PTFE gaskets and cold flow —
why chemical resistance
alone does not
guarantee a seal
Under bolt load, PTFE cold flows — it deforms plastically and permanently at the contact zone, reducing its thickness and losing bolt load over time. A joint sealed with a PTFE gasket may hold at first assembly and leak later if residual seating stress falls too far, even when the joint has not been disturbed. Chemical compatibility is one part of gasket selection. Mechanical sealing behaviour is another.
What cold flow is — and why PTFE is particularly susceptible
Cold flow is a form of time-dependent plastic deformation, often discussed together with creep relaxation, where the material slowly deforms under sustained load without fracturing. Most materials creep to some degree under load, particularly at elevated temperature. PTFE can creep measurably even at ambient temperature, which is why gasket grade and joint design matter.
When a PTFE gasket is installed and the bolts are tightened, the gasket is compressed between the two flange faces. This compression produces the seating stress that seals. Simultaneously, the PTFE begins to cold flow at the compressed zone — material flows outward at the edges of the contact area, reducing the gasket thickness. As the thickness reduces, the bolt stretch reduces, and the bolt load drops. The seating stress falls.
Schematic illustration only — not design data. Actual bolt load retention depends on gasket grade, dimensions, bolt load, temperature and flange stiffness. Curves are directional.
The seating stress that remains in the joint after cold flow relaxation — the residual seating stress — must still exceed the minimum needed for the service pressure and medium. If the cold flow loss is large enough to drop the residual stress below this minimum, the joint leaks — even though the bolts have not been loosened and the gasket has not been damaged in the usual sense.
What makes PTFE cold flow worse
- Higher bolt load: higher bolt load can accelerate cold flow in PTFE when the gasket is compressed toward the upper end of its useful seating range — the relationship is not simply linear and depends on grade, geometry and temperature. Overtightening a PTFE gasket to compensate for perceived leaking can accelerate the cold flow that is causing the leak in the first place.
- Elevated temperature: PTFE cold flows more rapidly at elevated temperature. A joint that is borderline at ambient may lose seating stress faster once the system reaches operating temperature. This is why PTFE gaskets in heated service may seal initially and then develop a leak after the first few operational cycles.
- Wider gasket contact width: a wide gasket contact area concentrates less stress per unit area at a given bolt load — and provides more material available to flow laterally. For a given bolt load, a narrower effective gasket area produces higher seating stress but also concentrates the cold flow at that narrower zone.
- Thicker gasket: a thicker gasket provides more material to cold flow and therefore loses more bolt load for the same stress condition. This is the opposite of the intuitive assumption that a thicker gasket provides more sealing material and is therefore more robust.
- Flanges with limited stiffness: a flange that deflects under bolt load — particularly smaller bore, thinner flanges or non-metallic flanges — may lose bolt load more rapidly as the PTFE cold flows and the flange springs back slightly, compounding the relaxation effect.
The three PTFE grades — and how they differ in sealing behaviour
PTFE vs compressed fibre — which problem is harder to manage
PTFE — the cold flow problem
Virgin PTFE resists a very wide chemical range and has no rubber binder to degrade. The sealing problem is mechanical: it cold flows under load and loses bolt load over time. Managing this requires adequate initial bolt load to account for relaxation, potentially shorter retorque intervals, and — for critical joints — joint design that accounts for the creep behaviour of the specific grade.
A PTFE gasket on an inadequately loaded flange, or in a service where retorque is not practical, may seal initially and fail progressively as bolt load declines.
Compressed fibre — the temperature and compatibility limit
Many correctly specified compressed fibre grades with rubber binders retain bolt load better than virgin PTFE sheet under comparable moderate conditions. The limitation is thermal and chemical: the rubber binder degrades at elevated temperature and may not be compatible with specific media. Above the continuous temperature rating or in chemically aggressive service, the fibre grade degrades and loses sealing performance.
For suitable water, heating and selected industrial fluid duties within the grade’s documented operating envelope, compressed fibre grades — including FLEXSEAL PRO 350 where its technical data and documentation match the medium, temperature and documentation requirement — may offer better sustained bolt load retention than virgin PTFE within their operating envelope.
PTFE's chemical resistance and its sealing performance are independent properties. A gasket that is chemically resistant to a medium does not automatically seal reliably in a flanged joint with that medium. The sealing performance depends on bolt load, cold flow behaviour, face condition, flange stiffness and operating temperature — none of which are determined by chemical compatibility alone. For media that both PTFE and compressed fibre grades can handle within their respective temperature limits, the compressed fibre grade may provide better long-term bolt load retention. For media that require PTFE, the cold flow behaviour must be managed through joint design rather than ignored because the material "should seal."
Field check: Before specifying PTFE, ask two separate questions. First: does the medium require PTFE-level chemical resistance? Second: can this joint keep enough seating stress after PTFE relaxation? If the answer to the first question is yes but the second is no, the material is chemically correct but mechanically wrong for the joint.
