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A Monte Carlo determination of the effectiveness of nanoparticles as spacers for optimizing TiO2 opacity

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Abstract

Optimal TiO2 opacity is achieved when the TiO2 particles are well spaced. Nanoparticle extenders are claimed to improve TiO2 spacing by positioning themselves between the larger TiO2 particles (0.25 μm) and preventing the TiO2 particles from approaching or touching one another. Claims have been made that this can as much as double the light-scattering efficiency of TiO2. This concept was tested by a combination of Monte Carlo simulations and image analysis techniques to determine computationally how the effectiveness of this approach is affected by TiO2 concentration, nanoparticle concentration, and nanoparticle size. Surprisingly, the results showed that nanoparticles did not, in fact, improve the spacing of TiO2 particles under any of the conditions examined (specifically, in the absence of any inter-particle interactions). Instead, TiO2 distributions and spacings were completely indifferent to the presence of smaller particles (however, large extender particles were found to cause TiO2 crowding, consistent with expectations and experiments). Explanation and implications of these findings are discussed.

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Notes

  1. The reader is cautioned that terminology of this phenomenon in the literature is somewhat confounding. In the early literature, this mechanism was termed “TiO2 spacing,”21 but this term was later changed to “TiO2 dilution”.11 This change was made because the original term was somewhat misleading, suggesting that extender particles push two nearby TiO2 particles apart and increase their separation distance.13 , 16 The term “TiO2 spacing” is now, in fact, used to describe this latter effect. The literature is further confused by one author’s unfortunate use of the term “TiO2 dilution” to describe the partial replacement of TiO2 pigment by small extender particles,24 rather than the replacement of large extender particles by small extender particles.

  2. Extender particles of intermediate size offer some light scattering advantages over large size extender particles, but still cause the pigment particles to crowd more than if the extender volume was instead occupied by resin. This can be accounted for in the Effective PVC calculation by including a variable termed “dilution efficiency” that modifies the magnitude of the contribution of extender volume to the PVC calculation.15

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  1. DuPont Titanium Technologies, Wilmington, DE, USA

    Michael P. Diebold

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  1. Michael P. Diebold
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Correspondence to Michael P. Diebold.

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This paper was awarded Second Place in the John A. Gordon Best Paper competition, at the 2011 CoatingsTech Conference, sponsored by ACA, March 14–16, in Rosemont, IL.

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Diebold, M.P. A Monte Carlo determination of the effectiveness of nanoparticles as spacers for optimizing TiO2 opacity. J Coat Technol Res 8, 541–552 (2011). https://doi.org/10.1007/s11998-011-9342-1

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