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I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Vol. 18, No. 12
Discussion and Summary
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equally in all directions. All the experimental work was carried out in a dark room maintained a t almost constant temperature. The weighings were made by the aid of a red electric bulb as a balance light. ACIDVALUE-The oil film was removed from the drying cabinet at regular intervals and weighed. The film was then placed in a flask and treated with a benzene-alcohol solvent on a steam bath for a definite time. The acid content was determined by titration with 0.02 N alcoholic potash (methanol) using phenolphthalein as indicator.
From Figure'l it is clear that the acid value of the oil containing varying concentrations of the cobalt acetate is an increasing function of the time. The increasing value of this function may be due to a number of causes, some chemical in their nature and others physico-chemical. Thus it may be due to a n acceleration of the following chemical changes: (a) the peroxide formation; ( b ) unsaturated acid formation; ( c ) the peroxide cleavage; (d) the oxidation of aldehydes to acids. It is also obvious that the increase in the acid value may be due to physical changes in the drying film of such a character that will allow an increased opportunity for the distinctly chemical changes to take place a t a greater speed. It is obvious that Figure 1 affords an opportunity of knowing the acid condition of the film a t any time during the drying process. Figure 2 gives the acid value of the drying film as a function of the cobalt acetate concentration. Through the kindness of Prof. C. C. Morris, Department of Mathematics of the Ohio State University, this relationship has been expressed by the following exponential equations: Let y = acid value of the oil, and X = the concentration of the cobalt acetate used. Time in hours 0.5 1.0 1.5 2.0 2.5
-
Log Y
+ 0.94427X + 0.70939 + 1.30518X + 0.74070 + 1.59750X + 0.78740 + 2.20544X + 0.82755 2.78269X + 0.88967
1.36540X' -1.38109X1 1.33300X* -2.01093XP -2.74584XZ $-
These equations have been derived by regarding the value 0.01 cobalt acetate as unity.
Protecting Wood from Moisture' By M . E. Dunlap FORBST PRODUCTS LABORATORY, MADISON, WIS.
OR the protection of wood from moisture too much confidence is usually placed in the ability of a coating to keep the wood from warping and twisting. It is not uncommon to find a painter or wood finisher who feels assured that after applying a coat of filler, primer, or linseed oil, no further moisture changes will take place. The pores in wood are sometimes considered to be so many tiny mouths through which must pass all the moisture which wood takes on or gives off. If this were the case closing these openings with a iiller should prevent these changes and stop the tendency of wood to change shape. Unfortunately, however, the wood substance itself also absorbs the moisture and simply the filling of the pores, which occupy a small part of the area, or applying a thin coat of paint is quite ineffective. I n this laboratory wood has been thoroughly impregnated with linseed oil and other materials in a n attempt to prevent moisture changes, but the results have been unsatisfactory. Filling the openings in wood with an oil does not break up the continuity of the wood fibers, so moisture enters through the surface fibers and passes on through the wood. Apparently it is necessary to supply a coating to restrict the entrance of moisture.
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Experimental Procedure
Early tests showed that there was not a very great difference between subjecting the panels to saturated air and free 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.
