THIXOTROPY IN PAINTS Influence on Packaging and Application
Properties of Flat Wall Coatings D. L. GAMBLE The New Jersey Zinc Company, Palmerton, Pa.
HE consistency characteristics of paints, aside from the problems of manufacturing and handling, are of prime importance because of the part they play in determining such properties as stability in the package, brushability, and leveling. Attempts to correlate ordinary consistency data (1, 16) with certain of the paint properties mentioned below have been successful only in a general way. The purpose of this paper is to discuss the nature of the so-called thixotropic characteristics of paints and their influence on paint properties with special reference to flat wall coatings.
Related Concepts of Plasticity and Thixotropy Paints are classified with that group of materials called “plastics” which in a broad sense are usually considered to have properties intermediate between those of a liquid and of a solid. Some investigators have gone so far as t o suggest that all paints are really pseudoplastics and that none possesses any real rigidity (yield value). This may possibly be true of some low-pigmented, highly dispersed enamels and of other paints immediately after vigorous mechanical agitation. However, experiments by Pryce-Jones (9), Green and Haslam @), and McMillen (7) indicated the existence of rigidity in some paints under certain conditions. In any case, most paints possess yield values in the practical sense that below a certain shearing stress the rate of shear becomes so small that it is insignificant when we consider the relatively short time required for some paint films to dry. Recently a rheological property referred to as “thixotropy”
and possessed by certain two-phase colloidal systems has received considerable attention. This term refers to the ability of these systems to undergo reproducible gel-sol transformations in which the gel is liquefied under mechanical agitation and, with time, reverts to the gel condition after the agitation ceases. This property is possessed to varying degrees by paints. Mchlillen (6, 7 ) was the first t o make a comprehensive study of this phenomenon as it exists in paints, and more recently Pryce-Jones (9) published results of a rather extensive investigation. The presence of plasticity in paints and other similar materials has been explained on the basis of the existence of structure within the system. The gradual breakdown of this structure under shear results in less resistance to flow and is responsible for the increase in fluidity observed as the rate of shear increases. Several investigators, including Reiner (IO), Peek and McLean (8), Cunningham (a), and McMillen ( 7 ) , have suggested that in plastic systems, such as paint, this structural resistance may be completely broken down at sufficiently high rates of shear, and the materia1 may exhibit true liquid flow. This possibility is based on the reasoning that fluidity cannot increase indefinitely with rate of shear but must approach a limiting value. Thus in the course of determining the degree of plasticity in this type of material by a flow-pressure curve, the material actually undergoes a change in state from one approaching that of a gel to one approaching that of a true liquid. Since this phenomenon is reproducible and caused by internal disturbances set up by the flow, it fits, in a broad sense, the not-too-well-defined concept of thixotropy. We see then, as McMillen (7) has already suggested, that plasticity and thixotropy are not readily separable phenomena. This state of affairs is further attested by the fact that both plasticity and thixotropy have been explained on the basis of the existence of an internal structure capable of being broken down under mechanical disturbance. Even identical natures of the structure have been postulated in explaining the two phenomena. Therefore, any real difference existing between these two phenoniena seems to lie primarily in the ease with which internal structure may be broken down and the relative times required for the structure to reform after mechanical disturbance is stopped. Those paints, which exhibit distinct rigidity when standing undisturbed in the can but which become definitely more fluid when stirred or brushed and remain so for an appreciable length of time, are known as “thixotropic” paints. It has not been generally recognized, however, that this property exists to varying degrees in practically all paints. McMillen (7) was the first to demonstrate this fact by studying the flow characteristics of paints at extremely low rates of shear. The concept of thixotropy as related to the consistency characteristics of paints has been of value primarily because it has called attention to the influence of the relative ease with which structure existing in a paint may be broken down, and the effect of the rate of reset upon other properties of the paint. The three consistency characteristics of a paint which have the greatest influence on both storage and application properties of the material are: 1. The degree of rigidity possessed by the paint as it stands undisturbed in the can. 2. The ease and extent t o which structure may be broken down and the fluidity increased,. 3. The rate at which the fluidity is decreased and rigidity developed as the structure reforms after cessation of mechanical disturbance.
