Title of Invention

AN ADHESIVE COMPOSITION AND A PROCESS FOR BONDING RUBBER TO METALS AND ALLOYS

Abstract

This invention relates to the composition and processing of a novel adhesive composition suitable for bonding vulcanized neoprene rubber to an alloy such as steel. The two-component adhesive composition consists of a primer to be applied on the metal/alloy surface and an overcoat adhesive to be applied on the primed metal/alloy surface and rubber. The primer is based on a nitrile-phenolic adhesive composition; whereas, the overcoat adhesive is based on a neoprene-phenolic adhesive composition. Curing of the adhesive can be effected at room temperature to provide a strong joint at the rubber-to-metal/alloy interface resulting in the required composite structure.

Full Text

This invention relates to an adhesive composition and a process of bonding rubber to metals and alloys, particularly Vulcanized neoprene rubber to metals and alloys. Background and Prior art: Adhesive bonding of rubber-to-metal has been widely used in the manufacture of different types of rubber-metal composites and structures for a variety of technical applications in aerospace, automobile and other industries. Characteristic hardness and reinforcement quality of metals can be combined with flexibility and vibration isolation properties of the rubber by joining them with a proper adhesive. Rubber-to-metal bonded components can be made by two methods: either by adhesively bonding the vulcanized rubber products to metal or by joining the rubber and metal using a bonding agent during the vulcanization process of the rubber. In either case, adhesives join the metal and rubber strongly by the establishment of interfacial contact at the molecular level through wetting and by physical and/or chemical links. Single-coat or two-coat (primer, overcoat) adhesive systems are used in rubber-metal bonding to achieve a reliable joint. The rubber-to-metal adhesive compositions are usually complex systems containing a number of ingredients. Generally, they contain a base polymer along with a number of other components incorporated into the base polymer in the required concentration in a suitable medium to meet specific end requirements. These components include modifying polymers, fillers, adhesion promoters, tackifiers, catalysts, crosslinkers, chain extenders, solvents, etc. Each component has a specific function and imparts special characteristics to the final product. When applied on the substrates, the adhesive has to wet, spread and establish strong interfacial forces on the metal surface, and then migrate into the rubber and crosslink into a mechanically strong film, producing a modulus gradient in the sequence, metal>adhesive>cured rubber without discontinuities in the bond region. It is essential that in forming a metal-rubber joint, a proper adhesive is selected that can transfer stress uniformly between the high-modulus metal and the low-modulus rubber without formation of stress-concentrated regions. This becomes all the more critical, if the adhesive joint is part of a structural assembly. Metal-to-rubber bonding is reported in a large number of patents. Some of them are on joining vulacanised rubber-to-metal; whereas, others are on joining rubber-to-metal during the vulcanisation process of rubber. Some patents describe specific methods for bonding the substrates whereas some others disclose the adhesive formulations suitable for selected substrates. US Patent 5268404 describes a one-coat adhesive for vulcanization bonding, which provides strong rubber-to-metal bonds with excellent environmental resistance. The adhesive composition consists of a halogenated olefin, an aromatic nitroso compound, metal oxide such as ZnO or MgO, vulcanizing agent, a phenolic epoxy resin and carbon black. An adhesive composition comprising of chlorinated natural rubber, brominated poly-2,3-dichlorobutadiene-1,3 and a dinitroso aromatic compound for vulcanization bonding of rubber to metal is detailed in US patent 3258389. GB 1523733 describes vulcanization bonding of the rubber material and metal by an adhesive composition comprising of an aqueous solution or dispersion of a carboxylic elastomer, a methylol compound, vulcanizing agent and vulcanization accelerator. GB 