An In-Situ Study of the Nucleation Process of Polyurethane Rigid Foam

After many studies into the various factors influencing the properties of polyurethane rigid foam, it is now widely accepted that the foam morphology has a major influence on the thermal conductivity [ 1 ], [ 2 ]. A large potential to reduce the thermal conductivity lies in the possibility of reducing the IR radiation contribution through the cell size. From these studies the importance of the understanding of the foam formation process, and in particular the nucleation process, has been highlighted. The nucleation process is considered as the process whereby the gas nuclei, which expand to form the cells of the foam, are formed. It has therefore a direct influence on the foam morphology, i.e. cell size and shape. Up until now there have been many theoretical descriptions of the nucleation process but very few detailed experimental research has been carried out. With the aim of gaining a deeper understanding of foam formation and the nucleation process, in order to develop possibilities in influencing it and to ultimately reduce the thermal conductivity, the nucleation process was systematically analysed. A method was developed whereby the nucleation process could be observed in situ and subsequently analysed by means of a PC-controlled camera attached to a stereo microscope. The increase in volume due to foam formation and its exothermic reactivity was taken into consideration. Thereby, the amount of cell nuclei in the initial phase was examinable and cell growth including coalescence could be followed. The initial nuclei compared favourably to the final number of cells and to their size, providing a consistency with real foaming conditions. Detailed research was carried out on the effect of various types and amounts of blowing agents (e.g. carbon dioxide, cyclopentane, perfluoralkanes), surfactants, catalysts and isocyanates on the nucleation process. Other important factors influencing the nucleation process are the component’s viscosity and surface tension and their relationship with each other during the foaming process. The surface tensions and viscosity of the various systems used were also investigated using several known methods. This provided additional information and backed up new theories on the nucleation process of polyurethane rigid foam.