Ozone attack compounds

Processing ndProperties. Neoprene has a variety of uses, both in latex and dry mbber form. The uses of the latex for dipping and coating have already been indicated. The dry mbber can be handled in the usual equipment, ie, mbber mills and Banbury mixers, to prepare various compounds. In addition to its excellent solvent resistance, polychloroprene is also much more resistant to oxidation or ozone attack than natural mbber. It is also more resistant to chemicals and has the additional property of flame resistance from the chlorine atoms. It exhibits good resiUence at room temperature, but has poor low temperature properties (crystallization). An interesting feature is its high density (1.23) resulting from the presence of chlorine in the chain this increases the price on a volume basis. 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

Retardation of oxidative degeneration and the effects of ozone attack can be mitigated but not totally overcome by the use of chemicals which, unfortunately, in the case of the most effective types, carry the penalty of causing staining of the rubber compound or surfaces with which it comes into contact. 

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

Major oxidation products of propanolol and metoprolol formed during ozonation in aqueous solution were investigated by Benner et al. [102, 103]. In the case of propanolol, the main ozonation product is a ring-opened compound with two aldehyde moieties, which results from ozone attack to the naphthalene ring [103]. Formation of aldehyde moieties was also one of the main oxidation routes during metoprolol ozonation, together with hydroxylation [102]. 

Ozone attacks C=C in unsaturated compounds including olefins, cycloolefins, pinenes, aromatics, and polybutadienes (for example, causing rubber to crack). 

Depending on the alkene cation radical nature, open-chain oxygenation and epoxida-tion take place as well as the formation of other trivial ozonolysis products. Alkylaromatic compounds are also oxidized by ozone via the ion radical mechanism. Ethylbenzene, for example, undergoes ozone attack on the ring (80%) and on the alkyl group (20%). According to kinetic studies, the ozone consumption obeys the chain law (Galstyan et al. 2001). 

In the case of benzene and substituted benzene, attack of a first molecule of ozone is slow but the resulting molecule reacts more rapidly since they belong to the group of olefins. At the end of the reaction, the peroxidic compounds are unstable and contain different products coming from more than one mode of ozone attack [40-43] and competing reaction leading to perox-... 

Localization of double bonds in unknown compounds has frequently been determined by ozonolysis. Unsaturated fatty acids of biological membranes are susceptible to ozone attack, but there are some important differences from autoxidation reactions. These include the fact that malonaldehyde is produced from linoleate by ozonolysis (53) but not autoxidation and also that ozonolysis does not cause double bond conjugation as judged by absorption at 233 nm (52). Reactions with the polyunsaturated fatty acids produce several possibilities for toxic reactions direct disruption of membrane integrity and toxic reactions caused by fatty acid hydroperoxides, hydrogen peroxide, and malonaldehyde. 

In contrast to this, very little is known about the ozonolysis of olefins which bear halogen substituents at the double bond. This is somewhat surprising since compounds containing vinyl halide moieties are important technical products whose properties could be adversely affected by ozone degradation. A case in point is neoprene rubber, whose performance as an elastomer could suffer considerably by ozone attack and concurrent crack formation (1). 

This result may also give a partial explanation for the known ozone stability of polymers containing vinyl halide moieties. Part of this inertness is undoubtedly the result of the slow ozone attack. However, this phenomenon can only impart a prolonged ozone resistance, while the conversion of the original double bond into single bonds could impart a permanent ozone resistance. We are extending our research to further model compounds of such polymer microstructures to find out whether... 

The second class of tests focused on ageing, oxygen and ozone attack, adhesion, resistance to wear and tear, and general fatigue. This class also includes tests under simulated service condition in laboratory environments. These two classes of tests are conducted on vulcanised rubber. For assessing the processibility of the compounded... 

Ozone attack on the oxidatively violated rubber surface is thus prevented (21, 27, 29, 30). Creation of the surface layer was confirmed using microscopy. The theory is in agreement with the observation that the initial rapid ozone consumption is stabilized rubber drops and may be renewed after mechanical break of the formed film 21). There is a chemical proof of the theory. Ozonation products of N,N -bis(l-methylheptyl)-l,4-PD (DOPPD) form a surface film on ozonized and DOPPD doped vulcanized NR (31) and carbon-black loaded NR (24, 32). In addition to unreacted DOPPD, many of the low-molecular weight compounds observed in the film were found also in the ozonation of pure DOPPD (24) the only difference was a... 

Ethylene-propylene rubber is particularly resistant to sun, weather, and ozone attack. Excellent weather resistance is obtained whether the material is formulated in color, white, or black. The elastomer remains free of surface crazing and retains a high percentage of its properties after years of exposure. Ozone resistance is inherent in the polymer, and for all practical purposes, it can be considered immune to ozone attack. It is not necessary to add any special compounding ingredients to produce this resistance. 

It is well-known that the cleavage of olefinic double bonds by ozone attack produces compounds having aldehyde, ketone and/or carboxyl end-groups (Scheme 7.8). 

Weather Resistance Many rubbers are affected adversely by outdoor exposure, particularly by the traces of ozone, which are always present in the atmosphere and which cause rapid cracking of lightly stressed rubber articles. Nitrile rubber is no exception and thus it is not suitable for use in exposed conditions. However, the incorporation of a proportion of PVC (Polyvinyl chloride) into the nitrile compound results in improved resistance until, with a blend of 70/30 nitrile/PVC, a material with almost complete resistance to ozone attack is produced. The presence of PVC produces stiffer compounds, with lower resiliance and inferior low temperature properties, but the oil and solvent resistance is enhanced. 

FKM vulcanizates have excellent heat resistance, giving continuous service for 1000 hr at 220°C. Useful service is even possible at 250°C. These elastomers are also highly resistant to weathering and ozone attack. FKM vulcanizates are resistant to swelling in hot oils and aliphatic compounds. They also are resistant to aromatics, chlorinated hydrocarbons, and motor fuels. In addition, they are very resistant to most mineral acids. The gas-permeability resistance of FKM vulcanizates even exceeds that of butyl rubber vulcanizates. 

Blooms and visually similar phenomena are sometimes seen on the surfaces of rubber mouldings. These may originate from the limited solubility of a particular component of the rubber compound, or from contamination, for instance, by a release agent from the mould. Some blooms are deliberately contrived - hydrocarbon wax gives some protection from ozone attack. Loadman and Tidd give a useful account of the classification and analysis of blooms on rubber. 

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