Wettability Reversal of Hydrophobic Pigment Particles Comprising Nanoscale Organosilane Shells: Concentrated Aqueous Dispersions and Corrosion-Resistant Waterborne Coatings.

We demonstrate the synthesis of polysiloxane-modified inorganic-oxide nanoparticles comprising a TiO2-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 the cross-linking monomer CH3Si(OH)3 onto the surface of Ti-Pure R-706 at pH 9.5 ± 0.2 in an aqueous suspension. After drying this surface-modified material in the presence of a polyanionic dispersant so as to preserve the primary particle size via dynamic light scattering, it is trimethylsilyl-capped with (CH3)3SiOH, which consumes some residual Si-OH functionalities, and washed to remove all dispersant and excess reagents. 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 rheological characteristics that are nearly the same as an aqueous dispersion of native unmodified hydrophilic Ti-Pure R-706 comprising an optimal amount of the organic anionic dispersant. It is also possible to synthesize dispersions without the use of an added surfactant and/or dispersant at even higher solid 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 a 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 wettability reversal in which the particles 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 of the polysiloxane coating for achieving this wettability reversal, a result that emphasizes the importance of surface reconstruction.

Stable Aqueous Dispersions of Hydrophobically Modified Titanium Dioxide Pigments through Polyanion Adsorption: Synthesis, Characterization, and Application in Coatings.

Polyanion dispersants stabilize aqueous dispersions of hydrophilic (native) inorganic oxide particles, including pigments currently used in paints, which are used at an annual scale of 3 million metric tons. While obtaining stable aqueous dispersions of hydrophobically modified particles has been desired for the promise of improved film performance and water barrier properties, it has until now required either prohibitively complex polyanions, which represent a departure from conventional dispersants, or multistep syntheses based on hybrid-material constructs. Here, we demonstrate the aqueous dispersion of alkylsilane-capped inorganic oxide pigments with conventional polycarboxylate dispersants, such as carboxymethylcellulose (CMC) and polyacrylate, as well as a commercial anionic copolymer. Contact-angle measurements demonstrate that the hydrophobically modified pigments retain significant hydrophobic character even after adsorbing polyanion dispersants. CMC adsorption isotherms demonstrate 92% greater polyanion loading on trimethylsilyl modified hydrophobic particles relative to native oxide at pH 8. However, consistent with prior literature, hydrophobically modified silica particles adsorb polyanions very weakly under these conditions. These data suggest that Lewis acidic heteroatoms such as Al(3+) sites on the pigment surface are necessary for polyanion adsorption. The adsorbed polyanions increase the dispersion stability and zeta potential of the particles. Based on particle sedimentation under centrifugal force, the hydrophobically modified pigments possess greater dispersion stability with polyanions than the corresponding native hydroxylated particles. The polyanions also assist in the aqueous wetting of the hydrophobic particles, facilitating the transition from a dry powder into an aqueous dispersion of primary particles using less agitation than the native hydroxylated pigment. The application of aqueous dispersions of hydrophobically modified oxide particles to waterborne coatings leads to films that display lower water uptake at high relative humidities and greater hydrophilic stain resistances. This improved film performance with hydrophobically modified pigments is the result of better association between latex polymer and pigment in the dry film.

Effects of freezing and thawing on the microstructure of latex paints.

Freeze-thaw (FT) cycles can aggregate particles in aqueous paint suspensions. To understand the mechanism of particle aggregation, cryogenic scanning electron microscopy (cryoSEM) was used to visualize the microstructure after the freezing and thawing steps of the FT cycle. After the freezing step, cryoSEM images show that the microstructure contains ice crystals and particle-rich regions. Adding propylene glycol, a FT stabilizing additive, leads to formation of larger ice crystals. After thawing, the dispersion structure revealed by cryoSEM shows that the particles redisperse only in the paint with the highest amount of propylene glycol. The other paints contain clusters that are different from the particle-rich regions found after the freezing step. Increasing the thawing rate leads to a more dispersed microstructure even in the absence of propylene glycol. Analysis of the cryoSEM results shows that particle aggregation into these clusters occurs during the thawing stage, and slow thawing conditions lead to more aggregation. The cryoSEM results reported here are used to propose a mechanism of aggregation of particles in the paint.

Spatially resolved solid-state 1H NMR for evaluation of gradient-composition polymeric libraries.

Polyurethane libraries consisting of films with composition gradients of aliphatic polyisocyanate and hydroxy-terminated polyacrylate resin were characterized using methods of (1)H NMR microimaging (i.e., magnetic resonance imaging, (MRI)) and solid-state NMR. Molecular mobilities and underlying structural information were extracted as a function of the relative content of each of the two components. Routine NMR microimaging using the spin-echo sequence only allows investigations of transverse relaxation of magnetization at echo times >2 ms. A single-exponential decay was found, which is likely due to free, noncross-linked polymer chains. The mobility of these chains decreases with increasing content of the aliphatic polyisocyanate. The concept of a 1D NMR profiler is introduced as a novel modality for library screening, which allows the convenient measurement of static solid-state NMR spectra as a function of spatial location along a library sample that is repositioned in the rf coil between experiments. With this setup the complete transverse relaxation function was measured using Bloch decays and spin echoes. For all positions within the gradient-composition film, relaxation data consisted of at least three components that were attributed to a rigid highly cross-linked resin, an intermediate cross-linked but mobile constituent, and the highly mobile free polymer chains (the latter is also detectable by MRI). Analysis of this overall relaxation function measured via Bloch decays and spin echoes revealed only minor changes in the mobilities of the individual fractions. Findings with respect to the most mobile components are consistent with the results obtained by NMR microimaging. The major effect is the significant increase in the rigid-component fraction with the addition of the hydroxy-terminated polyacrylate resin.