Comparison of rheological and electrical percolation phenomena in carbon black and carbon nanotube filled epoxy polymers

Epoxy nanocomposite suspensions including multi-wall carbon nanotubes (MWCNTs) and carbon black (CB) were produced and investigated by means of combined rheological and electrical analysis. The rheological percolation behaviour was compared to the electrical percolation behaviour. Due to similar dynamic agglomeration mechanisms the difference between the rheological and the electrical percolation threshold in the cured state is identical for MWCNT and CB filled systems. Non-covalent matrix-nanoparticle interactions in uncured epoxy suspensions are negligible since the onset of electrical and rheological percolation in the uncured state coincidence. Furthermore, the electrical percolation threshold in the cured state is always lower than in the uncured state because of the high tendency of CB and MWCNTs to form conductive networks during curing. The difference between rheological and electrical percolation threshold is dependent on the curing conditions. Thus, the rheological percolation threshold can be considered as an upper limit for the electrical percolation threshold in the cured state. Due to the formation of co-supporting networks multi-filler (MWCNTs and CB) suspensions exhibit a similar rheological behaviour as the binary MWCNT suspensions. For both types of suspensions a rheological percolation threshold of around 0.2 and 0.25 wt% was determined. Conversely, the binary CB nanocomposites exhibit a four-times higher percolation threshold of about 0.8 wt%. The difference between the binary MWCNT suspension and the ternary CB/MWCNT suspension in storage shear modulus at high filler concentrations (~0.8 wt%) turns out to be less than expected. Thus, synergistic effects in network formation are already present in the epoxy suspension and get more pronounced during curing.