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NIcCleary, who prepared the enamels and films described in Table I ; T. D. McKinley, who determined the refractive indices; and J. E. Booge. The writers are also indebted t o the R. T. Vanderbilt Company for the use of the constanttemperature constant-humidity room in their Norwalk Laboratory, and for their assistance in the preparation of many of the films. Literature Cited (1) Baltimore Paint & Varnish Production Club, Am. Paint J . , 24, Convention Daily 14-15, 16-17 (Oct 26, 1939). (2) Baltimore Paint 8: Varnish Production Club, Natl. Paint, Varnish Lacquer Assoc., Circ. 629,255-9 (1941).
Vol. 34, No. 12
(3) Bruce, H. D., Bur. Standards, Tech. Paper 306 (1926). (4)Bur. of Standards, Circ. 63 (1917). ( 5 ) Haslam, G.S., IND.ENG.CHEX., AXAL.ED.,2, 69-72 (1930). (6) Hunter, R. S.,A.S. T. M. Symposiumon Color, pp. 61-77 (1941). (7) Hunter, R. S., J . Research Natl. Bur. Standards, 25, 581 (1940). (8) Jacobsen, A. E., and Reynolds, C. E., Ixn.ENG.CHEX.,ANAL. ED.,6, 393-6 (1934). (9) Morrison, R. A., Oficial Digest Federation Paint & Varnish Production Clubs, No. 112, 745-9 (1932). (10) Pfund, .4.H., J . Franklin Inst., 196,77 (1923). (11) Pfund, A. H., J . Optical SOC.Am., 31, 679-82 (1941). (12) Sawyer, R. H., IND.ENG.CHEM.,ASAL. ED.,6 , 113 (1934). PRESENTED before the Division of Paint and Varnish Chemistry at the 103rd Meeting of the AMERICAN CHEMICAL SOCIETY, Memphis, Tenn.
INSECTICIDAL AEROSOL PRODUCTION SPRAYING SOLUTIONS I N LIQUEFIED GASES LYLE&). GOODHUE Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Reltsville, Md.
1
Baggage Compartment of an Airplane Being Treated w i t h the Aerosol t o Kill.Llangerous Mosquitoes (above), and a Cage of Insects Being Placed for Trial Exposure t o the Aerosol (below)
NSkCTICIDAL aerosols produced by spraying solutions of insecticides on a hot surface have been found t o be highly toxic (6, 14, 16) to many species of insects. This method was first used in a practical way to disperse nicotine by utilizing the heat of an internal-combustion engine, and a machine was developed (1, 2 1 ) for the control of the pea aphid, Macrosiphum p i s i (Kalt.). Sullivan et al. (14) found that even such unstable substances as pyrethrum and rotenone can be suspended in the air by this method with no apparent loss of toxicity. This method of applying insecticides makes possible the use of nonvolatile or slightly volatile substances as fumigants, and it appears promising for the control of such insects as mosquitoes, flies, and roaches. This volume condensation method of producingan aerosol,whichrequirestheuse of heat, is not always practical because of the lack of facilities or the danger from fire. To overcome
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INDUSTRIAL AND ENGINEERING CHEMISTRY
A new method of producing insecticidal aerosols has been developed. A solution in some low-boiling solvent, such as dichlorodifluoromethane or methyl chloride, is allowed to escape under its own pressure through a nozzle. An aerosol well adapted for the control of flies and mosquitoes in the presence of man is prepared by spraying a solution of purified pyrethrum extract and sesame oil in dichlorodifluoromethane. It is highly toxic to many species of insects and nontoxic to man, is nonflammable, and does not produce oily deposits. This solution is manufactured commFrcially and
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packaged in one-pound dispensers, but the entire output is being used by our armed forces. Other insecticides, such as nicotine, which themselves are toxic to man, can also be applied with methyl chloride, especially for greenhouse fumigation. These aerosols are much more finely divided than the mists produced by most spraying methods, and they settle more slowly. A method of studying and comparing the settling rates of mists and aerosols has been developed. A blue dye is included in the solution, and the settling rate can be quantitatively determined with a photometer.
