Light screens

Reactions (d) and (e) occur much less frequently than reactions (a)-(c) but do so at a sufficient rate to cause changes in most polymers. There are three methods available for combating such effects light screens, ultraviolet absorbers and quenching agents. 

Ultraviolet absorbers are a form of light screen which absorb primarily in the ultraviolet range. It is a requirement for most ultraviolet absorbers that absorption in the visible range should be negligible if this were not so the resultant instant colour formation could be worse than that experienced after prolonged exposure of the polymer. 

To improve the resistance to ultraviolet light carbon black is often useful as a light screen. Its use in fibres and films is clearly very restricted and in these instances ultraviolet absorbers and/or quenching agents are used. Recent developments include the greater use of hindered amine and nickel compounds. 

On the other hand, the alkoxide system presented several problems in formulation. The system first chosen as a model consisted of a trimethoxymethyl silane crosslinker, 8000 centistoke HEB siloxane, and a catalyst. A number of catalysts were used and each exhibited different cure rates and electrical properties. DuPont tetraalkoxytitante-Tyzor appears to he one of the better catalysts used in this type of curing system. Fillers are usually incorporated into the silicone formulation to improve mechanical properties, promote adhesion, and to serve as light screening and pigment agents. Cab-o-sil, a form of fumed silica, carbon-black, titanium dioxide and calcium carbonate are then used as RTV fillers. 

What does the light-screening factor, S(X), exactly describe For what can it be... 

Use Eq. 2 in Illustrative Example 16.1 to estimate the near-surface production rate of HO in the pond. To account for light attenuation in the water column, apply light-screening factors for the wavelength of maximum light absorption of NO (Amax = 320 nm) and NO (A = 360 nm) ... 

Apart from interest in its synthesis and properties, some of the work on indolizines has been concerned with the search for drugs, for dyestuffs and for light-screening agents used in photographic emulsions. 

In the case of the sample with PDK, acetophenone and dec-l-ene were isolated, showing that the Norrish type-II reaction is operating. A hypothesis that can be put forward is that, with different efficiency, the aniline and the ketone additives act essentially as a light screen without transfer of energy to the polymer. 

Inorganic pigments, such as titanium dioxide, zinc oxide or carbon black act as efficient light screens and thus protect plastics against photodegradation (Accorsi and Yu, 1998 Rabek, 1990 Allen, 1989). Colored iron oxides and organic pigments, such... 

The convenient UV absorption region of 2,5-diaryl-1,3,4-oxadiazoles enables them to be used as light-screening agents and optical bright-eners.8-9-12 13 174... 

In plants, flavonoids are suggested to be involved in various processes including plant-pathogen interactions, pollination, light screening, seed... 

Polarized Light Screen (PLS). The PLS technique (23,24) was used extensively to obtain Immediate information about the macroscopic phase behavior of solutions. Diffuse light is transmitted through a polarizer and an analyzer. The sample is placed between the two polarizers, and system behavior is observed as shown in Figure 1. 

Use UV light—screening agents, protective coatings, plasticizers. 

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