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August 1999 - High technology is weaving its way into the textile industry, now that researchers have devised a method for reinforcing thread that exploits the strange properties of a curious state of matter known as a supercritical fluid.

Chemist Alan Propp and engineer Mark Argyle are the first to use a supercritical fluid--a high pressure hybrid of liquid and gas--in a continuous manufacturing process. For their ingenuity, the researchers from the U.S. Department of Energy's Idaho National Engineering and Environmental Laboratory have won an R&D100 Award. The award, presented by R&D Magazine, recognizes Propp and Argyle's innovation as one of the most important of the preceding year.

Inventors Alan Propp and Mark Argyle.

Thread for weaving is coated with "size"--a substance such as starch or polyvinyl alcohol--to give it the strength it needs to stand up to the rigors of the loom. Traditionally, thread for weaving, properly known as yarn, runs through a mixture of size and water in a process called slashing. Slashing is an essential step in the production of textiles, for without size yarns catch and break when they rub against one another during weaving.

Propp and Argyle apply size with supercritical carbon dioxide instead of water. They have developed a method in which a single yarn runs through a pressure chamber roughly two feet long. As the yarn courses through the heart of the chamber, supercritical carbon dioxide and size swirl around it, coating it evenly and gluing down stray fibers. The technique, dubbed the Supercritical Fluid Slashing System, eliminates secondary waste, saves energy, and dramatically reduces the amount of equipment needed.

The new technology was born of the spirited chemist's passion for thermodynamics and the soft-spoken engineer's appetite for challenges. "Alan and I had pretty good synergy," says Argyle, whose surname could only be more fitting if Argyle material were woven, not knit

Warp speed

If you think you're overworked, be thankful you're not a yarn. In the process of being woven into cloth, yarns gets pulled and rubbed and tamped over and over again.

Photomicrograph showing an uncoated thread (top) and a coated thread (bottom).

On the loom, thousands of parallel yarns stretch out to form a vast sheet. Known as the warp, these yarns run through a series of harnesses, called heddles, that raise and lower the yarns and control the pattern of the fabric. Several heddles will rise, creating a gap between the yarns they control and the rest of the warp. The filling yarn, or weft, passes through this gap. The raised heddles then fall, closing the gap in the warp, and a comb called a reed comes forward to pack the trapped filling yarn into place. The process continues as other heddles rise. Modern looms operate at dizzying speeds. The heddles shift up and down as many as a dozen times each second, and the reed matches the pace.

The warp yarns rub repeatedly against the weft, one another, and against the reed. All of this rubbing can overwhelm a freshly spun yarn — a fuzzy filament with little strength and a penchant for entangling with its equally fuzzy brethren. Unfortified, the warp yarns snarl and break, bringing the weaving process to a halt.

Size helps prevent such problems by making warp yarns stronger. (Usually, the weft yarns are not sized.) Size also glues down stray fibers, making yarns smoother. Manufacturers wash most of the coating away once the fabric is off the loom.

The old-fashioned way

Conventional slashing technology employs a soak, squeeze, and steam strategy. Thousands of yarns roll off long spools known as section beams, dip into a vat of aqueous starch or polyvinyl alcohol, and emerge wet with size. To dry, the yarns run through a wringer and across a series of large steam-heated drums. The finished yarns roll onto a single long spool, called the loom beam, on which they are transported to the loom.

A conventional slasher at work.
(Photo courtesy of A. D. Cotney, West Point Foundry and Machine Company, West Point, GA)

Little changed for a century. Conventional slashing technology leaves something to be desired, or at least something to be disposed of. The process produces large quantities of water laced with size. This secondary waste — so called because it is a by-product of the process — needs to be treated because, released raw into streams or lakes, it causes bacteria to proliferate and gobble up oxygen, triggering environmental problems. Standard slashing also consumes considerable energy and contributes to air pollution, since steam for the drying drums usually comes from coal or oil-burning boilers.

