Over 75% of flexible foam products targeted at the transportation market are designed for seating within automobiles, buses, trucks, airplanes, etc. . Additional areas employing flexible polyurethane foams are instrument panels, headliners, door panels, arm and head rests, and also within structural members for the purpose of noise reduction. Each application has specific requirements that are met by specific and highly optimized formulations.
As mentioned in Chapter 6, the comfort aspects of seating are subjective, and different world geographies have consistently developed formulations that reflect these preferences. Thus, the North American market typically seeks greater softness in seating, translating to lower-density TDI-based foams. The European market has generally delivered firmer seats using higher-density foams based on MDI. The Asian market has generally sought a middle ground between these consumer expectations . In this regard, the aging demographics of the traveling public have evolved over time to seats that offer firmer support in seat backs. Due to the importance of seat comfort to the purchasing decision and the perception of delivered comfort, automobile manufacturers have specific standards that foams must deliver in every aspect of their static, dynamic, and aging properties. Furthermore, the specifications for driver, front passenger, and rear seats are usually different and in order of declining quality .
Given the proliferation of specialty polyurethane foams sought by manufacturers and consumers for transportation applications, the best that can be offered are representative recipes for foams that have a good chance of working in typical equipment. While some transportation applications such as carpet underlay and headliners can be made from slabstock foams, almost all seats are prepared by molded foam operations. The specificity of molding equipment is an additional overlay of complexity in creating useful foam since different molding equipment may produce somewhat different foams with the same polyurethane formulation [35-37]. Table 7.3 provides a representative molded foam formulation based on TDI, molding conditions, and properties for an automotive seat cushion that might be found on a car manufactured in the United States.
An MDI-based molded foam formulation is presented in Table 7.4. While the conditions and results are somewhat different than those of the TDI foam of Table 7.3, these distinctions should not be viewed as a general representation of the differences. The opening and demold times can be as much about arbitrary process differences as about demands resulting from the specific formulation. Molded foam operations also commonly use pre-polymers (Section 2.1.7) rather than pure pMDI/isocyanate monomer mixtures. The pre-polymer route may ease certain process requirements related to mixing, pumping, and mold filling, justifying the added complexity and expense of employing a prepolymer.
While the formulation choices and foam property variability can be mind-boggling, there are a few general rules for molded foam types and compositional choices (Table 7.5) and formulation variables and their effects on molded foaming and molded foams (Table 7.6).