(46b) Dispersibility of Carbon Black As Function of Dispersing Intensity Using Hansen Dispersibility Parameters – Evaluation of Nonideal Mixing Effects Based on Normalized Relative Sedimentation Time

Dispersibility of agglomerated/aggregated, i.e. complex solids is of key interest for application in various dispersion-based formulations. To make an economic use of raw materials in suspension-based formulations, a high degree of dispersion, ideally down to the level of primary particles has to be obtained. This is related also to practical aspects like achieving high hiding power at lower pigment concentration and of outstanding material properties in composites. Dispersibility and the resulting quality of dispersion are determined by several factors including structure/quality of the raw materials and dispersion media, dispersion process and additives. Hansen Solubility Parameters (HSP) have gained growing interest for selection, optimization and prediction of suitable dispersion media, e.g. [1]. HSP concept builds on Hildebrandt solubility parameter (square root of cohesive energy between molecules, related to vaporization) but is subdivided into a disperse, a polar and a H-bond contribution to account for different kinds of interaction. Due to its root (cohesive energy), HSP describe thermodynamic (equilibrium) not kinetic properties. Extending HSP concept to dispersions of particulates in liquids has to depart from this solid basis, however, it has been proven to be widely? applicable in various fields [2]. This is why we suggest using the term Hansen Dispersibility Parameters (HDP) whenever the HSP concept is applied to dispersions and for characterization of surface properties. Extending the use of solubility parameters to dispersibility and dispersion stability, i.e. going from solutions to dispersions, a boundary is crossed. Instead of only interactions between molecules, interactions between molecules, between solvent molecules and particle surface, as well as interparticle forces have to be considered. While solutions are thermodynamically stable, dispersions (interface dominated) are inherently thermodynamically unstable systems. Experimental evaluation of dispersion stability of agglomerated/aggregated solids goes with the necessity of dispersing solids first hand, i.e. dispersion stability and dispersibility are usually studied in one experiment. Nevertheless, media for optimum dispersibility and optimum dispersion stability may be different. In case of complex solid particulates, intensity of the dispersion process must have an effect on HDP characteristic for the solid phase. It is often suggested [2] to use blends of ‘green’ solvents instead of harmful solvents to match optimal HDP values assuming ideal mixing. However, beside prevalent nonideality of mixing solvents, potential preferential adsorption of one solvent compound should be considered. Deviation of solvent composition in bulk and at interface should be rather the rule than the exception similar to preferential solvation in solvent mixtures [3].

The paper intends to investigate this complexity for dispersibility of carbon black with the focus on effect of the dispersion process. HDP were determined based on experimental sedimentation behavior by means of automatically monitoring the integral extinction (IE) over sample height by STEP-Technology [4] for carbon black dispersed in different individual liquids of the 3D-Hansen space (chemical mapping). A modified relative sedimentation (standard) time (RST) taking into account acceleration and optical path was introduced [5]. Higher RST values indicate higher dispersibility. Obviously RST values will be affected in case of fast agglomeration when particles are not properly stabilized by the solvent. Degree of dispersion processing intensity was varied between simple vortex mixing and sonication with varied input of energy. As predicted, best liquid(s) and HDP exhibit remarkable dependence on dispersion intensity. H-bond contribution reduces markedly in case of sonication with highest intensity. Noteworthy, liquids with H-bond acceptor properties proved to be especially effective in dispersion and dispersion stabilization.

To investigate and quantify nonideality for dispersion in binary liquid mixtures (experiments with low mixing intensity) it proved convenient to directly use RST values determined as function of mixing ratio. Synergistic and antagonistic effects were clearly identified and dispersibility improved.

Acknowledgements

The authors want to thank the funding of Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence “Engineering of Advanced Materials”. Moreover, we acknowledge the Federal Ministry of Economic Affairs through the Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke” e.V. (AiF, project no. KF 2347922UW4).

References

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