806449 describes vulcanisation bonding of rubber-to-metal with a composition containing brominated poly-2,3-dichlorobutadiene-l,3 as an ingredient. US 2900292 deals with vulcanization bonding of the rubber to metal with an adhesive comprising of brominated poly-2,3-dichlorobutadiene-l,3 along with a metal adherent such as epoxies, phenolics, polyesters, etc. US 3998992 is about a process for vulcanization bonding of rubber (containing silica filler) to ferrous metal through and adhesive comprising of a conjugated diene and a heterocyclic nitrogen base. US 2037749 claims an improved rubber composition for forming a direct bond between rubber and ferrous metals. Here, a bond-forming ingredient selected from the carbosulfhydryl polysulfide class of organic vulcanization accelerators is incorporated in the vulcanization rubber composition to get a secure bond at the interface. US 220460 gives a method of bonding rubber to ferrous metals which comprises incorporating red phosphorous and a halogen-containing activator into a vulcanisable rubber composition and uniting it with metal during vulcanization of the rubber. US 2409759 relates to a process for producing rubber-metal articles wherein the rubber composition is pre-vulcanised, either partially or completely, and subsequently bonded directly to the metal with an adhesive composition containing rubber, MBT, stearic acid, ZnO, Iron oxide and sulfur. In US 2423755, a polymer of a substituted pentadiene is cyclized, dissolved in a suitable solvent, applied to metal or other surface, the solvent is volatilized out and the desired rubber body is cured against the cyclized polymer to adhere the rubber body to the metal. US 3342660 relates to a method of bonding a rubber composition to a metal surface. This comprises vulcanizing while in contact 4 with the metal surface, a vulcanisable rubber composition containing a minor proportion of an epoxidised rubber polymer. JP 2000034446 is about a method for fast bonding of vulcanized rubber-to-metal. The surface of the vulcanized rubber to be bonded is irradiated with uv rays and then bonded to metal with an epoxy type adhesive. Adhesives of phenolic resins with elastomers and epoxies find extensive use in bonding of rubber-to-metal. Usually, adhesion and durability are improved by the presence of phenolics. A resorcinol-pyrogallol-formaldehyde copolymer was found to provide good rubber-metal bonding even during exposure to boiling water. NBR-phenolic based adhesive was found suitable for bonding friction materials for disk brakes and they retained almost 50% of their room temperature strength at 200°C. A water-borne phenolic adhesive has been reported to be useful in the manufacture of metal-rubber composites. An adhesive based on NBR-epoxy-phenolic has been reported which can join metal-to-rubber at room temperature. Phenolic resins have been the important constituents of formulations for preparation of rubber composites for bellows of pneumatic suspension of vehicles, which retain good adhesion even at high rates of flexing. Another study deals with the use of alkyl phenol and phenol-formaldehyde oligomers along with chloroprene rubber as adhesive for improved adhesion of rubber-to-meal. Improvement in peel strength properties of Al-to-NBR vulcanization bonding has been reported using a NBR-PF adhesive with the incorporation of an interfacial agent. Mechanism of adhesive bonding of rubber-to-metal by nitrile phenolics, based on the formation of a cross-bridge between the elastomer and adhesive due to diffusion and interfacial reactivity, has also been proposed recently. Thus, it is seen that prior art literature provides many references on the bonding of rubber-to-metal. However, each one of them is specific to the substrate used, vulcanization process, testing procedure, exposure to environment, etc. The present invention is unique and differs from prior art in following aspects. This invention discloses a novel adhesive composition and a new process of bonding rubber to metals. The adhesive composition of the subject invention provides better and higher bonding strength when compared with that of the hitherto known compositions even in extreme environmental conditions. Tests have shown that the bonded articles exhibit better and higher