Nature of Structure in Paints Two main theories are designed to account for the structural effects in paint which are responsible for both the plastic 1204
>m(: expected t,o have a~~preci:hie elkct upon both the x-my pattern and the ilielcotrii: I'~,c,rrnx: I . 'l'oan,c,s l'XXI>,~,'UM iiropertics of tlie dispersing inedium. The preliminary rez i i l t i of tliese rather cursory irivestigations were not sofSclmvnii~lTiiud !vas uscd by hiin ill studying rigidity in gelati18 fieiently pro~iiisingto encoiirage further work. The fioccdnsols. More recently I'ryce-Jones (9) elaborated on tlie thin or particle grouping theory of structure serves i1.s well as original apparatus in am ingenious nianiier and referred to it tuiy to explain the consistency effects obtained in inmt cases its an "electromagnetic thixoirometer." The spparatub :ind is the more generally accepted, especially by those wlio (Figpircs 1 and 2) is simple in form but yields results of value have had occasion to carry out extensive inicroscopic exaniinain studying the consisteney properties of paints from a prections of paints. tical standpoint: Structure may d s o be imparted to a paint througli the use of Tlx initrurnent consists ui a liollow metal cylinder (or paddle) *naps. Relatively insoluble (colloidally undispersible) soaps cosxially suspended from a steel wire of known rigidity and pararo pictured as setting up a colloidal strncture ainiilar in tially immersed in the oaint. The otlrer end of the wire is atinany respects to t.hat set np by ttie floccidated pigment. This soap s t r i i o t i m i s nrrperiniposmi upon that of the pigment and may he brought tillout tiy introducing the soaps separately a tFue Newto&n liquid and the torsion heid is displaces tlirouglr or through reactioiis between pigment and vehicle. In a a givcrr ;trigle, the r,ylindor mill rotate tlrrougli t,l,e same angle to similar mny gel structure may be introduced through t l i e w e roach wiuilibrium. T h e angulur rotation of the cylinder ia n n d ligbl I)PILII~. In the c a w oi R sol p s of liiglily cooked oils wliich in tlreinselves corit.niii dispersed r n ~ x s o by ~ ~ ~mirror l
The general plastometric and thixotropic properties of paints are discussed. These properties are interrelated and not readily separable. The conception of thixotropy as related t o the consistency characteristics of paints is valuable because it focuses attention upon the influence of the relative ease with which structure existing in a paint may be broken down in application and upon the effect of the rate of reset upon other properties of the paint. The three consistency characteristics of a paint which have the greatest influence on both storage and application
properties are described. Correlation is shown between such properties as pigment settling tendencies, brushability, and leveling with the thixotropic characteristics of the paint. The application of the simple Schevedoff torsion pendulum to the measurement of rigidity and thixotropic properties in paints is demonstrated. The plastometric and thixotropic characteristics of various types of structure existent in paints of the flat wall type are discussed, and the advantages of giving proper consideration to these properties in formulating paints emphasized.
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3essing yield value, the cylinder will rotate through an angle smaller than the angular displacement of the torsion head. From the difference of these two angles the torque remaining in the wire may be calculat'ed and the yield value of the sol determined from the known dimensions of the cylinder. In operating the instrument the torsion head is displaced through a given angle (usually loo), and the rate of relaxation of the cylinder and its ultimate position of equilibrium are determined by recording the position of the light beam on the scale at definite time intervals. Curves of the type shown in Figure 3 are obtained by plotting scale reading against time. The curve eventually flattens out and becomes essentially horizontal. The point at which the curve becomes apparently horizontal is taken as a measure of the rigidity of the paint, the higher the scale reading at this point the less rigid the paint. The movement of t'he cylinder in approaching the equilibrium point. can be made quite slow through proper choice of the torsion wire and the angle of initial displacement,. Thus it is believed that the rigidity as measured in this way represents closely that which the paint possesses in an undisturbed condition. This simple instrument does not give highly accurate measurements of yield value in paints nor does it even prove the existence of real rigidity. It is believed, however, to yield relative results indicative of the amount of rigidity possessed by a paint in the broader sense discussed previously.
