[ SCIENCE/TECHNOLOGY Pressure Mounts As Search For Halon Replacements Reaches Critical Phase • Range of substitutes soughtfor firefighting applications as deadline nearsfor use of halons in military aircraft Michael Freemantle, C&EN London esearchers are urgently pressing forward on several fronts to find replacements for halons currently used in firefighting applications. The work is being spearheaded by the Department of Defense, which requires suitable alternatives for specific applications in military aircraft to be identified by October and the technology for their use to be in place by Sept. 30,1995. At present, the armed forces use halons for fire extinguishment and explosion suppression in helicopters, fixed-wing aircraft, ships, maritime craft, ground armored vehicles, and other applications. Halons also are used widely in commercial applications—to protect
R
Moore: low toxicity is essential
valuable electronics, in oil and gas production, and on civilian aircraft. The major halons used for fire suppression are CBrF3 (halon 1301) and CBrClF2 (halon 1211). When they ascend to the stratosphere, these compounds undergo photolysis to form bromine radicals. Bromine is much more efficient than chlorine in removing ozone in the stratosphere (C&EN, Aug. 15, page 8). So, although compared with chlorine the concentration of bromine in the stratosphere is relatively low—0.02 ppb—it makes a significant impact on the stratospheric ozone balance. On Jan. 1, the manufacture of halons 1301 and 1211 was banned under the latest revision of the Montreal Protocol on Substances That Deplete the Ozone Layer. The ban is spurring halon manufacturers and users to a frantic search for replacements or alternative methods of firefighting and explosion protection. There is no immediate shortage of halons, however. Existing supplies are being carefully husbanded (C&EN, Nov. 15, 1993, page 12). The nonprofit Halon Recycling Corp., based in Washington, D.C, is helping match companies that have excess halons with those that need them for critical uses such as aircraft protection. The Defense Logistics Agency, Alexandria, Va., has a similar halon bank for the military. The United Nations Environment Program estimates that current worldwide supply is large enough to last until the middle of the next century. Nevertheless, there is an urgency to find suitable halon replacements. It likely will take years to retrofit fire suppression systems on existing fleets of aircraft. And new generations of both military and civilian aircraft will need to rely on non-halon-based fire protection systems. The most ubiquitous halon in both military and civilian applications is halon 1301. It is used, for example, by the armed forces in handheld fire extinguishers and for explosion suppression in ground armored vehicles and air-
craft. As yet, no single replacement for this halon has been found. "There is no son of wonder gas," says Robert E. Tapscott, director of the Center for Global Environmental Technologies (CGET) at the New Mexico Engineering Research Institute, Albuquerque. 'There are no breakthroughs, no clear-cut winners, no all-encompassing solutions." This view was echoed by speakers at a session on "Halon Replacements: Technology and Science," which was part of a symposium on "Advances in Replacements for Ozone Depleting Compounds" sponsored by the Environmental Chemistry Division at the ACS national meeting in Washington, D.C. The consensus at the session was that halons will be replaced by a range of alternatives depending on specific applications. With respect to halons, Tapscott carefully distinguishes between the terms "replacement" and "alternative." He points out that replacements are halocarbon substitutes. First-generation halon replacement agents are generally "physical action agents." They operate primarily by heat absorption and include hydrochlorofluorocarbons (HCFCs),
Grosshandler: evaluate halon replacements SEPTEMBER 19,1994 C&EN 29
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hydrofluorocarbons (HFCs), and perfluorocarbons (FCs or PFCs). Second-generation replacement agents are all chemical action agents (CAAs) with low global environmental impacts. CAAs operate primarily by removing free radicals from flames. Most contain bromine or iodine, "one of which appears to be necessary for chemically active halocarbon fire extinguishment/' Tapscott says. These agents have short lifetimes in the atmosphere, and the bromine or iodine they contain do not reach the stratosphere. Halon alternatives, unlike replacements, are nonhalocarbon substitutes. They include water sprinklers, dry chemical extinguishants, carbon dioxide, misting systems, particulate aerosols, and inert gas blends. One possibility for certain halon applications is to abandon fire protection altogether. William L. Grosshandler, group leader at the Building & Fire Research Laboratory of the National Institute of Standards & Technology (NIST), Gaithersburg, Md., uses computers to illustrate the point. Fifteen to 20 years ago, a mainframe computer may have cost a company $1 million and would have been protected from fire by a halonbased system, Grosshandler points out. That computer now very likely has been replaced by a dozen much smaller computers, each costing less than $5,000. Installing a new non-halon-based system to protect each computer from fire would be much more expensive than simply replacing the computers if they are