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Auxetic Foams



Maro Encyclopedia


Patents with Abstracts




1. “Auxetics are materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures, which flex when stretched. Auxetic materials can be single molecules or a particular structure of macroscopic matter. Such materials are expected to have mechanical properties such as high energy absorption and fracture resistance. Auxetics may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.

Auxetics can be illustrated with an inelastic string wound around an elastic cord. When the ends of the structure are pulled apart, the inelastic string straightens while the elastic cord stretches and winds around it, increasing the structure's effective volume.

The term auxetic derives from the Greek word αὐξητικός (auxetikos) which means "that which tends to increase" and has its root in the word αὔξησις, or auxesis, meaning "increase" (noun). This terminology was coined by Professor Ken Evans of the University of Exeter.

Scientists have known about auxetic materials for over 100 years, but have only recently given them special attention. The earliest published example of a synthetic auxetic material was in Science in 1987, entitled "Foam structures with a Negative Poisson's Ratio"  by R.S. Lakes from the University of Iowa. The use of the word auxetic to refer to this property probably began in 1991.

Typically, auxetic materials have low density, which is what allows the hinge-like areas of the auxetic microstructures to flex.

Examples of auxetic materials include:
Certain rocks and minerals
Living bone tissue (although this is only suspected)
Specific variants of polytetrafluorethylene polymers such as Gore-Tex
Paper, all types. If a paper is stretched in an in-plane direction it will expand in its thickness direction due to its network structure.
Tailored structures designed to exhibit special designed Poisson's ratios."

(Wikipedia, Auxetic Materials, 10/5/2012)

2. “Auxetic materials, including foam, have a negative Poisson ratio whereby, when stretched in one direction by application of a tensile load, the material expands transversely to that direction. Alternatively, when compressed in one direction, the material contracts transversely to that direction.

Auxetic materials are available in the form of polymer gels, carbon fibre composites, honeycombs and microporous polymers.

Synthetic auxetic foams have been known since 1987. As described in WO88/00523, auxetic foams were prepared as open-celled polymeric foams. The negative Poisson ratio of these foams was obtained as a consequence of mechanical deformation of conventional foam by triaxial compression at a temperature above the softening temperature, followed by cooling below the softening temperature. Auxetic thermoplastic (polyester polyurethane), thermosetting (silicone rubber) and metal (copper) foams have been reported (E. A. Friis, R. S. Lakes, J. B. Park, J. Mater. Sci. 1988, 23, 4406).

Commonly available foam materials have a convex polyhedral cell shape. Conversion of these foam materials into auxetic foam materials is achieved by compressing the cell structure to a re-entrant and much more convoluted structure. The difference in the two cell structures can be seen in FIGS. 3a and 3b.

In the case of thermoplastic foams the transformation from conventional foam to auxetic foam is achieved by tri-axial compression followed by heating of the compressed foam to above the softening point. Finally the compressed foam is cooled.

In the case of thermosetting foams the processing route typically entails a two part elastomer base and catalyst mixture in accordance with conventional foaming techniques. The mixture is allowed to foam and set and then tri-axial compression is applied during curing.

The conversion in ductile metal foams typically takes place at room temperature and consists of applying uniaxial compression until yielding occurs. The final structure is then achieved by sequentially applying “

[Alderson et al, US Patent 8,277,719 (10/2/2012)]


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(RDC 6/5/2012)


Roger D. Corneliussen

Maro Polymer Links
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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is10/5/2012.