Document BRoOZedD0X46Bvy2KObpNVVMJ

3ucknan Laboratories, Inc Swelling of Paint Films in .Water V. Effects of Different Pigments1 F. L. BROWNE Chemist, Forest Products Laboratory,* Forest Service, U. S. Department of Agriculture Describes measurements of absorption, swelling, and related data for free films of single-pigment paints tested by soaking in distilled water for three days both before and after artificial weather ing for 15 days. Forty-four pigments tested and classified. O DETERMINE THE EFFECTS of dif Some pigments reduced the absorp Tferent pigments on the behavior of tion of water and swelling of free films free lilms of paint when soaked in below the absorption ana swelling of water, 44 single-pigment paints of 0.30 unpigmented linseed oil, whereas others pigment volume in linseed oil were increased absolution and swelling, made and tested before and after arti sometimes greatly. The effects of the ficial weathering for 15 days. pigments on absorption of water seem Changes in density during drying and weathering gave evidence of a varying effect of pigments on oxidation of linseed oil. Losses in weight during weathering were due, first, to leaching of soluble organic and inorganic mate rials and, later on, to mechanical losses by chalking. to be connected with their effects on the oxidation of linseed oil. Weathering tends to make some films porous so that part of the water they take up is free water in pores that causes no swelling. Measurements were made of swelling in volume, area, and thickness, and of shrinkage in volume, area, and thickness, from which further x A contiibuled paper. * Maintained at Madison. Wis. in cooperation with the University of Wisconsin, The Author, Frederick L. Browne, received B.Chem. decree from Cornell University, Ph.D. in colloid chemistry from University of Wiscon sin. He first joinea the Forest Products Labora tory staff in 1918. helped develop casein glues for wood airplanes of World War I. In 1921--22 he was a National Research Fellow at Wiscon sin. Since 1929, Dr. Browne has edited the Genera! and Physical Chemistry section of Chemical Abstracts. indications of the film structure and effect of weathering were obtained. Soluble substances derived from de composition of linseed oil and some times from the pigments, were extracted from paint films during the 3-day period of soaking in water. Previous papers in this series de scribed the technique of . preparing free films of weathered and unweathered house paints and of measuring their absorption of water and swelling when soaked in distilled water for 3 days, to gether with the changes in density, the shrinkage on redrying after soaking, the loss of soluble substances to the soaking water, and the losses in weight during artificial weathering in a Weatherometer (5).* The effects on absorption and swelling of such factors as time of soak ing in water, temperature and pH of soaking water, age of films before soak ing, thickness of films, pigment volume of the paints, and both artificial and natural weathering were reported. Data were given for some unpig mented paint vehicles and for laboratorymade and commercial paints of widely differeing compositions (4, 7). Absorp tion and swelling of free films of ve hicles and paints-when exposed to air of different relative humidities were measured (6). Dunn (10) has reported measure ments of the absorption of water, swell ing in area, and solubility of free films Numbers in parentheses refer to literature cited. Reprinted from the June, 1955, Forest Products Journal, pages 192-200 Forest Products Research Society, P. O. Box 2010, University Station, Madison 5, Wisconsin of single-pigment linseed oil paints made with white lead, zinc oxide, titanium dioxide, and magnesium sil icate, of multiple-pigment paints, and of unpigmented linseed oil and various resinous vehicles. His findings are sim ilar to those described in this series of papers. In house paints made with linseed oil, the most important factor deter mining the absorption and swelling in water proves to be the kind of pigments with which the paint is made. Pigments and Paints Tested All pigments except mica and alu minum, which are "leafing" pigments, were tested in linseed-oil paints of pig ment volume 0.30 made according to the following general formula: Pigment ---------------------------------------- 0.261 gallon Raw linseed oil_....-609 gallon Lead-manganese naphthenate liquid drier _______________ .020 gallon Mineral spirits -------...- .110 gallon Total ...1.000 gallon The aluminum paint consisted of 2 pounds of commercial aluminum paste (75 per cent aluminum powder, 25 per cent mineral spirits) in 1 gallon of bodied-iinseed-oil vehicle `(66.5 per cent bodied linseed oil, 33.5 per cent mineral spirits and lead-manganese drier, viscosity B). The mica paint con sisted of 2 pounds of a commercial surface-treated mica in 1 gallon of the bodied-linsecd-oil vehicle. In commercial practice, basic car bonate white lead is commonly used for house painting as a single-pigment paint, of which the paint tested is rep resentative. The aluminum paint was also a commercially practicable paint. All of the other pigments, however, are used in house paints only as admix tures with other pigments in making mixed-pigment paints. For the present study, however, the purpose was to learn the effect of each pigment by it self on the absorption and swelling of paint films. Although all of the pigments tested are purchasable under the names by which they are designated, they are not all single chemical compounds. The fol lowing are either "composite" pigments or naturally occurring mixtures. Leaded zinc oxides are mixtures of basic lead sulfate and zinc oxide; those tested were cofumed products, that is, mixtures condensed together from the gas phase in the process of manufacture. Basic silicate white lead is a composite of basic lead sulfate and silica said to consist of granules of silica firmly coated with the basic lead sulfate. Lfthopone is a mixture of zinc sulfide and calcium sulfate precipitated together from aqueous solution in manufacture. Titanium-calcium pigment is a mixture of rutile titanium dioxide precipitated in a slurry of calcium sulfate. Venetian 2 red is a mixture of ferric oxide and calcium sulfate made by calcining to gether ferrous sulfate and calcium car bonate. The manufactured and natural ironoxide reds contain siliceous minerals present in the ore from which the pig ments are made. Some of the titanium dioxides contain small additions of other substances deliberately incorporated to control the chalking properties of paints made with the pigments. Other pigments may contain small proportions of extraneous substances classifiable as impurities. Other comments about the pig ments tested are the following: An timony oxide is an opaque white pig ment used for house paints more widely in Europe than in the United States. Lead titanate is a slightly yellowish opaque white pigment formerly used in house paints but no longer available in the American market. Dibasic lead phosphite is an opaque white pigment recently developed primarily for use in vinyl resins and chloriated hydrocarbons but with properties that might well prove useful in house paints. Basic carbonate white lead Nos. 1 and 2 were products of the same manu facturer. No. 1 was higher in ratio of lead hydroxide to lead carbonate than No. 2. Of the two leaded zinc oxides tested, one is a pigment that has long been in use in house paints; the other, 18 per cent leaded zinc oxide, is a recent devel opment in which an unusually high pro portion of the lead component is said to be in a form available for reaction with acids formed in the paint vehicle. Four of the five lead-free zinc oxides tested