Flexible polyurethane foam with high water absorption

Abstract

The present invention provides a method for producing a hydrophilic and flexible polyurethane foam, comprising: (a) at least one polyurethane prepolymer having an isocyanate content of 5 to 10% by mass (the isocyanate content is based on the mass of the prepolymer (a) having the isocyanate); (b) at least one aqueous component; and mixing and reacting the polyurethane prepolymer (a) and the aqueous component (b) so that the mass ratio of the polyurethane prepolymer (a) and the aqueous component (b) is 3:1 to 1:1.2 to form the flexible polyurethane foam. The polyurethane prepolymer (a) is obtained by mixing and reacting at least one isocyanate (a1) with at least one polyetherol (a2), the isocyanate (a1) comprises methylene diphenylene diisocyanate; the polyetherol (a2) has a hydroxyl value of 30 to 60 mg KOH / g and is obtained by alkoxylating at least one difunctional and / or trifunctional initiator molecule with ethylene oxide and propylene oxide; the content of the ethylene oxide in the total mass of the alkylene oxide is at least 60% by mass (based on the total mass of the polyether polyol (a2)); the polyetherol (a2) is obtained by propoxylating the starter molecules in a first step to a hydroxyl number of 400 to 1200 mg KOH / g, followed by alkoxylating the propoxylated starter molecules with a mixture of ethylene oxide and propylene oxide, and finally ethoxylating the alkoxylated product thus obtained with 2 to 10% by weight of ethylene oxide (based on the alkylene oxide used to prepare the polyetherol (a2)), and The present invention relates to a method for producing a polyurethane foam, wherein the aqueous component (b) comprises at least 90% by weight of water (b1) and not more than 10% by weight of a silicon-free surfactant (b2). The present invention further relates to a flexible polyurethane foam obtainable by such a method, and to the use of such a flexible polyurethane foam for treating wounds.

Description

Technical Field

[0001] The present invention relates to a method for producing a hydrophilic and flexible polyurethane foam, comprising: (a) at least one polyurethane prepolymer having an isocyanate content of 5 to 10% by mass (where the isocyanate content is based on the mass of the prepolymer (a) having the isocyanate), and (b) at least one aqueous component, are mixed and reacted such that the mass ratio of the polyurethane prepolymer (a) to the aqueous component (b) is 3:1 to 1:1.2 to form the flexible polyurethane foam, The polyurethane prepolymer (a) is obtained by mixing and reacting at least one isocyanate (a1) and at least one polyether polyol (a2), The isocyanate (a1) contains methylene diphenyl diisocyanate, The polyether polyol (a2) has a hydroxyl value of 30 to 60 mgKOH / g and is obtained by alkoxylating at least one bifunctional and / or trifunctional starting molecule with ethylene oxide and propylene oxide, The content of ethylene oxide in the total mass of alkylene oxides is at least 60% by mass (based on the total mass of the polyether polyol (a2)), The polyether polyol (a2) is obtained by first propoxylating the starting molecule to a hydroxyl value of 400 to 1200 mgKOH / g, then alkoxylating the propoxylated starting molecule with a mixture of ethylene oxide and propylene oxide, and finally ethoxylating the thus obtained alkoxylated product with 2 to 10% by mass of ethylene oxide (based on the alkylene oxides used in the production of the polyether polyol (a2)), and The present invention relates to a method for producing the aqueous component (b) comprising at least 90% by mass of water (b1) and 10% by mass or less of a silicon-free surfactant (b2). The present invention further relates to a flexible polyurethane foam obtained by such a method, and to the use of such a flexible polyurethane foam for treating wounds. [Background technology]

[0002] The use of hydrophilic and flexible polyurethane foam to treat wounds is known and is described, for example, in WO2012055834, WO2004074343, EP2336211, and WO9429361.

