What are the two types of polyurethane?

The two main types of polyurethane are generally classified by their base solvent, oil-based polyurethane and water-based polyurethane, which is the most common distinction for wood finishes. Oil-based polyurethane is made with petroleum solvents, has a strong odor, adds an amber warm tone to wood, is thicker, more durable per coat, and takes longer to dry, about 24 hours between coats and 30 days to fully cure. Water-based polyurethane is made with water as the carrier, has low odor, low volatile organic compounds, dries clear without yellowing, dries fast in 2 to 4 hours, but requires more coats and is slightly less durable than oil-based. Another classification is by chemical structure, thermoset polyurethane which cures irreversibly and cannot be melted again, used for finishes and rigid foams, and thermoplastic polyurethane which can be melted and reshaped, used for phone cases, wheels, and flexible parts. In foam, there are rigid polyurethane foam for insulation and flexible polyurethane foam for mattresses and cushions. For finishes, there is also a distinction between aliphatic and aromatic polyurethane, with aliphatic being UV stable and non-yellowing for exterior use. But for most homeowners, the two types they choose between are oil-based and water-based.