OCTOBER, 1935
INDUSTRIAL I N D ENGISEERING CHEMISTRI-
tioii on the aluminum alloy surfaces than on the steel surfaces after almost 3 years of exposure; in fact, very lit'tle failure was noted on the structural aluminum. 90special surface preparation was employed in either case. Photographs of a iiuniber of these painted aluiniiiuni and steel sections are shown in Figure 1. Each exposure specimen consiqts of two 6-inch channels 24 inches long, bolted together nith aluminum, galvanized iron, and steel bolts. The four steel section' were coated with the same paint materials used on the aluiiiinuin alloy sections. Conipari~onshave also been made hetx-een the .-erviceability of paint coatings on >tee1and lis-T alloy iii sea water under conditions where they are completely and coiitiiiuowly submerged in sea Ti-ater, and where they are out of the water a t Ion- tide. Test. vere conducted in Black Rclck Harbor near Fairfield, Conn. The completely ininiersed panel. ihon-n in Figure 2 v-ere gireii two coats of an inhibitire primer followed by anticorroqive aiid antifouling paints. The 178-T panels were found to lie in excellent condition, h i t the steel panel had rusted quite badly, particularly near the edges, after 13 weeks of immersion. The alternately immersed panels are shown in Figure 3. The superior performance of the paint coatings on 17s-T (no surface treatment' given), as compared with steel, is evident. In t,hese tests also, the best results were obtained TTith a primer con-
1149
taining zinc chromate follolTed by two coats of aluminum paint, all made with synthet'ic resin-base vehicles. I t is quite important to select the paint' best suited for any given service, whether for aluminum or for steel. In the caw of structural aluminum alloys, exposure tests have shown that t'he best resulk are to be secured through the use of primers with a synthetic resin varnish base containing a substantial amount of zinc chromate in the pigment content. =Is in the case of structural steel, aluminum paint top coats were found to giye t,lie best service; good syrrthetic reqin \.ariiiJi wliicles are to be preferred. A n example of a practical u-e of such a teiii i.: to be found in the painting of the aluminurii floor section of the Smithfield Street, Bridge in Pitt+burgh, as dewihed by Reppert ( 5 ) . Literature Cited (1) Edwards. J. D.. U. 8 . P a t e n t 1,946,152 (,Feb. 6. 1034). ('2) Edwards, J. D.. and \Tray, R.I.,ISD. ESG.CHEM., 25, 2:3 (1933). 113) I h i d . , ;Inal. Ed., 7, 5 (1935). ( 5 ) E v a n s , U. R . , Trans. Electrociiem. Sac., 5 5 , 243 (1929). ( 5 ) R e p p e r t , C . 11..En7. .\-ews-Record, 111, 611 (1933). (6) Whitmore, hl. R . , ISD. E S G C " n f . , 25, 19 (1933).
RECEIVED April 19, 1935. Presented before the Division of Paint and Tarnish Chemistry at the 89th Meeting of the -4merican Chemical Society S e w York, N.I-., April 2 2 to 2 6 , 1935.
Painting Zinc and Zinc Alloy Surfaces J
H.IRLE1- -4. NELSON
T
WO years ago the author collahorated in
the preparation of a paper ( 6 ) dealing with the more practical aspects of the pretreatnient of zinc surfaces and illustrating the relative adherence qualities of some common structural painting systems on galvanized iron and sheet zinc. Since not much that is essentially new can be added to the practical discussion, the reader is referred to the above-mentioned article for details. In this paper, a brief resum6 nil1 be given to indicate the status of certain practical features, and some theoretical questions will be presented that should be brought' into the open for discussion by paint chemists. The corrosion life of the iiietal itself is generally not a serious consideration in the painting of zinc surfaces, although there is argument for taking a,dvantage of the added protection derived from a paint film. The real commercial problem is that of obtaining good adhesion life in the paint film, whether it is used for added protection or for decorative purposes.
The Sew Jersey Zinc Company (of Pa.), Palmerton, Pa.
This paper consists primarily of a discussion of the factors thought to influence the initial adherence and adherence retention of paint on zinc and zinc alloy surfaces. These are considered i n the light of surface relationships that may exist between the metal and the paint and the possible influence of changes in the chemical and physical nature of the paint film upon aging. Discussion of pretreatments for the metal surface and of painting procedure is also included.
