Continuous process for the production of isocyanate prepolymers and polyurethane foams prepared therefrom

Abstract: A continuous process for the preparation of an isocyanate-terminated prepolymer having an NCO content in the range from 2 to 15% by weight, said process comprising the steps of : (A) continuously delivering to a reaction zone an organic polyisocyanate composition and an isocyanate reactive compound having an average molecular weight of at least 1000 the relative proportions of isocyanate composition and isocyanate reactive compound being appropriate for the formation of a prepolymer having an NCO content in the indicated range; (B) allowing the isocyanate composition and the isocyanate reactive compound to react in the reaction zone to form an isocyanate-terminated prepolymer, and (C) continuously removing the prepolymer from the reaction zone.

Description: This invention relates to a method for the production of flexible foams. Isocyanate-containing polyurethane prepolymers obtained by reacting a stoichiometric excess of an organic polyisocyanate with an organic polyol are well known in the field of polyurethanes and have been used, for example, in the production of solid and microcellular elastomers, flexible and rigid foams, coatings, adhesives and the like. Thus, essentially difunctional prepolymers having low free isocyanate contents obtained by reacting tolylene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) with a polyester or polyether diol have been used in the preparation of elastomers. On the other hand, prepolymers having high free isocyanate contents obtained by reacting various diols with 4,4'-diphenylmethane diisocyanate have been manufactured as a means of providing this normally solid diisocyanate in a convenient liquid form. The conventional method of making prepolymers is to react the polyol and polyisocyanate batchwise at an elevated temperature, the reactants being used in appropriate proportions to provide a prepolymer of the desired free isocyanate content. The reaction between the polyol and the polyisocyanate is exothermic but, in order to achieve completion in an acceptable time, it is usual to heat the reaction mixture to a temperature in the range from about 40 to about 100°C for one hour or longer. It has also been proposed to employ catalysts to bring about prepolymer formation. European Patent Application 351852 is disclosing a method for preparing a foam by combining a polyol and a polyisocyanate in such a way that they do not react and to bring this combination into contact with the other ingredients necessary for preparing the foam. In Polyurethanes, Chemistry and Technology, Part II, Technology by Saunders and Frisch, 1983, pages 38-43 a process for the continuous production of a prepolymer is described using TDI and a polyol. Further a process for preparing a flexible foam is described using such prepolymer. The present invention is concerned with a method for the production of flexible foams comprising the steps of: (A) continuously delivering to a reaction zone a diphenylmethane diisocyanate or diphenylmethane diisocyanate composition and a polyol having an average molecular weight of at least 1000, with the proviso that these reactants are preheated to a temperature of at most 80°C if heating is applied, and a catalyst for urethane formation not exceeding the amount needed for foam production in step E below, the relative proportions of isocyanate and polyol being appropriate for the formation of a prepolymer having an NCO content in the range from 2 to 15% by weight; (B) allowing the isocyanate and the polyol to react in the reaction zone to form an isocyanate-terminated prepolymer; (C) continuously transferring the reaction mixture from the reaction zone to a foam-forming zone, while if necessary allowing the reaction mixture to mature to form a prepolymer having an NCO content in the range from 2 to 15% by weight; (D) continuously delivering the prepolymer to a foam-forming zone; (E) reacting the prepolymer in the foam-forming zone with an isocyanate-reactive component comprising water and (F) recovering the foam so formed. Suitable organic polyisocyanates include diphenylmethane diisocyanates (MDI) in their various pure, modified and crude forms and mixtures thereof with tolylene diisocyanates (TDI). Suitable isocyanates include "pure" MDI preferably containing at least 60% by weight of 4,4'-isomer. Other suitable isocyanates include the substantially pure 4,4'-isomer apd isomer mixtures containing that isomer and not more than 40%, preferably not more than 30%, and more preferably not more than 20%, by weight of the 2,4'-isomer and not more than 5% by weight of the 2,2'isomer. Still other suitable isocyanate compositions include modified forms of diphenylmethane diisocyanates, that is to say MDI modified in known manner by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues. These so-called MDI variants particularly include uretonimine-modified MDI having NCO contents of at least 25% by weight and polyether-based prepolymers having NCO contents of at least 20% by weight. Isocyanate compositions containing MDI and polymeric fillers may also be used in the preparation of the prepolymer. Such products include polyurea dispersions in MDI such as have been described, for example, in EP-A-O 103996. Other compositions containing polymeric fillers include prepolymers based on MDI and polymer polyols containing dispersed