Relation of Permeability to Moisture and Durability
of Paint Systems W. W. KITTELBERGER The New Jersey Zinc Company, Palmerton, Pa.
A
BOUT seven or eight years ago the attention of the
paint industry was focused on certain types of unsatisfactory paint service due to early adherence failure. Since a large number of these failures had been traced to abnormal moisture conditions (S), improved priming practices as well as mill and back priming of house siding were suggested as a means of overcoming these troubles. The value of such schemes was assumed to lie largely in their ability to exclude moisture from the wood and to prevent swelling and shrinkage phenomena which were thought to be the source of the difficulties. To test these ideas, an investigation was instituted in which an attempt was made to correlate permeability of paint systems with their ability to prevent these early adherence failures (8). Some of the results obtained in this investigation are being reported because of the recent interest in permeability measurements. A large number of methods and instruments have been proposed for measuring the moisture permeability of paint films. Muckenfuss (6) placed a film spread on paper, wire cloth, or other porous support, over a pan of water and under a dish containing calcium chloride; all parts were sealed with mercury in such a manner that the water had to pass through the film in order to reach the desiccant. Both fresh and exposed coatings were measured in this manner (7). Several years earlier Gardner (2) had evaluated the water-excluding efficiencies of various paints applied to white pine boards by
In an investigation of the possible relation between the permeability to moisture and the durability of various priming and three-coat painting systems on wood, it was found that the initial permeability to moisture alone cannot be used as a criterion of the protection rendered by such a system on prolonged exposure. The permeability to moisture of the ordinary multicoat paint system is low and, when weathered, was found not to increase appreciably until breaks in the film enabled moisture to enter the wood. > PERMEABILITY EXPRESSED A S A PERCENTAGE OF THE VALUE FOR UNPAINTED WOO0
-- 0
NO 3 7.5 NO 4 11.0
9.0
U
y
NO 5 11.5 NO 6
8.5
OF THREB-COAT SYSTEMS FIGURE2. CONDITION WITH RESPECT TO INTEGRITY FAILURES AFTER %YEAR EXPOSURE 45' SOUTH
FIGURE 1. TESTDISKIN RIMO F BOTTLE, AND METAL CLAMP FOR HOLDIXG DISKFIRM 328
The higher the number, the greater the failure. The.upper bar in eaeh group shows the initial permeability of the priming coat alone. The other three bars in each group show the permeabilities of three-coat systems consisting of primer and two coats of the same exterior house paint: bar 2, unweathered: bar 3 , weathered 1 year; bar 4, weathered 2 years.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
55.0
41.0
31.8
44.0
FIGURE 3.
64.3
72.1
329
97.0
83.1
RELATIONBETWEEN INTEGRITY FAILURES AND PERMEABILITY Permeability in per cent under each film.
measuring the gain in weight resulting from immersion in water for 3 weeks. He also determined the permeability of films tied over wide-mouth jars containing lime water by exposing them to an atmosphere of carbon dioxide. A method similar to Gardner's panel test was used by Browne (1) and Hunt (4) to determine the effectiveness of paint coatings in retarding the absorption of moisture by wood from saturated air. Walker and Hickson (IO)determined the progressive change in permeability on weathering by applying paint coatings to 100-mesh wire screens, fastening them to Petri dishes containing calcium chloride, and weighing the amount of moisture absorbed after exposure to a saturated atmosphere. The testing methods and instruments proposed for measuring the permeability of organic coatings are almost as numerous as the investigators of this field. Although they range from the simple panel test used by Gardner in 1910 to the complicated mercury-sealed test cup developed by Muckenfuss, all are based upon the same principle. The film or coating in question is arranged so that one face is in contact with a high-humidity and the other with a low-humidity atmosphere. Under these conditions water is absorbed into one face and, after passing through the film, evaporates from the opposite side (9). I n one group of testing methods, the low humidity is maintained on the interior of the test cell and the increase in the weight of the apparatus is taken as a measure of permeability (6). I n several other methods these conditions are reversed; the high humidity is maintained within the cell, and its decrease in weight is used as a measure of permeability. Most of the suggested methods utilize the former scheme which has certain disadvantages not found in the latter. At the start of a test the desiccant within the cup or cell creates an essentially dry atmosphere, but as it absorbs mois-
