Document omgd3NnXYrzV5ZNBmzroBK3pr

With the'Coapilaents of 646 3uektian Laboratories, Inc Swelling of Paint Films In Water IV. Effect of Thickness of Film and Pigment Volume of Paint F. L. BROWNE Chemist, Forest Products laboratory,1 Forest Service, U. S. Department of Agriculture Presents systematic data on how absorption af water, swelling, and solubility of free Alms in water vary with the thickness of Alms. Discusses how pigment volume affects absorption and swelling. h e a b s o r p t io n o f w a t e r , swell absorption and swelling of its films in Ting, and solubility of free films of water. This paper presents more sys house paints vary with the thickness of tematic data on the meets of film thick the films. The relations were studied ness and of pigment volume. The meth for 5 paints, each in 6 thicknesses be ods and paints used, and the calculation tween 2 and 7 or 8 mils. The thicker and expression of results were described films gave lower values than the thinner in detail in the second paper. films. Absorption and swelling vary also with the pigment volume of the Effect of Film Thickness paint. Results are reported for 3 paints, For the study of film thickness 5 each at pigment volumes ranging from paints were chosen, pure white lead 20 to 60 per cent at intervals of 5 per paint, L, p/nv 0.30; pure zinc oxide cent. faint, Z, p/nv 0.30; pure magnesium Absorption and swelling were mini mum near a critical pigment volume characteristic of the pigment. Unweath silicate paint, p/nv 0.30; and 2 tita nium-zinc paints differing in pigment volume, paints TZae> p/nv 0.30 and ered films of paints of pigment volume TZ,0, p/nv 0.40. Films of 6 different less than the critical point swelled 1 cubic centimeter in volume for each thicknesses were spread on gummed pa per with each paint by use of a Bird gram of water absorbed. Above the critical point, swelling was less than 1 doctor blade and suction plate together with suitable shims. Films approxi cubic centimeter per gram of water ab mately 2, 3, and 4 mils thidc were sorbed, presumably because the films contained voids that could hold water that did not cause swelling. Weather ing had the effect of shifting the critical spread in 1 coat; films of approximately 5, 6 and 7 mils thickness were spread in 2 coats with a day between for the first coat to dry. Films were stripped from point to lower pigment volume. The second paper in this series (3)1 described a method of measuring the volumetric swelling of free and bound the gummed paper when they were 6 days old, and me soaking tests were started when the films were 9 days old. Table 1 gives the data for free films films of house paint when soaked in of the different thicknesses. distilled water for 3 days together with determinations of the absorption of wa ter, the swelling of free films in area, During the initial drying or harden ing of the coatings after spreading the liquid paints, the density increased by the changes in dimensions on redrying about 8 per cent. For any one paint the after soaking, and the weight of soluble density reached a somewhat higher ingredients leached from the films by the soaking water. The first paper in the series (2) showed that absorption, swelling, and solubility are greater for thin than for thidc films and suggested that