Adherence of Paint
From a practical point of view, we may choose any coiiibina(1) use a primer tion of two alternative positions-namely, with mediocre adhering qualities aiid stress surface treatment of the zinc, or ( 2 ) use improved primers that make it unnecessary to stress surface treatment. Sou- that the paint chemist is giving inore serious attention to the subject of primer formulation, there has been a, distinct shift tom-ard the second position and me shall undoubtedly see more rapid progress in that direction, with less and less need for the precautions that have been advocated in the past. Apparently, litt'le trouble is encountered in the finishing of galvanized structural shapes: such as I-beams, channel bars, etc., provided precautions commonly recommended are taken
to obtain clean surfaces. The ream1 is that the conditions of galvanizing on such parts result in the formation of heavy zinc coatings with more or l e v rough surfaces, free froin glossy .-pangles. If the exposure conditions are extremely severe, an etching treatment may be in order, but care in selecting a good finishing system is always important. Galvanized metal of the usual smooth, spangled type requires more attention either to the surface coriditicni or to the inherent adherence properties of the priming coat. Only with a good primer is mere cleaning of the surface sufficient for best results. With the ordinary types of metal protective paints, it is desirable both to clean and etch, or treat, the surface. The preferred methods, which accomplish both
ISDUSTRIAL AND ENGINEERING CHEMISTRY
1150
cleaning and etching in one operation, aside from certain proprietary methods and other methods described in the patent literature ( 3 ) are: (1) Light blasting, preferably nith a fine iharp grit of about 50 t o 90 mesh. (2) Treating with an acidulated toluene-alcohol-carbon tetra-
chloride mixture, as described in the previous publication (6). (This mixture should preferably be made up and used as 1% anted.) (3) Natural reathering for 3 to 6 months, wheie conditions are favorable, as on roofs and exposed sides. The removal of water-soluble salts or loosely adhering deposits remaining from any etching treatment should always be emphasized. This is likely to be one seriouq weakness of the popular treatments which form deposits of other metals on the zinc surface. Since sheet zinc and zinc alloy sheet5 of the Zilloy type, when used outdoors, are seldom painted either for protective or decorative purposes, they need not be discussed here. Because of the rapidly increasing commercial significance of zinc-alloy die castings, finishing systems for these have received considerable attention and active investigativn of this subject i s under way in many laboratories. Since most diecast articles are designed for interior service, the quality may vary over a wide range, from very moderate adherence to the impact resistance required by practically constant handling. However, the rapid drying generally required of the finishes used on such surfaces makes the obtaining of good adherence somewhat difficult where air-drying must be used. Where forced drying and baking can be applied, superior adherence is more easily attained, and quality results can be obtained with a good primer without treatment other than cleaning. Nevertheless, articles intended for severe handling still deserve the most effective pretreatment (preferably blasting with fine grit)l plus a baked primer. I n choosing temperatures for the baking schedule, possible damage to the alloy by excess heat must be avoided, and the producer of the alloy should be consulted in case of doubt. Generally, 100" to 150' C. (212' to 302" F.) for a total time of 2 hours would be considered a safe limit.
VOL. 27, S O . 10
ucth. but the nature of the method makes it difficult to apply directly in the case of die-casting alloys. Laboratory weathering tests as well as outdoor exposures (45") can be used to advantage to accelerate tests on finishes for outdoor service. For interior adherence service tests, a special laboratory cycle has been used, which is helpful although not enough time has elapsed for a thorough correlation with service. This cycle is given in Table I. If initial adherence was a safe indication of quality, the range of acceptable finishing systems would be broad enough to satisfy anybody. Khile there are finishes that have very good adherence retention there are many more that have their hour of good initial adherence and then fail. Unfortunately, also, there is no known treatment for the metal that mill come completely to the rescue and improve the adherence retention of a finish that is inherently weak in this respect. Any attempt a t an explanation of initial adherence and adherence retention leads to involved theoretical consideration of (1) selective adsorption, (2) progressive changes in the paint film,(3) disturbance of the oxide film normally existing a t the surface of the metal, and (4)electrolytic attack of the metal. These topics cannot all be adequately covered in one paper, yet even limited discussion of them here may aid in giving direction to the work on finishing systems with improved initial adherence and adherence retention.
