Surf ace-Treated Calcium Carbonates in a High-Solid Paint System d
U
C. A. KUMINS Research Laboratories, Interchemical Corp., New York, N . Y .
T
HE incorporation of calcium carbonate in flat wall paint formulations must be carried out with care lest its hydrophilic surface interfere with the flow and leveling characteristics of the paint. Much work has been done in attempts t o overcome these difficulties by treatment of the material with f a t t y acids and phosphates so lt9 t o render the surface hydrophobic and less water sensitive (2, 6). Since the leveling of paints is a n outward manifestation of their rheological behavior, it was the purpose of this investigation t o study the effect of surface-treated calcium carbonates on the flow properties of a simplified flat wall paint formulation. Particular attention was paid t o the relationship between yield value of the paint and the degree of deflocculation of the calcium carbonate in a varnish vehicle.
The resulting pigment was filtered, washed free of salts, dried, and ball milled for 5 hours. The pigments and the treating agents are listed in the following table: Sample
D
E
FORNULATION. Since most paints are highly complex systems and consequently difficult to study, the simplified paint formulation described below was employed:
ieagent by precipitation of t h r sodium salt Kith calcium chloride.
W EIGtil-GM S . Figure 1. Plasticity Characteristics of Dispersions of Calcium Carbonate 0. Treated with 2 % sodium palmitate 0.
CaC08
152 grams 48 grams 63 grams
EXPERLMENTAL
PIGMENT TREATMENT. A master batch of calcium carbonate was divided into five 200-gram portions (samples A, B, C, D, and E ) and each slurried with 2000 ml. of water. Sample A was a blank, and the others were treated with 4 grams of the respective
Treating Agent Blank Sodium ricinoleate Sodium palmitate Sodium oleate Tetrasodium pyrophosphate
A B C
TiOz (anatase) Vehicle: Varnish (60% solids) Combination fish, tung, and linseed oils Mineral spirits
45 grams
so
CaC08 = 152 X 100 = 76% Because 76% of the total pigment solid was composed of calcium carbonate, the same weight per cent of vehicle was used in making the extender paste and the remainder, 24%, was milled with the titanium oxide. -4fter the properties of each calcium carbonate paste were studird separately, each was mixed with the titanium oxide dispersion and the characteristics of the resultant
WEIGHT-GMS. Figure 2.
Same plue TiOn
Plasticity Characteristics of Dispersions of Calcium Carbonate 0. Treated with 2 % sodium ricinoleate 0.
1844
Same plus Ti02
i
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1952
Substitution of these values in Equation 1 gives 2:
F = P , X 2
(3)
MEASUREMENT OF DEFLOCCULATION. The rheological properties of pigment pastes vary with the amount of particle deflocculation which has taken place in the vehicle employed. Therefore, in order t o arrive a t some quantitative value for this phenomenon, the calcium carbonate dispersions in the oleoresinous vehicle were subjected to a particle size analysis, using the centrifugal sedimentation technique of Martin (8). The results, expressed in terms of surface area, will be related t o the degree of dispersion, since the absolute surface of the base extender is constant, and any variation in this value among the treated carbonates as determined by sedimentation analysis, will be due t o flocculation. The actual mechanics of the method are carried out :LR dcscribed below :
' 6 5
8rc
1845
W v) I 40
ii paste containing 25% calcium carbonate in 75% varnish was ground on a three-roll paint mill. Then 124 grams of this mixture, which contained 31 grams of calcium carbonate, were slurried with 380 grams of mineral spirits and 1046 grams of varnish. This yielded 1550 grams of a 2y0 slurry of calcium carbonate in 7570 varnish and 2501, mineral spirits. Centrifugation was carried out for varying lengths of time. T h e supernatant liquid was then ashed and t h e residue titrated with standard
% 30
WEIGHT- GMS. Figure 3.
Plasticity Characteristics of Dispersions of Calcium Carbonate
0. Treated with 2%sodium oleate 0.
S a m e plus TiOn
paint, whose formula is given above, were then compared and measurements made.
DETERMINATION O F YIELD
VALUES AND PLASTIC VISCOSITIES.
