Thermogravimetric analysis (TGA) is widely used together with DSC, TMA, and DMA. TGA measures the mass of a sample while the sample is heated or cooled in a defined atmosphere. The main use of TGA is to characterize materials with regard to their composition.
A TGA/DSC instrument even allows you measure thermal events that do not produce a mass change such as melting, glass transitions, or other solid-solid transitions.
In this Webinar, we will discuss the basic principles of TGA/DSC and present some interesting applications.
The Webinar covers the following topics:
- Principles of TGA
- TGA/DSC 1
- Measurement possibilities
- Why use TGA?
- Industries and applications
The TGA technique measures the mass of a sample as it is heated, cooled or held at a constant temperature in a defined atmosphere. Thermogravimetric analysis (TGA) is ideal for characterizing the thermal properties of materials such as plastics, elastomers and thermosets, mineral compounds and ceramics as well as for chemical and pharmaceutical products.
TGA/DSC and hyphenated techniques
TGA is often used together with Differential Scanning Calorimetry (DSC) because the two techniques provide complementary information, which often facilitates the interpretation of a thermal analysis experiment.
The gaseous products that are evolved can be analyzed using hyphenated techniques, for example by coupling a Fourier transform infrared spectrometer (FTIR) or a mass spectrometer (MS) to the thermobalance of the TGA instrument.
The properties and behavior that can be measured by the TGA technique include composition, purity, decomposition reactions, decomposition temperatures, and absorbed moisture content.
This webinar presents several different application examples that demonstrate the analytical power and versatility of TGA/DSC.
Thermogravimetric Analysis (TGA)
Slide 0: Thermogravimetric Analysis (TGA)
Ladies and Gentlemen
Welcome to this seminar on Thermogravimetric Analysis – or TGA as it is usually called.
TGA is a technique frequently used in thermal analysis. Its main use is to characterize materials by measuring their change in mass as a function of temperature.
The properties and behavior that can be measured by TGA include composition, purity, decomposition reactions, decomposition temperatures, and absorbed moisture content.
TGA is often used together with Differential Scanning Calorimetry (DSC) because the two techniques provide complementary information which often facilitates the interpretation of a thermal analysis experiment.
Slide 1: Contents
In the course of the webinar, I would first like to explain the basic principles of TGAand DSC and then describe the METTLER TOLEDO TGA/DSC 1, a high-performance TGA instrument that can simultaneously perform TGA and DSC measurements on the same sample.
I also want to point out a number of important design features and options.
Finally, I will present several practical examples to illustrate different application possibilities that the TGA/DSC 1 instrument offers.
Slide 2: Principles of TGA
Thermogravimetric Analysis measures the mass of a sample as it is heated, cooled or held at a constant temperature in a defined atmosphere.
The picture on the left shows a match that has just been ignited. The burning process produces gaseous combustion products that are released into the atmosphere. The mass of the match steadily decreases finally leaving just ashes behind.
The burning process of a match can easily be measured by TGA. A typical mass loss curve of a polymer is shown on the right. The different steps are numbered one to five next to the curve:
One is the loss of volatile components such as moisture, solvents, and monomers;
Three, the atmosphere is switched from nitrogen to oxygen;
Four, combustion of carbon;
Five, inert inorganic residue of ash, fillers, or glass fibers.
The gaseous products that are evolved can be analyzed using hyphenated techniques, for example by coupling a Fourier transform infrared spectrometer (FTIR) or a mass spectrometer (MS) to the thermobalance of the TGA instrument.
The curve in the diagram shows only the mass loss. In fact, much more information can be obtained from a single measurement.
I will explain how this is achieved with the aid of the next slide.
Slide 3: Principles of TGA
We see a schematic view of the TGA/DSC 1 instrument.
It combines a special furnace shown in the middle and a highly sensitive balance on the right. In the TGA/DSC 1, the scale pan is replaced by a DSC sensor.
The balance cell on the right is thermostated to minimize environmental influences. The balance beam is connected to the TGA/DSC sensor. The support for the sample and reference crucibles is located in the middle of the furnace.
The horizontal design of the furnace helps to minimize possible turbulence due to thermal buoyancy and the purge gas flow.
