Hermetic pans

With some types of hermetic pans, the lid may be inverted and placed into the sample pan. This is ideal for running smaller samples as it produces an hermetic pan with a smaller dead volume above the sample, and improves heat transfer to the top of the sample. For very small samples two pans may be used, one inverted and pushed inside the other. This type of configuration is also good for sealing 2 to 3 mg of calibration sample and keeping as a retained standard. 

Liquid samples, if heated to too high a temperature, will burst the pan open when the pressure exceeds around -300 kPa for aluminum hermetic pans and -600 kPa for gold pans. If higher temperatures or pressures are required, the use of a large volume stainless steel capsule to hold the sample is recommended. Another solution to this problem is to pierce the lid of the pan to enable volatile components 

FIGURE 2.5 Comparison of DSC heat flow data for water obtained at 5°C/min in an open pan (dashed line) and a sealed hermetic pan with a 50-gm hole in the lid. 

The use of the hermetic pan with laser pinhole can be extended by using this configuration to measure boiling points of materials within a pressure DSC cell. If the boiling point is measured at a range of known pressures, the boiling point shifts can be used in the Clausius-Clapeyron equation to obtain quantitative vapor pressure data. 

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FIGURE 2.3 Comparison of three samples of indium, of varying mass, heated at 10°C/min. Hermetic pan with inverted lid. Nitrogen purge gas at 25 ml/min. Heat flow plotted against temperature. 

Typical volume is -100 pi If the sample reacts with stainless steel use an aluminum hermetic pan to hold sample alternatively, use a glass ampoule... 

FIGURE 2.14 DSC data for 2.64 mg of the liquid crystal azodianisol heated at 10°C/min in an hermetic pan with lid inverted. Nitrogen purge gas at 25 ml/min. Data are shown full scale (left-hand axis) and 20 times expansion (right-hand axis). 

Epoxy systems used in structural applications, whether as adhesives or the matrix of fibre-reinforced composites, are normally cured under some pressure and can be regarded as in closed containers. Studies on the kinetics and mechanisms of cure chemistry are often conducted without pressure in containers essentially open to the atmosphere. Extensive thermal analysis examinations at this Laboratory on a range of epoxy formulations have shown that for such fundamental quantities as the heat of reaction substantially different values can be obtained by using open or hermetic pans (Table 2). These differences are apparent with both the TDI-DMA adduct and dicyandiamide as curing agent but not with DDS. 

Figure 3. DSC curves of PPy/p-TS film at 10 C/min (a) open pan (b), hermetic pan, initial heating (c), reheat of (b). The small endotherm near 90 can be intensified by suitable annealing.

Some evidence of a Tg in this range has been obtained and this has been detailed elsewhere (9). The most persuasive evidence has been foimd in the DSC curves of some samples where there are endotherms which could be attributed to physical aging after the Tg had been lowered because of plastidzation by absorbed moisture. A typical result is seen in Figure 3 when the dehydration of PPy/p-TS is suppressed in hermetic pans a typical aging endotherm precedes the loss of water (Figure 3, curves b and c) and this can be enhanced by suitable annealing. Also, instead of the usual broad endotherm (Figure 3, curve a) obtained for fresher material in open pans, some other older samples of different PPy film have shown an additional sharp endotherm (between 60 and 90 O superimposed on the expected DSC curve. 

Figure 6. Friedman isoconversional kinetic parameters for DSC heat release from RX-55AE-5 heated at 0.1 and 1.0 C/min in a pinhole hermetic pan.

A TA Instruments DSC 2920 (TA Instruments, New Castle, Delaware) was used to study the thermal properties and crystallinity of the microgels. Microgels with excess squalane were sealed in an aluminum hermetic pan and cooled to 0 °C in the DSC cell before being heated at a rate of 10 °C/min to 155 °C. Following this, the sample was brought back to -40 °C with a cooling rate of 10 °C/min and this process was repeated for an additional seven cycles. 

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