basic lead carbonate
(Pb[OH]
2
· 2 PbCO
3
).
This is precipitated from dilute aqueous solutions of lead acetate and carbon dioxide
[130]
. Given the
right precipitation conditions, hexagonal flakes are formed (diameter 20 µm, thickness 50 nm)
that have a refractive index of 2.10. The particles are very sensitive to mechanical impact. As a
result, and since lead compounds are classified as toxic, the product no longer finds industrial
use. Global consumption is estimated to be less than 1,000 t
[130]
.
Bismuth oxychloride (BiOCl), which is prepared by hydrolysis of acidic bismuth chloride solu-
tions, forms flake-like pearlescent particles under specific reaction conditions. The product has
a refractive index of 2.15. It has low resistance to light and mechanical impact. Its sole use is in
cosmetics and the global market is estimated at about 400 t
[130]
.
Figure 3.7.12: Reactions which occur in the doping of mica surfaces
Figure 3.7.13: Colour effects of mica pigments doped with different layers of titanium dioxide
Figure 3.7.14: Colour effects of mica pigments doped with different layers of iron oxide
Automotive OEM coatings
151
Mica pigments
Eventually, pearlescent pigments were prepared from mica. Mica is potassium aluminium alu-
mino silicate, also known as the mineral muscovite (KAl
2
[OH]
2
[AlSi
3
O
10
]). It is often contaminated
with iron. Deposits of mica are numerous and so the raw material for pigments is relatively inex-
pensive. The mineral is conditioned by milling, cleaning, and classification. Mica is composed of
a thin-layer crystal lattice; it is easy to fragment into thin flakes that have thicknesses of 200 to
500 nm and mean flake diameters of 5 to 200 µm. Although the refractive index is relatively small
at 1.60, the flakes are pearlescent.
Mica pigments were described as long ago as 1942, but they were not introduced into the pig-
ment market until the late 1960s
[131]
. The first mica pigments conferred translucent pearles-
cence. On account of the low hiding power, such pearlescent basecoats had to be combined with
coloured primer surfacers, or more particularly with white primer surfacers. The first formula-
tions suffered from problems with levelling and topcoat hold-out. Not until it proved possible to
prepare particles with a reproducible narrow particle size distribution did the market for mica
pearlescent pigments grow. Since then, the mica pigments have usually been combined with
aluminium pigments and coloured pigments. However, the most important step in the develop-
ment of new effect pigments was the development and introduction of mica pigments which
were doped with thin layers of metal oxides
[132]
. Besides pearlescence, such pigments confer
colour effects (colour flop). The first colours developed were very “gaudy”. However, since the
mid-1980s, muted colours have also become available which better meet the tastes of car buyers.
At the moment, more than 50 % of effect basecoats contain pearlescent pigments – mostly based
on mica. Surface doping of the mica flakes is carried out in an aqueous dispersion. The most
popular doping layers consist of oxides whose higher refractive indices are much greater than
that of mica. These are usually titanium dioxide and iron oxide. The process converts the disper-
sion of mica flakes into a dilute aqueous solution of metal chlorides. Under carefully controlled
conditions (temperature, concentration, time), alkali hydroxide solution is added to precipitate
a thin layer of metal hydroxides on the mica particles. The particles are filtered, washed and
calcined at temperatures of 800 to 900 °C to yield the corresponding metal oxides. The reaction
is illustrated in Figure 3.7.12.
On account of the different refractive indices and the ability of metal oxides to absorb some wave-
lengths of visible light, colour effects (colour flop) are also obtained in addition to interference
effects. Since interference depends on the thickness of the layers and the absorption of light, the
colour effect also depends on the layer thicknesses. For example, doping with titanium dioxide
generates silver greys at 40 to 50 nm, yellows at 60 to 70 nm, reds at 80 to 90 nm, blues at 110 to
120 nm, and greens at 140 to 150 nm. Interference effects produced by titanium dioxide layers of
different thickness are shown in Figure 3.7.13.
Doping mica surfaces with iron oxide also yields colour effects that vary with the thickness of the
oxide layers. The light absorption properties of iron oxide cause all the colours to have a metallic
sheen. At layer thicknesses of 40 to 50 nm, the effect is that of bronze; at 60 to 70 nm, copper;
and at 80 to 90 nm, metallic red to gold. The dependence of colour effects on the layer thickness
of iron oxides is illustrated in Figure 3.7.14.
Combinations of different metal oxide layers (titanium dioxide together with iron oxide) leads
to other colour effects, mainly various shades of gold. If mica particles are treated with tita-
nium oxychloride and ammonia, the surface layer after calcination at 800 to 900 °C consists
of some titanium compounds in a low oxidation state and probably of compounds containing
nitrogen (TiO, TiO
x
N
y
, TiN) in addition to titanium dioxide. This type of mica pigment confers
bluish-silver effects.
Basecoats
152
As mica pigments have a tendency to settle, it is necessary to use wetting agents and suitable
rheological additives. The low hiding power of mica pigments is compensated by combining them
with aluminium pigments and coloured pigments. The degree of pearlescence increases with
increase in particle size of the mica pigments. Large mica pigments give rise to a sparkle effect
comparable to that of aluminium pigments.
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