Film-Continuity of Synthetic
Resin Coatings Test Methods
CONTINUITYTESTAPPARATUS
G. H. YOUNG AND G.W. GERHARDT Mellon Institute of Industrial Research, Pittsburgh, Pa.
I
Film continuity is a discontinuous function of coating weight. Further, it has been demonstrated that the threshold weight of coating material required to just produce continuous films varies directly with the activity of .the metal being coated. This work is being continued.
N THE application of synthetic resin coatings to the
problem of protection of metal surfaces against corrosive influences, a knowledge of the actual continuity of the protecting film is of fundamental importance. The desirability of knowing the actual coverage effectedwith an applied coating without involving a t least partial destruction or questionable degradation of the film in the process of determining continuity is obvious. Serious objection can be raised against use of stripped films and subsequent conventional dialysis technics for determination of ion permeability both on the score of actual mechanical rupture and of minute strains set up in the stripped film, regardless of the method of removal, I n addition, data presented in the present and in subsequent communications show clearly that film properties are fundamentally modified by the nature of the surface on which the film is deposited, so that films stripped from glass, for example, cannot and do not show the properties of a film of the same material deposited on an active metal surface. Serious objection may also be raised against the conventional copper sulfate test for location of pinholes and similar discontinuities. The appreciable time element involved before the plated copper spots became visible, with the accompanying degradation of the edges of the pinhole and undercutting of the adjacent film, usually leads to an exaggerated estimate of discontinuity, Considerations based on the corrosion phenomena actually operative against a metal coated with a protective resinous film have brought about the development of testing methods in this laboratory which are inherently superior to tests on stripped films in that they involve a test of the actual film in place, and are superior to plating methods in that no appreciable exposure time is required. The method is designed for determination of initial continuity only, is practically instantaneous, and in no way subjects the film to abnormal
degradation influences. Tests are carried out in the presence of the material or solution which the coating is designed to protect.
Test for Film Continuity Continuity tests were based on the premise that a resin film which is nonpermeable to positive or negative ions from a solution in contact with the film must of necessity be “continuous” against that solution. If the coated metal is assembled as an anode in a single-cell circuit, the contained solution acting as electrolyte, and a suitable inactive metal such as platinum is used as cathode, then instantaneous proof of discontinuity is given by easily observed deflections on a suitable galvanometer wired into the circuit, if such discontinuity exists. Conversely, the absence of deflections on the instrument with the circuit closed is instantaneous proof that the anode is completely insulated from the electrolyte by the protective coating, and this can be the case only with a continuous film coating. The potential which instantaneously develops within the assembled cell is basically a function of the anode and cathode metals, is independent of electrode areas, and is readily measured with the familiar Leeds and Northrup K-type potentiometer using a d’Arsonva1 galvanometer as null-point indicator. Although the development of a measurable potential under these test conditions serves as a completely authentic test for initial film continuity, no measure of extent of discontinuity, where such exists, can be had because the potential is independent of ex1277
INDUSTRIAL AND ENGINEERING CHEMISTRY
1278
posed area. The current which flows instantaneously, however, is a function both of potential and of electrode area, so that a suitable measure of current can be used for comparisons of exposed anode areas and thus of film porosities. Preliminary experiments showed that use of the familiar current-registering devices was unsatisfactory; the amount of anode exposed even in test sections several inches in area is so small compared to (bare) cathode area that the assembled cell reveals extremely rapid polarization rates, and microammeter readings are indeterminate. Accordingly, use is made of a conventional circuit for determining cell internal GALVANOMETER
VOL. 29, NO. 11
The electrolyte used for routine tests in this laboratory was a 0.1 per cent sodium chloride solution, which was 0.0001 normal in hydrochloric acid and had a pH of 4.2. A 6-inch 20-gage
copper lead was soldered to the coated test panel; contact to the platinum cathode was made through a mercury well. The cell cup consisted of a 3-inch open Pyrex glass tube, 1.78 inch i. d., sealed to the coated test panel with paraffin. The effective area of film exposed to electrolyte was 2.40 square inches; volume of electrolyte was 75 ml.; the area of platinum cathode immersed was 7 square inches; its lowest point was 0.5 inch above the coated test film. A minimum of 5 minutes was allowed to elapse after introduction of the electrolyte and before actually closing the circuit to permit complete wetting of the test film. All readings were repeated at least twice and usually three times to ensure constancy of deflection. The convention used in this laboratory was to measure internal resistance first, followed by the potentiometric determination. The complete wiring diagram or determining both potential and internal resistance is shown in Figure 1.
