INDUSTRIAL AND ENGINEERING CHEMISTRY
348
Vol. 21, No. 4 .-
A Study of Lithopone Darkening’ J. H. Goshorn and C. K. Black OHIO STATEHIGHWAY TESTINGLABORATORY, COLUMBUS, OHIO
T IS a well-known fact that lithopone pigments darken This shows that some of the sulfur is in a different state on exposure to ultra-violet light, especially in the presence than that of being combined with zinc as zinc sulfide. of moisture. It is also well known that these darkened The next experiments were made in an attempt to deterpigments will whiten again if kept in the dark. The current mine the free extractable sulfur in exposed lithopones. The explanation of this phenomenon as given by O’Brien,*Cawley,3 extractions were made with carbon tetrachloride in a Soxhlet D u r ~ t Job , ~ and E m s c h ~ i l l e r ,Weiser ~ and Garrison,G and extractor. A blank run on unexposed lithopone showed no others is that the zinc sulfide is reduced to metallic zinc, material extracted. Duplicate runs made on material excausing the dark gray color. This metallic zinc is in turn posed for 2 hours (stirred every 5 minutes) and then extracted oxidized to zinc oxide and thus the white color reappears. 7 hours gave very concordant results when made under It was observed in this laboratory that a panel painted with exactly the same conditions. The rapid gain in weight of lithopone paint and exposed in part to ultra-violet light the extraction thimbles in the air made a standard method showed the characteristic darkening on the exposed part. of procedure necessary. The approximate weights of the When kept in the dark the exposed part whitened again. If thimble and contents were placed on the balance pan. The the entire panel was exposed to ultra-violet light, that part thimble was heated in an oven a t 100” C. (212’ F.) for 10 which was previously darkened darkened even more readily minutes and then rapidly weighed, the process being repeated than the unexposed section. Ordinary zinc oxide does not until concordant results were obtained. darken on exposure to ultra-violet light, therefore this work Some metals are known to greatly influence darkening, so was originally undertaken to study this redarkening. in these experiments not only pure lithopone, but also lithopone containing varying amounts of lead sulfate were used. Apparatus and Materials In each case 4 grams of lithopone were used. To separate The source of ultra-violet light was a Cooper-Hewitt labora- samples were added 0.25, 0.50, and 1.0 gram lead sulfate, tory mercury arc lamp. A lithopone of standard make respectively. After exposure for 2 hours approximately a was used. The exposures of painted panels were made in a 2-gram sample was used for extraction. Below are the shallow pan containing sufficient water to cover the panels to percentages of sulfur obtained: a depth of 6 mm. inch). The non-paint mixtures used Per cent Lithopone 0.23 consisted of the dry powders, thoroughly mixed, covered with Llthopone + 0.25 gram PbSOl 0.15 Lithopone + 0.50 gram PbSOI 0.11 water, and exposed in 25.4-cm. (10-inch) watch glasses.
