Isothermal DSC experiments

To confirm that the matrix is amorphous following primary solidification, isothermal dsc experiments can be performed. The character of the isothermal transformation kinetics makes it possible to distinguish a microcrystalline stmcture from an amorphous stmcture assuming that the rate of heat released, dH/dt in an exothermic transformation is proportional to the transformation rate, dxjdt where H is the enthalpy and x(t) is the transformed volume fraction at time t. If microcrystals do exist in a grain growth process, the isothermal calorimetric signal dUldt s proportional to, where ris... 

Worked Example 11.1 TMRad and T,m from Isothermal DSC Experiments... 

The estimation of TMRld from isothermal DSC experiments is illustrated by an example taken from Figures 11.4 and 11.5. For calculation of the activation energy, two reference points, 500 Wkg1 at 200°C and 90Wkg 1 at 170°C, are taken ... 

Figu re 12.1 Comparison of autocatalytic (a) an nth-order (ri) reactions in an isothermal DSC experiment performed at 200°C. Both reactions present a maximum heat release rate of lOOWkg 1 at 200°C. The induction time of the autocatalytic reaction leads to a delay in the reaction course. 

Figu re 12.4 Comparison of a strongly autocatalytic reaction with a weak autocatalytic reaction (dashed line). Isothermal DSC experiment at 200°C. The heat release rate for the strong autocatalytic reaction is zero at the beginning of the exposure. 

Since this thermogram shows a steep peak, the autocatalytic nature of the decomposition is likely. Thus, two isothermal DSC experiments were performed at 240 and 250 °C, in order to confirm this hypothesis and to evaluate the probability of triggering the decomposition (Figure 12.13). The results can be summarized as follows at 240°C the initial heat release rate is 8.5 Wkg-1 and the maximum heat release rate 260 Wkg-1. At 250°C, the measured heat release rates are 15 and 360Wkg 1, respectively. 

A series of isothermal DSC experiments were performed on a sample. The samples were contained in pressure-resistant tight gold plated crucibles. The oven of the DSC was previously heated to the desired temperature with the reference in place. At time zero, the sample crucible was placed in the oven. The maximum heat release rate and the time at which it appeared were measured. The results are summarized in Table 12.2. 

A number of methods are available for deriving reaction kinetics constants from DSC thermograms (Wright, 1984). For example, the thermogram obtained during an isothermal DSC experiment at a temperature at which crystallization of a fat occurs can be analyzed in a way similar to that described earlier for the determination of solid fat content, but in this case the evolution of peak area (representing the formation of solid fat crystals) is related to time rather than temperature (Chong, 2001). 

Figure 6. Determination of the reaction order from the 135 C Isothermal DSC experiment.

To assess the potential safety problem, isothermal DSC experiments at different temperatures and after different amounts of conversion (Figure 2), preferably in presence of the catalyst, can be used to predict the behavior of the reaction mass in the event of a cooling failure. 

The reaction of a stoichiometric amount of these twelve acids with the epoxy resin was measured during a series of non-isothermal DSC experiments. These measurements were performed with a Perkin Elmer DSC-2C using a scanning rate of 10°C/ minute. Each experiment consisted of three heating scans ... 

Zhou and Hay [1993] investigated the crystallization behavior in LLDPE/PP blends. The crystallization rate of the LLDPE matrix, measured from isothermal DSC experiments, was not affected by the dispersed PP domains. However, its degree of crystallinity slightly decreased with increasing PP content in the blend. According to the authors, this could be ascribed to the lower degree of perfection of the LLDPE crystals. 

In certain situations it is possible to evaluate the kinetics of the runaway reaction using isothermal DSC experiments. The apparatus is similar to that described in Section 3.3.2 on page 28, but is run in an isothermal mode. 

The TMR can also be determined from isothermal DSC data - A test is initially carried out using scanning DSC to obtain the overall heat released, A//, and the specific heat, C. Several isothermal DSC experiments (Figure 4.30) are then carried out at different temperatures near to the exotherm onset temperature detected in the scanning DSC run. 

Reaction kinetics for bulk polymerizations can conveniently be determined from differential scanning calorimetry (DSC). The results of isothermal DSC experiments of butyl-methacrylate are shown in Fig. 3.1 as conversion versus time. In the curves the conversion is defined as... 

The dynamic cure profile of a formulation was obtained using a modulated Difforential Scanning Calorimeter (DSC), Model 2920 by TA Instruments. A sample of 10 mg was placed into a hermetic DSC sanq>le pan and heated in the DSC cell fix>m room temperature to 350 °C at a heating rate of 5 °C/min under N2 purge (40 ml/min). The heat of reaction was recorded as a function of temperature. The onset temperature and the peak temperature of the cure reaction were recorded. For kinetic study, isothermal DSC experiments were conducted. The sample was placed into a hometic DSC sanq le pan and kept at a certain temperature for a sufficient amount of time. For each formulation, four isothermal temperatures were chosen within 20 °C of the onset cure temperature. The heat of reaction was recorded as a function of time. 

First-order reaction rate constants were determined over a wide variety of temperatures, stoichiometries, and TPP concentrations by using the isothermal DSC experiments as described previously. For the sake of comparison, the first-order rate constants at 150 C are plotted with respect to the same scaling parameters as were used... 

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