Allophanates urethans

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). 

Catalysts such as dibutyl tin dilaurate or tertiary amines are added to promote the urethane reaction and/or subsequent moisture cure. Dimorpholine diethyl ether is particularly effective at promoting moisture cure without promoting allophanate side reactions at the application temperature (which leads to instability in the hot melt pot) [29]. 

The allophanate linkage is formed by the reaction of urethane with isocyanate, as shown in the fourth item of Fig. 1 [7], Isocyanates can react with many active hydrogen compounds. The active hydrogen of the urethane linkage is not very reactive, but if reaction temperatures get high enough (usually in excess of 100°C), or in the presence of certain allophanate catalysts, this reaction can actually become favored over the urethane reaction (see pp. 180-188 in [6]). 

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. 

The chemistry of the glycolysis of polyurethanes is complicated by the fact that there are additional groups in the polymer such as ureas, allophanates, and biurets, and die PURs may be crosslinked. In die presence of the appropriate glycols and at about 200°C, PURs undergo transesterification to form polyols. Under the same conditions, ureas undergo glycolysis to form urethanes and amines (Fig. 10.5). 

Lastly, of course, the main reaction of interest is the formation of urethane groups by reaction of isocyanate groups and hydroxy-groups of the polyester or polyether. Even these reactions do not exhaust the possibilities available to the highly reactive isocyanate group. It will then go on to react with the urethane links to form a structure known as an allophanate (see Reaction 4.13). 

Carbamates (substituted urethanes) are prepared when isocyanates are treated with alcohols. This is an excellent reaction, of wide scope, and gives good yields. Isocyanic acid HNCO gives unsubstituted carbamates. Addition of a second mole of HNCO gives allophanates. 

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

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... 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, i.e, die urethane group acts as a cocatalyst in the dimerization reaction. By combining cyclotrimeiization with polyurethane formation, processibility is improved, and the friability of the derived foams is reduced. Modification of cellular polymers by incorporating amide, imide, oxazolidinone, or carbodiimide groups has been attempted but only the urethane-modified isocyanurate foams are produced in the 1990s. PUIR foams often do not require added fire retardants to meet most regulatory requirements. A typical PUIR foam formulation is shown in Table 2. 

Figure 2.40 illustrates the formation of an allophanate bond formed between an isocyanate group and a urethane group. 

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 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... 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibility is improved, and the friability of the derived... 

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. 

There are a few other chemical reactions on the wood surface that could make important contributions. One is that of moisture on the surface of wood to form an unstable carbamic acid group that quickly decomposes to form a primary amine with evolution of carbon dioxide. The primary amine formed has active hydrogens reactive to isocyanate. Other successive reactions ensue leading first to disub-stituted ureas and then to biurets. Furthermore, isocyanate reaction with urethane to form allophanates, and trimerization of isocyanates to form isocyanurate are also possible to variable extents, under the conditions of bonding. The different reactions are summarized in Scheme 2. 

A special technique of trimerization has been described by Kogon 24, 25). Phenyl isocyanate reacts with ethyl alcohol to form a urethane (ethyl carbanilate). At 125° a substantial yield of ethyl a,7-diphenyl allophanate is observed as well as a small amount of phenyl isocyanate dimer. However, when A-methyhnorpholine (NMM) is added as a catalyst, the reaction is altered and the product is triphenylisocyanurate (isocyanate trimer) in high yield. The reaction sequence is believed to be ... 

Thus, as shown in detailed studies of the trimer, two isocyanate molecules are supplied by the dimer, and the third by the isocyanate portion of the allophanate. This mechanism applies specifically for the trimerization reaction of an isocyanate carried out in the presence of a tertiary amine catalyst and either an alcohol or a urethane. 

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