Bleaching reversal processing

The brown color of the Ni(n-Bu2 Dtc) J cation is photochemically and thermally bleached reversibly in CH3CN solution. Thiuram disulfide and the Ni(II) complex were identified as products of this bleaching process. The kinetics of the reappearance of color were reported to depend on the square of the concentration of the bleached nickel and were inversely proportional to the concentrations of thiuram disulfide and Br (231). 

In the Formula section of the Cookbook, only one bleach bath and one clearing bath formula is given, Bleach Bath DW-1 and Clearing Bath DW-2. Use both of these with any of the three developers given under Reversal Processing. 

A second important aspect of electrochromism is the temporal response under alternating potentials ( 0.55 V). The DG showed sharp and distinct transitions between the colored/oxidized and bleached/reduced state across the entire visible spectrum (Fig. 6.10b). This time-resolved switching behavior was analyzed in more detail at A = 630 nm (Fig. 6.10c). The DG device showed short characteristic response times of 53 ms for the bleaching step and 63 ms for the reverse process, determined by fitting exponential functions to the switching curves. These short response times which are close to video rate (24 frames per second) are enabled by the short ion diffusion distance through the only >= 5 nm thick NiO nanotube wall. 

The reverse reaction, B returning to A, can be driven either by thermal or photochemical energy, or both. When the reversion is photochemicaHy driven, the process is called optical bleaching. Optical bleaching is a general characteristic and is a factor in almost all photochromic systems, even those normally thought of as being thermally reversible. 

Plastics can be used to make erasable printing media by a number of different techniques. Photo changing dyes could be incorporated into the structure of the plastics. The printer could change the dye to the colored form to read, and the material can be bleached with another unit that would reverse the photo coloring process. An ionic type plastic can be incorporated into the plastics and used to color the printed area by the use of an indicator type reaction with an organic acid or base. Another method would be to use a thermal printer in conjunction with liquid crystal type materials that would alter the state of the liquid crystals in the printed areas. Applying heat and electrical fields to the printed sheet would erase the printing. 

Bleaching is reversed in the dark and the red-purple color of rhodopsin returns. This is thought to occur by the reduction of all-Pms-retinal to vitamin Ai (retinal), which diffuses from the rod into the pigment epithelium, where it is converted enzymatically to the 1 l-c isomer of vitamin At. The enzymatic isomerization is followed by diffusion back into the rod, oxidation to 11 -rfr-retinal, and combination with opsin to form rhodopsin. This process is shown schematically in Figure 12.5.[Pg.289]

Sulfite paper has a relatively short life span, since residual acid will continue to hydrolyze the cellulose and cause embrittlement. Further sources of acid include aluminum sulfate (which is added together with resin to suppress bleeding or feathering of ink into the paper) and S02 and NO from the atmosphere. Much of the world s library collections and archives will soon be lost as the paper crumbles. Various deacidification treatments (e.g., with ammonia, morpholine, cyclohexylamine carbamate, or diethyl-zinc) have been proposed and tried, but at best they can only halt the process of embrittlement and cannot reverse it.14 With the move to kraft pulping, alkaline peroxide bleaching, and increasing use of precipitated calcium carbonate as a filler, the high quality papers produced today are intrinsically acid free and should also resist subsequent acidification by S02-polluted air fairly well. 

The bleaching process also proceeded on steady-state irradiation with visible light in the presence of moderately reductive compounds.33 Since this oxidation-reduction reaction is not possible in the dark, the reactions have been well studied with a variety of combinations of reductants such as the Fe(II>—Fe(III) system in aqueous solutions.1 This section discusses reversible spectral changes using a combination of two functions, i.e., oxidative (thionine) and reductive ones in a polymer matrix.34,35... 

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