Biuret linkage

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

The situation is even more complex since the N—H bonds of both urethane and urea linkages add to isocyanate groups to form allophanate and biuret linkages, respectively. These... 

Subsidiary chemical reactions can take place. The major of these is the formation of an allophanate cross-link, as illustrated in Figure 2.6. This reaction normally needs a temperature of between 120 and 140°C to take place. The presence of a urea group at 100°C can react with the isocyanate group to form a biuret linkage. This is shown in Figure 2.7. 

Other reactions which lead to branching and cross-linking are the formation of allophanate and biuret linkages. The allophanate linkages occurs when the hydrogen on the nitrogen atom of the urethane group reacts with an isocyanate ... 

The biuret linkage occurring when the hydrogens on the nitrogen atoms in the disubstituted urea reacts with isocyanate ... 

Non-CFC-Blown Urethane-Modified Isocyanurate Foams. Recently, methods of making non-CFC-blown urethane-modified isocyanurate foams have been reported. These methods involve the partial replacement of water for CFC-11 (97). The methods however, have the disadvantages of (a) the higher thermal conductivity of the resulting foams due to the presence of carbon dioxide in the foam cells and (b) the higher friability of foams due to increased urea and biuret linkages (197). 

The reaction can continue between the NH groups and other isocyanate groups with the formation of biuret linkages. 

In addition Duff et al. also undertook a quantitative analysis by utilising the large difference in cross-polarisation time for protonated and nonprotonated nitrogen. In a related study a series of resins were examined in which biuret linkages predominated [18]. These were formed by the reaction of formic acid and 4,4 -methylenebis(phenyl isocyanate) (MDI). The principal reactions that occur in the resins are shown in Fig. 15.2.16. 

The C and N CP/MAS spectra showed that when the formic acid MDI ratio increased the biuret linkage predominates. The pathway for this was initially the formation of MDI-based urea and formic anhydride moieties which further reacted with isocyanate groups to form the biuret linkages and possibly diformyl imide groups. 

Urea and biuret type linkages were all characterised. Biuret structures were predominant when the moisture content was low, gradually being replaced by urea linkages when there was higher moisture content, the formation of urethane and amine moieties also occurred at intermediate moisture contents. This is in contrast to the previous study [17] where only a small amount of biuret linkages were detected. This may be due to the presence... 

For the NCO excess group consumption many more reactions have been suggested. Also evidenced was the spontaneous isocyanate group dimerization at room temperature [322, 323]. In the hot cure reaction, the principal process, admitted by A. Awater, was the appearance of the allophanate and/or biuret linkages [335]. K.A. 

Increased temperature also has an important effect on the properties of the cured polymer since both the allophanate and biuret linkages tend to dissociate at higher temperatures giving a more linear polymer. Some results are given in Table 4.5. 

Of the two reactions, isocyanurate formation is the most widely utilized. Diisocyanates can be converted via this reaction into trifunctional isocyanurate derivatives and subsequently used to introduce branching and crosslinking into a polyurethane.These crosslinks have greater thermal stability than either allophonate or biuret linkages, and hence they are more useful in elevated temperature applications. 

In the early 1950s, flexible foams were made by the prepolymer process. In this process, a prepolymer is prepared by reaction of a diisocyanate with a diol, usually a linear polyether diol similar to those described in Section IV.A for elastomers. The prepolymer polyether diisocyanate, usually a poly(propylene oxide) diisocyanate, has a molecular weight in the range of 1000 to 2000. Water is used as a chain extender in making the prepolymer and also to provide branch sites to increase the functionality of the prepolymer through formation of biuret linkages. 

The carbon dioxide thus liberated initially leaves the reaction mass, but with the progress of polymerization, the viscosity increases and the gas is trapped, giving a cellular structure. Finally, the urethane formed is not necessarily linear, but branches are generated through allophenate and biuret linkages ... 

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