Adhesion of Lacquers to Nonferrous L. REED BRANTLEY, ARTHUR WOODWARD',
AND
GORDON CARPENTER2
Occidental College, Los Angeles, Calif.
T""
adhesive forces. de Bruyne ( 3 )stated that most of the shrinkage took place while the film was still poft, but with other investigators (5, 8, I,$), he reported that considerable residual strain was eventually produced. Gourlay ( B ) , in his investigations of stresses in nitrocellulose films, reported that the stress was proportional to the film thickness and that it decreased on addition of plasticizer and increase of humidity. Andrevs, Hofman-Band, and Tobolsky ( 2 ) found in their study of relaxation of stresses a t constant extension that the rate of relaxation was inversely proportional to the molecular M eight and directly proportional to the temperature. Generally speaking, metals have low coefficients of thermal expansion compared to organic materials. If a metal coated with a high polymer were subjected to a continual rise and fall in temperature, stresses could reach a magnitude to rupture the adhesive bond (3,18). Many investigators have reported a polar attraction of the dipoles of the high polymer t o those of the metal substrate (15, 1 6 ) . Gerngross ( 4 ) postulated that the polymer chains mere oriented at right angles t o the surface and that adhesion resulted from the attraction of the end polar groups with the metal surface. Adams ( 1 ) went further to state that when the chain lengths were great, it was not only the end groups but alsothe sidegroups which were effective. Young ( 8 1 ) reported that resin molecules were inclined at an angle depending upon the number and distribution of the polar groups along the molecule. Hofrichter and McLaren (10) found an increase in adhesion with increasing amounts of maleic acid when they tested theadhesionof vinyl chloridev i q 1 acetate-maleic acid copoll mevs to cellulose films. Harvey and McSeshJ- ( 9 ) reported from adsorption studies that a generally greater attraction for the metal was indicated on increasing chain length of a hydrocarbon, on introducing double bonds or a halogen atom into the molecule, or, especially, on introducing a hydroxyl group. Tnoue ( 11 ) reported that the adhesion of nitrocellulose coatings to Duralumin decreased as the degree of nitration was increased. Figure 1. Two Views of Adherometer
4 nature of the adhesion of organic coatings to metals has been the subject, of much investigat,ion and many art,icles. The lack of satisfactory instruments for its measurement has detracted from the value of much of t,he previous experimental work published. With the recent development of instruments or their refinements designed specifically for the measuren1en.t oC adhesion, new impetus has been given to the study of adhesion. For t,he work reported in this paper, the ivory knife Adherometer (Figure 1) has been used as one of the most promising instruments commercially available. The particular advantage of the Adherometer, in addition to its ability to give reproducible quantitative measuremente, is its method of use by means of TThich several, if not all, of the film properties closely related to the measurement of adhesion, such as tear resistance and resistance to plafitic flow, can be corrected for ( 7 , 1 9 ) . I n this paper, the investigation of the contribution to adhesion of several factors in the make-up of prot'ective coatings, for which the Adherometer is suited, will be discussed. Since the purpose of this investigation was t,o study bhe principles of adhesion, no special attempt has been made to include systems because of their high adhesion alone. The work reported includes the effect on adhesion of the percentage of plasticizer, match of coefficient of expansion between coating and metal substrate, percentage of polar groups, and degree of polymerization in the film former. The organic coatings used contained only polymer, plasticizer, and solvent, in order to have as simple a system for study as possible. hfetal surfaces for use TT-ith the ivory knife have been limited to alloys as hard or harder than 24ST aluminum. When organic coatings are formed on metal surfaces by solvent evaporation, chemical change, or baking a t elevated temperatures, a shrinkage of the coating relative to the metal results. The resulting stresses can be expected to act in competition with the 1 Present address, Polytechnic Institute of Brooklyn, Brooklyn, N. Y . 2 Present address. Naval Civi Engineering Research and Evaluation Laboratory, Port Hueneme, Calif.
2386
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1952
Savage, von Fischer, and Prutton (80) reported that adhesion increased with molecular weight. Josefowitz and Mark (11) gave the optimum degree of polymerization as between 50 and 300. McLaren and Seiler ( 1 7 ) stated that adhesion was independent of molecular weight if the molecular weight was high enough to prevent chain slippage. Kline and Reinhart (IS) stated that adhesion should decrease for the high molecular weights. Inoue (11) reported that adhesion was independent of the degree of polymerization for nitrocellulose coatings on Duralumin.
