Protection against Light

Some polymers, such as n s-l,4-poly(isoprene) and poly(styrene), absorb UV light directly. In the case of poly(isoprene), this produces a resonance-stabilized allyl free radical through the elimination of hydrogen (2H- H )  

The recombination of these free radicals causes cross-linking of the poly-(isoprene). If the free radicals are only slightly resonance-stabilized, as, for example, in poly(methyl methacrylate), then disproportionation and hence chain degradation result  

Most polymers, such as poly(ethylene) and poly(propylene), should not contain absorbing groups. But most commercially available polymers do absorb ultraviolet light, and structural defects as well as impurities are responsible. The nature of these photoactive groups is not definitely es- 

Four different photolytically induced processes can occur with ketones. In what is known as the Norrish type I mechanism, a free radical reaction occurs, and this can be made harmless by adding free radical scavengers  

The Norrish type I reaction does not occur with aryl ketones, but it is very important with poly(a-olefins). 

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Diphenyl phosphorazidate is a colorless non-explosive oil that can be kept for a long time without decomposition if it is protected against light and moisture. 

Barbe, M.F., Tytell, M., Gower, D.J., Welch, W.J. (1988). Hyperthermia protects against light damage in the rat retina. Science 241,1817-1820. 

Titanium dioxide may be included to render the shell opaque. Opaque capsules may be employed to provide protection against light or to conceal the contents. 

Frankel and Tarassuk (1959) exposed a layer of raw skim milk 1 cm thick to direct sunlight at room temperature and noted a loss in lipase activity of 84% in 5 min and of 96% after 10 min. In diffuse daylight inactivation was reduced but 71% of lipase activity was lost in 1 hr. The loss of activity by light was independent of the temperature of the milk, equal losses being observed at 0°C and at 37°C. The enzymes were markedly protected against light inactivation by the presence of fat. 

Samples and SPE cartridges are processed in a completely enclosed system, protected against light and air. 

Figure 14 14. Biocatalytic optodes based on a bifurcated fiber bundle (top) or a single fiber (bottom). In the recognition part the indicator and an enzyme are immobilized. The reactive layer is permeable to the substrate but protected against light.

Stabilizers are intended to prevent degradation reactions (in the broadest sense) of plastics during processing (heat stabilizers against thermal decomposition) and in practical use (antioxidants and UV absorbers to protect against light and aging). They are added in amounts of 1 %. See Chapter 24 for their protection mechanisms. 

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