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Wettable latex makes for drier surroundings.

Diapers absorb urine, but they don't hold it very well. To keep babies' bottoms drier, one needs a material that likes being wetter.

Disposable diapers, like many consumer products, contain elastomer - a stretchable material made of chain-like molecules called polymers. Elastomers make room for urine as then baby wets. But because elastomer surface lack the energy to hold on to water, droplets bead up and spill off, and the diaper leaks.

Adding soap can lower the surface tension so that the drops stay on longer, but eventually the soap washes off. Thus, most elastomers are swellable but not "wettable," says Isao Noda, a polymer scientist with Procter & Gamble in Cinnati.

A new latex film promises to fix that, he says. In the March 14 Nature, Noda describes an elastomer that any water molecule can love. Unlike other rubber materials, its surface attracts water. So instead of rolling off, the droplets spread flat and stay put.

"Up to now, you've never had that property before [in rubber]," says Carl C. Gryte, a chemical engineer at Columbia University in New York City.

The secret lies in a hybrid molecule known as a block copolymer, which Noda mixes with latex particles when making his elastomer. This hybrid consists of two molecular chains attached at their tail ends. One, called the polar chain, sticks out of the finished elastomer film and contains lots of oxygen, which attracts water molecules. The other chain branches as the elastomer forms, entwining itself with the latex particles like an overgrown root.

Usually, such arrangements don't last because the molecules in rubber move around all the time and "the polar groups get swallowed up like quicksand," Noda explains. As a result, the water-loving property eventually disappears. Noda suspects his block copolymer molecule may be so large that the rubber can't swallow it. "It takes a tremendous amount of work to push this huge molecule inside the rubber matrix," he says.

The new material's water-loving property seems permanent. Noda reports washing the elastomer continuously for a week without rinsing off the polar surface, heating the material to 140 [degrees]F. and keeping it in dry air, all with no ill effects.

If placed in a diaper's absorbert layer, he says, the wettable film will suck in urine "almost like a pump."

Its potential applications reach well beyond the nursery, observes Mohamed El-Aasser, director of Lehigh University's Center for Polymer Science and Engineering in Bethlehem, Pa. Noda's invention may enable scientists to gain more precise control over the strength and placement of water-loving properties, El-Aasser suggests. And with the ability to control which parts of a material get wet and which stay dry, all sorts of product improvements become possible. Looking to the future, the and others envision rubber surfaces that hold paints or inks more tightly, fabricas that "breathe" better and medical devices more compatible with body fluids.
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Title Annotation:a new water-absorbing elastomer for use in diapers and other products
Author:Pennisi, Elizabeth
Publication:Science News
Date:Mar 16, 1991
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