Rayon, acrylic, polyester and polyurethane, four fibers with unique properties, can be blended into a fabric having multiple functions. We’ve made the fabric softer and thinner while keeping the air pocket structure.

Closeups of original weave and today’s weave show a reduction in napping. This yields a smoother texture.

Believe it or not, HEATTECH was conceived as a summer fabric. Debuting in Summer 2003 as men’s innerwear, it was originally a technical base layer made using a new synthetic moisture-wicking, quick-drying fabric from Toray Industries. Entering a market where 100% cotton innerwear was the norm, this synthetic alternative was a ground-breaking success. Pretty soon, we noticed a demand for a warm and soft winter version that wouldn’t feel sticky when you sweat. Thus began our design partnership with Toray Industries.

To ensure a natural fit, the fabric needed to be supple and stretchy, so that you barely noticed that you had it on. The first round of prototypes somehow didn’t feel right. Our curiosity led us to examine an experimental sample that Toray had sent over. The texture was exactly what we had in mind. A blend of micro acrylic and rayon, this fabric is the predecessor of the HEATTECH that we know today.

One of the big problems involved the dyeing process. The fabric blends four different fibers, each of which has a different absorption rate, resulting in a mix of tones. To solve this problem, engineers from Toray and designers from UNIQLO worked together on a variety of trials. Conventionally, creating fibers, weaving the yarn, and dyeing happen during separate processes. To solve this complex problem, we combined these steps in various ways. Through repeated testing we developed dyes and dyeing methods especially for HEATTECH. Our labors soon bore fruit and went into mass production ahead of the 2006 winter season. It took about a year and a half to resolve the color problem and develop scalable technologies.

The four different kinds of fibers of HEATTECH each contribute special characteristics: rayon, which creates heat by absorbing moisture; polyurethane, which has superior stretchability and elasticity; acrylic, which includes more air for space higher heat retention; and quick-drying polyester, for swiftly dispersing moisture. The polyurethane and polyester take the form of individual strands, while the acrylic and rayon are incorporated as a blended fiber that has heat-generating and insulating properties, HEATTECH’s greatest feature. Woven into yarn, these three strands make the fabric comfortable and warm.

So, how does HEATTECH warm itself up? The answer is the moisture that our body is perpetually releasing. The rayon fibers catch the moisture and convert kinetic energy into heat. This creates warm air, which the acrylic keeps close to the skin. HEATTECH uses micro acrylic fibers one-tenth the thickness of a human hair, forming a structure of insulating pockets that holds the warm air where you need it most. It’s the same as how a down jacket holds heat in the air pockets between feathers, only at a microscopic scale, in between strands of yarn on the inside of the fabric. That’s how it can be warm and thin at the same time.

We’re seeing a demand for all kinds of products. That’s why HEATTECH is constantly evolving. Over the past twenty years, the fibers have gotten thinner and more even. Opting for micro acrylic has allowed for increased softness and a thinner fabric while keeping the air pocket structure. Today, we’ve added two additional varieties of “extra warm” and “ultra warm” HEATTECH, for use in a wider variety of situations and environments.