Polyurethanes

Polyurethanes are prepared 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 aromatic and aliphatic. Aliphatic polyurethanes are more resistant to UV 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 tough and have better chemical resistance in immersion than aliphatic types, but will chalk rapidly in sunlight. The main hazard associated with the use of polyurethanes chemistries is the toxicity of the isocyanate component. Prolonged exposure to isocyanates can cause irreversible sensitization problems. Users must follow safety precautions identified in Material Safety Data Sheets and by their local safety officer.

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. Brush or roller application is normally used for touch up of small areas. Conventional airless spray systems are typically used for the coating of large surface 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 kept apart until they meet at the spray gun.