Polyurethane prepolymer composition

Abstract

A NCO-terminated polyurethane prepolymer composition based on the reaction of polymeric methylene diphenyldiisocyanate ("MDI") and / or polymeric methylene diphenyldiisocyanate ("pMDI") and polyols which further has a reduced amount of residual non-reacted diisocyanate monomers, as well as a process for preparing such polyurethane prepolymers.

Description

[0001] POLYURETHANE PREPOLYMER COMPOSITION

[0002] BACKGROUND OF THE INVENTION

[0003] Polyurethane prepolymers are widely used in various industries, including coatings, adhesives, sealants, and elastomers. For many adhesive applications it is preferable to provide polyurethane prepolymers with a low viscosity, a high NCO content and a low amount of residual diisocyanate monomers. As used herein, such prepolymers with residual non-reacted diisocyante monomers may be referred to as “low free” or “LF” prepolymers.

[0004] Methylene diphenyldiisocyanate (MDI) is a popular starting material for the production of polyurethanes prepolymers. As used herein “MDI” refers to the isomeric forms of monomeric MDI namely 4,4’- , 2,4’-, and 2,2’ -isomers of methylene diphenyldiisocyanate. As used herein polymeric methylene diphenyldiisocyanate (“pMDI”) is a mixture of monomeric (2-ring) MDI and polyaromatic (3-ring and higher) species of methylene diphenyldiisocyanates. Three ring and high species of MDI are represented by the formula I, where n is greater than or equal to 1.

[0005] (Formula I.) pMDI is often used for the production of rigid foam and as a binder in the wood-working industry.

[0006] Various routes have been pursued in order to provide LF MDI based prepolymers having low viscosity and higher NCO content. For example, Reese et al. (EP1518874) disclose a low- monomer polymethylenepolyphenylene polyisocyanate mixture formed by stripping a pMDI / MDI mixture alone to form a low-monomer pMDI, and then, in a second step, mixing the “low-monomer” pMDI / MDI mixture with polyurethane prepolymers to provide a foam.

[0007] A need has therefore been recognized in connection with providing further improved compositions and methods for preparing LF MDI polyurethane prepolymers with low viscosity, high NCO value and low amount of MDI monomers. It was surprisingly found that the drawbacks of the state of the art could be overcome by the reaction of pMDI with polyols performed with an additional distillation, which results in the formation of an N CO-terminated prepolymer having increased NCO content and having a lower viscosity then other known methods.

[0008] If not otherwise stated herein, it is to be assumed that all patents, patent applications, patent publications and other publications mentioned and cited herein are hereby fully incorporated by reference herein as if set forth in their entirety herein. As used in this description and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. For a better understanding of the present invention, together with other and further features and advantages thereof, reference is made to the following description; the scope of the invention will be pointed out in the appended claims.

[0009] BRIEF SUMMARY OF THE INVENTION

[0010] The present invention relates to an N CO-terminated polyurethane prepolymer composition comprising pMDI. The prepolymer composition has a low viscosity and increased NCO content and can be cured to produce a flexible and durable polyurethane adhesive material. The invention also provides a process for preparing such polyurethane prepolymers by reacting pMDI and polyols in a specific ratio and under specific reaction conditions or by reacting MDI and polyols in a specific ratio and under specific reaction conditions and admixing pMDI therewith.

