High colour purity iron oxide red - European-coatings.com

Quelle/Publication: European Coatings Journal 

Ausgabe/Issue: 04/2004 

Seite/Page: 20 

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 

Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000 

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.




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" 

Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000





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High colour purity iron oxide red - European-coatings.com

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