lap shear and peel strength. Further, the adhesive composition of the subject invention is applicable to a wide variety of metals, alloys and rubber. The main objective of this invention is to provide a novel room temperature curable adhesive composition and a process for bonding rubber to metals, particularly vulcanized chloroprene rubber to steel, so as to obtain leak proof adhesive joints having high peel and lap shear strength to withstand external environments including thermal, vibration and shock. This adhesive composition consists of mainly two components: The primer and the overcoat adhesive. The primer is based on a nitrile-phenolic adhesive composition, whereas, the overcoat is based on a neoprene-phenolic adhesive composition. Phenolic resins obtained by the condensation reaction of phenol with formaldehyde possess exceptional adhesive property due to the interaction of phenolic and methylol hydroxyl group. However, phenolics are rarely used alone as adhesives due to their brittleness. They are usually modified by reaction/blending with elastomers to enhance their flexibility and render them suitable for bonding low modulus substrates such as rubbers. In the present invention, nitrile rubber and chloroprene rubber were used to modify phenolic resin and the formulations so developed form the basis of the adhesive system. Detailed description of invention: The primer described here is a combination of nitrile rubber, paracresol-modified resorcinol-formaldehyde resin, paraformaldehyde, vulcanizing agents and fillers dispersed in a solvent and is capable of being cured under ambient conditions. The overcoat adhesive described here is a neoprene-phenolic adhesive formulation. This is based on polychloroprene (neoprene rubber) and alkylphenol resole with known additives in a known solvent, which exhibit good adhesion and tackifying effect. Neoprene-phenolic overcoat adhesive serves as a contact adhesive and can be used to bond neoprene rubber and a wide variety of substrates. The preparations of these components are described below. Preparation of Primer Composition : The nitrile-phenolic primer consists of two parts: Part A and Part B. Part A Part A is a solution of nitrile rubber along with other ingredients. It is prepared by mastication of rubber and the other ingredients in a two-roll mill, sheeting out, followed by dissolution in a suitable solvent to form a 7 homogenous solution. The formulation consists of 100 parts by weight (pbw) of nitrile rubber, 17-22 pbw of paraformaldehyde, 3-5 pbw of zinc oxide, 2-4 pbw of sulphur, 2-4 pbw of dibutyl phthalate and 5-15 pbw of carbon black. The masticated rubber is dissolved in a suitable solvent such as ethyl acetate to get a solution of approximately 8-12% solid content. A typical example is given here. 120 parts of nitrile rubber (of acrylonitrile content in the range 25-45%) is masticated in a two-roll mill with 24 parts of paraformaldehyde and then with 4.8 parts of zinc oxide, 3 parts of sulphur and 3 parts of dibutyl phthalate added together. Now the rubber is masticated with 12 parts of carbon black (previously sieved through 120 mesh and dried at 120°C for 2 h) and sheeted out to a thickness of less than 2 mm, after which a minimum of five passes are given through the roller to get a homogenous compound. It is then cut into small pieces and dissolved in 1200 parts of ethyl acetate by mechanically mixing in a flask for about 24 h. The black homogenous solution so obtained has a solid content (CI) in the range 10 ± 2 % by weight determined at 100-110°C. PartB Primer Part B is solution of phenolic resins, which is a blend of resorcinol-formaldehyde and paracresol-resorcinol formaldehyde resins and cure accelerator in a suitable solvent. The preparations of each of the phenolic resins and the blending process are detailed below under different steps. Step 1 Resorcinol-formaldehyde resin is prepared by the condensation reaction of resorcinol with formaldehyde at high temperature. For this, a resorcinol: formaldehyde molar ratio in the range 1:0.6 to 1: 0.85 is employed. The addition of formaldehyde is done in two steps and the reaction is carried out for a period of 1-2 hours at a temperature of 90- 98°C. After the reaction, the water present in the resin is removed