damaged or destroyed by fire. So the cost-effective solution is to leave the computers unprotected. Ronald S. Sheinson, a research chemist at the Naval Research Laboratory in Washington, D.C., points out that halon 1301 is widely used because it is an efficient, clean fire-extinguishing agent with negligible toxicity. "But it is overused," he says. "Only about 5% [of what is used] is necessary. Halon replacement starts with basic fire protection principles." At the meeting, Sheinson discussed the Navy's total-flooding halon replacement program. Total flooding is filling an enclosed space with an agent to the concentration required to suppress a fire or an explosion. Halon 1301 is used in total-flooding shipboard fire protection primarily for its effectiveness in extinguishing flammable spray and cascading liquid three-dimensional fires. The 30
SEPTEMBER 19,1994 C&EN
Gann (left): research is at critical phase; Sheinson: some halons are overused
replacement program is testing HFCs, that all halocarbon candidates produce FCs, trifluoromethyl iodide (CF3I), agent similar toxic combustion products, such blends, synthetic atmospheres, fine wa- as CO, HF, and COF2. ter mist, and fine dry powders. The Environmental Protection Agency Sheinson says that real-scale fire extin- requires acute toxicity, cardiac sensitizaguishment tests are currently under way tion, development toxicity, and subin an 850-cubic-meter space aboard the chronic toxicity testing. The Clean Air Navy's fire research ship, the former Act requires that EPA enact regulations U.S.S. Shadwell. He tells C&EN that "this making it unlawful to replace any chlois the largest and most heavily instru- rofluorocarbon or halon with any agent mented program for testing total- that may impact human health or the environment. EPA is also required to flooding halon replacements." In recent months, CF3I has been publish lists of both prohibited and acwidely acclaimed as a possible second- ceptable substitutes. Moore notes that total-flooding agents generation candidate for replacing halons. "At first, CF3I was viewed as and streaming agents require different great," says Susan Solomon, a senior toxicological considerations. Streaming scientist at the National Oceanic & At- agents are typically used in portable exmospheric Administration, Boulder, tinguishers to put out localized fires. Colo. "It was going to be our miracle Specific considerations include wind direplacement. It is an excellent fire ex- rection and obstructions. tinguishant, has a short lifetime in the Moore's studies show that several atmosphere, and has small ozone- candidates, including CHF3 (HFC-23), depleting and global-warming poten- CF3CHFCF3 (HFC-227ea), and the fluotials—all the right properties from the rocarbon C4F10, are suitable for use in environmental point of view. Unfortu- occupied areas from a toxicological point nately, I understand that it is not doing of view. well on toxicity tests." Low toxicity is one of four desirable Grosshandler says, in his opinion, "it characteristics used to assess halon reis unlikely that CF3I will be used in oc- placements. The others are cleanliness, cupied spaces in the same way that low global environmental impact, and halon 1301 has been used. However, it effectiveness as fire extinguishers. may be recommended for other appli- "Though it is very easy to find candidate cations such as putting out aircraft en- replacements that meet any three of gine fires, where possible toxicological these criteria, it has been difficult to find agents that meet all four," says CGET's impact is [negligible]." Ted A. Moore, an environmental engi- Tapscott. Second-generation agents, in particuneer at CGET, stresses that test species, exposure route, duration, and endpoints lar, present massive problems. For these must all be considered when comparing compounds, Tapscott comments, "we halon replacement candidates. He adds know little about manufacturability, tox-
icity, emissions, materials compatibility, and stability, and the market may not be sufficiently large to justify the cost of determining these unknowns." One of the main problems with firstgeneration halon replacements is the mass and volume of material needed to put out a fire. 'Tor most, but not all, applications, at least two to three times as much of any first-generation replacement will be needed to provide the same degree of protection as provided by the present halons," Tapscott observes. This issue is crucial for aircraft, where weight carried is a critical factor, and inside ground combat vehicles. "Unlikely as it may seem, all the space and weight allocations are spoken for, even in a 60-ton tank," observes Anthony E. Finnerty, a research chemist at the Army Research Laboratory, Aberdeen Proving Ground, Md. Finnerty reported at the meeting that the Army is engaged in research and development to identify a replacement for halon 1301 in the crew compartments of combat vehicles. He notes that for this application the replacement agent must be environmentally friendly, have low toxicity, be noncorrosive, and be effective in the temperature range -55 to 71 °C. In addition, any by-products of the fire-extinguishing process should not be injurious to crew members or hinder their exit from a damaged vehicle. Furthermore, any residue, after discharge, should be easy to clean up. Until a suitable agent is found, the Army will use halon 1301 from its reserves, Finnerty says.
