First normal stress difference in capillary extrusion flow of nanometer calcium carbonate‐filled PLLA biocomposites

The nanometer calcium carbonate (nano‐CaCO 3 )‐filled poly‐ L ‐lactide (PLLA) biocomposites were prepared using a twin‐screw extruder. The first normal stress difference of the composites were measured by means of a capillary rheometer under experimental conditions with temperatures ranging from 170 to 200°C and shear rates varying from 50 to 10 3 s −1 . The first normal stress difference ( N 1 ) increased roughly linearly with increasing shear stress (τ w ). The sensitivity of the N 1 to τ w increased with an increase of the die length–‐diameter ratio, and the N 1 value varied slightly with the filler weight fraction (ϕ f ) as test temperature was constant. When the shear stress was fixed, the N 1 reached a minimum value for ϕ f = 1%. The values of the N 1 of the composite melts decreased roughly linearly with a rise of temperature when the shear rate was constant. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers