FLEXIBLE POLYURETHANE FOAMS WITH HIGH WATER ABSORPTION CAPACITY.

Abstract

The present invention relates to a process for producing a flexible hydrophilic polyurethane foam, wherein (a) at least one polyurethane prepolymer having an isocyanate content of 5 to 10% by weight, depending on the weight of the isocyanate prepolymer (a), is mixed with (b) at least one aqueous component in a weight ratio between the polyurethane prepolymer (a) and the aqueous component (b) of 3:1 to 1:1.2 and allowed to react to form the flexible polyurethane foam, wherein the polyurethane prepolymer (a) can be obtained by mixing and reacting at least one isocyanate (a1) with at least one polyetherole (a2), wherein the isocyanate (a1) comprises a methylenediphenylene diisocyanate and the polyetherole (a2) has a hydroxyl number of 30 to 60 mg KOH / g and can be obtained by alkoxylation of at least one difunctional and / or trifunctional initiator molecule with ethylene oxide and hydrogen oxide. propylene and ethylene oxide content,based on the total weight of alkylene oxide, is at least 60% by weight, based on the total weight of the polyether polyol (a2), wherein the polyetherol (a2) can be obtained by propoxylation of the initiator molecules in a first step to a hydroxyl number of 400 to 1200 mg KOH / g, followed by alkoxylation of the propoxylated initiator molecule with a mixture of ethylene oxide and propylene oxide and, finally, ethoxylation of the alkoxylation product thus obtained with 2 to 10% by weight of ethylene oxide, depending on the alkylene oxide used to produce the polyetherol (a2), and the aqueous component (b) comprises at least 90% by weight of water (b1) and up to 10% by weight of non-silicone surfactants (b2). The present invention further relates to a flexible polyurethane foam that can be obtained by such a process and to the use of said flexible polyurethane foam for treating wounds.

