Method, composition and product with improved adhesion between a metal member and a contiguous cured rubber skim stock

This invention is directed to a method, a rubber skim stock and a product containing the skim stock having improved adhesion between a metal member and contiguous rubber skim stock. The invention lies in the discovery that improved rubber-to-metal adhesion can be obtained by adding to an otherwise conventional rubber skim stock composition appropriate amounts of a transition salt of p-aminobenzoic acid. The method of this invention comprises the steps of mixing the transition salt of p-aminobenzoic acid into a rubber composition, bringing this composition into contiguous relationship with a metal member in an unvulcanized product and vulcanizing the product 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. 
U.S. Pat. No. 3,340,214 teaches the use of benzoic acid or nitrobenzoic 
acid as an additive to reduce the resiliency of rubber as utilized, for 
example, in the production of tires. 
U.S. Pat. No. 3,993,847 teaches the use of cobalt salts in an adhesive 
formulation for bonding rubber to metal; in addition to cobalt salts, the 
presence of from 5 to 180 phr silica filler is required. The cobalt salt 
can be, for example, cobalt naphthenate. 
U.S. Pat. No. 2,935,485 teaches the use of magnesium aminostearate as a 
stabilizer for rubber compositions; the stabilization achieved is that 
against oxidative degradation and is effective in the presence of rubber 
or blends thereof that have contaminating iron salts; the rubber is that 
which has been oil extended. 
DESCRIPTION OF THE INVENTION 
This invention relates to a method, 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 metal member. The invention 
lies in the discovery that improved rubber-to-metal adhesion can be 
obtained by adding to an otherwise conventional rubber skim stock 
composition appropriate amounts of a transition salt of p-aminobenzoic 
acid. 
The method of this invention comprises the steps of mixing the transition 
salt of PABA into a rubber composition, as described below, bringing this 
composition into contiguous relationship with at least one metal member in 
an unvulcanized product and vulcanizing the product to yield the end 
product. 
The para-aminobenzoic acid used to illustrate the present invention was 
purchased from Aldrich Chemical Company, Inc. and was specified as having 
a molecular weight of 137.14, a melting range of 188.degree. to 
189.degree. C., and a density at its melting point of 1.374. It was 
classified as being 99% pure. 
The transition metal salt of p-aminobenzoic acid can be prepared by adding 
p-aminobenzoic acid (137g, 1 mole) to a solution of sodium hydroxide (40 
g, 1 mole) in 900 ml. of distilled water at room temperature with 
mechanical stirring. To the resulting brown solution of the sodium salt of 
p-aminobenzoic acid can be added slowly a solution of selected transition 
metal chloride (120 g, 0.506 mole) in 100 ml. of distilled water at room 
temperature with stirring. A reaction takes place immediately to form a 
precipitate. After the addition is complete, the reaction mixture is 
stirred at room temperature for an additional 3 hours. The solid is then 
collected on a filter, washed with a small amount of water and dried at 
120.degree. C. under vacuum overnight. After this drying step, solid 
transition p-aminobenzoic acid is obtained in high yield. 
The rubber compound is described with all ingredients based on the total 
rubber component in the compound being 100 parts by weight. 
The composition of this invention consists essentially of 100 parts rubber, 
40 to 70 phr carbon black, 4 to 10 phr zinc oxide, 10 to 30 phr silica, 
0.5 to 1 phr antioxidant, 0.5 to 1 phr stearic acid, 0.2 to 2.0 phr 
accelerator, 1 to 10 phr hydrocarbon resin, 2 to 6 phr resorcinol, 1 to 5 
phr Manobond C, 4 to 9 phr sulfur/oil, 80/20 4 to 9 phr melamine resin and 
0.5 to 8 phr of a transition salt of PABA. 
DETAILED DESCRIPTION OF THE INVENTION 
The following Examples are representative of the method and rubber 
composition of this invention. 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 
Natural rubber 60-80 parts 
Butadiene 40-20 parts 
Carbon black 40-70 phr 
Zinc oxide 4-10 phr 
Silica 10-30 phr 
Stearic acid .5-1 phr 
Antiozonant/antioxidant (i.e., 
Santoflex 13) .5-1 phr 
Hydrocarbon resin 1-10 phr 
Resorcinol 2-6 phr 
Manobond C 1-5 phr 
Final Mix 
Accelerator .2-2 phr 
Sulfur/oil 80/20 4-9 phr 
Melamine resin (i.e., Cyrez 963) 
4-9 phr 
Transition metal salt of PABA 
0.5-8 phr 
______________________________________

Specific detailed examples of useful compositions within the ranges set out 
above are: 
EXAMPLE 1 
The following composition was Banbury mixed at about 
280.degree.-340.degree. F. for seven (7) minutes, or 340.degree. F., 
whichever occurs first, and a rotor speed of 80 rpm; the resulting 
masterbatch was then dumped. The Banbury type was a Type B Internal Mixer 
(Farrel-Birmingham Company). 
