Reentrant foam

When compared to conventional foams, reentrant foams display superior resiliency and toughness. This structure has applications where the redistribution of stresses is desired in air filters, flexible fasteners, gaskets, sound-absorbing layers, fillers for highway joints, ankle wraps, wheelchair cushions, and many more. An air filter made of reentrant foam would, for example, address the problem of a pressure rise behind a clogged conventional filter. The pore space in the reentrant foam would open rather than close as the pressure increased, to prevent clogging. And if it were used in a flexible fastener, it would expand when the pull-out tension was applied. 

The transformation of closed cell polymethacryUmide and LDPE foams are described in an attempt to impose reentrant structures on them. New methods of achieving permanent, triaxial compression were developed. 9 refs. 

A polyester foam with a negative Poisson s ratio has been reported by Lakes. Its cells consist of reentrant faces. Figure 19.4 shows a cell with 24 faces. When the cell is extended its sides are moved inward and when it is compressed the walls move outward. The negativity of v increases with 9. 

Janowski et al. [36] Metallic coating of a foam substrate Irregular matrix of surface and reentrant cavities... 

This reentrant foam, as it is called by its inventor Roderic Lakes, a professor of biomedical engineering at the University of Iowa, is the only material thus far to exhibit negative action. The key to this behavior is in its microarchitecture. Whereas conventional foams have a convex cell structure, the ribs of each of the new foam s cells permanently protrude inward. When a tensile force is applied, the ribs push outward, causing this foam to expand laterally. With a compression force the ribs collapse into themselves, causing the material to contract laterally. 

Each reentrant foam starts out as a conventional plastic foam with a positive Poisson ratio and a convex, open-celled structure. The conventional foam is then compressed triaxially (that is, in three orthogonal directions) and placed in a mold. The mold is heated to a temperature slightly above its softening temperature, then cooled to room temperature. Any foam subjected to this conditioning that then possesses a permanent volumetric compression factor between 1.4 and 4.0 exhibits a negative Poisson ratio. 

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