Accurate thermal conductivity determination of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE), widely known under the trade name Teflon®, is a thermoplastic polymer with exceptional chemical and thermal resistance. Common applications range from cookware and electrical insulation to medical equipment, seals, and gaskets. When modified with fillers like glass fibers, PTFE’s properties can be tailored to meet even more demanding applications.

GHFM schematic (Figure: NETZSCH)

Understanding the thermal behavior of both unfilled and filled PTFE across the operating temperature range is critical for proper material selection and design. Precise determination of thermal conductivity is possible using the NETZSCH Guarded Heat Flow Meter (GHFM) TCT 716 Lambda.

Measurement Method
The GHFM is a steady-state technique where a sample of known thickness is placed between two plates held at different temperatures. The heat flow through the specimen is measured, and the thermal conductivity is calculated.

This method is especially suited for non-homogeneous or anisotropic materials, such as composites or layered structures — which often present challenges for other methods. In this study, it was used to test three PTFE samples:
• Two unfilled PTFE samples from different manufacturers
• One glass-fiber filled PTFE sample

All specimens were disks with a diameter of ~50 mm and a thickness of ~3 mm. The temperature range of the measurements expanded from approximately -10°C to 200°C. Calibration was performed using Vespel® SP-1, and a silicone-based thermal joint compound was applied to minimize interfacial resistance. A contact pressure of approximately 175 kPa was applied during testing.

Results and Observations
The thermal conductivity was plotted against temperature for all three specimens:
• The unfilled PTFE samples (Sample 1 and 2) showed results that matched literature values (~0.27 W/(m•K) at room temperature). The sample with higher density exhibited slightly higher thermal conductivity.
• The glass-fiber filled PTFE sample showed a clearly increased thermal conductivity, as expected.
• A solid–solid phase transition in PTFE near room temperature was detected — seen in the noticeable change in thermal conductivity. Above this region, temperature had little effect on conductivity.

Conclusion
The NETZSCH TCT 716 Lambda proved highly effective for characterizing the thermal conductivity of both filled and unfilled PTFE. The instrument’s ability to analyze difficult samples (such as fiber-filled polymers) makes it especially valuable for R&D and quality control applications in the polymer industry.

By Aileen Sammler