Description

Flexible polyurethane foams with high water absorption capacity The present invention relates to a process for producing a flexible hydrophilic polyurethane foam, wherein (a) at least one polyurethane prepolymer having an isocyanate content of 5 to 10% by weight, depending on the weight of the isocyanate prepolymer (a), is mixed with (b) at least one aqueous component in a weight ratio between the polyurethane prepolymer (a) and the aqueous component (b) of 3:1 to 1:1.2 and allowed to react to form the flexible polyurethane foam, wherein the polyurethane prepolymer (a) can be obtained by mixing and reacting at least one isocyanate (a1) with at least one polyetherole (a2), wherein the isocyanate (a1) comprises a methylenediphenylene diisocyanate and the polyetherole (a2) has a hydroxyl number of 30 to 60 mg KOH / g and can be obtained by alkoxylation of at least one difunctional and / or trifunctional initiator molecule with ethylene oxide and hydrogen oxide. propylene and ethylene oxide content,based on the total weight of alkylene oxide, is at least 60% by weight, based on the total weight of the polyether polyol (a2), wherein the polyetherol (a2) can be obtained by propoxylation of the initiator molecules in a first step to a hydroxyl number of 400 to 1200 mg KOH / g, followed by alkoxylation of the propoxylated initiator molecule with a mixture of ethylene oxide and propylene oxide and, finally, ethoxylation of the alkoxylation product thus obtained with 2 to 10% by weight of ethylene oxide, depending on the alkylene oxide used to produce the polyetherol (a2), and the aqueous component (b) comprises at least 90% by weight of water (b1) and up to 10% by weight of non-silicone surfactants (b2). The present invention further relates to a flexible polyurethane foam that can be obtained by such a process and to the use of said flexible polyurethane foam for treating wounds. The use of flexible hydrophilic polyurethane foams for wound treatment is known and described, for example, in WO 2012055834, WO 2004074343, EP 2336211 and WO 9429361. WO 2012055834 and EP 2336211 disclose the production of flexible hydrophilic polyurethane foams starting from aliphatic isocyanates. For this purpose, an isocyanate prepolymer reacts with an aqueous component. The disadvantages of such aliphatic isocyanates are their high volatility and, consequently, the resulting health risks during processing. Furthermore, aliphatic isocyanates are only weakly reactive during processing, resulting in slow and energy-intensive processes. Acceleration using catalysts is problematic, as these can migrate out of the foam, rendering such a foam unsuitable for wound care. WO 2004074343 and WO 9429361 describe flexible hydrophilic polyurethane foams produced by reacting polyurethane prepolymers based on MDI and hydrophilic polyetherols with an aqueous component. However, the processes described in these documents result in flexible polyurethane foams with water absorption properties that can be improved depending on the absolute amount of water absorbed and the rate of water absorption, and the wet tensile strength can also be enhanced. Therefore, the object of the present invention was to provide flexible polyurethane foams Αηηκηη / ι ζηζ / Β / γ that are suitable for treating wounds, where the mechanical properties, in particular the wet tensile strength, and water absorption are further improved compared to known hydrophilic flexible polyurethane foams. The object according to the invention is achieved by means of a flexible polyurethane foam, which can be obtained through a process in which (a) at least one polyurethane prepolymer having an isocyanate content of 5 to 10% by weight, depending on the weight of the isocyanate prepolymer (a), is mixed with (b) at least one aqueous component in a weight ratio between the polyurethane prepolymer (a) and the aqueous component (b) of 3:1 to 1:1.2 and allowed to react to form the flexible polyurethane foam, wherein the polyurethane prepolymer (a) can be obtained by mixing and reacting at least one isocyanate (a1) with at least one polyetherol (a2), wherein the isocyanate (a1) comprises a methylenediphenylene diisocyanate and the polyetherol (a2) has a hydroxyl number of 30 to 60 mg KOH / g and can be obtained by alkoxylation of at least one difunctional and / or trifunctional initiator molecule with oxide of ethylene and propylene oxide and the ethylene oxide content,The polyether polyol (a2) is at least 60% by weight of the total alkylene oxide, depending on the total weight of the polyether polyol (a2). The polyether polyol (a2) can be obtained by propoxylation of the initiator molecules in a first step to a hydroxyl number of 400 to 1200 mg KOH / g, followed by alkoxylation of the propoxylated initiator molecule with a mixture of ethylene oxide and propylene oxide, and finally, ethoxylation of the alkoxylation product thus obtained with 2 to 10% by weight