the pig value die thinner the coating. Thinner films perhaps became more thoroughly oxidized during the 9-day diying period than thicker films. After soaking die free films in water for 3 days, the density of the swollen ment volume of the paint may affect the films was always materially decreased; 1 Maintained at Madison, Wis., in cooperation with the University of WUcooiin. * Numbers in ureotheses refer to literature cited. Tk* A*ik*r, Frederick L Browne, received B.Chem. degree (tom Cornell Uahrtraity, Pn.D. in colloid enemijtnr from University of wiseon> sin. He first ioined the Forest Producer tabors* (tory staff tn 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 General and Physical Chemistry section of Chemical Abstracts. but on redrying afterward die density returned to die value just before soak ing or to a slightly higher value. Leach ing of soluble organic material by the soaking water would be expected to raise the density because die pigments are much heavier than the organic vehicle. Some paints not included in this series on film thickness lose greater pro portions of pigment by leaching or be come somewhat porous after rediying and therefore fail to return quite to the density they had immediately before soaking (3). For any one paint the absorption of water during die 3-day soaking period decreased significantly as the thickness of film increased. There were discrep ancies in the data but the trends were dear. The 5-mil films of paints L and Z were somewhat too high in absorp tion; and there were irregularities for magnesium silicate paint and for paint TZ,,,, p/nv 0.40, but not for paint TZ,0> p/nv 0.30. Some of the discrepancies were greater than die experimental errors; they suggest that there are minor factors affecting the absorption of water that have not yet been discovered and con trolled. Nevertheless, the general trend of diminishing absorption with increas ing film thickness is dearly revealed in Table 1. Films 7 to 8 mils thidc absorbed only 50 to 65 per cent of the water taken up by films of die same paint that were 2 mils thick, except for the magnesium silicate paint for which the variation with thickness was less than for the other paints. The volumetric swelling for all paints was nearly equal to the absorption of water and therefore showed similar variation with film thickness. Swelling efficiencies ranged between 99 and 106 per cent. Swelling in area and in thick ness followed much the same pattern. On redrying, the trend was for the thin ner films to shrink in volume somewhat more than the diidcer films, but the shrinking in area appeared to be nearly independent of film thickness. The weight loss by leaching during the 3-day soaking period was greatest for the thinnest films of any one paint and decreased as the film thickness in creased. The effect of film thickness on ab sorption, swelling, shrinking, and solu bility is important enough to be knit constantly in mind when comparing the behavior of different paints m soaking fists. But it is not yet possible to cal culate precise relations between thick ness and absorption, swelling, and sohi- Reprinted /tom the April, 1955, Forest Products Journal, page* 142-146 Forest Products Research Society, P. O. Box 2010, University Station, Madison 5, Wisconsin Table 1.