Http:// Joseph N. Grima, Daphne Attard and Ruben Gatt Department of Chemistry, University of Malta A novel chemo-mechanical process for.

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


Introduction

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


Properties and applications

• Auxetic nails

Gets shorter & thinner whilst ‘going in’

Gets longer & fatter whilst ‘going out’



Auxetic materials are harder to indent…

...In auxetics, the material tends to go towards the point of impact to become denser.



A saddle shape is formed when bending a conventional foam.



A dome shape is formed when bending an auxetic foam.



• Smart filters

PULL PULL

Smart filters are made of auxetic structures. The stress applied determines the pore size and thus which particles are filtered.



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.


Auxetic foams


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


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.


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


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


Result

JN Grima, D Attard, R Gatt and RN Cassar, Adv. Eng. Mater., 21 (2009)


Models


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.


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.)


Missing rib model

CW Smith, JN Grima and KE Evans, Acta Mater., 48 (2000) 4349.

Uniaxial

loading


Uniaxial

loadingconventiona

l

auxetic


Rotating rigid units

(a)

(idealised form … rotating triangles model)

Uniaxial

loading

Uniaxial

loading


conventional

auxetic

JN Grima, A Alderson and KE Evans, J. Phys. Soc. Jpn, 74 (2005) 1341.


Reconversions


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


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


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


Thank You !

Http:// Joseph N. Grima, Daphne Attard and Ruben Gatt Department of Chemistry, University of Malta A novel chemo-mechanical process for.

Documents

compression slide

introduction slide

x y slide

science technology slide

wetting foam

auxetic foams

applications auxetic

applications auxetic