of ethylene oxide, depending on the alkylene oxide used to produce the polyether polyol (a2). The aqueous component (b) comprises at least 90% by weight of water (b1) and up to 10% by weight of non-silicone surfactants (b2). The present invention further relates to a process for producing said flexible polyurethane foam and its use for treating wounds. The flexible foam according to the invention preferably has a density of 70 to 140 g / L, with particular preference from 80 to 120 g / L and especially from 85 to 115 g / L. In the context of the present invention, the density was determined in accordance with Annex C of the European standard EN 14315-2. Furthermore, the flexible polyurethane foams according to the invention preferably have a hardness of 1.0 to 5.0, with particular preference from 2.5 to 3.5. In the context of the present invention, the hardness was determined in accordance with Asker C ASTM D 2240. Flexible polyurethane foams according to the invention are also characterized by excellent water absorption capacity. This was determined as explained in the examples and is preferably greater than 10 g per gram of foam, with a particular preference for 12 to 20 g per gram of foam. To produce the flexible polyurethane foam according to the invention, at least one polyurethane prepolymer (a) having an isocyanate content of 5 to 10% by weight, preferably 6 to 9% by weight, and especially 6.5 to 8.5% by weight, depending on the weight of the isocyanate prepolymer (a), is mixed with at least one aqueous component (b) and reacted to form the polyurethane foam. The isocyanate content is known and can be determined, for example, by titration or spectrometry. In the context of the invention, this is specified as 0.1%. Ann^nn / i ζηζ / Β / γ in nearest weight. The isocyanate prepolymer is obtained in this case by mixing and reacting at least one isocyanate (a1) with at least one polyetherol (a2). The isocyanates (a) used are preferably aromatic di- and polyisocyanates; these are known in polyurethane chemistry and comprise, for example, toluene diisocyanate isomers and higher isomeric and polycyclic homologues of methylenediphenylene diisocyanate. It is essential to the invention that the isocyanates comprise methylenediphenylene diisocyanate (a1). Methylenediphenylene diisocyanate (hereafter also referred to as MDI) includes the isomers 2,2'-MDI, 2,4'-MDI, and 4,4'-MDI. The proportion of 4,4'-methylenediphenylene diisocyanate is preferably from 30 to 100% by weight and more preferably from 45 to 90% by weight, with particular preference from 55 to 85% by weight, and the proportion of 2,4'-methylenediphenylene diisocyanate is from 0 to 70% by weight and more preferably from 10 to 55% by weight and with particular preference from 15 to 45% by weight, in each case depending on the total weight of the isocyanate (a).In a particularly preferred embodiment, the isocyanate (a), based on the total weight of isocyanates (a), comprises less than 10% by weight, preferably less than 5% by weight, and especially no isocyanate other than 4,4'-methylenediphenylene diisocyanate and 2,4'-methylenediphenylene diisocyanate. In particular, the isocyanate (a) comprises less than 1% by weight, preferably less than 0.01% by weight, and more preferably no 2,2'-methylenediphenylene diisocyanate. The polyetherols (a2) used are one or more polyetherols having a hydroxyl number of 30 to 60 mg KOH / g, preferably 35 to 50, which can be obtained by alkoxylation of at least one difunctional and / or trifunctional initiator molecule with ethylene oxide and propylene oxide, wherein the ethylene oxide content, as a function of the total weight of alkylene oxide, is at least 60% by weight, preferably 65 to 95% by weight and particularly 70 to 80% by weight, and wherein the propylene oxide content is greater than 0 to 40% by weight, preferably 5 to 35% by weight and particularly 20 to 30% by weight. In the production of the polyetherol (a2), in addition to ethylene oxide and propylene oxide, less than 5% by weight and in particular no additional alkylene oxides are preferably used. Furthermore, according to the invention, the content of primary hydroxyl groups of the polyetherol (a2) is at least 90% by weight, preferably 95% by weight to 100% by weight, with particular preference 99% by weight to 100% by weight and in particular 100%, in each case depending on the total weight of the hydroxyl groups of the polyether polyol (a2). Polyetherols (a2) can be produced by known processes, for example, by anionic polymerization of alkylene oxides with the addition of at least one initiator molecule, preferably comprising 2 to 4, and particularly 2 to 3, reactive hydrogen atoms in bonded form in the presence of catalysts. Fractional functionalities can be obtained by using mixtures of initiator molecules with different functionalities. The nominal functionality ignores the effects on functionality due, for example, to