Ultraviolet attack

Such structural changes are a consequence of chemical reactions of which the most common are oxidation, ozone attack, dehydrochlorination and ultraviolet attack. (Reactions due to high-energy radiation or to high temperature are not considered here as causing natural aging.) Over the years many materials have been introduced as antioxidants, antiozonants, dehydrochlorination stabilisers and ultraviolet absorbers—originally on an empirical basis but today more and more as the result of fundamental studies. Each of these additive types will be eonsidered in turn. 

Other studies of interest on rubbers include weathering and photolysis in the far ultraviolet." The resistance of rubbers to ultraviolet attack has been found to decrease in the order neoprene > polybutadiene > polychloroprene." ... 

Additives required to facilitate processing include various lubricants and plasticizers, whilst those required to enhance durability during service include thermal stabilizers and carbon black which enhance the resistance to ultraviolet attack. PP is more prone to oxidation than PE and therefore the incorporation of antioxidants is an important issue. 

The ECPI approach has been adopted by the European Commission in their "Technical Guidance Document on the Risk Assessment of Notified New Substances" as the model for assessment of environmental exposure from additives in plastics. It is important to note, however, that due to the effect of ultraviolet degradation and microbial attack, a significant proportion of the emissions from flexible PVC consists of plasticizer degradation products. In these instances, therefore, the level of plasticizers appearing in the environment will be significantly less than indicated by the plasticizer loss data. 

Ultraviolet radiation causes cleavage of the aryl ether linkage (23). DMPPO undergoes oxidation when exposed to ultraviolet light and oxygen by direct attack on the aromatic ring to produce a variety of ring-cleaved and quinoidal stmctures (24). 

The oxidation of hydrocarbons, including hydrocarbon polymers, takes the form of a free-radical chain reaction. As a result of mechanical shearing, exposure of ultraviolet radiation, attack by metal ions such as those of copper and manganese as well as other possible mechanisms, a hydrocarbon molecule breaks down into two radicals... 

TFifluorothiolacetic acid adds to a senes of olefins under ultraviolet irradiation The addition appears to start by a CFjCOS radical attack, giving the more stable radical intermediate [14] (Table 2)... 

Because CFCs are highly resistant to chemical attack, they are stable in the lower atmosphere, where they can exist for up to 100 years. This stability gives CFCs time to diffuse up through the troposphere and into the stratosphere. There, CFCs absorb short-wavelength ultraviolet light from the sun that breaks carbon-chlorine... 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

Furthermore, the preparation and reactions of 2-methoxythiophene were studied by Sice (70). This compound was obtained by a copper catalysed Williamson synthesis. It was also found that iodothiophene reacted readily with sodium alkoxides, whereas bromothiophene reacted slowly and chlorothiophene did not react at all. Sodium iodide accelerated the reaction of bromothiophene. The ortho, para orienting alkoxy group on carbon atom 2 increased the directive influence of the sulphur atom to the 5 position but competed with it to induce some attack on the 3 position by electrophilic reagents (nitration, acylation). The acylation of 2-methoxythiophene with stannic chloride at low temperatures furnished a mixture of two isomers. The 5-methoxy-2-acetothienone was obtained in higher yield and was identified by its ultraviolet absorption spectrum. 

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