A measurement of the rigidity of a paint as it stands undisturbed in the can is desired primarily because of the influence of t'his property upon pigment-settling characteristics of the paint. However, as will be discussed in more detail later, a measurement of the rigidity existing immediately after vigorous mechanical agitation and of the rate a t which the structure resets after agitation ceases is also desirable. The torsion pendulum apparatus permits measurements of this type merely by determining, as described above, the rigidity of a paint as soon as possible after breakdown and at definite time intervals thereafter. Curves obtained in this m y are shown in Figure 5 and will be discussed later. This procedure amounts to a measurement of those characteristics which determine the degree and character of thixotropy possessed by a paint. The measurement is probably far from ideal from the standpoint of the interpretation of the results in a quantitative way in terms of other paint, properties. I n the first place, the degree of breakdown produced in the structure before measurement is arbitrary (usually vigorous prolonged stirring in the can) and may differ appreciably from that produced by brushing a film of the material. I n the second place, the rate of reset of the structure in mass may differ appreciably from that when the mate-
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TORSION HEAD B TORSION WIRE WEIGHT @ U-SHAPED PADDLE
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MIRROR PAINT CONTAINER LIGHT SOURCE SCALE
FIGURE 2. DIAC~RAM OF TORSION PENDULUM
rialis in the form of a relatively thin film. Measurements of this type, however, prove of value in evaluating qualitatively the thixotropic characteristics of various paints, the effects of both pigment and vehicle modification upon these characteristics, and the control of uniformity in this respect. This instrument has been used in the laboratory of the New Jersey Zinc Company for the most part in studying the consistency characteristics of flat wall paints. For this pur-
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pose the cylinder was replaced with a flat U-shaped paddle. The results were more consistent and reproducible. With the cylinder an apparent slippage phenomenon was frequently encountered, and in some cases the paint failed to wet the cylinder walls completely. The paddle also has the advantage that it can be introduced into the paint with a minimum amount of disturbance. The use of the paddle does not permit the calculation of the results in terms of absolute units, but this is of little consequence because relative results are sufficient from a practical standpoint, absolute results being primarily of academic interest. A steel torsion wire 20 em. long and 0.05 cm. in diameter (eight-gage music wire) is generally used with this type of paint.
Influence of Rigidity on Pigment-Settling Tendencies The degree of rigidity possessed by a paint as it stands undisturbed in the can is of prime importance because the internal structure responsible for the rigidity also tends to prevent settling and hard caking of the pigment. In some cases the flocculated pigment structure may be sufficiently great to support its own weight, so to speak, and no measurable settling will occur. Even when the interlocking structure of the flocculated pigment is not sufficiently great to prevent settling entirely, a sufficient degree of flocculation may still exist in the settled pigment to prevent hard, dry caking. It is generally recognized that highly dispersed systems, although slow settling, tend eventually to produce hard, dry, settled cakes, because of the close packing of the particles. Any modification in the paint which tends to promote flocculation or set up additional structure, such as the introduction of the proper type and amount of soap, will tend to prevent hard settling. It is desirable, therefore, that sufficient structure t o prevent undesirable settling should exist in the paint as it stands undisturbed in the can. Since rigidity serves as a measure of the internal structure existent in the paint, it is highly indicative of the nonsettling qualities of the paint and serves as a means of controlling uniformity in this respect. This assumes, of course, a stable system which will not suffer marked chemical or physical changes on aging. The following data illustrate the influence of rigidity upon settling properties of the paint. A series of eight flat wall paints was selected, all with the same consistency as measured on the mobilometer or vacuum plastometer. These paints were stable to the extent that there were nn appreciable changes in consistency after a 3-week aging period. The degree of rigidity possessed by each was determined as described with the torsion pendulum, and the paints were
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evaluated for settling tendencies both by accelerated and shelf tests. The correlation of the degree of rigidity existent in the original paint and the relative settling tendency is as folloli--q: Paint No
134 132 79 89
90 128 88 122
Rigidity (Torsion Pendulum) Dynes/sq. cm.
