Environmentally Benign Solution-Based Procedure for the Fabrication of Metal Oxide Coatings on Metallic Pigments

Article abstract of DOI:10.1002/open.202000223

Aluminum pigments were coated with Fe₂O₃ and CuO by solution-based thermal decomposition of the urea nitrate compounds hexakisureairon(III)nitrate and tetrakisureacopper(II)nitrate. The deposition process was optimized to obtain homo

Full text of DOI:10.1002/open.202000223

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Environmentally Benign Solution-Based Procedure for the
Fabrication of Metal Oxide Coatings on Metallic Pigments
[a]
[a]
[b]
[b]
Thorsten Bies, Rudolf C. Hoffmann, Matthias Stöter, Adalbert Huber, and
[a]
Jörg J. Schneider*
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Aluminum pigments were coated with Fe
O
and CuO by
urea molecules inside the coatings while the copper precursor
showed complete dissociation accompanied by in situ forma-
tion of amine complexes. The amount of organic residues
resulting from ligand fragments in the final oxide coatings
could be reduced to 22% for the iron oxide and 12% for the
copper oxide by further temperature treatment in solution
(259°C). Colorimetric investigations of the obtained pigments
revealed an excellent hiding power, outperforming the pig-
ments used in current state-of-the-art formulations.
2
3
solution-based thermal decomposition of the urea nitrate
compounds hexakisureairon(III)nitrate and tetrakisureacopper(II)
nitrate. The deposition process was optimized to obtain
homogeneously coated aluminum pigments. The growth of the
surface coatings was controlled by investigation with scanning
electron microscopy, energy dispersive X-ray spectroscopy and
static light scattering as well as infrared, X-ray diffraction and
thermogravimetric analysis. The iron precursor showed an
incomplete decomposition in solution, incorporating traces of
1. Introduction
Metal oxides have a long history as coloring agents in the form
[1–3]
of absorption pigments.
In the past decades, their applica-
tion has been extended, as they find usage in the formation of
special effect pigments, which are based on a multi-layer design
Figure 1. Schematic diagram showing interference-induced color change in
a multi-layer thin film arrangement. Diffuse reflection of light due to an
inhomogeneous surface is demonstrated on the right.
[4,5]
of materials with varying refractive indices.
If the core is a
metal effect pigment, e.g. aluminum, these pigments form a
mirror-like surface in a varnish and show targeted reflection of
light, leading to varying appearances when observed from
different angles (“flop-effect”). In order to obtain brilliant films,
it is important to synthesize homogeneous materials with most
perfect and smooth surfaces, as irregularities on the surface like
cracks or roughness lead to undirected reflection resulting in
[7]
thickness of only 1–50 nm. These types of pigments are used
e.g. in cosmetics, automotive coatings and secure document
[7]
applications. Owing to their multi-layer design, special effect
pigments like the oxide coated aluminum pigments, show
interesting optical phenomena. When they are irradiated, the
incoming light is diffracted at every layer into a reflected and a
transmitted part. Due to the resulting varying path difference
between the light waves, constructive and destructive interfer-
ence occur so that an angle dependent color change can be
observed (Figure 1). The main parameters influencing these
effects are the refractive index of the dielectric layer and its size
as these parameters explicitly define the path difference,
meaning that by defined choice of material and coating
thickness the properties and the appearance of the pigment
[6]
decreasing luster (Figure 1, right). Typically commercially used
aluminum base pigments which are widely used as substrates
for oxide coatings are the so called “silverdollar pigments”,
[7]
which have a thickness of about 150–300 nm. Aluminum
pigments accessible by physical vapor deposition (PVD) show a
[a] T. Bies, Dr. R. C. Hoffmann, Prof. Dr. J. J. Schneider
Department of Chemistry
Eduard-Zintl-Institut für Anorganische und Physikalische Chemie
Technische Universität Darmstadt
Alarich-Weiss-Straße 12
[4,7,8]
can be adjusted as desired.
Materials of choice for the layer
64287 Darmstadt (Germany)
formation are metal oxides with high refractive indices like
E-mail: joerg.schneider@tu-darmstadt.de
b] Dr. M. Stöter, Dr. A. Huber
Schlenk Metallic Pigments GmbH
Barnsdorfer Hauptstraße 5
[
9–11]
[12,13]
[14]
Cr O
and Co O
or mixed oxides like Bi Ti O
4 3 12
[
TiO2,
2
3
2
3
[
15,16]
and CoAl O .
Due to its absorption properties, an oxide
2
4
widely used for red and gold colored effect pigments is
91154 Roth (Germany)
[17,18]
Fe O .
2
3
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000223
The most common route for the coating of aluminum
©
2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
pigments with metal oxides is the sol-gel process which is
based on hydrolysis of metal salts (mainly alkoxides) in presence
of the substrate to be coated. In this process pH and temper-
ature must be controlled precisely to obtain a homogeneous
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any med-
ium, provided the original work is properly cited, the use is non-commercial
and no modifications or adaptations are made.
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film in desired thickness and smoothness, which is dried and
finally calcined in order to produce the desired oxide
order to be interesting and easy to apply under a potential
large-scale process. Furthermore, it should decompose at
relatively low temperatures with preferably low amounts of
organic residues. A promising group of such compounds
possessing all the desired characteristics are urea nitrate
complexes. Urea nitrate is a well-known compound that
combines an oxidizing agent (nitrate) with a fuel (urea), making
it a promising substance for the combustion synthesis of metal
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[19–21]
coating.
As an example, an oxidic layer can be deposited
by slow addition of a 3% FeCl3 solution into an aqueous
dispersion containing the substrates at 75°C and pH 3. The pH
is kept constant by simultaneous addition of NaOH. The coating
[20]
thickness can be controlled by the amount of iron salt added.
One major drawback of this procedure is that an aluminum
pigment used as substrate isn’t stable under the conditions
required for the precipitation, as it dissolves in both acidic and
[31]
oxides under controlled conditions (Figure 2). In this type of
reactions, the processing temperature needed for obtaining a
metal oxide can be drastically reduced e.g. compared to the
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[22]
basic media.
