Induction time effects

Induction time effects (autocatalysis) e.g., the development of thermal instability after prolonged storage... 

The isothermal version of this test can also be used to detect induction time effects. In this test the sample is held for long periods under conditions of minimal heat loss. This can be achieved by placing the tube in a Dewar flask, which is then placed in an oven held at a steady temperature. Sample temperature and pressure inside the tube are recorded continuously. A rise in either temperature or pressure indicates onset of decomposition. Usually it is found that ... 

In one experiment the effect of ppd assay was correlated to scorch safety. As the ppd degrades Hberate free amine, scorch time decreases and cure rate is faster. The degradation products apparentiy serve to activate the cure, since both the induction time, and cure time, decrease with decreasing ppd assay. However, the effect on unaged properties is minimal. 

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. 

To find the effect of reaction temperature and ultrasoimd for the preparation of nickel powders, hydrothermal reductions were performed at 60 °C, 70 °C and 80 °C for various times by using the conventional and ultrasonic hydrothermal reduction method. Table 1 shows that the induction time, when starts turning the solution s color to black, decreases with increasing the reaction temperature in both the method. The induction time in the ultrasonic method was relatively shorter, compared to the conventional one. It assumes that hydrothermal reduction is faster in the ultrasonic method than the conventional one due to the cavitation effect of ultrasound. 

T able 7.1 Effect of ultrasound on crystallite size, induction time and energy during carbonization... 

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. 

Table 4 The effect of antioxidants on induction time of oxidation of polypropylene determined at 150°C from chemiluminescence measurements...

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. 

Chemiluminescence is the emission of light in the 300-500 nm region due to exothermic chemical reactions. With a suitable detection system this can be used to monitor the chemical reactions that occur when materials age. This technique is sensitive enough to produce results more rapidly and at lower temperatures. Chemiluminescence has been used to investigate the effect of different additives on the oxidation of polymers as well as to determine oxidation induction times which are more meaningful than those obtained from the DSC method. 

The pressure dependence, as before, is derived not only from the perfect gas law for p, but from the density-pressure relationship in Z as well. Also, the effect of the stoichiometry of a reacting gas mixture would be in Z. But the mole fraction terms would be in the logarithm, and therefore have only a mild effect on the induction time. For hydrocarbon-air mixtures, the overall order is approximately 2, so Eq. (7.46) becomes... 

Today one of the most common chemical applications of ultrasound is the initiation of a reluctant Grignard reaction. The quantitative effects of ultrasound on the induction times for the formation of a Grignard reagent in various grades of ether is given in Tab. 3.2 [88]. 

The long-known stimulating effect of mono- and polynitro com-pounds on the onset of fermentation in yeast maceration juice has been reinvestigated by Vandendriessche. The induction time is shortened significantly by 2,4- or 2,5-dinitrophenol, while 2,6-dinitro-phenol did not show such an effect. The influence is evident when using as substrates the fermentable hexoses and D-fructose-6-phosphate, but not hexose diphosphate. According to MarkoviCev a stimulation of the oxidation processes can be proved thereby. It is probable that these effects are related to the known phytochemical reduction of nitro compounds (see pp. 98 and 99). 

The study of absorption, distribution, metabolism and excretion in toxicology studies, usually referred to as toxicokinetics, provides extremely useful information on the pharmacokinetics of high doses and of repeated doses of the compound. The dose dependency of the pharmacokinetics and the possible time effects, e.g. a decrease in exposure over time as a result of enzyme induction, is essential information for the interpretation of the toxicity findings as well as for the planned clinical studies. 

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