INDUSTRIAL AND ENGINEERING CHEMISTRY
1452
Assume S,, = 1.4 inch. From Figure 11 for Sa, = 1.4, 7 14 skirt clearance = 1 inch, and z ~ ( p ) " . ~ ( + ) = 0.67: Ahl = 0.52 inch Ah = 0.52
(go)(;) O.8Oinch =
0.80
Sa,= 1 + = 1.4inches 2 which checks the assumption. Therefore the build-up across the plate is 0.80 inch of water.
Conclusions I n order that liquid may flow across a bubble cap plate of the usual design, it is necessary that a hydrostatic head be set up between upstream and downstream ends of the plate. This differential of level can be so great that some upstream rows of caps are rendered inactive; all vapor is thus required to pass through the fewer caps which remain. Such an effect can lead to high pressure drop of vapor, bad entrainment, danger of flooding, and low efficiency. Hydraulic gradient, or build-up, is shown t o increase rapidly with liquid rate, expressed as volume per unit of time per unit of plate width. It is higher a t low skirt clearance and low seal, and increases with increasing vapor rate. For the caps discussed in the present paper, build-up is proportional to the number of rows normal to liquid flow and, therefore, increases with tower diameter. There is a definite relation between vapor rate and amount of liquid a plate can handle with all caps bubbling. If build-up is no greater than its equivalent in pressure drop up t o zero seal, it can be expected that all caps will be active. As Ap, exceeds one inch of water, the ratio Ah/Ap, can be greater than unity with safety. This criterion makes possible the prediction of plate stability if the separate values of Ah and of Apo (average for the plate) can be estimated. It can be concluded that plate design should involve keeping build-up at a minimum, and that when high liquid rates are to be handled on a large-diameter plate, caps should be a t a, sufficient height above the plate. Also if vapor rate is so low that Ap, is low, it may be advantageous to use fewer caps on wider spacing. This has the twofold advantage of reducing resistance to liquid flow as well as increasing a p ~ . The determination of stability should not be regarded as a substitute for other methods of establishing an optimum design. However, it is an important supplementary determination without which the usual calculations of pressure drop, entrainment, and plate spacing may have little value, since these usually assume a uniform plate. It is obvious that the use of the capacity chart alone could lead to serious overloading of a tower from the point of view of pressure drop, entrainment, and efficiency, since the provision for stability alone often permits extremely high liquid rates a t sufficiently high vapor rates. Even a stable plate can have such a high build-up a t higher vapor rates that distribution of vapor is poor enough to cause bad entrainment and poor conditions for vapor-liquid contact. Acknowledgment The authors wish t o express their indebtedness to W. A. Peters, Jr., of E. B. Badger 8: Sons Company, for his suggestions during the progress of the work reported in this paper. Nomenclature Ah = build-up or hydraulic gradient, in. of water N = number of caps per sq. ft,. of area upon which u is based
( N = 7.14 for exptl. plate)
Vol. 34, No. 12
A p = total pressure drop through plate, in. of water
pressure drop up t o zero seal or total pressure drop corrected for head of liquid above top of slots, based on average vapor rate through caps, in. of water S, = minimum seal, difference between level of clear liquid at downpipe and level of top of slots, in. A h / 2 in which Ah is exSa, = average seal, defined as S , pressed in inches of the liquid flowing, in. u = linear velocity of vapor based on superficial cross section of tower, ft./sec. p = density of vapor, lb./cu. ft. Apo =
+
Literature Cited (1) Carey, J . S., Chem. & M e t . Eng., 46, 314 (1939). ( 2 ) Chillas, R. B., a n d II'eir, H. M., Trans. Am. Inst. Chem. Engrs., 22, 79 (1929). (3) Rogers, M. c.,and Thiele, E. IF'., IXD. ENG.CXEM., 2 6 , 5 2 4 (1934).
PRESENTED before the Division of Petroleum Chemistry at the 104th Meeting of the ERICAN AN CHEMICALSOCIETY. Buffalo, s.Y.
