Practical Use of Differential Scanning Calorimetry for Thermal Stability

Article Cite This: Org. Process Res. Dev. XXXX, XXX, XXX−XXX

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Practical Use of Differential Scanning Calorimetry for Thermal Stability Hazard Evaluation Min Sheng,* Daniel Valco, Craig Tucker, Elizabeth Cayo, and Tyler Lopez

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Reactive Chemicals, Corteva Agriscience, Midland, Michigan 48667, United States ABSTRACT: Differential scanning calorimetry (DSC) is a common industry tool used in the assessment of thermal stability of materials. Despite widespread use of DSC for thermal stability hazard evaluation, mistakes in testing methodology or interpretations of data are common. To avoid these issues, a standard operating procedure and list of common practices utilized within our Corteva Agriscience Reactive Chemicals (RC) group is presented in this manuscript. Emphasis within our RC program is placed on device calibration and maintenance, selection of the appropriate sample container, and a unique sample preparation methodology. The use of glass capillary and glass ampoule sample containers for DSC testing is outlined, along with the unique flame-sealing procedure utilized to protect the sample. The results of the glass sample containers using di-tert-butylperoxide in toluene compared to gold pan are presented showing the effects that sample containers can have on results. Additionally, glass ampoule sample containers, containing ethylene glycol, are used in DSC testing to show their effectiveness for examining a sample’s oxidative nature. A discussion of the issues and shortcomings of the commonly used aluminum pans, for use with organic samples particularly, is also presented. All this DSC testing information provides insight into our group’s ability to work on a diverse array of samples and generate quality data for understanding the thermal stability hazards present within our company. KEYWORDS: differential scanning calorimetry, thermal stability, high-pressure DSC sample container, misuse of aluminum pan

1. INTRODUCTION For reactive chemicals (RC) hazard evaluation, differential scanning calorimetry (DSC) is one of the most commonly used thermal stability testing methodologies. DSC is primarily used as a screening tool to identify possible thermal instability hazards within our company, because the test is inexpensive, has a quick turnaround, is on a small scale, is experimentally simple, and provides accurate enthalpy measurement in comparison to other thermal stability testing options.1−3 Generally, a flat “boring” DSC curve, without any exothermic or endothermic event, can rule out the need for future advanced testing. Conversely, a DSC curve containing multiple exothermic and/or endothermic events in a temperature range of interest may trigger more sophisticated thermal testing (e.g., ARC, VSP, or RC1) and even thermal runaway evaluation using the “Heat Gain Versus Heat Loss” approach.4 Although the DSC instrument’s use is widespread for thermal stability hazard evaluation, mistakes in the testing methodology or interpretations of the data are common and may provide erroneous results. One of the most frequently encountered issues is the use of an unsuitable DSC sample container. A recent example happened where the onset of a significant exothermic event was detected by a DSC test on a material below its storage set point temperature. However, upon consulting an RC Subject Matter Expert (SME), it was discovered that this exotherm was caused by the interactivity of this material with the aluminum sample pan used in the DSC test. The actual storage conditions were appropriate, so the material was not in jeopardy of causing an incident. This article focuses on how DSC testing is performed within our company’s RC group to generate quality data for a diverse array of samples and to better evaluate thermal stability © XXXX American Chemical Society

hazards. Common misinterpretations of DSC data are also addressed in this manuscript, as a general guidance in assessing RC hazards.

2. DSC STANDARD OPERATING PROCEDURE IN RC GROUP The standard testing methodologies published by ASTM Committee E27,5 especially ASTM E967, E968, E4876 and E537,7 were used as a general guide for our standard operating procedure (SOP). 2.1. Instrument Calibration and Maintenance. The DSC instruments currently used in our RC group are Q-series from TA Instruments with a sensitivity of 0.2 μW. The Q2000 and Q2500 were used to perform all experiments presented in this paper, while the Trios software was used to analyze all DSC thermograms using an exotherm up (EXO up) designation. When the instrument is new, or after major equipment maintenance, a calibration for temperature and cell constant with a standard, such as indium, must be performed. The calibration should be done with a high degree of precision. Upon rescan, agreement with the standard should be within ±0.3 °C for temperature and within ±0.6 J/g for enthalpy. Generally, for each type of sample container used on the device and for all programmed heat rates, a specific calibration must be performed. A typical cell constant is between 0.9 and 1.5 under a nitrogen purge. A cell constant greater than 1.5 may indicate an instrument hardware problem. The baseline drift of Special Issue: Corteva Agriscience Received: June 13, 2019

A

DOI: 10.1021/acs.oprd.9b00266 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