4 7.3 5.1 5 5.3 9.5 20.6 40
120,
the latter, through its dispersing action, decreases the rigidity and tends to promote leveling. Why these two modifications affect the brushing properties thus is not obvious. We would be more inclined, from the previously discussed theory, t o expect their action t o be just the opposite of that actually produced.
Relative Settling Tendenciesa
Influence of Thixotropy on Leveling Properties Objectionable Equal to standard paint A Equal to standard paint A Slightly better than standard paint -4 Slightly poorer than standard paint B Equal to standard paint B Superior to standard paint B
a Measured in terms of two standard paints: ency: B = unusually good nonsettling.
A = normal settling tend-
With the exception of paint 132, the correlation between rigidity of the paint as it stands undisturbed and the settling tendency was good. The ordinary type of consistency measurement gave no indication as to difference in the paints in this respect. Differences in rigidity or body as judged by stirring in the can were not apparent.
Influence of Thixotropy on Brushability Given a paint which in the undisturbed condition possesses enough rigidity to prevent objectionable settling, it is desirable that the structure responsible for the rigidity possess another characteristic-namely, a certain degree of thixotropy. This characteristic when sufficiently pronounced has a marked influence upon both the “brushability” of a paint and its flow and leveling properties. Attempts to correlate plastometric data with brushability have met with little success. This may be due primarily to the fact that the consistency data have been confined to a range of shearing rates of a different order of magnitude than those actually involved in the brushing of a paint film. A study of the consistency characteristics of paints at these higher shearing rates is complicated by the difficulties of avoiding turbulence. The possibility that the plasticity characteristics of a relatively thin film of the paint possessing a free surface may be quite different from those possessed by the material in “mass,” along with the influences of such factors as the rate of evaporation of volatile matter and the ease with which the paint “wets” the surface to which it is being applied, all introduce complications which make the problem difficult. However, in the case of flat wall paints of the heavy body type the existence of thixotropy is definitely beneficial, as regards that property referred t o as “drag,” In the application of a paint, part of the work of brushing is consumed in overcoming viscous resistance t o flow, and part is used in breaking down structural resistance and decreasing the apparent viscosity of the paint. If the structure existent in the paint is broken down easily under the mechanical agitation of brushing and the fluidity is markedly increased, the energy involved in distributing the paint over the surface is less and manifests itself as a reduction in the so-called drag. Thus, two paints possessing equal consistency, as judged by inspection or by the usual type of measurement, will exhibit quite different degrees of brushability if one of the paints possesses a structure which can be more or less completely broken down a t the shearing rates involved in brushing, while the other does not. This simple theory cannot account for all observed differences in the brushing properties of different paints. It is common experience, for instance, that the addition of the proper amount of a stearate gel to a paint will often decrease drag whereas the addition of a bodied oil will develop drag. The former promotes rigidity in the paint and tends to cause poorer leveling;
Waring (13) made an analytical study of leveling and showed that surface tension is the prime force tending to cause brush marks to level out, and that this force is opposed by any rigidity possessed by the paint over and above a limiting value. The apparent fluidity of the paint under the extremely small shearing force tending to produce leveling will influence the rate a t which the film levels out but will affect leveling only if the viscosity is so great as to retard leveling t o the extent that the film sets before leveling is complete. Since the surface tension values of the various paint vehicles are of the same order of magnitude and can be modified only slightly, the consistency characteristics of the paint are the prime factors in determining leveling properties. Therefore, we are interested in consistency characteristics both immediately after vigorous agitation (brushing) and under very small rates of shear (leveling). For this reason the thixotropic characteristics of the paint are of great importance. In order t o exhibit good leveling, a paint must possess a relatively high fluidity under small shearing forces immediately after brushing, and the rate of reset of structural resistance to flow and the development of rigidity must not be so fast as to retard or prevent leveling before gelation of the film begins. On the other hand, proper I
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balance must be obtained, for, if the fluidity of the paint becomes too great upon agitation and the rate of development of structural resistance to flow too slow, sagging may occur. Outside of the influence of these direct consistency effects, such factors as the rate of loss of volatile matter and the rate of gelation brought about by the ordinary processes of drying are, in many cases, of considerable influence.