Minimum Coating Weights for Film Continuity
II ‘
‘ R-r-l
The question of film thickness, or weight per unit of area covered, in relation to ultimate film structure and consequent film properties has long been of interest. With the development of quantitative methods for evaluating continuity, it became possible to study intensively the phenomena associated with coverage on active metals. Such studies have
J
KI
FIQTJRE1. CIRCUITDIAGRAM
resistance by discharge through two different external resistances. By maintaining film test area, volume and concentration of a reference electrolyte, cathode area, and its distance from the coated anode constant, cell internal resistance is a function of exposed anode area, and the current which instantaneously flows can be calculated from the previously measured potential and this resistance by means of Ohm’s law :
where EO I di dz
R Ri
R2 Rint
current, am eres observed degection with Kt closed observed deflection with K1 and Kzclosed resistance factor for use in Equation 2 10,000 ohms fixed resistance 1000 ohms fixed resistance = internal resistance calculated from Equation 2
The circuit diagram is shown in Figure 1. For all practical comparisons it is satisfactory to consider R equal to Rz; a constant error of approximately 10 per cent is thus introduced. Equation 3 shows that, as R1 approaches infinity, R approaches Rz as a limiting value. Lead and galvanometer resistances are neglected as small in comparison with the magnitude of the fixed resistances, R1 and Rz, which were selected experimentally to give maximum deflections on the d’Arsonva1 galvanometer scale. Under these conditions the precision of the measurement is approximately 7 per cent, which is easily within the limits of reproducibility of commercial resin coatings. The accuracy of resistance values is subject to considerable doubt, since theoretical considerations would indicate that measured values are true internal resistances only when R1 approaches infinity as a limiting value.
COATING W E I G H T I N MGMS./SQ IN.
FIGURE 2. RESISTIVITY us. COATING WEIQKT FOR SIX METALS
NOVEMBER, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
1279
TABLB I. COATINQ WDIQHTS OF SIXMDTALS Sample No.
Coating wt. Potential
Mo./sn. an.
Voll
1
10.07
...
2
8.32
0.503
3 4 5 6 7 8 9 Bare metal
7.47 6.77 4.72 3.54 2.15 1.44 0.70 0.00
0.503 0.541 0.526 0.533 0.541 0.633 0.522 0.480
1
10.07
2
8.32
3
6.77
4
4.72
5
3.54
6 7 8 Bare metal
2.15 1.44 0.70 0.00 12.50
2 3 4 5
5.66 3.52 2.68 1.48 1.08 0.58 0.00
6
7
Bare'metal
Ohms Film continuous 7,680
Milliamp.
...
0.066
7,000 3,300 3,000 3,850 2 830 2'410 1'984 1:200
0.072 0.163 0.176 0.138 0.192 0.221 0.264 0.399
Tin Plate Filmcontinuous Film continuous Film con,. tinuous Film continuous , Filmcontinuous 0.562 9,280 0.568 7050 0.676 1:935 1,090 0.487
... ...
... .
...
...
...
...
..
,
..
0.142 0.227 0.182 0.190 0.183 0.224 0.202
CopperFilm continuous 11,000 6,340 8,500 6,000 5,333 1,845 2,020
Aluminum Coating Internal wt. Potentia1 resistance MB./W. Yell m. Ohms 10.07 Film continuous 6.77 Film continuous 0.860 4.72 16,000 0.843 3.54 6,250 2.15 0.842 4,670 0.732 1.39 5,690 0.70 0.818 3,130
. 7
E/R
...
