Effects of oxidation degradation

The principal effects of oxidative degradation of polymers are the decay of good mechanical properties (strength, elongation, resilience, etc.) and discoloration (mainly yellowing). 

The behaviour of polymers may vary widely. A polymer may be resistant to mechanical decay but not to colour decay, or the reverse. Often the two go together. Table 22.1 gives a survey of these effects for the different polymer families in the case of photodegradation. 

Meaning of symbols +, improvement 0, no change 1, slight detonation 2, moderate deterioration 3, strong deterioration 4, very strong deterioration. 

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Semisolid and Solid Dosage Forms Since most oxidative reactions occur in solution, there is not much effort to investigate the effect of oxidative degradation in solid dosage forms. Auto-oxidation of tetrazepam tablets has been described [31]. 

Historically, the bulk lubricant has been studied by dielectric spectroscopy and interpreted according to the Debye relaxation theory [3,4]. In impedance terms the system can also be represented according to a theory of colloidal dispersions or polycrystalline media composed of spheres of vastly different conductivities, where the contaminants become a more conductive phase suspended inside the less conductive additive/base oil matrix [6, 34]. Alternatively, when the contaminants are absent, the polar additives can be considered as a conductive discontinuous phase suspended inside insulating continuous base oil. Initially the description of the impedance representation of the fresh, uncontaminated oil will be provided, and then the effects of oxidative degradation and contaminants will be discussed. 

The effect of oxidative irradiation on mechanical properties on the foams of E-plastomers has been investigated. In this study, stress relaxation and dynamic rheological experiments are used to probe the effects of oxidative irradiation on the stmcture and final properties of these polymeric foams. Experiments conducted on irradiated E-plastomer (octene comonomer) foams of two different densities reveal significantly different behavior. Gamma irradiation of the lighter foam causes stmctural degradation due to chain scission reactions. This is manifested in faster stress-relaxation rates and lower values of elastic modulus and gel fraction in the irradiated samples. The incorporation of O2 into the polymer backbone, verified by IR analysis, conftrms the hypothesis of... 

Numerous studies have demonstrated that degradation products of (3-carotene exhibit deleterious effects in cellular systems (Alija et al., 2004, 2006 Hurst et al., 2005 Salerno et al., 2005 Siems et al., 2003). A mixture of (3-carotene degradation products exerts pro-apoptotic effects and cytotoxicity to human neutrophils (Salerno et al., 2005 Siems et al., 2003), and enhances the geno-toxic effects of oxidative stress in primary rat hepatocytes (Alija et al., 2004, 2006), as well as dramatically reduces mitochondrial activity in a human leukaemic cell line, K562, and RPE 28 SV4 cell line derived from stably transformed fetal human retinal pigmented epithelial cells (Hurst et al., 2005). As a result of degradation or enzymatic cleavage of (3-carotene, retinoids are formed, which are powerful modulators of cell proliferation, differentiation, and apoptosis (Blomhoff and Blomhoff, 2006). 

As already mentioned, van Kuijk and colleagues (Kalariya et al., 2008) tested the effects of oxidation products of [i-carotcnc, lutein, and zeaxanthin on the activation of redox-sensitive transcription factors, NF-kB, and AP-1 in cultured ARPE-19 cells. Degradation products of all three carotenoids induced activation of NF-kB and AP-1, and these effects were ameliorated by pretreatment of cells with 1 mM NAC. NF-kB is a major transcription factor that binds to promoter sites of many pro-inflammatory cytokines such as IL-1, IL-6, TNF-a, and iNOS. These results indicate that the degradation products of carotenoids can stimulate a pro-inflammatory pathway. 

Altogether, there is strong evidence that carotenoids can exert multiple effects via modulation of signaling pathways. Because oxidative stress can easily degrade carotenoids, it is not enough to investigate the effects of intact carotenoids, but it is necessary to elucidate the effects of their degradation products as well. 

The effect of heat, whether generated externally or by other mechanisms within the polymer itself, will be to raise the temperature. An increase in temperature will accelerate most of the degradation mechanisms listed, such as oxidation, chemical attack or mechanical creep. Temperature alone can cause thermal expansion, while at very high temperatures polymers will decompose, although this may be masked by the effects of oxidation. 

The effect of photodynamic degradation on the viscosity of DNA132 was shown to be the result initially of single chain breaks, followed eventually by double chain scission this is in agreement with the idea that guanine oxidation is the cause of the viscosity change. 

Seakins (16) has reported that the low temperature oxidation of propane is promoted by chloroform but not by carbon tetrachloride. Our studies, however, show that chloroform and carbon tetrachloride have generally similar effects on all preflame stages (Figure 3) and that their patterns of oxidative degradation are also similar (Figure 8). Under the conditions of Seakins experiments the following reaction, which he suggested, probably initiates the sequence of reactions responsible for promotion. 

To reduce the injurious effect of oxidative stress, cells are equipped with two major antioxidant defense systems. The first concerns numerous enzymes, which catalyze ROS degradation, such as superoxide dismutase (SOD), catalase or glutathione peroxidase (GPx) ... 

Stabilizer—A substance added to a polymer to inhibit the degrading effects of oxidizers, ultraviolet light, or electrical discharge. 

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