Control of carbon black aggregate size by using polystyrene-polyethylene oxide non ionic diblock copolymers

Abstract

The fine fraction of carbon black (CB) sample N234, initially prepared by the furnace process, was obtained by sieving and it was then dispersed in aqueous medium at various aqueous phase pH. Further, adsorption from water onto this non porous CB particles, of polystyrene-polyethylene oxide (PS-PEO), non ionic diblock copolymers, having molecular weights, Mw=2000 and Mw=4000g/mol, were investigated at ambient temperature. For the bare CB particles, prepared in the absence of copolymers, the TEM micrographs show aggregates and/or agglomerates having sizes depending of the aqueous phase pH, and formed from fused primary particles having sizes in the range 13–17nm. These aggregates develop intra-porosity that result from their branching shape. The CB bare particles prepared in acidic aqueous dispersion form mostly ellipsoidal agglomerates having sizes exceeding 500nm. However, by increasing the pH of the medium from pH=3 to pH=10, and after drying the CB dispersion, aggregates shaped as “bunch” of grapes and having smaller sizes are observed by TEM. The microelectrophoresis studies of these dispersions indicate that the CB particles are positively and negatively surface charged at pH values, respectively, below and above the Isoelectrical point (IEP) which is about pH∼6. Such electrokinetic behavior of the CB aqueous dispersion results from the presence on the CB surface of acidic and basic functional groups. However, upon PS-PEO copolymers adsorption onto CB, reduction in the magnitude of zeta potential, and shift of the Isoelectrical point (IEP) to lower pH values were observed. Such IEP shift is related to the copolymer adsorption on the Stern layer and to the position of the shear plane in the electrical double layer. In addition, the copolymer adsorption was found to reduce the CB particle size achieving a minimum value in water of about 200nm, resulting in sterically stable CB aqueous dispersions.