Paints to Reflect Ultraviolet Light DONALD F. WILCOCIC1 AND WALTER SOLLER University of Cincinnati, Cincinnati, Ohio
I
NTERIOR paints with high ultraviolet reflectances are required if efficient use is to be made of the convenient s o u r c e s of u l t r a v i o l e t light which have been developed to bring the benefits of summer s u n l i g h t i n d o o r s Luckiesh (3, 4) showed that commercial paints have low ultraviolet reflectances and that new paints must be made. Irradiation b y u l t r a v i o l e t rays sborter than approximately 3200 A. in wave length will prevent and cure rickets by production of vitamin Da in the skin, and other health benefits have also been attributed to s u c h r a d i a t i o n . The lower limit of symmer sunlight is about 2900 A., and irradiation by wave l e n g t h s s h o r t e r t h a n 2800 A. is harmful to the skin. C o n s e q u e n t l y , t o guide the search for new ultraviolet reflecting paints, measurements of reflectances were made in the region which Luckiesh called that of “maximum erythemal effectiveness”, from 2800 to 3200 8.
.
Measurement The apparatus designed for making these measurements differs principally from apparatus previously used for measurement in this region in the provision of a more sensitive and more stable means of measuring the phototube currents. It consists of a General Electric S-1 Sunlamp as the ultraviolet source, with current and temperature regulation, a 10-inch (25.4-cm.) sphere reflectometer, two phototubes in a balanced circuit, a n amplifier, and a sensitive galvanometer. Figure 1 is a diagram of the aEparatus. The S-1 Sunlamp emits little radiation below 2800 A. (11). The General Electric FJ-135 cadmium alloy phototubes doo not respond t o radiation of wave lengths longer than 3200 A. (IS). The band of measurement is thus limited to the desired range. The energy distribution within this band as measured by each of the two phototubes is shown in Figure 2. The sphere reflectometer was designed by Taylor (8, 12, 14) t o give an absolute measurement of reflection factor-that is, a measurement independent of the reflection factor of the sphere coating or any other “standard” substance. The inside surface of the sphere is coated with magnesium oxide. The balanced phototube circuit (Figure 3) was designed t o eliminate the effect on the precision of t h e measurements of small uncontrollable variations in the output of the Sunlamp. 1
In this circuit one phototube is placed beneath a window in the sphere to measure the light intensity within the sphere, and another is placed in the light beam so that it measures directly the output of the Sunlamp. The amplifier and galvanometer are used as a null-point instrument. The resistance, Rz, is adjusted until the potential difference between points A and C in the circuit is zero. This value of Rs is a measure of the rat,io of the light intensity within the sphere t o that in the direct light beam from the Sunlamp, and is not affected by variations in the Sunlamp output. The voltage difference across AC that must be detectable in order to obtain a precision of 0.1 per cent is volt. This sensitivity is o b t a i n e d w i t h t h e vacuum-tube amplifier (Figure 4) which is an adaptation of the Soller circuit (6,9)to the Western Electric D96475 electrometer tube, in conjunction with a Leeds & Northrup type R galvanometer having a sensitivity of 10-10 ampere per mm. Absolute measurement of reflection factors is described by Taylor (8, 12, 14). His method is to determine the ratio of the current flowing in the phototube beneath the sphere when the lieht beam is incident on the sample to that when it is incidenx on the sphere wall. The sphere is designed so that this ratio is the absolute reflection factor of the sample. In using the present balanced circuit, the ratio of the two values of RI(Figure 3) with the light beam in the positions mentioned above is determined. Since values of R1 represent ratios of sphere t o source intensities, the ratio of the two values of RPis the ratio of the sphere intensities, or the absolute reflection factor.
The development of paints reflecting up to 70 per cent in the ultraviolet is described. The stages of development are (a)testing of reflection or transmission of possible paint materials, ( b ) testing of the simplest possible combinations of pigment and vehicle to determine those suitable, ( c ) deterrnination of those resin-plasticizer combinations which form good films and have low ultraviolet absorption, and ( d ) accurate milling of the most likely formulations and testing of these for reflection and stability on exposure to ultraviolet light. For this purpose an apparatus was developed which measures accurately the integrated reflection factors and the transmission coefficients of substances in the band from 2800 to 3200 A. This apparatus balances out fluctuations in source intensity, and its readings are independent of the amplification of its amplifier. A n exact method of correcting for the multiple reflections between the surfaces of the windows in an absorption cell is described.
