Modern Thermogravimetry - Analytical Chemistry (ACS Publications)

Instrumentation Charles M. Earnest Perkin-Elmer Corporation Norwalk, Conn. 06856

Modern Thermogravimetry Although early activity in the devel opment of the thermogravimetric method began as long ago as 1903 (1, 2), thermogravimetry (TG) was slow coming into use as an analytical tool in some countries; serious work did not begin in the U.S. until the 1950s. The man who had the greatest impact on modern analytical thermogravimetry was C. Duval, a Frenchman who stud ied more than 1000 analytical precipi tates using the first commercial ther mogravimetric system, the Chevnard thermobalance. Recent years have seen major advances in this analytical technique through improved furnace technology, microcomputer control, and microcomputer data reduction. Today thermogravimetry is an impor tant analytical tool in the various branches of chemistry, geology, fuel science, ceramics, and material science. TG is a technique in which the mass of a substance is monitored as a func tion of temperature or time as the sample specimen is subjected to a con trolled temperature program in a con trolled atmosphere. Thermogravime try can be considered the modern-day frontier of the much older and wellestablished technique of gravimetry. TG and gravimetry differ in that in TG the sample mass is continuously monitored as it is heated; in gravime try, the sample is weighed after ex tended periods of isothermal heating. Conventional TG uses much smaller samples (milligram quantities) than those encountered in gravimetry (gram quantities). In TG, only those events associated with a change in mass will be ob served. Therefore, other thermal anal 0003-2700/84/A351-1471$01.50/0 © 1984 American Chemical Society

ysis techniques are necessary to study those physical phenomena, such as melting, crystallization, and glass transitions, that do not lead to a change in mass. Differential thermal analysis (DTA) is a technique in which the temperature difference (ΔΤ) between a sample and reference material subjected to a controlled temperature program is measured as a function of temperature. A closely re lated technique, differential scanning calorimetry (DSC), measures the dif ference in the rate of heat flow to (or from) the sample and reference mate rial as a function of either tempera ture or time. In thermomechanical analysis (TMA), either the dimension or deformation of a substance under nonoscillatory load is measured as a function of temperature. Dynamic me chanical analysis (DMA), or dynamic thermomechanometry, measures the dynamic modulus and/or damping of a substance under oscillatory load as a function of temperature. The fundamental definitions for all of the thermal analysis techniques mentioned here state that the mea surement is made while the specimen is subjected to a controlled tempera ture program. One might be misled by this commonality to think that in thermogravimetry the sample temper ature is always being elevated in a continuous fashion at a regular pro grammed rate. Although this is often the case, many important thermo gravimetric procedures involve iso thermal monitoring of the mass loss from a material. Thus, when one or more isothermal holds are involved in the total heating program, the mass loss is often monitored vs. time. One ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 · 1471 A

Figure 1. TG thermal curve with extrapolated onset temperature assignment for the decomposition of polytetrafluoroethylene

can see that it is with the isothermal thermogravimetric procedures that the closest kinship to conventional gravimetry lies. The technique of thermogravimetry is often referred to as thermogravimetric analysis or TGA. The name thermogravimetry and its abbreviated symbol TG were given to this technique by a nomenclature committee of the International Confederation for Thermal Analysis (ICTA) (3-5). This committee has made an effort to remove the term analysis from as many thermal methods as possible. The A in DTA remains because DTA is such an old and established technique. One reason the removal of the A from TGA has not been accepted by some is the possible confusion of TG with Tg, a glass transition commonly studied by the thermal analysis techniques of DSC, TMA, and DMA. This has caused problems in computerized literature searches in which the computer does not distinguish between TG and Tg. This committee also eliminated the use of thermogram in favor of thermal curve to refer to the graphical presentation of thermal analysis data. Figure 1 shows a typical TG thermal curve that gives the weight percent of the original sample vs. temperature for a polytetrafluoroethylene (PTFE) specimen that was heated in dynamic nitrogen at 20 °C/min. The two major items of information one obtains from such a curve are the extrapolated onset temperature (T onset ) and the percentage weight loss assignments. The data handling shown in Figure 1 describes the determination of the Tonset (606.7 °C) for the decomposition of this P T F E specimen. Since this

thermal curve was obtained in a dynamic inert atmosphere, the TOJ]8et is a measure of the thermal stability of the PTFE. One also notes that all of the original P T F E specimen is converted to volatile decomposition products. Organic polymers exhibiting this behavior may be quantified in the presence of thermally stable additives or fillers, such as carbon black and glass, by such pyrolytic separation. Instrumentation

