Standard Test Method for Thermal Stability of Chemicals by Differential Scanning Calorimetry

Importancia y uso:

5.1 This test method is useful in detecting potentially hazardous reactions including those from volatile chemicals and in estimating the temperatures at which these reactions occur and their enthalpies (heats). This test method is recommended as an early test (or a screening test) for detecting the thermal hazards of a chemical substance or a mixture where the thermal stability is poorly understood (see Section 8).

5.2 The magnitude of the change of enthalpy may not necessarily denote the relative hazard in a particular application. For example, certain exothermic reactions are often accompanied by gas evolution that increases the potential hazard. Alternatively, the extent of energy release for certain exothermic reactions may differ widely with the extent of confinement of volatile products. Thus, the presence of an exotherm and its approximate temperature are the most significant criteria in this test method (see Section 3 and Fig. 1).

5.3 If sample mass loss (the mass difference before and after DSC analysis divided by the tested sample mass before DSC analysis) is greater than 10 %, a leak likely has happened and the data is compromised. Sample leaking typically causes an artificial endotherm or cancels exothermic activities which could lead to an unsafe thermal stability evaluation.5 Therefore, it is highly recommended to discard such data and repeat the analysis.

5.4 When solid substances which will melt in the temperature range of a DSC analysis or liquids are being studied, it is important to perform the DSC analysis within a sealed high-pressure DSC sample container (able to hold at least 7 MPa or two times the vapor pressure at the highest temperature). Such sealed high-pressure containers could minimize the endothermic effect of vaporization. For example, 20 % by weight DTBP in toluene sample has been studied within a pinhole aluminum pan and a sealed aluminum pan in a temperature range from 0 °C to 400 °C. Both analyses had a sample mass loss of over 99 %. The pinhole aluminum pan one (Fig. 3) demonstrates a single endotherm peak due to the vaporization around the boiling point (boiling point: 111 ℃ for DTBP, 110.6 ℃ for toluene), while the sealed aluminum pan one (Fig. 4) shows rupture activities. Neither of them provides the exotherm information, as seen in Fig. 1.5

FIG. 3 DSC Curve Within a Pinhole Aluminum Pan; Tested Sample: 20 % by Weight DTBP in Toluene

FIG. 4 DSC Curve Within a Sealed Aluminum Pan; Tested Sample: 20 % by Weight DTBP in Toluene

5.5 The headspace gas in the sealed high-pressure DSC sample container may impact thermal stability evaluation. A reactive headspace gas may introduce an exothermic peak (for example, oxidation peak when using air) compared to an inert headspace gas, especially for organic samples.5 For example, Ethylene Glycol has been studied within a gold-plated DSC sample container with an air headspace (sealed on a lab bench) and a nitrogen headspace (sealed in an N2 glove box). As seen in Fig. 5, only the DSC test with air one shows an extra exotherm peak.

Note 2: The exotherm enthalpy of the air oxidation peak may be approximated if the available oxygen can be estimated. Oxygen based oxidation reactions are exothermic and release about –100 kcal/mole of oxygen.6 Assuming the sample is sealed at normal temperature and pressure (20 °C and 1 atm), this corresponds to 3.65mJ/µL of air. For example, in the test in Fig. 5, about 50 µL air was sealed in the container, and the energy release is expected to be –183 mJ or –56 J g^-1.

FIG. 5 DSC Curve of Ethylene Glycol in Gold-Plated DSC Sample Container (High-Pressure) with Different Headspace Gases; Black: Air Headspace; Green: Nitrogen Headspace

5.6 For some substances, the rate of enthalpy change during an exothermic reaction may be small when testing a mixture with a low concentration of the substance, making an assessment of the temperature of instability difficult. Generally, a repeated analysis at a higher concentration will improve the assessment by increasing the rate of change of enthalpy.

5.7 The three significant criteria of this test method are: the detection of a change of enthalpy; the approximate temperature at which the event occurs and the estimation of its enthalpy.

Subcomité:

E27.02

Referida por:

E2890-21, E2744-21, E1591-20, E1981-22, E2046-19, E2160-23, E2550-21, E1232-07R19, E1231-24, E3174-22, E0487-20, E2070-23, E2041-23, E2012-06R20, E1445-08R23

Volúmen:

14.01

Número ICS:

07.030 (Physics. Chemistry)

Palabras clave:

differential scanning calorimetry; hazard potential; thermal analysis; thermal hazard; thermal stability;