Stabilizer photosensitizing effect

The incorporation of P-1 into monofilament by compounding with PPH powder and extrusion was effective only partially in reducing draw-initiated oxidation. After X3 cold draw (at 25°C) the observed -OOH level was 0.3 X 10 3M, in the presence of 0.1 wt % P-1, in comparison with 1.1 X 10 3M for the stabilizer-free fiber. Furthermore, this lower level of draw-induced degradation still had a large photosensitizing effect, even in the presence of the stabilizer. After 200 hr of UV exposure in the xenon arc, the P-1 concentration was reduced by more than 20 fold in the cold-drawn filament and photooxidation was accelerating rapidly, whereas for the undrawn filament no P-1 loss was visible, and photooxidation was undetectable. 

The deleterious effect of photosensitizing products formed during processing was discussed earlier (Section 19.4.1.1). In some cases, however, transformation products which are formed can be more effective than the parent stabilizer initially added to the polymer. Rapid oxidation of the polymer commences only at the point where the effective stabilizer is fully exhausted. " Examples of UV stabilizers whose effectiveness depends on their transformation products are now briefly described (for detailed mechanisms, see later sections). 

As with drugs and purified biomarkers, thermal- and photostability of botanical products are the factors that must be considered. Commercial dried extract and capsules of SJW were evaluated under harmonized test conditions (25). Photostability testing showed all the constituents to be photosensitive in the tested conditions. However, different opacity agents and pigments influenced the stability of the constituents. Amber containers had little effect on the photostability of the investigated constituents. Long-term thermal stability testing showed a shelf life of less than four months for hyperforins and hypericins, even when ascorbic and citric acids were added to the formulation. 

Charged colloids and water-in-oil microemulsions provide organized environments that control photosensitized electron transfer reactions. Effective charge separation of the primary encounter cage complex, and subsequent stabilization of the photoproducts against back electron transfer reactions is achieved by means of electrostatic and hydrophobic interactions of the photoproducts and the organized media. 

The reaction of diethyl zinc with water produces zinc oxide, and then zinc carbonate, as the alkaline reserve. These chemicals have antiseptic properties which may also prevent the growth of mold in paper. They may also improve the brightness of treated papers. However, it is also known that zine oxide is a photosensitizer (56) which may trigger photo-oxidation of treated papers to initiate a chemical chain reaction that will lead eventually to the formation of acidic products (57). Moreover, the interaction of zinc oxide and zinc carbonate with copper, iron and cobalt present in the paper and their subsequent effects on paper stability have not been studied. 

Fig. 20. Photosensitized H2-evolution from an organized assembly composed of a Pt-colloid stabilized by a positively charged polymer matrice. Charge separation is effected by means of complementary electrostatic and hydrophobic interactions...

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