Present address, T h e Sherwin-Williams Company, Chicago, Ill.
Transmission measurements on liquid materials were made with a cell constructed from two polished fused quartz plates, a thin copper spacer t o hold the liquid, and a steel frame to clamp the assembly together. The cell was designed so that its reflection coefficient could also be obtained. It is customary t o use these transmission cells in such a way that the transmission of the cell containing a solution is compared to the transmission of the cell containing pure solvent; the effect of the cell is thus eliminated. In testing oils and solvents, this cannot be done. Furthermore, with certain liquids in the cell the per cent transmission is greater than that for the empty cell. Since these liquids are almost completely transparent to the ultraviolet and have a refractive index greater than that of air, they reduce the amount of light reflected from the Vitreosil-liquid interfaces and so increase the over-all transmission of the cell. This extreme case shows the necessity of correcting for the effect on transmission measurements of the multiple reflections from the four surfaces of the cell. 1446
NOVEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
L
-$.TAW
1447
GALV.
C W N T TUBES
OTOTUBE N O 2
TROMETER TUBE
FIGURE 1. DIAGRAM O F APPARATUS
I n order to analyze the effect of these multiple reflections, let: @ = incident light intensity R = total reflection from cell (measured) T = total transmission of cell (measured) t = fraction of light transmitted by material in cell x = fraction of light reflected from each fused quartz plate Then, considering each plate as a single reflecting surface (Figure 5 ) , the total reflection from the cell can be written as the infinite series: x
=
+ (1 - z)'~'z + (1 -
2)'t423
+ ...
Eliminating z between Equations 2 and 3 givesythe relation for t: t = -
(1
- R)' - T* +
-
2T
;;-
T']'
+1
(4)
Calculations on many samples, using Equation 4, have shown that 1 is usually about 10 per cent larger than T. Lambert's law-(t = e --kd) is used to obtain the absorption coefficient, k. I.
A
(1)
Similarly, the total transmission of the cell is: (3)
_,.
I
I
r;
I
FIGURE 3. PHOTOTUBE CIRCUIT
For a series of reflection tests on successive days employing the same sample, the reproducibility attainable with the a p paratus was found to be within 0.2 per cent, based on the incident light. As an independent check on the measurements made with this apparatus, A. H. Taylor of the Nela Park Laboratories of the General Electric Company was kind enough too measure the reflectances of several painted panels a t 3024 A. As Figure 2 shows, this is the most intense mercuIy line in the band measured. Taylor's measurements were made with a 4-inch (10.2-cm.) integrating sphere, a quartz spectrograph, and a quartz sodium photoelectric cell (14). The reflectances obtained by Taylor are compared to those recorded with the present apparatus in Table I; the differences are probably due to the different spectral absorption curves of the several paints.
125-
wloo
-
v)
z
8 W
75
-
W
m
c
E2
50-
25
I
TABLEI. COMPARISON OF REFLECTANCES
I
2750
XXO
ANGSTROMS
3250
FIGURE 2. RELATIVE RESPONSET O S-1 RADIATION OF PHOTOTUBES 1 AND 2 AS
MEASUREDBY MONOCHROMATOR, AMPLIFIER,AND GALVANOMETER
Panel G-5s
H-6
H-8 H-10
Reflectance for Band 2800-3200 A,, % 54.7 41.1 41.6 41.2
Reflection a t 3024 A,. % 53 42 43 45
INDUSTRIAL AND ENGINEERING CHEMISTRY
1448
C'
rJ
I
II
n
17a
FIGURE 4. AMPLIFIERCIRCUIT
Two principal advantages are inherent in the circuit and the arrangement of the apparatus. The effects of variation in the intensity of the light source are eliminated; and the two phototube currents are balanced before being amplified, so that changes in the amplification factor of the amplifier cannot affect the measurements. Furthermore, this circuit and arrangement may be adapted to afford a simple low-cost method of making precise reflection measurements, either band or (with the addition of a monochromator) monochromatic, in the visible and near infrared regions of the spectrum as well.