To perform experimental thermogravimetry, the instrument must be capable of both simultaneous heating

and weighing. Therefore, the instrument used for performing thermogravimetric measurements is often referred to as a thermobalance. This term originated with the early work of Honda (6), who described an apparatus with which he obtained TG curves for MnS0 4 -H 2 0, CaC0 3 , and CrC0 3 . The major instrumentation requirements for performing thermogravimetry include a sensitive recording microbalance; a furnace and an appropriate enclosure for heating the sample specimen; a temperature programmer, heater control circuitry, and associated electronics; a pneumatic system for dynamic purging of the furnace/ sample chamber; a data acquisition system; a purge gas switching device; and first-derivative capability. The latter two items are optional: The purge gas switching capability is needed only for those applications in which the purge gas is changed during the experiment, and the first-derivative computer is often eliminated when computerized data-handling systems are used. Many commercial TG systems are available. The major differences in these are in the furnaces (size, design, and positioning), degree of microcomputer control of the hardware, and in the capabilities of the data acquisition systems offered with the thermal analysis system. There is also some variation in the maximum balance sensitivity used in these systems. Figure 2 shows a TG thermal curve obtained for the thermal decomposition of nylon 6,6 at a constant heating rate of 20 °C/min. In many cases, the first-derivative trace of the weight loss profile is presented along with the TG

Figure 2. TG and DTG thermal curves for the decomposition of nylon 6,6 in dynamic nitrogen atmosphere

1472 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984

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Figure 3. Thermal curve (a) and derivative thermal curve (b) for selected Curie point standards (alumel, 163 °C; nickel, 354 °C; perkalloy, 596 °C; and iron, 780 °C)

thermal curve. This first-derivative trace corresponds to the technique of derivative thermogravimetry (DTG), the resulting DTG thermal curve representing the rate of mass loss vs. temperature. The DTG ordinate has units of mass per unit time (mg/min). The DTG thermal curve may be generated simultaneously with the TG thermal curve by electronic means. This may be achieved by a simple R-C circuit or by the more modern approach of an operational amplifier with active filtering by field effect transistors to eliminate or minimize noise. When a computer is used as the data-handling device for thermogravimetric studies, the DTG curve may be generated, after the TG data are stored or received, by a mathematical algorithm

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in the software. The TG and DTG thermal curves presented in Figure 2 for nylon 6,6 represent a hard-copy printout from a computerized thermogravimetric system. Thermal curves obtained from thermogravimetry offer what is possibly the most accurate ordinate scale (weight or weight percent) of all thermal analysis techniques. The temperature axis, on the other hand, is often less well defined. There are several problems that complicate the calibration of the TG temperature axis. First, since the thermocouple is generally close to but not in contact with the sample during analysis, the actual temperature of the sample is not being measured. The composition and flow rate of

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the dynamic gaseous atmosphere sur rounding the TG sample also influ ence the sample temperature. The composition of the purge gas is chosen to satisfy the need of the application or analysis. For those analyses made in inert atmospheres, such as pyrolysis, either nitrogen or argon is general ly used. Characterizations or analyses requiring oxidative environments are performed in dynamic air or dynamic oxygen atmospheres. As was pointed out years ago by Gray (7), an additional factor that can influence the true sample temperature in thermogravimetry is the effect of radiation. As temperatures in excess of 500 °C are reached in a thermogravimetric apparatus, radiation pro gressively becomes the dominant mechanism for heat transfer. When using an open-cup-type TG sample pan at these temperatures, radiation to and from the surface of the sample can become a significant factor. Thus, a black sample specimen such as coal can be at a different temperature than a white sample such as calcium car bonate or calcium oxide, all other con ditions being the same. The best approach to calibrating the actual sample pan temperature has been through the use of ferromag netic materials that exhibit known Curie point transitions. In this case, small pieces of the ferromagnetic metal (or metal alloy) are placed in the TG sample pan, and the actual mass of materials is either electroni cally tared or subtracted using a com puter software algorithm. A small per manent magnet is then placed either above or below the TG sample pan. The vertical component of the mag netic field causes the balance to read either more or less depending on the position of the magnet. When the magnet is placed below the TG sample pan, the magnetic force on the sample acts as an equivalent magnetic mass on the balance beam to increase the apparent sample weight. When heat ed, the magnetic domains in ferromag netic materials become disoriented and transform to the paramagnetic state at characteristic temperatures (Curie points). At these temperatures, the TG thermal curve will show an ap parent weight loss due to the loss of magnetic mass. Figure 3 shows the thermal curves obtained by the author using several Curie point standards. Since the Curie point transitions are reversible, the same standards may be reheated nu merous times for the purpose of tem perature calibration. Furthermore, in cases where one wants to minimize the effects of radiation, the standards may be buried in a specimen of the sample material for calibration purposes. This type of calibration procedure is

