I N 1) ti S T I( I A I, A N D 1’: N G I N 1:. I3 K I N G C H E M 1 S T I1 Y
32
used Rith paints U and C, which also represent high-grade priming paints for iron and steel.
TESTSFOB ADEIERENCE It is not proposed to enter into a detailed discussiuii ol the measurement of adherence. The impact test (S) undoubtedly offers a valuable grading method, particularly where the finished article is subject to handling, but i t seeins that a too literal interpretation of numerical values may well be avoided until more experience with the method is available. In the meantime, it might be suggested that the term “impact resistance” be used in connection with such results instead of labeling them as adherence. The use of qualitative observations to supplenieiit the impact test is worth while. Scratching the coating wit.h a knife and noting whether the paint film can be flaked clean from the metal, or if the surface of the base metal must be abraded in order to remove a complete cross section of the film, furnishes information to the experienced paint man which may be difficult to obtain by any other test.
SUMMARY P’rom the above review, which a t this time i s of necessity based on a relatively limited amount of experimental data,
Vul. 25, No. 1
it is apparent that there is still opportunity for investigation of the subject. This is particularly true now in view of recent developments in the production of new zinc alloys, as well as the much wider range of types of finishing materials obtainable. In so far as cleaning is concerned, zinc surfaces offer no special problems. Mechanical pretreating methods are quite satisfactory and the best chemical methods are undoubtedly equally satisfactory. Chemical etching can apparently be used to advantage to supplement mechanieal treatment. Factory-treated galvanized irons and the new zinc alloys are decided improvements from the standpoint of paintability. As for the selection of finishes, consideration of good practice in primer formulation is just as essential for obtaining good adherence on zinc surfaces as on any other metal. The new synthetic-resin products have shown superior results in many cases and the development of special primers will undouhtedly, sooner or later, eliminate or reduce to a large extent the necessity for the use of surface treatments.
r,r~l~EnaTunr: c ~ ~ , ~ : ~ ( I ) Graveil, J. H., U. S.Patent 1,221,048 (1917). (2) Metal Finishing Research Corp.. I3rit.ish l’aterrt 982,958 (1990) (3) Sohuh. A. E., IND. ENQ.CUEM.,23, 1840 (1931). Wr;cuiumo Auguel 29, 1932
Application of Finishing Materials on Zinc MILFORDIf. CORBIN,The Auk & Wiborg Varnish Works, Inc., Cincinnati, Ohio LTHOUGII zinc is one of the oldest metals, its use for
A
structural and ornamental purposes has been slow in developing. One of the factors responsible for this retardation is probably related to the difficulties presented in the application of finishing materials to its surface. Yyroxylin and oleoresinous types of finishing materials both exhibit certain signs of decomposition or breakdown in contact with rnetallic sine, losing their flexibility arid adhesion. This decomposition is proportional in extent to the time in contact. There have been many theories attempting to exnlain this behavior and various treatments offered to overcome the difficultis. Neverthelws, zinc still presents its age-old problems of finishing. Cadmium, which hes come into use much more recently, is so closely related to zinc that its behavior is similar and similar remedies may be applied. CAUSES01,’BILEAKDOWN OF PAINT FILMS
One theory, a t t e m p t i n g to explain the breakdown of paint films in contact with zinc, a t t r i b u t e s t h e c a u s e to occluded oxygen (or other gases) which, upon aging, comes to the surface and undermines the paint film, thus destroying its adhesion. In the case of oleoresinous matorials, a possible additional cause of breakdown of the film is further oxidat,iou, u-hich would tend to increase the brittleness of the film. If this were the case, it would l o g i c a l l y follow that paint would g r a d u a l l y lose its adhesion when applied to buffed met.al1ic
surfaces, since it has been proved that during the buffing operation a certain amount of oxygen is occluded by the surface. But this is not true. Paint films do not deteriorate more over buffed than they do over unbuffed surfaces. Another theory (2) mentions that zinc presents a smooth surface which offera no pits or pores into which a paint can anchor itself. This probably explains why it is difficult to make paints initially adhere to zinc surfaces, but bears no connection vith the apparent breakdown of the film. The relatively high coefficient of expansion is also offered as a cause (2) for-the lack of adhesion of paint films to zinc. Nevertheless, films possessing an unusually high dopee of elasticity still bcconie brittle and lose their adhesion when applied on zinc. Most t h e o r i e s are similar to those mentioned above, attempting to explain the difficulties upon physical grounds, whereas the action of a paint filin in contact with zinc indicates that a definite chemical reaction takes place. Zinc is a highly electropositive metal and, like aluminum, readily foni~san oxide when exposed to air and moisture. Since zinc oxide pigments, when incorporated into the usual type of oleor e s i n o u s vehicles, cause a hardening effect (decreasing the flexibility and distensibility of the film), there is some ground for tlie belief that. the action of zinc or some compound of zinc cause a similar behavior (whether of accelerated polymerization or otherwise) comparable to that of a metal d r i e r . The breakdown of an oil base material when
1 N D U S T R I A I,
January, 1933
A N D E N G I N E E R I N G C H EIM I S T R Y
applied over ziuc may tlius be a coiltinuation of such behavior. The fact that there is a definite chemical action taking place is evident both by the change effected in the zinc surface after being in contact with a paint film and the complete alteration in the characteristics of tlie film. I n the case of a pyroxylin material, there is apparently a decomposition of the nitrocellulose, and the free acid liberated reacts with the zinc surface. Zinc in this case nrobablv acts as a catalyst. Assumine that there is a ciiemical action takine Dlace betveen a p&t film and a zinc surface, the soluiiOn to the problem seems to be: (1) to incorporate an inhibitor into the pa.int or (2) to change the nature of the zinc surface.