Article

Figure 1. A standardized RC DSC thermograph; internal test ID 180310-001-01; (blue curve) the first scan; (green curve) the rescan of the same sample.

acceptable limits must be demonstrated before proceeding with any future analysis. The ruptured DSC test should be repeated either with a much smaller sample size or with a different sample container. 2.2. Standard DSC Test Conditions. The standard DSC test conditions used in our RC group are listed below: • Purge gas: N2 at 50 mL/min. • Instrument standby temperature: 40 °C. • Typical data sampling interval: ≤1.0 s/point. • Typical heat rate: 10 °C/min. • Typical temperature range: 30 °C or lower (0 °C in this paper) to 400 °C. • Always rescan the same sample with the same heat rate over the same temperature range. • Always weigh the sample container before and after the DSC test, to check for mass loss of the tested sample. Must retest if sample mass loss is greater than 10%. A longer data sampling interval is NOT recommended, except for long-term isothermal aging tests. Conversely, for fast reactions, a shorter data sampling interval may be needed for more data points. Given that most chemical reactions in our company are expected to occur well within the typical temperature range listed above, such a temperature range is adequate for most screening purposes. Weight loss in a tested sample greater than 10% should be considered as a leakage run, which requires cleaning the calorimeter as aforementioned and rerunning the test. Scanning the same sample twice allows a comparison between the heat flux signal from the first and second heat cycle. This “rescan” methodology can distinguish physical events (phase transitions) from chemical events and help better define the baseline of chemical event that occurred in the first cycle. An example of this is shown in Figure 1, which displays a standardized DSC thermograph for Chemical A (solid sample, internal test ID 180310-001-01). During the first scan, the sample melted at ∼94 °C and then underwent a decomposition event with a detected onset temperature of 134 °C and a total heat of −1586 J/g. The low integration limit of the peak is often used as the detected onset temperature in our RC group for conservative purposes when evaluating the thermal hazard, instead of the extrapolated onset temperature (180 °C in Figure 1). With assistance from the second scan, such detected onset temperatures can be determined more accurately. After the first scan, the actual materials in the DSC sample container are either decom-

an empty sample container in the typical temperature range (e.g., 30 to 400 °C) should be lower than 2 mW (excluding start and stop transients). While an analytical balance with 0.001 mg accuracy is preferred for DSC sample preparation, a Mettler Toledo XPR Microbalance is currently used in our RC group, which is capable of accurately measuring 0.0001 mg. For a regularly used DSC instrument, weekly indium checks should be performed to ensure the accurate measurement of temperature and enthalpy (at minimum every other week). Acceptable limits for indium melting are 156.6 ± 0.5 °C for extrapolated onset temperature and 28.6 ± 2.9 J/g (i.e., ±10%) for enthalpy of fusion.6 If both values are within the acceptable limits, proceed with the analysis. If either value is not within the acceptable limits, check for and correct any deviations from normal run conditions and perform the indium check again. If indium melting results are still outside of limits, the instruments must be recalibrated. Leakage from a poorly sealed sample container can cause contamination of the DSC sensor, which can cause baseline stability issues. Baselines that show significant curvature, spikes, peaks, or noise should be questioned. Deviations that result in a variation greater than 2 mW from the highest point to the lowest point of the baseline for an empty sample container (excluding start/stop transients) should be questioned. Occasionally, a sealed glass capillary or ampoule will rupture during a DSC run due to a buildup of excess pressure. Rupturing of the glass tubes can lead to deformation of the DSC sensor over time; so every rupture should be investigated for the cause, and then the necessary steps to prevent additional ruptures in the future should be determined. After ruptures and leaks, residues may remain on the sample holders, which can produce noisy baselines and in some instances result in “fake” peaks. For example, after the rupture of a sample container, a sample residue was deposited on the surface of sample holder. During the ensuing DSC tests, this residue would melt reproducibly creating a “false endothermic” peak on thermograms. Therefore, the DSC cell, sensor, sample holders, and reference container must be fully cleaned after a leakage or rupture. To clean the instrument, remove the debris with a vacuum and then clean the DSC sensors (and sample holders, if used) that are contaminated; they must be cleaned with a proper solvent and blown dry. After the instrument is clean, the DSC sensors (and sample holders, if used) are “baked out” by ramping to 300 °C under an air purge and held for 5−10 min at 300 °C before cooling. An indium check with B



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Figure 2. Pictures of three DSC sample containers currently used in our RC group; (a) sealed glass capillary with a liquid sample; (b) capillary holder; (c) three-dimensional draw of capillary in a holder; (d) sealed glass ampoule with a solid sample; (e) ampoule holder; (f) three-dimensional draw of ampoule in a holder; (g) Fauske gold-plated DSC crucible.

