High shear and transient viscosity of starch and maleated ethylene vinyl acetate copolymer blends

Blends of wheat starch and ethylene vinyl acetate copolymers with maleic anhydride functional groups, EVAMA, were melt blended in an extruder. The starch content in the blend was either 60 or 70% by weight. The steady shear viscosity, η, and the first normal stress difference, N₁, were evaluated at 150 and 170°C by extrusion feeding the material through a slit die. Both temperature and starch content affected η and N₁. Entrance pressure drop was obtained from a capillary rheometer from which extensional viscosity, η_u, was estimated. Transient data were obtained from stress growth and stress relaxation tests conducted using an RMS-800 (Rheometrics, Inc.) on compression molded samples of the blends. These blends displayed shear and extension thinning. η obtained by extrusion feeding the material through the slit die was lower than that obtained from the capillary rheometer indicating the effect of processing history on the rheology of these materials. The stress growth behavior of the blends showed strain and temperature dependence. Blends with higher starch content had nonlinear viscoelasticity compared to low starch content blends at the same shear rates. Blends with higher starch content relaxed faster both in stress relaxation tests and in the step-strain experiments. Increasing the temperature increased the relaxation behavior of blends with lower starch content but no appreciable effect on blends with higher starch content was ob-served. Both blends seem to have time-strain separability at long durations in the step-strain experiments. Blends with 70% starch did not show significant variations with temperature in the elongational, low shear viscosities, and relaxation modulus.