Extrusion and processing parameters for nanotube compounds

Nanotubes are of recent origin and need to be understood for their processing. Nanotube compounds require lower shear processing equipment for developing efficient conductivity and other properties. The three most basic aspects for successfully producing a polymer CNT compound are:
• Extruder and screw profile
• Compounds and formulation
• Extrusion parameters (in particular, the temperature profile)
Typically twin size co-rotating screw with extruder having L/D ratio between 36 and 48 is preferred. Uniform dispersion is generally more difficult to achieve with a shorter screw. Shorter screw gives inadequate intercalation (diffusivity of polymer melt into the free volume of CNT agglomerate).
The main factor that determines whether a compound can be produced with good conductivity depends on the compounding process and formulation. Along with the compounding methods, the electrical conductivity (EC) or electrical resistivity (ER) level of a CNT-based plastic product is also sensitive to the chosen mode and parameters of its conversion process. Typically, both the surface EC and volume EC can be achieved easily in a lower-shear process, such as extrusion blow moulding, cast and blown film extrusion, extrusion and rotational moulding. The EC of a compound typically increases with the extrusion temperature. This phenomenon is generally attributed to improved CNT dispersion delivering a more efficient wetting and melt intercalation. The relationships between extrusion screw speed and EC are in many cases non-linear. For most compounds, the optimum screw speed for EC usually falls at 250-350 rpm.