I
Lithopone
Method
Maas and Kempf7 explain darkening as due to the formation of metallic zinc and zinc disulfide. O’Brien2sags that metallic zinc and hydrogen sulfide are formed. Thus the question of the disposition of the sulfur seems to be unsettled. Ultra-violet light exposure tests were made on (1) pure zinc oxide, (2) zinc oxide intimately mixed with sulfur, and (3) zinc oxide covered with a hydrogen sulfide in water solution. I n the case of the pure zinc oxide no darkening occurred, while with the other two mixtures darkening did occur, being more intense with the zinc oxide and sulfur. As this result indicated that sulfur must play some important role in the re-darkening of lithopone, a n attempt was made to find a difference in the free-sulfur content of exposed and unexposed lithopones. Four grams of lithopone covered with water were exposed t o ultra-violet light for 10 hours. The amount of zinc sulfide remaining mas then determined analytically as in a paint pigment analysis, * and compared with the amount present in an unexposed sample. Below are the results: Per cent Unexposed Exposed
23 05
Difference
6.05
29 10
Received September 21, 1928. O’Brien, J . Phys. Chcm., 19, 113 (1915). * Cawley, Chcm. News, 44, 51, 167 (1888). 4 Durst, Z . angew. Chem., 35, 709 (1922). b Job and Emschwiller, Comfit. rend., 177, 313 (1923). e Weiser and Garrison, J . Phys. Chem., 31, 1237 (1927). 7 Maas and Kempf, 2. angew. Chem., 36, 293 (1923). 8 Gardner, “Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors,” p. 707. 1
2
+ 1.0
gram PbSOi
0.025
The extract liquor was evaporated to dryness each time and tested qualitatively for sulfur. From these results it appears that lead, when present, combines with the sulfur. Linseed-oil solutions of lead linoleate drier were made up in concentrations of 0.01, 0.02, 0.04, and 0.06 mol of drier per 1000 cc. oil. With these oils, and also a raw oil containing no drier, lithopone paints were made up containing 50 per cent pigment, and panels covered with these paints were partially exposed to ultra-violet for a period of 10 minutes. There was an increase in darkening with increase in amount of lead, which supports the theory that the darkening in this case is due to the formation of lead sulfide rather than any catalytic activity. The darkened paints containing lead will not whiten as will a pure lithopone paint. It was thought that possibly the zinc oxide in re-whitened lithopone could be separated by differences in solubility. Lithopone was exposed, allowed to whiten again, and then thoroughly extracted with water. A blank of unexposed lithopone was extracted in like manner and the respective filtrates from the exposed and unexposed samples were analyzed for zinc. Y o zinc could be detected with hydrogen sulfide in the unexposed sample, while the exposed sample gave the equivalent of 0.47 per cent zinc oxide. On the basis of the theory of lead sulfide formation, it was thought possible to explain darkening and protective action of other metals on the basis of sulfide formation. The salts of various metals were intimately mixed with lithopone and exposed in water to ultra-violet light for 30 minutes. I n the case of aluminum acetate no darkening occurred. Aluminum sulfide would not form in water solution, but aluminum hydroxide probably would. This material would effect a protective action. I n the case of cadmium chloride the mixture became extremely yellow accompanied by some zinc
I-VDUSTRIAL AiVD ENGINEERING CHEXISTRY
April, 1929
darkening. Calcium oxide showed no color and no darkening. Ferrous sulfate formed a yellowish green mixture. Ferrous sulfide is black, but ferric sulfide is yellowish green. Ozone present around the light has probably oxidized the material. Mercuric chloride caused the mixture to turn black after exposure. The mixture was yellow before exposure. From these examples it seems that the sulfide of the metal is formed when possible under the conditions of the experiment. If this sulfide is dark, it increases darkening, if light, it may partly overcome the darkening due to metallic zinc. The color of the mixture depends on two factors, color of sulfide and quantity of metallic zinc present. Conclusions
From the results of these experiments, in which the darkening of lithopone has been driven as near as possible to its
349
ultimate end by means of intense ultra-violet light in the presence of moisture, the following conclusions might be inferred: The darkening of lithopone is due to the reduction to metallic zinc and probably free sulfur. The whitening is due t o oxidation of zinc to zinc oxide. The redarkening may be due in part to the influence of the free sulfur, which causes the zinc oxide to darken, and in part to the decomposition of more zinc sulfide. Metals present tend t o form the sulfide and darken in the case of dark-colored sulfides, but protect in the case of light-colored sulfides. Such darkening does not bleach out as does zinc darkening. Quantities of metal used in driers (0.5 per cent or less) are sufficient to cause sulfide darkening. The darkening with a metal present is probably due to both causes, the formation of metallic zinc and a dark-colored sulfide.
Alkyd Resins as Film-Forming Materials‘ R . H. Kienle and C. S. Ferguson GENERAL ELECTRIC COMPANY, SCHENECTADY, N. Y.