Effect of ALBERT LIGHTBODY AND D. H. DAWSON,
WHITE paint depends upon the white pigment in its composition not only for its whiteness but also for its opacity or obscuring power. The effect of the vehicle on hiding power has been generally disregarded, since the refractive indices of the suitable binder solids differ only slightly. The fact that dark colored vehicles give higher hiding has been noted (9),and the variation of hiding with brightness has been shown in several instances (2, 6, 8, 10). The fact that the hiding power of pigments is dependent upon their concentration in the binder has been pointed out by Sawyer ( I f l ) , Jacobsen and Reynolds ( 8 ) , and others ( 1 ) . It has been shown in this laboratory and has been recognized by others that enamels made with certain types of binders give greater hiding or greater coverage a t a given hiding level than with other vehicles. The factors that cause these differences between binders and their individual importance in influencing these hiding power variations are the subjects of this study.
A
Method The method employed for the determination of hiding power is an incomplete dry hiding method adapted from the photometric method of the National Bureau of Standards (3, 4). The paint films were laid down over black and white lacquered paper, carefully dried or baked, and photometered by the Hunter reflectometer (7); and a definite area of film was stripped from the paper and ashed. From the weight of ash, the composition of the paint, and the area stripped, the wet film thickness and spreading rate per gallon of paint can be calculated. The contrast ratio was determined from the Hunter readings over the black and white areas. Throughout this study the doctor blade method of laying down films was used. This method makes possible films which are quite uniform in thickness and also allows handling of paints too thick to be applied by brushing, spinning, or similar methods. These doctor blades had clearances varying by 0.002 inch from 0.002 to 0.012 inch.
December, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
Binders for architectural enamels and industrial baking enamels, when utilized in practical compositions, cause large differences in hiding power. These are greater than the difference caused by a change in refractive index of the white pigment from 2.53 to 2.71. A large part of these differences is due to the necessity of using the short-oil binders at low nonvolatile contents, due to the high viscosity of the binder solids, and the necessity of preparing paints in a low, practical consistency range. The short-oil high-binder viscosity liquids lay down thinner films with greater pigment crowd-
ing and, consequently, less efficient utilization of the pigment. Even at equal pigment volumes in the film, however, differences in hiding power of the order of 20 per cent exist between vehicles. These differences are not due to binder refractive index, binder color (although binder color can cause differences of the same magnitude), or to solvent. They are attributed, in the absence of a better explanation, to differences in degree of deflocculation of the pigment. Efforts to vary the deflocculation by variations in conventional grinding methods have, however, been unsuccessful.
on Hiding Power of Enamels Krebs Pigment and Color Corporation, Newport, Del.
1453
.
The lacquered paper consisted of two black bands and two white bands, running across the paper. The white portion of the paper had 82.5-82.8 per cent brightness, using the green filter on the Hunter reflectometer, while the black portion had an apparent reflectance of 1.0. The paint was drawn down across these bands so that for each drawdown the paint film covered two black strips and two white strips. Each paint was drawn down twice with a given blade. The airdried paint films were allowed to dry in a room kept a t 70" F. and 50 per cent relative humidity. The baked films were drawn down in the constant-temperature constant-humidity room, allowed to set for 10 minutes, and placed in the oven. At the end of half the baking period, the panels were removed and all positions changed so that the top panels went to the bottom, the ones in front t o the back, etc., to eliminate the small variations in baking due to the positions of the panels in the oven. It was found difficult to obtain reproducible values with the baking enamels. This was shown to be due to the effect of the high-solvency thinners on the lacquer coating of the paper, resulting in penetration of the paint into the paper fibers. Selection of paper with heavy lacquer coatings eliminated this variation. Results were checked with films on black and white glass. The Hunter reflectometer was used throughout, inasmuch as the green filter, with the source and photoelectric cell (6), had been so selected that the readings are equal to the Y function of International Commission on Illumination observer, using illuminant C. This duplicates the luminosity function. Thus the Hunter readings, using the green filter and the proper scale corrections, give directly the luminous apparentreflectance values. Care was taken to check workingstandards frequently so that variations from them were held a t a minimum. The method of determining the spreading rate of a paint from the weight of ashed film has been used by Sawyer ( l a ) . As each paint was laid down at several different thicknesses,
...
it is possible to plot a smooth spreading rate-contrast ratio curve and determine from it the number of square feet a unit volume of paint will cover a t any given contrast ratio. I n general, contrast ratio values greater than 0.98 show wide variations because small differences in contrast ratio in that range accompany large variations in amounts of paints. Also, with very thick films, surface irregularities, such as wrinkling, etc., are apt to occur and cause variations in the brightness readings. At contrast ratio values of less than 0.88, the film thickness of a high hiding enamel is thinner than in practical applications. Consequently, in making comparisons between these types of paints, contrast ratio values of not less than 0.88 and not greater than 0.96 should be used. Actually, some commercial enamels were found to give 0.96 contrast ratio at a spreading rate of approximately 500 square feet per gallon.