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Figure 4. Block diagram for a microcomputer-controlled thermal analysis system

ideally suited for TG instruments that are constructed with a microfurnace inside the furnace tube, since a small horseshoe magnet may be placed around the narrow furnace tube. However, for those systems using larger furnaces external to the furnace tube, the positioning of the permanent magnet becomes much more awkward. An alternate approach, which uses a fusible link of a melting-point standard and seems to show some promise for temperature calibration of those instruments using external furnaces, has recently been published (8, 9). Microcomputer controllers

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The first step in the computerization of thermal analysis instrumenta-

tion was the introduction of microcomputer-based controllers in 1978. These controllers made possible multistep heating programs, automatic purge gas switching, methods storage and recall, and completely unattended operation. The precision of the power delivery to the furnace has also been greatly enhanced by the controllers. One such system includes an automatic temperature calibration routine, in which the programmer temperature and the actual thermocouple temperature are matched at three points along the total heating ramp. This has been extended to a nine-point matching program in one of the more recent commercial systems. Figure 4 shows a basic block di-

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Figure 5. Microcomputer-controlled TG proximate analysis of a bituminous coal specimen



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agram of a microcomputer-controlled thermal analysis system. The microcomputer controller is in communication with the heater control circuitry to control the voltage ramp to the furnace and is in direct communication with the thermal analyzer to continuously monitor the sample temperature. The emf-vs.-temperature table is contained in read-only memory (ROM) in the microcomputer controller for the particular thermocouple being used with the analyzer. The emf signal is converted to temperature and displayed on the microcomputer controller screen. More than one set of firmware components may be included in the controller so that more than one type of analyzer (DSC, TMA, etc.) can be used with the same controller. The analytical temperature program is developed and entered into random access memory (RAM) through a keyboard. Error messages, actual temperature, etc., are communicated to the user via the screen display. Figure 5 shows a multistep TG analysis with purge gas switching that can be performed automatically with a microcomputer controller. In this rapid proximate analysis procedure, a bituminous coal specimen is rapidly heated to 110 °C under flowing nitrogen and held for 5 min until all of the moisture is lost. This isothermal step is followed by dynamic heating of the sample at 100 °C/min to 950 °C; the temperature is then held isothermally until all of the volatile matter has been liberated. At this point, the purge gas is automatically switched from nitrogen to oxygen through the side arm of a two-arm furnace tube through the use of a gas-switching accessory controlled by the microcomputer. As a result, the final weight loss step assigns the percentage of fixed carbon for the coal specimen, and the remaining weight in the pan is read directly as the ash value. The system automatically cools down on completion of the heating program, and the purge gas is automatically changed back to dynamic nitrogen. Analyses such as this required total operator attention prior to the advent of microcomputer controllers. For details of the rapid proximate analysis of coals, see Reference 10. Other examples of TG multistep analyses include the compositional analysis of rubber and elastomer formulations, lube oils, and polymers filled with carbon black. The appropriate analytical heat-hold programs may be developed for these analyses and stored in the memory of the controller along with the valve times for purge gas switching. The box on p. 1482 A lists a number of applications that can be performed by thermogravimetry. For more de-

Some applications of thermogravimetry Environmental stability • Thermal stability in inert atmospheres • Oxidative stability Compositional analysis • Rubber and elastomer formulations • Proximate analysis of coals and coal products • Lubricants and lube oils • Polyethylene/carbon black Moisture determinations Residual solvent determinations Ash and inert filler determinations Characterization and analysis of clays and minerals, oil shales, tar sands, etc.

tails, consult recent literature reviews in this JOURNAL (11), selected reference books (12,13), and instrument manufacturers' literature. Microcomputer data acquisition