side reactions.The catalysts used may include alkali metal hydroxides, for example, sodium zinc hydroxide or potassium hydroxide, or alkali metal alkoxides, for example, sodium methoxide, sodium ethoxide, potassium ethoxide, or potassium isopropoxide. In the case of cationic polymerization, Lewis acids may be used as catalysts, for example, antimony pentachloride, boron trifluoride etherate, or bleaching earth. Amine alkoxylation catalysts may also be used, for example, dimethylethanolamine (DMEOA), imidazole, and imidazole derivatives. Furthermore, the catalysts used may include double metal cyanide compounds, known as DMC catalysts. Compounds containing hydroxyl groups or amine groups are suitable as starter molecules, for example, ethylene glycol, diethylene glycol, glycerol, trimethylolpropane, pentaerythritol, methylamine, ethylamine, isopropylamine, butylamine, benzylamine, aniline, toluidine, toluenediamine (TDA), naphthylamine, ethylenediamine, diethylenetriamine, 4,4'-methylenediamine, 1,3-propanediamine, 1,6-hexanediamine, ethanolamine, diethanolamine, and other dihydric or polyhydric alcohols or monoamines or diamines. The starting molecule for producing polyetherol (a2) is preferably selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol, trimethylolpropane and mixtures thereof, with particular preference given to the use of glycerol, especially exclusively glycerol, as the starting molecule. The polyetherol (a2) according to the invention can be obtained by propoxylating an initiator molecule in a first step to a hydroxyl number of 400 to 1200 mg KOH / g, preferably 450 to 800 mg KOH / g, followed by alkoxylation of the propoxylated initiator molecule with a mixture of ethylene oxide and propylene oxide, and by ethoxylation of the alkoxylation product thus obtained with 2 to 10 wt%, preferably 3 to 5 wt%, of ethylene oxide, depending on the alkylene oxide used to produce the polyetherol (a2). It is essential to the invention that, after propoxylation of the initiator molecule, it contains on average at least one molecule of propylene oxide. If the starter molecules containing propylene oxide are already in use, such as tripropylene glycol, for example, this is considered to be already propoxylated, but can be further propoxylated to a hydroxyl number of 400 mg KOH / g.As a result, a polyetherol (a2) is obtained comprising an ethylene oxide terminal group containing 2 to 10 wt%, preferably 3 to 5 wt%, of ethylene oxide, depending on the total weight of the alkylene oxide used. In this case, it is not necessary to purify the propoxylated initiator molecule after the propoxylation or alkoxylation of the propoxylated initiator molecule and the mixture of ethylene oxide and propylene oxide after alkoxylation and before ethoxylation. To monitor the reaction progress, the conversion of the alkylene oxide can be spectroscopically monitored, for example, by IR spectrometry. Before the addition of the final ethylene oxide, the conversion of the alkylene oxide is preferably verified by spectroscopic methods, so that essentially no unreacted propylene oxide is present in the reaction mixture.This means that the proportion of unreacted propylene oxide before the addition of ethylene oxide is less than 1% by weight, preferably less than 0.5% by weight, more preferably less than 0.1% by weight and especially less than 0.01% by weight, based on the total weight of alkylene oxide used up to this time point. The polyols (a2) according to the invention are preferably purified from any Ann^nn / i ζηζ / Β / γ alkoxylation catalyst still present after production when they are put in contact with an acidic ion exchange resin, for example, Amberlite. The alkali metal content of the polyol (a2) is preferably less than 50 ppm, with particular preference for less than 30 ppm and especially less than 20 ppm. The alkali metal content is generally measured in this case by titration. In addition to polyetherol (a2), it is preferred not to use additional compounds having isocyanate-reactive groups for the production of the polyisocyanate prepolymer (a). According to the invention, the aqueous component (b) used is a component comprising at least 90% by weight, preferably 94 to 100% by weight, with particular preference 96 to 99.5% by weight and especially 97 to 99% by weight of water (b1) and up to 10% by weight, preferably 0 to 6% by weight, with particular preference 0.5 to 4% by weight and especially 1 to 3% by weight of non-silicone surfactants (b2). The silicone-free surfactants (b2) are preferably compounds that are soluble in water at 20°C to a degree of at least 5% by weight and that have isocyanate-reactive groups. At a concentration of 1% by weight in water at 20°C, the surfactants (b2) preferably reduce the surface tension by at least 10 mNm, particularly preferably by 15 mNm and especially by 20 mNm, measured according to DIN EN 14370: 2004-11. The surfactants are preferably