by rotary vacuum drying at high temperature to a constant weight. The dried resin is then dissolved in a suitable solvent to get a solution of approximate solid content 55-70%. A typical example of preparation of resorcinol-formaldehyde resin is given below. Let C be the concentration of formalin, expressed in %wt /vol. Resorcinol (110 parts), distilled water (20 parts) and formalin solution (975/C ml) are charged into a three-necked RB flask fitted with a mechanical stirrer and a reflux condenser. The reaction mixture is stirred and heated in a water bath at a temperature of 95 ± 2°C for 15 minutes. Then a second lot of 975/C ml of formalin solution is added to the reaction mixture drop-wise through a dropping funnel over a period of 10 minutes and the reaction is continued for one hour at 95 ± 2°C. The reaction mixture is cooled to room temperature and transferred to a previously weighed single-necked RB flask. The water from the resin is distilled out using a rotary flash evaporator under reduced pressure level not higher than 30 mm of mercury at elevated temperature in a water bath, till constant resin weight is obtained. The weight difference shall be solution in methyl ethyl ketone is determined using Ostwald's viscometer. It should be in the range 0.02 -0.08 dl/g. Step 2 Paracresol-Resorcinol-formaldehyde resin is prepared by the condensation reaction of paracresol and resorcinol with formaldehyde at high temperature. For this, equimolar quantities of paracresol and resorcinol are reacted with stoichiometrically less amount of formalin. A phenol (p-cresol + resorcinol) : formalin molar ratio of 1: 0.7 to 1: 0.85 is employed. The reaction is carried out for 10-14 h at a temperature of 90- 98°C, in two steps. The first step is the p-cresol-formalin reaction and the second step is the resorcinol-formalin reaction. After the reaction, the water present in the resin is removed by rotary vacuum drying at high temperature to a constant weight. The dried resin is then dissolved in a suitable solvent to get a solution of approximate solid content 55-70%. A typical example of the preparation of p-cresol-resorcinol-formaldehyde resin is given below. Let C be the concentration of formalin expressed in % wt/vol. Para¬cresol (54 parts), 2250/C ml of formalin solution and 1 part of sodium hydroxide are charged into a three-necked RB flask fitted with a mechanical stirrer and reflux condenser. This is heated in a water-bath at 95 ± 2° under stirring for three and a half hours. It is then cooled to about 60°C and 90 ml of methyl ethyl ketone is added and stirred for about 2 h at that temperature. Now, 55 parts of resorcinol is added and the reaction is continued for 8 h keeping the bath temperature at 95 ± 2°. The reaction mixture is then cooled to room temperature and pH is adjusted in the range 7 to 8 with 0.75 g of sodium hydroxide dissolved in 10ml of distilled water. The resin is then transferred to a previously weighed single-necked RB flask. The water from the resin is distilled out using a rotary flash evaporator at 65 ± 5° under reduced pressure level not higher than 30 mm of Hg, at elevated temperature in a water bath, till constant 10 resin weight is obtained. The difference in weight shall be Step 3 The phenolic resins prepared as per steps 1 and 2 are to be blended along with accelerator to form Primer Part B. The formulation consists of 100 parts by weight of the solid phenolic resin (a 1: 1 by weight mixture of resorcinol-formaldehyde and p-cresol-resorcinol-formaldehyde resins), 1 -3 parts by weight of tetramethyl thiuram disulphide or mercaptobenzothiazole or their derivatives and the required quantity of solvent methyl ethyl ketone to get a solution of approximately 65%o solid content. The procedure for preparation of Primer Part B is given below. 