Regulation of cell structure in water blown rigid polyurethane foam

Abstract

Rigid polyurethane foams are generally "closed celled"with a gas entrapped in each cell. The properties of such foams are significantly affected by the cell size distribution and morphology of the cells. For example, in thermal insulation foams, small cells lead to lower thermal conductivity and in foams for buoyancy applications, stronger cell windows are resistant to ingress of water until higher hydraulic pressures. The effect of formulation parameters on the size and structure of rigid polyurethane foams was investigated. Foams were made with different surfactants, with variation of surfactant concentration at different blowing agent concentration, with variation of nucleating agent concentration and with variation of catalyst concentration. Micron sized silica particles were used as nucleating agent. The densities of the foams were in the range of 140 to 165 kg/m³. The cell window areas of different foams have been measured. The bubble size in the polyol depends on the surface tension lowering ability of a surfactant, and the entrainment of a large number of small bubbles during mixing leads to a foam with a small cell size and a narrow cell size distribution. Increasing the concentration of the surfactant reduces the cell size and narrows the distribution. Increasing the proportion of organometallic catalyst in the amount of total catalyst used, results in smaller cell sizes, which are narrowly distributed. Hydraulic resistance of the foams was monitored by measuring buoyancy losses at different hydraulic pressures. Hydraulic resistance of the foams can be improved by decreasing the area and increasing the thickness of the cell windows.