alkylene oxide block copolymers having a molecular weight of 800 to 15,000 g / mol, preferably from 1200 to 10,000 g / mol and especially from 2000 to 8000 g / mol.The block copolymers in particular preferentially have a core block of alkylene oxides having at least 3 carbon atoms, such as propylene oxide, butylene oxide, pentylene oxide, and mixtures of such alkylene oxides, preferably propylene oxide, and two end blocks, each of ethylene oxide. In this case, the weight ratio between the core block and the end blocks is preferably from 1:0.5 to 1:5. Such surfactants are available under the trade name Pluriol® from BASF. In addition to water (b1) and surfactants (b2), the aqueous component (b) preferably does not comprise any compound having isocyanate-reactive groups; component (b) particularly preferably does not comprise any amine catalyst and, in particular, no catalyst at all. In a further preferred embodiment, component (b) consists only of water (b1) and surfactants (b2). To produce the flexible hydrophilic polyurethane foams according to the invention, the polyurethane prepolymer (a) and the aqueous component (b) are mixed. The mixing is preferably carried out at a weight ratio of the polyurethane prepolymer (a) to the aqueous component (b) of 2.5:1 to 1.2:1, particularly preferably from 2.2:1 to 1.0:1 and especially from 0.5:1 to 0.8:1. The mixing is preferably carried out at water component temperatures above 0 to 25°C, preferably from 1 to 10°C and especially from 3 to 5°C, and isocyanate component temperatures preferably from 10 to 50°C, particularly from 15 to 40°C and especially from 20 to 35°C. Curing is preferably carried out in the absence of tertiary amines and, in In particular, the mixture of polyisocyanate prepolymer and aqueous component (b) does not comprise any additional substance. The reaction mixture is preferably applied continuously to a release paper and cured, preferably in an oven. The application is generally carried out in layer thicknesses of 1 mm to 10 mm. After curing, the hydrophilic flexible polyurethane foam according to the invention detaches from the release paper. The flexible polyurethane foam obtained according to the invention preferably has a density of 70 to 140 g / L, with particular preference 80 to 120 g / L and especially 85 to 115 g / L and a water absorption capacity of at least 8 g / g, preferably at least 10 g / g and especially 12 to 20 g / g. The water absorption capacity is determined as follows: A solution of 142 mmol of sodium chloride and 2.5 mmol of calcium chloride in 1 liter of demineralized water was used (according to method EN 13726-12002 (solution 1)). From the foam to be evaluated, a 50 mm x 50 mm foam sample is punched out of a 1.6 cm thick foam strip using a punch and immersed for 3 minutes in an aqueous test liquid according to EN 13726-1:2002. The foam is then carefully grasped at one corner without wringing it out, allowed to drain for 10 seconds, and weighed again. The resulting water absorption (the weight of the foam after drainage minus the weight of the dry foam sample) is divided by the weight of the dry sample and reported in g / g. The polyurethane foam according to the invention also exhibits a high water absorption rate.To determine the rate of water absorption in a dry foam sample, as is also used to determine water absorption, 2 mL of the above-specified aqueous test liquid are applied to the surface of the foam sample at room temperature and time is stopped when it has been completely absorbed by the foam sample. The polyurethane foam according to the invention also has very good mechanical properties, for example, high tensile strength in both the wet and dry states. The flexible, hydrophilic polyurethane foam according to the invention can preferably be used in the cosmetic field, for example, as cosmetic pads or wound dressings, or as shoulder pads in clothing. Furthermore, the foam according to the invention can be used for passive climate control in enclosed spaces, such as in vehicles or buildings. It can also be used for reversible liquid absorption, for example, in service carts. Additionally, the foam according to the invention can be used as hearing protection or for absorbing bodily fluids. The polyurethane foam according to the invention is particularly preferred for wound treatment, for example, as a wound dressing. The present invention is illustrated below with the help of examples. The following substances were used: Polyol 1: Glycerol-initiated polyether polyol having a hydroxyl number of 42 mg KOH / g, obtained by addition reaction of 6% by weight of propylene oxide in a first Αηη^ηη / ι ζηζ / Β / γ stage, a mixture of 69% by weight of ethylene oxide and 20% by weight of propylene oxide in a second stage and 5% by weight of ethylene oxide in a third stage by using KOH as a catalyst. Polyol 2: Diethylene-initiated polyether polyol having a hydroxyl number of 42 mg KOH / g, obtained by addition reaction of 6% by weight of propylene oxide in a first stage, a mixture of 69% by weight of ethylene oxide and 20% by weight of propylene oxide in a second stage and 5% by weight of ethylene oxide in a third stage by using KOH as a catalyst. Polyol 3: Glycerol-initiated polyether polyol having a hydroxyl number of 32 mg KOH / g, obtained by alkoxylation of glycerol with a mixture of ethylene oxide and propylene oxide, available under the trade name Voranol® CP-1421 through Dow Chemicals. Polyol 4: Glycerol-initiated block polyether polyol having a hydroxyl number of 42 mg KOH / g, obtained by addition reaction of 26% by weight of propylene oxide in a first step, 74% by weight of ethylene oxide in a second step using KOH as a catalyst. Auxiliary 1: Diglycol bis (chloroformate) Auxiliary 2: Block copolymer composed of a central polyoxypropylene block having a molecular weight of 1750 g / mol and two terminal polyoxyethylene blocks each having a molecular weight of 350 g / mol Auxiliary 3: Block copolymer composed of a central polyoxypropylene block having a molecular weight of 1750 g / mol and two terminal polyoxyethylene blocks each having a molecular weight of 1400 g / mol Aqueous component: Mixture of 98% by weight of water, 1% by weight of auxiliary 2 and 1% by weight of auxiliary 3 ISO 1: Mixture comprising 98.6% by weight of 4,4'-MDI and 1.4% by weight of 2,4'-MDI Iso 2: Mixture of 2.4% by weight of 2,2'-MDI, 48.6% by weight of 2,4'-MDI and 49.0% by weight of 4,4'-MDL Prepolymers were produced according to Table 1. To this end, the isocyanate was initially loaded, the polymer was added dropwise with stirring, and the mixture was stirred at 80°C for one hour. Unless otherwise stated, quantities are given in parts by weight. NCO content was determined in accordance with DIN EN ISO 14896. The viscosity of the prepolymers was determined using a Haake VT500 rotational viscometer in accordance with DIN EN ISO 3219. Ann^nn / i ζηζ / Β / γ Table 1 Prepolymer 1 2 3 4 5 6 7 8 9 10 1> 11 NCO Content 8.7% 12.2% 6.3% 4.6% 12.1% 8.2% 6.5% 4.6% 8.3% 7.2% 8.3%** Iso 1 [%] 31.3 42 2620.6 31.3 29 31.3 Iso 2 42.1 31.3 26 20.6 Auxiliary 1 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Polyol 1 68.69 57.99 73.99 79.39 57.89 68.69 73.99 79.39 Polyol 2 68.69 Polyol 3 70.99 Polyol 4 68.69* and 11 = comparative examples, Ex. 11: polyol 4 is a solid, mp: 35eC AnnAnn / i ζηζ / Β / γ According to Tables 2 to 5, the foams were produced from the prepolymers by mixing them with an aqueous component, where the procedure is as follows: The water component is initially loaded into a small PE beaker (V = 160 mL) at room temperature. The isocyanate is initially loaded into a prepared beaker (V = 550 mL) at room temperature. The Vollrath stirrer is set to its slowest speed. A disposable stirrer with a 65 mm diameter is used for mixing. The water component is then quickly transferred to the clear beaker containing the isocyanate. When stirring begins, the timer is also started. The starting materials are mixed at increasing speeds until a homogeneous mixture is formed. The stirring time depends on the reactivity of the system and should be a maximum of 80% of the creaming time. Table 2: Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Aqueous component (1) 15 18.75 25 30 37.5 45 50 Prepolymer 1 (2) 60 56.25 50 45 37.5 30 25 Mixing ratio (1):(2) 1:4 1:3 1:2 1:1.5 1:1 1:0.67 1:0.5 Density [g / L] 91 74 94 106 140 188 220 Water absorption [g of water per 1 g of foam] 3.1 10.4 15.0 14.1 10.9 8.0 7.1 Table 2 shows that a mixing ratio of the aqueous component (1) to the prepolymer (2) of 1:1 results in the best water absorption capabilities. According to Tables 3 and 4, the aqueous component and the prepolymer were mixed in a 1:2 weight ratio to produce a foam. The NCO content of the isocyanate was varied in Table 3, and different polyols were used to produce the prepolymer in Table 4. Tables 3 and 4 specify the density and water absorption capacity of the resulting foams. Table 3 Example 8 Example 9 Example 10 Example 11 Example 12 Example 13 Example 14 Example 15 Aqueous component 25 25 25 25 25 25 25 25 Prepolymer 2, NCO = 12.2 50 Prepolymer 1 NCO = 8.7 50 Prepolymer 3 NCO = 6.3 50 Prepolymer 4 NCO = 4.6 50 Prepolymer 5 NCO = 12.1 50 Prepolymer 6 NCO = 8.2 50 Prepolymer 7 NCO = 6.5 50 Prepolymer 8 NCO = 4.6 50 Density [g / L] 88 103 114 573* 76 90 132 544 Water absorption [g of water per 1 g of foam] 11.4 14.9 12.2 0.2 5.2 17.2 11.4 0.3 kηηκηη / ι znz / B / v thick, very hard cell structure Table 3 shows that optimum water absorption is obtained at a prepolymer isocyanate content of 6 to 9% by weight. Table 4 Example 16 Example 17 Example 18 Example 19 Aqueous component 25 25 25 25 Prepolymer 1 NCO = 8.7% 50 Prepolymer 9 NCO = 8.9% 50 Prepolymer 10 NCO = 7.2% 50 Prepolymer 11 NCO = 8.3% 50 Density [g / L] 103 108 135 109 Water absorption [g of water per 1 g of foam] 14.9 14.0 4.7 8.3