polymer particles. In these products, it is generally preferred that the dispersed polymer particles have an average size of less thant 50 microns. Further isocyanate compositions which may be used in the preparation of the prepolymer include compositions containing polymethylene polyphenylene polyisocyanates. Thus, mixtures may be used containing at least 70% by weight of pure MDI isomers and up to 30% by weight of the so-called crude MDI containing from 35 to 65% by weight of diisocyanates, the remainder being largely polymethylene polyphenylene polyisocyanates having isocyanate functionalities greater than 2. Mixtures may also be used of pure MDI and polymeric MDI compositions containing higher proportions (up to 100%) of the said higher functionality polyisocyanates. Suitable isocyanate-reactive compounds for the process of the present invention are polyols, which have molecular weights of at least 1000, preferably 2000-8000 and a functionality ranging from 1.1 to 5, preferably 2-4, more preferably 2-3. Suitable polyols and methods for their preparation have been described in the prior art and, as example of such polyols, there may be mentioned polyesters, polyesteramides, polythioethers, polycarbonates, polyacetals, polyolefins, polysiloxanes and, especially, polyethers. Polyether polyols which may be used include products obtained by the polymerisation of a cyclic oxide, for example ethylene oxide, propylene oxide, butylene oxide or tetrahydrofuran in the presence, where necessary, of polyfunctional initiators. Suitable initiator compounds contain a plurality of active hydrogen atoms and include water and polyols, for example ethylene glycol, propylene glycol, diethylene glycol, cyclohexane dimethanol, resorcinol, bisphenol A, glycerol, trimethylolpropane, 1,2,6-hexanetriol or pentaerythritol. Mixtures of initiators and/or cyclic oxides may be used. Especially useful polyether polyols include polyoxypropylene diols and triols and poly(oxyethylene-oxypropylene) diols and triols obtained by the simultaneous or sequential addition of ethylene and propylene oxides to di- or trifunctional initiators as fully described in the prior art. Mixtures of the said diols and triols can be particularly useful. Other particularly useful polyether polyols include polytetramethylene glycols obtained by the polymerisation of tetrahydrofuran. Polyester polyols which may be used include hydroxyl-terminated reaction products of polyhydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, 1,4- butanediol, neopentyl glycol, 1,6-hexanediol, cyclohexane dimethanol, bis(hydroxyethyl) terephthalate, glycerol, trimethylolpropane, pentaerythritol or polyether polyols or mixtures of such polyhydric alcohols, and polycarboxylic acids, especially dicarboxylic acids or their ester-forming derivatives, for example succinic, glutaric an adipic acids or their dimethyl esters, sebasic acid, phthalic anhydride, tetrachlorophthalic anhydride or dimethyl terephthalate or mixtures thereof. Polyesteramides may be obtained by the inclusion of aminoalcohols such as ethanolamine in polyesterification mixtures.Polyesters obtained by the polymerisation of lactones, for example caprolactone, in conjunction with a polyol, or of hydroxy carboxylic acids such as hydroxy caproic acid, may also be used. Polythioether polyols which may be used include products obtained by condensing thiodiglycol either alone or with other glycols, alkylene oxides, dicarboxylic acids, formaldehyde, amino-alcohols or aminocarboxylic acids. Polycarbonate polyols which may be used include products obtained by reacting diols such as 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol or tetraethylene glycol with diaryl carbonates, for example diphenyl carbonate, or with phosgene. Polyacetal polyols which may be used include those prepared by reacting glycols such as diethylene glycol, triethylene glycol or hexanediol with formaldehyde. Suitable polyacetals may also be prepared by polymerising cyclic acetals. Suitable polyolefin polyols include hydroxy-terminated butadiene homo- and copolymers and suitable polysiloxane polyols include polydimethylsiloxane diols and triols. Other polyols which may be used comprise dispersions or solutions of addition or condensation polymers in polyols of the types described above. Such modified polyols have been fully described in the prior art and include products obtained by the in situ polymerisation of one or more vinyl monomers, for example styrene and acrylonitrile, in polymeric polyols, for example polyether polyols or by the in situ reaction between a polyisocyanate and an amino-functional compound and/or low MW polyhydroxyls and/or amino-alcohols in a polymeric polyol. Mixtures of polyols may be used. Such mixtures may contain components differing in respect of molecular weight, functionality or polymer backbone. In the process of the present invention a catalyst for urethane formation is used. Catalysts for urethane formation which may be used are compounds which promote reaction between isocyanate and hydroxyl groups. Such catalysts have been fully described in the prior art (for example in Saunders and Frisch, Polyurethanes : Chemistry and Technology, Part I, Interscience, New York, 1962, pages 161-173) and include organic and inorganic salts of, and organometallic derivatives of, bismuth, lead, tin, iron, antimony, cadmium, cobalt, aluminium, mercury, zinc, nickel, cerium, molybdenum, vanadium, copper, manganese