ture during the test, the humidity tends to increase, with the result that the humidity differential between the two faces of the specimen decreases. Since we can maintain a constant low humidity outside more easily than inside the test cells, it follows that we can more easily maintain a constant humidity differential with the high-humidity atmosphere within the cell. Moreover, with the high humidity outside the cell, moisture condensing on the surface of the specimen will be a source of error, which is eliminated by reversing the humidity conditions. Various investigators have differed on another point; some used free films, whereas others employed films attached to a porous support of some kind. Although the use of free films may give more scientifically correct permeability data, their practical value is necessarily limited. Moreover, such specimens are difficult to handle, especially if they are aged or weathered for any great length of time. Obviously, permeability measurements on a film supported and weathered on the same type of surface as that for which the paint is intended should be of the greatest practical value. Fortunately this can be done with house paints which are used on wood surfaces because wood itself is quite permeable to moisture. Therefore, since these investigations were limited to exterior house paint systems, all coatings were tested on wood panels. Test specimens of this type may be sealed to the test cup with either the back of the panel or the paint film facing the high-humidity atmosphere within the cup, depending on the information desired. In the former case wet-wall conditions are simulated, but blistering is apt to complicate the results obtained. The latter arrangement yields more nearly true permeability values and measures the protection rendered by ilm against penetration of moisture into the wood. the f Since interest was primarily centered in studying the normal
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INDUSTRIAL AND ENGINEERING CHEMISTRY
330 Paint A
Paint B
Paint C
13.6
12.1 Unweathered Films
11.7
’
losses occurring after the second day in the cabinet. This seemed to allow sufficient time for the rate of passage of moisture through the painted wood disk to r e a c h an equilibrium value. A t least two, and generally four, unpainted disks were included in each set of t e s t s a s comparison standards.
. .. ,
.
5’ 1
45.9
.
.
41.0
16.2 Films Weathered 36 Months
t
.
T h e permeability results are reported as p e r c e n t a g e s of the v a l u e s obtained for the unpainted wood disks. T h u s t h e p e r m e a b i l i t y of a paint coating is equal to loss in weight through the painted disk X 100 loss in weight through the unpainted disk
84.6
9G.2
14.7 Films Weathered 43 Months
81.4
18.8
80.5
Films Weathered 50 Months
OF PAINT SYSTEMS AFTER WBATHERING FOR VARIOUS PERIODS FIGURE 4. PERMEABILITY
Exposed vertically south: permeability in per cent under each film.
conditions with the painted surface exposed to the highhumidity atmosphere, most of the tests were made with the paint film facing inward.
Testing Method The method used in this investigation was first employed in these laboratories in 1930 (8): Because of its high permeability a thin wood support exerts little or no influence on the permeability value of a paint film; therefore the test specimens were cut down from the back t o a inch (3.18 mm.), or the aints were directly apthickness of plied to panels af this thickness if weatfering was not contemplated. The actual test disks (15/8 inches or 4.13 cm. in diameter) were cemented with sealing wax to the rims of 4-ounce sample bottles containing 25 ml. of distilled water (Figure 1). Warping of the r l s and rupturing of the seals were revented by metal clamps ttin snugly over the mouths of the gottles. Wi% the clamps affixed, the bottles were weighed and pl3Fed in a cabinet held at 120’ F. (48.9’ C.) and a relative humidity of 17 to 20 per cent. Subsequent weighings, usually every other day, gave the amount of water which had escaped through the paint coating. All tests were made in duplicate. The permeability results reported were based on the weight
The reciprocal of this value might be termed the “moisture excluding efficiency’’ (1) or the “moisture i m p e d a n c e ” (11). The results may also be expressed in such absolute terms as the r a t e of p a s s a g e of moisture per square inch of c o a t i n g p e r day. The film area through which water c o u l d escape in the tests covered in this paper was 1.54 square inches (9.9 sq. cm.). This method yields results of satisfactory reproducibility :
Paint Film
Paint -Permeability, %Film -Permeability, YoTest 25 Test 15 Test la Test 20 No. 1 9.7 11.0 4 56.0 53.8 2 29.5 25.5 5 68.9 75.9 3 42.8 43.5 6 84.2 80.6 a Two disks were cut from the same panel and measured a t the same time and under the same conditions, NO.
In the case of the exposed films, the variation is partially due to the difficulty of selecting two or more strictly comparable test areas from a weathered panel. Nevertheless, it is not difficult to obtain reasonably close checks.
