Nco

The polyols HO—(—R2—O)—H popularly used are as follows:

PTG: poly(tetramethylene glycol) HO—(C4H8O—)„—H

The structural formulas of the common polyisocyanates used in the preparation of polyurethanes are shown above. The polyethers prereacted with excess isocyanate to yield an isocyanate end-capped prepolymer are stable viscous liquids under dry conditions. These mixed with water and catalysts produce a soft foam used in upholstery and other applications. The more popular and economic route to producing foam is the "one shot" process where the reactants, catalysts, and additives are all mixed together in a mixing head and spread onto a moving trough to obtain slabstock of the foam. Typically the mixing head is designed to accommodate several streams of feed: the polyols; the isocyanates; catalyst system, water, and additives; and the blowing agents.

Auxiliary Blowing Agents (ABA) Historically, CFCs, particularly CFC-11, CFC-12, CFC-113, and CFC-114, were popularly used in polyurethane foams. In 1992, the U.S. Environmental Protection Agency (EPA) issued its final rule implementing section 604 of the Clean Air Act Amendments of 1990. The rule required the production of class I substances (CFCs, halons, carbon tetrachloride, and methyl chloroform) to be gradually phased out completely by January, 2000 (2002 for methyl chloroform). The phase out now includes hydrochloroflu-orocarbon (HCFC) blowing agents presently used in some of the polyurethane foam products. These will be phased out completely in the United States by year 2030 and alternative ABAs that are either low or zero ozone depleting have been identified and put to use. Table 2.8 compares some of the conventional CFC blowing agents with selected alternatives. Those that have even a low ozone-depleting potential are yet likely to be regulated in the future. Roughly a decade after its peak use in 1989, the use of ozone-depleting chemicals in foam-blowing applications have now dropped by more than 75% worldwide.

In flexible slabstock and molded polyurethane foam production, methylene chloride, CO2 (liquid or gas injection), and water are used as ABAs. In rigid insulation foams CFCs have been replaced with HFC-134a (sometimes blended with HFC 152a) or liquid CO2. In integral skin polyurethane products, CO2, water, and hydrocarbons are used for the purpose.

2.8.2. Phenolic Foams

Another thermoset plastic that is used as a foam in building applications is phenol-formaldehyde resins. These foams can be manufactured with about the same range of densities as polyurethanes or polyisocyanurate foams and are good thermal insulators as well. The phenolic foams are self-extinguishing and emit relatively low smoke on burning. But, they are relatively more expensive to produce and are therefore of limited popularity. Phenolic resins are two-part systems consisting of a phenol-formaldehyde condensate [mainly consisting of bis(hydroxyphenyl methane) mixtures] and a hardening agent based on formaldehyde. HCFC-141b is the predominant blowing agent used with phenolic foams [32]. While pentanes can be used as a partial replacement as an ABA, this lowers the insulation efficiency and more importantly reduces the desirable fire performance of the foam. However, the amount of phenolic foams used worldwide is relatively small (less than 5%) and the release of ODS from this source is minimal.

2.8.3. Other Foams

Both polyethylene and polystyrene are used extensively as thermoplastic foam materials in packaging as well as in building applications. Expandable polystyrene

Table 2.8 Characteristics of Common CFCs, CHFCs, and Alternatives for Use with PUR Foams3

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