Polyurethane coatings are one or two-package systems. For two-package systems, one component is an isocyanate and the other a polyol component. Because of the reactivity of the isocyanate, polyurethanes are moisture sensitive, and the gloss may drop when the wet film is exposed to high humidity. One component types cure with moisture supplied from the atmosphere. Polyurethanes are chemical reaction coatings and available in a variety of formulations; giving rise to a variety of properties from very soft polymers to very hard cast materials.
There are two major types of polyurethane coatings: aromatic and aliphatic. Aliphatic polyurethanes are more resistant to UV attack and are typically used in exterior coating formulations. Aliphatic isocyanates, when used in the formulation of polyurethanes, provide coatings with excellent gloss and color retention. Aromatic polyurethanes are extremely tough and have better chemical resistance in immersion than aliphatic types, but chalks rapidly in sunlight. They are commonly used in thin film 1.5–2 mils (75– 100 microns) finish coats for exterior exposure in many industrial, marine, and commercial applications. Both types can formulated to be low in VOC’s.
The main hazard associated with the use of polyurethane chemistry is the toxicity of the isocyanate component. Prolonged exposure to isocyanates can cause irreversible sensitization problems. Users must employ personal protection equipment including respirators and follow safety precautions identified in the MSDS.
Polyurethanes are convertible coatings available with a variety of curing times, from less than one minute to several hours. Apply slow curing coatings with conventional or airless spray equipment or by brush or roller. Use brush or roller application for small area touch ups. Use conventional airless spray systems to coat large surface areas. It is typical to use plural-component spray systems to apply very rapid cure coating systems, particularly for coatings with pot lives of fewer than five minutes.
Advantages of polyurethane coatings include:
- Low in VOCs
- Good solvent resistance
- Good hardness or flexibility
- May have excellent gloss
- Good durability
- Good abrasion resistance
Areas of concern for polyurethane coatings include:
- Highly toxic; need personal protection equipment
- Moisture sensitive; gloss may drop
- Skilled applicator needed
- Limited pot life
- Blasted surface required
- High cost
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