[0003] WO2012055834 and EP2336211 disclose the production of hydrophilic and flexible polyurethane foams starting from aliphatic isocyanates. For this purpose, a prepolymer having an isocyanate is reacted with an aqueous component. The disadvantage of such aliphatic isocyanates is their high volatility, which can result in health hazards during processing. Furthermore, aliphatic isocyanates exhibit only weak reactivity during processing, resulting in energy-intensive and time-consuming processing. There are problems with accelerating the process by using a catalyst, as the catalyst may migrate from the foam, and such foams are not suitable for wound healing.

[0004] WO2004074343 and WO9429361 describe hydrophilic and flexible polyurethane foams obtained by reacting MDI and hydrophilic polyetherol-based polyurethane prepolymers with aqueous components. However, the processes described in these documents result in flexible polyurethane foams with water absorption that has room for improvement in terms of absolute water absorption and water absorption rate, as well as wet tensile strength that has room for improvement. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] WO2012055834 [Patent Document 2] WO2004074343 [Patent Document 3] EP2336211 [Patent Document 4] WO9429361 [Overview of the project] [Problems that the invention aims to solve]

[0006] Therefore, an object of the present invention is to provide a flexible polyurethane foam suitable for wound healing, in which the mechanical properties (particularly wet tensile strength) and water absorption are further improved compared to known hydrophilic and flexible polyurethane foams. [Means for solving the problem]

[0007] The objective of the present invention is, (a) At least one polyurethane prepolymer having an isocyanate content of 5 to 10% by mass (the isocyanate content is based on the mass of the prepolymer (a) having the isocyanate) (b) at least one aqueous component and The polyurethane prepolymer (a) and aqueous component (b) are mixed and reacted in a mass ratio of 3:1 to 1:1.2 to form the flexible polyurethane foam. The polyurethane prepolymer (a) is obtained by mixing and reacting at least one isocyanate (a1) and at least one polyetherol (a2), The isocyanate (a1) comprises methylenediphenylenediisocyanate, The polyetherol (a2) has a hydroxyl value of 30-60 mg KOH / g and is obtained by alkoxyly encapsulating at least one difunctional and / or trifunctional starting molecule with ethylene oxide and propylene oxide. The content of ethylene oxide in the total mass of alkylene oxide is at least 60% by mass (based on the total mass of the polyether polyol (a2)), The polyetherol (a2) is obtained by first propoxyling the initial molecule to a hydroxyl value of 400-1200 mgKOH / g, then alkoxyling the propoxyled initial molecule with a mixture of ethylene oxide and propylene oxide, and finally ethoxyling the alkoxylated product with 2-10% by mass of ethylene oxide (based on the alkylene oxide used in the production of the polyetherol (a2)), and The aqueous component (b) is achieved by a flexible polyurethane foam obtained by a manufacturing method, wherein the aqueous component (b) comprises at least 90% by mass of water (b1) and 10% by mass or less of a silicon-free surfactant (b2). The present invention further relates to a method for manufacturing such a flexible polyurethane foam and its use for treating wounds. [Modes for carrying out the invention]

[0008] The flexible foam according to the present invention preferably has a density of 70 to 140 g / l, particularly preferably 80 to 120 g / l, and especially 85 to 115 g / l. In the context of the present invention, the density was determined according to Annex C of European Standard EN14315-2. Furthermore, the flexible polyurethane foam according to the present invention preferably has a hardness of 1.0 to 5.0, particularly preferably 2.5 to 3.5. In the context of the present invention, the hardness was determined according to Asker C ASTM D 2240.

[0009] Furthermore, the flexible polyurethane foam according to the present invention is also characterized by its excellent water absorption capacity. This water absorption capacity is determined as described in the examples, preferably 10g or more per gram of foam, and particularly preferably 12 to 20g per gram of foam.

[0010] To produce the flexible polyurethane foam according to the present invention, at least one polyurethane prepolymer (a) having an isocyanate content of 5 to 10% by mass, preferably 6 to 9% by mass, and particularly 6.5 to 8.5% by mass (based on the mass of the isocyanate-containing prepolymer (a)) is mixed and reacted with at least one aqueous component (b) to form a polyurethane foam. The isocyanate content is known and can be determined, for example, by titration or spectroscopic analysis. In the context of the present invention, this is specified as the value closest to 0.1% by mass.