were chosen to represent the range in reactivity of the zinc oxide pig ments made in the United States. Ten widely used brands were tested for their rate of reaction with meso-fcartaric acid in aqueous medium. Zinc oxide No. 1 was the brand found slowest in reaction, zinc oxide No. 4 was the fastest in re action, and Nos. 2 and 3 were inter mediate. Relative rates- of reaction were expressible by the figures 1.65, 3.70, 5.15, and 7.10 for zinc oxides Nos. 1, 2, 3, and 4 respectively. Zinc oxide No. 1 was of large and No. 4 was of fine particle size. Zinc oxide No. 5 was an experimental prod uct made in accordance with a sugges tion by a manufacturer of silicone in the following manner: commercial zinc oxide No. 2 was slurried in a 1 per cent aqueous solution of vinylsilane contain ing 0.02 per cent of sodium hydroxide and was then filtered and dried at 105 C. The treatment was said to make zinc oxide water repellent. Titanium dioxide Nos. 1, 3, and 4 were products of one manufacturer, and Nos. 2 and 5 of another manufacturer. Nos. 1 and 2 were of the anatase crystal structure; 3, 4, and 5 of the rutile crystal structure. Nos. 1, 2, and 3 were said to contain no modifying agents, but Nos. 4 and 5 contained not more than a total of 6 per cent of alumina, silica, and zinc oxide incorporated in manu facture to give maximum resistance to chalking. Lead chromate and zinc chromate are bright yellow pigments. Basic lead chromate is bright orange in color. The ferric oxide pigments and Venetian red are red in color. Ferric oxide Nos. 1 and 2 were manufactured pigments from 2 different makers, and No. 3 was a naturally occurring iron ore known as Spanish oxide. Ferric hydroxide yellow was a manufactured pigment of dull yellow color. Zinc dust is dark gray in color without metallic luster, but alu minum pigments are light gray with high luster. Magnesium silicate Nos. l and 2 came from different producers. Calcium carbonate Nos. 1 and 2 were surfacetreated pigments from different makers. Calcium carbonate Nos. 3 and 4 were naturally occurring products of which No. 3 was of relatively coarse particle size and remarkably low oil absorption, and No. 4 was typical of the pigments called whiting. Diatomaceous silica is a porous pig ment consisting of the skeletons of diatoms found in what w?rc ocean beds in former geologic times. The treated bentonite was dimethyloctadecyl am monium bentonite made by cation ex change between the organic base and the naturally occurring montmoriUonite (day) known as bentonite; it is a recent development used as a gelling agent in paints. Magnesium silicate, calcium car- bonate, barium sulfate, calcium sulfate, silica, diatomaceous silica, treated ben tonite, and mica are all pigments of low opacity that make transparent or nearly transparent coatings when made into paints with linseed-oil vehicle. Preparation of Free Films and Tests To prepare unweathered films, the paints were spread on gummed paper by doctor blade and suction plate at a thickness designed to make coatings ap proximately 4 mils thick when dry. Ac tually 36 of the 44 films reported in Table 1 were within 0.5 mil of the de sired thickness. The films were stripped from the gummed paper when they were 7 of 8 days old, and the soaking tests were started when they were 10 days old. To prepare weathered films, the paints were spread on tared tinplate by doctor blade and suction plate to make coatings approximately 4 mils thick. When 10 days old, the coated tinplate specimens were weighed to find the weight of coating and were then sub GLD38006 jected to artificial weathering for 15 days in a Weatherometer. Distilled water was supplied to the Weatherom eter sprays. After weathering, the spec imens were weighed to determine the loss in weight of the coatings during weathering. The films were then stripped from the tinplate by amalga mation and promptly submitted to soaking tests. For details of the testing technique and methods of expressing results, ref erence should be made to a previous paper (5). Unweathered Films The results obtained with un weathered films are reported in Table 1. Density Changes: All paints in creased significantly in density when the liquid paint hardened to a dry coat ing. The increase in density, however, was always less than that calculated from the increase in density and in weight of the linseed oil in the paint, assuming that the changes in the oil of the paints are the same as for unpigmented linseed oil. Data of Carrick and Permoda (8) in dicate that 10-day-old linseed-oil films have a density of 1.12 and are 4.45 per cent heavier than the original, undried oil. Data in the second paper of this series (5) agree about the density of 1.12. If the oil in the antimony oxide paint of Table 1 had reached the same condi tion 10 days after spreading, the density of the film would have been 2.64, which exceeds the density of 2.54 actually found by 0.10. Similar calculations for all of the other paints of Table 1 reveal differences in the density, as calculated and as determined, that range from 0.01 to 0.26. Rhodes and coworkers (15, 16) showed that some pigments greatly alter the absorption of oxygen and evolution of volatile matter from paint films over a length of time that included the 10day period, whereas other pigments ex ert much less effect. Silica, iron oxide, and Venetian red were pigments that had little effect. For all of them the discrepancy between the calculated and determined density in Table 1 was less than 0.10. On the other hand, Rhodes found large effects for basic carbonate and basic sulfate white lead, lithopone, titanium-barium pigment, barium sul fate, leaded zinc oxide, and zinc oxide. For all of them the discrepancy between calculated and determined density in Table 1 was 0.10 or greater. It is, there fore, reasonable to attribute the differ ences between calculated density and density actually found to the effect of pigments on the oxidation of linseed oil. The pigments in Table 1 for which the difference between calculated and determined density of paint films was less than 0.10 were: basic silicate white lead, dibasic lead phosphite, 18 per cent leaded zinc oxide, zinc sulfide, zinc chromate, titanium dioxide Nos. 1, 2, and 3, titanium-calcium, all iron oxide and iron hydroxide pigments including Venetian red, both magnesium silicates, all calcium carbonates, calcium sulfate, silica, and treated bentonite. The alu minum and mica paints cannot be con sidered in this connection because they were made with bodied linseed oil and have a different paint structure. For paints made with all the other pigments, the differences between the calculated and determined densities were 0.10 or more. It is of interest that the 2 paints made with titanium dioxide Nos. 4 and 5, both of which contained small amounts of zinc oxide, exhibited higher density difference than the rarer titanium dioxides. The density of ad paint films de creased when they absorbed water dur ing die 3-day soaking period. After re drying, the density usually returned to a value slightly greater than that just before soaking. By that time, of course, the paints were 4 or 5 days older. Paints made with calcium sulfate, titaniumcalcium, and Venetian red, which con tained calcium sulfate, lost part of the calcium sulfate by leaching while soak ing, and on redrying they failed to at tain a density greater than that before soaking. Zinc oxide No. 5, which was treated with vinyl silane, and zinc chromate, ferric oxide No. 1, and calcium car bonate Nos. 1 and 2 all made paints that lost much of the soluble pigment ingredients during soaking. After redry ing, they remained lower, or at least no higher, in density than they were be fore soaking. The paints made with 35 per