Table 1. Typical Parameters of Three Sample Containers Currently Used in Our RC Group sample container

max temperature, °C

rated max pressure, psi

container mass, mg

holder mass, mg

internal volume, μL

sample size,a mg

glass capillary glass ampoule gold-plated DSC crucible

500 500 400

3000 1000 3000

25 35 980

320 285 N/A

5 25 20

1 1 3

a

Note: A higher sample size is not recommended for samples you do not know well.

In many cases, not all of these criteria are necessary for highquality work, and sample containers may be chosen with other factors in mind as well (cost, convenience, etc.). However, the capability to withstand pressure buildup is very important to consider when selecting a DSC sample container for thermal stability hazard evaluation. High-pressure, inert sample containers have been recommended by ASTM E5377 for thermal stability hazard evaluation. Since thermal stability hazard evaluation emphasizes postulating plausible process upset scenarios (e.g., loss of cooling), the final temperature may be significantly higher than the process temperature. Therefore, the tested samples need to be run at an elevated temperature in the DSC to check whether there is a decomposition reaction. Complicated reaction mechanisms of decomposition reactions always create a challenge in identifying the exact decomposition products, which leads to an unknown final pressure. In addition, the samples required to run for a large company are highly variable in composition, physical properties (e.g., vapor pressure), and reactivity. A reliable and robust high-pressure sample container is critical to obtain quality DSC data. There are commercially available inert, high-pressure DSC crucibles for the purpose of thermal stability hazard evaluation, such as gold-plated DSC crucible from Fauske LLC (Figure 2g). Generally, these are easy to use, but it is not an economically feasible option as a daily use sample container for a large company that runs many hundreds of DSC experiments each year. An economically feasible process was developed over 30 years ago utilizing flame-sealed glass capillary8 and glass ampoule9 as DSC sample containers to evaluate reactive chemicals thermal stability. The pictures of sealed glass capillary (DWK Life Sciences, 3450799), sealed glass ampoule (Wilmod-LabGlass, 529-A), and their holders are shown in Figure 2a−f. The typical parameters of the three sample containers currently used in our RC group are listed in Table 1.

position products, thermally stable chemicals, or a mixture of these, which normally do not exhibit any thermal activity events in the same temperature range; that is, only physical events may appear. The heat capacity difference between reactant and products is normally not significant, so that the effect on the DSC heat flux is negligible compared to that of the heat release of a decomposition reaction. Therefore, the rescan curve serves as a great reference for drawing the baseline of the chemical event in the first scan. The upward shift of the heat flux curves in both scans above 350 °C in Figure 1 is commonly observed. Either phase transition (entering a supercritical phase) or chemical reaction can cause this behavior. If it is due to chemical reaction, the reaction was incomplete at the end of first scan, so any remaining reactant continues being consumed around the same temperature range during the rescan. Normally, such upward shift behavior is far above the process temperature, and no future investigation is necessary for hazard evaluation. Otherwise, more sophisticated thermal testing is needed to better understand the potential hazard of such an event in the actual process. 2.3. Glass Capillary and Glass Ampoule. Because of a small sample size and a relatively high contact surface between test samples and the sample container used to run the DSC test, the proper selection of a sample container is crucial to the data quality. The desirable characteristics of a good DSC sample container are 1. Capable of withstanding pressure buildup. 2. Chemically inert in the desired test. 3. Simple to operate. 4. Provides stable baselines. 5. Exhibits fast thermal responses. 6. Has minimal headspace volume to reduce evaporation endothermic events. 7. Allows for easy means to change the atmosphere in the cell from inert or oxidizing. C



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cooling media and thereby prevents overheating of the sample inside it. To determine the actual sample temperature when a glass capillary with sample was cooled by the coldfinger, a fine thermocouple was inserted into a water sample loaded in a capillary, and then the capillary was transferred to the short end of the coldfinger. As shown in Figure 4, dry ice/acetone is

The holders are needed to accommodate the sample containers on the DSC sensor, while also providing rapid heat transfer between sample container and DSC sensor. In design of these custom sample holders, one must take into consideration using a highly conductive material, ensuring a good contact between all parts, polishing the bottom surface as flat as possible (to reduce the heat resistance between holder and DSC sensor), and making it as light as possible (to ensure a fast thermal response). The dimension of the capillary holder can be found in the original paper.8 The ampoule holder is made of a cylinder, Ø5.0 mm by 2.6 mm, with an Ø4.5 mm hemisphere hole on the top and an Ø1.9 mm half-hole on the wall. The original holders used in our reactive chemical group were made of aluminum, but it was discovered that certain aluminum alloys gave unacceptable baselines at temperatures above 350 °C. All current holders used in our RC group are made of 99.95+% pure silver, which not only offers much better heat transfer characteristics but also gives a stable baseline. The nature of the flame-sealed glass containers and their commensurate silver container holders meet, to a great extent, all seven criteria mentioned above with a few exceptions. One exception may be that the operation to flame seal a capillary tube, without overheating the sample, is not a simple task. In our RC laboratories, it has been demonstrated repeatedly that one with no experience can master this operation with some practice. The pictures of our newest flame sealer is shown in Figure 3a. The flame sealer consists of aluminum frame with a