water where a fairly effective coating was used; and since we were primarily interested in atmospheric moisture we continued to use the humidity test. All the specimens for standard tests have been of yellow X 4 X 8 inches with rounded edges. These panels birch are made up in quantity and stored at 60 per cent relative humidity until needed for testing. When a coating is to be investigated two panels are removed and coated on all surfaces with three coats of the material to be tested, and after properly drying they are returned to the 60 per cent room and kept there for 2 weeks and then exposed in a compartment maintained at a humidity of 95 to 100 per cent for 2 weeks. After exposure to the high humidity the panels are returned to the 60 per cent room for reconditioning and are later exposed for 6 weeks on the laboratory roof at a n angle of 60 degrees with the horimntal, facing south. The panels are passed through this cycle until the coating becomes badly weathered. The figure obtained for the absorption during the period of exposure to high humidity is used as a basis for the calculation of the efficiency of the coating by comparison with the absorption obtained with a similar panel completely uncoated and exposed to the same conditions. The figures are based upon the absorption over a unit of area, since it has been determined in previous tests that the rate of absorption is a function of the surface area and does not depend upon the weight or density of the wood provided the absorptions are relatively small. Our results as recorded represent the ability of the coating to keep out moisture; thus, a coating having an
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
December, 1926
efficiency of 70 per cent means that it was successful in excluding 70 per cent of the moisture that would have been absorbed had no coating been applied. Effect of Number of Coats
The greatest gain in moisture resistance is usually found in the first two coats that are applied and the gain in moisture resistance by each additional coat becomes small, as illustrated in Tables I and 11. of Two t o Twelve Coats of Varnish (First cycle) No. 1 No. 2 No. 3 Per cent Per cent Per cent 83.5 75.3 78.3 89.6 83.5 82.2 91.6 87.6 87.0 90.3 93.1 89.0 91.0 94.4 90.3 91.6 95.1 90.3
Table I-Efficiency
No. of coats 2 4 6 8 10 12
Table 11-Gain No. of coats 0 to 2
2 4 6 8 10
to to to to to
4 6 8 10 12
in Efficiency by Successive Coats of Varnish (First cycle) No. 1 No. 2 No. 3 Per cent Per cent Per cent 75.3 78.3 83.5 8.2 3.9 6.1 4.1 4.8 2.0 1.3 3.3 1.5 1.3 0.7 1.3 0.0 0.6 0.7
Relative Efficiency of Various Coatings
Linseed oil is commonly considered to be an excellent moisture-proofing material, but we have not found it so. We have tested it in many ways-by brush coating, dipping, soaking, and by impregnation under pressure. The results of a n impregnation treatment are shown in Figure 1 as 17 per cent, which is quite ineffective as compared with some other materials.
Figure 1-Moisture
Wax coatings similar t o those used on floors do not rate high in moisture-proofing value. The test recorded in Figure 1 gave an efficiency of only about 7 per cent. W i l e we have not made a detailed investigation of the effect of various constituents of coatings, some rather significant points are brought out in our studies: Spar varnishes
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are low in moisture resistance as compared with rubbing varnishes. For instance, the efficiency of spar varnish, when new averages about 57 per cent and that of rubbing varnishes, 88 p4r cent. Apparently the greater amount of gum present in a rubbing varnish accounts for the difference. The addition of a pigment to a varnish or oil increases its efficiency in keeping out moisture. An oil paint is far more effective than linseed oil alone and an enamel shows a marked improvement over varnish. Aluminum powder is very effective in keeping out moisture when used in the preparation of a coating. When freshly mixed with the vehicle it leafs out, forming a very effective barrier against moisture changes. Probably the most striking illustration of its value is in raising the efficiency of bronzing liquid on first exposure from about 12 per cent to 92 per cent, a gain of 80 per cent. Addition of aluminum powder also increases the efficiency of spar varnish by about 40 per cent. From 1 to 2 pounds of powder are required to the gallon. The majority of asphalt and pitch paints have good moisture resistance. Their efficiency, however, may also be improved by the addition of 10 to 20 per cent of aluminum powder. I n some cases the powder will leaf out and rise to the surface, hiding the black color. Probably the most effective protection against moisture changes is the aluminum-leaf coating. This process simply incorporates a layer of aluminum leaf in the coating and provides a very strong barrier to moisture. The efficiency of this coating is about 98 per cent. It was developed particularly for use on airplane propellers, where very slight changes in moisture content are important. The process consists of applying a filler for open-grain woods, a coat of varnish for a foundation coat, and a coat of varnish which is used as a size. This size coat is allowed to dry until it is almost dust-
Resistance of Coatings
free, when the leaf is applied over the surface, lapping the edges. It is smoothed down with cotton and a protective coating of spar varnish, enamel, or paint is usually applied over the surface. We have specimens under test which have two and three coats of aluminum leaf that have been exposed to the weather for two years and still maintain an efficiency of
Val. 18, KO. 12
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98 to 99 per cent. One coat of leaf maintained a high value for over one year. White lead paint is probably the best material to apply for durability. Spar varnish and cellulose lacquers do not maintain their efficiencies so well as the aluminum-leaf coating. Cellulose lacquers have an efficiency of a little over 70 per cent when new, and cellulose enamels about 5 or 6 per cent higher. Shellac has a remarkably high moisture-proofing value about 88 per cent-but does not stand up very well when subjected to the weather. It is important to know that none of our ordinary coatings are absolutely waterproof. All permit the passage of moisture to some degree, and if extreme conditions persist over a con-
siderable period of time injury may result to glue joints and woodwork which they cover. Prevention of Warping
Few manufacturers realize the effect that coatings may have on warping. I n many kinds of goods one surface of a panel is finished with a very effective coating while no attention is paid to the back. When moisture changes take place the gain or loss of moisture is greater from the exposed side than from the coated side and if the changes occur rapidly, warping is almost sure to result. Such difficulties may be avoided by applying t o the backs a cheap coating which will balance the face coating in moisture resistance. Such practice would reduce the rate of change in moisture content and permit better equalization of absorbed moisture.
