STUDIES IN T H E PLASTICITY OF PAINTS
w. J. .JEBENS' The plasticity of paints has been extensively studied in connection with their brushing characteristics. Among the various factors that affect the plasticity, the following are some of the more important ones-the volume concentration of pigment, the size and shape of particles, the extent to which the pigment is wetted by the vehicle, the extent to which the pigment is flocculated in the paint, the viscosity of the vehicle, the presence of soap, the presence of water, and the age of the paint. As the concentration of the pigment in the paint is increased, the mobility of the material is decreased and yield value is increased. Rhodes and Wells2 found this to be true with paints made with the following pigments: zinc oxide, lithopone, Dutch process white lead, basic sulphate white lead, electrolytic white lead, and titanox. Although theoretically the yield value is zero for values of concentration lower than that required for cubical packing of the pigment particles, a small yield value is obtained experimentally. The mobility of the suspension decreases linearly with the concentration of the pigment in the ~ a i n t . ~ . ~ For particles of fairly uniform size and shape the yield value increases and the mobility decreases with a decrease in the average diameter of the particles. Einstein5 in deriving his formula for the viscosities of dilute suspensions neglected the effect of the size of the particles as modified by adsorption. Hatschek6 called attention to the error introduced by this omission. However for paints prepared from zinc oxide and linseed oil, Green and Haslam' have shown that the yield value increases as the particle size decreased. Od&* found an increase in the viscosity for sulphur sols with increased dispersion of the sulphur, the effect being more pronounced the higher the concentration of the sulphur. I t has also been noted that the form of the size distribution curve for the pigment affects the plastic characteristics of the paint.g A third factor that may affect the plast,ic characteristics of a paint is the ease with which the solid is wetted by the liquid. Both the nature of the pigment and the nature of the vehicle affect the ease of wetting. I n general, an increase in the ease of wetting causes an increase in the mobility and a decrease in the yield value. In some instances, the wettability of the pigment varies greatly with apparently minor changes in the composition of the vehicle. BY F. K. RHODES A N D
DuPont Fellow in Chemistry at Cornell University. Ind. Eng. Chem., 21, 1273 (1929). Bingham, Bruce, and Wolbach: J. Franklin Inst., 195, 303 (1923). Bingham and Jacques: Ind. Eng.Chem., 15, 1033 (1923). Ann. Physik, (4) 34, 591 (1911). e Kolloid-Z., 1, 301 (1910). Ind. Eng. Chem., 19, 53 (192j). Z. physik. Chem., 80, 709 (1912). Ingalls: .4m. Paint Var. Mfg. Assocn. Circ. 135.
F. H. RHODES A S D W. J. J E B E N S
384
For instance, a small increase in the concentration of free fatty acids may cause the oil to wet the pigment much more readily. I t is well known that linseed oil will displace water from pulp-mixed white lead only if the acid value of the oil is above a certain minimum limit.1° The extent to which the pigment in the paint is flocculated depends upon t’he nature of the pigment and upon the character of the vehicle. In general, the deflocculation of the pigment is more nearly complete when the solid is readily wetted by the liquid. In some cases, the effect of the degree of flocculation upon the consistency of the suspension is very pronounced. The effect of the addition of a deflocculating agent upon the plastic characteristics of a suspension may be illustrated by adding a small amount of oleic acid to a mixture of lithopone and mineral oil. If dry lithopone is mixed with a considerable amount of mineral oil, the pigment remains in a highly flocculated condition, and the resulting mixture is a stiff paste which can be shaped and moulded like clay. If a few drops of a deflocculating agent-as, for example, oleic acid-are added, the lithopone is wetted and dispersed, and a mobile suspension results. The addition of the dispersing agent greatly increases the mobility but decreases the yield point only slightly. The flocculating power of a liquid is one of the important factors in determining its characteristics as a thinner for paints. All of the commonly used thinners-benzol, petroleum naphtha, and turpentine-have viscosities of the same general order of magnitude. Turpentine, however, tends to flocculate Certain pigments, and therefore in reducing the paint to the desired consistency it is necessary to add more of the turpentine than of any of the other thinners.2B*1 A decrease in the viscosity of the medium increases the mobility of the suspension but has little or no effect upon the yield point. In many cases the effect of the viscosity of the liquid is small in comparison with that due to its wetting power. The addition of soaps to a paint results in “false” body. Bingham and Jacques4 found that the addition of aluminum stearate to a paint made with lithopone and linseed oil increased the yield value markedly, but lowered the mobility only slightly. The effect of soaps in lubricating greases is much better known than their effect in paints, and it may be of interest to give some of the facts concerning them. The structure of a soap in a lubricating oil is probably much the same as that given by McBain for water solutions of soap.12 If the soap is quite soluble in the oil there is little increase in the consistency of the grease, This is true with greases made with calcium oleate where in order to obtain proper consistency it is necessary that water should be incorporated in the solution of soap in oil. Sodium soaps incorporated in a grease are practically completely dehydrated and are able to give a very stable structure in this state. Lead soaps are very much less soluble in the oil and tend to precipitate out if too much water is present. Zinc soaps are -
Smith: “The Manufacture of Paints” (1915). l1 Wolff: Chem.-Ztg., 48, 647 (1924). Kkingard: “Lubricating Greases” (1927).
S T U D I E S IN THE PLASTICITY O F PAISTS
385
never used alone in the manufacture of greases as t’hey have little ability to thicken the oil and are very unstable in the presence of moisture. The effect due to soaps in paints if similar to their effect in greases will vary widely depending on the type of soap and the conditions. Bingham and Jacques4 found that the addition of small amounts of water to a paint made with dry lithopone and linseed oil increases the yield value and decreases the mobility. I t does not follow, however that the presence of water always injures the brushing qualities of a paint; in fact, it is common experience that in many cases paints which contain small amounts of water “brush out” much better than do those made from absolutely dry materials.I3 The superior working qualities which are claimed for pulp-mixed white lead may be due in part to the fact that this material always contains a small amount of moisture.” In many instances, ready mixed paints tend to undergo changes in texture and plasticity on standing. The oil may hydrolyze to some extent, and the free acids thus formed may react with the pigment to form soaps.15 In certain cases, the formation of these soaps may tend to cause “skinning” and “puttying.” Furthermore, t,he slow reactions that occur during ageing may change, and in some instances may improve, the plastic characteristics. SabinlBstates that “The fluidity and working characteristics of white zinc paint are considerably improved with age.” Bingham and Jacques‘ found that with continued grinding the yield value a t first decreases and then remains constant, while the mobility a t first rises and later decreases. The effects may be due in part to changes which are analogous to those which occur during ageing. Experimental Procedure The linseed oil used in preparing the paints was pure refined linseed oil from h’orth American seed. The two lots of oil which were used were analyzed by the methods recommended by the A. C. S. Committee on Analysis of Commercial Fats and Oils with the following results: Lot
Acid number Saponification number Iodine number Specific gravity
I
0.459 189.800 194.300 0.9275
Lot
2
0.280 194.500 182.700 0.9276
The pigments were zinc oxide, Dutoh process white lead, aluminum powder, and iron oxide. I n the preparation of the paints, the following procedure was adopted: The required amount of pigment was weighed into a mortar, and sufficient ~~
Toch: “Chemistry and Technology of Paints” (192j). l4 Sabin: “White Lead: Its Uses in Paints” (1920). Ware and Christman: Ind. Eng. Chem., 8, 897 (1916). lei‘Technologyof Paints and Varnishes” (1917). l3
386
F. H. RHODES AND W. J. JEBENS
linseed oil was added to form a thick paste. The mixture was ground for thirty minutes. The remainder of the oil required to give a paint of the desired composition was then added slowly and with constant grinding, and the grinding was continued until the pigment was thoroughly incorporated in the vehicle and a homogeneous paint was obtained. The paint was passed through a zoo-mesh sieve and was then placed under a vacuum for several hours and stirred at intervals to eliminate bubbles of air. Paints free from moisture were prepared from dry linseed oil and pigment which had been dried for two hours at 120'C. In the preparation of the paints which contained water, the required amount of water was weighed into a mortar and a portion of the dry paint was added. The mixture was ground until homogeneous, then the rest of the dry paint was added with constant grinding and the grinding was continued until thorough incorporation of the water was attained. To remove any air which may have been introduced during this operation, the wet paint was again evacuated for thirty minutes with almost constant stirring. Some of the water in the paint was vaporized during the second evacuation. The water content could not therefore be calculated from the proportions used in preparing the paint and was determined by analysis. The same apparatus and procedure was used to measure the plasticity of. the paints as was used by Rhodes and Wells.2 The capillary used in this work was 6.05 cm long and 0.0247 cm in radius. The pressures were measured in centimeters of mercury, and are so expressed in all of the following tables. With the particular capillary which was used, a pressure of one centimeter of mercury is equivalent to a stress of 27.06 dynes per square centimeter. All yield points, as given, may be converted to the absolute basis by multiplying by this factor. The mobilities, as given, are expressed in terms of the slopes of the graphs obtained by plotting rates of flow, in cubic centimeters per minute, against the corresponding pressures in centimeters of mercury. While these units are perfectly satisfactory for purposes of comparison, they may be converted into absolute units by multiplying by the factor, 8L/R4Dg, in which, L is the length of the capillary in centimeters, R is the radius of the capillary in centimeters, D is the density of mercury (13.596), and g is the gravity constant (980.4). The numerical value of this constant, for the capillary used, was 52.03.
