ADHESIVE

Abstract: There is disclosed an adhesive for bonding elastomers with improved dynamic fatigue resistance comprising an halogenated polymer which can be cured and stabilised and which contains isocyanate with a functionality of between 2.1 and 2.6. The adhesive may be a bonding agent or coating for incorporation into a bonding process. It is particularly useful for bonding polyurethane tread or sidewall components to a tyre.

Description: ADHESIVE This invention relates to adhesives, particularly, but not exclusively to adhesives for bonding elastomers such as polyurethane, particularly to rubber, and to the manufacture of automobile tyres and retreads using such adhesives. Currently, retread tyres are made either by moulding a new rubber tread directly on to a buffed tyre casing or by bonding a pre-formed rubber tread on to the casing. Rubber treads have a limited life (hence retreads) though reasonably hardwearing and durable, and alternative materials having better performance are sought. Polyurethane is an ideal substitute. However, although polyurethane tread has been bonded to a rubber substrate, the bonding has proved unsatisfactory, with high failure rate and low dynamic fatigue resistance not to mention inadequate temperature resistance - tyres get hot when working. Commercially available polyurethane/rubber cements have low tack and low green strength. Tacky adhesives - so-called tack coat or tie coat - have achieved only limited success. In particular no method for attachment of polyurethane tread to rubber tyre casing has been disclosed to date which is capable of meeting high European and US technical standards for dynamic fatigue resistance and heat resistance. The present invention provides an adhesive for polyurethane and indeed for other elastomers which is capable of meeting those standards and moreover provides a polyurethane/rubber bonded system which produces tyres having good overall heat and dynamic fatigue resistance. The invention comprises an adhesive for bonding elastomers with improved dynamic fatigue resistance comprising an halogenated polymer which can be cured and stabilised and which contains isocyanate with a functionality between 2.1 and 2.6. By "adhesive" is meant to be understood also a bonding agent or coating for incorporation into a bonding process in any convenient manner. The adhesive may comprise a one part adhesive, which may comprise a chlorinated or brominated polymer and a curing agent, which may comprise p- dinitrosobenzene (DNSB). The polymer may be chlorinated or brominated rubber, CSM, partly halogenated butadiene polymers, halogenated polyurethane or mixtures of the aforementioned polymers. Of particular interest are highly chlorinated Diels- Alder adducts of hexachlorocyclobutadiene or hexachlorocyclopentadiene and dienophiles such as polybutadiene, butadiene copolymers and isoprene polymers. Of course, the adhesive may also be presented as a two-part adhesive. The adhesive may comprise an adhesion promoter, which may be adapted to achieve initial cure to the polyurethane elastomer and the rubber coating from the heat output by the exothermic reaction. This could eliminate the need to post-cure the composite polyurethane tread and facilitate continuous processing. The isocyanate may comprise a part-polymerised diisocyanate, or a quasi- prepolymer made from branched polyols and pure diphenyl-methane diisocyanate (MDI) or a quasi-prepolymer made from partially polymerised diisocyanates and polyols. The isocyanate content of the isocyanate molecule may be between 8 and 33%. The invention also comprises a method for bonding elastomer such as polyurethane with improved dynamic fatigue resistance using an adhesive, bonding agent or coating as specified above. The method may be used to bond an elastomer such as polyurethane of which the polyol has an overall functionality between 2.07 and 2.3. The isocyanate of the elastomer may have a functionality between 2.0 and 2.3. Such an elastomer may be thus bonded to a rubber substrate with improved dynamic fatigue resistance and heat resistance sufficient to meet the standards referred to above for tyres, and the method may in particular be used to bond a polyurethane casting or moulding to a tyre. The casting or moulding may comprise a tread and/or a protective sidewall, useful in, for example, mining where conventional vehicle tyre sidewalls are often badly scuffed and damaged in narrow roadways and galleries. Not only retreads may be made in this way - the method is well adapted to the production of new tyres with improved performance over conventional all-rubber tyres. Of course, many other items may be bonded. For example, polyurethane side walls may be bonded to rubber dinghies and rafts, especially those used in white water rafting and, in the production of fenders for marine