--EFFECT OF THICKNESS Of FILM ON CHANGES IN DENSITY, MSOIOTION Of WATER AND SWEUING DURING SOAK ING FOR 3 DAYS IN DISTIllED WATER, SHRINKING ON REDRYING, AND LOSS Of SOLUBLE UBSTANCES BY LEACHING FOR FREE FILMS OF FIVE PAINTS OF DIFFERENT COMPOSITION Itlat {denity of Deiulty 33 TUm :Aborp-J Bwlllcc iSvelllof: ftadriad eb*ae> iVftlfht Io m liquid) Dry l :Swollen:Bedried : in J : vater J nets T0 voter > la : la A7V. iy o Iir m : A*. * At. Xn ; clacy In t la t thlak-: AV.V,/A:valaaat inft mm : ATr AT, i la t : tivlek- 1 j BMI 2 ATy S X Dulc earbenate vfelte: 2.9S l 2.7* lfA, 1, p/nr 0.30 2.91 ! 2.7* (a-705) 2.89 : 2.75 2.88 5 2.73 2.87 : 2.7$ i 2.85 s 2.75 2.98 2.95 2.92 2.91 2.90 2.88 Zinc oxide, Z, p/a 0.30 (2.3*5) 2.5* : 1.8* 2.*9 s 1.8* J 2.*8 t 1.89 2.50 s 1.77 2.*6 : 1.91 s 2.*5 J 1.95 2.59 2.53 2.51 2-51 2.18 2.*7 ItosMslua aillcata, P/CT 0.30 (1.515) TltaaiUa-slSC, "* itl*? f 1.67 t l.*2 2 1.66 : l.*5 l 1.6$ t l.** l 1.6* : l.*2 I.63 : 1.** : 1.62 : 1.13 I 1.66 1 1.31 ft 1.86 1.86 : : I.32 1.35 tft 1.85 1.85 j ; l.*l 1.15 : I.85 1 i.*a 1.68 Z.68 J 1.67 1.65 t 1.61 I 1.61 * 1.89 * 1.87 1.86 1.87 1.86 t 1.86 Titaaiua-aiac, (1.975) 2.1* : 1.5* s 2.1* t 1.55 : 2.13 : 1.53 2.13 : 1.61 s 2.11 : 1.70 2.10 1 1.70 1 2.16 J 2.16 : 2.15 2.11 3 2a2 3 2.11 Kilft :Percent: Percent:Percent: hr* ileroect eeat : 1-9 12.8 s 12.6 3 8.6 2.7 11.7 1 U.7 1 7.3 3.5 9.* t X:l\ 5.6 *.9 10.* 1 6.2 6.1 8.1 t 8.1* *.7 8.1 7.2 7.* 1 *.* 1.8 85.5 \ 85.6 * 38.2 2.8 3 78.* 1 78.2 t 31.9 l 70.0 1 69.1 26.1 t 96.5 t 96.2 30.* 7.2 l 61.0 s 60.6 : 20.9 6.0 55.* 55.0 t 9-0 2.1 l 57.2 1 57.3 1 15.1 2.8 *5-7 * *5.6 j 1*.9 3.* 3 *7.* : *7.5 s 15.7 5.2 5*.7 1 5*.8 s 16.1 7.8 3 *3.5 *1.2 1 8.3 3 *5.5 s 16.0 : 2.1 168 l 167 ! 68.7 fX 153 s 155 s 65.* 1*5 1 116 56.0 5.0 3 109 1 9 t 58.8 6.0 89.2 1 88.6 s *2.5 6.8 80.9 s 80.3 : 39.0 1.T 3 92.2 s 97-9 1 50,2 2.2 3 105 I 10* I *7.8 2-t 111 $ 110 ! *3.6 1.1 3 86.5 J 85.8 1 *5.3 6.1 6.8 60.3 : 59.5 33: 27.2 26.1 3 1.0 : 1.9 i 2.3 1 2.7 1 2.0 : 1.9 s 99 100 iff 100 103 31.5 . .100 32.2 t 100 35-0 * 99 *8.1 1 100 31.0 i >.* > 99 99 32.5 1 23.6 < 23.7 1 31.1 1 25.8 : 26.O t IOO 100 100 100 101 am 55.0 t 51.6 1 56.3 1 29.0 t 30.2 1 27.7 1 99 }C9k 101 100 99 99 28.2 t 35.7 **.2 t 26.T t 2*.0 : 2**.6 1 s 106 99 99 99 99 99 P*r- 1percentiPereeat: Percent cm* j 2 36 1 -3.1 1 >2.5 1 -2.5 * -2,1 t -2.1 t -0.3 * 6 ; -.71 -9 * ...7 S -.7 -3.1 j 2.2 1 -1.1 1.* 1 1.1 1 -1.1 * (.a k.C 3.6 3.0 2.7 -5.0 -*.2 -3.9 1 -2.9 * -2.* I -1.9 5.6 s -9.8 j 7.* -10.9 5.9 1 -9.0 t *7.7 : -9.8 1 6.1 : -8.0 1 *.8 t -6.1 1 8.6 7.5 6.8 5.0 3.7 2.9 -5.0 s *.5 i -*.0 1 3.2 t -2.2 t -2.2 s 1.3 s *2.3 t +3.3 3.5 1 *2.6 > 2.7 J *6.5 2 6.5 j -7.* 1 6.1 -*.e t -*.2 1 7.6 6.0 5.0 3-9 1-9 1.7 -6.2 1 20.* S -2*.2 t -3.7 1 -1.9 -*.3 1 21.7 t -21.3 t -2.6 t -2.8 1 1*.* : -15.0 ! -5.11 8.8 ! -12.8 : -*.l I +9.6 t -11.6 : -*.6 > +5.* s -9.1 1 2.2 1 *10.8 1 -11.3 t -1.7 t 6.5 -7.5 -1.7 1 48.3 t -9*3 t t2 9.