Agglomeration process of surfactant-dispersed carbon nanotubes in unstable dispersion: A two-stage agglomeration model and experimental evidence

Dispersion of carbon nanotubes (CNTs) is key in the fabrication of CNT composites. Unstable dispersion of CNTs in matrix with Ca2+ and high alkalinity (pH > 12) hinders the development of CNT composites in the matrix. This study addresses this issue by advancing understanding of the agglomeration process of CNT via a new agglomeration model and experimental evidence. Naphthalene (NC) and lignosulfonate (SL) based surfactants are used to disperse CNTs in saturated Ca(OH)(2) solution. A continuous drop (over 50%) in the degree of dispersion is observed over 24 h. The model and experimental results suggest that there are two stages in this agglomeration process. The first stage is the formation of small CNT bundles with hydrodynamic size 710-950 nm, within 1 h after ultrasonication. Individual CNTs with hydrodynamic size 190-220 nm mostly disappear during this stage. The second stage is the formation of large 3D meshes (>4140 nm) by these CNT bundles. These small bundles can remain suspended for 2.5 h. The results show that the preferred' number of CNTs in these bundles is 2-3 and 3-6 for SL and NC, respectively. This study suggests that dispersion of small CNT bundles is more stable than individual tubes and the degree of dispersion should be considered a time-sensitive parameter. (C) 2016 Elsevier B.V. All rights reserved.