HIGH-SHEAR AND TRANSIENT VISCOSITY OF STARCH AND MALEATED ETHYLENE-VINYL ACETATE COPOLYMER BLENDS

Blends of wheat starch and ethylene vinyl acetate copolymers with male ic anhydride functional groups, EVAMA, were melt blended in an extrude r. The starch content in the blend was either 60 or 70% by weight. The steady shear viscosity, eta, and the first normal stress difference, N-1, were evaluated at 150 and 170 degrees C by extrusion feeding the material through a slit die. Both temperature and starch content affec ted eta and N-1. Entrance pressure drop was obtained from a capillary rheometer from which extensional viscosity, eta(u), was estimated. Tra nsient data were obtained from stress growth and stress relaxation tes ts conducted using an RMS-800 (Rheometrics, Inc.) on compression molde d samples of the blends. These blends displayed shear and extension th inning, eta obtained by extrusion feeding the material through the sli t die was lower than that obtained from the capillary rheometer indica ting the effect of processing history on the rheology of these materia ls. The stress growth behavior of the blends showed strain and tempera ture dependence. Blends with higher starch content had nonlinear visco elasticity compared to low starch content blends at the same shear rat es. Blends with higher starch content relaxed faster both in stress re laxation tests and in the step-strain experiments. Increasing the temp erature increased the relaxation behavior of blends with lower starch content but no appreciable effect on blends with higher starch content was observed. Both blends seem to have time-strain separability at lo ng durations in the step-strain experiments. Blends with 70% starch di d not show significant variations with temperature in the elongational low shear viscosities, and relaxation modulus.