The Analysis of Lacquers’ General Discussion By B. J. Oakes VALENTINS
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H E progress in lacquers in the last few years has been due mainly to research work on soluble nitrocellulose, and this field has by no means been exhausted. Fifteen years ago mixtures of nitrocellulose, pigments, and castor oil were used as coating materials, and during the war mixtures of nitrocellulose and plasticizers were furnished the Government for coating their silver reflectors, and pigmented lacquers were furnished for other work. The possibility of lacquers such as we have today was seen early, but the work on nitrocellulose had to come first. Since that time the work on solvents and plasticizers has kept pace with that of the nitrocellulose, and resins are receiving more and more attention. A simple mixture of nitrocellulose, a couple of resins, a vegetable oil, an animal oil, a couple of plasticizers, a stabilizer, and several pigments present a picture very like a varnish when viewed from the analyst’s point of view. Following good analytical procedure, the analyst will strive to make a separation of groups and then to make a group analysis. To carry out the group analysis, however, he must have indications of the materials present and these indications are best gained by a series of observations and by physical or chemical tests made on the original lacquers. This paper will deal with these observations and physical tests and give a partial separation of the nonvolatile constituents into groups. Preliminary Physical Tests
It cannot be too strongly emphasized that experience in lacquer composition and testing is a prime requisite in the lacquer analyst. This experience can be gained in the laboratory by making up experimental lacquers and subjecting them to a series of physical tests. The laboratory lacquers should include some which approach in characteristics those which it is hoped to analyze. A wide range in composition is possible and should be covered as thoroughly as conditions will permit. A well-organized study of the effects of each ingredient should also be made. Starting with a soluble nitrocellulose ‘kolution,” the other materials are added in turn and tests are made of films, etc. Given this experience, an analyst may face a lacquer analysis with some degree of confidence. 1 Presented as a part of the Symposium on “Analysis of Lacquers” a t the Midwest Regional Meeting and the Meeting of the Section of Paint and Varnish Chemistry of the American Chemical Society, hladison, Wis , May 27 to 29, 1926.
NEWYORK,N. Y.
Certain observations of the material, such as initial and residual odor, consistency, color, instructions for application and uses, are particularly helpful both as guides and as checks. The following physical tests give information as t o the lacquer composition. Films are poured on flexible metal plates and after they have dried some of them are placed in ovens maintained a t various temperatures. The behavior of the films a t these temperatures is a fertile source of information on composition. After cooling from the heat treatment, the plates are bent through various angles. I n every test such as the bend tests mentioned, before conclusions are drawn the operator should assure himself as to the adhesion of the material t o the plate. Faulty cleaning of the metal, for instance, may give a film entirely out of keeping with the true nature of the material. This point deserves great emphasis. A film should be poured on paper, and the drying time, residual odor, and the effect of the dried film on the paper noted. These observations give definite knowledge as to the possible ratio in which the components of the lacquer are present. However, a knowledge of color material is essential in drawing conclusions from any of these tests, for it plays an important part, often a determining part, in the manner the film reacts to the treatment studied. The effect of water on the film offers possibilities for information. Added to these tests are those of hardqess, brittleness, gloss, adhesion, and on materials for certain classes of work such tests as ease of polishing. Tests which have great possibilities in preliminary work are those determining tensile strength and elongation. These tests are carried out on strips of the dried film and usually are followed over quite a period of time t o determine the effect of age on these properties of the film. A great deal of work has been done on this problem a t the laboratories of the h’ew Jersey Zinc Company, and by H. A. Gardner of the Paint and Varnish Research Association. At present complete data are not available to translate results in terms of composition nor are there standard methods of the test or accelerations of the test. The literature of the subject gives additional tests designed for the same purpose and which need not be discussed here. The application of the findings of the physical tests to the analysis of the lacquer may be illustrated here. T h e
