Photodegradation degradation reactions

On the other hand, many excipients can act to chemically stabilize an API in the solid state and in solid dosage forms. The most common class of stabilizing excipients is cyclodextrins (36). Cyclodextrins can envelop the API in their hydrophobic cavities and shield it from common degradation reactions such as hydrolysis, oxidation, or photodegradation. Some excipients or additives may also act as complexing agents that provide hydrolytic (37) and oxidative (38) stabilization. Many excipients, such as cyclodextrins, dyes, and colored additives, are capable of providing extensive photostabilization in the solid state (39-41). 

It has been demonstrated that the sensitivity of the mass spectrometer makes it Ideally suited for application to the study of polymer photodegradation. The basic understanding of the system makes it possible to extract useful kinetic and chemical information about the degradation reactions. Work on the roles of temperature, chemical composition and network structure is in progress. 

The ketonic products can be expected to occur in trace amounts in pure polymers and are also components of the complex photo-oxidation products. Photolytic reactions of these ketones are important both in the very early stages of photo-oxidative degradation and in the continuation of an established photo-degradation reaction. The ketones may affect the source of the photodegradation by contributing to free radical processes, chain scission and energy transfer. 

Some weighting agents will act to accelerate the photo-deterioration of silks too. Metal ions, such as tin, adopt catalytic roles in degradation reactions, whereas tannin compounds can reduce the pH of the silk to the point where it becomes particularly susceptible to photodegradation. 

This degradation reaction, supplemented by various subsequent oxidation steps, has found renewed interest in the form of the introduction of photodegrad-able plastics as part of the campaign to reduce plastic litter from throwaway packaging. Although as yet there has been no demand for photodegradable mb-bers, the incorporation of a small percentage of a vinyl ketone into a rubber copolymer or homopolymer would open the way to a useful synthesis of block copolymers. 

To evaluate the persistence of compounds adequately, the various degradation pathways (Schwarzenbach, Gschwend and Imboden, 1993) that may be undergone have to be considered in combination. The extent, the rates and the byproducts of the individual processes should preferably be integrated. The predominant abiotic degradation reactions comprise reactions with reactive species in the atmosphere (so-called photodegradation), hydrolysis and photolysis (Figure 4.7). 

The degradation of polymers by light is not only a photodegradation, but rather a photooxidative degradation, since in practice the degradation occurs in the presence of air. Thus, secondary degradation reactions will occur where oxygen is involved. 

The photodegradation of PE has been extensively examined [58, 74, 75]. It has been postulated that carbonyl groups are the main light-absorbing species responsible for the photochemical-induced degradation reactions for UV-exposed PE [76]. 

As mentioned previously, the exposure of poly(vinyl chloride) to either ultraviolet light or heat leads to degradation, as a result of which discoloration and changes in mechanical properties can occur. The degradation of poly(vinyl chloride) has been the subject of extensive investigation and much information has accumulated. However, the reactions which take place are complex and are not completely understood at the present time. Whilst thermal degradation and photodegradation of poly(vinyl chloride) have much in common, they are distinct processes and are considered separately. 

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