238'2
20% nonvolatile matter, 85% of which was nitrocellulose and the 15% dibutyl phthalate, The solvent used was 75% butyl acetate and 25% ethyl alcohol. The ethylcellulose series contained 15% nonvolatile matter, 90% of which was ethylcellulose and 10% dibutyl phthalate. The solvent contained 80% toluene and 20% ethyl alcohol. The viscosity of the lacquers was determined by NITROCELLULOSE
ETHYLCELLULOSE
EXPERIMENTAL
PREPARATION OF TEsr PANELS. All solvents and plasticizers used were industrial grade. The nitrocellulose and ethylcellulose samples for the percentage polar groups and the degree of polymerization investigations were specially supplied by the Hercules Powder Co. The aluminum test panels used for a given set of experiments were cut from the same 24ST aluminum sheet and received the same solvent cleaning treatment in order to reduce surface differences to a minimum. The nitrocellulose series of lacquers used for the investigation of the effect of percentage of plasticizer on adhesion was prepared from RS l/a-second (11.8 to 12.2% nitrogen) nitrocellulose. The solvent made up 80% of the lacquer and contained from 86 to 93% butyl acetate, ethyl alcohol making up the balance. The 20% nonvolatile matter consisted of dibutyl phthalate plasticizer and nitrocellulose in the proportions shown in Table I. For the ethylcellulose lacquer series, Dow 20 centipoises type (46.5 to 48.5%ethoxy content) was used, with butyl acetate as the solvent. The 15% nonvolatile matter consisted of dibutyl phthalate plasticizer and ethylcellulose in the proportions shown in Table I. The test panels were dried a t room temperature for approximately 2 months before testing.
TABLEI. EFFECTOF PERCENTAGE PLASTICIZER Composition, % Plasticizer Polymer 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0
*
100.0 95.0 90.0 85.0 80.0 75.0 70.0 65.0 60.0 55.0
0
IO 20 30 PER CENT PLASTICIZER
Figure 2.
0
IO 20 30 PER CENT PLASTICIZER
Effect of Dibutyl Phthalate on Adhesion of Lacquers to Aluminum
the standard falling-ball method as the time in seconds required for a l/lG-inch diameter steel ball to fall 10 inches in a standard viscosity tube, 25 mm. inside diameter, thermostated a t 25" C. A dip method with a regulated speed of withdrawal was used to coat the aluminum test panels. The panels were redipped to prepare the thicker coatings. The coated panels were forced-air dried a t 65" C.
Adhesion, 105 Dynes/Mm. Nitrocellulose Ethylcellulose 0.29 0.35 0.46 0.93 0.67 0.49 0.44 0.10 0.09 0.08
,
0.13 0.23 0.44 0.24 0.13 0.12 0.09 0.09 0.06 0.02
An unplasticized ethylcellulose lacquer and a series of metals were used to investigate the effect of the difference in coefficients of expansion on loss of adhesion resulting from different drying treatments. The lacquer contained 1570 ethylcellulose, Dow 20 centipoises (46.5 to 48.5ojO ethoxy) grade, dissolved in n-butyl acetate. One group of test panels was dried at 40' C. for 14 days and tested 2 days later. The control group was dried a t room temperature for the entire 16 days. The lacquer films for the percentage plasticizer and coefficient of expansion experiments were spread by a motor-driven applicator with adjustable clearance drawn-down blades. To study the part played by polar groups in adhesion, the effect*of the per cent nitration of nitrocellulose and of the ethoxy content of ethylcellulose was investigated. The members of the nitrocellulose and ethylcellulose series were selected to provide a range in percentage of polar groups without differing appreciably from one another in degree of polymerization. Nitrocellulose and ethylcellulose lacquers were also used to investigate the effect of degree of polymerization on adhesion of lacquers. These materials were selected to provide as large a range of degree of polymerization values as possible, as shown by the viscosity of their solutions, without any appreciable difference from one another in percentage of polar groups. I n the case of nitrocellulose, the materials used ranged in degree of polymerization from approximately 50 to 200. The nitrocellulose series of lacquers contained
;i p
20
01
5.5
I I I I 1 5.6 5;1 5.8 5.9 6.0 DIFFERENCE IN COEFFICIENTS OF EXPANSION
x
I 6.1 105
I 6.2
Figure 3. Loss of Adhesion of an Ethylcellulose Lacquer to Metals Dried at 40' Compared to 20' C.