______________________________________ 
Parts 
______________________________________ 
(1) natural rubber 75 parts 
(2) butadine rubber 25 parts 
(3) carbon black (FEF) 40 phr 
(4) zinc oxide 4.0 phr 
(5) pelletized hydrated silica 
10 phr 
(6) stearic acid 1.0 phr 
(7) N-(1,3-dimethyl butyl)-N'-phenyl- 
1.0 phr 
p-phenylenediamine 
(8) hydrocarbon resin 5.0 phr 
(9) resorcinol (meta-dihydroxybenzene) 
2.5 phr 
(10) Manobond C 16 3.5 phr 
______________________________________ 
The hydrocarbon resin was in flake form, had a softening point between 
about 100.degree. to 110.degree. C., an iodine number of from 125 to 167, 
and, an ash content of 0.05%. The masterbatch resulting from the preceding 
was then final roll mill mixed for 4 to 8 minutes until dispersion was 
achieved at a mill speed of approximately 50 rpm and at a temperature of 
from 160.degree. to 180.degree. F.; the resulting final mix was then 
dumped. The composition components added to the masterbatch prior to final 
mill mixing were as follows: 
______________________________________ 
(a) N-tert-butyl-2-benzothiazole 
0.7 phr 
(b) sulfur oil, 80/20 6.0 phr 
(c) hexamethoxymethyl-melamine 
4.0 phr 
powder, and 
(d) Nickel PABA. 4.0 phr 
______________________________________ 
This product was cured for 30 minutes at 300.degree. F., a cure pressure of 
800-900 psi, and is the invention composition of Table I. The control is 
the identical composition without nickel para-aminobenzoic acid. 
Table I which follows illustrates the performance of our nickel PABA 
containing composition with bright steel (unplated) wire as it would be 
utilized. 
TABLE I 
______________________________________ 
Control + Control + 
2 PHR 4 PHR 
Test Control Ni-PABA Ni-PABA 
______________________________________ 
Unconditioned T Adhesion 
to (bright steel)* 
30'Cure, R. T. Test 
121(90) 130(90) 124(90) 
Conditioned T Adhesion 
to bright steel after 
1 hour in 300.degree. F. steam 
30'Cure at 300.degree. F. 
79(50) 94(80) 95(80) 
Tested at 230.degree. F. 
______________________________________ 
(T adhesion values in lb./in.; Coverages in percent) 
*Bright steelunplated 
EXAMPLE 2 
Substantially the same results of Example I are achieved when the following 
masterbatch and composition added prior to final mill mixing are 
substituted therein: 
______________________________________ 
Masterbatch 
(1) Natural rubber 75 parts 
(2) Butadiene rubber 25 parts 
(3) Carbon black (FEF) 55 phr 
(4) Zinc oxide 10 phr 
(5) Pelletized hydrated silica 
12.5 phr 
(6) Stearic acid 1 phr 
(7) N-(1,3-dimethyl butyl)-N'-phenyl- 
1 phr 
p-phenylenediamine 
(8) Hydrocarbon resin 5 phr 
(9) Resorcinol (meta-dihydroxybenzene) 
4 phr 
(10) Manobond C 16 3.5 phr 
Composition Added to Above Masterbatch 
(A) Sulfur/oil - 80/20 7 phr 
(B) Hexamethoxymethyl-melamine 
6 phr 
(C) N-oxydiethylene benzothiazole-2- 
.8 phr 
sulfenamide, and 
(D) Ni-PABA 4 phr 
______________________________________ 
The following transition metal salts of PABA, Cr, Mn, Zn, Mo, Cd, Zr, Ag 
and Ti PABA can be substituted in the preceding example formulations for 
Ni-PABA and will result in a significant improvement over the control 
formulation. 
The steel cord adhesion results set forth for bright steel wire (unplated) 
in the Examples above were determined by the following T adhesion 
procedure: 
T-ADHESION TEST 
1. Using a clicker machine and a 6.times.1/2 inch die, prepare an adequate 
number of experimental 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 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 a 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. adhesion per 
imbedded inch of wire. 
Characteristics of several of the ingredients set out in the examples are 
set out below. These definitions are to be considered by way of 
illustration and represent known materials that have proven useful in this 
invention. 
Preferably, the skim stock also contains a suitable proportion of a 
conventional organo-cobalt complex, such as a material which is 
commercially sold under the name "Manobond C". It is known that such 
materials, including Manobond C, facilitate rubber-to-metal adhesion. 
Manobond C is a commercially available source of a cobalt and boron 
containing additive that is compatible in our formulation; it is believed 
to have the structure: 
##STR1## 
wherein each "R" is an alkyl radical of from 9 to 12 carbons. Manobond C 
is available as a blue, viscous liquid; it contains 15.5 to 16.5% cobalt 
(Manobond C 16) or it contains 17.5 to 18.5% cobalt (Manobond C 18); it 
has a viscosity (at 25.degree. C.) of 3,000 to 9,000 cps. The ash content 
is from 22 to 25 weight percent. Manobond C is commercially available from 
Wyrough and Loser, Inc., Trenton, N.J. 
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 
commerically 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. 
The pelletized hydrated silica is commercially available, for example, from 
PPG, Industries, Inc., Pittsburgh, Pa. The preferred silica is identified 
as Hi-Sil 233. 
Commercially available hydrocarbon resins that can be utilized in the 
invention include, for example, Butaprene 105 (Reichhold Chemical) and 
Picco 14215 supplied by Hercules, Inc. Included are the intermediate and 
aliphatic hydrocarbon resins that are otherwise commercially available. 
The selected hydrocarbon resin will preferably have a softening point of 
from about 100.degree. to about 110.degree. C., an iodine number of from 
about 125 to about 167 and a maximum ash content of 0.05%. 
The melamine resin is preferably Cyrez 963, one of a family of melamine 
resins marketed by American Cyanamid. 
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 within the designated ranges specified. 
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. 
Acceptable results would be achieved on substituting brass or zinc plated 
steel wire for the bright steel wire of Tables I and II. The wire coated 
in the practice of our invention can be, for example, brass plated wire, 
i.e., 70% Cu, 30% Zn, zinc plated, or, bright (unplated) steel. The wire 
can be in the form of a strand, mat, web, ply or braid.