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.
鈥淭raditional energetic materials tend to be crystalline, like TNT,鈥� says
Marcia Hohn of Britain鈥檚 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鈥檚 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鈥檚 shape cannot be changed, it can be used only in the rockets it was
originally intended for.
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鈥淣ow we have a rubber which can be recycled,鈥� says Allan Fawcett, a chemist
at The Queen鈥檚 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 掳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. 鈥淸Conventional]
rubber tends to be inert, so you鈥檙e diluting the energy of the whole
formulation,鈥� 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鈥橦猫res, who has done similar work on polymers, notes that
Fawcett鈥檚 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鈥攑ipes, pen caps,
cups鈥攃ould now be made from Fawcett鈥檚 rubber. 鈥淚 don鈥檛 know why you would
want to do that,鈥� he admits. But, he says: 鈥淲hen bags were first made out of
plastic, people said, `Why don鈥檛 you just use paper?鈥� It takes time for anything
like this to get into use.鈥