INDEX oxidative degradation

There are some aspects in the raw dry NR grades for adhesive manufacturing to be considered. NR tends to suffer oxidative degradation catalyzed by metals (mainly copper). The susceptibility of NR to oxidation can be measured using the plasticity retention index. The better grades of rubber have the higher plasticity retention index. 

The relationships of oxidation potential to radical reactivity index Sr and nucleophilic reactivity index Sn illustrated in Figure 4 are very similar to those with antioxidation and antiozonization, where the maximum values were observed at 0.4 and 0.25 volt. Therefore, antioxidation seems to proceed by a radical mechanism in contrast to the nucleophilic type of antiozonization. Indeed, the antioxidation effect of amines toward NR, SBR, BR, and HR is well correlated with radical reactivity as shown in Figures 5-8. The protection of SBR solution by amines from oxidative degradation and the termination of chain reaction in the oxygen-Tetralin system are also shown as functions of Sr in Figures 9 and 10. 

Oxidative degradation appears to be the main mechanism governing humification. In determining the degree of decomposition or humification of peat materials the estimation of total fiber content correlates well with relative biodegradability and is a more reliable method than either the pyrophosphate index or bulk density values. 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

The OIT values in this case correspond to amounts of an antioxidant present in the material. When the OIT values are below 1 min, the plastic often is partially oxidized, degraded, its molecular weight shifted to lower values. This often leads to lower values of the power-law index, and for the first group of the (lower) OIT values, the average power-law index is 0.34 for the second (higher) OIT values, the average power-law index is 0.43. Hot melt in the first case is more sensitive to speed variations, that is, it is more likely to result in melt fracture (see the next section). 

We have found an extreme combustion behavior for a system polypropylene-polypropylene-co-polyethylene [238], It can be explained in terms of oxidative degradation of polymer materials. We studied the features of autooxidation and combustibility of blends of isotactic polypropylene (PP) and ethylene-propylene copolymers (PP-co-PE) as thin films (50-80 ). ASTM D2863 (ISO 4589 Part 2) was used to determine the oxygen index (LOI) for polymer films 140 by 52 mm. 

Figure 4.40 shows that, for a PE-HD, dispersion index and molecular weight increase due to thermal-oxidative degradation at high processing temperatures (in contrast to their behavior at low temperatures. Fig. 5.158). Radical chain reaction in polyethylenes with unsaturated vinyl groups leads to three- or four-pointed star configurations that cause an increase in shear viscosity. Here, chain branching predominates chain cleavage [37].

Influence of temperature on yellowing (evaluated limit value yellowness index = 10) in polycarbonate caused by thermal-oxidative degradation [673]... 

In products with a large wall thickness, the influence of weathering is often limited to a surface layer, either due to limited oxygen diffusion or to limited UV penetration. The depth of this layer may be small ( 0.5 mm) compared with the whole wall thickness ( 4 mm) but it can causes brittle fracture. Mechanical behavior depends on the oxidation (degradation) profile, a critical degradation profile accounted for the failure of samples, that is time of failure. Tensile tests were performed on films microtomed from the samples [39]. The density of weathered samples can increase because of chemicrystallization, increase in polar groups, oxygen uptake or the loss in volatile products. The decrease in the nominal strain corresponded to an increase in the vinyl index and in the density. The carbonyl index showed too much scatter. 

The plasticity retention index (PRI) is a measure of the susceptibility of the raw rubber to oxidative degradation. Oxidation is catalyzed by some metals such as copper, so that the PRI test gives an indirect indication of the presence... 

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