Determination of Oxidation Stability by Pressure-Dependent OIT Measurements
Introduction
The determination of the oxidation induction time (OIT) is a method frequently used to estimate the oxidation stability of materials.
This is done by first heating the sample to a sufficiently high temperature under inert conditions (purging the DSC cell with nitrogen). After a brief thermal equilibration time, the purge gas is switched from nitrogen to oxygen. If the temperature has been chosen correctly there is no immediate change in the DSC signal.
However, after a certain induction time, the exothermic oxidation reaction begins. The OIT is the time at which the onset of the oxidation reaction occurs measured from the point when the gas is switched from nitrogen to oxygen. The OIT depends very much on the current chemical state of a material. For example, chemical aging causes a significant decrease in the measured OIT.
An example of this is shown in Figure 1. This shows OIT measurements performed at 210 °C on polyethylene (PE) tubing that had been used to transport aqueous solutions of chemicals. Two samples, one from the outside and the other from the inside were measured.
The inside of the tubing had of course been in direct contact with the solutions. The OIT of the material from the outside of the tubing is significantly longer than that from the inside. The OIT therefore provides information on the extent of damage of materials.
Experimental Details
The measurements described in this study on the pressure dependence of OIT were performed using the new high pressure DSC (HP DSC827e ). The oxygen pressure and gas flow were set using a pressure and flow regulator. The gas flow was 40 mL/min and the oxygen pressure set to different values in the range 0.1 MPa to 10 MPa (1 to 100 bar). Since gas switching in pressure measurements is not always so easy, the oxygen pressure was set at room temperature and the sample heated rapidly at 50 K/min to the reaction temperature.
The chemiluminescence measurements were performed in an HP DSC827e using a CCD camera and the relevant accessories (see UserCom 20).
Pressure Dependence of OIT
Experimental Results
If the OIT is measured at constant reaction temperature but at different oxygen pressures, one sees that the OIT decreases with increasing pressure. This is illustrated in Figure 2 using mineral oil as an example. At an oxygen pressure of 0.5 MPa, the OIT is more than 120 min. In contrast, at 6.8 MPa the OIT is less than 40 min. Besides shorter measurement times, the measurement reproducibility also improves at higher pressure.
Figure 3a shows the pressure dependence of the OIT of two different mineral oils, A and B at a reaction temperature of 195 °C. The OIT of both oils decreases with increasing pressure, whereby the OIT values of oil B are always greater than those of oil A. In Figure 3b, the values have been plotted in a double logarithmic presentation in order to analyze the relationship between OIT and oxygen pressure more easily. In this presentation, the measured points lie on straight lines. It follows that the relationship between OIT (tOIT) and the oxygen pressure (p) can be described by a power law:
torr = A p -q (1)
Taking logarithms of both sides of the equation shows the linear relationship in the double logarithmic presentation:
log torr = log A - q log p (2)
The factor A is a measure of the rate of the oxidation reaction and the exponent q is the slope. The q values obtained for the two oils are 0.62 (oil B) and 0.45 (oil A).
The general validity of the potential law and the meaning of the exponent q must first be considered before the results can be assessed in more depth. This was done by performing OIT measurements on a number of different polymers. Some of the results obtained are shown in Figure 4.
Different reaction temperatures were chosen for each polymer so that the OIT values were all in a convenient time frame (between 20 and 300 min). This facilitated the analysis of the pressure dependence of OIT for the materials tested. The diagram shows that in double logarithmic presentation the relationship is linear for all the polymers investigated. The exponent q here lies between 0.6 (linear low-density polyethylene, LLDPE), and 0.3 (polypropylene, PP).
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Conclusions
The pressure dependence of OIT obeys a power law. The exponent q is mainly influenced through oxygen diffusion processes and radical formation in the material under test. Materials with small exponents tend to oxidize more readily than those with large exponents. The power law dependence of OIT on pressure presupposes a nucleation process before the reaction. This means that the oxidation does not begin throughout the entire sample, but just in a few isolated regions in the sample where conditions are favorable for diffusion (e.g. microfissures) or where there is a greater concentration of radicals. This nucleation mechanism in the oxidation reaction can be measured by chemiluminescence.
The pressure dependence of OIT provides additional information on the stability, previous damage, and inherent tendency for oxidation of materials.
It is important to remember when performing OIT measurements that the geometry of samples plays an important role because of its influence on diffusion processes. For this reason, it is best to use very thin samples with defined surface areas.
The author would like to thank his colleagues Dr. R. Riesen, M. Zappa and Dr. M. Schubnell for valuable help and discussions.
Determination of Oxidation Stability by Pressure-Dependent OIT Measurements | Thermal Analysis Application No. UC 265 | Application published in METTLER TOLEDO Thermal Analysis UserCom 26