November 1, 2005

Bumpers with more bounce

Automobile bumpers that deform and recover rather than crack and splinter may be possible if you can embed tiny functionalized rubbery particles in their plastic matrices.

"Plastics such as polypropylene, nylon, polycarbonate, epoxy resins, and other compounds are brittle and fracture easily," said T.C. Chung, Penn State University professor of materials science and engineering. "Usually, manufacturers take rubbery compounds and just mix them with the plastic, but there are many issues with this approach."

The problems include difficulty in controlling the mixing of the components and adhesion between plastic and rubber.

Chung and Usama F. Kandil, postdoctoral researcher in materials science and engineering, looked at another way to embed rubbery particles into a plastic matrix.

Researchers used polyolefin ethylene-based elastomer, a very inexpensive stable rubber that withstands exposure to ultraviolet radiation. Manufacturers use this rubber as the sidewall in automobile tires. Rather than simply produce micro particles of polyolefin, Chung and Kandil produced a core-shell particle structure with a tangle of polymerized polyolefin rubber forming a ball with functionalized groups hanging out like bristles.

"These functional groups can combine with the plastic and improve the adhesion of the rubber with the plastic," Chung said.

The rubber particles embedded in other materials absorb some of the energy of impact. Rather than the brittle portion breaking on impact, the rubber parts deform and absorb the energy without breaking. Chung and Kandil said if they can introduce the rubber particles into other materials, such as ceramics, the rubber would function in the same way, making resilient ceramics. Plastics and rubber are polymers but have one significant difference. Plastics have relatively high glass transition temperatures, the temperature at which the materials cease being pliable and become brittle like glass. Rubber, especially polyolefin, has very low glass transition temperatures.

"Tires never freeze above glass transition temperature," Chung said. "So the material is always in a pliable state at ambient temperatures. This can improve the toughness of any material."