Pore size distribution in tablets measured with a morphological sieve

Abstract

Porosity and pore structure are important characteristics of tablets, since they influence mechanical strength and many other properties. This paper proposes an alternative method for the characterization of pore structure based on image analysis of SEM micrographs. SEM images were made of sodium chloride tablets made with three different particle sizes. The pore size distribution in these images was determined with a technique referred to as a morphological sieve. The results were compared to the pore size distributions as obtained with mercury porosimetry. The SEM images display small cracks inside the grains and small ‘floating’ grains inside the pore space. As these artifacts are induced in sample preparation, they need to be identified and removed from the images before analysis. The influence of the size of the discarded structures on the total porosity and the pore size distribution was investigated. The small ‘floating’ grains prevented the determination of the size of large pores, but had a negligible effect on the porosity. The removal of small cracks inside the grains had no effect on the pore size distribution but a large effect on the porosity. Based on the comparison of these results with the experimentally determined porosity, a maximum size for the structures that were to be removed was determined. The resulting pore size distributions were in the same order of magnitude as the results obtained with mercury porosimetry. Both methods display a comparable relative shift of the pore size distributions to larger sizes for tablets with increasing particle size. Therefore, it can be concluded this image analysis technique is a good method for the characterization of pore structure.