Foam Formulation and Structure-Property Relationships
As discussed in Chapter 5, the properties of a foam are largely defined by the tests that are performed [29-33]. Two foams may feel identical to the touch but perform very differently in tests developed to measure the same property (such as compression set). In addition, subtle changes in test conditions can reveal significant differences in foam performance (Fig. 6.12). This unavoidable fact regarding foam properties
FIGURE 6.12 Compression set for three molded foams measured following 75% compression for 22 h (see ASTM 3574). All foams were four parts water and 105 index. The data shows the sensitivity of foam performance to structural details, as well as the importance of careful control of temperature when making these measurements since errors or large fluctuations can result in artifacts in the data. Reprinted with permission from Ref. . © Elsevier Pub.
FIGURE 6.13 Lack of correlation of 75% humid aged compression set (HACS) with 50% HACS for MDI molded foams. The two compression tests sample mostly different physical properties of the foam. Reprinted with permission from Ref. . © Elsevier Pub.
accentuates the need for very strict control of polymerization conditions to obtain optimized results. Additionally, it points out the need to make very careful measurements, especially with respect to temperature calibration since foam properties can have very nonlinear dependency.
It is generally conceded that the effect of formulation changes is the effect of those changes on the underlying structure as defined by the phase diagrams and phase separation mechanisms that determine that structure [27, 35]. Thus, if a test to measure humid aged compression set is performed at 50% compression, the result may not correlate at all with the same measurement at 75% compression. In fact, it has been demonstrated that the 50 and 75% compression tests do not correlate at all in molded foams (Fig. 6.13). While it is possible to pick points from the data set that appear to show correlation, it is clear from the analysis of large data sets that these measurements probe different aspects of polymer structure. A 50% humid aged compression set may probe the ratio of loss modulus to elastic modulus or the stickiness of the humid aged foam walls, while the 75% compression set measurement may probe the cocontinuity of the hard segment structure and its ability to regain its initial topology within the foam following severe deformation stresses. By the same token, different tests that do probe the same physical characteristics of the foam will correlate to each other, but not show correlation to a measurement that probes a different physical quantity. This is well illustrated by Figure 6.14 in which foam humid aged load loss (compressive softening to 75% compression after exposure to humid heat) correlates very well to 75% compression set but not at all to 50% compression set. As part of this analysis, it has been shown that the foam structural property that correlates to these measurements at high deformation stresses is in fact the hard segment thickness as measured by the SAXS-determined d-spacing (Chapter 5) (Fig. 6.15).
Finally, it is necessary to reiterate that the effect of formulation variables on foam properties is primarily a reflection on its affect on the foam's polymer structure. This
FIGURE 6.14 Correlation of humid aged load loss (HALL), a test that ages the foam in humid conditions and then tests compression resistance to 70% compression versus humid aged compression set (where the foam is maintained in a compressed state for an extended period of time and then removed from compressive strain and allowed to relax to an equilibrium thickness). The two tests that probe at similar strains are highly correlated to each other. Reprinted with permission from Ref. . © Elsevier Pub.
FIGURE 6.15 Correlation of humid aged load loss (HALL) to the small-angle X-ray-determined d-spacing (see Section 5.2.3). All foams have the same hard segment volume and demonstrate that the better the microphase separation, the better the aging properties when all other factors are controlled. Reprinted with permission from Ref. . © Elsevier Pub.
is easily demonstrated by the lack of relationship between the content of DEOA, a polymer cross-linker, and a foam aging property such as humid aged load loss in the absence of control of other structural variables (Fig. 6.16). While it may be possible to perform an experiment in which the progressive addition of DEOA can affect foam
FIGURE 6.16 Lack of correlation of diethanolamine (DEOA, a cross-linking agent that speeds gelation) content in foam formulation and humid aged load loss. DEOA is a casual, not a causative, factor to aging properties. Reprinted with permission from Ref. . © Elsevier Pub.
properties, it must be remembered that the effect of DEOA is actually a measure of its effect on the foam kinetics and resulting polymer structure, which in turn are directly responsible and not casually responsible for foam performance.