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.

1.1 This test method describes the ascertainment of the presence of enthalpic changes in a test specimen, using minimum quantities of material, approximates the temperature at which these enthalpic changes occur and determines their enthalpies (heats) using differential scanning calorimetry or pressure differential scanning calorimetry.

Note 1: Caution should be used if trying to use pressure differential scanning calorimetry to understand the thermal stability of a liquid or a solid (if it melts during the test or produces gas or liquid products), because the headspace volume to sample size is relatively large, so that the vaporization effect could significantly skew the results. Typically, the pressure differential scanning calorimeter would be limited to situations where a pressurized reactive headspace is required (that is, reactivity of a liquid or solid sample with a gas, more detail in Appendix X1). Sealed high-pressure sample containers are still recommended for the evaluation of intrinsic thermal stability of liquid or solid samples, even within a pressure DSC instrument.

1.2 This test method may be performed on solids, liquids, or slurries.

1.3 This test method may be performed in a sealed high-pressure sample container with an inert or a reactive headspace atmosphere with an absolute pressure range from 100 Pa through 30 MPa and over a temperature range from 273 K to 800 K (0 °C to 527 °C).

1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.4.1 Exceptions—Inch-pound units are provided as a courtesy to the user in X1.3.3, X1.4.1, and X1.4.2.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Specific safety precautions are given in Section 8.

1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.