Hermetically sealed pans

The sample containers most commonly used are cylindrical pans pressed from pure aluminium foil. Alternative materials are used for very high temperatures or corrosive substances, and hermetically sealed pans to withstand several atmospheres pressure can be used for volatile materials. Some heat flux DSC instruments are available which are capable of operation at high pressures, by means of containment of the DSC cell within a pressure vessel. 

The intensity of the 120°-130°C endotherm increases as stratum corneum moisture content increases there is a corresponding decrease in the high temperature meltings at 194° and 210°C (Figure 20). Highly hydrated corneum displays the 120°-130°C endotherm while below 15 to 20% water content, the transition is quite small. These DSC measurements were performed in hermetically sealed pans to avoid the endothermic loss of water. The transition in the presence of water is a cooperative one in that water facilitates the magnitude of the observed transition. The heat change associated with the 120°-130°C transition... 

All the normal considerations associated with DSC measurements are applicable to the study of hydrates, although particular care must be given to the choice of pan used. The use of hermetically sealed pans often leads to suppression of the water loss peak because of the buildup of headspace pressure within the vessel, which in turn alters the equilibrium of the dehydration or evaporation process. In this respect, several authors have recommended the use of open pans to study hydrates as the water loss peak is clearly seen under these circumstances. A further possibility is... 

The use of hermetically sealed pans gave valid data for only a few substances. It is dubious whether the data in Table 3-15 are true values of the activation energy and the frequency factor from pyrolysis reactions. For example, activation energy E for phenan-threne according to the literature is twice the value found by the experiment using sealed pans. Similarly, the value for tiiphenylmethane (E - 255 kJ/Mol) is markedly lower than the reference value E 347 kJ/Mol). However the values ascertained for the two methyl-pyrenes are in the order of magnitude of other alkane-substituted aromatics. 

Pan Type. The larger the contact area between the DSC pan and cell, the more reliable the data. So, whenever sublimation of the sample is not a factor, use of standard DSC pans rather than hermetically sealed pans is recommended. 

Often hermetically sealed pans must be used for various purposes. In such cases, the modulation period must be increased somewhat, because the contact area between the pan and the sensor for hermetically sealed pans is smaller than that for standard DSC pans. 

Modulated temperature DSC has also been used to study plasticization of poly(ethylene terephthalate) by water. PET is somewhat hygroscopic and absorbs moisture, so the glass transition temperature will decrease. Quasi-isothermal analysis was used and the glass transition temperature was found to increase with time as the water evaporated (Toda et al. 1997d). A similar analysis was performed to investigate the effects of water on Nylon 6. A hermetically sealed pan was used and the plasticized glass transition separated into the reversing heat flow (TA Instruments 1994). 

Hermetically sealed pans should be used for volatile (or liquid) samples. After the liquid sample is placed on the bottom of the DSC pan, it is covered with a lid, and the edges of the bottom and the lid are cold-welded together with a small instrument called a crimper. The process itself is called crimping. In addition to measurements on low-molecular-mass liquids, these sample pans should be used for liquid samples such as unreacted thermosets and when the sample contains volatile components such as residual solvent or low-mass additives whose evaporation one wishes to suppress (these pans are also used for polymers that can release degradation products). Evaporation of these materials is endothermic and may interfere with exothermic events such as... 

The difficulty in determining the amounts of the product is associated with the separation of the two endothermic peaks which are not sufficiently resolved to allow quantification. This is generally the case when thermal analysis (e.g., DSC) is carried out in an open system (unsealed pan). The dehydration in a hermetically sealed pan improves the resolution of the DSC peaks dramatically (Fig. 26). This improved resolution results because the atmosphere of the water vapor generated by the first step retards the onset of the second dehydration and moves the peak to a... 

Figure 26. DSC curve for hydrated portland cement in a hermetically sealed pan.t l...

Pressure build-up in an inappropriately chosen pan is a frequent cause of difficulties. It is important to determine whether the sample needs to be rim in a hermetically sealed pan or not dry samples imhkely to evolve significant amounts of volatiles below decomposition do not need to be sealed. Yet if run in a hermetically sealed pan then pressure will build up inside the pan as it is heated. Many hermetically sealed aluminium pans cannot withstand high internal pressure and will deform (not necessarily visible) and result in potential artefacts in the trace as heat transfer to the sample changes. Ultimately sample leakage and bursting can occur, which usually results in contamination of the analyser. The best solution for such systems is to work with crimped pans that do not seal, or use hds with holes in. If not available then it may be best to pierce the pan hd before encapsulation so that pressure does not build up. Sometimes one hole will block with sample (particularly if a hole is made after loading the sample) and back pressure will force sample out of the pan giving more artefacts, so it is better to have more than one hole in the lid. 

It has been foimd necessary to use hermetically sealed pans for this type of work. The decomposition products of the peroxide are appreciably volatile and if not contained, will volatilise, resulting in erroneously small peak integrations for the reaction. 

With hermetically sealed pans, the sample is sealed within a closed system such that the volatiles cannot escape and remain in the head space of the pan even after ejection from the sample. In this pan type, the melting peak of the solvate may be observed, but not the loss of volatiles. 

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