Method for producing re-expandable polycarbonate beads, and corresponding polycarbonate beads

A process for producing post-expandable foamed polycarbonate beads by means of

• a) continuous particle foam extrusion of a plastified mixture in an extrusion process,
• where the mixture contains the following components:
• i) aromatic polycarbonate, where the aromatic polycarbonate, based on the total weight of the aromatic polycarbonate, has a content of OH end groups of at least 350 ppm, determined by 1H NMR spectroscopy with dichloromethane as solvent at room temperature,
• and
• ii) a chain extender suitable for OH groups,
• wherein the aromatic polycarbonate and the chain extender suitable for OH groups are used in such a molar ratio that 0.64-1.10 mol of the reactive group of the chain extender is used per 1 mol of OH end groups of the polycarbonate,
• and
• iii) a physical blowing agent,
• at a temperature T of the plastified mixture of T1=(Tg+110° C.) to T2=(Tg+170° C.),
• where Tg is the glass transition temperature of the aromatic polycarbonate,
• wherein the dwell time of the aromatic polycarbonate and of the chain extender suitable for OH groups in the molten state is at least 4 minutes, and
• b) subsequent underwater pelletization of the blowing agent-containing melt at water temperatures T3=(Tg−90° C.) to T4=(Tg−55° C.), where Tg is the glass transition temperature of the aromatic polycarbonate,
• without pressurization or under pressurization of the water up to 3.5 bar.