NATIONAL BUREAU OF STANDARDS REPORT

5424

A METHOD OP MEASURING THE DISPERSIBILITY

OP

LIGHT COLORED POWDERS

by

C. So McCamy and T. G. Lee

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

A METHOD OF MEASURING THE DISPERSIBILITY OF

LIGHT COLORED POWDERS

by

C, S. McCamy and T. G. Lee

Bureau of Ships

Department of the Navy

Code 538

Index Number NS-I83 -OOI

IMPORTANT NOTICE

NBS PROJECT NBS REPORT

1002-30-4873 August 6 , 1957 5424

for

NATIONAL BUREAU OF 5 intended fur use within tli to additional evaluation an

listing of this Report, eithe the Office of the Director, however, by the Governme

to reproduce additional co t <NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
.
A METHOD OP MEASURING THE DISPERSIBILITY OF

LIGHT COLORED POWDERS

ABSTRACT

A small sample of powder was dropped in a specified

manner from a dispenser at the top of an enclosure. The

reflectance of the powder distribution at the bottom of

the enclosure was measured with respect to a layer of the

same powder in a specified form. One hundred times the

reflectance measured in this way was the measure of dispersibility, The dispersibility of powdered sodium bicarbonate was studied as a function of particle size and

the dispersibility was found to be a maximum for powder

with a mean particle diameter of 15 microns.

1. INTRODUCTION

When a small quantity of a powdered material is thrown

into the air, the particles may cling to one another or

they may become widely distributed. The characteristic

tendency of a powder to become widely distributed in the

air is referred to here as the dispersibility of the powder.

It is the purpose here to give a definition of this quantity in terms of a specific method of measurement.

The need for a precise definition of dispersibility

and a method of measurement arose in the course of a study

of the effectiveness of various fine powders as fire extinguishing agents r In a series of fire extinguishment

experiments, various powders were roughly graded according

to apparent dispersibility (l) .--- A pinch of powder was

dropped, its behavior was observed as it fell through the

air, and after several observations the powder was graded

A, B, or C. The fire test results showed a general correlation between the effectiveness of the powders and the

dispersibility rating. Further investigation of this

relationship and the relationship between particle size

and dispersibility required a more precise and objective

measure of dispersibility.

---Numbers in parentheses refer to references at the end of

this report.
- 2 -

The method developed by Puri and Keen (2) was not

directly applicable because it involves the preparation of

a dispersion of the powder in water and many of the materials of interest were soluble in water. The powders could

have been dispersed in a liquid medium in which they were

not soluble but the correlations if any, between the dispersibility in liquid and that in air was unknown. The

method developed appeared to measure directly the property

of interest in fire research work and required about onetenth as much time as the method of Puri and Keen,

Andreasen ( 3 ) measured dispersibility by dropping one

gram of powder about 2,5 meters through still air and noting

the weight of powder reaching the bottom, as a function of

time. Dispersibility was evaluated on the basis of the

percent by weight remaining suspended in the air after 6

seconds. This method makes no distinction between different

materials if all of the particles reach the bottom in less

than or more than 6 seconds. Even if the material remains

partly suspended at 6 seconds, this criterion would not take

into account the conditions prevailing at times before or

after 6 seconds. Thus, the observed dispersibility index

might be the same if 90 percent of the material reached the

bottom just after 5 seconds as it would be if 90 percent

reached the bottom in 1 second. It would seem that ideally

the effect of all parts of the sample should, somehow, be

taken into account. This ideal condition is more closely

approximated In the present method than in methods previ=

ously described.

2, PROCEDURE

The method developed was a quantitative version of the

original method of estimating dispersibility by dropping

powder and observing its behavior. The procedure consisted

of two parts, dropping a measured amount of powder under

controlled conditions and measuring the distribution of the

powder deposit.

