Pinhole Control in Thermal Analysis for Thermal Hazard Studies of Volatile Materials

摘要

Differential scanning calorimetry (DSC) has gained wide acceptance as a screening tool for preliminary assessment of the thermal hazards of materials. This screening for the presence of an exotherm in nonvolatile materials is generally straightforward. Complications can arise when a specimen is capable of generating a significant amount of vapor phase during the heat program of the DSC. This can lead to an appreciable loss of specimen in an unsealed container and therefore an underestimation of any enthalpy change. Alternatively, it can result in significant self-pressurization of a sealed specimen container resulting in analyses at unknown conditions, distortion of the specimen container with associated baseline noise, or catastrophic failure of the specimen container with its host of complications. Attempts to assess the volatility of materials such as through mass loss using thermogravimetry (TGA) often exhibit poor repeatability, are difficult to reproduce in other laboratories, and are unlikely to simulate reality. This stems from the fact that volatility is not a thermodynamic property but rather a complicated product of a thermodynamic property- vapor pressure- and numerousl extrinsic factors. Heretofore, use of DSC for measurement of gas phase reactions has had very limited success. Of particular interest with respect to thermal hazards is the consequences of vapors mixing with residual oxygen at conditions above ambient. Use of a hermetic scalable specimen container to which is added a specific dimension pinhole significantly improves the potential for thermal analytical studies to aid in the thermal hazard assessment of volatile materials. The one-way-like valve action of the pinhole allows for self-generated atmospheres to develop without significant over pressurization of the specimen atmosphere beyond that maintained in the test fixture while minimizing any loss of condensed material. These pinholes improve the use of DSC for vapor pressure and boiling point determinations. They have been successful in showing oxidation reactions with DSC, they negate the extrinsic factor effects for volatility, and they promote assessment of self-generated atmosphere influences on decomposition reactions. Examples of each these benefits imparted by use of pinhole control in thermal analysis studies will be provided in this presentation.