INDUSTRIAL AND ENGINEERING CHEMISTRY
890
Vol. 22. No. 8
The Adhesion-Tension Cell in Paint Investigations' Elliott L. McMillen THENEW JERSEYZINC COMPANY, PALMERTON, PA.
The factors influencing the accuracy of wettability refinements of more accurate N A recent paper (6) data measurements upon lithopone by means of the Bartellsurface-tension values, conwere presented correlatOsterhof cell have been investigated. The wettability stant temperature, s m a l l e r ing plasticity of l i t h o data indicate that poor wettability is not the cause of pigment additions and higher pone-liquid systems with the plasticity of solid-liquid systems. packing pressures (535 atmoswettability of lithopone as The various terms used in discussing wettability pheres), the data presented in measured by the B a r t e l l are defined and illustrated; their bearing on paint Table 1were obtained. The Osterhof cell. The generally problems is indicated. table offers a comparison with accepted ideas of plasticity the data previously published. associate low w e t t a b i l i t y The main error in the previous data for Nujol was in with the greater plasticity. The reported data showed the reverse to be true, the best flow characteristics being ob- the surface-tension determination. The first six liquids tained with those liquids having the lowest wettability. in the new data have been assumed to possess zero contact This reversal of the accepted theory for the cause of plas- angle with the lithopone although the pore radii, calculated ticity raises the question of the accuracy of the wettability from the various values, show a deviation of 2 per cent from measurements. I n the present paper the wettability measure- the average. The new values indicate that the liquids line ments have been rechecked and an investigation made of up in respect to their wetting power for lithopone in the same order as that indicated by the previous data, although the possible sources of error in the method. a different sample of lithopone and, in some cases, different Experimental samples of liquids were used. While the present data check in a general way the previous The capillary-rise method for surface-tension determination was substituted for the du Nouy ring method and data on the wettability of lithopone by various liquids, the values obtained checked satisfactorily with the Inter- complete relationship between consistency and wettability national Critical Tables. The effect of temperature was cannot be established until data are available upon the eliminated by enclosing the apparatus in a constant-tempera- wettability in the range where K , the adhesion constant, is ture cabinet (Figure 1) which could be maintained constant 1 or greater than 1. The Bartell-Osterhof method of diswithin 0.1 degree of 25" C. The normal moisture content placing one liquid with another is applicable in this deterof lithopone did not influence the measurement. Dis- mination, but considerable experimentation will be required placement-pressure measurements were the same regardless to develop satisfactory methods of packing the cell with a of which end of the pigment cake was wetted, either the end pigment wet by different liquids. Preliminary results indicate that a number of factors make i t impossible to packed first or that packed last.
I
Table I-Wettability
of L i t h o p o n e
I
PREVIOUS DATA(6)
LIQL-ID
Ratio Surface Dz:tte- A:1e tension to vol. pressure contact solid
,it':
29.0
Grams/ s q . cm. 0.901 4620
30.6 33.6 31.8 33.7 33.4
0.889 0.890 0.869 0,900 0.889
Dynes/ cm.
Benzene Carbon tetrachloride Toluene Acetone Carbon disulfide Nujol Linseed oil fatty acids Methyl ester of linseed oil fatty acid Alkalke refined linseed oil Triglycerides of linseed oil iatty acid a
4690 5110 4725 4725 4290
Adhesion
K
8
Dynes/ cm.
0'
15'50' 17'10' 21'15; 29'10 36'20'
Surface tension
tension
',C :Y:
I
>1
0.962 0.955 0.932 0,873 0.805
29.4 32.1 29.6 29.6 26.9
,
Ratio AdAdvoids vO1' ment Angle of 'Ore hesion hesion to vol. pressure contact radius constant tension solid Cos 8
Dynes/
Grams/ sq. cm.
cm.
28.21 26.04 27.89 22.76 31.58 29.70 33.10
0.678 0.667 0.676 0.679 0.673 0.664 0.702
6860 6335 6950 5720 7645 7210 63900
33.65 33.86
0.693 0.690
53350 42500
8
X 10-0
K
Cm.
8.38 8.38 8.18 8.11 8.42 8.40 49'40'5 59'10'0
Dynes/ cm.
>1 >1 >1 >1 >1 >1 0.786"
26.05
0.6480 21.7' 0.5110 17.3'
These values were calculated from tne initial displacement-pressure readings.
