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Application of the Statistical Method in Testing Paints for Durability’ By J. H. Calbeckl THE EAGLE-PICHER LEADCo.,
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HE durability of a paint is not only a difficult property to define but also a very difficult one to evaluate. Durability may be defined by the time of outside exposure during which the paint film will maintain its good appearance, protect the surface painted, and continually be in perfect condition for repainting. The durability of a paint may be determined by observations of the behavior of the paint in actual use or by comparison with other paints applied and exposed under similar conditions on a test fence or in a n accelerated weathering device. The durability of the different paints is judged by a comparison of the paint film, protection of the painted surface, and condition of the surface for repainting. Test fences are used to determine the extent to which the durability of the paint is affected by its composition. Among the variables other than the composition of the paint that may affect the durability of the paint film are: character of the surface painted, weather conditions a t the time of painting and during drying, method of application, number and thickness of coats applied. Some of these variables are interdependent, such as the thickness of the paint coat and the composition, or time of drying and weather conditions. I n constructing a test fence an effort is made to keep as many as possible of these variables constant. If all of them could be controlled, a test fence with relatively few panels would give data that would be scientifically accurate. As yet, however, no test fence has been able to meet this condition.. This criticism applies especially to the small fence that the average manufacturer has on his roof, where one or two panels of each formula are exposed more or less irregularly and the great variety of formulas advertised and sold today (as the result of scientific research) is the best evidence that our methods of outside exposure and especially our methods of interpreting and collecting data need to be improved. The realization of this condition has brought out a variety of accelerated weathering devices, but as yet these devices are subject to the same criticism as test fences. For this reason the adoption of statistical methods in the study of the exposures is the only way to learn the effect of one cause as segregated from all other causes. Unfortunately, such a method calls for considerable outlay of time and money and is out of the question for the ordinary paint man. A review of the results of the North Dakota test fences with reference to one or two of the variables that are not generally considered may therefore be of interest. The North Dakota test fences are unique in that the work has been conducted in a most precise and scientific manner and has been directed by outstanding paint technologists representing no particular industry or organization and with no purpose but to obtain the most accurate scientific data possible on the durability of paints. This work, extensive as it has been, gives hardly enough data to make conclusive the results shown in the paper and the tables are presented more to illustrate a method than to shorn conclusive evidence. 3 Presented as a part of the Symposium on “Testing Paints for Durability” before the Midwest Regional Meeting and the Meeting of the Section of Paint and Varnish Chemistry of the American Chemical Society, Madison, Wis., May 27 to 29, 1926. 2 Director of Research.
JOPLIN.
Mo.
Volume Ratio as an Index of Durability
It has long been recognized that the weight of a paint is a criterion of its quality. Instead of speaking of the weight of the paint it is more acceptable to speak of its pigment concentration and if consideration is given to the differences in specific gravity of the pigments the pigment concentration by volume should be considered. I n order to differentiate between the pigment concentration by volume of the liquid paint and the dried paint film-the term “volume ratio” will be used. The volume ratio of a paint is defined as t h e percentage by volume of the pigment phase in the dried paint film. For all practical purposes it is the ratio of the volume of the pigment to the volume of paint less the volatile component of the vehicle. Table I is a comparison of durability and the volume ratio of 120 panels exposed in the 1915 test fence of North Dakota. The data include all the exposures made on the west side, where the three coats have the same composition and t h e vehicle used mas the same. The durability was taken from the “reading” or grading under the heading “condition.” Each panel was then tallied in the square corresponding to its “condition” and volume ratio. I n this table all of t h e panels graded rrgood” and half of the panels graded “fair” have a volume ratio of 28 or above, and all but one of t h e panels that graded “bad” and 55 out of 66 of the panels which graded “poor” have a volume ratio of less than 28. Table I1 shows similar results for the third year of the 1921 fence. All of the “goods” and 10 out of 26 “fairs” have volume ratios greater than 28, and 15 out of 17 “bads” and 23 out of 23 “poors” have volume ratios of less than 28. Table I1 shows a greater percentage of “fairs” with a low volume ratio, as we should expect when comparing a 3-year exposure with a 5-year exposure. Of course, if tables were prepared on the second- and first-year inspections a greater number of “goods” and “fairs” would be numbered with t h e low volume ratios. It is interesting to note to what extent the 1921 test fence checks the 1915 fence. By combining the results of t h e third-year inspections of both the 1921 and 1915 fences we double the number of items and-obtain a more accurate chart for the third-year inspections. Table I11 shows the 1915-1921 combination. As before, the volume ratio seems to be a reliable index of durability, although the third-year inspection of the 1915 fence has added some exceptions i n the 12 “goods” with volume ratios of less than 28. Lead, Zinc, and Inert Pigments
The extent to which the lead, zinc, and inerts in the formula. affect the durability is a matter in which there is a variety of opinions. Tables IV and VI1 show the effect of the lead or zinc content as a percentage by weight of the total pigment content. The tables indicate that the higher the lead content and the lower the zinc content the better the condition of the paint after 5 years’ exposure. The North Dakota fences when reviewed by this method indicate that there is no optimum percentage of zinc, as many believe. Table VI11 shows the effect of inerts on durability and is difficult to interpret inasmuch as the actual percentage of
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inert does not seem to have much to do with the gradings. With one exception, all the volume ratios of those panels with inerts grading “good” and “fair” have ratios of above 28. Durability and Initial Chalking
One of the reasons for this review of the North Dakota fences was to use the results as a check on the results of an accelerated weathering machine which we have been using, There is a strong temptation to take initial chalking as an index of durability, especially in weathering machines using intense ultra-violet light. Tables V and VI show a comparison between the chalking that was observed a t the end of the first year’s exposure of the 1915 fence and the condition of the paint after 5 years’ exposure. I n both tables the different degrees of chalking are distributed rather uniformly between the four conditions. The totals show greater chalk-
Vol. 18, No. 12
ing on the west side than on the east side, as would be expected. On the west side none of the 28 “goods” showed any chalking a t the end of the first year and on the east side only 4 of the 27 “goods” showed any chalking after one year’s exposure. These tables show rather decisively that chalking should be expected of a durable paint and that the extent of chalking after the first year’s exposure cannot be made a measure of durability. Conclusion
Many similar tables might be prepared from these reports, but these few show how a statistical method may be applied to reports where a great many results are given. If accelerated weathering devices become standardized a good opportunity will be afforded to study durability by similar methods and on a greater scale.
