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aPOCP (photochemical ozone-creating potential) is the ozone-creating potential of a compound relative to ethylene, expressed as an index with the value for ethylene being 100.

aPOCP (photochemical ozone-creating potential) is the ozone-creating potential of a compound relative to ethylene, expressed as an index with the value for ethylene being 100.

remains in the air. Three key mechanisms serve as sinks for the VOCs remaining in the atmosphere: photochemical oxidation by hydroxyl radicals, direct photolysis by ultraviolet radiation, and reaction with ozone or other reactive species such as chlorine in the atmosphere. Table 1.7 shows the lifetimes of some common VOCs in the presence of these reactive species.

Paints and coatings can be formulated as water-borne or water-based compositions with relatively small amounts of organic volatile co-solvents in their formulations. Coating systems such as powder coatings commonly employed in original equipment manufacture or the UV-cured coatings used in consumer products do not involve the evaporation of solvents. Recent amendments to the Clean Air Act of 1990 led to the adoption of ceiling values for VOCs in coatings as well as in adhesives. The paint industry has responded to the restrictions on the use of VOCs by increased reliance on powder coatings, high-solids paints, and 100% solids coatings.

• Case: Fiber Spinning In the production of fibers melt spinning, wet spinning or dry spinning of a polymer are employed. Dry spinning, in which a solvent-borne polymer is extruded into a fiber, is no longer popular because it requires a solvent recovery step in the manufacturing operation to be in compliance with environmental regulations. Melt spinning in which the melt is extruded through spinnerets is the most convenient and the least polluting fiber-spinning technique. Polymers such as cellulose acetate, aramids, and rayon, however, cannot be spun from the melt and wet spinning has to be used.

In wet spinning, the polymer dissolved in a solvent is spun into a nonsolvent bath where the polymer coagulates into a fiber. The fiber is washed free of the bath solvent and dried during the drawing stage. The composition of the coagulation bath changes during the process, as the wet spinning of fibers slowly uses up the chemical solution. In the case of Kevlar manufacture, for instance, the aromatic polyamide solution (or the "dope") is spun into a sulfuric acid bath. The process results in waste sulfuric acid that is not economically recoverable. The same is true of the xanthate process used in the manufacturing of cellophane or rayon. Alkaline cellulose xanthate in carbon disulfide is extruded into a bath of dilute sulfuric acid where it regenerates the cellulosic fiber or film. Economical uses need to be found for the waste from these processes to avoid their accumulation in the environment. With products such as regenerated cellulose, the advantage of using these films over those made from conventional plastics (such as polyolefins) is their ready biodegradability.

• Case: Styrene Emissions from Composite Manufacture A sector of the plastics industry presently under review by the EPA is that of composites. When final, the regulatory action by the agency is expected to reduce related air emissions by 14,500 tons per year (a reduction of 65% over 1997 levels).

Unsaturated polyester thermoset systems use styrene as a reactive diluent (sometimes along with substituted styrenes such as vinyl styrene or methyl methacrylate). Some of the styrene evaporates in the process (the emission factor is estimated at about 1 -3% of the styrene for composites and a little higher for continuous lamination) and poses a threat as a hazardous air pollutant. The time-weighted average (TWA) for styrene is 50 ppm (or 213 mg/m3). In 1996 the Occupational Safety and Health Administration (OSHA) endorsed a proposal by the styrene industry to voluntarily adopt the 50-ppm exposure limit.

The composite industry was able to achieve low emissions by a number of changes, including the increased use of low-styrene resins of lower molecular weight and higher percentage of fillers in the compounds. Also employed were suppressants to minimize the surface evaporation of styrene during the curing stage of the resin. In addition to changes in the formulation, engineering improvements in spraying technology (e.g. the use of controlled spray atomizers) and improved plant design with better airflow characteristics can also be employed to reduce emissions.

• Case: Greener Route to Lactones Used in Synthesis of Polyols Low-molecular-weight (generally up to about 40,000) polyols used as prepolymers in the polyurethane industry can be made by ring-opening polymerization of lactones with titanium catalysts and di- or tri-hydroxy initiators. The lactam raw material for the reaction is made by oxidation of ketones (Bayer-Villiger reaction) using a peroxycarboxylic acid. The reaction produces equimolar quantities of the lactone and reduced acid waste:

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