Development of Ultraviolet-Reflecting P a i n t s
VOL. 32. NO. 11
because of its combination of low absorption and high refractive index. The measurements on colorei pigments agreed well with those of Stutz (IO) for 3024 A. wherever comparison was possible. No colored pigment was found which possessed a high ultraviolet reflectance. The absorption coefficients obtained for some common oils, resins, plasticizers, thinners, driers, and other liquids are listed in Table 111. Lambert's law (I = l o e c k d ) was used. The resins and solid plasticizers were tested in solution. Correction for the presence of solvent was made by assuming that the absorption coefficients are additive in proportion to the relative volume of each component, and that the volumes of resin and solvent are additive in solution. A material with a n absorption coefficient less than 60 to 70 inch-' (23.6 to 27.6 cm.-l) is fairly satisfactory as a binder for ultraviolet paints. It is evident from Table 111, A and D, that drying oils and driers cannot be used. The natural resins also have too much ultraviolet absorption; but among the synthetic resins, urea-formaldehyde, nitrocellulose, ethylcellulose, the methacrylate esters, and Vinylite are low in absorption. Of these, urea-formaldehyde is unsuitable since i t must be baked, and nitrocellulose deteriorates upon exposure to ultraviolet radiation. Adequate plasticizers with low absorption are available for use with the transparent resins.
TABLE11. ULTRAVIOLET REFLECTAXCES OF SOMEDRY PIGVEXTS Pigment Titanox B Lead titanate Titanox C Zirconium oxide, commercial Basic sulfate white lead Aluminum oxide Basic carbonate white lead (Dutch process) Aluminum hydroxide Zirconium oxide, c. P. Magnesium carbonate, commercial
Ultraviolet Reflection Factor, % 6
Three fundamental requirements must be met by the ma6 7 terials for a high-reflectance paint, whether i t is to reflect 41 visible, infrared, or ultraviolet light. First, the particles of 48 55 the pigment must be translucent or low in absorption, so that 62 67 by multiple reflections and refractions a large percentage of 78 the light striking it may be returned. The reflection factor of 81 the dry pigment is a measure of this property, although reflection also depends on the refractive index of the pigment. Metallic pigments are an exception to the first requirement, A typical complex paint formula is usually the result of the the requirement then being that reflection from each particle modification of a n originally simple formulation in order to be high. The second requirement is that the binder or vehicle give the paint various desirable properties such as brushmust be transparent to the radiation to be reflected so that ability, lack of settling, high hiding power, etc. I n develop light may pass between the pigment particles in the process of ing paints to reflect ultraviolet light, very simple formulations reflection. The third requirement, metallic pigments again were adhered to, so that the behavior of the individual comexcepted, is that the difference in refractive indices between ponents might be followed. Attention was paid chiefly to the pigment and the medium immediately surrounding it light reflection and film properties as the controlling factors. must be large so that the reflection and refraction of the light KOeffort was made to control other properties. at the pigment-medium interfaces will be appreciable. Pigments were tested for reflection by packing t i e m in a standardized manner in a shallow steel cell. The reflectances of the white pigments agreed qualitatively with the data which Pfund (7), Stutz (IO), Luckiesb (3, 4), and Goodeve (2) obtained for 3024 A. The ultraviolet reflection factors of a few of them are listed in Table 11. As Pfund pointed out, there is a n inverse relation between the reflection factor and the opacity (i. e., light absorption) of a pigment; and in comparison with ordinary pigments, "the inerts, as a class, are much less opaque, particularly in the short wave length region". Thus, with the exception of white lead, the common highhiding pigments (zinc oxide, zinc sulfide, litho\\\\\\ pone, and titanium dioxide) are opaque in the ultraviolet. If zirconium oxide could be obIN TRAXSMISSION OF LIQHT THROUGH FIGURE 5. MULTIPLEREFLECTIONS tained in a purer state commercially, i t would T W O PARALLEL SURFACES, EACH WITH REFLECT.4NCE 5 A N D TRANSMISSIOX (1 - 5) be an excellent pigment for ultraviolet paints
\\\\,\\\\\!,\\\\\\\\,{,\,\\\\\'
INDUSTRIAL AND ENGINEERING CHEMISTRY
h-OVERIBER, 1940
Inert pigments behave in much the same way in the ultraviolet as they do in the visible in that their reflection is much lower when mixed with a vehicle than when tested dry. The addition of small amounts of aluminum bronze to white pigments produced grays with relatively high ultraviolet reflection and little trace of specular reflection from the aluminum. Addition of colored pigments caused a rapid diminu-
T.4BLE
111.