Selective -Absorption It is probable that selective absorption of individual con-
stituents from the vehicle by the metal surface, or the oxide film generally conceded to exist on metals, mould have considerable bearing on initial adherence and, indirectly, some influence on adherence retention. Unless the ingredients of the vehicle have been polymerized together to form some uniformity of structure, such selective absorption might conceivably occur rather easily. The most obvious demonstrations of this are the experiments of Bulkleg and Snyder ( 1 ) with mixtures of mineral oils and fatty acids on polished metal surfaces. I n thi- case the selective adsorption of a monomolecular layer of fatty acid from the mixture actually resulted in disruption of the drop of the mixture, owing to changes in surface energy relationships. This example admittedly represents an extreme case of a mixture of dissimilar T.4BLE I. ACCELERATED AGINGCYCLE FOR TESTING -ADHERENCE compounds, but it illustrates a principle that RETESTIONOF FIXISHES o s ZINC FOR INTERIOR SERVICE must be encountered, more or less w h e r e v e r 110' F., loo,% Relatiye mixtures of oils, resins, and thinners, such as ordi120° F., 20%,Relative Day Humidity Humidity 32' F. nary mixed vehicles, are involved. 12:OO P. M. t o 8 : O O A . SI. 8:OO A . M .t o 1:OO P . M. 1:00 P . M . t o 2:OO P. M. Monday I n a paint vehicle, consisting of mixtures of ,.......... 2:oo P. M. t o 1a:oo P. I f . .......... 12:OO P. 11. t o 8:oO A . M. 1:OO P. M. t o 2:OO P. 11. 8:OO A . M. t o 1:OO P. M. Tuesday such constituents of different degrees of polarity, 2:00 P. M. to 12:00 P. M. i l o o ~ . ~ . s h o r t - c h a i n acids would be the most active. 12:OO P. M. t o 8:OO A. x. Wednesday ........... .......... According to the evidence, the particular constitu.......... 12:OO P.M. to 8 : O O A . 11. S:OO A . 11. t o 1:00 P. M . Thursday .......... 1:OO p . 11. to *:OO p . 3%. 2:00 P. M. t o 12:oo P. \I. ent of the mixture that happened to be adsorbed 1 2 : O O P. M. t o 8:oO A . M , 8 : O O A . 11. t o 1:00 P. 11. Friday could have considerable influence on the surface 2:oo P. M. t o 12:oo P . 31. 1:oo P : M.'G iloo P. M. ........... ,......... 8:M) A . M . to 1l:Oo A . M . 12330 Saturday energy relationships between the oriented mole11100 12:00 M , 12:oo M. t o 12:m P. M. ... 12:oo P. M. t o 12:oo P. M. Sunday cules a t the surface of the metal and the rest of the paint film; nor should it be out of the question to determine which constituent this would be, since, as in the case of the Bulkley and Snyder experiments, Zinc or alloy sheets subjected to drawing operations after the most active compound m-ould logically tend to monopofinishing require careful attention to cleaning and etching, lize the surface of the metal. This tendency should be parbut it is obvious that the ability of the system to withstand ticularly strong where the metal surface is covered by an drawing depends largely on the plastic qualities of the finish. active oxide film, as would exist on zinc. I n testing, much use has been made of an impact machine A well-known case of poor initial adherence to zinc, that of the Bell Laboratory type for measuring changes in adhermight be explained on this basis, is that of a straight (resinence, but the simplicity of the ordinary knife scraping test free) nitrocellulose lacquer. Yitrocellulose is relatively nonand the bend test encourages their use in spite of their obvious polar, while many of the solvents used in lacquers are rather limitations. The method recent'ly described by Courtney and polar, so that the lack of adhesion might be due t o concentraWakefield ( 2 ) should be useful for direct study of sheet modtion of the more polar thinners a t thesurface of the metal. 1 Recent reports indicate that commercially satisfactory results are being This leads to the further thought that the constituent of obtained n i t h some proprietary treatments, notably where a light phosphate the vehicle that is most polar to zinc (or zinc oxide) is the one treatment is applied under properly controlled conditions. .
