(220c) Time and Energy Dependent Properties of Stirred Wet Milled Alumina-Doped Titanium Dioxide, a Discretely Heterogeneous System

Time and Energy
Dependent Properties of Stirred Wet Milled Alumina-doped Titanium Dioxide, a
Discretely Heterogeneous System

David Austin^1*,
Timothy Hunter¹, Ali Hassanpour¹, Stephen Sutcliffe²,
John Edwards², David Harbottle^1†

1) School of
Chemical and Process Engineering, University of Leeds, United Kingdom; 2) Venator
Ltd.

Titanium
dioxide is widely used in coatings, inks and plastics due to the high
refractive index of the two polymorphs, rutile (R.I. = 2.74) and anatase (R.I.
= 2.54). Titania is often manufactured via the chloride process, during which,
up to 5 mol% alumina trichloride (AlCl₃) is added to promote the
formation of the rutile polymorph. This co-addition has led to questions around
the location of alumina in the primary particles. Garbassi et al.¹ determined the
alumina to be located in the bulk, not being observed via X-ray photoelectron
spectroscopy (XPS), or through acid leaching experiments, whilst Taylor et
al.² showed that with increased
dopant concentrations, both the surface and bulk concentrations increased, with
comparatively more alumina present at the surface.

  With the desired particle size for pigments being sub-micron,
d₅₀ ~ 0.3 mm, ultra-fine
grinding is achieved by stirred wet milling.  In the current study, we have
considered the surface properties of milled alumina-doped titania. By measuring
the zeta potential of the milled particles, we have shown that the particle
surface properties change during milling, with the particle iso-electric point
(i.e.p) shifting to higher pH values (Fig. 1). While there is good agreement
for milling speeds of 2500 to 6000 rpm, the pH i.e.p. for the particles milled
at 8000 rpm remains lower, albeit the specific surface area of the milled
particles is consistent with lower rpm values.  At 8000 rpm, reduced 1^st
order grinding kinetics were determined from laser diffraction analysis,
suggesting a change in the grinding behavior from bead-bead impact breakage
(lower rpm) to bead-bead and bead-wall shear breakage; a consequence of the
relationship between turbulent dispersion and centrifugal forces with milling
speed.  While the high shear forces may cause mechanochemical changes, X-ray
diffraction analysis showed no variance between samples (Figs. 2a and b).

Further
analysis of the milled samples using XPS revealed that the changing pH i.e.p. during
milling correlated linearly to the atomic % of surface alumina, ranging from
2.7 to 9.8 at. % over the pH i.e.p range of 5.7 to 8.1 (2500 – 6000 rpm). At
8000 rpm, dehydroxylation of surface sites is hypothesized, altering the zeta
potential behavior, as described in the high temperature treated surface
chemistry of α-alumina and rutile titania work of Yang³ and Sedev et
al.⁴ respectively –
this is an area of ongoing analysis.

This
study has demonstrated the evolving surfaces properties of alumina-doped
titania particles during milling, and work is ongoing to fabricate model,
nanoscopically smooth composite surfaces via atomic atomic layer deposition
(ALD), which can then be used to better understand the surface interactions with
milling dispersants and their influence on particle lubrication/dispersion.

References

1         F.
Garbassi, E. Melloceresa, E. Occhiello, L. Pozzi, M. Visca and D. M. Lenti, Langmuir,
1987, 3, 173–179.

2         M.
L. Taylor, G. E. Morris and R. S. C. Smart, J. Colloid Interface Sci.,
2003, 262, 81–88.

3         D.
Yang, M. Krasowska, R. Sedev and J. Ralston, Phys. Chem. Chem. Phys.,
2010, 12, 13724.

4         A.
Kanta, R. Sedev and J. Ralston, Langmuir, 2005, 21, 2400–2407.