Go for meltdown
By Lila Guterman ROCKET makers will have more freedom to experiment with their designs if they use a new type of rubber to make their solid fuel. The synthetic rubber, invented by a chemist in Northern Ireland, allows solid fuel to be melted down and reshaped. “Traditional energetic materials tend to be crystalline, like TNT,” says Marcia Hohn of Britain’s Defence Evaluation and Research Agency (DERA). Such propellants form powders that are difficult to handle because they are very sensitive and ignite easily, so they are normally stabilised by adding a rubbery polymer. As links form between the polymer’s long-chain molecules, the fuel solidifies. But solid fuel that is made this way does not melt. If something goes wrong during processing, there is no way to recover the materials. And because the fuel’s shape cannot be changed, it can be used only in the rockets it was originally intended for. “Now we have a rubber which can be recycled,” says Allan Fawcett, a chemist at The Queen’s University of Belfast. Fawcett, whose work was funded by DERA, started with a liquid polymer made up of molecules that include oxygen-containing rings called furans. He then added a compound that binds to the furan groups on different chains, linking the chains together. When Fawcett heated the rubber to 150 °C, the reaction reversed and the polymer molecules separated to form a liquid. Fawcett tried this procedure on a sheet of rubber folded into a Z-shape. When he cooled it down again, he found the three layers had joined without a seam. While a few companies have developed other meltable rubber materials, Fawcett says his furan-based system has the advantage that it will be easy to add energy-rich groups such as nitrates to the chain of the polymer itself. If this can be done, the rubber will both bind the fuel together and provide extra energy. Hohn thinks that this is a strength of the system. “[Conventional] rubber tends to be inert, so you’re diluting the energy of the whole formulation,” she says. No one has previously made rubber that can contain energetic molecules and has reversible cross-links, she says. Alessandro Gandini of the French Engineering School of Paper and Printing in Saint Martin d’Hères, who has done similar work on polymers, notes that Fawcett’s form of rubber can be melted only once. The furan rings seem to form permanent links between polymer chains after melting because of oxidation reactions. Fawcett believes this will be easy to get around by adding a stabilising chemical, but Gandini suggests the solution is to use a polymer with fewer furan rings. The new rubber may find numerous applications. Because it can be moulded and shaped like plastic, it could be squeezed through plastics processors, so anything that is currently made from plastic—pipes, pen caps, cups—could now be made from Fawcett’s rubber. “I don’t know why you would want to do that,” he admits. But, he says: “When bags were first made out of plastic, people said, `Why don’t you just use paper?