Analyzing the Layers of a Cable Used in the Automotive Industry using TG-IR. The aim of this analysis was to identify the polymer used for forming the cable’s external sheath.
The cable is composed of five concentric layers disposed in the following sequence: rubber, fabric, rubber, fabric, rubber. Given that the cable’s polymeric material is filled with carbon black, detecting this material with infrared (IR) spectroscopy was not applicable. The TG-IR method was used, which helps in identifying a material based on its decomposition products, thus providing a complete, interference-free analysis even when carbon is present.
TG-IR Analysis – Sample Preparation
The TG-IR analysis eliminates the need for a specific sample preparation. Instead, a small portion of the external rubber sheath is removed using a sharp scalpel. Normally, when gas emitted in small quantities needs to be identified, care must be taken to ensure that the sample mass used for TG-IR analysis ranges from 10 to 30mg. If the sample is too small, it can result in false analysis, and if the sample is too large, it can cause pyrolysis or incomplete combustion of that section of the sample, which is not experiencing the gas flow.
TG-IR Analysis – Experimental
First, the sample is heated at 20 °C/min under the nitrogen flow of 100mL/min. Throughout the TGA analysis process, spectra of the gas emitted by the sample are obtained with a spectral resolution of 4cm-1 and at a sampling frequency of 1 spectrum every 4 seconds.
IG-IR Analysis – Discussion of the Results
The intensity profile of the gases emitted by the sample as well as the TGA analysis is discussed in detail. Here, the following major weight losses were observed:
- The first around minute 15
- The second around minute 24
- The third around minute 29
Three TG-IR peaks are identified along with the weight loss. Therefore, as the sample releases the gas, it continues to degrade. Here, the details of the acquired spectra are thoroughly analyzed, followed by the detection of the emitted gases.
Ten minutes after start up of the analysis (T = 200 ˚C) there is a slight decline in the derived curve: from this point onward the spectra show the stretching band of the –CH2 and –CH3 (peak A) groups; a limited CO2 generation reaching a maximum at minute 15 (Figures 1 and 2) is also observed.
Figure 1. Released gases
Figure 2. Peak A shows the stretching of –CH2 and –CH3 groups; and CO2 peak.
It is possible that the CO2 peak derives from the CO2 loss of the polymer’s vinyl-acetic group, and the peak corresponding to the organic part could occur either due to a premature decomposition of the polymeric matrix or due to the evaporation of an additive. This last hypothesis would also appear to be supported by the fact that immediately after, a marked generation of HCl occurs.
Figure 3. Intensity profiles of released gases.
Figure 4. HCl spectrum superimposed on Peak A
Figure 5. HCl band after Peak A subtracted
The intensity profiles with respect to HCl and peak A are compared in Figure 3, and a HCl spectrum overlaid on the peak A as it appears prior to data processing is shown in Figure 4. The HCl band following the subtraction of peak A is shown in Figure 5. During analysis, the generation of HCl shows that the sample indeed contained chlorine.
The analysis of the gases that are produced during the second major weight loss is considered. This weight loss occurs at minute 24 at approximately 500°C. At this temperature, the polymer significantly decomposes, and releases different types of gases as a result of chain fragmentation. Here, the presence of ethylene, methane, and medium-short alkyl chain fragments can be observed. The spectra of these gases are partly overlapping. Identification of the chemical species can be seen in Figures 6 and 7.
Figure 6. Atlas of spectra
Figure 7. Gas profiles superimposed.
The spectra of the spectra atlas as well as the spectrum recorded at minute 24.4 are shown in Figure 6, while the profiles corresponding to individual gases are illustrated in Figure 7.
Figure 8. CO2 profile
Figure 9. Gas profiles of CO2 and traces of CO
As soon as the gas emission is completed, i.e., from 27 minutes to the end of the analysis, CO2 emission and CO traces can be identified (Figures 8 and 9). The Pyris™ 1 TGA was used to perform all these experiments. PerkinElmer provides both IR and Thermal Analysis for better and absolute characterization of samples from one supplier. Figures 10 and 11 show a few graphical representations of the analyses.
Figure 10. False color map relative to the generation of hydrochloric acid.
Figure 11. Stack plot centered on the band at 2950 cm-1, between 21.5 and 28.0.
This information has been sourced, reviewed and adapted from materials provided by PerkinElmer.
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