CURED RUBBER SKIM STOCK COMPOSITIONS HAVING IMPROVED METAL ADHESION AND METAL ADHESION RETENTION

Abstract: This invention is directed toward a rubber skim stock composition and a product containing the skim stock hav­ ing improved adhesion and adhesion retention with metal­ lic reinforcement cords such as brassed steel. The inven­ tion composition comprises a vulcanizable rubber skim stock and an organic salt of cobalt and an organic salt of nickel and is prepared by mixing the cobalt and nickel salts with the rubber skim stock prior to contacting the metallic reinforcement. Tests conducted with brass-plated and zinc-plated steel cords showed improved metal adhe­ sion and metal adhesion retention with use of the rubber skim stock of this invention when compared to existing formulations.

Description: CURED RUBBER SKIM STOCK COMPOSITIONS HAVING IMPROVED METAL ADHESION AND METAL ADHESION RETENTION Technical Field The present invention is directed toward improvement in the adhesion and adhesion retention between a rubber skim stock composition, used in the manufacture of tires, conveyor belts, hoses and the like, and metallic reinforcement cord such as steel wire and cable which commonly carries a protective coating of zinc or brass. In the manufacture of the foregoing rubber articles, particularly steel-belted bias and radial tires, it has become common to reinforce the rubber skim stock material with steel wire or cable. The metallic reinforced rubber is employed as a belt, one or more of which is circumferentially oriented beneath the tread stock to maintain the integrity and shape of the tire during inflation and subsequent load. In order for the belt to function effectively, it is imperative that adhesion between the rubber and the steel cord be effectively maintained. Because steel is prone to oxidation, which even in minor degree is highly deleterious to the necessary adhesion with the rubber skim stock, and it would be most impractical to incorporate a chemically clean, oxidation-free steel cord in the belt at the time of its manufacture, the steel cord is plated with zinc or brass thereby protecting it from oxidation until it can be used. While adhesion between zinc or brass-plated steel cord and rubber is generally far greater than that between the latter and oxidized steel, existing data determined from article life as well as modern testing techniques indicates that adhesion obtained between plated steel cord and the elastomer must be increased for improved article life and service. Adhesion must also be improved between the rubber skim stock and bright steel, i.e., surfaces of the cord which are neither plated nor oxidized, generally occurring when the steel cords and rubber skim stock are cut to size from larger, calendered sheets of the reinforced rubber stock. Background Art In order to promote adhesion between rubber and ferrous metals it is known to employ a variety of metallic salts as coatings to the metal or as an ingredient in a rubber composition. One early patent embodying the former technique is U.S. Pat. No. 1,919,718 which discloses a composite rubber product that adheres to ferrous metals. The invention requires the coating of the metal with a metallic salt such as that formed with copper, cobalt, manganese, iron or lead and a fatty acid of high molecular weight, e.g., stearic, palmitic, oleic, linoleic, lauric and the like. Alternatively, the metallic salt may be dissolved in a solvent or in a rubber cement which is then applied to the metal surface. The rubber layer is then applied and vulcanized in position. U.S. Pat. No. 2,643,273 discloses a rubber adhesive composition which contains a water soluble cobalt salt to increase the adherence of rubber to metal. The specification discloses incorporating cobalt chloride or sulfate in the rubber or dipping the metal in a solution of the salt. U.S. Pat. No. 2,912,355 is directed toward improving the adhesion between rubber and metal by the incorporation into a rubber composition of a partially oxidized metal salt of an aliphatic fatty acid compound, the metal being cobalt, copper, iron, lead, mercury, nickel or silver. U.S. Pat. No. 3,897,583 is directed toward the adhesion of metal to rubber by incorporating a cobalt salt in a rubber stock which contains an adhesive resin forming system based on a methylene donor which is a methylolated nitroalkane in combination with a resorcinol type