differences in color retention of the one- and two-coat systems are small, less than one NBS unit at all sites, they may prove significant after longer exposure times. The one-coat enamels had consistently higher color retentions than the two-coat enamels.

3.7. Correlation with Accelerated Tests

The correlation between color retention and the boiling acid solubility test is illustrated for Kure Beach and Los Angeles in figure 4. It can be seen that at the mild site, Los Angeles, the color retention remains high regardless of the boiling acid solubility, while at the severe site, Kure Beach, the color retention may be as low as 70 or as high as 98 for approximately the same value of boiling acid solubility.

The relatively poor correlation between color retention and boiling acid solubility led to the development of another test which could be useful in predicting the color retentions of these enamels. The new test uses the same apparatus as the boiling acid solubility test but the reagents and time of treatment were changed. Fifty ml of 12.5 percent aqueous cupric chloride solution are substituted for 150 ml of 6 percent citric acid. The specimen surface is exposed to boiling cupric chloride solution for one hour. The treated area is then rinsed with 10 ml of a 1 percent solution of oxalic acid to remove any green copper stains that may have accumulated. The specimen is removed from the apparatus, washed with a mild detergent solution, rinsed with tap water, distilled

water, and alcohol, and allowed to dry in a near vertical position. After the specimens are dry the color difference between the tested and untested portions of the specimens is determined and is used as a measure of the specimens' weather resistance. These values are plotted as a function of the color-retention values after three years' exposure in figure 5. The correlation is improved over that obtained for the boiling acid solubility as indicated by the smaller residual standard deviations (see table 7) for the fits of the least squares lines. Although the correlation between the color retention and the results of the boiling cupric chloride test is better than that with the boiling acid solubility test, it is felt that longer exposure times are necessary before any recommendations for specification changes are made. This increased exposure time is necessary because the color retention after three years' exposure of the enamels that passed the boiling acid solubility test is adequate (more than 94) at all sites except Kure Beach.

3.8. Correlation with Enamels in a Previous Exposure Test

Kure Beach, Washington, and Los Angeles are three sites common to both the current test and a test initiated in 1956 [11]. The average color-retention data for the enamels exposed at these sites in both tests are included in table 8. These data indicate that the enamels in the current test are as durable as the early porcelain enamels on aluminum at all sites, and at the milder sites, Washington and Los Angeles, are as good as the acid resistant enamels on steel. Since it was

3.9. Nature-Tone Enamels on Aluminum

Seven nature-tone or mat enamels on aluminum were added to the test program after the one-year inspection. The initial gloss, color, and thickness of these enamels are presented in table 9 while the colorretention and percentage gloss-retained data for these enamels are presented in table 10. These data indicate good gloss and color retention for most enamels at all sites. One enamel, AM-7, spalled enough to constitute failure at all sites.

TABLE 9. Initial data for nature-tone enamels on aluminum

Enamel

AM-2

Lt. brown_

19.5

2.3

10.0

AAAAABB

The 45-deg specular gloss was measured in accordance with ASTM designation C-346. b The thickness was measured in accordance with ASTM designation E-376. The boiling acid solubility was determined in accordance with ASTM designation C-283. The acid spot test ratings were determined in accordance with ASTM designation C-282.

The 3-year inspection of the enamels included in the 1964 Exposure Test of Porcelain Enamels on Aluminum indicated that:

1. The color retention of all colors except reds was good at all sites except Kure Beach.

2. There was no significant spalling on any of the enamels originally included in this test. One nature-tone enamel added to the test after one year's exposure did spall.

3. The atmosphere at Kure Beach was extremely aggressive on many of the enamels included in this test.

4. There were no significant effects attributable either to initial gloss or coating thickness.

5. The correlation between color retention and boiling acid solubility was poor.

6. The results of the boiling cupric chloride test correlated better with color retention than did those of the boiling acid solubility test.

[1] Rushmer, M. A., and Burdick, M. D., Weather Resistance of Porcelain Enamels: Effect of Exposure Site and Other Variables after Seven Years, Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 4, 16 pages (May 1966).

[2] Resistance of Porcelain Enameled Utensils to Boiling Acid, C-283 ASTM Book of Standards, Pt. 13, (1968).

[3] Acid Resistance of Porcelain Enamels (Citric Acid Spot Test), C-282 ASTM Book of Standards, Pt. 13, (1968).

[4] Recommended Practice for Measurement of Coating Thickness by Magnetic Field or Eddy Current (Electromagnetic) Test Methods, E-376 ASTM Book of Standards, Pt. 3, (1968).

[5] Gloss of Ceramic Materials, 45-Deg. Specular, C-346, ASTM Book of Standards, Pt. 13, (1968).

[6] Instrumental Evaluation of Color Differences of Opaque Materials, D-2244, ASTM Book of Standards, Pt. 21, (1968).

[7] Potter, A., Using an electronic computer to reduce weathering data, Proceedings of the Porcelain Enamel Institute Forum 20, 73 (1958).

[8] Test for Spalling Resistance of Porcelain Enameled Aluminum, C-486, ASTM Book of Standards, Pt. 13, (1968).

[9] Natrella, M. G., Some tests which are independent of the form of distribution, Chapter 16, Experimental Statistics, Nat. Bur. Stand. (U.S.), Handb. 91 (August 1963).

[10] Recommended Specification for Architectural Porcelain Enamel on Aluminum for Exterior Use, PEI: ALS-105, Porcelain Enamel Institute, (1969).

[11] Moore, D. G., and Potter, A., Effect of Exposure Site on Weather Resistance of Porcelain Enamels Exposed for Three Years, Nat. Bur. Stand. (U.S.), Monogr. 44, 13 pages (April 1962).

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