and zirconium, as well as phosphines and tertiary amines. One particularly important class of catalysts comprises tin salts and organotin salts, for example stannous acetate, stannous butyrate, stannous octoate, stannous laurate, stannous oleate, stannous stearate, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin mercaptopropionate and the like. Another important class of catalysts comprises tertiary amines in which the nitrogen atom is not directly attached to an aromatic ring. As examples of tertiary amine catalysts, there may be mentioned triethylamine, N, N, N', N'-tetramethylene-diamine, N-N,N',N'-tetramethyl-1,3- butanediamine, bis-2-dimethylaminoethyl ether, N,N-dimethylcyclohexylamine, N,N-dimethylbenzylamine, N-methylmorpholine, N-ethylmorpholine, 1,4 -diazabicyclo-[2.2.2] octane and the like. Polyols containing tertiary amino groups, for example polyethers obtained by the oxyalkylation of ammonia or triethanolamine, may be self-catalytic and may, in some cases, be reacted with the diisocyanate composition in the absence of additional catalyst. Combinations of one or more metal salts or complexes with one or more tertiary amines provide especially effective catalysts. The prepolymer should not contain more catalyst than is required in subsequent foam production. If desired or necessary, the catalyst may be introduced into the reaction zone in the form of a solution in an inert solvent. Persons skilled in the art, knowing the isocyanate content of the isocyanate composition and the functionality and molecular weight of the isocyanate-reactive compound, will have no difficulty in calculating the relative amounts of these reactants to be delivered to the reaction zone in order to provide a prepolymer having any desired NCO content. To achieve a final NCO content within the range 2 to 15% by weight, an initial ratio of isocyanate to isocyanate-reactive groups would typically be within the range from about 3:1 to about 20:1. Preferred prepolymers are made by reacting the starting materials at an initial ratio of isocyanate to isocyanate-reactive groups in the range from 3.5:1 to 15:1, especially 4:1 to 10.1 to give prepolymers having NCO contents of 4 to 12%, especially 5 to 10% by weight. The configuration of the reaction zone used in the method of the invention is not important, any conventional equipment of appropriate size being suitable. Similarly, conventional equipment and techniques may be used for delivering the isocyanate composition, the isocyanate-reactive compound and the catalyst, to the reaction zone and removing the prepolymer therefrom. In general, it is preferred to react the isocyanate composition and the isocyanate-reactive compound at or close to the ambient temperature no heat being applied to the reactants or to the reaction mixture other than to liquefy any reactants, for example 4,4'-MDI, which may be solid at the ambient temperature. In fact, cooling facilities may be required when the prepolymer is to be immediately subjected to further processing. If heat is to be applied it is to preheat the reactants up to at most 80°C and not to apply heat further down-stream in the process. The prepolymers prepared by the method of the invention are liquid compositions having low vapour pressures. They are used in the production of flexible foams, especially cold-curing, low density flexible foams having high resilience combined with excellent tear and tensile properties. To effects foam formation, the isocyanate-terminated prepolymer is reacted with water in the presence as necessary of conventional additives. Such additives include catalysts, for example the tertiary amines and tin compounds described above, surface-active agents and foam stabilisers, for example siloxane-oxyalkylene copolymers, chain extenders, for example low molecular weight diols, diamines and di-imines, crosslinking agents, for example triethanolamine, flame retardants, organic and inorganic fillers, pigments and internal mould release agents. Moulded or slabstock foam may be prepared from the prepolymers using continuous production techniques. The prepolymer preparation and the foam-forming process are linked in such a way that the whole process can be carried out continuously. The prepolymer may reside in a tank wherein the prepolymer may be allowed to mature under appropriate conditions such a tank providing for a residence time of the prepolymer, which time is sufficient to give the prepolymer the desired maturity. In operating the method for the production of foam, cooling of the prepolymer, for example to the ambient temperature, may be effected during transfer from the reaction zone to the foam-forming zone if desired. The foam-forming zone typically comprises conventional equipment, for example a conventional mixing head, used for foam production. In a preferred embodiment the prepolymer may be reacted with an isocyanate reactive component comprising at least 40% by weight of water, the rest of it being chain extenders, crosslinkers or conventional additives and less than 5% by weight based on the isocyanate reactive component, of a conventional polyol used for the formation of polyurethane foams. Typically a water content of at least 95% by weight may be achieved. In addition, there may also be delivered to the foam-forming zone one or more of the additives known per se in foam production, such additives having been described above. It is preferred, however, that