I
OCTOBER,-1935
ISDUSTRIAL AND ENGINEERISG CHEMISTRY
that determines the adhesion, not by any lack of adhesion of the adsorbed molecular layer to the metal or oxide film, but by the degree to which it represents or becomes associated with the longer chain compounds, which give to the paint film its real cohesive strength. That is, there may be undesirable concentration of the “weaker stuff” a t the metal surface. From this, we can go on to the idea that metal primer rehicles should contain a preponderance of long-chain compounds that are definitely polar. JTe are not lacking in experiinental evidence to support this idea, for the best adhesion and adhesion retention results on zinc surfaces point strongly to primer vehicleb formulated with a resin in which an acid (COOH) group is found a t the end of the long-chain polymer, with other points of polarity distributed throughout the molecule. At a previous meeting of the AMERICA^^; CHEMICAL SOCIETY the influence of metallic powders in primers on the adhesion to a metal similar to the powder, was commented upon by some one as a curious fact even in cases of metals other than zinc. It may seem far-fetched, but we might suggest that the notable improvement in adhesion obtained when metallic zinc powder (zinc dust, which will always have a film of zinc oxide on the surface) is used in oil primer. for zinc, is partly due t o selective adsorption on the metallic powder bf certain vehicle constituen& that are normally detrimental to adhesion if adaorbed on the metal. Thus the surface of the metal is left free t o take on more desirable constituent.. This might be popularly restated by saying that the zinc dust takes on the role of a “scavenger” in the paint film. There are other possible explanations for the peculiar effects of zinc dust, which will be mentioned later. The improvement of adhesion usually obtained when primers are baked on zinc surfaces (and on other metals) can be pictured by considering that the baking operation polymerizes the entire vehicle structure and knits the rest of the paint film to the oriented layer next to the metal. This would account for good initial adherence as well as adherence retention. The better adherence retention could also be accounted for on the basis of greater imperviousness of baked films, which would retard corrosion of the surface, as well as on the baqis of modification of the decomposition products formed m-ithin the film as the organic matter ages. Unfortunately, no data seem to be available on the amount and nature of the decomposition products formed in baked films, such as we have for air-dried oleoresinous films, but it is reasonable to believe that the baking operation “jumps” the most active oxidation stages, during mhicli tlie formation of moisture, acids, and peroxides are known to take place in air-dried films. More will be said about this later.
Disturbance of the Oxide Film In the field of metal corrosion, increasing importance is attached t o the nature of the film (4)that is geiierallyacknowledged as existing on every metal surface. This film, which i5 usually an oxide, may be more or less stable, depending upon its chemical nature (solubility) and the environment to which it is exposed; but much hinges on its relative stability. If it withstands tlie attacks of its environment, such as corrosive agents that penetrate the paint film from the outside, or the conditions set up by the presence of the paint film, or if the reaction of the paint with it increases its resistance t o attack, the oxide layer can protect such metal areas as would be subject to anodic attack if it is disturbed and thus retard, or inhibit, electrolytic action. This is known as anodic control of electrolytic corrosion, as contrasted to minimizing or preventing the action of oxidizing agents (depolarizers) a t cathodic areas. which is termed cathodic control.2 2 The author does not consider it nithin the scope of this paper t o discuss the details of the possible distribution of anodic and cathodic areas, or the
1151
T h e n this surface film normally consists of a chemically active oxide, as in the case of zinc, it is obviously sensitive to a n acid environment. Some investigators, in working 011 the development of pretreatments for zinc surfaces, have realized the significance of this and have sought to replace the oxide film with adherent films of greater stability in the presence of acids, but the general tendency evident in the patent literature is to emphasize insolubility of the coating in water, which obviously may not indicate any particular merit. Decomposition products formed as a paint film ages have already been mentioned. The author, in a recent paper (j), emphasized the fact that both acid decomposition products and hydrogen peroxide, an active cathodic depolarizer, are present. The first can promote corrosion a t the anode by increasing the solution pressure of the metal; the second by depolarizing hydrogen a t the cathode. The action of the acid decomposition products might also be looked upon merely as a case of continued chemical attack on the original oxide film, which attack finally breaks the bond between the oxide film and the metal. However, we must keep in mind that a normal priming paint carries what would seem to be sufficient free acid to react with the oxide film on a well-cleaned zinc surface and, furthermore, more acid vehicles should then show- Door initial adherence. On the other hand, there is no o6vious relation between the acidity of the vehicle and adherence, so it is probable that progressive loss of adherence involves some electrolytic attack of the metal itself, which tendency depends on the solution pressure of zinc in the material with which the metal comes in contact and is stimulated by any depolarization that mag take place a t cathodic regions. It is significant for this point of view that the one pigment that has the most beneficial effect on adherence retention, particularly of air-drying oil primers, in which acid decomposition products are known to be formed, is metallic zinc powder (zinc dust), which reacts with acids and moisture to form hydrogen. It could thus serve the dual purpose of (1) reacting with acid decomposition products and (2) increasing the hydrogen concentration in the paint film. The latter would, by mass effect, repress the discharging of hydrogen a t the cathode, as well as react with any peroxide in the film with which it comes in contact.
