Structure-property relationships of flexible polyurethane foams

Abstract

In this study the viscoelastic and compression set properties were studied as a function of temperature and humidity for a series of moulded foams based on toluene diisocyanate (TDI) and glycerol initiated ethylene-oxide-capped propylene-oxide. The results were compared to those obtained on conventional slabstock foams based on TDI and glycerol initiated propylene-oxide. These comparisons were made to delineate and clarify distinct differences between these two different but very important systems. It was found that high temperatures and humidities ‘plasticized’ the moulded foams to a greater extent than the slabstock foams. Moulded foams displayed a higher compression set value and higher load decay value in the viscoelastic measurements than slabstock foams. In an attempt to understand these dramatic differences, the two types of ‘cross-links’ (covalent cross-links and urea-based phase separated hard segment domains) were more directly compared. It was found that the hard segment domains in the slabstock foam had a much higher level of short range ordering and cohesiveness. This was first confirmed by comparing the wide-angle X-ray scattering (WAXS) patterns where the amorphous character was much more pronounced in the moulded systems of equal water/TDI content. The WAXS behaviour of the slabstock system distinctly displayed short range ordering of the TDI based units. Second, Fourier transform infrared ( FTi.r.) data from the carbonyl region showed that the slabstock foam had a much higher level of bidentate urea (strong hydrogen bonding) within the hard segments. Conversely, the moulded foam displayed no bidentate urea but only monodentate urea or weaker hydrogen bonding within the hard segments. This is not to imply that microphase separation did not occur since small angle X-ray scattering (SAXS) clearly showed evidence of a phase separated morphology for both types of foam systems. It is thus concluded that the dramatic differences between the mechanical properties of moulded and slabstock foams are due in part to differences in the covalent network but mostly due to the lower and weaker ordering of the hard segments in moulded systems making these physical cross-links more labile at higher temperatures and humidities. These morphological differences are believed to be due primarily to two differences in the formulation components and processing between the two studied systems. First, the ethylene-oxide capping used in the polyol of moulded foams to increase the reactivity is known to also increase the compatibility between the hard and soft segments. Second, the addition of diethanolamine (DEOA) added in the moulded foam formulation to decrease demould times by enhancing more rapid cross-linking has been shown to prevent the full local packing development of the hard-segment domains (physical cross-links).