Sunlight screen effect

The photochemical oxidation of PC can be characterized by the increase in absorbance at 3470 cm 1 versus irradiation time, as shown on Fig. 15.3. Oxidation occurred rapidly for uncoated PC. A decrease in the rate of oxidation was observed in the case of the coated samples. The more the ZnO layer is thick, the less is the oxidation rate. Similar conclusions were drawn when the absorbance was measured at 400 nm. The screening effect reduces therefore the PC photodegradation produced by the direct absorption of sunlight radiations. In addition, we have shown that the grain size and the density increase with the coating thickness. The coatings are denser and have lower micro voids. Consequently, they are more impermeable to oxygen and the oxidation is limited. 

Very often, HA(L)Ss are combined with UV absorbers to improve the effidency against the degrading effea of sunlight which can be explained by a synergy between UV absorbers and HA(L)S. In fact, the screening effect of the UV absorber diminishes the UV light intensity within the polymer and thus limits somehow the formation of radical intermediates, which finally leads to a reduced degradation of the polymer. 

Most of the reactions the inorganic chemist encounters in the laboratory involve ionic species such as the reactants and products in the reactions just discussed or those of coordination compounds (Chapter 13). However, in the atmosphere there are many free radical reactions initiated by sunlight. One of the most important and controversial sets of atmospheric reactions at present is that concerning stratospheric ozone. The importance of ozone and the effect of ultraviolet (UV) radiation on life has been much discussed. Here we note briefly that only a small portion of the sun s spectrum reaches the surface of the earth and that parts of the UV portion that are largely screened can cause various ill effects to living systems. 

To determine whether decreases in 0 observed in the napus crop in the field were the result of photoinhibition, a sample of plants, selected by a randomized design, were shaded from direct sunlight each morning. Shades consisted of vertical screens of 66 cm square, covered in reflective aluminium foil. Each day at dusk, shades were placed on the eastern side of the selected individuals. This ensured that each of these plants would be shaded from direct sunlight at dawn and during the morning. A second treatment in which shades were positioned for 6 day periods was used at intervals from January to March to examine the effect of successive days of photoinhibition. 

It is not always appreciated that some cable sheathing materials are adversely affected by the ultraviolet radiation in sunlight, and to counter this effect a black compound is added see Regulation 522-11-01. Unfortunately, this increases the absorption of infrared radiation and this solar heating can increase the cable temperature considerably, so outdoor cables should either be screened from sunlight or derated to allow for an increase in ambient temperature of about 20°C. 

The effects caused by screen glare due to direct light incidence are shown in Fig. 38. The image clearly shows the difference between the part of the monitor exposed to sunlight and the shadow of the A-piUar. The colours in the shaded part and the contrast are clearly much stronger. As a consequence, road side vegetation... 

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