With a spate of cold fronts bringing down temperatures across the country, and customers eager to wrap up against the inclement weather, garments claiming to be “thermogenic” — meaning that they generate heat — are flying off the shelves.
Unlike electric blankets that actively produce heat after being switched on, most of the so-called “thermogenic” clothes generate heat through the process of capturing moisture evaporating from the body and trapping it in their fibers, said Chiu Sheng-fu (邱勝福), a director at the Taiwan Textile Research Institute’s Department of Testing and Certification.
“However, the name ‘heat-generating clothes’ is bizarre, as the working theory of such products is based on the physical principle of moisture absorption and heat retention, making its heating effect a passive one [as opposed to an active one,]” Chiu said.
Most heat-trapping garments available in shops are able to reach a thermal limit of between 2°C and 5°C within the first 10 minutes after making contact with the wearer’s body, with the temperature gradually dropping over time.
Accordingly, products that are able to maintain a 1°C rise in body temperature after 15 minutes of wearing are considered to be of good quality.
According to experts, high- quality thermogenic clothes can be made from various textiles, ranging from cotton, wool, natural silk, polyethylene terephthalate (PET) and linen to nylon and acrylic fibers.
The warming effect of such products is determined by the water absorption rate of their constituent fabrics, Chiu said, adding that “the more moisture they absorb, the more heat they retain for the body.”
“Among commonly used textiles [for heat-trapping clothes,] wool has the highest rate of water absorption at 15 percent, followed by cotton at 8 percent, while PET is ranked bottom with only 0.4 percent,” Chiu said.
Chiu said that the theory of “far infrared heat generation” is also applied in the manufacturing of some types of heat-generating garments, in which an oxide is either embedded into the fabric or applied to its surface to help absorb far infrared rays.
Absorbed far infrared rays are then converted into heat and stored in the fibers of the fabric, helping to keep the body warm, Chiu added.
Touting the use of wool in clothing, Hsing Wen-hao (邢文灝), a professor at Chinese Cultural University’s Department of Textile Engineering, said that wool is one of the few natural materials that absorbs moisture effectively and retains warmth and can increase body temperature by up to 6°C.
“The only drawback is that it can be hard to dry once the fabric absorbs moisture,” Hsing added.
Sales figures of thermogenic clothing have steadily increased in recent years, with millions of such products flying off the shelves this winter season.
According to statistics from Taipei-based Ming Yao Department Store, heat-trapping garments sold by Japanese clothing chain Uniqlo and Far Eastern Group were big hits during the shopping mall’s 18-day anniversary sale between Nov. 15 and Dec. 2.
“Thermogenic clothes manufactured by the Far Eastern Group sold during the anniversary period accounted for as much as 60 percent of the brand’s total sales during the period,” the department store said.
Meanwhile, heat-trapping garments produced by Taiwan’s largest convenience store chain, 7-Eleven — priced between NT$359 and NT$399 — have seen sales of about 600,000 clothing items as of October, more than three times the number it sold during the same period last year.