The furnace temperature is controlled by the temperature sensor located below the furnace. The furnace is shielded on both sides by baffles and the entire volume is purged with a constant flow of gas. In addition, the sample can be purged with a reactive gas that flows through a capillary located near the sample crucible.
The volatile and gaseous combustion products from the sample as well as the purge gas and reactive gas leave the furnace through the gas outlet on the left. Analytical instruments can be connected to this outlet for evolved gas analysis.
Slide 4: Principles of DSC
I now want to look at the TGA/DSC sensors in more detail.
One of these sensors is shown in the bottom left corner of the slide.
The schematic diagram above displays a sensor with sample and reference crucibles. The temperature difference between the sample and the reference is measured by means of thermocouples located directly below the crucible support. The signal is calibrated and adjusted to produce a heat flow signal. The resulting signal is analyzed in the same way as for DSC.
The schematic curve in the lower right corner of the slide shows a typical melting peak. The curve provides valuable information such as the enthalpy of melting, the melting point, and the specific heat capacity.
As in a dedicated DSC instrument, the heat flow is calibrated and adjusted at different temperatures using certified reference materials.
Slide 5: TGA/DSC 1 Curves
The TGA/DSC 1 generates three different curves namely the weight loss curve, the derivative of the weight loss or DTG curve, and the heat flow curve.
The example shows the three curves obtained from the analysis of a sample of polyethylene terephthalate.
Slide 6: TGA/DSC 1: Sensors
The TGA/DSC 1 can be equipped with three different sensors:
The TGA/DSC sensor has six thermocouples located directly below a protective ceramic support which measure the sample and reference temperatures.
The TGA/DTA sensor has two thermocouples that measure the sample and the reference temperatures. The support is made of platinum. The differential measurement improves the signal-to-noise performance of the sensor compared to that of a sensor with only one thermocouple.
The TGA/SDTA sensor consists of a platinum support with a thermocouple that measures the sample temperature. A modeled reference temperature is used for the DSC signal.
All three sensors generate a DSC signal which provides additional information that is otherwise not revealed by TGA alone (for example solid-solid phase transitions). The DSC signal is generally less sensitive than the signal from a dedicated DSC instrument.
Slide 7: TGA/DSC 1: Balance
The TGA/DSC 1 uses a top-of-the-line METTLER TOLEDO ultra-micro balance to measure sample weight. The balance is unique in every respect:
It provides ultra-microgram resolution over the whole weighing range and measures up to 50 million resolution points continuously. This means that changes in mass of a five-milligram (5-g) sample are determined to zero-point-one microgram.
The balance uses automatic internal adjustment. External adjustment with certificated weights is also possible.
And finally, samples can be weighed in semi- or fully automatically, or using an external balance.
Slide 8: TGA/DSC 1: Crucibles
The type of crucible and the material used are very important for achieving optimum results. METTLER TOLEDO offers different types of crucibles for different sorts of samples and experiments. Some of the most commonly used crucibles are displayed in the slide.
The standard TGA alumina crucibles are available in different sizes. These are the most frequently used crucibles and can be reused after cleaning.
Platinum crucibles are used to achieve a high-quality DSC signal.
Aluminum crucibles are cheap and can be used for temperatures up to about six hundred degrees Celsius (600 °C).
Sapphire crucibles are used to measure liquid metals at high temperatures.
Other crucibles may also be used, depending on the sample and measurement requirements.
Crucibles are normally used without a lid. For applications that require a self-generated atmosphere, the crucible should be closed by a lid with a small hole.
Special aluminum lids are available to prevent contamination and evaporation before the measurement.
Slide 9: TGA/DSC 1: Options
The TGA/DSC 1 can be fitted with optional accessories for specific applications:
The upper part the slide shows three different versions of the TGA/DSC 1.
The standard version includes the SmartSens terminal shown on the top right-hand-side of all three instruments.
The picture in the middle shows an instrument fitted with a sample robot. The robot can process up to 34 samples even if every sample requires a different method and a different crucible.
In the picture on the right, we see a TGA/DSC 1 equipped with gas box. This can automatically switch, monitor and control gas flows and switch from an inert gas to a reactive gas atmosphere during a measurement.
The lower part of the slide shows different interfaces used for hyphenated techniques, namely, mass spectrometry (MS), Fourier transform infrared spectroscopy (FTIR), and Sorption.
