Measuring the rheology of thermoplastic polymer melts
Malvern uses rotational and capillary rheometry to characterize polymer melts
Rheology is the science of studying the flow and deformation of materials rooted in the laws of elasticity and viscosity proposed by Hooke and Newton in the late 17th Century. Thermoplastic polymer melts are widely used in many modern industrial processes to manufacture a multitude of objects. Polymers are used because they are relatively cheap to form into complex shapes in the molten state and therefore, we need to understand how they flow when being processed.
Polymers are complicated materials to characterize rheologically because there are many factors that influence their flow properties, such as, processing temperature, rate of flow, residence time etc. Furthermore the rheological properties of polymers lie between those of a liquid and a solid. This leads to time dependence of the flow properties and other important characteristics. 
Various rheological instruments are available for characterizing the flow and viscoelastic properties of polymers. Rotational rheometers are the preferred choice when the requirement is to obtain information concerning the molecular structure and how this affects processing characteristics. In particular, the ability to easily extract information about the average molecular weight and molecular weight distribution via measurement of the viscoelastic properties makes the rotational rheometer a powerful tool. The capillary rheometer extends the shear rate range attainable in the laboratory beyond that available in a rotational instrument and allows the flow properties to be measured under typical processing conditions. In addition, the ability to readily determine both the shear and extensional properties under real life conditions provide the polymer producer and processor with information that is vital to the successful use of a polymer melt. Finally, the capillary rheometer enables processing problems to be investigated in a controlled environment without the need to stop production on the factory floor.
In this white paper we discuss and compare rotational and capillary rheometry and the various tests that can be performed with each system to provide comprehensive insight into the structure, properties and processing characteristics of polymer melts.