Thermoplastic Starch Composites with Highly Exfoliated Nano-Clay Fillers and Excellent Barrier Properties

Thermoplastic starch (TPS) nanocomposites with unprecedentedly high loadings of up to 15 wt.% of the nano-clays Laponite (LAP; a synthetic product capable of good dispersion in suitable media) or Montmorillonite (MMT; modified with dialkyldimethylammonium chloride) were prepared by means of our new, two-step TPS preparation protocol. In both the TPS/LAP and TPS/MMT composites, we achieved perfect dispersion and extensive exfoliation of the nano-clays, resulting in pronounced improvements in mechanical performance (modulus increased up to one order of magnitude) and in excellent gas-barrier properties (extremely small permeabilities for O₂, CO₂, and even H₂). MMT, owing to its larger platelet size and to the formation of partially exfoliated multi-layer structures, generated a percolating filler network that provided particularly strong reinforcement, especially at 15 wt.% loading. LAP, though more completely exfoliated, generated a somewhat smaller mechanical reinforcement, but it more strongly increased processing viscosity due to its high specific surface area, which generated highly stable physical crosslinking that persisted even at processing temperatures of T ≥ 120 °C. Efficient matrix-filler interactions were confirmed by thermogravimetric analysis, where the better-exfoliated LAP generated a higher stabilization. The combination of strong mechanical reinforcement with outstanding gas-barrier properties makes the TPS/MMT and TPS/LAP nanocomposites attractive for food-packaging applications, where their natural origin, non-toxicity, bio-degradability, and abundance of nanocomposite components are an additional bonus.