PROCESS FOR CONTINUOUSLY PRODUCING THERMOPLASTICALLY PROCESSABLE POLYURETHANES

1 . Process for continuously producing thermoplastically processable polyurethane elastomers, by mixing and reacting the following in a first static mixer with a shear rate from >500 sec −1 to <50 000 sec −1 :

one or more polyisocyanates A and

a mixture B which comprises hydrogen atoms that have Zerevitinov activity and which is made of

B1: from 1 to 85 equivalent %, based on the isocyanate groups in A, of one or more compounds which have, per molecule, at least one 1.8 and at most 2.2 hydrogen atoms that have Zerevitinov activity, and which have an average molar mass M n of from 450 to 5000 g/mol, and

B2: from 15 to 99 equivalent %, based on the isocyanate groups in A, of one or more chain extenders which have, per molecule, at least one 1.8 and at most 2.2 hydrogen atoms that have Zerevitinov activity, and which have a molar mass of from 60 to 400 g/mol,

and also from 0 to 20% by weight, based on the total amount of TPU, of further auxiliaries and additives C,

where the NCO/OH ratio of components A and B used is from 0.9:1 to 1.1:1, and the resultant reaction mixture is then introduced into a second static mixer which has lower shear rate than the first static mixer, where the conversion in the first static mixer is from 10% to 90%, based on starting component A.

2 . Process according to claim 1 , wherein the flow channels of the first static mixer have, transversely with respect to the direction of flow, an interior diameter in the range from 0.1 to 30 mm.

3 . Process according to claim 1 , wherein the wall shear rate in the first static mixer is in the range from 100 sec −1 to 50 000 sec −1 .

4 . Process according to claim 1 , wherein the residence time in the first static mixer is in the range from 0.1 sec to 5 sec.

5 . Process according to claim 1 , wherein the length/diameter ratio of the first static mixer is from 5:1 to 60:1.

6 . Process according to claim 1 , wherein the conversion achieved in the first static mixer is from 20% to 80%, based on starting component A.

7 . Process according to claim 1 , wherein the wall shear rate in the second static mixer is in the range from 1 sec −1 to 10 000 sec −1 .

8 . Process according to claim 1 , wherein the residence time in the second static mixer is in the range from 1 sec to 120 sec.

9 . Process according to claim 1 , wherein the length/diameter ratio of the second static mixer is from 5:1 to 30:1.

10 . Process according to claim 1 , wherein, during the mixing of the components in the first and/or second static mixers, heat is introduced or dissipated.

11 . Process according to claim 1 , wherein the second static mixer is followed by a mixer which ensures cooling of the reactant composition within 10 sec, where the cooling takes place to <300° C.

12 . Process according to claim 1 , wherein the temperature of the reacting components at the entry to the static mixer is >180° C.

13 . Process according to claim 1 , wherein component A comprises diphenylmethane diisocyanate isomer mixtures having more than 96% by weight diphenylmethane 4,4′-diisocyanate content.

14 . Process according to claim 1 , wherein component B1 comprises polyester diols, polyether diols, polycarbonate diols, or a mixture thereof.

15 . Thermoplastically processable polyurethane elastomers produced by the process of claim 1 .

16 . Process of claim 13 , wherein said component A comprises diphenylmethane 4,4′-diisocyanate and naphthylene 1,5-diisocyanate.