Curing induction period

Clock-type induction periods occur in the spontaneous ignition of hydrocarbon-oxygen mixtures [2], in the setting of concrete and the curing of polymers [3]. A related phenomenon is the induction period exhibited... 

Fig. 5. Cure characteristics of accelerators A, thiuram B, dithiocarbamate C, sulfenamide D, thiazole and E, guanidine. The induction period represents...

To assist in control of the onset of vulcanization, a retarder or prevulcanization iuhibiter (PVI) is used. Retardation of the onset of cure does not mean that the rate of cure is slowed, in fact cure rate may actually be increased. Rather, there is an induction period prior to cure. 

Induction period. The curatives react with themselves in preparation for the cross-linking reaction. This period allows the ingredients to be safely mixed avoiding premature curing ( scorch ). 

The induction period observed at the very beginning of the irradiation is due to the well known inhibition effect of oxygen on these radical-induced reactions. Once it is over, after the, v10 ms needed to consume essentially all of the oxygen dissolved in the liquid film (19), the polymerization starts rapidly to reach 75 % conversion within 0.08 s. Further UV exposure leads only to a slow increase in the cure, mainly because of mobility restrictions in the rigid matrix, so that there still remains about 15 % of acrylic unsaturation in coatings heavily irradiated for 0.4 s. 

In all the above methods, it is necessary to cure specimens of test samples for each of a series of curing times and then perform the desired test on the vulcanizate. However, in the test for continuous measurement of vulcanization complete information could be obtained with saving in time. The mooney viscometer test approaches this objective. However a weakness of the mooney viscometer test is that the test is completed before a measurable modulus value after the scorch point has been obtained. This is because the test sample is destroyed after the induction period is passed due to tearing by continuous rotation of the rotor whether small or large. To overcome this deficiency and to provide a total cure curve for the entire vulcanization cycle, a series of instruments called cure meters was developed. In each of these instruments the stiffness or modulus of the compound was chosen as parameters for vulcanization continuously. The Vulkameter developed by Bayers, Germany was the first of the cure meters developed. 

The occurrence of self-acceleration during curing of epoxy resins and epoxy-based compounds was proven by rheokinetic and calorimetric methods.53 This phenomenon can be treated formally in terms of an induction period (when the reaction is very slow in the initial stage of a process), followed by a constant rate. However, it seems preferable to use a single kinetic equation incorporating the self-acceleration effect to describe reaction as a whole. Such a kinetic equation contains only a limited number of constants (K and co in Eq. (2.33)) and allows easy and unambiguous interpretation of their dependencies on process factors. 

INFLUENCE OF SHEAR RATE ON THE INDUCTION PERIOD DURING OLIGOMER CURING... 

The role of shearing is especially important in the kinetics of oligomer curing.112 114 It is related to the ability to maintain the reactive material in the fluid state for some finite time during the curing process. This time, t, called the induction period or "lifetime", is a ver important material property, since the material can be processed only during this period. At t >t, viscosity increases sharply, and the material becomes solid. 

Figure 2.31. Influence of shear rate on decrease of an induction period in oligomer curing. An arrow shows the direction of increase in shear rate.

The results of the calculations shown in Fig. 2.32 represent a complete quantitative solution of the problem, because they show the decrease in the induction period in non-isothermal curing when there is a temperature increase due to heat dissipation in the flow of the reactive mass. The case where = 0 is of particular interest. It is related to the experimental observation that shear stress is almost constant in the range t < t. In this situation the temperature dependence of the viscosity of the reactive mass can be neglected because of low values of the apparent activation energy of viscous flow E, and Eq. (2.73) leads to a linear time dependence of temperature ... 

The correspondence between the calculated results based on the model of heat dissipation due to viscous flow and the experimental data in the decrease of the induction period at high shear rates proves that the observed effect is adequately explained by this mechanism. The effects of shearing itself on the kinetics of curing are either absent or of secondary importance. If the experimentally observed decrease in the induction period is more pronounced than predicted by the dissipative model, then it is reasonable to consider additional heat sources, for example, the exothermal effects of a reaction. Heat flux from the surroundings can also influence the kinetics of... 

