Residual heat of reaction

Table V presents the results of least-squares fitting of straight lines to the 0.2, 0.3, 0.4, and 0.5 fractional heat points of the log-log plots (of Figures 7 and 8) of polymerization rates (-dH/dt). and residual heats of reaction for Series III, IV, and V data. The slopes, B, progressively Increase as Initial initiator concentrations decrease. However, restricting our consideration to the ten runs in which -AH 68 cal gm l the average B is 1.65. For the six runs (Series IV and Series V) for which -AH >68 cal gm"l and no free radical Inhibitor was initially present the average B Is 1.57. We therefore conclude, with considerable reservation, that -dH/dt CM] for LA where CM] Is the monomer concentration.

The baseline at the end of the reaction was extrapolated to determine the total area under the exotherm curve and hence the isothermal heat of cure q.. Dynamic DSC analyses were run at a heating rate of 2°C/min. f)e8ween 30 and 200°C on each isothermally cured sample to obtain the residual heat of reaction... 

Isothermal kinetic measurements fall into two categories method 1, in which the rate and extent of reaction at constant temperature are continuously monitored in the DSC and method 2, in which a partially cured sample is heated in the DSC to measure the residual heat of reaction. An advantage of method 1 is the simultaneous measurement of conversion and rate of conversion, which are necessary for some kinetic analyses. It should be noted that vitrification will occur during method 1 measurements if Tcure is less than Tg,. Method 2 has the advantage of simultaneous measurement of and conversion, from which the Tg-conversion relationship can be established. Both thermal and UV cure reactions can be measured by these methods. 

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.

If the reaction is incomplete after the first irradiation, a residual heat of reaction will be detected on the second irradiation. A good practice is to repeat irradiation cycles until two cycles are obtained that are indistinguishable from each other. This same methodology can be used to ascertain the completeness of cure of samples cured in laboratory and production processes, although additional measurement of Tg as an indicator of conversion is recommended. 

Figure 22 Differential scanning calorimetry profile of a silver-filled epoxy adhesive using a temperature ramp of 10°C min The area under the first scan (a) provides the total heat of polymerization A/f while the area under the second scan (b) gives the value of the residual heat of reaction of a partially cured material.

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