Therefore, it must be encapsulated in a
[32]
protective shell, normally SiO , prior to the desired metal oxide
widely used sol-gel type approach. The oxidizer-fuel combi-
nation leads to a strong exothermic decomposition when being
exposed to a thermal treatment. Thereby, it provides the overall
energy for the oxide formation and reduces the additional
external energy effort needed. This approach has been proven
suitable for the low temperature synthesis (200–300°C) of metal
oxide semiconductors starting from metal nitrate and
2
[7,23–26]
coating.
This additional layer renders the process more
difficult and is disadvantageous from a coloristic point of view,
as it causes the hiding power to decrease due to the higher
[27,28]
pigment thickness.
An alternative approach is based on a
variation of the chemical vapor deposition (CVD) technique,
where a metal oxide precursor e.g. Fe(CO) , is fed into a
5
[32]
fluidized-bed reactor together with the base pigment to be
coated and heated under an inert atmosphere. Through
controlled addition of oxygen or water as oxidizing agents the
precursor can be decomposed in a controlled manner (Eq. 1),
acetylacetone.
Furthermore, urea complexes are very versatile and have
[33–40]
been reported for a large variety of metals
thus making a
large group of metal oxides potentially accessible. It has already
been shown that indium, gallium and zinc oxides can be
deposited by use of these compounds as thin films by a spin
[29]
forming a homogeneous coating.
D; N2
[
31]
2
FeðCOÞ þ Al þ 1:5 O ��! Fe O =Al þ 5 CO2
coating technique for thin film transistor applications. In this
work we will devise a new route to obtain homogeneous metal
oxide coatings on thin physical vapor deposited metal effect
pigments, especially aluminum, in solution by use of the
thermal transformation of urea nitrate complexes resulting in
highly dense thin and smooth metal coatings.
5
2
2
3
core
(1)
Despite the good process control and the unnecessary
protective layer, there are still some downsides of this process
as the use of the precursor Fe(CO) generates severe safety
5
[29,30]
issues.
A further limitation is that ultrathin PVD pigments
are difficult to be completely fluidized owing to their high
surface which leads to agglomeration. It is therefore highly
desirable to overcome these current restrictions and to devise a 2. Results and Discussion
process which allows to generate even thinner pigments e.g.
resulting in improved hiding power.
2.1. Metal Oxide Precursor Synthesis and Characterization
Precursor compounds [Fe(urea) ](NO ) (1) and [Cu(urea) ](NO )
3 2
The aim of this work is to develop a synthetic approach for
the deposition of optically active metal oxide based thin films
on aluminum metal substrates that overcomes the drawbacks
of the established procedures described above. For the purpose
of reducing the processing effort and of preventing aluminum
corrosion forced by working in an acidic or basic solvent
medium, a new coating procedure based on the thermal
decomposition of a molecular single-source metal precursor
complex in an organic solvent is developed. This allows to
control the coating thickness and the optical parameters by
variation of the initial molecular precursor concentration. This
procedure also allows for the first time the direct coating of
ultrathin pigments e.g. obtained by PVD from solution without
any protective layer to shield the bare metal surface of the
aluminum base pigments towards corrosion. This new solution-
based approach could allow to reach better hiding power of
the coatings as compared to currently available established
special effect pigments based on a composite composition of a
metal substrate and an oxide. From a technological viewpoint
such a suitable precursor complex must fulfill certain require-
ments to be an appropriate candidate. It should be nontoxic,
stable under ambient conditions and most easy to synthesize in
6
3 3
4
(2) were synthesized by mixing stoichiometric amounts of metal
nitrates and urea in ethanol (1) or n-butanol (2) (Scheme 1)(all
spectroscopic data are given in the SI). Due to the coordination,
À 1
the CO stretching vibration shifted to 1625 cm in both
À 1
complexes compared to 1675 cm in uncoordinated urea. On
the other hand, the CN stretching vibration showed a
À 1
hypsochromic shift from 1459 cm in uncoordinated urea to
Figure 2. Schematic overview of the energy profile of a conventional thermal
pathway (blue) compared with an internal combustion reaction for the metal
oxide synthesis starting from a molecular precursor.
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Compound 1 decomposes in one steep step in the temperature
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[43,44]
range between 200–250°C.
The thermal decomposition of compound 2 is described
herein for the first time. 2 was found to decompose in three
main steps (Figure 3a). First, from 115°C to 160°C a mass loss of
4
% is found, followed by a second step from 160°C to 300°C
which generates 64% of the observed mass loss. In the last
step, up to 415°C 14% of the remaining mass loss is detected.
Figure 3 shows the ion currents of the corresponding fragments
as detected by TG-MS (b), the Gram-Schmidt intensities during
the decomposition process (c) and the IR spectra measured at
the maxima (d). The Gram-Schmidt curve shows that the gas
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Scheme 1. Reaction scheme of formation of the iron urea coordination
compound 1 using iron nitrate (a) or directly metallic iron (b). Formation of
the copper urea compound 2 using copper nitrate and urea (c).
evolving in the first step consists mainly of isocyanic acid
À 1
(
HNCO=2283 and 2252 cm ), which indicates a beginning
decomposition of the urea ligand. This is followed by decom-
position of urea nitrate which show signals corresponding to
-
1
À 1
HNCO (2283, 2252, 3535 cm ), N O (2243 and 2212 cm ), CO
2
2
À 1
À 1
[41–43]
-1
À 1 [45]
1
504 cm (1) and to 1508 cm (2).
In an alternative
(2358, 2327, 668 cm ) and NH (3331, 1625, 966, 930 cm ).
3
approach we have found that 1 is also accessible in a direct
reaction using elemental metallic iron, nitric acid and urea,
giving rise to the possibility of implementing that approach
into an even more eco-friendly procedure.
Thermogravimetric studies allow to control the purity of the
obtained precursors and to evaluate their thermal behavior in
the solid state (Figure 3a). This allows to elucidate the thermal
transformation and precipitation process of the oxide films.
