Pigments lightfastness

Although standard types resist light well, especially in full shades, they are not as fast as most monoazo yellow pigments. Lightfastness is lost rapidly with decreasing depth of shade and decreasing pigment area concentration, respectively. The commercially available types disperse easily in most media. 

Lightfastness is measured by exposing pigmented film to an artificial or natural, eg, Florida exposure, light for a predetermined time. It is a relative term where the color of a sample exposed to a known light source is compared to its original color values. To make it meaningful, aH conditions of the exposure have to be weH defined. 

These pigments are sensitive to heat and bleed ia most paint solvents. They are, however, resistant to acids and bases. Their tinctorial strength is considerably greater than that of inorganic yellows but they are weaker than the diaryUde yellows. They are used extensively ia emulsion paints, paper coating compositions, inks (qv), and, depending on particle size, can ia some cases be used outdoors because of excellent lightfastness ia full shades. 

Reds. Para Red or Pigment Red 1 [6410-10-2] is an intense, reasonably opaque red which shows poor lightfastness, particularly in tints. 

Another important pigment in this class is Pigment Red 170 [6985-95-17, which provides medium shades of red, and when particle-grown produces an opaque modification which shows improved migration resistance and lightfastness. Like some other Naphthol AS pigments it shows the phenomenon of polymorphism. 

The pigments are used primarily in plastics, including polypropylene fibers, because of very good bleed resistance, heat stabiUty, and lightfastness. The reds also find use in printing inks, primarily for high quaUty products. 

Present usage of triaryknethane dyes, ca 1996, is confined mainly to nontextile appkcations. Substantial quantities ate used in the preparation of organic pigments for printing inks, pastes, and for the paper printing trade, where cost and brilliance of shade are more important than lightfastness. 

Monoazo Pigments. In combination with other groups, the a2o linkage, —N=N—, imparts color to many dyes and pigments (see Azo Dyes). The simplest of these, ie, the Hansa yellows, toluidine reds, and naphthol reds, do not have the lightfastness and heat stabiUty required for plastics. Permanent YeUow FGL and Permanent Red 2B are stable enough for vinyls, polyethylene, polypropylene, and ceUulosics (11). Permanent Red 2B is available as the calcium, barium, or manganese salt. 

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