TGA-FTIR Combination for The Investigation of Sealing Rings - METTLER TOLEDO

TGA-FTIR Combination for The Investigation of Sealing Rings

The measurements show that a qualitative and quantitative classification with regard to the thermochemical properties of the materials is possible.

The combination of TGA and IR spectroscopy facilitates the comparison of materials with given tolerances in quality assurance and in the control of incoming material. The interpretation of the TGA curves is greatly enhanced by the (spectroscopic) separation of the evolved gases into individual components. 

In addition, this enables a separation of overlapping decomposition reactions to be made, which in turn allows a better insight into the mechanism and kinetics of the decomposition processes.

 

Introduction

Thermogravimetric Analysis (TGA) is a well-established method that is used in quality assurance and quality control for the characterization of the thermal behavior of a very wide range of substances. With TGA alone, however, it is not possible to learn anything about the composition of the volatile substances evolved from a sample. The online combination of a TGA with a Fourier Transform Infrared Spectrometer (FTIR), however, enables both quantitative (TGA) and qualitative (FTIR) analysis to be performed simultaneously. The technique allows the substances evolved to be identified and correlated with the weight-loss steps detected by the TGA.  

 

The TGA-FTIR Combination

The METTLER TOLEDO TGA/SDTA851e measuring module is interfaced to an FTIR spectrometer (Nicolet Analytical Instruments Nexus or Protege 460).

The interface consists of a thermostated gas transfer line that connects the TGA to a thermostated gas cell mounted in the spectrometer. The TGA purge gas flushes the gases evolved from the sample through the transfer line into the gas cell. The IR absorption of the gases is measured in the mid-infrared region. Only a small volume of gas is required for the measurement. This guarantees a high spectrometric detection sensitivity and ensures that the TGA and IR signals coincide.

Apart from the entire IR spectrum, characteristic wavenumber regions can also be selected [1,2], and the changes in the spectrum observed as a function of time or temperature. This method allows the gas mixture evolved to be examined selectively for different components. In this way, relative concentration profiles of the individual components (chemigrams) can be obtained.

The TGA/SDTA851e module allows a large variation of the purge gas flow rate. This enables the performance of the TGA-FTIR combination to be optimized. With an otimum setting of the flow rate, a good compromise between a sufficiently high product concentration and a short residence time of the gases in the cell can be achieved.

 

Application Example: The Thermal Stability of Sealing Rings

The task was to investigate the thermal degradation of sealing rings (elastomers) from two different manufacturers both qualitatively and quantitatively, in order to obtain information about the onset temperature of the initial degradation and any further degradation steps, and to identify the volatile components evolved. The samples were measured in the temperature range 30 °C to 900 °C at a heating rate of 10 K/min with a nitrogen purge gas flow rate of 40 ml/min. The sample weights used were 27.52 mg and 37.41 mg. The gas cell and the transfer line were thermostated at 200 °C. 

 

Figure 1 shows the results for sealing ring 1. In this example, the derivative of the weight loss curve, the DTG curve, was used to identify the individual TG steps. Each step in the TG curve gives rise to a peak in the DTG curve. The TG curve was found to consist of six steps namely at 130 °C, 320 °C, 460 °C, 510 °C, 650 °C and 880 °C.

The uppermost diagram shows the chemigrams of H2O, HCN, CO2 and cyclohexane. The very small loss of weight at about 130 °C is due to the evaporation of water (0.06%). Afterwards, the decomposition of the nitrile groups leads to the elimination of HCN (0.6%). At the largest decomposition step (53% at 460 °C) HCN, CO2 and above all cyclohexane are liberated.

The latter is formed as a cracking product of the thermal degradation of the polymer chain. The elimination of CO2 at 650 °C and 890 °C is a result of the decomposition of the filler components (dolomite).

The curves corresponding to the thermal degradation of sealing ring 2 are shown in Figure 2 (the presentation is analogous that in Figure 1). In this sample, only three significant decomposition steps at 220 °C, 330 °C and 470 °C are observed. Afterwards there is a continuous but very gradual loss of weight (1.2%). The chemigram of CS2 is shown instead of that of H2O. The very weak loss of weight detected in the TGA curve at 220 °C (1.3%) corresponds to the formation of carbon disulfide.

This results from the decomposition of sulfur compounds formed during vulcanization. HCN and CO2 are liberated during the second decomposition step (8.1% at 330 °C). Later on, the liberation of cyclohexane begins at about 390 °C in the third decomposition step (53% at 470 °C). A comparison of the results of the two sealing rings shows differences with regard to the composition of the elastomer and the fillers. The degradation of the polymer chain at about 480 °C is however almost identical in both samples. Carbon disulfide does not occur as a decomposition product of sealing ring 1. 

Summary

The measurements show that a qualitative and quantitative classification with regard to the thermochemical properties of the materials is possible. The combination of TGA and IR spectroscopy facilitates the comparison of materials with given tolerances in quality assurance and in the control of incoming material. The interpretation of the TGA curves is greatly enhanced by the (spectroscopic) separation of the evolved gases into individual components. In addition, this enables a separation of overlapping decomposition reactions to be made, which in turn allows a better insight into the mechanism and kinetics of the decomposition processes. 

TGA-FTIR combination for the investigation of sealing rings | Thermal Analysis Application No. UC 102 | Application published in METTLER TOLEDO Thermal Analysis UserCom 10