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L. REED BRANTLEY Department of Chemistry, Occidental College, Los Angeles 41, Calif.
Removal of Organic Coatings Importance of viscoelastic properties and use of work of hesion are shown by stripping studies. Effects of aging and composition of cellulosic coatings are related
IN
PREVIOUS investigations of the adhesion of organic coatings with the Interchemical Adherometer (6, 8, 77): the stripping force per unit width of coating removed, corrected for tear resistance and friction of the knife on the substrate, has been extrapolated to a hypothetical zero coating thickness. This limiting value of the stripping force, or intercept, has been reported to have a negative value in certain cases ( 2 - 4 ) . T o avoid the dilemma of a negative value for adhesion, the interpolated value of the stripping force for a coating of 1-mil thickness, or “mil-hesion,” has been used. Hesion was used in place of adhesion because in each case at least a partial monomolecular film was left attached to the substrate after removal of the coating, as detected by the lubrication of the Adherometer knife on a stripped surface compared to an uncoated surface ( 5 ) . Because of the uncertainty as to the component and the amount of the stripping force responsible for the removal of the coating from a surface, and to provide a common unit for the comparison of adhesion measurements made with different types of instruments, the results of the Adherometer measurements have been reported as “work of hesion,” the work required to remove unit volume of the coating (7).
Experimental
Rate of Stripping. The methods of preparation and testing of coatings with the Adherometer have been described (6, 8, 72). The Adherometer was modified to provide speeds of stripping below the 12 inches per minute rate for which the instrument was designed. For
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Editor’s Note Detailed data for this article including tables, graphs, and discussion appear in the Journal of Chemical and Engineering Data, Vol. 6 (April 1961).
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a series of baked alkyd coatings, the limiting value of the stripping force was found to decrease by 70Yc for a decrease in the rate of stripping from 2 to 0.2 inches per minute (7). This is in disagreement with previous work but in agreement with that of Wolf on nitrocellulose-resin coatings (72). Also in agreement with IYolf is the independence of the slope of the stripping forcecoating thickness curve on the rate of stripping. The intercept of the curves appeared to come closer to zero as the stripping speed was reduced. Consequently, since the mil-hesion approaches the numerical value of the slope for extremely slow rates of stripping, the work of hesion values are calculated from the slope in preference to mil-hesion. Temperature. Aluminum panels were dip-coated with tritolyl phosphate plasticized nitrocellulose, air-dried at 65’ C. for 60 hours and tested after conditioning at 25” C. and 50% relative humidity for different lengths of time. The work of hesion computed from the slope was found to decrease to onethird of its value in 60 days, with the effect more pronounced at first (7). T o determine the effect of the temperature at which the coating was tested, a tritolyl phosphate plasticized nitrocellulose coating was held at the prescribed temperature for 96 hours before testing. In contrast to previous results reported on the basis of limiting stripping force (6),the work of hesion was found to increase by 457, in the temperature range from 35’ to 65” C. ( I ) . Consistent results were obtained when the work was repeated with a different heating method for only one half of the time before testing. The results reported in the literature are in conflict (7, 9. 70). One explanation suggested for the increase in work of hesion with temperature is that as the temperature is raised the increased elasticity in the adhesive or coating allows the strain energy to spread over a larger volume of the coating. As a higher temperature is reached, other effects could be expected to produce a maximum and then a decrease. Plasticizer a n d Resin Content of the Coating. The effect of plasticizer content, as interpreted by the limiting strip-
I N D U S T R I A L AND E N G I N E E R I N G C H E M I S T R Y
ping force meThod, had indicated an optimum plasticizer content ( 6 ) in conflict with Wolf (72), who reported only a decrease upon addition of plasticizer. A reinvestigation showed 507, decrease in work of hesion of nitrocellulose to aluminum when the percentage of dibutyl phthalate was increased to 40%, and for both an ethylcellulose coating and one containing 2070 dammar, a decrease in work of hesion of as much as 75Yc was found (7). I n each case the addition of plasticizer caused the Iargest decrease in hesion when only a small amount was present but neither an optimum nor a minimum effect was found. Pigmentation. Varying proportions of talc and calcium carbonate, as precipitated chalk, were used to prepare ethylcellulose coatings to contain up to 2SY$ pigment (7). The work of hesion of ethylcellulose to aluminum was found to decrease upon addition of chalk and to increase upon addition of talc, in general agreement with the difference in shear strength of the two pigments ( I ) .
Literature Cited
(1) Brantley, L. Reed, J . Chem. Eng. Data
6, No. 2. (2) Brantley, L. K., Bills, Kenneth, Jr., Charnell, J. F.: Div. of Paint, Plastics
and Printing Ink Chemistry, ACS, Preprint Booklet, 15, No. 2, pp. 34-40 (September 1955). (3) Brantley, L. R., Charnell, J. F., Stott, Barbara, Bills, Kenneth, Jr., Zbid., 14, No. 2, pp. 46-48 (September 1954). (4) Brantley, L. R., Stabler, Reginald, Bills, Kenneth, Jr., Zbid., 13, No. 1, pp. 140-2 (March 1953). (5) Brantley, L. K., Stabler, Reginald, Charnell, J. F., Bills, Kenneth, Jr., Zbid.,14, No. 1, pp. 1-5 (March 1954). (6) Brantley, L. Reed, Woodward, Arthur, Carpenter, Gordon, IND.ENG.CHEM. 44, 2386 (1952). (7).Ele: D. D., “Adhesion and Adhesives, p. 26, Wiley, Yew York, 1954. (8) Green, H., Lamattina, T. P., Anal. Chem. 20, 523 (1948). (9) Lasoski, Jr., S. W.,Kraus, Gerard, J . Polymer Sci. 18, 359 (1955). (10) McLaren, A. D., Seiler, C. J., J. Polymer Sci. 4, 63 (1949). (11) Rolle, C. J., Dietrich, T. LA., Anal. Chem. 21, 996 (1949). (12) Wolf, K., FATIPEC Congress Rept., pp. 96-103, May 1953. RECEIVED for review July 5, 1960 ACCEPTED January 27, 1961 Division of Paint, Plastics and Printing Ink Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960. This investigation was assisted by the Office of Naval Research, Contract N9-onr-86701.
