Foam, advantages Blowing agents

Halocarbons have the further advantage of reducing the viscosity of the reaction mixture and, where used as the main blowing agent instead of the carbon dioxide produced by the isocyanate-water reaction, cheaper foams are obtained since less isocyanate is used. The reader should, however, note the comments made about the use of chlorofluoroearbons and their effect on the ozone layer made in Section 27.5.4. 

Several other blowing agents have been disclosed, and many of these have particular advantages. Neopentane (tetramethylmethane), b.p., 9.5°C., is retained well by polystyrene and generates low density foam of extremely fine cell size (100). Isopentane (b.p., 28°C.) also... 

CFCs. Table 6 shows major CFCs. CFC-11 has been the representative blowing agent for both flexible and rigid urethane foams (155). The use of CFCs brought significant advantages to both flexible and rigid polyurethane foams. However, ozone-depletion potential (ODP) in the stratosphere have led to the worldwide ban of the production and use of CFCs by 1995. 

Water as the sole blowing agent is not practical for producing these foams, because it results in extremely high-friability foams which make it very difficult to handle in practical applications. An alternative method proposed by Ashida involves the use of specific solvent blends, e.g., an 80/20 blend of methylene chloride/pentane (196). Advantages of this solvent blend are ... 

Foamed cellular plastics provide a number of advantages, however, they can also bring two important problems as well. These are, the problems of the residual chemicals that may be left in the system and the problem of the flammability and smoke evolution on combustion . In addition, there can certainly be residues of unreacted monomer(s), blowing agent(s), polyols, catalyst(s). Are retardants left if the system is not prepared carefully, all of these can be left or trapped in the system (inside the internal cells). These... 

There is no direct alternative blowing agent for polyurethane foams that combines the advantageous properties of CFCs and the trend has been to develop replacements specific to individual applications. Much has been achieved but the present blowing systems are widely regarded as interim solutions in the longer term, the solution is seen in pentane-based compounds. 

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