Effects evaluating oxidative changes

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

The main aim was, on the one hand, to ascertain the presence of a through-space interaction between the two nitrogen lone pairs and the role played by the Si—C and Sn—C bonds in the mechanism of this interaction, and on the other hand, to evaluate the effect of the change in tin oxidation state (+2 in 27, +4 in 30) on the electronic structure of these molecules. The He I and He II spectra were interpreted by comparison... 

P-450 and the activity of the three enzymes only in the 3-week-old rats. Six- and 10-week-old rats showed an inhibition of AHH and increased activity of aniline hydroxylase and ethylmorphine N-demethylase, which were lower than that seen after 7 days of exposure in their respective groups. The effect of the changes in the MFO enzymes on the liver is difficult to evaluate. Although the MFO system tends to process various foreign chemicals and thus be of benefit, some of the oxidized intermediary metabolites produced by the initial MFO reactions are more toxic than are the parent compounds. 

Thermoanalytical techniques such as differential scanning calorimetry (DSC) and thermogravi-metric analysis (TGA) have also been widely used to study rubber oxidation [24—27]. The oxidative stability of mbbers and the effectiveness of various antioxidants can be evaluated with DSC based on the heat change (oxidation exotherm) during oxidation, the activation energy of oxidation, the isothermal induction time, the onset temperamre of oxidation, and the oxidation peak temperature. 

The effects of temperature on carotenoid content can be considered from three perspectives (1) evaluation of stability or retention of carotenoids, (2) study of the chemical changes (isomerization, oxidation, epoxy-furanoid rearrangement), and (3) their effects on the nutritional value and other carotenoid actions in humans. The first two topics are discussed in the following sections. The third is presented in Section 3.2.4.1 of Section 3.2. 

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