Conventional slashing doesn't always apply size evenly and can even break the yarns it is supposed to reinforce. Yarns tend to stick together as they dry, so they run through a series of metal rods, known as leasing rods, and a large comb to separate them as they roll off the drying cans. The leasing rods and the comb can rub away the size, leaving yarns unevenly coated, and can snag and snap the yarns before they make it to the loom.

A supercritical improvement

Propp and Argyle may have solved many nagging problems in one fell swoop. They have devised a method for applying size to individual yarns that coats evenly, leaves no excess sizing solution, and saves energy and space by eliminating drying drums and other components of the traditional slashing system. To achieve such improvements, the INEEL researchers replaced the water used to dissolve the size with a high-tech, eco-friendly solvent known as supercritical carbon dioxide.

A supercritical fluid is a subtle state of matter that blurs the distinction between a liquid and a gas. At low pressure, a liquid that gets sufficiently hot will boil to produce a gas. For example, at atmospheric pressure water boils to make steam at 212 degrees Fahrenheit. Above a certain pressure and temperature, however, the distinction between liquid and gas disappears, and a substance enters a state known as a supercritical fluid. Water becomes a supercritical fluid at a pressure roughly 220 times greater than atmospheric pressure.

The supercritical fluid state bridges the divide between the liquid and gaseous states. If a supercritical fluid is cooled while pressure is maintained, it will become a liquid without condensing. If the pressure on a supercritical fluid is lowered while the temperature is maintained, it will become a gas without boiling.

A supercritical fluid is dense like a liquid, but has low viscosity and surface tension like a gas. Roughly speaking, viscosity is the runniness of a fluid: motor oil and syrup have higher viscosity than water. Surface tension is the tendency of a fluid to stick to itself: some insects are able to walk on water because the surface tension of the liquid forms a boundary layer just strong enough to hold them up. Supercritical fluids make good solvents. Because they are dense, they can take up hefty doses of solute. Because they have low viscosity and surface tension, they can get into tiny pores and crevasses to deposit or extract their cargo.

Also, supercritical carbon dioxide, the fluid used by Propp and Argyle, reverts to its gaseous state once the pressure on it is released. So at the end of an industrial process, supercritical carbon dioxide leaves behind no secondary waste, only a clean gas that can be recovered and reused.

The prototype supercritical slashing device is between two and three feet long and fits on a laboratory benchtop.

Supercritical fluids are already in use in a few industries. For instance, supercritical carbon dioxide is used to extract caffeine from coffee. However, until now, all applications of supercritical fluid technology have involved pressure vessels called autoclaves, which must be loaded and unloaded batch by batch. Because autoclaves are expensive and batch processes are slow and labor-intensive, supercritical technology has been reserved primarily for products with high unit value, like your eight-dollar-per-pound Javan decaf.

That's changing with Propp and Argyle's innovation. Their device coats yard after yard of yarn--a product of very low unit value — without so much as pausing to catch its breath. "We have shown that we can operate a supercritical fluid system in a continuous mode, as opposed to a batch mode," Propp says. "And that opens up the potential for all sorts of applications."

The essential twist

An expert on supercritical fluids, Propp saw an opportunity to apply his knowledge when a Department of Energy initiative to improve U.S. economic competitiveness targeted the textile industry. Argyle joined in, and the two attended a series of meetings of various textile industry associations, including a January 1995 workshop sponsored by the American Association of Textile Chemists and Colorists. "The device was generated after that conference," Argyle says. "We brainstormed in the car on the way back to the hotel."

From the beginning, the two knew they would have to go beyond the conventional batch-by-batch method of supercritical process. "It was obvious that we had to find a way to do it continuously or we were dead in the water," Propp says.

At the center of the Supercritical Fluid Slashing System, size enters a Venturi tube at an angle.

Putting their idea into practice involved several challenges. First, Propp and Argyle had to figure out how to run a yarn in one side of a high pressure chamber and out the other without squirting carbon dioxide and size through the inlet and outlet and spraying the entire room. To accomplish this, they employed a series of baffled compartments leading up to and away from the central high pressure cavity, each compartment pressurized slightly less than the next one closer to the center of the device.

