The reactions described earlier are those based on reactants and intended by the chemist. However, due to the reactivity of the isocyanate group, the adiabatic heat buildup in the reacting mass, and local inhomogeneities in reactant concentration, other reactions can and do occur. The reaction of an isocyanate and a urethane to form an allophanate is one such reaction (Fig. 3.14) . The active H on the urethane nitrogen is quite a bit less active than those available on water or alcohol resulting in relatively unfavorable kinetics of formation (Table 3.5). Allophanate formation can occur under conditions of elevated temperature and the presence of excess isocyanate in the immediate vicinity of the urethane linkage. However, like the urethane bond, at temperatures above 100-150 °C, the allophanate linkage will revert to the urethane and the free isocyanate reflecting the stability of the parent urethane . It is seemingly inevitable that allophanates form in the practice of making urethanes. The industrial approach has in this case made the best of it by formulating reactant concentrations to optimize the number of allophanate linkages that provides crosslinking to obtain the desired final polymer properties [57, 58]. The gradual increase of polyurethane tensile properties upon cooling over a period of days is sometimes considered a result of slow formation of allophanates (among other possibilities including maturation of phase morphology). Due to the thermal reversibility of the allophanate bond, these crosslinks do not have to negatively affect the processability of the polymer .
Figure 3.14 Formation of an allophanate by reaction of a urethane with an isocyanate. R group can constitute an additional isocyanate moiety resulting in crosslinking.
Table 3.5 Relative reaction rates of isocyanates with varying reaction partners