Figure 4. Measurement of actual sample temperature; (0 to 300 s) real capillary and no flame applied; (350 to 700 s) open end-capillary and flame applied at ∼530 s.

able to cool the sample to −49 °C and liquid nitrogen to −147 °C. If a capillary is directly immersed in liquid nitrogen (−196 °C), liquid oxygen (−183 °C) could be condensed from air onto the tested sample.10 A mixture of liquid oxygen with a lot of materials, such as hydrocarbons and other organics, can be highly explosive.11 We believe this was the cause for “glass capillary blasting” mentioned in the paper by Shimizu and Ueki.12 Obviously, the use of a coldfinger keeps the sample temperature above the boiling point of liquid oxygen and effectively prevents such a scenario from occurring. To confirm the effectiveness of the coldfinger design in preventing overheating of a sample inside a glass capillary, the actual sample temperature during the flame sealing was measured by another fine thermocouple. This thermocouple was inserted into a grease sample that was located at one of the open ends of a capillary (Figure 4), and then the capillary was flame-sealed, per the usual flame-sealing procedure. A side thermocouple was used to detect the duration of flame on the capillary by locating it at the sealing point, but outside of the capillary. With a propane/oxygen flame, it took ∼3 s to melt the glass so that the flat tweezer could pull away the excess glass, while an O-ring held the sealed bottom part of the capillary at the end of the coldfinger. The recorded max sample temperature rise, due to the heat from the flame sealing, was 20.2 °C for dry ice/acetone and 5.7 °C for liquid nitrogen. Both peak sample temperatures occurred ∼9 s after the flame was removed from the glass tubing. This result shows that both cooling media are acceptable for flame-sealing operation. Liquid nitrogen is preferred in our RC lab, since it gives much lower chance to have an overheated sample, while dry ice normally needs acetone (lowest sample temperature of −27 °C without acetone vs −49 °C with acetone) to improve the heat transfer in the Dewar, which introduces an additional flammability hazard. Even though liquid nitrogen is used as the cooling media, to have a reliable DSC measurement, careful attention is still needed during sample preparation. Listed below are several recommendations followed in our RC lab:

Figure 3. Pictures of flame sealer; (a) the newest flame sealer in our RC group; (b) coldfinger; (c) flame sealing of a capillary.

linear stage traveler on the top, an arm with a lifting flat tweezer, valve/flowmeter/tubing to feed N2 or air for container headspace, needle valve/tubing to feed propane and pure oxygen for the flame, a coldfinger (Figure 3b), and a glass Dewar. On the coldfinger there is a copper rod whose long end is immersed in the cooling media (dry ice/acetone or liquid nitrogen) in the glass Dewar, while the short end holds the capillary or ampoule during the flame-sealing operation. The copper rod conducts heat from the capillary or ampoule to the D



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basis of heats of formation, the decomposition heat for a stoichiometric mixture should be −231 kJ/mol DTBP or −1580 J/g DTBP. Therefore, decomposition heat of 20% DTBP in toluene is calculated to be −315.9 J/g. Since heat measurement from DSC tests allows 10% uncertainty, the acceptable range of decomposition heat for 20% DTBP in toluene is between −285 and −347 J/g in our RC lab. 44 such acceptable DTBP DSC results done with glass capillary (Cap) and N2 headspace per our standard DSC test conditions were found in our testing database over the last 10 years. As shown in Figure 5, the average decomposition heat is −307.7 J/g with

