January, 1927
I~VDi7STIZI.IL .LVD E.VGIiVEER1NG CHEMISTRY
51
PARTICLE SIZE, DISTRIBUTION AND ITS FUNCTION I N PAINTS AND ENAMELS A Relative Method for Determining Particle Size of Pigments' By G. F. A. StutzZ and A. 11. PfundJ
111s paper describes nn apparatus developed to mensure accurately tlie relative avorage particle size of :L pigment,, by detennining the opacil,y of 3 suspcnsioii of
T
the pigment. Previous ohscuriug power mctIiocls*.~liave used the depth of suspension necessary to ohscui.~a lninp filament as a measure of particle size. In tliis app:xatus die amount of light transmitted by a suspension is phot,omet,ered and used as a measure of tlie average prrrticle size of tlie suspended material. When the particle size is greater than the wave lonxth of light, tlie weakening of a beam of light passing through the suspension is due to reflection of the light by the surfaces of the pigment particles (a very efficient process). Decreasing the particle size increases the surface available, sild hence the amount of reflection, resulting i n s decrease in tho amount of light transmitted by the suspension. However, this cnndition does not hold if the particle size is small in comparison with the wave length of light. When a ray of light meets such small particle, the particle acts as a source and scatters some of the light. The intensity of this scattered light is proportional to the sixth power of the diameter of the particle. Now, if the particle size is further reduced, while the number of particles is greatly increased, the efticiency of the scattering process becomes so low that the amount of light transmitted b; the su8;ension is greatly increased. It is therefore evident that, starting with a particle &e large in comparison with
Apparatus
A photograph or the npparnlus is shown iii Fibl'ipure 2 gives ib ~ii:rpr:i~~lm:if.ic representation of tho iustmwit. ;\ r i l h i i R1:tnwnt l:~irp,L,, is connected to a B-volt w t i Lmttcry ~ ~ t,lmii:Ii a n titrimeter and variable resistance. ic ctirrctit llirwg!i the 1:mi;) ciin be kept constant, and at airy desired v:iliits. ovci' 1~1tig prriods of time. Light from this I m p is rendered psnllel hv means of the lens A , and passes through a susponsioiiof the pigment to he measured, contained iqtlic cell F. This cell is of brass, 3 inches long, with glass
Flgurs 2-Diagram
Figure I-Photograph
of Pswtlde Slze Apparatus
the wave length of light, and decreasing the particle fiiae continuously down to colloidal dimensions, the intensity of the light transmitted must. pass through a minimum for some definite part,icle size. At this "best" particle size the hiding power of the pigment is a maximum. L This investigation W V ~ Sbegun a t the Johns Hopkins Unlversify and completed at the Research Lnborstory of The New Jersey Zinc Compsny. 8 Investigator,Research Division, The New Jersey Zinc Company. Professor of Physics. Johns Hopkini University. L e a k and Baker, Ttrrs J o u e n ~ 12,830 ~. (1920). Vast, I n d i a Rllhb~rWorld, 66, 247 (1922).
of Parncle Sire AppaBrstus
windows cemented in each end. The parallel light transmitted through the cell is focused by means of the lens C in the plane of the filament of the lamp La. The total reflection prism, P, refieets the beam at right angles to its initial direction. The eye, placed at the aperture 8, focuses by means of the lens D on the filament of lamp L,, superimposed on the image of the filament of lamp L I . 9screen of red or green glass is introduced at the aperture S, to limit the wave length of the light used and eliminate color differences. Lamp Lz is the lamp from an optical pyrometer eyepiece. It is connected to a variable resistance, dry cells, and milliammeter, E. By rotating the disk at the side of the box E, the current through the filament of lamp LZmay be varied until the filament equals in brightness that of the image of the filament of lamp LI. The intensity of the light transmitted by the suspension is then measured in terms of milliamperes of current through the lamp La. The two objects to be matched in brightness are similarthat is, both are lamp filaments. There is consequently but little color difference and photometric settings can be made with ease and accuracy. The fact that the light heam, while passing through the suspension, is parallel makes each particle effective in reducing the light intensity, and is therefore more sensitive than the use of a diverging light beam.