All TGA/DSC 1 instruments can be connected online to a mass spectrometer or a Fourier transform infrared spectrometer to analyze gaseous decomposition products. This technique is known as Evolved Gas Analysis and yields additional information about the sample and so enables you to interpret measurements curves with greater certainty.
A humidity generator can be attached to the TGA/DSC 1 equipped with a large furnace in just a few minutes. This option allows materials to be analyzed under precisely defined conditions of relative humidity and temperature.
Slide 10: Measurement Possibilities
The next slide shows some of the different measurements possibilities that the TGA/DSC technique offers:
TGA/DSC measurements can be performed dynamically using a linear temperature ramp or isothermally.
Temperature ramps are used to investigate temperature-dependent processes such as the loss of moisture, composition, and chemical reactions.
Isothermal measurements are mainly used to determine the oxidation induction time of materials or to study the release or absorption of moisture.
The atmosphere is often switched from inert to oxidative to burn the carbon black and determine the ash or filler content.
Measurements under reduced pressure or vacuum are employed to separate overlapping effects of vaporization and decomposition.
The simultaneously measured DSC heat flow signal records exothermic and endothermic events such as the glass transition, melting, crystallization, chemical reaction and phase transitions.
In the following slides, I want to show examples of each of these four measurement possibilities.
Slide 11: Measurement Possibilities - Temperature ramp
Let me begin with the temperature ramp.
In this example, eleven milligrams (11 mg) of calcium oxalate monohydrate was heated at thirty Kelvin per minute (30 K/min) in a nitrogen atmosphere. The weight loss curve shows three well-separated steps that correspond to the stoichiometric decomposition reactions of the substance.
The first step is due to the loss of water of crystallization.
The second step is due to decomposition with the formation of calcium carbonate and release of carbon monoxide;
The final step is also caused by a decomposition reaction involving the formation of calcium oxide and the liberation of carbon dioxide.
Slide 12: Measurement Possibilities – Isothermal
Here we see an example of an isothermal measurement.
Zeolites that have been preconditioned with a defined amount of moisture are used to check and adjust the balances in infrared drying instruments. The amount of moisture can be determined in an isothermal measurement using the TGA/DSC 1.
The measurement was performed at one hundred degrees Celsius (100 °C) in a thirty-microliter (30-mL) alumina crucible. After 400 minutes the weight is stable. The mass loss is approximately eight percent (8%).
Slide 13: Measurement Possibilities – Atmosphere
The next example illustrates the use of gas switching at six hundred degrees Celsius (600 °C).
The upper diagram shows a sample of rubber that was first heated to six hundred degrees under inert conditions. In this temperature range, volatile liquid components such as plasticizers vaporize. This is followed by pyrolysis of the polymer shortly afterward at about four hundred degrees (400 °C).
At six hundred degrees (600 °C), the atmosphere was switched from nitrogen to oxygen, which results in combustion of the carbon black additive. Inorganic components remain behind as a residue at eight hundred degrees (800 °C).
Evaluation of the TGA curve gave the following sample composition:
six-point-four percent (6.4%) plasticizer,
sixty-eight-point-two percent (68.2%) polymer,
and twenty-one-point-eight percent (21.8%) carbon black,
and a residue (mainly zinc oxide) of three-point-six percent (3.6%).
The lower curve in the diagram is the DTG curve. In this case, the curve was used to set the limits for the overlapping effects of vaporization and decomposition.
Slide 14: Measurement Possibilities – Vacuum
Now let me describe an experiment that illustrates the importance of measurements under vacuum or reduced pressure.
Oils are often used as plasticizers in elastomers. Usually, decomposition of the elastomer begins before the oil has completely vaporized. The two effects overlap in this temperature range. This makes it difficult to quantify the oil content. In such cases, it is advantageous to measure the elastomer samples under vacuum in order to separate the two effects.
In this example, the dotted lines display the weight loss curves of SBR samples with and without oil at ambient pressure. The continuous lines show the corresponding curves measured under vacuum at a pressure of one-point-two kilo-pascals (1.2 kPa).
We notice that vacuum hardly influences the measurement curve of SBR without oil. In contrast, when SBR with oil is measured under vacuum, the vaporization of the oil and decomposition of the elastomer are almost completely separated.
