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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 Data obtained in this laboratory, as yet unpublished, strongly indicate that film continuity may be a combined function of metal-surface, degree of polarity of the resin, and its molecular weight.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 . . . Film con. .. 8.51 ... con... 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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INDUSTRIAL AND ENGINEERING CHEMISTRY
1281
In order to reconfirm the fact that, in so far as initial continuity is conPolished Iron Unpolished Iron cerned, the separating f i l m m a y b e Coating Potential Internal Coating Potential Internal either on the anode or cathode, cells Sample No, wt. Fe-Pt resistance E/R wt. Fe-Pt resistance E/R composed of polished iron cathodes and Mg,./sq. 'Volt Mg,/sQ. Volt Ohms Milliamp. zn. Ohms Milliamp. an. bare zinc anodes were tested. Data on 1 8.79 ., , Film con. . . 10.07 ... Fi!m conthe iron-copper and zinc-iron assemblies tinuous tinuous 7,680 0:066 8.32 0.503 0.030 20,000 2 7.56 0.599 are presented in Table I11 and Fig7,000 0,072 7.47 0.503 0.057 10,700 3 7.15 0.610 3,330 0.163 6.77 0.541 10,200 0.060 4.46 0.608 ure 1. 3,000 0.176 0.526 0.085 4.72 9,500 4.00 0.621 Comparison of the data in Table 111 3,850 0.138 0,532 0,087 3.54 5,760 2.00 0.502 2,820 0,192 7,830 1.17 0.510 0.065 2.15 with those in Table I1 on polished iron 0.54: 2,410 0.221 ... ... .... . ., 1.44 y.m< 1,984 0.264 seems to indicate that the only effect a 9 ... 0.70 u , 5x2 1,200 0.399 0.480 Baremetal 0.00 0:5iO 1,185 , 0'432 0.00 change in potential can have within the limits encountered in the series of metals originally studied is to displace the inWEIGHTSOF FILMS FOR IRON-COPPER AND ZINC-IRON SYSTEMS tercept on the E / R axis and introduce a TABLE111. COATING Iron-Copper .Zinc-Iron slight compensating change in slope. Coating PotenInternal Coating PotenInternal Sample No. wt. tial resistance E/R wt. tial resistance E/R In no case has the point of minimum Mg:/sq. voll continuity been displaced outside the Volt Ohms Milliamp. an. Ohms Milliamp. margin of error of the measurements; 1 8.79 ... Film con... 8.79 ... Film con... tinuous tinuous that is, this point continues to be re0.026 -0.314 12,000 0.025 7.15 0.330 13,400 2 7.15 producible to *0.5 mg. per square inch, 9,000 0.035 -0,314 0.045 4.46 3 4.46 0,439 9,800 0,038 -0,338 9000 4.00 9,000 0.052 4 4.00 0,368 regardless of effective potential or cell 7:lOO 0.074 2.00 -0.525 5 2.00 0,344 7,150 0.048 -0,509 7,000 0.073 0.046 1.17 6 1 . 1 7 0.334 7,220 assembly used to determine it. Baremetal 0.00 0.294 1,185 0.248 0.00 -0.487 1,185 0.412 Comparison of the data for polished iron, tin plate, and platinum, with effective Dotentials adiusted internallv to values between 0.4 and 0.5;olt, shows t6e points of minimum Data for both series of experiments are plotted in Figure 1. continuous coating weights to be : Of primary importance is the experimentally demonstrable fact (Table 11)that the points of minimum initial continuity AnodeCoating Weight Cathode are in excellent agreement, the two extrapolated points lying Mg./sq. in. between 8.5 and 9.5 mg. per square inch. It thus seems logical Polished iron 8.5-9.6 Zn-Fe to conclude that minor surface variations do not introduce an Tin plate 3.0-4.0 Pt-Sn Platinum 1.0-1.5 Sn-Pt appreciable factor in minimum continuity data. The experimental fact that the rate of decrease of cell P resistivity with decrease in coating weight is appreciably lower on polished iron than on black iron is significant; this 2 finding is in complete accord with predictions, since minute irregularities in the surface of the black iron, not present in 0 polished iron, should tend to increase the discontinuities as observed. A Finally, it is important that in no case do experimental points fall entirely outside a margin which completely in.2 dividualizes the given metal; that is, observed differences in continuity phenomena between platinum and iron, for exLT L o ample, cannot now be attributed to differences in surface regularity.
-
WEIGHTSOF FILMS ON IRON-PLATINUM SYSTEMS TABLE11. COATING . 7
--.
