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Isocyanate-terminated prepolymer having an NCO-value of 5-30% by weight and being the reaction product of an excessive amount of diphenylmethane diisocyanate comprising at least 80% by weight of 4,4′-diphenylmethane diisocyanate and a polyoxyethylene-polyoxypropylene polyol having an average

Isocyanate-terminated prepolymer having an NCO-value of 5-30% by weight and being the reaction product of an excessive amount of diphenylmethane diisocyanate comprising at least 80% by weight of 4,4′-diphenylmethane diisocyanate and a polyoxyethylene-polyoxypropylene polyol having an average molecular weight of 2000-10000, an average nominal hydroxy functionality of 2-6, an oxyethylene content of 21-45% by weight and a structure of the type -PO-PO/EO-EO wherein the PO block comprises 60-90% of the PO and the ratio of tipped EO:random is 3:1 to 1:3, and a process for preparing a flexible foam by reacting in a mould and at an index of 70-120, the above prepolymer or a composition comprising this prepolymer and b1) a polyoxyethylene-polyoxypropylene polyol, having an average nominal hydroxy functionality of 2-6, and a oxyethylene (EO) content of more than 50% by weight; and optionally b2) a polyoxyethylene-polyoxypropylene polyol, having an average nominal hydroxy functionality of 2-6, an EO content of between 20-50% by weight and a primary hydroxy content of at least 50%, calculated on the number of primary and secondary hydroxyl groups; and c) water; and optionally d) additives and auxiliaries known per se.

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What is claimed is: 1. A process for preparing a flexible polyurethane foam, comprising: a pour-in-place-process reaction of a composition in a mould, the composition comprising: a polyisocyanate composition comprising: 1-99 parts by weight of an isocyanate-terminated prepolymer having an NCO-value

