How does self-heating underwear generate heat out of thin air?

Key Points

★ Heat does not come out of nothing. Heat-generating fibers generate heat with the help of "external forces".

★ Self-heating underwear mostly uses "moisture absorption and heat generation", that is, your sweat provides you with energy.

In the winter warm clothing market, many underwear are labeled with the "self-heating" function, claiming that the clothing itself can generate heat to warm the human body. So is this "self-heating" function true?

From the perspective of the law of conservation of energy, heat cannot be generated out of thin air, so there is no such thing as "self-heating" clothing that can "create something out of nothing". However, there are indeed many types of heat-generating fiber materials that can provide us with a little extra warmth under certain conditions. Clothes made of these materials do not need to be particularly thick to be extra warm.

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So how do these magical materials generate heat? What should we pay attention to when wearing and washing these clothes? Let's reveal the secrets today!

01

Magic heating fiber

There is no doubt that all types of self-heating fibers actually generate heat with the help of external forces.

For example, light-energy heating fibers absorb sunlight and reflect human body heat radiation; electric-energy heating fibers heat up when electricity is applied; and phase-change fibers regulate temperature by releasing or absorbing latent heat during material phase change. However, the most commonly used heating mechanism in everyday clothing is "hygroscopic heating."

Moisture-absorbing and heat-generating fibers mainly absorb water vapor emitted by the human body, that is, sweat, and form hydrogen bonds to emit heat. In the structure of the fiber, there are some hydrophilic groups - which can be simply understood as "water-loving" atomic groups - that will combine with water molecules to generate heat, and the kinetic energy of water molecules will also be converted into heat.

Hydrogen bonds are a special type of chemical bond that lies between intermolecular forces and covalent bonds. They are usually found in water and will constantly break and reassemble between water molecules. | From the Internet

Common textile fibers have the ability to absorb moisture and generate heat, but different types have different heat generating abilities. Specialized self-heating fibers have more hydrophilic groups in their chemical composition, and the fibers have better hygroscopicity, and they generate more heat after absorbing moisture.

In order to improve the hygroscopicity of the fiber, not only can hydrophilic groups be added to the fiber for hydrophilization treatment, but the surface morphology of the fiber can also be changed to facilitate the adsorption of water molecules on the fiber surface.

For example, by increasing the number and depth of grooves on the fiber surface, the fiber's ability to absorb water vapor can be improved. In the figure below, the number and depth of the surface grooves of the two moisture-absorbing and heat-generating fibers on the right exceed those of the ordinary acrylic fiber on the far left.

The morphology of different fibers under scanning electron microscopy. | From the cited literature

Hygroscopic heat-generating fibers release heat by absorbing moisture, but after the fibers are "saturated" with moisture, their heat release reaction slows down, and the hygroscopic fibers will also discharge moisture at the same time, which in turn absorbs heat. If moisture cannot be discharged in time, the hygroscopic fibers will cause discomfort to the wearer.

Therefore, in order to improve this discomfort, many fabrics use moisture-absorbing and heat-generating fibers blended with other types of fibers to achieve a balance between moisture absorption and heat release and moisture dissipation and dryness, such as acrylic fibers and polyester fibers.

There are many types of self-heating underwear | Screenshot from an e-commerce website

So, faced with a wide variety of self-heating underwear products, how should we choose?

02

Self-heating clothing has "standards"

Although my country currently does not have mandatory standards for hygroscopic and heat-generating underwear, the industry standard FZ/T 73036-2010 "Hygroscopic and Heat-generating Knitted Underwear" can be used to evaluate hygroscopic and heat-generating underwear. This standard proposes two indicators for hygroscopic and heat-generating performance, one is that the maximum temperature rise value is greater than or equal to 4.0℃, and the other is that the average temperature rise value within 30 minutes is greater than or equal to 3.0℃.

When buying moisture-absorbing and heat-generating underwear, consumers should be careful to guard against exaggerated propaganda by merchants and choose regular channels and trustworthy brands. In addition, underwear that meets the standards will be clearly marked in the column of the implementation standard on the product tag, which can be used as a reference when purchasing.

For infants and young children with delicate skin, it is not recommended to wear moisture-absorbing and heat-generating underwear, because moisture-absorbing and heat-generating underwear goes back and forth in a cycle of moisture absorption-heat generation-moisture discharge-heat absorption, which can easily cause skin discomfort to infants and young children.

Standard description of a certain moisture-absorbing and heat-generating garment. | From the Internet

In addition, clothing containing electric heating fibers is also a good choice. Generally, there are metal fibers, carbon fibers, and composite heating fibers. The principle is similar to that of electric blankets: the heating fibers are woven into the fabric, or made into special heating units and sewn into the interlayer of clothes. Through continuous power supply from an external power source, electrical energy can be converted into heat energy and transferred to the human body.

Electric heating clothing generally uses an external power supply. As long as the power is not cut off, it can provide heat for a long time. It is very suitable for people who work outdoors for a long time in severe cold. Commercially available electric heating clothing is generally powered by a small power bank, which has a low voltage and is relatively safe. When wearing it, consumers should pay attention to not bending or folding the electric heating fiber to prevent local overheating and burns.

Electric heating clothing can generate heat for a longer period of time, making it very suitable for people working outdoors for long periods of time in severe cold weather. | Tuchong Creative

03

How to clean self-heating clothing?

The performance of moisture-absorbing and heating fibers will not be affected after daily washing, so they can be washed normally. However, since the fabrics of moisture-absorbing and heating underwear are often blended with a variety of fibers including moisture-absorbing and heating fibers, and some fabrics also use special weaving processes, it is recommended to wash them gently in warm water, avoid using high-temperature water and rubbing vigorously, and avoid exposure to the sun.

It is generally not recommended to wash electrically heated clothing, as washing will damage the electrically heated fibers or heating units, reducing the heating effect or posing a safety hazard.

To sum up, although qualified "self-heating" underwear cannot really generate heat out of thin air, it will indeed make you warmer when you wear it. You can choose according to your needs when buying it, and give priority to products that meet industry standards.

Author: Song Lidan, Consulting Engineer, Industrial Research Department, China Textile Construction Planning Institute

Review | Zhang Jie, Chief Engineer, China Textile Construction Planning Institute

Editor | Ding Zong Jiang Fan

Editor | Gao Peiwen

This article is produced by the "Science Rumor Refutation Platform" (ID: Science_Facts). Please indicate the source when reprinting.

The pictures in this article are from the copyright gallery and are not authorized for reproduction.

References:

Xiao Xueliang, Chen Tianjiao, Xie Yuntao, et al. Development and application of heat-generating clothing fabrics[J]. Journal of Clothing, 2021.