Biuret effect, urea

Not only N-H bonds from amines can participate in the aminolysis reaction, but also less nucleophilic urea, thiourea and biuret NH units can react with halophosphanes in an effective manner, forming the corresponding phosphinous amides with additional functionalities at the nitrogen atom [39-44]. 

As intended by the adhesive formulation, no parasitic side reactions such as formation of urea, new uretdione, isocyanurate, biuret, or allophanate are detected for the bulk and for thick films on Al, Cu, and Au (dpu>l pm cf Figs. 6.3, 6.7). Even ultrathin PU films on Au correspond to bulk properties to a very good approximation (Fig. 6.8), which is a recommendation for films on Au as references with respect to effects in the interphase on different metals. 

This non-success led them to attempt to combine phenylcarbamic acid azide CgHj.NH.CO—Nj, which Curtius and Hofmann and Curtius and Burkhardt had described in 1896 and 1898, with urea, but again the desired result was not achieved, nor could a combination of this compound with biuret be effected. 

A tailored modifier does not always have to be deliberately added to a crystallizing system it may already exist, e.g., as a synthesis by-product of a chemical reaction. If its presence causes a crystal habit problems, it must be removed or deactivated. On the other hand, it may have a beneficial effect. These are both commonly encountered cases in the manufacture of organic chemicals. A simple, but industrially important, example is the effect of biuret on the crystallization of urea (Davey, Fila and Garside, 1986). In the synthesis of urea (NH2CONH2) from ammonia and carbon dioxide a small amount of biuret (NH2CONHCONH2), a condensation dimer, is formed. The presence of biuret is actually beneficial because from pure aqueous solution urea crystals form as elongated [001] needles that are difficult to process. Biuret retards growth in the... 

Details of the TDI and MDI foam model systems have been previously published [2]. The models require the use of mono-functional reactants that are quantitatively analysed to correlate structure-activity relationships for various classes of catalysts. A realistic thermal profile is produced through the imposition of an external exotherm. Urethane, urea, allophanate and biuret reaction products are quantified by liquid chromatographic analysis of quenched reaction samples. The models effectively account for such nonideal conditions as reactant depletion at variable rates, temperature and concentration-dependent catalyst activity, and catalyst selectivity as a function of isocyanate distribution. 

This mechanism, however, can be considered as a single-step reaction only if the heating is carried out fast. A slow heating of the urea will produce side reactions, due to the high reactivity of isocyanic acid, with the production of undesirable chemical products such as biuret, trimet, and cyanuric acid [25]. Usually, the reaction is supposed to occur without any side effect and is endothermic (+185.5 kJ at standard conditions). 

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