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3.7.5 Water-borne basecoats
Although high-solid basecoats contain 30 to 40 % by weight application solids and save a lot of
solvent in comparison to low-solids basecoats, solvent emissions still amount to 60 to 70 % by
weight. In addition, high-solid basecoats have poor application robustness, and some colours and
effects are not available. Both these reasons led to the development of water-borne basecoats. The
components for water-borne basecoats were chosen in accordance with the experiences gained
with solvent-borne basecoats. For water-borne basecoats, too, the resin combination chosen con-
tained physically drying resins and crosslinkable resins with plasticizing properties. There was
also the need for very efficient rheological additives. In transferring the principles from solvent
phase into water phase, it proved necessary in some cases to change the class of product, but not
the function and properties.
3.7.5.1 Resins for water-borne basecoats
Resins for water-borne basecoats that boost physical drying are acrylic dispersions and poly-
urethane dispersions. Resins with plasticizing properties are water-thinnable, saturated poly-
ester or acrylic resins that contain hydroxyl groups which are crosslinked by water-thinnable
melamine resins.
Primary acrylic dispersions
There are different types of aqueous acrylic dispersions, namely primary and secondary. Primary
acrylic dispersions are prepared from acrylic monomers directly in the water phase, following
the emulsion polymerisation process. The process utilise water-soluble polymerisation initiators
(e.g. ammonium peroxydisulphate) and emulsifiers (e.g. alkyl sulphonates, ethoxylated alkyl
sulphonates, and alkyl ethoxylates). Emulsifier and initiator are dissolved in water. According to
the most widespread, classical theory of emulsion polymerisation
[143]
, the emulsifier forms micro-
micelles that contain initiator molecules. The aqueous phase is heated to about 80 °C, the reaction
temperature for polymerisation. The mixture of acrylic monomers is admixed to yield emulsions
with relatively large particles. Monomers which are mainly insoluble in water diffuse into the
micelles, starting a free-radical polymerisation process. The free-radical chain polymerisation
process is comparable to solution polymerisation (described in detail in Chapter 3.8.3.1) and
consists of the reaction steps: initiator decomposition, initiation, chain extension (propagation),
and chain termination through recombination of free-radicals or chain transfer. This process can
yield high-molecular polymers. The reaction enthalpy (exothermic reaction) is transferred into
water and removed by cooling the aqueous reaction mixture. The particle sizes are determined
by the type and quantity of emulsifier. The more emulsifier and the greater it efficiency is, the
smaller are the particles and the more stable is the emulsion. Attainable average particle sizes
range from 150 to 300 nm (diameter). Figure 3.7.16 (page 158) shows the principle behind emul-
sion polymerisation.
Basecoats
158
The physical properties of the polymer (hardness, flexibility, glass-transition temperature) depend
on the composition of the monomers. Theoretically, the same conditions are employed as for solu-
tion polymerisation products (see Chapter 3.8.3.1). The film-forming behaviour of such primary
acrylic dispersions is influenced by the interaction of the small particles. As already mentioned
under the general description of dispersions, the viscosity at low solids content is relatively low.
However, from a certain concentration on, the viscosity increases rapidly with increase in solids
content. The reason for this is that fine particles interact efficiently due to the large surface energy
possessed by small particles. This property provides strong support for film formation and the
immobilisation of effect substances. This behaviour is intensified when polymer dispersions are
used that have very large molecules or, even better, crosslinked molecules. Such dispersions
are able to form gels in the aqueous phase by interacting with the cosolvent. Such polymer gels
act like rheological additives. Partly crosslinked acrylic polymers bearing free carboxyl groups
can form gels if they are partly neutralised by amines. Specific quantities of such products in a
water-borne basecoat can have benefits for the application behaviour: immobilisation of the effect
substance, no sagging of the film on vertical parts of the coated object, support for physical dry-
ing, and no redissolving.
The particles of primary acrylic dispersion are able to form films only above a specific temperature
after water has evaporated. This temperature is called the minimum film-forming temperature
(MFT) and is a little bit higher than the glass-transition temperature (T
G
). Film forming of disper-
sions takes place by fusion of particles by their outer shells. As the film-forming temperatures of
basecoats are initially relatively low, acrylic polymers with low minimum film-forming temperatures
should be used. Unfortunately such polymers are relatively soft and not resistant to mechanical
impact, solvents or chemicals. To compensate, the dispersions chosen have polymers with higher
glass-transition temperature that confer hardness and resistance. The particles of such polymers
are embedded into the film matrix by the combination resins, which are not dispersions, but rather
are water-soluble and able to fill the hollow spaces between the said particles. So, at latest during
stoving with the clearcoat, the basecoat layer is fused very well, although it contains polymers with a
high MFT. However, it must be assumed that full interdiffusion does not occur. Domains of relatively
Figure 3.7.16: Principle behind emulsion polymerisation
Automotive OEM coatings
159
high-molecular acrylic polymers still remain. Such areas cannot contribute to the crosslinking proc-
ess, even though they may contain functional groups for crosslinking reactions. Normally, therefore,
the polymers of dispersion do not contain monomers with additional functional groups. The useful
properties possessed by acrylic polymer dispersions are their drying properties, influence on the
rheology, and physical resistance. They are less suitable for wetting substrates and pigments, pro-
moting flow and levelling, or supporting flexibility and crosslinking.
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