TiO2 PERFORMANCE | PIGMENTS
What makes TiO2
so special?
Lomon Billions Quality and Technical Manager Neil MacDonald, provided a primer on TiO2
at a recent pigments
event, explaining why it is so good at its job. This article summarises his key points
Two allotropes of TiO2 , rutile and anatase, are
commercially suitable for use as pigments. This is because they can be manufactured cost effectively, have very high refractive indices (RI), and scatter electromagnetic radiation efficiently - especially in the visible region of the spectrum. Figure 1 compares the RI of rutile and anatase
TiO2
with other materials that might be considered opacifying pigments. Although some materials have refractive indices approaching those of TiO2
,
the optimal combination of high refractive index and ease/cost of production is provided by TiO2
.
Figure 2 shows the difference in RI between the two allotropes and a number of common polymers (which determines effective reflectivity). To deliver opacity efficiently, a pigment must also be present in a well-dispersed particulate form with correct average particle size and narrow particle size distribution. For TiO2
pigments, the
optimal particle size for visible light scattering is about 0.25 microns. Ideally, all TiO2
particles would
be monodisperse with particle size of 0.25 microns. In practice, however, TiO2
is made with a range of
particle sizes centred on the optimal average size. The amount of oversize pigment particles must
be kept to the minimum if a high gloss surface finish is required. “Oversize” in this case means particles greater than about 0.4 microns. The
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presence of a high proportion of these particles, which can protrude from the surface of a blown film, for example, would make it difficult to obtain a high gloss.
Process effects TiO2
is made via two process technologies – Sulphate
and Chloride. In the Sulphate Process, ilmenite ore is purified by a sequence of crystallisation, filtration and washing techniques before being calcined to produce crystalline titanium dioxide. In the Chloride Process, pigmentary titanium dioxide pigment with extremely low levels of transition metal impurities is produced. The latter has benefits for the colour of the pigment – Chloride Process pigments generally have a cleaner, more blue tone than Sulphate Process pigments. The properties of most TiO2
pigments are further
tailored by application of coatings to the surface of the crystals. These coatings prevent compaction of the pigment during storage and transportation, encourage rapid redispersion during compound- ing, and minimise photochemical activity in its final application.
While the colour of Chloride Process pigments is generally preferred, Chloride TiO2
rutile pigments
are also more abrasive than Sulphate process rutiles. This can have an effect on compounding
October 2018 | COMPOUNDING WORLD 35
Main image: Neil MacDonald explains why TiO2 is such a powerful
addition to the range of plastic pigments
PHOTO: SHUTTERSTOCK
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