The cornea and the vitreous humor (ocular medium), which lie in front of the retina, transmit 400- to 1400-m/x radiation to the cornea. The eye doesn't transmit ultraviolet and far infrared, but lasers emitting at these wave lengths (such as the 10,600-m/x carbon dioxide laser) can cause severe burns on the cornea, Dr. Ham says. The absorption of the pigment epithelium, and hence the damage done to it, varies with wave lengths, he finds. A large spot on the retina causes more damage than a small one, if their energy densities are comparable. Spot size depends on the divergence of the laser beam and on the focusing ability of the individual's eye. Most lasers have divergencies of 1 to 10 milliradians. Because of diffraction, the human eye can't focus even a nondivergent beam to a spot smaller than 10 to 20 microns in diameter, Dr. Ham finds, although some laser burns are as large as 200 microns. Large spots are more damaging because the energy absorbed near the center of such a spot cannot be readily conducted away, Dr. Ham explains. Energy focused on a small spot can be conducted to nearby, cooler areas. Long exposures to radiation of lowenergy density are less damaging than short exposures to high-energy densities, Dr. Ham says. For exposures of less than one millisecond, all of the incoming energy is absorbed by the pigment epithelium before any of it can be conducted away. Q-switched lasers are dangerous not only because they emit high-power radiation over a short time, but also because their brief pulses cause sonic (shock) waves in the eye, he adds. On the other hand, longer exposures allow the energy to be conducted into the choroid behind the pigment epithelium. However, at high-energy levels, the choroid also can be damaged.
Epoxy asphalt used to pave bridge over San Francisco Bay When the new San Mateo-Hayward bridge that spans San Francisco Bay opens to traffic at month's end, many eyes will be turned toward its asphaltmodified epoxy pavement. If the pavement lives up to expectations, it could spark a sizable increase in demand for epoxy resin. "As a surfacing material for roadways, epoxy asphalt combines the many desirable properties of an epoxy and asphalt," notes Richard M. Lydon, technical sales manager for Adhesive Engineering Co., San Carlos, Calif., the company that paved the bridge. The product has a hardness value matching that of portland cement 24 C&EN OCT. 23, 1967
Paving San Mateo-Hayward Bridge With an unusual guarantee while its flexural strength equals asphalt's. When fully cured, it is thermoset material that won't melt even at 600° F. It doesn't "rut" or "shove" under traffic pressure, stands up to hydrocarbon solvents and a variety of corrosive chemicals, has a high degree of built-in skid resistance, and bonds to asphalt, cement, or steel. Epoxy asphalt (not to be confused with epoxy coal tar) isn't new. Shell Development's Dr. Warren C. Simpson at Emeryville, Calif., was awarded the first patent on the material in 1957 (U.S. 2,906,720). Since then, Shell Oil has cooperated with a number of paving contractors throughout the country who have used it. Typical of its limited commercial applications to date has been the paving of the maintenance area of United Airlines at San Francisco International Airport and the Homestead, Fla., base of the Strategic Air Command. But despite the excellent service epoxy asphalt has given under punishing conditions, the paving industry has been slow to adopt it. A big problem has been in handling a two-component system in which the exact ratio of epoxy to asphalt is a critical parameter for the success of the compounding and surfacing operations. Adhesive Engineering, a Bemis Co. subsidiary, now claims to have overcome the handling problems. The company has developed an electronically controlled device that automatically meters out a controlled amount of epoxy on the one hand and asphalt, with catalyst and a number of additives, on the other. These are blended together at 250° F., and fed to a pug mill for mixing with limestone or granite aggregate at the same temperature. The aggregate makes up 94% of the final composition. The bridge is of an orthotropic de-
sign, an engineering principle developed in Germany. It has a steel roadway deck that's part of its support structure. The paving bonds directly to the steel plates and bolt heads without need for a special bonding layer. Its low specific gravity is an important engineering feature. "We're so sure of the product that we have guaranteed the paving against cracking or failure for two years, an unusual move in this business," Mr. Lydon says. Adhesive Engineering, at present Shell's sole U.S. licensee for epoxy asphalt, foresees a variety of uses for the material other than for surfacing orthotropic bridges. Examples are the maintenance areas at airports where fuels, hydraulic fluids, and oil cause conventional asphalt to fail. A layer of the epoxy asphalt over concrete runways would protect them against the corrosive action of salt used to melt ice and snow. And at the intersection of city streets, where oil drippings and the heat of auto engines combine to eat into asphalt, epoxy asphalt would make a logical surfacing material.
