Flowability of powders

G. E. Amidon, D. P. Smith, E. N. Hiestand, and B. D. Tiffany, Flowability of Powders Using a Simplified Shear Cell, APhA National Meeting, November 1981. 

Irwin GM, Dodson GJ, Ravin LJ. Encapsulation of clomacron phosphate I. Effect of flowability of powder blends, lot-to-lot variability, and concentration of active ingredient on weight variation of capsules filled on an automatic capsule filling machine. J Pharm Sci 1970 59 547-550. 

So far, we understand that the flowability of powders depends on their failure stresses from the Mohr-Coulomb failure criterion. Therefore, analyses of powder flows... 

Particle sizes combined with shape factors have been the subject of many of the recent studies regarding flow of solids. Sphericity, circularity, surface-shape coefficient, volume-shape coefficient, and surface-volume-shape coefficient are some of the most commonly used shape factors. It is generally accepted that the flowability of powders decreases as the shapes of particles become more irregular. Efforts to relate various shape factors to powder bulk behavior have become more successful recently, primarily because of the fact that shape characterization techniques and methods for physically sorting particles of different shapes are... 

To improve the flowability of powders and granulations, a small amount of a flow agent, or glidant, is... 

Kaye, B.H. Characterizing the flowability of powder using the concepts of fractal geometry and chaos theory. Part. Part. Syst. Charact. 1997, 14, 53-66. 

The New FT3 Powder Rheometer—Freeman Technology. Freeman, R. The Flowability of Powders, International Conference on Powder and Bulk Solids Handling, 2000. Velasco, A. Munoz-Ruiz, A.J. Perales, M.C. Munoz, N. Jimenez-Castellanos, M.R. Evaluation of an adequate method of estimating flowability according to powder characteristics. Int. J. Pharm. 1994, 103, 155-161. 

Ploof DA, Carson JW. Quality control tester to measure relative flowability of powders. Bulk Solids Handling 1994 14(l) 127-32. 

One may compare the flowability of powders at similar pressures by comparing their unconfined yield stress at a single normal stress, or one point off a flow function. In this case one should clearly state the pressure of comparison. Flow indices have been defined to aid such one-point comparisons, given by the ratio of normal stress to strength, or... 

Other Methods of Flow Characterization A variety of other test methods to characterize flowability of powders have been proposed, which include density ratios, flow from funnels and orifices, angles of repose and sliding, simplified indicizer flow testing, and... 

It is generally agreed that one of the more important parameters of interest to formulators is the flowability of powdered solids.The processability of these materials is greatly affected by flowability concerns, because the materials invariably need to be moved from place to place. For example, when tablets are to be compressed at high speeds, the efficiency of the machine will be suitable only if the powder feed can be delivered at a sufficiently high rate. Whereas many pharmaceutical compounds are cohesive in nature, their flow characteristics tend to be undesirable. One of the aims of granulation is reduce the cohesive nature of the individual components to produce a uniformly blended material whose physical properties are more suitable for processing. 

In understanding, the greatest question to be yet resolved is what is the best, yet simple, test to be used to monitor the cohesivity or flowability of powders There are, of course, the direct (or almost direct) ways of measuring it (the cohesion tester or the tensile testers) but neither the tests nor the equipment are really simple. Angles of repose or spatula are simple but, unfortunately, become quite meaningless as the powder gets more cohesive. I personally think that the way powders compact can be directly related to cohesivity the recent evidence of the usefulness of the Hausner ratio in many different applications points in this direction. The compaction test should also be better defined and more widely used. 

Conversely, Ehlermann and Schubert (1987) sustained that compressibility results from materials of different composition cannot be compared and that flowability characterization through compressibility must be made specifically for each food variety. Moreover, confined uniaxial compression is a simple compression test that provides an approximate measure of the flowability of powders. Therefore, it is not suitable for silo design but may prove to be a convenient method for process control in any food laboratory (e.g., to evaluate particle cohesion). Table II offers a range value definition for flowability classification by comparing flow function (ratio between the maximum consolidation stress and unconfined yield stress) with compressibility. 

