New Varieties
tails of their respective formulations and polymerization techniques. Graft Polymers. Most of the ABS materials produced today are blends of a polybutadiene-acrylonitrile-styrene graft polymer and free styreneacrylonitrile (SAN) resin. Polybutadiene serves as the "spine" for the rubbery graft polymer, to which styrene and acrylonitrile are copolymerized as side chains. During the copolymerization, free SAN resin also forms. The butadiene-based portion is often called a terpolymer, though it isn't formed by simultaneous polymerization of all three monomers. The first commercial ABS materials were mechanical blends of SAN resin and nitrile rubber ( acrylonitrile-butadiene). The shift to graft-polymer ABS began in the mid-fifties, when it was found to be more compatible with SAN than is nitrile rubber. This compatibility generally gives a graft-polymer ABS a better combination of physical properties than those a mechanical-blend ABS with the same ratio of monomers would have. Free SAN in commercial ABS grades ranges from 50 to about 90%. Acrylonitrile-styrene ratios of from 25:70 to 30:70 are standard. At higher or lower ratios, copolymerization has been found harder to control. Homopolymers of styrene or acrylonitrile, if formed during the process, cut the strength and cohesiveness of the product. These homopolymers are successfully controlled to considerably less than 1% in practice—down to the parts-per-million level, according to one producer. First Step. The first step in making a typical ABS is butadiene polymerization. Since the reaction yields more than 500 B.t.u. per pound of butadiene polymerized, it is carried out in water inside a jacketed vessel. An emulsifier is used, so that the end product is a latex-like water emulsion of polybutadiene. Modifiers such as carbon tetrachloride are also added to the reactor to control the molecular
weight of the polymer. Polymerization won't go to completion. An arrester, such as sodium dithiocarbamate, is added at about 75% of completion to cut off the reaction entirely, and the latex is steam-stripped to recover butadiene. The polybutadiene is then added to a second reactor in which styrene and acrylonitrile are copolymerized. Some of the monomers copolymerize as side chains on the polybutadiene. Modifiers such as mercaptans are used to control the length of these chains. One producer says that the graft polymer can contain up to about 3 5 % SAN side chains without damaging the toughness of the finished ABS. Catalysts for the polymerization include persulfates and organic peroxides. This reaction also occurs in water emulsion, to help dissipate the heat generated. Reaction temperatures may range from about 80° to 200° F. Depending on the formulation, additives, and process techniques used, polymerization of a batch of ABS requires from several hours to several days. The resulting latex may be blended with SAN latex, if medium-impact grades are desired. The emulsion is broken in a flocculation tank, and the ABS is filtered and dried to a crumb form. Pigments and fillers are added in a Banbury-type mixer to produce finished resin. Research. Naugatuck Chemical introduced the first commercial ABS resin in 1948. However, there are still enough unknowns about the effects of changes of polymer structure and monomer ratios, producers say, to justify substantial research programs. One such area in the past has been heat resistance. In-use temperature limits of some grades now stand at about 225° F., making them sterilizable without heat distortion. Producers point out that a natural limit is imposed here by processing considerations. Â typical sterilizable grade must be heated for molding to about 475° F. for extrusion, or to about 550° F. for injection molding. If inuse temperature limits rise significantly higher, extrusion or injection temperatures must also rise and will approach decomposition temperatures. ABS is flammable, though slowburning. Self-extinguishing grades, filled heavily with inorganics, are in the development stage, but the effect on other desirable physical properties of such resins is still open to question.
