Allophanate formation

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. 

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... 

Several types of additives have been found which reduce or eliminate the effects of various catalysts in polyurethane formation. When the catalyst is a simple base, an acid or acid precursor such as an acid chloride will reduce the rate of reaction due to neutralization of the basic catalyst. This is often most desirable, since many of the bases stronger than tertiary amines promote side reactions such as trimerization, carbodiimide formation and allophanate formation. The beneficial effect of acid retarders in a variety of systems was illustrated by Heiss et al. [151], as well as by others. 

Diaryl allophantes as indicated by the above equation have been prepared with R = H, 0-CH3, p-Cl, p-CH R = H, p-CHs, m-OCHa R" = CjHs, CeHs. The allophanate formation fails to occur if the carbanil-ate or the aryl isocyanate is replaced by ethyl carbamate or by ethyl isocyanate, respectively. 

To draw a conclusion, one needs to observe the allophanate formation, but unfortunately there is not a test method available to identify this reaction. 

Due to the electron withdrawing effect of the carbonyl groups, the urethane group has a much lower reactivity than the aminic -N-H groups and in order to promote the allophanate formation higher temperatures are necessary greater than 110 °C. It is important to mention that the allophanate formation is a reversible reaction. 

Similarly to the allophanate formation, the -N-H groups of urea react with isocyanates, to generate a biuret [1, 3, 6-18, 21, 23-25] ... 

Also similarly to the allophanate formation, the reaction between urea and isocyanates is an equilibrium reaction and needs higher temperatures too ( > 110 °C). 

Thus, socydimitgroups exhibit different functionalities according to the reaction conditions they behave bifunctionally in addition polymerization [Equation (15-4)], monofunctionally in condensation polymerization [Equation (15-3)], and semifunctionally with respect to initially present hydroxyl groups in allophanate formation [Equation (15-5)]. Thus, the functionality is not an absolute property, but is a relative parameter which also depends on the reaction partner and reaction conditions. 

Acids influence the NCO/OH reaction by accelerating chain extension a little, and retarding crosslinking. If p-nitrobenzoylchloride is added to a urethane system in which active hydrogen compounds must be present, this additive has a mild catalytic effect on chain extension, no effect on allophanate formation, and a strong retarding effect on biuret formation. If water is present the reaction is strongly catalyzed. 

Isocyanates react with alcohols and phenols to form urethanes. In general, rates of urethane formation decrease in the following order primary alcohols > secondary alcohols > 2-alkoxyethanols > l-alkoxy-2-propanols. Isocyanates can react with urethanes to form allophanates. This reaction is much slower than the reaction of isocyanate with alcohol. Isocyanates react rapidly with primary and secondary amines to form ureas. The reaction is much faster than the reaction of isocyanates with alcohols. Isocyanates can react with ureas to form biurets. Biuret formation is slower than urethane formation, but faster than allophanate formation. Isocyanates react with water to form imstable carbamic acids, which dissociate into carbon dioxide and an amine. The amine is so much more reactive that it reacts with another isocyanate (in preference to water) to form mea. The reactivity of water with isocyanates is somewhat slower than that of secondary alcohols, but much more rapid than that of imcatalyzed reaction with methanes or ureas. 

DBTDL is soluble in a wide range of solvents, comparatively low in cost, colorless, and, in general, highly effective at levels of the order of 0.05 wt%. DBTDL promotes urethane formation without promoting allophanate formation (2) or trimerization (3). While aromatic isocyanates are more reactive than aliphatic isocyanates in uncatalyzed reactions with alcohols, the reactivity of aliphatics can be roughly equal with DBTDL. On the other hand, amine catalysts are more effective with aromatic than aliphatic isocyanates. Carboxylic acids inhibit catalysis by organotin compounds. 

Finally it is known that effects like molecular weight of the block-copolymer, allophanate formation, side reactions, and morphology also influence the TPU properties [1-4]. 

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