Material Characterization With Complete Compositional Analysis Using Thermogravimetry

Consistent high performance demanded from modern materials has been a topic of discussion for both technical and business individuals for many years. The globalization of the supply and manufacturing chains has increased the need to thoroughly characterize and define processing parameters for critical materials and parts in almost every field, albeit some more than others. Disappearing are the days of on-the-fly formulation adjustments, assumption of consistent quality ingredients, and simple random spot checks as the global marketplace pressures suppliers to deliver high-quality performance at a competitive price.

Evolution of compositional analysis

Along with these economic forces, consider the evolution of science and instrumentation of compositional analysis. In the early days, small muffle furnaces and offline weighing gave us simple ash tests. The space race of the 1960s and our dream to go to the moon fueled the development of the first commercially available thermogravimetric analyzer (TGA) in 1964, the METTLER TA 1 (METTLER Instrument GmbH, Greifensee, Switzerland). Clearly, this enabled researchers to assess a myriad of new materials that needed to perform in the harshness of space. Today it is the automated, robust, and versatile coupled TGA system with Fourier transform infrared (FTIR) analysis and mass spectrometry (MS) analysis that offers both the quantification and identification that makes complete compositional analysis a key technology tool in many industries.

Application areas for thermogravimetric analysis include plastics, elastomers, thermosets, minerals, and ceramics, as well as a wide range of materials in the chemical and pharmaceutical industries. The power of a TGA is only limited by its effective continuous resolution range to detect the smallest mass changes. The determination of trace compounds in bulk materials (e.g., 2% carbon black in polymer tubing) is usually a difficult and demanding task for most instruments on the market and definitely requires superior balance technology. Furthermore, in the pursuit of constantly increasing efficiency in the laboratory, reliable and robust automation has become quite common in the last two decades.

Thermogravimetric analyzer performance parameters

Figure 1 – METTLER TOLEDO TGA 1.

The METTLER TOLEDO TGA 1 (Figure 1) is the culmination of almost 50 years of innovation to successfully perform a variety of material characterization tasks in production, quality assurance, and research and development. There are a number of performance parameters that are critical to a superior performing thermobalance:

  • Extremely high resolution
  • High accuracy
  • Outstanding precision
  • Wide continuous measurement range
  • High capacity.

METTLER TOLEDO TGAs are the only thermogravimetric analyzers on the market that can measure up to 50 million resolution points continuously down to 0.1 μg for a 5-g sample weight. This means that small and large samples can be measured with the same high resolution without having to change the weighing range. Excellent measurement performance across the entire weighing and temperature range requires that the balance is housed in a well-insulated, gas-purged, temperature-controlled chamber, which is isolated from the furnace. Two internal calibration weights guarantee that the balance is always properly adjusted The TGAs feature a weighing accuracy of 0.005% and a weighing precision of 0.0025%.

An example that illustrates the importance of weighing accuracy is the measurement of mascara for quality control purposes in the cosmetics industry. Some formulations contain very small amounts of compounds that have to be quantified accurately if the complete composition needs to be determined. Figure 2 displays a high-sensitivity carbon black determination in a 1-mg sample of mascara, a well-known cosmetic product. The evaluation shows a weight loss of 23 μg after gas switching from nitrogen to air, which corresponds to the decomposition of carbon black into carbon dioxide. Clearly, this indicates that very low amounts could easily be detected.

Figure 2 – High-sensitivity carbon black determination in a mascara cake sample. Heating was from ambient temperature to 600 °C in nitrogen, then switching to air atmosphere. A weight loss of 23 μg was determined.

Wide continuous measurement range

The measurement of inhomogeneous or low-density samples often requires large sample amounts and correspondingly large sample volumes to be prepared. In order to be able to measure these, METTLER TOLEDO TGAs can be equipped with a large furnace that allows the use of high-volume crucibles. Figure 3 shows the compositional analysis of a very low-density material, i.e., silicified microcrystalline cellulose (SMCC) containing a nominal content of 98% microcrystalline cellulose (MCC) and 2% colloidal silicon dioxide (CSD) In order to measure a sufficiently large amount of this pharmaceutical excipient for incoming compound conformity tests, it was necessary to prepare the sample in a 900-μL aluminum oxide (Al2O3) crucible. The first weight loss step is due to evaporation of water; the second and third steps are due to decomposition of organic compounds. The residue at 950 °C is the SiO2 that remains as a residue.