Finnerty: space is a critical factor
Solomon (left): low environmental impact; Miziolek: simulate flame structures Grosshandler and coworkers at NIST are developing laboratory-scale facilities that can be used to systematically evaluate the behavior of possible replacements for halon 1301. Their research program—which is sponsored by WrightPatterson Air Force Base, the Naval Air Systems Command, the Army Aviation & Troop Command, and the Federal Aviation Administration (FAA) Technical Center—focuses on protection of the engine nacelle and dry bay. The engine nacelle is the portion of the airframe surrounding the main jet engines. Dry bays are closed spaces adjacent to flammable liquid storage areas in which a combustible mixture and an ignition source could coexist after penetration by an antiaircraft projectile. The flame suppression performance of candidates is measured in terms of the ratio of the candidate's volume to that of halon 1301 needed to put out the same fire. The ratio varies depending on the fire. The NIST group is currently recommending CHF 2 CF 3 (HFC-125), CF3CHFCF3 (HFC-227ea), and CF3CF2CF3 (FC-218) for full-scale testing for these specific aircraft applications. Grosshandler points out that "CF3I is also being looked at very seriously/' The compound has a volume factor on the same order as halon 1301, whereas for other candidates the ratio is between 1.5 and 3.0. Richard G. Gann, chief of NIST's fire science division, is leading a research program examining the compatibility of halon 1301 alternatives for in-flight fire suppression in engine nacelle and dry bay spaces. The program is spon-
sored by the armed forces and FAA under the direction of Michael Bennett at the Flight Dynamics Laboratory at Wright-Patterson Air Force Base. The program is evaluating 11 HFC, HCFC, and FC fluids and the dry powder NaHC0 3 with regard to their compatibility with the delivery system and their stability during storage under maximum in-flight conditions. The fluids are stored in stainless steel cylinders with an elastomer seal. Each cylinder contains a metal used in suppressant bottles. The fluids are stored in these cylinders at elevated temperatures and pressures and examined by Fourier transform infrared spectroscopy before and after incubation. Gann reported at the ACS meeting that none of the compounds showed any significant deterioration and that no by-products were detected. The metals are also tested for corrosion by exposing them to candidate r e placements for 30 days. Metal samples are then weighed and examined by optical microscopy, scanning electron microscopy, and metallography. Gann and his group are also examining the compatibility of suppressant fluids with representative elastomers and greases used in fire suppressant bottle seals. Yet another corrosion problem concerning halon replacements must be solved. Huorine-containing compounds produce the corrosive (and toxic) gas HF when used to extinguish flames. This is true for both halon 1301 and its fluorine-containing replacements. The replacements, however, are required in significantly higher concentrations, and so the inhibited flames may produce SEPTEMBER 19,1994 C&EN
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correspondingly higher concentrations of HE Gregory T. Linteris, a research engineer at NIST's fire science division, has been investigating this problem by determining the rate of acid gas production in inhibited flames. His experiments involve wet-chemistry analysis of the final products of the flames using ion-selective electrodes for F~. Andrzej W. Miziolek, research leader for applied photochemistry and laser spectroscopy at the Army Research Laboratory, who organized the symposium, and coworkers are also studying flames inhibited by halon replacements. The group aims to simulate flame structures with respect to concentrations of different chemical species. 'The studies tell us how much of the acid gases, primarily HF, are produced for a given amount of [inhibitor] as well as what the flame speed is," says Miziolek. "Flame speed is a measure of inhibition.,,
Miziolek adds that an understanding of flame inhibition and the chemical mechanisms involved will help the search for halon replacements. He comments that the ban on halon manufacture has spurred further research efforts in this field. NIST's Gann underlines the point that research on halon replacements is now taking place at a rapid pace. "We are now into the critical phase," he says, adding that DOD and FAA will decide in October on replacements for fire extinguishment in the engine nacelle and dry bay. "This is a big-time decision with longterm implications for both military and civilian aircraft," Gann says. "Our schedule has us taking data literally right up to a few days before that meeting." DOD recently asked Gann to put together a long-range research plan to develop new alternative technologies for all their current uses of halon 1301. •
Tomatoes illustrate safety issues for biotech foods At a symposium sponsored by the Agricultural & Food Chemistry Division during the recent ACS national meeting in Washington, D.C., scientists and government officials focused on the general issue of the safety of genetically modified foods and flavors. But for a variety of reasons, the subject of tomatoes kept coming up. Tomatoes and their relatives are
among the easiest plants to manipulate genetically, and as such, they have been the center of much basic research on plant genetics. Tomatoes also are a reasonably important cash crop, and genetically engineered versions of the fruit are making their way to the market. Indeed, earlier this year, Calgene's FlavrSavr tomato, engineered for longer shelf life, became the first whole food produced
1-HEPTYNE Variety of traits are being genetically engineered into foods
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Trait
Herbicide tolerance
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Farchan Laboratories P.O. Box 2430 • Gainesville, FL 32602 U.S.A. Tel: 904-378-5864 - Fax: 904-371-7571 CIRCLE 1 ON READER SERVICE CARD 32
SEPTEMBER 19, 1994 C&EN
Disease/pest resistance Viruses
Insects Other agronomic properties Cold tolerance Altered ripening Postharvest properties Increased starch Altered oil composition Altered amino acid content Source: Food & Drug Administration
Gene modification
Crop
Enzyme genes
Tomato, cotton, soybean, corn, rapeseed
Various viral coat proteins
Cantaloupe, squash, tomato, corn, alfalfa, potato Tomato, cotton, corn, rapeseed, rice, potato
Bacillus thuringiensis 5 endotoxin Fish "antifreeze protein' Polygalacturonase antisense gene
Tomato Tomato
Metabolic enzymes Oil-modification enzymes Seed storage proteins
Potato Rapeseed Corn, soybean, rice, sunflower
