Compaction behavior

A fundamental requirement in powder processing is characterization of the as-received powders (10—12). Many powder suppHers provide information on tap and pour densities, particle size distributions, specific surface areas, and chemical analyses. Characterization data provided by suppHers should be checked and further augmented where possible with in-house characterization. Uniaxial characterization compaction behavior, in particular, is easily measured and provides data on the nature of the agglomerates in a powder (13,14). 

As shown in Fig. 4.1, resin feedstocks have a considerable level of interparticle space that is occupied by air. This level of space and thus the bulk density of the feedstock depend on the temperature, pressure, pellet (or powder) shape, resin type, and the level and shape of the recycle material. For a specific resin feedstock, the bulk density Increases with both temperature and the applied pressure. Understanding the compaction behavior of a resin feedstock is essential for both screw design and numerical simulation of the solids-conveying and melting processes. Screw channels must be able to accommodate the change in the bulk density to mitigate the entrainment of air and the decomposition of resin at the root of the screw. Typically, screw channels are set by using an acceptable compression ratio and compression rate for the resin. These parameters will be discussed in Section 6.1. 

A. Gupta, G.E. Peck, R.W. Miller and K.R. Morris, Influence of ambient moisture on the compaction behavior of microcrystalline cellulose powder undergoing uni-axial compression and roller compaction a comparative study using near-infrared spectroscopy, J. Pharm. Sci., 94(10), 2301-2313 (2005). 

The effect of operating pressure on flux for various annealing treatments is shown in Figure 3. As is usually observed the optimum pressure, beyond which the curves flaten out, increases as the membranes become denser. Maximum flux of the untreated membrane at a pressure of 30 bar is around 10 m /m d (250 gfd) at 20 % brackish water rejection. The optimum operating pressure of annealed bentonite-containing membranes is lowered by about 10 bar, however, the compaction behavior is comparable to that of the reference membrane. 

Figure 7.26 Compaction behavior of KBr powder and spray-dried granules of alumina and clay tile as a function of punch pressure. From J. S. Reed, Principles of Ceramics Processing, 2nd ed. Copyright 1995 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

The compaction behavior of the preform differs a lot depending on the preforming method and the type of reinforcement that has been used. A typical compaction curve for a carbon-fiber-woven fabric is shown in Figure 12.4. The three curves correspond to... 

Figure 12.4 Compaction behavior of a woven carbon fiber fabric. The solid curve shows the behavior at room temperature the other two lines show the behavior at 100 C and 110°C. The fabric was factory treated with an epoxy preform binder...

An important observation is that the fiber volume fraction at zero compaction pressure can differ significantly between different fabrics [8], This represents a lower limit to the range of fiber volume fractions that are acceptable. A lower nominal fiber volume fraction can result in movement of the reinforcement during filling and incomplete impregnation. Another typical feature of the compaction behavior is that all fabrics behave like nonlinear (stiffening) springs and that the possible increase in fiber volume fraction from the value at rest is limited [8] (Fig. 12.4). 

The only real test to determine that the scale-up batch will run well on the selected tablet press in production is a use test i.e., the batch must be run. Although there is no completely accurate prediction of compaction behavior during scale-up, there are many excellent test methods that can provide an evaluation of specific material properties (flow, lubrication, etc.) and provide an understanding of the material properties of one s formulation. If proper science is applied, these measurements and approaches can provide assurance that scale-up can occur with a minimum of problems. 

All measured variables, namely force, time, displacement, and temperature, can be combined differently and can be analyzed afterward. From the functional relations, conclusions can be drawn about the compression and compaction behavior of the materials. 

Tsardaka, E. D. (1990), Viscoelastic properties and compaction behavior of pharmaceutical particulate material, Ph.D. Thesis, University of Bath, Bath, England. 

York, P. (1979), A consideration of experimental variables in the analysis of powder compaction behavior, J. Pharm. Pharmacol., 31,244-246. 

Moisute acts as a plasticizer and influences the mechanical properties of powdered materials for tablet compression. In the case of microcrystalline cellulose, at moisture levels above 5% the material exhibits significant changes consistent with a transition from the glassy state to the rubbery state [17]. The possible influence of moisture on the compaction behavior of powders was also analyzed by Gupta et al. [18]. This work evaluates the effect of variation in the ambient moisture on the compaction behavior of microcrystalline cellulose powder. 

The work conducted by Gustafsson et al. [19] evaluated the particle properties and solid-state characteristics of two different brands of microcrystalline cellulose (Avicel PH101 and a brand obtained from the alga Cladophora sp.) and related the compaction behavior to the properties of the tablets. The difference in fibril dimension and, thereby, the fibril surface area of the two celluloses were shown to be the primary factor in determining their properties and behavior. 

Gustafsson, C., Lennholm, H., Iversen, T., and Nystrom, C. (2003), Evaluation of surface and bulk characteristics of cellulose I powders in relation to compaction behavior and tablet properties, Drug. Dev. Ind. Pharm., 29(10), 1095-1107. 

Multipoint BET analysis showed that the surface area and average pore diameter of quartz are about two times higher than that of dolomite. These results are important especially in the random packing on the filtration and the compaction behavior of the cakes in shear strength measurements. 

Heckel considered the compaction of powders analogous to that of a first-order chemical reaction. The pores were the reactant and the densification of the material the product. The proportionality between the change of density with pressure and the pore fraction was the process kinetics.Heckel explained mathematical constants that described the compaction behavior of a given powder and developed a mathematical relationship, Eq. (2). ... 

Razaghi, A.M. Venkatesh, G., Fassihi, R., Consolidation and Compaction Behavior of Sorbitol in Pharmaceutical Technology A Compaction Simulator Study. AAPS Meeting, November 1997. 

Patel S, Kaushal A. Bansal A. Effect of particle size and compression force on compaction behavior and derived mathematical parameters of compressibility. Pharm. Res. 2007 24(1), 111-24. 

The type of roll surfaces has a marked effect on the overall production throughput of a roller compaction process (Fig, lO). A variety of roll surfaces and configurations are available depending on the compaction behavior of the material. For instance, powders that tend to stick or cling to the roll surface requires the use of smooth or circumferential grooved surfaces, while materials that release cleanly from the roil after compaction may be pressed with one of the pocketed design or rolls with grooves in the axial direction. 

Gupta APG. Miller RW, Morris KR. Effect of the variation in the ambient moisture on the compaction behavior of powder undergoing roller-compaction and on the characterLstics of tablets produced from the post-milled granules. J Pharm Sci 2005 94(I0) 2314-26. 

When interpreting compaction data it is important to know what compaction mechanisms operate over different levels of pressure thus, a good compaction plot should be able to describe material behavior and determine when material behavior changes. One of the earliest studies done to investigate the compaction behavior of powders was by Walker in 1923 he plotted the relative volume (V,.) of the powder compact against the logarithm of the applied axial pressure (F) using the relationship (69) ... 

Empirical evidence shows that materials with low mean yield pressures undergo plastic deformation, whereas, tableting materials with high mean yield pressure have a tendency to be brittle and consolidate via fragmentation. Based on Heckel plots and the compaction behavior of materials can be classified into three types. A, B, and C (Fig. 6) (88,89). 

Particle size distribution measurements should also be made from lab milled samples only. The particle size distribution influences the compacting behavior and green density as well as the sintering properties. 

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