Figure 3 – Analysis of very low-density material silicified microcrystalline cellulose (SMCC). In order to prepare enough material, a 900-μL aluminum oxide crucible is required. Step 1: evaporation of water; steps 2 and 3: decomposition of organic material; residue: silicon dioxide.

Evolved gas analysis

As stated above, complete compositional analysis with a coupled TGA system provides a clear view into not only the decomposition profile of materials, but also the chemical nature of the evolved gases. This is an extremely powerful analytical technique for determining the real cause of failure of materials as well as for competitive analysis.

The example given in Figure 4 involves coupling the TGA 1 to an infrared spectrometer (FTIR) via a heated transfer line and comparatively investigates two similar automotive sealing rings, one working well while the other failed. Although the recorded changes in mass as a function of temperature can be invaluable to fingerprint the composition, they do not offer insight into the complete chemical nature of the elastomer system. This additional information is obtained from a simultaneously recorded series of infrared spectra of the gas phase chemicals. It is these critical data that identify the inherent chemical differences that result from small changes in formulation, additives, or fillers.

Figure 4 – Analysis of two sealing rings: The TGA curves show similar volatile and polymer content, but an additional weight loss between 800 and 900 °C is detected in only one sample. The FTIR chemigrams confirm the presence of C–S bonds in the failed rings due to vulcanization, which is missing in the good rings.

The Gram-Schmidt curves shown in Figure 4 are the quantitative measure of the total IR absorption, and demonstrate how the concentration of the evolved gases varies with time. The chemigram for CS2 shows a clear difference in the type of cross-linking materials used in the sealing rings. The failed rings were vulcanized with sulfur, while the good rings were cross-linked with peroxide. Between 800 and 900 °C we see a decomposition step of an inorganic filler (i.e., chalk) present in the good sample, which is confirmed by the elimination of carbon dioxide as a peak in the IR chemigram CO2.

Efficient and reliable automation

The high throughput that is required for rapid quality control screening (e.g., in production environments) can be achieved by using an automatic sample changer. All METTLER TOLEDO TGAs can be automated with a 34-position autosampler. This robot has a simple and rugged z-axis design that guarantees error-free operation around the clock, day in and day out. This TGA sample robot is recognized industry-wide as the most reliable sample changer available on the market. Productivity is not the only reason automation has become commonplace over the last two decades.

Figure 5 – Automatic lid-piercing accessory. Schematic drawing showing the moment when the needle pierces the high lid of a hermetically closed crucible. The needle never comes in contact with the sample, thus avoiding contamination.

Furthermore, some samples lose moisture, take up moisture, or have a high tendency to oxidize between preparation and the start of measurement. In order to prevent this, a sample robot should be able to open the crucible immediately before measurement.

With a special accessory, the sample robot can remove the protective crucible lid from a ceramic crucible or pierce the lid of hermetically sealed aluminum crucibles immediately before measurement (see Figure 5). The efficient design and extra-high crucible lid stop the needle from coming into contact with the sample. This prevents possible contamination of the next sample.

Summary

As the pace of technology and the global market continue to accelerate, the best tools are required to quickly and efficiently solve complex material problems and maintain a competitive edge. Material characterization with complete compositional analysis using a coupled TGA/gas analyzer system is a key technology to overcome many technical and production challenges. This enables the user to efficiently and successfully perform verification of incoming materials, proper formulation, as well as competitive and failure analysis.

Dr. Matthias Wagner, Ph.D., is Product Manager, Materials Characterization, METTLER TOLEDO AG, Schwerzenbach, Switzerland. Michael R. Zemo is Market Manager, Materials Characterization, METTLER TOLEDO LLC, 1900 Polaris Pkwy., Columbus, OH 43240, U.S.A.; tel.: 614-438-4686; e-mail: Michael.Zemo@mt.com.

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