Exploring thermal conductivity testing applications, including thermal mapping of polymer composites, powder thermal conductivity, and thermal effusivity testing of textile products. 

Introduction

Thermal conductivity is a key property for materials across a wide range of applications- including those in your every-day life. A skillet must have high thermal conductivity to ensure food is cooked in a reasonable timeframe. Oven mitts must have a low thermal conductivity to ensure the hands inside do not get burnt. What a “good” thermal conductivity is will depend on the thermal conductivity application in question. Thermal conductivity is defined as the rate at which heat transfers through a material with a given temperature gradient. Thermal conductivity is often represented by k or λ, and has units of W/mK.

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Figure 1: Thermal Conductivity Concept Drawing

This post will highlight the use of the Modified Transient Plane Source technique in three thermal conductivity testing applications: thermal mapping in polymer composites, thermal conductivity testing of powders, and thermal comfort characterization in textiles. 

Modified Transient Plane Source

The Modified Transient Plane Source (MTPS) method is the most versatile method for measuring thermal conductivity. C-Therm’s single-sided sensor with patented guard ring technology can accommodate solids, liquids, powders, and pastes. Applicable for almost every field of study, the MTPS sensor has a wide measurement range of 0-500W/mK, and a temperature range of -50 to 500°C. Sample preparation for this test method is easy- simply ensure your sample has a flat edge, at least 18mm in diameter, to make adequate contact with the sensor (Figure 2).[1]

Figure 2: Loading a Sample onto an MTPS Sensor

Thermal Conductivity of Polymer Composites

Polymer composites are created to maximize the desirable properties in a sample. However, there are cases when the additive is not evenly distributed in the base polymer, which can lead to variable performance. The MTPS sensor is a valuable tool for quality control in the field of polymer composites. In Figure 3 below, the concentration gradient is easily visible (filler has settled to the bottom, causing a high concentration on the bottom and a low concentration on the top).

Figure 3: Filled Polymer with a Visible Concentration Gradient. 

Testing the thermal conductivity on both sides of this sample revealed the clear difference in filler concentration (Figure 4). In many cases, a concentration gradient will not be visible to the human eye, resulting in the need to test both sides of the sample with the MTPS sensor to quantify the filler/additive distribution.

Figure 4: Thermal Conductivity Results for Filled Polymer (Figure 3)

For more information on thermal mapping with MTPS, refer to this blog post on thermal mapping and additive settling detection in polymer composites.

Thermal Conductivity of Powders

The thermal conductivity of powders can be tested using the MTPS sensor with the addition of the liquids and powders accessory (Figure 5). If the end use of a sample is in powder form, it is important to understand the thermal properties of the samples in this form. When compared to the bulk (solid) state, samples such as metals have exceptionally low thermal conductivity values in powder form. This is due to the presence of air (k = 0.025W/mK), which drastically lowers the thermal conductivity of a sample. Compaction is an important factor when measuring the thermal conductivity of powder samples, and thus it is bets practice to employ the powder compacting weight (75g) to achieve consistent compaction. Varying compacting forces results in varying amounts of air in the sample, which can cause confusion when comparing data.

Figure 5: MTPS Sensor with Liquids and Powders Accessory and Compaction Weight

 Powders often have much lower thermal conductivity than the bulk material. To learn more about why, take a look at this blog post on why air dominates heat-transfer in powders. 

Thermal Effusivity

Thermal Effusivity is also measured by the MTPS sensor. This measurement quantifies how cool or warm materials feel to the touch when both the material and the hand are at the same ambient temperature. High thermal effusivity values correspond to materials that can draw heat away from your hand at a faster rate, resulting in the perception of cooler temperatures. Activewear tends to feel cool to the touch, and thus would have a high thermal effusivity value. On the other hand, a fluffy blanket feels warm and cozy to the touch, indicating a low thermal effusivity value. Quantifying this thermal property is invaluable for applications in clothing, bedding, diapers, and seating. Measuring thermal effusivity with the C-Therm MTPS sensor is the only way to conform to ASTM D7984 (https://www.astm.org/Standards/D7984.htm).

Thermal Effusivity of Diapers

When a baby is crying, one of the first things that parents do is check the diaper. A wet diaper feels cold and uncomfortable. Thermal effusivity testing has quantified this feeling (Figure 6). 

Figure 6: Thermal Effusivity Measurements of Diapers

When comparing the no name brand with the leading brand straight out of the box, the diapers perform incredible similarly. However, when wetted, the no name brand diapers show a large increase in thermal effusivity, resulting in a far colder and more uncomfortable feeling for the baby. Twelve hours after wetting, the diapers’ thermal effusivity lowers and the 2 brands return to a similar value. This experiment shows the leading brand performs significantly better at quickly absorbing moisture away from the skin and maximizing comfort for the baby. 

To learn more about measuring thermal effusivity and its role in thermal comfort, and how thermal effusivity can be used for claims validation, see this webinar. 

Contract Testing Services

Thermal Analysis Labs offers thermal conductivity and thermal effusivity testing with the TPS sensor, in addition to a variety of other thermal tests. Contact us here if you want more information about our testing services or to to get a quote for testing. 

[1] Sample size requirements for samples with thermal conductivity >90W/mK have different requirements (cylinder of diameter 17.76mm ± 0.05mm, height ≥38.1mm).