Investigating the Effectiveness of Dispersants for Graphitic Carbon Black Suspensions

Abstract

The dispersability of graphitic carbon black (Monarch 1000) selected as a model for carbon nanotubes has been investigated in aqueous and non aqueous media using rheological, conductivity measurements and atomic force microscopy. The effectiveness of eight dispersants used for water was investigated namely polyethylene oxide polypropylene oxide ABA copolymers (PE/F 103 with 2x16 ethylene oxide units and PE/F 108 with 2x148 ethylene oxide units), Triton X100 and Triton X405 which contains an alkyl (octyl) phenol group with 10 and 40 ethylene oxide groups attached respectively, Lugalvan BNO12 which is a Naphthol Ethoxylate with 12 ethylene oxide units, sodium dodecylsulfate (SDS) an anionic surfactant with a tail of 12 carbon atoms and sulphate group attached to the tail and Sodium dedecylbenzenesulfonate (SDBS) which contains benzene ring in its anchoring group and NPE1800 (nonyl phenyl polypropylene oxide-polyethylene oxide with 27 ethylene oxide units). While for non polar organic solvents three dispersants namely polyhydroxystearic acid (Hypermer LP1), PEG 30-dipolyhydroxystearic acid (Hypermer B246) and polyisobutylene succinimide (OLOA 11000) were used. Hypermer LP1 is homopolymer and Hypermer B246 is polyhydroxystearic acid/polyethylene oxide/polyhydroxystearic acid ABA block copolymer while OLOA 11000 has polar head group (polyamine) attached to a hydrocarbon chain (polyisobutylene). Two non polar organic solvents decalin and xylene were selected. Decalin is aliphatic in nature while xylene is aromatic and it was observed that dispersing carbon black in xylene was relatively easy but there was not much difference in results for either media, which showed that the role of aromaticity of medium in dispersing graphitic carbon black is not significant. Adsorption isotherms of all dispersants were studied. The adsorption isotherms of PE/F 103 in comparison with PE/F 108 and Triton X100 in comparison with Triton X405 revealed that in molar terms the adsorption decreases with increasing number of ethylene oxide units indicating that adsorption is governed by the size of PEO (polyethylene oxide) chain length. Triton X100, Triton X405, Lugalvan BNO12 and NPE 1800 contain aromatic rings in their anchor group and adsorbed more strongly and proved to be much more efficient stabilizers. SDBS also showed higher adsorption than SDS due to п-п interaction with the graphitic carbon black. In non aqueous media, adsorption is a minimum in molar terms for homopolymer Hypermer LP1 as compared to other polymers. As the whole polymer molecule has affinity to adsorb onto the surface and by consequence the whole molecule may lay flat onto the surface giving smaller adsorption amounts. While Hypermer B246 and OLOA 11000 both dispersants consist of an anchoring group which strongly adsorbs on the surface and stabilising chain which has good solubility in the solvent and extends sufficiently in the solvent to import stability. The relative viscosity-effective volume fraction curves were compared with the theoretical curves for the hard sphere dispersions calculated using Krieger-Dougherty equation and showed that Triton X100, Triton X405, Lugalvan BNO12, NPE 1800, SDS and SDBS dispersions could be prepared at much higher solid fraction than those dispersions stabilized by PE/F 103 and PE/F 108. The results indicate that the presence of aromatic groups in the hydrophobic group and sufficient number of ethylene oxide units in adsorbed layer of the surfactants is desirable in producing the stable dispersions for these graphitic carbon black dispersions and would be sensible choices in stabilising carbon nanotubes. In non aqueous media, Hypermer LP1 did not show good agreement with the Krieger-Dougherty equation; the viscosities were all slightly higher than that predicted by that equation. The other two dispersants Hypermer B246 and OLOA 11000 proved to be good stabilizers for crystalline graphitic carbon black as they made dispersions of lower viscosities. That means homopolymer Hypermer LP1 may be more suitable for polar particles but not effective for hydrophobic surfaces. For hydrophobic surfaces a dispersant with block copolymer structure is required rather than homopolymer. Oscillatory shear measurements showed high values of storage and loss modulus at high volume fractions indicating strong repulsive interactions between the carbon black particles. The effectiveness of all dispersants was investigated by measuring the electrical conductivity measurements of carbon black dispersions prepared by using polymers at their optimum concentrations. PE series and Hypermer LP1 produced flocculated dispersions of much higher electrical conductivity as compared to other polymers which might be due to less number of ethylene oxide units in adsorbed layer. The performance of polymers was also measured by atomic force microscopy which is a characterizing technique to evaluate the effectiveness of polymers by measuring the interaction forces (attractive or repulsive forces) between particles in the presence and in the absence of polymers. Spherical glassy carbon black (2-12 micron size) was used to model Monarch 1000 because a larger size carbon black particle was required in AFM and similar results were observed except PE/F 108. PE/F 108 showed repulsive forces on approach and separation which indicated it an effective stabilizer which was a contradiction with rheology and conductivity experiments. However PE/F 103 and Hypermer LP1 showed an attraction on approach and separation