In the Greek myth of Icarus, wings melting in the sun are a tragedy. However, for the military, flying vehicles that evaporate under the influence of sunlight can become a powerful weapon: imagine a drone that disappears without a trace after a secret delivery of supplies to a remote place.
Engineers at the Georgia Institute of Technology have developed a new type of plastic that can form both flexible sheets and rigid mechanical parts, and then disappear within minutes or hours under the influence of ultraviolet light or temperatures above 80 degrees Celsius. Previous attempts to make plastics that self-destruct when heated have had problems with the destruction of such materials even at room temperature. But the new techie from Georgia can remain stable for many years as long as it shuns the sun.
Such plastics can also be used to make environmental sensors and medical devices that dissolve after data collection or to create temporary adhesives that can be peeled off using a heat gun. “They’re great for situations where you want things to disappear quickly,” said Paul Kol, a professor of chemical and biomolecular engineering at the Georgia Institute of Technology.
Kohl’s presentation covered the most recent improvements that he and his colleagues made to his material, but his team worked on the formula of this substance for many years – and this is not the only attempt to create temporary plastics. The Defense Advanced Research Projects Agency (DARPA) has entire programs to fund research that could lead to the development of endangered aerial delivery devices, including Col’s.
This is a very difficult task. Plastic consists of synthetic polymers, long chains of smaller molecules linked by superstrong bonds. Returning them to their building blocks requires breaking each connection, akin to breaking the necklace bead after bead. Since moisture, acidity, temperature, and other factors in the environment can affect this process, destruction can take months or years — if at all. “There are many synthetic plastics that decompose,” says Kohl, “but it’s a slow and painful process.”
To create durable plastics that break quickly under certain conditions, the Kola team and some other researchers turned to a molecule called polyphthalaldehyde (PPHA). Like all polymers, it breaks down into building blocks when heated above a certain temperature. But unlike plastics used in bottles and bags that break down only at very high temperatures, PPHA can break down even below room temperature. This property makes it easy to destroy such a polymer, but this is still not quite what you need.
To make PPHA become pseudo-stable, Kol and colleagues “tied” his polymer chain into a ring. “Now it only depolymerizes from the end to the inside,” says Kol. “It’s like peeling a banana,” he explains. “It’s hard to do it from the middle, but easy from the end.” Although the circular structure prevents the destruction of the modified PPHA at ordinary room temperature if the trigger breaks at least one bond, “the whole molecule instantly depolymerizes and completely breaks up,” he says.
“It’s pretty clever because they designed a polymer that works on the principle of lightning: just pull the tongue and the strong connection until then breaks up right away,” says Andrew Dove, a chemistry professor at the University of Birmingham in England who was not involved in the new study. The Kol team also finalized the zipper process by adding a whole mix of different chemicals to the circular PPHA.
To cause the destruction of the polymer by ultraviolet radiation, the researchers added to the compound a substance that oxidizes in the presence of sunlight and then destroys the PPHA bonds. And in order to slow down the rate of this degradation, thereby providing a simple aircraft, such as a glider, enough time to complete their mission, they added additional alkaline substances to the material that react with the acid and weaken it.
However, even in its round shape, PPHA was initially not stable enough for long-term use. Kohl and colleagues found that impurities in the polymer — mainly traces of the boron trifluoride catalyst used to assemble the PPHA chain — were the main culprits. By removing all such impurities, they increased the shelf life of the material. “Now it can be stored for up to 20 years indoors at room temperature,” says Kolya. “Light from fluorescent lamps will not destroy it.”
Despite these improvements, the resulting plastic was difficult to use due to its relatively fragile structure. But at a meeting of the American Chemical Society, Kohl announced that he and his colleagues had solved this problem by adding ionic liquids — salts in liquid form — to plastic. They can greatly change the mechanical properties of PPHA: one type of ionic liquid makes the plastic more rigid and hard, ideal for glider wings, and the other soft and foldable, which is great for creating parachutes or packages.
The PPHA parachute decomposed in just half an hour.
DARPA has already used new plastic to make lightweight, durable gliders and parachutes. In October last year, the agency conducted a field test of one of these vehicles: at night, being dropped from a high-altitude balloon, the glider successfully delivered a kilogram package to a point at a distance of 150 kilometers. After four hours in the sun, he disappeared, leaving only an oily stain on the ground. Kohl says that this plastic can dissolve even faster under the bright midday sun, in some cases the process takes only five minutes.
However, the new plastic still has quite different problems. For example, Dove emphasizes that the degradation caused by light will limit its use. “This is a great idea,” he says, “but I’m not sure if it is suitable for consumer plastic.” What remains after the decay of this polymer also matters. “Nothing disappears completely,” says Ann-Christine Albertsson, a chemist at KTH Royal Institute of Technology in Sweden. “Decomposition products and their environmental impact are very important.”
In this case, the decomposition residues — the oily stain that used to be a test glider — are mainly composed of ionic liquids used by the researchers to process the polymer. These compounds are antimicrobial chemicals similar to those “used in hospitals with antibacterial wipes,” says Kol. “However, they are not suitable for human consumption and their contact with food is undesirable. In the future, we will try to make them evaporate. “