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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).
T
u
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).
INDUSTXIAL A N D ENGINEEEING CHEMISTRY
December, 1926
compensated for only by cracking or peeling. Should the temperature change extend sufficiently below freezing temperatures, the stress-strain effect may be further increased by a change of phase of adsorbed water in the film3 or hy a solidification of certain constituents of the film that are normally semiliquid or plastic at ordinary temperatures. Accelerated Weathering
Accelerated weathering is intended to he a speeding up of the weathering processes just outlined. Toward ac-
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Arc). The former has a line spectrum with approximately 58 per cent of its total intensity below 4100 A. and 36 per cent below 3100 A,, while the latter has a hand spectrum, more closely resembling that of the sun, with approximately 34 per cent of its intensity below 4100 and none below 3100 (These measurements were made with a Hilger Type F 110, linear thermopile and D’Arsonval type galvanometer). The short wave lengths of the spectrum are especially active iii inducing oxidation of oleoresinous films. Hence, although the total intensity of the mercury arc is only about 86 per cent as much as that of the carbon arc, visible deteriorations on
A.
A.
__ Figure I-Pian
for Accelerated Weathering Exposure Tank. in Which Either 6-Inch Mercury Are or the Carbon
Arc Can Be W e d
oornplishirig this acceleratiou of deterioration, we have a t our disposal, a t present., the following ngcncies:
or treated atmospheres (o;ygen- orbzone-enriched) as additionii deteriorating agents. WA’CZK-SPKAY WAsizmc-This factor accelerates weathering mainly in so far as i t dissolves water-soluble products previously formed a i d emulsifies the surface, as erempliiied in the whitening of some varnishes. It is true that some actual erasion takes ~ilace,but this effect is a rather small and constant factor as compred with the erosion that may take place under certain outdour exnosure conditions. KETKIGiRtdTloN-This C X ~ O S U I permits ~ temperatme changes of a wide range, depending on the capacity of the equipment available.
A description and discussion of an apparatus used in thiv laboratory for applying the above factors in routine testing were Dresented before the meetine of the American Societv for Tksting Materials in June, 1f26.6 The main features i f the light exposure apparatus will he briefiy outlined here. Figures 1 and 2 show a plan and a photograph of this part of the weathering system. Primarily, it is a circular exposure tank, 4 feet in diameter and 16 inches in height, with the destructive light source a t the huh. An air recirculatory system is used to facilitate the mainteriauce of a desired humidity and temperature conditions. A water spray, a silica-gel dehydrator, and heating and cooling coils are included in this air-recirculating system. Gas inlets and o u t lets are also provided to permit the introduction of gases, as oxygen or ozone. Refrigeration and water-spray exposures are carried on in separate units, but it is possible to use the water spray in the light-exposure chamber.
equidistant dest surfaces seem to be produced by-the former much more rapidly than the latter. For example, two sets of test panels, painted with three coats of a house paint, were exposed for approximately one week to a weathering cycle similar in all respects, except that the mercury arc was used as the light source in one case aiid the treated carbon arc in the other. I n the former exposure the surfaces lost 76 per cent of their original gloss (gloss readings are according to the new “percentage gloss scale” recommended by the Customs Department for defining gloss of paper stock) and chalked badly, while in the latter they lost 13 per cent in gloss and did not chalk. Although the mercury arc has been used throughout in this investigation, the fundamentals that hold for exposure systems using the mercury are should also apply in systems using the carbon arc, except in so far as the actual deteriorab ing effectivenesses of the two light sources may be different.
Exposures to Destructive Light and Its Auxiliary Deteriorating Agents
Figure 2-Photograph of Expoemre Tank with Carbon Arc in Place
No ideal destructive light source exactly comparable to sunlight is as yet available. The two light sources most generally used are the quartz mercury arc (Cooper-Hewitt “Uviarc”) and the flaming arc with chemically treated electrodes Devices Company “Fado-Meter”
Heat as an Auxiliary to Light
Nelson aod Schrnutz. Pioc. Am. So
