effect of glidant addition.pdf

J. Soc. Cosmet. Chem. 21 483-500 (1970) ¸ t970 Society of Cosmetic Chemists of Great Britain

The effect of glidant addition on the flowability of bulk particulate solids

T. M. JONES*

Presented at the symposium on "Powders", organised by the Pharmaceutical Society of Ireland and the Society of Cosmetic Chemists of Great Britain, at Dublin, on 17th April 1969.

Synopsis•Systems where the GLIDANT is chemically similar or dissimilar to the bulk SOLID are discussed, and it is shown that glidant efficiency is dependent upon the PARTICLE SIZE of both coarse and fine component and the diameter of the ORIFICE through which material is discharged. The improvement in FLOWABILITY of fine POWDERS on admixture with coarse material is also outlined.

It is suggested that glidants may act by one or more of the following mechanisms; reduction of interparticulate friction, change in surface rugosity, separation of coarse particles, reduction of liquid or solid bridging, and minimising static charge.

INTRODUCTION

In compressing coarse granular solids, lubricants are added to reduce friction between the punches and dies. In addition, some lubricants prevent the adhesion of powder particles to the punch faces and these have been termed anti-adherents. The term glidant was first introduced by Munzel (1) to describe those agents which added in small amounts improve the flow characteristics of granulations. Now many handling processes are concerned with the discharge of material from hoppers. In the compression of granules, however, the flow of bulk solids is further controlled by feed frames and hopper shoes so that flow into the die cavity is uniformly maintained. It

*Department of Pharmacy, University of Nottingham, Nottingham. 483

484 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

has been suggested therefore that the definition of a glidant must include flow under these conditions (2).

Several methods have been employed to assess the effect of glidant addition on flowability and these include rotational viscometry (3), angular characteristics (4-6), and gravity discharge from model hoppers (7, 8).

Whilst some of these tests are of limited application in the quantitative assessment of flowability (5, 9) they give an indication of possible effects of glidant addition. A more practical approach has been to investigate tablet weight variation (2, 10).

From the reported results it is possible to distinguish between two types of glidants:-

1. Materials chemically similar to the bulk solids to which they are added.

2. Materials chemically dissimilar to the bulk solids to which they are added.

It has also been shown that it is possible to improve the flowability of fine powders by the addition of these two categories of glidants (11) and it is suggested that this may be another division of their classification (12). The way in which these materials improve the flowability of bulk solids varies according to the material used. It is necessary therefore firs fly to outline the effects produced by the various types of glidant and then discuss the suggested mechanisms of action.

GLIDANTS ADDED TO GRANULAR SOLIDS

Before the effect of glidant addition can be assessed it is useful to understand the problems that may be encountered in the handling of granular solids. It is now well established that when considering the gravity discharge of a bulk solid, the rate of flow increases as the particle size is reduced until a size is reached below which flow becomes impaired by the action of interparticulate forces. Furthermore, it is generally accepted that difficulties may arise in flowability when the material is reduced in size to less tfian 1509m. Table I lists some of the critical particle sizes below which flow impairment has been reported. It can be seen that this critical size varies according to the particular material investigated. It may be that problems of flowability could be reduced by a judicious choice of particle size. However, it is often impracticable to use monosized systems. Further- more, it may be desirable to include fine material in a blend, e.g. the

EFFECT OF GLIDANT ADDITION ON FLOWABILITY OF SOLIDS 485

Table I

The particle size of bulk solids below which impaired flow occurs

Estimated critical Method of assessment

Material particle size (gm) Source of flowability Silica sand 204 (25) Flow through orifice

Quartz sand Sodium chloride Sodium carbonate Citric acid

N-cyclohexyl 2- benzothiazole-

sulphenamide Strontium nitrate Acetanilide Ballotini

Lactose

Light magnesia

Heavy magnesia

Quartz sand

Glass beads Sand Griseofulvin

Lactose

Sodium borate Boric acid

Calcium gluconate

Coal

Sulphathiazole

150 150 35O 150

175 4OO 35O

<5O

120 250

158

250

300 300

< 200

250

150-300 100-250

(26)

(27)

(28)

(29)

(3O)

(31)

(32)

250 (33)

lOO

4OO

(34)

(35)

Slide down a roughened inclined plane

Flow through orifice


Flow through funnel


Static angle of repose

Angular characteristics

Flow through orifices

Angle of repose

Angle of repose

presence of 'fines' in a tablet granulation and in these cases the addition of a flow-aid such as a glidant should be considered.