[0011] There is broadly contemplated, in accordance with at least one presently preferred embodiment of the present invention, a polyurethane prepolymer composition is prepared by a process comprising the following steps: (1) reacting a pMDI and a polyol to form a polyurethane prepolymer comprising residual monomeric MDI; and (2) reducing the amount of residual monomeric MDI of said polyurethane prepolymer. In another embodiment of the present invention there is a polyurethane prepolymer composition prepared by a process comprising the steps: (1) reacting monomeric MDI and a polyol to form a polyurethane prepolymer; (2) admixing pMDI to the prepolymer thereby forming a prepolymer mixture comprising residual monomeric MDI; and (3) reducing the amount of residual monomeric MDI of said polyurethane prepolymer mixture. Furthermore, the polyurethane prepolymer composition in once embodiment has a residual monomeric MDI content of 0.4 wt% or lower, preferably 0.1 wt% or lower, based on the total weight of prepolymers. In another embodiment, the polyurethane prepolymer composition is formed of a polyether polyol, preferably PPG, more preferably PPG with a molecular weight of 200 to 10,000 g / mol and more preferably 500 to 4000 g / mol and even more preferably 1500- 2500 g / mol. The polyurethane prepolymer composition is formed in one embodiment by reducing the content of residual monomeric MDI by means of distillation. In a further embodiment there is a process for preparing a polyurethane prepolymer composition, comprising the steps of: (1) reacting a pMDI and a polyol to form a polyurethane prepolymer comprising residual monomeric MDI; and (2) reducing the amount of residual monomeric MDI of said polyurethane prepolymer. Similarly, in another embodiment there is a process for preparing a polyurethane prepolymer composition, comprising the steps of: (1) reacting monomeric MDI and a polyol to form a polyurethane prepolymer; (2) admixing pMDI to the prepolymer thereby forming a prepolymer mixture comprising residual monomeric MDI; and (3) reducing the amount of residual monomeric MDI of said polyurethane prepolymer mixture. In an embodiment of the process the reducing of the amount of residual monomeric MDI is accomplished by means of distillation. In a further embodiment polyurethane prepolymer compositions described above are used for the production of adhesives and sealants.

[0012] The preceding summary is not intended to restrict in any way the scope of the claimed invention. In addition, it is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[0013] DETAILED DESCRIPTION OF THE INVENTION

[0014] The polyurethane prepolymer compositions of the invention comprise a prepolymer segment composed of specific diisocyanate units and a specific polyol units.

[0015] More specifically, in one embodiment, the prepolymer segment is formed of the reaction of (A) a methylene diphenyl diisocyanate (MDI) unit, and (B) a polyol unit. In another embodiment the prepolymer segment is formed of the reaction of (A) a polyphenylene polydimethylene polyisocyanates diisocyanate (pMDI) unit and (B) a polyol unit.

[0016] In the present specification, the compound A unit in a prepolymer refers to a unit derived from the compound A among the repeat units that constitute the polymer chain. That is, the compound A in the polymerization of a polymer would be one of the repeat units that constitute the polymer chain after a certain component or bond at the terminal is modified by a reaction such as condensation polymerization. The polymer thus produced will thus contain the compound A unit in the main chain or side chain.

[0017] The unit (A) and the unit (B) may be interconnected with each other to form a chain in the prepolymer segment. For example, the unit (A) and the unit (B) may be interconnected with each other via an NCO bond.

[0018] Generally, the prepolymers of the invention are made using standard reaction processes and conditions as known in the art for the production of prepolymers and polyurethanes generally. Illustrative processes are described by way of example in U.S. Pat. Nos. 4,832,098, 4,934,425, 4,921 ,029, 4,784,201 , and 5,605,657, and U.S. application Ser. No. 09 / 919,994, filed on Aug. 2, 2001.

[0019] There is no particular restriction on the prepolymer, or mixture of prepolymers, that can be used in the present invention, nor is there a particular restriction on the polyols or diisocyanate monomers or polyols that can be used in the preparation of the prepolymer.

[0020] The diisocyanate component for preparation of the prepolymer may include aromatic and aliphatic diisocyanate monomers or polymers. Aromatic diisocyanates are well known and are widely used in the preparation of polyurethanes.

[0021] Aromatic diisocyanate monomers useful in the practice of the present invention include 2,4'- and 4,4'-methylene-bis-(phenyl isocyanate) (MDI), 2,4'- and 2,6'-toluene diisocyanate (TDI), paraphenylene diisocyanate (PPDI), tolidene diisocyanate (TODI), naphthalene-1 , 5-diisocyanate (NDI), diphenyl-4, 4'-diisocyanate, and mixtures thereof.

[0022] Aliphatic diisocyanate monomers include 1,6-hexane diisocyanate (HDI), dibenzyl-4,4'- diisocyanate, isophorone diisocyanate (IPDI), 1 ,3'- and 1 ,4'-xylene diisocyanates, 1 ,6- hexamethylene diisocyanate, 1 ,3'- and 1,4'-cyclohexyl diisocyanate (CHDI), the three geometric isomers of 1 ,1 '-methylene-bis(4-isocyanatocyclohexane) (H12MDI), and mixtures thereof.

