Process for preparing granules of pigments by means of double extrusion

A process for the production of a particle composition containing at least one of aldehyde, ketone and acrylic resin, and at least one pigment, comprising the following steps:

• (a) extrusion, in a co-rotating twin-screw extruder, of a mixture containing said at least resin and said at least one pigment without induced heating, using only the heat developed naturally by the cutting and friction forces inside the extruder, wherein the temperature inside the extruder according to step (a) varies from 5 to 50° Celsius, and the screws of the extruder according to step (a) have a length 20-40 25-48 times the diameter, and wherein an extrudate leaves the co-rotating twin-screw extruder after step (a) and comprises a non-granulated homogenous mixture of the at least one resin and the at least one pigment;
• (b) subsequent extrusion, with induced heating, of the mixture obtained from the extrusion according to step (a), wherein the temperature inside the extruder according to step (b) varies from 80 to 130° Celsius, and the screws of the extruder according to step (b) have a length 30-60 times the diameter;
• and
• (c) granulation of the mixture thus obtained, wherein the extrusion of step (a) comprises a plurality of zones that are temperature controlled such that a temperature of at least one zone is set to a temperature within a range of from 10 to 15° C. and wherein the temperature of one or more zones of the first extrusion step is controlled with at least a cooling set, and
• wherein the extrusion of step (b) comprises a plurality of zones that are temperature controlled such that a temperature of at least one zone increases from a temperature in a previous zone.