Vol. 21, s o . 11
INDUSTRIAL A S D ENGINEERING CHEiIIISTRY
1118
Settling of Pigments in House Paint-11' H. L. Beakes DEVOE& RAYNOLDS CO.,
ISC.,
1 -''EST
Pigment Ratio Maintained
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HE writer pointed out in a previous paper ( 1 ) that the pigment ratio of lead to zinc in a two-pigment paint did not change during normal (not accelerated) settling. The same paint subsequently analyzed periodically for 6 months shows a minimum of 49.1 per cent and a maximum of 49.6 per cent zinc oxide. Table I gives analyses for six additional two-pigment paints, and confirms the belief that this ratio does not change, although the specific gravity of the pigments in some of the samples varies greatly. Table I-Pigment
Ratio Maintained, as Shown by Zinc Oxide Content TOP SECOND THIRD FOURTH BOTTOX
DATE OPENING OPENING OPENING OPENING OPEXINO Zinc Oxide and Barytes Paint Containing 44.25 Per Cent Zinc Oxide, M a y 11, 1928
70
%
so
%
%
44.4
44.25 44.4 44.1 44 25 43.7 43.7 43.3 43.4 .. 41.6 42.6 .. 44:6 44:7 45,o 44.6 .. .. 44.1 .. 43.5 .. .. .. 44.0 44.1 Carbonate White Lead-Zinc Oxide Paint Containing 50 Per Cent Zinc Oxide, March 26, 1928 3-31-28 49.9 , . 49.1 .. 49.2 4-14-28 49.6 .. 49.5 .. 49.4 4-16-28 49.6 , . 49.6 .. 49.5 4 - 2 3- 2 8 49.5 49.5 .. 49.6 5-17-28 .. 49:. .. .. 49.3 6- 7-28 .. 49.0 ,. , . 49.3 6-30-28 .. 49.0 49.0 8-23-28 .. .. 49:6 49: 8 49.6 9-25-28 .. .. 49.2 .. 49.1 Sublimed White Lead-Zinc Oxide Paint Containing 52 Per Cent Zinc Oxide, March 26, 1928 51.5 .. 52.25 , . 52.25 .. 52 5 52.5 52.5 .. 5i:4 52,5 .. 52.5 .. .. .. 22.7 52.3 52.5 .. .. .. d2.4 52.3 .. .. .. 52.2 52.0 .. .. .. 52.0 .. .. 32.7 52.4 52.0 52.2 .. , . 32.6 .. Red Lead-Asbestine Paint Started February 13, 1928 55.8 .. 56.6 56.6 .. 56 1 .. .. 57.2 57.6 .. pz.z .. 56.4 .. 3D.J , . , . 56.9 .. 55.5 55.8 56.5 56.8 .. .. , . 57.0 56.8 .. 57.0 56.3 56 7 5617 58.1 .. 57.0 57.0 Titanox-Zinc Oxide Paint Started March 2 6 . 1928 5-17-28a .. 48.8 48.8 48.8 .. 48 8 5-26-28 48.8 48.8 .. 48.6 48.6 7-20-28 48.6 .. 48.8 48.8 8-23-28 .. 48.8 8-31-28 .. 49.0 9-25-28 Titanox-Zinc Oxide (5% Leaded) Paint Started March 28, 1928 5-17-28b .. 48.2 47.6 48.2 48.0 3-25-28 .. 47.6 47.8 48.0 47.6 7- 2 0- 2 8 .. 48.2 47.8 47,5 48.2 8-23-28 .. .. 47.9 .. 47.6 .. .. 47.7 .. 47.8 9- 2 5- 2 8 a Determinations prior t o 5-17 were on insoluble which varied 2.8 per cent between high and low. b Determinations prior t o 5-17 were on insoluble which varied 2.07 between high a n d low. , .
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Accelerated Settling Test
An accelerated settling test' ( 2 ) was proposed. Dcterminations made on a two-pigment paint by heating in an oven overnight a t 72' C. and then centrifuging 20 minutes 1 Presented before t h e Division of Paint a n d Varnish Chemistry a t the 77th Meeting of t h e American Chemical Society, Columbus, Ohio, April 29 t o M a y 3, 1929.
