Automotive Coatings Formulation: Chemistry, Physics und Practices

Extenders
Versions
Density
[g/cm³]
Average particle 
size [µm]
Oil absorption 
value
barium sulphates
natural
4.0–4.5
2–15
9–19
precipitated
4.1–4.5
0.7– 3.0
11–20
aluminium silicates
kaolin
2.1–2.6
0.8–25
30–68
mica
2.8
1.5–50
37–75
feldspar
2.6–2.7
7–25
20–32
talc
2.7–3.5
0.8–25
20–60
synthetic
1.9–2.6
0.015–5.0
35–160
other silicates
wollastonite
2.85
3.5–25
80–90
calcium sulphate
2.5–2.9
10–12
15
calcium carbonates
chalk
2.7
1.0–10.0
15–20
calcite
2.7
0.8–25
15–25
dolomite
2.7–2.9
2–25
12–22
synthetic
2.7
0.02–1.0
22–56
aluminium oxide
2.4
0.7–15
–
aluminium hydroxide
2.1–2.4
0.1–2.2
60
silicium dioxide
cristobalite
2.35
3.5–22
20–30
kieselguhr
2.4–2.5
2–10
80–115
quartz
2.6–2.7
2–25
20–30
silica gel
2.1
2–10
80–300
precipitated
1.9–2.1
0.008–10
180–260
pyrogenic
–
0.007–0.2
120–350
Automotive OEM coatings

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95
influence the rheological behaviour of the paint material. They give rise to intrinsic viscosity, and 
so are used in limited quantities to avoid lowering the application solids and adversely affecting 
application behaviour (levelling, film gloss). They benefit primer surfacer formulations by prevent-
ing settling of dispersed pigments and sagging of paints applied to vertical surfaces.
In the past, as their name suggests, extenders were used make materials stretch further, primarily 
to reduce the overall material costs. It was subsequently discovered that a combination of pig-
ments of different particle sizes, used in defined pigment volume concentrations, imbues primer 
surfacer films with a great capacity to absorb mechanical impact energy. It is believed that the 
various particles alter their relative positions within the film matrix on impact. The impact energy 
is transformed into kinetic energy (i.e. is dissipated). This idea forms the basis for the afore-
mentioned analogy between a primer surfacer layer and an asphalt road. Figure 3.5.11 conveys 
an impression of a combination of barium sulphate extender and titanium dioxide with average 
diameters of 1 µm and 200 nm respectively. The weight ratios are approximately 1:1, while the 
pigment volume concentration in the film is about 20 %.
There is evidence that pigments with platelet particles help to absorb much more energy. How-
ever, as mentioned above, such extenders are not widely used due to the negative influences on 
viscosity, levelling, and topcoat holdout.