Automotive Coatings Formulation: Chemistry, Physics und Practices

3.7.6 Solid colour water-borne basecoats
Following the successful implementation of effect basecoats, the wet-on-wet process was extended 
to solid colour topcoats as well. This led to significant improvements in weathering resistance, resist-
ance to chemicals and, most important, gloss retention. However, there were application problems. 
Table 3.7.5: Formulation and characteristic values of a water-borne basecoat
Pos.
Components
Solid wt-%
Formulation wt-%
01
thickener dispersion (3 % in water)
0.75
25.0
02
polyurethane dispersion (25 % in water)
6.00
24.0
03
polyester (80 % in n-butanol)
4.00
5.0
04
DMEA
0.5
05
melamine resin (70 % in water)
1.40
2.0
06
aluminium paste (65 % in water)
3.25
5.0
07
butyl glycol
5.0
12
deionised water
33.5
sum
15.4
100.0
application solids: 
15,4 wt-%
application viscosity 
14“ (DIN 4/23 °C), equivalent to ISO cup 53 s
solvent amount 
6 wt-%
pos.
01 
dispersion of precipitated Hectorite (sodium-magnesium-silicate) 
02 
polyurethane dispersion, example in DE 3825278 
05 
polyester of neopentyl glycol, isophthalic acid and trimellitic anhydride
(acid number 39 mg KOH/g, average molecular weight 1439 g/mol) 
06 
stabilised aluminium pigment paste 65 % in water
Basecoats

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Perfect execution of the wet-on-wet process requires that the resultant film thickness of the 
basecoats must be relatively low. Increasing the film thickness of basecoats can lead to blistering 
due to evaporating components such as water and cosolvents (popping). There is also the risk 
of sagging on vertical parts of substrate surfaces. Furthermore, there will also be a tendency to 
redissolve. However, if solid colours are to provide sufficient hiding power, they must be applied 
in optimum film thicknesses. For conferring optimum hiding power, mainly in the case of yellows 
and reds, the film thicknesses must be greater than for black, blue or green colours or for effect 
basecoats. Hiding power depends not only on the film thickness but also on the pigment volume 
concentration in a coating layer. If basecoats do not have to be glossy, it is possible to further 
increase the pigment volume concentration as in the case of solid colour topcoats applied in one 
layer. However, there are limits to this in that the hiding power does not always increase with 
increase in pigment quantities as these have a tendency to flocculate. These problems are circum-
vented in various ways. Agents for preventing popping can be added, and also judicious quantities 
of cosolvent can serve to prevent formation of blisters during the film-forming process. Modifying 
the application conditions can also boost optimum application behaviour for thicker layers.
The choice of suitable pigments and, again, modifying application conditions can help to yield 
higher film thicknesses and to improve hiding power. Finally, there is the possibility of applying 
solid colour basecoats over primer surfacers with modified colours (see Chapter 3.5.3.4).