Here again, the steps are more clearly separated in the DTG curve. At ambient pressure, the two peaks overlap whereas under vacuum, the peaks are well separated.
Slide 15: Why Use TGA/DSC?
The four examples just presented illustrate the wide application range of the TGA/DSC technique. This slide summarizes the main areas of application.
One important application is compositional analysis, for example the content of volatiles, polymers, carbon black, or fillers in polymers.
In the polymer field, it is important to know the temperature range in which the polymer is stable and the range in which decomposition occurs.
The TGA/DSC technique provides information about the purity of the sample if the stoichiometry of the reaction is known.
The results of isothermal or temperature-ramped measurements can be used to determine reaction kinetics.
Other applications include the investigation of desorption or adsorption processes, evaporation behavior, and the influence of reactive gases.
Evolved gas analysis of decomposition products using hyphenated techniques such as mass spectrometry or Fourier transform infrared spectroscopy provides additional information about the sample.
Slide 16: Industries and Applications
TGA/DSC has very many potential applications and is used in a wide range of industries. The slide presents an overview of the different industries and applications.
Thermogravimetric analysis provides quantitative information on the composition and thermal stability of many types of materials. The method is fast and can be used with very small amounts of sample.
The DSC signal can also be quantitatively evaluated, allowing glass transition and melting temperatures to be determined.
The TGA/DSC 1 is an exceptionally versatile instrument for the characterization of physical and chemical material properties under precisely controlled atmospheric conditions. It provides valuable information for research, quality control, and development.
I would now like to present several different application examples that demonstrate the analytical power and versatility of the TGA/DSC technique.
Slide 17: TGA/DSC 1: Application 1 Pyrolysis, carbon black
Polymeric materials often contain small amounts of carbon black for coloration and stabilization. The carbon black content can be determined by switching to a reactive gas after pyrolysis of the polymer.
The example shows the analysis of a polyethylene sample. The atmosphere was switched from nitrogen to air at six hundred degrees Celsius (600 °C). Evaluation of the pyrolysis weight loss step yields a polymer content of ninety-seven-point-seven percent (97.7%). The carbon black content is two-point-one percent (2.1%).
The DSC curve shows the endothermic melting peak of polyethylene at about one hundred and thirty degrees (130 °C). Pyrolysis of the polyethylene produces an endothermic peak. This is followed by the strongly exothermic combustion of carbon black immediately after switching to oxygen.
Slide 18: TGA/DSC 1: Application 2 Kaolin
Kaolin is a white mineral used in the paper industry, as a filler in plastics, and for the manufacture of porcelain. The example shows the measurement of three samples of kaolin containing different contents of kaolinite.
Chemically, kaolinite is a hydroxide of aluminum silicate and is the main constituent of kaolin. Kaolinite dehydroxylates between four hundred and fifty and six hundred degrees Celsius (450 °C and 600 °C) and is responsible for the weight loss steps in the TGA curves.
The DSC curve of Kaolin A shows a small peak at about five hundred and seventy-five degrees (575 °C). This peak is characteristic for the solid-solid transition of alpha (a-quartz) to beta quartz (b-quartz). The exothermic peak at about one thousand degrees (1000 °C) is due to the formation of mullite.
Slide 19: TGA/DSC 1: Application 3 Pseudopolymorphism
In a hermetically sealed crucible, glucose monohydrate melts in its water of crystallization without weight loss. The first peak in the DSC curve resembles the melting of a eutectic mixture of glucose and water. The excess glucose then melts up until about one hundred and thirty degrees Celsius (130 °C).
In an open crucible, the substance behaves very differently: The water of crystallization vaporizes between fifty (50 °C) and one hundred and forty degrees (140 °C), resulting in a broad endothermic peak in the DSC curve. The anhydrous glucose then begins to melt at about one hundred and forty-five degrees (145 °C). The mass loss of nine-point-one percent (9.1%) corresponds to the stoichiometrically expected value.
Slide 20: TGA/DSC 1: Application 4 MaxRes
This slide shows an example of the use of the MaxRes software option.