Results of Permeability Tests To evaluate the moisture-excluding efficiency of special primers and one-coat systems recommended several years ago for back-priming lumber, twenty-four priming paints comprising four different vehicles and six pigments or pigment combinations were subjected to permeability tests. Three of the vehicles were also tested in the clear in order to determine the effect of pigmentation. Permeability measurements were made on all twenty-seven one-coat systems and on a series of three-coat systems consisting of primer and two
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INDUSTRIAL AND ENGINEERING CHEMISTRY
331
Paint D ,exposed 45' south Paint E, exposed 45' south Paint F , exposed vertically south coats of an exterior h o u s e p a i n t . The t h r e e - c o a t systems were t e s t e d b e f o r e a n d a f t e r 1- and 2year exposure a t 45" f a c i n g south. The results of these measurements as well as the final relative dura14.4 16.2 15.0 Unweathered Films bility ratings of the coatings are given in Figure 2. Tabulation of the r e s u l t s in Figure 2 brings out several interesting points. For example, the priming coats vary widely 'in their permeability to m o i s t u r e ; t h e exWeathered 7 months; 16.3 Weathered 7 months; 12.9 Weathered 9 months; 13.7 tremes are 8.5 and 72.0 per cent of the permeability of u n painted wood. Also, the variation in the permeability of t h e three-coat systems is s m a l l ; t h e two extremes are 5.0 and 9.5 per cent. There is no Weathered 14 months; 44.0 Weathered 12 months; 12.4 Weathered 24 months; 31.8 correlation between the permeability of the priming coats and that of the three-coat systems. The tabulated data s u g g e s t t h a t only negligible changes occurred in the permeability of these coatWeathered 24 months; 72.1 Weathered 19 months; 38.2 Weathered 31 months; 43.2 ings before integrity failures became evident. That there is only a rough correlation bet ween permeab i l i t y v a l u e s and durability gradings a t the end of 2-year exposure i s felt to be d u e t o t h e type of Weathered 35 months; 78.3 Weathered 30 months; 64.3 Weathered 48 months; 72.3 failure exhibited by a FIGURE 5. PERMEABILITY OF PAINT SYSTEMS AFTER WEATHERING FOR VARIOUSPERIODS number of these coatings. I n a good many Permeability in per cent under each film. c a s e s c h e c k i n g developed into a tfpe of failure known as alligatoring, which extended only through the graphs ( X 10) were taken to show the condition of these panels at the time of the test, and permeability measuretwo top coats of paint and led to more or less severe intercoat peeling, leaving the primer films intact. Thus some of the ments were made on the photographed areas. The results relatively impermeable coatings had to be given low duraare shown in Figure 3. There appears to be a close connecbility gradings. It was therefore considered desirable to tion between permeability of weathered panels and degree of investigate further the connection between permeability to integrity failure of the films. moisture and film integrity, particularly with respect to the Considerable support is given to this belief by the evidence more normal types of failure known as checking, cracking, presented in Figures 4 and 5 . Some of these coatings were and scaling. exposed a t an angle of 45' in a semi-industrial location and show many dirt particles which should not be confused with Moisture Permeability and Film Integrity integrity failures. Paint films freshly prepared and weathered A series of test fence panels showing various degrees and for varying periods of time were measured for permeability types of failure was selected for this study. Photomicroto moisture. The differences in the permeability of the three
Paint G
Paint H
15.8
9.8
Unweathered Films
Paint I
Paint J
12.0
10.9
Films Weathered 26 Months, 45' South BETWEEN PERMEABILITY AND FINAL DURABILITY FIGURE 6. RELATION
Permeabilities of unweathered films are given in per cent: permeabilities of weathered films were not measured. Paint K
Paint L
27.5
19.2
Paint M
15.1 Unweathered Films
Paint N
Paint 0
14.3
9.5
Films Weathered 18 Months, 45' South
FIGCRE 7.
RELATION BETWEEN PERMEABILITY AND FINAL DURABILITY
Permeabilities of unweathered films are given in per cent; permeabilities of weathered films were not measured.
332
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INDUSTRIAL AND ENGINEERING CHEMISTRY
unweathered paint systems shown in Figure 4 were insignificant. Paint systems A and C showed sudden increases in permeability after 36-month exposure-that is, after serious integrity failure had occurred. Paint system B was still intact after 50-month exposure and showed only slightly increased permeability over the freshly prepared a m . In Figure 5 paint system D showed a sudden jump in permeability from 16.3 to 44.0 per cent between the seventh and fourteenth month of exposure coincident with the development of serious integrity failure. A similar sudden increase in permeability coincident with the development of integrity failures occurred also in the case of paint systems E and F. On the basis of these results it appears that permeability measurements serve only to determine the amount of wood exposed by the more serious types of film integrity failure. Such failures, however, are usually sufficiently marked and characteristic so that they can be detected and rated by simple visual examination. The only advantage offered by permeability measurement over visual examination seems to be the fact that it makes possible a numerical evaluation of the degree of protection still rendered by the failing film. That there is little correlation between the original permeability value and the h a 1 durability of a paint coating is shown by Figures 6 and 7. With one exception the paints in this test series covered a relatively narrow range in initial permeability, but on exterior exposure they showed widely different degrees and types of failure which were not indicated by the initial permeability data. Paint K had a rather high
333
initial permeability value of 27.5 per cent, which is probably due to its low pigment concentration of 50 per cent by weight. These results, together with the observation that most threecoat systems are quite impermeable to moisture, seem to indicate that permeability measurements cannot serve as the sole criterion of the durability of a paint system. Moreover, it appears that significant changes in permeability occur only when a house paint coating has begun to show integrity failures.
