INDUSTRIAL A N D ENGINEERING CHEMISTRY
1356
Vol. 19. No. 12
Testing of Finished Lacquers' By J. D. Jenkins a n d P. R. Croll PITTSBURGH PLATEGLASSCOYPANY, MILWAUKEE, WIS.
T
HE purpose of testing protective coatings by physical, chemical, or exposure methods is to predict their
behavior under normal service conditions, and such test methods are of value only in so far as they permit the prediction of these properties. Tests of this nature may be grouped into three general classes: (a) normal exposure tests, (6) accelerated exposure tests, and (c) tests of physical and chemical properties of the film. This paper will be limited to the last two of these three groups and will include a description of the equipment and cycle in use a t the Milwaukee Laboratories of the Pittsburgh Plate Glass Company for accelerated exposure tests and a discussion of three physical tests on finished lacquer films. Accelerated Exposure Tests EQUIPMENT-The
accelerated exposure equipment used
at the Milwaukee Laboratories comprises a refrigeration room and a light and moisture tank (Figure 1). The refrigeration room proper is 9 by 12 feet and 8 feet high. It is insulated by a 2-inch layer of cork and provided with a double window and a refrigerator door. Within this room is a tank 5 feet high and 6 feet in diameter, supplied by an individual air duct from the refrigeration equipment. The light and water tank is also 5 feet high by 6 feet in diameter. Both tanks are provided with hexagonal racks rotated by motors operating through speed reducers and bevel gears and so arranged as to rotate once every 10 seconds. T h e p a n e l s a r e e x p o s e d in panel holders (Figure 2), which hold three 6 by 12-inch panels, bolted with brass bolts, one above another and spaced out I TAM. // from the holders by strips of beading inch square a t the top and bottom of each panel. This spacing p r o v i d e s
and adjusted to spray evenly on the three rows of panels from a distance of 2 feet. A 30-inch quartz mercury arc is suspended in the center of the tank through a 6-inch hole in the top. It is so hung that its center is on a level with the center of the middle panel. The distribution of light upon the fifteen panels on each side of the hexacronal rack is shown in Figure 3. It may be as-sumed that over a relatGely C 6 - 4 large number of changes, from the waterI light tank to the refrigerator tank and back, the law of chance will cause any one panel holder to occupy each of the five possible positions an equal number of times; consequently, the variation of intensity between the middle position for the panel holders and the outside positions will be offset. The top 38' and bottom panels, however, will always get less light than the corresponding center panel, about 90 per cent for the top and bottom as compared with 100 per cent for the center, and this can only be eliminated by changing the panels around. These values are all calculated figures using the formula:
177 I
I
.:
Intensity = A
L-A
10
:m
\----_I
-'r.
WI.Yi0W
M
Tuesday: 8:OO A.M.
11:30 2:OO 6:OO
P.M. P.M.
11:30
A.M.
11:30
A.M.
11:30
A.M.
A.M.
Wednesday: 2:M) P.M. 6:OO P.M.
1;
J
Figure 1-Plan
I I
of Weather R o o m
drainage and air circulation behind the panels. The holders are hung vertically from the top bar of the hexagonal rack by means of two small hooks on the back of the holder. The tanks are fitted with covers and the light tank is equipped with three spray nozzles set 1 foot apart on a vertical pipe Presented as part of the Symposium on Lacquers, Surfacers, and Thinners before the Section of Paint and Varnish Chemistry at the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16, 1927.
Thursday : 2:OO P.M. 6:OO P.M.
Friday:
to 8:oo to 11:30 to 2:00 to 6:OO
A.M. A.M. P.M. P.M.
Light Water Refrigeration Light and water
to 11:30 to 2:00 to 6:OO
A.M. P.M. P.M.
Light Refrigeration Light and water
to 11:30
A.M.
Water Refrigeration Light and water
to 11:aO to 2:OO
P.M.
to to
2:OO P.M. to 6:OO P.Y.
