Commercial linear PLA grades do not offer the processability needed for some polymer processing operations involving extensional flow (e.g., film blowing, stretch blow molding, foaming, fiber spinning, thermoforming) owing to their low melt strength and absent or weak strain hardening behavior. Recently, there have been several attempts to broaden the processability and the end-use properties of PLA by developing PLA/clay nanocomposites.
The present work focuses on the possibility of conveniently tuning materials in PLA based nanocomposites in order to improve their processability in manufacturing processes where extensional flow is mainly involved.
Nanocomposites at a constant silicate loading were produced by melt compounding with a twin screw extruder using two commercial polylactide grades (PLA 4032D and PLA 2003D) with two different organo-silicates (Cloisite 30B and Nanofil SE3010). The two type of polylactide acid were characterized by different optical purity but similar shear viscosities, whilst the nanoclays were prepared with different organo-modifiers, providing the natural montmorillonite with peculiar polarity and basal interlayer spacing.
FTIR analysis evidenced the strongest polymer/organoclay interactions for the system PLA4032D/C30B, resulting in a higher clay dispersion and exfoliation levels, as revealed by the morphological characterization of the hybrids in both solid (TEM analysis) and melt state (dynamic shear measurements). The extensional rheological response of the different nanocomposites was affected by the initial nanostructure and the different polymer-clay affinity. In particular, for NSE3010 based hybrids a marked extensional thickening was evidenced, while the stronger polymer–silicate interactions and the better clay dispersion, observed in presence of C30B, determine increments in viscosity, both in dynamic and elongational flow.
