Oxygen induction time

As indicated in the previous sections, the antioxidant content in plastic material is often determined by chromatographic methods. Another widely used technique for polymer characterization is thermal analysis with differential scanning calorimetry (DSC). When the oxygen induction time (OIT) for a sample containing a phenoHc antioxidant is measured, a significant oxidative exothermic response is obtained in the DSC when all the phenolic antioxidant in a sample is consumed. The OIT is thus directly related to the antioxidant content in the material and to the stabihzing function, i.e. the antioxidant efficiency in the sample, if the consumption of phenolic antioxidants obeys zero-order kinetics at the temperature used [44]. Table 1 shows the amount of the antioxidant Irganox 1081 in polyethylene (PE) determined by HPLC and extraction by microwave assisted extraction (MAE),... 

Commercial WPC boards, 40, 41 10,10 -Oxybisphenoxyarsine, 449 Oxygen Induction Time, 571 Oxygen intake, 58... 

Figure 9.6 Oxidative DSC curves (corrected for evaporation) for (1) 1,4-cis polyisoprene with added BHT, and (2) 1,4-cis polyiosprene incorporating BHT capsules. These results show that encapsulation of the BHT increases the oxygen induction time (see also Table 9.1) Reproduced from Keen and co-workers. Polymer Degradation and Stability [20]...

Table 9.1 Oxygen induction times (OIT) from DSC measurements ...

G. Del Alamo, F.A. Williams, A.L. Sancez, Hydrogen-oxygen induction times above crossover temperature. Comb. Sci. Technol 176, 1599-1626 (2004)... 

Table 10.8 Comparison oE antioxidants in polyethylene in both the absence and presence of copper powder and carbon black (data based on ICI literature). Induction time assessed from oxygen uptake measurements using a Barcroft manometer...

It is noteworthy that metallic copper or cuprous bromide used under nitrogen atmosphere shows only a very short induction time. This last result points out the inhibitor role of the oxygen of the air atmosphere and most likely the important role taken either by reduced species or by radical intermediates in the catalytic cycle. 

In comparison to the normal reaction without additive which affords a yield of 50.2 % in phenol, the additives on the right part of the table reduce the induction time probably by trapping more or less efficiently the air-oxygen inhibitor. 

Comparison of chemiluminescence isothermal runs with oxygen uptake and DSC measurements has been at the centre of interest since practical industrial applications of the chemiluminescence method were attempted. It is a fact that the best comparison may be achieved when studying polymers that give a distinct induction time of oxidation typical for autoaccelerating curves of a stepwise developing oxidation. This is the particular case of polyolefins, polydienes and polyamides. The theoretical justification for the search of a mutual relationship between the oxidation runs found by the various methods follows directly from the kinetic analysis of the Bolland-Gee scheme of polymer oxidation. 

The observation of an unusual effect occurring as a consequence of preannealing on the induction time of oxidation of PP stabilized with Irganox 1010 (Figures 19 and 20) needs to be taken into account when searching for correlations between induction periods from chemiluminescence and oxygen uptake for oxidation of PP stabilized with different types of antioxidants. 

Figure 20 An increase of induction time of oxidation of polypropylene stabilized by Irganox 1010 (points 1) due to prior sample annealing at 130°C (points 2) in oxygen. The induction time corresponds to the time of cross-section of the straight line passing the CL inflexion point and time axis. It was determined for an oxygen atmosphere and temperature 150°C.

The results of the CO He 02 experiment (22) are shown in Fig. 17. There is an induction time between the admission of O2 and start of the reaction of C0(a). Oxygen cannot react from the gas phase, and its adsorption must await the desorption of a trace of C0(a). Bonzel and Ku (J found a similar result, as shown in Fig. 18. Their induction times are much longer than those of Dwyer (22), perhaps because of the regularity of their (110) surface. 

Due to the accurate knowledge of the dose used, a control of the progressive extent of the radiolytic reduction was achieved. However, quite often, puzzling data were reported when the zero-valent metal was formed, such as an induction time for precipitation, a sensitivity of the radiolytic yields to the initial presence of added particles, and an unusually weak reproducibility [9,10]. Moreover, oxidation of silver atoms by molecular oxygen was observed [7], although the process was thermodynamically improbable for a noble metal such as silver. 

Induction Time The time period under given test conditions in which a petroleum product does not absorb oxygen at a substantial rate to form gum. It is measured as the time elapsed between the placing of a test bomb containing 50 mL of fuel and oxygen at 100 to 102 psi into a 212°F (100°C) test well and the point in the time-pressure curve preceded by two consecutive 15-minute periods of a pressure drop not less than 2 psi. 

Data on vibrational relaxation times from interferometric studies, reported by White and Moore (Ref 8), show the rapid decrease of relaxation time with rise of relaxation zone temperature. Addition of up to 1% H2 to C>2 is shown to reduce the relaxation time and accelerate the reaction, but not to affect the maximum density. At a pressure of 0.001 atm, about 0.8 tort, the relaxation times would be in milliseconds instead of microseconds. The induction times for exothermic reaction are inversely proportional to the square root of the product of the number of moles of oxygen ([O2]) and the number of moles of hydrogen (IH2I) Per Hter, over the entire C J/LC ] range, to a good approximation. 

Table III presents the reaction rate and the induction time for reaction (as defined in Figure 2) for the five coals studied. These data were taken under the following conditions 109.5°C., 1 N sodium hydroxide, and 8.5 atm. of oxygen partial pressure. The mesh size of the coals was through 60 mesh on 100 mesh. The reaction rates are based on the external area of the coal particles.

Figure 2. Determination of induction time and reaction rate concentration of humic acids proportional to 2-log (% transmission) Coal C —2-1-3 mesh, 1.16N NaOH 6.4 atm. Oi partial pressure, 109.2°C. = rate of oxygen uptake O = concentration of humic acids...

Coal % Carbon, (daf) Reaction Rate X 10 grams oxygen/hr-sq. cm. Induction Time, hours... 

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