Photodegradation accelerators

Supralittoral Beach above water line Photodegradation (accelerated)" 20% oxygen... 

Photodegradation of polyethylene waste can be markedly accelerated via its grafting with acrylamide [98], In contrast, photostabilization of polyethylene and polypropylene can be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2-hydroxy-propoxy) benzophenone using gamma radiation [ 147], In this case, the grafted compound acting as a U V stabilizer is chemically bound to the backbone chain of the polymer and its evaporation from the surface can be avoided. 

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. 

Arita, S., S. Ando, H. Hosoda et al. 2005. Acceleration effect of sulfides on photodegradation of carotenoids by UVA irradiation. Biosci. Biotechnol. Biochem. 69 1786-1789. 

Finally, the third major input information required is external (i.e., extrinsic to the compound itself) the environmental physical conditions (see Fig. 2). Temperature and water regimes are often the most determinant factors which affect the mobility of chemicals in the environment by accelerating volatilization or sorption processes. Solar radiation is also crucial in the chemicals fate since it is strongly related to photodegradation and volatilization processes as well. 

Water, methanol, and n-hexane do not influence the photooxidation of PVC (43), but the photodegradation is accelerated by ferric chloride (70,71) and certain other compounds containing iron (70,71,72). Purification of the polymer might be expected to enhance its photostability by removing deleterious impurities such as iron compounds that are derived from metal equipment. This type of result was obtained in one recent study (58) but not in others (30,59). In contrast, the photo-oxidative degradation of PVC should be enhanced by admixture of the polymer with materials that are unusually susceptible to photooxidation themselves. Such behavior has been observed for impact-modified PVC containing polybutadiene-based polyblends (69,73). 

Polymers which undergo accelerated photodegradation in the environment can be made by making a copolymer of the monomer with an unsaturated ketone. The principle of the method is shown in Scheme 9.6, using styrene as the monomer. 

Jang and McDow (1997) studied the photodegradation of benzo[a]anthracene in the presence of three common constituents of atmospheric aerosols reported to accelerate benzo [a] anthracene, namely 9,10-anthroquinone, 9-xanthone, and vanillin. The photo-degradation experiments were conducted using a photochemical reactor equipped with a 450-W medium pressure mercury arc lamp and a water bath to maintain the solution temperature at 16 °C. The concentration of benzo [a] anthracene and co-solutes was 10" M. Irradiation experiments were conducted in toluene, benzene, and benzene-c/e- Products identified by GC/MS, FTIR, and NMR included benzo[a]an-thracene-7,12-dione, phthalic acid, phthalic anhydride, 1,2-benzenedicarboxaldehyde, naphtha-lene-2,3-dicarboxylic acid/anhydride, 7,12-dihydrobenzo[a]anthracene, 10-benzyl-10-hydroan-thracen-9-one, benzyl alcohol, and 1,2-diphenylethanol. 

Figure 8. Biodegradation of photodegraded LDPE. Cpm released (as C-CO2) demonstrates decomposition of the low molecular weight fraction of the plastic. Line A LDPE with accelerant of photodegradation Line B LDPE without accelerant. Maximum cpm released is less than 5% of total cpm added as LDPE. Data are from (45), Table 5.

Similar chemistry leading to the formation of the products shown in Table 9.22 is discussed by Forstner et al. (1997b). Some of these products may accelerate the photodegradation of less reactive species in the condensed phase (e.g., McDow et al., 1996). 

There are many reports in the literature describing photodegradation sensitized by aromatic ketones and quinones. Beachell and Chang (24) showed that triplet excited benzophenone accelerates degradation of polyurethanes by hydrogen abstraction. Harper and McKellar (25) showed the same effect with benzophenone on opolypropylene, and Amin and Scott (26) on polyethylene. Rabek and Ranby (27) have demonstrated that triplet excited quinones enhance... 

The stability of some vitamins is influenced by aw. In general, the stability of retinol (vitamin A), thiamin (vitamin Bj) and riboflavin (vitamin B2) decreases with increasing aw. At low av (below 0.40), metal ions do not have a catalytic effect on the destruction of ascorbic acid. The rate of loss of ascorbic acid increases exponentially as aw increases. The photodegradation of riboflavin (Chapter 6) is also accelerated by increasing aw. 

The thermal and photochemical dehydrochlorination of the vinyl chloride—CO copolymer have been studied by two different groups56,57). The decomposition rate for the copolymer was significantly higher than that for poly(vinyl chloride), the rate increasing with increasing CO content of the copolymer. In addition, the thermal decomposition of the copolymer was accelerated in the presence of molecular 02 while the photodegradation was slowed down 57). As with poly(vinyl chloride), the dehydrochlorination of the copolymer resulted in the formation of polyene sequences. There was no appreciable decrease in molecular weight. 

The photodegradation of an aqueous solution of terbuthylazine was not only accelerated, but was also more extensive in the presence of humic acids isolated from soil (Mansour et al., 1997). In the absence of humic acids, only hydroxyterbuthylazine (OBET) was formed (Sanlaville et al., 1996), whereas in the presence of humic acids, dealkylated products (CBAT, CBDT, CEAT, CAAT, OAAT) were formed (Table 23.2) (Sanlaville et al., 1996 Mansour et al., 1997). In contrast, fulvic acids isolated from stream water slowed the photolysis of terbuthylazine, most likely reflecting differences in structure between the soil- and stream-derived materials. The photodegradation of atrazine and its initial photoproduct OEIT (Table 23.2) in artificial sea water containing humic acids was also accelerated compared to photolysis in distilled water (Durand et al., 1990,1991). 

An interesting outcome of photodegradation studies on polymers is the finding that tensile and shear stress can accelerate the rate of photochemical degradation.9,19,63 For example, recent studies of this phenomenon showed... 

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