What is hot? What is cold?

In very simple terms:
Heat is energy and cool is the absence of energy.

The human body generates heat and is constantly trying to regulate the core temperature in the range between 36.5–37.5°C (see Focus Topic „Thermal Technologies“, November 2017). Heat can be generated directly from the human metabolism (= metabolic heat) and by the effect of environmental conditions on the human body (like air temperature, radiation, humidity, wind). When more heat is generated, e.g. from sports activity, the body reacts by producing sweat to regulate temperature and to cool down.

The stages of cooling

How does cooling work?

Evaporative cooling is the most effective method to draw heat from the body. To conduct away body heat, i.e., to effect the cooling process, perspiration should evaporate directly on the skin surface. Only fabrics that have been properly engineered can help support the body‘s own cooling mechanism. The extent to which clothing is able to help the wearer stay cool and prevent overheating is the subject of physiological testing at renowned institutes like Hohenstein and Empa as well as by service providers like Inside Climate.

One test method for determining the moisture transfer of clothing systems and textile insulation systems is the sweating Torso at Empa. The Torso allows the degree of thermal insulation of surface materials as well as the cooling effect of perspiration to be determined. In this way, the thermal characteristics of single and multiple layer textiles can be studied and optimized for the corresponding application.

WATson is a measuring device developed at Hohenstein and based on the principle of physio-thermal regulation: The physical cooling performance of a textile can be quantified under actual use conditions (e.g. in tropical heat, cooler temperature zones and different wind speeds). WATson enables fast efficient testing of the „cooling effect“ of textile fabrics early in the development process without the need for cost intensive wearer trials in a climate chamber so that fabrics can be optimized and compliance with quality standards is ensured.

It is to be considered that with all tests, the cooling effect on the skin depends on many factors and is perceived differently by each individual.

For cooling purposes, the "Minus" (factors leading to heat loss) needs to outweigh the "Plus" (factors generating heat).


Absorption

A medium can absorb heat and sweat. In the case of textiles, the absorbent medium is a knit or woven fibre design. Tests have shown that the degree and speed of the moisture transport is critical: The body needs to have the moisture close to the skin for rapid vaporization/evaporation!

How is the principle used in textiles?
  • Sweat is transported by means of (fibre and fabric) designs. This direction of transport can be along the width of the fabric (2-dimensional/lateral wicking) and/or vertically, i.e., away from the skin to the outer surface of the fabric (3-dimensional).
  • Sweat can be repelled (hydrophobic) or attracted (hydrophilic) like a magnet. If a hydrophobe (moisture repellent) fabric on the skin is combined with a hydrophile (moisture absorbent) one on the outside, the moisture will effectively be transported to the outside. A fabric can have natural moisture transport properties or these can be created by the application of chemical treatments in or on the fibre.
  • For cotton, moisture settles in the crystal structure or in the core of the fibre and evaporates very poorly. Polyester retains only about 5% of its own weight as moisture; for the hydrophilic polyamide it is approximately 2-3% , and it is even < 1% for polypropylene.
    The term endothermic reaction applies when a fabric draws heat out of its environment (e.g. in contact with water). Wool is endothermic, but so is the sugar alcohol „xylit“, which draws heat from the environment in contact with moisture.*
    PCM (Phase Change Materials)* exchange heat and cold (latent heat storage).

 

* Endothermic + PCM effects are designed to keep the body in a comfortable temperature range to avoid sweating. They are (only) useful where the body is not yet generating enough heat to produce sweat. Therefore the effects are not measured as -xy°C cooler.


Convection

Warm air or water vapor is transported within or out of the clothing system. This type of transport can also be forced, e.g. by the motions of the wearer.

How is the principle used in textiles?
  • Use of ventilation slots or mesh.

Conduction or diffusion

Heat always flows only in the direction of the lower temperature – from warm to cold – unless the heat transport is prevented, e.g. by the insulating air.

How is the principle used in textiles?
  • Avoid heat retaining fibre cross sections (compare to November 2017 Focus
    Topic „Thermal Technologies“) and, instead, use fibres that are especially good at dissipating the heat like round or flat cross section fibres: non-textured,
    especially smooth twisted yarns give the feeling of being cool.
  • Use thermal conductor material like metals (silver, copper, aluminum) for prints. Important: Metals must have direct contact with each other, otherwise the flow is interrupted.
    Prints with carbon materials (graphite, graphene, jade) are also conductive.
    Unfortuantely, there are no measured values for a single print. Presumably
    these values are less than the ones of metals as metals are better conductors. Prints usually don't cover enough of the surface in order to conduct the warmth as well.

Evaporation/Vaporization

Liquids are transformed through the absorption of energy into the aggregate gaseous state, for example, perspiration into water vapor. The greater the contact area of the liquid on the heat source, the more sweat can evaporate, the required energy is drawn from the body and cooling is achieved. In accordance with our human physiology and physics, evaporating sweat is the best and most effective method to draw heat out from the body (2.4 kJ/g at skin temperature, Hohenstein Institute) – and that is why people sweat!

