Induction times

The induction time is the time involved between the instant where the sample reaches its initial temperature and the instant where the reaction rate reaches its maximum. In practice, two types of induction times must be considered the isothermal and the adiabatic. The isothermal induction time is the time a reaction takes to reach its maximum rate under isothermal conditions. It can typically be measured by DSC or DTA. This assumes that the heat release rate can be removed by an appropriate heat exchange system. Since the induction time is the result of a reaction producing the catalyst, the isothermal induction time is an exponential function of temperature. Thus, a plot of its natural logarithm, as a function of the inverse absolute temperature, delivers a straight line. The adiabatic induction time corresponds to the time to maximum rate under adiabatic conditions (TMRJ). It can be measured by adiabatic calorimetry or calculated from kinetic data. This time is valid if the temperature is left increasing at the instantaneous heat release rate. In general, adiabatic induction time is shorter than isothermal induction time. 

The induction time of a polymer is a very useful quantity in process design, process optimization, and troubleshooting. It represents the amount of time elapsed at a certain temperature and in a certain atmosphere before the effects of degradation become measurable. Essentially, the induction time indicates how long a polymer can be exposed to a certain temperature before it starts to degrade. 

In extrusion, one would like to make sure that the longest residence time in the machine at a certain process temperature is less than the Induction time at the same temperature. Thus, if one knows the residence times to be expected in the extrusion process and if one knows how the induction time varies with temperature, the process temperature at which degradation will be avoided can be accurately determined. This is a very useful tool in process engineering, particularly if one deals with a polymer of limited thermal stability. 

If degradation occurs during the extrusion process, there are two approaches that one can take to the problem. One is to modify the process so as to reduce the chance of degradation. The other approach is to modify the polymer to improve its thermal stability. The changes to the process should result in lower stock temperatures, and/ 

In some cases, the thermal stability of a polymer or a compound is so poor that it cannot be extruded without degradation under any conditions. This can be conclusively determined if induction time data are available. The process engineer can then go back to the polymer chemist and explain exactly what changes should be made to the polymer. This procedure eliminates the question of whether the problem is caused by the polymer or by the process. Thus, induction time data can act as a bridge between the process engineer and polymer chemist and allow them to communicate and cooperate in a useful fashion. 

It is clear that the induction time is a strong function of temperature. For many polymers, a plot of induction time against reciprocal absolute temperature will form approximately a straight line on semi-log paper, as shown in Fig. 6.30. 

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As appHed to hydrocarbon resins, dsc is mainly used for the determination of glass-transition temperatures (7p. Information can also be gained as to the physical state of a material, ie, amorphous vs crystalline. As a general rule of thumb, the T of a hydrocarbon resin is approximately 50°C below the softening point. Oxidative induction times, which are also deterrnined by dsc, are used to predict the relative oxidative stabiHty of a hydrocarbon resin. 

The onset of action is fast (within 60 seconds) for the intravenous anesthetic agents and somewhat slower for inhalation and local anesthetics. The induction time for inhalation agents is a function of the equiUbrium estabUshed between the alveolar concentration relative to the inspired concentration of the gas. Onset of anesthesia can be enhanced by increasing the inspired concentration to approximately twice the desired alveolar concentration, then reducing the concentration once induction is achieved (3). The onset of local anesthetic action is influenced by the site, route, dosage (volume and concentration), and pH at the injection site. 

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. 

The maximum rates of crystallisation of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallise rapidly and other fractions that do not crystallise at all. Poly(vinyhdene chloride) probably crystallises at a maximum rate at 140—150°C, but the process is difficult to foUow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallisation depends on both the type and amount of comonomer PVDC crystallises within minutes at 25°C. 

Table 10.8 Comparison oE antioxidants in polyethylene in both the absence and presence of copper powder and carbon black (data based on ICI literature). Induction time assessed from oxygen uptake measurements using a Barcroft manometer...

Adiabatic induction time Induction period or time to an event (spontaneous ignition, explosion, etc.) under adiabatic conditions, starting at operating conditions. 

