Polyurethane prepolymer composition

Abstract

The present invention provides an NCO-terminated polyurethane prepolymer composition produced by the reaction of methylenediphenyl diisocyanate (MDI) and / or polyphenylene polydimethylene polyisocyanate diisocyanate (pMDI) with a polyol, wherein the amount of residual unreacted diisocyanate monomer is further reduced, as well as a method for producing the polyurethane prepolymer.

Description

Background Art

[0001] Polyurethane prepolymers are widely used in various industries such as coatings, adhesives, sealants, and elastomers. In many adhesive applications, polyurethane prepolymers with low viscosity, high NCO content, and low residual diisocyanate monomer content are preferred. In this specification, such prepolymers with unreacted diisocyanate monomers remaining are sometimes referred to as "low free" or "LF" prepolymers.

[0002] Methylene diphenyl diisocyanate (MDI) is a starting material widely used in the production of polyurethane prepolymers. As used herein, "MDI" refers to isomers of monomeric MDI, namely the 4,4'-,-2,4'-, and 2,2'-isomers of methylene diphenyl diisocyanate. As used herein, polyphenylene poly(dimethylmethylene) polyisocyanate diisocyanate ("pMDI") means a mixture of monomeric (bicyclic) MDI and polyaromatic (tricyclic or higher) methylene diphenyl diisocyanate. Tricyclic or higher MDI is represented by formula I (n is 1 or more).

[0003] (Formula 1) TIFF2026514300000001.tif1742

[0004] pMDI is often used in the production of rigid foams and as a binder in the wood processing industry.

[0005] Various methods have been investigated to obtain LF MDI-based prepolymers with low viscosity and high NCO content. For example, Reese et al. (EP1518874) disclose a low monomer polymethylene polyphenylene polyisocyanate mixture in which a pMDI / MDI mixture is stripped to produce low monomer pMDI, and then in a second step this "low monomer" pMDI / MDI mixture is mixed with a polyurethane prepolymer to obtain a foam.

[0006] Therefore, there is a need for further improved compositions and methods for producing LF MDI polyurethane prepolymers having low viscosity, high NCO value, and low MDI monomer content. Surprisingly, it was found that the shortcomings of the conventional technique could be overcome by performing additional distillation in the reaction of pMDI with polyol. As a result, an NCO-terminated prepolymer with increased NCO content and lower viscosity than that obtained by other known methods was obtained.

[0007] In this specification, unless otherwise stated, all patents, patent applications, patent publications and other publications mentioned and cited herein are deemed to be fully incorporated herein by reference as if they were included in their entirety. To better understand the present invention, as well as its other features and advantages, please refer to the following description. The scope of the present invention is set forth in the attached claims. [Overview of the project]

[0008] This invention relates to an NCO-terminated polyurethane prepolymer composition containing pMDI. This prepolymer composition has low viscosity and a high NCO content, and when cured, it can produce a flexible and durable polyurethane adhesive material. The present invention also provides a method for producing such a polyurethane prepolymer by reacting pMDI with a polyol in a specific ratio and under specific reaction conditions, or by reacting MDI with a polyol in a specific ratio and under specific reaction conditions and mixing it with pMDI.

[0009] According to at least one preferred embodiment of the present invention, the polyurethane prepolymer composition is broadly envisioned to be produced by a process comprising the following steps: (1) reacting pMDI with a polyol to form a polyurethane prepolymer containing residual monomeric MDI; and (2) reducing the amount of residual monomeric MDI in the polyurethane prepolymer.

[0010] In another embodiment of the present invention, there is a polyurethane prepolymer composition produced by a process comprising the following steps: (1) reacting monomeric MDI with a polyol to form a polyurethane prepolymer; (2) mixing pMDI with the polyurethane prepolymer to form a polyurethane prepolymer mixture containing residual monomeric MDI; and (3) reducing the amount of residual monomeric MDI in the polyurethane prepolymer mixture.

[0011] Furthermore, in one embodiment, the polyurethane prepolymer composition has a residual monomeric MDI of 0.4% by weight or less, preferably 0.1% by weight or less, relative to the total weight of the prepolymer.

[0012] In another embodiment, in the polyurethane prepolymer composition, the polyol is a polyether, preferably polypropylene glycol (PPG), more preferably PPG with a molecular weight of 200 to 10,000 g / mol, more preferably 500 to 4,000 g / mol, and even more preferably 1,500 to 2,500 g / mol.

[0013] In one embodiment, the polyurethane prepolymer composition is formed by reducing the content of residual monomeric MDI by distillation.