[0023] In one embodiment, the diisocyanate monomers are aromatic diisocyanates namely a monomeric MDI. Monomeric MDI is, for example, selected from the group of diphenylmethane diisocyanate (MDI) with all its isomers (4,4'-diphenylmethane diisocyanate (4,4'-MDI), 2,4'-diphenylmethane diisocyanate (2,4'-MDI), 2,2'-diphenylmethane diisocyanate (2,2'-M DI)). In another embodiment, the diisocyanate component for preparation of the prepolymer is polymeric isocyanate for example pMDI. As explained earlier, pMDI is a mixture of monomeric (2-ring) MDI and polyaromatic (3-ring and higher) species of methylene diphenyldiisocyanates. These are, for example, the known industrial raw materials "crude MDI" and the "polymeric MDI" obtainable therefrom. The person skilled in the art understands the term crude MDI to mean the crude product obtained during the industrial synthesis of MDI after the phosgenation step, which is a mixture of the known binuclear MDI isomers and polynuclear (~3) oligomers. This is worked up by distillation in a further step to give so-called polymeric MDI, which is a crude MDI depleted in binuclear MDI isomers and other low-boiling by-products.

[0024] With respect to the polyol component of the prepolymer various polyols can be employed. Polyols include compounds having more than one hydroxyl, amino, or thiol functional groups or combinations thereof. The formation of such polyols is well known in the art. Such polyols may contain ester, ether, amide, aliphatic, acrylic, polylactic acid, polyglycolic acid, metal, metalloid and other functionalities as also known to those skilled in the art. In various embodiments, the polyol may comprise one or more of polyether polyols, polyester polyols, polycarbonate polyols, polycaprolactone polyols, polyacrylate polyols, polylactic acid, polyglycolic acid, and polyols containing blocks of different types of esters, ethers, amides, and / or other repeating groups or segments, and mixtures or combinations thereof. In some embodiments the polyols have a molecular weight ranging from 100 to 10,000, in one embodiment , from 225 to 6,000, and in another embodiment from 250 to 3,000. In this context, molecular weight refers to the number average molecular weight in Daltons which as used herein is calculated via Hydroxyl number measurement by ASTM E222-94 method.

[0025] In various embodiments, the polyol may comprise glycols, triols, and / or higher average hydroxyl functionality and having molecular weights ranging, for example those including from 50 to 600, and in another embodiment from 55 to 300, and in another from 60 to 200. Such polyols may include in one embodiment lower molecular weight polyols. The average hydroxyl functionality can range from about 2 to 8, preferably about 2 to 3 and more preferably from about 2 to 2.5. Such glycols or triols may include, for example, ethylene glycol, isomers of propylene glycol, isomers of butane diol, isomers of pentanediol, isomers of hexanediol, trimethylolpropane, pentaerythritol, poly(tetramethylene ether) glycol, poly(trimethylene ether) glycol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, and mixtures thereof. In another embodiment of the invention the polyols useful for making prepolymer are compounds of the polyester polyol type. These polyols are prepared by conventional methods using a combination of diacids and diols that are known in the art, for example succinate, adipate or other esters. The esters may also be prepared by the condensation reaction of hydroxyl carboxylic acids, for example lactic or glycolic acids. Esters may also be prepared from acyl chlorides. Illustrative of the polyester polyols are 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, copolymers (including block and random copolymers) and terpolymers thereof.

[0026] In one embodiment the use of PEAG polyol with an aromatic diisocyanate as the isocyanate component.

[0027] In one embodiment of the invention, the prepolymer segment comprises methylene diphenyl diisocyanate (MDI) as the diisocyanate unit (A). In another embodiment, the prepolymer segment comprises polyphenylene polydimethylene polyisocyanates (pMDI) as the polyisocyanate unit (A).

[0028] In a further embodiment, the prepolymer segment comprises PPG as the polyol unit (B).

[0029] Ratio NCO / OH

[0030] In the process for preparing the prepolymers of the present invention, the NCO:OH ratio is from 1 :1 to 10:1, preferably 3:1 to 8:1, more preferably 5:1 to 7:1 and most preferably 7:1.

[0031] Method of making LF-prepolymers compositions

[0032] Any process suitable for preparing a polyurethane prepolymer generally known can be used. In one embodiment of the present invention, the inventive polyurethane prepolymer composition is prepared by a process comprising the following steps: (1) reacting pMDI (A) and a polyol (B) to form a polyurethane prepolymer; and (2) distilling off monomeric MDI from said polyurethane prepolymer.