Correlation of Thixotropic and Paint Properties McMillen (6) demonstrated the importance of the thixotropic properties of enamel-type paints in regard to their leveling characteristics. The data presented here demonstrate the influence of the thixotropic properties of flat wall paints on the brushing and leveling characteristics of paints of this type. Figure 4 shows the plastic flow curves of a series of eight flat wall paints as obtained through the usual range of shearing rates on a vacuum plastometer. With the exceptions of B and C, these paints would be classed as heavy-bodied. In the following table these paints are
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arranged in relative order froin best to poorest, in regard t o brushability and leveling, as judged by paint-outs : Brushability -4, B,C
H, F
G D, E
Leveling 8 ,B C F G. H D
E
These paints exhibit good noiisettling properties with the exception of C and B , and in no case is settling objectionable. The paints are all similar as regards the general nature of the nonvolatile portion of the vehicle, the kind and amount of thinner, and drying time, so that the influences of factors other than the plastometric properties should be as nearly comparable as possible. We would be inclined, after inspection of the plastic flow curves, to group these paints as follows: A and C; B, F , G. and H ; D and E; and to expect the paints within one of these groups to exhibit similar brushing and leveling properties. This is actually found t o be the case only in a general way. Paints D and E are outstanding in that they exhibit markedly poorer leveling properties than any of the others, with the possible exception of paint G, and possess by far the greatest amount of drag under the brush. Paints A and C are similar in that they both possess good leveling ( A is outstanding in this respect) and excellent brushing. However, the marked difference in body of these two paints as they stand undisturbed in the can is not apparent from the flow curves. In the case of the intermediate group the discrepancies in brushing and leveling properties within the group are pronounced. Paint B possesses leveling properties slightly better than C and equal t o 4 , and exhibits no more drag under the brush than either A or C. On the other hand, paints G and H are only slightly better in leveling properties than D and E. Paint G also possesses rather marked drag and is not a great deal better than D and E in this respect; thus, within this group brushing properties vary from very good to definitely poor. Figure 5 shows the thixotropic characteristics of these same paints as obtained in measurements made ~ i t hthe torsion pendulum. Only a section of the curve as obtained in Figure 2 is plotted here. In each case the intercept of the curve with the heavy vertical line represents roughly the point a t which the curve becomes essentially horizontal The curves marked G (gel state) in each case indicate the rigidity possessed by the paint in an undisturbed condition after prolonged standing in the can. Those marked S (sol state) indicate the degree of rigidity existent after vigorous mechanical agitation within the time required to make the measurement. Rlo (recovery after 10 minutes) represents a measurement started 10 minutes after agitation was ceased. This time interval was chosen because it was observed that 10 minutes after application no further leveling was detectable in this group of paints. -4study of the thixotropic characteristics of these paints as indicated by the simple measurements described will reveal quite good correlation between these characteristics and the brushing and leveling properties. Paints A , B , and C appear t o be easily brushable because they are capable of being reduced through mechanical agitation to a state of relatively high fluidity with little structural resistance t o flow.. The good leveling properties would appear to be due both to the relatively high fluidity obtained immediately after agitation and the relatively slow rate of reset of structural resistance to flow. The relatively poor leveling properties of paints G, F , and H in comparison to A , B , and C are probably, for the most part, due to the rather rapid rate of reset of structure in the paints after agitation is stopped.
