Concentration of Antioxidants

About 40 years ago, D. Christiansen [57] and H. Bakstrom [58] explained the inhibition of the oxidation reaction by small impurities, assuming that oxidation is a chain process, and impurities terminate the reaction chains. This theory was developed by N. N. Semenov in his monograph Chain Reactions [5]. 

It is customarily believed that carbon chain polymers are oxidized according to the mechanism of chain reactions with degenerate branches, proposed by Semenov [3]. Initiation occurs as a result of attack on the RH molecule by oxygen according to the reaction 

Moreover, molecules of the hydroperoxide ROOH are formed as the primary reaction product they are capable of decomposing, thus giving rise to branching  

It is assumed that the antioxidants JH react with active RO2 radicals, forming inactive products  

Semenov showed that the presence of a critical concentration of the antioxidant [6] follows from the mechanism cited, described by the differential equations 

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Aminophenols and their derivatives are of commercial importance, both in their own right and as intermediates in the photographic, pharmaceutical, and chemical dye industries. They are amphoteric and can behave either as weak acids or weak bases, but the basic character usually predominates. 3-Aminophenol (2) is fairly stable in air unlike 2-aminophenol (1) and 4-aminophenol (3) which easily undergo oxidation to colored products. The former are generally converted to their acid salts, whereas 4-amiaophenol is usually formulated with low concentrations of antioxidants which act as inhibitors against undesired oxidation. 

Antioxidants are used to retard the reaction of organic materials with atmospheric oxygen. Such reaction can cause degradation of the mechanical, aesthetic, and electrical properties of polymers loss of flavor and development of rancidity ia foods and an iacrease ia the viscosity, acidity, and formation of iasolubles ia lubricants. The need for antioxidants depends upon the chemical composition of the substrate and the conditions of exposure. Relatively high concentrations of antioxidants are used to stabilize polymers such as natural mbber and polyunsaturated oils. Saturated polymers have greater oxidative stabiUty and require relatively low concentrations of stabilizers. Specialized antioxidants which have been commercialized meet the needs of the iadustry by extending the useflil Hves of the many substrates produced under anticipated conditions of exposure. The sales of antioxidants ia the United States were approximately 730 million ia 1990 (1,2). 

Table 3 UV-Embrittlement Times (EMT) of PP and LDPE Films Containing Different Concentrations of Antioxidants (processed in an internal mixer at 190°C and 150°C, respectively, and exposed to UV light in an accelerated sunlamp-blacklamp UV aging cabinet)...

Repeated periods of exercise reduce the likelihood of damage to skeletal muscle during subsequent bouts of the same form of exercise and this appears to be associated with an increase in the activity of muscle SOD (Higuchi et al. 1985), a reduced level of lipid peroxidation products during exercise in trained rats (Alessio and Goldfarb, 1988), and a modification of the concentration of antioxidants and activity of antioxidant enzymes in trained humans (Robertson etal., 1991). Packer and colleagues (Quintanilha etui., 1983 Packer, 1984) have also examined the exercise endurance of animals of modified antioxidant capacity and found that vitamin E-deficient rats have a reduced endurance capacity, while Amelink (1990) has reported that vitamin E-deficient rats have an increased amount of injury following treadmill exercise. 

In summary, therefore, the evidence seems convincing that exercise modifies circulating and tissue concentrations of antioxidants and enzyme activities. It is much less certain that the fatigue or damage to skeletal muscle associated with various forms of excessive or unaccustomed exercise is initiated by free radical-mediated degradation. Considerably more work is required in this area to clarify the underlying pathogenic mechanisms. 

This reaction is slow due to a high activation energy (see Chapter 15). However, at elevated temperatures and sufficiently high concentrations of antioxidant and hydroperoxide, this reaction becomes fast and, hence, can accelerate the rate of oxidation. As a result, the rate of initiation increases v = vi0 + enkn[ROOH] [InH] (en is the probability of the appearance of active radicals in the bulk). From the other side, this reaction shortens the induction period (T0=/[InH]0/vl0). 

Equations for the Rate of Oxidation v, Critical Concentration of Antioxidant [lnH]cr, and Quasistationary Concentration of Formed Hydroperoxide [ROOH]s at Common Action of Antioxidant and Quinone Q or Nitroxyl (>NO ) [4]... 

Various aromatic secondary amines, substituted phenols, and pyrazoli-dones (3) that function as traps for the propagating peroxy radicals gave dead-stop induction periods when used at a concentration of 50 p.p.m. An indication of the ease of oxidation of chloroprene is that 50 p.p.m. of 2,6-di-ferf-butyl-4-methylphenol gave an induction period of only 15 minutes, while the same concentration of antioxidant prevented n-hexadecane from oxidizing for 2 hours at 160°C. 

Several homemade creams, with different concentrations of antioxidants, are studied. Current and charge densities increase (Fig. 9.5) ... 

Fig. 9.5. First cyclic voltammograms recorded on a 2-mm platinum disk (scan rate 50 mV s-1) in homemade o/w (40/60) emulsions influence of the presence and the concentration of antioxidants.

Most of the data are presented as plots of the time to failure for the polypropylene as a function of the size of the alkyl group derived from the a-olefin. These comparisons are made on an equal antioxidant weight basis, and as the size of the alkyl group is increased, the molar concentration of antioxidant decreases. Presented in this customary way the data are weighted in favor of lower molecular weight compounds because their molar concentration is higher. 

AA = / (time or rate temperature substrate concentration of antioxidant concentration of other substances" partitioning behavior)... 

Organotin compounds are effective catalysts for the isocyanate-hydroxyl reaction. Tin catalysts have a slight odour, and low amounts are required to achieve a high reaction rate. Examples of organotin catalysts are stannous octoate, stannous oleate, dibutyltin dilaurate and dibutyltin di-2-ethylhexoate. They are often used in conjunction with small concentrations of antioxidants such as tertiary-butyl catechol resorcinol and tartaric acid. 

Ciclosporin can cause cholestasis and cellular necrosis by an inhibitory effect on hepatocyte membrane transport proteins at both sinusoidal and canalicular levels. It induces oxidative stress by accumulation of various free radicals. Ademetionine (5-adenosylmethionine) is a naturally occurring substance that is involved in liver detoxification processes. The efficacy of ademetionine in the treatment and prevention of ciclosporin-induced cholestasis has been studied in 72 men with psoriasis (89). The patients who were given ciclosporin plus ademetionine had low plasma and erythrocyte concentrations of oxidants and high concentrations of antioxidants. The authors concluded that ademetionine may protect the hver against hepatotoxic substances such as ciclosporin. 

The loss of effective stahlllzatlon Is predominantly physical, not chemical, as Identification of stabilizer on the surface of aged polyolefins has shown.(6, ) The concentration of antioxidant dissolved in the polymer and the resulting oxidative stability decrease with time and approach equilibrium for the temperature of concern. The entire process Is complex, with stabilizer solubility only a few parts per million at room tem-peratureC ) and diffusion of stabilizer from the polymer rapid. 

Utilizing a voltammetric measurement technique, the RULER quantitatively analyses the relative concentrations of antioxidants (hindered phenolic and aromatic amine) in new and used oils. This data can be trended to determine the depletion rates of the antioxidant protection package in the oil provided the instrument has been calibrated for that oil type. From pre-established limits, proper oil change cycles, potential interval extension or timely antioxidant replenishments can be determined. 

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