Oxidation induction time temperature dependence

As a resnlt, incoming plastics, as pellets, powder, or flakes, to a composite-making plant show variable life span, depending on the manufacturer of the plastic (see Table 15.2). This life span is often measnred in the OIT values, or the oxidative induction time. This will be explained in detail below in this chapter, and essentially the OIT is a lifetime of the material in a chamber filled with pure oxygen and heated to a certain temperatnre, typically to 180-200°C. At this temperature the material is oxidized by about 1-10 million times faster than in ambient conditions. 

Apparently, this can explain an unusually steep temperature dependence of the OIT of polyethylene degradation, described in Application News T95 of Shimadzu DSC, entitled Measurement of Oxidizing Induction Time of PE by DSC. The authors have obtained the following data for the oxidation of polyethylene in the presence of an antioxidant ... 

Lugao and his group had introduced a new procedure to determine OIT in non-stabilized, stabilized, irradiated, and nonirradiated PP. The new procedure was based on two main features (1) starting the oxidation on melted samples at temperatures as low as possible and (2) oxidation under slow heating conditions. Since each sample has a set of two values of time and temperature, it is called as temperature-dependent oxidative induction time. This new method is found to be reproducible, sensitive (to small changes in additive compositions), simple, and inexpensive (Lugao et al. 2002). 

The time from the start of an isothermal DTA experiment to the beginning of exothermal decomposition is the so-called oxidation induction time (OIT). After this period, which depends on the antioxidant concentration, effectiveness, and temperature used, autocatalytic oxidation produces an exothermal peak [99-102]. The time from the start of the test to the maximum of this peak is the so-called oxidation maximum time (OMT) [103], which means the complete consumption of antioxidants and the loss of thermal stability of polymer. At elevated test temperatures, corresponding to short reaction times, it was difficult, or even impossible, to determine the OIT in the usual manner. For... 

OIT is a widely used screening parameter for the oxidative stability of polymers, edible oils, and lubricants, which is typically used as a quality control tool to rank the effectiveness of various oxidation inhibitors. It is a kinetic parameter (i.e. dependent on both time and temperature) and not a thermodynamic property. As a parameter dependent on test time and temperature, the OIT value appears to be decreasing with time but in a well-behaved and predictable manner. OIT is either a measure of the amount of antioxidant present in the polymer or the effectiveness of the particular AO used. If the amount of AO in the polymer is known, then OITime or lOTemperature allow monitoring residual AO contents and calculation of the linear rate of AO consumption. A major limitation of DSC-OIT is that if the isothermal test temperature is lowered below the standard 200° C temperature to reveal small differences in AO concentration at low levels, the polymer s exothermic oxidation rate may decrease below the limits of DSC detectability. Lugao et al. [120] have recently introduced a temperature dependent oxidative induction time (TOIT) in order to cope with some limitations of the traditional OIT method. 

The oxidative induction time (OIT) is measured by first heating the polymer sample while keeping it in a nitrogen atmosphere to a high temperature, typically 200°C for polyethylene. After the establishment of constant temperature, the atmosphere is switched to oxygen and the time (OIT) to the start of an exothermic (oxidation) process is measured (Fig. 10.23). It has been shown that the OIT exhibits an Arrhenius temperature dependence and that there is a linear relationship between OIT and the content of efficient antioxidant. This follows from the fact that... 

Radical trapping. To allow for stabilizaton by this mechanism, another reaction (number 49) was included to allow easy abstraction of a hydrogen atom from an additive (QH) by a peroxy radical to form a hydroperoxide and a harmless adduct. With the same value of the rate constant as for energy transfer and for concentrations as low as 10 M, the photooxidation process was efficiently slowed. Figure 9 shows the linear dependence of the time to failure (5% oxidation) as the concentration of QH is altered. Note that the trap is consumed in the process and the apparent induction time is associated with its removal. The stabilization is less effective for higher intensity (and probably higher temperature) because the faster photo (or thermal) decomposition of ROOH continues the degradation process. 

These results may indicate the kinetic dependence of induction time wherein the time is lowered with an increase in reaction temperature. Further, it has been earlier envisaged by Germain et al. [106] that addition of catechol or hydroquinone removes the induction period for faujasitc KAU 2,5 (Si/AI = 2.5) and this was attributed to initial oxidation of the additive in generating an autocatalytic quinonic redox couple. However, when we added catechol (20 1 mole ratio of phcnohcatechol) to the initial reaction mixture (in an anticipation to sec whether it has any influence on the induction time), no reduction in the induction time was... 

The lack of a stable complex explains why the room temperature rate is much smaller than that of the isoelectronic reaction of CH2+O2 to form the CH3OO complex (1 ), which is stable by 26 kcal-mole". The endothermicity of 30 kcal-mole for reaction (5) (no barrier should exist on the exit channel) is consistent with both the temperature dependence of 37.7 2.6 kcal-mole" fit to induction times of various shock tube experiments ( ) and with the need for a large activation energy for the reaction of NH2+O2 used in various models of ammonia oxidation (1-3,5,6). The direct hydrogen abstraction reaction... 

Fig. 82 Stabilization effects of the addition of carbon black into HDPE. Temperature 180°C.Environment air. The data were taken from [03J2]. (a) CL curves CB concentration (1) free, (2) 0.075 %, (3) 0.15 %, (4) 0.25 %. (b) dependence of the induction time of oxidation on carbon black concentration.

Recently, Fujii et al. (1981) reviewed previous work on NH3 oxidation and conducted new experiments using uv absorption to determine induction times for NH3 removal in various NH3,02,H2,H20, and Ar mixtures. The experiments were conducted using reflected shock waves over a wide range of temperature and pressure (760 reaction mechanism was employed to model the ammonia induction times and to argue that reaction path (3) was consistent with their observations. Values of were inferred for the temperature range 1550-1800 K, but some dependence on pressure, attributed to the formation of NH2O2 as an intermediate complex, was observed. At low pressures, of the order of 1 atm, the reaction appeared to proceed directly with the apparent elementary reaction rate constant... 

S02 is a very stable oxide and its thermal decomposition is only measurable at the very high temperatures attained in a shock tube. A study357 of the time-dependence of light emission from shock-heated S02/Ar mixtures in the region of 3000 °K has shown that S02 is removed in accordance with a sigmoid-shaped concentration-time curve typical of a chain or autoaccelerated reaction. The induction period observed357 prior to the onset of detectable decomposition corresponded closely with the time for the formation of a fixed concentration of O (or SO) calculated from the rate expression (Table 24) for the unimolecular decomposition... 

Figure 5 shows the variation of time to failure (5% oxidation) with temperature. The decrease in lifetime with no stabilizer is more or less as expected, ranging from a few months in hot tropical weather, 310K (100°F), to almost two years in cool weather, 280K (45°F). An attempt at a typical Arrhenius plot (Figure 6) shows an "apparent net activation energy" of 10-16 kcal/mol near atmospheric temperatures (280-310K). Experimental values of 16-35 kcal/mol for the dependence of the induction period in polyethylene oxidation have been reported by Wilson (29) and Blum et al. (30) at temperatures above 380 K. For thick films the observed value is as low as 10 kcal/mol (31). 

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