The reasons for the outstandingly poor brushing and leveling properties of paints D and E are obvious. Paints A , B , C, G, P , and N would be considered thixotropic paints in the common usage of the term as applied to flat wall paints, with the possible exception of C because of its relatively low rigidity as it stands undisturbed. Paint. D and E, on the other hand, are more or less typical of the so-called false-body paints. A "false-body" paint is characterized by a relatively high apparent viscosity a t extremely low rates of shear, which decreases rapidly as the rate of shear is increased so that at moderate shearing rates it becomes relatively fluid. Such paints possess considerable rigidity as they stand undisturbed in the can but, when stirred a t a normal rate, offer relatively little resistance to the stirring. Once the stirring is stopped, however, the high degree of rigidity of the paint in the undisturbed condition is immediately apparent. The false-body paint is therefore thixotropic in the broad sense of the definition, but it differs primarily from those paints which are usually referred t o as thixotropic in that the rate of reset of structural resistance t o flow and the development of rigidity after agitation are rapid. The (5'curves for paints D and E are not as indicative of the degree of fluidity exhibited by these paints immediately after agitation as they might be, merely because the initial rate of reset of structure within the paint is so rapid. This is probably also true t o a lewer degree of paints H , F , and PAINTS
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S - IMMEDIATELY AFTER VIGOROUS MECHANICAL 4 i i T A T I O N .
Rlo- RECOVERY IO MINUTES AF-ER CESSATION O f MECHANICAL AClTATlOH
, FIGURE5. RELAXATION CURVESREVEALINGTHrsorRoPIc PROPERTIES OF PAISTS A
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INDUSTRIAL AND EXGINEEHING CHEMISTRl
G. The structural resistance to flow of those paints classified as having false body uwally cannot be reduced through agitation t o the extent that it can be in the pronouncedly thixotropic paints. Actually, then, the real difference betn-een false-body paints and so-called thixotropic paints is a matter of degree; therefore, there can be no very definite classification. Paint G probably represents a border line case between the two general types. Because of the rapid rate of reset of structure after agitation, the false-body type of paint normally exhibits poor leveling. Paints A , B , and C in the good-leveling group exhibit surprisingly little rigidity immediately after agitation, as evidenced by the fact that a scale reading of 24 (designated by the broken horizontal line, Figure 5) represent,q zero rigidity. The fact that paint A possesses leveling properties equal to the other two in this group indicates that a degree of rigidity lower than that possessed by this paint immediately after brushing is not essential for satisfactory leveling. A lower degree of rigidity might tend to produce sagging under certain conditions of application. I n this same group paint d has a degree of rigidity as it stands undisturbed in the can comparable with the paints in the other two groups and therefore possesses equally excellent nonsettling characteristics. However, because of its marked thixotropic properties, its brushing and leveling characteristics are comparable to the lower bodied paints B and C. The del-elopment, of a high degree of thixotropy in the heavy-bodied type of good, nonsettling, flat wall paint represents the case in which the beneficial effects are most obvious. By virtue of the thixotropy characteristics a lesser degree of compromise is necessitated between nonsettling and leveling properties, and the ideal in both respects can be much more nearly attained in the same paint.
Thixotropic Characteristics of Structural Types As has already been pointed out, rigidity in a paint may be developed readily by fostering flocculation of pigment particles through control of pigment-vehicle wetting relations and through the introduction of superimposed soap or gel structures. Obtaining a relatively high degree of rigidity of the type which possesses marked thixotropy is, however, not so simple. Thixotropy requires a type of structure which is metastable in its nature. The forces tending to hold the structural units-that is, pigment particles, soap, or gel micellae-into a more or lese continuous structure must not be so great as to prevent dispersion of these units when mechanically agitated, nor must they be so great as t o cause too rapid reformation of the continuous structure when agitation is stopped. In attempting to promote marked thixotropy through the ordinary type of flocculated pigment structure, impractical pigment-nonvolatile ratios are usually required because of the high pigment content needed t o obtain the relatively high rigidity in the undisturbed condition. Under these conditions the pigment structure is not completely broken down with agitation, and the rate of reset is normally rapid so that the tendency is t o produce paints of the false-body type rather than of the highly thixotropic type. It is not impossible to obtain a reasonable degree of thixotropy in this way, but the condition is unusual. Through the use of superimposed soap structure, rigidity in the undisturbed condition can generally be obtained a t lower pigmentation than when relying on flocculated pigment structure alone. The quantity of soap required to promote amreciable riniditv is usuallv in excess of t h a t formed with the normal pigments and vehicles. A higher degree of structure is obtained through (a) direct addition of soap (such as a stearate gel), (ti) the use of a highly reactive pig.A
