Reaction between Paint Films and It had been noticed in many cases in which the paint film fell away from the galvanized surface in large sections that there seemed to be a very thin layer of white material adhering to the paint film. Preliminary tests seemed to indicate compound formation between the paint film and the zinc (9). However, no systematic work had been carried out to prove or disprove the presence of reaction products at the interface between the zinc and the paint. This study was made with the idea of definitely settling this question.
The failure of paints to adhere properly to zinc or galvanized surfaces may be due to a combination of causes. I t has been assumed that one of the possible causes was reaction between the decomposition products, formed in the drying of the paint film, and the zinc surface, producing compounds at the interface which destroyed adhesion. This study has shown definitely that reactions can take place between zinc and the oxidation products formed in the drying of oil or varnish type finishes, and that compounds formed as a result of this reaction are actually present on paint films which peel from galvanized surfaces. Zinc formate has been definitely identified as one of these compounds.
I N ORDER to observe small effects and to isolate any Q reaction products, very thin zinc mirrors on glass were
HENRY J. WING' Antioch Industrial Research Institute, Yellow Springs, Ohio
OG
ALVANIZED iron or sheet zinc presents a difficult problem to the paint manufacturer. Paint finishes on such surfaces have long been known to have a useful life of unpredictable length but usually much shorter than the life of the same finishes on other surfaces. A number of different theories have been advanced to explain the loss of adhesion of paint to zinc surfaces. The theory which has been tacitly accepted by the trade is that the zinc surface is too smooth and that, if it were rougher, better adhesion would be attained ( l a ) . A second explanation, which has received less support in practice, assumes that the lack of adhesion is due to the formation of a compound a t the interface between zinc and the coating (4). A third explanation is that the zinc destroys the normal orientation of the molecules toward the metal surface with a resultant loss in adhesion (12). Most of the remedies which have been proposed in hopes of prolonging the life of paint on zinc depend on roughening the zinc surface. These range from the familiar vinegar rinse to compliced chemical, electrochemical, and heat treatments (13). However, a t the time of undertaking this study, no completely successful method or material had been developed for treating galvanized iron so that a paint would have as long a useful life on it as on any other surface. Many times finishes over galvanized iron will seemingly stay in good condition for comparatively long times and then suddenly lose all adhesion. This was particularly noticeable with a wide variety of paint materials over a large section of the eastern United States in the winter of 1933-34 (7). The unusual condition seemed to be a period of high precipitation followed by a period of very low temperatures. 1 Preaent address, E. I. du Pont de Nemours & Company. h a . . Parlin, N. J:
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made by high-vacuum distillation (1, 2, 6, 10, 16-20). It was discovered that by the use of t h e proper vacuum (about l o w 4mm. of mercury without freezing out the mercury vapor, using only water cooling on the mercury vapor trap) mirrors could be made very rapidly, the actual distillation taking place in less than half a minute. Figure 1 is a drawing of the high-vacuum chamber. This chamber was sealed to the vacuum system consisting of a mercury vapor trap, mercury vapor pump, and oil forepump, together with McCleod gages for measuring the high vacuum and forevacuum. By use of a megavac forepump and a three-nozzle mercury vapor pump (8), the whole system could be pumped down to the required vacuum in less than 10 minutes. This method differed from that reported by van Wijk (17) in that the distillation was carried out a t a comparatively high pressure (only water cooling was used on the mercury vapor trap) and that the distillation was very rapid. The mirror formation was dependent on a high concentration of zinc vapor, which method van Wijk found unsuitable for his apparatus. The mirrors were not of uniform thickness but this was no disadvantage in these experiments. The average t h i c k n e s s was of the order of mm. Each mirror was formed from 1.0 to 2.0 mg.