ULTRAVIOLET
70 Soln.
Material
Tested
A.
Oils
Oiticica Tung Heat-bodied linseed Refined perilla Alkali-refined linseed Heat-bodied Derilla
.... ....
.... .... ....
.... ....
.... .... .... .... B. Resins Modified phenolic (B. R . 2963 Bakelite) Modified phenolic (Amberol F 7 light) Maleic-rosin ester (801 Amberol) Limed rosin V-13005 light stable phenolic Kopol 500 (fused Congo-glycerol ester) Ester gum A. V. 5-7 Ter ene resin (550 Durez) Lig& stable phenolic (XR4036 Bakelite) 1007 phenolic (XR820 Bakelite) 100% phenolic (2000 Super Beckacite) Cumar W 1 / ~ Batavia dammar 100% phenolic, heat-reactive (Varcurn 250-F) 4 X Manila Hvdrocarbon resin (Nevillite) Gfvcervl Dhthalate varnish B
C. Plasticizers 4,4’-Dichlorodiphenyl Methyl abietate Hydrogenated methyl abietate Di-(o-xeny1)monophenyl phosphate (Dow plasticizer 6) Diphenyl mono-(o-xenyl) phosphate (Dow plasticizer 5 ) Benzyl benzoate Methyl ricinoleate Santicizer 8 Dibutyl phthalate Santicizer B-16 Butyl stearate Tricresyl phosphate Triphenyl phosphate Di-(p-tert -butylphenyl) monophenyl phosphate (Dow plasticizer 2) n-Butyl-d-tartrate Tri-(p-tert-butylphenyl) phosphate (Dow plasticizer 7) Tripropionin
> 2 . 5 x 10; 1 . 6 X 10 1 . 6 X 102 1 . 6 X 102 1 . 5 x 102 1 . 5 x 102 1 . 2 x 102
x
102
95 74 62
65 10 50 50 18 Pure 20 25 20 50 20 20 20 18
1 . 4 X 103 1.4 x 103 1 . 3 x 103 1 . 3 x 103 1 . 3 x 103 1 . 2 x 103 1.1 x loa 9 . 6 X 102 8.8 X 102 5 . 1 x 102 6 . 8 X 102 6.3 X 102 5 . 8 X 102 5.3 x 102 4 . 5 x 102 2.3 109 2 . 2 x 102 1 . 7 X 102 1 . 4 X 102 75 64 52 48.8 46 40.5 40b 30 25 24.8 21.8 20.0 20
4.5 Pure Pure
1 . 9 x 103 2 . 3 x 102 1 . 6 X 102
Pure
1.2
Pure Pure Pure Pure Pure Pure Pure Pure 50
1 . 2 x 102 1 . 2 x 102 1.0 x 102 60 49 37 37 30 20
20 20 20 20 67 20 20 20 20 20 20 20 20
20 20
20 65 65
Methyl methacrylate Urea-formaldehyde E t h y l methacrylate Nitrocellulose lacquer Acryloid B-7 (methacrylates) Urea-formaldehyde soln. B Prouvl methacrvlaie BuCyi methacrylate Isobutyl methacrylate Vinylite AY-AA
Absorption Coefficienta, Inches-’
1.2
Pure Pure 41 Pure
x
x
....
6070 ethylcellulose, 40% Dow Plasticizer 7 80% ethylcellulose, 20% Vinylite 88% isobutyl methacrylate, 12% butyl stearate 6570 isobutyl methacrylate, 35y0 Dow plasticizer 2
Paints containing each of these four combinations as vehicle were then milled. The solvent used was 80-20 toluene-alcoho1 for the ethylcellulose vehicles and toluene for the isobutyl methacrylate vehicles. All the materials for each paint were milled together in an 8-inch (20.3-em.) porcelain pebble mill for 70 hours a t 90 r. p. m. The compositions and reflectances of typical paints which were studied are shown in Table IV. TABLE
G-5
Pigments, 7o 60 B. C. white lead 40 Mg carbonate Same a s G-4
G-6
Same as G-4
No.
