Evaluation of Paint Films The Interchemical Adherometer CLIFFORD J. ROLLE A
RICH, Interchemical Corporation, New York, N. 1
A method is presenzeeo *or resolving rne stripping force measurements of organio coatings on metal surfaces into their hasio factors of plasticity and adhesion. The application of this method should m a k e the adherometer a useful research tool in the evaluation and formulation of organio finishes.
G
REEN and Lamsttina. ( 1 ) recently described
a new instrument for measuring the adherence of organic coatings to metal surfaces. They stated that “adherence” or “stripping force” as measured by this instrument is not synonomom with “adhesion”; other factors involved include tear resistance, plastic resistance, and mechanical entrapment of the paint. The instrument, now called the Interchemical adherometer, measures the force required for a sharpened ivory knife t o r e move or strip a 4 m m . width of film from the test panel. Smoothsurfaced test panels must be used, and i t is not the purpose of this instrument to determine the force with which a coating is held mechanically in pits or pores of a roughened surface. The stripping force, F, as measured on the instrument can be expressed in t e r m of its three component forces by the equation
1‘ i
F=T+P+A Figure 2
These individual forces may he identified as:
T
tear resistance of f l m an either side of knife as i t cuts its path through coating P = plrtstic resistance of film to pushing action of cutting face of knife. This might also be considered a measure of the toughness of the film. A = force of adhesion a t metal-paint interface =
Figure 1.
simultaneous equations and eliminate T (W, and W2 being the
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Film Stripping -.
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‘Lhe hgure tar ‘I
The previous paper described the method of determining the value of T by shifting the cutting knife exactly 4 mm. laterally, and measuring tbe force used in stripping a second band of film of the same width, which in this case has only one edge subject to tearing. However, because it is difficult to move the knife by precisely this amount, a more accurate procedure has nov been adopted. The knife is shifted somewhat less than 4 mm. for the second cut. By stopping this second cut a little short of the first one, as shown in Figure 1, the width of both cuts can be accurately measured, using a low-power microscope with hairline eyepiece. A sliding stage is provided, whose motion is controlled by a micrometer reading to 0.01 mm. By taking readings as the hairline falls consecutively on the first edge, the stepped-in edge, and the last edge of the cut, the accurate width of both strips is obtained. This has the added advantage of saving the rather timeconsuming job of grinding the ivory knives to exactly 4-mm. width. It is evident that component force T is independent of strip width whereas P and A are directly proportional thereto. Therefore, as a result of the two readings, i t is possible to set up tN0
. . . ... . . . + A here obtained will be that for strip width
W,, from which i t readily follows that the value for a unit width of 1 mm. will be ( P A)/Wl, or (ZF, - F I ) / ( ~ W -ZW I ) . All methods of measuring the stripping force of paint films which have been used up to this time, including the time-honored thumbnail test, must necessarily result in a value which is the sum of these two hasio factors. This, however, does not provide the fullest inform&ion for a true evaluation of the film. A highly plasticized film might have good adhesion with a low toughness value. I n a different formulation the reverse might be true, yet the sum of the two, as represented by the measured stripping force, could be the same in the two cases. Either condition might he entirely desirable for the specific application far which i t was designed, but separate values for each component factor would be of greatest importance to the formulator. With the present instrument, it is possible to do this on the following basis:
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Adhesion factor $,is purely an, ;yterfa+d ~~~~,~~~ attraction between ~ 1 1 1 ~ ~ ~ m e upper s u m m 01 me memi ma zne unaer ~urrzoe UI m e palm film. It should bo independent of the film thickness of the applied paint.