Next, Propp and Argyle had to prove that supercritical carbon dioxide would hold enough size to coat the yarn. This presented the two with a challenge, since the solubility of polyvinyl alcohol in carbon dioxide is supposed to be low. Argyle ran a series of tests with a single length of yarn held stationary in their device. Early one morning, after several weeks of effort, he got the size and the supercritical fluid to cooperate. "I stayed all through the night," he says, "and on the last run it finally worked."

Finally, the researchers had to show they could run yarn through the device at high speed, coating it evenly with size without shredding it. They realized they could accomplish this task better if they employed a Venturi tube — a flared cylindrical tube that looks something like the horns of two trumpets joined by their narrow ends — as the heart of the device.

Inside the Venturi tube, supercritical carbon dioxide and size swirl around the yarn.

The pressure in a fluid drops as the fluid passes through the constriction in the middle of a Venturi tube. Propp and Argyle realized they could use this pressure drop to encourage the size to drop out of the supercritical carbon dioxide and on to the yarn, giving them better control over the coating. Moreover, they realized that if the stream of size entered the Venturi at an angle, it would swirl about the yarn, coating it evenly and gluing down stray fibers.

The next step

Having proved the concept, Propp and Argyle hope to license and fully develop their technique. A sizing system that can reduce waste, save money, and maintain weaving efficiency should attract the attention of textile manufacturers, says Don Cotney, president of West Point Foundry and Machine Co., which makes sizing and warping equipment. "Textile manufacturers are always looking for better production and techniques to improve weaving efficiency," he says.

However, Cotney says to convince manufacturers to retool, Propp and Argyle will first have to show that the Supercritical Fluid Slashing System can match the output of conventional slashing rigs, which size as many as 15,000 yarns at a time. He also warns the two researchers may encounter a certain amount of inertia in an industry in which "change is more evolutionary than revolutionary."

Propp and Argyle see no problem in running many pressure chambers in parallel to process multiple yarns. Moreover, they have already shown they can compete with a conventional slashing machine's output for a single yarn — 100 yards per minute — and believe they can increase their throughput to 300 yards per minute.

And they are open to applying their technique in other settings and industries. "This technology is flexible," Propp says, "so it may find a number of other uses."

The end of our yarn

Propp, a physical chemist, enjoyed thinking about the chemical and physical interactions that make the Supercritical Fluid Slashing System possible. Having worked on subject ranging from the corrosion of construction materials to the incineration of garbage to generate electricity, he characterizes his career as "everything from soup to nuts." But he stresses the common thread that runs through all of his endeavors. "Thermodynamics is my biggy," Propp says unabashedly. "I love thermodynamics."

Argyle, an engineer who has also worked on petroleum plant design, designed distributed control systems, studied methods for stripping metallic coatings, and participated in environmental initiatives, embraced the challenge of developing a basic idea into a fully functioning machine. While striving to put principle into practice, he had to overcome a number of problems, often with little time and no assistance. "To me it was amazing that just when it seemed like I wouldn't get past the next big hurdle, I'd find the solution," he says. "It almost seemed like the project drove itself."

Engineer Mark Argyle tunes the Supercritical Fluid Slashing System.

As for the R&D100 Award, Propp says that his experience with supercritical slashing may help him realize an ambition from his youth. He recalls reading textbooks as a student and admiring the scientists who had managed to get their names associated with an axiom, effect, or process. "I would like to be able to look back eventually and say, yes, this process has my name associated with it," he says. "I think that would be very satisfying."

For Argyle, the award affirms the usefulness of his efforts. "It means that industry recognizes the value of something you worked on, and that feels good," he says. Argyle also hopes the award will speed the further development of the Supercritical Fluid Slashing System. "It gives us a chance to get news of this technology out to the people who might be interested in it," he says. "I hope we might get some people calling."

In September, Propp and Argyle will travel to Chicago to receive their awards. No matter what they wear to the banquet, they will have the nicest threads in the place.