Electron beams rub rubber the right way.

Electron beams rub rubber the right way

Each of the two revolving tracks on a modern military tank consists of 100 or so metal slabs strung together with a rubber pad attached to each slab. The impact-buffering pads, which take a beating beneath the 70-ton behemoths, typically need replacement after 700 miles. "It's the most expensive element in the operation of a tank," says materials scientist Joseph Silverman of the University of Maryland in College Park, who notes that taxpayers shell out $100 million each year to keep U.S. tanks fitted with working pads.

Working on a federal contract to design longer-lasting rubber pads, Silverman says he has come up with a process for making pads that can last for 2,000 miles and can resist environmental wear up to 10 times longer than conventional pads.

His technique combines standard processing steps in non-standard sequences. He begins with commercial, uncured styrene-butadiene rubber (SBR), then uses a heat-and-sulfur treatment to establish enough chemical cross-links between the rubber's polymer molecules to make the material rigid enough to handle. But most of the cross-linking, or curing, takes place when this intermediate material gets blasted with a powerful electron beam. In the thermochemical process, most cross-links develop in the thermochemical step, and the remainder form in a preliminary radiation step.

SBR cured by either method has the same tensile strength, stiffness and number of cross-links. But Silverman's process yields materials that resist tearing at high temperatures and last far longer in laboratory degradation tests. Field tests at an Army proving ground show that pads made of Silverman's rubber last up to three times as long as standard pads.

While potentially reducing tank maintenance costs, the improved rubber could also prove useful for nonmilitary applications such as roofing material, Silverman says.

The surprising properties of Silverman's rubber suggest to him that the standard picture of rubber degradation could use some refining. For example, he says, the standard view might explain that aging windshield wipers stiffen and leave water channels behind their sweeps because environmental exposure increases the amount of cross-linking among the rubber molecules. Silverman speculates that rubber cured by his process might counter the stiffening process with a bond-breaking brehanism, keeping the total number of cross-links the same. No one really knows how rubber goes bad, he adds.