Calcium

Calcium RELATION OF PARTICLE SIZE TO PHYSICAL PROPEHTIES OF PAINTS HERBERT W. SIESHOLTZ' AND LEONARD H. COHAN IFYtro Technical Service Laboratory, ilTeU:Y o r k 17, ,Y.Y . cium carbonates have been placed one above the other in Figure I . These tracings were obtained by means of the direct recording Geiger counter x-ray spectrometer shown in Figure 2. Comparison of the curves reveals t,hat all the materials are similar and that all are essentially pure calcite, although slight amounts of what is presumably dolomite are indicated bp the line a t 28 = 31.2". The oil absorption values plotted in Figure 3 increase regularly with surface area, showing t'hat' surface area rather than tlifferences in the nature of the surface, which might be indicated by the pH variations in Table I, is the dominant factor.

Five calcium carbonates were formulated and the eflcci

of particle size on the optical properties of the dried paint films and on the rheological properties of the fluid paint was investigated. A regular increase in hiding power, lightness, and gloss of a paint film with decrease in particle size of the calcium Carbonate is shown. Improvement in the above properties bj the ultrafine particle precipitated calcium carbonates appears to be a result of improved dispersion of the hiding pigment brought about by the calcium carbonate particles, whirh m a y function as grinding or antiflocculatinp agents. This improved dispersion increases hiding power of the paint despite the fact that the ultrafine parEicle calcium carbonates have less opacity in oil than the coarser carbonates. Better dispersion of the hiding pigment is probably also partly responsible for improved gloss and lightness.

FORMULATIONS

Composition and means of dispersion of the formulations testctd are listed in Table 11. Sincc the pigments vary in pH, a linseed oil vehicle of l o x acid value was included in order to br sure of having a t least one svstem in which possible interaction betn eeri the pigment and the acid constituents in the oil could not obhcure the effect of particle size on paint properties. The agreement hr-

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110 ' classes of pigments used in paints are the high refractive index, hiding pigments and the low refractive index, extender pigments. The latter are used t o increa3e the pigment volume and modify various properties of a paint. An important class of extender pigments is the calcium carbonate class. These calcium carbonates are commercially available in particle sizes ranging from relatively coarse, ground, natural products t o ultrafine, precipitated materials. The particle size of an extender pigment can have a n important influence on the characteristics of a paint. I t is generally known, for example, that, compared to a coarse material, a fine particle size calcium carbonate increases the consistency ( 1 ) and dry hiding (IO) of a paint. The gloss of a paint is higher mith a fine ground than with a coarse ground calcite extender (8). While these specific comparisons have been made, no data have been presented showing the variation of paint properties over a wide particle size range. For this purpose, five calcium carbonates, the properties of which appear in Table I, xere formulated in the WET paint compositions given in Table 11, and the effect of particle size on the optical properties of the dried paint filnis and on the rheological properties of the fluid paint was investigated.

X-RAY SPECT R 0METER C H A R T CALCIUM CARBONATES

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PIGMENT PROPERTIES

As indicated in Table I, the calcium carbonates cover a particle size Iange fiom 0.05 to 3.9 microns, corresponding to specific surface areas from 32 to 0.55 square meter per gram. The x-ray (CuK a: radiation) spectrometer tracings for all of the cal1 Present address, Paint and Aberdeen Pioving Ground, l f d .

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INDUSTRIAL AND ENGINEERING CHEMISTRY

February 1949

Figure

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Direct

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Recording Geiger Spectrometer

Counter X-Ray

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i " OIL A B S O R P T I O N OF

