Investigation of shear thinning behavior and microstructures of MWCNT/epoxy and CNF/epoxy suspensions under steady shear conditions

This report investigates the steady-state viscosities of multiwall carbon nanotube (MWCNT)/epoxy and carbon nanofiber (CNF)/epoxy suspensions with varying filler concentrations under different shear rates at various temperatures. In situ observation of filler networks suggests the build-up of shear induced MWCNT and CNF agglomerates at low shear rates, which correlates with the measured shear thinning behavior. The agglomeration process in MWCNT/epoxy suspensions is enhanced at lower shear rates in the case of higher temperatures, whereas, at high shear rates, both nano-fillers show good dispersion. Shear thinning behavior is observed for both types of fillers, and shear thinning exponential parameters are evaluated as a function of filler content. The shear thinning exponent increases in conjunction with increase in filler content, but it is found to saturate at a specific value, independently of filler material. Finally, the micromechanical elasticity-based analogy model is applied to the prediction of steady state shear viscosity of suspensions at higher shear rates with the assumption of complete dispersion and alignment of individual nano-fillers in suspensions. The predicted viscosities and the experimental data at higher shear rates are compared. The results conclude that fairly good agreement can be seen for the cases of CNF/epoxy suspensions at lower temperatures, whereas MWCNT/epoxy suspensions and CNF/epoxy suspensions at higher temperatures show discrepancy between the prediction and the experimental data. (C) 2012 Elsevier Ltd. All rights reserved.