A brand provided a detailed description of the functions of its thermal underwear
As Beijing experienced its first snowfall of the winter and temperatures plummeted, various cold-weather clothing items became hot commodities. In addition to common thermal underwear, undergarments claiming to generate heat have started gaining popularity among consumers. Do thermal underwears really produce heat as advertised? What are they actually made of? And how do they differ from regular thermal underwear?
The main material of thermal underwear is moisture-absorbing and heat-retaining fiber
Reporters visited several large shopping malls and supermarkets in Beijing and found that many retailers were selling thermal underwear. The main components listed on the labels were polyester and acrylic fibers, along with small amounts of viscose and spandex. Reporters also noticed that many thermal underwears claim that the fibers themselves can absorb water vapor evaporated from the body and convert it into heat energy, causing the material to warm up.
“The concept of thermal underwear originated in Japan. The earliest thermal underwears were improved versions of chemical fiber materials, primarily using Japanese synthetic fibers. This is because such fibers have relatively good moisture absorption and heat generation capabilities. In fact, there are many ways to generate heat, including heat from moisture absorption, electrical heating, and converting solar energy into heat. Currently, the most common thermal underwears on the market achieve warmth mainly through the principle of moisture absorption and heat generation,” Wang Zhi, director of the China Knitting Technology Committee, told reporters. In reality, all fibers have some degree of heat generation capability, but the level and amount of heat produced vary. In terms of heat generation performance, natural materials such as wool and viscose generally have higher moisture absorption and thus generate more heat than ordinary fibers. Research has found that in terms of integral heat (i.e., the heat generated by 1 gram of dry fiber after fully absorbing moisture), wool produces 26.9 calories, making it quite suitable as a material for thermal underwear.
Heat-generating fibers were first used by mountaineers
Gong M?, a professor at the College of Materials Science and Engineering at Beijing Institute of Fashion Technology, explained that the so-called new material for thermal underwear is a special type of fiber. This type of fiber is relatively expensive and thermal underwear is usually made by adding a small amount of heat-generating fiber to other fibers. Most of these fibers are polyester, nylon, spandex, or modal. This type of fiber was first used for military uniforms in Japan and later became popular in general consumer markets, mainly used for thermal underwear. “For example, ‘Warm and Soft’ actually adds a certain amount of heat-generating powder to the fiber, giving it heat-generating properties. The so-called heat-generating fiber is actually a high-functionality fiber that can release heat during friction or movement. It can store heat in the gaps between fibers and release it when the temperature drops. Therefore, it has a heat-generating effect. These fibers are high-performance materials and can effectively retain heat when worn close to the skin,” Gong said.
“In recent years, abroad, new types of fibers that can generate heat on their own have been developed and are used as materials for thermal underwear. The main method for manufacturing thermal and heat-generating fibers is to add special chemical components to the fibers, which trigger chemical reactions under specific conditions to produce heat. Examples include Thermotron fiber – a solar heat storage and thermal insulation fiber, Outlast air-conditioning fiber – a phase-change heat storage fiber, Eks moisture-absorbing and heat-generating fiber, Textcare nano bamboo charcoal fiber, and Ceramino infrared heat storage and thermal insulation fiber,” Yu Kun, a senior engineer at the Tianjin Textile Fiber Inspection Institute, told reporters. Currently, domestic thermal underwear manufacturers mainly use moisture-absorbing and heat-generating fibers as raw materials. By varying the ratios and production techniques, they develop knitted fabrics that change the traditional bulky and heavy form of thermal fabrics, giving them the characteristics of being light, thin, soft, and warm.
Thermal underwear cannot replace cotton coats
Ms. Li recently bought two pieces of thermal underwear, but the so-called warming sensation was not noticeable. In response, Wang Zhi explained that when a person wears thermal underwear, the hydrophilic groups in the fiber molecules bind with moisture emitted by the body, reducing the movement of body moisture and thus minimizing heat loss. The kinetic energy is converted, actively raising the temperature by 2°C to 4°C. However, it should be noted that even genuine thermal underwear only provides a limited temperature increase, and people should not use it as a substitute for cotton outerwear.
■ Purchasing Tips
Recognize the “yellow background with black text” thermal underwear label
In December 2010, China implemented an industry standard for moisture-absorbing and heat-generating thermal underwear, and there are specialized instruments to test the standards for such underwear. According to the industry standard, there are two main criteria to determine whether underwear qualifies as thermal underwear: first, the peak temperature increase should be at least greater than 3°C, and second, considering factors like heat dissipation, the average temperature increase within 30 minutes should be greater than 2°C. These are the minimum thresholds.
Gong M? reminded that in the domestic market, regarding so-called moisture-absorbing and heat-generating thermal underwear, especially those sold online, the regulations on product quality and performance standards are difficult to enforce in actual purchases. China’s consumer product quality supervision departments are also researching and formulating relevant quality and function standards for different types of functional fibers, aiming to establish quality standards for different functional categories. When consumers purchase moisture-absorbing and heat-generating underwear, they should check whether the product has the official “moisture-absorbing and heat-generating fiber” label printed on it. Look for information on the heat-generating fiber, heat-generating effect, and additive ratio. In addition, it is best to try on the actual product in person.