It has long been known that linseed and other drying oils give off various acid products during the drying process ( 3 , 6 , 1 1 ) . In order to learn something of the reactivity of these vapors and to endeavor to identify the products of the reaction with zinc, two coats of clear spar varnish mp were brushed on a Cellophane sheet 30 cm. square. After drying 24 hours, this coated sheet was cut into strips and the strips were for the rioo. coiled i n s i d e a 10-cm. a-~i~~~p/a+e, c r y s t a l l i z i n g dish. A lor Mirror mirror, s u p p o r t e d zinc side up, was placed in the center of the dish. This dish was then placed ins i d e a n o t h e r 1 -cm. FIGURE 1. HIGH-VACUUM crystallizing dish.yac>e CHAMBER annular space betwedn'the two dishes was filled to a depth of about 1.5 em. with distilled water. The whole was closed with a flat glass plate and placed in a constant temperature oven a t 40' C. In
Zinc Surfaces this way the varnish film was exposed to a high-humidity atmosphere a t an elevated temperature. This was done to accelerate the oxidation and decomposition of the film. Within 2 hours the a.ttack on the zinc could be noted by the gradual disappearance of the mirror. In 24 hours or less the zinc had conipletely disappeared and a white deposit was left. This deposit was peculiar in that it usually did not occupy the whole urea of the original mirror and in many cases exhibited a beautiful dendritic growth. Figure 2 shows a photograph of a typical deposit and a photomicrograph between crossed nicol prisms of the crystal growth. The white deposit wa.s soluble in water and charred and burned when heated. Since it was undoubtedly a zinc salt of one of the low-nrolecular-weight aliphatic acids, its identification was accomplished by comparing its properties with those of the zinc salts of the lower fatty acids. Its melting point was greater than 250" C. Those of laboratory preparations of various zinc salts were: zinc propionate, 192197' C.; zinc scet,ate, 238-240O C.; and zinc formate, >250° C. A microscopic examination showed crystals very similar to those of zinc formate. Figure 3 shows a photomicrograph of the recrystallized zinc formate aiid of the unknown salt. These show a great similarity.
HOWEVFR the hest proof of the identity of this salt Q with . zinc foriimte was a chemical test. An examination .
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of the lit,eratwe (f4)and actual tests on the zinc salts of formic, acetic, propionic, and n-butyric acids showed that the formate was the only salt of the lower fatty acids which reacted rapidly with sulfiiric acid-potassium dichromate solutions. This reaction was easily carried out by placing a small drop of concentrated sulfuric acid on a slide and placing beside it a similar drop of saturated potassium dichromate solution. These two were mixed, and small portions of the salt under examination were added to the drop. If the salt was a formate, a reaction took place. Cas bubbles were given off and the drop became green in color. Salts of the other acids did not show thisreaction. This test could be used as a chemical microscopic test by adding a very small drop of the mixed solution to the solid m a t e r i a l and then observing the gas evolution t h r o u g h a microscope. In this way it was possible to show the orwnce of
white product formed by the reaction of the gaseous products froin spar varnish with the zinc mirrors, enough material could be easily collected so t h a t microscopic observation was unnecessary. This white material exhibited definitely the zinc formate reactions. Tests were also carried out in which the varnish film was in d i r e c t contact with the zinc surface. Here, too, zinc formate was identified by its reaction with the potassium dichromate reaaent. This test vas a p plied tosmall sections of p a i n t films, both those which failed on galvanized iron a n d those which failed on plain iron that carried no zinc. The side of the films which had been next to the galvanized iron gave a good r e a c t i o n f o r formate by this test, but neither the other side nor either side of t h e filii1 f r o m t h e
FROM . these. tests . it' may be concluded, when paints of the oil or oil-resin type are applied to a zinc surface, that a reaction may he set up in which one of the principal products is zinc formate. The presence of this material a t the interface is probably one of the causes of thc failure of many paints on galvanized iron.
Literature Cited
A,, Natl. Psiit, Varnish Lacquer .4ssoo.. Circ. 451,
Ho, T.L.,Rev. Sei. Instrurnats, 3, 133 (1932). Iliff. J. W.,private communication. Knudsen, M., Ann. Physik, 50, 472 (1916). Long, Reineok. and Ball, IND.ENQ.Camw., 25, 1086 (1933). Nelson, H. A., Ibid., 27, 1149 (1935). Nelson, E. A,. and Kittleberger,Bid., 25, 27 (1933). Polinski, M., O h m . A i d y s t . 9, 4 (1920). Ribchl, R., Z. Physik, 69, 578 (1931). Tiede and Birnbrauer, 2.m w g . Cham.. 87, 129 (1914). Wijk,W.R. vm, IND.ENQ.CHEU.,And. Ed., 7,48 (1935) Williams,R. C..Sci. Monthly, 39, 274 (1934). imj Williams and Sabine. Astrwkt,8. J.. 77, 316 (1933). Phil. Mag.. I61 32, 364 (1916). (W)Wood,R. W., Rmcmveo AugUst 6. 1936. Presented beiote the Uiviaioa of Paint sad varniah Chemistry at the 90th Meeting o i the American Chemioal Sooiety. Sam Franoisoo, Calli.. August 19 to 23. 1935. This w r k wv88 done under B mant fiom E. I. du Pont de Nemaurs & Corn~any,Ino.. to ths Xntiocb Industrid RePearoh Iastitute.
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