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I N D U S T R I A L A N D E.VGINEERING CHEiWISTRY
guards and windows on special test equipment. The uses developed during the war in the naval and air services also continue, and considerable multiple-layer laminated glass 1 to 11/4 inches in thickness is used as bullet-proof glass. All of these applications are based on the safety feature of laminated glass. A new field is now being developed, however, in which this feature is secondary and the emphasis is upon the artistic effects which may be obtained by using color and design in the sheet of plastic. Table tops, tray bottoms, mirrors, wall paneling, soda fountains, display signs and novelties, art placques and silhouettes designed in mirror, and many other products of extremely attractive appearance have already been placed on the market. Celluloid can now be printed satisfactorily in four colors, with future prospects of
Vol. 23, KO.5
additional variety if desired, so that marbles and grained and inlaid woods can be secured which rival the natural materials. The opportunities for new and striking artistic effects through the medium of laminated glass are numerous and future development of the industry may be quite as much along this line as along the established one of insuring safety. Literature Cited (1) Benedictus, E.,Glaces e l w r i e s , No. 18, 9 (Oct., 1930). (2) Benedictus, E.,U. S. Patents 1,098,342(May 26,1914); 1,128,094(Feb. 9,1915); 1,206,656(Nov. 28. 1916). (3) Benedictus. E.,U.S.Patent 1,182,739(May 9,1916). (4) Woods, J. C . , U S . Patent 830,398(Sept. 4, 1906).
Energy-Emission Data of Light Sources for Photochemical Reactions' C. E. Greider RESEARCH LABORATORY, NATIONAL
CARBON COMPANY, ISC
, 1280
\VEST
73RD
ST, C L E V E L A N D ,
OHIO
Data are presented giving the amount and spectral IGHT sources, in the Light Sources distribution of the radiant energy from a number of ordinary meaning of typical carbon arcs, most of which are in common use the term, are now beThe sources commonly used at the present time. This information is essential ing used for many purposes for the production of visual for the selection of the most suitable light source for other than the production of light, as well as ultra-violet a reaction involving the use of ultra-violet or visual visual light. Physicians and and infra-red, fall into two light. It also illustrates the wide variety of types of clinical workers make use of classes. The first is represpectral-energy distribution that can be obtained from them for various therapeutic sented by incandescent solids, different types of carbon arcs. mmoses. lawnen for health which show an essentiallv con- maintenance, while technical tinuous spectrum of which the workers use light sources for a wide variety of photochemical conimonest illustration is the ordinary tungsten-filament lamp. and analytical reactions influenced by radiation in various The second is the discontinuous or line spectrum caused by the parts of the spectrum. thermal or electrical excitation of gas molecules, commonly In order to determine the effectiveness of a given source produced by an arc or other electrical discharge. of radiation for a specified purpose, it is not sufficient to Both types of radiation are found in the carbon arc. The measure its intensity simply in the broad bands of the spec- crater or electrode tip is heated to incandescence and shows trum designated as ultra-violet, visual, or infra-red, since the typical continuous spectrum of an incandescent solid, many of the effects of radiation are produced by relatively while the arc stream, between the two electrodes, givesoff the narrow portions of these bands. As an example, the actira- characteristic line or band spectrum of the molecules and tion of ergosterol to form vitamin D is brought about on$ atoms present in .the arc. The relative amounts of the two by ultra-violet of shorter wave length than about 3100 A. types of radiation can be varied a t will in accordance with the Since this in many sources constitutes only a small fraction purpose to which the arc is to be put; thus, the radiation of the total ultra-violet and may be practically absent in from the crater of the plain or projector carbon is almost some sources which contain appreciable amounts of ultra- all the continuous spectrum of the incandescent solid, while violet of longer wave length, a determination of ultra-violet from a true flame arc i t is practically all of the second type. intensity tells nothing of the value of a light source for this In this case the incandescent solid electrodes produce only a very small fraction of the total ultra-violet and visual light, purpose. The evaluation of a source of ultra-violet or visual light for most of which comes from the arc stream between the two any of its various purposes may be obtained from measure- electrodes. The difference between the two types of arc is ments of the intensity and distribution of energy throughout clearly shown in Figure 1, A and B. An arc between two pieces of pure carbon will show, in the spectrum. When the response curve of a reaction under consideration is known, the effectiveness of a given source addition to the radiation from the electrodes themselves, can be determined directly from its spectral-energy distribu- certain lines and bands, chiefly in the violet and near ultration curve by determining the intensity in that range of the violet, characteristic of the carbon arc in air. This distribuspectrum to which the material responds. When the response tion of energy can be modified by incorporating other matecurve is not known, information regarding it can be obtained rials in the carbon which are vaporized by the arc and add by comparing the effectiveness of two different sources whose their characteristic spectrum to that of the carbon itself. Such materials may be distributed throughout the electrode, spectral-energy distribution curves are known. but are more commonly concentrated in the center or core. 1 Received March 4, 1931. Presented before the Division of InThe flame carbon arc thus offers a very versatile source of dustrial and Engineering Chemistry at the 81st Meeting of the American radiation, for by changing the nature of the material in its Chemical Society, Indianapolis, Ind., March 30 t o April 3, 1931.