How is the principle used in textiles?
  • Combine the benefits of absorption/wicking and evaporation with good
    breathability.

(Thermal) radiation

Heat is thermal energy in the form of electromagnetic waves. The wavelength can vary. The shorter the wavelength, the more energy it has (Joule) – thermal radiation may extend from short wavelengths (NIR) over mid-range (MIR) to long wavelengths (FIR). The strength of this radiation/the wavelength is variable.

How is the principle used in textiles?
  • Avoid the use of NIR/FIR (Near and Far-Infra-Red) reflection.
  • Ensure the material does not warm up/absorb heat in the range of visible light (MIR), but rather reflects light with its heat energy. The influence of UV protection from the fabric and/or from treatment on the core body temperature is unknown.

Respiration/Breathing

Becomes a relevant factor only below minus 10°C.

How is the principle used in textiles?
  • Fabrics (e.g. mask in front of mouth) shields the body from breathing in too cold air.

Product examples from the Performance Forum

To see the chart in full size, please download the PDF-file


What is the best way to keep the body cool?

The measurement values chart above from Institute Hohenstein shows:

A

For ambient temperatures of ca. 20–31°C, radiation is the strongest cooling factor.
--> Ensure the use of fabrics that reflect visible light (MIR, not UV light) and do not heat up.

B

For ambient temperatures of ca. 31–35°C, a combination of radiation, convection, and evaporation produces the greatest cooling effect.
--> Ensure the use of fabrics that reflect visible light (MIR, not UV light) and do not heat up.
--> Sweat in the form of water vapor passes through the textile thanks to ventilation slots and mesh.
--> Rapid and wide distribution of sweat close to the skin surface creates more area for evaporation.

C

Above an ambient temperature of 35°C, evaporation is really the only relevant factor.
--> Rapid and wide distribution of sweat close to the skin surface creates more area for evaporation.
The cooling effect of max. 0.08°C heat loss refers to the core body temperature – not the skin surface temperature!

 


What is important?

  • A cooling effect occurs when the heat loss factors are greater than the heat generation.
  • Physiologic/physical cooling effects are measurable and should be measured.The key is to define how and what to measure. Figures cannot be compared unless tested to the same standard. That is why the fabrics at the Jury‘s Pick Table are not ranked – the measuring methods are not comparable. A value like 0.05°C can be the change in body core temperature or the skin temperature. A loss of 0.05°C body core temperature is quite a bit, while 0.05°C skin temperature is nothing. Always ask what (DIN) ISO or test method was used for the testing.
  • PCMs are tested according to DIN 16 806-1 which measures the heat storage and release capacity, so not the cooling effect in -/+xy°C. The idea behind PCMs is rather to keep the body temperature stable by absorbing and remitting heat, so avoiding over-heating and sweat production.
  • The haptic of the fabric might be misleading: the ambient and body temperature when sourcing the fabric will be different from when the finished garment is actually used. It is neccessary to know in which environment the use is intended, e.g. the ambient temperature in which the cooling needs to be achieved. The technology(ies) similar to the graphic should be intelligently chosen and combined.
  • For a short period of several minutes, sensory cooling (triggering of the skin receptors) or the Cool Touch feel may be experienced, but this does not lead to long term intensive heat dissipation and is not suited for metabolic activities
    (for the body to generate heat).
  • Above an ambient temperature of 35°C, evaporation is most effective, but requires perspiration on the skin.
  • The weight of a fabric influences its cooling intensity and duration:
    Light fabric --> shorter but more intensive cooling
    Heavy fabric --> longer but less intensive cooling
  • Be aware: The performance of tested fabrics can be affected by washing, especially when the cooling effect is based on a finish applied to the fabric‘s surface (not in the fibre).
  • Next to the measurement values, the comfort of the wearer needs to be considered as well. As always, people experience sensory perception and psychological interpretation differently. What is measured in the lab or what is felt/thought by one, may not be the same for another person. Therefore, the recommendation for clothing is: do both! Test in the lab and on people under controlled and realistic conditions.
  • Post-exercise chill effect: A soon as the body cools down, it stops perspiring and no new sweat is produced. However, although cooling is no longer necessary, there may still be some sweat that continues cooling through evaporation. The body chills down to the corresponding outside temperature if no other technology is present (e.g. an exothermic effect or PCM) or, the person takes a shower, dries off and puts on appropriately warm and dry clothes.

A special „Thank you“ for great support to:

  • Dr. Jan Beringer, Hohenstein Institute
  • Dr. Simon Annaheim, Swiss Federal Laboratories for Materials Science and Technology (Empa)
  • PERFORMANCE JURY April 2018 including Puma, Odlo & Elke Schimanietz
  • And all April 2018 PERFORMANCE DAYS exhibitors

Did you know about these Other Focus topics?