Oxidation stability induction times

It is therefore desirable to have a method for the evaluation of chemiluminescence results which will provide information on induction time, oxidation rate, and extent of oxidation, relevant to the thermal oxidative stability of materials. It is also advantageous to have an instrument which would be able to analyze numerous polymer samples simultaneously. 

Because the chemiluminescence method is a tool frequently used to qualify the stabilizing effect in polypropylene of various hindered amine, the analysis of CL kinetic parameters (oxidation induction time, oxidation rate, activation energy of oxidation, CL intensity) could provide reliable information related to the photodegradation state of polypropylene modified with different HALS compounds [76-78]. 

Oxidation stability (gasoline) (induction period) NFM 07-012 ISO/DlS 7536 ASTM D 525 Time necessary for a sample bomb under oxygen pressure to reach the critical induction point... 

Thermoanalytical techniques such as differential scanning calorimetry (DSC) and thermogravi-metric analysis (TGA) have also been widely used to study rubber oxidation [24—27]. The oxidative stability of mbbers and the effectiveness of various antioxidants can be evaluated with DSC based on the heat change (oxidation exotherm) during oxidation, the activation energy of oxidation, the isothermal induction time, the onset temperamre of oxidation, and the oxidation peak temperature. 

IV, CCR and oxidation stability are three strictly co-related parameters. As a general rale, the reduction of IV (on the same feedstock) dramatically improves the oxidation stability. On the contrary the distillation step removes the main part of naturally occurring antioxidants. For this reason, even after hydrogenation the Rancimat induction time (as measured according to the EN 14112 standard) of the hydrogenated sample does not fulfill the EN 14214 requirement for oxidation stabihty (6 hours at 110°C), 4 hours being the measured induction period. 

DSC can yield both qualitative and quantitative information, and is a relatively fast technique, with typical heating rates being about 10°Cmin 1. It is routinely used to determine oxidation-induction time, see Section 2.1, which can be a useful parameter in assessing the thermal stability of a series of materials [103]. 

On well characterised non-stabilized PP samples [48] having molar mass within 45-180 kg/mol with differing tacticity and crystallinity, we can see that the increasing molar mass leads to an increase of induction time and reduction of the maximum chemiluminescence intensity (Figure 14). The polymer with higher average molar mass appears to be more stable than that with lower molar mass. This may be ascribed to the effect of increased concentration of more reactive terminal groups, which promote initiation of thermal oxidation. 

The observation of an unusual effect occurring as a consequence of preannealing on the induction time of oxidation of PP stabilized with Irganox 1010 (Figures 19 and 20) needs to be taken into account when searching for correlations between induction periods from chemiluminescence and oxygen uptake for oxidation of PP stabilized with different types of antioxidants. 

Figure 20 An increase of induction time of oxidation of polypropylene stabilized by Irganox 1010 (points 1) due to prior sample annealing at 130°C (points 2) in oxygen. The induction time corresponds to the time of cross-section of the straight line passing the CL inflexion point and time axis. It was determined for an oxygen atmosphere and temperature 150°C.

A widespread method for determining the induction period for autoxidation of oils and fats consists of passing a continuous stream of air through the heated sample and collecting the volatile acids evolved in a water trap, where they are determined on a real time basis. The time plot usually presents a flat appearance for a certain period and then takes off in an accelerated manner. This test is the basis of several national and international standards (e.g. AOCS Cd 12b-92—oil stability index" ISO 6886—accelerated oxidation test for oxidative stability of fats and oils ) and the design of the Rancimat equipment, where the end determination is based on conductivity measurements . In addition to oxidation stability as determined by the Rancimat method and POV, which negatively affects virgin olive oil stability, other nonstandard properties were proposed for better assessment of the quality of this oil, namely LC determination of Vitamin E (21), colorimetric determination of total polar phenols and UVD of total chlorophyll. ... 

Chemiluminescence of oxidized luminol has been the basis of several lumino-metric methods of estimation of TAC (Table 1). The mostcommon is to measure the induction time of the reaction. Often the chemiluminescence is first induced by an oxidant and then attenuated by addition of a sample, and the time to recover the initial fluorescence is measured. The enhanced chemiluminescent assay introduced a decade ago is based on the oxidation of luminol by perborate or by hydrogen peroxide in a reaction catalyzed by horseradish peroxidase. Enhancement (and stabilization) of luminescence is achieved by addition of p-iodophenol. The original procedure used a commercial reagent kit (ECL Anti-oxidant Detection Pack... 

Stavinoha and Kline (2001) adapted ASTM method D 6186 (Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry [P-DSC]) for analyzing the oxidative stability of SME treated with antioxidants. This report concluded that isothermal P-DSC analysis is suitable for screening the effectiveness of antioxidants for treating biodiesel. 

Values denoting oxidative deterioration increase gradually during the shelf-life of an oil. Determinations of induction times in rapid-aging tests also reflect the oxidative stability of commercial refined oils. Grompone et al. (84) studied the stability of commercial regular sunflower oils. The oil samples were stored at ambient conditions in their original PET bottles for a one-year period. The OSI induction time... 

Oxidative stability index (hrs) Time (in hours induction period) for fats/oils to begin to undergo rapid acceleration in oxidation Cd 12b-92... 

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... 

Radical trapping. To allow for stabilizaton by this mechanism, another reaction (number 49) was included to allow easy abstraction of a hydrogen atom from an additive (QH) by a peroxy radical to form a hydroperoxide and a harmless adduct. With the same value of the rate constant as for energy transfer and for concentrations as low as 10 M, the photooxidation process was efficiently slowed. Figure 9 shows the linear dependence of the time to failure (5% oxidation) as the concentration of QH is altered. Note that the trap is consumed in the process and the apparent induction time is associated with its removal. The stabilization is less effective for higher intensity (and probably higher temperature) because the faster photo (or thermal) decomposition of ROOH continues the degradation process. 

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