Abstract
This paper investigates the characterisation of alumina-doped titania nanoparticles, milled under high-shear over time, in the presence of sodium hexametaphosphate (SHMP) dispersant. Transmission electron microscopy (TEM) indicated that prolonged milling times led to the formation of 10 nm particle fines which were electrostatically attracted to larger particles, where no change in the crystal structure was observed. Primary particle sizes measured by dynamic light scattering (DLS) and TEM were in agreement and showed no change in primary particle size (∼250 nm) with respect to milling time, however, there was a clear reduction in the magnitude of the slow mode decay associated to aggregates. The TiO2 was found to have an isoelectric point (iep) in the range of pH 3–4.5, where an increase in milling time led to a lower pHiep, indicative of an increase in SHMP coverage, which was further supported by an intensification in phosphorus content measured by X-ray fluorescence (XRF). Phosphorus content and zeta potential analysis before and after centrifugal washing showed that SHMP was partially removed or hydrolysed for the longer milled pigment samples, whereas no change was observed for shorter milled samples. Relaxation NMR was also performed, where enhanced relaxation rates at longer milling times were associated partially to increases in surface area and exposure of Al sites, as well as physicochemical changes to SHMP density and structure. It is thought that extended milling times may lead to hydrolysis or other structural changes of the dispersant from the high energy milling conditions, allowing easier removal after washing for longer milled pigments.
Highlights
Titanium dioxide (TiO2), commonly called titania, occurs in three abundant crystallographic forms, rutile, anatase and brookite [1,2], and is a key pigment used in a wide range of consumer products such as paints, cosmetics, pharmaceuticals and plastics [3,4,5,6]
Transmission electron microscopy (TEM) highlighted the presence of $20 nm fractured fines in samples milled for 32 min that were found to have dspacings correlating to a rutile crystal structure, implying that milling did not induce changes in crystal structure
The appearance of fractured TiO2 fines is even more significant, as many pigment production processes have similar unit operations and it could be assumed that many TiO2 pigment samples may contain these fractured fines if the TiO2 reactor discharge is milled for long times
Summary
Titanium dioxide (TiO2), commonly called titania, occurs in three abundant crystallographic forms, rutile, anatase and brookite [1,2], and is a key pigment used in a wide range of consumer products such as paints, cosmetics, pharmaceuticals and plastics [3,4,5,6]. Commercial pigment type rutile titania is primarily produced by the chloride process, which was commercialised by DuPont in the 1950’s [6,8]. Small quantities of AlCl3 are added to the reactor feed to control pigment aggregation and favour the rutile phase [2,8,9]. Later in the production process, titania is subjected to various chemical surface treatments to improve durability and dispersibility, before being wet milled to control the particle size distribution and dried. The final quality of TiO2 in consumer products is dependent on a number of these variables where, in particular, the size of the milled dispersed crystallite aggregates is a key physical parameter that dictates overall whiteness and brightness [6,8,12]
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