The Stormer viscometer, equipped with rotating cylinder, waa used to determine the flow properties of the paints. This was accomplished by determining the time in seconds for 100 revolutions of the cylinder under given driving stresses. I n order t o eliminate thixotropic effects, the weight was allowed t o drop the equivalent of 100 revolutions before the second 100 rotations were timed. The reciprocals of the time were then plotted against the weight to give curves shown in Figures 1 t o 6. In the range of higher shearing stress, the curves are virtually straight lines. Extension of these t o the 2: axis determines the apparent yield values of the paint. The actual yield values in dynes per square centimeter (IO)are calculated by means of Equation l.
f = P,g E,,
c/2'
(1)
WEIGHT- GMS. Figure 4.
Plasticity Characteristics of Dispersions of Calcium Carbonate 0. Untreated 0.
Plastic viscosities were obtained from Equation 2:
(P
U =
- Po)Rap X
T (2)
where 18 =_
R2h
SURFACE AGENTSON DISPERSION AND TABLE I. EFFECTOF VARIOUSHYDROPHOBIC RHEOLOGICAL CHARACTERISTICS OF CALCIUM CARBONATE IN PAINTS
S X 9.55
l / t X 60 X
R2,1
4Irh
S a m e plus Ti08
Treatment Phosphated Oleated Palmitated Ricinoleated TiOn Untreated
CaCOs Paste Yield value, Visoosity, dynes/sq. am. poises 130 1.29 224 1.42 446 1.07 1040 3.16 350 1.08 356 1.52
CaCOe Paint Yield value, Viscosity, dynes/sq. cm. poises 170 1.41 206 1.61 350 1.13 790 1.97
ieb
....
1.64
Calculated % ' Deviation Yield Value, from Calculated Area, DynedSq. Cm. Values Sq. M. 169 0.5 369 247 15.2 293 429 18.4 399 916 13.8 189
...
357
....
3.9
...
166
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
1846
IP
I
2oi-
c
h’ !7
‘41
/”
36
I 11,
Vol. 44, No. 8
I
I
I
0
100
Figiire 5.
200
1
I
I
300
400
500
WEIGHT- GMS. Plasticity Characteristics of TiOl Paste
hydrorhloric acid After blanh coiiectioricl were made, the total eurfare area was calculated arcording to the formulas below The general sedimentation equation as given by Martin is. =
A-T\(
(4)
iTdF-d2
Figure 6.
(51
6 Z A / D sq. inetcis Mhen A = Eiactional weight of particles in range D1 t o I>?--i e , 1) = average diameter.
The r c d t i n g data are listed in T,tblrs I and I1 and Figures 7 to 12 DISCUSSION OF R E S ~ - L T S
Before a correlation of the rhological and dispersion data wa8 attempted, a test panel of three experienced painters was asked to judge the five paints containing the various calcium carbonates, for their leveling and flow properties. Their results are indicated in the following table in whirh the paints are arranged in orr1c.r of derreasing desirahilitv
leaves inuch to be desired a; far as Iavclirig is concerned. On the other hand, the tetrasodium pyrophosphated extender is also excellently dispersed and exhibits a low yield value and extremely good leveling and flow properties. However, a detailed study of thc (lata produces t,he following results: from the paint forinulation used, the relative volumes of calcium carbonate and titaniiiiii oxidc are calculated to be 56.6 and 12.3 ml., respectively. 152 .__
wt. _ _ _of_ oalciiiin _ ~ ~ ~ _cai,hunnte _ ~- = 56.6 sp. gr.
2.685
48 -
3.9
1111.
wt, of ti&iiiurn oxide = 12.3 mi. ,511. gr.
Thus, 82.2% of the total pigment volume is occupied bj. the calcium carbonate and the remaining 17.8% by the titanium oxide.
Mean Particle S i z e Dist,ribution, % Diameter, 0-1.011 1 , O - 1 . 5 , 1 . 6 - 2 . 0 ~ 2 . 0 - 2 . 5 ~2 . 5 - 8 . 0 ~3 0 - 4 0p t 4 . O p Microns ~
‘Treatineiit
pyrophvnpbatr
Same plus Ti02
0.