DETERMINATION OF ADHESION WITH THE ADHEROMETER. For each of four or more thicknesses of a given lacquer on a test panel, the force of stripping the film per millimeter width of the strip removed is measured several times and averaged. T o eliminate tear resistance from the stripping force readings of the Adherometer, alternate strips down the panel can be removed in advance of testing, so as to leave attached film strips slightly less wide than the ivory knife. The stripping force, adhesion plus resistance to plastic flow, is then plotted against the corresponding film thickness. A straight line drawn through these points is extrapolated to zero thickness. The intercept, or limiting stripping force for zero thickness, is taken as the force of adhesion (7, 19). SPECIALTESTMETHODS. To shorten the drying period, an accelerated drying procedure was developed. Coated panels were
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2388
TABLE 11. EFFECT ON ADHESION OF THE DIFFERENCEIN EXPANSION COEFFICIENTS OF AN ETHYLCELLULQSE LACQUER AND VARIOUS METALS Metal Aluminum
(24ST)
Brass
Ex ansion CPoeff./ 'F. X l o a n
Diff. in Exp. Adhesion*1 8 D Y n d M m . Lossin Coeff./ Dried at Dried a t Adhesion, O F . X 106 20° C. 40' C. % 8.5 8.6 5.7
1.2 1.1 0.m
0.19 0.17 0.24
0.40
0.41 0.85
53 89
is
Copper Stainless steel 302 0.96 5.7 1,13 0.35 Stainless 6.2 0.04 0.00 steel 430 0.51 0 Expansion coefficient of ethylcellulose is 6.7 X 10-6/' F.
09 100
dried overnight a t room temperature, then forced dried for 48 hours in a stream of air heated to 65" C. Before testing, all panels were suspended in a 50% relative humidity chamber for a minimum of 24 hours. It was found that if stripping force measurements were attempted immediately, the values obtained were often unusually low. If these were plotted against film thickness and the line extrapolated to zero thickness, the extrapolated value of the stripping force obtained was very low or in some cases negative. Upon retesting, the values of the stripping force were found to gradually increase with time until after a period of 1 or 2 weeks constant values for each of the original panels were reached. When these stripping forces were plotted against film thickness, satisfactory values were obtained for the limiting stripping force or adhesion. This effect was related to the drying temperature used, the difference in coefficient of expansion, and the rate of cooling. If lacquered panels were allowed t o cool more slowly or from a lower temperature, this phenomenon was less pronounced and the panels reached the adhesion values of samples dried a t room temperature. This demonstrates the necessity of carefully controlled drying conditions for adhesion investigation experiments.
Vol. 44, No. 10
treated the same except for the drying temperature, variations in the surface conditions between the different metals used was minimized. The effect of percentage of polar groups on the adhesion of lacquers to aluminum is shown in Table I11 for nitrocellulose and ethylcellulose lacquers. The curves in Figure 4 show that both series of lacquers show an increase in adhesion with increase in per cent polar groups. I n the nitrocellulose series the adhesion is more than doubled over the range of per cent nitration used. An increase of adhesion with percentage polar groups is shown for ethylcellulose, although it is not as pronounced as in the nitrocellulose series. Table IV and Figure 5 show the results of the experiments designed to investigate the effect on adhesion of changes in the degree of polymerization, as measured hy viscosity of the solution. The adhesion was plotted against the square root of the viscosity in order to show the effect of the lower molecular weight polymers more clearly. An increase in adhesion with increase of
TABLE111. EFFECTOF PERCENTAGE OF POLAR GROUPS Nitrogen, %
Type SS 1/2 seo. *4S '/e sec. R8 l/n see. RS ' / z seo.
NITROCELLULOSE LACQUERS 11.09 13.1 11.57 9.3 14.4 11.98 12.3 12.10 Ethoxy, % '
Type
Viscosity, Sec.
Viscosity, Bec.
