How it works
The chemistry, simply
Both coatings start with the same ingredient. What it reacts with decides which coating you get and how it behaves.
The shared A side
Isocyanates carry reactive groups that link with other molecules. Both polyurea and polyurethane use them, usually as the A side of a two part system or as the reactive part of a single component product.
Amines make urea
Amine terminated resins react with isocyanate very quickly to form urea links. No catalyst is needed, which is why spray polyurea gels within seconds and shrugs off moisture during cure.
Polyols make urethane
Polyols react more slowly to form urethane links, often with a catalyst to control the speed. Water can compete for the isocyanate and release gas, which is why polyurethanes are more sensitive to humidity and damp surfaces.
Aromatic or aliphatic
The structure of the isocyanate decides how a coating handles sunlight. Aromatic types are tough and common but discolor outdoors. Aliphatic types hold color and gloss and are often used as topcoats.
Why it matters on site
Chemistry explains the practical differences: fast set systems need equipment that mixes at the gun, slower ones give time to work, and every system needs the prep and safety steps on its data sheet.
General information only. Isocyanates need proper protective equipment. Follow the safety data sheet for every product.