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Propellants are used to disperse aerosols, powders and other materials in a wide variety of applications. Consumers are familiar with products such as cleaners, waxes, and spray paints that are packaged in "aerosol spray cans" that use propellants. In addition, many medical, commercial and industrial products use propellants. These products include adhesives; document preservation sprays; portable fire extinguishing equipment; insecticides; sterilants; animal repellants; medical devices such as anti-asthma inhalers and topical anesthetic applicators; lubricants, coatings and cleaning fluids used in the electrical, electronic, and aerospace industries; and blowing agents and mold release agents used in the production of foams, plastic and elastomeric materials. In many cases, the propellant pressurizes, atomizes and delivers the product as an aerosol spray. In other cases, the propellant itself is the sole or major "active ingredient" of the product, usually for solvent and/or cleaning applications.

Aerosol sprays and other dispersions made possible using propellants can form very small and uniform droplets with controllable properties. These characteristics are desirable or necessary in critical applications where a very thin layer or coating must be applied to a surface, e.g., as a lubricant coating in an aerospace application. Additionally, the small aerosols can be inhaled deeply into the lung and the droplet itself may evaporate quickly, depending on the product. Such characteristics help optimize the delivery of drugs using medical inhalers.

The ideal propellant should generate sufficient volume and pressure of vapor or gas for a particular application, should be safe to humans and the environment , inert with respect to reactions with other materials, miscible or highly soluble in the product, easy to handle, and inexpensive. Each chemical has its own physical and chemical properties that affect the selection of a propellant for a given product and application. Important properties are solvency, performance, cost, and environmental considerations. Many of these properties are well characterized. For example, the solvency of a propellant can be measured as its solubility in water or other chemicals . Performance properties include the amount and pressure of vapor generated from the liquid, measured as vapor pressure, the volume of vapor generated per mass or volume of the liquid, and the ratio of the volume of gas to the volume of liquid. Performance also includes the range of temperatures where the propellant will function adequately, measured as the boiling and freezing points of the propellant, and as the vapor generation rates at different temperatures. The flammability of the propellant is another often critical property, measured as the flammability limits in air and the flash point. Additional properties that may be important include the density of the liquid and gas, the critical temperature and pressure, the specific heat of gas and liquid, the heat of vaporization, the viscosity of the liquid propellant, the coefficient of liquid expansion, and the surface tension. Environmental characteristics of propellants include the toxicity (often measured as cancer causing potency), the ozone depleting potential, the greenhouse warming potential, and the reactivity (in terms of forming ground level ozone). It is the environmental considerations that have brought propellants forward in the last 20+ years as an national and international environmental issue.

Restrictions on the Use of Propellants

Prior to the late 1970s, aerosol propellants were frequently chlorofluorocarbon (CFC) propellants, and this use constituted over 50% of total U.S. CFC consumption. At the time, the most widely used CFCs were CFC-11 and CFC-12. (These chemicals were also used in refrigeration, foam blowing, and sterilization). Following concerns raised in 1974 regarding possible stratospheric ozone depletion resulting from CFCs, the U.S. Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA) acted on March 17, 1978, to ban the use of CFCs as aerosol propellants in all but essential applications. This reduced aerosol use of CFCs by approximately 95% and achieved nearly a 50% reduction in the U.S. consumption of CFCs. This reduction was largely accomplished without economic penalty as consumers voluntarily responded to advertising for "ozone safe" substitutes. Also in the late 1970s, Canada and a few Nordic nations banned or restricted aerosol propellant uses, also resulting in a sharp drop in their total CFC use. Later in the 1980s, however, increases in other uses of CFCs, largely as solvents, offset the earlier decreases.

The 1978 ban specifically exempted certain products based on a determination of essentiality. Also excluded were products where the CFC itself was the active ingredient or sole ingredient in an aerosol or pressurized dispenser products, as this did not fit the ban's narrow definition of an "aerosol" propellant. These restrictions remained in effect until 1990. Title VI of the 1990 Clean Air Act Amendments (CAAA) included provisions relevant to use of ozone depleting chemicals used as propellants and expanded restrictions for propellants. EPA's response is the "Significant New Alternatives Policy" (SNAP) Program which includes the evaluation of alternatives to ozone-depleting substances, including an assessment of their ozone layer depletion potential, global warming potential, toxicity, flammability, and exposure potential (section 612 of the CAAA; 59 FR 13044).

In general, ozone-depleting substances are divided into two classes with different product bans (section 610 in the CAAA): Class I comprises CFCs, halons , carbon tetrachloride, methyl chloroform (MCF), hydrobromofluorocarbons and methyl bromide; and Class II comprises solely of hydrochlorofluorocarbons (HCFCs). EPA is to prohibit the sale or distribution of certain "nonessential" products (as determined by Congress and EPA) that release class I substances, mandate the phaseout of class I and class II substances (sections 604 and 605), review substitutes (section 612), and prohibit the sale of certain nonessential products made with class I and class II substances (section 610). After its review, EPA issued regulations on January 15, 1993 (58 FR 4767) which banned CFC propellants in aerosols and other pressurized dispensers (and also in some other products such as flexible and packaging foam). Exceptions were made for a number of products, including certain medical devices; lubricants for pharmaceutical and tablet manufacture; gauze bandage adhesives and adhesive removers; topical anesthetic and vapocoolant products; lubricants, coatings and cleaning fluids for electrical and electronic equipment and aircraft maintenance that contain CFC-11, CFC-12 or CFC-113; release agents for molds used to produce plastic or elastomeric materials that contain CFC-11 or CFC-113; spinnerette lubricant/cleaning sprays used to produce synthetic fibers that contain CFC-114; CFCs used as halogen ion sources in plasma etching; document preservation sprays that contain CFC-113; and red pepper bear repellant sprays that contain CFC-113. However, these products are not exempted from the phase-out requirements for CFCs.

