WO2025195909A1

WO2025195909A1 - Process for the production of a polyurethane prepolymer

Info

Publication number: WO2025195909A1
Application number: PCT/EP2025/056998
Authority: WO
Inventors: Stefan Bokern; Hendrik KATTNER; Thomas Rene WIEGMANN; Juergen Roeder; Tobias Gienau
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2024-03-22
Filing date: 2025-03-14
Publication date: 2025-09-25
Anticipated expiration: 2026-09-22

Abstract

The present invention relates to a Process for the production of a polyurethane prepolymer comprising mixing (a) at least one polyisocyanate, (b) at least one compound comprising at least one group reactive towards isocyanates and (c) at least one acidic phosphoric acid ester to form a reaction mixture and reacting the mixture to obtain the polyure- thane prepolymer, wherein the aromatic polyisocyanate (a) and the compound comprising at least one group reactive towards isocyanates are reacted to form the polyurethane prepolymer in presence of the phosphoric acid ester (c) and wherein the at least one the phosphoric acid ester is present in an amount of 0.0001 to 0.5 wt.-%, based on the total weight of compounds (a) to (c).

Description

Process for the production of a polyurethane prepolymer

The present invention relates to a process for the production of a polyurethane prepolymer comprising mixing (a) at least one aromatic polyisocyanate, (b) at least one compound comprising at least one group reactive towards isocyanates and (c) at least one acidic phosphoric acid ester to form a reaction mixture and reacting the mixture to obtain the polyurethane prepolymer, wherein the aromatic polyisocyanate (a) and the compound comprising at least one group reactive towards isocyanates are reacted to form the polyurethane prepolymer in presence of the acidic phosphoric acid ester (c) and wherein the at least one acidic phosphoric acid ester is present in an amount of 0.0001 to 0.5 wt.-%, based on the total weight of compounds (a) to (c).

Prepolymers comprising polyurethane groups, hereinafter referred to as polyurethane prepolymers, are important intermediates for the production of polyurethanes. They have been known for a long time and have been described many times in the literature. They are produced by reacting compounds with at least one hydrogen atom reactive with isocyanate groups, in particular polyols, with polyisocyanates.

During prepolymer synthesis, aromatic isocyanates are typically acidified for example with acyl chlorides, benzoyl chloride, sulfonyl chloride, or phosphoric acid. Acidification will keep the reaction in a safe region of slight acid excess to avoid critical runaway polyisocyanurate trimerization, which is catalyzed by alkaline substances like carboxylate salts or alkali metal hydroxides. Compared to aromatic isocyanates, reactivity of aliphatic isocyanates is much lower, therefore, during classical prepolymer production based on aliphatic isocyanates, the critical runaway polyisocyanurate trimerization will not occur.

Such alkaline substances are often used to catalyze polyol synthesis and often remain in the polyol due to only partial neutralization in the polyol production step. Especially for polyether poylols with secondary hydroxyl groups, the prepolymer synthesis reaction acidified with acyl chlorides is relatively slow or requires high temperature, which causes additional cost and can lead to undesirable side reactions. It is therefore desirable to accelerate the prepolymer reaction.

The acceleration of the prepolymer synthesis reaction by catalysts such as bismuth catalysts or tin catalysts is well known, also in combination with acidifiers and for example disclosed in DE 102007037641. Depending on the dosing technology and the miscibility of the components, it is possible to achieve an extremely high concentration of catalysts in a local area within the reactor. In such cases, undesired isocyanurates can easily be formed in an exothermic reaction, which can also lead to a runaway of the reaction. By nature, these catalysts do not self-limit their catalytic activity and could therefore lead to an exothermic reaction that is too rapid and uncontrollable at these sites. It would be desirable to provide a process that can maximize production capacity by optimally controlling the reaction rate of the prepolymer-polyurethane reaction without having to face the risk of uncontrollable reaction rates. It is well known that phosphoric acid has a significant accelerating effect on the reaction of the isocyanate group and the hydroxyl group, this is for example disclosed in DE1593665A1. However, technical-grade phosphoric acid contains 15% water by weight. The water-isocyanate reaction produces urea groups, which, even in small amounts, reduce the quality of the prepolymer in terms of durability, turbidity and mechanical properties, and at the same time increase its viscosity in an undesirable way.

Anhydrous polyphosphoric acid derivatives are not a technical solution due to their extremely high hygroscopy and viscosity and are therefore difficult to handle in production. Additionally, it is obvious to a person skilled in the art that due to poor solubility of inorganic acids like phosphoric acids in organic aprotic liquids like isocyanates, phosphoric acid is usually pre-mixed with the polyol before addition of the polyol to the isocyanate. Polyols contain polar end groups like hydroxyl groups and therefore inorganic acids have a significantly higher solubility in polyols than in isocyanates. This pre-mixing of acids with polyols is undesired in large scale production due to the need of an additional blend vessel. It would be preferred to have a process utilizing an acidifier which can be dosed directly into the isocyanate and dissolves rapidly for quick and homogenous acidification of the reaction mixture.

