We define hierarchical composites (HC) as composites consisting of at least two reinforcements of very different sizes embedded in a matrix. The two main routes for reinforcing continuous carbon fibre reinforced polymers with carbon nanotubes (CNTs) involve either growing or attaching CNTs to the fibres or dispersing them into the matrix. Although both methods have had some success in improving matrix dominated properties, the latter process was proven to be more amenable for scale-up as conventional composite manufacturing processes can be adapted. Attempts to produce hierarchical composites by further reinforcing the matrix with CNTs have been hindered by processing difficulties caused by the increased matrix viscosity caused by the presence of CNTs and self-filtration.

Multiwalled nanotubes (MWNTs) were shear mixed with a solid epoxy resin with an uncured glass transition temperature of 39°C in concentrations ranging from 1 to 20 wt%. This process was optimized for maximum dispersion and distribution, which should, in turn, eliminate stress concentrations and maximize mechanical performance. The resulting nanocomposite was milled into a fine powder. This powder was used in a wet powder impregnation process to produce fibre reinforced thermosetting nanocomposite prepregs with a fibre volume content of 58%. The HC prepregs were laid up and press-claved to produce HC laminates, which were characterized with respect to their electrical conductivity and mechanical properties.

Matrix dominated properties, such the interlaminar shear strength, compression strength and through thickness electrical conductivity, improved significantly with the introduction of nanoreinforcement. Furthermore, we will show that composite toughness can be improved by creating hybrid composites with consisting of unreinforced matrix zones and nanoreinforced matrix regions. Electron microscopy shows not only that the presence of CNTs induced toughening mechanisms, but also the inhomogeneity of the matrix.