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INDUSTRIAL AND ENGINEERING CHEMISTRY
varnish had been cooked to the appearance of a satisfactory “string,” it wm allowed to cool and was then diluted to 50% solids with a solvent mixture comprising 40% mineral spirits, 40% VM&P naphtha, and 20% xylene (by volume). A naphthenate drier, consisting of 0.5% lead, 0.05% cobalt, and 0.025% manganese on the basis of drying oil, was added 24 hours prior to testing. Films were cast on 3 X 5 inch glass plates a t 0.003-inch wet thickness with a Bird applicator, and drying times were determined under conditions of 70’ F. and 65% relative humidity. Films for alkali testing were prepared by dipping a 1 X 6 inch test tube into the varnish and permitting it to dry in an inverted position for 1 week a t 70” F. and 40% relative humidity. The results of these tests, presented in Table 111, show that a satisfactory varnish can be made from the inositol ester. According to these findings the inositol varnish possesses drying characteristics comparable to sorbitol, pentaerythritol, or linseed varnishes. I t s alkali resistance is equal to that of pentaerythritol and decidedly superior to that of linseed oil. However, the poor alkali resistance of the sorbitol ester is probably not normal behavior, since Brandner and cc-workers ( g ) obtained alkali resistance equal to that of the pentaerythritol esters. It is believed that this investigation demonstrates that inositol
Vol. 42, No. 8
esters of linseed fatty acids can be made that have unique and perhaps valuable properties for certain purposes, although at present the cost of inositol discourages its general use in the manufacture of drying oils. ACKNOWLEDGMENT
The authors wish to express their appreciation to the Corn Products Refining Company, donor of the multiple fellowship a t Jlellon Institute on R hich this work was done. LITER’ATURE CITED
(1) Bollev. D. S.. IND.ENG.CHEM..41. 287 (19491. (2) Bran&er, J. D., Hunter, R. H., Brewstor, hi. D., and Botiner, R. E., Ibid., 37,809 (1945). (3) Burns, Brit. Patent 408,597 (-4pril 6, 1934). (4) Burrell, H., IND. EKG.C m x , 37, 86 (1945). (5) Dangschat, G., and Fischer. H. 0 . L., Saturwissenschu/’ten, 30,
146 (1942). (6) Posternak, T., Hela. C h i m . Acta, 25, 746 (1942). (7) T o n Mikusch. J. D., ISD. ENG.C H E x f . , 32, 1061 (1940). R E C E I V E D December 29. 1949. T h e experimental work presented in this paper was carried out in 1945-4F with the hope of preparing a drying oil with properties superior t o those of linseed for the purpose of partially replacing the then scarce t u n g oil.
Frosting and Gas-Checking of Conjugated Drying Oils HANS DANNENBERG, J. IC. KAGERS, AND T. F. BRADLEIShell Development Company, Emerycille, Calif. T h e tendency of drying oils and resins containing fatty acid radicals of conjugated unsaturated structure to undergo “frosting” or “checking” during their drying and film formation has been subjected to new experimental investigations in an attempt to reach better understanding of this phenomenon, its cause, and means of correction. This effect has been found to be greatly influenced by atmospheric dust contamination. Fine dust particles, such as fibers or soot which may occur in the atmosphere and settle on the drying films, are significant factors tending to induce frosting and “gas-checking.” The soot generated by the kerosene lamp of the commonly employed gas-checking tester appears to have a similar effect. Small amounts of certain surface-active agents, such as calcium octoate and the calcium salts of phenolic resins, have been found to control frosting and checking. Nevertheless, it is recommended that the industry devote particular attention to the removal or minimization of dust contamination in the atmosphere in which coating compositions are applied and dried.