* 6.3 IS 1:1 8.5 6.9 7.2 ft 3.0 biiity because the data aie not repro ducible closely enough for that purpose. However, the variations with film thick ness, and the limitations of reproduci bility are small in comparison with the differences among paints. In comparing different kinds of paints, uncertainties can arise only if the paints are similar enough in absorptive properties to make distinction between them of little con sequence or if the films differ greatly in thickness. Effect of Pigment Volume For the study of the effect of pig ment volume of paint on the absorption and swelling of free films, three kinds of single-pigment paints were chosen, made respectively with basic carbonate white lead, zinc oxide, and anatase titaium dioxide. The first was a kind of paint that is very low in absorption and swelling, the second very high, and the third intermediate. For each kind of pigmentation a series of 9 paints, varying in pigment volume from 0.20 to 0.60 in steps of 0.05 were made. Each paint was spread by doctor blade, suction plate, and suitable shims at a wet-film thickness that was intended to make a coating approximately 4y2 mils thick when dry. The thicknesses actually pro 2 duced, however, varied from the intent somewhat more than might be desired, but not sufficiently to obscure the gen eral trend in the relations between pig ment volume and absorption. Each paint was spread on gummed paper to provide samples of free films for test before weathering. Each paint was spread also on previously weighed tinplate to furnish samples for exposure in artificial weathering for 15 days in a Weatherometer. Unweathered films were 17 days old when submitted to die soaking test. Coatings on tinplate were 17 days old when exposure began in the Weatherometer. Hard water was used in the Weatherometer sprays. Results with unweathered films are reported in Table 2 and those with weathered films in Table J. Coatings of white lead paints dried with the degree of gloss typical of house paints at all pigment volumes below 0.60, At p/nv 0.60 white lead paint dried flat, that is, without gloss. Zinc oxide paints dried glossy at p/nv of 0.45 ana less, with low gloss at p/nv 0.50, and without gloss at p/nv 0.55 and 0.60. Titanium dioxide paints dried glossy at p/nv of 0.40 and less, with low gloss at p/nv 0.45, and with out gloss at higher pigment volume. Asbedc and Van Loo (1) showed that the pigment volume at which tran sition from die glossy to the glossless condition occurs is a critical point at which marked changes occur in other properties of the paint such as sensi tivity to moisture blistering, permeabil ity to moisture, and protective power against rusting of metal They therefore oiled the point die critical pigment vol ume. Presumably it is the pobt at which the film contains just enough vehicle to cover die surfaces of the particles of pigment and to fill the voids between them. Thus in these tests the critical pig ment volume was between 0.55 and 0.60 for white lead paint, between 0.45 and 0.50 for zinc oxide paint and between 0.40 and 0.45 for anatasetitanium-dioxide paint. The results of the tests of absorption and swelling indicate that absorption and swelling may now be added to the paint properties that are known to change sharply at die critical pigment volume. Films Before Weathering Unweathered films of all 3 kinds of paint (Table 2) exhibited a minimum absorption at or just below the pigment GLD38001 TabU 2 --UNWEATHERED TAINTS--EFFECT OF FIGMENT VOLUME OF FAINT ON THE ABSORPTION OF WATER AND SWELLING DURING SOAKING FOR 3 DAYS IN DISTILLED WATER, SHRINKING ON REDRYINO, AND LOSS OF SOLUBLE SUBSTANCES Y LEACHING FOR FREE FILMS OF PAINTS MADE WITH THREE DIFFERENT KINDS OF FIGMENTS Faint ;>i lt t sPin*nt:Inltlal:Aborp-: BweUlns tSmiling: IWdrled :Uei*ht leee :roluae :thick- I tltc :-------- ---1 am- t.........