When MaxRes is activated, the heating rate varies automatically depending on the rate of change of weight. This enables overlapping weight loss steps to be optimally separated in the shortest possible time. The example shows the dehydration of copper sulfate pentahydrate. At a constant heating rate of twenty-five Kelvin per minute (25 K/min), the first two weight loss steps are not properly separated. Using MaxRes, the separation is better in a shorter measurement time than at the relatively low heating rate of five Kelvin per minute (5 K/min).
MaxRes saves time and is a good tool for method development.
Slide 21: TGA/DSC 1: Application 5 T up to 1600 °C
In this application example, the full temperature range up to sixteen hundred degrees Celsius (1600 °C) was used.
In the TGA curve, we see that gypsum (calcium sulfate dihydrate) loses its water of crystallization below three hundred degrees Celsius (300 °C) to form calcium sulfate. The calcium carbonate present as an impurity decomposes at about 700 °C. Decomposition of the calcium sulfate occurs in several steps from about 1200 °C onward.
The simultaneously recorded DSC curve shows two additional effects due to solid-solid transitions. These are the transitions from gamma to beta calcium sulfate at about three hundred and ninety degrees (390 °C) and from beta to alpha calcium sulfate at twelve hundred and thirty six degrees (1236 °C). The latter compound melts slightly below 1400 °C and is observed as a sharp endothermic peak.
Slide 22: TGA/DSC 1: Application 6 Mass spectrometry
This slide shows an example from the pharmaceutical industry that uses mass spectrometry.
Many active pharmaceutical substances are recrystallized from solvents. As a result, residues of solvents often remain in the product. A hyphenated technique such as a TGA-MS combination is ideal for detecting and identifying such undesired residues.
In this example, the active substance was recrystallized from methanol and acetone. The presence of these two solvents is confirmed by the peaks in the em-over-zee forty-three (m/z 43) and em-over-zee thirty-one (m/z 31) emm ess (MS) fragment ion curves.
The results indicate that the weight loss step at 200 °C is almost entirely due to the elimination of acetone.
Slide 23: TGA/DSC 1: Application 7 Curie transitions
The upper and lower diagrams illustrate the two main methods used to calibrate and adjust the TGA/DSC 1.
The upper diagram illustrates temperature and heat flow adjustment. This is normally performed using certified pure metals such as indium, zinc and aluminum. Gold and palladium can be used to calibrate and adjust the temperature and heat flow up to the maximum temperatures specified for the TGA/DSC 1 furnaces (1100 °C and 1600 °C).
The lower diagram shows the use of the Curie transition temperatures of certain ferromagnetic metal or alloy reference materials to calibrate the TGA temperature scale. The reference material is measured in a magnetic field and shows an apparent loss of weight at the Curie temperature.
Slide 24: Summary: TGA/DSC 1
Thermogravimetric Analysisis an excellent technique for characterizing the thermal properties of materials such as plastics, elastomers and thermosets, mineral compounds and ceramics as well as for chemical and pharmaceutical products.
The slide summarizes the features and benefits of the TGA/DSC 1. The METTLER TOLEDO TGA/DSC 1 instrument simultaneously measures weight loss and heat flow.
For high-throughput applications, a sample robot allows easy automation of entire sample series. Even different types of crucibles can be measured with individual temperature programs.
The modular concept of the TGA/DSC 1 permits options such as the sample robot, gas box or hyphenated techniques to be added later on if required.
Flexible calibration procedures allow the instrument to be calibrated and adjusted over the entire temperature range from room temperature to 1600 °C.
The TGA/DSC 1 is the ideal instrument for manual or automated operation in production, quality assurance or research and development.
Slide 25: For More Information on TGA/DSC
Finally, I would like to draw your attention to information about thermogravimetric analysis that you can download from the Internet.
METTLER TOLEDO publishes articles on thermal analysis and applications from different fields twice a year in UserCom, the well-known METTLER TOLEDO biannual technical customer magazine. Back issues can be downloaded as PDFs from www.mt.com/usercoms.
The slide lists a number of UserCom articles on thermogravimetric analysis.
In addition, you can download information about webinars, application handbooks or information of a more general nature from the Internet addresses given at the bottom of the slide.
Slide 26: For More Information on TGA
Information about application handbooks, webinars, training, or thermal analysis in general can be obtained from the internet addresses given on this slide.
Slide 27: Thank You
This concludes my presentation on thermogravimetric analysis.
Thank you for your interest and attention.