[0033] In another embodiment, the inventive polyurethane prepolymer composition prepared by process comprising the following steps: (1) reacting MDI (A) and a polyol (B) to form a polyurethane prepolymer; (2) admixing pMDI to the prepolymer thereby forming a prepolymer mixture; and (3) distilling off the monomeric MDI from said polyurethane prepolymer mixture.

[0034] LF technology

[0035] Any process suitable in reducing the amount of free diisocyanate monomer in the polyurethane prepolymer composition to the low levels of the present invention may be employed. A variety of methods is known for reducing the residual isocyanate content of diisocyanate monomers to a minimum such as wiped film evaporation, solvent aided distillation / co-distillation, molecular sieves, and solvent extraction. Distillation under reduced pressure is preferred, in particular thin film or agitated film evaporation under vacuum.

[0036] Amount of free NCO monomers

[0037] The NCO-terminated polyurethane prepolymers according to the invention have a content of monomeric, unreacted MDI monomer of less than 1 wt. %, preferably less than 0.5 wt. %, particularly preferably less than 0.2 wt.%, and most particularly preferably less than 0.1 wt.%.

[0038] The number of NCO groups per molecule is between 1 and 5, preferably 2 to 4, and in particular, exclusively reactive aromatic isocyanate groups are contained. The reaction products contain at least two urethane groups in the molecule.

[0039] Curative

[0040] The polyurethane prepolymers of the present invention can be cured with moisture or water. Suitable further curatives for the polyurethane prepolymer composition of the present invention include polyamines, polyols, or blends thereof. Suitable polyamines for the polyurethane prepolymer composition of the present invention include both aromatic and aliphatic diamines, primary and secondary amine terminated polyether polyols, and difunctional, trifunctional, and polymeric amines. Suitable of polyols for the polyurethane prepolymer composition of the present invention include polyester or polyether polyols, which can be diols, triols and tetrols, having primary, secondary and / or tertiary alcohol groups. These polyols may be mixed with diamines. Polyols are typically preferred over polyamines. In a preferred embodiment, the curative is 1 ,4-Butanediol

[0041] Additives

[0042] The polyurethane prepolymer composition of the present invention optionally comprises, where appropriate, further additives such as stabilizers, thickening agents, tackifying resins, fillers, plasticizers, thixotropic agents, color agents, pigments, solvents and / or drying agents. Stabilizers for the purposes of this invention refers on the one hand to stabilizers which have a viscosity-stabilizing effect on polyurethane prepolymers during production, storage and use. These are for example monofunctional carbonyl chlorides, monofunctional high reactivity isocyanates, but also non-corrosive inorganic acids, examples being benzoyl chloride, toluenesulfonyl isocyanate, phosphoric acid or phosphorous acid. Useful stabilizers for the purposes of this invention further include antioxidants, UV stabilizers or hydrolysis stabilizers. The selection of these stabilizers depends not only on the main components of the composition but also on the application conditions and the likely destabilizing stresses on the cured product. When the polyurethane prepolymer is predominantly constructed from polyether building blocks, there is mainly a need for antioxidants with or without UV protectants. Examples thereof are the commercially available sterically hindered phenols and / or thioethers and / or substituted benzotriazoles or the sterically hindered amines of the HALS (Hindered Amine Light Stabilizer) type.

[0043] Use of polyurethane prepolymers

[0044] The low-free pMDI polyurethane prepolymer of the present invention are typically used in applications such as coatings, adhesives, sealants or elastomers. In a preferred embodiment, the low-free pMDI polyurethane prepolymer of the present invention are used for adhesive applications.

[0045] Although the preferred embodiments of the present invention are described herein, it is to be understood that the invention is not limited to that precise embodiment, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. Hereinafter, the present invention is explained in more detail by the following examples. However, these examples are set forth to illustrate the present invention, and the scope of the present invention is not limited thereto.

