Wettability reversal of hydrophobic pigment particles comprising nanoscale organosilane shells: concentrated aqueous dispersions and corrosion-resistant waterborne coatings.

We demonstrate synthesis of polysiloxane-modified inorganic-oxide nanoparticles comprising a TiO-based pigment (Ti-Pure R-706), which undergo drastic wettability reversal from a hydrophilic wet state to a hydrophobic state upon drying. Furthermore, the dry hydrophobic pigment particles can be reversibly converted back to a hydrophilic form by the application of high shear aqueous milling. Our synthetic approach involves first condensing crosslinking monomer CHSi(OH) onto the surface of Ti-Pure R-706 at pH 9.5 ± 0.2 in aqueous suspension. After drying this surface-modified material in the presence of a polyanionic dispersant so as to preserve primary particle size via dynamic light scattering, it is trimethylsilyl-capped with (CH)SiOH, which consumes some residual Si-OH functionality, and washed to remove all dispersant and excess reagent. Transmission electron microscopy demonstrates a ~6 nm polysiloxane coating uniformly surrounding the surface of the pigment particle. A 70 wt. % (37 vol. %) concentrated aqueous slurry of the hydrophobically modified pigment particles prepared in the absence of dispersant exhibits viscosity and shear stress as a function of shear rate that are nearly the same or smaller than that of the native unmodified hydrophilic Ti-Pure R-706 in aqueous suspension, with the latter suspension containing an optimal amount of organic anionic dispersant. It is also possible to synthesize dispersions without the use of added surfactant and/or dispersant at even higher solids concentrations of up to 75 wt. % (43 vol. %) in water, conditions at which even the hydrophilic native Ti-Pure R-706 oxide pigment yields a gel-like paste in the absence of dispersant. Films prepared by drying an aqueous suspension of these pigment particles exhibited a hydrophobic contact angle of ~125°. When acrylic-based waterborne coatings were prepared comprising these surface-modified Ti Pure R-706 pigments, they showed excellent corrosion protection of a mild steel substrate. These data point to a shear-induced wettability reversal, in which the particles can change from hydrophobic to hydrophilic upon high-shear aqueous milling, and vice versa upon drying. 29Si CP/MAS NMR spectroscopy highlights the importance of flexibility in the polysiloxane coating for achieving this changing surface wettability.