[0014] In a further embodiment, a method for producing a polyurethane prepolymer composition comprises the following steps: (1) reacting pMDI with a polyol to form a polyurethane prepolymer containing residual monomeric MDI; and (2) reducing the amount of residual monomeric MDI in the polyurethane prepolymer.

[0015] Similarly, in another embodiment, a method for producing a polyurethane prepolymer composition comprising the following steps: (1) reacting monomeric MDI with a polyol to form a polyurethane prepolymer; (2) mixing pMDI with the polyurethane prepolymer to form a polyurethane prepolymer mixture containing residual monomeric MDI; and (3) reducing the amount of residual monomeric MDI in the polyurethane prepolymer mixture.

[0016] In one embodiment of this method, the reduction of the amount of residual monomeric MDI is achieved by distillation. In a further embodiment, the polyurethane prepolymer composition described above is used in the manufacture of adhesives and sealants.

[0017] The above summary does not limit in any way the scope of the invention as described in the claims. Furthermore, the general description above and the detailed description below are for illustrative and explanatory purposes only and do not limit the invention described in the claims. Detailed Description of the Invention

[0018] The polyurethane prepolymer composition of the present invention comprises a prepolymer segment consisting of specific diisocyanate units and specific polyol units. More specifically, in one embodiment, the prepolymer segment is formed by the reaction of (A) methylenediphenyl diisocyanate (MDI) units and (B) polyol units. In another embodiment, the prepolymer segment is formed by the reaction of (A) polyphenylene polydimethylene polyisocyanate diisocyanate (pMDI) units and (B) polyol units.

[0019] In this specification, the compound A unit in the prepolymer refers to a unit derived from compound A among the repeating units that constitute the polymer chain. That is, in polymer polymerization, compound A is a specific component or bond at the terminal end, for example, a condensation polymerization. The polymer produced in this way will contain compound A units in its main chain or side chains. Unit (A) and unit (B) may be linked to each other in the prepolymer segment to form a chain. For example, unit (A) and unit (B) may be linked via an NCO bond.

[0020] Generally, the prepolymers according to the present invention are produced using standard reaction processes and conditions known in the art for the production of prepolymers and polyurethanes. The specific process is illustrated in U.S. Patents Nos. 4,832,098, 4,934,425, 4,921,029, 4,784,201, and 5,605,657, as well as U.S. Patent Application No. 09 / 919,994 filed on 2 August 2001.

[0021] There are no particular restrictions on the prepolymer or mixture of prepolymers that can be used in the present invention, nor are there any particular restrictions on the polyol or diisocyanate monomer or polyol that can be used in the production of the prepolymer.

[0022] The diisocyanate components used in the manufacture of prepolymers include aromatic diisocyanate monomers or aliphatic diisocyanate polymers. Aromatic diisocyanates are well-known and widely used in the manufacture of polyurethanes.

[0023] Aromatic diisocyanate monomers useful for the implementation of the present invention include 2,4'- and 4,4'-methylenebis(phenyl isocyanate) (MDI), 2,4'- and 2,6'-toluene diisocyanate (TDI), para-phenylene diisocyanate (PPDI), tolidine diisocyanate (TODI), naphthalene-1,5-diisocyanate (NDI), diphenyl-4,4'-diisocyanate, and mixtures thereof.

[0024] Aliphatic diisocyanate monomers include 1,6-hexane diisocyanate (HDI), dibenzyl-4,4'-diisocyanate, isophorone diisocyanate (IPDI), 1,3'- and 1,4'-xylene diisocyanate, 1,6-hexamethylene diisocyanate, 1,3'- and 1,4'-cyclohexyl diisocyanate (CHDI), three geometric isomers of 1,1'-methylenebis(4-isocyanatocyclohexane) (H12MDI), and mixtures thereof.

[0025] In one embodiment, the diisocyanate monomer is an aromatic diisocyanate, namely monomeric MDI. Monomeric MDI is selected from the group of, for example, diphenylmethane diisocyanate (MDI) and all its isomers (4,4'-diphenylmethane diisocyanate (4,4'-MDI), 2,4'-diphenylmethane diisocyanate (2,4'-MDI), 2,2'-diphenylmethane diisocyanate (2,2'-MDI)).

