Degradation under the influence of light

Only the part of the radiation that is actually absorbed by the material can become chemically active. Most pure, organic synthetic polymers (polyethylene, polypropylene, 

Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly (phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. UV-protection of polymers can be well achieved by the use of additives (e.g. nickel chelates) that, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene). 

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CdS can be degraded under the influence of light in the presence of dissolved oxygen. The action of oxygen consists of an oxidation of sulfide radicals arising through the formation of hole-anion pairs at the particle surface [170] ... 

The abundance of natural and man-made polymers provides a wider scope for the choice of shell material, which may be made permeable, semi-permeable or impermeable. Permeable shells are used for release applications, while semi-permeable capsules are usually impermeable to the core material but permeable to low molecular-weight liquids. Thus, these capsules can be used to absorb substances from the environment and to release them again when brought into another medium. The impermeable shell encloses the core material and protects it from the external environment Hence, to release the content of the core material the shell must be ruptured by outside pressure, melted, dried out dissolved in solvent or degraded under the influence of light (see Chapter 7). Release of the core material through the permeable shell is mainly controlled by the thickness of the shell wall and its pore size. The dimension of a microcapsule is an important criterion for industrial applications the following section will focus on spherical core-shell types of microcapsules (Fig. 1.8). 

When primaquine degrades under the influence of light (photodegradatiOTi) degradation products may be formed that are more toxic than primaquine itself [5]. 

The most damaging atmospheric factor is light radiation. Photo-oxidative, as well as photolytic reactions are the basis for plastics degradation under the influence of light [38]. 

When polybutylene terephthalate and polyethylene terephthalate degrade under the influence of light, photo-induced reactions are observed such as Norrish type I and type II (see Figure 5.12) and photo-oxidative processes. Figure 5.88. The formation of m-biphenyl structures can also be detected [38]. 

Sodium nitroprusside has long been known to degrade under the influence of fluorescent lighting or sunlight [ 13]. Initial reaction [ 14] follows the equation ... 

Ethylene propylene copolymers and their blends exhibit diverse degradation behavior under the influence of light, heat and radiation. In spite of many papers in this area, little, if any, mechanistic data on degradation and stabilization of this important class of materials is available in the literature. The present paper reviews the published literature in this area organised under five distinct class of materials, namely, thermoplastic, elastomeric, and heterophasic copolymers, thermoplastic elastomer and blends. Of this, elastomeric ethylene-propylene copolymers appears to have been most exhaustively studied. Very few studies have reported on thermoplastic copolymers, both random as well as heterophasic as well as thermoplastic elastomers and blends. Specific mechanisms of degradation and stabilization of each of these classes of materials are discussed. 

Besides mixtures of polymers or copolymers, a very common procedure to improve desired characteristics of polymers is that of including additives. The additives are typically nonpolymeric molecules. Among the properties that are modified with additives are cost, mechanical properties, color, degradation rate under the influence of light or gases from atmosphere, odor, adhesion to processing equipment, etc. Some polymer properties modified with additives are shown in Table 1.5.1 [1]. 

As was noted in Chapter 4, polyesters such as polyethylene terephthalate (PET) degrade mainly in a thin surface layer under the influence of light and oxygen. This suggests that photostabilisation in the bulk of an article will not be optimally used at best, portions of the additive distributed in the interior will act as a reservoir to replace stabiliser lost in the outer regions during any of the chemical or physical processes under way. 

As many preparations will show degradation under the influence of (day) light, most ear drops should be kept away from light. In some cases the protection against light by a brown bottle will not be enough, for instance for chloramphenicol. 

Under the influence of light chloramphenicol degrades into p-nitrosanilline, which is carcinogenic. 

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