this objection a' cold method has been developed and applied to the dispersion of insecticides. The process is very simple. It is necessary only to release a solution of the insecticide in a suitable liquefied gas through a spray nozzle, and the aerosol is formed by the rapid evaporation of the solvent in much the same way as when higher boiling solutions are sprayed on a hot surface. The heat of vaporization in this case is supplied from the atmosphere. Recent advances in the development of new refrigerants have made available several low-boiling solvents that are adaptable to the dispersion of insecticides. The most promising one appears to be dichlorodifluoromethane, but , methyl chloride can be used for many purposes. Propane is too flammable to be used alone, but mixtures with dichlorodifluoromethane are practical. Ethyl fluoride and other halogenated compounds are being studied, as well as liquid carbon dioxide. Sulfur dioxide has been used by Jander and Wygasch (7) to make aerosols of potassium iodide, but the application of this solvent to the production of insecticidal aerosols would be limited. Mixtures of liquid carbon dioxide and acetone or alcohol produce a wet spray. To avoid a coarse wet spray, a solvent whose components are gaseous considerably below the spraying temperature must be used.
Apparatus for Applying Aerosols Little equipment is needed for the production of the aerosols by this method. The pressure of the solvent is used for spraying, and the only equipment in addition to the container is a nozzle like those commonly used on oil burners. If the tank is to be used in an upright position, it must have a liquid delivery tube extending to the bottom, since the solution and not the gas must be sprayed. An oil-burner nozzle designed to deliver 2 gallons per hour at 100 pounds per square inch is suitable. A screen to prevent clogging is used. The orifice is the smallest constriction, so that no expansion takes place until the solution is sprayed into the atmosphere. A satisfactory apparatus is shown in Figure 1.
Pyrethrum-Sesame Oil Aerosol An aerosol of pyrethrum extract and sesame oil (4) produced by spraying a solution of these materials in dichlorodifluoromethane has shown considerable promise. This solution is prepared by dissolving 5 grams of a purified pyrethrum
FIGURE 1. CONTAINER WITH NOZZLE SPRAYINGSOLUTIONS OF INSECTICIDES IN LIQUEFIED GASES
FOR
1. Tank Liquid-delivery tube 3. Valve 2.
4.
Nipple
Nozzle Adapter 7. Screen 6. 5.
extract containing 20 per cent total pyrethrins and 2 grams of refined sesame oil per 93 grams of dichlorodifluoromethane. The solvent, which has a pressure near 90 pounds per square inch a t room temperature, is an odorless, nontoxic (IO, l a ) , nonflammable gas (8),and the resulting aerosol may be used with safety in the presence of man and animals. Entomologists have tested this aerosol and find it very toxic to mosquitoes (IJ), especially yellow fever mosquitoes, Aedes aegypti (L.), which are killed in 2 minutes when as little as 5 mg. of total pyrethrins and 10 mg. of sesame oil are suspended in 1000 cubic feet of air. Houseflies and other species of flies frequently found around livestock barns are easily controlled (IS).
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prepared by a cold method of extraction is now on the market, and aerosols of this material are satisfactory. Since the irritant does not appear to be the pyrethrins, purified extracts still retain their initial toxicity. The question of the most suitable concentration of pyrethrum and sesame oil in dichlorodifluoromethane was also studied. Tests were made with 2.5 to 6 mg. of total pyrethrins per gram of solution and twice as much sesame oil as pyrethrins. As much as 12 mg. of pyrethrins per gram of solution was used in a special test on cockroaches. Since the particle size of the aerosol becomes larger as the concentration of the insecticide increases, there is an upper limit of concentration for the production of a good aerosol. At low concentrations the solvent is too expensive in proportion to the insecticide. A satisfactory concentration is 10 mg. of pyrethrins and 20 mg. of sesame oil per gram of solution.
Methyl Chloride as a Solvent Methyl chloride is weakly combustible and slightly toxic to man and animals (IO), but it will dissolve many more insecticides than dichlorodifluoromethane. It is also less expensive and has a relatively low density (0.9 gram per cc. a t 25' (3.). It boils a t -23.7' C. and has a pressure of about 80 pounds per square inch a t room temperature. For the dispersion of nicotine, thiocyanates, rotenone, and other insecticides or fungicides that are not generally considered tolerable to man in aerosol form, methyl chloride has been
FIGURE 2.
