Maintaining Water Repellency with an Inverted Structure! Toray's Advanced Material Powering the "SAKASANANO" Umbrella

Umbrella brand Waterfront, celebrating its 40th anniversary, has launched a new product called SAKASANANO, aimed at solving long-standing issues with umbrellas by leveraging its years of expertise. Although priced at a high 14,300 yen (tax included) for an umbrella, the first batch sold out in roughly two weeks after release.

Combining Toray's advanced NANODESIGN® technology with an inverted folding structure that keeps the wet side on the inside, how does this umbrella achieve and maintain its impressive water repellency? We spoke with Yuki Oshima from Waterfront Co., Ltd. to find out.

Challenging the Revival of "Ultra-Water-Repellent Umbrellas" Vanishing from the Market

In fact, umbrellas boasting high water repellency have reportedly been disappearing from the market in recent years. This trend is driven by tightening global regulations on per- and polyfluoroalkyl substances (PFAS) due to concerns over environmental damage and health risks.

"In the past, water-repellent treatments mostly involved coating fabric with fluorine. Following the regulations, various manufacturers struggled to develop new water-repellent materials. As a result, while products with standard water repellency still exist today, fewer items claim the ultra-high water repellency of the past."

On the other hand, with torrential downpours becoming increasingly common, the demand for water-repellent umbrellas felt higher than ever. Could they create an umbrella with top-tier water repellency without relying on conventional fluorine-based treatments? That question sparked the development.

A Breakthrough Powered by Toray's Nanotechnology

Traditional fluorine-based treatments achieved water repellency by creating microscopic bumps on the fabric surface with fluorine, supporting water droplets at minimal contact points.

"So initially, we explored whether a similar effect could be achieved through a different post-processing method on the fabric. However, after repeated prototyping, we realized it was extremely difficult."

That was when they encountered NANODESIGN®, a technology announced by Toray, a company celebrating its 100th anniversary.

"Instead of a post-treatment, this revolutionary technology designs the yarn itself at the nano level. Traditional synthetic fibers are created by extruding molten raw material through round holes. With NANODESIGN®, however, those holes can be divided and controlled freely at the nano scale. For example, yarns with star-shaped or elliptical cross-sections, or yarns with hollow centers, can be crafted at nano-scale fineness. We realized that by leveraging this technology, we could produce the ultra-water-repellent fabric we were looking for."

The material used in SAKASANANO intricately combines yarns of various thicknesses and cross-sectional shapes to form microscopic surface bumps invisible to the human eye. Rather than relying solely on chemical water repellents, this design minimizes the contact area with water droplets to the absolute limit, allowing them to roll off like beads of water for top-tier performance.

Creating a structure that enhances water repellency in a microscopic world visible only under a microscope—this approach stemmed from a concept entirely different from conventional water-repellent processing.

An "Inverted Structure" to Maintain High Water Repellency Over Time

Have you ever felt your umbrella's water repellency fading over years of use?

"As mentioned earlier, water repellency originally relies on tiny bumps on the fabric's surface. However, as you use the umbrella, oils from your hands transferred when closing it fill in those microscopic grooves, causing performance to drop. Pressure applied when rolling up the umbrella can also crush these tiny ridges, contributing to the loss of water repellency."

The key to solving this issue lay in the "inverted structure" umbrella that Waterfront had been offering for about six to seven years. Folding inward so the wet surface stays on the inside, this structure was previously seen more as a novelty item emphasizing uniqueness rather than performance. However, if hand contact was a major reason for declining water repellency, a structure that allows folding without touching the wet fabric could be the ultimate solution.

By fusing the cutting-edge NANODESIGN® material with this inverted structure, long-lasting ultra-water repellency was born.

"However, since NANODESIGN® wasn't originally a fabric designed for umbrellas, other issues surfaced—most notably, water resistance," says Oshima.

He explains that water repellency and water resistance sound similar, but are actually quite different.

"Water repellency is the ability to repel water droplets, whereas water resistance measures how well the fabric can withstand water trying to seep through from the surface to the underside without letting it penetrate."

In a water resistance test, for example, fabric is clamped between cylinders, and water is applied from above. The test measures how much water pressure the fabric can take before water begins dripping into the lower cylinder. If water resistance is low, heavy rain will force water droplets right through the fabric, causing leaks on the inside.

"The original material had extremely low water resistance, making it totally unusable for umbrellas. So we pushed the thread density to its absolute limit, successfully boosting its water resistance."

Solving Structural Challenges with Years of Expertise

There were structural challenges, too. Increasing the thread density made the fabric unexpectedly thick, which prevented the umbrella from closing completely. To fix this, an internal spring was added to assist with closing force. As a bonus, this modification also enhanced the umbrella's stability when standing on its own.

"Also, because the SAKASANANO uses a dual-layer fabric construction, the two layers tended to stick together when opening and closing. So we added ventilation holes to the inner fabric layer."

Determining the placement and size of these holes sparked a millimeter-by-millimeter negotiation with product designer Yota Kakuda.

"From a design perspective, Kakuda wanted as few holes as possible. But for quality control, a certain number was essential. We checked and adjusted by mere millimeters over and over again to find the sweet spot where design and functionality could coexist."

Bonus Benefits of the Inverted Design

While the inverted structure was initially adopted to preserve water repellency, keeping the wet surface on the inside offers plenty of practical perks.

"For instance, if you hate getting your car interior wet when getting in, or if you dislike getting your clothes wet when holding the umbrella between your legs on a train, this umbrella will eliminate that daily frustration."

Another plus is that it stands on its own, so finding a place to put it down is never a hassle.

Furthermore, thanks to the dual-layer structure, the outer fabric is designed to be detachable and replaceable. No matter how superior a water-repellent fabric is, degradation from UV rays and dirt over time is inevitable. With the SAKASANANO, however, you can simply replace the outer fabric with a new one—allowing you to keep using the same umbrella for years while maintaining top-notch water repellency. Waterfront refers to this concept as "an umbrella you raise over time."

"Instead of a throwaway cycle where you toss the umbrella as soon as it breaks or loses its repellency, we wanted people to restore its performance and enjoy using it over a long period. In the future, if even higher-performance fabrics are developed, users can simply swap out the outer layer. We're also planning to expand our lineup of design variations."

Pro Tips to Restore Your Umbrella's Water Repellency

While SAKASANANO is engineered to stay water-repellent for a long time, there's actually a trick to restoring water repellency on any umbrella you already own.

"Water repellency drops when the microscopic bumps on the fabric surface wear down. To restore it, open the umbrella and rinse off surface dirt with a warm shower at 38–40°C (100–104°F). After air-drying it in the shade, hold a hairdryer 3–5 cm away from the fabric to apply heat, or gently press it with an iron set to medium heat on the dry setting. Using steam will make the fabric absorb moisture, so keeping it completely dry is key."

Because rainwater carries various pollutants and debris, thoroughly drying off any moisture after use is equally important.

"When shaking off excess water, close the umbrella, point the tip downward, hold the handle, and tap it lightly toward the ground. This lets you easily shake off water droplets without taking up space or splashing people around you."

With just a little daily care, you can keep your umbrella repelling water for much longer.

By directly tackling the common flaws of conventional umbrellas, the SAKASANANO has achieved sales far beyond expectations. Rethinking not just the material but the underlying structure itself, it stands as a product that completely redesigns umbrella usability from the ground up.

Text by Junko Hayashida