Hydrophobic Inorganic Oxide Pigments via Polymethylhydrosiloxane Grafting: Dispersion in Aqueous Solution at Extraordinarily High Solids Concentrations

Abstract

Building on the recent demonstration of aqueous-dispersible hydrophobic pigments that retain their surface hydrophobicity even after drying, we demonstrate the synthesis of surface-modified Ti-Pure R-706 (denoted R706) titanium dioxide-based pigments, consisting of a thin (one to three monolayers) grafted polymethylhydrosiloxane (PMHS) coating, which (i) are hydrophobic in the dry state according to capillary rise and dynamic vapor sorption measurements and (ii) form stable aqueous dispersions at solid contents exceeding 75 wt % (43 vol %), without added dispersant, displaying similar rheology to R706 native oxide pigments at 70 wt % (37 vol %) consisting of an optimal amount of conventional polyanionic dispersant (0.3 wt % on pigment basis). The surface-modified pigments have been characterized via Si andC cross-polarization/magic angle spinning solid-state NMR spectroscopy; infrared spectroscopy; thermogravimetric and elemental analyses; and ζ potential measurements. On the basis of these data, the stability of the surface-modified PMHS-R706 aqueous dispersions is attributed to steric effects, as a result of grafted PMHS strands on the R706 surface, and depends on the chaotropic nature of the base used during PMHS condensation to the pigment/polysiloxane interface. The lack of water wettability of the surface-modified oxide particles in their dry state translates to improved water-barrier properties in coatings produced with these surface-modified pigment particles. The synthetic approach appears general as demonstrated by its application to various inorganic-oxide pigment particles.

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

“…Depending on the particular conditions, the Si–H functionality in water can slowly liberate hydrogen and react to synthesize the Si–OH functionality . Hydrogen release is unacceptable for coating applications, and the SiOH functionality synthesized in the process opens new questions about how significant this surface reactivity may be to the observed surface wettability changes in these materials . Here, for the first time, we demonstrate similar high solid concentrations of aqueous dispersions comprising initially hydrophobic pigment particles, which avoid Si–H on Si.…”

“…Hydrophobic modification of the R-706 surface was performed by condensing MeSi­(OH) 3 (K-MTHS) to the surface of amine-pretreated R-706 ( A-R-706 ), as shown in Figure . We chose to condense organosilane to A-R-706 rather than native R-706 because our previous work demonstrated the benefit of triethylamine (TEA) as a promoter for synthesizing uniform and robust polysiloxane nanoscale coatings on the R-706 surface . The most crucial aspect of this synthesis is control of the pH, which is critical to have in the range of 9.5 ± 0.2.…”

“…Capillary-rise measurements were performed using dried powder of materials, and the advancing water/air contact angle was determined using Washburn’s equation based on the reasonable assumption of a water contact angle for native R-706 of 50°. , For hydrophobically modified materials K-MTHS-A-R-706 and TMS-K-MTHS-A-R-706 , capillary-rise measurements were performed with the as-synthesized material after 48 h of drying and with the as-synthesized wet material that underwent a high-shear mixing stress test followed by 48 h of drying. For the capillary-rise test, a 2.5 mM bromothymol dye solution that was stabilized at pH 8 in an aqueous HEPES buffer was used.…”

“…To endow aqueous dispersibility to hydrophobic inorganic-oxide particles, research has previously focused on pigment encapsulation, such as with surfactants, , polymer shells, , adsorbed latices, or multilayers . Our research group has previously reported the synthesis of hydrophobic surface-modified pigment particles comprising adsorbed polysiloxane layers, , which most recently have been demonstrated to disperse at the high solid concentrations that are typically required for coating applications of up to 43 vol % (75 wt % pigment) solids . Although these aqueous dispersions overcome the need for stabilizing additives such as dispersants and surfactants, which are known to degrade water-barrier properties of the final coating, they have until now required the use of polymethylhydrosilane (PMHS) for surface modification, which consists of a reactive Si–H functionality.…”

Gravity-Driven Separation of Oil/Water Mixture by Porous Ceramic Membranes with Desired Surface Wettability

“…On the other hand, the hydrophilicity of the ceramic membranes has traditionally made them unsuitable for the separation process where a small amount of oils needs to be removed from the water. Various materials have been used to modify ceramic membranes to make them hydrophobic and oleophilic [19][20][21]. Among these materials, fluoroalkylsilane (FAS) is mostly used as a surface coupling agent for ceramics.…”