Isocyanates, especially polyisocyanates, are highly identified with polyurethane chemistry. While isocyanates are capable of undergoing many reactions, their significance is usually in the context of their relationship to polyurethane polymerization and structure. Isocyanates represent a class of chemicals that are characterized by high reactivity and versatility. This combination of positive attributes has contributed greatly to the broad application of polyurethane materials but is part and parcel of the complications associated with isocyanates . The most chemically relevant attribute of isocyanate chemistry is its reactivity with molecules having active hydrogens. Such active hydrogens are typically found on molecules having alcohol and amine functionalities and also water . The basic isocyanate structure is given in Figure 2.46.
Since addition polymerization requires that monomers be able to propagate a chain by undergoing multiple reactions, polymerizable isocyanate monomers have at least two isocyanate functionalities. The growth of polyurethane technology has been associated with the ability to produce polymerizable isocyanate monomers at low cost. The low-cost requirement has driven the chemistry to employ available and low-cost building blocks, and industrial production has been optimized to an exceptional extent. The two highest volume isocyanates are based in one case on toluene to make toluene diisocyanate (commonly referred to as TDI) and in the other case aniline and formaldehyde to make methylenebis (phenyl isocyanate) (commonly referred to as MDI). The preparation of each of these important molecules and their characteristics will be handled separately. Polyurethanes also function in any number of applications requiring good weatherability. Good weatherability can be hindered by aromatic ring structures, and entirely aliphatic polyisocyanates have been developed for these uses and they will be handled as a class in this chapter.
As discussed in greater detail in Chapter 3, the utility of isocyanates is a result of their electronic structure. As illustrated in Figure 2.47, the unique N=C = 0 triad results in an electrophilic carbon and a relatively nucleophilic nitrogen . Without going into detail here, this creates an advantageous condition for addition of active hydrogen molecules and formation of urethanes, ureas, allophanates, biurets, and other structures.
The benefits of this seemingly simple reaction have resulted in an industry representing the sixth most common plastic material produced . Much of this is owed to the ability to produce isocyanates from low-cost feedstocks using procedures that
FIGURE 2.46 The isocyanate function.
FIGURE 2.47 Assembling the reactants into a transition state geometry for urethane formation.
even if complex could be optimized to an extent that cost is not a barrier to use. As will be discussed at length in the following, the ability to produce isocyanates with attractive economics is because the pathways to product are well established. The result of this is that many isocyanates can be imagined and have been produced using these processes. Regardless, 90% of all isocyanates produced are based on aromatic feedstocks, and of these, there are only two substrates for polyisocyanates: one beginning with toluene and the other beginning with aniline . From these substrates are produced TDI and MDI (or pMDI). The other 10% of isocyanate volume is based on one of several aliphatic substrates .
As commodity chemicals, the isocyanate segment of the polyurethane industry is closely associated with the equilibrium between industrial capacity and consumption . Unlike the polyols market of which there are many varieties and continuous innovation based on structure, the isocyanates market is reasonably defined by the TDI and MDI markets and their component feedstocks. Figure 2.48 describes 2011 estimates for the balance between isocyanate industrial capacity to produce and industrial consumption. The ratio of consumption to capacity was about ca. 0.8, suggesting a significant amount of overcapacity in world markets. To an extent, this reflects the downturn in western construction markets and overbuilding of capacity in some developing economies that occurred prior to the economic downturn of 2008. Significant capacity increases in the Middle East are planned under the premise of future economic growth and adoption of rising living standards requiring polyurethane products. The expected annual growth rate is 4.6% for TDI, 5.2% for MDI, and 4.0 for aliphatic isocyanates. The reasons for these disparate growth rates reflect the differing growth rates for the underlying applications and increasing industrial preference for MDI and its preferred occupational health and safety profile.