[0026] In another embodiment, the diisocyanate component for producing the prepolymer is a polymeric isocyanate such as pMDI. As described above, pMDI is a mixture of monomeric (bicyclic) MDI and polyaromatic (tricyclic or higher) methylene diphenyl diisocyanate. These are, for example, "crude MDI", a known industrial raw material, and "polymeric MDI" obtained therefrom. Those skilled in the art will understand that the term “crude MDI” refers to the crude product obtained after the phosgenation step in the industrial synthesis of MDI, i.e., a mixture of known dinuclear MDI isomers and polynuclear (approximately 3) oligomers. This is then subjected to further distillation to remove dinuclear MDI isomers and other low-boiling by-products, resulting in crude MDI, also known as polymeric MDI.

[0027] Various polyols can be used as the polyol component of the prepolymer. Polyols include compounds having multiple hydroxyl groups, amino groups, or thiol functional groups, or combinations thereof. The formation of such polyols is well known in the relevant field. Such polyols may contain esters, ethers, amides, aliphatic groups, acrylics, polylactic acids, polyglycolic acids, metals, metalloids, and other functional groups known to those skilled in the art.

[0028] In various embodiments, the polyol may include one or more polyols comprising different types of esters, ethers, amides, and / or other repeating groups or segments, as well as mixtures or combinations thereof.

[0029] In some embodiments, the molecular weight of the polyol is in the range of 100 to 10,000, in one embodiment it is in the range of 225 to 6,000, and in another embodiment it is in the range of 250 to 3,000. The molecular weight referred to herein is the number-average molecular weight expressed in Dalton terms, which in this specification is calculated by hydroxyl value measurement using the ASTM E222-94 method.

[0030] In various embodiments, the polyol may include glycols, triols, and / or polyols with a high average hydroxyl functional value and a molecular weight in the range of, for example, 50 to 600, in another embodiment 55 to 300, and in yet another embodiment 60 to 200. Such polyols may include low molecular weight polyols in one embodiment. The average hydroxyl functional value may be in the range of about 2 to 8, preferably about 2 to 3, and more preferably about 2 to 2.5. Examples of such glycols or triols include isomers of ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, trimethylolpropane, pentaerythritol, poly(tetramethylene ether) glycol, poly(trimethylene ether) glycol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, and mixtures thereof.

[0031] In another embodiment of the present invention, the polyol useful for the production of the prepolymer is a polyester polyol type compound. These polyols are produced by conventional methods using combinations of diacids and diols known in the art, such as succinic acid esters, adipic acid esters, or other esters.

[0032] Esters can also be produced by the condensation reaction of hydroxycarboxylic acids, such as lactic acid or glycolic acid. Esters can also be produced from acyl chlorides.

[0033] Examples of polyester polyols include poly(adipate) glycol, poly(hexamethylene adipate) glycol, polyethylene adipate glycol (PEAG), poly(diethylene adipate) glycol, poly(ethylene / propylene adipate) glycol, poly(trimethylolpropane / hexamethylene adipate) glycol, poly(ethylene / butylene adipate) glycol, poly(butylene adipate) glycol, poly(hexamethylene / neopentyl adipate) glycol, poly(butylene / hexamethylene adipate) glycol (PBHAG), poly(neopentyl adipate) glycol, and mixtures thereof, copolymers (including block copolymers and random copolymers), and terpolymers. In one embodiment, a PEAG polyol containing an aromatic diisocyanate as the isocyanate component is used.

[0034] In one embodiment of the present invention, the prepolymer segment contains methylenediphenyl diisocyanate (MDI) as a diisocyanate unit (A). In another embodiment, the prepolymer segment comprises polyphenylene. Polydimethylene polyisocyanate (pMDI) is used as the polyisocyanate unit (A).

[0035] In further embodiments, the prepolymer segment contains PPG as a polyol unit (B).

[0036] (NCO / OH ratio) In the method for producing the prepolymer of the present invention, the NCO:OH ratio is 1:1 to 10:1, preferably 3:1 to 8:1, more preferably 5:1 to 7:1, and most preferably 7:1.

[0037] (Method for producing LF prepolymer composition) Any suitable method commonly known for producing polyurethane prepolymers can be used. In one embodiment of the present invention, the polyurethane prepolymer composition of the present invention is produced by a method comprising the following steps: (1) reacting pMDI (A) with a polyol (B) to form a polyurethane prepolymer; and (2) distilling off monomeric MDI from the polyurethane prepolymer.

[0038] In another embodiment, the polyurethane prepolymer composition of the present invention is produced by a method comprising the following steps: (1) reacting MDI (A) with polyol (B) to form a polyurethane prepolymer; (2) mixing pMDI with the prepolymer to form a prepolymer mixture; and (3) distilling monomeric MDI from the polyurethane prepolymer mixture.

