PTFE Gasket Compression Creep Testing: Standards, Methods and Performance Evaluation

PTFE gasket compression creep testing measures the compressibility, load retention and long-term dimensional stability of PTFE sealing materials under sustained compressive stress and elevated temperature. Based on established methods such as ASTM F36, ASTM F38 and EN 13555, the results are decisive for preventing bolt load loss and leakage in bolted flanged joints. This article explains the tested sample types, measured parameters, test procedures, equipment and typical acceptance criteria in detail.

Technical Overview

PTFE (polytetrafluoroethylene) is one of the most widely used static sealing materials in industry, thanks to its nearly universal chemical resistance, broad service temperature range of approximately −200 °C to +260 °C, low friction and non-aging surface. Its principal mechanical limitation, however, is a pronounced tendency to cold flow: under a sustained compressive load, PTFE continues to deform over time, a phenomenon known as creep.

In a bolted flanged joint, gasket creep directly reduces the residual bolt load and the contact stress acting on the gasket. Once the contact stress falls below the minimum value required to maintain tightness, leakage and fugitive emissions occur, particularly at elevated temperature. Compression creep testing therefore has a twofold objective:

• To quantify the short-term compressibility, meaning the deflection under the assembly load, and the elastic recovery of the gasket material.
• To quantify the long-term load loss, known as creep relaxation, and the irreversible thickness reduction, known as creep deformation, under sustained stress at operating temperature.

The resulting data are used to compare and qualify gasket grades, to define assembly bolt stresses and permissible operating envelopes in pT diagrams, and to provide the gasket input parameters required for flange calculations according to EN 1591-1 in combination with the characteristics determined by EN 13555.

Testing Samples

Compression creep behavior is strongly dependent on the PTFE grade and construction. Testing programs therefore cover all material variants supplied by a manufacturer, using specimens cut from production sheets or from finished gaskets.

Virgin PTFE Gaskets

Unfilled, sintered PTFE offers the highest chemical purity but exhibits the highest creep of all grades. Virgin PTFE samples serve as the reference baseline and represent the most demanding case for load retention testing.

Filled PTFE Gaskets

Compounds containing glass fiber, typically 15 to 25 percent, carbon, graphite, bronze, barium sulfate or silica fillers show substantially improved creep resistance and compressive strength. Each compound formulation must be tested individually, because filler type and content change the relaxation curve significantly.

Expanded PTFE Gaskets

Uniaxially or biaxially expanded PTFE tape and sheet materials are highly conformable and seal at low assembly stresses. Their anisotropic, porous microstructure requires dedicated compression and relaxation testing, including careful evaluation of thickness recovery after unloading.

Modified and High-Performance PTFE

Copolymers such as modified PTFE containing perfluoromethyl vinyl ether combine the chemical resistance of standard PTFE with markedly lower creep, improved permeation resistance and weldability, and are tested at correspondingly higher qualification temperatures.

PTFE Envelope and Composite Gaskets

For envelope gaskets consisting of a PTFE jacket over corrugated metal, graphite or elastomer inserts, creep testing is performed on the complete composite in order to capture the mechanical interaction between the envelope and the insert.

Specimen Geometry and Conditioning

• Ring or annular specimens are used for creep relaxation testing according to ASTM F38, while square or strip specimens are used for compressibility and recovery according to ASTM F36.
• Specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 percent relative humidity, and the initial thickness is recorded under a defined minor seating load.
• Surface finish, thickness tolerance and flatness are documented, because they directly influence the measured compression values.

Testing Items

A complete compression creep evaluation of a PTFE gasket comprises the following measured parameters:

• Compressibility: the percentage thickness reduction under the specified major compressive stress, indicating how well the gasket conforms to flange surface irregularities.
• Recovery: the percentage of the compressed thickness regained after load removal, indicating the elastic springback of the material.
• Creep relaxation: the percentage loss of the initial bolt load or gasket stress over time, with the gasket held between rigid platens; this is the key long-term parameter.
• Creep deformation: the irreversible thickness reduction that accumulates over time under sustained stress at temperature.
• Creep relaxation factor: the ratio of residual to initial gasket stress as defined by EN 13555, used directly in EN 1591-1 flange design calculations.
• Compression strength and crush resistance: the maximum stress the gasket withstands before yielding or extrusion.
• Temperature dependence: repetition of the tests at 23 °C and at elevated temperatures, commonly 100 °C, 150 °C and up to 200 °C or higher, in order to establish the pressure-temperature capability of the material.
• Tightness verification: optional leak rate measurement for the tightness classes referenced by VDI 2440 and TA Luft, performed in parallel with relaxation testing.

Testing Methods

Specimen Preparation and Conditioning

• Cut specimens from representative production material, verify the dimensions and record the free thickness.
• Condition the specimens at 23 ± 2 °C and 50 ± 5 percent relative humidity before measurement.
• Measure the initial thickness with a calibrated micrometer or displacement transducer under a minor seating load.

Compressibility and Recovery, ASTM F36

The specimen is placed between parallel platens and loaded to the specified major compressive stress. The thickness is measured under a minor seating load before loading, under the major load, and again under the minor load after unloading. From these three values, compressibility and recovery are calculated as percentages. The test is short term and characterizes the conformity behavior relevant to sealing against flange surface imperfections.

