Abstract
The physical properties corresponding to the sensory values “hardness” and “body” of urethane foams were discussed in the previous reports1), 2) and it was reported mainly that the gradient of compressive stress in the first linear part on S-S curves (young's modulus) is representative of the sensory value “hardness”. Therefor, young's modulus seems to be an important physical property on “handling” in this material.
In this report, a model cell structure of flexible polyurethane foam was constructed through microscopical observations, and the theoretical calculation of compressive young's modulus is discussed.
The results obtained as follows:
(1) In general, the size (l2) of the cell in parallel to rising direction is larger than that (lx) in perpendicular to rising direction.
(2) In the model proposed in this report, a foam is composed of many uniform cells, shaped rectangular parallelopiped. Twelve edges of this rectangular parallelopiped are equal to cell skeletons of the foam. The following equations, showing the compressive young's modulus (Yz) in parallel to rising direction and (Yx) in perpendicular to rising direction, were derived theoretically.
Where ρ is the apparent specific gravity of foam, ρo the specific gravity of cell skeleton and Yo the compressive young's modulus of the cell skeleton (solid elastomer).
The results of the calculations on the above equations showed a good agreement with the observed values
of compressive young's modulus.
(3) The anisotropy of compressive young's modulus of foam between in parallel and in perpendicular to rising direction may be explained by the above equations, as Yz and Yx include lz and lx as the factors. The anisotropy depends upon lz and lx in the form of the equation Yz/Yx=lz/lx.
