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 reduc- tion 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