Nature and Constitution of Shellac XI. Rates of Adsorption of Moisture by Shellac Films' Il-
WILLIAM HOWLETT GARDNER AND WILLIAM KAPPENBERG Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
spaces as being continuous, anymore than we would necessarily expect them to be in the solid phase of a jelly. Undoubtedly a few of the channels have external openings on both sides since some varnish films show a definite porosity ( I , 9, 3, 5 , 8,9), but it is clear from the studies made that the pores which do exist are usually extremely fine (1). Swelling by various liquids may, however, produce a permanent increase in permeability since varnish films would be of the elastic type of gels. The adsorptive forces involved when a liquid or vapor wets the film should be extremely large. Shellac flms should therefore adsorb moisture without transmission in much the same manner as any other adsorbent, unless attendant swelling effects should alter the structure of the capillary spaces. A study, therefore, of moisture adsorption a t low humidities where whitening does not take place might explain why shellac films are one of the best known protective coatings as long as they remain intact, a fact which is frequently overlooked in practical application (4).
FIGURE 1. DRYING APPARATUS Large funnel B . Small funnel C . Shellac film D. Mercury E . Crystallizing dish A.
F. f3. H. J.
Drying tubes (Dehydrite) Drying tower (calcium chloride) Reservoir Cloth filter in water outlet
T
HE degree to which a shellac (7) or an oil varnish (IO)film may whiten or bloom will depend not only upon conditions of exposure to which the dry film is subjected but also upon the conditions prevailing during their application and drying in addition to the amount of solvent which may be retained in the film. I n the case of shellac a t least, whitening is purely Essential Characteristics of Films physical and does not involve any chemical change (4). In preparing varnish films for experimental studies, prefAlthough adsorption of moisture by these films has not been erence is usually given to free films since they eliminate any clearly demonstrated before, the belief was expressed in a effect which the undersurface may have upon their properties, previous article (4) that whitening resulted primarily from Such films may be obtained by applying them to sizes which adsorption of water in pores or interspaces of the films and can be3dissolved when the varnish film is dry ( 3 ) . For exthat this caused swelling to take place which produced a disentangling of the enmeshed molecules pictured as forming the film. I n other words, the author h a s s i m p l y asAdsorption of water by shellac films is undoubtedly an important sumed a gel structure for the factor in contributing to their whitening or bloom. The true charfilm which is very similar to other gels f o r m e d b y t h e acter of this adsorption, however, can be observed only when films coalescence of single moleare exposed to atmospheres of relatively low humidity since at high cules or colloidal aggregates. humidities effects are masked by attendant swelling. In developing such a theory This preliminary paper, the first of a more extended study of the for the structure of films, it is not necessary to picture all structure of films, describes how free films of shellac when carefully the pores or i n t e r m i c e l l a r dried will rapidly adsorb moisture during the initial periods of their 1 Other articles in this series apexposure, and how they then slowly lose weight until a stable state peared in thia journal in 1929,1931, of equilibrium is reached. This reduction in adsorptive power, re1933, and 1935, and in the Analytioal Edition of INDUSTRIAL AND ported here for the first time, is probably due to a swelling of the ENQINIIRINQCEIMISTRYin 1929, films and suggests that the films may have a gel type of structure. 1932, 1933, and 1934. 437
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FIGURE 2. DRYING CURVES FOR VARNISHFILMS
I
FIGURF 3 VOLUME
OF AIR
5L.= I HR. 120 139
FIGURE3. ADSORPTIONBY VARNISH FILMS AT 25 PERCENTRELATIVE HUMIDITY
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- LITERS
ample, shellac fdms niay be released from a copal size with turpentine in this manner. Similarly, galvanized steel can be used as a base and the zinc coat dissolved by acid (3). Films can also be prepared b y them on a b s o r b i n g pregnated with water g&ss ( B ) , or, since shellac does not adhere to Cellophane, this material may also be used. For adsorption studies, however, in each of these cases the films may be subjected to liquids which may change their characteristics during the dissolving of the size, or in the case of Cellophane there may be a small adsorption of glycerol. In addition, other factors must be considered. From a practical standpoint a very large continuous surface should be taken in order to measure accurately the a m o w t of water absorbed. The extreme hardness and hence the brittleness of free shellac films would not permit of their being handled as experimental material in this manner. It would also be difficult to obtain uniform thickness, although this is also considered to be an essential prerequisite (1). Therefore, it would appear that in the case of shellac each and every method suggested might be precluded. However, if a study is confined to measurements of rates of adsorption without attempting in a preliminary study such as this to obtain quantitative comparisons between films from different varnishes, valuable information could be obtained concerning the exact nature of what takes place when dry shellac films are placed in humid atmospheres. All that is now necessary is to obtain a large surface area; even if the film is broken into small pieces, the variations which may be caused by differences in surface of the very thin sides may be neglected as inconsequential in affecting the
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FIGURE4. ADSORPTION B Y VARNISH FILMS AT 4.4 PER CENT RELATIVEHUMIDITY
general character of the measurements. I n this case the shellac films may be prepared on a large pool of mercury where they can be dried under constant conditions. That no contamination takes place as a result of any possible corrosive action by the shellac varnish on the metal was proved by preparing films one foot in diameter, drying them, and carefully digesting them with nitric acid. Analysis showed them to contain no traces of mercury.
