Light intensity: degradation with time

The relationship of degradation to light intensity is certainly nearer to linear than is the case with temperature. This implied time scale would add considerably to the effort needed to collect data at a series of acceleration levels. Also, only very recent weathering apparatus would be capable of operating over a range of irradiance levels. The result, in practice, is that it is extremely uncommon for more than one irradiance level to be used and degradation is assumed to be proportional to irradiance. By this is meant irradiance of the same spectral distribution. Different distributions, particularly in the UV regions, will inevitably produce different results. 

Figure 14. (a) Photocatalytic degradation of methanol with different initial concentrations as a function of UV illumination time (light intensity 2095 juW/cm2 at a wavelength of 254 nm) at a H20 concentration of 0.38 mol/m3 and a reaction temperature of 45 °C. (b) Initial reaction rates r0 as derived from the data in (a) versus the initial methanol concentrations the solid line is a fit according Eq. (9). Data from [388], S. B. Kim and S. C. Hong, Appl. Catal. B Environ. 35, 305 (2002). 

Obviously, diindenoperylene shows almost no degradation in solution even if exposed to UV light at 366 nm. In eontrast, the intensity of the tetraeene absorption peaks decreases with time and the proeess can be drastically accelerated by UV illumination. The photographs of the cuvettes in the inset show that the yellowish colour of the solution at the beginning almost vanishes after 20 h. The final spectral signature with an absorption maximum around 390 nm (red curve) can be identified with the stable oxidation product 5,12-tetracene-quinone. 

Illuminated control experiments containing seawater medium were conducted in parallel with the batch TNT uptake and metabolism experiments. The TNT concentration decreased over time due to transformation. The abiotic TNT oxidation product 1,3,5-trinitrobenzene was found in the seawater liquid medium, consistent with previous studies for the photolytic degradation of TNT in water [57], The apparent first-order rate constants for abiotic degradation of TNT in seawater under -150 pE nr2 s 1 light intensity were 0.0023-0.0028 h at initial TNT concentrations ranging from 1.0 to 50 mg L 1, which were 15-25 times smaller than the apparent first-order rate constants for TNT removal in the uptake and metabolism experiments using 1 g of biomass. 

The relationship of degradation with light intensity is certainly nearer to linear than is the case with temperature. This implied time scale adds considerably to the effort needed to collect data at a. series of acceleration levels, and in fact it is extremely uncommon for this to be done. 

Figure 5.24 Degradation of light intensity with time. (Lumen Maintenance Projection for White LXM3-PWxl at 55 °C and 0.3A [Tjunction 68 °C, Normalized to 1 at 24 hours]). Copyright( ) 2010 Philips Lumileds Lighting Company, reproduced with permission.

The correlation between outdoor UV degradation and accelerated ageing in the laboratory has been an important research topic for many years [12]. Several projects ended with the conclusion that there is no unique acceleration factor characteristic for a particular laboratory tester (e.g. a Xenon-tester 1200). This conclusion might have been reached beforehand i.e. without testing [13-14]. For polyolefines, the UV-stimulated oxidation can be described by the classical scheme of Bolland [15]. There is a prime production rate [R] of radicals ([R] is the radical concentration, a dot means differentiation with respect to time) which is proportional to the UV-light intensity I. However, the breakdown reaction rate [R] depends on the stabilization system. If the radicals recom-bine with each other, [R] will be proportional to [R]. If the radicals are scavenged by, e.g., an excess of stabilizer, the rate [R] will be proportional to [R]. This implies that the steady-state radical concentration [R] will be proportional to depending on the stabilizer in... 

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