G-4
G-7
Same a s G-4
G-5
Same as G-4
G-9
M g oxide (Baker)
G-loa G-11
M g carbonate B . C. white lead
+
Iv.
h f I L L E D PAINTS
Resin and Plasticizer, 70 60 ethylcellulose 4- 40 Dow 7 80 ethylcellulose 4- 20 Vinylite 65 isobutyl methacrylate 35 Dow 2 88 isobutvl methacrvlate 12 butyl stearate 50 ethylcellulose Jr 40 Dow 7 10 Vinylite 60 ethylcellulose 4- 40 Dow 7 Same a s G-9 Same as G-9
Vol. yo ReflectPigance, ment yo 30.0
+
4
+
62.1
50.8
51.6
50.0
54.3
50.0
60.8
45.0
56.5
40.0 40.0 40.0
47.3 31.6 55.5
It was found that in these vehicles a combination of white lead and an inert, such as magnesium carbonate, produces a paint with better adhesion than white lead paints and with less tendency to crack than paints containing only a n inert. Figure 6 shows the rapid increase in ultraviolet reflection which occurs when the pigment volume per cent is increased.
102
18
15 8.1 2.8
D. Thi nners, Driers, and Other Liquids 6% cobalt Nuodex .... z.2.5 x 102 6 % manganese Nuodex .... : > 2 . 5 X 101 24% lead Nuodex . . . . 1 . 7 X 102 Acetone . . . . 1 . 2 x 104 Sulfate wood turpentine .... 76 Casein s o h . 57 Mineral spirits 44 Solvesso 2 .... 28 Commercial xylene .... 24 Toluene . . . . 9.5 Butanol .... 9.2 Ethylene dichloride .... 3.9 Glycerol .... 3.5 Absolute alcohol . . . . 1.1 Distilled water .. 0.4 4 Absorption coefficient is a quantity independent of the sample thickness, d , and is related t o the transmission b y t h e relation k = - ( l / d ) In t. b I n solution.
....
tion of the ultraviolet reflection factor. The logarithm of the reflection factor decreased approximately linearly with increase in the volume per cent of colored pigment added. I n order to determine which plasticizer-resin combinations possessed good film properties within the limits of compatibility and low ultraviolet absorption, a series of unpigmented films was prepared. The best compositions were found to be: 1. 2. 3. 4.
ABSORPTIOXCOEFFICIENTS
1449
60
P
Y
B
55
L 0
2 50 3 IW
:
45
n
aa .
45 PIGMENT VOLUME PER CENT IN DRIED FILM
50
FIGURE 6. ULTRAVIOLET REFLECTION vs. PIGMENT VOLUMEPER CENTFOR MILLEDPAINTS (TABLE
Iv)
INDUSTRIAL AND ENGINEERING CHEMISTRY
1450
/G-IO
/G-4
G-6
I
I 5
10
I
I5 DAYS EXPOSED IO
I 20
VOL. 32, NO. 11
Sunlamp. A comparison of the Cooper-Hewitt arc and the S-1 Sunlamp on the basis of data by McAllister (5) and Barnes (1) shows that those ultraviolet lines from the CooperHewitt arc which pass through the Corex D glass are ten to twenty times as intense as those from the S-1, so that 25-day exposure in these tests is approximately equivalent to continuous service for one year, one foot from an S-1Sunlamp, or to several years of ordinary service. The effect of ultraviolet radiation on the reflection factors of the exposed panels is shown in Figure 7, and the effect of aging without irradiation on duplicate panels kept in the dark is shown in Figure 8. The small initial differences in reflection between each pair of duplicate panels are attributed to slight differences in film thickness. The ethylcellulose and the isobutyl methacrylate paints which contained a Dow plasticizer decreased on exposure 25 and 50 per cent a t first and then gradually increased to a reflectance almost as high as the un-
I 25
FIGURE 7. VARIATIONIN ULTRAVIOLET REOF MILLEDPAINTS ON CONTINUED EXPOSURE TO S 1 RADIATION
FLECTION
The limit to the per cent pigment which can be used is set by the desired film properties, since too high a pigment content results in a brittle inelastic film. For an inside wall finish between 40 and 50 per cent pigment by volume gives a film with satisfactory properties, using either ethylcellulose or isobutyl methacrylate vehicles. The ethylcellulose paints not only had higher reflectances (Figure 6) but also made less brittle, more cohesive, and more “flat” films than paints containing methacrylate resin.
