December, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
excess or deficiency of any one weathering factor. The most promising medium for a,ccomplishing such increased speed in deterioration seems to be the use of special atmospheres. T a b l e 111-Comparison of R e s u l t s O b t a i n e d by Accelerated a n d O u t d o o r T e s t s on R e p r e s e n t a t i v e H o u s e P a i n t s --ACCELERATED~-OUTDOOR--No. of No. of Approx. Av. days panels Av. days panels rate of Paint required observed required observed acceleration Firsl Evidence o f Removable Chalk 4 65 5 1 to 16 1 4 12 105 3 1 to 13 2 8 6 115 3 1 to 13 3 9 First Evidence of Checking 1 21 4 300 12 1 to 14 400 4 1 t o 13 2 30 12 3 39 6 500 4 1 t o 13 The light exposures in these tests were made with a 30-inch mercury a r c in a n atmosphere enriched t o 30 per cent oxygen.
I n routine testing, the writers have standardized upon the cycle given in Table 11. By comparison of results obtained under this cycle with those obtained on the test fence, they have been able to arrive a t an approximate figure for the acceleration available in this system. A comparison of some of these results is given in Table 111. On the basis of these data, one month in the accelerated system should approximate 14 months' exposure outdoors in the vicinity of Palmerton.
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This ratio is about double that previously observed with a system in which no addit'ional oxygen was used in the lightexposure chamber.' Figure 11 indicates the general similarity of the failures obtained in the laboratory and outdoors. Conclusion The general conclusion from this work is that the balance of the weathering factors used in any accelerated system must a t least roughly approximate the order and relative magnitude of these factors as they appear in the outdoor weather conditions that are to be simulated. Failure to take this into consideration in designing a weathering cycle might easily cause serious lack of agreement between laboratory and outdoor results where different types of materials are being tested. Acknowledgment The writers wish to express appreciation to D. L. Gamble and other members of their research organization for coljperstion and assistance in this work. 7 Gardner, "Physical and Chemical Examination of Paints, Etc.," Chap. IX b y Nelson, 1926, p. 74; also Nelson a n d McKim, Drugs, Odr Paints, January, 4i (1926); Can. Chem. M e l , 10 (1926).
Cause and Prevention of Staining on White Paints' By H. T. Morgan and J. H. Calbeck THEEAGJ.E-PICHERLEADCo.,
ISCOLORATIONS covering entire painted surfaces caused by soot and dirt are probably the most commen of the various types of staining. Sulfur gas from railroads and industries offers another type. Water washing off roofs and porches often stains white paint surfaces, apparently due to dirt adhering to the paint film. But the type of staining considered in this paper is that due to discoloiations on white paint surfaces caused by corrosion of iron and copper screen wire. These stains, being dark brown to black, are very unsightly. They are not found on surfaces painted with white lead only but surfaces painted with mixed paints show varying amounts of discoloration. I n face of these facts experiments were conducted to determine why some paint films should be stained by corroded iron and copper while white lead films were not.
D
Comparative Staining of Various Paints Selected pine panels were painted with basic carbonate white lead and linseed oil and with four mixed paints of different composition, as follows: (1) Titanium oxide 50, zinc oxide 30, barytes 10, silica 10 per cent (2) Titanox 66, zinc oxide 30, magnesium silicate 4 per cent ( 3 ) Basic carbonate white lead 100 per cent (4) Titanox 60, zinc oxide 40 per cent ( 5 ) 1,ithopone 40, zinc oxide 40, barytes 20 per cent
On the left side of each of these panels was tacked a piece of iron screen wire and on the right-hand side a piece of copper screen wire. The panels were placed on the roof of the laboratory and exposed for one month. At the end of this time they were examined and photographed (Figure 1). Panels 1, 2, 4, and 5 are badly stained while panel 3, painted with 100 per cent basic carbonate white lead, shows only very slight 1
Presented before t h e Midwest Regional Meeting and the Meeting
of the Section of Paint and Varnish Chemistry of t h e American Chemical
Soriety, Madison, Wis., M a y 27 t o 29, 1026.