DE1251018B mentions the use of benzoyl chloride for pH control. Benzoyl chloride, as is typical for acyl chlorides, does not have an accelerating effect on prepolymer synthesis.

EP0856551 A1 describes the additivation of a synthesized prepolymer with anhydrous polyphosphoric acid. Polyphosphoric acid is difficult to handle on an industrial scale due to its high hygroscopy, and does not readily dissolve in substances with low polarity like isocyanates.

US2002/0153507 discloses a polyurethane prepolymer with improved pot life, wherein the prepolymer contains plasticizers and acidic phosphoric acid esters in addition to the reaction product of polyisocyanate and polyol. However, the synthesis of the prepolymer is described to always take place in the presence of acyl chlorides. In the examples the acidic phosphoric acid ester is only added to the reaction mixture after the prepolymer synthesis step has already taken place, so the accelerating effect is not taken advantage of.

EP 3589672 discloses a composition containing isocyanate and acidic phosphoric acid esters, the mass ratio of isocyanate to phosphoric acid esters not exceeding 3.5 : 1. The composition has an extended pot life and good mold release properties and can be used, for example, as a coating composition.

US5962561 disclosed a composition containing isocyanate and mixtures of phosphate acid esters for mold release properties of optical materials. The described produced materials are fully cured plastics and not NCO-containing prepolymers, and kinetic effects are not described.

US2011/251301 discloses an organic glass and the synthesis of the organic glass by polymerizing two components (A) and (B), wherein component (A) contains at least one cyclo-aliphatic diisocyanate monomer or a mixture of one cyclo-aliphatic diisocyanate monomer and a pre-polymer obtained by reaction between said cyclo-aliphatic diisocyanate monomer and one or more polyols and the second component (B) contains one or more polyols. Instead of conventional catalysts acid phosphate esters are used as polymerization catalysts to obtain the organic glass. In case that component (A) comprises a prepolymer based on aliphatic isocyanate, this prepolymer is obtained in the presence of the acid phosphate ester catalyst. The acid phosphate ester catalyst for production of the organic glass is used in an amount of 0.2 to 3 % by weight, based on the total weight of the composition. and results in a long open time compared to conventional catalysts.

It was object of the present invention to find a process for the production of polyurethane prepolymers which allows the acceleration of the production compared to state-of-the-art acyl chlorides under safe conditions, without introduction of significant amounts of water into the system. This acceleration has to be well controllable, due to the fact that the Urethane formation is an exothermic reaction and a too fast reaction may lead to a too strong rise in temperature and a runaway reaction. The prepolymer should be easily applicable to amine-catalyzed follow-up reactions with common ingredients in polyurethane formulations like water or polyols, so the acidification should not cause a significant decrease or increase in reactivity in the polyurethane formulations formulated with the described prepolymers. Preferably, the acidification agent is also readily doseable into the pure polyisocyanate and does not require preblending with polyol components.

The object of the present invention has been solved by a process for the production of a polyurethane prepolymer comprising mixing (a) at least one aromatic polyisocyanate, (b) at least one compound comprising at least one group reactive towards isocyanates and (c) at least one acidic phosphoric acid ester to form a reaction mixture and reacting the mixture to obtain the polyurethane prepolymer, wherein the aromatic polyisocyanate (a) and the compound comprising at least one group reactive towards isocyanates (b) are reacted to form the polyurethane prepolymer in presence of the acidic phosphoric acid ester (c) and wherein the at least one acidic phosphoric acid ester (c) is present in an amount of 0.0001 to 0.5 % by weight, based on the total weight of compounds (a) to (c).

Polyurethane prepolymers according to the present invention are liquid at a temperature of 80 °C and have a viscosity of less than 10.000 mPas at 80°C. In a preferred embodiment the polyurethane prepolymers obtained by the process according to the invention typically have viscosities at 25°C in the range from 60 mPas to 500 Pas, preferably in the range 80 mPas to 200 Pas, and particularly preferably in the range 90 mPas to 100 Pas (measured according to DIN EN ISO 3219 and with plate/cone measurement geometry at a shear rate of 40 1/s). In a preferred embodiment the isocyanate prepolymer according to the invention can be produced by reaction of a polyhydroxyl compound with an excess of a aromatic polyisocyanate. In a preferred embodiment the NCO-content of the isocyanate prepolymer according to the present invention is 1 to 40 % by weight, more preferred 2 to 30 % by weight, even more preferred 5 to 25 % by weight and especially preferred 7 to 15 % by weight. As aromatic polyisocyanates (a) all aromatic isocyanates known in the field of production of polyurethane prepolymers can be used. MMDI is particularly suitable as aromatic polyisocyanate (a) for the process according to the invention.

Suitable aromatic diisocyanates are especially naphthylene 1 ,5-diisocyanate (NDI), tolylene 2,4- and/or 2,6- diisocyanate (TDI), diphenylmethane 2,2'-, 2,4'- and/or 4,4'-diisocy anate (MMDI) or else mixtures thereof and higher homologs (PMDI), 3,3’-dimethyl-4,4’-diisocyanatodiphenyl (TODI), p-phenylene diisocyanate (PDI), diphenylethane 4,4'-diisocyanate (EDI), diphenylmethane diisocyanate, 3,3’-dimethyldiphenyl diisocyanate, diphenylethane 1,2- diisocyanate and/or phenylene diisocyanate.

