Antioxidant depletion time

In this respect 6,2 is called antioxidant depletion time and E the apparent activation energy of antioxidant depletion. Again values for E in the vicinity of 50kJ/mol are experimentally found. As discussed in Sect. 5.1, the Arrhenius equation leads to the old mle of thumb that an increase in temperature by about 10 °C doubles or trebles the depletion rate. This rule can also be used as an initial guide to estimate service life. If, after ageing of geomembrane samples in water or air at 80 °C test temperature over a period of at least one year, effective stabiliser substance can still be found, for example by OIT-measurements, a service lifetime of many decades can be expected at normal ambient temperatures. 

However, none of these experiments were pursued to the end of the depletion process and oxidative degradation was not actually achieved, since this requires very long test times even at high temperatures. Therefore, the depletion rates have to be considered with caution because they have been estimated from short-term experiments, and the assumption that the antioxidant depletion time is the most relevant part of the service lifetime, needs experimental justification. 

The oxidative induction time is used to determine the quantity and types of antioxidants and to evaluate antioxidant depletion times. 

Fig. 2 presents the analysis based on OIT data and the linear extrapolation of these data to longer times. The time to reach depletion of the antioxidant system can thus be predicted even after relatively short testing times (see insert figure in Fig. 2). Data by Hassinen et al. (//) for the antioxidant concentration profiles taken from high-density polyethylene pipes exposed to chlorinated water (3 ppm chlorine) at different temperatures between 25 and 105°C followed the Arrhenius equation with an activation energy of 85 kJ mol (0-0.1 mm beneath inner wall surface) and 80 kJ mol (0.35-0.45 mm beneath the inner wall surface). It is thus possible to make predictions about the time for antioxidant depletion at service temperatures (20-40°C) by extrapolation of high temperature data. However, there is currently not a sufficient set of data to reveal the kinetics of polymer degradation and crack growth that would allow reliable extrapolation to room temperature.

Under normal conditions, oxidative reactions progress extremely slowly. Correspondingly, the associated oxidative stabiliser consumption is also slow. However, not only oxidation, but other chemical degradation processes can also destroy antioxidants. For example, phosphates and other antioxidants are hydrolysis-sensitive (Gugumus 1990). In addition to such chemical antioxidant depletion , there are physical depletion processes The concentration of the added antioxidants is reduced by extraction and migration processes while the plastic is stored and used (Pfahler and Lotzsch 1988). Such processes are usually the main cause of gradual loss of stabilisers at application temperatures. All these depletion processes determine induction time t 2 and thus service lifetime of the plastic. They... 

The velocity of extraction or migration processes and consumption by oxidation or other chemical degradation processes or the antioxidant depletion rate can be approximately assumed to be proportional to the available amount of stabiliser. The antioxidant content [ 4] as a function of time can then be described by an exponential function, already introduced in Eq. 5.1 ... 

We call kefflliQ antioxidant depletion rate and its reciprocal %the depletion time constant. Both are characteristic parameters of the depletion process. Constant hff as a function of temperature T is given by the Arrhenius law Eq. 5.3. 

What conclusion about service lifetimes can be drawn from these longterm test results For our discussion it is crucial that the change in OIT value provides at least a rough estimation for the depletion time t2. This assumption was established for the antioxidant package commonly used in HOPE geomembranes and it explains the oxidation behaviour in the OIT measurement itself as described in Sect. 3.2.7. In the following we will show that a consistent explanation of all our experimental results is possible within this interpretation scheme. 

We thought that the resumption of oxidation after a few min was likely due to the depletion of NO since subsequent additions of NO inhihited oxidation with kinetics similar to the first addition. In order to prove this, we repeated the experiment, hut this time determined the [ NO] at periodic time intervals. We initiated the experiment with 20 pM and then added 0.9 pM NO 1 min later. At this 1 min time point, oxidation was inhihited (Figure 3). This inhibition continued until about 4.5 min, which is about when the [ NO] fell to below the limit of detection. At this time there was still sufficient Fe (7.2 pM) to reinitiate oxidation. These results demonstrate that NO is acting as a chain-breaking antioxidant during cellular lipid peroxidation. 

Utilizing a voltammetric measurement technique, the RULER quantitatively analyses the relative concentrations of antioxidants (hindered phenolic and aromatic amine) in new and used oils. This data can be trended to determine the depletion rates of the antioxidant protection package in the oil provided the instrument has been calibrated for that oil type. From pre-established limits, proper oil change cycles, potential interval extension or timely antioxidant replenishments can be determined. 

It was pointed out by Ashby (136) that in the presence of stabilizers (antioxidants), ihe OL of the polymer was changed. Using polypropylene containing a 1 1 mixture of the stabilizer, 4,4 -thiobis(6-terr-butyl-o-cresol) with dilaurylthiopropionate. he greatly reduced the OL intensity for an initial time interval and the extent of this reduction was determined by the concentration of the stabilizer. When the stabilizer concentration was depleted, the OL returned to its original intensity. This suggested that the intensity of OL is reduced as the rate of oxidation was reduced by the sta-... 

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