Oxidation induction time measurements

S.M. Marcus and R.L. Baine, Estimation of bias in the oxidative induction time measurement by pressure DSC, Application note TA-228, TA Instruments, Inc., New Castle, DE, USA. 

A series of high pressure oxidative induction time measurements (HPOIT) were conducted on a PE geomembrane sheet in order to investigate the interaction of the pressure and temperature variables on the induction time. The experiments consisted of determining the EffOIT at constant cell volume employing a wide operational range of pressure and temperature values. The HPOIT test results were found to be inversely related to both variables, with temperature being the predominant factor (Tikuisis et al. 1985). 

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. 

Table 9.3 ASTM international oxidative induction time measurements...

Thermal analysis measurements were made using a duFont 990 Thermal Analyzer using aluminum pans and a heating rate of 10 C/ min. Melting endotherms were determined In the presence of helium. Oxidative Induction times were measured Isothermally at 200 C In the presence of oxygen. A detailed description of this technique Is given in reference 4. 

The oxidation induction time can also be used as an indication of oxidation stability. No correlation has been determined between the results of this test and service performance. In the test (ASTM D-5483), a small quantity of grease in a sample pan is placed in a test cell. The cell is heated to a specified temperature and then pressurized with oxygen and is held at a regulated temperature and pressure until an exothermic reaction occurs. The extrapolated onset time is measured and reported as the oxidation... 

In reactive processing, such as reactive extrusion of thermoplastics, the use of DSC may be seen in determining the stability of the polymer formulation by measuring the oxidation induction time (OIT) (Bair, 1997). This is an isothermal experiment in the presence of air... 

The isothermal measurement under oxygen showed good correlations between chemiluminescence parameters and the concentration of stabilizers. The oxidation induction times have been seen to increase with the ratio of Irgafos 168/Irganox 1330 (Fig. 3.10). In general, the OITs increased with the transition temperature determined... 

J.E. Volponi, L.H.I. Mei, and D. dos Santos Rosa, The use of differential photocalorimetry to measure the oxidation induction time of isotatic polypropylene, Polym. Testing 2004, 23, 461 165. 

Until recently, the effects of these factors and their qnantitative manifestation were practically nnknown and not even recognized, neither in the WPG indnstry nor in academic research in the area. That is why the acnte deterioration and crumbling of some WPG boards tnrned ont to be qnite nnexpected and puzzling, and resulted in some cases in an avalanche of warranty claims. These cases will be considered in detail in chapter 15. It tnrned ont that the progressive deterioration and crumbling have resnlted from WPG oxidation, and as soon as it was recognized, measures were taken. The OIT (oxidative induction time) parameter was introduced into characterization of WPG products and evaluation of their lifetime in the real world, on real decks. 

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 ... 

The oxidative stability of polyethylene has been reviewed by Collins and Wendlandt (197), Charsley and Dunn (198,199), and others (200). One of the most common measurements of the oxidative stability of polyolefin resins is the oxidative induction time (OIT) utilizing DSC. There are several versions... 

Duske [7] also discussed several other techniques of value for this material. DSC can be used to check the oxidation resistance of the wire insulation by determination of the oxidation induction time (OFT) — an isothermal technique to determine when the oxidation will occur. Thermogravimetry (TG) has also been used to examine the decomposition of the insulation and the effectiveness of flame retardants. TMA is also useful for measuring the diermal expansion of the wire insulation and this technique is also applicable to multi-layered coatings. 

Finally, the researchers considered the PO s contact with copper wire over a cable s 50-year lifetime, and how this contact can contribute to "metal catalyzed thermal failures " Although the real-world tendency of a cable to oxidize over the long term is difflcult to measure in testing, these research ers evaluations showed that phenolic AOs, combined with a metal deactivator or chelator (see Table 3.1, for example), are effective in providing long oxidative Induction times at high temperatures. 

Oxidation induction time (OIT) measurements are less commonly performed, but they do provide some information about the oxidative stability of the UHMWPE. In this test, the DSC is heated to 200°C, then it is held isothermally. The purge gas, which is normally nitrogen, is then switched to pure oxygen. The time to the start of the degradation exotherm caused by oxidation is reported as the OIT. Open sample pans are used in this test technique. 

Fig. 3.8. OIT values of an HDPE geomembrane in the state of delivery (specimen 48), measured at different Off measiuement temperatures Tm after oven ageing in air at 80 °C over 6 years (specimen 48a) and over 8 years (specimen 48b) (Mueller and Jakob 2003) and after an exposure to a mixture of liquid hydrocarbons over 2977 days at room terrrperatme (specimen 48c) (Kalbe et al. 2000). The data show an approximately linear relatiortship between the logarithm of oxidative induction time and the inverse absolute measuring temperatrrre (1/7m)...

Schwartz and co-workers [97] used isothermal differential thermal analysis to study the diffusion of Irganox 1330 (1,3,5 tris (3,5 di-tor -butyl-4-hydroxyl benzyl) mesitylene) in extruded sheets of isotactic polypropylene (iPP). Studies were conducted over the temperature range 80-120 °C. The measurements showed a clear relation between oxidation induction time and oxidation maximum time [both determined by isothermal dynamic thermal analysis (DTA)] and the concentration of stabiliser. It was possible to calculate the diffusion coefficients and the activation energy of diffusion of Irganox 1330 in iPP by measuring the oxidation maximum times across stacks of iPP sheets. 

The oxidative induction time (OIT) is determined from data recorded during the isothermal test. The elapsed time between time zero and the extrapolated onset of oxidation is recorded as a measure of oxidative stability. A dynamic heating method is also used. In this method, the sample is heated at a rate of 10 °C min from ambient temperature in an air or oxygen atmosphere. The temperature at the onset of oxidation is taken as a measure of oxidative stability. When... 

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