Experimental Results Eflect of Water. Paints were prepared from dry neutral linseed oil (Lot I) and each of the following pigments:' zinc oxide, white lead, mixtures of white lead and zinc oxide, and iron oxide. To each of the dry paints, various amounts of water were added, and the plasticities of the resulting paints were determined. The results are summarized in Table I. The addition of water to paints made with oil of low acid value increases the yield and lowers the mobilityi.e., it renders the paint considerably stiffer and less fluid, In general, the effect of the addition of water upon the mobility appears to vary qualitatively with the relative ease with which the
STUDIES IS T H E PLASTICITY O F P A I S T S
TABLEI Effect of Water on Paints prepared from Xeutral Linseed Oil 70 by weight Water Yield Pigment
of Pigment
Zinc Oxide
30%
Content
Value 1.5
o ,0306
0.40 0.89
4.3 5.4 6.9 8.2
0.0190
I .84
4.75 7.77 Zinc Oxide
40
1 0 .o
4.9
0.00
0.99 White Lead
60
35.2
0 .oo
I .o 4.5 6.o 7 .o
0.42 0.76 I .83 4.87 Mixture: 65Ycwhite lead 35YCzinc oxide
Iron oxide
50
40
Mobility
o.oo"?c
0.0190 o ,0168 o ,0168 0.0147 0.0240 0,0147 o ,0226 0.0139 0.0127
0.0116
10.8
o ,0089
0.oo
1.5
0.0272
0.50
4.3
I .02
5.5
0.0138 0.0126
I .86 4.92 7.78
6.7 7.8 10.2
0 .oo
0.44 0.96 1.79 4.54 7.38 The results are shown graphically in Figs.
I .o
and
0.0110
0.0114 0.0221
2.1
0.0212
5.0
0.0196
6.2 9.8
o ,0188
12.2 I
0.0120
0.0160 0.0126
2.
pigment is wetted by water. In the paints made with white lead, which is very much less readily wetted by water than by oil, the first small additions of water very greatly decrease the mobility; with larger amounts of water the effect is still apparent, although much less pronounced. Somewhat similar results were obtained with the paints made with mixtures of white lead and zinc oxide. Paints containing zinc oxide alone, which is somewhat more readily wetted by water than is white lead, also showed a rather marked decrease in mobility with the first additions of water, but the effect was somewhat less pronounced. With iron oxide, which is rather readily wetted by water, the decrease in mobility was less pronounced and was more nearly linear throughout the entire range of concentrations.
