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Requirements
The requirements on coatings for larger vehicles concern the durability of the objects, even after
particular exposure. The most important durability property is corrosion resistance. After that, the
coating films must be resistant to chemicals, solvents and water. Only in a few cases is decoration
a consideration.
Application
Not only must the coating materials for larger vehicles be economical, but the application process
must also be simple (easily to apply), efficient and cheap. One way to meet these demands is to
maximise transfer efficiencies. The coating materials are therefore applied by means of airless
spray equipment that offers very high transfer efficiency (see Chapter 3.3). Pneumatic spray
guns are used only decorative effects are required, for example, for coloured topcoats. Since
most objects are relatively large, common stoving coatings cannot be used; however, very often
film forming at elevated temperatures is possible. For this technology segment, physical drying
paints, oxidatively drying paints and two-component paint systems are in use. Most of the coating
systems are comparable to the systems which are used for repair coatings (see Chapter 4). Small
metal parts are primed with electrodeposition primers (see Chapter 3.4.6) and then coated with
powder topcoats applied by electrostatic spray guns. As already mentioned, both coating systems
form films in stoving processes at relatively high temperatures.
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Primers
Of course, the primary task of primers is provide corrosion protection. Substrates such as steel,
galvanised steel and aluminium must be protected. In this technology field, oxidatively drying
corrosion protection primers are still in use. These consist of
alkyd resins
modified with fatty
acids containing two double bonds and more; the most important ones employed are linseed fatty
acids. Besides coloured pigments, the primer surfacers contain extenders, which mainly confer
corrosion protection by forming a barrier to water and moisture. However, active corrosion-protec-
tion pigments, e.g. zinc phosphates, are also employed. It is necessary to add siccatives to catalyse
the oxidative film-forming process. Most of the products are solvent-borne. To meet the current
VOC regulations, high-solids versions are available which achieve application solids of 65 % by
weight and higher.
Optimum products for this application field are two-component primers containing aromatic
epoxy resins (see Chapter 3.4.5.1) as binders and polyamine adducts or polyamide amines that
still contain free primary or secondary amine groups as crosslinkers (hardeners). The same type
of primer is used as that for repair coatings already described in Chapter 4.2. Also available are
water-borne primers based on the same resin groups. These contain epoxy resin dispersion and
water-tolerable amine components. Although the corrosion protection afforded by such primers is
excellent, there are some disadvantages, namely the products are not easy to sand and the films
are not weathering-resistant. Parts of large vehicles which are primed, assembled later and then
coated with topcoats, require a minimum weathering resistance for storage and transport until
assembly is started. Such elements are primed with two-component systems which contain satu-
rated polyesters bearing hydroxyl groups and polyisocyanate adducts as crosslinkers. It is pos-
sible to choose polyesters which confer higher hardness and also a suitable pigment combination;
this renders the films much easier to sand. Water-borne systems containing the aforementioned
product classes are currently available.
For special parts of large vehicles, thick-layer primers are applied for optimum corrosion resist-
ance; these parts include such building elements as frames for the vehicle floor, wheelhouses,
and floor panels. In such cases, solvent-borne systems fail because they do not form homo-
geneous films when applied in thick layers. The plastisols suitable for thick-layer coatings of
passenger cars (see Chapter 3.4.8.1) require too high a temperature for optimum film forming
(gelation), which is not available for the application conditions here. Therefore, liquid 100 %
systems are applied that contain liquid polyols (polyether polyols, as well as castor oil), which
are crosslinked with liquid aromatic polyisocyanates, mainly MDI derivatives (highly functional
isomers and derivatives of 4,4’-diisocyanatodiphenylmethane). By way of alternative, thick-layer
systems can also be prepared with the aid of aqueous dispersions containing epoxy resins and
water-tolerable amine adducts. All the aforementioned products are applied by airless spray
methods.
It has already been mentioned that small parts for large vehicles may be coated first with elec-
trodeposition primers. It is possible to choose the same materials as are used for the OEM proc-
ess for passenger car bodies that contain amine-modified, plasticised epoxy resins and blocked
polyisocyanates as crosslinker (cationic ED primers, see Chapter 3.4.5).
Finally, there are also water-borne one-component primers for small objects that contain mainly
water-dilutable epoxy esters and urea resins as crosslinker and have to be stoved.
Putties
After assembly of large vehicles from primed parts, it may be necessary to smooth structures such
as joints, weld seams, and bolted or riveted joins. Suitable for this purpose are the putties that
have already been described for repair coatings (see Chapter 4.3).
Coatings for heavy loader
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Coatings for heavy loader
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