Polyurethane, sometimes referred to just as ‘urethane’ is one of the most widely used materials for seals and gaskets. It’s inexpensive and has characteristics that make it very useful. Just be careful not to use urethane gasketing foam when you need a polyurethane seal! A polyurethane foam gasket is different from a polyurethan seal.
Polyurethane is formed by reacting isocyanate with polyol. Isocyanate is a simple compound of nitrogen oxygen and carbon while polyols are formed from oxygen, hydrogen and carbon. Put them together and you get materials that make excellent seals and gaskets.
Polyurethane seals work well from around -65°F up to 210°F. Below this minimum they become brittle while at higher temperatures they melt. Polyurethane is tough with good impact resistance and load bearing capacity. It’s also resistant to attack by petroleum and hydraulic oils.
Polyurethane is produced in a range of hardnesses, although the bulk of the material used for seals is around 95 Shore A. It’s tough, stronger than natural rubber, and abrasion resistant.
Polyurethane becomes a lot softer when foamed. This entails adding a foaming agent to the isocyanate/polyol reaction to create a gas. As the liquid solidifies this leaves bubbles dispersed throughout the material, which gives it a springy resilience.
Applications for polyurethane foam
Cushioning is probably the biggest application. Polyurethane foam is used in furniture, shoes and anywhere else that compliance and recovery are sought after. Gasketing is another common use. Here it’s compressibility cushions against impacts and takes up manufacturing tolerances.
One time not to use polyurethane foam is when sealing is needed. Most polyurethane foams have an open cell structure, so fluids can pass through.
Polyurethane seal applications
With it’s excellent abrasion resistance, polyurethane is often used in situations where there’s relative movement. Seals around rods in hydraulic cylinders is one example, where the oil resistance also helps ensure long life. Pipe seals are another common application, as are wipers and shaft seals.
Buy the right polyurethane!
Polyurethane, or just plain urethane, is a good choice for many seal and gasket applications. Just remember that in it’s foamed state it may not provide much of a seal!