A novel chemomechanical process for making auxetic foams

A novel chemo-mechanical process for making auxetic foams and for their reconversion to * conventional form Joseph N. Grima, Daphne Attard and Ruben Gatt Department of Chemistry, University of Malta * 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

Introduction http: //www. auxetic. info

Auxetics Poisson’s ratio: Conventional Auxetic vs. PULL y x +ve Poisson’s ratio http: //www. auxetic. info -ve Poisson’s ratio 3

Properties and applications • Auxetic nails Gets shorter & thinner whilst ‘going in’ Gets longer & fatter whilst ‘going out’ http: //www. auxetic. info 4

Properties and applications Auxetic materials are harder to indent…. . . In auxetics, the material tends to go towards the point of impact to become denser. http: //www. auxetic. info 5

Properties and applications A saddle shape is formed when bending a conventional foam. http: //www. auxetic. info 6

Properties and applications A dome shape is formed when bending an auxetic foam. http: //www. auxetic. info 7

Properties and applications • Smart filters PULL Smart filters are made of auxetic structures. The stress applied determines the pore size and thus which particles are filtered. http: //www. auxetic. info 8

Properties and applications • Smart dressings a bandage made from an auxetic microstructure impregnated with a healing drug. 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. http: //www. auxetic. info 9

Auxetic foams http: //www. auxetic. info

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 http: //www. auxetic. info 11

Typical Procedure Starting from: Reticulated 30 ppi polyester polyurethane • Cut conventional foam in the shape of a cuboid of size 35 mm x 105 mm long; • Press sample into a mould of dimensions 25 mm x 75 mm (28. 6 % strain along each axis); • Heat at 200 °C for 10 minutes, Remove from mould x 2 Stretch Replace in the mould. • Cool to room temperature • Heat for 1 hour at 100 °C 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. http: //www. auxetic. info 12

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 http: //www. auxetic. info 13

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 40 mm and length 84 mm • Wet the foam with acetone • Remove excess solvent • Press sample into a mould of diameter 26 mm and length 55 mm (~35 % strain along each axis); • Allow the sample to dry completely before removing from mould http: //www. auxetic. info 14

Result JN Grima, D Attard, R Gatt and RN Cassar, Adv. Eng. Mater. , 21 (2009) http: //www. auxetic. info 15

Models http: //www. auxetic. info

Re-entrant structures Uniaxial loading conventional Compression/ heating process Uniaxial loading 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. http: //www. auxetic. info 17

3 D Re-entrant structures conventional Re-entrant dodecahedron foam models (KE Evans, MA Nkansah and IJ Hutchinson, Acta Metall. Mater. , 2 (1994) 1289) tetrakaidecahedron foam models (JB Choi, RS Lakes, J Compos. Mater. , 29 (1995) 113. ) http: //www. auxetic. info 18

Missing rib model Uniaxial loading conventional Compression/ heating process Uniaxial loading auxetic CW Smith, JN Grima and KE Evans, Acta Mater. , 48 (2000) 4349. http: //www. auxetic. info 19

Rotating rigid units (a) Uniaxial loading conventional Compression/ heating process Uniaxial loading auxetic (idealised form … rotating triangles model) JN Grima, A Alderson and KE Evans, J. Phys. Soc. Jpn, 74 (2005) 1341. http: //www. auxetic. info 20

Reconversions http: //www. auxetic. info

Process • Expose auxetic foam to solvent • Resulting foam is conventional with comparable dimensions to the original foam • Auxetic foams made by thermal method also lose their auxeticity when in contact with a solvent http: //www. auxetic. info 22

Result and Implications Conversion to auxetic Re Conversion to conventional • 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 http: //www. auxetic. info 23

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 http: //www. auxetic. info 24

Thank You ! http: //www. auxetic. info