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INDUSTRIAL A N D ENGINEERI,VG CHEMISTRY
Vol. 19, No. 9
A Principle for Testing the Durability of Paints as Protective Coatings for Wood‘ By F. L. Browne E.
s FORESTP R O D U C T S
L A B O R A T O R Y , hIaDISoS,
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Paint Protection a Matter of Resistance to Moisture ROTECTION and decoration are usually given as the Movement two principal reasons for painting exterior woodwork. The paint industry considers protection so important The weathering of wood and its prevention by paint that its cooperative advertising slogan is based upon that coatings were first studied theoretically by TiemannZ and idea. Yet, in the many paint exposure tests on wood that experimentally by the a ~ t h o r . Weathering ~ is attyibuted have been conducted during the last quarter-century, the principally to the disintegrating effect of internal stresses investigators have been content with describing roughly set up in the wood as the result of fluctuating moisture conthe changes in such properties of the paint film as gloss, tent of those portions exposed directly to the weather. opacity, dirt collection, fading, chalking, checking, cracking. The hygroscopic, swelling character of wood and the slow flaking, and scaling. Attempts to integrate the results into transfusion of moisture through i t give rise to a “working” a general judgment of the condition of the coating and to of the wood near the exposed surface against the more slugfix the end point of its service life have failed of acceptance gishly changing interior. Other factors. such as mschanical because of the inability of abrasion and photochemical different workers t o agree on oxidation of the cellulose. the significance and relative doubtless play parts in the The purpose of this paper is to set forth the basic importance of the several weathering proceqs, b u t principle of a technic for measuring, independently of factors involved. No seriswelling and shrinking in the personal bias of the operator, the degree of protecous effort has been made to response to changing atmostion afforded by paint coatings against the weathering determine experimental!y pheric c o n d i t i o n s a r e of wood and the change in their protective power as the how long coatings continue thought to take the leading coatings themselves deteriorate during exposure. Illusto protect wood against depart. trative data are taken from experiments, some of which Paint c o a t i n g s protect terioration. On t h e conwere started more than six years ago with equipment wood against weathering by trary, paint technologists in and routine designed for a somewhat different purpose. retarding the passage of their tests are prone to Details of the experimental procedure are being immoisture into or out of the ignore completely surh eviproved materially in the work now in progress, but the wood sufficiently to damp dences of wood-weathering fundamental principle remains the same. Discussion as wood checks, loose grain, out extreme fluctuations in of minor changes in method and comparison of results moisture content near the cupping, warping, a n d obtained with paints of specified composition are reloosening of nail fastenings, s u r f a c e . They need not served for future publication: for the present purpose on the ground that they prevent transfusion comthe composition of the three paints used in obtaining pletely. It is impossible to are “wood defects,” not the illustrative data is unimportant. say as yet just how retard“paint defects.” Even the ant to moisture coatinas failure of paint over the summust be; this probab!y demerwood earlier than that over the neighboring springwood is sometimes dismissed as pends upon various extrinsic factors, such as species of woad “wood defect” and, when traditional painting methods are and climatic conditions. But, as will be seen, it is not necesfound to give less satisfactory coatings on some kinds of sary to fix the degree of retardation exactly in older to wood than on others, the poor results are likely to be ascribed apply the technic suggested herein to the comparison of the to the “refrartory nature of the wood” rather than t o in- protective values of different paints when applied to wood. appropriate painting procedure. Measuring Moisture-Retarding Effectiveness The writer does not purpose to exaggerate the importance of protection as the object in painting wood. Perhaps Thus moisture-retarding effectiveness of paint coatings decoration is a motive so much more urgent that paint is the physical property upon which their protective value technologists may be justified in paying very little attention for wood depends, and a means for following changes in that in their tests to protection; that is a point to be decided by property during the life of coatings should constitute a psychological rather than by technological inquiry. But measure, a t least for comparative purposes, of their durability as protective agents. Dunlap‘ has described such as long as the paint industry insists upon and the paint user acquiesces in the view that protection is one of the major, a means which, essentially, is the technic of the investigation if not the major, reason for painting, it seems to be incumbent here reported. In carrying out this technic, wood specimens “8 by 4 by upon technologists conducting paint tests to devise some method for measuring the length of time coatings guard 8 inches (1.6 by 10 by 20 em.), with all sharp edges and corners rounded off, are seasoned in a room a t 60 per cent relative wood against the elernents. humidity and 80’ F. (27” C.)and are coated similarly on all The principle for paint testing herein suggested, together with a part surfaces. They are then subjected repeatedly t o the follow-
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of the data, was 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. The first draft of a paper on the subject was received in the office of THIS JOURNAL, June 11, 1926.
2 Sci. A m . , 130,314 (1924); seealsoBrowne, A m .Paint J . , 9, 56 (1925); and Drugs, Oils. Painl.~,41, 88 (1926). 8 Paint Mfrs.’ Assocn. U . S.,Tech. Circ. 238,289 (1925). 4 THIS JOURNAL, 18, 1230 (1926); Mech. Eng., 48, 1457 (1926).