The addition of glidant material of similar chemical constitution to the bulk solid

When fine particles of size less than the optimum for flowability are added to a bulk solid of similar chemical constitution there is often an

improvement in the rate of flow through an orifice (7-9). The effect is demonstrated in Fig. 1 for systems of heavy grade magnesia. The improvement is dependent upon the size and concentration of the fine particles;


1400

1200

iooo

8OO

6OO

4oo I I I I I 0 20 40 60 80 I00

% w/w odded fine powder

Figure 1 The effect of size and concentration of fine particles on the flow rate of magnesia (851 gm) through a circular hopper orifice 11.4 mm diameter.

the smaller the particles the lower the concentration required to produce an increase in flow but not necessarily a greater flow rate. The effect has also been shown for lactose (Table II).

Table II

The effect of size and concentration of fine lactose particles on the flowability of lactose granules (1 242gm). Results interpreted from (7).

Arithmetic mean size in micrometres of added fine

particles

626 335 213 163 111 -74

Estimated percentage fine material required to produce optimum flow

Indeterminate 75 50 40 25 15

Rate of flow at optimum

Rate of flow of plain granule

1.15 1.32 1.38 1.44 1.38 1.19

The concentration of fine material that is required to produce a flow rate maximum is, however, strongly dependent upon the orifice diameter of the hopper; the required concentration of glidant increases as the orifice


size decreases. It has been shown that these variables can be related in an

empirical equation (12). 71p. m

253p. m Do • 0.60.3 cm 851p. m

71p. m

253pm Do = 0.740 cm 851p. m 71p. m

600• -6ø0

Flow rates, g min-I 7oo-8o• / / ,,,•oo-•oo/•<•oo

2531J, m I)o=0,898 r.m 851p, m 711•m 71p. m

/ ,•',oø-:.•:C,• <'•øø .%- ½ • --,-

Figure 2 The effect of orifice diameter (Do) on the flow rate of multicomponent mixtures of magnesia. Flow rates in g min-t

A similar improvement in flowability occurs when fine material is added to binary mixtures of coarse components. Again the particle size of the fine component is an important variable in determining the optimum


flow conditions. Fig. 2 illustrates the effect of orifice diameter on flow rate in ternary mixtures and it can be seen that as the orifice size increases, the percentage fines required to produce optimum flow decreases.

In compression processes, gravity discharge through orifices studies are of limited value since it is the die filling capacity of the formulation which is the important parameter. It has been shown that these particle

Table III

Glidants reported to have been added to bulk solids.

Bulk solid

Approx. Method of assessmenl Material arithmetic Source of flowability

mean size

(gm)

Lactose 950 -> -149 (7) Flow through orifice Lactose 805 (8) Flow through orifice S.D. -177 (5) Angle of repose and Lactose- flow through orifice Aspirin 541 Calcium

sulphate 541 (1)

(39) Sulpha- thiazole 213 (35) Angle of repose

Lactose 950 --> -149 (7) Flow through orifice S.D.

Lactose -177 (5) Angle of repose and Aspirin 541 flow through orifice Calcium

sulphate 541 Various

tablet (40) Vibrating funnel diluents

Lactose 950 --> -149 (7) Flow through orifice S.D.

Lactose -177 (5) Angle of repose and Aspirin 541 flow through orifice Calcium

sulphate 541 Sulpha- 335 thiazole 163 (35) Angle of repose

(1) Various

tablet (40) Vibrating funnel diluents

Glidant

Concentration

for optimum Material flowability

% w/w

0.5

up to 3

Talc 1

No improvement

No improvement up to 4

0.5

up to 1 No improvement

No improvement Starch No improvement

5 delayed flow

No improvement

0.5 0.25

0.25

Magnesium 0.25-1 stearate

No improvement

No improvement Delayed flow

Polyethylene glycol 4000

Delayed flow

Improved flow tablet ' (40) Vibrating funnel diluents

(1)


Table III--continued

Glidants reported to have been added to bulk solids.