Since various powders were to be compared with respect

to their tendency to become spacially distributed, comparisons were made on the basis of equal volumes (0.7 cm3) of

solid crystalline material. Because weighing is simpler and

more precise than measuring the volume of Irregular solids,

the volume of each sample used was measured indirectly by

weighing.
-3

-

The weighed sample of powder was placed in a brass

cylinder with an inside diameter of l/8 in, and a length

of 1 1/4 in. The cylinder was open at one end and was

equipped at the other end with a close-fitting piston and

a thumb screw having 1+0 threads per inch to advance the

piston. A wire 0,0179 in diameter (25> A.W.G.) was placed

diametrically across the open end of the cylinder to break

up the powder stream. The cylinder was held in a fixed

position with its axis and the cross-wire horizontal and

the screw was turned by hand. The powder advanced toward

the opening until a small amount projected over the edge

of the cylinder , then a section of the powder would fall

off. The size of the pieces which fell varied over a wide

range with any given powder but seemed to be independent

of the rate with which the screw was turned from intermittent operation to l5 rpm. This type of behavior served

to make the method insensitive to variations in the discharge rate. The rate used was about 15> rpm.

To prevent the influence of extraneous air currents,

the powder was dropped within an enclosure. The enclosure

was a cylindrical fiberboard drum with a diameter of 17

in. and a height of 32 in. 9 fitted with a thin metal top

and bottom. The powder dispenser was placed in a holder

at the center of the top of the enclosure. A glass window

in the holder permitted observation of the falling powder.

The enclosure could be opened to permit the removal of

powder after each run. To allow time for the enclosed air

to become still, the dropping of powder was not started

until one minute after the drum was closed.

The reflectance of the powder distribution relative

to a standard surface of the same powder was taken as a measure of dispersibility. The enclosure was designed so

that the reflectance measurements could be made on the

powder where it fell. After the powder was dropped and

one minute was allowed for the last of the powder to reach

the bottom, the powder dispenser was replaced by a photovoltaic cell, Weston type M-856. The photocell was mounted

in one end of a cylinder 1 7/32 in. in diameter and 5 l/4 in°

long painted flat black inside and a 3/8 in. aperture was

placed at the other end to limit the field of view. The

mirror filled the field of view of the photocell and reflected only the black interior of the enclosure and the

image of the photocell. For the reflectance measurements,

the powder distribution was illuminated through two Lucite

windows of 2-in, diameter located on opposite sides of the

enclosure with centers 4 in. above the mirror surface.

Light was provided at each window by a 3^0 watt photoflood
-4

lamp operated at constant voltage . The positions of the lamps

were adjusted slightly when necessary to provide the same illumination from one test to the next. For this purpose, the

illumination was measured on a matte-white enameled metal disk

4 in, in diameter placed at the center of the mirror. The

output of the photocell was measured with a Leeds and Northrup

Microvoltmeter. The response was practically linear in the

range used. When the response to the dispersed powder distribution had been obtained, the powder was removed, the mirror

was cleaned, and the response to a standard distribution of

the same kind of powder was obtained. The standard distribution was a uniform layer of the powder I/I4

. in* thick in a circular pan with a diameter of 3 3/8 in. placed at the center

of the mirror. One hundred times the ratio of the response

with the dispersed powder distribution to the response with

the standard distribution was designated the "dispersibility"

of the powder.

3. RESULTS

Most of the measurements have been on commercially

prepared sodium bicarbonate in the form known to fire protection engineers as "dry chemical" and on the separate

fractions of this material obtained by sieving or elutriation. For this material, some correlation has been found

between mean particle diameter as determined by an airpermeability technique (Fisher Sub Sieve Sizer) and the

dispersibility. The dispersibility was a maximum for powder

with a mean particle diameter of about 15> microns. Powders

having about half that particle diameter or about twice

that particle diameter had a dispersibility only about half

as great, Andreasen found a similar correlation for a ceramic powder, although different particle diameters were

involved and the definition of dispersibility was different.

These results and the values of dispersibility of several

other kinds of powder are given in Table 1. In the table,

the notation "-L|. 00" means the material passed a Ip 00 mesh

sieve (ASTM Standard Specification for Sieves for Testing

Purposes, Ell-39) and the notation "+I4.OO" means the material was retained on a 499 mesh sieve. All particle

diameters were obtained by the air-permeability technique.

The reported dispersibility is the mean of three to five

determinations except in one case where only one determination was made. The mean deviations are given in all

cases but this one. All determinations were made at a temperature of 70 to 72° F. All of the measurements were

made at a relative humidity between 50 and 60 percent except in the case of the material designated "Silica gel No.