------- --.----sunn* nceke i of : mi< l A/V. 1 In : In 1 la $ tinner 1 la t Zb i; In t toe : paint ! T l : p/nv s tvolvae 2 area tthick-l AVT-VA it o Iw h : wee :ithick- t AT, 49, Mil t AT, * AO, "* * 0 :: At, t >1 1 ATr t Sonic eertcoate nfalte lead, i, p/sr 0.30 : : mie iPercentiPercent: Per- t Per- sPweent t per- Per- j Per- t Percent :t 1 t cent : cent 1 t cent t cent 1 cent t 1s 2 lt t X : -0.60 : $.0 12.2 1 6.7 t 3.0 t 3A : 55 J 0 -0.5 j *0.5 2*6 : .55 : 6.1 5.2 1 3.7 s 2.2 1 1.3 * 71 1 -1.5 t -.5 > -1.0 1 BA : .50 > 6.2 A.? 1 *.5 s 2A 1 2.0 : 96 : -1.5 1 -.6 : -.9 i 1.9 : .*5 5-8 A.7 1 A.9 t 2.9 : 2.1 : 10k t -1.8 1 -A : -l.e : 2.0 s M : A.6 5.6 : 5.8 3.5 2.2 : 103 t -2.7 -A : -2.3 1 2.8 > .35 ! 5-8 5.5 1 5-6 : 3.1 : 2.3 1 102 : -2.J t 0 : -2.3 2.6 > .30 : h.l 6.A > 6.5 > AA : 2.0 1 102 : -3.S t -A 1 -2.9 : 3.7 ! -25 *. 3.9 6.9 : 7.3 : A.6 : 2.6 1 106 : -3.8 t -.3 t -3.5 J A.2 : .20 : A.l t 6.8 1 6.9 i 5-2 : lA : 102 1 -3.6 1 -.6 : -3.0 3.9 Zinc oxide, Z, pfur 0.30 1 %60 ! 6.1 33.5 1 25-9 s 6.6 \ 18.0 I 77 : S'.55 t 5.9 32.6 : 22.3 : 9.3 1 12.0 : 66 s -.50 : 5.7 t 3A.7 : 33.1 i 15.1 1 15.5 95 s A5 : 3.7 t 65.5 63.3 33.2 t 22.5 : 97 : .Ao s *.3 X 71.6 1 70.1 1 35A 25.6 : 96 : .35 : A.7 X 76.3 75.5 ! 39.9 t 25.5 1 99 s .30 : M : 71.6 t 7lA : 33.1 28.7 t 100 : .25 s 3.7 * lO :U9 : 67.3 J 31.0 > 98 : .20 : A.O : 121 121 : 66.2 t 38.9 * 100 s1 S : t i t Anntone titanic* dioxide,: r.60 : 5.1 t 26.6 : 21.0 : ofi : 20.8 : 79 t--. p/nr O.JO 480 : f.55 : 4-9 16.5 t 13.0 1 A : 12,3 : 79 : i.50 : 5-k 17.1 : 1A.1 : 3.0 : 11.1 1 82 : : A.2 a 11.3 : U.2 : 6.1 : A.6 i 99 s ,AO : A.O i 13.1 ; 13.1 t 8.1 > A.5 J 100 : : .35 : .30 : >-5 A.O X 15.2 j 15*1 : 16.1 j. 16.3 : 9A t 9.0 j 5.2 1 6.7 1 99 101 : .25 : 3-9 : 16.2 1 16,V : 9.6 : 6.2 : 101 : .20 : 3.7 z IS.* t: 16.6 X 9.3 i 8.6 x 102 t: t j 45A 1 +1.9 : +3*5 : 1 42.1 41.5 s 4.6 : j -.9 t 4I.3 > -2,0 : t -1.0 : 4*.5 : -5.3 > 1 -.6 t 46.2 : -6.5 1 j -1.5 48A t -9.2 : t >2.5 > 4IH.8 : .15.0 : t -.9 t 4S3.O : -19.5 : t -2.7 1 423A : -a.2 l ttxt : 41.7 -.1 1 41.8 s > -lA 1 -.2 j -1.2 1 -.6 1 -.3 t -.3 t t -l.o 1 -1.0 : -3.0 t i -3.1 -.8 t -2.3 : < -2.5 -.7 1 -1.8 1 -3.2 0 : -3.2 J 1 -3.2 : 4.1 t -3.3 1 t *3.7 x *.l * *3*8 j 1t : : l*V 6.0 1.8 *8 2.7 8.9 1.1 1.0 A.2 3.7 A.l 3.0 A.5 3-3 3.0 3.6 3.9 A.3 ~lbeae paint* dried without any (loee. 8 --Ibe.e pelnte dried with lov (Lots. volume of transition from gloss to flat. The volumetric swelling reached a minimum at the same or the next step in pigment volume above that of mini mum absorption. The swelling efficiency was always dose to 100 per cent (between 97 and 106 per cent) at pigment volumes from 0.20 to the critical pigment volume and was always less than 97 per cent, some times as low as 5 5 per cent, at higher pigment volumes. Inis is strong con firmation of the interpretation of low swelling efficiencies made in a previous paper (3), namely, that an efficiency less than 100 per cent by more than rea sonable experimental error indicates that the film contains voids capable of hold ing absorbed water that does not cause swelling. Films of paint of pigment volume higher than critical necessarily contain