2 5 30 I.0 0,
0
I 20
I
-
1
40 €0 DAYS IN DARK
I 80
FIGURE8. VARIATIONIN ULTRAVIOLET REFLECTION OF MILLED PAINTS ON AQINGIN THE DARK
An ultraviolet reflecting paint must be designed to retain its high reflectivity as long as possible, since i t will be subjected in service to prolonged exposure to ultraviolet radiation. Hence, the four types of paint, G-4, G-5, G-6, G-7, representing each of the four satisfactory resin-plasticizer combinations, and G-9, G-10, and G-11, representing a single vehicle combination milled with three different pigments, were exposed to the radiation from a Cooper-Hewitt mercury arc (rated 0.4 ampere at 200-250 volts). The panels were placed 12 inches (30.5 cm.) from the arc and were protected by sheets of ‘/&nch (1.6-mm.) Corex D glass from short wave length radiation not present in the output of the S-1
HOURS EXPOSURE TO S-I
SUNLAMP
FIGURE 9. EFFECT OF EXPOSURE TO S-1 RADIATION ON ABSORPTION COEFFICIENT OF Dow PLASTICIZER 2 exposed duplicate panels, whereas the paints containing either resin but no Dow plasticizer increased almost 20 per cent in reflectance on exposure. The deterioration in reflectance due to the presence of the Dow plasticizers is accompanied by a permanent yellow discoloration. The behavior of a Dow plasticizer alone, exposed in the transmission cell to continuous irradiation by the S-1 Sunlamp, is shown in Figure 9. It is similar to that of the paints in the exposure test. Paints G-9, G-10, and G-11 show the influence of each of three different pigments on this deterioration with Dow plasticizer 7. The reaction is greatest with magnesium oxide. These exposure tests clearly show that although the substituted triphenyl phosphates (the Dow plasticizers) impart excellent film properties to the paints, they are not suited to use in paints exposed to ultraviolet light. Vehicle formulations such as those used in paints G-5 and G-7, however, which after an exposure equivalent to approximately one year of continuous exposure one foot from an S-1 Sunlamp have actually increased in ultraviolet reflectance while panels kept in the dark have diminished, are suitable basic formulations for an ultraviolet-reflecting paint.
Acknowledgment The writers wish to thank The Lowe Brothers Company for its support of the fellowship which made this work possible and for permission to publish the results. Grateful thanks are due F. F. Heyroth and P. J. Flory of the Basic Science Research Laboratory for their counsel and
NOVEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
advice. The writers are indebted to E. E. Ware of the Sherwin-Williams Company, and to D. A. Kohr, E. W. Fasig, J. M. Purdy, and R. W. Kewish of The Lowe Brothers Company for their interested assistance and advice. The cooperation of A. H. Taylor of the Lighting Research Laboratory of the General Electric Company is appreciated.
Literature Cited (1) B a r n e s , B. T., Phys. Rev.,36, 1468 (1930). (2) G o o d e v e , C. F., Trans. Faraday SOC.,33, 340 (1937). (3) L u c k i e s h , M., Rev. Sci. Instruments, 19, 1 (1929). (4) L u c k i e s h , M., and Holladay, L. L., J . Franklin Inst., 212, 787 (1931). ( 5 ) McAllister, E. D., S n i i t h s o n i a n Inst. Pub., Misc. Coltections, 87, No. 17 (1933). (6) P e n i c k , D. B., Rev. Sci. Instruments, [N. S . ] 6, 115 (1935).
1451
Pfund, A. H., Proc. Am. SOC. Testing Naterials, 23, 11, 369 (1923).
