Isocyanurate ring

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

Catalysis is usually accompHshed through the use of tertiary amines such as triethylenediamine. Other catalysts such as 2,4,6-/m(/V,/V-dimethylaminomethyl)phenol are used in the presence of high levels of cmde MDI to promote trimerization of the isocyanate and thus form isocyanurate ring stmctures. These groups are more thermally stable than the urethane stmcture and hence are desirable for improved flammabiUty resistance (236). Some urethane content is desirable for improved physical properties such as abrasion resistance. 

Whilst reaction can take place in the absence of catalysts it is more common to use such materials as tetra-alkylammonium halides and tertiary amines such as triethylenediamine. A major side reaction leads to the production of isocyanurate rings, particularly in the presence of tertiary amines. 

The polycyclotrimerization of difunctional isocyanates (or NCO-terminated prepolymers) produces polymer networks containing heterocyclic, thermostable per-hydro-1,3,5-triazine-2,4,6-trione (isocyanurate) rings as crosslinks ... 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

Figure 9.7. Comparison between IR spectra of Z1031-I (upper) and Z1030/1073 (lower) coatings (a) carbonyl of urethane groups (b) carbonyl of isocyanurate ring and urethane by-products (c) NH urethane and urethane by-products (d) CH2, CH3 (e) CFo, COC, backbone (f) isocyanurate ring (g) carbonyl of ureas and urethane by-products).

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. 

Earlier we have studied the reactions of fullerene Cgo with the isocyanurato-, phosphoryl-, pyrimidino-, nitropyrimidino-, quinoxalin-, benzopyrazine substituted azides [18-24]. As a result, both 6,6-, 5,6-closed and 5,6-open organofullerens were synthesized, and the series of them were reduced electrochemically easier than the pristine fullerene C6o- Now the reactions of fullerene C6o with isocyanurato- and nitropyrimidine substituted azides were used for the search of the answer to delivered question. For solving the problem the substituents in positions 1 and 3 of the isocyanurate ring and the number of methylene units between ring and azide group in isocyanurate containing azides were varied. 

By changing the ratio epoxy/isocyanate, the cured resins are composed of oxazolidone and isocyanurate rings in a varying ratio. 

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. 

The isocyanurate rings are thermally stable (decomposition started at temperatures above A00°C (5)) and therefore cyclotrimerization of difunctional isocyanates can be utilized for preparation of thermally stable polymers. (2 J3)... 

Preparation and Properties of Polycocyclotrimers of Isocyanates. The formation of isocyanurate rings in the polycyclo-trimerization or polycocyclotrimerization reaction of isocyanates is characterized by the disappearance of the NCO groups (v(N=C=0) 2280 cm ) and by the appearance of a new IR absorption band at 1700 cm-l, assigned for the stretching vibration v(C=0) of the isocyanurate ring (12). 

At this moment, in spite of their high application potential, the triazinic polyols are only used to a very small extent for rigid PU foam production. The structure of these triazinic polyols has many similarities to the structure of urethane - isocyanuric foams, the difference being that the triazinic rings (isocyanuric rings) are introduced into the PU structure with the polyol and in urethane-isocyanuric foams they are generated in situ (by trimerisation of -NCO groups). 

The highly crosslinked structure is not derived from polyester polyol, which has a low functionality (f = 2-3 OH groups/mol), but is derived from the isocyanurate rings generated by the trimerisation of the excess of -NCO groups. 

Urethane - polyisocyanuric foams (PU/PIR) foams) contain isocyanuric rings. Isocyanuric rings are glycolysed at lower reaction rates than the urea or urethane groups, with the formation of hydroxyalkyl carbamates and an amine (reaction 20.7) [11] ... 

This triisocyanate has been reported to impart good light stability and weather resistance when used in urethane coatings and is among the most widely used aliphatic isocyanates (23. 24). More recently Bayer A. G. Introduced another product based on HDI. By partial trimerization of HDI in the presence of a catalyst, an isocyanurate ring containing Isocyanate is formed that exhibits superior thermooxidative stability and weatherability ... 

A number of other aliphatic diisocyanates that impart excellent color stability to urethane coatings have been introduced into the U.S. market. Foremost among these are 4,4 -methylene bis(cyclohexyl isocyanate) (H12MDI) and Isophorone diisocyanate (25. 26). An isocyanurate ring containing isocyanate produced by trimerization of IPDI is also commercially available (T-1890). 

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