In a more preferred embodiment of the present invention the aromatic polyisocyanate (a) is at least one isocyanate selected from the group consisting of diphenylmethane 2,2'-, 2,4'- and/or 4,4'-diisocyanate (MMDI), naphthylene 1,5- diisocyanate (NDI), tolylene 2,4- and/or 2,6-diisocyanate (TDI), 3,3‘-dimethyl-4,4‘-diisocyanatodiphenyl (TODI), p- phenylene diisocyanate (PDI), hexamethylene 1 ,6-diisocyanate (HDI), 1-isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI) and methylenedicyclohexyl 4,4'-, 2,4'- and/or 2,2'- diisocyanate (H12MDI), or derivatives of these isocyanates.

It is also possible in the context of the present invention to use higher-functionality isocyanates, by way of example triisocyanates, for example triphenylmethane 4,4’, 4”-triisocy anate, and also the cyanurates of the aforementioned diisocyanates, and the oligomers obtainable by partial reaction of diisocyanates with water, for example the biurets of the aforementioned diisocyanates, and additionally oligomers obtainable by controlled reaction of semiblocked diisocyanates with polyols having an average of more than two and preferably three or more hydroxyl groups.

In an especially preferred embodiment, the isocyanate (a) comprises at least 90 % by weight, based on the total weight of isocyanate (a) of MMDI or PMDI or mixtures thereof, more preferred of MMDI, more preferred of diphenylmethane 2,4'- and/or 4,4'-diisocyanate and most preferred diphenylmethane 4,4'-diisocyanate.

As compound comprising at least one group reactive towards isocyanates (b) all isocyanate reactive compounds known in the field of production of polyurethane prepolymers can be used. In one preferred embodiment of the process according to the invention polyether polyols and/or polyester polyols are used as compound comprising at least one group reactive towards isocyanates (b).

The spectrum of employable polyols ranges from polyether polyols based on for example propylene glycol (PPGs) or butanediol (PolyTHF) through to polyester polyols. In accordance with the invention polymeric polyols as polyetherols are suitable, but so are polyesterols, block copolymers and hybrid polyols such as for example poly(ester/amide) or poly (ester/ether) as well as low molecular weight polyols. Preferred polyols are polytetramethylene ether glycol, polyethylene glycols, polypropylene glycols, polyadipates, polycarbonates/polycarbonate diols and polycaprolactone. In addition to polymeric polyols also low molecular polyols, often referred to as chain extenders and crosslinkers can be employed as polyol. The number-average molecular weight of the polymeric polyols employed according to the invention is preferably 0.3x10³g/mol to 8x10³ g/mol, preferably from 0.4x10³ g/mol to 5x10³ g/mol (the number-average molecular weight is determined according to DIN55672-2; calibration is carried out with PMMA). The hydroxyl numbers of the polyols employable according to the invention are from 5 to 1500 mg KOH/g, preferably 10 to 800 mg KOH/g and especially preferred 20 to 600 mg KOH/g (according to DIN 5342 [1971-12]). The employed polyols/the polyol composition preferably has/have an average functionality of 1 to 6.5, particularly preferably of 1 .5 to 4.5, more preferred 1 .8 to 3.5 and especially preferably of 1 .9 to 3.0.

The polyether polyols are produced by known processes, for example by anionic polymerization of one or more alkylene oxides containing 2 to 4 carbon atoms containing ethylene oxide, with common catalysts such as alkali hydroxides, such as sodium or potassium hydroxide, alkali alcoholates, such as sodium methylate, sodium or potassium isopropylate, or amine alkoxylation catalysts, such as dimethylethanolamine (DMEOA), imidazole and/or imidazole derivatives, under the use of at least one starter molecule or starter molecule mixture. In a preferred embodiment, the starter molecule or starter molecule mixture has in average < 3.0 and > 2.0 and preferably exactly two reactive hydrogen atoms bonded.

In addition to the anionic polymerization of the starter molecules, the production can also be carried out by means of cationic polymerization, whereby Lewis acids such as antimony pentachloride, boron fluoride etherate or bleaching earth are used as catalysts.

Preferred alkoxylation catalysts are alkali hydroxides, especially preferred KOH. Instead of alkali hydroxides, amine alkoxylation catalysts can also be used. Preferred aminic alkoxylation catalysts are selected from the group containing dimethylethanolamine (DMEOA), imidazole and imidazole derivatives as well as mixtures thereof, especially imidazole.

Suitable alkylene oxides are, for example, ethylene oxide, tetrahydrofuran, 1 ,3 or 1 ,2-propylene oxide, 1,2- or 2,3- butylene oxide, styrene oxide and preferably 1 ,2-propylene oxide. Especially preferred as alkylene oxides are ethylene oxide, 1 ,2-propylene oxide and tetrahydrofuran. The alkylene oxides can be used individually, alternately one after the other or as mixtures.