Most reproducible displacement-pressure readings resulted from the use of a more densely packed pigment cake, obtained by 3-gram additions during each step of the packing process. No change in surface tension of the liquid after coming in contact with the pigment could be detected. Substitution of highly purified liquids for the ordinary c. P. materials did not change the results. Finally it was found that more reproducible results were possible by the use of a higher packing pressure. Using these Received April 23, 1930. Presented before the Division of Paint and Varnish Chemistry a t the 79th Meeting of the ~~~~i~~~ Chemical Society, Atlanta, Ga., April 7 to 11, 1930. 1
PRESENTDATA Packing pressure-535 atmospheres Size of pigment additions-3 grams Av. pore radius--8.31 X 10-6 cm.
racking pressure-214 atmospheres Size of pigment additioas-about 12 grams Av. pore radius-1.28 X 10-6 cm.
obtain the same pore radius when the same packing pressure and procedure are adopted for wet-packing as are used in packing dry pigment. I n view of the apparent discrepancy between the data here presented and the accepted theory of the cause of the plasticity in paint and because of the indiscriminate use of terms describing wetting characteristics, a discussion of terminology and theory is considered necessary and desirable. Discussion of Terminology
Many of the terms used in discussing wettability Of solids are quite general and in some cases subject to several inter-
August, 1930
INDUSTRIAL A N D ENGINEERING CHEMISTRY
pretations. Thus, the term “wetting” may imply to different persons the ideas of the process of wetting, the ease of wetting, or the speed of wetting. Such a term should be used cautiously. The term “wettability” is more definite, implying the ability of a speciiic solid to be wet by a specific liquid. The wettahility of a solid depends, not only upon the forces of adhesion between solid and liquid, but also upon the physical condition of the solid and the surface energy of the liquid. Thus neither wetting nor wettability can be assigned definite numerical values. Wetting power, like wettability, refers to the power of a speciiic liquid to wet a specific solid. As wetting power depends upon the physical condition of the solid, the term can be used only in a qualitative manner; that is, one liquid may be said to have a greater wetting power for a given solid than a second liquid. “Wetting force” refers to the actual force of attraction between solid and liquid which is responsible for wetting. Again, since wettability varies with the physical condition of the solid, it is meaningless to attempt to assign exact numerical values of wetting force. “Force of adhesion” implies a very definite idea of the actual force of attraction between the solid surface and the liquid. Unfortunately, force of adhesion cannot be measured directly and can be assigned numerical values only by assuming that these attractive forces are effective for a definite distance from the solid surface. Then by dividing the work of adhesion, which can be measured, by this arbitrarily assumed distance, the force of adhesion can be calculated. In contrast to this group of terms, which imply more or less definite ideas but cannot be assigned definite numerical values, we have a group of terms implying very definite concepts which can be measured and assigned definite numerical values. “Surface tension” of a liquid or solid is the force necessary to extend a surface of the liquid or solid whose width remains 1 cm. To do this it is necessary to bring liquid or solid from the interior to the surface. Surface tension, expressed in dynes per centimeter, is numerically equal to surface energy, which is expressed in ergs per square centimeter. Surface energy is the amount of energy needed to create 1 sq. cm. of surface by bringing liquid or solid from the interior to the surface. The surface tension of liquids may be measured quite accurately by a number of methods. The measurement of solid surface tension presents more difficulty. Although theoretieally a number of methods are possible, only one method (6)has yielded results which can be considered trustworthy, and this method is applicable only to soluble solids. “Interfacial tension” between solid and liquid is theoretically the force necessary to extend an interface whose width remains 1 em. Its measurement also presents experimental difficulties. If we had accurate knowledge of surface tensions of a solid and a liquid and the interfacial tension between the solid and liquid, i t could easily be predicted how well the liquid would wet the solid. Since determinations of solid surface tension and solid liquid interfacial tension are so difficult as to be considered practical impossibilities, a more direct method of measuring wettability must be employed. Such a method, in fact the only method available for powdered solids, is that of Bartell and Osterhof (3)in which the force with which a liquid advances into and wets a compressed cake of the powdered solid is measured. The BartellOsterhof method measures adhesion tension. Adhesion tension is subject to the same misinterpretation as is surface tension. Surface tension can best be thought of, not as a tension in the surface, but as the force necessary to lengthen a surface of liquid whose width remains 1 cm., or as numerically equal to the work needed to bring sufficient liquid from the interior of the liquid to form 1 sq. em. of surface. Ad-
