Quelle/Publication: European Coatings Journal
06/2005
Ausgabe/Issue:
48
Seite/Page:
Obtaining opacity for orange and red organic
pigments
Gerhard Wilker, Gerd König.
A 1993 European Regulation made it mandatory to label
industrial and automotive coatings containing heavy metals.
As a consequence, coating manufacturers had to replace
lead-, chromate, and molybdate-containing pigments with
safe alternatives. Since then, numerous organic pigments in
brilliant colours and with outstanding fastness properties,
excellent flowability, and optimized particle size distribution
have been developed, and existing pigment grades have
been improved. In spite of this, it remained difficult to create
organic pigments, especially yellow and red ones, that offer
the same opacity as inorganic pigments. The physical
reasons for this are well known: unlike blue or green
shades, light absorption of yellow, orange and red organic
pigments is low, particularly for short wavelengths. Also, the
refractive index as a material constant cannot be modified in
these pigments.
One possibility to increase the opacity is to add inorganic
colour pigments. Due to their higher refraction indices, these
show a higher light scattering than optimised organic
pigments, ultimately leading to a coating with improved
opacity. This addition, however, may also lighten or dull the
original shade, affecting brilliance and purity.
Systematic pigment evaluation with all practical
parameters
Thus, are there any pigment grades requiring only a small or
no addition of colour pigments or light-absorbing pigments to
obtain the desired opacity? To answer this question,
comprehensive tests were undertaken to study the opacity
of suitable red and orange pigments. Practical parameters,
such as flowability and pigment load, as well as the change
in the coloristic potential, were systematically examined.
In the study, the binder of choice was a conventional system
consisting of a non-drying alkyd resin, based on a synthetic
fatty acid, combined with a highly reactive melamine. For the
coloration, various established pigment grades with
optimized particle sizes were selected (Table 1).
For each test, the binder and one pigment were mixed. The
pigment concentration was set at a constant mill base
viscosity of 1.5 Pa × s, ensuring smooth processing of the
paints. The pigment content, however, varied for the
different pigment types (Table 1). The colour mixture was
then diluted with suitable solvents to obtain a spraying
viscosity, the pigment content was determined (Table 1),
and the paint was applied to steel panels by automatic
spraying equipment.
After stove-drying, the film thicknesses and the contrast (∆E
in CIELAB units) were measured. Figure 1 shows that C.I.
Pigment Red 254 and C.I. Pigment Orange 36 offer good
opacity already for standard film thicknesses. C.I. Pigment
Orange 74 and 73, on the other hand, reach the desired
opacity only with a film thickness of 150 µm.
To improve the opacity, the organic C.I. Pigment Green 36
was then added. This is a pigment that generates
significantly purer shades than carbon black, usually used
for tinting. The resulting coloristic changes were quantified
by using the method described above.
pigment for an opaque coating film of 55 µm. As a result,
their coloristic quality is significantly reduced (Figure 2,
Table 2).
The diketopyrrolopyrrole pigments and C.I. Pigment Orange
36, however, show an entirely different picture. Adding only
small amounts of Pigment Green 36 is sufficient to produce
a 55 µm coating with good opacity. As a consequence, the
coloristic properties of these pigments show no or only
negligible change (Figure 2, Table 2). These pigments
therefore offer the best conditions to produce excellent,
coloristically pure, and brilliant coatings, not only in the
laboratory, but also in practical applications.
Although all of these details might be already known, this is
the first time that the results of this research are presented
as a systematic study of all practical parameters. The
objective assessment of the coloristic potential of a pigment
is only possible when all important aspects of the process
are taken into consideration, including viscosity and pigment
concentration. Thus, assessing Pigment Orange 36 from
this perspective lifts it into the rank of the best pigments in
this colour range, although some paint manufacturers view it
as being rather on the dull side.
The authors:
-> Dr. Gerd König, born in 1951, studied Chemistry at
Stuttgart University where he obtained his Doctorate in
Organic Chemistry in 1978 and subsequently worked as an
Assistant Lecturer. His industrial career began in 1980 as
Laboratory Director R&D at Riedel de Haën AG, Hannover.
Two years later he transferred to what was then Hoechst
AG, Frankfurt, where he worked in various positions. Today
Gerd König is the Head of Technical Marketing, Coating
Business, within Clariant´s Division Pigments & Additives.
-> Dipl. Ing. Gerhard Wilker, was born in 1947. After his
apprenticeship as chemical laboratory technician he studied
chemical engineering at the University of Applied Sciences
Darmstadt. In 1969 he worked in the application technology
pigment unit of the Hoechst AG. Since 1993 he has worked
as Head of the Referate Pigments for automobile lacquers,
first in the Hoechst AG and, since 1997, for Clariant GmbH,
Frankfurt. Today he is Senior Technical Manager for
Automotive Coatings within Clariant´s Division Pigments &
Additives.
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DPPs and P.O. 36 require very little tinting to become
opaque
C.I. Pigment Orange 73 and 74, both initially pure and
brilliant shades, require relatively high amounts of a tinting
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Quelle/Publication: European Coatings Journal
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Figure 1: Determination of opacity.
Figure 2: Colour changes in red and orange pigments after adding Pigment Green 36.
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Quelle/Publication: European Coatings Journal
06/2005
Ausgabe/Issue:
48
Seite/Page:
.
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Quelle/Publication: European Coatings Journal
06/2005
Ausgabe/Issue:
48
Seite/Page:
.
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