1. Minimize the chance to have sample adhering to the internal surface around the sealing location of the capillary. Either liquid sample or solid sample should be introduced to the bottom of capillary/ampoule with micro syringes (Hamilton Syringes 1701N for liquid sample, Trajan SI-7 Solids Syringe 009980 for solid sample). Clean the internal surface with a Kimwipe or Paper Point as needed, since sample contamination at the sealing location of the capillary normally results in a poor quality of the final seal. 2. Control the purge gas flow rate under 3 cc/min. A high purge gas flow may remove high vapor pressure volatiles in a sample and change the sample composition. It is a good practice to wait several seconds after the capillary is positioned in the coldfinger and then insert the purge gas into capillary so that the sample has been frozen before purging. Our measurements (Figure 4) show that it takes ∼6 s to cool the capillary to −90 °C if liquid nitrogen is used and ∼10 s to −20 °C for dry ice. 3. Avoid overheating the capillary. If the flame is applied for too long during the flame sealing, it can cause the sample in the capillary to heat up and evaporate volatile contents or even cause thermal decomposition to occur. It is necessary to wait for the formation of white frost on the outside the capillary before applying flame (Figure 3c). Once the glass melts by the initial flame (∼3 s) and tweezers pull away the top glass piece, remove the flame from the capillary immediately. Wait for the white frost to reform on the capillary remaining in the coldfinger, and then flame polish the capillary by quickly passing the flame back and forth over the top of capillary a few more times. 4. Visually check the quality of the seal under microscope. Open orifice, crack, connected bubble trail, or any suspect seal should be discarded and sample preparation repeated. 5. Normally, the tested sample mass in a sealed capillary or ampoule is obtained by subtracting the previously recorded empty capillary mass from the mass of sealed capillary and the top glass piece that has been cut off by the flame. Note that loss of glass during the sealing of capillaries is rare, but can occur, which will result in an incorrect mass of the tested sample. Therefore, attention should be paid to avoid any loss of the glass during the flame sealing. After one discards the top glass piece, weigh the sealed container with sample and record mass. Compare it with the mass post DSC run to determine the mass loss of the tested sample during the DSC run.

Figure 5. Decomposition heat of 20% DTBP in toluene for 44 DSC run with glass capillary and N2 headspace.

a standard deviation of 16.1 J/g. This experimental average decomposition heat is in good agreement with the theoretical value, as only 2.60% error was calculated from the theoretical value. These 44 DSC results give an average extrapolated onset temperature of 164.7 °C with a standard deviation of 1.0 °C and a peak temperature of 192.5 °C with a standard deviation of 0.7 °C. An example of a N2 capillary DSC thermogram for 20% (wt) DTBP (Aldrich 98%) in toluene (Aldrich 99.8%) is shown in Figure 6. As mentioned above, the rescan curve serves as a great reference to draw the baseline for the peak integration of the first scan. On the basis of the separation and overlay of the first scan (solid curve) with the rescan (dot curve), it is easy to determine the integration limit. With the shape of the rescan, a linear baseline was selected for integrating the DSC test conducted using a capillary and N2 headspace. For comparison, as shown in Figure 6, a Fauske gold-plated DSC crucible (GP) was also run on the same instrument, using the same sample per our standard DSC test conditions. Inerting the headspace of the GP sample container without sealing it in a nitrogen box is difficult; therefore, the GP DSC test was performed with an air headspace. The oxidation reaction due to presence of oxygen in air headspace causes a small shoulder peak at ∼150 °C, which results in a slightly larger total heat (−316.49 vs −292.60 J/g). Also apparent is that the decomposition peak on the GP DSC has shifted slightly to a higher temperature. As a result, the extrapolated onset temperature and peak temperature of the decomposition peak with GP are higher than the typical values found using a glass capillary container. This peak shift may be the result of the DSC heat flow mode (T1 for GP vs T4P for capillary and Amp) and/or the increased mass of the sample container. In general, such a small peak shift can be considered negligible from a thermal stability hazard evaluation perspective. However, this cannot be ignored, if such a DSC peak is used

3. EXAMPLE OF DSC FOR THERMAL STABILITY HAZARD EVALUATION 3.1. Di-tert-butyl peroxide in Toluene. Di-tert-butyl peroxide (DTBP) in toluene is commonly used to check the DSC performance in our RC group. Since pure DTBP tends to degrade, even at ambient temperature, it should be always stored in a freezer. A degraded DTBP sample should not be used to verify the DSC performance. This may be one of the likely causes why a wide range of decomposition heats for DTBP (−171 to −332 kJ/mol13−15), measured by DSC, have been reported. Thermal decomposition of DTBP in toluene produces diphenyl ethane, methane, t-butanol, and acetone.16 On the E



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Figure 6. DSC thermograms for 20% DTBP in toluene; (blue) sealed glass capillary with nitrogen headspace; (green) Fauske gold-plated DSC crucible with air headspace; (red) sealed glass ampoule with nitrogen headspace.

