Cold wave is coming! Take a look at "self-heating underwear": Is it me or the clothes that are heating up?

In the cold weather, warm-keeping products are hot-selling, especially "self-heating underwear". Can underwear really heat itself? Let's find out>>>

Written by reporter Zhao Tianyu Edited by Liu Zhao

New Media Editor/Chen Xuanzhi

Interview experts

Wang Baojun (Researcher at the National Textile Products Quality Supervision and Inspection Center)

As the end of the year approaches, a new wave of cold waves is coming, and the temperature in many places has even broken the historical extreme. People have begun to focus on warming products, especially self-heating underwear, which has remained hot this year.

"Significant temperature increase", "37-degree constant temperature", "German black technology"... Open the shopping website platform, and you will be dazzled by the variety of light and thin self-heating underwear products. They have been hailed by many businesses as a "winter magic weapon."

Can a piece of underwear that is not too thick really generate heat on its own?

Secret: There is no self-heating material in the world

Most thermal underwear merchants claim that their thermal underwear contains a layer of "high-tech" material that can generate heat on its own and has a warmth-keeping effect several times greater than traditional pure cotton thermal underwear. It is "black technology" and so on.

But the actual effect is that some self-heating underwear is indeed a little warmer, but there is still a gap between several times warmer; some self-heating underwear is simply not warm, and is not even as good as thicker autumn clothes.

Wang Baojun, director of the National Cotton Textile Product Quality Inspection Center, told reporters that from the perspective of the law of conservation of energy, self-heating is not scientific, because energy needs a source and heat cannot be generated out of thin air. At present, there is no material in the world that can truly generate heat by itself, that is, there is no real self-heating clothing.

"Moisture wicking and warmth retention"

So are self-heating underwear a complete scam?

This is certainly not the case. Wang Baojun further explained that it is just a change of concept of the principle of "moisture absorption and heat generation" of fiber, giving the public the illusion of "keeping warm and generating heat".

As we all know, the stratum corneum on the surface of human skin can expel water. When the human skin emits water vapor, the water vapor will dissipate heat when it liquefies, which is an important reason why we feel cold in winter.

So, how can we better resist the cold? The first is to wear thicker clothes. Still air has the lowest thermal conductivity. Under the premise of no air flow, the more air the fiber layer can hold, the better the warmth retention of the clothing.

(Picture from the Internet)

But this will bring some challenges, just like in the winter of the 1960s and 1970s, people wore bulky cotton trousers. Although they could keep warm, they were not only unsightly but also inconvenient to move around in.

With the advancement of technology and the development of the times, clothing manufacturers began to work hard on fabrics: on the one hand, the material must be thin and light enough, fit well and easy to put on and take off. On the other hand, the thermal insulation performance must be good enough, and fiber materials are a good choice.

Wang Baojun said that when the fiber absorbs water, the hydrophilic groups in the fiber molecules combine with the water molecules, the kinetic energy of the water molecules (water vapor) is reduced, and at the same time converted into heat energy and released, which is hygroscopic heat. The higher the hygroscopicity of the fiber, the more heat is dissipated during the hygroscopic process.

Therefore, self-heating underwear is actually underwear made of special fiber materials and has the effect of "moisture absorbing and keeping warm".

How does 1+1 equal 5?

In fact, whether it is natural fibers such as wool and silk, or artificial fibers such as polyester, most fibers have the function of "absorbing moisture and generating heat". The difference lies in that different fibers have different costs or prices, and the effects of absorbing moisture and generating heat are also different.

For example, polyester fiber, which is actually "polyester" and "Dacron", is the most common chemical fiber with good elasticity and wrinkle resistance. Viscose fiber, also known as viscose, has very good moisture absorption and good warmth retention, but its cost is relatively high.

▲Promotional picture of a certain brand of self-heating underwear (picture from the Internet)

For example, acrylic has excellent warmth retention properties and some people think it is warmer than wool; while spandex has good elasticity. The Leica fabric that we often hear about is a type of spandex.

Judging only from the names of these fiber fabrics, they are indeed a bit far from "black technology", but as the saying goes "1+1 equals 5", Wang Baojun said that if these fiber materials are combined in a certain proportion, different effects can be obtained, achieving the purpose of "moisture absorbing and keeping warm".

