ISDUSTRIAL A S D ESGISEERISG CHEMISTRY
7i2
Vol. 22. s o . 7
Yellowing of Interior Gloss Paints and Enamels’ S . Werthan, A. C. Elm, and R. H. Wien THEPiEIV
D
ISCOLORATIOS o r “yellowing” of organic s u b s t a n c e s occurs quite frequently as a conseq u e n c e of aging processes. The yellowing of wood and paper, of ivory and celluloid, of varnished furniture and oil paintings is well known. W e a r e u s e d t o seeing a d i s c o l o r e d spot on a wall w h e n removing pictures or furniture, and this effect is m o s t n o t i c e a b l e and annoying in the case of white or light tinted enamels. Literature
JERSEY Z I S C C o l l ~ h N YP.ALMERTON, , Pa
This paper is a preliminary report covering an investigation of the yellowing of interior gloss paints and enamels. A brief survey of the literature is given. Comparisons of accelerated tests with normal service results are discussed. Color changes are measured with a Pfund colorimeter. The effects of exposure conditions, such as length of exposure, temperature, humidity, kind of atmosphere (oxidizing or inert), color or wave length of light, and absence of light have been investigated. The influence of the various constituents of the enamel is considered. These include such factors as the effect of pigment, volatile thinner, drier, kind of oil, and treatment of oil. Owing to these many variables, a solution of the problem necessitates a fundamental investigation of the chemical reactions occurring in a pure vehicle. This phase of the subject, using pure trilinolenic glyceride, is being investigated.
Many investigators have studied various phases of this problem. Thompson (19) ascribes the yellowing of interior enamels to the action of ammonia on the film, while Sniith (18) states that heat and absence of light are responsible. Toch (20) is of the opinion that, “in as much as water colors, pastel, and tempera painting do not show this particular deterioration, it is quite evident that the cause does not lie in the pigment itself, nor in the substratum which is painted, but in the medium which is used to bind the pigment to its foundation,” and comes to the conclusion that the chlorophyll contained in the vegetable oils is to be blamed for the yellowing of white enamels. Rebs (1.5) claims that a natural dyestuff in the oils, bleached during the drying, forms again later and thus is the chief cause of the yellowing. This theory is refuted by Eibner ( 5 ) and his pupils (2, 11 , 16), who, based on extensive research, conclude that the highly unsaturated compounds in the oils forin yellow or brown oxidation products. They arc inclined to accept as the cause of the yellowing the ketoxy compounds which Fahrion (6) and Holde and Rlnrcusson (9) say are formed during the oxidation of the films. Scheiber ( 1 7 ) and Sauroy (121 ascribe it to the formation of chromophoric systems of the type of mono-, di-, or tri-ketones. Rforrell and Marks (1‘~) believe that the yellowing is due to the formation of oxy groups. Since the data on the subject are incomplete and the conclusions not entirely in agreement, a general investigation was undertaken a t this laboratory. This preliminary report covers a discussion of the methods being used and some of the conclusions. Test Methods
The usual procedure has been to expose partially covered panels t o diffused light under ordinary room conditions. Tests of this type usually require several months. To accelerate the yellowing, panels are often placed in a dark box, drawer, or cupboard. This procedure is unsatisfactory because the conditions are not controlled; the air becomes stagnant, and comparative exposures t o diffused light are not 1 Received April 1.5, 1930. Presented before the Division of Paint and Varnish Chemistry a t the 79th Meeting of the AmericJn Chemical Society, Atlanta, Ga , Apiil 7 t o 11. 1930.