CALCIUM CARBONATE S-

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Figure 3

twecn trends obtained with the low acid value linseed oil formulation and the other formulations leads to the conclusion that the surface differences indicated by the pH of the calcium carbonates in Table I are slight enough so as not to interfere seriously with the influence of TABLE r. PHYSICAL PROPERTIES OF PIGMENTS particle size. The variations observed in the Average Surface properties of these formulations when the calcium Particle Area, carbonates in Table I are used as extenders may Diameter, Sq. M. LightSludge, Oil Specific Pigment Type Micronsa per G. nessb pHC Abs0rp.d Gravity therefore be attributed primarily t o particle size, Witcarb R, Pptd. the composition and surface nature of the carbonCaC03 ultrafine 97.9 11.3 38 2.65 Pptd. Fine particle ates being sufficiently similar not to alter the 95.9 10.6 34 2.68 CaCOa CaC03 trends. Pptd. Witcarb Regular, CaCO8 medium fine 99.1 9.1 a.68 30 Natural Water-ground CaCOa calcite 91.0 8.6 15 2.73 Katural Dry-ground CaCOa calcite 90.5 8.8 11 2.73 Titanium dioxide Anatase 0.33h 5,9h Q8 1 6.9 21 3.90 a Linear mean diameter, 500 particles counted. b Dry, compacted powder measured with Hunter multipurpose reflectometer, green filter, MgO = 100. 0 Sludge from suspension of 5 grams CaCO3 in 50 ml. water, boiled 15 minutes and cooled t o room temperature. d A.S.T.M. Method D 281-31. e Electron microscope. i X-ray patterns. Recent results have given lower values, about 0.03 micron for Witcarb R and about 0.04 mioron for the fine particle CaCOs. Q Calculated from particle size distribution curves obtained by electron microscope measurements. A Light microscope. Considerably smaller values would probably be obtained with the electron microscope, 6 Size distribution gives coincidental agreement between this value and the surfaoe mean diameter.

TABLE 11. FORMULATIONB Formulation

A

B

C Enamel nonacid vehicle, extended 54 68 32 46 Linseed

D

Enamel, ndnacid vehicle, unextended 57 100

Flat Enamel Type Pigment, % by weight 63 30 Titanium dioxide 25 85 Calcium carbonate 75 15 Vehicle, yo by weight 37 70 43 Oil Bodied linseed Linseed Linseed .... Resin Estergum Alkyd Oil length 37 gal. 22 gal. .... .... Volatile, % ' 64.2 47 0.0 0.0 Acid No. 7 3 0-0.25 0-0.25 Total pigment volume on film solids, 60 22 24 24 Calcium carbonate volume on film solids, % 49 4.7 9.7 0.0 Grind Pebble milla 3-roll millb 3-roll millb %roll mill b a Porcelain ball grinding charge in 2-quart size porcelain jar. All paints were ground at the same consistency for 24 hours. Additional vehicle requiremerats were made u p after grinding. b Laboratory mill a t constant roll setting of approximately 0.3 mil. Pastes were prepared a t the same grinding consistency and were given two passes. Remaining vehicle requirements were mixed in after the grind. .

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HIDING POWER

Films were applied on No. 03-B Morest charts with doctor blades of nominal 3.5,3.0-, and 2.5-mil clearance. Thicknesses of dry films were determined to 0.01 mil with a dial niicrometer gage. From these values, wet film thicknesses were calculated and plotted against contrast ratios determined with a Hunter inultipurpose reflectometer. Contrast ratios a t a wet film thickness of 3.0 mils were obtained from the plot and were corrected for reflectance values using the Judd graph of the Kubelka-Munk equation assuming constant scattering power for any given paint a t a constant film thickness ( 8 ) . Hiding power, as expressed by contrast ratio at a constant film thickness, increases with increase in surface area of the calcium carbonate extender (Figure 4). The increase is greatest in the flat paint formulation A. The difference in degree of improvement between the enamels and the flat formulation may be due in part to greater dry film hiding effects a t the higher pigment volume (11). The hiding porn-er charts prepared with formulation A (Figure 5 ) show clearly the improved hiding power of the paints prepared with the fine particle size extenders. Two factors on which the effect of the calcium carbonate on the hiding power of the paint film depends are 'the opacity of the calcium carbonate itself, and the influence of the calcium carbonate on the other pigments in the paint. In order to evaluate the first factor, each of the calcium carbonates was ground by itself in &the low acid value linseed oil.

