Such studies follow up worldwide cell works on the photoelectric effect: Light knocks electrons loose near the toxicology tests that make methylene p-n junction, forming electron-hole chloride one of the "most investigated pairs. Electrons liberated on the p- solvents in the whole world," ac side are drawn to the η-side; holes on cording to one Dow scientist. One the η-side are drawn to the p-side. reason for all this interest is that, The result is an electric current. D acting as solvent, vapor pressure de pressant, and flame suppressant, methylene chloride substitutes nicely in aerosol products for the contro Isocyanate needs price versial chlorofluorocarbons 11 and hike to spur expansion 12. Unlike the chlorofluorocarbons, A 55% jump in the U.S. price of tolu methylene chloride is not "implicated ene diisocyanate (TDI), the most in the ozone issue," Dow's Hugh A. important raw material of polyure- Farber notes. Not only does meth thanes, is needed to justify additional ylene chloride degrade rapidly in the plant investment by 1982. This is the lower atmosphere, it doesn't generate gist of what Donald D. Threlkeld, photochemical oxidants there, he director of administrative services at adds. Upjohn's La Porte, Tex., polymer In some ways, methylene chloride chemicals division, told the Chemical is more attractive than chlorofluoro Marketing Research Association at a carbons for aertfsols—even without IBM's Woodall (left) and Hovel prepare to meeting in New York City last the ozone controversy. For example, week. methylene chloride is cheaper, and it test experimental solar cell Threlkeld says that TDI would could reduce the cost of hair sprays Since gallium arsenide cells are not have to jump from its present 55 cents 40% and antiperspirants 70%, ac yet made commercially, their price per lb to at least 85 cents per lb in five cording to a Dow spokesman. One problem, however, which cannot be compared directly with years before U.S. producers would that of silicon. But their materials find it economically practical to might have kept methylene chloride from gaining early acceptance in cost alone would likely make them build. more expensive, say Woodall and Threlkeld told the meeting that aerosol products, is the compound's Hovel. On the other hand, gallium isocyanate prices, despite recent price tendency to hydrolyze. Because it arsenide cells have two advantages hikes, have not kept pace with in forms hydrochloric acid in small creases in manufacturing costs. If this amounts, containers tend to corrode. that could make them competitive. One advantage is their efficiency. gap does not close, he says, the iso However, Dow's formulation of The other is their ability to operate at cyanate industry "will not be able to methylene chloride has specific cor high temperatures. A relatively cheap attract the capital necessary to ex rosion inhibitors to minimize this system of mirrors and lenses thus pand capacity and growth of demand problem. could be used to concentrate sunlight will be limited by supply capabili Regardless of such potential utility, up to a thousandfold on the expensive ties." methylene chloride's acceptance by cells. With silicon, the concentration According to Threlkeld, actual ca the aerosol industry, which is already factor is limited to a few hundred. pacity of TDI plants is about 85% of sensitive to such issues, must depend Furthermore, the gallium arsenide nameplate capacity. Although ca on a full assessment of the com cells can be produced in a simple, pacities have been underutilized for pound's safety. Little was known of one-step process. In fact, devising this most of the past 15 years, this year its carcinogenic potential when Dow process was the key to Woodall and producers are operating at close to the began a 24-month inhalation study last year. Hovel's latest breakthrough. A 1 cm practical capacity. square wafer of gallium arsenide is The tests, involving nearly 2000 He adds, "Barring governmental covered with a molten mixture of price controls and other overrestric- animals which were exposed to gallium, aluminum, arsenic, and zinc. tive constraints, world consumption methylene chloride levels as high as A half-hour later it is removed, and of urethane polymers should grow 3500 ppm, so far give the solvent a except for some auxiliary operations about 10% per year from about 5.3 remarkably clean bill of health. like the attaching of electrodes, it is billion lb this year to more than 8 Though the usual variety of sponta ready to take its place in the sun. billion lb in 1982. About 3 billion lb of neous lesions are found in both con In the melt, several things happen isocyanates [both TDI and methylene trols and exposed animals, "none can at once. The gallium arsenide wafer diphenyl diisocyanate] will be re be associated with the exposure," says D Dr. Larry W. Rampy, group leader of was originally an η-type semicon quired to meet this demand." Dow's toxicology laboratory. ductor; that is, it had a surplus of One other potential problem with electrons. In the melt, zinc diffuses methylene chloride use involves its into the wafer, changing a layer sev Methylene chloride metabolism to carbon monoxide, eral micrometers thick into a p-type which binds to hemoglobin in blood. semiconductor (with a deficiency of passes early tests However, at the permitted levels of electrons, or equivalently, a surplus of "holes")· Thus the wafer now con Rarely are scientists excited about exposure—currently 500 ppm, tains a p-n junction. negative findings, but toxicologists at though this might be stiffened to 200 Meanwhile, impurities (thought to Dow Chemical seem delighted that ppm—methylene chloride elevates be copper) migrate out of the wafer methylene chloride is proving so in blood carbon monoxide slightly, if at into the melt, as an 0.2-micrometer nocuous. Interim results of a two-year all. Hence any risk, even to aerosol layer of zinc-doped gallium aluminum inhalation study indicate "no evi "sniffers" hoping for highs or to car arsenide is deposited on the surface. dence of cancer in test animals" ex diovascular patients who are more • What does all this mean? A solar posed to methylene chloride vapors. sensitive, is minimal. 6 C&ENMay9, 1977