The obtained mass spectra of the evolved gases show the
expected signals for the urea decomposition, which are
+
+
ammonia (m/z 17), water (m/z 18), carbon monoxide/nitro-
+
+
+
gen (m/z 28), nitric oxide (m/z 30), isocyanic acid (m/z 43),
+
+
carbon dioxide (m/z 44) and nitrogen dioxide (m/z 46). Peaks
observed in the IR spectra at 1599 cm could not be assigned
À 1
unambiguously, however are most likely to be attributed to
carboxylic compounds formed also during urea decomposition.
2
Figure 3. Coupled IR/MS-thermogravimetric mass loss (a) of compounds 1 (black) and 2 (red) under O atmosphere. Shown are ion currents of the mass
fragments detected during decomposition of 2 (b); intensity of m/z+ 43 multiplied by 10 for enhanced clarity, that of m/z+ 46 by 100; and the Gram-Schmidt
intensities during decomposition of 2 (c) and the corresponding IR signals of the maxima (d).
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This is substantiated by the fact that these bands only appear in
maxima iii and v, which are the maxima of the second and third
decomposition step observed in the TG. The same could be
true for the signal at 1363 cm , that is due to some NO
containing residue. It could thus be concluded that the
decomposition starts with slow decomposition of urea around
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À 1
1
40°C. At about 226°C, most of the urea and nitrate are
released, leaving behind a composite composed of CuO and
remaining organic residues. During the decomposition of urea
and urea nitrate released isocyanic acid reacts with remaining
urea to form biuret, which is known to undergo further
polymerization reactions to form compounds like ammelide,
ammeline, cyanuric acid and melamine which finally decom-
1
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[45–47]
pose at T>400°C.
These processes are thus well reflected
in the observed TG curve.
2
.2. Fabrication of Oxide Coated Aluminum-Based Pigments
To obtain a high-quality oxide coated aluminum base pigment,
it is important to generate a homogeneous oxide coating on
the aluminum base pigment. Therefore, the deposition param-
eters must be optimized in order to minimize additional surplus
oxide precipitation so that a smooth oxide coated pigment not
containing any side precipitation on the base pigment’s surface
is obtained. For the parameter optimization a silverdollar-type
base pigment was used, which has a thickness of 150–300 nm
[7]
(
Figure 4a). However, the final optimized coatings were then
applied on a physical vapor deposited (PVD) aluminum base
[7]
pigment with a thickness of only about 1–50 nm (Figure 4b).
The diameter of both base pigments is about 22 μm, which is
why the PVD base pigments have a strong tendency towards
agglomeration when dried due to the higher aspect ratio. Both
substrates show homogeneous native surfaces without signifi-
cant defects.
Any surplus precipitation during the coating process must
be avoided since this would increase the amount of diffuse
reflected light and therefore directly impair performance
parameters like gloss and flop effect. For the evaluation of
optimal coating parameters, the pigments were dispersed in a
precursor solution in varying ratios and refluxed while monitor-
ing sample composition in certain intervals by EDX which gives
the reported medium values and associated standard deviations
of the precipitated coatings (Figure 5). Depending on the
reaction time, the amount of iron oxide deposition on the
aluminum base pigments during decomposition of 1 grows
linearly, until a saturation is reached after 100 minutes,
indicating that the precipitation is finished at this point
Figure 4. SEM images of a native uncoated silverdollar-type (a) and a native
physical vapor deposited aluminum base pigment (b). The silverdollar base
pigment particles are visible mainly as isolated particles, while the thinner
PVD obtained base pigment particles show a strong surface agglomeration
giving rise to a sheet like arrangement.
available substrate as it directly influences nucleation taking
place on the pigment surface and agglomerate formation of the
coated pigments. The EDX analysis of iron oxide coated
pigments obtained from decomposition of 1 reveal that the
precipitation already takes place at low precursor concentra-
tions compared to 2 but remains constant with only small
variations up to a ratio of 1. At this point the resulting Fe/Al
ratio starts increasing however accompanied by increasing
inhomogeneity of the deposition. At a ratio of about 2.5, this
effect further increases significantly. The findings from the EDX
investigations are confirmed by the SEM images shown in
Figure 6 a-c. At a precursor/aluminum substrate ratio of 0.7
(Figure 6a), the surface looks smooth and no additional particles
(Figure 5a). In comparison, the amount of precipitated copper
oxide had already reached a maximum after 30 min and
remains constant during ongoing reaction times, indicating a
different decomposition mechanism and/or differing reaction
kinetics of 2 as compared to 1. This fact has a major influence
on the coating process as a faster precipitation means that the
deposition process must be strongly controlled in order to
avoid co-precipitation. The main factor influencing the coating
quality is the ratio of the amount of transformed precursor and
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increasing the tendency to form inhomogeneous precipitates.
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However, the coating thicknesses obtained under such con-
ditions are only a few nanometers, which is insufficient for
obtaining the desired interference color changes. Therefore, the
coating of the Al base pigment has to be at least 20–30 nm in
[5]
thickness. In order to make these discrete dimensions
accessible, a sequential coating technique with periodical
renewal of solvent was studied (Figure 6e). The effect observed
is significant as the amount of side precipitation for sequentially
coated pigments is much higher when the solvent is not
exchanged during the process (Figure 6f). Therefore, ideal base
pigment coatings could be obtained when (i) using low
precursor/aluminum ratios, (ii) employing repetition of the
coating process for several times and (iii) which are accompa-
nied by a periodic exchange of the solvent to remove any side
precipitates. This procedure is maintained until an observable
change in the hue of the coated base pigments from silver to
gold appears. An SEM image of the obtained optimized
pigments from decomposition of 1 is shown in Figure 6g. The
mean iron/aluminum ratio in the coated base pigments was
determined to be 0.596 with a calculated standard deviation of
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Figure 5. Development of the ratio metal(Fe, Cu)/aluminum vs. deposition
time for the decomposition of 1 (a) and 2 (b) and dependence of the ratio
precursor/aluminum base pigment for 1 (c) and 2 (d), as determined by EDX.