,-1
Iron. Internal resistance
0.061 0.081 0.349 0.437
... ...
o:st)2 -Zinc
19.9
...
...
... i:oi
793
-
(Galvanieed Iron) Thermallv de-
0.083 0.132 0.226 1.61 1.19 1.69 1.91 2.53
..,
12.50
...
0.013 0.036 0.021 0.032 0.034 0.121 0.100
6.66
...
been in progress in this laboratory during the past year and a half, and the data presented are representative of hundreds of individual determinations on thermoplastic resins. Studies on thermosetting films will be reported later. Six different metals were selected for the investigationgalvanized iron, aluminum, iron, copper, tin plate, and platinum. The solution used for film deposition was a typical vinyl thermoplastic resin dissolved to approximately 20 per cent solids in methyl ethyl ketone and thinned with toluene. Coatings were applied to the freshly cleaned panels by slow automatic withdrawal from the solution; weight taken up was controlled by decreasing solids content through dilution with solvent-thinner mixtures. Solution and panels were placed in a glass container fitted with a dust filter and drying tower through which a slight current of air was circulated; this enclosure ensured moisture- and dust-free film conditions. Coating weights were determined by weighing a measured area on duplicate tin plate panels, stripping with solvent, and reweighing. Because of the known differences in thermal stability of vinyl resins when baked on different metals, it was necessary to bake the test films a t varying temperatures to ensure equivalent adhesion and freedom from incipient thermal decomposition on the more active metals. Accordingly, the samples were baked for 10 minutes on the various metals a t the following temperatures: galvanized iron (zinc), 270" F.; iron, 290 O ; tin plate, 300 O ; copper, 325 O ; aluminum, 340 O ; platinum, 350'. Preliminary tests showed that under these conditions the films showed comparable adhesion and general stability. Film continuity was determined as described above; all potentials and resistivities were measured against a platinum cathode except in the case of coated platinum, where bare tin plate was used as the anode, the film acting as a cathode
0.054 0.135 0.180 0.126 0.261
....
9.96 5.66 3.52 2.88 1.48 1.08 0.58 0.00
3.52
...
...
....
... ...
0.00
2.68
E/R Milliamp.
...
., .
1.48
...
1.08 0.58 0.00
0.328 0.345 0.622
Film continuous Film continuous Film continuous Film continuous Film continuous 9,000
8,000 1,370
... ...
... ... 0.036 0.043 0.453
coating. Calculated currents were expressed in milliamperes. No attempt was made to evaluate exposed areas directly. Calculated current data are entirely satisfactory for comparative purposes over narrow experimental ranges where the areas exposed are directly proportional to these currents. Data obtained for various coating weights on the six metals are presented in Table I. Curves showing the relation between instantaneous current and coating weight are shown in Figure 2.
Discussion of Data Of major importance is the established experimental fact that film resistivity is a discontinuous function of coating weight, and that for every metal surface there exists some finite coating weight which must be exceeded before the familiar characteristics of amorphous films are encountered and before continuous film membranes are formed (Table I, Figure 2). Of almost equal importance is the experimentally demonstrated fact that this minimum weight of basic understructure is not constant for all metals but varies widely with the surface being coated. If the six metals are arranged in the order of their position in the conventional electromotive force series, together with the corresponding coating weight required to just produce complete continuity, with only one exception (copper), coating-weight figures fall into line with this order: Metal Surface (in Order of Increasing Activity) 1. 2. 3. 4. 5. 6.
Platinum Copper Tin Iron Zinc (galvanized iron) Aluminum
Min. Coating Wt. Mo./sq. in. 1-1.5 6-8 3-3.5 9.5-10 16-18 5.5-6.5
Approx. Thickness
Mil. 0.05-0.07 0.30-0.40 0.15-0.18 0.48-0.50 0.80-0.90 0,28-0.33
,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
No tenable explanation for the anomalous behavior of copper can be advanced a t present. Additional experiments with metals near copper in the periodic table are being carried out. Results obtained with aluminum are more easily explained when it is recalled that in ordinary experiments with aluminum surfaces we are dealing not with active metallic aluminum, but with an adsorbed oxide film, so that this highly saturated surface behaves as if we were dealing with an entirely different metal entity. Similar results have been obtained with chromium plate and other stainless steels where surface adsorption phenomena modify the behavior of the sorbing metal.
