Bog'liq Automotive Coatings Formulation Ulrich Poth - Chemistry, Physics und Practices (2008, Vincentz Network) - libgen.li
4.7 Clearcoats The most important demands imposed on repair clearcoats are optimum application behaviour
(application robustness) and optimum resistance (durability). Repair clearcoats are applied to
pre-dried basecoats and must form optimum film properties at ambient or elevated temperatures.
Film forming can be forced by stoving for up to 30 minutes at 60 °C. IR emitters are also suitable
for supporting film formation by repair clearcoats. Clearcoats must wet the basecoat surfaces
perfectly without giving rise to any re-dissolving effects. They must provide optimum levelling
and high gloss. Repair clearcoats must confer virtually the same properties as OEM clearcoats as
regards weathering resistance, non-yellowing, resistance to chemicals, solvents, and mechanical
impact, e.g. scratching.
In the past, repair clearcoats were used that were based only on physical drying acrylic resins
(TPA, thermoplastic acrylics), but such clearcoats are unable to meet the aforementioned resist-
ance requirements. Nowadays, nearly all repair clearcoats consist of hydroxy acrylic resins (see
Chapter 3.8.3.1), which are crosslinked by polyisocyanate adducts. Repair clearcoats are two-com-
ponent systems and are comparable to two-component clearcoats for automotive OEM application,
which are described in Chapter 3.8.3. However, the choice of acrylic resins here is influenced
by the other film forming conditions (lower crosslinking temperatures). For optimum resistance
properties, the acrylic resins contain a balanced ratio of hard and plasticizing monomers. The
resins have high hydroxyl numbers. The preferred crosslinkers are isocyanurate adducts of hex-
amethylene diisocyanate, which are more reactive than others and also generate lower solution
viscosities for higher application solids. To maximise application solids, adducts of polyisocy-
anates are chosen that have a particularly low viscosity
[182]
. The low-viscosity isocyanate adducts
are achieved by generating particularly narrow molecular weight distributions of isocyanurates,
or by combining isocyanurate oligomerisation with the formation of allophanates.
The application viscosity of repair clearcoats is typically 60 s (ISO 4231, cup 4 mm, 23 °C). Cur-
rent VOC regulations allow the mixture of base colours and hardener to emit 420 g solvent per
litre coating material, i.e. the solids content must be higher than 60 % by weight. For optimum
crosslinking at temperatures up to 60 °C, catalysts must be added. Until now, the preferred cata-
lysts have again been organic tin compounds (e.g. dibutyltin dilaurate). It must be remembered
that the use of tin compounds will be restricted in the near future.
As the VOC regulations can be adequately met at the moment, there has been little effort so far
to develop water-borne repair clearcoats. However, there are other industrial applications which
use water-borne two-component clearcoats that are based on secondary acrylic dispersions and
hydrophilic polyisocyanate adducts.
Due to the fundamental concerns surrounding the use of isocyanates, attempts are underway
to introduce other crosslinking mechanisms into the repair coat application (NISO or non-iso-
cyanate technologies). Examples here are acrylic resins containing acetoacetate groups that are
crosslinked by ketimimes
[184]
and acrylic resins crosslinked by siloxanes
[185]
. However, these
crosslinking methods have not played any role in repair clearcoats to date.
It is likely that the market volume for repair coatings will decrease. This assumption is based on
the observation that more and more car body parts are being made from plastic and that will obvi-
ate the need for repair coating. However, there is sure to be some demand for individual additional
coatings and special designs in the future, as well.
Clearcoats