use, particularly for harbour installations and road barriers, where the resilience of the rubber is combined with the hardwearing nature, durability and resistance to corrosion of the polyurethane. The invention is primarily of importance, however, where dynamic fatigue resistance, possibly combined with heat resistance, is a requirement. The elastomer may in any event comprise a polyurethane elastomer with a hardness range from 50° to 95° Shore A, preferably 60° to 75° Shore A. The elastomer may be injection moulded by a high or a low pressure technique, especially for a tyre tread, or it may be cast. The elastomer may be only partially cured or may be post cured before bonding. The elastomer may comprise an ester prepolymer or ester quasi-prepolymer or may comprise a polycaprolactone prepolymer or a polycaprolactone quasi-prepolymer. The elastomer may comprise a mixed diol adipate with a molecular weight between 1000 and 5000, preferably between 1500 and 2500. The elastomer may comprise an ether prepolymer, or an ether quasi- prepolymer system. The elastomer may comprise extender material such as reclaimed rubber crumb, rubber granules, rubber powder, polymeric fibres including polyurethane chopped fibres, and mineral fillers, which may be treated. Extender material may be present in an amount from 2 to 60% w/w of the polyurethane. The polyurethane may, in particular, comprise reinforcing extenders which may include precipitated or fumed silicas, modified silicas where there may be reactive groups grafted onto the silica or the silica may be modified in terms of its hydrogen bonding capability. One modification is to chemically graft a silica to the polyol backbone, where the polyol may be an ester, a caprolactone, an ether or polycarbonate using reactive silanes of the type containing epoxy groups, thiol groups, amine groups or other reactive groups. The reaction is carried out using a high shear mixer in a heated reactor. The silica is dispersed in the polyol or part of the polyol mixture using high shear and the silane added. Heat is applied to 70-90°C and a catalyst or initiator added. After grafting for a required period, the mixture is vacuum stripped and then converted to the preferred polyurethane elastomer. The level of silica in the polyol may be between 0% and 10% and the preferred range of silica is 5-10%. The level of silane on the silica may be up to 5% but the preferred level is 0.5-2.5% on the silica. A rubber coating may be bonded to the elastomer and then bonded to a rubber substrate, such as a tyre carcass. The rubber coating may comprise a curable cushion gum or a solvent-based rubber solution. The adhesive may cure and bond at a temperature between 60 and 130°C and curing and bonding may proceed for between 30 minutes and 3 hours. The elastomer may be cured using a catalyst such as a heavy metal complex such as a tin mercaptide or another complex of tin, titanium, zirconium, iron, lead or mercury. The elastomer may be cured using amine catalyst or combinations of amine catalysts and precious metal complexes. Adhesives and adhesive/elastomer systems according to the invention will now be described with reference to the accompanying drawings, in which : Figure 1 shows a polyurethane tyre bonded to a rubber carcass; Figure 2 shows a sidewall of polyurethane bonded to a tyre; Figure 3 is a cross-section through a tread region of a tyre. Preparation of one part adhesive : The one part adhesive is a cement, based on CSM chlorosulphonated polyethylene, other chlorinated polymers, particularly highly chlorinated Diels-Alder adducts of hexachlorocyclopentadiene and polybutadiene with a chlorine content above 50%, DNSB, carbon black and suitable solvents. Modified MDI of a narrow functionality range 2.1 to 2.6 is added to give an NCO content of between 3.5 and 7.0%. The range of functionality is required to give the correct level of cross linking to the polyurethane and give the correct flexibility and hence dynamic fatigue resistance within the adhesive layer. Polyurethane elastomer preparation : (a) a polyol blend was produced from the following ingredients a mixed glycol adipate polyester with a functionality of 2.07 - 2.30 and a molecular weight of 1500 - 2000 and sufficient low weight molecular extender, such as butane diol, diethylene glycol, ethylene glycol or similar diols was used to give the correct hardness when cured with the MDI or an MDI quasi-prepolymer. The polyol blend was homogenised by mixing at an elevated temperature of up to 70 °C under moderate vacuum conditions. A molecular sieve is used to absorb unwanted moisture and sufficient defoamers used to ensure bubble free castings. (b) a quasi-prepolymer was made from a similar mixed glycol adipate and 4,4 diphenyl methane diisocyanate (MDI) by mixing under vacuum conditions at 70 °C until the