Powders flow and compaction

Table 1 lists a number of factors that may influence powder flow and compaction. The list is long and includes physical and mechanical properties as well as... 

Table 1 Factors That Influence Powder Flow and Compaction...

As we just suggested, particle size and shape are important physical properties influencing powder flow and compaction. Particle size is a simple concept and yet a difficult one to quantitate. Feret s diameter, Martin s diameter, projected area diameter, specific surface diameter, Stokes diameter, and volume diameter are but several of the measurements that have been used to quantify particle size using a variety of methods. 

The mechanical properties of a material play an important role in powder flow and compaction by influencing particle-particle interaction and cohesion, that is to say, by influencing the true area of contact between particles. For example, Hertz [26] demonstrated that both the size and shape of the zone of contact followed simply from the elastic properties of a material. Clearly then, the true area of contact is affected by elastic properties. From the laws of elasticity, one can predict the area of contact between two elastic bodies. More recent work has demonstrated, however, that additional factors must be taken... 

Hancock BC, Carlson GT, Ladipo DD, Langdon BA, Mullarney MP. The powder flow and compact mechanical properties of two recently developed matrix-forming polymers. J Pharma Pharmacol 2001 53 1193-1199. 

In essence, the test battery should include XRPD to characterize crystallinity of excipients, moisture analysis to confirm crystallinity and hydration state of excipients, bulk density to ensure reproducibility in the blending process, and particle size distribution to ensure consistent mixing and compaction of powder blends. Often three-point PSD limits are needed for excipients. Also, morphic forms of excipients should be clearly specified and controlled as changes may impact powder flow and compactibility of blends. XRPD, DSC, SEM, and FTIR spectroscopy techniques may often be applied to characterize and control polymorphic and hydrate composition critical to the function of the excipients. Additionally, moisture sorption studies, Raman mapping, surface area analysis, particle size analysis, and KF analysis may show whether excipients possess the desired polymorphic state and whether significant amounts of amorphous components are present. Together, these studies will ensure lotto-lot consistency in the physical properties that assure flow, compaction, minimal segregation, and compunction ability of excipients used in low-dose formulations. 

Mullarney, M. P., Hancock, B. C., Carlson, G. T., Ladipo D. D., and Langdon, B. A. (2003), The powder flow and compact mechanical properties of sucrose and three high-intensity sweeteners used in chewable tablets, Int. J. Pharm., 257, 227-236. 

Because of powder feed variability at the nip and in the roll gap regions, powder leakage is produced during the compaction process. This situation produces excessive fines and possible undesirable processed material. Usually, this problem is caused by uneven powder flow and compact formed when the powder is fed towards the middle of the roll width. Granules produced under these conditions are typically not optimal for further pharmaceutical processing. 

In order to produce an adequate tablet formulation, certain requirements, such as sufficient mechanical strength and desired drug release profile, must be met. At times this may be a difficult task for the formulator to achieve, due to poor flow and compactibility characteristics of the powdered drug. This is of particular importance when one only has a small amount of active material to work with and cannot afford to make use of trial-and-error methods. The study of the physics of tablet compaction through the use of instrumented tableting machines (ITMs) enables the formulator to systematically evaluate his formula and make any necessary changes. 

The most important characteristics of the final formulation to be compacted are particle size and particle size distribution, density and/or porosity, powder flow, cohesiveness, and lubrication. Particle size, particle size distribution, and density and porosity of the formula will not be addressed here because they are the result of other operations in the scale-up sequence, such as granulation and milling. They should be evaluated as part of those specific operations. It should be noted, however, that the influence of particle size on powder flow and, therefore, on uniform die fill is very important to the compaction operation, but is not a result of it. The one consideration to keep in mind during scale-up is the speed of the press, which will directly affect the time available for the die filling to occur. This is an important parameter to observe carefully. 

Mannitol Freely soluble, used particularly for chewable tablet powder form has poor flow and compaction granular form has good flow... 

The influence of sorbed moisture on chemical stability and the flow and compaction of powders and granulations is well established. The moisture content and hygroscopicity of excipients is particularly important as total product processing as well as finished product stability can be affected. Hygroscopicity, moisture-sorption isotherms, and equilibrium moisture content can be determined by thermogravimetric analysis and Karl Fisher titration methods. 

Pressure compaction Extrusion Roll press Tablet press Molding press Pellet mill >0.5 >1 10 High to very high Up to 5 tons/h Up to 50 tons/h Up to 1 ton/h Very narrow size distributions, very sensitive to powder flow and mechanical properties Often subsequent milling and blending operations Pharmaceuticals, catalysts, inorganic chemicals, organic chemicals, plastic preforms, metal parts, ceramics, clays, minerals, animal feeds... 

Many important pharmaceutical unit operations handle and process powders in various ways, and these unit operations may be analyzed and modeled by using computational mechanics. Examples include powder compression and compaction, powder flow, fluidization, mixing and segregation, packing, and milling. 

Crystal morphology or habit is important, since it can influence many properties of the compound. For example, powder flow properties, compaction and stability have all been found to be dependent on crystal morphology. It has been shown that tolbutamide B (platelike)... 

Onwulata, C.L, Smith, P.W., and Holsinger, V.H. 1995. Flow and compaction of spray-dried powders of anhydrous butteroil and high melting milkfat encapsulated dissacharides. J. Food Sci. 60,... 

Dominant analytical techniques Spectroscopy X-ray diffraction (via the supramolecular slate) X-ray diffraction thermal analysis and microcalorimelry, pycnometry, spectroscopy, solubility Microscopic techniques micromeritics Micromeritics powder flow characterization, compaction studies... 

The flow and compaction nature of fine powders is therefore sensitively influenced by even minor variations of moisture content Fine products that are hygroscopic can present special difficulties in their handling, as can condensation in equipment prone to present contact surfaces that fall below dew point in temperature. 

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