Blocked Polyisocyanates for Polyurethane Coatings

General Aspects of Blocked Polyisocyanates

For technical and economic reasons, blocked polyisocyanates are being used where a one-package coating system is required, the presence of free isocyanate must be excluded, heat curing is possible, and the high performance of polyurethane coatings is desired.  Bayer MaterialScience markets blocked polyisocyanates under the Desmodur® BL trademark.

A blocked polyisocyanate can be defined as an isocyanate reaction product which is stable at room temperature but dissociates to regenerate isocyanate functionality under the influence of heat (Figure 1). Temperatures between 120°C and 250°C are necessary to release the blocking groups which usually volatilize from the coating. The resulting polyisocyanates can react with other active hydrogen-containing compounds to form more thermally stable urethane or urea linkages.

The dissociation temperatures of the labile bond of a blocked polyisocyanate depend on the structures of the polyisocyanates and the blocking groups utilized. Blocked polyisocyanates based on aromatic polyisocyanates dissociate at lower temperatures than those based on aliphatic ones. The dissociation temperatures of blocked polyisocyanates based on commercially utilized blocking agents decrease in this order: alcohols>e- caprolactam>phenols>methyl ethyl ketoxime>active methylene compounds.

Blocked polyisocyanates undergo alcoholysis (or aminolysis) in the presence of coreactants at temperatures lower than their dissociation; the curing temperature of the formulated coating system being dependent on the type of coreactant utilized. Curing cycles are shorter when aliphatic amines are used compared to those utilizing hydroxy-functional compounds.

Although it is highly desirable to have coatings systems which cure with minimal energy input, the storage stability of the formulated coating generally decreases with lower curing temperatures. For example, alcohol blocked aromatic polyisocyanates combined with polyether coreactants are storage stable for years at room temperature; however, temperatures in excess of 200°C are necessary to effect cure of the films. In contrast, phenol blocked aromatic polyisocyanates react with aliphatic diamines at room temperature on the substrate as well as in the can. The ramifications of these observations must be taken into account. One cannot formulate a one-component, one-phase system which is infinitely stable at room temperature but still cures rapidly at slightly elevated temperatures.

Blocked polyisocyanates can be used to cross-link both solventborne and waterborne resins. The blocked polyisocyanates offer wide formulation latitude. They can be added to the coreactant resins providing one-package coatings with excellent shelf life. These combinations result in cross-linked films within reasonable curing cycles. Coatings obtained show high-performance with the unique combination of high hardness and good flexibility. Coatings based on water-dispersible blocked polyisocyanate cross-linkers and suitable waterborne polymers approach the performance levels previously obtained only by solventborne coatings.

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