Modified isocyanurate foams

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. 

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. 

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 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... 

A one-shot method for oxazolidone-modified isocyanurate foams was proposed by Hayash et al (129). The process, however, resulted in a high-exotherm reaction which caused thermal degradation of the resulting foams. 

Recently, oxazolidone-urethane-modified isocyanurate foams made by the one-shot process have been reported by Fuzesi et al (128, 209, 210). This process consists of a three-component system. Unmodified oxazolidone foams will be described in the following section. 

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. 

Filled Isocyanurate Foams. The addition of specific inorganic powders, such as graphite and talc, to urethane-modified isocyanurate foams, has been jH oven to jH oduce high temperature- and flame-retardant insulation materials (59). 

Figure 46. Results of fire test of a composite composed of U-modified isocyanurate foam and mortar (79). (Courtesy of Nisshin Spitming Co. Ud.)...

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. 

Matrix Plastic Foams. The matrix for plastic foams includes rigid polyurethane foam, urethane-modified isocyanurate foam, unsaturated polyester-polyurethane hybrid foam, and vinyl ester-polyurethane hybrid foam. 

Urethane-Modified Isocyanurate Foams. Urethane-modified isocyanurate foams are prepared by the trimerization of a polyisocyanate in the presence of a polyol, a trimerization catalyst, a blowing agent, and a surfactant. The foams have high flame and temperature resistance. The combined use of an isocyanurate foam and glass fiber not only improves the physical properties, e.g., flexural strength, friability, etc. but it also improves the flame resistance because the char formed from the foam acts as thermal barrier and protects it from flame and heat. This type of composite, therefore, is widely used for buUding applications in the U.S.A. Urethane-modified isocyanurate foam systems have also been used in the SRIM process (26, 36, 37). 

Examples of the matrix resins employed for SRIM include rigid polyurethane (7,15, 16), or urethane-modified isocyanurate foam (9, 26, 35, 36). The matrix resins for foamed composites are required to have low viscosity and relatively slow cream time for better impregnation into fiber mat and fast-cure cycle time for higher j oductivity. 

Kuyzin et al discussed a low-density SRIM made by using glass-fiber reinforcement and a urethane-modified isocyanurate foam system (26), or by using polyamide-coated polyester fibers and a urethane-modified isocyanurate-foam system (36). 

Celotex Corp. (35) has commercialized foamed composites consisting of a urethane-modified isocyanurate foam and continuous-glass-strand mat. The trade name of the composite is Thermax. Some physical properties of Thermax are shown in Table 52. The physical properties of a low-density SRIM system of urethane-isocyanurate foam for interior-trim applications are shown in Table 48. 

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