Water blowing agent

Figure 7.7 Schematic representation of the process for the preparation of WEPS (a) emulsification of water (blowing agent) in a styrene-PS mixture and (b) suspension polymerization of styrene-PS droplets containing emulsified water. Reproduced from reference 57 by permission of Elsevier.

Polyethylene. This is essentially a closed-ceU insulation manufactured at 448 2 K by an extmsion process. A blowing agent and nucleating agent are employed to control the ceU size, and primary use is in insulating pipelines for hot and chilled water lines, air conditioning, and processing systems. 

Research and development programs have been initiated by the cellular plastics industry to develop viable substitute blowing agents. These must have similar or improved properties to their CFC counterparts at a reasonable cost. Emphasis was initially placed on HCFC 123 and HCFC 141b, both having much shorter lifetimes and considerably less effect (up to 50 times) on o2one layer depletion (22). However, various options, including gas mixtures, water, or CO2 blown foams, continue to be studied ultimately to eliminate all CFCs and HCFCs. 

Although over 1.1 x 10 t of phosphoms pentoxide was produced in 1992, 99.5% of this material was converted immediately into phosphoric acid. The amount of phosphoms pentoxide produced and sold as such in the United States in 1994 was 6200 tons. The typical 1994 price was 1.6—1.7/kg. The only U.S. producer is Rhc ne-Poulenc at Mt. Pleasant, Teimessee. Phosphoms pentoxide, which is used to make asphalt-blowing agents and in water treatment, amounts to 1.0 x 10 t in 1992. 

Formulations for one-shot polyether systems are similar to those used for flexible foams and contain polyether, isocyanate, catalyst, surfactant and water. Trichloroethyl phosphate is also often used as a flame retardant. As with polyesters, diphenylmethane di-isocyanate is usually preferred to TDI because of its lower volatility. Tertiary amines and organo-tin catalysts are used as with the flexible foams but not necessarily in combination. Silicone oil surfactants are again found to be good foam stabilisers. Volatile liquids such as trichlorofluoro-methane have been widely used as supplementary blowing agents and give products of low density and of very low thermal conductivity. 

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. 

Amongst the catalysts used or the polymerisation-trimerisation reactions are alkali metal phenolates, alcoholates and carboxylates and compounds containing o-(dimethylaminomethyl)phenol subgroups. Fluorocarbons such as trichloro-fluoromethanes are used as the sole blowing agents in the absence of any isocyanate-water reaction. 

The major commercial applications of hydrazine solutions are as blowing agents ( 40%), agricultural chemicals ( 25%), medicinals ( 5%), and — increasingly — in boiler water treatment now as much as 20%. The detailed pattern of usage, of course, depends to some extent on the country concerned. 

The tank is typically about sixteen inches in diameter and about four to five feet tall. The top of the tank is domed upward and the bottom of the tank is also domed upward in a concave manner. The outside of the tank is insulated with a polyurethane foam insulation that is squirted into the gap between the tank and a thinner sheet metal jacket. The polyurethane is made of two different components that react and harden when mixed. Included in the mixture is a blowing agent that causes the polyurethane to expand in a foam-like manner. Prior to about 1980, water heaters were insulated with fiberglass insulation. The foam insulation process was developed to allow automation and increased manufacturing speed and reduced costs. A side benefit was improved insulating ability leading to a slight increase in efficiency. 

Polyurethanes (PURs) are usually described as being prepared by the reaction of diols with diisocyanates. However, this is an oversimplification because often water is deliberately added in the production of flexible polyurethane foams. Unreacted isocyanate groups react with water to form carbon dioxide and urea groups in the polymer chain. The carbon dioxide acts as a blowing agent in the production of PUR foams. Also, polyurethanes can be formed by the reaction of bischloroformates with diamines. 

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