Systems Recover Chromate from Cooling Tower Water Discharge Separate systems from Nalco and Crane use ion exchange resin to save costs, prevent pollution Existence of chromate chemicals in blowdown wastes from recirculating water systems treated with the chemicals may not be a problem much longer. At least two companies are preparing to attack it directly. Nalco Chemical has developed an ion exchange method for recovering and reusing the chromate and says field testing is indicated. Meanwhile, Crane Co. has field tested and has started to offer skid-mounted ion exchange units to do the job. The two developments are independent. Although Crane gets some of its ion exchange resin from Nalco, the company chose to develop its own system using them. Both are separately pursuing patents on the systems, and are currently negotiating to get some kind of working agreement. Chromate chemicals are widely used in cooling-tower recirculating water systems as a rust inhibitor for the distribution lines and equipment. Such systems, however, continuously discharge about 1% of their flow to waste (blowdown) to prevent solids build-up in the water with the resulting scaling of equipment. This volume is then made up with fresh water. Thus, if the system uses chromate treatment, the chromate chemicals are continuously discharge in the blowdown. With ever-tightening pollution regulations the discharge of chromate-containing effluent into public waters is no longer allowed in many areas. But with the new systems, chromate can still be used in these areas, since it's removed from the discharge. The systems could also expand application of chromate treatment to plants where it is not now excluded. The systems could also save the cost of replacing much of the chromate lost in blowdown. Blowdown wastes usually contain from 15 to 300 p.p.m. chromate as C r 0 4 - 2 ion. The lower concentrations are more common, since most chromate is now used with synergistic ingredients. Other chemicals in these mixtures reduce the amount of chromate from what would be needed if it were used by itself to give corrosion protection. Nevertheless, chromate content of MAR. 30, 1964 C&EN
43
SELECTIVE REMOVAL. Alfred W. Oberhofer, Nalco research chemist, shows James C. Hesler, Nalco products application director, a water sample taken from test apparatus which demonstrates the selective removal of chromate from cooling tower blowdown water by a strong base anion resin
the blowdown can be sizable, James C. Hesler, products application director for Nalco's ion exchange division, points out. Even moderately sized cooling towers recirculating 10,000 gal. per minute with a blowdown rate of about 100 gal. per minute, and operating with a chromate residual of 30 to 35 p.p.m., will discharge more than 14,000 lb. of chromate ion annually. Process. Mr. Hesler gave details
of the Nalco development of its new process at the recent conference of the National Association of Corrosion Engineers in Chicago. The method was developed by Alfred W. Oberhofer, Nalco research chemist. All the work has been done on a laboratory scale. The process consists of passing these blowdown wastes through standard ion exchange equipment containing Dowex SBR, a conventional strong base anion resin. Chromate is selec-
Chromates: Why and How They Are Used Chromate inhibitors are used in open recirculating cooling systems to prevent corrosion of ferrous metals in the system. This type of cooling system is used in the petroleum, petrochemical, and chemical industries where it is necessary to economize on the volume of water taken into the system. The system usually includes either mechanical- or natural-draft towers to expose the circulating water to the atmosphere, reducing the water temperature. In this exposure, the circulating water picks up oxygen. When the water is returned to the system, the oxygen in the water sets up a corrosive condition in the presence of ferrous metals. Chromâtes, phosphates, silicates, and some alkalies are used to control this condition. The most favored inhibitors are sodium chromate and complex chromate salts. The amount of chromate used varies from 30 to 100 p.p.m. and will generally reduce corrosion by 90 to 9 8 % . Test coupons, usually low-carbon steels, are periodically tested as guides to chromate addition. Complex chromate inhibitors (for example, sodium polyphosphate-sodium chromate) exhibit a synergistic effect.
44
C&EN
MAR.
3 0,
1964
tively retained and chromate-free effluent is discharged. The chromate is then recovered by regenerating with a solution of sodium chloride and hydroxide and returned to the cooling system. No special preparation or treatment of the Dowex SBR resin is necessary, Nalco says. When converted to the chloride salt form it preferentially absorbs the chromate from the wastes although other anions, such as sulfate, are present in much higher concentrations. Because of the large volume of these blowdown wastes and the very low chromate concentration, a high flow rate through the resin is needed in order to avoid very large equipment. The pH of the wastes is critical in this area. At a pH of about 4.5 to 5.0, high flow rates are possible. Under these conditions less chromate leaks through the columns before they absorb their full capacity of chromate. With less acidic wastes of about 6.0 pH, more will leak through. Hence lower pH means faster flow rates. The pH also determines the capacity of the resin for chromate. For example, reducing pH from 6.0 to 4.8 increases the resin's chromate capacity from about 2.5 lb. per cubic foot to 5.8. Because cooling waters usually have a pH of 5.5 to 7.0, acidifying the blowdown water with sulfuric acid may be necessary in some cases, Mr. Hesler points out. The Nalco experiments show that the resin gives good removal of chromate and low leakage with flow rates of about 2 gal. per minute per cubic foot of resin. Regeneration. The chromate is regenerated from the resin in two stages. The first solution used contains 10% sodium chloride and 0.5% sodium hydroxide. When passed through the columns, it removes about 90% of the chromate on the resin. The resulting solution is returned directly to the cooling system. Stage two uses a straight sodium chloride solution, which removes almost all the remaining chromate from the resin. This solution is kept and used as the primary régénérant for the next cycle, after being made up with sodium hydroxides. If sodium chloride alone were used for the complete regeneration of the resin a very large excess would be necessary. Adding sodium hydroxide to the first solution converts acid chromate on the resin to the natural state.
