Use of Aliphatic Isocyanates
The primary uses of aliphatic isocyanates are in coating applications (see Chapter 10) because of their preferred weathering performance relative to aromatic building blocks [165-167]. However, along with weathering, coatings often must also provide other properties such as optical clarity, a high glass transition temperature, and barrier to a variety of chemicals the coating may encounter in use. The ability of isocyanates to form cross-linking structures by self-reaction is made use of in this instance [168, 169]. The mechanism of these cross-linking reactions is covered in detail in Chapter 3. The most common cross-linking applied to aliphatic isocyanates and particularly for coating applications is their processing to isocyanurates (Fig. 2.81) [170-172].
Unlike dimer formation, the trimer is a nonthermoreversible structure. Trimer formation is highly exothermic with a heat of reaction estimated to be 42kcal/mol of trimer. Since trimerization of difunctional isocyanates leaves free isocyanates to participate in further trimerization, the conditions of reaction have to be controlled to limit oligomerization. Along with providing increased cross-link density, it enhances flame retardancy , thermal resistance , and chemical resistance ; enhances film forming ; and improves worker safety  by reducing isocyanate vapor pressure. By virtue of the cross-link density, polyurethane-phase separation processes are inhibited, and desirable amorphous coatings are promoted.
FIGURE 2.81 Tnmenzation of aliphatic isocyanates for commonly preceding their use in coatings formulations.
Distinguishing the aliphatic isocyanate monomers based on structure is difficult because HDI is seldom used as a monomer but is usually used in the trimer form, so its application is substantially as a cyclic. IPDI is used both as a monomer and in its trimer form, and H12MDI is used almost exclusively in its monomer form. As such, there is a significant overlap in the resulting polyurethane properties, and customers will often use one or the other based on preference. It is also quite common to mix aliphatic isocyanates for formulation optimization . Forming of mixed trimers has also been reported. In this case, the mixture randomization is achieved by staged addition of isocyanates to compensate for the differences in isocyanate reactivity (for instance, IPDI is somewhat more reactive than HDI).
Although the great majority of aliphatic isocyanates are produced via phosgene chemistry, the higher prices commanded by aliphatic isocyanates has allowed non-phosgene routes to diisocyanates to realize industrial reality. Conversion of IPDA to
FIGURE 2.82 Conversion of isophorone diamine to isophorone diisocyanates using urea and aliphatic alcohol reagents to form products. Reaction pathways are speculative.
IPDI via reaction with DMC to form the biscarbamate followed by thermolysis to the isocyanate and methanol (see Section "Thermolysis of Carbamic Acid,AyV'-(4-Methyl-l,3-Phenylene)Bis-,C,C'-Dimethyl Ester Made from the Reaction of Toluene Diamine with Methyl Carbonate") has been practiced commercially by Daicel Chemical (Japan). Degussa has produced IPDI by reaction of IPDA with urea and alcohol to form the diisocyanates and ammonia by-product (Fig. 2.82). The mechanism of this route is potentially complex with many possible routes to go from products to reactants. Problems associated with this route have been relatively slow rates at low temperatures and the formation of poly ureas at high temperatures. A series of patents document procedures for efficiently producing high yields of the desired diisocyanates [179-181]. A series of reaction pathways or reactants to products is offered in Figure 2.82 .
An additional route involves the formation of a carbamate by reaction of urea with an alcohol (see Chapter 12) and substitution of the target amine for the unsubstituted carbamate.