Study of Cell Opening in Flexible Polyurethane Foam

Abstract

The time to onset of cell opening and the modulus around the event were measured using a parallel-plate rheometer for flexible polyurethane foams. Cell opening time was compared with the visual blow-off time. The onset of cell opening occurs several seconds after phase separation, and the visual blow-off occurs several seconds (sometimes more than ten seconds) after the onset. Two series of formulations were studied, one in which the factors other than surfactant (tin catalyst level, TDI index, polyol type) were changed, and the second series in which only the surfactant was changed. In the first series, the modulus at cell opening did not change much with different formulations. However, the modulus development rate during the cell opening period (between the onset and the visual blow-off) changed significantly. Air flow was lower with higher modulus development rate. In the second series, the modulus development rate or the timing of onset of cell opening did not change even when the air flow changed considerably. The fact that the timing did not change and it is several seconds after the phase separation implies that the cell opening is triggered by sudden urea precipitation. Light interferometry showed that the average value of the remaining cell window thicknesses in low air flow foam is higher than that of the high air flow foam. Scanning electron microscopy (SEM) of the final foam products showed that the cell windows are in a variety of stages. The windows were classified into four stages-fully open, partially open, pin holes and closed. Air flow is directly proportional to the effective fraction of open windows. SEM also showed that the cell windows are in several different states. Some window films were totally missing and others have left torn-off films. The possible mechanism of cell window rupture is discussed with respect to the results.