[0011] In this case, the prepolymer having an isocyanate is obtained by mixing and reacting at least one isocyanate (a1) and at least one polyetherol (a2).

[0012] The isocyanate (a) used is preferably an aromatic diisocyanate and polyisocyanate, which are known in polyurethane chemistry and include, for example, isomers of toluene diisocyanate and isomers and higher-order polycyclic homologs of methylenediphenyl diisocyanate. The inclusion of methylenediphenyl diisocyanate (a1) is essential to the present invention. Methylenediphenyl diisocyanate (hereinafter also referred to as MDI) includes the isomers 2,2'-MDI, 2,4'-MDI, and 4,4'-MDI. The proportion of 4,4'-methylenediphenylenediisocyanate is preferably 30 to 100% by mass, more preferably 45 to 90% by mass, and particularly preferably 55 to 85% by mass, and the proportion of 2,4'-methylenediphenylenediisocyanate is preferably 0 to 70% by mass, more preferably 10 to 55% by mass, and particularly preferably 15 to 45% by mass (based on the total mass of isocyanate(a) in each case). In a particularly preferred embodiment, isocyanate(a) includes less than 10% by mass, preferably less than 5% by mass (based on the total mass of isocyanate(a)) of isocyanates different from 4,4'-methylenediphenylenediisocyanate and 2,4'-methylenediphenylenediisocyanate, and particularly does not include isocyanates different from 4,4'-methylenediphenylenediisocyanate and 2,4'-methylenediphenylenediisocyanate. In particular, isocyanate (a) contains less than 1% by mass, preferably less than 0.01% by mass, of 2,2'-methylenediphenylenediisocyanate, and more preferably does not contain 2,2'-methylenediphenylenediisocyanate.

[0013] The polyetherol (a2) used is one or more polyetherols having a hydroxyl value of 30-60 mgKOH / g, preferably 35-50, which are obtained by alkoxylyzing at least one difunctional and / or trifunctional initiating molecule with ethylene oxide and propylene oxide. The ethylene oxide content is at least 60% by mass, preferably 65-95% by mass, and particularly preferably 70-80% by mass, based on the total mass of alkylene oxides. The propylene oxide content is 0-40% by mass, preferably 5-35% by mass, and particularly preferably more than 20-30% by mass. In the production of polyetherol (a2), it is preferable to use less than 5% by mass of alkylene oxide in addition to ethylene oxide and propylene oxide, and in particular, it is preferable not to use any further alkylene oxides other than ethylene oxide and propylene oxide.

[0014] Furthermore, according to the present invention, the content of primary hydroxyl groups in the polyether polyol (a2) is at least 90% by mass, preferably 95% to 100% by mass, particularly preferably 99% to 100% by mass, and especially 100% (in each case, based on the total mass of hydroxyl groups in the polyether polyol (a2)).

[0015] The polyether polyol (a2) can be produced by an anionic polymerization of an alkylene oxide to which at least one starting molecule (preferably containing 2 to 4, particularly preferably 2 or 3 reactive hydrogen atoms) is added by a known method, for example, in the presence of a catalyst. By using a mixture of starting molecules having different functionalities, it is possible to obtain only slight functionality. The nominal functionality ignores, for example, the influence on functionality due to side reactions. The catalysts used are alkali metal hydroxides (e.g., sodium hydroxide or potassium hydroxide), or alkali metal alkoxides (e.g., sodium methoxide, sodium ethoxide, or potassium ethoxide, or potassium isopropoxide), or, in the case of cationic polymerization, Lewis acids (e.g., antimony pentachloride, boron trifluoride etherate, or bleaching earth) can be included as catalysts. Also, amine-based alkoxylation catalysts (e.g., dimethylethanolamine (DMEOA), imidazole, imidazole derivatives) can be used. Further, the catalyst to be used can include double metal cyanides known as DMC catalysts.