such voids because there is insufficient vehicle to fill all spaces between partides of pigment. Experimental evidence of porosity above tne critical pigment volume and absence of pores below that point was published by Eckhaus, Wolode, and Harris (4) on the twists of measurements of permeability to mois ture and surface roughness or free films of paint made with titanium dioxide and with magnesium silicate pigments. Swelling in area was approximately parallel to the volumetric swelling, ex cept that, on passing from die critical to higher pigment volume, swelling in volume increased from the minimum, but swelling in area either continued to decrease or increased very little. On redrying, white lead pint of p/nv 0.60 shrank to its initial volume before soaking. All other white lead paints shrank to less than the initial vol ume, increasingly so the lower the pig ment volume. Thus there was no evi dence of a change in shrinking at a critical point. With zinc oxide paints the films of paints of p/nv 0.60 and 0.55 failed to return on redrying to the Initial volume, whereas films of paints of lower p/nv shrank below the initial volume, in creasingly so as p/nv decreased further. Films of titanium dioxide of p/nv 0.60 after redrying remained somewhat above their initial volume, but those of p/nv 0.55 or less shrank to less than the ini tial volume. On the whole, the relation between shrinking on redtying after soaking and pigment volume is much less sharply defined than the relation between swelling and pigment volume. The loss in weight from leaching of soluble substances during soaking seems to be nearly independent of pigment volume. Weathered Films Use of hard water in the Weatherometer sprays may be responsible for ir regularities in the data for loss of weight during weathering, particularly with the zinc oxide paints (3). Never theless, there was dearly a trend to higher losses at low than at high pig ment volume, but no evidence of a minimum at a critical pigment volume. The absorption of water on soaking was minimum at die critical pigment volume for the weathered just as it was for the unweatheued films of white lead paints and zinc oxide paints. But for the weathered films of titanium dioxide paints die minimum of absorption was at P/nv 0.35, much below the pigment volume of minimum absorption for the unweathered films. All titanium dioxide paints were chalking very freely by the end of the 13-day weathering period, but those of p/nv 0.35 and higher were chalking much more seriously than those of lower pigment volume. The volumetric swelling of weathered films was minimum for all three kinds of paint at the same pigment volume at which absorption was minimum. At pigment volumes above that of the minimum the swelling increased less than die absorption did. 3 GLD38002 Table 3.--WEATHERED PAINTS--EFFECT OP MOMENT VOtUME Of PAINT ON THE IOSS Of WEIGHT WHILE WEATHEtING. ABSORPTION OF WATER AND SWELLING DURING SOAKING FOR 3 DAYS IN DISTILLED WATER, SHRINKING ON REDRYING, AND LOSS OF SOLUBLE SUBSTANCES BY LEACHING FOR FREE FILMS OF PAINTS MADE WITH THREE DIFFERENT KINDS OF PIGMENTS Paint Pignent: Height 2lnltiel:Al>ori>-2 Swelling Swelling Redrlelt volume : lo* S' .hick- : tlon am- of sveetber*.