Degradation by Light

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. 

An important exception of this view in the photodegradation im-der the conditions where space crafts are operating. In contrast, the type of radiation is more severe still, since under normal atmospheric conditions a lot of hard radiation is filtered by the atmosphere. 

When light enters a surface of a material, it may by either reflected from the surface, scattered, pass unchanged, or absorbed in the interior of the material. 

Photochemistry teaches that absorbed light causes photochemical reactions, such as crosslinking or degradation. 

Chromophores may not necessarily be attached to the basic structure of the polymer under consideration. Chromophores may be introduced as impurities in the course of manufacture. Examples are catalyst residues and other processing aids. 

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Choose a suitable colorant by also considering degradation by light and staining of fabrics or surfaces. 

Halothane was introduced into clinical practice in 1956. It was not the first fluorinated anaesthetic— fluoroxene (Fluoromar) holds that distinction—but it was the first to achieve widespread acceptability. Halothane is a fluorinated alkane 1-bromo, 1-chloro -2,2,2-trifluoroethane (Figure 3.2). It has a characteristic odour, similar to chloroform, and requires a stabiliser, thymol (0.01%), to prevent degradation by light. Halothane has a blood/gas partition coefficient of 2.4 able 3.2) but its lack of irritant qualities makes possible the use of relatively high inspired concentrations (2-4%). For that reason, inhalation induction is characteristically smooth and rapid. Compared to sevoflurane, and possibly isoflurane, recovery from halothane anaesthesia is delayed. 

The azomethine dye (102) (76BRP1451000) and the nickel chelates (103) and (104) have also been claimed to have activity in protecting image dyes from degradation by light (79BRP2022U9, 80BRP2026518). 

Elements Ge, Sn, and Pb form many interesting varieties of organometallic compounds, some of which are important commercial products. For example, EuPb is an anti-knock agent, and organotin compounds are employed as polyvinylchloride (PVC) stabilizers against degradation by light and heat. Some examples and advances are discussed in the present section. 

When the dispensers are exposed, the pheromone gradually diffuses out through the outer layers of the laminate and is thereby slowly released to the atmosphere in a manner similar to that of an insect secreting its lure into the air to attract a mate. Since most pheromones are potent, very little pheromone is needed, and the dispensers can frequently contain enough pheromone to last an entire season. Location of the bulk of the pheromone in the inner layer of the plastic protects it from degradation by light, air and weather. This feature can be important because some pheromones are rather unstable (e.g., aldehydes), and they have to be protected until released,... 

In an effort to determine the effect of aqueous acid on nylon in the presence of light, Zeronion et al (12) submerged nylon fabric in 20% sulfuric acid at 50 C in a flint glass jar and exposed it to irradiation from a 275 watt sunlamp at a distance of 6 inches from the fabric. The nylon showed more polymer chain scission and greater [NH ] than nylon degraded by light and SO gas both with and without a water spray. From these experiments, it was concluded that if sulfuric acid was present in the atmosphere, its attack on nylon was accelerated by the presence of light. 

It is well known that polypropylene is easily degraded by light in the presence of air. The ESR spectrum observed after UV irradiation of polypropylene at —196° C has been reported by several workers (41, 59-61, 77—79). Ranby et al. (59) and Browning et al. (60) reported the spectrum observed after UV irradiation of polypropylene at —196° C as shown in Fig. 7. The spectrum shown in Fig. 7a was observed immedia-... 

Organo-compounds of the group 14 elements include some important commercial products, and we have already discussed silicones in Section 13.10 and Box 13.12. Organotin compounds are employed as polyvinylchloride (PVC) stabilizers (against degradation by light and heat), antifouling paints on ships, wood preservatives and... 

D-ribo-2,3,4,5-tetrahydroxypentyl)isoalloxazine and 7,8-dimethyl-10-ribityhsoalloxazine its formula is C17H20N4O6. Riboflavin has a molar mass of 376.37 grams (13.3 ounces). It is heat-stabile but easily degraded by light. Riboflavin was referred to as vitamin G in the early part of the twentieth century because it was recognized as a dietary factor needed for growth. Riboflavin was first isolated in 1879, and its chemical structure was determined in 1933. 

D-glucose to degradation by light from an unscreened, quartz lamp when the acids are irradiated at pH 7.2. 

Lignin is easily degraded by light with a wavelength <350 nm. Significant color buildup or formation of chromophoric groups occurs. 

However, there are two related areas of technology, lithography and microlithography, which make positive use of polymer degradation and for which polymers are designed to be as readily degraded by light as possible. 

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