INDUSTRIAL AND ENGINEERING CHEMISTRY
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experiments will be necessary to determine under what conditions the use of ultra-violet glass in winter in New York or Chicago will be of value in protecting against rickets. Summary
1-This study shows that when albino raB are exposed to Chicago sunshine a t the street level during the month of February and behind commercial ultra-violet glass or ordinary glass, they are not protected against rickets; whereas similar rats under similar conditions during a March-April period are fully protected against rickets when behind the ultra-violet glass, but not protected when behind ordinary glass. Z-Spectrophotographs comparing the transmitting e 5 ciency of the ultra-violet glasses used in the previously mentioned rat experiments, with a variety of other samples of commercial ultra-violet glass show that in the region 3000 to 3100 A. the transmission is between about 30 per cent for the poorest to about 52 per cent for the best of the glasses which
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were used; and that, therefore, there are among these glasses some which are average and representative of commercial ultra-violet glass. 3-These results indicate that there is a critical difference between the rickets-preventing efficiency of February sunshine and March-April sunshine behind commercial ultraviolet glass in Chicago at street level. ~
Literature Cited (1) Blunt and Cowan, “Ultra-Violet Light and Vitamin D in Nutrition,” p. 81, Univ. Chicago Press, 1930. (2) Bundesen, Lemon, Falk, and Coade, J . A m . Med. Assocn., 59,187 (1927). (3) Eddy, Institute of Research, Practical Arts Division, Teachers College, Columbia Univ., New York, Special Bull., May, 1930. (4) Eddy, “Report of Vitaglass Studies.” Contract 128, Institute of Research, Practical Arts Division, Teachers College, Columbia Univ., New York, October, 1929. (5) Fleming, Military Surgeon, 62, 692 (1928); 63, 658 (1928). (6) Stockbarger and Johnson, J . Franklin Insl., 210, 455 (1930). (7) Tonney, Hoeft, and Somers, J . Prm. Med., 4, 139 (1930).
Nature and Constitution of Shellac 111-Some Observations on Development of Opacity in Clear Varnish Films’sa Wm. Howlett Gardner SHELLAC RESEARCH BUREAU. UNITEDSTATES IMPORTERS’ ASSOCIATION, THE POLYTECHNIC INSTITUTE OF BROOKLYN, BROOKLYN, N. Y.
Some of the conditions which cause clear varnish dry films are under a cerfilms t o become opaque are reviewed and discussed. tain amount of tension and cause clear v a r n i s h These phenomena are divided into three classes i n any change that lowers the films t o become accordance with the different causes. A few examples elasticity of the material of opaque is a subject of interest are cited, as a means of illustration, which are of the film will manifest itself to every userof p r o t e c t i v e interest especially t o users of spirit varnishes. by p h y s i c a l defects in the coatings. The development A theory is advanced t o account for some of the film. It m a y be d e m o n of opacityis not confined to results observed i n the many varied experiments upon s t r a t e d that films can be any one type of varnish film which this paper is based. The results of a preliminary rendered opaque and be disbut is found to occur under study of the effect of water and other non-solvents on i n t e g r a t e d by purely mecertain conditions with widely shellac is cited and interpreted i n the light of this chanical means by stretching differing compositions, such theory. t h e m b e y o n d their elastic as oil varnishes, nitrocellulose The whitening of shellac films is shown t o be purely limits. lacquers, and spirit varnishes. physical in nature. The ultra-violet part of the The Cailaes for the producspectra in the majority of tion of this omcitv are many. Sometimes only t h e decorkive effects are impaired, while cases appears to produce the maximum amount of deterioration. in other cases actual disintegration of the film may take place, For this reason the mercury arc is frequently used as a means of obtaining accelerated effects for comparing different films which may be mechanical or chemical in nature. The phenomenon of films becoming opaque may be con- (11, 16,20). The method of exposure is very important, and veniently divided into three classes: (1) opacity resulting there may be different time factors for obtaining the same from the action of the actinic rays of the sun on a film; result, depending on the method employed. Continuous (2) opacity caused by the production of heterogeneity in a exposure to light is usually recommended ( 3 ) . Although a large amount of work (11, 20) is being done in film during drying through the precipitation of solid material; and (3) opacity produced by the adsorption and condensation this field, there is still much to be desired from studies of of substances of different refractive index in the pores of a dry chemical constitution and absorption spectra, coupled with film. By keeping these three classes distinctly and separately durability, of various varnishes. Until such information is in mind, one may more readily determine the real source of available generalizations cannot be drawn. For example, the the trouble, and not become confused by results which appear addition of varied substances markedly influences the deterioration. It is well known that pure nitrocellulose films are to be the same but have totally different origins. extremely sensitive to ultra-violet radiation, but that the Opacity from Action of Actinic Rays addition of a resin, as shellac, will materially retard its deThe effect of the actinic rays of the sun on 6lms varies terioration. Pure shellac films themselves are remarkably widely. It appears to depend to some extent upon the nature resistant to sources of deterioration of this type (5). of the solids, their combinations in the varnish films, and Blooming their absorption spectra. Unquestionably in many cases The development of opacity during the drying of films is chemical changes take place during the exposure to light, but certain physical or mechanical changes also transpire. Many often spoken of as blooming. In some cases it will disappear on further drying. Blooming probably is most frequently I Received September 10, 1931. Presented before the Division of encounteredwith nitrocellulose compositions but is not entirely Paint and Varnish Chemistry at the 82nd Meeting of the American Chemical confined to this field. It is usually considered to be caused August 31 to September 4, 1931. Society, Bu5al0, N. Y., by the precipitation of insoluble solid material in the drying 1 Contribution No. 3 from the Shellac Research Bureau.
HE conditions which
December, 1931
INDUSTRIAL A N D ENGINEERIRG CHEMISTRY
films, which may be pictured as very viscous solutions. However, with oil varnishes, blooming appears to be the result of the formation of large air spaces (12, 22). In either case, light striking the heterogeneous particles of the structure is scattered within the film producing the opacity. The precipitation of solid material in a drying film may be caused by a rapid evaporation of one or more of the solvent carriers, leaving a liquid in which a solid component of the varnish is insoluble. This rapid evaporation may also give rise to a heterogeneous mixture of two solutions (9). On the other hand, precipitation can be caused by the condensation of moisture on a film. Being the non-solvent in this case, moisture is absorbed by the solvent with a lowering of its solvent power. These difficulties may be overcome in many cases by giving careful consideration to the rates of evaporation of the solvents or their mixtures, and to the solubility of the solids in the different solvent mixtures during evaporation (1’5). Many solvents form minimum boiling mixtures which have higher rates of evaporation than either solvent alone. The solids in a composition also affect the rates of evaporation (IO). This field of study of evaporation and solubility is necessarily very large when the many complex compositions that exist are considered. It is unfortunate that more data of the type of Hofmann (9) are not available for the user’s guide. For this reason, elimination of blooming most often requires methods of trial and error. The condensation of moisture on a film may be caused by the too-rapid evaporation of the solvent. This cools the surface of the film below its surrounding temperature, producing the condensation. This is naturally more marked on days when the humidity of the atmosphere is high. I n some cases it may be eliminated by using higher boiling solvents. When this is done, care must be taken not to add a solvent of which traces will tend to remain in the film, for with many solvents these last traces themselves will absorb moisture and cause blooming. It is well to remember that it is not the solubility of a solvent in water but the solubility of water in a solvent that determines its tendency to take up moisture. Many solvents are practically insoluble in water and yet are capable of dissolving considerable amounts. For all these reasons, experience has shown that a good grade of denatured alcohol is the best all-round solvent for