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i l The excessive presence of these hydrogen bonds in low density foams cause a high degree of fatigue and softening, front edge set and cushion distortion. We have discussed the chemistry of urethane foams. Now let us turn to the examination of their physical properties. The first important physical property is density. The remarkable capabilities of the basic urethane polymer linkage is more clearly understood when you consider the following. Solid urethane rubber will have a density of approximately 68 lbs. per cu. ft. A typical urethane foam has a density of 1.5 lbs. per cu. ft. You can see the rubber has been expanded to approximately 45 times its normal volume by the expanding gases in the reaction. Dens- ity is controlled by the amount of water and/or gas producing fluoro- carbon in the formulation. The performance of the urethane molecule is rather remarkable when you consider the amount it has been stretched. �l Density is an easy property to measure, requiring only a scale and ruler and some simple arithmetic. Frequently density is confused with firmness. Here we have two samples with the same firmness but one sample has a density of one pound per cu. ft. The other has a density of 1.80 lbs. per cu, ft. It is likewise possible to have the same density and a wide variation in firmness, as illustrated by these samples, both at a 1.25 lbs. per cu. ft. density, one sample is firm ( 32 lbs. ILD ) and the other soft `. (14 lbs. ILD). Before specifying the density of a foam for an application, the in- tended use must be known. If it is a soft foam to be used in the back of a chair, then a density of 1.10 lbs. per cu. ft. can be satisfactory. These foams are generally formulated as is a 1.50 density foam. Ad- ditional fluorocarbon is added to reduce this density to 1.10 lbs. per cu. ft. However, if the same degree of softness and luxury is required for a seat cushion, then a density of 1.8 lbs. per cu. ft. should be specified to obtain satisfactory performance. The hydrogen bond con- tent for this material is relatively low. In a seat cushion the stress is much greater than in a back cushion. Therefore, the hydrogen bond content in the seat cushion should be low to obtain the necessary performance. The next important property of urethane foam tries to define the "Feel Of Comfort" obtained from a furniture cushion. What do you specify when you want a cushion to have a certain feel in a particular chair or sofa? The cushion can not be too firm i 5