Isocyanurates, formation

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. 

Isocyanurate formation (cyclotrimerization catalyzed with N,N, N"-tris(dimethylaminopropyl) hexa-hydrotriazine) -... 

Temperatures of reaction can, of course, be important. At up to SC C the linear chain-forming reaction predominates but as higher temperatures (up to 150°C) are reached then biuret and isocyanurate formation become effective and branching occurs. At above 150°C some of the less stable links are affected and reversion or degradation can then take place. It must be stressed that the isocyanate reactions are highly exothermic, and under conditions where heat transfer is slow appreciable temperature rises can be experienced care is necessary to minimize their occurrence, otherwise deterioration in properties results. 

Second, for the preparation of ISOX resins, the preliminary finding is that EMI, being an effective catalyst for the formation of both oxazolidone and isocyanurate, is the preferred catalyst. This conclusion is based on finding that DABCO is a strong catalyst for isocyanurate formation and HEXA a good catalyst for oxazolidone formation. This latter conclusion is to be regarded as preliminary. 

Of the two reactions, isocyanurate formation is the most widely utilized. Diisocyanates can be converted via this reaction into trifunctional isocyanurate derivatives and subsequently used to introduce branching and crosslinking into a polyurethane.These crosslinks have greater thermal stability than either allophonate or biuret linkages, and hence they are more useful in elevated temperature applications. 

Isocyanates also react with each other to form dimers (metdiones) and trimers (isocyanurates). Formation of metdiones is catalyzed by phosphines. Formation of isocyanurates is catalyzed by quaternary ammonium compounds trimerization of aromatic isocyanates is catalyzed by tertiary amines. Uretdiones decompose thermally to regenerate isocyanates and are used as blocked isocyanates. Isocyanurates are stable and isocyanurates of diisocyanates are extensively used as multifunctional isocyanates. 

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. 

Also, the presence of strong bases, even in trace amounts, can promote the formation of isocyanurates or carbodiimides. In the event of gross contamination, the exothermic reaction can sharply increase the temperature of the material. Normally, the trimerization reaction occurs first and furnishes heat for the carbodiimide reaction. The carbodiimide reaction Hberates carbon dioxide and forms a hard soHd. The Hberation of carbon dioxide in a sealed vessel could result in overpressurization and mpture. 

Urethane network polymers are also formed by trimerization of part of the isocyanate groups. This approach is used in the formation of rigid polyurethane-modified isocyanurate (PUIR) foams (3). 

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

When chlorine is employed for outdoor swimming pool sanitation, it is relatively rapidly decomposed by sunlight. Isocyanuric acid stabilizes chlorine by formation of photostable chloroisocyanurates (12). By contrast, bromine is not stabilized by isocyanuric acid. 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

The formation of a cyclic species (isocyanurate from 3 molecules of MDI) is shown in Equation 2. This cyclotrimerizatlon reaction is highly exothermic (-95 KJ/mol in diglyme ( )) and is believed to be the major source of heat during the exothermic reaction of MDI. 

Peroxides decompose when heated to produce active free radicals which in turn react with the mbber to produce cross-links. The rate of peroxide cure is controlled by temperature and selection of the specific peroxide, based on half-life considerations (see Initiators, free-radical Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triallyl isocyanurate, and diallyl phthalate, act as coagents in peroxide cures, they are not vulcanization accelerators. Instead they act to improve cross-link efficiency (cross-linking vs scission), but not rate of cross-link 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. 

Dicyanates react with diisocyanates. The poly addition results in the formation of crosslinked polymers with cyanurate and isocyanurate rings [32]. The polymers have high deformation and decomposition temperature in the case of a crosslinked copolymer from 9,9-bis(4-cyanatophenyl)fluorene the corresponding values are 480 °C and 440 °C (in air), respectively. 

The NHCs were found to react with isocyanates to afford isocyanurates (Eq. 9) [27,28]. Although SIPr was found to be an effective catalyst for iso-cyanurate formation (for a wide variety of isocyanates), no isocyanurate was observed in most Ni-catalyzed cycloaddition reactions of diynes and isocyanates (Eq. 10). Furthermore, isocyanurates were not formed reversibly during the course of the reaction since no pyridones were obtained when isocyanurates were used as the sole source of isocyanate. These data further highlight the efficacy of the Ni/NHC catalyst system. 

Three isocyanate groups can react to form a trimer or substituted isocyanurate ring. Phosphines or bases such as sodium acetate or sodium formate can catalyze this reaction. The isocyanurate ring is thermally stable, has good chemical resistance, and can enhance the resistance of a urethane adhesive to aggressive environments. 

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