The technological importance of the crystallographic and surface properties of copper phthalocyanine pigments

Abstract

AbstractNew and previously reported results are combined to emphasise the importance of fundamental crystallographic and surface chemical properties on the technology of copper phthalocyanine pigments. Particular attention is paid to undesirable aggregation of pigment crystals and to the changes in aggregate structure and crystal phase which occur during ageing. The two most commercially significant forms of copper phthalocyanine are the α‐ and β‐polymorphs. The crystals of both consist of parallel molecular stacks. In β‐phase crystals, the coordinate bonding between adjacent molecules within the stacks promotes crystal growth in the direction of the stacks, producing commonly observed rod‐shaped crystals. Moreover, the molecular stacks of the β‐phase, being interlocked, are not readily distorted. In contrast, the distortion of lattice planes in the α‐phase can accommodate mismatch between contiguous crystals, and promote their fusion into strongly coherent aggregates. Although the strength of aggregation is thus influenced by the crystal lattice, the geometric structure of aggregates is largely determined by crystal shape, brick‐shaped crystals being more prone to aggregation than rod‐shaped crystals. Aggregates of brick‐shaped crystals are porous, and solvent molecules can penetrate some way into their internal structure. Ageing may then occur. Changes in crystal phase and aggregate structure are discussed at some length. New evidence is presented for the existence of an intermediate metastable state in the α‐→β‐phase transformation induced by solvents. The β‐→α‐phase change induced by mechanical grinding is shown to start at the surface of β‐phase crystals, and then to extend gradually throughout their bulk. Examples are given of different kinds of ageing of aggregate structures. Ageing of aggregates of α‐phase crystals in liquid toluene involves not only Ostwald ripening and a transformation to the β‐phase, but also crystal fusion (as occurs during ageing in toluene vapour). In liquid n‐propanol, on the other hand, the aggregates are converted to highly consolidated structures.