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Figure 4

The resulting -~difpersion c3ontaincd 9 7% carbonate by Vi)!unie, the same concentration as in lormulation C. I n E'igui.~~ 6, hiding pov-er of the calcium carbonate-oil mixtures incrcass n-itli decreasing particle size do\m to the medium fine calciuin carbonate (\Tit,carb Regular, average diameter approximately 0.14j micron) and then decreases with further reduction in particle size. This is in accord wit,h the existence of an optimum opacity n-hich, for n h i w hiding pigments, occurs in the neighborhood of 0.2 micron, Since hiding power of the calcium carbonatc itself actually falls off as particle size decreases from 0.145 micron, the increase in hiding power of paints containing pstenders fiiicr than t,his value can certainly not be csplaiiied on the ba.sis of the first factor above. Turning to the consideration of the second factor, it is possible that calcium carbonate particles of diameter around 0.05 micron aid in the dispersion of the titanium dioxide or other hiding pigment. They may function as a grinding aid t o break u p ' t h e aggregates of titanium dioxide or act as a barrier to reflocculation of the titanium dioxide once the latter is dispersed. \Ye should expect the hiding power to be increavd lyhen flocculation of the

Figure 5 .

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hiding piginent is decreased ( i ). \t-ith either mechanism the effectiveness of the calcium carbonate \\-ill increase as the number of available particles increases and as the specific surface a w i increases. In other words, the ability of the calcium carbonate l o improve hiding power should increase as its part,icle size dcc~c~;lscs. This explanation finds further confirmation in the improwmciit, of tinting strength of titanium dioxide and Inany c o l o i ~ pigmclrits ~J when t,hep arc ground Lyith ultrafine calcium carbonate e x t e i i c l ( ~ i ~ ~ . Tinting strength of titanium dioxide is increased xhen it is ground in oil with a n ultrafine particle calcium carbonate such a? \Til ciiib R, but only a slight increase occurs \Then the titanium dioxide and Kitcarb R are ground sepa.ratcly and then mixed with a spatula (Table 111). The apparent increase in tinting st,rcngt,h of the epatula mixture of titanium dioxide and Witcarb R is duo t o the dilution effect of the increased oil content, as is shoxvn by comparison Jvith unextcnded titanium dioxide at, the same oil cont>cnt. Similar results have bccii obtained with colored pigments ( 1 % ) . The increase in tinting strength obtained by grinding titanium dioxide with Witcarb I2 appears, in both cases, to be best esplained by assuming improved dispersion of t,he tinting pignicnt. When the pigments listed in Ta,ble I11 were ground on a laboratory automatic niuller instead of EL %roll inill, anomalous result's occurred. With this intense grind, the spatula mixture of titanium dioxide ground on the niuller and Witcarb R ground 011 the niuller had even greater tinting strength than the Witcarh it-titanium dioxide ground together. With both procedures, the tinting strength of titanium dioxide containing 10% Witcarb I t was about 15% greater than for t,itanium dioxide paste containiiig 110 Witcarb It. While the discrepancies between roller mill grinding and muller grinding are not fully understood, i t may be that the grinding together of Witcarb R and titanium dioxide oil :i muller resulted in an overgrind. GLOSS .AND SHEEN

Gloss measurements 1%-eremade on films applied on Xorest chart's wit,h a 3-mil doctor blade and aged 4 days. Values wcrc determined over the white portion of the chart, with the Hunter multipurpose reflectoineter and expressed on the basis of 1000 for a perfect mirror. Corrections for diffuse reflectance were n o t made.

These properties are oitcil associated nTith the roaghness or inhomogeneity of the film. Large narticles in the aaint film which project above the surface should decrease gloss-for exaniple, large particle size natural ground limestone gives paint films of lower gloss than more finely ground limestone (Si. I n Figure 7 , the gloss of enamel formulation I3 increases as the average particle diameter of t h e calcium carbonat,e extender pigment decreasw -that is, as the specific area goes from 0.55 to 32 square meters per gram. L.ilrewise, in Figure 8, both the gloss and the sheen ( r tion ai an angle of 76 ") of the flat,paint formulation A increase as the surface arca increascs. These results may be explained by the Cm1 that, as the average particle diameter oi t h e calcium carbonate extender decreases, the ovwall average diameter of the particles in the pailit film decreases, n-hich leads to n smoother, nloro regular film. Another factor is the effect of the calciuin carbonate on t,he dispersion of the hiding pipment itself. As discussed under hiding p o ~ ~ r , it would seem liltely t,hat,, as a result of thc iniproved dispersion effected by the finer partirlo size calcium carbonates, t h e gloss of the pairit v-ould also be improved. Figure 7 shons l,llat the gloss of the paints containing the tL7-o firwst calcium carbonate extenders was actualiy superior to that of the unextended paink. Hiding Power Charts Prepared with Formulation A I

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H I D I N G POWER C A L C I U M CARBONATES IN OIL

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it Figure 7.

