Atmospheric oxidation, stability

The greatest source of contamination is extraneous matter. Atmospheric dirt, for example, is always a serious threat. It can enter the oil system through vents, breathers, and seals. Its primary effect is equipment wear, but plugging of oil lines and ports, and reduced oxidation stability of the oil are also serious effects. 

Impurities in mineral fillers can have serious effects. Coarse particles (grit) will lead to points of weakness in soft polymers which will therefore fail under stresses below that which might be expected. Traces of copper, manganese and iron can affect the oxidative stability whilst lead may react with sulphur-containing additives or sulphurous fumes in the atmosphere to give a discoloured product. 

The di- and monoalkyltin compounds are considered to be effective as stabilizers because they (i) inhibit the onset of the dehydrochlorination reaction by exchanging their anionic groups, X, with the reactive, allylic chlorine atoms in the polymer (ii) react with, and thereby scavenge, the hydrogen chloride that is produced and that would otherwise induce further elimination (jii) produce the compound HX, which may also help to inhibit other undesirable side reactions and iiv) prevent breakdown of the polymer initiated by atmospheric oxidation, i.e., by acting as antioxidants. 

Measuring the relative oxidative stability of polymers is important. Measurements can be used to determine dependencies on structural and molecular weight/weight distribution or the effectiveness of an antioxidant, or to perhaps assess the amount present in a polymer sample, etc. The preferred and commonest method consists in raising the sample temperature to a predetermined level, while in an inert atmosphere, then switching the atmosphere to air or oxygen. The time to the onset of exothermic reaction is measured. 

Allylic boronates are more stable to atmospheric oxidation and are thns mnch easier to handle than the corresponding allylic boranes. The stability of the boronate reagents arises from the partial donation of the lone pairs of electrons on the oxygen atoms into the empty p-orbital of boron. This mesomelic effect is responsible for the npfield shift of the boron atom in NMR compared to that of allylic boranes (compare allylboronate 31 and allylborane 32). ... 

Polyacetylene has good inert atmospheric thermal stability but oxidizes easily in the presence of air. The doped samples are even more susceptible to air. Polyacetylene films have a lustrous, silvery appearance and some flexibility. Other polymers have been found to be conductive. These include poly(p-phenylene) prepared by the Freidel-Crafts polymerization of benzene, polythiophene and derivatives, PPV, polypyrrole, and polyaniline. The first polymers commercialized as conductive polymers were polypyrrole and polythiophene because of their greater stability to air and the ability to directly produce these polymers in a doped form. While their conductivities (often on the order of 10" S/m) are lower than that of polyacetylene, this is sufficient for many applications. 

As mentioned earlier PMR polyimide thermosts are used as matrix resins for glass- and carbon fiber composites, mainly in aeroengine applications. At this point it has to be mentioned that the thermal oxidative stability of a PMR composite is dependent on the type of fiber used (113) and the cure conditions (time/temperature/atmosphere) employed for molding. Very interesting is the observed higher thermal oxidative stability of PMR-II composites when cured/-... 

Finely divided stoichiometric Fe304 pigments are not stable to atmospheric oxidation. They are therefore stabilized by partial oxidation or by complete oxidation to y-Fe203 below 500 °C. 

The solubility of air in fat is 8.7 ml/100 g, of oxygen 2.8 ml/100 g and of nitrogen 5.9 ml/100 g at room temperature and atmospheric pressure. Liquid fat in contact with air contains 0.004% oxygen. These values are related to the oxidative stability and effectiveness of the packaging of stored milk fat. 

Since TG and DTA complement each other, it is an obvious move to attempt both investigations simultaneously [173]. TG-DTA measures mass and energy changes as a function of temperature or time. Depending on the atmospheric conditions (vacuum, inert or air conditions) thermal or oxidative stability is measured. Typical TG-DTA... 

The common oxidation states of iron are + 2 and + 3. The relative stability of the two oxidation states in acid aqueous solution is defined by the standard electrode potential of + 0.77 V for the Fe3+/Fe2+ couple.1 This potential is such that the hydrated Fe11 cation is thermodynamically unstable with respect to atmospheric oxidation (equation 1). The oxidation is even more favourable in basic solution (equation 2). It is apparent, therefore, that the chemistry of iron, including its... 

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