891
hesion tension must be thought of in the same respect. It is numerically equal to the amount of work evolved in replacing 1 sq. em. of solid gas surface with 1 sq. cm. of solid liquid surface, the amount of free liquid surface remaining unchanged. Such a process would be to immerse a solid particle, having 1 sq. om. of surface, beneath the surface of a liquid. Tension suggests a force, and hence adhesion tension suggests a force of adhesion. However, “adhesion tension’’ is a term exactly analogous to “surface tension.” Just as surface tension can be thought of as the force neces-
Figure i-Conatant-Temperature Nr Cabinet. Adhealon-Tension Celt Assembly
sary to lengthen a liquid surface 1 om. wide, so adhesion tension can he thought of as the force necessary to hold a rod of 1 em. circumference from being pulled into a liquid in which its lower end dips. This example serves only in ease there is a finite contact angle between the liquid and solid. When a liquid is placed upon a plane surface of a solid there is a definite tendency for the liquid to spread or contract, depending upon how well the liquid wets the solid. When a drop of liquid spreads upon a plane solid surface, the angle between the free surface of the liquid (near the solid) and the solid surface decreaaes. As the angle becomes smaller spreading slows down, until finally spreading ceases a t some definite angle between the free liquid surface and the solid surface. This equilibrium angle which is characteristic of this liquid and this solid is known as the “angle of contact” between this liquid and this solid and depends only upon the ability of the liquid to wet the solid. This characteristic abdity of a given liquid to wet a given solid is what is designated by the term “wettability.” When this contact angle between liquid and solid has been attained, the following relation holds (8): Y, - rSL= y r . COS 8 = surface tension of solid yaL = interfacial tension between solid and liquid = surface tension of the liquid yL a = angle of contact
(1)
y,
The “work of adhesion” is the work necessary to separate a solid liquid interface of 1 sq. em. area creating 1 sq. em. of solid surface and 1 sq. cm. of liquid surface. Work of adhesion = Adhesion tension =
In case B
=
y, y,
0 degrees,
- ysL + yL = - ysL = cos Ey‘
+
Work of adhesion = (1 K)y, Adhesion tension = K y ,
(1
+ cos Eh,,
(2)
INDUSTRIAL AND ENGINEERING CHEMISTRY
892
K (2) is known as the “adhesion constant.” For all cases where there is a definite contact angle, K = cos 8, but if 0 = 0 degrees, K may be unity or greater depending upon the adhesion between solid and liquid. It is seen that there is not the same relation between adhesion tension and work of adhesion that there is between surface tension and surface energy. The work of adhesion is always greater than adhesion tension by an amount equal to the surface tension of the liquid. It is of interest to compare values of work of adhesion, adhesion tension, and adhesion constant for the different conditions of wettability corresponding to angles of contact of 180, 90, and 0 degrees. ANGLEOF CONTACT
180’ Adhesion constant Adhesion tension Spreading tendency Work of adhesion Force of adhesion
- 1 or less y L or less Negative 0 0
-
900
0 0
0
yL
Positive
0” 1 or greater y L or greater Positive 2 Y or greater Positive
Vol. 22, N o . 8
characteristic contact angle which depends only upon how well the liquid wets the solid. It is true that a knowledge of the value of adhesion tension does not tell the complete story of how well the liquid wets the solid. Suppose we imagine the case of two liquids, water and benzene, each possessing an adhesion tension of 50 dynes per centimeter for the same solid. Since y L for benzene equals 28.2 dynes per centimeter and y L for water equals 72 dynes per centimeter, it is apparent that benzene possesses a zero contact angle with the solid and the value of K in Equation 5 is equal to 50/28.2 = 1.77, while in the case of water K = 0.705 and the angle of contact between the water and the solid is 45 degrees. The work of adhesion of benzene for the solid equals (1 1.77) 28.2 = 78.2 ergs per square centimeter, while in the case of water with the solid the work 0.705) 72 = 122 ergs per square of adhesion equals (1 centimeter. Although the adhesion tensions are equal and the work of adhesion is greater in the case of water than for benzene, still benzene will spread indefinitely over the solid while water will spread only slightly until the equilibrium contact angle of 45 degrees is reached. The reason for this phenomenon is that the work of cohesion of the liquid which opposes spreading and equals 2 y L is 56.4 ergs per square centimeter for benzene and 144 ergs per square centimeter in the case of watter. Thus the value of the adhesion tension telb us whether the liquid will spread upon and wet the solid only when we also take into consideration the surface tension of the liquid. The adhesion constant, K , is really a much better measure of the spreading ability of the liquid upon the solid.