Rosa, E. B., and Taylor, A. H., Bur. S t a n d a r d s , Sci. Papm 18, No. 447, 281 (1922). Soller, W a l t e r , Rev. Sci. Instruments, [N.S . ] 3, 416 (1932); U. S. P a t e n t 2,104,211 (Jan. 4, 1938). S t u t z , G . F. A., J . Franklin Inst., 200,S7 (1925). T a y l o r , A . H., J. Optical SOC.Am., 21,20 (1931). Ibid., 21,776 (1931). Ibid., 23,60 (1933). T a y l o r , A . H., Trans. Illum. Eng. SOC.(N.Y . ) ,15, 811 (1920). PRESENTED before the Division of Paint a n d Varnish Chemistry a t the 99th Meeting of the American Chemical Society, Cincinnati, Ohio. P a r t of a dissertation submitted by Donald F. Wilcock (Lowe Brothers Cooperative Fellow) t o the faculty of t h e Institute of Scientific Research, University of Cincinnati, in partial fulfillment of the requirements for the degree of doctor of engineering science.
High-Vacuum Distillation of the Steroids
KENNETH C. D. HICKMAN
Distillation Products, Inc., Rochester, N. Y.
Sterols can be separated from the natural oils in which they occur by distillation in the molecular still at 100-220" C. The steroids which are not oil soluble are preferably separated by crystallization. If it is desired to distill them, they are best handled as solids in the molecular pot still or in stills of the diffusion type. Even the sterols which occur naturally in oil are only partially oil soluble. They separate as crystals from the enriched distillates, and after separation of the crystals these distillates can be returned to the molecular still to give new distillates, from which a further crop of crystals can be extracted. The antirachitic materials formed by the irradiation of sterols are more soluble in oil. They can be separated from the parent sterol by a combination of distillation and crystallization, the antirachitic material remaining in the soluble portion of the distillate. The molecular still is useful for purifying steroids and waxes isolated in biological research. Small quantities are generally all that are available, and these are handled in the pot still with a detachable head. Although distillation effects concentration of the steroids, it is seldom able to separate them in absolute purity. It must be assisted by saponification and crystallization.
D
URING the early stages of molecular distillation of animal and vegetable oils, solids may appear on the walls of the condenser. This is certain to happen with cod or tuna liver oils, or with corn oil, from whose distillates crystals separate in characteristic rosettes. These rosettes are sterols. The molecular still is thus admirably fitted to handle the sterols; and it is difficult to distill a natural oil without making provision to remove the sterols. The sterols collect under molecular vacuum between 100220° C., although there are occasions where distillations a t lower and higher temperatures are feasible. The sterols in general are oil soluble. The broader class of steroids becomes
decreasingly soluble in oil as the oxygen, particularly the carboxyl oxygen, content of the molecule increases. Each carboxyl group appears to raise the distillation temperature about 40" C. so that the steroid acids, in addition to becoming oil insoluble, become less easily handled by distillation. The commoner sterols are excellently suited to separation by distillation because of their solubility and volatility characteristics. Cholesterol (6),for instance, occurs in fish oils to the extent of 0.1 to 2 per cent, and sitosterol (6) and stigmasterol occur in vegetable oils in similar concentration. These sterols evaporate at 140-170O C., their volatility comparing approximately with diethyl-, dipropyl-, and diamylaminoanthraquinone pilot dyes. Most of the glycerides distill at a temperature 50-100° C. higher, so that a fair separation of sterols from the fats may be made in the early fractions distilled from natural oil. Again the sterols volatilize 30" or more above free vitamin A. They are, however, not so readily separated by distillation from free vitamin D and the tocopherols. The sterols mentioned are only moderately soluble in oil, from which they crystallize in well-defined needles or rosettes, easily recognizable and readily filtered. They withstand the action of alkali unchanged, so that they may be handled in oil solution or distilled in admixture with oils, and the residual glyceride can be removed by saponification. Solvent extraction of the soaps yields a solution of sterols which can be crystallized nicely in a single operation. By another procedure the saponification process can be dispensed with or its use minimized. Soybean oil, for instance, can be distilled and the sterols allowed to crystallize as far as they will from the distillate. The filtrate is returned to the still, a new distillate removed from which sterols are crystallized, the residue returned to the still, and so on, until substantially all of the sterol has been obtained in crystalline form. A final saponification or even a plain crystallization from solvent will yield the sterols uncontaminated with fat. The less soluble steroids cannot be handled in this manner, and if crystallization is not indicated, i t is necessary to sublime the solids. Sublimation from crystals cannot be recommended since the degree of purification produced is small, particularly if well-defined crystal specimens are used.