methylene acceptor. Cobalt salts disclosed include those of ali phatic or alicyclic carboxylic acids having 6-30 carbon atoms. U.S. Pat. No. 3,903,026 discloses a rubber composition containing cobalt carboxylate and magnesium oxide to improve the adhesion of the rubber to zinc or zinc alloy plated steel. U.S. Pat. No. 3,936,536 discloses the method of adhering rubber to metal by coating the metal with rubber containing a small amount of Co(NH3)2.C12. U.S. Pat. No. 3,993,847 discloses a process of bonding a rubber to a ferrous metal and such a laminated article. Adhesion is improved by employing an adhesive comprising a conjugated diene and a heterocyclic nitrogen base and containing about 5-180 parts of silica filler per 100 parts of the diene-heterocyclic nitrogen interpolymer. The patent states that adhesion is further improved by using a rubber containing small amounts of cobalt metal. While others have sought to enhance adhesion between rubber compositions and metals by employing certain metallic salts and complexes with or without other compounds such as resins, the art of which we are aware has not disclosed a satisfactory metal salt that can complement the presence of a cobalt salt when amounts of the latter are decreased and yet maintain adhesion properties as good as when the cobalt salt is utilized as the sole metal compound to improve rubber-to-metal adhesion. Disclosure of Invention It is therefore an object of the present invention to provide a rubber skim stock exhibiting better humidity aged adhesion and adhesion retention with metallic reinforcement such as unplated steel and brass-plated or zinc-plated steel which can be in the form of, for example, wire or cord. It is another object of the present invention to provide a rubber skim stock containing organic cobalt and nickel salts for improving the adhesion and adhesion retention properties between the rubber and metallic reinforcement embedded therein. It is yet another object of the present invention to provide a tire having improved adhesion and adhesion retention between the rubber and steel reinforcing elements embedded therein. These and other objects, together with the advantages thereof over the prior art, which shall become apparent from the specification which follows, are accomplished by our invention as hereinafter described and claimed. In the practice of the present invention certain organic cobalt and organic nickel salts are mixed with a rubber skim stock which is, in turn, subsequently reinforced with metallic cords. The rubber skim stock composition of our invention exhibits enhanced adhesion and adhesion retention with metallic reinforcement such as steel, plated with brass or zinc or unplated, and comprises a vulcanizable rubber skim stock having an organic salt of cobalt in an amount of from about 0.25 to about 5.50 parts by weight, with about 0.50 to about 2.50 being preferred, and an organic salt of nickel in an amount of from about 0.50 to about 12.0 parts by weight, with about 1.0 to about 4.8 being preferred. Both additives are present in parts per hundred parts of rubber (phr) based upon the weight of the elastomer in the rubber skim stock. Preferred Mode for Carrying Out the Invention To illustrate our invention, a typical rubber skim stock is presented hereinbelow which is suitable for preparation of rubber articles such as tires. Adhesion between this stock with steel reinforcement subsequent to vulcanization has been measured and is also presented hereinbelow. It is to be understood that the composition of the rubber skim stock provided is not part of the present invention and that it has been presented solely to enable those skilled in the art to have at least one rubber skim stock with which to practice the invention. The ingredients utilized in the practice of the present invention include organic salts of the metals with aliphatic fatty acids, having from two to about 20 carbon atoms. The salts are prepared by the combination of the metal hydrate and the fatty acid. Suitable aliphatic fatty acids used to practice the present invention would be preferably, i.e., acetic, propionic, butyric, valeric, undecanoic, lauric, palmitic, stearic, nonadecanoic and the like. The metal used to prepare the salt of the aliphatic fatty acid could be selected from the group consisting of cobalt, nickel, copper, barium, iron, mercury, tin, zinc, calcium, magnesium and silver, with