Polyurethanes are formed by the chemical reaction between a polyol and an isocyanate. Properties of 100 percent solid polyurethanes vary from very soft polymers to a very hard cast material. The two major types of polyurethanes are aliphatic and aromatic. Aliphatic polyurethanes are more resistant to ultraviolet (UV) radiation attack and are therefore 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 durable and have better chemical resistance in immersion than aliphatic types, but will chalk rapidly in sunlight.
Polyurethane coatings are available with a variety of curing times, from less than one minute to several hours. Slow curing coatings are typically applied with conventional airless spray equipment or by brush or roller. Conventional airless spray systems are typically used for the coating of large surface areas, while brush or roller application is normally used for touch up of small areas. Plural component spray systems are typically used to apply very rapid cure coating systems. This is typical for coatings with pot lives of less than five minutes. The advantage of plural component spray equipment is that the coating components are not mixed until they combine at the spray gun.
Aliphatic polyurethanes are extensively used on aircraft and Marine Corps vehicles for protection from UV degradation. Polyurethane is authorized by the Navy as a topcoat on submarine hulls (MIL-PRF-24631).
The main hazard associated with the use of polyurethanes is the toxicity of the isocyanate component. During the mixing, application, and curing of a polyurethane, isocyanate vapors can be released. Exposure to isocyanates can cause irritation to the skin, eyes, and respiratory system. Furthermore, exposure may result in irreversible sensitization problems that may trigger severe allergic reactions and produce asthma-like symptoms. Per Naval Ships Technical Manual: Chapter 631 (NSTM 631), Preservation of Ships in Service – General, application of polyurethane coatings is depot level maintenance work. Other than a submarine hull topcoat option specified in NAVSEA Standard Item 009-32, polyurethanes are not recommended for Navy use, as they can’t be maintained by ship’s force due to the toxicity of the isocyanates.