Film Continuity of Synthetic Resin Coatings Effect of Composition and Molecular Weight on Minimum Coating Weights for Thermoplastic
Resin Films G.H. YOUNG,G. W. GERHARDT, W. K. SCHNEIDER, AND 0 . W. SEAGREN Mellon Institute, Pittsburgh, Pa.
I
I’;THE first paper in this series (3)a method for the rapid
was constructed, a 10,000-ohm resistance was the maximum available for use as R1 in the resistance circuit shown in Figure 1. Preliminary tests with resistances varying from 1 to 1000 ohms for Rz gave optimum results using 1000 ohms for
determination of film continuity and for the quantitative evaluation of effective coverage was described. Data were presented showing that film continuity of a typical vinyl thermoplastic was a discontinuous function of coating thickness, and that the specificity in continuity characteristics might be related to the activity of the metal on which the coating was deposited. A tentative explanation was advanced in the second paper (4) to account for this specificity in terms of partial orientation within the film, conditioned by the substratum metal. In the present paper results of a study of the effect of resin composition and of molecular weight on continuity are p r e sented. For this purpose a series of five experimental vinyl thermoplastic copolymers of varying vinyl chloride content and of varying molecular weight, as determined by viscometric methods (g), were employed, together with a series of three polyvinyl acetates. The resins are described as follows : %
Vinyl
Sample Chloride A 65 B 75-79 C 87 D 87
%
Vinyl Acetate 35 25-21 13 13
R8.
With increased emphasis on wider utilization of this method for film evaluation, it was thought desirable to obtain quantitative data on the actual relation between measured and true resistances. Accordingly, a series of known fixed resistances covering the range from 1000 to 1,000,000ohms was obtained, and the apparatus was calibrated over its entire working range.
Calibration of Apparatus with Previous Resistances in Circuit The instrument was first calibrated by means of the previous circuit, R1 = 10,000 ohms, and Rz = 1000 ohms. For this purpose known resistances were placed in series with a 1.5-volt dry battery (internal resistance, 0.05 ohm), and
Av. % % Av. Mol. Vinyl VinyI Mol. Wt. Sample Chloride Acetate Wt. 13 28,000 28,000 E 87 100 8,000 28,000 F 0 100 13,000 7,000 G 0 100 27,000 10,000 H 0
I Minimum coating weight studies on eight experimental thermoplastic vinyl resins of varying compositions and of varying molecular weights show that the coating weight required to just produce complete film continuity varies directly with the vinyl acetate content and inversely with the average molecular weight. This observation seems to be in complete agreement with a tentative theory involving partial orientation conditioned by the activity of the metal being coated and by the effective polarity of the thermoplastic resin.
The experimental apparatus used in indirect measurement of current flow through discontinuous films was previously described (3). Factors governing the magnitude of calculated internal resistances were pointed out, and a clear distinction was made between such “observed” resistances and true values. Until the time of this writing no intensive study of the relation between measured resistivity and true resistivity has been presented. Preliminary results showed satisfactory agreement in the lower resistance ranges with the more familiar though less precise Wheatstone bridge method. Within .the same range, currents calculated from resistance measurements were in reasonable agreement with those obtained by direct readings using a Weston microammeter. As prev.iously stated, however, the actual “internal resistance” of films which are just discontinuous has long been subject to considerable error. At the time the writers’ apparatus 685
INDUSTRIAL AND ENGINEERING CHEMISTRY
686 P
TABLEI. DATAOBTAINEDWITH Rl = 10,000 OHMS Measured Known Deflection Deflection Internal Resistance dl d2 Resistance Ohms Ohms 1,000 13.9 7.68 834 1,500 13.25 6.0 1,210 2,500 12.3 4.25 1,890 5,000 10.27 2.43 3,220 10,000 7.64 1.31 4,830 20,000 5.22 0.70 6,460 50,000 14.4 1.55 8,290
Measured Known Deflection Deflection Interaal Resistance dl dz Resistance Ohms Qhms 100,000 7.82 0.77 9,150 200,000 11.7 1.14 9,270 300,000 7.9 0.72 10,100 400,000 7.7 0.71 9,850 500,000 6.67 0.65 9,580 1,000,000 3.23 0.30 9,770
TABLE11. DATAOBTAINEDWITH R1 = 50,000 7
R1
= 50,000 Ohms
Measured Known Deflection Deflection Internal Resistance dl dl Resistance Ohms Ohms 1,000 16.8 8.76 920 1,500 16.7 7.14 1,340 16.4 5.06 2,240 2,500 5,000 15.7 2.97 4,260 14.38' 1.63 7,720 10,000 20,000 12.4 0.82 14,120 1.06 22,900 50,000 25.4 100,000 21.6 0.63 33,300 200,000 12.62 0.31 39,800 300,000 8.96 0.21 41,700 0.15 46,000 400,000 7.05 0.12 48,300 500,000 6.0 0.06 49,000 1,000,000 3.0
AND
100,000 Owns
RI = 100,000 Ohms Measured Known Deflection Deflection Internal Resistance dl da Resistance Ohms Ohms 1,500 3.18 1.3 1,445 2,500 3.15 0.9 2,500 5,000 3.17 0.33 4,980 10,000 3.14 0.32 8,800 20,000 3.10 0.17 17,250 50,000 3.02 0.09 32,300 100,000 2.90 0.04 71,500 500,000 2.20 0.01 219,000
the internal resistance of the combinations was determined in the usual manner. Data are tabulated in Table I. The data in Table I clearly show that the original method for determining cell internal resistances ( I ) is accurate only in the low ranges; the deviation from the true value increases with increasing magnitude, and the value of the measured resistance approaches a constant value fixed by the magnitude of R1 (10,000 ohms in this case).