cobalt and nickel being preferred. Particularly useful as the cobalt salt is cobalt propionate prepared from propionic acid and cobalt hydroxide. Cobalt propionate, as used in the experimental work discussed hereinbelow, is a purple powder having about 29% cobalt and a specific gravity of 1.68 + 0.1 at 250 C. Particularly useful as the nickel salt is nickel octoate prepared from octanoic acid and nickel hydrate. Nickel octoate, as used in the experimental work discussed hereinbelow, is a viscous green liquid having 12% nickel and a specific gravity of 1.10 at 250 C. Amounts of the cobalt salt employed can range from about 0.25 to about 5.50 parts per hundred parts of rubber (phr) with 0.80 parts phr being preferred. Similarly, the amount of nickel salt employed can range from about 0.50 to about 12.0 parts phr with 3.20 parts phr being preferred. Other cobalt or nickel salts of the foregoing fatty acids can be employed in substantially the same amounts. In order to determine the improvement in adhesion obtained when the cobalt and nickel salts are added to the rubber skim stock, T-adhesion tests (rubber-to-steel cord) were conducted according to the procedure which follows The test utilized T-adhesion pads prepared by placing 60 gauge slabs of uncured fully compounded rubber skim stock on 51 gauge fabric reinforced rubber backing. Commercial brass-coated cables (1 x 5 x 0.25 mm diam.) were placed between two pads of the reinforced skim stock with the wires in contact with the uncured rubber skim at 1.25 cm intervals. The width of each adhesion pad was 1.25 cm. The pads were placed in a curing mold and were cured for 30 minutes at 1490 C. Testing was done on a Model 1130 Instron Universal Tester at a crosshead speed of 25.4 cm per minute and 1100 C. The T-adhesion pads were preheated in the 1100 C oven for 20 minutes prior to testing. Oven aging of the cured T-adhesion pads was done in a forced air oven at 1210 C for two days. Steam bomb aging of the cured samples was done in a pressure tight bomb for one hour at 1490 C, in a saturated steam atmosphere. Detailed T-Adhesion Test Procedure 1. Using a Clicker machine and a 15.24 x 1.25 cm die, prepare an adequate number of calendered and control stock samples of T-adhesion pad building. 2. Use one piece of calendered fabric reinforced rubber backing (0.1295 cm). 3. Ply one piece of 60 gauge control rubber stock (0.1524 cm) onto the fabric backing. 4. Place sample in building jig with fabric side down. 5. Place ten cords (of brass or zinc coated wire) approxi mately 17.78 cm 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 the 2 layers of rubber skim stock to be tested. 7. This assembly should now fit snugly into the cavity of the curing mold. 8. Adhesion pads shall be cured for 30 minutes at 1490 C and then allowed to equilibrate for 24 hours before testing. 9. Testing Machine: Model 1130 Instron Universal Tester. 10. Test speed 25.4 cm/minute; testing temperature, 1100 C after 20 minutes 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 wires protruding. The bottom grip should be a wedge type, designed to exert increasing tightening as each wire is pulled from the cured sample. 12. Record 10 wire pull-outs and average. Multiply average pull-out force value by 0.3572 to obtain kilograms per centimeter. In the tests which follow, a rubber skim stock, Stock A, was prepared to which the ingredients of the present invention were added in varying amounts. Formulations for each are as follows with all parts given on the basis of parts per hundred parts of rubber (phr) by weight, unless otherwise specified. Compounding Ingredients Stock A Natural Rubber 100 HAF carbon black 60 Zinc Oxide 7.5 Stearic Acid 0.5 Hydrocarbon Resins 2.0 Ajone DD*1 2.0 2 NOBS Special* 0.8 Santoflex 13*3 1.0 Process Oil 2.0 Sulfur MB*4 7.5 Santogard PVI* 0.4 *1 6-dodecyl-1,2-dihydro-2,2,4-trimethylquinoline *2 N-oxydiethylene benzothiazole 2-sulfenamide *3 N-(1,3 dimethylbutyl)-Nl-phenyl-p-phenylenediamine *4 80% Insoluble sulfur and 20% process oil *5 N-(cyclohexylthiol)phthalimide In the tests which follow, cobalt propionate and nickel octoate were added to Stock A in amounts ranging from 0.81 to 1.62 parts phr of the cobalt salt and from 1.985 to 3.97 parts phr of the nickel salt. A control was also provided consisting of Stock A alone without the adhesion promoting additives of the present invention. Five sets of tests were conducted for