shellac. With spirit varnishes it is also advisable to use thin “cuts” where possible, since concentrated solutions give films that retain solvent. Shellac varnishes are usually thinned t o 2 pounds per gallon before application. One advantage of spirit varnishes is their rapid drying property. This necessitates the use of relatively rapid evaporating solvents, which, as mentioned in the foregoing, will cause condensation of moisture. It therefore follows that, on applying varnishes as shellac on damp days, every precaution should be taken to exclude as much moisture as possible from the place of a p plication. It is wise to close the windows and to see that the articles to be coated have been thoroughly dried. Many important examples of blooming known to the spiritvarnish application are invariably the fault of the object being coated and not of the varnish. This fact is too often overlooked by users of varnishes. For example, turpentine is a non-solvent for shellac. It may be added to a clear French varnish without causing clouding, but, when the film dries, the alcohol evaporates more readily, and the coating has a permanent bloom or opacity. Turpentine is never added to shellac varnishes, except possibly in low-grade substitutes, but solvents of its type are sometimes used in varnish removers. Unless the cleaned object is completely freed from these solvents, the alcohol of the shellac varnish will absorb them when it is applied, and blooming will result. Articles often appear to be in the right condition for the application of a
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shellac varnish but will have their pores filled with nonsolvents. This difficulty is most frequently met when applying shellac to damp floors. In some cases the floor may rest on a damp foundation and water diffuses through the wood into the drying film. Opacity from Adsorption of Liquid in Pores of Film All films which have not been rubbed down unquestionably contain minute pores. The protective property of coatings depends on the size of these pores. When they are filled with a medium of widely different refractive index from the materials comprising a film, the light striking it is scattered in a multitude of directions within the film, and one has the sensation of the film’s appearing white. This may be entirely a physical or optical effect, and the pores may be of such a shape that a maximum scattering of light is obtained, while the protective property of the film is in no way changed. If the pores are filled with a liquid of the same refractive index as the solids, the film will appear translucent. It can thus be seen that a measure of the whiteness of a film is in no way a measure of its utility as a protective coating, even if its decorative effect may be temporarily destroyed. For example, an oil-varnish film applied over a shellac filler on the deck of a yacht may turn white. The shellac filler is usually what is condemned as being the source of the trouble, while the actual facts of the case clearly show that the oil varnish is very porous or the moisture would never reach the filler. If shellac had not been used as the filler, this moisture would have been taken up by the wood with subsequent warping before it might have been noticed. Gardner (6) has shown by soaking panels in water that, although shellac films whiten more readily than those of oil varnishes, so long as the coatings remain intact, the wood coated with shellac absorbs less moisture than those coated with oil varnishes. Varnish films are put on in many coats, sandpapered, and rubbed down in order to help fill the pores. A Possible Structure for Varnish Films From recent studies of the structure ( I , 17) of materials, such as rubber (4), cellulose ( I S , 18), synthetic resins (2, 8), and shellac (7, 2 I ) , it appears that many of the protective coating materials have chain-like structures. It is not unreasonable, therefore, to picture films of these substances as being composed of enmeshed fibers, some molecular in size. This picture would be similar to the macro-phenomenon of pressed felt fibers. The chemical constitution and size of the molecular chain might influence the ability with which these molecular.fibers would be able to be compacted in a film. The submicroscopic spaces between the enmeshed fibers would account for the porous structure and the ability of solvents to more or less completely leave a drying film. The complex (probably both ring and chain) structure of shellac molecules of relative small molecular weight (21) may account for the unusual protective properties of films of this