Each single point reflects 5 independent measurements with error bar.
0
.048, what is a relatively low value confirming the good
can be observed, while at a value of 1 (Figure 6b) first surplus
particle deposition occurred.
homogeneity of the obtained coating. SEM images of obtained
pigments using the optimized parameters for the decomposi-
tion of 2 are shown in Figure 6h. The mean copper/aluminum
ratio is 0.569, the standard deviation is 0.134. This error is
higher compared with the optimized coatings obtained from 1,
but is still much lower than the deviations using a non-
optimized procedure for 2.
For precursor 2 the inhomogeneities during transformation
into the oxide at a precursor/substrate ratio of 0.5 are still
relatively small but start to increase rapidly at values of 0.7 and
higher. SEM images for a ratio of 0.5 show that the coated
pigments still possess homogeneous surfaces (Figure 6c). At a
ratio of 1, however, the existence of massive precipitates in the
range of several micrometers can be observed (Figure 6d).
Hence, the development of smooth surfaces with no defects
requires a slower coating speed with less amount of precursor 2
compared to the deposited oxide films derived from precursor
1
. This can be explained by faster decomposition kinetics of 2,
Figure 6. SEM images of coated Al base pigments obtained from 1 and the silverdollar base pigment with ratios of 0.7 (a), 1 (b), and from 2 and base pigment
ratios of 0.5 (c) and 1 (d). Additionally, base pigments sequentially coated are shown in (e) with and in (f) without exchange of solvent and the pigments
coated using the optimized procedure from decomposition of 1 (g) and 2 (h).
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2
.3. Spectroscopic Characterization of the Solution
obtained oxide material and to investigate its remaining
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Precipitated Pigment Coatings
decomposition under further thermal treatment (Figure 8a,
black trace). The primary precipitate obviously undergoes a
multi-step decomposition process. The first decomposition step
confirms a slight mass loss of 3% in the temperature range
from 100 to 140°C, followed by a major mass loss of 30% from
140 to 265°C. From 330 to 390°C an increase of about 2% is
observed which can be explained by the oxidation of the
formed iron species. Between 390°C up to 495°C again a mass
The results of the TG analysis reveal that the transformation of
the precursor compounds 1 and 2 into the oxide coatings in
solution is not complete under the chosen conditions (1-
methoxy-2-propanol, 120°C). The infrared spectrum of the
product obtained from the thermal decomposition of the iron
urea nitrate compound 1 in absence of aluminum base pigment
is shown in Figure 7. The observed significant number of
intense bands confirms the expected incomplete decomposi-
loss of about 11% is observed. The total mass loss observed
during transformation of the solid obtained from the decom-
position of 1 is 42%, indicating a massive organic residue under
these conditions. The species comprising this residue can be
concluded to mainly consist of undecomposed urea, as the
1
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4
4
4
4
4
5
5
5
5
5
5
5
5
À 1
tion of the precursor. The signal at 1630 cm has a far higher
intensity than in urea. This can be assigned to either nitrate
valence vibrations,
[
48–50]
or vibration deformation modes origi-
[51,52]
À 1
nating from FeOOH (δ , β-FeOOH).
The band at 1292 cm
Additional vibration modes
at 1152 correspond to δ , (γ-FeOOH), at 1013 to δ , (γ-FeOOH)
observed mass vs. temperature profile (Figure 8a, black) is
OH
[49,50]
[46,47,56]
corresponds solely to ν in NO .
typical for the pyrolysis of urea.
In addition, the infrared
s
3
spectrum reveals the presence of urea in the pyrolysis product
from solution decomposition (Figure 7). Such a high organic
content might influence the refractive index of the oxide
pigment coating and therefore also the color saturation of the
final coated pigment. To furthermore increase the purity of the
coating layer, small amounts of water as oxidizing agent have
been added to the precursor solution. As the thermogravimetric
measurements show (Figure 8a), the presence of water in the
precursor solution has a massive influence on the decomposi-
tion behavior, as the mass loss can be reduced by about 15%
OH
À 1
OH
[51–54]
and at 890 and 790 cm respectively, to δ (α-FeOOH).
Additional bands at 1091 and at 1410 cm have also been
reported to be characteristic for α-Fe O
additional nitrate impurities.
OH
À 1
[55]
3
[51]
and FeOOH or
2
[52]
A thermogravimetric measurement of the oxide precipitate
obtained from thermal decomposition of 1 in methoxypropanol
(
120°C, 2 h) under oxygen atmosphere was performed in order
to determine the quantity and nature of organic content in the
from 42% to 27% by addition of 1% H O. Additionally, the high
2
temperature mass loss at 400°C arising from polymerization
reactions between isocyanic acid released during urea decom-
[45–47]
position and still undecomposed urea disappears.
Water
inhibits these reactions as it reacts with isocyanic acid under
[47]
formation of evaporating NH and CO . In addition, even the
3
2
formation of isocyanic acid is prevented as urea can be
hydrolyzed directly to form ammonium carbonate, which
[
44]
decomposes immediately into ammonia and carbon dioxide.
These effects are detectable in the infrared spectrum of the
precipitate (Figure 7). The intensity of the urea bands is reduced
À 1
and the ammonia bands around 3300 cm become slightly
obstructed by an intense OH valence vibration. The intensities
À 1
of the bands at 1659 and 1410 cm attributable to FeOOH are
Figure 7. Infrared spectra of the precipitates obtained from refluxing 1 in 1-
methoxy-2-propanol for 2 hours with no addition (black) and 1% (green)
addition of H O.
2
increased. This signal increase, while the NH vibration at
À 1
1
588 cm is only existent as a weak signal, can also be
Figure 8. TG measurements of the obtained iron oxide precipitate under oxygen atmosphere obtained from refluxing of 1 in 1-methoxy-2-propanol for
2
hours with water content of 0% (black), 0.5% (red) and 1% (green) (a) and after following heat treatment in phenyl ether for 5 minutes at 259°C (b).
