POROSITY OF PAINT FILMS *
Adsorption Studies of Unsupported Linseed Oil Films IRVIN WOLOcI(. AND B. L. HARRIS Johns Hopkins University, Baltimore 18, Md. Surface area measurement by adsorption of krypton has been extended to samples of total surface area equal to 1000 sq. cm. and applied to the study of unsupported linseed oil films. The observance of very low roughness factors leads to the conclusion that there is no appreciable quantity of discrete fine pores-i.e., the order of several molecular diameters-in such films. Extraction of the films with acetone resultedin approximately the same weight loss for films of different thicknesses and very little increase in surface area.
E
XTENSTVE studies have been made of the permeability of ‘ paint films to water vapor in an attempt to evolve a theory of the mechanism of permeation. It has been shown that the mechanism is a very complicated one and it has been postulated that one or more of the following mechanisms are responsible: diffusion through discrete pores, solution in the film and subsequent diffusion, and diffusion through or between micelles (8-18, 14). As an alternative attack on the problem of paint film structure, it was decided to investigate the possibility of discrete pores by a method other than permeability studies. According to the statement of the Federation of Paint and Varnish Production Clubs ( 6 ) , such a possibility does exist. There has been little actual work on the existence of pores in films. Sheppard and Newsome (13) showed that, although submicroscopic pores exist in the surface of cellulose films, these pores do not penetrate the film completely, inasmuch as the rate of permeation of water was independent of the hydrostatic head applied. This work, however, has not been applied to drying oil films nor to paint films. This study was therefore undertaken to obtain positive information on the presence of physical pores in linseed oil films. It was decided to test for the existence of pores by measuring the total surface area of samples of unsupported film of known geometrical area, because the presence of fine pores will increase the total area over that measured for the same film without pores. The technique chosen for measurement of the area was that of gaseous adsorption according to the general method of Brunauer, Emmett, and Teller (BET, 3 ) as modified for small total areas by Beebe, Beckwith, and Honig ( I ) , using krypton as the adsorbate. In considering this method, the questions arose as to (1) whether the krypton molecule could adsorb in a pore just large enough t o allow the transmission of water molecules; and (2) whether the presence of pores would increase the surface area of the film enough so that a porous film could be positively differentiated from a nonporous film. Because the diameter of the adsorbed krypton molecule is about 5.1 A. ( I ) , and that of a water molecule about 2.8 A. ( 2 ) , the krypton molecule might be adsorbed in any capillary with a diameter twice that of a water molecule. It was believed that pores too small to adsorb krypton would not contribute markedly to the permeability of water vapor because gaseous diffusion in such pores is inappreciable and surface diffusion is thought to be slow. Gaseous diffusion is appreciable when the pores are the order of the mean free path of the gas. This was calculated and found to be about 10-6 om. for water vapor under usual atmospheric conditions, which is twentyor thirtyfold as great as the diameter of the pore smaller than a kryptop molecule. Very little work has been done on surface diffusion of molecules through capillaries the order of their own diameters. The work of Emmett and DeWitt seems t o indicate
that the diffusion of anv molecule down such caaillaries is a slow process and one that may well entail a considerable energy of activation ( 5 ) . As to the question of increase in surface area due to porosity, a film 2 mils thick containing pores 10 A. in diameter spaced on 500 A. centers (50 times the pore diameter) would give a calculated thirtyfold increase in surface area over the geometrical external area. The presence of a few very large pores would not be detected by the present method, because such pores would not cause an appreciable increase in surface area of the films. PREPARATION O F FILMS
The linseed oil used in this work was Spencer Kellogg S: Sons “Superior” varnish oil having the following specifications :
