Expandable vinylaromatic polymers and process for their preparation

A process for preparing, in mass and continuously, an expandable vinylaromatic polymer which comprises, in series:

• i. feeding a vinylaromatic polymer to an extruder, together with a carbon black filler, having an average diameter ranging from 30 to 2000 nm, a surface area ranging from 5 to 40 m2/g, a sulfur content ranging from 0.1 to 2000 ppm and an ash residue ranging from 0.001 to 1%;
• ii. heating the vinylaromatic polymer to a temperature higher than the relative melting point, to obtain a molten polymer;
• iii. injecting an expanding agent and optionally an additive, into the molten polymer before extrusion through a die;
• and
• iv. forming expandable beads, through a die, in a substantially spherical form with an average diameter ranging from 0.2 to 2 mm.