Rigid polyurethane foams for buoyancy applications: Hydraulic resistance

Polyurethane foams used in floatation applications must have a high `hydraulic resistance', which is the resistance to penetration of water into the foam cells at elevated hydraulic pressures. Experiments are carried out to study the effect of different surfactants on the structure and hydraulic resistance of rigid polyurethane foams with densities in the range 100-200 kg/m3. The hydraulic resistance is evaluated in terms of the weight of water absorbed per unit weight of the foam at a fixed hydraulic pressure, and the cell and strut sizes of the foams are measured by microscopy and image analysis. The hydraulic resistance of the foams increases with the increase in density of the foams. The compression strength of the foam is significantly higher than the corresponding hydraulic pressure the foams can withstand, indicating that the hydraulic resistance is determined by the existence and strength of cell windows. The water absorbed by the foams increases with the hydraulic pressure. Among the different surfactants used, the foams made with Tegostab B8404, Atsurf 3800 or Cresmer 3845 surfactants have a low loss in buoyancy (less than 20%) at 3 bar hydraulic pressure. These foams also have smaller cell windows compared to foams with lower hydraulic resistance.