Preparation of Films The apparatus which was used for preparing such films is shown in Figure 1:
It consisted of two funnels, one placed inside the other. The outer funnel rested in the pool of mercury and surrounded the film of shellac varnish which has been allowed to spread on the surface of the pool. The rim of the inner funnel just cleared the surface of the varnish film and permitted dry air to be drawn downward and across the film by means of a large reservoir containing water. Magnesium perchlorate was employed as desiccant t o remove all traces of moisture from the incoming air, and a large tower of anhydrous calcium chloride placed in series between the outer funnel and the reservoir was effective in preventing any back flow of moisture from the latter source. The rate of flow of air through the apparatus was controlled by a capillary tube attached to the siphon of the reservoir. This was found to be mors convenient than a pet cock. Films from approximately 1, 2 , 3, and 4 pounds (0.5, 0.9, 1.4, and 1.8 kg.) of orange shellac varnishes [l to 4 pounds of shellac
APRIL, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
per gallon (3.78 liters) alcohol] were prepared by pouring them upon the surface of the mercury and permitting the alcohol to evaporate in the dry atmosphere obtained in the apparatus until they were free from tackiness. The were then removed from the mercury, with care that no metaradhered, and were placed in large weighing bottles. These bottles were then placed in the apparatus without their covers, and the drying was continued. Films were weighed at definite intervals and were allowed to remain in the apparatus until they had obtained constant weight. The drying curves for the four films studied are shown in Figure 2.
Rate of Adsorption of Moisture These bottles were then placed in a chamber similar to the one used in their preparation, and air containing a definite humidity was passed over them a t the rate of 5 liters per hour. This was accomplished by substituting wash bottles containing the requisite aqueous solution of sulfuric acid for the magnesium perchlorate tubes shown in Figure 1. The exact amount of water vapor in the air was determined by placing weighing bottles containing magnesium perchlorate in the chamber and measuring their increase in weight for definite periods. This was done before each series of measurements on the films. The precision with which atmospheres of a desired humidity may be obtained in this manner is shown as follows : A 70 per cent sulfuric acid solution with a specific gravity of 1.6059 should give a humidity of 4.4 per cent at 25" C.; the actual gain in weight of the desiccant after the passing of 14.5 liters of air during a period of 3 hours was 0.0151 gram, or an error of 3.8 per cent from the calculated gain of 0.0157. The rates at which shellac films of different cuts adsorb moisture from atmospheres containing 25 and 4.4 per cent relative humidities a t 2.5" C. are shown in Figures 3 and 4, respectively. Curves were duplicated in all cases for adsorption of moisture under identical conditions. Films in no case showed any signs of whitening. There was a rapid initial adsorption of water in all instances, but after continued exposure the films, instead of continuing to gain in weight a t a decreasing rate as would be expected of an adsorbent, actually lost weight in most instances, until a h a 1 state of equilibrium was reached for each set of conditions. The initial adsorption was most marked and reached the greatest proportions for films from varnishes of the lowest poundage. This behavior is interesting since this loss in weight might possibly have represented a displacement of alcohol retained in the film by the adsorbed water, but it is known that the amount of alcohol retained in shellac films is far less in coatings from 1- and 2-pound cuts than by those from heavier viscosities. The curves show the 1-pound cut a t 4.4 per cent relative humidity to give an abrupt peak with a comparatively large loss in weight; for the films from the 3- and 4-pound cuts the curves show only a slight maximum and not as pronounced a loss. At the higher humidity none of the curves has a break, but the same tendencies are manifest. The 4-pound cut film gives a uniform curve showing no evidence of a maximum. Films of different cuts cannot, however, be compared for specific adsorption since these curves are based upon the adsorption per gram of fdm, and the surface areas do not bear the same ratios to each other as those of the poundages. This characteristic is due to the differences in surface tension between the mercury and the varnishes of different concentration. They represent, however, the normal type of films
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such as would be obtained from these varnishes and may be legitimately compared for differences in adsorption characteristics since they bear a qualitative relation to each other, even though they cannot be compared quantitatively.
