EFFECT OF GLIDANT ADDITION ON FLOWABILITY OF SOLIDS 497 (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.

498 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 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 investi- gation 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