resin. When substances of this class are placed in contact with liquids that wet the surface of the solid, liquid will be condensed and compressed in the minute pores as a result of the adsorptive forces, such as observed with charcoal or silica gel. This compressed liquid, because of the mechanical stresses set up, will tend to cause the enmeshed mass to swell and become less compact. The smaller thc pores, the greater will be the adsorptive forces, and the more liquid compressed. I n extreme cases where there is great adsorption, the whole mass may become disintegrated in relieving the stresses set up by adsorption. On the other hand, i t is possible to have films with pores SO large that the amount of liquid condensed in them is comparatively so small that none of the foregoing disruptive effects take place. The size, shape, and general structure of the pores should
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
determine, for the most part, the effect liquids will have on films, assuming of course that the liquid wets the solid, For solids of the same adsorption or wetting power, a measure of the amount of non-solvent taken up by a film would be a measure of the structure of that film, and not any physical or chemical property of the substances comprising it. For this reason the author can not concur with members of the Indian Lac Research Institute in their view that the method of measuring the increase in weight of dry shellac films soaked in water (19) is in any way a measure of any property of a particular sample of shellac. The slight variations in conditions during the preparation of shellac films will materially affect the amount of moisture adsorbed (16). This is also true even for the structure of oil-varnish films (22). Unless a careful study has been made of all of the conditions effecting structure, it is very difficult to interpret results. A solvent which appears to give the best results under one set of conditions may not be the best varnish-carrier under .other circumstances. The use of solvent mixtures for shellac, such as recommended by Venugopalan and Rangaswami ( I @ , is not to be advocated until considerably more study has been given them. Effect of Water on Shellac Shellac has been shown to be an ester condensation product
(Wi), and it was f i s t thought that the whitening of shellac films by soaking them in water might, in some manner, result from a partial hydrolysis of these ester linkages. When esters are hydrolyzed by water, constituent acids are formed. An increase of acid number can therefore be taken as a criterion of the extent of such a hydrolysis. The increase of acid number is also directly proportional to the increase in weight by the addition of water in the formation of the products, These possibilities were therefore investigated. Since the amount of material contained in any convenient size of film is necessarily very small, flaked shellac was first studied. There is no reason to believe that its structure should materially differ from that of a thoroughly dried film, except that the effect of water would take longer because of its greater thickness. A good grade of orange shellac was chosen for study. After finely grinding, 5-gram samples were soaked in distilled water for periods of six months. At the end of this time the shellac was removed by filtration and air-dried. The properties of this whitened material were determined, as well as of larger flakes which had been similarly treated. Treatment with water for six months had caused the translucent orange flakes to become opaque and have a white or light buff color. Some evidence of what might be interpreted as swelling could be noted. Instead of the hard vitreous 'consistency of the original lac, the treated material had what might be described as a woody texture. When viewed under the microscope, the swollen structure was very visible. The material appeared as a mass of white globules surrounding clearly defined pores. Light was reflected in all directions by the minute aggregated particles. Drying at 80' C . did not affect the whitening. However, when this material was fused, the orange color was restored. Soaking in solvents, as ether and esters, which partially dissolve and soften shellac, destroyed the opacity. The whitened material dissolved in alcohol to give the usual orange solution.