SETTLINGCHAMBERFOR AEROSOLS
6. Thin cardboard strips
This aerosol is particularly well adapted to airplane fumigation for the control of malaria and yellow fever mosquitoes. It answers a need for a nonflammable, nonpoisonous insecticide that is easily applied without the use of supplementary power. Tests in cooperation with the United States Public Health Service have been made on several types of airplanes with good results. The ability of the aerosol to remain suspended and effective over longer periods than sprays, its greater penetration, and its freedom from oily deposits are important advantages. The pyrethrum-sesame oil aerosol has also been found very toxic (2) to the adult cheese skipper, Piophila casei (L.), the larva of which is a pest of stored meat and cheese. Being nonpoisonous, this aerosol is applicable to treatment of such stored-food products. Two families of flies, Sciaridae and Phoridae, that are pests of growing mushrooms can be killed with about 400 mg. of total pyrethrins and 800 mg. of sesame oil per 1000 cubic feet, as shown by preliminary tests conducted in a small 254-cubicfoot fumigation chamber. When applied in aerosol form, the ordinary commercial pyrethrum extracts cause some irritation to the nose and throat. A study has been made to determine whether the pyrethrins or some impurity is responsible. Martin and Hester (9) believe the substance causing irritation is an oil volatile with steam and is acidic enough to be extracted with dilute alkali. Samples purified in this laboratory by molecular distillation are less irritating than the original, and nearly all of the impurity is removed when the distilled sample in ether is extracted with dilute alkali. A relatively pure product
TIME IN MINUTES
FIGURE 3. COMPARATIVE SETTLING RATES OF Two AEROSOLS AND A MIST FORMED B Y SPR.4YING A KEROSENE SOLUTION
given attention. Aerosols of various insecticides produced with this solvent have been tested in the greenhouse against the cyclamen mite, Tarsonemus pallidus Banks. The amount of methyl chloride required does not appear to injure plants.
Settling Rates of Aerosols I n numerous experiments a high degree of persistence of these aerosols has been noted. I n some experiments a noticeable amount was present after 5 hours. To obtain more accurate information on the settling rate, a special procedure was developed: A Pyrex jar, 16 X 29 inches, was placed upside down on a small table (Figure 2). A 4-inch hole was cut in the table, and a friction-lid tin can with the bottom cut out was placed upside down in the hole. The friction cover gave a good means of closing the opening, and the can prevented possible large droplets from spattering onto the floor of the chamber. The top of the table was covered with blotting paper, and ten 2 X 3 inch glass slides were distributed evenly around the can. Each of these slides
,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Method of Producing Aerosols by the Use of Liquefied Gas
except one was covered with a strip of thin 4 X 7 inch cardboard. When the jar was in place, the end of each cardboard strip extended out from under the edge, so that it could be withdrawn to uncover the glass plate. A solution of pyrethrins (1 per cent) and sesame oil (2 per cent) in dichlorodifluoromethane that had been dyed blue with oil-soluble du Pont anthraquinone blue AB base was then sprayed in all directions up through the tin can for 30 seconds. The lid of the can was quickly put in place, a time clock was started, and the cover over glass slide No. 2 was removed. The rest of the covers were removed after 5, 10, 20, 40, 80, and 160 minutes, respectively. The ninth and tenth slides were left covered and were used as blanks to compensate for any amount of aerosol that might diffuse under the cardboard covers. The aerosol was allowed to settle completely, the deposit on each slide was removed with acetone and made up to a volume of 10 eo., and the percentage transmission of the colored solution was determined in a Brice photoelectric photometer (3)with a filter whose absorption peak is at 5800 A. The photometer was adjusted to read zero with the blank in place. The absorption by the deposit on slide 2 was taken as 100 per cent. There was usually about 10 per cent more on slide 1. This amount shows how much settling takes place before the complete cloud has been prepared. The logs of the reciprocals of transmission are directly proportional to the amount remaining in suspension after any time interval. The percentages can be obtained directly from a plot on semilogarithmic graph paper (6). The blue dye mentioned is well suited for the purpose because it has a high absorption in the yellow and follows Beer’s law closely. Duplicate runs checked to 5 per cent or less in any fraction.
