Why Does PTFE Deform Under Load Understanding Cold Flow and How to Mitigate It

The Question

Engineers frequently ask: We selected PTFE for its outstanding chemical resistance and low friction, but our gaskets and seals are deforming over time under load. Is this normal? Can it be prevented?

The short answer: yes, it is normal and it has a name. It is called cold flow, also known as creep, and it is the single most cited limitation of PTFE in structural and sealing applications.

What Is Cold Flow?

Cold flow is the permanent deformation of a material under sustained mechanical stress at temperatures below its melting point. PTFE is notoriously susceptible because of its molecular structure. The carbon-fluorine backbone forms a smooth, low-energy surface, and the polymer chains slide past one another with minimal resistance. This gives PTFE its legendary non-stick and low-friction properties but it also means the material cannot hold its shape under prolonged compressive or tensile load.

Unlike metals that yield plastically only above a threshold stress, PTFE creeps at any stress level. The strain increases logarithmically with time, following a characteristic creep curve: an initial rapid deformation, a steady-state secondary creep, and eventually a tertiary acceleration before failure.

Key Factors That Influence PTFE Creep

• Temperature: Creep rate increases dramatically with temperature. At 23C, PTFE under 7 MPa may deform 5-8% in 24 hours. At 100C, the same load can cause deformation exceeding 20%.
• Load magnitude: Higher stress equals faster and larger deformation. Even modest loads (2-3 MPa) produce measurable creep over weeks.
• Crystallinity: PTFE with higher crystallinity (typically above 65%) exhibits better creep resistance. Fine-powder PTFE processed with slow cooling tends to have higher crystallinity than granular PTFE.
• Filler additions: Adding fillers such as glass fiber (15-25%), carbon graphite, bronze, or MoS2 dramatically reduces creep while preserving most of PTFE chemical and thermal advantages.
• Part geometry: Thin-walled components and wide, flat gaskets deform more than thick, confined designs. Confinement such as a gasket in a groove restricts lateral flow and reduces net deformation.

Practical Mitigation Strategies

1. Use Filled PTFE Compounds

Glass-filled PTFE (15-25% glass fiber) reduces creep by 50-70% compared to unfilled PTFE. Carbon-filled and bronze-filled grades offer similar improvements with added benefits in wear resistance and thermal conductivity. For chemical sealing, glass-filled PTFE is often the best compromise between creep resistance and chemical inertness.

2. Design for Confinement

Always seat PTFE gaskets in properly dimensioned grooves. A confined gasket cannot flow laterally, which limits total deformation. Follow ASME B16.20 or DIN groove standards, and avoid overly wide, unconfined flange faces.

3. Re-torque After Installation

PTFE gaskets lose bolt load rapidly in the first 24-48 hours due to initial creep. A scheduled re-torque after 24 hours recovers much of this lost load and significantly extends seal life. Document this as a mandatory step in maintenance procedures.

4. Consider Alternative Materials When Creep Is Critical

For applications where dimensional stability under load is non-negotiable, consider PEEK (creep resistance 10x better than PTFE), PCTFE (low-temperature, low-creep fluoropolymer), or expanded PTFE (ePTFE) gasket tape, which has a microporous structure that resists cold flow differently than solid PTFE. PEEK is often the upgrade path when PTFE creep causes recurring failures, though it sacrifices some chemical resistance.

5. Reduce Operating Temperature Where Possible

Since creep rate is strongly temperature-dependent, even a 10-15C reduction in operating temperature can halve the creep rate. Insulation, heat sinks, or process temperature optimization may yield disproportionate improvements in seal life.

Quick Reference Table

Bottom Line

Cold flow is not a defect it is an intrinsic property of PTFE. The engineer job is not to eliminate it but to manage it through material selection (filled grades), design (confinement), and procedure (re-torque). When creep remains unmanageable despite these measures, it is time to graduate to PEEK or PCTFE. Understanding creep behavior upfront prevents costly seal failures and redesign cycles downstream.