Rheology to understand and optimize processibility, structures and properties of starch polymeric materials

This paper reviews the state of the art in the field of the rheology of starch polymers, including specially designed rheometric techniques and complex rheology as influenced by different conditions. In terms of rheometric techniques, off-line extruder-type capillary/slit rheometers are commonly used but subsequent changes during measurement often occur as starch structures are highly sensitive to thermomechanical treatment. An in-line rheometer set-up with a double-channel die incorporated to the processing extruder is a direct and effective method to minimize the processing history change at different testing shear rates. In addition, pre-shearing, multipass, and mixer-type rheometers are also suitable for starch polymers. The rheological behavior of starch polymeric materials can be greatly impacted by their formulation (botanical source, plasticizer and additive type and content, and the structure related to blend or composite) and processing conditions (temperature, mechanical energy, etc.). Starch polymer melts exhibit shear-thinning and extension-thinning behaviors, and shows strong elastic properties. A wide range of rheological models, considering formulation and processing conditions, have been reviewed for different multiphase systems. The rheological behavior can also be related to the compatibility (blends, composites), expansion/foaming properties, film blowing properties, etc. The significance of processing rheology of starch polymers lies in characterizing the complex melting and flow behaviors, characterizing the viscoelastic properties, determining optimal processing method and conditions, and better controlling the quality of the final products.