The aim of the current experiment was to show the use of the evolved gas profile and evolved group profile in the identification of degradation products.
The sample used was BHET (bis-hydroxylethyl terephthalate) commonly synthesized for industrial use by esterifying terephthalic acid (TPA) with ethylene glycol (EG). BHET typically polymerizes to polyethylene terephthalate (PET).
The measuring cell was a TGA/SDTA851e coupled to a Nicolet Nexus FTIR
Pan: Alumina without lid, 70 μl
Sample preparation: None. 18.122 mg
TGA measurement: heating from 50 °C to 900 °C at 10 K/min, with corrected blank curve
Atmosphere: Nitrogen at 80 ml/min
The degradation of BHETin two steps caused the appearance of two maxima in the DTG curve at approximately 300 °C and 450 °C, which were correlated to the peaks in the Gram-Schmidt curve.
Evolved Gas Profile
The evolved gas profile was used here to find out why the first weight loss occurred (as seen by the DTG peak at 300 oC). The FTIR spectrum at 300 oC was directly compared with the spectra from the database and this showed that the first component evolved upon heating was pure EG. A doublet seen between 2340 and 2360 cm-1 (A) was caused by the carbon dioxide residue in the system. Absorption bands seen between 4000-3500 cm-1 and 2000-1200 cm-1 are due to background moisture, or in other words, water.
Functional Group Profile
When the spectrum at 450 oC is directly compared with those in the database no single decomposition product could be picked out. The functional group profiles of several decomposition products that could possibly be evolved are shown above. The peaks, expectedly, correlate to those of the DTG curve.
Degradation products are identified by the absorption band measurement that corresponds to different chemical groups, as for instance those that are produced by C-O in esters, alcohols and carboxylic acids. Aromatic ring and ketone profiles failed to produce detectable or significant signals on measurement. The BHET side chains undergo cleavage when the temperature rises above 400 oC to yield hydroxy formic acid ester (CH-CO-O-(CH2)2-OH).
The weight loss steps lie close together are impossible using TGA alone, but TGA-FTIR allows the decomposition process to be studied with regard to the chemicals produced. When the analysis is performed at 300 oC, the spectrum shows that EG is lost leading to the first decomposition step. The second step is due to the cleavage of the side chains of the aromatic group by the elimination of the hydroxy formic acid ester. The current experiment shows how the evolved gas profile and the functional group profile techniques can be applied in thermal analysis to obtain complementary information.
This information has been sourced, reviewed and adapted from materials provided by Mettler Toledo - Thermal Analysis.
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