F. H. RHODES ASD W. J. JEBENS
388
With each of the paints, the first small additions of water increased the yield values markedly, further increase in the content of water had relatively less effect. -411 of the dry paints except that one which contained forty per cent of zinc oxide had yield values of the same general order of magnitude, and with all of these paints the effect of the addition of water was quite similar. The initially high yield value of the paint which contained forty per cent of zinc oxide was increased very rapidly indeed by the addition of small amounts
Figure
L
1
Wofer Content VJ
Yle/d Vu/ue
I
i
i
l
I
l
of water, so that at first glance it would seem that the effect of water upon the plasticity of this paint is unusually great. It will be observed, however, that the proportional increase in yield value which is brought about by the addition of water to the dry paint is approximately the same with all of the paints. The increase in the consistency of all these paints is probably due to the action of the water in tending to flocculate the pigment and to form an emulsion of water in the paint. I n either case a structure is built up in the paint with the resulting increase in yield value and decrease in mobility. The E$& of Soaps. The effect of the addition of soap upon the plastic characteristics of paints was studied for the two soaps, sodium oleate and
STUDIES I T THE PLASTICITY O F PAISTS
389
calcium oleate. These particular ones were selected because they are typical of the two principal classes of soaps: sodium oleate is soluble in water and tends to aid in the formation of emulsions of oil in water, while calcium oleate is soluble in oil, and tends to produce emulsions of water in oil. In preparing these paints, an amount of soap equal to ten per cent of the weight of the combined oil and pigment was placed in a mortar, and to it was added a small amount, of the linseed oil, After grinding the soap and the oil
Par C e n t
Water
together thoroughly, the pigment and the remainder of the oil were added and the paint prepared in the usual way. A11 of the paints contained thirty parts by weight of zinc oxide to seventy parts of linseed oil (Lot 2 ) . Mixtures containing less than ten per cent of soap were made up by blending portions of this paint with the proper quantities of paint which contained no soap. The results of the plasticity determination are given in Table I1 and are shown graphically in Figs. 3 and 4. These results are qualitatively significant, but may not be extremely ac . curate quantitatively. The soaps-and particularly the sodium oleateprobably exist in the paint in the form of micelles or minute threads which give the mass a “false body”. The mere stirring of the solution, or the dis-
F. H. RHODES A S D W . J. JEBENS
390
Per
Cent
Soop
TABLE 11 Effect of Soaps on Paints prepared from Seutral Linseed Oil Soap
Calcium Oleate
Per Cent of Soap 0.00
I,
18
Mobility
4.3
0.0322
5 . 2
0 .0 3 I 8
2.72
j.6
0.0306
5.19
6.9 8.5
0.0280
10.00
Sodium Oleate
Yield Value
0.0220
0.00
4.3
0.0322
1.02
0,0274
2.01
8.5 10.0
5.10
13.3
0.0182
10.00
15.2
0.0132
0.0212
STUDIES I N THE PLASTICITY O F P A I S T S
391
tortion which occurs when the mass flows through the outlet tube of the plastometer, may disrupt this structure to some extent, and may therefore cause a change in plasticity during the determination. It is well known that a change in the plasticity of a lubricating grease-a suspension of soap in mineral oil-does occur when the grease is passed through a capillary tube, and it is to be expected that a similar change will occur with a suspension of a soap in a vegetable oil. 082
02 d
a24
011
f
O2i
026
678
Ub
U4
The addition of either sodium oleate or calcium oleate increases the yield point and decreases the mobility of the paint, but the effects are much more pronounced in the case of the sodium soap. This is to be expected. The calcium oleate is quite soluble in the soil, and probably exists either in true solution or in a highly dispersed condition, while the sodium oleat,e is much less soluble in linseed oil, and probably exists principally in the form of thread-like micelles. Joint Effect of S o a p and Water. Various amounts of water were added to paints which were made up with 30 parts of zinc oxide, 70 of linseed oil and z of soap. The results are shown in Table I11 and Figs. j and 6.
