Butylated hydroxytoluene , radical inhibition

The solution oxidation technique allows the study of polyolefin autoxi-dation under conditions where the temperature, concentration of reactants, and rates of radical initiation can be controlled. The results should be considered as a useful prelude to any fundamental understanding of the autoxidation processes which occur in neat polymers where the effects of very high viscosity, partial crystallinity, and oxygen diffusion rates are included. The objective of our work was to determine the kinetics and stoichiometry of the inhibited autoxidation of polypropylene in solution. A relatively detailed study of the oxidation of polypropylene inhibited by 2,6-di-terf-butyl-4-methylphenol [butylated hydroxytoluene (BHT)] has been made for comparison with data obtained in polypropylene oxidations inhibited by a variety of other stabilizers which include commercial polyfunctional antioxidants. Singly hindered phenols appeared to be superior in the inhibited-solution oxidation of polypropylene, and the application of this finding to stabilization technology was investigated briefly. 

This sequence of reactions is affected by prooxidants and by anti-oxidants. The former-frequently metals or radical-producing species-encourage initiation whilst anti-oxidants usually shorten the propagation sequence by promoting the termination reactions. Other compounds (synergists), added to anti-oxidants to enhance their activity, are mainly metal-chelating compounds such as citric acid, phosphoric acid, ascorbic acid or ethylenediamine tetra-acetic acid which inhibit the metal-catalysed initiation. Anti-oxidants include natural compounds like the tocopherols or synthetic materials such as butylated hydroxy-anisole (BHA), butylated hydroxytoluene (BHT), propyl gallate or tertiary butyl hydroquinone. 

As an example a model of die liquid-phase oxidation of the ethylbenzene in the presence of inhibitors, the iora-substituted phenols and the butylated hydroxytoluene, was selected. The identified dynamics of die value contribution of steps in the reaction mechanism is complicated. The dominant steps for die different time intervals of the inhibited reaction were determined. The inhibition mechanism of die ethylbenzene oxidation by sterically unhindered phenols is conditioned by establishing equilibrium (7.24) in the reaction of the chain carrier, the peroxyl radical, with the inhibitor s molecule (within sufficiently wide interval of the inhibitor s initial concentration), followed by the reaction radical s quadratic termination with the participation of the phenoxyl radical. The value analysis has established that the efficient inhibitor with low dissociation energy of the phenolic 0-H bond promotes shifting the mentioned equilibrium from the chain carrier to the direction of the phenoxyl radical formation. 

Although LDL is well protected against oxidation in blood plasma by an ample supply of endogenous antioxidants and metal-binding proteins, this protection may not be adequate in the arterial wall where the oxidative modification of LDL is induced by endothelial cells. One hypothesis proposed is that natural antioxidants may be depleted within the arterial sub-endothelial space where oxidation takes place. The oxidative modification of LDL by endothelial cells can be completely inhibited in the presence of sufficient vitamin E or other antioxidants, such as butylated hydroxytoluene. These antioxidants can inhibit two steps in the free radical oxidation pathway, by reacting with the peroxyl... 

Hindered phenols and aromatic amines are the two major groups of chain-breaking antioxidants that inhibit free radical chain reactions. Commonly used antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and vitamin E are hindered phenols. Because the O-H bonds of phenols and the N-H bonds of aromatic amines are very weak, the rate of oxidation is generally higher with the antioxidant than with the polymer (Crowley et al. 2007). 

The mechanism of thermal oxidation in PP is through the formation of free radicals which react with environmental oxygen to produce peroxides. It could also occur due to radiation, light or the presence of metal residues. Primary antioxidants inhibit the oxidation reaction by combining with free radicals. Hindered phenolics (e g., butylated hydroxytoluene (BHT)) are commonly used as primary antioxidants. BHT is FDA approved for many applications but suffers from high-temperature volatility. Furthermore, phenolics suffer from oxidation in the presence of metal residues left from catalysts and produce a yellow colour. Different residues or different amounts of... 

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