Shrinkage of the Paint Film One other important factor in adhesion retention is shrinkage of the paint film. There ha> been an inclination t o charge loss of adhesion on zinc to the higher coefficient of expansion of the metal. However, this could hardly be much of a factor on galvanized iron, for example, and it is more likely that the blame must rest with the paint film, because of oxidation sh,inkage of the vehicle and temperature effects on hardness. It has also been suggested that it is still worth while to give attention to the old-time principle of building up the finishing system on the basis of a flexible primer and a harder intermediate coat, followed by a more flexible finish coat. Unfortunately, there is little opportunity in the modern scheme of finishing t o utilize such three-coat systems. Finally, as for selection of finishes and finishing systems, consideration of good practice in primer formulation is just as essential for obtaining good adherence on zinc surfaces as on any other metal. In any case, a better understanding of what goes on within that interesting region between the paint film and the metal is es.entia1 in developing primers with better adhesion. possibility of areas changing from anodic to cathodic and vice versa, depending on changes in the environment a t the surface of the metal as corroaion progresses. The reader is referred to the recent literature on corrosion.
\-oL.2 7 . NO. 10
INDUSTRIAL AND ENGIXEERING CHEMISTRI-
1152
Literature Cited
Gravell, [hid., 2,005,780 ( J u n e 25, 1935). (4) Hedges, "Protective Films on Metals," London, C h a p m a n and H a l l , 1932. ( 5 ) Xelson, ISD. ESG. C H E ~ I 27, . , 35 (1935~. ( 6 ) Nelson and Kittelherger, I b i d . , 25, 27 ( 1 9 3 3 ) .
Ihid., 1,973.Gl3 (Sept. 11, 1934) :
(1) Bulkley a n d Snyder, J . Ana. C h e m . Soe., 55, 194 (1933). (2) Courtney a n d Wakefield, J. IXD.Exc. C'HEZI., 6,470 (19341. (3) Gravell, J. H., U. S. P a t e n t 1,231,046 (April 3 , 1917); T r i m s , JT. W.,British P a t e n t 362,956 (August 8, 1930); P a r k e r R u s t proof Company, Ibid., 346,401 ( J a n . 10, 1 9 2 9 ) ; Green a n d Eokelman, Canadian P a t e n t 314,036 (March 4, 19311; Pyrene C o m p a n y , Ltd., British P a t e n t 394.211 (Jan. 21, 1 9 3 2 ) ; T a u n e r , U. S. P a t e n t 1.911,537 ( M a y 30, 1 9 3 3 ) ; Corhin,
R E C E I V EM D a y 13, 1935. Presented before the Dirision of Paint and Varnish Chemistry a t the 89th Meeting o f the .irnerican Chemical Society. New lork T I.ipril 1 2 t o 2 6 , 1933.
T
HE tobacco industry has a rich background of empirical knowledge and information which awaits correlation with more exact scientific criteria such as bhe chemical composition, physical properties, and physiological behavior of tobacco as a plant. The ektablishment of such critdria is essential to the development of a worthwhile program looking to the improvement of the plant through cultural practices, selection, and fertilization. This paper presents in condensed form the results of a detailed survey, extending over five years, of fluecured tobaccos from representative tobacco markets in the entire flue-cured tobacco region from southern Virginia to Georgia. I n quantity and in rnoney value flue-cured tobacco is our most important tobacco crop. As material for this survey samples of a medium grade of tobacco used in cigarette manufacture were chosen. Their prior history is briefly as follows: After being harvested green by the farmer, the leaves are subjected to the thermal process known as flue-curing ( 3 ,6 ) by the farmer and are then Rold on
TYPE-AREAS OF THE FLUE-CURED
Flue-Cured Tobacco Factors Determining Type and Seasonal Differences
,. F. R. DARKIS, L. F. D I X O S , AND
P. RI. GROSS
Duke University. D u r h a m , N. C.
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STATE BOUNDARIES TYPE-AREA L I N E S PHYSIOGRAPHIC DIVS.------
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THERMAL LINES RAINFALL L I N E S
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the market by him. The purchaser subjects them to further heat treatment and drying. This latter operation is known as redrying. Both the flue-curing and redrying operations are reasonably well standardized in practice. After being subjected to these processes, the tobacco is in the condition in which it is stored for aging. S a m p l e s taken a t this stage were utilized for this survey. Although the coniposition of the tobacco in this state differs considerably from that of the uncured green leaf, it represents the stabilized form or condition of the tobacco which most closely approaches that in which it is empirically judged as an agricultural commodity. T h e c o m p o s i t i o n a t t h i s stage can be related to the growing conditions, the soil, and the seasonal and climatic factors which influence e a c h t o b a c c o type.
