Physical Properties and Characterization Indicators of Polymer Materials for Seals

PTFE and POM engineering plastics, together with NBR and FKM rubbers, are the most widely used polymer materials in the sealing industry. Whether discussing formulations, shop-floor production, technical communication with customers, or professional exchanges with peers, familiarity with these technical terms is essential. Below is an introduction to the most common properties and characterization indicators.

Hardness:The most fundamental characterization indicator for sealing materials, representing the material’s resistance to surface indentation and deformation. The industry adopts separate test standards: Shore D hardness for plastics and Shore A hardness for rubbers. Hardness directly determines the seal’s extrusion resistance, load-bearing capacity and assembly conformability. NBR and FKM offer a wide adjustable hardness range. Higher hardness suits high-pressure anti-extrusion applications, while lower hardness improves sealing contact and sealing completeness.

Tensile Strength and Elongation at Break: Characterize structural strength and toughness. Tensile strength indicates the maximum stress the material can withstand before tensile rupture or burst failure. Elongation at break reflects deformation tolerance, which governs crack resistance under reciprocating motion and pressure shocks. POM delivers high tensile strength yet low elongation and relatively high brittleness. PTFE features excellent toughness and high elongation, resisting brittle fracture. NBR balances strength and toughness for general dynamic sealing; FKM shows minor tensile property degradation at elevated temperatures, ensuring superior structural stability under severe working conditions.

Tear Strength: Measures the material’s ability to resist crack propagation and edge tearing. It is a critical indicator for resistance to scratching and assembly damage. Rubbers are highly sensitive to this parameter. NBR exhibits excellent tear strength at room temperature and good resistance to knocks and notch cracking. FKM maintains stable tear performance with better tear resistance under high temperature. PTFE has good toughness and is less prone to tearing failure.

Molding Shrinkage: Is the core indicator for controlling the dimensional accuracy of seals. It refers to the dimensional reduction of the finished part relative to the mold after molding and cooling, directly affecting tolerance matching and assembly precision. Sintered PTFE has the largest molding shrinkage and poses the greatest challenge for dimensional control. POM has moderate injection molding shrinkage with good forming stability. NBR and FKM have the smallest vulcanization shrinkage, delivering high dimensional accuracy for precision sealing structures.

Friction and Wear Performance:Serves as a key characterization indicator for dynamic seals, evaluated by friction coefficient and wear loss. It determines service life and running smoothness. PTFE has outstanding self-lubricity, an extremely low friction coefficient and excellent wear resistance, making it the preferred material for low-friction anti-extrusion backup rings. NBR and FKM rubbers lack self-lubricating properties and have relatively high friction coefficients. They suffer greater wear during dynamic reciprocating movement and rely on lubricating media to reduce friction loss; otherwise surface abrasion, whitening and powdering failure may occur.

Fluid Swell Resistance and Thermal Aging Performance: Determine long-term service life. Swell rate describes the volume and weight change of specimens immersed in oils or chemicals. Excessive swelling causes deformation and leakage. PTFE has nearly zero swell and the best chemical resistance. POM is not resistant to hot water and strong acids. NBR is compatible with mineral oils; FKM achieves the best comprehensive resistance to oil and chemicals. Thermal aging evaluates property degradation after high-temperature exposure. FKM stands out in high-temperature resistance. PTFE has a broad working temperature range. NBR is suitable for medium-temperature conditions, while POM tends to degrade under prolonged high temperature.

Compression Set: The percentage of permanent residual deformation relative to the original compression after the material is held under constant compression at a set temperature for a period then released. It is the core indicator for rubbers, measuring resilience after compression relief and serving as the decisive parameter for O-ring sealing performance.

Compressive Creep: The slow, irreversible plastic deformation of a material over time under constant pressure and temperature. It is the key indicator for engineering plastics (PTFE / POM). Under sustained pressure, PTFE exhibits obvious creep, impairing the long-term limiting function of backup rings. POM has better creep resistance than neat PTFE, yet its creep rises significantly at high temperatures.