Stabilization radical scavenging

Exposure to solar irradiation (sunlight) may alter their chemical integrity and in due course some physical properties such as mechanical strength (e.g. brittleness) or color (e.g. fading) as well. Additives like industrial ultra violet (UV)-absorbers, hindered amine light stabilizers, radical scavengers and antioxidants are incorporated or applied in these products to effectively reduce or delay such radiation induced deteriorations. 

Figure 3.181 ionol (BFIT) C,5hl240, M 220, CAS 128-37-0. Occurrence antioxidant in plastics, stabilizer (radical scavenger) for ethers, THF, dioxan, technical mixture of isomers. 

Vinyhdene chloride polymers containing stabilizing features have been prepared. In general, these have been polymers containing comonomer units with functionahty that can consume evolved hydrogen chloride and do so in such a manner that good radical scavenging sites are exposed (145,146). 

Aromatic Amines. Antioxidants derived from -phenylenediarnine and diphenylamine are highly effective peroxy radical scavengers. They are more effective than phenoHc antioxidants for the stabilization of easily oxidized organic materials, such as unsaturated elastomers. Because of their intense staining effect, derivatives of -phenylenediamine are used primarily for elastomers containing carbon black (qv). 

Stabilization of Fuels and Lubricants. Gasoline and jet engine fuels contain unsaturated compounds that oxidize on storage, darken, and form gums and deposits. Radical scavengers such as 2,4-dimethyl-6-/ f2 butylphenol [1879-09-0] 2,6-di-/ f2 -butyl-/)-cresol (1), 2,6-di-/ f2 -butylphenol [128-39-2], and alkylated paraphenylene diamines ate used in concentrations of about 5—10 ppm as stabilizers. 

FIGURE 15.4 Primary stabilization via radical scavenging by hindered pbenobcs. 

Most kinetic treatments of the photo-oxidation of solid polymers and their stabilization are based on the tacit assumption that the system behaves in the same way as a fluid liquid. Inherent in this approach is the assumption of a completely random distribution of all species such as free radicals, additives and oxidation products. In all cases this assumption may be erroneous and has important consequences which can explain inhibition by the relatively slow radical scavenging processes (reactions 7 and 9) discussed in the previous section. 

An unsubstituted hydroxylamine is a powerful hydroperoxide decomposer and peroxyl radical scavenger, and could play an important role in photo-stabilization even if present at only a low concentration after dark intervals. 

Regarding the proposed mechanisms of carvedilol antioxidative activity, membrane stabilization through the biophysical interaction of carvedilol with the membrane seems to be the most reliable one. However, a higher antioxidant activity of the metabolite SB 211475 leads to another explanation. In contrast to the parent carvedilol, SB 211475 has the active free radical scavenging phenolic hydroxyl, which is apparently responsible for its enhanced antioxidant activity. Thus, we may suggest that the in vivo antioxidant activity of carvedilol is due to its converting into active metabolites, which, for example, may be formed in the reactions with primary free radicals such as hydroxyl radicals. 

It should be mentioned that the commercially available VC usually contained a small percentage (<2%) of prohibitors that stabilized the reactive VC from polymerizing upon storage. These compounds were usually radical scavengers such as 2,6-di-tert-butyl-p-cresol (DBC) or butylated hydroxy toluene (BHT). Recently, reports pointed out that the presence of these prohibitors actually had a negative impact on the anodic stability of VC on various cathode materials, and VC of high purity was of vital importance in obtaining reproducible performance in lithium ion cells. 

Traditional hindered phenol and phenylenediamine free-radical scavenger type antioxidants do not usually provide the stability performance desired in distillate fuel. Stabilizer formulations containing components which provide a wider range of performance are often required. Some of the components commonly used in distillate fuel stabilizers are described as follows ... 

Guyot and coworkers have also noted that monomers stabilize poly(vinyl chloride) during mastication, probably because they act as free radical scavengers with the exception of basic monomers, such as vinyl pyridine, B. In this case dehydrochlorination reaction... 

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