The sample was supplied by the National Paint, Varnish and Lacquer Association and contained drier to give a concentration of 0.3% lead, 0.03% cobalt, and 0.03% manganese. The linseed oil films were prepared by coating a panel of tincoated sheet metal and stripping the films by amalgamation with mercury. The tin panel was f i s t cleaned with solvent, and then a strip of Scotch tape waa laid across the top. The panel was then coated by dipping it vertically in the linseed oil and removing it very slowly at constant speed by means of a Fisher-Payne dip coater. Theoretically, the dip coater should remove the panel slowly enough so that there is no run-down. Actually, however, the viscosity of the linseed oil was not great enough to prevent run-down, so in order to minimize the unevenness produced in the thickness, the anels were inverted for alternate coats. Each coat was oven-8ied 24 hours at 110’ C. before the next coat was applied. After the required number of coats had been applied, the Scotch tape was separated from the metal by means of a razor blade, and mercury was inserted in the opening. I n a short time, the mercury worked its way between the film and the panel, and the stripped film could be lifted off intact. Panels measuring approximately 7.5 cm. by 8.5 em. were used. providing finished films of about 50 sq. em. geometric area, or a total plane surface area of 100 sq. cm. (both sides of film). Ten of these films were used for a sample, giving a plane area of 0.1 sq. meter. KRYPTON ADSORPTION METHOD
The measurement of small surface areas by gaseous adsorption of krypton at -195’ C. has been described by Beebe and coworkers ( I ) and Davis and co-workers ( 4 ) . Briefly, this is done by measuring the amount of gas the sample physically adsorbs a t varying relative pressures a t a temperature near the boiling point of the gas. From these data, the B E T plot is made, from which, in turn, the volume equivalent to a monolayer on the surface of the sample can be obtained. If the area occupied by an
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1348
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
ton isotherm a t 68' K, gave a U K ~of 21.6 sq. A. Although the nitrogen isotherm and the krypton isotherm were run at different temperatures, the coefficient of expansion of glass was calculated and found to be too small to make any temperature correction for the surface area necessary.
01
a. y 03
WEASUREMEh-T OF SURFACE AREA O F LIh-SEED FILMS
0
cl
0
n
W
.02 0 0 0
a W
5i >" .Ol
0
Vol. 42, No. 7
The areas of linseed oil filnB of three thicknesses-approxiniately 0.75, 1.5, and 3 mils-were measured. They were made up of two, four, and eight coats, respectively. It was found t o be extremely difficult to strip a single-coat film without its tearing and also rolling back and sticking together; so no one-coat films were measured. Even the thicker films had a tendency to roll back and stick together, making their handling rather difficult. A t first, the films for a sample were cut in half, placed on top of one anot'her in a pile, and then rolled up and placed in the sample tube, which had a diamet'er of about 15 mm. It was found, however, that the surface area obtained from such a sample was too small, indicating that sonic of the films were sticking together, preventing the krypt'on molecules from adsorbing on the surface. Thereafter, the films were cut into small pieces, and each individual piece xas dropped into the sample tube, with very little packing pressure applied, to prevent sticking together. The presumption that the latter niet.hod kept sticking to a minimum was confirmed in several ways by the data. First, one of the samples was removed from the sample tube, repacked, and rerun (runs 2LE10 and 2LE20, Table I). Although the second run gave an area about 5% greater than the initial run, this variation was not great enough to affect the conclusions from these experiments. Secondly, two different untreated samples (runs 2L20 and 3LlO) of approximately the same geomet,ric area agreed very closely in area when measured by krypton, Thirdly, an acetonc-extracted film, when re-extracted and rerun (runs lLElO and 1LE20), gave an area very close t o the one obtained after the first extraction, which can be interpreted as a check on the effect of repacking, assuming the area did not change appreciably. The untreated eight-coat sample (Sample I ) was placed in the sample tube in a roll, giving a very low value for the surface area. This was not realized until after the film had been extracted, but because the values for the surface area of the untreated four-coat sample and the untreated t,wo-coat sample checked so closely, it was deemed unneccssary to make up another eight-coat sample in order to check its initial area. The untreated area for Sample 1 wm estimated by assuming a value of 1.14 for the roughness factor, which is the average of that for Samples 2 and 3. The t,wo-coat films (Sample 3) stuck together so badly after extract,ion that it was impossible to separate the pieces in order to run the surface area.
