What Are Sealing Materials?

Sealing materials are substances used to prevent the passage of fluids, gases, or contaminants across the interface between two mechanical surfaces. They function by filling microscopic surface irregularities under applied contact pressure, establishing a barrier that resists leakage across a pressure differential or environmental boundary. The selection of a sealing material governs whether a joint, fitting, or rotating shaft assembly will maintain containment across a design lifetime under specified conditions of temperature, pressure, chemical exposure, and mechanical motion.

The field draws from polymer science, tribology, and mechanical engineering. Most sealing materials are elastomers or polymers, though metal and ceramic seals are used at temperature and pressure extremes beyond the range of organic materials. The difference between static and dynamic applications is fundamental: static seals join surfaces that do not move relative to each other, while dynamic seals must accommodate rotary or reciprocating motion while maintaining a functional barrier.

Material Types and Composition

The most widely used sealing materials are elastomers, polymeric networks with the capacity for large reversible deformation. Common formulations include nitrile butadiene rubber (NBR), valued for resistance to petroleum-based oils; ethylene propylene diene monomer (EPDM), suited for steam, hot water, and outdoor weathering; fluoroelastomers such as Viton, used where resistance to fuels, acids, and high temperatures is required; and silicone rubber (VMQ), which retains flexibility across a wide temperature range from below minus 60 degrees Celsius to over 200 degrees Celsius. Research on elastomer O-ring material compatibility documents how chemical swelling and hardness change under immersion in specific fluids, which governs seal suitability for a given service environment. PTFE (polytetrafluoroethylene) and other fluoropolymers are used as static gaskets and backup rings where chemical inertness is essential and creep resistance is acceptable. NIST's historical research on elastomeric seals at cryogenic temperatures identified the vulnerability of standard elastomers to glass-transition embrittlement, driving the development of formulations specifically for low-temperature service.

Performance and Failure Mechanisms

The load-carrying and sealing performance of elastomeric seals depends on contact stress distribution across the interface. O-rings in most industrial applications use compounds with a Shore A hardness of 70 to 80, providing enough compliance to conform to mating surface roughness while resisting extrusion into the gap between components at operating pressure. Higher hardness grades, around 90 Shore A, offer better extrusion resistance in high-pressure dynamic applications but reduce initial conformability, increasing the risk of leakage at low temperatures or on rough surfaces. Failure modes include compression set, where the material loses its ability to recover after prolonged compression; chemical degradation from solvent absorption or oxidative attack; thermal aging; and abrasive wear in dynamic applications. Studies of O-ring performance under high-pressure hydrogen environments have documented rapid gas decompression damage, in which dissolved hydrogen expands within the elastomer matrix when pressure is released quickly, causing internal blistering and cracking, a failure mode of particular concern in hydrogen fuel systems.

Applications

Sealing materials have applications in a wide range of disciplines, including:

• Hydraulic and pneumatic systems in industrial machinery
• Aerospace fuel, hydraulic, and environmental control systems
• Automotive engines, transmissions, and braking systems
• Nuclear power plant coolant systems and pressure boundaries
• Food and pharmaceutical processing equipment requiring clean-in-place compatibility