JOPLIN,
Mo.
staining. It is also noticeable that the stains under the strips of iron screen are more pronounced than under the c o p per, this no doubt being due to the rapidity with which iron Lorrodes. This experiment checks in every respect the complaints as received. Determination of Staining Component An attempt was then made to determine why mixed paints should be the offenders and whether the difficulty could be attributed to any one component. Another set of panels was painted with the following paints: (1) Basic carbonate white lead 40, zinc oxide 40, barytes 20 per cent (2) Basic carbonate white lead 100 per cent (3) Zinc oxide 100 per cent (4) Super-suhlimed white lead 100 p r cent ( 5 ) Basic sulfate white lead 40, zinc oxide 40, barytes 20 per cent (6) Sublimed white lead, 100 per cent (7) Titanox paint containing 20 per cent zinc oxide
The results of this test (Figure 2) show that sublimed white lead surfaces and corroded white lead surfaces are not stained by iron and copper rust. Of the seven panels the one painted with 100 per cent zinc oxide showed the most staining, while all those with lesser amounts of zinc oxide showed almost as much staining. The panel painted with a Titanox paint containing 20 per cent zinc oxide showed considerable staining. This experiment indicated that zinc oxide was the source of all the trouble. Another experiment was carried out to show that iron and copper rust washed off of white leaded surfaces but is retained by white paint surfaces containing zinc oxide. Strips of weather boarding were painted with the following paints and arranged horizontally in the order given: Basic carhonate white lead 100 per cent ( 2 ) Zinc oxide 100 per cent
(1)
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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Super-sublimed white lead 100 per cent Titanox paint containing 30 per cent zinc oxide Basic carbonate white lead 100 per cent A lead and zinc paint containing 30 per cent zinc oxide
On the upper panel painted with basic carbonate white lead were tacked strips of iron and copper screens. Figure 3 shows that the panels containing zinc oxide are stained while the panels painted with 100 per cent lead are not stained.
VOl. 18, No. 12
precipitate the iron as the flocculent hydroxide although the zinc oxide continues to do so. One theory that explains the corrosion of iron is the colloidal theory of Friend. According to this theory iron is first oxidized, forming colloidal ferrous hydroxide, which is immediately oxidized to ferric hydroxide, remaining colloidal. The rust, then being colloidal, is free to wash away as rapidly as it is formed. The peculiar property that zinc oxide possesses, and which no other pigment has, is that it precipitates colloidal hydroxides from solution. This can be shown by the following experiment : Small amounts of the following pigments are placed in separate beakers. (1) Basic Carbonate white lead (2) Super-sublimed white lead ( 3 ) Zinc oxide (4) Titanox ( 5 ) X mixture containing 7 parts of basic carbonate white lead and 3 parts of zinc oxide
Figure 1
Theory of the Staining Property of Zinc Oxide Paints
Why should zinc oxide paint films have this property while no others do? Is it because of their physical properties, being hard and brittle, or is it due to their chemical properties? One explanation offered is that lead films, being soft and chalky, absorb the stain while with the hard zinc film the rusts are not absorbed and remain on the surface of the paint. If a small amount of freshly prepared ferric chloride solution is added to a mixture of zinc oxide and water, the iron is immediately precipitated as the hydroxide and a corresponding amount of zinc goes into solution as the chloride. 2 FeCI,
+ 3 ZnO + 3 HzO = 2 Fe(OH), + 3 ZnCL
This a t first seemed to be the explanation of the trouble, although corroded white lead, being basic, will also precipitate
A little water is added to each and the mixture stirred and allowed to settle. Equal amounts of a colloidal ferric hydroxide solution are added to each beaker and the contents again stirred. After settling, the mixtures are filtered and the filtrates tested for iron with a potassium ferrocyanide solution. The filtrates from 1, 2 , and 4 give positive tests, showing that the ferric hydroxide has remained in solution and free to wash through the filter paper. Filtrates from 3 and 5 give negative tests for iron, showing that the iron had been precipitated and on filtering remained behind with the pigment. The same thing was found to hold true in the case of copper. These experiments are analogous to what happens when iron and copper corrode and their solutions wash over white paint surfaces. When the colloidal hydroxides of these metals penetrate zinc oxide surfaces, however, they are precipitated within the film. As the paint film dries the hydroxides of these metals lose water and form their corresponding oxides, leaving the paint film stained with their respective colors. e
Prevention of Staining
The corrosion of screen wire can be eliminated by frequent painting and this would do away with stains of this type. However, it is hardly in order to suggest painting copper
Figure 3 Figure 2
ferric hydroxide from a freshly prepared solution of ferric chloride. However, if the ferric chloride solution is boiled or allowed to stand for some time until the iron is present as colloidal ferric hydroxide, basic carbonate white lead will not
screen mire, as the chief argument for its use is that of resisting corrosion without a protective coat. Instead, i t seems more practical to suggest the use of white lead in oil to paint window frames and other surfaces on which stains from corroded metals are liable to occur.