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attributed to carboxylates that were formed during urea
hydrolysis. A further benefit of the reduction of organic residues
by the addition of water is that no significant influence on the
topography of the coatings formed on aluminum base pig-
ments could be observed (S2). Removal of the remaining
organic species accompanied by formation of hematite through
calcination at 370°C seems possible (S3). However, it is not
suitable for the coated pigments as agglomeration occurs
during such a heat treatment under ambient atmosphere (S4).
That is why a solution-based approach was chosen to further
reduce these impurities. Due to solvent interaction thermal
processing at 259°C for 5 minutes in phenyl ether leads only to
minor particle aggregation when compared to a thermal
annealing of the solid (S4). The organic content could thus be
massively reduced. The thermogravimetric measurements of
the precipitate obtained after refluxing in phenyl ether without
addition of water (Figure 8b) reveal a reduced mass loss of 12%
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
(from 42% to 30%) and for the precipitate obtained after
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
addition of 1% water of 10% (from 27% to 17%). The latter is
the lowest value for the obtained iron species. However, a
major effect is already seen for a sample obtained after usage
of 0.5% water with a mass loss of 22%. As it is also desired to
use as low water as possible to avoid oxidation of the aluminum
base pigment, a reduced water content of 0.5% is supposed to
be the optimum in regard of the overall performance. Fig-
ure 10a shows an XRD of a base pigment coated under these
optimal conditions, revealing that the temperature is high
enough to observe a beginning crystallization of γ-Fe O (JCPDS
2
3
card no. 25-1402) maghemite, with broad reflexes visible at 35
Figure 9. Scheme of the processes described herein starting from 1.
Figure 10. XRD analysis of the base pigments (a) coated by thermal decomposition of 1 and heat treatment in phenyl ether compared with metallic aluminum
(red)(JCPDS 04-0787) and maghemite (blue)(JCPDS 25-1402), (b) coated by thermal decomposition of 2 in 1-methoxy-2-propanol, 120°C, 2 h, compared with
tetraamminecopper(II) nitrate (green)(JCPDS 51-0271), diaminecopper(II) nitrate (blue) (JCPDS 48-1187), copper nitrate hydroxide (cyan)(COD 9012715) and
aluminum (red)(JCPDS 04–0787). (c) coated starting from 2 and heat treated in phenyl ether compared with metallic aluminum (red)(JCPDS 04-0787) and CuO
(blue)(JCPDS 80-0076).
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and 63 °2θ besides reflexes of the aluminum substrate (JCPDS
card no. 04-0787).
card no. 51-0271) and diamminecopper(II)nitrate (JCPDS card
no. 48-1187) is confirmed. Some additional small reflexes
cannot be assigned in detail but are most likely to belong to
some additional amine/hydroxo/nitrate phases. The mass loss
measured by TG for the copper precipitate in absence of
aluminum is 67.6% (Figure 11), lying between the expected
values for [Cu(NH ) ](NO ) (64.1%) and [Cu(NH ) ](NO )
(68.9%), so that the amino nitrates can be regarded as the
dominant transformation products. As heat treatment in
solution already showed promising results during the process-
ing of the coated base pigments obtained from 1, this approach
was also further studied for the coated base pigments using
precursor 2. The TG studies (Figure 11) reveal that the mass loss
after heat treatment is reduced drastically from 64.3 to 12.0%,
showing that most of the organic species could be eliminated
using this process. Besides the aluminum phase of the base
pigment the formation of crystalline CuO (JCPDS card no. 80-
0076) is detected (Figure 10c).
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
The oxidizing step and the high temperature mass loss
around 400°C disappear after treatment in phenyl ether no
matter if water had been used during the precipitation in 1-
methoxy-2-propanol. We speculate that during heat treatment,
FeOOH is transformed into iron oxide accompanied by release
of water. Under ambient atmosphere, water evaporates imme-
diately, and urea decomposes under formation of larger
molecules with higher thermal stability like cyanuric acid,
3
2
3 2
3 4
3 2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[45–47]
ammelide, ammeline and melamine.
In solution however,
water could stay in the solvent medium for a longer time,
thereby leading to the positive effects described afore. With
increasing water content in the initial precursor solution, the
content of FeOOH rises thereby increasing this effect and
reducing the final mass. A summary of the transformation
processes, proven intermediates and final products is presented
in Figure 9.
While 1 shows incomplete decomposition in solution (Fig-
ure 9) and urea remains intact to a large extent, in the case of 2
decomposition of the urea and formation of ammonia com-
plexes can be observed (Figure 12). XRD analysis of the
decomposition product of 2 reveals several products after
refluxing a solution containing 2 for two hours (Figure 10b).
Besides signals originating from the aluminum base pigments,
the existence of mainly tetraamminecopper(II)nitrate (JCPDS
In conclusion, compound 2 shows an advantageous decom-
position behavior in solution as compared to 1. The energetic
driving force for the decomposition of the precursors is
formation of the amino nitrate complexes, which are unstable
[
57]
under heat treatment and tend to decompose vigorously.
These are not existent with iron as cation, which is why urea
still remains intact to a large extent after decomposition of 1.
Another positive aspect of the copper coated pigments is that
only minor particle agglomeration can be observed during the
process (S4).
2
.4. Colorimetric Investigations of the Final Iron Oxide and
Copper Oxide Coated Aluminum Base Pigments
Table 1 provides an overview over the pigments investigated
herein, which are the commercial coated aluminum based
pigments Paliocrom® Brilliant Gold L 2050 (“Palio” a silverdollar-
type Al base pigment coated with Fe O using a fluidized bed
2
3
Table 1. Overview over the metal oxide coated pigments investigated in
this work. Shown are the type of Al base pigments, short processing
information corresponding to the sample names used. Degree of
pigmentation of the investigated varnishes containing the commercial
pigments was 7% and the pigments synthesized herein 3%.