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VOL. 30, NO. 6
the magnitude of R1. Precision has been increased, since the magnitude of deflection dz is increased. Finally, data obtained when R1 = 50,000 ohms and Rz = 10,000' ohms are summarized in Table IV. The precision has again been increased because of the increase in Rz, with a corresponding increase in the magnitude of deflection dz. Inasmuch as the values o b t a i n e d with R1 = 50,000 ohms and RS = 10,000ohms are of satisfactory precision, and enable a calibration curve to be used over all r a n g e s o r d i n a r i l y encountered in film continuity work, the continuity test apparatus was changed accordingly. All resistance r e a d i n g s reported in the present contribution, therefore, are corrected values obtained by use of a calibration curve.
Preparation of Samples Each of the vinyl chloride-vinyl acetate copolymers was dissolved in a mixture of 4 parts of methyl isobutyl ketone and 1 part of toluene. The final concentrations varied from 13 per cent solids for the high molecular weight resins to 33 per cent solids for those of low GALVANOMETER
Calibration of Apparatus Using High Values for R1 In view of the fact that the inherent accuracy of this method (in contradistinction to its precision) depends upon the abPOTENTIOMETER 0 OHMS solute magnitude of resistance R1, calibration experiments were repeated with R1 = 50,000 ohms, R1 = 100,000ohms; Rz was maintained at 1000 ohms in both cases. The data are given in Table 11. KI In general, as the magnitude of R1 is increased, inherent accuracy increases over a wider range. On the other hand, FIGURE1. CIRCUIT DIAGRAM as R1 increases, the magnitude of deflections dl and dz decreases, with accompanying decrease in precision. TABLE111. DATA OBTAINBD WITH R1 = 50,000 AND R2 = 5000 OHMS Under actual laboratory c o n d i t i o n s , R1 = 100,000 ohms is already impracMeasured Measured Known Deflection Deflection Interns1 tical since the error introduced by lack Defl$ftion Deflection d¶ Resistance Interm' Resistance dl d? Resistance of precision in r e a d i n g dt p r e v e n t s Ohms Ohms Ohms Ohms accurate comparisonsbetween test films. 1,500 2.7 2.12 1,368 100,000 21.5 2.88 32,300 12.53 1.35 41,400 2,500 2.62 1.82 2,200 200,000 Use of R1 = 50,000 ohms, however, is 5,000 15.48 8.9 0.93 42,800 . 8.3 4,330 300,000 7.05 0.66 48,500 10,000 14.15 5.38 8,140 400,000 indicated as practical, with corrected 6.01 0.6 48,500 20,000 3.22 13,950 500 000 12.21 values obtained by reference to a suit30,000 8.61 0.28 49,300 1,000:000 3.05 1.45 24,700 able calibration curve. After a possible optimum value for TABLBIV. DATAOBTAINED WITH R1 = 50,000 AND R z = 10,000 OHMS R1 was found, the problem was whether Measured Measured increased precision could result from Known Deflection Deflection Internal dl Resistance increasing Rz. Table I11 gives the data Deflz:tion dl Resistance Internal Resistance dl obtained when R1 = 50,000 ohms and Ohms Ohms Ohms Ohms 5.2 31,300 100,000 21.5 1,380 R2 = 5,000 ohms. 1,500 16.45 14.45 12.6 2.55 39,400 200,000 13.2 2,240 2,500 16.15 These data are in excellent agree5,000 15.4 9.0 1.7 42,900 10.76 4,330 300,000 7.1 1.3 44,600 400,000 7.8 8,220 10,000 14.2 ment with those listed in Table I1 and 6.05 1.07 46,600 500,000 5.12 13,800 20,000 12.2 50,000 8.62 2.48 24,800 1,000,000 3.02 0.52 48,100 clearly indicate that measured values are airnost exclusively dependent upon a
~Z.(azz&~
~zzz&~
INDUSTRIAL AND ENGINEERING CHEMISTRY
JUNE, 1938
molecular weight. The vinyl acetate polymers were dissolved in a mixture of 4 parts of methyl isobutyl ketone and 1 part of methyl ethyl ketone; the concentration varied from 30 to 40 per cent solids, depending on the molecular weight. All the samples were dip-coated on freshly cleaned tin plate under dust- and moisture-free conditions, as described previously (3). The weight taken up was controlled by decreasing the solids content through dilutions with 4-1 solvent-thinner mixture. The coated panels were baked for 15minutes a t 290 F. Coating weights were determined as before by weighing measured areas cut from duplicate panels, stripping with acetone, and reweighing. O
Effect of Composition on Film Continuity I n order to study the effect of variation in composition of the resin copolymers on the coating weight required to produce continuous films, samples A, R, and E were selected; these samples represent a spread in vinyl chloride content from 65 to 87 per cent a t a constant average molecular weight of 28,000. The data obtained are summarized in Table V.
