Expanded PTFE (ePTFE) has revolutionized the sealing industry. By modifying traditional Polytetrafluoroethylene (PTFE) through a biaxial stretching and lamination process, ePTFE retains PTFE’s legendary chemical resistance while addressing its two biggest weaknesses: high creep (cold flow) and low tensile strength.
Today, expanded PTFE gasket materials are the gold standard for critical applications in petrochemicals, food & pharmaceuticals, and increasingly, nuclear power plant water systems (specifically seawater systems). To guarantee seal integrity and corrosion resistance in these high-stakes environments, understanding the mechanical behavior of ePTFE is important.
This article presents a comprehensive study of ePTFE sheets (1-5mm) that analyzes their tensile strength, creep behavior, and long-term sealing performance.
1. Material and Rigorous Testing Methodology
To ensure data reflects real-world industrial conditions, our testing went beyond standard protocols to simulate long-term usage.
Materials: expanded PTFE sheets in thicknesses of 1, 2, 3, 4, and 5mm.
Tensile Testing: Conducted per ASTM D638 using an MTS high-low temperature tensile machine (Rate: 500mm/min). Creep Testing: Utilized a Dynamic Mechanical Analyzer (DMA, TA-Q800).
Stress: 0.2 MPa; Duration: 4800s at 30°C; Sealing Performance: Modeled after DIN 3535/6.
Critical Adjustment: Standard tests measure leakage instantly. However, PTFE creeps. To reflect true long-term sealing, we allowed the ePTFE gaskets to creep for 1.5 hours under load before measuring leakage.
Leakage Analysis: We tested gaskets both “as-is” and “coated with sealant” to distinguish between body permeation (through the material) and interface leakage (between gasket and flange).
2. Tensile Performance: The “3mm Sweet Spot.”
Our research reveals that expanded PTFE exhibits anisotropy—its strength differs in the transverse and longitudinal directions. Unlike solid materials that snap, ePTFE undergoes strain hardening. As stress increases, the multi-layered membrane structure prevents instant catastrophic failure.
The Thickness Effect:
Surprisingly, thicker is not always stronger. Peak Performance at 3mm: The Maximum Tensile Strength increases with thickness up to 3mm. The Decline: Beyond 3mm (4mm and 5mm), tensile strength begins to decrease.
Why? This is due to the multi-layer lamination structure. Up to 3mm, the interaction force between layers reinforces the material. However, as layers multiply beyond this point, weak interface defects accumulate. Without effective force transmission across these numerous interfaces, the structural integrity dips slightly.
3. Creep Behavior & Microstructure Analysis
The unique “Node and Fibril” structure of expanded PTFE is the secret to its compressibility. Under load, the fibers stretch and lock, providing a “locking function” that prevents unlimited deformation.
To quantify this, we utilized the KWW (Kohlrausch-Williams-Watts) Equation to model Creep Compliance ($J(t)$). The data showed a near-perfect fit, confirming KWW is ideal for characterizing ePTFE viscoelasticity.
The Role of Relaxation Time ($\tau$)
The parameter $\tau$ (Tau) represents the time required for the material’s molecular chains to relax toward an equilibrium state.
High $\tau$ (Longer Relaxation): Found prominently in 3mm sheets. This indicates restricted chain movement due to strong interlayer forces.
Low $\tau$ (Shorter Relaxation): Found in very thin or very thick sheets (>3mm). In thicker sheets, interface defects allow faster relaxation.
Key Takeaway: The microstructure provides expanded PTFE with a high free volume for compression, yet the fibrous locking mechanism ensures superior creep resistance compared to virgin PTFE.
4. Sealing Performance: Interface vs. Permeation
Understanding how a gasket leaks is just as important as knowing if it leaks. Our study yielded two critical insights for engineers designing flange systems.
A. The 90/10 Rule
By comparing coated vs. uncoated gaskets, we discovered:
Interface Leakage: Accounts for ~90% of total leakage; Body Permeation: Accounts for only ~10%.
Implication: The unique microporous network of expanded PTFE effectively blocks permeation. The primary challenge is always the seal at the flange face.
B. The 40MPa Threshold
Leakage rates ($L_v$) follow a negative exponential relationship with gasket stress.
< 30 MPa: Leakage is higher and depends heavily on thickness (thicker gaskets = more potential leak paths).
>40 MPa (The Sweet Spot): Once the initial seating stress exceeds 40MPa, the ePTFE densifies sufficiently. Leakage channels are blocked, surface defects are filled, and performance stabilizes across all thicknesses.
5. The Correlation: Creep Properties & Leakage
Is a gasket with high relaxation time ($\tau$) better? Not necessarily for sealing stability.
Our analysis of Leakage Rate ($L_v$) versus Relaxation Time ($\tau$) reveals a direct correlation:
Smaller $\tau$ = Lower Leakage.
Larger $\tau$ = Higher Leakage.
The Science: A smaller $\tau$ means the material reaches a stable state quickly. It adapts to the load and maintains a steady clamping force. A larger $\tau$ implies the material stays in a state of “creeping” for a long time, leading to a continuous, slow loss of bolt load (relaxation). This instability in clamping force leads to higher interface leakage. Therefore, Characteristic Relaxation Time ($\tau$) is a vital parameter for evaluating expanded PTFE gasket performance.
Conclusion: Summary for Engineers
Thickness Matters: Tensile strength and creep resistance are heavily influenced by the number of laminated layers. 3mm represents a structural inflection point where strength peaks before interlayer defects begin to reduce mechanical properties.
Predictable Behavior: The KWW equation accurately predicts ePTFE creep. The “Lock-and-Key” fibril structure gives ePTFE its unique balance of high compressibility and low cold flow.
Sealing Strategy: ePTFE leakage is primarily interfacial. To ensure a tight seal, design calculations should aim for an initial gasket stress of 40 MPa or higher. Furthermore, materials with a shorter characteristic relaxation time ($\tau$) offer better long-term sealing stability.
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