the Control and Examples 1-4 and these are reported in Table I. The tests included T-adhesion determination for vulcanized specimens -- unaged or unconditioned, Test A; oven aged, Test B; and, steam bomb aged, Test C. The force necessary to pull or remove the metallic reinforcement from the vulcanized rubber skim stock is given first, in kg/cm, followed by the percent of rubber skim stock remaining on the surface of the metallic reinforcement. The amount of rubber skim stock remaining on the metallic reinforcement was determined by visual examination and has been reported as % rubber coverage. TABLE I Effects of Different Levels of Cobalt and Nickel in Stock A Control Ex. 1 Ex. 2 Ex. 3 Ex. 4 Cobalt propionate, phr --- 1.62 1.62 0.81 0.81 Nickel octoate, phr --- 3.97 1.985 3.97 1.985 Test A Unaged or Unconditioned Plated Steel Corda Brass-plated Kg/cm 26.36 26.29 26.36 31.61 30.79 % rubber coverage 70 90 80-85 95 95 Zinc-plated Kg/cm 2.95 30.25 31.11 31.93 30.75 % rubber coverage 0 95 95 80-85 95 Test B Oven Aged Plated Steel Cordb Brass-plated Kg/cm 15.64 14.18 19.29 19.25 19.75 % rubber coverage 90-95 80 85-90 90 100 Zinc-plated Kg/cm 3.00 18.57 17.29 19.72 17.04 % rubber coverage 0 90 85 90 80 TABLE I (Continued) Control Ex. 1 Ex. 2 Ex. 3 Ex. 4 Test C Steam Bomb Aged Plated Steel Cordc Brass-plated Kg/cm 29.11 9.11 18.61 12.29 12.93 % rubber coverage 95 5 30 15 15-20 Zinc-plated Kg/cm 3.21 18.97 22.32 19.72 19.61 % rubber coverage 0 60-65 75-80 70 60-65 a) T-adhesion pads cured 30 minutes at 149 C and tested at 110 C. b) T-adhesion pads cured 30 minutes at 149 C, heat aged in a forced air oven for 2 days at 121 C and tested at 110 C. c) T-adhesion pads cured 30 minutes at 149 C, steam bomb aged 1 hour at 149 C and tested at 110 C. As can be determined from Table I, Test A, adhesion of the control stock to brass-plated steel cord was significantly better than to zinc-plated steel cord. The addition of the cobalt and nickel salts greatly improved adhesion in all examples with the zinc-plated cord and were best with brass-plated cord for examples 3 and 4 which contained greater amounts of the nickel salt than the cobalt. In Test B, oven aging, adhesion of the stocks with the zinc-plated wire was again comparable to the adhesion with the brass-plated wire after the cobalt and nickel salts were incorporated, whereas with the control stocks, adhesion to the zinc-plated wire was quite inferior to the adhesion with the brass-plated wire. The oven aging test is an accelerated heat aging test and is significant in determining the effect of heat on the thermal stability of the chemical bonds formed between the rubber skim stock and the metallic reinforcement during vulcanization. In Test C, steam bomb aging, adhesion of the stocks of examples 1-4 with zinc-plated wire was significantly better than the values obtained with the brassplated wire. Steam bomb aging for one hour at 1490 C is an accelerated humidity aging test and is significant in determining the chemical stability of the chemical bonds formed between the rubber skim stock and the metallic reinforcement when exposed to moisture or steam under pressure. In the second series of tests, reported in Table II, the effect of long term humidity chamber aging is presented for periods of time of 0-210 days between the control, Stock A and Examples 1-4, with brass-plated steel cord. TABLE II Effects of Different Levels of Cobalt and Nickel in Stock A Control Ex. 1 Ex. 2 Ex. 3 Ex. 4 Test D 0 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 23.39 23.40 27.33 32.15 27.51 % rubber coverage 60 65 90 100 100 Test E 30 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 25.71 20.36 24.11 31.43 28.45 % rubber coverage 65-70 60 75 90-95 95 Test F 60 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 23.93 21.25 22.36 24.47 23.40 % rubber coverage 65 75-80 85 90-95 90-95 Test G 90 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 18.86 18.65 23.58 17.86 19.57 % rubber coverage 65 75 75 70-75 80-85 TABLE II (Continued) Control Ex. 1 Ex. 2 Ex. 3 Ex. 4 Test H 120 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 21.14 18.79 19.11 19.15 19.04 % rubber coverage 60-65 80 75-80 90 80-85 Test T 150 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 19.32 17.15 20.36 20.54 20.86 % rubber coverage 35 50-55 70-75 80 85 Test J 180 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 17.96 13.04 17.82 16.29 18.18 % rubber