Various fluids and polymer systems
Single and twin screw extruder have been used as reactors for various fluids and polymer systems. Reactions which have been conducted within extruders extend from controlled degradation reduction of molecular weight of polymer to grafting reactions or side group modifications and in-situ polymerizations.
An important variable when using a single screw extruder as a reactor is the mean residence time and the residence time distribution of the materials within a selected section of an extruder. By "mean residence time" we mean the average time of all fluid materials spent in a particular section of the extruder or of any process vessel. By the "residence time distribution" we mean the distribution of residence times about the mean which each fluid material spends in the extruder or process vessel.
Generally, extruders have been used in connection with reactions or operations which do not require a residence time distribution similar to that which would be obtained in a continuous stirred reactor. For example, in a polycondensation reaction, molecular weight build-up is achieved by the gradual coupling of A-B type monomeric units into dimers and trimers. Molecular weight is achieved by the coupling of two smaller chains to form one larger one. In this case, such polycondensation reaction favors a plug flow distribution because in a plug flow distribution, there is no back-mixing and instead, the material which enterssheet production extruder first also leaves the extruder first. Thus, in a polycondensation reaction, a plug flow distribution prevents the established chains from mixing with fresh monomer and instead, allows the chains to couple and build molecular weight.
There are however, certain types of reactions for which back-mixing is considered critical in order to obtain the desired product. For example, when one polymerizes methyl methacrylate to obtain poly (methyl methacrylate), (a free radical reaction) it is important that the molecular weight of the polymer be built and that the molecular weight distribution be narrow. As an initiated polymer chain begins to grow, it requires a constant fresh supply of monomer in order to build the molecular weight. This of course is unlike a polycondensation reaction which requires the coupling of dimers and trimers. In a plug-flow type of arrangement in an extruder, there would be no or little opportunity for fresh monomer to come into contact with the initiated polymer chain in order to continue to build its molecular weight. Instead, the initiated chains would continue through the extruder while fresh monomer would be added to the entry point to the extruder and would initiate to form other initiated chains of small molecular weight.