TGA

 Thermogravimetric analysis (TGA) is a common thermal analysis technique that provides composition information for polymeric materials. Most often, we associate TGA with quantitative data. However, I was reminded during a recent material analysis that TGA can also provide insight into the qualitative analysis.

I was analyzing a rubber O-ring. My first test, as usual, was Fourier transform infrared spectroscopy (FTIR). The FTIR indicated that the material was a nitrile rubber (NBR) compound. Additional absorption bands associated with aluminum silicate clay were also present. Weak bands indicated an ester-based plasticizer. The general form of the spectrum was suggestive of a moderate loading of carbon black.

I conducted the TGA analysis and found all of the expected weight loss events for the quantification of the plasticizer, polymer, and carbon black, as well as the residue for the mineral filler. However, one additional weight loss stood out. A weight loss of 4.4% centered at approximately 280 C. The temperature and relatively sharp nature of the weight loss were characteristic of dehydrohalogenation – in this case, the evolution of HCl from poly(vinyl chloride) (PVC). PVC is often added to NBR rubber compounds to modify the mechanical properties of the rubber and increase the ozone resistance and improve the weathering resistance.

This example illustrates the power of thermogravimetric analysis, both quantitative and qualitative.