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Aliphatic and aromatic isocyanates are the two classes of isocyanates used in polyurethane coatings.
Aromatic isocyanates are used where oxidative discoloration on exposure to UV radiation is not an issue. Polyurethane coatings made from aromatic polyisocyanates are
very sensitive to oxydation and, as a result, is prone to degradation in direct exposure to sunlight.
On the opposite, the main use of aliphatic isocyanates is in the manufacture of light-stable coatings. They are used when
UV or light stability is a must as it is the case in automotive clearcoats and in many water-based recipes.
 ALIPHATIC ISOCYANATES
Figure 1: Aliphatic di-isocyanates
The use of Aliphatic polyisocyanates in polyurethane coating formulations will provide
exceptional chemical resistance and good
weathering properties. Thanks to the lack of phenyl group, the use of
aliphatic polyisocyanates will ensure durability of the adhesion in severe conditions.
AROMATIC ISOCYANATES
Figure 2: Aromatic di-isocyanates
Derivatives of these two products will lead to coatings with poor weathering resistance (yellowing) and with lower chemical resistance than aliphatic derivatives (especially poorer saponification
resistance). Thus aromatic polyisocyanates are mainly used for indoor applications (floor coatings, tank coatings, etc.) or for primers. Even for primers, they are used less and less
in automotive applications since yellowing of the primer can affect the color of the topcoat and lead to delamination between layers.
In general, the main applications of aromatic polyisocyanates are not in coatings. For instance, 80% of the production of TDI is used to prepare soft foams and 65% of
MDI rigid foams.
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