INDUSTRIAL AND ENGIR’EERING CHEMISTRY
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catalytic, coupled with isomerization, alkylation, polymerization, aromatization, etc., are developing rapidly, and are increasing both yield and quality of gasoline. The very magnitude of the petroleum industry makes such developments gradual. KO matter how valuable a new refining process may be, considerable time must elapse before it can play a suhstantial part in the industry. The writer visualizes a gradual development of the refining art, with a corresponding gradual increase in the quality of the marketed products, and with no apparent limit on this quality. A corresponding gradual increase in the efficiency of the automotive engine may likewise be expected. What part will tetraethyllead play in this development? Perhaps we should say “antiknock compounds” instead of “tetraethyllead” since we are considering the future, and particularly since some mixtures of lead alkyls are already known which possess advantages over tetraethyllead. The writer believes that antiknock compounds will continue to play much the same part in the future that they have in the past. First, wherever it is more economic to obtain any given octane level by the use of antiknocks, they will continue to be so used; secondly, they will always be added to the best available refinery product to produce continually the “fuel of the future” for which the automobile of the future will be developed.
VOL. 31. NO. 12
It is sometimes argued that improvements in the quality of gasoline are uneconomic. The definition of the word ‘(uneconomic” is somewhat open to question. Certainly it must be admitted that the oil industry is entitled to a profit, and that the selling price of improved products should justify any increased cost. But whether or not improvement in quality necessarily involves increased cost is problematical as we look into the future. I n any case, it can hardly be said that variations in the cost of manufacture of gasoline over the last few years have been the primary factor in the profit or loss of the petroleum industry. The thin dividends of the petroleum industry cannot be laid a t the door of the refinery technologist! In summary, we can say that the discovery of the antiknock value of tetraethyllead has played a n important role in the development of the petroleum and automotive industries; that many problems of manufacture and utilization have been solved, and it can be expected that others will be solved as they arise; that after sixteen years we find tetraethyllead a well established factor in the economic life of the petroleum industry; and finally, that there is every expectation that it will continue to play its part in the fuel and automobile of the future, either unchanged or in the form of a n improved substitute. PREBENTED before the meeting of the Institute of Petroleum, Chioago, 111.
Moisture Peeling of House Paints Mechanism of Moisture Peeling a.
J. W. ILIFF AND R. DAVIS E. 1. du Pont de Nemours and Company, Inc., Philadelphia, Penna. HE previous paper in this series1 presented data on the geographical distribution of the moisture peeling of paint based on observations of a large number of house tests. Figure 1 of that article showed that moisture failure is much more common in the colder and damper climates than in the warmer and drier climates. This naturally suggests that a low outside temperature in combination with moisture is a necessary condition for this type of failure. Three sources of moisture are conceivable which can cause this type of failure-condensation, direct water contact on the back of the painted surface, and driving of water against the exterior of the painted surface.
T
Condensation Observations of a considerable number of practical cases have shown that condensation is a n important cause of moisture failure in house paint8. I n conducting a preliminary study of this problem, six small buildings were constructed; each was approximately 3 feet square and 6 feet high. The exteriors were covered with ordinary house siding painted in the usual manner. An interior wall of Celotex was included i n half of the buildings, the others were unlined. The buildings 1
IND. ENQ.CHEW.31, 1407
(1939).
were electrically heated and were humidified by pans of water placed on the floor. Temperatures were controlled through suitable thermostatic equipment, and air circulation was provided by electric fans. The buildings were operated a t different temperatures. The three lined buildings (Figure 1) were operated a t 50”, 60”, and 70” F., respectively. The unlined buildings were operated a t 40”, 50°, and 60” F. Conforming to field observations, operation of the buildings was started late in the fall. Some of the results obtained in this preliminary study are shown in Figure 2. Failure occurred only in the lined buildings. Moreover, the most rapid failure appeared in the building operated a t the highest temperature. These results were checked in larger buildings erected subsequently. Figure 3 shows the details of the lined and unlined sections of one of these buildings. Figure 4 shows differences in paint failure between the lined and unlined adjacent sections; the building was operated a t 70‘ E’. The following explanation is offered for the absence of failure in the unlined section: Apparently the temperature of the unprotected siding in the unlined section was so high that no condensation took place throughout its thickness. Obviously, lower external temperatures than those encountered during
DECEMBER, 1939
INPUSTRIAL AND ENGINEERIR'G CHEMISTRY
The three sources of moisture are discussed : condensation, direct water contact, and driving of water directly through the paint film from the outside. The condition of the paint film and its relation to moisture failure are described. Of the three sources of moisture, condensation and direct water contact are by far the most important. A temperature gradient through the wall with paint on the cold side is necessary for failure resulting from condensation. A temperature gradient through the wall with the paint on the cold side accelerates failure
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by direct water contact. Moisture failure rarely occurs until the wood a t the paintwood interface approaches or reaches saturation. Moisture failure does not occur with a film whose integrity is low enough so t h a t moisture will pass out as rapidly as it appears. The permeability of none of the common paint types is sufficient to prevent failure. New and flexible paint films fail by blistering, and as the rigidity of the film increases, blistering becomes peeling. Repaint jobs over old paint for this reason usually show only peeling failure.
