Sunlight attack

Light-induced oxidation of proteins has been shown to lead to off-flavors and destruction of essential amino acids in milk. Patton (1954) demonstrated that sunlight attacks methionine and converts it into methional (( -methylmercaptopropionaldehyde), which can cause a typical sunlight off-flavor at a level of 0.1 ppm. It was later demonstrated by Finley and Shipe (1971) that the source of the light-induced off-flavor in milk resides in a low-density lipoprotein fraction. 

Polymers that are resistant to sunlight attacks outdoors, like polyethylene and poly(methyl methacrylate). 

Limitations Not resistant to weather and sunlight attacked by organic solvents. 

Chloroform slowly decomposes on prolonged exposure to sunlight in the presence or absence of air and in the dark in the presence of air. The products of oxidative breakdown include phosgene, hydrogen chloride, chlorine, carbon dioxide, and water. At 290°C, chloroform vapor is not attacked by oxygen. In contact with iron and water hydrogen peroxide is also produced, probably by the following reaction sequence (2) ... 

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

Other electrophilic substitutions proceed with difficulty, or not at all. Nitrosation and diazo coupling require the presence of the strongly activating dimethylamino group (see Section VIII). Bromine adds, in the presence of sunlight, to give tetrabromotetrahydrobenzofuroxan (48) the initial attack is probably free-radical in nature. The product can be dehydrobrominated to form 4,7-, or a mixture of 4,5- and 4,6-dibromobenzofuroxan, depending upon the conditions. More conventional electrophilic bromination conditions have been tried in an attempt to obtain a monosubstituted product, but without success. 

Natural rubber is resistant to dilute mineral acids, alkahes, and salts, but oxidizing media, oils, and most organic solvents will attack it. Hard rubber is made by adding 25 percent or more of sulfur to natural or synthetic rubber and, as such, is both hard and strong. Chloroprene or neoprene rubber is resistant to attack by ozone, sunlight, oils, gasoline, and aromatic or halogenated solvents but is... 

The chain length, i.e. number of RH —> RC1 conversions per Cl produced by photolysis, is wlO6 for CH4, and the reaction can be explosive in sunlight. Chlorination can also be initiated thermolytically, but considerably elevated temperatures are required to effect Cl2 — 2C1, and the rate of chlorination of C2H6 in the dark at 120° is virtually indetectable. It becomes extremely rapid on the introduction of traces of PbEt4, however, as this decomposes to yield ethyl radicals, Et, at this temperature, and these can act as initiators Et- + Cl—Cl —> Et—Cl + Cl. Chlorination of simple alkanes such as these is seldom useful for the preparation of mono-chloro derivatives, as this first product readily undergoes further attack by the highly reactive chlorine, and complex product mixtures are often obtained. 

An ethanolic solution of the hypnotic, methyprylone (269), was irradiated with simulated sunlight [164]. Solvent attack and ring opening gave the amidoester (270) [165]. 

Bunce, N.J., Nakai, J.S., and Yawching, M. A model for estimating the rate of chemical transformation of a VOC in the troposphere by two pathways photolysis by sunlight and hydroxyl radical attack, Chemosphere, 22(3/4) 305-315, 1991. 

PP is now used in indoor or outdoor carpets. However, the first PP products deteriorated rapidly when subjected to sunlight because of the presence of tertiary hydrogen atoms present. Explain. Which is more resistant to attack by microorganisms (a) PVC or (b) plasticized PVC ... 

Like ldpe, polybutadienes are resistant to most nonoxidizing acids, alkalis, and salts. However, because they are unsaturated, the polyalkadienes are attacked by hydrochloric, hydrobromic, and hydrofluoric acids, as well as by hydrogen and chlorine. The reaction products, which are thermoplastic, have been used as commercial nonelastomcric plastics. NR and other diene elastomers are also attacked by peroxides and ozone. In the absence of an tioxidants and carbon black filler, these unsaturated elastomers are degraded in the sunlight. 

These ABA copolymers have an index of refraction of 1.5 and water absorption of about 0.2%. Unless hydrogenated to saturated block copolymers, these unsaturated unstabilized plastics are degraded in sunlight. The polybutadiene domains are attacked by aliphatic hydrocarbon solvents, such as hexane, and the polystyrene domains arc attacked by aromatic hydrocarbon... 

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