F. H. RHODES AND W . J. J E B E S S
Per Cent
k&&r
TABLE I11 Joint Effect of Soap and Water on Paints prepared from Keutral Linseed Oil Soap
Calcium Oleate
Per Cent of Water 0.00
1.19
Sodium Oleate
Yield Value
Mobility
5.5
0.0300
11.2
0.0220
2 2j
12.0
0.0219
5.62
14.0
0.0216
10.31
18.j
0.0200
0.00
8.j
0.02jO
0.48
IO.j
0.0264
1.36
16.5
0.0240
2.05
'7.5
0.0234
STCDIES IT T H E PLASTICITY O F PAISTS
393
With the paints containing sodium oleate, difficulty was experienced in obtaining complete and permanent emulsification when the water content was over two per cent. h very thick mixture was obtained when the water was first emulsified but in a few minutes the zinc oxide agglomerated in curdy masses which settled very quickly. Sodium oleate tends to give emulsion of oil in water and it is not surprising therefore that emulsions of water in oil are rendered less stable. On the other hand, water could be very readily
I 2
I 4
Pry
I
I
I
6 Cent
a
/o
I
Wder
emulsified in the paints containing calcium oleate. In this case, both the paint and the calcium oleate tend to give emulsions of water in oil with the result that a stable emulsion is formed. The water increases the yield value of these paints. The effect is qualitatively similar to that observed with paints which contained no soap. The first additions had a greater effect than later ones. For the same water content, the paints made with soap have a greater yield value than those without. That is, the effect due to the water and that due to soap are qualitatively additive. The mobilities of these paints are lowered by water, the effect being more pronounced with the first additions. The water does not have
F. H . RHODES AND W . J . JEBENS
394
I
fie&
82
I
I
Fiqure
7
Vuhe vs
prr zbl
cent
woter
for a m r 4 C&On*;nP & p S r d 9 a,%&,nZ
I
to
I
I
I
I
I
I
The acids thus obtained were but slightly darker than the original oil and had an acid value of 197.1. This free fatty acid was mixed with various portions of the linseed oil to give oils of the desired acid values. The pigment used for this work were: zinc oxide, white lead, and aluminum powder. The paints made with zinc oxide contained 3 0 parts of the pigment to 7 0 of the vehicle, those with white lead contained 60 parts of the pigment to 40 of the vehicle, while those made with aluminum powder contained 30 parts of the pigment to 7 0 of the oil. The first method of incorporating the water was found satisfactory if the water was readily emulsified in the paint and was therefore adopted in the first part of the experimental work. I t was found, however, that water could
STUDIES I N T H E PLASTICITY O F PAINTS
395
be emulsified somewhat more readily by shaking the mixture of paint and water than by merely grinding the two together. Accordingly, the following procedure for introducing water into these paints was adopted: About 30 to 40 cubic centimeters of the paint was placed in a IOO cc bottle, the desired amount of water was added, the mixture thoroughly shaken, and then allowed to stand for about one hour before being tested in the plastometer. The effect
due to the air that may have been introduced into the paint during the shaking process was assumed t o be negligible and paints prepared in this way were not evacuated a second time. The results are shown in Table I V and in Figs. to 12. The effect of water in these paints is more complex than in those paints made with the practically neutral linseed oil. Paints made with zinc oxide and low acid value oil showed an increase in the yield point and a decrease in mobility as the water content was increased. The effect of the free fatty acid alone in the linseed oil is to increase the yield value greatly. The presence of both water and free fatty acid results in changes in the plasticity depending on the conditions. If the acid value of the oil is 2.66, the effect of water is
396
F. H. RHODES A S D TV. J. JEBEKS
STUDIES IS T H E PLASTICITY O F PAISTS
397
398
F. H. RHODES A S D W. J. JEBEKS
TABLE IV Effect of Water on Paints prepared from Linseed Oil of Varying Acid Value Pigment
Zinc Oxide
Acid Value of Oil 0.28
Per Cent of Water 0.00 0.51 I 2
2.66
5.69
8 .so
White Lead
0.28
.03 .oo
4.40
0.0222
15
.o
IS
.8
0.0324 0.0218
0.0218
17.2
0.0214
9.89
19.6
0.0204
0.00
10.9
o ,0242
I .oo
11.3
0.0235
2.23
11 . 6
0.0234
4.32 9.57
12.5
o ,0230
13.2
0.0226
0.00
25
0.99 4.91 9.90
11.5
13.2 13.4
0 .oo
28
I .06
I 2 .o
0.0192
2.24
14.0
0.0184
4.65
15.5
0.0174
.o
.o
0.0156 0.0246 0.0242 0.0234 0.0102
.oo
17 .o
0.0236 0.0184 0.0176 0.0140
4.62
22.5 28 .o
0.0102
0.00
I
.o
0.22
IO .o
0.49
12.8