Material

Calcium

phosphate

Fly ash

Microcell

Zinc stearate

Lithium stearate

Calcium stearate

Aluminium stearate

Calcium silicate

Fumed silica dioxide

Pyrogenie silica

Silico- aluminate

Glidant

Concentration

for optimum flowability

%

Improved flow

20

3.6

0.5

0.25

2 0.25

Delayed flow

0.25

0.5 0.25

No improvement

No improvement

0.1-0.5

0•1-0.5

Bulk solid

Material

Ottawa sand

Ottawa sand

Sponge Iron

Sponge Iron

Lactose

sponge iron

Various tablet diluents

Sponge iron

Thermo-

plastic Powder

Lactose

Aspirin S.D. Lactose Calcium

sulphate

Micro-

crystalline cellulose

S.D. Egg yolk

Micro-

crystalline cellulose

S.D. egg yolk

Approx. arithmetic mean size

(•m)

711

711

950 -+ -149 541

-177

541

Method of assessment

of flowability



Hall flowmeter

Hall flowmeter

Hall flowmeter

Vibrating funnel

Hall flowmeter

Flow through orifice Angle of repose and flow through orifice

Tablet weight variation

Tablet weight variation


size distribution effects are relevant to the tabletting process (8, 13) and that provided segregation is not significant the presence of a large quantity of fine material is not necessarily unacceptable (14).

Addition of material of dissimilar chemical constitution to the bulk solid

In some cases, even though an optimum particle size and size distribution is achieved, the bulk solid may still not possess the desired flow properties. Furthermore, it may be that a monosized system is required by a specification. In these circumstances a material chemically dissimilar to the bulk can be added to improve flowability. In this context it has been reported that the inclusion of lubricants in a tablet granulation may improve or impair its flow properties (1).

The literature contains many different types of material that have been used as glidants in this category and Table III summaries some of these.

It is obvious that the glidants differ not only in chemical properties but also in their physical characteristics such as size, frictional properties, crystalline structure and density. It can also be seen that the concentration of glidant varies with the material to which it is added and that in some cases there is some doubt as to their efficiency in improving the flow properties of the bulk solid. In order to explain these apparent anomalies in glidant efficiency a preliminary study has been carried out using a model system.

Fig. $ illustrates the effect of particle shape and concentration of glidant on the flow rate of mixtures of magnesium stearate with lactose. The lactose was granulated with 15% w/v PVP in 50% alcohol and the variation in shape produced by passing coarse equidimensional lactose granules through a dry granulator so that fracture occurred.

It is clear that for both particle forms, the rate of flow is improved by the addition of magnesium stearate (-66•m) up to a limiting concentration of glidant. Above this concentration (between 0.25 and 1%) flow rate is not significantly changed until an excessive amount of glidant is added.

At the lower concentration, the results are in good agreement with those of Gold, Duvall, Palermo and Slater (7) but these authors report the results of investigations using only one orifice diameter.

Table VI presents the data of the present investigation in terms of a glidant efficiency factor f

where f = Rate of flow in presence of glidant Rate of flow in absence of glidant


900 --

800 -- e• e

Y'• 600 -- 50C L •

400 •

•00

200 'mm mmm/m

I 00• o 2 4 6 8 I0 12

ø/o w/w magnesium stearate

Figure $ The effect of magnesium stearate 66pro (-230 mesh) on the flow rate of lactose 951gin (14/22 mesh) granules.

Particle shape Orifice diameter Regular Irregular

8.67mm ß [] 11.58mm ß 12.62mm ß

It can be seen that the efficiency of magnesium stearate decreases as the orifice size increases.

Furthermore a closer inspection of these systems (Fig. •t) suggests that the optimum glidant concentration is also dependent upon orifice size, i.e. a situation analogous to the addition of fine to coarse matehal of similar chemical constitution.

This could offer some explanation of the apparent disagreement between the reported effects of the use of magnesium stearate as a glidant in tablet-


Table IV

The efficiency of magnesium stearate (-66[tm) as a glidant when admixed with lactose (951 gm --14/22 mesh)

Orifice

diameter (mm)

8.67 11.58 12.62

8.67 11.58 12.62

Shape of granules

Regular

Irregular

f value

Glidant concentration % w/w

0.25 0.5 0.75 1.0 2.0 4.0 8.0

1.098 1.157 1.133 1.151 1.181 1.163 1.309 1.093 1.151 1.117 1.116 1.105 1.162 1.229 1.095 1.078 1.038 1.066 1.016 1.062 1.184

1.112 1.205 1.175 1.177 1.195 1.185 1.347 1.085 1.103 1.118 1.138 1.123 1.148 1.226 1.062 1.061 1.076 1.081 1.081 1.121 1.205

ting since in some published reports magnesium stearate is claimed to improve the flow rate of granules whereas other authors maintain that it has no glidant properties (1, 7, 10). The orifice diameters used in the laboratory assessments are comparatively small when considering those used in manufacturing plants and therefore differences may be expected in the efficiency of any particular concentrations of glidant.

