Supports - Preventive Biomechanics - page 153

Fig. 4.2 Scanning electron microscope photographs of a open-cell PU foam and b closed-cell

PE foam

4.2.1.1 Material Characterization

Microstructure: Soft polymeric foams belong to a material class generally termed
cellular materials. Their microscopic structure consists of long, randomly oriented
molecular chains, which form an interconnected (cross linked and end linked)
three-dimensional network of struts, either open-cell, or plates, or closed-cell, or
both. The struts and plates form the edges and faces of the cells, whereby the cell
shape is most commonly polyhedral. The dilatational response and rubber-like
elasticity come from the material properties (flexible behavior) and the interlinked
micro-structure of the particular foam material, as well as the behavior of the
single struts and plates under loading.
An important feature of such materials is their relative density (ratio of foam
density to a reference density, usually the density of the solid foam material) which
can be as low as q R ¼ 0 : 001 for low-density foams. The degree of density is
determined by the technique used in the polymer production process. The low
stiffness of foams makes them ideal for cushioning since the porosity, especially in
open-cell foams, permits large volumetric change.
For illustrative purposes, in Fig. 4.2 two images, adopted from (Mills 2000),
obtained from scanning electron microscope (SEM) of open-cell and closed-cell
polymeric foams are shown. In Fig. 4.2 a, the open-cell structure with single cell
ligaments forming the cell edges and their cross links at the vertices can be
distinguished. Within the cell framework, air passes freely under compression,
thus not contributing significantly to the dilatational response, when strain rates
remain moderate.
In closed-cell foams, besides cell edges and vertices, material is contained in
the cell face as well, sealing each cell off from its neighbour (cf. Fig. 4.2 b). As a
consequence, gas or liquid is trapped within the cells, generally providing a stiffer
response under loading.
Different mechanical behaviour may be observed subject to the loading
direction, Fig. 4.3 a, b, (Zhu 1997) in individual cells from a compression loaded
open-cell foam structure, depending on the foam generation process.