The Use Of Dual Capillary Rheometer Techniques As An Aid To Determining Optimum Processing Conditions In Multilayer Extrusion

Multilayer co-extrusion of plastics is fast becoming a very cost effective method of improving the barrier properties of plastic products. In this process individual polymers are melted and conveyed by separate extrusion systems, into a common distribution block and through a forming die where the polymer melts merge to form an integral multilayer structure. However, these multilayer structures are proving difficult to extrude because of their widely different temperature profiles required during extrusion, and the fact that all the melts enter a common die which can only be maintained at one particular temperature.

The melt rheological characteristics of a range of commercially available barrier materials, polyvinylidene fluoride (PVDF), a terpolymer of vinylidene fluoride, tetrafluoroethylene and hexafluoropropylene (THV), plasticised Nylons and tie layer materials have been studied using Dual Capillary Rheometric techniques. The relative change in shear viscosity with temperature, up to 270°C and shear rates up to 10^-5 sec^-1 have been investigated, for these materials, in order to determine optimum extrusion parameters during manufacture. The findings are confirmed by pilot plant tube extrusion trials using various multilayer structures and Arrhenius activation energies are also reported for the various polymers over a range of shear rates.