L
tcriris of ab&ute energy u&s aiid screens;if kiitnvii spcctral traiisinissioii. T h e First iiionocliromator, of wliicli thi, optietil parts are OS crystalline quartz, is ea$-ated to mcarure rarliation over tlic range %dOO to iOO0 A. which includes inmt of the ultra-violet and visual light. Its operatioil has been previously described (8, 8). The second is an infra-red spectrorailiorneler made by t,he Gaertmer Scientific Corpnrat.ion of the type of the Wadswortli constantdeviation p e t r o scope. This was used for t,lie range 6000 to 18,000 A. The operatiun of this instrument is essentially the same as that of the spectroradiometer for the ultra-violet and visual witli
*-
i
.'I
- D.c. projccts~carbnn arc
a
Sunshine Rnmr arc
*'t*ure 1
wliich regioii quite satidactory agrtwiecit is s l ~ o w nby tlie two methods of measurement. It is believed that the range OS wave lerigtlis chosen covers the entire region of interest to either biological or teclinical worhrs. Tlie stiiirt-wa,uc-lcn~tlilimit of 2300 A. in the ultraviolet is (iiirly rlwr to the: limit of transmission of air, which, cxceirtoiii vrry tliiii layi:rs, may be t:rkeii an approximately 2000 8. l~"urthcrmore,comioercially available sources of nltra-violet do not ernit appreciable aniounts of cnergy of shorter wave length than 2300 1. On j the long-wave-length sidc infra-red radiation of longer wave length than 14,000 1.is completely t :ilm~rlxxi by even a relatively thin layer of water. 1b.i spectral d i s t r i h t i w i s of relatively little imI portance for most purposes, altirougli its total smoont may be significant as i n d i c a t i n g the amount of heat given off by the light source. Discussion of Results
so
~
,,a%
FMure Z
ndditional correct,iori for tlie variation iii t.he reflecting puwer of the gold niirrors througiiout the region measured. Both these instrurrieiits give relative energydistriimtioii curves only. By means OS a procedure that has beeii deserihcd elsewhere (8) they are converted into curvcs expressed in absolute energy units. Briefly, this consists in isolating by means of suitable screens the aniount of energy in wave bands corresponding to those covered by the speetroradiometer for any given light source, measuring this in absolute iinits by means of a galvanometer and thermopile, mid correlating tlic corresponding areas under the energy-distribution ciiryes to correspond to tho almlote energy values so obtained. By this means curves are obtained in which unit area,s represent unit amounts of energy, so that the intensity between any two wave-length limits may be deterniined simply by measuring the corresponding area u n d e r t h e curve. The two sets of curves obtained overlap in the region 6000-7000 I . , in aii
/.olo
Figure 2 shows the energy-distribution curve for a typical high-intensity carbon arc such as i s used in srarclilights and in motion-picture projection and p h o t o g r a p h y . On this is superposed the energy-distribution curve for norriial sunlirht (8). The radiation from the arc :is measured is the radiation from the positive crater only, as in most a p p l i c a t i o n s of this
510
ILVDUSTRIALA N D ENGINEEIiING CHEMISTRY
type of light source this is all that can be utilized. The curve shows the rather striking agreement between this light source and natural sunlight through the entire range of the spectrum, as has been previously reported by others (1, 2, 5 ) . The chief difference bztween the two sources is the radiation shorter than 2900 A. which is present in the arc but absent in natural sunlight as received a t the earth’s surface. Although this ultra-violet of short wave length is small in amount, its effects in some cases are considerable, and for the duplication of the effects of sunlight it may be necessary to remove it by means of a suitable filter.