Figure 11. Thermogravimetric measurements under oxygen atmosphere of
the oxide precipitates obtained from refluxing 2 in 1-methoxy-2-propanol
for 2 hours (black) and after following treatment in phenyl ether for 5
minutes at 259°C (red).
Sample
name
Pigment
type
Fabrication
Palio
Ale
Silver
dollar
PVD
Paliocrom®; coated with Fe
reactor
Alegrace®; wet chemically coated with SiO and
2
2 3
O in a fluidized bed
2
Fe O3
1
-nopt
PVD
thermal decomposition of 1, not optimized proce-
dure without solvent exchange during the coating
steps
1-nopt-
calc
PVD
PVD
PVD
PVD
thermal decomposition of 1, not optimized proce-
dure; heat treated in phenyl ether
thermal decomposition of 1, optimized procedure
with solvent exchange during the coating steps
thermal decomposition of 1, optimized procedure,
heat treated in phenyl ether
1
-opt
1-opt-
DPE
2-opt-
Figure 12. Scheme of the processes described herein starting from 2, leading
to copper oxide with a purity of 88%.
thermal decomposition of 2, heat treated in
DPE
phenyl ether
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reactor) and Alegrace® Aurous A 21/71-1 White Gold (“Ale” a
PVD Al base pigment, wet chemically coated with SiO2 and
Fe O ). The herein synthesized pigments are coated Decomet®
pigments obtained from decomposition of 1 using (i) an
unoptimized procedure without exchange of solvent (1-nopt),
optical appearance of a sample when observed from different
viewing angles.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
The color distance dE describes the distance of two samples
2
3
[58,59]
in the CIELAB color space.
Here, it is used to determine the
hiding power, as the same sample is measured on a black and a
white background. When the hiding power is high, the dE
should be close to 0, meaning that the underground is
optimally covered by the pigments. In the case of a weak hiding
power the influence of the underlying background becomes
(
ii) calcined at 400°C for 4 h under ambient atmosphere (1-
nopt-calc), (iii) using the optimized procedure (1-opt) and finally
(
iv) an optimized pigment (1-opt) which is additionally heat
treated in phenyl ether (1-opt-DPE) employing the strategies
described before. Moreover, the copper oxide coated Al base
pigments (2-opt-DPE) were obtained from decomposition of 2
followed by additional heat treatment in phenyl ether (see
table 1).
The corresponding optical spectra (S5-9) and a table with
the numeric values of the optical parameters (S10) derived from
the spectra are given in the supplementary information.
Figure 13 shows a comparison of the characteristic coloristic
values for the differently coated aluminum base pigments (see
table 1). Gloss describes the intensity of the reflectance of the
material from the glancing angle. The color saturation S is
important for the optical performance of a pigment, as it relates
the chromaticity and the lightness. If the saturation is high, the
color of the material looks intense, while it appears pale for
lower values. The flop index is a measure for the change of the
[58,60,61]
dominant and dE rises.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
For the coloristic investigations, all pigments were dispersed
in a varnish and deposited on a black/white card by doctor
blading technique. A comparison of the not optimised pig-
ments obtained without exchange of solvent during the coating
steps (1-nopt), and the same pigments calcined at 400°C (1-
nopt-calc), shows that the color saturation reaches the highest
values of all pigments synthesized in this work after calcination
(Figure 13a). This can be attributed to the most complete
removal of organic residues originating from the precursor
molecule 1 and almost complete oxidation of the iron species.
However, the coloristic measures as gloss, flop index and hiding
power are drastically diminished (Figure 13b–d). This can be
attributed to the observed particle agglomeration (S3) which
hinders a homogeneous distribution of the pigments in the
deposited film while increasing the inhomogeneity of the
Figure 13. Comparison of the colorimetric results of the obtained pigments at different stages of the process compared with Paliocrom® Brilliant Gold L 2050
“Palio”) and Alegrace® Aurous A 21/71-1 White Gold (“Ale”). Bold numbers indicate the complex used for the precipitation. Shown are results for color
(
saturation (a), gloss (b), flop index (c) and the color distance dE45° between black and white background from 45° (d).
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Figure 14. Optical reflectance spectra of pigment samples coated by thermal decomposition of 1; a) from 45 , shown are the calcined (400 C, 4 h) sample on
white (red trace) and black (black trace) background and the uncalcined pigment sample on white (blue trace) and black (green trace) background; b) shows
spectra on black background of the calcined sample from 15° (black trace) and 110° (red) and for the uncalcined sample from 15° (blue trace) and 110° (green
trace).
°
°
coated pigments which causes an increased diffuse reflectivity
in the particles and can obviously only be obtained when the
pigment is nicely dispersed. Figure 14a, shows the spectra of
the samples before and after ambient calcination. An optimal
hiding power is characterized by a low color distance between
the pigments measured on a black and a white background
from a viewing angle of 45°. The dE45° should be ideally below
residues (Figure 8). However, a drawback are the reduced gloss
and flop index, which can be attributed to changes in the
surface morphology of the varnish. After precipitation of the
coatings at 120°C, the organic residues are most likely to be
evenly distributed in the deposited film. During heat treatment,
these residues evaporate, leaving micro voids in the surface. In
addition, the oxidic phases start to contract during a beginning
crystallization process. SEM images show indeed a reduced
homogeneity after heat treatment (Figure 16b). This observed
inhomogeneity results in a lowered gloss and flop of the CuO-
coated pigments (Figure 16c), while the surface of the Fe O -
coated pigments appears significantly smoother (Figure 16d).
However, both Al base pigments show dense and homoge-
neous oxide coatings with no visible macroscopic cracks.
Compared with the two commercial aluminum base pig-
ments Paliochrom® and Alegrace®, all pigments synthesized
herein, except the one calcined in ambient atmosphere, show
1
. The spectra of the uncalcined sample show nearly the same
habitus, while in the case of the calcined samples a relatively
large difference can be observed. In addition, the flop decreases
drastically after calcination. This value depends mainly on the
difference between the lightness at 15° and 110°; the bigger
2
3
the difference the higher is the flop. The spectra of the samples
before and after ambient calcination on a black background
measured from an angle of 15° and 110° reveal that the
difference in the spectra of the sample before calcination is
significantly higher than after heat treatment (Figure 14b).