2 " , which was measured at a relative humidity of about l£
-5

percent. This material was in every other respect the same

as that designated "Silica gel No. 1 " . The dispersibility

was greater in the drier air. The lowest dispersibility

measured in this first series was Z\\. 9 for starch and the

highest in this series was 79.8 for the fraction of a 00mmercial dry chemical passing a I4.OO mesh sieve*

l*, DISCUSSION

When the development of a method of measuring dispersibility was undertaken, the view was held that dispersibility Implied a "spreading out" and it was the objective to devise a measure of "how much powder fell how far

from the center" when dropped in a specified manner. In

the first procedure employed* a long narrow glass plate was

placed at the bottom of the enclosure to catch powder. The

distribution of powder with respect to the distance from

the center was Obtained by measuring the transmittance of

the powder deposit on the plate along one diameter of the

distribution. For the purpose of analyzing the data, and

with some logical justification* the distribution was considered as a Gaussian error function and the probable error

of the distribution, so considered, was adopted as the measure of dispersibility. The method distinguished between

powders of high and low dispersibility and consistently

ranked materials of high dispersibility but the results

were not consistently reproducible with powders of low

dispersibility. In the case of powders of low dispersibility, agglomerations of considerable size sometimes fell

quite a distanoe off center without disintegrating. With

a bright light placed In the enclosure, it was possible to

observe the powder falling from the dispenser to the plate

and it was found that agglomerations in the form of thin

plates or wedges reached considerable distances from the

center by gliding through the air„ This observation brought

out the fact that materials may disperse in more ways than

one and that the mere spreading out of material in any manner whatsoever does not in itself constitute dispersion in

the sense intended here. The "error function" method rated

materials which dispersed as aggregations as more dispersible than was desirable since it gave a measure of "how much

powder fell how far from the center" . It was to eliminate

this interpretation of the dispersal of aggregations that

a change in the method was desirable.

The basis for the method adopted was essentially the

theory of hiding power and opacity of paints. Finely

divided matter can cover a greater area than the same weight

of material in large lumps. It is a corollary that lightcolored powders on a dark background reflect more light if

they are well dispersed than if they are distributed in

large agglomerations.
- 6 -

The mirror was chosen for a background because with the

mirror facing the unilluminated black surfaces on the interior of the enclosure it reflected less light than any black

material illuminated by the lamps. Furthermore, between

experiments the glass mirror could be cleansed of all powder

more easily than most black surfaces.

The measurement of reflectance with respect to a thick

layer of a certain area of the same powder rather than a specified standard white surface cancelled the effect of the

color of the powder. The standard layer chosen reflected

about one -fourth more light to the photocell than the powder

distribution resulting from dropping the most dispersible

powders tested. Though comparison of the distribution to a uniform layer covering the whole field might have been a less arbitrary procedure, keeping the two values of luminous

flux in the same range minimized photometric errors. In

either case the procedure would be arbitrary because the

amount of powder dropped and the height of drop were arbitrarily selected.

All of the powders of interest in the work for which

this method was developed were near white in color and the

method was particularly designed for light-colored materials.

It would appear that if it were necessary to work with dark

materials, the mirror could be replaced by a clear or translucent plate and the transmittance rather than reflectance

of the deposit could be measured by a suitable photometric

system. Such a system might be applied equally well to light

and dark powders and the correlation between dispersibilities

determined by transmittance and reflectance could be obtained

for the intercomparison of data.

The method described is rapid and easy to use. It does

not require a very large amount of powder nor does it involve

very elaborate or delicate apparatus. It has sufficient precision for the intended purpose.
-7-

5^ REFERENCES

(1) McCamy, C, S„ , Shoub, H.* Lee, T, G.j Fir© extinguish

ment by means of dry powder,. Sixth International

Symposium on Combustion, August 1956 (proceedings in

publication)

(2) Puri, A, N*, and Keen, Bernard A.J The dispersion of

soil in water under various conditions. J. Agr, Sci

12* 147-161 (1925)

(3) Andreasen, A. K. M, Hofman-Bang, N*, and Rasmussen,

No H$ Uber das Staubungsvermogen der Stoffe, Kolloid~

Zeits, 86, 70 (1939)
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U. S. DEPARTMENT OK COMMERCE

Sinclair \\ ccks, Srcrvturv NATIONAL IIUKLAU Ol STANDARDS

A. V. A slin. Director THE NATIONAL BUREAU OF STANDARDS

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