Both white oils "on spec"--but only one processes right (Here's where Humble's experience can help you) In today's critical new processes, just meeting white oil specifications does not always insure processability. Here's where you need Humble's experience and tech nical resources. Chances are the Humble Representa tive can iron out your difficulty. And if not, he has the world's leading petroleum research organization to back him up. We've helped industry pioneer the use of white oils in everything from polyolefin reactions
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45
In this condition it is more readily removed from the resin. Economics. The economics of the Nalco process depend to a large extent on the size and nature of each plant in which it may be used. But they look attractive, the company says. Even if pollution control is not a factor, the economics are favorable. According to Mr. Hesler, the cost of the chemicals used to recover 1 lb. of chromate in the Nalco process is about 4 cents per pound. Replace ment cost of chromate is 20 cents per pound. Other costs must be added to Nalco's 4-cent chemical cost but the picture still looks good for all but the very smallest plants, Nalco says. If pollution control is important, the Nalco system looks even better, accord ing to Mr. Hesler. A current disposal method consists of reducing the chro mate ion to a trivalent chromium salt and then removing it as a sludge. The cost of this operation must be added to that of chromate replacement. Hence in these cases the Nalco proc ess could offer a double advantage, since it saves the cost of both destroy ing and replacing chromate. As an example of the cost of operat ing one of its plants, Crane gives an example of an industrial plant operat ing a cooling tower with a chromate level of 50 p.p.m. in the recirculating water. For this system, the blowdown rate is 200 gal. per minute. At $75 per year per gallon per minute blowdown, Crane says that an annual sav ings of $15,000 will be realized. Since Crane offers units of various sizes, a unit costing about $10,000 will meet this plant's needs, the firm says. Crane estimates that under these conditions the unit will pay for itself in 8 months.
BuMines Working on Process To Upgrade Iron Ores A system for using scrap iron as a re ducing agent to process off-grade iron ores, such as hematite and nonmag netic taconite, to produce a material suitable for blast furnaces is being de veloped by the Bureau of Mines' Min neapolis, Minn., Research Center. Successful commercial development of the technique could mean a new out let for the scrap industry, which has suffered market losses as steelmakers have increasingly substituted molten pig iron for scrap. It could also have economic significance in areas contain ing off-grade nonmagnetic iron ores. 46
C&EN
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The economics of the process are not clear yet. It would seem, how ever, that the price to beat is that of high-grade iron ore, which contains about 63% iron and sells for about $17 per ton. Research at BuMines is currently di rected toward using scrap turnings and borings. These are on the market for $10 to $20 per ton. Other types of scrap, such as sheet steel clippings, sell for $30 to $50 per ton, and are therefore not being con sidered at present. In the bureau's process, nonmag netic iron ore is mixed with scrap and heated in a carbon dioxide atmos phere (or water vapor, or both). During the reactions that follow, the iron in the ore loses some of its oxygen to the scrap metal. The oxygen loss produces magnetic iron oxide. Secretary of the Interior Stewart L. Udall says the bureau's studies are aimed at demonstrating the practica bility of its process, which, once pat ented, would be available for licens ing on a royalty-free basis.