* ne# s A/V0 In : In 2 In cleoey Zn i in : paint : Ins p/nv : Bx/V0 % : : :vol\ae: area : thick-: AV.Vq /A volute 2 area :thick : AQ- i nett Ar 2 AO, 2 net* s t 2 AT, eight low ilia soak- IhR 8j A0 :Percent . Ml f ;Percent: Peri cent Per* cent Per* :Percent cent cSeEn?t ;1 Per* cent : 2 Percent Percent Bade carbccate white lend, 1, p/nv, 0.30 ^0.60 : 9.8 .55 ! 5.1 .50 6.6 7.2 2 15.8 6-3 18*.34 2 2 8.8 8.9 s 6.1 7.8 3.9 : 2.3 2.1 2 4.0 2.5 2 5.1 .45 : 18.8 : <2.5 t 84.0 82.9 3.6 2 18.6 .40 s 17.7 3-9 2 36.5 : 35.0 3.1 t 31.0 .35 2 18.8 7-7 2 11.5 : U.O 2.1 2 8.7 .30 . 87.0 4.3 2 16.7 16.7 3.3 2 13.0 .85 2 39.8 8.8 : 58.5 * 56.7 i 3.1 t 51.8 .80 2 81.7 4.8 2 18.8 s 17.8 t 3.4 2 14.0 * l *r.Zlac oxide, l, p/nv 0.30 T.60 2 18) 3V.5505 t 5.9 : 40.6 5.8 : 5.8 : 85.0 84. 81.6 6.1 17.1 8.8 11.8 &M 8.1 s 4.8 : 37.6 .to 11.3 s 4.4 : 43.1 16.8 11.9 a.i 16.0 .35 9.9 : 4.7 i 44.4 to.5 : 82.t lt.8 .30 14.0 *. 3-3 : 75.8 69.1 2 a.6 39.8 9.7 46.7 45.0 17.1 .20 : U-5 36 2 67.0 163.4 2 31+8 : 2t,0 1 ts Anataee tltanliat dioxide, T^Z)' 8/"* 0,3 T.fiO ; r.tt : .50 2 *.45 2 (k) .9 31.9 , 4.4 2 83.8 2 18.7 5 1.5 U.O 2 2-9 : 22.2 J 8.5 2 1.5 > 12.6 2 M 1 87.6 3.7 2 18.4 2 6.9 2 1.3 t W.9 * .35 2 86.0 .30 2 38.3 3.7 2 6.8 2 4,0 2 3.4 .5 2 8.8 : 12.6 2 9.8 2 5.1 : 4.2 t 1------------------------------ .85 2 44.2 8.9 t 13.1 r 11.5 2 5.4 2 5.8 2 .20 : 51.0 3.1 2 17.8 2 13.8 2 5.9 2 7.0 : : f : : : 5? "The*. paint* dried without any gloee. 2 -rile too brittle to be (tripped trm tinplate after weathering, 40 95 96 96 100 106 9 60 86 86 i ?! 95 53 3B 37 65 78 68 77 +2.9 2 +1.8 2 1.7 -.5 2 -.7 2 .2 0 2 *,6 s +.7 +8.8 1 -1.0 2 +3.8 ,6 1 -1.8 2 1.8 -9 ! -.3 2 *.6 -2.1 1 -1.1 2 -1.0 -2.1 2 -8.0 I -.1 -1,8 : -.7 ! -.1 ti 6.5 1.3 -7-7 -1.6 2 +.5 -2.1 -3.3 8.7 -5.9 -1.6 t +5.8 1 -6.5 +.8 z +6.9 1 -6.1 1.3 2 -+6.,t : -4.8 +.8 -2*2 t tt 1 t -2.5 2 -.5 : -2.0 2 t +1.6 1 0 5 1.6 i : +.7 2 -.7 s 0 2 1.8 J *2.2 S 4fc.O t : -1.3 -1.6 2 +.3 2 t -2.6 2 -1.1 2 -1.5 2 j +1.8 2 -1.3 2 +2.5 2 ftt 2.0 1.2 1.0 1.1 1.6 1.7 2.6 4.0 2.5 18.8 8.1 6.6 3.9 3.5 6.0 1.8 7.3 4.8 3.7 3.2 6.0 4.4 4.5 4.6 paint* dried with low gloee. -Tllne eracked and scaled fran tinplate during weathering. For all weathered films of zinc oxide paints and titanium dioxide paints and For white lead paints of p/nv 0.35 or higher, the swelling efficiency was less than 97 per cent. Moreover, the trend was from higher swelling efficiency at low to lower efficiency at high pigment volume. Weathering, then, seems to re duce swelling efficiency and to reduce it more rapidly at high than at low pig ment volume. Weathering markedly increased the absorption and volumetric swelling (but not the swelling in area) of the white lead paints at or below the critical pig ment volume. Similar results were re ported for artificial weathering but not for natural weathering in a previous pa per (3). A subsequent paper will show that artificial weathering reduces rather than increases the absorption and swell ing of white lead paint when distilled water is used instead of hard water for the Weatherometer sprays. Weathering decreased the absorption and swelling of the zinc oxide and the titanium di oxide paints at or below the critical pig ment volume. Swelling in area of the weathered films paralleled the volumetric swelling fairly closely, except that swelling in 4 area increased little if any at pigment volumes above