=
At first glance a consideration of this table shows n o ohvious relationship which would account for the flow properties of the paints. The best disp ersion--namely, the p a1 m i t a t e- t r e a t ed calcium carbonate-gives one of the high& plastic yield values and
Plasticity Characteristics of Dispersions of‘ Calcium Carbonate
0. Treated with 2 % tetrnmdium
D = 1454;
Leveling 1. Pyrophosphated CaCQ 2. Oleated CaCOZ 3. Palmitated CaC03 4. Untreated CaC03 5 . lticinoleated CaCO3
200
WEIGHT- GMS.
, 217 In R I S
Sul~stiiutionof values in Equation 4 gixes.
,ireti per ml.
too
0
Untreated 2% sodium ricinoleate 2 % tetrasodium pyrophosphate 2% sodium oleate 2% sodium palmitate Untreated dispersed in water with 1% ammonical casein
.. . I
~~
Surface Area, 6q. Meters per M I .
7.0
19.0
48 0
14 3
6 .3
5.0
2.04
2.939
11 5
32 5
43 6
2 5
5 .0
5 0
1 .so
3,336
..
28 5
49 0
17.5
5 0
2 2 .o
17.5
16.0
41 0
39.0
2i.5
24.0
5 5
4.0
24.5
20.3
20.9
13 1
10.8
3.5
.. 10.4
1.32
0,924
6.492
1,160
5.180
0.852
7,044
1.32
August
INDUSTRIAL AND ENGINEERING CHEMISTRY
1952
1847
SEDIMENTATION TIME- MIN. Figure 7.
Centrifugal Sedimentation Curve Treated CaCOa dis ersed in varnish and mineral spirits X 2 % Na oleate CaCOa 0. 2% T.s.P.P. 8 a c o s A. 2 % Na palmitate CaCOa 0.
.
+.
2 % Na ricinoleate CaCOa
I t was believed that if the same relative volumes of titanium oxide and calcium carbonate pastes are mixed (1, S), the flow properties and yield values should be additive-provided no other reactions, mechanical or otherwise, take place. I n other words, if the apparent plastic yield value of a calcium carbonate paste is multiplied by the volume per cent of the calcium carbonate present in the paint formulation, and the product is then added t o an analogous figure for the titanium oxide, a number which is equal to the observed apparent yield value of the paint should be obtained. Expressed mat hemat i call y :
f
= fCsCO3
x
VCRCOJ
f .fTiOZ x
(6)
VT102
Table I illustrates the results obtained. I t can be seen that Equation G holds for the untreated and the phosphate-treated calcium carbonate only. The organic surface-
0
I
2
3 MICRONS
Untreated CaC0a
treated materials all yield a higher calculated value than actually observed, and of the three, the one having the greatest apparent surface exhibits the largest per cent deviation (pa1mitated)namely, 18.4%-and the carbonate which is deflocculated least (the rincinoleated one) has the lowest deviation of 13.8%. The deviation from Equation 6 may be explained by a consideration of the surface of the fatty acid-treated carbonates in which the organophilic molecules are so oriented that the hydrophilic group (COOCa/2) is closest t o the crystal interface of the calcium carbonate, in a situation analogous to that described by Langmuir ( 4 ) in his classical work with fatty acid films on water. The f a t t y acid portion of the molecule exerts an influence upon t h e similar molecules of the vehicle tending t o restrict their free movement. When it is recalled that we are dealing with a flat wall paint system in which the amount of vehicle is at a minimum
4
MICRONS
Figure 8. Particle Size Distribution of Calcium Carbonate i n Varnish
Figure 9. Particie Size Distribution of Calcium Carbonate i n Varnish
Treated with 2% sodium palmitate
Treated with 2 $'6 tetrasodium pyrophosphate
1848
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
MICRONS Figure 10. Particle Size Distribution of Calcium Carbonate in Varnish Treated with 2 Isodium ricinoleate (33%), it is easy t o see that any icitiictionP of molecular movement will tend t o set up a rhrologicall- noticeable structure. as evidenced by a n increase in the plastic yield value. When the titanium oxide portion of the paint is added, the situation is changed. A suspension is being added in which t h r vehicle molecules are not restricted b~ an oriented surface Con-
-
Vol. 44, No. 8
