Polyurethane Characteristics
In the hierarchy of industrial protective coatings, polyurethanes typically occupy the crucial role of the "topcoat." While zinc primers provide galvanic protection and epoxies provide barrier protection, polyurethanes provide aesthetic durability and environmental shielding against ultraviolet (UV) radiation.
Aliphatic vs. Aromatic Polyurethanes
Contractors and specifiers must distinguish between the two primary classes of polyurethanes, as their performance profiles differ significantly.
Aliphatic Polyurethanes
These are the standard choice for exterior architectural and industrial steel applications. Aliphatic structures are highly stable when exposed to sunlight. They offer excellent color retention, gloss retention, and chalking resistance over extended periods. If you see a brightly colored water tower or bridge, the visible layer is almost certainly an aliphatic polyurethane.
Aromatic Polyurethanes
Aromatic formulations cure faster and are often more chemical resistant, but they degrade rapidly under UV light, ambering (turning yellow) and chalking. They are typically reserved for interior applications, tank linings, or as primers where aesthetics are irrelevant.
Application Considerations
Applying polyurethanes requires strict adherence to environmental controls. The curing mechanism is highly sensitive to moisture.
- Moisture Sensitivity: The isocyanate hardener reacts preferentially with ambient moisture (humidity) over the polyol resin. If applied in high humidity, the film can micro-blister or cure with a cloudy, reduced-gloss finish (known as "amine blush" or "blooming").
- Recoat Windows: Polyurethanes must be applied within a specific timeframe over epoxy primers. If the epoxy cures beyond its "recoat window," it becomes too hard for the polyurethane to chemically cross-link, requiring mechanical abrasion (sanding/sweep blasting) before application.
- Health Hazards: As detailed in our safety section, atomized isocyanates are hazardous. Specialized respiratory protection is non-negotiable during spray application.