Figure 5.8 Induction time-supersaturation plot for cyanazine in 70% wjw aqueous ethanol at 20°C Hurley etal., 1985)...

Induction period measurements can also be used to determine interfacial tensions. To validate the values inferred, however, it is necessary to compare the results with an independent source. Hurley etal. (1995) achieved this for Cyanazine using a dynamic contact angle analyser (Calm DCA312). Solid-liquid interfacial tensions estimated from contact angle measurements were in the range 5-12 mJ/m which showed closest agreement with values (4—20mJ/m ) obtained from the log-log plots of induction time versus supersaturation based on the assumption of — tg. 

In addition to induction time measurements, several other methods have been proposed for determination of bulk crystallization kinetics since they are often considered appropriate for design purposes, either growth and nucleation separately or simultaneously, from both batch and continuous crystallization. Additionally, Mullin (2001) also describes methods for single crystal growth rate determination. 

Schreiner etal. (2001) modelled the precipitation process of CaC03 in the SFTR via direct solution of the coupled mass and population balances and CFD in order to predict flow regimes, induction times and powder quality. The fluid dynamic conditions in the mixer-segmenter were predicted using CFX 4.3 (Flarwell, UK). 

Calculation of the induction time is crucial, since gaining a stable and continuous process requires residence times in the mixer < precipitation induction times in order to prevent incrustation in the mixing device. The induction... 

Figure 8.37 CaC03 induction times (Schreiner etal., 2001)...

N02 addition [both (1.3 2,6) x lCf4 moles] acts the same way. In the absence of salt, however, 2.6 x lO 4 moles of N02 first slow down the 171° decompn, and then make it faster than first order. With 1.3 x 1CT4 moles of added N02, the decompn (lower dashed line in Fig 13) is pseudo first order with an apparent induction time, or more probably a much slower decompn in its initial stages... 

Then the reciprocal of the maximum amplitude factor, r l/Pmax is thought to express the induction time for pit generation. Some calculated values are shown in Table 2. 

The investigation on the influence of the temperature on the hydrolysis rate of the ortho-bromophenol into the catecholate shows that the induction time depends strongly on the temperature from about 5 hours at 135°C to 1 hour at 165°C and 1/2 hour at 180°C (Fig. 17). 

In the same way, the correlation of the nature of the copper catalyst with the rate of the hydrolysis of bromobenzene exhibits in all cases an induction time of about 1 hour, and a transformation time of 10 to 40 minutes (Fig. 18). 

It is noteworthy that metallic copper or cuprous bromide used under nitrogen atmosphere shows only a very short induction time. This last result points out the inhibitor role of the oxygen of the air atmosphere and most likely the important role taken either by reduced species or by radical intermediates in the catalytic cycle. 

In comparison to the normal reaction without additive which affords a yield of 50.2 % in phenol, the additives on the right part of the table reduce the induction time probably by trapping more or less efficiently the air-oxygen inhibitor. 

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. 

Comparison of induction time and the properties of samples prepared in the conventional... 

Method Reaction temp. Induction time Particle size Tap density... 

The spherical fine nickel powders have been prepared fiom aqueous NiCU and hydrazine hydrate at various temperatures wife ethanol-water solvent by the conventional and ultrasonic hydrothermal reduction method. The induction time decreased wife inrareasing fee reaction temperature in both fee method, but was relatively shorter in fee ultrasonic mefeod. Compared to the conventional one, the surface morphology and particle size of fee sample obtained by the ultrasonic method was much smooth and regular in spherical shape and was much small, respectively. Therefore, the tap density of the sample obtained by fee ultrasonic mefeod was relatively higher than feat obtained by fee conventional one. 

KNUDSEN J c, ANTANUSE H s, RisBO j and SKIBSTED L H (2002) Induction time and kinetics of crystallization of amorphous lactose, infant formula and whole milk powder as studied by isothermal differential scanning calorimetry, Milchwissenschaft, 57, 543-546. 

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