Touch cotton. It feels warm because it has a low thermal effusivity.
Touch. It’s one of our critical senses in perceiving the world – everything from the clothes we wear to the bedding we sleep in. The thermoreceptors in our skin monitor and relay information to our brains about everything we come in contact with. This helps us make decisions about what feels pleasant to the touch and keeps us safe. Human skin is very good at detecting differences in a material’s ability to transfer heat, such as the warmth of a fleece sweater compared to the coolness of leather. This material property is known as thermal effusivity – as a metric, it can be used to quantify a textile’s ability to exchange thermal energy between skin and fabric. Why is this important? Because human test panels have established a positive correlation between our touch perception of the warmth or coolness of a textile, to its thermal effusivity. In other words, our perception that certain materials will keep us warm or help us cool down is quantifiable. We know this is important in quantifying performance in a wide range of applications – including diapers, activewear, personal protective clothing, upholstery, neoprene dive suits, and bedding. Our skin’s thermoreceptors are giving us good information, though subjective. The C-Therm Tx Platform–quantifies warm and cool feel for you, making what was previously subjective into a metric that can be quantitatively measured. It provides accurate measurements across a range of real world scenarios, including higher humidity environments and under varying compressive loads. The C-Therm Tx measures the science of touch – so you can provide the comfort.
THE WARM FEEL – COOL TOUCH
PRODUCT PERFORMANCE INDEX
PRINCIPLE OF OPERATION
The C-Therm TCi employs the patented Modified Transient Plane Source (MTPS) technique. The one-sided, interfacial heat reflectance sensor applies a momentary constant heat source to the sample. Thermal conductivity and effusivity are measured directly, providing a detailed overview of the thermal characteristics of the sample.
1) A known current is applied to the sensor’s spiral heating element, providing a small amount of heat.
2) The sensor’s guard ring is fired simultaneously supporting a one-dimensional heat exchange between the primary sensor coil and the sample. The current applied to the coil results in a rise in temperature at the interface between the sensor and sample, which induces a change in the voltage drop of the sensor element.
3) The increase in temperature is monitored with the sensor’s voltage and is used to determine the thermo-physical properties of the sample. The thermal conductivity is inversely proportional to the rate of increase in the sensor voltage (or temperature increase). The voltage rise will be steeper for lower thermal conductivity materials (e.g. foam) and flatter for higher thermal conductivity materials (e.g.metal).
Bureau Veritas Guangzhou
We offer ASTM D7984 testing with the TCi in our lab. We were impressed with the service and support provided by C-Therm."
An outdoor retailer was interested in assessing how the feelings of “warmth” changed with increased moisture content. Water has a very high thermal effusivity ( ~1600 Ws½/m²K) compared with that of dry textiles (100- 200 Ws½/m²K). Notice the thermal effusivity of the cotton textile is twice the effusivity of the micro-fibre material when moderately wet with 10 sprays of water. Incidentally, when fully saturated both materials have an effusivity very close to that of water itself. This reinforces that if you fall in a cold lake, the best thing you can do to improve your comfort is to remove your clothing upon exiting. However, technical apparels such as this micro-fibre material can provide substantial improvements in comfort under moderate wetting conditions.
Thermal Conductivity Vs. Compression
Fabrics are typically compressible and it is important to control the compaction of the material in the test method applied for measuring either the thermal conductivity or the thermal effusivity. There is no “universal” level of compression recommended as it is application-dependant. For example, in measuring the insulation quality of a down jacket minimal compression would be applied. Conversely, in measuring a down sleeping bag, considerable compression force would be applied to mimic the “real-world” application conditions. With the C-Therm CTA accessory researchers can reproducibly create such user conditions in testing the performance of the materials.
Characterizing The "Warm Feel" Of Activewear Textiles
A major Canadian retailer employs the C-Therm Tx to measure the “Warm Feel” of candidate textiles. As a producer of high performance thermal underwear and activewear, the retailer measures the thermal effusivity of candidate textiles in evaluating product performance. Effusivity quantifies a textile’s ability to exchange thermal energy between skin and fabric. As a material property, thermal effusivity is a key metric in selecting fabrics for either cool or warm environment applications. In terms of a scale, higher thermal effusivities indicate that it feels cooler to the touch, while lower values feel warmer. The table below highlights results from 4 candidate materials in terms of their thermal effusivity.
Characterizing The "Cool Touch" Of Bedding Textiles
A US manufacturer of finishings for woven and knitted mattress fabrics partnered with C-Therm to study the “Cool Touch” of candidate textile finishings. As a supplier to mattress manufacturers, the US company measures the thermal effusivity in characterizing the effect of different coatings designed to improve the cool touch property. Thermal effusivity quantifies a textile’s ability to exchange thermal energy between skin and fabric, which is directly related to thermal comfort. This supports the supplier in recommending the optimal finishings to the major mattress manufacturers and retailers. If the thermal effusivity is higher, the textile or fabric feels cooler to the touch. Conversely, if the thermal effusivity is lower, the textile or fabric feels warmer to the touch. The chart below highlights results from candidate materials in terms of their thermal effusivity.
Tx Touch Experience Platform for testing warm feel / cool touch via ASTM D7984
Thermal Effusively Range
5 to 1,600 Ws½/m²K
Specific heat capacity (derived)
-50º to 200ºC
0% – 90% Relative Humidity (non-condensing)
Better than 1%
Better than 5%
0.8 to 3 seconds
Minimum Sample Size
Min. diameter of 18 mm
Min. thickness is dependent
on the thermal effusively.