Photodegradation experimental studies

A feature of this ABC, 2s, 2kc kinetic scheme is its cyclic nature photoproduct C regenerates the starting compound A. This enables it to operate in a closed system, since it does not photodegrade. Experimental photochromic, photodegradable systems involving similar processes are thus likely to exhibit similar behavior, but they need to be studied in an open system. An illustative case is the photochromism of the triphenylimidazolyl dimer (TPID). 

The technology and role of photodegradable plastics is considered and computer models are described to evaluate strategies for litter abatement, and experimental studies of the synthesis and biodegradation of conventional and photodegradable polyethylene, polypropylene, poly(ethylene terephthalate), and polystyrene. 

The photodegradation of para-aramid in an 0 atmosphere allows the differentiation between the accelerated experimental photooxidative conditions from its usual daylight exposure effects. This study illustrated an estimation of the rates of photooxidation of a commercial para-aramid product (i.e., DuPont s Kevlar-29 woven fabric) based on the oxygen-18-labelled carbon dioxide ( CC and CC ) decarboxylated from the sample. The oxygen-18-labelled atoms, which are inserted in the macromolecules, were analyzed for the photodegradation processes. This technique also allows the radial l O-distribution measurement from the fiber surface toward the fiber center. 

On the other hand, radicals are undoubtedly involved in the photodegradation of PVC under some experimental conditions. Recent ESR studies have provided evidence for the formation of alkyl and allyl-type radicals during the low-temperature UV irradiation of the polymer (43,72,87). Peroxy radicals were also observed when molecular oxygen was present (43,87). Other ESR work has shown convincingly that the radical -CHCI-CH2-CH-CH2-CHCI- results from the irradiation of PVC at liquid-nitrogen temperature (61,93) and is converted into a -CH2-CC1-CH2- radical at -110°C (93). 

Fukuda et al. (1988) studied the photodegradation of anthracene and other polycyclic aromatic hydrocarbons in distilled water and artificial seawater using a high-pressure mercury lamp. Based upon an experimentally rate constant of 0.660/h, the photolytic half-life of anthracene in water is 1 h. 

Subsequently, Fan and co-workers (1996a) conducted an experimental and modeling study of the reactions of 03 and N02 with nitro-PAHs on heterogeneous soot particles. They concluded that while photodegradation is the major daytime loss process for the nitro-PAH, during the night, particle oxidation by 03 may be the most important decay pathway. 

Presently the most common experimental approach to study photodegradation of organic compounds on soils is to expose a series of thin, spiked soil layers (thickness typically between 0.25 and 2 mm) to a light source. The overall disappearance rate... 

The main focus of this chapter is on two well-studied series of photochromic compounds, spiropyrans and spirooxazines, and the comparison of their photo-reactive excited states and molecular structures. The discussion of their photodegradation takes into account the results obtained from spectrophotometric measurements together with the identification of the fragments resulting from photodegradation. Different mechanisms are proposed for the photooxidation reaction. The experimental apparatus and procedures for photodegradation studies are also described. 

Fig. 19 A representative terfluorene used to study photodegradation, with a number of monomers showing the types of defect sites found experimentally after intense photoirradiation...

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