10.22
White Lead
Mobility
4.8 12.4
5.27
2
2.40
Yield Value
0.00
7.2
1.14 2.06 4.59 8.21
8.5 IO .o
0.00 I .OI
2.06
5 .oo 9.95
12.7
18.5
5.4 5.8 6.2 8.3 12.8
0.0120
0.0194 0.0218
o ,0203
0.0192 0.0174 0.0204 0.0224 0.0213
0.0195 0.0176
STUDIES IS T H E PLASTICITY O F PAIXTS
3 99
TABLE I V (Continued) Effect of Water on Paints prepared from Linseed Oil of Varying Acid Value Pigment
White Lead
Acid Value of Oil
6.48
Per Cent
of Water
.oo 0.49 I .03 0
2.02
4.81 9.28 8.35
0 .oo
0.58 I .08 2.10
4.92 9.94
Aluminum Powder 0.28
6.48
Yield Value
4 .O 4.5 4.8 5.6 7.2 10.2
3.4 3.6 4.0 4.4 5.4 7.2
Mobility
0.0226 o ,0238 0.0232
0.0230
0.0204 0.0188 0.0226 0.0240 0.0230
0.0226 0.0202
0.0179
0 .oo
0 .o
0.0330
0.51
0.8
0.0318
.a
0.0310
I
.04
I
2
.oo
4.0
4.90 9.60
16.0
0.0306 0.0290 0.0280
0 .oo
0.2
0.0322
0.57 0.98 2.16 5.09
I .o
0.0296 0.0292 0.0290 0.0274 0.0266
10.25
I 1 .o
2.6 4.8 11.4 18.2
to increase but slightly the relatively high yield value due to the free fatty acids. However for paints made with linseed oil with an acid value of 5.69 and 8.80, the first addition of water has a very pronounced effect in lowering the yield value. A minimum value is obtained a t about one per cent of water and further additions increase the yield value. For paints made with zinc oxide and a practically neutral linseed oil, the effect of the water alone or of fatty acids alone is to increase the mobility. With an acid value of 2.66 for the oil, the mobilities are decreased but slightly on addition of water with the result that the mobilities are higher for these paints for the same water content than with those made with the low acid value oil. When the acid value of the oil is 5.69 the first additions of water tend to increase the mobility to a maximum value when about one per cent of water is present. Further additions lower the mobility again slightly. The paints made with this concentration of free fatty acid show the highest
400
F. H. RHODES AND W .J. JEBESS
mobilities for any given content, of water. When the acid value of the oil is 8.80, there is again a maximum in the mobility curve when about one per cent of water is present, but these paints all show a relatively low mobility. With paints made with white lead, the effect of free fatty acids alone on the yield point is to increase it to a maximum at an acid value of 2.40 and then to decrease it continuously on addition of more free fatty acid. The lowest yield value is shown by the paint made with neutral oil and no water, For acid values of the oil of 2.40 and above, the effect of the water is to increase the yield value slightly, and for the same concentration of water, the yield value decreases as the acid value increases. The effect of free fatty acids on the mobility of paints made with dry white lead is to decrease it to a minimum with an oil of acid value of 2.40 and then to increase it slightly with further additions of free fatty acid. For paints made with linseed oil of 2.40 and higher, the mobility increases to a maximum on addition of about one-half of one per cent of water and then decreases again. The mobilities are higher for the same water content in those paints made with oil of higher acid value. The paints made with aluminum powder had practically the same yield value for the same amount of water when made with linseed oil of acid value of 0.28 or 6.48. Water increased the yield value in both cases. The mobilities of these paints decrease as more water is added, the values for the paints made with oil of acid value of 0.28 being slightly higher than those made with oil of acid value of 6.48. Efect of Heating the Paint. The effect due to heating the paint was studied with mixtures made up with 30 per cent of zinc oxide and 7 0 of linseed oil (Lot 2 ) . The paints were prepared as described and then heated for half an hour in an oil bath kept a t a temperature of 150 to 160'c. To prevent any oxidation of the oil during the heating, a slow stream of nitrogen was passed through the flask. The following results were obtained. T.4BLE
v
Effect of Heating the Paint Acid Value of oil
Heat Treated '
Yield Value
0.28 0.28
NO
4.8
0.0324
Yes
5.4
0.0332
8.35 8.35
NO
Yes
31 . o
6.0
Mobility
0.0136 0.0274
With the paints prepared from the neutral linseed oil, the effect of heating the paint resulted in a slight increase in yield value and mobility. A very pronounced change occurs however with paint made with the oil of acid value of 8.35. The yield value is reduced almost to that for the paint made with the neutral oil while the mobility is increased to double its initial value.
