Automotive Coatings Formulation: Chemistry, Physics und Practices

Metallic basecoats
Medium-solid
Water-borne
components
density
wt-%
vol-%
wt-%
vol-%
aluminium pigment
2.70
3.0
1.0
3.0
1.2
resins
1.20
19.0
14.6
19.0
16.4
solids
22.0
22.0
solvents-1
0.85
78.0
84.4
solvents-2
0.90
10.0
11.6
water
1.00
68.0
70.8
sum
100.0
100.0
100.0
100.0
layer thickness
15 µm
15 µm
g solid/m
2
21.1
21.1
g solvent/m²
69.0
8.9
Table 3.9.3: Calculation of solvent emission of solvent-borne and water-borne primer surfacers
Primer surfacer
Medium-solid
Water-borne
components
density
wt-%
vol-%
wt-%
vol-%
pigments
4.00
28.0
8.7
20.0
6.1
resins
1.20
32.0
33.0
25.0
25.4
solids
60.0
41.7
45.0
31.5
solvent
0.85
40.0
58.3
co-solvent
0.90
10.0
13.6
water
1.00
45.0
54.9
sum
100.0
100.0
100.0
100.0
layer thickness
35 
µm
35 
µm
g solids/m²
87.7
85.6
g solvent/m²
41.6
13.5
Automotive OEM coatings

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203
systems. The two-layer systems offered better appearance and weathering resistance. However, 
introducing an additional coating layer would also increase the VOC values of the entire coating 
system if it contained solvent-borne materials. Therefore, it was necessary from the outset to 
introduce water-borne solid colour basecoats in order that the VOC requirements could be met. 
High-solid solid colour basecoats do not meet current requirements.
In the past, conventional automotive OEM clearcoats contained acrylic resins of average solution 
viscosity and high-molecular, more reactive melamine resins. The application solid were 40 to 
45 % by weight. The application solid were increased to 48 to 50 % by weight through the use 
of low-viscosity acrylic and melamine resins. The use of HMMM resins can even push the solid 
content up to 55 to 60 % by weight. However, in that case, it becomes necessary to use stronger 
acids, e.g. sulphonic acids as crosslinking catalysts, and that has some disadvantages. These 
clearcoats are in competition, mainly in Europe, with two-components clearcoats that contain 
low-viscosity acrylic resins and aliphatic and cycloaliphatic polyisocyanate adducts as crosslinker 
(see Chapter 3.8.3.4). Naturally, the application of such clearcoats requires two-component spray 
equipment. However, these clearcoats feature greatly improved weathering and chemical resist-
ance in comparison to clearcoats crosslinked by melamine resins. Such two-component clearcoats 
have application solid of 55 % by weight and more. Table 3.9.4 shows the compositions of various 
clearcoats and the calculated solvent emissions.
While one-component, high-solid clearcoats are preferred in the USA, water-borne clearcoats have 
been developed in Europe. Water-borne clearcoats offer more scope than high-solid clearcoats 
for reducing VOC emissions, even though they still contain some cosolvents. One-component 
clearcoats 
[164]
and two-components clearcoats 
[165]
have been developed as well. However, both 
technologies have so far failed to conquer the automotive coatings market. The main reasons are 
problems in application behaviour. They suffer from the tendency to form blisters (popping) dur-
ing drying and crosslinking due to the anomalous behaviour of water as a solvent (high evapora-
tion enthalpy). Studies on preventing popping through addition of specific additives have shown 
that other film properties can be adversely affected, e.g. gloss and levelling. These are the very 
properties that clearcoats are expected to meet very well.
If solvent emissions are to be totally prevented, one possibility is to use clearcoats containing liq-
uid film formers. Such compositions are then UV cured. However, even UV clearcoats have some 
restrictions on use due to problems with application behaviour. These are:
• 
inhibition of effective crosslinking by atmospheric oxygen
• 
different radiation density on three dimensional objects
• 
shadow zones and interior parts are not reached by UV light
No automobile plant has so far installed UV-curing in an automotive OEM application line. How-
ever, many trials aimed at overcoming the aforementioned problems are underway 
[173]
.
Table 3.9.4: Compositions of the different clearcoats and calculation of solvent emission