Microcharacterisation of halogenated copper phthalocyanines using transmission electron microscopy
Halogenated copper phthalocyanines are widely used in the pigmentary form to colour paints, plastics, dyes and printing inks. Their colouring properties are strongly dependent on the size, shape and chemical composition of the individual pigment particles which have dimensions of typically 50nm. Batches of pigments, produced using nominally the same industrial process, can exhibit differences in their colouring properties. Electron microscopy can provide the high spatial resolution required to analyse both the physical and chemical properties of the pigment particles and thus help to determine the reasons for their differing colouring properties. Various techniques, available in electron microscopy, were used in attempt to characterise as fully as possible samples of highly chlorinated CPC's. Low magnification images revealed differences in the particle size, shape and particle size distribution, between pigment samples, which would explain differences in their performance. These images also showed that the pigments consisted mainly of small aggregates with dimensions of the order of several pigment particles. Diffraction studies of these aggregates suggested that most of them were pieces of pigmentary material which had not been broken down by the pigmentation process. Further diffraction studies also suggested that the particles were faceted and that these facets coincided with preferred crystallographic planes. Lattice images showed that many particles deviated considerably from perfect crystallinity. The modified differential phase contrast imaging technique allowed simultaneous topographic and lattice images to be recorded from the same area, distinguishing aggregates which had not been broken down by the pigmentation process. The chemical composition of highly chlorinated CPC material was obtained using electron energy loss spectroscopy. An accuracy of ±0.7C1 atoms per molecule could be obtained from a diameter of ~35nm on epitaxially grown thin films. However, the actual value obtained had a small dependence on thickness. With individual pigment particles, other effects were apparent, preventing a precise determination of the composition. The technique showed that with further refinements, it might be possible to analyse accurately single pigment particles.