Oxidative-induction time, OIT

Figure 1. Schematic representation of the method to predict lifetime ofpipes from oxidation induction time (OIT) data. Drawn after a chart presented in Viebke and Gedde, (10).

Figure 2. Oxidation induction time (OIT not the log-scale) for samples taken at a certain depth (0.9-1.0 mm from the inner wall from PB pipes pressure-tested at 90 C with internal chlorinated water (0.5, 1.0 and 1.5 ppm Cl) as a function of the exposure time. The line is a linear fit of the data. The insert figure shows the results of extrapolation to 01T= 1 min based on OIT data from exposures shorter than tfnax- From Lundback et al.(12) and with permission from Elsevier, UK.

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 high efficiency of Irganox 1010, as a long-term thermal stabilizer in polyethylene film, was evidenced by the increased oxidation induction time (OIT) (Table 3.4). 

The CL of thermally degraded PECT samples was undertaken under oxygen at 150°C, to evaluate the effectiveness of antioxidants incorporated into the copolymer (Fig. 3.12). The maximum of the initial intensity of CL emission (/maxi) and the oxidation induction time (OIT) were determined (Table 3.11). 

Figure 1.45 The oxidative induction time (OIT) values for commercially available WPC deck boards. The manufacturers and board names are numbered in the order of increasing of the OIT values.

Ground rice hulls, being added to HDPE, remarkably increases oxidative induction time (OIT) of the plastic (Table 3.23). Biodac also increases the OIT of... 

Solar radiation, effect of, 503 Stress, effect of, 502 Temperature, effect of, 503 Oxidative depolymerization, 205 Oxidative induction time (OIT), 39, 114,... 

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... 

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. 

DSC and TG can be used to determine the thermal/oxidative stability of PP, PE and their blends. The oxidation induction time (OIT) and the oxidation temperature (T j ) provide relatively, rapid information about the total amount of effective antioxidants in the reprocessed resin, which is important to establish the need for re-stabilisation or upgrade of the resins. 

Oxidative induction time (OIT) provides an index useful in comparing the relative resistance to oxidation of a variety of hydrocarbon materials. The OIT procedure was first developed in 1975 by Gilroy and coworkers at Bell Laboratory as a test procedure to screen polyethylene insulation used in telephone wire and cable for its oxidation resistance. The method first became available as a Western Electric Specification and later as ASTM Test Method for Copper-Induced Oxidative Induction Time of Polyolefins. Polyolefin manufacturers quickly embraced the procedure and began to apply it to other applications including raw resins, finished pipes, wire and cable insulation, and, most recently, geosynthetic waste pit liners (ASTM D3895 2009). 

The antioxidant performance of carvacrol and thymol in the developed active films was also studied by DSC by determining their oxidation induction time, OIT (min) (Pospisil et al., 2003 Archodoulaki et al., 2006). The OTT value is defined as the time to the onset of an exothermic oxidation peak in oxidative atmosphere and it was determined by using oxygen and air, as the results obtained can be dependent on the type of atmosphere used for the analysis. 

The time from the start of an isothermal DTA experiment to the beginning of exothermal decomposition is the so-called oxidation induction time (OIT). After this period, which depends on the antioxidant concentration, effectiveness, and temperature used, autocatalytic oxidation produces an exothermal peak [99-102]. The time from the start of the test to the maximum of this peak is the so-called oxidation maximum time (OMT) [103], which means the complete consumption of antioxidants and the loss of thermal stability of polymer. At elevated test temperatures, corresponding to short reaction times, it was difficult, or even impossible, to determine the OIT in the usual manner. For... 

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... 

In the past, oxygen-uptake measurements at elevated temperatures were used to determine oxidation resistance. Today, other types of testing, such as oxidative induction time (OIT), by differential scanning calorimetry or chemiluminescence, are used. 

Another relatively recent development in thermal technique for polymers is equipment for reliable measurement of chemiluminescence. The luminescence is thought to be from the drop of an excited carbonyl group back to the neutral state after excitation from the termination of two peroxy radicals. Its use is primarily in determination of oxidation stability of polymers to determine an oxidative induction time (OIT) (194,195). It has also been used to monitor oxidation of polymers subjected to stress (196) and simulated weathering (197). 

Fortunately, the large amount of impurity in the broader melting lot of hindered phenol does not have a deleterious effect on the additive s oxidation induction time (OIT), as compared to the OIT of the higher-purity antioxidant. 

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... 

Oxidative degradation is a process easily detected by DSC. Many industrial test specifications exist such as the DSC based Underwriters Laboratory test [118]. Oxidative induction time (OIT) is defined as the time to the onset of oxidation of a test specimen, exposed to an oxidising gas at an elevated isothermal test temperature. Bair [119] has described the details of the technique, using DSC, DTA, and TG. A sample is brought to the preselected isothermal (preferably in a N2 stream), the atmosphere is changed to O2 at the same flow-rate (zero time of the experiment), and the delay before the oxidation starts (detected as an abrupt departure from the baseline) then serves as an indication of the relative ox-idisability of the polymer (Fig. 2.2). It is recommended that OIT experimental conditions are selected so that OIT values are between 15 and 100 min. In dynamic DSC scans the onset temperature of the exotherm transition (Tonset) is obtained. Dynamic OIT (temperature) or OOT (oxidation onset temperature) is quicker. 

Simultaneous DSC-ICL experiments (with an astronomy CCD image) for oxidative induction time (OIT) studies for PP and PVC samples have been reported [606]. Close correlations between DSC-OIT and ICL-OIT data were observed. Dudler et al. [604] have studied the relative stabiliser effectiveness of some phenolic AOs (Irganox 1010/1076/1330) by ICL and oven ageing. OIT at 150°C, measured on thin films of PP in pure oxygen, scales with the AO concentration in PP films and correlates linearly with the embrittlement time observed in the universal oven ageing test. 

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 oxidative induction time (OIT) is measured by first heating the polymer sample while keeping it in a nitrogen atmosphere to a high temperature, typically 200°C for polyethylene. After the establishment of constant temperature, the atmosphere is switched to oxygen and the time (OIT) to the start of an exothermic (oxidation) process is measured (Fig. 10.23). It has been shown that the OIT exhibits an Arrhenius temperature dependence and that there is a linear relationship between OIT and the content of efficient antioxidant. This follows from the fact that... 

---