A Comparison of Three Extrusion Systems - Pharmaceutical Technology

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A Comparison of Three Extrusion Systems
The authors conducted an experiment to determine the type of extrusion that provides the best productivity and pellet quality. This article contains online bonus material.


Pharmaceutical Technology
Volume 35, Issue 1

Results and discussion




Design of experiments interpretation. The mathematical model generated for each response Y was a quadratic model with first-order interactions, designed according to the following equation:

where X i and X j represent the levels of the factors; a 0 is the intercept representing the mean of the measured response data; a i and a j , a ii and a jj, and a ij correspond to the coefficients of first-order terms, the coefficients of second-order quadratic terms, and the coefficient of second-order interaction terms, respectively. The coefficient corresponding to a factor or interaction shows its importance to the studied response. The symbol e represents pure error.




To simplify the design-of-experiments interpretation, the coefficients of second-order quadratic terms are not presented. Results are expressed according to the following equation:


Table I: Significant effects for the main factors and interactions for various responses Yx.
For the multilevel categoric factor C presenting three levels, the software calculated two coefficients, C(1) and C(2), corresponding to the difference between the high level and the average, and the difference between the low level and the average, respectively. The coefficient values were expressed in coded units to compare each one's relative effects to those of the others. Analysis of variance was performed to determine the significance of the model. A term that had a probability value lower than 0.05 was considered a significant effect. A probability value greater than 0.10 was regarded as insignificant. Table I summarizes the p values for the coefficients of factors for the different responses tested. Graphical analyses of responses were performed to determine the most important factors for a response by comparison with the other factors. Figure 4 shows the effects of factors and interactions on the responses. The red bars correspond to the factors that influenced the response. When an interaction between water quantity and extrusion system is underlined, the mean response value according to the extrusion system is shown according to three water-quantity values (i.e., minimum, medium, and maximum).

Pellet characteristics. The pellet-morphology study showed great differences between the three extrusion systems. At the lowest water quantity (i.e., 22.5%), pellets created after radial extrusion presented a wide size distribution with many small particles, compared with pellets created after axial extrusion, which presented a majority of rods. Only dome extrusion provided pellets of the correct specifications. The pellets' morphology is linked to extrudate characteristics and the spheronization step. At low water quantities, radial extrudates were not densified enough and broke during the spheronization step, in contrast with axial extrudates, which were well densified but did not have enough plasticity to deform during the spheronization step. An increase in the water quantity improved the morphology of all pellets excepted those produced by the dome system.




The following equations obtained for pellet size, pellet dispersion, and usable yield, in which significant terms are shown in bold type, are represented in Figure 4:


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