N. S. Allen

Fluorescence, phosphorescence and light fastness of hydroxyanthraquinones

Abstract The fluorescence and phosphorescence spectra and quantum yields of nine 1-and 2-substituted hydroxyanthraquinones were determined in isopropanol and EPA glass respectively. Their light fastness on polyester fabric were also measured. Fluorescence is observed from all the hydroxy- anthraquinones whereas phosphorescence is observed only from the 2-hydroxy derivatives. The fluorescence quantum yields of the 1-hydroxy derivatives are much higher than those of the 2-substituted derivatives These results are discussed in relation to the light fastness of the hydroxy-anthraquinones on polyester fabric.

Fluorescence and light fastness of aminoanthraquinones

Abstract The fluorescence emission spectra and quantum yields of thirteen 1-amino-2-amino- and 1,4-diamino-anthraquinones in a variety of solvents have been determined. The quantum yield values for the 1-amino derivatives are decreased on N-alkylation and also by the N-substitution of acyl or aryl groups. Both emission wavelength maxima and quantum yields of the 2-substituted derivatives are markedly sensitive to solvent polarity. These observations are discussed in relation to the light fastness of the aminoanthraquinones on polyester fabric.

Luminescence and degradation of nylon polymers II: quenching of fluorescent and phosphorescent species

Abstract The effect of various photostabilisers on the fluorescent and phosphorescent species in nylon-6,6 polymer has been studied and related to their known light-stabilising activity in the polymer. Alkali metal halides quench the fluorescent species in the polymer in the order I > Br > Cl > F and this follows the same order as their light stabilising activity. Alkali and transition metal salicylates quench the phosphorescent species in the polymer and are effective photostabilisers, while phenyl salicylate, which is a poor stabiliser, exhibits weak quenching and salicylic acid itself, which is a non-stabiliser, exhibits no quenching. The sensitising action of the anatase form of titanium dioxide pigment, added as a fibre delustrant, is normally counteracted by coating the pigment particles with a manganese compound. The effect of this treatment is to quench the phosphorescence emissions from the pigment and the polymer.

Stabiliser interactions with hindered piperidine compounds in polypropylene: Influence of processing

Abstract The effect of two hindered piperidine compounds on the photo-stability and light stabilising performance of a 2-hydroxy-benzophenone and a 2-hydroxybenzotriazole stabiliser in polypropylene film has been examined. The piperidine compounds—a stable N -oxy radical and a hindered amine—inhibit the photolysis of both the benzophenone and the benzotriazole stabilisers, the amine being more effective. Processing reduces the protective efficiency of the piperidine compounds. The light stability of the polymer was, however, variable, both antagonism and synergism being observed. The processing operation had a dramatic effect on the light stabilising performance of the hindered amine systems whereas it had virtually no effect on the performance of the N -oxy radical systems. The inhibition process is attributed to a regeneration process involving the N -oxy radical and the effects of processing are attributed to the formation of hydroperoxides.

Luminescence properties and photo -activity of sulphate-processed rutile (titanium dioxide) pigments in commercial polyethylene

The low temperature infra-red emission spectra of five commercial rutile pigments prepared by the sulphate process have been recorded and compared with that of a pigment prepared in the laboratory by the same method. All the pigments, dispersed in low density polyethylene, exhibit emission at 815 nm whereas a different type of emission is observed from the laboratory prepared pigment. The photoactivity of all the commercial rutile pigments in polyethylene have been examined and found to correlate well with their corresponding emission intensities at 815 nm.