150x(65AVi) parts of resorcinol formaldehyde solution prepared as per Step 1 and 150x(65/w2) parts of paracresol-resorcinol-formaldehyde solution prepared as per Step 2 (Wi and W2 are their respective solid contents) are taken in a single necked flask and diluted with calculated quantity of methyl ethyl ketone to get the total weight of the solution as 300 parts. Now, 2.63 parts of tetramethyl thiuram disulphide is added and the solution is stirred thoroughly with a mechanical stirrer at room temperature for about 8 h. The resin thus 11 prepared has a solid content (C2) in the range of 60-70% determined at 100-110°C. Preparation of Overcoat Adhesive The overcoat adhesive is based on neoprene-phenolic formulation. The formulation consists of 100 parts by weight (pbw) of neoprene rubber, 17-23 pbw of magnesium oxide, 4-6 pbw of zinc oxide, 140-150 pbw of alkylated phenolic resin, and a mixture of the solvents toluene and ethyl acetate so as to get a solution of approximate solid content 22-30%. The procedure consists of the mastication of neoprene rubber with the vulcanising agents in a two-roll mill, sheeting out, followed by dissolution in a solvent mixture along with phenolic resin. The detailed procedure is given below with a typical formulation. 200 parts of neoprene rubber (of chlorine content 30% minimum) is masticated in a two-roll mill along with 40 parts of magnesium oxide and 10.64 parts of zinc oxide and sheeted out to a thickness of less than 2 iran. Then a minimum of five passes is given through the roller to get a uniform compound. The masticated rubber is then cut into small pieces and dissolved in a mixture of 200 ml toluene and 800 ml ethyl acetate by mechanically stirring the contents in a single-necked RB flask. 292.8 parts of alkylated phenolic resin is powdered and dissolved in 534 ml of toluene separately. This phenolic solution is filtered to remove any undissolved impurities and added to the rubber solution. Stirring is continued for about 8 h to get a homogenous solution. The exact solid content of the solution is determined by evaporating a weighed quantity of the resin solution to a constant weight in a hot air oven at 100-110°C. The solid content 12 must be adjusted with solvent in the range 22 to 30%, to get the B-4 Ford cup flow time in the range 40 to 120 seconds. Bonding Procedure The surfaces of the metal and vulcanized neoprene rubber to be bonded are wiped with trichloroethylene and then roughened with emery paper. The surfaces are again cleaned with methyl ethyl ketone and dried at room temperature for about half an hour. Weigh (lOxlO/Ci) parts by weight of Primer Part A and (2.5x65/C2) parts by weight of Primer Part B (Ci and C2 are the exact solid contents of Part A and Part B respectively) into a container and mix thoroughly for about 10 minutes with a rod. The flow time of the mixed primer is now determined using B-4 Ford cup viscometer, it should be in the range 20 to 80 seconds. The mixed primer is applied uniformly with a brush on the cleaned metal surface. The mixed primer has about 5 h pot life, if kept tightly closed in a container. The overcoat adhesive is mixed thoroughly in its container and is used as such. It is applied on the cleaned rubber surface using a brush and allowed to dry at room temperature for about 10 minutes. A second coating is given with the overcoat over the primed metal surface and over the coated rubber surface, dried for about 10 minutes and assembled. It is then pressed with a roller and kept for curing under a pressure of 2-3 kg/cm at room temperature for 72 h. This invention relates to an adhesive composition for bonding rubber to metals and alloys comprising a primer and an overcoat, said primer having two parts in combination wherein part A is a solution of nitrile-rubber and paraformaldehyde in addition with conventional fillers and additives; part B is a blend of resorcinol-formaldehyde and para-cresol-resorcinol-formaldehyde resin combined with known cure accelerators in a sohent medium and wherein said overcoat comprising a combination of neoprene rubber and alkylated phenolic resin with known fillers, additive and vulcanizing agents, in a solvent medium. The invention also relates to a process for bonding rubber with metals/alloys comprising the steps of coating the metal surface with a primer mixture such as herein described and subsequently overcoating the same with an overcoat composition as herein described, coating the rubber surface with atleast one layer of said overcoating composition, bringing the two coated surface together and curing the same in a known manner. Lap shear strength and Peel strength of the joint are dependent on the type of the metal and vulcanized neoprene rubber substrates used. The values specified are generated using AISI 304 stainless steel and nylon-reinforced vulcanized neoprene rubber as substrates. This adhesive system is useful for bonding vulcanized chloroprene rubber to a wide variety of substrates for applications in aerospace, automotive, building construction, shipping, consumer goods, and electrical and electronic industries. This composition is used to join metals with rubber surface. It is particularly usefully in binding vulcanized neoprene rubber with steel. 