Question

Why do some plastics foams have anisotropic properties?

Answer 1

Polymer foams made by pouring the polymer plus a hardener and a foaming agent into a mould (so that it rises like a loaf of bread) usually have cells which are elongated in the rise direction and equiaxed in the plane normal to it, giving an axisymmetric structure. The size of the cells in such materials often varies with direction and because of this, the properties of the material, too, varies with direction: the material is said to be anisotropic. The anisotropy in cell shape can conveniently be measured by the ratio of the mean intercept length in the rise direction to that in the perpendicular plane; we call this the shape-anisotropy ratio, R. The value of R for polymer foams is typically about 1.3; it varies from 1 for an isotropic foam to 10 or more for one which is very anisotropic.