Evaluation of the Objective Properties of the Cool or Warm Touch of Textiles
The measurement of thermal effusivity allows quantifying a subjective notion, a sensation felt upon touch. This enables scientific validation and quantification of comfort through a comparative study since, depending on the use of the textile, a warm or cool touch sensation will be preferred.
Warm or Cool Touch… What Are We Talking About?
Thermal effusivity, or thermal inertia, expresses the ability of a material to absorb or release thermal power, i.e., heat. It is directly related to the temperature difference between two surfaces in contact, for example, between a textile and the human body. It can also be described as the rate at which a material’s temperature adapts to that of the body. The higher the effusivity of a material, the faster the thermal fluxes are transported and conducted to the amount of matter it contains without significantly raising the temperature, creating a cold touch, also known as “cool touch”, thus providing a sensation of coolness.
Conversely, low thermal effusivity characterizes a material whose temperature rises quickly to balance with that of the contact surface, thus creating a warm touch, also known as “warm feel,” providing a sensation of warmth.
Influencing Factors:
The nature of textile fibers, ambient temperature, humidity, and the density of the textile product (weaves, thickness, textures…), finishing, and treatments are variables that influence the thermal effusivity of fabrics.
When a textile contains moisture, its thermal effusivity will be altered: water is characterized by an effusivity of 1600 W. s1/2 /m2.K. The presence of moisture in the textile will notably increase the measured value, giving a sensation of coolness. Therefore, it is interesting to study the impact of sweat or rain on technical sportswear, for example. It is also important to study the water-repellent properties and drying speeds of textiles.
How to Measure it:
There is and equipment and a measurement method: ASTM D7984 “Standard Test Method for Measurement of Thermal Effusivity of Fabrics Using a Modified Transient Plane Source (MTPS) Instrument.”
The apparatus consists of a temperature sensor, a heating surface, a constant pressure applicator, and a data acquisition system. An insulated ring is placed around the temperature sensor to ensure one-dimensional heat propagation across the entire surface of the sensor. The sample (minimum diameter of 18mm) is placed on the heating surface, and constant pressure is applied. The heating surface will cause a temperature rise of about 1 to 3°C in just a few seconds.
Once this temperature impulse is emitted, the measured voltage is analyzed to quantify the surface temperature change between the sample and the plate.
The temperature rise on the surface is inversely proportional to the thermal effusivity of the sample material. A faster temperature rise will be measured for a textile with low thermal effusivity (i.e., Warm Feel), while a gentler slope will be observed for a textile with high effusivity (i.e., Cold Touch).
Thus, it is possible to measure a temperature difference that, depending on the textile, will take more or less time to balance with the body. The equipment also measures the thermal conductivity of a textile. Knowing that effusivity is the square root of the product of the following three values: heat capacity, conductivity, and density, it is possible to determine the heat capacity of the textile. Therefore, the advantage of the MTPS equipment is its ability to measure several properties defining the thermal characteristics of a textile precisely and efficiently.
For your developments, the CTT team can measure your products according to the standard method and following protocols adapted to your needs: humidity, controlled environment, etc.
CTT Group, at the heart of the matter
Thermal Effusivity Scale
Results thermal effusivity textile samples
