Fading Stability

The various factors affecting the light-fading stability of color prints have been reviewed. The various issues of inkjet printed materials made with dye-based as well as pigmented inks, and traditional chromogenic color prints have been discussed. The predicted years of display before noticeable fading occurs with color prints have been listed for various ink types and printers (4). 

Inkjet photographic prints, especially those printed with dye-based inks on porous media, are subject to rapid dye fading when exposed to ambient levels of ozone (5). In contrast, the fading rate is much slower when dye-based inks are printed on sweUable media and moreover, when pigment-based inks are used. High concentrations of ozone under controlled conditions have been used for testing in order to establish a standardized test method for accelerated aging. There is a relationship between the concentration of ozone and the time of exposure. 

The first ANSI standard for testing the permanence of color photographs was ANSI PHl.42-1969 (6). This standard did not contain tests for the prediction of the stability. 

The ANSI Standard IT9.9-1996 (7) is commonly recommended for predicting the dark storage print life for color photographic materials (8). 

This standard is based on Arrhenius methodology and calls for measuring changes in print density that occur as a function of temperature at 50% + 3% relative humidity. The objective was to examine the dark keeping properties of inkjet photographic prints as a function of constant absolute humidity at various temperatures. The primary focus is on commercially available dye-based inks printed onto both porous and nonporous photo-quaUty papers (9). 

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Dye Stability. The dyes used in photographic systems can degrade over time, both by thermal reactions and, if the image is displayed for extended periods of time, by photochemical processes. The relative importance of these two mechanistic classes, known as dark fade and light fade. 

The blue-violet stain which forms on thin-layer chromatograms when amino acids are stained with ninhydrin is only stable for a short time. It rapidly begins to fade even on cellulose layers. The stability can be appreciably enhanced by complex formation with metal ions [3]. 

Most published data deals with model solutions to assess the major factors influencing betalain stability, among which pH and temperature are most frequently addressed. Until recently, total color loss was assessed by spectrophotometric monitoring of the decline at the wavelength of maximum absorption. To predict color fading over time, kinetic data were derived therefrom, most often obeying first-order decay principles. 

Common additives in food are sulfur S(IV) oxospecies. Quinoline Yellow and Tartrazine show excellent stability toward S(IV) oxospecies while erythrosine. Red 2G, and Green S show good stability. - All other colorants show fair stability, except indigotine, which fades. In the presence of metabisulflte. Sunset Yellow FCF is degraded to a lemon yellow compound identified as l-(4 -sulfo-l-phenylhydrazo)-keto-3,3,4-trihydronaphtalene-4,6-disulfonic acid by NMR and FAB-MS techniques. ... 

The substituent effect in the phenyl group at the 3-position is also observed in the benzothiazoline series.2,17 The thermal fading rate increases with the bulky group at the 3-position, but the colored forms of 3-methoxy and 3-phenoxy derivatives are largely stabilized by an intramolecular interaction with 5-hydrogen atom.2... 

New developments are hydroxylamines and lactones (for processing stability), which operate at an earlier stage during stabilisation. Lactone (benzofuranone) chemistry has been identified as commercially viable, and marks a revolutionary advance in comparison to hindered phenols and phosphites [18]. New lactone chemistry (Figure 10.1) provides enhanced additive compatibility, reduced taste and odour (organoleptics), resistance to irradiation-induced oxidation, and inhibition of gas fade discoloration. The commercial introduction of fundamentally new types of stabilisers for commodity and engineering polymers is not expected in the near future. 

No such fading in methanol is observed with the ion XVIII, in which the additional phenyl group extends the unsaturated system and stabilizes the ion XVIII, as compared with the ion XIX, in which the nitrogen atom insulates the two parts of the molecule. 

The other stability problem with curcumin is sulfur dioxide. If the sulfur dioxide level is above 100 ppm then the colour will fade. 

The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. 

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