Figure 7. DSC thermograms for 50% DTBP in toluene; (blue) sealed glass capillary with nitrogen headspace; (green) Fauske gold-plated DSC crucible with air headspace; (red) sealed glass ampoule with nitrogen headspace.

to fit the kinetic parameters of the decomposition reaction. Either signal deconvolution of the DSC peak or a much slower scan rate is necessary to obtain good kinetic parameters for samples tested with GP containers. A glass ampoule (Amp) containing the same 20% DTBP in toluene solution also underwent DSC testing on the same instrument per our standard DSC test conditions, as shown in Figure 6. The Amp container has a similar internal volume as the GP, but the sample size needs to be lower, since the rated max pressure is much lower. Generally, an Amp sample container is used to confirm whether there is an oxidation reaction of a sample by running the DSC with an air

headspace. Here, DSC of 20% DTBP in toluene was performed within an Amp container, but using a N2 headspace for comparisons. Unlike the other two sample containers, the use of an Amp container causes a significant difference between the first scan curve and the rescan curve prior to the DTBP decomposition peak. This difference is caused by the large headspace volume to sample volume ratio, which creates an endothermic effect from vaporization of the liquid sample, thereby influencing the DSC heat flow signal and causing a downward baseline shift. As the substances in the sample container change after the decomposition reaction, different vapor−liquid equilibrium behavior may introduce a significant F



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Figure 8. DSC thermograms for ethylene glycol with a sealed glass ampoule and air headspace.

difference between the first scan and the rescan. Since the rescan curve does not overlay well with the first scan before the decomposition reaction, the onset of the decomposition is not as clear as the other two sample containers; therefore, a σ-type baseline is used to integrate this peak. The decomposition heat is much lower in the Amp than the value obtained with Cap sample container (−242.24 vs −292.60 J/g). The cause of this difference can be uncertainty of the baseline, slow gas phase reaction for vaporized DTBP, and/or ongoing vaporization behavior of the mixture during the decomposition reaction. On the basis of those possible factors, an Amp is not preferred to run under N2 headspace but is acceptable to run with an air headspace to examine any oxidation reaction between a sample and air. 50% DTBP in toluene is another sample used to check the DSC instrument, resulting in a higher max heat flow and a larger decomposition heat. As shown in Figure 7, DSC tests of 50% DTBP in toluene were also conducted using Cap, GP, and Amp sample containers per our standard DSC test conditions. For all three sample containers, the decomposition heat of 50% DTBP is ∼2.5 times the 20% DTBP decomposition heats, as expected based on the concentration of DTBP, while the peak heights of 50% DTBP are all higher than that of 20% DTBP. The GP DSC again exhibits a small shoulder peak due to the oxidation reaction (sample sealed in air) and a slight peak shift toward higher temperatures. The Amp DSC also shows a difference between first scan and rescan for the temperature range before decomposition peak and a low decomposition heat. When examining the results of the DSCs and taking into account the economics of GP versus glass sample containers, our RC lab prefers using glass capillaries with N2 headspace for thermal stability hazard evaluation and glass ampoules with an air headspace to confirm whether there is an oxidation reaction in a sample. 3.2. Ethylene Glycol in Glass Ampoule with Air Headspace. Generally, glass ampoule containers are chosen to investigate the oxidative stability of a sample by running under an air headspace. The larger volume of air present in the ampoule ensures enough O2 is present to register a detectable DSC signal if the sample oxidizes, although there is a potential sacrifice to the baseline signal being altered by evaporative effects. For most organics, there is an oxidation peak in the temperature range from 150 to 250 °C. Normally, this oxidation reaction is limited by the amount of oxygen present inside the sealed container. On the basis of our experience, the

total heat of oxidation of an organic sample by running a DSC with glass ampoule and air headspace is ∼150 mJ. Ethylene glycol is a standard chemical selected by our lab to demonstrate oxidation stability by running a glass ampoule DSC under an air headspace. 57 ethylene glycol DSC results conducted in a glass ampoule and air headspace per our standard DSC test conditions were found in our testing database over the last 10 years. The average decomposition heat is −57.8 J/g with a standard deviation of 34.1 J/g and onset temperature of 220.5 °C with a standard deviation of 13.1 °C. As an example, a DSC thermogram of ethylene glycol (Aldrich 99%) is shown in Figure 8. Since the internal volume of an ampoule sample container is significantly larger than a capillary, the ampoule container suffers much more from the vaporizing endothermic effect as the temperature ramps up. Such an endothermic effect causes a significant negative baseline shift from 20 to 300 °C. This baseline drift terminates at ∼320 °C, because all the liquid has been vaporized, or sometimes the temperature reaches the critical temperature of the tested material. Since only a small part of the tested sample has been consumed by the oxidation reaction due to limited oxygen present, the rescan overlays reasonably well with the first scan. 3.3. Glass Reactive Chemicals. As a cardinal rule, glass reactive chemicals should not be tested in glass DSC sample containers. Most common glass is “silicate glass”, which is based on the chemical compound silica or silicon dioxide (SiO2). It is well-known that silica reacts with strong bases, such as NaOH and KOH.17 At ambient temperature, this reaction is slow or not favorable. However, at elevated temperatures, this reaction will be favorable and accelerate, especially for high concentrations of strong bases. DSC data in Table 2 show there is a significant heat release for a highly concentrated strong base solution by running in glass sample containers. Table 2. DSC Summary Data for Strong Bases in Glass Sample Containers

