Pigments, coloristic properties

Bismuth V n d te. The use of lead chromate pigments has been slowly phased out of many appHcations. Thus a search for more environmentally acceptable relatively inexpensive yeHow pigments having exceHent coloristic properties has been ongoing. 

Dyes, on the other hand, ate colored substances which ate soluble or go into solution during the appHcation process and impart color by selective absorption of light. In contrast to dyes, whose coloristic properties ate almost exclusively defined by their chemical stmcture, the properties of pigments also depend on the physical characteristics of its particles. 

Publications have over the course of the years proposed several classification systems for organic pigments. Basically, it seems appropriate to adopt a classification system by grouping pigments either by chemical constitution or by coloristic properties. Strict separation of the two classification systems is not very practical, because the categories tend to overlap however, for the purposes of this book it is useful to list pigments according to chemical constitution. 

Fig. 38 Dispersion of Pigment Blue 15 6 in an alkyd-melamine resin baking enamel. Effect of the dispersion time and the dispersion temperature on the coloristic properties (depth of shade). White reduction 1 50 Ti02.

Fig.51 Effect of the hardening system in an epoxide resin powder coating on the coloristic properties of the paint. Pigment Dibromoanthanthrone (Pigment Red 168). Cross-linking conditions 15 minutes at 180°C, respectively.

It has often been observed that the coloristic properties of an organic pigment are a function not only of the size of particles but also of their shape. This is due to the anisotropy of the optical properties in different crystallographic directions within the crystal forms of a pigment. In 1974 [5, 6], it was demonstrated that of the equally sized but differently shaped particles of beta copper phthalocyanine blue, the almost completely cubic, i.e., more or less isometric form produces greenish blue shades, while acicular forms are responsible for reddish blue hues. The optical behavior of ordered pigment particles in systems has been reported in the literature [7, 8]. 

P.O.5 is one of the most significant organic pigments. Two product lines with different particle sizes are available which differ considerably in their coloristic properties. The varieties with coarser particle sizes and specific surface areas between ca. 10 and 12 m2/g are much more reddish and duller than the types with somewhat finer particle sizes and specific surface areas between 15 and 25 m2/g. 

P.Y.139 is a reddish yellow pigment, used in plastics, paints, and printing inks. The commercial types exhibit a wide variety of particle size distributions and accordingly demonstrate very different coloristic properties, which is especially true for the hiding power. The opaque version is considerably redder. Incorporated in a paint, it is less viscous, which makes it possible to increase the pigment concentration without affecting the gloss of the product. 

P.R.149 also lends color to cast resins made from materials such as unsaturated polyester or methacrylic acid methylester, which are polymerized with peroxide catalysts. P.R.149 is equally lightfast in these media. In polycarbonate, the pigment tolerates exposure to more than 320°C. This is an asset in view of the fact that polycarbonate shows high melt viscosity and is thus processed at up to 340°C. The list of applications also includes other media, such as PUR foams and elastomers, for which P.R.149 is recommended because of its good heat stability and its coloristic properties. 

Its coloristic properties make P.B.l a suitable and widely used colorant for special printing inks. The pigment may be incorporated without problems in oily binders, i.e., in letterpress and offset inks. It is equally often used in publication gravure printing inks based on toluene and in packaging printing inks based on NC. Problems which may arise as P.B.l is incorporated in these vehicles are mentioned under P.V.2. Besides, P.B.l lends color to paper, wallpaper, typewriter ribbons, and other media, although it is often difficult to disperse. 

P.Y.192 is also used in other polymers which are processed at high temperature. According to the manufacturer, no coloristic changes are observed if P.Y.192 is incorporated in polyester the pigment maintains its coloristic properties even during the condensation process (Sec. 1.8.3). 

Coloristic Variations on the Basis of Mica. Mica platelets can be coated with a variety of compounds to produce novel pigments. Solid state reactions and CVD-process enlarge the possibilities for the synthesis of mica pigments. In addition, the calcination of the materials in the presence of inert (e.g. N2, Ar) or reactive gases (e.g. NFI3, H2, hydrocarbons) allows the formation of phases which are not producible by working in air. Table 54 contains a summary of nacreous mica pigments with special coloristic properties. 

Table 54. Examples of mica-based pearlescent pigments with special coloristic properties...

Extender pigments are low-cost, generally colorless or white pigments with a refractive index less than 1.7. Sometimes diese pigments are also referred to as fillers. Many extenders are derived from natural sources and display many diverse properties. They are added to various formulations to improve technical and application properties and to reduce costs, Like pigments, extenders are dispersed in media in which they do not dissolve, but compared to pigments they do not have any significant coloristic properties. 

Table 5.3 Coloristic properties of pigments synthesized in two different microreactors compared to the batch standard.

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