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Tiny bubbles help plastics take a pounding.

If you have ever sat in a canoe as it snagged on a rock and split open, you know the sinking feeling of discovering that a material couldn't quite do its job, Now, research into the mechanism of fracturing promises to aid engineers in their quest to balance weight, cost, toughness and flexibility for longer-lasting products -- including, perhaps, indestructible boats.

Manufacturers usually assess the toughness of a compound by measuring the amount of energy needed to break a sample in half. But a plastic's toughness also depends on how it reacts to stresses leading up to the fracture, and engineers should consider this in designing new products, assert researchers from the University of Rochester (N.Y.) in the June JOURNAL OF MATERIALS RESEARCH.

James C.M. Li and two graduate students studied acrylonitrile-butadiene styrene (ABS), a plastic commonly used in canoes, recreational vehicles, bathtubs, pipes and even TV cabinets. Companies tailor ABS for its various uses by adjusting the amount, size and structure of the particles of butadiene, a rubber mixed in to make ABS less brittle.

The Rochester group's findings about prefacture stress should help materials scientists achieve a more reasonable trade-off between toughness and other chaacteristics, says David E. Henton, an engineer with Dow Chemical Co. in Midland, Mich., which helped fund the work.

When the researchers analyzed what makes microscopic cracks worsen, they found that ABS materials vary in the amount of stress they absorb before they break. "You must put energy into it before the crack starts to go," Li explains.

ABS polymers can take up energy in any of three ways. Microscopic bubbles may form in the rubber particles. Sometimes the polymer molecules shear, slipping slightly away from those above and below. Also, a network of very fine cracks, called crazes, can develop, says Li.

He and his co-workers discovered that bubbles reduce stresses most effectively and that ABS polymrs with larger rubber partcles form more bubbles. This finding will aid in improving ABS because "you want to maximize the efficiency of the rubber phase," says Henton, who notes that rubber is the material's most expensive component.

Royalte Thermoplastics in Mishawaka, Ind., which makes ABS polymer for canoes, takes a different approach. Rather than anter the rubber component.

Royalite Thermoplastics in Mishawaka, Ind., which makes ABS polymer for canoes, takes a different approach. Rather than alter the rubber component, the company sandwiches an ABS foam between two thin ABS layers, creating a material with more rubber per unit weight than a single thick layer would have, says Victor W. Lee, a technical manager with Royalite.

The resulting material satisfies most customers, according to canoe manufacturers. Nevertheless, "we keep telling them we want it stronger, stiffer and cheaper," says Wendall Easler, vice president of operations for Old Town Canoe in Old Town, Maine.

With the new report, says Henton, "one begins to better understand the critical interplay between these molecular variables." This knowledge, he says, might one day lead to a tougher ABS canoe with less rubber and perhaps a lower cost.
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Title Annotation:materials research into the mechanism of fracturing
Author:Pennisi, Elizabeth
Publication:Science News
Date:Jun 22, 1991
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