HCFCs can make excellent propellants, but their use has been limited due to their cost and restrictions regarding the class II ozone-depleting substances. In 1993, as a result of increased taxation and CFC phase-out, HCFCs were temporarily economically viable in applications where flammability is a concern and cheaper alternatives (hydrocarbons ) could not be used. However, section 610(d) of the CAAA prohibits the sale or distribution of aerosol or foam products that contain or are manufactured with class II substances after January 1, 1994. Again, exceptions and exclusions from this ban have been made for "essential uses," i.e., if necessary due to flammability or worker safety issues and if the only available alternative is the use of a class I substance. On December 30, 1993, EPA published a final rule (58 FR 69637) which exempted medical devices, lubricants, coatings or cleaning fluids for electrical, electronic equipment, and aircraft maintenance, mold release agents used in the production of plastic and elastomeric materials and synthetic fibers, document preservation sprays containing HCFC-141b or HCFC-22, portable fire extinguishing equipment sold to commercial users, owners of boats, noncommercial aircraft, and wasp and hornet sprays for use near high-tension power lines. However, no other exceptions for class II propellants were made since substitutes are available.

Propellant Alternatives and Substitutes

Due to the restrictions on propellants use, a large number of propellant alternatives and substitutes have been investigated. Both EPA and industry have been active in this area.

Many products can be effectively packaged, distributed and used without employing a propellant. For example, some products are now sold for direct application as liquids or use manually operated finger and trigger pumps, two-compartment aerosol mechanism, mechanical pressure dispenser systems, and nonspray dispensers (e.g., solid stick dispensers). The elimination of propellants altogether can be viable option for some products and applications. In other cases, however, this may not provide proper dispersal or accurate application of the product. Also, persons using manual pumps or sprays may become fatigued with the constant pumping motion and thus produce poor product performance. Because propellants are considered essential in a variety of uses, there remains a need to find propellants that are not environmentally damaging. Unfortunately, no propellant exists which has all favorable properties. For example, some CFC propellant substitutes, e.g., ammonia, butane and pentane, have problems with toxicity and flammability. There may be some essential applications for CFCs propellants for which no practical substitutes exist, and the use of CFC propellants in anti-asthma inhalers is a frequently cited example.

A variety of propellants are being considered or are being used as alternative propellants for class I and II controlled substances. Each alternative propellant has its own physical and chemical characteristics that influence its suitability for a given application. The primary substitutes for aerosol propellant uses of CFC-11, HCFC-22 and HCFC-142b are saturated hydrocarbons (C3-C6); dimethyl ether; compressed gases; and HFCs. A few EPA-approved alternative propellants are discussed below.

Of the hydrocarbons, butane, isobutane and propane may be used singly or in mixtures. All have low boiling points, are relatively nontoxic, inexpensive and readily available. (As with essentially any propellant, very high concentrations may result in asphyxiation because of the lack of oxygen.) However, these propellants are flammable and, for example, should not be used around electrical equipment if sparks could ignite the hydrocarbon propellant. To reduce product flammability, hydrocarbons can be used with water-based formulations. In the United States, nearly 50% of aerosol propellants were using hydrocarbon propellants prior to 1978, and nearly 90% in 1979 as a result of the CFC ban. Propane/butane propellants have been used since 1987 in the Scandinavian market.

Dimethyl ether (DME) is a medium pressure, flammable, liquefied propellant generally used in combination with other propellants. Its properties are similar to the hydrocarbons.

Compressed gases, including carbon dioxide , nitrogen , air, and nitrous oxide , are used in applications where a nonflammable propellant is necessary. These gases are inexpensive, readily available, nonflammable (although certain temperatures and pressures of nitrous oxide may create a moderate explosion risk), relatively nontoxic, and industrial practices for using these substitutes are well established. Since these gases are under significantly greater pressure than CFCs and HCFCs, containers holding these gases must be larger and bulkier, and safety precautions are necessary during filling operations. Often, modifications must be made to use compressed gases as they require new dispensing mechanisms and stronger containers due to the greater pressure; their low molecular weights restrict certain applications; high pressure compressed gases dispel material faster which may waste product; compressed gases cool upon expansion; and finally, they are not well suited for applications that require a fine and even dispersion. At present, about 7-9% of the aerosol products use compressed gases, and their use is expected to grow.

Hydrofluorocarbons (HFCs) such as HFC-134a, HFC-125 and HFC-152a are partially fluorinated hydrocarbons, developed relatively recently and currently priced significantly higher than HCFC-22. HFCs are less dense than HCFC-22, but can provide good performance in applications (but not products such as noise horns, which require a more dense gas). HFC-134a and HFC-125 are nonflammable and have very low toxicity. HFC-152a is slightly flammable. All three HFCs have zero ozone depletion potential, but are potential greenhouse gases although their atmospheric residence times are short. Additionally, these HFCs are chemically reactive and they contribute to the formation of tropospheric ozone. In ozone nonattainment areas, state and local controls on VOCs may restrict the use of these products. HFCs may be combined with flammable propellants to reduce the flammability of the mixtures. HFC-134a and HFC-227a are possible alternative propellants in medical applications (currently using CFC-12 and CFC-114). Approvals from both FDA and EPA are needed for these applications.

[Stuart Batterman ]