As the physical property requirements for molded seating foam increase in severity, the desired humid aged properties become more difficult to achieve. Thus a knowledge of the factors influencing these properties becomes increasingly valuable. This paper describes the initial part of a study into the effects of conventional polyol and isocyanate structures on the durability and humid aging of flexible molded foam.

A series of conventional polyols was synthesized and evaluated in an all-TDI molded foam formulation (32 kg/M³ density), with emphasis on compression set, dynamic fatigue (constant force pounding) and humid aging. FTIR was used to measure the effect of humid aging on the hard domain microstructure. The compression set data suggest that increasing the polyol ethylene oxide (EO) cap (endblock) size accelerates humid aging, and the overall EO content (including EO added randomly to the internal block) is important to longer term humid aging (such as during the "wet set" test). The effects of nominal functionality, equivalent weight and unsaturation (monol) content were also measured. Dynamic fatigue (hardness loss) increased with the amount of cross linking in the foam, which increases with decreasing equivalent weight, decreasing unsaturation level and increasing functionality. Humid aging had little effect on the dynamic fatigue.

Additionally, several different isocyanates (including monomeric MDI, a modified monomeric MDI and polymeric MDI) were evaluated in a similar molded foam formulation. This series of foams was subjected to alternating cycles of humid aging and pounding, and a series of physical properties was measured following each cycle. The sensitivities of peak tensile and wet compression set to the amount of humid aging and fatigue correlated with the type of isocyanate. Higher isocyanate functionality decreased sensitivity to humid aging, due to formation of a permanent covalently bound network. Humid aging sensitivity was also affected by the packing behavior of the urea hard domains, which is controlled by the isocyanate structure. The effect of isocyanate on hard domain microstructure is further illustrated using dynamic mechanical thermal analysis (DTMA).