For example, the main materials of a Japanese brand of self-heating underwear are polyester and viscose. Acrylic is responsible for warmth; viscose is mainly responsible for breathability and sweat removal, and also assists in warmth; polyester is responsible for wrinkle resistance; spandex is responsible for making the clothes fit better. As the "heating capacity" increases, we can see that the total proportion of viscose and acrylic is also roughly increasing.

For example, the very popular De velvet this year is actually a chemical fiber fabric, mainly made of acrylic, blended with a certain proportion of spandex and polyester. Some De velvet fabrics are also blended with a certain proportion of viscose fiber, in which the acrylic is modified and is a moisture-absorbing and heat-generating fiber.

(Picture from the Internet)

As for why it is called "Delphin", it is because the inventor of this fabric is the German Bayer company, but this does not mean that it must be a product made in Germany. Although some low-priced textiles can also be called Develvet fabrics, they may not be of good quality.

There is also an infrared self-heating product, that is, some infrared radiation materials with good heat radiation absorption are added to the infrared fiber, which can absorb the infrared rays emitted by the human body and then feed them back to the human body in the form of infrared rays to prevent the loss of human body heat. But this is essentially improving the heat preservation ability, and the cost of infrared radiation materials is relatively high, so there are not many similar products on the market.

▲Graphene is currently the best thermal conductive material in the world, but it is expensive and difficult to commercialize on a large scale (Picture from the Internet)

In addition, a kind of self-heating underwear made of "graphene material" has recently appeared on the market. From the perspective of materials science, graphene is the material with the highest thermal conductivity at present, but its price is also very expensive. If it is used as a raw material for keeping warm in underwear, it may not even recover the cost. Therefore, the self-heating of graphene is probably just a gimmick at present.

It’s possible to relieve the cold, but it’s difficult to resist the cold winter!

In summary, after the fiber materials and components are re-"combined", self-heating underwear is indeed warmer than ordinary autumn clothes and autumn trousers, but how warm is it? Can you survive the winter by wearing self-heating underwear? Wang Baoguo believes: "It is possible to use it (self-heating underwear) to alleviate the cold shock caused by sudden changes in the environment, but it is really a bit difficult to resist severe cold."

Wang Baoguo explained that the reason for this conclusion is that, first, self-heating underwear is affected by the environment. In a very dry environment, it has no ability to absorb heat. For example, in the northwest and north China regions where it is dry and cold in winter, the effect of self-heating underwear is definitely not as good as in the Yangtze River Basin.

Second, the fiber's moisture absorption and heat release is a dynamic process, so it is not easy to maintain a constant temperature. "When the fiber reaches saturation, the less water it absorbs, the less heat it generates."

Third, all manufacturers, including the manufacturer, focus on promoting "moisture absorption and heat generation", but in fact, according to the principle of conservation of energy, the moisture absorbed by the fiber material needs to be released, which is a process of "absorbing heat and dissipating moisture". In other words, self-heating underwear is actually constantly circulating in the process of moisture absorption-heat release-moisture release-heat absorption.

This feeling of alternating hot and cold is not very friendly for people with sensitive skin.

▲Although there are industry standards for hygroscopic and heat-generating products, the data are values ​​under extreme laboratory conditions and have no reference value for use in real life.

In terms of industry standards, the reporter learned that there is currently no international standard for knitted heating underwear, only one textile industry standard FZ/T 73036-2010 "Moisture-absorbing and heating knitted underwear". It stipulates that "the maximum temperature rise value shall not be less than 4°C; and the average temperature rise value within 30 minutes shall not be less than 3°C". Only products that meet these two requirements are considered qualified products.

▲Different self-heating underwear materials have different composition ratios. Some low-cost self-heating underwear is not as good as thickened autumn clothes and autumn trousers.

However, Wang Baoguo also admitted that this data was only generated in the laboratory after simulating extreme conditions, and it is difficult to achieve that kind of warming effect in real life. In short, there is a gap between the test report and the actual wearing experience, and this gap is not small.

So from the current perspective, although the starting point of self-heating underwear is good, it is still at the stage of "concept outweighs reality". Wang Baoguo suggested that consumers should not be blinded by the so-called "black technology" gimmick, and rational consumption is the most important. "If you really want to keep warm, you can wear wool pants, cotton and linen pants or even down pants. The warmth-keeping effect of this kind of clothing is much better than that of self-heating underwear."

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