G

sample description

sample container and headspace

endo or exo

detected onset, °C

peak, °C

end, °C

heat, J/g

10% NaOH 50% NaOH

DSC in Cap with N2 DSC in Amp with Air

exo

75

180

220

−144

exo

125

200

268

−569



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Figure 9. DSC thermograms done with sealed Al pans; (blue) deionized (DI) water; (green) 20% DTBP in toluene; (red) 50% DTBP in toluene.

Figure 10. DSC thermograms run with Al pan and a pinhole lid; (blue) 20% DTBP in toluene; (green) 50% DTBP in toluene.

decomposition of certain peroxides. In addition, the rated max pressure of sealed Al pans is relatively low. A sealed pan tends to rupture at elevated temperatures for most liquid and organic samples. For instance, a TA Al T0 pan with a T0 hermetic lid is rated to hold ∼43.5 psi (3 bar) internal pressure, while the vapor pressure of pure water reaches that pressure at 144 °C. Assuming the rupture pressure is 2 times the rated pressure, the vapor pressure of water will cause a rupture of a sealed Al pan at a temperature slightly higher than 144 °C. This is confirmed in Figure 9, by a water sample run in an Al pan, which shows a pan rupture at 163 °C. When the DSC pan ruptures, the pressure suddenly decreases, and supersaturated water quickly vaporizes and escapes, which results in a large endothermic event or, even, a negative DSC sensor temperature drift. Normally, such a sharp, large endothermic event is caused by liquid vaporization and is easy to identify from other thermal activities. After the rupture, there is virtually no water left in the pan, and no further thermal activities are detected. The measured mass loss of water sample after the DSC run was 99.6%. DSC tests are also shown in Figure 9 with a sealed Al pan for samples of 20% and 50% DTBP in toluene that ruptured at ∼180 °C. With these two samples, the vapor pressure of the sample and the gas generation from the decomposition contribute to the overpressure of the Al pan. Since the sample containers rupture and most of the test sample escapes during the decomposition event, the DSC results do not provide a full understanding of the decomposition reaction, which could be incorrectly interpreted. Most organic samples will generate some pressure at elevated temperatures, because of vapor pressure and/or

Other chemicals that can react with glass in the temperature range of interest (0 to 400 °C) include hydrogen fluoride (HF) and anything that would produce a reactive fluoride species below 400 °C, such as ClF3, FOOF, etc. Silanes are another chemical class that can react with glass. For all glass reactive chemicals, a gold-plated DSC crucible should be used as the alternative sample container.

4. MISUSE OF ALUMINUM PAN FOR THERMAL STABILITY HAZARD EVALUATION Aluminum (Al) pan is a popular sample container that is widely used in DSC testing. For example, for determining polymer glass transition temperature (Tg), aluminum pans (crimped or uncrimped) have found wide acceptance, since the pans are cheap, easy to load, and chemically inert to the polymer, which has insignificant vapor pressure. Also, the degradation of polymers is typically well-above the temperature range of interest in Tg determinations. However, there are many times when researchers in our company do their own thermal stability hazard evaluation based on DSC tests with an aluminum pan in their local laboratories. Very often we found that this is an unacceptable approach. Extra caution is needed if an Al pan is used for thermal stability hazard evaluation. There are various reasons that Al pan may result in a misleading DSC thermogram. First of all, many materials are well-known to react with aluminum. These materials include oxidizers, acids, strong bases, and halogenated organics. Also, aluminum (like many other metals) may catalyze chemical reactions, for example, the H