-
I
Figure 4
The energy distribution from the crater of a plain carbon arc is shown in Figure 3. This differs from the curve in Figure 2: first, in that the maximum intensity occurs a t a longer wave length, corresponding to a lower temperature; second, that the total intensity is lower; and third, that superimposed upon the radiation from the incandescent solid there appears a larger amountof the lineor band spectrum due to the arc itself. T h i s is especially noticeable in the ultra-violet a t about 2500 .&.and again near 3900 A,, which is near the commonly accepted lower limit of the visual portion of the spectrum. Energy-distribution curves for other incandescent solids such as the tungsten-filament incandescent lamp will follow the same general shape as this, except that, since they operate a t lower temperatures, the maximum of the curve will be shifted further toward the infra-red; such a light source will contain relatively less ultra-violet and visual light and more infra-red or heat radiation. It is seen from these curves that, even when the radiation to be utilized is that of the cont i n u o u s s p e c t r u m from the electrode tip or craters of the arc, a small amount of the discontinuous or line spectrum from the arc itself is also present. Similarly, when the radiation from the arc stream is desired, some light from the craters is also present, although in most cases this is but a small fraction of the total. Figure 4 shows radiation measurements for arcs of which the p r i n c i p a l light source is the arc itself rather than the crater. Three t y p e s of carbons are shown in this curve. The first is a “neutral core” or plain carbon similar to that shown in Figure 3, except that the radiation is measured from the side instead .of facing the positive crater. The result is that the radiation from the craters is smaller in amount and is a t a lower average temperature, so that the maximum of the curve is further
’
Vol. 23, No. 5
toward the infra-red. Also, the amount of the characteristic radiation of the arc stream is increased. This is the radiation characteristic of the plain carbon arc, to which no flame material has been added. “Sunshine” carbons, which contain cerium salts in the core, give an entirely different energy-distribution curve. The same bands are present as before, but in addition the presence of this flame material causes an emission of energy in the ultra-violet, @creasing gradually in intensity from about 2900 to 4000 A,, which approximates the increase in intensity of natural sunlight itself throughout this region. There is also a high and fairly uniform intensity throughout the visual part of the spectrum, exI tending to about 8000 8.in the infra-red. Most of the energy through this region is caused by spectrum-emission lines so closely packed together as to approximate the effects gf a continuous spectrum. Beyond about 8000 A. the intensity falls off rapidly, until a t longer wave l e n g t h s practically the only radiation given off is that from the incandescent electrodes. I n the same figure is shown the energy distribution of “C” carbons, which contain a mixture of metals, including iron, in the core. Radiation from this carbon is characterized by a series of groups of very intense lines throughout the ultra-violet, and a relatively low intensity of energy in the visual part of the spectrum. In the infra-red the radiation is chiefly that from the incandescent craters. Through this region it follows the general shape of the curve for the carbon containing no flame material, but is lower in intensity. This is due to the fact that a flame carbon dissipates more of its energy in the arc stream and less a t the craters than does a plain carbon arc. The above curves show the characteristics of the two types of flame carbon arc most commonly used, the Sunshine carbon for the production of relatively large amounts of visual light and infra-red together with moderate amounts of ultra-
Figure 5
violet, corresponding to that found in sunlight itself, and the l‘C’l carbon for producing high intensities of ultra-violet with only moderate intensities in the visual and infra-red portions of the spectrum. Figure 5 shows some of the possible effects that can be ob-
May, 1931
INDUSTRIAL A1YD ENGINEERI-VG CHEMISTRY
tained by using other materials in the arc. For producing other, and sometimes unusual, distributions of energy in the ultra-violet, various metals are especially suitable. The carbon shown in the curve containing cobalt as the principal flame material is an illustrationoof this, giving a very hi h intensity of radiation near 2400 A,, also a peak near 3600 . with a low intensity of energy in the infra-red and visual regions of the spectrum. Calcium compounds have been used in the carbon arc for many years. The energy-distribution curve of an arc containing this material is shown also in Figure 5 . It contains relatively little energy in the ultra-violet, fairly strong emission in the violet, and another maximum extending from the green to the orange. This consists principally of two bands, one in the green and one in the orange, which give the light from these carbons a yellowish color. Another instance of usual energy distribution is shown with carbons containing strontium compounds in the core, shown in the same figure. These are characterized principally by a very strong band centered around 6500 A. in the red with a few lines and bands throughout the rest of the visual spectrum, having relatively little energy in the ultra-violet. Except in the case of the high-intensity arc, which is normally operated a t much higher currents, the energy-distribution curves all represent arcs operated a t 30 amperes and 50 to 55 volts. Arcs of this type can be operated Euccessfully a t currents of 8 amperes or less to 100 amperes or more. In order to operate satisfactorily, it is, of course, necessary to use carbons of small diameter for the lower currents and larger carbons, sometimes copper-coated, for the higher currents. The effect of variation in current on the energy emitted throughout the ultra-violet and visual is shown in Figure 6, in which energy-distribution curves are given for “C” carbons a t different currents. It is seen from these curves that, while the characteristic radiation is qualitatively the same for the different currents, it increases quantitatively with increasing current, the radiation of shorter wave length increasing more rapidly. The “C” carbon arc a t high currents offers a practical method for obtaining the extremely high intensities of ultra-violet necessary for commercial photochemical applications.
