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The urea linkage is a short length polymer which cross links the <br /> urethane chains to contribute stiffness or firmness to the foam. (Re- <br /> fer to Figure Number 3.) <br /> URETHANE POLYMER STRUCTURE <br /> POLYOL <br /> JUREA =() <br /> =UREA <br /> FIGURE 3 <br /> Both the urethane and urea linkages are considered primary in <br /> nature and are difficult to destroy. <br /> The urea linkages create hydrogen bonds that serve as a source <br /> for additional firmness. These hydrogen bonds are secondary and <br /> are the result of weak chemical-electrical attraction between molec- <br /> ular groups that contain hydrogen and oxygen. The primary urea <br /> linkages have a number of oxygen groups and in such a position <br /> hydrogen bonds can easily form with adjacent hydrogen molecules. <br /> These hydrogen bonds contribute materially to the firmness of foam. <br /> They do not have however, the permanent strength of the primary <br /> urea and urethane linkages. These hydrogen bonds are relatively <br /> quickly and easily broken but are not quickly reformed. <br /> How are these hydrogen bonds broken? A physical working of the <br /> foam when sitting on a cushion will stretch, twist and compress the <br /> molecules. This stresses the bonds and some of them will break. <br /> It is possible to produce very firm low density foams because of <br /> these contributing factors from the urea groups. As the water con- <br /> tent of the formulations is increased, more gas is generated. This gas <br /> produces a lower density product. The firmness of the foam is main- <br /> tained by the firming influence of the hydrogen bonds and urea cross <br /> links. <br /> 4 <br />