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The chemistry of isocyanates

Polyurethane coating chemistry is based on the combination of polyisocyanates with polyols. As crosslinking agents, polyisocyanates play a crucial role with respect to processing, curing and the resulting coating properties. Polyisocyanates are obtained by modification of diisocyanates[3]. Depending on the desired property, either aliphatic or aromatic components are used. The most important representatives in industry are shown in Fig. 2.

The chemistry of isocyanates
Fig. 2: All industrially important aromatic (top) and aliphatic diisocyanates (bottom),
apart from HDI, exist as a number of isomers or oligomers (MDI)

A fundamental feature of isocyanates is their high reactivity. The reaction of the isocyanate groups with one another or with small NCO-reactive molecules (for example water or short-chain mono- or polyhydric alcohols) is utilized in the modification reaction for the preparation of polyisocyanates, the crosslinking agents for coatings. Figs. 3 and 4 below illustrate the extraordinary versatility of isocyanate chemistry and, consequently, the many ways of controlling the properties of polyurethane coatings.


Formation of
Reaction equation
Linear polymer (α-Nylon)
Carbodiimide
Uretonimine
Fig. 3: Cyclopolymerization and polymerization of isocyanates (diagrammatic)


Reaction with:
Reaction equation
Alcohol to Urethane:

Urethane to allophanate:

HDI-allophanate

IPDI-Allophanate
Water to urea
Amine to urea:
Urea to biuret
Carboxylic acid to amide
Amide to acyl urea
Anhydride to imide
Epoxide to oxazolidone
Oxime to oxime carbamate
Carbon dioxide to oxadiazinetrione
Fig. 4: Reactions of isocyanates (diagrammatic)

References:
[3] H. J. Laas, R. Halpaap and J. Pedain, J. Prakt. Chem./Chem. Ztg. 336, 185 (1994)

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