33
over a zinc strip, a section of wliicli w&s scratch-brushed with a fine wire brush. The portion of the film over the scratched surface took considerably longer to break down than that portion over the untreated section. However, in repeating the experiment and allowing an interval of several days be-
USE OF INHIBITORS Work with inhibitors in this lahoratory has beeii iiioatly confined to the pyroxylin type of materials. The addition of a small amount of amyl borate to a lacquer lias been found to retard greatly its decomposition when applied over zinc. This effect is shown by Figure 1. Half of this panel (R) is coated with a regular nitrocelliilose lacquer, and tlie other half ( A ) is coated wit11 the same material t o wliicli about 1
fore applying the lacquer, little difference was noted in the time for the breakdown of the respective films.
FIGURE
2.
ErFECT OF
CHEMICAmY
PRIMING ZINC SURFACE
per cent of amyl borate has been added. It can be seen by bending the ends of this panel that the film containing the amyl borate has remained flexible with good adhesion,’whereas the other film has become quite brittle. It is possible that by the use of such an inhibitor nitrocellulose lacquers may be prepared that will sliow entirely satisfactory adhesion over untreated zinc surfaces. However, to date it is practically accepted that zinc surfaces must be treated eitlier mechanically or chemically before the application of finishingmaterials.
MECEANICAL TaEATMEhT The meclianical method generally used is sand-blasting. This operation roughens the surface, forming pits to which a paint can anchor itself more firmly. The initial adhesion is considerably improved by this method. However, unless the paint is applied immediately after tlie operation, the decomposition is not retarded. This fact is illustrated by the following experiment: A pyroxylin lacquer was applied
TREATMENTS CHEMICAL The same roughening of the surface is acconiplished chemically by etching. Various solutions are used for this purpose, each claiming certain advantages and effects. Perhaps the most effective of these solutions is a dilute chromic acid bath containing a small amount of nitric acid. A practical formula is: 135 grams sodium dichromate, 400 cc. nitric acid, 600 cc. sulfuric acid, and 20 liters water. Contrary to most etching solutions, tliis gives an even crystalline ground which will not show under a paint. .4 brown scum usually appears on the surface when the metal comes from this solution. However, immersion for about a minute in a dilute nitric and sulfuric acid solution readily removes this scum. The plate is then waslied free from acid and dried. This drying is important. The water must either be wiped off by means of sawdust or any other absorbing medium, or be displaced by dipping the plate into a lacquer thinner that is sufficiently miscible with water so as to allow the plate to dry free from contact with water. This process has the same disadvantage as sandblasting in that it is often quite impractical to apply the finishing material immediately after treatment. Another method of t.reating zinc surfaces is by the deposition of E metal or a metal compound on the surface. By this method the paint film is prevented from coming in direct contact with the zinc. The deposition is accomplished either by electroplating or by simple immersion. The immersion method which takes advantage of the potential difference set up in the solution is considerably less expensive and simpler, and for these reasons is most widely used. The most popular solution of this type is a dilute copper sulfate solution. It can be brushed over the zinc surface, or the object can be immersed in the solution, The copper in this case is replaced by the zinc (forming zinc sulfate) and the reduced copper deposits on the surface. Salts of antimony, arsenic, and nickel are also used for this
I N D U S ' L " I A L A N 1)
3,1-
E N G I N E15 I1 1 N G C H E M I
purpose arid are marketed under various trade names. Their effectiveness,altl~ouglisatisfactory .i\.ltilc it lasts, is not permanent. Tlre nretal deposited is ercntunlly a1isorI)ed or gradually iilloys with tlie otlier parts of t.lie zinc, forming a uniform alloy Iraving :i high zinc rat,io. Althoiigli ill a less de-
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FIGUHL: 1. S'AXEL Sr~rr.atrTO Frcuin.. EX(:BPT ?'HZAT C ~ m nL~cuusnDOES NOT CONTAINAwunrn
3
gree, t.liis alloy effects a decomposition of the paint film in the same >say as zinc itself. Brass, electrodeposited over zinc, has been known to (li*awcw awrirently o w a lJeri')fi of Yo far, the most clicrtire metliod is tlie dcliuaitimr of a metal coirinound whieli will n u t allov with the sine and at the same time will be inert. to paint films. Nickel s111fide is a
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No. 1
Sree from oil atid grease before it receives tiis treatment. This can be done in various ways. A caustic soda solution, vapor bath, and cleaning with lacquer t.liinners all give satisfactory results as prelirriinary treatments to the ehemicalpriiiiing solution. The clieiriical-1)riiriing deposit has excellent adliesioii and flexibility, provided it is not put on too thick. A satisSactorJ. deposit
