Process for making a polymer polyol

A continuous process for making a polymer polyol, the polymer polyol comprising:

• particles of a thermoplastic styrene-acrylonitrile-copolymer (TP);
• at least one polyol (P);
• and
• a stabilizer (S);
• wherein the stabilizer (S) comprises:
• from 10 to 70% by weight, based on the sum of all components, of at least one polyol P2, and
• at least one polyol CSP which is a reaction product of at least one macromere M, styrene and acrylonitrile in P2, optionally with an initiator and/or a chain transfer agent,
• wherein the macromere comprises one or more polymerizable double bonds and one or more hydroxyl-terminated polyether tails,
• a content of the macromere M in the stabilizer (S) is between 30-70 wt %, based on the sum of all components of the stabilizer,
• wherein the polyol CSP comprises two or more —OH groups,
• the process comprising:
• continuously feeding the TP, the P and the S to an extruder (E) having a plurality of process zones,
• preparing an initial dispersion of the TP, the P and the S in the extruder (E);
• continuously passing the initial dispersion obtained in the extruder into at least one rotor-stator device (RS) decoupled from the extruder comprising at least one rotor-stator combination;
• and
• cooling the rotor-stator treated dispersion below the glass transition temperature (Tg) of the thermoplastic styrene-acrylonitrile-copolymer (TP) after passing all of the rotor-stators (RS) to obtain the polymer polyol,
• wherein in the preparation of the initial dispersion, TP is continuously fed into a first process zone Z1 of the extruder E, S is continuously fed into a second process zone Z2 or a later process zone, and P is continuously fed into a process zone following the process zone of addition of S, wherein the terms “first” and “second” refer to a direction of flow of the reaction mixture through the extruder E.