Amines biurets

An excess of phosgene is used during the initial reaction of amine and phosgene to retard the formation of substituted ureas. Ureas are undesirable because they serve as a source for secondary product formation which adversely affects isocyanate stabiUty and performance. By-products, such as biurets (23) and triurets (24), are formed via the reaction of the labile hydrogens of the urea with excess isocyanate. Isocyanurates (25, R = phenyl, toluyl) may subsequendy be formed from the urea oligomers via ring closure. 

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). 

Amines, too, possess active hydrogens in the sense required for reaction with an isocyanate group. Thus the products of Reaction 4.10 react further to yield substituted ureas by the process shown in Reaction 4.11. Reaction can proceed still further, since there are still active hydrogens in the urea produced in Reaction 4.11. The substance that results from the reaction between an isocyanate and a urea is called a biuret (see Reaction 4.12). 

Not only N-H bonds from amines can participate in the aminolysis reaction, but also less nucleophilic urea, thiourea and biuret NH units can react with halophosphanes in an effective manner, forming the corresponding phosphinous amides with additional functionalities at the nitrogen atom [39-44]. 

Lindoy et al. [16], have recently investigated the perturbation to the biuret lattice (Fig. 1) resulting from the introduction of [Cu(dmca)2]2+ cations [dmca= di(methoxycarbimido)amine], Br anions, MeOH and MeCN molecules. An alternating layered structure is formed in which one layer [stacked parallel to (0 0 1) at zlc=0.00 and 0.50] comprises bu molecules and Br anions (Fig. 9) and the other [stacked parallel to (0 0 1) at z/c=025 and 0.75] comprises [Cu(dmca)2]2+... 

The extent of crosslinking in polyurethanes depends on a combination of the amount of polyfunctional monomers present and the extent of biuret, allophanate, and trimerization reactions [Dusek, 1987]. The latter reactions are controlled by the overall stoichiometry and the specific catalyst present. Stannous and other metal carboxylates as well as tertiary amines are catalysts for the various reactions. Proper choice of the specific catalyst result in differences in the relative amounts of each reaction. Temperature also affects the extents... 

Isocyanates are produeed almost exclusively by the reaction of amines with phosgene (COCy, with the speeifie reaetion eonditions varying particularly for aromatic and aliphatic isocyanates (Chadwick and Cleveland 1981 Codd et al. 1972 Ulrich 1989). Aliphatic diisocyanates are produced by reaction of phosgene with either a slurry of the carbamate salts obtained in the reaction of the aliphatic diamines with earbon dioxide, or with a slimy of the amine hydrochloride (Ulrich 1989). Hexamethylene diisocyanate (HDI) is produeed by the reaction of phosgene with the amine salt (Chadwick and Cleveland 1981). The trimerie HDI biuret (HDI-BT), which has a low monomer content and is widely used in the formulation of exeeptionally high quality polymer coatings, is produced by controlled reaction of HDI with water, a water generator, or an amine (Chadwick and Cleveland 1981). 

Amines produce polyurethanes with better mechanical properties than when diols are used for curing. Amines produce polyurethanes with a lower temperature resistance than when diols are used. The use of catalysts has been found to direct the cross-linking reactions away from the biuret to the allophanate reactions. 

Evidence for the diiocnc structure of the compounds was originally adduced by the raaotion with amines to form triaubstituUKl biurets ll, loi (Eq. 85) although the disubetituted urea could aleo he... 

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. 

Substituted ureas and biurets undergo analogous reactions with water to yield amines and C02 ... 

CP/MAS NMR. In the first of these papers the structures within the resins were examined as a function of cure temperature [17]. The chemistry of the resins is very complex but one of the principle reactions is the formation of stable isocyanurate structures from three isocyanate units. Other species are also present e.g., amine, urea and biuret. These are formed by the reactions shown in Fig. 15.2.13. CP/MAS NMR spectra of the resins indicated that the optimum cure temperature was 120°C at which most of the isocyanate groups were converted to isoeyanurate (Fig. 15.2.14). 

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... 

It is clear that an admixture of significant amounts of amines, amine-generating water, or carboxylic acids gives rise to hybrid urethane-urea or even urethane-urea-biuret products. The extent to which such hybrid macromolecules are formed will affect the thermomechanical properties of the end products. 

The analogous biuret structure can be produced when moisture decomposes some isocyanate to the corresponding amine and the latter adds to two intact isocyanates ... 

Structure 4.1) [7,8,42] as well as absence of unstable biuret and allophanate units [9] seem to be responsible for increased thermal stability and chemical resistance to nonpolar solvents. The reactivity of amine is shown in Scheme 4.2 [42],... 

Diisocyanates are an important class of chemicals of commercial interest, which are frequently used in the manufacture of indoor materials. such as adhesives, coatings, foams and rubbers (Ulrich, 1989). In some types of particle board, the diisocyanates have replaced formaldehyde. Isocyanates are characterized by the electrophilic -N=C=0 group, which can easily react with molecules containing hydroxy groups, such as water or alcohols. On hydrolysis with water, primary amines are formed, while a reaction with alcohols leads to carbamates (urethanes). Polyurethane (PUR) products are then obtained from a polyaddition of diisocyanate and diol components. Compounds commonly used in industrial surface technology are 4,4 -diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI). The diisocyanate monomers are known as respiratory sensitizers and cause irritation of eyes, skin and mucous membrane. Therefore, polyisocyanates such as HDI-biuret and HDI-isocyanurate with a monomer content <0.5 % are used for industrial applications, and isocyanate monomers will not achieve high concentrations in ambient air. Nevertheless, it is desirable to measure even trace emissions from materials in private dwellings. 

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