Method, Composition and Product Employing a Tetracarboxylic Dianhydride to Improve Adhesion Between a Metal Memeber and a Contiguous Rubber Skim Stock

United States Patent [19] Ravagnani et al. [54] METHOD, COMPOSITION AND PRODUCT EMPLOYING A TETRACARBOXYLIC DIANHYDRIDE TO IMPROVE ADHESION BETWEEN A METAL MEMBER AND A CONTIGUOUS RUBBER SKIM STOCK [75] Inventors: Frederick J. Ravagnani, Uniontown; Steven E. Schonfeld, Akron, both of Ohio The Firestone Tire & Rubber Company, Akron, Ohio [21] Appl. No.2 139,302 [22] Filed: Apr. 11, 1980 [51] Int. Cl.3 ........................ .. B60C 1/00; B29H 9/02; - CO9J 3/00; CO8K 5/09 [52] U.S. Cl. ............................. .. 152/359; 156/110 A; 156/330; 428/462; 428/465; 428/418; 524/112 [58] Field of Search ............. .. 152/330 R, 356 R, 359i 156/110 A, 110 C, 110 MD, 124, 330; 428/462, 465, 470, 414, 418; 260/3, 23.7 M, 31.2 R, 31.2 T, 31.4 EP, 31.8 E, 32.8 EP, 33.6 EP, 37 EP, 45.85 T; 526/335 [73] Assignee: [56] References Cited U.S. PATENT DOCUMENTS 3,715,266 2/1973 Winters et al. .................... .. 428/465 3,847,727 11/1974 Kindle et al. ..................... .. 152/359 [11] 4,324,281 [45] Apr. 13, 1982 3,948,849 4/1976 Barie, Jr. ...................... .. 260/37 EP 4,022,732 5/1977 Schwarcz_ ...... .. 4,041,004 8/1977 Barie, Jr. et al. .......... .. 260/33.6 EP OTHER PUBLICATIONS Skeist, Irving, Epoxy Resins, Reinhold Publishing Corp., 1958, Chapter 3, pp. 21~58. Primary Exam1'ner—-Edward C. Kimlin _ Assistant Examiner——Lois E. Boland [57] ABSTRACT 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 con- tiguous 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 tetracarboxylic dianhydride. The method of this invention comprises the steps of mixing a tetracarboxylic dianhydride into a rubber com- position, bringing this composition into contiguous rela- tionship with a metal member in an unvulcanized prod- uct and vulcanizing the product to yield the end prod- uct. 9 Claims, No Drawings 1 METHOD, COMPOSITION AND PRODUCT EMPLOYING A TETRACARBOXYLIC DIANHYDRIDE TO IMPROVE ADHESION BETWEEN A METAL MEMBER AND A CONTIGUOUS RUBBER SKIM STOCK BACKGROUND OF THE INVENTION 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 prod- uct. 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. Maxi- mum reinforcement of the rubber is obtained when maximum adhesion is produced and retained between the laminate of rubber and the metal reinforcing ele- ment as used to form a unitary structure. Of equal im- portance is the requirement that, for example, the lami- nate of the reinforcing metal element and rubber remain in a bonded relationship with each other throughout the useful life of the reinforced structure in which the lami- nate is used. PRIOR ART In U.S. Pat. No. 3,517,722 to Endter et al., a rubber- metal adhesion system is described which involves for- mationof a resorcinol-formaldehyde resin at the inter- face between the rubber and the metal, thereby bonding these members together. In forming the resin, com- pounds capable of liberating the methylene precursor are added to resorcinol in the rubber stock. Upon vulca- nization, 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 con- densation product of resorcinol and acetaldehyde in the rubber prior to application to the metal and vulcaniza- tion 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, magnesium oxide (0.1 to 4 phr) and rubber. Rubber Age, October 1975, pages 31 to 36, “Evalua- tion of Resotropin” by Cunningham and Hart. Resotro- pin, 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 resis- tance. ‘ ‘ U.S. Pat. No. 4,022,732 discloses, in part, the prepara- tion of reaction products of tetracarboxylic dianhy- drides and primary polyamides and their use as release agents in the manufacture of pressure-sensitive adhesive tapes. U.S. Pat. No. 4,041,004 discloses (including refer- ences cited therein) the preparation and use of tetracar- boxylic dianhydrides (and other dianhydrides) as hard- 4,324,281 5 l0 15 20 25 30 35 40 45 50 55 60 65 2 ening agents with selected polyepoxides used to prepare reinforced laminates. ’ 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. US. Pat. No. 2,581,920 also describes the use of haloge- nated polymers to bond rubber to metal. It is common practice in the art to increase the strength of rubber products such as a rubber belt or a pneumatic tire by incorporating therein a zinc plated or a copper-zinc (brass) alloy plated steel cord. Such prior art rubber-steel cord composites must retain satisfactory adhesion of the steel cord to the vulcanized rubber, particularly when such skim stock steel cord composite has been stored for a long period of time in a humid atmosphere prior to fabrication with the rubber. The rubber component can be a single component or a blend of one or more