available. Of the various cabinets that have been designed, one type consists of a dark box with slits in the top in which the panels are inserted, a portion remaining exposed to the diffused light of the room. The presence of water in the bottom of the box provides a practically saturated atmosphere. Although comparative results are obtained, the exposures in the dark and to diffused light are not under comparable conditions. Gardner ( 7 ) has described a yellowing cabinet consisting of two coniDartments. of which one is painted white and illuminated &le the other is black and kept dark. Duplicate panels are exposed to obtain the contrast between light and dark exposure. It was believed that by masking part of the panel a single white illuminated cabinet could be inade to suffice and in this manner standardized conditions be more readily maintained and the necessity of painting duplicate panels eliminated. A cabinet was built’(Figure I ) with capacity for exposing thirty-three 8l/* by 41/*inch panels, the size of the panels being governed by the desire to have a n area 4 inches in diameter for the color analysis. The interior of the cabinet is painted white and is illuminated by three i5-watt Mazda daylight bulbs. Practically saturated humidity is maintained by cotton wicks suspended over carbon lamps and dipping into water. Perforations in the walls provide for the entrance of fresh air, Ivhich is circulated by means of electric fans. A temperature of approximately 40” C. is maintained in the cabinet. Since high humidity and high temperature accelerate yelloiving, the conditions in the cabinet are suitable for accelerated tests and still compatible with possible normal exposures. One-half of each panel is covered, using three sheets of paper-white, black, and white. If a single sheet of black paper is used, during the exposure the temperature of the masked portion of the film is found to be higher than that of the unmasked portion and difficulties due to adherence and staining from the black paper are encountered. The OSS SIbility that the masking of the panels prevents volatilization of certain reaction products, thus resulting in abnorinal discoloration, was investigated. However, panels that had been masked and exposed were subsequently submitted to a vacuum of 10 mm. of mercury, but no change in the degree of discoloration could be detected. A series of tests was made t o determine whether the results obtained in the cabinet were comparable with actual room exposures. Duplicate panels were painted with a large number of white gloss paints and enamels, well representing the various types of products. One set was fastened to boards, lapped so that half of each panel was covered by the preceding panel and exposed in a representative room; the other set was exposed in the cabinet. Visual comparison after exposure shoived excellent agreement of the yarious
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I.VDUSTRI.41, A.VD E.VVGI.VEERI.VG CHEMISTRY
July. 1930
duplicate panels. Seven days' exposure in the cabinet produced yellowing comparable t.o that resulting from 6 to 9 months' exposure in the room. Although discolorat.ion usually progresses with length of exposure (Figure 3) it is most rapid at first, but on the basis of the satisfactory cotnparison just mentioned 7 days was adopt,ed as the st,andard cabinet exposure. Discoloration Measurement
It was early realized that, a numerical measure of the discolorat i o n w o u l d be necessary. This was cornplicated hy t h e f a c t that the discolorations iibtained cover a wide range of yellows and b r o w n s . The logical procedure was to make an actual color ~leterttiinatiiino f t h e I c o a t , i n g b e f o r e the panel was exposed and of the two p o r t . i o n s , e x p o s e d t u diffusrd light. ~ n dcovered, on removal froin the calliFigure 1--Yellowing Cabinef net,. Attempts were made to use a Keufft.1 & Esser color analyzer (81, but with this instrmnent t,he sample is exposed to an intense light, and ifie bleaching power of t,his liglit was found t,o lie suffieieiitly rapid to affect, the color of the panel during the time required for the determination. The Pfund colorimeter (fS),by utilizing t,lie principle of niitltiple reflection and thereby Rmriding the n posing the sample to a light of such high intensity, provides a tiieaiis of olitaining Sitirly accurate color analyses of substances deviating only slightly from the wtiit,e. This instmrnent gives readings in the IAie, grceii, ycllow, and red regions, with the dornimant, wave lengths at 4800, 5400, 5900, and 6300 A, respectively. A good white e i i ~ ~ i ihas e l apriroxirrratcly the same readings i n all four colors (Figure a), and (lisculoration due to exposure resulis in lower readings in greeu aiid blue than in yellow and rcd. Tlie excess of red and yellow ovcr blue and green dividcd by the green giws a numerical value approxiniately proprtiotial to the t,rue yellow a.nd red saturatioii. The followinc rnaq be used as an exriression of the color saturation, ii:
,
K
= per cent brightness in red Y = per cent brightness in yellow B = per cent brightness in blue G = per cent brightness in greeir
Numerous tests were made in which sets of pariels wcre independently graded and classified by the visual inspection of several investigators and also graded by means of the Pfund colorimeter, using the above formula. The agreement between these two methods proved the reliability of tile instrurrieiit and of tlic saturatioii forniula. Warmth and Humidity
There are temperature and humidity conditions which do iiot prodme yellowing. White enarnel panels exposed at
room temperature in an absoliitely dry atnrosphcre both to diffused light and in the dark showed practically no yellowing tendencies, which confirined Eihner's results. A set, of white enamel panels exposed for 4 inonths t o saturated humidity a t freezing tempera1,ures in the presence and absence of light showed practically no discoloration (Figure 4), although at room tempcratriro these panels ycllnwed within 1 week. Tliese results indicate that moisture and a temperature above freezing are TI ssary for t,lie reactions which lead to the fonnation of t,he yelliw compounds and suggest a solut,ion Sor some of the difficulties encoiintered in storing oilcloth and floor covering. Pigment Since it is generally accepted tliat the changes causing yellowing occur in the organic binder, an investigation of t,he subject sheuld deal primarily wit.lt the vehicle. Still the results of numerous tests indicate that l.lie pigment influences the discoloring tendencies. For instance, i t has heen observed that often the normal tendency of an oil vehicle to change very sliglit,ly when exposed to diffiised light and to discolor in the dark also occurs with zinc oxide enarriel prepared from the vehicle, aIt,liough a marked decrease ill tlie yellowing tendency results Srom the addibion of the zinc oxide. When a t,itauium pigment is incorporaled in tlie same vehicle, the resulting enarnel may yellow inst.ead of bleach when exposed to lislrt. Because of the complicat,ions due to the presence of pigment, several series of tests were made using the clear vehicles applied to opalite glass p~iiels,which made it possible t,o determine t,he color changes.