Literature Cited (1) Browne, F.L., IND.ENQ.CHEM.,19, 982 (1927);25, 835 (1933). (2) Gardner, H. A.,“Paint Technology and Tests,” pp. 71 and 73, New York, McGraw-Hill Book Co., 1911. (3) Hartwig, 0. R.,Am. Paint Varnish Mfrs. Assoc., Circ. 355, (1929). (4) Hunt, G.M., U. S . Dept. Agr., Circ. 128 (Oct., 1930). (5) Jacobsen, A. E., Oficial Dioest Fed. Paint & Varnish Production Clubs, No. 146, 215 (1935). (6) Muckenfuss, A. M., J. IND.ENQ.CHEM., 5 , 535 (1913). (7) Muckenfuss, A. M., Proc. Am. SOC.Testing Materials, 14, 361 (1914). (8) Nelson, H.A., Am. Paint S., 15, 20 (April 27, 1931). (9) Payne, H.F.,and Gardner, W. H . , IND.ENQ.CHEM., 29, 893 (1937). (10) Walker, P. H., and Hickson, E. F., Bur. Standards J. Research, 1, 1 (1928). (11) Wray, R. J., and Van Vorst. A. R., IND. ENQ.CHEM.,25, 842 (1933). RECEIVED September 23, 1937. Presented before the Divlsion of Paint and Varnish Chemistry at the 94th Meeting of the American Chemical Society, Rochester, N. Y., September 6 to 10, 1937.
Nature and Constitution of Shellac’ Separation of the Constituent Acids BENJAMIN B. SCHAEFFER2A N D WM. HOWLETT GARDNER Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
A
LMORE complete study of methods for the quantitative separation of the acids forming the components of the resinous portion of shellac is an essential step in the advancement of our knowledge of its constitution. Investigators have met with only partial successes in their attempts to isolate these acids in pure form. This paper describes a new method, the details of which were adopted after an extensive study of many possible procedures. Further applications of this method in the study of the constitution of shellac will be described in future papers. It will be shown also that the speculations made (6, 9) regarding possible structure of this resin have but limited significance.
Known Constituent Acids
A new method is presented for fractionating the mixture of constituent acids obtained upon saponifying shellac ; only those operations are employed which give quantitative separations. Evidence is presented to show that shellac may contain three acids which have not been previously reported and that some of the constituents may be isomers of the others already described.
1 Other articles in this series appeared in this journal in 1929, 1931, 1933, 1936. and 1936, and in the Analytical Edition of INDUSTRIAL AND ENQINEERINQ CHEMISTRY in 1929, 1932, 1933, and 1934. 1 Shellac Research Fellow, 1934-36; present address, United Gas Improvement Company, Philadelphia, Pa.
Tschirch and Farner (13) were the first to isolate a constituent from the resinous portion of lac. They obtained an acid melting a t 101.5” C. by subjecting the part of the sticklac which is soluble in diethyl ether to hydrolysis with steam for several weeks. They named it “aleuritic” acid and suggested that it might be a dihydroxytridecylic acid, C13H2604. However, as they have since conceded ( I C ) , it could well have been the 9,10,16-trihydroxypalmitic acid, CleHa20s,which Harries and Nagel found later in the part of the resin that is insoluble in diethyl ether. This latter acid was also called “aleuritic acid” (7). It had a melting point of 100’ to 101O C. when pure. Endemann (3) isolated another acid melting a t 100’ to 101O C., which he judged to be 10,11,15-trihydroxypalmitic acid, since it gave &-caproicand sebacic acids upon oxidation. Later, Rittler (12) apparently the g,g,16-trihydroxypahitic acid Ivhich yielded Only suberic acid as an oxidation product. He obtained his acid as plates which