Inco has stainless steel with strength of structural steel International Nickel has developed a stainless steel that has a strength twice that of conventional steels and equal to that of structural steels. It is a direct result of fundamental studies on superplasticity at the company's Paul D. Merica research laboratory, Sterling Forest, N.Y. Superplasticity is the ability of a metal to stretch, usually at high temperature, without breaking. Most metals break before adding 50% to their original length. Superplastic
metals, however, may stretch to more than 1000% of their original length without breaking, according to Inco's Dr. J. H. Brophy, Dr. H. W. Hayden, and R. C. Gibson, inventors of the new stainless. But until now, superplasticity has been limited to alloys such as aluminum-zinc, tin-bismuth, and leadtin, which are not strong enough for structural use. Nor have conventional stainless steels been able to match the strength of structural steel until now. Conventional stainless steels yield at 3500 to 4500 p.s.i.; Inco's new stainless, called IN 744X, yields at about 75,000 p.s.i., which is in the structural steel range, the company says. The main difference between IN 744X and a high-strength, ductile steel developed at Lawrence Radiation Laboratory recently is that Inco's product comes from the mill with high strength. The Lawrence laboratory's steel must be warm-worked to attain its special properties. Key to the phenomenon appears to be an ultrafine-grained matrix, which is prevented from coarsening at high temperature by a finely dispersed second phase. Dr. Brophy and his coworkers have coined a metallurgical term—microduplex—to describe this kind of structure. They then developed a method for making nickel-containing steels with the microduplex structure and found that these materials exhibited superplasticity over a range of composition. The next step was to test the material for exceptional room-temperature properties which might be of commercial interest. The Inco scientists found
Welding IN 744X Unusual strength
unusual strength and excellent corrosion resistance in the new family of superplastic stainless steels. The first of the family to be introduced, IN 744X, is 26% chromium and 6.5% nickel. It consists of about 40% of a fine precipitate of austenitic stainless (face-centered cubic structure) in a matrix of ferritic stainless (body-centered cubic structure). The grain size in IN 744X is 2 to 3 microns. Conventional stainless steels are single-phase alloys with grain sizes exceeding 50 microns. Although IN 744X is still in the development stage, steel companies have already expressed interest in it, Inco says. The company has applied for patents covering the new family of steels and will license the process nonexclusively. Inco says IN 744X will not compete against conventional stainless steels. Instead, it will complement existing types by extending the use of stainless steel qualities. IN 744X combines the appearance and corrosion resistance of stainless steel with structural strength and could find applications in bodies for transit vehicles or supports for traffic signs, Inco says. No price has yet been given for the new material.
Chemical-biological process removes phosphorus Scientists throughout the nation are bending with a will to the task of finding cheap and reliable ways to take phosphorus out of municipal wastes. Last week, for example, Dorr-Oliver entered the race with a method involving both chemical and biological processes. The first plant to use the method is being built in Michigan. Meanwhile, at the Water Pollution Control Federation meeting in New York City, a full-house symposium heard of research and development progress with other combined chemical-biological methods. But one speaker claimed that, with relatively minor adjustments, maybe 30% of the activated sludge sewage treatment plants in the country could achieve 80%-or-better phosphorus removal without resorting to chemical methods. With the example of already critically polluted Lake Erie before them, many people are striving to save U.S. lakes and waterways from ruination caused by "overfertilization" with phosphate nutrient. So far, they are not agreed on any best way of going about it. Chemical methods, biological methods, and methods involving both are in operation or being studied. A conventional sewage treatment plant consists of a primary settling step followed by secondary treatment
involving aeration and then solids precipitation. The aeration allows microorganisms in the sewage to break down organic wastes. Such plants rarely remove more than about 20% of the phosphorus in sewage. One way to remove almost all phosphorus is by a purely chemical tertiary stage. With this method phosphate in secondary effluent is precipitated by lime or other chemicals. This, however, is an expensive approach—up to $120 per million gallons—as it involves large amounts of chemicals and an additional facility. With the Dorr-Oliver method, called the phosphate extraction process (PEP), lime is added to the primary settling tank to raise the pH to between 9.5 and 10.0. This removes 85% of the suspended solids and from 65 to 75% of the biological oxygen demand of the sewage. Without the lime only about 60% of the solids and 30% of the BOD are removed. The lime addition also cuts phosphate content from a typical 20 to 30 mg. per liter to 3 to 6 mg. per liter. This enhanced primary step greatly reduces the load on the secondary, biological stage of the process. This can now remove almost all the remaining phosphate. Among other advantages claimed by the Stamford, Conn., process equipment maker for PEP are low chemical costs—30 to 40% of those for tertiary treatment—and cheaper sludge handling due to the preponderance of easy-to-handle primary sludge. At the WPCF meeting, E. F. Barth of the Federal Water Pollution Control Administration's research laboratory in Cincinnati, Ohio, outlined progress on 100 gallon-per-day pilot-plant tests checking the effect of adding chemicals directly to the aeration chamber. He finds sodium aluminate the most effective. When added at approximately one part of aluminum for every one part of phosphorus in the influent, it removes 90% or better of the phosphorus. In a typical run, influent phosphorus concentration of 10.5 mg. per liter is cut to 8.5 mg. per liter by primary treatment, and then to 0.3 mg. per liter for the final effluent of the plant by secondary treatment (boosted by adding 10 mg. per liter aluminum to the aerator). Without the aluminum addition, only about 40% of the phosphorus is removed. W. A. Eberhardt of Pennsylvania State University told the meeting that aluminum sulfate has proved effective in lab-scale treatment of primary effluent. Added to a mixer just prior to aeration it helps cut phosphorus content more than 95%. The best results are with an aluminum-to-phosphorus ratio of more than two. Both Mr. Barth and Mr. Eberhardt estimate OCT. 23, 1967 C&EN
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