Pigments light fastness

Pigments for finishes are selected on the basis of their colour, but special attention must be paid to inertness in the chosen binder and stability and light fastness under the conditions of application and exposure. Flake pigments such as aluminium and micaceous iron oxide give finishes of lower moisture-vapour permeability than conventional pigments, and consequently contribute to better protection. 

Although it might seem at first sight that dyestuffs are merely held mechanically within the pores, and this view is probably correct in the case of inorganic pigments, there is some support for the opinion that only those dyestuffs which form aluminium/metal complexes produce really light-fast colorations. 

For the synthesis of these pigments the dye molecules must possess a high light fastness. The colors are not very bright, and because of the thin dye layer these pigments are more susceptible to an oxidative photo degradation than normal pigments. 

Many other dyes, including tartrazines and eosines, have been made into pigments in the past for use in printing inks, in which their soft texture and bright hues were much valued. Their light fastness was generally only 1 on the ISO scale and they are now obsolete. 

Unfortunately, the fluorescent effect is not directly proportional to the concentration of colorant present, since there is considerable quenching if quite low concentrations are exceeded. The light fastness of the fluorescent pigments is also less than that of many other organic pigments now available, but improvement can be achieved using overlayers containing ultra-violet absorbers. This is an area in which further research will clearly be needed. 

In general, metal complexes formed from bidentate azo chromogens are little used as dyes but do find important applications as pigments (section 2.3.2). Rare exceptions exist, however, such as the nickel(II) complex of p-nitroaniline— BON acid (5.11). This has been used for bordeaux prints of high light fastness on cotton fabrics. Two possible modes of bidentate attachment to the nickel atom can be envisaged (Scheme 5.2). 

The light fastness requirements of plastic colorations are met by many organic pigments but are similarly dependent on the ratio of colored pigment to titanium dioxide. The stability of the plastic used should also be taken into consideration here. 

Standards exist for determining the light fastness of colored pigmented plastics in daylight and in xenon arc light, i.e. in accelerated exposure equipment, and for determining the weathering fastness. 

Pigment Red 8 affords clean, bluish shades of red. It is primarily used in the printing ink industry. P.R.8 exhibits high tinctorial strength and produces brilliant prints. Commercially available types with specific surface areas between about 50 and 60 m2/g afford transparent prints. P.R.8 is used in prints which require no particular solvent resistance. However, the pigment tolerates solvents much better than the yellower P.R.7 in this respect, P.R.8 matches the yellower, but more light-fast P.R.5. The prints are fast to soap but not entirely stable to butter and paraffin. P.R.8 is sensitive to clear lacquers coatings and to sterilization. It tolerates exposure to 140°C for 30 minutes. 

P.V.50, produced in Japan, is only of limited regional importance, since it fails to satisfy the increasingly stringent application requirements for organic pigments. Its fastness to light, for instance, is particularly poor 1/3 SD and 1/1 SD letterpress proof prints equal only step 2 on the Blue Scale. Compared to P.V.23, Dioxazine... 

As a result of their salt character, (3-naphthol pigment lakes are faster to solvents and more resistant to migration than (3-naphthol pigments, but also less light-fast. They are only moderately fast to alkaline agents. The polar character of these pigments is responsible for their good heat stability. 

P.Y.154 is a useful pigment for the printing ink industry wherever high light-fastness is required. Letterpress proof prints up to 1/25 SD equal step 6-7 on the Blue Scale for lightfastness, which is at least 11/2 to 2 steps above that of similarly colored diarylide yellow pigments or representatives of the monoazo yellow pig-... 

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