Film Continuity of Synthetic Resin Coatings J
J
Applications of the Impressed-Voltage Breakdown Test' G. H. YOUNG, G. W. GERHARDT, W. IC. SCHNEIDER, AND G. W. SEAGREK Mellon Institute, Pittsburgh, Penna.
breakdown is inversely proportional to the film thickness above the threshold value for complete continuity. Table I presents the results on a series of tin plate panels dip-coated with 5-7 second nitrocellulose. Coating weights were varied by altering the solids content of a dibutyl-phthalate-plasticized formulation in 5 parts toluene, 3 parts ethyl acetate, and 2 parts butyl acetate. All the coatings were baked for 25 minutes a t 200" F. The actual test panels appear in Figure 1. Broadly speaking, the nitrocellulose offers relatively high resistance to electrolytic degradation. Only as coating weights approach the range of 4 to 5 mg. per square inch is the rate of accelerated breakdown appreciable. It may or may not be coincidental that the experimentally determined minimum coating weight of unplasticized nitrocellulose of this viscosity grade is approximately 4 mg. per square inch on tin plate. We have not yet accumulated data on a sufficient number of resins, over large coating-weight ranges, to report definitely that the breakdown rate is profoundly different on very light films as compared with the rate on heavier films of the same resin. This question is being investigated further.
Resistance to film breakdown by impressed voltage potentials appears to vary directly with the film thickness above the threshold thickness for complete continuity, and is a characteristic of both thermoplastic and thermosetting resin films. Data are presented to show the utility of impressed voltage tests for determining the effects of added materials and of special treatments on resin a m s .
A
PREVIOUS contribution in this series ( 2 ) described an accelerated test for comparing the resistance of resin
films to degradation by electrolyte-containing solutions. The method involved electrolyzing a galvanic cell ( I ) , in which the coated metal panel is the anode, with a 25-volt impressed potential and determining the extent of film discontinuity (1) a t regular time intervals. We showed that t h e r a t e of f i l m breakdown for vinyloid resins under impressed voltages is dependent upon the initial film thickness (2). The present paper gives results of similar studies on nitrocellulose and on oil-modified alkyd resin films over a wide range of coating weights. A 0.1 per cent sodium chloride solution was adjusted to pH 4.2 with hydrochloric acid as the electrolyte. The data confirm the previous conclusions and reveal that, in general, the rate of film 1 The five previous papers in this series appeared in 1937, 1938, 1939, and January, 1841.
FIGURE
LACQUER 1. NITROCELLULOSE
550
AFTER
25
VOLTS IMPRESSED VOLTAGE
INDUSTRIAL AND E N
April, 1941
NEERING CHEMISTRY
551
Resistance to Electrolyzing Voltage Breakdown of Heatand Oxygen-Converting Resins
FIGURE 2.
OIL
MODIFIEDALKYDVARNISH AFTER 25 VOLTS IMPRESSED VOLTA5E 1
The presence of the plasticizer introduces variable initial film discontinuities a t weights where films containing no nonfilm-forming ingredients would be “continuous”. Similar effects have been noted with other thermoplastic resins containing nonfilm-forming modifiers. Similar breakdown studies were conducted on a number of alkyd resin films. Table I1 records the results obtained on a phthalic-glycerol alkyd containing 50 per cent soybean oil as modifier. Dip coatings were applied from solutions in xylene, allowed to air-dry for 30 minutes, and then baked at 400” F. for 15 minutes. Here again a general increase in breakdown rate with decrease in coating weight was observed. The panels were photographed after the conclusion of the test (Figure 2).
TABLE11. IMPRESSED VOLTAGEBREAKDOWN OF MODIFIEDALKYDRESIN Panel No. 1
Coating Weight Mg./Sq. fn. 27.7
2
19.1
3
11.2
4
7.9
5
VOLTAGEBREAKDOWN OF A NITROCELLUTABLE I. IMPRESSED LOSE LACQUER Coating Weight, Panel No. Mg./Sq. In. 7.4 1
2
6.4
Cell. Potential, Volts 0.568
0.456
Cell Internal E/R Resistance, Ohms Microakp. 53,000 11 12 46,000 40,000 14 64,000 9 55,000 10 40,000 11 40 000 11 53:OOO 9 40,000 11
Exposure Time,
6
3.3
8
2.7
TABLE111.
2 4.1
0.427
6
3.4
0.402
7
2.2
0.439
8
1.4
0.456
32,000 26,000 8,000 43,000 28,000 12,000 15,000 3,000 950 9,000 830
13 16 54 9 14 33 29 146 460 51 550
4.3
7
Sample No. 1
5
5.5
Hr.
0 0.5 1 2 4 0 0.5 1 2
0 0.5
1 0 0.25 0.5 0 0.25 0.5 0 0.5
Electrolyzing voltage breakdown tests are of assistance in determining optimum film weights and baking schedules for heat- and oxygen-convertible resins of the phenol-aldehyde, alkyd, urea, and coumarone-indene types. The breakdown resistance is a direct function of the degree of cure up to a maximum or optimum point; further cure leads to excessive brittleness, f ilm degradation, and ultimately to complete film failure. The rates of impressed voltage breakdown in general show the same trend with degree of cure or “conversion”. Data from a typical voltage breakdown study on a complex converting cyclopentadiene polymer are summarized in Table 111. The increase i n resistance to degradation with increased baking temperature is clearly
3
4
Cell Cell Internal Potential, Resistance, Volts Ohms Film continuous Film continuous Film continuous Film continuous 0.457 70,000 40,000 26,000 Film continuous 0.443 54,000 92,000 43,000 28,500 Film continuous 0.372 100,000 29,000 29,000 877 Film continuous 0.409 26,500 5,500 267 0.417 >500,000 47,000 23.000 560 0.374 >500,000 1,380 0.562 28,500 1,160
E/R
Microakp.
... ...
... ...
7 11 17
...8
5 10 16
...4
13 13 425
...16
75 1500