I .20 .40 60 E
Figure 1. Adsorption Isotherms of Krypton 011 Linseed Oil Films at 68.6' K. Upper. Extracted film, r u n 2LElO Lotoer. Unextracted film, r u n 2L20
adsorbed molecule in the monolayer is known, the surface area of the sample can then be calculated. The area of the sample used in the current investigation was of the order of 0.1 sq. meter, appreciably smaller than the samples used by the above investigators. It was found to be impossible to measure this area accurately a t -195' C. Because it was impractical to increase the size of the sample appreciably, it was decided to work at lower temperatures, thus decreasing the saturation pressure and, consequently, the dead volume correction. This was done by maintaining the pressure of the boiling liquid nitrogen bath a t a constant value below atmospheric pressure and hence controlling the temperature a t a lower value than - 195' C. To accomplish this, a vacuum pump was connected to the bath through a Cartesian manostat (Emil Greiner Company). At first, it was decided to decrease the saturation pressure, p o , of krypton 100-fold. This meant a-orking with liquid nitrogen a t a bath pressure of 75 mm. of mercury, corresponding to 61.9' K., and a p o value for krypton of 0.027 mm. At this pressure, however, equilibration of the first point on the isotherm took several hours, possibly because the film was loosely packed in the sample tube and it took this long to reach temperature equilibrium a t the low gas pressure. Because of this and because a small variation in the liquid nitrogen bath pressure during the course of a run resulted in an appreciable change in the saturatlon pressure, it was necessary to use a somewhat higher temperature. The next pressure tried was about 235 mm , corresponding to 68.6' K., and a liquid krypton p 0 of 0.26 mm. At this pressure, a small variation in bath pressure produced only a small change in p o , and equilibration was very much faster. This pressure proved satisfactory throughout the runs reported. It was decided to check the published values for U K ~ ,the area occupied by the adsorbed krypton molecule (as compared t o the nitrogen molecule), using a sample of borosilicate glass spheres for the correlation. The nitrogen determination a t 77' K. gave a value of 4.17 meters per gram for the surface area of the spheres, checking a previously determined value. On this basis, the kryp-
TABLE I. SURFACE ARE.4
LICASUREMENTS O F
Geometrical
LINSEEDOILFILMS
Tima
(BZT), ' S.T.P.
Area by Krypton, Sq. M.
Roughness Factorb
lLElO 1LE20
Sample 1, 8 coats, 3 mils thick Untreated 0.1058 ,., , , Extracted 24 hours ,.,, 0.02455 Extracted 48 hours , . .. 0,02472
(0,121) C 0.1433 0.1442
(1.14) 1.36 1.37
2L20 2LE10 2LE2O
Sample 2 . 4 coats, 1.5 Untreated 0.0976 Extracted 24 hours ,. ,, Repacked ,. ,
3L10
Samyle 3, 2 coats, 0.75 mil thick Untreated 0.0941 0,01882
.$rea,
~~~n KO.
1L10
Treatinent
Sq. AI.
mils thick 0.01856 0,02000
0,02116
0 1082
1.11
0.11&3 0.1234
1.20
0.1098
1.17
1 26
Calculated from statistical average BET plot from data for several rum. b Ratio of area by krypton t o geometrical area. For & plane surface, value would be 1.0 if krypton area was true total area. 0 Estimated assuming roughness factor of 1.14, average of values for untreated samples 2 a n d 3. d r e a measured o n this salnple was smailer, owing to faulty packing. a
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
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The weight losses on extraction by acetone were measured on different films from those used for the surface area determinations. This was done because the unextracted film used for surface area picked up a small amount of mercury and this might have affected the weight loss on extraction, if a small amount of this mercury had been lost in handling.
TABLE 11. ACETONE EXTRACTION OF LINSEEDOILFILMS Number of Coats
Initial Weight,
in Film 2 4 8
G. 0.2466 0.4124 0.8150
Weight Loss for 24-
Hour Extraction G. % 0.0366 14.8 0.0352 8.5
....
..
