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3.7.3.4 Rheological additives
As already described, rheological additives are necessary for immobilising the orientation of alu-
minium flakes during film forming. Preferred additives are those that do not generate a certain
degree of structural viscosity until physical drying starts. The advantage is that such additives do
not increase the application viscosity or lower the application solids. The viscosity curves shown
in Figure 3.7.7 as a function of the shear rate of basecoats containing optimum additives illustrate
how structural viscosity arises and increases during physical drying.
Various classes of materials make suitable rheological additives: modified bentonites (swellable
by solvents), polyethylene wax dispersions (prepared by precipitation in solvents of hot wax
solutions), crystalline ureas (low-molecular weights, precipitated
in situ
from primary amines
and polyisocyanates during formation in a resin solution), polyamides and polyurethanes which
are partly crystalline.
3.7.3.5 Effect substances
The first effect substances to be used for coatings and which are still the most important are
alu-
minium flakes
. These pigments contain highly pure aluminium.
Physical data of pure aluminium for pigments
[127]
:
Melting temperature
660.2 °C
Boiling temperature
2495 °C
Density (20 °C)
2.699 g/cm³
Density (liquid, at 660.2 °C)
2.357 g/cm³
Crystal structure
Cubic, lattice length 4.0496 * 10
-10
m at 298 K
Electrical resistance
2.5 µ
Ω
cm
Aluminium is essentially obtained
from pure aluminium oxide (Al
2
O
3
).
Pure aluminium oxide is rare in
Nature but is found in the minerals
corundum and natural emery. Corun-
dum is also the essential ingredient
of rubies and sapphires. Relatively
high quantities are contained in
bauxite (50 to 60 % by weight Al
2
O
3
,
plus silica). However, various ben-
tonites lend themselves to the
preparation of aluminium metal as
well. A number of dressing proce-
dures are employed to separate the
aluminium oxide from by-products
such as silica and iron oxides.
The aluminium metal is prepared
from aluminium oxide by elec-
trolysis in a fused state. Since pure
Figure 3.7.7: Change in viscosity of basecoats containing
rheological additives during physical drying
Automotive OEM coatings
147
aluminium oxide has a melting temperature of 2,050 °C, a flux has to be used. A suitable flux is
cryolite (Greenland spar, sodium hexafluoroaluminate, Na
3
AlF
6
) which in the past was mined in
Greenland, but now is made synthetically by making aluminium oxide react with hydrofluoric acid
and sodium carbonate. The melting temperature is 1,010 °C. The eutectic mixture of cryolite (81.5 %
by weight) and aluminium oxide (18.5 % by weight) melts at 935 °C. The electrolysis is initiated
by adding aluminium metal to increase the conductivity and is carried out at 940 to 985 °C. The
cryolite dissociates into sodium and hexafluoroaluminate ions. Hexafluoroaluminate reacts with
aluminium oxide to form fluoroxoaluminates. All aluminate ions are decomposed to aluminium
ions and fluoride ions. The theoretical decomposition voltage is 2.2 Volts, but to overcome the elec-
trical resistance a higher voltage of 6 to 7 Volts is chosen. The excess current power is converted
into heat to boost the melting process. The discharged aluminium is deposited on the cathode,
which is the floor of the electrolysis cell. The sodium is not discharged. When the voltage rises,
more aluminium oxide is added. The molten aluminium is covered and protected from oxidation
by the overlying melt, and can be removed from the cell. The electrodes are made of carbon. Since
oxygen is formed at the anode, the carbon is oxidized and must be replaced. Aluminium prepared
by this process has a purity of more than 99.9 % by weight.
Aluminium of such high purity is suitable for the preparation of aluminium pigment flakes. The
pure metal is easy to shape (it is ductile). In addition, there is the advantage that pure aluminium
is much more resistant to chemicals (acids) and more weather resistant than aluminium that
might contain small quantities of silica or iron oxide. The pigments are produced by atomising
melted metal in a cyclone. The resultant powders are passed through screens. Wetting materi-
als, e.g. fatty acids and aromatic hydrocarbons to act as solvent, are added to batches of defined
particle size which are then shaped in ball mills. The resultant slurries are screened again and
the excess solvent is removed. In a kneader, other solvents and additives are added. The delivery
form is a paste containing mainly 65 % by weight aluminium and 35 % aromatic hydrocarbons,
along with small quantities of additives.
Aluminium pigments are either leafing or non-leafing, the difference arising from the degree
of surface treatment. Leafing types are covered with wetting agents which are solid, very non-
polar, and not easy to remove from the flake surface (e.g. stearic acid). After application of paints
containing leafing pigments, the aluminium flakes float on the surface to form a mirror-like layer
(like leaves on a lake). Non-leafing effect pigments are covered with liquid, more polar additives
that are removable (e.g. oleic acid or fatty amines). Such pigments are distributed within the film
layer, and do not float on the surface.
Leafing types are suitable for heat-resistant paints and corrosion-protection paints. They can
also be used to create coating layers with a so-called chrome effect. Such effect coatings are very
sensitive and cannot be recoated. Non-leafing types are suitable for metallic basecoats because
distribution of the particles is one of the preconditions for the flip-flop effect. Different influences
govern the effect created by aluminium flake pigments. Most important – of course – are optimum
distribution in the basecoat layer, efficient wetting of the particle surfaces, and good compatibility
with the resin matrix. Also important are the viscosity, the application solids, the application
process and its conditions, and the type of clearcoat and its method of application. Shrinking and
the effect of rheological additives during film forming by physical drying are important for immo-
bilising the aluminium particles to achieve the requisite metallic effect. The effect also depends
on the particle size of the aluminium pigments, the particle size distribution, and the shape of the
particles. The conventional process of atomising and milling aluminium yields irregularly shaped
flakes. These are called “corn-flake” aluminium
[128]
because of their shape under a microscope.
Figure 3.7.8 (page 148) shows scanning electron microscope (SEM) micrographs of conventionally
prepared aluminium pigments after atomization and milling.
Basecoats
148
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