Structural and viscoelastic studies of flexible polyurethane foams
In this study, the viscoelastic and morphological characterization of molded foams was the main focus. A series of molded foams based on toluene diisocyanate (TDI) and glycerol initiated polyethylene-oxide-capped propylene-oxide was studied in terms of the structure property features. The results were in many instances 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 and very important systems. It was found that high temperatures and humidities "plasticized" the viscoelastic behavior of molded foams to a greater extent than that of slabstock foams; the molded foams displayed higher load decay values 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 evaluated. It was discovered that the structure of the hard segment domains dictated the foam's behavior, especially at elevated temperatures and humidity. Furthermore, it was found that the hard segment domains in slabstock foams had a much higher level of short range ordering. This was confirmed by wide-angle x-ray scattering (WAXS) and fourier transform infrared (FTIR) which revealed that the conventional slabstock foams had much more organized hard segment domains. It is thus concluded that the dramatic differences between the mechanical properties of molded and slabstock foams are due to the lower and weaker ordering of the hard segments in molded systems making these physical "cross-links" more labile at higher temperatures and humidities. These morphological differences were shown to be due primarily to differences in the formulation components between the two studied systems. First, the ethylene-oxide capping used in the polyol of molded foams to increase the reactivity is known to also increase the compatibility between the hard and soft segments thus promoting some phase-mixing. Second, the addition of diethanolamine (DEOA) added in the molded foam formulation to decrease demold times by enhancing cross-linking clearly resulted in the prevention of the full development of the hard-segment domains.
It was also found that the copolymer polyol particles (CPP), added to molded foams to increase load bearing capabilities, had a negative effect on the viscoelastic properties. The viscoelastic properties of the CPP containing foams were more time-dependent than those of the foams lacking these particles. As expected, the incorporation of these particles increased the initial load and decreased the initial strain over the foams lacking the particles suggesting that the initial stiffness of these materials was increased. However, over a period of time, the amount of this initial load that decayed was greater for the CPP containing foams and furthermore, at elevated conditions, the load decreased to levels below those of the CPP lacking foams.
A series of slabstock foams was also studied to evaluate the effect of toluene diisocyanate (TDI) index on the physical properties, and morphology of the foams. Extraction experiments using dimethyl formamide (DMF) showed that increasing the index increased the level of covalent cross-linking with perhaps a maximum being reached at an index of 100. Viscoelastic measurements also supported the claim of increased crosslinking with TDI index. The initial load in load relaxation experiments systematically increased with increasing TDI while the percent decay in a three hour period decreased. Temperature and/or humidity "plasticized" the load relaxation behavior in all the foams studied indicating that the hard segment domain physical "cross-links" play a significant role in the properties of these materials. The morphology of the foams was also found to be influenced by the TDI index. Small angle x-ray scattering (SAXS), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) all provided evidence that an increase in the TDI index promoted phase mixing. FTIR and WAXS showed that the short range ordering within the hard segment domains displayed a maximum at an index of 100 and decreased as the index was increased.
Finally, the influence of transient moisture conditions on the viscoelastic behavior was also investigated. In creep extractions, as the moisture conditions were cycled from low to high humidity while maintaining constant temperature, the compressive strain increased in subsequent steps where the strain levels under cyclic moisture conditions surpassed those observed at the highest constant relative humidity. This overall phenomenon of enhanced creep under cyclic moisture levels was attributed to water interacting with the hydrogen bonded structure within the foam. These hydrophillic interactions, principally promoted within the hard segment regions due to high hydrogen bonding, are disrupted causing slippage and increases in strain. As the foam is rapidly dried, regions of free volume are induced by the loss of water thus causing further increases in strain prior to the re-establishment of well ordered hydrogen bonding.