STUDIES I N THE PLASTICITY O F PAISTS
401
That this effect is not due to any change in the linseed oil was shown by preparing paints from linseed oil that had been heated as described, cooled, and then used in preparing these paints. The plastic constants of these paints differed but little from those of paints prepared from untreated linseed oil of the same acid value. As described in the first part of the experimental work, the effect of adding one per cent of water to a paint made with 30 parts of zinc oxide and 7 0 of linseed oil with acid value of 8.35was to decrease the yield value and increase the mobility. This effect is very similar to that due to heat although not as marked. If the effect due to the water is the same as that due to heating the paint, the addition of water to a paint that has been heated should not result in lowering of the yield value and an increase in the mobility when approximately one per cent water is added. Water was added to a paint made with linseed oil of acid value of 8.35 and zinc oxide and heated as described above. The water had a similar effect in increasing the yield value and decreasing the mobility as in those paints made with the neutral oil. E$ect of Zznc Sulphate. The effect due to zinc sulphate in paints made with zinc oxide was studied with mixtures prepared with one part of anhydrous zinc sulfate, thirty of zinc oxide, and seventy of linseed oil of various acid values. The plastic constants for these paints were found t o be practically independent of the zinc sulphate content for paints made with linseed oil of acid value as high as 8.35. Efect of Free Fatty Aczd and Soap. Paints were prepared with thirty parts of zinc oxide, seventy of oil, and two of calcium oleate as described previously. The results are shown in Table VI.
TABLE VI Effect of Calcium Oleate on Paints prepared from Linseed Oil of Various Acid Values Acid Value of oil
Parts of Soap added
0.28
0.0
0.28
2.0
2.40 2.40
0.0
8.35 8.35
0.0
2.0
2.0
Yield Value
4.8 5.5
Mobility
0.0324 0.0300 0 . 0 2 so
10.4 6.0
0.0308
31.0 6.2
0.0136 0.0288
The effect of the soap in the paint made with the oil of low acid value is to increase the yield value slightly and lower the mobility. However in the paint made with the oil of acid value of 2.40 the soap decreased the yield value and increased the mobility. The same thing was true in the case of the paints
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F. H, RHODES AND W. J . J E B E S S
made with the oil of 8.35 although the effect was very much more marked. When this last paint is heated, the yield value is lowered still more and the mobility increased to practically the same value as those for the paints made with neutral oil and no water.
Discussion of Results The results obtained with paints made with practically neutral linseed oil can be readily explained qualitatively. The effect of the water in always increasing the yield value and lowering the mobility is due to its tendency to emulsify in the paint and thus build up a structure in the paint. The amount of the increase in consistency depends on the type of pigment, the concentration of the pigment, and the concentration of water. The effect of sodium oleate and calcium oleate is to increase the yield value and decrease the mobility due to their gel structure in the paints. The sodium oleate has a more pronounced structure than the calcium oleate and so has a greater effect in increasing the consistency. The presence of free fatty acids in the vehicle may affect the plasticity of the paints in various ways. With paints made with zinc oxide, the effect of the free fatty acids in the linseed oil is to increase the consistency of the paint very markedly. This is probably due to the interaction of the zinc oxide and the free fatty acids to give zinc soaps. The addition of anhydrous zinc sulphate to these paints has no appreciable effect; although as this is a soluble zinc salt, one would expect it to form soaps more readily than the zinc oxide and thus increase the consistency to a greater extent. Apparently the free fatty acids are able to react with the dry zinc oxide rather completely. When the acid value of the oil is 8.3 j, there are several factors that can decrease the initial high consistency due to the free fatty acid. Water in small amounts tends to do this. Heating the paint for thirty minutes a t a temperature of ISOT brought the plastic constants very close to that for the paints made with the neutral oil. The addition of two parts of calcium oleate was also found to have approximately the same effect as that due to heat. Another factor that tends to do the same thing is ageing which lowers the consistency. With paints made with zinc oxide and linseed oil of high acid value, soaps are formed in relatively large amounts. The zinc soaps are insoluble in the linseed oil, but are probably dispersed to some extent by the free fatty acids present. That free fatty acids are adsorbed by the soap micelles has been shown by Arsen.17 The high consistency of paints made with a high acid value oil is due to the soap micelles which are present as loose ramifying aggregates. Anything that will tend to precipitate out the soap and thus destroy the structure of the soap should result in a lowering of the viscosity of the paint. '1
Ind. Eng. Chem., 18, 157 (1926).
