Sequence showing how a complex-shaped object is made by successively deforming and heating it. CNRS Photothèque/ESPCI/Cyril FRÉSILLON |
A
common feature of sailboards, aircraft, and electronic circuits is that
they all contain resins used for their lightness, strength, and
resistance. However, once cured, these resins can no longer be reshaped.
Until now, only certain inorganic compounds, including glass, offered
this possibility.
Combining
such properties in a single material seemed impossible until a team led
by Ludwik Leibler, CNRS researcher at the Laboratoire “Matière Molle et
Chimie” (CNRS/ESPCI ParisTech), developed a new class of compounds
capable of this remarkable feat. Repairable and recyclable, this novel
material can be shaped at will and in a reversible manner at high
temperature. And, quite surprisingly, it also retains certain properties
specific to organic resins and rubbers: it is light, insoluble and
difficult to break. Inexpensive and easy to produce, this material could
be used in numerous industrial applications, particularly in the
automobile, aeronautics, building, electronics, and leisure sectors. This
work is published on Nov. 18, 2011, in Science.
Replacing
metals by lighter but just as efficient materials is a necessity for
numerous industries, such as aeronautics, car manufacturing, building,
electronics, and sports industry. Due to their exceptional mechanical
strength and thermal and chemical resistance, composite materials based
on thermosetting resins are currently the most suitable. However, such
resins must be cured in situ, using from the outset the definitive shape
of the part to be produced. In fact, once these resins have hardened,
welding and repair become impossible. In addition, even when hot, it is
impossible to reshape parts in the manner of a blacksmith or glassmaker.
The material can take various forms. CNRS Photothèque/ESPCI/Cyril FRÉSILLON |
This
is because glass (inorganic silica) is a unique material: once heated,
it changes from a solid to a liquid state in a very progressive manner
(glass transition), which means it can be shaped as required without
using molds. Conceiving highly resistant materials that can be repaired
and are infinitely malleable, like glass, is a real challenge both in
economic and ecological terms. It requires a material that is capable of
flowing when hot, while being insoluble and neither as brittle nor as “heavy” as glass.
From
ingredients that are currently available and used in industry (epoxy
resins, hardeners, catalysts, etc.), researchers from the Laboratoire “Matière Molle et Chimie” (CNRS/ESPCI ParisTech) developed a novel
organic material made of a molecular network with original properties:
under the action of heat, this network is capable of reorganizing itself
without altering the number of cross-links between its atoms. This
novel material goes from the liquid to the solid state or vice versa,
just like glass. Until now, only silica and some inorganic compounds
were known to show this type of behavior. The material thus acts like
purely organic silica. It is insoluble even when heated above its glass
transition temperature.
Remarkably,
at room temperature, it resembles either hard or soft elastic solids,
depending on the chosen composition. In both cases, it has the same
characteristics as thermosetting resins and rubbers currently used in
industry, namely lightness, resistance, and insolubility. Most
importantly, it has a significant advantage over the latter as it is
reshapeable at will and can be repaired and recycled under the action of
heat. This property means it can undergo transformations using methods
that cannot be envisaged either for thermosetting resins or for
conventional plastic materials. In particular, it makes it possible to
produce shapes that are difficult or even impossible to obtain by
molding or for which making a mold is too expensive for the envisaged
purpose.
A strip of material is deformed in an oven. It is subjected to torsion stress, clearly visible in bright colors under polarized light. These colors fade away within a few minutes when hot: the material has taken a new, permanent shape. CNRS Photothèque/ESPCI/Cyril FRÉSILLON |
Used
as the basis of composites, this new material could therefore favorably
compete with metals and find extensive applications in sectors as
diverse as electronics, car manufacturing, construction, aeronautics, or
printing. In addition to these applications, these results shed
unexpected light on a fundamental problem: the physics of glass
transition.
This work was supported in particular by CNRS, ESPCI ParisTech, and the Arkema group.
Silica-Like Malleable Materials from Permanent Organic Networks