INDUSTRIAL AND ENGINEERING CHEMISTRY
52
Procedure
Thus far the instrument has been used chiefly to measure zinc oxides. For this material the suspension, which contains 0.0000694 gram of zinc oxide per cubic centimeter, is made as follows: 0.25 gram of zinc oxide, 0.10 gram of gum arabic, and 0.05 gram of saponin are ground for 8 minutes with 1 cc. of 0.05 N potassium ferrocyanide solution, in a glass mortar with a glass pestle; the paste is then diluted with 90 cc. of water and shaken vigorously. Five cubic centimeters
Vol. 19, No. 1
The pigments used in determining the curves are tabulated in Table I. All readings were taken with a green glass screen, transmitting an average wave length of 5400 A. Table I-Zinc
Oxides of Different Particle Size8
AVSRAOB PIGWNT
~ a o c s s sOF MANUPACTURB
A B
C D E P
c H
J
Reheated American Reheated American Reheated American Reheated American Special American French French Kadox Kadox
METHOD
' ~ ~ s ~ OF , "SIZE " c
DBTN.
1.18 0.679 0.484 0.436 0.276 0.246 0.238 0.180
Green Green Green Green Green Green Green
0.125
Count Count
Discussion
' I I .4 d PART~CLE SIZE
U35d
1
I
'
.3
Curve 1-Particle
-6
.7
In
.8
I
1
.9
10
If
MU (.001m m.1
1.2
The opacity of a suspension of fine material is dependent upon the particle size of the material, the difference in refractive index of dispersed material and dispersing medium, the degree of absorption of light by the particles of the material, and the wave length of the light used in the opacity measurement. Therefore, in measuring particle size by the present method, it is only possible to compare different samples of the same material. A light-absorbing material, such as l a m p black, cannot be compared with a light-transmitting pigment, such as zinc oxide; nor can zinc oxide, having a refractive index of 2.01, be compared with zinc sulfide, having a refractive index of 2.37. A separate calibration curve, similar to Curve 1, is necessary for each kind of pigment to be measured. However, once this curve is established a measure of average size can be made in less than ten minutes.
Size-Relationship, Refractive Index of Vehicle 1.33
of this suspension are diluted to 200 cc. with water, and this latter is used in the determination. It is found that two observers can check one another to within one division (2 milliamperes) on the same suspension. Furthermore, two separate suspensions made of the same material will check within one and one-half divisions (3 milliamperes). In order to keep the instrument in constant adjustment, it is only necessary to keep the current through the lamp L1 constant. However, it is more satisfactory to make use of a constant standard of turbidity, and to adjust the current through L1 until this standard gives a definite reading. As a standard, a piece of Corning turbid glass (G632J) approximately 3 mm. thick is placed in the cell and the cell filled with clear water. Results
Curve 1 shows the results obtained on a series of zinc oxides dispersed in water (refractive index 1.33). The ordinates are the readings on the opacity instrument in milliamperes of current through lamp LP. The abscissas are particle size readings in microns. For materials over 0.2 micron the particle size was determined by the method of Green.6 For samples under 0.2 micron the particle size was determined by the count method using a dark-ground illuminator and counting chamber. The diameter so determined is based on the number of particles per unit volume and is not entirely comparable with the diameter obtained by Green's method. However, it is the only satisfactory absolute measurement that has been obtained for this very fine material. Curve 2 shows the same series dispersed in a mixture of 50 per cent glycerol and 50 per cent water, having a refractive index of 1.40, while Curve 3 shows the oxides dispersed in pure glycerol, having a refractive index of 1.47. 4 J . Franklin Ins;., 194, 637 (1921). The writer is indebted to Mr. Green, who supplied these samples of zinc oxide measured by his method.
