Optimization of Melt Compounding Processing Conditions of Thermoplastic Polyurethanes Nanocomposites

In this work thermoplastic polyurethane (TPUs) nanocomposites incorporating carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) were prepared via melt blending and compression molding and the compounding process was optimized taking into account different physical properties of 1D CNTs and 2D GNPs. Long tortuous 1D CNTs can get easily entangled with each other, making mixing more efficient when the filler is added into the mixer with solid TPU granules. In this case, the high internal friction between TPU granules and rotating screws favors CNT disentanglement and improves filler dispersion within the matrix. On the other hand, 2D GNPs characterized by strong Van der Waals forces on the surface, tend to aggregate layer by layer. In this case, internal friction is not effective in exfoliating GNPs but will rather accelerate their aggregation. Therefore it will be more effective to mix GNPs within the TPU melt during compounding.

Since shear mixing was not effective enough to disentangle CNTs during melt mixing, CNTs were first treated with the surfactant BYK333 via ultra-sonication. Nanocomposites made with the surfactant treated CNTs exhibited significantly enhanced tensile properties and viscoelastic behavior by comparison with nanocomposites prepared without the surfactant, due to more homogeneous filler dispersion, as indicated by scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) shed some light on a second mechanism of improvement in the mechanical properties: the well-dispersed nanofillers can also favor higher phase separation in the TPUs, leading to a better microstructure able to enhance the load transfer and maximize the mechanical properties.