Effect of Pigmentation Four binders were selected to give variations in properties and use. Two air-dry brushing vehicles, an oleoresinous type and a long-oil alkyd, were used as suitable for architectural enamels while a short-oil alkyd and a urea-alkyd mixture represented industrial baking enamels. The composition of each vehicle was kept constant throughout each series, while the pigmentation varied from approximately 2 to 8 pounds of titanium dioxide per gallon. Both rutile and anatase titanium dioxides were used. To eliminate errors due to differences in formulation, a thick paint base was made with each pigment and composite vehicle by grinding over a tight-set five-roll laboratory paint mill. This base was reduced with calculated amounts of the composite liquids to give the desired pigment concentrations. The consistencies of the paints were thus allowed to vary from very thin a t low pigmentations to very thick a t high pigmentations. Table I presents some of the significant properties of these paints, and Figure 1 shows the relative hiding powers obtained using
INDUSTRIAL AND ENGINEERING CHEMISTRY
1454
TABLE I.
PROPERTIES O F PAINTS VEHICLE A, AIR-DRY OLEOREsINOUS TYPE (55.0% nonvolatile b y wqight, 48.4% by volume weight 3 7.45 lb./gal.; refractive index of dried film 1.504; solveht = mineral spirits)
-
Ti02 lb /gal. Conshdncy. Krebs units Max. apparent reflectance
Rutile Titanium Dioxide 2 3 4 5 68 61 66 70 88.5 90.5 90.6 90.4
6 81 89.7
7 98 90.0
117 89.5
TiOs,. lb./gal. Consistency, Krebs units Max. apparent reflectanoe
Anatase Titanium Dioxide 4 5 2 . 3 66 73 57 61 88.7 90.2 90.8 91.1
6 87 91.2
7 108 91.2
131 90.2
8
8
V ~ H I C L EB, AIR-DRYLONQ-OIL ALKYDTYPE (55% nonvolatile by we.ight, 47.7% by volume, weight P 7.56 lb./gal.; refraotive index of dried films 1.507; solvent mineral spirits)
-
-
Rutile Titanium Dioxide 2 3 4 5 61 65 70 73 91.4 92.4 92.6 92.0
Ti02 lb./gal. Conshenay, Krebs units Max. apparent reflectance
6 81 91.9
7 90 91.2
102 90.4
6
7 9s 90.7
8 112 90.0
8
Anatase Titanium Dioxide 2
TiO,,. lb./gal. Consistency, Krebs units Max. apparent reflectance
63 91.2
3
68 92.2
4
72 92.9
5
79 91.9
87 92.4
--
VEHICLEC, BAKED SHORT-OILALKYDTYPE (40% nonvolatile by weight,,33.3% by volume, weight 7.77 Ib./gal.: refractive index of dried film = 1.495; solvent toluene) TiO2,. lb./sal. Consistency Krebs units Max. appardnt reflectance
Rutile Titanium Dioxide 1.58 2.40 3.20 4.07 4.92 68 71 75 49 61 91.6 92.5 92.9 92.6 92.0
TiOa l b / g a l Con;ist;ncy,'Krebs units Max. apparent reflectance
Anatase Titanium Dioxide 1.58 2.40 3.20 4.07 4.92 62 68 72 76 60 91.0 92.1 92.7 92.7 92.3
5.75 89 91!2
6.65 108 90.4
5.75 6.65 95 121 91.5 91.0
VEHICLED , BAKEDUREA-ALKYD TYPE(60% ALKYDRESIN) (40% nonvolatile b y weight, 32.2% by volume, weight 8.65 lb./gal * refractive index of dried film 1.513; solvents 40% xylene-60% but'dnol)
-
-
-
Rutile Titanium Dioxide 2 3 4 5 57 59 61 63 91.8 92.3 92.2 91.6
Vol. 34, No. 12
volume of pigment X 100, divided by total volume of solids) is plotted against spreading rate in square feet per gallon divided by pounds of titanium dioxide per gallon. The curves show that comparisons a t equal amounts of pigment and of vehicle display small differences between the various liquids in hiding. The largest variations are of the order of 20 per cent. To check the conclusion that equal weights of pigment a t equal volumes of binder give nearly equal hiding, a series of paints were formulated a t equal pounds of titanium dioxide per gallon and a t equal pigment volumes. Rutile and anatase titanium dioxides were incorporated ' a t 3 pounds per gallon and 20 per cent pigment volume in six air-dry brushing type enamel liquids. The vehicles were all alkyd type vehicles; one was a phenol-modified alkyd of rather dark color. Table I1 shows these results. Here again equal hiding is not attained with different binders, although the variation of binder volume has been removed. The refractive indices were calculated from the Brewsterian angles of the dried unpigmented films measured according to Pfund (11). They are nearly the same for all vehicles tested and would not explain the differences in hiding power observed. The dark colored vehicle H gave high hiding as expected but was not appreciably different from vehicle G, a light colored long-oil alkyd.
