Automotive Coatings Formulation: Chemistry, Physics und Practices

Solvents
Reason
methanol
toxic
ethylene glycol methyl- and -ethyl ether
teratogenic
ethylene glycol methyl- and -ethyl ether acetates
teratogenic
dimethyl formamide
toxic
n-methyl pyrrolidone
hazardous
isophorone
hazardous
halogenated hydrocarbons
toxic, photolytic
benzene
toxic, cancerogen
toluene
hazardous
methyl ethyl ketone
unpleasant odour
methyl isobutyl ketone
unpleasant odour
Table 2.2: Suitable solvents for automotive paints
Solvents
Boiling 
temp.
[°C]
Evaporation-
rate 
(ether=1)
Vapour
pressure
[hPa] (20 °C)
Density 
(20 °C)
[g/cm
3
]
Flashpoint 
[°C]
xylene
137–142
17
90
0.874
25
aromatic 100
155–182
43
3
0.877
41
aromatic 150
178–209
120
1
0.889
62
n-butanol
116–118
33
5.6
0.811
34
isobutanol
106–107
25
9.5
0.802
28
ethylhexanol
183–185
600
0.5
0.833
76
methoxy propanol
119–123
25
13.3
0.934
38
butyl glycol
167–173
163
0.9
0.901
67
butyl acetate
123–127
11
12.5
0.880
25
methoxy propyl acetate
143–149
33
3.4
0.965
45
butyl glycol acetate
190–198
250
0.4
0.945
75
Composition of paints

---

28
Water as solvent or dispersing agent
Water acting as a solvent or dispersing agent perfectly meets the requirements for cutting down 
on volatile organic compounds in coating systems. There are numerous ways to convert various 
polymers into aqueous phase with a view to creating the optimum application form (see Chapters 
2.2.2 and 2.2.3). But the use of water creates some problems. Table 2.3 
[23]
shows the physical 
constants of water in comparison of those of typical organic solvents. Worth noting are the higher 
evaporation enthalpy, higher density, higher polarity, and higher conductivity of water. Water 
lends itself to the preparation of colloidal solutions and various polymer dispersions. When it 
comes to formulating water-borne paints, allowance must be made for its specific properties. The 
stability of colloidal solutions is often supported by adding organic solvents, also known as co-
solvents. These also boost uniform evaporation of water during the film forming process. Special 
wetting agents are added to water-borne systems to ensure optimum wetting of various surfaces 
and pigments. Since water can exhibit anomalous viscosity behaviour and has a relatively low 
evaporation rate, rheological additives must be added if sagging and other film defects are to be 
avoided. The slow evaporation is also the reason why high air circulation rates and higher tem-
peratures are used for the flash-off process.