December 5, 2007

NC State Engineers Develop Nano-Scale Fabric Modification Technique

Transmission electron microscope cross-sectional image of cotton fiber coated with 200 cycles of Al2 O3 ALD demonstrates uniform, conformal coating of fiber surface. (Photo: courtesy of Dr. Kevin Hyde)

Imagine a simple cotton shirt. Now imagine it stronger, antibacterial, antimicrobial, waterproof.

According to Dr. G. Kevin Hyde, a postdoctoral research associate in the Department of Chemical and Biomolecular Engineering at North Carolina State University, these and many more practical qualities can be added to fibers and fabrics without changing the look and feel of the original materials.

Hyde has been working with a team of researchers at NC State to show that the innovative application of atomic layer deposition (ALD) to alter the properties of fibers now can have a level of precision and control previously unknown in research or industry. With the results of their work published in Langmuir in August, Hyde and his group have shown that, with this technique, the surface properties of fabrics can be custom tailored to fit an ever-increasing range of practical applications.

The roots of the project stretch back to fall 2005 when Hyde began his doctoral project on a new type of surface modification and was introduced to Dr. Gregory N. Parsons, professor of Chemical and Biomolecular Engineering at NC State. Hyde, who finished his Ph.D. in fiber and polymer science in the Department of Textile Engineering, Chemistry, and Science at NC State in August 2007, said he was interested in researching new techniques for modifying fabric finishing.

“We're looking into new ways to finish these fabrics and fibers, looking for what new properties we can give them,” said Hyde. “With ALD we can modify these fibers on an atomic scale — sub-nanometer. It's a scale not used very much in the textile industry. But we can impart some unique properties to these materials with this coating, and we can do it with substances usually not considered, such as metal oxides and metal nitrides.”

Atomic layer deposition is a technique derived and modified from chemical vapor deposition. The process involves a precursor and a reactant, and the reaction usually occurs in a low-pressure vacuum. It can be done at a range of temperatures, generally aiming for the lowest temperatures possible for more energy efficiency. It has even been done, Hyde said, as low as room temperature.

The process is fairly straightforward, and the emphasis is on control. A swatch of fabric is loaded into the reactor — in this case, a reactor Hyde and his colleagues built specifically for this purpose — and then a precursor gas is pumped in. For the initial demonstrations, Hyde's group used cotton as a substrate and trimethyl aluminum gas as a precursor.

“The neat thing about the ALD process is that it is a self-saturating reaction,” Hyde said. “It's a cyclic process. The trimethyl aluminum only has so many sites where it can react, and once those sites are filled the reaction stops.” After this cycle, there is an atomic layer on the surface of the material. Then the reaction chamber is purged of all the excess precursor, using Argon gas in this demonstration, and water vapor is pumped in. This reacts with the methyl groups from the trimethyl aluminum and turns that atomic layer on the cotton into aluminum oxide.

“Traditional textile processes use wet chemicals and a lot of energy,” said Hyde, “they tend to be environmentally unfriendly with lots of waste — lots of water waste.”

But because reactants in ALD are delivered to the surface as a gas, there is complete control over the volume of precursors and reactants. Excess and waste can be easily minimized, and the byproducts are all non-dangerous.

After each cycle there is an atomic layer of aluminum oxide, uniform across the material and of predictable and controllable thickness, and it is this level of control that makes the technique so useful to researchers and industries. It's a level of precision not offered by any other textile finishing techniques.

According to Hyde, the technology this group uses already exists and is used in many industries — just not on fabrics and fibers, and not with this level of control. ALD is most commonly used on silicon chips, but silicon is planar and simple to work with. Fabrics, however, are made up of three-dimensional fibers with very complex surfaces.

“With these fibers,” Hyde said, “very high surface area structures can be fabricated. Our specialty is the process of treating these complex surfaces.”

For the first published experiment, Hyde and his group used cotton because it is “still the most-used material in the world, textile-wise,” and because it is a renewable resource. He also explained that “aluminum oxide is probably the most well-known ALD process,” so the group opted for a substance familiar to industry to showcase the technique.

The group is currently expanding their research to work with other chemicals and materials, looking into even more practical applications for this technique. These thin layers of inorganic materials can add a variety of new properties to surfaces.

For example, while cotton is known for its absorption, applying a substance such as aluminum oxide completely changes the surface energy of fibers. Hydrophilic cotton becomes hydrophobic on the surface, so that water droplets will roll right off. While this technique probably isn't the most cost-effective way to waterproof T-shirts, the team is working with many other substances to alter and improve the properties of fabrics.

There are photocatalytic materials, substances activated by UV light, which can be applied to fibers and increase the active surface area, opening up possibilities for fabric-based catalytic mantles. This finishing technique can apply substances that improve the fabric's physical strength, actual breaking strength, durability, conductivity and much more. Because the additions are made on the nano-scale, certain surface properties can be changed without necessarily altering the original properties of the fabrics, such as the softness and breathability of cotton.

“It doesn't change the feel,” Hyde said. “It doesn't cause a lot of stiffening of the fibers, so it still feels natural.” And aluminum oxide is transparent, so it doesn't even alter the color.

“This project really brought together two very different research areas,” Hyde said, bringing researchers from textiles and chemical engineering together in the group. “Now everyone in the group is interested in doing work on fibers and using them, adapting them, for new applications. It's really opened up a wide range of research projects.”

— anselm —

For more information on the team's research, read:

Atomic Layer Deposition of Conformal Inorganic Nanoscale Coatings on Three-Dimensional Natural Fiber Systems: Effect of Surface Topology on Film Growth Characteristics.
Hyde, G.K., Park, K.J., Stewart, S.M., Hinestroza, J.P., and Parsons, G.N.
Langmuir, 23, 19, 9844 - 9849, 2007, 10.1021/la701449t

http://pubs.acs.org/cgi-bin/article.cgi/langd5/2007/23/i19/html/la701449t.html

Technical Contact:
Dr. G. Kevin Hyde, gkhyde@unity.ncsu.edu

Media Contact:
Kathi McBlief, 919-515-2283, kathi_mcblief@ncsu.edu

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