Until recently, potentiometric recorders were used with thermal analysis equipment. These varied in type from time base strip chart recorders to dual-pen XY1Y2 recorders. Due to the ease and speed of microcomputer data systems, the potentiometric recorder is rapidly disappearing from the thermal analysis laboratory. Commercially available instruments containing either built-in or stand-alone microcomputer data acquisition, reduction, and storage became available as early as 1979. There are presently at least six different stand-alone computers and one intelligent recorder that are marketed with, or for use with, thermal analysis equipment. A wide variety of software packages are available with these data systems, easing tasks that were once time-consuming. Among these, differences in both computer hardware and software capabilities exist. These include the presence or absence of a CRT screen, whether or not the data system is operator programmable, use of floppydisk and/or hard-disk storage, and whether or not the system is capable of multi-instrument operation. Most of these differences depend on the size of the central processing unit (CPU) and operating system used. The major advantage of a CRT screen is the ability to optimize the data and perform data analysis on the screen prior to obtaining a hard copy of the results. Generally speaking, in commercial systems, the raw data are smoothed by boxcar averaging. This means that

Microdistillation of petroleum and syncrude fractions Sublimation Plasticizer content of polymers Sorption studies Catalytic activity Decomposition kinetics Flame retardancy of polymer additives Vacuum TG studies Process simulation Loss on ignition of industrial raw materials Establishment of stoichiometries of dehydration Characterization of cements

each data point saved in the data set is a result of the averaging of many raw data points. One of the greatest advantages that accrues from the use of these systems is that the operator will seldom, if ever, have to repeat a thermal analysis experiment because of improper choice of ordinate scale sensitivity. In all of the commercial software packages, the operator may rescale the ordinate axis (as well as the temperature axis) after the data for the entire analysis have been received by the CPU. In some commercial systems, the ordinate axis is automatically rescaled upon receipt of the last data point for

the run. To estimate the time saving, one can calculate that a TG characterization run conducted at 10 °C/min from ambient to 1025 °C consumes 100 min, and the rerun of such an experiment would require a minimum of 2 h if the cool-down time and sample reloading time are included. Another important feature of these data stations is storage of the original data in a file for recall at a later date. The concept of multitasking

It has recently become possible to operate and take data from more than one thermal analyzer both simultaneously and independently. Figure 6 is a block diagram of a system that is capable of multi-instrument operation. This capability depends not only on the size of the CPU but also on the operating system used. As can be seen in the diagram, the CPU undergoes two-way communication with two separate intelligent controllers via RS-232C communication interfaces and data buses, each of which controls a thermal analyzer. These intelligent instrument controllers should not be mistaken for simple communication interfaces; they are microcomputer-based systems themselves. The thermal analyzer units, in this particular case, may be two TG units, two DSCs, or one of each. This system uses the concept of information windows on the video display unit (VDU) to communicate the progress of an analysis being performed by the second analyzer while the other analysis is being monitored. Such a totally computerized system has essentially eliminated control knobs from ther-

Figure 6. A computerized thermal analysis system with multi-instrument operating capability


Figure 7. Coupling of TG systems for EGA or EGD

mal analyzers. All control parameters, temperature calibration, etc., are entered from the keyboard.

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EGD and EGA

The analysis of the dynamic purge gas exit stream of DTA, DSC, and TG analyzers has become increasingly popular in recent years. The concept of evolved gas detection (EGD) differs from evolved gas analysis (EGA) in that in the former the absolute identity of the component or components in the exit gas stream need not be determined. EGD and EGA are used for the simulation of many industrial processes in which the materials are heated, for establishing decomposition mechanisms, and for determining the stoichiometric relationships for such decompositions. Figure 7 shows a simplified block diagram for performing either EGD or EGA by coupling an appropriate analyzer to the thermogravimetric system. As in any of the hyphenated analytical techniques, the coupling interface often requires careful attention and consideration in design and implementation. Among the most popular and powerful combinations are T G -

mass spectrometry (TG-MS) and TG-tandem mass spectrometry (TG-MS-MS). In these combined systems, the evolved gas from the TG furnace may be sampled with a capillary or a jet separator. In most cases, a heated line is needed in the coupling to avoid condensation of volatile matter. This need is minimized by keeping the distance between the TG exit system and the analyzer small. Two recent reviews (14,15) contain details of the TG-MS technique. The schematic given in Figure 8 shows the interfacing of a TG furnace tube to a commercial triple quadrupole mass spectrometer that utilizes an atmospheric pressure chemical ionization (APCI) ion source. This T G MS-MS interface has been used (16) with both internal microfurnace TG systems as well as those employing external furnaces. The ion source uses air as the chemical ionization reagent. Molecular species from the TG effluent enter the ion source from the heated transfer line and are converted to their quasi-molecular or molecular ions via well-characterized APCI reactions. The ions are then focused into the high-vacuum analyzer portion for

