FLEXIBLE FOAM FORMULATION AND STRUCTURE-PROPERTY RELATIONSHIPS
The preparation of flexible foams will usually begin with screening of formulations. It is not uncommon for initial attempts to fail spectacularly with small foam (100 s of grams of material) made in a box rising a few inches and then collapsing. As the iterative process continues in which catalysts, surfactants, components, and their amounts are adjusted, the results will usually improve. At that point, it is necessary to obtain quantitative data on the screening studies by which to compare relatively similar results. Among the techniques are measurement of the time dependence of the foam rise and the temperature rise during the experiment. While the measurement of foam rise is relatively straightforward, the measurement of temperature requires the use of a low mass thermistor or thermocouple placed strategically in the foaming mass. The type of data obtained is typified by that shown in Figure 6.11. Within
FIGURE 6.11 Measured properties from small-scale polyurethane foam screening experiments, (a) Foam height change with time, (b) Foam temperature change with time, (c) Rheology change with time—the inflection at ca. 150 s may reflect the precipitation of the hard segment. Reprinted with permission from Ref. . © Elsevier Pub.
the height data, one can see that for some of the foams, there is a distinct shrinkage that occurs shortly after reaching full height. This effect is termed "sigh back" in the industry. It is a result of the loss of foam pressure holding the inflated foam coupled with insufficient polymer strength to hold the weight of the polymer. While this is not always the point of highest chemical exotherm as measured by the foam's internal temperature, it is at elevated temperature and sets a relatively difficult to achieve criterion for the required polymer properties to maintain foam height. The temperature data is a direct measure of the rate of reaction, which is of course influenced by the formulation. It allows one to determine catalyst levels to meet the required timing of events as previously described. It is not by coincidence that in the illustrative data set, the foam exhibiting the lowest temperature, increasing at the slowest rate, also had the lowest foam rise and most severe sigh back afterward. Additionally, one can obtain a measure of the changing foam rheology with time using one of the several rheometers engineered for the purpose. These instruments typically use a vibrating probe that is submerged into the foaming mass. The change in polymer rheology affects the amount of energy needed to maintain the resonant energy or affects the frequency itself . This is a notoriously tricky measurement, and the experimentalist must get numerous measurements on a single formulation to be satisfied that there is interpretable data.