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| Home : Volume 22 : Summer 1996 : | |
| COE's rheology and polymer research goes with the flow | |
Wouldn't it be nice to know how a material would flow just by looking at its molecular structure?" asks Associate Professor of Mechanical Engineering A. Jeffrey Giacomin, director of the college's Rheology Research Center. Predicting rheological properties--the way materials flow and deform--is a cornerstone of the research programs for many of the center's 11 faculty members. And although not all center members are investigating simulations, one single characteristic is common to all of their research. "Every member of the center is doing work that is relevant to industry," Giacomin says. With funding from such companies as the 3M Corporation, the Kimberly-Clark Corporation and the Dow Chemical Company, center researchers are investigating topics as disparate as plastic body panels for automobiles to fluids that quickly thicken when placed in a strong electrical or magnetic field.
A. Jeffrey Giacomin confers with graduate student Joe Yosick on the insertion of a plastic plaque into one of the world's few sliding-plate rheometers. The UW-Madison is the only university in America equipped with this type of rheometer, which measures the properties of molten plastics under extreme conditions of stress and strain.
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Partially through cooperation and partially through kismet, the work of some of the center's newest members dovetails to reveal the inner workings of rheological materials. Giacomin, for example, is developing ways to measure flow behavior of materials under extreme conditions. That work helps other researchers verify their simulations, providing powerful new tools for developing polymers. Ideally, he explains, an engineer who wanted to find the perfect polymer for a given product could select characteristics of different molecular combinations and would then be able to view on screen how the resulting plastic would behave for a given application and without the expense of producing the actual substance.
"That's what I'm trying to do: go all the way from atoms to the factory," says Juan J. de Pablo, an assistant professor of chemical engineering. Currently, de Pablo is developing computer models that describe how individual polymer molecules, which are long chains of atoms, fit together and interact. As these models evolve, he hopes to link them with models that other center researchers are developing to provide a turn-key simulation that could follow the manufacturing process all the way from the molecular level to final product.
Currently, de Pablo is concentrating on modeling thermophysical properties of fluids and solids. His first step is to simulate the interaction of as many molecules as possible of pure polymer. (Increasing the number of molecules makes the models more accurate, but can bog down the computer, and slow the simulation.) Once he has a good representation of how the material behaves, he can simulate conditions that would be difficult or impossible to measure in the real world. He can, for example, predict how materials flow at pressures of thousands of atmospheres or how well they lubricate in a space just a few millionths of an inch across. This software, Giacomin says, would let researchers "tinker with molecular structures until they figured out which one would give them the best flow behavior. It would really change the way new polymer products are invented."
Another application of de Pablo's simulations is predicting the effects of imperfections in manufactured materials. Manufactured polymers always have inconsistencies and contaminants that can alter the materials' behavior. And because these flaws usually appear in combinations and can't be entirely eliminated, it is difficult to gauge their effects. But after de Pablo has generated a computerized version of a pure material, he can follow its subtle changes as he adds individual flaws. "An advantage of the simulation is that I have perfect control over the molecule, which is difficult to achieve in real life," he says.
Just as important as predicting the behavior of polymer molecules is understanding how different combinations blend. Because creating polymers from scratch is expensive, most "new" plastic materials are blends of established polymers. And most of this mixing is in machines that were designed through trial and error almost a century ago. To improve the mixing process, researchers need to understand what exactly is happening inside these devices. "Now, through simulation, for the first time we can peek inside and see how things flow in these mixers," says Tim A. Osswald, an associate processor of mechanical engineering.
Osswald's simulations provide striking, real-time images of how different types of plastics combine synergistically to produce blends that out-perform their composite parts. To find the most efficient process, the models permit the researcher to vary virtual components--such as mixing pins and different types of mixing heads--and factors such as viscosity ratio, surface tension, temperature dependence and viscous heating to accurately project how the materials will interact.
The resulting patterns, however, are extraordinarily complex, and would normally tax even the fastest computer workstation. Osswald, however, accelerates and simplifies his models with mathematical strategies that use a sampling of data from the mixture's edges to portray the entire batch. "With that mathematical trick," he explains, "we eventually have a governing equation that only needs information on the boundaries, but is still representative of what's happening inside."
