Isothermal DSC measurements

Comparative studies [1127] of the kinetics of decomposition of similar salts containing related pyridine ligands have been used to investigate the strength of M—N bonds in coordination compounds. Non-isothermal DSC measurements were used to determine values of E for the reactions... 

Future work in this area will involve the extension of these techniques to other temperatures in an effort to better characterize the overall reaction kinetics of these two processes. In addition, degree of cure obtained through isothermal DSC measurements will be compared with the fraction of acetylene consumed as measured by isothermal FTIR experiments for the same temperature and time. Also, the effect of the incorporation of metal fillers on the isomerization and crosslinking reactions will be addressed. 

Isothermal DSC measurements were made with a Perkin Elmer DSC-2C apparatus, modified for UV irradiation (Figure 1). The aluminum sample holder enclosure cover contains two windows, one for the sample and one for the reference compartment. The windows consist of cylindrical quartz cuvettes which have been evacuated in order to prevent moisture condensation. The windows were mounted by using a thermally cured epoxy adhesive. 

To obtain a more realistic estimation of the behavior of an autocatalytic reaction under adiabatic conditions, it is possible to identify the kinetic parameters of the Benito-Perez model from a set of isothermal DSC measurements. In the example shown in Figure 12.11, the effect of neglecting the induction time assumes a zero-order reaction leading to a factor of over 15 during the time to explosion. Since this factor strongly depends on the initial conversion or concentration of catalyst initially present in the reaction mass, this method must be applied with extreme care. The sample must be truly representative of the substance used at industrial scale. For this reason, the method should be only be applied by specialists. 

Isothermal DSC measurement of UPE samples. Courtesy of GenCorp Research,... 

Both decomposition steps are exothermic. For the first reaction step a consistent value of ArH=—21.7 1.2 kj (mol AB) was obtained from isothermal DSC measurements [61]. For the second decomposition step, depending on the heating rate, values from —15 to —24kJ (mol AB) were obtained [63]. The variation may be explained by the rate-dependent product spectrum generated. Figure 8.1a displays... 

Figure S. Isothermal DSC measurements at the epoxy diacrylate resin stabilized with HALS-1 at various temperatures.

Figure 4. Log fractional reaction rate vs. log unreacted fraction taken from isothermal DSC measurement of heat released during urethane polymerization (extended formulation) 32° C.

Bohmhammel, K., Christ, B. and Wolf, G. (1998) Kinetic investigations on the basis of isothermal DSC measurements of hydrogenation and dehydrogenation of magnesium hydride, Thermochimica Acta, 310, 167-171. 

Fig. 3-6. Typical traces of isothermal DSC - measurements for an n-th order kinetic reaction mechanism n>l...

To further understand the effect of the interaction between the nanoparticles and the epoxy resin, the curing reaction kinetics of the composites have been studied. Based on non-isothermal DSC measurements and the Kissinger equation, the activation energy E, the pre-exponential factor A, and the reaction order , of the curing kinetics are obtained (Table 4). Comparison of the kinetic data suggests that the presence of the nanoparticles in epoxy does not change the overall reaction... 

Figure7.11 Typical isothermal DSC measurements of the setting behaviour of brushite-forming calcium phosphate cements based on cement ( 3-tricalcium phosphate/monocalcium monophosphate) at 37°C, showing (a) the normalised heat flow of the exothermic setting reaction with time for fixed PLR 2.0 g/mL and (b) three different citric acid retardant concentrations and fixed retardant concentration of 800 mM citric acid and three different PLRs. While the citric acid concentration had an effect on the setting times, in terms of the time at which the maximum heat of reaction took place, the PLR did not. (From [24], Copyright 2006. Copyright John Wiley Sons Limited. Reproduced with permission.)...

In a study on the thermal and UV ageing of two commercial polyfoxymethy-lene) (POM) samples, one of which was a copolymer (see related study discussed later under Section 4.3, thermogravimetric analysis (TGA)), used in car interior applications, involving both DSC and TGA, isothermal OIT measurements were made at several different temperatures [8]. One conclusion from this study was that "extrapolation of the OIT data from high temperatures (molten state) to ambient temperatures in the solid state does not reflect effective antioxidant performance at room temperature", and thus measurements close to the melting point are not appropriate for reliable lifetime estimations. 