GLIDANTS ADDED TO FINE POWDERS

The removal of superfine and ultrafine powder from fine and granular powders often improves their flow properties. Farley and Valentin (15) have demonstrated the effect of particle size distribution on the cohesion and tensile strength for a number of inorganic materials and from their results it can be seen that the cohesion of a bulk solid may be significantly reduced by careful control of the size range of the material.

In some cases, however, it may not be possible to vary the size distribution of the material. It has been reported that the addition of coarse granular solids to such systems improves their handling characteristics (16), for example, the addition of about 1% of zinc oxide, kaolin or heavy magnesium carbonate significantly improves the flowability of sulphani- lamide powder (17, 18).

Fig. 5 illustrates the effect of adding a coarse, free flowing powder to a binary mixture of two powders which have impaired flow properties. It can be seen that an improvement in flow rate can be achieved by a suitable combination of the component size fractions.

To produce gravity discharge of superfine powders, however, a vast excess of coarse material is required (16, 19) and clearly in many cases this

EFFECT OF GLIDANT ADDITION ON FLOW-ABILITY OF SOLIDS 493

.

Jolnõa•


561/zm

600-650

650-700

600-650

500-600

400- 500

300-400

200- 300 48/.t.m 90/.t.m

trigure &. The effect produced on flow rate by varying the composition of a ternary mixture of magnesia. Flow rates in g rain-1. Orifice diameter 8.98 mm

is undesirable in the final product or difficult to handle due to segregation. The addition of small quantities of fine glidants such as A erosil, magnesium oxide and corn starch can be shown to improve the flow properties of mildly cohesive powders (4, 6, 36-38).

MECHANISM OF GLIDANT ACTION

An improvement in flowability of bulk solids is produced by the addition of many types of glidants and several mechanisms of action may be involved.

Frictional effects

Since many of the materials used as glidants are also efficient lubricants (1) a reduction of interparticulate friction may be involved. The reduction may take place in two ways. Firsfly, when fine material adheres to the surfaces of an irregular shaped but equidimensional coarse fraction the


reduction in surface rugosity will minimise the mechanical interlocking of the particle and thereby reduce the rolling friction. This would be parti- cularly relevant when flow improvement is caused by the addition of material of similar chemical constitution to the original granulation. Secondly, the added material may possess a coefficient of friction which is lower than that of the bulk solid to which it is added and therefore decrease

interparticle friction. It has been suggested that the glidants which possess laminar crystalline forms, e.g. talc, graphite, roll up under low shear stresses to produce a 'ball bearing type' action (20). In addition, it may be that some substances are acting as boundary lubricants between the particles but it is considered that the low shear stresses involved in most handling processes do not seem great enough to place too much emphasis on this mechanism.

Separation effects

Although glidants may possess a minute crystallite size (of the order of several nm) on addition to bulk which the individual particles may become aggregated (sizes up to several [tm) (2).

The aggregates increase the interparticulate distances of the coarse material and may reduce the forces of attraction between the surface asperities. This is also the case for fine material added to coarse material of similar chemical constitution but under these conditions the separation is much greater. Thus, during the gravity discharge of a bulk solid from a hopper, the points of slippage in a free fall arch over the orifice are increased by the interposition of the fine particles and flow may be increased (21). Many of the glidants used are also flow conditioning or anticaking agents, e.g. calcium phosphate, silico aluminates.

The physical separation of the coarse particles which is produced when these materials adhere to their surfaces is thought to reduce the action of capillary adhesion forces and also prevent the formation of solid bridges between particles (22, 23). This is of importance where bin residence time is prolonged or the formulation subjected to a variable environment. The particle size of the added material is important since it is the ability of the additive to coat the surface of the coarse material that determines its

efficiency (2, 19). The combination of both frictional and separation effects should therefore produce a useful improvement in flowability and this can be demonstrated by the improvement in flow produced on the addition of fines and talc to a tablet granulation {8).