VOL. 29, NO. 11
That the experimental differences between metals cannot be attributed to differences in surface irregularities has been demonstrated in this laboratory. The consistency with which such data can be reproduced and the specificity of behavior on different metals suggest that the basic film structure of thermoplastic resins may not be entirely amorphous or "brush heap" in make-up, as has been generally supposed. Data obtained in this laboratory, as yet unpublished, strongly indicate that film continuity may be a combined function of metal-surface, degree of polarity of the resin, and its molecular weight. RBCBIYBD August 8, 1937.
Effect of Surface Variation on Minimum Coating Weights for Synthetic Resin Films 0. H. YOUNG, W. K. SCHNEIDER, AND G. W. GERHARDT
The effect of surface irregularities on minimum coating weight is to increase the discontinuity on very light films, but there is little effect on the point of minimum continuity. Primarily, the nearer the surface being coated approaches true optical planeness, the less will be the deviation from a linear relation between continuity and coating weight in the transition zone within the film. Magnitude of effective potential between the limits operative in the determination of minimum continuity does not alter the point of minimum continuity; as would be expected, however, slope and intercept on the E / R axis do vary with the effective potential. A tentative theory involving partially oriented film understructure is suggested to account for differences in film continuity and in thermal decomposition phenomena on various metals.
E
XPERIMENTS on air-dried finishes reported in detail in the present contribution show little difference in initial continuity characteristics between baked and airdry thermoplastic films. Six sets of duplicate coated tin-plate panels representing coating weights varying from 2.15 to 10.52 mg. per square inch were dip-coated with a typical vinyl thermoplastic resin formulation. One series of panels was baked 15 minutes a t 300' F.; the other was air-dried for 18 hours. Initial continuity was determined in the manner described in the preceding article. Data obtained on the films are summarized in Table I.
Effect of Surface on Film Continuity
Frequently it has seemed possible that pronounced differences in actual surface characteristics may have a profound influence on film continuity, particularly on light films, Time did not permit a detailed study of specific surface variations for all metals, but these effects were determined on a t least one metal in the experimental series given in the preceding article. I n all previous experiments the panels used were solventcleaned commercial metal sheet, with black iron showing the greatest surface roughness and irregularity in the series of metals investigated. Accordingly, representative panels of the same black iron were selected for the surface study. Surfaces were brought to a high polish by continued buffing with Norton No. 2/0 Metallography paper cemented to a wood-faced block r o t a t i n g a t 1750 r.p.m. ApproxiWEIGHTS OF AIR-DRIED AND BAKED FILMS ON TIN PLATE TABLE I. COATING mately 30-minute polishing was required ,-Air-Dried Film Baked Film to bring each specimen to a mirror Coating PotenInternal Coating PotenInternal Sample No. wt. tial resistance E/R wt. tial resistance E/R finish. The Danels were dip-coated Mq./sq. Mg./sn. a t the indicated coating weighis under Volt Ohms Ohms Milliamp. m. Milliamp. an. Bolt moisture- and dust-free conditions, and Film con10.52 . . . Film con. . 10.52 ... ... baked 10 minutes a t 300" F. Coating tinuous tinuous 8.51 con. . . Film con. .. 8.51 ... ... Film weights were determined in the usual tinuous tinuous Film con7.20 ... Film con... 7.20 ... ... manner on tin plate dipped simultanetinuous tinuous ously. Data on initial continuity for the 6.38 . . . Film con... 5.36 ... Film con5 3 66 . . . Film tinuous con. . 3 86 0.505 tinuous 13,000 039 indicated coating weights using a platitinuous num cathode are summarized in Table 6 2 15 0 492 11,150 0 043 2 15 0 484 9,110 0 061 Bare metal 0 00 0 487 1,090 0 437 0 00 0.487 1,090 0 437 11, as well as data previously obtained on' unpolished iron, for comparison.
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