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ment (such as special lithopones purposely designed to be reactive), or (c) the use of a fatty acid which will react with one of the pigment constituents. The latter two methods are often more efficient in promoting structure because of the pigment-flocculating tendencies of certain soaps when formed in excessive amounts at the pigment-oil interface. The colloidal dispersibility of the soap in the vehicle seems to be a prime factor in determining the nature of the structure. An intermediate degree of dispersibility seems necessary if sufficient rigidity is t o be obtained to prevent objectionable pigment settling and a t the same time produce the best possible leveling. In any case, it is extremely difficult to obtain a high degree of leveling with this type of structure if marked rigidity is developed. Highly thixotropic paints have been produced through the use of gel structure promoted by the introduction of highly cooked oils into the paint. The greatest difficulty experienced with this type of structure is the lack of stability as regards aging characteristics. One of the most effective ways of producing marked thixotropic properties in a paint is through the introduction of water. Rhodes and Jebens (11) studied the effects of water additions to several paint systems on the plastometric properties and found a general tendency for water, even in relatively small quantities, to increase markedly the yield value and to decrease fluidity. Ryan, Harkins, Gans (12) and others pointed out the marked flocculating action of water on pigment suspensions in various organic media. When water is added to a paint, two main phenomena may occur: (a) a straight emulsification of the water in the organic vehicle and (b) flocculation of the pigment particles. These two phenomena may occur alone or together, the relative predominance of one over the other depending primarily upon the pigment-vehicle wetting relations of the paint system. The straight emulsification phenomenon predominates in those systems in which the pigment is well wetted by the organic medium (low interfacial tension-high adhesion tension), and tends t o increase the yield value or rigidity and decrease the fluidity of the system. I n such a system relatively large amounts of ~ a t e (2 r to 3 per cent or greater) are required to produce appreciable changes in consistency. .Is the wetting of the pigment by the vehicle becomes poorer (that is, in teriis of relatively high interfacial tension-1oTv adhesion tension), the second phenomenon becomes more and more predominant, and the tendency is to produce a false-body type of consistency. If pignieritvehicle wetting relations are properly adjusted, the introduction of small amounts of vater (1 per cent or less of the weight of the vehicle) will promote the formation of a type of flocculated pigment structure of marked thixotropic characteristics. In order to produce such a system nitli available vehicles, a pigment ( I d ) of definite but not too great hydrophilic tendencies seems to be required. Studies of these systems indicate that the introduction of mater into the system promotes a flocculation of the pigment due. iii part a t least, t o the tendency of the hydrophilic pigment to crowd into the water-oil interfaces. If the hydrophilic tendency of the pigment is not too great with respect to the organic vehicle, mechanical agitation will cause the pigment particles t o redisperse in the oil phase, thus destroying to a large extent the rigidity due to the flocculated pigment structure which exists in the undisturbed condition. Khen agitation is stopped, the pigment will gradually reflocculate. Thus, conditions are set up which tend t o develop the metastable type of structure required to produce marked thixotropic characteristics. A pigment too organophilic in nature will not produce the type of structure discussed above, even in the poorer wetting types of vehicles, because it possesses little or no tendency to qeek the water phase
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With a pigment of definite hydrophilic properties, the relative hydrophilic and organophilic tendencies with respect to the water and oil phases may be so balanced by manipulation of the wetting properties of the latter phase as t o produce varying degrees of thixotropy in the system. The water is usually introduced into these paints in the form of a dilute solution of a soap or electrolyte. Although the presence of the soap or electrolyte no doubt has some influence on the tendency of the water phase to replace the oil a t the pigment interface, the main function of these agents appears to be merely to aid in distributing the water more uniformly through the system by means of the emulsifying action.
General Discussion An attempt has been made to point out the nature of thixotropy as it exists in paints with special reference to the flat wall type. Because of the marked influence these thixotropic characteristics have on certain paint properties, they should be given consideration in the formulation of paints not only of the heavier bodied, flat wall type but in other classes as well. With the development of pigments of widely different properties and with the increase in the types of vehicles used, oil absorption values give no indication of the type of consistency produced and can be taken as only a very general measure of the relative consistencies two pigments may produce in a given vehicle. Also, the rheological properties of paints are somewhat complex, and more detailed consideration must be given them if a complete understanding of the influence which they have on other paint properties is to be had. The control of these properties is, to a large extent, a matter of the formulator's art,
but a better understanding not only of the factors determining the rheological characteristics of a paint but also of the influences these factors have on other paint properties will certainly lead to greater latitude in paint formulation.
