While it is a nearly hopeless task to provide specific guidance for making foams due to the huge number of possible components and desired properties, it is possible to understand the basis for the foam formulator or designer to choose components. Regardless of the foam production format (i.e., slabstock or molded), the same physical processes occur, and the timing and intensity of the chemical reactions while the foam is forming must be controlled. Figure 6.4 shows a timeline for the various individual processes that the choice of components, catalysis, surfactancy, temperature, and myriad process details can affect.
FIGURE 6.4 Timeline for the physical and chemical processes occurring in polyurethane foaming. Reprinted with permission from Ref. . © John Wiley & Sons, Inc.
Along the timeline, individual processes can be codependent (for instance, bubble entrainment, C02 generation, and bubble expansion), can be competitive (for instance, the water reaction vs. the polyol reaction with isocyanate), and can be consistently coincidental, but with no clear connection (for instance, urea precipitation with cell opening) .
As indicated in Figure 6.4, the first process is mixing, which involves bubble entrainment. Surprisingly, this is a critical aspect of the foaming process since it has been shown that no new bubbles are formed after this step, and all subsequent processes are bubble growth and coalescence . Thus, the work put into this step by mixing is a critical aspect, and mixer technology is a proprietary and protected intellectual asset. While C02 produced by the isocyanate-water reaction can in principle nucleate and form a bubble that can expand, this is in fact a highly unlikely event. The reason is because the bubble nucleus will of necessity be very small and the increase of free energy (AF) to form the bubble has the following relationship to the radius of the bubble (Eq. 6.1):
where y is the interfacial surface tension, p is the gas density, and r is the bubble radius. The very small size of a nucleating bubble insures that the energy input to form those bubbles spontaneously will be immense [13, 14]. Video recording experiments have been performed, which seem to confirm that no new bubbles are formed in the foaming process after mixing. C02 produced from the isocyanate-water reaction diffuses to the already existing bubbles and solely affects the growth of those bubbles. Reacting polyurethanes that do not have bubbles produced by the mixing process are not observed to foam in any substantial way [15-17].
Following the timeline of Figure 6.4, with the initial rush of bubble expansion (and the increasing system temperature associated with the water-isocyanate reaction), the polyol-isocyanate reaction accelerates, and the system viscosity increases due to increasing molecular weight. With increasing viscosity, the system develops a viscoelastic response to the upward pressure of the expanding gas bubbles. As the gas bubbles contact each other, their initial contact points preferentially increase in size transforming the spherical bubbles into polyhedra. If unimpeded, the gas bubbles grow, coalesce, and burst prior to the system developing sufficient elasticity to stabilize the foam structure. If viscosity increases too quickly such that the cell windows cannot thin and retract to the plateau borders (Fig. 6.5a), the cell windows will remain in place resulting in closed-cell or partially closed-cell foam (Fig. 6.5b). Flexible foams with a large number of unruptured cell walls will present a hydraulic resistance to foam compression (see Section 126.96.36.199 for a quantitative treatment), resulting in the foam being termed "tight."
As discussed in Chapter 4, at some time, the polyurea hard segment will phase separate from the chain extending soft segment to assemble into a microphase structure dependent on the phase incompatibility (quantified by the Flory % value) and the molecular weight of the phase. This precipitation event is often witnessed to occur just prior to the simultaneous cell wall breakages and foam reticulation into an open-cell foam. The reticulation event can in some cases be observed as an effervescence of gas off the top of the foam bun (termed "blowoff'). The coincidence of these events has led some researchers to speculate that the precipitation is itself the determining event for the timing of foam reticulation. However, arguments have been made that the timing of the events is coincidental and not physically dependent .
A method of visualizing and timing the events of polyurethane foaming is to perform an experiment as shown in Figure 6.6 . In this experiment, all of the isocyanate-reactive components and the catalysts are placed in one beaker and mixed with a high-shear mixer. For this experiment, no surfactant is included so that the bubbles grow but collapse rapidly resulting from rapid drainage of the polymer bubble structure. The isocyanate is then poured into the first beaker and the polyurethane formulation again mixed with high shear for a short time (ca. 10 s), and the whole formulation then poured into a disposable plastic bag as shown in Figure 6.6. Following the timeline of Figure 6.4, all of the processes are visible except the growth of the foam into a bun. The bubbles grow and escape through the top of the bubbling mixture. At some point, the polyurea hard segment exceeds its compatibility with the soft segment and precipitates as a white mass at the bottom
FIGURE 6.5 Scanning electron microscope images of flexible foams, (a) An open-cell foam that has had nearly complete drainage of the window materials into the bubble walls (termed the plateau borders (struts)). Remaining bubble windows are indicated, (b) A flexible foam showing the effect of kinetic imbalance such that the chemical processes were truncated before cell windows (lamellae) were drained into the plateau borders.
of the bag. When this experiment is performed carefully, the time reproducibility of the events is within 1%. This experiment is especially useful when comparing foaming kinetics with a new foam component to a well-known benchmark foam composition (Table 6.9).
FIGURE 6.6 A useful experimental technique for evaluating the timing of urea precipitation in a foam formulation. Top left: the experiment is performed in a well-operating fume hood. Top right: after mixing of the components and dumping into the disposable plastic bag (see text), the water reaction to urea and C02 formation are readily visible. Bottom left: the water reaction begins to slow and the urethane reaction begins to accelerate. There hasn't been any microphase separation as observable by the lack of precipitate in the bottom clear liquid. Bottom right: urea precipitation is clearly visible marking the beginning of microphase separation. Reprinted with permission from Ref. . © John Wiley & Sons, Inc.
TABLE 6.9 Effects of foam raw material structure variation on foaming and foam properties