Residence time distribution: An old concept in chemical engineering and a new application in polymer processing

Polymer blending is a very important polymer processing process. It aims at preparing new polymer materials by blending existing polymers. However, most polymers are mutually immiscible, and their blends are thermodynamically unstable. Therefore, emulsifiers such as block or graft copolymers are often used for improving the adhesion of the interfaces and control the morphology of the resulting blends. The efficiency of a copolymer as an emulsifier is often evaluated by a so-called emulsification curve. The latter essentially follows the evolution of the dispersed phase domain size of the polymer blend as a function of the copolymer concentration. Polymer blending processes use mainly batch mixers and continuous mixers of type screw extruder especially twinscrew extruders. The former are mainly used in the laboratory and the latter both in the laboratory and for production purposes. An emulsification curve in a batch mixer can be built up in the following manner. Given compositions of the polymer components of the blend as well as the copolymer are charged to the mixer. After a certain elapse of time, the process reaches a steady state and samples are then taken from the mixer. The size of the dispersed phase domains of the corresponding blend is measured. This makes up a point on the emulsification curve. The earlier process is repeated upon varying the copolymer concentration. An emulsification curve is then builtup for the said composition of the blend. Correspondence concerning this article should be addressed to G. H. Hu at hu@ensic.inpl-nancy.fr or F. L. Feng at fenglf@zju.edu.cn.