Making Fibers Softer and More Absorbent - using supercritical carbon dioxide - Brief Article

USA Today (Society for the Advancement of Education), June, 1999

Researchers at Ohio State University, Columbus, have found that a supercritical fluid form of carbon dioxide could deposit a chemical additive into a sheet of polymer fibers to make the material more absorbent. Supercritical carbon dioxide is composed of [CO.sub.2] molecules heated under pressure and held in a state somewhere between a liquid and a gas. The supercritical carbon dioxide carries additives deep into materials like cloth, plastic, and paper, just as hazardous organic solvents do, but without diminishing the material's appearance or strength and without creating hazardous by-products.

The researchers drove supercritical carbon dioxide into a polymer, a material in which the stringy molecules wrap around each other like a tangled rope. The carbon dioxide molecules wedged between the strings, swelling the polymer matrix. Once the polymer swelled, other molecules could enter it much more easily through the enlarged holes between the strings.

They then added molecules of detergent to the swollen polymer to make it absorb water better. After they removed the supercritical carbon dioxide and the polymer shrunk back down to its original size, the detergent molecules remained trapped inside the polymer. "We didn't change the chemical nature of the polymer, and we didn't change its strength," explains David Tomasko, assistant professor of chemical engineering. "We changed the surface properties by adding the detergent. Now, instead of beading up on the surface of the polymer, water molecules spread out and wet the surface."

In the future, this technique could make polymer fibers pick up moisture faster and more easily in diapers and feminine hygiene products. A polymer treated with detergent and supercritical carbon dioxide is stronger, lighter, and less bulky than cellulose, the traditional absorbent material in these products. A different type of additive could make facial tissues softer.

"A really good example of the effects of liquid solvent on a material is what happens when you get a piece of paper wet. Water penetrates the fibers of the paper, so the paper swells a little bit. And when the water evaporates out of the paper, it pulls those fibers and twists them, so the paper crinkles. A supercritical fluid wouldn't do that," notes Tomasko.

Now Tomasko wants to find out how fast the process works. He and the other researchers are constructing a device that will contain the polymer at high pressure and time how long it takes for the supercritical carbon dioxide and additive to penetrate the material.