Discussion of Results The limited range of data which has been obtained heretofore from studies of permeability and other properties of varnish films, has led some investigators to hold the view that films such as these are similar to semi-permeable membranes. They would therefore picture the adsorption of moisture by shellac as a solution of water in the fdm in much the same manner that pyridine or benzene is supposed to be taken up by rubber. This explanation would have been plausible even in this study if the investigation had been confined to adsorption a t high humidities, as can be seen from the shape of the 4-pound curve in Figure 3; but it would not explain the shape of the other curves, even if the loss in weight was assumed to be due to a displacement of retained alcohol. It does not account for the observed fact that the thinner fdms show the comparatively greater losses, nor does it account for the sharp peaks of these curves in Figure 4. I n contrast, a nonrigid gel picture for the structure of varnish coatings will not only account for the initial rapid adsorption of moisture by these films, but it will also predicate their ability to swell and to lose adsorbed water and retained solvent. The fact that films from heavier cuts and those exposed to the higher humidity do not show as prominent a maximum for the adsorption is probably due to the fact that swelling takes place more rapidly, accelerated by the greater ability of these films to adsorb water. The potential adsorption is not attained in these cases since it has been inhibited by the lack of retention developed as a result of the swelling. This behavior is in accord with certain permeability data which have been obtained for a variety of varnish films of different types, and further confirmation of these views will be presented in subsequent articles now in preparation. Literature Cited (1) Bauer, K. H., and Gonser, K., Chem. Umschau Fette, OZe, Wachse Harm, 31, 197 (1924). (2) Browne, F. L., IND.ENG.CHEM.,25, 835 (1933). (3) Gardner, H. A., "Physical and Chemical Examination of Paints, Lacquers, and Colors," 5th ed., 775 (1930). (4) Gardner, W. H., IND.ENG.CHEM.,23, 1402 (1931). (5) Kirsch, A,, Korrosion u. Metallschutz, 3, 227 (1927). (6) 0 1 - und Farbfilm-Akt.-Ges., German Patent 281,594 (Dee. 12, 1913). (7) Paisley, J. W., IND.ENG.CHEM.,23, 1402 (1931). (8) Toeldte, W., Farben-Ztg., 36, 364, 409, 518 (1930). (9) Wilkinson, J. A., and Figg, E. F., J . Oil Colour Chem. Assoc., 7, 232 (1924). (10) Wolff, H., Farben-Ztg., 36, 505 (1930). (11) Wray, E. I., and Van Vorst, A. R., IND. ENG.CHEM.,25, 842 (1933). REC~IVBID November 20, 1935. Presented before the Division of Paint and Varnish Chemistry at the 13th Midwest Regional Meeting of the American Chemical Society, Louisville, KY., October 31 to November 2, 1936. This paper is based upon part of the thesis mbmitted by William Kappenberg in partial fulfillment of the requirements for the degree of bachelor of chemical engineering at the Polytechnic InRtitute of Brooklyn, June, 1933. Contribution 34, from the Shellac Research Bureau and from the Department of Chemical Engineermg of the Polytechnic Institute of Brooklyn cooperating with the United States Shellac Importers' Association.