Antistatic effects

Many powders acquire a static charge during their handling and it has been shown that the addition of løfo or more of magnesium stearate, polyethylene glycol 4 000 or talc effectively lowers the accumulation of static charge in a number of pharmaceutical formulations (24).

(Received.' 2nd January 19(;9)

REFERENCES

(1) Strickland, V• •. A. Jnr. Drug Cosmetic Ind. 8õ 318 (1959). (2) Augsberger, L. L. and Shangraw, R. F..[. Phar•n. $ci. õõ 418 (1966). (3) Harwood, C. F. and Pilpel, N. Lab. Pract. 17, 1236 (1068). (4) Tawashi, yon R. Pharmazeutische Technologie, 2 64 (1063). (5) Gold, G., Duvall, R. N., Palermo, B. T. and Slater, J. G. J. Pharm. Sci. õõ 1201 (1966). (6) Craik, D. J. and Miller, B. F. J. Pharm. Pharmacol. 10 136T (1958). (7) Gold, G., Duvall, R. N., Palermo, B. T. and Slater, J. G. J. Pharm. Sci., õ,1 667 (1968). (8) Hammerness, F. C. and Thompson, H. O. J. Am. Pharm. Assoc. Sci. Ed. 4'1 58 (1958). (0) Jones, T. M. Mfg. Chemist 30 38 (March 1068).

(10) Hansen, G. Arch. Pharm. Chemi. 01 632 (1954). (11) Bulsara, P. U., Zenz, F. A. and Eckert, R. A. IEC Proc. Des. and Der. 3 348 (1964). (12) Jones, T. M. and Pilpel, N.J. Pharm. Phar•nacol. 18 429 (1966). (13) Sumner, E. D., Thompson, H. O., Poole, W. K. and Grizzle, J. E. J. Pharm. Sci. õ5

1441 (1966). (14) Tucker, S. J. and Hays, H. M. J. Am. Pharm. Assoc. Sci. Ed. 48 362 (1959). (15) Farley, R. and Valentin, F. H. H. Powder Technol. I 344 (1967/68). (16) HawksIcy, P. G .W. Inst. Fuel Conf. on Pulv. Fuel 656 (1947). (17) Shotton, E. and Simons, F. M. J. Pharm. Pharmacol. 2 231 (1950). (18) Davis, H. Pharm. J. 150 118 (1943). (19) Irani, R. R. and Callis, C. F. Particle Size; Measurement, Interpretation and Application

3 (1963). (John Wiley, N.Y.). (20) Train, D. and Hersey, J. A. J. Pharm. Pharmacol. 12 97T (1960). (21) Jones, T. M. J. Pharm. Sci. 57 2015 (1968). (22) Hardesty, J. O. and Kumagai, R. Agr. Chem. 7 (2) 38 (1952). (23) Whynes, A. L. and Dee, T. P. J. Sci. Food Agr. 8 577 (1957). (24) Gold, G. and Palermo, B. T. J. Pharm. Sci. 54 1517 (1965). (25) Rose, H. E. and Tanaka, T. Engineer, 208 465 (1959). (26) Burak, N. Chem. Ind. London 844. (1966). (27) Jones, T. M. and Pilpel, N.J. Pharm. Pharmacol. 17 440 (1965). (28) Jones, T. M. Ph.D. thesis University of London (1967). (29) Smalley, I. J., Hearer, A. A. and McGrath, L. Trans. Inst. Mining Met. (Sect. C

Mineral Process Extr. Met.) 76 183 (1967). (30) Harwood, C. F. and Pilpel, N. Chem. Process Eng. 49 92 (July 1968). (31) Segovia, E. Acta Pharm. Suecica, 4 171 (1967). (32) Kaneniwa, N., Ikekawa, A. and Aoki, H. Chem. Pharm. Bull. 15 1441 (1967). (33) Egrova, V. I. Med. Prom. SSSR. 20 (11) 47 (1966). (34) Brown, R. L. S.C.œ. Monograph No. 14 150 (1961). (35) Nelson, E. J. Am. Pharm. Assoc. Sci. Ed. 44 435 (1955). (36) Gstirner, F. and Pick, C. Arch. Pharm. 300 757 (1967). (37) Czetsch Lindenwald, H. v. E1 Khawas, F. and Tawashi, R. J. Soc. Cosmetic Chemists,

16 251 (1965).