Accelerated Weathering’ A Consideration of Some Fundamentals Governing Its Application By H. A. Nelson and F. C. Schmutz PAINT
SECTION, RESEARCH DIVISION, THENEWJERSEYZINC CO.,PALMERTON, PA
0 BE of practical value, due to shrinkage, or both, and The time required for testing paint, varnish, and any system for testthen of (3) revealing these nitrocellulose lacquer products in service makes the ing the endurance of we a k n e s s e s t h r o u g h the development of acceptable laboratory accelerated paint, varnish, and nitrocellubreaking up of the surface. service tests a problem of major importance to the lose lacquer finishes must give This view of the weatherindustry. A number of such testing schemes have results which can be intering process does not take into already been described. Some are specialized tests preted in terms of actual, or account d e t e r i o r a t i o n b y intended to indicate only the relative resistance of the a t least relative, life under erosion. This varies so much product to the action of some major weathering factor, use. Some users of accelerin its relative importance that as light or light in combination with moisture. These ated weathering systems have no attempt has yet been made tests are useful, but are also limited in their scope, and reported that the results obto standardize it as a factor care must be exercised in interpreting them; especially tained with one type of finish in routine laboratory testing, if materials of widely different physical properties are do not bear the same ratio of although in a few cases, where being compared. accelerated deterioration as it is particularly important, The ideal test is one so designed as to reproduce the those of another type when some special scheme of abraactual weathering that occurs in service. The latter both results are c o m p a r e d sion has been applied-as, for has the advantages of wider application and direct with the deterioration under example, the rubber abrasion interpretation in terms of the ordinary visible results. actual use. For example, one machine used in conjunction However, the formulation of a properly balanced labweek’s exposure of a nitrowith accelerated weathering oratory weathering cycle is no simple matter and decellulose lacquer surface in for testing the durability of mands careful consideration of certain general printhe w e a t h e r i n g u n i t m a y highway marking paints.2 ciples, some of which are discussed in this paper. mean 30 weeks’ outdoor exThe Drocess of reducing the distensibility of oleoresinous3 posure in this vicinity, while one week’s exposure of an oleoresinous house paint may mean and nitrocellulose4 films may involve oxidations, condensaonly 10 weeks under actual use. Even such contrary results tion, and chemical decomposition, individually or collectively. as very poor house paints showing better endurance than very The formation of water-soluble reaction products (acetic acid, good nitrocellulose lacquers have been reported. Naturally, soaps, and oxides of nitrogen) generally accompanies the in order to understand the possible causes of these erratic re- decomposition process. It has been observed that in the sults, we must first understand some of the general principles case of painted surfaces exposed to industrial conditions, that govern the application of an accelerated weathering test. where considerable amounts of gases such as carbon dioxide A study of these fundamentals has been under way in this and sulfur dioxide are present in air, the formation of waterlaboratory for some time, and some of the results and con- soluble material very often becomes a major reaction. Xext, temperature changes, even though rather limited as clusions arrived a t to date will be briefly presented here. to actual degrees of change, set up expansion or contraction Analysis of Weathering Process stresses and strains (1) within the film, (2) between coats of I n a primary sense, the weathering of a paint, varnish, or a finish, or (3) between the surface to which it is applied lacquer surface may be considered a process of (1) rendering and the finish itself, which in a nondistensible film can be the film indistensible or (2) bringing about internal strains 2 Nelson and Werthan, THIS JOURNAL, 18, 965 (1926).
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1 Presented before the Midwest Regional Meeting and the Meeting of the Section of Paint and Varnish Chemistry of the American Chemical Society, Madison, Wis., May 27 to 29, 1926.
8 Stutz, Nelson, and Schmutz, THIS J O U R N A L , 17, 1141 (1925), and eccompanying references. 4 Weisel, Paint, 011Chem R e v , 79 (1925).