isocyanate (NCO) content stabilised at 15- 33% NCO. The two components (a) and (b) were mixed in a ratio chosen to give NCO:OH equivalent ratio of between 0.98 and 1.05. It is important to ensure the NCO:OH equivalent ratio is maintained in the given range to guarantee the correct cross link density in the PU. EXAMPLE 1 A mixed diol (butane diol/ethylene glycol) adipate polyester was produced to a molecular weight of 2000 and a functionality of 2.15, using a triol as the branching mechanism. This polyester was converted into a 2 part PU system by making a quasi- prepolymer with pure MDI and a catalysed polyol component using catalyst and a molecular sieve and butane diol to control the hardness. The 2 part MDI quasi ester polyurethane system as described above was metered mixed and cast into a flat mould to produce a tyre tread of the correct length and width for a commercial tyre size. The polyurethane yielded a 65 Shore A elastomer. After demoulding and post curing to achieve maximum physical properties, the upper (flat surface) was buffed and solvent degreased using methyl ethyl ketone or toluene. A coating of the one part adhesive (described above) was applied to the upper surface of the tread and allowed to dry. An extruded cushion gum was applied to the adhesive using a roller technique and the whole assembly cured at 95 °C for 2 hours in a vacuum bag or under a low pressure heat platen to ensure that there was intimate contact between the cushion gum and the polyurethane. This tread was packaged and used to build a precured retreaded tyre by the conventional method. EXAMPLE 2 A 2 part quasi MDI PU tread was produced as in Example 1. One coat of the one component adhesive was applied and dried. When dry a solution of the cushion gum in toluene and naphtha was applied to give a high build coating on top of the adhesive. When dry, the tread was packaged. To a buffed casing a layer of extruded cushion gum was applied by the normal methods and the tread applied to this cushion gum coated buffed casing. EXAMPLE 3 A blend of mixed polycaprolactones of functionality 2.0 and a highly functional mixed diol/triol polycaprolactone was made to give an average functionality of 2.2. A quasi-prepolymer was made from one of the above polycaprolactone polymers and pure MDI so that in the final polyurethane the polyester branching is approximately 2.10. A one part, one coat adhesive was prepared in toluene and xylene from 16% brominated poly 2,3 dichloro polybutadiene CSM, highly chlorinated paraffin, 65% chlorinated polyisoprene, p-dinitrosobenzene (DNSB) and carbon black and a lead stabiliser. To this was added an isocyanate quasi-prepolymer based on MDI and a branched polyester giving a functionality of 2.3 and an isocyanate content of 12%. The amount of the isocyanate quasi-prepolymer added to the adhesive gave an isocyanate content of ca. 2.5%. Other curing agents were added to accelerate the cure of the adhesive. The 2 part PU was machine mixed accurately to give an NCO/OH cure index of 1.02 and cast into a flat mould. When demoulded the upper flat surface was buffed and a single generous coat of the adhesive applied and fully dried. An extruded cushion gum was applied to the coated adhesive tread using pressure. This tread was packaged and used to build a precured retreaded tyre by the conventional method. EXAMPLE 4 A grafted silica was produced in situ as follows: 50kg of a blend of PTMEG polyether of overall Mwt of ca. 2500, functionality of 2.17 and water content 0.1% was pumped into a reactor and heated to 70°C. Whilst shear stir mixing 3.5kg of standard fumed silica (typically Cabosil M5 or Aerosil 200) was added into the blend. Once fully dispersed as shown by Hegman Grind <5 micron, 65gm of thiol containing silane was added and the mixture stirred at 70°C for 3 hours. The mixture was vacuum degassed to <0.05% water and made into a 2 part polyurethane with remaining 150kg of the PTMEG polyether blend, butane diol, molecular sieve and a mixed amine and tin catalyst added to control the reaction rate. A quasi prepolymer was made from the same PTMEG polyether base and a pure MDI/dimerised MDI mixture to give a mobile liquid. The one part, one coat adhesive from example 3 was used to coat a buffed tyre casing and the casing dried 15 minutes and then sited in a mould and the whole heated to 60°C in an oven prior to removal and injecting the PU system. The two part PU as described above, was meter mixed at an NCO/OH index of 1.025 and injected at low pressure <2 bar into the mould and sealed. After 20 minutes the mould was opened and the retreaded tyre removed and post cured at ambient temperature for 4 days prior to use. EXAMPLE 5 A two part polyurethane as described in Example 3 was adjusted to give a mixed gel time of about 6 minutes. With this gel time the exotherm gives a temperature