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colour management
of plastic topcoats. As already mentioned,
there must be no difference in colour or effect on the different exterior parts of car bodies. For
some time, car customers prefer to have all body parts and attached parts (e.g. even mirror hous-
ings) in the same colour shade and effect as the rest of the car. This requirement is unlikely to
change much in the future. Not only that but the construction joints are made as narrow as pos-
sible to convey the impression of body surface homogeneity, even for small cars. This may be the
reason that small colour differences are easily noticed since the human eye is very sensitive when
Coatings for attached parts
220
it comes to comparing colour shades. To achieve the goal of minimising differences in shade, it is
not enough to choose the same pigmentation for all topcoat systems (basecoats for metal part and
plastic parts). First of all, the two coating systems may contain the same types and quantities of
pigments but different resin systems, which confer different wetting properties on pigments and
effect substance, and thus lead to differences in shade. However, the most important influence is
the different film-forming behaviour. Metal parts and plastic parts have totally different specific
heat capacities and the film forming conditions (mainly the temperatures) are totally different.
Both conditions influence the viscosity during film forming and thus the effect substance orienta-
tion, along with the re-flocculation of pigments dispersed in the paint.
There are principally two ways to manage these problems: The first is to adjust the formulation of
the different systems – i.e. besides the different resin compounds, to use different recipes, quanti-
ties and, in extreme cases, different pigment types or effect substances, and production processes
– with the aim of achieving the same colour and effect on all parts, even though the film forming
conditions differ. These demands can be met with the help of computer-aided colour analysis and
formulation guidelines for standard tinting coatings. In addition, there is the possibility of produc-
ing large series of attached parts for car bodies and to choose individual pieces from the series
on the basis of best colour match. The second strategy is based on on-line application of attached
parts. All attached parts, primed only, are fitted to the car body, and the entire car body is coated
with the same material. Of course, the chosen coatings must confer universal properties. The must
meet the relatively high demands on flexibility matching that of plastic parts, and also meet the
resistance requirements imposed for OEM coatings on metal parts of car bodies. In particular, it
is very difficult to balance the flexibility and solvent-resistance properties (e.g. low swellability) of
such universally applied topcoat systems. Since elevated temperatures may promote crosslinking
to meet the mentioned requirement combination, the plastic materials chosen are more resistant
to elevated temperatures (e.g. polycarbonates, polyamides, and SMC), a fact which means that
stoving coatings may be used. Unfortunately, such plastic materials are more expensive (material,
production process, moulding process) than other plastic grades. On the other hand, the automo-
tive industry has demanded for some time that the stoving temperatures of all OEM coatings be
reduced with a view to saving on energy costs and to rendering the application methods more
versatile (to enable earlier use of compounds and materials which are less stable). Therefore, one
of the key objectives for application of OEM coatings besides reductions in VOC emission is to
provide coating materials which on one hand are stable in storage, but on the other are able to
crosslink efficiently at lower temperatures than is now the case, will save on energy and can be
applied to all the various substrates found in car bodies. Such universal coating systems must
bestow all the necessary properties (colour and effect reproduction, resistance to weathering,
chemicals, solvents, water, and mechanical impact).
Radiation curable coatings are also used for plastic substrates. Until now, such coating materials
have only been used for flat objects and not for pigmented systems. There are clearcoats available
which crosslink in UV light or even electron beams. Important examples are clearcoats for wheel
covers. In addition, front light reflectors are protected with UV coatings. This is also planned for
plastic windows (polycarbonate) (see Chapter 3.8.7).
Plastic coatings for automobiles
221
Coatings for heavy loader
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