Arrhenius plots oxidative induction time

Figure 3. Arrhenius plot of the oxidation induction time for PE in oxygen excess (Po2 2 10 Pa). Points Experimental data. Continuous line kinetic model based on equations 5—9.

It is believed that copper or copper oxide is a catalyst for the oxidation of polyalkenes. For this reason, polymers to be used for coating copper wire should be run in copper or oxidized copper sample pans. Coating for aluminum wire should be run in aluminum pans. From an Arrhenius plot of the logarithm of oxidative induction time against reciprocal temperature, the oxidation energies were obtained as 110.9 kj mol" for an aluminum pan and 37.2 kJ mol" for a copper pan [4-6]. 

Unfortunately, any attempt to predict the long-term stability of polyethylene based on an Arrhenius plot of high-temperature oxidative induction times measured above the melting point fails when projected to lower temperatures where polyethylene is a semicrystalline solid (Bair 1973 Chan et al. 1978). The reasons for this nonlinear behavior appear to be associated with complex chemical and physical interactions that behave differently in the solid state than in the melt. 

A plot of oven resistance times compared to oxidative induction time (OIT) in an Arrhenius diagram for PE-HD [86]... 

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). 

Figure 7.30 Induction time of oxidation determined by (a) DSC and (b) Arrhenius plot for polyethylene [32]...

The DSC curve shows an exothermic peak due to absorption of oxygen after a certain induction time. The induction time obviously decreases for the Cu crucible, showing distinct catalytic oxidation. From Arrhenius plots constructed from data for induction times at different temperatures. The activation energies of oxidation for polyethylene were obtained as 110.9 kj mol for the A1 crucible and 37.2 kJ mol" for the Cu crucible. 

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