Reactions of leuco dyes on acid activated clays

Wang, Liangcheng (1993) Reactions of leuco dyes on acid activated clays. Doctoral thesis, University of Central Lancashire.

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The colour formers used in carbonless copying paper consist of admixtures of primary and secondary dyes. Work was carried out in this programme both on retarding the photofading of the primary dyes and on improving the reactivity of the clay minerals towards the development of the secondary dye.
The photolytic fading behaviour of S primary dyes were examined both in solution and on solid clay surfaces, and the intermediacy of singlet oxygen in the fading processes of the dyes examined by the DPBF trapping method. The retardation of dye fading was accomplished using bis(dithiocarbamato)nickel(II) complexes and it was found that some hydroxy-possessing species conferred improved photo-stability upon the developed dyes, especially towards fluorans, and to a lesser degree towards phthalides.
The dark development of carbazolyl blue was examined on a series of cation exchanged and acid activated clays. There appeared to be two main mechanisms governing S-RB development in the dark; the first was catalysis by co-ordination to the Lewis acid sites on the clay edges, whilst the second was a redox process probably associated with the Fe 3 cations present in the clays.
Several treatments were applied to modify common commercial days. It was found that introducing Fe 3 and Cu2 into the clay profoundly improved the reactivity of the day to the secondary leuco dye. Cation exchange of clay with some transition metal ions such as CO 2 and Ni2 also showed a beneficial effect on the colour formation.
The Langmuir isotherm type plots were used to examine cation adsorption behaviour for 9 types of montmorillonite clays. It was found that cation exchange of clay occurs at at least two different sites; the tetrahedral silica layer sites and the intact montmorillonite interlayer cation exchange sites. It was also found that the silica layer sites are the predominant ones responsible for S-RB development, with interlayer sites providing a much lesser contribution. Adsorption of aquated metal salts makes virtually no further contributions to the leuco dye reaction.

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