TABLE OF CONTENTS The solid polymer that makes up the struts and membranes of an FPF is called polyurethane by convention, but is really much more than that. It is a polymer with ester and/or ether bonds from the polyol portion, and urethane and urea bonds from the reactions of isocyanate. Polyurethane foam starts out as a liquid mixture of three reactants with a number of additives. Within a few minutes it becomes a solid network of polymer that can have 99% of its volume filled by air. Despite the complexity of the system, the chemistry used to describe it is actually quite simple.
Isocyanates are involved in all of the chemistry that occurs during polyurethane foam manufacturing. The two major reactions are known as the gel and blow reactions. The gel reaction takes place between isocyanate and hydroxyl groups to yield a urethane linkage that builds molecular weight and gels the polymer. The blow reaction occurs in two steps and consumes one water molecule and two isocyanate groups. Water first reacts with isocyanate to produce a carbamic acid intermediate that quickly decomposes to give an amine and carbon dioxide. Carbon dioxide is the blowing gas which fills the cells. The amine reacts with a second isocyanate to form a urea linkage. Both the gel and blow reactions build molecular weight and therefore cause the polymer to gel, but the blowing gas is only produced from the water reaction. These reactions are shown in Figures 5.1 and 5.2.