rubbers. Rubbers that can be utilized include natural rubbers, synthetic rubbers and blends thereof. The particular rubber composition se- lected is not critical and can be selected from, for exam- ple, natural rubber, synthetic polyisoprene, SBR and polybutadiene. The extender oil, when utilized can be, for example, any known medium process oil, aromatic or naphthenic hydrocarbon derived. The ‘antioxidant selected is not critical; it can be, for example, N-(1,3-dimethyl butyl)-N’—phenyl-p- phenylenediamine, known in the trade as Santoflex 13; or, 6—dodecyl-1,2-dihydro-2,4-trimethyl quinoline, known in the trade as Santoflex DD, sold by Monsanto Chemical Company. Dibutyl-p-cresol and quinones can also be used. Accelerators that can be utilized include: Altax benzothiazyl disulfide by R. T. Vanderbilt Com- Pan)’ NOBS Special N-oxydiethylene benzothiazole 2—sul- fenamide by American Cyanamid Company VULKACIT DZ N,N-dicyclohexyl-2-benzothiazyl sulfenamide by Mobay Chemical Company Any known rubber reinforcing carbon black can be used, such as HAF carbon black and the curative means can be that achieved through use of sulfur, peroxide or irradiation. Typical peroxides that can be utilized in- clude dicumyl peroxide and 2,5-dimethyl-2,5-di(t-butyl peroxy)—hexane. 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 con- tiguous 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 other- wise conventional rubber skim stock composition, wherein said cured rubber skim stock of conventional composition does not include an epoxy resin adhesive, appropriate amounts of a tetracarboxylic dianhydride. The method of this invention comprises the steps of mixing the selected dianhydride into a rubber composi- tion, as described below, bringing this composition into ' contiguous relationship with at least one metal member 4,324,281 3 in an unvulcanized product and vulcanizing the product to yield the end product. The skim stock of the present invention can be ap- plied by use of calendering means, spray means or other known application techniques. Areas of significant util- ity include, but are not limited to, radiator hose, pneu- matic 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, our skim stock composition can be used to increase adhesion retention properties between brass coated metal and rubber. Our preferred dianhydride adhesion retention pro- moter is 3,3’-4,4’-benzophenone tetracarboxylic dianhy- dride; this compound is supplied by the Gulf Oil Chemi- cals Company, Houston, Tex. and has the following structure: 0 O \\\C 1‘; C/ / \ O\ /O E K 0 0 it is known by the designation BTDA. DETAILED DESCRIPTION OF THE INVENTION The following Examples are representative and com- position component parts are expressed in parts per hundred rubber, phr, unless otherwise specified. EXAMPLE I The following composition was blended in a 1100 cc. Banbury mixer for about 7 minutes with a final Banbury temperature of 325° F. ‘ (1) 100 phr natural rubber (2) 50 phr carbon black (3) 7.5 phr zinc oxide (4) 3 phr antioxidant (l,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° F. (a) 2.8 phr sulfur (b) 1.0 phr N-(cyclohexylthio)-phthalalimide as re- tarder (c) 0.6 phr N-oxydiethylene-benzothiazole-2-sulfena- mide as accelerator (d) 4.0 phr BTDA as adhesion retention promoter (e) 0.7 phr extender oil, and (I) 2.2 phr 95/5 hexamethylenetetramine/stearic acid preblend. T-adhesions to brass coated wire for the resulting mill mix were determined at 230° F.’The samples were cured 30' at 300° F. Percent coverage* was determined visu- ally. The control of Table I was the above masterbatch which had been roll mill mixed without BTDA. TABLE I Control +4 phr Control BTDA Monsanto Rheometer - 300° F. 10 15 20 25 30 35 40 45 50 55 60 65 4 TABLE I-continued Control +4 phr Control BTDA Time to TS (2) 5.3 5.3 Time to TC (90) 20.7 26.2 Ultimate Torque, dN . M 50.1 45.0 R. T. Ring Tensile - 23' Cure 300% Modulus, (PSI) 3070 2520 Tensile Strength, (PSI) 3190 2850 Elongation, (%) 310 330 R. T. Ring Tensile - 23’ Cure - Aged 4 Days at 212° F. Tensile Strength, (PSI) 2000 1580 Elongation, (%) 150 160 Firestone Flexometer ASTM D 623/Method B Initial Shore A 75 71 Hot Shore A 71 65 % Deflection 10.7 13.3 Running Temp., ("F.) 225 225 T-Adhesion to W152 Brass at 230° F. Unconditioned 135 (90)* 96 (70)* Steam Bomb - 1 Hr., 300° F. 111 (80) 154 (100) Humidity Chamber - 14 Days ' 54 (20) 92 (70) 30 Days 26 (0) 100 (70) 60 Days 28 (0) 109 (70) 90 Days 37 (0) 82 (30) 120 Days 40 (0) 78 (so) 180 Days 31 (0) 101 (70) This Example I composition components can be var- ied within the following ranges: l00 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 .1 to 5 phr resorcinol 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, BTDA as dianhydride, and 0.3 to 2.0 phr extender oil. Masterbatch &“1_1‘4_iP