Disclosed is a process to increase adhesion between two adjacent layers. The process comprises roughening the surface of one layer followed by chlorination of the roughened surface and joining of the layers, or treating the surface of one layer with a silicone-based adhesion promoter and joining the layers. In one preferred embodiment the process comprises roughening the surface of one layer, followed by chlorination of the roughened surface, joining of the layers and post-treatment of the molded layers at an elevated temperature for a predetermined time.