Curve 2-Particle
Size-Opacity Vehicle Relationship, 1.40 Refractive Index of
Curve 3-Particle
Size-Opacity Relationship. Refractive Index of Vehicle 1.47
Since the calibration curve is in the shape of a loop, it is possible for two pigments of different particle size to have the same reading. For example, reading 410 corresponds to a particle size of 0.48~and also of 0 . 1 4 ~ . Microscopic examination will usually determine on which side of the curve a pigment is. A more rapid method, however, is to replace the green screen in the eyepiece by a red one, transmitting light of average wave length 6200 b. If the pigment is on the large size portion of the curve, it will give a lower reading-that is, it is more opaque, If the pigment is on the small-size portion
January, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
of the curve, it is more transparent to red than to green light and consequently will give a higher reading. The relationship drawn between particle size and opacity (Curve 1) indicates that there is one particle size a t which the opacity of a pigment attains a maximum. It is seen that this size corresponds to that of French process zinc oxide. Any oxide larger than this decreases in opacity, and any oxide smaller than this also decreases in opacity. On the right, or large-size, arm of the curve the particles cause opacity by reflecting light from their surfaces. Therefore the smaller the particle size the greater the surface available and hence the greater the opacity. However, below the particle size giving maximum opacity the particles are so small that they n o longer can reflect the light, but instead only scatter or diffuse the light, with the result that the smaller the particles
s3
become the less will be their opacity until, for very small sizes, the suspension will be optically clear. Curves 2 and 3 clearly show that the pigment having a particle size of 0 . 2 4 will ~ have a maximum opacity, no matter what the relative refractive indices of pigment and vehicle. Therefore, zinc oxide, having this particle size, will give a maximum opacity when the pigment is dispersed in linseed oil (refractive index 1.475). Again, any white (transparent) pigment having this particle size will show the maximum opacity obtainable for that pigment. This may not be true for an absorbing material such as a black. Evidently, a very definite relation exists between the size of the particles and the wave length of the light for which the pigment will show a maximum opacity. This is being made the subject for further investigation.
The Relation of Yield Value to Particle Size By Henry Green and George S. Haslam THENEWJERSEY ZINC Co., PALMERTON, PA.
HE success of an investigation of yield value in its rela-
T
tion to particle size depends largely on the correctness with which the investigator visualizes the nature of the pigment vehicle structure. For this purpose, the use of the microscope is a sine qua non; attempts at visualization without the aid of microscopy usually lead to misconceptions such as the possibility of cubical and trigonal packing. Unfortunately, the nonuniformity of particle size, the irregularity of particle shape, and the force of flocculation make symmetry of structure (of the cubical and trigonal kind) impossible, and so the investigator is compelled to work with a model that is dficult to treat theoretically.
Figure 1-Diagram of Structure of Paint The pigment particles are grouped into flocculates and the flocculates, being in contact with one another, give a "continuous" structure to the mass.
Assume that there exists a mass of spheres or particles of equal diameter. These spheres can be packed cubically or trigonally. If we now imagine that each particle changes in size so that ultimately no two of them are exactly of equal ,diameter, then symmetrical arrangement becomes impossible; but if we next assume that each sphere changes into some irregular shape or form, and that these forms are suspended in a liquid medium where flocculation must occur, then the utter impossibility of symmetry becomes quite apparent. The act of incorporating pigments into a vehicle necessarily produces motion in the form of currents, so it is evident that if the pigment-vehicle ratio is sufficiently great the particles will collide, and when this happens the flocculating force will cause them to adhere. This random adherence of the particles into groups or flocculates produces a heterogeneous
structure-that is, one where the particle density (number of particles per unit volume of mixture) varies throughout the mass-the density being greatest in the flocculate and perhaps zero in the spaces between the flocculates. In a plastic mixture the flocculates are so close that they touch or interlock, thus making the structure continuous, though still heterogeneous. Nofe-A flocculate must not be thought of as a hard aggregate, but rather as a loose cluster of particles easily dispersed by stirring, and easily reformed again as motion ceases.
Figure 1 is a diagram of the structure of paint in cross section. This much the microscope can reveal, but it can tell us nothing about the nature of the flocculating force, or even indicate where it resides. About this force we can only speculate. Sulmanl has given a clear-cut description in which he shows that interfacial tension is sufficient to cause flocculation. This theory will be adopted here as a working hypothesis. Flocculation produces structure; the resistance to shear offered by this structure causes yield value;* that yield value is finite-that is, yield does not take place under infinitely small pressure as in liquids-is due, presumably, to the fact that the interfacial tension is finite. In the ultimate analysis there can be very little doubt that yield value arises from interfacial tension, but there is an intermediate step where it becomes necessary to think of this resistance as a quasifrictional phenomenon.
Sketch in Cross Section of T w o Layers of Particles When these layers are in contact (adjacent) a quasi-frictional resistance to shear arises.
Figure 2-Diagrammatic
The particles in each layer in Figure 2 are held together by the force of flocculation. Assume that the layers come together so that the projecting particles of a fit into the depressions of b. If a shearing force is applied so that a moves relative to b, in the direction of the arrow, then there will be a collision of particles which will stop flow unless the applied force is great enough to cause a rearrangement of the particles. 1 Bull.
I n s f . Mining M e f . , 18a (1919).
* Green, THISJOURNAL, 16, 122 (1923).
