Formation of Biurets
Just as isocyanate in the presence of urethane at elevated temperatures can create allophanates, isocyanate in the presence of urea can create a biuret linkage (Fig. 3.15). The hydrogen on the urea nitrogen is slightly more reactive than that of the urethane resulting in the biuret linkage having slightly more thermal stability than the allophanate (Table 3.5). Regardless, the biuret linkage is thermo-reversible and has temperature-dependent effects on overall polymer properties [60, 61].
Formation of IJretidione (isocyanate dimer)
The dimerization of isocyanate to form a uretidione is a slow reversible reaction (Fig. 3.16) . Uretidione structures have found industrial use as a crosslinking structure within urethane powder coatings [63, 64], but is otherwise most notable as a problem related to MDI storage. Due to fortuitous packing of MDI in the crystal, the formation of MDI dimers occurs more rapidly in the solid state than in the melt . In fact the rate of formation of MDI dimer is most rapid at normal room temperatures converting approximately 0.01% (wt/day) at 21 °C. For this reason, it is common practice to store MDI either under refrigeration or in the melted state to increase shelf-life and to filter the molten MDI prior to use . Dimer has a nearly zero heat of formation and the disassociation occurs at relatively low temperatures (80-100 °C). This permits uretidione to serve as a protected isocyanate group in a manner similar to that described previously with phenols. This is particularly useful for 2,4 TDI protection since there is a substantial difference in the reactivity of the isocyanates, and the formation of trimer does not present itself as a competitive reaction .
Figure 3.15 Formation of a biuret linkage by reaction of urea with isocyanate. The R group can constitute an additional isocyanate moiety resulting in crosslinking.
Figure 3.16 Dimerization of isocyanate to form uretidione.