(38) Okada, J., Matsuda, Y. and Wada, Y. Yakugaku Zasshi, 88 647 (1968). (39) Hersey, J. Rheol. Acta, 4 235 (1965). (40) Leoveanu, O., Zaharia, N. and Pilea, V. Rev. Chim. 17 112 (1966). (41) Jones, W. D. Fundamental Principles of Powder Metallurgy (1960). (Edward Arnold,

London). (42) Maly, J. Acta Fac. Pharm. Bohemoslovenicae, VIII 81 (1963). (43) Berry, F. and Payne, M. Paper presented to Institution of Chemical Engineers Sym-

posium on Aggregation (28th March 1968). (44) Forsythe, R. E., Scharpf, L. G. Jr. and Marion, W. W. Food Technol. 18 153 (1964).

DISCUSSION

MR. J. C. WILLIAMS; In view of the difficulties of assessing the effect of glidants on the flowability may I ask whether you have considered the use of a shear cell as a means of assessing the flowability of your materials?

THE LECTURER: I have done some work on a shear cell; unfortunately the price of these instruments is somewhat high. At the moment we are concerned with the rate of flow improvement rather than the absolute conditions at the surface of the particles, so perhaps the straightforward flow measurement is more relevant.

gIR. R. CHUDZIKOWSKIi It seems to me that by restricting yourself to discussing the effect of "glidants" only, you have (somewhat) obscured the overall picture of "flowability" of bulk particulate solids. This becomes more clear when viewed in the light of the basic equation for any flow (material, electricity, heat), viz:

driving force flOW

sum of resistances

which, in this particular case will become:

gravity Flow of particulate solids

Sum of frictional, cohesive, adhesive, etc., forces.

Thus, properties pertaining to the driving force of gravity, will be apparent density of the bulk, its head, etc. and the "resistances" can be differentiated into interparticulate friction, friction between the particles and the hopper, "packing" at the orifice, various forces of cohesion (Van der Waals', electrostatic, etc.), "sticking" due to moisture, etc. All, or some of these, forces contribute to the overall resistance, and in

certain conditions one, or some of them, may become the governing factor. Its diag- nosis will then suggest a remedy. The condition of flow is that the driving force must exceed the resistances, and this may be achieved either by increasing the numerator, or decreasing the denominator.

When, for instance, frictional forces are the governing factor small quantities of fines have a beneficial effect by reducing rugosity of the particles. When, however, the percentage of fines is greatly increased, various cohesive forces come increasingly into play (at the same time reducing the bulk density), and they may in turn become the governing factor of an impaired flow.

Such interpretation also helps to explain apparent paradoxes of borderline cases where, for instance, an addition of a coarse powder will make a "fluffy" bulk flow, by increasing its apparent density, while an addition of an otherwise most effective "glidant" will have an adverse effect, by further reducing it.


TI•E LECTURER: This paper does not attempt to consider the overall picture of flowability of bulk particulate solids but is an attempt to clarify some of the confusion that exists in the literature concerning the use of gildants. For example, Strickland (1) states that "magnesium stearate which is an excellent lubricant at the tablet-die wall interface actually tends to retard the flow of granularions", yet Gold and Palermo (24) show that it increases the flow rate of granularions.

DR. M. AllMAD: In Fig. 3 you have compared glidant efficiency of regular and irregular particles. I consider it essential that you should give some estimate of the shapes of the "regular and irregular" particles that you have used, otherwise this simply adds to the volume of ill-defined literature which cannot be used for reference. In your opening remarks, you said "equidimensional regular particles". It is, however, a very well-kno•vn fact that cubes and spheres, for example, covered by the said phrase have very different flow rates.

TIlE LECTURER: I admit that this is a very preliminary investigation of this phenomenon and we have gone onto measuring the shapes of the particles.

A •MEMBER OF TIlE AUDIENCE: It is known that the flow rates out of hoppers vary •vith the angle of the hopper and the orifice. You do not say how these measurements were carried out and •vhat apparatus you used. You talk about fine particles, but you go very fine in your experiments. I would also like to emphasise that humidity can affect the results very considerably; •ve do our testing in humidity controlled rooms, •ve could not do it otherwise.