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paint. (Daylight, 45 O , 0 ' apparent reflectance was determined using the Hunter multipurpose reflectometer. Films were applied to No. 03-B Morest charts with a doctor blade to give 98 to 99% contrast, and measurements were made over the white portion of the chart.) In a white paint, an off-color pigment or a coarse pigment having low light reflecting and scattering power will decrease lightness. The abrasive effect of a coarse calcium carbonate upon the steel rolls or balls of the grinding equipment will also produce discoloration (mill stain) (9). LIGHTNESS OF PAINTS As the average particle size of a calcium carbonate decreasesthat is, as the specific surface area increases-lightness of the Brightness or lightness, as determined by A.X;T.M. Procedure D 771-441', depends primarily on the pigments present in the paint increases; and, for the finest size materials, lightness is higher than for the paint containing no extenders (Figure 9). The improved lightness of the paint containing Witcarb R compared to the unextended paint may be a result of the better dispersion of TABLE 111. TINTING STRENGTH OF TITANIUM DIOXIDEQ the titanium dioxide, as discussed previously. Oil in White The effect may also be a result of the possibility Witcarb Paste Total Tinting White TiOz, Regular, LampGrind, Linseed Strength, that Witcarb R is even less abrasive than the titaPsate Pigment G. G. black, G. G. Oil, G. 95 nium dioxide and t,hat, therefore, less mill stain A TiO2, unextended 3.00 0.0 0.060 1.32 1.50 100 is produced in the paint in which part of the titaB TiOr, ground with Witcarb R 3.00 0.300 0.060 1.55 1.73 115 nium dioxide is replaced by Witcarb R. C TiOz, ground in oil plus Witcarb R Reflectivity of the dry calcium carbonate also ground in oil mixed affects the lightness of the paint. However, parwith spatula 3.00 0.300 0.060 1.32 1.73 105 D TiO2,unextended: 3.00 0.0 0.060 1.32 1.73 105 ticularly for the smaller particle size calcium carE TiOz, unextended 3.00 0.0 0.060 1.55 1.73 104 bonates in Figure 9, the effect of reflectivity ap5 Pigments ground on 3-roll mill. Pastes A B C, and D have same grinding consistency. pears to be subordinate to that of particle diamWhit? pastes mixed on glass plate with lamphiack paste (0.18 g. oil per 0.06 g. black) using spatula. eter. For example, Witcarb Regular, which has b Paste D obtained by adding linseed oil to paste A after grinding. the highest reflectivity (Table I), does not produce Tior ground with increased oil content. the lightest paint.

Some caution should be observed in applying the above results in the case of enamels which contain a critical amount of oil and in which the gloss is sensitive to slight changes in the oil content. Gloss will also depend upon the amount of oil absorption by the pigments present, and the introduction of a fine particle size oalcium carbonate with a high oil absorption might not necessarily result in improved gloss.

