TEM studies of the crystal growth of indanthrone pigments
The aim of this work was to study the crystal growth of indanthrone during the pigmentation process. The colouring properties of a pigment are dependant on the chemical and crystallographic structure of the pigment. However, other factors are known to affect these properties including particle size, particle size distribution and level of dispersion in the chosen application medium. The parameters which affect the growth of the pigment particles were investigated with the emphasis placed on the mechanism by which growth took place. The final form of the crystals after growth was also investigated in some detail. Various electron microscopy techniques were employed in the investigations in this thesis. High and low magnification imaging and diffraction were studied on the CTEM (conventional transmission electron microscope) whilst PEELS (parallel electron energy loss spectroscopy) and DPC (differential phase contrast) studies took place on the VG HB5 STEM (scanning transmission electron microscope). In addition to these studies, x-ray diffraction and surface area analysis techniques were employed.
The low magnification CTEM work gave good information on the size, shape and size distribution of the pigment particles and enabled detailed analysis of the level of growth attained under varied reaction conditions. Parameters varied during these reactions included choice of solvent, solvent concentration, reaction time and the method used for removing the solvent. Methyl benzoate and nitrobenzene were found to be effective in promoting crystal growth in indanthrone whilst isopropanol proved to be extremely ineffective. The rate of growth was found to be affected by, among other things, the concentration of the solvent and the time it was in contact with the pigment. The most likely method of growth has been identified as ripening followed by coalescence. Initially, larger particles grow at the expense of the smaller particles - the smaller ones go into solution and are then able to aid the growth of the larger particles. As they increase in size coalescence is more likely as the surface area along the side of the particles is greater - therefore adhesion by the adjoining particle is more likely. Heat is also an important factor as it promotes the ripening part of this process as this increases both solubility and energy leading to faster ripening.
Diffraction showed that growth is strongly promoted along the same direction (b-axis) in the majority of crystals. Lattice imaging studies confirmed this orientation as the most favoured and also gave detailed structural information. This technique also gave evidence for coalescence as a secondary growth mechanism and illustrated strained crystals.
Annular dark field (ADF) imaging on the VG HB5 indicated that some particles were not of uniform thickness. PEELS confirmed that thickness variations were present in numerous particles due to irregular growth and/or coalescence. Differential phase contrast (DPC) imaging was used to give a topographical image of certain samples and confirmed the presence of undulations on the surface of the particles.