Isocyanuric foams

J. S. Newman, "Smoke Characteri2ation of Rigid Polyurethane/Isocyanurate Foams," 34th SPl Annual Polyurethane Technical Marketing Conference, New Orleans, La., Oct. 21—24, 1992, pp. 307—311. 

Modification of cellular polymers by incorporating amide, imide, oxa2ohdinone, 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 (34). A typical PUIR foam formulation is shown in Table 6. 

Typical properties of isocyanurate foam are given in Table 27.5. 

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. 

Rigid insulation foam, 206 Rigid isocyanurate foams, 226 RIM. See Reaction injection molding (RIM)... 

The polyisocyanates which can be used for preparing isocyanate-based foams are mainly aromatic compounds and some aliphatic or aralkyl polyisocyanates. TDI (toluene diisocyanate) is widely used for flexible foams. Pure MDI (diphenylmethane diisocyanate) is used for elastomers and coatings. Modified TDI and modified MDI are used for high-resilience flexible foams. Polymeric isocyanates (polymeric MDI or oligomeric MDI) are mostly used for preparing rigid urethane and isocyanurate foams, and in part, for preparing flexible and semi-flexible foams. 

Modification of the poly(carbodiimide) foams, using polyols as comonomers, is possible, but the excellent thermal and flammability properties are reduced. Poly(carbodiimide isocyanurate) foams can also be continuously produced using 1,3,5-tris-(3-dimethylaminopropyl)-hexahydro-s-triazine as the cocatalyst. Other trimerization catalysts, such as potassium 2-ethylhexanoate and quaternary ammonium carboxylate (Dabco TMR 2) are also used as cocatalysts in the formation of poly(carbodiimide isocyanurate)... 

In addition to these unmodified foams, many modified or hybrid foams have appeared in the literature, e.g., urethane-modified isocyanurate foams, isocyanurate-modified urethane foams, urea-modified isocyanurate foams, unsaturated poIyester-poI)rurethane hybrid foams, etc. 

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. 

Buricus (201) and Nicholas and Gmitter (58) reported on TDI prepolymer-based isocyanurate foams prepared by trimerizing NCO-terminated jn-epolymers. TGA data (58) showed that there was no... 

Substantially flame-retardant and high-temperature-resistant isocyanurate foams were first invented independently in 1965 by Ashida (39) and in 1966 by Haggis (199). Since 1966 a number of papers and patents regarding modified isocyanurate foams have appeared. A few reviews on urethane-modified-isocyanurate foams are also available (144, 202, 203, 204). 

The above modification philosophy, i.e., lowering crosslink density, has been extended to various modified-isocyanurate foams. The methods for modifying isocyanurate foams are summarized in Table 26. 

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). 

The commercialization of simUar foams was followed by Hexaform (trade name ICl) in 1968 (57). The commercialization was later followed by Upjohn Company (trade name Kode 25) and Jim Walter Corp. (trade name Thermax). Foaming methods of modified-isocyanurate foams include slabstock, laminate, block, pour-in-place and spraying. 

Principles of Urethane Modification. The flame retardance and temperature resistance (or flame endurance) of modified isocyanurate foams are affected by the following factors ... 

In contrast, TDl-based modified isocyanurate foams did not show any increase in bum-through time, even if the NCO/OH equivalent ratio was increased. This significant difference could be attributed to the difference in flash point of the two isocyanates. The flash points of polymeric isocyanate and liquid-modified MDI oligomers are >200°C, and that of TDl is 135 C. 

In contrast, polymeric isocyanate does not immediately ignite, because its decomposition temperature is lower than its flash point. Accordingly, only polymeric isocyanates or liquid oligomeric MDI should be used for urethane-modified isocyanurate foams having substantial flame retardance. 

The second factor important in obtaining highly flame-retardant isocyanurate foams is the NCO/OH equivalent ratio. Fire endurance, i.e., flame retardance) and temperature resistance, can be increased with increase of the NCO/OH equivalent ratio when polymeric isocyanate is used as the polyisocyanate component. 

The third factor important in obtaining high-flame-endurance foams is the choice of molecular weight, functionality of polyol and aliphaticity index (65). The polyol content, which is largely responsible for the low flammability of isocyanurate foams, is expressed by the aliphaticity index, as defined by the following equation ... 

The fifth factor important in obtaining high-flame-endurance foams is the choice of polyol structure. Since the beginning of the isocyanurate-foam industry, the major polyols used for modification have been polyether polyols (65, 66), which include polyols having a variety of functionalities (e.g., 2 to 8) and varying equivalent ratios. 

The polyols which function as modifiers include ethylene glycols, 1,4-butanediol, polyether polyols and polyester polyols. In recent years aromatic polyesters prepared from reclaimed PET (polytetraethylene terephthalate) or the distillation residue of DMT (dimethylterephthalate) have appeared as modifiers for urethane-modified isocyanurate foams (73, 78). These aromatic polyesters are produced by the transesterification of reclaimed PET or DMT distillation residue. 

Preparation. Urethane-modified isocyanurate foams are mostly fH epared by the one-shot process based on the jH inciple discussed previously in the urethane modification section. The semi-prepolymer process is used only in limited cases because of viscosity problems. This section describes several examples of formulations for producing block foams, slabstock foams, laminate foams, and spray foams. 

Urethane-modified isocyanurate foams using polyols of functionality of at least 3 were described by Ashida et al in 1967 (71). Other types of urethane-modified isocyanurate foams are shown below. 

Sorbitol Polyol-Modified Isocyanurate Foam (71). This foam was prepared according to the following procedure. Potassium 2-ethylhex-anoate containing 4% water was used as the catalyst (soluble in polyether polyols). 94 g of sorbitol-based polyether polyol having a hydroxyl number of 490, 8.5 g of the potassium 2-ethylhexanoate, 120 g of tris(chloroethyl) phosphate, 5 g of silicone surfactant, and 90 g of... 

Properties. The physical strengths of modified isocyanurate foams are proportional to foam densities. The values are about the same as those of urethane foams, as discussed in the previous chapter. Typical jn-operties of modified isocyanurate foams to be discussed are flame endurance and firiabUity. Figure 32 shows the relationship between OH/NCO equivalent ratio (inverse equivalent ratio of NCO/OH) and fiiabUity in terms of the ASTM C-421 tumbling test (16). 

The major application fields of urethane-modified isocyanurate foams lie in budding aj lications, such as warehouses, high-rise buildings and residential houses. In the USA, laminates are one of the major applications of urethane-modified foams. Figure 35 shows an example of the laminate process (72). 

The spraying process is carried out at ambient temperatures. The spraying of urethane-modified isocyanurate foam is not as easy as urethane-foam spraying because the cyclotrimerization reaction of isocyanate groups requires relatively higher temperatures than for urethane foams. An example of the spraying of urethane-modified isocyanurate foams was reported (198). The spraying was conducted with formulations at a low-NCO/OH equivalent ratio. 

The frothing process is widely used in rigid urethane foam pour-in-place applications. The frothing process of urethane-modified isocyanurate foams has been used for the insulation of petrochemical plants, e.g., spherical tanks, reaction towers, etc. (79). An example of the frothing system is shown below (71). 

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). 

An example of the surface flammability in terms of the Butler chimney test of oxazolidone-modified isocyanurate foams is shown in Figure 40. The effect of equivalent ratio on friabUity is shown in Figure... 

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