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Figure 10

Consistency (Figure 10) increases regularly with increase in surface area of the calcium carbonate extender. (Consistency values were determined a t 25' * 0.2" C. with a Stormer viscometer using a cvlindrical rotor on paints aged one week in closed, filled containers.) The increase is most pronounced for the flat paints which have the highest pigment volume loadings. The force-flow curves in Figure 11 show that, for the low acid linseed oil paint formulation C, mobility decreases and yield value increases with increasing surface area of the calcium carbonate extender. The hysteresis loops indicate a thixotropic structure for the formulas containing the fine and ultrafine particle calcium carbonates. Although hysteresis is not wident in the paints containing the coarser calcium carbonates, it may exist to a degree not detectable by the standard Stormer viscometer used in determining the data. The effect of calcium carbonates on the consistency of a paint is analogous to ,their behavior in rubber, where plasticity and modulus normally increase with surface area (Z), volume loading, and asymmetry of the pigment particles (3-6). Although the theoretical equations apply only to low pigment loadings, consistency of paints and plasticity and modulus of rubber increase with pigment volume a t most commercial loadings. That these properties should also depend on surface area might be explained by the adsorption and immobilization of vehicle a t the pigment surface, resulting in an apparent increase in pigment volume. This increase in pigment volume v-ould also explain the increase in yield value v-ith surface area. An alternative qualitative explaiiation of the increase in paint consistency with decreasing average diameter of the pigment extender can be obtained if one assumes suspensions of the solids to behave t o some extent like a framework of fixed solid particles interspersed through the liquid. In this case, the radius of the capillary spaces through which the suspension medium flows !\ ill depend on the number of particles present. Assuming uniform packing, the mean distance between particles, d, is proportional t o the average diameter, D. Whether one considers the flow of oil through the particle framework as flow through capillaries, orifices, or other configurations, the resistance to flow should increase Kith decreasing particle diameter, as indicated by the experimental results (Figure 10). Assuming that the flow through the partiole framework is analogous to flow through a system of capillaries of radius d / 2 and length D, the resistance to flow through one of the capillary fpaces is :

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RHEOLOGICAL PROPERTIES

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VISCOSIMETER

Figure 11

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In a layer of thickness d D the number ( p ) of paths or capillaries through which the liquid can flow increases as the 2/3 power of the total number of particles, N:

p

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Rt, the resistance to flow through all the layers, is number of layers, L, vhere

times the

so that

The above equation should apply only if the pigment particles arc rigidly fixed or if the degree to which the particles lag behind the liquid does not depend on their size-that is, if the velocity of the liquid relative to the particles is the same for particles of all sizes. At concent,rations such that the particles are independent of each other, the velocity, V,,of the liquid relative to the particles might be expected to decrease as the mass, m, of tho individual particles decreases and to increase as the projected area, A , perpendicular to the direction of flow decreases. In other viords,

Vf

N

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Equation 5 was derived for the case where the velocity of the liquid relative t o the particles was equal to the actual velocity of the liquid, V L . Since the resistance is proportional to the relative velocity if this velocity is I',

(7) Therefore, the consistency should increase with specific surface aiea. The data in Figure 10 do show an increase which is approximately linear, as predicted by Equation 7, for the alkyd enamel, which has the lowest pigment volume ratio. The more rapid increase in consistency for the other two paints, &hich have higher pigment volume ratios, may be related to the formation of more nearly fixed interlocked structures, to which Equation 5 rather than Equation 7 should apply.

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INDUSTRIAL AND ENGINEERING CHEMISTRY

The authors hope that the above discussion may stimulate someone better qualified mathematically to make a more rigorous analysis of the effect of degree of subdivision on consistency of concentrated suspensions. SUMMARY

The particle size of a calcium carbonate extender influences the optical properties of a dry paint film and the rheological properties of the fluid paint. Hiding power, lightness, and gloss of a paint film increase regularly with decrease in particle size of the calcium carbonate. Improvement in the above properties by the ultrafine particle precipitated calcium carbonates appears to be due to improved dispersion of the hiding pigment brought about by the calcium carbonate particles, which may function as grinding or antiflocculating agents. This improved dispersion increases hiding power of the paint despite the fact that the ultrafine particle calcium carbonates have less opacity in oil than the coarser carbonates. Better dispersion of the hiding pigment is probably also partly responsible for improved gloss and lightness. Decreasing particle size of the calcium carbonate extender is accompanied by a regular increase in consistency and yield values of the paint. These results may be caused by an increase in volume of relatively rigid dispersed phase due to immobilization of vehicle by adsorption on the surface of the calcium carbonate. The increase in the total number of particles in suspension resulting from reduction in average particle diameter may also explain qualitatively the inrrease in consistency.

