Shear Rheology with a Wink to Polymer Processing
An interesting group of materials are polymers, like polyethylene’s, because their time constants of flow are in the same magnitude as their processing times for extrusion and injection moulding. At very short processing times polymers behave as solids while long processing times polymers may behave as fluids. This behaviour of polymers is referred to as visco-elastic behaviour. As processors strive to cut costs, processing times in extrusion and injection moulding have become shorter and shorter. As a consequence polymeric materials behave in a more elastic manner which has a negative effect on the process ability of these materials in injection moulding and extrusion processes. The increased elasticity will cause for example more frozen-in molecular orientation during the rapid cooling of the polymeric materials in the injection mould.
It is therefore very important to control the rheology of polymeric materials. The ultimate goal is to modify the structure of the polymers in such a way that the process ability of these materials increase without sacrificing the properties of the end product (tensile strength, impact resistance, hardness and so on). Rheology is the desired technique due to its sensitivity to changes in the structure of polymers. This paper describes a number of rheological testing techniques available at Intertek. Three different types of polyethylene, HDPE, LDPE and LLDPE, were studied applying these rheological testing techniques.
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