Automotive Coatings Formulation: Chemistry, Physics und Practices

OEM-clearcoat
Low-solid
Medium-solid
High-solid, 2 K
components
density
wt-%
vol-%
wt-%
vol-%
wt-%
vol-%
resins
1.20
42.0
33.9
48.0
39.5
55.0
46.4
solvents
0.85
58.0
66.1
52.0
60.5
45.0
53.6
sum
100.0
100.0
100.0
100.0
100.0
100.0
layer thickness
40 
µm
40 
µm
40 
µm
g solids/m²
50.4
50.4
50.4
g solvent/m²
69.6
54.6
41.2
Reductions in solvent emissions

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204
Figure 3.9.1: Composition of clearcoats and solvent emissions
Table 3.9.5: Reductions in VOC emissions from automotive OEM coating systems at the various development stages
Coatings
system
emission
g solvent/m²
1 A-EDP
LS primer
surfacer
LS basecoat
LS clearcoat
2 C-EDP
LS primer
surfacer
MS basecoat
MS clearcoat
3 C-EDP
MS primer
surfacer
W basecoat
MS clearcoat
4 C-EDP
W primer 
surfacer
W basecoat
HS clearcoat
5 C-EDP. new
W primer 
surfacer
W basecoat
P/D clearcoat
primer
18 
µm
10.0
2.0
2.0
2.0
0.0
powder surfacer 
35 
µm
50.4
50.4
33.4
14.4
14.4
basecoat
15 
µm
130.5
69.0
8.9
8.9
8.9
clearcoat
40 (65) 
µm
69.6
54.6
54.6
41.2
0.0
sum
260.5
176.0
98.9
66.5
23.3
A-EDP = anodic electro deposition primer 
HS = high solid 
C-EDP = cationic electro deposition primer 
W = water-borne 
LS = low solid 
D = dispersion, solvent-free 
MS = medium solid
Automotive OEM coatings

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205
There are also interesting alternatives in the guise of powder clearcoats. At least one major 
European car maker has introduced these into several assembly plants 
[176]
. By contrast, powder 
clearcoats are not used in the USA, where they are considered to be inferior. The main problem of 
powder clearcoats (as described in Chapter 3.8.7) is that smooth and glossy films with optimum 
levelling are achieved only at relatively high layer thicknesses of more than 60 µm. This increases 
paint consumption and car-body weight. Trials involving complex and expensive methods for 
producing smaller particles yielded thinner films that were smooth and glossy, but fluidising 
such particles proved difficult. 
The problem has been solved by preparing dispersions of powder clearcoats in water. Such dis-
persions make it easy to generate much lower particle sizes by milling. Powder slurries of this 
kind 
[170]
yield optimum clearcoat films with a layer thickness of about 40 µm, equivalent to that 
of conventional clearcoats (see Chapter 3.8.8 and 3.8.7). However, the overall production process 
is complex and expensive.
Finally, solvent-free clearcoats based on secondary dispersions have been developed 
[177]
. These 
secondary dispersions are notable for the fact that they contain homogeneous particles which 
contain all the film forming ingredients (resins, crosslinker, and some additives). Figure 3.9.1 
shows the composition of the various clearcoat technologies, along with the respective solvent 
emissions at various film thicknesses (g/m
2
).
Total solvent emissions from the various development stages of automotive coating systems from 
the 1970s to the present day are shown in Table 3.9.5.
All charts and calculations of solvent emissions presented so far are based on silver-grey base-
coats (containing mainly aluminium pigments). Other colours and effects may yield other abso-
lute figures, but the general trend is the same. They also ignore the possible complex geometry 
of coated objects and transfer efficiency. As already described (see Chapter 3.3), major steps have 
been taken to optimise spray equipment to increase transfer efficiency, and these will make an 
important contribution to reducing solvent emissions.
Other steps aimed at reducing VOC emissions seek to reduce layer thickness, e.g. of primer surfac-
ers (see Chapter 3.5.7). Although there are regional differences in legislation around the world, 
there are various coating systems available that enable car makers to meet the different stipula-
tions on VOC emissions 
[163]
. Most legislative restrictions can be met with high-solid coatings. It 
is expected that legislative restrictions will be progressively tightened. Experience suggests that 
coating systems based on aqueous dispersions and systems with reactive diluents crosslinked by 
UV light have every chance of meeting future requirements, without loss of film properties.
Reductions in solvent emissions

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