Characterization of the Composition and Decomposition of Carpet

Across the globe each year, billions of pounds worth of carpet is generated – a significant percentage of which is sent to landfills. This is an issue as nylon is not biodegradable.

The detriment caused to the environment by disposing of carpet waste in this manner, as well as limitations in the capacity of landfills, have contributed to the growing importance of recovering nylon from carpet waste.

Characterizing waste carpet composition as well as its thermal decomposition profile is pivotal to the recycling process. This is because the composition of carpets can differ significantly. They can contain Nylon-6 and/or Nylon-6, 6, in addition to a range of other materials like latex adhesives, polymer fibers (such as PE, PP, or polyester), inorganic fillers (such as CaCO₃ and BaSO₄)¹ and dyes.

It is possible to analyze a material’s thermal mass loss profile whilst simultaneously identifying the gaseous species evolved during the decomposition. This is courtesy of Thermogravimetric analysis (TGA) combined with evolved gas analysis (EGA) by MS, GC-MS, or FT-IR.

This study analyzed material which was recovered from carpet waste using TGA-MS, TGA-GC-MS, and TGA-FT-IR. The purpose was to compare the ability of each of these methods to identify the recycled carpet’s composition.

Experimental

With the use of a NETZSCH TG 209 F1 Libra thermogravimetric analyzer (TGA), which was paired with a BRUKER Optics TENSOR FT-IR spectrometer and a NETZSCH QMS 403 Aëolos quadrupole mass spectrometer (Figure 1), TGA-MS and TGA-FT-IR were performed.

Figure 1. NETZSCH STA 449 F3 Jupiter^® instrument simultaneously coupled to a BRUKER Optics TENSORTM FT-IR spectrometer and a NETZSCH QMS 403 C Aëolos^® quadrupole mass spectrometer.

Additionally, the NETZSCH TG 209 F1 Libra was paired with an Agilent Technologies 7890A gas chromatograph – which was fitted with an Agilent 5975C quadrupole mass spectrometer (QMS) as shown in Figure 2 – in order to perform TGA-GC-MS measurements.

Figure 2. NETZSCH TG 209 F1 Libra^® TGA instrument coupled to the Agilent 7890A gas chromatograph equipped with an Agilent 5975C quadrupole mass spectrometer (QMS).

In the thermobalance, the samples of recycled carpet were heated from 25 to 600° at 10 K/min under either helium (65 ml/min; TGA-GC-MS) or nitrogen (40 ml/min; TGA-MS and TGA-FT-IR).

A transfer line heated at 220 °C was used to pass evolved gases to the EGA analysis instrument from the thermobalance for the FT-IR and MS coupling.

For the GC-MS coupling, the transfer line was heated at 300 °C and every four minutes the gases were sampled and injected onto an Agilent HP-5MS column. This column was eluted with a helium gas flow of 2 ml/min and held at 150 °C.

In the case of the FT-IR and MS measurements, the gases were continuously introduced into an IR gas cell which was maintained at 200 °C. Alternatively, they were introduced directly into the MS analyzer.

Results and Discussion

TGA-FT-IR

In Figure 3, Mass loss (TGA) and mass-loss rate (DTG) curves are shown. There are also curves for the integrated intensity of the CO₂ asymmetric stretching band and the total integrated IR absorption (Gram Schmidt).

Figure 3. Results of the TGA-FT-IR analysis showing TGA (green), DTG (red), Gram Schmidt (black), and CO₂ IR absorption (pink) curves, and melting peak (blue) determined with c-DTA^®.

The peak rate of the single mass-loss step was observed at 436.6 °C. The CO₂ and DTG curves’ peaks are almost coincident, and are closely followed by the Gram Schmidt curve’s peak. A melting endotherm at 220 °C is also shown, which was determined with the NETZSCH patented c-DTA analysis.

Figure 4 depicts a 3-dimensional plot of the FT-IR spectra of the evolved gases for the duration of the thermal decomposition. A database of IR spectra was used as a standard against which to compare individual extracted spectra, as a means of identifying species evolved at varying temperatures during the thermal decomposition.

Figure 4. 3-D plot of FT-IR spectra of evolved gas from the sample pyrolysis.

As Figure 5 illustrates, the FT-IR spectra of gases which evolved at 460°C were consistent with those for Nylon-6 (PA6) and Nylon-6, 6 (PA66).

Figure 5. Results of database search of extracted FT-IR spectrum (red) of gases evolved at 460°C showing matches with PA66 (blue) and PA6 (purple).

