Degradation induction period

After the induction period, when DPs are mineralized, total organic compound (TOC) mineralization is the sum of the degradation of different compounds. Under these conditions, the evolution of the reaction can be described as apparent zero-order kinetics ... 

One particular phenomenon, often shown with oxidising reactions in polymers, is an induction period for ageing, whereby relatively little change is seen during this period but then the rate of degradation increases abruptly. It is easy to see how this can happen in materials protected by antioxidants in that with time the protective additives will be consumed. 

It is important not to assume that a lack of change or a very slow change will continue forever. One should not ignore the possibility that an induction period exists, particularly when durability is being assessed on the basis of degradation after a single time period (single-point data). 

Oxidising reactions in polymers often have an induction period for ageing (see Section 4.12.1) during which relatively little change is seen, but after which the rate of degradation increases abruptly. In materials protected by antioxidants the induction time ends when the antioxidants have been consumed and oxidation begins in earnest. 

The behaviour of chain-length distribution is non-characteristic during thermal-oxidative degradation of isotropic PP films the degradation shifts towards decrease in molecular mass, while during oxidation in deformed films it shifts towards increase in molecular mass. It means that with oxidation in isotropic samples in the induction period the destruction of molecules prevails. On the other hand, with oxidation in deformed samples attachement and cross-linking dominate. 

Since we still observed increased rates in the absence of inhibitor and in the presence of ultrasound (Fig. 5.35) we have explained the reduction or lack of an induction period in the presence of ultrasound in terms of two factors namely, a greater radical production both from enhanced initiator breakdown [74], and/or degradation of the polymer, and also from the creation of a far more stable emulsion. This latter point was confirmed visually. Depending upon the irradiation power and surfactant level employed, we were able to observe up to 40 % increase in initiator breakdown and a de-... 

Transition metals (iron, copper, nickel and cobalt) catalyse oxidation by shortening the induction period, and by promoting free radical formation [60]. Hong et al. [61] reported on the oxidation of a substimted a-hydroxyamine in an intravenous formulation. The kinetic investigations showed that the molecule underwent a one-electron transfer oxidative mechanism, which was catalysed by transition metals. This yielded two oxidative degradants 4-hydroxybenzalde-hyde and 4-hydroxy-4-phenylpiperidine. It has been previously shown that a-hydroxyamines are good metal ion chelators [62], and that this can induce oxidative attack on the a-hydroxy functionality. 

Absorption of polar degradation products is noticed immediately. Whereas the AOM method determines the time for the triglyceride to reach a specific level of oxidation (100 meq peroxide value), OSI determines the induction period (time required to exhaust the antioxidant properties), but not oxidation progress in the oil. OSI values always are lower (less time) than AOM values. 

During the start-up of an alkylation process that employs fresh sulfuric acid as the catalyst, an apparent induction period occurs (13). During this period, considerable acid-soluble hydrocarbons are produced, but relatively little alkylate is produced. A somewhat similar induction period also sometimes occurs, when TMP s are degraded in the presence of sulfuric acid... 

According to Waller and coworkers, the loss of strength on heating the yarn in the presence of oxygen and 12 % of moisture is not uniform with time, and appears to go through three distinct phases. These are an induction period, an interval of rapid degradation, and, finally, a phase of rather slow deterioration. The existence of an induction period which increases as the temperature is decreased indicates that the reaction products accelerate the degradation process. The effect of the decomposition products, accumulated in... 

Steps (1) and (4) are slow, so thermal oxidation is usually associated with an induction period. Additionally, thermal oxidation with O2 can be slowed with the use of an antioxidant, which competes with R H for hydro peroxy radicals. When atomic oxygen is present, however, oxidation begins immediately, without evidence for an induction period, and the erosion rate is unaffected by the addition of antioxidants. Therefore, the initiation of degradation was proposed to involve reaction with atomic oxygen ... 

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