cent leaded ztnc oxide and with treated bentonite were anomalous in that the density after redrying was no higher than before soaking, although the leaching losses were only moderate. Absorption of Water: In the sec ond paper of this series (5) the absorp tion of water by unpigmented raw lin seed oil on soaking for 3 days was re ported as approximately 20 per cent. Since the paint films reported here con tained 70 per cent linseed oil by volume, they would be expected to absorb 14 per cent of water if the pigment present had no effect beyond reducing the volume of oil in 1 cubic centimeter of film. Tab'c 1 shows that paints made with the following pigments were signifi cantly less absorptive than unpigmented linseed oil: antimony oxide, lead titanate, the white leads including the basic carbonates, basic sulfate, and basic silicate white lead, dibasic lead phos phite, lead chromate (but not basic lead chromate), barium sulfate, silica, mica, and aluminum. In addition, paints with the following pigments were either slightly less, or only slightly more, absorptive than un pigmented linseed oil: basic lead chro mate, titanium dioxide Nos. 1, 2, and 3, which contained no zinc oxide, ferric oxide No. 2 fbut not Nos. 1 or 3), ferric hydroxide, and coarse particlesize caldum carbonate No. 3 (put not calcium carbonate Nos. 1, 2, and 4). All other pigments made paints of greater absorptiveness than unpig mented linseed oil. Zinc oxides, leaded zinc oxides, and zinc chromate made paints of very high absorption, from 74.5 to 117 per cent. Other pigments that made highly absorptive paints were ferric oxide No. 1, Venetian red, and calcium carbonate Nos. 1, 2, and 4. Zinc sulfide, lithopone, and zinc dust made paints that absorbed 39-9 to 45.5 per cent of water--significantly mote than unpigmented linseed oil but only about half as much as zinc oxide paints. Among the titanium dioxide pigments the 2. (Nos. 4 and 5) in which small quantities of zinc oxide were incorporated by the maker to re duce chalking made paints of distinctly higher absorptiveness than the 3 in which there were no such additives. Calcium sulfate and the pigments that contained caldum sulfate, titaniumcaldnm, and Venetian red, made paints of moderately high (about 35 per cent) or very high (87.5 per cent) absorptiveness. Magnesium silicate paint films absorbed 56.2 to 69.6 per cent of water. The extreme variation in absorptiveness among the 3 ferric oxide pigments, 22.4, 31.4, and 110 per cent, respec tively, is as yet unaccountable, as are . the differences among the 4 calcium carbonates, from 24.5 to 117 per cent absorption. Diatomaceous silica made paint of slightly higher absorption than unpigmented linseed oil, whereas ordi nary silica gave low absorption. It is not yet possible to relate the effect of pigments on the absorption of water by paint films to other physical or themial properties of the pigments. Chemical reactivity in the sense of soap formation between basic pigments and fret adds or add decomposition prod ucts of the oil is not a determining fac tor because unreactive silica or barium sulfate, as well as reactive white leads, reduce absorption below that of unpig mented linseed oil; whereas unreactive magnesium siliates and titanium diox ides, as well as reactive zinc oxides, increase absorption. Furthermore, the great variation in chemical reactivity of the 4 untreated zinc oxides was not paralleled by sim ilar variation in absorption. In fact, the 3 GLD3B007 A Toblo 1.--DATA TO* TUI FILM* OF SINGLE-FIGMENT FAINTS EEFOR7 WEATHERING--CHANGES IN DENSITY; ASSOIFTION OP WXTil AND SWELLING IN VOLUME, AREA, AND THICKNESS WHEN SOAKED IN WATER FOR THREE DAYS; SWELLING EFFICIENCY; SHRINKING IN VOLUME, AREA, AND THICKNESS WHB4 REDRIED, AND LOTS IN WEIGHT RY LEACHING DURING THE SOAKING FERIOD Pmlat Denali? of; Daultj of tin : Xaltlml AVoorpUec i Dtlekaeae of water in liquid t tlqr ; Bmllea:Bodrlods *a v. la point 1 la water: t rol\m J t AT, BweUi* 1 8wliuc , artrM I* IMl 1* s wr.r. ,TdUw . Area **. Ittlltami --r-8 t *T,, I 1 **. : t *, xtM la wMH VbdcBuii t ** i* af*. II. l, HUB Ferceat hKWt! rwww Usas. iwe^ianmi toraat 'BBXStt Aottacejy cgclte Lead tita&ate Hale e*rbao*t ehlta lead Vo. 1 Buie cartceate felt* lead *o. t Male eul/mt* vfclto )*d Buie illlcftto Alto lead Blbule lead fhoephlte Load chrsate Bute looA ehramto 35 percent looted sloe oxide 8.38 t *2 2.52 2.6* *? 2.69 : 8.67 ? 2.8* 2.71 * 2.TO : 2.6? 2.7* *.57 > 2.7* 8.59 1 1.86 i 2.01 1.92 t 9.78 1 8.97 2.76 r 2,*o ; 8.58 2.** 2.72 : 2.76 2.*5 * 9.85 : 8.5# 1.76 9.61 1 3.0* 1 2.90 t 8.90 > 2.781 2.03 t 3.01 , 2.6l 1 *.79 *.*2 l 3-8 4.0 5.9 3.8 u *.2 3.6 4.1 *.L 8.1 5.5 7.6 10.7 6.9 9.7 10.1 U.T 11.4 24.0 111 5.8 7-5 10.6 9.3 11,6 10.5 12.1 U.5 *3-3 106 *9 0.3 8.7 .7 6.0 1.6 2.1 5.1 7.8 1*7 5.9 1 2.2 38 6.7 6.1 2.4 13-7 7-2 61.3 *9.0 .109 -1.* 96 ( -2.0 t :l o i 1 -3*3 X 1.0 10* I -2.5 t 1 120 t -3.2 < -.7 -3.6 2.* 105 x -3.0 1 1.* -*.3 103 t -1.5 1 .3 101 1 -2.9 1 -.2 -2.6 -2.7 97 X -1.6 1 41.5 3.1 97 : +*.8 1 85.8 -U.2 si* y 3.8 y i:j l6 percoot luted Uee caddo (reactive) SIbc oxlte Vo. 1 : Zlae oxide Wo. 2 i SiBe oxlte Vo* 3 : 21Be oxlte Vo. * tiae oxlte (troeted) Vo. J Zlac eulflda UUsopoM Zlac cfercwfct* ZLoe duet Tltttlts dioxide (aaataee) Xo. 1 Tltealua dioxide iintt*M) So, 2 Titantua dioxide i rutile) Vo. J Tlteatue dioxide (rutile) Vo. L TiUoiot dioxide (rutile; Vo* 5 9.38 8.38 9.33 2.3* 2.3* *.33 1.96 1.* 1.69 8.76 r.aa 1.89 1.9* 1.9* 1.91 2.55 : 2.*6 > 2.** : 2.*7 1 2.*7 : 9.10 : 9.00 < 2.07 > 1.8) : 5.68 * * 8: 12-.086 : 2.03 : 2.06 1.89 1.66 1.80 1.72 1.75 1.66 1.73 1.1* l.*3 2.66 1.63 1.79 1.9* i:S *.59 > 2.53 1 2.V7 t 2.*6 1 2.*8 1 22..1072 2.10 1.62 3-70 1.99 2.02 2.06 2.06 2.11 3-8 3.T 3-7 4.1 4.1 1:1 3.7 1:2 3.7 3.5 1.0 *.0 3.5 78.5 78.6 -5 106 62.0 117 !; *3.8 17.3 26.6 1S.5 3p.| 3*.9 7**5 78.6 81.6 05 87.3 U2 *0.J *5.9 67.* *3-3 17.9 37.2 17.2 30.7 35.5 *7.0 *2.7 26.* 70.6 22.9 a50*..&5 *r.i *5.1 85*9 9.8 13.5 9S 16.5 17.1 16.8 20.0 39.* 1?.* 38.8 *1.0 U.2 m12.* *.0 8.2 11*20...5** 100 1 -3.* 1 .18,1 -18.1 100 x -3.9 1 9.*' -12.0 98 > -3.6 > 45.0 8.0 99 t -2.0 , 32.5 -26.1 106 J -2.9 x -3.3 .5 96 101 7.2 j 20.5 2.7 J +3.8 -1-60..09 101 -8.5 t +5.7 -7.3 99 -*.7 * lj.* -17.5 100 -2.5 * *2.3 -8.7 103 102 -3.T * -*.9 i i:l --68..59 10* -2.6 x 0 -8.5 101 -2.7 : 1.6 -8.0 102 5-5 -7.1 5J4 5.6 5.0 6.% 7.8 9.7 *. 99 3.6 3-7 1:1 Tttaalsa-Calelia X Vorrlc ootldo rod (usufactured Vo. 1 : Ferrte oxlte red faaaufaetured Vo. 2 : Ferric oxlte rod (natural) Vo, 3 x TeaetUa rd Ferric hydroxide yellow Xtofneeitw ellle*te Vo. 1 MMteBiva ellleBU Vo. 2 Calclta eubOBBte Vo. 1 Calelia cubOBBte Vo. 2 1 t X CelciUB carbonate Vo. 3 Caleiua carbonate Vo. * Varivax eulfate Caleiua aulfato Silica I.63 2.06 2-16 1.99 1.62 1.87 1.51 1-51 1.46 1.46 1.47 1.46 1.97 1.36 1.45 X 1.75 2.23 X 2.32 X 2.14 1.99 ; 2.00 X 1.66 1.62 3 1.56 * 1.56 \ I.60 * 1.5* 2.06 1.47 1.56 1.5* 1*59 2.11 1.8* l-*9 1.86 1.** 1.37 1.33 1.38 l.*9 1.26 1.96 1.19 1.52 1.75 t 2.22 : 2.38 s 2.16 1 1.83 3? 3.0 3.7 3.9 4.6 35.* uo 22.4 31.* 87.5 2.0* S 3.3 16.4 1.70 t 2.9 X 56.2 1.63 1 3.7 69.6 1.51 * 3.8 t 71.6 I.JS 1 4.1 X 10* 1.62 : *.5 X 2*.5 1.55 t 3.6 X U7 2.09 4.7 t 11.4 1.23 t 3.6 5 36.8 1.57 x 4.6 X 9.3 55.6 : 17.6 108 t 58.4 *a.9 5.5 < > *`2 17*8 81.0 1 48.3 16.8 I T.l 56.1 1 13.7 69.5 1 20.4 66.5 t *6.9 1*5 1 57.9 *8.7 ! 12.7 107 1 64.5 11.1 t 8.6 53.8 s 10.7 9-7 > 5.5 10.7 29.6 9.1 13.5 22.2 5.7 37.3 39*2 26.4 7.5 23.1 0 6.3 1.9 101 x -5.3 : 2.7 x -7.4 98 t -2.5 x 12.3 x -13.5 102 1 -4.1 : -.2 x -3.5 U6 t -3.1 2.6 -5.6 93 t 0 : *9.6 1 -9.1 100 I -3.