rise from 23 °C to about 70°C on the surface of a cast tread. A two part adhesive was prepared. The resin component was made from a mixture of a 55% chlorine content Diels-Alder adduct of hexachlorocyclopentadiene and natural rubber and CSM in toluene and xylene stablised with a lead compound. To this was added carbon black and the whole mixture was finely ground. The curing system was produced from a suitable solvent, DNSB, carbon black and a quasi prepolymer based on MDI and a branched polyether to give a functionality of 2.2 and an isocyanate content of 20% for the quasi prepolymer. The two components were mixed in equal quantities by weight and used to apply to the cast tread. The 2 part PU was machine mixed accurately to give an NCO/OH cure index of 1.03 and cast into a cold flat mould. As soon as the PU had gelled and produced a solid surface the mixed 2-part adhesive was applied to the exothermed hot surface. Within one minute the surface was overcoated with a solution of cushion gum in toluene and naphtha, dried and packaged. To a buffed casing, a layer of extruded cushion gum was applied by the normal methods and the tread applied to this cushion gum. Exposed cut ends of the PU tread were given a thin coating of the adhesive described in Example 3 and the two ends separated by a layer of cushion gum. The tread was compressed onto the casing and the ends stapled prior to placing in a vacuum bag and curing as normal. It is also possible to apply the extruded cushion gum to the PU tread after the application of the cushion gum solution and apply such a tread to a buffed casing which has been coated with a rubber tack cement. The drawings illustrate, in Figure 1, a polyurethane tyre tread 11 bonded to a rubber coating 12 which may be a curable cushion gum or a solvent-based rubber solution. This in turn is bonded as a normal rubber-to-rubber bond to the tyre carcass 13. Figure 2 shows a side wall protector 21 bonded to a tyre, as may readily be achieved using the adhesive and methods of the invention. Figure 3 shows a novel tyre design in which a rubber carcass 31 has an annular groove 32 between road-engaging rubber shoulders 33, a polyurethane tread 34 being bonded into the groove 32 using an adhesive and method as hereinbefore described.

Claims: CLAIMS 1. An adhesive for bonding elastomers with improved dynamic fatigue resistance comprising an halogenated polymer which can be cured and stabilised and which contains isocyanate with a functionality between 2.1 and 2.6. 2. An adhesive according to claim 1, of which the polymer is chlorinated. 3. An adhesive according to claim 1 or claim 2, of which the polymer is brominated. 4. An adhesive according to any one of claims 1 to 3, being a one-part adhesive. 5. An adhesive according to claim 4, comprising a chlorinated or brominated polymer and a curing agent. 6. An adhesive according to claim 5, in which the curing agent comprises p- dinitrosobenzene. 7. An adhesive according to claim 5 or claim 6, in which the polymer is chlorinated or brominated rubber. 8. An adhesive according to claim 5 or claim 6, in which the polymer is CSM. 9. An adhesive according to claim 5 or claim 6, in which the polymer comprises a partly halogenated butadiene polymer. 10. An adhesive according to claim 5 or claim 6, in which the polymer comprises a halogenated polyurethane. 11. An adhesive according to claim 5 or claim 6, in which the polymer comprises a mixture of two or more of chlorinated or brominated rubber, CSM, partly halogenated butadiene polymer, and halogenated butadiene. 12. An adhesive according to any one of claims 5 to 11, in which the polymer comprises at least one highly chlorinated Diels-Alder adduct of hexachlorocyclobutadiene or hexachlorocyclopentadiene. 13. An adhesive according to any one of claims 5 to 12, in which the polymer comprises a dienophile. 14. An adhesive according to claim 13, in which the dienophile comprises polybutadiene, a butadiene copolymer or an isoprene polymer. 15. An adhesive according to any one of claims 1 to 14 comprising an adhesion promoter. 16. An adhesive according to any one of claims 1 to 15, adapted to achieve initial cure from the heat output from an exothermic reaction. 17. An adhesive according to claim 16, in which the adhesion promoter is adapted to achieve the initial cure by exothermic reaction heat. 18. An adhesive according to any one of claims 1 to 17, in which the isocyanate comprises a part-polymerised diisocyanate. 19. An adhesive according to any one of claims 1 to 18, in which the isocyanate comprises a quasi-prepolymer made from branched polyols and pure diphenyl-methane diisocyanate (MDI). 20. An adhesive according to any one of claims 1 to 19, in which the isocyanate comprises a quasi-prepolymer made from partially polymerised diisocyanates and polyols. 