Powder compaction mechanisms

Rue, P.J. Rees, J.E. Limitations of the Heckel relation for predicting powder compaction mechanisms. J. Pharm. Pharmacol. 1978, 30, 642-643. 

Sintering. Basically a soHd-state process, sintering transforms compacted mechanical bonds between the powder particle into metallurgical bonds (23,34—38). 

Conventional Sintering. Ceramic sintering is usually accompHshed by heating a powder compact to ca two-thirds of its melting temperature at ambient pressure and hoi ding for a given time. Densification can occur by soHd-state, Hquid-phase, or viscous sintering mechanisms. 

The success of compression agglomeration depends on the effective utilization and transmission ofthe applied external force and on the ability of the material to form and maintain interparticle bonds during pressure compaction (or consolidation) and decompression. Both these aspects are controlled in turn by the geometiy of the confined space, the nature of the apphed loads and the physical properties of the particulate material and of the confining walls. (See the section on Powder Mechanics and Powder Compaction.)... 

Perhaps the most definitive result to come from the early nickel-aluminia synthesis work was the thermal analysis investigation of Hammetter [88HO 88W01], which showed explicit data on substantial changes in the shockec-but-unreacted mixtures. Differential thermal analysis was carried out on th -starting powder compacts of both the mechanically mixed and composite powders. Shocked and unreacted powders were compared to provide direc evidence for substantial changes introduced by the shock process. 

Other investigators have evaluated the potential for these indices. In their studies, Williams and McGinnity have concluded that evaluation of single-material systems should precede binary or tertiary powder systems [29]. A full discussion of compaction mechanisms is given later in this chapter. 

Studies involving instrumented compaction equipment can be extremely useful in the development of dosage forms, especially when the amount of drug substance is limited in quantity. Marshall has described a program in which dynamic studies of powder compaction can be used at all stages of the development process to acquire formulation information [63]. The initial experiments include a determination of the intrinsic compactability of the compound. In subsequent work, simple tablets are prepared, and tested for dissolution, potency, and content uniformity. Through studies of the compaction mechanism, it becomes possible to deduce means to improve the formulation under study. 

Rowe, R. C., Roberts, R. J. Mechanical properties. In G. Alderbom, C. Nystrom (Editors), Pharmaceutical Powder Compaction Technology, Marcel Dekker, New York, pp. 283-322. 

The use of compaction simulators was first reported in 1976. Since then, a variety of simulators have been developed. Hydraulic simulators, as well as mechanical simulators, are available to characterize raw materials, drug substances, and formulations, as well as to predict material behavior on scale-up. The appeal of simulators is due to the fact that they purport to provide the same compaction profile as experienced on a tablet press while using only gram or even milligram quantities of powders. Compaction simulators can achieve high speeds, as would be experienced on a production tablet press, and can be instrumented to measure a variety of parameters, including upper and lower punch force, upper and lower punch displacement, ejection force, radial die wall force, take-off force, etc. Summaries on the uses of simulators and tablet press instrumentation can be found in (19,20). 

Intuitively, SF is a measure of the degree of compression because it increases with applied pressure. Mechanical properties of powder compacts are dependent on SF and best compared at the same SF. The authors laboratory selected 0.85 SF as its standard because it approximates the midpoint of the range typical of immediate release tablets (0.8-0.9). 

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. 

Dry-pressing is carried out in a die with movable top and bottom punches (Fig. 3.2). A cavity is formed with the bottom punch in a low position and this is filled with free-flowing granulated powder (Fig. 3.2(a)) which is then struck off level with the top of the die. The top punch then descends and compresses the powder to a predetermined volume (Fig. 3.2(b)) or, in more elaborate presses, to a set pressure (75-300 MPa or 750-300 atm). Both punches then move upwards until the bottom punch is level with the top of the die and the top punch is clear of the powder-feeding mechanism (Fig. 3.2(c)). The compact is then removed, the bottom punch is lowered and the cycle is repeated (Fig. 3.2(d)). 

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. 

Nystrom, C., Alderborn, G., Duberg, M., and Karehill, P-G. (1993), Bonding surface area and bonding mechanism—Two important factors for the understanding of powder compactability, Drug Dev. Ind. Pharm., 19, 2143-2196. 

DeCrosta, M. T., Schwartz, J. B., Wigent, R. J., and Marshall, K. (2000),Thermodynamic analysis of compact formation compaction, unloading, and ejection. I. Design and development of a compaction calorimeter and mechanical and thermal energy determinations of powder compaction, Ini. J. Pharm., 198,113-134. 

The demonstration of the validity of the continuum-based modelling approach to tablet compaction requires familiarity with fundamental concepts of applied mechanics. Under the theory of such a mechanism, powder compaction can be viewed as a forming event during which large irrecoverable deformation takes place as the state of the material changes from loose packing to near full density. Moreover, it is important to define the three components of the elastoplastic constitutive models which arose from the growing theory of plasticity, that is the deformation of materials such as powder within a die ... 

The phenomenon studied with the help of instrumentation is the so-called lag time (the time difference between peak of compression and maximum punch penetration). The extent of this lag is indicative of compaction mechanisms of the powder being compressed. ... 

---