Not all plastics are created equal. Malleable thermoplastics can be easily melted and reused in products such as food containers. Other plastics, called thermosets, are essentially stuck in their final form because of cross-linking chemical bonds that give them their strength for applications such as golf balls and car tires.
"Nobody takes a thermoset and recycles it like you would a water bottle," said Orlando Rios from the Department of Energy's Oak Ridge National Laboratory.
Rios and a team of researchers from Washington State University and the University of Idaho have developed a process to make a thermoset that can be reshaped and reused. The team's study, published in the journal Macromolecules, is featured on the current issue's cover.
The new plastic is a shape-memory polymer, so named because the material can "remember" its original shape and return to it after being deformed with heat or other forces. The materials exhibit triple-shape memory behavior, meaning that the polymers can transform from one temporary shape to another temporary shape at one temperature, and then back to a permanent shape at another temperature.
Rios explains that although researchers have been interested in making use of shape-memory polymers' intriguing characteristics, managing the shape-shifting behavior has been a struggle.
"One big issue that has limited their use is controlling the transformation temperatures and their properties," he said. "We give a recipe where you can adjust the transformation temperature and shift the performance of the material."
Changing the ratio of ingredients allows the researchers to control the overall properties of the material. The team's method also uses off-the-shelf chemicals that can be easily scaled up to manufacture the material in bulk.
"We've taken it from somewhat of a scientific curiosity or fundamental research material to something that can be produced in larger volumes," Rios said.
Mixing the shape-memory polymers with other materials could produce stronger and stiffer composite parts that can later be recycled or reprocessed. Recyclable carbon fiber and glass fiber composites, for instance, are in high demand in the automotive industry.
"The ability to control the shape-memory behavior of the material provides great design flexibility," said Yuzhan Li of Washington State University.
The material could also be used as binding glue for new types of rare earth-free magnets made from powders. The team is already experimenting with 3-D printing powder-based magnets with shape-memory polymers.
"The applications for these materials are very broad, since the shape-shifting temperatures for these materials can be finely tuned by controlling the ratio of the chemicals used in their synthesis," said Michael Kessler of Washington State University.
The team has filed for a patent on its method. Coauthors of the Macromolecules study are Washington State's Yuzhan Li, Cole Pruitt, Mitch Rock and Michael Kessler; ORNL's Rios and Jong Keum; and University of Idaho's Liqing Wei and Armando McDonald.
The researchers used resources at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. The project was supported by the Air Force Office of Scientific Research and the Critical Materials Institute, an Energy Innovation Hub funded by the Department of Energy's Office of Energy Efficiency and Renewable Energy.
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