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Figure 8. A TG-MS-MS interface (from Reference 16, with permission)



MS or MS-MS analysis. One of the best examples of EGD is the use of a flame ionization detector (FID) in the TG analysis and charac terization of source rock materials. For example, the Green River oil shales of the western U.S. contain a sizable carbonate component. When performing TG pyrolysis studies on these shales one may follow the pyrol ysis, particularly at temperatures of 500 °C or greater, by connecting the TG dynamic exit stream to an FID. In this case, the volatile organic pyrolysis products may be monitored vs. either sample temperature or time, and the carbon dioxide due to decomposition of the inorganic carbonate component will be ignored by the FID. The concept of EGD and EGA is one of the most active areas in thermal analysis. There are some who feel that when studying new materials TG and DTG thermal curves represent incom plete data without the inclusion of EGA. Quantitation of sample compo nents by conventional TG may be made only if a stoichiometric relation ship exists between the gaseous de composition products and the original material. Obviously, quantitation is much simpler when the component is volatilized without decomposition, such as in the assignment of moisture content or residual solvent.

References (1) Nernst, W.; Riesenfeld, Ε. Η. Ber. Dtsch. Chem. Ges. 1903,36, 2086. (2) Wendlandt, W. In "A History of Ana lytical Chemistry"; Laitinen, Η. Α.; Ewing, G. W., Eds.; American Chemical Society: Washington, D.C., 1977; p. 48. (3) MacKenzie, R. C. J. Thermal. Anal. 1975,8,197. (4) MacKenzie, R. C. Thermochim. Acta 1979,28, 1-6. (5) Lombardi, G. "For Better Thermal Analysis," 2nd éd.; International Confederation for Thermal Analysis: Rome, 1980. (6) Honda, K. Sci. Rept. Tohoku Univ. 1915,4,97. (7) Norem, S. D.; O'Neill, M. J.; Gray, A. P. Thermochim. Acta 1970,1, 29. (8) McGhie, A. R. Anal. Chem. 1983,55, 987. (9) McGhie, A. R. et al. Thermochim. Acta 1983 67 241 (10) Earnest, C. M.; Fyans, R. L. In "Thermal Analysis, Proc. of the 7th International Conference on Thermal Analysis"; Miller, B., Ed.; Wiley-Heyden: New York, N.Y., 1983; Vol. 2, pp. 1260-69. (11) Wendlandt, W. Anal. Chem. 1984,56, 250-61R. (12) Wendlandt, W. "Thermal Methods of Analysis," 2nd éd.; Wiley-Interscience: New York, N.Y., 1974. (13) Turi, E. "Thermal Characterization of Polymeric Materials," Academic Press: New York, N.Y., 1981. (14) Dollimore, D.; Gamlen, G. Α.; Taylor, T. J. Thermochim. Acta 1984, 75, 59. (15) Holdiness, M. R. Thermochim. Acta 1984, 75, 361. (16) Sushan, B.; Davidson, B.; Prime, R. B. In "Analytical Calorimetry"; Johnson,

J. F.; Gill, P. S., Eds. Plenum Press: New York, N.Y., 1984; pp. 105-11.

Charles M. Earnest is senior re searcher for the Thermal and Ele mental Analysis Department at the Perkin-Elmer Corporation, Norwalk, Conn. Prior to joining Perkin-Elmer in 1978, he was associate professor of chemistry at Northeast Louisiana University, Monroe, La., and also served on the faculties of the Univer sity of Pittsburgh at Johnstown and Stillman College. He received both his BS in chemistry and his PhD in analytical chemistry from the Uni versity of Alabama. Earnest's re search efforts have been in energy and environmental analyses, materi als characterization, and in various aspects of chromatographic science.

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Modern Thermogravimetry - Analytical Chemistry (ACS Publications)

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