Examples of starter molecules include water, organic dicarboxylic acids such as succinic acid, adipic acid, phthalic acid and terephthalic acid, or preferably two or polyhydric alcohols, such as ethanediol (MEG), propanediol-1 ,2 and - 1 ,3, diethylene glycol (DEG), dipropylene glycol (DPG), butanediol-1 ,4, hexanediol-1 ,6, glycerin, trimethylolpropane, bisphenol-A, bisphenol-F and pentaerythritol. Diethylene glycol, mono-ethylene glycol, propandi-ol-1 ,2 and glycerol as a starter molecule, are particularly preferred.

According to the present invention, the functionalities of the polyether polyols are the nominal functionalities of the starter molecules. If mixtures of starter molecules with different functionalities are used, fractional functionalities can be obtained. Influences on the functionality, for example through side reactions, are not considered in the nominal functionality.

In a particularly preferred embodiment, the polyetherols for use in the present invention are prepared using alkaline catalysts, more preferred alkali hydroxides and in particular potassium hydroxide. Especially preferred polyether polyols are selected from the group, consisting of polyethylene glycol, poly propylene glycol, polytetramethylene oxide ore mixtures thereof.

Suitable polyester polyols, especially polyester diols, may be produced, for example, from dicarboxylic acids having 2 to 12 carbon atoms, preferably 4 to 10 carbon atoms, and polyhydric alcohols. Examples of useful dicarboxylic acids include: aliphatic dicarboxylic acids, such as succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid and sebacic acid, or aromatic dicarboxylic acids, such as phthalic acid, isophthalic acid and terephthalic acid. The dicarboxylic acids may be used individually or in the form of mixtures, for example in the form of a mixture of succinic acid, sebacic acid and adipic acid. For production of the polyester diols, it may possibly be advantageous to use, rather than the dicarboxylic acids, the corresponding dicarboxylic acid derivatives such as carboxylic diesters having 1 to 4 carbon atoms in the alcohol radical, for example dimethyl terephthalate or dimethyl adipate, carboxylic anhydrides, for example succinic anhydride, glutaric anhydride or phthalic anhydride, or carbonyl chlorides. Examples of polyhydric alcohols are glycols having 2 to 10, preferably 2 to 6, carbon atoms, for example ethylene glycol, diethylene glycol, butane-1 ,4-diol, pentane-1,5-diol, hexane-1 ,6-diol, decane-1, 10-diol, 2,2-dimethylpropane-1 ,3-diol, propane- 1 ,3-diol, 2-methylpropane-1 ,3-diol, 3-methylpentane-1 ,5-diol or dipropylene glycol. The polyhydric alcohols may be used individually or as mixtures, for example in the form of a butane-1, 4-diol and/or propane-1, 3-diol mixture. In addition, it is also possible to include small amounts of up to 3% by weight of the total reaction mixture of higher- functionality polyols of low molecular weight, for example 1,1,1 -trimethylolpropane or pentaerythritol.

Besides polymeric polyols or preferably in addition to polymeric polyols chain extenders may be used, for example, be commonly known aliphatic, araliphatic, aromatic and/or cycloaliphatic compounds having a molecular weight of 50 to 299 g/mol, preferably bifunctional compounds, for example alkanediols having 2 to 10 carbon atoms in the alkylene radical, for example diols selected from the group consisting of C2- to Ce-diols, preferably butane-1, 4-diol, hexane-1, 6-diol and/or di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona- and/or decaalkylene glycols having 3 to 8 carbon atoms, preferably unbranched alkanediols, especially propane-1, 3-diol, butane-1, 4-diol and hexane-1, 6-diol. Also suitable as chain extenders within the context of the present invention are branched compounds such as cyclo- hexyl-1,4-dimethanol, 2-butyl-2-ethylpropanediol, neopentyl glycol, 2,2,4-trimethylpentane-1 ,3-diol, pinacol, 2- ethyl hexane-1, 3-diol, cyclohexane-1, 4-diol or N-phenyldiethanolamine. Compounds having OH and NH groups are also suitable, such as 4-aminobutanol for example. It is also possible in accordance with the invention to use mixtures of two or more chain extenders.

It is especially preferred that the at least one compound comprising at least one group reactive towards isocyanates (b) comprises polyether polyols and more preferred consists of polyether polyols. Especially preferred polyether polyols are polypropylene glycols, polyethylene glycols and polytetramethylene glycols, most preferred polypropylene glycols and/or polyethylene glycols.

The acidic phosphoric acid ester (c) according to the invention comprises at least one OH-group bound to a phoshorous atom and can be obtained from reacting hydroxyl containing compounds with phosphoric acid or phosphoryl chloride. Preferably the acidic phosphoric acid ester is described by formula (I): where R¹ is hydrogen or an organic residue; and R² is an organic residue. If R¹ is an organic residue, it may be identical to or different from the organic residue R². The organic residues R¹ and R² in formula (I) are preferably alkyl residues. They preferably consist of branched or unbranched alkanes with 2 to 18 carbon atoms. It does not matter whether a primary or secondary carbon atom of an alkane is bound to the oxygen atom of the phosphoric acid residue. Particularly preferred organic residues in formula (I) are selected from the group consisting of propyl, isopropyl, butyl, isobutyl, pentyl, isopentyl, hexyl, isohexyl, heptyl, isohepyl, octyl, isooctyl, nonyl, decyl, isodecyl, dodecyl and tridecyl residues. Residues selected from the group consisting of octyl, decyl and dodecyl residues are particularly preferred. Furthermore, according to the invention, the organic residues R¹ and R² can also contain ether groups or halogens.

