Enthalpy of cure

AH enthalpy of cure, in Equations 1.14-1.16 E activation energy of the cure reaction... 

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. 

A calorimeter C 80 (SETARAM) is run in scanning mode with a heating rate of 0.2°C/min for measuring the enthalpy of cure. 

It is also of interest to evaluate the effect of the thickness of the rubber sheet when the heating system is on the external surface of the mold in contact with the surroundings, as shown in Figure 4.10 for the temperature-time history and in Figure 4.11 for the state of cure-time history. These curves are drawn as they are calculated at the mold temperature of 180°C with the rubber compound containing 2% sulfur responsible for an enthalpy of cure of 14.3 J/g. 

V. Comparison between the curves of the Figures 4.6,4.14, and 4.15 obtained with the same thickness of the rubber (2 cm) shows that the profiles of state of cure are quite different, except at the very beginning of the process, for times lower or equal to 15 minutes. For times higher than 20 minutes, the values of the state of cure largely depend on the value of the enthalpy of cure, following the statement the higher the enthalpy of cure, the higher the value of the state of cure. 

FIGU RE 4.18 Temperature-time history at the surface of the rubber in contact with the mold (x = L dotted line) and at the mid-plane of the rubber (x = 0 full line) for various values of the enthalpy of cure. Mold temperature = 180°C. Thickness of rubber = 2 cm. Heating system on the face of the mold is in contact with the rubber. 

The effect of the mold temperature is studied by considering the profiles of temperature and of the state of cure developed through rubber sheets of the same thickness (2 cm) with the same percent curing agent and the same enthalpy of cure (14.3 J/g). The heating system is placed at the rubber-mold interface, and thus the temperature at the rubber surface attains instantaneously that of the mold. 

For calculation, the thermal and kinetic parameters of the rubber are collected in Table 5.1 with 14.3 J/g for the enthalpy of cure. The heating system of the mold is placed on the surface of the mold in contact with the rubber. 

In order to be as informative as possible, the results are presented in terms of the profiles of temperature and of state of cure (e.g., the thickness of a sheet during the process). Much better than other curves such as the tanperature—or state of cure—time histories, these profiles are able to give a fuUer insight into the process. Moreover, the temperature-time history at the mid-plane looks not too informative because of the low enthalpy of cure. As a matter of fact, the processes of heat transfer and cure reaction are intimately bound together, and thns, the profiles of temperature and of state of cure should be shown in parallel. 

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