Process for incorporating pigments into thermoplastics, and adhesion promoters for use in this process

In a process for incorporating a pigment into a thermoplastic polymer, comprising mixing granules of the polymer, an adhesion promoter and the pigment whereby said mixture is treatable under high temperature operating conditions, the improvement wherein about 0.05 to 2% by weight of the adhesion promoter, based on the weight of polymer and pigment, is added in said mixture and wherein the adhesion promoter consists essentially of 95-99.9% by weight of an organic solvent compatible with the polymer and which is a plasticizer therefor and 0.1-5% by weight of a surfactant which is structurally stable under said high temperature operating conditions.