Measured induction periods properties

The concept of a measurement of an induction period is based upon determination of critical time of the reaction progress that leads to the catastrophic change of the useful properties of the material. This may be demonstrated on the... 

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. ... 

Oil (Aa 4-38) determines oil content in a dried sample of oil-bearing material by extraction with petroleum ether. This method is specific for cottonseed, which first must be fumed with hydrochloric acid to prevent oil adsorption to the fiber. Additional methods exist for other oilseeds. Oxygen Stability Index (OSI) (Cd 12b-92) measures the oxidation induction period of fat sample (essentially the time for a sample to exhaust its antioxidant properties) under conditions of the test. 

The above outlined measurements of the solution property change to determine the induction period may be complemented by the count of the number of nuclei formed within unit solution volume to assess nucleation rates. The simplest but somewhat tedious method of nuclei count is that via a hematocytometer (Burker cell) (Nielsen and Sohnel 1971). Sampling, sample handling, and sample quenching are critical in order to obtain a reliable count. A particle counter with a well-defined optical volume (sensing zone) can also be used. A less accurate procedure is based on determination of the number of nuclei from the mean size of a known mass of precipitate. In such experiments, size can be determined either from optical or electron photomicrographs or measured by an appropriate partiele sizer. 

As the induction period can be affected profoundly by so many external influences, it cannot be regarded as a fundamental property of a system. Nor can it be relied upon to yield basic information on the process of nucleation. Nevertheless, despite its complexity and uncertain composition, the induction period has frequently been used as a measure of the nucleation event, making the simplifying assumption that it can be considered to be inversely proportional to the rate of nucleation ... 

The autoxidation of trilinolein was more complicated than that of the simple fatty esters and did not follow the same rate equation. The order of the reaction was about 0.84 compared to 1.0 for the simple fatty esters. The efficiency of the initiators used, DMVN, was increased to 100% in the trilinolein compared to about 75% in the simple fatty esters. The difference in kinetic behavior between trilinolein and methyl linoleate was attributed to the tendency of the triacylglycerol to form aggregates. The kinetic induction period approach used to measure oxidizability may be subject to errors because of the changes in efficiency of some of the artificial initiators used according to the system and the lipid substrate. Phenolic antioxidants such as a-tocopherol are also affected by the colloidal properties of the lipids used in the oxidation test system employed (Chapter 9). 

The chemical oscillations start with a short induction period that undergoes for few second to minutes. Depending upon the conditions and the nature of the reactions, specific method has been used to measure the oscillating behaviors. Typical example of a reaction system with different oscillating property has been presented in Fig. 1.4. 

In selecting stabilizers for practical application, an important factor is a knowledge of the effectiveness of compounds of various classes. Unfortunately, there are no data on series of antioxidants in the published literature as applied to concrete examples. Comparative data on the effectiveness of a limited number of antioxidants and their mixtures for polyethylene and polypropylene are cited only in [29, 41, 63, 64, 79, 80], However, even in these works the researchers limited themselves to a measurement of the induction period of the oxidation process, without investigating widely the change in properties of the polymer. 

For oxygen-uptake experiments, the time required to absorb a specified volume of oxygen is an indication of failure when the change in the rate of oxygen absorption is not sufficient to permit an accurate measurement of the induction period. However, the amount of oxygen absorbed and the loss of desired properties must be correlated. This type of test is not reliable for estimating service life under conditions in which the pol5uner is exposed to air movement. 

In the absence of light, most polymers are stable for very long periods at ambient temperatures. However, above room temperature many polymers start to degrade in an air atmosphere even without the influence of light. For example, a number of polymers show a deterioration of mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g., polyethylene, polypropylene, poly(oxy methylene), and poly(ethylene sulfide)). Measurements have shown that the oxidation at 140 °C of low-density polyethylene increases exponentially after an induction period of 2 h. It was concluded that thermal oxidation, like photooxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydroperoxide is now activated by heat. The primary reaction can be a direct reaction with oxygen (Van Krevelen and Nijenhuis 2009) ... 

The value of the induction period of the oxidation reaction (Ip) is a measure of antioxidant efficiency. Figure 10.1 shows the general trend of the effect of different antioxidants on oxidation kinetics of polyolefins. Obviously, oxidation progresses in the same manner as for unstabilized polymer when all the antioxidant has been consumed. In many cases, physical properties of the polymer change quite seriously even in the absence of oxidation during the induction period. However, it is necessary to know the effect of antioxidants on physical properties. 

The photo-oxidation of polypropylene is characterized by an induction period in which oxygen uptake (whether measured directly or by carbonyl index) occurs at a very low rate, followed by autoacceleration to a steady rate of oxidation. Measurements of mechanical properties and polymer molecular weight [2187] have shown that extensive degradation occurs in the induction period and the useful life extends little beyond this point [1443]. The surface of photo-oxidized polypropylene is highly degraded to a depth of 0.5 ym during the induction period, leading to microcracks and loss of mechanical properties [390]. 

This definition is intended to apply to the study of chemical reaction kinetics. The concept of induction time has also been used, and will continue to be used, to describe the period before a marked and relatively rapid change in a physical property occurs, but the author has suggested that the designation apparent induction time should be used for all situations other than those that fit the precise definition presented above (14). Thus, when the chemical processes that are occurring are not being specifically monitored, the adjectival modifier apparent should be used to indicate the lack of precise measure of a specific chemical reaction. 

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