ETHYLCELLULOSE LACQUERS 46.2 94.8 46.3 61.4 47.9 50.7 50.0 49.7
G K N T
Adhesion, 105 Dynes/Mm. 0.40 0.50 0.65 0.90
Adhesion, 103 Dynes/Mm. 0.65 0.50 0.80 1.00
RESULTS AND DISCUSSION
The results of the experiments to investigate the effect of change in the percentage of plasticizer content of lacquers on their adhesion to aluminum are shown in Table I. An optimum percentage of plasticizer is shown in Figure 2 for the adhesion of nitrocellulose and ethylcellulose lacquers to aluminum. The effect of the difference in expansion coefficients of an unplasticized ethylcellulose lacquer and various metals on adhesion is shown in Table 11. The table shows the percentage decreasc or loss in adhesion of the coatings dried for 14 days a t 40' compared to 20' C. Figure 3 shows that for an uiiplasticixed lacquer, for which a condition of stress is more likely to occur, the comparative loss in adhesion increases with the difference in the coefficient8 of expansion of thc coa,ting and the substrate. Since the adhesion values obtained with the 40" C. drj-ing procedure are in each case compared to controls of thc same metal NITROCELLULOSE
IID
11.5
12.0
PERCENTAGE
Figi1r.e 4.
NITROGEN
45
46 47 PERCENTAGE
48
49
Typen
ySy,2,5,;ps. RS 5-6
SOC.
KS 13-20 sec. Typea
Nitrogen, %
Ethoxy, %
Square Root of Viscosity, Seo.
ETHYLCELLULOSE LACQUER 47.9 2.4 X.50 47.9 7.0 N-100 47.9 10.0 N-100 47.8 10.1 Hercules Powder Co. designation. N-10
a
Square Root of Viscosity, Sea.
NITROCELLULOSE LACQUER 12.03 1.5 12.05 4 6 12.12 I1 9 12.07 23 6
NITROCELLULOSE
E T h Y L CELLULOSE
12.5
TABLEIV. ADHESIONTO ALUMINUM OF LACQUERS DIFFERING IN THE DEGREEOF POLYMERIZATION OF THE POLYMER USED Adhesion,
105 Dynes/Mm.
0.10 0.65 1.05 1.55
Adhesion
106 D y n e s / & h .
0.15 0.25 0.30 0.25
E T H Y L CELLULOSE
50
ETHOXY GROUPS
Effect of Per C e n t Polar Groups on Adhesion of Lacquers to A l u m i n u m
SQUARE ROOT OF VISCOSITY, SEC.
SQUARE ROOT OF VISCOSITY,
SEC.
Figure 5. Effect of Degree of Polymerization on Adhesion of Lacquer to A l u m i n u m
October 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
polymerization is shown to occur for both the nitrocellulose and ethylcellulose systems. As observed in the percentage polar group studies, the increase in adhesion was largest for the nitrocellulose lacquers. ACKNOWLEDGMENT
This investigation was assisted by the Office of Naval Research, Contract N9-onr-86701. The assistance of the Los Angeles Paint, Varnish, and Lacquer Association in providing the Adherometer and raw materials, and of the Hercules Powder Co. in furnishing special types of ethylcellulose and nitrocellulose samples, is also gratefully acknowledged. LITERATURE CITED
(1) Adams, N. X., “Physios and Chemistry of Surfaces,” p. 92,
London, Oxford University Press, 1938.
(2) Andrews, R. D., Hofman-Band, N., and Tobolsky, A. V., J .
(3) (4) (5) (6) (7)
Polymer Sci., 3,669 (1948). de Bruyne, N. A., J . Sci. Instruments, 24,29 (1947). Gerngross, O., 2. angew. Chem., 44, 232 (1931). Gourlay, J. S., Aircraft Eng., 18, 296 (1946). Gourlay, J. S., J.Oil & Colour Chemists’ Assoc., 29, 94 (1946). Green, H., and Lamattina, T., Anal. Chem., 20, 523 (1948).
