ABSTRACT
In the automotive seating industry, convention holds that the density of a polyurethane (PU) foam pad has a strong relationship to its overall durability. As foam density is increased in a given seat system, durability performance with time is thought to improve. Thereby, seat systems suppliers attempt to control long-term complete seat comfort and appearance by constraining foam density. This density to performance trend is generally true; however, recent studies have shown that foams of the same density may have different performance based on chemistry and processing [1]. This leads to a problem for seat manufacturers that receive foam from several different production facilities with different line sizes, catalyst amounts, cure times, and other fundamental processes. Also, as advances in PU formulation are made, or as process conditions change with time, foam durability may fluctuate at a constant density.
These discoveries led to the postulate that although foam density is a large factor in determining mechanical durability, density alone does not quantify other factors that affect foam durability. The hysteresis loss test may better determine foam durability. This test measures the percent of energy not recovered during a compression cycle of the foam. In fact, the word hysteresis means mechanical lost energy. McEvoy has previously presented the hysteresis loss test method [1]. This measure suggests that a lower number is better for foam durability, in that, the foam itself absorbs less energy at a lower hysteresis loss. It will be shown that the hysteresis loss test is a composite measure of several factors affecting durability including density and foam formulation. Therefore, the belief is that this composite measure will better predict long-term mechanical durability than will the density test.