weakening a more rigid structure than one in which the net,work il; not so well developed. The character of the Ikal sediment in the paint providuti further evidence for this theory. Thus, a very well-dispersed pigment will settle in time t o a hard-packed and dense cake. This is due to the fact that each particle settles as a unit. When the pigment reaches the bottom it tends to occupy the nearest vacant space. After all the particles have settled, thew arc ft:w or no vacant spaces b e h e e n them, and B closely packed nntl dense sediment is obtained. In the case of a flocculated syst,em rach flocculate encloses sonic. vehicle in the interstitial spaces, and this produces a. typtl of settling which is characteristically loosely packed and voluminous. Examination of the settled paints with these facts in mind revealed that the one containing the untreated calcium carbonatr settled in a manner characteristic of a, flocculated system. The dispersion data in Table I1 agree with the observed result. The paint made with the pyrophosphated calcium carbonate settles, as would be expected from t,he disperfiion data, into a hard, closely packed mass. The best dispersed extendcr was the palmitated calcium c w bonate, and it is noteworthy t,hat t,his exhibited soft settling a n d was easily stirred into suspension. This may be accounted for by what has been said previously. Though each particle settles as a unit, (since it is well disperscd) its surface contains both the precipitatcd and oriented palmitate radicals as &-ellas some of the similar vehicle moleculcs for whic*h the protruding palmitate radicals have great affinity. Thus, the, final sediment is loosely packed and easily worked back into t,hc paint. Upon this supposit.ion, it is possible t,o declare t h a t very littlv or no adsorption of the vehicle takes place on the untreated and the pyrophosphated extender. In the latter case, the yield value is low, its degree of deflocculation is excellent, and the calculated yield value as obtained from Equation 6 agrees with the observed figure within experimental error. I n the former case, while
3
MICRONS Figure 11. Particle Size Distribution of Calcium Carbonate i n Varnish Treated with 2 TOsodium oleate 0
sequently, the mixture of the two will affect the apparent yield value since there are now molecules free t o move between the structures and thus t o aeaken the entire network (Figure 13). That this may be so is indicated by the actual plastic yield values which are lower than the calculated figures. Another factor pointing in this direction may be observed from the data in which thc degree of deviation from Equation (6) varies directly with the surface area of the organic-treated extender. This result may be anticipated, since the more surface exposed, the greater will he the number of vehicle molecules entrapped, with consequent increase in yield value. Any increase in the number of free molecules TVNtend t o have a proportionately greater effect upon
I
2
3
4
5
MICRONS Figure 12. Particle Size Distribution o f Calcium Carbonate in Varnish Untrcnted
the degree of deflocculation is poor (total area 166 bq. meters) t h r yield value, 356, is far lowrr than that of the palmitated extender which has a large surface area value of 399 sq. meters and a yield value of 446 drnes per 49. cm. From the above data it would appear t h a t the forces of flocculation in the untreated carbonate are not nearly so great as those
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1952
f
ORIENTED VEHICLE MOLECULES %
1849
of another. This is indicated by the fact that the saponification number is about 200, whereas its acid number may range from 130 to 140. This would give rise to an exceptionally long molecule which would more readily form a strong network and give rise to a higher than usual yield value. The exceptionally high viscosity for the ricinoleated paint as indicated in Table I may be attributed to the increased frictional resistance caused by molecules of longer than usual chain length. CONCLUSIONS
Surface-treated calcium carbonates were analyzed in a highsolid paint system for their effect on the rheological and dispersion properties of the pastes. The variation in yield values among the different carbonates in a system where the amount of vehicle is kept a t a very low value, may be attributed t o the orientation of the fatty acid molecules on the crystal surfaces. Where no orientation exists, as in the phosphated material, the yield value is low. Where the yield value is high, 346 dynes per sq. cm., as in the untreated extender, it is still lower than that obtained with the two normal fatty acid-treated whites and it may be attributed to poor dispersion. Paste viscosities show all to be within the same order of magnitude, though the ricinoleated calcium carbonate gave the highest viscosity paste. This inordinately large value has been attributed t o the increased chain length of the fatty acid brought about by hydrogen-bonding and/or self-esterification.
A
-
NOMENCLATURE dl
Figure 13. A.
