TABLE OF CONTENTS There are five ingredients that are common to all flexible polyurethane foams: isocyanate, polyol, water, catalyst, and surfactant. However, few formulations are actually that simple. There are a number of other additives that can be used to improve processing, performance, appearance, or stability of the foam. By choosing the raw materials and process conditions carefully, the manufacturer controls the physical properties, mechanical properties and appearance of the foam [7].
As previously mentioned, all of the reactive chemistry that occurs during the foam manufacturing process involves isocyanate groups. Several isocyanates are commercially used in the polyurethane industry, but the majority of slabstock foams (those made in long, continuous blocks) are made with toluene diisocyanate (TDI) (see Figure 5.3). This is a low viscosity aromatic isocyanate that has hindered and unhindered isocyanate groups that react at different rates. The most common grade is known as TDI-80, which has an 80/20 mole ratio of the 2,4 and 2,6 isomers of TDI. TDI-65 is also popular in Europe, but infrequently used in the rest of the world. With its higher content of the 2,6 isomer, TDI-65 gives a stiffer foam with a more open cell structure.
Isocyanates derived from methylene diphenyl diisocyanate (MDI) are used in both moulded (those made in a closed mould) and slabstock [8] foams. These are also preferred for low resilience viscoelastic foams [9]. MDI types cover a number of related chemical structures. Pure MDI is not often used...
