As soon as the temperature dips below 0ºC, you will find me covered head to toe in thermal clothing. I am not picky when it comes to brands, although I do love Uniqlo’s HEATTECH. As long as I am warm, I am game. However, when it comes to shopping for these products, the magic word among the stock seems to be “moisture-wicking.” It turns out the science behind moisture-wicking plays a pretty significant role in why certain fibers and fabrics are off the table when it comes to layering up.
To Be Moist or Not to Be Moist
Spoiler alert: moisture-wicking is important. If you are doing any kind of outdoor activity or just running to catch your bus in the dead of winter, chances are you’ll break a sweat. What’s important here is that when you find yourself in these scenarios, you’ll want to avoid clothing that will just soak up your sweat, like cotton. You don’t want your base layer to be a sponge – especially in an unforgiving winter. This is where moisture-wicking comes in. When wearing these base layers, the fibers pull sweat away from your body, and the moisture evaporates into the air. This technology primarily relies on a process called capillary action. Capillary action is the tendency of a liquid to flow in narrow spaces such as clothing fibers. The key here is that this can be done with or without gravity.
How does capillary action work on a molecular level?
Adhesion, cohesion, and surface tension are our three helpers here. Adhesion describes how well liquid molecules stick to solids. Cohesion is when liquid molecules stick together, and surface tension is what keeps this liquid column intact – it is the interface between the liquid and the air. Capillary action is present in other aspects of our daily life. Remember seeing those commercials on TV about how well certain paper towel brands could clean up messes over their competitors? That is thanks to capillary action. This is the primary technology behind moisture-wicking clothing, and most thermal clothing. Instead of paper towels, it is the yarns and fibers forming microscopic channels for moisture (our sweat) to move away from our skin.
It is useful to think of this technology akin to that of a suction system, rather than a sponge. As a result, the structure and fiber thickness of certain yarn materials play a substantial role in effective moisture-wicking. Having breathable fabric structures are preferred when developing these articles of clothing as this allows for longer capillary “highways,” which, in turn, result in more movement of water (sweat) away from your body and into the air. It is in this stage of the fiber-design process where brands attempt to engineer the “best” moisture-wicking product.
So what fibers are the best at moisture-wicking? Synthetic ones. Fibers such as polyester and nylon are naturally hydrophobic, meaning they do not like to absorb water. Because they absorb almost no moisture, our sweat has no choice but to travel straight into the established capillary pathways and evaporate. While a great technology for keeping us warm, these synthetic fabrics do have their downsides. Hydrophobic fibers tend to hold onto skin oils that are beneficial for odor-causing bacteria. Additionally, these fibers, when washed, may shed microplastics. However, unless you are exposed to large amounts of polyester or live near a landfill – you are unlikely to experience significant health impacts from these microplastics.
Interestingly, wool, a natural fiber, is also considered to be moisture-wicking. While it is not fully hydrophobic, meaning it absorbs a small amount of liquid, it is still able to wick out the moisture through small capillaries present in the fibers.
Moisture-Wicking Isn’t Everything: HEATTECH
As I mentioned earlier, my brand of choice is Uniqlo’s HEATTECH line. I figured it would only be right to delve into the brand’s specific technology to investigate what’s really going on under my winter puffer.
So, how does HEATTECH work? HEATTECH is comprised of four fibers, which are knitted together in a complex structure: rayon, acrylic, polyester, and polyurethane. Toray, the Japanese chemical engineering company that has partnered with Uniqlo on this project emphasizes not just the important moisture-wicking properties of their technology, but also of their heat generation technology.
In HEATTECH clothing, rayon fibers are hygroscopic, meaning they attract water molecules. When water molecules bind to a surface, they release energy that they were previously using to move about. This release is in the form of heat. While the amount of heat released by this process is small, it happens continuously while moisture is being absorbed. The effect is noticeable.
The fine acrylic fibers contain tiny pockets that then trap the generated warm air, providing a protective barrier between our skin and the great outdoors. Polyester wicks moisture away through is fiber gaps and capillary action, while the polyurethane adds the iconic stretch.
While I am a proponent for HEATTECH, my obsession mainly lies in how fashionable the product is. The bottom line? When shopping for thermal layers, products labeled as moisture-wicking are sure to be your friend – and make the best base layers! So don’t sweat the technology, and shop smart.
Eva Kellner is a recent graduate from the Faculty of Arts and Science, with a major in Environment. Her research interests include urban green spaces, urban agriculture, and outdoor community spaces - all as promoters of climate resilience among city-dwellers.
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