Pigments in paints, polymers and inks or on tapes, discs, etc. must fulfil various tasks which vary according to the particle properties of the powder. The extent to which these tasks can be fulfilled depends not only on the powder itself but also on the techniques used to incorporate it into the medium of use and on the interfacial properties of the particle/vehicle interface. The dominating properties are chemical constitution, crystal modification, particle size and shape, particle size distribution, absorption and scattering of electromagnetic radiation, dispersibility in very different vehicles, and fastness and insolubility qualities with respect to different environments and conditions. To achieve these different goals with one and the same chemical bulk species, it is necessary to surface treat the pigments at different steps in the production and application processes.
As far as dispersion is concerned, several methods of surface treatment are known which offer pigment manufacturers the possibility of modifying the surface character of a pigment in a desired manner. Thus, the original pigment surface can either be modified or replaced by a completely new surface by precipitating appropriate substances on to the pigment. The nature and amount of coating agent used depends on the type of pigment, as well as on the nature of the application medium. In some cases, it is enough to cover part of the pigment surface with a monomolecular layer of the coating agent, whereas in other cases complete coating with a multimolecular layer may be more advantageous.
Traditional methods for the surface treatment of organic pigments were designed for solvent borne coatings. The surfaces of both untreated and conventionally treated organic pigments tend to be hydrophobic. However, because of the trend away from solvent-based coatings toward waterborne coatings, surface treatments that allow the organic pigment to be more readily dispersed and stabilised in waterborne systems would be desirable. That is, a more hydrophilic pigment surface is desired.
Pigment Treatments
The surface treatment of pigment particles plays an ever more important role in the pigment industry, especially in paint and ink applications. The use of pigment derivatives shows some strong limitations with respect to bleeding and colouristic. Novel treatments of pigments with inorganic materials are presented. This type of colourless and insoluble treatment has a marked positive effect on the rheological behaviour of paints without changing the other pigment properties. These treatments allow a broad modification of the surface characteristics of the pigment particles which leads to novel improved products.
Phthalocyanine Pigments
Pigment Green 7 is treated with naphthalene sulfonic acid - formaldehyde - polycondensate as sodium salt CAS Number: 9084-06-4:
This is the main anionic surfactant used it gives a very distinctive smell to the pigment. This anionic surfactant is used to improve the dispensability of the pigment. It is often the reason when formulating aqueous dispersions for coatings that nonionic surfactant only is required to produce a stable dispersion.
Phthalocyanine Green treated with Nonionic
The surface treatment of phthalocyanine green in chlorobenzene has been studied by use of span, tween and fatty amines. The lengths of hydrophobic chains have a slight effect on the wetting critical surface tension and the dispersion component of the surface tension of the treated pigment. The distribution of the size of the treated pigment moves from big size region to small one. The polar component of surface tension decreases obviously with the increasing of length of hydrophobic chains. The disperse medium used for treating pigment has an evident effect on the lypohydrophilic character. When water is taken as the dispersing medium, the obtained pigment has a good dispersibility in water and when oil is taken as the medium, the obtained pigment has a good dispersibility in oil.
Azo Pigments
The treatment of azo pigment production both anionic and nonionic surfactants are used to treat azo pigments, most used to alter the crystal shape and particle size most are added after diazotisation and are usually washed off the pigment in the final stages of production.
The main purpose of the surfactant is to make the pigment brighter and to shift the hue i.e. make a red pigment yellower or bluer depending on the customer requirements.
PEGMono Oleate alkoxylated nonionic, used with red azo pigments:
Increases brightness and yellow hue.
Rosin Treated Pigments
Rosin ester used in pigment synthesis and surface treatment, can improve azo pigment colour rendering and dispersibility, get much better colour retention, thermostability than traditional rosin.
Rosin ester is also used in azo pigment synthesis, can be used without saponification. Improve the fluidity of the offset ink, decrease the viscosity of the offset ink. Improve the compatibility, wettability and dispersibility between pigment and medium.
Pigment Red 57.1 can contain up to 10% rosin.
Many azo pigments for ink applications are treated with Rosin.
Rosinationis the oldest form of pigment surface treatment.
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