This treated shellac, when air-dried a t 43" C . , had the same weight as the original sample dried a t this temperature, when corrections were made for the small amount of water-soluble albuminous material always extracted from this grade of lac. The acid number was the same as the original. The experimental data are given in Table I, which clearly shows that no chemical change had taken place as a result of the whitening. Films of orange shellac were found to behave in similar manner. Even here, in many cases, it took several days to weeks to produce whitening. In these experiments the effect of water appeared to take place in two stages. If removed
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from the water when the whitening first occurred, the &s on drying became translucent in the same manner as observed by Venugopalan and Rangaswami. On the other hand, long exposure to moisture produced permanent opacity. In all cases films increased in weight when soaked in water, but this water was completely lost when they were air-dried. By brushing these films with any of the shellac solvents, their transparency could be restored. Restored films appeared to have the same properties as the original, Table I-Showing
No Chemical Change of Shellac Soaked in Water
Weight original dry shellac Weight whitened shellac Weight water-soluble from whitened shellac Total weight whitened shellac Difference in weight after soaking in water Acid number, original Acid number, whitened
Grams 4.9400 4.8920 0.0475 4.9395 -0.0005 64.65 64.40
Grams 4.9190 4.8595 0.0600 4.9195 +0.0005 64.60 63.30
These experiments show that the effect of water on shellac is purely physical in nature, and that with dry films the production of opacity belongs to the third class discussed previously. Since shellac contains a high oxygen content and many free hydroxyl groups, excellent wetting by water might be expected. The apparent extreme fineness of the pores of its films, which gives it its great protective qualities, would cause this water to condense in them. This water would be compressed in the pores and not pass through the film. From general knowledge of adsorption, this compressed water would naturally set up tremendous mechanical stresses, which under the extreme conditions of the previously mentioned experiments would tend to disrupt any known material, especially that of any enmeshed mass of which these films may be composed. That this phenomenon is not confined to water was demonstrated by soaking dry films in benzene where the same effects were produced. It would appear from this discussion that problems con-, cerned with the production of opacity in clear dry varnish films by the effect of liquids involves studies of adsorption and gel structure. Further experiments are in progress which will be reported in subsequent papers. Literature Cited (1) Carothers, (V. H., J . A m . Chem. SOL., 11, 2548 (1929). et al., Ibid., Sa, 314, 3292, 3470, 5289,5307 (1930). (2) Carothers, W.H., (3) Crane, C. I,.. Bur. Standards J . Research, 4,247 (1930). (4) Freudenberg, K.,and Braun, E., Ann., 460, 288 (1928). (5) Gamble, D. S..and Stutz, G . F. A., IND. END.CHEW.,21, 330 (1929). (6) Gardner, H. A., "Papers on Paint and Varnish and the Materials Used in Their Manufacture," P. H. Butler, Washington, D. C.,1920. (7) Gardner, W. H., and Whitmore, W. F., IND. END.CHLM.,91, 226 (1929). (8) Hoenel, H., Paint Oil Chem. Rev., 91, 19 (1931). (9) Hofmann, H. A., IN^. ENG.CHLM.,23, 127 (1931). (10) Hofmann, H.A.. and Reid, E.W., Ibid., 21, 955 (1929). (11) Kessler, E, H., Am, Paint Varnish Mfrs.' Assocn., Cirt. 370, 548; A m . Painf J . , 14, No. 52C. 25; Painf Oil Ckcm. Rev., 90, No. 18, 14; Oil, Paint Drug Repfr., 118, No. 19, 83 (1930). (12) Rrumbhaar, W., Am. Paint Varnish Mfrs.' Assocn., Circ. 370, 520; A m . Paint J . , 14, No. 52A, 30; Paint Oil Ckem. Rev., BO, No. 17, 59; Oil, P a i d Drug Repfr., 118, No. 19,59 (1930). (13) Meyer, K.H., and Mark, H., Be?., 61, 693, 1932, 1939 (1928). (14) Paisley, J. W., Paper presented before Division of Paint and Varnish Chemistry, 82nd Meeting of AMERICAN CHEMICAL SOCIETY, Buffalo. N.Y., Aug. 31 to Sept. 4, 1931. (15) Reid, E. W.,and Hofmann, H. E . , IND. END. CHBM.,20, 497 (1929). and Gamble, D. L., Ibid., Anal. Ed.. 1, 83 (1929). (16) Schmutz, L. C., (17) Shepard, S. E., IND. ENG.CHEM.,23, 781 (1931). (18) Staudinger, H . , et al., Helv. Chim. Acta, 8, 785 (1922); 8, 65,67 (1925); 9, 629 (1926); 11, 1047, 1052 (1928); Ber., 13, 1073 (1920); S7, . 1203 (1924); 19, 3019 (1926); 60, 1782 (1927): 61, 2427 (1928); 61, 241. 263 (1929). (19) Venugopalan, M., and Rangaswami, M., IND. ENQ. CHEM.,22, 911 (1930). (20) Walker, P. H., and Hickson, E. F., Bur. Slandards J . Research, 1, 1 (1928). (21) Whitmore, W.F.,Weinberger, H., with Gardner, ,W. H., IND. END. CHEM.,Anal. Ed.; 4 (1932). (To be published.) (22) Wolff, H., Farben-Zfg., 36, 505 (1930).