CONTINUITY us. RESINCOMPOSITION TABLE V. COATING. Vinyl Coating Chloride Wt. PoBential % Mo./sq. in. Volt 14.40 65 10.72 0:549 6.74 4.54 0.554 3.94 0.509 0.444 1.57 0.75 0.470 0.51 0.459 75-79 6.08 3.74 01433 0.538 2.19 1.06 0.530 0.50 0.516 87 6.82 3.57 2.23 0:475 0.96 0.533 0.88 0.528 0.00 0.487
Sample No.
...
A1 2 3 4 5 6 7 8
B1 2 3 4 5 El
.. .
2
3 4 5 Bare. metal
Cor. Internal Resistance E/R Ohms Milliamp. Fjlm oontinuous Film continuous o:Oi2 45,000 0.061 9,000 0.017 0.028 0.338 0.389 Film continuous 7,000 0:oez 1,625 0.333 1,475 0.359 1,215 0.425 Film continuous Film continuous 1,350 0:353 2,000 0.267 n 815 -1.67'5 ,___ 1,090 0.437
...
...
Curves showing the relation between film continuity and coating weight are plotted in Figure 2. The extrapolated points for coating weights to just produce complete continuity are given in Table VI.
TABLEVI.
COATING WEIGHTSTO JUST PRODUCE CONTINUITY
Sample
Av. Mol. Wt.
A B
28,000 28,000 28,000
Vinyl Chloride
%
E
65 75-79 87
Vinyl Acetate
% 35 21-25 13
Min. Coating Wt.
iMo./sa. in. 7.0-8.0 5.0-6.0 3.0-4.0
Approx. Thickness Mil 0.35-0.40 0.25-0.30 0.15-0.20
The relation between vinyl acetate content and the weight required to just produce continuity is approximately linear between the relatively narrow limits investigated.
Effect of Molecular Weight on Film Continuity Resin samples C, D, and E, containing 87 per cent vinyl chloride, and samples F, G, and H, containing only polyvinyl acetate, were selected for the study on the effect of variation in molecular weight on continuity characteristics. Data on initial continuity for the copolymers and the unmodified polyvinyl acetates are summarized in Table VII.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
688
TABLEVII. COATINGCONTINUITY v S . MOLECULAR WEIGHT Av. Mol. Sample No.
wt.
c1
7,000
2 3 4 5 6 7 D1 2 3 4 5 6
El
10,000
28,000
2 3 4 5 Bare metal
F1
2 3 4 5
6,000
6
7 8 G1 2 3 4 5
13,000
6
7
8
9 10 11 12 Bare metal
... :
...
...
...
...