[0046] EXAMPLES

[0047] Material

[0048] Ongronat® TR 4030 by BorsodChem (Mixture of monomeric MDI mixed isomers and polymeric MDI; NCO=30.7-32.7; f=2.5)

[0049] Ongronat® CO 4150 by BorsodChem (Mixture of monomeric MDI mixed isomers and polymeric MDI; NCO=30.8-32.8; f=2.5)

[0050] Ongronat® 2100 by BorsodChem (Mixture of monomeric MDI mixed isomers and polymeric MDI; NCG=30.0-32.0; f=2.6-2.7)

[0051] Ongronat® XP 1043 by BorsodChem (Mixture of polymeric MDI, 4,4'-M DI and 4,4'-MDI / PPG; 1.5% PPG; NCO=30.1-31.1 ; f=2.4)

[0052] PPG-500 by Lupranol® 1200 polypropylene glycol; PPG-500; Mw=500 g / mol; CAS

[0053] No.: 25322-69-4; (Commercially available from BASF)

[0054] PPG-1100 by Lupranol® 1100 / 1 polypropylene glycol; PPG-1100; Mw=1100 g / mol;

[0055] CAS No.: 25322-69-4; (Commercially available from BASF)

[0056] PPG-2000 by xyz (polypropylene glycol)

[0057] TPG by Tripropylene Glycol Regular Grade tri(propylene) glycol isomer mixture; TPG; CAS No.: 24800-44-0; (Commercially available from DOW)

[0058] BD 1,4-butanediol; CAS No.: 110-63-4; (Commercially available from Sigma

[0059] Aldrich)

[0060] Characterization of pMDI mixtures

[0061] Table 1. Total MDI isomer ratio by NMR Table 2. Free MDI (average) by HPLC

[0062] Non-polymeric MDI Comparative Example 1 (LFM G600)

[0063] The prepolymer was prepared as follows. The 4,4’-MDI (68.3%), available from Covestro under the trademark Mondur® M, is agitated and heated to 50°C under a blanket of nitrogen. Thereafter, charges of 26.4% PPG-1100 and 5.4% PPG-500 were added, and the reaction temperature was held at 70 °C for 4 hours.. Finally, excess residual 4,4’-MDI monomer was removed by thin-film distillation under reduced pressure from the reaction mixture to a level of less than 0.1 wt % residual 4,4’-MDI, and total % NCO content of 6.0%.

[0064] Preparation of pMDI prepolymers

[0065] In the following inventive examples, 4,4’-MDI of the Comparative Example is replaced by the pMDI mixtures.

[0066] Inventive Example 1

[0067] The same process was used as described before for Comparative Example 1, with the exception that Ongronat® TR4030 was used instead of 4,4’-MDI.

[0068] Inventive Example 2

[0069] The same process was used as described before for Comparative Example 1, with the exception that Ongronat® CO 4150 was used instead of 4,4’-MDI.

[0070] Inventive Example 3

[0071] The same process was used as described before for Comparative Example 1, with the exception that Ongronat® 2100 was used instead of 4,4’- DI .

[0072] Inventive Example 4

[0073] The same process was used as described before for Comparative Example 1, with the exception that Ongronat® XP 1043 was used instead of 4,4’-M DI . Inventive Example 5

[0074] The prepolymer was prepared as follows. The pMDI (63.4%), available from BorsodChem under the trademark Ongronat® CO 4150, is agitated and heated to 50°C under a blanket of nitrogen. Thereafter, a charge of 36.6% PPG-1100 was added, and the reaction temperature was held at 70 °C for 4 hours. Finally, excess residual 2,4’-M DI and 4,4’-M DI monomer was removed by thin- film distillation under reduced pressure from the reaction mixture to a level of less than 0.1 wt % residual 2,4’-MDI and 4,4’-MDI.

[0075] Inventive Example 6

[0076] The prepolymer was prepared as follows. The pMDI (48.3%), available from BorsodChem under the trademark Ongronat® CO 4150, is agitated and heated to 50°C under a blanket of nitrogen. Thereafter, a charge of 51.7% PPG-2000 was added, and the reaction temperature was held at 70 °C for 4 hours. Finally, excess residual 2,4’-M DI and 4,4’-M DI monomer was removed by thin- film distillation under reduced pressure from the reaction mixture to a level of less than 0.1 wt % residual 2,4’-MDI and 4,4’-MDI.

[0077] Inventive Example 7

[0078] The same process was used as described before for Inventive Example 6, with the exception that the pMDI charge was 40.0%, and the PPG-2000 charge was 60.0%.

[0079] Inventive Example 8

[0080] First, the prepolymer was prepared as follows. The 4,4’-MDI (38.5%), available from Covestro under the trademark Mondur® M, is agitated and heated to 50°C under a blanket of nitrogen. Thereafter, a charge of 61.5% PPG-2000 was added, and the reaction temperature was held at 70 °C for 4 hours. Secondly, after completion of the prepolymer preparation, 70.0% prepolymer was blended with 30.0% pMDI available from BorsodChem under the trademark Ongronat® CO 4150. Finally, excess residual 2,4’-MDI and 4,4’-MDI monomer was removed by thin-film distillation under reduced pressure from the reaction mixture to a level of less than 0.1 wt % residual 2,4’-MDI and 4,4’-MDI.