The photos of the obtained pigments dispersed in a varnish
visualize the effects of the different treatments. It can be clearly
seen that the gloss of the sample calcined at 400°C (Figure 15b)
is much lower than the values of the others, which possess
extremely high glance. On the other hand, the 400°C calcined
sample has by far the best color saturation; all the other
varnishes appear comparatively pale. A small improvement of
the color saturation can be observed after treatment in phenyl
ether (Figure 15d) due to the lowered content of organic
Figure 15. Photos of the varnish pigments applied on a cardboard with black
and a white background. The pigments are deposited by the doctor blading
technique. The cardboards are viewed from the glancing angle (top line) and
directly from above (bottom line); a, 1-nopt; b, 1-nopt-calc; c, 1-opt; d, 1-opt-
DPE; e, 2-opt-DPE.
Figure 16. SEM images of the surfaces of the coated pigments obtained
from precipitation of 1 before (a) and after (b) treatment in phenyl ether at
259°C, 5 min. Lateral view of the CuO-coated (c) and Fe
2
3
O -coated (d)
pigments.
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similar or even higher gloss values, indicating that they match
or outperform the state of the art. However, due to the
remaining organic residues, the samples appear comparably
pale. A further major advantage of the samples can be observed
as far as the hiding power is concerned. The pigments
reduced material demand in automobile paint lacquers could
lead to significant saving of costs and contributes to a
protection of the environment.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
synthesized herein show a much better hiding strength Experimental Section
compared to the state-of-the-art pigments (Figure 13). Although
the degree of pigmentation in the formulation is only 3%, the
pigment sample synthesized from precursor compound 1
before calcination under optimized conditions has a dE45° of
only 0.2. For Paliocrom® Brilliant Gold L 2050 it is 7.7 even at a
higher degree of pigmentation of 7%. For a material with a
sufficiently good hiding power, a dE45° value of not more than
Materials
Iron(III) nitrate nonahydrate, ethanol and 1-butanol were purchased
from VWR, copper(II) nitrate trihydrate and 1-Methoxy-2-Propanol
from Roth, urea and phenyl ether from Merck. Alustar®, Decomet®
and Alegrace® Aurous A 21/71-1 White Gold were obtained from
Schlenk Metallic Pigments GmbH and Paliocrom® Brilliant Gold L
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
2050 from BASF.
1
is desirable. The reason for the extraordinary hiding power
may be due to the optimized coating process using the
molecular precursors 1 and 2 which allows a smooth deposition
of the molecular precursor followed by a transformation into
the oxidic pigments. This allows a very narrow and smooth
coverage of the ultrathin physical vapor deposited aluminum
base pigments. It is known that the thinner a base pigment is,
the lower is the amount needed for a hiding coverage in a
Precursor Synthesis
Hexakis(urea)iron(III)nitrate (1) was synthesized according to a
[43]
literature procedure : Iron nitrate nonahydrate (4.85 g, 12 mmol)
was dissolved in 250 mL EtOH and urea (6.006 g, 100 mmol) was
dissolved in 100 mL ethanol. Both solutions were mixed and stirred
for 2 hours. The precipitation of a yellow product was observed,
which was separated by filtration, washed in ethanol, and dried in
vacuo (5.29 g, 73%). Anal. Calcd (%) for C H FeN O (601.8
[27]
varnish.
6
24
15 15
À 1
gmol ) C 11.97 H 3.99 N 34.92; found C 12.40 H 3.93 N 35.46. TG
ceramic yield (%) Calcd 13.26, found 13.77. H NMR ([D ] dimethyl
1
6
3. Conclusions
°
sulfoxide, 25 C) δ =5.43 ppm (br, NH ). IR ῦ=3437 (s, νNH, in phase),
2
3
1
7
339 (s, ν
), 3233 (s, νNH, in phase), 1625 (s, ν ), 1551 (s, δNH),
NH, in phase
CO
504 (s, ν ), 1332 (s, νNO), 1148 (m, 1NH), 1031 (m, 1NH), 828 (w, ωNH),
A solution based environmentally benign procedure for coating
of aluminum metal effect pigments using a single source
precursor route is presented. It was possible to synthesize
special effect pigments based on ultrathin physical vapor
deposited aluminum flakes (Decomet®). All reagents used
herein are safe to handle, nontoxic, environmentally friendly
and allow an uncomplicated adaptation towards technological
scale up. Due to the single source precursor approach the
reaction control is straightforward as the metal oxide source,
oxidizing agent and fuel for the combustion reaction are
combined in one molecular compound. Interestingly, the iron
precursor for the iron oxide coating is also available in a direct
approach using iron metal and urea. Because of the versatility
of the urea ligand in coordination chemistry the single source
precursor approach offers access to a wide variety of other
CN
60 (w, ωNH), 609 (m, δ ), 537 (m, δ ), 429 (m, δ ).
CN
CN
CN
In an alternative approach, 1 was synthesized directly from iron
metal. Iron powder (0.089 g, 1.6 mmol) was dispersed in 5 mL
ethanol and 308 μL nitric acid (65%) added. After stirring for 1 day,
unsolved iron was removed. Additionally, 0.59 g urea (9.8 mmol)
was dissolved in 20 mL ethanol. Both solutions were mixed and
stirred for 2 hours. The resulting yellow precipitate was separated
by filtration, washed in ethanol, and dried in vacuo (0.364 g, 38%).
À 1
Anal. Calcd (%) for C H FeN O (601.8 gmol ) C 11.97 H 3.99 N
6
24
15 15
3
1
4.92; found C 11.63 H 4.74 N 34.46. TG ceramic yield (%) Calcd
1
3.26, found 13.45. H NMR ([D ] dimethyl sulfoxide, 25°C) δ=
6
5.39 ppm (br, NH ). IR ῦ=3436 (s, ν
), 3335 (s, ν
NH, in phase
),
NH, in phase
2
3225 (s, νNH, in phase), 1625 (s, νCO), 1549 (s, δNH), 1501 (s, νCN), 1331 (s,
νNO), 1148 (m, 1NH), 1030 (m, 1NH), 828 (w, ωNH), 759 (w, ωNH), 610 (m,
δ ), 538 (m, δ ), 424 (m, δ ).
