© 2006 by Taylor & Francis Group, LLC

Composition of the Anticorrosion Coating 19
FIGURE 2.6 Some typical isocyanate reactions. A-hydroxyl reaction; B-amino reaction;
C-moisture core reaction.
2.2.3 POLYURETHANES
A. R NCO + HO R′ R N
H
O
C OR′
(Urethane)
B. R NCO + H2N R′ R N
O
C NR′
H H
(Urea)
C. R NCO + HOH R N
H
O
C OH R NH2 + CO2 (Carbamic acid)
Polyurethanes as a class have the following characteristics:
• Excellent water resistance [1]
• Good resistance to acids and solvents
• Better alkali resistance than most other polymers
• Good abrasion resistance and, in general, good mechanical properties
They are formed by isocyanate (R–N=C=O) reactions, typically with hydroxyl
groups, amines, or water. Some typical reactions are shown in Figure 2.6. Polyurethanes
are classified into two types, depending on their curing mechanisms: moisturecure
urethanes and chemical-cure urethanes [1]. These are described in more detail
in subsequent sections. Both moisture-cure and chemical-cure polyurethanes can be
made from either aliphatic or aromatic isocyanates.
Aromatic polyurethanes are made from isocyanates that contain unsaturated
carbon rings, for example, toluene diisocyanate. Aromatic polyurethanes cure faster
due to inherently higher chemical reactivity of the polyisocyanates [8], have more
chemical and solvent resistance, and are less expensive than aliphatics but are more
susceptible to UV radiation [1,9,10]. They are mostly used, therefore, as primers or
intermediate coats in conjunction with nonaromatic topcoats that provide UV protection.
The UV susceptibility of aromatic polyurethane primers means that the time
that elapses between applying coats is very important. The manufacturer’s recommendations
for maximum recoat time should be carefully followed.
Aliphatic polyurethanes are made from isocyanates that do not contain unsaturated
carbon rings. They may have linear or cyclic structures; in cyclic structures, the ring
is saturated [11]. The UV resistance of aliphatic polyurethanes is higher than that of
aromatic polyurethanes, which results in better weathering characteristics, such as
gloss and color retention. For outdoor applications in which good weatherability is
necessary, aliphatic topcoats are preferable [1,9]. In aromatic-aliphatic blends, even
small amounts of an aromatic component can significantly affect gloss retention [12].
© 2006 by Taylor & Francis Group, LLC