Preferably, the acidic phosphoric acid ester (c) contains at least one phosphoric acid ester selected from the group consisting of phosphoric acid mono-n-octyl ester, phosphoric acid mono-n-decyl ester, phosphoric acid mono-iso- decyl ester, phosphoric acid mono-n-dodecyl ester, phosphoric acid mono-n-octadecyl ester, phosphoric acid di-n- octyl ester, phosphoric acid mono-octylmono-n-decyl ester, phosphoric acid di-iso-decyl ester and phosphoric acid di-n-octyldecyl ester. In another preferred embodiment of the invention, the acidic phosphoric acid ester (c) is a mixture of different acidic phosphoric acid esters, each of which is distinguished by its different organic residues R¹ and/or R².

Especially preferred the acidic phosphoric acid ester (c) is selected from ethyl hexyl-dihydrogen phosphate, bis(2- ethy lhexy l)hydrogen phosphate or mixtures thereof.

It has been found that an excellent control of the reaction speed of the Isocyanate/Polyol reaction in a prepolymer synthesis is possible when the acidic phosphoric acid ester is present during the reaction at concentrations of preferably 0.5 % by weight and below, more preferred below 0.3 % by weight, even more preferred 0.2 and below and especially 0.15 % by weight and below. At higher concentrations the control of the reaction speed is not possible, and the final synthesized prepolymer becomes unsuitable for follow-up uses in polyurethane formulations due to antagonistic interaction of the acidic phosphoric acid ester with amine catalysts contained in the majority of polyurethane formulations. In addition, the acidic phosphoric acid ester increases the viscosity of the aromatic polyisocya- nate. Concentrations of more than 0.2 % by weight, based on the total weight of the aromatic isocyanate and the acidic phosphoric acid ester, result in a considerable increase of the viscosity. Especially at concentrations of more than 0.5 % by weight and even more of more than 1 % by weight result in undesirable high viscosity of the aromatic isocyanate. Therefore, the content of the at least one acidic phosphoric acid ester (c), based on the total weight of compounds (a) to (c), is preferred to be 0.0001 % by weight to 0.2 % by weight, preferably 0.001 % by weight to 0.1 % by weight and especially preferred 0.01 % by weight to 0.08 % by weight.

In one embodiment of the process in addition to at least one aromatic polyisocyanate (a), at least one compound comprising at least one group reactive towards isocyanates (b) and at least one acidic phosphoric acid ester (c), optionally additives as stabilizers, surface active substances, chain extenders, fillers, flame retardants, dyes, pigments, water scavengers, IR-absorbing materials, plasticizers, antistatic agents, fungistatic agents, bacteriostatic agents, hydrolysis inhibitors, antioxidants, acyl chlorides, as diethylene glycol-bis-chloroformiate (DIBIS) or benzoyl chloride, acids as phosphoric acid, cell regulators and mixtures thereof may be added.

Examples of stabilizers are benzoyl chloride and paratoluenesulfonyl isocyanate. Stabilizers, as preferably benzoyl chloride, can be added to inhibit the reaction of the isocyanate group with water in the ambient air. Due to its negative properties, the amount of benzoyl chloride is preferably limited to a minor amount. The benzoyl chloride generally is present in amount of from 0 to 1 % by weight based on the overall weight of the isocyanate prepolymer composition.

Examples of surface-active substances include for example those that promote homogenization of the starting materials and are optionally also suitable for regulating the cell structure of the ultimately produced plastics. Examples include for example emulsifiers, such as sodium salts of castor oil sulfates or of fatty acids and salts of fatty acids with amines, for example diethylamine oleate, diethanolamine stearate, diethanolamine ricinoleate, salts of sulfonic acids, for example alkali metal or ammonium salts of dodecylbenzene- or dinaphthylmethanedisulfonic acid and ricinoleic acid; foam stabilizers, such as siloxane-oxyalkylene copolymers and other organopolysiloxanes, ethoxylated alkylphenols, ethoxylated fatty alcohols, paraffin oils, castor oil esters or ricinoleic esters, turkey red oil and peanut oil, and cell regulators, such as paraffins, fatty alcohols and dimethylpolysiloxanes. Silicone stabilizers are particularly preferred.

In one embodiment the addition of additives, added substances or auxiliaries is carried out after the manufacture of the prepolymers before or after any cooling step but before containerizing.