[0016] Suitable starting molecules include compounds containing a hydroxyl group or an amine group (e.g., ethylene glycol, diethylene glycol, glycerol, trimethylolpropane, pentaerythritol, methylamine, ethylamine, isopropylamine, butylamine, benzylamine, aniline, toluidine, toluenediamine (TDA), naphthylamine, ethylenediamine, diethylenetriamine, 4,4'-methylenedianiline, 1,3-propanediamine, 1,6-hexanediamine, ethanolamine, diethanolamine, and other divalent or polyvalent alcohols, monoamines, or diamines). The starting molecule for producing the polyether polyol (a2) is preferably selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol, trimethylolpropane, and mixtures thereof, and particularly preferably, glycerol (particularly only glycerol) is used as the starting molecule.

[0017] The polyetherol (a2) according to the present invention is obtained by propoxylyzing an initial molecule to a hydroxyl value of 400 to 1200 mg KOH / g, preferably 450 to 800 mg KOH / g, in the first step, then alkoxylyzing the propoxylyzed initial molecule with a mixture of ethylene oxide and propylene oxide, and further ethoxyling the resulting alkoxylyzed product with 2 to 10% by mass, preferably 3 to 5% by mass of ethylene oxide (based on alkylene oxide used in the production of polyetherol (a2)). In the present invention, it is essential that, on average, at least one propylene oxide molecule is present after propoxyling of the initial molecule. If an initial molecule containing propylene oxide (e.g., tripropylene glycol) has already been used, it is considered to have already been propoxylyzed, but it can be further propoxylyzed to a hydroxyl value of 400 mg KOH / g. As a result, a polyetherol (a2) having an ethylene oxide-terminated group is obtained, containing 2-10% by mass, preferably 3-5% by mass, of ethylene oxide (based on the total mass of alkylene oxides used). In this case, it is not necessary to purify the propoxylated starting molecule after propoxylation, or the alkoxylation product of the propoxylated starting molecule and the mixture of ethylene oxide and propylene oxide, after alkoxylation and before ethoxylation. To track the progress of the reaction, the conversion of alkylene oxide can be monitored spectroscopically (e.g., by near-infrared spectroscopy). Before adding the final ethylene oxide, it is preferable to confirm the conversion of alkylene oxide by spectroscopy to ensure that there is essentially no unreacted propylene oxide in the reaction mixture. This means that the percentage of unreacted propylene oxide before adding ethylene oxide (based on the total mass of alkylene oxides used up to this point) is less than 1% by mass, preferably less than 0.5% by mass, more preferably less than 0.1% by mass, and particularly less than 0.01% by mass.

[0018] The polyol (a2) according to the present invention is preferably purified from the alkoxylation catalyst still present after production by contacting it with an acidic ion exchange resin (for example, Amberlite).

[0019] The alkali metal content of the polyol (a2) is preferably 50 ppm or less, particularly preferably 30 ppm or less, and particularly preferably 20 ppm or less. In this case, the alkali metal content is usually measured by a titration method.

[0020] In addition to the polyether polyol (a2), it is preferable to produce the prepolymer (a) having a polyisocyanate without further using a compound having a reactive group with respect to the isocyanate.

[0021] According to the present invention, the aqueous component (b) used is a component containing at least 90% by mass, preferably 94 to 100% by mass, particularly preferably 96 to 99.5% by mass, especially 97 to 99% by mass of water (b1) and up to 10% by mass, preferably 0 to 6% by mass, particularly preferably 0.5 to 4% by mass, especially 1 to 3% by mass of a non-silicone surfactant (b2).

[0022] The silicone-free surfactant (b2) is preferably a compound that dissolves in water at 20°C in a range of at least 5% by mass and has an isocyanate reactive group. The surfactant (b2), at a concentration of 1% by mass in water at 20°C, preferably reduces the surface tension to at least 10 mNm, particularly preferably 15 mNm, and especially preferably 20 mNm (measured according to DIN EN 14370:2004-11). The surfactant is preferably an alkylene oxide block copolymer having a molecular weight of 800 to 15000 g / mol, preferably 1200 to 10000, and particularly 2000 to 8000 g / mol. The block copolymer is particularly preferably having a central block of alkylene oxide having at least 3 carbon atoms (e.g., propylene oxide, butylene oxide, pentylene oxide, and mixtures of these alkylene oxides, preferably propylene oxide), and two terminal blocks (each of which is ethylene oxide). In this case, the mass ratio of the central block to the terminal block is preferably 1:0.5 to 1:5. Such a surfactant is a commercially available product from BASF under the trade name Pluriol®.

