent coverage was determined visually. The following 
T-adhesion data was determined to brass plated wire: 
______________________________________ 
A B 
______________________________________ 
Unconditioned 153(100) 157(100) 
After 1 hr. in 300.degree. F. Steam 
57(0) 44(0) 
After conditioning in the cured 
state in 90% R. H., 95.degree. F. environment 
14 days 163(100) 169(100) 
60 days 104(80) 109(80) 
90 days 81(60) 96(80) 
120 days 73(30) 107(70) 
150 days 95(70) 125(100) 
______________________________________ 
This Example III composition components can be varied within the following 
ranges: 
Masterbatch 
100 parts rubber 
20 to 90 phr carbon black 
3 to 12 phr zinc oxide 
0.1 to 5 phr antioxidant 
0 to 5 phr stearic acid 
0 to 20 phr pine tar 
Mill Mix 
3 to 10 phr of an 80/20 sulfur/oil preblend 
0 to 4 phr of an antioxidant 
0 to 2 phr of a vulcanization retarder 
0.2 to 2 phr of an accelerator 
0.5 to 15 phr of copper sulfide 
The retarder component functions as a prevulcanization inhibitor and can 
be, for example, Santogard PVI by Monsanto, 
N-(cyclohexylthio)-phthalimide. 
The testing of our stabilized and adhesion retaining skim stocks was done 
on T-adhesion pads prepared by placing 60 gage slabs of uncured skim stock 
on 51 gage fiber reinforced backing. Sample width was 1/2 inch. Clean 
brass coated wires were placed between two pads of the reinforced skim 
stock with the wires in contact with the uncured skim at 1/2 inch 
intervals. The pads were placed in a mold and were cured for 30 minutes at 
300.degree. F. Testing was done on an Instron Universal Tester at 10 
inches per minute and 230.degree. F. Steam bomb aging of the samples was 
done in a pressure tight bomb at 1 atmosphere pressure, 300.degree. F., 
and a saturated steam atmosphere. 
The unconditioned T-adhesion results set forth in the preceding examples is 
in lbs./in. and % coverage as visually determined. The conditioned 
T-adhesion results are determined after 1 hour in 300.degree. F. steam in 
lbs./in. with % coverage being determined visually. 
DETAILED T-ADHESION TEST PROCEDURE 
1. Using a Clicker machine and a 6.times.1/2 inch die, prepare an adequate 
number of calendered and control stock samples for pad building. 
2. Use one piece of calendered fabric backing (0.051"). 
3. Ply one piece of control rubber stock (0.060") onto the fabric backing. 
4. Place sample in building jig with fabric side down. 
5. Place ten cords (of brass coated wire) approximately 7" in length 
equally spaced on top of the two piece assembly. 
6. Invert another 2 ply assembly, made as in items 1, 2 and 3, on top of 
cords so that cords are between 2 layers of stock to be tested. 
7. This assembly should now fit snugly into the mold. 
8. Adhesion pads shall be cured for 30 minutes at 300.degree. F. and then 
allowed to equilibrate for 24 hours. 
9. Testing Machine: 1130 Instron Universal Tester. 
10. Test speed 10"/minute; temperature 230.degree. F. after 20' preheat. 
11. The top grip shall be of a special holder made for the cured sample, 
with a slot in the bottom to permit the sample to be inserted with the 
wire protruding. The bottom grip shall be a wedge type, designed to exert 
increasing tightening as the wire is pulled. 
12. Record 10 pulls and average. Multiply by 2 to get lbs. per inch. 
The preceding Examples can be varied within the scope of our total 
specification disclosure, as it would be understood and practiced by one 
skilled in the art, to achieve essentially the same results. Equivalent 
reactants can be used. 
When our skim stock is used in the conventional steel cord tire 
construction, for example, it is extremely important that the bond between 
the rubber ply stock and the wire fabric also be as flexible and as strong 
as possible for efficient use under operating conditions; this is 
especially important in the case of truck tires which are subjected to 
high loads and speeds with consequent heat buildup due to the rapid 
flexing of the plies. 
The rubber to be used in the practice of this invention includes 
vulcanizable rubbers. Rubbers that can be utilized include natural 
rubbers, synthetic rubbers, polyisoprene, polybutadiene, copolymers of 
butadiene and styrene and the like, and blends thereof. The particular 
rubber composition selected is preferably a blend of natural rubber and 
polybutadiene. An extender oil, when utilized, can be, for example, any 
known medium process oil, aromatic or naphthenic hydrocarbon derived. 
The antioxidant selected can be, for example, N-(1,3-dimethyl 
butyl)-N'-phenyl-p-phenylenediamine, known in the trade as Santoflex 13; 
or other phenyl-p-phenylenediamine derivatives. 
The accelerator preferably utilized in the practice of our invention is 
N-oxydiethylene benzothiazole-2-sulfenamide; this accelerator is 
commercially available from American Cyanamid and is known as NOBS 
Special. Other accelerators such as N-t-butyl-2-benzothiazole-sulfenamide 
can also be utilized; the particular accelerator selected is not critical. 
Any known rubber reinforcing carbon black can be used, such as the known 
FEF, ISAF and other carbon blacks. Curing is preferably achieved through 
use of sulfur as a sulfur/oil blend preferably utilized at a ratio of 
80/20. The use of FEF carbon black is preferred. Curing can also be 
achieved through known peroxide or irradiation means. 
Compounding ingredients customarily employed in the rubber compounding art 
can be added to our skim stock composition and include accelerators, 
antioxidants, bactericides and the like, color pigments, extenders, 
reinforcing pigments, softeners, vulcanizing agents, etc. The compounding 
ingredients are used in the amounts necessary to achieve the desired 
properties in the resulting vulcanizate as is well known to those skilled 
in the art. 
The skim stock of the present invention can be applied by use of 
calendering means, spray means or other known application techniques. 
Areas of significant utility include, but are not limited to, radiator 
hose, pneumatic tires, air ride springs, metal reinforced products such as 
rubber bumpers and sporting goods grips such as golf club handles, in each 
of these representative areas of utility, the skim stock composition can 
be used to increase adhesion and adhesion retention properties between 
metal and rubber, including use in operation when bright steel surfaces 
are present. 
When the skim stock of this invention is used in steel cord tire 
construction, for example, it is extremely important, both in new tire 
construction and retread or repair operations, that the bond between the 
rubber ply stock and the wire fabric be as flexible and as strong as 
possible for efficient use under operation conditions; this is especially 
important in the case of truck tires which are subjected to high loads and 
speeds with consequent heat buildup due to the rapid flexing of the plies. 
The present invention also finds utility in, for example, metal-rubber 
articles such as motor mounts, cutless bearings, torsilastic springs, 
power belts, printing rolls, metal wire reinforced or braided hose, 
electrical deicers, shoe heels, and wherever it is desired to secure 
rubber to plated or unplated metal to provide a flexible and strong bond 
between the same. 
The coated wire utilized in the practice of our invention can be, for 
example, brass plated wire, i.e., 70% Cu, 30% Zn steel. The wire can be in 
the form of a strand, mat, web, ply or braid.