Claims

1. A process for producing a flexible hydrophilic polyurethane foam, wherein (a) at least one polyurethane prepolymer having an isocyanate content of 5 to 10% by weight, depending on the weight of the isocyanate prepolymer (a), is mixed with (b) at least one aqueous compound in a weight ratio of polyurethane prepolymer (a) to aqueous component (b) of 3:1 to 1:1.2 and allowed to react to form the flexible polyurethane foam, wherein the polyurethane prepolymer (a) can be obtained by mixing and reacting at least one isocyanate (a1) with at least one polyetherol (a2), the isocyanate (a1) comprising methylenediphenylene diisocyanate, and the polyetherol (a2) having a hydroxyl number of 30 to 60 mg KOH / g can be obtained by alkoxylation of at least one difunctional and / or trifunctional initiator molecule with ethylene oxide and propylene oxide, and the content of ethylene oxide, as a function of the total weight of alkylene oxide,is at least 60% by weight, depending on the total weight of the polyether polyol (a2), wherein the polyetherol (a2) can be obtained by propoxylation of the initiator molecules in a first step to a hydroxyl number of 400 to 1200 mg KOH / g, followed by alkoxylation of the propoxylated initiator molecule with a mixture of ethylene oxide and propylene oxide and, finally, ethoxylation of the alkoxylation product thus obtained with 2 to 10% by weight, depending on the alkylene oxide used to produce the polyetherol (a2), and the aqueous component (b) comprises at least 90% by weight of water (b1) and up to 10% by weight of non-silicone surfactants (b2), in each case depending on the total weight of component (b).

2. The process according to claim 1, wherein the isocyanate content of the isocyanate prepolymer (a) is 6 to 9% by weight, depending on the weight of the isocyanate prepolymer (a).

3. The process according to claim 1 or 2, wherein the isocyanate (a) comprises from 30 to 100% by weight of 4,4'-methylenediphenylene diisocyanate and from 0 to 70% by weight of 2,4'-methylenediphenylene diisocyanate, in each case depending on the total weight of the isocyanate (a).

4. The process according to any of claims 1 to 3, wherein the isocyanate (a) comprises less than 10% by weight of isocyanates other than 4,4'-methylenediphenylene diisocyanate and 2,4'-methylenediphenylene diisocyanate.

5. The process according to any of claims 1 to 4, wherein the isocyanate (a) comprises less than 1% by weight of 2,2'-methylenediphenylene diisocyanate.

6. The process according to any of claims 1 to 5, wherein the weight ratio of the polyurethane prepolymer (a) to the aqueous component (b) is from 2.5:1 to 1.2:

1.

7. The process according to any of claims 1 to 6, wherein the polyurethane prepolymer (a) and the aqueous component (b) react to form the flexible polyurethane foam in the absence of tertiary amines.

8. The process according to any of claims 1 to 7, wherein the polyetherol (a2) has a primary hydroxyl group content of 95 to 100%.

9. The process according to any one of claims 1 to 8, wherein the initiator molecule for producing the polyetherol (a2) is selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol, trimethylolpropane and mixtures thereof.

10. The process according to any one of claims 1 to 9, wherein the polyetherol (a2) can be obtained by propoxylation of an initiator molecule in a first step to a hydroxyl number of 450 to 800 mg KOH / g, followed by alkoxylation with a mixture of ethylene oxide and propylene oxide and ethoxylation of the alkoxylation product thus obtained with 3 to 5 wt% ethylene oxide, depending on the alkylene oxide used to produce the polyetherol (a2).

11. The process according to any of claims 1 to 10, wherein, in addition to the polyetherol (a2), no additional compounds with isocyanate-reactive groups are used to produce the polyisocyanate prepolymer (a).

12. The process according to any of claims 1 to 11, wherein the density of the flexible polyurethane foam is 80 to 120 g / L.

13. A flexible hydrophilic polyurethane foam obtainable by a process according to any of claims 1 to 12.

14. The use of a flexible hydrophilic polyurethane foam according to claim 14 for treating wounds.