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ACKNOWLEDGMEYT

The authors wish to express appreciation ’to J. L. Abbott and W. T. Scovil, Jr., of North American Philips Company, for the x-ray spectrometer charts of the various calcium carbonates. They also wish to thank T. J. Starkie, Mrs. T. Newman, J. E. Cunningham, and A. B. Craig, Jr., for suggestions and assistance in preparing this paper. LITERATURE CITED (1) Battline, F., Paint, Oil, Chem. Rev., 107, No. 2, 9-12 (1944). (2) Cohan, L. H., and Spielman, R., IND.ENC. CHEW 40, 2204 (1948). (3) Einstein, A., Ann. Phvsik, 19, 289 (1906). Ibid., 34, 591 (1911). Guth, E., J. Applied Phvs., 16, 20-25 (1945). Guth, E., and Gold, O., Phys. Rev., 53, 322 (1938).

Hoagland, Stewart, “The Rheology of Surface Coatings,” 1st ed., pp. 54-7, Bound Brook, N. J., R-B-H Dispersions, Inc., 1946. Judd, D. B., Paper Trade J., 101,No. 5, T S 4 0 (1935). Lukens, A. R., Cummings, C. L., and Babel, V. J., Oficial Dioest Federation Paint & Varnish Product& Clubs, 228,

306-26 (1943). (10) Matiello, J. J., “Protective and Decorative Coatings,” 1st ed., 2d printing, Vol. 11, p. 443, New York, John Wiley & Sons, 1944. (11) Sawyer, R. H., IND. ENC.CHEM., ANAL.ED., 6, 113 (1934). (12) Witco Chemioal Co., New York, “Witoarb R as an Extender of Tinting Colors,” Technical Service Report P-I, 1946. RECEIVED September IS, 1947. Presented before the Division of Colloid CHEMICAL SOCIPTY, Chemistry at the 111th Meeting of the AYERICAN Atlantic City, N. J.

Mercaptan Formation in Acid Treatment of Pressure Distillate -

C. R. WILBE

WILLIAM JAMES WRIDEl

University of California, Berkeley, Calif.

State College of Washington, Pullman, Wash.

E x p e r i m e n t s are described showing the formation of appreciable q u a n t i t i e s of mercaptans in the acid t r e a t m e n t of pressure distillate containing hydrogen sulfide. The data indicate the addition of hydrogen sulfide to olefins w i t h the acid acting a s a catalyst. Mercaptan formation as h i g h as 969 mg. of sulfur per liter occurred in pressure distillate initially containing 1430 mg. of hydrogen sulfide per liter d u r i n g t r e a t m e n t w i t h 5 pounds of 97.470 sulfuric acid per barrel. Formation of disulfides was also observed. Similar results were obtained w i t h caprylene-Skellysolve mixtures. The effects of temperature, acid strength, a n d acid q u a n t i t y were studied over a range of conditions used in industrial practice.

lates, for improvement of color and stability is widely used (8,111. The oil is usually brought into contact with concentrated sulfuric acid in an agitator or by flow of the mixture through a high speed centrifugal pump or other mixing device. The acid forms an immiscible sludge which is removed by settling. The oil is subjected to washing and neutralization, and in many cases to distillation, before blending into finished gasoline. Interest in the present problem developed when it was observed that pressure distillate which had been acid treated had a higher mercaptan content than the original untreated material. Test data for a commercial continuous acid treating unit are given in Table I. The feed to the plant was a gasoline fraction produced by thermal cracking containing rather large quantities of hydrogen sulfide. Because cracked distillates contain relatively large amounts of olefins and other unsaturated compounds, it seemed desirable to investigate the possibility and extent of the formation of mercaptans by combination of hydrogen sulfide with olefins during acid treating. A series of experiments was conducted in the laboratory in which samples of a typical pressure distillate were acid treated under varying conditions of hydrogen sulfide content, acid strength, acid quantity, and temperature. I n another series of experiments solutions of caprylene in Skellysolve were acid treated under varying conditions of hydrogen sulfide content and olefin content. This work is discussed in detail.

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H E presence of mercaptans (thiols) in gasoline stocks has long been a source of difficulty and expense to petroleum refiners. The requirement that finished gasoline give a negative doctor test has led to the adoption of various sweetening processes in which mercaptans are converted to disulfides. In recent years the demand for high octane motor fuels has emphasized the deleterious effect of mercaptans and disulfides on octane number and tetraethyllead susceptibility, and has led to increased application of mercaptan extraction processes. Acid treatment of crude gasoline, particularly cracked distil1

Present address, Iowa State College, Ames, Iowa.