1
Applications
The application of photochemistry to industrial chemical reactions has been summarized by Ellis (6),and more recently by Dorcas (4) and by Frankenburger ( 7 ) . It has been claimed that industrial photochemistry has not made the progress which a few years ago it had been expected to make. This is undoubtedly due in part to the fact that insufficient attention has been paid to the spectral composition of the radiation used and to the response of the reaction in question to different wave lengths. Future progress in this field should depend to a considerable extent upon determination of the most favorable part of the spectrum for carrying out the reaction, and selection of the light source which will give the maximum of intensit)y in that region. Furthermore, portions of the spectrum other than the one desired may actually be harmful to the reaction in question, This has been investigated in the case of the activation of ergosterol to form vitamin D (9 to 12). In this case the desired reaction proceeds most satisfactorily Kith ultraviolet between the wave-length limits of approximately 2700 and 3100 A. Longer wave lengths are not effective, while radiation of shorter wave length than about 2700 A. ia actually harmful, causing undesirable side reactions. Another instance is the duplication in accelerated weather-
51 1
ing tests of the weathering produced by sunlight. This is a very complex phenomenon, produced in most cases by the entire ultra-violet spectru? from the lower limit of the sun’s spectrum a t about 2900 A. up to the beginning of the visual spectrum a t 4000 A. and, possibly, by the violet and blue of visual light. In cases in which light is an important factor in accelerated weathering tests, it is desirable to duplicate as closely as possible the effects of sunlight. To do this, the light should not contain radiation of shorter wave length than found in the sun, as this radiation is known to produce entirely different results; and all the wave lengths present in sunlight which contribute to the production of the observed effect should be present in the artificial light source. SPECTPAL ENERGY DISTRIBUTION IURVf C ’ CARBONS SOVOLTS A C
Figure 6
The fact that ultra-violet in many cases causes fluorescence, of which both the intensity and color depend on the material investigated, has been used as a basis for a great many proposals for its use in analytical determinations. The limited practical success that has been achieved in this field may be due in part to the fact that insufficient attention has been paid to the spectral composition of the light source used and to the variation in the response of the material with wave length. The most familiar photochemical reaction is the synthesis of carbohydrates by living plants, in which case light furnishes all the energy for the reaction. A much more promising field for the commercial application of photochemistry, however, seems to be its use, not as the source of energy for a reaction, but as a catalyst, in which case it fills all the functions of the ordinary material catalysts with which chemists are familiar. The second use, which seems a t present to be of the greater importance, is the production of changes in a constituent, present only as a very small fraction of a mixture, resulting in a considerable improvement in the quality of the product. Literature Cited (1) Bassett Trans. A m . Elecfrochem. Sob., 44, 153 (1923). (2) Benford, Trans. SOC.Motion Piclure Eng., 24, 71 (1926). (3) Coblentz, Dorcas, and Hughes, Bur. Standards, Sci. Paper 639 (1926). (4) Dorcas, IND.END. CHEM.,24, 1244 (1930). (5) Downes and Joy, J. SOC.Motion Picture Eng., 14, 291 (1930). ( 6 ) Ellis and Wells, “Chemical Action of Ultra-Violet Rays,” Chemical
Catalog, 1925. (7) Frankenburger, 2. angew. Chem., 43, 797 (1930). (8) Greider and Downes, Trans. Illum. Eng. SOL.( N . Y.),48, 378 (1930). (9) Morton, Heilbron, and Kamm, J . Chcm. Soc., 1927, 2000. (lo) Pickering, J. Am. Pharm. Assocn., 18, 359 (1929). (11) Reerink and Van Wijk, Biochem. J., 43, 1294 (1929). (12) Reiter, Nafur~’issenschaflen,17, 867 (1929).