TIlE LECTURER: The method and techniques are well reported elsewhere (12). As far as the gildants, which are chemically similar to the fine component, are concerned, once we get down to superfine powders of this nature, surface adhesion occurs and I have some results which show that the improvement from these compounds is virtually non-existent since both glidant and fine particle have poor flow properties. The small variation in humidity in the laboratory whilst these results were determined •vas shown to produce no change in flow rate in the systems investigated.

DR. ]N. A. ARMSTRONG: From the line in Fig. I represented by the black circles, I note that the flow rate increases above about 80% of fine powder. Would you care to comment?

THE LECTURER: One of the possible reasons at this,level is segregation. We start off with a uniformly packed, uniformly mixed, incrementally packed bed so that during flow, if segregation occurs, it would perhaps be reflected in that sort of change.

MR. F. F. ADEY: Why was magnesium stearate chosen as a material for investigation when it is well known as a lubricant and not as glidant? Pyrogenie silica is well known as a glidant without lubricant properties, and it might have been a better choice.

TIlE LECTURER: I picked magnesium stearate because, as I have already stated, in the literature this confusion exists, some people say it does not work as a glidant, others say it does. This is an attempt in the first instance to see exactly what is happen- ing with magnesium stearate. Gold and Palermo (24) have produced results showing this improvement of flow which tended to contradict a number of earlier studies. It has •ot been looked at since then and I am not claiming that magnesium stearate always


acts as a glidant; I would not advocate its use as such. I think the addition of these types of material to pharmaceutical formulations is to be discouraged when they can be avoided, i.e. if they are unnecessary.

MR. G. DUNCALF: One practical effect of glidant addition is a marked change in case of aeration of some mixtures. The degree of aeration and therefore flowability is considerably dependent upon the degree and type of agitation given to the mixture before use.

Equally important is the rate of deaeration. Practical tests have shown that with some mixtures there are marked differences in rate of flow (in this case measured by dispersing from a pack with 4-8 orifices, of say, 6 mm diameter) depending upon whether the tests are carried out within minutes of mixing or some time later. With some mixtures the rate of deaeration and reduction of flowability may be significant in less than 15 min and in other cases not for many hours.

Is it possible that some of the discrepancies in effect of glidants noted in the literature could have been due to the fact that this aeration and flowability factor had not been considered, and that the degree of agitation and time elapsed between mixing and testing had not been standardised? Judging by comments already made, similar variations in flow behaviour are often encountered under practical conditions and the same com•nents might apply.

40C

0.1 0.2 03 0.4 0'5

Particle size, mm

Figure 6 The effect of change in bulk density on the flow rate of various size fractions of magnesia.

T•E LEcxu•: The effect can be illustrated by referring to the changes that occur in flow rate for different initial states of packing (Fig. •). These are some results for magnesia flowing through a circular orifice 7.4 mm in diameter. As the particle size decreases the flow rate increases to a maximum. Below this particle size, flow is impaired because of the influence of interparticulate forces.

ß •00 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

The two sets of points indicate that for non-cohesive material the effect of the initial state of packing is not very significant, perhaps because the flow limiting condition is bed dilation to a characteristic bulk density at the orifice region. Once interparticulate forces become relevant, the initial state of packing seems to be quite important because the bed is trying to dilate before it flows.

A MF•MUF•R OF T•F• AUX)IF•NCF•: We have also found a discrepancy between results, and having done some work in this I would agree that one can very easily be misled by making the mix in the morning and leaving the sample, doing it later in the day and getting a quite different set of results, i.e. until one realised that there had been a dramatic change in bulk density and therefore a move away from flowability.

THF• LF•CTURE•: I wonder whether a lot of the problems here may be environ- mental changes. You may get surface adsorption of moisture and capillary adhesion, electrostatic changes, etc.

A MF•MBF• OF ZHF• AUX)IF•NCF•: I wonder if one can get back to the original set of conditions? If this dramatic change is not appreciated, you might well get conflicting results.

T•u LECZU•F•R: In my experience with those materials not subject to any interparticulate forces their flow rate is not considerably affected by bulk density (Fig. •). A change in bulk density in the hopper has not made much difference in the eventual rate of flow from that hopper.

effect of glidant addition.pdf

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