PANEL TEST EVALUATION OF
EXTERIOR HOUSE PAINTS G. W. ASHMAN
The New Jersey Zinc Company, Palmerton, Pa.
The advantages of panel tests for the evaluation of exterior house paint are well known, but the limitations and disadvantages are not so generally appreciated. An investigation of the various limiting factors has been in progress for several years and has shown at least twenty variables in the testing procedure which can definitely affect durability. The relative influence of each variable changes with the type of formula being tested, and increases or decreases according to the test conditions. This changing degree of importance probably explains some of the contradictions in results. Another major influence is the accelerated nature of panel testing, a condition that may overemphasize certain characteristics and minimize others. An exact interpretation of panel tests results, in terms of probable durability on a house, must, therefore, correct for this condition. Panel exposure tests are capable of a high degree of accuracy and offer a convenient, speedy, and economical method of paint evaluation, if the necessary safeguards are taken.
thousand exterior tests on panels, and several hundred house tests. The factors influencing results may be classified according to ( A ) preparation and application conditions, (B) exposure conditions, and (C) interpretation of results. The differences between small batch production and commercial production are a source of possible error but these effects are not considered here. Also, it is assumed that the formulation variables (raw materials, pigment-nonvolatile ratio, thinning, and drier) have been controlled or evaluated. A group classification of the factors for which definite effect is found is as follows: A . Preparation and Application Conditions: 1. Method of mixing and grinding of paint. 2. Degree of paste or paint aging before application on the panel. 3. Weather conditions during test preparation. 4. Drying period between coats. 5. Indoor preparation of the tests. 6. Aging period of the film before exposure. 7 . Quality and type of wood used in the panels. 8. Type of formula and condition of the weathered paint, if a repaint test. 9. Back-painting of the panels. 10. Skill of the painter. 11. Color of the paint. B. Panel Exposure Conditions: 1. Month of exposure. 2. Weather conditions at time of exposure. 3. Climate of exposure site (amount and season of rainfall, range and rate of change in humidity and temperature, and amount of direct sunlight). 4. Atmospheric conditions at the exposure site (rural, urban, or industrial; amount of vegetation in the vicinity or amount of dirt in the air). 5. Position of test after exposure (inclination of the panel, direction of exposure, amount of protection from the elements, and influence of adjacent tests). C. Interpretation of Results: 1. Effect of moisture. 2. Inability to duplicate on a panel the wide variety of conditions occurring in a house test. 3. Accelerated nature of most panel tests as compared to house tests. 4. Relative importance of various film characteristics and types of film failure.
HE final criterion of paint quality is the result obtained in actual service on a house. Unfortunately, the same paint applied on different houses may show different degrees of durability. For this reason numerous house tests must be made before a definite evaluation of quality can be obtained. The painting of a large number of houses is too involved and too expensive to be used for general evaluation purposes. Therefore, panel exposure tests are widely used. This procedure for testing paints is generally accepted as a means for determining paint quality, and, although the advantages are well recognized, it is probable that the disadvantages of panel exposure tests are not as generally appreciated. The conflicting results obtained in this laboratory and the contradictory conclusions reported by different investigators for tests on similar formulations are disconcerting. These differences emphasize the urgent need for a clearer understanding of the limitations of panel testing. In the Paint Testing Laboratories of The New Jersey Zinc Company a definite evaluation program has been followed for many years. This program has been greatly expanded within the past ten or twelve years and now includes several carefully calibrated laboratory testing procedures, about five
This list is large but not complete. The type of brush, panel design, panel size, and method of shielding are a few of the additional variables that may affect the results. The factors listed have varying degrees of importance, and their relative effect upon results obtained seems to vary according to the type of formula, type of test, and exposure conditions obtaining. Paint quality is determined by its protective value and by its attractiveness. The factors listed may affect either or both of these functions. For simplification, only the effects upon durability will be considered here, since they are most easily illustrated. It must not be inferred, however, that the effects upon paint attractiveness are absent or are of lesser importance. 934
AUGUST. 1936
INDUBl'ItIAL AUD ENGINEERING CHEM ISTHY
935
Method of Paint Preparation Figure 1 illustrates the difference in durability that can result from variation in the paste preparation procedure; two laboratory batches (one pint each) of identical formulas n w e prepared on the same day. l'he pigment and oil of one was incorporated to form a soft paste (hut not too soft to he properly niilled); and in the other a lower proportion of oil n.as used and a stiffer paste obtained. Both pastes were theii ground through the same mill, and thesubsequent preparation, application, and cxp s u r e procedures ivere idcntical. Thephotographs show the two tests after 27 months of vertical south exposure on selected first-grade white pine clapboards. Three sets of tests show this same wide difference in durability. The degree of this effect is surwising - and, as not been satisfactorily explained.
Ingiedienta mired to stiff pasre before minding
1. ErrECT
FIGORE
Ingredients mixed to Bolt pes& beiow grinding
O F DIFFERENT PASTE PREPARATION
in the second test. Figure 2 shows the present degree of separation of the finish paint from the primer applied 6 months earlier. The present condition of the second test i8 still perfect. This emphasizes the need for a standard drying schedule in the preparation of test panels.
Aging Paint Agiilg of the liquid paint changes its physical properties. Similarly, aging of the liquid paint resiilts in a change of durability. This change is sometinies beneficial and sometimes detrimental, the magnitude of the effect and the direction of the change being dependent upon the type of formula. Three sets of tests, thirty-six in all, were exposed at irregular int.ervals and after different periods of aging of the liqiiid paint. Several different formulas were included in each type of paint. All paints a-ere used without further treatment, and in no mise had tlie storage period impaired their working properties. In tn-o of the three groups of tests, the aging period varied between 16 and 26 months, but in the third the aged paints had been stored for only 6 months. In each series of tests, siniilar fresh paints were prepared and were held until the initiiil consistency changes had been complet,cd, a period of not more than 10 days. The respective comparisons m r e then applied simultaneously and were exposed vertically under ideritical conditions on edge-grain western red cedar panels. The results for the respective types of paint in each of tlie three series of tests were in good agreement. The differences between the aged and the fresh paint.s were most definite in those samples stored for the 2-year intervals but were significant enough to be important after 6 months of aging. Tlritli one type of paint the aging was found to be l~eneficial,with another it was detrimental. These iudications are based upon tire panel conditions after 3 years of vertical exposure and illustrate the pmsi1,ility of error in results that may be cnuseil by
