The steady shear viscosity is the primary rheological property measured for plasticized starch melts. An increase in temperature or plasticizer content results in a lower viscosity of plasticized starch melts. The elastic component of plasticized starch melts can be characterized by analyzing the entrance and exit pressures data in a die viscometer to measure the extensional viscosity and the first normal stress difference. The Trouton ratio is commonly used to evaluate the elasticity of a material, and high Tr values indicate viscoelastic fluids. Understanding the elastic properties of plasticized starch is important because they are closely related to certain processing techniques such as film blowing and foaming.
Viscous Properties of Plasticized Starch Melts
The steady shear viscosity is the primary rheological property measured for plasticized starch melts because it is essential to understand the material’s melt-flow curve for practical production. The capillary/slit die viscometer is the most common method for studying the steady shear viscosity of plasticized starch, and some interesting rheometers are also used to understand the viscous properties of plasticized starch. The structural modification of starch during extrusion processing depends on several factors, such as SME, feed rate, screw speed, screw profile, and barrel pressure in an extruder. An increase in temperature or plasticizer content results in a lower viscosity of plasticized starch melts, while both higher amounts of water and glycerol result in a lower viscosity, but they may have different plasticizing effects on starch.
Elastic Properties of Plasticized Starch Melts
Plasticized starch melts are often considered viscoelastic, but their elastic properties are not commonly studied due to measurement difficulties (Dealy & Wissbrun, 1999). However, the elastic component of these melts can be characterized by analyzing the entrance and exit pressures data in a die viscometer to measure the extensional viscosity (ηe) and the first normal stress difference (N1) (Martin et al., 2003; Senouci & Smith, 1988). Martin et al. (2003) found a strong elastic behavior in plasticized starch melts based on the ηe and N1 results. The Trouton ratio (Tr), which measures the elasticity of a material, is commonly used, and Tr values > 3 indicate viscoelastic fluids (Jones, Walters, & Williams, 1987). Bhattacharya, Padmanabhan, and Seethamraju (1994) reported Tr values ranging from 25 to 50 for maize meal in uniaxial extensional flow, indicating a significant elastic behavior. Understanding the elastic properties of plasticized starch is important because they are closely related to certain processing techniques such as film blowing and foaming (Xie et al., 2012).