What is claimed is: 1. A process for preparing a flexible polyurethane foam, comprising: a pour-in-place-process reaction of a composition in a mould, the composition comprising: a polyisocyanate composition comprising: 1-99 parts by weight of an isocyanate-terminated prepolymer having an NCO-value of 5-30% by weight and being the reaction product of: an excessive amount of diphenylmethane diisocyanate comprising at least 80% by weight of 4,4′-diphenylmethane diisocyanate, and a first polyoxyethylene-polyoxypropylene polyol having an average molecular weight of 2000-10000, an average nominal hydroxy functionality of 2-6, an oxyethylene content of 21-45% by weight and from two to six -PO-PO/EO-EO tail structures connected to an initiator, wherein each structure linked to said initiator is a -PO-PO/EO-EO structure consisting of a first block of PO that is attached to the initiator, the first block comprises 60-90% of the PO, a second block of randomly distributed PO and EO connected to the PO block, and a third block of EO connected to the block of randomly distributed PO and EO that is the terminus of the -PO-PO/EO-EO structure, the ratio of third block EO to random EO being 3:1 to 1:3; and 1-99 parts by weight of an other polyisocyanate, calculated on 100 parts by weight of the polyisocyanate composition; 30-100 parts by weight of a second polyoxyethylene-polyoxypropylene polyol having an average nominal hydroxyl functionality of 2-6 and an oxyethylene (EO) content of more than 50% by weight; 0-70 parts by weight of a third polyoxyethylene-polyoxypropylene polyol having an average nominal hydroxy functionality of 2-6, an EO content of between 20-50% by weight and a primary hydroxyl content of at least 50%, calculated on the number of primary and secondary hydroxyl groups therein, and wherein the amounts of the second polyoxyethylene-polyoxypropylene polyol and the third polyoxyethylene-polyoxypropylene polyol being calculated on 100 parts by weight of the second polyoxyethylene-polyoxypropylene polyol and the third polyoxyethylene-polyoxypropylene polyol; and a blowing agent comprising water, wherein the mould has a foam barrier disposed on an outer layer and there is an absence of penetration of the outer layer by the flexible polyurethane foam. 2. The process according to claim 1, wherein the polyisocyanate composition comprises 10-90 parts by weight of the isocyanate-terminated prepolymer and 10-90 parts by weight of the other polyisocyanate. 3. The process according to claim 1, wherein the other polyisocyanate is selected from the group consisting of: 1) diphenylmethane diisocyanates (MDI), 2) crude MDI, 3) polymeric MDI, 4) modified variants of these MDIs containing urethane groups, obtained by reacting these MDIs with polyols having a molecular weight of at most 500, and/or carbodiimide groups, and/or uretonimine groups, such variants having an NCO-value of at least 20% by weight, and 5) mixtures of any of 1) through 4). 4. The process according to claim 2, wherein the other polyisocyanate is selected from the group consisting of: 1) diphenylmethane diisocyanates (MDI), 2) crude MDI, 3) polymeric MDI, 4) modified variants of these MDIs containing urethane groups, obtained by reacting these MDIs with polyols having a molecular weight of at most 500, and/or carbodiimide groups, and/or uretonimine groups, such variants having an NCO-value of at least 20% by weight, and 5) mixtures of any of 1) through 4). 5. A process for preparing a flexible polyurethane foam, comprising: a pour-in-place-process reaction of a composition in a mould and at an index of 70-120, the composition comprising: a) an isocyanate-terminated prepolymer having an NCO-value of 5-30% by weight and being the reaction product of: (i) an excessive amount of diphenylmethane diisocyanate comprising at least 80% by weight of 4,4′-diphenylmethane diisocyanate, and (ii) a polyoxyethylene-polyoxypropylene polyol having an average molecular weight of 2000-10000, an average nominal hydroxy functionality of 2-6, an oxyethylene content of 21-45% by weight and from two to six -PO-PO/EO-EO tail structures, each -PO-PO/EO-EO tail structure of said polyol consisting of a PO first block linked to an initiator, a second block of randomly distributed PO and EO linked to the PO first block, and a third block of EO linked to the second block of randomly distributed PO and EO and that the EO third block comprises the end of the tail structure, the first block of PO containing 60-90% of the PO and the ratio of third block EO to random EO is 3:1 to 1:3; or a polyisocyanate composition comprising: (i) 1-99 parts by weight of an isocyanate-terminated prepolymer having an NCO-value of 5-30% by weight and being the reaction product of: (A) an excessive amount of diphenylmethane diisocyanate comprising at least 80% by weight of 4,4′-diphenylmethane diisocyanate, and (B) a polyoxyethylene-polyoxypropylene polyol having an average molecular weight of 2000-10000, an average nominal hydroxy functionality of 2-6, an oxyethylene content of 21-45% by weight and from two to six -PO-PO/EO-EO tail structures, each -PO-PO/EO-EO tail structure of said polyol consisting of a PO first block linked to an initiator, a second block of randomly distributed PO and EO linked to the PO first block, and an EO third block linked to the second block of randomly distributed PO and EO and that the EO third block comprises the end of the tail structure, the first block of PO containing 60-90% of the PO and the ratio of third block EO to random EO is 3:1 to 1:3; and (ii) 1-99 parts by weight of an other polyisocyanate, calculated on 100 parts by weight of the polyisocyanate composition; b1) 30-100 parts by weight of a polyoxyethylene-polyoxypropylene polyol having an average nominal hydroxy functionality of 2-6 and an oxyethylene (EO) content of more than 50% by weight; b2) 0-70 parts by weight of a polyoxyethylene-polyoxypropylene polyol having an average nominal hydroxy functionality of 2-6, an EO content of between 20-50% by weight and a primary hydroxy content of at least 50%, calculated on the number of primary and secondary hydroxyl groups in polyol b2), the amounts of b1) and b2) being calculated on 100 parts by weight of b1) and b2); and c) water; and optionally; d) additives and auxiliaries known per se, wherein the mould has a foam barrier disposed on an outer layer and there is an absence of penetration of the outer layer by the flexible polyurethane foam. 6. The process according to claim 5 wherein polyol b2) has the structure PO-EO/PO-EO wherein the PO block comprises 60-90% by weight of the PO and wherein the weight ratio of third block EO to random EO is 1:3 to 3:1. 7. The process according to claim 5, wherein no polyol is used that has an average molecular weight of 2000 or more and an oxyethylene of less than 20% by weight. 8. The process according to claim 6, wherein no polyol is used that has an average molecular weight of 2000 or more and an oxyethylene of less than 20% by weight. 9. The process according to claim 5, wherein the process is conducted in a closed mould. 10. The process according to claim 6, wherein the process is conducted in a closed mould. 11. The process according to claim 7, wherein the process is conducted in a closed mould. 12. The process according to claim 8, wherein the process is conducted in a closed mould. 13. The process of claim 1 wherein the polyoxyethylene-polyoxypropylene polyol reacted with the diphenylmethane diisocyanate is the sole polyol to react with the diphenylmethane diisocyanate to produce said prepolymer. 14. The process according to claim 1, wherein the polyoxyethylene-polyoxypropylene polyol having EO content of between 20-50% by weight comprises an average molecular weight of 2000-10000, an average nominal hydroxy functionality of 2-6, an oxyethylene content of 21-45% by weight and from two to six -PO-PO/EO-EO tail structures connected to an initiator, wherein each structure linked to said initiator is a -PO-PO/EO-EO structure consisting of a first block of PO that is attached to the initiator, the first block comprises 60-90% of the PO, a second block of randomly distributed PO and EO connected to the PO block, and a third block of EO connected to the block of randomly distributed PO and EO that is the terminus of the -PO-PO/EO-EO structure, the ratio of third block EO to random EO being 3:1 to 1:3. 15. The process according to claim 1, wherein the flexible polyurethane foam comprises an overpack from 5% to 100%. 16. The process according to claim 1, wherein the mould comprises an upholstery material. 17. The process according to claim 5, wherein the mould comprises an upholstery material. 18. The process according to claim 5, wherein the mould has an outer layer. 19. The process according to claim 5, wherein the flexible polyurethane foam comprises an overpack from 5% to 100%.