Cure exotherm reaction conversion

Part cures were characterized by exothermic reaction wave propagation. Figures 6a-9b show the development of the reaction waves. The waves propagate from the walls of the part towards the center. A comparison of the temperature and epoxide conversion profiles revealed that the highest temperature corresponded to the highest conversion. As the part initially heats the resin/glass matrix nearest the walls heats fastest however, as the part exotherms the temperatures in the interior of the part exceeded the wall temperatures. The center temperature does not become the hottest temperature until the waves intersect. It must be noted that the hottest temperature does not always occur at the center of the part. The wave velocities are proportional to the wall temperatures. In Figures 6a to 9b the mold temperature was 90 C and the press temperature was elevated to 115 C. Since the press does not heat the part until after it is wound, the press temperature was elevated to accelerate the reaction wave from the press so that the waves would intersect in the center of the part. 

Baseline for Isothermal and Non-Isothermal (MT)DSC Cure The calculation of conversion [Eq. (7)] and conversion rate [Eq. (8)] requires the numerical integration of the (partial) areas of exothermic reaction peaks (Figure 2.1) and therefore the need to draw a baseline. 

Liu et al. (2004, 2005) examined a three-dimensional non-linear coupled auto-catalytic cure kinetic model and transient-heat-transfer model solved by finite-element methods to simulate the microwave cure process for underfill materials. Temperature and conversion inside the underfill during a microwave cure process were evaluated by solving the nonlinear anisotropic heat-conduction equation including internal heat generation produced by exothermic chemical reactions. 

The measurement of the degree of conversion by enthalpy measurement applies to elastomer cure also, since the vulcanization reaction is highly exothermic. Unlike for thermosets, Tg is not a measure of degree of cure for elastomers. Sircar discusses this and numerous other aspects of the characterization of elastomeric materials using thermal analysis in chapter 5 of Reference 5. 

In order to determine the fraction of epoxy reacted from the reaction exotherm, the heat from the exotherm was scaled to the ideal heat of reaction expected from complete conversion of the epoxy. It is assumed throughout this paper that the epoxyphenol reaction and epoxy-secondary hydroxyl reaction generate the same amount of heat per reacted epoxy. The ideal heat of reaction for epoxy with hydrojQd groups was found by Hale to be 85.8 kJ/mole epojQ. This value was tested and confirmed by running ramp-cures of ECN-PN formulations consisting of 1 part epoxide to 2 parts phenolic hydroxyl (0.50 stoichiometry). Ample excess phenohc novolac ensured complete reaction of the epoxy with the phenolic. The results from these tests were found to be consistent with the hypothesis that epoxy reacted completely with the phenol accordingly, the value found by Hale was confirmed. 

Figure 2.72 graphically illustrates the relationship between the glass transition temperature and conversion measured from the residual exotherm of the DSC curves of the epoxy-amine system shown in Fig. 2.70. Several workers have shown that for most thermoset systems there is indeed a unique relationship between the chemical conversion of a thermoset and its glass transition temperature, independent of the cure temperature and thermal history [see, e.g., Pascault and Williams (1990), Hale et al. (1991), Venditti and Gillham (1997), and, for a general overview. Prime (1997)]. It is good practice to establish the Tg-conversion relationship as part of a cure study. Conversion can be difficult or impossible to measure directly, for example, for systems where mass loss accompanies cure. In these cases this relationship may be invoked in order to use Tg as a measure of cure. As Fig. 2.72 suggests, may be preferred as a measure of cure for the final 5-10% of cure where it is usually more sensitive than the residual heat of reaction.

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