Effect of Solvent Release and Pigment Dispersion
It is conceivable that differences in hiding power might be due to the differences in solvent release of the vehicles. In order to check Anatase Titanium Dioxide this, the same alkyd resin was dissolved in two 2 3 4 5 6 7 8 TiOz lb/gal different solvents-toluene with a high rate and Con&tdncy,'Krebs units 57 60 62 64 71 81 82 mineral spirits with a slow rate of evaporation. Max. apparent reflectance 91.7 92.1 92.4 91.7 90.0 88.8 88.1 The enamels formulated with equal volumes of pigment, binder, and thinner show no differences in hiding due to the different solvents. 0.96 contrast ratio. These curves confirm the findings of Rate of evaporation as deterkned by temperature differences Sawyer (12) that the hiding power of titanium dioxide also appear ineffective in changing the hiding, as enamels enamel may actually decrease with increasing pigmentation. made with vehicle C showed no differences in hiding between The increase in hiding observed a t even higher pigmentations air-dried and baked films. Therefore it seems improbable can be explained by the development of flat hiding or surface hiding, which is normal in highly pigmented flat paints and is found a t the excessive pigmentations of 8 pounds of titanium dioxide per gallon. The point of maximum hiding is apparently about 5 pounds of titanium dioxide in these vehicles. At 3 pounds of titanium dioxide per gallon of paint, all of these enamels represent usable products, but the spreading rates a t 0.96 contrast ratio vary from 310 to 465 square feet for anatase titanium dioxide, depending upon the vehicle used, and from 415 to 570 square feet for rutile. These differences between vehicles with equal weights of pigment exceed even the large differences between the rutile and the anatase types of titanium dioxide. The factors which could be assumed to have these large effects on the hiding power of equal amounts of titanium dioxide are: volume of solid binder, refractive indices of the g j VI I vehicles, color of the vehicles, differences in solvent release 2 . 0 3.0 4.0 5.0 6.0 7.0 of the films, and dispersion of the pigment in the vehicles. POUNDS OF TaOz PER G A L L O N To translate the results from Figure 1 t o give comparable results a t equal pigmentation and equal amounts of binders, FIGURE1. COMPARISON OF SPREADIXG RATESOF TITANIUM Figure 2 was drawn. Here per cent pigment volume (i. e., DIOXIDE IN VARIOUS VEHICLES TiOn lb/gal Cons'istdnoy,'Krebs units Max. apparent reflectance
6 68 90.7
7 78 89.2
8 82 88.5
December, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1455.