7
MtL.sq,
v)
Relation between Effective Potential and Continuity It is possible that the effective operating potential of the test cell may have some influence on the point of minimum continuous coating weight; during the brief period of cell discharge while reading internal resistances, a film continuous against 0.2 volt, for example, might become discontinuous against 1.0 volt (operative in the case of galvanized iron). The data presented in the preceding article on tin plate and on platinum serve to answer this question, since the same cell anode-cathode couple was used, in one case the insulating film being deposited on tin plate and in the other on platinum; however, it was decided to investigate the behavior of films on polished iron under various effective potentials. Preliminary consideration showed the impracticality of adjusting potentials outside the cell; by use of metals other than platinum as cathodes in the cell assembly, however, it was possible to select an effective potential between rather wide limits; copper gave an excellent potential a t approximately 0.33 volt, for example.
4 .
e 0 0
2
4 6 8 10 12 COATING WEIGHT IN MGMS./SP.IN.
14
16
FIGURE 1. RESISTIVITY vs. COATINGWEIGHT FOR
VARIOUSMETALSYSTEMS
Theory of Oriented Film Structure The data presented, when combined with results on the studies of synthetic resins on metalsurfaces, show a specificity conditioned by, and perhaps directly attributable to, the metal. Intensive studies of continuity phenomena as yet unpublished, using thermoplastic resins of wide differences
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manifest. Finally, it is obvious that the minimum weight of coating material which should completely “satisfy” a given metal surface may be distinctly deficient on another. The data presented in the previous contribution completely confirm such specificity of behavior. From experimental results the existence of a third or sensitive zone might be inferred, representing a transition condition in which orientation forces may be vanishingly small but still operative, so that minute ZONE OF RANDOM MOLECULAR A R R A N G E M E N T variations in relative polarity of the coating ION OR SENSITIVE Z O N E material, slight thermal differences, etc., may or may not produce a tendency to orientation in this region. Such differences would be insensitive to the usual coating weight determinations, so that within this zone samples of identical coating weight might and do show wide differences in continuity. The various relations discussed are shown schematically in Figure 2. Metal A may be considered as relatively inactive; only a slight weight of coating material is required to satisfy Z O N E OF REGULAR ?l:LECULAR ORIENTATION the surface unbalance, and amorphous characteristics develop early. Metal C , on the other FIGURE 2. HYPOTHETICAL FILMSTRUCTURES hand, has a large surface sphere of activity and a much greater proportion of coating material goes into a partially oriented structure, so that discontinuities deed, for fundamental differences in thermal stability on various metal surfaces. Assuming that electrode potentials continue a t coating weights which can be shown experimentally to be continuous on metal A. may be directly related to surface activity and, of more importance, to the sphere of such activity, it would seem to be Relation between Surface Activity and Thermal broadly true that very active metals should show high spheres Decomposition of surface force phenomena, whereas inactive metals should show relatively low spheres of force phenomena. I n general, Taking into consideration the fundamental concepts with we would expect that the surface with the greatest unbalanced regard to orientation phenomena on active surfaces, a rational surface activity should exert the strongest attractive force for explanation for thermal decomposition differences on difa dissimilar body of opposite charge. If the latter body is ferent metals can be advanced. Of primary importance is polar or semipolar in nature, it should orient itself upon the the experimentally demonstrable fact that such decomposition attracting surface in such a manner as to yield a bonded is catalyzed by metal surfaces. An initial hypothesis in surstructure in which the residual force is a minimum. It folface catalysis is that the catalyst may function as an orienting lows that the stronger the attractive force, the more complete medium, bringing the sorbed molecule into correct position should be the orientation. to be most readily attacked by true reaction forces. Thus One of the essential characteristics of an oriented or “cryswe see that, all other factors being held constant, the metal talline” arrangement would be the more or less uniform dissurface producing the maximum orientation should be the tribution of regularly spaced interstices between molecules, strongest catalyst of a thermal decomposition, and vice versa. so that any oriented portion of the film can contribute little or This inference agrees quantitatively with the experimental nothing to film continuity; it represents a foundation strucfacts. Vinyl films of identical composition and weight, ture which must be completely built up from the active surface baked for the same length of time, will be completely decombefore the actual “amorphous film” can be deposited. Thus, posed on zinc a t 290” F. but will be completely stable on platian experimentally continuous film might be made up of a t num a t 360” F. or higher. Each metal is characterized by a least two zones: one upon and up from the metal surface finite and rather narrow temperature range above which whichmay be more or less oriented, to a height perhaps directly thermal decomposition invariably takes place. related to the sphere of surface activity of that metal; and the other, amorphous in character, where the true film propRBCEIVED August 5, 1937. Contributions from the Stoner-Mudge, Ino., Industrial Fellowship at Mellon InatitUte. erties of elasticity, continuity, and other properties should be in average molecular weight and in molecular weight distribution as well as of varying polarity, indicate that film continuity may be a combined function of metal surface and of degree of polarity of the resin. Such considerations lead us to suggest a tentative explanation involving partial orientation in the resin film which may account for the wide variations in coating weight required to deposit continuous films and, in-