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With time more of the free fatty acids react with the zinc oxide to form soaps. The result is a gradual decrease in the concentration of the free fatty acids. The vehicle then loses its power to disperse the soaps which are precipitated out. The gel structure of the soap in the paint is thus broken down with a resulting decrease in the yield value and increase in the mobility. This is probably the change that occurs during the ageing of paints prepared with zinc oxide. Water is known to precipitate soaps from solution in organic liquids and its action in reducing the consistency of paints prepared from linseed oil of high acid value and zinc oxide can be explained on this basis. Thus calcium soaps are precipitated out as a flaky material from a clear benzene solution in the presence of a trace of moisture.** Also lead and zinc soaps in greases are unstable and will tend to precipitate out if water is present. The effect of heating the paint is due to the increase in the rate with which the free fatty acids react with the pigment to form soaps. The action is thus similar to that which occurs during ageing. The calcium oleate is effective in lowering the concentration of the free fatty acid due to its ability to adsorb these acids. The calcium soaps apparently have but little effect in changing the consistency even when dispersed in the vehicle. This change in the structure of the soap micelles may also explain why turpentine is less effective in reducing the consistency of paints than some of the other thinners. It is known that zinc soaps are insoluble in neutral linseed oil and in naphtha and are soluble to some extent in free fatty acids and in turpentine.I6 The turpentine apparently has the power to disperse the zinc soaps into loose ramifying aggregates which tend to enmesh the vehicle and thus increase the consistency. With paints prepared from white lead, the effect due to any free fatty acid is very much less marked than with paints made with zinc oxide. An increase in the acid value of the oil from 0.28 to 2.40 resulted in a slight increase in consistency probably due to the formation of a small amount of soap. Any further increase in the acid value of the oil resulted in a decrease in consistency due to the better wetting and dispersing power of the vehicle. Moreover the lead soaps differ from the zinc soaps in that they do not have the same tendency to coagulate or “lump”.15 The result is that no very marked structure is built up in the paint with a resulting increase in consistency. Some soaps are however formed. With paints prepared with linseed oil of acid value of 2.40 or higher, the addition of one-half of one per cent of water results in an increase in mobility while further additions lowered the mobility. The ability of water to increase the mobility of these paints is substantiated by the fact that paints prepared from pulp-mixed white lead are said to have better working properties than those prepared from pan-dried white lead. Wellman and Tartar: J. Phys. Chem., 34, 379 (1930).
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summary Water added to a paint prepared from a practically neutral linseed oil increases the yield value and decreases the mobility: the amount of the effect depending on the type and concentration of pigment. 2. Sodium oleate and calcium oleate increase the yield value and decrease the mobility of a paint prepared from a neutral linseed oil, the effect being more pronounced for the sodium oleate. 3. Water added to a paint prepared from a practically neutral linseed oil and two parts of sodium oleate or calcium oleate increases the yield value and decreases the mobility. 4. With paints prepared from linseed oil and dry zinc oxide, the effect of free fatty acid is to very markedly increase the consistency. Small additions of water tend to offset part of this increase in consistency, the effect being most marked when one per cent of water is present and the acid value of the oil is relatively high. 5 . With paints prepared from dry white lead and linseed oil, the effect of free fatty acids is relatively small. With paints prepared with oil of acid value of 2 . 4 0 or higher, the mobility has a maximum value when about onehalf of one per cent of water is present ; while the yield value increases slightly but continuously with water content. 6 . The presence of anhydrous zinc sulfate has but little effect on the plastic constants of paints prepared from zinc oxide. 7. The increase in consistency in a paint prepared from zinc oxide due to the free fatty acids present in the vehicle can be practically completely offset by heating the paint for one-half hour a t I jo°C. The same result can also be obtained by the addition of two parts of calcium oleate in the paint. I.
Cornel2 University, Ithaca, New York.