HE tendency of drying oils, such as tung, oiticica, dehydrated castor oil, isomerized (partially conjugated) linseed or soybean oils, and varnishes or alkyd resins made from these oils, to dry with frosted, finely wrinkled or finely checked surfaces is a well known and troublesome phenomenon which constitutes one of the major objections to the use of such products for many applications. Although it has been recognized that this tendency is largely confined to compounds which contain conjugated carbon-to-carbon double bonds and that it may be modified or eliminated b y the use of sufficiently high varnish-cooking temper-
atures or by the addition of large percentages of certain phenolformaldehyde resins, relatively little has been learned concerning the cause of this phenomenon and its true nature. Previous studies of the subject have followed devious paths and have led to a number of explanations. The cause has variously been held to involve the transformation of or-eleostearin or its equivalents to their p-isomers ( 7 ) , the presence of moisture ( l 7 ) , the influence of light radiation ( 2 , 7 ) ,unequal oxygen absorption ( 7 ) , and the influence of nitrogen dioxide (3, 12). The most commonly accepted view is that the phenomenon (often termed gas-checking because it can be induced in gas-heated ovens) is caused by improper ventilation or circulation of fresh air, by gas fumes which reduce the oxygen content of the air, or by a cold draught blowing on the film before it is dry and hard ( 1 6 ) . Federal varnish specifications often call for, and most laboratories apply, a so-called gas-check test to varnishes made from conjugated drying oils in which varnished panels are exposed to the atmosphere within a bell jar in which a kerosene lamp is burned until extinguished by its consumption of the oxygen of the contained air. Under these conditions frosting or checking 1s often induced in coatings which would not frost in normal air. Uncooked tung oil, however, will frost or check even under the latter conditions, and, according to Chatfield, it crystallizes a t the slightest provocation (j)., One of the authors, together with former associates, had p r c viously observed and reported a significant difference in the apparent mode of oxidation of conjugated as compared t o unconjugated drying oils ( d ) , an observation which appears to have been further strengthened by the work of Farmer and his associates ( 8 , 9). Although the mode of oxidation may eonstitute the basic cause for the confinement of gas-checking to the conjugated
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1950
A.
D.
Full Gloss
Checking and Frosting
Figure 1.
B.
C.
Slight Cheoking
E. Frosting and Checking Checking and Frosting of Varnishes
systems, some of the factors involved in this problem are still obscure, and the authors have left this phase for future study. Instead they undertook a more direct approach to the study of the phenomenon of gas-checking by experiments hereinafter described, which led to new conclusions. EXPERIMENTAL
The appearance of checked or frosted films is familiar to everyone in the varnish industry. However, for a precise illustration of the phenomena a series of photographs is presented in Figure 1. The pictures were taken by the method of Luck and Archibald (16)which enhances the structure of the surface. The first panel has full gloss, the second shows a slight case of checking or crow-footing, the third a severe case of checking, the fourth and fifth, combinations of checking and frosting, and the last, complete frosting.
PREPARATION OF FILMS.The study of conditions that induce checking and frosting was conducted with several materials which under unfavorable circumstances had been observed to exhibit these effects. Among these were a 25-gallon tung oilester gum varnish, a commercial alkyd resin on a dehydrated castor oil base, and a laboratory-made experimental varnish on a dehydrated castor oil base; the last-named varnish was prepared in a special manner (low temperature of reaction) to accentuate the effects under study. INFLUENCE OF PREDHYING PERIOD.At the outset of the work, it was found quite difficult to duplicate these effects consistently. I n order to achieve this, it was necessary to investigate a large number of variables-even those that were less likely to have an influence on the structure of the surface. These studies were made with the experimental dehydrated castor oil varnish. The initial experiments were influenced by the common belief that checking is caused or aggravated by the presence of combustion
1595
Severe Checking
F. ("6
Frosting
Natural Size)
gases in gas-heated baking ovens. However, experiments in an electrically-heated oven, which p a s filled with pure nitrogen or carbon dioxide, gave the same results as experiments in air. The next finding was that the predrying period, before baking, had a strong influence and actually decided whether a smooth surface would or would not be obtained on subsequent baking. Table I shows that predrying periods below 0.5 hour yielded smooth films, those between 0.5 and 1 hour resulted in checking, and those from 1 to 4 hours resulted in frosting when the panels were baked. Drying periods of over 4 hours resulted in spontaneous frosting without baking. Considering the importance of the predrying period, the next step was the investigation of the presence of various gases during the predrying period. The freshly coated panels were placed in individual glass jars through which streams of various gases were sent a t a rate corresponding to a complete renewal of the atmosphere inside the jar every 4 minutes; the panels stayed in these jars for 2 hours. Experiments of this kind were carried out with
TABLE I. INFLUENCE OF PREDRYING CONDITIONS ON CHECKING AND FROSTING OF FILMS FROM EXPERIMENTAL DEHYDRATED CASTOR OIL VARNISH (Appearance of films baked for 30 minutes at 150° C. after predrying) Predrying Conditions Open laboratory air Laboratory air sucked through Compressed cylinder air Oxygen Nitrogen Carbon dioxide
Predrying Time, Minutes 120 Checking Frosting
730 Smooth
... .. . .. ... .. .
