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3.8.3.3 Amino resins
Theoretically, the melamine resins that are used for topcoats and basecoats are suitable for clear-
coats. However, regional preferences exist with regard to the melamine preferred. For medium-
solid, one-component clearcoats that are used in Europe, the chosen melamine resins are relatively
low-molecular, highly etherified and contain free imino-groups; they exhibit adequate reactivity
and do not require catalysts under the right automotive stoving conditions. The application solids
content achieved with suitable acrylic resins is about 50 % by weight. During crosslinking, some
methylene groups form by self-crosslinking. Methylene groups impart relatively good weathering
resistance and better resistance to chemicals (acids) than methylene ether groups.
One-component, high-solid clearcoats (solids content up to 60 % by weight) contain mainly
HMMM resins (hexamethoxymethylmelamine). These are particularly important in the USA
(see Chapter 3.8.4).
3.8.3.4 Polyisocyanates
Automotive clearcoats that feature optimum weathering require the use of aliphatic and
cycloaliphatic polyisocyanates as crosslinker. Since isocyanate groups react at ambient tempera-
tures with hydroxyl groups, the clearcoat with resins containing hydroxyl groups and additives
(base lacquer) has to be delivered separately from the isocyanate crosslinker (hardener). Such
clearcoats are therefore classified as two-component coatings. Both components are mixed just
before application. Ready-mixed clearcoats have to be use within a certain period of time known
as the pot life. During this time, the viscosity undergoes exponential growth that ultimately leads
to application problems, e.g. loss of flow and levelling properties and forming of specks in the
clearcoat layer. For the application of two-component clearcoats in automotive coating processes,
the problem of pot life is resolved by using two-component spray guns. This application system
has separate feed lines for the base lacquer and hardener. The two are mixed just prior by a static
mixer just prior to reaching the gun and the mixture is sprayed immediately. Consequently, the
pot life has no impact on the application process. It is only necessary to ensure that the spray gun
is cleaned, mainly during breaks.
Diisocyanates are toxic (by inhalation) because they have a measurable vapour pressure at ambi-
ent temperature. They therefore make unsuitable hardeners. To render them suitable, they are
transformed into oligomers by various oligomerisation methods. Such oligomers still present
Clearcoats
180
physiological concerns, due to the quantity of free reactive isocyanate groups, but they are not
toxic by inhalation on account of their resin-like properties, with practically no vapour pressure
at ambient temperatures. Nonetheless, users must ensure to avoid direct contact between such
products and skin or mucous membranes. The oligomerisation reactions are nearly exclusively
based on hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI). They involve
the formation of different adducts, such as urethanes, allophanates, biurets, uretdiones and, most
commonly, isocyanurates (see Figure 3.8.11).
These polyisocyanate adducts react much more clearly with hydroxyl groups from partner resins
than with the functional groups of melamine resins. Consequently, the mixing ratios for resins
containing hydroxyl groups and polyisocyanate adducts are calculated stoichiometrically. The
optimum is an equivalent ratio of both components (n
NCO
= n
OH
), but it is possible to vary the
ratios within certain limits, without loss of properties. Only the reaction of isocyanates with
atmospheric moisture need be taken into consideration. This side-reaction leads to the formation
of urea groups. For stoving coatings, such as automotive OEM clearcoats, this side-reaction is not
very extensive.
As already mentioned, isocyanates can react at ambient temperatures with resins containing
hydroxyl groups. However, the gradient of the reaction rate as a function of temperature (Arrhe-
nius plot) is much smaller than that for the reaction of functional groups of melamine resins. It
makes sense, then, to crosslink two-component clearcoats containing polyisocyanate adducts at
the same temperatures as the one-component clearcoats containing melamine resins with a view
to obtaining efficient crosslinking efficiently and perfect film properties.
Compared with films resulting from melamine resin crosslinking, films containing polyisocyanate
adducts are much more homogeneous and the molecular network is more extended. This explains
the better chemical resistance, better resistance to chemicals and better flexibility. However, as
a rule, the network arc length is greater than for the networks in films crosslinked by melamine
resins. That leads to lower hardness and opens the possibility of diffusion effects. However, both
groups of polyisocyanate adducts behave in totally different ways. Crosslinkers based on hexam-
ethylene diisocyanate are distinguished by high reactivity and crosslink optimally to afford rela-
tively flexible and weathering-resistant clearcoat films that can be swollen by strong chemicals
and specific solvents. IPDI adducts are less reactive and lead to less efficient crosslinking than
Figure 3.8.11: Isocyanurate adduct of hexamethylene diisocyanate
Automotive OEM coatings
181
HDI adducts. However, due to their cycloaliphatic structure, the resultant films are much harder
and more resistant to diffusion processes, which can cause swelling. Therefore, the films offer
better chemical and solvent resistance, but are also less flexible.
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