Skin development in free rise, flexible polyurethane foam

The behavior of top skin layer in reacting flexible polyurethane foam was studied to interpret its role in determining the properties of the foam. Temperature profile near the top skin rises much slower than that at the center of the foam bun. Parallel plate rheometer and infrared spectroscopy showed that the material near the top skin is still liquid after the onset of cell opening at the center of the bun, due to the slow temperature increase. The cells near the top skin are supposed to still be closed at visual blow-off. The visual blow-off time changed with different silicone surfactant structure used in the formulations, although the onset of cell opening time did not change. This indicates that surfactants are playing some role to prevent CO₂ gas from coming out by breaking the top skin and cell layers under it. Cell opening was shown to be continuously occurring between onset of cell opening and visual blow-off time. Delaying the visual blow-off was efficient to get higher foam bun height and could affect air flow rate. However, the visual blow-off time alone cannot account for the difference in the air flow of final foam products.