Surfactancy and Catalysis
The art of polyurethane foam formulation is balancing the competing processes by mixing complementary components such that all the processes of Figure 6.4 occur in amount and timing to result in foam of the correct structure and properties. Thus, along with the choice of polyol, isocyanate, amount of water, and any chain extender, the foam designer must also choose catalyst or often several catalysts and a surfactant. Once the foam designer has chosen a surfactant and suite of catalysts, the foam must then be screened by foaming at a small scale (typically 100 s of grams of raw materials poured into a cardboard box with a polyethylene or Teflon film interlayer). If the choice of surfactant and catalysis is not correct, the screening foams may evidence poor foam openness (see Fig. 6.5b), shrinkage upon cooling, collapse during foaming, poor thermal properties, or even poor aging properties. If the screening foams are acceptable, there is almost invariably additional iteration when the foam is scaled to larger volumes due to the different chemical energy release and foam geometry.
The structure and mechanism of action of amine, organometallic, and trimerization catalysts are presented in detail in Chapter 3. Their incorporation into foam formulations is usually a matter of trial and error in which certain process and foam property specifications are iteratively approached. Along with the representative but far from exhaustive catalogue presented in Chapter 3, there is a growing industrial move away from utilizing organometallic catalysts and toward using reactive catalysts or autocatalytic polyols. The purpose of this industry shift is to minimize contact with fugitive heavy metals and to reduce the volatile organic components emanating from foam in day-to-day use.
Reactive catalysts are typically tertiary amines connected by a spacer group to an isocyanate-reactive group like a primary hydroxyl group [18,19]. They are added to the formulation as one would add a conventional catalyst but typically in larger amounts. The increased usage rate is because of the lower tertiary amine equivalence per gram of catalyst and because the catalyst becomes less effective once the reactive group has been incorporated into the polymer network and is no longer able to diffuse freely. Examples of reactive catalysts are provided in Figure 6.7. Each example has isocyanate-reactive groups. The reactive group chosen for a catalyst has a significant effect on the catalyst function. For instance, a catalyst with secondary hydroxyls will react more slowly into the network than one with primary hydroxyls, so it may maintain catalytic activity for a longer period of the foaming reaction, but risks not being incorporated at all and can result in a volatile amine. A reactive catalyst with amine groups is almost certain to react into the polymer network but may rapidly lose effectiveness due to its inability to diffuse to new reactive sites. Some catalyst designs will ensure reaction into the polymer network by using a highly reactive group, but put a large spacer between the reactive group and the catalytic function that can diffuse through a relatively large spherical or conical volume even after integration into the network. These large elastically ineffective pendant groups have the potential to effect foam properties, especially aging properties.
FIGURE 6.7 Examples of reactive catalyst structures. The tertiary amines are catalytic for the polyurethane reaction.
In addition to reactive catalysts, manufacturers, especially polyol manufacturers, have attempted to incorporate catalytic activity into the polyol backbone [20-22]. A relatively straightforward way is to take a reactive catalyst and grow an alkoxylate chain off of the reactive group using PO or block-polymerizing PO and EO. An example of such a procedure is shown in Figure 6.8.
The effectiveness of these reactive polyols in a foaming reaction is dictated by the same fundamental issues reflected in the behavior of any polyurethane catalyst with the additional complication of steric hindrance of alkoxylate chains and their temperature-dependent interaction with the catalytic center.
In choosing a catalyst, the foam designer must consider the characteristics of the catalyst in light of all of the processing and property attributes required by the customer. This does not always dictate the catalyst with the fastest or most balanced (blow/gelation) attributes. Table 6.10 indicates the effect of catalyst speed on the foaming process. One can see that if the preference is for a soft comfortable flexible foam, one may choose a microcellular foam with good airflow. This would dictate a certain glass transition and a certain hardness dictated by the polyol, the amount of isocyanate, and the amount of water (to control density). It might also dictate a catalyst that gels quickly to promote rapid gelation of the polymer prior to cell expansion, as well a blow catalyst that could develop sufficient pressure to burst the cells at early reaction times. If the molded foam producer needs to increase his/her throughput, he/ she may want to increase his/her overall catalysis and especially his/her blow catalyst to increase the early buildup of heat, but this may dictate that the mold close much sooner than normally practiced since the foam rise may be much faster than normally accommodated.