6
7 8 9 10 11 12 13 14 H1 2 3 4 5
Coating Cor, Internal Wt. Potential Resistance E/R Mg./sq. in. Volt Ohms Milliamp. 87% Chloride Copolymer Film continuous 9.62 Film continuom 5.93 0 : 033 13,500 0 464 4.45 0.095 6,000 2.35 0.569 0.029 15,000 1.56 0.441 0.477 1,130 0.542 1.11 0.286 1,620 0.464 0.81 6.22 Film continuous 500,000 0:OOl 0:4i)5 3.56 30,000 0.019 2.89 0.531 2,100 0.228 0.476 1.79 1,800 0.299 0.540 1.38 1,950 0.282 0.550 0.91 6.82 .., Film continuous 3.57 Film continuous 2.23 01475 1,350 0:353 0.96 0.533 2,000 0.267 0.88 0.528 1.675 0.315 0.00 0.487 1,090 0.437 100% Acetate Polymer Film continuous ,. 10.20 ... Film continuous . 8.75 Film continuous 8.05 48,ooo o:oii 0 544 7.90 380,000 0.001 6.85 0.535 150,000 0.003 0.503 5.12 16.000 0.033 ~. 2.32 0.533 5;500 0.089 0.488 1.98 Film continuous .. 10.60 Film continuous . 8.66 Film continuous . 7.00 Film continuous . . ... 6.50 Film continuous ... 6.32 Film oontinuous 6.08 0:004 135,000 0 520 5.96 500,000 iapprox.) 0.001 0.501 5.82 0.003 180,000 0.500 5.64 0.001 470,000 0.524 5.31 0,588 500,00O(approx.) 0.001 5.23 0.006 100,000 0.563 3.75 0.021 23,000 0.485 2.65 0.027 17,000 0.462 1.65 Film continuous , 10.30 Film continuous 8.44 Film continuous . 6.98 Film continuous 6.45 500,000(approx.) 0:001 0 : 657 6.16 165000 0.003 0.512 5.93 500,000(approx.) 0.001 0.550 5.80 70,000 0.007 0.501 4.15 61,000 0.008 0.488 3.00 19,000 0.031 0.592 2.07 13,000 0.041 0.536 1.34 8,440 0.059 0.496 0.82 1,090 0.437 0.487 0.00
. ..
... ...
:
27,000
... ... ...
..
.. ...
Curves showing the relation between film continuity and average molecular weight are presented in Figures 3 and 4. The extrapolated points for coating weights which just produce continuity are given in Table VIII. I n general, comparison of the data in Table VI11 shows that, a t approximately equivalent molecular weights, the high-chloride polymers have minimum coating weights appreciably lower than those for 100 per cent acetate. The data clearly indicate that for a given type of polymer the minimum coating weight varies inversely with the average molecular weight. The phenomenon seems to be more pro-
VOL. 30, NO. 6
nounced with the copolymers than with the pure polyvinyl acetates. This mav be fortuitous. however. Relatively small ;mounts of vinyl acetate in chloride-acetate copolymers apparently exert a profound influence on continuity characteristics. Thus, comparison of the data in Table VI11 with those in Table VI indicates that a 35 per cent content of vinyl acetate already confers a filming behavior on the copolymer approaching that of pure acetate, and this is in general agreement with actual operating experience. TABLEVIII. EXTRAPOLATED POINTS FOR COATING WEIGHTS PRODUCE CONTINUITY WHICHJUST Sample
Vinyl Chloride
Vinyl Acetate
%
%
87 87 87 0 0 0
13 13 13 100 100 100
C D E F
G H
Av. Mol.
Wt.
7,000 10,000 28,000 6,000 13,000 27,000
Min. Coating Wt. Mg./sq. in. 5.0-6.0 3.5-4.5 3.0-4.0 7.5-8.5 5.5-6.5 5.5-6.5
Approx. Thickness
Mil 0.25-0.30 0.18-0.23 0.15-0.20 0.38-0.43 0.28-0.33 0.28-0.33
The minimum coating weight for resin H, a t 5.5 to 6.5 mg. per square inch, is not in agreement with the extrapolated value calculated from the data in Table VI; the latter value would predict a minimum coating weight of a t least 15 to 16 mg. per square inch if the apparent linear relation is valid over the entire range of possible copolymers. Unfortunately, copolymers of greater than 35 per cent vinyl acetate content are not available a t present. A study of such copolymers with vinyl acetate contents exceeding 50 per cent would aid in clarifying this point. The data here presented perhaps indicate that the expected relation between acetate content and minimum coating weight is exponential rather than linear; additional data must be accumulated before a final decision can be made.
Acknowledgment The experimental resins used in this research were obtained through the courtesy of the Carbide and Carbon Chemicals Corporation, whose cooperation is greatly appreciated.
Literature Cited (1) Carhart, H. S., and Patterson, G. W., “Electrical Measurements,” pp. 98-100, Boston, Allyn and Bacon, 1895. (2) Douglas, S. D., and Stoops, W. N., IND.ENG.CHEM.,28, 1152 (1936).
(3) Young, G.H., and Gerhardt, G . W., Ibid., 29, 1277 (1937). (4) Young, G. H., Schneider, W. K., and Gerhardt, G W., Ibid., 29, 1280 (1937).
RECEIVED December 21, 1937. Contribution from the Stoner-Mudge, Inc., Industrial Fellowship a t Mellon Institute.
