Antioxidants diffusion

Figure 3. CL upon introduction ("on") of 19.8 mg of 250- jm polypropilene film containing 2,6-di-f-butyl-4-methyl-phenol [2.18x10 mol/kg] (curve 1) concentration of the antioxidant diffused into probe solution amounts to 8.65x10 M. Curve 2 refers to CL response to the injection of the equal amount of the same antioxidant taken from liquid (PhCl) solution. CL was generated in oxidation of PhEt (50% in PhCl) initiated by AIBN at 333 K (initiation rate was 2.52x10 Ms ).

It is well known from pipe pressure tests that immersion in water enhances antioxidant migration in comparison to air, as was observed in our experiments. Smith et al. attributed this to the influence of carbon black (Smith et al. 1992). Antioxidants are adsorbed at the earbon black surface, whereby the migration process is retarded considerably. During immersion in water some moisture is also present in the HDPE bulk material and this is preferentially adsorbed by the carbon black aggregates. The adsorption of water thereby supersedes that of the antioxidants and decreases the retardation coefficient and increases the apparent diffusion coefficient of the latter. In addition, it was suggested that the antioxidant forms loosely bonded clusters within the dry PE bulk material which may disperse slowly in contact with diffusing water thus enhancing the antioxidant diffusion coefficient (Le Poidevin 1977). 

Physical losses occur because of volatilization, poor solubility, diffusion, leachability, blooming, etc. Efficiency means inherent activity and physically retained by polymers. There are still less information about chemical nature and migration of transformation products of additives. The antioxidants diffuse in PE hot water pipes leading to their consumption and pipe deterioration [71]. 

EB irradiation of polymeric materials leads to superior properties than the 7-ray-induced modification due to the latter having lower achievable dose rate than the former. Because of the lower dose rate, oxygen has an opportunity to diffuse into the polymer and react with the free radicals generated thus causing the greater amount of chain scissions. EB radiation is so rapid that there is insufficient time for any significant amount of oxygen to diffuse into the polymer. Stabilizers (antirads) reduce the dose-rate effect [74]. Their effectiveness depends on the abUity to survive irradiation and then to act as an antioxidant in the absence of radiation. 

Ubiquinone, known also as coenzyme Q, plays a crucial role as a respiratory chain electron carrier transport in inner mitochondrial membranes. It exerts this function through its reversible reduction to semiquinone or to fully hydrogenated ubiquinol, accepting two protons and two electrons. Because it is a small lipophilic molecule, it is freely diffusable within the inner mitochondrial membrane. Ubiquinones also act as important lipophilic endogenous antioxidants and have other functions of great importance for cellular metabolism. ... 

As diffusion to the surface of a polymer is one of the limiting steps in extraction, the particle size or film thickness of a sample is also important [278,333,337-340]. With the typical diffusion coefficients of additives in polymers a particle diameter of about 0.3 mm is required for an extraction time of about 1000 s at 40 °C. An exception to this is the extraction of thin films and foams, for which the shortest dimension is small. It is not surprising that no more than 50 % of antioxidants could be extracted from PP pellets as opposed to 90 % recoveries from the same polymer extruded into film [341]. Grinding of the polymer is usually an essential step before extraction. Care should be taken to avoid loss of volatile additives owing to the heat generated in such processes. Therefore, cryogrind-ing is preferred. 

Sawada, G. A., Williams, L. R. Lutzke, B. S., Raub, T. J., Novel, highly hpophilic antioxidants readily diffuse across the blood-brain barrier and access intracellular sites, J. Pharmacol. Exp. Then 288, 1327-1333 (1999). 

T. J., Increased lipophilicity and subsequent cell partitioning decrease passive transcellular diffusion of novel, highly lipophilic antioxidants,... 

Oxidative ageing of rubbers is limited by the rate of diffusion of oxygen into the rubber product and is usually confined to the outer 3 mm. Antioxidants are used to protect rubbers from the effects of thermal oxidation and the vast majority of compounds will contain one or more. Peroxide vulcanisates are usually protected with dihydroquinolines. Other antioxidants react adversely with the peroxide inhibiting the crosslinking reaction. 

The mechanism of antioxidant action on the oxidation of carbon-chain polymers is practically the same as that of hydrocarbon oxidation (see Chapters 14 and 15 and monographs [29 10]). The peculiarities lie in the specificity of diffusion and the cage effect in polymers. As described earlier, the reaction of peroxyl radicals with phenol occurs more slowly in the polymer matrix than in the liquid phase. This is due to the influence of the polymeric rigid cage on a bimolecular reaction (see earlier). The values of rate constants of macromolecular peroxyl radicals with phenols are collected in Table 19.7. 

Such a dependence was interpreted within the scope of the model of chain oxidation with diffusionally controlled chain termination on the surface of solid antioxidant (for example, Mo or MoS2). According to the Smolukhovsky equation, the diffusion velocity of radical R02 at the distance //2 is v = 0.2DkS13, where S is the surface of the solid inhibitor and k is the coefficient of proportionality between the surface and number n of the solid particles (S=k x ). The function F for such diffusionally controlled chain termination is the following ... 

As mentioned earlier, ascorbate and ubihydroquinone regenerate a-tocopherol contained in a LDL particle and by this may enhance its antioxidant activity. Stocker and his coworkers [123] suggest that this role of ubihydroquinone is especially important. However, it is questionable because ubihydroquinone content in LDL is very small and only 50% to 60% of LDL particles contain a molecule of ubihydroquinone. Moreover, there is another apparently much more effective co-antioxidant of a-tocopherol in LDL particles, namely, nitric oxide [125], It has been already mentioned that nitric oxide exhibits both antioxidant and prooxidant effects depending on the 02 /NO ratio [42]. It is important that NO concentrates up to 25-fold in lipid membranes and LDL compartments due to the high lipid partition coefficient, charge neutrality, and small molecular radius [126,127]. Because of this, the value of 02 /N0 ratio should be very small, and the antioxidant effect of NO must exceed the prooxidant effect of peroxynitrite. As the rate constants for the recombination reaction of NO with peroxyl radicals are close to diffusion limit (about 109 1 mol 1 s 1 [125]), NO will inhibit both Reactions (7) and (8) and by that spare a-tocopherol in LDL oxidation. 

Sawada GA, Barsuhn CL, Lutzke BS, Houghton ME, Padbury GE, Ho NFH, Raub TJ (1999) Increased lipophilicity and subsequent cell partitioning decrease passive transcellular diffusion of novel, highly lipophilic antioxidants. J Pharmacol Exp Ther 288 1317-1326. 

Salvia miltiorhiza Bunge. Tan Seng (Red-rooted sage) (root) Tanshinone, cryptotanshinone, isocryptotanshinone, miltirone, tanshinol, salviol, acetylsalicylic acid 33.226.235,428,429 Treat angina pectoris, cerebral atherosclerosis, diffusive intravascular clotting, thrombophlebitis, antioxidant. 

Reactions Limited by Rotational Diffusion in Polymer Matrix Antioxidants Reacting with Peroxyl Radicals Antioxidants Reacting with Alkyl Radicals Cyclic Chain Termination in Oxidized Polymers... 

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