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As the Hysteresis loss value is reduced, the ability of the foam to recover its firmness after being stressed is increased. When you are sitting in a cushion, the foam having the lower hysteresis value will give you more support and a better "Feeling Of Comfort." Hysteresis is controlled by the available number of hydrogen bonds. As the number of hydrogen bonds increases, the hysteresis will also increase. As you sit in a cushion having a high hysteresis value, the hydrogen bonds break and the cushion becomes softer. The percent loss of a 1.25 density product will be 35 to 40 percent, a 1.50 density from 30 to 35 percent and a heavy density reinforced foam 21 to 26 percent. The absolute value is dependent on the variables that affect the ILD measurements and will also vary with the thickness. Therefore, it is important when comparing this value that the measurements are taken by the same method and on the same thickness samples. The constant relative difference, however, between low and higher density foams will be the same. The need for an accelerated test to more closely measure the performance of a foam within the laboratory, before allowing it to go into the market place is quite obvious. Our industry had devised many but has not standardized on any. There are many methods of running accelerated fatigue tests. Seven methods of testing are now being used. It is very difficult to relate results of these tests to actual field performance. The numerical loss from the various methods are difficult to correlate. The only safe observation we make is, a foam giving a lower fatigue loss by an accelerated test will generally give better service performance. Metic- ulous care must be taken when all ILD measurements are being made to assure reliability and the test repeated to assure reproducibility. In review, the major physical properties and measurements con- tributing to cushion performance are density, 25% ILD, Compression Modulus and Hysteresis. I4 In the time remaining, we will briefly examine the other physical properties. The mechanical properties of urethane foam are composed of tensile strength, elongation and tear strength. The foam must have sufficient mechanical properties to allow the user to easily handle and fabricate. Almost all types of urethane foam have mechanical strength values to satisfy these requirements. The importance of mechanical properties can be overstressed. The mechanical properties of water-blown low density foams are excellent. These foams, however, do not give the high total per- formance that one could infer from their high mechanical properties. 9