Further particulars on auxiliaries and additives may be found for example in Kunststoffhandbuch, Volume 7, "Polyurethane", Carl-Hanser-Verlag Munich, 1st edition, 1966, 2nd edition, 1983 and 3rd edition, 1993

In a preferred embodiment of the present invention the reaction mixture is reacted at temperatures of 40 to 140 °C, more preferred 50 to 120 °C and most preferred 60 to 100 °C. The reaction time is preferably at least 10 minutes, more preferred 15 to 120 minutes and especially preferred 20 to 80 minutes. In a preferred embodiment the reaction is conducted in a large vessel having a capacity of at least 100 kg, more preferred 200 to 20000 kg and especially preferred 300 to 10000 kg of the reaction mixture. In a preferred embodiment the reaction vessel is equipped with a mechanical stirrer.

It was found that the process according to the present invention accelerates the polyurethane prepolymer reaction compared to the state-of-the-art acyl chlorides under safe conditions without introduction of significant amounts of water or miscibility issues, in a well-controllable way. The acidic phosphoric acid esters (c) are easy to use due to being liquid, readily miscible with isocyanates, and still give a well-controllable boost in reactivity during prepolymer synthesis, despite being an acid. A pre-blending with the used polyol was not required, as it is common for phosphoric acid. Depending on acidic phosphoric acid ester concentration and base prepolymer formulation, the boost in reaction speed was about 50-2000%, which significantly increases the capacity of the prepolymer plants, without reducing safety, product quality, or significantly interacting with amine catalysts in a follow-up polyurethane synthesis in a strong antagonistic way.

The polyurethane prepolymers produced by the process according to the invention may be used for producing polyurethanes, in particular polyurethane foams, for example rigid foams, flexible foams, semi-rigid foams or integral foams, adhesives such as for example packaging adhesives and hotmelt adhesives, sealants, elastomers, coatings, thermosets and thermoplastics. Such polyurethane foams according to the present invention can be used for example in automotive interieur or for the production of shoe soles.

The invention will be exemplified by the following examples:

Used materials:

Acidifiers:

DBP: Dibutylphosphate: Technical grade, purity >97%

M2EHP: Phosphoric acid, 2-Ethylhexyl ester (EXOfos PA080S, PCC)

DI BIS: Diethylene glycol-bis-chloroformiate

BZCL: Benzoyl chloride, purity >99%

PA: Phosphoric Acid, technical grade, 85 wt.-% purity, water content 15 wt.-%

PPA: Polyphosphoric acid, technical grade, 115 % H3PO4-basis (acidimetric determination)

DBPHOSPHIT: Dibenzoylphosphite

B2EHP: Bis(2-Ethylhexyl)phosphate DPPHOSPHAT: Diphenylphosphate DBPHOSPHAT: Dibenzoylphosphate THCL: Thionyl chloride Other materials:

ISO 1 : 50/50 blend of 2,4-Methylenediphenyldiisocyanate and 4,4'-Methylenediphenyldiisocyanate

ISO 2: 4,4'-Methylenediphenyldiisocyanate

POLY 1 : Difunctional Polypropylene glycol, molecular weight Mw = 2000 g/mol, KOH-catalyzed

POLY 2: Difunctional Polypropylene glycol capped with ethylene glycol block, OH-value 29 mgKOH/g, KOH- catalyzed

POLY 3: Difunctional Polytetramethyleneetherglycol, Mw=2000 g/mol

POLY 4: Trifunctional polyether polyol based on polypropylene glycol with glycerine as starter molecule, KOH- catalyzed, OH-value 400 mgKOH/g

CAT: Triethylene diamine, 33 wt.-% dissolved in dipropylene glyol

Methods:

Viscosities are measured according to DIN EN ISO 3219 and with plate/cone measurement geometry at a shear rate of 40 1/s at 25 °C.

The weight fraction of NCO groups in the materials is determined by titration in accordance with DIN EN ISO 14896:2009

Experimental procedure for Trials 1 - 39:

The required amount of Isocyanate ISO1 or ISO2, preheated to 60°C, is charged into a 2 liter glass vessel equipped with a heating mantle and stirrer, and stirred constantly with a paddle stirrer at 120 rounds per minute under a blanket of continuously added dry nitrogen gas with a volume stream of 1 liter per minute until the end of the experiment. The acidifier is added, and homogenized with the isocyanate under constant stirring for a minute. In case of multiple polyols, the polyols are pre-mixed at room temperature. The polyol or the polyol mixture is added completely into the glass vessel within 10 seconds under constant stirring. All quantities of the ingredients are expressed in parts by weight unless otherwise specified.

The beginning of polyol or polyol mixture charging is defined as starting time of the reaction. After charging the polyol or polyol mixture, the mixture is heated within 10 minutes to the reaction temperature RTEMP and kept at this temperature for 300 minutes, under constant stirring.

Samples of 10 ml for viscosity determination and NCO determination are taken 0 minutes, 2 minutes, 6 minutes, 10 minutes, 14 minutes, 18 minutes, 23 minutes, 30 minutes, 30 minutes, 60 minutes, 90 minutes, 120 minutes, 180 minutes, 240 minutes and 300 minutes after beginning of the addition of the polyol or polyol mixture to the vessel.

