Film Continuitv of Svnthetic Resin Coatings J
J
Effect of Composition and Chain Length on Minimum Coating Weights for Methacrylate Polymers
I
G. H. YOUNG, G. W. GERHARDT, W. IC. SCHNEIDER, AND G. W. SEAGREN
N T H E first paper in this series ( 2 ) a method for the rapid determination of the ‘(continuity” of resin films was described. I n this and later contributions (3, 4) it E F F E C T I V E C I R C U I T WITH K O P E N was shown that t h e minimum thickness of film required to produce such continuity might be related to the activity of the metal being coated, to t h e average molecular weight of the resin, and to its E F F E C T I V E C I R C U I T W I T H K CLOSED composition. T h e experiFIGURE 1. EFFECTIVE CIRCUITS mental techn i a u e a n d aDparatus were discussed previously ( 2 ) . It was pointed out that the use of the equation given by Carhart and Patterson ( 1 ) for the measurement of high internal resistances introduces inaccuracies in “observed” resistances which increase with the magnitude of the resistances being measured; it was shown (8, 3) that this is to be expected when Carhart and Patterson’s limiting assumptions are considered. At that time the apparatus was calibrated against known resistances over its usual working range, and corrected internal resistance values were obtained by use of the calibration curves. The approximated equation which Carhart and Patterson suggest for internal resistance measurements is derived by them as follows (1):
Mellon Institute of Industrial Research, Pittsburgh, Penna.
d1:da = E:E’ = (Rz
+ r):Rz
(1)
When the key K is open, the P. D. at the battery terminals, measured by dl, is the entire e. m. f. of the cell if its internal resistance is negligible in comparison with the high resistance in circuit; and when K is closed, the P. D. measured by dz is the fall of potential over the external resistance R. Now if the e. m. f. of the cell does not change immediately on closing K , then the fall of potential over the entire resistance, R r , is the e. m. f. of the cell. We may, therefore, put the two deflections proportional to the resistances (Equation l). From Equation 1 by subtraction,
+
(dl
whence
r
- dz):dz = r:Ra di - dz = Rzda
(2)
(3)
An earlier paper (3) showed that Equation 3 gives accurate values only in low resistance ranges, and that only as the magnitude of R1 approaches infinity can the equation be considered quantitative.
Corrected Equation for Calculating Internal Resistances Although the use of calibration curves for the several combinations of R1 and RQ employed in this laboratory to determine actual resistances from values obtained from Equation 3 is satisfactory, the desirability of expanding the equation to
Minimum coating weight studies on three experimental methacrylate polymers of varying composition and varying average chain lengths show that the coating weight required to just produce continuity seems to vary inversely with the chain length and directly with the effective polarity. This relation was previously shown to be valid for vinyl acetate and vinyl chloride copolymers.
Let dl be the deflection of the galvanometer when the circuit is closed through the high resistance (R1),the key K being left open; and let dz be the deflection when key K is closed (see Figure 1). The two deflections are proportional to the currents through the galvanometer, and therefore to the P. D.’s at the terminals of the shunt, with K open and closed, respectively. Since the shunt bears a constant ratio to the entire resistance in circuit, the deflections dl and dz are proportional t o the P. D.’s at the battery (cell) terminals in the two cases. Hence, 719
INDUSTRIAL AND ENGINEERING CHEMISTRY
720
VOL. 31, NO. 6
TABLEI. COATINGCONTINUITY us. FILMWEIGHT -Internal ResistanceCor. by Calcd. from curve Equation 10 E//Roalcd. Ohms -Ohms iiwoamp. Methyl Methacrylate c1.0 Deflection too low to read >500,000 0.663 1.4 0.3 137,500 4.8 145,000 0.584 2.8 1.3 15,000 38.9 17,500 2.9 1.6 61.0 9,000 0.591 9,720 1.6 3.1 11,530 50.4 11,000 0.582 2.2 0.7 40,000 17.5 0.655 37,500 3.1 1.7 10,000 9.875 66.2 0.653 3.2 1.7 10,500 10,725 68.0 0.729 3.5 2.1 ' 7.680 110.0 7,500 0.843 3.6 2.2 7,000 104.5 0.761 7,290 9.0 7.5 2,900 2,975 257.0 0.764
Sample Coating No. Wt. Potential M g . / s q . in. VOll
5.07 4.38 3.20 2.45 2.21 1.70 1.48 1.18 0.80 0.62 0.00
dl
dr
.....
n-Propyl Methacrylate
9.24 6.74 6.02 4.61 3.62 2.81 1.68 0.00
13.77 12.00 10.95 8.75 6.84 4.33 3.20 1.45 0.00
... ...
Film continuous Film continuous Film continuous Diflection too low to read >500,000 Deflection too low to read >500,000 0.783 1.9 0.6 40,000 1.1 17,000 0.659 2.5 9.6 7.5 2.900 0.764
..... ..... ..... ..... .....
38,200 17,100 2.975
...
... ...