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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T‘ol. 22,
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eliminate those driers which contain constituents besides the metal that in themselves are active or possess yellowing tendencies. Since the drying properties of lead, manganese, and cobalt increase in the order named, minimum discoloration, for a given drying time, results from the use of a cobalt drier. As in the case of the volatile thinner, to eliminate complications a specific high-grade cobalt drier was used throughout the investigation. Standardization Difficulties
Having selected the type of panels, a cabinet, the conditions of exposure, the method of recording the discoloration, and the thinner and drier, it was believed that standardized conditions had been established, but during the investigation other modifying factors developed. Such points as thickness of film, age when exposed, time elapsed between exposure and reading of the color, etc., were found to affect results and necessitate further standardization. Therefore, some of the data collected were not completely reliable. Still certain general information and conclusions are warranted. la2
Influence of Atmosphere 75
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There seems to be little doubt that the yellow compounds are oxidation products of the oils, so the following tests were carried out to determine whether discoloration requires an oxidizing atmosphere. The same dry white enamels were exposed to high humidity a t room temperature in ozonized air (1.5 mg. 0 3 per liter), in hydrogen, and in nitrogen. The enamels yellowed to approximately the same degree irrespective of the atmosphere in which they were exposed. (Figure 5) Therefore, the direct cause of yellowing is not further atmospheric oxidation of the film, but some other reaction dependent upon the presence of moisture.
G
0
4800
5M)
5470
5700
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630
Wave Lengfb AU
Vehicle Constituents
The constituents of enamel vehicles are the oils, resins, driers, and volatile thinners. It is general knowledge that the resins vary in their tendencies to discolor on exposure: for example, a straight gloss oil enamel yellows markedly more than a similar spirit dammar enamel. Owing to the complicated compounds formed in the cooking, the investigation of oleoresinous vehicles would be very involved; hence, only oil vehicles were considered in the present investigation. Any discoloration effects of the volatile thinners are due to the presence of compounds reactive with other constituents of the enamel, such as sulfur compounds, and to the non-volatile residues. By the selection of a completely volatile mineral spirit, practically free of sulfur, and the use of the same lot throughout the investigation, effects due to the volatile thinner were eliminated. The common drying metals-lead, manganese, and cobaltmarkedly accelerate the yellowing tendencies of the oils ( I , 5 , 6a, 14). Although the tests made have neither covered a sufficiently wide range of oils nor been sufficiently positive to warrant definite conclusions, the indications are that the degree of discoloration is approximately proportional to the number of mols of drying metal present. Thus, a decrease in drying time produced by increasing the percentage of drier results in greater yellowing tendency. The effect depends on the amount of drying metal, and not on how it is incorporated or the form in which it is added. This does not
i
F i g u r e 6-Cabinet
f o r E x p o s u r e t o C o l o r e d Light
C-Color filter L-Daylight bulb
TY-Water cell P-Panel holders
Wave Length of Light
Certain peculiar results of early experiments suggested that the color of the light might influence the yellowing tendencies. Panels were exposed in a high humidity cabinet t o light filtered through color filters.* (Figure 6) The intensity of the light incident upon the filter was so adjusted that the total 2, Blue-theater blue, Corning Glass Co. Catalog A’o 9 , red-sharp cut-off type, Corning Glass Co. Catalog h-0. 1, green-Eastman gelatin type, Catalog No Wratten Filter KO.58B2 (green) cemented between “B” glass 4 X 4 inches.