* ! -.6 I -2.4 100 t -3.8 2.2 x -5.6 99 x -2.2 ; 4.3 i -6.2 93 x -2.9 t 17.6 t -17.3 99 * -4.0 x 14.9 x -16.2 101 x -3.4 ! X .5 1 -3.6 92 1 -4.6 t 18.2 x 19.2 97 s -2.7 * .9 x -1.4 91 s -20.4 : -4.7 x 16.2 104 t -2.6 t -.5 x 2.1 9.9 7*5 4.2 4.2 15.9 4.4 4.5 3.0 11.6 5.8 4.1 6.3 2.2 50*5 2.1 PUtcaaeeoue elite* Tretted beatoolte met in bodied oil AliMtaa la bodied oil 1.25 1.19 1.16 l.U x 1.31 : 1.26 t 1.30 : 1.21 1.29 1.20 1.27 1.20 1.31 * 1.26 x 1.38 x 1.23 X 4.7 33 3.0 2.7 28.7 30.9 11.6 9.3 25.1 t 10.0 10.7 30.9 : 17.2 9.2 12.4 x .9 7.0 10.1 x 1.6 1.5 J1- r -3.2 : 2 X -3.4 100 X -2.9 1 3.2 x -5.8 107 : -4.6 : .1 X -5,4 109 t -7.2 x -.4 x -6.9 most rapidly reactive zinc oxide, No. 4, made paint little more absorptive than the least reactive zinc oxide, No. 1, and less absorptive than No. 2, which was about half as rapidly reactive. On the other hand, basic carbonate white lead No. 1 contained a higher proportion of reactive lead hydroxide and made paint slightly more abso-ptive than basic carbonate white lead No. 2, and basic lead chromate made more ab sorptive paint than neutral lead chro mate. Solubility of pigments or pigment in gredients may tie a minor factor that increases absorption, as in the cases of calcium sulfate, Venetian red, calcium carbonate, and zinc chromate, but such nearly insoluble pigments as the tita nium dioxides increase absorption at least moderately. The dominant effect of pigments, in cluding nonreactive and insoluble pig ments, indicates that the interface be tween pigment and oil is the seat of the unknown mechanism by which absorp tion is chiefly governed. But the area of interface, which increases greatly as the size of the pigment particles diminishes, seems to be of secondary importance. 4 Zinc oxide No. 4, which was of exceed ingly fine particle size (about 0.X2 micron), made paint less absorptive than zinc oxide No. 2, which was much coarser, and little more absorptive than the zinc oxide No. 1, which was of relatively coarse particle size (about 1 micron). The titanium dioxides, though finer in particle size than zinc oxide No. 1, made much less absorptive paints. The white lead pigments were comparable in particle size to zinc oxide No. 1, but made paint of much lower absorption. Among the calcium carbonate pigments, the one of coarsest particle size, No. 3, made the least absorptive paint, but it was higher in absorption than such paints as those made with the white leads and lead titanate with much finer particle size. Likewise, particle shape seems to be unimportant because the paint made with markedly acicular zinc oxide No. 2 fell between the paints made with dis tinctly nodular zinc oxide Nos. 3 and 4, and the paints with acicular magnesium silicates fell within the range of absorp tiveness of the calcium carbonate paints with nodular pigments. It is reasonable to suppose that water absorbed by pigmented paints in excess of that taken up by unpigmented lin seed oil is held chiefly at the interface between pigment and oil, at least when the pigments do not react to form oilsoluble products diffused through the vehicle. If so, a relation might well be expected between absorption of water by films and adsorptive properties of pigments, or between absorption of water by films and the interfacial ten sions between pigment and water and pigment and oil. Gardner (11) pub lished data on die adsorption of methylene blue from aqueous solution by pigments; and Gardner (12) and MacGregor (14) reported tests of the distribution of pigments between water and organic liquids, including linseed oil, but no significant parallels can be found between either or these pigment properties and the absorption of water by paint films in the present tests. Bartel! and coworkers (1, 2) studied the interfacial tensions between pig ments and liquids in great detail, but again there seems to be no dear relation between their findings and the effects GL038008 of pigments on the absorption of water by paint films. The lack of agreement between the surface properties of pigments and the absorptiveness of paints made from them may be due to the change in chemical nature of liquid linseed oil as it is transformed into the linoxyn gel derestimation of the absorptions, which were small, from loss of moisture while weighing the wet films in air. The film of paint made with iron oxide No. 3 gave high swelling effi ciency with moderately high absorption, which suggests more serious experimen tal error. Low swelling efficiencies, less all other pigments in Table 1, the ratio was 1.0 to 1.2, except for titanium dioxide No. 2 and treated bentonite, /or which the ratio was 0.9. Shrinkage on Redrying: All films except those of the paints made with 35 per cent leaded zinc oxide, treated zinc oxide No. 5, and Venetian red of dried paint films (3) and to the effect of pigments on that change. than 96 per cent, were found for films containing Venetian red, calcium car shrank to less than their initial volume when they were redried after soaking in The work of Rhodes and coworkers (15, 16), which has already been bonate Nos. I and 4, and calcium sul fate, all of which lost substantial pro water. The 3 exceptions were films for which the sweHing efficiency was less * shown to be related to the increase in density during the drying of paints ob portions of pigment by solution in the soaking water, and diatomaceous silica, than 100 per cent, indicating the devel opment of some degree of porosity, but served in these experiments, also proves roughly indicative of the absorption of which has a highly porous pigment structure (7). not all films of low sweHing efficiency remained swoUen or failed to shrink on water by the paint films. Basic car Swelling in area was always less than redrying. bonate white lead, basic sulfate white swelling in volume. AH films, there Shrinkage to less than the initial lead, and barium sulfate increased the fore, swelled also in thickness. The ratio volume on redrying may be attributed consumption of oxygen and evolution of swelling in area to sweHing in thick primarily to loss of soluble ingredients of volatile matter in Rhodes' experi ness, however, varied from 3-7 for zinc to the soaking water. The film of cal ments, and they caused a reduction of oxide No. 3 to 0.4 for zinc oxide No. 4, cium sulfate paint, which lost 50.5 the absorption of water by films in the if only those pigments are considered grams of sohiable material per 100 cubic present tests. Zinc oxide and leaded for which the swelling was large centimeters, chiefly calcium sulfate, zinc oxide markedly reduced the con enough to give confidence in the sig shrank 20.4 per cent in volume when sumption of oxygen and evolution of nificance of the ratio. The swelling in redried. For the other paints the Vol volatile matter and greatly stimulated absorption of water by films. area exceeded the swelling in thickness for 36 of the 48 pigments tested. There umetric shrinkage was 1.5 to 7.2 per cent. Lithopone and titanium-barium pig ment moderately reduc'd the consump tion of oxygen and evolution of volatile matter, and lithopone and titanium dioxide paint films exhibited moderate increase over unpigmented linse'd oil in amount of water absorbed. Silica, which had little effect in Rhodes' ex periments, made paint films of low ab sorption. There were variations among iron oxide pigments in both Rhodes' experiments and these. Venetian red re duced consumption of oxygen and evo seems to be no recognizable connection between the ratio of areal to thickness swelling and other properties of the pigments. In the first paper of this series (4), it was shown that most paints swell more across the grain of the paint than along the grain, especially when the paint contains markedly acicular pig ments. The grain of the paint is deter mined by the direction of last flow of the liquid paint before it sets and begins to harden. The ratio of swelling in In area, 33 of die 44 films remained larger after redrying