Surfactant level influences on structure and properties of flexible slabstock polyurethane foams

Abstract

A series of flexible polyurethane slabstock foam samples were prepared with varying surfactant concentration. Several samples were also prepared by quenching small pieces into liquid nitrogen during the foaming process. The morphology of these materials was characterized at many length scales via scanning electron microscopy (SEM), transmission electron microscopy (TEM), wide-angle and small-angle X-ray scattering (WAXS and SAXS), tapping-mode atomic force microscopy (AFM), and Fourier transform infrared spectroscopy (FTIR). AFM was also utilized to probe trends in the mechanical stiffness of hard domains in the polyurethane foam. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were applied to examine the thermal and viscoelastic properties of these foams. It was shown in this study that collapse of the cellular structure of a foam prior to the point of urea precipitation alters the aggregation behavior of the hard domains and influences their ultimate properties. Samples without surfactant quenched in liquid nitrogen exhibited urea-rich aggregations on the order of 2–4μm with similar sized urea-poor regions. Equivalent samples with surfactant showed no such aggregations, suggesting that surfactant does play a principal role in the way that urea precipitates in these materials. DMA and DSC revealed that all samples of any surfactant concentration which spontaneously collapsed or were quenched or crushed prior to completely curing had a polyol glass transition 3–5° higher and somewhat broader than any foam sample which maintained its cellular structure until cured. This is interpreted to mean that the polyol matrix of the collapsed, crushed, or quenched materials is not as pure as the cellular samples, indicating that the presence of the cellular morphology plays a significant role in the microphase separation behavior of the solid-state at the molecular level. This hypothesis is supported by the results of WAXS, FTIR, SAXS, and AFM. The WAXS results demonstrate that at no surfactant concentration is the ordering, or hydrogen bonding, within the hard domains being significantly altered; however, in the lower range of the concentrations studied here, the FTIR results show that the surfactant level in the formulation does play a significant role on the amount of bidentate hydrogen bonded hard domains that organize locally. Further, as shown by SAXS, the surfactant concentration influences the mean chord length across the hard domains. These changes in structure and domain size distribution lead to the properties investigated via AFM, where the relative hardness of the hard domains was noted to initially increase as the surfactant concentration increases and then levels off above a certain concentration. The surfactant is thus suggested to play a secondary role in the development of the hard domains by maintaining the cellular structure in the foam as the phase separation occurs and at least until the polyurethane foam has more fully organized hard segment domains.