Allophanate and Biuret Formation

The initial step to prepare polyurethane polymers for solution wet or dry spinning includes reaction of 1000-3500 molecular weight macroglycol with a diisocyanate at molar ratios of between about 1 1.4 and 1 2.0. Reaction conditions must be carefully selected and controlled to minimize side reactions, eg, allophanate and biuret formation, which can result in trifunctional branched chains and ultimately to insoluble cross-linked poljuners. For the prepolymer reaction, poly(tetramethylene ether) glycol [25190-06-1] and bis(4-isocyanatophenyl) methane [101-68-8] are currently the most commonly used macroglycol and diisocyanate. Several types of polyester-based macroglycols are included in spandex producers product lines, but with the exception of Dorlastan, made by Bayer AG in Germany, the polyester-based products represent only a minor part of their spandex fiber production. 

In reality, allophanate and biuret formation is reversible at temperatures above 130°C [252]. Formation of isocyanurates causes a reorganization of the CLD akin to what happens in reversible polycondensations because of exchange reactions. 

Model urethane and urea linkages were created using a prepolymer process. The small molecules were characterized using solution and solid state NMR spectroscopy. The results of the project were identified peaks on the NMR spectra, reactions times, and a study of allophanate and biuret formations. 

It follows from the structure of cured KL-3 that secondary reactions are possible in the formation of the network manifested in the formation of allophanate and biuret... 

With an excess isocyanate in the above systems, allophanate and biuret reactions take place (Eqs (2.25) and (2.26)), resulting in further cross-linking. When increased rigidity and high-temperature performance are desired, further crosslinking may be accomplished via isocyanurate formation (Eq. (2.29)). Base catalysts such as alkoxides, quaternary ammonium or phosphonium, etc., promote this reaction. Aromatic isocyanates give iso-cyanurates far more easily than aliphatic ones. 

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 ... 

Formation of allophanates and biurets in polyurethane chemistry, especially when an excess of isocyanate is used, is in fact a supplementary source of crosslinking. 

To compensate for hydroxyl groups that are untreated due to imperfect mixing, or for excess isocyanate groups consumed in allophanate or biuret formation, it is standard practice to combine the isocyanate and polyol... 

The formation of allophanates and biurets are important in that they will lead to branching in the polymeric network. 

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. 

The basic polymer structure, which results from the formation of urethane groups and may be linear or crosslinked, largely determines the polymer s properties. However, other than these primary reactions, secondary reactions such as biuret, allophanate, dimer and trimer formation also have an effect on the properties of the polymer and their presence can also be detected by IR techniques. 

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. 

The structural analysis of the samples PP ADMH = 2 1 obtained at 60 and 80°C in the absence of catalysts shows the formation of a branched structure at temperatures lower than those required for the formation of allophanate (120-140°C) and biuret (100°C) structures [1]. The ability of the ADMH to form hydrazinium cations leads to the assumption that the elastomer synthesis with the participation of ADMH involves formation of reactive centers of ionic nature, similarly to the Ritter reaction, where the synthesis of the N-substituted amides of carboxylic acids passes through a stage of carbonium ion formation... 

Reaction speeds and NCO presence were documented at the end of each reaction and were determined by NMR spectra. The 1 mole DMPA reaction was completed within 3 hours, and showed residual isocyanate groups. The 3 moles DMPA reaction showed no residual isocyanate after 40 hours of reaction time. The 1 and 3 mole(s) hexanol reactions were both completed as soon as the temperature stabilized from the exothermic bond forming phase, and only the 1 mole hexanol reaction showed unreacted isocyanate at the end of 20 hours. The 1 and 3 mole hexylamine reactions were completed as soon as the temperature stabilized from the exothermic bond forming phase with the 1 mole hexylamine reaction showing unreacted isocyanate after 20 reaction hours. Finally, after studying the and spectra for all the reactions, there was no significant formation of allophanates or biurets detected. 

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). 

Using controlled reaction conditions such as the temperature profile and rate and time of the addition of polyols, more uniform materials can be produced. The correct spacing of the hard segments required to produce the physical properties can be obtained. The controlled conditions will also help prevent the formation of undesirable side products such as allophanate, biuret, and trimers. These reactions will give branching of the polymer chains. 

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. 

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. 

This is an interesting comparison with regard to control of polymer crosslinking by biuret- or allophanate-forming side reactions. For example, 1,5-naphthalene diisocyanate showed the greatest tendency toward alloph-anate formation, and hexamethylene diisocyanate the least. 

Scheme 2. Typical reactions of isocyanates leading to the formation of substituted carhamic acid fXIIj, stihstituted urea fXIIIj, biuret ("XIV allophanate fXV), and isocyanurate fXVl).

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. 

As observed in the first stage of our study, the considerable decrease of the rate of maturation process at room temperature in the absence of humidity shown in Fig. 4.67 from above [331], suggested that in our case the allophanate, biuret, uretidine-dione and isocyanurate group formation equations (4.22 (d), (e) and (h) should play only a minor role in the PUs consolidation). 

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