Introduction
PTFE (polytetrafluoroethylene), also known as Teflon®, is a thermoplastic polymer known for its excellent resistance to chemicals and heat. It is commonly used in various applications such as cookware, electrical insulation, medical and laboratory equipment, lubricants, seals, and gasket materials. Additionally, fillers can be incorporated into PTFE to modify its properties. For example, glass fillers are often added to enhance its thermal and mechanical characteristics. Therefore, understanding the thermal behavior of both unfilled and filled PTFE throughout its range of operating temperatures is essential.
Experimental
Thermal conductivity was determined using the TCT 716 Lambda Guarded Heat Flow Meter (GHFM). This steady-state technique involves placing a sample of known thickness between two plates maintained at different temperatures, enabling heat to flow through the sample. The heat flow through the sample's thickness is measured, and Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity is then calculated.
The GHFM method sets itself apart from other methods because it is particularly effective for traditionally challenging specimens, for example non-homogeneous, anisotropic materials, such as multi-layer and composite specimens [1]. In addition to more standard homogeneous materials, the GHFM is also able to accurately determine Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity of layered or filled materials (e.g., glass-filled polymers).
For this study, PTFE samples (table 1) were obtained from two different manufacturers, including both an unfilled and a glass-fiber filled PTFE sample from one of the manufacturers. Each test specimen had a diameter of approximately 50 mm and a thickness of 3 mm. A summary of the sample information is provided in the table below. Measurements were conducted over a temperature range of approximately -10°C to 200°C, and calibration was carried out using Vespel® SP-1. A thin layer of silicone thermal joint compound was applied between the specimens and instrument plates to minimize interfacial resistance. A pressure of approximately 175 kPa was applied to the specimens during testing.
Table 1: Test Specimens
| Sample 1 | Sample 2 | Sample 3 | |
|---|---|---|---|
| Material | Unfilled PTFE | Unfilled PTFE | Glass-fiber filled PTFE |
| Manufacturer | A | B | B |
| Sample thickness | 2.90 mm | 3.20 mm | 3.15 mm |
| Sample DensityThe mass density is defined as the ratio between mass and volume. density | 2.118 g/cm³ | 2.166 g/cm³ | 2.172 g/cm³ |
Results and Analysis
The apparent Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity versus temperature results for the tested samples are presented in figure 1. The unfilled samples from manufacturers A (blue curve) and B (orange curve) align with the expected values from literature, which is approximately 0.27 W/(m·K) at room temperature [2]. Additionally, Sample 2 has a higher DensityThe mass density is defined as the ratio between mass and volume. density than Sample 1, leading to a corresponding increase in Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity. As anticipated, the sample with glass fiber filler exhibits a significantly higher Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity. Furthermore, PTFE is known to undergo a solid-solid Phase TransitionsThe term phase transition (or phase change) is most commonly used to describe transitions between the solid, liquid and gaseous states.phase transition at room temperature [3], which is evident in the noticeable change in apparent thermal conductivity in this temperature region. (It should be noted that during this Phase TransitionsThe term phase transition (or phase change) is most commonly used to describe transitions between the solid, liquid and gaseous states.phase transition region, heat is absorbed by the material, the effects of which are not within the scope of this application note). Above this Phase TransitionsThe term phase transition (or phase change) is most commonly used to describe transitions between the solid, liquid and gaseous states.phase transition region, the effect of temperature increase on thermal conductivity is minimal [4].
Summary
The results showed that the unfilled samples from both manufacturers aligned with the expected thermal conductivity values of unfilled PTFE based on literature sources. The sample with higher DensityThe mass density is defined as the ratio between mass and volume. density exhibited higher thermal conductivity, and the glass-fiber filled sample demonstrated an increased thermal conductivity. Additionally, PTFE underwent a solid-solid Phase TransitionsThe term phase transition (or phase change) is most commonly used to describe transitions between the solid, liquid and gaseous states.phase transition at room temperature, which was evident in the change in thermal conductivity. Above this Phase TransitionsThe term phase transition (or phase change) is most commonly used to describe transitions between the solid, liquid and gaseous states.phase transition, the effect of temperature on thermal conductivity was minimal. The results of this study demonstrate that the TCT 716 Lambda is highly effective for analyzing the thermal properties of both unfilled and filled PTFE.