Weight Loss for 48-
Hour Extraction G. %
0.0379 0.0420 0.0453
15.4 10.2 5 .,56
The average thickness of the films was calculated from the geometric area and the weight, using a value of 1.0 for the specific gravity of the dried linseed film. On some of the samples, krypton runs on the same sample on successive days produced BET plots which varied significantly. The samples were not removed between runs, but merely pumped overnight t o degas for the next day's run. No explanation could be found for the variation, so in order to evaluate the data better, the method of least squares was used to calculate the slopes and intercepts of all B E T plots. A typical adsorption isotherm and B E T plot for the extracted and unextracted four-coat film is given in Figures 1 and 2. SUMMARY
By working a t a temperature of 68.6" K. (-204.5" C.)) it was possible to measure accurately surface areas of the order of 1000 sq. em. (0.1 sq. meter). A sample of smaller size can probably be measured with reasonable accuracy. Although there was significant variation in individual runs on a single sample of linseed films, the statistical average values checked well from one sample to another of the same geometric area, or for the same sample after repacking. Each average was for at least two separate runs on a given linseed oil film. Two runs on duplicate samples of glass spheres checked to within 1%, indicating that the variation in results on linseed films may be due to nature of the film itself rather than the method. The value of 21.6 sq. A. determined for the area of the adsorbed krypton molecule on glass spheres was higher than the average values of 19.5 and 20.8 sq. A. reported by Beebe and co-workers ( 1 ) and by Davis and co-workers (4), respectively. The values reported by these authors are average values for several adsorbents. It is well known that adsorption areas of molecules vary with the adsorbent (1, 4, 7). The present authors' value agrees well with the value of 21.8 sq. A. on porous glass reported by Beebe and co-workers and the values of 21.9 and 20.4 sq. A. on glass spheres reported by Davis and co-workers. The surface area data of the linseed oil films measured indicate that there is no appreciable quantity of fine pores in the film large enough to allow physical transmission of water vapor. The average roughness factor of 1.14 for the untreated films is even less than the value of 1.3, usually reported for smooth metal surfaces. The areaa were calculated using a value of 21.6 sq. A. for U K ~ even , higher than the average values of 19.5 and 20.8 sq. A., reported by Beebe and Davis, respectively. The latter values would have given roughness factors of 1.03 and 1.10, respectively. The low roughness may be due to the concept of films as semifluid, consisting of a liquidlike phase, which would tend to decrease the surface roughness. Extraction of the films with acetone resulted in approximately the same total weight loss for all three samples, with a slight increase in roughness factor (Table 11). This seems t o indicate that the extraction merely removed some of the surface layer, effecting
0
.05
.I5
.IO
.20
.25
p/
5
Figure 2. BET Plots of Krypton on Linseed Oil Films at 68.6" K. Upper. Unextractcd film, runs 2L20 and 2L21 Lower. Extracted film, runs 2LE10 to 2LE15
a slight increase in the roughness. The increase was small enough to rule out the possibility of the creation of pores. ACKNOWLEDGMENT
This problem is part of a general project sponsored by the Federation of Paint and Varnish Production Clubs. The authors wish to acknowledge the aasistance of that group and of the Baltimore Paint and Varnish Production Club for information and a grant-in-aid of the research. LITERATURE CITED (1) Beebe, R. A., Beckwith, J. B.,and Honig, J. M., J. Am. Chem. Soc., 67, 1554 (1945). (2) Bernal, J. D., and Fowler, R. H., J . Chem. Phys., 1, 518 (1933). (3) Brunauer, S.,Emmett, P. H., and Teller, E., J . Am. Chem. SOC., 60,309 (1938). (4) Davis, R., DeWitt, T. W., and Emmett, P. H., J. Phus. Colloid Chem., 51, 1232 (1947). (5) Emmett, P. H., and DeWitt, T. W., J. Am. Chem. Soc., 65, 1253 (1943). (6) Federation Paint & Varnish Production Clubs, Oficial Digest Federation Paint & Varnish Production Clubs, No. 268, 278 (1947). (7) Harris, B. L., and Emmett, P. H., J. Phys. Colloid Chem., 53, 811 (1949). (8) Kienle, R. H., J . SOC.Chem. Ind., 55, 229 (1936). (9) Payne, H. F., New York Paint & Varnish Production Club, Circ. 10 (1937). (10) Payne, H. F., Official Digest Federation Paint & Varnish Production Clubs, 8, No. 159, 297 (1936). (11) Payne, H. F., and Gardner, W. H., IND. ENG.CHEM.,29, 893 (1937). (12) Sheppard, S. E., and Newsome, P. T., J . Phys. Chem., 33, 1817 (1929). (13) Ibid., 34, 1158 (1930). (14) Taylor, W. E., Hermann, D . B., and Kemp, A. R . , ISD. ENG. CHEM.,28, 1255 (1936).
RECEIVED October 23, 1949.