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Since some convection currents cannot be avoided in the settling chamber, tests were made to determine the uniformity of settling. Slides placed in different quadrants did not vary by more than 3 per cent in the amount of deposit collected. An effort was made to equalize the temperature on all sides of the settling chamber without insulation. By the use of an atomizer mounted inside the chamber it was possible to study the settling rate of aerosols produced by the volume condensation method. A hot plate a t 300’ C. was held against the opening in the tin can only long enough to complete the spraying (15 seconds). A similar solution of pyrethrum and sesame oil in deodorized kerosene containing the same blue dye was used. The rest of the procedure was identical with the method previously described. The same solution was also sprayed into the chamber with a standard Peet-Grady nozzle a t 12.5 pounds pressure, and the rate of settling of the mist was determined. For settling mists, shorter time intervals were used. Fifty per cent of the material settled even before 3 cc. had been sprayed, and the remainder settled rapidly. The results are shown graphically in Figure 3. During the formation of the cloud, 2 per cent of the aerosol produced by heat, 15 per cent of the aerosol produced by spraying insecticides in volatile solvents, and 50 per cent of the mist settle before the determination can be started. The aerosol produced by the condensation method settled more slowly than that prepared by spraying solutions in dichlorodifluoromethane, but both aerosols remained suspended much longer than the mist. Different typm of nozzles, methods of spraying, and materials influence the settling rates of both the mist and the aerosol, and many other factors enter into the final result. The procedure described gives a comparatively simple method of studying settling rates in a practical way. No attempts have been made to calculate the particle-size distribution.
Acknowledgment Preliminary tests with pyrethrum-sesame oil aerosol were made against the yellow fever mosquito by B. V. Travis and against the mushroom flies by J. D. DeCoursey, and with insecticide-methyl chloride aerosols against cyclamen mite by Floyd F. Smith, all of the Bureau of Entomology and Plant Quarantine.
Literature Cited (1) ArnoId, R. B., U. S. Patent 2,120,225(1938). (2) Billings, 5. C., Goodhue, L. D., and Sullivan, W. N., J . Econ. EntomoZ., 35, 289 (1942). (3) Brice, B. A., Rev. Sei. Instruments, 8,279 (1937). (4) Eagleson, Craig, U.S. Patent 2,209,145(1941). (5) Goodhue, L. D.,and Haller, H. L., IND.ENQ.CHEW.,ANAL.ED., 12,652 (1940). (6) Goodhue, L. D., and Sullivan, W. N., Soap, 17 (8),98 (1941). (7) Jander, G.,and Wygasoh, E., Kol2oid.-Z., 85,1 (1938). (8) Jones, G. W., and Perrott, G. St. J., U. 5. Bur. Mines, Rept. Inwestigation3042 (1930). (9) Martin, J. T., and Heater, K. H. C., Brit. J . Dermatology, 53,127 (1941). (10) Nuokolls. A. H..et al.. Underwriters’ Lab., Miscellalteous Ha* urds 2375 (1933). Savage, J. C., U. S. Patent 1,842,844(1932). Sayers, R. R., Yant, W. P., Chornyak, J., and Shoat, H. W.. U. S. Bur. Mines, Rept. Investigation 3013 (1930). Sullivan, W. N., Goodhue, L. D., and Fales, J. H., J . Econ. Entomol., 35,48 (1942). Sullivan, W. N., Goodhue, L. D., and Fales, J. H., Soap, 16 t6), 121 (1940). Sullivan, W. N., MoGovran, E. R., and Goodhue, L. D., J. Ecm. Entomol., 34,79 (1941). PRESENTED before the Division of Agrioultural and Food Chemistry at the 104th Meeting of the AMERICAN C H ~ M I C SOCIETY, AL Buffalo,N. Y.