: partial gelation and c umps

"Time to Viscosity of >2400 mPas at 25 °C” describes the time frame between start of the reaction at polyol addition until the first sample being taken that reaches a viscosity above 2400 mPas, measured at 25 °C

"Time to NCO-content <10.00 wt.-%” describes the time frame between start of the reaction at polyol addition until the first sample being taken that reaches a NCO content below 10.00 wt.-%

Both values are a measure of the conversion rate of the reaction. Final viscosity at 25°C for trials 1 to 22 is measured after 300 minutes reaction at 80°C (400 minutes in the case of trial 3 and 4) and additional storage at 25°C for one day. Due to the fact that it is in the range of 2450 mPas to 3000 mPas measured at 25°C for all trials 1 to 22, the value of >2400 mPas measured at 25 °C is an indicator of nearly complete conversion at which fill-off operations can begin.

Final NCO content for trials 1 to 22 is measured after 300 minutes reaction at 80°C (400 minutes in the case of trial 3 and 4) and additional storage at 25°C for one day, and is in the range of 9.5 to 9.8 wt.-% for all trials, making the value of <10.00 wt.-% an indicator of nearly complete conversion.

While acyl chlorides like DIBIS, as shown in Trial 2 to 4, lead to a desirable low final viscosity that indicates a lack of undesired crosslink side reactions or urea formation, no acceleration of the prepolymer reaction is observed at any concentration.

Phosphoric acid, as shown in trial 13 to 15, leads to a significant acceleration of the reaction, but also causes a significant undesired increase in the final viscosity with increasing concentration due to the contained water content and thus resulting in the formation of urea groups in the prepolymer, which have a high tendency to form hydrogen bonds.

Trial 17 and 18 are showing that a free OH group is needed in the acidic phosphoric acid ester to observe any accelerating effect. The phosphite, being a phosphorous compound without free acid group, does not lead to any acceleration of the prepolymer reaction.

Trial 5 to 12 show that acidic phosphoric acid esters not only allow an acceleration of the prepolymer reaction, but also a good control of the reaction speed by the added amount, which means that the reaction speed and therefore the resulting thermal output of the reaction within a specific time frame can be tailored to the cooling capacity of a specific reactor by the amount of added acidic phosphoric acid ester to maximize reactor efficiency within the cooling capabilities of the reactor. This is especially important for large-scale as ton-scale reactors with limited cooling capabilities, to avoid any thermal runaway reaction.

Trial 10 to 12 show that the acceleration by the acidic phosphoric acid ester is not unlimited. Between 0.09 % by weight and 0.15 % by weight, there is only a minor further increase of the reaction speed, making it technically non- advantageous to exceed these concentrations. This also shows that the acceleration is self-limiting, making it highly desirable for a safe and stable synthesis of prepolymers. Locally high concentrations due to insufficient homogenization will not lead to a runaway reaction.

Trial 16 shows that anhydrous polyphosphoric acid is not an adequate solution due to its poor solubility in nonaqueous media. The formation of inhomogenous gel clumps was observed.

"Time to Viscosity of >5800 mPas at 50 °C” describes the time frame between start of the reaction at polyol addition until the first sample being taken that reaches a viscosity above 5800 mPas, measured at 50 °C

"Time to NCO-content <6.00 wt.-%” describes the time frame between start of the reaction at polyol addition until the first sample being taken that reaches an NCO content below 6.00 wt.-%

Both values are a measure of the conversion rate of the reaction. Final viscosity at 50°C for trials 23 to 29 is measured after 300 minutes reaction at 80°C and additional storage at 25°C for one day. Due to the fact that it is in the range of 5842 mPas to 6961 mPas measured at 50°C for all trials 1 to 22, the value of >5800 mPas measured at 50 °C is an indicator of nearly complete conversion at which fill-off operations can begin. Final NCO content for trials 23 to 29 is measured after 300 minutes reaction at 80°C (400 minutes in the case of trial 3 and 4) and additional storage at 25°C for one day, and is in the range of 5.6 to 5.9 wt.-% for all trials, making the value of 6.00 wt.-% an indicator of nearly complete conversion.

**: Trial 29 is a comparison example to trial 28, but unlike described in the experimental procedure for trial 28, in trial 29 the acidifier DBPHOSPHAT is added after 300 minutes after polyol dosage into the reaction mixture, instead of addition into the isocyanate before the polyol is added as in trial 28. No acceleration effect compared to the Acyl Chloride benchmark in trial 23 is observed, showing that the presence of the acidic phosphoric acid ester in the reaction mixture is needed during the reaction to yield any acceleration effect.

***: Material was visibly turbid after 1 day

Inventive trials 25, 26 and 28 also show in this prepolymer, which significantly differs from the compositions of the prepolymers in trials 1 to 22, that the presence of acidic phosphoric acid esters during the prepolymer formation reaction has a significantly accelerating effect. This shows the broad applicability in polyurethane prepolymers of this invention.

While the accelerating effect is even stronger with the non-inventive phosphoric acid used in trial 24, it causes side reactions due to its water content which increase final viscosity at 50°C significantly in an undesired way, and also leads to turbidity which is not observed in all other trials, making it undesirable as an acidifier that is dosed directly into the isocyanate.