than they were be fore soaking. In thickness, all films shrank after redrying, sometimes con siderably, except that of zinc oxide No. 4. Its 0.5 per cent increase was small enough to be within experimental error. In general, therefore, unweatbered free films after sweHing seem to be able to shrink in thickness more readily than in urea. That appears to be particularly true: of films for which die sweHing in water is large. lution of volatile matter only slightly, width (across the grain) to swelling in Low in Weight While Soaking: but it made highly absorptive films. length, AB/AL, was greater than 2 All films lost weight during the 3 days Iron oxides increased consumption of for the foUowing pigments in Table 1: of soaking in water. Unpigmented films oxygen and evolution of volatile matter dibasic lead phosphite, zinc oxide No. of linseed oil lose 5 to 6 grams per 100 slightly to moderately, but they made 2, ferric hydroxide yellow, magnesium cubic centimeters of film (5), much moderately to highly absorptive paints. silicate Nos. 1 and 2, and calcium sul but not aU of which can be recovered But with the exception of the variable iron oxide pigments, the most anom fate, all of which have decidedly needleshaped crystals. by evaporating the soaking water to dryness. Volatile organic solutes, of alous one of which was not available at Among Hie zinc oxides, No. 2, with course, are lost during evaporation and the time of Rhodes' work, th-re is a acicular particles, gave a ratio AB/AL drying at 105 C. reasonably close parallel between the of 4.0, but for zinc oxide Nos. 1, 3, Films of most of the paints in Table effect of pigments on the oxidation of and 4, with nodular particles, the ratios 1 lost 1.3 to 5.3 grams per 100 cubic linseed oil and the absorption of water were 1.2,1.0, and 1.0, respectively. But centimeters of film. When the films con by the films they produce. High ab when acicular zinc oxide No. 2 was tained insoluble, nonreactive pigments sorption of water apparently comes treated with vinyl silane, making zinc such as siUca, magnesium silicate, ba from use of pigments that markedly re oxide No. 5, the ratio AB/AL fell rium sulfate, titanium dioxide, and lead duce the consumption of oxygen and to 1.1. The treatment could not have titanate, the soluble ingredients con evolution of volatile matter when the altered the shape of the particLs, but it sisted entirely of organic matter from paint has dried. probably resulted in clumping of (he die linseed-oil vehicle. * Swelling: For most of the un acicular crystals into larger aggregates likewise, die leadiings from films i weathered films, the volumetric swell of effectively spherical shape that were containing any of the white leads or ing during 3 days' soaking in water not dispersed by grinding into paint. basic lead chromate contained organic was nearly equal to the absorption of In conformity with that view, the oil matter with no more than a trace of water. The swelling efficiency was there absorption of the pigment was very soluble lead compounds. Any salts of fore close to 100 per cent, a little greatly increased by the treatment lead formed by reaction with adds greater than 100 per cent more often Pigments for which the ratio from decomposition of the linseed oil than less. The nigh efficiencies for AB/AL was greater than 1.2 but less must, therefore, be nearly insoluble in paints made with antimony oxide, basic than 2 were: antimony oxide (1.3), water. sulfate white lead, and aluminum in titanium-caldum (1..4), diatomaceous The films of paint made with calcium bodied oil could result from slight un silica (1.5), and aluminum (1.8). For sulfate lost 50.5 grams per 100 cubic 5 GL038009 Tobl* 2--DMA. FOR me HIM* OF SINGLE-FIGMENT FAINTS Am* ARTIFICIAL WEATHERING FO* U DAYS--LOSS IN WEIGHT WHIIE WEATHERING; CHANGES IN DENSITY, ABSORFTION OF WATE* AND SWELLING IN VOLUME, AREA, AND THICKNESS WHEN SOAKED IN WATER FOR THREE DAYS, SWELLING EFFICIENCY, SHRINKING IN VOLUME, AREA, AND THICK NESS WHEN REDRIED, AND LOSS IN WEIGHT BY LEACHING DURING THE SOAKING FERIOD Mat tt loot: Bntlti tt 1 1 Iftittel lAboorptlou mlltaf 1 s iwniif MrM s tkM U *t t Bn iVoUia iVoArloAt * t siBiourt ft "*A. X tlA ami x imli-- Aft. ifttrtwtftw safe 1 iVolwt 1 . j A6, . i I ^ 6 t t1 .AT, 1 ^ t % t Intlmay oadAfe Lm4 UUMU iMle ewbcMtt Alt* 1mA lo. 1 iMlc emrtonaU vfett* 1m6 Jto* 2 Hole aulftt* vfalW 1mA ftuie lUlttte *iu UU DltaMle 1mA phorpblt* U*A c&raftta Buie 1*4 efcra*W 35 ptreut tlac partA* t ferecat t t ; 34\4 > 12.8 *. a,7 : a.o ti i 2.94 t : 3.3 * i 3.11 : 3.S64 2.9T 3.4 3.U 3.0ft 1 36.3 t 3*37 > 3.2T ft S4. ! 11,1 : 10.1 l 16.3 t 10.1 1 2.87 'i s 3.2T J : 2.76 1 s 3.1* t 1 2.60 1 2.24 l.a 2.73 2.99 a.53 l6 parent ImM list oacUa (mtlTt) t Zinc axlAt Be. 1 t UM OSlift VO. * s Zlae ad4a Vo. 3 : llae cntU* Vo. 4 ft t Ubc alia (tmttd) Vo. 5 t Ztac tulflA* I Llt&OpCM ttJte rBirr--ito ft Hoc tu*t i tltaaliB ilcadd* (r z*Um) Vo . 1 Tlueiia Alamo (MftlMoj Bo. 2 Tltoalvn 41cm* irutUo} lo. 3 !ltui Sltulia Alamo Hallo c(nruittUil*o)l Vo. Vo. 4 5 Tlto&lwfColelui Vonrlc as&Ao rod (aouftetaai) Vo. 1 Tarrle caaAo rod (MsafoeturoA) Vo. t ferric catfAo red {afttural) Vo. 3 T<wu rot * t * t I < t t 1 l6.1 12.1 H.T 19.6 40.8 10.6 H.0 27.2 9.2 T.S 55.1 53.S 32.1. 19.0 24.1 31.5 22.4 24.0 16.1 IB.) < 2.60 1 1 2.r > : 2.6J t t t.n t : 2.6 2.51 2.32 2.10 2. IT 2.44 '1 2.66 ! 1 2.20 ft ft 2.27 * ft 1.9^ ft 3.901 1.74 1.99 2.19 1.88 3-23 t 2.3 \ : 2.36 * t 2.39 t t 2.2ft t ft 2.34 1 2.J6 2.36 2.3ft 2.24 2.3* 1 1.67 > 1 2.5* r t 2.6ft t t 2.3ft ft t 2. 1 1.35 2.22 t.5T 2.10 2.00 ferric hydraoldt joUoo l*Vim tilicmU to, 1 Vpioliq ciUcatc Vo. V Colelav eaMnti Vo. 1 Cclolua Vo. 2 Cdeiuu cataite Vo. 3 Cileia ooftKMftto Vo. 4 krlai culfttc Oftleiai oal/Vto 2111c* BlKtaoMoa oUleo fyntod bostadto Met 1b tolled oU JUalttS lo MM oil t 19.9 t 19.0 t 26.3 i 42.J t 2.21 1 : 1.77 1 * *'P 1 l,8j 1 2.17 l.fil 1.72 1.61 1.74 i &5 : 61.6 i 93* i 51.5 11.901 ft UTf i 1 2.71 t > 1.66 1 1 1.76 1 1.75 1.68 *.ft7 1.12 1.81 ) 31.8 ft 1. t 1.40 1 63.8 1 1.11 > 1.37 t 2! 34.1 20.1 1 1.10 t * 1.30 l 11..199 -I____ L t t 2.97 * J.M : 3.16 1 3.U t 3.36 2.23 1 J.2T 1 2.79 3.14 t 2.76 1 .60 2.67 2.61 s 2.65 1 t 2.87 * 2.80 ft 2.3ft i. 3. t &! 2.3ft 1 2.301 2.37 1 1.97 1 2.52 2.6t f<t 2.37 1 2.00 1 2.JJ 1 l.9i 1.76 1 i:g: 1.89 * 0.A0 > 1.24 ) 1.78 1 l.lfi 1.43 1* 1.4T 1 1.32 t 1 BK 2.9 9.9 3.? 2.9 3.6 5.0 3.1 5.2 3.* 5.* *1 3.8 3-6 3-9 3.1 a.ft 3.3 l:i 3- PT^C1 n TA 9.9 T.5 6.4 32 7.2 16.0 20.0 3:`* 29.3 )fi.O 132 5.0 lft.1 8.3 33.6 t 2.3 1 .9 t 4.9 t KfcM t HrMt 9 8.3 -0.7 1.3 1 . 8.2 ft 1.0 .7 ft 6.6 * W 84 U 8.0 t 3-4 9 69 thitttltl wvtft ! i:5; -0.8 1 *.7 ft -2.4 ft -1.1 1 > *t.s> -.7 > ft -1.1 ft -.71 t) -0.3 3 -2*6 2.1 3 1.9 3 8.0 8.9 2.3 2.8 I !.r J .8 62 3-3 > 1.4 ft 1.3 79 3.2 s l.ft t M 100 7.5 * 4.8 1 5.5 104 14.7 1 ft.O < 6.6 92 ! -.6 > -3 -1.0 1 -.6 1 -.2 9 1 -.7 -1.3. -.7 * -9 * -43 t -3.7 1 1.3 .6 8.7 .7 9 1 18.31 10.3 t 6.6 1 93 3:3; 13-4. 7.4 1 12.2 5.0 1 93 92 11.6 1 *UJ s 1?T 95 ; -.31 1.T I > -.1 s 4.2 1 0 s 1.4 t . +2.3 1 3.3 -2.0 -4,2 -.3 2.0 .9 .6 .8 .7 X< 12L t 17.3 1 69.0 t 98 23-9 1 14.9 1 n.j 1 90 v>\ T. 1 8.7 95 6.3 1 1 100 .9 t 12.9 ee 1 410.8 1 -2.0 1 ft -5.8 1 *t *7.7 < l 7.0 1 +2.1 t 5 9 -.6 t *9.1 3.9 -2.1 -5.7 -.7 *.3 9.4 6.6 6.1 1.1 3-T 2.7 . 3.0 9.5 3-1 9.6 > ! 2.8 4.ft 6.3 i:i! 8.8 ft 2.7 2.2 1.0 1 1 .3 1 8.8 ft 2.5 * 4.31 4.3 l 3.5 3-7 9.6 1 1.9 t u 88 n 69 i -.6 1 -1.4 t . -1.. -1.1 9 ft +.4 t 9 t -1.1 s -1.4 s s -1.8 t -1.3 -.7 -.3 7 -.9 0 3-6 1.,49 .6 1.0 39 a 4.0 3.1 5ft,.8l 5.1 4*ft 12.T -4.9 t 29.9 5.0 1 4.9 1 5.2 6.6 ! 12.5 ft 2.9 1.7 4.3 1 7.3 4.2 1. .......... i 97 99 . 1 wr ft 69 t -6.6 i -1.3 -.9 1 -*2..71 * ft -.5 * ::I| "-*1 .47 0 *3 -2.3 T.8 2.6 1.3 .5 23.3 3.0 5.0 5.0 t 2.9 1 n i 100 -1.71 -1.6 1 0 1.1 2.4 3.9 3.2 2.3 7.5 7.6 19.T 1J.0 9.8. 