Vibrational characterization of various polyurethane foams employed in automotive seating applications

A current trend in the automotive seating industry is to move to the so called deep foam concept and away from the metal spring suspension foam pad design. The driving force for this change is a reduction in the number of individual components, overall weight and cost, coupled with increased ease of disassembly and recycling. A key concern with the deep foam design is the ability to tune the vibrational or harmonic characteristics of the resultant automotive seat. With the more conventional spring and foam design, both the spring stiffness as well as the foam contribute to the overall vibrational character of the seat. This vibrational character can therefore be tuned by changing the spring stiffness to give the desired assembled seat vibrational character. In the deep foam design, however, the foam vibrational characteristics tend to dominate the assembled seat's vibrational behavior, and therefore foam harmonics become important. The key challenge is to lower the foam resonance to a frequency below the critical human discomfort region while avoiding the natural frequencies of the vehicle. A laboratory scale method has been developed to measure foam vibrational transmissivity and has proven effective in comparing the transmissivity performance of different foam systems. The test employs a constant peak acceleration frequency sweep over frequencies critical to automotive seating. While the test does not predict the actual seat dynamics which are geometry and mass specific, it does provide a method to quickly compare foams and performance trends which will correlate to in-use seating vibrational character. Compared in this study are the vibrational characteristics of the three common polyurethane foam technologies, i.e., TDI HR, TDI hot cure, and MDI HR. Variables explored, where applicable to the specific technology, included all or part of the following: overpack level, isocyanate index, polymer solids, formulation water level, and polyol structure. Vibrational characteristics were correlated to these variables within a specific foam technology using regression analysis. Trends of vibrational behavior with the variables are presented for comparison. Generally, TDI chemistry provided the desired lower natural frequency response and was the least sensitive to processing variables. Correlations of vibrational behavior to key physical properties such as indentation force deflection (IFD) were also included for the three foam technologies.