[0039] (LF Technology) Any method suitable for reducing the amount of free diisocyanate monomer in the polyurethane prepolymer composition to the low levels of the present invention can be used. Various methods are known to minimize the residual isocyanate content of diisocyanate monomers, including the wiped film evaporation method, solvent-assisted distillation / co-distillation method, molecular sieve method, and solvent extraction method. Distillation under reduced pressure is preferred, and thin-film evaporation or stirred-film evaporation under vacuum is particularly preferred.

[0040] (Amount of free NCO monomers) The NCO-terminated polyurethane prepolymer according to the present invention has a monomeric unreacted MDI monomer content of less than 1% by weight, preferably less than 0.5% by weight, particularly preferably less than 0.2% by weight, and most preferably less than 0.1% by weight. The number of NCO groups per molecule is 1 to 5, preferably 2 to 4, and it contains only highly reactive aromatic isocyanate groups. The reaction product contains at least two urethane groups in the molecule.

[0041] (Hardening agent) The polyurethane prepolymer of the present invention can be cured with moisture or water. Further curing agents suitable for the polyurethane prepolymer composition of the present invention include polyamines, polyols, or mixtures thereof. Suitable polyamines for the polyurethane prepolymer composition of the present invention include aromatic and aliphatic diamines, primary and secondary amine-terminated polyether polyols, and bifunctional, trifunctional, and polymeric amines.

[0042] Suitable polyols for the polyurethane prepolymer composition of the present invention include polyesters and polyether polyols, which may be diols, triols, and tetraols having primary, secondary, and / or tertiary alcohol groups. These polyols may be mixed with diamines. Generally, polyols are preferred over polyamines. In a preferred embodiment, the curing agent is 1,4-butanediol.

[0043] (Additives) The polyurethane prepolymer composition of the present invention may further contain additives such as stabilizers, thickeners, tackifying resins, fillers, plasticizers, thixotropic agents, colorants, pigments, solvents, and / or drying agents, as needed.

[0044] In this invention, the term "stabilizer" refers to a stabilizer that has the effect of stabilizing the viscosity of the polyurethane prepolymer during manufacturing, storage, and use. These include, for example, monofunctional carbonyl chlorides, monofunctional highly reactive isocyanates, and non-corrosive inorganic acids such as benzoyl chloride, toluenesulfonyl isocyanate, phosphoric acid, or phosphorous acid. Useful stabilizers in the present invention further include antioxidants, ultraviolet stabilizers, or hydrolysis stabilizers.

[0045] The selection of these stabilizers depends not only on the main components of the composition but also on the application conditions and the potential destabilizing stresses that may occur in the cured product. When the polyurethane prepolymer is mainly composed of polyether components, antioxidants (with or without UV protection) are primarily required. Examples include commercially available sterically hindered phenols, thioethers, substituted benzotriazoles, or HALS (hindered amine light stabilizers) type sterically hindered amines.

[0046] (Use of polyurethane prepolymer) The low-free pMDI polyurethane prepolymer of the present invention is typically used in applications such as coatings, adhesives, sealants, or elastomers. In preferred embodiments, the low-free pMDI polyurethane prepolymer of the present invention is used in adhesive applications.

[0047] While preferred embodiments of the present invention have been described herein, it will be understood that the present invention is not limited to such embodiments, and that various modifications and changes can be made by those skilled in the art without departing from the scope or spirit of the invention.

[0048] The present invention will be described in more detail below with reference to examples. However, these examples are for illustrative purposes only, and the scope of the present invention is not limited thereto. [Examples]

[0049] (material) Ongronat® TR4030, BorsodChem (Monomeric MDI mixed isomers and polymeric MDI; NCO = 30.7~32.7; f = 2.5) Ongronat® CO4150, BorsodChem (Monomeric MDI mixed isomers and polymeric MDI; NCO = 30.8~32.8; f = 2.5) Ongronat (registered trademark) 2100, BorsodChem (Monomeric MDI mixed isomers and polymeric MDI; NCG = 30.0~32.0; f = 2.6~2.7) Ongronat® XP1043, BorsodChem (Polymer MDI, 4,4'-MDI and 4,4'-MDI / PPG; 1.5% PPG; NCO=30.1~31.1; f=2.4) PPG-500, Lupranol® 1200 polypropylene glycol; PPG-500; Mw = 500 g / mol; CAS number: 25322-69-4; (Commercially available from BASF) PPG-1100, Lupranol® 1100 / 1 polypropylene glycol; PPG-1100; Mw = 1100 g / mol; CAS number: 25322-69-4 (commercially available from BASF) PPG-2000, xyz (polypropylene glycol) TPG, Tripropylene Glycol, Regular Grade, Mixture of Tri(propylene) Glycol Isomers; TPG; CAS Number: 24800-44-0; (Commercially available from Dow) BD, 1,4-butanediol; CAS number: 110-63-4; (Commercially available from Sigma-Aldrich)

[0050] (Characteristics of pMDI mixtures) Table 1 shows the isomer ratios determined by NMR.