***: Material was visibly turbid after 1 day *: partial gelation and clumps

In trials 29 to 39, it is seen that only in the presence of the inventive acidic phosphoric acid esters the polyurethane prepolymer formation reaction is adequately accelerated without decrease in the product quality as it is the case with Trial 31 (Phosphoric Acid I PA) and Trial 39 (Polyphosphoric Acid I PPA)

Experimental procedure for Trials 40 to 46 75.89 grams of a prepolymer synthesized according to one of the Trials 1 to 22 was charged at room temperature into a round polypropylene cup with 7 cm diameter. 24.11 gram of a previously homogenized mixture of 99% POLY4 and 1% CAT was added, the reaction mixture was immediately well homogenized for 20 seconds at 1950 rounds per minute (RPM) with a tumbler mixer (Speedmixer TM).

The polypropylene cup was placed immediately into a Shyodu Gel-Time, Type 100, Version 2012 and the reaction mixer was stirred with the associated wire mixer at 20 RPM until the viscosity of the mixture exceeded the stirring power of the gel timer. This time is given as the "open time” of the system.

The trials 40 to 46 show that it is not desirable to increase the concentration of the used acidic phosphoric acid ester as high as possible, to achieve as high reaction speeds as possible in the prepolymer synthesis step. In this exemplary amine-catalyzed compact 2 component system, the acidic phosphodic acid ester show to have a negative pro- longing effect on the open time due to the antagonistic interaction of the acidic phosphoric acid ester with the alkaline amine catalyst. Despite only a minor difference in the acceleration of the prepolymer synthesis in Trial 9 and Trial 12, the 2 component systems based on these prepolymers, Trial 45 and 46, show a drastic difference in open time, showing that concentrations of acidic phosphoric acid esters significantly exceeding these described concentrations have neither a technical advantage regarding acceleration of the prepolymer synthesis step, nor are they desirable for follow-up use in amine-catalyzed polyurethane systems.

This makes it desirable to use acidic phosphoric acid esters only in a specific concentration range in the prepolymer production step to avoid significant negative interaction especially with amine catalysts in follow-up use in 2 component polyurethane formulations.

Trial 42 shows that phosphoric acid as an accelerating acidifier for the prepolymer synthesis is an inferior solution to acidic phosphoric acid esters due to observed foaming in this exemplary compact two-component system.

Claims

1 . Process for the production of a polyurethane prepolymer comprising mixing a) at least one aromatic polyisocyanate, b) at least one compound comprising at least one group reactive towards isocyanates and c) at least one acidic phosphoric acid ester to form a reaction mixture and reacting the mixture to obtain the polyurethane prepolymer, wherein the aromatic polyisocyanate (a) and the compound comprising at least one group reactive towards isocyanates are reacted to form the polyurethane prepolymer in presence of the acidic phosphoric acid ester (c) and wherein the at least one acidic phosphoric acid ester (c) is present in an amount of 0.0001 to 0.5 % by weight, based on the total weight of compounds (a) to (c).

2. Process according to claim 1 characterized in that the mixture is heated to a temperature of 40 to 140 °C for a period of at least 20 minutes.

3. Process according to claim 1 or 2, characterized in that the polyurethane prepolymer obtained has an NCO content of 1 to 40 % by weight.

4. Process according to any of claims 1 to 3, characterized in that the at least one compound comprising at least one group reactive towards isocyanates (b) comprises polyetherpolyols.

5. Process according to claim 4 characterized in that the polyetherpolyol is selected from the group consisting of polypropylene glycol, polyethylene glycol, polytetramethyleneether glycol or mixtures thereof.

6. Process according to any of claims 1 to 5, characterized in that the aromatic polyisocyanate (a) comprises at least 90 % by weight, based on the total weight of isocyanate (a) of an isocyanate, selected from the group consisting of diphenylmethane 2,2'-diisocyanate, diphenylmethane 2,4'-diisocyanate, diphenylme-thane 4,4'- diisocyanate, higher homologues of diphenylmethane diisocyanate and mixtures of two or more thereof.

7. Process according to claim 6, characterized in that the aromatic polyisocyanate (a) comprises at least 90 % by weight, based on the total weight of isocyanate (a) of diphenylmethane 4,4'-diisocy anate.

8. Process according claim 7, characterized in that the isocyanate (a) is 4,4'-MDI.

9. Process according to any of claims 1 to 8, characterized in that the acidic phosphoric acid ester comprises 2-ethylhexyldihydrogenphosphate and/or bis(2-ethylhexyl) monohydrogenphosphate.

10. Process according to any of claims 1 to 9, characterized in that the process is conducted in a reaction vessel having a capacity of at least 100 kg of the reaction mixture.

11. Process according to any of claims 1 to 10, characterized in that the polyurethane prepolymers obtained have a viscosity of 10 to 10.000 mPas at 25 °C.

12. Polyurethane prepolymer obtained by a process according to any of claims 1 to 11 .

13. Use of a polyurethane prepolymer according to claim 12 for the production of polyurethanes.

14. Use according to claim 13 wherein the polyurethane is selected from the group consisting of polyurethane foams, adhesives, sealants, elastomers, coatings, thermosets and thermoplastics.