Cure exotherms

Commercial use of triallyl cyanurate is severely limited by the high price and the high curing exotherm of polyester-triallyl cyanurate systems. The exotherm... 

The applications of the unsaturated polyester resins were increased in the late 1960s by the introduction of water-extended polyesters. In these materials water is dispersed into the resin in very tiny droplets (ca 2-5 p.m diameter). Up to 90% of the system can consist of water but more commonly about equal parts of resin and water are used. The water component has two basic virtues in this system it is very cheap and because of its high specific heat it is a good heat sink for moderating cure exotherms and also giving good heat shielding properties of interest in ablation studies. 

The NCO—CH2(CF2) CH2—OCN monomer series have been characterized by DSC, IR, H-NMR, 19F-NMR, 13C-NMR, and elemental analysis.8 Table 2.1 summarizes the characterization most pertinent to these cyanate ester monomers. The n = 5,1, and 9 members are missing. This is a reflection of the difficulty in obtaining the odd hydrocarbon diol precursors. The trend of a rapid melting point increase with increasing fluoromethylene sequence length is an indication that monomers with n > 10 will probably not be melt-processible since the onset of the cure exotherm in most purified monomers occurs at 200°C. 

In order to study their cure behavior, samples of each oligomer were heated for 10, 20, and 30 min at 300°C under nitrogen. The DSC thermograms of samples heated for 10 min did not contain curing exotherms, and were essentially identical to those of samples heated for 30 min (Figure 1). In fact, the thermogram of a sample of 3b that... 

Other fibers. The other major class of synthetic fibers, the polyacrylonitriles (orlon, acrilon, etc.) like the cellulosics (rayon, cotton) show no thermal activity up to 300 °C. Above these temperatures degradation of sample accompanies any characteristic transitions or curing exotherms. To minimize this effect, the samples are run in an inert environment such as N2, as seen in figure 16. Under these conditions reproducible characteristic endotherms were obtained for identifying wool, cotton and rayon. In roughly the same temperature region,... 

The actual rate of reaction is given by the instantaneous heat flow, so that at any point on a cure exotherm in Figure 3.3 the reaction rate dpldt is given by... 

The isothermal DSC curve is a plot of AH At against time so that, from Equation (3.4), it may be integrated to determine the extent of reaction and so analyse the kinetics of the system. There is still the necessity to perform a scanning DSC experiment in order to determine the residual exotherm from the sample due to the cessation of reaction at the cure temperature as the Tg of the resin reaches the isothermal cure temperature. This just becomes a correction in the form of a scaling factor for the entire curve. A further requirement in this analysis of the isothermal cure exotherm is the determination of the form of the baseline for the integration. In many cases the simple assumption is to choose a flat baseline. However, this ignores the fact that the heat capacity, Cp, of the system will... 

We have carried out some preliminary mathematical modeling of these cures based on our tentative conclusions on the mechanistic complexity. The results do not mirror the experiments well. In particular, it is difficult to explain the apparent sequential nature of the third maximum from the aromatic hexaacrylate resin cures. This third maximum appears even in flash excitation although less prominent than with continuous illumination. In Figure 8 we show the latter part of the cure exotherm with flash excitation illustrating this third maximum. Interestingly this third maximum occurs later in the flash excitation (at - 2 minutes) than with continuous excitation (1.45 minutes). 

Figure 8. Aromatic urethane hexaacrylate cure exotherm from 5 second flash excitation at 50° C.

DSC CURE -EXOTHERM OF A POWDER COATING WTTH AN ANHYDRIDE BASED CURING AGENT ... 

We have to understand that if the heat flux defined by Equation 3.5 at the rubber sensor is of interest, because it provides the intensity and shape of the cure exotherm, the other taking place in the air, expressed by Equation 3.6, is characterized by the loss in heat for the cure reaction. From a first approach, based on a logical consideration, this loss in heat affects essentially the values of the enthalpy of cure which is reduced somewhat. The shape of the exotherm, which gives rise to the kinetics of the reaction, should not be affected as in Equation 3.6, the loss in heat is proportional to the difference in temperatures, in the same way as for the gradient of temperature shown in Equation 3.5. The loss in heat could follow kinetics similar to that observed on the sensor of the calorimeter. 

Thixotropic epoxies are convenient when the adhesive has to be applied from below or on vertical surfaces (Fig. 1, left). Epoxy grouts are employed to fill large volumes and should, therefore, be able to eliminate trapped air bubbles, should not stratify, should exhibit alow cure exotherm and be self-levelling (Fig. 1, right). 

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