Automobile bumpers that deform and recover rather than crack andsplinter, computer cases that withstand the occasional rough encounter,and resilient coatings that can withstand the ravages of the sun, mayall be possible if tiny functionalized rubbery particles are imbeddedin their plastic matrices, according to Penn State materials scientists.
"Plastics such as polypropylene, nylon, polycarbonate, epoxy resinsand other compounds are brittle and fracture easily," says Dr. T.C.Chung, professor of materials science and engineering. "Usually,manufacturers take rubbery compounds and just mix them with theplastic, but there are many issues with this approach."
The problems include difficulty in controlling the mixing of the twocomponents and adhesion between the plastic and rubber. Chung, and Dr.Usama F. Kandil, postdoctoral researcher in materials science andengineering, looked at another way to embed rubbery particles into aplastic matrix. They described their work today (Aug. 29) at the 230thAmerican Chemical Society National Meeting in Washington, D.C.
The researchers used polyolefin ethylene-based elastomer, a veryinexpensive stable rubber that withstands exposure to ultra violetradiation. This rubber is often used as the sidewall in many automotivetires. However, rather than simply produce micro particles ofpolyolefin, Chung and Kandil produce a core-shell particle structurewith a tangle of polymerized polyolefin rubber forming a ball withfunctionalized groups hanging out like bristles.
"These functional groups can combine with the plastic and improve theadhesion of the rubber with the plastic," says Chung. The rubberparticles embedded in other materials absorb some of the energy ofimpact. Rather than the brittle portion breaking on impact, the rubberparts deform and absorb the energy without breaking. Chung and Kandilbelieve if they can introduce the rubber particles into othermaterials, such as ceramics, the rubber would function in the same way,making resilient ceramics. Plastics and rubbers are both polymers, buthave one significant difference. Plastics have relatively high glasstransition temperatures -- the temperature at which the materials ceasebeing pliable and become brittle like glass. Rubbers, especiallypolyolefin, have very low glass transition temperatures.
"Tires never freeze above glass transition temperature," says Chung."So the material is always in a pliable state at ambient temperatures.This can improve the toughness of any material."
The functionalized groups on the outside of the rubber balls can betailored to join with any plastic or ceramic, solving the problems ofadhesion found when using only untailored rubber particles. These coreand shell particles range in size from 30 nanometers to 10 micrometers.
The researchers manufacture their tiny rubber balls in a one-potprocedure that causes the rubber components to cross-link into theshape of a tiny rubber ball with their functional groups intact.Addition of a surfactant -- a soap-like compound -- causes the polymersto entangle into a ball with some of the functional groups sticking outfrom the surface. By controlling the process, the researchers cancontrol the size of the particles from micron-sized to nano particles.
The researchers have applied for a provisional patent on this work.
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