July, 1930
I S D U S T R I A L A-VD E S G I N E E R I S G C H E X I X T R Y
0
48m
51/00
%GO
5760
6000
63w
1
I
Wave L eng?hA l l
I
OL 4800
I
I
I
5/00
5400
5700
I
6ow
I a 0 0
I
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FIGURE 10
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Wave i englh .AU energy transmitted as measured by a thermopile was identical and constant in all cases. The blue light produced the least discoloration, the green light slightly more, but the red light generally resulted in markedly the greatest discoloration. (Figure 7 ) I n some cases the enamels had yellowed to a greater degree in red light than in complete darkness. Simple tests show that under ultra-violet light marked bleaching occurs and that exposure to infra-red radiations results in pronounced yellowing, greater than that produced by the visible red. These results indicate that the reactions are influenced by the wan’ length of the light. It has not been established whether this phenomenon is due to tht. fact that with increasing wave length light accelerates yellowing or whether with decreasing u-ave length it counteracts yellowing by bleaching or accelerating further decomposition of the yellow compounds. Properties of the Oil
Oils are not chemical individuals, but are inipurch mixtures consisting of large amounts of glycerides of unsaturated fatty acids, siiiall amounts of glycerides of saturated fatty acids, free fatty acids, and traces of unsaponifiable matter. Athough impurities iiiay tend to increase the yellowing tendencies of oils, they are not the chief yellowing constituents. Complete removal of the impurities does not result in a non-yello~virigproduct, as illustrated by the fact that pure synthetic t rilinolenic glyceride yellows badly. Furthermore, the yellowing tendency of an oil is independent
of its free acid content. Tests using oil enamels in which the acidity of the vehicles was varied through a range of acid values from 4.3 t o 16.0 clearly indicate that the fatty acid content of theoil doesnot affect its yellowing tendency. (Figure 8) Eibner (3) had arrived at the same conclusions on the basis of theoretical considerations. The relative susceptibilities of the various vegetable oils to yellowing are apparently proportional to the degree of unsaturation. Thus, soy bean yellows less than linseed and linseed less than perilla oil. Although strictly comparable tests could not be made with straight Chinawood oil. comparative tests of combinations of the various oils indicate that China wood oil, as would be anticipated, yellows more than perilla. (Figure 9) Other things being equal, the least unsaturated oil-that is, the oil with the lowest iodine numberhas the best color retention. Heat bodying or polymerization of oils reduces their iodine numbers and makes them less susceptible to yellowing. This is especially true if the oil is heated in a n inert atmosphere, such as nitrogen or carbon dioxide, so as to prevent the formation of oxidation products. (Figure 10) Eibner and others believe that the yellowing of oils is a function of the amount of free or esterified a-linolenic and a-linolic acid present. The hexabromide number of the liberated fatty acids is considered to be a reliable quantitative expression of the ainount of a-linolenic acid derivatives present in the oils, while the iodine number measures quantitatively the total unsaturation. I n spite of numerous careful
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experiments, the authors were unable to find a definite simple proportionality between these constants and the degree of yellowing of single oils and their combinations. The fact that the synthetic triglyceride of the linolenic acid yellows far less than Tvould be expected on the basis of Eibner’s theory lends further support to the belief that there are other factors entering into this reaction. From all the data so far collected on the subject it appears rather certain that the final solution will not be found through a study of commercial oils, because the variables and unknowns are far too numerous. The reactions causing yellowing are probably purely chemical, and the final solution of this problem must, therefore, be expected to come from a fundamental study of the chemistry of the drying and aging processes of oils under various conditions. J. S. Long has supplied the writers with a quantity of pure trilinolenic glyceride for this work and an extensive study of these reactions has been started, the results of which will be reported as the work progresses.