2:OO P.M. 6:OO P.M. A.M.
6:OO P.M.
Light Refrigeration Water
~~
Saturday: 1o:OO
A.M.
to 1O:OO 12:oo
A.M.
Light Refrigeration
to 11:30
A.M.
Light
t0
12:Oo
Monday:
1
It is very similar to the one used by H. A. Nelson, of the New Jersev Zinc Company, due to differ- - . with some changes -
December, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ences in equipment. No attempt has been made to enrich the air in the light tank with oxygen, nor to raise the temperature of the light tank during the irradiation. The refrigeration temperature has averaged about -5" F. CoNcLusIoNs-Most of the work with this equipment during the last 9 months has been with lacquer-base materials. Here it has been found very useful for comparative tests between relatively similar materials. Attempts to correlate the results of these tests with outside exposure panels, however, have been less fruitful. The reasons for this are fairly obvious. In the f i s t place, the normal manner of failure of such Ikishes as automobile e n a m e l s a n d clear lacquers is by chalking or disintegration by light. However, the variation in the a m o u n t of sunlight and its content of the a c t i v e s h o r t wave lengths, or ultraviolet light, is very .79 marked between summer and winter. As a result, lacquers will be much longer lived d u r i n g t h e winter than during the summer m o n t h s on exterior exposure. In Figure &Percentage Distribution of products which fail by Light on Fifteen Panels on One Side of cracking or checking Hexagonal Rack the type of failure can usually be approximated, but the time ratio- of accelerated to normal exposure varies somewhat. These results are, of course, preliminary and incomplete. Further data will be presented as they are obtained.
1357
Then the length of the arc across the top of the bulge, measured along a diameter of the clamping ring from one side to the other, is given by the expression Lengthof arc
=
2h -arc tan d
and since the length of the chord is, of course, the diameter of the clamping ring, the 9 2h Per cent elongation = dP + 4ha arc tan - - 1 ~
2hd
d
The per cent elongation is given in t h e following table, calculated as a function of h/d: h/d
PER CENTELONGATION
0.0 0.1 0.2
h/d
0.0 2.7
10.4
PERCENTELONGATION
0.3
22.5
0.4
38.3 57.1
0.5
The nature of the stress applied to the film in this test is of interest. The edges of the circle are subject to a bending action and to elongation practically all in one dimension. The section of film near the center of the circle, however, is subject to elongation in two dimensions-i. e., to an increase in area. Consequently, the results of this test cannot, perhaps, be compared directly with the results obtained on the type of machine which stretches a strip of material in one direction only. Whether the results can be translated into per cent linear elongation or not, the test is a useful, duplicable, and reliable one for control work. The bursting
Tests on the Film
An apparatus for the control of the elongation at rupture and the tensile strength of lacquer films has been, in daily use testing automobile enamels for nearly a year. This apparatus is an ordinary Mullen paper tester with a small, very simple attachment for indicating the elongation. The principle of this indicator will be plain from the diagram (Figure 4). The lever arm, L, rests on the surface of the fdm, which is clamped over the rubber membrane. When the handwheel on the instrument is turned so as to force the plunger into the glycerol chamber and distend the rubber membrane, the film is bulged up and consequently stretched. This degree of bulging is a measure of the amount of stretch undergone by the film. The distention of the film moves the lever arm, L, up, causing the pointer, P, to travel across the scale S, which is calibrated directly in per cent elongation. The calibration of the elongation scale is somewhat arbitrary. The elongation is calculated as the per cent increase in the length of the arc along a diameter of the circular opening in the clamping ring, as the film is bulged out in the form of a segment of a sphere. The elongation is expressed as length of arc - length of chord Per cent elongation = length of chord The calculation of this scale giving the elongation as a function of the vertical movement of the top of the bulge is given below. Let d = inside diameter of clamping ring h = heightLof bulging of film
I
GAUGE
-/--I