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ISDUSTRIAL AND ENGILVEERINGCHEMISTRY
ing, cycle of conditions, being weighed before and after each exposure to step (2) : (1) 60 per cent relative humidity, 80” F., for 2 weeks (2) 95 t o 100 per cent relative humidity, 80” F., for 2 weeks (3) 60 per cent relative humidity, 80” F., for 2 weeks (4) Outdoor exposure facing south, a t an angle of 60 degrees5 to the horizontal, for 6 weeks
Step (4) secures the actual weat’hering; step (2, ptovides the means for measuring the result; steps (1) and (3) merely return the specimens to a standard moisture cont’ent before their reexposure to the conditions of the other two parts of the cycle. The gain in weight during step (2) is the amount of moisture absorbed through the coating. Plotting moisture absorbed against time of exposure to the outdoor weathering conditions gives a graphical representation of the history of the coating, and comparison of the moisture absorbed through different coatings shows their relative effectiveness in protecting wood against weathering. As will be shown later, the moist.ure absorption by an adequately coated panel is a characteristic of the coating and not of t,he wood, a t least until the coating has begun to disintegrate seriously, so that within the experimental error a given paint allows the same absorption regardless of the wood on which it is applied. .l’ofe-Dunlap computes the “efficiency” of t h e coating by subtracting t h e absorption by t h e coated panel from t h a t b y uncoated panels of the same wood, expressing t h e difference as a percentage of t h e absorption b y uncoated wood. T h a t procedure is more elegant t h a n t h e method employed here and is reasonably convenient when studying different coatings on t h e same kind of wood. I t becomes cumbersome a n d confusing, however, when t h e same coating is studied on different moods, because, although t h e permeability of the coating niay be independent of the wood, the base of t h e percentage calculdtion is not.
Typical Results
Figure 1 is constructed from data obtained by Dunlap6 in the course of a study of moisture-retardant finishes for aircraft propellers. Yellow birch panels were employed, two of them painted with three coats of a widely used type of white house paint which will be designated paint A , two painted with paint A using slightly different reductions with oil and thinner for the priming and second coats, and two painted with three coats of another white paint representat’ive of common practice, which is designated as paint B. The panels with paint A were in the test cycle from October, 1921, to June, 1926; those with paint B from Sovember, 1921, to November, 1926. Of that period of approximately 5 yeais about 21/2 years were spent in step (4) of the test cycle, the rest of the time in steps (l),(2), and (3). Figure 2 show> the exposed faces of the pane!s a t the end of the test. Both types of paint a t first became less permeable to moist,ure on aging, and a t their maximum effectiveness (the minimum of the curves) they differed from each other only very slightly in this quality. Paint B retained its maximum resistance to the passage of moisture for a fairly long time, however, whereas paint A soon began to fall off and continued to become increasingly permeable during the rest of its history. Chalking became noticeable earlier in the case of paint A than in that of paint B , but in both cases it antedated materially the beginning of deterioration in effectiveness of the coating as measured by the moisture absorption. Disintegrat’ion of the coating. leaving bare areas of wood readily visible t o the unaided eye, also began earlier with paint A than with paint B. With paint B disintegration was a close precursor of loss in moisture-retarding effectiveness, but with paint A the resistance of the coat,ing to moisture decreased materially before disintegration began. 6
4 5 degrees would be better; see Walker, THISJ O U R N A L , 16, 528 (1924). Unpublished d a t a of t h e Forest Products Laboratory.
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In December, 1921, 84 panels, 4 panels of each of 21 different species of wood, 2 painted with paint A and 2 with paint C, which was similar but not quite identical in composition with paint B , were started in their first test cycle. The results obtained so far are given in Figure 3. The woods are divided, on the basis of present information regarding their painting characteristics, into three groups, which will be called the “cedar group” (7 woods), the “white pine group” (8 woods), and the “yellow pine group” (6 woods). The graphs show the average absorption of all the panels in each of these groups, which were painted with paint A or with paint C, respectively. Thus each point in the figure represents an average value for 14, 16, or 12 test panels depending upon whether it is the (‘cedar,’’ “white pine,” or “yellow pine” group. As far as they have gone, the new experiments give results very similar to the old ones. Both paints reached a minimum moisture absorption, of roughly 6 grams per panel, after about 24 weeks’ exposure. Paint A then began to become more permeable while the permeability of paint C remained practically unchanged. The species of wood, therefore, seems to affect the resistance of the coating to moisture only very slightly, if a t all, during the early part of its history. which is in confirmation of Dunlap’s previous findings. Later, however, the influence of the wood apparently begins to express itself and the three wood groups give evidence of separating, the decline in resistance of the coating to moisture being more rapid on the yellow pine group than on the white pine group, and least rapid on the cedar group. Some such development should be expected siiice we know that the common painting method employed for all the woods is best adapted to the characteristics of the cedar group and least well adapted t o those of the yellow pine group. I
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some paints just as certainly fail to furnish adequate protection for the wood long before coating disintegration becomes noticeable. If paints are regarded seriously as protective coatings for wood, their durability in that respect must not be left to deduction from assumptions, but must be measured definitely. Although the routine herein suggested may be cumbersome, the principle involved is sound and a more convenient and more rapid technic embodying i t is being developed in this laboratory. Conclusions
1-Since paint coatings protect wood against weathering by retarding the exchange of moisture between wood and air, their durability as protective coatings can be measured by observing their effectiveness in retarding the absorption of moisture from saturated air by painted wood panels, a t intervals during the exposure of the panels to the weather. 2-Paints of different composition may have very different life histories with respect to moisture-excluding effectiveness
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and neither the time of initial chalking nor of initial exposure of wood through coating disintegration can be relied upon as a general indication of the durability of the coating's effectiveness. 3-During the early part of the life history of an initially adequate protective coating the amount of moisture absorbed by coated wood panels is a characteristic of the coating rather than of the wood; that is, the absorption is about the same regardless of the kind of wood coated. During the latter part of the life history of the coating the influence of the wood becomes noticeable. 4-The observations of the change in moisture-excluding effectiveness during exposure confirm inferences previously drawn by the writer as R result of his inspections of test fences on which panels of the same woods and paints were exposed. I n the test-fence results indications of the deterioration in moisture-excluding effectiveness of the coatings were given by the obvious beginning of wood-weathering under coatings that still remained intact.