these tests would cause failure even on the unlined surface. From the results in Figure 2 it is evident that the most rapid failure occurs under the effect of the highest temperature gradient. I t further appeared probable that moisture actually
4 0. 1
FIGURE 1. LINEDTESTHOUSE
migrated under the effect of a temperature gradient within a piece of wood. This was checked experimentally. A piece of wood was first subjected to a high humidity until a moisture content of 22 per cent was obtained. It was then completely sealed by the application of three coats of aluminum paint to its exterior surface. One side was then exposed to a temperature of 25" F., the other side to a temperature of 77" F. The results are shown in Figure 5 . The moisture content of the cold side increased while that of the warm side decreased. Since there is a temperature gradient between the inside and the outside surfaces of the building, Condensation will necessarily take place a t the dew point of the interior air. If the wood composing the siding is all below this dew point, condensation will take place on the interior surface of the siding. On the other hand, if the temperature a t which condensation takes place is within the siding, water will start to collect a t that point. These two examples of condensation location are illustrated in Figure 6. I n case A condensation first took place on the interior surface of the siding. Approximately 3
weeks elapsed before the moisture content of the wood immediately behind the paint film began to rise. I n case B condensation took place within the siding and apparently very close to the wood-paint interface. The wood a t this point reached saturation before the moisture content of the wood on the interior surface of the siding began to rise. The term "saturation" refers here to the moisture content above which the measuring instrument fails to give a specific reading. All moisture contents of approximately 28 per cent or over are considered saturation. In practically every instance the wood a t the wood-paint interface reached 25 per cent moisture content or approached saturation before failure took place.
FIGURE3. DETAIL O F LINED AND SIDEWALLS
USLINED
Figure 7 is a typical example of the time relation of the appearance of the paint failure and the moisture content of the wood a t the wood-paint interface. As a general rule, if the weather conditions were such that the exterior temperature of the wood was maintained below freezing, paint failure would
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1NDUSTRIAL AND ENGlNEERlNG CHEMISTRY
not take place until the weather moderated. Obviously. in such cases the condensed moisture was frozen, and consequently, failure not could take place.
U
VOL. 31, NO, 12 I
r
Direct Water Contact “Direct water contact” here means that the water reaches the painted wood in the form of a liquid. For example, through leaks in the exterior surface, such m insufficiently flashed windows or open joints, etc., the term also includes wicked-up moisture, which results from placing the wood on surfaces which become wet or in contact with the ground. n
io
20
So
40
30
60
DAYS
FIGURE 6 . CONDENSATION FAILURE SI.
(Con. = considerable;
Unlined
Lined
FIGURE 4. DIFFERENCES IN PAINTFAILURE BETWEEX LINEDAND UNLINED ADJACENTSECT,IONS
Unheated buildings are much less likely to show moisture failure of the paint than are heated buildings. This appears to be true with water resulting from condensation and also from direct contact. Liquid water was allowed to drip down an inside corner of each of the small buildings previously described. Failure first took place in the building operated a t the highest trmperature.
Further tests on direct water contact were carried out in the larger buildings. Water was sprayed directly onto the backs of the test panels. Figure 8 shows a comparison of results between condensation moisture arid direct water coatact moisture undcr otherwise identical conditions. Spraying the backs of the panels 8 hours a day for 5 days a week caused failure to appear more rapidly than did condensation. On the other hand, spraying them only twice a week caused failure to appear at approximately the same time as did coudensation. Here again, approximate saturation was approached before failure of the paint occurred. 4 number of
slight1
typical examples of failure on actual houses are shown in Table I. In every case the moisture content of the wood at the point of failure was higher than that of portions of the house showing no failure. In a number of cases the moisture content at the point of failure was not close to saturation. This was due to the fact that the conditions which caused failure were not existent a t the time the measurements were made.
Water through Paint Film I t is oiten claimed that moisture appearing in painted siding has been driven through the paint itself. An attempt was made to obtain paint failure by direct spraying of the paint film. This could be considered analogous to a continuous driving rain. These experiments were again carried out on
0
FIQURE 5. MOISTUREMIGRATION UNDER TEMPERATURE GUADXEXT
=
LO
4G
60
DO
80
‘0
Y5
FIGURE7. TYPICAL MOISTURE C~NTENT-FAILURE CURYES oue of the test buildings by spraying sections of painted siding with apparatus specially provided for the purpose. The results are shown in Figure 9. An exccssive amount of spraying was necessary to cause any blistering. It appears, therefore, that this is an extremely unlikely cause of paint failure.
Condition of Paint Film Field observations have often indicated that the physical characteristics or condition of the film at the time of failure
DECEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
1449
1,'
..,
40
A
A-.
60
80
I
,
, ,., I
0
D A Y S
20
D
JO
JO
40
A YS
FIGURE9. FAILURE RESVLTIXG FROM EXTERIOH SPRAYING OF FILM
P A N E L BACK8
may affect the probability of failirre or the type of failure; hence a study along these lines was indicated. F I L M PERMEABILITY. Paint failure occurs when moisture accumulates behind the film and in the painted wood. It appears obvious that moisture will not accumulate a t this point if the filmis sufficiently permeable to allow the moisture to evaporate as rapidly as it appears. Kone of the common types of paint are free from moisture failure, The pcrmeahility of four cOmmonand widely differing paint compositions is shown in the following table. These permeability measurements are based on a method in which the film is exposed to liquid water on one side and desiccated air on the other.
IO
0
FIGURE 8. FNLURE RESULTING FBOM SPRAYING
This permeability range covers that of the corninon paints on the market: Paint
k C
D
a
.\IC.
Peimesbility 1 Mo. d t e i E r ~ o s u i e n 0.14
0.20 0.20 0.08
01 wster per 89. cm. per hour at 3 mils film thiokness.
Figure 10 shows the actual failure of the four paints in question. Obviously the permeability of none of these com-
FIGURE10. FAILVICE OF FOUR TYPICAL PAINTS