5.0 ft 130 ii-j: 8.1 ft 10.6 > 5.0 16.9 1 96 14-7 > ft.3 1 U.1 t 96 1 -2.1 1 -1,3 1 s -4.2 t -3.61 -3.4 09 -a1..99*i A -1.7 -4.0 -3.6 8.8 8.1 7*9 3.1 2.7 19.0 5-?' a.7 15.2 5.5 .T 2.3 iX 4.0 1 -3.9 s.* -ft ft -42..18 .? 8.7 3*T 1.7 1 2.8 32..3ft 6#.* 13.6 22.R 1 1.5 ft 18.7 ua < 9.9 1 5.5 *.* 2.1 19-9 9.7 w:t; 1.1 9 ITS l.ft t 5,0 J. X____ L 1 t 101 I 0 ! 1,9 1 4.6 9 K 96 1 -4.4 t -.5 * -4.0 6.1 1ft. ...5..1... 1 * *.t s -35.7 t -.1 9 0 *9.6 > -89.2 9 l.t 66.9 1 1 -1.7 1 -+1 -1.5 2.3 ft 47 t 102 ft | 202 97 ! +4.8 1 1 -3.2 . t -2.6 i ft -1.1 ft -.6 1 ---...4371.i 4.8 -2.5 2.6 -1.4 4.9 8.2 1.9 .2 ____ L JL centimeters of films. Upon evaporation of the soaking water and drying, 39.6 grams of calcium sulfate crystals and 0.9 gram of organic nutter were re covered. Likewise, calcium sulfate was found in the soaking water from the films containing titanium-calcium and Venetian red, for which the solubility losses were 9.9 and 15.9 grams per 100 cubic centimeters, respectively. Films of zinc chromate paint lost 9 9 grams per 100 cubic centimeters to the soaking water and colored the water strongly with dissolved zinc chromate. But the water from the films of lead chromate paint, with a loss of 3.6 grams, remained colorless. Leaching from films of ail paints containing fer ric oxide were colorless and gave no test for soluble compounds of iron. The high Joss in soaking of the film with ferric oxide No. 1 may be due to soluble ingredients in the pigment that were not identified. Leachings from all paints containing 2inc oxide, leaded zinc oxide, zinc sul fide, or zinc chromate contained sub stantial proportions of soluble zinc salts from which zinc sulfide could be read 6 ily precipitated by hydrogen sulfide. Presumably, zinc oxide and zinc sulfide reacted with organic adds from the lin seed oil to form soluble zinc soaps. Similarly, all calcium carbonate paint films yielded leachings from which the soluble calcium was precipitated as cal cium oxalate in amounts that were pro portional to the losses during soaking. Weathered Films The results obtained with films that were subjected to artificial weathering for 15 days before testing are reported in Table 2. Weight Loss During Weathering: All paints lost weight during weather ing. The losses ranged from 7.6 grams per 100 cubic centimeters of weathered film for zinc-dust paint to 61.4 gtams for barium sulfate. Zinc pigments generally exhibited low losses, from 7.6 to 27.2 grams per 100 cubic centimeters, except that zinc oxide No. 4, with very fine particle size, lost 40.8 grams. Zinc sulfide and lithopone lost more than any other zinc pig ments except zinc oxide No. 4. Dibasic lead phosphite, lead titanate, die lead chromates, and leaded zinc oxides had low losses of the same order as zinc oxide Nos. 1, 2, 3, and 5, but the white leads lost somewhat more--about as much as zinc sulfide. Losses by the titanium dioxide paints ranged from 19.0 to 55.1 grams per 100 cubic centimeters. The freely chalking anatase pigments lost more than 50 grams, much of which may have been in the form of chalk mechan ically eroded from the surface of the films. Pure rutile titanium dioxide No. 3 lost 32.1 grams, and the zinc-contain ing, highly chalk-resistant titanium di oxide Nos. 4 and 5 lost only 19.0 and 24.1 grams, nearly all of which was probably in the form of water-soluble teachings. Titanium-calcium lost no more than titanium dioxide No. 3 despite its content of calcium sulfate. The iron oxide pigments, including Venetian red, aluminum, and mag nesium silicate, exhibited losses of about the same order of magnitude as the lead and zinc pigments. But barium sulfate, calcium sulrate, silica, mica, and treated bentonite gave high losses. Losses by calcium carbonates varied from 26.5 to 51.8 grams per 100 cubic centimeters. GL038010 It is not yet possible to explain sat isfactorily the variation in weathering losses by paints of differing pigmrntation. Probably several factors are in volved. It has already been shown (4) that much of the loss occurs early in the weather exposure before chalking sets in. leaching of water-soluble ingredients presumably accounts for the first por tion of the loss on weathering. It is, in turn, regulated in amount by the effect of the pigments on the disruptive oxidation of the oil vehicle and by the solubility of any soaps formed by reac tion between chemically active pigments and acid decomposition products of the oil vehicle. Later on there may be addi tional mechanical loss by chalking from the surface if a condition of free chalking sets in. Density Changes: Weathering in creased the density of all paint films ex cept those of paints made with diatomaceous silica, for which the density dropped from 1.31 before weathering to 1.28 afterward. The anomalous be havior of the diatomaccous silica paint may be connected with the porosity of the film due to the porous nature of the pigment itself. The swelling efficiency of the 10-day old films was 87 per cent and that of the weathered films 47 per cent. Per haps the increased porosity on weather ing was sufficient to diminish the den sity. For all other paints the increase in density during weathering was in ac cord with the contraction in volume normally expected in linseed-oil paints (9). The density of the weathered films usually decreased when they were swol len in water, just as was always the case with unweathered films But with weathered films there were seven excep tions to the general rule. Films of the paints with basic carbonate white lead No. 1 and titanium dioxide No. 1 were not changed measurably in density. Films of antimony oxide, lead titanate, and silica paints increased slightly in density when swollen in water. These were all films of low absorption of wa ter, low volumetric swelling, and low swelling efficiency after they had been weathered. Where most of the water absorbed is held in pores and little water enters the vehicle to cause swelling, the density when wet should exceed the density when dry. The film of diatomaccous silica paint, which was decidedly porous even before weathering, showed this effect even though it absorbed a rela tively large amount of swelling water in addition to the free water in pores. The increased density of the films of titanium-calcium paint after soaking cor responded to an actual loss in volume of 4.9 per cent during the soaking pe riod. This loss was caused by leaching of the calcium sulfate in the pigment, which left a higher proportion of the heavier titanium dioxide in the leadhed film. This effect did not appear in the unweathered films because it was over balanced by the much greater absorption of swelling water. On redrying after soaking, the den sity usually returned to the density be fore soaking or to a slightly higher density unless the film remained swollen in volume, as with basic carbonate white lead No. 2, zinc oxide Nos. 3, 4, and 3, zinc dust, titanium dioxide No. 3, barium sulfate, and diatomaccous silica, or unless there was a large loss of sol uble ingredients during the soaking period as with calcium sulfate and Venetian red. Absorption of Water: Weathered films of all but five of the paints ab sorbed less water than the unweathered films. The five exceptions can be ex plained in part on the basis of differ ence in film thickness because the weathered films were all thinner than the unweathered films. With die' diatomaceous silica paint, and perhaps to a less extent with the paints made with basic carbonate white lead No. 2 and treated zinc oxide No. 5, the increased porosity of the weathered films was a contributing fac tor by permitting the weathered films to take up more free water in pores. The other two exceptions were the mica and the aluminum