In the world of sealants, caulk is a multipurpose tool. It is used in weatherization, plumbing, tiling and other projects in and around the home. Two materials used often in caulk tubes are silicone and polyurethane. Despite their common use and other similarities, there are some major differences between polyurethane caulk and silicone caulk that dictate their uses for household projects and others.
Although they are commonly identified as sealants, silicone and polyurethane have two different ways of getting the job done. Silicone works by bonding the surfaces it is applied to. When used as a caulk, the silicone binds to the window seal, for example, creating a barrier between the outside air and the inside. Silicone works best as a gasket between two surfaces or as a joint compound. Polyurethane, however, functions like a true sealant and is most often used as a barrier between two surfaces.
Silicone is best at drying quickly. Polyurethane remains wet or "tacky" for up to 24 hours after the silicon has dried, according to Dow Corning Corp. The use of silicone in a high traffic area is a mistake, however, because it does not hold up to abrasion as polyurethane does.
Material Bond Limits
Silicone is the best sealant for glass, tile and metal, while polyurethane works well on natural products such as wood. Silicone fails when applied to wood. Polyurethane succeeds with this material as well because it dries hard. The hard drying makes polyurethane a lot more difficult to apply as it stiffens when out of the tube. Silicone has a much easier application. It cures soft and is therefore softer when being applied.
Silicone is the best sealant for the colder temperatures. The cold does not affect application. Polyurethane, however, becomes even harder to apply and does not make a permanent bond when the temperatures are low. Silicone remains flexible unlike the polyurethane.