[0051] [Table 1]

[0052] Table 2 shows the average free MDI rate obtained by HPLC.

[0053] [Table 2]

[0054] (Non-polymerized MDI Comparative Example 1 (LFM G600)) The prepolymer was prepared as follows: 4,4'-MDI (68.3%), available from Covestro as Mondur M®, was heated to 50°C under a nitrogen atmosphere with stirring. Subsequently, 26.4% of PPG-1100 and 5.4% of PPG-500 were added, and the reaction temperature was maintained at 70°C for 4 hours. Finally, excess residual 4,4'-MDI monomer was removed from the reaction mixture by thin-film distillation under reduced pressure, resulting in a residual 4,4'-MDI content of less than 0.1% by weight and a total NCO content of 6.0%.

[0055] (Preparation of pMDI prepolymer) In the following example of the invention, the 4,4'-MDI in the comparative example was replaced with a pMDI mixture.

[0056] (Example of Invention 1) The same method as described in Comparative Example 1 was used, except that Ongronat TR4030® was used instead of 4,4'-MDI.

[0057] (Example of Invention 2) The same method as described in Comparative Example 1 was used, except that Ongronat CO 4150 (registered trademark) was used instead of 4,4'-MDI.

[0058] (Example of Invention 3) The same method as described above in Comparative Example 1 was used, except that Ongronat 2100® was used instead of 4,4'-MDI.

[0059] (Example of Invention 4) The same method as described in Comparative Example 1 was used, except that Ongronat XP 1043 (registered trademark) was used instead of 4,4'-MDI.

[0060] (Example of Invention 5) The prepolymer was prepared as follows: pMDI (63.4%) is available from BorsodChem as Ongronat CO 4150®. This was heated to 50°C with stirring under a nitrogen atmosphere. Subsequently, 36.6% of PPG-1100 is added, and the reaction temperature is maintained at 70°C for 4 hours. Finally, excess residual 2,4'-MDI and 4,4'-MDI monomers were removed from the reaction mixture by thin-film distillation under reduced pressure, and the mixture was concentrated until the residual 2,4'-MDI and 4,4'-MDI content was less than 0.1% by weight.

[0061] (Example of Invention 6) The prepolymer was prepared as follows: pMDI (48.3%) (available from BorsodChem as Ongronat CO 4150®) was heated to 50°C with stirring under a nitrogen atmosphere. Subsequently, 51.7% of PPG-2000 was added, and the reaction temperature was maintained at 70°C for 4 hours. Finally, excess residual 2,4'-MDI and 4,4'-MDI monomers were removed from the reaction mixture by thin-film distillation under reduced pressure, adjusting the mixture until the residual 2,4'-MDI and 4,4'-MDI content was less than 0.1% by weight.

[0062] (Example of Invention 7) The same process as in Example 6 was used, except that the amount of pMDI added was 40.0% and the amount of PPG-2000 added was 60.0%.

[0063] (Example of Invention 8) First, the prepolymer was prepared as follows: 4,4'-MDI (38.5%), available from Covestro as Mondur M®, was stirred and heated to 50°C under a nitrogen atmosphere. Subsequently, 61.5% of PPG-2000 was added, and the reaction temperature was maintained at 70°C for 4 hours. Next, after the prepolymer preparation was complete, 70.0% of the prepolymer was mixed with 30.0% of pMDI, which is available from BorsodChem under the trademark Ongronat CO 4150 (registered trademark). Finally, excess residual 2,4'-MDI and 4,4'-MDI monomers were removed from the reaction mixture by thin-film distillation under reduced pressure until the residual 2,4'-MDI and 4,4'-MDI content was less than 0.1% by weight.

[0064] [Table 3]

[0065] All four resulting prepolymers were stable, dark in color (very dark brown), more than twice the target %NCO6%, and the batch yield was higher than expected. Analysis of Examples 1-4 in Table 3 shows that while excess 2,4'-MDI and 4,4'-MDI are removed by distillation, excess pMDI is not removed and remains in the prepolymer residue. Therefore, the resulting product is a prepolymer consisting of pMDI / 2,4'-MDI / 4,4'-MDI / PPG + "free" pMDI.

