Biurets special

Commercial urea generally contains a small amount of the compound biuret [108-15-0] NH2CONHCONH2. For fertilizer use other than foflar apphcation, biuret contents of about 1.5% or less are not harmful, and under usual production conditions it is not difficult to avoid higher biuret levels. For most fohar apphcation, however, the level should not exceed 0.1%, and special production modifications are usually requited to ensure such a low level. 

In the majority of cases the addition product is stable, but in some special cases it is only moderately stable and may either dissociate to form the initial reactants again or decompose to other products. Secondary reactions of isocyanates that are important in the formation of urethane polymers are those with urea and urethanes. These reactions result in the formation of biuret and allophanate, respectively (Figure 2.19). The relative reaction rates of active-hydrogen-bearing compounds with isocyanate are given in Table 2.7. 

The titration of small volumes requires special biurets. Modified stopcock burets have been described for use in the microliter range, but the accuracy is never greater than 1 (il (P4). Manipulation is very diflBcult and not suited for the routine laboratory. 

Isocyanates have several more reactions that are important in some more specialized applications (Fig. 3.2). Cyclotrimerization produces the isocyanurate ring, which is extremely stable, and can be used to build more heat resistance into polyurethanes. Excess isocyanate can react with the N-H group in polyurethanes to produce allophanate crosslinks, which add to the cure of the polyurethane. And excess isocyanate can similarly react with the N-H groups in polyureas to produce biuret cross-links, which add to the cure of the polyurea. 

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