paints, both of whidi contained much less pigment than the other paints, and in both of which the pigments concentrated largely at the sur face of the film, leaving nearly a pure vehicle in the underlying parts or the film. A previous paper (3) reported that bodied linseed oil absorbs more water after weathering than before. Thus, the mica and aluminum paints seem to act much like unpigmented oil. If so, the smaller increase in absorption by alu minum than by mica paint is due to the high opacity of aluminum and the transparency or mica to visible and ultraviolet light. Weathering reduced the absorption of water by films of paint containing zinc oxides and leaded zinc oxides from the level of 73 to 110 per cent to the order of 16 to 32 per cent. But the zinc oxide treated with vinyl silane absorbed even more water after than before weathering. Zinc sulfide, lithopone, and zinc-dust paints still absorbed from 16 to 33 per cent of water after the films were weathered. Films containing titanium dioxides after weathering tell in the low-absorption class with 2.8 to 9.6 per cent absorption. Weathered iron oxide films, with the exception of ferric oxide No. 1, and magnesium silicate films also were low in absorption. Swelling: Weathering reduced volumetic swelling as much or more than it did the absorption of water. Films for which the swelling efficiency remained 96 per cent or higher were those made with the lead chromates, zinc oxide Nos. 3 and 4 (but not Nos. 1, 2, or 3), zinc duomate, ferric oxide Nos. 1,2, and 3, ferric hydroxide, mag nesium silicate, calcium carbonate Nos. 2, 3, and 4 (but not No. 1), treated bentonite, mica, and aluminum. For all other films the swelling efficiency after weathering was' less than 96 per cent, indicating the development of porosity. Films for which the swelling effiden<y after weathering became less than 80 per cent were: lead titanate, all white leads except basic carbonate No. 2, dibasic lead phosphite, titanium dioxide Nos. 1 and 3, Venetian red, barium sulfate, silica, and diatomaceous silica. Swelling of weathered films in area was less than the swelling in volume except for films containing antimony oxide, lead titanate, and titanium diox ide No. 4, for whidi the swelling was small enough for experimental error to . account for the anomaly; and for cal cium sulfate and titanium-calcium, for which the leaching of calcium sulfate caused abnormalities. Weathered films, In contrast to the unweathered films, showed a strong tendency to swell more in thickness than in area. For 22 of the 40 weathered films for which the data are available, the swelling in thickness exceeded the swelling in area, whereas the proportion for unweathered films was 8 of 44. The ratio of swelling in area to swell ing in thickness ranged from 0.02 to 2.3 for weathered films compared with 0.4 to 3-7 for unweathered films, if 1 or 2 films with swelling too small for accurate measurement are excluded from each set. Evidently the denser, harder, mote brittle films produced by weathering find it easier to change in thickness than in area. Calcium sulfate and titanium-calcium paint films shrank in thickness during the soaking period, but they lost much of their caldum sulfate by itaching. Shrinking on Redrying: On redrymg after soaking, mast weathered films shank to a smaller volume than they had before soaking, but such residual shrinkage was usually less for the weathered than for the unweathered films;. The decrease in residual shrink age after weathering was due largely to a concomitant decrease in the leaching loss during soaking. Films of lithopone, Venetian red, and calcium sulfate paint shrank more and lost more by leaching after weathering than before. But films of titanium- GLD38011 7 calcium, calcium carbonate No. t, and treated bentonite paint shrank slightly more despite slightly lower leaching loss after than before weathering; ana weathered films of 35 per cent leaded zinc oxide paint shrank much more despite much lower leaching loss than the unweathered films. The weathered films showed less tendency to shrink more readily in thick ness than in area than was the case for unweathered films. Thus, only 10 weathered films, compared with 33 un weathered films, retained larger area after redrying than before soaking; and 10 weathered films, compared with only 1 unweathered film, were thicker after redrying than before soaking. Embrittlement by weathering ap parently renders the films less capable of accomplishing their volumetric shrink age more by change in thickness than by change in area. No doubt this property is closely connected with the fact that paints seldom develop checks or cracks from their own internal stresses until they have become em brittled by weathering. Loss in Weight While Soaking: Weathered films of all but eight paints lost less in weight during soaking than the corresponding unweathered films. The unweathered films probably contain water-soluble decomposition products of linseed oil that are leached out dur ing weathering. In addition, it may be that weathered linoxyn is less subject to hydrolysis during soaking than is unweathered linoxyn. The eight paints that lost more in weight during soaking after than be fore they were weathered were: the paints made with dibasic lead phos phite, zinc sulfide, lithopone, zinc dust, Venetian red, calcium sulfate, silica. The leaching losses by films of dibasic lead phosphite paint and the silica paint were so small both before and after weathering that the differences may be of no significance. The losses by die films of zinc-dust paint were slightly larger but the differ ence between the weathered and un weathered films was small enough to be of doubtful significance. The increase in porosity of the diatomaceous silica paint after weathering may have allowed the soaking water to have had longer access to the inner portions of the films than before weathering. The other anomalous paints are ones for which the leach ings include much sol uble material derived from the pigment, and for which the increased porosity on weathering may well render the pig ment more accessible to the soaking water. The calcium sulfate paint and the Venetian red paint yielded much cal cium sulfate to the soaking water. The titanium-calcium paint, which also con- 8 tained calcium sulfate, had high leach ing loss after weathering, even though the loss was slightly less than before weathering. In the films' of zinc sulfide and lithopone paints, the soluble zinc salts probably are formed by reaction be tween zinc sulfide and organic adds from decomposition of the oil. Ap parently such reactions go on in weathered films more readily with zinc sulfide^han with zinc oxide, either be cause zinc sulfide reacts with weaker acids than zinc oxide does or because the pigments have different effects on the formation of the organic acids from linoxyn. Conclusions 1. Linseed-oil paints containing pig ments increased m density when the liquid paint hardened to a dried coat ing. But the increase in density was less than that calculated on the assumption that the oil in the paints gains as much in weight and density as unpigmented linseed oil docs in the same length of time (10 days). The discrepancy be tween calculated and observed density differed for different pigments and seemed to be related to the effect of die pigments on the absorption of oxygen and loss of volatile material by linseed oil as reported in the literature. There was a further increase in density of films during artificial weathering except for diatomaceous silica paint, which contained a structurally porous pigment, and which may have increased in pore volume during weathering. 