GRAVITATIONAL EFFECTS ON CLOSED-CELLULAR-FOAM MICROSTRUCTURE

Polyurethane foam has been produced in low gravity for the first time. The cause and distribution of different void or pore sizes are elucid ated from direct comparison of unit-gravity and low gravity samples. L ow gravity is found to increase the pore roundness by 17% and reduce t he void size by 50%, The standard deviation for pores becomes narrower (a more homogeneous foam is produced) in low gravity. Both a Gaussian and a Weibull model fail to describe the statistical distribution of void areas, and hence the governing dynamics do not combine small void s in either a uniform or a dependent fashion to make larger voids. Ins tead, the void areas follow an exponential law, which effectively rand omizes the production of void sizes in a nondependent fashion consiste nt more with single nucleation than with multiple or combining events.