Slight checking Smooth Smooth Smooth Smooth
Checking a n d frosting Smooth Smooth Smooth Smooth
_.. 240
Frosting Frosting Smooth Smooth Smooth Smooth
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ROLE OF hITROGEN DIOXIDE
The results of the work described above appeared t o be in disagreement with the work of Hyman and Greenfield ( l a ) , who showed that the presence of minute amounts of nitrogen dioxide caused frosting of varnishes. The possibility that, in the work of these authors, the spark plugs used to generate nitrogen dioxide had inadvertently served as a source of finely dispersed metal was considered. However, experiments carried out in an air stream which had been treated with a spark and then filtered through a quartz filter bed confirmed the findings of I-Iyman and Greenfield. Kext, an attempt was made to ieconcile the nitrogen dioxide effect and the dust effect by the assumption that, in many cases, the nitrogen dioxide coiltained in the air would be adsoi bed to the surface of dust particles, so that the dust and soot in the air would serve as a carrier for nitrogen dioxide and that, Figure 2. Frosted Films of T u n g: O i l C o n t a i n i n g 0.05% Cobalt (SOX) thus, the dust effect would be a Frosting induced by dusting surface with silica powder while film was drjing in purified air modified nitrogen dioxide effect. Indeed, analytical examination of a sample of soot which had been prepared air (from a compressed-air cylinder) as well as with nitrogen, by burning a kerosene lamp in a confined space shon-et1J L conoxygen, and carbon dioxide. The result was that all gases taken siderable amount, (0.8% by weight) of nitrogen dioxide. from cylinders, including air, gave smooth films, whereis laboraOn the ot,her hand, experiments with dust's that were free from tory air, when sucked through t,he jar :it the same rate, invariabl>nitrogen-dioxide contamination resulted nevertheless in frosted caused frosting on subsiquent h k i n g . films. These experiments \yere carried out with a material which INFLUEXCE OF FILTRATIOX. Purthcr invcpt'igation gavc the is highly susceptible to frosting-namely, raw t,ung oil to \I-hich surprising result that the difference bet\veon laborat,oi,y air and 0.05% cobalt in the form of octoate had been added. Films on cylinder air consisted in the presenve of particles of solid matter in glass plates were dried (1) in open laboratory air; (2) in a jar the former. Properly filtered air did not induce any checking or through which purified air (filtered through cotton and activated frosting under the described condit,ions. Table I1 shows the carbon) was passed at a controlled rate; and (3) similar to (2) but results of some of theve tests which differed only by the treatment with provisions to sprinkle small quantkies of finely divided of the air to which the panels were exposed during the predrying solids on the specimen a t an appropriate time, mostly 2 hours period. Filtration of the air through cotton, glass wool, sintered after the oil had been flowed out. In the first case, completely glass, and filter paper was not effective, whereas filtration through frost,ed films were obtained, in the second case smooth films tubes (200 mni. long, 18 mni. wide) containing granular materials resulted, and in the third case frosted or partly frosted films were such as activated carbon, Drierit,e, and Ascarite, stopped frosting completely. *4n experiment in which the filler tube contained pieces of quartz (8-14 mesh), \~liichwas selected as a nonrcactive T 4 A J 2 ! % 11. I S F L U E N C E O F P R E D R Y I N G CONDITIONS ON CHECKING and nonadsorhnt material, \vas especially significant. This AND FROSTING quartz filter was effective in eliminating frosting and checking of The varnishes, obviously by removal of solid particles. (It a-a,s found by experiment that this filter does not retain a detectable Condition of . i i r Appearance of F i l m amount of nitrogen dioxide.) Lahoratory air Not treated Frosting These experiments showed that S.i.rrraE OF SOLIDPARTICLES. Washed with water Frostin the deposition of solid particles of small size on t,he n e t film, Washed with 1% NaOIl Slight ciecking Filtered through or on a film that is just beginning to set, induces checking and Glass wool Frosting Cotton w ~ o l Frosting frosting. The fact that, cotton and paper did not retain the Filter paper S o . 