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Molybdate Red and Molybdate Orange
Molybdate Red and Molybdate Orange
Molybdate red and molybdate orange [12656-85-8], C.I. Pigment Red 104:77605, are
mixed-phase pigments with the general formula Pb(Cr,Mo,S)O4 [3.115]. Most commercial
products show a MoO3 content of 4–6%, a refractive index of 2.3–2.65 and densities of about 5.4–6.3 g cm−3). Their hue depends on the proportion of molybdate,
crystal form, and particle size.
Pure tetragonal lead molybdate, which is colorless, forms orange to red tetragonal
mixed-phase pigments with lead sulfochromate. The composition of molybdate red
and molybdate orange pigments can be varied to give the required coloristic properties;
commercial products usually contain ca. 10% lead molybdate. Lead molybdate
pigments have a thermodynamically unstable tetragonal crystal modification that can
be transformed into the undesirable stable yellow modification merely by dispersing
[3.129]. This is especially true of the bluish varieties of molybdate red which have
larger particles whose color can be changed to yellow by shear forces. The tetragonal
modification of the lead molybdate pigments must therefore be stabilized after
precipitation [3.130, 3.131].
The fastness properties of the molybdate orange and molybdate red pigments are
comparable with those of the chrome yellows. As with the chrome yellows, the
pigment particles can be coated with metal oxides, metal phosphates, silicates, etc.,
to give stabilized pigments with high color brilliance and good fastness properties, as
well as highly stabilized grades with very good resistance to light, weathering, sulfur
dioxide, and temperature, and with a very low content of acid-soluble lead (DIN 55770,
1986 or DIN/ISO 6713, 1985).
The colors of lead molybdate pigments vary from red with a yellow hue to red with
a blue hue. Since chrome orange is no longer available (see Section 3.4.3), molybdate
orange has become much more important.
Uses
Molybdate orange and molybate red are mainly used in paints, coil coatings, and for
coloring plastics (e.g., polyethylene, polyesters, polystyrene). The temperature-stable
grades are the most suitable for coil coatings and plastics.
Molybdate orange and molybdate red feature a low binder demand, good dispersibility,
hiding power, and tinting strength, combined with very high lightfastness
and weather resistance. Stabilization (see “Production”) also yields high-grade pigments
with good resistance to sulfur dioxide and high temperature.
Like chrome yellows, molybdate reds are used to produce mixed pigments. Combinations
with organic red pigments provide a considerably extended color range.
Such combinations have very good stability properties because the lightfastness and
weather resistance of many organic red pigments are not adversely affected by molybdate
pigments.
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