996
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V O L U M E 21, NO. 8, A U G U S T 1 9 4 9
991
Plastic resistance factor P is an intrinsic property sf the R h regardless of tlie metal on which it is applied. The value obtained, however, should be directly proportional to the thickness of the film being stripped. The validity of these assumptions was proved over a wide range of typical organic coatings by preparing a set of four or five panels of each formula with a successively increasing number of applied coats in order to obtain a varying set of film thicknesses. The metal used t,liroughout was 24gage cold-rolled steel. The stripping forces for these films were now measured and the value of P A was calculated as previously described. If these .4 are now plotted against the correspoiiding values for P film thicknesses, they should show a straight-line relationship, due only to, the varying values of the P factor. The A factor, remaining constant throughout the series, should merely cause a lateral shift in the curve to the right of the origin. Its exact value will then be represented by the distance from the origin to the intercept of the curve with the horizontal axis (Figure 2). These results are in close agreement with the previously outlined theories. In order to eliiiiinate any personal factor, tlie
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LINSEED MODIFIED ALKYD 4.0
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/
BAKING ENAMEL-ALKYD-UREA
A I
3
6
9
1 2 1 5 i s 2 1 IPSA1 x los CLEAR PYROXYLIN LACOUER
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30-
ul Y
Y
Y I-
-: In
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A* - 3 3 P/FT=4.22
A * 49 P/FT. 1.22
0
l
l
3
6
l 9
l 12 iP+A)
l IS
l 18
l 21
~ 0
I
105 DYNES
Figure 3
I
I
I
3
6
9
4.64 1 13 5 56 3.06 0 17
x
106 X 106 X 10’ X 106
X 106
P / F T , Dynea 6.73 x 10’ 5 . 6 2 X 10b 6 . 2 9 X 10’ 5 65 X 106 3 . 1 4 x 105
location of the curve n hich best fits the recorded points is calculated from the data by the method of least squares. After the value of the adhesion factor, -4,is determined thp value of P easily tollo~vsby subtraction of d from each value of ( P A ) . The final step is to reduce the varying values of P to its specific value for a unit film thickness of 1 mil-in other words, calculate the average value for P IFT ( F T representing film thickness). This term, vihich is actually represented by the slope of the curve, can be considered a measure of the film’s toughness or resistance to deformation. The values sh0Tv.n in Figure 3 are for strips of 4-mm. width. A further confirmation of the theory is attained by extending the investigation to films on other metals besides the cold-rolled steel used in the previous work The case of a single paint formulation applied to a variety of metals may be considered. For each metal, a set of panels is again prepared by varying film thickness, so that in each case the separate values of A and P can be calculated as above. I t is to be expected that the adhesion factor will now show a varying set of values as we pass from one metal surface to another. On the other hand, the plasticity or toughness factor, being a specific property of the film composition, should not be affected by the nature of the surface on which the paint is applied: hence it should maintain a constant value. An air-drying, gray pigmented, alkyd aint was selected, and sets of test panels were prepared on colcfrolled steel, hot-rolled steel, stainless steel, 24s-T aluminum alloy, and galvanized iron. Stripping force measurements were carried out, and from the data the values of A and P/FT were calculated. The results are shown in Table I.
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ACKNOWLEDGMENT
FORMULA N 0 . 2
FORMULA NO. 1
Stripping Force .4, Dynes
The plasticity value of the film on the first four metals maintains a very uniform figure, averaging 5.80 X 106dynes, while the force of adhesion covers a range from 1.13 to 5.56 X lo5 dynes, as we pass from one metal to another. The results are, therefore, entirely in agreement with the deduced theory. The set of galvanized panels, however, presents a value for P / F T which is out of line with the other readings. Were we are dealing with an extremely low adhesion factor. The result was that, in the stripping operation, the cohesion of the film was so much greater than the adhesion, that portions of the film were loosened and flaked off for appreciable distances beyond the sides of the cutting knife, producing jagged edges and a strip of uneven width. A true value for PIFT is thus not obtainable in this particular case. The authors believe, however, that this does not invalidate the theory, as evidenced by the results with other metals which show a reasonable degree of adhesion for the paint film.
RESIN EASE
REO PIGMENTED
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Table I. Metal Cold-rolled steel Hot-rolled steel Stainless steel 24ST aluminum alloy Electrolytic galvanized iron
I 12 IPtA)
I
I
I
15
18
21
The work described in this paper is part of a general investigation of the adhesion of organic coatings on metal surfaces, conducted by Interchemical Corporation Research Laboratories under contract with the U. S. h’avy Department, Washington, D. C., for the Bureau of Ships. The authors gratefully acknomledge permission granted by the U.S.Kavy Department to publish this work. The opinions herein expressed are those of the authors, and are not to be construed as reflecting the views of the Navy Department. LITERATURE CITED
(1) Green and Lamattina, ANAL.CHEM..20, 523 (1948). RECEIVEDNovember 4, 1948. Presented before t h e Division of Paint, Varnish, and Plastics Chemistry at t h e 114th Meeting of t h e AMERICAN C H E W C ASOCIETY, L Washington, D C
