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aUPA/ton = units of polluted water associated with the production of a ton of plastic. UPA/ton = units of polluted water associated with the production of a ton of plastic.

bSteel plate about 0.5 mm thick coated with a layer of tin on both sides. Most of the energy is for steel production process.

cEmissions include those associated with extraction of raw materials. dGlass made from 43,2% cullet.

aUPA/ton = units of polluted water associated with the production of a ton of plastic. UPA/ton = units of polluted water associated with the production of a ton of plastic.

bSteel plate about 0.5 mm thick coated with a layer of tin on both sides. Most of the energy is for steel production process.

cEmissions include those associated with extraction of raw materials. dGlass made from 43,2% cullet.

Those ranking high in each column are particularly good candidates for recycling, assuming that the energy and emissions associated with reprocessing are not excessive. Units of polluted water (UPW) and air (UPA) used in the table are measures of emissions. UPW is the number of cubic meters of water polluted up to the European drinking water standard by the production of 1 tonne of the material. UPA is a similar measure for air and gives the cubic meters of air needed to dilute the emissions to the European maximum acceptable concentration (MAC).

In terms of emissions, plastic materials compare favorably with competing materials as seen in Table 1.6. Another set of data illustrating the same compares plastics with other materials including bleached and unbleached paper. The comparison is illustrated in Figure 1.9 on the air and water pollution associated with different materials. Interestingly, in these studies, recycled aluminum (and perhaps even recycled glass) appears to be a low-energy and low-polluting competing material compared to virgin plastics. Individual estimates of energy as well as emissions vary according to the mix of energy sources used (therefore by the country of origin) as well as with the type of raw materials and the process employed. The life-cycle analysis methodology used and the assumptions therein can also affect the numbers. However, a valid approximate comparison might be made (as in Fig. 1.9) using data from a single published source.

Chapter 4 includes detailed inventories of emissions associated with the production of common thermoplastic materials. The potential for emissions, however, does exist in the synthesis, processing, and use of polymer-based products.

Critical volume of air (1000 m3/kg)

Figure 1.9. Air and water emissions associated with the production of various materials including common thermoplastics.

Critical volume of air (1000 m3/kg)

Figure 1.9. Air and water emissions associated with the production of various materials including common thermoplastics.

It is important to quantify any pollutants associated with the plastics industry, assess their impacts on the environment, and adopt pollution prevention measures to minimize the impact of such releases on the environment.

For instance, the monomer raw materials for common plastics such as polystyrene are indeed volatile organic compounds and hazardous air pollutants. Also, polyolefins manufacture in liquid slurry or solution processes use alkane solvents. Exposure to heptane, hexane, or butane solvents used in these processes can be potentially hazardous and threshold limit values (TLVs) for these chemicals are published by the ACGIF.38 The newer installations, using gas-phase polymerization processes for polyethylene and polypropylene in fluidized beds or continuous-flow stirred-bed reactors do not employ solvents. These processes are less polluting and are more energy efficient as no energy is spent on recovering the solvent and drying the polymer. Some of the processes used in the manufacture of PVC, polystyrene (PS), as well as the many specialty polymers also involve the use of solvents. Improved engineering designs and management practices, however, have generally ensured that these solvents are used in closed systems with only minimal releases into the environment. Recovery of residual solvents from the resin product can require several steps in the manufacturing process and adds to the cost of the final product [38]. Despite increased costs, most of

38 The American Conference of Governmental Industrial Hygienists, publishes the TLV in an annually revised publication. ACGIH, 1330 Kemper Meadow Drive, Cincinnati, OH 45240.

the technology for solvent removal and containment within the manufacturing environment are presently implemented.

The following examples highlight selected polymer-based industries with a particularly high potential for atmospheric emissions. These selections do not, of course, present a comprehensive coverage of all such applications.

• Case: Paints and Adhesives Paints and adhesives are essentially concentrated solutions (or in some instances latices) of polymers in a suitable mix of solvents.

Products, such as varnishes, lacquers, latex paints, and glues, have large amounts of solvents that are invariably volatilized into the atmosphere. In the United States, annual associated VOC load on the atmosphere is about 25 million tons [39]. In western Europe alone, 4 million tons of solvents are used annually with about a quarter of the annual total VOC (including biogenic emissions) attributed to solvent use. A conservative estimate of the VOC emissions from a single class of paints, the solvent-based paints used as U.S. highway markings, amounted to about 40 million pounds annually [40].

Film formation in paints and adhesives depends on the evaporation of these solvents into the environment. A well-known effect of VOCs in the atmosphere is the propensity for smog formation. The VOCs encourage the formation of photochemical oxidants such as ozone at ground level in environments already affected by NOx pollution.39 Some natural processes also contribute to smog formation (as is apparent in the morning mists of the beautiful Blue Ridge Mountains in North Carolina). Biological VOCs from vegetation (forests account for over 90% of the biological emissions such as terpenes) or from soil, combined with NOX generated during lightning photoreact under exposure to solar ultraviolet radiation to produce this "natural" smog. High levels of ozone at ground level are particularly harmful to human health as well as to the biosphere. (High ozone levels in the "ozone layer" in the stratosphere, however, are protective and desirable.) The VOCs could be toxic or carcinogenic and may directly affect the health of populations routinely exposed to them (such as paint crews). While some of the VOCs partition into bodies of water or are deposited in the soil, most of it

39 Ozone is formed photochemically in the troposphere by photolysis of NO2 in a equilibrium process described in the following simplified scheme:

The presence of VOCs in the atmosphere produce peroxy radicals that compete in and interfere with the last reaction increasing O3 levels:

Table 1.7 Lifetimes (in years) of Some Common VOCs in Presence of Selected Reactive Species (OH radicals and Ozone) in Atmosphere [41, 42]

Compound

POCPa

OH Radicals

Ozone

Propane

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