2389
(8) Grinsfelder, H., Oficial Digest Federation Paint & Varnwh Production Clubs, No. 312, 42 (1951). (9) Harvey, E. N., Jr., and McNesby, J. R., paper presented before the Division of Paint, Varnish, and Plastics Chemistry, 114th Meeting AM. CHEM.SOC.,Washington, D. C., 1948. (10) Hofrichter, C. H., and McLaren, A. D., IND.ENG.CHEM.,40, 329 (1948). (11) Inoue, Yukihiko, J . C h a . SOC.Japan, Ind. Chem. Sect., 51, 161 (1948). (12) Josefowitz, D., and Mark, H., India Rubber World, 106, 33 (1944). (13) Kline, G. M., and Reinhart, F. W., Mech. Eng., 72, 717 (1950). (14) Littleton, J. T., J. Am. Ceram. Soc., 18, 239 (1939). (15) McBain, J. W., et al., Repts. 1 , 2, S, Adhesive Research Committee, London, H.M. Stationery Office, 1922, 1926, 1936. (16) McLaren, A. D., J . Polymer Sci., 3, 652 (1948). (17) McLaren, A. D., and Seiler, C. J., Ibid., 4, 63 (1949). (18) Meynis de Paulin, J. J., Rev. aluminium, 24, 309 (1947). (19) Rolle, C. J., and Dietrich, T. L., Anal. Chem., 21, 996 (1949). (20) Savage, R. L., von Fischer, W., and Prutton, C. F., Oficial Digest Federation Paint & Varnish Production Clubs, No. 220, 591 (1942). (21) Young, G. H., J . Am. Chem. SOC.,60,2825 (1938). RECEIVED for review September 7, 1951. ACCEPTEDMay 3, 1952. Paper presented before the XIIth International Congress of Pure and Applied Chemistry, Elastomers and Plastomers Section, New York, Sept. 11, 1951.
Surface Active Agent Phenomena in Dust Abatement P. L. WALKER, JR., E, E. PETERSEN, AND C. C. WRIGHT Pennsylvania State College, State College, Pa.
F
OR many years surface active agents have been used in water sprays for the abatement of dust. The principal benefit claimed for their use has been that they enable water to wet the dust more readily-a factor believed t o be advantageous in knocking the dust out of the air more quickly. The increased wetting power of a surface active agent-water solution is based not only on the reduction of surface tension of the solution but also upon the reduction of the interfacial tension between the solid and liquid. Despite the theoretical reasons why utilization of a surface active agent (commonly called wetting agent) should improve dust abatement, conflicting reports on their merits have come to the attention of the authors, particularly with reference to dust abatement in coal mines and at coal cleaning plants. A considerable amount of work has been done on the use of wetting agents for dust abatement, much of which has been reviewed in recent publications by the U. s. Bureau of Mines (11, 13, 91, 24). A recent report by Merritts and Reay (91) deals with the use of wetting agent sprays for abatement of dust during continuous miner operations. They report that up t o 90% of the dust present when water alone is used is knocked out of the air when slightly higher than 0.01% wetting agent solutions are employed. The authors thoroughly investigated a wide variety of promising surface active agents in order to determine the most economical ones in terms of efficiency and cost. The relative merits of these agents were determined by gently dropping coal particles, ranging in equivalent diameter from 25 to 100 microns, individually, onto the surface of solutions of wetting agents of various concentrations contained in 250-ml. beakers. preliminary tests were made with 16 sille fractions separated from a bituminous coal sample. These fractions had equivalent
diameters ranging progressively from 25 to 44 microns u p to 840 to 1000 microns. Results indicated that, for the procedure employed, the rate of disappearance was not a measurable function of particle size. For the standard test, therefore, the procedure adopted consisted of dropping coal particles, individually, approximately 1 cm. onto the surfaces of progressively more dilute solutions until a dilution was found a t which the particles were not instantaneously wetted. For Tergitol this concentration was found to lie between 0.0030 and 0.0025 weight %. The time of wetting was measured as the time for the particles to disappear from the surface of the solution. This test was adapted from the widely used Draves test (6, 6) for the wetting of fabrics. If the time of wetting of the coal particles was virtually instantaneous, the agent was classified as satisfactory. The various agents were tested in distilled water and in the presence of acids, bases, and salts a p t to be found in typical mine waters. Although the data herein reported are for distilled water solutions, the particular agent used was found equally effective in the typical mine water solutions. I n this way a wide variety of surface active agents was evaluated on a relative basis, even though specific data could not be secured regarding the effective concentrations necessary when the agents were used in sprays. SURFACE ACTIVE AGENT PHENOMENA
To comprehend more clearly the possible implications of laboratory wetting results and to permit use of such data in specific spray application problems, a study was made of the phenomena of surface active agent solutions and the physical changes involved in going from large volume solutions to small droplets. From this study, it would appear possible to use wetting results obtained with large volume, dilute or bulk solutions, t o predict wetting with spray droplets, if it is realized that in spray droplets the ratio of the surface area to volume is much greater