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at 1400 r. p. in. showed that the pigment ratio ib not necessarily maintained. In the case of the red lead-asbestine paint described in the previous work, which contained 56.5 per cent of red lead, the upper third of the settled pigment using this accelerated test contained 40.6 per cent red lead, the lower third 62.7 per cent red lead. Effect of Charging Pigment
A 440-d. c. generator was connected to an insulated threeroll Day laboratory mill. The paste was first mixed by hand, and was then ground through the charged mill. After thinning, the samples were placed in quart glass jars to observe the settling. With a positive charge the settling of red lead was hastened, while silica treated in a similar manner stayed in suspension better. (Table 11) After the test had progressed one month, the silica paints were remixed and the percentage of pigment checked; 61.9 per cent was found for both the positive and the negative. Other pigments similarly charged showed no marked difference in settling rates. Table 11-Effect
of Charging the Pigment as Shown by Content of Supernatant Liquid RED LEAD SILICA
Negative charge
Date 10-15-28 10-19-28 10-23-28 10- 26- 28 10-30-28 11- 7-28 11-13-28 11-19-28
Positive charge
"0
o/o
5.2 8.9 11.8 14.8 17.0 23.0
6.6 11.7 15.4 18.3 21.2 27.0 30.7 33.5
26.7 29.6
Date
S'egative charge
7c 10-15-28 10-19-28 10-23-28 10-26-28 10-30-28 11- 7-28 11-13-28
4.41 6.62 9.57 12.50 15.40 22.0 24.2
Positive charge
9% 2.94
4.4 5.14 6.25 7.35 8.83 10 30
Constancy of Pigment Concentration
Silica in linseed oil, when varied from 65 per cent to-40 per cent by weight, settled in all cases to the same (71 per
1119
I S D C S T R I A L A S D E-VGI,YEERIA'G CHEMISTRY
November, 1929
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by centrifuging was stiffer than a inill-ground paste of the same analysis and that it became softer as it was worked under a spatula, which suggests that the form of packing in these cases is different. Samples were withdrawn periodically froin the iniddle and the bottom opening of the single pigment-silica paint previously described ( I ) . The percentage of pigment found was plotted against time in days, forming a settling curve. per ccwt to 55 per (Curve No. 111) The curve for the middle opening shows cent by weight, acted in thr same maniler. (Curve S o . 11) that 71 per cent is a very definite and characteristic perThis indicates that each pigment settles to a definite con- centage of pigment that is maintained for a considerable centration and. since the length of time for settling is a con- length of time. Aant, the original percentage of pigment in the ready-mixed To determine whether other paints show definite points paint merely controls the rate of settling. of pigment concentration analagous to that of (71 per cent To determine whether the figure (71 per cent) for silica for) silica, retained samples of eight paints that had settled concentration in linseed oil was a definite point in its settling, 10 months were stirred up and centrifuged as above. The a silica-linseed oil paint containing 55 per cent pigment was concentration of the pigment in this settled paste was coincentrifuged one hour a t 3000 r. p. m. a t a radius of 8 inches. pared with that of the same paint settled in a 10-gallon conilnalysis of the settled paste showed 70 per cent of pigment. tainer sampled from the bottom opening. (Table 111) It was noticed that the consistency of the paste obtained The agreement in five of the paints was satisfactory. In
when concentrating to 71 per cent. Concentrations greater than 71.5 per cent or above the flat part of the curve might be expected to be harder
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Table V-Effect of Composition of Liquid5 (Volume of silica, 35.9%; volume of liquid, 64.1%;) BUTYL TCRPEX- OELLOTIME WATER BUTANOLTOLUEKE TINE !SOLVE Hours % % 5% 70 2 3.1 Trace Trace Trace Trace 18 6.3 7.6 Trace 2.2 7.5 Days 2 7.3 17.4 Trace 2.2 16.0 4 7.3 22.8 Trace 2.2 25.6 6 7.3 22.8 2.2 2.2 25.6 S 7.3 23.9 2.2 2.2 26.6 10 7.3 23.9 2.2 3.3 26.6 31 7.3 23.9 2.2 3.3 26.6
Cellosolve. This is shown in another way by Curve No. XI, where the volume of the pigment is constant and the liquid varied from 100 per cent kerosene to 100 per cent linseed oil. Apparently the volume of the settled pigment paste depends on the composition of the liquid. Literature Cited (1) Beakes, IND. ENO. CEEM.,80, 732 (1928).
The Mobility-Concentration Intercept' J. T. Baldwin SANDURA COYPANY, INC., PAULSBORO, h'. J.
Effect of Wetting Power The mobility-concentration curves of castor oil, of Vehicle China wood oil, linseed oil, and lubricating oil with concentration i n t e r silica and drop black are studied. The results agree cept" will be used to Since the degree of disperwith Bingham's prediction that the mobility-concendenote the point a t which the sion and the pore space of a tration intercept is independent of the wetting power pigment are dependent on the mobility would become zero of the liquid used, and that the mobility of a pigmentwhen the mobility is plotted w e t t i n g power or adhesion vehicle system i s a linear function of the viscosity of a g a i n s t the percentage by tension of the vehicle, deterthe vehicle and the concentration of pigment. Thus weight of the pigment in a minations were made using mobility for any concentration of pigment may be p i g m e n t - v e h i c 1e mixture. vehicles of diverse wetting calculated if the viscosity of the vehicle and the moThe word "mobility" is used powers to see if the mobilitybility-concentration intercept are known. Oil abto denote the ability to flow concentration intercept would sorption varies with the wetting power or adhesion tenafter the yield point (mobilbe in any way affected. It is sion of the liquid. ometer intercept) is reached known that the oil absorption and zero mobility then means varies indirectly with the weta state of immobility. This is the reciprocal of 1,he may in ting power of the oil and that mobility is above zero a t the which the paint man uses the word in connection with the ratio of oil to pigment at the oil absorption point, so that mobilometer. The mobilometer intercept is the point a t which variation of the intercept would be expected. Linseed oil, the prolongation of the straight part of the pressure versus the China mood oil, lubricating oil, and castor oil were ground reciprocal of time curve intercepts the pressure axis. in a laboratory edge-runner mill with silica and drop black, The mobility-concentration curves of clay suspended in and the mobility was determined with a Gardner-Parks mobilaqueous media (3) and of lithopone and linseed oil (6) have ometer. The mobility was calculated from the mobilometer been studied by Bingham, who concludes that the mobility readings by the formula: reaches zero a t a finite concentration of pigment which seems 100,000 M = intimately related to the pore space in the disperse phase, and ( w tlW - t M ) that according to this view the mobility-concentration curve tl - t 2 is completely defined by the fluidity of the medium and the where M = mobility pore space of the disperse phase.
HE t e r m "mobility-
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Presented before the Division of Paint and Varnish Chemistry at the 77th Meeting of the American Chemical Society, Columbus, Ohio, April 29 to May 3, 1929.
time for plunger t o fall, in seconds pressure on plunger, in grams Subscripts 1 and 2 represent runs in which the time is appreciably longer in 1 than in 2. t = w =