VERY material t h a t In discussing the use of alkyd resins as film-forming of the film-forming possibilimaterials, differentiation is made between three general ties of these resins. They approduces a transparent, homogelleous film types, namely, heat-non-convertible, heat-convertible, p a r e n t l y were content to and oxygen-convertible. The preparation of satisfactory study the reaction as such and upon e\Taporation of the solvents from its solution sooner solutions of the last two types and the properties of the were satisfied when they got or later comes under investiresultant films are described. It is shown that the a resin. oxygen-convertible resins bear serious consideration as D u r i n g 1911-1915 Callagation as to its applicability in the paint, v a r n i s h , a n d film-forming materials as they possess both the quickhan7 a t the Pittsfield works drying characteristics of nitrocellulose lacquers and the l a b o r a t o r y , together with l a c q u e r industries. If the film-building properties of oil-base varnishes. A r s e m , * D a w ~ o n , a~n d material is a resin 01: possesses Howelllo a t the Schenectady resinous characteristics, it is customary to examine it as a film-forming material (1) when research laboratory of the General Electric Company, carried used alone; ( 2 ) in conjunction with other resins or gums; (3) out an extensive investigation into the glycerol-phthalic blended with drying oils; (4)as an ingredient in nitrocellu- anhydride reaction, and as a result new and useful resins were lose lacquers. The alkyd resins are no exception and are made. They became particularly interested in the resins bebeing examined from all these angles. However, a t this time cause of their heat irreversibility. they will be discussed only from the standpoint of their filmCallahan devoted most of his attention to the glycerolforming ability when used by themselves. phthalic anhydride reaction and its possible application. Alkyd resins include all those complexes resulting primarily hrsem and his co-workers studied the alkyd reaction as a from the inter-reaction of a polyhydric alcohol and a poly- whole and the preparation of numerous other resins based basic acid, and the term has therefore a definite scientific on this reaction-i. e., they replaced the phthalic anhydride meaning. in whole or in part with other polybasic acids and in part with some monobasic acids. In addition they studied flexiHistorical bilization. working chieflv with castor oil as the flexibilizine The formation of alkyd resins has long been known. Ber- agent. They ascertained many characteristics of the resins and pointed out the possibility of using them as film-forming zelius reported a resin from tartaric acid and glycerol. Van materials. They found the resins to be extraordinarily good Bemmelnz probably did the first systematic work. He prepared resins from succinic acid, from citric acid, from a mix- stickers and, working with solutions of the resins, they obture of benzoic and succinic acid, by heating with glycerol. tained very adherent, tough, varnish-like films on metals if Other early investigators were D e b ~ sL, o~~ r e n c oand , ~ Funaro these films were properly baked. They only worked with and Danesi.j Smith6 was the first to prepare a resin from a few simple solvents, such as acetone, alcohol-benzene, and glycerol and phthalic anhydride. His product, however, on coal-tar oil-alcohol. With these solvents films of poor bodyheating puffed up into a brittle, vesicular mass, which was ing characteristics and with decided tendencies to pull up, owing to high surface-tension effects, were the best they obuseless from an industrial standpoint. tained. In only one case did they obtain a satisfactory reNone of these investigators seem to have been cognizant sult. Dawson, working with alcohol-benzene solutions of a Presented before the Division of Paint and Varnish Chemistry at glycerol-phthalic anhydride-oleic acid resin, was able to the 76th Meeting of the American Chemical Society, Swampscott, Mass., obtain smooth, tough, adherent films on metals, but the filmSeptember 10 to 14, 1928. building properties of this varnish were poor. Neverthe2 Van Bemmeln, J . prakt. Chem., 69, 84 (1856).
E
Y
1
Mag., 4, 1 6 ; Jahrb. for!. Chem.. 1856, 431. Lourenco, A n n . chim phys., 8, 63, 313 (1883). 5 Funaro and Danesi, Jahrb. for!. Chem., 1880, 799. ‘ S m i t h , J . SOC.Chem. Znd., 20, 1073 (1901). a Debus, P h d .
4
7 Callahan, U. S. Patent 1,108,329 (1914), et al. SArsem, U. S. Patent (1914). Dawson, U. S. Patent 1,141,944 (1915). lo Howell, U. S. Patent 1,098,728 (1914).
v
