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Abstract

Basic concepts of photochemistry are outlined.

Detailed descriptions of the morphology of wool, its chemical composition and the probable mechanism towards its photodiscolouration (yellowing) process are discussed. The use of fluorescent whitening agents (FWAs) to brighten photoyellowed wool and some processes for the prevention of photoyellowing are briefly outlined.

Phthalimides, as FWAs for wool, have been synthesised. They have been shown to behave in a similar fashion to other conventional FWAs, when applied to wool, despite the high fluorescence quantum yields observed with some of them in solution. Some drawbacks from their use as whiteners for wool have also been encountered.

Potential FWAs containing halogen atoms which intramolecularly quench fluorescence have been synthesised and applied to wool in a number of ways. Application by cold pad-batch methods revealed that the complete displacement of all halogen atoms by nucleophili groups in wool did not take place. Treatment of these materials with morpholine, sodium carbonate or water was necessary to develop the full potential fluorescence yield of the FWA on the fabric. That the fluorescence of FWAs can be quenched by the heavy atom effect was shown by the fact that no fluorescence was observed when commercial FWAs were applied to brominated wool. If the FWA treated brominated fabrics were subjected to a reduction treatment then the bromine atoms were removed and the fluorescence of the FWA was restored. Application of the potential FWAs by exhaustion at high temperatures always led to strong fluorescent fabrics implying that the halogen atoms were completely displaced using this application method but the question remains as to how much of the displacement leads to covalent bonds between the fabric and the FWA.

Several stilbenes have been shown to sensitise the formation of singlet oxygen which is capable of oxidising indole residues, e.g. tryptophan. Some stilbenes were found to quench singlet oxygen, e.g. disodium 4,4'-diaminostilbene-2,2'-disulphonate. Such materials when applied to wool sensitised the photoyellowing of wool and proved to be no more stable than conventional stilbene fluorescent whitening agents.

Different chemically treated wools have been exposed to UV radiation in the presence of different aqueous reductive bleaching agent solutions. It was observed that thiourea dioxide gave the best performance, and in the case of wool serge, led to a fabric having a better light fastness. An interesting beneficial synergistic effect was uncovered when the reducing agents were used in conjunction with formaldehyde. Biphenols (models for dityrosine) have also been padded onto wool and exposed to light in the wet state to mimic the photoyellowing of dityrosine. o,o'-Biphenol treated
wool was exposed to light in the presence of the reducing agents and these were found to decrease the amount of discolouration produced by degradation of o,o'-biphenol.

Both intramolecular and intermolecular fluorescence quenching by disulphide bond containing compounds have been studied in order to further establish the view that the disulphide bond containing cystine residue of wool quenches the fluorescence of the excited states of some closely located amino acid residues, e.g. tryptophan.

Intermolecular quenching of fluorescence, in solution, has been found to be dependent on the lifetime of the excited singlet state of the fluorescer. Strong quenching of the intramolecular type, in solution, has been clearly demonstrated. Further quenching studies of the intermolecular type were also carried out on wool. Neither the fluorescence of a conventional FWA or of kynurenine, a photo-oxidation product of tryptophan, have been observed to be quenched by cystine residues, many of which lie on the surface of the wool. Application, under basic condition in air, of polyethylene glycol dimercaptoacetate onto the surface of wool pre-treated with the above conventional FWA led to strong fluorescence quenching of the applied FWA. Some future work remains to be performed in order to make some firm conclusions on this particular study.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry T Technology > TS Manufactures T Technology > TT Handicrafts Arts and crafts
Departments:	School of Science & Technology School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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