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by J. Michael Quante, AATCC

"Research is to see what everybody else has seen, and to think what nobody else has thought."
-Albert Szent-Györgi

Last month, we explored the world of naturally-occurring materials, such as plant leaves and bird feathers, that keep themselves clean by allowing water to bead and easily roll off of the surface. These materials provided scientists with models useful for developing self-cleaning textiles that mimic their counterparts in nature.

Studies of these systems revealed the various properties that contribute to the phenomenon of very strong water-repellency or superhydrophobicity (sometimes referred to as ultrahydrophobicity or UH). These were high contact angle (greater than 150°), low roll-off angle (less than 5°), fabric roughness, and low surface energy. Hoon Joo Lee at the College of Textiles, North Carolina State University (NCSU) has developed superhydrophobic textile fabrics having contact angles as large as 178° (the theoretical maximum is 180°). Let's take a look at the science behind designing and creating our own self-cleaning fabric.

Do the Math...

The properties of self-cleaning fabrics are understood well enough to be modeled mathematically. The material/fabric is hydrophobic (or made hydrophobic) and some degree of roughness can be introduced to increase the contact angle.

Two things must be known to determine whether a fabric is self-cleaning: the contact angle—the angle formed where a liquid/vapor (or liquid/air) interface meets a solid surface—of the water droplet on the surface and the surface tensions of the liquid droplet and fabric. The contact angle can be determined by experiment. The surface tension is simply the amount of energy needed to disrupt the surface of the material. The relationship between them is described by Young's equation (Eq. 1):

θ is the contact angle and γ are the solid-air (S), solid-liquid (SL), and liquid-air (L) surface tension values. Eq. 1, however, is not appropriate for textiles because it only describes the behavior of flat surfaces. Flat surfaces can be made hydrophobic, but the maximum contact angle achievable is 119°, smaller than the 150° angle needed for a self-cleaning fabric.

Roughness is the critical missing factor needed to achieve self-cleaning (superhydrophobic) capability. Two equations relate θ obtained from Young's equation with an apparent contact angle of a water droplet on a rough surface having low surface energy. These provide models for self-cleaning behavior on textiles. The Wenzel approach (Eq. 2) describes the liquid interaction with the surface as filling the surface air pockets with the liquid (Fig. 1a):

r is the roughness. Later work revealed a more general relationship that can describe the system as a liquid drop resting on the air-fabric surface (Fig. 1b). It is known as the Cassie-Baxter approach (Eq. 3):

f₁ is the surface area of the liquid contacting the solid divided by the projected area. f₂ is the surface area of the liquid contacting air divided by the projected area. Compare Figs. 1a and 1b to see how the models provide different interpretations of the solid-liquid-air interactions.

Superhydrophobicity model

According to Lee, Eq. 3 offers the best general model for textiles, although both the Wenzel and Cassie-Baxter approaches are useful to researchers. The surface roughness can be modeled mathematically and the results plugged into these equations to give an indication of the material's self-cleaning ability when it is actually prepared. For more details, see the references at the end of this article.

...and Then the Chemistry

For a textile material to be self-cleaning, it must be made hydrophobic so that the contact angle is greater than 90° prior to roughening or flocking. This is where chemistry plays a most important role. For example, a hydrophillic substrate can be made hydrophobic by grafting its reactive groups (e.g., hydroxyl) to long-chain fluorohydrocarbons or fluoroamines. If the fiber does not contain a sufficient number of reactive groups (e.g., nylon), the reactive groups can be created by chemical means prior to treatment with the hydrophobic reagent. Finishing agents are available that impart self-cleaning properties.

One of the first companies to incorporate UH finishes into consumer products was Nano-tex. Today, many clothing manufacturers use its technology for imparting water/oil repellency and stain resistance to fabrics.

Because of environmental problems with the use of long-chain fluorohydrocarbons, alternative reagents such as alkylamines are being studied and some short-chain fluorinated hydrocarbons or amines may be substituted. Scholler Technologies AG has developed C6 (short-chain) fluorocarbon technology in its NanoSphere finish to replace the C8 (long-chain) product. The new product is free of biopersistent PFOS (perfluorooctane sulfonate) and PFOA (perfluorooctanoic acid) contaminants. The newly-formulated product also meets bluesign standard specifications. According to Deborah Richert, Scholler sales manager, North America, the new NanoSphere product "improves the performance in abrasion resistance and oil repellency. Improvement was also seen in soil and water repellency."

This technology will benefit textile products for workwear, outdoor use, and military clothing. Lee leads a research team developing ultraoleophobic, self-decontaminating textile fabric that applies superhydrophobic technology. The resulting fabric can be used in military wear such as battle dresssed uniforms (BDU), providing protection against chemical/biological (CB) warfare agents. "Someday, this cutting-edge technology will make our everyday life easier," says Lee.

As the technology matures, look for more self-cleaning textile products to appear in the marketplace. And, who knows, easy clothes laundering may soon be just a rain shower away!

References

Lee, H. J. and S. Michielsen, Journal of the Textile Institute, Vol. 95, No. 5, 2006, pp455-462.

Lee, Hoon Joo and Stephen Michielsen, Journal of Polymer Science: Part B: Polymer Physics, Vol. 45, 2007, pp253-261.

Michielsen, Stephen and Hoon J. Lee, Langmuir, Vol. 23, 2007, pp6004-6010.

Note: Lotus leaf computer graphic on newsletter cover attributed to William Thielicke and its use licensed under GNU and Wikimedia Commons: http://en.wikipedia.org/wiki/Image:Lotus3.jpg.