the foam-forming reaction mixture is substantially free of halocarbon blowing agent, that is to say that the foam formulation should contain less than 1%, preferably less than 0.5% of halocarbon blowing agent, for example trichlorofluoromethane, so that less than 10% of the blowing is due to the halocarbon. In a particularly preferred method of making foam, a second isocyanate composition is delivered to the foam-forming zone in addition to the prepolymer. Isocyanate compositions which may be used in conjunction with the isocyanate-terminated prepolymer include the various types of diphenylmethane diisocyanate compositions described above in relation to the preparation of the prepolymer provided they are liquids at the operating temperature. Thus, suitable compositions include MDI isomer mixtures, preferably containing at least 60% by weight of the 4,4'-isomer, MDI variants and crude or polymeric MDI. It is preferred that the second diphenylmethane diisocyanate composition has an NCO content of at least 25%, especially at least 30% by weight. In especially preferred foam formulations, the prepolymer is obtained from a diphenylmethane diisocyanate composition and is used in conjunction with a second diphenylmethane diisocyanate composition having an NCO content of at least 25% by weight, the weight ratio of prepolymer to second MDI composition being in the range from 2.3:1 to 100:1, the prepolymer and second MDI composition together containing from 20 to 40% by weight of methylene-bridged polyphenyl poly-isocyanates or reacted residues thereof and from 80 to 60% byweight of reacted residues of isocyanate-reactive species. Most preferably the isocyanate reactive component comprises at least 95% of water and the weight ratio of prepolymer and second MDI species to aqueous isocyanate-reactive component is in the range from 100:10 to 100:1.

Claims: Method for the production of flexible foams comprising the steps of: (A) continuously delivering to a reaction zone a diphenylmethane diisocyanate or diphenylmethane diisocyanate composition and a polyol having an average molecular weight of at least 1000, with the proviso that these reactants are preheated to a temperature of at most 80°C if heating is applied, and a catalyst for urethane formation not exceeding the amount needed for foam production in step E below, the relative proportions of isocyanate and polyol being appropriate for the formation of a prepolymer having an NCO content in the range from 2 to 15% by weight; (B) allowing the isocyanate and the polyol to react in the reaction zone to form an isocyanate-terminated prepolymer, (C) continuously transferring the reaction mixture from the reaction zone to a foam-forming zone, while if necessary allowing the reaction mixture to mature to form a prepolymer having an NCO content in the range from 2 to 15% by weight; (D) continuously delivering the prepolymer to a foam-forming zone; (E) reacting the prepolymer in the foam-forming zone with an isocyanate-reactive component comprising water and (F) recovering the foam so formed. A method according to claim 1 wherein the isocyanate-reactive component comprises at least 40% by weight of water, the rest of it being chain extenders, crosslinkers, conventional additives and less than 5% by weight of a conventional polyol used for the formation of polyurethane foams. A method according to claims 1-2 wherein the isocyanate-reactive component comprises at least 95% by weight of water. A method according to claims 1-3 wherein the foam-forming reaction mixture is substantially free of halocarbon blowing agent. A method according to claims 1-4 wherein a second isocyanate composition is continuously delivered to the foam-forming zone in addition to the prepolymer. A method according to claim 5 wherein the second isocyanate composition has an NCO content of at least 25% by weight. A method according to claims 5-6 wherein the second isocyanate composition is delivered to the foam-forming zone independently from the prepolymer. A method according to claims 1-7 wherein the prepolymer is prepared from a diphenylmethane diisocyanate composition and is used in conjunction with a second diphenylmethane diisocyanate composition having a NCO content of at least 25% by weight, the weight ratio of prepolymer to second MDI composition being in the range from 2.3:1 to 100:1, the prepolymer and second MDI composition together containing from 20 to 40% by weight of methylene-bridged polyphenyl polyisocyanates or reacted residues thereof and from 80 to 60% by weight of reacted residues of isocyanate-reactive species, the isocyanate-reactive component comprising at least 95% by weight of water and the weight ratio of prepolymer and second MDI species to aqueous isocyanate-reactive component being in the range from 100:10 to 100:1. A process according to claims 1-8 wherein the polyol has an average nominal hydroxyl functionality of 2 to 3 and a molecular weight of 2000-8000. A process according to any preceding claim wherein no heat is applied to the prepolymer-forming reactants or reaction mixture other than to liquify solid reactants. A process according to any of the predeeding claims wherein steps E and F are conducted continuously.

Inventors: MULLER LOUIS, PARFONDRY ALAIN, VERHELST GABRIEL, MULLER, LOUIS, PARFONDRY, ALAIN, VERHELST, GABRIEL
Applicants: ICI PLC, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1991
Main classification: C08G18
IPC codes: C08G18