New Ultrasonic Transducers Offer Increased Bandwidth Diffusion-layer and evaporated-layer ultrasonic transducers using piezo electric semiconductors have been de veloped at Bell Telephone Labora tories, Murray Hill, N.J. According to Bell Telephone, the devices are more efficient and have a greater band width than do other transducers cur rently-available in the 100- to 1000megacycle range. The properties of the transducers should make them useful in ultrasonic delay lines, which are used to store information similar to the memory core element in a computer. Transducers also create ultrasonic waves. By meas uring the way attenuation changes with frequency and temperature in a material, some insight about the struc ture of the material can be obtained. Therefore, Bell Telephone says, the transducers will be useful in research to determine the microstructure of solid-state materials. Both types of transducers, developed by Bell Telephone's Dr. Norman F. Foster, are made of cadmium sulfide. The diffusion-layer transducers have bandwidths up to 200 megacycles and fundamental frequencies to 1000 megacycles. The diffusion-layer transducers are
made by diffusing copper from an evaporated copper electrode into sam ples of conductive cadmium sulfide. By providing trapping sites for the conduction electrons, the copper im purity in a thin-surface layer produces a high resistivity region across which the electric field necessary for piezo electric transducer action may be de veloped. The evaporated-layer transducer is formed by depositing a thin layer (up to 7 microns thick) of cadmium sulfide on a suitable ultrasonic propagation material such as quartz. The cad mium sulfide film is evaporated in vacuum onto the heated surface of a quartz bar which has been previously plated with a conductive coating such as copper. The conductivity of the vacuum-deposited film of cadmium sulfide is reduced by overplating with copper and then subjecting the films to a heat treatment. Low-loss transducers which act at frequencies between 100 and 1000 megacycles have been made from evaporated cadmium sulfide films. These film transducers are effective for both longitudinal and transverse waves, Bell Telephone says. They may be deposited on any suitable de lay medium in contrast to the diffusionlayer transducers. Both types of transducers avoid the problem of grinding a conventional transducer to the thickness of a half wavelength of a specific frequency, as is the case with transducers using insulating materials. The grinding technique puts a practical upper fre quency limit of about 100 megacycles on fundamental mode ultrasonic trans ducers, Dr. Foster says. The funda mental frequency of both the diffusionlayer and the evaporated-layer trans ducers is determined by the thickness of the resistive layer and is independ ent of the physical size of the sample.
NEW EQUIPMENT Diaphragm compressors for use with toxic, flammable, and explosive gases at pressures from 150 to 30,000 p.s.i.g. are available from Pressure Products Industries, Inc., Hatboro, Pa. The 3000 Series compressors have displace ments varying from 9.2 cu. ft. per minute at 150 p.s.i.g. to 0.0217 cu. ft. per minute at 30,000 p.s.i.g. Sug gested uses include compression of oxygen, hydrogen, neon, carbon mon oxide, and other gases. Ε 30
Gas chromatograph
calibration
kits
are available from PolyScience Corp., Kenilworth, 111. The GC Qual and GC Quant kits consist of 10 pure standards of one compound type, and five mixtures containing homologs and isomers of this type. The kits are useful for calibrating other analytical instruments, the firm says. Ε 31
Multichannel analyzer operating at 10 nanoseconds per channel has been de veloped by Radiation Instrument De velopment Laboratory, Melrose Park, 111. The 256-channel, thin-film, memory analyzer, Model 34-26, operates 50 times faster than con ventional multichannel analyzers, the firm says. A high-speed logic which uses all-silicon semiconductors is in cluded. Ε 32
NEW CHEMICALS An additive to give superior low-tem perature properties to vinyl plastic and synthetic rubber compounds has been developed by the organic chemicals division of FMC Corp., New York, N.Y. The material, Kroniflex TOF, is used in blends with general-purpose plasticizers. The material is tri-(2ethylhexyl) phosphate. C1
Polyamide resin system that is soluble in alcohol-type solvents is available from Lawter Chemicals, Inc., Chicago, 111. Polymid 1155 is compatible with nitrocellulose and has high resistance to gellation at low temperatures, the firm says. It is recommended for use in flexographic inks, gravure inks, coatings, and adhesives. C2
Organic acid phosphate surfactants with more than 90% active phosphated ester are available from Wayland Chemical Co., Lincoln, R.I. Accord ing to the company, Alkapents M60 and M100 show strong synergism with nonionics and other anionics at high temperatures and in the presence of alkaline electrolytes. The firm sug gests that the materials can be used in metal- and hard-surface cleaners, dry cleaning detergents, emulsion polymerization, and pesticide emul sions. C 3
