Polyester and polycarbonate will react with each other in the extruder (transesterification) thus forming block copolymers of polyester and polycarbonate that compatibilise the blend. However, you may need a long residence time in the extruder or more than one pass to get enough reaction. In a former job we added some catalyst to the polyester / polycarbonate blend to speed things up. Normal transesterification catalysts like antimony trioxide work as do many others.
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