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http://www.auxetic.info
Joseph N. Grima, Daphne Attard and Ruben Gatt
Department of Chemistry, University of Malta
A novel chemo-mechanical process for making auxetic foams and for their reconversion to conventional form*
* Project led by the University of Malta in collaboration with Methode Electronics (Malta) Ltd. Finance by the Malta Council for Science & Technology
http://www.auxetic.info 3
Auxetics
Conventional vs. Auxetic
PULL
PULL
PULL
PULL
+ve Poisson’s ratio -ve Poisson’s ratio
Poisson’s ratio:y
xyx
x
y
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Properties and applications
• Auxetic nails
Gets shorter & thinner whilst ‘going in’
Gets longer & fatter whilst ‘going out’
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Properties and applications
Auxetic materials are harder to indent…
...In auxetics, the material tends to go towards the point of impact to become denser.
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Properties and applications
A saddle shape is formed when bending a conventional foam.
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Properties and applications
A dome shape is formed when bending an auxetic foam.
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Properties and applications
• Smart filters
PULL PULL
Smart filters are made of auxetic structures. The stress applied determines the pore size and thus which particles are filtered.
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Properties and applications
Smart dressings are a smart way to administer drugs to freshly made wounds. When wounded a smart dressing is applied, as the wound swells it pulls the bandage, opening the microstructure and thus releasing the medicine found inside it. As the wound cures the swelling decreases releasing less medicine.
• Smart dressingsa bandage made from an auxetic microstructure impregnated with a healing drug.
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Process
• First manufactured by Rod Lakes, University of Wisconsin, Madison, (R. Lakes, Science, 235 (1987) p. 1038-1040.)
• Produced from commercially available conventional foams through a process involving:– Volumetric compression of ~30% in volume– Heating to the polymer’s softening temperature– Cooling whilst remaining under compression
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Typical Procedure
Starting from: Reticulated 30 ppi polyester polyurethane
• Cut conventional foam in the shape of a cuboid of size 35 mm x 35 mm x 105 mm long;
• Press sample into a mould of dimensions 25 mm x 25 mm x 75 mm (28.6 % strain along each axis);
• Heat at 200 °C for 10 minutes, Remove from mould Stretch Replace in the mould.• Cool to room temperature• Heat for 1 hour at 100 °C
x 2
Taken from: Smith, Grima, Evans, Acta Mater. 48 (2000) p.4349-4356.Technique adapted from: Chan and Evans, J. Mater. Sci., 32 (1997) p. 5945-
5953.
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New approach
• Uses solvent instead of heat
• Process involves– Wetting foam with appropriate solvent– Compressing the foam volumetrically by 30%– Allowing the foam to dry well
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Typical Process
• Starting from: Reticulated 30 ppi polyurethane foam (Dongguan Dihui Foam Sponge, China)
• Cut conventional foam in the shape of a cylinder of diameter 40mm and length 84mm
• Wet the foam with acetone• Remove excess solvent• Press sample into a mould of diameter 26 mm and length
55mm (~35 % strain along each axis);• Allow the sample to dry completely before removing from
mould
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Result
JN Grima, D Attard, R Gatt and RN Cassar, Adv. Eng. Mater., 21 (2009)
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Re-entrant structures
Uniaxial
loading
Uniaxial
loading
Compression/heating process
conventional
auxetic
LJ Gibson and MF Ashby, Cellular Solids, Cambridge Uni. Press, 1997.IG Masters and KE Evans, Composite Struct, 35 (1996) 403.KE Evans, A Alderson and FR Christian, J. Chem. Soc. Faraday Trans., 91 (1995) 2671.
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3D Re-entrant structures
dodecahedron foam models
tetrakaidecahedron foam models
conventionalRe-
entrant
(KE Evans, MA Nkansah and IJ Hutchinson, Acta Metall. Mater., 2 (1994) 1289)
(JB Choi, RS Lakes, J Compos. Mater., 29 (1995) 113.)
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Missing rib model
CW Smith, JN Grima and KE Evans, Acta Mater., 48 (2000) 4349.
Uniaxial
loading
Compression/heating process
Uniaxial
loadingconventiona
l
auxetic
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Rotating rigid units
(a)
(idealised form … rotating triangles model)
Uniaxial
loading
Uniaxial
loading
Compression/heating process
conventional
auxetic
JN Grima, A Alderson and KE Evans, J. Phys. Soc. Jpn, 74 (2005) 1341.
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Process
• Expose auxetic foam to solvent
• Resulting foam is conventional with comparable dimensions to the original foam
• Auxetic foams made by the thermal method also lose their auxeticity when in contact with a solvent
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Result and Implications
• The conversion / re-conversion process can be repeated for a number of times
• Auxetic foams should not be used in applications where they come into contact with solvents
Conversion
to auxetic
Re -Conversion
to conventional
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Acknowledgments…
The financial support of the Malta Council for Science and Technology and of the Malta Government Scholarship Scheme (Grant Number ME 367/07/17) is gratefully
acknowledged.
We also thank the ICMAT 2009 Organising committee for their financial assistance