+ +
The value of adhesion tension between solid and liquid tells us whether a liquid will advance in a horizontal capillary tube of the solid. Adhesion-tension values also tell us whether or not one liquid will displace another liquid from a compressed cake of the powdered solid. It is customary to speak of a liquid which spreads upon a solid as being able to wet the solid; one that does not spread is not able to wet the solid. The value of the adhesion constant tells us how great the tendency is for the liquid to spread upon and wet the solid. When the adhesion-constant value lies between 0 and 1, spreading is only partial and will cease when the characteristic contact angle is reached, Application to Paint Problems and the liquid is a poor wetter for the solid. If the value of adhesion constant is negative, no spreading, but contracIt has been pointed out by Davidson (3) that the wetting tion, of the liquid will occur, and the liquid is not a wetter which paint men are interested in may be a different quanfor the solid. I n case the value of the adhesion constant is tity than adhesion tension. An attempt will be made to unity or greater, spreading will continue indefinitely or show in what way adhesion tension and these related quanuntil a very thin layer of the liquid upon the solid is reached, tities apply to the paint man’s problems. One of the factors and the liquid is a good wetter for the solid. Harkins’ ( 4 ) influencing the mixing of pigment and vehicle to form a “spreading coefficient” ( y ~ ~ S L y ~ has ) a positive paste is the wettability of the pigment by the vehicle. If value only when the angle of contact is 0 degrees and does the pigment sample is a tightly compressed mass, the value not indicate the partial spreading that occiirs when the of adhesion tension will tell us whether the liquid will advance into and n e t the pigment. If adhesion tension has angle of contact lies between 0 and 90 degrees. “Work of adhesion” gives us an idea of the actual forces a positive value, no matter how small, then the vehicle of adhesion operating between solid and liquid. If these will advance into and wet the tightly packed pigment mass. forces can be assumed always to act for the same distance The higher the value of adhesion tension the faster the vehicle from the solid surface, then force of adhesion would be will wet the pigment. If, however, the pigment mass is directly propoAional to work of adhesion. The measure- ,just loosely piled together, the final stage of the liquid surment of adhesion tension in the Bartell-Osterhof cell enables rounding each particle is one of spreading of the liquid on the calculation of values of work of adhesion, contact angle, the far side of the particle. I n this case, for the liquid to advance into and wet the loosely piled pigment, the value and adhesion constant. A study of the preceding table reveals several interesting of K must be 1 or greater and the angle of contact, 0. Adfacts. The work of adhesion, and hence force of adhesion, hesion tension may have a positive value when K lies bemay have a positive value and the liquid still fail to spread tween 0 and l and the contact angle is between 90 and 0 upon and wet the solid. This occurs in the range of wetta- degrees, and still the liquid will not completely spread around bility between contact angles of 180 and 90 degrees, where and wet each pigment particle and advance into the loosely both the adhesion tension and spreading tendency are less piled pigment mass. When the adhesion constant lies b e than zero. One can think of the force of adhesion tending tween 0 and 1 it is possible, by means of mechanical agitation, to cause spreading and wetting being opposed by the co- to mix the pigment and vehicle so that finally each pignent hesive forces within the liquid. If the contact angle is particle is completely wet. However, the greater the value between 90 and 0 degrees, spreading will be only partial, of the adhesion constant the easier it will be to mix pigment since the cohesive forces within the liquid still exceed the and vehicle. The apparent wettability of a pigment will adhesive force between solid and liquid. Spreading will depend upon whether the pigment is loosely piled or tightly continue indefinitely only when the force of adhesion is packed. For example, water when brought into contact equal to or greater than the force of cohesion of the liquid, with the end of a packed cake of graphite will advance into a condition that occurs when the contact angle is 0 degrees. the graphite, displacing the air and completely wetting the It is well to bear in mind that one liquid and one solid can- graphite. If some of the same graphite just loosely piled not assume any of the contact angles mentioned above, but has water poured upon it, the graphite and water remain that for each liquid with a certain solid there is a definite separate and no apparent wetting occurs. The angle of
August, 1930
ISDUSTRIAL AA'D ENGINEERIAVGCHE.ZIISTRY