5 Frosting particles, diereas granular filter beds did, indicates that the size Sintered glass, fine Frosting Smooth Tube containing activated carbon of the particles in question is very small, in the range of aerosolsSmooth Tube containing Drierite (10-20 mesh) Smooth Tube containing Ascarite (8-20 mesh) that is, below 1 micron. In other words, it appears t>obe what Smooth Tube containing quartz chips (8-14 mesh) normally is called dust or smoke. Outside air, after rain Checking and frosting .4ir irom cylinder The physical and chemical nature of the particles that induce Smooth ?&tened with water Smooth checking or frosting has not been studied in detail. A few Contaminated with att,empts to induce frosting were made with art,ificial dusts and Smooth Acetone Frosting NO2 smokes. These are likewise shown in Table 11. Lycopodium Smooth Lycopodium dust Paraffin-wax smoke Smooth powder and paraffin-wax smoke were ineffective; carbon black Carbon black powder" Checking or slight frosting dust, and ammonium chloride smoke induced slight frosting. NHaCl smokea Washing the air with a 1%sodium hydroxide solution purified it a Predrying time, 160 minutes. partially, but moisture and light generally were ineffective.
1
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MECHANISM OF CHECKING AND FROSTING
BELL JAR TRIPOD SPECIMEN KEROSENE LAMP E . GAS OUTLET
A. 8. C. D.
Figure 3.
F. GAS RETURN G, ,G2. FILTER TUBES H. SMALL JAR I. SPECIMEN J. MAGNETIC PUMP
Modified Bell-Jar Test for Gas-Checking of Varnishes
It should be clearly understood that the described action of dust and soot is not considered to be the cause of checking and frosting but only a factor t h a t induces this effect when the tendency already exists. Other factors may exist which have a similar inducing influence, or, in severe cases, the frosting may orcur spontaneously. In this connection, it appeared of interest to have some insight into the mechanism of the checking and frosting phenomenon and to see how the deposition of particles mould fit into such a mechanism. The literature offers numerous descriptions and several photographs of the wrinkled or folded appearance of frosted and checked films (13, 19). There are statements that the film cracks (6, 7 , 10) and forms double-lipped joints ( 7 ) . Observations during this work confirmed these statements and added certain details.
obtained. The frosting pattern, on microscopic examination, was found to be arranged around solid particles, as is shown in Figure 2. This effect was independent of the nature of the solid particle: it was found with lampblack which contained noticeable amounts of nitrogen dioxide, with thermal black containing only traces of nitrogen dioxide, and with silica gel powder, which had been heated in vacuo at 300" C. for 2 hours and was completely free from nit,rogen dioxide, as indicated by a test with the highly sensitive GriessLunge reagent. These experiments indicated that the frosting and checking induced by dust as described here is independent of frosting that may occur under the influence of nitrogen dioxide. SIGNIFICANCE OF BELL-JAR TEST
In the light of these findings, the bell-jar test for gasproofness of varnishes, according to Federal
tains the specimen of oxygen, makes u t o induce checking.
