Infrared Spectroscopy of Site-Dependent Inhomogeneities in Conventional Flexible Polyurethane foam

An IR-microscope was used to study the cell membranes of a flexible PU foam prepared with cell dimensions up to 2.5 mm. Along the membrane's long axis IR-spectra up to 25 overlapping spots with diameters of 200 μm were measured. Varying absorbance values of the IR-signals along the membrane axis indicate variations in the membrane thickness. It was possible to correlate the absorbance values of the methyl-signals with absolute thicknesses calculated from the interference patterns observed in some spectra. In general, membranes elongated in the rise direction were thinner in the upper part. This can be explained by the influence of gravitation during the foaming process when the material is still able to flow. The detailed evaluation of the spectra shows varying concentrations of urea and urethane species across the membrane. Manually separated membranes and struts were investigated by the conventional KBr-disc technique. Statistical treatment of the data obtained shows no significant difference between both sample types. Consequently the reaction process in struts and membranes must be very similar.