[0023] The aqueous component (b) contains water (b1) and a surfactant (b2), but preferably does not contain a compound having an isocyanate reactive group. Component (b) does not contain an amine catalyst, and it is particularly preferable that it does not contain any catalyst at all. In a more preferred embodiment, component (b) consists only of water (b1) and a surfactant (b2).

[0024] To produce a hydrophilic and flexible polyurethane foam according to the present invention, a polyurethane prepolymer (a) and an aqueous component (b) are mixed. The mixing is preferably carried out in a mass ratio of 2.5:1 to 1.2:1, particularly preferably 2.2:1 to 1.0:1, and especially preferably 0.5:1 to 0.8:1 of polyurethane prepolymer (a) to aqueous component (b). The mixing is preferably carried out at a temperature of the aqueous component higher than 0 to 25°C, preferably 1 to 10°C, particularly 3 to 5°C, and at a temperature of the isocyanate component preferably 10 to 50°C, particularly preferably 15 to 40°C, and especially preferably 20 to 35°C. Curing is particularly preferably carried out in the absence of tertiary amines, and in particular, the mixture of the polyisocyanate prepolymer and aqueous component (b) does not contain any further substances.

[0025] The reaction mixture is applied continuously to the release paper and preferably cured in an oven. The application is usually carried out in layers with a thickness of 1 mm to 10 mm. After curing, the hydrophilic and flexible polyurethane foam according to the present invention is peeled off the release paper.

[0026] The flexible polyurethane foam obtained according to the present invention has a density preferably of 70-140 g / l, particularly preferably of 80-120 g / l, and especially of 85-115 g / l, and a water absorption capacity of at least 8 g / g, preferably at least 10 g / g, and especially of 12-20 g / g. The water absorption capacity is measured as follows: A solution prepared by dissolving 142 mmol of sodium chloride and 2.5 mmol of calcium chloride in 1 liter of desalinated water was used (according to the method of EN 13726-12002 (Solution 1)).

[0027] From the foam to be tested, a 50mm x 50mm foam sample is punched out from a 1.6cm thick foam strip using a punch, and immersed in the aqueous test solution for 3 minutes (according to EN 13726-1.2002 1). Then, without squeezing the foam, carefully grasp one corner and drain for 10 seconds, and measure the mass again. The obtained water absorption rate (mass of foam after draining minus mass of dry foam sample) is divided by the mass of the dry sample and reported in g / g. Furthermore, the polyurethane foam according to the present invention also has a high water absorption rate. To determine the water absorption rate of the dry foam sample, 2 ml of the aqueous test solution described above, which was also used to determine the water absorption rate, is applied to the surface of the foam sample at room temperature, and time is stopped when it is completely absorbed by the foam sample.

[0028] Furthermore, the polyurethane foam according to the present invention has excellent mechanical properties, for example, it has high tensile strength in both wet and dry conditions.

[0029] The hydrophilic and flexible polyurethane foam according to the present invention can preferably be used in the cosmetics field (for example, as cosmetic pads or wound dressings, or as shoulder pads for clothing). Furthermore, the foam according to the present invention can be used for passive climate control in enclosed spaces (for example, vehicles (e.g., automobiles) or buildings). The foam according to the present invention can also be used as a reversible liquid intake device (for example, in a serving trolley). Furthermore, the foam according to the present invention can be used for hearing protection or for absorbing bodily fluids. The polyurethane foam according to the present invention is particularly preferably used for wound treatment (for example, as a wound dressing). [Examples]

[0030] The present invention will be described below with reference to examples. In the following examples, prepolymers 1, 3, 6, 7, and 9 in Table 1 correspond to prepolymer (a) of the present invention, and examples 2 to 5, 9, 10, 13, 14, 16, and 17 in Tables 2 to 4 correspond to examples of the present invention.