2. Unweathered films always de creased in density as they absorbed water and, on redrying, returned to a density at least as high as that before soaking, unless the soaking water leached out soluble material derived from the pigment to cause a lowering of the density. Most weathered films behave similarly, but if weathering made the film sufficiently po'ous to hold enough free water, which caused no swelling, the density when soaked sometimes exceeded the density when dry. 3. All pa:nts lost in weight during weathering, but the weight loss varied widely with different pigments. Part of the loss occurred early in the weathering period and was probably water-soluble material extracted by rain, the amount of which may be governed by the effect of pigments on the disruptive oxidation of linseed oil. Later in the weathering period, paints that chalked freely lost sub stance by mechanical erosion from the surface. 4. Some pigments reduced th$ ab sorption of water by free films below the absorptiveness of unpigmented lin seed oil. In this class were the white leads and other lead pigments, anti mony oxide, barium sulfate, silica, mica, and aluminum. Little effect on ab:o:ption was exerted by zinc-free titanium dioxides, some iron oxides, and calcium carbonate of coarse particle size. Zinc oxides, leaded zinc oxides, zinc chromate, one iron oxide, Venetian red, and most calcium carbonates made paint films highly absorptive. Other pigments such as zinc sulfide, lithopone, zinctreated titanium dioxides, and magne sium silicates made paint films of greater absorptiveness than unpigmented lin seed oil but less than zinc oxide paints. Efforts to relate the effect of pig ments on the absorption of water to such surface properties as the relative wetting by water and organic liquids or the interfacial tension between pig ments and liquids were fruitless. Butthere does seem to be a relation to the effect of pigments on the absorption of oxygen and evolution of volatile sub stances from linseed oil while drying. 5. Weathering reduced the absorp tion of water by paint films materially, unless it greatly increased the porosity of the films or unless the pigments were of the leafing type (mica and alu minum), so that most of the underly ing portions of the oil film contained little or no pigment and acted like unpigmented linseed oil. 6. The volumetric swelling of un weathered free films was nearly eq-al to the absorption of water; that is, the swelling efficiency was dose to 100 per cent, unless the films contained pig ments from which the water extracted much soluble matter. Weathering diminished the volumet ric swelling proportionately to the ab sorption, leaving the swelling efficiency near 100 per cent, for films of paint made with the lead chromates, zinc chromate, some zinc oxides, some iron oxides? ferric hydroxide, magnesium silicate, some calcium carbonates, treated bentonite, mica, and aluminum. Films contain:ng any other pigments de creased in swelling efficiency and pre sumably became porous on weathering. 7. Unweathered films always swelled less in area than in volume and usually swelled still less in thickness. Weathered films, in contrast, tended strongly to swell more in thickness than in area. 8. Films of paints made with adcular pigments swelled more, sometimes 4 to 5 times as much, across the grain than along the grain of the paint. The grain is determined by the direction of last flow of the liquid paint before the coating hardens. 9. Uhweathered films, on redrying after waking, shrank to a volume slightly smaller than that before soak ing unless the swelling efficiency indi cated that the film was somewhat porous. The residual shrinkage was due chiefly to extraction of soluble sub stances by the soaking water. Often the GL038012 area remained enlarged after drying, but the thickness was always dim.nished. Thus, unweathered films seem to shrink in thickness mote readily than they do in area. 10. For weathered films redried after soaking, the residual shrinkage in vol ume was usually less, but so was the loss of soluble matter, than that of un weathered films. Weathered films proved much less likely to shrink pro portionately more in thickness than in area than was the case for unweathered films. 11. Films of all paints lost weight from extraction of soluble matter when soaked in water. When the pigments were insoluble and nonreactive, the material dissolved consisted entirely of organic matter derived from the lin seed-oil vehicle, much of which could be recovered by evaporating the leach ing water to dryness. When the pigments contained watersoluble ingredients or formed soluble salts by reaction with organic acids from decomposition of the oil, the losses were larger and the leachings con tained the inorganic constituents from the pigments in addition to organic matter from the oil. Weathering re duced the amount of soluble organic matter leached from the films, but it sometimes increased the amount of inor ganic extract. References 1. Bartel), F. E., and Hershberger, A. 1930. Liquid absorption by pigments of different types of organic liquids. 1. Relation of liquid absorption with a series of organic liquids to interfacial tension of these liquids against water. Industrial & Engineering Chemistry 22:1304-9. 2. --------------- , and Bartell, L. S. 1934. Quantitative correlation of interfacial free surface energies. Journal of the .American Chemical Society 56:2205- 10 3. Browne, F. L. 1935. The effect of change from linoxyn gel to xerogel on the behavior of paint. Colloid Symposium Monograph 11:211-22. 4.---------------- 1953. The absorption of water, swelling, and solubility of free films of paint. Journal of the Forest Products Research Society 3 (No. 5) :108-24. 5.---------------- Swelling of paint films in water. II. Absorption and volu metric swelling of bound and free films before and after weathering. Journal of the Forest Products Re search Society. 6.--------------- - Swelling of paint films in water. HI. Absorption and volu metric swelling of bound and free films from air of different relative hu midities. Forest Products Journal V (1) :92. 7.---------------- Swelling of paint films in water. IV. Effect of thickness of film and pigment volume of paint. Forest Products Journal V (2). 8. Carrick, L. L,, and Permoda, A. J. 1951. Shrinkage of some organic film forming materials during aging. Offi cial Digest, Federation of Paint and Varnish Production Clubs, No. 322: 692-700. 9. Clark, G. L., and Tschentke, H. L. 1929. Physico-chemical studies on the mechanism of the drying of linseed oil. I. Changes in density of films. Industrial tc Engineering Chemistry 21:621-7. 10. Dunn, E. J,, Jr. 1954. Moisture re sistance of paint films. Paint Industry Magazine 69 (No. 9):I3-16, 20-22. 11. Gardner, H. A. 1926. The color num ber of dry pigments and experiments on the selective absorption of various pigments and liquids. American Paint and Varnish Manufacturers' Associa tion, Scientific Section Circular 295. 12. ---------------- 1932. Notes on colloidal w phenomena in paint and varnish prod ucts. Colloid Chemistry by Jerome Alexander, Vol. IV, 477-94. 13. Dim, J. S,, Rheineck, A. B., and Ball, G. L. 1933. Studies in the drying oils. XVII. Influence of several factors on the mechanism of drying of oil films. Industrial & Engineering Chemistry 25:1086-91. 14. MacGregor, J. R. 1952. The unex plored field of exterior paint. Official Digest, Paint and Varnish Production Clubs No. 333:869-80. 13. Rhodes, F. H., and Van Wirt, A. E. 1923. Effect of various pigments upon the rate of oxidation of linseed oil. Industrial tc Engineering Chemistry 15:1135-40. 16.------------ Burr, C. R., and Web ster, P. A. 1924. Effect of iron oxide pigments on rate of oxidation of lin seed oils. Industrial tc Engineering Chemistry 16:960-2. GLD38013