Thermal Analysis Oxidation Induction Time

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. 

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. 

Standard Test Method for Oxidative Induction Time of Polyolefins by Thermal Analysis Standard Method for Assessing the Thermal Stability of Chemicals by Methods of Differential Thermal Analysis... 

Most thermal analysis methods for studying polymeric stabilizer systems are based on the antioxidant s ability to delay the oxidation process. Usually a sample is heated to a specified temperature and the induction time, or period of time before the onset of rapid thermal oxidation, is determined [see discussion of oxidative induction time (OIT) in Section 3.4.2 of this chapter]. The end of the induction period is marked by an abrupt increase in the sample s temperature, evolved heat, or mass and can be detected by DTA, DSC or TGA, respectively (Bair 1997). The effect of antioxidant structure and its concentration on prolonging a sample s induction period can be used to determine the most effective antioxidant system for a polymer such as polyethylene. Extensive data have shown that thermal information such as this can be used successfully to estimate the lifetime of polyethylene at processing temperatures (Bair 1997). 

Thermal analysis is another technique which can be used to determine the antioxidant concentration in a polymer sample. The measurement of the oxidative induction time (OIT) of a sample determined with a differential scanning calorimeter (DSC) is a popular method. The OIT of a plastic material is determined by the thermo-analytical measurement of the time interval to the onset of exothermic oxidation of the sample at the specified temperature in an aerobic atmosphere (either air or... 

Modem methods of thermal analysis can significantly reduce the time of polymer tests, and informative results, their accuracy and capability to forecast the coating lifetime are significantly improved and expanded [5-7]. In particular, the determination of oxidation induction time (OIT) and the oxidation onset temperature (OOT) by differential scanning calorimetry (DSC) is effective for the accelerated study of thermal-oxidation polymers stability. 

The method oxidation induction time (OIT) - ISO 113557-6 of the test specimens/samples was determined by differential scanning calorimetry method (DSC) with Pyris 6 DSK thermal analysis instrument from PerkinElmer, USA, at 210°C and oxygen flow of 20 ml/min. 

Using isothermal differential thermal analysis for the characterization of the long-term oxidation of poly(butene) and crosslinked poly(ethylene), it has been pointed out that straight line extrapolations from short-term experiments at elevated temperatures to low temperatures and long times are not possible (3). Also, oxidation induction times introduce another difficulty for interpretation of the experiments. In contrast, for isotactic poly(propylene) the application of the relationship of Arrhenius does not make problems (4). 

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),... 

The main reason for using induction time data for the determination of antioxidant concentration in polymers is the frequently observed linear relationship between induction time and antioxidant concentration [131]. In view of the aforementioned considerations great caution should be exercised in quantitative estimation of antioxidant levels in polymers. Wight [181] and others [143] have used quantitative differential thermal analysis (QDTA), in particular for determining the degree of oxidative stability of polyolefins for QC purposes in the wire and cable industry in lieu of a direct antioxidant analysis. Application of the basic purpose of a QC test... 

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