Photo-oxidation of commercial polyethylene containing titanium dioxide (rutile)/ antioxidant systems

The weathering performance of various grades of untreated (or uncoated) and treated (or coated) rutile pigments in commercial polyethylene in the absence and presence of primary and secondary antioxidants is examined. Using carbonyl index as a monitor of the rate of photo-oxidation, untreated rutile sensitizes the photo-oxidation of polyethylene, whereas the treated grades act as photostabilisers. The photosensitizing effect of untreated rutile is markedly inhibited by a synergistic combination of a hindered phenol (Topanol CA) and a sulphur containing antioxidant (DLTDP). Further, the order of weathering stability of the treated rutile pigments in the absence of any antioxidant in the polymer is altered significantly when an antioxidant system is present. The imporance of pigment/antioxidant interactions is discussed. The photo-oxidation of polyethylene containing the treated rutile pigments is also significantly improved by the presence of a primary antioxidant or a primary/secondary antioxidant system compared with a secondary antioxidant alone. These results indicate that peroxides play only a minor role in the photo-oxidation of TiO2 (rutile) pigmented polyethylene.

Interaction between antioxidants and hindered piperidine compounds in the photostabilisation of polypropylene: Influence of processing history

Abstract The effect of various antioxidants on the photostabilising performance of hindered piperidine compounds in polypropylene has been examined using infra-red, ultra-violet, and ESR spectroscopic techniques. Processing history is shown to play a dominant role in controlling the photostabilising performance of these systems. The destruction of the nitroxyl radical during processing is inhibited by the presence of an antioxidant, and prooxidant activity is assessed by the melt flow index. Antioxidants appear to compete effectively with the nitroxyl radicals for macroradicals. The results are discussed in terms of the importance of antioxidant oxidation versus the formation and destruction of hydroperoxides during processing.

Luminescence and degradation of nylon polymers: Part I: Photo-oxidation processes involving phosphorescent species

Abstract The thermal and photochemical oxidation of nylon polymers has been studied by luminescence spectroscopy. All the nylon polymers exhibit phosphorescence emission in the wavelength region 400 – 500 nm which is concluded to originate from impurity oxidation products formed during polymerization and processing. Mild oxidation of model amide compounds produces a phosphorescent species whose emission spectrum closely matches that of the polymer. The phosphorescence from the polymer and model amides exhibits several characteristic features that indicate their source to be α,β-unsaturated carbonyl groups. From a comparison of the phosphorescence excitation spectrum of the oxidized model amides with the absorption spectra of two possible general structural types of model α,β-unsaturated carbonyls, the phosphorescence is concluded to originate from dienone chromophoric units. These species appear to play an important role in the sunlight-induced oxidation of the polymer.

Structural influences on the light stability of 2-substituted aminoanthraquinones and hydroxyanthraquinones in man-made polymers

Abstract The fluorescence and phosphorescence spectra and quantum yields of 2-amino-, 2-hydroxy- and 2-amino-3-hydroxyanthraquinones have been recorded in a variety of solvents and these data have been linked to studies on the light stabilities of the compounds in man-made polymers. Strong fluorescence is observed only from the 2-amino derivative and strong phosphorescence only from the 2-hydroxy derivative while the 2-amino-3-hydroxy derivative is non-luminescent. On flash photolysis in 2-propanol, the 2-amino and 2-hydroxy derivatives gave strong transient absorption due to the radical anion A− whereas the 2-amino-3-hydroxy derivative gave strong transient absorption due to the semiquinone radical AH·. These results are discussed in relation to the photostabilities of the compounds in polyester and nylon 6,6 film and fabric.

Photochemical fading and photostabilization of the crystal violet lactone colour former system

Abstract The photochemistry and photofading of the leuco and dye forms of the crystal violet lactone colour former system have been examined under various conditions using UV absorption, flash photolysis and high performance liquid chromatography techniques as well as hydroperoxide analysis. Colour production in solution is very much dependent on the acid concentration attaining a maximum at 90 vol.% acetic acid in acetonitrile. The leuco-dye equilibrium is an important factor in controlling the light stability of the system; photofading is shown to be essentially a photo-oxidative free-radical process followed by oxygen attack generating peroxides. The latter are effectively destroyed by a well-known hydroperoxide decomposer, nickel dialkyldithiocarbamate, which thus imparts stability to both the leuco and the dye forms of the colour former. Mechanisms are proposed to account for the results and the commercial implications of the light stability data to the practical use of colour formers are discussed.