Acknowledgment The author wishes to acknowledge the assistance of L. D. Grady, Jr., in the preparation of this paper, and the criticisms of other members of the Research Division of the New Jersey Zinc Company. He is indebted to Victor Bachman who made the rheological measurements.
Literature Cited (1) Booge, Bingham, and Bruce, Proc. Am. SOC.Tevfzng Materiala, 22,Pt. 11,420 (1922). (2) Cunningham, J . Phys. Chem., 35, 796 (1930). (3) DeWaele and Lewis, Kolloid-Z., 54, 175 (1931). (4) Green, I N D . E N G . CHEII., 15, 122 (1923). ( 5 ) Green and Haslam, Ibid., 17, 726 (1925). (ti) McMillen, Ibid., 23,676 (1931). (7) McMillen, J. Rheol., 3,Nos. 1 and 2 (1932). (8) Peek and hlclean, Ibid., 2, 370 (1931). (9) Pryce-Jones, J . Oil Colour Chem. Issoc., 17,305 (1934) (10) Reiner, Kolloid-Z., 54, 175 (1931). (11) Rhodes and Jebens, J . Phys. Chem., 35,383 (1931j. (12) Ryan, Harkins, and Gans, IKD.EKG.CHEM., 24, 1288 (1932). (13) Waring, J . Rheol., 2,No.3 (1931). (14) Werthan, S.,Oficial Digest Fed. Paint &. Varnish Production Clubs, March, 1936; Am. Paint J.,20, 18 (March 16, 1936). (15) Williamson, IND.ENG.CHEx., 21, 1108 (1929). (16) Williamson, Patterson, and Hunt.,Ibid., 21, 1111 (1929). (17) Wolff and Zeidler, Farben-Ztg., 38, 1495-7 (1933). R E C E I V ~April D 21, 1936. Presented before the Division of Paint and Varnish Chemistry at the 91st Meeting of t h e American Chemical Society, Kansas City, hlo., .4pril 13 t o 17, 1936.
DISTILLATION EFFICIENCY IN 3-AND 6-MM. FRACTIONATING ARTHUR ROSE The Pennsylvania State College, State College, Pa.
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HE experiments described here consisted in the determination of the number of theoretical plates in glass columns, 3 and 6 mm. in diameter and 30.3 em. (one foot) high, a t all vapor velocities from nearly flooding to as low as possible, when the columns were both empty and packed. Figure 1 indicates the construction of the columns. The mixture used was benzene-carbon tetrachloride, and analyses were made by refractive index. The number of theoretical plates was obtained by reference to the vapor-liquid equilibrium diagram for the system (1, 2 ) . One bheoretical plate was subtracted to correct for the fact that samples were taken from the still rather than the bottom of the column. The rate of flow of liquid down the column was determined sometimes by counting the drops per minute falling from the calibrated tip a t the bottom of the column, and sometimes by the heat input. In the latter case it was necessary t o correct for heat loss from the still. Samples from the still were withdrawn by applying a slight vacuum to the still take-off line until liquid was drawn above the side arm on thib line. The vacuum was then re-
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leased, and the few drops of liquid t r a p p e d in the side arm were taken off and analyzed. Samples from the top of t h e c o l u m n w e r e withdrawn simply by opening slightly the stopcock on the side arm there. No g r e a s e was used on any of the stopcocks, and the first two drops of a sample were d i s c a r d e d to avoid possibility of contamination. The rate of taking a sample was sometimes d e t e r m i n e d , not by the position of the stopcock plug in its socket, but by the slope of the take-off line and the rate of flow of l i q u i d p a s s i n g the side arm. The rate of taking a sample was always so slow that the reflux ratio was greater than 70 t o 1.