contact between graphite and water is approximately 80 degrees, the value of adhesion tension is about 14 dynes per centimeter, and the value of K , the adhesion constant, is roughly 0.2. This difference in wettability of packed and loose pigment may offer some explanation of the efficiency of various paint mixers and paste mills. Any mixer with a tendency to pack the pigment particles more closely together-for example, a kneading action-will render the pigment more easily wet by the oil. The action of mixing equipment can be thought of as being effective in two ways: (1) tumbling the pigment particles about so that they ultimately come into contact with the oil; (2) compressing the pigment mass so that the oil easily penetrates and wets it. The mixers which give the kneading, compressing action are far more effective in forming pastes than a mixer that merely stirs pigment and oil together. The paint man is also interested in how the wettability of pigment by vehicle influences the plasticity of the resulting paint. Heretofore it has been customary to observe the plasticity of a pigment-liquid mixture and estimate wettability of the pigment by the liquid from the observed plasticity. It has been assumed that a liquid producing plasticity was a poor wetter for the pigment and one producing a fluid mixture was a good wetter for the pigment. This assumption was supported by the theory that poor wetting, with its high interfacial tension between liquid and solid, resulted in a definite tendency to diminish the free energy a t the liquid-solid interface by flocculation or grouping of the particles, such flocculation being thought to produce the structure giving rise to plasticity. No one thought to test the theory except by visual observations of wettability when pigment and oil were stirred together. The visual observation of wettability is influenced to a large extent by the viscosity of the liquid, and if structure viscosity results, with either a poor or good wetter, such structure slows down the process of wetting additional pigment and renders the visual observation of wettability a very inaccurate test. The advent of the adhesion-tension cell made possible the testing of this theory by enabling actual measurements of adhesion tension, adhesion constant, and work of adhesion to be compared with the experimentally observed plasticity of pigment-liquid mixtures. It was found (6) that plasticity was more pronounced the better the liquid wetted the pig-
893
ment. These data applied only to the range of wettability between contact angles of 90 and 0 degrees or adhesion constants of 0 to 1. More complete data, especially in the range of wettability where the adhesion constant is greater than unity, will be necessary before the complete relation between wettability and plasticity is known. These data indicate, however, that the cause of plasticity lies in the force of adhesion between liquid and solid. Adhesion between solid and liquid is thought to result in an adsorbed liquid layer upon the solid whose dimensions may greatly exceed that of the familiar mono-molecular layer. De Waele ( 7 ) believes that the source of plasticity lies in the adsorbed liquid layers about the solid particles. He believes that the greater the force of adhesion the more tenaciously the adsorbed layer is held; but that the adsorbed layer is thicker the weaker the force of adhesion between solid and liquid. He believes that plasticity or, more specifically, rigidity is proportional to the thickness of the adsorbed layer. Rigidity or yield value of pigment-liquid systems has in the past been attributed to flocculation of the pigment particles, by which is meant the bunching together of particles to form groups which link up together and impart structure to the system. Bartell and Greager (1) have shown that the liquidabsorption value of a pigment is less the greater the adhesion tension, indicating that strong forces of adhesion between solid and liquid tend t o pull the particles of the solid more closely together when they are wetted by the liquid. Undoubtedly wettability of the pigment by the vehicle is a major factor governing the consistency of pigmentvehicle mixtures. Whether plasticity observed in mixtures of pigment with good wetting liquids is to be explained upon the basis of adsorbed liquid layers or of flocculation remains to be seen. The Bartell-Osterhof cell is the weapon which will make possible the experimental correlation of wettability data with plasticity data. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8)
Bartell and Greager, IND.ENG.CHEX.,21, 1248 (1929). Bartell and Osterhof, Ibid., 19, 1277 (1927). Davidson, Paint Oil Chem. Rev., 89, 12 (1930). Harkins and Feldman, J . A m . Chem. Soc., 44, 2666 (1922). Lipseet, Johnson, and Maass, Ibid., 49, 926 (1927). McMillen, I N D . ENG. CHRM.,21, 1237 (1929). Waele, de, and Lewis, Kolloid-Z., 48, 126 (1929). Young, Phil. Trans., 1806, 65.
Flash Points of Mixed Solvents' Foster D. Snell 130 CLINTONSr., BROOKLYN, N. Y
T
HE practice of adding non-inflammable solvents to inflammable ones to raise the flash point of the mixture or to render the mixture non-inflammable is in general use. The products so produced are marketed as fabric cleaners, volatile suspension media for abrasives, larvacides, and in many other ways. This class includes various mixtures, many of them complex, having as the inflammable ingredients naphthas, benzene, toluene, xylene, acetone, ethylene dichloride, and many others. The outstanding flameproofing agent is carbon tetrachloride, although some trichloroethylene is used for the purpose. Unless molecular compound formation occurs between the solvents mixed, as is sometimes indicated in mixtures of two or more solvents, it is to be expected that the solvent with 1
Received July 7, 1930.
the lower boiling point will evaporate more rapidly. If, in a non-inflammable mixture, the non-inflammable solvent has a boiling point higher than the other solvent or solvents, it is to be expected that the mixture will evaporate without giving an inflammable vapor a t any time. If the boiling points are approximately the same, a similar result is to be expected. If, however, the inflammable solvent has a boiling point higher than that of the non-inflammable solvent, it is to be expected that the mixture after partial evaporation will be inflammable. Mixtures Used To indicate the extent to which this occurs in practice, mixtures were prepared to represent cases where the inflammable solvent was less volatile and where it was more