TGA-MS

MS analysis was used to identify the evolution of CO₂, however, a search of extracted mass spectra from the acquisition in the NIST mass spectral library did not successfully identify organic species with any confidence.

Regardless of this, the peaks shown in Figure 6 of ion currents for mass numbers 55, 41, and 15, are consistent with Nylon-6. Furthermore, peaks in the ion currents of mass numbers 54 and 17 concur with Nylon-6, 6.

Figure 6. Overlay of TGA curve and MS ion currents for mass numbers 15, 17, 30, 41, 42, 44, 54, 55, 84, 97, and 113.

Figure 6 also shows currents for ion masses 44, 30, and 27. During the process of decomposition, these also exhibit peaks. However, the ions produced are common to both polymers.

Although no peaks were seen in the ion current for masses 84 (cyclopentanone) or 113 (caprolactone), this was anticipated as these ions are not normally produced when using electron impact mass spectrometric (EIMS) analysis.²

TGA-GC-MS Analysis

A quasi-continuous manner of sampling was performed during the TGA-GC-MS analysis. This was ensured by sampling gases which evolved during the pyrolysis of the sample at four minute intervals.

An overlay of the total ion chromatogram (TIC) from the GC-MS measurement is shown in Figure 7 with the thermal mass loss curve. In Figure 8, an expanded view of the TIC is displayed. The peak identifications illustrated were established via library searches of the extracted mass spectra.

Figure 7. TGA curve (green) and TIC (red) from quasi-continuous mode GC-MS analysis of evolved gases.

Figure 8. Expanded view of TIC from GC-MS analysis with labeling of identified peaks.

A major component of the evolved gases was caprolactam, which is a major decomposition product of Nylon-6. Caprolactam began appearing in gas sampling at roughly 400 °C and continued appearing in pulses until approximately 500 °C.

The presence of CO₂ in gas samples from 400 °C to 480 °C was consistent with the results produced by both the TGA-MS and TGA-FT-IR methods.

A range of different organic species which were not located by either FT-IR or MS analysis were able to be identified during GC-MS analysis. This was courtesy of the chromatographic separation of gaseous components during the process, as shown in Figure 9. Cyclopentanone is a thermal decomposition product which is particularly characteristic of Nylon-6,6.

Figure 9. Results of library searches of mass spectra extracted from peaks in GC-MS. Extracted spectra are in red and library spectra are in blue.

Conclusion

There are strengths and weaknesses specific to each of the evolved gas analysis methods outlined above. These factors make each method more suited to a specific application.

Of all the methods, GC-MS is in general the most informative. This is because of the chromatographic separation of gaseous components, which allows for their individual identification.

GC-MS provided the clearest identification of caprolactam in this study, delivering confirmation that the material primarily consisted of Nylon-6. It was also able to identify nitrile and cyclopentanone products which are more characteristic of Nylon-6, 6.

This study also provided the first identification of a range of other cyclic organic species which are potentially products of Nylon-6,6.

The presence of both nylon polymers in the recycled carpet material was confirmed by the FT-IR and EIMS (electron impact mass spectrometry) results. EIMS also identified molecular ion masses which are characteristic of both Nylon-6 and Nylon-6, 6.

Both polymers were identified as potential components of the material by the FT-IR method. However, this evolved gas analysis method was the least definitive at identifying which specific nylon polymers were actually present, as a result of the similarities between the spectra.

As this study demonstrates, the combination of evolved gas and thermogravimetric analyses (TGA-EGA) constitutes a time-saving, informative analytical tool. This is useful for simultaneously determining the materials’ chemical compositions and thermal decomposition profiles. It can also establish which chemical processes are responsible for thermal mass loss by identifying the corresponding evolved gas species.

References and Further Reading

1. C. Mihut, D. K. Captain, F. Gadala-Maria, and M.D. Amiridis. “Review: Recycling of Nylon from Carpet Waste”, Polymer Eng. Sci., Vol. 41(9), pp. 1457-1470, 2001.
2. T. Arii, K. Motomura, and S. Otake. “Evolved Gas Analysis Using Photoionization Mass Spectrometry, EGA-PIMS: Characterization of Pyrolysis Products from Polymers”, J. Mass Spetrom. Soc. Jpn., Vol. 59(1), pp. 5-11, 2011.
3. S. V. Levchik, E. D. Weil, and M. L. “Review: Thermal decomposition of aliphatic nylons” Polym. Int., 48(7), pp. 532-557, 1999.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.