Isocyanurate linkage

The isocyanurate reaction occurs when three equivalents of isocyanate react to form a six-membered ring, as shown in the fifth item of Fig. 1. Isocyanurate linkages are usually more stable than urethane linkages. Model compound studies show no degradation of the trimer of phenyl isocyanate below 270°C [10,11]. Catalysts are usually needed to form the isocyanurate bond. Alkali metals of carboxylic acids, such as potassium acetate, various quaternary ammonium salts, and even potassium or sodium hydroxide, are most commonly used as catalysts for the isocyanurate reaction. However, many others will work as well [12]. 

This method, however, did not result in improved flame retardance because the isocyanurate content was too low. In recent years, however, high-functionality and low-viscosity polyether polyols have become available, and therefore the above method has become less important. Even so, the method was sometimes used to make foams having increased crosslink density to improve dimensional stability or chemical resistance. Therefore, a higher isocyanate index, e.g., 150 to 200, was used to incorporate isocyanurate linkages in rigid urethane foams. 

It should be noted that the incorporation of isocyanurate linkages in small quantity does not improve high-temperature resistance nor flame retardance of the resulting foam. Urethane-modified isocyanurate foams which have isocyanate indices of more than 300 show outstanding high-temperature resistance and flame retardance (71). These foams will be discussed in the next section. 

In contrast, the isocyanurate linkage is thermally stable, as determined by TGA, as shown by a model compound study (58) and produces less combustible gases. Accordingly, unmodified isocyanurate-based polymers, e.g., resins and foams, are thermally stable, and therefore, temperature- and flame-resistant. In other words, the unmodified polyisocyanurates decompose at higher temperatures than the polyurethanes, and generate lower amounts of combustible gases than polyurethanes. 

The isocyanurate linkage is obtained by the cyclotrimerization of isocyanate groups, as shown in the following model reaction. 

Oxazolidone-Modified Isor anurate Foams. The 2-oxazolidone, or 2-oxazolidinone, linkage is considered to be a cyclic urethane linkage, but its thermal stability is much higher than that of a urethane linkage. Kordomenos et al (207) compared the thermal stabilities of urethane, oxazolidone and isocyanurate linkages in terms of activation energy by using model compounds. The results obtained were as follows. 

The predominant structure in these resins is the isocyanurate linkage. These are relatively stable and it is the chemistry of the residual isocyanate groups that dominate the formation of new bonds in the system during the... 

Besides urethane bonds, other chemical functionalities are present due to reaction of isocyanate groups with water and other compounds. In fact, urea and isocyanurate bonds are more abundant than the urethane linkages in some commercial products that are designated as polyurethanes. Allophanate, biuret, urea, carbodiimide, and isocyanurate linkages may be found. These bonds certainly affect the properties of the final polymer, but unfortunately it is almost impossible to determine them quantitatively (Figure 6). 

Polyurethane ionomers, 25 460 Polyurethane-modified isocyanurate (PUIR) foams, 25 455, 456 Polyurethanes (PUs), 9 564. See also Polyurethane (PU) antioxidant applications, 3 121-122 cardiovascular device applications, 3 720 CASE, 25 474-477 commercial block copolymers, 7 648t containing sulfur linkages, 23 742-745 dyeing, 9 204... 

Modified polyisocyanates are prepared by incorporating at least one linkage into monomeric polyisocyanates. Such linkages include urethane, carbodiimide, allophanate, biuret, amide, imide, isocyanurate, and oxazolidone. These modifications provide some advantages, e.g., lower vapor pressure, increased viscosity, and controlled reactivity. 

Side reactions, e.g., formation of allophanate, biuret, isocyanurate, or catbodiimide linkages, may be formed, depending upon the reaction conditions. 

The modification linkages include urethane, amide, imide, carbodiimide and oxazolidone linkages. A urethane-modified isocyanurate foam (trade name Airlite Foam SNB, Nisshinbo, Ind. Inc.) was first applied to the petrochemical industry as a seamless fire-resistant insulant in 1965 (39). 

Non-CFC-Blown Urethane-Modified Isocyanurate Foams. Recently, methods of making non-CFC-blown urethane-modified isocyanurate foams have been reported. These methods involve the partial replacement of water for CFC-11 (97). The methods however, have the disadvantages of (a) the higher thermal conductivity of the resulting foams due to the presence of carbon dioxide in the foam cells and (b) the higher friability of foams due to increased urea and biuret linkages (197). 

Amide-Modified Isocyanurate Foams. The amide linkage is a thermally stable, difunctional linkage. Amide-modified isocyanurate foams can be prepared by using carboxylic acids having at least two carboxylic groups as modifiers. 

Imide-Modified Isocyanurate Foams. The imide linkage is a thermally stable linkage, and therefore, imide-modified isocyanurate foams have higher thermal stability and flame retardance than urethane-modified isocyanurate foams. R. Grieve (114) prepared such foams in a one-shot process by reacting a polycarboxylic acid anhydride with an organic polyisocyanate in the presence of a catalytic amount of a monomeric homocyclic polyepoxide and a tertiary amine. 

The oxazolidone linkage is a thermally stable and difunctional linkage that has been used for modifying isocyanurate foams (61, 63). Unmodified oxazolidone foams are expected to have improved properties such as temperature stability, flame retardance, and low friability. An attempt to obtain unmodified polyoxazolidone foams was reported (98). The formulation employed is shown in Table 31. 

Figure 22.1 Basic reactions of isoq nate with different reactants (a) hydroxyl compound to give urethanes, (b) amines to give ureas, (c) water to form an unstable compound, carbamic acid, which decomposes and produces an amine, (d) amines and excess isocyanate produces a urea linkage, (e) urea linkage and one molecule of isocyanate produces biuret, (f) urethane and asocyanate produce allophanates. Secondary reactions of isoc3canate self-condensation produce (g) uretidione ring (dimer-), (h) isocyanurate (trimer-), (j) carbodiimide (reprinted with permission of Chattopadh3ray et al., 2007, Elsevier [15]).

Oxazolidone-isocyanurate polymers were prepared by reacting five glycidyl ethers with 4, 4 -diisocyanato-diphenylmethane in the presence of 2-ethyl-4-methylimidazole. The degree of cure of the resins was followed by IR spectroscopy by measuring the fraction of the isocyanurate to oxazolidone linkages (377). 

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