Prepolymers for crosslinking solventborne, waterborne resins
With inherent wide formulation latitude, blocked polyisocyanates can be used to crosslink solventborne and waterborne resins. When added to co-reactant resins, they provide one-component (1K) coatings without any shelf life. After baking in crosslinked films, the coatings display high performance, durable hardness and excellent flexibility.
A blocked polyisocyanate is an isocyanate reaction product that is stable at room temperature but dissociates to regenerate isocyanate functionality when heated. Temperatures between 120° and 200°C are needed to release the blocking agents, which usually volatilize from the coating. The resulting polyisocyanates can react with other active hydroxyl-containing compounds to form 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 agents utilized. Blocked polyisocyanates based on aromatic polyisocyanates dissociate at comparable temperatures to those based on aliphatic polyisocyanates.
For technical and economic reasons, blocked polyisocyanates are ideal for a heat curing coating system that requires:
- Exclusion of free isocyanates
- 1K technology (no potlife)
- High performance of polyurethane coatings quickly after cool down
Key advantages include ease of handling, broad choices of baking temperature and less sensitivity to atmospheric humidity.
Covestro supplies blocked isocyanates and prepolymers for coatings under the trade names Desmodur® BL, Desmocap® and Sumidur® BL.