165
f4 0
11s 90
6s
4
10
IS
20
25
3 0 35 40 45 PERCENT PIGMENT VOLUME I N DRY FILM
Anatase Rutile FIGURE 2. SPREADING RATE OF ONE POUND OF TITANIUM DIOXIDE that difference in solvent release plays an important part in determining the hiding power of a white enamel. Another factor, dispersion of the pigment in the solid vehicle, can be defined as lack of flocculated particles, with uniform distribution of dispersed particles throughout the film. An attempt to determine the effect of pigment deflocculation upon the hiding power was made, assuming that
since more vehicle is incorporated with less pigment. Thus the rutile has two advantages over anatase in the formulation of high-hiding enamels. First, its high refractive index gives high hiding power to the enamel, and secondly, the amount of pigment may be reduced for a given volume of binder so t h a t the pigment performs more efficiently. Consequently, while it is usually stated that rutile will yield 20-35 per cent more
TABLE11. VARIATIONS BETWEEN ENAMELS Vehicle Description
(All enamels pigmented at 20 per cent pigment volume and 3 pounds Ti03 per gallon) B C E F G Long-oil Short-oil MediumMediumVery-longalkyd alkyd oil alkyd oil alkyd oil alkyd
Solvent Phthalic anhydride, %
Mineral spirits 23.9
Stormer consistency. Krebs units Max. apparent reflectance Spreading rate sq. ft./gal. 0.96 contraa't ratio 0.90 contrast ratio
478 880
Stormer consistency, Krebs units Max. apparent reflectance Spreading rate, sq. ft./gal. 0.96 contrast ratio ' 0.90 contrast ratio
61 89.4 390 693
57 90.3
Toluene
Mineral spirits 39.5 34.9 Rutile Titanium Dioxide 96 80 90.0 90.8
456
479
800
868
Anatase Titanium Dioxide 105 95 89.6 90.4
357 659
degrees of grinding would vary the degree of deflocculation. The lowest hiding vehicle, C, was used. Four paints were formulated alike and ground under exactly similar conditions in laboratory pebble mills for 4, 16, 48, and 96 hours, respectively. The hiding powers on these four paints ground for different lengths of time were equal within the limits of experimental error. Further efforts to determine the reasons for these unexplained differences, attributed here to dispersion differences, have not yet been successful. Table I11 shows that the relation in hiding power between t h e rutile and the anatase titanium dioxide is maintained even when the level of hiding with both pigments varies widely. When rutile is substituted at equal pigment volume for anatase, a greater weight is required due to the greater density of rutile. Consequently, the common method of substitution of rutile for anatase is at equal or less than equal weight. This permits the rutile to perform even more efficiently than it does a t equal pigment volume substitution
384 695
Mine+ spirits 30.4
Mineral spirits 10.0
65 90.4
57 90.4
H Medium oilphenolic kmyOded al-
-
Mineral spirits
....
104 87.9
475 870
487 920
54s 950
80 89.8 400 720
59 90.0 425 790
124 87.4 446 776
hiding than anatase in a comparable enamel when anatase is replaced with solid binder and rutile, the amount of rutile required to give equal hiding to the original enamel is often as little as half the weight of the anatase. TABLE 111. RATIOOF SPREADING RATESOF RUTILETO ANATASE TITANIUM DIOXIDE Contrast Ratio 0.96 0.90
(Equal weight of Tioi at equal pigment volume) Vehicle Vehicle Vehicle Vehicle Vehicle B C E F G 123 128 125 117 115 127 122 121 SI7 121
Vehiole
H
121 122
Acknowledgment The advice and assistance of the following persons are acknowledged: W. E. Brophy, of the R. T. Vanderbilt Company, who collaborated in outlining this research; R. L.
1456
INDUSTRIAL AND ENGINEERING CHEMISTRY
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 P a i n t & Varnish Production Club, Am. Paint J . , 24, Convention Daily 14-15, 16-17 (Oct 26, 1939). (2) Baltimore P a i n t 8: Varnish Production Club, Natl. P a i n t , 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 ) H a s l a m , G.S., IND.ENG.CHEX., AXAL.ED.,2, 69-72 (1930). (6) H u n t e r , R. S.,A.S. T. M. Symposiumon Color, pp. 61-77 (1941). (7) H u n t e r , R . S., J . Research Natl. Bur. Standards, 25, 581 (1940). (8) Jacobsen, A. E., a n d Reynolds, C. E., Ixn.ENG. C H E X . ,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) P f u n d , 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 B u r e a u of Entomology a n d P l a n t Q u a r a n t i n e , U. S. D e p a r t m e n t of A g r i c u l t u r e , Reltsville, Md.
1
Baggage Compartment of an Airplane Being Treated with 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