the critical. Shrinking on redxying weathered films after soaking seemed to follow no consistent pattern. With all three kinds of paint, some films failed to shrink back to the initial volume before soak ing and others shrank to slightly smaller volume. Loss in weight from leaching during soaking was independent of pigment volume and was about the same for weathered as fot unweathered films. Conclusions 1. Film thickness at which paint coatings are spread has the following effects: Density increases slightly less during initial drying of wet paint the thicker the film. Absorption of water and volumetric swelling of free films during 3 days* soaking in distilled water decrease as film thickness in creases; but unweathered films, regard less of thickness,- usually swell abort 1 cubic centimeter for each gram of water absorbed. On redrying after soaking, thin films shrink in volume mote than thick films, but the shrinking in area is independent of thickness. Weight losses by leaching of soluble substances are greater for thin than for thick films. 2. Although the effect of film thick ness on absorption and swelling mast be kept in mind when comparing differ ent paints, it is not so great as to ob scure differences between paints unless the film thicknesses are far apart or die paints arc too nearly alike in absorption and swelling for difference between diem to have much significance. 3. Absorption of water and swelling of free films of unweathered paints are minimum at a critical pigment volume that is characteristic of the kind of pig ment and vehicle in die paint The critical pigment volume is well above the pigment volume at which house paints are usually made. 4. At and below the critical pigment volume unweathered films swell 1 cubic centimeter fot each gram of water ab sorbed, but above the critical point they swell less than 1 cubic centimeter per gram of water. Since the films of higher than critical pigment volume contain less than enough vehide to completely fill the spaces between partides of pig ment, a condusion previously drawn, that less than a cubic centimeter of swelling for a gram of absorbed water GLD38003 indicates the presence of voids in the film, is confirmed. 5. Loss of weight of paint coatings during artificial weathering is greater at low than at high pigment volumes, but there is no evidence of a minimum of loss at a critical pigment volume. 6. For weathered films there are min ima of absorption and swelling at an intermediate pigment volume, but the minima occur at lower pigment volumes in weathered than in imweathcred films. Perhaps it may fairly be concluded that weathering lowers the critical pigment volume of paints. References 1: Asbeck, W. K. and Van Loo, M. 1949Critical Pigment Volume Relationships, tad. Eng. Cbm. 41:1470-5. 2. Browne, F. L. 1953. The Absorption of Water, Swelling, and Solubility of Free Films of Faint. /. of Fortit Products Re search Society HI (No. 5); 108-24. 3. Browne, F. L. 1954. Swelling of Paint Films in Water, II. Absorption and Swelling of Bound and Free Films Be fore and After Weathering. /. of Forest Products Research Society. IV, No. 6: 391-400. 4. Edchaus, S., Wolock, 1, and Harris, B. L. 1953. Porosity of Faint Films. Watervapor Adsorption and Permeability, lad. Eng. Cbm. 45:42(3-8. GLD3800A 5