CN
CN
CN
Tetrakis(urea)copper(II)nitrate (2) was synthesized according to a
[40]
metal oxides, too. The synthesized pigment coatings show a
homogeneous surface leading to excellent coloristic values for
gloss (45–100) and flop indices (20–30). The color saturation is
comparatively weak (0.2–0.3), probably due to minor amounts
of residual organics still remaining in the film (~15%). Solid
state calcination in ambient atmosphere is challenging due to
particle agglomeration during oxide film formation. However, it
was possible to largely reduce the content of organic impurities
by heat treatment in solution. It was possible to coat aluminum
base pigments directly without application of a protective layer
as it is needed in the currently commercially established sol-gel
coating procedures to avoid pigment decomposition. The
coated aluminum base pigments show an excellent hiding
power exceeding that obtained in current state of the art
pigments by far with dE45° of only ~0.3 for varnishes with a
low degree of deposited pigment of only 3%. This drastically
[62]
literature procedure with small adaptations
:
Urea (5 g,
8
3.3 mmol) was dissolved in 300 mL n-butanol at 50 C. After
°
complete dissolution, copper nitrate trihydrate (4.82 g, 20.0 mmol)
was added and the mixture stirred for 2 hours. The resulting blue
precipitate was separated by filtration, washed with n-butanol and
2-Propanol and dried in vacuo (6.33 g, 74%). Anal. Calcd (%) for
À 1
C
H
4
16CuN10
O
10 (427.8 gmol ) C 11.22 H 3.74 N 33.25; found C 11.87
1
H 3.74 N 32.74. TG ceramic yield (%) Calcd 18.59, found 18.68. H
NMR ([D ] dimethyl sulfoxide, 25°C) δ=6.16 ppm (br, NH ). IR ῦ=
6
2
3470 (s, νNH, in phase), 3400 (s, νNH, in phase), 3335 (s, νNH, in phase), 3233 (s,
ν
), 1621 (s, ν ), 1567 (s, δ ), 1508 (s, ν ), 1385 (s, ν ),
NH, in phase
CO NH CN NO
1329 (s, νNO), 1151 (m, 1NH), 1053 (m, 1NH), 1028 (m, 1NH), 815 (w,
ωNH), 757 (w, ωNH), 608 (m, δ ), 540 (m, δ ), 426 (m, δ ).
CN
CN
CN
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Pigment Coating
5205 (Micromeritics) with isopropanol as solvent. TG measurements
were performed using a 209-Iris (Netzsch) coupled with a QMS
03 C mass spectrometer (Netzsch) and an infrared spectrometer
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For the process optimization, the silverdollar-type aluminum base
pigment Alustar® (thickness 150–300 nm) supplied by Schlenk
Metallic Pigments GmbH was used. In the general coating
procedure, the aluminum base pigment was dispersed in 1-meth-
oxy-2-propanol. The precursor compound 1 or 2 was added in the
desired ratio and the mixtures refluxed (120 C) for 2 hours. For the
investigation of the coating materials alone, the whole procedure
was done in the same way in absence of aluminum pigments. For
the final samples for the coloristic investigations the physical vapor
deposited aluminum base pigment Decomet® (thickness 1–50 nm)
supplied by Schlenk Metallic Pigments GmbH was used. The
synthesis conditions of the coated pigments investigated there are
as follows:
4
Nicolet iS10 (ThermoScientific) under oxygen atmosphere using
aluminum crucibles. For the colorimetric investigations, the pig-
ments were dispersed in a nitrocellulose/polycyclohexanone/poly-
acrylic varnish and applied on a black/white DIN A5 card (TQC)
using an automatic system from Zehntner with a 38 μm spiral
blade. The coloristic values were measured using the device Byk-
mac i (Byk) except the gloss, which was determined with the
system Byk micro-TRI-gloss (Byk). The illumination spectrum used
was D65 norm light.
°
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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5
5
5
5
5
1
-nopt: 1 g of the pigment paste was dispersed in a mixture of Acknowledgements
100 mL 1-methoxy-2-propanol and 1 mL water. 1 g 1 was dissolved
therein and refluxed for 2 hours. After cooling down, 1 more g 1
was added and again refluxed for 2 hours. The process was
repeated one more time. The resulting coated substrates were
isolated by filtration, washed with isopropanol and dried for
Support through the Technical University of Darmstadt is acknowl-
edged with gratitude. We gratefully acknowledge F. Burger
(Schlenk Metallic Pigments GmbH) for the coloristic measurements
1
6 hours at 50°C.
and S. Heinschke (TUDa) for performing XRD measurements.
1-nopt-calc: The sample was synthesized in the same way as 1-
nopt, but in the end additionally calcined at ambient atmosphere
in a muffle furnace at 400°C for 4 h.
Conflict of Interest
1-opt: 1 g of the Decomet® paste was dispersed in a mixture of
The authors declare no conflict of interest.
1
00 mL 1-methoxy-2-propanol and 0.5 mL water and coated in 6
steps consisting of addition of 0.5 g 1 and refluxing for 2 h. After
each second step, the solvent was renewed. The resulting pigments
were filtered off, separated by filtration, washed in isopropanol and
redispersed in isopropanol.
Keywords: copper · iron · metal oxide thin films · special effect
pigments · urea-nitrate compounds
1
-opt-DPE: The sample was synthesized under the same conditions
as 1-opt following heat treatment in phenyl ether (259°C, 5
minutes). The resulting pigments were separated by filtration,
washed in ethanol, acetone and diethyl ether and redispersed in
isopropanol.
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3
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Manuscript received: August 11, 2020
Revised manuscript received: November 4, 2020
Thermochim. Acta 2004, 424, 131.
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