Figure 2.33. Experimental data illustrating the effect of a decrease in the induction period during curing of a phenolic-based compound at different shear rates 0.015 s 1 (curve 1) 0.95 s 1 (curve 2) 3.75 s 1 (curve 3) ...

Figure 2.34. Comparison of theoretical predictions (curve, calculated from Eq. (2.8S) according to the dissipative model of non-isothermal curing) with experimental data on the decrease of the induction period at high shear rates for phenolic-based compounds (vertical bars) and silicon-based composites at different initial temperatures 150°C (1) 170°C (2) and 190°C (3).

Isothermal kinetic runs for the same system were also analyzed by Mauri et al. (1997) and Galante et al. (1999). An excellent fit of a first-order kinetics after an induction period was reported, with E lying in the range of 70-75 kJ mol-1. Obviously, the overall behavior, including the induction period, is autocatalytic. However, for conversions higher than about 0.15, a first-order kinetics provided an excellent agreement, particularly at low cure temperatures and up to vitrification. This behavior is partially explained by Eq. (5.29) the function f(x) exhibits a behavior close to (1 — x) in the 0.4—0.8 conversion range. 

Also important is that the induction period, which was about 3 min in the ESR experiment, was found to increase to about 30 min in isothermal DSC scans performed at the same cure temperature (Tollens and Lee, 1993). This is possibly due to the presence of dissolved oxygen (coming from air) in the DSC samples. Oxygen is a known inhibitor of the UP-S free-radical polymerization. This is a very important fact rate equations determined... 

The time to tQ is the time for the wood-monomer mass to reach oven or curing temperature at T5. During the period of constant temperature, the induction period, the inhibitor is being removed by reaction with the free radicals. Once the inhibitor is eliminated from the monomer and wood, the temperature rises to a maximum which corresponds to the peak of the exothermic polymerization reaction. Polymerization continues to completion although at a decreased rate and the temperature returns to that of the curing chamber. The time to the peak temperature depends upon the amount of catalyst present, the type of monomer, the type of crosslinker, and the ratio of the mass of monomer to that of the wood. The wood mass acts as a heat sink. Figure 4 illustrates the effect of increased Vazo catalyst on the decrease in time to the peak temperature, and the increase in the peak temperature(10)... 

The induction period values for the curing process of the HDDA/BA mixture in the presence of either polymeric or low-molecular-weight photoinitiators based on benzoin methyl ether moieties are found to be quite similar (Table 21) and one order of magnitude shorter than those reported for the systems based on 0-acyloxime moieties (Table 15). 

Induction period of the curing process Half-time of the process... 

All the MBA/A(,iV-dialkylamino acrylate copolymers behave similarly to poly(MBA-co-MtA)s (Tables 21 and 25), thus suggesting that the replacement of MtA by A,A(-dialkylamino acrylate co-units does not markedly affect the photoinitiation activity of the system. Accordingly, BMI/A,A(-dialkylamino isobutyrates mixtures exhibit substantially the same activity as MBI alone [118]. Similar results have previously been obtained for 2,2-dimethoxy-2-phenyl acetophenone (DMPA), when additioned with diethylmethylamine, in the UV initiated polymerization of -butyl methacrylate [113]. However, a remarkable shortening of the induction period (to) of UV curing is observed for all the polymeric photoinitiators in the presence of tertiary amines as compared with the low-molecular-weight MBl/A,A(-diatkylamino isobutyrates systems, the maximum effect resulting in the case of MBA/A(,iV-dialkylamino acrylate copolymers (Table 25). 

As reported in Table 26, poly(MAPO) and poly(MAPO-co-MMA) systems are found [124] to display a higher photoinitiation activity in clear coating formulations, expressed in terms of half-time of the curing process, essentially due to their shorter induction period, as compared with the structural model PIVPO. The polymeric systems, therefore, appear to be preferable for their higher productivity in clear coatings formulations. 

Induction period of the curing process Time required to obtain 50% conversion of the HDDA/BA mixture Maximum polymerization rate expressed as percentage of monomer to polymer conversion over time... 

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