[0031] The following substances were used.

[0032] Polyol 1: A polyether polyol derived from glycerol with a hydroxyl value of 42 mg KOH / g (obtained by adding 6% by mass of propylene oxide in the first step, a mixture of 69% by mass of ethylene oxide and 20% by mass of propylene oxide in the second step, and 5% by mass of ethylene oxide in the third step, using KOH as a catalyst).

[0033] Polyol 2: A polyether polyol derived from diethylene glycol with a hydroxyl value of 42 mg KOH / g (obtained by addition reaction using KOH as a catalyst, with 6% by mass of propylene oxide in the first step, a mixture of 69% by mass of ethylene oxide and 20% by mass of propylene oxide in the second step, and 5% by mass of ethylene oxide in the third step).

[0034] Polyol 3: A polyether polyol derived from glycerol with a hydroxyl value of 32 mg KOH / g (obtained by alkoxylation of glycerol with a mixture of ethylene oxide and propylene oxide; commercially available under the trade name Voranol® CP-1421 from Dow Chemicals).

[0035] Polyol 4: A blocked polyether polyol derived from glycerol with a hydroxyl value of 42 mg KOH / g (obtained by adding 26% by mass of propylene oxide in the first step and 74% by mass of ethylene oxide in the second step, using KOH as a catalyst).

[0036] Auxiliary agent 1: Diglycol bis(chloroformate) Auxiliary agent 2: A block copolymer consisting of a central polyoxypropylene block with a molecular weight of 1750 g / mol and two terminal polyoxyethylene blocks (each terminal having a molecular weight of 350 g / mol). Auxiliary agent 3: A block copolymer consisting of a central polyoxypropylene block with a molecular weight of 1750 g / mol and two terminal polyoxyethylene blocks (each terminal having a molecular weight of 1400 g / mol). Aqueous component: A mixture of 98% by mass of water, 1% by mass of additive 2, and 1% by mass of additive 3. ISO 1: A mixture containing 98.6% by mass of 4,4'-MDI and 1.4% by mass of 2,4'-MDI. ISO 2: A mixture of 2.4% by mass of 2,2'-MDI, 48.6% by mass of 2,4'-MDI, and 49.0% by mass of 4,4'-MDI.

[0037] Prepolymers were prepared according to Table 1. For this purpose, isocyanate was added first, followed by the addition of polymers dropwise while stirring, and the mixture was stirred at 80°C for 1 hour.

[0038] Unless otherwise specified, quantities are given in parts by mass. NCO content was measured in accordance with DIN EN ISO 14896.

[0039] The viscosity of the prepolymer was measured using a Haake VT500 rotational viscometer in accordance with DIN EN ISO 3219.

[0040] [Table 1]

[0041] Table 2~ 4 The foam is manufactured from the prepolymer by mixing it with an aqueous component, and the procedure is as follows:

[0042] The water component is first added to a small PE beaker (V=160mL) at room temperature. The isocyanate is first added to a beaker (V=550mL) prepared at room temperature. Set the Vollrath stirrer to the lowest speed. Use a disposable stirrer with a diameter of 65mm for mixing. Quickly transfer the water component to the clear beaker containing the isocyanate. Start the stopwatch when stirring begins. Mix the starting materials at high speed until a homogeneous mixture is formed. The stirring time depends on the reactivity of the system and should be a maximum of 80% cream time.

[0043] [Table 2]

[0044] Table 2 shows the mixing ratio of aqueous component (1) to prepolymer (2) at 1: 2 This indicates that the best water absorption capacity is obtained in this case.

[0045] Tables 3 and 4 show foams prepared by mixing aqueous components and prepolymers in a mass ratio of 1:2. In Table 3, the NCO content of the isocyanate was varied, and in Table 4, different polyols were used to prepare the prepolymers. Tables 3 and 4 show the density and water absorption capacity of the obtained foams.