14 Though this invention has been described herein above with a specific embodiment, aheration and modifications are within. 15 WE CLAIM ; 1. An adhesive composition for bonding rubber to metals and alloys comprising a primer and an overcoat, said primer having two parts in combination wherein part A is a solution of nitrile-rubber and paraformaldehyde in addition with conventional fillers and additives; part.B is a blend of resorcinol-formaldehyde and para-cresol-resorcinol-formaldehyde resin combined with known cure accelerators in a solvent medium and wherein said overcoat comprising a combination of neoprene rubber and alkylated phenolic resin with known fillers, additive and vulcanizing agents, in a solvent medium. 2. The composition as claimed in claim 1, wherein part A of the primer comprises a solution of nitrile rubber, paraformaldehyde, zinc oxide, sulphur, dibutyl phthalate and carbon black in ethyl acetate. 3. The composition as claimed in claims 1 & 2, wherein said part A composition has 100 parts by weight of nitrile rubber, 17-22 pbw of paraformaldehyde, 3-5 pbw of zinc oxide, 2-4 pbw of dibutyl phthalate and 5-15 pbw of carbon black. 4. The composition as claimed in claim 3, wherein said part A composition has 8-12 % solid content. 5. The composition as claimed in claims 1 to 4, wherein said nitrile rubber is masticated prior to addition of other components. 6. The composition as claimed in claim 1, wherein said part B of the primer is a blend of resorcinol-formaldehyde resin, para-cresol-resorcinol formaldehyde, accelerators such as tetra methyl thiuram disulphide, mercaptobenzo thiazole or their derivatives in methyl ethyl ketone. 7. The composition as claimed in claim 6, wherein said part B composition has 100 parts by weight of solid phenolic resin such as 1:1 by weight mixture of resorcinol formaldehyde and p-cresol-resorcinol-formaldehyde resins, and 1-3 parts by weight of tetra methyl thiuram disulphide in methyl ethyl ketone which has about 65 % of solid content. 8. The composition as claimed in claim 1, wherein said overcoat comprises 100 parts by weight of neoprene rubber 17-23 parts by weight of magnesium oxide, 4-6 pbw of zinc oxide, 140-150 pbw of alkylated phenolic resin, dissolved in solvents like toluene and ethyl acetate. 9. The composition as claimed in claim 8, wherein said overcoat composition has 22 to 30% solid content. 10. The composition as claimed in claim 9, wherein said neoprene rubber has a chlorine content of atleast 30% and is masticated prior to addition with other components to obtain a homogeneous mixture. 11. A process for bonding rubber with metals/alloys comprising the steps of coating the metal surface with a primer mixture such as herein described and subsequently overcoating the same with an overcoat composition as herein described, coating the rubber surface with atleast one layer of said overcoating 17 composition, bringing the two coated surface together and curing the same in a known manner. 12. The process as claimed in claim 1, wherein said rubber and metal surfaces are cleaned prior to coating said coated surface are dried at room temperature before second coatings are appUed, and then cured under pressure of 2- 3 kg/cm^ at room temperature for about 72 hrs.

Documents:

0015-che-2005 abstract 11-01-2005.pdf

0015-che-2005 claims 11-01-2005.pdf

0015-che-2005 description(complete) 11-01-2005.pdf

0015-che-2005 form- 2 11-01-2005.pdf

15-CHE-2005 AMENDED PAGES OF SPECIFICATION 19-04-2013.pdf

15-CHE-2005 AMENDED CLAIMS 19-04-2013.pdf

15-CHE-2005 FORM-1 19-04-2013.pdf

15-CHE-2005 OTHER PATENT DOCUMENT 19-04-2013.pdf

15-CHE-2005 CORRESPONDENCE OTHERS 05-06-2013.pdf

15-CHE-2005 CORRESPONDENCE OTHERS 31-05-2013.pdf

15-CHE-2005 EXAMINATION REPORT REPLY RECEIVED 19-04-2013.pdf