21. An adhesive according to any one of claims 1 to 20, in which the isocyanate content of the isocyanate molecule is between 8 and 33%. 22. A method for bonding elastomers such as polyurethane with improved dynamic fatigue resistance comprising using an adhesive according to any one of claims l to 21. 23. A method for producing a bonded polyol - elastomer/substrate system such as a polyurethane/substrate system with improved dynamic fatigue resistance comprising using an adhesive according to any one of claims 1 to 22 and an elastomer such as polyurethane of which the polyol has an overall functionality between 2.07 and 2.3. 24. A method according to claim 22, in which the isocyanate of the elastomer has a functionality between 2.0 and 2.3. 25. A method according to any one of claims 22 to 24, in which the elastomer is bonded to a rubber substrate. 26. A method according to any one of claims 22 to 25, in which the elastomer is bonded to an elastomer substrate. 27. A method according to claim 26, used for bonding an elastomer (such as polyurethane) casting or moulding to a tyre. 28. A method according to claim 27, in which the casting or moulding comprises a tyre tread. 29. A method according to claim 27 or 28, in which the casting or moulding comprises a protective side-wall. 30. A tyre comprising a casting or moulding adhered to a carcass by an adhesive according to any one of claims 1 to 21 or by a method according to any one of claims 12 to 19. 31. A tyre according to claim 30, in which the casting or moulding comprises the tyre tread. 32. A tyre according to claim 31, in which the carcass has an annular groove between road-engaging shoulders and the cast or moulded tread is adhesively secured in the groove. 33. A tyre according to any one of claims 30 to 32, of which the carcass is of rubber and the tread is of polyurethane. 34. A method according to any one of claims 22 to 29, or a tyre according to any one of claims 28 to 31, in which the elastomer comprises a polyurethane elastomer in the hardness range 50 °C Shore A to 95 °C Shore A. 35. A method or a tyre according to claim 34, in which the elastomer is in the range 60° Shore A to 75° Shore A. 36. A method or a tyre according to any one of claims 22 to 35, in which the elastomer is injection moulded. 37. A method or a tyre according to any one of claims 22 to 36, in which the elastomer is only partly cured before bonding. 38. A method or a tyre according to any one of claims 22 to 37, in which the elastomer is post-cured before bonding. 39. A method or a tyre according to any one of claims 22 to 38, in which the elastomer comprises an ester prepolymer. 40. A method or a tyre according to any one of claims 22 to 39, in which the elastomer comprises a polycaprolactone prepolymer. 41. A method or a tyre according to any one of claims 22 to 40, in which the elastomer comprises a polyol with a molecular weight between 1000 and 5000. 42. A method or a tyre according to claim 41, in which the polyol comprises a mixed diol adipate. 43. A method or a tyre according to claim 42 in which the molecular weight is between 1500 and 2500. 44. A method or a tyre according to any one of claims 22 to 43, in which the elastomer comprises an ether prepolymer. 45. A method or a tyre according to any one claims 22 to 44, in which the elastomer comprises a quasi-prepolymer system. 46. A method or a tyre according to any one of claims 22 to 45, in which the elastomer comprises extender material. 47. A method or a tyre according to claim 46, in which the extender material is present in an amount between 2 and 60% w/w of the polyurethane. 48. A method or a tyre according to any one of claims 22 to 47, in which a rubber coating is bonded to the elastomer and is then bonded to a rubber substrate. 49. A method or a tyre according to claim 48, in which the rubber coating comprises a curable cushion gum. 50. A method or a tyre according to claim 49, in which the rubber coating comprises a solvent-based rubber solution. 51. A method or a tyre according to any one of claims 27 to 50, in which the adhesive cures and bonds at a temperature between 80 and 130°C. 52. A method or a tyre according to any one of claims 22 to 51, in which curing and bonding proceeds for between 30 minutes and 3 hours. 53. A method or a tyre according to any one of claims 22 to 52, in which the elastomer is cured using a catalyst. 54. A method or a tyre according to claim 53, in which the catalyst comprises a heavy metal complex.

Inventors: GOMBERG EDWARD NICHOLAS, WORTHINGTON KEITH, GOMBERG, EDWARD, NICHOLAS, WORTHINGTON, KEITH
Applicants: COMPOUNDING INGREDIENTS LIMITE, GOMBERG EDWARD NICHOLAS, WORTHINGTON KEITH, COMPOUNDING INGREDIENTS LIMITED, GOMBERG, EDWARD, NICHOLAS, WORTHINGTON, KEITH
Application year: 1997
Main classification: C08G18
IPC codes: B60C1, B60C11, C08G18, C09J123, C09J175