Common types of thermal underwear on the market
Moisture-Absorbing and Heat-Generating
The mechanism behind moisture-absorbing and heat-generating fibers is that when the fibers absorb moisture, the hydrophilic groups in the fiber molecules bind with water molecules, reducing the kinetic energy of the water molecules, which is then converted and released as heat energy. Typically, the moisture-absorbing and heat-generating performance is closely related to the moisture regain rate. Fibers with a high moisture regain rate have better moisture-absorbing and heat-generating properties, while those with a low rate perform poorly. For example, wool and modal fibers have good moisture-absorbing and heat-generating effects, whereas ordinary acrylic and polyester fibers have poorer performance.
Solar Heat Storage
This method involves adding special substances to the fibers to absorb solar energy and convert it into heat energy, emitting long-wavelength far-infrared rays that warm the inside of the garment.
Phase-Change Heat Storage
This involves adding thermosensitive phase-change materials to the fibers, which can absorb, store, and release heat in the form of latent heat. These materials can transform between solid and liquid states with temperature changes, thereby achieving heat absorption and heat generation effects.
(Classified by different heat generation principles)
■ Thermal Underwear vs. Thermal Insulation Underwear
Thermal insulation underwear has poor breathability
Both thermal insulation underwear and thermal underwear provide warmth. Thermal underwear generally uses moisture-absorbing and heat-generating fibers as raw materials, actively generating heat by absorbing moisture emitted by the human body. Gong M? explained that the difference from thermal insulation underwear is that most thermal insulation underwear materials are regular chemical fibers, which generally do not have heat-generating properties.
Yu Kun told reporters that regular thermal insulation underwear reduces heat loss and increases warmth by increasing thickness and altering the fiber structure (such as making it hollow). Moreover, the fabric itself does not have any heat-generating function. Wang Zhi pointed out that thermal insulation underwear achieves warmth by reducing the exchange of heat between the human body and the external environment. Regular thermal insulation underwear contains some static air, which has very low thermal conductivity. This static air reduces heat conduction and radiation, thereby minimizing heat loss from the body. As a result, thermal insulation underwear generally has poor breathability.
Hot Topics Q&A
No research has been conducted on the relationship between thermal underwear and skin allergies
Q: Is it true that thermal underwear made from moisture-absorbing and heat-generating fibers may cause skin discomfort with prolonged wear? Is it suitable for people who are prone to allergies?
A: Gong M... said that the materials used in thermal underwear are composites of various types of fibers, not achieved through some special functional layer. Generally, there won’t be issues such as allergies or irritation due to the material itself.
Yu Kun: The raw materials for new types of thermal underwear are generally chemical fibers that have undergone special processing, blended with acrylic and cellulose fibers. Chemical fibers usually do not perform as well as natural fibers in terms of comfort. People who are prone to allergies should still choose based on their own characteristics, but currently there is no research on this.
Wang Zhi: For people with skin allergies, it cannot be generalized whether thermal underwear is suitable, because the material composition of different thermal underwears varies, individual skin tolerance differs, and different people have different allergens. Some people are allergic to synthetic fibers, acrylic, etc. Therefore, citizens with skin allergies can pay attention to whether the product labels contain materials they are allergic to when purchasing thermal underwear.
There is no clear indication on how washing frequency affects heating performance.
Q: Thermal underwear is made from moisture-absorbing and heat-generating fibers. Are there any special requirements for washing it? Can it only be hand-washed or dry-cleaned? Will repeated washing affect its heating effect?
A: Gong M... said there are generally no special requirements for washing thermal underwear. However, it should be noted that some thermal underwear products may specify certain washing instructions on the label, and the heating effect may vary depending on the number of washes. Currently, there is no clear research or data on the relationship between the number of washes and the heating effect. Relevant national standards may also have gaps in this area. He also mentioned that repeated washing might reduce the heating performance of thermal underwear. This is also one aspect that China's environmental standards committee is considering in the research of functional textile standards.
Q: A market survey by reporters found that different brands of thermal underwear indicate different material compositions and proportions. Does this affect their heating performance?
A: Wang Zhi: Since many materials have moisture-absorbing and heat-generating properties, though with varying insulation effects, various types of thermal underwear are made by combining different materials while meeting industry standards. Therefore, the material composition ratios of different thermal underwears sold on the market also vary.
It should be noted that, according to product labels, many thermal underwears contain common components found in regular clothing, such as acrylic, which are also the main components of thermal underwear. Although all are labeled as acrylic, it is an improved type of acrylic with better moisture-absorbing and heat-generating properties. However, since the main component remains unchanged, it can only be labeled as acrylic.