Quelle/Publication: European Coatings Journal
04/2004
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20
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High colour purity iron oxide red
Uwe Hempelmann, Volker Schneider.
Derived via calcination of a special, near-spheroidal iron
oxide yellow precursor, a new iron oxide red pigment has
been developed that features excellent colouristic as well as
application properties.
Although iron oxide pigments have been used in paints for
many thousands of years and synthetic iron oxides have
been produced on an industrial scale for more than 75
years, not all market requirements can yet be satisfied. In
particular, the brilliance of iron oxide red pigments is
restricted by natural causes and does not always meet user
requirements. Thus, the optimization of colour purity offers
scope for new developments.
Particle shape determines the properties
Many types of iron oxide pigment are available in three
basic colours: yellow, red and black. These have goethite (α
-FeOOH), hematite (Fe2O3) and magnetite (Fe3O4)
structures respectively and differ in their particle size and
shape. Chemically speaking, the basic colours are identical.
The tint is largely determined by the mean particle size and
the purity of colour by the particle size distribution width and
the trace element content. The physical properties, e.g.
influence on viscosity in pigment preparations, are largely
dependent on the particle shape.
Three synthesis routes are normally used in the
manufacture of red hematite pigments:
- Controlled oxidation of metallic iron in a two-stage process
(Laux process) in which the first stage yields a magnetite
intermediate which is then calcined to form the iron oxide
red end product;
- Direct precipitation of α-Fe2O3 from iron(II) salt solutions
under oxidizing conditions;
- Calcination of iron oxide yellow pigments.
To a certain extent, the synthesis routes also determine the
properties of the pigments. The Laux process yields
high-quality iron oxide pigments that combine grinding
stability, thermal stability and an isomorphic particle
geometry. The almost cubic structure is associated with a
relatively low viscosity in coatings. However, not all colour
loci can be achieved by this route.
Hematite pigments manufactured by the precipitation
process typically display good colour purity. However, they
are sensitive to grinding, i.e. the colour values change
noticeably after intensive dispersion. Moreover, their thermal
stability is often lower than that of pigments manufactured
by processes in which temperatures exceed 600 °C.
Calcined pigments retain the shape of their precursors
Iron oxide red pigments manufactured by calcining yellow
pigments display several unfavourable characteristics
associated with the hematite yellows used as intermediates.
For instance, the typical acicular (needle-like) structure of α
-FeOOH causes the much higher viscosity of iron oxide
yellow pigment preparations and highly pigmented coatings,
and it also acounts for an optical anisotropy, which is
responsible for the silking effect that occurs when a coating
is applied by brushing.
The acicular crystal structure of the yellow intermediate
remains virtually intact after calcination (pseudomorphic
transformation). Thus, the resulting red pigments also lead
to a higher viscosity in pigment preparations and, to some
extent, cause silking.
Highly branched pigments lead to novel properties
By modifying the manufacturing process it is possible to
produce highly branched primary particles that do not
display the unfavorable characteristics of acicular pigments.
One product manufactured in this way is "Bayferrox 915", a
yellow pigment. Figures 1 and 2 shows electron
micrographs of two different yellow precursor pigments
(Figure 1 a and b) as well as two red pigment test products
(TP 1 and TP 2) manufactured from these different yellow
intermediates (Figure 2a and b).
The influence of the different structure is revealed by
comparison of the viscosity of pigment preparations
containing various iron oxide pigments. Thus, the pigments
were tested at a pigment concentration of 60% in a
multipurpose shading paste. At this concentration, TP 2
could not even be incorporated into the dispersion, therefore
the maximum concentration of 51% was used. Table 1
shows the findings for the two test products and three
conventional pigments. The performance of TP 1 is almost
equivalent to that of the products manufactured by the Laux
process.
Table 2 and Figure 3 show the colour values measured in
an emulsion paint and in "Alkydal F 48", both for the full
shade and for a 1:5 reduction with titanium dioxide.
Compared with the "Bayferrox" standards, TP 1 yields more
saturated colours, whereas the acicular TP 2 yields no
significant improvement.
Once product optimization and scale-up to full production of
TP1 will be completed, preparations will be made for the
market launch of the new product.
Results at a glance
- Existing standard red pigments obtained by transforming
yellow pigments retain the unfavourable properties of their
acicular intermediates.
- A new test product TP 1 has been produced by calcination
of a highly branched, almost spherulitic iron oxide yellow
intermediate, which has been optimized for the subsequent
transformation.
- The new red pigment shows positive colouristic and
application properties, combining the colour purity and good
dispersion properties of pigments manufactured by the
precipitation process with the positive characteristics of
pigments produced by the Laux process and by calcination
of yellow pigments, e.g. thermal and grinding stability, as
well as low viscosity in pigment preparations.
The authors:
>
Dr.
Uwe
Hempelmann,
Bayer
Chemicals,
studiedchemistry at the University of Bielefeld, Germany. He
joined Bayer AG in Krefeld-Uerdingen in 1989 to work in the
field of inorganic pigments, especially iron oxides. Within
Bayer Chemicals AG he is now responsible for the
application of iron oxide pigmented paints and coatings as a
manager in the competence center paint. Dr. Hempelmann
is an active member of several national and international
working groups in the field of pigments and extenders.
> Dr. Volker Schneider, Bayer Chemicals, studied chemistry
at the University of the Saarland, Germany, where he also
obtained his Doctorate. He joined Bayer AG in 1988 as
research manager in the coatings raw materials division,
became technical marketing manager for concrete
protection systems in 1995 and since 2000 has been
technical marketing manager, competence center paints, in
the inorganic pigments group of Bayer Chemicals AG,
responsible for the application of iron oxide pigments in
paints and coatings.
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To figure 1 a-b, PDF p. 3 and 4:
Figure 1:
Electron micrographs of a) an acicular yellow pigment
("Bayferrox 3920"); b) a highly branched, almost spherulitic
yellow pigment ("Bayferrox 915")
To figure 2 a-b, PDF p. 5 and 6:
Figure 2: Electron micrographs of:
a) the red pigment TP2 yielded by calcination of the acicular
yellow pigment shown in Figure 1 a);
b) the red pigment TP1 with near-spherulitic structure, made
by calcination of the yellow pigment shown in Figure 1 b)
To figure 3 a-d, PDF p. 7 and 8:
Figure 3: a* and b* colour values of "Bayferrox 110 M", "120
M" and "130 M", and the two test products TP 1 and TP2:
a) full shade, in an emulsion paint;
b) reduced 1:5 with titanium dioxide, in an emulsion paint;
c) full shade, in "Alkydal F 48"; d) reduced 1:5 with titanium
dioxide, in "Alkydal F 48"
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