Oxidation induction time general

As appHed to hydrocarbon resins, dsc is mainly used for the determination of glass-transition temperatures (7p. Information can also be gained as to the physical state of a material, ie, amorphous vs crystalline. As a general rule of thumb, the T of a hydrocarbon resin is approximately 50°C below the softening point. Oxidative induction times, which are also deterrnined by dsc, are used to predict the relative oxidative stabiHty of a hydrocarbon resin. 

The isothermal measurement under oxygen showed good correlations between chemiluminescence parameters and the concentration of stabilizers. The oxidation induction times have been seen to increase with the ratio of Irgafos 168/Irganox 1330 (Fig. 3.10). In general, the OITs increased with the transition temperature determined... 

It is general knowledge that there is no correlation between oxidative induction time and long-term properties, because the OIT procedure is performed far above melt temperature. These high temperatures cause chemical reactions that are not activated at lower temperatures in addition, some additives may dissipate at these temperatures [177]. Stabilizers accumulate in the amorphous zones of semicrystalline thermoplastics. Subsequent to crystallite melting, their mobility and solubility increase abruptly. They spread out over the entire mass, while local concentration decreases [178]. 

The onset of the oxidative exotherm due to thermal degradation can be used to obtain a measure of the stability of a polymer. An oxidative atmosphere is used and the sample is either heated at a constant rate to a given temperature, or heated isothermally at a specified temperature. In the former case the temperature at the onset point is used as the endpoint, and in the latter the time is used. A common test to determine the stability of plastics is the OIT (oxidation induction time) test. The method to be used for plastics in general is described in ISO 11357-6 (2002), and methods for polyolefins are described in the standards EN 728 (1997) and ASTM D3895 (2003). Schmid and Affolter [82] have reported on the results of an interlaboratory study investigating the use of DSC for the determination of OIT. 

Alia- testing, the following test methods were performed on the selected tested specimens Visual examination of the specimens was performed to identify the modes of failure. Oxidation induction time (OIT) was performed in general accordance with ISO 11357-6-2002 (E) [10] at 200 °C. Specimens wo-e taken liom the inner and outer surfaces as well as liom the bulk pipe wall and Micro-attenuated total reflection Fourier Transform Infrared Spectroscopy (micro-ATR) was performed. The inner surface and the fracture surface were examined. Scaiming Eleetron Microscopy (SEM) coupled with Energy Dispersive X-ray analysis (EDX) was performed on the iimer surface and the fiacture surface. 

IV, CCR and oxidation stability are three strictly co-related parameters. As a general rale, the reduction of IV (on the same feedstock) dramatically improves the oxidation stability. On the contrary the distillation step removes the main part of naturally occurring antioxidants. For this reason, even after hydrogenation the Rancimat induction time (as measured according to the EN 14112 standard) of the hydrogenated sample does not fulfill the EN 14214 requirement for oxidation stabihty (6 hours at 110°C), 4 hours being the measured induction period. 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

The location of the CO peak indicates the temperature regime for onset of fast oxidation for each hydrocarbon. At the reaction conditions of the flow reactor, the characteristic temperature regime for oxidation differs widely between fuels. Compared to methane, the C2 hydrocarbons are consumed at much lower temperatures at a given reaction time. This is consistent with the general observation that the C2 hydrocarbons have quite different ignition characteristics compared to methane [427]. As a consequence the presence of ethane (C2H6) and higher hydrocarbons in natural gas has a considerable influence on induction times. 

The effect of various metals on the rate of oxidation and on the induction time is shown in Table 15.3. This table suggests the following general rules ... 

Another explanation was the following. The organomontmorillonite used was a natural montmorillonite that contained iron. Chemical analysis of the clay confirmed the presence of a low amount of iron. It was recalled that iron and, in more general terms, metals are likely to induce the photochemical degradation of polymers. Iron at low concentration had a prooxidant effect that was due to the metal ion of iron that can initiate the oxidation of the polymer by the well-known redox reactions with hydroperoxides [93]. It was concluded that the transition metal ions, such as Fe, displayed a strong catalytic effect by redox catalysis of hydroperoxide decomposition, which was probably the most usual mechanism of filler accelerating effect on polymer oxidation. A characteristic of such catalytic effect was that it did not influence the steady-state oxidation rate, but it shortened the induction time. 

Though it is scientifically accepted that lipid oxidation generally proceeds slowly in the initial stages, after a certain time (widely known as induction time) the oxidation rate starts increasing exponentially due to the fact that more unstable hydroperoxides are produced and subsequently are more easily broken down. - Induction time is an important parameter for the quality of the lipid... 

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