so. 7
Literature Cited Amsel, Farben-ZLg., 34, 23, 1372 (1929). Brosel, Dissertation, Munich, 1927. Eibner, Farbi Lack, 1914, 310. Eibner, “ u b e r fette Ole,’’ p. 69, hlunich, 1929. Emery, Paint Oil Chem. Rev., 87, 3 (January 17, 19291. Fahrion, Z . angew. Chem., ZS, 722 (1910). Farbe L a c k , 1929, 166. Gdrdner, Paint hffrs. Assocn. S.,Tech. Circ. 162 (19222). Gardner, I b i d . , 191, 86 (1923). Holde and hlarcusson, Ber., 36, 2637 (1903). hlorrell and Marks, J . Oil Colour Chem. Assocn., 12, 183 (1929). Munzert, Dissertation, Munich, 1925; Farben-Ztg., 33, 46, 2849 (1928). S a u r o y , “Peintures, Pigments, Vernis,” p. 997 (1929). Pfund, Proc. .Am. Soc. Tei!ing A\!~le7ialr, 2 0 , 11, 440 (1920). Pickard, Monograph: “Driers,” Am. Paiizt J . , p. 79 (1925). Rebs, Farben-Ztg., 26, 242 (1921). Reithler, Dissertation, Nunich, 1926. Scheiber, “Lacke und ihre Rohstoffe,” p. 173. Leipzig, 1926. Smith, “Paint and Painting Defects,” London, 1912. Thompson, “Painting Defects. Their Causes a n d Prevention” (1914). Tach, “hlaterials for Permanent Painting,” p. 32 (19111.
u
Pressure-Aging of Duralumin’ Leopold Pessel ”01
xI.%XHEIM
S T , PHIL4DELPHIA. Pa.
HE prevention of intergranular corrosion of duralumin is still one of the main problems of the practical metallurgy of this alloy. A considerable amount of work
T
has been done in studying this question from many angles ( I ) , the most recent and comprehensive being that done by Rawdon (3). The \+-eight of opinion indicates that the best method for decreasing the susceptibility of duralumin towards intergranular embrittlement lies in cold quenching and aging at room temperature. Although the duralumin treated by this method s h o m a better corrosion resistance than hot-quenched and hot-aged material. there is enough of this undesirable tendency left to warrant further attempts to decrease it by treatment of the material itself without resorting t o protective coatings, such as sprayed molten a l ~ m i n u m the . beneficial effects of which are well known. Theory of Corrosion of Duralumin
The structural causes of the susceptibility of duralumin and siniilar alloys to intergranular corrosion are not exactly known. As it is fairly well established that the copper content of these alloys has much to do with the intergranular failure under corrosion attack, the prevailing opinion is that the loosening of the crystal bond is effected by electrolytic action between the aluminum-copper compound particles and the grains of the alloy itself. Without any definite proof-on account of the submicroscopic dimension of the hardening particles-it is assumed that these particles accumulate along the grain boundaries and open the path for progressive electrolytic action. It is contended that the improvement of the resistance against this form of corrosion by cold-water quenching is due to the fact that the quickness of cooling prevents the migration of the particles towards the grain boundaries ( 2 ) . As a consequence of the theory that the maximum of corrosion resistance would be obtained in a material that has the hardening particles distributed as evenly as possible and shows a minimum of accumulation along the grain boundaries, the question arose whether there could not be found 1
Received March 12, 1930.
a way to influence this submicroscopic structure by treatment after quenching. It seems clear that the precipitation, growth, and migration of the particles is going on long after quenching and that the sum of these phenomena constitutes what is known as aging. If the particles have a tendency to migrate towards the grain boundaries, their mobility will be accelerated by an increase in temperature. This holds true also during the period of aging. As mentioned above, experience has shown that aging a t increased temperatures decreases the resistance against corrosion embrittlement. TThile it seems that aging at lower temperatures improves this resistance. there is set a practical lower limit on account of the slowness of aging a t very low temperatures. Aging at room temperature will not completely prevent accumulation of the particles along the grain boundaries. I t seems that there is little hope in attempting any form of thermic treatment during the aging. The possibility occurred t o the writer that another way of influencing the migration of the hardening particles might lie in the application of pressure during the period of aging. Khile pressure might have a retarding effect upon the precipitation itself, it was held to be more plausible that the increased rigidity of the compressed material would tend to keep the particles in their place of precipitation and counteract their migration. It was also thought that lines of strain might be set up within the grains, which might tend to attract the particles and prevent them from reaching the grain boundaries. The experiments described below are an outcome of these theoretical considerations. Their undertaking mas stimulated by the fact that the influence of pressure during aging upon the corrosion properties of duralumin has not yet been investigated. Experimental
The duralumin used was obtained from the iiluniinum Company of America and was in form of 17SO sheet, l / inch (1.6 mm.) thick, fully annealed. The alloy had the following approximate analysis: copper 4.0, magnesium 0.5, silicon 0.35, manganese 0.6, iron 0.5, balance aluminum.
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