Urethane-isocyanurate system

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. 

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). 

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. 

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). 

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). 

N. P. Vespoli ancTL. M. Alberino, "Computer Modeling of the Heat Transfer Processes and Reaction Kinetics of Urethane Modified Isocyanurate RIM Systems", AICHE Diamond Jubilee Meeting, Washington, D.C. No.v. 1 2, 1983. 

Isocyanurates are usually more thermally stable than urethanes since they contain no labile hydrogen in the isocyanurate ring system. Hipchen [32a] reported the comparative thermogravimetric analysis of pure polyisocyanurate, pure urethane, and urethane-modihed polyisocyanurates. The modified isocyanurate showed a total wt loss at 300 C of 9% versus 25-30% for the urethane and 7% for the pure isocyanurate. 

The trimerization of isocyanates to form isocyanurate rings is particularly important in producing iire-resistant rigid foams, and a large volume of work has appeared on this topic. Much of it has been done by Kresta and co-workers, who have discussed the general mechanism, the co-catalytic effect of urethane groups, and the use of cyclic sulphonium zwitterions and aminimides as catalysts. Model systems have been used for these studies, but attention has also been paid to the kinetics and mechanism of commercial polyisocyanurate systems. ... 

As mentioned in Section 8.4.3.1., the majority of hydroxy groups in a polyether triol are secondary groups and are comparatively unreactive towards isocyanates. It is therefore necessary to select a catalyst which favours the formation of urethane links relatively more than the formation of gas by the reaction of isocyanate and water. Tin compounds (e.g., stannous octoate and dibutyltin dilaurate) are particularly effective in this respect (cf.. Table 14.3) and are very widely used. In addition to the primary isocyanate-polyol and isocyanate-water reactions, several secondary reactions occur during the preparation of foam. As shown in Section 14.4, the final product may contain allophanate, biuret, isocyanurate and uretidione links. It will be appreciated that in a polymeric system, which is based on a diisocyanate, all of these links (except uretidione) represent points of branching or cross-linking. These secondary reactions are particularly favoured by tertiary amines (e.g., triethylenediamine and 4-dimethylaminopyridine) and these catalysts therefore contribute to the final cross-linking of the foam and hence to the achievement of, for example, a low compression set. Mixtures of tin compounds and tertiary amines are more... 

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