Granulating Is not primarily a method to improve flowability of powders and formulations in the pharmaceutical industry but also in the fertilizer and bulk chemical industries as well as for carbon black, silica fume, and many other materials. 

As fumed silica enhances several different properties in formulation, such as thickening liquids or improving flowability of powders, it is used in a variety of applications [63], including plastics, rubbers, coatings, cosmetics, adhesives, sealants, inks, and toners. 

Geldart, D., Mallet, M. F., and Rolfe, N. 1990. Assessing the flowability of powders using angle of repose. Powder Handling and Processing 2 341-345. 

The specific surface of copper powder is related to the overpotential of electrodeposition. Also, it is shown that the apparent density is a function of the specific surface of powder, while the flowability of powder is a function of the apparent density. In this way, the most important properties of the powder are related to both the specific surface and the conditions of electrodeposition. Additionally the shape of particle size distribution curve is also discussed qualitatively. 

The most complicated technique for the determination of powder flowability is that proposed by Jenike (Section 1.6.1), in which the ratio of the maximum compaction stress, <7, to an unconfined stress, /c, at a specified shear stress of 3 kPa - the flow function, ff- is calculated to predict the ease of powder flow. This Jenike flow function, = cti//c, has remained the main consensual parameter designated for mass flow hopper design and the flowability of powders. 

Although the Hausner ratio or FR is a reliable indicator for the prediction of the flowability of powders the ratio may not represent the actual compaction behaviour a powder may... 

Jenike failure function (Jf) This is the reiationship between the unconfined yield strength (fa) and the major consoiidation stress (cti ). A plot of the values fa versus cti shows the possible relationship of the rate of flow of powdered material out of hopper orifices. Jenike could classify the flowability of powders from selected ratios of these values. 

The shear flowability index, n, was found, from past observations (Farley Valentin 1965, 67/68), to be independent of the bulk density of sheared compacted powder. Because of this independence of particle size from bulk density it is now realised that the shear flowability index, n, from the Warren Spring equation and the Jenike internal angle of friction may be the preferred parameters to eharacterise and quantify the flowability of powders. Jenike and others (Williams et al. 1970/71 Williams Birks 1965 Hill Wu 1996 Cox Hill 2004) selected the Jenike failure function to be one of the best indicators to predict the ease of powder movement and powder flowability. 

Kurz and Minz (1975) investigated the flowability of powders in terms of the relationship between the unconfined yield strength, /c, and the major consolidation, tri - the failure function of Jenike — for different-sized limestone powders (3.1-55.0 um) having either a narrow or a wide particle size distribution. The width of the distribution was defined by a variation coefficient, Cv, where Cv = cTstatAi.3, with agtat as the standard deviation of the particle size profile and X1.3 the average particle size of the number-volume diameter distribution of the limestone particles. A narrow distribution was considered to have a Cv < 0.5 while a wide distribution had a Cv > 0.5. 

In a survey of the various bulk powder tests developed and in use for the determination of the failure properties and measurement of the degrees of flowability of powders, Schwedes (2003) indicated that the equipment available varied from highly theoretical and technical... 

The science of soil mechanics was integrated with the related field of powder mechanics and reduced to industrial practice by Jenike [1] in 1964. Since then, it has been possible for industry to reliably measure the flowability of powders and relate the measurements, in engineering units, to the design requirements for silo flow. However, Jenike s publication was neither the first effort to quantify flowability nor the last. New testing methods continue to be introduced, with varying degrees of success. In many cases these alternative measurement methods are the result of an industrial necessity and reflect some shortcoming of the Jenike method. In other cases, they exist because the Jenike method is not known to the people involved or is not relevant to their problem. Business value can be derived from many different types of measurements. 

Ploof, D.A. and J. W. Carson Quality Control Tester to Measure Relative Flowability of Powders, bulk solids handling 14(1994), 127/132... 

Ease and completeness of dispersion and intensity of colour are the primary application requirements of organic pigment powder (OPP). In order to meet these requirements, pigment manufacturers can vary the crystal size and surface treatment. Such variation can influence the flowability of powders and thereby affect handling processes at both manufacturers and consumers plants. 

---