Automotive Coatings Formulation: Chemistry, Physics und Practices

2.2 Application form
Paints are made in different physical forms for application. These are
• 
organic solutions
• 
aqueous solutions
• 
aqueous dispersions
• 
non-aqueous dispersions (NADs)
• 
100 % systems
• 
powder coatings
2.2.1 Organic solutions
The most important characteristic of organic solutions is their viscosity 
[25]
. Viscosity (
η
) is the spe-
cific coefficient of shearing stress and shear gradient in liquid phases (
/D). Shearing stress has 
the same units as pressure; the vector is oriented not vertically towards the surface of the liquid, 
but horizontally ( = F / A). The shear gradient is the differential of the velocity and the thickness 
of the liquid (D = d[v] /d[l]) (see Fig-
ure 2.6). The unit of viscosity is Pa s
(1 Pa s = N s/m²).
The viscosity of a colloidal solution 
varies with the type and viscosity 
of the solvent, the number of par-
ticles (concentration), the particle 
size, the interaction of polymer 
and solvent, and the temperature. 
Solutions of low molecular prod-
ucts have viscosities which are 
independent of shear gradients 
(viscosity behaviour is said to be 
Newtonian if it follows Newton’s 
law of viscosity). On account of 
the specific particle interactions of 
polymers, the viscosities of most 
colloidal solutions vary with the 
shear gradient. With a few excep-
tions, the viscosity decreases with 
increase in shear rate. This behav-
iour is known as structural viscos-
ity (pseudoplasticity) and is due to 
the stepwise decrease in particle 
interaction with increase in shear 
rate over time. If particle interac-
tion of particles is very extensive, 
a minimum shear rate is often 
needed before a shear gradient can 
be created. The minimum shear 
rate is also called the yield point. 
Solutions with a viscosity below 
the yield point are gels. The viscos-
ity of a colloidal solution may also 
Figure 2.6: Shear gradient of a solution as a measure of viscosity
Figure 2.7: Viscosity of different types of solutions as a function
of shear rate and time
Application form

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be influenced by the shear time. The reason is that some time is needed for particle interactions 
to diminish as well as for them to increase again. Such viscosity behaviour is called thixotropy. 
Figure 2.7 shows various viscosity curves as a function of shear rate and time.