Powder Structure Control

Publication Reference: 
Author Last Name: 
R. Kohlus
Report Type: 
Research Area: 
Particle Formation
Publication Year: 
Publication Month: 

The project powder structure control is concerned with the definition and calculation of mathematical descriptors of granule and powder structure. The additional goal is to give the structure descriptors a physical meaning by linking them to product properties e.g. dissolution behaviour and mechanical strength.

This report covers the last year (Dec. 2011 - Nov. 2012) of work on the powder structure control project. In this period the focus has been on the development of a software package for structure evaluation. The other main part of work was concerned with a literature review on stereological methods in practical application and on the relationship between process, structure and properties.

The software package includes first and second-order stereological methods. The volume fractions of the three phases and the interfaces between those have been calculated for a test image of spheres and for exemplary images of granules. Results are in good agreement with the known test image and with visual examination of the granule images. For the evaluation of the spatial arrangement of granules second-order methods are necessary. The chord length distribution, the volume weighted star volume and the covariance function deliver results that are plausible regarding the exemplary images. In the literature review publications of al disciplines have been screened and sorted into 3 different groups: description or derivation of stereological methods, practical application of stereological methods and relationship between process, structure and properties of granules. As the stereological investigation of structure is known for a long time a lot of review publications are available, especially for first-order stereology [9, 10, 12, 15, 21, 50]. Second-order stereology has been in the focus of recent publications, especially the normalized covariance function and the pair correlation function have been of interest.

Applications of these methods could be found in different fields of research and for different spatial materials like biological tissue, concretes and porous materials. Granular materials play an important role in industry and the control of their behaviour and structure is of interest. Different approaches can be made to correlate structure, process properties and product properties. The relationship between process and product properties is very common in literature. Process properties like impeller speed of a granulator or the recipe of the raw materials are varied and product properties like strength, porosity or dissolution behaviour are compared to each other [23, 37, 39]. Further publications are concerned with the relation between structure and granule properties [2, 5, 11]. Thereby porosity and primary particle size distribution has been a frequently used parameter.

Based on these findings the next step is a structure analysis that includes spatial arrangement of structure elements because first-order measures like porosity are not suitable for every issue. When the structure can be fully described by one or more of the mathematical descriptors it should become possible to predict product properties. Process parameters as well as the raw materials will be altered systematically to generate a variety of structures.

The investigation of the functional relationship between process, structure and properties shall be examined in the following year of this project. The implemented structure measures shall be tested and optimised by using experimental input for further interpretation and understanding. Different structure generation and different recipes are planned to achieve a good variability of internal structures of the granules.