About 500 million pounds of aliphatic isocyanates was consumed in 2011, and the expected growth rate for this industrial segment is between 3 and 4% per annum. The vast majority of aliphatic isocyanates are used for coatings. This application particularly takes advantage of the reduced weather-induced coloring of aliphatic-based urethanes versus aromatic-based polyurethanes (Fig. 2.74). Additionally, a small amount of aliphatic isocyanates is used in thermoplastic polyurethane films that are also used for weatherable applications. About 40% of aliphatic isocyanates are consumed in Asia, about 34% in Europe, and 24% in North America. The growth
FIGURE 2.73 Measured percentage molar conversion of MDI monomer to dimer per day as a function of temperature.
rate is highest in the developing economies, but does not mirror the difference in overall economic growth with growth somewhat slower than expected on macroeconomics . Aliphatic isocyanates are typically characterized in contrast to aromatic isocyanates by their superior weatherability, by their relatively slow reactivity, and by their being substantially more expensive.
Three aliphatic isocyanates represent more than 95% of the total aliphatic isocyanate world production. They are in order of size hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and the hydrogenated version of MDI, 4,4'-diisocyanatodicyclohexylmethane (H12MDI). Approximately half of the total used is HDI, 30% is IPDI, and the rest is H12MDI. Almost 90% of aliphatic isocyanates are used in coatings and 6% in elastomers. Almost half of the polyurethane coatings that use aliphatic isocyanates are automotive related. The other volumes are spread disparately over many relatively smaller applications. The structures of the three main isocyanates are shown in Figure 2.75.
The need for aliphatic isocyanates stems from the photooxidative instability of aromatic isocyanate structures. The photooxidative instability mechanisms have been well studied and are, of course, complex . MDI-based urethanes tend to show an increased tendency to yellow upon light exposure. It is believed that this is due to formation of the quinonediimide product shown in Figure 2.74 [153, 154]. It is also shown that under other circumstances, a photo-Fries-type rearrangement can occur resulting in anilide formation, which is known to oxidize and discolor upon further oxidation (Fig. 2.74). In addition, it has been shown that the soft segment can have a significant influence on photodegradation with increased degradation occurring with increased soft segment flexibility .
FIGURE 2.74 Oxidation processes of isocyanates leading to color in polymers. The intrinsic nature of these reactions leads to the favored uses of aliphatic isocyanates in weathering applications.
FIGURE 2.75 Structures of common and commercially available aliphatic isocyanates.
FIGURE 2.76 Process to produce hexamethylenediamine from adipomtrile.
FIGURE 2.77 Phosgenation of hexamethylenediamine to hexane diisocyanates.