When PTFE is the right choice — and what the joint needs to support it
PTFE — particularly filled or expanded grades — is appropriate where:
- The medium is chemically incompatible with rubber binders or fibre reinforcement — strong acids, alkalis, solvents or oxidising media outside the range of standard compressed fibre grades
- Purity requirements preclude materials with rubber content — pharmaceutical, food contact, or ultra-pure water applications
- The flange face has minor serviceable irregularities and conformance at lower bolt load is needed — ePTFE can be useful where the product documentation supports that duty
- Glass-lined or non-metallic flanges require low-bolt-load sealing without face damage
When PTFE is specified for these reasons, the joint design should account for cold flow:
- Initial bolt load above minimum seating stress to provide margin for cold flow relaxation
- Retorque provision — considered only where the applicable joint procedure, gasket system and service conditions specifically allow it, and where controlled reloading after early bolt load loss is appropriate
- Grade selection: filled or expanded PTFE rather than virgin sheet where cold flow resistance is important and chemical compatibility still allows it
- Thinner gasket where face condition allows — a thinner gasket cold flows less than a thicker grade of the same material at equivalent seating stress
A PTFE gasket that has cold flowed significantly should not simply be retorqued to the original torque value without reassessment. If significant cold flow has occurred — the gasket has reduced in thickness — the torque-to-load relationship for the bolt has changed. Applying the original torque to a bolt in a joint where the gasket has thinned substantially may produce a different bolt tension than at original assembly. For joints requiring careful load management, bolt elongation or load-indicating methods provide more reliable control than torque alone when retorquing after cold flow. Retorquing should only be done where the joint procedure, equipment manufacturer and site safety rules allow it. Do not retorque a live, pressurised, hot or hazardous-media joint unless the governing procedure explicitly permits it.
Frequently asked questions
What is cold flow in PTFE gaskets?
Cold flow is time-dependent plastic deformation under sustained compressive load. In PTFE gaskets it can occur even at ambient temperature, without heating. Unlike elastic deformation that recovers when load is removed, cold flow is permanent. A PTFE gasket under bolt load progressively reduces in thickness as the material flows outward at the contact zone. This reduces bolt stretch and therefore reduces bolt load. The effective seating stress on the gasket face drops over time. If the residual seating stress falls below the minimum needed for the service pressure, the joint may begin to leak even though the bolts remain at their original torque.
What is the difference between virgin PTFE, filled PTFE and expanded PTFE gaskets?
Virgin PTFE is the base polymer in its pure form — broad chemical resistance but high cold flow susceptibility. A virgin PTFE gasket under bolt load may cold flow significantly, particularly at elevated temperature. Filled PTFE incorporates reinforcing fillers — glass fibre, graphite, barium sulphate or other materials — into the PTFE matrix. The fillers reduce cold flow and improve creep resistance while broadly maintaining the chemical resistance of PTFE, though some fillers affect resistance to specific chemicals. Expanded PTFE (sometimes called ePTFE) is produced by stretching PTFE during manufacture, creating a microporous structure. This structure can conform to surface irregularities at lower bolt loads and generally shows improved cold flow behaviour compared with virgin PTFE sheet, depending on the specific product form, density and grade, while retaining good chemical resistance. The specific grade, filler system and manufacturing process determine the sealing behaviour — the material designation alone does not fully specify the performance.
What bolt load and face conditions does a PTFE gasket need to seal reliably?
A PTFE gasket requires adequate initial bolt load to seat the gasket — compress it sufficiently to conform to the flange face — and sufficient residual bolt load after cold flow relaxation to maintain the sealing contact stress under operating pressure. Because PTFE cold flows and loses bolt load over time, the initial bolt load must be higher than the minimum seating stress to account for the relaxation that follows. The flange face condition is also significant — PTFE, particularly ePTFE and softer grades, can conform to moderate face irregularities, which is one reason it is sometimes used on faces with minor serviceable irregularities or glass-lined flanges. However, this conformance requires adequate bolt load. On flanges with limited bolt load — small bolt sizes, few bolts, or low bolt material strength — achieving both adequate seating and acceptable residual stress after cold flow may be difficult. The specific bolt load and face finish requirements depend on the gasket grade, dimensions, flange standard and operating conditions.
Chemical resistance is a necessary condition for PTFE gasket selection. It is not a sufficient one.
PTFE cold flows under bolt load, reducing seating stress over time. A joint that seals at assembly may leak in service as the residual stress drops below the minimum for the service pressure. The grade matters: virgin PTFE generally cold flows most; filled PTFE less; ePTFE often performs better again, depending on the specific product form and grade. The joint design matters: bolt load, retorque provision, gasket thickness and flange stiffness all affect how much bolt load remains after the first operational period. For service conditions where compressed fibre grades are chemically and thermally suitable, they may offer better bolt-load retention than virgin PTFE under comparable moderate conditions. Where PTFE is genuinely required, its cold flow behaviour must be managed — not overlooked because the material is chemically correct.