Due to the production process, the nanocarbons are mainly present as agglomerates or aggregates. To allow their outstanding properties to fully develop, it is necessary to disperse them homogeneously and as gently as possible in the medium or in one of its components. Different technologies are used depending on the viscosity and nature of the medium.
Homogeneous distribution of CNTs (MWCNT: Nanocyl 7000) in an epoxy resin (Dow Epoxy DER 324) after the 1st run (left) and 4th run (right), device: Exakt Three Roll Mill
In high-pressure shear dispersion, which is especially suited to aqueous and low-viscosity media, the mixture is pumped under high pressure through a narrow nozzle. The resulting shear forces pull in principle the agglomerates apart. Hence this method is often used for MWCNT agglomerates.
Ultrasound dispersion is also well suited to aqueous and low-viscosity media, and especially for aggregated nanoparticles. Here, the forces break the aggregates apart during cavitation of the gas bubbles. With strongly entangled agglomerates, e.g. MWCNTs, the method is often insufficient on its own.
The ball mill is also well-suited to low-viscosity media, provided correspondingly small balls are used. Since the viscosity rises strongly as dispersion increases, certain limits are set here with regard to concentration. Since it is possible to apply relatively large amounts of energy using ball mills, care must be taken to ensure that the possible material damage is not too great.
The three-roll mill has proven to be effective in the dispersion of nanocarbons, especially MWCNTs, in low- to medium-viscosity media and also in high-viscosity pastes. Since the final particle size can be predefined via the roller spacing, it is possible to configure especially narrow particle-size distributions without residual agglomerates, which are often bothersome. In practice, this device is primarily used to disperse nanocarbons in reaction resin components.
Twin-screw extruders have proven to be especially well-suited to dispersing nanocarbons in molten thermoplastics. In order to ensure the highest possible shear-energy input, as is necessary for dispersing MWCNT agglomerates, it is recommended to feed the nanoparticles in directly together with the granulate at the start of the screw.
The twin-screw kneader has proven to be effective for the production and analysis of mixtures primarily consisting of elastomers, but also of thermoplastics, thermosets or ceramic moulding compounds with fillers, pigments and many other plastic and plasticisable substances in realistic conditions. A homogeneous mass is achieved through optimum mixing using tangentially interlocking rotors.