Actually seeing what's happening inside a mixing machine, however, isn't that easy. The center's battery of polymer processing equipment--which represents all of the usual manufacturing methods used in industry--is designed for production rather than observation. To check his simulations and watch how his polymers were mixing, Osswald's group constructed a single-screw extruder with a transparent shell. Based on his simulations and observations, companies such as 3M are expected to design a whole new generation of processing equipment.
Chemical engineering professors Daniel J. Klingenberg (left) and Sangtae Kim collaborate on the construction of a flow-manifold model.
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Osswald is also focusing on a specific manufacturing process: how sheets of plastic shrink and warp when compressed in large molds. "This is a problem that has given people headaches for 50 years," Osswald says. But now auto-parts manufacturers are using simulation software that his group developed to fine-tune the production of plastic body panels, which are made by squeezing layers of plastic in hot molds. Stringent ISO 9000 quality standards require that these panels deviate by no more than a single millimeter, a difficult-to-reach goal that demands precise control of the production process. By simulating the patterns that the polymer fibers form, how the materials solidify and how heat travels through the material in the molds, then varying such factors as the mold's temperature and the placement of the plastic sheets, Osswald has been able to accurately predict how well the plastics will conform to the molds. "Our models can predict the filling and they agree quite well with experiments," he says.
This combination of simulation and experimental work is often seen in the Rheology Research Center. Working independently, Assistant Professors of Chemical and Biological Engineering Michael D. Graham and Daniel J. Klingenberg, for example, are inventing computer models and laboratory fixtures to both improve vexing industrial process problems and develop new materials.
One of the products Graham is investigating is polymer fibers, which are made by pulling a thread from a pool of polymer through a die. Manufacturers want to produce polymer strands--such as optical fibers--that have smooth surfaces. That isn't difficult if the thread is pulled very slowly. But working slowly is expensive. Speeding up the process, however, causes distortions in the fiber's surface. "There is some interaction between the polymer and the die's metal surface that is important, somehow," he says. Working with Giacomin, Graham is creating computer models that represent these interactions and is experimenting with factors, such as the elasticity of the polymer, the smoothness of the die and additives to the polymer, that can affect the process.
Professor Michael D. Graham and a research assistant examine instabilities in flow between independently rotating concentric cylinders.
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Graham is also applying these techniques to improve the process of making sheets of polymer or paper. Like fibers, sheets of polymer or paper develop imperfections if processed too quickly. "In either case we think the elastic nature of the liquids is leading to these problems," he explains. "In both cases the liquids are elastic. In one case it's molten plastic. In the other case the suspension of paper fibers also has some elasticity."
Although all of the center's scientists investigate ways to control the rheological properties of materials, Klingenberg investigates ways to control them very quickly using powerful electrical or magnetic fields. "How many ways could you use a fluid where you could turn it solid and back to liquid again in a hurry?" he asks. Truck shock absorbers that can be matched quickly to a varying load or a varying road, temporary molds for casting limited-run items, continuously variable transmissions, dampers for medical lasers and flexible barriers to control the flow of liquids are all applications that he envisions for "electrorheological" materials.
To a limited extent, any material will change viscosity in an electrical or magnetic field. But certain fluids--most often suspensions of metals in oils--can be shifted precisely and continuously from liquid to solid states. Klingenberg is both creating computer models of idealized systems and conducting experiments--for example adding materials such as milk proteins and devising new polarization strategies--to enhance the capabilities of these fluids.
No other single research facility, Giacomin says, has generated such a broad range of rheology simulation and experimentation, ranging from models of molecular structures to simulations of polymer processing. As computer technology evolves and researchers continue to make breakthroughs, he expects that soon--perhaps in just five years--a single set of linked simulations will allow engineers to choose among molecules on a computer screen and immediately see how the resulting material would perform. "That is the cutting edge of polymer technology," he says. "It's been a longtime dream."
By Scott Fields
Content by perspective@engr.wisc.edu
Date last modified: Friday, 28-Jun-1996 12:00:00 CDT
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