[0046] [Table 3]

[0047] Table 3 shows that the optimal water absorption rate is obtained when the isocyanate content of the prepolymer is between 6 and 9% by mass.

[0048] [Table 4]

Claims

1. A method for producing hydrophilic and flexible polyurethane foam, (a) At least one polyurethane prepolymer having an isocyanate group content of 5.0 to 10% by mass (the isocyanate group content is based on the mass of the polyurethane prepolymer (a) having a structure derived from methylenediphenylenediisocyanate) (b) at least one aqueous component and The polyurethane prepolymer (a) and aqueous component (b) are mixed and reacted in such a mass ratio of 3:1 to 1:1.2 to form the flexible polyurethane foam. The polyurethane prepolymer (a) is obtained by mixing and reacting at least one isocyanate (a1) and at least one polyetherol (a2), The isocyanate (a1) comprises methylenediphenylenediisocyanate, The polyetherol (a2) has a hydroxyl value of 30 to 60 mg KOH / g and is obtained by alkoxyling a bifunctional initiating molecule, a trifunctional initiating molecule, or a bifunctional initiating molecule and a trifunctional initiating molecule with ethylene oxide and propylene oxide. The content of ethylene oxide in the total mass of alkylene oxide is at least 60% by mass. The polyetherol (a2) is obtained by first propoxyling the initial molecule to a hydroxyl value of 400 to 1200 mg KOH / g, then alkoxyling the propoxyled initial molecule with a mixture of ethylene oxide and propylene oxide, and finally ethoxyling the alkoxylated product with 2 to 10% by mass of ethylene oxide (based on the alkylene oxide used in the production of the polyetherol (a2)), and A method for producing the aqueous component (b), wherein the aqueous component (b) comprises 90 to 100% by mass of water (b1) and 0 to 10% by mass of a silicon-free surfactant (b2) (in either case, based on the total mass of component (b)).

2. The method according to claim 1, wherein the isocyanate group content in the polyurethane prepolymer (a) is 6 to 9% by mass (based on the mass of the polyurethane prepolymer (a)).

3. The method according to claim 1 or 2, wherein the isocyanate (a1) comprises 30 to 100% by mass of 4,4'-methylenediphenylenediisocyanate and 0 to 70% by mass of 2,4'-methylenediphenylenediisocyanate (in either case, based on the total mass of the isocyanate (a1)).

4. The method according to any one of claims 1 to 3, wherein the isocyanate (a1) contains less than 10% by mass of an isocyanate different from 4,4'-methylenediphenylenediisocyanate and 2,4'-methylenediphenylenediisocyanate, or does not contain an isocyanate different from 4,4'-methylenediphenylenediisocyanate and 2,4'-methylenediphenylenediisocyanate.

5. The method according to any one of claims 1 to 4, wherein the isocyanate (a1) contains less than 1% by mass of 2,2'-methylenediphenylenediisocyanate, or does not contain 2,2'-methylenediphenylenediisocyanate.

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

1.

7. The method according to any one of claims 1 to 6, comprising reacting a polyurethane prepolymer (a) and an aqueous component (b) to form a polyurethane foam in the absence of a tertiary amine.

8. The method according to any one of claims 1 to 7, wherein the polyetherol (a2) contains primary hydroxyl groups in an amount of 95 to 100% (based on the total mass of hydroxyl groups of the polyetherol (a2)).

9. The method according to any one of claims 1 to 8, wherein the starting 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 method according to any one of claims 1 to 9, wherein the initial molecule is propoxylated in the first step to a hydroxyl value of 450 to 800 mg KOH / g, subsequently alkoxylated with a mixture of ethylene oxide and propylene oxide, and the alkoxylated product thus obtained is ethoxylated with 3 to 5% by mass of ethylene oxide (based on the alkylene oxide used in the production of the polyetherol (a2)) to obtain the polyetherol (a2).

11. The method according to any one of claims 1 to 10, wherein the polyetherol (a2) is used, but no further compound having a reactive group for isocyanate is used to produce the polyurethane prepolymer (a).

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

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