[0066] [Table 4]

[0067] The prepolymer manufactured using Ongronat CO 4150 (registered trademark) exhibited an optimal balance of high NCO value and low viscosity.

[0068] (Effect of polyol molecular weight) The effect of molecular weight (Mw) of polyether polyols (PPGs) on viscosity was evaluated.

[0069] Table 5 shows a comparison of the viscosities of various LF prepolymer compositions.

[0070] [Table 5]

[0071] The results in Table 5 show that removing free monomers more effectively increased the NCO ratio (%) and decreased viscosity than conventional methods. The NCO value increased by lowering the molecular weight of the polyol or by adding a low molecular weight diol. However, the viscosity increased when a low viscosity diol such as tripropylene glycol was added.

[0072] (Effect of NCO:OH ratio) The effect of the NCO:OH ratio on viscosity was evaluated. Table 6 shows a comparison of different NCO:OH ratios.

[0073] [Table 6]

[0074] Table 6 shows that although the viscosity profiles of the obtained LF prepolymers are similar, the higher the NCO:OH ratio, the higher the %NCO.

[0075] (Cast polyurethane elastomer) In Example 8 of the present invention, a 4,4'-MDI / PPG-2000 prepolymer was produced in the first step. After the reaction, 30% pMDI was added, and finally, mold release was performed.

[0076] [Table 7]

[0077] The results in Table 7 show that the viscosity was reduced by the manufacturing method of Example 7 of the present invention, even with the same NCO content. Furthermore, the color was also improved.

Claims

1. A polyurethane prepolymer composition manufactured by a method including the following steps: (1) A step of reacting pMDI with a polyol to form a polyurethane prepolymer containing residual monomeric MDI: (2) A step of reducing the amount of residual monomeric MDI in the polyurethane prepolymer.

2. A polyurethane prepolymer composition manufactured by a method including the following steps: (1) A step of reacting monomeric MDI with a polyol to form a polyurethane prepolymer containing residual monomeric MDI: (2) A step of mixing pMDI into the polyurethane prepolymer to form a polyurethane prepolymer mixture containing residual monomeric MDI: (3) A step of reducing the amount of residual monomeric MDI in the polyurethane prepolymer mixture.

3. A polyurethane prepolymer composition according to claim 1 or 2, wherein the amount of residual monomeric MDI relative to the total weight of the prepolymer is 0.4% by weight or less, preferably 0.1% by weight or less.

4. A polyurethane prepolymer composition according to claim 1 or 2, The polyurethane prepolymer composition wherein the polyol is a polyether, preferably PPG, more preferably PPG with a molecular weight of 200 to 10,000 g / mol, and even more preferably 500 to 4,000 g / mol.

5. A polyurethane prepolymer composition according to claim 1 or 2, A polyurethane prepolymer composition wherein the NCO:OH ratio of the polyurethane prepolymer is 1:1 to 10:1, preferably 3:1 to 8:1, more preferably 5:1 to 7:1, and most preferably 7:

1.

6. The polyurethane prepolymer composition according to claim 1 or 2, wherein the reduction step is performed by distillation.

7. A method for producing a polyurethane prepolymer, including the following steps: (1) A step of reacting pMDI with a polyol to form a polyurethane prepolymer containing residual monomeric MDI: (2) A step of reducing the amount of residual monomeric MDI in the polyurethane prepolymer.

8. A method for producing a polyurethane prepolymer, including the following steps: (1) A step of reacting monomeric MDI with a polyol to form a polyurethane prepolymer containing residual monomeric MDI: (2) A step of mixing pMDI into the polyurethane prepolymer to form a polyurethane prepolymer mixture containing residual monomeric MDI: (3) A step of reducing the amount of residual monomeric MDI in the polyurethane prepolymer mixture.

9. A method for producing a polyurethane prepolymer according to claim 7 or 8, wherein the NCO:OH ratio of the polyurethane prepolymer is 1:1 to 10:1, preferably 3:1 to 8:1, more preferably 5:1 to 7:1, and most preferably 7:

1.

10. The method for producing a polyurethane prepolymer according to claim 7 or 8, wherein the reduction step is performed by distillation.

11. Use of the polyurethane prepolymer composition according to claim 1 or 2 for the manufacture of adhesives and sealants.

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