[0081] Table 3.

[0082] All four resulting prepolymers are stable with a high color (very dark brown), have more than double the target %NCO of 6% and have higher than expected batch yields.

[0083] Analysis of Inventive Examples 1 to 4 in Table 3 shows that the excess 2,4’-M DI and 4,4’-M DI is removed by distillation, however, the excess pMDI is not and remains with the prepolymer residue. Thus the resulting product is a prepolymer of pMDI / 2,4’-MDI / 4,4’-MDI / PPG + “free” pMDI.

[0084] Table 4.

[0085] The prepolymer prepared by using Ongronat® CO 4150 shows the best balance of high NCO and low viscosity.

[0086] Effect of polyol molecular weight

[0087] The effect of the molecular weight (Mw) of the polyether polyol (PPG) on the viscosity is evaluated.

[0088] Table 5. Comparison of viscosity of various LF prepolymer Compositions

[0089]

[0090] The results in Table 5 show that removal of free monomer more effectively increases % NCO and lower viscosity by previous means. The NCO value was raised either by lowering the molecular weight of the polyol or by incorporating low MW diol. However, adding low viscosity diols, such as tripropylene glycol, increases the viscosity.

[0091] Effect of NCO:OH ratio

[0092] The effect of the NCO:OH ratio on the viscosity is evaluated.

[0093] Table 6. Comparison of different NCO:OH ratios

[0094] Table 6 shows that the resulting LF prepolymers had similar viscosity profiles, however the higher NCO:OH ratio had a higher %NCO.

[0095] Cast polyurethane elastomers

[0096] In Inventive Example 8, a 4,4’-MDI / PPG-2000 prepolymerwas produced in a first step. Then, 30% pMDI were added after the reaction and then finally stripped.

[0097] Table 7.

[0098] The results in Table 7 show that method of preparation for inventive Example 7 lowered the viscosity for the similar % NCO. In addition, the color is also better.

Claims

CLAIMSWhat is claimed is:

1. A polyurethane prepolymer composition prepared by a process comprising the following steps:(1) reacting a pMDI and a polyol to form a polyurethane prepolymer comprising residual monomeric MDI; and(2) reducing the amount of residual monomeric MDI of said polyurethane prepolymer.

2. A polyurethane prepolymer composition prepared by a process comprising the following steps:(1) reacting monomeric MDI and a polyol to form a polyurethane prepolymer;(2) admixing pMDI to the prepolymer thereby forming a prepolymer mixture comprising residual monomeric MDI; and(3) reducing the amount of residual monomeric MDI of said polyurethane prepolymer mixture.

3. The polyurethane prepolymer composition according to claims 1 or 2, wherein the amount of residual monomeric MDI is 0.4 wt% or lower, preferably 0.1 wt% or lower, based on the total weight of prepolymers.

4. The polyurethane prepolymer composition according to claims 1 or 2, wherein the polyol is a polyether, preferably PPG, more preferably PPG with a molecular weight of 200 to 10,000 g / mol and more preferably 500 to 4000 g / mol.

5. The polyurethane prepolymer composition according to claims 1 or 2, wherein the prepolymer NCO:OH ratio is from 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 claims 1 or 2, wherein the reducing step is performed by distillation.

7. A process for preparing a polyurethane prepolymer composition, comprising the steps of: (1) reacting a pMDI and a polyol to form a polyurethane prepolymer comprising residual monomeric MDI; and(2) reducing the amount of residual monomeric MDI of said polyurethane prepolymer.

8. A process for preparing a polyurethane prepolymer composition, comprising the steps of:(1) reacting monomeric MDI and a polyol to form a polyurethane prepolymer;(2) admixing pMDI to the prepolymer thereby forming a prepolymer mixture comprising residual monomeric MDI; and(3) reducing the amount of residual monomeric MDI of said polyurethane prepolymer mixture.

9. The process according to claims 7 or 8, wherein the NCO:OH ratio is from 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 process according to claims 7 or 8, wherein the reducing step is performed by distillation.

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

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