coverage 30 40 65-70 80 85 Test K 210 Day Humidity Chamber Test Brass-Plated Steel Cordd Kg/cm 17.32 13.25 18.97 18.43 16.36 % rubber coverage 30 25 75 70 80 % coverage retention 50.0 38.5 83.3 70.0 80.0 d) T-adhesion pads cured 30 minutes at 149 C, aged in humidity chamber at 90% relative humidity and 35 C and tested at 110 C. Tests D through K, representing the effect of humidity chamber aging, show that the control Stock A gradually lost adhesive properties with aging. The effect of humidity aging was somewhat minimized for the stocks of examples 2-4 which had particularly high percent rubber coverage retention after 210 days when compared to the control. Significance of the humidity chamber tests is again in determining the chemical stability of chemical bonds formed between the rubber skim stock and the metallic reinforcement when exposed to conditions of high relative humidity and high temperature, as compared to ambient, and extended periods of time. Based on the foregoing results reported in Tables I and II, we consider the presence of cobalt propionate and nickel octoate in a rubber skim stock to be effective in promoting adhesion between the rubber skim stock and metallic reinforcement. As stated hereinabove, 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 steel. The wire can be in the form of a strand, mat, web, ply or braid. The present invention also finds utility in, for example, brassed 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 metal or provide a flexible and strong, thermally stable bond between the same. Thus, it can be seen that the disclosed invention carries out the objects set forth hereinabove. As will be apparent to those skilled in the art, the composition of the rubber skim stock can be varied within the scope of our total specification disclosure by the selection of the various organometal salts as well as the amounts thereof, and it is believed that the preparation and use of these according to the preferred method of the invention can be determined without departing from the spirit of the inven tion herein disclosed and described, the scope of the invention being limited solely by the scope of the attached claims.

Claims: CLAIMS 1. Cured rubber skim stock compositions having improved metal adhesion and metal adhesion retention propertied said composition comprising: 100 parts of a vulcanizable rubber stock; from about 0.25 to about 5.50 parts by weight of an organic salt of cobalt per 100 parts of the rubber component in said rubber stock; and, from about 0.50 to about 12.0 parts by weight of an organic salt of nickel per 100 parts of the rubber component in said rubber skim stock. 2. Cured rubber skim stock compositions, as set forth in claim 1, wherein said organic salts of cobalt and nickel contain fatty acid compounds, said fatty acids having from two to about 20 carbon atoms. 3. Cured rubber skim stock compositions, as set forth in claim 2, wherein said cobalt salt is cobalt pro pionate. 4. Cured rubber skim stock compositions, as set forth in claim 2, wherein said nickel salt is nickel octoate. 5. Cured rubber skim stock compositions, as set forth in claims 1 or 2, wherein said metal is present in a metallic reinforcing cord selected from the group consisting of steel cord, brass-plated steel cord and zinc-plated steel cord. 6. A tire having metallic reinforcing elements embedded in vulcanized rubber skim stock, said skim stock being characterized by having incorporated therein: from about 0.25 to about 5.50 parts by weight of an organic salt of cobalt per 100 parts of the rubber component in said rubber skim stock; and, from about 0.50 to about 12.0 parts by weight of an organic salt of nickel per 100 parts of the rubber component in said rubber skim stock. 7. A tire, as set forth in claim 6, wherein the metallic reinforcing elements are selected from the group consisting of steel cord, brass-plated steel cord and zinc-plated steel cord and wherein said organic salts of cobalt and nickel contain fatty acids having from two to about 20 carbon atoms. 8. A tire, as set forth in claim 7, wherein said cobalt salt is cobalt propionate. 9. A tire, as set forth in claim 7, wherein said nickel salt is nickel octoate.

Inventors: DAVIS, JAMES A., KANG, JUNG W., KOCH, ROBERT C.
Applicants: THE FIRESTONE TIRE & RUBBER COMPANY
Application year: 1981
Main classification: C08J5
IPC codes: C08L21, C08L7, C08K5, B60C1, C08J5