Comparison of chemiluminescence isothermal runs with oxygen uptake and DSC measurements has been at the centre of interest since practical industrial applications of the chemiluminescence method were attempted. It is a fact that the best comparison may be achieved when studying polymers that give a distinct induction time of oxidation typical for autoaccelerating curves of a stepwise developing oxidation. This is the particular case of polyolefins, polydienes and polyamides. The theoretical justification for the search of a mutual relationship between the oxidation runs found by the various methods follows directly from the kinetic analysis of the Bolland-Gee scheme of polymer oxidation. 

Isothermal differential scanning calorimetry (DSC) measurements were carried out to investigate the curing kinetics [85]. Conversion vs time curves of DGEBPA-PACP systems prepared with 1 wt % of catalyst and without catalyst at identical curing temperature are overlaid in Fig. 31. 

Network formation by photopolymerization has been studied for tetraethyleneglycol diacrylate (TEGDA) using isothermal calorimetry (DSC), isothermal shrinkage measurement and dynamic mechanical thermal analysis (DMTA). Due to vitrification the polymerization does not go to completion at room temperature. The ultimate conversion as measured by DSC seems to depend on light intensity. This can be explained by the observed delay of shrinkage with respect to conversion. 

To obtain the cure kinetic parameters K, m, and n, cure rate and cure state must be measured simultaneously. This is most commonly accomplished by thermal analysis techniques such as DSC. In isothermal DSC testing several different isothermal cures are analyzed to develop the temperature dependence of the kinetic parameters. With the temperature dependence of the kinetic parameters known, the degree of cure can be predicted for any temperature history by integration of Equation 8.5. 

Via DSC measurements it was shown that the new bis(vinylbenzyl)ethers can undergo a homopolymerization and a copolymerisation with BMI. In a BMI/-divinylbenzylether blend the Diels-Alder copolymerization is favoured over the divinylbenzylether homopolymerisation. The Tgs of the new copolymers are well in excess of 270 °C when the BMI/divinylbenzylether molar ratio is 1 1 (Table 9). Isothermal weight loss studies over a period of 4000 hours indicate that the BMI-MDA/divinylbenzylether copolymer is the most stable system of this family. 

Clear indications of the induction period and of an increase in the reaction rate after copolymerization has started were found for isothermal runs by DSC measurements by Peyser and Bascom 941 even for melt copolymerization. According to the copolymerization mechanism, the induction period is interpreted as a gradual increase in the concentration of active centres45,52 and is identical with the time for reaching the maximum on the conductivity curves57). An induction period has also been established by other measurements 39,40>73.90.95), where it is often considered as an imprecision in the determination of the monomer concentration, mixing effect, temperature establishement, or it is not considered at all. 

Figure 4.8 Extrapolation of isothermal DSC data and confirmation by TAM-measurement at low temperature.

Figure 11.4 Isothermal DSC thermograms recorded at 170, 180, 190, and 200°C. The measured maximum heat release rate are 90, 160, 290, and 500Wkg . ...

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. 

Figure 2.27 Degree of cure as a function time for an epoxy resin measured using isothermal DSC.

Isothermal crystallization Injection molded sPS/PPE blends having different composition [19], melted at Tmax = 300 °C for tmax = 5 min and then crystallized isothermally for 60 min at various Tc (from 232 to 244 °C), were investigated by means of DSC and WAXD and compared with pure sPS. DSC measurements do not show a melting peak for sPS < 40 wt%, suggesting the absence of crystallinity. In contrast, at higher contents (80 wt%) three separate melting endotherms (labeled I, II and III) between 260 and 271 °C are clearly found, as in pure sPS (Figure 20.2a). 

Induction times determined by pNMR, turbidity, and light-scattering measurements are compared to those determined using PLM in conjunction with image analysis. Isothermal DSC was attempted as a fifth method for comparison. However, because of the inherent lack of sensitivity at the high cooling rates required to obtain isothermal crystallization conditions, it was abandoned. This research was carried out in the context of our milk-fat minor components study (2). 

In differential scanning calorimetry, the selected chemical reaction is carried out in a crucible and the temperature difference AT compared to that of an empty crucible is measured. The temperature is increased by heating and from the measured AT the heat production rate, q, can be calculated (Fig. 3.19). Integration of the value of q with respect to time yields measures of the total heats released, Qr, and Qd, from which AHr and AHp, as well as ATadi and ATad2 can be calculated from Fig. 3.18. The estimation of the activation energy, Ea, is possible by the use of isothermal DSC based on the following equation... 

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