Ureas allophanate

Glycolysis is the most promising approach for the chemical recycling of polyurethanes.1 The chemistry of PUR depolymerization is complicated by the presence of other chemical groups in the polymer, such as ureas, allophanates, and biurets. 

Figure 38. Perspective view of a portion of the wide urea-allophanate ribbon linked by hydrogen bonds in (rvCjH7)4N+NH2CONHCOi-3(NH2)2CO (1.9).

In one of the two urea-allophanate compounds (1.8, Figure 11), hydrogen-bonded zigzag ribbons are built up from repetition of the allophanate ion and one independent urea molecule joined together by a pair of N-H -O hydrogen bonds (LF type), whereas wide puckered urea and allophanate ribbons connected by hydrogen bonds exist in the other (1.9, Figure 38). 

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. 

Figure 8.29 Crystal structure of 14 showing two series of urea-allophanate ribbons running parallel to the [110] and [HO] directions and their interlink es by hydrogen bonds to form channels extending parallel to the [101] direction...

C2H4N2O3, NH2CONHCOOH. Unknown in the free state as it breaks down immediately to urea and COi- The NH4, Ba, Ca, K and Na salts are known and are prepared by treating ethyl allophanate with the appropriate hydroxide. The esters with alcohols and phenols are crystalline solids, sparingly soluble in water and alcohol. They are formed by passing cyanic acid into alcohols or a solution of an alcohol or phenol in benzene. The amide of allophanic acid is biuret. Alcohols are sometimes isolated and identified by means of their allophanates. 

Not only are these reactions of importance in the development of the cross-linked polyurethane networks which are involved in the manufacture of most polyurethane products but many are now also being used to produce modified isocycuiates. For example, modified TDI types containing allophanate, urethane and urea groups are now being used in flexible foam manufacture. For flexible integral foams and for reaction injection moulding, modified MDIs and carbodi-imide MDI modifications cU"e employed. 

The water reaction evolves carbon dioxide and is to be avoided with solid elastomers but is important in the manufacture of foams. These reactions cause chain extension and by the formation of urea and urethane linkages they provide sites for cross-linking, since these groups can react with free isocyanate or terminal isocyanate groups to form biuret or allophanate linkages respectively (Figure 27.5). 

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]. 

Reactions of urethane and urea groups to form allophanates and biurets. 

Examples of non-urethane linkages derived from isocyanates a) urea, b) urea, c) biuret, d) amide, and e) allophanate... 

One can observe positive deviations in the region of rH < 1 (excess of isocyanate groups) which are due to side reactions (allophanate, urea and biuret groups). In the region of rH > 1 the agreement of wg values is good. In the case of i e, the predicted curves depend not only on the results of the branching theory but also... 

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... 

This addition reaction proceeds readily and quantitatively. Side reactions can give amide, urea, biuret, allophanate, and isocyanurate groupings, so that the structure of the product can deviate from that above such side reactions are sometimes desired (see Sect. 4.2.1.2). 

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. 

The isocyanate (2270 cm"1) uretedinedione ring carbonyl (1780 cm"1) and urea carbonyl (1660 cm"1) groups can usually be identified. Carbonyls from ester, urethane, allophanate, isocyanuric acid ring and Biuret groups all absorb near 1730 cm 1 and are difficult to distinguish. Hydrogen bonds which can function as physical crosslinks in PU have been... 

Allophanate esters H2NCONHCOOR are among the oldest organic compounds recorded in the literature. The parent allophanic acid, H2NCONHCOOH, is not known in the free state, whereas inorganic allophanate salts are unstable and readily hydrolyzed by water to carbon dioxide, urea, and carbonate. However, the elusive allophanate anion can be generated in situ and stabilized in the following three inclusion compounds ... 

Part of the host framework in [(CH3 )4N]+[H2NC0NHC02] - -5(NH2)2CO showing the double ribbon constructed by allophanate anions and one of the independent urea molecules. From T. C. 

The extraction with ligroin may be completed satisfactorily by means of the apparatus described in Org. Syn. 2, 48 (1922). Butyl allophanate is not soluble in ligroin and, if present, would remain with the cyanuric acid. However, when the material which is insoluble in ligroin is washed with hot acetone the washings yield no appreciable residue on evaporation. This is evidence that butyl allophanate and urea are not present in the cyanuric acid. 

Fusion of (279) with urea or ethyl allophanate or reaction with excess ethyl chloroformate yielded the 3-hydroxy derivatives (91). Urea seems to be the reagent of choice (66JOC251) and involves a semicarbazide intermediate. The spedral data for these compounds indicate that they exist predominantly as the keto tautomer (282). 

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