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The Photochemistry of Dyes. IV—The Role of Singlet Oxygen and Hydrogen Peroxide in Photosensitised Degradation of Polymers

“…The action of sunlight on textile fibres initiates chemical reactions that lead to a deterioration in the physical properlie\ of the fibre, and to changes in the nature and degree of polymerisation of the polymer molecule. The degradation of vat ious undyed fibres on exposure to sunlight, either in the open or behind glass, has been studied by several workers (8)(9)(10)(11). Some fibres are more affected than others, and the order of degradation is not the same behind glass as in the open.…”

“…Vat Yellow 2) are shown in When different parameters are used to assess fibre degradation it does not follow that similar conclusions of the nature of the change will be derived. With vat-dyed cotton exposed to sunlight a linear relation between change in degree of polymerisation, as assessed by fluidity in cuprammonium solution, and loss in tensile strength is found in both dry and moist air (8). However, with dyed and undyed silk, a different relation between fluidity and tensile strength exists for exposures to sunlight according to whether they are made in dry or in moist atmospheres (13).…”

“…(h) the extent to which singlet oxygen ('Z'tU and 'Ao) may be deactivated under the experimental conditions. In this respect the ob5ervation (8,13,14) that photosensitised degradation of textile materials in dry air or oxygen requires intimate contact of sensitiser and polymer molecules is of obvious importance; and (c) the oxidative etfect of singlet oxygen (l&' and la,) OII the piwticular polymer. This will vary considerably with ditlerenl polyniei s, its does its oxidative eEwt on various organic con~pounds.…”

II—Evidence for the Formation of Singlet Oxygen by Phototendering Vat Dyes

“…There appear to be two main schools of thought, and the situation is complicated by the evident multiplicity of the mechanisms involved. Thus it seems that the degradation reaction can depend on the wavelength of light used [ l , 21, and on the presence of water vapour [3]. If one considers phototendering by visible light in the absence of water vapour, the most probable mechanism then appears to be the formation of singlet oxygen ('Ag or ' X + g ) by energy transfer from an excited dye molecule to ground-state oxygen, followed by degradation of the polymer by the action of one or both of these forms of oxygen [ 1 , 4 ] .…”