B.
Schematic Diagrams
Effect of surface treatment i n orienting vehicle molecules Effect of TiOz paste containing excess vehicle molecules upon addition to A
d,
D j
fl
7
which exist around a particle surface in contact with an oriented film. A considerable variation in surface area and yield values exists among the three fatty acid-treated carbonates. For example, both the oleated and palmitated extenders are excellently dispersed in the varnish system but the latter has a much higher yield value. On the other hand, the ricinoleate-treated material is poorly deflocculated and has an inordinately high yield value. The difference may be attributed to the molecular structure of the fatty acids. The low yield value of the oleated extender is probably a result of the fact that, instead of the surface existing as a condensed film, as in the case of the palmitate, it actually is present as an expanded film occupying a greater area and projecting about one half the length of the saturated palmitic acid. 4 s pointed out by Rideal, Langmuir ( 5 ) found that on hater, oleic acid occupied an area of 46 sq. A. and had a chain length of 11.2 A., whereas palmitic acid covered 21 sq. A. and was 24 A. in length. The shortened length would decrease the tendency to form a network structure and, in addition, the fact that the film is expanded would imply that the van der Waals forces responsible for the orientation of the vehicle molecule are satisfied by the bending of the oleic acid molecule back onto the surface of the calcium carbonate. The high yield value and poor dispersion of the ricinoleic acidtreated calcium carbonate may be partly attributed to the presence of the hydroxyl group. As suggested by Marsden and Rideal ( 7 ) , adjacent molecules may be joined by means of hydrogen bonds and thus give rise to a structure above and beyond that which is obtained b r mere orientation, This would account for an abnormally high yield value. Another explanation may reside in the fact that ricinoleic acid does not normally exist as such but as self-ester in which the cnrboxyl group of one molecule is esterified by the hydroxyl group
N P
= specific gravity of CaC03 = 2.685 grams per ml. = specific gravity of suspending medium= 0.861 gram per = = = = = = =
Po =
R
=
Rb
=
R, = R,, = t
T
= =
l/t
=
B
=
ml . diameter, microns yield value acceleration due to gravity height of bob = 3.5 em. viscosity of suspending medium = 0.09441 poise number of revolutions per minute = 500 attached weight intercept on x axis distance from axis of rotation to bottom of centrifuge cup = 27.14 cm. radius of bob = 1.58 cm. radius of cup = 1.74 cm. radius of drum t o which weight is attached = 1.43 cm. time rotation in minutes gear ratio between drum and shaft of bob = 11.4 number of revolutions/time of revolutions volume per cent ACKNOWLEDGMENT
The author wishes to express his appreciation t o the Wyandotte Corp. where some of the earlier work was carried out and to Interchemical Corp. for its kind assistance and encouragement in rechecking and expanding the data. LITERATURE CITED
(1) Bingham, E. C., “Fluidity and Plasticity,” pp. 220, 232-3, New York, McGraw-Hill Book Co., 1922. (2) Church, J. W., and McClure, R. R., U. S. Patent 2,034,797 (1938). (3) Green, H., Interchem. Reo., 3, No. 4, 109 (1944). (4)Langmuir, I., J. Am. Chem. Soc., 39, 1848 (1917). (5) Langmuir, I., PTOC. Natl. Acad. Sci. U . S., 3, 351-7 (1917). (6) McCleary, R. L., U. S. Patent 2,266,233 (1942). (7) Nlarsden, J., and Rideal, E. K., J . Chem. Soc., 1938, 1163-71. ENG.CHEM.,A 4 ED., ~ ~ 11, ~ 471 . (1939). (8) Martin, S.’W., IND. (9) Rideal, E. K., “An Introduction to Surface Chemistry,” p. 93, London, Cambridge University Press, 1930. (10) Weltmann, R. N., Interchem. Rea., 2, No. 3, 51 (1943). RECEIVED October 13, 1951. ACCEPTED April 28, 1952. Presented before the Division of Paint, Varnish, and Plastics Chemistry a n d Division of Colloid Chemistry, Pigment Characteristics Symposium, a t the 120th Meeting of the AMERICAX CHEMICAL SOCIETY, New l i o r k , N. Y