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liquid nitrogen coldfinger to protect the sample has been successfully demonstrated. In addition, the importance of having a sealed sample and weighing before and after testing to check for mass loss is stressed. Results from DSC testing using our group’s method were also shown for 20% DTBP in toluene and ethylene glycol. The theoretical decomposition heat of 20% DTBP in toluene is calculated to be −315.9 J/g. Average heat of decomposition calculated from 44 DSC tests run in a glass capillary in N2 yielded a value of −306.6 J/g, only 2.3% error from the theoretical value. 20% DTBP in toluene was also run in gold pan and glass ampoule sample containers. The gold pan yielded acceptable results but also contained an oxidative “shoulder peak” and was slightly shifted to higher temperatures (onset was 3 °C higher). The glass ampoule in N2 did not yield acceptable results, caused by the large headspace volume to sample volume ratio, which creates an endothermic effect from vaporization of the liquid sample, thereby influencing the DSC heat flow signal and causing a downward baseline shift. However, the increased headspace in the ampoule provides a benefit when running samples under an oxidative atmosphere. Ethylene glycol in air was run in the glass ampoule, which showed an oxidative peak. Our RC group has shown that the total heat of oxidation of an organic sample by running a DSC with glass ampoule and air headspace is ∼150 mJ and generally occurs between 150 and 250 °C. The important features that need to be examined for selecting a sample container when conducting a DSC test were also discussed, stressing the ability of the container to withstand pressure and be chemically inert to the sample. One of the most commonly used pan types, aluminum pans, can have many issues. Mishaps of using aluminum pans and the shortcomings they pose from a thermal stability hazards analysis were presented, focusing on these two important features. The glass sample containers commonly used in our group are inert to most chemicals, but gold pan is used as a backup in cases where reactivity is a concern (strong bases and fluorinated compounds). All this practical DSC testing information provides insight into our group’s ability to work on a diverse array of samples and generate quality data for understanding the thermal stability hazards present within our company.

decomposition products, as exhibited by the DTBP in toluene samples. Unfortunately, many (probably the majority) of process streams in a chemical synthesis company contain some organic chemicals. The use of Al pan in DSC testing for organic samples is unreliable and should be avoided for thermal stability hazard evaluation. Moreover, a rupture of sample containers inside the DSC instrument may cause damage to the DSC sensor, which is not desired for long-term reliability of the instrument. Even if there is no damage to the sensor, a rupture of sample containers still causes DSC downtime to clean and recalibrate the instrument. Open Al pans or Al pans with a pinhole lid are also commonly used for DSC testing, which can avoid DSC sensor damage by mitigating the container rupture. However, on the basis of our experience, DSC data generated by using unsealed pans is typically not suitable for thermal stability hazard evaluation. For liquid samples or solid samples above their melting point, the endothermic effect from vaporization may cancel decomposition exothermic events, to some degree, and will eventually dominate the heat flux signal as the test approaches the sample’s boiling point. Above the boiling point, the sample will quickly vaporize, until there is no sample remaining. As a result, no information about sample stability will be recorded in the DSC test postboiling point. As an example, DSC tests of 20% and 50% DTBP in toluene were performed in an Al pan with a pinhole lid, shown in Figure 10. There are big endothermic events around the boiling points of the mixtures, but no decomposition peak is detected by DSC test in contrast to the glass capillary testing. This can be extremely dangerous, if one concludes that no thermal stability hazards are present for these samples based on these DSC results done with an unsealed Al pan. Some mixtures do not even exhibit such large endothermic behavior, since there may only be partial vaporizing of the sample. The endothermic event from partial vaporization can cancel decomposition exothermic event(s), and thus the DSC may record a relatively flat thermogram. In this example, the DSC results could easily mislead the investigator to the conclusion that no thermal stability concern exists. Even if there is no decomposition during the partial vaporization of a sample, after the light compound(s) boils off, the reaction rate of any decomposition of the remaining compounds may be significantly affected because of the concentration change and alteration in the reaction mechanism. Because of those reasons, DSC data generated by using unsealed pans should be avoided for the purpose of thermal stability hazard evaluation.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +01 989-638-2206. E-mail: [email protected].

5. CONCLUSIONS DSC is an important tool used in conducting thermal hazard evaluation on materials to determine safe operating limits for materials in multiple industries. Despite DSC being a widely used tool, there are still misuses in the methodology or misinterpretation of data. For successful DSC testing, good habits with instrument maintenance need to be adapted, which has been utilized in our RC group for years. Routine calibrations should be performed on the DSC instrument to ensure the device is registering heat gains and losses properly. In addition, in the instance of sample leakage or a container rupture, cleaning of the device and conducting a root cause investigation to avoid those mishaps in the future to increase the longevity and not deteriorate the performance of the DSC is paramount. Our RC group’s unique sample preparation methodology of flame-sealing glass sample containers using a

ORCID

Min Sheng: 0000-0002-5321-2627 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Reactive Chemicals Group in The Dow Chemical Company (their heritage company) for sharing of testing methodologies and historical data.

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REFERENCES

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Practical Use of Differential Scanning Calorimetry for Thermal Stability

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