tinguished by lack cient soot generator
the fumes. Such an experiment was carried out in the apparatus shown in Figure 3. The usual bell-jar test was employed, but the fumes generated by the kerosene lamp were recycled by means of a magnetic pump through two tubes filled with quartz chips, then through a smaller jar, and back into the bell jar. The spec be tested were prepared in duplicate, one being exposed in the bell jar, the other in the auxiliary jar. Experiments were carried out with the experimental varnish on dehydrated castor oil base and with a 25-gallon tung oil-ester gum varnish containing 0.05% cobalt in form of the naphthenate as a drier. The tung oil-ester gum varnish was prepared by heating 392 grams of raw tung oil with 200 grams of ester gum in a carbon dioxide atmosphere to 260' C. in 22 minutes, holding for 13 minutes, cooling to 249' C., and diluting with mineral spirits to 50% solids. The product had a Gardner-Holdt viscosity of E-F. In both cases, the specimen in the bell jar frosted within a few minutes, whereas the one in the auxiliary jart h a t is, exposed to the filtered fumes, remained smooth.
A.
C.
Surface Recedes from Dust Particles
Cracks Widen and Form New Skin
F i g u r e 4.
B.
Fine Surface Cracks Form, Starting a t Particles
D.
Secondary Cracks Form and Wrinkling Appears
Stages in Gas-Checking of Films
Experimental varnish o n dehydrated castor oil base (SOX)
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 42, No. 8
film, but the cffect, especially the changes of the surfare layel, can be observed more easily. The following sequence of events ivvas observed and is rel)iesented approsimately by the microphotographs in Figure 1.
I
C (ri J.
20
Figure 5 .
I00 120 140 AGE OF SURFACE IN MINUTES
60
40
80
160
I80
200
220
Surface Tension of Tung Oil in Contact with Air
A. The surface skin breaks a t a fern points, mostly where foreign bodies such as dust specks or tiny air bubbles are present, and pulls away from these (Figure 4-4). B. Breakage of the skin proceeds (Figure 413) in the form of cracks emanating radially from these centers, sometimes in such a way that these cracks connect two or more centers. Thus, the typical check pattern is initiated. C. The widening gap (Figure 3 C ) is next filled by liquid Tarnish which forms a new, local surface skin: this in turn mav crack again and repeat the procedure. The scars that remah after the cracks have been filled in, appear as slightly raised lines, the normal check pattern, after completion of the drying. D. The skin may also break or become perforated a t numerous points simultaneously. Again, liquid varnish will ooze through the openings, viiden the breaks, and shove the original skin away until it piles up in wrinkles. ;Z microphotograph 15 hich shows this condition is presented in Figure 4D.
~
STAINED SURB'ACES. The following staining technique was found helpful in this work: The varnish is flowed out on glass plates and air-dried for a suitable period of time until it is close to its setting point-that is, susceptible to wrinkling when induced. At this point, a 0.5% solution of basic fuchsin in methanol is poured over the film and allowed to drain and dry rapidly, leaving the top layer of the film in a stained condition. Checking and wrinkling will occur soon in a manner similar to the unstained
FLEXIBLE SUBSTRATES.The evisterice and extent of the force,\ that push the skin into folds were demonstrated by experiments with varnish films on flexible substrates. Thin metal foils (platinum, 0.1 mil thick) or inert plastic film (polyethylene, 2 mils thick) were coated on one side with an oil having a strong tendency to frost (tung oil containing 0.05C;O cobalt in the form of octoate), dried in hanging position, and matched for bending or curling of the substrate. Curling started after about 2 how. of air-drying, just after the wrinkle pattern had been formed, and increased during the folloiving hour and a half The direr-
. . :