Creep Relaxation, ASTM F38

The ring specimen is assembled between two platens and loaded through calibrated, strain-gauge instrumented bolts to the specified initial gasket stress, which is approximately 34.5 MPa or 5000 psi in the standard method. Because the joint length is fixed, any creep of the gasket produces a measurable decay of the bolt load. The residual load is recorded at defined time intervals, commonly up to 24 hours, and creep relaxation is reported as the percentage loss of the initial stress. For qualification programs, extended durations of 100 to 1000 hours may be applied.

Elevated Temperature Testing

The complete test assembly is placed in a temperature-controlled oven so that relaxation and deformation are measured at the intended service temperature. Elevated-temperature results typically dominate material selection, because PTFE creep accelerates significantly above approximately 150 °C. Tests are frequently run at several temperatures in order to generate the relaxation versus temperature curve used for pT diagrams.

European Approach, EN 13555 and DIN 28090

EN 13555 defines the gasket parameters required for flange calculations according to EN 1591-1, including the creep relaxation test at operating temperature from which the relaxation factor is derived, the minimum gasket stress for defined tightness classes, and the maximum permissible pressure-temperature combination. The predecessor standard DIN 28090 followed the same principle of separating tightness testing from relaxation testing. Certified values obtained under these standards are required for gaskets used in fugitive emission control applications governed by TA Luft and VDI 2440.

Equipment Used

• Creep relaxation test rigs with calibrated instrumented bolts and proving rings in accordance with ASTM F38.
• Universal testing machines or dedicated compression platens with parallelism control for ASTM F36 testing.
• Forced-convection laboratory ovens and temperature chambers with verified temperature uniformity.
• Precision displacement measurement using LVDT transducers, dial gauges and calibrated micrometers.
• Data acquisition systems for continuous logging of load and deformation as a function of time.
• Optional helium mass spectrometer leak detection for tightness classification.

Calculation and Reporting

Creep relaxation in percent is calculated as the initial gasket stress minus the residual stress at time t, divided by the initial stress and multiplied by 100. Compressibility and recovery are calculated analogously from the three thickness measurements taken during the F36 procedure. The test report includes the material grade and lot, specimen dimensions, initial stress, temperature, duration, load-time curve, calculated relaxation and creep values, and any deviation from the standard conditions.

Typical Results and Acceptance Values

As indicative ranges, high-performance filled or modified PTFE grades typically exhibit ASTM F38 creep relaxation of roughly 8 to 15 percent at room temperature and moderately higher values at elevated temperature, while virgin PTFE may exceed 20 to 25 percent, especially above 150 °C. Expanded PTFE combines high compressibility with good recovery but must be qualified for load retention separately. Specifiers should always rely on manufacturer-certified values for the specific grade and thickness.

Applications

Compression creep test data for PTFE gaskets underpin material selection and joint design across a wide range of industries:

• Chemical and petrochemical processing, where aggressive acids, alkalis and solvents can be sealed reliably only with fluoropolymers.
• Oil and gas, including refineries and hydrocarbon service subject to fugitive emission monitoring.
• Food, beverage and pharmaceutical manufacturing, where FDA-compliant, non-contaminating seals are required in sterile and clean-in-place service.
• Semiconductor fabrication and ultra-pure chemical distribution using high-purity, low-outgassing grades.
• Pulp and paper, chlorine and bleach production plants.
• Power generation and water treatment systems.
• Regulatory applications requiring gasket qualification under TA Luft, VDI 2440 and VDI 2200, as well as BAM approvals for oxygen service.

Whenever bolted joints operate at elevated temperature, under pressure cycling or in emission-monitored service, certified creep relaxation data form the basis for assembly stress selection, re-torque scheduling and maintenance planning.

FAQ

What is the difference between compression, creep and relaxation?

Compression is the immediate thickness reduction under the assembly load. Creep is the additional, time-dependent deformation that occurs under sustained stress. Relaxation is the associated loss of bolt load in a rigid joint: as the gasket creeps, the joint cannot follow the deformation, so the stress falls. ASTM F36 addresses the first behavior, while ASTM F38 and EN 13555 address creep and relaxation.

Why does PTFE creep more than other gasket materials?

PTFE molecular chains are held together only by weak van der Waals forces, so the chains slip past one another under sustained load. Without reinforcing fillers or copolymer modification, the material cold-flows, and the effect intensifies strongly with temperature.

Which standards govern the testing?

ASTM F36 for compressibility and recovery, ASTM F38 for creep relaxation, and EN 13555 for the gasket parameters used in EN 1591-1 flange calculations, with DIN 28090 as the historical predecessor. Tightness requirements for emission control reference VDI 2440 and TA Luft.

What load and temperature are used in the test?

ASTM F38 applies an initial gasket stress of approximately 34.5 MPa, equivalent to 5000 psi. Tests are normally performed at 23 °C and repeated at the intended service temperature, commonly between 100 °C and 200 °C depending on the grade.

How long does a creep test take?

Standard screening is typically 24 hours per temperature level. Qualification and certification programs may extend the duration to 100 or 1000 hours in order to demonstrate long-term load retention.

How can creep-related leakage of PTFE gaskets be reduced?

• Select filled or modified PTFE grades with certified low relaxation values.
• Use thinner gaskets where the flange design allows, because creep loss increases with gasket thickness.
• Assemble to the bolt stress recommended by the gasket manufacturer and consider a re-torque after initial thermal cycling.
• Apply live loading with disc springs on critical joints to compensate for load decay.

Do PTFE envelope gaskets need separate creep testing?

Yes. The PTFE envelope and the insert interact mechanically, so relaxation must be evaluated on the complete composite rather than on the PTFE skin alone.