Taking a fresh look at optical crowding of TiO_2: the role of nanocarbonate as synergistic optical extender

摘要

The light scattering potential of titanium dioxide (TiO_2) is maximised when the TiO_2 particles are correctly spaced in relation to the size of the particles, their spacing and the refractive index contrast with the surrounding medium. Nanoparticle extenders are claimed to improve TiO_2 spacing due to the small particles acting to separate larger TiO_2 particles (separation ～0.2 μm) and so preventing the TiO_2 particles from reaching optical crowding. This concept has been challenged by previous work [Diebold 2011] using a combination of Monte Carlo simulations and image analysis techniques. Surprisingly, the results showed that nanoparticles did not, in fact, space TiO_2 particles under any conditions examined. Instead, TiO_2 distributions and spacings were completely indifferent to the presence of smaller particles. This current paper undertakes to study why in certain cases extenders do in fact deliver significant synergy allowing TiO_2 levels to be reduced in practice. The use of nano calcium carbonate is demonstrated to provide a wide range of synergistic extension of TiO_2 when in combination with synthetic latex binder, but, as found in the previous theoretical work, only a limited synergistic range in direct pigment packing studies using slurry pigment mixes only. A schematic model of the action of calcium carbonate containing a high nanoparticle fraction in the presence of binder is proposed, and is related to a combination of the potential reduction of contact between latex and TiO_2 and an increase in effective pigment volume fraction. This model is supported by electron microscopy images, and a measure of the "hardness" of the skeletal elements making up the coating structure, via bulk modulus determination using mercury intrusion porosimetry, enables the effective incompressible pigment volume fraction in relation to compressible binder to be identified.