O x y g e n Required for the Propagation of Hydrogen, Carbon Monoxide, and Methane Flames' By G. W. Jones and G. St. J. Perrott PITTSBURGH EXPERIMENT STATION. U. S. BCREAU OF MINES,PITTSBURGH, PA.
ENERALLT speaking, three factors are essential for
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the development of gas or dust explosions: (1) combustible substances, (2) oxygen supply, and (3) igniting source. The elimination or proper control of any one of these may be used as a means of preventing explosions. The combustible substances usually contain hydrogen, or combinations of the two, and may be in the form of gases or dusts, Sulfur, usually in the form of a sulfide, and the metallic dusts, such as aluminum powder, are also capable of developing explosions. The oxygen supply is generally that furnished by the air present. Normal air consists of 20.93 per cent oxygen, 0.03 per cent carbon dioxide, and 79.04 per cent nitrogen by volume. The nitrogen includes small amounts of the rare gases argon, neon, and krypton. The igniting sources are gaseous or solid substances which are maintained a t a temperature sufficiently high for a period long enough to ignite the gas mixture in question. A heated substance, gaseous or solid, several hundred degrees above the ignition temperature of the gas under investigation will fail to ignite the gas if the period of application of this heat is too short. On the other hand, a heat source a t or only a few degrees above the ignition temperatures will ignite the mixture, provided the application of the heat is sufficiently long in duration. Ignition by flames is one of the most certain and positive methods for igniting gas mixtures. Other sources of ignition are electric, sparks, arcs, and incandescent materials. Any of these, if of sufficient temperature and duration, will ignite explosive mixtures. Methods of Preventing Explosions
Confining our attention to gases, and more especially those found in mines, the engineer has the choice of three methods of preventing explosions: (1) He may eliminate the accumuPresented before the Division of Gas and Fuel Chemistry at the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16, 1927. Published with approval of the Director, U. S. Bureau of Mines.
lation of gas so that the proportions present never reach a percentage high enough to become explosive; ( 2 ) he may remove all possible sources of ignition; and (3) he may control the amount of oxygen present so that explosions become impossible, irrespective of the amount of combustible gas that may be present. I n general, the first and second requisites are followed in practice, thus rendering the control of oxygen supply unnecessary. VENTILATION OF MIms-Under normal conditions of coalmining, methane is the only combustible gas that is liberated in appreciable amounts by the coal strata. Ventilation is usually controlled so as to keep the amount present in the air below the lower inflammable limit, but cases may arise where the accumulation of methane cannot be prevented. These conditions arise in old workings or gobs where ventilation is inadequate. These areas are usually sealed off and explosive hazards prevented by oxygen control. This is automntically taken care of by combination of the oxygen in the sealed area with the coal. The amount of oxygen in the atmosphere is thereby reduced so that the mixture is rendered non-explosive. As will be shown later, when the oxygen present in atmospheres containing methane is reduced to 12 per cent, all mixtures of methane are rendered non-explosive. Under abnormal conditions in mining-as, for example, after mine explosions, mine fires, or through liberation of gases from explosives used in mining-other combustible gases are added to the mine atmosphere. Hydrogen and carbon monoxide may be present as partial combustion products along with varying proportions of black damp (carbon dioxide and nitrogen). The question arises -to what extent must the oxygen in such atmospheres be reduced to render the atmosphere non-explosive? This is of particular importance in the case of mine fires where sections of a mine are sealed, where fire is in progress, in order that safe and efficient recovery work of the sealed area may be carried out. These gases from mine fires may contain rather large proportions of hydrogen and carbon monoxide, especially soon after closing the area under fire.
