Continuous preparation of thermoplastically processable polyurethanes

1. A multi-stage process for the continuous preparation of thermoplastically processable polyurethane elastomers (TPU) with tensile strengths of >30 MPa (measured in accordance with EN ISO 527-3), comprising

a) preparing a prepolymer I by reacting

A) at least one organic diisocyanate, with

B) a polyol 1 having on average at least 1.8 and not more than 3.0 Zerewitinoff-active hydrogen atoms and a number-average molecular weight {overscore (M)} n of 450 to 10,000;

b) reacting said prepolymer I prepared in a) with

C) a polyol 2 comprising an organic phosphorus-containing compound, which is different than polyol 1, wherein said polyol 2 has on average at least 1.5 and not more than 2.5 Zerewitinoff-active hydrogen atoms and a number-average molecular weight {overscore (M)} n of 100 to 5.000, and polyol 2 being present in an amount of 0.01 to 50 wt. %, based on the total amount of TPU,

 to yield a prepolymer II, wherein an equivalent ratio of NCO groups to the sum of NCO-reactive groups of from 1.2:1 to 10:1 is established, based on reaction components (A), (B) and (C);

c) reacting, in a high-viscosity reactor operating with a high shear energy, said prepolymer II prepared in b) completely with:

D) at least one low molecular weight polyol or polyamine having on average at least 1.8 and not more than 3.0 Zerewitinoff-active hydrogen atoms and a number-average molecular weight {overscore (M)} n of 60 to 400 as a chain lengthener;

wherein steps a) to c) are optionally carried out in the presence of F) catalysts, and optionally, with the addition of E) 0 to 20 wt. %, based on the total amount of TPU, of further auxiliary substances and additives, with the overall equivalent ratio of NCO groups to the sum of NCO-reactive groups being from 0.9:1 to 1.2:1, based on the sum of all the reaction components of steps a) to c).

2. The process of claim 1 , wherein B) said polyol 1 and C) said polyol 2, both of which contain Zerewitinoff-active hydrogen atoms, are selected from the group consisting of (i) polyester-polyols, (ii) polyether-polyols, (iii) polycarbonate-polyols, (iv) polyols which contain nitrogen, phosphorus, sulfur and/or silicon atoms and (v) mixtures thereof.

3. The process of claim 1 , wherein D) said low molecular weight polyols containing Zerewitinoff-active hydrogen atoms comprises ethylene glycol, butanediol, hexanediol, 1,4-di-(β-hydroxyethyl)-hydroquinone, or 1,4-di-(β-hydroxyethyl)-bisphenol A.

4. The process of claim 1 , wherein A) said organic diisocyanate comprises an aromatic diisocyanate.

5. The process of claim 4 , wherein said aromatic diisocyanate comprises a diphenylmethane-diisocyanate isomer mixture having a 4,4′-diphenylmethane-diisocyanate content of >96 wt. %.

6. The process of claim 1 , wherein steps a) and b) are carried out in separate reactors.

7. The process of claim 1 , wherein step c) is carried out in a separate reactor than steps a) and b).

8. The process of claim 1 , wherein step c) is carried out in a multi-screw extruder.