Testing.3 – Thermal Conductivity Testing on High Temperature Polymers –Torlon (PAI)
Torlon (polyamide-imide) is the first sample of high-temperature polymers in our high temperature polymer series, which was tested for thermal conductivity and thermal diffusivity using the Hot Disk Transient Plane Source (TPS) 2500 S instrument. This instrument has the capability of measuring absolute bulk and directional thermal conductivity and thermal diffusivity for solids, liquids, pastes and powders without the need for calibration or contacts agents. Thermal conductivities ranging from 0.001 W/m•K to 1800 W/m•K are measureable using this technique.
In addition to Torlon, other possible polymers to be tested are: fluorinated ethylene propylene, poly(etherether-ketone), polysulfone, polyphenyl sulfide, poly(phenylene-sulfone), polytetrafluoroethylene, polyamide-imide, and polyetherimide.
Thermal property measurements were performed using the convenient Hot Disk TPS single-sided sensor. The TPS single-sided sensor is ideal when only one sample piece is available or when testing large hard to handle samples.
Transient PlaneSource Bulk Measurements:
Experimental set up is extremely easy, and achieved by placing the material sample on top of the sensor. A weight is added on top of the sample to ensure good sensor to sample contact. This unique sensor offers excellent accuracy and repeatability without the requirement of two pieces of sample. The TPS single-sided sensor may also be inverted for larger samples.
Parameters of test time for Torlon: 20 seconds @ 0.1 W were selected to ensure representative 3-dimensional heat penetration, without testing beyond any of the dimension boundaries of the sample. This is easily determined as the Hot Disk TPS measures the thermal diffusivity (mm2/s), thus the penetration depth is calculated and displayed. It is important to make sure the penetration depth from measurement is less the available probing depth, this will result in a clean signal free of any reflections.
Once measurement is complete, the contact resistance between sensor and sample may be removed by simply selecting and removing the portion of time, which represents the non-linear portion of the temperature vs time curve. This is easily accomplished with the easy to use Hot Disk TPS 7.2.1 Software.
After calculations, the testing results of thermal conductivity (W/m•K) and thermal diffusivity (mm2/s) for the Torlon sample are displayed (See Figure 2).
Figure 2. Bulk thermal conductivity and thermal diffusivity for Torlon PAI
|Bulk Thermal Conductivity
|Bulk Thermal Diffusivity
( for λ*)
When compared to our reference value obtained by ASTM C177  (0.540 W/m•K), the measured bulk value was found to be quite high. An important consideration when comparing bulk to through-thickness values, like measured values by ASTM C177 , is the possibility that the materials may be anisotropic. It was then decided the Torlon sample, should be investigated for anisotropy.
The anisotropic testing module allows you to measure directional thermal transport properties of anisotropic materials such as layered materials, and fiber-reinforced plastics. When using the Hot Disk transient plane source (TPS) system to evaluate materials with direction dependent thermal properties, the following assumptions must be made:
- The main directions in the material to be investigated are orthogonal and can be described in terms of in-plane, out-of-plane, and through-plane (x-, y-, and z-axes).
When performing measurements using the anisotropic module, it was particularly important to orient the sample and the TPS sensor correctly according to the main directions in the material. This is to say that the plane of the sensor containing the heat source (double Nickel spiral) would be placed so that it would be perpendicular to the normal direction (x-axis) and parallel with the planar direction (the plane formed by the y- and z-axes). Experimental set-up using this module was identical to the isotropic set-up.
Figure 3. Directional thermal conductivity and thermal diffusivity for Torlon PAI
|Through-Thickness Thermal Conductivity
As the through-thickness measurement of thermal conductivity and thermal diffusivity from the TPS anisotropic measurements showed good correlation to the through-thickness ASTM C177  result (0.540 W/m•K), it was confirmed the Torlon sample was anisotropic
1 Percent difference between reference value and experimental anisotropic value
2 ASTM C177 standard test method for steady-state heat flux measurements and thermal transmission properties by means of the guarded-hot-plate apparatus
* where λ is thermal conductivity